STUDIES ON LASER ABLATION OF POLYMER COATED PROPELLANT FILMS 
 
 
 
Except where reference is made to the work of others, the work described in this thesis is 
my own or was done in collaboration with my advisory committee. This thesis does not 
include proprietary or classified information. 
 
_______________________ 
Artem Dyachenko 
 
 
 Certificate of Approval: 
 
 
 
 
German Mills 
Associate Professor 
Chemistry and Biochemistry 
 
Rik Blumenthal, Chair 
Associate Professor 
Chemistry and Biochemistry 
 
 
Andreas J. Illies 
Professor 
Chemistry and Biochemistry 
 
Stephen L. McFarland 
Acting Dean 
Graduate School 
STUDIES ON LASER ABLATION OF POLYMER COATED PROPELLANT FILMS 
 
 
 
 
 
Artem Dyachenko 
 
 
 
 
 
A Thesis 
Submitted to 
 The Graduate Faculty of 
 Auburn University 
in Partial Fulfillment of the 
Requirements for the 
Degree of 
Master of Science 
 
 
 
 
 
 
 
 
 
Auburn, Alabama 
August 7, 2006
 iii
STUDIES ON LASER ABLATION OF POLYMER COATED PROPELLANT FILMS 
 
 
 
 
 
Artem Dyachenko 
 
 
 
 
 
Permission is granted to Auburn University to make copies of this thesis at its discretion, 
upon request of individuals or institutions at their expense. The author reserves all 
publication rights. 
 
 
 
 
 
 
  
Signature of Author 
  
  
Date of Graduation 
 
 iv
VITA 
 
 
 
 Artem Aleksandrovich Dyachenko, son of Alexander and Elena, was born in 
Moscow, Russia on December 22, 1979. Upon graduation from High School in June, 
1996, he entered the Physical Chemistry Department at Russian University of Chemical 
Technology where he graduated from in March, 2003 with a Bachelor of Science degree 
in Chemical Engineering. Artem began his graduate studies at Auburn University, 
Auburn, Alabama in August, 2003. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 v
THESIS ABSTRACT 
 
STUDIES ON LASER ABLATION OF POLYMER COATED PROPELLANT FILMS 
 
 
 
Artem Dyachenko 
 
 
Master of Science, August 7, 2006 
(B.S., Russian University of Chemical Technology, 2003) 
 
 
69 Typed Pages 
 
 
 
Directed by Rik Blumenthal 
 
 
 
In artillery, safety issues make propellants highly preferable as compared to 
ordinary explosives. However, the same properties that make propellants safer, such as 
their low shock sensitivities, also make them more difficult to ignite directly. In large 
bore artillery shells, ignition of the propellant is achieved using a small amount of an 
ordinary explosive, such as lead azide, as the primer. This sequential ignition process 
results in irreproducible delays that prevent targeting of fast moving objects. Numerous 
attempts have been made to develop improved ignition processes. Laser ignition would 
overcome the ignition delay problem and be practical. Unfortunately, all of the 
propellants used today cleanly ablate under laser irradiation, and ignition cannot be 
 vi
achieved. Electrothermal Chemical (ETC) ignition has proved capable of overcoming the 
ignition delay problem, but it is not practical. In this method, a large capacitor is 
discharged across a small piece of plastic, generating a high-pressure, high-temperature 
plasma that ignites the propellant after a short and highly reproducible delay. 
Unfortunately, the capacitor and related power supply to charge it are too large and heavy 
for practical application of ETC ignition. In this work, a new approach to ignition is 
developed based on the generation of a high-temperature, high-pressure plasma, directly 
at the propellant surface, that has a chemical composition similar to that of the ETC 
igniter, using laser ablation of the plastic coating on the propellant. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 vii
ACKNOWLEDGMENTS 
 
 First of all, I would like to express my deepest appreciation to my advisor Dr. Rik 
Blumenthal who provided me with valuable knowledge, academic guidance and support. 
I would like to thank my group members, especially Rodney Valliere who has been a 
great help, as well as Huijiao Sun who recently joined our group. I thank James Black for 
his assistance with polymer coating and Dr. German Mills for providing his laboratory 
facilities. I acknowledge Army Research Office for financial support of this project. I 
thank all my committee members for taking their time reading this thesis. Last but not 
least, I am grateful to the Faculty of the Department of Chemistry and Biochemistry for 
their teaching and for giving me the opportunity to gain valuable teaching experience. 
 
 
 
 
 
 
 
 
 
 viii
Style manual or journal used: American Chemical Society style 
Computer software used: MS Word 2003, MS Excel 2003, Adobe Photoshop CS2 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 ix
TABLE OF CONTENTS 
 
 
 
 
LIST OF FIGURES ???????????????????????... xi
I. INTRODUCTION 
1. Propellants??????????????????.......................... 1
2. Ignition Techniques..??????..???????............................ 3
3. Laser Ablation 
1. Introduction 10
2. Laser Ablation Mechanisms????????????????. 12
3. Laser Ablation of Polymers????????????????.. 15
4. Laser Ablation of RDX Films 20
4. Thin Plastic Films 
1. Deposition Methods...??.??????????????....?.. 22
2.   Thin Film Characterization ?????????.............................. 25
II. EXPERIMENTAL SECTION 
1. Thin Plastic Films Preparation 
1. Introduction ??????????????.................................. 27
2. Experimental Setup...???????????????????. 30
 x
3. Sprayed-on Films...??????????????????..? 32
4. Spin-coated Films...??????..................................................... 36
2. Laser Ablation of RDX Films 
1. Deposition of RDX Films....??????????........................ 40
2. Calibration??????????????.................................... 42
3. Experimental Setup???????????????????.. 45
4. Results and Discussion?????????.????????.... 48
REFERENCES...????????????????????????.... 54
 
 
 
 
 
 
 
 xi
LIST OF FIGURES 
 
 
1.1   a) RDX molecule; b) HMX molecule................................................................... 2
1.2   Ordinary fuse cap.................................................................................................. 3
1.3   Large bore artillery shell....................................................................................... 5
1.4   Energy-level diagram for a hypothetical bond A-B.............................................. 13
1.5   Various energy pathways in UV processing of polymers..................................... 14
1.6   Laser Ablation Plume............................................................................................ 16
1.7   Schematic impact of laser pulse on polymer surface............................................ 17
1.8   a) Ablation of material; b) Sample after ablation.................................................. 20
1.9   Spin-coating technique.......................................................................................... 22
1.10 Contact angle......................................................................................................... 25
2.1   Water droplets placed on a microscope slide covered with a polycarbonate 
        film........................................................................................................................ 28
2.2    Polycarbonate film with pinholes......................................................................... 28
2.3   SEM images of polyethylene films....................................................................... 29
2.4   Sprayer.................................................................................................................. 30
2.5   Spraying technique................................................................................................ 31
 xii
2.6   Spin-coater WS-400 Process Controller............................................................... 32
2.7   Polyethylene films sprayed at room temperature and at ~100 ?C........................ 33
2.8   The thickness of sprayed on polyethylene films as a function of spraying 
        time....................................................................................................................... 33
2.9   Polyethylene film alone and polyethylene film with underlying RDX film 
        on a glass slide...................................................................................................... 34
2.10 Thickness of sprayed-on polycarbonate film as a function of time...................... 34
2.11 Polycarbonate film alone and polycarbonate film with underlying 
        RDX film on a glass slide..................................................................................... 35
2.12 Spin-coating from a molten polyethylene............................................................. 36
2.13 Molten polyethylene spin-coated on a glass slide................................................. 37
2.14 Spin-coated polycarbonate film partially removed from 
        the glass slide........................................................................................................ 37
2.15 Thickness of polycarbonate films as a function of revolution rate....................... 38
2.16 Thickness of spin-coated polycarbonate films as a function of 
        concentration......................................................................................................... 39
2.17 Polycarbonate film alone and polycarbonate film with underlying  
        RDX film placed on a glass slide.......................................................................... 39
2.18 Nebulizing Sprayer................................................................................................ 42
2.19 Polycarbonate film hole width as a function of lens to film distance.................... 43
2.20 Number of shots necessary to break though a 0.254 mm thick 
 xiii
        film as a function of distance from lens to sample............................................... 44
2.21 Polycarbonate films ablated under different laser focusing conditions................ 45
2.22 Laser ablation experimental setup........................................................................ 47
2.23 ?Sandwich? structure samples.............................................................................. 48
2.24 Ablated RDX film sprayed on a glass substrate................................................... 49
2.25 Pressure as a function of time for ablation of polycarbonate film........................ 50
2.26 Pressure as a function of time for ablation of RDX film...................................... 51
2.27 Pressure as a function of time for ablation of polycarbonate coated 
        RDX film.............................................................................................................. 52
 
 
 
 
 
 
 1
I. INTRODUCTION 
I.1 PROPELLANTS 
A propellant is defined as a rapid burning charge that propels a bullet, shell, 
rocket, or missile.
1
 Unlike primary explosives, propellants are temperature and pressure 
dependent and therefore require the use of detonator and also possibly a booster. A 
detonator contains a primary explosive as an essential element; however, it may be more 
complex. This is true particularly of military detonators, or fuses and delay blasting caps 
in which timing, safety, and other mechanisms are also built into the detonator. A booster 
is a sensitive secondary explosive which reinforces the detonation wave from the primary 
explosive, or detonator, and delivers thereby a more powerful detonation wave to the 
main (secondary) explosive charge. Examples of primary explosives include lead azide, 
mercury fulminate and nitromannite. Primary explosives are very sensitive to heat, 
impact or friction and detonate or burn very rapidly. Nearly all combustible gases and 
dusts can be explosive when mixed with air in certain proportions, e.g.: 
      CH
4
 + 2O
2
 ? CO
2
 + 2H
2
O + heat                          (1) 
Many gaseous and dust-air or dust oxygen explosives are, in fact, primary explosives, 
since they are readily detonated in a manner characteristic of primary explosives, 
frequently with exceedingly small sources of energy. For this reason, combustible gases 
and dusts are extremely dangerous explosion hazards and have been responsible for
numerous residential and industrial accidents and fatalities. Primaries often need O
2
 gas 
as a reactive. Grain dust such as lycopodium powder is not especially flammable but 
when grain is dumped into a grain silo, some of the finer dust particles can remain 
suspended in air surrounded by oxygen. This mixture can be ignited by a spark, resulting 
in an explosion. Primary explosives, detonate when initiated, but they are extremely 
sensitive  and, as a class, have less power than secondary explosives such as TNT, RDX 
and HMX which have the highest energy outputs of any explosives.
2, 3
 The energy output 
is proportional to the volume of the gaseous products of explosion, e.g. during detonation 
of RDX, a relatively large number of net gas molecules formed: 
C
3
H
6
O
6
N
6
 ? 3CH
2
N + 3NO
2                               
(2) 
 Propellants have a number of advantages over primary explosives. They are less 
sensitive to heat and shock than primary explosives: most simply burn rather than 
explode when ignited in air, and can be detonated only by the nearby explosion of a 
primary initiator. Two common propellants used by the Army are RDX (Research 
Department Explosive) and HMX (High Melting Explosive), see Figure 1.1. 
 2
O 
N 
C 
H
 
O 
N 
N 
 
O 
O 
CHHC 
N 
N
N 
O 
O 
2 2
2
N C 
N 
C 
N C 
N
C
N 
N 
N 
N 
O
O
O
O 
O 
O 
O 
O 
H
2 
H
2 
H
2 
H
2
a b 
Figure 1.1 ? a) RDX molecule; b) HMX molecule 
           
 
 
 
           
                                                                                        
Propellants are used by military as fillings for bombs and shells with only a small amount 
of primary explosive needed. This makes the explosive devices safer and less susceptible 
to accidental explosions. The initiation of secondary explosives generally requires the 
shock wave energy from a primary explosive. Secondary explosives will not detonate 
when subjected to a spark, flame, or a hot wire as will a primary explosive. 
 
I.2 IGNITION TECHNIQUES 
 
 Conventional ignition requires the use of primary explosive such as a military 
fuse to initiate the explosion of a less sensitive secondary explosive. The simplest fuse is 
a length of combustible material which burns from the free end, through a small opening 
in the casing, into the explosive charge, where it then ignites the explosive material, see 
Figure 1.2. 
 
A typical large bore shell contains an ordinary fuse cap, primary and 
secondary explosives. The black powder in an ordinary fuse cap refers to a low explosive 
such as sodium nitrate, sulfur or charcoal and serves as a safety fuse. 
 
 Primary Explosive 
((Pb(N
3
)
2
) 
Fuse with Black Powder Core 
 
 
 
 
Figure 1.2 - Ordinary fuse cap.
 
 
 3
In the ordinary fuse cap, a primary explosive such as lead azide is used alone to fulfill the 
threefold primary purposes of the cap: ignition of pre-detonation explosion, creation of 
the detonation wave, and delivery of sufficiently intense detonation wave to a secondary 
charge to detonate this main charge.
2
 Lead from the primary explosive is released in the 
barrel of the gun and then deposits on the interior of the barrel and must be removed later 
as it builds up. Lead also is released into the air where it may result in a hazardous 
exposure to the troops operating the gun. One motivation to replace conventional ignition 
is the elimination of lead. 
 A typical primary explosive used in the Army is environmentally hazardous lead 
azide (Pb(N
3
)
2
) which can be avoided by using plasma ignition technique. The interest of 
this research is the process of plasma ignition which removes the need for lead azide 
completely. In plasma ignition systems, also referred to as electrothermal chemical (ETC) 
ignition, an ordinary fuse cap is replaced by a high temperature and pressure plasma, 
generated by the application of high-current, short-lifetime electrical discharge to a 
polymer capillary, see Figure 1.3. 
Interest in plasma ignition of propellants began in the early 1990?s and was 
primarily focused on liquid propellants.
4
 Interest in the plasma ignition of solid 
propellants did not start to develop until several years later, when an enhanced burn rate 
was thought to have been discovered.
5
 Despite the fact that the existence of an enhanced 
burn rate is still questionable today,
6 
it did produce a substantial interest in ETC ignition. 
 
 
 4
 
 
 5
 
 
 
 
 
 
 
 
 
 
Conventional Ignition 
Lead Azide 
Plasma Ignition 
 
Capacitor 
+ 
- 
Propellant (RDX) 
Propellant (RDX) 
Figure 1.3 ? Large bore artillery shell. 
Plastic tubing 
 
Further investigations resulted in the observations of significant advantages of the 
plasma ignition such as: a short and highly reproducible ignition delay
7
 and the 
possibility of temperature compensation using a trivial adjustment of the electrical energy 
needed to initiate combustion.
8
 Subsequent simulations of the ETC igniter
9
 did not 
explain the benefits, but yielded many insights into the process which was not possible to 
obtain from the more prevalent studies of pressure transients.  
Recent work at the Army Research Laboratory in Aberdeen, MD, has showed 
numerous observations that may provide significant insight into the process of plasma 
ignitions. Namely, the studies have showed that the choice of a polymer liner plays an 
important role in the ignition process. Polyethylene liners, used in the early work, were 
discovered to leave a residual ?soot? on the grains at the time when the detonation was 
 6
terminated, while plasma ignition achieved with Mylar (polycarbonate) liners left a 
?clean? surface. Studies of compositional and morphological changes in grains exposed 
to the ignition source have proved that pits, gouges, blisters and wormholes are formed 
on the surface of the propellant grains.
10, 11, 12, 13
  Moreover, the normal melt layer was 
found to be either very thin or absent on the plasma-treated grains and the observed 
denitration was extending 0.5-0.75 mm into the surface depending on the chemical 
composition of the propellant. All existing experimental data suggest an ablative, rather 
than a combustive, process occurring during the initial stages of plasma ignition where 
increased surface area may be an explanation of the increased burn rate. 
Attempts to determine the mechanism of plasma ignition include large variety of 
techniques which test the influence of specific components of the plasma. Some of these 
studies are as follows: Andreasson and Carlson
14
 built a closed vessel cell in which an 
electrical current is passed through a test charge while ignited and burned. Despite a 
number of difficulties coming from a poorly reproducible ignition delay, they did 
demonstrate an existence of a burn rate enhancement with the application of external 
electrical power. Katulka et al.
15, 16
 investigated the power dependence of ignition for a 
various propellant compositions. They also demonstrated the effects of Mylar and 
aluminum films on the samples to reduce exposure of the samples to the UV and ion 
radiation coming from the plasma. The results of these studies are clouded by the 
reactivity of some of the films, however, some important observations were still made 
such as the presence of metal clusters from the plasma source on the samples, radiation 
transmitted through polyethylene films changed the chemical composition of some 
 7
propellant mixtures, and all the effects depended strictly on the chemical nature of the 
propellant. Others focused on heat loss,
17
 and modeling of the ignition process.
18, 19
 Modeling of a standard plasma ignition system has showed that the flux of 
species arising from the plasma ignition source is mostly hydrogen and carbon atoms 
with their ions.
9
 Besides the work at Aberdeen, the Army has also supported several 
groups that are working in collaboration with the ARL group. For example, at 
Pennsylvania State University a ?standard? ETC igniter has designed and constructed. 
Moreover, their mass spectrometric measurements of the plasma composition as it 
expands out of the plasma source are providing experimental results which can be 
compared directly with the modeling.
20
 Another group, at North Carolina State 
University, uses optical probing of the ETC igniter pulse to investigate the shape and 
temperature profiles of the expanding plasma pulse.
21
 Meanwhile, the group at the 
University of Texas uses optical probing technique to image individual chemical 
constituents of the pulse.
22
In order to understand the process of erosion of propellant films in the plasma it is 
important to understand the physical processes taking place in plasma environment. It is 
not unreasonable to assume that physical sputtering is responsible for erosion of 
propellant films in plasma. At the microscopic level physical sputtering can be 
considered as a result of fast moving ions bombarding a surface which leads to a transfer 
of entire energy and momentum to the lattice.  This process is well known for many types 
of materials,
23
 however propellants are unusual sputter targets. First, they are molecular 
solids, meaning they are composed of molecular units that are held together by relatively 
 8
weak Van de Waals forces as compared to the covalent forces which bind the atoms of 
the more traditional sputter targets, such as metals and semiconductors. Due to the 
weaker bonding between the units, molecular solids tend to have higher sputter yields. 
Second, propellants can spontaneously combust, if sufficient pressure and temperature 
are achieved. When a propellant surface is impacted by an ion, combustion may become 
an additional and likely outcome. The mechanism of ion stimulated propellant 
combustion can be understood through either an atomistic or a continuum model of the 
interaction. From atomistic point of view, the glancing angle collisions of an ion with a 
propellant molecule can transfer sufficient amount of energy and momentum to the 
vibrational and electronic energy manifolds of the propellant molecule to initiate 
combustion. From a continuum viewpoint, the ion bombardment event may be seen as 
the source of a short-lived, local pressure and temperature spike as the ion transfers its 
momentum and energy to the surface. When the spike in pressure and temperature are 
large enough, local conditions suitable for the spontaneous combustion of propellant 
molecules can result within a few nanometers of the ion impact point. 
Ion energy plays a large role in determining the outcome of the ion bombardment 
process. The resulting combination of effects as a function of ion energy is the logical 
consequence of competing factors in the bombardment process: the energy dependent, or 
more specifically the velocity dependent cross-section for collision and the average 
energy transferred to target atoms. At very low ion energies (0-50 eV), the ions move 
relatively slowly and the collision cross-section with surface layer atoms is very large. 
Hence, all of the ions undergo ?hard? collisions with the uppermost surface atomic layer, 
where they transfer most of their energy to target atoms. However, with little energy in 
 9
the ions, the energy transferred to individual target atoms is most likely below the 
threshold for ejection of target atoms, and only a small fraction of the atoms are ejected. 
The result is a low sputter yield. As the ion energy is increased within the low energy 
regime, the velocity of the ions goes up, and thus, the cross-section for collisions with the 
uppermost layer goes down. In this regime, ions can penetrate as much as a few 
nanometers into the surface before suffering a hard collision with a target atom. 
Meanwhile, the average energy transferred to the target atoms also rises and an increasing 
fraction of the target atoms now have enough energy to be ejected. In the low ion energy 
ion regime, the sputter yield reaches a maximum. As the ion energy is further increased 
to the medium and high energy regime, a large fraction of the ions begin to penetrate 
deep into the surface layer before suffering a hard collision. Despite the fact that the 
average energy transferred to target atoms is large, they are too deep to be able to escape 
the solid and the energy is dispersed within the lattice. As a result, the sputter yield 
decreases. In the medium to high energy regime, implantation of ions takes place over 
sputtering and other surface effects. 
The modeling of the plasma-propellant interactions (PPI) using laser ablation is 
based on the fact that the laser ablation of plastics, especially the ones used as liner in 
ETC igniters produces plasma that in many ways resembles the output pulse of the ETC 
igniters. By laser ablating a polymer film of proper thickness deposited directly on top of 
a propellant film, a miniature replica of the ETC igniter pulse can be created at the 
surface of the propellant. As laser ablation process is highly controllable and can be done 
on a large variety of sizes by focusing and adjusting power of the laser pulse, the 
 10
advantage of this approach is obvious since actual ETC igniters cannot easily be 
miniaturized.
24
 
I.3. LASER ABLATION 
I.3.1 INTRODUCTION 
 
Ablation, in the broadest sense, is removal of material by incident light. In most 
metals and glasses/crystals the removal is by vaporization of the material due to heat. In 
polymers the removal can be induced by photochemical changes which include a 
chemical degradation of the polymer, akin that employed in photolithography. If the 
removal is by vaporization, special attention must be given to the plume. The plume will 
be a plasma-like substance consisting of molecular fragments, neutral particles, free 
electrons and ions, and chemical reaction products. The plume will be responsible for 
optical absorption and scattering of the incident beam and can condense on the 
surrounding material and/or the beam delivery optics. Normally, the ablation site is 
cleared by a pressurized inert gas, such as nitrogen or argon. If the material to be ablated 
has a poor absorption, such as diamond, a thermally converted form of the material, such 
as graphite which has relatively good absorptivity, is used to cover the diamond surface 
with a thin coating. The laser beam will ablate the graphite and in doing so the surface of 
the underlying diamond will be converted to graphite allowing efficient absorption. 
Sequentially, graphite is ablated and each newly formed layer of diamond is converted to 
graphite. The ability of the material to absorb photons limits the depth to which that 
energy of light can perform useful ablation. Ablation depth is determined by the 
 11
absorption depth of the material and the heat of vaporization of the substrate. The depth is 
also a function of beam energy density, the laser pulse duration, and laser wavelength. 
Laser energy per unit area of substrate is measured in terms of the energy fluence.
25
The peak intensity and fluence of the laser beam is given by: 
Intensity (Watts/cm
2
) = peak power (W) / focal spot area (cm
2
)  
Fluence (Joules/cm
2
) = laser pulse energy (J) / focal spot area (cm
2
) 
while the peak power is 
Peak power (W) = pulse energy (J) / pulse duration (sec)  
There are several key parameters to consider for laser ablation. The first is 
selection of a wavelength with a minimum absorption depth. This will help ensure a high 
energy deposition in a small volume for rapid and complete ablation. The second 
parameter is short pulse duration to maximize peak power and to minimize thermal 
conduction to the surrounding work material. This is analogous to a vibrating system 
where the mass is large and the forcing function is of high frequency. This combination 
will reduce the amplitude of the response. The third parameter is the pulse repetition rate. 
If the rate is too low, all of the energy which was not used for ablation will leave the 
ablation zone allowing cooling. If the residual heat can be retained using a high repetition 
rate, thus limiting the time for conduction, then ablation will be more efficient. More of 
the incident energy will go toward ablation and less will be lost to the surrounding work 
material and the environment. The fourth parameter is the beam quality. Beam quality is 
 12
measured by the brightness (energy), the focusability, and homogeneity. The beam 
energy is of no use if it can not be properly and efficiently delivered to the ablation 
region. Further, if the beam is not of a controlled size, the ablation region may be larger 
than desired with excessive slope in the sidewalls. 
Typical laser sources commonly used in surface modifications include Nd:YAG 
and excimer lasers. Since this work is focused on laser ablation of polymers 
(polyethylene and polycarbonate) which both absorb light in the UV region, only 
ultraviolet laser ablation mechanisms are discussed hereafter. 
I.3.2 LASER ABLATION MECHANISMS 
Laser-induced ablation results from the conversion of an initial electronic or 
vibrational photoexcitation into kinetic energy of nuclear motion, leading to the ejection 
of atoms, ions, molecules, and even clusters from a surface. Laser ablation is a sputtering 
process in which material removal rates typically exceed one-tenth monolayer per pulse; 
the surface is structurally or compositionally modified at mesoscopic length scales; and 
particle yields are superlinear functions of the density of excitation. The formation of an 
ablation plume (a weakly ionized, low-to-moderate density expanding gas cloud) adds to 
laser ablation the complications of plasma-surface interactions, gas dynamics, and laser-
induced photochemistry. 
Lasers deposit energy in irradiated surfaces and the near-surface region of the 
bulk material down to a penetration depth that is characteristic of the laser frequency (or 
wavelength) and the material. The energy may be deposited either by exciting free 
electrons or by exciting electronic or vibrational transitions in atoms, ions, molecules, or 
optically active defects. The mechanism, density, and lifetime of the induced excitation 
depend on the electronic structure, composition, surface topography, and defect 
populations of the irradiated solid as well as on the laser frequency and pulse duration. 
There are two steps in the mechanism by which UV laser pulses bring the etching of 
polymer surfaces with a minimum of thermal damage to the substrate. 
 
Ener
gy
 
Interatomic Distance 
a
b
 
 
 
 
 
 
 
 
 
  
 
Figure 1.4 - Energy-level diagram for a hypothetical bond A-B. The lower broken line represents 
the ground electronic state; the upper broken line and the solid line represent excited 
states. 
 
The principal reaction steps that have been proposed can be understood by referring to an 
energy diagram, Figure 1.4. It is generally accepted that the absorption of UV photons 
results in electronic excitation (step a). The excited electronic state can undergo 
decomposition in that state, which would be a purely photochemical reaction, or, if the 
 13
excited molecule undergoes internal conversion (step b) to a vibrationally excited ground 
state, any subsequent decomposition can be considered to be the equivalent of a thermal 
process. This is the so-called photothermal mechanism in which the photons merely act 
as a source of thermal energy.
26
  
In Figure 1.5, a schematic diagram of the energy pathways present in ultraviolet 
processing of polymers is shown. Photochemical and photothermal mechanisms can both 
lead to decomposition as well as changes in absorption properties. As polymer ablation 
and modification involves actual chemical transformation of the material, absorption can 
change with laser processing. Some polymers which are initially only lightly absorbing 
become absorbing after the first few laser pulses and ablation does not begin until 
?incubation? is accomplished.
25 
The details of surface alteration by laser ablation ? such 
as large-scale material removal ? will also be influenced by surface morphology, by 
surface roughening and instabilities, and by the ambient atmosphere. Finally, the amount 
Photochemical 
Process 
Photothermal 
 Process 
Decomposition Electronic States 
Electronic 
Excitation 
Radiative 
Transitions 
UV 
Laser 
Radiationless 
Transitions
Incubation 
Incubation 
Phase 
Transition 
Kinetic 
Energy
Dissociation 
Figure 1.5 ? Various energy pathways in UV processing of polymers.
25
 14
 15
of light absorbed at the surface may be influenced by laser interactions with the ejected 
material and, in case of thin irradiated samples, by internal reflections and phase changes 
at internal boundaries.  
  
I.3.3 LASER ABLATION OF POLYMERS 
 
It is commonly accepted that laser ablation involves photothermal and/or 
photochemical processes, depending on the nature of the polymers used and the 
experimental conditions, for example laser fluence, wavelength, and pulse duration. The 
photothermal process involves the absorption of photons, followed by the release of the 
photon energy into the polymer matrix via vibrational cooling. This induces a rapid 
temperature rise in the bulk material leading to the thermal decomposition of the 
polymer. If the vibrational energy attains a particular fluence threshold, then bonds in the 
polymer will break, resulting in a phenomeneon known as photofragmentation. These 
fragments typically occupy a larger volume compared to the surrounding material.  The 
increase in volume, if confined by non-irradiated material below and adjacent to the 
irradiated material, leads to a sharp increase in pressure and a forward ejection of ablated 
material. The photochemical process involves the breaking of chemical bonds due to 
interaction with nanosecond (or shorter), high power, UV pulses yielding gaseous 
photoproducts. During this process, thermal and mechanical damage to the surrounding 
polymer is minimal and more precise control over the ablated region is realized. 
The ablation of the surface of a polymer by a UV laser pulse is a function of the 
energy deposited in the solid in unit time. If a typical UV pulse has a full width at half-
maximum (FWHM) of 20 ns and an energy of 450 mJ and the size of the beam at the 
polymer surface is 1.5 cm
2
, the fluence at the surface will be 300 mJ/cm
2
 and the power 
density will be 15 MW/cm
2
. When this pulse strikes the surface a loud audible sound will 
be heard and depending upon the wavelength, 0.01-0.1 micron of the material can be 
removed with a geometry that is defined by the light beam. If this experiment is 
performed in air, a bright plume will be ejected from the surface and will extend to a few 
millimeters. Figure 1.6 shows images of laser ablation plumes from polycarbonate films 
using different laser focusing conditions. In Figure 1.6 (a), the ablation plume can be 
readily seen. The mushroom cloud like appearance of the plume is the result of emission 
from the ablated species and the fireball that results when the plume materials combust in 
the air. In Figure 1.6 (b), the bright spot in the air is light from the ionization of air at the 
focal point of the laser. 
a b
Figure 1.6 ? Laser Ablation Plume. 
 16
Typically, UV laser ablation is carried out with a succession of pulses. R. 
Srinivasan and Bodil Braren
27
 have shown that the depth etched is a linear function of the 
number of pulses and that there is a very long extrapolation between the origin (zero 
pulses) and the first data point.  
Laser Beam 
Mask 
Organic Polymer 
Long-Chain 
Molecules 
Irradiation 
(a) 
 
Absorption 
(b) 
 
Bond 
Breaking 
(c) 
 
Ablation 
Figure 1.7 ? Schematic impact of laser pulse on polymer surface. 
 
 
 
A pictorial representation of the interaction of a laser pulse with a polymer 
surface is shown in Figure 1.7. As shown in Figure 1.7 (a), the stream of photons from a 
single laser pulse falls on the polymer and is absorbed in a depth that can he as little as a 
 17
 18
fraction of a micron for intense absorbers, to many tens of microns for weakly absorbing 
polymers. Obviously, weak absorption and strong absorption refer to specific 
wavelengths so that the same polymer can absorb weakly at one laser wavelength and 
strongly at another. Figure 1.7 (b) shows that within the absorption depth, there are 
numerous bond breaks. In Figure 1.7 (c), the fragments are shown to be ejected from the 
surface, leaving an etched pit behind. 
The general features of UV laser ablation of polymers are summarized as follows:  
? Polymer ablation takes place within 10 to 100 nanoseconds. 
? The threshold energy fluence, defined as the fluence at which the etch depth is 
0.05 ?m per pulse, is low for polymers (typically in the range 10 to 100 
MJ/cm
2
).  
? For fluences near or below the threshold, the etch depth follows Beer-Lambert's 
law (photo-chemical, linear absorption). For fluences above the threshold, 
thermal effects contribute to the etch depth. In addition, the longer the 
wavelength, the stronger are the thermal effects. 
? Wavelength affects absorption and threshold fluence. The etch depth per pulse 
(lower absorption coefficient) is larger for a weaker absorber than for a stronger 
absorber.  
? The formation and expansion of the plasma plume during the laser pulse 
characterize the rapid etching process. The etch depth per pulse increases with 
 19
energy fluence until the phenomenon of saturation is reached. "Saturation" is a 
mechanism involving the blocking of the trailing part of the laser pulse by both 
the plume and the excited polymer species generated by the leading part of the 
pulse. This occurs only at high energy densities and prevents additional material 
removal.  
? Ablation is accompanied by an acoustic signal that decreases with increasing 
laser wavelength. 
? Ablation creates numerous products, which may include atoms, monomers and 
fragments normal to the surface. The velocities of ablation products are high, up 
to 101 m/s. The velocity distribution of ejected material is not dependent on the 
energy fluence. 
? Ablation takes place in the temperature range 400 to 800 
0
C. 
? The small absorption depth coupled with short laser pulses and low thermal 
conductivity of polymers restricts the extent of heat transfer, leading to precise 
material removal and a small heat-affected zone.  
 
 
 
 
 
I.3.4 LASER ABLATION OF RDX FILMS 
When a sample is illuminated with a short pulse of intense laser radiation, a few 
monolayers of material may be "cleanly" removed from the surface, see Figure 1.8. 
 
Substrate 
Pulsed 
Laser 
Plasma 
Plume 
Plasma 
Emission 
Substrate 
Atomic layers of 
material to be ablated 
a
b
 
Figure 1.8 ? a) Ablation of material; b) Sample after ablation. 
 
 
In laser ablation, the rapid adsorption of laser energy generates a high-pressure, 
high-temperature plasma that then extinguishes rapidly as it expands into the ambient 
environment, see Figure 1.8 (a). Experimental measurements and theoretical studies
 
of 
laser ablation both indicate that pressures of several MPa and temperatures of several 
 20
 21
thousand degrees are typically realized in the ablation process.
28,29 
Hence, it is no 
coincidence that images of laser ablation plumes
 
closely resemble the plumes recently 
observed by Varghese
30
 observed to emerge from an ETC ignition source. 
In this work, the laser ablation of thin films of ETC liner material deposited 
directly onto propellant samples is studied. Polyethylene and polycarbonate films are 
typically used by the Army as ETC liners and have been shown to be effective igniters 
when ablated, consequently they have been chosen for study herein. Ablation of both of 
these polymers can be achieved with UV irradiation with the fourth harmonic of an Nd-
YAG laser. Ablation of deposited layers of these polymers will produce short-lived 
plasma very similar to those found at the exit ETC igniter. 
Finally, it should be noted that the ablation process removes only a few 
monolayers (see Figure 1.8) of material at a time and that the deposition of the films will 
most likely result in thickness on the order of microns. Therefore, a single pulse ablation 
experiment will simply measure the ablation products of the bulk films. However, 
repeated ablation of the same region, using a laser spot size of ~ 1mm as compared to the 
micron thickness, will successively thin the film until pure propellant is revealed. When 
the polymer film thickness exactly matches the ablation depth, the surface of the 
propellant will be presented with plasma comparable in composition and pressure to the 
output of an ETC igniter. Under these conditions, the propellant is expected to burn at 
least until the pressure of the ablation pulse dissipates. For identically ablated structures 
where the polymer film is 10-20% thicker, the ablation will not reach the surface of the 
propellant and only the pressure of the pulse, but not the chemical species of the plasma, 
will reach the surface of the propellant. In this case, the propellant film is expected to 
remain mostly intact after the laser pulse. For structures that have polymer films 10-20 % 
thinner than the ablation depth, some of the propellant will be expected to ablate directly, 
a process that has been shown not to be capable of igniting the propellant.
16 
 
I.4 THIN PLASTIC FILMS 
 
 
I.4.1 DEPOSITION METHODS 
 
 
Spin-coating has been extensively exploited by the microelectronics industry for 
depositing layers of photoresist films on to silicon wafers. 
31
 The various steps involved 
in the process are illustrated in Figure 1.9. A quantity of a polymer solution is first placed  
 
Apply Spread Spin 
Vacuum 
Low rpm High rpm 
Figure 1.9 ? Spin-coating technique. 
 
 
on the substrate (typically semiconductor wafer), which is then rotated at a fixed speed of 
several thousand rpm (or the solution can be applied while the wafer is slowly rotating). 
 22
 23
The resist solution flows radially outwards, thus reducing the fluid layer thickness. 
Evaporation of the solvent results in a film of uniform thickness. The initial stage 
involves delivering a quantity of solution to the surface of the substrate. The polymer 
viscosity (dependent on the concentration of the starting solution) and final film speed are 
both important process parameters. An increase in angular velocity decreases the film 
thickness; an inverse power-law relationship usually holds for the thickness dependence 
on the final spin speed. For a given speed, the film thickness decreases rapidly at first, but 
then slows considerably at longer times. A simple theory
32
 predicts the following 
relationship between the thickness of the spun film, d, the viscosity coefficient of the 
solution, ?, its density, ?, the angular velocity of the spinning, ?, and the spinning time, t: 
d = (?/(4? ? ?
2
)
1/2
t
-1/2
         
                  (3) 
Organic compounds that have been successfully deposited by spin-coating include 
electrically insulating polymers such as poly (vinylidene fluoride), conductive polymers 
and dyes developed for electroluminescent displays, and certain phtalocyanine 
materials.
33, 34, 35, 36, 37 
Although spin-coating is expected to produce films in which 
individual molecules are relatively disordered, this is not always the case. For example, 
organized phtalocyanine layers have been deposited.
35
 The way in which the order is 
achieved is not fully understood but may result from the centrifugal forces acting upon 
the individual molecules during spinning. Spin-coating is the preferred method for 
application of thin, uniform films to flat substrates. 
Other methods of thin film formation include dip-coating, spraying, painting, and 
screen printing. Most of these techniques are relatively easy to carry out and require a 
minimum of equipment. Polymer films of materials such as polypropylene, polystyrene 
 24
and poly(vinyl chloride) (PVC) can be obtained by the technique of direct isothermal 
immersion of a substrate into a suitable solution of the polymer (e.g. PVC in 
cyclohexanone). Material will be deposited on the immersed substrate until equilibrium is 
reached between the deposition rate and the re-solution rate. Satisfactory films can also 
be obtained by solution casting ? allowing the evaporation of a polymer-containing 
solution placed on a substrate (e.g. polystyrene in chloroform). 
The technology of screen printing offers a further inexpensive method for the 
preparation of films. The process consists of dispensing a paste (the ink) of the material 
to be deposited on a mesh-type screen on which a desired pattern may be defined 
photolithographically. The substrate is placed at a short distance beneath the screen. A 
flexible wiper then moves across the screen surface, deflecting it vertically and bringing 
it into contact with the substrate. This forces the paste through the open mesh areas. The 
substrate is allowed to stand at ambient temperature for some time in order to enable the 
paste to coalesce to form a coherent film.
32
In the spraying method, a polymer solution is typically sprayed onto a pre-heated 
substrate. The thickness of the coating is governed by the total exposure time and the 
concentration of the spraying solution. In our case, the film thickness is controlled by the 
number of repeated passes of the spray across the substrate. A large particle size 
originating from insufficiently soluble polymers or a wide molecular weight distribution 
may facilitate the formation of numerous heterogeneities within the microstructure of the 
coating. In this work, polyethylene powder is only partially dissolved in a solvent 
(toluene) to form a saturated solution which is then used to spray on to a pre-heated 
substrate. The resulting thin films often have a rough surface with inconsistent thickness 
measured in different spots. In fact, the thickness can vary over the surface the film by as 
much as 50 %. 
 
I.4.2 THIN FILM CHARACTERIZATION 
 
Two main characteristics of polymer thin film quality are film hydrophobicity and 
uniformity. The first parameter can be estimated by measuring a contact angle (the angle 
at which liquid/vapor meets the solid surface), see Figure 1.10. 
 
 25
? 
 
 
? 
Figure 1.10 - Contact angle. 
a b 
 
 
                                                                  
 
 
Figures 1.10 (a) and 1.10 (b) demonstrate a difference in wettability. Figure 1.10 (a) 
shows how a water droplet might appear on a hydrophobic surface such as wax. Figure 
1.10 (b) shows how a water droplet might appear on a hydrophilic surface. 
To evaluate the thin film uniformity, various techniques such as optical 
microscopy, scanning electron microscopy (SEM), transmission electron microscopy 
(TEM) as well as scanning probe microscopes ? AFM, STM, etc. can be employed. In 
 26
this work, to evaluate the surface uniformity, a water droplet is placed on a surface of 
polymer coated glass slide: if the surface is uniform, the droplet is observed to roll freely 
over the entire surface of the film as the slide is tipped. To confirm the reliability of the   
technique, SEM images were also obtained.                                                                        .                         
 27
II. EXPERIMENTAL 
 
II.1 THIN PLASTIC FILMS 
II.1.1 INTRODUCTION 
Electrothermal Chemical (ETC) ignition is a new technology based on the 
generation of a high temperature and pressure plasma by the capacitive ablation of a 
polymer liner. Polyethylene and polycarbonate are two of the more common polymers 
used in ETC research. Efforts to develop spray-on technologies of polymer films have 
been focused on the preparation of polyethylene and polycarbonate films, to correlate 
with the ETC igniters used by the ARL (Army Research Lab) group and Varghese,
30
 
respectively. In the spraying method, a polymer solution is sprayed on to a pre-heated 
substrate and the thickness of the coating is governed by the number of repeated passes of 
the spray across the substrate. Another common technique of forming thin polymer films 
is spin coating. In the spin coating method, a quantity of polymer solution is placed on 
the substrate which is then rotated at a fixed speed. To prepare smoother, more uniform 
polymer thin films, it would be preferable to develop spin-coat technologies for both 
polyethylene and polycarbonate.  
Film quality was judged by several standards. The first standard was the 
hydrophobicity determined by the contact angle of 10 ?L water droplets placed on the 
films. Figure 2.1 shows 10 ?L water droplets placed on a microscope slide covered with 
polycarbonate film (right) and on a droplet placed on a microscope glass slide without 
film (left). 
 
Figure 2.1 ? Water droplets placed on a microscope slide covered with polycarbonate film 
(right) and on a droplet placed on a microscope glass slide without film (left). 
 
 
 
 
 
 
In this work, the contact angle is defined as the angle between the surface and the rising 
edge of the droplet. A film judged as good is less than 90 degrees, while, in poor quality 
films, the angle may be as large as 140 degrees, see Figure 2.1.  The second standard was 
the presence of pinholes. This was determined in one of two ways.  If pinholes exist, the 
droplet is observed to adhere to an individual point on the surface as the sample is tipped.  
If pinholes are not present the droplet freely rolls across the surface as the slide is tipped.  
Another test for pinholes is the stability of the surface to the presence of water drops.  If 
pinholes exist, water seeps through them and below the film to wet the hydrophilic glass 
surface. This undermines the film and in a matter of minutes the film tears and the droplet  
 
 
 
Figure 2.2 ? Polycarbonate film with pinholes 
 
 28
appears to be below the fragments of the film, see Figure 2.2. To confirm the reliability 
of these tests, SEM images were also taken, see Figure 2.3. 
 
 
 29
 
 
 
 
 
 
                              
a b 
Figure 2.3 ? SEM images of polyethylene films: a) film with pinholes; b) film without pinholes 
 
 
The thickness of the films has been measured by using a Tencor Alpha-Step 200 surface 
profiler. The device measures surface profiles by means of scanning a mechanical stylus 
across the sample. The small motions of the stylus are amplified and displayed as a chart 
on the computer. The thickness of the film is then determined as a difference between 
two points: the surface height of the glass slide covered with polymer and the surface 
height of the glass slide alone. 
 
 
 
 
 
 
 
 
 
II.1.2 EXPERIMENTAL SETUP 
 
 
SPRAYING TECHNIQUE 
A 50 ml round bottom flask is attached to the sprayer unit using a standard 14/20 
ground glass joint and is secured by rubber bands see Figure 2.4. The sprayer has a thick 
wall 1.5 mm I.D. feed tube that extends ~ 40 mm from ground glass,
 
is bent 90 degrees 
and terminates in a 0.5 mm capillary. The capillary feeds through the wall of a 15 mm 
I.D. closed-end tube that extends ~ 60 mm above the joint.  The outer closed-end tube has 
both a feed port for N
2
 pressurization and an opening in the side for pressure relief. No 
sheath gas is designed into the sprayer as the polymer solutions are at, or near, saturation 
and any significant loss of solvent would inhibit flow of the droplets once on the 
substrate surface, resulting in rougher films. 
 
            
Spraying port 
N
2
feed port 
Flask with solution 
to spray 
Point of pressure relief 
Figure 2.4 ? Sprayer. (Note that the food dye was added to allow the reader to see the normally 
colorless solution)
 
 
 
 
 30
To spray a film, the round bottom flask is filled with polymer solution, the sprayer is 
attached, a N
2
 flow is applied, and the pressure relief opening is covered with a finger, 
see Figure 2.5. The glass substrate is heated to ~ 100 ?C. 
 
 
 
 
N
2
 hose 
Sprayer 
Glass slide
Heater
 
Figure 2.5 ? Spraying technique. 
 
SPIN-COATING TECHNIQUE 
 
To prepare thin films by spin coating, a Laurell WS-400 B spin-coater device was 
used. After a substrate is loaded on to the chuck, vacuum hold-down is engaged from the 
side mounted control panel and the lid is closed, a pre-programmed process is selected 
and then initiated. A 1 ml of polymer solution is placed on the substrate. The spin-coater 
was programmed to operate in 2 stages. Stage one is a spin coating with a revolution rate 
of 500 rpm for 5 seconds. Stage two was programmed to spin coat for 45 seconds with a 
 31
revolution rate of 1500 rpm. The first stage was necessary to insure that the polymer 
solution coats uniformly and is not lost to walls of the 
device. The program panel is shown on Figure 2.5. 
 
 
 
Figure 2.6 ? Spin-coater WS-400 
                 Process Controller. 
 
 
 
 
II.1.3 SPRAYED-ON FILMS 
 
Polyethylene films were sprayed from a nearly saturated solution, typically 
0.01364 g/ml, in toluene. The solutions were prepared by adding a pre-weighed quantity 
of polyethylene powder (Alfa Aesar A10239 and Acros 178505000) to 30 ml of toluene, 
and refluxed for several hours or overnight to insure dissolution of the solid.  
Polyethylene solutions with concentrations significantly higher than 0.014 g/ml were too 
viscous to spray and therefore were not used. Solutions with concentration significantly 
lower than 0.0135 g/ml formed discontinuous films that had numerous pinholes. Images 
of 8.2 ?m thick polyethylene films sprayed-on at room temperature and at ~100 ?C are 
shown in Figure 2.6. Polyethylene films sprayed onto room temperature glass slides did 
not pass any of the tests for a high quality film. These films appeared an almost opaque 
 32
white. They were insufficiently hydrophobic and riddled with pinholes. 
 33
  Figure 2.7 - Polyethylene films sprayed at room 
temperature (left) and at ~100 ?C (right). 
 
Films sprayed onto glass slides heated to ~ 100 ?C, as measured with an optical 
pyrometer, appeared more transparent and proved to be very hydrophobic, and displayed 
no evidence of pinholes. Spraying at temperatures significantly above 100 ?C resulted in 
0
2
4
6
8
10
12
14
10 15 20 30 45 60
T
h
i
ckn
es
s,
 u
m
 
Time, s
Figure 2.8 - Thickness of sprayed-on polyethylene films as a function of spraying time. 
instant melting of the polymer film. Figure 2.8 is a plot of the thickness of the resulting 
films as a function of spraying time. 
 
Figure 2.9 ? Polyethylene film 
alone (top) and polyethylene 
film with underlying RDX film 
(bottom) placed on a glass slide. 
 
 34
10
20
30
40
50
60
70
10 15 20 30 45 60
Time, s
T
h
i
c
kn
ess,
 u
m
 
Figure 2.10 - Thickness of sprayed-on polyethylene films as a function of spraying time
 
 
 
Polyethylene films have also been sprayed onto glass slides where RDX films had 
previously been prepared. The spraying of the polyethylene films had no visually 
detectable effect on the morphology of the underlying RDX film, see Figure 2.9. 
 
 
 
 
 
 
Polycarbonate films were prepared from a pre-weighed amount of cut 
polycarbonate sheet (McMaster Carr 10mil, 85585K13) that was dissolved in 99.5% 
methylene chloride (UN 1593 Acros) to yield a solution of 0.023 g/ml. Solutions with 
concentrations significantly smaller than 0.023 g/ml failed to form a good quality film, 
while solutions with concentrations significantly higher than 0.023 g/ml were too viscous 
to spray. Pinhole free, hydrophobic films of polycarbonate can be prepared by spraying 
onto a glass slide heated to ~ 100 ?C. Room temperature polycarbonate films were 
significantly less hydrophobic and had a large number of pinholes. Figure 2.10 is a plot 
of the thickness of the resulting films as a function of spraying time. The morphology of 
a sprayed-on polycarbonate film closely resembled the morphology of the polyethylene 
films sprayed onto a ~ 100 ?C substrate, see Figure 2.11. Polycarbonate films have also 
been sprayed onto glass slides where RDX films had previously been deposited. The 
spraying of the polycarbonate films had no visually detectable effect on the morphology 
of the underlying RDX film, see Figure 2.11. 
 
 
 
 35
Figure 2.11 - Polycarbonate film 
alone (top) and polycarbonate 
film with underlying RDX film 
(bottom) placed on a glass slide. 
 
 
II.1.4 SPIN-COATED FILMS 
 
None of the efforts to spin-coat polyethylene films have been successful. Spin 
coating from solution resulted in highly discontinuous films that appeared to have 
crystallized at the surface of evaporating solvent droplets.  This is primarily due to the 
low solubility of polyethylene in all solvents investigated.  In an attempt to spin-coat 
molten polyethylene, a custom top cover for a spin coating device was made, see Figure 
2.12.  Heater wire was connected to form spirals around the glass tube to ensure melting 
of the polyethylene. Once the melting point is reached the polyethylene granules, which  
Figure 2.12 ? Spin coating from a molten polyethylene. 
Low MW 
polyethylene 
Heating 
electrodes 
Teflon 
valve 
Program 
panel 
 36
are supplied from the top opening, start to melt down the tube and the Teflon valve is 
opened allowing molten polymer to spin coat the glass slide. 
All attempts to spin-coat from molten polyethylene produced only thick string-
like deposits (see Figure 2.13), presumably due to the high viscosity of the polymer, even 
using the low molecular weight (M.W. 50,000 Acros) material. 
 
 
 
 
 
 
 
 
 
 
Figure 2.13 - Molten polyethylene spin coated on a glass slide. 
Polycarbonate films were readily prepared by spin-coating from methylene 
chloride solutions of various concentrations. The resulting films were smooth, 
continuous, hydrophobic and pinhole free, see Figure 2.14.  
 
Figure 2.14 ? Sp
polycarbonate partially rem
the glass slide. 
in-coated 
oved from 
 
 37
The film thickness can be varied by controlling the rate of revolution, see Figure 2.15. 
 
Figure 2.15 ? Thickness of polycarbonate films as a function of revolution rate. 
0
10
20
30
40
50
60
200 500 700 1000 1200 1500 2000
RPM
T
h
i
ckn
e
ss,
 u
m
 
 
 
 
 
 
   
 
In order to choose the optimum revolution rate, a polycarbonate solution with a 
concentration 0.005 g/ml was chosen to spin-coat at various rpm settings, see Figure 
2.15. Solutions with concentrations significantly smaller than 0.005 g/ml failed to form a 
thin film, and the solution simply was wasted on the walls of the device. The thinnest 
polycarbonate film yielding a thickness of 3 ?m was obtained at 1500 rpm, see Figure 
2.16.  Once the optimum rate of revolution of 1500 rpm was determined, the effect of 
concentration was investigated, see Figure 2.16. The spin coater was programmed to 
perform an operation in two steps. First step is to spin coat for 5 seconds at a rate of 800 
rpm, and then to spin coat for 45 seconds at a constant rate of 1500 rpm. The first, 
reduced-rate step is necessary to avoid wasting of the solution on the walls of the device. 
So the revolution rate of ~ half of the constant rate was chosen for the first step. 
 38
 
 39
0
5
10
15
0.005 0.01 0.015 0.02 0.025 0.03
Concentration, g/ml
Thi
c
k
ne
s
s
,
20
25
30
 um
  
Figure 2.16 ? Thickness of spin-coated polycarbonate films as a function of concentration.
 
 
 
 
 
 
 
Polycarbonate films have also been spin-coated onto glass samples where RDX 
films had previously been prepared. The spin-coating of the polycarbonate films had no 
visually detectable effect on the morphology of underlying RDX films, see Figure 2.17. 
 
Fig. 2.17 - Polycarbonate film 
alone (top) and polycarbonate 
film with underlying RDX film 
(bottom) placed on a glass slide. 
  
 
 
 
 40
CONCLUSION 
 
 
Overall, the spin coating method resulted in higher quality polycarbonate films in 
terms of uniformity and hydrophobicity as opposed to the spraying technique. The films 
were significantly thinner (~ 10 fold) with virtually constant thickness over the surface of 
the film and pinholes free. The thickness of polyethylene sprayed-on films was 
comparable to the one of spin-coated polycarbonate films. However, due to inaccuracy of 
manual spraying, uniformity of the sprayed-on films remains an issue. The thickness of 
the sprayed-on films measured by profilometer varies over the surface of the film by ~ 
50%. 
The possibility of enhancing the film properties by adding commonly used 
plasticizers such as phthalate esters to the polycarbonate solutions was also investigated. 
No difference in film quality or thickness was observed after adding a plasticizer. Since 
spin-coated films proved to be of a higher quality than sprayed on films, only spin-coated 
polycarbonate thin films are used in this work in laser ablation experiments. 
 
II.2 LASER ABLATION OF RDX FILMS 
 
II.2.1 DEPOSITION OF RDX FILMS 
 
Initial attempts at preparing thin film samples by filling a reservoir with an 
analytical solution of RDX and evaporating the solvent resulted in very uneven films that 
tended to be very thick at the edges of where the final drops of liquid solution existed, 
 41
and nonexistent on other parts of the sample holder. It was decided that a spray 
deposition system would be superior in both film uniformity and quantification of the 
amount of film deposited. The nebulizing spray system
38
 designed was based on the 
design of a typical nebulizer found in an electrospray mass spectrometer. In this design, a 
solution is slowly driven through a thin tube that is run down the center of a larger 
diameter tube that carries a much faster moving sheath gas. As the solution emerges from 
the small tube, the much faster moving sheath gas breaks it down into small droplets. As 
the droplets are carried to the sample in the sheath gas, a significant fraction of the 
solvent evaporates into the "dry" sheath gas. Under the deposition conditions described 
below, the RDX appears to be deposited almost completely dry. This is interpreted to 
mean that, as the solvent is removed in the sheath, the solution concentration increases to 
the solubility limit and crystallization may begin within the sheath as the droplet is 
carried to the sample. 
 
EXPERIMENTAL 
 
A nebulizing sprayer was constructed from a standard 1/8" swagelock Male Run 
Tee NPT adapting tee, see Figure 2.18. The noncollinear swagelock connection is 
attached to source of nitrogen and is used to provide the gas sheath. A rubber septum is 
attached to the 1/8" NPT connector and is secured with a twisted piece of wire. A 1.5" 
piece of 1/16" tubing is epoxied into one end of a 1.5" length of 1/8" steel tubing such 
that ~1/8" sticks out, and the other end is attached to the collinear swagelock connector. 
One end of a one foot length of 32 gauge (0.009" O.D., 0.004" I.D.) tubing is inserted 
 
Solution
 
 
 
 
 
 
 
N
2
in 
 
Figure 2.18 ? Nebulizing Sprayer.
 
 
into a 26 gauge syringe needle with epoxy and the other end is inserted through the 
septum using a second syringe needle which is withdrawn back onto the tubing after the 
tubing has been fed ~ 1/8" out of the end of the 1/8"-1/16" tube on the other end of the 
adapter. A 1 ml syringe is then filled with 1000 ?g/ml RDX in acetonitrile solution. The 
solution is then driven through the 32 gauge tubing at a rate of 1.5 ml/hour as the N
2
 
sheath gas is flowed from a pressure of 40 psi. 
 
II.2.2 CALIBRATION 
 
A Continuum Inlite Laser Nd-YAG, 4 ns pulse, 50 mJ/pulse @ 266 nm is used in 
ablation experiments, 10 mil (0.254 mm) thick films of polycarbonate were obtained 
from McMaster Carr (85585K13), and 1000 ?g/ml analytical samples of RDX in 
 42
acetonitrile obtained from Cerilliant.   In terms of ablation of polycarbonate, the laser was 
more than adequate. Laser fluence at the surface of the film, the critical parameter for 
ablation, was adjusted at full laser power (a Q-switch delay of 190 ?s and 1300V on the 
flashlamps) by simply focusing the beam by adjusting the distance between the sample 
and a 110 mm focal length lens.  Polycarbonate films could be ablated from full focus, 
110 mm separation, up to a spot size of ~1 mm, which was ~1/2 the beam size at 
unfocused separation.  The unfocussed laser beam, or laser focused to a spot size of >1 
mm resulted in no ablation or other effect on the film. The size of the ablation pits and 
the number of pulses necessary to break through the polycarbonate film were determined 
as a function of separation between the sample and the lens, see Figures 2.19 and 2.20.  
The data were taken using a 190 Q-switch microsecond delay. 
 
0
100
200
300
400
500
600
700
800
900
90 95 100 105 110 115 125
Distance from Lens to Film, mm
H
o
le
 W
id
t
h
, u
m 
 43
Figure 2.19 ? Polycarbonate film hole width as a function of lens to film distance. 
 
The optimal focus conditions were found at a lens to sample distance of 110 mm. At this  
0
100
200
300
400
500
600
700
800
90 95 100 105 110 115 125
Distance from Lens to Film (mm)
# o
f
 sh
o
t
s
 
t
o
 b
r
eak t
h
r
o
u
g
h
  
  
 44
Figure 2.20 ? Number of shots necessary to break though a 0.254 mm thick polycarbonate film 
as a function of lens to film distance. 
 
distance the smallest number of shots (20) is necessary to breakthrough the film, which 
also produced the smallest hole of 0.25 mm. 
Figure 2.21 is a photograph of ablated polycarbonate films. The discoloration 
observed on the film around the ablation pits is soot from the fireball created by the 
ablation event that can be wiped from the surface with methanol.  For the purposes of this 
picture the soot has been left on the film to help provide image contrast. 
 
 
 
 
 
Figure 2.21 ? Polycarbonate films ablated under different laser focusing conditions. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
II.2.3 EXPERIMENTAL SETUP 
 
To study the ablation of polycarbonate coated RDX films, the following 
experimental setup shown in the Figure 2.22 was constructed. The core of the system is a 
 45
 46
stainless steel cross with a 15 mm interior diameter. The front port of the chamber is 
fitted with a UV-grade quartz window to allow laser irradiation of the sample. A 10 x 10 
mm sample is attached to a stainless steel stub at the opposite end of the cross from the 
window with adhesive tape. An MKS Baratron pressure gauge with a working range of 
1000 Torr is attached to one of the perpendicular ports (see Figure 2.22) to monitor the 
pressure in the chamber.  It is connected to an MKS PDR-2C power supply digital 
readout which is in turn connected to an NB-MIO-16 I/O Board in an Apple Macintosh 
computer. The system also can be connected to a vacuum pump via the remaining port on 
the cross by a valve. In this work, laser ablation is conducted at ambient pressure with the 
valve closed. The pressure (mm Hg) reading data as a function of number of points is 
collected by means of a Lab View 5.1 interface with the rate of 250 points/s. Laser 
irradiation is achieved using the fourth harmonic output (266 nm, 4 ns) of a Nd-YAG 
laser (Continuum Inlite-III-10). Laser fluence at the surface of the sample is controlled by 
adjusting the distance between the sample and 140 mm focal length lens mounted on an 
optical rail. An IBM compatible computer is used to control various laser parameters 
such as flahlamp voltage and frequency, and the Q-switch delay and divider through a 
serial port connection using hyper terminal. All experiments were conducted with a 190 
microsecond Q-switch delay and 1300 V, 10 Hz and the Q-switch divider was used to 
allow laser pulsing at an integer fractions of the 10 Hz flash lamp frequency. The laser 
focus was set at 100 mm to insure that multiple laser shots were required to ablate 
completely through a standard RDX film.  The standard RDX film was prepared by 
spraying 0.8 mg of RDX solution from a sprayer to sample distance of 80 mm, resulting 
in a spot that was 5 mm in diameter.                             .                        
 
Fi
gu
re 2.
2
2
 ? Laser 
a
b
l
a
t
i
o
n
 expe
ri
me
nt
al
 
s
e
t
up. 
 47
II.2.4 RESULTS AND DISCUSSION 
 
To investigate whether the ablation plasma generated in the polymer film can 
create a short-lived ignition of the propellant, ?sandwich? structures of the polymer on 
propellant have been prepared, see Figure 2.23 (a).  
 48
Su
b
s
t
r
a
t
e
Pr
o
p
e
l
l
a
n
t
Po
l
y
me
r
S
ubst
r
at
e
P
r
o
p
ella
n
t
Po
l
y
me
r
Su
b
s
t
r
a
t
e
Po
l
y
me
r
Su
b
s
t
r
a
t
e
Pr
o
p
e
l
l
a
n
t
Po
l
y
me
r
Su
b
s
t
r
a
t
e
Pr
o
p
e
l
l
a
n
t
Po
l
y
me
r
S
ubst
r
at
e
P
r
o
p
ella
n
t
Po
l
y
me
r
S
ubst
r
at
e
P
r
o
p
ella
n
t
Po
l
y
me
r
S
ubst
r
at
e
P
r
o
p
ella
n
t
Po
l
y
me
r
Su
b
s
t
r
a
t
e
Po
l
y
me
r
Su
b
s
t
r
a
t
e
Po
l
y
me
r
Figure 2.23 ? Sandwich structure samples: a) before ablation; b) after 1
st
 shot; c) after 2
nd
 shot 
a b c
 
RDX solution was sprayed with a nebulizer on a 10 x 10 mm glass substrate, and 
a polycarbonate film was spin coated over the propellant film. In Figure 2.23 (b), a 
schematic illustration of a ?sandwich? sample after exposure to a number of laser pulses 
is shown.  Since the valve is closed, a pressure change in the vessel is anticipated with 
each laser pulse as the polycarbonate film is ablated.  The magnitude of the pressure 
change should be the same as those observed for the ablation of bulk polycarbonate films.  
After the polycarbonate film has been ablated to a thickness of less than the thickness 
ablated by a single laser pulse, see Figure 2.23 (c), the following pulse will create a 
polycarbonate ablation plasma in contact with the underlying RDX layer. If ignition is 
achieved, the entire RDX film will erode in a single laser shot and a pressure change 
larger than that observed for a single laser shot on a pure RDX film will be observed.  If 
ignition is not achieved, multiple laser pulses will be required to ablate through the RDX, 
and the magnitude of the pressure changes observed for each shot should be the same as 
that observed for a pure RDX film. 
1mm 
Figure 2.24 ? Ablated RDX 
film sprayed on a glass 
substrate. 
Before the experiments with ?sandwich? 
samples were conducted, pure RDX films as well as 
polycarbonate films were ablated. The focusing lens 
to sample distance was chosen such that RDX film 
ablates in several shots. At a focusing distance of 100 
mm, it was found that it takes 3 laser shots to break 
through the RDX film. The spot formed as a result of 
ablation is 1 mm in diameter (see Figure 2.24) so that 
the amount of material removed per each shot is 3.2x10
-5
 g. 
Figure 2.25 is a plot of the pressure change observed as a 27 micron thick spin 
coated polycarbonate film is ablated. The ablation was conducted with a frequency of 1 
Hz. The expected number of laser pulses to break through the 27 ?m thick polycarbonate 
film is 40 (see calibration curve, Figure 2.20). No pressure rise beyond the noise level is 
observed. This result may be explained by considering both the amount of polycarbonate 
ablated and the details of the chemical processes involved in polycarbonate combustion. 
 
 49
750.0
751.0
752.0
753.0
754.0
755.0
756.0
757.0
758.0
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90
Time, s
P
r
es
su
re
,
 mm Hg
 
1
st
pulse
 50
Figure 2.25 ? Pressure as a function of time for ablation of polycarbonate film. 
At a lens to sample distance of 100 mm a single laser pulse results in a pit that is 500 ?m 
in diameter and 0.64 ?m deep, see results in Figures 2.19 and 2.20. The total ablated 
volume is then 1.25 x 10
5
 ?m
3
. Using the density of polycarbonate (1.2 g/cm
3
), a mass of 
1.5 x 10
-7 
g or 5.9 x 10
-10
 moles of the monomer are ablated in each laser shot.  The 
complete combustion of polycarbonate is: 
C
16
H
14
O
3 (s)
 + 18O
2 (g)
           16CO
2 (g)
 + 7H
2
O 
(l)
                 (i) 
Since the pressure measurement is made on the millisecond timescale, and the air within 
the vessel is at ambient temperature before and after the ablation event, the water is 
expected to condense and the only gas product is CO
2
.  In this reaction, more O
2
 is 
consumed than CO
2
 is produced so a net pressure decrease might be expected.   
Assuming a volume of the vessel (including the Baratron gauge) of approximately 3.96 x 
10
-5
 m
3
, a pressure drop of only 2.73 x 10
-4
 mm Hg would be anticipated, which would 
not be detectable. In addition, complete combustion is not typically observed in 
polycarbonate combustion, meaning that CO is anticipated as a product 
39
 along with the 
soot deposited around the ablation pits. The balanced reaction for the production of only 
CO 
(g)
 in the combustion of polycarbonate is:   
C
16
H
14
O
3 (s)
 + 10O
2 (g)
           16CO 
(g)
 + 7H
2
O 
(l)
                   (ii) 
This reaction results in a net production of 6 gas molecules and a net increase in pressure 
that would be three times larger than the loss pressure for the complete combustion, but 
that would still be below detectable levels.  The production of soot, primarily C
x
, requires 
no consumption of O
2 (g)
 and produces no gas products and so is expected to have no 
influence on the total pressure.  Since the actual combustion of the polycarbonate is some 
combination of the three processes, it does not seem unreasonable to believe that no net 
pressure increase might be observed even if its magnitude were not below the detection 
limit of the Baratron gauge used. 
Figure 2.26 is a plot of pressure change as an RDX film is ablated.  The ablation 
was conducted with a frequency of 0.2 Hz. The expected number of laser pulses to break 
 51
756
757
758
759
760
761
0 5 10 15 20 25 30 35 40 45 50 55 60
Time, s
P
r
e
ssu
re,
 mm Hg
  
Figure 2.26 ? Pressure as a function of time for ablation of RDX film. 
1
st
 shot 
3
rd
shot
2
nd
shot
 52
753
754
755
756
757
758
20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100
Time, s
P
r
essu
r
e
, m
m
 H
g
  
  
 
Figure 2.27 ? Pressure as a function of time for polymer coated RDX film. 
39
th
pulse
through the RDX film is 3. The reaction products of RDX are not well known, but a 
simple model of the reaction might be expected to result in the net formation of at least 6 
molecules by either simple fragmentation (iii) or complete combustion to stable products 
(iv): 
C
3
H
6
O
6
N
6
 
(s)
 ? 3CH
2
N 
(g)
 + 3NO
2 (g)                                
(iii) 
             2C
3
H
6
O
6
N
6
 
(s)
 ? 6HCN 
(g)
 + 3H
2
O 
(l)
 + 3NO 
(g) 
+ 3NO
2 (g)      
(iv) 
The amount of RDX removed per each shot is 3.2x10
-5
 g which makes 1.44 x 10
-7
 
moles of RDX. Thus, 8.64 x 10
-7
 total moles of gas formed during combustion. This 
accounts for a pressure rise of 0.4 mm Hg per each shot. From the Figure 2.23, first two 
shots each raised the pressure by ~ 0.4 mm Hg which is in close agreement with 
calculated pressure change. The 3
rd
 shot resulted in larger pressure rise of 0.7 mm Hg. 
The experimental error is ? 0.2 mm Hg. 
Figure 2.27 is a plot of pressure change as a polycarbonate coated RDX film is 
ablated. The ablation was conducted with a frequency of 0.2 Hz. The expected number of 
 53
laser pulses to break through the 27 ?m thick RDX film is 40. The pressure rise of ~ 1 
mm Hg is observed as the 39
th
 laser pulse reaches the underlying RDX film and 
completely breaks through it. The result is within the experimental error of ? 0.2 mm Hg 
as compared to calculated total pressure increase of 1.2 mm Hg. 
It is not unreasonable to believe that this 39
th
 laser pulse generated a high 
pressure, high temperature polycarbonate ablation plasma in contact with the underlying 
RDX film and the entire RDX film has eroded in one shot resulting in ignition. More 
experiments at various laser ablation conditions need to be done to prove that laser 
ignition of propellants can be achieved. 
 
 
 
 
 54
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