Thermoplastic polymer nanocomposites

Abstract

This thesis describes the use of non-covalent functionalization of nanomaterials and pre-processing methods for improving thermal and mechanical properties of melt processed thermoplastic polymer nanocomposites. The effects of non-covalent functionalization and pre-processing methods were studied for thermoplastic polymers: polypropylene (PP), poly(ethylene-co-vinyl alcohol) (EVOH) and polylactic acid (PLA); using a combination of morphology, thermal and rheological methods. The effects of non-covalent functionalization and pre-processing methods were evaluated by incorporating polyvinylpyrrolidone (PVP) and sodium dodecyl sulfate (SDS) functionalized single-walled carbon nanotubes (SWNT) in polymer in conjunction with the rotary evaporation or spray freeze drying methods employed prior to melt processing. The initial investigation of PP nanocomposites through rotary evaporation method revealed that the PVP functionalized SWNT dispersed poorly in the PP matrix compared to pristine SWNT. This was the result of the incompatibility between the dispersing agent and polymer matrix as well as the aggregation caused by capillary and van der Waals forces during rotary evaporation. SWNT dispersion state was slightly improved in the case of PP/SDS-SWNT, due to compatibility between SDS and PP. On the other hand, the spray freeze drying method produced porous nanotube bundles reducing the aggregation effect of capillary and van der Waals forces and resulted in a more uniform dispersion state of SWNT in PP as well as EVOH nanocomposites. Consequently, significant changes in thermal and rheological properties of PP and EVOH nanocomposites were observed. For 0.75 vol.% PP/SDS-SWNT, largest increase of 38 °C was observed in thermal degradation temperature along with maximum increase (~3 orders of magnitude) in storage modulus. 0.75 vol.% EVOH/PVP-SWNT showed similar increase in storage modulus. Moreover, a noticeable difference between storage modulus and complex viscosity of EVOH/PVP-SWNT and EVOH/SWNT was observed as a result of the compatibility between PVP and EVOH. Direct comparison of spray freeze drying with some other promising pre-processing methods (including masterbatch mixing, spray drying, and freeze drying) was carried out for PLA/cellulose nanocrystals (CNC) nanocomposites. Poor dispersion of CNC in PLA using spray drying and freeze drying methods did not cause any noticeable change in rheological behavior of PLA. Improved dispersion state of spray freeze dried CNC in PLA caused a larger increase in storage modulus compared to other pre-processing methods which further showed the effectiveness of spray freeze drying method. Additionally, it further demonstrated that the extent of changes in rheological properties of nanocomposites did not necessarily correlate with the changes in their thermal properties. The results of this research highlight the importance of spray freeze drying as a pre-processing method for producing melt processed nanocomposites and its effectiveness in translating the uniform dispersion of non-covalently functionalized/pristine nanomaterials in solvent to final nanocomposites.