Biobased and Biodegradable Polymer Blends and Nanocomposites with Superior Dispersion and Physical Properties Created Using Solid-State Shear PulverizationPublic Deposited
Growing concerns associated with global warming and the depletion of oil reserves have motivated significant research in support of sustainable polymers systems. In this thesis, biobased and biodegradable polymer blends and nanocomposites were created using a novel processing method, solid-state shear pulverization (SSSP). This process utilizes a modified twin-screw extruder which is cooled to maintain the polymer below its glass transition temperature or melt transition temperature. A combination of high shear and compressive forces results in repeated fragmentation and fusion during pulverization. This novel process provides an opportunity to tune and improve the mechanical, thermal, and barrier properties of biodegradable polymer systems. Pulverization of biodegradable homopolymers led to substantial changes in the behavior of the polymers. When polycaprolactone (PCL) was pulverized, the resulting materials showed a doubling of the tensile modulus, a 30% increase in tensile strength, and a 30% reduction in oxygen permeability compared to the neat PCL homopolymer. These improvements are associated with the combined effect of a reduction in molecular weight and an enhancement in crystallization kinetics of the PCL following pulverization.\ Another method for utilizing biodegradable materials is incorporation of biobased fillers into a polymer matrix. Blends containing starch-based fillers and cellulose were created using pulverization. When just 5 wt% cellulose was added to polypropylene (PP), the tensile modulus increased by 60% while the tensile strength remained unchanged. When the cellulose increased to 20% of the composite, the modulus doubled and the oxygen permeability coefficient was decreased by over 25%. Due to the inexpensive and green nature of the cellulose filler, these composites could be an attractive alternative to the PP homopolymer or more expensive glass filled composites. Pulverization also provides an opportunity to disperse nanofillers, such as silicates. In layered silicates the ideal platelet dispersion, exfoliation, is often difficult to achieve using conventional polymer processing techniques. The unique mixing capabilities of SSSP result in tunable levels of platelet dispersion, even achieving well-exfoliated nanocomposites in systems which were previously impossible to exfoliate using conventional methods. The high shear forces in pulverization resulted in exfoliation of pristine clay in PCL for the first time.