Structure and relaxation of polymers, polymer-based nanocomposites, and biopolymers

Polymer nanocomposites have generated significant attention in both the academic and the industrial areas of materials science and engineering. They provide both interesting and improved physical properties compared to the conventional polymers. Polymer nanocomposites are simply the mixture of the conventional polymer matrix and several kinds of nanofillers, including nanoclay, carbon nanotube and nanoparticles. The most common reason for adding nanofillers, such as nanoclay and carbon nanotubes, into a conventional polymer matrix is the mechanical reinforcement. The enhancement of the stiffness and elasticity and the reduction of the coefficient of thermal expansion (CTE) of the polymer matrix are the direct results of the addition of nanofillers. Other than the mechanical reinforcement, the polymer nanocomposites show some other improvements compared to the conventional polymer matrix. Properly aligned nanoclay can reduce the gas permeability to a great extent and improve the barrier properties, which has potential application in the industrial packaging field. Finally, polymer nanocomposites have applications in biomedical engineering and fuel cell technology. The incorporation of nanoclay can introduce some "nano-effect" into the polymer matrix. The crystallization kinetics and the glass transition temperature Tg can be altered due to the interaction between nanoclay and the matrix polymer chains.In this thesis, several topics and issues are presented and discussed to show various effects of nanoclay on the crystal structure, physical properties and the crystallization process of the polymer matrix. In Chapter IV, the change of crystal phase of electrospun poly(vinylidene fluoride) (PVDF) nanofibers with the addition of nanoclay is investigated. Neat electrospun PVDF nanofibers contain non-polar alpha phase and polar beta phase crystals and the nanoclay induces more polar beta phase. An ion-dipole interaction between the negatively charged nanoclay platelet and the partially positive CH2 group in PVDF is utilized to explain this phenomenon and the existence of this interaction is proved by Fourier transform infrared spectroscopy.The effect of nanoclay on the relaxation behavior of PVDF nanocomposite film was investigated in Chapter V using dielectric relaxation spectroscopy (DRS) and wide and small angle X-ray scattering. The addition of this nanoclay to PVDF also results in preferential formation of the polar beta-crystallographic phase. The relaxation rates for processes termed alphaa (glass transition, related to polymer chain motions in the amorphous regions) increases with the concentration of nanoclay because of the reduction of intermolecular correlations between the polymer chains. These are caused by the presence of nanoclay silicate layers, which segregate polymer chains in the amorphous regions. The alphac relaxation rate (related to polymer chain motions in the crystalline regions and fold surfaces) increases with concentration of nanoclay in all nanocomposite samples. The DC conductivity is compared between neat PVDF and nanocomposites samples and 10 wt% nanoclay increases the DC conductivity by almost four decades compared with neat PVDF.X-ray scattering and dielectric relaxation spectroscopy (DRS) were used in Chapter VI to investigate the effects of nanoclay on the structure and relaxation dynamics of poly(vinylidene fluoride-co-trifluoroethylene) [P(VDF-TrFE)]/Nanoclay nanocomposites. The electrical properties of the neat P(VDF-TrFE) and nanocomposites with nanoclay were studied during the crystallization process which includes the melt-to-paraelectric and paraelectric-to-ferroelectric transitions. In Chapter VII, silk fibroin protein was studied with dielectric relaxation spectroscopy (DRS). Two emerging dielectric relaxations were observed in silk fibroin protein with bound water compared to that without bound water. Isothermal crystallization of silk fibroin protein is investigated by real-time DRS methods and the silk fibroin protein shows a different crystallization mechanism compared to conventional thermoplastic polymers.Finally, .m files for two MATLAB programs, "Real Time Dielectric Application" and "DFit", which are used for dielectric relaxation spectroscopy (DRS) data gathering and analysis, are shown in the Appendix.