An analysis of the amorphous phase in polymers
An accurate and quantitative description of the amorphous phase is necessary to understand a correlation of structure with properties for polymeric materials, especially as these evolve with time. Due to the lack of long-range order, the analysis of the amorphous phase in polymers has been difficult and has yielded only limited information. Flory's rotational isomeric states (RIS) model provides a useful characterization of the short-range order for chains in the amorphous phase, and provides semi-quantitative information for an entire amorphous chain. In this research, experimental spectroscopic methods are combined with simulation of spectra based on superposition of normal coordinate analysis of RIS generated chains as applied to Poly(lactic acid) (PLA) and Poly(vinylidene fluoride-hexafluoropropylene) [P(VDF-HFP)]. Due to the unusually high strain-induced crystallization in PLA, it is of great importance to understand the structural changes that accompany deformation. The change in conformational distribution induced by deformation of PLA is estimated based on comparison of changes in the observed Raman spectra with those in the calculated. Analysis of the amorphous phase of PVDF homopolymer and P(VDF-HFP) copolymer is motivated by the increasing use of these polymers in biomedical devices. Spectroscopic features that characterize the conformational structure were established by comparing experimental and simulated Raman spectra of these materials. Different RIS models were investigated and compared with the experimental spectra, leading to an assessment of the distribution of conformation in the amorphous phase.
The dynamic behavior of the amorphous phase under external excitation is also of great interest, as this behavior is closely related to the physical properties of the polymer. In this research, the dynamics of amorphous chains were investigated through the response to microwave frequency excitation. Eleven different polymeric systems were studied as a function of temperature by microwave frequency dielectric spectroscopy. Relaxation processes were observed in polymer with low glass transition temperatures (Tg) and characterized on the basis of the temperature dependence of the relaxation. These relaxation processes all followed an Arrhenius temperature dependence. The activation energy was determined and compared to Tg, which strongly suggested that relaxation was by the Johari-Goldstein β process.