NONLINEAR MECHANO-OPTICAL BEHAVIOR OF PHENYLALANINE-BASED POLY(ESTER UREA): LOCAL ORDER IN AMORPHOUS POLYMERS

摘要

The uniaxial mechano-optical behavior of amorphous phenylalanine-based poly(ester urea) (PEU) of varying diol chain lengths was studied in the rubbery state. A real-time measurement system that can capture true stress, true strain and real-time birefringence during uniaxial stretching was used. When subjected to deformation at low temperatures above Tg, materials exhibit photoelastic behavior contributed by the initial glassy component as segment- segment rigid contacts dominate. At higher temperatures above Tg, PEU with relatively shorter diol chain length shows the liquid-liquid (T_(ll)) transition at about 1.06 Tg (K), in which the material transforms from a "liquid of fixed-structure" to a "true liquid" state and the initial photoelastic behavior disappears and initial slope becomes independent of temperature signifying true liquid state. The Fourier transform infrared spectroscopy (FTIR) spectra indicates that the strength of hydrogen bonding diminishes with increasing temperature. In the case of PEUs with the longest diol chain length, integration of the area associated with N-H stretching region exhibits a linear dependence of temperature. However, the integrated area of N-H stretching region changes with temperature in three stages for PEU with shorter diol chain length. The presence of hydrogen bonding enhances the segmental correlations between adjacent chains in the PEU structure. It was found that the photoelastic constant decreases with increasing hydrogen bonding strength.