A rheo-optic study of hydrogen-bonded polymers

The influence of hydrogen bonds on polymer mechanical properties was examined using rheo-optic techniques. To isolate the hydrogen bond effect, poly(vinyl alcohol) (PVOH), which has a high hydrogen bond density, was studied during stress relaxation, a process dominated by intermolecular hydrogen bonds.;To vary the degree and strength of hydrogen bonding, copolymers containing 2.7, 5.7 and 12.8% poly(vinyl acetate) were prepared by reacetylation of PVOH. Samples were also annealed, which increased the crystalline fraction of these semi-crystalline polymers, thereby affecting hydrogen bond strengths.;The OH stretching frequency in the infrared (IR) spectra of these polymers is a function of hydrogen bond strength. IR spectroscopy was used to measure samples' molecular response during stress relaxation.;Samples were subjected to deuterium exchange reaction before testing; amorphous regions undergo this exchange process preferentially to crystalline regions. Because IR vibration is a function of mass, deuteration resulted in separation of crystalline and amorphous responses to stress in the spectra: OH stretching frequency represented crystalline response, OD the amorphous.;Prior to rheo-optic testing, polymers were characterized by differential scanning calorimetry. Analysis of these data lead to the identification of a previously improperly assigned endotherm near 410 K. This endotherm was shown to be caused by a transition in crystalline regions to a lower energy state.;Three types of stress relaxation tests were performed: high strain level, low strain level, and strain and recovery sequences. Correlations between stress and IR peak position and band distribution were found during the application of large strain, though during subsequent relaxation these correlations were not as pronounced.;During low strain level tests, each stress-sensitive band was divided into increments, which effectively separated the various hydrogen bond strengths. The proportion of total peak area in each increment was plotted, along with relaxation modulus, against time. These results indicate that stress relaxation occurs by a redistribution of hydrogen bond strengths, and that stress is borne differently in crystalline and amorphous regions.;Strain and recovery test results emphasized the correlation between stress and hydrogen bond strengths. These data indicated crystalline response to stress is "stiff", whereas amorphous response is "viscous".