No abstract

Azochromophores are used as molecular labels in the main chains of polyurethane to study molecular motion and physical aging in amorphous, single-phase matrices. Kinetics of both trans cis photoisomerization and cis -* trans thermal isomerization are analyzed for dilute solutions as well as in the solid state covering the rubbery and glassy states. Photoisomerization in dilute solution occurs by a single-rate process with little temperature dependence in the range -10 to +40 °C. In solid films analogous photoisomerization seems to proceed by two separable processes, one as fast as in dilute solution and a slower one. The fractional amount of the fast process, a, increases slowly with temperature but shows a transition around Tg. With increasing aging time, a decreases in the glassy state with approximately the same time scale as enthalpy relaxation. At lower temperature, a decreases more slowly with aging time, a trend consistent with enthalpy or volume relaxation. When the film is plasticized or stretched at a temperature just below Tg, a increases. These trends suggest that a is a sensitive characteristic of free volumes or holes present in the solid matrices under investigation. A plausible way to interpret these results is to assume that a is related to the fraction of free volumes which are greater than a critical size or to the amount of the high segmental mobility associated with less dense regions in solids. Thus, the photochromic labels can provide some quantitative information on the structural changes occurring in amorphous polymeric solids. Thermal isomerization exhibits first-order kinetics either in dilute solution or in the rubbery state with an activation of about 18 kcal/mol. Deviation from first-order kinetics is observed in glassy films. The effect of physical aging on thermal isomerization seems rather small, probably due to the slower time scale of thermal isomerization.

No abstract

Curing bis(maleimide)/diallylbisphenol A (BMI/DABPA) results in the formation of a high-temperature thermoset resin. FT-IR, fluorescence, and UV-reflectance spectroscopy were used to investigate the cure behavior of this material under three different cure schedules. Fluorescence signals were quenched before curing due to the BMI component but increased and eventually leveled off as cure time increased. The largest fluorescence intensity increases occurred after 80% of the phenylmaleimide units were converted to phenylsuccinimide. Fluorescence signals were observed in both short-wavelength and long-wavelength regions. Model compound studies indicated that the phenolic portion of the BMI/DABPA resin has a higher quantum yield for fluorescence at a shorter wavelength than phenylsuccinimide derivatives. Therefore, fluorescence emission observed near 356 nm during curing is attributed to phenolic structures. FT-IR was used to quantify the extent of succinimide formation and to identify cross-linking processes which occurred during high temperature curing (250−260 °C). High-temperature curing processes were also identified by UV-reflection spectroscopy. Various reaction pathways are discussed in terms of their consistency with the spectroscopic data.

No abstract

Intrinsic fluorescence during the reactions between methylene 4,4‘-diphenyl diisocyanate (MDI) and 1-butanol or hydroxy-terminated poly(propylene oxide) was investigated to characterize a model urethane reaction and polyurethane formation both in dilute solution and in bulk. Fluorescence intensity was found to increase about 87 times when MDI was converted to diurethane, with the emission maximum around 315 nm. The rate constants and the activation energies for the model reaction and polyurethane formation were obtained from the fluorescence results and compared with the IR results. The correlation between the fluorescence intensity and the extent of the reaction determined by IR has been established.

No abstract

Curing bis(maleimide) (BMI) with diallylbisphenol A (DABPA) results in the formation of a high-performance thermoset resin. A variety of reactions in which maleimide units are converted to succinimide moieties have been proposed. In order to make spectral assignments for the fluorescence behavior observed during the cure of BMI/DABPA resin and to assess the likelihood that certain types of reactions take place during resin cure, several succinimide model compounds were synthesized from N-phenylmaleimide (NPM) and characterized. These model compounds gave fluorescence signals which were red-shifted by 40 nm or more from the emission maximum in DABPA resin, while no fluorescence was observed from the BMI. The BMI was found to quench the fluorescence from DABPA and a Stern−Volmer quenching constant was determined for this pair. Relative fluorescence quantum yields were determined for the model compounds. The DABPA resin component was found to have the highest quantum yield and is likely to be responsible for most of the fluorescence near 356 nm when the resin is excited near 280 nm. A succinimide derivative which arises from a Diels−Alder−Ene reaction sequence was found to have a higher quantum yield than other succinimides which were investigated. This type of structure might be responsible for most of the fluorescence observed in the long wavelength regions. Fluorescence peak shapes and peak positions were found to have a concentration dependence.