Kinetic effects on the multiphase structure of segmented polyurethanes.

摘要

This dissertation was aimed at elucidating the structural changes of a few series of segmented polyurethanes as a function of temperature and thermal treatment from a kinetic point of view by using a combination of differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), thermally stimulated discharge current (TSDC), stress-strain test, small-angle light scattering (SALS), polarized light microscopy (PLM), scanning electron microscopy (SEM), and synchrotron small-angle X-ray scattering (SAXS). SAXS studies showed that the phase-separation process behaved like a relaxation, and was very slow at most of the annealing temperatures owing to the high system viscosity and the low hard segment mobility. The phase-separation rate was temperature dependent while the interdomain spacing remained unchanged throughout the whole process. Once the structural development reached an equilibrium and the interaction among the hard segments reached a certain level, post-annealing had very little effect. The structural changes were irreversible. The relations between the microphase structure and the macroscopic structure were investigated by using SALS, SAXS, PLM, SEM, and DSC. The spherulite structures were observed for the first time from melt-quenching. Four temperature regions were proposed to explain this behavior.; This work showed that the kinetic factors, i.e., the hard segment mobility, the system viscosity, and the hard segment interaction, are the three controlling factors for the structural development in segmented polyurethanes. They are generally more important in formulating a better understanding on the structure-property relationships of segmented polyurethanes. The thermodynamic point of view, prevalent in the past, appears to be overemphasized. This is further proved by a series of experiments on the samples with different hard segment mobilities. In addition, a long time controversy on the theoretical prediction of phase separation degree as a function of segment length could be readily explained based on the mobility-viscosity-interaction proposal.