Crystallization and birefringence studies on fast structural changes followed by non-contact spectral birefringence and Raman spectroscopy techniques

The structural characteristics of the manufactured polymers such as crystallinity and orientation are controlled by the thermal-deformation history imposed by the processing machinery. Therefore it is essential to monitor these structural characteristics and use the information advantageously to optimize the properties of interest by controlling the process conditions. The focus of this study is to develop robust automated techniques to monitor birefringence and crystallization in real-time.; In many of the polymer processing operations involving film stretching where the polymer undergoes a series of thermal deformation treatments, the quantitative changes that take place during the course of these processing operations is poorly understood partly as a result of very rapid structural changes that occur when the polymer's initial condition is not isotropic melt of isotropic solid. There is a great need to quantify the kinetics of the structural changes from preoriented/partially-crystallized states. As part of this dissertation, the birefringence development of preoriented polyethylene terephthalate (PET) films is monitored on-line by the developed spectral birefringence technique with the intent of simulating the heat-setting stage of a tenter film stretching process. Theoretical improvements, software development and optimization, and the equipment design and construction are some of the major tasks accomplished in this dissertation. The automated spectral birefringence technique not only allows the measurement of retardations to very high values, but also enables one to detect the changes in the trend in birefringence (such as a decrease as a result of relaxation, or an increase as a result of crystallization) and to investigate changes that occur very rapidly in the order of a few hundred milliseconds.; The effects of stretching conditions on the kinetics of the structural changes in PET films were also investigated using characterization techniques such as differential scanning calorimetry (DSC), wide-angle and small-angle X-ray scattering. In general, it was found that the heat setting for samples stretched to low stretch ratios (2.0X) results in partial or complete relaxation of orientation. This relaxation disappears above certain conditions (≥2.5X) followed by rapid increase in birefringence and long-term slow increase. At higher stretch ratios where the precursor films attain high crystallinities, the rate of structural changes slows down. These results are attributed primarily to the establishment of highly constrained network structure where nodes are composed of crystallized regions formed during stretching stage. The measurements are performed at temperatures, where the crystallization rates are the fastest and our technique performed exceptionally well to follow these structural changes.; As part of this dissertation, we also developed another robust, non-contact on-line measurement technique to monitor crystallinity of semicrystalline polymers (low-density polyethylene and polyoxymethylene) using laser Raman spectroscopy. In this technique, Raman instrument is calibrated off-line by partial least squares calibration method using the data from independent dynamic techniques such as small-angle light scattering, depolarized light intensity and DSC. The utility of this technique as an on-line measurement technique was illustrated by its application on the melt spinning of LDPE tape. In these experiments, the non-contact Raman spectroscopy technique successfully predicted the development of crystallinity along the spinline.