| This research provides an in-depth analysis on the effects of organoclay inclusion in PLA film on the evolution of structural hierarchy during constant and variable rate uniaxial and biaxial stretching processes. At lower deformation rates and/or higher temperatures PLA exhibits three regime stress-optical behavior: In Regime I, the material remains amorphous while obeying the traditional linear Stress Optical Rule (SOR), in Regime II strain crystallization occurs in conjunction with a positive deviation from SOR, and in Regime III, birefringence increases slowly with stress as the chains reach their finite extensibilities or are locked in place by an oriented crystalline network.;Application of very low stretch rates early on in the deformation process (Exponential (EXP) profile) was found to delay the strain crystallization mechanism which leads to rapid orientation development in the material. For the same value of strain, the highest birefringence values are evidenced in the Logarithmic (LOG) profile, whereas the lowest values are seen in the EXP profile. Furthermore, higher crystallinity values for the same strain values are obtained in the LOG profile, while lower values are obtained in the EXP profile. This implies that the chains orient and crystallize more efficiently when high rates are applied early on in the deformation process. Intermediate levels of crystallinity are induced when Linear (constant stretch rate) or Sigmoidal (high stretch rates at intermediate stages of deformation) deformation profiles are applied. Incorporation of Cloisite 30B organoclay speeds up the oriented crystallization process, bringing about significantly higher crystallinity levels at the lower deformation levels. However, as the deformation proceeds to high extension levels, higher crystallinity levels are noted in neat PLA.;Films stretched using simultaneous equal biaxial (SEB) mode, in which the film is stretched under identical rates to the same stretch ratios in the MD and the TD, exhibit optically uniaxial symmetry with the two out-of-plane birefringences increasing monotonously with deformation. Furthermore, in-plane isotropy, as indicated by near zero value for in-plane birefringence Deltan 12, is evident throughout the entire deformation process in both neat and filled PLA material regardless of process conditions, as the in-plane crystal and molecular orientation is equal at all equal draw ratios. Diffraction rings of much higher intensity are noted in neat PLA, indicating higher strain crystallization levels as proven by DSC. Additionally, despite the formation of a crystalline phase, out-of-plane Regime II stress-optical behavior is not observed during SEB stretching at all conditions. Furthermore, more intense diffraction spots, as well as higher % crystallinity values, are observed during UCW stretching, as stress relaxation leading to 3D oriented and ordered chains is harder to achieve and is of lower magnitude during SEB stretching.;Another part of the research involves the effects of molecular weight and rubbery state uniaxial stretching conditions on the mechano-optical behavior of Poly(m-xylylenediamine adipamide) (Nylon MXD6) films. At low stretching temperatures and high rates the stress optical behavior is found to start with an initial glassy photo-elastic behavior. Decreasing stretching rate or increasing processing temperature is found to eliminate this glassy photo-elastic regime leading to the observation of a linear initial stress optical behavior past a temperature Tll of 95°C for both M.W. materials. Following this initial regime, the behavior is controlled by the competition between orientation and relaxation during deformation. If the chain orientation relaxation is not suppressed by increasing the stretching rate and/or the molecular weight or by decreasing temperature, the material strain crystallizes.;The influence of relaxation and forced retraction from different extension levels on the structure and mechano-optical behavior of Nylon MXD6 is also investigated. When relaxation takes place at the early stages of Regime I, or from the amorphous state, the stress relaxation at the processing temperature accompanies a decrease in birefringence following the linear stress-optical behavior and the material remains amorphous. In contrast, when the material is deformed and relaxed from various points along the Regime II and the Regime III, an increase in birefringence is observed during the holding stage, as a result of the local, microscopic spontaneous deformation process of the macromolecular chains under constrained engineering strain. Whereas forced retraction from within Regime I results in a gradual decrease in birefringence, retraction from within Regime III does not yield any significant changes in structure or crystallinity level even when retraction as high as 30% is applied. (Abstract shortened by UMI.). |
