The Crystallization Behavior and Rheological Properties of Poly(lactic acid) at Quiescent State and under Continuous Shear

• Main Authors: WU, CHIA-HSUEH, 吳佳學
• Other Authors: YANG, I-Kuan
• Format: Others
• Language: zh-TW
• Published: 2014
• Online Access: http://ndltd.ncl.edu.tw/handle/34522585481548731163

博士 === 東海大學 === 化學工程與材料工程學系 === 103 === The purpose of this research is to investigate the crystallization and rheological behavior for poly(lactic acid) or PLA at quiescent and continuous shear flow. Results of RDA II, POM and SAXS reveal that at quiescent the evolution of storage modulus is divided into three stages, the formation of embryos or nuclei, crystal growth and exponential growth. A simple model, combining the kinetics of embryo population and the Palierne Equation for suspension, is proposed to describe the evolution of the storage modulus along the process. The model is extremely successful in portraying the rheological behavior. Rate constants related to the nucleation and the formation and dissociation of the embryo, as well as the Avrami parameter, can therefore be determined by the rheological tool. Although linear viscoelasticity is frequency dependent, the rate constants are found to be essentially independent of the frequency applied. The Avrami parameter is found to have a threefold change over a frequency range of more than two orders of magnitude, which could be an important reference for the frequency selection when implementing the rheological tool in the investigation of polymer crystallization. Upon continuous shear flow, the crystallization is clear divided into four stages, incubation time, nucleation time, crystal growth and exponential growth. The collations of three investigative tools reveal that cylindrical ordered structure, including shish-kebab and cylindrically stacked lamellae can form, survive and grow under flow as long as a critical rate is exceeded. Time at the emergence of cylindrical structure is shortened as shear rate increased and the relation between the time and the flow strength can be described by Arrhenius equation. Lamellae in crystallites formed under shear exist a longer period and higher thickness than those from quiescent crystallization. The period and thickness are found independent of shear life and shear strength, implying that nucleus determine the size of epitaxially grown lamellae. Finally, from the total scattering intensity and the crystallinity, weak continuous shear flow is favorable to the degree of crystallinity and instead, strong shear flow can reduce the crystallinity.