| Abstract: | 本研究探討聚乳酸高分子於靜態與連續剪切流場下之結晶行為與流變性質。由RDA II、POM與SAXS的結果發現在靜態時的儲存模數曲線可以被區分成晶胚或晶核的生成、晶體的成長和以指數型式的成長等三個區段。我們提出一個具有晶胚數目的動力學和描述懸浮物之Palierne Equation 的方程式來敘述靜態結晶過程的儲存模數曲線。此方程式非常成功的描述流變行為。因此，流變工具是可以量測出PLA靜態結晶過程的速率常數，此包括晶核成長參數、晶胚的生成與熔解參數和Avrami參數。雖然線性黏彈性是與頻率有關的，但是晶體速率常數在本質上應與頻率無關。當頻率增加兩個量級時，我們卻發現Avrami參數有三倍的改變量。因此在使用流變工具描述聚合物的結晶過程時，Avrami參數可能就是頻率範圍選擇的重要依據。另外，在連續剪切下，結晶過程中的流變曲線可被區分成晶胚的孵化時間、晶核的生成時間、晶體的成長時間和黏彈性曲線以指數型式成長等四個區段。由SAXS、光學顯微鏡和流變數據的整合，當剪切速率達一臨界值時，有序柱狀結構(shish-kebab和cylindrically stacked lamellae)可能會隨著流場有生成與成長的行為。柱狀結構出現的時間會隨著剪切速率的增加而縮短且其時間與流場間的關係可被Arrhenius Equation所描述。在剪切時，晶體中是由較長的長週期之層狀結構所組成且晶層厚度是大於晶態結晶時所觀測的。此時晶層中的長週期與晶層厚度是與剪切周期和剪切強度無關的，這暗示著晶核的尺寸決定著層狀結構的生成尺寸。最後，從SAXS中所測量到聚乳酸的總散射光強與DSC所測量到的結晶度做分析，在較弱連續剪切流場下，剪切流場是有利的結晶度。相反地，在較強剪切流場時，剪切流場是會降低結晶度。
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. |
