Characterization On The Chain Heterogen-of PLLA Homopolymer And L-lactide Based Copolymer And Its Influence On Polymer Thermal Behavior

Commercial poly(L-lactide)(PLLA) is typically heterogeneous in chain structure due to the existence of a small amount of D-lactyl units that are produced by the racemization reactions during the synthesis. Further, different polymerization conditions may also result in changes to the structure of poly(L-lactide) since the racemization reactions may be affected by many factors, such as reaction temperature, pressure, time and types of catalyst. For this reason, two commercial poly(L-lactide) samples, one with relatively high stereoregularity and the other with low stereoregularity were selected as the research sample in this study. The stereochemical heterogeneity of two commercial samples was investigated with temperature rising elution fractionation (TREF) and successive self-nucleation/annealing (SSA) thermal fractionation.For both samples three fractions were collected and characterized with rotatory power analysis. The collected fractions show distinct optical purity, which reflect the structure differences among them directly. DSC results of the PLLA fractions were compared. It was found that the reduction in the stereoregularity had a strong inhibition effect on both crystallization and reorganization abilities of the PLLA fractions, which led to distinct cold crystallization and melting behavior. In the self-nucleation (SN) analysis, self-nucleation domain (Domain Ⅱ) was carefully determined in the region of164-165℃for J-PLLA110as an example. The locations of Domain Ⅱ for the rest of the TREF fractions were determined in the same way. The first self-nucleation temperature (First Ts) to be used in the SSA method was carefully chosen in Domain Ⅱ for all fractions. After SSA treatment, the appearance of multiple melting behavior for each fraction indicated that the chains of fractionated sample contained a heterogeneous inter-and intramolecular distribution of stereodefects. Additionally, analysis of the lamellar thickness showed that as the optical purity decreased, both the amount of thicker lamella and the range of lamella thickness decreased gradually. Furthermore, the molecular chain heterogeneity of L-lactide based copolymers was also investigated using TREF and SSA. Three copolymers (PLGA95, PLC95and PLC90), one with5mol%glycolide (GA) and the other two with5and10mol%ε-caprolactone (ε-CL) compositions were prepared and fractionated by using TREF. For each sample, three fractions were collected and characterized with nuclear magnetic resonance spectroscopy (1H NMR) and gel permeation chromatography (GPC). The1H NMR results showed that there exists obvious differences in the comonomer content among the fractions. It was also found, with GPC method, that for the fraction of higher elution temperature, the number average molecular weight (Mn) was higher and the distribution was narrower. The data obtained suggested that the fractionation was mainly the result of the difference of the chain tacticity. Similar to the PLLA fractions, the copolymer fractions also showed multiple endotherms after SSA treatment, which indicated that a heterogeneous inter-and intramolecular distribution of defects still existed in the chains of copolymer fractions.The effect of molecular structure distribution on crystallization and melting behavior of PLGA95and PLC95fractions were studied using DSC and1H NMR.1H NMR results showed that the fractions of PLGA95and PLC95have similar comonomer content but different comonomer distribution. DSC results showed the difference in comonomer distribution resulted in a marked difference in the crystallization and melting characteristic of TREF fractions for the two polymers. The crystallization and reorganization abilities of PLC95fractions decreased markedly with increasing comonomer content, whereas these properties of PLGA95fractions only changed slightly. This indicated that the comonomer distribution had a more significant effect on the crystallization and melting behavior than the comonomer content.Finally, the spherulite growth behavior and mechanism of J-PLLA, PLGA95and PLC95fractions were studied using polarization optical microscopy. It was found that the incorparation of comonomer units reduced the radius growth rate of spherulite. Such effect became remarkable with the content of comonomers. In the whole crystallization temperature range, the disturbance effects of comonomers on the spherulite growth decreased in the following order:s-caprolactone> glycolide> D-lactide. Moreover, the spherulite growth kinetics of the fractions of three polymers was analyzed by using the second nucleation theory. The energetic parameters, such as the nucleation constant Kg, the fold surface free energy σe, and the chain fold work q were estimated. The values of Kg, σe and q increased with increasing comonomer content, which indicated the fraction with lower stereoregularity need to overcome a higher energy barrier for nucleation and growth.