Influence Of Self-nucleation And Nucleating Agents On The Crystallization Behavior Of Poly (Lactic Acid) Probed By Real-time Infrared Spectroscopy

Poly(lactic acid)(PLA)is an important biodegradable and biocompatible crystalline polyester,derived from renewable resources.Due to its intrinsic slow crystallization rate,molded PLA products are usually amorphous,limiting its wider application.Self-nucleation(or memory effect)and adding nucleating agent have been demonstrated to be two effective ways in improving the crystallization rate of PLA.In this thesis,the influence of self-nucleation,and two kinds of nucleating agents including multiamide(TMC)and zinc phenylphosphonate(PPZn)on the crystallization behavior of PLA have been investigated.The main results are as follows.(1)The effect of self-nucleation(or memory effect)on crystallization behavior of PLA.Fourier transform infrared spectroscopy(FTIR)was used as a probe of real-time vibrational changes associated with inter-and intra-chain interactions during self-nucleated PLA crystallization.The melt with different degrees of order was prepared by partial melting at various temperatures.The results are as follows.Compared with the sample completely melted at 190 oC,partially ordered melt(melted at 170 and 168 oC)could serve as heterogeneous nucleation sites,and shorten the induction period of crystallization and accelerate the crystallization rate.In the initial period of crystallization,the order of intensity changes is as follows: 1458 cm-1 > 1210 cm-1 >> 921 cm-1,1458 cm-1 > 1210 cm-1 > 921 cm-1,and 1458 cm-1 > 1210 cm-1 ~ 921 cm-1 for PLA melted at 190,170,and 168 oC,respectively.This indicates that with increasing the degree of order,both the formation of skeletal conformational-ordered structure and,especially,the 103 helix structure are accelerated.In addition,the influence of self-nucleation on the crystallization kinetics was also discussed based on the Avrami model.It is found that the value of the Avrami exponent for partially melted PLA is lower than that of the completely melted material.This can be attributed to residual crystals or ordered structures existing in the low-temperature melt of PLA.(2)The effect of nucleating agent TMC on crystallization behavior of PLA.The effect of TMC on the crystallization behavior of PLA has been investigated by time-resolved FTIR and wide angle X-ray diffraction(WAXD).The results are as follows.A small amount of TMC can significantly increase the crystallization peak temperature and the crystallization rate of PLA,although the crystal form of PLA is not changed.In the initial period of crystallization,the order of intensity changes is as follows: 1458 cm-1 > 1210 cm-1 >> 921 cm-1,1458 cm-1 ~ 1210 cm-1 > 921 cm-1,and 1458 cm-1 ~ 1210 cm-1 ~ 921 cm-1 for neat PLA,PLA containing 0.1 wt% and 0.3 wt% TMC,respectively.This indicates that TMC can accelerate both the formation of skeletal conformational-ordered structure and the 103 helix one,and the formation of ordered structures caused by intermolecular interactions,especially.As a result,the crystallization induction period is shortened.This can be explained on the basis of surface-induced conformation order and size-dependent soft epiphytic theory.Moreover,the different crystallization behavior between PLA containing 0.1 wt% and 0.3 wt% TMC can be ascribed to the more efficient nucleating ability of PLA with higher TMC content.(3)The effect of nucleating agent PPZn on crystallization behavior of PLA.The effect of PPZn on the crystallization behavior of PLA has been investigated by time-resolved FTIR and WAXD.The results are as follws.A small amount of PPZn can significantly increase the crystallization peak temperature and the crystallization rate of PLA,although the crystal form of PLA is not changed.In the initial period of crystallization,the order of intensity changes is as follows: 1458 cm-1 > 1210 cm-1 >> 921 cm-1,and 1458 cm-1 > 921 cm-1 > 1210 cm-1 for neat PLA and PLA containing 1.0 wt% PPZn,respectively.This suggests that PPZn can accelerate both the formation of skeletal conformational-ordered structure and,especially,the 103 helix structure,and then shorten the crystallization induction period.This can also be explained by surface-induced conformational order and size-dependent soft epiphytic theory.