Low-Temperature Nutleation Of Polylactide And Its Effect On Structure And Morphology

As a renewable bio-based polymer material,polylactide(PLA)not only has good biodegradability and biocompatibility,but also has mechanical properties and processability comparable to traditional petroleum-based materials.PLA is thus widely used in many fields,such as biomedical materials and tissue engineering.However,the slow crystallization rate of PLA limits its further application.Similar to other semicrystalline polymers,the macroscopic physical properties of poly(L-lactide)(PLLA)materials strongly depend on their microscopic crystal structure/morphological parameters.It is of great significance to regulate and control the formation and evolution of crystalline structure of PLLA.Polymer crystallization can be divided into two processes:nucleation and growth.However,understanding the crystal nucleation is still elusive.This thesis aims to study the low-temperature crystal nucleation behavior of different PLA systems under atmospheric pressure or carbon dioxide atmosphere by tracking molecular conformation and surface morphology changes.1.Crystal Nucleation behavior of PLLA with low optical purity.Using infrared(IR)spectroscopy and in-situ IR spectroscopy,the effect of physical aging(50-70?for different times)on the conformational changes of PLLA during physical aging and on subsequent crystallization behavior of PLLA at high temperatures were studied.The promotion effect of previous crystal nucleation on the subsequent PLLA crystallization was gradually enhanced upon increasing aging time.The results of in-situ IR spectroscopy showed that the physical aging process increased the molecular ordered conformation and decreased the disordered conformation.Upon physical aging,the AFM results indicated that the local ordered structures with a nodular morphology were formed,which was futher transformed into the aggregation of needle-like ordered structures on the surface of the samples after annealing at higher tempretures.The results confirmed that low-temperature physical aging treatment enhanced the crystallization of PLLA with low optical purity by promoting the crystal nucleation.2.Diluted effects on the crystal nucleation of PLLA at low tempreture.Four groups of PLLA/PDLLA blends with blending ratios of PDLLA with 0%,10%,25%,and 40%were prepared by solution blending.Their nucleation and crystallization behavior after pretreatment around the glass transition temperature(60?)for different times were studied.IR Results indicated that the previous pretreatment promoted the crystallization of samples at 130 ?.The related content of the ordered structure increased with increasing pretreatment time and decreased with increasing blending ratio.Combining with the in-situ IR results,it was found that the blending effect not only suppress the formation the local ordered structure in the melt-quenched samples but also affected the formation of ordered conformations during the pretreatment(i.e.,physical aging),thereby exerting a profound impact on the file the degree of ordering of the crystallized samples that annealed at high temperature.Furthermore,AFM results confirmed the blending effect of PDLLA as a diluent on the suppressed formation of local ordered structures during the pretreatment and the resulting crystalline morphology of the crystallized samples.These results deepened the understanding into the effect of the blending effect on the nucleation and crystallization of PLLA.3.The CO2-induced structural ordering in glassy PLLA/PDLLA blends(four groups with blending ratios of 100/0,75/25,50/50,and 25/75)was investigated to reveal the blending effect of PDLLA on the structural ordering of optical pure PLLA.The results showed that no crystallization occurred after CO2-treatment under 0?/2MPa even for 9 h;instead,the mesophase structure was formed.The content of the mesophase structure in all of four groups of blends increased with increasing of pretreatment time.Moreover,the blending of PDLLA clearly retarded the mesophase formation of PLLA in PLLA/PDLLA,which may be mainly due to the restriction effect of the noncrystalline PDLLA chains on the movement of the PLLA chains.