Natural Degradation Of Poly(l-lactic Acid)/montmorillonite Nanocomposites By Ftir Spectroscopy And Mapping Coupled With Two-dimensional Correlation Spectroscopy

Poly(L-lactic acid)(PLLA)is a semi-crystalline polymer with outstanding mechanical properties,biocompatibility and biodegradability.It is a green plastic material that meets the needs of national development.Due to the relatively low crystallinity and low crystallization rate of pure PLLA,modification of it by adding nanomaterials can promote some of its properties.In this dissertation,the crystalline polymorphs of PLLA and its nanocomposite,the crystallization behavior of PLLA during the heating process and the natural degradation of the nanocomposites were systematically studied by FTIR spectroscopy and mapping coupled with two-dimensional correlation spectroscopy(2DCOS)and perturbation-correlation moving-window two-dimensional(PCMW2D)analysis.First,the evolutions of the crystalline polymorphs of PLLA at 85℃ and 145℃ were respectively studied by FTIR coupled with 2DCOS and PCMW2D.Four stages were identified for crystallization at 85℃,in which the transformation of amorphous PLLA to α’-PLLA was found in the initial stage(0-30 min),while some of α’-PLLA was also changed to α-PLLA in the growth stage(30-150 min),in the final stage(150-250 minutes),the transformation from amorphous to crystalline is almost completed,but there is still a slight transition from intermediate and α’-PLLA to α-PLLA.For isothermal crystallization at 145℃,the amorphous and the intermediate PLLAs were first changed to the crystalline forms in the initial period(0-30 min),then alternate changes between α-,α’-and intermediate phases occurred in the other periods with the extension of crystallization.Second,organic modification of montmorillonite(MMT)was performed by using cetyltrimethylammonium bromide(CTAB)and PLLA/MMT nanocomposites were prepared by solvent casting.The crystallization behavior of PLLA/MMT nanocomposites during heating was studied by variable temperature FTIR(VT-FTIR)combined with difference spectroscopy,PCMW2D and 2DCOS analysis.The curves of absorbance difference versus temperature directly reflected the effect of the amounts of MMT on the crystallization of PLLA,i.e.,a small amount(1%)of MMT may restrict the crystallization,while the addition of 5%of MMT may increase the crystallization of PLLA.The results of 2DCOS and PCMW2D indicate that the addition of MMT does not change the transformation sequence of PLLA states of order during the heating process.However,less amount of MMT(≤3%)will delay the crystallization of PLLA,while more amount of MMT(≥5%)will not delay it and even leads to the cold crystallization being advanced.Finally,the natural degradation of PLLA/MMT nanocomposites was studied by FTIR mapping.The results of univariate analysis and principal component analysis(PCA)of FTIR images show that the degradation of the nanocomposites was alleviated after adding MMT and the degradation of 5%PLLA/MMT was less than the one of 3%PLLA/MMT.By further analyzing PC1 loadings from PCA results using PCMW2D and 2DCOS analysis,we found that the degradation of the nanocomposites involved the swelling and the recrystallization of PLLA molecular chains.These two processes competed with each other and alternately changed,and dominated the whole degradation process at varied stages.In this work,FTIR spectroscopy and mapping coupled with multivariate analysis and two-dimensional correlation spectroscopy were used to analyze the crystallization polymorphs of PLLA nanocomposites,which provides a new research way to explore the natural degradation of PLLA nanocomposites.This hyphenation method can also be applied to other polymer materials.