Study Of The Structural Polymorphs Of Poly(l-Lactic Acid) And Its Cellulose Nanocrystal Composites By Attenuated Total Reflection Infrared Mapping

Currently,Poly(L-lactic acid)(PLLA),which is called "green"eco-friendly material.However,some shortcomings of pure PLLA,such as slow crystallization rate and poor thermal deformation resistance,limit its applications in certain fields.Researchers found that the addition of nanomaterials can effectively improve the performance of PLLA.In this thesis,FTIR mapping was used to systematically study PLLA/ACNC nanocomposites.Firstly,FTIR combined with XRD analysis method was applied to systematically study the influences of different solvents on the structural polymorphs of PLLA casting film.It is found that the Hansen solubility parameters of different solvents are the internal determinants of the structural polymorphs of solvent-cast PLLA films,while the casting conditions are the external factors.The stronger the interaction between the solvent and the PLLA,the better the solubility.The order of the solubility of the five solvents to PLLA is DCM>TCM>DCE>DOX>THF.Both PLLA films cast by DCM and TCM are amorphous,while the ones cast by DCE,DOX and THF are crystalline.When the evaporation temperature T is less than Tg(～50℃),PLLA molecular chains are rigid and their free diffusions are hindered during film formation,and PLLA is mainly composed of relatively unstableα’-crystals.When T is larger than Tg,PLLA molecular chains become soft and can diffuse freely,therefore they can spontaneously rearrange into more ordered α-crystal structure with lower potential energy.The PLLA films cast by THF at room temperature are preferentially crystallized along the(0010)crystal plane of the α’-form.However,increasing the evaporation temperature to 60℃ can eliminate this preference.Secondly,the CNC was acetylated and modified,and the PLLA/ACNC nanocomposite was prepared by casting with chloroform and 1,4-dioxane.It is found that the compatibility of acetylated CNC in PLLA has been significantly improved by using FTIR mapping.For ACNCs with a degree of substitution of 1.17 and 1.60,good dispersion can be obtained when the amount of addition is 2-6 phr.However,when the ACNC added is increased to 8 phr,the compatibility of the composite material is significantly worse and an agglomeration occurs.The principal component analysis(PCA)is used to process FTIR images by studying the intensity changes of the load spectra I1384/I1361 and I1211/I1186,We found that addition of ACNC will prevent the crystallization of PLLA by studying the intensity ratios I1384/I1361 and I1211/I1186 of the load spectra.However,the addition of ACNC can lead the crystalline regions of PLLA to transform to a more regular crystal form.Finally,the visualized degradation kinetics of PLLA/ACNC nanocomposites and the evolution of PLLA’s structural polymorphs during degradation were systematically studied by FTIR mapping.The FTIR images based on the Al057/A1133 ratio directly shows that the degradation kinetics of PLLA/ACNC nanocomposite is affected by the degradation temperature,the amount of ACNC and the type of casting solvent.When the degradation temperature is larger than Tg,the polymer degradation rate is fast;in turn,the degradation rate is slow.In addition,the crystallinity of PLLA is affected by the amount of ACNC and the type of casting solvent,i.e.,PLLA with low crystallinity degrades faster.When PCA was used to process the FTIR images and the intensity ratio(I1384/I1361 and I1211/I1186)changes of the load spectra were studied,we found that the amorphous PLLA molecular chains can be rearranged to form a crystalline state and a transformation between α-andα’-forms of the PLLA films cast by TCM and DCM is occurred when the degradation temperature is larger than Tg.For the degradation temperature less than Tg,the transformation between amorphous and crystalline states is not obvious,while the transformation between α-and α’-forms is still observed.The work in this thesis shows that both the casting solvents and the adding nanomaterials have an important influence on the structural polymorphs of PLLA and its nanocomposites,which determine their performance.The FTIR mapping method developed in this thesis can sensitively found the detailed structure changes of PLLA nanocomposites,which can be extended to the researches of more polymer nanocomposites and provide a great new ideas and methods for study of such nanocomposites.