Preparation And Properties Of Poly(L-lactic Acid)/Montmorillonite Composites

Increasingly depleted oil resources and environmental problems caused by thewhite pollution urging people to look out for polymers that can replace petroleum-based plastics. Poly(L-lactic acid)(PLLA) has attracted much attention due to itsexcellent biodegradability and biocompatibility in recent years. However, PLLA haspoor thermal resistance,low crystallinity and weak barrier properties that have limitedits application in packaging materials.This study aims at broadening its application inthe field of packaging through incorporating nanofiller organo-montmorillonite(OMMT) to PLLA and making a comparision between the two processes. Thefollowing two aspects were included in the experiments.(1)PLLA/OMMT composites were prepared through melt mixing, and tensiletesting machine, transmission electron microscopy and differential scanningcalorimeter were used to investigate its properties. Tensile strength, flexural strengthand modulus has been reduced with the addition of OMMT.In contrast, impactstrength of PLLA/OMMT has been slightly improved when compared with purePLLA. It is worth noting that the elongation at break has been significantly improvedwith increasing OMMT content. Subsequent TEM test has shown that only a smallportion of OMMT sheets have been peeled into PLLA matrix, and the presence ofholes in the system has proved poor interaction between PLLA and OMMT. Bothpure PLLA and PLLA/OMMT composites crystallized at120℃were observedwith polarized microscope.When compared with pure PLLA, the composites systemshowed decreased spherulite size, lower degree of order and more grains.In addition,the crystallinity of PLLA/OMMT composites are higher than pure PLLA, and thedegree of crystallinity were gradually increased with increasing OMMT loading，indicating OMMT has played a role in nucleating agent.Thermal properties werestudied and analyzed with DSC and TG.OMMT was found to has improved thermalstability of PLLA by comparing the half life and the initial weight losstemperature,and reached to the top as OMMT content come to1wt%. (2)PLLA/OMMT composite films were prepared through solution castedmethod, in which chloroform was used as solvent. Mechanical, thermal andcrystallization properties of pure PLLA and PLLA/OMMT composites films werestudied as well as morphology through scanning electron microscope. Mechanicaltests showed that tensile strength of PLLA/OMMT composite films graduallyincreased with increasing OMMT content,but reduced when more OMMT powderwere added. Loading of2wt%of OMMT seemed to benifit PLLA in tensile strengthmost.With the increase of montmorillonite content, the elongation at break of PLLAdecreased first and then reached a steady level. Both pure PLLA and PLLA/OMMTcomposite films crystallized at120℃were observed with polarizedmicroscope.When compared with pure PLLA, the composite system showeddecreased spherulite size, lower degree of order and more grains. SEM wereperformed to investigate the structure of composites.It is clear that OMMT wereuniformly dispersed in the polylactic acid matrix, ie no serious agglomerationoccurred.But when OMMT content comes to3wt%, some OMMT layers stacked toform clusters, which is consistent with the mechanical test results.Thermal analysishas proved that the addition of OMMT greatly enhanced crystability and thermalstability of PLLA.The results show that poly（L-lactic acid）/organo-montmorillonite compositefilms prepared by solution mixing has better comprehensive mechanical performancethan that of melting mixing, which can be explained by the former process haspromoted OMMT’s dispersion in the PLLA matrix more effectively than the meltingshearing process. Besides,the two processes have the same effect on crystallizationand thermal behavior of PLLA.