Experimental And Mechanism Investigation Of Polylactide/layered Silicate Nanocomposites

In recent years, polylactide (PLA)/layered silicate nanocomposites have raisedgreat interest from worldwide research institutes and enterprises due to the remarkableimprovement of mechanical, thermal and barrier properties. But it remains animportant subject to fabricate polylactide nanocomposites with clay mineralsdispersed uniformly in the polymer matrix successfully. In addition, probing the microstructure and interaction mechanism within these materials from molecular level is ofgreat significance to predict, design and prepare polylactide/layered silicatenanocomposites with excellent and controllable properties.In this paper, layered silicate minerals (montmorillonite and attapulgite) wereorganic modified firstly, and were used to prepare polylactide nanocomposites bysolution casting methods. The effects of different organic modifiers on the claydispersion status, arrangement structure and thermal properties of polylactide wereinvestigated using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction(XRD), transmission electron microscopy (TEM), and thermogravimetric analysis(TGA) in detail. The results indicate that12-aminolauric acid (ALA) has distincteffects on the dispersion of montmorillonite (MMT) platelets into PLA matrix, wherepartial exfoliated as well as intercalated structures have been obtained. However,organoclay with ordinary modifier-cetyltrimethyl ammonium bromide can not beexfoliated and dispersed uniformly into the matrix. TGA data verifies that PLAnanocomposites with ALA-MMT organoclay display enhanced thermal stability. Theoptimal clay loading of ALA-MMT occurs at3%wt, leading to the best compromisebetween clay dispersion and thermal properties. Attapulgite (ATP) can be disperseduniformly into the polylactide matrix whether it is organic modified or not. However,the thermal stability has been improved more by introducing natural ATP than thatwith organic ATP. Meanwhile, molecular dynamics simulations have been performed to investigatethe adsorbing behavior of novel bifunctional Gemini surfactants at the interface.Effects of the structure on the aggregation and interfacial properties of surfactantshave been studied in detail. The results testify that the existence of irregular Geminisurfactants monolayer. The interfacial thickness has been significantly increased andthe interface reaches the most stable status when the spacer of Gemini surfactants hassix carbons. Besides, when the spacer has three or six carbons, the interaction betweenwater molecules and surfactants becomes strongest and has greatest impact on thediffusion coefficient of water molecules. In PLA/MMT system, the binding energybetween modifiers and MMT surface, as well as the basal spacing of MMT, exhibitsan increasing tendency with the increasing chain length of organic modifiers. On thecontrary, the binding energy between polylactide and MMT shows the opposite trendcomparing to the binding energy between modifiers and MMT surface. Incorporatingthe carboxyl group into the organic modifiers will further enhance the above effects.When MMT was modified with positively charged amino acids, larger spacing roomand stronger interaction between components of it have been obtained when comparedwith those ones modified by ordinary quaternary ammonium with the same chainlength. In the polylactide/ATP system, organic modifiers will hinder the interactionbetween polylactide molecules and ATP surface, thus preventing the improvement ofthermal stability of polylactide.