Structure-property relationship of polylactide/organoclay nanocomposites

Numerous studies of polylactide (PLA) nanocomposites have been reported. However, the role and importance of processing conditions in preparing the nanocomposites is the subject of very few papers. The effect of filler loadings on nanocomposites also has been widely reported but previous studies either use a narrow range of filler or report a limited range of properties.<br>In this study, PLA/organoclay nanocomposites with concentrations of 2-10 wt % of montmorillonite (MMT) were prepared by melt intercalation technique and tested for a wide range of properties. One novel contribution of this study is the optimisation of processing conditions by using statistical analysis. A Box-Behnken Design (BBD) of experiments was applied to investigate three variables; processing temperature, rotor speed and mixing time. Results showed that the optimum processing conditions were found to be at 175 °C, 100 rpm and 7 min. These nanocomposites exhibit significant improvement of practical materials properties, such as Young’s modulus and thermal stability, as compared to the neat PLA.<br>Mixing of the PLA with Cloisite® 30B (the MMT modified with a quaternary ammonium salt) resulted in formation of a nanocomposite material (PLA-30B), with characteristic intercalated peaks seen in small-angle x-ray scattering (SAXS) results and high aspect ratio tactoids or exfoliated layers in transmission electron microscopy (TEM). Rheological behaviour of the PLA nanocomposites showed effective interaction between polymer and organoclay. The rheological properties correlated with the nanostructure evolution and material property enhancement observed in other tests. The percolation threshold was reached around 4.2 wt % filler. Measurement of thermal properties showed that increasing clay content did not influence the glass transition temperature (Tg) significantly compared to the pure matrix. However, the melt temperature (Tm) and degree of crystallinity (χc) increased significantly in the presence of organoclays, indicating the filler acts as a nucleator. An increase in thermal stability with the increase in clay loading level was also observed for PLA nanocomposites. The Young’s modulus of the PLA nanocomposites improved very significantly. At 10 wt % of filler, the increase in Young’s modulus compared with the unfilled PLA was around 54 %. Tensile strength and elongation at break decreased, attributed to presence of agglomerates. Overall the properties of the nanocomposite PLA-30B were consistent with an exfoliated/intercalated morphology, with good dispersion and interfacial adhesion.<br>A further novel contribution of this work is the comparison of these nanocomposites to microcomposites made with natural MMT. Microcomposites were made with two types of natural MMT, Cloisite® Na+ and Cloisite® Ca++ DEV, to produce samples PLA-Na and PLA-Ca. The main difference between these two types of MMT is that the Na+ grade is water dispersible whereas the Ca++ grade is not. Results observed by SAXS shows the composite has a morphology that is neither exfoliated nor intercalated. Scanning electron microscopy (SEM) suggested the morphology has a predominance of agglomerates and aggregates while agglomerates were also observed in TEM. This confirmed that the morphology of these samples was that of a micro- rather than a nanocomposite.<br>In contrast to PLA-30B, both PLA-Na and PLA-Ca show a decrease in G′, G″, and η* in the terminal region (low frequency) compared to neat PLA. This suggested poor dispersion and/or interfacial adhesion between polymer and filler. For the microcomposite samples, the percolation threshold was much higher, around 9 wt %. This is consistent with the lower aspect ratio of filler in these samples compared to a nanocomposite. TGA curves recorded for PLA filled with Cloisite® Na+ and Cloisite® Ca++ show that increasing the filler content triggered a substantial decrease in thermal stability, also suggesting poor dispersion. The addition of unmodified clay provided only a modest improvement to the Young’s modulus of PLA. At the same 10 wt % filler loading, PLA-Na and PLA-Ca improved modulus by 18 % and 17 %, respectively. The inferior performance of the micro filler was attributed to the low interaction between the low aspect ratio microparticles and surrounding polymer matrix, as well as the creation of aggregates and/or agglomerates as shown by SEM and TEM. Overall the properties of the microcomposite were consistent with an agglomerated morphology, poor dispersion and interfacial adhesion.<br>Finally, the composite theory of Halpin–Tsai was applied to predict the Young’s modulus of the nano and microcomposite as a function of the organoclay concentration and aspect ratio. The higher the filler concentration and aspect ratio, the higher the modulus. The objective was to evaluate whether the Young’s modulus of PLA composites (both nano and micro) can be modelled using the Halpin-Tsai micromechanical model.<br>A comparison of the experimentaldata of Young’s modulus with values predicted from a model suggested that the aspect ratio of the clay layer is about 4 for Cloisite® 30B, and about 2 for Cloisite® Na+ and Cloisite® Ca++. The model predicted the behaviour of PLA composites satisfactorily with the appropriate material parameters obtained from the literature. However, a more accurate estimate of aspect ratio by quantitative examination of multiple TEM micrographs is recommended.<br>Other areas suggested for further work include evaluation of barrier properties and bio degradability, whether mixing could be optimised better with a statistical analysis based on a different design of experiments, as well as comparison of properties with other micromechanical models.<br>