Singling out the Role of Molecular Weight in the Crystallization Kinetics of Polyester/Clay Bionanocomposites Obtained by in Situ Step Growth Polycondensation

The isothermal crystallization kinetics of a set of bio-nanocomposites produced by in situ catalytic step growth polycondensation of adipic acid and 1,4-butanediol in the presence of Moroccan clay beidellite (BDT) organo-modified with hexadecyltrimethylammonium bromide (cetyltrimethylammonium bromide, CTA) was investigated and compared with that of the parent poly(butylene adipate) (PBA) matrices from which the clay had been extracted. In situ bio-nanocomposites had different contents (0-5 wt %) of CTA/BDT nanofillers characterized by different extents of organo-modification (CTA/BDT equivalent ratios from 0 to 5). The isothermal crystallization rates of the bio-nanocomposites and of the parent PBA matrices were investigated by differential scanning calorimetry (DSC) at 45, 40, and 37 °C and analyzed according to the Avrami model. The bionanocomposites with an intermediate (2 wt %) concentration of organoclays with a higher CTA/BDT ratio (3 and 5) showed the highest exfoliation degree, along with an increase in the crystallization rates, compared to those of the parent PBA matrices, which was larger than that in the other nanocomposites. The lack of a simple correlation between the nanoclay content/composition and crystallization kinetics was ascribed to the molecular mass, an additional variable for in situ bio-nanocomposites as compared to nanocomposites prepared by simple physical blending of nanoclays with a single polymer matrix. The specific contribution of the molecular mass to the crystallization kinetics was untangled from those of the organoclay content and CTA/BDT ratio by comparing each bio-nanocomposite with its parent polymer matrix. The crystallization rate of the nanocomposites was always found to reach a maximum within an intermediate range of molecular weights of the polymer matrix, a behavior previously reported only for pure polymers. Such differences in the crystallization rate of in situ bio-nanocomposites may affect the crystalline phase morphology and, in polymorphs such as in PBA, phase composition, with consequent effects on properties that may be of interest for specific applications.