Isothermal crystallization in PCL/clay nanocomposites investigated with thermal and rheometric methods

Nanocomposites based on biodegradable poly(epsilon-caprolactone), (PCL) and organically modified layered silicates (organoclay) were prepared by melt mixing. The isothermal crystallization of PCL/clay nanocomposites at different clay concentrations (from 0.1 to 10 wt%) were investigated by differential scanning calorimetry (DSC) and rheometry in dynamic tests. The results showed that the well dispersed organoclay platelets acted as nucleating agents in the PCL matrix, as confirmed by a remarkable reduction of the crystallization half-time, t(1/2). This effect was most pronounced at very low clay concentrations. In particular, in composites with 0.4% of clay, t(1/2) was one order of magnitude lower than the pure polymer. The heating thermograms, performed on PCL and on PCL/clay after isothermal crystallization showed a reduction of the melting temperature with the increase of the clay content, suggesting that the degree of perfection of the crystals and the degree of crystallinity were affected by the restricted mobility of the chains, which did not allow the growth of well developed lamellar crystals. The rheological measurements confirmed the effect of clay on crystallization kinetics. The evolution of G' and G" vs. time was correlated to the clay concentration and to the development of the crystalline phase during isothermal experiments.