Chain conformational transformations in syndiotactic polypropylene/layered silicate nanocomposites during mechanical elongation and thermal treatment

Abstract

Polymer nanocomposites prepared by melt-mixing syndiotactic polypropylene (sPP) with a quaternary modified montmorillonite have been studied with FT-IR and XRD spectroscopic techniques. FT-IR spectroscopic analysis has shown that the addition of the nanoclay results in a higher helical content for the syndiotactic polypropylene matrix. Furthermore, FT-IR spectroscopy showed that the presence of the nanoclay hinders the polymeric chains from achieving the degree of transformation from helical to trans-planar form during the application of mechanical stress compared to the neat sPP case. Accordingly, the sPP nanocomposites show a higher tendency relative to neat sPP to return to the initial helical conformation upon either releasing the applied mechanical tension or upon exposing to heat at 120 °C. Additionally, XRD patterns provided evidence that the use of low concentration of nanoclay (1%) resulted in partially exfoliated nanocomposites, while only intercalated nanostructures were produced at high nanoclay contents (10%). However, the application of stress can improve the degree of exfoliation of an sPP nanocomposite. In addition, linear dichroic infrared measurements which allow the monitoring of the influence of the nanoclay on the orientation of the polymeric chains during the application of stress showed that the trans-planar infrared bands exhibit lower orientation in comparison to the same bands in neat sPP, while the addition of nanoclay has no particular influence on the orientation of the infrared bands that are related to helical conformations. Finally, dynamic mechanical analysis (DMA) verified the enhanced mechanical properties of the sPP nanocomposites relative to neat sPP, whereas differential scanning calorimetry (DSC) depicted a slight increase in the glass transition temperature of the polymeric matrix in these nanocomposites, especially for low clay concentrations.