The Influence of Graft Length and Density on Dispersion, Crystallisation and Rheology of Poly(ε-caprolactone)/Silica Nanocomposites.

Different techniques of grafting polymer chains to filler surfaces are often employed to compatibilise filler and polymer matrices. In this paper the influence of graft length and graft density on the state of dispersion, crystallisation and rheological properties of poly(ε-caprolactone) (PCL)/silica (SiO2) nanocomposites are reported. Grafted silica nanoparticles were prepared through polymerisation of PCL from the nanoparticle surface. Graft length was controlled by the reaction time, while the grafting density was controlled by the monomer-to-initiator ratio. Grafted nanoparticles were mixed with PCL of different molecular weights and the state of dispersion was assessed. Different matrix-to-graft molecular weight ratios resulted in different states of dispersion. Composites based on the higher molecular weight matrix exhibited small spherical agglomerates while the lower molecular weight matrix revealed more sheet-like microstructures. The state of dispersion was found to be relatively independent of graft length and density. Under quiescent conditions the grafts showed increased nucleation ability in the higher molecular weight PCL, while in the lower molecular weight matrix the effect was less pronounced. Rheological experiments showed an increase in viscosity with increased filler content, which was beneficial for the formation of oriented structures in shear-induced crystallisation.

Deformation mechanisms of sub-micrometer thermoplastic vulcanizates obtained by reaction-induced phase separation of miscible poly(ε-caprolactone)/dimethacrylate systems.

The micromechanical deformation mechanisms of sub-µm thermoplastic vulcanizates (TPVs) based on poly(ε-caprolactone) (PCL) and cross-linked methacrylate rubbers were studied by time-resolved small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) measurements in order to better understand the underlying deformation mechanisms responsible for the high elongation at break and good elastic recovery of sub-µm TPVs. It is demonstrated that, in contrast to neat PCL, the interlamellar void formation in the PCL matrix and subsequent coalescence of voids is suppressed by the presence of the rubber (nano)particles in these TPVs. The deformation of the TPVs under tensile conditions is dominated by yielding of the PCL matrix, which is initially localized at the equatorial regions of the rubber particles and progresses towards the polar regions at higher strains. Re-ordering of the crystal structures is both time and stress dependent, and stress relaxation of the TPV under tension is primarily governed by the break-up of the crystal lamellae at the equatorial regions of the rubber particles. This study demonstrates that the rubber particle size as well as chemical grafting of thermoplastic polymer chains onto the surface of cross-linked rubber particles are important parameters to control the mechanical deformation behavior of TPVs.