Investigations on mechanical properties of PI-PS multigraft copolymers

The stress-strain behaviour of multigraft copolymers consisting of a polyisoprene (PI) backbone and grafted polystyrene (PS) arms have been characterized by applying models of rubber elasticity such as Mooney-Rivlin, slip-tube and the extended non-affine tube model. Additionally, the range of low deformation has been investigated by relaxation tests for determining the stress relaxation. Multigraft copolymers show high strain at break and low residual strain caused by the large number of physical cross links resulting from several grafted PS side chains. From the model fits the material parameters G_c^ST and G_e^ST of the slip-tube model, representing the influence of chemical cross links and entanglements effects, respectively, and the n_e/T_e-value (n_e - number of statistical segments between two successive entanglements, T_e - Langley trapping factor) of the extended non-affine tube model, are used to describe the tensile behaviour of these thermoplastic elastomers. The PS content was considered as filler phase taking into account the effect of hydrodynamic amplification. The influence of functionality and the number of branch points per molecule on the strain at break and the tensile strength is explained by the model parameters describing the stress-strain curve at low to medium (≤400%, slip-tube, Mooney-Rivlin) and low to high (≤900%, extended non-affine tube) elongations. It was observed that for the material with a spherical morphology G_e^ST is increasing with the number of branch points β (each branch point consists of a PI backbone segment, depending on the functionality one, two or four grafted PS arms). For cylindrical and lamellar morphologies the G_e^ST was decreasing with increasing β, which could be reconfirmed by applying the extended non-affine tube model where the n_e/T_e-parameter is increasing with β.