Macromolecules, Vol.43, No.16, 6782-6788, 2010
Step-Cycle Mechanical Processing of Gels of sPP-b-EPR-b-sPP Triblock Copolymer in Mineral Oil
Gels of syndiotactic polypropylene-b-ethylene-propylene-rubber-b-syndiotactic polypropylene (sPP-EPR-sPP) were prepared by dissolving similar to 6 wt % of the triblock copolymer in mineral oil at 170 degrees C and then cooling to room temperature in several steps to crystallize the sPP block. The gel was subjected to step-cycle processing by first extending it to a given maximum tensile strain, followed by decreasing the load to zero. The cycle was then repeated to a higher maximum strain and so on until the sample either failed or it reached an ultimate predetermined strain. The true stress and true strain epsilon(H) during each cycle were recorded, including the true strain at zero load epsilon(H,p) after each cycle that resulted from the plastic deformation of the sPP crystals in the gel. The initial Young's modulus E-init and maximum tangent modulus E-max each cycle undergo dramatic changes as a function of epsilon(H,p), with E-init decreasing for epsilon(H,p) <= 0.1 and then increasing slowly as epsilon(H,p) increases to 1 while E-max increases rapidly over the entire range of epsilon(H,p) resulting in a ratio of E-max/E-init > 1000 at the highest maximum (nominal) strain of 20. On the basis of small-angle X-ray scattering patterns from the deformed and relaxed gels, as well as on previous results on deformation of semicrystalline random copolymers by Strobl and co-workers, we propose that the initial decrease in E-init with epsilon(H,p) is due to a breakup of the network of the original sPP crystal lamellae while the increase in E-max with epsilon(H,p) is caused by the conversion of the sPP lamellae into fibrils of an aspect ratio that increases with further plastic deformation. The gel elastic properties can be understood as those of a short fiber composite with a highly deformable matrix. At zero stress the random copolymer midblock chains that connect the fibrils cause these to make all angles to the tensile axis (low E-init) while at the maximum strain the stiff crystalline sPP fibrils align with the tensile axis producing a strong, relatively stiff gel.