DYNAMIC MECHANICAL MEASUREMENT OF CRYSTALLIZATION-INDUCED GELATION IN THERMOPLASTIC ELASTOMERIC POLY(PROPYLENE)

The thermoplastic elastomeric poly(propylene)(TPE-PP) of this study has a linear structure comprised of multiple blocks of stereoregular and stereoirregular sequences. Above a melting temperature of about 65-degrees-C, the TPE-PP is a liquid. In our experiments, the samples were first melted by heating to a prescribed temperature T(p) (75, 100, or 150-degrees-C) and then cooled to 40-degrees-C and held there for observation of the crystallization. Upon this cooling into a supercooled state, TPE-PP undergoes a gradual liquid-solid transition as the stereoregular sequences crystallize and hence form physical network junctions. At the gel point of this physical gelation process, the TPE-PP exhibits the typical scaling behavior expressed in power law relaxation, G(t) = St-n with a prefactor S of about 2.0 MPa s(n) and a relaxation exponent n close to 0.13. S is constant, while the value of n decreases slightly with the degree of crystallinity at the gel point. The degree of crystallinity at the gel point depends on the thermal history. If T(p) is too low (e.g., 75-degrees-C), some of the microphase-separated solid-state structure persists into the melted state and the subsequent gelation is characterized by the absence of scaling behavior.