Long Memory in Large Molecules: Self-Nucleation in Long-Chain Branched Polypropylene Melts

We demonstrate that long-chain branched polypropylene (LCB PP) exhibits pronounced melt memory above the equilibrium melting point due to topological constraints and gel-like structures that slow the relaxation of non-equilibrium chain states. These states, formed during synthesis and subsequent melt processing, act as self-nuclei in differential scanning calorimetry (DSC) experiments and persist beyond the conventional 5 min equilibration used in standard DSC protocols. The decay of self-nuclei, monitored via the isothermal crystallization rate, follows a power-law dependence with the equilibration time. This behavior agrees with diffusive relaxation of non-equilibrium clusters in a topologically complex environment. Morphological analyses show that self-nucleation produces nucleation densities (Nd) up to 1011 cm–3, which suppresses spherulitic growth and induces anisotropic lamellar textures. The self-nuclei are most effectively reduced in a prior solution treatment of the polymer, which decreases Nd and crystallization rate but also increases the viscoelastic relaxation time, demonstrating the link between melt structure and crystallization.