The semi-crystalline nature of polyolefins gives rise to superior mechanical properties of everyday plastic products. With linear low-density polyethylenes (LLDPE), chemists can manipulate the structure and size of crystalline domains through control of the molecular weight distribution and short-chain branching (SCB) content of the constituting polymers. However, when solidified during processing, the semi-crystalline morphology is also sensitive to the complex rheology of the LLDPE, which in turn affects the material properties. The changes in relaxation times and the magnitudes of viscous and elastic moduli during crystallization can be linked to the structure of developing crystalline domains. Additionally, the rheology during crystallization is crucial for polymer manufacturing optimization studies.
We first show that LLDPEs with different SCB exhibit dissimilar rheology in the crystallizing melts, as measured by rheo-Raman experiments. Using the slip-link model, the differences in rheology are interpreted as different fractions of bridging and dangling chains in the uncrystallized portion of a partially crystallized entangled polymer fluid. For crystallizing LLDPEs, the model implies different crystalline structures, which are consistent with expectations from SCB distribution. We then incorporate experimentally-measured comonomer content distribution and molecular weight distribution and their cross-correlations explicitly into the slip-link model. When applied to the rheo-Raman measurements, this refined slip-link model explicitly connects the SCB distribution to the rheology and network structure of crystallizing LLDPEs. This model offers a more detailed perspective on the relationship between molecular structure and product performance for LLDPE materials.
