(74e) Crystallization of Copolymers with Short-Chain Branching

Short-chain branching (SCB) is commonly used to tune the mechanical properties of industrial LLDPE. These materials are semi-crystalline, and crystallization of the polymer chain is inhibited by the presence of comonomers. The fraction of such comonomers and their placement modify crystallite stem length, crystallite dimensions, and overall crystallinity. However, the mechanical response of semi-crystalline material is also dependent on the uncrystallized segments of polymer chains, which lie in amorphous domains and can bridge between different crystallites intramolecularly. The network comprising crystallites connected by bridge chains percolates throughout the material and renders it an elastic solid. These mechanics of the network are sensitive to the lengths of bridges. Therefore, the molecular weight distribution of polymer chains and the SCB distribution are manipulated together to achieve desired mechanical properties. However, knowledge of the molecular weight distribution (MWD) of bridging segments would provide more direct control of the mechanical response.

To address this question, we developed a Kinetic Monte-Carlo model to study the crystallization of copolymers with SCB. As opposed to earlier works that focused on chain folding and the real space assembly of crystallized segments of polymer chains into lamellar crystallites, our approach operates in the sequence space of the chain backbone, similar to Flory’s model for copolymer crystallization, allowing us to track the length of uncrystallized segments as a function of crystallization time. We calculate the dependences of average crystallite stem length and overall crystallinity on branched comonomer fraction, bulk, and surface terms of crystallization free energy. The importance of chain-folding, stem length, and entanglements are investigated. Most importantly, we evaluate the molecular weight distribution of uncrystallized segments and connect it to the mechanical response of material through the rheological slip-link model. Our model thus incorporates information about MWD and SCB distribution in the dynamic modulus of semi-crystalline LLDPE.