Branching-Density Dependent Chain Relaxation and Orientated Crystallization Behavior of the Stretched Polylactic Acid Melt

The branching density of long-chain branched polymers is crucial for polymer crystallization and orientation during melt processing, but there are few reports on related quantitative research. In this work, poly(lactic acid) (PLA) was used as an example, and long-chain branched PLA (LCB-PLA) with different branching densities (φ) was constructed by melt blending with epoxy copolymers. The influence of branching density on the chain relaxation, orientation, and crystallization of stretched PLA melt was quantitatively studied. The long-chain branched structure effectively inhibited the chain relaxation of PLA melt, promoting the orientation and crystallization of PLA. The orientated crystallization behavior of LCB-PLA is closely related to the branching density, and there are two critical branching densities. When φ ≤ 2.57 mol/104 mol C, an effective strong molecular chain network that inhibits the chain relaxation of PLA has not yet been formed, and the orientation and crystal structure are hardly formed. When φ is in the intermediate stage (2.57 mol/104 mol C < φ < 6.47 mol/104 mol C), a local strong chain network is formed, which is able to inhibit the relaxation of the oriented chains to some extent. At a higher φ (≥6.47 mol/104 mol C), chain slip and chain mobility are enormously reduced due to the complex topological constraints of adjacent chains, forming a strong global chain network that effectively restricted the relaxation of oriented chains during melt stretching (increasing relaxation time by 3 orders of magnitude), As a result, the orientation degree and crystallinity of stretched LCB-PLA increased to 56% and 36%, respectively. This work provides valuable insights into the structural evolution and performance improvement of long-chain branched polymers during melt processing.