Development of Polyolefin Elastomers with Distinct Dual Long-Chain Branch Structures

Long-chain branched (LCB) structures significantly impact the processability and mechanical performance of polyolefin materials. Comb-branched polyolefin elastomer (CPOE) represents an emerging class of polyolefin thermoplastic elastomers composed of an amorphous ethylene/1-octene copolymer backbone and crystalline polyethylene (PE) LCBs (T-type). While CPOEs exhibit higher melting points and improved thermal stability compared to conventional polyolefin elastomers, achieving both high mechanical strength and excellent elastic recovery remains a key challenge. In this study, a second LCB structure was strategically introduced into CPOEs to develop a dual-branched polyolefin elastomer (DPOE). In addition to the existing crystalline T-type LCBs, which act as reversible physical cross-linking sites, covalently bonded H-type LCBs were generated through the incorporation of 1,7-octadiene. The resulting DPOEs possessed 0.5–0.6 T-type LCBs and 0.5–1.6 H-type LCBs per polymer chain, forming an integrated dual cross-linking network. This dual-LCB design led to a remarkable enhancement in mechanical properties and thermal performance. Compared to CPOEs, the DPOEs exhibited an increase in tensile strengths from 9.4 to 16.6 MPa, elongations at break from 975% to 1085%, toughness from 70.1 MJ/m³ to 105 MJ/m³, and elastic recoveries from 65.8% to 73.7%. Furthermore, the DPOEs maintained a storage modulus plateau of 0.2–0.3 MPa across the 150–200 °C range. Notably, the materials also showed outstanding reprocessability, retaining up to 96% of their original mechanical strengths and nearly unchanged elongations at break after five hot-pressing cycles. The introduction of dual LCBs offers a powerful design strategy for producing high-performance, thermally stable, and reprocessable polyolefin thermoplastic elastomers, expanding their potential for advanced industrial applications.