Synthesis and characterization of side-chain liquid-crystalline polymers and study of their role in the crystallization of high-density polyethylene

Side-chain liquid-crystalline polymers (SCLCPs) as nucleating agents for high-density polyethylene (HOPE) were investigated. For this purpose, the molecular architectures of four different vinyl monomers with liquid-crystalline properties were designed and prepared with 1-butanol, 1-pentanol, 4-hydroxybenzoic acid, hydroquinone, and acryloyl chloride as the starting materials through alkylation and acylation reactions. The corresponding polymers were synthesized by homopolymerization in 1,4-dioxane with benzoyl peroxide as the initiator at 60°C. Both the monomers and the synthesized polymers were characterized with elemental analysis, Fourier transform infrared, and ¹H NMR measurements. Differential scanning calorimetry, thermogravimetric analysis, and hot stage polarized optical microscopy were employed to study the phase-transition temperature, mesophase texture, and thermal stability of the liquid-crystalline polymers. The results showed that all the polymers had thermotropic liquid-crystalline features. Being used as nucleating agents, SCLCPs effectively increased both the crystallization temperature and rate and, at the same time, raised the crystallinity for HDPE. In comparison with common small-molecule nucleating agents, such as 1,3:2,4-dibenzylidenesorbitol, SCLCPs are more efficient and are indeed excellent nucleating agents for HDPE.