Differences in Chemical Structures between High Density Polyethylene and Low Density Polyethylene

From the perspective of chemical structural units, the structural units of high-density polyethylene are mainly linear, with approximately 5-20 short branches per 1000 carbon atoms; The number of short branches per 1000 carbon atoms in low-density polyethylene is approximately 20-50, which directly affects their physical properties. The molecular chain linearity of high-density polyethylene is high, and the distance between molecular chains is relatively small, which makes it have a high density, generally between 0.941-0.965g/cm ³; Low density polyethylene has a high density of 0.910-0.940g/cm ³ due to the presence of many longer branches and larger distances between molecular chains. In terms of spatial structure, high-density polyethylene molecular chains have low steric hindrance, allowing them to be densely packed; The branched chains on the molecular chains of low-density polyethylene cause significant steric hindrance, resulting in loose stacking of the molecular chains. The main chain of high-density polyethylene has fewer methyl and other substituents, which facilitates van der Waals forces between molecular chains and strengthens their interactions; There are many substituents on the main chain of low-density polyethylene, which weakens the van der Waals forces between molecular chains.
From the perspective of chemical bonding, high-density polyethylene molecular chains are connected by strong intermolecular forces, and the molecular chains are relatively stable; Due to the presence of branched chains, the regularity of molecular chains in low-density polyethylene is disrupted, resulting in relatively weak intermolecular forces. In the chemical structure of high-density polyethylene, the spacing between adjacent molecular chains is uniform and small, which ensures its high strength and rigidity; The molecular chain spacing of low-density polyethylene varies, resulting in good flexibility but relatively low strength. The arrangement of carbon atoms on the molecular chain of high-density polyethylene is almost linear, making it easy for the molecular chain to crystallize in parallel; Low density polyethylene molecular chains have irregular arrangement of carbon atoms due to interference from branch chains, resulting in relatively fewer crystalline regions. From the perspective of polymerization reactions, high-density polyethylene is mostly polymerized using Ziegler Natta catalysts or chromium based catalysts, which results in a more regular growth of molecular chains; Low density polyethylene mainly adopts free radical polymerization, which is prone to producing many branched structures.
In the structure of high-density polyethylene, the degree of entanglement of molecular chains is relatively low because its regular structure makes it difficult for molecular chains to entangle with each other; Low density polyethylene molecular chains have a high degree of entanglement due to the presence of branched chains.