低密度聚乙烯基料链结构对黏弹特性的影响

As high voltage cable transmission systems develop to higher voltage levels and longer distances, the thickness and length of cable insulation layers are increasing, which places higher demands on the processing properties of crosslinked polyethylene insulating materials. The viscoelasticity of low-density polyethylene (LDPE) directly affects the processing performance of the insulating materials and ultimately determine the molding quality of cable insulation. While the molecular structure of LDPE is the key to determine its viscoelasticity. However, due to the complex branching architectures and broadly distributed chain structure of LDPE, it is challenging to establish the relationship between a single molecular chain structural parameter and viscoelasticity. Therefore, accurate obtainment of the influence of LDPE molecular chain structural characteristics on the viscoelasticity is significant for cable insulation optimization and upgrading. In this paper, preparative temperature-rising elution fractionation (P-TREF) was carried out to separate a tubular LDPE and an autoclave LDPE into fractions with different average molecular weight. For each LDPE sample, two fractions with high content and large molecular weight differences were selected for subsequent experiments. Molecular structures of the original LDPE samples and their fractions were characterized by gel permeation chromatography and their viscoelastic parameters such as complex viscosity, storage modulus and loss modulus, were measured using an electromagnetic rheometer. The results show that the LDPE resins prepared by different process exhibit significantly different molecular chain structure characteristics. The shape of molecular weight distribution for the autoclave LDPE is bimodal, with more molecules of higher molecular weight and a wider molecular weight distribution. It has higher long-chain branching degree and its molecular chain architecture is closer to a sphere. It is closer to star branched polymer. While the tubular LDPE features a unimodal molecular weight distribution curve with a smaller average molecular weight and a narrower molecular weight distribution. It has fewer long chain branching per molecule and its molecular chain architecture is closer to a rod. At the same weight average molecular weight, the zero-shear viscosity of autoclave LDPE is comparatively lower than that of tubular LDPE. For the two LDPE resins, the relationship between zero-shear viscosity and weight average molecular weight is as follows: for the autoclave LDPE, η₀ ∝ M_w^3.4 , while for the tubular LDPE, η₀ ∝ M_w^6.7 .The research in this paper also found that when the long chain branching topology is similar, the larger the weight average molecular weight, the more probable to create entanglement between molecular chains. Thus, the viscosity and moduli are higher and the shear thinning phenomenon appears at lower frequency. When the weight average molecular weight is close, the zero-shear viscosity has a non-monotonic relationship with the degree of long-chain branching. Excessive long-chain branches will weaken the physical entanglement between molecular chains, resulting in a decrease in the zero-shear viscosity. Based on the structure-activity relationship of viscoelasticity of LDPE, and combined it with cable extrusion molding process, it is feasible to design more suitable viscoelasticity parameters of LDPE insulating materials, and optimize the chain structure of LDPE according to this. This is the future direction of the research and development of advanced cable insulating materials.