Low-temperature properties of high-density polyethylene (HDPE), low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), and their blends were studied. The analyzed low-temperature mechanical properties involve the deformation resistance and impact strength characteristics. HDPE is a bimodal ethylene/1-hexene copolymer; LDPE is a branched ethylene homopolymer containing short-chain branches of different length; LLDPE is a binary ethylene/1-butene copolymer and an ethylene/1-butene/1-hexene terpolymer. The samples of copolymers and their blends were studied by gel permeation chromatography (GPC), differential scanning calorimetry (DSC), ¹³C NMR spectroscopy, and dynamic mechanical analysis (DMA) using testing machines equipped with a cryochamber. It is proposed that such parameters as “relative elongation at break at −45 °C” and “Izod impact strength at −40 °C” are used instead of the ductile-to-brittle transition temperature to assess frost resistance properties because these parameters are more sensitive to deformation and impact at subzero temperatures for HDPE. LLDPE is shown to exhibit higher relative elongation at break at −45 °C and Izod impact strength at −20 ÷ 60 °C compared to those of LDPE. LLDPE terpolymer added to HDPE (at a content ≥ 25 wt.%) simultaneously increases flow properties and improves tensile properties of the blend at −45 °C. Changes in low-temperature properties as a function of molecular weight, MWD, crystallinity, and branch content were determined for HDPE, LLDPE, and their blends. The DMA data prove the resulting dependences. The reported findings allow one to understand and predict mechanical properties in the HDPE–LLDPE systems at subzero temperatures.

The compositions based on bimodal high-density polyethylene (HDPE, copolymer of ethylene with hexene-1) and in mixture with monomodal tercopolymer of ethylene with butene-1/hexene-1 (LLDPE, low-density polyethylene) have been studied. Phase equilibrium, thermodynamic parameters of interdiffusion in a wide range of temperatures and ratios of co-components were identified by refractometry, differential scanning calorimetry, optical laser interferometry, X-ray phase analysis. The phase state diagrams of the HDPE—LLDPE systems were constructed. It has been established that they belong to the class of state diagrams of “solid crystal solutions with unrestricted mixing of components”. The paired parameters of the components interaction and their temperature dependences were calculated. Thermodynamic compatibility of α-olefins in the region of melts and crystallization of one of the components has been shown. The kinetics of formation of interphase boundaries during crystallization of α-olefins has been analyzed. The morphology of crystallized gradient diffusion zones has been analyzed by optical polarization microscopy. The sizes of spherulites in different areas of concentration profiles and values of interdiffusion coefficients were determined.

Rheological, molecular, thermal, and mechanical properties of binary blends based on bimodal high-density polyethylene (HDPE; ethylene–1-hexene copolymer) and unimodal linear low-density polyethylene (LLDPE; ethylene–1-butene–1-hexene terpolymer) were studied. Six blends with different HDPE/LLDPE weight ratios, 80/20, 70/30, 60/40, 50/50, 40/60, and 20/80, were prepared by blending in a double-worm extruder. The correlation of the rheological properties of the blends, analyzed by the capillary and rotation methods, with the melt flow index and molecular characteristics was demonstrated. LLDPE with increased flowability (MFI = 3 g/10 min) as a polymer additive is an effective plasticizer allowing control of the melt viscosity in a wide range and choice of the optimum target value of the melt flow index of the blend. The activation energy of the melt flow was calculated, and the relationship between this quantity and the content of short-chain branchings in polyethylene blends was revealed. The blends obtained are homogeneous, but an increase in the content of LLDPE in HDPE leads to a change in the polyethylene structure, which is confirmed by a decrease in the crystallinity and in the lamella thickness. The strength values at the yield point and at break decrease with an increase in the fraction of LLDPE in HDPE. Changes in the properties of these blends in their processing into a film by flat-die extrusion were studied.