Effect of molecular structure and morphology on the mechanical properties of blown high density polyethylene films

Abstract

High density polyethylene(HDPE) resins with unimodal and bimodal molecular weight distribution have been fractionated according to crystallizability using preparative temperature rising elution fractionation. The molecular structure and thermal properties of the fractions with their whole polymers have been characterized using $^{13}C$ NMR spectroscopy, gel permeation chromatography, and differential scanning calorimetry. The average short chain branching content of the fractions obtained ranged from 0 to 8 branches per 1000 carbon atoms while that of the whole polymers is about 2 branches per 1000 carbon atoms. The TREF calibration curve for the HDPEs is significantly different from that of the conventional linear low density polyethylene(LLDPE) resins due to the different comonomer blockiness. The short chain branching content of the HDPEs decreases with increasing the molecular weight up to 50000 g/mol and then levels off. The bimodal resins have a slightly higher frequency of short chain branch in higher molecular weight species than in those of the unimodal resins. The short chain branching distribution as well as the low molecular weight species in the fractions seem to be important parameters to determine thermal behaviors of the fractions. The fractions with the short chain branching content above 3 branches per 1000 carbon atoms showed a significantly different DSC thermal behavior from those with less than 3 branches per 1000 carbon atoms. The blown films of the HDPEs were prepared under several processing conditions using a 40 mm Yoo Jin Engineering tubular blown equipment, and their morphologies such as crystallinity, preferred orientation, lamellar stacked crystalline morphology, and intercrystalline connectivity have been extensively characterized using differential scanning calorimetry, wide angle X-ray diffraction pole figure analysis, polarized infrared spectroscopy, scanning electron microscopy, and low temperature brittle fracture stress ana...