The morphology of crystalline structures in polyethylene obtained from dilute solutions

Abstract

THE STUDY of crystallization of linear polyethylene from dilute solution in toluene and xylene has led to the discovery of monocrystals of this polymer, and at the same time to the development of the theory of a folded conformation of the polymer chains. Further study showed the existence of intermediate crystalline forms of polyethylene, including fibrils [1], strips, helical formations, hexagonal monocrystals and needles [7]. Kargin and his collaborators put forward the hypothesis of the bundle structure of polymers [8-12] by which the processes of formation of the various crystalline forms are explained. I t can be shown also that crystals of similar types are also formed during the crystallization of polyethylene from the melt [13, 14]. The existence of different types of crystalline formation produced from solution is associated with the thermodynamic state of the crystalline polyethylene [15]. The purpose of the present work was to study the morphological types of polyethylene formed during crystallization from dilute solutions and their dependence on the degree of branching of the polymer, the concentration of the solution and the conditions of crystallization. The materials chosen for the study were a linear polyethylene of Polish origin (molecular weight ~ 35,000) and a high-pressure polyethylene of British origin (Alkathene HD). Crystallization was carried out in an ultrathermostat that provided temperature control with a precision of :[:0.I °. The low-pressure polyethylene was crystallized from xylene solutions at concentrations from 0.001 to The first series of morphological types of crystalline structure observed were produced by crystallization from solution at temperatures from 82-100 ° for definite periods of time, usually from 1 to 6 hr. After the end of the crystallization period the solutions were cooled uniformly at a rate of 10 ° per hour to 40 °. Drops of the cold solution were doposited on a carbon supporting film and studied in a Tesla BS-242 electron microscope. The main type of crystalline structure obtained from solutions of concentration below 0.01% was the fibrillar type in the form of long, fine filaments of uniform thickness. In addition small crystals in the form