Molecular Weight Dependence of the Crystallization Kinetics of Linear Polyethylene. I. Experimental Results

Abstract

The results of a dilatometric study of the crystallization kinetics of molecular weight fractions of linear polyethylene are reported. These data encompass t?ie molecular weight range 4 X 103-8 X 106 7. Although the usual deviations from the Avrami or Goler-Sachs free-growth formulations are observed, these deviations are systematic with molecular weight and become more pronounced as the molecular weight is increased. There is a one to one correlation between the level of crystallinity at which these deviations occur and the final level of crystallinity that can be attained for each molecular weight. These kinetic results in turn explain the very wide range in values that is observed for all properties. To explain the isotherm shape, we note that over the region of experimental adherance the Avrami exponent is an integral number and varies from four to two as the molecular weight increases. The exponent is independent of temperature except for the very highest crystallization temperatures. At the very highest crystallization temperatures, closer adherance to theory is found, with the implication that if the experiments could be conducted at still higher temperatures the Avrami-type theory would be obeyed. At a fixed temperature the crystallization rate goes through a maximum as a function of molecular weight and the location of the maximum is dependent on the undercooling. Although the very marked negative temperature coefficient gives strong support to nucleation control, it is demonstrated that the rudimentary nucleation theories that have been proposed are not obeyed. irtually all thermodynamic, spectral, mechanical and physical properties of linear polyethylene are dependent on molecular weight and on the crystallization conditions.2-7 A wide range in the values of any specific property is observed for the same chemically constituted polymer. For

Effect of molecular weight and temperature on the isothermal crystallization of poly(oxetane)

“…This conclusion has been obtained previously in other polymers and, specifically, in polyethylene [9] and polyethers [13,14]. However, the total crystallinity varies with molecular weight.…”

“…The crystallization kinetic studies at low undercoolings were carried out using dilatometric techniques previously described [8,9]. About 200 mg of bubble-free polymer films were used in the dilatometers, filled with triply distilled mercury.…”

Nucleation in Polymer Crystallization

“…As mentioned above, crystallization proceeds with the initial formation of isolated spherulites, which then grow until their mutual impingement with further slow crystallization. Time (t)-dependent measurements [26] of the density of the crystallizing melt at different temperatures are given in Figure 1.3 as a plot of degree of crystallinity versus logarithm of time. A decrease in the crystallization temperature results in a rapid increase in crystallization rate.…”

The Relation between Structure and Properties of Crystalline Polymers

“…I, 1985 by theory to corresponding supermolecular forms. 8 1.8 2 The structures observed vary from welldeveloped spherulites, to rod and sheet like aggregates as well as to a random collection of lamellae. The latter clearly do not represent any definite morphological form.…”