Twisting orientation and the role of transient states in polymer crystallization

Studies of the crystallization, melting, and morphology of random ethylene/1-octene copolymers by a combination of differential scanning calorimetry and atomic force microscopy are presented. Two d...

Influence of Structural and Topological Constraints on the Crystallization and Melting Behavior of Polymers. 1. Ethylene/1-Octene Copolymers†

On transcrystallinity in semi-crystalline polymer composites

This article reviews the various theories of polymer crystallization which have been proposed over the last thirty years. Their background in thermodynamics and crystal growth theories of simple substances is laid out in some detail. This provides the basis for a discussion of the application of surface nucleation and surface roughening concepts to polymer crystallization. The widely used nucleation model of Lauritzen and Hoffman is examined in depth with emphasis on its consistency with experimental evidence and including its recent modifications and extensions. Alternative approaches are also analyzed, in particular Point's ‘multipath’ model and the ‘roughness/pinning’ model by Sadler and Gilmer, but also ‘molecular nucleation’ and ‘chain sliding’. The aim of this article is to lead to a better understanding of the basic hypotheses of the various models and to give guidance to the theorist in improving the models as well as to the experimentalist in interpreting his/her data. It will hopefully make the subject more accessible and stimulate discussion and further research directed at resolving the controversial issues.

Polymer crystallization theories

The tensile behaviour of linear polyethylene was examined over a wide range of temperatures. Samples were prepared from low and medium molecular weight polymer with different morphologies, by varying the initial crystallization conditions. It was found that the temperature of the ductile-brittle transition was markedly different for different samples. In particular, low molecular weight polymer crystallized at a low degree of of supercooling, showed brittle behaviour over most of the temperature range, with a ductile-brittle transition near to room temperature. Rapidly quenched material, where the degree of supercooling is high, showed a very low ductile-brittle transition temperature, especially in high molecular weight polymer. The reasons for these differences in behaviour are discussed both at a phenomenological level and in terms of known structural differences between the different materials examined.

The influence of morphology and molecular weight on ductile-brittle transitions in linear polyethylene

Study of crystallization and isothermal thickening in polyethylene using SAXD, low frequency Raman spectroscopy and electron microscopy

Different samples of high-density polyethylene provided different nucleation densities which were assessed by optical microscopy and light scattering. Crystallizations were carried out under controlled cooling rates using a microscope hot stage and DSC from which crystallization temperatures were obtained. It is shown that the effective crystallization temperature is a function of the nucleation density, and a quantitative fit is found with theoretical predictions based on crystallization kinetics. The melting point was, in turn, found to be dependent on nucleation density, through the agency of lamellar thickness as assessed by low-frequency Raman spectroscopy. The effect of cooling rate on melting point is also explained in terms of variation of supercooling. The implications of these effects together with considerations of surface and interior nucleation and sample size are discussed for the industrial processing of polyethylene.

The influence of nucleation density and cooling rate on crystallization of polyethylene from the melt

Following up the previous studies on this topic [1] it is verified that in the course of crystallization of polyethylene melts taking place during cooling, the number of predetermined nuclei initiating crystallization influences not only the scale of the spherulitic texture in the fully crystallized sample, but also the thickness of the lamellae and through it the melting behaviour of the final product and this in a predictable manner. Extending the scope of the previous study [1] these results were now obtained in samples in which the nucleation density was systematically altered, with all other relevant variables kept constant. The investigation thus demonstrates the previously unsuspected subtleties which can influence the structure and properties of samples which have been crystallized non-isothermally. It also provides an example for the complementary use of different techniques such as light scattering, optical microscopy, DSC calorimetry and Raman spectroscopy, for the purposes in question.

The effect of nucleation density on the kinetics of crystallization and on the resulting structure and thermal properties of polymers crystallized during cooling

G. V. Fraser

Papers

Study of crystallization and isothermal thickening in polyethylene using SAXD, low frequency Raman spectroscopy and electron microscopy

The influence of nucleation density and cooling rate on crystallization of polyethylene from the melt

The effect of nucleation density on the kinetics of crystallization and on the resulting structure and thermal properties of polymers crystallized during cooling