Crystallization and melting of polyethylene copolymers: Differential scanning calorimetry and atomic force microscopy studies

Abstract

ABSTRACT:The HoffmanLauritzen theory of secondary, surface nucleation and growth was primarily relied upon for about 40 years after its introduction in about 1960 to rationalize the crystallization of flexible chain polymers into lamellar crystals. However, in about 1998, Strobl and coworkers introduced a different model for crystallization, based on the stagewise formation of lamellae. Two major components of this model were as follows: (1) the concept of the formation of a mesomorphic melt as a precursor to crystallization and (2) the control of the melting temperature range of lamellar crystals of homogeneous polyolefin copolymers by an inner degree of order or perfection rather than on the crystal thickness. The first concept is in disagreement with the HL theory and the second with the Gibbs-Thomson theory, which associates melting temperature with lamella thickness. In the present study, differential scanning calorimetry and atomic force microscopy were successfully employed to monitor the in situ quiescent crystallization of polyethylene homopolymer and copolymer. In the present study, evidence was not found to support the concept of lamellae with equal thickness melting over a broad temperature range. Some evidence was found that might be interpreted to support the concept of a mesomorphic melt as a precursor to crystallization. At present, the model promoted by Strobl and coworkers appears to be at an uncertain stage at which strong proof or disproof are not available. However, this alternative model has injected a new vitality into the study of crystallization of flexible chain polymers.

Polymer structure and morphology of low density polyethylene filled with silica nanoparticles

“…Considering that α varies from 20° to 40° and taking d FFT = 49 nm, an average lamellar thickness between 16.7 nm and 31.5 nm may be estimated that matches the values reported [40].…”

“…Average distances for the periodical features of nonmilled LDPE of 49.2 nm and for the 1‐h milled sample of 49.8 nm were obtained, confirming that the main periodical characteristics of LDPE do not significantly change with HEBM for 1 h. Typical values for lamellar thickness in polyethylene determined from small‐angle X‐ray scattering ( SAXS ) analysis were found to be between 17.5 nm and 27.4 nm [40] that is roughly half of the values obtained after the FFT analysis, d FFT . One possible explanation for these results is that the periodical spacing determined from the image analysis is not the real value of lamellar thickness.…”

Perfectly Controlled Lamella Thickness and Thickness Distribution: A Morphological Study on ADMET Polyolefins

“…By correlating DSC data with direct observation using an atomic force microscopy (AFM), Mirabella determined that ethylene copolymers with a homogeneous intermolecular composition distribution also possess a broad lamella thickness distribution. [16][17][18] The broadening of the lamella thickness distribution was estimated to occur as a result of stepwise lamella growth based on the ethylene sequence length distribution.…”

Density Fluctuations during the Early Stages of Polymer Crystallization: An Overview

“…In a series of SALS studies on the cold crystallization of PET,15, 21 syndiotactic polystyrene (sPS)22 and isotactic polystyrene (iPS),23 the logarithmic increase with time of the orientation fluctuations in the induction period of polymer crystallization could be described by spinodal decomposition kinetics, again supporting the presence of an ordered melt before nucleation. A recent study using atomic force microscopy (AFM) showed that in a branched PE large‐scale structures are formed before the formation of any local crystalline structure 24. In 1998, Olmsted4 outlined a theory based on spinodal decomposition of a melt, to explain the previous observations of such kinetics in SAXS experiments before the emergence of a crystalline structure.…”