Effect of annealing time and molecular weight on melt memory of random ethylene 1‐butene copolymers

Abstract

The effects of annealing time and molecular weight on the strong melt memory effect observed in random ethylene 1‐alkene copolymers are analyzed in a series of model ethylene 1‐butene copolymers with 2.2 mol% branches. Melt memory is associated with molten clusters of ethylene sequences from the initial crystals that remain in close proximity and are unable to diffuse quickly to the randomized melt state, thus increasing the recrystallization rate. Melt memory persists even for greater than 1000 min annealing indicating a long‐lived nature of the clusters that only fully dissolve at melt temperatures above a critical value (>160 °C). Below the critical melt temperature, molecular weight and annealing temperature have a strong influence on the slow kinetics of melt memory. For the copolymers analyzed, slow dissolution of clusters is experimentally observed only for Mw < 50 000 g mol−1. More stable clusters that survive higher annealing temperatures display slower dissolution rates than clusters remaining at lower temperatures. The threshold crystallinity level to enable melt memory (Xc,threshold) decreases with increasing molecular weight and decreasing annealing temperature similarly to the variation of the chain diffusivity in the melt. The process leading to melt memory is thermally activated as the variation of Xc,threshold with temperature follows Arrhenius behavior with high activation energy (ca 108 kJ mol−1) that is independent of molecular weight. © 2018 Society of Chemical Industry

Melt Memory Effects in Poly(butylene succinate) Studied by Differential Fast Scanning Calorimetry

“…Concerning the time effect on melt memory, different behaviors have been reported in the literature [3,[5][6][7][14][15][16]. In a recent paper published by some of us [4], it is concluded that depending on the temperature range, different trends can be observed: for materials in Domain III, no effect of time has been observed.…”

“…All these works have focused on the erasure of melt Melt memory effects are considered to have a kinetic nature, since the temperature regions of the different Domains depend on the conditions employed to perform the experiment [2][3][4]. Although some aspects, such as the effect of time spent at T s temperature [3,7,[14][15][16], have attracted attention in the literature, other aspects, such as the effect of cooling and heating rates have not been studied. In the following sections, the different parameters that affect the self-nucleation Domains are studied systematically to provide new insights into the control of melt memory effects by taking advantage of its kinetic nature.…”

“…The nature of the melt memory effect has attracted much attention in the last few years, but it is still under debate [5][6][7][8][9][10][11][12]. In a recent review [4] some of us have analyzed the different theories and the parameters that affect melt memory, such as the molecular weight of the material [3,7,9,13], chain topology [13], the time spent at the self-nucleation temperature [3,7,[14][15][16] and the effect of chemical structure [17] or confinement [18][19][20], among others. The new evidence about the nature of melt memory has been also summarized, which have been obtained taking advantage of new combined techniques for its study such as rheology [21][22][23], dielectric spectroscopy [24] or infrared spectroscopy [25,26].…”

“…However, a thorough study analyzing systematically the different parameters that affect this phenomenon is missing, so far. In the literature, considerable work has been devoted to the effect of time spent at the self-nucleation temperature [3][4][5][6][7][14][15][16], with the aim of erasing the self-nuclei by spending long times at the T s temperature. Despite this, the effect of the time spent at a given T s temperature on self-nucleation Domains, and that of the cooling/heating rates employed, are not known.…”

Special issue session dedicated to the retirement of Prof. Vincent Mathot: polymer thermal analysis and crystallization

“…The fourth paper was contributed by Prof. Rufina Alamo's group with a deep analysis on the strong memory effects of crystallization unique for random copolymers. The memory is long living at low annealing temperatures and exhibits an activation barrier independent of molecular weight.…”