We report the effect of molecular weight and comonomer content on melt crystallization of model random ethylene 1-butene copolymers. A large set of narrowly distributed linear polyethylenes (PE) was used as reference of unbranched molecules. The samples were crystallized from a melt state above the equilibrium melting temperature and cooled at a constant rate. The exothermic peaks of the melt-solid transition are reported as the crystallization temperatures (Tc). Following expectations, the Tc of unbranched PE samples was constant and independent of the initial melt temperature. The same independence was observed for copolymers (2.2 mol % ethyl branches) with molar mass below 4500 g/mol. Moreover, the Tc of copolymers with higher molar mass depends on the temperature of the initial melt, Tc increases as the temperature of the melt decreases. We attribute the increase in Tc to a strong crystallization memory in the melt above the equilibrium melting, and correlate this phenomenon with remains in the melt o...

Strong Memory Effect of Crystallization above the Equilibrium Melting Point of Random Copolymers

Over the past two decades, biomass-derived and biodegradable polylactide (PLA) has sparked tremendous attention as a sustainable alternative to traditional petroleum-derived polymers for diverse applications. Unfortunately, the current applications of PLA have been mainly limited to biomedical and commodity fields, mostly because the poor heat resistance (resulting from low melting temperature) and hydrolysis stability make it hard to use as an engineering plastic. Stereocomplexation between enantiomeric poly(l-lactide) (PLLA) and poly(d-lactide) (PDLA) opens a new avenue toward PLA-based engineering plastics with improved properties. The formation, crystal structure, properties, and potential applications of stereocomplex-type PLA (SC-PLA) are summarized by some research groups. However, since it is challenging to achieve full stereocomplexation from high-molecular-weight PLLA/PDLA blends and to avoid serious thermal degradation of the PLAs after complete melting, the advances and progress in the processing of SC-PLA into useful products are quite rare in open publication. In this review, some important strategies for enhancing stereocomplex crystallization in practical processing operations are presented and recently developed processing technologies for SC-PLA are highlighted, such as low-temperature sintering. Furthermore, major challenges and future developments are briefly discussed. This review is expected to potentially open up new research activities in the processing of SC-PLA.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1002/marc.201700454

Recent Advances in Processing of Stereocomplex-Type Polylactide.

Self-nucleation (SN) is a special nucleation process triggered by self-seeds or self-nuclei that are generated in a given polymeric material by specific thermal protocols or by inducing chain orientation in the molten or partially molten state. SN increases the nucleation density of polymers by several orders of magnitude, producing significant modifications to their morphology and overall crystallization kinetics. In fact, SN can be used as a tool for investigating the overall isothermal crystallization kinetics of slow-crystallizing materials by accelerating the primary nucleation stage in a previous SN step. Additionally, SN can facilitate the formation of one particular crystalline phase in polymorphic materials. The SN behavior of a given polymer is influenced by its molecular weight, molecular topology, and chemical structure, among other intrinsic and extrinsic characteristics. This review paper focuses on the applications of DSC-based SN techniques to study the nucleation, crystallization, and morphology of different types of polymers, blends, copolymers, and nanocomposites.

Self-Nucleation of Crystalline Phases Within Homopolymers, Polymer Blends, Copolymers, and Nanocomposites

The crystallization behavior of a butene-1/ethylene random copolymer with 9.88 mol % ethylene counits was investigated by means of differential scanning calorimetry, wide- and small-angle X-ray scattering, and polarized optical microscopy. Unlike in its homopolymer counterpart which crystallizes always into a metastable form II from the melt state, the random copolymer was found to crystallize either into form II or stable form I′ directly from its melt state after being cooled down. The occurrence of either crystalline form only depended on the temperature where the crystalline material was molten before cooling down. Even though the material was brought to temperatures higher than the equilibrium melting temperature, heterogeneities of segmental segregation character were preserved which promoted massive nucleation of form I′ crystallites, which makes it possible that the material is able to crystallize into pure form I′. Only if when the melt temperature was high enough where all heterogeneities of the...

Direct Formation of Different Crystalline Forms in Butene-1/Ethylene Copolymer via Manipulating Melt Temperature

A series of low polydispersity cyclic PCL samples (C-PCLs), as well as their linear analogs (L-PCLs), were synthesized by click chemistry in a number average molecular weight ( M n ) range of 2–22 kg/mol. They were investigated by Polarized Light Optical Microscopy (PLOM) and Differential Scanning Calorimetry (DSC). The nucleation and overall crystallization kinetics were studied, as well as their self-nucleation behavior and SSA (Successive Self-nucleation and Annealing) thermal fractionation. Cyclic PCLs were found to nucleate and crystallize faster than linear PCLs due to: (a) faster diffusion of C-PCL chains and (b) larger supercoolings of C-PCLs at any given crystallization temperature, as compared to L-PCLs. A bell shape curve was obtained when the overall crystallization rate was examined as a function of M n , this effect is probably due to a competition between nucleation and diffusion. It was found for the first time, that since cyclic molecules have lower entanglement densities, they can quickly recover their pseudo-equilibrium compact coil conformations upon melting and therefore exhibit much smaller crystalline memory effects than their linear counterparts of identical chain lengths. SSA revealed that C-PCLs are more sensitive to annealing than L-PCLs because their ring topology and limited lamellar chain folding facilitates crystal thickening.

Nucleation, crystallization, self-nucleation and thermal fractionation of cyclic and linear poly(ε-caprolactone)s

Origin of the melt memory effect in polymer crystallization

To provide insight into the formation of shear-induced precursor structures, three apparently unrelated subjects are analyzed and discussed: the saturation of crystallization from sheared polymer melts, evaluated with a new shear DTA instrument, the steady state in steady shear, and the entanglement-disentanglement transition. It is shown that the same large strains that saturate crystallization also lead to a reversible steady state in steady shear, where the viscosity of the sheared melt is constant in time, and their magnitude is only determined by the temperature of the sheared melt. Features of the melt morphology at this state are discussed, and their importance is highlighted to understand the possible mechanisms behind the formation of shear-induced precursors. Measurements of the reptation time for unsheared samples, and samples sheared up to the steady state, allowed the quantification of the entanglements loss during the transition between these two states (around 2/3), which is interpreted as an entanglement-disentanglement transition. The relevance of this result on assumptions of flow models, particularly the constant number of topological constraints, convective constraint release process, and chain stretch, is discussed.

Saturation of Shear-Induced Isothermal Crystallization of Polymers at the Steady State and the Entanglement-Disentanglement Transition

Shear-induced nonisothermal crystallization of low-density polyethylene

The effect of melt memory on shear-induced crystallization of a low-density polyethylene melt is evaluated with a new shear DTA instrument. A critical strain is identified as the controlling factor for saturating the crystallization. Variation of this strain with the temperature of the sheared melt is established, and the melt state responsible for saturating the crystallization is identified by shear-stress growth experiments at the steady-state. Reptation times have been evaluated in both viscoelastic nonlinear and linear regimes, and it is shown that small-angle oscillatory shear experiments cannot be used to probe effectively the effect of shear flow on the overall crystallization kinetics.

Evaluation of the Effect of Melt Memory on Shear‐Induced Crystallization of Low‐Density Polyethylene

This work aims to verify the impossibility mentioned in the literature of saturating the crystallization kinetics of pre-sheared metallocene polyethylene melts. Similarly to results reported for other materials, and contrary to other published works, an acceleration of crystallization kinetics with the increase of shear strain and its saturation at large strain values was found. Similar strain values, with the same temperature variation, were evaluated with independent experiments using different devices, which allowed us to identify the steady state as the melt state responsible for the saturation of crystallization kinetics. Since this is a partially disentangled melt state, with viscosity lower than that of fully entangled (relaxed) melts, we assign the acceleration of crystallization kinetics by application of shear, and its saturation, mainly to the facilitated diffusion of chain segments to the lamellae growth front. This conclusion is supported additionally with the experimental results of other authors.

Weidong Zhang

Papers

Origin of the melt memory effect in polymer crystallization

Saturation of Shear-Induced Isothermal Crystallization of Polymers at the Steady State and the Entanglement-Disentanglement Transition

Shear-induced nonisothermal crystallization of low-density polyethylene

Evaluation of the Effect of Melt Memory on Shear‐Induced Crystallization of Low‐Density Polyethylene

Effect of melt viscosity on the crystallization kinetics of pre-sheared metallocene polyethylene melts