Enhancing the melt crystallization of polymers, especially slow crystallizing polymers like PLLA and PET

Abstract

Over 20 years ago we demonstrated that it is possible to control the melt crystallization of polymers via self‐nucleation. Non‐covalently bonded inclusion compounds (ICs) can be formed with cyclodextrins (CDs) or urea (U) when these small host molecules crystallize around guest polymers and form narrow parallel host channels, where the isolated and stretched guest polymer chains are included. Careful removal of the host crystalline lattice yields a neat coalesced (c) guest polymer sample with less entangled chains that are packed to a higher density than bulk samples of the same polymer obtained from its melt or solutions. Consequently, the reorganized c‐polymer samples behave distinctly, with higher glass‐transition temperatures for amorphous polymers and enhanced crystallizabilities for semi‐crystalline polymers. Remarkably, c‐polymer samples retain their distinct reorganized structures and behaviors even after extended periods of melt annealing. When a small amount of the rapidly crystallizable c‐polymer is homogeneously dispersed in a large quantity of the same as‐received (asr) polymer, and it is melted and then cooled, the c‐polymer chains crystallize first and nucleate the crystallization of the asr‐polymer chains, so that the entire sample (nuc‐polymer) crystallizes at nearly the same temperature as the neat nucleant (c‐polymer), including slow melt‐crystallizing polymers, such as PET and PLLA.

Self-Nucleation Effects on Polymer Crystallization

“…More recently, a low concentration of strongly aligned polymer chains, obtained by coalesced inclusion compounds of various polymers with cyclodextrins molecules, has been shown to efficiently self-nucleate an isotropic melt of the same polymer. 157,158 These experiments corroborate the interpretation of Lorenzo et al, 47 although the role of partial disentangled melt topology cannot be ruled out. In the next section, we will show some convincing evidence that residual interactions between chain segments can constitute a kind of noncrystalline melt structure that strongly influences memory effects.…”

“…Such small bundles of aligned chains act as self-nuclei upon re-crystallization. More recently, a low concentration of strongly aligned polymer chains, obtained by coalesced inclusion compounds of various polymers with cyclodextrins molecules, has been shown to efficiently self-nucleate an isotropic melt of the same polymer 157,158 . These experiments corroborate the interpretation of Lorenzo et al 47 , although the role of partial disentangled melt topology cannot be ruled out.…”

Fractionated crystallization in semicrystalline polymers

“…This can be detected by the increase in the crystallization temperature in comparison with the standard crystallization temperature. The nature of the self-seeds is controversial [313][314][315][316][317][318][319][320][321][322][323][324][325][326][327][328][329]. Our group has proposed a division of Domain II into two sub-Domains [325].…”