Low-molar-mass cyclic poly(oxyethylene)s studied by Raman spectroscopy, X-ray scattering and differential scanning calorimetry

Cyclodextrin-Induced Suppression of the Crystallization of Low-Molar-Mass Poly(ethylene glycol)

We examine the effect of alpha-cyclodextrin (αCD) on the crystallization of poly(ethylene glycol) (PEG) [poly(ethylene oxide), PEO] in low-molar-mass polymers, with M w = 1000, 3000, or 6000 g mol-1. Differential scanning calorimetry (DSC) and simultaneous synchrotron small-/wide-angle X-ray scattering (SAXS/WAXS) show that crystallization of PEG is suppressed by αCD, provided that the cyclodextrin content is sufficient. The PEG crystal structure is replaced by a hexagonal mesophase of αCD-threaded polymer chains. The αCD threading reduces the conformational flexibility of PEG and, hence, suppresses crystallization. These findings point to the use of cyclodextrin additives as a powerful means to tune the crystallization of PEG (PEO), which, in turn, will impact bulk properties including biodegradability.

State Transitions and Crystalline Structures of Single Polyethylene Rings: MD Simulations

This study investigates the structural changes of cyclic polyethylene (PE) single chains during cooling through molecular dynamics simulations. The influence of topological constraint on a ring is examined by comparing it with the results of its linear counterpart. A pseudo phase diagram of state transition for PE rings based on length and temperature is constructed, revealing a consistent chain-folding transition during cooling. The shape anisotropy of short crystallized cyclic chains exhibits oscillations with chain length, leading to a more pronounced odd-even effect in single cyclic chains compared with the linear ones. A honeycomb model is proposed to elucidate the odd-even effect of chain folding in crystalline structures of single linear and cyclic chains, and we discuss its potential to predict surface tension. Analyses of the tight folding model and the re-entry modes demonstrate that a cyclic chain possesses a shorter average crystalline stem length and a more compact folded structure than its linear counterpart. The findings highlight the impact of topological change on crystallization and the odd-even effect of chain length, providing valuable insights for understanding polymer crystallization with different topologies.

Accelerating the crystallization kinetics of linear polylactides by adding cyclic poly (L-lactide): Nucleation, plasticization and topological effects

Polylactide is one of the most versatile biopolymers, but its slow crystallization limits its temperature usage range. Hence finding ways to enhance it is crucial to widen its applications. Linear and cyclic poly (L-lactide) (l-PLLA and c-PLLA) of similarly low molecular weights (MW) were synthesized by ring-opening polymerization of L-lactide, and ring-expansion methodology, respectively. Two types of blends were prepared by solution mixing: (a) l-PLLA/c-PLLA, at extreme compositions (rich in linear or in cyclic chains), and (b) blends of each of these low MW materials with a commercial high MW linear PLA. The crystallization of the different blends was evaluated by polarized light optical microscopy and differential scanning calorimetry. It was found, for the first time, that in the l-PLLA rich blends, small amounts of c-PLLA (i.e., 5 and 10 wt%) increase the nucleation density, nucleation rate (1/τ₀), spherulitic growth rate (G), and overall crystallization rate (1/τ_50%), when compared to neat l-PLLA, due to a synergistic effect (i.e., nucleation plus plasticization). In contrast, the opposite effect was found in the c-PLLA rich blends. The addition of small amounts of l-PLLA to a matrix of c-PLLA chains causes a decrease in the nucleation density, 1/τ₀, G, and 1/τ_50% values, due to threading effects between cyclic and linear chains. Small amounts of l-PLLA and c-PLLA enhance the crystallization ability of a commercial high MW linear PLA without affecting its melting temperature. The l-PLLA only acts as a plasticizer for the PLA matrix, whereas c-PLLA has a synergistic effect in accelerating the crystallization of PLA that goes beyond simple plasticization. The addition of small amounts of c-PLLA affects not only PLA crystal growth but also its nucleation due to the unique cyclic chains topology.

Well-defined cyclic polymer synthesis via an efficient etherification-based bimolecular ring-closure strategy

An alternative method to synthesize well-defined cyclic polymers via combination of anionic polymerization high vacuum techniques and Williamson etherification reaction in moderate dilution and up to 1 g scale.

Influence of chain topology on polymer crystallization: poly(ethylene oxide) (PEO) rings vs. linear chains

The absence of entanglements, the more compact structure and the faster diffusion in melts of cyclic poly(ethylene oxide) (PEO) chains have consequences on their crystallization behavior at the lamellar and spherulitic length scales. Rings with molecular weight below the entanglement molecular weight (M < M_e), attain the equilibrium configuration composed from twice-folded chains with a lamellar periodicity that is half of the corresponding linear chains. Rings with M > M_e undergo distinct step-like conformational changes to a crystalline lamellar with the equilibrium configuration. Rings melt from this configuration in the absence of crystal thickening in sharp contrast to linear chains. In general, rings more easily attain their extended equilibrium configuration due to strained segments and the absence of entanglements. In addition, rings have a higher equilibrium melting temperature. At the level of the spherulitic superstructure, growth rates are much faster for rings reflecting the faster diffusion and more compact structure. With respect to the segmental dynamics in their semi-crystalline state, ring PEOs with a steepness index of ∼34 form some of the "strongest" glasses.

Photoinduced topological transformation of cyclized polylactides for switching the properties of homocrystals and stereocomplexes

A new methodology for a stimuli-responsive polymer was proposed on the basis of cyclization and photocleavage. This requires only a single reaction per polymer molecule.