Crystallization-Induced Self-Assembly of Poly(ethylene glycol) Side Chains in Dithiol-yne-Based Comb Polymers: Side Chain Spacing and Molecular Weight Effects.

The chain architecture and topology of macromolecules impact their physical properties and final performance, including their crystallization process. In this work, comb polymers constituted by poly(ethylene glycol), PEG, side chains, and a dithiol-yne-based ring polymer backbone have been studied, focusing on the micro- and nanostructures of the system, thermal behavior, and crystallization kinetics. The designed comb system allows us to investigate the role of a ring backbone, the impact of varying the distance between two neighboring side chains, and the effect of the molecular weight of the side chain. The results reflect that the governing factor in the crystalline properties is the molar mass of the side chains and that the tethering of PEG chains to the ring backbone brings important constraints to the crystallization process, reducing the crystallinity degree and slowing down the crystallization kinetics in comparison to analogue PEG homopolymers. We demonstrate that the effect of spatial hindrance in the comb-like PEG polymers drives the morphology toward highly ordered, self-assembled, semicrystalline superstructures with either extended interdigitated chain crystals or novel (for comb polymers) interdigitated folded chain lamellar crystals. These structures depend on PEG molecular weight, the distance between neighboring tethered PEG chains, and the crystallization conditions (nonisothermal versus isothermal). This work sheds light on the role of chain architecture and topology in the structure of comb-like semicrystalline polymers.

Origin of Melt Memory Effects in Poly(ethylene oxide): The Crucial Role of Entanglements.

The melt memory effect on crystallization is an intriguing phenomenon displayed by semicrystalline polymers, as opposed to low molar mass molecules. It concerns the effect of melt temperature on nucleation upon recrystallization. Typically, polymer crystals must be considerably superheated to erase the effect of previous morphology on the subsequent crystallization, avoiding an acceleration of the process. Despite being known for decades, its origin is still not fully understood. Investigating model poly(ethylene oxide) covering a wide range of molar mass, it is demonstrated that melt memory originates from topological constraints among the chains, i.e., entanglements, for PEO in which weak intermolecular interactions are present due to the ether groups. In fact, no memory is observed for samples below the critical molar mass for the formation of entanglements (about 1 kg mol-1). The increase in molar mass raises the number of entanglements and induces the formation of folded chains crystals, both factors leading to a topologically complex amorphous phase, enhancing the melt memory effect. The molecular origin of the melt memory effect in polymers with weak intermolecular interactions is thus ascribed to a slower isotropization in the melt of the chain segments originally contained in the crystals, due to the presence of entanglements among the chains. This study defines the distinction between small molecules and polymers from the point of view of melt memory.