Konstantinos Ntetsikas

Model systems incorporating dynamic covalent bonds, like star-shaped polymers, are ideal for gaining insights into the role played by the bond exchange in the dynamics.

Well-defined poly­(isoprene)-b-poly­(l-lactide) (PI-b-PLLA) diblock copolymers were synthesized by ring-opening polymerization of l-lactide using as macroinitiator hydroxy-terminated polyisoprene (PI–OH), synthesized by anionic polymerization using high-vacuum techniques. The resulting copolymers exhibited strong melt-state microphase segregation, with their nanostructures retained upon rapid melt quenching. Upon heating between 70 and 100 °C, confined crystallization of PLLA occurred, forming an unconventional β form along with the α­(α′) forms. This process increased microdomain thickness from 27 ± 0.5 to 32 ± 0.5 nm. At higher temperatures, 110–140 °C, the metastable γ form appeared along with the α­(α′) and β forms, achieving maximum crystallinity. During this stage, breakout crystallization overwrote the existing microphase-separated lamellar morphology due to the increased crystallinity. Notably, between 130 and 150 °C, the metastable β and γ forms transformed into the more stable α form just before melting. We propose that confinement imposed by the covalently linked PI chains, in the presence of preformed α­(α′) and β crystals, promoted the formation of these unconventional metastable crystalline forms (β and γ forms). These insights are crucial for understanding the competitive self-organization processes in semicrystalline block copolymers and the constrained crystallization behaviors within phase-separated structures.

A well‐defined α,ω‐dialdehyde polyisoprene‐ b ‐polystyrene block copolymer, synthesized using anionic polymerization high‐vacuum techniques, is employed to prepare vitrimers with tris(2‐aminoethyl)amine as the cross‐linking agent. The vitrimer network, featuring dynamic imine cross‐links, results in robust, flexible, and solvent‐resistant films, which are applicable in thin film composite membranes. These vitrimer membranes, with molecular weight cut‐offs in the nanofiltration range, are successfully used for organic solvent separation and evaluated for gas separation. The cross‐linking density, controlled by the cross‐linker, affects the material's gas permeability and affinity for CO₂. The dynamic nature of the imine cross‐links enables the vitrimer's self‐healing ability, activated by heat treatment at temperatures as low as 50 °C. Additionally, the vitrimer membranes can be reprocessed through solvent dissolution in the presence of the excess cross‐linking agent.

Polymer vitrimers are a new class of materials that combine the advantages of thermoplastics and thermosets. This is due to the dynamic nature of the chemical bonds linking different chains. However, how this property affects the polymer dynamics at different length scales is still an open question. Here, we investigate the dynamics of model vitrimers based on well-defined polyisoprene (PI) chains using broadband dielectric spectroscopy. In this way, we study the polymer dynamics from the segmental to the whole chain scale, taking advantage of the fact that PI belongs to the class of molecules that exhibit a net dipole moment associated with the end-to-end vector. Three distinct relaxation phenomena are identified. The fastest relaxation is attributed to the segmental PI dynamics with a small influence of the cross-linking. An intermediate relaxation attributed to the dipolar character of the cross-linker is also observed. The slower identified relaxation component, corresponding to limited fluctuations of the end-to-end PI chains, is found to be determined by the dynamics of the clusters formed by the cross-linkers with an average time scale orders of magnitude faster than that of the terminal relaxation as inferred from the viscous flow.

The design of functional polymeric materials with tunable response requires a synergetic use of macromolecular architecture and interactions. Here, we combine experiments with computer simulations to demonstrate how physical properties of gels can be tailored at the molecular level, using star block copolymers with alternating block sequences as a paradigm. Telechelic star polymers containing attractive outer blocks self-assemble into soft patchy nanoparticles, whereas their mirror-image inverted architecture with inner attractive blocks yields micelles. In concentrated solutions, bridged and interpenetrated hexagonally packed nanocylinders are formed, respectively, with distinct structural and rheological properties. The phase diagrams exhibit a peculiar re-entrance where the hexagonal phase melts upon both heating and cooling because of solvent-block and block-block interactions. The bridged nanostructure is characterized by similar deformability, extended structural coherence, enhanced elasticity, and yield stress compared to micelles or typical colloidal gels, which make them promising and versatile materials for diverse applications.