Phase Separation and Self-Assembly in Vitrimers: Hierarchical Morphology of Molten and Semicrystalline Polyethylene/Dioxaborolane Maleimide Systems

Abstract

Vitrimers -a class of polymer networks which are covalently crosslinked and insoluble like thermosets, but flow when heated like thermoplastics -contain dynamic links and/or crosslinks that undergo an associative exchange reaction. These dynamic crosslinks enable vitrimers to have interesting mechanical/rheological behavior, self-healing, adhesive, and shape memory properties.We demonstrate that vitrimers can self-assemble into complex meso-and nanostructures when crosslinks and backbone monomers strongly interact. Vitrimers featuring polyethylene (PE) as the backbone and dioxaborolane maleimide as the crosslinkable moiety were studied in both the molten and semi-crystalline states. We observed that PE vitrimers macroscopically phase separated into dioxaborolane maleimide rich and poor regions, and characterized the extent of phase separation by optical transmission measurements. This phase separation can explain the relatively low insoluble fractions and overall crystallinities of PE vitrimers. Using synchrotronsourced small-angle X-ray scattering (SAXS), we discovered that PE vitrimers and their linear precursors micro-phase separated into hierarchical nanostructures. Fitting of the SAXS patterns to a scattering model strongly suggests that the nanostructures -which persist in both the melt and amorphous fraction of the semi-crystalline state -may be described as dioxaborolane maleimide rich aggregates packed in a mass fractal arrangement. These findings of hierarchical meso-and nanostructures point out that incompatibility effects between network components and resulting self-assembly must be considered for understanding behavior and the rational design of vitrimer materials.Thermoplastics and thermoplastic elastomers are soluble in good solvents and flow when heated above the glass or melting temperature. Thermosets and rubbers do not flow and are not soluble.Vitrimers, a class of polymers introduced by Leibler and collaborators in 2011, flow when heated, but remain insoluble. 1 Vitrimers are made of polymer networks which contain covalent crosslinks that undergo dynamic associative exchange reactions. The covalent crosslinks in a vitrimer maintain network connectivity at all times and temperatures. Unlike materials employing dissociative crosslinking mechanisms, 2 vitrimers cannot be completely dissolved -even in good solvents. 1,3 Associative exchange reactions permit the network topology to fluctuate and the system to flow when stress is applied, and exchange reaction kinetics control the vitrimer relaxation dynamics and viscosity. 2,4,5,6 The initial reports of vitrimer systems focused on epoxy networks that reorganized via metal-catalyzed transesterification. 1,4,6 Today, the library of dynamic exchange reactions has expanded to include chemistries that are catalytically-controlled (olefin metathesis and transcarbonation) 7,8 or catalyst-free (transamination, 9,10,11 trans-N-alkylation, 12,13 reversible addition of thiols, 14 imine exchange, 15,16 addition-fragmentation chain transfer, 17,18 boronic est...

Linear Viscoelasticity and Flow of Self-Assembled Vitrimers: The Case of a Polyethylene/dioxaborolane System

“…The molecular weight and dispersity of this commercial polymer (i 8 = 11 kg/mol and Đ = 7.5) were reported previously. 56 Figure 3A features the horizontal shift factors (@ A ) determined by shifting the PE SAOS tan E data ( Figure S4C) However, the ∆J &2( determined from creep is 16 ± 1 kJ/mol, matching the value of the virgin polymer ( Figure S14D). These findings are consistent with the observations of van Gurp and Palmen.…”

“…Ricarte, Tournilhac, and Leibler showed that polyethylene/dioxaborolane vitrimers self-assemble into nanostructures. 56 Chen, Li, and Torkelson further observed microphase separation in molten polybutadiene and polydimethylsiloxane vitrimers with hydroxyurethane cross-links. 57 In this work, we show that self-assembly and phase separation have a significant impact on the rheology and, consequently, processability of vitrimers.…”

“…The cross-links rearrange by engaging in a dioxaborolane metathesis exchange reaction. 15,21,56 Previously, we observed that strong interactions between the polymer backbone and cross-links induce PE vitrimer to adopt hierarchical meso-and nanostructures ( Figure 1). Essentially, PE vitrimer may be described as a phase separated blend with two distinct macrophases.…”

“…phenyl-1,3,2-dioxaborolan-4-yl)methyl]-1H-pyrrole-2,5-dione (i.e., dioxaborolane maleimide) and 2,2'-(1,4-phenylene)-bis[4-methyl-1,3,2-dioxaborolane] (i.e., bis-dioxaborolane cross-linker) were conducted according to schemes reported in Röttger et al 21 For reactive mixing followed by extrusion of graft-functionalized PE (PE-g) and PE vitrimer (PE-v), the scheme reported by Ricarte et al was used. 56 Table 1 details the measured graft/ethylene and cross-linker/graft molar ratios for PE-g and PE-v. The functionalization and dynamic cross-linking of PE were achieved using a DSM Explore batch twin-screw extruder with a 5 cm 3 capacity, co-rotating conical screw profile, and recirculation channel.…”

High Glass‐Transition Temperature Polymer Networks Harnessing the Dynamic Ring Opening of Pinacol Boronates

“…[14] Recently,N icolaþ, Leibler, and co-workers disclosed fascinating covalent exchangeable vitrimers crosslinked by dynamic exchange of boronate ester linkages,a lthough the actual exchange mechanism still remains unsolved. [24][25][26] To the best of our knowledge,n one of the dynamically crosslinked materials described so far reach the very high glass transition temperatures (T g above 200 8 8C) required for adhesives,p rotective coatings,l ow-wear,o rl ow-friction materials when exposed to high temperatures (contact with hot parts,c omposites for aerospace,e lectronics,b earings). Fors uch demanding applications,o nly af ew thermoplastics constituted of very rigid segments,w ith various degrees of solubility and processability are commercially available (for example,p olyaryletherketones,p olyethersulfones,p olyaramides,p olyetherimides,o rp olybenzimidazoles).…”