Conformational Entropy as a Means to Control the Behavior of Poly(diketoenamine) Vitrimers In and Out of Equilibrium

Abstract

Control of equilibrium and non‐equilibrium thermomechanical behavior of poly(diketoenamine) vitrimers is shown by incorporating linear polymer segments varying in molecular weight (MW) and conformational degrees of freedom into the dynamic covalent network. While increasing MW of linear segments yields a lower storage modulus at the rubbery plateau after softening above the glass transition (Tg), both Tg and the characteristic time of stress relaxation are independently governed by the conformational entropy of the embodied linear segments. Activation energies for bond exchange in the solid state are lower for networks incorporating flexible chains; the network topology freezing temperature decreases with increasing MW of flexible linear segments but increases with increasing MW of stiff segments. Vitrimer reconfigurability is therefore influenced not only by the energetics of bond exchange for a given network density, but also the entropy of polymer chains within the network.

The effect of polarity on the molecular exchange dynamics in imine-based covalent adaptable networks

“…[8][9][10] By stimulating the exchange reactions of the dynamic covalent bonds, for example by introduction of heat or light, a molecular flow is introduced in the material. Several types of bond exchange reactions have been studied over the last decade, such as transesterifications, 1,12 boronic ester exchange, [13][14][15][16] 1,2,3-triazolium exchange, 17,18 diketoenamine exchange, 19,20 (alkyl)urea exchange, [21][22][23] urethane exchange, [24][25][26][27][28] thiol-ene/yne exchange, 29,30 and imine exchange. [31][32][33][34] Individually, each type of bond exchange reaction possesses different dynamics according to the mechanism of the exchange reaction.…”

Cross-linker control of vitrimer flow

“…Strategies to control the rate of vitrimer flow primarily rely on the effect of catalysts, 36,37 with isolated examples examining macromolecular architecture 38 and strand flexibility. 39 In a pioneering study, Leibler and co-workers demonstrated that changing the catalyst type and concentration affects flow activation energies for ester-based vitrimers. 40 Bates and co-workers later identified a surprising inverse relationship between the pKa of Brønsted acid catalysts and flow activation energy.…”

Stimuli‐responsive mechanical properties in polymer glasses: challenges and opportunities for defense applications

“…204 Transamination. Although similar to transesterification, transamination reactions include transamination of vinylogous urethanes, 225,226 diketoenamines 207,208 and imines. [220][221][222][227][228][229][230] Typically considered an associative dynamic reaction (Scheme 3), free primary amines attack either vinylogous urethane ⊎-carbons, diketoenamines or imine functional groups in a mechanism similar to that of transesterification.…”

“…124,126,133,205,206 The T v transition can also be tailored with molecular weight of flexible or rigid polymer chains between crosslinks. 207,208 Transesterification. One of the most prominent dynamic crosslinking chemistries in the vitrimer field continues to be the transesterification of aromatic and aliphatic ester groups with alcohols.…”

“…A wide range of amine hardeners appear in the literature producing a library similar to traditional epoxyamine systems, where concentration (network density) and chemical composition (energetics of bond exchange and entropy of polymer chains) affect T g and T v . 207,208 With the incorporated amine-based dynamic exchange reactions, these networks produce versatile, robust, reproducible and commonly catalyst-free vitrimer systems. New areas of interest include converting high-performance polymers like polysulfones to vitrimer or reversible dynamic systems.…”