Vitrification and plastic flow in transient elastomer networks

Pritchard, Robyn H.; Redmann, Anna-Lena; Pei, Zhiqiang; Ji, Yan; Terentjev, Eugene M.

doi:10.1016/j.polymer.2016.04.060

Cited by 43 publications

(44 citation statements)

References 27 publications

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“…The region where the data deviates from the initial rubber-elastic decreasing slope is identified as the transition to plastic flow, the vitrification point Tv: apparently it does not depend on the applied stress. 51 One should expect some creep under stress in network with siloxane-exchange above 140-150C, although the rapid flow only sets in at a much higher temperature (over 250C).…”

Section: Resultsmentioning

confidence: 99%

Siloxane crosslinks with dynamic bond exchange enable shape programming in liquid-crystalline elastomers

Saed

Terentjev

2020

Sci Rep

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Liquid crystalline elastomers (LCEs) undergo reversible shape changes in response to stimuli may enable a wide range of smart applications, such as soft robot, adhesive systems or implantable medical devices. In this manuscript, we introduce new dynamic covalent chemistry based on siloxane equilibrium exchange into the LCEs to enable processing (director alignment, remolding, and welding). Unlike, the traditional siloxane based LCEs, which are produced by a other reaction schemes with irreversible bonds (e.g. hydrosilylation), here, we use a much more robust reaction (thiolacrylate/thiol-ene 'double-click' chemistry) to obtain highly uniform dynamically crosslinked networks. Combining the siloxane crosslinker with click chemistry produces LCEs with tunable properties, low glass transition (-30˚C), controllable nematic to isotropic transition (32 to 75˚C), and a very high vitrification temperature (250˚C). Accordingly, this class of dynamically crosslinked LCEs shows unprecedented thermal stability within the working temperature range (-50 to 140 ˚C), over many thermal actuation cycles without any creep. Finally, multiple LCEs sharing the same siloxane exchangeable bonds can be welded into single structures to allow for materials that sequentially and reversibly undergo multiple phase transformations in different sections of the sample.

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Section: Resultsmentioning

confidence: 99%

Siloxane crosslinks with dynamic bond exchange enable shape programming in liquid-crystalline elastomers

Saed

Terentjev

2020

Sci Rep

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“…2,3,58 Following these initial investigations, several groups employed constitutive equations to describe vitrimer rheology and, in particular, plastic flow at intermediate time scales. 34,35,59,60,61,62,63,64,65,66,67 The full and simplified theories developed by Terentjev and coworkers can even describe the stress relaxation of vitrimers bearing both exchangeable and nondynamic cross-links. 34,35,68,69 Complementing the continuum model approaches, Sciortino et al used molecular dynamics simulations to study vitrimer relaxations at the microscale.…”

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confidence: 99%

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

Ricarte

Tournilhac²,

Cloître³

et al. 2020

Preprint

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For vitrimer systems obtained by dynamic cross-linking of polymer chains, incompatibility effects between the cross-links and polymer backbone can lead to microphase separation, resulting in a network made of cross-linked aggregates. Additionally, when there is a wide distribution of the number of cross-links per chain, macrophase separation can occur, mostly due to entropic effects.Here, we investigate the linear viscoelasticity and flow of a polyethylene (PE) vitrimer that has cross-linkable dioxaborolane maleimide grafts, which self-assemble into a hierarchical nanostructure. To elucidate the role of self-assembly, we first studied dioxaborolane graft functionalized PE that was non-cross-linked. It had a terminal relaxation time that was orders of magnitude larger than both neat PE and partially peroxide cross-linked PE. When dioxaborolane cross-linker was added to form the vitrimer, the resulting material could not achieve terminal relaxation within 8 h. The soluble and insoluble portions of the PE vitrimer were then isolated and characterized. The soluble portion, which was graft-poor, expressed similar flow behavior as neat PE, while the insoluble portion -which was a graft-rich network of cross-linked aggregatesrelaxed very little over 8 h. When the insoluble and soluble portions were blended, the rheological behavior of the original vitrimer was basically recovered, showing that the soluble portion acts as a lubricant. When the insoluble portion was blended with neat PE, the material relaxed much more stress, but still did not reach steady-state flow within 8 h. When high stresses were applied, however, PE vitrimer flowed. Nonlinear rheology experiments revealed melt fracture at high strains and suggested that flow is enabled by rapid healing, which follows fracture events. The presence of macroscopic phase separation facilitated flow. IntroductionVitrimers are covalent networks that engage in thermoactivated associative exchange reactions. 1,2,3 Under cooling, they undergo a reversible topology freezing transition, analogous to the glass transition of amorphous silica. In contrast to dissociative cross-links, the associative cross-links of vitrimers preserve network connectivity at all times and temperatures below degradation conditions, yet still enable the network topology to fluctuate. 1,2,3 The consequences of the topology freezing transition on materials properties were investigated for epoxy/acid systems that underwent catalyst-driven transesterification. 1 Since then, the literature has focused on altering the exchange reaction (either catalytically controlled or catalyst-free) and adapting chemistries to different polymer backbones. Other studies explored the properties of vitrimer composites and the addition of nondynamic cross-links to the network. 14,28,29,30,31,32,33,34,35 These research efforts established that modifying vitrimer chemistry provides a pathway for tuning mechanical properties, 36,37,38 stress relaxation, 39,40,41,42 shape memory, 43,44,45 and the ability to self-heal and adhe...

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“…Concurrent to the examinations detailed here, a parallel community has developed focused on so‐called “covalent adaptive networks.” These materials are also crosslinked and have functional groups that allow for bond breaking and reformation. Recently, the two communities have begun to merge evident in a number of recent reports …”

Section: Outlook and Conclusionmentioning

confidence: 99%

“…A number of other groups have reported on a variety of reaction chemistries in the recent literature . Many recent papers also report on chemistries with reconfigurable covalent and adaptive networks and employ them to post‐process the material after preparation …”

Section: Introductionmentioning

confidence: 99%

Photomechanical effects in liquid crystalline polymer networks and elastomers

White

2018

J Polym Sci B Polym Phys

101

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Liquid crystals are widely employed as stimuliresponsive materials. Liquid crystallinity can be retained in polymeric form. Photoinduced mechanical effects in liquid crystalline polymer networks and elastomers have been a topic of considerable recent research. This review details the historical underpinnings and recent advances in the synthesis and the corresponding photomechanical response of these materials. In nearly all cases, the conversion of light into mechanical work has employed azobenzene as either a guest additive or covalently attached to the network. V C 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018, 56, 695-705 KEYWORDS: azobenzene; liquid crystal; liquid crystal elastomer; liquid crystal polymer; photochemistry; shape change; shape programing INTRODUCTION Liquid crystalline materials are synonymous with their use in the displays that surround us in our daily lives. The widespread application of liquid crystals in display applications is enabled by their unique assimilation of properties: the long range fluidity of liquids combined with the anisotropic optical properties of crystalline solids. In displays, the relationship of orientation of the liquid crystal director and birefringence has been leveraged by concurrent advances in thin film transistors to locally control the transmission and spectral output (color) of light visualized in a pixel. The display community, in response to seemingly insatiable consumer demand, is currently focused on improvements in resolution and speed as well as realization of flexible form factors.

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Vitrification and plastic flow in transient elastomer networks

Cited by 43 publications

References 27 publications

Siloxane crosslinks with dynamic bond exchange enable shape programming in liquid-crystalline elastomers

Siloxane crosslinks with dynamic bond exchange enable shape programming in liquid-crystalline elastomers

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

Photomechanical effects in liquid crystalline polymer networks and elastomers

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