Elsevier

Polymer

Volume 205, 28 September 2020, 122837
Polymer

Effect of gold and graphene oxide nanoparticles on the thermo- and photo-actuation of monodomain liquid crystal elastomers

https://doi.org/10.1016/j.polymer.2020.122837Get rights and content

Highlights

  • Monodomain liquid crystal elastomers (m-LCEs) carrying benzoate and azobenzene-type mesogens were synthesized.

  • Small amounts of Au or GO nanoparticles were dispersed into m-LCE aiming to enhance the response to external stimuli.

  • A good polymer-particle interaction is favorable for thermo-actuation but detrimental for photo-actuation.

  • A new model is proposed for explaining the enhancement in thermal contraction.

Abstract

Monodomain liquid crystal elastomers (m-LCEs) were synthesized from polymethylhydrosiloxane (PMHS), two distinct vinyl-functionalized mesogens (benzoate and azobenzene) and one crosslinking monomer, using the hydrosilylation reaction. Gold or amino-functionalized graphene oxide nanoparticles (NP) were in-situ incorporated into the m-LCEs, aiming to enhance their response to thermal and optical stimuli. All m-LCEs exhibited a degradation temperature of ~310 °C and a mesomorphic behavior over a wide temperature range (clearing temperature around 100 °C). The inclusion of nanoparticles stiffened the m-LCEs, although they induced a highly randomized ordering of mesogens at the nematic to isotropic transition, resulting in a higher specimen contraction; a schematic model is proposed in this concern. The photo-actuation (bending) of the m-LCEs/NP composites resulted rather diminished as they became stiffer than the neat m-LCEs, although the increasing content of azobenzene units allowed higher bending angles that could be suitable for remote actuation applications.

Introduction

m-LCEs carrying side mesogenic groups are unique among the shape memory materials (SMMs) as they are able to exhibit fast and large reversible response to electrical, thermal and luminous stimuli [[1], [2], [3]]. In these liquid-crystalline materials the response to stimuli relies on the nematic ↔ isotropic transition, where the orientation of mesogens (tightly coupled to the polymer network) turns from unidirectional to randomized and vice versa. In this reversible order to disorder transition, the m-LCEs undergo contraction followed by expansion, thus recovering their original shape. Contraction is due to the conformational change of the polymer chains that is triggered by the difference in entropy between the aligned (low entropy) and coiled (high entropy) chains [4]. Expansion is triggered by enthalpy (mesogens self-assemble into a monodomain), which is higher than entropy in the liquid crystal state (low temperature). This fully reversible order-disorder transition of mesogens, along with the rubber elasticity (aligned and coil conformations) and mechanical resistance of polymer chains, all three reflected at macroscopic scale, makes the m-LCEs ideal SMMs for systems requiring fast and large actuation [[5], [6]], as for instance, microelectromechanical systems (MEMS) [7], artificial muscles [8], smart textiles [9], light-activated motors [10], micro-fluidic valves [11], soft robots [12], 4D printed actuators [13], and bio-inspired actuators [14].

The m-LCEs can be prepared through a variety of synthesis methods such as conventional free radical polymerization, controlled or living radical polymerization using chain transfer agents [15], thiol-ene coupling [16], condensation reactions [17], hydrosilylation coupling [6], among others. The hydrosilylation coupling is a platinum-catalyzed reaction between Si–H and unsaturated (CH2double bondCH–R) groups that has been regularly used for the preparation of PMHS-based m-LCEs because it produces high yields and proceeds via anti-Markovnikov addition. This means that the silicon atom reacts with the terminal carbon atom of the vinyl (or ethenyl) group. So, the vinyl-terminated mesogens can be inserted into polymers carrying S–H groups like the PMHS whose low glass transition temperature and flexibility are ideal to prepare SMMs. The hydrosilylation reaction has been successfully used for the insertion of vinyl-functionalized mesogens into the PMHS chains while the latter are in-situ crosslinked with low amounts of a divinyl monomer [5]. Both, the mesogens and crosslinking monomer might possess one or more flexible groups (spacer) to decouple the rigid aromatic cores from the polymer backbones, and thus facilitate the mechanical response to external stimuli. The actuation properties of the resulting LCEs mainly depend on the chemical structure of mesogens, frequency and distribution of mesogens along the chains, crosslinking degree, and orientation of mesogens [2]. Side-chain LCEs often show different mechanical properties to those shown by main chain LCEs [18]. The inclusion of small amounts of azobenzene-type mesogens allow the LCEs to respond to UV light through the trans-to-cis isomerization, which produces a change in their chemical geometry (linear to bent shape), and this in turn disrupts the mesomorphic order, including that of the neighboring mesogens without azo group. Such disruption is reflected in a macroscale bending of m-LCEs [[19], [20]]. The photo-actuation of m-LCEs carrying azo groups is promising as it permits remote manipulation, fast response, and high control, among others [1].

On the other hand, previous studies on m-LCEs have demonstrated that the thermo- and photo-actuation can be induced, tuned or even enhanced by embedding or dispersing small amounts of nanofillers such as metallic and carbonaceous (graphene, carbon nanotubes, etc.) nanoparticles [21]. For instance, in dispersing gold nanoparticles (AuNPs) in nematic LCEs, Montazami et al. observed a significant improvement in actuation speed (>100%), although they noted a reduced strain that they associated to the elastomer stiffening produced by nanoparticles [22]. They also noted that the actuation speed was raised only when the AuNPs allowed the formation of a thermal conducting network. Gold nanoparticles can also induce actuation through the photo-to-thermal conversion produced by surface plasmon resonance (SPR) on the nanoscale particles irradiated with UV light. For instance, Xu et al. prepared polysiloxane-based m-LCE/Au-NP composites, which exhibited an important contraction (third of the original length) at the nematic-to-isotropic transition triggered by the SPR photo-thermal effect [23]. Li et al. also explored this effect in m-LCE/graphene oxide composites, which showed a contraction of around 50% [24]. As mentioned and explained above, light stimuli can trigger an order to disorder transition that is reflected in the actuation of m-LCEs as well. For example, Bi et al. prepared azobenzene-based m-LCE/carbon nanotubes composites that showed light-driven deformations; tested in light micro-valve actuators, these materials showed a fairly good dimensional stability after multiple opening-closing cycles [25].

Herein, PMHS-based m-LCEs, carrying azobenzene and benzoate type mesogens, were synthesized and their thermotropic behavior as well as their thermo- and photo-actuation was evaluated. Gold nanoparticles or amino-functionalized graphene oxide nanoparticles were introduced at low loadings into m-LCEs aiming to improve both the thermo and photo actuations.

Section snippets

Materials

4-aminobenzonitrile, 1-bromoundecene, dichloro(1,5-cyclooctadiene) platinum (II) (Pt-cat), hydrochloric acid, hydroquinone, phenol, PMHS (1700–3200 g mol−1), potassium carbonate, potassium iodide, sodium nitrite, 1-undecene, toluene, dichloromethane were all purchased from Sigma-Aldrich and used as received. 4-Methoxyphenyl-4-(3-butenyloxy)benzoate (Bz-C4) from TCI chemicals was used as received. 4-((Undec-10-enyl-1-oxy)-phenyl)-azo)-benzonitrile (Az-C11) was synthesized as described in a

Synthesis of the m-LCEs

The hydrosilylation reaction between the Si–H groups of PMHS and the vinyl groups of Bz-C4 and Az-C11 allowed the functionalization of the PMHS chains with side mesogenic groups. It also allowed the PMHS chains to be in-situ crosslinked with HQ-2C11. The success of these coupling reactions was confirmed by the presence of the stretching band of the C–Si group at 810 cm−1 in the FTIR spectra (Fig. 1). The m-LCEs were synthesized using the exact molar amount of monomers (mesogens and crosslinking

Conclusion

Monodomain liquid crystal elastomers (m-LCEs) carrying benzoate and azobenzene mesogens were prepared by applying uniaxial stress during the crosslinking process. Small amounts of gold or graphene oxide nanoparticles were included aiming to enhance their response to thermal and light stimuli. The m-LCEs and m-LCEs/NP composites developed nematic and/or smectic type arrangements with a clearing temperature (Ti) of around 100 °C. The increase in the azo-mesogen content (from 10 to 20 mol%)

Funding

This research was supported by CONACYT Mexico through projects 157652-Y and 258195.

CRediT authorship contribution statement

Marco A. De Jesús-Téllez: Conceptualization, Methodology, Investigation. Isaura Felix-Serrano: Investigation, Validation. Rosa Julia Rodríguez-González: Methodology, Investigation. Dámaso Navarro-Rodríguez: Visualization, Writing - review & editing. Leticia Larios-López: Writing - original draft, Project administration, Funding acquisition.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

Financial support from CONACYT Mexico, through the projects 157652-Y and 258195, and the grant allowed to MAJT to pursuit his PhD studies, is gratefully appreciated. Authors also appreciate the technical assistance of G. Méndez, J. A. Mercado, R. Rangel, C. Gallardo, E. Diaz, A. Y. Ruiz, and J.A. Cepeda from CIQA.

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