Preparation of Monodomain Liquid Crystal Elastomers and Liquid Crystal Elastomer Nanocomposites

LCEs are shape-responsive materials with fully reversible shape change and potential applications in medicine, tissue engineering, artificial muscles, and ...

Rice University

Liquid crystals are often combined with polymers to influence the liquid crystals' orientation and mechanical properties, but at the expense of reorientation speed or uniformity of alignment. We introduce a new method to create self-assembled nematic liquid-crystal gels using an ABA triblock copolymer with a side-group liquid-crystalline midblock and liquid-crystal-phobic endblocks. In contrast to in situ polymerized networks, these physical gels are homogeneous systems with a solubilized polymer network giving them exceptional optical uniformity and well-defined crosslink density. Furthermore, the unusually high-molecular-weight polymers used allow gels to form at lower concentrations than previously accessible. This enables these gels to be aligned by surface anchoring, shear, or magnetic fields. The high content of small-molecule liquid crystal (>/=95%) allows access to a regime of fast reorientation dynamics.

Self-assembled liquid-crystalline gels designed from the bottom up.

In a nematic gel we observe a low-energy buckling deformation arising from soft and semisoft elastic modes. We prepare the self-assembled gel by dissolving a coil-side-group liquid-crystalline polymer-coil copolymer in a nematic liquid crystal. The gel has long network strands and a precisely tailored structure, making it ideal for studying nematic rubber elasticity. Under polarized optical microscopy we observe a striped texture that forms when gels uniformly aligned at 35 degrees C are cooled to room temperature. We model the instability using the molecular theory of nematic rubber elasticity, and the theory correctly captures the change in pitch length with sample thickness and polymer concentration. This buckling instability is a clear example of a low-energy deformation that arises in materials where polymer network strains are coupled to the director orientation.

Buckling instability in liquid crystalline physical gels.

Well-defined liquid crystal networks with controlled molecular weight between cross-links and cross-link functionality were prepared by "click" cross-linking of telechelic polymers produced by ring-opening metathesis polymerization (ROMP). The networks readily swell in a small molecule liquid crystal, 5CB, to form LC gels with high swelling ratios. These gels exhibit fast, reversible, and low-threshold optic switching under applied electric fields when they are unconstrained between electrodes. For a given electric field, the LC gels prepared from shorter telechelic polymers showed a reduced degree of switching than their counterparts made from longer polymer strands. The reported approach provides control over important parameters for LC networks, such as the length of the network strands between cross-links, cross-linker functionality, and mesogen density. Therefore, it allows a detailed study of relationships between molecular structure and macroscopic properties of these scientifically and technologically interesting networks.

Well-defined liquid crystal gels from telechelic polymers.

We investigate a series of poly(amidoamine) starburst dendrimers (PAMAM) of different generations in acidic, aqueous solutions using small-angle neutron scattering (SANS). While the overall molecular size is found to be practically unaffected by a pD change, a strong generational dependence of counterion association is revealed. Upon increasing the dendrimer generation, the effective charge obtained from our SANS experiments only shows a small increase in contrast to the nearly exponential increase predicted by a recent atomic simulation. We also find that with the same degree of molecular protonation the specific counterion association, which is defined as the ratio of bound chloride anions to positively charged amines in solutions, is larger for higher-generation PAMAM dendrimer. The associated counterion density also increases upon increasing generation number.

Structural investigation of PAMAM dendrimers in aqueous solutions using small-angle neutron scattering: effect of generation.

A novel dye-sensitized solar cell (DSSC) structure using vertically aligned single-walled carbon nanotubes (VASWCNTs) as the counter electrode has been developed. In this design, the VASWCNTs serve as a stable high surface area and highly active electrocatalytic counter-electrode that could be a promising alternative to the conventional Pt analogue. Utilizing a scalable dry transfer approach to form a VASWCNTs conductive electrode, the DSSCs with various lengths of VASWCNTs were studied. VASWCNTs-DSSC with 34 μm original length was found to be the optimal choice in the present study. The highest conversion efficiencies of VASWCNTs-DSSC achieved 5.5%, which rivals that of the reference Pt DSSC. From the electrochemical impedance spectroscopy analysis, it shows that the new DSSC offers lower interface resistance between the electrolyte and the counter electrode. This reproducible work emphasizes the promise of VASWCNTs as efficient and stable counter electrode materials in DSSC device design, especially taking into account the low-cost merit of this promising material.

Vertically aligned single-walled carbon nanotubes as low-cost and high electrocatalytic counter electrode for dye-sensitized solar cells.

Graphene oxide nanoribbons (GONRs) are wide bandgap semiconductors that can be reduced to metallic graphene nanoribbons. The transformation of GONRs from their semiconductive to the metallic state by annealing has attracted significant interest due to its simplicity. However, the detailed process by which GONRs transform from wide-bandgap semiconductors to semimetals with a near zero bandgap is unclear. As a result, precise control of the bandgap between these two states is not currently achievable. Here, we quantitatively examine the removal of oxygen-containing groups and changes in the bandgap during thermal annealing of GONRs. X-ray photoelectron spectroscopy measurements show the progressive removal of oxygen-containing functional groups. Aberration-corrected scanning transmission electron microscopy reveals that initially small graphene regions in GONRs become large stacked graphitic layers during thermal annealing. These structural and chemical changes are correlated with progressive changes in the electrochemical bandgap, monitored by cyclic voltammetry. These results show that small changes in the thermal annealing temperature result in significant changes to the bandgap and chemical composition of GONRs and provide a straightforward method for tuning the bandgap in oxidized graphene structures.

Quantitative analysis of structure and bandgap changes in graphene oxide nanoribbons during thermal annealing.

Biological tissues have the remarkable ability to remodel and repair in response to disease, injury and mechanical stresses. Synthetic materials lack the complexity of biological tissues, and man-made materials that respond to external stresses through a permanent increase in stiffness are uncommon. Here we report that polydomain nematic liquid crystal elastomers increase in stiffness by up to 90% when subjected to a low-amplitude (5%), repetitive (dynamic) compression. Elastomer stiffening is influenced by liquid crystal content, the presence of a nematic liquid crystal phase and the use of a dynamic as opposed to static deformation. Through rheological and X-ray diffraction measurements, stiffening can be attributed to a mobile nematic director, which rotates in response to dynamic compression. Stiffening under dynamic compression has not been previously observed in liquid crystal elastomers and may be useful for the development of self-healing materials or for the development of biocompatible, adaptive materials for tissue replacement.

Dynamic self-stiffening in liquid crystal elastomers.

Les tissus biologiques ont la capacit&eacute; remarquable de remodeler et de r&eacute;parer en r&eacute;ponse &agrave; la maladie, les blessures et les sollicitations m&eacute;caniques. Les mati&egrave;res synth&eacute;tiques n&#39;ont pas la complexit&eacute; des tissus biologiques, et des mat&eacute;riaux synth&eacute;tiques qui r&eacute;pondent aux sollicitations ext&eacute;rieures gr&acirc;ce &agrave; une augmentation permanente de la rigidit&eacute; sont rares. Nous rapportons ici que polydomain n&eacute;matique le cristal liquide &eacute;lastom&egrave;res augmentent rigidit&eacute; jusqu&#39;&agrave; 90 % lorsqu&#39;il est soumis &agrave; une faible amplitude (5 %), compression (dynamique) r&eacute;p&eacute;titif. Raidissement de l&#39;&eacute;lastom&egrave;re est influenc&eacute;e par le contenu de cristaux liquides, la pr&eacute;sence d&#39;une phase de cristaux liquides n&eacute;matiques et l&#39;utilisation d&#39;une dynamique par opposition &agrave; une d&eacute;formation statique. Par rh&eacute;ologiques et mesures de diffraction des rayons x, des raidisseurs peuvent &ecirc;tre attribu&eacute;s &agrave; un directeur n&eacute;matique mobile, qui tourne &agrave; la suite de la compression dynamique. Renfort sous compression dynamique n&#39;a pas &eacute;t&eacute; observ&eacute;e auparavant dans les cristaux liquides &eacute;lastom&egrave;res et peut &ecirc;tre utile pour le d&eacute;veloppement de l&#39;autor&eacute;paration des mat&eacute;riaux ou pour le d&eacute;veloppement de mat&eacute;riaux biocompatibles, adaptatifs pour le remplacement des tissus.

Dynamique self-stiffening dans les cristaux liquides &eacute;lastom&egrave;res.

Tessuti biologici hanno la capacit&agrave; notevole di ritoccare e riparare in risposta alla malattia, infortunio e sollecitazioni meccaniche. Materiali sintetici mancano la complessit&agrave; dei tessuti biologici e materiali artificiali che rispondono alle sollecitazioni esterne attraverso un aumento permanente di rigidit&agrave; sono infrequenti. Qui riportiamo quello polydomain nematic liquid crystal elastomeri aumentano della rigidit&agrave; fino al 90% quando &egrave; sottoposto a una bassa ampiezza (5%), compressione ripetitiva (dinamica). Irrigidimento di elastomero &egrave; influenzata dal contenuto di cristalli liquidi, la presenza di una fase di nematic liquid crystal e l&#39;uso di una dinamica rispetto a deformazione statica. Attraverso reologiche e le misure di diffrazione a raggi x, di rinforzo possono essere attribuite a un direttore nematic mobile, che ruota in risposta a compressione dinamica. Irrigidimento sotto compressione dinamica non &egrave; stata osservata in precedenza in elastomeri di cristalli liquidi e pu&ograve; essere utile per lo sviluppo di materiali di auto-guarigione o per lo sviluppo di materiali biocompatibili, adattivi per la sostituzione di tessuto.

Dinamica self-stiffening in elastomeri di cristalli liquidi.

Organic electronic materials have the potential to impact almost every aspect of modern life including how we access information, light our homes, and power personal electronics. Nevertheless, weak intermolecular interactions and disorder at junctions of different organic materials limit the performance and stability of organic interfaces and hence the applicability of organic semiconductors to electronic devices. Here, we demonstrate control of donor-acceptor heterojunctions through microphase-separated conjugated block copolymers. When utilized as the active layer of photovoltaic cells, block copolymer-based devices demonstrate efficient photoconversion well beyond devices composed of homopolymer blends. The 3% block copolymer device efficiencies are achieved without the use of a fullerene acceptor. X-ray scattering results reveal that the remarkable performance of block copolymer solar cells is due to self-assembly into mesoscale lamellar morphologies with primarily face-on crystallite orientations. Conjugated block copolymers thus provide a pathway to enhance performance in excitonic solar cells through control of donor-acceptor interfaces.

Conjugated block copolymer photovoltaics with near 3% efficiency through microphase separation.

Monodomain liquid crystal elastomers (LCEs) are shape-responsive materials, but shape changes are typically limited to simple uniaxial extensions or contractions. Here, we demonstrate that complex surface patterns and shape changes, including patterned wrinkles, helical twisting, and reversible folding, can be achieved in LCE-polystyrene (PS) bilayers. LCE-PS bilayer shape changes are achieved in response to simple temperature changes and can be controlled through various material parameters including overall aspect ratio and LCE and polystyrene film thicknesses. Deposition of a patterned PS film on top of an LCE enables the preparation of an elastomer that reversibly twists and a folding leaf-like elastomer, which opens and closes in response to temperature changes. The phenomena are captured through finite element simulations, in quantitative agreement with experiments.

Shape-responsive liquid crystal elastomer bilayers.

We explore the phase behaviour, solution conformation, and interfacial properties of bottlebrush polymers with side-chains comprised of poly(N-isopropylacrylamide) (PNIPAAM), a thermally responsive polymer that exhibits a lower critical solution temperature (LCST) in water. PNIPAAM bottlebrush polymers with controlled side-chain length and side-chain end-group structure are prepared using a "grafting-through" technique. Due to reduced flexibility of bottlebrush polymer side-chains, side-chain end-groups have a disproportionate effect on bottlebrush polymer solubility and phase behaviour. Bottlebrush polymers with a hydrophobic end-group have poor water solubilities and depressed LCSTs, whereas bottlebrush polymers with thiol-terminated side-chains are fully water-soluble and exhibit an LCST greater than that of PNIPAAM homopolymers. The temperature-dependent solution conformation of PNIPAAM bottlebrush polymers in D2O is analyzed by small-angle neutron scattering (SANS), and data analysis using the Guinier-Porod model shows that the bottlebrush polymer radius decreases as the temperature increases towards the LCST for PNIPAAM bottlebrush polymers with relatively long 9 kg mol(-1) side-chains. Above the LCST, PNIPAAM bottlebrush polymers can form a lyotropic liquid crystal phase in water. Interfacial tension measurements show that bottlebrush polymers reduce the interfacial tension between chloroform and water to levels comparable to PNIPAAM homopolymers without the formation of microemulsions, suggesting that bottlebrush polymers are unable to stabilize highly curved interfaces. These results demonstrate that bottlebrush polymer side-chain length and flexibility impact phase behavior, solubility, and interfacial properties.

Thermoresponsive PNIPAAM bottlebrush polymers with tailored side-chain length and end-group structure.

All-conjugated block copolymers are an emerging class of polymeric materials promising for organic electronic applications, but further progress requires a better understanding of their microstructure including crystallinity and self-assembly through micro-phase segregation. Here, we demonstrate remarkable changes in the thin film structure of a model series of all-conjugated block copolymers with varying processing conditions. Under thermal annealing, poly(3-hexylthiophene)-b-poly(9',9'-dioctylfluorene) (P3HT-b-PF) all-conjugated block copolymers exhibit crystalline features of P3HT or PF, depending on the block ratio, and poor π-π stacking. Under chloroform solvent annealing, the block copolymers exhibit lamellar ordering, as evidenced by multiple reflections in grazing incidence wide- and small-angle X-ray scattering (GIWAXS and GISAXS), including an in-plane reflection indicative of order along the π-π stacking direction for both P3HT and PF blocks. The lamellae have a characteristic domain size of 4.2 nm, and this domain size is found to be independent of block copolymer molecular weight and block ratio. This suggests that lamellar self-assembly arises due to a combination of polymer block segregation and π-π stacking of both P3HT and PF polymer blocks. Strategies for predicting the microstructure of all-conjugated block copolymers must take into account intermolecular π-π stacking and liquid crystalline interactions not typically found in flexible coil block copolymers.

Lamellar and liquid crystal ordering in solvent-annealed all-conjugated block copolymers.

Bottlebrush polymers are a type of branched or graft polymer with polymeric side-chains attached to a linear backbone, and the unusual architectures of bottlebrushes provide a number of unique and potentially useful properties. These include a high entanglement molecular weight, enabling rapid self-assembly of bottlebrush block copolymers into large domain structures, the self-assembly of bottlebrush block copolymer micelles in a selective solvent even at very low dilutions, and the functionalization of bottlebrush side-chains for recognition, imaging, or drug delivery in aqueous environments. This review article focuses on recent developments in the field of bottlebrush polymers with an emphasis on applications of bottlebrush copolymers. Bottlebrush copolymers contain two (or more) different types of polymeric side-chains. Recent work has explored the diverse properties and functions of bottlebrush polymers and copolymers in solutions, films, and melts, and applications explored include photonic materials, bottlebrush films for lithographic patterning, drug delivery, and tumor detection and imaging. We provide a brief introduction to bottlebrush synthesis and physical properties and then discuss work related to: (i) bottlebrush self-assembly in melts and bulk thin films, (ii) bottlebrushes for photonics and lithography, (iii) bottlebrushes for small molecule encapsulation and delivery in solution, and (iv) bottlebrush micelles and assemblies in solution. We briefly discuss three potential areas for future research, including developing a more quantitative model of bottlebrush self-assembly in the bulk, studying the properties of bottlebrushes at interfaces, and investigating the solution assembly of bottlebrush copolymers.

Structure, function, self-assembly, and applications of bottlebrush copolymers.

Actuating materials. Shape-shifting liquid crystals.

Correction for 'Structure, function, self-assembly, and applications of bottlebrush copolymers' by Rafael Verduzco et al., Chem. Soc. Rev., 2015, 44, 2405-2420.

Correction: Structure, function, self-assembly, and applications of bottlebrush copolymers.

Mechanically robust battery electrodes are desired for applications in wearable devices, flexible displays, and structural energy and power. In this regard, the challenge is to balance mechanical and electrochemical properties in materials that are inherently brittle. Here, we demonstrate a unique water-based self-assembly approach that incorporates a diblock copolymer bearing electron- and ion-conducting blocks, poly(3-hexylthiophene)-block-poly(ethyleneoxide) (P3HT-b-PEO), with V2O5 to form a flexible, tough, carbon-free hybrid battery cathode. V2O5 is a promising lithium intercalation material, but it remains limited by its poor conductivity and mechanical properties. Our approach leads to a unique electrode structure consisting of interlocking V2O5 layers glued together with micellar aggregates of P3HT-b-PEO, which results in robust mechanical properties, far exceeding the those obtained from conventional fluoropolymer binders. Only 5 wt % polymer is required to triple the flexibility of V2O5, and electrodes comprised of 10 wt % polymer have unusually high toughness (293 kJ/m(3)) and specific energy (530 Wh/kg), both higher than reduced graphene oxide paper electrodes. Furthermore, addition of P3HT-b-PEO enhances lithium-ion diffusion, eliminates cracking during cycling, and boosts cyclability relative to V2O5 alone. These results highlight the importance of tradeoffs between mechanical and electrochemical performance, where polymer content can be used to tune both aspects.

Rafael Verduzco

.css-o250i3{font-size:18px;font-family:Open Sans,sans-serif;line-height:21.6px;}Chemical and Biomolecular Engineering,Materials Sciences and NanoEngineering

Rice University

Preparation of Monodomain Liquid Crystal Elastomers and Liquid Crystal Elastomer Nanocomposites

Chemical and Biomolecular Engineering,
Materials Sciences and NanoEngineering