Pixelated Polymers: Directed Self Assembly of Liquid Crystalline

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Pixelated Polymers: Directed Self Assembly of Liquid Crystalline Polymer Networks Benjamin A. Kowalski,†,‡,§ Tyler C. Guin,†,‡,§ Anesia D. Auguste,†,§ Nicholas P. Godman,†,‡,§ and Timothy J. White*,† †

Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson Air Force Base, Dayton, Ohio 45433-7750, United States ‡ Azimuth Corporation, 4027 Colonel Glenn Highway, Beavercreek, Ohio 45431, United States ABSTRACT: Polymeric materials are pervasive in modern society, in part attributable to the diverse range of properties that are accessible in these materials. Polymers can be stiff or soft, dissipative or elastic, adhesive or nonstick. Localizing the properties of polymeric materials can be achieved by a number of methods, including self-assembly, lithography, or 3-d printing. Here, we detail recent advances in the preparation of “pixelated” polymers prepared by the directed self-assembly of liquid crystalline monomers to yield cross-linked polymer networks (liquid crystalline polymer networks, LCN, or liquid crystalline elastomers, LCE). Through the local and arbitrary control of the orientation of the liquid crystalline units, monolithic elements can be realized with spatial variation in mechanical, thermal, electrical, optical, or acoustic properties. Stimuli-induced variation of these properties may enable paradigm-shifting end uses in a diverse set of applications.

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The extent of cross-linking dictates the glass transition temperature (Tg), defining the material as a glass or elastomer (commonly referred to as a liquid crystalline elastomer, LCE). We will use the term LCN to generally refer to the class of materials (both glasses and elastomers), with specific qualification when appropriate. A number of synthetic methods have been used to prepare these materials. The most established method is hydrosilylation of vinyl liquid crystalline monomers and silicone precursors to form liquid crystalline elastomers with polysiloxane backbones.7 Another common method to prepare LCN (both glasses and elastomers) is by free radical polymerization of (meth)acrylate monomers and oligomers (Figure 1b,c).20 Strategies utilizing thiol-X step-growth polymerization have also been reported.21 Figure 1 illustrates the various network chemistries utilized, mesogen attachment, and chemical structures of common liquid crystalline monomers. Polymerization of liquid crystalline monomers without external conditions enforcing alignment yields polymeric materials with organization within micron-sized domains but absent macroscopic orientation (polydomain). A number of alignment approaches have been established to form single crystal or monodomain or other aligned orientations. Some of these methods are illustrated in Figure 2. The most common alignment method, particularly for LCEs prepared via hydrosilylation reactions, is the use of mechanical force in what is referred to as the “Finkelmann method.”7 External magnetic (Figure 2a) or electric fields can also enforce alignment in both hydrosilylation and free radical/thiol-X reaction schemes.22,23 This Viewpoint will focus on materials prepared from surface alignment, which has proven to be a powerful method to prepare these materials with the spatial

olymeric materials and composites are widely utilized in both functional and structural applications.1,2 Various stimuli (biochemical,3 thermal,4 photonic,5 electrical6) can initiate property changes within polymeric materials. Stimuliresponsive property variation within these materials may enable paradigm-shifting applications in medicine, automobiles, consumer products (particularly sporting goods), optics, and aerospace. One burgeoning area of macromolecular research is focused on programming mechanical responses in these materials. Spatial or hierarchical variation (localization) of the mechanical properties (response) is necessary to induce shape change,9 surface reconfiguration,10 or other functional properties such as optical,11 acoustic,12 or thermal.13 Toward this end, a variety of approaches have been pursued including polymer/polymer composites,14 polymer nanocomposites,15 hydrogels,16−18 or covalent networks.19 This Viewpoint reviews the directed self-assembly of liquid crystals to prepare “pixelated” polymers, for example, polymeric materials with programmed local functionalities. The materials and methods to prepare them merge bottoms-up self-assembly with top-down patterning. We will introduce the materials, discuss recent developments, and conclude by assessing future opportunities for these materials over the horizon.

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SYNTHETIC APPROACHES AND ALIGNMENT METHODS

Received: February 17, 2017 Accepted: March 27, 2017

Liquid crystalline networks (LCNs) are cross-linked polymers prepared from monomeric precursors exhibiting liquid crystallinity. © XXXX American Chemical Society

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Figure 1. Liquid crystalline polymer networks and elastomers. (a) Siloxane-based LCEs (in this illustration, with side-chain mesogens) can exhibit in-plane strains of as much as 400% (inset) when heated through the nematic−isotropic transition temperature.7 (b) Glassy LCNs prepared from liquid crystalline monomers exhibit strains of approximately 5%, but can be locally aligned to yield periodic deformation (inset).8 (c) Chain-extension reactions are another approach to prepare main-chain LCEs that are also conducive to alignment and allow for large-scale, out-of-plane deformation (inset).9 (d) Representative liquid crystalline monomer structures.

Figure 2. Directed self-assembly methods to align LCNs. (a) Alignment induced by an externally applied magnetic field, either uniform (left)22 or structured (right).25 (b) Alignment templated by “command surfaces” with molecular-level anisotropy, induced by either mechanical rubbing (left)26 or photoinduced reorientation of photoalignment layers upon exposure to a low-power polarized light source (right).9 (c) Alignment derived from surface topography, that is, microchannels inscribed with AFM probe tip (left)27 or prepared with lithography (right).28

precision necessary to allow complex and arbitrary director profiles to be imprinted into these materials (Figure 2b,c).9

A variety of optical schemes have been used to address these alignment layers with polarized light. Foundational work used a series of mask exposures with successive adjustment to the linear polarization.33 Specialized optical schemes have been developed for specific patterns such as turntables for radial patterns10 and interference holography for small-pitch patterns.34 The patterning scheme of refs 9 and 35, in which a focused laser beam with controllable polarization is rastered across the alignment surface, has moderate throughput, but has the advantage of being dynamically reconfigurable. Other exposure schemes are also dexterious and capable of generating fully arbitrary polarization (alignment) profiles. For example, a plasmonic mask can transmit locally modulated polarization in a single exposure, based on the local orientation of subwavelength apertures.36 Recent work has used digital micromirror devices (DMD) or spatial light modulators (SLM) to simultaneously pattern across a large area, achieving high throughput along with versatility.37,38

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DIRECTED SELF-ASSEMBLY VIA SURFACE ALIGNMENT Spatial control of the orientation of the alignment of liquid crystals was initially demonstrated via localized mechanical scribing of microgrooves into an alignment layer using a probe tip (Figure 2c).24 Higher throughput techniques for fabricating microscopic surface morphology have been developed, including interference holography29 and photolithography28 (Figure 2c). In all of these cases, the microgrooves template the liquid crystal orientation in order to minimize elastic strain.30 Among the most versatile approaches for encoding fully arbitrary patterns at sufficiently high resolution is the photoalignment of a so-called “command” surface (Figure 2b). Photoalignment materials and methods have been reviewed in refs 31 and 32. Two distinct approaches have been employed in the recent literature. The more common method utilizes alignment that is mediated by surface interactions with molecular-level anisotropy in photoresponsive surface chemistries. In general, this anisotropy is induced by exposing the command surface with linearly polarized light. This can trigger stochastic reorientation through photoisomerization of small molecules as well as polymers. A related approach uses orientationally selective photopolymerization (referred to as linearly polymerized photopolymers, LPP).

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PROGRAMMING SHAPE OR STRETCH IN LIQUID CRYSTALLINE NETWORKS AND ELASTOMERS Building from these methods and materials conducive to them have allowed for the realization of dramatic shape change or surface reconfiguration in both glassy and elastomeric LCNs (Figure 3). This convergence has perhaps most profoundly been exercised by imprinting director patterns described by 437

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Figure 3. Nonlinear elastic deformation (e.g, “soft elasticity”) of LCEs can be localized. (a) The reorientation of the nematic director of a LCE to a directionally applied stress is illustrated and associated with the nonlinearity in a notional stress−strain curve. (Adapted with permission from ref 45. ACS Copyright 2007.) (b) Left: the orientation of the director of the LCE to the direction of the applied stress result in distinct stress−strain curves. Localizing the alignment in monolithic LCE can confines the nonlinear deformation of the material (in this case, “AFRL”),46 measured via digital image correlation.

Figure 4. Extending functionality with nanoinclusions. (a) CNT/LCE bilayer (adhered to PDMS) moves across a surface by irradiation to patterned IR light.64 (b) Contractile strain of a carbon black/LCE composite to an applied AC voltage.6 (c) Controlled shape-change of a LCN nanocomposite (gold nanoparticles), initiated by patterned white light exposure.6,62,64

particles,56 graphene,53 carbon nanotubes,57 or carbon black.58 In particular, the strong absorption of carbon based nanoinclusions have been employed to photosensitize the thermomechanical response of LCN.57,59−61 Examinations seeking electromechanical deformations have been undertaken in CNT based nanocomposites with LCE.6 Optical functionality has been enhanced by the addition of nanoparticle additives. For instance, gold nanoparticles exhibit strong optical anisotropy in the visible and NIR.53,62 Magnetic nanoparticles, such as Fe3O4, can be aligned to encode permanent anisotropic magnetic effects.63 Little if any work to date has reported on the preparation nanocomposites prepared from LCNs and LCEs with arbitrary control of the director profile accompanied by the alignment of the nanoinclusion. Ongoing experimentation in our laboratory indicates this could enhance mechanical performance, especially toughness, which will be critical for many applications as well as opening up new opportunity space for nanocomposite with programmed thermal, electrical, or optical functionalities.

topological defects. Defect charges ranging from the familiar charges (±1/2, ±1) to the exotic (as much as ±10) have been prepared. The deformation of ±1 defects closely match those expected from theory.39 New topologies, such as the spindle patterns of ref 40, are particularly intriguing in the realization of reconfigurable topographical surfaces. Another salient feature of LCEs is their nonlinear elastic deformation under load. The “soft elastic” plateau in the stress−strain curve of LCEs is attributable to director rotation without distorting the chain distribution.41−44 Localization of soft elasticity in a uniquely prepared monolithic element enables tailored mechanical responses, which were previously accessible only through multimaterial composite fabrication.46,47 Patterned LCEs are composed of singular compositions, absent of multimaterial interfaces, adhesives, or pores. We believe that these designer materials could find utility in applications such as flexible hybrid electronics, where the substrate would be designed to pair with the device itself to produce a desired stretchability and conformality while preventing local deformation on and near sensitive traces and components.

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DYNAMIC CHEMISTRY A number of recent papers have assimilated advances in reconfigurable network chemistries with the anisotropy of LCNs. These materials employ “dynamic chemistry” in which adaptive, exchangeable cross-links are integrated into the polymer network. An initial report describes vitrimeric LCNs, in which the material behaves as a classic thermosetting plastic at low temperatures but a viscoelastic material at high temperature, enabled by bond breaking/reforming through a reversible chain-transfer reaction (transesterification)65,70 Crucially, this enables an LCN to be prepared in one step, using common bulk processing, and subsequently programmed and potentially reprogrammed.67,71 Conceivably, localization of alignment can be induced by magnetic fields or light to facilitate directed self-assembly of the material at higher resolutions.

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NANOINCLUSIONS A strategy to further expand the functionality is the use of LCNs as a host to dictate the orientation of nanoinclusions.48,49 A number of reports have detailed the integration of various nanomaterials into these materials motivated by the potential to further improve mechanical,50 electrical,6 thermal,51 magnetic,52 or optical53,54 properties (Figure 4). Most, if not all, of these examinations have focused on samples with uniform alignment, typically in polysiloxane-based LCE aligned by mechanical strain.55 The nanoinclusions have included metallic nano438

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ACS Macro Letters These materials can be made orthogonally responsive to a broad range of stimuli in addition to heat. For example, photoinduced mechanical responses have been demonstrated in azobenzene-functionalized xLCNs,72 and chemically triggered responses have been demonstrated in Bip-containing xLCEs that are sensitive to the presence of metal ions (Figure 5).69

Figure 6. Complex topologies through printing. Localized alignment of cellulose fibers via extrusion shear (top) enables designed actuation into complex bioinspired 4D shapes (bottom).77

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SUMMARY “Pixelated” polymers can be prepared by the directed selfassembly of liquid crystalline monomers to form LCN (including LCE). Advances in materials chemistry that are now conducive to established alignment techniques have made these materials more accessible, which has compelled a number of groups to initiate research in this area. LCNs hold promise as shape programmable materials, designer substrates for flexible hybrid electronics, or someday as 3-d printed shape changing elements. Before these applications can materialize, advances are necessary in further deepening both the fundamental understanding as well as performance of in metrics such as robustness (toughness, strain to failure). Further exploration and exploitation of this foundational knowledge is necessary to scope the bounds of how effectively the local optical, thermal, electrical, or acoustic properties of these materials can be varied.

Figure 5. Examples of reconfigurable LCEs (xLCEs) (a) produced using vitrimers: (i) Diagram of mesogen realignment in response to heat and an applied load.65 (ii) Demonstration of the reprocessability of xLCEs.66 (iii) Chemical diagram depicting the formation of a monodomain xLCE in response to an external force.67 (b) (i) Molecular depiction of change in mesogen alignment of Bip containing xLCEs in response to metal ions.68 (ii) Actuation of an Bip containing xLCE lifting a 10 g mass in response to Fe(OTf)2.69

The merger of covalent adaptive networks with the anisotropy inherent to LCN may enable new possibilities beyond the paradigm of surface-aligned films inherently limited to 2-d. Hybrid techniques that combine bulk mechanical processing with photoalignment, interference lithography, or soft lithography could have promise to creating hierarchically stratified and aligned materials.73

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AUTHOR INFORMATION

Corresponding Author

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*E-mail: [email protected]. ORCID

THIRD DIMENSION The most obvious method to realize 3-d construction of a material is through printing.74 The primary hurdle to realization is integrating methods to enforce alignment at the macroscale. Conceivably, alignment of the material could occur concurrently to placement by rheology or external magnetic or photonic fields. 33,75 The promise of this approach is demonstrated by printed cellulose hydrogels, which have been shown to exhibit predictable, reversible shape change.76 Control of alignment via extrusion shear provides another degree of design freedom that enables even more complex predictable bioinspired shape morphing (Figure 6).47,76−79

Timothy J. White: 0000-0001-8006-7173 Author Contributions §

These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was enabled by financial support from the Materials and Manufacturing Directorate of the Air Force Research Laboratory and the Air Force Office of Scientific Research. A.A. acknowledges support from the Air Force Office of Scientific 439

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Research through a National Research Council (NRC) Postdoctoral Research Associateship.

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DOI: 10.1021/acsmacrolett.7b00116 ACS Macro Lett. 2017, 6, 436−441