Liquid crystalline elastomers (LCEs) are widely recognized for their exceptional promise as actuating materials. Here, the comparatively less celebrated but also compelling nonlinear response of ...these materials to mechanical load is examined. Prior examinations of planarly aligned LCEs exhibit unidirectional nonlinear deformation to mechanical loads. A methodology is presented to realize surface‐templated homeotropic orientation in LCEs and omnidirectional nonlinearity in mechanical deformation. Inkjet printing of the homeotropic alignment surface localizes regions of homeotropic and planar orientation within a monolithic LCE element. The local control of the self‐assembly and orientation of the LCE, when subject to rational design, yield functional materials continuous in composition with discontinuous mechanical deformation. The variation in mechanical deformation in the film can enable the realization of nontrivial performance. For example, a patterned LCE is prepared and shown to exhibit a near‐zero Poisson's ratio. Further, it is demonstrated that the local control of deformation can enable the fabrication of rugged, flexible electronic devices. An additively manufactured device withstands complex mechanical deformations that would normally cause catastrophic failure.
The synthesis of liquid crystal elastomers (LCEs) in the homeotropic orientation enables omnidirectional nonlinearity in mechanical deformation. Locally directing the self‐assembly of the orientation of the LCEs generates films of continuous composition but spatially distinguished mechanical responses. Local control of the mechanical deformation of the LCEs has functional benefits in realizing near‐zero Poisson's ratio or by ruggedizing flexible electronic devices.
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.
Layer-by-layer (LbL) assembly is a promising technique for depositing multi-functional thin films from dilute aqueous solutions. These films have found use as environmentally-benign flame retardants, ...replacing halogenated flame retardants, and as flexible gas barrier thin films, replacing vacuum-deposited inorganic oxide thin films. Unfortunately, LbL assembly has drawbacks that have not been adequately addressed, such as stiffening of coated substrates and the high number of deposition steps required. Thin films of chitosan and poly(sodium phosphate) were deposited on cotton fabric via LbL assembly to reduce flammability. The fabric was rinsed in an ultrasonication bath between deposition steps to improve the softness (i.e., hand) of the coated fabric. Ultrasonication is believed to remove weakly adhered polyelectrolyte and eliminate bridging of individual fibers, preventing the fabric from becoming stiff while improving the flame retardant behavior. Incorporating amine salts into the cationic polyelectrolyte and its associated rinse enables LbL clay-containing films to achieve large thickness (>1 μm) with relatively few deposition steps. This technique is potentially universal, exhibiting thick growth with multiple types of nanoclay, including montmorillonite and vermiculite, a variety of amine salts (e.g., hexylamine and tris), and a host of cationic polyelectrolytes, such as poly(allylamine) and chitosan. The characteristic ordered structure found in LbL-assembled films is maintained despite the increased thickness. These films display extraordinary gas barrier and flame resistance with fewer than 8 deposition cycles.
Layer‐by‐layer (LbL) assembly has emerged as the leading non‐vacuum technology for the fabrication of transparent, super gas barrier films. The super gas barrier performance of LbL deposited films ...has been demonstrated in numerous studies, with a variety of polyelectrolytes, to rival that of metal and metal oxide‐based barrier films. This Feature Article is a mini‐review of LbL‐based multilayer thin films with a ‘nanobrick wall’ microstructure comprising polymeric mortar and nanoplatelet bricks that impart high gas barrier to otherwise permeable polymer substrates. These transparent, water‐based thin films exhibit oxygen transmission rates below 5 × 10‐3 cm3 m‐2 day‐1 atm‐1 and lower permeability than any other barrier material reported. In an effort to put this technology in the proper context, incumbent technologies such as metallized plastics, metal oxides, and flake‐filled polymers are briefly reviewed.
This Feature Article reviews multilayer thin films, deposited layer‐by‐layer to produce a ‘nanobrick wall’ structure that imparts high gas barrier to permeable polymer substrates. These transparent, water‐based thin films exhibit a lower permeability than any other barrier thin film material.
•Nanoindentation and nanoscratch behavior of submicron (100–200 nm) films.•Both stiff and compliant substrates can substantially affect indentation results.•True film properties require less than 4% ...indentation depth.•Film adhesion strength and delamination behavior.
Polymer thin films are deposited onto rigid materials for testing mechanical properties. However, the functionality of nanocomposites typically requires compliant substrates. Hence, it is important to investigate the nanomechanical properties of these films deposited on both substrates. This study compares the nanoindentation and nanoscratch behavior of Polyvinylamine (PVAm)/ Graphene oxide (GO) nanocomposite films on rigid silicon and compliant Polyethylene Terephthalate (PET) substrates. Contrary to the indentation rule of adhering to 10% of film depth, we obtain different measured hardness and reduced modulus on each film/substrate system with reduced modulus and hardness values of the PVAm/GO film on Silicon being 2 and 1.3 times higher than the film on PET at 10% film thickness. Experimenting on both substrates showed that extremely shallow indentation depths (< 4% of total film thickness) are necessary to measure comparable substrate independent film properties. The results also indicated that compliant substrate-based films exhibit better scratch resistance and higher adhesion strength of the film. Additionally, finite element analysis showed that using a rigid substrate concentrates the stress and deformation near the film surface. Using the compliant PET substrate results in larger elastic strain zone indicating deformation of the film and substrate alike.
Thin films of environmentally benign polyelectrolytes, cationic chitosan (CH) and anionic poly(sodium phosphate) (PSP), were deposited on cotton fabric via layer-by-layer (LbL) assembly to reduce ...flammability. This CH–PSP nanocoating promotes charring of the cotton, rendering the fabric self-extinguishing. The coated fabric was rinsed in an ultrasonication bath between deposition steps to improve the softness (i.e., hand) of the coated fabric. Ultrasonication is believed to remove weakly adhered polyelectrolyte, preventing the fabric from becoming stiff, while improving anti-flammable behavior at a given coating weight. At 17 bilayers, only 9.1 wt% was added to the cotton, yet the coated cotton consistently passed vertical flame testing. Electron microscopy provides evidence of intumescence and confirms the cleaner deposition afforded by ultrasonication. The reduction in peak heat release rate and total heat release, as measured by micro cone calorimetry, were 73 and 81 % respectively, which is a new benchmark in LbL flame retardant coating on cotton. The mechanical properties of the fabric were measured using the Kawabata evaluation system, which showed that ultrasonication rinsing significantly improved the hand. The ability to render cotton fabric self-extinguishing, while maintaining a soft hand, marks a major milestone in the development of these environmentally-benign nanocoatings.
Liquid crystal elastomers (LCE) are soft materials which anisotropically shape morph in response to external stimuli. Herein, a method to produce large‐area carbon nanotube–LCE nanocomposite films ...with exceptional electrostrictive properties is examined. A methodology to produce telechelic precursor oligomeric composites at scale (>100 g) is presented along with the continuous casting and photocuring process. The carbon nanotubes are well‐aligned and well‐dispersed in the films, which exhibit exceptional anisotropic thermomechanical, optical, and thermal shape change characteristics. When an electric field is applied through the film thickness, the material quickly and reversibly contracts along the alignment direction. As an example, a compliant carbon nanotube–LCE film contracts >4% against a 140 kPa load, roughly comparable to the blocking force of many natural muscular tissues. Furthermore, it is demonstrated that this tunable contraction is dependent upon the electric field strength, and that the contraction mimics the form function of the input electric field (e.g., a sine wave). These compliant actuators are excellent candidates for incorporation into soft and/or biological systems.
A scalable process for producing large area, highly aligned carbon nanotube–liquid crystal elastomer films is presented. The films can reversibly contract over 4% against a 140 kPa load, and the response is highly tunable with applied voltage. The outstanding performance makes these films attractive candidates for incorporation as actuators into soft or rubbery systems.
Recent work with multilayer nanocoatings composed of polyelectrolytes and clay has demonstrated the ability to prepare super gas barrier layers from water that rival inorganic CVD-based films (e.g., ...SiO x ). In an effort to reduce the number of layers required to achieve a very low oxygen transmission rate (OTR (<0.01 cc/m2·day·atm)) in these nanocoatings, buffered cationic chitosan (CH) and vermiculite clay (VMT) were deposited using layer-by-layer (LbL) assembly. Buffering the chitosan solution and its rinse with 50 mM Trizma base increased the thickness of these films by an order of magnitude. The OTR of a 1.6-μm-thick, six-bilayer film was 0.009 cc/m2·day·atm, making this the best gas barrier reported for such a small number of layers. This simple modification to the LbL process could likely be applied more universally to produce films with the desired properties much more quickly.
Layer‐by‐layer (LbL) assembly is a powerful and versatile technique to deposit functional thin films, but often requires a large number of deposition steps to achieve a film thick enough to provide a ...desired property. By incorporating amine salts into the cationic polyelectrolyte and its associated rinse, LbL clay‐containing nanocomposite films can achieve much greater thickness (>1 μm) with relatively few deposition cycles (≤6 bilayers). Amine salts interact with nanoclays, causing nanoplatelets to deposit in stacks rather than as individual platelets. This technique appears to be universal, exhibiting thick growth with multiple types of nanoclay, including montmorillonite and vermiculite (VMT), and a variety of amine salts (e.g., hexylamine and diethanolamine). The characteristic order found in LbL‐assembled films is maintained despite the incredible thickness. Films assembled in this manner achieve oxygen transmission rates below 0.009 cc m−2 d−1 atm−1 with just 6 bilayers (BLs) of chitosan/VMT deposited. These thick clay‐based thin films also impart exceptional flame resistance. A 2‐BL film renders a 3.2 mm polystyrene plate self‐extinguishing, while an 8‐BL film (3.9 μm thick) prevents ignition entirely. This ability to generate much thicker clay‐based multilayers with amine salts opens up tremendous potential for these nanocoatings in real world applications.
The incorporation of amine salts is shown to generate relatively thick clay nanocomposites via layer‐by‐layer assembly. These relatively thick nanofilms display extraordinary gas impermeability and flame resistance. A 6‐bilayer (BL) polymer/clay film has an oxygen permeability equivalent to SiOx thin films, while an 8‐BL film prevents ignition of a thick polystyrene plate entirely in a flame‐through test.
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