Enhanced Dynamic Adhesion in Nematic Liquid Crystal Elastomers

Abstract

stimuli, including thermal, pH, electric, chemicals, etc. [3] Many of them rely on the interfacial properties of the contact, such as surface topography and chemical functionality. The former directly affects the contact area A to be detached and, therefore, the adhesion can indeed be altered by changing the surface topography. [4][5][6][7] The latter mainly affects the interfacial energy density via the bare surface tension of each material or chemical/physical bond/ entanglements. [8][9][10] However, less attention has been paid to the use of the bulk viscoelastic characteristics that are especially important in dynamic applications toward stimuli-responsive adhesives. [11,12] To our knowledge, no experimental study exists of the switchable adhesion of the nematic liquid crystal elastomers (LCEs), which possess unique bulk mechanical properties.LCEs are a unique class of elastomers, in which the nematic director is coupled to the polymer network in both static and dynamic manner. [13] In the macroscopically nonaligned LCE (called "polydomain"), the local uniaxial nematic order Q(T) develops in a usual manner of the frustrated first-order phase transition from the isotropic phase, [14] but the nematic regions of a characteristic size of a micrometer are randomly misaligned in the bulk material. As a result, the polydomain LCE is macroscopically isotropic, [15] but retains all its characteristic nematic LCE parameters (such as the elastic modulus and loss factor, both relevant to this study). In contrast, the aligned ("monodomain") nematic LCE shows the uniform macroscopic anisotropy Q(T) across the whole sample, and the characteristic effect of spontaneous shape change [16,17] on increasing temperature T across the nematic-isotropic transition temperature, T NI (often called "thermal actuation"). This effect makes LCE highly attractive for applications [13,[18][19][20][21][22] involving soft actuators, shape-memory materials, and others.The other unique characteristics of nematic LCE, on which we especially focus here, is the effect of "soft elasticity," which in dynamic-mechanical setting manifests itself in a large increase in the loss factor tanδ, and the decrease in the storage moduli E′ and G′. [13,[23][24][25] This effect shows a great potential in LCE as energy damping materials. [23,26] The higher internal viscosity originates from the additional nematic (orientational) interaction between polymer chains. This adds resistance against the local rotation of the liquid crystalline director relative to the network in response to the finite strain rate (similarly to the higher viscosity in the ordinary liquid crystals).

Dynamic Manipulation of Friction in Smart Textile Composites of Liquid‐Crystal Elastomers

“…Liquid crystal elastomers (LCEs) are very promising systems to induce very large topographical changes via phase transitions [ 17–25 ] that can be triggered by multiple stimuli, such as temperature, light, and chemicals. In addition, the LCEs show drastic changes in the bulk viscoelasticity [ 18,26–30 ] which allows to tune friction via the F 0 , α, and ϕ entries in the basic Equation (). However, the development of the dynamic frictional system through the combination of these effects remains an open challenge.…”

“…Our system is the composite ( Figure ) of a nematic LCE [ 23,25,30 ] (Figure S1, Supporting Information) and a plain‐weave textile, in which the volume other than the fiber components within the thickness of the textile is almost fully filled with the LCE (Figure 1b), as in the rubber‐coated textiles. The basic properties of the present LCE have been characterized using the neat LCE samples, and listed in the Supporting Information: polymerization reactions (Figure S2, Supporting Information), nematic‐isotropic transition T NI (Figure S3, Supporting Information), LC phase (Figures S3 and S4, Supporting Information), thermal actuation (Figure S5, Supporting Information), and viscoelastic response (Figure S6, Supporting Information).…”

“…In brief, the LCE has T NI ≈ 35 °C and the glass transition temperature of ≈−40 °C, s pontaneous and reversible changes of the shape with the uniaxial strain of at least 30%, and of the soft viscoelasticity with the high loss factor tanδ in the nematic phase. [ 30 ]…”

“…In general, F S at L = 0 is of the same order of magnitude as the adhesive strength, σ ad . Since σ ad between the glass surface and the LCE at RT after the dwell time of ≈30 s is in the order of 0.1 MPa, [ 30 ] F S at L = 0 with the apparent area of interface ( A = 133 mm 2 ) can be estimated as αAσ ad ≈ 1 N, where α ≈ 0.1 is the typical ratio of the contact area at L = 0 to A . This corresponds roughly to the nonzero values of the fitted line on the experiments at RT on 0 and 10 wt% solvent systems at L = 0, giving ≈0.9 N (Figure S17a, Supporting Information) and ≈0.4 N (Figure 4a), respectively.…”

Switchable bumps of a bead-embedded elastomer surface with variable adhesion

“…In previous studies, we and other groups have made developments via the former strategy; creating shape-tuneable surfaces, using wrinkles [7][8][9][10][11] with a wavy topography, which spontaneously appear because of the strain-induced buckling of a hard skin layer formed on a soft elastic substrate. The use of shape-tuneable wrinkles [12][13][14][15][16][17][18][19][20][21] has demonstrated that friction [22][23][24] and adhesion 25 can be switched depending on the strain applied to the wrinkle substrate. Although various structural designs that realize nonlinear shape-tunability 26,27 exist, designs that target the tribological requirements have not been proposed, other than the wrinkle system.…”