Buckling Instability in Liquid Crystalline Physical Gels

Abstract

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 polymercoil 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 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. DOI: 10.1103/PhysRevLett.96.147802 PACS numbers: 61.30.ÿv, 61.41.+e, 82.70.Gg Liquid-crystalline elastomers and gels combine the orientational order of liquid crystals and the translational order of a polymer network. The coupling of these two effects in liquid-crystalline elastomers and gels gives rise to unique phenomena such as soft elasticity in which certain director rotations and network strains cost no free energy in the ideal case. Soft elastic modes are predicted and well understood theoretically by a molecular theory of nematic rubber elasticity [1][2][3]. However, a significant limitation in testing existing theories is the difficulty in preparing well-characterized networks. Many approaches have been undertaken to prepare liquid crystal elastomers, including free-radical polymerization of bulk liquid crystal acrylates and diacrylates [4], free-radical polymerization of diacrylates in the presence of liquid crystals [5,6], and side-coupling reactions with poly(methylsiloxane) [7]. Unfortunately, these approaches result in nematic elastomers and gels with heterogeneities in the polymer network and, in the case of many gels, phase separation of the polymer network from the liquid crystal host [8,9]. More recently, self-assembly of block copolymers has been used as an approach to create model liquid crystal networks [10 -12]. We have utilized self-assembly of a triblock copolymer that has a narrow molar mass distribution and a very long midblock to produce well-defined nematic gels [10,11]. This gel facilitates comparison to present theories which assume the material is lightly cross-linked and single phase.In this Letter, we present a study of the effects of nematic rubber elasticity in thin layers of uniformly aligned gels contained between rigid parallel plates. Using polarized optical microscopy, we observe a banded texture that arises in response to small temperature changes. We propose that this texture represents a buckling deformation made possible by the presence of soft and semisoft elastic modes. We formulate a description based on the molecular theory of nematic elastomers that incorporates soft and semisoft elasticity as well as l...

Bleaching and stimulated recovery of dyes and of photocantilevers

“…Features are slightly shifted to smaller w in comparison with the β = 0 case of [16]. With two regions of compression and two of elongation, one expects subtle behavior when considering compression-induced director rotation [24] in such cantilevers, to which we return elsewhere.…”

Rheological study of structural transitions in triblock copolymers in a liquid crystal solvent

“…Furthermore, the birefringence, dielectric anisotropy, diamagnetic anisotropy, and orientational elasticity inherent to the LC solvent give rise to electrooptic and mechano-optic phenomena in LC gels that are forbidden by symmetry in isotropic systems. 12 The LC gels are formed by dissolving an ABA triblock copolymer having polystyrene (PS) endblocks (''A'') and a very long side-group liquid crystalline polymer (SGLCP) midblock (''B'') in 4-pentyl-49-cyanobiphenyl (5CB), a wellcharacterized nematic LC. 13,14 The SGLCP midblock is designed to be soluble in both the isotropic and nematic phases of the solvent.…”

Cholesteric colloidal liquid crystals from phytosterol rod-like particles

“…The large pitch size compared to particle length is consistent with the presence of a cholesteric phase. Buckled nematic liquid crystals with a stripe pattern which resembles the cholesteric phase are found, for instance, when samples are exposed to an external field 31 or in nematic gels, 29 however, both systems are entirely different from the one discussed here. The cholesteric LC phase persists despite the presence of a neutral surfactant layer adsorbed as steric stabilizer on the particles surface pointing to modulated long range electrostatic repulsions.…”

“…These structures are characteristic for the cholesteric, [8][9][10][11] smectic phase, 28 and buckled nematic liquid crystal phases. 29 In the smectic liquid crystalline phases the pitch size is on the order of the rod length. 30 In our experiments the pitch size is constant for all the samples in bulk and in the tactoids and was measured about 15 mm in the case of pure water (no added salt), a distance much larger than the average particles length (2-3 mm).…”