Main-chain liquid crystalline polymers that form low-temperature smectic mesophases are prepared by linking terephthalic acid, bis(4-allyloxyphenyl) ester (PPT) mesogens with 1,1,3,3,5,5-hexamethyltrisiloxane (F3) spacers via Pt-catalyzed hydrosilylation. Significant differences in thermal behavior and mesomorphic ordering are found between the polymer having unsubstituted PPT mesogens (F3-PPT-H) and the polymer having methyl substituents on the terminal rings of the mesogens (F3-PPT-CH 3 ). Combined evidence from polarized light optical microscopy, differential scanning calorimetry, and X-ray diffraction reveals S CA ordering in both polymers. Smectic elastomers are prepared by nonlinear polymerization of PPT-H or PPT-CH 3 mesogens with F3 spacers and a tetrafunctional cross-linker, tetrakis(dimethylsiloxy)silane (A 4 ). The dynamic mechanical response of unoriented, polydomain elastomers is characterized in small-strain, oscillatory shear. A prominent peak in tan δ (≡ G′′(ω)/ G′(ω)) associated with the smectic-isotropic clearing transition dominates the mechanical loss spectrum. Mesogen ring substitution is a simple synthetic approach to tuning the dynamic mechanical response of smectic elastomers for possible applications in vibration isolation or impact absorption.

IntroductionOsteoarthritis (OA) is a progressively debilitating disease that affects mostly cartilage, with associated changes in the bone. The increasing incidence of OA and an ageing population, coupled with insufficient therapeutic choices, has led to focus on the potential of stem cells as a novel strategy for cartilage repair.MethodsIn this study, we used scaffold-free mesenchymal stem cells (MSCs) obtained from bone marrow in an experimental animal model of OA by direct intra-articular injection. MSCs were isolated from 2.8 kg white New Zealand rabbits. There were ten in the study group and ten in the control group. OA was induced by unilateral transection of the anterior cruciate ligament of the knee joint. At 12 weeks post-operatively, a single dose of 1 million cells suspended in 1 ml of medium was delivered to the injured knee by direct intra-articular injection. The control group received 1 ml of medium without cells. The knees were examined at 16 and 20 weeks following surgery. Repair was investigated radiologically, grossly and histologically using haematoxylin and eosin, Safranin-O and toluidine blue staining.ResultsRadiological assessment confirmed development of OA changes after 12 weeks. Rabbits receiving MSCs showed a lower degree of cartilage degeneration, osteophyte formation, and subchondral sclerosis than the control group at 20 weeks post-operatively. The quality of cartilage was significantly better in the cell-treated group compared with the control group after 20 weeks.ConclusionsBone marrow-derived MSCs could be promising cell sources for the treatment of OA. Neither stem cell culture nor scaffolds are absolutely necessary for a favourable outcome.Cite this article: Bone Joint Res 2014;3:32–7.

The mechanical response and the evolution of director orientation are characterized in a smectic, main-chain liquid crystalline elastomer (LCE) as it undergoes the familiar polydomain−monodomain (P−M) transition. Under uniaxial tension, the LCE behaves like an ordinary rubber-like network at low strains, and local director rotations are shown to slightly favor the perpendicular (“anomalous”) orientation of chain axes with respect to the draw direction. As strain increases, a well-defined yield stress is observed due to the onset of a necking instability. Macroscopic elongation proceeds by growth of the necked monodomain region, which appears to consume the non-necked polydomain region(s) at its boundaries. Within the necked region, the parallel (“normal”) orientation of chain axes with respect to the draw direction is strongly favored. The P−M transition is attributed to a change in the conformation of the elastic polymer backbones from hairpinned coils to extended chains. Under the conditions of temperature and strain rate studied, buckling of the smectic layers is observed as the fully necked monodomain state is approached.

ABSTRACT:We examine the influence of structural imperfections on mechanical damping in polydomain smectic main-chain liquid crystalline elastomers (MCLCE) subjected to small strain oscillatory shear. The mechanical loss factor tan d ¼ G@(x)/ G 0 (x) exhibits a strong maximum (tan d % 1.0) near the smectic-isotropic (clearing) transition. ''Optimal'' elastomers that exhibit minimal equilibrium swelling in a good solvent are compared with highly swelling ''imperfect elastomers'' that contain higher concentrations of structural imperfections such as pendant chains. For the imperfect elastomers, tan d is markedly enhanced in the isotropic state because of relaxation of pendant chains and other imperfections. However, within the smectic state, the magnitude of tan d and its temperature dependence are similar for optimal and imperfect elastomers at x ¼ 1 Hz. The prominent loss peak near the clearing transition arises from segment-level relaxations that are insensitive to the details of chain connectivity. Smectic MCLCE can be tailored for applications as vibration-damping materials by manipulating the clearing transition temperature through the backbone structure or by deliberate introduction of structural imperfections such as pendant chains.

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Smectic main-chain liquid crystalline elastomers (MCLCE) with polydomain morphology are rare examples of elastomers that can form a neck and undergo cold drawing under tension. However, not all previous studies of the mechanical behavior of smectic MCLCE reported neck formation. The mechanical response of a polydomain smectic MCLCE has therefore been characterized by elongation at varying strain rates and temperatures to identify factors favoring mechanical instability. Yielding and neck formation are increasingly favored as the strain rate increases at constant temperature, or as the temperature decreases toward T g . As cold drawing pro-ceeds, significant creep occurs continuously within the neck, in contrast to the behavior of certain linear polymers that exhibit a ''natural'' draw ratio. Thermal imaging during elongation indicates that viscous heating is not a prerequisite for neck formation. Rather, inherent softening of the material during yielding due to morphological changes leads to an enhanced rate of deformation and contraction at the neck. V C 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 591-598, 2011

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