In‐situ photopolymerization of oriented liquid‐crystalline acrylates, 3. Oriented polymer networks from a mesogenic diacrylate

Abstract

Synthesis, mesomorphism, orientation and photo‐initiated chain crosslinking of the liquid‐crystalline diacrylate 1,4‐phenylene bis{4‐[6‐(acryloyloxy)hexyloxy]benzoate} (1) are studied. Monomer 1 exhibits a broad nematic phase between 108 and 155°C and a monotropic smectic phase below 88°C. The monomer is uniaxially oriented in its nematic phase at a substrate which has been coated with polyimide and unidirectionally rubbed with tissue. At the transition temperature to the smectic phase the order parameter is measured to be 0,7. During polymerization, the ordering of the mesogens is frozen‐in, yielding a uniaxially crosslinked network. The clear films of oriented poly(1) exhibit a birefringence Δn between 0,12 and 0,15, depending on the polymerization temperature. In the highest oriented state of 1 a small reduction of the degree of order is observed during the crosslinking reaction, whereas at higher temperatures and lower ordering of 1, the uniaxially orientation increases upon reaction. A special feature of the oriented networks is that the ordering is maintained while heating at high temperatures. The polymerization of the acrylate groups in the mesomorphic phases proceeds fast and to high conversion. Below 90°C the polymerization behaviour is similar to that of conventional isotropic diacrylates. Above 90°C the polymerization reaction of the liquid‐crystalline diacrylate proceeds faster than that of an isotropic diacrylate.

Curvature by design and on demand in liquid crystal elastomers

“…Here, we use a recently reported approach [20] to prepare LCEs that exhibit large deformations upon relatively modest temperature changes. The synthesis of these materials is largely based on commercially available liquid crystalline monomers (diacrylates) that homopolymerize to form glassy liquid crystalline films with good uniformity [21]. To these materials, a minimal amount of chain transfer agent has been shown to dramatically reduce the crosslink density and yield LCEs with actuation strain of nearly 200% and soft elasticity that rivals polysiloxane LCE ( Figure 1).…”

Measurement of Degree of Order in Mixed Polarised Fluorescent Polymer Liquid Crystal Films

“…It has been shown previously 9 that the temperature of photopolymerisation affects the order in C6H films. The polymerisation temperature is usually expressed in terms of 'reduced' temperature (T r ):…”

“…In order to produce a low viscosity nematic phase, 5% by weight of FLLC was mixed with 1,4-phenylene bis{4-[6-(acryloyl-oxy)hexyloxy]benzoate} (C6H), a previously studied cross-linkable nematic liquid crystal. 9 The same proportion of photoinitiator and thermal inhibitor as before was then added. This new mixture exhibits a crystalline-nematic phase transition at 116 o C and a nematic to isotropic transition at 150 o C. Glass cells containing this mixture were found to exhibit long range in the nematic phase.…”

Molecular ordering in a liquid crystalline material visualized by scanning electron microscopy

“…No structural reorganization is observed in a polarizing microscope [22]. Dichroic measurements only reveal a small change in the order parameter, which even reduces to zero for a good choice of the polymerization temperature [21,22]. The UV-polymerized LC diacrylate films have an optically clear appearance.…”

“…It is based on the fixation of the director profile of an LC diacrylate by UV polymerization [21]. In the pre-polymerization state, the diacrylate molecules behave like ordinary low molar mass LC materials, possessing a nematic and smectic phase and being oriented by surface interaction or by an external electric or magnetic field.…”