Thermotropic side chain polyacrylates with azobenzene mesogenic groups: Synthesis and thermal behaviour

Abstract

Four side chain liquid crystalline (LC) polyacrylates containing substituted (methyl/long alkyl) azobenzene mesogens attached directly to the polymer backbone were synthesised. The polyacrylates were synthesised by free radical polymerization of the respective monomers, 4‐[(4‐acryloyloxyphenyl)azo]benzoic acid, 4‐[(4‐acryloyloxy‐3‐methylphenyl)azo]benzoic acid, 4‐[(4‐acryloyloxy‐2‐methylphenyl)azo]benzoic acid and 4‐[(4‐acryloyloxy‐2‐pentadecylphenyl)azo]benzoic acid. The monomers were obtained by acryloylation of the azo derivative prepared by the diazo coupling reaction between p‐aminobenzoic acid and the respective phenolic derivatives with varying hydrocarbon substituents. All polymers were soluble in polar solvents. The inherent viscosities of the polymers in DMSO at 28°C were in the range of 0.55–0.62 dL/g. The monomers and their polymers were characterised by elemental analysis, IR, UV‐vis, 1H and 13C NMR spectroscopies and WAXD. The thermal and phase transition behaviour of the polymers were investigated using DSC and a hot stage polarized light microscope (PLM). The polyacrylates exhibited stir opalescence and low viscosity characteristic of nematic phase on melting. However, the polymer with the long alkyl side chain (C15H31) was polymorphic with a smectic phase besides a nematic phase. The methyl substituted polymers showed only nematic phases. The effect of regioisomers on transition temperatures was significant. The carboxylic acid groups of the azophenyl mesogen are found to exist as hydrogen bonded dimers as indicated by IR, NMR and finally confirmed by WAXD. It is proposed that the long alkyl substituent might be involved in the stabilisation of the mesophase through its alignment parallel to the long axis of the molecule and might probably be supporting the formation of a smectic phase by reducing the transition temperature enough to maintain the layered arrangement of hydrogen bonded dimers.

Self-assembly and supramolecular liquid crystals based on organic cation encapsulated polyoxometalate hybrid reverse micelles and pyridine derivatives

“…It should be noted that the mismatched thermotropic phase transitions between heating and cooling runs do not mean that the phase behavior in the cooling process is in disaccordance with that of the heating process, because the corresponding phase transition that was not detected by DSC in the cooling run can be observed through POM measurements. A possible reason may be sourced from the dynamic nature of non-covalent interactions between the carboxylic acid and pyridine derivatives directed by intermolecular hydrogen-bonding at LC states [40]. In accordance with the POM results, the peak at high temperature can be clearly assigned to the transition of LC phase to isotropic state and the other peaks appeared in heating process could be assigned based on the XRD results.…”

“…1730 cm À1 . And two halo bands that represent intermolecular hydrogen bond emerge at 1922 and 2483 cm À1 , obviously confirming that the hydrogen-bonding between benzoic acid and pyridine derivatives still exists in the high temperature region, where LC states locate at [32][33][34][35][36][37][38][39][40][41]. In addition to the well maintained hydrogen-bonding, the vibrations of CH 2 antisymmetric and symmetric stretching move to the higher wavenumber somewhat, implying the alkyl chains of the assembly in disordered state.…”

Nonlinear Optical Properties and Liquid-Crystalline Behavior of New Polyesters with Dipole Moments Aligned Transverse to the Backbone

“…[39,41] In the case of polymer PPAR, the hydrogen atom in place of the nitro group may not be able to give the stabilization required for the formation of LC phase. So, the occurrence of anisotropic ordering might be possibly due to the contribution of the nitro groups, stabilizing the azobenzene moiety, which is well known to exhibit LC properties.…”

Challenges for Natural Monomers and Polymers: Novel Design Strategies and Engineering to Develop Advanced Polymers

“…As they combine the properties of anisotropy with photo responsive behavior in a single polymer, they find applications in areas such as optical storage devices, data processing and light modulation [203][204][205][206][207][208][209][210][211][212][213][214]. It is possible to bring down the transition temperature and increase solubility in common solvents by introducing long flexible alkyl side-chain laterally to the backbone of linearly rigid rod azobenzene moiety [203][204][205][206]. It was shown that side-chain azobenzenebased polymers derived from cardanol (Fig.…”

“…It was shown that side-chain azobenzenebased polymers derived from cardanol (Fig. 7) exhibited both LC phases and NLO properties and that the presence of the liquid crystalline phase enhanced the NLO properties [203,204].…”