ABSTRACT:Molecular mechanisms were proposed to account for extraordinary optical activities of polyisocyanates containing chiral moieties (chiral monomer or initiator fragment) reported by Green et al. and by Okamoto et al., where the polyisocyanate molecule was modeled by a helical chain consisting of an alternating sequence of right-handed and left-handed helices with helix reversals in between. Theoretical predictions based on this model were consistent with remarkable dependence of optical rotation on monomer composition for random copolyisocyanates of chiral and achiral monomers and enantiomorphous copolymers and chain length dependence of optical rotation for achiral polyisocyanates with a chiral initiator fragment on the chain end. The helix reversal was shown to be the key factor for all the cases and more frequent for aromatic sidechain polymer poly(m-methyl phenyl isocyanate) than for aliphatic side chain polymers such as poly(butyl and hexyl isocyanate)s.KEY WORDS Chiral / Optical Activity / Polyisocyanate / Helical Conformation / Helix Reversal / Statistical Mechanics / Copolymer / Amide bonds of partial double-bond nature in a polyisocyanate chain tend to be coplanar, but they are distorted in a helical conformation due to steric interactions between the main chain and side chains. 1 -1° For this reason the global conformation of the polymer chain is well modeled by wormlike chains. 7 -9 • 11 -18 However the sense of the helix is not determined for an achiral polyisocyanate because both the right-handed and lefthanded helices are symmetric. This symmetry is broken by a subtle chiral perturbation within a given molecule and this molecule becomes optical active. [19][20][21][22][23][24][25] showed that incorporation of a very small amount of chiral monomers into an achiral polyisocyanate in a random fashion gives rise to remarkable optical activity, which increases disproportionately with the mole fraction of chiral monomer. A similar disproportionate change in optical activity has been found in copolymers of enantiomorphous isocyanates (R-S copolymers). 25 On the other hand Okamoto and collaborators 26 • 27 introduced a chiral perturbation into polyisocyanate chains in a different way; they synthesized polyisocyanates with chiral initiators, which were attached to a chain end, and showed that those polyisocyanates exhibited optical activity changing remarkably with length and temperature. The present study was motivated by these findings and tries to explore the molecular mechanisms behind the optical activities in these particular polymers. Green et al. 20 called chiral entities sergeants and achiral entities soldiers. Thus we here deal with sergeants-soldiers polymers. Theoretically these polyisocyanates are regarded as typical linear cooperative systems just as chiral polyisocyanates and a-helical polypeptides. Therefore we analyze their conformational properties including optical activity by the well-established statistical mechanical procedures based on the matrix method. 28 -30 A preliminary...

Acetoxypropylcellulose (APC) and propionic, n‐butyric, isobutyric, valeric, isovaleric, hexanionic, and heptanionic acid esters of hydroxypropylcellulose (HPC) (respectively PPC, BPC, iBPC, VPC, iVPC, HexPc, and HepPc) were prepared and characterized by differential scanning calorimetry, chromatography, polarizing microscopy, chemical methods, and spectroscopy. All these esters form thermotropic cholesteric liquid crystalline phases. The glass (Tg) and clearing (Tc) transition temperatures were determined. The stability interval of the mesophases appears to be greater in the case of longer‐linear side chains. The mesophases of APC, PPC, iBPC, VPC, and iVPC exhibit reflection bands in the visible region, at wavelengths that depend on temperature, moisture content, size and number of substituents, and degree of polymerization ( \[ \overline {{\rm DP}} \] ). The pitch of the cholesteric helical structure increases with side chain length and with increasing temperature. At room temperature, a fingerprint‐like pattern can be observed for HepPc. No reversal in the sense of the pitch with temperature variations was observed. The pitch of BPC increases with moisture content and with decreasing values of the degree of esterification ( \[ \overline {{\rm DE}} \] ) and \[ \overline {{\rm DP}} \] . A theory for cholesteric mesophases composed of helical rod‐like species and a model of elastic bend chain have been compared to the experimentally observed changes in the pitch with temperature and with the length of the side chain substituents (for iBPC, iVPC, BPC, PPC, and APC) and \[ \overline {{\rm DE}} \] and \[ \overline {{\rm DP}} \] (for BPC). © 1994 John Wiley & Sons, Inc.

SUMMARY:In liquid-crystal elastomers, the simultaneous presence of rubber elasticity due to the crosslinked backbone chains and of optical birefringence due to the mesogens in the side chains lead to exceptional physical properties. An elastic deformation of the network influences the order of the mesogens and, therefore, the optical properties. A theory based on a Landau-de Gennes expansion of the free energy is proposed. In the opaque polydomain phase, the local orientation is given by a compromise between the external mechanical field and a local anchoring interaction. As the field is increased, it becomes energetically favorable for the mesogens to align parallel to the mechanical field, and a transition to a transparent monodomain structure occurs. Results for the average orientation, the stress and the chain conformation are given.

Phase and orientational ordering of low molecular weight rod molecules in a quenched liquid crystalline polymer matrix with mobile side chains

SYNOPSISSome esters of (2-hydroxypropyl) cellulose have been prepared and studied. Their cholesteric properties change with ageing and may disappear. Accelerated aging has been performed in order to analyze degradation processes which are relevant to the scission of the polymeric main chain and to the hydrolysis of the ester bonds which are mainly due to traces of water. The necessity of carefully elaborate purification is highlighted.

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In the past few years, there have been important developments concerning the role and the use of chirality in liquid crystals. An example which is now relatively well known is the chiral smectic C* low molecular weight liquid crystal (LMWLC) which lowers the response time of displays by a factor between one hundred and one thousand [1-3]. In chiral liquid crystal polymers (cholesteric or twisted nematic and smectic C*), it is necessary to understand the relation between the chemical structure and the properties of mesophases in liquid and solid polymer. Furthermore, chirality is also found in abundance in biopolymers and may be important for life processes. Applications in industry (high modulus fibers, sensors, non linear optics...) have already been realised or are being pursued. The aim of this paper is to emphasize the polymer character of these liquid crystals. What is the influence of the molecular weight, the degree of substitution, the viscoelasticity and the existence of a glass transition? How to use the polymer character to obtain new informations on the liquid crystal state or to make new devices? This rapid overview roughly covers the following parts: (1) Formation and general properties of cholesteric mesophases: we give a rapid discussion of thermotropics and lyotropics, of the influence of degree of polymerisation, degree of substitution, concentration, temperature..., on the cholesteric-isotropic transition and on the order observed by textures or as described more quantitatively by the order parameter. (2) Optical properties: we give a brief presentation of a theoretical work concerning the pitch prediction taking into account the asymmetric term in the interaction potential and the comparison with experimental results. (3) Textures and defects which are mainly observed in these mesophases. (4) Flow properties and rheological behaviour which are fundamental for processing. (5) Blue phases near the isotropic-cholesteric transition which are discussed with emphasis on the specificity of the polymer state. It is usual to distinguish main chain and side chain liquid crystal polymers [4, 5], which may be modelled as indicated in Fig. 1. Cellulosic derivatives are good examples of main chain cholesteric polymers [6]. Cellulose is a very abundant polymer: each year, biosynthesis produces about 1011 tons of cellulose. It readily accepts grafting of substituents, thus producing derivatives like cellulose acetate of hydroxypropylcellulose. Examples of side chain liquid crystal polymers are those obtained with homo- or copolymers of polyacrilate, polymethacrylate or polysiloxane.

A review is given on the use of the bend elastic chain model for the calculation of physical properties of polymers.