Influence of molecular weight on phase transitions and alignment of a thermotropic nematic polyester

Abstract

We have investigated oligomers, polymers, and a model of poly(4,4'-dioxy-2,2'-dimethylazoxybenzenedodecanedioyl). The molecular weights ranged from approximately 700 to 19000. In the oligomeric range, the kinetics of melt and cold crystallization, as well as the kinetics of macroscopic sample alignment in a magnetic field of ~1.8 T, is controlled by the aromatic end-group distribution. Phase transition temperatures increase with increasing molecular weight. A biphasic region (nematic + isotropic) is observed for all oligomers and polymers. Its range of thermal stability shrinks from ~40to ~5 °C as the molecular weight increases. The value of the glass transition temperature is nearly independent of molecular weight or development of crystallinity. The values of phase transition entropy at the N -* I transition and of the nematic order parameter increase with increasing molecular weight to reach a plateau at average chain lengths of 8-10 repeating units. Development of a polymeric mesophase, as characterized by maximum local chain extension, appears to require cooperativity between 8-10 repeating units per chain. The semicrystalline state of higher molecular weight samples is represented in terms of a fringed micelle model, with extended-chain segments forming cybotactic nematic domains.

Influence of Polymerization Degree and Its Distribution on Thermal Behavior of Thermotropic Nematic Model Compounds

“…Though composed mainly of (B8hB, which is monotropic, the oligomer-b showed an enantiotropic nematic phase over a wide range of 13°C (Figures 1 and 2a), suggesting that a small amount of other large mers can have a large effect on the development of the liquid crystalline phase. We mixed the pure (B8) 2 (B8hB in different mixing ratios and investigated the transition behavior. Figure 5 illustrates the DSC heating and cooling curves of a typical (B8)zB-(B8hB mixture (mo!…”

“…Blumstein et al insist that this effect of polymerization degree is attributable to the cooperativity between repeating units in the nematic phase. 2 Actually, the distribution of polymerization degree and end groups also seem to add complicated effects. To reveal the This oligomer consists of an ester (repeating unit) of 4,4'-dihydroxybiphenyl and sebacic acid, and p-phenylphenol (end group), which has a similar structure as the mesogen group.…”

Nematic Order in Polyesters with Flexible Moieties in the Main Chain

“…; and 3) PMR: the biphase is characterized by the nematic fraction fN obtained on slow cooling from the isotropic phase. 4 One can easily see from Figure 1 that the range of the biphase delineated by the NMR experiment (interval between the two vertical lines) is larger than the range delineated by DSC (T1N"-T 1 N.) or polarizing microscopy (T,-T 2 ). One can note, however, that the maximum on the DSC curve corresponds to the temperature of disappearance of isotropic droplets T 2 as well as to the inflection point on thefN(T) curve from PMR and is therefore adopted as the temperature of isotropic-nematic transition T1N.…”

“…4 Comparison of properties of liquid crystalline oligomers is delicate and should always consider the nature of end groups.…”

Morphology of thermotropic polyesters with azoxybenzene moiety examined by optical microscopy.

“…Blumstein et al (1)(2)(3)(4) described the synthesis and thermal properties of main-chain nematic polyesters of poly(4,4'-dioxy-2,2'-dimethylazoxybenzene alkanedioyl). The polymer consists of a regular sequence of rigid azoxybenzene mesogenic core (mesogen 9) and flexible alkanedioate "spacer" group as shown below.…”