Configuration of a thermotropic polyester from small-angle neutron scattering

Abstract

Small-angle neutron scattering has been used to ascertain the configuration of a random thermotropic terpolyester at three temperatures between the glass transition and the melting point on four fractions of different molecular weight. Specimens were prepared by casting films from solutions and subsequent annealing at selected temperatures for 1 h. All samples have a small amount of crystallinity, but at the lowest annealing temperature, 448 K, the molecules have a random-coil configuration. At the two higher annealing temperatures, 493 and 523 K, the molecules become increasingly stiff. This is observed as an increase in the persistence length from 9 Á at 448 K to ca. 44 A at 523 K. The variation of the ratio (s2)w/Mw with Afw for the two higher temperatures displayed the characteristics of a Kratky-Porod wormlike chain. Persistence lengths and shift factors were obtained from an extrapolation procedure based on an approximation for the dependence of the radius of gyration of a wormlike chain on the contour length and persistence length. The value of the persistence length obtained at 523 K is in good agreement with a value obtained from scanning tunneling microscopy elsewhere. These data suggest that the transformation to the nematic mesophase at temperatures above the melting point is aided by stiffening of the molecule at lower temperatures.

A small-angle neutron scattering study of a semiflexible main-chain liquid crystalline copolyester

“…Lower values of the persistence length were reported by Richards et al 35 for a main-chain LCP prepared from hydroxybenzoic acid, hydroquinone, and isophthalic acid.…”

“…At first a procedure similar to the one adopted by Olbrich et al 28 and Richards et al 35 in treating the scattering from a main-chain liquid crystalline polyester was considered. The weight-average radius of gyration, (fi2)»1/2, was derived from the z-average (eq 6) by means of the relation36 <7> which applies to unperturbed Gaussian coils.…”

Molecular dimensions and melt rheology of an all‐aromatic liquid crystalline polymer

“…Given their unique rheo‐optical behaviors, excellent flow characteristic, and many physical properties superior to those of flexible polymers, thermotropic liquid crystalline polymers (TLCP) have been drawing much attention from both academia and industry. The molecular dimensions1–6 and the melt rheology7–14 of various thermotropic liquid crystalline polymers have been extensively studied over the last two decades. However, less work has been done on the effect of the molecular dimensions on the melt rheology of TLCPs.…”

“…However, less work has been done on the effect of the molecular dimensions on the melt rheology of TLCPs. Typical commercial TLCPs posses semirigid chain structures with a relatively long persistence length in the range of 50–300 Å 1–6. The rigid molecular structure extremely restricts the rotational motion of the polymer chain and permits almost no perpendicular motion of the chain confined in the tube constructed by neighboring molecules.…”

Liquid-crystalline polymers in good nematic solvents: free chains, mushrooms, and brushes

“…The theoretical picture summarized above dates from the seminal paper of de Gennes.39'41 Experimental verification was slow to materialize. The most detailed probe of macromolecular configurations, small angle neutron scattering (SANS), is difficult to implement in this case: 22,23,42 since main chain LCPs are synthesized by condensation polymerization, it is difficult to obtain samples with high polymerization degrees and low polydispersity. This difficulty is compounded by the necessity to deutrate the polymers and to obtain uniform mixture of the deutrated and hydrogenated chains.…”