Genaro A. Gutierrez

The structure of liquid crystalline aromatic copolyesters prepared from 4-hydroxybenzoic acid and 2-hydroxy-6-naphthoic acid

Abstract X-ray methods have been used to investigate the structure of liquid crystalline copolyesters prepared from 4-hydroxybenzoic acid (HBA) and 2-hydroxy-6-naphthoic acid (HNA). Fibre diagrams and meridional diffractometer scans have been obtained for melt-spun specimens of five HBA/HNA ratios: 25 75 , 30 70 , 50 50 , 58 42 and 75 25 . The data indicate a high degree of axial orientation. The lateral packing is more irregular but the observation of some equatorial and off-equatorial Bragg reflections indicates the presence of some three-dimensional order. The patterns differ in the number and position of the meridional maxima, and these can be predicted accurately by a point model for aperiodic chains of totally random sequence. The sensitivity of the data to monomer ratio is illustrated by calculations across the range of composition from 100% HBA to 100% HNA.

The structure of copoly(4-hydroxybenzoic acid/2-hydroxy-6-naphthoic acid): 2. An atomic model for the copolyester chain

Abstract X-ray fibre diagrams of fibres drawn from the (liquid crystalline) melts of copoly(4-hydroxybenzoic acid/2-hydroxy-6-naphthoic acid) show meridional maxima that are aperiodic. The positions of these maxima are reproduced in calculations of the meridional intensities for completely random copolymer sequences in which the monomers are approximated to points separated by the appropriate monomer lengths. These calculations have now been extended to consider an atomic model for the chains. Comparison of the calculated meridional transforms with diffractometer scans for five monomer ratios shows good agreement, not only for the positions but also for the relative intensities of the observed and calculated maxima. The linewidth of the maximum at d ≅2.1 Ǎ depends on the chain length used for the calculations and presents a method for determination of the persistence length of the extended chain conformation.

The structure of liquid crystalline copolyester fibers prepared from p-hydroxybenzoic acid, 2,6-dihydroxy naphthalene, and terephthalic acid

Abstract X-ray methods are being used to investigate the physical structure of high strength melt-spun liquid crystalline aromatic copolyester fibres prepared from the following monomers: p-hydroxybenzoic acid (B), 2,6-dihydroxy naphthalene (N), and terephthalic acid (P). The X-ray data for fibres prepared for B/N/P ratios of 60/20/20, 50/25/25, and 40/30/30 show variations in the positions and intensities of the maxima suggestive of a truly random copolymer structure rather than extensive block copolymer character. We have predicted the intensity distribution on the meridian of the X-ray patterns by modelling random chains by a linear sequence of points separated by distances corresponding to the lengths of the B, N and P residues. The positions of the maxima in the averaged squared Fourier transforms of random monomer sequences are in good agreement with those for the three observed strong meridional intensities, and the calculations reproduce the observed shifts in the positions of these meridionals as the monomer ratio is varied.

The structure of liquid crystalline copolyesters prepared from 4-hydroxybenzoic acid, 2,6-dihydroxy naphthalene, and terephthalic acid: 2. Atomic models for the copolymer chains

Abstract The X-ray fibre patterns from copolyesters of 4-hydroxybenzoic acid, 2,6-dihydroxynaphthalene, and terephthalic acid show meridional maxima that are aperiodic. The positions of these peaks can be reproduced by calculation of the scattering characteristics of copolymer chains of random sequence modelled as linear arrays of points, where each point represents a monomer separated from adjacent points by the appropriate monomer lengths. By introducing atomic models for the monomers, good agreement is also obtained between the observed and calculated intensities. It is shown that the intensity agreement is improved by refinement of the orientation of the monomers with respect to the fibre axis.

X-ray analysis of the structure of liquid-crystalline copolyesters

The X-ray analyses of the structures of a group of aromatic copolyesters that form liquid-crystalline melts is described. X-ray patterns of melt-spun fibres of wholly aromatic copolymers prepared from p-hydroxybenzoic acid (HBA) and 2-hydroxy-6-naphthoic acid (HNA) show a high degree of axial orientation and there is also evidence for three-dimensional order. The meridional maxima are aperiodic, and it is shown that their positions and intensities are predicted by a model consisting of an assembly of parallel chains of completely random monomer sequences. Analyses of the scattering by aperiodic chains are summarized, first for point monomers and subsequently for an atomic model. From the breadth of some of the meridional maxima it is possible to estimate the persistence or correlation length for the stiff-chain conformation, which is found to be between 9 and 13 monomer units, depending on the HBA/HNA ratio. Determination of the three-dimensional structure is initiated via calculation of the cylindrically averaged transform for the random chain, followed by consideration of the interference effects caused by chain packing.

X-Ray Studies of the Structure of HBA/HNA Copolyesters

X-ray diffraction patterns of fibers spun from liquid crystalline melts of ρ-hydroxybenzoic acid (HBA) and 2-hydroxy-6-naphthoic acid (HNA) show a high degree of axial orientation. Several meridional maxima are detected which are aperiodic and also change in position and number with the monomer composition. The positions of these maxima can be predicted by calculating the theoretical scattering of random copolymer chains, in which the residues are represented by points separated by the monomer lengths. Both peak positions and intensities are reproduced when intraresidue interferences are allowed for in an atomic model for the random chains. This procedure also allows determination of the stiff-chain persistence (or correlation) length from the breadth of the maximum at d =2.1A which increases from 9 to 13 residues as the HBA content is increased from 25 to 75%.

X-ray studies of the structure of thermotropic copolyesters

Abstract This paper describes x-ray analysis of the structure of a group of wholly aromatic thermotropic liquid crystalline copolyesters, focusing on two systems: condensation copolymers of 4-hydroxy-benzoic acid (HBA) and 2-hydroxy–6-naphthoic acid; and of HBA, 2,6-dihydroxy-naphthalene, and terephthalic acid. The fiber diagrams of these copolymers contain a series of aperiodic meridional maxima that shift in position with the monomer ratio. We have shown that these characteristics are predicted by a structure consisting of an array of chains of completely random monomer sequence. The positions of the maxima are predicted by a simple model in which the monomers are represented by points, separated from their neighbors by the appropriate monomer lengths. Use of atomic models for the monomers leads to prediction of both the positions and the intensities of the maxima. Calculations for nonrandom sequence distribution show that all but minimal blockiness can be ruled out. From the breath of the meridional ma...

X-ray Studies of Thermotropic Aromatic Copolyesters

X-ray diffraction has been used to investigate the solid state structure of aromatic copolyesters that can be spun as high strength fibers from thermotropic melts. Copolymers of 4-hydroxybenzoic acid (HBA), 2,6-dihydroxynaphthalene and terephthalic acid give fiber diagrams that indicate a high degree of axial orientation. Much of the structure has disordered lateral packing but there is some three-dimensional order. The meridional maxima are aperiodic and shift in position with changes in the monomer composition. The positions of these maxima can be reproduced by a model for chains of random monomer sequence, in which the residues are represented by points separated from their neighbors by the appropriate monomer lengths. Extension of these calculations to an atomic model for the chains allows for comparison of the observed and calculated X-ray intensities. We find that refinement of the model by tilting the monomer residues with respect to the chain axis leads to good agreement between the observed intensities and those calculated for a completely random monomer sequence. An indication of the chain packing in the ordered regions comes from study of a second set of copolymers prepared by modification of poly(ethylene terephthalate) by incorporation of HBA. The latter copolymers are found to contain ordered regions with a structure similar to that for the high temperature hexagonal form of homopoly(HBA).