Highly oriented polymeric products have been produced over the past fifteen years by two very different processing routes; from conventional polymers processed to highly oriented extended chain structures, and from “rod-like” polymers which exhibit liquid crystalline behaviour. Gel spun polyethylene is an example of such a conventional polymer. There are three main types of liquid crystalline polymers (LCP) which have high orientation and modulus: lyotropic aramids, such as poly(ρ-phenylene terephthalamide) (PPTA); lyotropic, aromatic heterocyclic polymers, or “ordered polymers”; and the family of thermotropic aromatic copolyesters. Extensive characterization of the thermotropic copolyesters has resulted in the delineation of a fibrillar, hierarchial structural model which accounts for the structures observed in a broad range of oriented fibres, extrudates and moulded articles. Three distinct fibrillar species are observed: microfibrils that are about 50 nm, fibrils about 500 nm, and macrofibrils about 5μm, in size. Superimposed on the structural hierarchy is a defect hierarchy, defined by the regular meander of the molecular chain and a localization of defects within a microfibril at about a 50 nm periodicity. Orientational variations, layering and skin core structures, in thick specimens, are the result of local flow fields on the basic structural units during solidification. The fibrillar textures appear to be present prior to any preparation for microscopy. A wide range of specimen preparation methods, i.e. fractography, sonication, microtomy and etching, and microscopic techniques, i.e. optical, scanning and transmission electron microscopy, were applied to the characterization of the aromatic copolyesters and PPTA. Interestingly, the same basic hierarchy is observed for both the lyotropic and the thermotropic LCPs and the microfibrillar structures of all the highly oriented polymers, including polyethylene, appear quite similar.

The structure of thermotropic copolyesters

Thermotropic liquid-crystalline polyesters are a new class of polymeric materials which have unique molecular and solid-state structures, flow characteristics and mechanical properties. Injection-moulded plaques were found to consist mainly of three highly anisotropic, flow-induced macrolayers: two skins with a core in between. By using fractographic methods, the internal solid-state structure of the macrolayers was elucidated. A hierarchical structure has been proposed describing the observed levels of organization. The skin macrolayer has a distinct structural gradient comprised of three subdivisions from the surface inward: a highly oriented top-layer, several oriented sublayers and a less oriented inner zone. The top layer is fibrillar in nature and the sublayers consist of stacks of interconnected microlayers. In the core, no well-defined substructure was observed, yet molecular orientation perpendicular to the injection direction represented the localized flow patterns.

Hierarchical structure in a thermotropic liquid-crystalline copolyester

This brief history and review of liquid crystalline polyesters (LCPs) is limited to all-aromatic LCPs except for a description of the discovery of the first thermotropic polymer to be injection molded or melt spun and characterized as being liquid crystalline: an Eastman PET/PHB (p-hydroxybenzoic acid) copolyester (first reported as being liquid crystalline in 1974). In the same year a Carborundum terephthalic acid/4,4′-biphenol/PHB all-aromatic copolyester was reported to be injection moldable, and later this polymer was shown to be liquid crystalline. In 1975 various Du Pont LCPs prepared with substituted hydroquinones were disclosed, and in early 1978 the first Celanese LCPs based on 2,6-oriented naphthalene monomers were disclosed. The development and characterization of all-aromatic LCPs up to 1987 are described, with the LCP classification based on the following monomers: 4,4′-biphenol; asymmetrically substituted monomers; 2,6-oriented naphthalene monomers; 3,4′-oriented benzophenone and di...

Liquid Crystal Polymers. XI. Liquid Crystal Aromatic Polyesters: Early History and Future Trends

Blends of a thermotropic liquid-crystal polyester with poly(phenylene sulphide)

It is well known that the structure of highly oriented liquid crystalline polymers (LCPs) can be characterized by a hierarchical fibrillar structural model. Structure models were first developed for the lyotropic aramid fibres in the late 1970s and a structural model was developed for the thermotropic copolyesters in the mid-1980s. Recently, imaging techniques with higher potential capability and resolution have been applied to assess the size, shape and organization of microfibrillar structures observed in LCPs. Field emission scanning electron microscopy and scanning tunnelling microscopy permit imaging of regions from 1 nm to many micrometres. As a result, the nature of the microfibrillar hierarchy has been further clarified and the macromolecular size has been shown to be less than 2 nm. The shape of the microfibrils has been shown to be tape-like. The LCP structural model, consisting of elongated well-ordered microfibrils continues to be consistent with measured properties: high anisotropy, very high tensile modulus and strength and poor compressive properties. A more detailed structural model is proposed to describe the macromolecular microfibril size, shape and organization for comparison with polymer composition and mechanical property evaluation.

The fibrillar hierarchy in liquid crystalline polymers

Micromorphologie d'un copolyester d'acide paracetoxybenzoique et d'acide acetoxynaphtalique par diffraction RX et microscopie electronique a balayage

H. Thapar

Papers

The micromorphology of an injection moulded thermotropic liquid crystal polymer