The fibrillar hierarchy in liquid crystalline polymers

Abstract

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.