Melt blending of a variety of conventional isotropic polymers (both crystalline and amorphous) has shown that considerable reinforcement is obtained from the inclusion of thermotropic liquid crystalline polymers (LCP). When the LCP is a minor component it forms highly elongated domains parallel to the flow direction. The intrinsically high strength and stiffness of the LCP improve the mechanical properties of the resulting blend. This approach is distinguished from the common practice of filling polymers with chopped glass and carbon fibers by the fact that the reinforcing component comes into existence during processing (molding or extrusion). Many of the problems associated with fibrous fillers are avoided. For example, viscosity of the “in situ composite” is actually lower than that of the base polymer and wear on the compounding and processing equipment is reduced.

In situ composites: blends of isotropic polymers and thermotropic liquid crystalline polymers

The applicaiton of a tensile stress to a monocrystalline fiber of a polydiacetylene, an extended‐chain polymer, causes the Raman‐observed C≡C and C=C in‐chain vibrational frequencies to decrease linearly with elongation. The polymer studied, poly‐HDU, results from the solid state polymerization of R–C≡C–C≡C–R, where R=–CH2OCONHC6H5. This experimental relationship between bond frequency shifts and deformation is analyzed by calculating bond force constant changes as a function of macroscopic stress. It is shown that possible effects due to stress‐induced changes in electron delocalization in this fully conjugated polymer are negligible and the principal contributions to the bond frequency shifts are due to bond anharmonicity. Models which assume that the entire effect is due to bond anharmonicity produce calculated dependencies of ν (C=C) and ν (C≡C) upon fractional elongation, which are in good agreement with that observed.

A laser Raman study of the stress dependence of vibrational frequencies of a monocrystalline polydiacetylene

Micro Raman spectroscopy (MRS) can be used to measure strain in crystalline and semicrystalline materials with urn scale spatial resolution. It has been employed as a tool for measuring micromechanical behaviour in composites, and has contributed significantly to understanding of composite behaviour. An overview of the fundamentals of Raman spectroscopy and strain measurements using Raman spectroscopy as well as the instrumentation required is given. Studies using MRS to gain insight into the structure, elastic moduli, and failure modes of several types of fibres are presented. The micromechanical behaviour of single filament model composites is examined in detail including: residual stresses caused by processing; the stress transfer behaviour from the matrix to the fibre during loading; and the interfacial behaviour as a function of tensile and compressive loading, interface surface treatment, and environmental exposure. Finally, recent studies using MRS to study the fibre-fibre interaction and h...

Fundamentals and applications of micro Raman spectroscopy to strain measurements in fibre reinforced composites

Raman spectroscopy of stressed high modulus poly(p-phenylene benzobisthiazole) fibres

The kinetics of single‐phase solid‐state polymerization reactions are evaluated using two different approximations for the effect of structural changes at intermediate conversions. Conversion curves are calculated for monomer phases which undergo large dimensional changes in the reaction direction (crystal strain approximation) and for monomer phases which are essentially dimensionally invariant during reaction (nearest neighbor approximation). Using the crystal strain approximation and experimentally derived elastic constants, the different autocatalytic reaction curves observed for thermal and radiation‐induced polymerization of a diacetylenic monomer are successfully predicted. The dramatic autocatalytic effect for thermal polymerization can be explained quantitatively by a strain‐dependent rate of chain initiation and chain propagation, while the weaker autocatalytic effect for γ‐ray induced polymerization suggests that radiation‐induced chain initiation is relatively insensitive to crystal strain. Chain propagation lengths as a function of conversion and the effects of external stress and impurities on reaction kinetics are also predicted using the model.

Solid‐state reaction kinetics in single‐phase polymerizations

The mechanical properties (Young's modulus, ultimate tensile strength, deformation processes) of extended-chain polydiacetylene crystals are investigated. The properties observed are similar to those of metal and ceramic whiskers. The elastic modulus is strain-dependent and the ultimate tensile strength increases with decreasing crystal size. The maximum tensile strength observed was 1700 Nmm−2. The ultimate tensile strength seems to be controlled by the presence of a small number of defects near the surface at which fracture nucleates. Irreversible deformation of the crystals was observed to occur by crack propagation normal and parallel to the direction of the macromolecules. The observed mechanical behavior corresponds to exceptionally high per-chain properties. The per-chain modulus obtained for these crystals is nearly as high as that of diamond. A chain-aligned polyethylene fiber with the same per-chain mechanical properties would have an ultimate strength as high as 0.9 × 104 Nmm−2.

N. Sendfeld

Papers

Deformation and microstructure of extended‐chain polydiacetylene crystals