A Raman microscopy study of stress transfer in high-performance epoxy composites reinforced with polyethylene fibers

Raman spectroscopy is shown to be a very powerful method for the study of stress transfer in epoxy composite materials reinforced with high-performance polyethylene (PE) fibers. We found that the stress transfer length as determined by Raman spectroscopy is substantially shorter for a plasma-treated fiber than for an untreated one. A shorter stress transfer length indicates stronger interactions between fiber and matrix. Furthermore, the stress transfer length was higher for a PE fiber/epoxy matrix cured at a higher temperature.     

The Effect of Degradation and Stabilization on the Mechanical Properties of Polymers Using Polypropylene Blends as the Main Example

Degradation can result from a variety of chemical, physical and mechanical mechanisms, most of them involving a reduction of molecular weight and thus a decrease in the mechanical performance of the degraded polymer. A clear understanding and control of these mechanisms is absolutely essential: without stabilization some polymers (e.g. PVC, polyolefins) would not survive their processing undamaged. In this paper an overview of the different degradation mechanisms, their effect on molecular chains, and the methods used to characterize the extent of degradation will be given. Subsequently we establish some fundamental relationships between the microstructure and the mechanical performance (of thermoplastic polymers) using differently aged and stabilized polypropylene (PP) - EPR compounds. In particular we investigate the influence of two types of heat stabilizers (phenolic antioxidants and hindered amine stabilizer HAS) on the degradation behaviour of test specimens thermally aged at 120 and 135°C respectively. From an investigation of the changes with aging time in structure and low-strain properties (yield stress, strain at yield, tensile modulus) and from the differences in the evolution of the fracture properties a molecular model of the chain scission mechanisms and of inter-lamellar connectivity (through tie-chain molecules) has been established, which allowed an explanation of the gradual change of the dominant deformation mechanism from cold drawing to crazing and brittle fracture.     

Deformation and entanglement in semicrystalline polymers

Transmission electron microscopy (TEM) of amorphous and semicrystalline isotactic polystyrene (iPS) thin films deformed well below T _g, suggests the same crazing mechanism to operate in both cases. Therefore, by analogy with the amorphous case, highly entangled semicrystalline polymers, such as poly(ether ether ketone) (PEEK) should craze less readily in the glassy-semicrystalline state than iPS, which has a low degree of entanglement. Since this is confirmed by observation, it is reasonable to extend this analogy, and invoke entanglement in order to account for the high fracture resistance of highly entangled semicrystalline polymers, such as polyoxymethylene, using models previously applied to glassy amorphous polymers. There are nevertheless often significant decreases in fracture toughness in polyoxymethylene as the crystallization temperature is raised and/or the molecular weight is reduced, which we attribute to entanglement loss during lamellar folding.     

Literatur

Ohne Zusammenfassung     

The role of chain length and conformation in stress-transmission and fracture of thermoplastic polymers

In this paper, a review of the molecular aspects of fracture is given, a subject that was pioneered by S. N. Zhurkov and his colleagues. Particular attention will be paid to the mechanisms of stress transfer onto straight chain segments, the role of chain interpenetration in establishing interfacial strength during crack healing, to the concept of taut tie molecules, to stress distribution in UHMWPE fibers, and to the possible role of chain ruptures in the deformation process of fibres. Using Raman microscopy, it is observed that some chains are exposed to stresses of up to 10 GPa, which is close to their estimated strength. From these experiments, a mechanical model of the organization of almost fully oriented UHMWPE fibers is developed accounting also for the presence of numerous and dispersed defects. The principal deformation mechanisms are chain slippage, crystal plasticity, and intra-and intermicrofibrillar slippage.     

Chain scission in transient extensional flow kinetics and molecular weight dependence

Recent experimental investigations have shown that large macromolecules can be fully stretched and fractured in an extensional flow. In this situation, the critical strain-rate for bond scission was found to depend on molecular weight as (M_W)⁻², in agreement with the theoretical predictions of the bead---rod model. One of the conditions prerequisite to full chain extension is that the residence time at the appropriate strain-rate must be much larger than the terminal relaxation time of the macromolecule. If this requirement is not fulfilled, chain fracture could still occur at sufficiently high strain-rate, but in a partially uncoiled state. In the present studies we have measured the critical strain-rate for chain scission in transient extensional flow of extremely sharp PS fractions dissolved in dekalin at the θ-temperature. The molecular weight range investigated varied from 2.86 × 10⁶ to 426,000. The critical strain-rate for chain scission was found experimentally to scale as (M_W)^−0.95 instead of (M_W)⁻² as predicted for stagnant extensional flow. Our results are in good accord with a recent theory for rupture of partly extended coils. Even in the partially uncoiled state, the degraded macromolecules showed a remarkable propensity for chain halving, indicating that midchain scission in flow is a general property that is not uniquely reserved to the fully extended chain.