Paul D. Garrett

Deformation mechanism and fibre toughening of nylon 6,6

Abstract The effects of glass-fibre reinforcement on the fracture toughness, KIC, of nylon 6,6 were examined and the deformation mechanisms of unreinforced nylon 6,6 were studied by varying the deformation rate, by dilatational measurements and by i.r. spectroscopy. In the unreinforced nylon 6,6 a flow stress plateau was observed in the stress-strain behaviour prior to the onset of necking. Of the 25–30% inelastic strains stored in this plateau a substantial portion appears to be related to a crystallographic plastic deformation due to the crystalline segments in nylon 6,6. In glass-fibre-reinforced nylon 6,6 a brittle to ductile transition was found to occur when the mean fibre-end spacing was less than a critical value. The observed brittle-ductile transition was found to originate from an observed enhanced matrix plasticity at fibre ends when the glass fibres are sufficiently closely spaced. Such enhanced localized plasticity at fibre ends was suggested to result from interactions of stress fields with nearby fibre ends when the fibre-end spacing is less than the critical value. It is further postulated that the enhanced localized fibre-end plasticity is made possible due to the ability of the matrix to exhibit a large degree of crystallographic plasticity. The toughening behaviour of fibre-reinforced nylon 6,6 was also compared qualitatively to that of rubber-toughened nylon 6,6 and a general principle for microstructural toughening in nylon 6,6 was addressed. Strategies for fibre toughening in fibre-reinforced nylon 6,6 were also discussed.

Fracture resistance of a glass-fibre reinforced rubber-modified thermoplastic hybrid composite

Toughening mechanisms in a hybrid amorphous thermoplastic composite containing both distributed rubber particles and rigid glass fibres have been investigated. Tensile properties were measured for a range of materials with varying rubber particle and glass-fibre contents, and different rubber particle sizes. Fracture toughness was characterized by separating the overall fracture into its initiation and propagation components. Deformation and fracture modes at crack tips were optically characterizedin situ during loading. The results indicate that both initiation and propagation toughness are enhanced by rubber particle additions to the glass-fibre reinforced composite. Synergistic effects between glass fibres and rubber particles are identified: for example, glass fibres inhibit crazing at rubber particles, and rubber particles tend to promote crazing at fibre/matrix interfaces and also void initiation at fibre ends. Toughening mechanisms are discussed in the light of available models.

Analysis of adhesion and interface debonding in laminated safety glass

Abstraet-A tension adhesion test is developed and analyzed to characterize polymer/glass adhesion in laminated safety glass. Laminated safety glass typically consists of two sheets of glass bonded by a polymer adhesive interlayer, usually plasticized poly(vinyl butyral). In the tension test, a rectangular laminate specimen is cracked on both glass sides, and loaded under a remote tension P at a constant displacement rate. The load on the specimen typically rises to a peak or deflection point load, P*, and the value of P* depends on the adhesion between the polymer interlayer and the glass. The peak load P* can be used as a characterization of the adhesion strength of the interface, but does not give a sample geometry-independent value for the adhesion. However, using a micromechanical model of debonding and a detailed finite element computation, the experimental data can be analyzed to determine the fracture toughness, and shear and normal strengths of the interface. The relationship between the peak load...

Effect of glass-fibre reinforcement and annealing on microstructure and mechanical behaviour of nylon 6,6

The effects of glass-fibre reinforcement and annealing on the deformation and fracture behaviour of nylon 6,6 were investigated. The roles of glass fibres were examined by varying the glass fibre content and the fibre length, and by in situ fracture studies in front of crack tips. The effects of microstructural changes were investigated by imposing various annealing conditions on the specimens. The results indicated that the fracture toughness showed a sharp decrease due to stress concentrations at fibre ends when the fibre volume fraction was small. Above a critical fibre volume fraction, it was found that the fracture toughness can be substantially increased by enhanced localized matrix plasticity at fibre ends. The competing roles of glass fibre ends were consistent with microstructure sensitive fracture mechanics models of failure based on the attainment of a critical stress or strain over a critical microstructural distance in the crack-tip region. Upon annealing above a critical annealing time the unreinforced nylon 6,6 showed a drastic decrease in the strength and ductility, corresponding to a loss of the constant-load deformation region prior to necking. However, the fracture toughness of unreinforced nylon 6,6 was only moderately reduced by annealing. On the other hand, the fracture toughness of the composites showed a significant increase upon annealing. The combined effects of glass fibres and annealing on microstructures and overall property optimization of the composites are also discussed.

Adhesion enhancement in immiscible polymer bilayer using oriented macroscopic roughness

The use of oriented macroscopic roughness to enhance the effective adhesion between two immiscible polymers was demonstrated. Bilayer specimens of polycarbonate and poly(styrene-co-acrylonitrile) (SAN) were produced with rough interfaces by scribing grooves of varying depths and spacings into the polycarbonate (PC) before joining the layers. The SAN, having a significantly lower glass transition temperature than polycarbonate, flows into the grooves during annealing at temperatures just over the PC glass transition, creating a mechanically interlocking interface. Subsequent measurements of bilayer interfacial fracture toughness showed up to a twenty-fold increase from that of a smooth interface when grooves were oriented perpendicular to the direction of interfacial crack propagation. The increase in toughness was shown to be greater as grooves were spaced closer together, and as groove depths were increased. Propagation of interface cracks followed a stick-slip mechanism, slowing considerably at each groove. Analysis of fracture surfaces indicates the increase in toughness to be mainly due to cohesive failure and deformation of the polymers at the grooves. Interface toughness was also enhanced with grooves scribed parallel to the propagation direction, but to a lesser degree than with perpendicular orientation.

Diffusion in blends of poly(methyl methacrylate) and poly(styrene-co-acrylonitrile)

Forward recoil spectrometry was used to obtain the tracer ( D *) and mutual ( D ) diffusion coefficients in a miscible polymer blend of poly(methyl methacrylate) (PMMA, T g =136°C) and poly(styrene- co -acrylonitrile) with ∼23 wt% acrylonitrile content (SAN, T g =112°C). For blends with SAN weight fraction w of 0.2 and 0.5, the temperature dependence of D* for both species was nearly identical. Tracer diffusion coefficients D * PMMA and D * SAN were determined for matrices consisting of 176 000 molecular weight SAN ( w =0.5) and various molecular weights of PMMA ranging from 27 000 to 840 000. The results were consistent with those expected from the mechanisms of reptation and constraint release. Analysis of the tracer diffusion coefficients D * showed that SAN has a monomer friction coefficient, ζ 0,PMMA , about five times smaller than that of PMMA (ζ 0,PMMA ) in a matrix of pure PMMA, but the difference decreased monotonically as w increased, so that the ζ 0 values were nearly equal when w =1. Corresponding to this relative change in ζ 0 , the glass transition process is broad for PMMA-rich blends and narrow for SAN-rich ones, raising the possibility that the difference in ζ 0 in the PMMA-rich blends is due to the existence of two local glass transitions, one for each species in the blend. For blends at the composition of w =0.5, the D was measured as a function of PMMA molecular weight. The data followed closely the predictions of the fast theory, the result expected if D is ultimately controlled by the diffusion of the faster moving species. From the measurements of D and D * at 187°C, the composition dependence of the Flory interaction parameter was also obtained, which showed good agreement with the recent small angle neutron scattering results by Hahn et al .

Surface segregation in blends of styrene-acrylonitrile copolymers

We investigated the kinetic as well as thermodynamic aspects of surface segregation in blends of random copolymers of styrene and acrylonitrile (SAN) with different acrylonitrile contents (22.5 and 27.2 wt%) annealed at 163°C. A comparison was made with equilibrium data collected by Mansfield et al. on the same system using neutron reflectivity. The lower-AN-content SAN, which is labelled with deuterium (d-SAN), was observed to segregate to the vacuum/polymer interface. Time-of-flight and conventional forward recoil spectrometry were used to determine the depth profile of the deuterated component. The apparent diffusion coefficient D app controlling the segregation kinetics at a bulk volume fraction ω of d-SAN of 0.34 was obtained. It was smaller by a factor of approximately 1.5 than the mutual diffusion coefficient D app at =0.34 calculated without taking the expected thermodynamic slowing of diffusion into account. However, D app was larger than the measured D at =0.34 by a factor of approximately 3. The mean-field theory of Schmidt and Binder was employed to extract the Flory interaction parameter χ , which was found to be smaller than but close to its critical value χ c . The value of D computed using this χ value was in good agreement with the independently measured D at= 0.34.