K. L. Devries

Reinforcement and environmental degradation of nylon-6/clay nanocomposites

Hybrid organic/inorganic nanocomposites are being developed to improve the physical and mechanical properties of polymeric materials without adversely effecting their processing characteristics. One such nanocomposite developed by Toyota and commercialized by Ube Industries is the nylon-6/montmorillonite clay nanocomposite. The mechanism of reinforcement in nylon-6/clay nanocomposite materials is investigated through tensile experiments, infrared absorption spectrography, and dynamic mechanical analysis. 200% improvements in modulus and 175% improvements in yield stress are attributed to the complexation of mid-chain carbonyl groups with the exfoliated clay lamellae. Because of the initial use of these materials in automotive components, and the known deleterious effects of the air pollutant NOx on nylon-6, the degradation of the nanocomposites in NOx was examined through post-exposure tensile experiments. It was found that NOx degrades the mechanical performance of the nanocomposites regardless of the constraining effect of clay lamellae.

Reaction‐Rate Model for Fracture in Polymeric Fibers

Electron paramagnetic resonance (EPR) techniques were used to determine the number of free radicals produced during deformation leading to fracture of nylon 6 fibers. A reaction‐rate molecular model is proposed to explain some of the deformation and bond‐rupture behavior leading to fracture. High‐strength polymer fibers are assumed to consist of a sandwich structure of crystalline‐block and amorphous‐flaw regions along the fiber axis. In the flaw regions, tie chains connecting the crystalline blocks are assumed to have a statistical distribution in length. These chains are, therefore, subjected to different stresses. The length distribution was determined by EPR. The probability of bond rupture was assumed to be controlled by reaction‐rate theory with a stress‐aided activation energy and behavior of various loadings determined by numerical techniques. The model is successfully correlated with experimental stress, strain, and bond‐rupture results for creep, constant‐rate‐of‐loading, and cyclic‐stress tests.

Molecular aspects of high polymer fracture as investigated by ESR-technique

The investigation of stress induced chain scission by the electron spin resonance (ESR) technique has led to detailed information on number, location, and production kinetics of molecular fracture points. These data are reported here. The possible impact of these ESR results on existing statistical, mechanical, kinetic, and continuum-mechanical fracture models is discussed.

Second normal stress difference of a Boger fluid

Abstract Measurement of the second normal stress difference for a highly elastic, constant viscosity ‘Boger fluid’ is reported. Two very different experimental techniques have been used: (1) measurement of the height of the free surface in rod-climbing flow; (2) measurement of the pressure distribution in cone-and-plate shearing flow. The second normal stress difference is at least 30 times smaller in magnitude than the first normal stress difference, and opposite in sign. The results should prove useful in distinguishing between the various constitutive equations proposed for Boger fluids.

The Effects of Plasticity in Adhesive Fracture

Abstract From the viewpoint of continuum mechanics, and particularly the energy concept of fracture, adhesive and cohesive failures are similar. The essential difference involves the interpretation of the energy required to create new (adhesive or cohesive) surface area. This fracture mechanics approach has in the past been applied to a number of different elastic problems. In this investigation an elastic-perfectly plastic analysis for adhesive failure of a beam is presented. This analysis accounts for the energy dissipated during plastic bending. Experimental results with 6061-T6 alumimum are presented as evidence of the validity of the approach.

Free radicals and new end groups resulting from chain scission: 1. y-Irradiation of polyethylene

Abstract Measurements of the concentrations of free radicals by electron spin resonance (e.s.r.) and of new chemical species by Fourier transform infrared spectroscopy (FT i.r.) were carried out on polyethylene specimens exposed to γ-irradiation at dosages from 3 to 50 Mrads both in the presence and absence of oxygen. The improved signal-to-noise capability of the FT i.r. method permitted a direct comparison of the free radical concentration and the resultant concentration of new chemical groups. It was found that approximately 10 carbonyl groups and 2 carbon-carbon double bonds were formed per free radical. These results are comparable with previous estimates and form the basis for an investigation of chemical species formed during mechanical deformation and fracture which will be discussed in the second paper of this series.

The influence of loading direction upon the character of adhesive debonding

Abstract Adhesive fracture surfaces have a topology that is affected by the type or mode of applied loading that induced the separation. In fracture mechanics these modes are commonly designated as I, II, and III, identified primarily with normal separation, sliding shear, and rotational shear, respectively. When the energy per unit area required to create new fracture surface, e α (ergs/cm 2 ) is measured, it is common practice to associate this area with the projected fracture area on the fracture plane irrespective of the actual surface topography. Hence the ratio of actual to projected fracture area introduces a perhaps artificial variation into the deduced value of fracture energy, which otherwise might turn out to be more of a universal (time-temperature dependent) quantity, independent of mode of loading. The paper describes the qualitative SEM fractrographic analysis conducted for adhesive debonds between polyurethane and polymethylmethacrylate. This analysis tends to explain part of the apparent differences in e α when deduced from different loading modes, and hence should put us closer to relating the continuum mechanic deductions for e α to more fundamental parameters of the molecular systems.

Free radicals and new end groups resulting from chain scission: 2. Mechanical degradation of polyethylene

Abstract The number of chain scissions accompanying mechanical degradation of polyethylene has been estimated from i.r. analysis of new end group concentrations. Polyethylene specimens fractured in tensile deformation and ground under liquid nitrogen were examined. The results are compared to the number of free radicals generated during mechanical degradation and measured by e.s.r. In comparison with previous results in the literature our results are lower by 1–2 orders of magnitude and in better agreement with estimates of the number of chain scissions from viscosity measurements. An ultra high molecular weight polyethylene was examined as a control specimen containing few end groups. The change in the number of vinyl groups resulting from grinding of this specimen were estimated to be at least an order of magnitude lower than that found for lower molecular weight polyethylenes. This finding suggests that large errors may be introduced into the determination of concentrations of end groups through subtraction of relatively intense absorption bands.

Structure changes caused by strain annealing of nylon 6 fibers

Abstract Experiments are described in which nylon 6 fibers are annealed while subjected to a constant stretch (or slack). Subsequent mechanical and structural measurements are described and analyzed. A paracrystalline structure model is proposed in which folded chains, fully extended chains, partially folded, and partially extended chains coexist in the highly drawn high strength fiber. An explanation of structural changes occurring during the thermal-mechanical treatment is that the folded and partially folded chains are arranged randomly in staggered fashion in small units throughout the structure. During slack annealing, the chains become more folded and shrinkage occurs. Some of the chain refolding will be permanent and may act as new defect sites thereby reducing fracture stress. During annealing in the presence of comparatively high tensile stresses the folded chains are unfolded to some extent, but not completely, and the load-carrying chains in the structure are more uniformly loaded. At the highe...

Macroscopic, microscopic and molecular aspects of fracture in polymers

Abstract The distribution of stress at macroscopic and molecular levels can dramatically affect mechanical properties. This paper explores both these aspects. In the first part, quenching operations for polycarbonate and polystyrene were shown to develop favorable residual stresses as well as structural alterations (as manifested by changes in density, hardness, DSC results, etc.). The changes in these glassy polymers can be accompanied by as much as an order of magnitude increase in impact strength and fatigue life. In the other phase of our study, various analytical methods were used to investigate phenomena associated with fracture in oriented semi-crystalline polymers. In the studies reported here, the combined effects of stress and environmental agents on mechanical strength of nylon, polyethylene, and Kevlar fibers were measured. These results, in conjunction with investigations of bond rupture kinetics, suggest that fracture in these materials involves thermally activated chain scission in which the activation energy is aided by stress and the chemical environment. Different mechanisms appear to dominate fracture in spherulitic forms of chemically similar polymers.