Application of activated barrier hopping theory to viscoplastic modeling of glassy polymers

YesAn established statistical mechanical theory of amorphous polymer deformation has been incorporated as a plastic mechanism into a constitutive model and applied to a range of polymer mechanical deformations. The temperature and rate dependence of the tensile yield of PVC, as reported in early studies, has been modeled to high levels of accuracy. Tensile experiments on PET reported here are analyzed similarly and good accuracy is also achieved. The frequently observed increase in the gradient of the plot of yield stress against logarithm of strain rate is an inherent feature of the constitutive model. The form of temperature dependence of the yield that is predicted by the model is found to give an accurate representation. The constitutive model is developed in two-dimensional form and implemented as a user-defined subroutine in the finite element package ABAQUS. This analysis is applied to the tensile experiments on PET, in some of which strain is localized in the form of shear bands and necks. These deformations are modeled with partial success, though adiabatic heating of the instability causes inaccuracies for this isothermal implementation of the model. The plastic mechanism has advantages over the Eyring process, is equally tractable,and presents no particular difficulties in implementation with finite elements.F. Boutenel acknowledges an Erasmus Programme Scholarshi

Double torsion testing of prescription lenses

In previous work, a modified double torsion test was developed to measure the fracture toughness of polymeric ophthalmic lenses with parallel surfaces. The method used production lenses so that the toughness of the manufactured articles could be measured directly. This method has now been extended to measure the K1c of powered lenses in which the thickness of the sample varied with both X and Y coordinates instead of being constant. A computer model of the sample's breaking load was constructed and the predicted values from this were compared to experimental results and a good correlation was obtained. In addition, the computer-generated correction factors were validated using the more rigorous compliance curve technique

The effect of curative on the fracture toughness of PTMEG/TDI polyurethane elastomers

The mechanical properties and fracture behaviour of polyurethanes with two hardness levels have been measured. The polyurethanes were based on polytetramethylene ether glycol (PTMEG) and toluene diisocyanate (TDI) and two different curatives, or chain extenders - methylene-bis-orthochloroaniline (MBOCA) and dimethylthiotoluene diamine (Ethacure E300). The polyurethanes made using MBOCA as the curative were found to have a lower initial modulus and less permanent set than polyurethanes of similar hardness but made using Ethacure E300. Moreover, the Ethacure E300 materials showed crack growth when the strain energy density in the sample was low, and a period of slow stable crack advance. The MBOCA cured material of lower hardness also showed slow crack growth, but crack growth started at much higher strain energy densities than for the Ethacure E300 materials. The harder MBOCA cured material did not show slow crack growth and failed rapidly. The strain energy density to cause fracture was higher than that required to initiate slow crack growth in the other materials. The reasons for the differences in the fracture behaviour are discussed in terms of the structure of the polyurethanes

The tensile deformation behaviour of a transparent ABS polymer

A transparent ABS polymer has been tested in uni-axial tension over a range of strain rates. The effect of strain rate on the yield stress has been explored and the magnitude of the activation volume obtained. By simultaneously monitoring volume strain and longitudinal strain, the contribution of crazing to the total deformation has been derived and shown to vary with strain rate. A detailed optical and electron microscopy study of the crazing and fracture behaviour of this material has also been conducted

High resolution transmission electron microscopy (HRTEM) and image analysis of uPVC thin films

The current model for the fine scale microstructure of unplasticized PVC (uPVC) has been built up using various quantitative techniques, such aa wide and small angle X-ray diffraction. The use of transmission electron microscopy (TEM) as a more qualitative technique has been previously attempted (Meyer et al., 1978), revealing only amorphous detail on the scale of the expected crystallinity. For this study, a combination of high resolution TEM (HRTEM) and digital image processing techniques was utilized in order to verify the 'micro-domain' model proposed by Summers (1981). The techniques utilized here are particularly applicable in examining the fine-scale texture of oriented polymers

The fracture behaviour of ABS polymers

The fracture behaviour of three different grades of ABS has been investigated over a range of temperature and strain-rates. A two-stage fracture process has been observed consisting of a slow crack propagation region followed by catastrophic failure. The surface features in these two regions have been examined in detail using scanning electron microscopy. In all cases, the fracture initiation site was located at either a surface defect or at an internal inclusion much larger than the average rubber particle size

The fracture of notched tensile specimens of a transparent ABS polymer

The fracture of notched samples of a transparent ABS polymer has been studied. The role of crazing in the fracture of these samples has been investigated by examining transverse and longitudinal sections of the specimens before and after fracture, and by following crack growth with a 16 mm movie camera. The fracture surfaces have also been studied by SEM. A three-stage fracture process has been found. Initial crack growth has been observed through a craze in the craze bundle which formed at the notch, producing a highly whitened region on the fracture surface. The crack continually accelerated and, at a later stage in the fracture, was shown to jump between crazes in the craze bundle, leaving islands of whitened material on the fracture surface. When the crack appeared to catch up with the tip of the craze bundle, a third banded region has been observed on the fracture surface

Effect of flyash content and coupling agent on the mechanical properties of flyash-filled polypropylene

Flyash to 20 wt% was added to polypropylene and an ethylene/propylene copolymer with and without the addition of a maleic anhydride grafted polypropylene coupling agent. The tensile mechanical properties were assessed and the fracture toughness measured by using notched impact specimens. The modulus of the materials increased with the addition of the filler and further increased with the addition of the coupling agent while the yield stress decreased with the addition of 10 wt% filler and no coupling agent but was unchanged from the matrix when the coupling agent was added. Elongation to break was significantly less in those composites to which the coupling agent had been added. Scanning electron microscopy showed that bonding between the matrix and the filler was significantly improved by the addition of the coupling agent and this appeared to hinder the drawing mechanisms in the polymer. The fracture toughness in the copolymer based composites was seen to decrease with filler content

The enzymatic hydrolysis of starch-based PVOH and polyol plasticised blends

Thermoplastic starch (TPS) materials present several advantages to the plastic industry and when blended with other materials they can exhibit improved mechanical and moisture sensitivity properties compared to pure TPS materials. However, the biodegradability of these blends, through such processes as enzymatic degradation, needs to be characterised to ensure the beneficial properties of TPS are not compromised. The aims of the study were to investigate the effect of varying polyvinyl alcohol (PVOH) content and polyol type within the TPS blends on the rate and extent of starch enzymatic hydrolysis using enzymes alpha-amylase and amyloglucosidase. The results of this study have revealed that TPS:PVOH blends with a PVOH content at 50 wt% exhibited a significantly reduced rate and extent of starch hydrolysis. The results suggest that this may have been attributed to interactions between starch and PVOH that further prevented enzymatic attack on the remaining starch phases within the blend. The extent of starch hydrolysis was not significantly affected by polyol type, however, the rate of starch hydrolysis from the maltitol blend was significantly reduced compared to sorbitol and glycerol substrates. Crown Copyright (C) 2009 Published by Elsevier Ltd. All rights reserved

Inverse gas chromatography for natural fibre characterisation: Identification of the critical parameters to determine the Brunauer-Emmett-Teller specific surface area

Inverse gas chromatography (IGC) is an alternative technique to determine the specific surface area of natural fibres. Natural fibres have a complex surface chemistry and unique microstructure that challenge the current capabilities to perform surface characterisation. This study investigated the influence of multiple parameters on the measured Brunauer–Emmett–Teller (BET) specific surface area for samples of flax, kenaf and BioMid cellulose fibres using IGC. The BET surface area of kenaf and flax differed with 0.51\ua0m\ua0g and 1.35\ua0m\ua0g respectively, the former being similar to the cellulose fibres (0.54\ua0m\ua0g). The data was calculated under conditions where the BET equation showed good linearity (R\ua0⩾\ua00.995). Repeatability was excellent so that two runs sufficed to obtain representative BET surface area values. The findings showed the choice of solvent was important for all specimens to avoid any misleading data comparison due to molecular orientation effects that impact the adsorbent–adsorbate interactions. The higher surface area of the flax sample, and its higher variability, was correlated with a higher surface roughness observed under optical microscopy. Packing the chromatography column with long or chopped fibres produced results that were statistically insignificant