Determination of the plastic behaviour of solid polymers at constant true strain rate

The methods of conventional tensile testing as applied to solid polymers are compared and reviewed critically. Experiments were performed using these techniques, and it is shown that large variations in local strain rate occur while necking and cold-drawing take place. A new tensile testing method is described in which the samples are tested atconstant local true strain rate. This technique is based on the use of a diameter transducer, an exponential voltage generator and a closed-loop testing machine. Flow curves for poly(vinyl chloride) and high density polyethylene were determined at room temperature over the strain rate range of 10^–1 to 10^–4 sec^–1. It is shown that the flow behaviour of these two polymers can be approximated by the constitutive relation: , whereK and are constants andm, the rate sensitivity, is in the range 0.02 to 0.06. It is concluded that the positive curvature of the log flow curve is responsible for the stabilization of flow localization associated with cold drawing, and that the rate sensitivity plays a much smaller role.     

Synthesis and application of functionalised acrylonitrile-butadiene rubber for enhancing recyclability of poly(vinylchloride) (PVC) and poly(methylmethacrylate) (PMMA) in recycled blends

The current investigation deals with the synthesis and application of functionalised acrylonitrile-butadiene rubber (NBR-g-MAH) as an impact modifier and compatibilising agent in the recycled blend of poly(vinyl chloride) and poly(methylmethacrylate). The performance of NBR as an impact modifier was improved by grafting with maleic anhydride. The varied concentration of NBR-g-MAH/NBR mixture has differed effects on the mechanical, thermal and morphological properties of the recycled blend. The recycled blend with 9 wt% of NBR-g-MAH/NBR mixture shows optimum impact strength and elongation-at-break. Moreover, Fourier-transform infrared spectroscopy analysis confirms interactions with polar functionalities in the recycled blends. Differential scanning calorimetry and scanning electron microscope analyses suggest partial miscibility as well as compatibility of the polymeric constituents in the blend. Additionally, thermal gravimetric analysis shows higher thermal stability for the modified recycled blend as compared to its parent blend.     

On the generality of discontinuous fatigue crack growth in glassy polymers

Fatigue fracture surface characteristics of five commercially available amorphous polymers [poly(methylmethacrylate) (PMMA), polycarbonate (PC), poly(vinyl chloride) (PVC), polystyrene (PS), and polysulphone (PSF)] as well as bulk-polymerized PMMA prepared over a wide range of molecular weights were studied to determine if common mechanisms of fatigue crack propagation prevail among these glassy polymers. In those polymers with viscosity-average molecular weight ¯M _v2×10⁵, the macroscopic appearance of the fracture surface showed the presence of a highly reflective mirror-like region which formed at low values of stress intensity and high cyclic test frequencies (100 Hz). The microscopic appearance of this region revealed that many parallel bands exist oriented perpendicular to the direction of crack growth and that the bands increase in size with K. In all instances, the crack front advanced discontinuously in increments equal to the band width after remaining stationary for hundreds of fatigue cycles. Electron fractographic studies verified the discontinuous nature of crack extension through a craze which developed continuously with the load fluctuations. By equating the band size to the Dugdale plastic zone dimension ahead of the crack, a relatively constant yield strength was inferred which agreed well with reported craze stress values for each material. At higher stress intensity levels in all polymers and all values of ¯M _v, another series of parallel bands were observed. These were also oriented perpendicular to the direction of crack growth and likewise increased in size with the range in stress intensity factor, K. Each band corresponded to the incremental advance of the crack during one load cycle, indicating these markings to be classical fatigue striations.     

Electrical conductivity of iodine adducts of nylon 6 and other non-conjugated polymers

Iodine-nylon 6 adducts containing 70 to 90wt% iodine have been prepared by heating iodine and nylon 6 at 115 and 145° C. The electrical conductivity () of the adduct increases with increase in the iodine content and the iodine-nylon 6 adduct containing 90wt% iodine and prepared at 145° C gives = 10^–3 S cm^–1 at 25° C. Infrared, nuclear magnetic resonance (¹H and¹³C{1 H}), and powder X-ray diffraction analysis of the adduct show a profound change of the structure around the amide group of nylon 6 and suggest the formation of a-C=NH⁺-group in the reaction of nylon 6 with iodine. The temperature dependences of of the idoinenylon 6 adducts prepared at 115° C give activation energies of 51 to 92 kJ mol^–1 depending on the iodine content. Addition of carbon powder to the iodine-nylon 6 adduct causes an increase in electrical conductivity. Other polymers (aliphatic and aromatic nylons, poly(vinyl alcohol), poly (tetrahydrofuran), poly(N-vinylpyrrolidone), poly (4-vi nylpyridine), and poly(acrylonitrile)) which have lone pair or -electrons also form iodine adducts containing 70 to 95 wt % iodine and the adducts show an electrical conductivity in the range of 10^–5 to 10^–2 S cm^–1. Among the iodine adducts, those of poly(vinyl alcohol) and poly (tetra hydrofuran) show electrical conductivities as high as 1.5 X 10^–2 S cm^–1 when the adducts contain about 90 wt% iodine.     

Time-domain dielectric relaxation in polyamide 6, poly(vinyl chloride), ethylene-vinyl acetate copolymer and poly(vinylidene fluoride)

The time-domain dielectric responses of polyamide 6 (PA6), poly(vinyl chloride) (PVC), ethylene-vinyl acetate copolymer (EVA) and poly(vinylidene fluoride) (PVDF) to a voltage step were measured at different temperatures. From the variation of the sample capacitance,C, with time, the ratioF _d/C was determined, whereF _d = (dC/dlnt)_max is the maximum (inflexion) slope of the capacitance versus log(time) dipole response curve, and C is the difference between the initial and the extrapolated equilibrium capacitance values. A modified KohIrausch-WiIIiams-Watts (stretched exponential) function provided a good fit to the measuredC(t) data. For low temperatures, typically below – 20°C,F _d/C, is about 0.1, characteristic of highly cooperative relaxation, while at higher temperatures the ratio approaches 1/e, characteristic of nearly independent (Maxwellian) relaxation. This is in contrast to corresponding analyses of mechanical relaxation in solids for which the constant is almost always near 0.1 at room temperature.     

Crystallisation and miscibility of poly(ethylene oxide)/poly(vinyl chloride) blends

Isothermal crystallisation of blends of Poly(ethylene oxide) and Poly(vinyl chloride), PEO/PVC, was analysed by differential scanning calorimetry (DSC). The influence of the amorphous polymer, PVC, on crystallisation rate of PEO was investigated using pure PEO as reference. Pure PEO and PEO/PVC blends were submitted to different crystallisation temperatures (from 40 to 58°C) and crystallisation times (from 1 to 72 h). Using the Hoffman-Weeks plot procedure, the equilibrium melting temperature, T _m°, was determined for pure PEO and for PEO/PVC blends with compositions (in wt%): 90/10, 80/20, 70/30, 60/40, 50/50, 40/60, 30/70 and 20/80. The lamellar thickness factor of PEO crystals for pure PEO and for the blends showed a strong decrease when the PVC content was higher than 60 wt%. A small depression in T _m° was verified as the composition of PVC was increased. From the depression in T _m° the polymer-polymer interaction parameter, ₁₂, was evaluated using the Nishi-Wang equation. The results indicate that the miscibility between PEO and PVC in the molten state depends on the blend composition. The crystallisation rate also depends on the blend composition: the richer in PVC is the blend, the slower the crystallisation process.     

Effect of hydrolysis on the properties of the copolymers of ethylene-co-vinyl-acetate: DSC and permeation

As soon as they are separated from their roots, fruits and vegetables exhibit multiple biochemical reactions consuming O₂ and producing H₂O and CO₂. To obtain a controlled atmosphere favorable for food conservation, packages must have high H₂O/CO₂ and O₂/CO₂ selectivities. It has been demonstrated that poly(ethylene-co-vinyl-acetate) films (EVA) of various vinyl acetate quantities are promising materials. By changing the content of vinyl-acetate (VA) in polyethylene (PE), it has also been observed that a composition of 70% w/w of VA in PE leads to a completely amorphous material. In this work, pseudo << bi-layer>> membranes, based on poly(ethylene-co-vinyl acetate) (EVA) containing 70 wt% of vinyl acetate, have been prepared by a treatment of unilateral hydrolysis using solutions of sodium hydroxide dissolved in a mixture of water and methanol. The influence of this hydrolysis treatment on O₂ and H₂O permeation properties is investigated. The existence of structural changes as a function of the hydrolysis duration (0–16 h) is checked by means of DSC. Without hydrolysis, a glass transition at T_g = –20 °C with C_p 0.56 J/g/K is obtained. By increasing the hydrolysis time, we find that T_g = constant, C_p decreases and an endothermic melting peak (125 °C) appears. These results lead to the conclusion that a crystalline phase occurs through hydrolysis. On the other hand, the O₂ permeability decreases with the reaction time, while the H₂O permeability passes through a maximum for a hydrolysis lasting 30 min.     

Fibre formation in the hydrostatic extrusion of linear polyethylene — sodium lignosulphonate blends

A blend consisting of equal amounts (by weight) of linear polyethylene and a technical grade of sodium lignosulphonate (a water soluble substance), was processed using hydrostatic extrusion with extrusion ratios varying from 5 to 20. The resulting extrudate contained thin (2 to 5m) fibres of the linear polyethylene in a matrix of the lignosulphonate. The fibres were basically continuous throughout the extrudate. Their stiffness was of the same order as that observed for pure hydrostatically extruded polyethylene. The fibre phase was easily isolated by dissolving the matrix material in water.     

Cell response of cultured macrophages, fibroblasts, and co-cultures of Kupffer cells and hepatocytes to particles of short-chain poly[(R)-3-hydroxybutyric acid]

The known biodegradability of poly[(R)-3-hydroxybutyric acid] (PHB) in certain biological environments has lead to its proposed use as biodegradable, biocompatible polymer. Recently, a new, rapidly biodegradable blockcopolymer has been synthesized that contains crystalline domains of PHB blocks. During degradation of these polymers, the PHB-domains are transformed in a first step into small crystalline particles of short-chain PHB. Therefore, particles of short-chain poly[(R)-3-hydroxybutyric acid] (M_n2300) (PHB-P), as possible degradation products, are investigated here for their effects on the viability and activation of macrophages, fibroblasts, and co-cultures of rat Kupffer cells and rat hepatocytes. Results obtained in the present study indicate that phagocytosis of particles of short-chain poly[(R)-3-hydroxybutyric acid] at high concentrations (higher than 10 g/ml) is dosedependent and associated with cell damage in macrophages but not in fibroblasts. At low concentrations, particles of PHB-P also failed to activate macrophages and are biocompatible. Besides the PHB phagocytosis by Kupffer cells, treatment of co-cultures of Kupffer cells and hepatocytes with 1 g PHB/ml showed neither cytotoxic (lactate dehydrogenase activity) effects nor any change in albumin secretion by hepatocytes.     

Effects of surfactants on the precipitation and properties of colloidal particles from forced hydrolysis of FeCl3-HCl solution

The forced hydrolysis of FeCl₃-HCl solution in the presence of surfactants such as sodium dodecyl sulphate (SDS), cetyltrimethyl ammonium chloride (CTAC), polyoxyethylene (20) nonylphenyl ether (NP-20) and poly(oxyethylene-oxypropylene) (PE-64) has been carried out and the precipitated particles were characterized by various techniques. No effect on the morphology of the precipitates was observed for CTAC and NP-20. The addition of a small amount of PE-64 progressed the particle growth but PE-64 yielded small lemon-shape -Fe₂O₃ particles at high concentration. On the other hand, the morphological uniformity of -Fe₂O₃ particles was impaired and they changed to needleor rod-like -FeOOH particles with increase in the SDS concentration. The -Fe₂O₃ particles formed with SDS showed a high selective adsorption of H₂O and CO₂, although -FeOOH particles have no such selectivity. The -FeOOH particles thus prepared exhibited a low porosity on the heat treatment in vacuo.     

The response of macrophages to particles of resorbable polymers and their degradation products

Alpha polyesters such as poly(L-lactide) and poly(glycolide) are biodegradable materials used in fracture fixation and they need to be assessed for problems associated with their degradation products. This study has compared cell responses to low molecular weight poly(L-lactide) particles, lactate monomer, poly(glycolide) particles and glycolic acid at cytotoxic and sub-cytotoxic concentrations. Murine macrophages were cultured in vitro and the release of lactate dehydrogenase (LDH), prostaglandin E₂ (PGE₂ and interleukin-1 alpha IL-1 was measured following the addition of particles or monomer. Experiments revealed that both the poly(L-lactide) and poly(glycolide) particles gave rise to dose dependent increases in LDH release and an increase in IL-1 and PGE₂ release. Comparisons of the poly(L-lactide) particles to the poly(glycolide) particles did not reveal any differences in their stimulation of LDH, IL-1 and PGE₂ release. The lactate and glycolate monomers did not increase PGE₂ or IL-1 release above control levels. There was no difference in biocompatibility between the poly(L-lactide) and poly(glycolide) degradation products both in particulate and monomeric form.     

Spherical and rod-like zinc oxide microcrystals: morphological characterization and microstructural evolution with temperature

Spherical ZnO microcrystals obtained by spray pyrolysis and thermal decomposition methods as well as rod-like ZnO particles (prismatic and needle shaped) prepared from precipitation in aqueous solutions, have been characterized by electron microscopy, X-ray diffraction and infrared spectroscopy. Very different sizes of ZnO particles were obtained from spray pyrolysis. However, only the larger particles (0.7 m) were found to be slightly deformed by infrared spectroscopy. From thermal decomposition of zinc acetate, fine particles of average size 0.05 m, rather spherical and agglomeration free were obtained. The role of initial size and morphology in the thermal evolution is fundamental: very fine spherical particles (0.01–0.02 m), can be sintered to give particles of 0.1–0.3 m at 875 °C with unchanged morphology. When the temperature induces a change in spherical shape, the first microstructural changes appear to take place through the crystallographic c-axis. However, for rod-like particles, changes begin from the a, b axes, being faster for needle-shaped microcrystals.     

Viscoelastic modelling and strain-rate behaviour of plasticized poly(vinyl chloride)

The effect of four different types of plasticizers and four strain-rates on the tensile behaviour of poly(vinyl chloride) (PVC) has been studied. di(2-ethylhexyl phthalate), benzyl butyl phthalate, epoxidized soyabean oil and chloroparaffin were mixed at different ratios and were used as plasticizers in concentration levels of up to 77% of the PVC weight. The plasticized and unplasticized PVC were processed into sheets by compression moulding. Tensile tests were conducted at different strain rates. It was found that tensile modulus increases with increasing strain rate while it decreases with increasing plasticizer concentration. The rate of variation of tensile modulus either as a function of strain rate and/or the plasticizer concentration was, in all cases, dependent on the mixing ratio of the different types of plasticizers. Assuming the material to be a linearly viscoelastic one, a simple viscoelastic model along with a least-squares-based computer procedure was applied which enabled us to fit the experimentally obtained curves with the respective theoretical predictions as well as to study the strain-rate effect on the relaxation spectrum, H(), of the material under consideration.     

New fast method for determination of number of UHMWPE wear particles

Ultra-high molecular weight polyethylene (UHMWPE) wear particles are the major cause of total joint replacement (TJR) failures because the wear particles, released from TJRs, cause bone loosening. To simplify the study of the relationship between numbers of particles at various locations around TJRs and extent of bone loosening at these locations, the authors of this work tried to develop a new method for easy and fast determination of number of wear particles. The method, called LSC (Light Scattering with Calibration spheres), is based on light scattering of a suspension of wear particles and calibration spheres, and yields relative numbers of particles. A modified LSC method, called LSCm, requires one additional experiment, a gravimetric analysis of a mixture of all studied samples, to determine absolute numbers of wear particles. LSC and LSCm methods are easy and fast, which make them suitable for processing and comparing high number of samples.     

Preparation of rod-shaped BaTiO3 powder

Rod-shaped BaTiO₃ powder particles have been prepared from rod-shaped TiO₂ ·nH₂O and BaCO₃ in molten chloride. The morphology of BaTiO₃ particles was studied referring to the effects of the chemical species of the starting titanium compound, amount of chloride, particle size of the titanium compound and reaction conditions, and the preparation condition of rod-shaped BaTiO₃ has been determined: i.e., large TiO₂ ·nH₂O particles were heated at 700°C in molten salt with an equal amount of BaTiO₃. This condition was effective in suppressing the formation of BaTiO₃ by a solution-precipitation process as well as the deformation of either TiO₂ ·nH₂O or BaTiO₃, which are responsible for the formation of equiaxed BaTiO₃ particles. The obtained rod-shaped BaTiO₃ particles had a cubic symmetry. Electron diffraction analysis showed that the following topotactic relation is retained; 0 1 0_{potassium tetratitanate} 0 1 0_{hydrated titania} 1 0 0_anatase 1 0 0_{barium titanate}     

Fatigue of rubber-modified epoxies: effect of particle size and volume fraction

A change in crack-tip plastic zone/rubber particle interactions induces a transition in the fatigue crack propagation (FCP) behaviour of rubber-modified epoxy polymers. The transition occurs at a specific K level, K _T, which corresponds to the condition where the size of the plastic zone is of the order of the size of the rubber particles. At K>K _T, rubber-modified epoxies exhibit improved FCP resistance compared to the unmodified epoxy. This is because the size of the plastic zone becomes large compared to the size of the rubber particles and, consequently, rubber cavitation/shear banding and plastic void growth mechanisms become active. At K>K _T, both neat and rubber-modified epoxies exhibit similar FCP resistance because the plastic zone size is smaller than the size of the rubber particles and hence, the rubber cavitation/shear banding and plastic void growth mechanisms are not operating. As a result of these interactions, the use of smaller 0.2 m rubber particles in place of 1.5 m rubber particles results in about one order of magnitude improvement in FCP resistance of the rubber-modified system, particularly near the threshold regime. Such mechanistic understanding of FCP behaviour was employed to model the FCP behaviour of rubber-modified epoxies. It is shown that the near threshold FCP behaviour is affected by the rubber particle size and blend morphology but not by the volume fraction of the modifiers. On the other hand, the slope of the Paris-Erdogan power law depends on the volume fraction of the modifiers and not on the particle size or blend morphology.     

The effect of compatibilizers on the morphology of isotactic polypropylene/linear low-density polyethylene blends

The morphology of isotactic polypropylene (iPP)/linear low-density polyethylene (LLDPE) blends, compatibilized with ethylene-propylene block copolymer (EP) and two types of styrene-ethylene/butylene-styrene triblock copolymer (SEBS), one containing maleic anhydride, the other no reactive sites, has been investigated by using small-angle X-ray scattering by evaluating their interface distribution functions. To characterize the crystallization behaviour of the blends, their spherulitic growth rates have been measured under the polarizing microscope and nucleation and crystallization kinetics data have been evaluated. The addition of LLDPE to iPP alone has a pronounced effect on the lamellar morphology of the iPP. Adding compatibilizer to the iPP/LLDPE blend leads to a further decrease of the lamellar thickness. Concurrently the nucleation density increases while the Avrami exponent drops from n2.3 for iPP to n=0.74 for the iPP/LLDPE/SEBS blend. It is concluded that the compatibilizer causes the polyethylene component to become more highly dispersed in the polypropylene matrix.     

Effect of impact modification on slow crack growth in poly(vinyl chloride)

The effect of impact modification on slow crack growth in a poly(vinyl chloride) (PVC) compound was examined in order to test a methodology for predicting long-term creep fracture from short-term tension-tension fatigue tests. In all cases the crack propagated in a stepwise manner through a crack tip craze zone. Step length was analyzed in terms of the Dugdale model for a crack tip plastic zone. The overall crack growth rate in fatigue and creep followed the conventional Paris power law with the same power 2.7, da/dt = A _f K _I ^2.7 and da/dt = BK _I ^2.7,respectively. The effects of frequency, temperature, and R-ratio (the ratio of the minimum to maximum stress intensity factor in the fatigue loading cycle) on the Paris prefactor were determined. Crack growth rate was modeled as the product of a creep contribution that depended only on the maximum stress intensity factor and a fatigue contribution that depended on strain rate da/dt = B _f K _I,max ^2.7 (1 + C , where C is a coefficient defining the strain rate sensitivity. A linear correlation allowed for extrapolation of the creep prefactor B _f from fatigue data. Impact modification decreased B _f but had no effect on C.     

VAE/PVC-VDC共混乳液的研究

此共混乳液采用乙烯－醋酸乙烯共聚物（ＶＡＥ）乳液和氯乙烯－偏氯乙烯共聚物（ＰＶＣ－ＶＤＣ）乳液以机械共混方法制得。考察了其微观形态、流变性及其成膜后的力学性能等，发现共混乳液具有一定的相容性，其拉伸强度与ＶＡＥ胶膜相比有很大的提高。     

Morphology of single crystals of poly (hydroxybutyrate) and copolymers of hydroxybuty rate and hydroxyvalerate

Single crystals of poly(hydroxybutyrate), and copolymers of hydroxybutyrate and hydroxyvalerate, have been grown from a variety of solvents and their morphology studied. In all cases the crystals appear elongated, with the crystallographic a axis along the long axis of the crystals. Screw dislocations (which act as branch points and can lead to large crystal aggregates) have been observed, and solvents, e.g. octanol in which more of these dislocations occur, have been identified. Decoration of the crystals with polyethylene shows that the fold plane is the (0 1 0) plane, while crystallographic and crystallinity arguments require successive adjacent folds to be along 1 1 0 directions. It is argued that folding is along [1 1 0] on the top, and [1 ¯1 0] on the bottom surface of a crystal.