A. Hiltner

Oxygen barrier properties of crystallized and talc-filled poly(ethylene terephthalate)

The improvement in oxygen barrier properties of poly(ethylene terephthalate) (PET) by incorporation of an impermeable phase such as crystallinity or talc platelets was examined. Crystallinity was induced by crystallization from the glassy state (cold crystallization). Microlayering was used to create talc-filled structures with controlled layer architecture. The reduction of permeability in crystallized and talc-filled PET was well described by Nielsens model. Changes in permeability of crystalline PET could not be ascribed to the filler effect of crystallites only. Our data on solubility, obtained on the basis of measurements of the oxygen transport coefficients, confirmed a previous finding that the amorphous phase density of PET decreases upon crystallization. The data were amenable to interpretation by free volume theory. Talc-filled materials processed by different methods showed the same permeability; however, much better mechanical properties were achieved by microlayering.

Thickness effects in microlayer composites of polycarbonate and poly(styrene-acrylonitrile)

Coextruded microlayer sheet consisting of alternating layers of polycarbonate (PC) and styrene-acrylonitrile copolymer (SAN) exhibits improved properties such as toughness and ductility as the number of layers is increased. In this study, the composition was kept essentially constant, as was the sheet thickness at 1.2 mm, and the layer thickness was changed by varying the total number of layers from 49 to 776. All the compositions exhibited macroscopic yielding in uniaxial tension but the fracture strain, which represents neck propagation, increased with the number of layers. The increased ductility was attributed to a transition in the microdeformation behaviour observed when microspecimens were stretched in the optical microscope. When the layers were thicker, individual layers exhibited behaviour characteristic of the bulk, that is SAN crazed or cracked while shear bands initiated in PC from the craze tips. As the layer thickness decreased, crazing or cracking of the SAN was suppressed and shear bands that extended through several layers produced shear yielding of both PC and SAN. Calculations showed that when the layer thickness is sufficiently small, impingement of a PC shear band on the interface creates a local shear stress concentration. As a result the shear band continues to grow through the SAN layer and subsequently, at the point of instability, shear yielding can occur in both PC and SAN layers.

Oxygen-barrier properties of oriented and heat-set poly(ethylene terephthalate)

The improvement in the oxygen-barrier properties of poly(ethylene terephthalate) (PET) by orientation and heat setting was examined. Orientation was carried out at 65 °C by constrained uniaxial stretching to a draw ratio of about 4. Heat setting was performed at temperatures from 90 to 160 °C with the specimen taut. Orientation decreased the permeability of PET to almost one-third that of the unoriented, amorphous polymer because of decreases in both the diffusion coefficient and the solubility coefficient. The proposed two-phase model for oriented PET consisted of a permeable isotropic amorphous phase (density = 1.335 g/cm3) with ethylene linkages predominately in the gauche conformation and an impermeable oriented phase (density = 1.38 g/cm3) with ethylene linkages that had transformed from the gauche conformation to the trans conformation during stretching. Chain segments in the trans conformation did not possess crystalline order; instead, they were viewed as forming an ordered amorphous phase. Crystallization by heat setting above the glass-transition temperature did not dramatically affect the permeability. However, a decrease in the diffusion coefficient, offset by an increase in the solubility coefficient, indicated that crystallization affected the barrier properties of the permeable amorphous phase. Analysis of the barrier data, assuming a two-phase model with variable density for both the permeable and impermeable phases, revealed that the impermeable phase density increased during crystallization, approaching a value of 1.476 g/cm3. This value is consistent with previous measurements of the density of the defective crystalline phase in PET. The density of the permeable amorphous phase decreased concurrently to about 1.325 g/cm3, indicating the appearance of additional free volume.

Ductile-to-brittle transition of rubber-modified polypropylene: Part 1 Irreversible deformation mechanisms

The irreversible deformation mechanisms of polypropylene (PP) blended with an ethylenepropylene rubber (EPR) were investigated in the region of the ductile-to-brittle (D-B) transition. The nature of the D-B transition over the composition range of 0 to 25% EPR was studied as a function of temperature and strain rate. Optical microscopy and scanning electron microscopy were used to examine the irreversible microdeformation processes in the fractured specimens. At −40° C, the controlling irreversible deformation process in PP was crazing. In the blends, two kinds of damage zones were observed: a diffuse zone due to voiding at rubber particles and an intense damage zone due to craze-like damage and deformation bands. In general, the size and density of the damage zones increase in a gradual manner through the D-B transition whether examined as a function of temperature, strain rate or blend composition.

Covalent and Non-Covalently Coupled Polyester-Inorganic Composite Materials

Abstract Two types of organic/inorganic materials were synthesized via sol–gel reactions for tetraethylorthosilicate (TEOS) and organo-alkoxsilane monomers in the presence of poly(e-caprolactone) (PCL): (1) non-covalent hybrids, in which PCL and silicate, and PCL and organically modified silicate (ORMOSIL), phases are coupled by non-bonded interactions; (2) covalent hybrids in which triethoxysilane–telechelic PCL molecules form chemical bonds with a sol–gel-derived silicate phase. Chemical structures were verified using FTIR and 13 C NMR spectroscopies and MALDI-TOF mass spectrometry. The constant PCL phase T g for the silicate–PCL hybrids of (1) implies poor organic/inorganic mixing, but dual-melting endotherms varied with silicate content. TGA revealed significant elevation of degradation onset temperature ( T d ) of (1) and suppression of the low temperature chain scission process. No PCL glass transition is seen for ORMOSIL–PCL hybrids where diethoxydimethylsilane is the co-monomer and melting occurs in one step, and there are significant increases in T d . Likewise, no glass transition is seen when acetoxypropyltrimethoxylsilane is the semi-organic co-monomer, but there are dual-melting endotherms. Triethoxysilane-endcapped PCL was synthesized and its microstructure verified by GPC, MALDI-TOF mass spectrometry, FTIR and NMR spectroscopies. T g for this telechelic PCL that was reacted with a small fraction of TEOS increases relative to hydroxy-telechelic PCL due to formation of phase-linking Si–O–Si bonds through end groups. The temperature/magnitude of the melting transition decreased upon inorganic modification. TGA showed appreciable increase in T d relative to PCL and both the high and low temperature degradation processes were hindered. Silane–telechelic PCL films have oxygen permeability values less than that of pure PCL, which is totally attributed to a decrease in diffusion coefficient.

Deformation behaviour of coextruded multilayer composites with polycarbonate and poly(styrene-acrylonitrile)

The tensile properties of coextruded multilayer composites comprised of predominantly 49 alternating layers of polycarbonate (PC) and polystyrene- acrylonitrile (SAN) were investigated in the bulk and microscopically. The bulk was characterized by three types of behaviour: brittle fracture at low strains, ductile yielding with fracture during neck formation, and formation of a stable neck followed by drawing to high extension. Optical microscopy was utilized to correlate deformation mechanisms within each phase to the observed modes of deformation in the bulk. Optical microscopy showed that in all cases the initial irreversible deformation event was the formation of cracks or crazes in the SAN layers. Good adhesion between the layers resulted in the subsequent initiation of shear bands in the polycarbonate layers at the craze tips. Interaction of crazes and shear bands produced an expanded damage zone ahead of the propagating crack which delocalized the stress and delayed fracture. The ultimate mode of fracture depended on the relative thickness of the SAN and PC layers, as determined by the composition, and the strain rate.

Ductile-to-brittle transition of rubber-modified polypropylene

The deformation behaviour of blends of polypropylene (PP) with ethylene-propylene rubber was studied as a function of temperature and composition under tension. The damage ahead of the deliberately introduced defect was traced quantitatively as a function of external load with the aid of intensity analysis. In unmodified PP or in rubber-modified blends, no stable crack growth was obtained up to 99% of the maximum stress. However, a hierarchy of failure events was observed. First, there was a gradual occurrence of a fan-shaped damage zone. Then an intense damage zone initiated. Finally, a slow tearing mode of crack growth occurred at the maximum stress and the sample failed. The size and shape of the damage zone were influenced by temperature, composition and the artificially introduced stress-raiser. Voids were dominant in the fan zone with some crazes close to the intense damage zone. In the intense damage zone, the crazes coalesced to form a network of deformation bands. The description of the fan zone was achieved by a non-linear or elastoplastic failure approach, analogous to the Hilton-Hutchinson formalism, and the analysis of the intense zone by a critical stress instability criterion. There was also a fair correlation with the Dugdale model for the growth of a wedge-like intense damage zone in the specific case of unmodified PP at −40° C.

Shear yielding modes of polycarbonate

The modes of shear yielding in edge-notched sheets of polycarbonate have been studied under slow tensile loading. Optical microscope techniques were used to characterize the flow lines through the thickness of the plastically deformed region. Three modes are observed, namely core yielding, hinge shear and intersecting shear. Core yielding consists of two families of shear flow lines contained in the centre region where the stress is highest. In the nearly plane strain condition, the dominating shear mode is hinge shear which is through-thickness yielding on inclined planes above and below the notch. Intersecting shear dominates in the nearly plane stress condition. In this case, yielding occurs through the entire thickness by slip along planes parallel to the width direction that make an angle with the plane of the sheet. It produces a necking effect in front of the notch. The thickness dependent transition from hinge shear to intersecting shear follows conditions suggested by Hahn and Rosenfield.

The role of surface stresses in the deformation of hard elastic polypropylene

Abstract The phenomenon of stress depression (Δσ) observed when hard elastic polypropylene (HEPP) is exposed to certain liquids has been utilized to probe the role of surface stresses in hard elastic behaviour. Stress depressions were measured in non-interacting liquids and vapours as a function of interfacial surface tension, strain and vapour pressure. Results indicate that surface stress on the microfibres is a significant component of the restoring force. At strains up to 10%, surface stresses from a large number of microfibres contribute about 40% to the restoring force. Above 10% strain, there is no change in the magnitude of the surface contribution. A constitutive equation has been developed from which the fibril diameter can be calculated. Calculated average fibril diameters show that stress-induced subfibrillation is the mechanism responsible for the gradual increase in the surface stress up to 10% strain. Subfibrils are stable only under an imposed stress, the smallest fibril diameter obtained with this material is approximately 20 A.

Correlation of fatigue crack propagation in polyethylene pipe specimens of different geometries

Correlation in mechanisms and kinetics of step-wise fatigue crack propagation in polyethylene pipe specimens of different geometries is studied experimentally. It is shown that crack propagation in a non-standard specimen cut from a real pipe and conserving the pipe geometry can be effectively simulated using a standard compact tension specimen. Good correlation in both kinetics of step-wise crack propagation and fractography between the specimens is achieved if experimental conditions are chosen to assure equal values of (a) stress intensity factor and (b) stress intensity factor gradient at the initial notch tips. These results extend previous technique of fatigue accelerating slow crack growth used to predict lifetime of polyethylene pipes.