Showing papers in "Polymer Engineering and Science in 1993"

Compatibilizing agents in polymer blends : interfacial tension, phase morphology, and mechanical properties

Abstract: The interfacial tension, phase morphology, and phase growth was determined for four polymer blend systems: polyethylene/polystyrene, polyethylene/polyamide-6, polystyrene/polyamide-6, and polystyrene/poly(ethylene terephthalate). Generally, high interfacial tension correlates with coarse phase morphology and rapid phase coalescence. The addition of various potential compatibilizing agents to these binary blend systems results in lowered interfacial tension, finer and stabilized phase morphologies. The characteristics of different compatibilizing agents were compared for several of the blend systems. We also look at the influences of compatibilizing agents on mechanical properties of the blend systems. Some compatibilizing agents are able to produce substantial improvements in ultimate properties.

Comparison of microwave and thermal cure of epoxy resins

Abstract: Stoichiometric mixtures of DGEBA (diglycidyl ether of bisphenol A)/DDS (diaminodiphenyl sulfone) and DGEBA/mPDA (meta phenylene diamine) have been isothermally cured by electromagnetic radiation and conventional heating using thin film sample configurations. Fourier transform infrared spectroscopy (FTIR) was used to measure the extent of cure. Thermal mechanical analysis (TMA) was used to determine the glass transition temperatures directly from the cured thin film samples. Well-defined glass transitions were observed in the TMA thermograph for both thermal and microwave cured samples. Significant increases in the reaction rates have been observed in the microwave cured DGEBA/DDS samples. Only slight increases in the reaction rates have been observed in the microwave cured DGEBA/mPDA samples. Higher glass transition temperatures were obtained in microwave cured samples compared to those of thermally cured ones after gelation. The magnitude of increases of glass transition temperature is much larger for the DGEBA/DDS system than DGEBA/mPDA system. The microwave radiation effect was much more significant in DGEBA/DDS system than in DGEBA/mPDA system. DiBenedetto's model was used to fit the experimental Tg data of both thermal and microwave cured epoxy resins.

Basic ideas of microwave processing of polymers

Abstract: The objective of this effort has been to investigate the relationship between polymer structure and microwave absorptivity. Dielectric loss factor, e″, loss tangent, tan δ, and oscillator strength, (eS −- e∞), were used to evaluate potential material processability under applied microwave radiation. Numerous polymeric materials varying in chemical and physical structures were irradiated in a low power (≤ 100W) electric field at 2.45 GHz. Electromagnetic radiation was applied as either traveling or resonant wave modes in cylindrical and rectangular waveguides. In general, heatability was found to be a direct function of the dielectric loss dispersion dependence on temperature and frequency. The dielectric loss factor obtained at low frequency measurements was found to be directly proportional to the heatability of polymers. A WLF plot was used to predict the shift of dielectric loss maxima into or out of the microwave frequency range.

Shear effects on thermoplastic foam nucleation

Abstract: Polyethylene foam experiments were conducted on a twin screw foam extruder to study the process effects upon foam nucleation. The results indicate that, other than quantity of nucleators, throughput rate and die opening are important process parameters, underscoring the importance of shear in the die on nucleation. It is also found that shear effects increase as nucleator level increases. A modified cavity model shows at least qualitative agreement with experimental results. Dimensionless Capillary number, ratio of shear force to surface tension force, provides evidence that shear force rather than shear rate alone is the principal parameter affecting foam nucleation.

The use of cryogenically ground rubber tires as a filler in polyolefin blends

Abstract: The effects of cryogenically ground rubber tires (CGT) on some of the mechanical properties of blends with linear low density polyethylene (LLDPE) and high density polyethylene (HDPE) are presented. Precoating the CGT particles with an ethylene-acrylic acid copolymer is shown to overcome most of the deleterious effects of adding CGT to LLDPE, while still retaining composite processability. A blend of 40 wt% EAA coated CGT particles with LLDPE is shown to have impact and tensile strengths that are 90% of those for the pure LLDPE, representing increases of 60 and 20%, respectively, over blends with uncoated particles. Blends of LLDPE with ground tire bladders demonstrate that even better mechanical properties can be obtained with similar large rubber particle size but somewhat better adhesion. For HDPE, however, it is shown that with large rubber particles, moderate adhesion is not sufficient to produce useful composites.

Factors influencing the formation of elongated morphologies in immiscible polymer blends during melt processing

Abstract: The morphology of the dispersed phase in immiscible polymer blends plays an important role in the determination of the final physical properties. This paper considers factors that influence the final state of deformation of the dispersed phase, and in particular, the formation of fibers and lamellae. Blends of polyethylene and nylon-6 were extruded by ribbon extrusion at different draw ratios. Prior to single-screw extrusion the materials were blended in a co-rotating twin-screw extruder, and the size of the dispersed phase was studied as a function of the viscosity ratio. As the blends are extruded into ribbons and drawn through the calender rolls, the morphology of the dispersed phase undergoes drastic transformations. The fiber formation is enhanced by increasing the draw ratio. At high draw ratios, long thin fibers are observed. Some biaxial deformation is obtained for the noncompatibilized systems when the extruded materials enter the calender with the maximum closing pressure applied to the rolls. The same effect is observed for the compatibilized systems with lower values of the viscosity ratio. As a general rule, it has been observed that the final dispersed phase deformation is diminished in interfacially compatibilized systems.

Effect of adhesion on the fracture toughness of calcium carbonate–filled polypropylene

Abstract: The effect of adhesion on the strain energy release rate (Gc) and Charpy notched impact strength (NIS) of calcium carbonate (CaCO3)-filled polypropylene (PP) at room temperature is investigated over a wide interval of particulate filler volume fractions. The concentration dependence of Gc and NIS are discussed in terms of competition between the effects of increasing stiffness, decreasing effective matrix cross section, and the transition from a plane strain to a plane stress mode of failure. In all cases the plane stress and plane strain limits of the critical strain energy release rate for initiation of cracks were not affected by the presence of the filler and are the same as those for neat matrix. In the case of no adhesion between components, the size of the crack tip plastic zone increases with increasing filler volume fraction (vf) because of the reduction of the material yield strength. In the region 0 < vf < 0.12, there is a mixed mode of failure, and the measured value of Gc for crack initiation increases steadily as the sample cross section approaches a fully plane stress state. The reduction in yield strength also results in the increase in Gc for crack propagation as reflected by an increase in NIS. Above vf= 0.12, the specimen cross section is in a fully plane stress state, and further increase in filler volume fraction (decrease in matrix effective cross section) has the net effect of reducing both Gc and NIS. In the case of “perfect” adhesion, the yield strength increases only slightly with vf. In the region 0 < yr < 0.05 there is also a mixed mode of failure, but the increase in Gc is much less than that for the no-adhesion case since the size of the plastic zone in front of the crack is much smaller. Above vf= 0.05, the combined effects of increasing stiffness, reduction of the size of the plastic zone, and decreasing matrix cross section dominate the behavior, causing a steady reduction in both Gc and NIS. Good agreement was found between experimental data and calculations based on fracture mechanics principles.

Free radical degradation of polypropylene : random chain scission

Abstract: Free-radical reactions, which are relevant in the chemical modification of polymeric materials, are discussed in relationship to their influence on random chain scission. A mathematical model is developed to calculate the entire molecular weight distribution (MWD) with the degree of chain scission as the sole parameter. This expression is identical to the one derived by Saito for the case of scission initiated by irradiation. An expression for the variation of molecular weight averages with the degree of chain scission is derived and compared with those calculated with the methods of moments found in the literature. The validity of empirical closure conditions used with moment methods is discussed. Experimental data for polypropylene (PP), degraded with organic peroxides in a single screw extruder, are compared with predictions when fitting the degree of chain scission. For a specific degradation process, the degree of chain scission is found to be proportional to the initiator level.

Effect of drawing on structure and properties of a liquid crystalline polymer and polycarbonate in - situ composite

Abstract: Fibers (strands) with various draw ratios were spun from the liquid crystalline state of a pure aromatic liquid crystalline copoly(ester amide) and the melts of its blend with polycarbonate. Scanning electron microscopy (SEMI, wide angle X-ray scattering (WAXS), and differential scanning calorimetry (DSC) were employed to investigate the structure and properties of the resulting fibers. Mechanical properties of the fibers were also evaluated. It was found that both the crystallite size and heat of fusion of the liquid crystalline polymer (LCP) increase steadily with draw ratio. However, the crystal-nematic transition temperature of the LCP is virtually unaffected by drawing. Moreover, heat of fusion of LCP is much smaller than that of isotropic condensation polymers despite the presence of very sharp dmraction peaks in WAXS measurements. These results are ascribed to the (semi)rigid rod nature of the LCP chains and the persistence of an ordered structure in the LCP melt, i.e., entropy effect. It was further observed that tensile modulus and tensile strength along fiber axis rise with draw ratio for the composite fibers. The elastic modulus of the composite fibers were found to be as high as 19 GPa and tensile strength reached 146 MPa with draw ratios below 40 and an LCP content of 30 wt%. Compared with the thermoplastic matrix, the elastic modulus and tensile strength of the in-situ composite have increased by 7.3 times and 1.4 times, respectively, with the addition of only 30 wt% LCP. This improvement in mechanical properties is attributed to fibrillation of the LCP phase in the blend and the increasing orientation of the LCP chains along the fiber axis during drawing.

Suspension, dispersion, and interfacial polycondensation: A methodological survey

Abstract: This review presents a general picture of suspension, dispersion, and interfacial polycondensation processes employed for the preparation of beaded resins, dry powders, and high solid dispersions. The polymer systems covered include polyamides, polyesters, polycarbonates, polyurethanes, and phenol-formaldehyde and urea-formaldehyde resins. Basic features of heterogeneous polycondensation processes are outlined, and different mechanisms of particle formation in suspension, dispersion, and interfacial polycondensation are discussed. Effects of manufacturing parameters such as feed ratio, droplet/particle stabilizer, and stirrer speed on product characteristics are also briefly covered.

The origin of β relaxations in segmented rigid‐rod polyimide and copolyimide films

Abstract: Most aromatic polyimides and copolyimides show a β relaxation process below their α relaxation processes in dynamic mechanical and dielectric measurements. The origin of this β relaxation has been under active discussion. In order to gain a better understanding of the origin of the β relaxation process, the dynamic mechanical behavior of a series of segmented rigid-rod polyimide and copolyimide films with different dianhydrides and diamines has been systematically studied. For some of the polymers, crystallinity and orientation effects on the β relaxation process were also investigated. It is found that the β relaxation process in the unoriented films can mainly be attributed to a relatively noncooperative motion in uncrystallized diamines. However, this noncooperative nature may be gradually lost by increasing the crystallinity and orientation. Molecular modeling and computation have led to a discussion of possible diamine motion in the β relaxation process.

Temperature dependence of the nucleation effect of sorbitol derivatives on polypropylene crystallization

Abstract: The temperature dependence of the nucleation effect of three sorbitol derivatives on the crystallization of isotactic polypropylene (iPP) was studied by means of isothermal crystallization kinetic analysis. Isothermal crystallization thermograms obtained by differential scanning calorimetry (DSC) were analyzed based on the Avrami equation. The Avrami analysis for the nucleated iPP was carried out with DSC data collected to 35% relative crystallinity, and the rate constants were corrected assuming the heterogeneous nucleation and three dimensional growth of iPP spherulites. A semi-empirical equation for the radial growth rate of iPP spherulites was given as a function of temperature and was used to determine the number of effective nuclei at different temperatures. The number of effective nuclei in the nucleated samples was estimated to be 3 × 102 ∽ 105 times larger than that in the neat iPP. The logarithmic numbers of the effective nuclei decreased linearly with decreasing degree of supercooling in the range of crystallization temperatures tested. The temperature dependence of the effect of the nucleating agents on iPP crystallization was given quantitatively in terms of the deactivation factor defined as a fraction of the particles that are active at a particular temperature but inert at the temperature one degree higher. The nucleation activity and its temperature dependence are considered to be cooperative effects of many factors, including the dispersion and the physical or chemical nature of the agent as well as the interaction between the agent and the polymer. It is suggested that the temperature dependence of the effect of a nucleating agent should be treated as a characteristic of a given polymer/ nucleating agent mixture.

Percolation behavior in morphology and modulus of polymer blends

Abstract: The tensile property of a plastic/rubber blend depends critically on the morphology and connectivity of the two phases. At low plastic volume fractions, the plastic phase forms isolated domains in the matrix of rubber phase, and the tensile property of the blend is largely controlled by the continuous rubber phase. As the plastic volume fraction increases, the plastic phase gradually connects into a pervasive network that eventually dominates the tensile and shear properties of the blend. The transition of the blend from a rubber-dominated to a plasticdominated behavior is a manifestation of percolation transition. The plastic volume concentration at which the transition takes place is the percolation threshold. Its dependence on morphology is discussed by contrasting the behaviors of anisotropic injection-molded specimens vs. isotropic compression-molded specimens of the two-phase blends of an amorphous thermoplastic polyester, PETG, and an ethylene-propylene-diene rubber, EPDM. It is found that the tensile modulus just above the percolation threshold obeys a power law as a function of the plastic volume concentration in excess of the percolation threshold. By analyzing the longitudinal tensile modulus of injection-molded PETG/EPDM specimens just above the threshold, it is shown that the scalar elastic percolation theory of de Gennes is at work here. For compression-molded PETG/EPDM specimens, it is found that the isotropic tensile modulus over the entire composition range obeys the symmetric effective medium theory.

Morphology and permeability in extruded polypropylene/ethylene vinyl‐alcohol copolymer blends

Abstract: Studies of the morphology of extruded polymer blend systems have shown that it is feasible to produce a laminar structure of an ethylene vinyl-alcohol copolymer (EVOH) dispersed phase in a polypropylene (PP) matrix phase. The laminar structure forms in the core of the extrudate when a slit die is incorporated into the extrusion process. Morphological studies, including a study of morphology development inside the die and studies of the effect of processing conditions on the morphology of the final product, revealed that the laminar structure is a result of die design. Processing conditions influence mainly the shape and dimensions of the laminar core region of the extrudate. Oxygen permeation tests have shown that the blend exhibits lower oxygen permeability than pure PP, when EVOH is incorporated as a dispersed phase into the system. Oxygen transmission rates obtained with a blend system approach those obtained with a multilayer coextrusion product, although only at high EVOH concentrations. Comparison of experimental data with theoretical permeation predictions shows that, up to 20 wt% EVOH, the reduction in oxygen transmission rate follows the prediction for a homogeneous system. At 25 wt%, a considerable decrease in oxygen transmission rate is noticeable, and the trend for higher EVOH contents is towards the behavior of a multilayer system.

A predictive model for the mechanical behavior of particulate composites. Part I: Model derivation

Abstract: A method for predicting the highly nonlinear stress-strain behavior and dilatation induced by cavitation of highly filled particulate composites from constituent properties has been developed. The approach presented uses a variation of linear elasticity throughout and has no adjustable parameters, unlike the methods currently used, which require large numbers of fitting factors and complicated nonlinear analyses. An energy balance derived from the first law of thermodynamics calculates critical strain values at which filler particles will debond when subjected to deformation. Repeated calculations of critical strain values using re-evaluated material properties accounting for the damage caused by debonding give very nonlinear stress-strain and dilatation curves. Experimentally observed dependencies on particle size, filler concentration, adhesion, and matrix and filler properties are correctly predicted. The method can be generalized for any state of stress or particle shape. Comparisons of experimental data with the model results give good agreement.

Melt blending of ultra high molecular weight and high density polyethylene: The effect of mixing rate on thermal, mechanical, and morphological properties

Abstract: The blends of two different ultra high molecular weight polyethylenes (UHMWPE) and high density polyethylene (HDPE) were prepared in melt at different compositions and mixing rates in Brabender Torque Rheometer. The temperature build-up due to the internal friction during melt blending was recorded and evaluated with respect to the change in the torque. The temperature at maximum torque was considered the fusion point temperature of the UHMWPE in the blend. This fusion point temperature was found to depend on the composition, mixing rate, and molecular weight. The effect of mixing rate on the mechanical properties (measured as yield and tensile strengths and elongation at break), thermal oxidative degradation and melting behavior were studied. The morphology of the blends were investigated by optical microscopy.

Reaction mechanisms of polyacrylonitrile on thermal treatment

Abstract: A high-temperature radical scavenger, 2,2-diphenyl-l-picryl hydrazyl (DPPH), has been used to study the reaction mechanisms of polyacrylonitrile (PAN) on thermal treatment. The effect of DPPH on the cyclization reaction of PAN in both air and nitrogen, investigated by differential scanning calorimetry (DSC), helped to verify the proposed reaction mechanisms, i.e., the free radical and the ionic ones. For PAN homopolymer, the peak temperature of the reaction exotherm shifted to higher temperatures and the heat of reaction was decreased with increasing DPPH concentration. For PAN copolymer with methylacrylate and itaconic acid, however, the effects of DPPH on DSC thermograms were insignificant. The effects of IPPH suggest that the reaction of the nitrile groups proceeds by free radicals for the homopolymer while by ions for the copolymer. The activation energies for the thermal reactions of PAN in both air and nitrogen were also estimated by the dynamic DSC method, and they proved to be highly dependent on reaction mechanism, environment of thermal treatment, and DPPH concentration.

Mechanical properties of in‐situ composites based on partially miscible blends of glass‐filled polyetherimide and liquid crystalline polymers

Abstract: The use of inorganic (glass) fiber reinforcement to enhance the mechanical properties and reduce the anisotropy of in situ composites based on blends of liquid crystalline polymers (LCPs) with polyetherimide (PEI) is discussed. It was found that the tensile and flexural moduli are increased and the anisotropy is reduced with increasing grass content (when compared at equivalent LCP weight fractions). The creep compliance of the PEI/LCP composites is reduced upon the addition of glass fibers. However, the disadvantage is that the processability worsens upon addition of glass fibers to the PEI/LCP in situ composites. The effect of adding glass reinforcement on the ultimate tensile strength is less clear, because the data do not show any consistent trend. Similarly, the elongation at break and toughness do not show any consistent improvement upon addition of glass reinforcement. Morphological studies show that there is considerable difference between the size and texture of the reinforcing glass fibers and LCP microfibrils.

Effect of catalyst on the reactive processing of polyesters with epoxy‐functional polymers

Abstract: The objective this work was to determine the effects of selected polyester catalysts on the reaction of a polyester with epoxy functional polymers. Polyesters containing various catalyst metals were melt blended with either an ethylene-co-glycidyl methacrylate or a styrene-co-glycidyl methacrylate copolymer. The viscosities of the blends were monitored as a function of mixing time using torque rheometry. In addition, the molecular weight distributions of selected samples were analyzed using gel permeation chromatography. Both the torque rheometry and the gel permeation chromatography results indicate that the polyester reacts with epoxy functional polymers. This reaction occurs under conditions and at processing times which are readily obtainable in conventional melt processing equipment. Furthermore, the reaction kinetics of polyesters with glycidyl methacrylate copolymers are dramatically affected by the nature of the catalyst system used to prepare the polyester. Under the conditions used, antimony catalysts are particularly effective at promoting the reaction between polyesters and the epoxy functionality and the activity of the catalysts studied appears to decrease in the following order: antimony > gallium > tin ≃ titanium > germanium. Manipulation of the polyester catalyst system may offer a method to control the extent of reaction obtained in reactive processing of polyesters with epoxy functional compounds.

Studies on miscibility of blends of ethylene methyl acrylate and polydimethyl siloxane rubber

Abstract: Blends of ethylene methyl acrylate (EMA) and poly(dimethylsiloxane) rubber (PDMS) are demonstrated to be miscible. The miscibility results in a single and composition-dependent glass transition temperature. IR spectra of the blends provide direct evidence of chemical reaction between EMA and PDMS rubber.

Process rheometers for molten plastics: A survey of existing technology

Abstract: There are many operations in the plastics industry where it is useful to be able to monitor the rheological properties of a melt as it is being processed. Such operations include polymerization, blending, compounding, and reactive extrusion. The design of such instruments poses major challenges, as there is a conflict between the need for rapid sample renewal and the desire for measurement accuracy. In-line types, which are installed directly in the process flow, have a rapid response but must operate in an environment where temperature, pressure, and flow rate fluctuate in response to process upsets. On-line types, which make use of a side stream and gear pump to feed the rheometer, allow for more freedom in the design of the rheometer but have an inherent signal delay due to the time required for melt to reach the rheometer. A variety of rheological sensors have been described in the patent and research literature, each with its unique advantages and disadvantages, and some of these are finding increased use in the plastics industry to improve product quality and process efficiency.

Deformation and fracture of glass bead/ CTBN‐rubber/epoxy composites

Abstract: Carboxyl-terminated butadiene-acrylonitrile-rubber decreases modulus and yield stress of the studied epoxy but increases fracture toughness. The addition of glass bead compensates for the loss in modulus but has little effect on yield stress. However, it significantly contributes to the fracture toughness by providing additional mechanisms for toughening of both the unmodified and rubber-modified epoxy. For the toughened epoxies studied, fracture surfaces gave only limited information on fracture mechanisms since significant energy absorption also occurs in the material below the fracture surface. Suggestions for suitable material compositions for fiber composite matrices are given.

Blends of a thermotropic liquid crystalline polymer and a thermoplastic elastomer. I: Mechanical properties and morphology

Abstract: Blends of a thermotropic liquid crystalline polymer (LCP), Vectra A900, and a thermoplastic elastomer, Kraton G1650, were made on a single screw extruder. During extrusion, fibers of the LCP are formed under influence of shearing and elongating forces. The stiffness and tensile strength of the elastomer are greatly improved by the addition of the LCP. The modulus of elasticity of blends containing up to 20% LCP can be described well with the Halpin-Tsai equation. Differential scanning calorimetry and dynamic mechanical thermal analysis (DMTA) measurements show that the polymers are immiscible, but the DMTA results show a shift of the glass transition temperature of the elastomeric block of the Kraton polymer. This shift may be attributed to a layer of elastomer adsorbed on the LCP particles.

Reactive processing of polymer blends: Analysis of the change in morphological and interfacial parameters with processing

Abstract: The change in morphology and interface quantities have been analyzed for an immiscible polymer blend during reactive processing. A model polymer/polymer combination, hydroxy-terminated poly(e-caprolactone)/liquid rubber with α, ω-carboxy groups, was employed. The blend was subjected to light scattering measurements, ellipsometry, and gel permeation chromatography (GPC). Size reduction of the dispersed phase during processing was followed by a systematic decrease in the correlation distance ζ and an increase in the specific interfacial area Ssp, both by the Debye-Bueche plot of light scattering profiles. Also observed was the time variation of the volume fraction of interface Vλ estimated as a product of the Ssp and the interfacial thickness by ellipsometry. The changes in ζ, Ssp, and Vλ with processing were accelerated when a coupling agent,γ-aminopropyltriethoxysilane (APS), was added. The amount of block copolymer formed in-situ in the APS-loaded system was estimated by GPC with RI and UV detectors. For the size reduction kinetics in both APS-loaded and -unloaded systems, Rittinger's law was found to be applicable.

Non-isothermal crystallization of isotactic polypropylene in dotriacontane. II: Effects of dilution, cooling rate, and nucleating agent addition on growth rate

Abstract: Non-isothermal crystallization growth rates of nucleated and non-nucleated isotactic polypropylene (iPP) in dotriacontane were determined experimentally by thermal optical microscopy. Adipic acid was used as the nucleating agent. The non-isothermal growth rates of the nucleated and non-nucleated systems were compared with experimentally determined isothermal growth rates. The Lauritzen and Hoffman growth rate equation, originally developed for isothermal crystallization, was modified to describe the non-isothermal growth. The modified Lauritzen-Hoffman equation was used to predict isothermal growth rates from non-isothermal crystallization for the nucleated and non-nucleated polymer-diluent mixtures. This study is the first to deal with polymer-diluent-nucleating agent systems vital to membrane production.

Development of crystallinity in NEW-TPI polyimide

Abstract: Development of crystallinity in NEW-TPI semicrystalline polyimide has been studied using differential scanning calorimetry (DSC), wide (WAXS), and small angle X-ray scattering (SAXS). Crystallinity of the fully imidized powder, pellet, or film processed NEW-TPI can occur from the melt, and depends upon the holding temperature of the melt. Repetitive exposure to elevated temperatures supresses the development of crystallinity from the melt state. In amorphous pellets and film, crystallinity can also develop by cold crystallization from the rubbery amorphous state. SAXS results show that during cold crystallization, NEW-TPI develops a periodic structure consistent with formation of alternating crystalamorphous stacks, but with crystals only a few molecular repeat units thick. Kinetics of nonisothermal crystallization were studied as a function of heating rate and could be described using the Ozawa analysis. Non-isothermal crystallization proceeds at a slower rate in NEW-TPI than in other high temperature thermoplastics such as PEEK, and with a much narrower processing window. The maximum degree of crystallinity that could develop during heating was 0.34, which occurred at a rate of 5°C/min. Similar degrees of crystallinity could be introduced by heating amorphous NEW-TPI film in N-methylpyrrolidone.

Dielectric characterization of water sorption in epoxy resin matrices

Abstract: Water sorption in epoxy based materials was analyzed by dielectric spectroscopy using remote sensors. A method to determine diffusion coefficients from the changes in permittivity during water absorption is proposed and was verified experimentally by comparison with standard water gain measurements. Although the technique is limited by electrode polarization phenomena, it is sensitive to the presence of water molecules and is capable of detecting different levels of water concentration as a function of frequency. The utilization of remote sensors demonstrated the capability of dielectric analysis to be applied both in the laboratory environment, and on a larger scale, as a nondestructive technique for monitoring environmental changes in actual polymer matrix composite parts.

Mechanical hysteresis of a polyether polyurethane thermoplastic elastomer

Abstract: The mechanical hysteresis of a polyether polyurethane thermoplastic elastomer was studied as a function of temperature, percent strain, and deformation energy. Hysteresis values remained small at low temperatures when the extent of the sample deformation did not disrupt the glassy matrix. This was readily evident at temperatures below the glass transition temperature, Tg of the polymer where the material did not formally yield. At temperatures above the Tg of the polymer, hysteresis remained small even at substantial strains levels and demonstrated the capabilities of the hard segment domains to act as physical crosslinks. At elevated temperatures, percent hysteresis increased as the hydrogen-bonded hard segment domains weakened. When mechanical hysteresis was considered on the basis of constant deformation energies, hysteresis values reached a maximum in the vicinity of the Tg of the polymer. These maxima existed as a consequence of two opposing trends: the decreasing resiliency of the polymer as it becomes a glass and the increase in the resistance of that glass to undergo deformations sufficient to cause plastic flow. Finally, a hysteresis response surface constructed as a function of deformation energy and temperature was found to be sensitive to both the strain-induced crystallization of the rubbery soft segment matrix and to the strain-induced yielding of the glassy soft segment matrix.

Buildup and relaxation of molecular orientation in injection molding. Part I: Formulation

Abstract: A numerical simulation model for the analysis of the buildup and, the relaxation of molecular orientation in injection-molded products has been developed. The constitutive equations of the material are described using an extended version of the viscoelastic material model developed by Leonov. The volumetric responses are described using two different equations of state, the Tait equation and an equation developed by Spencer and Gilmore. A WLF-type of equation is used for the temperature and pressure dependence. Stresses calculated with the model are coupled to birefringence by means of the stress-optical rule. Birefringence, in turn, is used to characterize the molecular orientation. The model is used to investigate the influence of the processing conditions, the mold elasticity, and the pressure dependence of the material functions on the pressure and birefringence profiles. The material data used are for polystyrene, PS 678E, and are determined experimentally.

The mechanism of skin‐core morphology formation in extrudates of polycarbonate/liquid crystalline polymer blends

Abstract: The mechanism of skin/core morphology development and LCP (liquid crystalline polymer) fibril formation in polycarbonate/LCP blends was studied A certain minimum concentration of the LCP phase must be present for the formation of continuous LCP fibrils in the extrudates A skin-core morphology characterizes the PC/LCP extrudates Short LCP fibrils are formed in the capillary converging entrance section, through the elongation of LCP domains and their coalescence Continuous fibrils were formed in the skin of extrudates emerging from cylindrical capillaries, through the coalescence of the short fibrils, provided the shear stresses are high enough and the LCP viscosity is equal or lower than that of PC Increasing capillary length enhances the LCP lateral migration and fibrils formation The high interfacial tension stabilizes the LCP fibrils In the core region the short fibrils recoil or breakup, resulting in spherical or elongated droplets Long and continuous fibrils cannot be formed in a zero length capillary, even at high flow rates