Tensile behavior of quasi-unidirectional glass fiber/polypropylene composites at room and elevated temperatures

In the present study, the tensile behavior of quasi-unidirectional glass fiber/polypropylene composites at room and elevated temperatures were investigated by both micro- and macromechanical test methods. In the micromechanical studies, a single fiber fragmentation test was employed for measuring the interfacial shear strength at fiber-polypropylene interface in the temperature range from 23 °C to 90 °C. The results show that interfacial shear strength decreases with increasing testing temperature. In the macromechanical studies, experimental results show that the elastic modulus of polypropylene and transverse elastic modulus of composites are sensitive to the testing temperature. The weakened fiber-polypropylene interface due to elevated temperatures led to the vanishing of “knee” in transverse tensile stress-strain curves. A function was proposed to evaluate the dependence of the elastic modulus of quasi-unidirectional glass fiber/polypropylene composites on the testing temperatures and tested against experimental data. Tensile failure mechanisms of composites were demonstrated to evolve with the testing temperature.     

Dielectric and impedance properties of Sr(Sm0.5Nb0.5)O3 ceramics

Perovskite types Sr(Sm_0.5Nb_0.5)O₃, (SSN) ceramics have been prepared through solid state reaction route. The scanning electron microscopy provides information on the quality of the samples and uniform grain distribution over the surface of the samples. The field dependence of the dielectric response was measured in a frequency range from 50 Hz to 1 MHz and in a temperature range from 60 °C to 420 °C indicates polydispersive nature of the materials. An analysis of the dielectric constant (?′) and tangent loss (tanδ) with frequency is performed assuming a distribution of relaxation times as confirmed by the scaling behavior of electric modulus spectra. The frequency dependence of the electric modulus peak is found to obey Arrhenius law with activation energy of ∼0.026 eV. The complex plane impedance plot shows the grain boundary contribution for higher value of dielectric constant in the law frequency region. The frequency dependence of electrical data is also analyzed in the framework of conductivity and electric modulus formalisms. Both these formalisms show qualitative similarities in relaxation times. The scaling behavior of imaginary part of electric modulus M″ suggests that the relaxation describes the same mechanism at various temperatures in SSN.     

Fabrication of open‐porous PCL/PLA tissue engineering scaffolds and the relationship of foaming process,morphology, and mechanical behavior

Tissue engineering scaffolds should provide a suitable porous structure and proper mechanical strength, which is beneficial for the delivery of growth factor and regulation of cells. In this study, the open‐porous polycaprolactone (PCL)/poly (lactic acid) (PLA) tissue engineering scaffolds with suitable porous scale were fabricated using different ratios of PCL/PLA blends. At the same time, the relationship of foaming process, morphology, and mechanical behavior in the optimized batch microcellular foaming process were studied based on the single‐factor experiment method. The porous structures and mechanical strength of the scaffolds were optimized by adjusting foaming parameters, including the temperature, pressure, and CO₂ dissolution time. The results indicated that the foaming parameters influence the cell morphology, further determine the mechanical behavior of PCL/PLA blends. When the PCL content is high, with the increase of temperature and time, the cell diameter and the elastic modulus increased, and the tensile strength and elastic modulus increased with the increase of the average cell size, and decreased as the increase of the cell density. While when the PLA content was high, the cell diameter showed the same trend, and the tensile strength and elastic modulus were higher, and the elongation at break was lower, and tensile strength and elastic modulus decreased with the increase of the average cell size and increased with the increase of cell density. This work successfully fabricated optimized porous PCL/PLA scaffolds with excellent suitable mechanical properties, pore sizes, and high interconnectivity, indicating the effectiveness of modulating the batch foaming process parameters.     

Thermoplastic elastomers by hydrogen bonding 1. Rheological properties of modified polybutadiene

Polybutadiene of narrow molecular weight distribution was modified using 4-phenyl-1, 2,4-triazoline-3,5-dione. The degree of modification was 1% and 2% with respect to the repeating units. Hydrogen bonding between the highly polar urazole groups thus incorporated into the polymer gives rise to the formation of a thermoreversible elastomeric network. Dynamic mechanical measurements in the temperature range between 220 and 330 K support the picture of the thermoreversible hydrogen bond interaction. The rubber elastic plateau is shifted to higher temperatures and lower frequencies. The increase in the plateau modulus cannot be attributed solely to the contribution of the network structure but is mainly a consequence of the broadening of the relaxation time spectrum in the modified samples. From the temperature dependence of the shift factors log(a _T ) it is concluded that the general WLF approach fails. The strong temperature dependence of the apparent activation energy of flow is a consequence of the temperature dependence of the hydrogen bond interaction.     

Charge effect of the cation on the structure and the molecular mobility of polymer electrolytes

A study of the thermal and mechanical characteristics of polyethylene oxide-Fe(SCN)₃ complexes shows the existence of structures which can be described by considering a crystalline phase of pure polyethelene oxide (PEO) and an amorphous phase of PEO with dissolved salt. Increasing salt contents decrease the content of crystals, favoring the formation of complexes with a high degree of noncrystallinity as a result of strong polymersalt interactions which tend to enhance the distortion degree of the polymeric skeleton. The microscopic homogeneity of the complexes is also confirmed by the presence in the thermal and mechanical spectra of single glass transitions, which shift to higher temperatures with increasing salt content. Two molecular relaxations are present in the mechanical behavior of these systems, the γ-and the α_a-processes at low and high temperatures, respectively, and show a relaxation strength which increases with decreasing degree of crystallinity of the polymer up to a X ～ 0.10 molar fraction. Both relaxations exhibit a marked nonexponentiality which has been well accounted for in terms of a gaussian distribution of relaxation times for the γ-process and of the Kolrausch-Williams-Watt exponential function for the α_a-process. In the glassy region, the elastic modulus E' reveals a linear temperature dependence which has been interpreted as arising from the anharmonicity of vibrational modes. Increasing noncrystallinity of the system gives rise to an increase of the anharmonicity parameter, which has been ascribed to the influence of the “free volume” in determining the thermal expansivity. ©1995 John Wiley & Sons, Inc.     

Maximum tensile properties of oriented polyethylene,achieved by uniaxial drawing of solution‐grown crystal mats: Effects of molecular weight and molecular weight distribution

The effects of molecular weight (MW) and MW distribution on the maximum tensile properties of polyethylene (PE), achieved by the uniaxial drawing of solution‐grown crystal (SGC) mats, were studied. The linear‐PE samples used had wide ranges of weight‐average (M_w = 1.5–65 × 10⁵) and number‐average MWs (M_n = 2.0–100 × 10⁴), and MW distribution (M_w/M_n = 2.3–14). The SGC mats of these samples were drawn by a two‐stage draw technique, which consists of a first‐stage solid‐state coextrusion followed by a second‐stage tensile drawing, under controlled conditions. The optimum temperature for the second‐stage draw and the resulting maximum‐achieved total draw ratio (DR_t) increased with the MW. For a given PE, both the tensile modulus and strength increased steadily with the DR_t and reached constant values that are characteristic for the sample MW. The tensile modulus at a given DR_t was not significantly affected by the MW in the lower DR_t range (DR_t < 50). However, both the maximum achieved tensile modulus (80–225 GPa) and strength (1.0–5.6 GPa), as well as those at higher DR_ts > 50, were significantly higher for a higher MW. Although the maximum modulus reached 225 ± 5 for M_n ≥ 4 × 10⁵, the maximum strength continued to increase with M_n even for M_n > 4 × 10⁵, showing that strength is more strongly dependent on the M_n, even at higher M_n. Furthermore, it was found that each of the maximum tensile modulus and strength achieved could be expressed by a unique function of the M_n, independently of the wide variations of the sample MW and MW distribution. These results provide an experimental evidence that the M_n has a crucial effect on the tensile properties of extremely drawn and chain‐extended PE fibers, because the structural continuity along the fiber axis increases with the chain length, and hence with the M_n. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 153–161, 2006     

Superdrawing of polytetrafluoroethylene nascent powder by solid‐state coextrusion

A film of nascent powder of polytetrafluoroethylene (PTFE), compacted below the ambient melting temperature (T_m, 335 °C), was drawn by two‐stage draw techniques consisting of a first‐stage solid‐state coextrusion followed by a second‐stage solid‐state coextrusion or tensile draw. Although the ductility of extrudates was lost for the second‐stage tensile draw at temperatures above 150 °C due to the rapid decrease in strength, as previously reported, the ductility of extrudates increased with temperature even above 150 °C when the second‐stage draw was made by solid‐state coextrusion, reflecting the different deformation flow fields in a free space for the former and in an extrusion die for the latter. Thus, a powder film initially coextruded to a low extrusion draw ratio (EDR) of 6–20 at 325 °C was further drawn by coextrusion to EDRs up to ～?400 at 325–340 °C, near the T_m. Extremely high chain orientation (f_c = 0.998 ± 0.001), crystallinity (96.5 ± 0.5)%, and tensile modulus (115 ± 5 GPa at 24 °C, corresponding to 73% of the X‐ray crystal modulus) were achieved at high EDRs. Despite such a morphological perfection and a high modulus, the tensile strength of a superdrawn tape, 0.48 ± 0.03 GPa, was significantly low when compared with those (1.4–2.3 GPa) previously reported by tensile drawing above the T_m. Such a low strength of a superdrawn, high‐modulus PTFE tape was ascribed to the low intermolecular interaction of PTFE and the lack of intercrystalline links along the fiber axis, reflecting the initial chain‐extended morphology of the nascent powder combined with the fairly high chain mobility associated with the crystal/crystal transitions at around room temperature. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3369–3377, 2006     

Photon correlation spectroscopy of bulk poly(n-hexyl methacrylate) near the glass transition

Slowly relaxing longitudinal density fluctuations in an optically perfect sample of bulk poly(n-hexyl methacrylate) (PHMA) have been studied by photon correlation spectroscopy in the temperature range 10–36°C. The glass transition temperature for this sample was measured to be T_g = −3°C by differential scanning calorimetry. The optical purity of the sample was verified by Rayleigh-Brillouin spectroscopy and the Landau-Placzek ratio was observed to be 2.3 at 25°C. Light-scattering relaxation functions were obtained over the time range 10⁻⁶-1 s. The shape of the relaxation functions broadened as the temperature was lowered towards the glass transition. Quantitative analysis of the results was carried out using the Kohlrausch-Williams-Watts (KWW) function to obtain average relaxation times, 〈τ〉, and width parameters, β. The width parameter decreased from 0.43 to 0.21 over the temperature interval, as suggested by visual inspection. Average relaxation times shifted with temperature in a manner consistent with previous mechanical studies of the primary glass-rubber relaxation in PHMA. The relaxation functions were also analyzed in terms of a distribution of relaxation rates, G(Γ). The calculated distributions were unimodal at all temperatures. The average relaxation times obtained from G(Γ) were in agreement with the KWW analysis, and the shape of the distribution broadened as the sample was cooled. The rate at which G(Γ) displayed a maximum correlated well with the corresponding frequency of maximum dielectric loss for PHMA. The temperature dependence of these two quantities could be reproduced with an Arrhenius activation energy of 21 Kcal/mol. A consistent picture of the molecular dynamics of PHMA near the glass transition requires a strong secondary relaxation process with a different temperature dependence from the primary glass-rubber relaxation. The present results suggest that the behavior of PHMA is similar to the other poly(alkyl methacrylates). © 1996 John Wiley & Sons, Inc.     

Rheological properties of poly(2,6-dimethylphenylene oxide)—polystyrene blends

The viscoelastic (VE) response of freeze-dried blends of polystyrene (PS) and poly-(2,6-dimethyl phenylene oxide) (PPO) has been studied as a function of composition, frequency, and temperature to examine the degree of rheological compatibility. When blended together, the relaxation processes of both molecular species exhibit the same temperature dependence. However, the temperature dependence of the VE response is a function of composition. It is shown that this behavior can be predicted from the measured glass transition temperatures by assuming the additivity of the free volumes of the components. The properties of the blends are compared at equal free volumes. The effective segmental friction factor is found to be independent of composition while the modulus of the rubbery plateau increases with PPO concentration. This result is interpreted as a change in the entanglement molecular weight M_e of the blends. When the changes in M_e are considered, the relationship between the zero-shear viscosity η0 and the 3.4 power of the weight-average molecular weight, commonly found for high molecular weight homopolymers, predicts the compositional dependence of η0 for the PPO–PS blends. It is concluded that the PPO–PS system forms a rheologically compatible blend.     

How does tack depend on time of contact and contact pressure?

The tack of polymer melts on rigid substrates under conditions of short contact times and low pressures is examined. The substrate is modeled as a random rough surface with a distribution of asperities heights. The true contact area between the adhesive and the substrate is calculated for a given total load and elastic modulus of the substrate. The dependence of tack on contact time is accounted for by introducing the relaxation of the adhesive through a time-dependent elastic modulus. For relatively high pressures the tack is predicted to scale with 1/E so that for short contact times, t_c, the tack is predicted to scale with (t_c/τ_e)^1/2, where τ_e is the entanglement time. For lower pressures this simple scaling law is no longer valid and we predict a complex variation of tack with contact time and molecular parameters. © 1996 John Wiley & Sons, Inc.     

Effects of the strain rate and temperature in stress–strain tests: study of the glass transition of a polyamide-6

In this work new insights are presented on the measurement of the tangent and secant moduli from stress–strain curves in polymeric systems. Expressions for the strain-rate and strain dependence of both moduli are derived for systems characterised by a distribution of relaxation times. The equivalent frequency of the stress–strain experiments is shown to be dependent on the strain rate and on the strain at which the measurements are carried out. Such considerations enable using quasi-static tensile stress–strain tests to study relaxational processes in polymeric materials. The tensile behaviour of a 30% glass fibre reinforced polyamide 6 was characterised at different strain rates and temperatures, covering the glass transition region. A master curve of the tangent modulus as a function of strain rate was successfully constructed by simple horizontal shifting of the isothermal data. The temperature dependence of the shift factors was well described by the WLF equation. It was also possible to fit the master curve considering a polymeric system with a distribution of relaxation times, relevant parameters such as the KWW β parameter being extracted. The results were found to be consistent with dynamic mechanical analysis results.     

Mechanical and dielectric studies of carrageenan sols and gels

The temperature dependence of the dynamic shear modulus, strain optical coefficient, DC conductivity, and complex dielectric spectrum of κ- and ι-carrageenan aqueous solutions with K, Ca, Cs, and Na were measured in order to clarify the formation process of the cross-linking region and the gel network structure. From the correlation analysis between the shear modulus and the strain optical coefficient, we found that the stress inducing unit orientation increases with decreasing temperature, which strongly suggests that the branching number in a cross-linking region increases with decreasing temperature, which depends on counterion species. In terms of the correlation parameters, an increasing scheme of the branching number depends on counterion species. Just below the coil-helix transition temperature, dielectric relaxation arises, with relaxation time ∼100μs and relaxation strength ∼10³. Dielectric relaxation can be assigned to the counterion fluctuation in the parallel direction to the helical axis. The fluctuation distance of the counterion estimated from the relaxation time increases sharply in the initial stage of gelation and gradually reaches a constant value. We concluded that the longitudinal length of the aggregated region increases sharply at the initial state of gelation while the number of helical molecules bundled in a cross-linking region increases successively with decreasing temperature.     

Viscoelasticity of oriented poly(ethylene terephthalate)

Time–temperature superposition can be successfully applied to both the stress relaxation and dynamic mechanical properties of oriented PET fibers. Two curves result; one is the time dependence of the modulus at constant temperature, while the other is the shift, log aT, of this curve along the time scale as a function of temperature. This temperature dependence is less than that for both unoriented PET and typical amorphous polymers above T_g. It is about the same as that for oriented nylon 66 and unoriented glassy poly(methyl methacrylate). The isothermal modulus has the same time dependence as that of the unoriented PET; however, it is a factor of 3.3 larger. The modulus curve is almost identical in both shape and magnitude with that of oriented nylon 66. However, a temperature of 82°C. is required to place the viscoelastic dispersion region of PET at the same time scale as nylon 66 at 25°C. This temperature increase is the major difference in viscoelasticity between these two oriented polymers.     

Dynamic parameters for some poly (n-alkyl methacrylates)

The dielectric relaxation data of poly (n-hexyl methacrylate) and poly (n-octyl methacrylate) by Strella and Chinai are represented in terms of the relaxation function proposed by Havriliak and Negami using the multiresponse techniques developed by Havriliak and Watts to evaluate the parameters and their dependence on temperature. The parameter α which represents the width of the distribution of relaxation times was found to be dependent on temperature whereas the parameter β representing the skewness of the distribution function was found to be independent of temperature. The relaxation process parameters are discussed in terms of Mansfield's jump model.     

Thermostimulated depolarization currents in thermorheologically simple materials

A theory is presented which makes possible the calculation of the dielectric parameters for a distributed dipole relaxation from thermostimulated depolarization current (TDC) data. The theory is applicable to dielectrics which obey the time–temperature superposition principle, i.e., for thermorheologically simple materials. The shift factor, the activation energy, the dielectric relaxation strength, the density of the isothermal displacement current, and the distribution function of relaxation times of the β relaxation in poly(methyl methacrylate) are calculated. The TDC investigations were carried out over the temperature range of ?136 to 90°C. The values for the activation energy U = 26.4 kcal/mole and the dielectric relaxation strength Δ_?β = 2 are in good agreement with values obtained from dynamic measurements. A criterion for checking the validity of the time–temperature superposition principle by TDC is suggested.     

Ab initio Predictions of Structural and Thermodynamic Properties of Zr2AlC Under High Pressure and High Temperature

The structural and thermodynamic properties of Zr₂AlC at high pressure and high temperature are investigated by first principles density functional theory method. The calculated lattice parameters of Zr₂AlC are in good agreement with the available theoretical data. The pressure dependences of the elastic constants, bulk modulus, shear modulus, Young's modulus, and Vickers hardness of Zr₂AlC are successfully obtained. The elastic anisotropy is examined through the computation of the direction dependence of Young's modulus. By using the quasiharmonic Debye model, the thermodynamic properties including the Debye temperature, heat capacity, volume thermal expansion coefficient, and Grüneisen parameter at high pressure and temperature are predicted for the first time.     

The mechanical moduli of lamellar semicrystalline polymers

Bounds on the elastic constants are derived for semicrystalline polymers whose local morphology is lamellar. Local response matrices (stiffness and compliance) are formulated in three dimensions that simultaneously incorporate uniform in-plane strain and additive forces from layer to layer of crystalline and amorphous phases and uniform stress and additive displacements normal to the lamellar surfaces. Spatial averaging of the stiffness and compliance matrices under the assumption of axially symmetric orientation gives the upper and lower bounds on the longitudinal and transverse tensile moduli and the axial and transverse shear moduli as functions of the separate phase elastic constants, the volume percent crystallinity, and the moments of the orientation 〈cos²θ〉 and 〈cos⁴θ〉. The bounds are much tighter than the Voight upper and Reuss lower bounds that do not recognize phase geometry. Using the known crystal elastic constants of polyethylene, sample calculations on isotropic unoriented materials show that the divergence of bounds at high crystallinity necessitated by the extreme crystal anisotropy shows up only at very high crystallinity. At low temperature the bounds are tight enough to specify G₁, the amorphous modulus, from the measured G and the known crystal elastic constants. At higher temperatures and lower G, the bounds are not tight enough for this purpose but the shear modulus versus crystallinity and temperature data are well fitted by the lamellar lower bound using a temperature-dependent, crystallinity-independent G₁.     

Enhanced translational diffusion of rubrene and tetracene in polysulfone

Translational diffusion of tetracene and rubrene in bisphenol A polysulfone (T_g = 460 K) was measured using a holographic fluorescence recovery after photobleaching (FRAP) technique. In the temperature range from 493 to 462 K, probe translation was diffusive and the translational diffusion coefficients varied from 10⁻⁸ to 10⁻¹³ cm²/s. Surprisingly, the observed translational diffusion coefficients showed a weaker temperature dependence than the rotational correlation times of the same probes. Rotational correlation times have the same temperature dependence as the viscoelastic relaxation times characteristic of the rubberlike modulus, while translational relaxation times decouple from the viscoelastic relaxation times. On average, probe molecules are translating larger and larger distances per probe rotation time as the temperature is lowered to T_g. These results can be explained qualitatively in terms of spatially heterogeneous segmental dynamics in the polysulfone matrix. © 1996 John Wiley & Sons, Inc.     

A dielectric study of molecular relaxation in oxidized and chlorinated polyethylenes

A study was made of the dielectric relaxation in polyethylenes rendered dielectrically active through oxidation (0.5–1.7 carbonyls/1000 CH₂) and chlorination (14–22 Cl/1000 CH₂). Both linear and branched polymers were studied. All of the relaxations between the melt and ?196° were studied in the frequency range 10 Hz to 10kHz (100 kHz in the chlorinated samples). In the linear samples a wide range of crystallinities was studied (55% in quenched specimens to 95% in extended-chain specimens obtained by crystallization at 5 kbar). As is consistent with its being a crystalline process, the α peak was found to discontinously disappear on melting of the samples and reappear on recrystallizing on cooling. The disappearance of the smaller crystals before the larger ones appeared to be evident in the isothermal loss versus frequency curves. The relaxation strength of the α process increases with crystallinity. The measured relaxation strength is less than that expected on the basis of direct proportionality to the crystalline fraction with full contribution of all dipoles in the crystalline material. However, the intensity is not sufficiently low for the process to be interpreted in terms of reorientation of localized conformational defects in the crystal. The variation of intensity with crystallinity is best interpreted in terms of full participation of crystalline dipoles but with selective partitioning of both carbonyls and chlorines favoring the amorphous domains. A strong correlation of the α loss peak location (T_max at constant frequency or log f_max at constant T) with crystallinity for both carbonyl and chlorine containing polymers was found. This variation is interpreted in terms of chain rotations in the crystal where the activation free energy depends on crystal thickness. The dependence of log f_max and T_max on lamellar thickness as well as a comparison with the loss peaks of ketones dissolved in parafins indicates that the chain rotation is not rigid and is accompanied by twisting as the rotation propagates through the crystal. In agreement with previous studies the β process is found to be strong only in the branched polymers but can be detected in the chlorinated linear polymer. The β process was resolved from the α in the branched samples by curve fitting and its activation parameters determined. The γ relaxation peak in oxidized polymers including its high asymmetry (low-temperature tail) and increasing ε_max with increasing frequency and temperature when plotted isochronally can be interpreted in terms of a simple nearly symmetrical relaxation time spectrum that narrows with increasing temperature. No increase in relaxation strength with temperature was found. The chlorinated polymers behave similarly but appear to have some Boltzmann enhancement (450–750 cal/mole) of relaxation strength with temperature. The dependence of relaxation strength on crystallinity indicates that the process is an amorphous one. Further, no evidence of relaxation peak shape changes with crystallinity that could be interpreted in terms of a crystalline component in addition to the amorphous one was found. The comparison of the γ relaxation strength with that expected on the basis of full participation of amorphous dipoles indicates that only a small fraction (～10% in oxidized linear polymers) of them are involved in the relaxation. Thus it would seem that a glass–rubber transition interpretation is not indicated but rather a localized chain motion. It is suggested that the γ process, including its intensity, width, and activation parameters, can be interpreted in terms of an (unspecified) localized conformational (bond rotation) motion that is perturbed by differing local packing environments. The thermal expansion lessens the effects of variations in packing and leads to narrowing with increasing temperature. The conformational motion itself leads to increase in thermal expansion and hence a transition in the latter property. Some previously proposed localized amorphous phase conformational motions appear to be suitable candidates for the bond rotation motion. A weak relaxation peak found at temperatures below the γ and at 10 kHz may possibly be the dielectric analog of the δ cryogenic peak found previously mechanically at lower frequencies.     

Elastic modulus of poly(isobutylene oxide) crystals parallel to the chain axis

The elastic tensile modulus of the crystalline regions parallel to the chain axis E₁ for poly(isobutylene oxide) (PIBO) was determined by an x-ray diffraction method. The stress-strain (σ—ε) curve calculated from the meridional reflection of (002) shows an inflection point at about 75 MN/m² or an extension of 0.25%. The observed moduli below and above this point are 29 and 47 GN/m², respectively, at room temperature. The initial lower modulus agrees well with the value, 29.7 GN/m², calculated on the basis of the double zigzag structure of PIBO chain determined by Kaji and Sakurada. The higher modulus may be due to an intrachain steric hindrance between the hydrogen atoms of the methylenic group in the main chain and those of the two side methyl groups.