Effect of polymer matrix on the rheology of hydroxyapatite‐filled polyethylene composites

The effect of matrix polymer and filler content on the rheological behavior of hydroxyapatite‐filled injection molding grade high‐density polyethylene (HDPE) has been studied. Studies of the flow curves revealed that the matrix and the composite exhibit three distinct regions in the flow curve, namely, a pseudoplastic region at low to moderate shear rates, a plateau and a second pseudoplastic region at high shear rates. The shear stress corresponding to the plateau (τ_c) is dependent on both the filler concentration and the melt temperature. Addition of HA in the HDPE matrix increases the value of τ_c and decreases compressibility of the melt. An increase in temperature also raises the value of τ_c. From the nature of flow curves it is concluded that the matrix polymer largely decides the rheology of the composite.     

Nonlinear numerical simulation on the onset of Soret-driven motion in a silica nanoparticles suspension

The onset of buoyancy-driven convection in an initially quiescent, horizontal silica nanoparticle suspension layer heated from above is analyzed theoretically. In this thermally-stably stratified fluid layer the Soret diffusion can induce buoyancy-driven motion for the case of the negative separation ratio. For the high Rayleigh number the convective motion sets in during the transient diffusion stage and the onset time of this motion is analyzed by employing the nonlinear numerical simulation. It is interesting that the convective motion is very weak and the diffusional process is dominant even after the onset time of convection, τ _c , and the nonlinear effects are manifested from the time τ _m (>τ _c ). The present τ _m explains the existing experimental results quite well.     

Onset of buoyancy-driven convection in the horizontal fluid layer subjected to ramp heating from below

The onset of buoyancy-driven convection in an initially isothermal, quiescent fluid layer heated from below with a constant heating rate is analyzed by the propagation theory. Here the dimensionless critical time τ_c to mark the onset of convective motion is presented as a function of the Rayleigh number Ra_φ and the Prandtl number Pr. The present stability analysis predicts that for a given large Ra_φ, τ_c decreases with increasing Pr and it is independent of the conditions of the upper boundary. For deep-pool systems, the deviation of the temperature profile from conduction state occurs starting from a certain time τ_o≅4τ_c. The present predictions are compared with other models and existing experimental results in the whole time domain.     

The onset of Taylor-Görtler vortices during impulsive spin-down to rest

The onset of hydrodynamical instability induced by impulsive spin-down to rest in a cylinder containing a Newtonian fluid is analyzed by using the propagation theory. The primary transient swirl flow is laminar but for an initially high rotating speed secondary motion sets in at a certain time. It is found here that the critical Reynolds number Re_c=320, below which the flow is unconditionally stable. For Re>Re_c the dimensionless critical time τ_c to mark the onset of a fastest growing instability is presented as a function of the Reynolds number Re. Available experimental data and also predictions show that deviation of the velocity profiles from their primary ones occurs starting from a certain time τ≈4τ_c. This means that secondary motion is detected experimentally at this characteristic time. It seems that during τ_c?τ?4τ_c secondary motion is relatively very weak.     

Mobility and e.s.r. spectra of alkyl free radicals trapped in irradiated mats of solution-grown polyethylene

E.s.r. studies of alkyl free radicals,a trapped in irradiated mats of solution-grown polyethylene crystals have been studied. The crystal mat was aligned so that the crystallites c-axes were perpendicular to its surface. When the angle between the direction of applied magnetic field and the c-axis of the crystallite was varied, the observed e.s.r. spectra showed apparent anisotropy due to α-proton hyperfine splitting. The principal values and directions of the $\text{A}\text{?}$ tensor due to the α-proton at 77 and 254K were determined using the line shapes of uniaxially-oriented samples. The principal axis directions, A₁ were assigned along the polymer chain axis and A₂ and A₃ were perpendicular to the chain axis. The maximum principal value A₁ did not change with temperature, but the lower principal values, A₂ and A₃ averaged out at high temperatures. Hence, it was concluded that the temperature dependence of the $\text{A}\text{?}$ tensor reflects hindered oscillation around the main chain axis at high temperature. A twisted configuration for the methylene group with 4.5° deviation from the trans-trans configuration was also evident for the rigid state configuration of the alkyl radicals at 77K. Temperature dependences of the hyperfine splitting widths due to β-protons were observed and analysed in terms of hindered oscillations of the β-methylene group.     

Influence of irradiation crosslinking on time dependences of conduction and self‐heating in acetylene carbon black filled high‐density polyethylene composites

The time dependences of electrical conduction and self‐heating in high‐density polyethylene/acetylene carbon black composites crosslinked with electron beam irradiation at three different dosages are studied in relation to voltage and ambient temperature. The characteristic decay current constant (τ_i) and the exponential growth time constant for self‐heating (τ_g) are determined for the samples under voltages (U) above the onset voltage (U_c) of self‐heating. The influence of crosslinking on the current decay dynamics, self‐heating process, and amplitude of the resistance switching under field action are discussed. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 4418–4422, 2006     

Melting and annealing effects on molecular motion of spin-labelled molecules at the polymer chain-end located on the polypropylene surface

The samples of spin-labelled molecules at the polymer chain-ends located on the polypropylene surface were annealed and melted. The e.s.r. spectra of spin-labelled molecules were observed at various temperatures. A long layer distance was obtained by small angle X-ray scattering spectroscopy. The plot of extrema separation (W) with temperature of the sample shows two steps of narrowing. In the first step (I, Iow temperature region), melting of the sample increases both the value of activation energy (ΔE) and the characteristic temperature (T-I); so the mobility of the spin labelled molecules trapped in the interior of bulk is less than on the surface. These increases are ascribed to more crowded surroundings. The effect of annealing on the motion is dependent on annealing temperature. In the second step (II, high temperature region), annealing at high enough temperatures increases the value of the characteristic temperature (T-II). These increases are ascribed to an inclusion of the spin-labelled molecules in the crystal. The first step (I) shows the influence of trapping site on mobility of spin-labelled molecules, and the second step (II) shows the influence of a crystal.     

Study on the foaming of crosslinked polyethylene

The processing of crosslinked polyethylene foam, which has a closed‐cell structure, has been investigated. In this study, two types of linear low‐density polyethylene (LL) produced by a metallocene catalyst were crosslinked by dicumyl peroxide (DCP). The expansion ratio of the foams decreases with increasing the DCP content, which is due to the enhancement of the elastic modulus. Moreover, the crystallization temperature T_c of the foams is also responsible for the expansion ratio. After expansion, the crosslinked foam with lower T_c shrinks to a great degree prior to the crystallization, which is attributed to the volume reduction of the gas in the cells. As a result, the expansion ratio decreases. The degree of shrinkage decreases with increasing the T_c, because immediate crystallization prevents the shrinkage. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 79: 2146–2155, 2001     

Permeable domains of segmented polyurethanes studied with paramagnetic spin probe

Studies on the permeable regions of the dense polyurethane-based membranes were performed by electron spin resonance spectroscopy (ESR) using TEMPO spin probe incorporated into the membrane via diffusion from the vapour phase. The ESR spectra were measured as a function of temperature and microwave power for polyurethanes (PU) varying in the molecular structure and morphology. It was found that the TEMPO spin probe exhibited anisotropic rotation whose anisotropy increased as temperature decreased and was more pronounced for PU with shorter soft segments. The simplified method was used to obtain apparent correlation time τ_c enabling the comparison of the polyurethanes studied. This approach was based on the Arrhenius relation of τ_c vs. 1/T determined from motionally narrowed ESR spectra and on the assumption that this behaviour prevails over a broader temperature range at temperatures generally greater than T_g of a given polymer.     

Electrical properties of composites in the vicinity of the percolation threshold

The electrical response of thermoplastic composites composed of carbon black and high‐density polyethylene near the electrical percolation threshold (p_c) has been investigated through the study of the volume resistivity and complex permittivity. The change in conductivity beyond p_c exhibited a critical exponent that was greater than predicted from percolation theory. Composites with carbon black contents slightly larger than p_c exhibited the greatest sensitivity in volume resistivity with temperature variations under the melting point of polyethylene. In addition, percolating composites with low carbon black contents exhibited significant “negative temperature coefficient” (NTC) effects and improvements in conductivity with annealing. Maxwell–Wagner interfacial polarization resulted in moderate increases in both the permittivity (ϵ′) and dielectric loss factor (ϵ″) below p_c, while at percolation, an abrupt and dramatic increase was observed for both components of the complex permittivity. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 1573–1582, 1999     

Dependence of relaxation processes in a low-density polyethylene with different crosslink densities investigated by fluorescence spectroscopy

Relaxation processes of one type of low-density polyethylene with three different degrees of crosslinking were studied using the fluorescence of 1-pyrenyl groups attached covalently to polymer chains. The average molecular weights between crosslinks (M_c) of the polyethylenes were 125, 76.5, and 7 kg mol⁻¹. As assessed by changes of the integrated fluorescence intensity and the full width at half maximum of the 0-0 emission band with temperature, chain mobility decreases with increasing crosslinking density. Chain mobility in the polyethylene with the smallest M_c value appears to be strongly inhibited up to 220 K, while motions of small segments of the polymer chains were observed for samples with lower crosslinking degrees at temperatures similar to those of the pristine (non-crosslinked) polymer. These data are discussed in terms of local versus bulk properties of the films.     

Short-time Monte Carlo study on the phase transition of a ferromagnetic polymer chain model

Short-time dynamic scaling analysis method is applied to investigate the phase transition of a ferromagnetic Ising chain model on the simple cubic lattice. The phase transition temperature T_c is determined from the time evolutions of magnetization M(t) and square magnetization M²(t) at temperatures near T_c. The resulted T_c is consistent with that obtained from the annealing Monte Carlo method. The short-time dynamic behaviors of magnetizations near the transition temperature are also studied. We find both M(t) and M²(t) show power law decay near T_c after a very short time period τ_mic≈100 Monte Carlo steps.     

E.s.r. studies of peroxy radicals in polyethylene: 2. Peroxy radicals in urea — polyethylene complexes

Temperature dependent e.s.r. spectra of peroxy radicals in urea—polyethylene complex materials (UPEC) were discussed. Peroxy radicals in UPEC were much more stable than in normal polyethylene. Change of anisotropic g-values of the radicals with temperature were investigated and it was found that the g₁-axis, which was assigned to be parallel to the chain axis, seemed to be almost unchanged throughout the temperature range in the present study, but the g₂- and g₃-axes changed drastically. Using these results, it was concluded that the main motion of the radical sites was rotation around the chain axis and this motion was found to be much faster than in normal polyethylene, which we have already studied and which we reported recently. This is a reflection of the properties of the materials studied, i.e. interaction between molecular chains in UPEC must be much weaker than in normal polyethylene because of the presence of the wall of urea molecules in the former.     

BiFeO3 ceramics synthesized by spark plasma sintering

Phase-pure BiFeO₃ particles were synthesized by an improved solid state technique. High density BiFeO₃ ceramics were prepared using these particles by spark plasma sintering (SPS). The dielectric permittivity and loss of SPS samples were measured as functions of sintering temperature, frequency, and annealing conditions. Dielectric spectra of the ceramics annealed at 650 °C were characterized in a broad range of temperature (300–725 K) and frequency (100 Hz to 20 MHz). Two kinds of dielectric relaxation following the Arrhenius law were detected in low and high temperature ranges, respectively. The low temperature dielectric relaxation could be almost completely removed by annealing in vacuum and it should be assigned to be a valence fluctuation of Fe ions, while the high temperature dielectric relaxation was proposed to stem from the short-range motion of oxygen vacancies.     

X‐ray diffraction studies on reverse‐annealed polyethylenes

Linear low and high density polyethylene sheets were compression molded and crystallized at a 5–10°C/min cooling rate. Parts of the sheets were annealed at different temperatures up to 2°C below the melting temperature. The small angle X‐ray scattering (SAXS) and the wide angle X‐ray scattering intensities of the annealed samples were studied. SAXS intensities showed particle scattering with a bimodal size distribution. The estimated radii of gyration were 15–17 nm and 5–7 nm, respectively. The crystallinity and the radius of gyration increased slightly with increasing annealing temperature for some samples; others did not show any change. No peaks characteristic of intercorrelated lamellar crystallinity in the SAXS intensities developed during the annealing. The original broad peak of high density polyethylene disappeared from the SAXS recordings on annealing. The length of the perfect chain versus melting temperature was calculated by the Thomson‐Gibbs formula and Flory's concept of melting temperature depression where methyl groups and tertiary carbon atoms at the branches were regarded as second components (solvent). Linear relationships were found for both cases. Experimental data for a linear low density polyethylene obtained from the literature were in between the two functions. A lamellar model of crystallization corresponding to the data is proposed. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 340–349, 2001     

Modeling attempt of shape memory performance of epoxy resin with experimental methods

Epoxy resin samples with low cross-linking density were prepared, and their shape recovery rate and glass transition were studied. The results showed that the shape fixity ratio was over 99 % for all of the samples. Without constraint, the final recovery ratio was approximate to 100 % for all of the samples. The temperature with rapid recovery rate for different samples changed in accordance with T _g. Under a constant temperature, the folding angle for the samples of EP80 decreased with the increase in time rapidly, at first, and then tended to level off. Curves can be fitted with the formula of $ y + A_{0} = A/(1 + \exp ((t - t_{0} )/\tau )) $ with R ² higher than 99.9 %. The fitting results demonstrated that the value of τ decreased from 15.1 to 6.39 when the temperature increased from 88 to 98 °C for EP80. The recovery rate decreased a little by extending the holding time from 10 to 60 s. By keeping the testing temperature constant, glass transition temperature (T _g) decreased with the increase in curing agents, and the value of τ reduced with the decrease in T _g. Usually, when temperature was close to T _g, segments of macromolecules were idle to move, and then, the relaxation process, τ, was lengthened and the shape recovery rate decreased accordingly. In a word, τ showed the similar change rules with that of relaxation process of polymers; therefore, the shape recovering process could be predicted with the model of relaxation time and modulus according to relaxation formulas.     

Passivation mechanism of thermal atomic layer-deposited Al2O3 films on silicon at different annealing temperatures

Thermal atomic layer-deposited (ALD) aluminum oxide (Al₂O₃) acquires high negative fixed charge density (Q_f) and sufficiently low interface trap density after annealing, which enables excellent surface passivation for crystalline silicon. Q_f can be controlled by varying the annealing temperatures. In this study, the effect of the annealing temperature of thermal ALD Al₂O₃ films on p-type Czochralski silicon wafers was investigated. Corona charging measurements revealed that the Q_f obtained at 300°C did not significantly affect passivation. The interface-trapping density markedly increased at high annealing temperature (>600°C) and degraded the surface passivation even at a high Q_f. Negatively charged or neutral vacancies were found in the samples annealed at 300°C, 500°C, and 750°C using positron annihilation techniques. The Al defect density in the bulk film and the vacancy density near the SiO_x/Si interface region decreased with increased temperature. Measurement results of Q_f proved that the Al vacancy of the bulk film may not be related to Q_f. The defect density in the SiO_x region affected the chemical passivation, but other factors may dominantly influence chemical passivation at 750°C.     

Rheological and thermal properties of m-LLDPE blends with m-HDPE and LDPE

The dynamic and steady state behavior of metallocene linear low density polyethylene (m-LLDPE) blended with metallocene high density polyethylene (m-HDPE) and with low density polyethylene (LDPE) were measured in parallel plate rheometer at 160, 180, and 200 °C. The composition dependence of zero shear viscosity η₀, the characteristic relaxation time τ₀ and the characteristic frequency ω₀ of m-LLDPE/m-HDPE blends show a linear variation in the whole range of weight fraction, which indicates that m-LLDPE/m-HDPE blends are miscible blend. At the same time, m-HDPE showing a ‘dissident’ rheological behavior should possess a certain very low degree of LCB. Two calculation methods of LCB verify this point. In contrast, the composition dependence of zero shear viscosity η₀ of m-LLDPE/LDPE blends shows a positive deviation from the log-additivity rule, which can be well fitted by using the immiscible blend equation of Utracki. The characteristic relaxation time τ₀ and the characteristic frequency ω₀ have a sharp variation with the small amounts of LDPE in the blends, which also indicates a phase separation in the system. The thermal properties of m-LLDPE/m-HDPE blends are very similar to a single-component system. However, m-LLDPE/LDPE blends are immiscible in both melt and crystal states. DSC results are consistent with the rheological properties of these two series of blends.     

Structure–Property–Process Relationship for Blown Films of Bimodal HDPE and Its LLDPE Blend

Blown films of bimodal‐high‐density polyethylene (HDPE) (BPE) and its blend containing 40 wt% of linear low‐density polyethylene (LLDPE) are prepared in various neck‐heights (NHs). The crystal structures of both films are investigated in detail using small‐angle X‐ray scattering and wide‐angle x‐ray diffraction techniques. The results show that the blending of LLDPE notably modifies the crystal structure of BPE, including crystal density (ρ_c), crystallite size of the 110 plane (〈L₁₁₀〉), thickness of the lamellar crystal (L_c), and grain widths of the lamellae. The relationships between NH, crystal structure, and the resistance of dart‐drop impact (DDI) are investigated for both BPE and BPE/LLDPE films. The results indicate that the reorientation of lamellae might be a primary factor responsible for the DDI property. However, large values of ρ_c, L_c, and 〈L₁₁₀〉 are required for the film to achieve high DDI.     

Relaxation spectrometry of polyethylene in urea-polyethylene complex by BL n.m.r. and e.s.r. measurements

The molecular motions of polyethylene in urea-polyethylene complexes have been studied by e.s.r. and BL n.m.r. The e.s.r. spectra of alkyl free radicals located along the polymer chains were recorded and the temperature dependences of hyperfine splitting widths due to the β-protons (ΔH_β1, ΔH_β2) and linewidths were estimated. A decrease in (ΔH_β1 ? ΔH_β2) and narrowing (which appears over a wide temperature region) was found. The motional narrowing of line widths in the _BL n.m.r. spectra were also found. Correlation times for the molecular motion of polyethylene molecules were calculated from this magnetic resonance data, taking into account the distribution of relaxation times according to Miyake's equation. It was concluded that the molecular motions studied by e.s.r. and n.m.r. were the same. Relaxation time spectra indicated higher activation energies (10 kcal) than those calculated for a system with a single correlation time.