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Using the peeling-induced splitting method, the effect of the irradiation of ultraviolet (UV) light on the formation of surface gratings on the surface of PMMA(Poly(Methyl Methacrylate))-based films is investigated at room temperature. The thickness of the PMMA-based films is in range of 236–534 nm, and the irradiation dose is in range of 0–5 J/cm2. Surface gratings are formed on the surface of the irradiated PMMA-based films. The spatial wave length is a linear function of the film thickness, independent of the UV doses used. The peeling-induced splitting process introduces compressive stress on the surface of the PMMA-based films, much larger than the corresponding surface energy. The magnitude of the apparent surface force increases with the increase of the film thickness. All the surface gratings formed have amplitudes approximately in the same range.
The effect of gamma radiation in vacuum on the isothermal crystallization kinetics of syndiotactic polystyrene (sPS) was investigated via differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), and x-ray diffraction (XRD). Amorphous sPS samples were irradiated in vacuum, heated to 310 °C, cooled down to crystallization temperatures (Tcs) from 220 to 260 °C, and annealed for different times. Upon reheating, overlapping endothermic melting peaks depicted the various crystallization forms, α, β, and β′. The endotherms were resolved using Gaussian functions relating enthalpy changes to the endothermic envelope. Isothermal crystallization kinetic data were analyzed using Avrami's model with Gaussian functions. The extent of crystallization of β and β′ forms increased with increasing crystallization time and temperature, while that of α form decreased. Crystallization half-time followed a modified Arrhenius equation. Crystallization activation energies for the β and β′ forms of sPS increased with increasing radiation doses. The results are compared to those of air irradiated sPS reported in the literature.
Understanding solvent transport in polymers is of practical importance for the applications of polymers in the fields of food packaging, biomedical bandages, materials engineering, etc. We studied one-side desorption in poly(methyl methacrylate) (PMMA). Experimental results showed that methanol desorption in PMMA depended on temperature and the initial distribution of concentration. The diffusion coefficient in PMMA and the evaporation rate of methanol across the PMMA surface followed the Arrhenius relation. The activation energies for the diffusion and the evaporation of methanol are 18.3, 42.6 and 8.6, 18.3 kJ/mol for the specimens with the ratio of initial mass to the equilibrium saturated absorbed mass, Mi/M∞, being 14.6% and 35.3%, respectively. The partial molal volume increased with the increase of the desorption temperature for Mi/M∞ = 14.6%, while it had an opposite trend for Mi/M∞ = 35.5%. The chemical stresses developed in PMMA during the desorption were also studied.
Isothermal crystallization kinetics of gamma-irradiated syndiotactic polystyrene (sPS) has been investigated by differential scanning calorimetry. Amorphous sPS samples were irradiated in air with gamma ray at various doses from 0 to 800 kGy, at a rate of 30 kGy/h, and melt-crystallized at different temperatures and times. Kinetics parameters were determined using Avrami's model with Gaussian functions and a modified Arrhenius equation. Isothermally crystallized sPS irradiated in air with gamma ray exhibited multiple endothermic melting peaks corresponding to various crystalline forms, and the radiation dose had a strong effect on their melting enthalpies, crystallinities, and crystallization kinetic parameters. The amount of the α-crystalline form increased with increasing crystallization time and those of the β- and β′ forms had an opposite trend. Both crystallization half time and crystallization activation energy of the α form in gamma-irradiated sPS increased with increasing radiation dose.
This study focuses on nanoindentation creep in polycarbonate (PC) and syndiotactic polystyrene (sPS) throughout the transient and steady-state regions. The viscoelastic Burgers model is used to explain transient creep data, while the power-law creep model is used to interpret steady-state creep data. The Newtonian shear viscosity of the Maxwell element and Young’s modulus of the Kelvin element are greater for the creep period than for the preload period, and an opposite trend is noted in the Newtonian shear viscosity of the Kelvin element and Young’s modulus of the Maxwell element. The fact that the Young’s moduli of Maxwell and Kelvin elements in the creep period are different from those in the preload period implies that a stress-induced mesomorphic structure forms or that crystallization occurs in nanoindentation creep. While the strain rate increases with decreasing preload period, the stress exponent factor is almost the same for all preload periods.
The exact solution of viscoelastic stresses in the bilayer system due to thermal and/or lattice mismatch is derived if both layers are Maxwell materials. When the thickness of one layer is much smaller than that of the other layer, the viscoelastic stresses in the bilayer system can be reduced to that of the thin film/substrate system. The relative film thickness and the position in the thin film/substrate systems are included in this solution. The average film stress decreases with increasing the normalized time and finally approaches zero in a long time. As the relative film thickness is equal to or less than 0.001, the average film stresses of the zeroth-order approximation, first-order approximation, and Hsueh and Lee model [J. Appl. Phys.91, 2760 (2002)] are close to that of exact solution. Nevertheless, as the relative film thickness is larger than 0.001, the accuracies of the zeroth-order approximation, first-order approximation, and Hsueh and Lee model are dependent on the normalized time and relative film thickness.
Microcantilevers fabricated by microelectromechanical system processes were used to study the residual stresses in the film/substrate systems. Aluminum films were deposited on silicon nitride substrates by thermal evaporation at room and elevated temperatures, and residual stresses were characterized from the deflection profiles of the Al/SiNx microcantilevers. The Al/SiNx microcantilever beam made of room-temperature-deposited Al film was found to deflect toward the substrate side, which in turn resulted in compressive residual stress in the film. In contrary, the microcantilever of Al film deposited at 105 °C was found to deflect toward the side of Al film when the thickness ratio of film to substrate was greater than 0.31 and the residual film stresses were tensile. The axes with zero bending strain component and zero stresses, i.e., the bending and the neutral axes in the film/substrate system were also investigated. The results can be applied to the arm of the atomic force microscope to characterize its deflection and stresses.
The relationship between atomic force microscopy probe-sample adhesion force and relative humidity (RH) at five different levels of surface free energy (γs) of an organic self-assembled monolayer (SAM) has been investigated. Different γs levels were achieved by exposing a patterned SiO2/CH3-terminated octyldimethylchlorosilane SAM sample to an ultraviolet (UV)/ozone atmosphere. A model consisting of the Laplace-Kelvin theory for capillary condensation for nanosized probe and probe-sample molecular interaction was derived to describe the adhesion force as a function of RH from 25 to 90% for different SAM γs values. The equations were solved analytically by using an equivalent curvature of the probe tip shape. Experimental results show that the adhesion force increases slightly with RH for nonpolar SAM. However, for polar SAM surfaces, it increases at first, reaches a maximum, and then decreases. Both the rate of increase and the maximum of the adhesion force with humidity are γs-dependent, which is in good agreement with theoretical prediction. The large rise in the adhesion force in this RH range is due to the capillary force.
The kinetics of methanol transport in 2-hydroxyethyl methacrylate (HEMA) homopolymer and 75/25 and 50/50 mol fraction HEMA/DHPMA (2,3-dihydroxypropyl methacrylate) copolymers at five different temperatures has been investigated using the sorption experiment technique. A combined case I and case II diffusion model was used to describe the transport processes. Four replicates for each temperature of each material having a nominal thickness of 0.1 mm were immersed in methanol maintained at 35, 40, 45, 50, and 55 °C, and the mass uptake as a function of time was measured gravimetrically. Experimental results are found to be in good agreement with model prediction at all temperatures and for all three materials. Both the diffusion coefficients of case I transport and velocity of case II transport increase with increasing temperature. D values at low temperatures (35 and 40 °C), which are in the 10−9 cm2/s range, of the HEMA homopolymer are less than those of the copolymers. On the other hand, the activation energies of case I transport of the copolymers are substantially higher than those of the HEMA homopolymer; however, the level of DHPMA loading in the copolymer does not seem to affect the activation energy. In addition, thermodynamic heat and free energy of mixing values indicate heat is released when HEMA/DHPMA copolymers are exposed to methanol and that the solvent/copolymer systems exist as a continuous phase. In contrast, the methanol/HEMA homopolymer system exists as separate phases.
Thermoset acrylic–melamine resins are widely used for automobile exterior coatings. These materials are formulated by reacting an acrylic polyol with an alkylated melamine. Because the reactions are reversible, acrylic–melamine coatings tend to hydrolyze during exposures in moist environments. During hydrolysis, water in the coating film is consumed. To keep the moisture content in the film in equilibrium, water must be transported from regions of high water concentration to regions of low water concentration. An approach based on Fourier transform infrared (FTIR) spectroscopy analysis of the coating degradation fitted to a transport model is presented to estimate the diffusion coefficients and velocities of water transport during the hydrolysis of an acrylic–melamine coating exposed to different relative humidities (RHs). Theoretical prediction agreed well with the experimental FTIR data of coating hydrolytic degradation. Generally, both the diffusion coefficient and velocity of water transport in the coating increased with increasing RH. Since water transport resulting from the hydrolysis reactions is a very slow and complex process, the approach presented here provides a viable means for obtaining valuable data for quantitative analyses of coating hydrolytic degradation at different RHs.
Wet oxidation in the AlAs layer sandwiched between two GaAs plates was investigated for the temperature range of 400 to 480 °C. The oxidation rate increased with increasing thickness of the AlAs layer. Theoretical analysis based on the boundary layer diffusion was performed to account for the thickness effect. The theory is in excellent agreement with the experimental measurement.
A fatigue damage accumulation model based on the Paris law is proposed for strain-rate-sensitive polymer composite materials. A pre-exponent factor c2/f and strain-rate-sensitive exponent n are introduced. Numerical analysis of the model was performed using experimental data obtained in the literature. Both factors were found to enhance fatigue damage accumulation. The analysis also revealed that the extent of damage increases with decreasing frequency and that the damage rate is more sensitive to the applied maximum stress than to the stiffness of the material.
Transmission losses were monitored in the ultraviolet-visible spectra of irradiated hydroxyethyl methacrylate (HEMA) copolymer at elevated temperatures. The transmission in irradiated HEMA in the ultraviolet and visible wave length range was almost the same for doses 400 kGy ≤ Φ ≤ 1000 kGy, but was smaller than that of the nonirradiated HEMA copolymer. The reduction in transmission in the irradiated specimens was attributed to the presence of color centers. The concentration of color centers was enhanced by thermal annealing. The transmission data (or absorption data) at 467 nm was found in good agreement with the theoretical model in which the color center production followed a first-order kinetic process. The rate constant satisfies the Arrhenius equation, and the corresponding activation energy is 17.37 kJ/mol and is independent of the dosage. The results were compared with those reported in the literature.
A microscopic theory of thermally induced crack healing in poly(methyl methacrylate) is presented. Both laser-induced cylindrical cracks and knife-induced surface cracks were analyzed. For a given temperature, the crack closure rate was constant for both types of cracks. However, the crack closure rate was lower for samples with cylindrical cracks than for those with surface cracks. The former exhibited higher activation energy for crack closure than the latter, because the knife-induced cracks had sharper crack tips. Fracture stress was proportional to surface crack healing time to the one-fourth power for thermal healing at a given temperature. Based on the reptation model of polymer chains, the activation energy of chain diffusion was calculated. The healing process was monitored via fractography and crack closure was confirmed. The results were compared with solvent healing and thermal healing in the literature.
The effect of buffer and γ irradiation on the optical properties of hydroxyethyl methacrylate (HEMA) copolymer was investigated. The transmission of HEMA copolymer decreased with the increase of irradiation dose and/or pH value of the buffer. The cutoff wavelength of HEMA copolymer exhibits a bathochromic shift as the γ-ray dose and/or pH value of buffer increases. The influence of atmosphere during γ-ray irradiation on the optical properties of HEMA copolymer was investigated. The change of optical properties of HEMA copolymer irradiated in air was more pronounced than that irradiated in vacuum. Light was scattered by holes in the polymer. The relationship between scattering intensity (Is) and incident wavelength (λ) can be described by the formula Is ∝ λ−n. The span of holes increases with the irradiation dose regardless of radiation atmosphere and pH value in the range of 4.1–6.5. A boundary between the inner and outer layers of HEMA copolymer irradiated in air was observed, separating two differential morphologies of holes.
Chemical stresses induced by grain-boundary diffusion in thin films were analyzed. The stress distribution consisted of both tension and compression fields, and its characteristics were similar to those obtained for a semi-infinite solid. At a given time, the maximum stress (tension or compression) increased with increasing film thickness for both constant and instantaneous sources; it was generally higher than that in the semi-infinite system. The maximum stress (tension or compression) decreased as the diffusion time increased and at a given time and film thickness it increased with decreasing diffusivity ratio. The buildup of local stress is likely to cause damage and malfunctions of the film when used in an electronic device.
In the present work we studied the depth of damage layer in machined silicon wafers that was incorporated with chemical etching using micro-Raman spectroscopy. Subsurface damage causes changes in the shape and intensity for the shoulder (450–570 cm−1) of the most intense band (519 cm−1) and the second band (300 cm−1) regions of the Raman spectrum. Etching reduces the thickness of the damage layer and, hence, the intensities at the shoulder and the second band. The intensities at the shoulder and the second band become stable when the damage layer is completely etched out. The shoulder consists of two Gaussian profiles: the major and the minor. The band for the major profile is independent of etching depth, but the band for the minor profile shifts toward the longer wave numbers with increasing etching period until the damage layer is completely etched out. The depth of the damage layer is determined by the profiles of the shoulder and the second band and confirmed by the band shift of the minor profile. Transmission electron microscopy (TEM) further verified the results with respect to the depth of the damage layer. TEM observation showed that dislocations and stacking faults are responsible for the subsurface damage.
The solvent-induced stresses in glassy polymers were investigated. The mass transport accounts for case I, case II, and anomalous transport. Case I transport is attributed to the concentration gradient, whereas case II transport is attributed to stress relaxation. Anomalous transport is the mixture of case I and case II. Both one-side and two-side mass transports with the boundary condition of constant surface concentration are considered. The stresses and longitudinal displacement arising from the mass transport are formulated based on the linear elasticity theory. The maximum stress is always located at the surface at the initial time. The stresses are a function of the partial molal volume, Young's modulus, and Poisson's ratio. From the longitudinal displacement data, the partial molal volume was determined.
The water vapor and gas transport in polyimide films were analyzed using Harmon's model with accounts for case I transport and case II transport. Harmon's model was in good agreement with the experimental data. The diffusion coefficient obtained by Harmon's model was smaller than that obtained by using the short-time slope of mass uptake versus time with the exception of CO2 in polyimide. A comparison of the present model and the dual-mode sorption model, in which populations follow Henry's law and Langmuir type, was made.
The effect of thickness on methanol transport in fourteen-year-old crosslinked poly(methyl methacrylate) was investigated. The samples studied here are from the same primary source of those used by a study made fourteen years earlier. The sample was encapsulated by a plastic bag and maintained in a desiccator at room temperature. Four thicknesses, 0.6, 1.0, 1.5, and 1.9 mm, were examined. Methanol sorption data were fit to a model in which the mass sorption is a combination of case I, case II, and anomalous sorption. The diffusion coefficient for case I transport increases with increasing thickness, but the velocity for case II transport does the opposite. The diffusion coefficient for case I transport and the velocity for case II transport exhibit the Arrhenius behavior. The activation energies for case II transport are 18.9, 16.3, 14.6, and 13.4 kcal/mole, corresponding to the thicknesses 0.6, 1.0, 1.5, and 1.9 mm, respectively. The activation energies for case I transport are 24.7, 24.2, 21.7, and 21.9 kcal/mole, corresponding to the thicknesses 0.6, 1.0, 1.5, and 1.9 mm, respectively. For thickness 1.5 mm the activation energies for case I and case II transport are 21.7 and 14.6 kcal/mole for this study and 24.9 and 17.3 kcal/mole obtained fourteen years ago.