Diffusion of water in a glassy epoxy resin has been studied by the sorption method. Film thickness scaling is used as a diagnostic tool for non-Fickian sorption processes. A sorptiondesorption cycle effectively increases the preexisting free volume in the resin and renders the subsequent sorption process different from the original. The effect is understood by considering the time scale for diffusion and that for molecular relaxation of the glassy polymer network. A non-Fickian sorption process is observed for an insufficiently cured resin and is caused by the resin undergoing further postcuring in the sorption process. It is shown that oxidation of the resin during sorption also gives rise to non-Fickian processes previously described in the literature as diffusion anomalies for epoxies. It is further shown that the difference between sorption and resorption curves is not caused by irreversible damage to the resin but due to reversible changes in network conformations. 相似文献
Theoretical equations have been developed for crystal growth rate in layers of small frit (glass) particles during firing. Throughout the process, the crystalline and the glassy phases have different compositions; therefore, the system can be considered a pseudo-two-component system consisting of a crystallizable component (structural unit) and a noncrystallizable mixture of several components. The concentration of the crystallizable component decreases in the residual glassy phase during the crystal growth process, on integrating at the surfaces of crystals having the same composition. Throughout the crystal growth process, a concentration gradient of the crystallizable component is therefore produced in the glassy phase, which results in mass transport by diffusion of this component from the bulk residual glassy phase to the surfaces of the crystals. Equations have been derived assuming that the diffusion step of the crystallizable component through the residual glassy phase is the overall crystal growth process rate-controlling step. 相似文献
The environmental stress crazing and cracking (ESC) behavior of an aromatic polyetherimide (PEI) has been characterized in a wide spectrum of organic liquids and compared to the behavior of several other glassy thermoplastics. PEI's response is qualitatively similar to that of the other resins for each of which ESC resistance reaches a minimum in solvents having solubility parameters close to that of the resin. Taken as a whole, the ESC resistance of PEI is found to be quantitatively superior to that of any other glassy resin for which similar data are available for comparison. 相似文献
Summary Dense polymer membranes were made by mixing aqueous solutions of hydrophilic polymers poly(vinyl alcohol) (PVA) and polyethyleneimine
(PEI) in different ratios for investigating the separation of Tetrahydrofuran (THF)/water azeotropic mixtures by pervaporation
(PV). In order to gain a more detailed picture of the molecular transport phenomenon, we have performed sorption gravimetric
experiments at 30 °C to compute diffusion, swelling, sorption and permeability coefficients of PVA/PEI membranes in the presence
of THF and water. The membranes were found to have good potential for breaking the azeotrope of THF at 6% concentration of
water. An increase in PVA content in the blend caused a reduction in the flux and an increase in selectivity. Among the blends
tested in the study, the 5:1 PVA/PEI blend membrane showed the highest separation factor of 181.5, exhibited a flux of 1.28 kg/m2h for THF respectively at azeotropic feed composition. 相似文献
Volume relaxation of glassy polymers has been followed by diffusion of simple alkane permeants. The changes in diffusion rates allow direct monitoring of changes in segmental mobilities of the polymers. In general, the diffusion rates decrease linearly with annealing time on double-logarithmic plots and then level off as equilibrium is reached. The reduction in diffusion rates, however, can be recovered by raising the temperature above the glass transition temperature and then cooling to the same annealing temperature. Such behavior indicates that the predominant molecular process responsible for changes in mechanical properties of glassy polymers that accompany volume relaxation is not “structure formation” but is the loss of segmental mobilities arising from a reduction of the total free volume in the polymers. 相似文献
The kinetics of diffusion of water from different activity vapors and liquid phase have been investigated in glassy amorphous poly(aryl ether ether ketcne) (PEEK) films at the temperature of 60°C and in glassy semicrystalline PEEK sheets at different temperatures, respectively. In the case of the amorphous PEEK films (250 μm thick) the data at low activity levels were interpreted by means of a purely Fickian mechanism. At higher activity levels the material has shown the presence of a relaxation process; in this case the data have been interpreted using a model proposed by Berens and Hopfenberg. Equilibrium sorption isotherm is also reported. Liquid water sorption in semicrystalline (30%) PEEK sheets (2 mm thick) has been determined to follow the classical Fickian mechanism. The water uptake values obtained for both amorphous and semicrystalline PEEK, confirm the good moisture and liquid water resistance of this kind of high performance thermoplastic polymer. 相似文献
This work deals with the study of the durability, in a hot/wet environment, of structural adhesively bonded polyetherimide (PEI) assemblies used in power electronics packaging technology. An overall approach is proposed, for which the epoxy joint-PEI substrates assembly on the one hand, and the adhesive system components (substrate surface and bulk adhesive) on the other hand, are studied separately with different analytical techniques. The first part of this work was devoted to the substrate surface state and to its modification using a cold plasma treatment of the PEI surface. Then for chosen parameters (power, duration) contact angle measurements indicated an increased surface tension resulting from surface decontamination (removal of release agent and carbon contaminants) and from the creation of polar species, such as esters or carboxylic acid groups, on the PEI surface (XPS analyses). The second part of this study concerned the bulk adhesive ageing in an ethylene glycol-water solution at 70°C. Mass uptake measurements versus time showed the liquid diffusion in the bulk adhesive associated with a microscopic damage of the epoxy system. An overall plasticizing of the adhesive with a considerable decay of the α-transition temperature of one of the two adhesive epoxy-amine networks (TGDDM-BAPP) was also highlighted using rheometry. However, in these ageing conditions, the adhesive glassy modulus decreases slighty because of the thermomechanical stability of the other epoxy network. In the third part, the asymmetric wedge test showed the beneficial effect of the cold plasma treatment on the epoxy/PEI interface durability in the aggressive medium. 相似文献
Case II diffusion in glassy polymers is considered in terms of a deformation model. The process is controlled by the mechanical response of the glass just ahead of the sharp front to an osmostic swelling stress. The important factor governing the establishment of the sharp solvent front is taken to be the concentration dependence of the creep response time. These ideas are set in the context of existing models for diffusion in glassy polymers, and experimental evidence is presented to support the deformation mechanism. 相似文献
A comprehensive thermal and rheological characterisation of a commercially important aerospace resin system modified with a thermoplastic toughener is presented here. The primary focus has been the understanding of how the cure kinetics and mechanism relate to other processes such as phase separation during cure. Differential scanning calorimetry has shown that thermoplastic modification does not affect the cure mechanism significantly and that the process was dominated by an autocatalytic process up to vitrification after which diffusion‐controlled processes dominate regardless of thermoplastic concentration. Thermoplastic addition at 10 wt.‐% PEI was found to increase the final cure conversion compared with the neat resin and inhibit the onset of diffusion control below a cure temperature of 160 °C. At 20 wt.‐% PEI concentration the final conversion and the onset of diffusion control were similar until 150 °C, after which they displayed lower values than the neat resin. Rheological analysis showed that the phase separation process could be followed conveniently through the changes in viscosity and exhibited effects consistent with expected variations in morphology. The 10 wt.‐% PEI system displayed abrupt increases in viscosities which were indicative of a dispersed particulate morphology, while the 20 wt.‐% PEI system displayed more complex behaviour consistent with a phase‐inverted or co‐continuous structure. The gelation process was shown to obey a power law model although the local environment or morphology of the reactive epoxide group at higher PEI concentrations was shown to affect the fit to the model.
A model is proposed for the diffusion mechanism of water in glassy epoxy resins. The polymer network is assumed to consist of two regions in which water molecules possess different mobilities. By considering the distribution of water molecules among these regions it is possible to describe the concentration dependency of the diffusion coefficient in the sorption and resorption processes. The diffusion coefficient becomes constant when the sorption temperature is close to the effective glass transition temperature of the epoxy-water binary mixture. An explanation of this effect is also provided by the model. 相似文献
The sorption rate of swelling penetrants in glassy polymers has been considered as controlled both by the swelling kinetics and the penetrant diffusion through the swollen layer. The stress exerted on the glassy core at the moving boundary is the driving force for the swelling, and results from an osmotic stress and a differential swelling stress contribution. During the sorption process, the osmotic stress at the moving boundary decreases, due to the increasing diffusion resistance; the differential swelling stress, on the contrary, increases giving rise to an acceleration of the front velocity (Super Case II).
The particular case of negligible diffusion resistance in the swollen region is here considered in more detail. It is shown that the rheological properties of both swollen and glassy phases crucially enter the mass transport problem; thestress relaxation in the swollen region must be taken into account in order to obtain a thickness dependent Super Case 11 effect 相似文献