Water Sorption of a Polyurethane/Bentonite Composite: Effects of Bentonite Content and Drying Conditions

In the present work, compression molded polyurethane/bentonite composites were manufactured and their water sorption characteristics were determined as a function of filler content and drying conditions. The matrix was a polyurethane derived from castor oil and the filler was a commercially available sodium bentonite. Filler contents varied from 0 to 10% by weight. The effects of three filler drying conditions, that is, 1 h at 200°C; 1 h at 110°C, and 24 h at 110°C, on the water sorption behavior of PU/bentonite composites with 5% w/w filler loading were also analyzed. The experimental results show that the water sorption behavior of the composites could be satisfactorily modeled by the Fickian model, and that the amount of absorbed water increases with the filler loading. It was also observed that, within the first 200 h of immersion, composite water sorption was lower than that of the polymeric matrix alone. The data evince that although only minor differences were noted, water sorption is reduced when the filler was dried at higher temperatures.     

Transport properties of dichloromethane in glassy polymers. II. Amorphous syndiotactic polystyrene films

Sorption and diffusion of dichloromethane vapor were measured in amorphous syndiotactic polystyrene films, obtained with different cooling conditions and after controlled aging times at different temperatures. The diffusional behavior, at the temperature of 25°C, was characterized by three stages, depending on penetrant activity. In the first stage, at low activity, the diffusion coefficient was independent of vapor concentration; the second stage was characterized by concentration-dependent diffusion, whereas in the third stage, at high activity, the strong interaction solvent–polymer increased the mobility, allowing the polymer crystallization. The different cooling conditions neither have an effect on the diffusional behavior nor on the sorption curve. The aging, both at room temperature and at 70°C, did not change the diffusion parameters, but led to the appearance of more and more anomalous sorption behavior. The sorption curve, as a function of vapor activity, did not show any difference for the fresh and the aged-at-room temperature samples, whereas the samples aged at 70°C presented a lower sorption at low activity. The presence of ordered domains, impermeable to the penetrant at low activity, was suggested on the basis of sorption results. The solvent-induced crystallization was investigated for all the samples. Crystallization was induced at an activity of 0.45 for the fresh and the aged-at-room temperature samples; at variance, the samples aged at 70°C crystallize at a slightly higher activity, reaching, nevertheless, a higher level of crystallinity. © 1993 John Wiley & Sons, Inc.     

聚氨酯大分子表面活性剂组成结构对WPUA胶粒结构及其膜性能的影响

设计了聚合度相同,但组成结构不同的聚氨酯大分子表面活性剂（PUS）,并将此大分子表面活性剂用于甲基丙烯酸甲酯（MMA）的聚合,制备出乳胶粒结构不同的水性聚氨酯-丙烯酸酯乳液（WPUA）,乳液经干燥成膜后得到不同的样膜。应用透射电镜（TEM）、Zeta纳米粒度仪及旋转黏度计对WPUA乳胶粒形貌、粒径和乳液黏度进行了测试与表征,通过ATR-FTIR、吸水率测试、万能材料试验机和紫外-可见光谱仪对WPUA膜的结构、耐水性、力学性能和物理老化行为进行了研究。结果表明,PUS组成结构会影响WPUA乳胶粒的结构,进而影响形成的WPUA膜的性能。WPUA-PCDL乳胶粒的核壳结构明显,其膜的力学和耐老化性能较好。     

SrTiO3薄膜的自组装生长

利用激光分子束外延技术在LaAlO3(100)单晶基片表面生长SrTiO3(STO)薄膜。通过反射式高能电子衍射原位实时监测STO薄膜自组装生长过程，采用原子力显微镜分析自组装生长演化过程，并利用X射线衍射分析薄膜结构及其生长方向。对不同生长条件下薄膜的生长的研究发现：在较低生长温度时，STO薄膜在(200)方向上以三维岛状模式进行生长，小岛在单位原胞尺度呈波浪状周期性排列，即出现自组装生长；而在较高的生长温度时，薄膜以层状模式进行生长自组装行为被抑止。并据此讨论了STO多元氧化物薄膜自组装生长条件及生长机理。     

Dynamic Viscoelasticity of Wood After Various Drying Processes

In this study, specimens of heartwood from Chinese fir (Cunninghamia lanceolata [Lamb.] Hook.) plantation trees were dried by high-temperature drying (HTD), low-temperature drying (LTD), and freeze vacuum drying (FVD), respectively. The dynamic viscoelastic properties of dried and untreated wood specimens with various moisture contents were investigated in the temperature range between ? 120 and 40°C at 1 Hz using a dynamic mechanical analysis (DMA). The results indicated that the relative storage modulus and relative loss modulus were both the highest for HTD wood and the lowest for FVD wood, and that two mechanical relaxation processes developed. The α relaxation process in the higher temperature range was presumably a result of surpassing the glass transition of hemicelluloses with low molecular weight, whereas the β relaxation process occurring in the lower temperature range was most probably due to the motions of both methyl groups in the amorphous region of wood cell wall and adsorbed water molecules in wood. As moisture content increased, the decrease of relative storage modulus with increasing temperature became more dramatic, and the loss peak temperatures of the relaxation processes shifted to lower temperature range. The difference of dynamic mechanical behavior among untreated and dried specimens reduced with the increase of moisture content.     

Sequential biaxial drawing of polypropylene films

Polypropylene cast films were drawn successively in two perpendicular directions at temperatures near the melting point. In order to investigate the structure formation along the deformation path the transverse draw ratio λ_TD was varied between 1 and 9. Quantitative X-ray texture analysis, small angle X-ray scattering and atomic force microscopy were used to characterize the structure of the films on a molecular and supermolecular level. Additionally, the melting behavior and the mechanical properties were investigated.The results of the experimental investigations show, that the first deformation step (MDO Process) transforms the initially spherulitic morphology of the cast film into a stacked lamellae morphology (shish kebab) by partial melting. This stacked lamellae morphology is deformed during the transverse drawing (TDO process) into a fibrillar network under the action of crystallographic slip processes ((010)[001] and (110)[001] slip). Further deformation orients the fibrils towards the actual draw direction. The observed structural changes correlate with the melting behavior and the mechanical and thermomechanical properties of the films.     

Dynamic mechanical properties of bacterial cellulose nanofibres

In this work, dynamic mechanical properties of the grown bacterial cellulose (BC) nanofibers were investigated. BC pellicles were fabricated using bacterial fermentation (Gluconacetobacter xylinus). The morphology results confirmed that the dried BC at ambient conditions could be categorized as a xerogel. The thermal dynamic mechanical analysis results indicated that the bound water in bacterial cellulose structure had a very significant effect on thermal and dynamic mechanical properties of BC pellicles. The results of dehydration kinetics study showed that the main mechanism governing water loss of BC was Fickian diffusion. The glass transition temperatures (T_g) of the BC dried at 25 °C (ambient temperature) and 105 °C were estimated ??33 and ??18 °C, respectively. This discrepancy can be attributed to the plasticizing effect of the bound water of BC dried at ambient temperature. Furthermore, the results indicated that its modulus drop smaller than one order of magnitude can be attributed to its high crystalline nature. The storage modulus versus frequency increased due to the limitation of the relaxation process of the polymer chains. Moreover, the relaxation time distribution was achieved from the slope of the modulus master curve versus logarithmic frequency. As a result, BC exhibited a solid-like behavior.     

Gas sorption in poly(butylene terephthalate). II. Influence of crystallinity and molecular orientation

Sorption measurements of CO₂, Ne, and Ar in poly(butylene terephthalate) (PBTP) films were studied by the gravimetric method with a recording microbalance at 298 K in the pressure range of 1–22 bar. The semicrystalline samples were annealed and oriented at 373 K. The sorption isotherms for CO₂ in PBTP films in the glassy state can be well described by the dual-sorption theory. The nonlinear sorption behavior of Ne and Ar can be satisfactorily analyzed using the sorption model developed for noble gases in PBTP. For the undrawn annealed films, it has been found that the increment of crystallinity leads to the reduction of the equilibrium gas concentration. For the oriented films, the gas concentration rises with increasing draw ratio. It appears that the sorption behavior for all tested gases in the oriented PBTP films does not depend on the changes of crystallinity and crystalline morphology under extension. The difference of the critical pressure p^* indicates the change in the size of the frozen microvoids existing in the noncrystalline phase, which was altered by annealing and drawing. © 1993 John Wiley & Sons, Inc.     

Effect of Drying Conditions on the Mechanical and Barrier Properties of Films Based on Chitosan

Films plasticized with glycerine were prepared using chitosan with two different molecular weights (Mw), 780 and 430 kDa. Films were obtained by drying at 80 and 40°C at 20 and 40% relative humidity (RH) in a climatic chamber. Drying kinetics were established by the measurement of the evolution of the actual temperatures of the film forming solutions. Chitosan Mw did not show any significant influence on drying kinetics. Drying temperature affected drying kinetics in a more intense way than drying RH. Maximum tensile strength (86 MPa) and elongation at break (56.5%) were obtained with slow drying cycles (lower drying T) and when higher Mw chitosan was employed. Minimum water vapor permeability (0.59 g · mm/kPa · h · m) was achieved for films dried at faster drying cycles (higher drying T). Chitosan Mw was not a significant factor affecting water vapor permeability.     

Transport properties of dichloromethane in glassy polymers. I. Atactic polystyrene films

Sorption and diffusion of dichloromethane vapor were measured in atactic polystyrene films, obtained with different cooling conditions and after controlled aging times at different temperatures. The diffusional behavior is characterized by three zones, depending on temperature and penetrant activity. In Zone I, at low activity, the diffusion coefficient is independent of vapor concentration; Zone II is characterized by concentration-dependent diffusion, whereas in Zone III structural transformations are possible. The study of diffusion at three temperatures allowed building of a temperature-penetrant concentration diagram, which is very useful to visualize the different zones of behavior. The aging at room temperature has no effect on the curve of sorption as a function of vapor activity, whereas it determines two effects on the diffusion: a decrease of the zero concentration diffusion coefficient and a more and more anomalous behavior with the aging time. The aging at 70°C produces the same effects on the diffusion behavior as the aging at room temperature; in addition, a decrease sorption is observed at low penetrant activity. The possible presence of ordered domains, impermeable to the vapor, in the samples stored at 70°C, was suggested on the basis of sorption results.     

Equilibrium disperse dye sorption by drawn polypropylene films

Isotactic polypropylene (PP) films drawn at various temperatures ranging from 100 to 155°C have been dyed at 80°C with p-aminoazobenzene (PAAB) or C.I. Disperse Yellow 7 (Y-7). The equilibrium dye sorption (M₀) of PAAB obtained for these films decreased with an increase in draw ratio (λ) of the films. The M₀ values of Y-7, however, increased with increasing in λ similarly the crystallinities of these films increased slightly with draw ratios. By use of fine structural data of these films, the change in M₀ of Y-7 for drawn PP films were analyzed in terms of a mosaic-block structural model. The increase in M₀ with increasing in λ was thus ascribed to enhanced sorption by the amorphous side region between crystalline cores located parallelly in the direction to the molecular axis. The side regions of the drawn systems become rich in interfibrillar tie chains in a higher range of λ. The side region seem to have a strong affinity particularly to a long rodlike dye molecule such as Y-7; this feature is probably associated with the amount of the amorphous chains having extended chain conformation, the fraction of which increases during drawing.     

Residual porosity in polymeric latex films

Solubility and diffusivity measurements of a probe gas (CO₂), which has an inherently low solubility in the polymer, have been used to characterize residual porosity in polymeric latex films. Sorption isotherms resembling those of a glassy polymer were obtained, even though the glass transition temperature of the polymer was 1°C, about 30°C below the experimental temperature. Solvent cast films of the same polymer exhibited much lower solubilities, and followed the expected Henry's law behavior. CO₂ solubility and diffusivity were found to decrease with aging time for the latex films, but did not quite reach the values of the solvent cast films, even after 75 days at room temperature. The sorption data could be described by the dual-mode sorption model, which is commonly employed in the analysis of glassy polymer sorption isotherms. Estimates of the amount of porosity were made from the sorption data, and values ranging from 0.6–0.03% were obtained for latex films aged from 62 h to 75 days, respectively. Our results suggest that permeability differences noted by others for latex and solvent cast films of the same polymer are due to the substantial solubility differences for low-solubility penetrants in these two types of films.     

Sorption kinetics and equilibria in annealed glassy polyblends

The sorption kinetics and equilibria of n-hexane in glassy polyblends of polystyrene and poly(2,6 dimethyl-1,4 phenylene oxide) were studied as a function of annealing conditions. Cast film samples were annealed 20°C above their respective glass transition temperatures for two hours and twenty-four hours. The rate of relaxation-controlled (Case II) sorption of n-hexane in these films was reduced markedly consequent to annealing. The effect of annealing on the sorption kinetics and the independently determined film densities was more pronounced for the poly(phenylene oxide)-rich samples. Although sorption rates were reduced by as much as a factor of 100, the sorption equilibrium was insignificantly affected by annealing. Super Case II transport was observed for the slow absorbing annealed samples whereas the more rapid sorption in the unannealed samples followed ideal Case II kinetics. The more pronounced effects of annealing for the poly(phenylene oxide)-rich samples on sorption rates and film densities were explained by considering the increasing difference between the film T_g and the drying temperature used in the original film preparation for the poly(phenylene oxide)-rich samples. These results suggest that glassy polymers, cast and dried well below their glass transition temperatures, will be subject to large long-term reductions in absorption rates and specific volume. Moreover, residual, excess free volume significantly affects relaxation-controlled absorption of vapors in partially annealed glassy polymers.     

Carbon Cloth Reinforced Carbon Aerogel Films Derived from Resorcinol Formaldehyde

Carbon cloth reinforced RF (Resorcinol Formaldehyde) aerogel films have been produced with extremely high RC ratio (molar ratio of resorcinol to catalyst) or with no catalyst at all. The gels were subcritically dried. Carbon aerogel films were obtained by pyrolysis of the RF aerogel films. The structure of the composite porous films was investigated using infrared spectrometer, optical and electron scanning microscope. The IR measurements indicated that the bottom surface reflectance of the films is higher than the top surface. The microscopical pictures revealed that the bottom surface of the films has a denser structure than the top surface, and the presence of carbon cloth in the sol-gel process strongly influences the structure of the films. The carbon cloth fibers act as catalytic sites. The particles are more likely to form around the fibers instead of within the sol. This leads to a somewhat reduced specific surface area as was shown via N₂ sorption and BET analysis.Using the 4-probe method the electrical conductivity of the carbon cloth reinforced carbon films was found to be higher than that of pure carbon aerogel films.     

An Investigation of the Nanomechanical Properties of 0.5Ba(Ti0.8Zr0.2)O3–0.5(Ba0.7Ca0.3)TiO3 Thin Films

For practical application, the functional piezoelectric film in microelectromechanical systems should meet the requirement of physical properties, as well as the mechanical properties. In this article, 0.5Ba(Ti_0.8Zr_0.2)O₃–0.5(Ba_0.7Ca_0.3)TiO₃ (0.5BZT–0.5BCT) thin films with varied properties were prepared on (100) Si substrates via a sol–gel technique at different annealing temperatures. The effects of the annealing temperature on the morphology, piezoelectricity, hardness, and elastic modulus were studied. Particular attention was paid to the surface frictional behavior of films, and the changes in the friction force can be radically explained in terms of differences in the hardness/elastic modulus ratio and the residual stress of films. And, it reveals that the higher ratio of hardness to elastic modulus and tensile residual stress can contribute to a lower friction force for 0.5BZT–0.5BCT film during sling friction.     

Simultaneous and sequential micro-porous semi-interpenetrating polymer network hydrogel films for drug delivery and wound dressing applications

Novel semi-interpenetrating polymer networks (so-called simultaneous SIPNs) of various compositions were synthesized using segmented polyurethane urea (SPUU), N-isopropylacrylamide (NIPAM), acrylic acid (AA), and butylmethacrylate (BMA), resulting in an SIPN film denoted as SPUU/poly(NIPAM-co-AA-co-BMA). The resulting simultaneous SIPN films were neutralized by exposure to pH 7.4 phosphate buffer solution (PBS). The neutralized films were dried and then characterized by differential scanning calorimetry (DSC). The DSC results showed that the T_g of the SIPNs depends mostly on SPUU content and on the composition of the acrylate monomers. PNIPAM was incorporated as a second network in one composition of simultaneously prepared SIPN film through a sequential polymerization method (so-called sequential SIPNs). Both simultaneous and sequential SIPN films were examined by scanning electron microscopy (SEM) after freeze drying at their equilibrium states. The SEM study revealed that simultaneous SIPNs had a porous morphology in the absence of BMA, but the porosity disappeared at higher BMA content. The morphology of the sequential SIPN film was almost similar to that of pure PNIPAM. Finally, simultaneous and sequential SIPNs were used for swelling and drug release studies at different pH values and at different temperatures to determine the environmental sensitivity of these gels. Simultaneous SIPNs absorbed more water than sequential SIPNs, but both had a poor rate of water absorption at pH 1.2. In the drug release study, a higher thermosensitivity was observed for sequential SIPNs than for simultaneous SIPNs.     

溶胶-浸渍提拉法制备ZnO薄膜

利用溶胶-凝胶浸渍提拉法在导电玻璃(FTO)基板上制备了氧化锌(ZnO)薄膜.利用X射线衍射仪(XRD)、扫描电子显微镜(SEM)、紫外分光光度计对ZnO薄膜的结构、形貌以及透过性能进行了分析.结果表明,当溶胶陈化时间为26h,煅烧温度550℃时,ZnO薄膜呈现棒状颗粒,缺陷密度低,从而使ZnO薄膜具有较高的透过率.     

Adverse effect of substrate surface impurities on O2 sensing properties of TiO2 gas sensor operating at high temperature

It is well known that surface state of substrates greatly influences the sensing characteristics of gas sensors. In this work, alumina (Al₂O₃) substrates with different distribution of surface impurity concentrations were utilized to prepare TiO₂ based lambda sensors. Several methods, including XPS, XRD and SEM were employed to exam the substrate surface impurity contents, as well as the structure of the sensing films and the interface morphology between the substrates and sensing films. The results indicated that the gas sensors prepared on alumina substrates containing higher Ca impurities displayed lower oxygen sensitivities and longer response times at high operating temperatures above 600 °C. Therefore, it was concluded that Ca impurities on alumina substrates had to be limited to lower possible levels when fabricating TiO₂ lambda sensors.     

Synthesis and physical properties of Curdlan branched Ester derivatives

Curdlan is a high-molecular-weight linear β-1,3-glucan synthesized by microorganisms. A series of curdlan branched esters with a degree of substitution of three were synthesized and their physical properties and structures were compared with those of curdlan linear esters. Thermal degradation temperatures of all the curdlan branched esters were ca. 360 °C; almost the same as those of curdlan linear esters. The curdlan branched esters had melting temperatures (T _m ) higher than those of the corresponding curdlan linear ester with the same side-chain carbon number. In particular, comparing T _m of curdlan propionate, curdlan isobutyrate, and curdlan pivalate, the latter two had high T _m of over 335 °C, suggesting that the degree of branching of the side chain affects the stability of molecular chains with helix structure in their crystals. Highly transparent films were prepared from the curdlan branched esters. These films exhibited higher Young’s modulus and tensile strength compared with those of films composed of the linear equivalents with the same side-chain carbon numbers. These results indicate that curdlan branched esters are promising thermoplastics with favorable thermal and mechanical properties because of the closer packing structure of their molecular chains than that of the corresponding curdlan linear esters.     

Influence of Composition,Humidity, and Temperature on Chemical Aging in Epoxies: A Local Study of the Interphase with Air

The developing chemical depth profile in an epoxy adhesive bulk (with varying amine content) is monitored during aging by FTIR microspectroscopy on sample cuts prepared with low angle microtomy. Three aging regimes are applied in order to separate the role of temperature and water: dried or moist air (90% rel. humidity) at 60°C and dried air at 120°C for up to 300 days. Quantitative evaluation of the IR spectra shows: thermo-oxidative aging (= dried air) is controlled by the diffusion of atmospheric oxygen. It affects a gradient region of more than 200 μm in depth. At given aging time, the depth profiles depend on temperature, humidity, and on the epoxy-amine ratio. Humidity mainly affects the IR band intensities. The plasticizing effect of water promotes the loss of small network fragments. At 120°C, autoxidation of α-CH₂ at ether and amine groups and the oxidative attack on tertiary amines dominate aging. At 60°C in dried air, these processes proceed only very slowly. In the case of amine excess, aging is extended by the additional oxidation of primary and secondary amines to carbonyls. Carbonyls undergo consecutive reactions, especially in the presence of water. Hence, increasing temperature does not simply accelerate the aging mechanisms but it reduces their selectivity and changes their hierarchy. Thus, the long-term aging behavior at moderate temperatures cannot be predicted safely from accelerated aging tests.