Dried Gel Films Research Articles

Gelation of poly(vinyl alcohol)/water/dimethylformamide solutions and properties of dried films

The properties of solutions for syndiotacticity-rich poly(vinyl alcohol) (s-PVA)/dimethylformamide (DMF)/water systems, the gelation of the s-PVA solutions, and the properties of the dried s-PVA gel films were examined. From the results of the dissolution temperature of the polymer, the gelation temperature of the solutions, the melting temperature of the gels, and the compressive modulus of the gels, the solubility of the polymer was the highest at DMF contents of 10–20 vol %. The maximum dynamic tensile modulus of the drawn (×18) films obtained from the dried gel films with a DMF content of 10 vol % was 54.9 GPa at 20°C. The orientation of the polymer chains in the amorphous regions was higher than that in the crystalline regions. The orientation of the polymer chains in the amorphous regions for the drawn films with a DMF content of 10 vol % was higher than that for the drawn films with a DMF content of 60 vol %. The orientation of the polymer chains in the amorphous regions was considered to play an important role in the properties of high strength and high modulus. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 70: 1661–1667, 1998

Development of high ductility and tensile properties by a two-stage draw of poly(acrylonitrile): Effect of molecular weight

Ultradrawing of atactic poly(acrylonitrile) (PAN) was investigated for a Mv series, ranging 8.0 × 104–2.3 × 106. Samples for the draw were prepared from 0.5–30 wt % solutions of PAN in N,N′-dimethylformamide. The solutions were converted to a gel by quenching from 100 to 0°C. The dried gel films were initially drawn uniaxially by solid-state coextrusion (first-stage draw) to an extrusion draw ratio (EDR) of 16, followed by further tensile draw at 100–250°C (second-stage draw). The maximum total draw ratio (DRt,max) and tensile properties achieved by two-stage draw increased remarkably with sample Mv. Other factors affecting ductility were the solution concentration from which gel was made and the second-stage draw temperature. The effects of these variables became more prominent with increasing Mv. The temperature for optimum second-stage draw increased with sample Mv. Both the initial gel and the drawn products showed no small-angle X-ray long period scattering maximum, suggesting the absence of a chain-folded lamellae structure, which had been found in our previous study on the drawing of nascent PAN powder. The chain orientation function (fc) and sample density (ρs) increased rapidly with DRt in the lower range (DRt < 30) and approached constant values of fc = 0.980–0.996 and ρs = 1.177–1.181 g/cm3, respectively, at higher DRt > 30–100. The tensile modulus also showed a similar increase with DRt. The tensile strength increased linearly with DRt, reaching a maximum, and decreased slightly at yet higher DRt. The highest modulus of 28.5 GPa and strength of 1.6 GPa were achieved with the highest Mv of 2.3 × 106. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 629–640, 1998

Preparation of Thick PZT Ceramic Film by an Interfacial Polymerization

The formation of a PZT film of 10–20 μm in thickness via an interfacial polymerization of metal-organic precursors has been studied. A commercially available PZT precursor solution developed for dip- or spin-coating was diluted with hexane containing acetylacetone as chelating agent. The solution was poured on water contained in a teflon reaction container of 88 mm inside diameter. A translucent gel film was formed at the interface of the two immiscible liquids. While floating on water it shrank as the evaporation of solvent proceeded until its diameter decreased to about 45% of its initial value. The dried gel film was almost pore free and its thickness was estimated to be about 10 μm.

Lithium borate gel films prepared from aqueous solution

Transparent, durable films of lithium diborate gel can easily be prepared on a variety of substrates from supersaturated aqueous solutions of lithium hydroxide monohydrate and boric acid by spin-coating. The gel to dried gel to glass to crystalline lithium tetraborate (Li2B4O7) transformation has been studied by infrared spectroscopy of thin films cast on potassium bromide windows. Examination by scanning electron microscopy disclosed that distinctly different crystal morphology was associated with a difference in the transformation rate observed between gel and dried gel films. Films of 250 nm to 30 μm in thickness have been produced.

Relationship between drawability of poly(vinyl alcohol) films prepared from semi-dilute solutions and phase separation of the solutions studied in terms of stereo-regularity and degree of polymerization

The phase separation of poly(vinyl alcohol) (PVA) in dimethyl sulfoxide/water mixtures was investigated by using light scattering techniques in terms of the degree of polymerization and stereo-regularity. Atactic (at-) and syndiotactic (st-) PVA, the degrees of polymerization ( P) being 17 900 and 1980, respectively, were used as test specimens. The analysis was mainly performed in comparison with previous results obtained for at-PVA ( P = 2000) whose degree of polymerization is similar to that ( P = 1980) of st-PVA, but lower than that ( P = 17 900) of at-PVA used in the present work. The phase separation occurred rapidly when the content of dimethyl sulfoxide in the mixed solvent was 50–70vol%. This tendency was found to be significant for st-PVA with high stereo-regularity, but to be almost independent of the degree of polymerization of at-PVA. The logarithm of scattered intensity from at- and st-PVA solutions increased linearly with time in the initial stage of the phase separation. If this phenomenon could be analysed within the framework of the linear theory of spinodal decomposition proposed by Cahn ( J. Chem. Phys., 1965, 42, 93) the phase diagram of st-PVA was classified into four regions: (I) a homogeneous sol region, (II) a sol region under spinodal temperature, indicating only spinodal decomposition, (III) a gel region under the spinodal curve, indicating simultaneous advance of both gelation and spinodal decomposition, and (IV) a gel region above the spinodal line, independent of liquid—liquid separation. On the other hand, for at-PVA, region II could not be observed and region IV existed only in a very narrow temperature range, which is similar to the result observed for at-PVA ( P = 2000) solutions in previous work. The drawability of the dried gel film was affected by the composition of the solvent mixture and by quenching temperature of the solutions, but it was hardly affected by the stereo-regularity. This interesting phenomenon is discussed in terms of morphology of gels and films, as studied by crystallinity, X-ray diffraction, birefringence and small angle light scattering under Hv polarization condition. As the results show, it turned out that the greatest significant drawability is related to a continuous tissue with honeycomb-like structure associated with rapid phase separation of PVA solutions due to thermodynamic instability.

Development of High Ductility and Tensile Properties upon Two-Stage Draw of Ultrahigh Molecular Weight Poly(acrylonitrile)

Solutions (1−5 wt %) of ultrahigh-molecular-weight atactic poly(acrylonitrile) (UHMW-PAN) in N,N‘-dimethylformamide were crystallized to gel by quenching from 100 to 0 °C. The dried gel films exhibited poor ductility on straight tensile draw. However, they were ductile on solid-state coextrusion to an extrusion draw ratio (EDR) of 8−16 (first-stage draw). Thus, the initially coextrusion drawn films were further drawn by a tensile force (second-stage draw). Drawability of the gel film was strongly influenced by several factors, including the solution concentration from which gel was made, first-stage EDR, and second-stage draw temperature. With increasing prior solution concentration, the ductility of a gel film decreased. The maximum achieved total draw ratio (DRt,max) by the two-stage draw and the uniformity of drawn products increased with first-stage EDR. Further, the ductility increased rapidly above ∼150 °C, reaching a maximum at ∼190 °C, and decreased at yet higher temperatures. A DRt,max of ∼110 wa...

Preparation and properties of a humidity sensor based on LiCl-doped porous silica

The importance of humidity control has been recognized in many fields, such as the electronics industry, food manufacturing processes, hospitals and, in everyday life [1]. A wide variety of ceramic, polymeric and composite sensors are being produced to serve these applications. With the development of silicon micromachining and on-chip integration technology, miniaturization of sensors is increasingly required. Recent progress in ceramic deposition technologies, mainly by the continuous advancement of microelectronics, provides new possibilities for the improvement of ceramic active elements in the field of chemical sensors. The sol–gel method is very attractive for the preparation of very uniform ceramic powders with high purity and a very homogeneous distribution of the components on the atomic scale. Also several ceramic oxides have recently been studied in thin film form prepared by the sol–gel process [2, 3]. This method starts with liquid precursors and converts the liquid into useful solids through chemical reactions at low temperatures. Some research on sol–gel-derived silica films has been done and the results provide some useful guidelines for obtaining successful films [4]. The sol–gel silica films were possibly used as humidity sensors because of their large pores, but had a high intrinsic resistance [5]. So, in this report, LiCl is employed as a dopant to modify the humidity sensitivity of the porous silica thin film. In addition, preparation by the dip-coating method and properties of the materials are described. Reagent grade tetraethyl orthosilicate (TEOS), ethyl alcohol (C2H5OH) and deionized water (H2O) were used to prepare a silica sol solution. The optimized composition of the solution is TEOS: C2H5OH:H2O ˆ 1:4:4(mol). LiCl was dissolved in deionized water to form another solution. These two solutions were mixed by magnetic stirring for 30 min. Dilute hydrochloric acid (HCl) at above 1% in molar ratio was added into the mixed solution as a catalyst when the TEOS hydrolized. Four specimens, with the proportions of TEOS:LiCl ˆ 100:0, 90:10, 80:20, and 70:30 (mol), respectively, were studied in this report. Al2O3 plates, with a Ag– Pd integrated electrode, were used as substrates. After cleaning, the substrates were immersed in the solution for 15 s and then withdrawn at a rate of 2 cm miny1. Coated samples were air-dried for a least 24 h and then calcined in air at over 400 8C. The thickness of the silica thin films with or without LiCl is 0.1–0.2 μm. The microstructural characteristics of the films were studied by using an Amray 100 type scanning electron microscope. Several saturated salt aqueous solutions were used to provide ambient atmospheres with humidities from 11 to 95%RH. The temperature was kept at 25 8C throughout the measurement process. The resistance of the samples at different relative humidities were measured with a HP 4274A impedance analyser. Fig. 1 indicates the typical microstructure of the silica film with 20% LiCl treated at 400 8C. It shows that the film is very uniform. At the same time, it can be observed that the sol–gel-derived silica films have a lot of pores. This result suggests that the films could be suitable for adsorbing water under ambient atmospheric conditions, that is to say, they are sensitive to relative humidity. Fig. 2 shows the humidity sensitivity of the films at different %RH at 1 KHz. It is found that the silica films with LiCl have a lower impedance than the pure silica ones, and the impedance of the dried gel films (not treated at an elevated temperature) is less than that of those films calcined at 400 8C. With increasing LiCl content, the conductance of the films increases by one to three orders of magnitude at different humidity regions. The impedance–humidity characteristic of pure silica films indicate that the gel sample is sensitive to humidity at the higher %RH region while that treated at 400 8C shows no sensitivity. Because the silica gel has many surface Si–OH hydroxyl groups [6], which adsorb water vapour physically through hydrogen bonding, this physisorbed water layer is localized by hydrogen bonding of a single water molecule to two surface hydroxyls, and a proton may be transferred from a

Morphology of poly(vinyl alcohol) gels assuring the greatest significant drawability

Atactic poly(vinyl alcohol) (at-PVA) and syndiotactic poly(vinyl alcohol) (st. PVA) prepared by gelation/crystallization using dimethyl sulphoxide/water mixtures were drawn in a hot oven at 160 °C under nitrogen. The degrees of polymerization of at- and st-PVA were 2000 and 1980, respectively. The drawability of at- and st-PVA films was affected by the composition of the solvent mixture as well as by quenching temperature. The drawability of at- and st-PVA films prepared by using the solvent mixture containing 60% of dimethyl sulphoxide and 40% of water became more pronounced as the temperature of gelation/crystallization decreased and the draw ratio reached maximum value at −80 °C. Namely, the greatest significant drawability was the same condition for at- and st-PVA films in spite of the different stereo-regularity. Even in this common best condition for significant drawability, however, the morphological properties of swollen gels and of the resultant dry gel films are different each other, dependent upon the tacticity. For at-PVA, small-angle light scattering under Hv polarization condition could not be observed in the swollen gels and in the dry films when the solutions were quenched at temperatures <−10 °C. In contrast, for st-PVA, the X-type scattering pattern from swollen gels became clearer as the temperature decreased but the pattern became indistinct under drying process at ambient condition. On the other hand, the fibrillar textures within the at- and st-PVA dry films became finer and the orresponding crystallinity became lower as the temperature of gelation/crystallization decreased. Thus it turned out that the morphological properties of the swollen gels and of the dried films play an important role to assure the greatest significant drawability.

Morphology and molecular mobility of several kinds of polyethylene specimens by 13C solid-state NMR and x-ray diffraction techniques

The morphology and molecular mobility of polyethylene were investigated by X-ray diffraction and 13 C solid-state NMR as a function of temperature. The test specimens chosen are melt films, single-crystal mats, undrawn dry gel films, and ultradrawn films with a draw ratio of 300. The semilogarithmic plot of the 13 C magnetization of the peak height of the line at 33 ppm indicated the existence of three different kinds of spin-lattice relaxation time (Tie) : a slow decay time, an intermediate decay time, and a rapid decay time for the orthorhombic crystal form. Among the test specimens, the decay time of the ultradrawn gel film was the longest. The spin-spin relaxation time (T 2C ) indicated that the noncrystalline component consists of an interfacial region and a rubber-like amorphous region with extensive molecular mobility. For single-crystal mats and dry gel films, the lenght of the interfacial region calculated using both the mass fraction of the interfacial region by 13 C NMR and the long period by X-ray scattering was almost equal to the length of the interfacial region estimated by analyzing a systematic deviation of the X-ray scattering curves at large angle tail from Porod's law. Furthermore, the crystallinity at room temperature estimated by 13 C NMR was also in good agreement with the results by density and X-ray diffraction. With increasing temperature, T 1C for the crystalline region became shorter, while T 2C for the noncrystalline region became longer, indicating more active molecular mobility. The crystallinity estimated by 13 C NMR was confirmed to decreased with increasing temperature. This tendency was in good agreement with the results obtained by X-ray diffraction using Ruland's method.

Thermal and photochemical formation of small gold colloids in sol-gel films

The formation of small gold colloids in SiO 2TiO 2 sol-gel films on soda-lime glass was investigated using ultraviolet-visible (UV-VIS) sectroscopy and electron microscopy. Conventional thermal reduction of gold ions in sol-gel films tended to form large particles due to growth or aggregation causing a hazy appearance in the high concentration range. It was found that a haze-free deep blue tinted film could be prepared by UV irradiations of the dried gel film before annealing at 500°C. Colloids with a narrow size distribution were formed in this film. Sufficient photoreduction of metallic species from gold salts is a key factor for the excellent coloration.

Ultradrawing of ultrahigh molecular weight poly(acrylonitrile)

Dry gel films of ultrahigh molecular weight poly(acrylonitrile) homopolymer, having strong intermolecular forces, have been drawn by solid‐state coextrusion in the temperature range from room temperature to 260 °C. The effects of solution concentration from which gel was made and extrusion temperature on the extrusion behavior and the structure and properties of drawn films have been studied. The drawability of each gel increased with extrusion temperature above the Tg (80 °C), reached a maximum at 160 °C, and then decreased rapidly above 160 °C. The drawability of gel also increased with decreasing the solution concentration from which gel was generated. This indicates that the ductility of poly(acrylonitrile), which has strong intermolecular forces and exhibits no crystalline relaxation, is also controlled by the entanglement density, like polyethylene having weak intermolecular interactions. Gel prepared from the lowest concentration of 2 wt % in this work could be extrusion‐drawn to draw ratios ≤82 at 160 °C. Extrusion temperature markedly affected not only the ductility of gel, but also the efficiency of draw and the formation of flaws at high draw ratios. Thus, the maximum tensile modulus and strength achieved in this work were 20 and 1.0 GPa, respectively.

Drawability of ultrahigh molecular weight polyethylene single-crystal mats

Single-crystal mats of ultrahigh molecular weight polyethylene were successfully prepared from p-xylene solution for studies of the influence of chain entanglement on the drawability of these mats. In the microscopic levels observed for SAXS and SEM, films prepared from the mats had a morphology similar to that of dried gel films that were prepared by the usual procedure. The maximum attainable draw ratio was well in excess of 200 if the specimen was annealed at 130-140°C and stretched at the same temperature. But it was only about 10 when the specimen annealed at 140°C, slightly above the melting point, was stretched at 20°C

Thermoreversible gelation of poly(vinylidene fluoride) in γ-butyrolactone solution

Poly(vinylidene fluoride) gels were formed in γ-butyrolactone; the critical polymer concentration for gel formation was 4.5 g per 100 cm 3. Gelation was caused by liquid-liquid phase separation, even if solid-liquid phase separation by crystallization occurred in the late stage of gelation, as shown by the formation of transparent gel, a relatively small enthalpy of gel formation and the existence of two types of crystal structures. In the dried gel films many spherulites connected by tie molecules were observed by scanning electron microscopy; the existence of such tie molecules is likely to make the formation of the gel films possible.

Fourier transform infra-red spectroscopic studies of gelation and crystallization of polystyrene/carbon disulfide solutions

Dried gel and solution-cast films of atactic (at-) and isotactic (iso-)polystyrene/carbon disulfide ( PS CS 2 ) systems were investigated by Fourier transform infra-red spectroscopy ( FTi.r.) and the relation between gelation and crystallization was studied. Iso-PS gel films dried at gelation temperature were less turbid than homogeneous solution-cast films made at room temperature. No difference was found in the i.r. spectrum of dried gel and solution-cast films of at-PS CS 2 . However, the i.r. spectrum of iso-PS dried gel film was different from that of solution-cast film. Comparison of these spectra indicated that spectral changes occurred at the bands near 1200, 560, 1085–1052 and 907–922 cm −1. There were two bands at 909 and 922 cm −1 for solution-cast films, while for dried gel films there was only one which shifted from 912 to 921 cm −1 with increasing polymer concentration (10 to 40 gl −1) of the original gel. I.r. spectra of dried gel films of low concentration gels were like those of solution and quenched iso-PS samples. The intensity of the crystalline band at 983 cm −1 decreased with decreasing iso-PS gel concentration. This means that gel containing no crystals should be formed at very low polymer concentration; this is consistent with our previous conclusion that gelation of the PS CS 2 system is independent of crystallization.

Effect of electron beam irradiation on simultaneously biaxially drawn ultra-high molecular weight polyethylene dried gel films

The effect of electron beam irradiation on the morphology and mechanical properties of biaxially drawn ultra-high molecular weight polyethylene dried gel films was investigated as a function of irradiation dose in terms of cross-linking and main-chain scission. Tensile strength and elongation decrease drastically, even with small irradiation doses, while the storage modulus increased with increasing dose up to 20 Mrad over a temperature range from 0 to 120°C. This indicates that electron beam irradiation causes cross-linking and main-chain scission; the former improves heat resistance and the latter causes deterioration of tensile strength.

Molecular-weight dependence of morphology and mechanical properties of ultrahigh-molecular-weight polyethylene gel films

Films of three kinds of ultrahigh-molecular-weight polyethylenes (UHMWPE) with molecular weights of 6 × 10 6, 3 × 10 6 and 1 × 10 6 were produced by gelation/crystallization from dilute solutions. The dried gel films were stretched up to their maximum draw ratios. The molecular-weight dependences of morphology and mechanical properties of the drawn films have been examined using wide-angle X-ray diffraction, small-angle X-ray scattering, birefringence, crystallinity and stress relaxation modulus. It turned out that the long period, molecular orientation and crystallinity were hardly affected by molecular weight, while the stress relaxation modulus was strongly affected. The master curves indicated that the decrease in the stress relaxation modulus was more pronounced as the molecular weight of the test specimens decreased. This was thought to be due to a drastic increase of the slippage of molecular chains, leading to the disappearance of tie molecules and/or entanglement meshes under the external applied stress.

Synthesis of an alumina coating from chelated aluminium alkoxides

Chelating agent, such as acetylacetone (AcAcH) and ethylacetoacetate (EAA) are used to avoid the undersirable precipitation of gelatinuous aluminum hydroxides during hydrolysis of aluminum-sec-butoxides, Al(OBu s). This procedure leads to alumina sols with particle size distributions in the range of 1 to 15 nm which depend on the type of chelating agent as well as on the molar Al(OBu s) 3/chelating agent ratio. Generally, EAA leads to particles smaller than AcAcH. Because of the small particle size, gel powders are X-ray amorphous and do not crystallize at temperature < 750°C. Crystallization of α-alumina starts between 950 and 1000°C. Complete transformation to α-alumina is achieved between 1050 and 1100°C. Coating experiments with different sols on silica glass lead to nearly crack-free dried gel films with AcAcH system. An ultrafine, dense microstructure is obtained.

Development of fibrillar texture during simultaneous biaxial drawing of ultra-high-molecular-weight polyethylene dried gels

Simultaneous biaxial drawing of dried gel films of ultra-high-molecular-weight polyethylene (UHMW-PE) was followed by scanning electron microscopy (SEM) and other structural investigation techniques. The average width of fibrils estimated by SEM was an effective measure of the development of fibrillation. Regular microfibrillation almost finished at a draw ratio of (10 × 10) and further drawing seemed to be accomplished by slippage, entanglement and interception of microfibrils, breakage of some entangled microfibrils, as well as drawing of each fibril. These SEM observations correspond well to the changes in nitrogen gas permeability coefficient and in thermoluminescence glow with biaxial drawing. A field-emission SEM observation of the fracture face perpendicular to the film surface showed that thin layers less than 1 μm were formed during drawing, suggesting that biaxial extension was performed in each layer.

Structural Study of Ultradrawn Polyethylene Films by High-Resolution Solid-State &lt;SUP&gt;13&lt;/SUP&gt;C NMR spectroscopy

High-resolution solid-state 13C NMR analyses have been carried out for dry gel films and their ultradrawn films of high molecular weight polyethylene in terms of 13C chemical shifts, spin-lattice and spin-spin relaxation parameters. In the dry gel films, the noncrystalline region consists of the crystalline-amorphous interphase with the thickness of about 20 Å and the rubbery amorphous component is completely devoid. This characteristic structure is subjected to a drastic change by annealing at 145°C; a large amount of rubbery component appears with thickening of the crystalline lamella. Although such a structural change will also occur prior to and during drawing at 145°C, ultradrawing is possible up to 150 times under the condition used in this work. The effects of drawing appear mainly in the noncrystalline region at lower draw ratios; the rubbery component considerably decreases in mass fraction and the halves of the decreased fraction are transformed to the interfacial component and the monoclinic crystalline component. At draw ratios higher than 100 the noncrystalline chains seem to undergo further extension and the orthorhombic crystals increase possibly as a result of the transformation of the rubbery component→the interfacial component→the orthorhombic crystals.

Vanadium dioxide films grown from vanadium tetra-isopropoxide by the sol-gel process

Vanadium dioxide thin films have been grown from vanadium (IV) tetra-isopropoxide by the sol-gel process. Films were deposited by dipcoating glass slides from an isopropanolic solution. The dried gel films were annealed at elevated temperatures under nitrogen to form polycrystalline VO 2. These gel-derived VO 2 films undergo a reversible semiconductor-to-metal phase transition near 67 °C, exhibiting characteristic resistive and spectral switching. The electrical resistance decreases by one to two orders of magnitude and the near-IR transmittance approaches zero as the material undergoes this phase transition. These results are comparable with near stoichiometric VO 2 films prepared on non-crystalline substrates by other techniques.