Structural and compositional effects on thermal expansion behavior in polyimide substrates of varying thicknesses

Polyimide films with thicknesses ranging from 6 μm to 80 μm were prepared with a solvent casting method to explore film thickness effects on the in-plane thermal expansion coefficient (CTE). In the case of polyimide films composed of bulky and flexible molecular units, CTE is consistent regardless of film thickness. In contrast, films with rigid and planar molecular structure show CTE increase according to the increase of film thickness up to 40–50 μm, which then plateau for thicker films. It is apparent that the film thickness dependent thermal expansion originates from complex effects of molecular orientation, charge transfer complex formation, and crystal formation as a function of film thicknesses, through characterization on UV–Vis absorption, crystalline structure, glass transition behavior, and optical retardation. These results provide insight into the design of polymer structures for flexible display substrates that require appropriate CTE values.     

Thin films of nylon 13,13 casted on water: Morphology and properties

Substrate-free thin film of Nylon 13,13 has been prepared continuously by casting the polymer on a water surface from m-cresol solution. The uniform thickness of film is ca. 500 A, as measured with a Profilometer. Film morphology and properties have been characterized by transmission electronic microscopy (TEM), x-ray photoelectron spectroscopy, wide-angle x-ray scattering, differential scanning calorimetry and by contact angle measurements. Results show that there is no preferential accumulation of the amide group toward the water surface. A spherulite structure exists in this film with a crystallinity of 26% by DSC. Fibers were drawn from the films which had a tensile modulus of 5 GPa, stiffer than reported for this nylon as prepared by other means. Other members of the nylon series, such as Nylon 6, Nylon 12, have also been prepared in films by the same casting method under modified conditions. © 1995 John Wiley & Sons, Inc.     

Printable optical sensors based on H-bonded supramolecular cholesteric liquid crystal networks

A printable H-bonded cholesteric liquid crystal (CLC) polymer film has been fabricated that, after conversion to a hygroscopic polymer salt film, responds to temperature and humidity by changing its reflection color. Fast-responding humidity sensors have been made in which the reflection color changes between green and yellow depending on the relative humidity. The change in reflection band is a result of a change in helix pitch in the film due to absorption and desorption of water, resulting in swelling/deswelling of the film material. When the polymer salt was saturated with water, a red-reflecting film was obtained that can potentially act as a time/temperature integrator. Finally, the films were printed on a foil, showing the potential application of supramolecular CLC materials as low-cost, printable, battery-free optical sensors.     

Water in polymer membranes. Part I : Water sorption and refractive index of cellulose acetate

The dependence of water absorption of 2 to 4 μm thick membranes of cellulose acetate on relative humidity was determined by measuring small changes in their asymmetric waveguide properties in the visible spectral region. Simultaneous measurements of changes in film thickness provide a direct method for obtaining the change in film volume as a function of water concentration and a new measure of polymer porosity. Data are presented for a typical film fabricated from cellulose acetate (CA398-30) which illustrate the usefulness of studying water absorption by integrated optics techniques.     

Effect of water contact on the density distributions of thin supported polymer films investigated by an X-ray reflectivity method

The diffusion processes of water molecules into polymer films (PMMA/PS homopolymers and random copolymers) in contact with liquid water were investigated using gravimetric methods and X-ray reflectivity (XRR) analysis. Methods of water contact and XRR measurement were designed for studying the systems in the nonequilibrium state of diffusion. Gravimetric measurements confirmed the Fickian diffusion behavior of films in contact with water. Vertical density distributions in PMMA and methylmethacrylate-rich copolymer films demonstrate the existence of a water-rich layer at the interface. However, with further absorption of water into the film, the overall density increased throughout the film. The results suggest that the diffusion of water into the polymer film occurs to recover density uniformity with a high concentration of water molecules at the surface. Some XRR data for the PS- and styrene-rich copolymer films could not be fit and converted to a vertical density distribution because of their huge diffusion coefficients. However, the reflectivity curves for these films and the vertical density distribution after sufficient water contact suggested that the surfaces of these films were commonly diffused after water contact. Atomic force microscopy (AFM) analysis demonstrated that the surface roughness of these films actually increased with water content.     

The effect of anhydrous ammonia on the crystalline-state deformation of nylons 6 and 6,6

The aliphatic polyamides nylon 6 and 6,6 have been cold drawn by using the new concept of reversible plasticization with NH₃. Two aspects were investigated: the effect of NH₃ plasticization on the stress-strain behavior of nylon 6,6, and the extent of NH₃ absorption as determined by x-ray analysis for the structurally simpler nylon 6. X-ray analysis shows that ammonia is absorbed preferentially by the amorphous phase and that the plasticization has a significant effect on the unit-cell orientation, the small-angle void scattering, and the interplanar spacings between sheets of hydrogen-bonded chains. The effect of plasticization was seen in the lower yield stress, the higher draw ratio (up to 8X) and tensile modulus (10 GPa) not attainable with unplasticized specimens.     

Effect of molecular orientation on the swelling of nylon 6

The effect of molecular orientation on the linear swelling of nylon 6 caused by absorption of water was studied using two kinds of oriented films, melt drawn and cold drawn. The molecular orientation causes anisotropy in swelling at all humidities such that the swelling is larger in the orientation direction than in the directions perpendicular to it. The large contribution of crystalline orientation to this phenomenon was expected for the melt drawn film which has practically no amorphous orientation. An analysis with a two-phase morphological model reveals that the distance between the crystallites is a prominent factor controlling the degree of linear swelling, and that the anisotropy of swelling arises from the change in the distribution of crystallites due to orientation. By comparing the result for the melt drawn film with that for the cold drawn film, which has considerable amorphous orientation, it was proven that molecular orientation in the amorphous phase where swelling actually occurs does not depend so much on the degree of swelling as on the distribution of crystallites.     

Effect of orientation,anisotropy, and water on the relaxation behavior of nylon 6 from 4.2 to 300°K

The relaxation behavior of nylon 6 from 4.2 to 300°K was investigated as a function of orientation, anisotropy and moisture content by using an inverted free-oscillating torsion pendulum. Three new relaxations, δ at 53°K, ? below 4.2°K, and ζ at 20°K, were discovered. The characteristics of these new relaxations strongly depend on the orientation anisotropy, and concentration of adsorbed water in the specimens. The results suggest that the mechanism of the γ process is associated with the motions of both the polar and methylene units. The mechanism of the β relaxation is postulated to originate with motions of both non-hydrogen-bonded polar groups and polymer—water complex units. The behavior of the α peak is consistent with the hypothesis that it originates with the rupture of interchain hydrogen bonding due to the motions of long-chain segments in the amorphous regions. Finally, the data strongly support the proposition that two types of water, tightly bound and loosely bound, exist in nylon 6.     

Ultraviolet derivative absorption spectra of nylon 6,6: Effect of photolysis versus photo-induced oxidation

The irradiation of nylon 6,6 film with light of wavelengths greater than 250 nm and 300 nm has been studied using uv derivative absorption spectroscopy. Initially, the polymer showed a strong absorption at 290 nm which is associated with an α,β-unsaturated carbonyl species. Changes in the absorption band were found to be dependent on the atmosphere and are discussed in terms of a reversible photolysis versus a photo-induced oxidation process.     

Kinetics and equilibria associated with the absorption and desorption of water and lithium chloride in an ethylene—vinyl alcohol copolymer

Sorption and desorption equilibria and kinetics for LiCl and H₂O in an ethylene—vinyl alcohol copolymer film containing 70 mole percent vinyl alcohol were investigated at 25°C. The swelling behavior of water in the polymer was characterized by vapor and liquid sorption experiments over a range of water activities. p]The effects of LiCl content on the water sorption kinetics and equilibria in the films are presented and discussed. The kinetics and mechanism of LiCl sorption have also been studied. The amount of salt sorbed into the polymeric films increases linearly with the salt concentration in the external aqueous solutions. Both the rate and the amount of sorbed water increase significantly as the LiCl content increases. p]The desorption of LiCl, previously sorbed into the polymer, was characterized for different salt loadings. The rate of fractional salt release is independent of LiCl concentration in the film. Initially, the salt release is controlled by the nearly constant-rate absorption of water. The salt release, at long times, lags behind the swelling-controlled water uptake, indicating that the salt release is not completely controlled by the water sorption and that diffusion in the swollen polymer matrix contributes significantly to the long term elution of LiCl. Independent thermal analysis experiments suggest the formation of a metal salt—poly(ethylene—vinyl alcohol) complex.     

Apparatus for infrared measurement of sorption/desorption in strained polymeric films

A new apparatus has been developed for optical measurement of sorption/desorption in transparent polymer films at a given strain or stress. The technique utilizes a chosen infrared absorption frequency of the diffusing vapor in a spectral region where the film has negligible absorption. From the time dependence of the IR absorption at this frequency the sorption/desorption behavior of the film may be determined at any strain or stress. The simultaneous measurement of mechanical relaxation as a function of the amount of sorbed vapor is also possible. The results presented here show the applicability of the apparatus for determining the transport and mechanical properties of a low-density polyethylene film in ethyl acetate vapor at 30°C.     

Confirmation of universality of time–humidity superposition principle for various water‐absorbable polymers through dynamic viscoelastic measurements under controlled conditions of relative humidity and temperature

Dynamic viscoelastic measurements were made for storage (E′) and loss (E″) tensile moduli of water‐absorbable polymers such as nylon‐6, nylon‐66, poly(vinyl alcohol), ethylene–vinyl alcohol copolymers, and regenerated cellulose under the control of stepwise scanned relative humidity at constant temperature by changes in the strain frequency over a wide range. Smooth master curves of log(E′) and log(E″) plotted against log(frequency) were successfully obtained for all samples. The evaluated shift factors changed with the relative humidity and could be interpreted well on the basis of a concept of free volume in the amorphous region. The free volume was affected sensitively, depending on the heat‐treatment condition and the types of polymers used. For nylon‐66 film, the dynamic viscoelastic measurements were made at different humidities and temperatures, from which one smooth master curve was obtained. This experimental result is important in realizing the similar effect of relative humidity and temperature on the expansion of free volume of the amorphous phase: the shift factor change induced by the relative humidity change of about 30% was equivalent to the shift factor change induced by the temperature change of about 30 °C. That is, the time–humidity superposition principle and the time–temperature superposition principle were applicable as equivalent contributors to the mechanical property of water‐absorbable polymers. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1638–1650, 2001     

Relaxation of polymer thin films in isothermal temperature‐jump measurements

The dynamic behavior of thin polymer films is of interest in the fabrication of microelectronics and optoelectronics and in the coatings industry. It is known that polymer relaxation is affected by film thickness and the particular substrate/polymer pair. We previously used a spectroscopic ellipsometer to investigate the glass transition in thin films. In addition to information on the modification of thermal transitions such as the glass‐transition temperature, the speed of data acquisition in an automated, spectroscopic ellipsometer, operated at a single wavelength of 780 nm, allows for the direct observation of the isothermal dimensions of a thin polymer film as a function of time after a rapid temperature change. In this article, we discuss recent results from the observation of the time dependence of film‐normal thickness and normalized, in‐plane, lateral dimension as well as simple fits to this relaxation behavior in terms of a normalized viscosity and relaxation time. The results support a highly asymmetric initial thermal expansion normal to the film followed by close to isotropic relaxation and anisotropic “flow” (the flow in response to the vanishingly small shears of thermal expansion). These features may clarify issues involving the observation of chain confinement in thin polymer films in terms of potential differences between equilibrium and dynamic measurements. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2929–2936, 2000     

Corrosion behavior of AZ91D magnesium alloy in distilled water

The corrosion of AZ91D magnesium alloy has received extensive attention due to the continuous expansion of its application field in recent years. However, the corrosion of AZ91D magnesium alloy in distilled water is relatively few. In this paper, the corrosion behavior of AZ91D magnesium alloy was studied in distilled water by electrochemical tests in combination with weight loss and surface analysis methods. The results indicated that the corrosion rate of AZ91D magnesium alloy increased with the increase of temperature and immersion time. The increase of the corrosion rate of AZ91D magnesium alloy with the increase of immersion time might be attributed to the damage of the structure of corrosion product film by hydrogen evolution, significantly accelerating the anodic process of AZ91D magnesium alloy. It was interesting that, in distilled water, the EIS of AZ91D magnesium alloy excluded an inductive arc in the low frequency region, which indicated that there was no the adsorption and desorption of aggressive ions or the damage and repair of film. The corrosion product film of AZ91D magnesium alloy in distilled water was composed of a compact inner corrosion product film and a loose outer corrosion product film.     

Structural characteristics and moisture sorption behavior of nylon‐6/clay hybrid films

The structure of nylon‐6 hybrids with synthetic or natural clays was investigated for melt‐pressed films with Fourier transform infrared spectroscopy, wide‐angle X‐ray diffraction, and differential scanning calorimetry in comparison with the nylon‐6 homopolymer. In contrast to the development of familiar α‐form crystals in plain nylon‐6 film, the hybrid films produced γ‐form crystals when nylon‐6 was conjugated with synthetic mica, whereas the hybridization with natural montmorillonite gave rise to both α‐ and γ‐crystalline modifications. The degree of crystallinity of the nylon‐6 hybrid with synthetic mica was the highest of the three series. Moisture sorption isotherms obtained for these nylon‐6‐based films were all typically sigmoid‐shaped, although the prevalence of a higher crystallinity in the hybrid samples lowered the degree of moisture regain. The sorption behavior was analyzed well in terms of the parameters of a Brunauer–Emmett–Teller multiplayer adsorption model and a Flory–Huggins treatment. It was also observed that the cluster formation of the water adsorbed into the nylon‐6 matrix tended to be restricted by the hybridization with clay. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 479–487, 2002; DOI 10.1002/polb.10106     

In situ study of the thermoresponsive behavior of micropatterned hydrogel films by imaging ellipsometry

A patterned hydrogel was immobilized on a polymer substrate by low-pressure argon plasma treatment using a masking technique. The polymer sample showed a thermoresponsive aggregation behavior in the region of 35-37 degrees C. The micropatterned, thermoresponsive hydrogel film has been characterized with imaging ellipsometry. The characterization was carried out on the dry film as well as on a swollen sample in water. The thermoresponsive behavior was studied in deionized water by temperature-dependent measurements in a solid-liquid cell. Through imaging ellipsometry, it was possible to distinguish the different regions of interest on a micrometer scale and to follow the swelling of the hydrogel part as a function of the temperature. It was possible to visualize the swelling as 3D profiles of Delta at various temperatures. Long-term changes of the sample could also be detected, which cannot be picked up by conventional ellipsometry.     

Effect of Electron Beam Irradiation on the Kinetics of Non-isothermal Crystallization of Nylon 66

The non-isothermal crystallization kinetics was studied by differential scanning calorimetric analysis on nylon 66 and e-beam irradiated nylon 66 at different cooling rates. The Modified Avrami equation, the Ozawa equation and the Combined Avrami-Ozawa equation were applied to study the kinetics of non-isothermal crystallization of nylon 66. The crystallization behavior of pristine nylon 66 polymer was compared with that of e-beam irradiated nylon 66 and observed that the kinetics of non-isothermal crystallization of nylon66 was affected largely upon e-beam irradiation. E-beam irradiation not only decreased the crystallization temperature of nylon 66, but influenced the mechanism of nucleation and crystal growth and reduced the overall crystallization rate of nylon 66 also. The crystallization activation energy calculated by the Kissinger method for irradiated nylon 66 was lower than that of pristine nylon 66.     

Modeling Equilibrium Behavior of Nylon 6, Nylon 6,6 and Nylon 6/6,6 Copolymer

Nylon 6 and nylon 6,6 reaction equilibria depend in a complex way on water concentration and temperature. For example, data sets from six research groups reveal that the apparent equilibrium constant for polycondensation increases with water at low water concentrations, reaches a maximum, and then decreases as the water concentration rises further. In this article, semi‐empirical expressions are proposed to describe the experimentally observed equilibrium behavior for the five main reactions that occur during nylon 6 and nylon 6,6 polymerization. Nine side reactions involving amidine ends, cyclopentanone ends, and hydrated carboxyl ends are used to develop expressions that account for the influence of both water and temperature on these equilibrium constants. Excellent fit to the data, over the entire range of the available nylon 6 and nylon 6,6 literature data, suggests that the proposed equations will be helpful for modeling reaction equilibria for nylon 6/6,6 copolymerization.     

Differential permeation and absorption of water vapor in polyacrylamide film

An investigation has been made of successive differential absorption and differential permeation of water vapor in polyacrylamide a t 30°C. The successive differential ab sorptions showed two types of non-Fickian anomalies: sigmoid type and two-stage type curves. The experimental data have been analyzed in terms of the Fick diffusion equation assuming a time-dependent approach of the surface concentration. The calculated family of absorption curves agreed with the experimental results. The permeation curves in the region of high and low pressure increments were apparently normal, but at medium pressures they showed anomalous behavior. It was found that in the differential type of permeation experiment the stress effect induced by a concentration gradient between the surfaces of the film was eliminated. By assuming the time-dependent approach of the equilibrium surface concentration, we calculated the time lag as a function of film thickness and applied the theory to the data for permeation through polyacrylamide film with different film thicknesses a t relatively small pressure intervals. The rate parameter calculated from permeation data was found to be in good agreement with that from successive differential absorption data.     

Ultrathin conjugated polymer network films of carbazole functionalized poly(p-phenylenes) via electropolymerization

Ultrathin films of a cross-linked and chemically distinct conjugated poly(p-phenylene) network via electropolymerization are described. The amphiphilic network precursor was synthesized by incorporating the alkoxy carbazole group (-O(CH2)5Cb) to a poly(p-phenylene) (C6PPPOH) backbone. In order to investigate the combined thin film electrochemical and photophysical properties of poly(p-phenylene)s and polycarbazole conjugated polymers, C6PPPC5Cb was deposited on substrates using the Langmuir Blodgett Kuhn (LBK) method. The monolayer isotherm of the polymer, C6PPPC5Cb, showed a liquid expanded region slightly different from the parent polymer C6PPPOH. Multilayers (up to 30 layers) were transferred to different substrates such as quartz, gold coated LaSFN9 and ITO substrates for analysis. For conversion to a conjugated polymer network (CPN) film, the electroactive carbazole group was electropolymerized using cyclic voltammetry (CV) resulting in polycarbazole linking units. The differences in the film properties and corresponding changes in the electrochemical behavior indicate the importance of film thickness and electron/ion transport process in cross linked network films. From the electrochemical studies, the scan rate was found to have a considerable effect on electropolymerization with higher oxidation and reduction peak values found for the rigid network polymer compared to the uncrosslinked polymers.