Laser interferometric technique for measuring polymer cure kinetics

An optical interference apparatus for the analysis of polymer curing kinetics is described. The device employs a helium‐neon laser and a home‐built Michelson interferometer to measure refractive index variations and film thickness variations during film polymerization. The refractive index variation provides a phenomenological measure of the extent of cure. The technique is demonstrated by collecting interferometric curing profiles for a UV photocurable acrylate resin formulation. Systematic comparisons of curing profiles are made as a function of UV illumination intensity and photoinitiator concentration. Several aspects of the curing process are characterized by these interferometric measurements, such as film shrinkage, gel point formation, and the kinetics of photopolymerization inhibition. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 2133–2142, 1999     

Fluorescence based measurement of temperature profiles during polymer processing

We describe a novel noninvasive method to measure temperature profiles during processing. A key feature is the use of a temperature-sensitive fluorescent dye that is incorporated into the polymer resin at dopant levels. By monitoring spectral features of this dye, we can effectively measure its temperature. A focusing technique that we call “confocal fluorescent optics” then allows us to measure the temperature as a function of position into the resin. We present results of critical tests of the device. Namely we test the spatial resolution and its ability to measure a linear temperature profile in a quiescent polymer. Finally, we present the first on-line temperature profile measurements, carried out in a circular die at the exit of a twin screw extruder. Temperature gradients on the order of 5°C/mm in a polyethylene resin are observed between the outer wall and the center of the die.     

Substrate thickness: An effective control parameter for polymer thin film buckling on PDMS substrates

Buckling patterns of polymer thin films on plasma‐treated poly(dimethylsiloxane) (PDMS) substrates were sensitively affected by the thickness of the substrate in addition to the substrate modulus. On highly crosslinked PDMS substrates, the buckling wavelength of polymer thin films sharply increased as the thickness of the substrates were raised and approached a plateau value when the substrate was 2.5 mm‐thick. On weakly crosslinked PDMS substrates, the wavelength still increased even when the substrate was thicker than 20 mm. The high dependence of the buckling on the substrate thickness has not been reported before and is unexpected from the current predictions. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Laser interferometry: A measurement technique for diffusion studies in thin polymer films

Laser interferometric techniques have many applications in the area of thin film science and technology due to their sensitivity, simplicity, and low cost. Typically, changes in thickness and/or refractive index of a transparent polymer film sample are detected from changes in film reflectivity, as measured by variations in the intensity of reflected laser light. Diffusion related properties such as solvent uptake/swelling, dissolution, and drying rates are accessible with laser interferometry since each of these processes involves a change in the film's thickness and/or refractive index. The principles and basic experimental considerations common to these three applications will be discussed. Illustrative examples from our lab will include n-methyl-2-pyrrolidone diffusion in polyimide, and methanol diffusion in poly(methyl methacrylate).     

Fluid transport in thin film polymer processors

An analysis of fluid transport in a polymer processor consisting of a hollow vertical cylinder with rotating blades is presented. The action of the rotating blades causes a thin film to be deposited on the inner surface of the processor and a bow wave to form in front of the blade. Fluid transport through the processor depends upon flow within the bow wave. Three types of flows are identified as contributing to the fluid transport: flow under the influence of a pressure gradient, flow due to gravitational force, and drag flow due to the action of the rotating blades. Equations describing these flows are derived as well as an equation for the bow wave profile. Experimental data in good agreement with the theoretical development are presented.     

Surface grafting of polymer onto carbon thin film

The surface grafting of polymers onto carbon thin film deposited on a glass plate was achieved by two methods: the graft polymerization initiated by initiating groups introduced onto the surface; and the trapping of polymer radicals by surface aromatic rings of the thin film. It was found that the radical and cationic graft polymerization of vinyl monomers are initiated by azo and acylium perchlorate groups introduced onto the surface, respectively, and the corresponding polymers are grafted onto the surface: the surface grafting of polymers were confirmed by the contact angle of the surface with water. In addition, the anionic ring-opening alternating copolymerization of epoxides with cyclic acid anhydrides was found to be initiated by potassium carboxylate groups on the carbon thin film to give the corresponding polyester-grafted carbon thin film. On the other hand, polymer radicals formed by the decomposition of azo polymer, such as poly(polydimethylsiloxane-azobiscyanopentanoate) and poly(polyoxyethylene-azobiscyanopentanoate), were successfully trapped by the surface aromatic rings of carbon thin film and polydimethylsiloxane and polyoxyethylene were grafted onto the surface. © 1995 John Wiley & Sons, Inc.     

Pattern formation by dewetting of polymer thin film

Strategies for the utilization of dewetting of polymer thin film to fabricate ordered patterns are reviewed. After a brief introduction to the polymer thin film dewetting theory, simulation results of pattern formation induced by physically and chemically patterned substrates, and physical confinement are then summarized. Experimental results including the mechanisms behind and the conditions for good quality of pattern formation based on the dewetting of polymer thin film induced by physical, chemical heterogeneous substrates, topographic structure on film surface, physical confinement and the movement of three-phase line are then discussed. A short introduction to the application of fabricated patterns is also discussed.     

SMA polymer thin film electrodeposition on carbon particles

Electrodeposition of styrene-co-maleic anhydride (SMA) polymer, as thin films on carbon particle substrates, was carried out in a fluidized electrode bed reactor (FEBR). Feeder current, time of deposition, flow rate of anolyte (i.e., bed expansion or bed porosity), concentration of SMA in the anolyte, and pH of the anolyte were the key parameters investigated. The film characteristics were evaluated through SEM and FTIR analyses, the amounts determined by weighing. The effect of these parameters on the electrodeposition process is discussed and optimum conditions for deposition are proposed. Also, a possible mechanism for electrodeposition, particularly for the SMA–carbon system, is discussed. Furthermore, where relevant, the parameters and mechanism are compared with those for our parallel work on the ethylene-co-acrylic acid (EAA)–carbon system.     

Influence of the thin‐film thickness and crystallization temperature on the spherulitic structure of polymer thin films

The spherulitic structure and morphology of poly(3‐hydroxybutyrate) (PHB) thin films crystallized from the melt were observed with a polarizing optical microscope. Depending on the thickness of the PHB thin film and crystallization temperature, banded and nonbanded spherulites could form. Reducing the thin‐film thickness and crystallization temperature was favorable for the formation of the banded structure. The morphology transition from banded spherulites to nonbanded spherulites was related to the ratio of the crystallization rate to the diffusion rate. The formation mechanism of the banded structure was examined with the discontinuity growth theory. A depletion zone was considered to appear periodically at the crystal growth front because of the slow diffusion rate, and this may have resulted in the banded spherulites. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Electrochromism of nickel oxide thin film with polymer gel electrolyte

Polymer gel electrolytes were investigated for an electrochromic device (ECD) using nickel oxide thin film. Poly(ethylene oxide) (PEO) derivatives were cross‐linked and swelled in KOH–aqueous solution giving a hydrogel electrolyte. The ECD containing the uniformly cross‐linked hydrogel showed good result in electrochromic switching performance. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 1305–1308, 2002     

Liquid crystal polymer/polyethylene blends for thin film applications

A series of liquid crystal polymer/polyethylene (LCP/PE) blends have been studied to determine the potential of such a system to produce a high modulus film material which retains fabrication and low temperature characteristics of some current PE films. The subject of liquid crystalline polymer blends has been the focus of significant attention for the last decade due to the novel rheological and mechanical properties of this class of polymers. It has been demonstrated that if an LCP blend is processed under elongational flow conditions, the partially ordered LCP meso-phase intermediate allows the development of an oriented fibrillar morphology which is retained upon solidification. In this study, blown films of blends of 5 and 15% LCP in PE have been produced which show an enhancement in modulus over the neat PE matrix. These results are discussed in terms of processing conditions, LCP reinforcement aspect ratio, fibril diameter, and LCP/PE modulus ratio.     

Oriented structure formation during polymer film extrusion

An experimental technique is described for producing fiberreinforced polymer films by inserting needle-like obstructions in a film-extrusion die. The, needles act as nucleation sites, generating a highly extensional local flow field, which causes sufficient orientation to induce the formation of oriented fibrillar crystallites embedded in a much less oriented matrix. To study the effectiveness of the above technique, linear low density polyethylene (LLDPE) and blends of linear low density with high density polyethylene (HDPE) were extruded through a film die with converging walls, with one or five needles inserted parallel to the extrusion direction, Microscopy observations, birefringence, and differential scanning calorimetry (DSC) measurements performed on the produced films showed in all cases that the presence of the needle induced the formation of a more oriented phase, which in most cases had a birefringence at least an order of magnitude higher than the film matrix. The best results were obtained in the case when blends of HDPE and LLDPE were extruded. The oriented structures obtained in this case consisted of HDPE and exhibited not only high birefringence but melting point elevation as well, indicating their fibrous nature.     

Regulation of film thickness, surface roughness and porosity in thin film growth using deposition rate

This work focuses on distributed control of film thickness, surface roughness and porosity in a porous thin film deposition process using the deposition rate as the manipulated input. The deposition process includes adsorption and migration processes and it is modeled via kinetic Monte Carlo simulation on a triangular lattice with vacancies and overhangs allowed to develop inside the film. A distributed parameter (partial differential equation) dynamic model is derived to describe the evolution of the surface height profile of the thin film accounting for the effect of deposition rate. The dynamics of film porosity, evaluated as film site occupancy ratio, are described by an ordinary differential equation. The developed dynamic models are then used as the basis for the design of a model predictive control algorithm that includes penalty on the deviation of film thickness, surface roughness and film porosity from their respective set-point values. Simulation results demonstrate the applicability and effectiveness of the proposed modeling and control approach in the context of the deposition process under consideration.     

Reduced temperature-dependent thermal conductivity of magnetite thin films by controlling film thickness

We report on the out-of-plane thermal conductivities of epitaxial Fe₃O₄ thin films with thicknesses of 100, 300, and 400 nm, prepared using pulsed laser deposition (PLD) on SiO₂/Si substrates. The four-point probe three-omega (3-ω) method was used for thermal conductivity measurements of the Fe₃O₄ thin films in the temperature range of 20 to 300 K. By measuring the temperature-dependent thermal characteristics of the Fe₃O₄ thin films, we realized that their thermal conductivities significantly decreased with decreasing grain size and thickness of the films. The out-of-plane thermal conductivities of the Fe₃O₄ films were found to be in the range of 0.52 to 3.51 W/m · K at 300 K. For 100-nm film, we found that the thermal conductivity was as low as approximately 0.52 W/m · K, which was 1.7 to 11.5 order of magnitude lower than the thermal conductivity of bulk material at 300 K. Furthermore, we calculated the temperature dependence of the thermal conductivity of these Fe₃O₄ films using a simple theoretical Callaway model for comparison with the experimental data. We found that the Callaway model predictions agree reasonably with the experimental data. We then noticed that the thin film-based oxide materials could be efficient thermoelectric materials to achieve high performance in thermoelectric devices.     

Use of laser scanning confocal microscopy for characterizing changes in film thickness and local surface morphology of UV-exposed polymer coatings

Laser scanning confocal microscopy (LSCM) has been used to characterize the changes in film thickness and local surface morphology of polymer coatings during the UV degradation process. With the noninvasive feature of LSCM, one can obtain thickness information directly and nondestructively at various exposure times without destroying the specimens or deriving the thickness values from IR measurement by assuming uniform film ablation. Two acrylic polymer coatings were chosen for the study, and the physical and chemical changes of the two systems at various exposure times were measured and analyzed. Those measurable physical changes caused by UV exposure include film ablation, formation of pits and other surface defects, and increases in surface roughness. It was found in both coatings that changes in measured film thickness by LSCM were not correlated linearly to the predicted thickness loss using the changes in the CH band obtained by the Fourier Transform Infrared (FTIR) spectroscopy measurements in the later degradation stages. This result suggested it was not a uniform film ablation process during the UV degradation. At later stages, where surface deformation became severe, surface roughness and profile information using LSCM were also proven to be useful for analyzing the surface degradation process Presented at the 81st Annual Meeting of the Federation of Societies for Coatings Technology, November 13–14, 2004 in Philadelphia, PA.     

Phenomena affecting the interferometric determination of concentration profiles of micromolecules diffusing along a stiff-chain polymer film

A detailed examination of the interferometric determination of penetrant concentration profiles during longitudinal penetration of micromolecular liquids in glassy polymer films, nonrigidly clamped between glass plates, is presented. Penetration of methylene chloride and methyl alcohol (representing strong and relatively weak swelling agents, respectively) into cellulose acetate films, characterized, respectively, by a degree of acetylation of 2.45 (CA-2.45) and 2.0 (CA-2.0) acetyl groups per monomer unit, was studied, and the importance of certain factors, which should be taken into account in order to obtain reliable concentration profiles, is demonstrated. In particular, conversion of the interferometrically determined optical path difference profiles to concentration profiles requires taking account of (i) the deviation from linearity of the refractive index–concentration relation, due to the filling of the excess free volume of the polymer by the penetrant and (ii) swelling of the clamped polymer film in the thickness direction, with particular attention to deviations from uniform specimen thickness. In the example of strong swelling agent penetration (CA-2.45/MC), no detectable increase in film thickness over that of the original dry film was detected, as a result of the fact that the polymer becomes sufficiently plasticized to deform plastically along the penetration axis under the clamping pressure. During weak swelling agent penetration (CA-2.0/MA), on the other hand, significant swelling in the thickness direction was observed, because the polymer is less plasticised and can undergo plastic deformation as above only to a limited extent. In this case, it is important to guard against deviations from uniform specimen thickness. The relevant strain birefringence profiles provided significant information which helped to establish the above picture. The calculated concentration profiles are discussed in the light of the measured penetration kinetics and in relation to the magnitude of the clamping pressure. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65:317–327, 1997     

Reversible thickness control of polymer thin films containing photoreactive coumarin derivative units

The reversible control of the thickness of polymer thin films was investigated using (meth)acrylic polymers containing photoreactive coumarin derivative units in the side chain. Coumarin derivative units underwent dimerization and the reverse-dimerization by photoirradiation and were used as a reversible cross-linking point. The homopolymer of 7-methacryloyloxy-4-methylcoumarin (T_g = 194 °C) did not cause changes in film thickness after photoreactions. The homopolymer of 7-(2′-acryloyloxyethoxy)-4-methylcoumarin (AEMC) (T_g = 89 °C) decreased 19% of film thickness by photodimerization and 73% of the decreased thickness was recovered after the reverse-dimerization and the subsequent thermal annealing at 130 °C. The reverse-dimerization of the copolymer of AEMC and n-butyl acrylate (AEMC content = 19 mol%, T_g = 11 °C) resulted in 53% of recovery from the decreased film thickness without annealing. The mobility of polymer main-chain was revealed to be essential factor to change film thickness by photoreactions. Photodimerization of coumarin derivative units in low glass transition temperature (T_g) tended to proceed faster than in high T_g polymers and resulted in larger decrease in film thickness.     

Laser heating of fibres during continuous processing

An x-ray method of determining the yarn temperature in laser heating of fibres during their continuous processing was developed. The calculated and experimental yarn temperatures were compared. The causes of the differences in the temperatures were discussed. The experimental and calculated dependences of the yarn temperature on the yarn speed were obtained and can be used to predict the temperature conditions and select the technological parameters of laser processing.St. Petersburg State University of Design and Technology. Translated from Khimicheskie Volokna, No. 3, pp. 10–12. May–June, 1996.     

Prediction of the film thickness distribution and pattern change during film insert thermoforming

Various surface process methods have been developed to decorate plastic or metallic products. Film insert molding (FIM) is one of the methods that enhance the functional and/or aesthetic qualities of a product's surface. However, the drawbacks of FIM are that the thickness of the film can change, depending on the product configuration, and further, the pattern of the decorated film may change. Therefore, this article attempts to quantify the changes in the thickness and in the pattern of the decorated film during the FIM process. G'Sell's viscoelastic constitutive law was adopted to describe the rheological behavior of polymer film. A constant‐velocity uniaxial tensile test at high temperature, which is a new method proposed in this research, was used to obtain the rheological parameters. We also suggested a visual method for predicting pattern change, which was validated by comparing analytical results with those of real products. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Simplified processing of anode-supported thin film planar solid oxide fuel cells

The concept of anode-supported planar solid oxide fuel cells (SOFCs) involves different fabrication procedures and several sintering steps. This contribution is an attempt towards minimizing the number of sintering steps. By adopting an improved technique a pre-firing step was eliminated. Cells obtained following this co-firing technique showed relatively higher power density (∼1 A cm⁻² at 0.7 V) at an operating temperature of 800 °C compared to the cells prepared by the conventional technique. In the present investigation, cells as large as$\text{10}\text{cm}\text{×10}\text{cm}$ were prepared and characterized following the new co-firing approach. Detailed microstructural, leak-rate testing (gastightness of the sintered electrolyte film) and electrochemical characterizations were carried out.