Low-temperature reactive coupling at polymer-polymer interfaces facilitated by supercritical CO2

Supercritical CO₂ (scCO₂) has been used to facilitate reactions in thin film bilayers between functionalized polystyrene and poly(methyl methacrylate) at temperatures far below the glass transition temperatures of the respective polymers. Secondary ion mass spectrometry (SIMS) is used to monitor the reaction progression directly by measuring the interfacial excess of deuterated PS. Complementary X-ray reflectometry (XR) yields the interfacial width and surface roughness of bilayer films for reactive systems with and without the addition of scCO₂, and comparisons are made with unreactive reference systems. From XR and SIMS analyses, the interfacial width and roughness have been found to be effectively independent of the reaction conditions employed here, and the primary impact of incorporated scCO₂ is enhanced mobility of the reactive polymer chains. The use of scCO₂ can change polymer mobility significantly enough over a very small temperature range (ΔT∼15 °C) so that both diffusion- and reaction-controlled type behavior can be observed for otherwise identical systems.     

Measurement of interfacial tension in PS/LDPE melts saturated with supercritical CO2

The effect of supercritical carbon dioxide (scCO₂) on the interfacial tension between a polymer melt pair—polystyrene (PS) and low density polyethylene (LDPE)—was studied using the pendant drop method at temperatures varying from 200 to 240°C and CO₂ pressures up to 18 MPa. The LDPE melt was prepared in a high pressure optical cell and the PS pendant drop was injected into the LDPE melt with a special high pressure syringe. For measurements with scCO₂, the optical cell was first pressurized with scCO₂ and measurements were taken after the saturation of scCO₂ into both polymer melts. Excellent agreement was found with literature data for the same system without using scCO₂. For the polymer pair saturated with scCO₂, it was found that the interfacial tension decreases significantly with increasing CO₂ pressure and appears to level off at higher CO₂ pressures.     

Surface modification of polymeric nanocomposite thin films using supercritical carbon dioxide

We report on an efficient and environmentally friendly means to modify surface properties of polymer films supported for nanoparticles. Ultrathin polystyrene (PS) films (<300 Å), in which inorganic nanoparticles were embedded, were exposed to supercritical carbon dioxide (scCO₂). The swollen structure was then preserved by quickly evaporating CO₂. X-ray reflectivity (XR) results showed that this procedure produced polymeric nanocomposite films with a low-density region of about 150Å at the polymer/air interface. The formation of the low-density layer was independent of the nature of the particles, indicating that the surface modification through exposure to scCO₂ may be a universal phenomenon regardless of a choice of nanoparticles.     

Structure,permeability, and rheology of supercritical CO2 dispersed polystyrene-clay nanocomposites

Improvement in clay dispersion and clay-polymer interfacial interactions are keys to producing superior nanocomposites. A supercritical CO₂ (scCO₂) processing method was utilized to pre-disperse commercial organic clays, for further solvent mixing with polystyrene (PS) to form nanocomposites with significant dispersion and interfacial enhancement. The effect of scCO₂ processing on clay pre-dispersion, and clay dispersion and interfacial interaction in nanocomposites were investigated. SEM and WAXD of the clays indicated that after scCO₂ processing the clays lose their long region ordered layer structure appreciably, associated with reduction in particle size. WAXD and TEM of the PS/clay nanocomposites showed that the polymer penetrated into the pre-dispersed clay, leading to a disordered intercalated/exfoliated structure with improved interfacial interaction rather than a disordered intercalated structure as seen with as-received clays. Relationships between those structures, rheological and barrier properties were investigated. The scCO₂-processed nanocomposites showed a plateau in the low-frequency storage modules and increased complex viscosity, each associated with significant clay dispersion in the nanocomposite. With only 1.09% volume fraction of clay, significant reduction (∼49%) of oxygen permeation was achieved.     

Effect of molecular weight on the dissolution properties of polystyrene latex films

An in situ steady-state fluorescence (SSF) technique was applied in order to study the dissolution process of polystyrene (PS) latex films. The effect of the molecular weight M _w of the PS on the dissolution rate was investigated. The PS chains were copolymerized with (1-pyrene)methyl methacrylate in order to make use of pyrene (P) as a fluorescent probe to monitor the dissolution process. Seven different films were prepared from P-labeled PS latex dispersions with different molecular weights at room temperature. These films were then annealed at 200 °C for 15 min to complete the film formation process before dissolution. The dissolution of PS films in a toluene (70 %)–cyclohexane (30 %) mixture was monitored in real time by watching the change in the fluorescence intensity of P, I _P. We used a model that included both case I and case II diffusion kinetics to interpret the results of the dissolution experiments. The relaxation constants k ₀ and the dissolution coefficients D _d of the polymer chains were measured. Two different dissolution coefficients were obtained, which were attributed to the small and long polymer chains in the film, considering the high polydispersity of the polymer. It was also found that both of the D _d values scaled with M _w according to the law D _d ≈ M ^?n .     

Effects of tension on mesomorphic transitions and mechanical properties of oriented poly(ethylene terephthalate) fibers under supercritical CO2 exposure

Samples of partially oriented yarn PET fibers were uniaxially drawn below the glass transition temperature (cold-drawing) before exposure to supercritical carbon dioxide (scCO₂) in the absence and presence of tension at a temperature of 80 °C and a pressure of 220 bar. The effects of draw ratio, scCO₂ exposure, and tension on structural changes and on mechanical properties in particular were investigated using differential scanning calorimetry, tensile deformation, and birefringence and density measurements. A good correlation was obtained among the results obtained from various techniques. Results indicate that exposure to scCO₂ not only induces structural changes but also develops crystallization in the samples. Tension under scCO₂ exposure also produces significant effects in terms of causing structural changes and transforming the oriented chains in the mesophase into the crystalline or noncrystalline domains. PET fibers exposed to scCO₂ under no tension yields lower values of crystallinity, orientation, tenacity, and Young’s modulus but higher values of breaking elongation compared to samples exposed under tension. This is an indication of higher plasticity of the chains in the amorphous domains in samples exposed under no tension. It is also found that mechanical measurements confirm the structural changes taking place in exposed PET samples.     

Fabrication of Three-Dimensionally Ordered Macroporous TiO2 Films with Enhanced Photovoltaic Conversion Efficiency

Negative-charged polystyrene (PS) microspheres were prepared through a soap-free emulsion polymerization method using potassium persulfate as initiator. Three-dimensionally ordered macroporous TiO₂ films were fabricated using the high-quality PS colloidal crystals templates obtained via a horizontal deposition method. The as-prepared macroporous TiO₂ films were applied as the photoanodes in dye-sensitized solar cell (DSSC). The microstructure of the products were characterized by X-ray diffractometer, fourier transform infrared spectroscopy, scanning electron microscopy, transmission electron microscopy and nitrogen adsorption–desorption analyzer. The results showed that the macroporous TiO₂ films replicated well the 3D ordered structure derived from PS colloidal crystal templates and revealed a relatively large specific surface area (69.99 m²/g), which could increase the capacity of TiO₂ film anode for absorbing dyes and scattering light. The photocurrent–voltage (I–V) characteristics of DSSC were measured by a digital source meter under simulated solar light. The results indicated that the introduction of an ordered macroporous TiO₂ interfacial layer increased the photovoltaic conversion efficiency, which was improved by 68 % from 3.61 to 6.08 %, as compared to a device using a bare P25 TiO₂ photoanode. Our results showed that the hierarchically ordered macroporous TiO₂ bilayer films photoanode for DSSC could be helpful to improve the photovoltaic conversion efficiency.     

Cyclophosphazene-containing Polymers as Imprint Lithography Resists

We present the evaluation of a cyclophosphazene-containing polymer as a patternable resist for imprint lithography. Hexamethacryloxybutoxycyclotriphosphazene layers containing small amounts of photoinitiator can be applied to silicon wafers substrates by spin coating techniques and cured photochemically to give tough, network polymer thin films. The films were characterized by FT-IR. Thin films approximately 200 nm in thickness were subjected to anisotropic O₂ and CF₄ plasmas and the etch rates were determined. The polymer films etch at a rate of 21 Å/s in CF₄ plasma, and as low as 1.6 Å/s in O₂ plasma, which is comparable or lower than the rates observed with commercially available silicon-containing photoresists. The surface chemical composition was surveyed by X-ray photoelectron spectroscopy, which gave results consistent with the formation of an etch resistant phosphorus-rich layer during reaction with O₂ plasma. The polymer was processed by nano-contact molding imprint lithography and replicated 200 nm period test patterns. This report is the first demonstration of a cyclophosphazene-containing polymer as a resist candidate for high-resolution lithography.     

Effect of interfacial area on heterogeneous free radical grafting of vinyl monomers in supercritical carbon dioxide: Grafting of acrylic acid on poly(vinylidenefluoride) nanoparticles

The role of the polymer interfacial area on free radical grafting of acrylic acid (AA) onto poly(vinylidenefluoride) (PVDF) was studied at 65°C using supercritical carbon dioxide (scCO₂) as a solvent and swelling agent, benzoylperoxide (BPO) as chemical initiator and PVDF nanoparticles as polymer matrix. Under adopted conditions PVDF particles do not melt neither dissolve in the reaction medium and FTIR analyses performed on carefully washed nanoparticles confirmed the achievement of high grafting levels. The mass fraction of grafted AA increased with the grafting time and the BPO concentration while it decreased when the density of the fluid phase was enhanced. Collected results suggest that the grafting level obtained by free radical grafting of vinyl monomers onto solid polymer in scCO₂ can be significantly enhanced by increasing the interfacial area of the matrix. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41541.     

Thermal and morphological studies on γ‐Fe2O3 polystyrene composites and the affect of additives

γ‐Fe₂O₃polystyrene (PS) composite films were prepared by a gel‐casting technique to obtain monodisperse composite films. To understand the effect of additives on the prepared composite films, additives such as rice husk ash and thiourea were made to disperse into the PS matrix. The as‐prepared γ‐Fe₂O₃ PS composite films, along with their additives, were subjected to characterization and study by X‐ray diffraction, scanning electron microscopy, thermal, IR, and dielectric measurement techniques. These studies showed monodisperse and chemically homogenous composite films with an increase in thermal behavior. An interesting self‐assembly of rod‐like nanoparticles of γ‐Fe₂O₃ particles into the polymer matrix, which formed spherical packets, was observed for the γ‐Fe₂O₃PS composite film. The electrical behavior of these films was interesting, as some showed conduction whereas others showed an increase in dielectric behavior. This nature was explained by the dielectric measurements. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 778–788, 2004     

Ordered Oxide Surfaces on Metals: Chromium Oxide

We present X-ray photoelectron spectroscopy (XPS) and X-ray photoelectron diffraction (XPD) investigations of ordered chromium oxide ultrathin films prepared on a Pd(111) single-crystal surface. The films where grown by thermal evaporation of Cr under an oxygen atmosphere and sample temperature of 600 K. The ordered films produced are strongly dependent on the film thickness and annealing treatment. Films with thickness below 5 Å, produced at low coverages, display a p(2 × 2) structure relative to Pd(111). Thicker films (thickness > 10 Å) always have a $ \left( {\sqrt 3 \times \sqrt 3 } \right)R30^{ \circ } $ structure. The photoemission measurements were done using conventional X-ray sources as well as synchrotron radiation taken at the Laboratório Nacional de Luz Síncrotron. The XPD data were interpreted through the use of a multiple scattering calculation approach combined with a genetic algorithm for surface structure optimization. Combining the information from XPS, low energy electron diffraction and XPD measurements we have determined the surface structure of the $ \left( {\sqrt 3 \times \sqrt 3 } \right)R30^{ \circ } $ phase. Elsewhere, we report on the structure of the p(2 × 2) phase.     

Studies of the development of fibrillar structure in nylon 6

Very thin films of poly(vinyl alcohol) could be prepared by utilizing the adsorption of polymer molecules at air/water interface from the aqueous solutions of the poly(vinyl alcohol) derived from vinyl trifluoroacetate. The films prepared by the bubble method were thinner than those obtained by the frame method. The minimum thickness of the former films was 260 Å and that of the latter was 1800 Å. These very thin films resisted water at temperatures below 55°C. The maximum Young's modulus of the drawn/annealed films prepared from these samples was 30 GPa. The permeability of water, J_w/δP, was 2–6 × 10^?3 cm · s^?1 atm^?1 (0–55°C) for the untreated film (thickness: 1800 Å) prepared by the frame method and 0.8–2.2 × 10^?2cm · s^?1 · atm^?1 (5–55°C) for the untreated film (360 Å) prepared by the bubble method, and depended on the thickness of film.     

Poly(methyl methacrylate) as masking material for microelectromechanical system (MEMS) fabrication

In the present study direct current (dc) sputtered poly(methyl methacrylate) (PMMA) films deposited on silicon substrates were evaluated as masking materials for anisotropic etching of silicon in aqueous potassium hydroxide (KOH) and tetramethyl ammonium hydroxide (TMAH) solutions. Sputtered PMMA films were characterized by FTIR to ascertain the bonding, by X‐ray photoelectron spectroscopy (XPS) for the elemental composition, and by the contact angle for measuring the adhesion of the film with the substrate. FTIR and XPS data showed the presence of a poly(tetrafluoroethylene)‐like film on the silicon substrate. The interfacial tension was calculated from the contact angle value, which was 0.82 dyne/cm, confirming good adhesion of the film and the substrate. A pattern was lithographically transferred through the masking material on the silicon substrate, and the etch rate of the masking layer was calculated from the masking time data of the films. The etch rate value of 4 Å/min obtained for the masking material is low compared to the etch rate of the conventional masking materials (60 Å/min for SiO₂ and 8 Å/min for Si₃N₄). © 2006 Wiley Periodicals Inc. J Appl Polym Sci 102: 2094–2098, 2006     

Effect of drawing temperature on mesomorphic transitions of oriented poly(ethylene terephthalate) fibers exposed to supercritical CO2

Samples of partially oriented yarn (POY) PET fibers were uniaxially drawn at 23, 68 °C (cold drawing) and 100 °C (hot drawing) and then exposed to the supercritical carbon dioxide (scCO₂) without tension at a temperature of 80 °C and a pressure of 220 bar. The effect of drawing temperature on the evolution of mesomorphic phases and the structural changes under exposure to scCO₂ were investigated by differential scanning calorimetry (DSC), Fourier transform infrared (FTIR), and wide-angle X-ray diffraction. The orientation factor of the samples was measured using a polarizing microscope. A good correlation was obtained between the results of various analyses. The results illustrate that evolution of structure strongly depends on both process temperature and post-treatment by scCO₂ exposure. The latter process leads to plasticization and reduced glass transition temperature of the samples, thus inducing structural changes in the fibers.     

Study of film formation from PS latex/TiO2 nanocomposites; Effect of latex size and TiO2 content

In this work, we investigated the film formation from polystyrene (PS) latex/TiO₂ nanocomposites using the steady state fluorescence (SSF) and UV–vis (UVV) techniques depending on PS particle size and TiO₂ content. The structural properties of films were characterized by scanning electron microscope (SEM). The films were prepared from pyrene (P)‐labeled PS particles (SmPS:203 nm; LgPS:382 nm) by covering them with different layers of TiO₂ by dip‐coating method and then annealed at elevated temperatures. Two film series (SmPS/TiO₂ and LgPS/TiO₂) were prepared and seven different films were studied in various TiO₂ contents for each series. Scattered (I_sc), fluorescence (I_P), and transmitted (I_tr) light intensities were measured after each annealing step to monitor the stages of film formation. Results showed that, SmPS/TiO₂ films undergo complete film formation independent of TiO₂ content. However, no film formation occurs above a certain TiO₂ content in LgPS/TiO₂ films. SEM images showed that SmPS/TiO₂ films have highly well‐ordered microporous structures with increasing TiO₂ content after extraction of PS polymer whereas LgPS/TiO2 composites show no porous structure for high TiO₂ content. Our experiments also showed that porous TiO₂ films with different sizes could be successfully prepared using this technique. POLYM. COMPOS., 35:2376–2389, 2014. © 2014 Society of Plastics Engineers     

Using supercritical carbon dioxide in preparing carbon nanotube nanocomposite: Improved dispersion and mechanical properties

Improvements in carbon nanotube (CNT) dispersion and subsequent mechanical properties of CNT/poly(phenylsulfone) (PPSF) composites were obtained by applying the supercritical CO₂ (scCO₂)‐aided melt‐blending technique that has been used in our laboratory for nanoclay/polymer composite preparation. The preparation process relied on rapid expansion of the CNTs followed by melt blending using a single‐screw extruder. Scanning electronic microscopy results revealed that the CNTs exposed to scCO₂ at certain pressures, temperatures, exposure time, and depressurization rates have a more dispersed structure. Microscopy results showed improved CNT dispersion in the polymer matrix and more uniform networks formed with the use of scCO₂, which indicated that CO₂‐expanded CNTs are easier to disperse into the polymer matrix during the blending procedure. The CNT/PPSF composites prepared with scCO₂‐aided melt blending and conventional melt blending showed similar tensile strength and elongation at break. The Young's modulus of the composite prepared by means of conventional direct melt blending failed to increase beyond the addition of 1 wt% CNT, but the scCO₂‐aided melt‐blending method provided continuous improvements in Young's modulus up to the addition of 7 wt% CNT. POLYM. COMPOS., 2012. © 2012 Society of Plastics Engineers     

Optical,Electrochemical, and Structural Properties of Spray Coated Dihexyl Substituted Poly (3,4 Propylene Dioxythiophene) Film for Optoelectronics Devices

The dihexyl substituted poly (3,4-propylenedioxythiophene) (PProDOT-Hx₂) thin films uniformly deposited by cost effective spray coating technique on transparent conducting oxide coated substrates. The electro-optical properties of PProDOT-Hx₂ films were studied by UV-Vis spectroscopy that shows the color contrast about 45% with coloration efficiency of ～ 185 cm²/C. The electrochemical properties of PProDOT-Hx₂ films were studied by cyclic voltammetry and AC impedance techniques. The cyclic voltammogram shows that redox reaction of films are diffusion controlled and ions transportation will be faster on the polymer film at higher scan rate. Impedance spectra indicate that polymer films are showing interface charge transfer process as well as capacitive behavior between the electrode and electrolyte. The XRD of the PProDOT-Hx₂ thin films revealed that the films are in amorphous nature, which accelerates the transportation of ions during redox process.     

Ionic Conductivity and Discharge Characteristic Studies of PVA-Mg(CH3COO)2 Solid Polymer Electrolytes

Ion conducting solid polymer electrolytes based on a polymer polyvinyl alcohol (PVA) complexed with magnesium acetate (Mg(CH₃COO)₂) were prepared by solution cast technique. Various experimental techniques, such as XRD, DSC, composition-dependent conductivity, temperature-dependent conductivity, and transport number measurements are used to characterize these polymer electrolyte films. The transference number data indicated the dominance of ion-type charge transport in these polymer electrolyte systems. An electrochemical cell with the configuration Mg/(80PVA + 20Mg(CH₃COO)₂)/(I₂ + C + electrolyte) has been fabricated and its discharge characteristics were studied. The Open Circuit Voltage (OCV) is 1.84 V.     

Moisture absorption into ultrathin hydrophilic polymer films on different substrate surfaces

Moisture absorption into ultrathin poly(vinyl pyrrolidone) (PVP) films with varying thickness was examined using X-ray reflectivity (XR) and quartz crystal microbalance (QCM) measurements. Two different surfaces were used for the substrate: a hydrophilic silicon oxide (SiO_x) and a hydrophobic hexamethyldisilazane (HMDS) treated silicon oxide surface. The total equilibrium moisture absorption (solubility) was insensitive to the surface treatment in the thickest films (≈150 nm). However, strong reductions in the equilibrium uptake with decreasing PVP film thickness were observed on the HMDS surfaces, while the SiO_x surface exhibited thickness independent equilibrium absorption. The decreased absorption with decreasing film thickness is attributed a depletion layer of water near the polymer/HMDS interface, arising from hydrophobic interactions between the surface and water. The diffusivity of water decreased when the film thickness was less than 60 nm, independent of the surface treatment. Changes in the properties of ultrathin polymer films occur even in plasticized films containing nearly 50% water.     

Purification of Waste Cooking Oils via Supercritical Carbon Dioxide Extraction

Purification of waste cooking oils (palm oil and soybean oil) using supercritical carbon dioxide (scCO₂) extraction has been investigated. The purified oils were characterized by their acid value, conjugated diene value, total polar compound measurements, and high-performance size exclusion chromatography. Using optimal extractions conditions of 353.15 K, 20 MPa, and CO₂ flow rate of 40 g/min, 80% of the oil was recovered and the purified oil compositions and properties were very close to those of the fresh oils. At higher pressures or lower temperatures, the separation efficiency of the scCO₂ extraction was significantly reduced.