Spinodal clustering induced dewetting and non-monotonic stabilization of polymer blend films at high nanofiller concentrations

We examine the effects of high fullerene nanoparticle (f-NP) concentrations, ?_f-NP ∼ (10–20) mass% on polystyrene (PS)/polybutadiene (PB) blend thin film stability. Dewetting of the polymer blend around spinodally clustered f-NPs in this high concentration limit leads to a spinodal like dewetting morphology. This is in contrast to our previously observed results on the stabilization effects of f-NPs on PS/PB blend thin films in the intermediate f-NP concentration range of 7–10 mass%, wherein, after saturating the polymer–blend interface, the NPs stabilize the film by segregating to the blend–substrate interface. We determine three regimes of polymer blend film stability as a function of filler concentration: a) ?_f-NP < 7 mass% where preferential segregation of the f-NPs to the polymer–polymer interface leads to macroscopic dewetting, b) ?_f-NP ∼ (7–10) mass% where PS/PB blend films exhibit complete film stability, and c) ?_f-NP ∼ (11–20) mass%, where spinodal clustering of the f-NPs gives rise to polymer–NP phase exclusion and subsequent dewetting.     

Multilayer formation via spinodal decomposition in TiO2-VO2 epitaxial films on sapphire substrates

We present multilayer formation via spinodal decomposition in rutile TiO₂-VO₂ (TVO) epitaxial films on sapphire substrates. (001)- and (101)-oriented TVO solid-solution films are grown epitaxially on TiO₂/Al₂O₃ using a pulsed laser deposition technique and annealed inside the spinodal region. X-ray diffraction measurements and scanning transmission electron microscopy (STEM) observations show that the films are phase-separated along the [001] direction and lamellar structures are formed in a parallel or slanted direction to the sapphire substrates depending on the film orientation. The results indicate the multilayer formation via spinodal decomposition in the TVO films. STEM investigations also reveal a relatively high Ti concentration in the decomposed phases, reflecting the influence of lattice deformation on the phase decomposition in the films. Our work shows that spinodal decomposition is a promising approach for the formation of a multilayer structure in TVO films and helps deepen understanding the spinodal decomposition in TVO system.     

Theoretical representation of shear and pressure influence on the phase separation behavior of PVME/PS mixture

In the framework of lattice fluid model, the Gibbs energy and equation of state are derived by introducing the energy (E_s) stored during flow for polymer blends under shear. From the calculation of the spinodal of poly(vinyl methyl ether) (PVME) and polystyrene (PS) mixtures, we have found the influence of E_s on equation of state in pure component is inappreciable, but it is appreciable in the mixture. However, the effect of E_s on phase separation behavior is extremely striking. In the calculation of spinodal for the PVME/PS system, a thin, long and banana miscibility gap generated by shear is seen beside the miscibility gap with lower critical solution temperature. Meanwhile, a binodal coalescence of upper and lower miscibility gaps is occurred. The three points of the three-phase equilibrium are forecasted. The shear rate dependence of cloud point temperature at a certain composition is discussed. The calculated results are acceptable compared with the experiment values obtained by Higgins et al. However, the maximum positive shift and the minimum negative shift of cloud point temperature guessed by Higgins are not obtained. Furthermore, the combining effects of pressure and shear on spinodal shift are predicted.     

Controlling morphology of polystyrene/poly(vinyl methyl ether) blends undergoing spinodal decomposition process by photo-crosslinks

The morphology developing during the spinodal decomposition process of polystyrene (PS)/poly(vinyl methyl ether) (PVME) blends was successfully controlled by photo-crosslink reactions between PS chains. The crosslink reaction was carried out by taking advantage of the photodimerization of anthracene moieties that are labeled on PS chains. Effects of photo-crosslinks on the morphology induced by temperature jumps (T-jump) from the one-phase region into the spinodal region were examined under several experimental conditions such as T-jump depths and irradiation times. It was found that the concentration fluctuations developing during the spinodal decomposition process were efficiently frozen upon irradiation using a XeF excimer laser as well as a mercury (Hg) lamp. Furthermore, these ordered structures are quite stable upon annealing. These results demonstrate that the morphology developing during the spinodal decomposition process can be well controlled by easily accessible light sources such as high pressure mercury lamps. Thus the photo-crosslink reaction described in this work can provide the basis for a potential technique to design multiphase polymer materials with controllable ordered structures.     

Kinetics of pressure-induced phase separation in polystyrene + acetone solutions at high pressures

The kinetics of pressure-induced phase separation in solutions of polystyrene (M_w = 129,200; PDI = 1.02) in acetone has been studied using time- and angle-resolved light scattering. A series of controlled pressure quench experiments with different quench depths were conducted at different polymer concentrations (4.0%, 5.0%, 8.2% and 11.4% by mass) to determine the binodal and spinodal boundaries and consequently the polymer critical concentration. The results show that the solution with a polymer concentration 11.4 wt% undergoes phase separation by spinodal decomposition mechanism for both the shallow and deep quenches as characterized by a maximum in the angular distribution of the scattered light intensity profiles. Phase separation in solutions at lower polymer concentrations (4.0, 5.0 and 8.2 wt%) proceeds by nucleation and growth mechanism for shallow quenches, but by spinodal decomposition for deeper quenches. These results have been used to map-out the metastable gap and identify the critical polymer concentration where the spinodal and binodal envelops merge.The time scale of new phase formation and growth as (accessed) from the time evolution of scattered light intensities is observed to be relatively short. The late stage of phase separation is entered within seconds after a pressure quench is applied. For the systems undergoing spinodal decomposition, the characteristic wave number qm corresponding to the scattered light intensity maximum Im was analyzed by power-law scaling according to qm∼t−α and Im∼tβ. The results show β≈2α. The domain size is observed to grow from 4 μm to 10 μm within 2 s for critical quench, but about 9 s for off-critical quenches. The domain growth displays elements of self-similarity.     

Orientational phase-separated domains in a polyolefin blend under a temperature gradient field

We investigate the effect of a temperature gradient on the orientation of phase-separated structures in a polyolefin blend system. Phase contrast optical microscopy (PCOM) has been used to measure the morphology of phase separation via spinodal decomposition as a function of phase separation time and temperature gradient. The bicontinuous and interconnected tubelike structure, the characteristic morphology of the spinodal decomposition process, exhibits a preferential alignment along the direction of temperature gradient after phase separation. The orientation of the bicontinuous and interconnected tubelike structures gradually increases with phase separation time and temperature gradients. Also the orientation of phase-separated domains can respond really quickly to the change in the direction of external temperature gradient field. The results suggest that “thermal force” induced by the temperature inhomogeneity might play an important role in aligning phase-separated domains preferentially along the temperature gradient direction.     

Light scattering behavior and the kinetics of pressure-induced phase separation in solutions of poly(ε-caprolactone) in acetone + CO2 binary fluid mixtures

The miscibility and the kinetics of pressure-induced phase separation in solutions of poly(ε-caprolactone) (PCL) in acetone + CO₂ binary fluid mixtures have been studied at pressures up to 28 MPa and temperatures up to 410 K using a unique high pressure view-cell equipped with a dual set of pistons and dual set of sapphire windows. One set of the windows separated by 25.4 mm allows the assessment of the phase state and is used to monitor the transmitted light intensities. The second set of windows separated by 50 μm is used to monitor the scattered light intensities over a wide range of scattering vector q (from 0.35 to 4 μm⁻¹) which allows the assessment of the mechanism of phase separation. Investigations have been carried out for a wide range of polymer concentrations, from 2.0 to 34.9 wt%, while holding the acetone-to-CO₂ (wt:wt) ratio in each solution at a constant value of 2:1. The dual set of pistons that are employed which are synchronized and motorized create a churn-like action in the cell insuring effective mixing, even at the high polymer concentrations by translating the cell content across a magnetically-coupled rotating mixer impeller. The piston actions assure also that the solution is effectively introduced into the narrow gap between the scattering windows, and refreshed. The solutions at off-critical concentrations undergo pressure-induced phase separation via nucleation and growth mechanism which shows circular symmetric patterns in their light scattering patterns. For these solutions, the Debye–Bueche type scattering function was used to analyze the domain size of the new phase that forms and develops after a pressure quench. The phase separation in solutions at or near the critical polymer concentrations (9.0–15.0 wt%) proceeds via spinodal decomposition which is characterized by the formation and evolution of the spinodal ring patterns corresponding to a maximum in the angular variation of the scattered light intensities. The results in early stage of the spinodal decomposition were described by the linearized Cahn theory. The variation of the scattered light intensity maximum I_m and its location in scattering vectors q_m with time in the later stage of the spinodal decomposition obey power-law scaling according to I_m ∼ t^β and q_m ∼ t^−α. The results for the 9.0 and 12.0 wt% solutions show that β/α changes its value from β/α > 3 to β/α ≈ 3 with time, indicating the progression of the spinodal decomposition from intermediate to late stage.     

Phase Separation of Alkoxy-derived (Ti,Sn)O2 Oriented Thin Films

Oriented (Ti,Sn)O₂ thin films with modulated microstructure were successfully synthesized on sapphire substrates by using sol–gel processing combined with spinodal decomposition. The degree of orientation of (Ti_0.5Sn_0.5)O₂ thin films increased in the following order: sapphire (0001), (11     0), and (01     2). (Ti_0.5Sn_0.5)O₂ thin films underwent spinodal decomposition at 900°C by annealing. The variation of the 2theta value of the 202 reflection of (Ti_0.5Sn_0.5)O₂ films showed the typical behavior of spinodal decomposition. The rate of spinodal decomposition of the (Ti_0.5Sn_0.5)O₂ films on sapphire (11     0) was faster than that on sapphire (01     2) substrates. The characteristic modulated microstructure was observed for the spinodally decomposed (Ti_0.5Sn_0.5)O₂ films on sapphire (01     2) substrates by transmission electron microscopy. (Ti_0.3Sn_0.7)O₂ films on sapphire (01     2) substrates were binodally decomposed during annealing at 1300°C.     

PROBING HIDDEN POLYMERIC INTERFACES USINGIR-VISIBLE SUM-FREQUENCY GENERATION SPECTROSCOPY

Understanding the molecular-level processes underlying interfacial phenomena is important in the area of adhesion. We briefly introduce IR-visible sum-frequency generation spectroscopy (SFG) using a total-internal-reflection geometry for the study of polymer-air, polymer-solid, and polymer-polymer interfaces. The following examples, predominantly of work done in our lab, illustrating differences in molecular structure and dynamic properties at interfaces are presented: the air- and solid-interface structure of an amorphous polystyrene (PS) and a semicrystalline polymer with side-chain crystallinity, poly(octadecyl acrylate) (PA-18); structure of a polymer-polymer interface between thin films of a semicrystalline polymer with side-chain crystallinity, poly(vinyl-N-octadecylcarbamate- co-vinyl acetate), and an amorphous PS; thermal order-to-disorder transitions of the air and solid interface of PA-18, and the interface of this polymer with PS; and dynamic surface-relaxation studies of a rubbed PS film.     

PROBING HIDDEN POLYMERIC INTERFACES USINGIR–VISIBLE SUM-FREQUENCY GENERATION SPECTROSCOPY

Understanding the molecular-level processes underlying interfacial phenomena is important in the area of adhesion. We briefly introduce IR–visible sum-frequency generation spectroscopy (SFG) using a total-internal-reflection geometry for the study of polymer–air, polymer–solid, and polymer–polymer interfaces. The following examples, predominantly of work done in our lab, illustrating differences in molecular structure and dynamic properties at interfaces are presented: the air- and solid-interface structure of an amorphous polystyrene (PS) and a semicrystalline polymer with side-chain crystallinity, poly(octadecyl acrylate) (PA-18); structure of a polymer–polymer interface between thin films of a semicrystalline polymer with side-chain crystallinity, poly(vinyl-N-octadecylcarbamate- co-vinyl acetate), and an amorphous PS; thermal order-to-disorder transitions of the air and solid interface of PA-18, and the interface of this polymer with PS; and dynamic surface-relaxation studies of a rubbed PS film.     

Dynamics of the leveling process of nanoindentation induced defects on thin polystyrene films

Using Atomic Force Microscopy (AFM) we study the effect of nanoindentation induced defects on 50 and 120 nm thick unentangled polystyrene (PS) films, spin cast on silicon (Si) substrates. Indents with residual depths of penetration less than the film thickness level upon heating above the glass transition temperature (T_g) of bulk PS. The resulting leveling process is discussed in terms of a diffusion process driven by the curvature gradient. Calculated diffusivity values are close to the self-diffusivity of bulk PS.     

Morphology of poly(styrene‐block‐dimethylsiloxane) copolymer films

The morphologies of poly(styrene‐block‐di‐methylsiloxane) (PS‐b‐PDMS) copolymer thin films were analyzed via atomic force microscopy and transition electron microscopy (TEM). The asymmetric copolymer thin films spin‐cast from toluene onto mica presented meshlike structures, which were different from the spherical structures from TEM measurements. The annealing temperature affected the surface morphology of the PS‐b‐PDMS copolymer thin films; the polydimethylsiloxane (PDMS) phases at the surface were increased when the annealing temperature was higher than the PDMS glass‐transition temperature. The morphologies of the PS‐b‐PDMS copolymer thin films were different from solvent to solvent; for thin films spin‐cast from toluene, the polystyrene (PS) phase appeared as pits in the PDMS matrix, whereas the thin films spin‐cast from cyclohexane solutions exhibited an islandlike structure and small, separated PS phases as protrusions over the macroscopically flat surface. The microphase structure of the PS‐b‐PDMS copolymer thin films was also strongly influenced by the different substrates; for an asymmetric block copolymer thin film, the PDMS and PS phases on a silicon substrate presented a lamellar structure parallel to the surface at intervals. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 1010–1018, 2007     

Dependence of thin polystyrene films stability on the thickness of grafted polystyrene brushes

We investigate the stability of thin polystyrene (PS) films on chemically identical grafted brushes of various thickness and grafting density. We observe an essential influence of the brush thickness on the stability of the PS films. For brushes with a thickness of 20-35 nm no de-wetting of the PS film occurs, while considerably thicker or thinner PS brushes lead to de-wetting of the PS top layer. We suggest that in the thin brush-like layers, the unfavorable interactions with underlying silica favor de-wetting. The tendency to de-wet is reduced once the brush is sufficiently thick to insulate the free PS layer from the surface. Beyond that point, the de-wetting process speeds up as the brush becomes thicker and has a higher grafting density with a substantial increase of the interfacial tension between the brush and the free polymer.     

Benzo[1,2-b:4,5-b′]dithiophene-based copolymers applied in bottom-contact field-effect transistors

Three copolymers of benzo[1,2-b:4,5-b′]dithiophene and 3,3′-bis(alkyl)-5,5′-bithiophene (dodecyl, tetradecyl and hexadecyl side chains) have been synthesized through Stille copolymerization. The polymers have number-average molecular weights over 20 kg/mol, are well-packed in the bulk and thin film, and possess an ionization potential of −5.1 eV in thin film, which offers stability versus oxidation in environmental conditions. The thin film packing of the polymer with dodecyl side chains leads to an excimeric emission upon excitation, which is not observed for longer side chain lengths. The presence of the dimers responsible for this excimer formation results in a device performance improvement as well. Field-effect transistors fabricated from these copolymers have On/Off ratios >10⁷, equal saturation and linear hole mobilities above 10⁻² cm²/Vs, almost no hysteresis and turn-on voltages around 0 V in bottom-contact devices.     

Morphology development and characterization of the phase-separated structure resulting from the thermal-induced phase separation phenomenon in polymer solutions under a temperature gradient

This paper studied the morphological development during the fabrication of anisotropic polymeric materials using the thermal-induced phase separation phenomenon (spinodal decomposition) in a model binary polymer solution under a linear spatial temperature gradient using mathematical modeling and computer simulation. The model incorporated the non-linear Cahn-Hilliard theory for spinodal decomposition and the Flory-Huggins theory for polymer solution thermodynamics. Moreover, the slow mode theory and Rouse law were used to account for polymer diffusion. The two-dimensional numerical results showed that an anisotropic morphology was developed when a temperature gradient was imposed along the polymer solution sample. The droplet size and droplet density decrease as temperature increases during the intermediate stage of spinodal decomposition. The spatial temperature gradient, however, had insignificant effect on the droplet shape.     

Phase separation and dewetting in polystyrene/poly(vinyl methyl ether) blend thin films in a wide thickness range

Phase separation and dewetting processes of blend thin films of polystyrene (PS) and poly(vinyl methyl ether) (PVME) in two phase region have been studied in a wide film thickness range from 65 μm to 42 nm (∼2.5R_g, R_g being radius of gyration of a polymer) using optical microscope (OM), atomic force microscope (AFM) and small-angle light scattering (LS). It was found that both phase separation and dewetting processes depend on the film thickness and were classified into four thickness regions. In the first region above ∼15 μm the spinodal decomposition (SD) type phase separation occurs in a similar manner to bulk and no dewetting is observed. This region can be regarded as bulk. In the second region between ∼15 and ∼1 μm, the SD type phase separation proceeds in the early stage while the characteristic wavelength of the SD decreases with the film thickness. In the late stage dewetting is induced by the phase separation. In the third region between ∼1 μm and ∼200 nm the dewetting is observed even in the early stage. The dewetting morphology is very irregular and no definite characteristic wavelength is observed. It is expected that the irregular morphology is induced by mixing up the characteristic wavelengths of the phase separation and the dewetting. In the fourth region below ∼200 nm the dewetting occurs after a long incubation time with a characteristic wavelength, which decreases with the film thickness. It is considered that the layered structure is formed in the thin film during the incubation period and triggers the dewetting through the capillary fluctuation mechanism or the composition fluctuation one.     

Verification of numerical simulation of the self-assembly of polymer-polymer-solvent ternary blends on a heterogeneously functionalized substrate

The directed self-assembly of polymer-polymer-solvent ternary blends on heterogeneously functionalized substrate is investigated with a three dimensional numerical model. The numerical simulation results are quantitatively verified by the experimental results. The phase separation of PS-PMMA-DMF blends are spin coated on a substrate functionalized by ODT/₂NH on Au surface. While many simulation parameters are set to the experimental conditions, other unmeasurable material constitutive model parameters are estimated from the real experiment observations. The effects of the spin speed, pattern periodicity, PS/PAA composition ratio, and the PAA molecular weight are investigated in both the experiments and numerical simulation. The simulation results are verified by comparison to the experimental results. During the verification process, numerical optimization methods are employed to determine the unmeasurable physical parameters. Quantitative methods are introduced for assessment of the results.     

Enhanced thermal properties of PS nanocomposites formed from inorganic POSS-treated montmorillonite

We have prepared polystyrene/clay nanocomposites using an emulsion polymerization technique. The nanocomposites were exfoliated at up to a 3 wt% content of pristine clay relative to the amount of polystyrene (PS). We used two different surfactants for the montmorillonite: the aminopropylisobutyl polyhedral oligomeric silsesquioxane (POSS) and the ammonium salt of cetylpyridinium chloride (CPC). Both surfactants can intercalate into the layers of the pristine clay dispersed in water prior to polymerization. Although the d spacing of the POSS-intercalated clay is relatively smaller than that of the CPC-intercalated clay, PS more easily intercalates and exfoliates the POSS-treated clay than the CPC-treated clay. IR spectroscopic analysis further confirms the intercalation of POSS within the clay layers. We used X-ray diffraction (XRD) and transmission electron microscopy (TEM) to characterize the structures of the nanocomposites. The nanocomposite prepared from the clay treated with the POSS containing surfactant is exfoliated, while an intercalated clay was obtained from the CPC-treated surfactant. The molecular weights of polystyrene (PS) obtained from the nanocomposite is slightly lower than the virgin PS formed under similar polymerization conditions. The value of T_g of the PS component in the nanocomposite is 8 °C higher than the virgin PS and its thermal decomposition temperature (21 °C) is also higher significantly. The presence of the POSS unit in the MMT enhances the thermal stability of the polystyrene.     

Crystallization of polycarbonate induced by spinodal decomposition in polymer blends

We found that amorphous polycarbonate (PC) can be crystallized in several minutes by blending poly(ethylene oxide) (PEO). When the blends were annealed in the two-phase region below the upper critical solution temperature, highly interconnected two-phase structure characteristic of the spinodal decomposition was developed and then the crystallization occurred in the PC-rich phase during the spinodal decomposition. As the molecular weight of PEO decreased, the crystallization rate decreased and the crystallizable temperature became narrower in spite of the acceleration of the polymeric segmental motion. These results suggest that the crystallization of the PC is not induced by the acceleration of the polymeric segmental motion, but by the up-hill diffusion of the liquid-liquid phase separation via spinodal decomposition. Owing to the competitive progress of the crystallization and the spinodal decomposition, the melting peak of the PC crystallites shifted to lower temperature with increasing annealing temperature.     

Effects of molecular weight distribution on the spinodal temperature of polymer mixtures

The effects of molecular weight distribution on the phase stability of polymer mixtures were explored theoretically and experimentally. Based on the lattice‐fluid theory and volume‐fluctuation thermodynamics, the spinodal conditions for a lattice‐fluid mixture of two polymers with molecular weight distribution were derived. The results indicated that the phase stability of a polymer mixture decreases by increasing the molecular weight distribution of polymers in the blend. To confirm the theoretical results with experiments, the changes in the spinodal temperatures of poly(ethyl methacrylate)/polystyrene (PEMA/PS) blends and tetramethyl polycarbonate/polystyrene (TMPC/PS) blends were examined when each component has a different molecular weight distribution. When the weight‐average molecular weight of each component is the same, a blend composed of polymers having broad molecular weight distribution always exhibited lower phase separation than that composed of polymers having narrow molecular weight distribution at the same blend composition. Copyright © 2004 Society of Chemical Industry