Affine deformation of single polymer chain in poly(methyl methacrylate) films under uniaxial extension observed by scanning near-field optical microscopy

The conformation of single poly(methyl methacrylate) chain in the uniaxially stretched film was observed by the combination of the fluorescence labeling technique and scanning near-field optical microscopy (SNOM). The dimension of the individual probe chain (M_w = 1.99 × 10⁶) along the extension axis was evaluated from SNOM image. In the high molecular weight matrix (M_w = 1.89 × 10⁶) the average extension ratio at the single chain level coincided with the macroscopic extension ratio. The distribution of the chain conformation was in good agreement with that of the freely jointed chain followed by affine deformation. On the other hand, the probe chain embedded in the low molecular weight matrix (M_w = 1.76 × 10⁵) showed the smaller extension than that expected from the affine deformation. This suggests that the conformation of the probe chain is affected by the relaxation of the short surrounding chains through disentanglement.     

Low frequency ac conduction and dielectric relaxation behavior of solution grown and uniaxially stretched poly(vinylidene fluoride) films

The measurements of ac conductivity [σ_m(ω)], dielectric constant [?′(ω)] and loss [?″(ω)] have been performed on solution grown (thickness ∼85 μm) and uniaxially stretched (thickness ∼25, 45 and 80 μm) films of poly(vinylidene fluoride) (PVDF) in the frequency range 0.1 kHz-10 MHz and in the temperature range 77-400 K. The σ_m(ω) can be described by the relation σ(ω) = Aω^s, where s is close to unity and decreases with increase in temperature. Three relaxations, observed in the present investigation, have been designated as the α_c-, the α_a- and the β-relaxations appearing from high temperature side to the low temperature side. The α_c-relaxation could not be observed in the case of uniaxially stretched poly(vinylidene fluoride) films. The α_c- and α_a-relaxations are associated with the molecular motions in the crystalline regions and micro-Brownian motion in the amorphous regions of the main polymer chain, respectively, whereas the β-relaxation is attributed to the rotation of side group dipoles or to the local oscillations of the frozen main polymer chain.     

Anisotropic actuation of mechanically textured polypyrrole films

Free-standing polypyrrole films have been stretched or cold rolled to produce uniaxially and biaxially textured films. The oriented films show an increase in conductivity up to 3× when compared to unprocessed films, due to polymer chain alignment. Highly anisotropic active stress and strain are observed for the textured films, with up to 7× larger active strains transverse to the orientation direction. This anisotropy results in a 100% increase in active stress and 70% increase in active strain when compared to unprocessed films, and active strains over 10% (when cycled at 0.1 V/s) have been achieved with this method.     

Temperature dependence of surface composition and morphology in polymer blend film

Thin films of poly(methyl methacrylate) (PMMA) and poly(styrene-ran-acrylonitrile) (SAN) blend can phase separate upon heating to above its critical temperature. Temperature dependence of the surface composition and morphology in the blend thin film upon thermal treatment was studied using in situ X-ray photoelectron spectroscopy (XPS) and in situ atomic force microscopy (AFM). It was found that in addition to phase separation, the blend component preferentially diffused to and aggregated at the surface of the blend film, leading to the variation of surface composition with temperature. At 185 °C, above the critical temperature, the amounts of PMMA and SAN phases were comparable. At lower temperatures PMMA migrated to the surface, leading to a much higher PMMA surface content than in the bulk. The migration and preferential segregation of a blend component in thin films demonstrated here are responsible for the great difference between in situ and ex situ experimental (not real quenching or annealing) results of polymer blend films, and help explain the slow kinetics of surface phase separation at early stage for blend thin films reported in literature. This is significant for the control of surface properties of polymer materials.     

In situ monitoring of thermal transitions in thin polymeric films via optical interferometry

A novel, low-cost, rapid, accurate, non-invasive and high throughput method based on the principles of Optical Interferometry (OPTI method) has been developed and applied for the in situ monitoring in one simple run of first (melting) and second (glass transition) order transitions as well as of the thermally induced decomposition of various thin polymeric films spin coated on flat reflective substrates (untreated silicon wafers). The new method has been applied successfully for measuring the glass transition, melting and decomposition temperatures of six commercially available polymers [poly(methyl methacrylate) (PMMA), poly(2-hydroxyethyl methacrylate), (PHEMA), poly(vinyl acetate-co-crotonic acid), (PVACA), poly(vinyl pyrrolidone) (PVP), poly(vinyl chloride-co-vinyl acetate) (PVCVA) and crystalline poly(vinylidene fluoride-co-hexafluoropropylene) (PVFHP)] of known T_gs or T_ms. The recorded interferometric signals were identified and characteristic signal patterns were qualitatively correlated to specific transitions. The monitoring of first and second order transitions in thin polymeric films is based on detectable differentiations of the total energy of a fixed wavelength laser beam incident almost vertically (angle of incidence <5°) onto a thin polymeric film spin coated on a flat reflective substrate. These differentiations are caused by film thickness and/or refractive index changes of the polymeric film both resulted from the significant change of the polymer's free volume taking place on the transitions. For film thicknesses over approx. 200-250 nm, the T_g or T_m of the polymeric films measured with the OPTI method were in excellent agreement with the corresponding values of the polymer, measured by DSC. An investigation on the trends of the T_g of PHEMA and PMMA films in a wide thickness range (30-1735 nm) was also carried out. Ultra-thin (∼30 nm) films of PMMA and PHEMA showed significant increase in their T_g values by approx. 30 °C upon comparing to their corresponding bulk T_gs. This behavior was attributed to an enhanced polymer-surface interaction through hydrogen bonding and/or to changes in the tacticity of the polymer.     

Rim instability in polymethylmethacrylate films on self-assembled monolayers with the hydrophilicity

The solvent-induced dewetting of polymethylmethacrylate (PMMA) films prepared on self-assembled monolayers (SAMs) supported by Si substrates was investigated with the hydrophilicity of the SAMs. Dewetting of the PMMA film was characterized as a function of the surface energy of the SAMs. At lower surface energies, the rim instability was found to be of type B. Type B instability created undulations in the rims of holes. As the holes merged, fingers and then droplets formed. At higher surface energies, the rim instability of the PMMA films was of type A. Type A instability did not form fingers. The formation of PMMA droplets was dominated by the total spreading coefficient (S) of the PMMA film on the SAMs, which can be expressed as the sum of the dispersion (S^d) and the polar components (S^p) of the spreading coefficient. S^d of the PMMA films decreased, whereas S^p increased as ultraviolet (UV) irradiation for periods of time between 0 s and 100 s, respectively. This result suggests that S^d destabilized but S^p stabilized the PMMA films.     

Diffusion of hairy polymeric micelles in a homopolymer solution

Polystyrene-block-poly(4-vinylpyridine) (PS-b-PVP) forms hairy micelles with PVP and long PS block as the core and corona in toluene, respectively. Diffusion of the micelles in solution in the presence of poly(methyl methacrylate) (PMMA) or polystyrene homopolymer (h-PS), from dilute to semidilute, has been investigated by laser light scattering (LLS). Our results indicate the micelles only exhibit translational diffusion with characteristic Γ = Dq² in PMMA dilute and semidilute solutions, where Γ, D and q are characteristic line width, translational diffusion coefficient and scattering vector, respectively. PMMA concentration dependence of D reveals that the micelle diffusion follows a “stretched exponential” scaling law, similar to that of a hard sphere in the presence of matrix polymer. This is because the PS corona is incompatible with PMMA and no entanglement between them occurs. In contrast, in h-PS solution, due to the overlap and entanglement between the PS corona and h-PS matrix, the micelles exhibit diffusion with characteristic of Γ ∝ q^α, where α = 2-2.6. For the same matrix polymer concentration, the micelles exhibit a faster diffusion in PMMA solution than that in h-PS solution, especially in semidilute solutions. The fact further indicates that the overlap and entanglement between the corona and h-PS matrix restrict the micelle motion.     

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.     

Highly ion conductive flexible films composed of network polymers based on polymerizable ionic liquids

Ionic liquid-type polymer brushes having different hydrocarbon (HC) chain lengths between polymerizable group and imidazolium ring were synthesized. When the carbon number of HC chain was 6, the ionic liquid-type polymer brush exhibited the highest ionic conductivity of 1.37×10⁻⁴ S cm⁻¹ at 30 °C, reflecting low T_g of −60 °C. Moreover, for the first time, we succeeded in obtaining transparent and flexible films without considerable decrease in the ionic conductivity as compared with that of corresponding monomers by using suitable cross-linkers. The most ion conductive (1.1×10⁻⁴ S cm⁻¹ at 30 °C) film was obtained when tetra(ethylene glycol)diacrylate was used 0.5 mol% to ionic liquid monomer as the cross-linker. This film is one of excellent conductive films among single-ion conductive materials.     

Light reflection at polyaniline films and its application to a kinetic study of polymer chain conformation

Light reflection at polymer-coated electrodes is studied for polyaniline, poly(o-methylaniline), and poly(o-methoxyaniline). Reflected light intensity is found to be affected greatly by the applied potential for the two reasons: one is absorption of light due to coloring of the oxidized polymer film and the other is light scattering which is concerned with a polymer chain conformation upon oxidation. A new method based on the potential dependence of light reflection is proposed for studying kinetics of conformational changes of polymer chains. The rate constants evaluated are in the range of 0.05-8 s⁻¹ at room temperature, depending on the sort of polymers, the film thickness, and pH of the solution. Irrespective of the sort of polymers, the increase in film thickness or solution pH leads to the decrease of the rate constant. It is found that a film morphology has a significant effect on the rate constant, as confirmed by a comparison of rate constants observed with polyaniline films grown at different rates.     

Vapor sorption in thin supported polymer films studied by white light interferometry

In the present study, we apply a white light interferometric methodology to study sorption of moisture and methanol vapor in thin films of poly(2-hydroxyethyl methacrylate) [PHEMA] and poly(methyl methacrylate) [PMMA], supported on oxidized silicon wafers. The measured equilibrium thickness expansion of each film, exposed to different activities of the vapor penetrant, is used to determine the sorption isotherm of the system. Results for relatively thick films (100 nm < L_o < 600 nm) are compared with corresponding literature data for bulk, free-standing films, obtained by direct gravimetric methods. Furthermore, PMMA films of thicknesses lower than 100 nm were employed in order to study the effect of the dry film's thickness, and of substrate, on fractional swelling.     

Thickness-dependent crystal orientation in poly(trimethylene 2,6-naphthalate) films studied with GIWAXD and RA-FTIR methods

The thickness dependence of the crystal orientation of poly(trimethylene 2,6-naphthalate) (PTN) films was clearly demonstrated using the methods of two-dimensional grazing incidence wide angle X-ray diffraction (2D GIWAXD) and grazing incidence reflection absorption FTIR (RA-FTIR) spectroscopy. The 2D GIWAXD results showed that for films thicker than 200 nm, the “c” axis (main chain direction) and “b” axis of crystal unit cell are almost parallel to the sample surface, whereas for thin films the “c” axis is preferentially perpendicular to the film plane in the crystalline phase of isothermally crystallized PTN films. The anisotropic orientation of the naphthalene rings in the isothermally crystallized PTN film was also confirmed. By analyzing the relative absorbance of the parallel band (1602 cm⁻¹) to the one of perpendicular band (917 cm⁻¹), the thickness dependence of the crystal orientation suggested by the GIWAXD results was also confirmed. Furthermore, the naphthalene rings in the isothermally crystallized thick films were found to lie flat on the film plane. The chain orientations derived from the GIWAXD and RA-FTIR results in this work were found to be consistent with the “flat-on” and “edge-on” lamellar orientation for the thin and thick films, respectively, which has previously been reported in many polymer systems.     

Physical aging of thin glassy polymer films monitored by gas permeability

The physical aging at 35 °C of three glassy polymers, polysulfone, a polyimide and poly(2,6-dimethyl-1,4-phenylene oxide), has been tracked by measurement of the permeation of three gases, O₂, N₂, and CH₄, for over 200 days. Several techniques were used to accurately determine the thickness of films (∼400 nm-62 μm) in order to obtain absolute permeability coefficients and to study the effects of film thickness on the rate of physical aging. Each film was heated above the polymer T_g to set the aging clock to time zero; ellipsometry revealed that this procedure leads to isotropic films having initial characteristics independent of film thickness. A substantial pronounced aging response, attributed to a decrease in polymer free volume, was observed at temperatures more than 150 °C below T_g for thin films of each polymer compared to what is observed for the bulk polymers. The films with thicknesses of approximately 400 nm of the three polymers exhibit an oxygen permeability decrease by as much as two-fold or more and about 14-15% increase in O₂/N₂ selectivity at an aging time of 1000 h. The results obtained in this study were compared with prior work on thickness dependent aging. The effects of crystallinity on physical aging were examined briefly.     

Stretched PEO-LiCF3SO3 films: Polarized IR spectroscopy and X-ray diffraction

The effect of tensile stretching on pure PEO and P(EO)_xLiCF₃SO₃ (x = 10 and 20) films was studied using polarized FTIR spectroscopy, wide angle scattering X-ray scattering (WAXS) and small angle X-ray scattering (SAXS). The polarization effects observed in the spectra are readily explained by the selection rules governing IR transitions and symmetry-based vibrational mode assignments of PEO and the 3:1 compound, P(EO)₃LiTf. The degree of polymer chain orientation in the stretched samples as measured by the average of the second Legendre polynomial was calculated from the dichroic ratio for the PEO helices in both pure PEO and the 20:1 film. The degree of chain orientation was markedly higher in the 20:1 film than in the pure PEO film; however it was difficult to calculate the degree of orientation in the 10:1 film.The addition of LiTf causes a slight increase in the fractional crystallinity of the samples as determined from the WAXS data. Orientation of the films did not significantly affect the fractional crystallinity for the 10:1 and 20:1 samples, whereas a slight decrease in the fractional crystallinity was noted for pure PEO upon stretching. The SAXS data suggested that the average crystal size becomes smaller with the addition of lithium triflate to PEO. The SAXS data also indicated that substantial morphological changes occur upon stretching, since the Bragg peak due to the crystalline-amorphous phase separation disappeared upon stretching. These data, taken together, suggest that in the 10:1 film, the tensile stress tends to form small, unoriented domains of the 3:1 compound.     

The effect of high-pressure carbon dioxide treatment on the crystallization behavior and mechanical properties of poly(l-lactic acid)/poly(methyl methacrylate) blends

The purpose of this study is to investigate the effect of carbon dioxide (CO₂) on the crystallization behavior and the mechanical properties of PLLA/PMMA blends with various weight fraction of PMMA. PLLA/PMMA blends can be crystallized even at a low temperature of 0 °C under high-pressure CO₂. The films treated with high-pressure CO₂ at 0 °C have about three times larger strain at break than that of the amorphous and cold-crystallized film. The size of spherulites in the CO₂ treated film is considered to be smaller than the wavelength of the visible light because of a good transparency. The improvement of the strain at break is attributed to the reduction of the stress concentration during the deformation.     

Microphase behaviour of ferrocenyldimethylsilane-b-methylmethacrylate diblock copolymers

A series of narrowly distributed diblock copolymers from poly(1,1′-ferrocenyldimethylsilane) (PFS) and polymethylmethacrylate (PMMA) was prepared via anionic polymerisation and characterised with respect to their thermal and morphological behaviour in the bulk. TGA and DSC experiments reveal a high thermal stability of the block copolymers up to temperatures of above 220 °C. Homogeneous films (ca. 1 mm in thickness) were prepared from these materials by standard film-casting procedures followed by annealing at elevated temperatures. These films were characterised by transmission electron microscopy (TEM). Depending on the PFS volume fraction, ?_PFS (0.25≤?_PFS≤0.48), the three classical microphase morphologies were found, i.e. the spherical, hexagonal and lamellar ones. Due to the strong segregation tendency of PFS and PMMA, no complex morphologies such as the gyroidic were found for the rather high-molecular-weight samples under investigation here. Instead, either the direct transition from the hexagonal into the lamellar morphology was observed when the PFS content crosses the critical value of ?_PFS=0.40, or the coexistence of these two morphologies was found.     

Synthesis and photophysics of new highly luminescent poly(alkylthiophene) derivatives with pyridine in the backbone

Starting from 2,6-bis-(3-octylthiophene-2yl)-pyridine, two new poly(alkylthiophene) derivatives, POTPyOT and POTPy, containing pyridine in the backbone were prepared from nickel(0)-mediated Stille coupling or by palladium-catalyzed Yamamoto coupling. These polymers exhibited good solubility in common organic solvents, thermal stability up to 400 °C, and facile film formation. They were amorphous and give strong luminescence both in CHCl₃ solution and solid state film. The polymers emitted blue light in solution with photoluminescence (PL) emission maximum at 420-484 nm and green light with PL emission maximum at 500-514 nm in thin films. These polymers showed a reversible redox reaction at potential from 0 to 1.3 V (vs. SCE). Nevertheless, the reduced form of the polymer was very unstable; it decomposed in the presence of oxygen or water. The emission and UV-vis absorption of the polymer were influenced by the solvent polarity, protonation, and acid-base treatment. These may be the results of the stabilization of the polar excited state by solvation and the change of the conformation in polymer backbone. Electroluminescence (EL) was achieved from a single-layer PLED with the configuration of ITO/POTPyOT/Al. The turn-on voltage of the device is 10 V and the λ_max (550 nm) of the EL is voltage independent.     

The effects of the selectivity of the toluene/ethanol mixture on the micellar and the ordered structures of an asymmetric poly(styrene-b-4-vinylpyridine)

The effects of the solvent selectivity of toluene/ethanol mixtures on the micellar and ordered structures of an asymmetric diblock copolymer of PS(19.6 K)-b-P4VP(5.1 K) in the dilute (1 wt%) and semi-dilute (8 wt%) solutions, as well as in the gel and solid films, were studied using small angle X-ray scattering (SAXS), generalized indirect Fourier transform (GIFT), and transmission electron microscopy (TEM) methods. The solvent selectivity was controlled by ? (weight percentage of ethanol in toluene/ethanol mixture). Individual micelles, space-filled micellar structure (without three-dimensional order), and three-dimensionally ordered gel and solid structures were observed from the 1 and 8 wt% solutions, the gel, and the solid film, respectively. In the 1 wt% solution, the individual micellar structures were strongly dependent on ?; the spherical micelles with P4VP core at ? = 0, the unimer state at 10 ≤ ? ≤ 50, the spherical micelles with PS core at ? = 60, the cylindrical micelles with PS core at ? = 70 and 80, and precipitation at ? = 90 and 100 were observed. The 8 wt% solution was close to overlap concentration with the unimer state in the regions of 20 ≤ ? ≤ 40. In the gel, the ordered structure was observed in the sequence of bcc, hexagonal, gyroid, lamellar, reverse hexagonal and random as ? increased, and could be explained by the change of the relative volume fraction of each block as ? changed, similar to the phase sequence in the phase diagram of the diblock copolymer. The solid films showed the various kinetically frozen ordered microstructures such as randomly packed sphere, hexagonal, gyroid, hexagonally perforated lamella, reversed hexagonal, and randomly packed cylinder, which were controlled by the solvent quality in the gel before solidification. We believe that these results can be applied to photonic crystals, self-assembled nano-patterning, and functional nanoparticles in which the structural control is most important.     

Miscibility and viscoelastic properties of acrylic polyhedral oligomeric silsesquioxane-poly(methyl methacrylate) blends

We investigate the miscibility of acrylic polyhedral oligomeric silsesquioxanes (POSS) [characteristic size d≈2 nm] and poly(methyl methacrylate)(PMMA) in order to determine the effect of well-dispersed POSS nanoparticles on the thermomechanical properties of PMMA. Two different acrylic POSS species (unmodified and hydrogenated) were blended separately with PMMA at volume fractions up to ?=0.30. Both POSS species have a plasticizing effect on PMMA by lowering the glass transition temperature T_g and decreasing the melt-state linear viscoelastic moduli measured in small amplitude oscillatory shear flow. The unmodified acrylic-POSS has better miscibility with PMMA than the hydrogenated form, approaching complete miscibility for loadings ?<0.10. At a loading ?=0.05, the unmodified acrylic POSS induces a 4.9 °C decrease in the T_g of PMMA, far less than the 17.4 °C decrease in the glass transition temperature observed in a blend of 5 vol% dioctyl phthalate (DOP) in PMMA; however, the decrease in the glass transition temperature per added plasticizer molecule is nearly the same in the unmodified acrylic-POSS-PMMA blend compared with the DOP-PMMA blend. Time-temperature superposition (TTS) was applied successfully to the storage and loss moduli data and the resulting shift factors were correlated with a significant increase in free volume of the blends. The fractional free volume f₀=0.046 for PMMA at T₀=170 °C while for a blend of 5 vol% unmodified acrylic-POSS in PMMA f₀=0.057, which corresponds to an addition of 0.47 nm³ per added POSS molecule at ?=0.05. The degree of dispersion was characterized using both wide-angle X-ray diffraction (WAXD) and dynamic mechanical analysis (DMA). Diffraction patterns for both blend systems show clear evidence of phase separation at ?=0.20 and higher, but no significant phase separation is evident at ?=0.10 and lower. The storage modulus measured in DMA indicates appreciable phase separation for unmodified acrylic POSS loadings ?≥0.10, while no evidence of phase separation is present in the ?=0.05 blend in DMA.     

High-pressure solution blending of poly(?-caprolactone) with poly(methyl methacrylate) in acetone + carbon dioxide

The miscibility of blends of poly(?-caprolactone) (PCL, M_w = 14,300) with poly(methyl methacrylate) (PMMA, M_w = 15K or 540K) in acetone + CO₂ mixed solvent has been explored. The liquid-liquid phase boundaries at different temperatures have been determined for mixtures containing 10 wt% total polymer blend, 50 wt% acetone and 40 wt% CO₂. The PCL and PMMA contents of the blends were varied while holding the total polymer concentration at 10 wt%. The polymer blend solutions all displayed LCST-type behavior and required higher pressures than individual polymer components for complete miscibility. Complete miscibilities were achieved at pressures within 40 MPa. The DSC scans show that the blends are microphase-separated. The blends display the melting transition of PCL and the glass transition temperature of the PMMA phases. The presence of PMMA is found to influence the crystallization and melting behavior of PCL in the blends. The DSC results on heat of melting and the FTIR spectra, specifically the changes at 1295 cm⁻¹ band show the changes (decrease) in overall crystallinity of the blend upon addition of PMMA.