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We investigated the segmental dynamics of asymmetrically confined polymer films and report an unusual phenomenon in which the presence and thickness of a soft confining layer are responsible for significant changes in the segmental dynamics of the confined films. Specifically, the segmental dynamics of poly(vinyl alcohol) (PVA) thin films asymmetrically confined between hard aluminum (Al), and soft polystyrene (PS) films are shown to shift by as much as half an order of magnitude upon changes in the thicknesses of the confining PS layer. These effects are more significant than those due to symmetric confinement between hard Al substrates or exposure to a free surface. These observations, partially rationalized in terms of recent simulations and theory, implicate the role of the moduli of the confining layers.

We show evidence of thickness-dependent elastic mechanical moduli that are associated largely with the effects of architecture (topology) and the overall shape of the macromolecule. Atomic force microscopy (AFM) based nanoindentation experiments were performed on linear chain polystyrene (LPS) and star-shaped polystyrene (SPS) macromolecules of varying functionalities (number of arms, f) and molecular weights per arm Mwarm. The out-of-plane elastic moduli E(h) increased with decreasing film thickness, h, for h less than a threshold film thickness, hth. For SPS with f ≤ 64 and Mwarm > 9 kg/mol, the dependencies of E(h) on h were virtually identical for the linear chains. Notably, however, for SPS with f = 64 and Mwarm = 9 kg/mol (SPS-9k-64), the hth was over 50% larger than that of the other polymers. These observations are rationalized in terms of the structure of the polymer for high f and sufficiently small Mwarm and not in terms of the influence of interfacial interactions.

In thin film A/B polymer/polymer mixtures, the formation of a layer at the free surface, with average composition that differs from the bulk, due to the preferential segregation of the lower cohesive energy density component, is well understood. While much is also understood about this surface layer formation and growth to date, virtually nothing is known about the surface dynamics of the chains in such mixtures. Questions about the surface chain dynamics in relation to the bulk have remained unanswered. With the use of X-ray photon correlation spectroscopy (XPCS) we show that the dynamics of poly(vinyl methyl ether) (PVME) chains at the free surface of polystyrene (PS)/PVME thin film mixtures can be orders of magnitude larger than the PVME chains in the bulk. These dynamics manifest from differences between the local compositions of the blend at the free surface and the bulk, as well as film thickness constraints.

The performance of conjugated polymer (CP)-based electronic devices relies on optimal charge carrier mobilities, which are determined by monomeric architecture, degree of polymerization, chain conformation, and the nano- and mesoscale morphologies. With regard to the latter, we discuss two effects that have received limited attention in the literature, yet important for device performance optimization: (1) the role of morphological disorder and of CP/substrate interactions on the in-plane and out-of-plane carrier transport in CPs; (2) the impact of morphological disorder on charge transfer at the CP/substrate interface. The emergence of film thickness-dependent carrier mobilities, varying over two orders of magnitude within a length scale of 200 nm, and band-bending phenomena, extending tens of nanometers within the CP, are associated with these effects. These findings suggest areas for further research in order to enable widespread applications of next-generation CP-based devices. Graphical abstract

Upon quenching a polymer melt to a temperature T below its glass transition temperature Tg, structural relaxations, physical aging, enable the material to return to equilibrium. Whereas the physica...

Spatial compositional heterogeneity strongly influences the dynamics of the A and B components of bulk miscible blends. Its effects are especially apparent in mixtures, such as poly(vinyl methyl ether) (PVME)/polystyrene (PS), where there exist significant disparities between the component glass transition temperatures (Tgs) and relaxation times. The relaxation processes characterized by distinct temperature dependencies and relaxation rates manifest different local compositional environments for temperatures above and below the glass transition temperature of the miscible blend. This same behavior is shown to exist in miscible PS/PVME films as thin as 100 nm. Moreover, in thin films, the characteristic segmental relaxation times τ of the PVME component of miscible PVME/PS blends confined between aluminum (Al) substrates decrease with increasing molecular weight M of the PS component. These relaxation rates are film thickness dependent, in films up to a few hundred nanometers in thickness. This is in rema...

Incorporating nanoparticles (NPs) within a polymer host to create polymer nanocomposites (PNCs) while having the effect of increasing the functionality (e.g., sensing, energy conversion) of these materials influences other properties. One challenge is to understand the effects of nanoparticles on the viscosity of nanoscale thick polymer films. A new mechanism that contributes to an enhancement of the viscosity of nanoscale thick polymer/nanoparticle films is identified. We show that while the viscosities of neat homopolymer poly(2-vinylpyridine) (P2VP) films as thin as 50 nm remained the same as the bulk, polymer/nanoparticle films containing P2VP brush-coated gold NPs, spaced 50 nm apart, exhibited unprecedented increases in viscosities of over an order of magnitude. For thicker films or more widely separated NPs, the chain dynamics and viscosities were comparable to the bulk values. These results—NP proximities and suppression of their dynamics—suggest a new mechanism by which the viscosities of polymer...

The segmental relaxation times τA and τB of the A and B components, respectively, of an A/B polymer/polymer blend typically exhibit dissimilar temperature dependences and can differ by orders of magnitude, thereby manifesting the influence of spatial compositional heterogeneity. We show that for weakly miscible A/B blends the relaxations of the faster A component occur via separate and distinct mechanisms. In the melt state, τA increases in a nonlinear manner as temperature T decreases toward blend glass transition temperature Tg(blend) (or toward the local effective glass transition temperature of its component Tg(A)); this is the typical α relaxation process. For temperatures below the transition, 1/τA exhibits an Arrhenius temperature dependence; this is identified as the α′ process. A third relaxation process, a so-called α0 process, also occurs in the melt state; it is slower than the α process and exhibits a significantly stronger dependence on temperature. Each relaxation process, characterized by ...

Structural relaxations of a substance quenched to a temperature Tage below its glass transition temperature Tg enable the structure of the substance to approach equilibrium. This phenomenon, also known as physical aging, has been studied for many decades in bulk linear-chain polymer systems, where the aging rates are generally, to first order, independent of chain length. More recently, the phenomenon has been of keen interest in thin films, where the aging rate is shown to be film thickness H dependent, for films in the thickness range of nanometers to a few hundred nanometers. We show here, based on a study of polystyrene star-shaped polymers of a wide range of functionalities 2 ≤ f ≤ 64 and arm molecular weights Marm, that in the limit of sufficiently large values of Marm the aging behavior is similar to that of linear chains—independent of Marm and f. More importantly, in the limit of sufficiently small Marm and large f, the aging rate is independent of film thickness. Otherwise, the rate is a nonmono...

The potential for polymer thin films to be used as coatings or dielectrics is limited by their low thermal conductivity, κ. However, there have been very limited studies on κ enhancement for polymer thin films. In this work, we show that the out-of-plane κ of a poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) thin film can be enhanced when it undergoes the appropriate thermal annealing conditions. When the film is annealed above its melting temperature, we achieve a 300–400% increase in κ across the measured temperature range, compared to the as-cast film. We attribute this enhancement to the extensive ordering of polymer backbone chains perpendicular to the substrate during the melt-recrystallization process.

The performance of power conversion devices is impacted by the energy level alignment at the interface between the conjugated polymer and conductive substrate. While band bending has been known to vary between conjugated polymers, we show that the degree of band bending within the same polymer can be just as significant with morphology change. Specifically, a significant band bending effect, studied via Kelvin probe force microscopy (KPFM), was exhibited by poly(3-hexylthiophene) (P3HT) films fabricated using matrix assisted pulsed laser evaporation (MAPLE) in contrast to the conventional spin-cast P3HT films. This finding is associated with a broadening of the density of states (DOS) in the MAPLE-deposited P3HT films, originating from the more disordered structure of the film. These findings, to the best of our knowledge, illustrate for the first time a strong connection between morphology, energy level alignment, and bulk transport in conjugated polymer films.

The morphological structure of poly(3-hexylthiophene) (P3HT) thin films deposited by both Matrix Assisted Pulsed Laser Evaporation (MAPLE) and solution spin-casting methods are investigated. The MAPLE samples possessed a higher degree of disorder, with random orientations of polymer crystallites along the side-chain stacking, π–π stacking, and conjugated backbone directions. Moreover, the average molecular orientations and relative degrees of crystallinity of MAPLE-deposited polymer films are insensitive to the chemistries of the substrates onto which they were deposited; this is in stark contrast to the films prepared by the conventional spin-casting technique. Despite the seemingly unfavorable molecular orientations and the highly disordered morphologies, the in-plane charge carrier transport characteristics of the MAPLE samples are comparable to those of spin-cast samples, exhibiting similar transport activation energies (56 vs. 54 meV) to those reported in the literature for high mobility polymers. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 39–48

The frequency ω dependent storage G′(ω) and loss G″(ω) moduli of star-shaped polystyrene (SPS) molecules of a range of functionalities f and molecular weights per arm Ma were measured under small amplitude oscillatory shear conditions. Star-shaped macromolecules are composed of an inner region, core, where the chain segments are stretched and the “packing” density is higher than that of the outer region, corona. The frequency dependencies of G′(ω) and G″(ω) for low functionality molecules (f < 8) with long arms Ma are well described by the model of Milner and McLeish, indicating that the translational dynamics are facilitated by an arm retraction mechanism. With increasing values of f and decreasing Ma the model fails—the arm retraction process is no longer valid—due largely to the increasing size of the core in relation to the overall size of the molecule. The molecules exhibit evidence of spatial structural order due to entropic, intermolecular interactions, and the translational dynamics of these molec...

Nanoindentation measurements of the elastic moduli Er of thin polymer films supported by stiff substrates with moduli Es ≫ Er show an increase of Er with decreasing h, for h less than a threshold thickness ht. In the thickness range h < ht , the value of the modulus manifests the influence of various interactions associated with the “stiff” substrate. We show that ht is a function of composition for the miscible blend of polystyrene (PS) and tetramethyl bisphenol-A polycarbonate (TMPC). The modulus Er,TMPC of TMPC films supported by SiOx increases for h < ht(TMPC) ∼ 300 nm while ht(PS) ∼ 450 nm for the modulus Er,PS of PS films, supported by the same substrate. The threshold thicknesses of the blends and the moduli of the blends appear to be reasonably described by an effective medium approximation. This behavior is rationalized in terms of the vibrational force constants f, determined using incoherent neutron scattering experiments, of the materials.

We show that the vitrification of star-shaped polystyrene (PS), of functionality f and molecular weight per arm Mwarm, thin films supported by silicon oxide, SiOx, is strongly dependent on Mwarm and f. When f is small, the vitrification behavior is similar to that of linear-chain PS where the average glass transition, Tg, decreases with decreasing film thickness (ΔTg 0. We show that the overall vitrification behavior of these thin film star-shaped polymers is due to compet...

The ability to control the wetting properties of a polymeric liquid on a given surface is important for several emerging technological applications including protective coatings, lubricants and sensors. Here we show that star-shaped polystyrene (PS) molecules exhibit notably different wetting properties than their linear analogues of the same chemical structure and their wetting properties can be controlled through changes of their functionality f (number of arms per molecule). Unlike linear chains, the wetting of star-shaped macromolecules is determined by the competition between entropic forces. Wetting is enhanced due to reductions in the loss of entropy upon adsorption of the stars with increasing f; soft colloidal-like entropic repulsion effects suppress the ability of the stars to exhibit a high degree of surface adsorption and to efficiently pack, for sufficiently large values of f. This phenomenon is manifested in the existence of a minimum in the macroscopic contact angle and other related micros...

A study of the poly(vinyl methyl ether) (PVME) segmental dynamics of bulk miscible blends of polystyrene (PS) and PVME reveals that while at high temperatures, T, there is evidence of a single α-relaxation process, at lower T, two separate dominant relaxation processes, associated with the change in structure of the blend with decreasing T, emerge. One relaxation process decreases with a much stronger dependence on T and “freezes” at a temperature comparable to the glass transition temperature, Tg, of the blend measured using differential scanning calorimetry. The other exhibits a weaker T dependence and persists at much lower T, becoming Arrhenius (the so-called α′-process) at sufficiently low T. In thin PVME/PS films confined between aluminum substrates, a new relaxation process, αint, associated with PVME chains that preferentially segregate to the substrates, emerges. These observations are considered in light of the influence of spatial compositional heterogeneities on blend dynamics.

The segmental dynamics of a low glass transition temperature Tg polymer under different conditions of morphological confinement are shown to manifest the influence of the frustration of the local “packing” of chain segments at interfaces of varying curvatures, unique to the morphology. Of particular interest are the polyisoprene (PI) segmental dynamics in polystyrene-b-polyisoprene (PS-b-PI) micelles and in layered onion-like structures in high Tg, “frozen,” polystyrene (PS) hosts. At temperatures close to the Tg of PI domains, the segmental relaxation times of PI chains in the micellar structures, τmicelle, are more than 1 order of magnitude shorter than those of PI chains in the onion-like structures, τonion. These rates are appreciably faster than those of homopolymer PI and of PI in the neat copolymers possessing lamellar and gyroid phases, τhomo ∼ 0.5τlam ∼ 0.5τgyroid, respectively. At high temperatures τhomo ∼ τlam ∼ τgyroid ∼ τonion ∼ τmicelle. These observations are readily reconciled with trends ...

We study the formation and organization of micelles, of polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP) diblock copolymer chains, in thin film hosts composed of (1) blends of long and short chain polystyrenes, PSL and PSS, of degrees of polymerization PL and PS, respectively (PS < PL), and (2) blends of PSL and a homopolymer with which it is compatible, tetramethyl bisphenol-A polycarbonate (TMPC), of degree of polymerization PTMPC (PTMPC < PL). The role of competing entropic and enthalpic interactions on the block copolymer micelle formation and organization is examined. We show that the average size of the micelle cores, Dcore, decreased with increasing weight fraction of the shorter chain PS component in PS(PS = 125)/PS(PL = 15400) mixtures, for PL≫NPS, NPS is the degree of polymerization of the PS block (the micelle corona). Dcore also decreased with increasing weight fraction of TMPC in TMPC(PTMPC = 122)/PS(PL = 15400) mixtures. The values of Dcore in the TMPC/PSL hosts were smaller than those in PSS...

An important class of functional hard/soft hybrid nanocomposites is formed when nanoparticles (NPs) are induced to self-assemble within the structures of ordered A-b-B diblock copolymers (BCPs). Ba...

The structure of A-b-B diblock copolymer (BCP) thin films is often exploited for “tailoring” the self-assembly of nanoparticles into various geometries, characterized by long-range order. Mechanistically, the nanoparticles are sequestered within the A or B domains of the copolymer, and the domains act as scaffolds to direct the assembly of the nanoparticles. We show that in BCP thin films, which order at suboptimal thicknesses, dislocations play a dominant role in determining the spatial organization of sufficiently large nanoparticles. The nanoparticles preferentially segregate to the core of edge dislocations, which are ubiquitous in these materials. To this end, the two-dimensional planar shape of the islands and holes determine the self-assembled geometry of the nanoparticles. We also show that the so-called “healing length”, λ, increases with film thickness, h, in a manner consistent with λ ∝ h1/2, in accordance with theory. In films of thickness between L < h < 3L, where L is the domain spacing, λ l...

The phase behavior of supported thin film mixtures (h ∼ 120 nm thick) of polystyrene (PS) brush-coated spherical nanoparticle and PS homopolymers is characterized by three regimes, depending on P, the degree of polymerization of the PS host, and N, the degree of polymerization of the grafted chains. Phase separation between the nanoparticles and the host chains occurs in samples for which N N*, preferential segregation of the grafted nanoparticles to the interfaces is accompanied by a structural instability (surface roughening). We identify this as regime I and the former as regime II. The system is miscible in regime III (P N*); the nanoparticles are dispersed throughout the film. There exists a region of partial miscibility that separates regimes I and III. The characteristics of regime I are reminiscent of phase separati...

The physical properties of thin polymer films are often thickness, h, dependent, influenced by confinement and by interfacial interactions between the chains and the external interfaces. We show that the magnitude and film thickness dependence of the average glass transition temperature, Tg, of the polystyrene−silicon oxide (PS/SiOx/Si) system are influenced appreciably with the addition of polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) diblock copolymers. The Tg can be “tailored” to increase, or decrease, with decreasing h or to remain independent of h. Tg shifts of as much of 35 °C are obtained for films of h ≈ 20 nm. Additionally, we report that the critical micelle concentration, φcmc, of the copolymer in thin films is considerably larger than for the bulk; specifically, micelles form only beyond a critical film thickness, determined by the size of the chains and by the number of chains in the system. The h dependence of Tg is not influenced by the ϕcmc or by the number of micelles in this system.

Thin film functional hybrid materials composed of inorganic nanocrystals sequestered within a self-assembled template are important for a diverse range of applications, from sensors to device electronics. The properties of these materials can be “tailored” by control of composition over various length scales; the major processing challenges are associated with understanding and controlling external factors, such as confinement and interfacial interactions, that affect the self-organization process. Spin-cast polystyrene-b-poly(1,1‘,2,2‘-tetrahydroperflurooctyl methacrylate) (PS-b-PFOMA) diblock copolymer films can form a micellar structure, with a PFOMA core (PS corona), and are induced to undergo a transition by annealing in supercritical CO2; consequently, the PS segments form the core with a PFOMA corona. We show that functionalized Au nanocrystals, initially dispersed within the corona (the PS phase) of the micelles, follow the morphological inversion and become sequestered within the core, now compos...

Poly(methyl methacrylate) (PMMA)−C60 nanocomposites, with compositions in the range 0 ≤ φC60wt ≤ 0.05, are shown to exhibit systematic increases in dynamic shear moduli, in glass transition temperature (Tg), and in the longest relaxation time of the polymer (τR) with increasing fullerene concentration. We show that while the φC60wt dependence of the plateau modulus can be reconciled with a conventional “filler” effect, the systematic increases in Tg and in τR are associated with specific interactions between the C60 and the polymer segments. In the melt, these segment−C60 interactions are proposed to reduce polymer segmental mobility in the vicinity of the particle surface and ultimately suppress polymer dynamics, as measured mechanically, in a manner consistent with an increase in the polymer segmental friction coefficient.

It has recently been shown that thin polymer films in the nanometer thickness range exhibit anomalous swelling maxima in supercritical CO2 (Sc-Co2 )i n the vicinity of the critical point of CO2. The adsorption isotherm of CO2 on carbon black, silica surfaces, porous zeolites, and other surfaces, is known to exhibit anoma- lous maxima under similar CO2 conditions. It is believed that because CO2 possesses a low cohesive energy density, there would be an excess amount of CO2 at the surfa- ces of these materials and hence the CO2/polymer interface. This might cause excess CO2 in the polymer films near the free surface, and hence the swelling anomaly. In addition, an excess of CO2 would reside at the polymer/substrate and polymer/CO2 interfaces for entropic reasons. These interfacial effects, as have been suggested, should account for an overall excess of CO2 in a thin polymer film compared to the bulk, and would be responsible for the anomalous swelling. In this study, we use in situ spectroscopic ellipsometry to investigate the role of interfaces on the anoma- lous swelling of polymer thin films of varying initial thicknesses, h0, exposed to Sc- CO2. We examined three homopolymers, poly(1,1 0 -dihydroperflurooctyl methacrylate) (PFOMA), polystyrene (PS), poly(ethylene oxide) (PEO), that exhibit very different interactions with Sc-CO2, and the diblock copolymer of PS-b-PFOMA. We show that the anomalous swelling cannot be solely explained by the excess adsorption of CO2 at interfaces. V C 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1313-1324, 2007

The dynamic mechanical properties of nanocomposites of functionalized single-walled carbon nanotubes dispersed in polystyrene are reported as a function of temperature. For nanocomposites containing less than 0.1 wt% nanotubes, the enhancement of the magnitude of the complex modulus | E ∗ | = E ′ 2 + E ″ 2 , where E′ and E″ are the elastic and loss moduli, respectively, exceeded the predictions of the Halpin–Tsai model (this model is often used to predict the properties of macroscopic composites). In this concentration range, however, the loss tangent, tan δ = E″/E′, of the nanocomposite remained comparable to that of pure PS, i.e., comparable elasticity. At larger concentrations the mechanical response became more elastic than PS, but the magnitude of the complex modulus fell appreciably below predictions based on the Halpin–Tsai model. An alternate hypothesis that relies on the role of attractive interactions between the nanotubes and polystyrene chains, and consistent with previous Raman scattering and glass transition temperature data, is proposed to describe the observed dynamic response.

We examine the morphological structures of asymmetric poly(ethylene oxide)-b-poly(1,1'-dihydroperflurooctyl methacrylate) (PEO-b-PFOMA) thin films upon annealing in a compressible fluid, supercritical CO2 (Sc-CO2). The strong affinity between PFOMA and CO2 is found to induce phase segregation when annealing PEO-b-PFOMA films at the same temperature as compared with vacuum. In vacuum, PEO-b-PFOMA films remain disordered from 80 to 180 degrees C, whereas, in Sc-CO2 at 13.9 MPa, an upper order-disorder transition (UODT) between 116 and 145 degrees C is found. In Sc-CO2, the observed ordered structure is layers of PEO spheres embedded in the matrix of PFOMA, followed by a brush layer, in which PEO wets the substrate. The swelling isotherms of PFOMA and PEO in CO2 are correlated with the Sanchez-Lacombe equation of state (SLEOS) to estimate the interaction parameters, XPFOMA-CO2 and XPEO-CO2. The phase segregation (order) induced by CO2 relative to vacuum at a given temperature is explained in terms of two factors: (1) copolymer volume fraction upon dilution with CO2, phi, and (2) the relative interaction parameter, DeltaX= XPEO-CO2 - XPFOMA-CO2. The latter factor favors order and is dominant at low temperatures over the phi factor, which always favors disorder. At high temperatures (above the T(ODT)), the preferential swelling of PFOMA by CO2 is less pronounced ( DeltaX decreases), and the copolymer is disordered.

We show that while polystyrene (PS) thin films are structurally unstable on oxidized silicon wafers, the addition of as little as a few weight percent tetramethylbisphenol A polycarbonate (TMPC) has a stabilizing effect on the topographical structure of the films. The stabilization is evident from the existence of a threshold TMPC concentration, φt, and a threshold thickness, ht, beyond which films do not dewet. The concentration threshold occurs for φTMPC ≤ 0.10. An examination of the effective interface potential, which accounts for short- and long-range intermolecular interactions, indicates that this dewetting inhibition is metastable.

Polystyrene-b-poly(1,1‘,2,2‘-tetrahydroperflurooctyl methacrylate) (PS-b-PFOMA) thin films, cast from a cosolvent mixture of Freon 113 and toluene onto Si/SiOx substrates, form spherical micelles; the cores are composed of PFOMA chains with a PS corona. Upon exposing the films to supercritical CO2 (Sc-CO2), the morphology inverts, wherein the core is composed of PS chains and the PFOMA chains constitute the corona. In each case, the free surface and polymer/substrate layers are enriched with PFOMA. The size of the PS cores is found to increase with decreasing Sc-CO2 activity. This size variation is discussed in light of recent theoretical developments that account for the effect of Sc-CO2 activity on PS−CO2 interfacial tension and chain stretching of the corona versus the core.

Through control of the temperature, T, and film thickness, h, the relative influence of forces associated with crystallization, long-range van der Waals forces, and block-copolymer ordering were manipulated to control the structure of films of asymmetric polyethylene-b-poly(styrene-r-ethylene-r-butene) (E-b-SEB) diblock copolymers. The bulk equilibrium structure of this copolymer consists of spheres of the crystallizable E block embedded in the amorphous SEB matrix. In thin films, the E component resides at the free surface and the SEB block resides in contact with the substrate. Within the temperature range T > Tm, where Tm is the melting temperature, yet below the order−disorder transition (ODT) temperature, TODT, all films of thickness h > 12 nm were ordered throughout. The thickness of the brush layer L0, in contact with the substrate, of these films was 12 nm; this thickness is about one-half of the intersphere spacing of the bulk copolymer. Films equal to or thinner than 12 nm dewet, forming droplet...

Thin polystyrene (PS) liquid films supported by oxidized silicon (SiOx/Si) substrates may be unstable or metastable, depending on the film thickness, h. In the metastable thickness regime, holes nucleate throughout the surface of the films and subsequently grow under the action of capillary forces. Recent studies show that the hole growth rate in thin PS films is suppressed with the addition of small concentrations of C60 fullerenes, due to pinning at the line of contact. We examined the hole growth dynamics in thin film polystyrenes with functionalized single-walled carbon nanotubes (PS-SWNT) supported by SiOx/Si substrates. The hole growth velocities in PS films containing 0.75 wt % functionalized single-walled nanotubes, VPS-SWNT, were appreciably slower than VPS, the hole-growth velocities of holes in polystyrene films of the same thickness. Moreover, VPS-SWNT and VPS decreased with decreasing film thickness, for h < 50 nm, with thickness dependencies which exceed theoretical predictions. In addition,...

The effect of CO2 on the welding kinetics for a PS colloidal crystal was determined by in-situ measurement of Bragg diffraction and by ex-situ SEM microscopy. The welds grow tangentially from the points of contact of the spheres in all directions transforming the spheres into a continuous film. The dissolved CO2 provides the mobility for the polymer chains at the interface needed for welding, which removes the energetically unfavorable interfacial area between the polymer and CO2. At high pressures where CO2 is relatively incompressible, the relaxation time for welding increased by 6 orders of magnitude with an increase in thermal energy, and a glass transition temperature was observed. At constant temperature, an anomalous increase in the welding rate by 1−2 orders of magnitude with a decrease in pressure was observed in the region where CO2 is highly compressible. It is likely that this increase in welding dynamics is influenced by the excess CO2 in the interfacial region.

Linear viscoelastic stress relaxation and calorimetric measurements were performed on a series of mixed alkali tellurite glasses of composition 0.3([xNa2O+(1−x)Li2O])+0.7TeO2 at temperatures near and above the glass transition temperature, Tg. The stress relaxation data were well described by the stretched exponential function, G(t)=G0exp[−(t/τ)β], where τ is the relaxation time, β is the distribution of relaxation times and G0 is the high frequency modulus. The fragility, determined from the temperature dependence of τ, exhibited a minimum in the middle of the mixed alkali composition. A possible connection between the kinetic and the thermodynamic dimensions of this system was established, wherein the heat capacity change at the Tg, ΔCp(Tg), and the fragility are correlated.

Dynamic mechanical analysis, nuclear magnetic resonance, and thermogravimetric analysis experiments were performed on pure poly(methyl methacrylate) and on in situ polymerized single-walled carbon nanotube (SWNT)/PMMA nanocomposites. The addition of less than 0.1 wt % SWNT to PMMA led to an increase in the low-temperature elastic modulus of approximately 10% beyond that of pure PMMA. The glass-transition temperature and the elastic modulus at higher temperatures of the nanocomposites remained unchanged from those of pure PMMA. These changes were associated with excessive cohesive interactions between the large-surface area nanotubes and PMMA and were not due to changes in the microstructural features of the polymer during synthesis.

The glass-transition temperatures (Tg's) of nanocomposites of polystyrene (PS) and single-walled carbon nanotubes were measured in the bulk and in thin films with differential scanning calorimetry and spectroscopic ellipsometry, respectively. The bulk Tg of the nanocomposites increased by approximately 3 °C and became much broader than that of PS. For the nanocomposite films thinner than 45 nm, Tg decreased with decreasing film thickness [i.e., ΔTg(nano) < 0]. This phenomenon also occurred in thin PS films, the magnitude of the depression in PS [ΔTg(PS)] being somewhat larger. The film thickness dependence and the differences in the magnitude of ΔTg in the two systems were examined in light of current theory, and a quantitative comparison was made. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 3339–3345, 2003

The stability of a sufficiently thin, supported, homopolymer film against the development of local thickness fluctuations which can become amplified, eventually leading to structural destabilization of the film, is typically determined by long and short-range intermolecular forces. In A-B diblock copolymers, the connectivity between the blocks, the preferential attraction of one block to an external interface, combined with an incompatibility between the A-B segments, the situation is very different. Two cases, largely dictated by χN, wher χ is the Flory-Huggins interaction parameter and N is the degree of polymerization, can arise in thin copolyme films. When χN is large, thin films exhibit comparatively stable topographical structures, where the dimensions of the topographies normal to the substrate reflect a natural length-scale associated with phase separation in the material. In the other situation, where χN is sufficiently small, the copolymer bulk structure is homogeneous. An ordered structure can be induced into the otherwise compositionally homogeneous structure in the vicinity of a substrate. Here, depending on film thickness, a series of transient and stable topographies can develop. Wetting, early stage structural destabilization dynamics leading to the formation of droplets, and late stage coarsening of the droplets are discussed. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2219–2235, 2003

The CO2 permeability, solubility, and dilation of 6FDA-DAM:DABA 2:1 polyimide membranes exposed to pure CO2 at 40 atm and 35 °C were investigated over long times (up to 20 days). Swelling and sorption measurements were performed on 120 nm films with in situ spectroscopic ellipsometry. The sorption measurements by ellipsometry are compared with sorption into thick films (40−70 μm) by the pressure-decay technique. There is a strong correlation between the CO2 diffusion coefficient and the film swelling. Solid-state covalent cross-linking of the membranes with 1,4-cyclohexanedimethanol and thermal annealing of the polymer films with free acid groups leads to significantly improved ability to maintain a relatively constant CO2 diffusion coefficient over long times. Annealing at temperatures between 100 and 295 °C leads to similar sorption behavior but significantly different diffusion and swelling. Relaxation functions are fit to the permeation, sorption, and swelling data to quantify the relationships betwee...

The application of membranes in gas separation and pervaporation requires materials that are resistant to plasticizing feed streams. We demonstrate the relationship between CO2 sorption, permeability, and film swelling of a polyimide gas separation membrane and how these properties are affected by systematic changes to the polymer structure induced by thermal annealing and covalent cross-linking. Dilation of polyimide thin films (∼120 nm) exposed to high-pressure CO2 (up to 100 atm at 35 °C) was measured by in situ spectroscopic ellipsometry to decouple the effects of thermal and chemical treatments on the film swelling. The refractive index of the CO2-swollen polymer is also used to estimate the CO2 sorption for comparison against that measured on thick films (∼50 μm) by the pressure-decay method. Differences in sorption levels in thin and thick films appear to be related to accelerated physical aging of the thin films. Both thermal annealing and covalent cross-linking of the polyimide films reduce polym...

Polystyrene (PS) and poly(tetramethyl bisphenol polycarbonate) (TMPC) form thermodynamically compatible mixtures below a lower critical solution temperature. In this regime, the PS-component in thin PS−TMPC films, supported by SiOx/Si substrates, preferentially enriches the free surface, forming a wetting layer, whereas the TMPC component enriches to the polymer−substrate interfacial region. We examined the dependence of the effective glass transition temperature, Tg, on film thickness, h, for 50/50 wt % mixtures on SiOx/Si substrates. In these experiments, the molecular weight, MPS, of the PS component was varied by over 2 orders of magnitude (4 kg/mol< MPS

The line of contact at the moving front of a simple liquid on a substrate can become unstable toward the formation of fingers under the action of an external force, such as a temperature gradient or a mechanical force. During the dewetting of entangled polymeric liquid thin films, the moving front (rim) of a growing hole can become unstable toward the formation of fingers in the absence of an external driving force. In this paper we show that instabilities develop at the line of contact, leading to the formation of fingers, in films below a critical film thickness, hc. The extent of fingering decreases as the film thickness increases and virtually disappears for h > hc. This critical film thickness increases with increasing molecular weight, approximately as h ∼ M3/7. Our observations are rationalized in terms of a rough scaling argument.

Carbon dioxide produces an anomalous maximum in the swelling of poly(methyl methacrylate) (PMMA) thin films that is not present in bulk films, as shown with in-situ spectroscopic ellipsometry. This maximum and a corresponding minimum in refractive index are observed in regions of pressure where CO2 is highly compressible near the critical point. An effective excess thickness, determined from the height of the anomalous swelling maxima, increases proportionally with increasing initial film thickness, h0, in the range studied of 85 nm < h0 < 325 nm. Therefore, the anomalous swelling maxima suggest concentration inhomogeneities in the thin film, i.e., the onset of polymer/CO2 phase separation, that are influenced by the compressibility of the system and the confinement of the film. The PMMA swelling isotherms are insensitive to changing the substrate from silicon to gallium arsenide.

The thickness and refractive index of poly(dimethylsiloxane) (PDMS) thin films on silicon, exposed to high-pressure carbon dioxide, have been measured in-situ by spectroscopic ellipsometry. The swe...

The stability of thin film nanocomposites on silicon substrates, formed from mixtures of a symmetric diblock copolymer blended with layered silicate nanocomposites, was investigated using a combination of optical microscopy, atomic force microscopy (AFM), and X-ray diffraction (XRD). Two cases examined are polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) blended with montmorillonite, modified with stoichiometric amounts of alkylammonium surfactant chains, OLS(S), and PS-b-PMMA blended with montmorillonite, modified with excess alkylammonium, OLS(E). While the phase behavior of the OLS(S)/copolymer is similar to that observed in bulk studies, that of the OLS(E)/copolymer system is different. We show that an exchange occurs between surfactant molecules in the OLS(E) system with copolymer chains and that the surfactant molecules form a separate layer on the substrate, resulting in a destabilization of the film.

Linear viscoelastic stress relaxation and oscillatory shear experiments were performed on a mixed alkali ultraphosphate of composition 0.2Na2O+0.2Li2O+0.6P2O5 at temperatures in the vicinity of the glass transition temperature, Tg. The stress relaxation data were well described by a stretched exponential function G(t)=G 0 exp [−(t/τ)] β . β decreased with increasing temperature. Oscillatory shear experiments were performed to determine the complex mechanical moduli, G*(ω)=G′(ω)+iG′′(ω), as a function of angular frequency, ω. G′(ω) is the elastic modulus and G′′(ω) is the loss modulus. The breadth of G′′(ω), which is a direct measure of the distribution of relaxation times, increased with increasing temperature in the same temperature range. The temperature dependence of β and of G′′(ω) indicates a failure of thermorheological simplicity in this glass. The shear viscosities of the glass determined from G(t) and the zero shear viscosities determined from G′′(ω) were found to be in agreement.

We examined the effect of the addition of homopolymers tetramethylbisphenol A polycarbonate, TMPC, and poly(norbornene−methyl-d3-carboxylate), NBMC, on the interlamellar spacing and the phase stability of symmetric poly(styrene-b-methyl methacrylate) diblock copolymer thin films on silicon substrates. The films were of thickness h, L 0.2 the morphology of the diblock copolymer changed to accommodate higher volume fractions of TMPC. This behavior is contrasted with earlier observations in the PS-b-PMMA/PS homopolymer system wher...