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The addition of nanofillers to rubber matrices is a powerful route to improve the mechanical properties. Here, we focus on a molecular understanding of basic mechanisms that are important for the r...

A better understanding of the aggregation states of polymer chains in thin films is of pivotal importance for developing thin film polymer devices in addition to its inherent scientific interest. Here we report the preferential orientation of the crystalline lamellae for isotactic polypropylene (iPP) in spin-coated films by grazing incidence of wide-angle X-ray diffraction in conjunction with sum frequency generation vibrational spectroscopy, which provides information on the local conformation of chains at crystal/amorphous interfaces buried in a thin film. The crystalline orientation of iPP, which formed cross-hatched lamellae induced by lamellar branching, altered from a mixture of edge-on and face-on mother lamellae to preferential face-on mother lamellae with decreasing thickness. The orientation of methyl groups at the crystal/amorphous interfaces in the interior region of the iPP films changed, accompanied by a change in the lamellar orientation.

We report in situ nanostructures and dynamics of polybutadiene (PB) chains bound to carbon black (CB) fillers (the so-called “bound polymer layer (BPL)”) in a good solvent. The BPL on the CB fillers was extracted by solvent leaching of a CB-filled PB compound and subsequently dispersed in deuterated toluene to label the BPL for small-angle neutron scattering and neutron spin echo techniques. The results demonstrate that the BPL is composed of two regions regardless of molecular weights of PB: the inner unswollen region of ≈ 0.5 nm thick and outer swollen region where the polymer chains display a parabolic profile with a diffuse tail. In addition, the results show that the dynamics of the swollen bound chains can be explained by the so-called “breathing mode” and is generalized with the thickness of the swollen BPL.

We report that the nanometer-scale architecture of polymer chains plays a crucial role in its protein resistant property over surface chemistry. Protein-repellent (noncharged), few nanometer thick polymer layers were designed with homopolymer chains physisorbed on solids. We evaluated the antifouling property of the hydrophilic or hydrophobic adsorbed homopolymer chains against bovine serum albumin in water. Molecular dynamics simulations along with sum frequency generation spectroscopy data revealed the self-organized nanoarchitecture of the adsorbed chains composed of inner nematic-like ordered segments and outer brush-like segments across homopolymer systems with different interactions among a polymer, substrate, and interfacial water. We propose that this structure acts as a dual barrier against protein adsorption.

We here report that adsorbed chains composed of one of the constituent blocks can be used as a new surface modification approach to induce perpendicularly oriented lamellar microdomains in block copolymer thin films. A nearly symmetric polystyrene-block-poly(methyl methacrylate) (PS-block-PMMA) diblock copolymer was used as a model. Densely packed PS- or PMMA-adsorbed chains of about 2–3 nm in thickness (“polymer nanocoatings”) were deposited on silicon (Si) substrates using a solvent-rinsing approach. Spin-cast films of 40 or 60 nm-thick PS-block-PMMA (equivalent to two or three interdomain spacings) were subsequently deposited onto the PS or PMMA nanocoatings. Grazing incidence small-angle X-ray scattering experiments revealed the formation of perpendicularly oriented lamellar microdomains within the entire films at 200 °C, where balanced interfacial interactions at the polymer–air interface were achieved. Additionally, X-ray photon correlation spectroscopy studies demonstrated the dynamics of the fully...

We report the in situ structures and dynamics of hydrogenated polybutadiene (PB) chains bound to carbon black nanoparticle surfaces in polymer solutions composed of deuterated PB and deuterated toluene using small-angle neutron scattering and neutron spin-echo techniques together with molecular dynamics (MD) simulations. The experimental results showed that the swollen bound polymer chains exhibit the collective dynamics (the so-called breathing mode) at polymer concentrations (c) below and above the overlap polymer concentration (c*) (i.e., 0.61 < c/c* < 1.83), where the concentration profiles of the bound polymer remained unchanged with the different c values. Interestingly, the collective dynamics slowed down by a factor of 2 compared to that in pure d-toluene when the chain lengths of the bound polymer and matrix polymer were equal. However, when the free polymer chains were longer than the bound polymer chains, the decrease in collective dynamics was not as significant. MD simulations were performed ...

Here we report a link between the interfacial structure and adhesive property of homopolymer chains physically adsorbed (i.e., via physisorption) onto solids. Polyethylene oxide (PEO) was used as a model and two different chain conformations of the adsorbed polymer were created on silicon substrates via the well-established Guiselin's approach: "flattened chains" which lie flat on the solid and are densely packed, and "loosely adsorbed polymer chains" which form bridges jointing up nearby empty sites on the solid surface and cover the flattened chains. We investigated the adhesion properties of the two different adsorbed chains using a custom-built adhesion testing device. Bilayers of a thick PEO overlayer on top of the flattened chains or loosely adsorbed chains were subjected to the adhesion test. The results revealed that the flattened chains do not show any adhesion even with the chemically identical free polymer on top, while the loosely adsorbed chains exhibit adhesion. Neutron reflectivity experiments corroborated that the difference in the interfacial adhesion is not attributed to the interfacial brodening at the free polymer-adsorbed polymer interface. Instead, coarse-grained molecular dynamics simulation results suggest that the tail parts of the loosely adsorbed chains act as "connector molecules", bridging the free chains and substrate surface and improving the interfacial adhesion. These findings not only shed light on the structure-property relationship at the interface, but also provide a novel approach for developing sticking/anti-sticking technologies through precise control of the interfacial polymer nanostructures.

It is known that when nanoparticles are added to polymer thin films, they often migrate to the film–substrate interface and form an “immobile interfacial layer”, which is believed to be the mechanism behind dewetting suppression. We here report a new mechanism of dewetting suppression from the structural aspect of polymer chains accommodated at the film–substrate interface. Dodecanethiol-functionalized gold (Au) nanoparticles embedded in relatively low molecular weight PS thin films prepared on silicon (Si) substrates were used as a model. We mimicked the previously reported conditions, where the nanoparticles preferentially migrate to the substrate, and successfully stabilized the PS thin films via thermal annealing. A suite of surface-sensitive techniques including atomic force microscopy, grazing incidence small-angle X-ray scattering, X-ray/neutron reflectivity, and sum frequency generation spectroscopy in conjunction with the established solvent leaching process enabled us to unveil the polymer chain...

We report the effects of compressed CO, molecules as a novel plasticization agent for poly(3-hexylthiophene) (P3HT)-conjugated polymer thin films. In situ neutron reflectivity experiments demonstrated the excess sorption of CO, molecules in the P3HT thin films (about 40 nm in thickness) at low pressure (P = 8.2 MPa) under the isothermal condition of T = 36 degrees C, which is far below the polymer bulk melting point. The results proved that these CO2, molecules accelerated the crystallization process of the polymer on the basis of ex situ grazing incidence X-ray diffraction measurements after drying the films-via rapid depressurization to atmospheric pressure: both the out-of-plane lamellar ordering of the backbone chains and the intraplane pi-pi stacking of the side chains were significantly improved, when compared with those in the control P3HT films subjected to conventional thermal annealing (at T = 170 degrees C). Electrical measurements elucidated that the CO2-annealed P3HT thin films exhibited enhanced charge carrier mobility along with decreased background charge carrier concentration and trap density compared with those in the thermally annealed counterpart. This is attributed to the CO2-induced increase in polymer chain mobility that can drive the detrapping of molecular oxygen and healing of conformational defects in the polymer thin film. Given the universality of the excess sorption of CO2 regardless of the type of polymers, the present findings suggest that CO2 annealing near the critical point can be useful as a robust processing strategy for improving the structural and electrical characteristics of other semiconducting conjugated polymers and related systems such as polymer:fullerene bulk heterojunction films.

We report the structural relaxation process of irreversibly adsorbed polymer chains via thermal annealing that lie flat on a solid (“flattened chains”). Amorphous polystyrene and quartz, which together constitute a weakly attractive system, was used as a model where the local chain conformations of the flattened chains were investigated by sum frequency generation spectroscopy (SFG). Two different film preparation processes (i.e., spin coating and dip coating methods) were utilized to create different initial chain conformations. The spin-coated and dip-coated PS thin films were annealed at a temperature far above the bulk glass transition temperature to reach the “quasiequilibrium” state and subsequently rinsed with chloroform to uncover the buried flattened chains. The SFG results revealed that the backbone chains (constituted of CH and CH2 groups) of the flattened PS chains preferentially orient to the weakly interactive substrate surface via thermal annealing regardless of the initial chain conformati...

The glass transition of supported polystyrene (PS) and poly(2-vinylpyridine) (P2VP) thin films in the vicinity of the substrate interface was studied by using a nanoplasmonic sensing (NPS) method. This “nanocalorimetric” approach utilizes localized surface plasmon resonance from two-dimensional arrangements of sensor nanoparticles deposited on SiO2-coated glass substrates. The NPS results demonstrated the existence of a high glass transition temperature (Tg,high) along with the bulk glass transition temperature (Tg,bulk ≈ 100 °C for PS and P2VP) within the thin films: Tg,high ≈ 160 °C for PS and Tg,high ≈ 200 °C for P2VP. To understand the origin of the Tg,high, we also studied the thermal transitions of lone polymer chains strongly adsorbed onto the substrate surface using solvent rinsing. Interestingly, the NPS data indicated that the Tg,high is attributed to the adsorbed polymer chains. To provide a better understanding of the mechanism of the Tg,high, molecular dynamics simulations were performed on a...

The re-entrance sites, successive chain-folding number ⟨n⟩, and chain-folding fraction ⟨F⟩ of the chain-folding (CF) structure of 13C CH3-labeled isotactic poly(1-butene) (iPB1) with an weight-averaged molecular weight (⟨Mw⟩ = 37 K g/mol) in solution- and melt-grown crystals as a function of crystallization temperature (Tc) were determined using solid-state (SS) NMR. The solution- and melt-grown crystals possessed adjacent re-entry structures between the right- and left-handed stems along the (100) and (010) planes, which were invariant as a function of Tc. The adjacent re-entry structures in the former exhibited long-range order (⟨n⟩ ≥ 8) compared with that in the latter (⟨n⟩ ≥ 1.7–2). These results indicated that the concentration and entanglement of polymers play significant roles in the CF process and structural formation during the initial stage of crystallization, whereas kinetics does not. Transmission electron microscopy (TEM) revealed well-defined hexagonal and circular crystals grown from the so...

We report a mobility gradient of polymer chains in close proximity of a planar solid substrate in compressed carbon dioxide (CO2) gas. A series of bilayers composed of bottom hydrogenated polystyrene (h-PS) and top deuterated PS (d-PS) layers were prepared on Si substrates. A high-pressure neutron reflectivity (NR) technique was used to study the diffusive motion at the h-PS/d-PS interface as a function of the distance from the substrate interface. The results reveal that the interdiffusive chain dynamics gets strongly hindered compared to the bulk when the distance from the substrate is less than 3R(g) (R-g is the radius of polymer gyration of the h-PS). At the same time, by utilizing rapid quench of CO2 and subsequent solvent leaching, we reveal the presence of the CO2-induced polymer adsorbed layer on the substrate. We postulate that loop components in the adsorbed polymer chains provide a structure that can trap the neighboring polymer chains effectively, hence reducing the chain mobility in the close vicinity of the solid substrate even in the presence of the effective plasticizer.

Attaching dendritically branched side chains to each repeat unit of a linear polymer produces molecular building blocks of nanometer-sized dimensions called dendronized polymers. The structure of these complex molecular architectures is highly tunable and, therefore, of interest for a wide range of potential applications. The first examples of dendronized polymers prepared by living ring-opening metathesis polymerization of oxanorbornenedicarboximide macromonomers with poly(alkyl ether) dendrons are reported. Small-angle X-ray scattering experiments on bulk samples confirm that the diameter of the individual cylindrical polymers can be tailored by the choice of dendron generation or the length of the hydrocarbon peripheral group. Analysis of the SAXS data based on a core-shell model indicates that although the diameter of the cylinder increases with generation, the size of the core does not change; this suggests that these dendrons only loosely encapsulate the polymer backbone. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 3221–3239

Thermal annealing is one of the most indispensable polymer fabrication processes and plays essential roles in controlling morphologies and properties of polymeric materials. We here report that thermal annealing also facilitates polymer adsorption from the melt on planar silicon (Si) substrates, resulting in the formation of a high-density polymer nanolayer with flattened chain confirmations. Three different homopolymers (polystyrene, poly(2-vinylpyridine), and poly(methyl methacrylate)), which have similar inherent stiffness and bulk glass transition temperature (Tg), but have different affinities with Si substrates, were chosen as models. Spin-cast films (∼50 nm in thickness) with the three polymers were prepared on cleaned Si substrates and then placed in a vacuum oven set at a temperature far above the bulk Tg. In order to monitor the polymer adsorption process at the solid-polymer melt interface during thermal annealing, we used the protocol that combines vitrification of the annealed films (via rapi...

Numerous studies have shown that the glass transition temperature, Tg, of polymers may change visibly upon addition of even small amounts of nanoparticles. In this study, we examine the effect of polymer–nanoparticle interfacial energy on this phenomenon in nanocomposite films containing polystyrene-grafted nanoparticles mixed with polystyrene homopolymer at low particle loadings. A previous experiment showed that the interfacial energy of this system modulates with the grafting density of the grafted polymer and the molecular weight ratio of the matrix and grafted polymers. We find that the changes in Tg vary with both parameters in manners that quantitatively agree with the interfacial energy.

Various properties of ultrathin polymer films differ substantially from their bulk values. However, the critical question is whether the unusual properties are uniform throughout the films. To explore the presence of heterogeneous viscosity distributions, we have established grazing-incidence X-ray photon correlation spectroscopy for single polymer films with embedded metal nanoparticles that act as markers. In addition, the combined use of resonance enhanced X-ray scattering enables us to intensify the probing electrical field in the regions of interest within a single polymer film.

Polymer properties, such as their mechanical strength, barrier properties, and dielectric response, can be dramatically improved by the addition of nanoparticles. This improvement is thought to be because the surface area per unit mass of particles increases with decreasing particle size, R, as 1/R. This favorable effect has to be reconciled with the expectation that at small enough R the nanoparticles must behave akin to a solvent and cause a deterioration of properties. How does this transition in behavior from large solutes to the solvent limit occur? We conjecture that for small enough particles the layer of polymer affected by the particles (“bound” polymer layer) must be much smaller than that for large particles: the favorable effect of increasing particle surface area can thus be overcome and lead to the small solvent limit with unfavorable mechanical properties, for example. To substantiate this picture requires that we measure and compare the “bound polymer layer” formed on nanoparticles with th...

We have shown that the addition of nanoclays is an effective means for enhancing the flame retardant properties of polymer blends. Polymer blends are difficult to render flame retardant even with the addition of flame retardant agents due to dispersion and phase segregation during the heating process. We show that the addition of 5% functionalized Cloisite 20A clays in combination with 15% decabromodiphenyl ether and 4% antimony trioxide to a polystyrene/poly(methyl methacrylate) blend can render the compound flame resistant within the UL-94-V0 standard. Using a variety of micro-characterization methods, we show that the clays are concentrated at the interfaces between the polymers in this blend and completely suppress phase segregation. The flame retardant (FR) is absorbed onto the clay surfaces, and the exfoliation of the clays also distributes the FR agent uniformly within the matrix. TGA of the nanocomposite indicates that prior to the addition of clay, the dissociation times of the individual components varied by more than 20 °C, which complicated the gas-phase kinetics. Addition of the clays causes all the components to have a single dissociation temperature, which enhanced the efficacy of the FR formula in the gas phase. Cone calorimetry also indicated that the clays decreased the heat release rate (HRR) and the mass loss rate (MLR), due to the formation of a robust char. In contrast, minimal charring occurred in blends containing just the FR. SEM examination of the chars showed that the clay platelets were curved and in some cases tightly folded into nanotube-like structures. These features were only apparent in blends, indicating that they might be associated with thermal gradients across the polymer phase interface. SEM and SAXS examinations of the nanocomposites after partial exposure to the flame indicated that the clays aggregated into ribbon-like structures, approximately microns in length, after the surfactant thermally decomposed. Thermal modeling indicated that these ribbons might partially explain the synergy due to better distribution of the heat and improve the mechanical properties of the melt at high temperatures, in a manner similar to the one reported for carbon nanotubes.

Using a combined ultra-small-angle and small-angle scattering (CSAS) method of neutrons and X-rays, we investigated hierarchical structures of carbon black (CB) highly loaded in polyisoprene (PI) and poly(styrene-random-butadiene) copolymer (SBR) under mechanical field (defined respectively as CB/PI and CB/SBR) as well as in toluene under a sonic field (defined as CB/toluene). In order to analyze each structure level comprising the hierarchical structures of CB from the CSAS profiles, we employed the unified Guinier/power-law approach proposed by Beaucage (J. Appl. Cryst. 1995, 28, 717). Furthermore, in order to extract not only sizes but also shapes of the structure elements, we developed a modified approach, in which the Guinier scattering function utilized in the Beaucage approach was replaced by a form factor of the corresponding structure. Comparison of the scattering profiles from CB/PI and CB/SBR with CB/toluene clarified that (i) the smallest structure elements of CB (that further form mass-fracta...

We have investigated the effect of the length of multiwalled carbon nanotubes (MWNT) on the dewetting dynamics of thin polymer films. The results indicated that long nanotubes were much more effective than short nanotubes in stabilizing the films against dewetting. The diffusion of polymer chains in the filled matrices was measured using neutron reflectivity, and the result indicated no significant effect on the diffusion coefficient by either short or long nanotubes. We therefore proposed a model whereby the nanotubes did not interact with the individual polymer chains. On the other hand, the long nanotubes formed an effective entangled network, which prevented long-range motion of the polymer films upon dewetting. This model was supported by rheological experiments on bulk samples where the nanotubes had a strong effect on G‘ of the polymer and only a negligible effect on G‘ ‘.

The stability or wettability of thin polymer films on solids is of vital interest in traditional technologies as well as in new emerging nanotechnologies. We report here that nanoscale structures of polymer chains adsorbed onto a solid surface play a crucial role in the thermal stability of the film. In this study, polystyrene (PS) spin-cast films (20 nm in thickness) with eight different molecular weights prepared on silicon (Si) substrates were used as a model. When low molecular weight (Mw≤ 50 kDa) PS films were subjected to thermal annealing at temperatures far above the bulk glass transition temperature, dewetting occurred promptly, while high molecular weight (Mw≥ 123 kDa) PS films were stable for at least 6 weeks at 150 °C. We reveal a strong correlation between the film stability and the two different interfacial structures of the adsorbed polymer chains: their opposing wettability against chemically identical free polymer chains results in a wetting-dewetting transition at the adsorbed polymer-free polymer interface. This is a unique aspect of the stability of polymer thin films and may be generalizable to other polymer systems regardless of the magnitude of solid-polymer attractive interactions.

This paper reviews our recent experimental results that address the effects of solvent density inhomogeneities in supercritical carbon dioxide (scCO(2)) on polymer thin film processing. The key phenomenon is excess sorption of CO(2) molecules into polymer thin films even when the bulk miscibility with CO(2) is very poor. We have found that the amount of the excess sorption is attributed to the large density fluctuations in scCO(2) near the critical point. Further, taking advantage of the vitrification process of polymer chains through quick evaporation of CO(2), we can preserve the "expanded" structures as they are. The resultant films have large degree of molecular-level porosity that is very useful in producing coatings with low dielectric constants, enhanced adhesion, and metallization properties. These characteristics can be achieved in an environmentally "green" manner, without organic solvents, and are not specific to any class of polymers.

We synthesized C18-functionalized gold and palladium nanoparticles with average diameter size of 10 and 3 nm, respectively, and carried out a systematic study of the effect of nanoscale metallic fillers on the dewetting dynamics of PS/PMMA bilayer substrates. Optical and atomic force microscopies were used to study the hole growth and determine the viscosity of the films as a function of PS molecular weight, particle radius, and concentration. Neutron reflectivity was used to measure the effects of the nanoparticles on the tracer diffusion coefficient. X-ray reflectivity and TEM microscopy were used to study the distribution of the particles within the film and ensure that no segregation or clustering occurred. The results indicated that the dynamics are a sensitive function of the ratio between the filler radius, Rparticle, and the polymer radius of gyration, Rg. The data were found to collapse on a universal curve where the relative velocity of the filled system was faster than that for the unfilled system when Rg/Rparticle > 4 and slower when Rg/Rparticle 4 and unchanged when Rg/Rparticle < 4. The results were interpreted in terms of an increase in the local excluded volume and possible elastic distortions of the polymer matrix.

We have measured the viscosity of thin polymer films as a function of film thickness using three independent techniques. The results of all methods indicated that the viscosity of the film increases about two orders of magnitude near the solid substrate. Measurements performed on split layer substrates indicated that a layer of polymer chains remained permanently adsorbed at the Si substrate. This layer was responsible for trapping subsequent layers, and propagating the effect of surface interactions to chains without direct contacts to the solid substrate. If this layer was applied prior to the rest of the film, it can screen the surface interactions and even initiate auto-dewetting of other chemically identical layers above it.

Supercritical CO2 (scCO2) has been used to facilitate reactions in thin film bilayers between functionalized polystyrene and poly(methyl methacrylate) at temperatures far below the glass transition temperatures of the respective polymers. Secondary ion mass spectrometry (SIMS) is used to monitor the reaction progression directly by measuring the interfacial excess of deuterated PS. Complementary X-ray reflectometry (XR) yields the interfacial width and surface roughness of bilayer films for reactive systems with and without the addition of scCO2, and comparisons are made with unreactive reference systems. From XR and SIMS analyses, the interfacial width and roughness have been found to be effectively independent of the reaction conditions employed here, and the primary impact of incorporated scCO2 is enhanced mobility of the reactive polymer chains. The use of scCO2 can change polymer mobility significantly enough over a very small temperature range (DTw15 8C) so that both diffusion- and reaction-controlled type behavior can be observed for otherwise identical systems. q 2005 Elsevier Ltd. All rights reserved.

We report an environmentally “green” method to improve adhesion at a polymer/metal interface by using supercritical carbon dioxide (scCO2). Spun-cast polystyrene (PS) and poly(methyl methacrylate) (PMMA) thin films on cleaned Si wafers were used for this study. Film thicknesses of both polymer films were prepared in the range of 100 A to 1600 A. We exposed the films to scCO2 in the pressure-temperature (P–T) range corresponding to the density-fluctuation ridge, where the excess swelling of both polymer films occurred, and then froze the swollen structures by quick evaporation of CO2. A chromium (Cr) layer with film thickness of 300–400 A was deposited onto the exposed film by using an E-beam evaporator. X-ray reflectivity (XR) measurements showed that the interfacial width between the Cr and exposed polymer layers increased by a factor of about two compared with that without exposure to scCO2. In addition, the large interfacial broadening was found to occur irrespective of the thickness of both p...

We report on an efficient and environmentally friendly means to modify surface properties of polymer films supported for nanoparticles. Ultrathin polystyrene (PS) films (

By using in situ neutron reflectivity, we measured the swelling behavior of two types of polymer brushes, deuterated polystyrene with a trichlorosilane end group and deuterated polystyrene-block-poly(4-vinylpyridine) block copolymer, in supercritical carbon dioxide (scCO2). The measurements were conducted in the pressure range of 0.1–20 MPa at 36 °C. The pressure dependence of the brush height clearly showed an anomalous peak at the density fluctuation ridge (pressure = 8.2 MPa) that defined the maximum long-range density fluctuation amplitude in the pressure–temperature phase diagram of carbon dioxide (CO2). The density profile of the brush, which could be approximated by a simple step function, and the magnitude of the brush height both indicated that the solvent quality of scCO2 for the deuterated polystyrene brushes was still poor even at the density fluctuation ridge. In addition, atomic force microscopy images for the frozen polystyrene brush prepared by the rapid drying of CO2 showed a phase-separated structure, as predicted from the numerical calculations of Grest and Murat, as a function of the variable Nσ, where N is the polymerization index and σ is the grafting density. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3282–3289, 2004

Supercritical carbon dioxide (scCO2) is being used increasingly as a green solvent in polymer processing. The major disadvantage thus far is that only a limited class of polymers, such as fluorinated or silicone-based polymers, can be dissolved in CO2. Here I show that large density fluctuations in scCO2 can significantly enhance the solubility of scCO2 in polymer thin films even when the bulk polymers have very poor miscibility with CO2. By using in situ neutron reflectivity, I found that a wide variety of polymer thin films can swell as much as 30-60% when exposed to scCO2 within a narrow temperature and pressure regime, known as the 'density fluctuation ridge', which defines the maximum density fluctuation amplitude in CO2. Furthermore, the swollen structures induced by the density fluctuation could be frozen by a flash evaporation of CO2 via the vitrification process of the polymer without a formation of void structures. X-ray reflectivity clearly showed that the scCO2 process could be used to produce uniform low-density polymer thin films. I also found that other properties of the vitrified films, such as index of refraction, dielectric constant and glass transition, were correlated with the low-density density profile.

In situ neutron reflectivity was used to investigate the effects of density fluctuations on the solubility of supercritical carbon dioxide (scCO2) in polymer thin films. Deuterated polystyrene, deuterated polybutadiene, and the corresponding random copolymer, deuterated styrene-random- butadiene copolymer, as well as deuterated poly(methyl methacrylate) were investigated. Data were obtained as a function of pressure under two isothermal conditions (T ) 36 and 50 °C). All the polymer films used showed anomalous swelling and CO2 sorption on the density fluctuation ridge in the P-T phase diagram of CO2. We found that the magnitude of the swelling was a function of the elasticity of the films rather than the bulk solubility of CO2. The enhanced miscibility of the rubber/CO2 systems, which are very poor in bulk, was found to be almost identical to that of the silicon rubber/CO2 mixture, which is one of the highly miscible polymeric materials under moderate CO2 conditions.

Solid-polymer interfaces play crucial roles in the multidisciplinary field of nanotechnology and are the confluence of physics, chemistry, biology, and engineering. There is now growing evidence that polymer chains irreversibly adsorb even onto weakly attractive solid surfaces, forming a nanometer-thick adsorbed polymer layer (“adsorbed polymer nanolayers”). In this Chapter, we review our recent experimental results on the structures and dynamics of the adsorbed polymer chains. Furthermore, we shed light on the mechanism giving rise to the extraordinary properties of the adsorbed nanolayers and on how these properties can propagate into the film interior over distances of several tens of nanometers, resulting in heterogeneities of local crystalline structures, viscosity, interdiffusive motions (in the melts and a solvent) of supported polymer thin films.

The solid–polymer melt interface is of great scientific interest due to its vital importance in governing a wide array of physical and mechanical properties of polymer thin films. Recent studies have elucidated the coexistence of two different chain conformations of polymer chains adsorbed on a solid (i.e., loosely adsorbed chains and flattened chains). In this work, film stabilities of the polystyrene (PS) “interfacial sublayer” (composed of outer loosely adsorbed chains and inner flattened chains) and flattened layer (composed of the lone flattened chains) prepared on silicon (Si) substrates are investigated. The atomic force microscopy studies reveal that the as-rinsed PS flattened layer is subjected to spinodal-like dewetting during a post-thermal annealing process even at temperatures below the bulk glass transition temperature. Furthermore, it is found that the surface morphology of the flattened layer can be reversibly changed from a homogeneous pattern under good solvent conditions to spinodal-like droplets under poor solvent conditions. By contrast, it is found that the PS interfacial sublayer remains stable under both good and poor solvent conditions. These findings illuminate the role which density variations within the adsorbed layers play in the mechanism behind the wetting-dewetting transition.

A water-soluble polythiophene (PT) was prepared by the self-assembling complex with a glycopolymer. The glycopolymer of poly(N-p-vinylbenzyl-D-lactonamide) (PVLA) formed self-assembling cylindrical structure based on the amphiphilicity even after the complexation with PT. We confirmed the improved optical functionality of PT due to the longer conjugated {pi}-orbital. It suggested that PT behaved like molecular nanowire with the self-assembled structure in the hydrophobic core of PVLA. PVLA-PT also showed specific biorecognition against corresponding lectin. These results suggested that the bioactive nanowire formation of PT with the glycopolymer was developed.

A series of giant polymer-dendron conjugates with a dendron head and a linear polymer tail were synthesized via"click" chemistry between azide-functionalized polystyrene (PS(N), N: degree-of-polymerization) and t-butyl protected, alkyne-functionalized second generation dendron (tD), followed by a deprotection process to generate a dendron termini possessing nine carboxylic acid groups. The molecular structures were confirmed by nuclear magnetic resonance, size-exclusion chromatographic analyses, and matrix-assisted laser desorption ionization time-of-flight mass spectra. These well-defined conjugates can serve as a model system to study the effects of the molecular geometries on the self-assembly behaviour, as compared with their linear analogues. Four phase morphologies found in flexible linear diblock copolymer systems, including lamellae, bicontinuous double gyroids, hexagonal packed cylinders, and body-centred cubic packed spheres, were observed in this series of conjugates based on the results of small angle X-ray scattering and transmission electron microscopy. All of the domain sizes in these phase separated structures were around or less than 10 nm. A 'half' phase diagram was constructed based on the experimental results. The geometrical effect was found not only to enhance the immiscibility between the PS(N) tail and dendron head, but also systematically shift all of the phase boundaries towards higher volume fractions of the PS(N) tails, resulting in an asymmetrical phase diagram. This study may provide a pathway to the construction of ordered patterns of sub-10 nm feature size using polymer-dendron conjugates.

It is shown that low-density polymer layers created by supercritical carbon dioxide (scCO2) processing can be exploited to enhance metallization of thin polymer films. Spun-cast thin films were exposed to scCO2 within the narrow temperature and pressure regime known as the `density fluctuation ridge', where excess swelling of the polymer thin films was induced, and the swollen structures could be subsequently preserved by quick evaporation of CO2. X-ray reflectivity (XR) measurements proved that the `expanded' films had a broader interfacial width between the polymer and deposited metal layers, regardless of the polymer film thickness or choice of polymers. Furthermore, standard peel test showed a drastic enhancement in adhesion between the polymer and metal layers, which correlates with the XR result: the broader interfacial width indicates increased adhesion.

The effects of CO2 annealing on the melting and subsequent melt crystallization processes of spin-cast poly(ethylene oxide) (PEO) ultrathin films (20-100 nm in thickness) prepared on Si substrates were investigated. By using in situ neutron reflectivity, we found that all the PEO thin films show melting at a pressure as low as P = 2.9 MPa and at T 48 degrees C which is below the bulk melting temperature (T-m). The films were then subjected to quick depressurization to atmospheric pressure, resulting in the non-equilibrium swollen state, and the melt crystallization (and/or dewetting) process was carried out in air via subsequent annealing at given temperatures below T-m. Detailed structural characterization using grazing incidence X-ray diffraction, atomic force microscopy, and polarized optical microscopy revealed two unique aspects of the CO2-treated PEO films: (i) a flat-on lamellar orientation, where the molecular chains stand normal to the film surface, is formed within the entire film regardless of the original film thickness and the annealing temperature; and (ii) the dewetting kinetics for the 20 nm thick film is much slower than that for the thicker films. The key to these phenomena is the formation of irreversibly adsorbed layers on the substrates during the CO2 annealing: the limited plasticization effect of CO2 at the polymer-substrate interface promotes polymer adsorption rather than melting. Here we explain the mechanisms of the melt crystallization and dewetting processes where the adsorbed layers play vital roles.

We report a method for producing stable low-density polymer films by using supercritical carbon dioxide (scCO(2)). Two different molecular weight polystyrene films with various thicknesses were exposed to scCO(2) along the density fluctuation ridge in P-T phase diagram. The swollen structures could be then frozen by flash evaporation of CO2 without forming additional voids. X-ray reflectivity data clearly showed that exposure to scCO(2) could be used to produce uniform low-density films of about 2R(g) thick or less, where R-g is radius of polymer gyration. (C) 2003 American Institute of Physics.

We report the chain conformations of polymer molecules accommodated at the solid-polymer melt interfaces in equilibrium. Polystyrene ``Guiselin'' brushes (adsorbed layers) with different molecular weights were prepared on Si substrates and characterized by using x-ray and neutron reflectivity. The results are intriguing to show that the adsorbed layers are composed of the two different nanoarchitectures: flattened chains that constitute the inner higher density region of the adsorbed layers and loosely adsorbed polymer chains that form the outer bulklike density region. In addition, we found that the lone flattened chains, which are uncovered by the additional prolonged solvent leaching ($\ensuremath{\sim}120$ days), are reversibly densified with increasing temperature up to $150\text{ }\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. By generalizing the chain conformations of bulks, we postulate that the change in probabilities of the local chain conformations (i.e., trans and gauche states) of polymer molecules is the origin of this densification process.

We report the origin of the effect of nanoscale confinement on the local viscosity of entangled polystyrene (PS) films at temperatures far above the glass transition temperature. By using marker x-ray photon correlation spectroscopy with gold nanoparticles embedded in the PS films prepared on solid substrates, we have determined the local viscosity as a function of the distance from the polymer-substrate interface. The results show the impact of a very thin adsorbed layer (~7 nm in thickness) even without specific interactions of the polymer with the substrate, overcoming the effect of a surface mobile layer at the air-polymer interface and thereby resulting in a significant increase in the local viscosity as approaching the substrate interface.

By using in-situ neutron reflectivity, we have investigated swelling isotherms of solvophilic and solvophobic end-grafted/non-grafted polymer chains on solid substrates in supercritical carbon dioxide and supercritical ethane. It was found that anomalous expansion of the polymer chains associated with excess absorption of the fluid molecules occurs in the large compressible regions of both supercritical fluids (SCFs) regardless of the polymer-fluid interactions. In addition, we found that the excess expansion of the solvophobic polymer chains in both SCFs collapse onto one master curve under the same magnitude of density fluctuations in the fluids. A simple thermodynamic two-state model along with the experimental results proposes that polymer chains are expanded independently of the polymer fluid interactions to further change solvent density fluctuations around the polymer chains, thereby lowering the free energy of the polymer/SCF systems. (C) 2011 Elsevier Ltd. All rights reserved.

A high-pressure dynamic light-scattering (DLS) technique has been utilized to study the behavior in solution of poly(1, 1-dihydroperfluorooctylacrylate) and poly(vinyl acetate) (PFOA-b-PVAC) in supercritical carbon dioxide. The hydrodynamic-radius distribution for each species, such as unimers, micelles, and large aggregates, were determined under both isobaric and isothermal conditions over a pressure range of 9-55.2 MPa, and a temperature range of 25-75 degrees C, respectively. The DLS results clearly showed both pressure-induced and temperature-induced dissolution and association behavior for the copolymer in supercritical carbon dioxide. Also presented are some preliminary experimental results for the micellar self-assembly of a fluorinated block copolymer, poly(2-tetrahydropyranyl methacrylate)-b-poly(1h, 1h-perfluorooctyl methacrylate) (THPMA-b-F7MA), in supercritical carbon dioxide by use of a new high-pressure cell that allows us to conduct simultaneous small-angle x-ray scattering and DLS measurements.

We investigated an effect of CO2 sorption on the compatibility of immiscible polystyrene (PS) and polybutadiene (PB) bilayers by using in situ neutron reflectivity. By labeling either polymer with deuterium, we found that the excess CO2 molecules were adsorbed to both top PS and bottom PB layers when the bilayers were exposed to CO2 at the narrow T and P regime near the critical point of pure CO2. Furthermore, we clarified that this excess sorption of CO2 molecules increased the interfacial width between the layers up to 100 angstroms even near room temperature, while the interfacial width without CO2 exposure has been reported to be at most 40 A even at the highest temperature (T congruent with 175 degrees C).

We have performed x-ray specular and diffuse scattering on liquid polymer films and analyzed the spectra as a function of film thickness and molecular weight. The results show that films whose molecular weight is close to the entanglement length behave as simple liquids except that the shortest wavelength is determined by the radius of gyration (${R}_{g}$) rather than the monomer-monomer distance. When the molecular weight is higher than the entanglement length, the strong deviations from liquidlike behavior are observed. We find that the long wavelength cutoff vector, ${q}_{l,c}$, scales with film thickness, $d$ as ${d}^{\ensuremath{-}1.1\ifmmode\pm\else\textpm\fi{}0.1}$ rather than the usual ${d}^{\ensuremath{-}2}$ expected for simple liquids. If we assume that these deviations are due to surface pinning of the polymer chains, then our results are consistent with the formalism developed by Fredrickson et al. to explain the capillary wave spectrum that can propagate in a polymer brush.

We report an anomalous swelling of polymer thin films in carbon dioxide (CO(2)) which is associated (in both locus and form) with the density fluctuation ridge that forms along the extension of the coexistence curve of gas and liquid in the P-T phase diagram. Neutron reflectivity results showed that CO(2) could be sorbed to a large extent ( approximately 60%) in thin polymer films even when the bulk miscibility of the polymer with CO(2) is very poor. The anomalous swelling is found to scale with the polymer radius of gyration (R(g)) and extends to a distance approximately 10 R(g).

We report a carbon dioxide-based approach to induce highly ordered molecular aggregation states of perfluoroalkyl (Rf) chains of densely-grafted poly{2-(perfluorooctyl)ethyl acrylate} (poly(FA-C8)) brush in place of conventional thermal annealing. Poly(FA-C8) brush films of 40 nm thickness were prepared by surface-initiated atom transfer radical polymerization. In-situ neutron reflectivity measurements for the poly(FA-C8) brush films under the isothermal condition of T = 309 K, which is below the bulk melting temperature (Tm = 348 K), elucidated large expansion of polymer chains due to sorption of CO2 molecules. Comparison of the swelling behavior with an amorphous poly{2-(perfluorobutyl)ethyl acrylate} brush thin film clarified that the sorption of CO2 molecules results in the melting of the semicrystalline poly(FA-C8) brush at P > 4.1 MPa. In addition, by using out-of-plane grazing incidence wide-angle X-ray diffraction, it was found that subsequent slow quench from P > 4.1 MPa induces rearrangement of the rigid rod-like Rf groups, forming highly ordered molecular aggregation structures similar to those via a conventional thermal process. The appropriate CO2 process conditions for the effective induction of the highly ordered structures of the rigid Rf groups are discussed in detail.