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

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

Adsorption-mediated nonlinearity of critical cracking thickness in drying nanoparticle–polymer suspensions

We examine the critical cracking thicknesses of as-dried colloidal TiO₂ suspensions containing water-soluble polymeric additives, above which cracks spontaneously propagate. In contrast to common observations, the critical cracking thickness increases in a nonlinear, stepwise manner as increasing the concentration of the polymeric additive in the suspension. The critical thickness diverges when the polymer content increases above a certain threshold. We demonstrate that the first and second critical cracking thickness increments are attributable to the onset of polymer adsorption and a subsequent transition in the conformations of the adsorbing polymer chains, respectively. The critical thickness profiles with respect to normalized polymer adsorption amounts at different particle diameters converged into a single master curve. These results illustrate nontrivial evaporation processes in which the various roles of polymeric additives delay cracking into thicker films.     

Conformation of single PMMA chain in uniaxially stretched film studied by scanning near-field optical microscopy

The conformation of the single polymer chain in uniaxially stretched poly(methyl methacrylate) (PMMA) films was studied by scanning near-field optical microscopy (SNOM), which enabled us to observe the elongated conformation of each polymer chain embedded in the film. The conformation of the individual PMMA chains was quantitatively evaluated from the fluorescence intensity distribution. Observation of films with different strains showed that the microscopic strain of the single chain was smaller than the macroscopic strain (?) of the film. Especially at the later stage of the deformation (? > 1.5), the PMMA chain was not stretched with the increase in the macroscopic elongation of the film, suggesting the presence of slipping of polymer chain on the course of stretching.     

The effect of grafting density on the crystallization behavior of one-dimensional confined polymers

Grafting density and confinement scale of the nano-area are significant elements affecting the crystallization behavior of polymers. In this work, the crystallization processes of confined and unconfined polymer systems with different grafting density were systematically studied by MC simulation. The results show that when the grafting density of confined systems is low, the crystallization rate is faster and the final crystallinity is higher. However, the crystallization ability is reduced for higher grafting density. We found that for this confined system, the segmental density of interfacial region is larger, and the movement of chain segment is lower. Due to the higher grafting density, the crowding effect of polymer chains is strong, which leads to the intermolecular nucleation. The critical nucleation free energy barrier is higher. Moreover, only homogeneous nucleation occurs in confined polymer systems. With the increasing grafting density, the crystals change from lying on the substrate surface to being perpendicular to the substrate surface in unconfined polymer systems. The crystals are mainly lying on the substrate surface in confined polymer systems. These simulation results are helpful to understand the microscopic mechanism of crystallization behaviors of polymer nanocomposites and provide a theoretical basis for the design of nanocomposites with excellent physical properties.     

One-dimensional confinement in crystallization of P(VDF-TrFE) thin films with transfer-printed metal electrode

The crystallization of thin polymer film depends on surface/interface properties, due to the fact that molecular chain motion is affected by the presence of the surface. In this work, we measured the ferroelectric properties, crystallinity, chain conformation and surface morphologies of one-dimensionally confined P(VDF-TrFE) thin films using transfer-printed Au film, annealed at elevated temperatures, from just below melting temperature up to 200 °C. Crystallization at low temperature, i.e., below melting temperature, the confinement effect has been found to be negligible. At high temperatures, however, confined crystallization has led to superior ferroelectric properties, compared to samples annealed without confinement. These observations have led to two- or three-layer model for those crystallized thin films with or without confinement, respectively. Further, the transfer-printing of metal as a confining surface has been found to be beneficial, compared to vacuum evaporation, due to deposition-induced damages on organic polymer. This confinement-induced retention of ferroelectricity in P(VDF-TrFE) thin films above its melting temperature can extend processing temperature in organic devices using the ferroelectric polymer, such as non-volatile organic memory devices.     

Effect of film thickness on auto-oxidation in cobalt-catalyzed 1,4-polybutadiene films

Oxygen mass uptake was measured in 1,4-polybutadiene (PB) films undergoing cobalt-catalyzed oxidation in air. Films thicker than approximately 50 μm showed an increase in oxygen uptake per unit polymer mass as film thickness increased, while oxygen uptake per unit film area remained independent of thickness, suggesting that oxidation was heterogeneous and proceeded essentially as an oxidized front penetrating into the film from the surfaces exposed to oxygen. In contrast, oxidation in films thinner than about 28 μm proceeds homogeneously, with oxygen uptake per unit mass being essentially independent of thickness. In oxidized samples, oxygen and nitrogen permeability decreased by more than two orders of magnitude relative to permeability values in unoxidized samples. In thicker films, a two-phase model, based upon high levels of oxidation in a thin skin at the surface of PB and relatively low levels of oxidation in the core of the films, was used to describe gas permeability data and estimate the oxygen and nitrogen permeability in fully oxidized PB.     

可逆失活自由基界面聚合

界面聚合通常指限定在液-液或液-固界面上进行的聚合反应,散见于少数高活性的缩聚反应体系。20世纪90年代,以RAFT聚合、ATRP等为代表的可逆失活自由基聚合（reversible deactivation radical polymerization,RDRP）因其兼具传统自由基聚合和活性阴离子聚合的优点,广泛用于聚合物链结构的可控制备。另一方面,RDRP已被用于构建更为普适的界面聚合反应,基于RDRP的界面聚合已发展成为一种可控制备具有精准纳米结构的功能性聚合物产品的新方法。本文以RAFT液-液界面聚合为主,阐述了RAFT法和ATRP法在液-液界面、液-固界面进行“活性”聚合的反应机理,总结了该领域的研究进展。在此基础上,重点介绍“活性”界面聚合在构建纳米（中空）胶囊、纳米界面工程与纳米分散以及纳米聚合物刷表面等方面的潜在应用前景。     

Surface phase morphology and composition of the casting films of PVDF-PVP blend

Films of a binary polymer blend comprising of hydrophobic poly(vinylidene fluoride) (PVDF) and hydrophilic poly(vinylpyrrolidone) (PVP) have been prepared by solution-casting. The dependence of surface structure and composition of the films on the PVP content in the blend was investigated by using atomic force microscope (AFM), XRD, XPS, SEM and differential scanning calorimeter (DSC). It has been found that the interaction between the two homopolymers prevents PVDF from crystallization in the blend, the net result of which has a primary effect on the surface properties of the films. PVP has a greater concentration at the surface than in the bulk as long as PVDF crystallizes in the bulk during the film formation process, which leaves a thermodynamically non-equilibrium surface state. On the other hand, with an increase in the PVP content, the interaction between segmental PVDF and PVP in the beginning transforms the crystalline state of PVDF from α to γ phase, and finally results in the disappearance of crystalline PVDF phases. A meager crystallization of PVDF segments could still carry on at the surface of a film with a miscible (or an amorphous) bulk; this occurrence makes the surface more hydrophobic than its bulk phase.     

Relative ultimate strain in white pigmented polymers after ageing

Three series of pigmented polymeric films of various pigment volume fractions were prepared. The first series of unsupported films was exposed to ultraviolet radiation and the second to demineralized water, the third series of films being unexposed for comparison. The ultimate tensile properties of all three film series were measured according to ASTM D 2370 standard method. The obtained experimental data follow the equation containing a pigment-polymer interaction parameter based on the Ziegel's concept. In vinyl acetate-butyl acrylate copolymer system, the ultraviolet radiation causes an increase of relative ultimate strain at lower pigment volume fractions, where the interaction parameter diminishes. On the contrary, water provokes a decrease of relative ultimate strain in this system by magnifying the interaction parameter at low pigment concentration. In ethyl acrylate-methyl methacrylate copolymer system, neither ultraviolet radiation nor water affect greatly the relative ultimate strain and the interaction parameter. The results obtained prove good resistance to ageing of this copolymer system, where the thickness of immobilized interfacial polymer region represents a logarithmic function of pigment volume fraction. On the other hand, in poly(vinyl acetate) system both ultraviolet radiation and water diminish the interaction parameter at low pigment concentrations, the thickness of immobilized interfacial polymer region being still the logarithmic function of pigment volume fraction in first approximation. It can be concluded that ageing effects on the relative ultimate strain in white pigmented polymers depend not only upon pigments and their concentration but also upon the nature of polymer matrix.     

Metalizable Polymer Thin Films in Supercritical Carbon Dioxide

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 Å to 1600 Å. 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 Å 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 polymer films. After the XR measurements, the dewetting structures of the PS/Cr films induced by additional annealing were characterized by using atomic force microscopy, showing improved surface morphology in the exposed films. Contact angle measurements showed that a decrease in interfacial tension with exposure to scCO₂ accompanied the increase in interfacial width.     

Dynamic mechanical property and interlaminar shear strength of the carbon fabric reinforced polyetherimide composite based on the polyetherimide resin films

In this study, the laminates reinforced with the concentrated nitric acid or acetone treated carbon fabrics were prepared based on polyetherimide films with different thickness including 30 μm and 50 μm, respectively. 4 Harness Satin (4HS) and 5 Harness Satin (5HS) carbon fabrics were used as the reinforcements of the composite laminates. Three impact factors, including the polyetherimide film thickness, fabric treatment method and fabric type, were considered in this study. Interlaminar shear strength (ILSS) measurement shown that the thicker polyetherimide film (50 μm), 4HS carbon fabric and the nitric acid treatment of the fabric could be used to increase the ILSS value because of the improvement of the interfacial property for the laminate. The 50 μm thickness polyetherimide films used in the laminates improved the storage modulus, and decreased the glass transition temperatures (T _gs) by DMA or DSC. It was because that the better interfacial property and the stronger mobility of the polymer chain under the greater residual internal stress in the laminate was obtained with the increase of the polyetherimide film thickness. The nitric acid treatment of the fabric increased the T _gs measured by DMA and DSC, and decreased the tan δ peak values of the laminates because of the stronger interfacial adhesion between the fiber and the resin and the decreased mobility of the polymer chain. In addition, the effects of above three impact factors on the ILSS, storage modulus, loss modulus, tan δ and the T _g of the laminate were discussed in detail by ILSS, DMA and DSC measurement, respectively. POLYM. COMPOS., 38:663–672, 2017. © 2015 Society of Plastics Engineers     

Relationship between thickness of polymer films and their oxidation on copper substrate

This research was undertaken to understand how the thickness of polyethylene films oxidized on a copper substrate influences the accumulation of carbonyl groups (measured by an IR‐spectroscopy technique) and of metal from the substrate (determined by polarography analysis). It was found that the whole polymer became inhibited by the time the copper stopped transferring into the specimen. Plots of copper concentration versus film thickness have two thickness sections: section I is found between 0 and 70 μm and section II between 80 and 170 μm. Between these two sections the metal concentration varies drastically. This situation can be explained by two schemes by which PE changes to inhibited condition. According to Scheme I (for section I, short oxidation time) this change has only one step: the inhibited layer gradually becomes thicker beginning from the interface and moving toward the outer surface. The second scheme (for section II) shows that the polymer becomes inhibited in two steps. It is typical of thicker films. In this case the oxidation process shifts and localizes in the outer surface because of longer treatment. As a result, transfer of metal and formation of an inhibited layer are interrupted for some time. The metal accumulation in the film only resumes when low‐molecular‐weight products of thermooxidative degradation—formed in the specimen outer surface—enter the region of adhesional contact. A so‐called second transfer stage for metal is realized during which the whole polymer becomes inhibited. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 671–675, 2003     

Actuation and blocking force of stacked nanocarbon polymer actuators

We have developed stacked nanocarbon polymer actuators that are composed of several nanocarbon polymer actuator films using nonwoven fabric as insulation layers. The nonwoven fabric prepared through electrospinning methods has extremely-low-density structures, which do not significantly prevent the motions of each nanocarbon actuator layer. Therefore, stacking several thin nanocarbon polymer actuators using nonwoven fabric as insulation layers is expected to increase generated force without decreasing the displacement of a one-layer actuator. We have prepared stacked actuators with one, two, three, four, and seven layers using this method. The displacement and blocking force of these actuators are measured and compared with those of one-layer actuators of different thicknesses. Displacement is weakly dependent on the thickness of the actuator films of the stacked actuators. On the contrary, it decreases considerably as the thickness of the actuator film of the one-layer actuator increases. In both cases, blocking force is proportional to the thickness of actuator films. We have developed a stacked actuator model based on a trilayer actuator model and confirmed the experimental results using the model.     

Effect of ultrasonication and other processing conditions on the morphology,thermomechanical, and piezoelectric properties of poly(vinylidene difluoride‐trifluoroethylene) copolymer films

This study is carried out on the effect of processing conditions including preparation methods, ultrasonication, film thickness, and thermal annealing on the thermal, mechanical, and piezoelectric properties of copolymer of poly(vinylidene difluoride‐trifluoroethylene) (P(VDF‐TrFE)). Free‐standing films and films on substrates are prepared by solvent casting and spin‐coating, respectively. The results obtained by differential scanning calorimetry show that the Curie transition of the copolymer films was influenced by the preparation methods, with very little difference in the melting transition. The difference was attributed to a less uniform distribution of TrFE comonomer units within the crystalline and amorphous regions of the spin‐coating films. However, no effect of the preparation methods on the piezoelectric properties of the films was observed. It is also found that the short ultrasonication, film thickness higher than critical value of crystallization of P(VDF‐TrFE) (about 100 nm), and drying before annealing did not have a significant effect on the properties of the films. Surprisingly, the ultrasonication had a clear impact on the relaxations at high temperature of the polymer chains. In addition, this study indicates that a short annealing by 10 min at 140°C was enough to obtain well‐crystallized films, which is interesting from the industrial point of view. POLYM. ENG. SCI., 54:1280–1288, 2014. © 2013 Society of Plastics Engineers     

Impact of polymer modulus/chain mobility on water accumulation at polymer/metal oxide interfaces

Moisture is known to accumulate at the interface between polymers and metal oxides, leading to detrimental effects on physical properties such as modulus and adhesion. Direct measurement of the interfacial moisture profile has been carried out with neutron reflectivity, while thickness dependent swelling of a thin film series has also been also utilized to indirectly assess the interfacial moisture content. In this work, the moisture adsorption on the clean surfaces is compared to that observed when the surface is coated with a series of polymer films. The mechanical properties of the polymer appear to impact the quantity of moisture adsorbed at the interface; surprisingly less moisture accumulates at the interface if the coating is rubbery (larger intrinsic mobility of polymer chains). For glassy polymers, the total accumulation at the interface is identical to the adsorption on the clean metal oxide surface. This result potentially provides an understanding of solvent distribution in glassy nanocomposites by measuring moisture adsorption onto the bare filler materials prior to incorporation into the polymer matrix.     

Polymer-polymer interdiffusion during coextrusion

Polymer-polymer interdiffusion during coextrusion is considered theoretically from the point of view of interdiffusion between two melt streams flowing side by side in a film or sheet coextrusion die. It has been found that three factors, namely, molecular weight, the interaction parameter, and polydispersity have a profound influence on the interfacial layer thickness and adhesive bond strength of coextruded films and sheets. It is pointed out that when interdiffusion between two chemically dissimilar polymers takes place In stratified two-phase flow, the chain orientation in the flow channel greatly influences the rate of interdiffusion, and consequently the interfacial layer thickness and adhesive bond strength. Orientation factors for macromolecular chains in a steady shear flow field are expressed in terms of shear stress at the interface and the plateau modulus of each of the respective polymers being coextruded. As an illustration, we have used information on the self-diffusion coefficients for poly(vinylidene fluoride) and poly(methyl methacrylate) in the molten state, to estimate the interfacial layer thickness and adhesive bond strength of coextruded two-layer sheets, as affected by processing variables.     

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.     

Specular X-ray reflectivity analysis of adhesion interface-dependent density profiles in nanometer-scale siloxane-based liquid films

Nanometer-scale thick liquid films of poly(methylhydro-dimethyl)siloxane copolymer (PMDMS) deposited on hydrophilic and hydrophobic solid organic films have been studied using synchrotron X-ray specular reflectivity (XRR). The physico-chemical properties of liquid PMDMS at the interfacial level are controlled by the nature of the solid surface. Detailed analysis of the XRR-data revealed the formation of a low-density region in the liquid PMDMS film in the vicinity of the hydrophobic surface, whereas a densely packed molecular layer is formed at the liquid PMDMS-hydrophilic substrate interface. Non-covalent polymer chains are ‘frozen’ at the solid-liquid interfaces in the confined liquid films and interactions with the substrate surfaces (i.e. hydrogen bonding) are responsible for distinctly different density profiles.     

Aspects of interactions in complex polymer coating formulations

Acid–base interaction parameters have been measured by inverse gas chromatography for mixed stationary phases of film‐forming polymers and pigments. The quantities of adsorbed polymer required fully to coat the pigment surfaces were established, and rheological measurements were used to evaluate the thickness of polymer barriers generated by the adsorption process. Both the barrier thickness and the critical amount of polymer needed to overcoat the pigments were found to be dependent on acid–base interactions. Acid–base considerations also determined the rate of material redistribution when a third component was added to premixtures of two‐component polymer/pigment combinations. Time‐dependent variations in the surface energies of polymer films were attributed to the component redistribution process. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 1378–1386, 2000     

Nanoscale localization of poly(vinylidene fluoride) in the lamellae of thin films of symmetric polystyrene-poly(methyl methacrylate) diblock copolymers

We employed thin film blends of diblock copolymers with functional homopolymers as a simple strategy to incorporate organic functional materials into nanodomains of diblock copolymers without serious synthesis. A blend pair of polystyrene-poly(methyl methacrylate) (PS-PMMA) diblock copolymers and poly(vinylidene fluoride) (PVDF) was selected as a model demonstration because PVDF is a well-known ferroelectric polymer and completely miscible with amorphous PMMA. Thin films of symmetric PS-PMMA copolymers provided the nanometer-sized PMMA lamellae, macroscopically parallel to the substrate, in which PVDF chains were dissolved. Thus, amorphous PVDF chains were effectively confined in the PMMA lamellae of thin film blends. The location of PVDF chains in the PMMA lamellae was investigated by the dependence of the lamellar period on the volume fraction of PVDF, from which we found that PVDF chains were localized in the middle of the PMMA lamellae. After the crystallization of PVDF, however, some of PVDF migrated to the surface of the film and formed small crystallites.