Creep behavior of ultra‐thin polymer films

A novel microbubble inflation method has been used to determine the creep compliance of poly(vinyl acetate) and polystyrene ultra‐thin films (13–300 nm thick) at temperatures from below to above the glass temperature. We present results that suggest that time‐temperature and time‐thickness superposition hold in the glassy relaxation regime. Although time‐temperature superposition is found for the entire response curve for each thickness, we also find that time‐thickness superposition fails as the long‐time compliance is approached. This effect occurs because of a strong stiffening as the film thickness decreases. We also show first evidence of stiffening in the glassy regime of free standing films of polystyrene. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1952–1965, 2008     

Surface stress of polydimethylsiloxane networks

For thin elastic films of crosslinked polydimethylsiloxane (PDMS), the tensile modulus was found to be an increasing function of reciprocal thickness over the whole range of elongations. PDMS films between 0.052 and 0.018 mm were investigated. With decreasing film thickness, surface properties may be expected to increasingly contribute to the measured modulus. For small elongations, surface tension is expected to have no effect or to decrease the measured modulus compared with that of a bulk sample. If a surface layer with a modulus greater than that of the bulk modulus is assumed to exist, then the observed increase in modulus with decreasing film thickness can be explained. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 2391–2396, 1997     

Size asymmetry in diblock copolymers of cylindrical morphology in thin films

We calculate the free energy of an AB diblock copolymer thin film of cylindrical morphology under confined geometry and find that the size of the cylinder can be asymmetric, depending on the film thickness and surface tension. The size of the cylinder right above the surface is slightly smaller than that of the other cylinders. The equilibrium period in this thin film is different from that in the bulk because of the surface effect. The tendency toward asymmetry diminishes as the film thickness increases and the interfacial tension between the major block (A) and the substrate decreases. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2217–2224, 2001     

Microdomain morphology of lamella‐forming diblock copolymer confined in a thin film

We report the self‐consistent field theory (SCFT) of the morphology of lamella‐forming diblock copolymer thin films confined in two horizontal symmetrical/asymmetrical surfaces. The morphological dependences of thin films on the polymer‐surface interactions and confinement, such as film thickness and confinement spatial structure, have been systematically investigated. Mechanisms of the morphological transitions can be understood mainly through the polymer‐surface interactions and confinement entropy, in which the plat confinement surface provides a surface‐induced effect. The confinement is expressed in the form of the ratio D/L₀, here D is film thickness, and L₀ is the period of bulk lamellar‐structure. Much richer morphologies and multiple surface‐induced morphological transitions for the lamella‐forming diblock copolymer thin films are observed, which have not been reported before. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1–10, 2009     

Thickness dependence of thermally induced changes in surface and bulk properties of Nafion® nanofilms

Thermally induced changes in surface wettability, dewetting behavior, and proton transport of “self‐assembled” nanothin Nafion^® films (4–300 nm) on SiO₂ substrate is reported. Thermal annealing induces switching of the surface wettability of 55 nm and thinner films from hydrophilic to super‐hydrophobic. Thickness dependence of this behavior is observed with higher annealing temperature required for lower thickness films, indicating highly restrictive mobility of Nafion^® ionomer as film thickness decreases. Dewetting is only observed for 4‐nm thin film. Significant suppression in proton conductivity upon thermal annealing was noted. Similarly, two other bulk properties, water uptake and swelling, were found to decrease upon annealing. This work reports a systematic examination of the thickness dependence of thermally induced changes in both surface and bulk properties of ultra‐thin Nafion^®. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1267–1277     

Streamlined ellipsometry procedure for characterizing physical aging rates of thin polymer films

Existing studies in the research literature showing conflicting changes in physical aging rates with decreasing film thickness in nanoconfined polymer films highlight the need for a single experimental technique to efficiently characterize physical aging rates in thin polymer films of varying chemical structure. To that end, we have developed a streamlined ellipsometry procedure to measure the structural relaxation of thin glassy polymer films. We evaluate different methods of calculating a physical aging rate β from the measured thickness h(t) and index of refraction n(t) data. We present extensive measurements of β as a function of aging temperature and aging time for polystyrene (PS) films supported on silicon, and determine that the physical aging rate β can be easily and reliably determined from β = −1/h₀ dh/d(log t), where h₀ is the initial measure of the film thickness at an aging time of 10 min. We have also carried out oxygen permeation studies on poly(methyl methacrylate) (PMMA) films from 800 μm down to 190 nm in thickness, and find no change in the permeability with film thickness or physical aging at room temperature for up to 65 days, which suggests that gas permeation may be insensitive to physical aging in such low free volume polymers. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2509–2519, 2009     

Viscoelastic properties of ultrathin polycarbonate films by liquid dewetting

A liquid dewetting method for the determination of the viscoelastic properties of ultrathin polymer films has been extended to study thickness effects on the properties of ultrathin polycarbonate (PC) films. PC films with film thicknesses ranging from 4 to 299 nm were placed on glycerol at temperatures from below the macroscopic glass transition temperature (T_g) to above it with the dewetting responses being monitored. It is found that the isothermal creep results for films of the same thickness, but dewetted at different temperatures can be superposed into one master curve, which is consistent with the fact of PC being a thermorheologically simple material. Furthermore, the results show that the T_g of PC thin films is thickness dependent, but the dependence is weaker than the results for freely standing films and similar to literature data for PC films supported on rigid substrates. It was also found that the rubbery plateau region for the PC films stiffens dramatically, but still less than what has been observed for freely standing polycarbonate films. The rubbery stiffening is discussed in terms of a recently reported model that relates macroscopic segmental dynamics with the stiffening. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1559–1566     

Submicron films prepared from aqueous dispersions of nanoscale polymer crystals

Thin and ultrathin films of polyethylene of variable thickness are obtained from aqueous dispersions of prefabricated nanoscale crystals by spin‐coating. Continuous films with a thickness of only 15 nm, up to 220 nm, homogeneous over hundreds of μm, or assembled discontinuous monolayers of flat‐on lamella particles were prepared, depending on the solids content of the dispersion employed, as revealed by AFM and TEM. The morphology of melt‐recrystallized films was not affected by the surfactant present. Homogeneous continuous films without undesirable dewetting were retained upon melting and recrystallization of the films upon cooling, composed of polygonal spherulites for a thicker film (220 nm), randomly grown edge‐on lamella for a 40 nm film, and dominant flat‐on lamella for a 15 nm thick film. Annealing below T_m resulted in lamella thickening, without changes of crystal orientation or structure of the particle assemblies for discontinuous monolayers. Surfactant adsorbed to the nanocrystals in the aqueous dispersion desorbs at least partially during formation of the nascent films, and upon annealing below the melting point surfactant migrates to the film‐air interface to form aggregates, which can be removed by rinsing, during which the film stays intact and structurally unaltered as revealed, amongst others, by water contact angles. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 6420–6432, 2009     

Surface and bulk ordering in thin films of a symmetrical diblock copolymer

Amphiphilic diblock copolymers have the ability to adapt their surface's molecular composition to the hydrophilicity of their environment. In the case of about equal volume fractions of the two polymer blocks, the bulk of these polymers is known to develop a laminar ordering. We report here our investigation of the relationship between bulk ordering and surface morphology/chemical composition in thin films of such an amphiphilic diblock copolymer. Upon annealing in vacuum, the expected lamella ordering in the bulk of the film is observed and we find the morphology of the film surface to be defined by the thickness of the as‐deposited film: If the as‐deposited thickness matches the height of a lamella stack, then the film exhibits a smooth surface. Otherwise, an incomplete lamella forms at the film surface. We show that the coverage of this incomplete layer can be quantified by X‐ray reflectivity. To establish the lamella ordering in the bulk, the film needs to be annealed above the glass temperature of the two blocks. Molecular segregation at the film surface, however, is already occurring at temperatures well below the glass temperature of the two blocks. This indicates that below the glass temperature of the blocks the bulk of the thin film is “frozen,” whereas the polymer chains composing the surface lamella have an increased mobility. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys., 2013 , 51, 1282–1287     

On the extreme depth dependence of the hardness of PDMS rubber: A problem of false surface detection

There is significant interest in nanoindentation of materials yet mixed results for material properties have been reported, including modest depth dependence of the surface stiffness in metals and other crystalline materials and polymer glasses, as well as stiffening of several orders of magnitude in some studies of soft materials such as poly(dimethylsiloxane) (PDMS) rubber. At the same time, there are reports that suggest that the observed extreme stiffening in soft materials might be an artifact, and that such materials at most exhibit only mild stiffening. Unfortunately, a quantitative model of potential artifacts has not been provided. In the present work, we examine the problem of one potential artifact in the testing of soft materials, that of the difficulty of detecting the surface or “true zero” in the indentation test. We provide a quantitative estimate of the effect of error in surface detection on the measured force–displacement curves for the Berkovich tip geometry and find that the observed apparent stiffening is in agreement with our analysis. The significance of the results for testing of soft materials by nanoindentation is discussed. It is also shown that for hard/stiff materials the induced errors are smaller, but may still be significant in some circumstances. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 30–38     

Characterization of thin polymer‐like films formed by plasma polymerization of methylmethacrylate: A neutron reflectivity study

The microstructure of the plasma‐polymerized methylmethacrylate (ppMMA) films is characterized using neutron reflectivity (NR) as a function of the plasma reaction time or film thickness. Variation in the crosslink density normal to the substrate surface is examined by swelling the film with a solvent, d‐nitrobenzene (dNB). In the presence of dNB, uniform swelling is observed throughout the bulk as well as at the air surface, and silicon oxide interfaces. The results indicate that the MMA film prepared by plasma polymerization (ppMMA) has a uniform crosslink density from air surface to substrate surface. Additionally, the scattering length density of the plasma‐polymerized MMA film (SLD ≈ 0.750 × 10⁻⁶ Å⁻²) is much lower than that of a conventional PMMA film (SLD = 1.177 × 10⁻⁶ Å⁻²). The increase in film thickness following dNB sorption is 7.5% and at least 36% for the ppMMA and PMMA films, respectively. This suggests that the films formed by plasma polymerization are different from conventional polymers in chemical structure. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2522–2530, 2004     

Transparent conductive films based on polymer composites containing the mixed‐valence tetrathiafulvalene nanofibers

We report the facile preparation of the conductive polymer composites containing the mixed‐valence tetrathiafulvalene (TTF) nanofibers and their applications as all‐organic transparent conductive materials. TTF can be used as a nanofiller for transforming conventional polymers to conductive materials. Self‐assemble nanofibers of the neutral and radical cation of TTF can be formed in the polymer solutions during the film deposition, and the resulting composite films with several micron thickness can serve as the conductive material with high transparency. Several kinds of conventional polymers, such as polystyrene, poly(methyl methacrylate) (PMMA), and poly(vinylpyrrolidone), can be used as a polymer matrix of the composites. The conductivities of the PMMA film containing 35 mol % of the mixed‐valence TTF and the PEDOT–PSS film showed similar values (2.8 × 10^–2 and 5.4 × 10^–1 S/cm, respectively). In contrast, the normalized transmittance of the PMMA film by 1‐μm thickness greatly increased (96%/μm) when compared with that of the PEDOT–PSS film (10%/μm). In addition, the degradation of the conductivity of the nanofibers by heating and aging was effectively suppressed in the composite samples. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 6441–6450, 2009     

Versatile route for tuning optical properties of poly(2‐methoxy‐5‐(2′‐ethylhexyloxy)‐1,4‐phenylenevinylene)

In this contribution, we report a versatile method for tuning optical properties of poly(2‐methoxy‐5‐(2′‐ethylhexyloxy)‐1,4‐phenylenevinylene) (MEH‐PPV) in its solution with 1,2‐dichloroethane, accomplished by reacting with pyridinium formate (PF), a volatile organic salt. We can systematically control the positions of absorption and photoluminescent (PL) spectra of MEH‐PPV by adjusting the concentration of PF in the solution. The addition of 10 vol % PF caused a blue‐shift in the absorption spectra by about 65 nm. When the concentration of PF decreased to 0.1 vol %, the blue‐shift occurred to a lesser extent, about 25 nm. The measurements of PL spectra showed similar behaviors. The λ_max shifted from 558 nm to 546 and 552 nm when 10 and 0.1 vol % of PF were added, respectively. The changes of PL colors from orange to yellow and green, respectively, were observed by naked eyes. Structural investigation by nuclear magnetic resonance and Fourier‐transformed infrared spectroscopy indicated that the changes of the optical properties were due to chemical modifications along the main chain and the side groups of MEH‐PPV. These results implied a simple route for engineering the HOMO–LUMO energy gap of MEH‐PPV, which could be utilized in advanced applications such as organic light‐emitting devices and solar cells. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 696–705, 2009     

Crystal growth rate in ultrathin films of poly(ethylene oxide)

Many dynamical properties of polymers, including segmental relaxation and chain diffusion, exhibit anomalies in thin‐film samples. We extend the studies of thin‐film dynamics to the case of semicrystalline polymers and present a study of the crystal growth rate for thin films of poly(ethylene oxide). We used optical microscopy and quartz crystal microbalance techniques to characterize the kinetics of crystallization for films with thicknesses from 40 to 1000 nm for a range of temperatures near the melting point. A remarkable slowing down of the crystal growth is observed at all temperatures studied for films with a thickness of less than ～100 nm. The results can be used to suggest reductions of the mobility of chains at the crystal/amorphous interface. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2615–2621, 2001     

Effect of fluorosurfactant on capillary instabilities in nanoimprinted polymer patterns

Surface forces play a paramount role in most aspects of Nanoimprint Lithography. In particular, subjecting nanoimprinted patterns to moderate heating allows surface tension to smooth out undesirable roughness and defects in the patterns, but this “thermal reflow” treatment can induce structural decay or even collapse of the patterns by capillary instability if this process is not carefully controlled. Adhesion between the mold and polymer film can also cause the imprinted structure to tear or fracture. Fluorinated surfactants (FS) are attractive for reducing mold adhesion, yet the effects of these additives on nanostructure stability during thermal reflow are not well understood. Here we present thermal stability studies of line-space grating patterns created by Thermal Embossing Nanoimprint Lithography (TENIL) on model polystyrene (PS) films with FS additives. As expected by energy considerations, FS segregates to the air interface, where it seems to facilitate mold release. This also reduces the surface energy and thus reduces the driving force for pattern “slumping” (height decay). However, the beneficial effects of the surfactant are counterbalanced by the fact that the FS decreases the effective film viscosity, which accelerates nanopattern leveling. The net effect is that the pattern height decay is strongly a function of FS concentration. This enhanced film fluidity in the presence of FS also makes the pattern more susceptible to an undulatory capillary instability under thermal reflow conditions. Surface phase segregation of FS and PS is also observed in conjunction with both slumping and lateral capillary instabilities, which may be useful for producing chemically patterned surfaces. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2591–2600, 2009     

Thickness of the soft layer on glassy polystyrene surfaces

The flow behavior of the near-surface region of polystyrene films was studied via the decay kinetics of imprinted corrugation gratings at elevated temperatures. The speed of the decay is the result of a balance between surface tension, on the one hand, and viscous drag, on the other. Depth profiling is possible because the penetration depth of a surface wave is proportional to its wavelength. From the dependence of the decay rate on the penetration depth one concludes that the surface region displays an increased molecular mobility. The soft surface layer has a thickness of about 20 nm. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3031–3036, 2006     

Apparent depth‐dependent modulus and hardness of polymers by nanoindentation: Investigation of surface detection error and pressure effects

Nanoindentation is a widely used technique to characterize the mechanical properties of polymeric materials at the nanoscale. Extreme surface stiffening has been reported for soft polymers such as poly(dimethylsiloxane) (PDMS) rubber. Our recent work [J. Polym. Sci. Part B Polym. Phys. 2017 , 55, 30–38] provided a quantitative model which demonstrates such extreme stiffening can be associated with experimental artifacts, for example, error in surface detection. In this work, we have further investigated the effect of surface detection error on the determination of mechanical properties by varying the sample modulus, instrument surface detection criterion, and probe geometry. We have examined materials having Young's moduli from ∼2 MPa (PDMS) to 3 GPa (polystyrene) using two different nanoindentation instruments (G200 and TI 950) which implement different surface detection methods. The results show that surface detection error can lead to apparent large stiffening. The errors are lower for the stiffer materials, but can still be significant if care is not taken to establish the range of the surface detection error in a particular experimental situation. We have also examined the effect of pressure beneath the probe on the nanoindentation‐determined modulus of polystyrene with different probe geometries. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 414–428     

Viscoelasticity of nanobubble‐inflated ultrathin polymer films: Justification by the coupling model

The nanobubble inflation method is the only experimental technique that can measure the viscoelastic creep compliance of unsupported ultrathin films of polymers over the glass–rubber transition zone as well as the dependence of the glass transition temperature (T_g) on film thickness. Sizeable reduction of T_g was observed in polystyrene (PS) and bisphenol A polycarbonate by the shift of the creep compliance to shorter times. The dependence of T_g on film thickness is consistent with the published data of free‐standing PS ultrathin films. However, accompanying the shift of the compliance to shorter times, a decrease in the rubbery plateau compliance is observed. The decrease becomes more dramatic in thinner films and at lower temperatures. This anomalous viscoelastic behavior was also observed in poly(vinyl acetate) and poly (n‐butyl methacrylate), but with large variation in the change of either the T_g or the plateau compliance. By now, well established in bulk polymers is the presence of three different viscoelastic mechanisms in the glass–rubber transition zone, namely, the Rouse modes, the sub‐Rouse modes, and the segmental α‐relaxation. Based on the thermorheological complexity of the three mechanisms, the viscoelastic anomaly observed in ultrathin polymer films and its dependence on chemical structure are explained in the framework of the Coupling Model. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013     

Roughness correlation and interdiffusion in thin films of polymer chains

The surface morphology of thin bilayer polymer films on top of glass substrates was investigated. The bilayer consists of a blend film of protonated and deuterated polystyrene and an underlying deuterated polystyrene film. Choosing the thickness of the top film larger than 8 times and smaller than 2 times the radius of gyration of the chains enables the determination of film thickness and confinement effects. With diffuse neutron scattering at grazing incidence in the region of total external reflection, a depth sensitivity and a contrast even at the internal polymer–polymer interface was achieved. The underlying film is conformal to the substrate, and depending on the thickness of the top film two different types of roughness correlations are observed. Thin confined films nestle to the underlying polymer films, while the stiffness of thicker bulky films provides an independent morphology. In both cases, annealing above the glass-transition temperature yields an interdiffusion at the internal polymer–polymer interface, and the polymer–air surface remains essentially unchanged with respect to roughness correlations. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2862–2874, 1999     

The Role of inclusion size in toughening of epoxy resins by spherical micelles

We report our finding of an optimal length scale for toughening of epoxies using spherical micelles formed by block copolymers. The amphiphilic diblock copolymer poly(hexylene oxide)‐poly(ethylene oxide) (PHO‐PEO) with 30 wt % PEO self‐assembled to form spherical micelles in a bisphenol A epoxy resin with a phenol novolac hardener. We systematically increased the size of the spherical micelles from 20–30 nm to 0.5–10 μm by swelling their PHO core using PHO homopolymer. Although all the blends were tougher than the unmodified epoxy, the largest enhancement of fracture resistance was measured in blends containing 0.1–1 μm spherical inclusions. This enhanced toughness was correlated with plastic deformation by shear banding in tensile test and greater roughness of the fracture surface. Smaller micelles neither induced plastic deformation nor contributed to surface roughness significantly whereas larger micelles acted as local defects resulting in early failure. These findings provide a framework in assessing the toughening effects of blended block copolymers on epoxy resins. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1125–1129, 2009