Free-standing Polymer Films Research Articles

Ultrathin Free-Standing Polymer Films Deposited onto Patterned Ionic Liquids and Silicone Oil

We studied the vapor deposition of polymers onto the surfaces of silicone oil and imidazolium-based ionic liquids (ILs). We found that the deposition of poly(2-hydroxyethyl methacrylate) (PHEMA) and poly(N-isopropylacrylamide) (PNIPAAm) resulted in polymer particles on silicone oil whereas continuous polymer skins formed on 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]), 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]), and 1-ethyl-3-methylimidazolium tetrafluoroborate ([emim][BF4]). The silicone oil and ILs were patterned onto a common substrate by exploiting their different wetting properties. Ultrathin free-standing PHEMA and PNIPAAm films of different shapes were produced by confining the shape of the IL within a wax barrier, surrounding it with silicone oil, and then depositing the polymer. The silicone oil prevented the polymer film from connecting to the underlying substrate and maintained the shape of the polymer film during deposition. Our process allows for multidimensional control over the resulting free-standing film: the area of the shape can be controlled by patterning the IL, and the thickness of the film can be controlled by adjusting the duration of polymer deposition. The films are highly pure and do not contain any residual monomer or solvent entrapment which extends their potential applications to include in vivo biomedical research.

Two Simultaneous Mechanisms Causing Glass Transition Temperature Reductions in High Molecular Weight Freestanding Polymer Films as Measured by Transmission Ellipsometry

We study the glass transition in confined polymer films and present the first experimental evidence indicating that two separate mechanisms can act simultaneously on the film to propagate enhanced mobility from the free surface into the material. Using transmission ellipsometry, we have measured the thermal expansion of ultrathin, high molecular-weight (MW), freestanding polystyrene films over an extended temperature range. For two different MWs, we observed two distinct reduced glass transition temperatures (T(g)'s), separated by up to 60 K, within single films with thicknesses h less than 70 nm. The lower transition follows the expected MW dependent, linear T(g)(h) behavior previously seen in high MW freestanding films. We also observe a much stronger upper transition with no MW dependence that exhibits the same T(g)(h) dependence as supported and low MW freestanding polymer films.

Fabrication of free-standing conductive polymer films through dynamic three-phase interline electropolymerization

Poly(3,4-ethylenedioxythiophene) (PEDOT) and polypyrrole (PPy) free-standing conductive polymer films are prepared through a dynamic solid/liquid (oil)/liquid (water) three phase interline electropolymerization (D3PIE) process. The oil phase (dichloromethane) contains only hydrophobic monomer (3,4-ethylenedioxythiophene or pyrrole), while the aqueous phase contains only dopant electrolytes. A platinum wire is immersed vertically across the O/W interface and is acted as working electrode. The electropolymerization starts from the three phase interline junction of water, dichloromethane and Pt wire and expands along the O/W interface so that a free-standing film forms. Furthermore, both the PEDOT film and PPy film exhibit heterogeneous microstructure because of this unique electropolymerization process. The novel D3PIE method proves to be applicable to the preparation of a variety of free-standing conductive polymer films with controllable size and microstructure.

Nanoengineered Films via Surface‐Confined Continuous Assembly of Polymers

A highly generalizable, surface-confined, continuous polymer assembly process, amenable to various substrates, reaction conditions and macromolecules, enables the synthesis of a range of nanoscale supported and freestanding cross-linked polymer films. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Molecular Motion in Free-Standing Thin Films of Poly(methyl methacrylate), Poly(4-tert-butylstyrene), Poly(α-methylstyrene), and Poly(2-vinylpyridine)

A photobleaching technique has been used to measure the reorientation of dilute fluorescent probes in free-standing polymer films from 7 nm to 20 μm thick. Similar to our previous report on free-standing polystyrene films, two subsets of probes with distinct dynamics were observed in free-standing poly(methyl methacrylate) films. The more mobile subset of probes is consistently interpreted in terms of a surface layer, and the thickness of the mobile surface layer is shown to increase with temperature. Poly(methyl methacrylate) has a mobile surface layer thickness of 4 nm at the bulk Tg. Similar measurements on free-standing poly(2-vinylpyridine) and poly(4-tert-butylstyrene) films indicate mobile surface layer thicknesses from 7 to 10 nm at their bulk Tg values, similar to the 7 nm value previously reported for polystyrene. In contrast to the other four polymers, free-standing poly(α-methylstyrene) films show no evidence of a mobile surface layer. We interpret these results in terms of contributions from ...

Vapor-Phase Free Radical Polymerization in the Presence of an Ionic Liquid

An ionic liquid (IL) was introduced into a vapor-phase free radical polymerization process for the first time. The deposition of poly(2-hydroxyethyl methacrylate) (PHEMA) and poly(1H,1H,2H,2H-perfluorodecyl acrylate) (PPFDA) was studied in the presence of 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) droplets. The polymerization occurred either at the vapor−IL interface or within the IL depending on reaction conditions such as the duration of deposition, the stage temperature, and the monomer solubility. A variety of polymeric architectures such as polymer skins that completely encapsulated the droplet, free-standing polymer, and polymer films that float freely on the surface of the IL were formed. The results from this study will facilitate the design of new polymer−IL composite materials for use in fuel cell and battery applications.

A Multiswitchable Poly(terthiophene) Bearing a Spiropyran Functionality: Understanding Photo- and Electrochemical Control

An electroactive nitrospiropyran-substituted polyterthiophene, poly(2-(3,3''-dimethylindoline-6'-nitrobenzospiropyranyl)ethyl 4,4''-didecyloxy-2,2':5',2''-terthiophene-3'-acetate), has been synthesized for the first time. The spiropyran, incorporated into the polymer backbone by covalent attachment to the alkoxyterthiophene monomer units, leads to multiple colored states as a result of both photochemical and electrochemical isomerization of the spiropyran moiety to merocyanine forms as well as electrochemical oxidation of the polyterthiophene backbone and the merocyanine substituents. While electrochemical polymerization of the terthiophene monomer can take place without oxidation of the spiropyran, increasing the oxidation potential leads to complex electrochemistry that clearly involves this substituent. To understand this complex behavior, the first detailed electrochemical study of the oxidation of the precursor spiropyran, 1-(2-hydroxyethyl)-3,3-dimethylindoline-6'-nitrobenzospiropyran, was undertaken, showing that, in solution, an irreversible electrochemical oxidation of the spiropyran occurs leading to reversible redox behavior of at least two merocyanine isomers. With these insights, an extensive electrochemical and spectroelectrochemical study of the nitrospiropyran-substituted polyterthiophene films reveals an initial irreversible electrochemical oxidative ring-opening of the spiropyran to oxidized merocyanine. Subsequent reduction and cyclic voltammetry of the resulting nitromerocyanine-substituted polyterthiophene film gives rise to the formation of both merocyanine π-dimers or oligomers and π-radical cation dimers, between polymer chains. Although merocyanine formation is not electrochemically reversible, the spiropyran can be photochemically regenerated, through irradiation with visible light. Subsequent electrochemical oxidation of the nitrospiropyran-substituted polymer reduces the efficiency of the spiropyran to merocyanine isomerization, providing electrochemical control over the polymer properties. SEM and AFM images support the conclusion that the bulky spiropyran substituent is electrochemically isomerized to the planar merocyanine moiety, affording a smoother polymer film. The conductivity of the freestanding polymer film was found to be 0.4 S cm(-1).

Electrical field directed electropolymerization of free-standing film of polypyrrole and poly(1-(2-carboxyethyl)pyrrole at the air/liquid interface

Thin flexible films of conductive polymers are useful in biosensors and flexible batteries. These films are typically prepared through electropolymerization on the surface of electrode and are subsequently peeled off. The present work reports a novel approach of synthesizing free-standing conductive polymer films of polypyrrole and poly(pyrrole-co-(1-(2-carboxyethyl)pyrrole)) at the air/liquid interface under the control of electrical field. The films grew along the direction of the electrical field and showed unique morphology at the air-facing side of the film. SEM, XPS, ATR-FTIR, WAXD and cyclic voltammetry were used to characterize the films. A mechanism of film growth was proposed with the important experimental parameters discussed. This approach may be used for other types of conducting polymers and also for the monomers sensible to electrical field.

Effect of various parameters on the conductivity of free standing electrosynthesized polypyrrole films

Electrical characteristics of polypyrrole films electrodeposited in different aqueous electrolyte solutions including p-toluenesulfonate, naphtalenesulfonate, nitrate, tetrafluoroborate, and perchlorate anions were investigated using the Van der Pauw procedure. The polymer films were synthesized by electrochemical oxidation at a fixed potential. Experimental parameters including the pyrrole concentration, electrolyte, applied potential and substrate were shown to affect the electrical conductivity σ of polypyrrole films. Since the substrate contributes significantly to the overall conductivity of polypyrrole-coated electrodes, the results obtained with free standing polymer films appeared more reliable. The results indicated that the p-toluenesulfonate doped PPy film showed the highest average conductivity ( σ 293 K = 4.5 × 10 5 S m −1) whereas the perchlorate doped one produced the lowest of all the films prepared ( σ 293 K = 2 × 10 4 S m −1).

Effect of Pd Nanoparticles on Thermal Degradation Kinetics of iPP/Pd Nanocomposite

Palladium (Pd) nanoparticles were incorporated into free-standing polymer films, where isotactic polypropylene (iPP) was used, by a one-step dry process involving simultaneous vaporization, absorption and reduction schemes of palladium(II) bis(acetylacetonate), Pd(acac)2 at 180oC, used as a precursor. iPP film was exposed to the sublimed Pd(acac)2 vapor in a glass vessel with nitrogen atmosphere heated at 180oC. The exposing time was 30 min and the Pd nanoparticle contents in polymer films were estimated from ash contents in a sample of about 5 mg by pyrolysis of the films at 800 oC for 1 h in an electric furnace of the TGA apparatus under dry argon atmosphere. The sensitivity of the TGA apparatus was 0.2 mg, and thus the minimum content to be measured was 0.004 wt% of a 5 mg sample. The reduced Pd nanoparticles were observed by transmission electron microscope (TEM), and it was found that metal nanoparticles were selectively loaded into the amorphous regions between the lamellae of crystalline polymers having higher melting temperatures than the processing temperature (180 oC). In order to measure the thermal degradation rate, TGA data measured by the heating rates of 5, 10, 15 and 20 oC /min at the nitrogen atmosphere of 200 ml/min. The TGA data was introduced to the Ozawa equation and the degradation activation energy was calculated according to the degradation ratio.

Measurement of Oxygen Diffusivity and Permeability in Polymers Using Fluorescence Microscopy

A simple fluorescence microscopy technique is developed and presented to investigate heterogeneities in emission intensity and quenching responses of luminescence sensors and to measure diffusion and permeation coefficients of oxygen in polymers. Most luminescence oxygen sensors do not follow linearity of the Stern-Volmer (SV) equation due to heterogeneity of luminophore in the polymer matrix. To circumvent this limitation, inverted fluorescence microscopy is utilized in this work to investigate the SV response of the sensors at the micron scale. It was found that intensity is higher in regions where the luminophore is aggregated, but the response is poorer to oxygen concentration. In contrast, the nearly homogeneous regions exhibit linearity with high SV constants. In these diffusion experiments, oxygen concentration was measured by luminescence changes in regions with high SV constants and good linearity. Two diffusion experiments were performed-termed film-on-sensor and accumulation-in-volume techniques. A new Fick's law based quasi-steady-state diffusion model was developed and combined with the SV equation to obtain effective permeation coefficients for the accumulation-in-volume technique. Using these experimental techniques, oxygen diffusion properties in free-standing Teflon polymer films, cast silicon elastomers, and cast polydimethylsiloxane films containing different weight percentages of zeolite were determined with good precision.

Synthesis of a Photoresponsive Liquid‐Crystalline Polymer Containing Azobenzene

The synthesis of an oriented liquid-crystalline photoresponsive polymer, prepared by polymerization of mono- and di-acrylates, both of which contain azobenzene chromophores, is reported. The prepared free-standing polymer film shows strong reversible photoinduced deformation upon exposure to unpolarized UV light at 366 nm, as a result of an optically induced isomeric change of the azobenzene moieties in the polymer network. The synthesis process is relatively simple and more efficient compared to conventional ones, and can be used to synthesize other liquid-crystalline photoresponsive polymers. The use of this photoresponsive polymer film as an optical high-pass/low-pass switch under UV or natural light irradiation for a laser beam is demonstrated. This photoresponsive polymer may have applications in robotic systems, artificial muscles, and actuators in microelectromechanical systems (MEMS) and labs on chips.

Molecular dynamics simulations of concentrated polymer solutions in thin film geometry. II. Solvent evaporation near the glass transition

We perform molecular dynamics simulations of a coarse-grained model of a polymer-solvent mixture to study solvent evaporation from supported and freestanding polymer films near the bulk glass transition temperature T(g). We find that the evaporation process is characterized by three time (t) regimes: An early regime where the initially large surplus of solvent at the film-vapor interface evaporates and the film thickness h varies little with t, an intermediate regime where h decreases strongly, and a final regime where h slowly converges toward the asymptotic value of the dry film. In the intermediate regime the decrease of h goes along with an increase of the monomer density at the retracting interface. This polymer-rich "crust" is a nonequilibrium effect caused by the fast evaporation rate in our simulation. The interfacial excess of polymer gradually vanishes as the film approaches the dry state. In the intermediate and final time regimes it is possible to describe the simulation data for h(t) and the solvent density profile phi(L)(y,t) by the numerical solution of a one-dimensional diffusion model depending only on the y direction perpendicular to the interface. The key parameter of this model is the mutual diffusion coefficient D(L) of the solvent in the film. Above T(g) we find that a constant D(L) allows to describe the simulation data, whereas near T(g) agreement between simulation and modeling can only be obtained if the diffusion coefficient depends on y through two factors: A factor describing the slowing down of the dynamics with decreasing solvent concentration phi(L)(y,t) and a factor parametrizing the smooth gradient toward enhanced dynamics as the film-vapor interface is approached.

Low bending loss metal waveguide embedded in a free-standing multilayered polymer film

Very low vertical bending loss is demonstrated in a flexible metal waveguide. The waveguide consists of an 8 nm-thick and 68 mm-long Ag strip embedded in a free-standing multilayered low-loss polymer film. The polymer film is composed of a 10 microm-thick inner cladding with a refractive index of 1.524, and a pair of 20 microm-thick outer claddings which both have a refractive index of 1.514, resulting in a total thickness of 50 microm. The measured vertical bending loss is lower than 0.3 dB/180 masculine at a wavelength of 1310 nm for the bending radii down to 2 mm.

A tensile test technique for the freestanding PMMA thin films

Abstract A simple tensile test technique for thin freestanding polymer films was introduced. The freestanding polymer film specimens were prepared by spin coating of PMMA solutions onto silicon substrates, removing the solvent, and then releasing the film safely by means of distilled water. Preliminary tensile tests were successfully done for the PMMA films made from 4 wt% concentration of solutions of two different molecular weights M ¯ w = 350,000 and 996,000 g/mol with thicknesses of 1.46 and 1.72 μm, respectively. The stress–strain behavior under the strain rates of 10, 100, and 1000 μe/s exhibited that Young’s modulus, yield strength, and tensile strength increased linearly as a function of the logarithm of strain rate. The effect of molecular weight on the mechanical properties was discussed. The tensile test method proposed in this study was proven to be simple, reliable, and easy to use.

Multi-scale mechanical characterization of a freestanding polymer film using indentation

Abstract A thin circular film is clamped at the periphery. An external mechanical force is applied to the film center via either a shaft with a spherical cap or a ball bearing with a specific weight. The deformed geometry allows the materials parameters of the film to be assessed for a wide range of length scales. Solid mechanics models are derived for a multi-scale deformation range as the film experiences (i) indentation or Hertzian contact, (ii) plate-bending, (iii) membrane-stretching, and (iv) large deformation. The method caters to ultrathin membranes where conventional macroscopic tensile tests might potentially damage the sample films. Thin film adhesion-delamination can also be characterized by the same set-up. Experiments were conducted for a range of polymer films.

Annihilation characteristics of positrons in free-standing thin metal and polymer films

Annihilation characteristics of positrons and positronium (Ps) in thin metal and polymer films were studied. Monoenergetic positrons were implanted into free-standing thin W and Au films and the annihilation γ-rays of positron–electron pairs were measured as a function of the incident energy of positrons. At the front-side surfaces of the films, an emission of Ps into vacuum and a resultant self-annihilation of ortho-Ps (o-Ps) were observed. At the backside surfaces, the Ps emission was found to be enhanced by an increase in the numbers of epithermal positrons and/or secondary electrons introduced by the impact of energetic positrons. For thin polymer films (polyester and polystyrene), the emission rate of o-Ps from the backside surfaces was higher than that from the metal films, which was attributed to the out-diffusion of o-Ps formed in the films. Those results suggested that the emission rate of Ps into vacuum was sensitive to the Ps formation process in the bulk and at the surface.

Electrochemical actuation properties of a novel solution-processable polythiophene

A solution-processable polythiophene, poly(( E)-4,4″-didecoxy-3′-styryl[2,2′:5′,2′′]terthiophene) demonstrated large electrochemically induced strain up to 11.5%. Free-standing polymer films were characterised using four-point probe conductivity measurements, cyclic voltammetry and electrochemical actuation measurements. Conductivities of ∼6 × 10 −5 S/cm (reduced state) or ∼1–2 S/cm (oxidised state) were measured. Well-defined polymer oxidation–reduction responses were observed in both the propylene carbonate and acetonitrile electrolytes, with electrochemical efficiency of >80% observed under ideal conditions. Results obtained suggested that the actuation strain approximately correlates with the size of the anion (i.e. TFSI > PF 6 − > ClO 4 −) used in the electrolyte. The largest strain ∼11.5% was obtained in an electrolyte solution consisting of 0.1 M Li.TFSI in acetonitrile. The maximum strain attainable increased with an increase in the anodic potential applied and decreased with an increase in stimulation frequency or increasing mechanical load. Such functionalised polythiophene material has the combined advantage of solution processability and the ability to produce large strain.

Optical power limiting in the femtosecond regime by silver nanoparticle–embedded polymer film

A simple methodology is developed for the fabrication of freestanding polymer films with embedded silver nanoparticles grown in situ. Strong nonlinear absorption, positive nonlinear refraction, and efficient optical limiting in the femtosecond regime are demonstrated with these films. Additional investigations with supported films as well as parallel studies in the nanosecond regime are also presented. The freestanding nature of the films is not only of potential interest from the application perspective but also facilitates the unambiguous determination of the nonlinear coefficients of the metal nanoparticles embedded in polymer matrix without complications arising due to the contributions from the substrate.

Slow dynamics and glass transition in simulated free-standing polymer films: a possiblerelation between global and local glass transition temperatures

We employ molecular dynamics simulations to explore the influence that the surface of afree-standing polymer film exerts on its structural relaxation when the film is cooledtoward the glass transition. Our simulations are concerned with the features of acoarse-grained bead-spring model in a temperature regime above the critical temperatureTc of mode-coupling theory. We find that the film dynamics is spatially heterogeneous. Monomersat the free surface relax much faster than they would in the bulk at the same temperatureT. Thefast relaxation of the surface layer continuously turns into bulk-like relaxation with increasing distancey from the surface. This crossover remains smooth for allT, butits range grows on cooling. We show that it is possible to associate a gradient in critical temperaturesTc(y) with the gradient in the relaxation dynamics. This finding is in qualitativeagreement with experimental results on supported polystyrene (PS) films(Ellison and Torkelson 2003 Nat. Mater. 2 695). Furthermore we show that they dependenceof Tc(y) can be expressed in terms of the depression ofTc(h)—the globalTc for a film of thicknessh—if we assume thatTc(h) is the arithmetic meanof Tc(y) and parameterizethe depression of Tc(h) by Tc(h) = Tc/(1+h0/h), a formula suggested by Herminghaus et al (2001 Eur. Phys. J. E 5 531) for the reduction ofthe glass transition temperature in supported PS films. We demonstrate the validity of thisformula by comparing our simulation results to results from other simulations andexperiments.