Photocrosslinking of thin polymer films – materials for sensors and actuators

Polymers with temperature dependent degrees of swelling, especially polymers which exhibit lower critical solution temperature (LCST) behaviour in aqueous solutions, are of interest for applications in microsystems. For these applications it is necessary to form and pattern thin films in the μm-range. This has been achieved through photocrosslinking of linear prepolymers. Copolymers based on N-isopropylacrylamide (NIPAAm) were modified with a stilbazolium salt chromophore to yield photocrosslinkable temperature sensitive polymers. The chromophore reacts via a [2+2]-cycloaddition under irradiation, this can be used to crosslink the polymer. The photocrosslinking properties were studied by UV irradiation of thin films and measuring the changes in UV absorption spectra. Through irradiation of thin films through a mask it was possible to obtain patterned networks in the μm-range (20 μm space width and > 50 μm line width). The polymers exhibited LCST behaviour, which was measured using DSC. The resulting patterned networks had temperature dependent swelling properties in aqueous media.     

Functional PEG-modified thin films for biological detection

We report a general procedure to prepare functional organic thin films for biological assays on oxide surfaces. Silica surfaces were functionalized by self-assembly of an amine-terminated silane film using both vapor- and solution-phase deposition of 3'-aminopropylmethyldiethoxysilane (APMDES). We found that vapor-phase deposition of APMDES under reduced pressure produced the highest quality monolayer films with uniform surface coverage, as determined by atomic force microscopy (AFM), ellipsometry, and contact angle measurements. The amine-terminated films were chemically modified with a mixture of carboxylic acid-terminated poly(ethylene glycol) (PEG) chains of varying functionality. A fraction of the PEG chains (0.1-10 mol %) terminated in biotin, which produced a surface with an affinity toward streptavidin. When used in pseudo-sandwich assays on waveguide platforms for the detection of Bacillus anthracis protective antigen (PA), these functional PEG surfaces significantly reduced nonspecific binding to the waveguide surface while allowing for highly specific binding. Detection of PA was used to validate these films for sensing applications in both buffer and complex media. Ultimately, these results represent a step toward the realization of a robust, reusable, and autonomous biosensor.     

Photoactive additives for cross-linking polymer films: Inhibition of dewetting in thin polymer films

In this report, we describe a versatile photochemical method for cross-linking polymer films and demonstrate that this method can be used to inhibit thin polymer films from dewetting. A bifunctional photoactive molecule featuring two benzophenone chromophores capable of abstracting hydrogen atoms from various donors, including C-H groups, is mixed into PS films. Upon exposure to UV light, the bis-benzophenone molecule cross-links the chains presumably by hydrogen abstraction followed by radical recombination. Photoinduced cross-linking is characterized by infrared spectroscopy and gel permeation chromatography. Optical and atomic force microscopy images show that photocrosslinked polystyrene (PS) thin films resist dewetting when heated above the glass transition temperature or exposed to solvent vapor. PS films are inhibited from dewetting on both solid and liquid substrates. The effectiveness of the method to inhibit dewetting is studied as a function of the ratio of cross-linker to macromolecule, duration of exposure to UV light, film thickness, the driving force for dewetting, and the thermodynamic nature of the substrate.     

Non-fickian diffusion in thin polymer films

Diffusion of penetrants through polymers often does not follow the standard Fickian model. Such anomalous behavior can cause difficulty when designing polymer networks for specific uses. One type of non-Fickian behavior that results is so-called case II diffusion, where Fickian-like fronts initially move like √t with a transition to a non-Fickian concentration profile and front speed for moderate time. A mathematical model is presented that replicates this behavior in thin polymer films, and an analysis is performed that yields relevant dimensionless groups for study. An unusual result is derived: In certain parameter ranges, the concentration profile can change concavity, reflecting Fickian behavior for short times and non-Fickian behavior for moderate times. Asymptotic and numerical results are then obtained to characterize the dependence of such relevant quantities as failure time, front speed, and mass transport on these dimensionless groups. This information can aid in the design of effective polymer protectant films. © 1996 John Wiley & Sons, Inc.     

The dynamics of thin polymer films

An increasing amount of experimental data now supports the idea that the dynamics of thin polymer films is different from bulk. An experimental consensus now supports the previously controversial view that glass transition temperatures of thin polymer films on weakly interacting substrates are reduced from bulk values, but evidence for whether the surface has a higher mobility than the bulk is still contradictory.     

Nonsolvent-induced dewetting of thin polymer films

The process of nonsolvent-induced dewetting of thin polystyrene (PS) films on hydrophilic surfaces at room temperature has been studied by using water as a nonsolvent. It is observed that the process of nonsolvent-induced dewetting is greatly different from other previous dewetting processes. The PS film is found in nonviscous state in our study. A mechanism of nonsolvent-induced dewetting is deduced in an order of penetration, replacement, and coalescent, and it is different from other previous dewetting mechanisms. The results of experiments are analyzed from thermodynamics and dynamics to support the hypothetical mechanism.     

Concentric-ringed structures in polymer thin films

Novel polymer crystalline structures containing micrometer-sized concentric rings (or bands) were observed in thin poly(bisphenol A hexane ether) (BA-C6) films. The origin of the banded structures was found to be different from that of traditional banded spherulites in polymer systems. Analyses based on optical microscopy (OM) and atomic force microscopy (AFM) revealed that the banded structures contained alternating ridge and valley bands of polymer crystals in the flat-on orientation. No lamellar twisting was observed within the concentric-ringed structures, which were developed as a result of the formation of a depletion zone during crystallization. The formation of a depletion zone was determined to be caused by the specific volume decrement between the crystal and the melt and by the diffusion of polymer chains to the fold surfaces of the flat-on lamellae. The height of the ridges and the interband widths could be adjusted by controlling the diffusion rate. Time-of-flight secondary ion mass spectrometry ion images showed higher concentrations of low-molecular-weight polymer chains on the surfaces of the ridges than in the valleys.     

Development of miniaturized pH biosensors based on electrosynthesized polymer films

A new type of pH biosensor was developed for biological applications. This biosensor was fabricated using silicon microsystem technology and consists in two platinum microelectrodes. The first microelectrode was coated by an electrosynthesized polymer and acted as the pH sensitive electrode when the second one was coated by a silver layer and was used as the reference electrode. Then, this potentiometric pH miniaturized biosensor based on electrosynthesized polypyrrole or electrosynthesized linear polyethylenimine films was tested. The potentiometric responses appeared reversible and linear to pH changes in the range from pH 4 to 9. More, the responses were fast (less than 1 min for all sensors), they were stable in time since PPy/PEI films were stable during more than 30 days, and no interference was observed. The influence of the polymer thickness was also studied.     

Modification and characterization of thin polymer films for electrochemical applications

Proton-conducting polymer membranes are utilized as the solid electrolyte in low temperature polymer electrolyte fuel cells (PEFC), which are efficient energy converters. We have selected the process of radiation grafting and subsequent sulfonation to prepare novel membranes because of its feasibility as a low cost production method. Investigations of the two first preparations steps, i.e., irradiation and grafting, lead to insight concerning the optimization of these two steps and the dependence of the final membrane properties on the various preparation parameters.     

Focused laser-induced marangoni dewetting for patterning polymer thin films

Highly-localized focused laser spike (FLaSk) heating of polymer thin films is a resist- and developer-free alternative to two-dimensional (2D) laser direct write for creating patterns on the single micron or, by exploiting overlap effects, submicron scale. The massive temporal and spatial thermal gradients and resulting thermal Marangoni stresses generated by FLaSk are an effective means for the directed dewetting and patterning of such films. Here, the general applicability of this technique to glassy amorphous polymer thin film systems is investigated through systematic investigation of film thickness, glass transition temperature, and polymer mobility. The results reveal that the important parameters are the film thickness (coupled to the optical heating effects through anti-reflection coating effects) and the high-temperature polymer melt mobility, allowing for generation of single features with linewidths of down to ∼1 μm. Further, the introduction of spatial mobility variations by using polymer brushes, bilayers, and microphase separated block copolymers leads to additional profile manipulation effects (i.e. spontaneous 2D pattern generation and flattened top profiles). © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 225–236     

Preparation and surface characterization of polymer nanoparticles designed for incorporation into hybrid materials

We prepared water dispersions of poly(n-butyl methacrylate-st-butyl acrylate) crosslinked core-shell nanoparticles functionalized with different amounts of trimethoxisilane (TMS) groups in the outer shell. The purpose of the TMS groups is to chemically bind the rubbery particles to a nanostructured silica network, using sol-gel copolymerization. Here, we present nanoparticles containing 13 mol % and 30 mol % of TMS groups in the outer shell and compare their surface morphology with particles that do not contain TMS. The particles are prepared by a two-step seeded emulsion polymerization technique at neutral pH. In the first step, we obtained crosslinked seed particles (44 nm in diameter) by a batch process. In the second step, we used a semi-continuous emulsion polymerization technique under starved feed conditions to obtain monodispersed particles of controlled composition and size (ca. 100 nm in diameter). Fluorescence decay measurements were performed in situ on the dispersions, using a pair of cationic dyes adsorbed onto the surface of the nanoparticles: rhodamine 6G as the energy transfer donor and malachite green carbinol hydrochloride as the acceptor. The kinetics of F?rster resonance energy transfer (FRET) between the dyes is sensitive to the donor-acceptor distance, allowing us to obtain the binding distribution of the dyes at the nanoparticle surface. For the unmodified nanoparticles, we found a dye distribution that corresponds to an average interface thickness of delta = (5.2 +/- 0.2) nm. For the samples containing 13 mol % and 30 mol % of TMS groups in the outer shell we obtained broader interfaces, with widths of delta = (6.2 +/- 0.2) nm and delta = (6.5 +/- 0.1) nm respectively. This broadening of the distribution with the surface modification is interpreted in terms of the increase in free volume of the shell caused by the TMS groups. Finally, we studied the effect of temperature on the water-polymer interface fuzziness, in order to evaluate the accessibility of the TMS groups during the sol-gel synthesis of nanostructured hybrid materials.     

Wetting-dewetting transition line in thin polymer films

Thin polymeric films are increasingly being utilized in diverse technological applications, and it is crucial to have a reliable method to characterize the stability of these films against dewetting. The parameter space that influences the dewetting of thin polymer films is wide (molecular mass, temperature, film thickness, substrate interaction) and a combinatorial method of investigation is suitable. We thus construct a combinatorial library of observations for polystyrene (PS) films cast on substrates having orthogonal temperature and surface energy gradients and perform a series of measurements for a range of molecular masses (1800 g/mol < M < 35 000 g/mol) and film thicknesses h (30 nm < h < 40 nm) to explore these primary parameter axes. We were able to obtain a near-universal scaling curve describing a wetting-dewetting transition line for polystyrene films of fixed thickness by introducing reduced temperature and surface energy variables dependent on M. Our observations also indicate that the apparent polymer surface tension gamma(p) becomes appreciably modified in thin polymer films from its bulk counterpart for films thinner than about 100-200 nm, so that bulk gamma(p) measurements cannot be used to estimate the stability of ultrathin films. Both of these observations are potentially fundamental for the control of thin film stability in applications where film dewetting can compromise film function.     

Viscoelastic dewetting of constrained polymer thin films

Thin films of fluids are playing a leading role in countless natural and industrial processes. Here we study the stability and dewetting dynamics of viscoelastic polymer thin films. The dewetting of polystyrene close to the glass transition reveals unexpected features: asymmetric rims collecting the dewetted liquid and logarithmic growth laws that we explain by considering the nonlinear velocity dependence of friction at the fluid/solid interface and by evoking residual stresses within the film. Systematically varying the time so that films were stored below the glass-transition temperature, we studied simultaneously the probability for film rupture and the dewetting dynamics at early stages. Both approaches proved independently the significance of residual stresses arising from the fast solvent evaporation associated with the spin-coating process. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3022–3030, 2006     

Patterning of spontaneous rolling thin polymer films for versatile microcapillaries

We investigate the spontaneous rolling of polydimethylsiloxane (PDMS) thin films and demonstrate the fabrication of capillaries with topographical and chemical patterns on the inner wall. Thin films of PDMS are either coated by a layer of hard material or have their surface hardened by plasma oxidation. They are then driven out of equilibrium by selective solvent swelling in vapor phase resulting in a tubular rolled‐up system. The inner diameter of those is measured as a function of layer thickness for different solvents and capping types. Those results are shown to be in good agreement with Timoshenko theory. Before rolling, the future inner surface can be characterized and functionalized. We demonstrate topographical and chemical patterning, respectively by embossing and microcontact printing. These methods are very simple and can easily produce cylindrical capillaries with inner diameter between 20 and some hundreds of microns with fully functionalized inner surface, overcoming many difficulties encountered in conventional soft lithography techniques. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 721–728     

A model for charge transport in semicrystalline polymer thin films

A model for simulating the charge transport properties of semicrystalline polymer (SCrP) using Monte Carlo simulation is reinvented. The model is validated by reproducing the experimentally observed field and temperature dependence of mobility in Poly(3‐hexylthiophene‐2,5‐diyl) (P3HT) thin films. This study also provides a new physical insight to the origin of much debated negative field dependence of mobility (NFDM) observed at low electric field strengths in P3HT thin films. The observed NFDM, which is not explainable with the mechanisms proposed earlier, is attributed to the weak dependence of transit time on the applied electric field strengths. In the semicrystalline films, the charge transport takes place mostly through the crystalline regions, in which the charge transport is weakly dependent on the strength of the applied electric field. In addition, a possible explanation for the origin of Arrhenius temperature dependence of mobility (lnμ ∝ 1/T) commonly observed in SCrP thin films is also proposed. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 137–141     

Nonlinear optical properties of polymers and thin polymer films

An introduction to nonlinear optics (NLO) and the major challenges in the field of NLO-properties of polymeric materials, is given in this paper. Methods for the investigation of nonlinear optical properties of [4-(2-methacryloxyethyl)methylamino]-4′-cyanoazobenzene copolymers with methyl methacrylate (MMA) are demonstrated: Electric Field Induced Second Harmonic Generation (EFISHG), Hyper Rayleigh Scattering (HRS), Second Harmonic Generation (SHG) in floating monolayers and SHG in poled polymer films. An example of phase-transition analysis by SHG is given.     

Cluster assembled thin films of covalent materials

Cluster assembled thin films of covalent materials are studied by Raman spectroscopy. The main properties are discussed in terms of frustration through several examples (carbon, silicon, and boron nitride). Some applications such as luminescence are discussed.     

Novel optical techniques for the analysis of polymer surfaces and thin films

Summary We report on novel optical techniques, based on evanescent waves, for the characterization of polymer surfaces and thin films. We first describe photo-ablation studies with polysilane films investigated by surface plasmon microscopy, a technique which is particularly well-suited for ultrathin samples. Thicker films that are homogeneous enough to carry optical waveguide modes can be characterized with high lateral resolution by the recently developed waveguide microscopy. We demonstrate this for a thin film of a solid polyelectrolyte. Finally, we report on surface plasmon field-enhanced Raman-spectroscopic and -imaging investigations of ultrathin Langmuir-Blodgett-Kuhn-layers of cadmium arachidate.W. Hickel is now with HOECHST AG, Angewandte Physik, W-6230 Frankfurt 80