Flat Polymer Research Articles

Preparation of flat and dense conjugated polymer films from dilute solutions by means of electrophoretic deposition

Some experimental results on the electrophoretic deposition of conjugated polymer are reported. The scanning electron micrographs clearly indicate that the the morphology of filmsof a fluorene-based polymer PDOF-MEHPV by the electrophoretic deposition strongly depends on the toluene/acetonitrile ratio of the parent suspensions. It is also shown that the suspension prepared from a 0.1 g/l toluene solution of the polymer, can give flat and dense polymer films with a thickness in the order of 100 nm by using the electrophoretic deposition. The electrical current originating from the movement of colloial particles has been measured for the suspensions containing polymer more than 0.2 g/l. However, the current due to impurities in the solvent makes it difficult to detect the electrical current corresponding to the particle movement in the suspensions containing smaller amount of the polymer. Polymer light-emitting devices having PDOF-MEHPV films with various thickness prepared by electrophoretic deposition have also been demonstrated.

Shrugging Off Grime

Materials Science![Figure][1] Flat (top) and wrinkle-protected samples.CREDIT: EFIMENKO ET AL., ACS APPL. MATER. INTERFACES 1 , 10.1021/AM9000562 (2009) To prevent the accumulation of unwanted microorganisms, plants, and animals on surfaces exposed to a marine environment, coatings are applied to the submerged surface. One challenge in creating such a coating is that the critical length scales involved in organism attachment range from hundreds of nanometers to centimeters. Efimenko et al. have developed polymer coatings that possess a hierarchical wrinkled structure. They stretched and then cross-linked the surface of poly(dimethylsiloxane), after which they applied a fluorinated silane monolayer. On gentle relaxation of the stress, a rippled surface layer formed, wherein each wrinkle had smaller-scale wrinkles on top of it that themselves bore even smaller wrinkles, proceeding over five generations. In seawater tests, flat polymer films showed fouling after a few weeks, whereas the wrinkled polymers resisted barnacle accumulation over a period of 18 months. In tests on the adhesion of green algae zoospores, the wrinkled films performed less well, as the spores could nestle and be protected from shear flows and physical contacts within the wrinkles. However, a combination of topology and the right surface chemistry conferred improved resistance, pointing toward development of a nontoxic universal antifouling coating. ACS Appl. Mater. Interfaces 1 , 10.1021/am9000562 (2009). [1]: pending:yes

Application of Flat Polymer Membrane Microextraction for the Continuous Detection of Trihalomethanes in Aqueous Matrices

A module was developed for the continuous monitoring of volatile halogen organic pollutants in complex matrices. The module utilized a polymer membrane for extraction and a capillary gas chromatography-mass spectrometry (GC-MS) for detection. The extraction of analytes (CHCl(3), CHCl(2)Br, CHClBr(2) and CHBr(3)) from samples was performed in a flow system mediated by a non-porous flat membrane. The effects of the phase-volume ratios, flow conditions, kinetics, analyte properties and matrices were assessed and optimized. The limits of detection were at low microg L(-1) to ng L(-1) levels, with GC-MS analysis under selected ion monitoring (SIM), whereas enrichment factors between 6 and 30 were achieved. Precision values calculated as repeatability and reproducibility with a relative standard deviation (%RSD, n = 5) of 7.2 to 9.5 after 30 min were obtained. The performance of the continuous membrane assisted solvent extraction system was compared with classical liquid-liquid extraction. The method was applied to the extraction of trihalomethanes in chlorinated drinking-water samples.

Tribological behaviour of the 304L/HDPE contact: application for the knee prostheses

A specific wear and friction device was used in this work to analyse the tribological behaviour of the tibial insert used in total knee arthroplasty (TKA). This wear and friction device reproduced the movements of flexion extension (F/E) and anterior posterior translation (A/P). Two sinusoidal movements with frequency 1Hz and amplitudes 8.185° and 5 mm were imposed respectively on a metal cylinder and a flat polymer sample which were in contact under a constant normal load. Each one of these movements was simulated separately. The polymer used was the HDPE (high density polyethylene). Two values of the normal load (Fn=500N and Fn=1000N) were considered. For each one of these two values, tests with different numbers of cycles were carried out. The tribological response of the contact was analysed through the evolution with the number of cycles of the friction coefficient, the loss in mass of the samples and the wear zone with scanning electron micrograph observation. The results indicate that the tribological behaviour of the HDPE depends on the type of the imposed movement.

Effects of sliding kinematics and normal load on the tribological behavior of metal/polymer contact

Wear and friction simulator with metal cylinder on flat polymer was developed to analyze the tribological behavior of tibial insert used in Total Knee Replacement (TKR). For the kinematics, flexion/extension motion (F/E) was applied to the metal cylinder and anterior/posterior translation (A/P) was applied to the flat polymer. Tests were first carried out with polymethylmethacrylate polymer (PMMA) for which the tribological behavior has been well developed. High density polyethylene (HDPE) was also characterized. In fact HDPE has been firstly used in the tibial insert before the use of ultra high molecular weight polyethylene (UHMWPE). Sinusoidal motions were considered for the F/E and A/P with 8.185 and 5 mm amplitude respectively. F/E and A/P motions were generated separately under a constant applied normal load and 1 Hz frequency. For each test condition, the damage of polymer surfaces was analyzed by following the evolution with the cycle numbers of the friction coefficient, the surface roughness and the micrograph of the wear scar. Whatever the sliding kinematics is, the initial average value of the friction coefficient measured is 0.3 for the steel/PMMA contact and 0.07 for the steel/HDPE contact. These values are similar to those presented in the literature. Specific tribological behavior, which present direct dependence on the sliding kinematics, was identified for each steel/polymer contact.

New Investigations on Ferrofluidics: Ferrofluidic Marbles and Magnetic-Field-Driven Drops on Superhydrophobic Surfaces

The motion of ferrofluidic marbles on flat polymer substrates is reported. Nanopowders of polyvinylidene fluoride and gammaFe2O3 were used for the preparation of ferrofluidic marbles. The marbles are activated easily with an external magnetic field. A microfluidic device based on ferrofluidic marbles (the ferrofluidic bearing) is described. Velocities of marbles as high as 25+/-3 cm/s were registered. The sliding of ferrofluidic drops on superhydrophobic surfaces was studied. It was demonstrated that the threshold magnetic force necessary for the drop displacement depends linearly on the drop radius, thus the motion of the drop is defined by the processes occurring in the vicinity of the triple line only.

Local deposition of SiOx plasma polymer films by a miniaturized atmospheric pressure plasma jet (APPJ)

An atmospheric plasma jet (APPJ, 27.17 MHz, Ar with 1% HMDSO) has been studied for the deposition of thin silicon-organic films. Jet geometries are attractive for local surface treatment or for conformal covering of 3D forms, e.g. inner walls of wells, trenches or cavities, because they are not confined by electrodes and their dimensions can be varied from several centimetres down to the sub-millimetre region. Deposition experiments have been performed on flat polymer and glass samples with a deposition rate of 0.25–23 nm s−1. The knowledge of the static deposition profile of the plasma source (footprint) is essential to allow for a controlled deposition with the source moving relative to the substrate. By adjusting the plasma parameters (RF power and gas flow) to the geometry (i.e. electrode configuration, tube diameter, relative tube position, substrate distance) the footprint can be shaped from a ring form reflecting the tube dimension to a parabolic profile. Next to the conventional stochastic mode of operation we observe a characteristic locked mode—reported here for the first time for an RF-APPJ which can improve the film deposition process distinctively. The experimental results of the local film distribution agree well with an analytical model of the deposition kinetics. The film properties have been evaluated (profilometry, XPS, FT-IR spectroscopy and SEM) for different deposition conditions and substrate distance. The FT-IR spectra demonstrate dominating SiO absorption bands, thus providing an indication for the prevailing (inorganic) SiOx character of the films. HMDSO molecules disintegrate to a sufficient degree as proved by the absence of CH2 absorption in the spectra. XPS measurements confirm the local dependence with a slightly increased organic character a few millimetres away from the maximum in the deposition profile. The substrate distance and the source direction both seem relevant and require consideration during coating of 3D objects.

Advances in Molecular Design and Synthesis of Regioregular Polythiophenes

Regioregular poly(3-alkylthiophene)s (rrP3ATs) are an important class of pi-conjugated polymers that can be used in plastic electronic devices such as solar cells and field-effect transistors. rrP3ATs can be ordered in three dimensions: conformational ordering along the backbone, pi-stacking of flat polymer chains, and lamellar stacking between chains. All of these features lead to the excellent electrical properties of these materials. Creative molecular design and advanced synthesis are critical in controlling the properties of the materials as well as their device performance. This Account reports the advances in molecular design of new functional polythiophenes as well as the associated polymerization methods. Many functionalized regioregular polythiophenes have been designed and synthesized and show fascinating properties such as high conductivity, mobility, chemosensitivity, liquid crystallinity, or chirality. The methods for the synthesis of rrP3ATs are also applicable to other functional side chains. Di- and triblock copolymers consisting of rrP3AT and polyacrylate or polystyrene have also been successfully synthesized, which can facilitate the assembly of the polythiophene segments. The synthesis of rrP3ATs has evolved into a simple and economical system in which the synthesis can be carried out quickly at room temperature and is thus suitable for large-scale manufacturing. Intensive study has revealed that the regioregular polymerization of 3-alkylthiophenes proceeds by a chain-growth mechanism and can be made into a living system. This feature enables precise control of the molecular weight and facile end-group functionalization of the polymer chains, leading to tailor-made regioregular polythiophenes for specific applications. In addition, researchers have recently designed and synthesized regiosymmetric polythiophenesthese are regioregular but not coupled in a head-to-tail fashionby various methods. These reports indicate that these regiosymmetric polymers show very high mobilities when used in field-effect transistors due to their highly ordered structure. The remarkable performance of regioregular polythiophenes in recent years has allowed for the rapid development in printable electronics and seems destined to lead to further advances in this field.

Interactions between polymer brushes in solvents of variable quality: A density functional theory study

We present a density functional theory study of interactions between sterically stabilized colloidal particles in solvents of variable quality. Both flat and spherical polymer brushes are considered, as well as both monatomic and polymeric solvents. It is shown that the interaction between sterically stabilized particles can be tuned from repulsive to attractive by varying the solvent quality, the relative length of free and grafted chains, and by employing a mixed brush consisting of both well and poorly solvated chains.

Preparation of Poly(acrylic acid) Nano‐films by In‐situ Polymerization of Acrylic Acid Macroclusters on Silicon Oxide Surfaces

AbstractA new method for preparing poly(acrylic acid) (PAA) films on silicon oxide surfaces with smooth morphology has been developed. Acrylic acid (AA) was preferably adsorbed on silicon oxide surfaces in AA/ chloroform binary liquids and formed a hydrogen‐bonded organized structure, which was called molecular macrocluster. AA macroclusters on silicon oxide surfaces were in‐situ polymerized to obtain molecularly flat polymer films with thickness up to 10 nm. In‐situ polymerizations were conducted by photo‐irradiation in the presence of a photo initiator, 2,2‐dimethoxy‐2‐phenylacetophenone (DPA). As a reference, the adsorption of PAA polymerized in the bulk solution was examined on silicon oxide surfaces. A series of techniques such as attenuated total reflection–FTIR (ATR‐FTIR) spectroscopy, ellipsometry and atomic force microscopy (AFM) was utilized for characterizing two types of films. It was found that flat PAA films with linear hydrogen‐bonded COOH could only be obtained by in‐situ polymerization, which demonstrated this method was an effective way for preparing molecularly uniform polymer films. The surface morphology and thickness of obtained PAA films were found to be dependent on the monomer concentration, initiator amount and photoirradiation time. Molecularly uniform and flat PAA films were obtained after 5 min irradiation at 0.8 mol% AA in the presence of 5 wt% DPA.

Amphiphilic Diblock Copolymers on Mica: Formation of Flat Polymer Nanoislands and Evolution to Protruding Surface Micelles

We show that the deposition of poly(isoprene-b-ethylene oxide) block copolymer (PI−PEO) micelles on freshly cleaved mica produced ultraflat, polymer nanoislands of well-defined monomolecular thickness. The islands were partly adsorbed and partly floating on the substrate, and their morphology changed over time (under ambient conditions). Their structural evolution was recorded and analyzed using atomic force microscopy. Larger islands evolved to high globular aggregates while smaller islands behaved in distinctly different manner becoming shorter; in both cases, the final structures had a surface micelle shape and organization. The time-dependent behavior originates from a decrease of the amount of water adsorbed to the mica surface, due to the fact that freshly cleaved mica under ambient conditions changes its surface character from highly hydrophilic to less hydrophilic with increasing exposure time. The combination and competition of strong affinity of the PEO block with the adsorbed water and flexibil...

Determination of wettability of surface‐modified hot‐embossed polycarbonate wafers used in microfluidic device fabrication via XPS and ToF‐SIMS

AbstractThe wettability of the surfaces inside the microchannels of a microfluidic device is an important property considering a liquid flows through them. Contact angle measurements usually applied to test the wettability of surfaces cannot be used for an analysis of microchannel walls within microfluidic devices. A workaround is the use of surface analytical methods, which are able to reach points of interest in microchannels and may provide information on the surface chemistry established there. In calibrating these methods by using flat polymer wafers, where the contact angle can be measured as usual, data measured in real microchannels can be evaluated in terms of wetting properties. Reference wafers of bisphenol‐A polycarbonate, a polymeric material that is often used in fluidic microdevice fabrication, were treated under different oxygen plasma conditions. The modified surfaces were characterized by using XPS, time of flight (ToF)‐SIMS and atomic force microscope (AFM). Surface chemistry and surface topography have been correlated with contact angle measurements. In addition, effects of ageing or rinsing after plasma treatment have also been investigated. Copyright © 2008 John Wiley & Sons, Ltd.

Moulding technique demonstrates the contribution of surface geometry to the super-hydrophobic properties of the surface of a water strider

Water striders (Insecta, Heteroptera, Gerridae) have a complex three-dimensional waterproof hairy cover which renders them super-hydrophobic. This paper experimentally demonstrates for the first time the mechanism of the super-hydrophobicity of the cuticle of water striders. The complex two-level microstructure of the surface, including the smallest microtrichia (200–300 nm wide, 7–9 μm long), was successfully replicated using a two-step moulding technique. The mould surface exhibited super-hydrophobic properties similar to the original insect surface. The average water contact angle (CA) of the mould was 164.7°, whereas the CA of the flat polymer was about 92°. These results show that (i) in water striders, the topography of the surface plays a dominant role in super-hydrophobicity, (ii) very low surface energy bulk material (typically smaller than 0.020 N m −1) is not necessary to achieve super-hydrophobicity; and (3) the two-step moulding technique may be used to mimic quite complex biological functional surfaces.

Formation of a Flat Conductive Polymer Film Using Template-Stripped Gold (TSG) Surface and Surface-Graft Polymerization for Scanning Multiprobe Data Storage

A thin conductive polymer recording medium with angstrom-scale surface roughness has been formed on template-stripped gold (TSG) surface by surface-graft polymerization for scanning multiprobe data storage. A gold film is deposited on a mica plate, which is bonded onto the silicon substrate with a gold film using gold-gold direct bonding technique. By removing the mica plate, gold surface with the angstrom-scale surface roughness is formed. A conductive polymer film as a recording medium is deposited on the TSG surface by the surface-graft polymerization. As a result, the polymer film with small roughness is obtained. Thus, the use of the TSG surface is efficient way to form the flat conductive polymer film. Finally, reversible electrical modification on the polymer film is demonstrated using atomic force microscopy. These results show the possibility that this formation method of the conductive polymer film provides rewritable recording media with small roughness for scanning multiprobe high-density data storage. [DOI: 10.1380/ejssnt.2008.202]

Tungsten atomic layer deposition on polymers

Tungsten (W) atomic layer deposition (ALD) was investigated on a variety of polymer films and polymer particles. These polymers included polyethylene, polyvinyl chloride, polystyrene, polycarbonate, polypropylene and polymethylmethacrylate. The W ALD was performed at 80 °C using WF 6 and Si 2H 6 as the gas phase reactants. W ALD on flat polymer films can eventually nucleate and grow after more than 60 AB cycles. X-ray photoelectron spectroscopy studies of W ALD on polystyrene after 50 AB cycles suggested that tungsten nanoclusters are present in the W ALD nucleation regime. The W ALD nucleation is greatly facilitated by a few cycles of Al 2O 3 ALD. W ALD films were grown at 80 °C on spin-coated polymers on silicon wafers after 10 AB cycles of Al 2O 3 ALD. The W ALD film was observed to grow linearly with a growth rate of 3.9 Å per AB cycle on the polymer films treated with the Al 2O 3 ALD seed layer. The W ALD films displayed an excellent, mirror-like optical reflectivity. The resistivity was 100–400 µΩ cm for W ALD films with thicknesses from 95–845 Å. W ALD was also observed on polymer particles after W ALD in a rotary reactor. Without the Al 2O 3 ALD seed layer, the nucleation of W ALD directly on the polymer particles at 80 °C required > 50 AB cycles. In contrast, the polymer particles treated with only 5 AB cycles of Al 2O 3 ALD were observed to blacken after 25 AB cycles of W ALD. W ALD on polymers may have applications for flexible optical mirrors, electromagnetic interference shielding and gas diffusion barriers.

Fabrication and Characterization of Plasma Processed Surfaces with Tuned Wettability

Engineered surfaces with controlled hydrophilic/ hydrophobic character have been fabricated by tailoring the substrate topography and chemistry. In this method, the substrate to be treated was first coated by a photoresist, which was then surface-roughened using SF6 plasma etching. The resulting rough texture was then transferred to the underlying silicon surface by over-etching of the photoresist. At this point, the topographically modified surface was modified chemically by controlled deposition of a thin polymer layer using plasma processing. In this way, both the surface texture and the surface chemistry could be varied independently, producing surfaces with variable wetting character, including super-hydrophilicity and super-hydrophobicity, depending on the choice of plasma polymer deposited. Chemical characterization demonstrates a correlation between the surface chemistry and the wettability of the samples after etching. The surface elementary composition contained more C-F groups as the measured contact angle increased, indicating that the change of wettability is due to both the roughness and the surface energy of the deposited photoresist. In the case of materials deposited on the plasma-treated rough surfaces, the strengthening of the wetting character is only due to the created surface roughness, as XPS analyses showed no significant chemical difference as compared to the flat polymer.

Improvement of Light Extraction From Patterned Polymer Encapsulated GaN-Based Flip-Chip Light-Emitting Diodes by Imprinting

To improve surface light extraction of GaN-based flip-chip light-emitting diodes (FC-LEDs), we employed an imprint approach of thermosetting polymer for patterning microscale surface grating on the polymer encapsulant. One-dimensional (1-D) and two-dimensional (2-D) taper-like polymer gratings with a period of 6 mum were successfully realized on encapsulant above the sapphire backplane of GaN LED. By adopting the 1-D and 2-D taper-like grating encapsulant, the improvement of light extraction from the 1 mm times 1 mm FC-blue LED with a reflective Ag film on the p-side was about 18.5% and 31.9% compared to the LED encapsulated by flat polymer, respectively. To evaluate the concept of a diffraction grating in enhancement of light extraction, we performed a simulation of diffraction based on 1-D rigorous coupled wave analysis with the supporting experiments.

Insects did it first: a micropatterned adhesive tape for robotic applications

Based on the structural and experimental studies of more than 300 insect species from different lineages, we have developed and characterized a bioinspired polymer material with the ability of multiple glue-free bonding and debonding. The material surface is covered with a pattern of microstructures, which resembles the geometry of tenent hairs previously described from the feet of flies, beetles, earwigs and other insects. The tape with such a microstructure pattern demonstrates at least two times higher pull-off force per unit apparent contact area compared to the flat polymer. Additionally, the tape is less sensitive to contamination by dust particles than a commercially available pressure-sensitive adhesive tape. Even if the ‘insect tape’ is contaminated, it can be washed with a soap solution in water, in order to completely recover its adhesive properties. We have successfully applied the tape to the 120 g wall-climbing robot Mini-Whegs™. Furthermore, the tape can be used for multiple adhering of objects to glass surfaces or as a protective tape for sensitive glass surfaces of optical quality. Another area of potential applications is gripping and manipulation of objects with smooth surfaces.

An energetic criterion to compare the evolution of thermally-excited surface disturbances and nanoindentation-induced defects on thin polymer films

A criterion is developed to predict the resulting evolution process of the following surface defects on thin (17 nm) polystyrene (PS) films on silicon (Si): (i) nanoindentation-induced indents which grow after being heated above the glass transition temperature of PS, T g, leading to dewetting; (ii) nanoindentation-induced indents which level at temperatures above the T g, resulting in a flat polymer surface and (iii) indents which are formed and grow spontaneously by thermal treatment above the T g (thermal film break up). The criterion is based on the concept of the excess surface energy, Δ F γ , which was introduced in previous reports for cases (i) and (ii). Here, a similar energetic term is used which corresponds only to the effect of the depressions, Δ F γ(D) . The effect of the rims which surround the depressions in cases (i) and (ii) is not taken into account. Measurements of Δ F γ(D) , performed by atomic force microscopy, prior to any treatment above the T g suggest that growing depressions (cases i and iii) correspond to Δ F γ(D) > 1.5 × 10 −16 J while for healing depressions (case ii) Δ F γ(D) < 1.8 × 10 −16 J. A critical region of Δ F γ(D) exists from 1.5 × 10 −16 J to 1.8 × 10 −16 J. Depressions which correspond to this, rather short, region can either grow or heal.

Is the Kramers-Kronig causal relationship between ultrasonic attenuation and dispersion maintained when phase aberrations distort the field incident on a phase sensitive aperture?

The objective of this investigation was to determine whether the causality-induced link between ultrasonic attenuation and dispersion remains valid when the measured signal loss arises as a consequence of phase cancellation at the face of a phase-sensitive receiver rather than from intrinsic losses within the medium under study. The frequency-dependent apparent attenuation and phase velocity were obtained from through-transmission measurements of two flat and parallel plastic polymer plates, PlexiglasTM and LexanTM, exhibiting an approximately linear with frequency attenuation coefficient and logarithmic with frequency phase velocity. Phase distortion was achieved by machining a step into one side of each plastic plate. Through-transmission measurements were performed in a water tank using 5 MHz center frequency single element planar transmitting and receiving transducers. The causal (Kramers-Kronig) link between apparent phase velocity and the apparent attenuation coefficient in the presence of phase distortion was examined for both plastics over a bandwidth ranging from 3 to 7 MHz. Results demonstrate that the Kramers-Kronig link between the apparent attenuation coefficient and apparent phase velocity dispersion remains causally consistent even in the presence of aberrating media.