Polymer Film Growth Research Articles

Effect of Cross-Linking on the Structure and Growth of Polymer Films Prepared by Interfacial Polymerization

Interfacial polymerization of tri- and bifunctional monomers (A3B2 polymerization) is investigated by dissipative particle dynamics to reveal an effect of cross-linking on the reaction kinetics and structure of the growing polymer film. Regardless of the comonomer reactivity and miscibility, the kinetics in an initially bilayer melt passes from the reaction to diffusion control. Within the crossover period, branched macromolecules undergo gelation, which drastically changes the scenario of the polymerization process. Comparison with the previously studied linear interfacial polymerization (Berezkin, A. V.; Kudryavtsev, Y. V. Linear Interfacial Polymerization: Theory and Simulations with Dissipative Particle Dynamics J. Chem. Phys. 2014, 141, 194906) shows similar conversion rates but very different product characteristics. Cross-linked polymer films are markedly heterogeneous in density, their average polymerization degree grows with the comonomer miscibility, and end groups are mostly trapped deeply in the film core. Products of linear interfacial polymerization demonstrate opposite trends as they are spontaneously homogenized by a convective flow of macromolecules expelled from the reactive zone to the film periphery, which we call the reactive extrusion effect and which is hampered in branched polymerization. Influence of the comonomer architecture on the polymer film characteristics could be used in various practical applications of interfacial polymerization, such as fabrication of membranes, micro- and nanocapsules and 3D printing.

Solvent-related effects in MAPLE mechanism

To study the role of the solvent and of the laser fluence in the matrix-assisted pulsed laser evaporation (MAPLE) process, we used a soft polymer (polydimethylsiloxane—PDMS) as “sensing surface” and toluene as solvent. Thin films of the PDMS polymer were placed in the position of the growing film, while a frozen toluene target was irradiated with an ArF laser at the conventional fluences used in MAPLE depositions (60–250 mJ/cm2). Apart the absence of solute, the MAPLE typical experimental conditions for the deposition of thin organic layers were tested. The effects on the PDMS films of the toluene target ablation, at different fluences, were studied using atomic force microscopy and contact angles measurements. The results were compared with the effects produced on similar PDMS films by four different treatments (exposure to a drop of the solvent, to saturated toluene vapors and to plasma sources of two different powers). From this comparative study, it appears that depending on the MAPLE experimental conditions: (1) the MAPLE process may be “semidry” rather than purely dry (namely the solvent is likely to be present in the deposition environment near the growing film), (2) the solvent, if sufficiently volatile, is in form of vapor molecules (neutral, ionized and probably dissociated) rather than in liquid phase near the substrate and (3) at relatively high laser fluences (>150 mJ/cm2), the formation of an intense plasma plume results which can damage/affect a soft substrate as well as a growing polymer film.

A study of the initial film growth of PEG-like plasma polymer films via XPS and NEXAFS

The chemistry of substrate–film interface (underside) of di(ethylene glycol) dimethyl ether plasma polymer (DGpp) films has been studied directly and compared to the top layer of the film (topside). By depositing the plasma polymer films onto indium tin oxide (ITO) glass, the films were easily delaminated from the substrate. The top- and underside of the films were examined by X-ray photoelectron spectroscopy (XPS) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. It was found that a rapid increase in pressure during plasma polymerization results in steep chemical gradients in the films, while small pressure changes do not lead to chemical gradient formation. These observations validate the findings of previous neutron reflectometry modeling studies of this class of plasma polymer thin film. In addition, subtle variations in plasma polymer film chemistry were observed between different substrates they were deposited onto. This approach will allow additional studies on the mechanisms of early plasma polymer thin film formation with various monomers.

Gold nanorods as nanotransducers to monitor the growth and swelling of ultrathin polymer films

In this work, we demonstrate that plasmonic nanostructures can be employed as nanoscale transducers to monitor the growth and phase transitions in ultrathin polymer films. In particular, gold nanorods with high refractive index sensitivity (∼150 nm/refractive index unit (RIU)) were employed to probe the growth and swelling of polyelectrolyte multilayers (PEM). By comparing the wavelength shift and extinction intensity increase of the localized surface plasmon resonance (LSPR) of the gold nanorods coated with PEM in air and water, the swelling of PEM was estimated to be 26% ± 6%. The swelling was quantitatively confirmed with independent thickness measurement of PEM in dry and swollen states using AFM. The deployment of shape-controlled metal nanostructures with high refractive index sensitivity represents a novel and facile approach for monitoring the phase transition in polymers with nanoscale resolution.

Surface Morphology in the Early Stages of Plasma Polymer Film Growth from Amine‐Containing Monomers

AbstractThe manner by which plasma polymers grow in the very first stages of deposition is a topic which has been almost overlooked. We show using atomic force microscopy (AFM) and X‐ray photoelectron spectroscopy (XPS) that in the early stages of plasma deposition there are significant differences in the way plasma polymers grow from two amine‐containing compounds onto silicon wafers. By AFM it is shown that films grown from n‐heptylamine (HA) initially show ‘island‐like’ growth before a continuous smooth film is formed. In contrast, films from allylamine grow smoothly from the very earliest stages. XPS data show substantial differences of plasma polymer chemistry in close proximity to the silicon surface manifested in the formation of ${\rm NH}_{{\rm 3}}^{{\rm + }} $ and NOx species which are more abundant in films of HA. We present a possible explanation for these results based upon post‐plasma surface phenomena in the case of HA. magnified image

Scaling of surface roughness and polymer structure in a model for film growth and polymerization

We study a model of growth of polymer films using numerical simulations and scaling concepts. During the deposition, each new monomer flows in a direction perpendicular to the substrate, aggregates at the first contact with the deposit and executes up to G steps along the polymers, propagating an existing chain or nucleating of a new polymer. Some qualitative results agree with those of a previous model for vapor deposition polymerization (VDP) with collective diffusion, such as the roughness increase and density decrease with G . This supports the interpretation of G as a ratio between diffusion coefficient and monomer flux. We perform a systematic study of scaling properties of the outer surface roughness and of polymer size and shape. For large G , the polymers are stretched in the direction perpendicular to the substrate and have typical size increasing as G(1/2). This is explained by the solution of the problem of random walk trapping, which illustrates the connection of surface processes and bulk properties. The distributions of polymer sizes are monotonically decreasing for all G and very broad, thus a large number of small chains and of chains much larger than the average is found in typical samples. The outer surface roughness obeys Kardar-Parisi-Zhang scaling, in contrast to the apparent anomalous scaling of previous VDP models with oblique monomer flux. However, the calculation of reliable exponents requires accounting for huge finite-size corrections. Possible applications and extensions of this model are discussed.

Electrochemical synthesis of polypyrrole layers doped with glutamic ions

AbstractElectrochemically synthesized polypyrrole thin films doped with glutamic ions were investigated as interesting materials for potential use as molecularly selective surfaces. Pyrrole and glutamate interact in aqueous solution, resulting in the formation of a prominent band at 240 nm in the absorption spectra of the solution. This feature was assigned to the formation of a complex, which significantly influences the electrochemical deposition of polypyrrole films doped with glutamate. It was also detected that oxidation of these complexes leads to the appearance of a shoulder in the absorption spectrum at 270 nm, which corresponds to the yellowish color of pyrrole aged under influence of air. A mechanism is proposed for polymer film growth in aqueous solutions containing monosodium L‐glutamate and pyrrole. It takes into account the observed influence of the pH of the solution, the concentration of reagents, the zwitterionic properties of glutamate on deposition, and results from spectroscopic measurements. It is also shown that glutamic ions can electrochemically be released from the synthesized polymer layers. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Growth of thin polymer films containing side-chain azo-dye analyzed by atomic force microscopy

Thin films containing both the azo-dye disperse red 1 (DR1) and the poly(methylmetacrylate) (PMMA) or the poly(phenylenevinylene) (PPV) are deposited by Langmuir–Blodgett and spin coating techniques on various substrates. The morphology, surface structure and growth pattern of the azo-dye-polymers were studied by atomic force microscopy. Nucleation and growth processes are responsible for the generation of different structures. The spin coated films were additionally oriented using the electric field poling method (corona poling). The film homogeneity is improved by this method in all cases. Due to the polar character of DR1 molecules, rod-like sub-structures are observed after the corona poling process in DR1–MMA films. The indium tin oxide surface enhances reorientation of the azo-dye-polymer molecules during corona poling and results in a significant reduction of the surface roughness.

Nano-layer growth of new anthracene polymer film through redox-induced polymerization

Abstract π-Conjugated new anthracene polymer film was prepared in a nanometer level from 9,10-(3,4-ethylenedioxythiophen-2-yl)anthracene [BEDOT-Ant] via direct electrochemical deposition on an ITO electrode. The growth of the polymeric nano thin film could be controlled by the potential cycle numbers. Having electro activity plus optical property, the new polymer film was electroactive and showed photoconductivity. The cyclic voltammogram (CV) for the polymer film showed new redox peaks at 0.9 and 0.5 V (vs. Ag/AgCl). The polymer film with 170 nm in thickness generated photocurrent when exposed to UV light. The polymeric thin film showed reversible photocurrent switching property.

Scaling analysis of the morphology of nanostructured poly(p-xylylene) films synthesized by vapor deposition polymerization

The surface morphology of poly(p-xylylene) thin films prepared by the vapor deposition polymerization technique in the range of polymerization temperatures from −23°C to +35°C was studied by atomic force microscopy. Silicon (100) single crystal wafers and freshly cleaved mica were used as substrates. To analyze the roughness of polymer films, we studied the height-height correlations for pairs of points on the surface as a function of the distance between them using the scaling approximation. An increase in the substrate temperature results in a decrease in the roughness exponent at temperatures higher than the poly(p-xylylene) glass transition temperature, leading to an increase in the surface irregularity. At polymerization temperatures of +20°C to +35°C and a low rate of polymer film growth on Si substrate of ∼0.5–1 nm/min, the roughness exponent α = 0.73 ± 0.02 can be interpreted as a result of bulk diffusion of the monomer. The effect of the nature of the substrate, growth rate of the polymer film, and temperature of the substrate on scaling coefficients of the synthesized films was discovered. An increase in the growth rate of the polymer film results in an increased roughness exponent.

Pulsed laser thin film growth of di-octyl substituted polyfluorene and its co-polymers

Matrix-assisted pulsed laser deposition (PLD) allows a controlled layer-by-layer growth of polymer films. Di-octyl substituted polyfluorene (PF8) and its copolymers were deposited as thin films using matrix-assisted PLD by employing a KrF excimer laser with a fluence of 125 mJ/pulses. The optical and structural properties of these films are compared with spincoated films via Raman spectroscopy, absorption and photoluminescence. The Raman spectra of both PLD and spincoated films are similar indicating that the polymer films deposited via PLD maintain their molecular structure. Both the spincoated and the PLD grown PF8 films that were cast from toluene show the presence of the β phase. Benzothiadiazole substituted PF8 (F8BT) and butyl phenyl-substituted PF8 (PFB) PLD grown films show a slightly broader emission compared to the spincoated films, which is attributed to an enhanced intermolecular interaction in the PLD grown thin films.

In situ transient study of polymer film growth via simultaneous correlation of charge, mass, and ellipsometric measurements

Abstract Using three synchronized, in situ, nonintrusive, real-time characterization techniques to conduct transient observations, we revealed mechanistic details of a polymer film growth. A thin methylene green (MG) polymer coating (of the order of 35 nm) was used as a model system in this electrochemical microgravimetric imaging ellipsometry (EmIE) investigation. The direct correlation of changes in mass (via quartz crystal microbalance, QCM), ellipsometric angles (via imaging ellipsometry) with electrochemical conditions (in cyclic voltammetry, CV) provides discrete temporal and spatial information to help us decipher the underlying steps, from which we were able to separate adsorption, reduction, oxidation, desorption, and polymerization regimes involved in the deposition process. The evidence revealed in this study could have broad impact on the general understanding regarding how a film is deposited onto a metal surface.

The preparation and characterization of poly( o-phenylenediamine)/gold nanoparticles interface for immunoassay by surface plasmon resonance and electrochemistry

The poly- o-phenylenediamine (P oPD) nonconducting film and gold nanoparticles (AuNPs) were combined to fabricate AuNPs/P oPD film, which is used as a novel biocompatible interface for the immobilization of antibody and develop a simple and sensitive label-free immunoassay for the detection of the related antigen (human immunoglobulin G (IgG)). Surface plasmon resonance (SPR) and electrochemical methods were used to provide the real-time information about the polymer film growth, assembling of various sizes of gold nanoparticles, anti-human IgG antibody (anti-hIgG) immobilization and the antigen–antibody interaction. The microstructures of the P oPD and AuNPs/P oPD films were characterized by atomic force microscopy (AFM). These results demonstrated that AuNPs were uniformly dispersed on the porous surface of P oPD film, which formed a nano-structure biocompatible AuNPs/P oPD interface. The use of gold nanoparticles and P oPD film could enhance the immunoassay sensitivity and anti-nonspecific property of the resulting immunoassay electrode. Additionally, the reproducibility and preliminary application of anti-hIgG/AuNPs/P oPD/Au electrode for SPR detection of hIgG was also evaluated.

Visualization of local ethylene polymerization activity on a flat CrO x /SiO2/Si(100) model catalyst

Flat model catalysts give access to fundamental aspects of olefin polymerization over heterogeneous catalysts. They are especially suited for the investigation on the early stages of polymer film growth employing scanning probe and electron microscopy. The polymerization centers are confined to a well defined two dimensional plane that remains constant during the polymerization. Using the Phillips (CrO x /SiO2) catalyst as an example we demonstrate how this approach can be used do visualize the lateral distribution of polymerization activity in the initial stages of polymer growth.

The Effect of Bias Power on the Etching Rate and Uniformity of Silicon Dioxide for N-Slot Inductively Coupled Plasma in TFT Application

The etching in SiO2 is a crucial step in fabricating thin film transistor. For large area etching, high density plasma which independently controls ion energy and ion flux is preferable than conventional RIE etcher for its high etching capability. In an attempt to understand how the bias power of N-slot ICP affects the etching rate and uniformity, we study the correlation among bias power, inductive power, etching rate and uniformity. The results show that the etching rate is proportional to the bias power up to 800 W which has the best uniformity. Beyond that power, the etching rate enters the different slope and the uniformity become worse. This phenomenon might be attributed to the combined effects of resist etching and polymer film growth. For N-slot ICP system, high etching rate and good uniformity can be obtained only when the bias power is in the moderate range.

Molecular Layer Deposition of Nylon 66 Films Examined Using in Situ FTIR Spectroscopy

Molecular layer deposition (MLD) techniques can utilize sequential, self-limiting surface reactions with bifunctional monomers to grow polymer films. In this study, transmission in situ Fourier transform infrared (FTIR) investigations were utilized to monitor the growth of nylon 66 MLD films using adipoyl chloride (AC) and 1,6-hexanediamine (HD) as the bifunctional monomers. In situ FTIR measurements on high surface area SiO2 powders monitored the linear growth of the integrated absorbance of the N−H, C−H, C−N, and CO stretching vibrations and the CO−N−H bending vibrations versus the number AC and HD sequential exposures. The growth of the integrated absorbance for the nylon 66 features was also self-limiting as a function of the individual AC and HD exposures. In addition to the vibrational modes expected for nylon 66, vibrational features were also observed that were consistent with some ammonium chloride salt incorporated in the film during nylon 66 MLD. The temperature dependence of the nylon 66 growth rate for constant exposures revealed that the highest growth rates were observed at the lowest sample temperature of 62 °C. This temperature dependence was consistent with a precursor-mediated adsorption model. Nylon 66 MLD was studied with and without surface functionalization of the SiO2 substrate using (3-aminopropyl)triethoxysilane (APS). The APS functionalization facilitated the reaction of AC with the amine-terminated surface. The growth rate of the nylon 66 MLD films was estimated using in situ transmission FTIR measurements on flat KBr substrates with an amine-terminated Al2O3 ALD overlayer. The integrated absorbance of the C−H vibrational stretches was utilized to estimate the nylon 66 MLD growth rate per AC/HD reaction cycle. The observed growth rates were both above and below the expected growth rate for one nylon 66 bilayer. This range of growth rates illustrates the difficulties involved in using bifunctional monomers for MLD.

In Situ Characterization of Electrochemical Polymerization of Methylene Green on Platinum Electrodes

A novel in situ characterization technique using imaging ellipsometry in conjunction with cyclic voltammetric (CV) deposition has been applied to correlate polymerized methylene green (poly-MG) film thickness and morphology, with charge transferred during CV cycles. We also report values for the refractive index and extinction coefficient of the monomer solution and the poly-MG film, for the first time. These values have been derived from measurements that employ multiple angles of incidence in the ellipsometry and analysis using an optical model based on Fresnel theory. The in situ characterization of film growth with thickness progression provided comprehensive information on the deposition process that could not be obtained using ex situ techniques such as scanning electron microscopy or optical microscopy. The results demonstrate the value of imaging ellipsometry as a quantitative characterization technique to monitor polymer film growth in situ and to characterize the quality of the film in terms of surface morphology and homogeneity as a function of film thickness. Comment is also provided on how the technique can be combined with other characterization techniques to deliver a more comprehensive analysis and control of the electrochemical deposition process.

Ferroelectric Poly(vinylidene fluoride) Thin Films Grown by Low‐Pressure Chemical Vapor Polymerization

AbstractFerroelectric poly(vinylidene fluoride) polymer (VDF2)n thin‐film formation by a low‐pressure chemical vapor polymerization (CVP) process is described. The process involves polymerization of an isotropic vinylidene fluoride monomer condensate layer by cleavage of C double bonds using :CF2 initiators derived from hot‐filament, thermochemical decomposition of the hexafluoropropylene oxide C3F6O. Two different growth modes on the film surface and in the bulk are identified by Fourier transform infrared (FTIR) and X‐ray photoelectron spectroscopy (XPS) studies. Surface impingement of :CF2 induces lateral growth of molecular chains in trans conformation. Due to the initiation of polymerization at random surface sites, amorphous β‐phase growth is also observed. Bulk growth of polymer film involves chain formation dominated by gauche conformations having a predominance of the α‐phase. Ferroelectric investigations of (VDF2)n films show spontaneous polarization and hysteresis effects, which improves on crystallization. Typical polarization values of 3.5 and 1.5 μC cm–2 in the planar and normal directions, respectively, are observed after crystallization. Structural defects in the molecular chains give rise to space charge conduction which affects polarization switching properties. The low‐pressure CVP technique is compatible with sub‐micrometer semiconductor device fabrication and also provides a novel method for incorporating additional functional units into the molecular chains to control the film properties.

Surface-enhanced Raman spectroscopy of the growth of ultra-thin organosilicon plasma polymers on nanoporous Ag/SiO2-bilayer films

A new method for the synthesis of thin bilayer films as surface-enhanced Raman spectroscopy (SERS) active substrates was developed which is based on the combination of plasma polymerization, plasma calcination and Ag-film deposition by means of physical vapor deposition. The surface morphology of prepared substrates was characterized by field emission scanning electron microscopy, atomic force microscopy and electrochemical impedance spectroscopy. These substrates lead to high surface enhancement factors proven by the spectroscopic analysis of adsorbed Trans-1,2 bis-(4-pyridyl) ethylene molecules. By this preparation technique, SERS-active films can be deposited on any substrate. The new SERS substrates were successfully applied to study the growth of ultra-thin hexamethyldisiloxane plasma polymer films. The Raman intensity of the CH-stretching vibration was studied as a function of the film thickness. The surface enhancement decreased sharply at about 20 nm. The resulting increase in the intensity of Raman peaks for thin adsorbed plasma polymer films was observed to be a combination of the electromagnetic enhancement mechanism and the high surface area increase of the rough Ag-surface.

DNA-Accelerated Atom Transfer Radical Polymerization on a Gold Surface

A significantly increased polymer growth rate was observed during a surface-initiated ATRP reaction in the presence of DNA molecules. To investigate this phenomenon, thiolated single-stranded DNA molecules (ssDNAs) with ATRP initiators coupled at the distal point were used as the model molecule in the study. In comparison, a small molecule, HS-(CH(2))(11)NHCOC(CH(3))(2)Br, was used to provide a less-polar surface coating for polymer grafting. 2-Hydroxyethyl methacrylate (HEMA) and monomethoxy-capped oligo(ethylene glycol) methacrylate (OEGMA) were used as the ATRP monomers. The polymer growth rates were monitored by measuring the thickness of the polymer films formed at different times. Our results show that the presence of DNA molecules, although at a less-than-1% surface coverage, significantly accelerated the growth rates of both poly(2-hydroxyethyl methacrylate) (PHEMA) and poly(oligoethylene glycol methacrylate) (POEGMA) at the beginning of the ATRP reactions. This accelerating effect was suspected to be a combined result of the highly charged sugar-phosphate backbones of DNA molecules and the formation of Cu complexes with DNA bases. After the initial polymer growth, a smaller yet constant polymer growth rate was observed, suggesting the reduced influence of DNA molecules as the ATRP reaction centers moved farther away from the surface. Similar to conventional ATRP reactions, the polymer growth from surface-anchored DNA molecules was found to be strongly dependent on the composition and the concentration of the catalysts used. Specifically, a catalyst mixture of CuCl/30% CuBr(2)/bpy with 23 mM CuCl was found to provide the optimal reaction condition to yield the fastest polymer film growth among the conditions tested.