XPS and wettability characterization of modified poly(lactic acid) and poly(lactic/glycolic acid) films

Poly(lactic acid) (PLA) and poly(lactic/glycolic acid) copolymers (PLGA) are biodegradable drug carriers of great importance, although successful pharmaceutical application requires adjustment of the surface properties of the polymeric drug delivery system to be compatible with the biological environment. For that reason, reduction of the original hydrophobicity of the PLA or PLGA surfaces was performed by applying a hydrophilic polymer poly(ethylene oxide) (PEO) with the aim to improve biocompatibility of the original polymer. PEO-containing surfaces were prepared by incorporation of block copolymeric surfactants, poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (Pluronic), into the hydrophobic surface. Films of polymer blends from PLA or PLGA (with lactic/glycolic acid ratios of 75/25 and 50/50) and from Pluronics (PE6800, PE6400, and PE6100) were obtained by the solvent casting method, applying the Pluronics at different concentrations between 1 and 9.1% w/w. Wettability was measured to monitor the change in surface hydrophobicity, while X-ray photoelectron spectroscopy (XPS) was applied to determine the composition and chemical structure of the polymer surface and its change with surface modification. Substantial reduction of surface hydrophobicity was achieved on both the PLA homopolymer and the PLGA copolymers by applying the Pluronics at various concentrations. In accordance with the wettability changes the accumulation of Pluronics in the surface layer was greatly affected by the initial hydrophobicity of the polymer, namely, by the lactide content of the copolymer. The extent of surface modification was also found to be dependent on the type of blended Pluronics. Surface activity of the modifying Pluronic component was interpreted by using the solubility parameters.     

Raman characterization of orientation in poly(lactic acid) films

Poly(lactic acid) is a new biopolymer material which is marketed by Cargill Dow Polymers under the tradename Nature Works*. One major application for this material is biaxially oriented films for food packaging because it possesses excellent barrier for flavor constituents, deadfold and heat sealability. Shrinkage must be minimized when the film is heat sealed for these applications and, therefore, characterization of the orientation of the amorphous phase of PLA films is necessary. Raman spectroscopy methodology has been developed to quantify orientation in PLA films. Bands were assigned to crystalline and amorphous phases of PLA such that orientation in both phases could be monitored. Raman depolarization ratios were used to characterize uniaxial systems but were insufficient for most biaxial draws. A new phenomenon for oriented films involving Raman band shifts was observed in these systems, and was shown to be capable of determining orientation, even for symmetrical biaxially drawn films. The origin of these shifts, as well as their use for the quantification of orientation will be discussed. Further, since the line widths of the bands could be used to quantify crystallinity, both crystallinity and orientation could be determined with one measurement.     

Electrochemical decomposition of layer-by-layer thin films composed of TEMPO-modified poly(acrylic acid) and poly(ethyleneimine)

The decomposition of layer-by-layer (LbL) thin films composed of 2,2,6,6-tetramethylpiperidine-1-oxyl free radical-appended poly(acrylic acid) (TEMPO-PAA) and poly(ethylenimine) (PEI) was studied by using a quartz crystal microbalance (QCM) and cyclic voltammetry. The electrode potential of the (PEI/TEMPO-PAA)₄/PEI film-coated Au resonator was scanned from +0.2 to +0.8 V vs Ag/AgCl. The CV showed that the oxidation peak current decreased as the number of scans increased. The change in the resonance frequency of the QCM increased after electrolysis, indicating that the film was decomposed by electrolysis. The positive charges originating from the oxoammonium ions probably destabilized the (PEI/TEMPO-PAA)₄/PEI film. Furthermore, the release of 5,10,15,20-tetraphenyl-21H,23H-porphine tetrasulfonic acid (TPPS) from TPPS-loaded (PEI/TEMPO-PAA)₄/PEI-coated ITO electrodes was investigated. TPPS was released at electrode potentials greater than +0.6 V by the decomposition of the film. The results suggest that TEMPO-PAA/PEI LbL films are suitable for electrochemically controlled drug delivery systems.     

Surface morphology of poly(cyano-p-xylylene) thin films

The surface morphology of poly(cyano-p-xylylene) thin films of different thicknesses (25–1500 nm or more than 5 μm) that were synthesized by vapor-deposition polymerization on the substrate surface in the temperature range from −22 to +35°C has been studied by atomic force microscopy. The surface topography is quantified through analysis of the height-height correlation function. The surface of all films is characterized by a similar granular morphology with a transverse size of granules of 50–500 nm. The surface morphology changes with the polymerization temperature (the substrate temperature) and the film thickness. The effect of film annealing on its surface morphology is considered. It has been established that annealing at 200°C leads to a change in the surface morphology of the films. Original Russian Text ? A.I. Buzin, D.S. Bartolome, K.A. Mailyan, A.V. Pebalk, S.N. Chvalun, 2006, published in Vysokomolekulyamye Soedineniya, Ser. A, 2006, Vol. 48, No. 9, pp. 1640–1646. This work was supported by the Russian Foundation for Basic Research (project nos. 03-03-32665 and 03-03-32634) and the Russian Science Support Foundation.     

Thermoelectric properties of nanocomposite thin films prepared with poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) and graphene

Carbon nanotubes (CNTs), either single wall carbon nanotubes (SWNTs) or multiwall carbon nanotubes (MWNTs), can improve the thermoelectric properties of poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT?:?PSS), but it requires addition of 30-40 wt% CNTs. We report that the figure of merit (ZT) value of PEDOT?:?PSS thin film for thermoelectric property is increased about 10 times by incorporating 2 wt% of graphene. PEDOT?:?PSS thin films containing 1, 2, 3 wt% graphene are prepared by solution spin coating method. X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy analyses identified the strong π-π interactions which facilitated the dispersion between graphene and PEDOT?:?PSS. The uniformly distributed graphene increased the interfacial area by 2-10 times as compared with CNT based on the same weight. The power factor and ZT value of PEDOT?:?PSS thin film containing 2 wt% graphene was 11.09 μW mK(-2) and 2.1 × 10(-2), respectively. This enhancement arises from the facilitated carrier transfer between PEDOT?:?PSS and graphene as well as the high electron mobility of graphene (200,000 cm(2) V(-1) s(-1)). Furthermore the porous structure of the thin film decreases the thermal conductivity resulting in a high ZT value, which is higher by 20% than that for a PEDOT?:?PSS thin film containing 35 wt% SWNTs.     

Synthesis and characterization of poly(lactic acid) and glucosamine-formaldehyde/dodecylbenzenesulfonate composite films

The composite films of poly(lactic acid) (PLA) doped with glucosamine(Gluc)-formaldehyde(FA) polymer/sodium dodecylbenzenesulfonate (SDBS) complexes at 1–5 wt% were synthesized to demonstrate striking improvement of their structural and mechanical properties. The polymer complexes were obtained by the hydrothermal polymerization of Gluc and FA at a molar ratio of 1:2 in the presence of SDBS. The atomic ratios of S in to N in (=S/N) in the polymer complexes limitedly range from 0.52 to 0.69, indicating that the complexation develops through the nonstoichiometric reaction between groups of (Gluc-FA) polymer and ones of SDBS and 31–48% of the groups remain unbound. The PLA composite film doped with 1 wt% (Gluc-FA)/SDBS showed the elongation-at-break of as large as 194% compared with 37% for PLA film, together with an appreciable increase of the crystallites size (D ₂₀₀) of PLA from 21.8 to 33.3 nm.     

Transcrystallinity of thin films of isotactic poly(propylene)

The isothermal crystallization behavior of thin films of isotactic poly(propylene) between plates coated with chromium, Teflon® or gold is studied by polarized light microscopy. At all contact surfaces between poly(propylene) and the other materials, transcrystallinity can be observed and competes with the growth of entities which originate in the bulk phase. The shape of these entities can be described by two intersecting paraboloids.     

Chemical surface modification of poly(p-xylylene) thin films

Electrophilic aromatic substitution reactions were studied at poly(p-xylylene) (PPX) film surface-reaction medium interfaces. The extent of the reactions (depth of penetration and degree of substitution) was determined by the interaction of the polymer with the reaction solution. Reaction with chlorosulfonic acid to produce sulfonyl chloride and sulfone functionalities occurred readily in the bulk of PPX, and yields were sensitive to time and temperature. Confinement of this reaction to the PPX surface was achieved by controlling the concentration of the acid. Functionalization of PPX with N-methylol-2-chloroacetamide in sulfuric acid to produce the chloroamidomethylated derivative occurred in high yield and was confined to the surface region of PPX. Hydrolysis of the amide to generate aminomethylated PPX was assessed by XPS and a derivatization reaction. Friedel-Crafts type chemistry (acylation and alkylation reactions) also produced functionalized surfaces, but with lower degrees of substitution than the other two reactions and was strictly surface-confined.     

Study of electrochemically deposited thin poly(N-vinylcarbazole) films

In this paper, the influence of the water content in the acetonitrile/LiClO(4) system on the electrosynthesis and the properties of poly(N-vinylcarbazole), PNVC, films is examined. By using conventional resonant frequency and impedance measurements of an electrochemical quartz crystal microbalance (EQCM), information about the electrochemical, morphological and adhesive properties of the thin conducting films were obtained. By changing the water content of the background electrolyte, the degree of cross-linking (through the vinyl group), the doping level and the morphology of PNVC films vary simultaneously. Two limiting cases of film properties were observed: for less than 10 Vol.% water, a highly doped, porous and cross-linked polymer is synthesized. Above 10 Vol.% water content, a dense and smooth film is deposited. The growth at a constant potential was found to be limited by the diffusion of monomers to the electrode. Films grown from a system containing 20 Vol.% water exhibit better adhesive properties to the substrate than those grown from 2 Vol.% water.     

The synthesis and characterization of monodisperse poly(acrylic acid) and poly(methacrylic acid)

A number of polyacrylic (PAA) and polymethacrylic (PMAA) acids have been synthesized by living anionic polymerization of the monomeric tert-butyl esters followed by subsequent hydrolysis of the corresponding polyesters. The necessary precautions were taken in order to assure good molecular weight control, as well as high yields in the polymerization reactions. The intermediate and final polymers were characterized by gel permeation chromatography and NMR-H¹ spectrometry.     

Substitution and elimination reactions of poly(epichlorohydrin) and poly(2-chloroethyl vinyl ether) using phase transfer catalysis

The reactions of poly(epichlorohydrin) (PECH) and poly(2-chloroethyl vinyl ether) (PCEVE) with various reagents were investigated using phase transfer catalyst (PTC) such as tetra-n-butylammonium bromide (TBAB), 18-crown-6 (CR6), and dicyclohexyl-18-crown-6 (DCHC) is a solid—liquid two-phase system. Although the reactions of these polymers hardly occurred without PTC in nonpolar solvents such as toluene and diglyme under mild conditions, the addition of PTC caused the reactions to proceed smoothly under the same conditions. In addition, the reactions of PECH and PCEVE with a strong base such as potassium hydroxide proceeded selectively through β-elimination reaction to produce the polymers with pendant vinyl groups. These results suggested this method is useful for the syntheses of functional polymers. On the other hand, it turned out that quaternary ammonium salts such as TBAB have higher catalytic activity than crown ethers such as CRG and DCHE in these reactions. Furthermore, the catalytic activity of quaternary ammonium salts was strongly influenced by their chain length and the structure of the polymers.     

Protein adhesion on silicon-supported hyperbranched poly(ethylene glycol) and poly(allylamine) thin films

Hyperbranching poly(allylamine) (PAAm) and poly(ethylene glycol) (PEG) on silicon and its effect on protein adhesion was investigated. Hyperbranching involves sequential grafting of polymers on a surface with one of the components having multiple reactive sites. In this research, PAAm provided multiple amines for grafting PEG diacrylate. Current methodologies for generating PEG surfaces include PEG-silane monolayers or polymerized PEG networks. Hyperbranching combines the nanoscale thickness of monolayers with the surface coverage afforded by polymerization. A multistep approach was used to generate the silicon-supported hyperbranched polymers. The silicon wafer surface was initially modified with a vinyl silane followed by oxidation of the terminal vinyl group to present an acid function. Carbodiimide activation of the surface carboxyl group allowed for coupling to PAAm amines to form the first polymer layer. The polymers were hyperbranched by grafting alternating PEG and PAAm layers to the surface using Michael addition chemistry. The alternating polymers were grafted up to six total layers. The substrates remained hydrophilic after each modification. Static contact angles for PAAm (32-44 degrees) and PEG (33-37 degrees) were characteristic of the corresponding individual polymer (30-50 degrees for allylamine, 34-42 degrees for PEG). Roughness values varied from approximately 1 to 8 nm, but had no apparent affect on protein adhesion. Modifications terminating with a PEG layer reduced bovine serum albumin adhesion to the surface by approximately 80% as determined by ELISA and radiolabel binding studies. The hyperbranched PAAm and PEG surfaces described in this paper are nanometer-scale, multilayer films capable of reducing protein adhesion.     

Gelatin/poly(aniline) composite films: Synthesis and characterization

In this work, glyoxal (Glox) - crosslinked gelatin (Gel) films have been loaded with aniline molecules, followed by their in-situ oxidative polymerization to yield Gel/poly(Ani) composite films. The films, so prepared, have been characterized by FTIR, XRD, TGA and AFM analysis. The water absorption of these films has been studied in the physiological fluid of pH 7.4 at 37°C.The dynamic water uptake data has been interpreted by various kinetic models such as power function model and Schott kinetic model. The various diffusion coefficients have also been evaluated.     

Glass transition of thin films of poly(2-vinyl pyridine) and poly(methyl methacrylate): nanocalorimetry measurements

Glass transitions were observed in thin films of poly(2-vinyl pyridine) (P2VP) and poly(methyl methacrylate) (PMMA) using a scanning nanocalorimetry technique which has both high sensitivity (10⁻⁹ J/K) and high scan rates (10⁴-10⁵ K/s). Samples were deposited by the spin-cast method. The thickness of samples was 100-400 nm. Glass transition temperature, obtained by nanocalorimetry, is shifted toward higher temperatures by 10-20 K and activation enthalpy of glass transition is shifted to lower values by factor of 2-4. The glass transition characteristics of both polymers are discussed in terms of the standard Tool-Narayanaswamy-Moynihan (TNM) multi-parameter model.     

Thermal and environmental stability of poly(4-ethynyl-p-xylylene-co-p-xylylene) thin films

The aim of this paper was to test the thermal and environmental stability of poly(4-ethynyl-p-xylyleneco-p-xylylene) thin films prepared by chemical vapor deposition(CVD) and to optimize the reaction conditions of the polymer.Fourier transformed infrared spectroscopy(FTIR),thermogravimetric analysis(TGA) and fluorescence microscopy were employed to investigate the stability of the reactive polymer coatings in various environmental conditions.Chemical reactivity of the thin films were then tested by Huisgen 1,3-dipolar cycloaddition reaction(‘‘click' reaction).The alkyne functional groups on poly(4-ethynyl-p-xylylene-co-p-xylylene) thin films were found to be stable under ambient storage conditions and thermally stable up to 100 8C when annealed at 0.08 Torr in argon.We also optimized the click reaction conditions of azide-functionalized molecules with poly(4-ethynyl-p-xylylene-co-p-xylylene).The best reaction result was achieved,when copper concentration was 0.5 mmol/L,sodium ascorbate concentration to copper concentration was 5:1.In contrast,the azide concentration and temperature had no obvious effect on the surface reaction.     

Voltammetric and waveguide spectroelectrochemical characterization of ultrathin poly(aniline)/poly(acrylic acid) films self-assembled on indium-tin oxide

We report on the spectroelectrochemical characterization of conducting polymer (CP) films, composed of alternating layers of poly(aniline) (PANI) and poly(acrylic acid) (PAA), deposited on ITO-coated, planar glass substrates using layer-by-layer self-assembly. Absorbance changes associated with voltammetrically induced redox changes in ultrathin films composed of only two bilayers (ITO/PANI/PAA/PANI/PAA) were monitored in real time using a unique multiple reflection, broadband attenuated total reflection (ATR) spectrometer. CP films in contact with pH 7 buffer undergo a single oxidation/reduction process, with ca. 12.5% of the aniline centers in the film being oxidized and reduced. The ATR spectra indicate that during an anodic sweep, the leucoemeraldine form of PANI in these films is oxidized to generate both the emeraldine and pernigraniline forms simultaneously. A comparison of the behavior observed during anodic and cathodic sweeps suggests that the rate of oxidation is limited by structural changes in the polymer film originating in electrostatic repulsion between positively charged PANI chains.     

Crystallization of isotactic poly(methylmethacrylate) in monolayers and thin films

Langmuir-Blodgett monolayers of isotactic PMMA exhibit a pressure-induced transition upon compression, that can be described in terms of a two-dimensional crystallization process, analogous to a normal melt crystallization. These water surface crystallized monolayers can be used to prepare highly crystalline thin films of isotactic PMMA with tailor-made orientational characteristics.     

Layer-by-layer assembly of UV-resistant poly(3,4-ethylenedioxythiophene) thin films

The layer-by-layer assembly technique was used to create electrically conductive films with poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) (PEDOT-PSS) and branched polyethylenimine (BPEI). Titanium dioxide (TiO(2)) and carbon black were used to prevent UV-degradation of these PEDOT-PSS thin film assemblies. Film growth and conductivity were studied, while varying composition and examining the effect of UV absorbing particles on the electrical conductivity. All films showed similar initial sheet resistances, but after exposure to 365 nm UV light for 9 days (correlating to approximately 4 years of sunlight), the films containing TiO(2) were up to 250 times more conductive. Additionally, the TiO(2) containing films were 27% more optically transparent than films made with PEDOT in the absence of TiO(2). The addition of colloidal titania allows the useful life of the PEDOT films to be extended without the detrimental effects of decreased transparency. Doping the PEDOT with dimethylsulfoxide produced eight bilayer films that were almost 6 times more conductive. However, the degradation rate for the doped PEDOT films without TiO(2) was 10 times greater than the doped films with TiO(2).     

Properties of chlorine-doped poly(N-vinylcarbazole) thin films

Thin films of poly(N-vinylcarbazole) (PVK) have been obtained by thermal evaporation under vacuum. The chain length of the polymer is shortened by this deposition technique, which induces a strong reactivity between chlorine and the PVK films. After chlorine doping, there is complex salt formation as shown by electron spin resonance spectroscopy (ESR) and X-ray photoelectron spectroscopy (XPS). However, the major part of the chlorine has reacted with PVK. The thermal evaporation induces amorphization of the PVK, while chlorine doping induces polymer degradation with NH₄Cl formation. Because of this degradation the carriers detected by ESR are strongly localized on carbazole radicals, thereby explaining the small increase in the conductivity of PVK films even after chlorine doping.