Role of nanoclay in determining microfibrillar morphology development in PP/PBT blend nanocomposite fibers

In this work, a new and highly efficient method of surface-initiated free radical graft-polymerizations on the surfaces of silica gel particles was put forward, and the graft-polymerization of methacrylic acid (MAA) was conducted. This method was convenient, feasible and highly effective. Coupling agent ??-mercaptopropyltrimethoxysilane(MPTS) was first bonded onto the surfaces of silica gel particles, obtaining the modified particles MPTS-SiO₂, onto which mercapto groups were chemically attached, so a redox initiation system of graft-polymerization was constituted by the mercapto group on the surfaces of MPTS-SiO₂ particles and the cerium (IV) salt in the solution. And then the surface-initiated free radical graft-polymerization of MAA on the surfaces of silica gel particles was carried out, resulting in the grafted particles PMAA/SiO₂ with a very high grafting density (35?g/100?g) of PMAA. The grafted particles PMAA/SiO₂ were characterized by infrared spectrum (FTIR), scanning electron microscope (SEM) and thermogravimetric analysis (TGA). The effects of the main factors on the new surface-initiated graft polymerization were emphatically examined, and the corresponding mechanism of the graft-polymerization was investigated in depth. The experimental results show that the mercapto group-cerium salt system analogous to the hydroxyl group-cerium salt system, can also effectively initiate vinyl monomers to be graft-polymerized on the surfaces of solid particles, and furthermore, it is a highly effective surface-initiated graft-polymerization method. In this graft-polymerization system, several factors such as sulfuric acid concentration, the used amount of cerium salt and the reaction temperature affect the grafting density greatly. For the graft-polymerization of MAA, the appropriate reaction conditions are as follows: reaction time of 3?h, reaction temperature of 50?°C, cerium concentration of 5.0?×?10^?3?M, acid (H⁺ ion) concentration of 0.15?M and MAA concentration of 0.5?M.     

非水介质中表面引发接枝聚合法制备接枝微粒PMAA/SiO2及其对阿魏酸的氢键吸附性能

以偶联剂γ-巯丙基三甲氧基硅烷(MPMS)对微米级硅胶微粒进行表面改性,制得表面键合巯基的改性微粒MPMS-SiO2. 在非水溶剂二甲基甲酰胺(DMF)中使偶氮二异丁腈(AIBN)与MPMS-SiO2表面的巯基(-SH)构成表面引发体系(-SH/AIBN),实现甲基丙烯酸(MAA)在非水介质中的表面引发接枝聚合,制得接枝度为20 g/100 g的接枝微粒PMAA/SiO2,对其进行了表征,考察了主要因素对MAA表面引发接枝聚合的影响规律,研究了PMAA/SiO2对酚酸化合物阿魏酸的氢键吸附作用. 结果表明,-SH/AIBN可顺利引发MAA在非水介质中的接枝聚合,适宜条件为75℃,AIBN用量为单体质量的1.5%. 在PMAA/SiO2与阿魏酸分子之间可产生强氢键作用,包括高强度的多位点常规氢键与p型氢键,导致PMAA/SiO2对阿魏酸有强吸附力,吸附容量达266 mg/g. 质子性溶剂甲醇中的溶剂竞争吸附作用使阿魏酸的吸附容量降低,升高温度吸附容量降低.     

Graft polymerization of vinyl acetate onto silica

The free‐radical graft polymerization of vinyl acetate onto nonporous silica particles was studied experimentally. The grafting procedure consisted of surface activation with vinyltrimethoxysilane, followed by free‐radical graft polymerization of vinyl acetate in ethyl acetate with 2,2′‐azobis(2,4‐dimethylpentanenitrile) initiator. Initial monomer concentration was varied from 10 to 40% by volume and the reaction was spanned from 50 to 70°C. The resulting grafted polymer, which was stable over a wide range of pH levels, consisted of polymer chains that are terminally and covalently bonded to the silica substrate. The experimental polymerization rate order, with respect to monomer concentration, ranged from 1.61 to 2.00, consistent with the kinetic order for the high polymerization regime. The corresponding rate order for polymer grafting varied from 1.24 to 1.43. The polymer graft yield increased with both initial monomer concentration and reaction temperature, and the polymer‐grafted surface became more hydrophobic with increasing polymer graft yield. The present study suggests that a denser grafted polymer phase of shorter chains was created upon increasing temperature. On the other hand, both polymer chain length and polymer graft density increased with initial monomer concentration. Atomic force microscopy–determined topology of the polymer‐grafted surface revealed a distribution of surface clusters and surface elevations consistent with the expected broad molecular‐weight distribution for free‐radical polymerization. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 300–310, 2003     

Hydrophilic and adhesive properties of methacrylic acid-grafted polyethylene plates

Methacrylic acid (MAA) was grafted onto polyethylene (PE) surfaces by simultaneous irradiation with UV rays in the liquid phase to estimate hydrophilic and mechanical properties for MAA-grafted PE plates. The amount of grafted MAA increased sigmoidally with UV irradiation time, and the higher grafted amount was obtained at higher monomer concentrations. With an increase in grafting of MAA, the wettabilities from the contact angles of water were enhanced and the refractive indices from the ellipsometry decreased. Though the contact angles remained constant above the grafted amount of 0.02 mmol/cm², the refractive indices approached the value of PMAA around 0.009 mmol/cm², indicating that the PE surfaces were fully covered with grafted PMAA chains. Then, at a fixed grafted amount, the grafted layer can absorb more water and the grafted PE plates possessed higher tensile shear adhesive strength, in case grafting was carried out at lower monomer concentrations. Surface properties depended on the density of carboxyl group at the surfaces of grafted layers, whereas adhesive properties depended on the structural properties of grafted chains as well as on the density of carboxyl group of the whole grafted layers.     

Designing and preparing of quercetin surface‐imprinted material and its molecular recognition characteristics

Quercetin is an important compound of flavonoids. In this work, quercetin molecule surface‐imprinted material with high performance was prepared using a novel surface‐imprinting technique of “synchronously graft‐polymerizing and imprinting.” The modified micron‐sized silica gel particles containing amino groups were used as matrix, methacrylic acid (MAA) was used as functional monomer, and N,N′‐Methylenebisacrylamide (MBA) was used as crosslinker. In dimethyl formamide solution of quercetin, MAA molecules arranged automatically around the template quercetin molecule by right of hydrogen bonding interactions of two type, ordinary hydrogen bond and π‐type hydrogen bond. By initiating the surface‐initiating system of – , the graft/cross‐linking polymerization of MAA on SiO₂ particles and the quercetin molecule surface‐imprinting were simultaneously carried out, forming quercetin molecule surface‐imprinted material MIP‐PMAA/SiO₂. With another two flavonoids, rutin and genistein, as contrasting substances, the molecule recognition character of the quercetin molecule surface‐imprinted material MIP‐PMAA/SiO₂ was investigated with batch and column methods. The experimental results show that the imprinted material MIP‐PMAA/SiO₂ possesses special recognition selectivity and excellent binding affinity for quercetin molecule. The binding capacity of MIP‐PMAA/SiO₂ for quercetin is 0.325 mmol/g, and its selectivity coefficients for quercetin relative to rutin and genistein are 7.69 and 4.40, respectively. The main conditions of imprinting process affect the property of MIP‐PMAA/SiO₂ greatly, and the optimal molar ratio of monomer MAA to crosslinker MBA is 7 : 1 and appropriate molar ratio of monomer MAA to template quercetin is equal to 6 : 1. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41112.     

Synthesis of syndiotactic poly(methacrylic acid) by free‐radical polymerization of the pseudo‐divinyl monomer formed with methacrylic acid and catechol

Syndiotactic poly(methacrylic acid) (Syn‐PMAA, r diad = 91 mol %) was synthesized by free radical polymerization of methacrylic acid (MAA) with catechol. The pseudo‐divinyl monomer was formed with one catechol and two MAA molecules by the hydrogen bonding between OH groups of catechol and COOH group of MAA. When the free radical polymerization of the pseudo‐divinyl monomer was carried out, intra‐ and intermolecular addition proceeded with racemic addition. The hydrogen bonding was the driving force to control tacticity. We discussed the effects of solvent, temperature, and the concentrations of MAA and catechol on the pseudo‐divinyl monomer formation. The highly syndiotactic PMAA was successfully obtained. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Synthesis of lactose‐containing glycopolymer‐grafted silica gel particles

The surface free‐radical graft polymerization of 2‐O‐meth‐acryloyloxyethoxyl‐(2,3,4,6‐tetra‐O‐acetyl‐β‐D ‐galactopyranosyl)‐(1‐4)‐2,3,6‐tri‐O‐acetyl‐β‐D ‐glucopyranoside onto silica gel particles has been carried out with azobis (isobutyronitrile) as initiator. The grafting reaction conditions and the glycopolymer‐grafted silica gel particles have been investigated in detail. Chromatographic experiments have been attempted on glycopolymer‐modified silica gel particles as a stationary phase under normal phase conditions, and it was found that a certain separation effect of the quercetin and its derivant isorhamnetin was achieved. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Photografting of methacrylic acid onto hydroxyapatite particles surfaces

The unsaturated functional groups (C?C) were introduced onto hydroxypatite (HAP) particles surfaces by esterification reaction between hydroxyl groups on the HAP surface and methacrylic acid (MAA). Then, graft polymerization of MAA onto the esterified HAP particles surfaces was carried out by ultraviolet light (UV) initiation. The structure of the surface‐esterified and grafted HAP was studied by ESCA, FTIR, Scanning Electron microscopy, laser particle sizes analyze, esterification, and graft degrees tests. The results show that poly(methyl acrylic acid) (PMAA) is successfully inserted onto HAP particles surfaces by covalent bond linkage. The esterification degree between MAA and HAP particles increases with reaction temperature and time, and tends to level off at about 6 h under 90°C. The graft degree of MAA onto HAP particles with 1.14% esterification degree and its average particle sizes increase with UV initiation polymerization time, and to reach a maximum at 25 min under 40°C, then decrease as irradiation time further increases. The surfaces of the grafted HAP particles become rough and loose. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

A novel method of surface-initiate atom transfer radical polymerization of styrene from silica nanoparticles for preparation of monodispersed core-shell hybrid nanospheres

A new kind of initiator, 3-(2-bromo-2-methylacryloxy)propyltriethysiliane (MPTS-Br), was prepared with a simply hydrobrominated commercial silane coupling agent (3-methacryloxy-proplytriethysilane, MPTS). It has been one-step self-assemble onto the surface of silica nanoparticles, and by using this initiator-modified nanoparticle (SiO₂-MPTS-Br) as macroinitiator for atom transfer radical polymerization (ATRP). Structurally well-defined homopolymer polystyrene (PS) and block polymer poly(styrene-b-methyl methacrylate) (PS-b-PMMA) chains were grown from the nanoparticles surface to yield individual particles composed of silica core and thick-coated polymer shell. The graft parameters could be calculated from the elemental analysis (EA) results, and linear plots of percentage of grafting (PG%) and conversion of monomer (C%) versus polymerizing time were achieved, respectively. Narrow molecular weight distribution (M_w/M_n) for the graft polymer samples were characterized by the gel permeation chromatography (GPC). The graft polymerizations exhibited the characteristics of the controlled/“living” polymerization. The glass transition temperature (T_g) of SiO₂-g-PS after polymerizing time of 24 h was found about 133 °C which was different from the polymer not grafted on the silica at 102 °C by the differential scanning calorimetry (DSC) analysis. The products were also characterized by FT-IR, XPS and TEM. The robustness and simplicity of this method may make large-scale manufacture of these polymer-coated nanospheres possible.     

Block copolymer grafted-silica particles: a core/double shell hybrid inorganic/organic material

Hybrid inorganic/organic materials consisting of a poly(n-butyl acrylate)-b-poly(styrene) diblock copolymer anchored to silica particles were synthesized via ‘grafting from’ technique using a controlled/living free radical polymerization named stable free radical polymerization. XPS and FTIR analysis were used to control the effectiveness of the chemical modification of the silica particles. Thermal characterizations were performed by thermal gravimetric analysis (TGA) and by differential scattering calorimetry (DSC). The TGA permitted the determination of the quantity of grafted polymer and thus the grafting density; DSC was used to study the influence of the silica and blocks of the copolymer on their thermal behaviors. The glass transition temperature of the grafted copolymers was compared to these of free polymers or copolymers homologues.     

Preparation and characterization of polystyrene grafted magnesium hydroxide nanoparticles via surface-initiated atom transfer radical polymerization

Polystyrene grafted magnesium hydroxide nanoparticles [PS–Mg(OH)₂] were prepared by the followed two steps: first magnesium hydroxide nanoneedles modified by α-bromoisobutyric acid [BA–Mg(OH)₂] were synthesized through injecting alkaline into magnesium chloride solution; then polystyrene grafted magnesium hydroxide particles [PS–Mg(OH)₂] were successfully prepared by the surface-initiated atom transfer radical polymerization (SI-ATRP) of styrene from the macroinitiators, BA–Mg(OH)₂, in toluene system with the catalysts of 2,2′-bipyridine and Cu(I)Br. The obtained PS–Fe₃O₄ nanoparticles were characterized by Fourier transform infrared spectroscopy (FT-IR), elemental analysis (EA), transmission electron microscope (TEM), sedimentation test, and viscosity test. The FT-IR analyses showed that the polystyrene had been covalently grafted onto the surfaces of the nanoneedles [BA–Mg(OH)₂]. The graft polymerizations exhibited the characteristics of the controlled/“living” polymerization. TEM showed that grafted polymer chains on nanoparticles could prevent the aggregation of PS–Mg(OH)₂ nanoparticles markedly in toluene and improve their compatibility with organic phase. The graft parameters could be calculated from the elemental analysis (EA) results, and linear plots of percentage of grafting (PG %) and conversion of monomer (C %) versus polymerizing time were achieved, respectively. Narrow molecular weight distribution (M _w/M _n) for the graft polymer samples were characterized by the gel permeation chromatography (GPC).     

Preparation of polymer latex particles carrying salt-responsive fluorescent graft chains

We prepared the novel fluorescent polymer latex particles which can change their fluorescence intensity in response to the increasing NaCl concentration in water. Core polymer latex particles were synthesized by emulsifier-free emulsion polymerization of styrene and 2-(2-chloroisobutyroyloxy)ethyl methacrylate. Hydrophilic polymer chains containing epoxy groups were grafted from the core particles by surface-initiated atom transfer radical copolymerization of methoxy polyethyleneglycol methacrylate (MEO_xMA, x = 4 or 9) and glycidyl methacrylate in aqueous media. After azidation of epoxy groups in graft chains, a water-soluble fluorescent dansyl derivative was successfully coupled with the graft chains by copper-catalyzed azide-alkyne cycloaddition in aqueous media. The wavelength of maximum fluorescence intensity of polymer particles carrying graft chains with longer PEG side chains (x = 9) was slightly blue-shifted (7 nm) and the fluorescence intensity increased (1.35 times) with an increase in NaCl concentration as opposed to polymer particles with shorter PEG chains (x = 4).     

A comparative study on effects of two kinds of polymerization methods on grafting of polymer onto silica surface

In this article we present the result of a comparative study of two kinds of polymerization methods—solution polymerization (sol. poly.) and dispersion polymerization (dis. poly.) for grafting polymer onto silica. As a model for the grafting polymerization reaction, styrene was chosen as the monomer and azo diisobutyronitrile (AIBN) as the initiator. The study aims at supplying theoretical reference for better selecting polymerization method to graft polymer on the silica particle surface. First, monolayers of 3‐methacryloylpropyl trimethoxysilane were chemically bonded onto the surfaces of micrometer‐sized silica gel particles, and so double bonds were immobilized onto the silica surface. Second, the copolymerizations between the immobilized double bonds and the monomer styrene were carried out, homopolymerizations of styrene followed, and finally polystyrene was grafted to the silica surfaces. Two kinds of polymerization methods, sol. poly. and dis. poly., were adopted respectively, and the effects of polymerization methods on grafting process were examined mainly. At the same time, the effects of different polymerization conditions on the grafting degree were researched. It was found that in the dis. poly. system the grafting degree is obviously higher than that in the sol. poly. system under the same polymerization conditions, and the grafting degree can go up to 47%, i.e. 47g/100g. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 5808–5817, 2006     

Towards controlled cationic polymer growth from inorganic oxide defects: Directing the mechanism of polystyrene grafting from γ-irradiated silica

Most studies on surface-initiated controlled polymerizations for the synthesis of polymeric covalent organic-inorganic hybrid materials focus on chemical methods requiring specific modifications of the inorganic substrate. Few mechanistically-aware approaches have been undertaken towards exploiting the reactivity of defects induced by physical techniques such as ionizing radiations or UV–Vis light. Within this framework, we take grafted polymerization of styrene from γ-irradiated silica as a mechanistic testing ground where para- and diamagnetic silica defects are present, and polymerization proceeds through both radical and cationic mechanisms, resulting in a bimodal molecular weight distribution. We show that these mechanistic intricacies can be sorted out by resorting to the chemical arsenal developed in the last decades for controlled polymerizations. Specifically, we obtained a silica-polystyrene grafted material by cationic grafting from at 30 °C, a unimodal molecular weight distribution, and a relatively high molecular weight (Mn = 7.4 kDa) with a PDI of 1.68.     

‘Graft from’ polymerization on colloidal silica particles: elaboration of alkoxyamine grafted surface by in situ trapping of carbon radicals

We report in this paper an original method for the synthesis of polybutylacrylate grafted on silica particles. First, we use the Stoeber method to synthesize silica particles with a narrow size distribution. An initiator for radical-chain controlled polymerization is then grafted on the silica surface in two steps. Trimethoxysilylpropyl methacrylate (TPM) is first grafted on silica by simple condensation. Then, a alkoxyamine initiator N-tert-butyl-N-1-diethylphosphono2,2-dimethylpropyl-0,1-methoxycarbonyl ethylhydroxylamine (MONAMS) reacts with the CC double bond of the TPM to form the grafted initiator of radical polymerization. The last step is the grafting of butylacrylate. For this, we use living free radical polymerization that allows to control the molecular weight and the polydispersity of the polybutylacrylate chains. We show that this synthesis method makes it possible to obtain a colloidal suspension of silica particles having a mean size of about 88 nm, a weak polydispersity and an important grafting density of polybutylacrylate (PBA).     

Graft polymerization of polyvinylpyrrolidone onto silica

A new technique is reported for the surface grafting of polyvinylpyrrolidone (PVP) to hydroxylated solid surfaces. Free radical graft polymerization of vinylpyrrolidone onto vinylsilane–modified impermeable silica particles was carried out in an aqueous slurry reactor. The graft yield and the monomer conversion were determined by thermogravimetric analysis and UV spectroscopy, respectively. The graft yield was shown to increase by increasing the initial monomer concentration. A comparison of the graft polymerization reaction with the adsorption of polyvinylpyrrolidone onto untreated, vinylsilane-modified, and PVP-grafted silica was performed.     

Chemical Functionalization of Graphene Oxide by Poly(styrene sulfonate) Using Atom Transfer Radical and Free Radical Polymerization: A Comparative Study

Poly(sodium styrenesulfonate)-functionalized graphene was prepared from graphene oxide, using atom transfer radical polymerization and free radical polymerization. In atom transfer radical polymerization route, the amine-functionalized GO was synthesized through hydroxyl group reaction of GO with 3-amino propyltriethoxysilane. Atom transfer radical polymerization initiator was grafted onto modified GO (GO-NH₂) by reaction of 2-bromo-2-methylpropionyl bromide with amine groups, then styrene sulfonate monomers were polymerized on the surface of GO sheets by in situ atom transfer radical polymerization. In free radical polymerization route, the poly(sodium 4-styrenesulfonate) chains were grafted on GO sheets in presence of Azobis-Isobutyronitrile as an initiator and styrene sulfonate monomer in water medium. The resulting modified GO was characterized using range of techniques. Thermal gravimetric analysis, scanning electron microscopy, transmission electron microscopy, and atomic force microscopy results indicated the successful graft of polymer chains on GO sheets. Thermogravimetric analysis showed that the amount of grafted polymer was 22.5 and 31?wt% in the free radical polymerization and atom transfer radical polymerization methods, respectively. The thickness of polymer grafted on GO sheets was 2.1?nm (free radical polymerization method) and 6?nm (atom transfer radical polymerization method) that was measured by atomic force microscopy analysis. X-ray diffractometer and transmission electron microscopy indicated that after grafting of poly(sodium 4-styrenesulfonate), the modified GO sheets still retained isolated and exfoliated, and also the dispersibility was enhanced.     

Radiation-initiated graft copolymerization of individual monomer and comonomer onto polyethylene and polytetrafluoroethylene films

Preparation and some properties of the graft copolymers obtained by radiation-induced graft polymerization of methacrylic acid (MAA) and (acrylonitrile/MAA) comonomer onto polyethylene and polytetrafluoroethylene films, were investigated. The effect of reaction conditions, solvent, monomer, and inhibitor concentration, comonomer composition, and comonomer concentration, on the graft copolymerization process was studied. The grafting process was enhanced in the presence of comonomer (AN/MAA) as compared with individual grafting of MAA or acrylonitrile (AN). The optimum comonomer composition, at which the highest grafting yield was obtained, was found to be (80/20) wt % of (AN/MAA) comonomer. The graft copolymerization of (AN/MAA) comonomer was enhanced in presence of AN due to its higher polarity strength. The electrical and swelling properties of the graft copolymers were greatly affected by the contents of PAN and PMAA graft chains. Mechanical properties of the graft copolymers were significantly changed with the grafting yield.     

High performance biocompatible cellulose‐based microcapsules encapsulating gallic acid prepared by inverse microsuspension polymerization

In this study, the preparation of biocompatible cellulose‐based microcapsules encapsulating gallic acid (GA), an important antioxidant of Bambara groundnut extracts, by water‐in‐oil inverse microsuspension polymerization was studied. GA and carboxymethyl cellulose (CMC) were selected as core and shell materials, respectively. For high encapsulation efficiency, CMC was firstly modified (modified‐CMC (m‐CMC)) with 3‐(trimethoxysilyl)propyl methacrylate (MPS) as a silane coupling agent. It was subsequently polymerized with methacrylic acid (MAA) monomer through a radical route, forming a PMAA grafted m‐CMC (m‐CMC‐g‐PMAA) biocompatible polymer shell. Using CMC:MPS in a ratio of 75:25 (w/w %), highly water‐soluble m‐CMC containing a C=C bond for further radical polymerization was obtained. After inverse microsuspension polymerization at various ratios of m‐CMC:MAA, highly stable spherical m‐CMC‐g‐PMAA microcapsules encapsulating GA were formed in all ratios. It was observed that the encapsulation efficiency increased with increase in MAA content. m‐CMC:MAA in a ratio of 33:67 (w/w%) presented the highest encapsulation efficiency which may due to the increase of hydrophilicity of the aqueous phase. It also presented rapid release and non‐cytotoxic characteristics, suited for use in cosmetic products. © 2018 Society of Chemical Industry     

N-[Salicylidene-1,2-ethanediaminoethyl]-2-bromoisobutyramide as initiator for atom transfer radical polymerization and surface-initiated ATRP of methyl methacrylate on copper

N-[Salicylidene-1,2-ethanediaminoethyl]-2-bromoisobutyramide (SEB) was synthesized and characterized by elemental analysis, FT-IR and ¹H NMR. It had been successfully used as a bidentate initiator for the atom transfer radical polymerization (ATRP) of methyl methacrylate with CuBr/2,2′-bipyridine as the catalyst and N,N-dimethylformamide as the solvent at 70 °C. The kinetics was first order in monomer and the number-average molecular weight of the polymer increased linearly with monomer conversion, indicating the ‘living’/controlled nature of the polymerization. The polymerization reached high conversions producing polymers with a low molecular weight distribution (M _w/M _n = 1.34). The obtained poly(methylmethacrylate) (PMMA) functionalized with salicylidene-1,2-ethanediaminoethyl and ω-Br as the end groups were characterized by FT-IR spectroscopy. They can be used as macroinitiators for chain-extension reaction. Then, PMMA coatings were grafted from copper substrates by surface-initiated ATRP from a surface-bound SEB initiator. The electrochemical impedance spectroscopy measurements and potentiodynamic electrochemical experiments confirmed the successful grafting of the polymer coatings. Greatly improved short-term anticorrosive properties for PMMA modified electrodes were demonstrated by substantially increased resistance of the film for a period of 24 h as compared to bare copper.