DSC study of polyhydroxyethylmethacrylate filled with modified silicas

Effects of the nature of functional groups (namely, hydroxyl, methyl, silicon hydride, amino, and vinyl) on the surface of pristine and modified silicas on polymerization of 2-hydroxyethylmethacrylate (HEMA) and on structural characteristics of the filled composites have been studied. DSC, FTIR spectroscopy and equilibrium water sorption (ESI) techniques were applied for the composites characterization. Results obtained testify that the chemical nature of the grafted groups has a strong influence on the monomer orientation in the surface layer of the filler. More uniform and cross-linked structures were detected in the composites with particularly methylated silica. Filler with chemically active silicon hydride groups promotes formation of ordered structure with rigid macromolecules. The presence of amino and vinyl groups on the silica surface results in formation of flexible polymer chains with a low cross-linking density or with a low polymerization degree, even at 2?wt% filling degree. Water uptake for composites with vinyl- and amine-containing silicas was low, indicating the close-packing of polymeric molecules in the filled polyHEMA.     

Microcalorimetric Study of Methanol Adsorption on Initial Silica and Silica Modified with Polyfluoroalkyl Groups

The adsorption of methanol on initial silica and modified silica samples containing large mesopores is studied by the adsorption–calorimetric method. The grafted tridecylfluoroalkyl groups have a tilted orientation on the silica and physically screen the part of the surface OH groups that have not been involved in the reaction with a modifier. Adsorbed methanol makes the modifying layer looser, thus facilitating the accessibility of methanol molecules to these hydrophilic adsorption sites. Concentrations of OH groups involved in the chemical interaction with molecules of the modifier, OH groups physically screened by its organofluoric radicals, and OH groups located on the surface areas free of the modifier are quantitatively estimated. An additional silanization of the modified silica leads to coverage of silica surface areas that are free of organofluoric modifier with trimethylsilyl radicals. The heat of interaction between the methanol molecules and silica surface hydroxyl groups is determined; it is equal to 60 kJ/mol. The structure of the modifying organofluoric layer and changes in this structure that resulted from additional silanization of the surface and from the methanol adsorption are discussed.     

Polysaccharides at interfaces 2. Surface potential of adsorbed cholesteryl-pullulan monolayers at the solution-air interface

The surface potential of adsorbed monolayers of cholesteryl-pullulan (CHP) derivatives has been determined by the ionizing differential electrode method. It has been found that this potential is highly dependent on the degree of cholesterol grafted onto pullulan, and that the native polysaccharide displays neither surface activity nor surface potential. As the disordered structure of the non-ionic polysaccharide unit generates a random orientation of intrinsic dipole moments, it has been considered that its contribution to the measured surface potential is rather small, compared to the cholesteryl group dipolar contribution. The surface densities of cholesteryl groups of adsorbed CHP molecules have been determined from the relationship between the surface potential and the surface density of spread cholesterol molecules. The assessment of these quantities was essential, as the determination of the surface tension data for the CHP derivatives with low cholesteryl content (CHP_45−0.6 and CHP_50−0.9) was difficult to achieve (Part I of this work [B. Deme´, V. Rosilio and A. Baszkin, Colloids Surfaces B: Biointerfaces, 4 (1995) 357]). These results complement those from the surface tension measurements, and confirm that in the surface layer of the adsorbed polysaccharide the ordered cholesteryl groups are oriented towards the air phase and the disordered polysaccharide is immersed in the aqueous subphase. Proposed models for semi-organized adsorbed CHP layers are discussed.     

Effective grafting of polymers onto ultrafine silica surface: Photopolymerization of vinyl monomers initiated by the system consisting of trichloroacetyl groups on the surface and Mn2(CO)10

The effective grafting of vinyl polymers onto an ultrafine silica surface was successfully achieved by the photopolymerization of vinyl monomers initiated by the system consisting of trichloroacetyl groups on the surface with Mn₂(CO)₁₀ under UV irradiation at 25 °C. The introduction of trichloroacetyl groups onto the surface of silica was achieved by the reaction of trichloroacetyl isocyanate with surface amino groups, which were introduced by the treatment of silica with 3‐aminopropyltriethoxysilane. During the polymerization, the corresponding polymers were effectively grafted onto the surface, based on the propagation of polymer from surface radicals formed by the interaction of trichloroacetyl groups and Mn₂(CO)₁₀. The percentage of poly(methyl methacrylate) grafting onto the silica reached 714.6% after 90 min. The grafting efficiency (proportion of grafted polymer to total polymer formed) in the polymerization of methyl methacrylate was very high, about 80%, indicating the depression of formation of ungrafted polymer. Polymer‐grafted silica gave a stable colloidal dispersion in good solvents for grafted polymer. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2157–2163, 2001     

Hydrolysis and grafting of dimethylalkoxysilanes onto stainless steel

The grafting of trialkoxysilane molecules should also give rise to the formation of a siloxane network at the substrate's surface when trialkoxysilanes are used. Other candidates that might be able to act as adhesion promoters at metallic surfaces are dimethylalkoxysilanes. The advantage of dimethylalkoxysilanes is that only one silanol group is produced during the hydrolysis step, leading to the formation of a grafted monolayer onto the steel. Moreover, the chemical grafting of stainless steel, which exhibits a low surface reactivity, is of great interest for industrial applications such as adhesive bonding or coatings. The objective of this work was to chemically graft dimethylalkoxysilanes onto AISI 316L stainless steel and to analyze the grafted layer by X‐ray photoelectron spectroscopy (XPS). Investigation of the hydrolysis of these molecules in aqueous solutions was also performed by proton nuclear magnetic resonance spectroscopy (¹H NMR). The grafting of 3‐(ethoxydimethylsilyl)propylamine (APDES) and 3‐glycidoxypropyldimethylethoxysilane (GPDES) was achieved onto stainless steel after a controlled hydrolysis reaction. A pH inferior or equal to 5 was necessary to obtain a sufficient hydrolysis of silanes. XPS results have evidenced the grafting of the silanes onto stainless steel. The signal of the Si 2p peak clearly showed the formation of a covalent bond between APDES and the stainless steel surface through the O atoms giving rise to a uniform layer of adsorbed molecules. Moreover, this grafted layer is strongly stable as no removal of the alkoxysilane was observed after immersion in hot water which is very critical for these molecules. Copyright © 2015 John Wiley & Sons, Ltd.     

Chemical reactions in dense monolayers: in situ thermal cleavage of grafted esters for preparation of solid surfaces functionalized with carboxylic acids

The thermodynamics of a chemical reaction confined at a solid surface was investigated through kinetic measurements of a model unimolecular reaction. The thermal cleavage of ester groups grafted at the surface of solid silica was investigated together with complementary physicochemical characterization of the grafted species. The ester molecules were chemically grafted to the silica surface and subsequently cleaved into the carboxylic acids. A grafting process of a reproducible monolayer was designed using the reaction of monofunctional organosilane from its gas phase. The thermal deprotection step of the ester end-group was investigated. The thermal deprotection reaction behaves in quite a specific manner when it is conducted at a surface in a grafted layer. Different organosilane molecules terminated by methyl, isopropyl and tert-butyl ester groups were grafted to silica surface; such functionalized materials were characterized by elemental analysis, IR and NMR spectroscopy, and thermogravimetric analysis, and the thermodynamic parameters of the thermal elimination reaction at the surface were measured. The limiting factor of such thermal ester cleavage reaction is the thermal stability of grafted ester group according to the temperature order: tert-butyl < i-propyl < methyl. Methyl ester groups were not selectively cleaved by temperature. The thermal deprotection of i-propyl ester groups took place at a temperature close to the thermal degradation of the organofunctional tail of the silane. The low thermolysis temperature of the grafted tert-butyl esters allowed their selective cleavage. There is a definite influence of the surface on the reaction. The enthalpy of activation is lower than in the gas phase because of the polarity of the reaction site. The major contribution is entropic; the negative entropy of activation comes from lateral interactions with the neighbor grafted molecules because of the high grafting density. Such reaction is an original strategy to functionalize the silica surface by carboxylic acid groups by means of a simple, reproducible, and efficient process involving in situ thermolysis of ester groups.     

A surface functional monomer-directing strategy for highly dense imprinting of TNT at surface of silica nanoparticles

This paper reports a surface functional monomer-directing strategy for the highly dense imprinting of 2,4,6-trinitrotoluene (TNT) molecules at the surface of silica nanoparticles. It has been demonstrated that the vinyl functional monomer layer of the silica surface can not only direct the selective occurrence of imprinting polymerization at the surface of silica through the copolymerization of vinyl end groups with functional monomers, but also drive TNT templates into the formed polymer shells through the charge-transfer complexing interactions between TNT and the functional monomer layer. The two basic processes lead to the formation of uniform core-shell TNT-imprinted nanoparticles with a controllable shell thickness and a high density of effective recognition sites. The high capacity and fast kinetics to uptake TNT molecules show that the density of effective imprinted sites in the nanoshells is nearly 5 times that of traditional imprinted particles. A critical value of shell thickness for the maximum rebinding capacity was determined by testing the evolution of rebinding capacity with shell thickness, which provides new insights into the effectiveness of molecular imprinting and the form of imprinted materials. These results reported here not only can find many applications in molecularly imprinting techniques but also can form the basis of a new strategy for preparing various polymer-coating layers on silica support.     

Characterization of poly(sodium styrene sulfonate) thin films grafted from functionalized titanium surfaces

Biointegration of titanium implants in the body is controlled by their surface properties. Improving surface properties by coating with a bioactive polymer is a promising approach to improve the biological performance of titanium implants. To optimize the grafting processes, it is important to fully understand the composition and structure of the modified surfaces. The main focus of this study is to provide a detailed, multitechnique characterization of a bioactive poly(sodium styrene sulfonate) (pNaSS) thin film grafted from titanium surfaces via a two-step procedure. Thin titanium films (～50 nm thick with an average surface roughness of 0.9 ± 0.2 nm) prepared by evaporation onto silicon wafers were used as smooth model substrates. X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) showed that the titanium film was covered with a TiO(2) layer that was at least 10 nm thick and contained hydroxyl groups present at the outermost surface. These hydroxyl groups were first modified with a 3-methacryloxypropyltrimethoxysilane (MPS) cross-linker. XPS and ToF-SIMS showed that a monolayer of the MPS molecules was successfully attached onto the titanium surfaces. The pNaSS film was grafted from the MPS-modified titanium through atom transfer radical polymerization. Again, XPS and ToF-SIMS were used to verify that the pNaSS molecules were successfully grafted onto the modified surfaces. Atomic force microscopy analysis showed that the film was smooth and uniformly covered the surface. Fourier transform infrared spectroscopy indicated that an ordered array of grafted NaSS molecules were present on the titanium surfaces. Sum frequency generation vibration spectroscopy and near edge X-ray absorption fine structure spectroscopy illustrated that the NaSS molecules were grafted onto the titanium surface with a substantial degree of orientational order in the styrene rings.     

IR Spectroscopic Study of Ethylene Adsorption and Oligomerization on the Hydrogen Form of Mordenite

Diffuse-reflectance IR spectroscopy is used to study adsorption and oligomerization of ethylene on the hydrogen form of mordenite at room temperature. Ethylene adsorbs on bridging acid hydroxyl groups of the zeolite and forms -complexes with a firm hydrogen bond. The interaction with hydroxyl groups most strongly excites composite vibrations in adsorbed molecules. These vibrations are a combination of the stretching vibration of a double bond and the deformational vibrations of the CH₂ group. A conjecture is drawn that these composite vibrations correspond to the reaction coordinate of ethylene transformation to the ethoxy groups. Their further reactions with weakly adsorbed molecules result in ethylene oligomerization. A linear oligomer is formed, grafted on the zeolite surface and filling the pores of zeolites.     

A novel method for preparation of exfoliated UV-curable polymer/clay nanocomposites

The half adduct of isophorone diisocyanate and 2-hydroxyethyl acrylate (IPDI-HEA), as a reactive organic modifier, was used to functionalize Na-montmorillonite (Na-MMT) clay. Unlike the electronic interaction in the conventional cation-exchange method, the driving force for the organic modification came from the chemical reaction between IPDI-HEA and framework hydroxyl groups on the surface of clay. With high degree of organic modification (48%), the d-spacing of clay layer was greatly enlarged to 3.32 nm, and the clay became more organophilic. After in situ photopolymerization among the IPDI-HEA grafted MMT clay, monomers and oligomers, the exfoliated polymer/clay nanocomposites were obtained. X-ray diffraction and transmission electron microscopy were used to detect the structure and morphology of the clay dispersed in the polymer matrix. Compared with the pure polymer materials, the exfoliated polymer/clay nanocomposites exhibited enhancements in mechanical and thermal properties.     

Mechanism of the grafting of organosilanes on mineral surfaces I. Nature and role of the hydrolysis products of the methylvinyldichlorosilane in the grafting of silicates in hydrochloric acid and isopropanol

Summary The hydrolysis of methylvinyldichlorosilane in a hydrochloric acid-isopropanol mixture gives an oil whose major components have been isolated and identified as methylvinylcyclosiloxanes with 4 to G silicon atoms in the molecule. The grafting of the sepiolite by methylvinyldichlorosilane in a hydrochloric acid-isopropanol mixture results in a vinylic derivative of the mineral. The kinetics of the reaction shows that both the silane and its hydrolysis products are grafted. Furthermore, other cyclosiloxanes can be grafted on different minerals; for example, octamethylcyclotetrasiloxane has been grafted on chrysotile fibers, giving a methylated derivative.

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Zusammenfassung Es werden die Graftpolymere des Silicates Sepiolith mit Methylvinyldichlorosilan untersucht.

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With 3 figures and 3 tables     

pH responsive adhesion of phospholipid vesicle on poly(acrylic acid) cushion grafted to poly(ethylene terephthalate) surface

Polymer-supported lipid bilayer is a key enabling technology for the design and fabrication of novel biomimetic devices. To date, the physical driving force underlying the formation of polymer-supported lipid bilayer remains to be determined. In this study, the interaction between dipalmitoylphosphocholine (DPPC) vesicle and poly(ethylene terephthalate) [PET] surface with or without grafted poly(acrylic acid) [PAA] layer is examined with several biophysical techniques. First, vesicle deformation analysis shows that the geometry of adherent vesicle on either plain PET or PAA-grafted PET surface is best described by a truncated sphere model. At neutral pH, the degree of deformation and adhesion energy are unaltered by the grafted polymerization of acrylic acid on PET surface. Interestingly, the average magnitude of adhesion energy is increased by 185% and −43% on PAA-grated PET and plain PET surface, respectively, towards an increase of pH at room temperature. Our results demonstrate the possibility of tuning the adhesive interaction between vesicle and polymer cushion through the control of polyelectrolyte ionization on the solid support.     

Suppression of surface pattern formation in spin‐coated polymer films by the addition of polydimethylsiloxane‐grafted silica nanoparticles

Polydimethylsiloxane (PDMS)‐grafted nanoparticles and PDMS were added, respectively, to inhibit the dewetting of polymer films and the formation of surface patterns in spin coating. Uniform and flat films were successfully achieved with the addition of PDMS‐grafted silica nanoparticles or PDMS. Time‐of‐flight secondary ion mass spectrometry depth profiling indicated that PDMS‐grafted silica nanoparticles and PDMS preferentially segregated to the surface. A high concentration of bromine end groups was observed at the interface. The surface layer of PDMS or PDMS‐grafted silica nanoparticles can decrease the surface tension of the polymer solutions and reduce the evaporation rates of the solvents, providing more time for the bromine end groups to anchor themselves at the silicon substrates. Copyright © 2016 John Wiley & Sons, Ltd.     

Tuning Bis(imino)pyridyl Iron‐Catalyzed Ethylene Oligomerization by Modification of MAO with p‐BrPhOH

To reduce the simultaneous production of insoluble polymers during the bis(imino)pyridyl iron‐catalyzed ethylene oligomerization, in this study, p‐BrPhOH (4‐bromophenol) has been chosen as the most optimal modifier for the production of linear α‐olefins. It is found that the polymer share in the total products is largely reduced with the use of p‐BrPhOH as the modifier. The catalytic system also possesses a high activity with the liquid production maintained high level of linearity. Moreover, the introduction of p‐BrPhOH promoted the high‐temperature stability of the catalytic system, leading to the enhanced oligomerization activity as the catalytic system can catalyze ethylene oligomerization at higher temperatures. A characterization of the catalytic system with electron paramagnetic resonance shows that introduction of p‐BrPhOH significantly inhibits the formation of ferric ions, which can be the main active centers responsible for generating undesired insoluble polymers, thus this can largely retard the production of insoluble polymers during ethylene oligomerization.     

Pervaporation of ethanol-water mixtures through temperature-sensitive poly(vinyl alcohol-g-N-isopropyacrylamide) membranes

Novel temperature-sensitive membranes have been synthesized by grafting poly(N-isopropyacrylamide) (poly(NIPAAm)) onto a poly(vinyl alcohol) (PVA) backbone using hydrogen peroxide-ferrous ion as initiator. Due to the grafting of poly(NIPAAm), the hydrophilic/hydrophobic balance and the polarity of the pendent groups within the membranes are modified. Significant temperature sensitivity of the grafted membranes is observed close to the LCST of linear poly(NIPAAm) in the pervaporation processes for ethanol-water separation. Both the pervaporation and sorption selectivities for water show a maximum value in the vicinity of 30–32°C for an ethanol content of 75 and 80%. The temperature sensitivity of the grafted membranes also depends on the ethanol concentration. The maxima of pervaporation and sorption selectivities disappear when the ethanol content is lower than 75% because the much larger degree of swelling reduces the size screening effect of the membranes.     

Nanoscale patterning of flat carbon surfaces by scanning probe lithography and electrochemistry

We report the formation of carbon surfaces patterned at the nanoscale with organic functionalities. Thin (<10 nm) films are covalently grafted to the surface via the electrochemical reduction of aryl diazonium salts. Areas of the film are removed with an AFM tip, and a second modifier is electrochemically grafted to the exposed surface. The pattern can incorporate different chemical functionalities, or alternatively topographical patterns can be assembled, where the same functionality is present throughout the pattern.     

Role of the concentration and nature of grafted groups in the adsorption of hydrocarbon vapors on silica modified by monofunctional polyfluoroalkylsilanes

The role of the grafting density of monofunctional polyfluoroalkylsilanes of the C _n F_{2n − 1}(CH₂) _m Si(CH₃)₂Cl general formula (where n = 3, 4, and 6; and m = 2 and 3) and their composition in intermolecular interactions of the molecules of saturated and aromatic hydrocarbons with a surface of chemically modified silica is studied by means of IR spectroscopy and adsorption-static and gas chromatography. It is shown that the higher the concentration and the shorter the length of the grafted chain, the greater (by a factor of 2 to 25) the drop in the adsorption values of hydrocarbons as a result of modifications, due to an increase in the degree of oleophobization of surface upon the formation of polyorganofluorine coatings. The high specificity of the surface with respect to benzene, which is due to the active participation of the polar fragment of a grafted chain in adsorption process, is related to the features of a relatively low-density sample with a concentration of grafted perfluorobutyl groups of 1.7 nm⁻². It is shown that the thermodestruction of polyfluoroalkyl silica remains virtually unobserved upon heating to 523 K in an argon flow.     

Cationic Grafting of Vinyl Polymers onto a Carbon Whisker,Initiated by Acylium Perchlorate Groups Introduced onto the Surface

Abstract

The cationic graft polymerization of vinyl monomers onto a carbon whisker, vapor-grown carbon fiber, initiated by acylium perchlorate groups introduced onto the surface, was investigated. The introduction of acylium perchlorate groups onto a carbon whisker was achieved by the treatment of a carbon whisker having acyl chloride groups, which were introduced by the reaction of surface carboxyl groups with thionyl chloride, with silver perchlorate in nitrobenzene. It was found that the cationic polymerization of vinyl monomers, such as styrene, indene, N-vinyl-2-pyrrolidone, and n-butyl vinyl ether, is initiated by acylium perchlorate groups on a carbon whisker. In the polymerization, the corresponding vinyl polymers were grafted onto a carbon-whisker surface based on the propagation of polymer from the surface: the percentage of grafting of polystyrene and polyindene reached 42.5 and 100.3%, respectively. The percentage of polystyrene grafting decreased with increasing polymerization temperature because of preferential chain transfer reactions at higher temperatures. Polymer-grafted carbon whisker gave a stable colloidal dispersion in a good solvent for grafted polymer.     

Supercrystal structures of polyhedral PbS nanocrystals

We analyzed the interaction between chemically grafted polysaccharide layers in aqueous solutions. To fabricate such layers, an end-terminated dextran silane coupling agent was synthesized and the polydextran was grafted to oxidized silicon wafers and to silica particles. This resulted in the formation of a 28 nm thick layer (in air) and a grafted amount of 40 mg/m(2) as determined by ellipsometry. The physical properties of the grafted layer were investigated in aqueous solutions by atomic force microscope imaging and colloidal probe force measurements. Surface and friction forces were measured between one bare and one polydextran coated silica surface. A notable feature was a bridging attraction due to affinity between dextran and the silica surface. Surface interactions and friction forces were also investigated between two surfaces coated with grafted polydextran. Repulsive forces were predominant, but nevertheless a high friction force was observed. The repulsive forces were enhanced by addition of sodium dodecyl sulfate (SDS) that associates with the tethered polydextran layers. SDS also decreased the friction force. Our data suggests that energy dissipation due to shear-induced structural changes within the grafted layer is of prime importance for the high friction forces observed, in particular deformation of protrusions in the surface layer.     

Photografting of polymers onto nanosized silica surface initiated by eosin moieties immobilized onto the surface

The photograft polymerization of various vinyl monomers onto nanosized silica surfaces was investigated. It was initiated by eosin moieties introduced onto the silica surface. The preparation of the silica with eosin moieties was achieved by the reaction of eosin with benzyl chloride groups on the silica surface.These were introduced by the reaction of surface silanol groups with 4‐(chloromethyl)phenyltrimethoxysilane in the presence of t‐butyl ammonium bromide as a phase‐transfer catalyst. The photopolymerization of various vinyl monomers, such as styrene, acrylamide, acrylic acid, and acrylonitrile was successfully initiated by eosin moieties on the silica surface in the presence of ascorbic acid as a reducing agent and by oxygen. The corresponding polymers were grafted from the silica surface. The grafting efficiency (percentage of grafted polymer to total polymer formed) in the photoinitiation system was much larger than that in the radical polymerization initiated by surface radicals; these radicals were formed by the thermal decomposition of azo groups introduced onto the silica surface. It was found that the polymer‐grafted silica gave stable dispersions in good solvents of grafted polymer and the wettability of the surfaces can be easily controlled by grafting of polymers. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 600–606, 2005