Surface free energies of silica fillers and their relation to the adsorption of poly(ethylene terephthalate)

The surface free energy of modified silica as well as of PET oligomers was evaluated through measurements of specific retention volumes of several probe molecules by use of the adsorption and adhesion principles in inverse gas chromatography. The nondispersive component of surface free energy of most silica fillers was larger than the dispersive component and the acidic component was much larger than the basic one, which indicated that the surfaces of most silica fillers were rather acidic. These methods were also applied to PET oligomer and it was found that the surface free energy of PET oligomer, regardless of preparation method, consisted of an almost dispersive component, suggesting that the surface of PET was neutral. The amount of PET oligomer adsorbed for the heat-treated silica fillers in acidic solvent increased linearly with increased acidic component of the surface free energy, which indicates that the acidic component of the surface free energy may be responsible for the adsorption. However, the adsorption amount on modified silica is much smaller than that for the heat-treated silica fillers because of steric hindrance caused by the attached organic chain, suggesting that the adsorption cannot be determined only by the surface free energy.     

Inverse Gas Chromatographic Characterization of Aluminosilicates as Fillers for Abrasive Articles

The surface properties of aluminosilicates (perlites and zeolites)—potential fillers in abrasive articles—have been examined by inverse gas chromatography. The dispersive component of the surface free energy ( $ \gamma_{\text{S}}^{\text{D}} $ ) and K _A and K _D describing the acidity and basicity, respectively, of the fillers were used to express the surface activity of examined materials. The Flory–Huggins parameter, $ \chi_{23}^{'} , $ characterized the interactions between polymer and filler, i.e., interactions between phenolic resin and filler. Zeolites were found to be materials with an active surface layer and relatively high ability to participate in filler–resin interactions. Therefore, they might be considered as potential “replacement material” for standard fillers. The IGC-derived values mentioned above provided useful information about the behavior of fillers during the manufacture and use of abrasive articles.     

Surface characterization of poly(methyl methacrylate-co-n-butyl acrylate-co-cyclopentylstyryl-polyhedral oligomeric silsesquioxane) by inverse gas chromatography

The surface properties of poly(methyl methacrylate-co-n-butyl acrylate-co-cyclopentylstyryl polyhedral oligomeric silsesquioxane) (poly(MMA-co-BA-co-styryl-POSS)) were studied by means of inverse gas chromatography (IGC) using 10 non-polar and polar solvents as the probes. Thermodynamic parameters of adsorption, e.g., specific retention volume, the dispersive component of the surface free energy, the specific interaction contribution to the free energy of adsorption and the acid/base constants were obtained to investigate the interactions between the surfaces of the copolymers and different solvents. It was found that incorporation of styryl-POSS into polymer resulted in increasing interactions between polymers and solvents, dispersive component of surface free energy of polymer and acidity of the surfaces of the polymers. The more the styryl-POSS were embedded, the stronger the interaction between the polymer surface and solvent, the dispersive component of the surface free energy and the acidity of the polymer surface were.     

Effect of silane coupling agent on mechanical interfacial properties of glass fiber‐reinforced unsaturated polyester composites

A silane coupling agent, γ‐methacryloxypropyltrimethoxysilane, for the surface modification of glass fibers was varied between 0.1 and 0.8 wt %. To understand the role of interfacial adhesion of glass fiber/unsaturated polyester composites, contact angles of the silane‐treated glass fibers were measured by the wicking method on the basis of the modified Washburn equation with deionized water, diiodomethane, and ethylene glycol as testing liquids. As a result, silane‐treated glass fibers led to increased surface free energy, mainly because of their increased specific or polar component. The mechanical interfacial behaviors based on the interlaminar shear strength (ILSS) of the composites determined by short‐beam tests and the critical stress‐intensity factor (K_IC) were also improved in the case of silane‐treated composites. The surface free energy and the mechanical interfacial properties especially showed the maximum value in the presence of 0.4 wt % silane coupling agent. It revealed that the increase of a specific component of the surface free energy or hydrogen bonding between the glass fibers and the coupling agents plays an important role in improving the degree of adhesion at interfaces in a composite system. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 55–62, 2003     

Determination of the surface properties of untreated and chemically treated kaolinites by inverse gas chromatography

Inverse gas chromatography is used to study the surface properties of the untreated and chemically treated kaolinite samples. Changes in the enthalpy of adsorption for a variety of probes and in the surface energy of clays are measured and the effect of modification of the natural clay after chemical treatment with Na₂CO₃ is determined. The surface energy of the natural clay increased by the modification due to an increase in the surface area. It can be concluded that the dispersive component of surface free energy, γ _s ^d , decreases with temperature in the 200–275°C temperature range for both samples. Temperature coefficients of γ _s ^d for untreated and modified kaolinites are −0.1185 and −0.3966 mJ/(m² °C) with the correlation coefficients (R ²) of 0.8479 and 0.965, respectively. From the retention data for polar probes at infinite dilution, information on the accessibility of surface sites to the probes and on the acid-base character of the surface is obtained. The specific free energy of adsorption, the specific enthalpy of adsorption (ΔH ^sp), and the specific entropy of adsorption of polar probes on initial and modified kaolinites are determined. The ΔH ^sp values correlated with the donor numbers and modified acceptor numbers of the probes to quantify the acidity (K _A) and basicity (K _D) parameters of clay surfaces. The values of K _A and K _D for initial and modified kaolinites are determined to be 0.1202 and 0.2803; 0.0130 and 0.0408 with the correlation coefficients of −0.9805 and −0.9782, respectively. The unmodified clay sample indicated a more acidic character, while the modified clay sample conferred a largely basic character. Consequently, the predominant surface basicity of the modified kaolinite agrees with expectation, bearing in mind the treatment with Na₂CO₃, taking into consideration that such a modification contributes to a decrease in the hydrophilicity of the surface and also results in the surface showing only weak Lewis acidity. The text was submitted by the authors in English.     

The effect of filler surface modification and processing conditions on distribution behaviour of silica nanofillers in polyesters

Dispersion and distribution of nanosized precipitated amorphous silica particles in poly(ethylene terephthalate) PET and poly(butylene terephthalate) PBT matrices have been studied. The effect of silane coupling agent as well as processing conditions has been analysed on the basis of microscopic analysis of sample morphology. N-2-(aminoethyl)-3-aminopropyltrimethoxysilane (A-1120) has been used as a silica surface modifier. The effect of the extrusion process performance with a single and a twin screw extruder has been tested. It has been found that the processing conditions are more important factors determining fine dispersion and homogenous distributions of filler particles in the matrix than the filler surface modification. The results obtained have revealed that single screw extrusion is preferred only for processing the composites comprising the silica modified with aminosilane, while the application of twin screw extrusion leads to homogenous dispersion and fully deagglomeration of filler particles without silane treatment. It has been established that when the concentration of silica filler increased from 3 up to 7% by weight, the secondary process of particle aggregation occurs.     

Studies on the Surface Free Energy of Carbon-Carbon Composites: Effect of Filler Addition on the ILSS of Composites

The effect of an inorganic oxidation inhibitor, such as MoSi(2), on the surface energetics of carbon-carbon composites has been studied. According to contact angle measurements based on a linear fit method obtained from multiple testing liquids, such as water, diiodomethane, ethylene glycol, and glycerol, it is observed that increasing the MoSi(2) filler content of the composites leads to an increase of the surface free energy, mainly due to the increase of the London dispersive component of surface free energy, even if its specific or polar component is decreased. As an experimental result, the London dispersive component is strongly dependent on the results of the interlaminar shear strength of the composites. In this work, it has been possible to show that the results obtained from the linear fit method correlate with those determined from a two-liquid geometric mean when water and diiodomethane testing liquids are considered as a two-liquid pair. Copyright 2000 Academic Press.     

Resistivity control in the semiconductive region for carbon-black-filled polymer composites

It is known that the electrical volume resistivity of insulating polymers filled with conductive fillers, such as metal particles and/or carbon black (CB) particles, suddenly decreases at a certain content of the filler. Therefore, it is very difficult to control the resistivity in the semiconductive region for the CB-filled composites. We examined two effects to control the electrical volume resistivity in the semiconductive region for CB-filled polymer composites. One is the effect of fluorination of the CB surface on the percolation behavior using surface-fluorinated CB particles as a filler. The other is the effect of copolymerization of polyethylene (PE) with a vinyl acetate (VA) functional group on the percolation behavior using poly(ethylene-co-VA) (EVA) as a matrix. By immersion heat measurements, it was found that the London dispersive component turned out to be the predominant factor of the surface energy of fluorinated CBs. The London dispersive component of the surface energy significantly decreased, while the polar component slightly increased on increasing the fluorine content. The resistivity of fluorinated a CB-filled low-density PE composite showed that the percolation threshold increased, and the transition from the insulating state to the conductive state became sluggish, on increasing the fluorine content. In the case of using EVA as a matrix, on the other hand, the percolation curve was moderated with the increase in the VA content. Therefore, copolymerization of PE with VA is also suitable for the design of a semiconductive polymer composite as well as for fluorination of the CB surface. The total surface area per unit mass of dispersed CB particles in the EVA matrix estimated from small-angle X-ray scattering decreased with increasing CB content. Further, the decrease in the surface area is moderated with an increase in VA content. It was found that the difference in the percolation curve is due to the difference in the dispersive state of CB particles.     

Characterisation of the surface of a cellulosic multi-purpose office paper by inverse gas chromatography

The surface of multi-purpose cellulosic office paper has been analysed by inverse gas chromatography (IGC). The parameters determined were the dispersive component of the surface free energy, the enthalpy of adsorption and the entropy of adsorption of polar and apolar probes, the Lewis acidity constant, K _a, and the Lewis basicity constant, K _b. It can be concluded that the dispersive component of the surface free energy, _s ^d decreases with temperature, in the range 50–90°C. The temperature coefficient of _s ^d, d_s ^d/dT, is –0.35 mJm ^–2K^–1. The values of K _a and K _b were determined to be 0.11±0.011 and 0.94±0.211, respectively. The predominant surface basicity agrees with expectation, bearing in mind the presence of calcium carbonate, and of a styrene-acrylic copolymer, in the surface sizing formulation. It is thought that during the drying stages following the surface sizing treatment, the starch used as the binder migrates to the interior of the surface sizing layer and then to the paper bulk itself. This migration contributes to a decrease in the hydrophilicity of the surface, and also results in the surface showing only slight Lewis.     

Surface acidity and basicity of functionalized silica particles

Tetraethoxysilane has been co-hydrolyzed with functionalized organosilanes in a modified Stöber process to produce silica particles with amino, carboxylate or dihydroimidazole groups on the surface. The effects of reaction conditions and the loading of the functionalized organosilane on particle size was examined by TEM. Fluorescence spectroscopy of the surface amino groups covalently modified with fluorescamine, and the surface carboxylate groups with 4-bromomethyl-6,7-dimethoxycoumarin, demonstrated that these functional groups were accessible for further reaction. Changes in surface acidity and basicity caused by the presence of functional groups (amine, dihydroimidazole, carboxylate) on the particle surface were determined using an indicator titration technique. Particles with surface imidazole and amine groups and particles with surface carboxylate groups have enhanced basicity and acidity, respectively. Dihydroimidazole-modified silica had greater surface basicity than the amine-modified silica. The effect on basicity and acidity increases as the amount of added functionalized silane increases. However, this increase is nonlinear with respect to the increase in added functionalized silane. Particles with both surface dihydroimidazole and carboxylate groups demonstrated reduced surface basicity and acidity.     

Study of a core-shell type impact modifier by inverse gas chromatography

Inverse gas chromatography (IGC) has been used to study the Lewis acid-base properties of a technologically and commercially important core-shell type elastomer (MBS rubber). The parameters determined were the dispersive component of the surface tension, the surface free energy, the enthalpy and the entropy of adsorption of polar and apolar probes, the surface Lewis acidity constant (Ka), and the surface Lewis basicity constant (Kb). The results show that the MBS rubber is amphoteric but strongly Lewis basic. It is weakly Lewis acidic. The results are in accord with the analysis of the molecular structure of PMMA, the shell component of this impact modifier (IM). The interactivity of this elastomer with the remaining materials in multicomponent polymeric systems is expected to be strongly influenced by the particular surface energetic properties of the MBS rubber. The results presented would contribute to the interpretation, forecast and optimization of the adhesion properties and phase preferences shown by this impact modifier when incorporated in such complex polymeric systems as polymer blends and composites.     

Determination of the surface characteristics of particulate fillers by inverse gas chromatography at infinite dilution: a critical approach

CaCO3 fillers were investigated by inverse gas chromatography (IGC) to determine the dispersion component of their surface tension as well as their acid-base character. Because of the high energy of the filler surface, it readily adsorbs water, thus the parameters measured by IGC depend on the conditioning temperature, as well as on the measurement conditions. As a consequence, the determined surface characteristics are not material constants; different fillers or the effect of coating can be compared only under standard conditions. The use of the same conditioning and measurement temperature eliminates the effect of measurement time. Under appropriate standard conditions the acid-base characteristics of the filler can be determined reliably. However, the accuracy of the determination and the value of the derived parameters depend very much on the selected approach and on the acid-base constants used for the probe molecules. A critical analysis of the approaches used in the current literature pointed out those that yield the most reasonable and accurate values. The results prove that the surface of CaCO3 is strongly basic in character. Coating significantly reduces basicity. Surprisingly, the filler coated with an amount of stearic acid resulting in minimum surface tension showed relatively strong acidity, which indicates a coating exceeding monolayer coverage and/or the uneven distribution of the surfactant on the surface.     

Influence of drawing and heat treatment on surface free energies of polyethylene terephthalate fibers

The surface free energies of polyethylene terepthalate fibers with different draw ratios were experimentally determined by contact angle measurements inn-alkane/water systems. The dispersive component of the surface free energy increased with increasing draw ratio, whereas the nondispersive one remained almost constant. After heat treatment, the dispersive surface free energy increased, but was reduced above 140°C. The nondispersive component increased by heat treatment at 190°C. The increases in the density and birefringence of the fibres due to the drawing and heat treatment suggested that the increase in the dispersive surface free energy was caused by the increase in the atomic density at the fiber surface due to drawing and heat treatment. ESCA results indicated that the increment in the nondispersive surface free energy due to heat treatment was caused by the addition of functional groups to the fiber surface due to heat treatment.     

Study of modified silicas by inverse gas chromatography. Influence of chain length on the conformation of n-alcohols grafted on a pyrogenic silica

Summary A fumed silica surface was systematically modified by the grafting of n-alkyl chains with increasing carbon numbers. The samples were characterized by the dispersive component of the surface energy, their specific interaction potential and enthalpies of adsorption of polar, in particular, alcohol probes. It is shown that the variation of the surface properties and adsorption capacities depend on the chain length of the graft. For instance, minimum values are recorded when the surface coverage by methylene groups corresponds either to one or two CH₂ surface layers. This behaviour is related to the mobility of the grafted alkyl chains, mobility which was examined by solid state NMR.     

How silanization of silica particles affects the adsorption of PDMS chains on its surface

A series of six fumed silica types, with different surface areas in the 50–400 m²/g range, were modified by grafting with trimethylchlorosilane. The grafting reaction was controlled by elemental analyses, surface hydroxyl titration, and combustion techniques. The silica surface energy was determined as a function of silanization degree by inverse gas chromatography (IGC). Adsorption of a series of poly(dimethylsiloxane) elastomers with molecular weights ranging between 4 and 420 kg/mol on silica was followed using flow microcalorimeter (FMC). IGC results show that free adsorption energies of two series of alkanes and siloxanes as well as the dispersive component of the surface energy were found to decrease monotonously with surface silanization and so does the polymer molar heat of adsorption. FMC results indicate, however, that the conformation of the macromolecules on silica depends on the silica surface area but remains unaffected by the surface treatments. A given polymer chain was found to remain adsorbed on the surface preserving its same conformation until its molar heat of adsorption falls bellow a critical value. These findings offer a better monitoring of surface–polymer interactions as it defined a comprehensive relationship between the degree of modifications of the filler surface and polymer adsorption conditions. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2010     

Understanding rice hull ash as fillers in polymers: A review

The polymer industry has a newfound interest in fillers from industrial by-products and other waste materials having potential recyclability. This new class of fillers includes fillers from natural sources (e.g., natural fibers), industrial by-products (e.g., saw dust, rice husks) and a recent entry in the form of silica ash – an industrial waste material –obtained by burning rice husks. Rice hulls possess an unusually high percentage of `opaline silica'. Its annual worldwide output is more than 80 million tons, which corresponds to 3.2 million tons of silica. Silanol groups present on the surface of rice hull ash can positively influence its reinforcing character ash as a filler, however, being hydrophilic, it suffers fromfiller-aggregation and moisture absorption. Present article reviews the performance of rice husk ash, or silica ash, in polymeric composites. This paper emphasizes the need for better characterization of silica ash to obtain an in-depth understanding of its behaviour with the view to identifying suitable modifications to improve its performance as a filler. It is emphasized that poor understanding of silica ash as a filler is linked to the lack of surface characterization, since its behaviour is significantly linked to its surface properties. Based on this analysis, a new approach to silica ash modification is proposed.     

Surface characterization of cellulose fibres by XPS and inverse gas chromatography

Inverse gas chromatography (IGC) was used to determine the dispersive component of the free energy as well as the acid-base properties of cellulose fibre surfaces, before and after modification by corona treatment. It was found that the corona treatment increases both the dispersive contribution to surface energy and its acidic character, whereas only a slight increase in its basicity was observed. It was also found that some chemical degradation of the surface occurs at high corona currents. The extent of modification of the surface properties, as revealed by IGC, was correlated to the surface chemical composition deduced from XPS analysis as well as with the electrical conductance and the pH of the water suspensions of the cellulose fibres.     

Development of surface-SFED models for polar solvents

We developed surface grid-based solvation free energy density (Surface-SFED) models for 36 commonly used polar solvents. The parametrization was performed with a large and diverse set of experimental solvation free energies mainly consisting of combinations of polar solvent and multipolar solute. Therefore, the contribution of hydrogen bonds was dominant in the model. In order to increase the accuracy of the model, an elaborate version of a previous hydrogen bond acidity and basicity prediction model was introduced. We present two parametrizations for use with experimentally determined (Surface-SFED/HB(exp)) and empirical (Surface-SFED/HB(cal)) hydrogen bond acidity and basicity values. Our computational results agreed well with experimental results, and inaccuracy of empirical hydrogen bond acidity and basicity values was the main source of error in Surface-SFED/HB(cal). The mean absolute errors of Surface-SFED/HB(exp) and Surface-SFED/HB(cal) were 0.49 and 0.54 kcal/mol, respectively.     

Surface‐dependent effect of functional silica fillers on photocuring kinetics of hydrogel materials

Two types of silica: precipitated (P, prepared in non‐polar media, a new type, submicrometer sized) and fumed (F, nanosized), both unmodified and surface modified are investigated as functional fillers for potential applications in nanocomposites with poly(2‐hydroxyethyl methacrylate) matrix. Special attention is paid to the kinetics of composite formation in an in situ photopolymerization process. Silica‐containing formulations polymerize faster; this effect is much stronger for silica P having much larger particle size than silica F. Surface treatment leads to further acceleration of the polymerization in case of silica P but to retardation in case of silica F; the effect of modification of the filler surface on properties of composites is different for each of the silicas. The obtained results are discussed in terms of effects of curvature of silica particles, surface properties, solvation cell, interphase region, viscosity changes, and morphology of the resulting composites. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 3472–3487     

Effect of surface oxyfluorination on the dyeability of polyethylene film

The effect of surface oxyfluorination on low-density polyethylene (LDPE) film was studied in terms of surface functionality and surface energetics of the film surfaces, which can be attributed to improvement of the dyeability. The growth of functional groups and surface free energy was confirmed by FTIR-ATR, XPS, and contact angle methods. As a result, the total surface free energy was increased with oxyfluorination time, as a progressive increase of the polar component together with a small decrease of the dispersive component of surface free energy. From the dyeability test using the Kubelka-Munk equation, it was found that the oxyfluorination treatment plays an important role in the growth of oxygen-containing functional groups of LDPE film, resulting in improving the dyeability with a basic dyeing agent. A direct linear relationship is shown between the specific component of surface free energy and the K/S value for this work.