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390. Wu, S., “Estimation of the critical surface tension for polymers from molecular constitution by a modified Hildebrand-Scott equation (notes),” J. Physical Chemistry, 72, 3332-3334, (1968).

The paper proposes a modified Hildebrand-Scott equation to estimate the critical surface tension of polymers based on their molecular constitution.

391. Wu, S., “Surface and interfacial tensions of polymer melts, II. Poly(methylmethacrylate), poly(n-butyl methacrylate), and polystyrene,” J. Physical Chemistry, 74, 632-638, (1970).

392. Wu, S., “Calculation of interfacial tension in polymer systems,” J. Polymer Science, 34, Part C, 19-30, (1971).

We propose an equation, based on “reciprocal” mean and force additivity, for calculating the interfacial tension between polymers or between a polymer and an ordinary liquid:

mathematical formula
where γ12 is the interfacial tension; γi the surface tension; γ and γ the dispersion and polar components of γi, respectively. This equation is shown to predict accurately the interfacial tension between polymers or between a polymer and an ordinary liquid. Fowkes' equation or Fowkes' equation with a geometric-mean polar term 2(γiPγ2p)1/2 is not applicable to polarlpolar systems. The interfacial tension arises mainly from disparity in the polarities of the two phases. The above equation can also be used to calculate the surface tension and polarity of polymers or organic solids from contact angle data.

393. Wu, S., “Interfacial and surface tensions of polymers,” J. Macromolecular Science, C10, 1-73, (1974).

Interfacial and surface tensions of polymers are important in the technology of plastics, coatings, textiles, films, and adhesives through their roles in the processes of wetting, adsorption, and adhesion. Because of experimental difficulty due to high viscosity, however, reliable measurements of these quantities were not reported until 1965 for surface tensions [l, 2] and 1969 for interfacial tensions [3, 4]. A body of scattered data has been accumulated in the literature. This review will evaluate, compile, and interpret these results.

600. Wu, S., “Polar and nonpolar interactions in adhesion,” J. Adhesion, 5, 39-55, (1973).

Equations for polar and nonpolar interactions across the interface are developed by using energy additivity concept in a semi-continuum model. Interfacial and surface tensions of molten polymers are measured directly and used to test the resulting equations:

The first expression may be called the harmonic-mean equation preferred for low energy systems such as organic liquids, water, polymers, and organic pigments. The second may be called the geometric-harmonic-mean equation preferred for high energy systems such as mercury, glass, metal oxides and graphite. The third may be called the geometric mean equation which is found unsatisfactory. The harmonic-mean equation is used to obtain the “optimum” wettability condition for adhesion. The importance of polar interactions and matching of the polarity are analyzed and emphasized.

657. Wu, S., “Interfacial energy, structure and adhesion between polymers,” in Polymer Blends, Vol. 1, Paul, D.R., and S. Newman, eds., Academic Press, 1978.

Most small-molecule organic liquids are mutually miscible and their mixtures do not form stable interfaces. Polymers are, however, usually immiscible and their mixtures form multiphase structures with stable interfaces. The dispersion, morphology, and adhesion of the component phases are affected by the interfacial energies, which thereby play an important role in determining the mechanical properties of a multiphase polymer blend. This chapter discusses interfacial energy, structure, and adhesion between polymers as related to the properties of polymer blends. It provides an overview of surface tension and interfacial tension, and discusses various theories of interfacial tension such as Antonoff's rule, theory of Good and Girifalco, theory of fractional polarity, and lattice theories. The interfacial tension varies more slowly with temperature than the surface tension and arises mainly from the disparity between the polarities of two phases. The extent of non-polar (dispersion) interaction between the two phases does not vary greatly from system to system but that of polar interactions can vary greatly. The polar interactions largely determine the magnitude of the interfacial tension. The polarity of a polymer can be calculated from the interfacial tension between the polymer and a non-polar polymer such as polyethylene, which can be regarded to be completely nonpolar. Thus, if the interfacial tension of a polar polymer against polyethylene is known, the polarity of the polar polymer can be calculated. Adhesion is the bonding or joining of dissimilar bodies, while autohesion or cohesion refers to joining of identical bodies. Several adhesion theories have been proposed. They have been a subject of much controversy. Most theories deal with the formation of the adhesive bond. The chapter reviews various factors affecting the formation and the fracture of adhesive bonds.

658. Wu, S., Polymer Interface and Adhesion, Marcel Dekker, 1982.

915. Wu, S., “Notes - Surface tension of solids: an equation of state analysis,” J. Colloid and Interface Science, 71, 605-609, (Oct 1979).

1772. Wu, S., “Surface and interfacial tensions of polymer melts I: Polyethylene, polyisobutylene, and polyvinyl acetate,” J. Colloid and Interface Science, 31, 153-161, (Oct 1969).

The surface tensions of polyethylene, polyisobutylene, and polyvinyl acetate, and the interfacial tensions of polyethylene/polyvinyl acetate and polyisobutylene/polyvinyl acetate systems have been measured by the pendent drop method in the temperature range up to 200°C. The results are analyzed in terms of the equations of Fowkes and of Girifalco and Good, and suggest that the conformational restriction of polymer molecules imparts a limitation on the extent of interfacial contacts and sharp phase boundaries in these systems. Several quantities of interest in adhesion, such as contact angle, spreading coefficient, and work of adhesion are also discussed.

1774. Wu, S., “Surface and interfacial tensions of polymers, oligomers, plasticizers and organic pigments,” in Polymer Handbook, 3rd Ed., Brandrup, J., and E.H. Immergut, eds., VI: 414-426, Wiley-Interscience, 1989 (also in Polymer Handbook, 4th Ed., J. Brandrup, E.H. Immergut, and E.A. Grulke, eds., p. VI: 521-535, John Wiley & Sons, Jul 2003).

2300. Wu, S., “Surface tension of solids: Generalization and reinterpretation of critical surface tension,” in Adhesion and Adsorption of Polymers, Part A, Lee, L.-H., ed., 53-65, Plenum Press, 1980.

The concept of critical surface tension is generalized and reinterpreted in terms of a proposed equation of state. The equation defines a spectrum of critical surface tensions for a given surface, and provides a method by which the surface tension can be accurately determined from the contact angles of a series of testing liquids. The surface tensions obtained for solid polymers, organic solids and monolayers by this method agree remarkably well with those obtained from melt data (temperature dependence), liquid homologs (molecular-weight dependence) and the harmonic-mean equation. In contrast, those obtained by the geometric-mean equation and Zisman’s critical surface tensions are often too low. These results also support the validity of the harmonic-mean equation.

2329. Wu, S., “Polar and nonpolar interactions in adhesion,” J. Adhesion, 5, 39-55, (1973) (also in Recent Advances in Adhesion, L.-H. Lee, ed., Gordon and Breach, p. 45-63, 1973).

Equations for polar and nonpolar interactions across the interface are developed by using energy additivity concept in a semi-continuum model. Interfacial and surface tensions of molten polymers are measured directly and used to test the resulting equations: The first expression may be called the harmonic-mean equation preferred for low energy systems such as organic liquids, water, polymers, and organic pigments. The second may be called the geometric-harmonic-mean equation preferred for high energy systems such as mercury, glass, metal oxides and graphite. The third may be called the geometric mean equation which is found unsatisfactory. The harmonic-mean equation is used to obtain the “optimum” wettability condition for adhesion. The importance of polar interactions and matching of the polarity are analyzed and emphasized.

2009. Wu, S., and K.J. Brzozowski, “Surface free energy and polarity of organic pigments,” J. Colloid and Interface Science, 37, 686-690, (Dec 1971).

The surface free energy and polarity values of a number of organic pigments are obtained from contact angle measurements and the interfacial tension equation of Wu(1). The pigment types studied are phthalocyanine, quinacridone, toluidine red, isoindolinone, indanthrone, β-oxynaphthoic acid derivative, and thioindigoid red. The surface free energies obtained agree reasonably well with those predicted from parachor and density values.

707. Wu., D.Y., W.S. Gutowski, and S. Li, “Surface engineering of polymers for enhanced adhesion,” Presented at First International Congress on Adhesion Science and Technology, Oct 1995.

1326. Wulf, M., K. Grundke, D.Y. Kwok, and A.W. Neumann, “Influence of different alkyl side chains on solid surface tension of polymethacrylates,” J. Applied Polymer Science, 77, 2493-2504, (2000).

Low-rate dynamic contact angles on poly(t-butyl methacrylate) (PtBMA) were measured by an automated axisymmetric drop shape analysis profile (ADSA-P). The solid surface tension of PtBMA is calculated to be 18.1 mJ/m2, with a 95% confidence limit of ±0.6 mJ/m2. This value was compared to previous results with different homopolymeric polymethacrylates [poly(methyl methacrylate) (PMMA), poly(ethyl methacrylate) (PEMA), and poly(n-butyl methacrylate) (PnBMA)] and with copolymeric polymethacrylates {poly(methyl methacrylate/ethyl methacrylate, 30/70) [P(MMA/EMA, 30/70)] and poly(methyl methacrylate/n-butyl methacrylate) [P(MMA/nBMA)]}. It was found that increasing length and size of the alkyl side chain decrease the solid surface tension, as expected. Comparison with pure alkyl surfaces suggests that the surface tension of PtBMA is dominated by the very hydrophobic t-butyl group. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 2493–2504, 2000

1318. Wulf, M., S. Michel, K. Grundke, O.I. del Rio, D.Y. Kwok, and A.W. Neumann, “Simultaneous determination of surface tension and density of polymer melts using axisymmetric drop shape analysis,” J. Colloid and Interface Science, 210, 172-181, (1999).

By employing a new strategy, we show that axisymmetric drop shape analysis (ADSA) can be used to determine simultaneously the surface tension and the density of polymer melts from sessile drops at elevated temperatures. To achieve this, two developments were necessary. First, the ADSA algorithm had to be modified to replace the density by the mass of the drop as an input parameter. Since ADSA also yields the volume, the density became output rather than input. Second, a closed high-temperature chamber whose temperature could be precisely controlled and a sample holder that allowed the formation of highly axisymmetric sessile drops at elevated temperatures had to be developed. For a typical polymeric material (polystyrene), it is demonstrated that measurements with sessile drops yield essentially the same surface tension values and temperature coefficients as measurements with pendant drops. The densities determined with ADSA are comparable to independent PVT results.

1387. Xia, Z., R. Gerhard-Multhaupt, W. Kunstler, A. Wedel, and R. Danz, “High surface-charge stability of porous polytetrafluoroethylene electret films at room and elevated temperatures,” J. Physics D: Applied Physics, 32, 83-85, (1999).

Porous polytetrafluoroethylene films were positively or negatively corona-charged at room or elevated temperatures and their charge-storage behaviour was investigated by means of isothermal surface-potential and thermally stimulated discharge-current measurements. In addition, electron micrographs of the sample morphology were taken and the influence of high humidities on the surface-charge decay was investigated. For comparison, nominally non-porous polytetrafluoroethylene films were studied in the same manner. It was found that porosity may lead to significantly enhanced surface-charge stability for both polarities if the relative humidity is not too high. Further investigations are under way in order to better understand this behaviour and to employ it for electret applications.

983. Xiao, G.Z., “Effects of solvents on the surface properties of oxygen plasma-treated polyethylene and polypropylene films,” J. Adhesion Science and Technology, 11, 655-663, (1997).

The effects of solvents on the surface properties of oxygen plasma-treated polyethylene and polypropylene films have been studied by ESCA, contact angle measurement, and adhesion testing. The results show that oxygen plasma treatment produces some low molecular weight materials (LMWM) on the treated surfaces, which can be removed, to some extent, by solvents. It seems that the LMWM has different solubilities in different solvents. Among the solvents (water, acetone, and 2-propanol) used, acetone has the most significant effect. The removal of LMWM considerably reduces the wettability of the treated materials, but does not impair the adhesion increased by the plasma treatment.

1914. Xie, X., T.R. Gengenbach, and H.J. Griesser, “Changes in wettability with time of plasma-modified perfluorinated polymers,” J. Adhesion Science and Technology, 6, 1411-1431, (1992) (also in Contact Angle, Wettability and Adhesion: Festschrift in Honor of Professor Robert J. Good, K.L. Mittal, ed., p. 509-529, VSP, Nov 1993).

Treatment of fluorinated ethylene propylene (FEP) and polytetrafluoroethylene (PTFE) by plasmas established in water vapour or ammonia gas enabled the rapid and facile modification of their surface chemistries. Under comparable plasma conditions, ammonia plasma exposures produced considerably lower air/water contact angles than water vapour plasmas. On storage of samples in air at ambient temperature, contact angles increased markedly within a few days on ammonia plasma-treated samples but remained constant over many weeks on water plasma-treated surfaces. Angle-dependent X-ray photoelectron spectroscopy (XPS) demonstrated a very low depth of modification in the case of ammonia plasma exposure, whereas the oxygen content of water plasma-treated samples was invariant with depth within the XPS analysis region. The long-term stability of water plasma-treated fluorocarbon polymer surfaces is believed to be due to this deep modification which prevents polymer chain reorientation, whereas the shallow modification in ammonia plasmas allows the rapid partial burial of the newly attached chemical groups inside the polymer. When ammonia plasma-treated samples stored in air were immersed in water, the contact angles remained constant, suggesting that the buried groups could not resurface. Contact angle measurements provided a simple and sensitive method for studying the time-dependent reduction in plasma treatment effects and the segmental mobility of modified fluorocarbon polymer surfaces; very shallow reorientation movements can be detected.

2727. Xiong, L., P. Chen, and Q. Zhou, “Adhesion promotion between PDMS and glass by oxygen plasma pre-treatment,” J. Adhesion Science and Technology, 28, 1046-1054, (2014).

Polydimethylsiloxane (PDMS) and glass are among the most widely used materials in biomedical and microfluidic applications. In this paper, oxygen plasma exposure was used to improve the adhesion properties of PDMS and glass. The effect of bonding quality parameters such as RF power, time of activation and oxygen flow was investigated. Bonding area and strength, two main indicators of bonding quality, were detected using manual peel and mechanical shear tests, respectively, to optimize the bonding parameters. It was observed that increase in activation time and RF power increased the bonding strength considerably. The oxygen flow had a slight influence in increasing the bonding strength. The application of this bond has also been demonstrated in PDMS–glass micropump, so this technique can be potentially applied for fabrication of PDMS–glass-based microfluidic and biomedical devices.

2901. Xiu, Y., L. Zhu, D.W. Hess, and C.P. Wong, “Relationship between work of adhesion and contact angle hysteresis on superhydrophobic surfaces,” J. Physical Chemistry, 112, 11403-11407, (Jul 2008).

Low contact angle hysteresis is an important characteristic of superhydrophobic surfaces for nonstick applications involving the exposure of these surfaces to water or dust particles. In this article, a relationship is derived between the surface work of adhesion and the dynamic contact angle hysteresis, and the resulting predictions are compared with experimental data. Superhydrophobic surfaces with different contact angles and contact angle hysteresis were prepared by generating silicon pillars with varying pillar size and pitch. Surfaces were subsequently treated with fluoroalkyl silanes to modify further the hydrophobicity. The three-phase contact line established for such systems was related to the Laplace pressure needed to maintain a stable superhydrophobic state.

394. Yagi, T., A.E. Pavlath, and A.G. Pittman, “Grafting fluorocarbons to polyethylene in glow discharge,” J. Applied Polymer Science, 27, 4019-4028, (1982).

A systematic surface fluorination of high-density polyethylene was carried out using CF4, CF3H, CF3Cl, and CF3Br, in a radio-frequency glow discharge. Based on ESCA and wettability measurements, all of these compounds provided a fluorocarbon layer on high-density polyethylene surface, but the fluorine to carbon ratio and extractability of the films were strongly dependent on the starting materials and the location of the sample specimen in the reactor chamber as well as the duration of the reaction. The results with vertically held, CF3H-treated samples showed a high level of nonextractable surface fluorination and very little change in wetting properties before and after extraction with CF2ClCFCl2.

933. Yalkowski, S.H., and Y. He, Handbook of Aqueous Solubility Data, CRC Press, Apr 2003.

1030. Yamaguchi, M., “Effect of molecular structure in branched polyethylene on adhesion properties with polypropylene,” J. Applied Polymer Science, 70, 457-463, (Oct 1998).

Adhesion properties between branched polyethylene (PE) and isotactic polypropylene (PP) were studied by a peel test and scanning electron microscopy. In this study, two types of branched PEs were used; one is a linear low density polyethylene (LLDPE) and the other is a high pressure low density polyethylene (LDPE). The adhesive strength of the LLDPE/PP is much higher than that of LDPE/PP. Furthermore, the formation of PE influxes between PP spherulites has a small effect on the adhesion. The dynamic viscoelastic measurements for the binary blends composed of branched PE and PP were also carried out to estimate the interfacial tension by using a rheological emulsion model proposed by Palierne. The interfacial tension is 1.0 mN for LLDPE/PP and 2.1 mN for LDPE/PP, suggesting that the interfacial thickness of LLDPE/PP is about twice that of LDPE/PP. The adhesive strength between branched PE and PP will be determined by the interfacial thickness, which represents the entanglements between two polymers. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 457–463, 1998

2216. Yang, W., and N. Sung, “Adhesion promotion through plasma treatment in thermoplastic/rubber systems,” in Proceedings of the ACS Division of Polymer Materials: Science and Engineering, Vol. 62, 0, American Chemical Society, 1990.

1885. Yao, Y., X. Liu, and Y. Zhu, “Surface modification of high-density polyethylene by plasma treatment,” J. Adhesion Science and Technology, 7, 63-75, (1993).

The extent of the surface crosslinking of high-density polyethylene (HDPE) under various plasma treatment conditions was investigated. The plasma modification efficiency was studied by surface energy and adhesive bond strength measurements. The results show that the surface crosslinking of HDPE takes place as soon as the HDPE is exposed to the plasma and that the crosslinking rate is a function of the plasma conditions. The surface energy and the adhesion of HDPE are greatly increased by the plasma treatment and these improvements are independent of the depth of surface crosslinking. Based on these results and our previous studies on the surface chemical composition and free radical density on the surface of HDPE after plasma treatment, the relationships among various surface changes and the surface modification efficiency are discussed.

1106. Yasuda, H., Luminous Chemical Vapor Deposition and Interface, Marcel Dekker, 2005.

1748. Yasuda, H., “Modification of polymers by plasma treatment and by plasma polymerization,” Radiation Physics and Chemistry, 9, 805-817, (1977).

Plasma chemistry of polymers may be categorized into two major types of reactions as (1) surface reaction of polymers and (2) polymerization of monomers by plasma. So far as these two types of reactions are concerned, plasma is very similar to other ionizing radiation, such as ..gamma.. radiation, x radiation, UV radiation, and high-energy electron beams, which can (1) initiate polymerization of certain monomers, and create free radicals on polymer exposed, which lead to (2) crosslinking of the polymer and/or (3) degradation of the polymer, or can be further utilized as the initiation sites of (4) graft copolymerization. The characteristic features of plasma are (1) the radiation effect is limited to the surface, and the depth of the layer affected by the plasma is much smaller than that by other more penetrating radiation, and (2) the intensity at the surface is generally stronger than that by the more penetrating radiations. Therefore, plasma treatment provides an ideal means of modifying surface properties of polymers. Examples are presented and discussed.

395. Yasuda, H.K., “Plasma for modification of polymers,” J. Macromolecular Science, A10, 383-420, (1976).

The effect of nonpolymer-forming plasma (e.g., plasma of hydrogen, helium, argon, nitrogen) can be viewed as the following two reactions: 1) reaction of active species with polymer, and 2) formation of free radicals in polymer which is mainly due to the UV emitted by the plasma. The incorporation of nitrogen into the polymer surface by N2 plasma and the surface oxidation by O2 plasma are typical examples of the first effect. The latter effect generally leads to incorporation of oxygen in the form of carbonyl and hydroxyl and to some degree of cross-linking depending on the type of substrate; however, the degradation of polymer at the surface manifested by weight loss occurs in nearly all cases when polymers are exposed to plasma for a prolonged period of time. The effects of polymer-forming plasma is predominated by the deposition of polymer (plasma polymer); however, with some plasma-susceptible polymer substrates the effect of UV emission from polymer-forming plasma cannot be neglected. The mechanism of polymer formation can be explained by the stepwise reaction of active species and/or of an active specie with a molecule, and the chain addition polymerization of some organic compounds (e.g., vinyl monomers) is not the main route of polymer formation.

Plasma polymers contain appreciable amount of trapped free radicals; however, the concentration is highly dependent on the chemical structure of the monomer. In plasma polymerization, 1) triple bond and/or aromatic structure, 2) double bond and/or cyclic structure, and 3) saturated structure are three major functions which determine the rate of polymer formation and the properties of plasma polymers. The changes of some properties of plasma polymers with time are directly related to the concentration of trapped free radicals in plasma polymers. The amount of trapped free radicals in a plasma polymer is also influenced by the conditions of discharge; however, the UV irradiation from the polymer-forming plasma is not the main cause of these free radicals. Excess amount of free radicals are trapped during the process of polymer formation (rather than forming free radicals in the deposited polymer by UV irradiation). The properties of a plasma polymer is generally different from what one might expect from the chemical structure of the monomer, due to the fragmentation of atoms and/or functions during the polymerization process. This is another important factor to be considered for the modification of polymer surfaces by plasma polymerization.

603. Yasuda, H.K., Plasma Polymerization, Academic Press, 1985.

864. Yasuda, H.K., “Surface dynamics and plasma polymers,” in Plasma Processing of Polymers (NATO Science Series E: Applied Sciences, Vol. 346), d'Agostino, R., P. Favia, and F. Fracassi, eds., 149-164, Kluwer Academic, Nov 1997.

According to the concept described by Langmuir in 1938 [1], the surface properties of a solid are determined by the surface-configuration (spatial arrangement of atoms at the interface) rather than the configuration of molecules which occupy the top surface region. In other words, whether a polymer surface is hydrophilic or hyrophobic cannot be predicted by the presence or absence of hydrophilic moieties in the molecules, but is determined by whether or not the hydrophilic moieties are located at the interface. In recent years, it has been recognized that the surface of a solid, particularly polymeric solid, is very different from what can be anticipated from the bulk characteristics of the same material. This discrepancy has been a focal point of the general phenomena recognized by the terms surface dynamics, surface reconstruction, etc., which deal with the change of chemical and morphological properties of polymer surface due to the change of the surrounding medium [2-14]. The surface dynamic change depends on the reference state from which the change takes place, and if one cannot define the reference state, the surface dynamics cannot be dealt in a generic sense. This problem was indeed found with moderately hydrophilic copolymer of ethylene/vinyl alcohol. The reference state depends on the history of a sample, and the change cannot be reproduced without precise knowledge of the history of a sample [15]. According to the view that a polymeric surface is an ever-changing entity depending on the surrounding medium [16], the restructured surface is not necessarily the final one to stay, ie, restructuring of once restructured surface or multiple repeated restructuring occur with highly perturbable polymeric surfaces. Therefore, the term" surface reconstruction" is intensionally avoided in the discussion of surface dynamics in this article. The change of surface is expressed by the change of surface-configuration.

397. Yasuda, H.K., A.K. Sharma, and T. Yasuda, “Effect of orientation and mobility of polymer molecules at surfaces on contact angle and its hysteresis,” J. Polymer Science Part B: Polymer Physics, 19, 1285-1291, (1981).

The contact angle of a water droplet on the surface of a solid polymer or hydrogel (water-swollen three-dimensional network) depends on whether a hydrophilic moiety of the polymer molecule is oriented towards the air interface or towards the bulk of the solid, but not on the hydrophilicity of the molecule. Therefore, the short-range rotational mobility of a polymer molecule has a major influence on the apparent hydrophilicity of a polymer surface as measured by the contact angle of water. By the came principle, the abnormally large hysteresis effect observed in advancing and receding contact angles of water on some polymer surfaces can be attributed to the reorientation of hydrophilic moieties of polymer molecules at the surface. These factors are demonstrated by selected polymer surfaces with different degrees of mobility at the polymer-air interface.

398. Yasuda, H.K., D.L. Cho, and Y.-S. Yeh, “Plasma-surface interactions in the plasma modification of polymer surfaces,” in Polymer Surfaces and Interfaces, Feast, W.J., and H.S. Munro, eds., 149-162, John Wiley & Sons, 1987.

396. Yasuda, H.K., H.C. Marsh, S. Brandt, and C.N. Reilly, “ESCA study of polymer surfaces treated by plasma,” J. Polymer Science Part A: Polymer Chemistry, 15, 991-1019, (1977).

Surfaces of polymers [polyethylene, polystyrene, poly(ethylene terephthalate), poly(oxymethylene), cellulose acetate, polyacrylonitrile, nylon 6, and polytetrafluoroethylene] treated with argon (inert) and nitrogen (reactive) plasma were examined by ESCA (electron spectroscopy for chemical analysis). Argon plasma treatment generally introduces oxygen functionalities into the polymer surface. Nitrogen treatment generally incorporates nitrogen and oxygen functionalities into the treated surface. The extent of oxygen incorporation is typically less than that produced by argon plasma. When nitrogen and oxygen functional groups are already in a polymer structure, the extent of additional incorporation of these two elements as a result of plasma treatment is very much less than with other polymers. Polymers which contain only one of the elements tend to incorporate the other element to much the same degree as polymers without either element initially present.

913. Yasuda, H.K., ed., Plasma Polymerization and Plasma Treatment of Polymers: Applied Polymer Symposia 42, Wiley - Interscience, Apr 1987.

604. Yasuda, T., K. Yoshida, T. Okuno, and H.K. Yasuda, “A study of surface dynamics of polymers, III. Surface dynamic stabilization by plasma polymerization,” J. Polymer Science Part B: Polymer Physics, 26, 2061-2074, (1988).

As demonstrated in Part II of this series of studies, the hydrophobic character of CF4 plasma-treated Nylon 6 and poly(ethylene terephthalate) (PET) decay with time of water immersion, and the rate of decay can be used as a measure for the surface mobility of (substrate) polymers. The same method of using fluorine-containing moieties introduced by CF4 plasma treatment as surface labeling is applied to investigate the influence of a thin layer of plasma polymer of methane applied onto the surface of those polymers. An ultrathin layer of plasma polymer provides a barrier to the rotational and diffusional migration of the introduced chemical moieties from the surface into the bulk of the film. The influence of operational parameters of plasma polymerization on the surface dynamic stability are examined by measuring the decay rate constants for (subsequently) CF4 plasma-treated samples. The rate constant was found to decrease sharply with increasing value of plasma energy input manifested by J/kg monomer, and no decay was observed as the energy input reached a threshold value (about 6.5 GJ/kg for PET, about 7.0 GJ/kg for Nylon 6), indicating that unperturbable surfaces can be created by means of plasma polymerization.

2108. Yasuda, T., M. Gazicki, and H. Yasuda, “Effects of glow discharges on fibers and fabrics,” J. Applied Polymer Science, 38, 201-214, (1984).

399. Yasuda, T., T. Okuno, K. Yoshida, and H.K. Yasuda, “A study of surface dynamics of polymers, II. Investigation by plasma surface implantation of fluorine-containing moieties,” J. Polymer Science Part B: Polymer Physics, 26, 1781-1794, (1988).

Macromolecules at the surface of a polymeric solid have considerable mobility, and the specific arrangement of functional groups of macromolecules at the surface is dictated by the environmental conditions in which the surface is placed. Consequently, the change of environmental conditions, such as immersion in water or placement in a biological surrounding, could cause a cosiderable degree of change in the surface characteristics of a polymer from those evaluated in the laboratory against ambient air. The mobile nature of a polymer surface can be investigated by surface-implanting fluorine-containing moieties, mainly—CF3, by the plasma implantation technique and following the disappearance and reappearance of fluorine atoms on the surface. The disappearance rates (based on the immersion time in water at room temperature) of ESCA F1s signals, the decay rates of (advancing) contact angle of water, and the recovery of these values on heat treatment of water-immersed samples were measured as a function of crystallinity of polymer samples (at three levels of crystallinity) for poly(ethylene terephthalate) and nylon 6.

1823. Yasuda, T., T. Okuno, and H. Yasuda, “Contact angle of water on polymer surfaces,” Langmuir, 10, 2435-2439, (Jul 1994).

1292. Yetka-Fard, M., and A.B. Ponter, “Factors affecting the wettability of polymer surfaces,” J. Adhesion Science and Technology, 6, 253-277, (1992).

The inconsistencies in contact angle data presented in the literature can be attributed to a number of factors. The awareness of these factors would allow novice researchers to make meaningful contact angle measurements and interpretations. In this survey the effects of surface roughness and heterogeneity, surface preparation and the presence of contaminants, the vapor environment, pressure and temperature, drop size, electrical charge, and heat transfer on the wettability of polymer surfaces were examined.


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