Surface composition, fluorine distribution, and morphology were determined for polyimide films modified downstream from microwave plasmas containing CF₄/O₂. Complementary analytical techniques including x‐ray photoelectron spectroscopy, Rutherford backscattering spectroscopy, and scanning electron microscopy yielded a more complete understanding of polyimide fluorination and subsequent etching of the modified film. Depth of fluorination increased nonlinearly with treatment time for films exposed downstream from a CF₄‐rich plasma. Exposure downstream from an O₂‐rich plasma resulted in a reduction of thickness in both the fluorinated layer and the unmodified polyimide during etching. Finally, a model for fluorination of polyimide and subsequent removal is proposed.

Oxidation of a polyethylene (PE) surface by corona discharge and the subsequent graft polymerization of acrylamide (AAm) were studied. The maximum amount of peroxides introduced by corona treatment at a voltage of 15 kV was about 2.3 × 10⁻⁹ mol cm⁻². The decomposition rate of peroxide and the dependence of graft amount on the storage period of the corona-treated PE films showed that there were several kinds of peroxides, the labile one being mainly responsible for the initiation of graft polymerization. When the corona-treated film was brought into contact with a deaerated aqueous solution of AAm, graft polymerization took place more strongly with the treatment time, but was reduced after passing a maximum. Although the x-ray photoelectron spectroscopic analyses of the corona-treated PE films showed homogeneous oxidation of the outer polymer surface by corona discharge, optical microscopy on the cross section of the grafted film revealed the graft polymerization to be limited to a very thin surface region.

The ESCA spectra of Kapton polyimide film exposed to atomic oxygen O(3P), either in low earth orbit (LEO) on the STS-8 Space Shuttle or downstream from a radio-frequency oxygen plasma, were compared. The major difference in surface chemistry induced by the two types of exposure to O(3P), both of which caused surface recession (etching), was a much larger uptake of oxygen by Kapton etched in the O2 plasma than in LEO. This difference is attributed to the presence of molecular oxygen in the plasma reactor and its absence in LEO: in the former case, O2 can react with radicals generated in the Kapton molecule as it etches, become incorporated in the etched polymer, and thereby yield a higher steady-state ‘surface oxidation’ level than in LEO.

X-ray photoelectron spectroscopy is used to measure the surface composition of polycarbonate/ poly(butylene terephthalate) blends. The blend surface is enriched in PC compared to the bulk, with the surface PC/PBT ratio equal to about 1.6 times to bulk formation. For blends containing an impact modifier as a third component, the XPS spectra of the molded surface indicates that no impact modifier is present within the XPS sampling depth. A spectral simulation scheme improves the accuracy of the computed PC/PBT ratio over conventional data reduction schemes involving curve fitting.

ESCA and contact-angle measurements were used to characterize the surfaces of polystyrene films exposed to SF₆, CF₄, and C₂F₆ plasmas. SF₆ plasmas cause loss of aromaticity in the polystyrene surface region via saturation of the phenyl ring and/or carbon-bond breakage and subsequent fluorination. C₂F₆ plasmas graft CF_x radicals directly to the polystyrene surface without necessarily destroying the aromaticity of the polymer. CF₄ plasmas appear to be intermediate in character between SF₆ and C₂F₆ plasmas.

A mean field theory is presented to describe surface ordering phenomena in diblock copolymers near the microphase separation transition (MST). We consider a near-symmetric diblock melt in the vicinity of a solid wall or free surface, such as a film-air interface. The surface is allowed to modify the Flory interaction parameter and the chemical potential in the adjacent copolymer layer. The composition profile normal to the surface is investigated both above and below the MST. In contrast to the surface critical behavior of binary fluids or polymer blends, we find interesting oscillatory profiles in copolymers that arise from the connectivity of the blocks. These composition profiles might be amenable to study by ellipsometry, by evanascent wave-induced fluorescence, or by scattering techniques. Wetting and other surface phenomena and transitions in block copolymers are briefly discussed.

There is provided a series of testing formulations or solutions which enable one to determine the critical surface tension or wettability of various solids, semi-solids and viscous liquids by application of the testing solutions to the surfaces of such materials or substrates. The solutions consist of a main solution which is 45 percent purified water and 55 percent of a dihydric alcohol such as ethylene glycol. This basic combination has a wetting tension of 56 dynes and has a completely neutral ph, and a neutral relative polarity. To vary surface tension, a non-ionic surfactant is added in ranges from about 1.5 percent to 0.001 percent by weight of the solution. In this manner the surface tension of each solution is varied in equal or predetermined increments. A dye is also employed to provide good contrast when the solution is placed on a test substrate. The percentage of dye can vary between 0.1 to 1 percent by weight depending upon the particular dye used. Due to the nature of the test solution provided, one can now perform wettability tests on various surfaces which could not be accommodated by prior art techniques. Each of the solutions employed have identical ph, identical viscosity, while further possessing the same neutral relative polarity.

The methods to estimate the surface tension of polymer solids using contact angles have been reviewed in the first part. They are classified into the following three groups depending on the theories or the equations applied: (1) the methods using the Young's equation alone, (2) the methods using the combined equation of Young and Good-Girifalco, and (3) the methods using the equations of work of adhesion. Some notes and comments are given for each method and results are compared with each other. The two-liquids method for rather high energy surface is also introduced.

Next, some new possibilities to evaluate the surface tension of polymer solids are presented by our new contact angle theory in consideration of the friction between a liquid drop and a solid surface. The advancing and receding angles of contact (θ_a and θ_r) are explained by the frictional tension γ_F and accordingly two kinds of the critical surface tension γ_C(γ_Ca and γ_Cr) are given.

This work has shown that one of the recommendable ways to evaluate γ_S is either the maximum γ_LV cos θ_a or the maximum γ_C using the advancing contact angle θ_a alone, and another way is the arithmetic or the harmonic mean of the γ_Ca and γ_Cr. A depiction to determine the γ_C such as ln(1 + cos θ₀) vs. γ_LV with cos θ₀ = (cos θ₀ + cos θ_r)/2 has also been proposed.

Any desired surface wettability of a plastic surface can be produced by changing the concentration of the plasma gas, which here is a mixture of oxygen and a compound which includes fluorine. In the plasma treatment, the use of a third electrode consisting of a metal mesh for ion trapping can significantly decrease the etching effect. The plastic surface wettability, given by the contact angle of a water drop, does not have any direct relationship with the surface roughness due to etching in this experiment.

As discussed in Chapter 1, it has been recognized for many years that the establishment of intimate molecular contact is a necessary, though sometimes insufficient, requirement for developing strong adhesive joints. This means that the adhesive, and primer if one is employed, needs to be able to spread over the solid surface, and needs to displace air and other contaminants that may be present on the surface.

ESCA and contact angle measurements were used to characterize the surfaces of Polyethylene and polypropylene films exposed to SF₆, CF₄, and C₂F₆ plasmas. None of these gases polymerized in the plasma. However, all plasma treatments grafted fluorinated functionalities directly to the polymer surfaces. SF₆ plasmas graft fluorine atoms to a polyolefin surface. CF₄ plasmas also react by a mechanism dominated by fluorine atoms, but with some contribution from CF_x-radical reactions. Although C₂F₆ does not polymerize, the mechanism of grafting is still dominated by the reactions of CF_x radicals. For all gases studied, the lack of polymerization is attributed to competitive ablation and polymerization reactions occurring under conditions of ion bombardment.

Following the development of a methodology for determining the apolar components as well as the electron donor and the electron acceptor parameters of the surface tension of polar surfaces, surfaces of a number of quite common materials were found to manifest virtually only electron donor properties and no, or hardly, any electron acceptor properties. Such materials may be called monopolar; they can strongly interact with bipolar materials (e.g., with polar liquids such as water); but one single polar parameter of a monopolar material cannot contribute to its energy of cohesion. Monopolar materials manifesting only electron acceptor properties also may exist, but they do not appear to occur in as great an abundance. Among the electron donor monopolar materials are: polymethylmethacrylate, polyvinylalcohol, polyethyleneglycol, proteins, many polysaccharides, phospholipids, nonionic surfactants, cellulose esters, etc.

Strongly monopolar materials of the same sign repel each other when immersed or dissolved in water or other polar liquids. The interfacial tension between strongly monopolar surfaces and water has a negative value. This leads to a tendency for water to penetrate between facing surfaces of a monopolar substance and hence, to repulsion between the molecules or particles of such a monopolar material, when immersed in water, and thus to pronounced solubility or dispersibility. Monopolar repulsion energies can far outweigh Lifshitz-van der Waals attractions as well as electrostatic and “steric” repulsions. In aqueous systems the commonly observed stabilization effects, which usually are ascribed to “steric” stabilization, may in many instances be attributed to monopolar repulsion between nonionic stabilizing molecules. The repulsion between monopolar molecules of the same sign can also lead to phase separation in aqueous solutions (or suspensions), where not only two, but multiple phases are possible. Negative interfacial tensions between monopolar surfactants and the brine phase can be the driving force for the formation of microemulsions; such negative interfacial tensions ultimately decay and stabilize at a value very close to zero.

Strongly monopolar macromolecules or particles surrounded by oriented water molecules of hydration can still repel each other, albeit to an attenuated degree. This repulsion was earlier perceived as caused by “hydration pressure”.

A few of the relevant colloid and surface phenomena are reviewed and re-examined in the light of the influence of surface monopolarity on these phenomena.

Contact angle measurements on polymer hydrogels were performed at various temperatures, and we obtained the dispersive (γ_s^d) and nondispersive (γ_s^p) components of the surface tension of polymer hydrogel at each temperature. Utilizing the temperature dependence values of γ_s^d and γ_s^p, we obtained the surface entropies of polymer hydrogels. The polymer hydrogels used were isotactic and syndiotactic poly(2-hydroxyethyl methacrylate) (HEMA), poly(2-hydroxyethyl methacrylate + aminoethyl methacrylate) (HEMA + AEMA), poly(2-hydroxyethyl methacrylate + N-vinyl pyrrolidone) (HEMA + VP), poly(2-hydroxyethyl methacrylate + methyl methacrylate) (HEMA + MMA), poly(2-hydroxyethyl methacrylate + methoxyethyl methacrylate) (HEMA + MEMA), and poly(2-hydroxyethyl methacrylate + methoxyethoxyethyl methacrylate) (HEMA + MEEMA), respectively. The contact angles were also measured by using droplets of water-immiscible liquids under conditions in which the polymer hydrogel was fully hydrated.

X-Ray photoelectron spectroscopy (XPS) was used to determine plasma induced chemical species on the surface of polyethylene (PE). Argon plasmas were found to have no detectable chemical effect on the PE surface, whereas oxygen and nitrogen plasmas created new chemical species which altered the chemical reactivity of the PE surface. Oxygen plasmas were found to react more rapidly with the PE surface than nitrogen plasmas. The degree of incorporation of new chemical species in the near surface region is approximately 20 at. % at the saturation level for both oxygen and nitrogen plasmas. Core level spectra for oxygen and nitrogen plasma treated PE suggest the formation of primarily C-O-C species in the former and C-N species in the latter. Angle-resolved XPS measurements indicate that the depth of incorporation of new chemical species is confined to the top 25 A.

The equilibrium at the triple line where a liquid and a fluid (either vapour or a second liquid immiscible with the first) meet on a solid surface was originally described nearly two centuries ago¹. By using a simple vectorial argument, the well-known Young equation may be obtained by resolution of the three interfacial tensions, γ, parallel to the solid surface:

$${\gamma _{s2}} = {\gamma _{s1}} + {\gamma _{12}}\cos \theta$$
(1)

where 1, 2 and S represent respectively the liquid, the fluid and the solid and θ is the contact angle measured in phase 1. Nevertheless, an objection has on occasion been presented. Although everything is balanced parallel to the solid surface, nothing would seem to counteract the vertical component γ₁₂ sin θ.^2,3 When the solid is treated as perfectly rigid, it is possible to apply variational calculus and the criterion of minimum free energy at equilibrium. The result is that equation 1 is perfectly correct.^4–9 When the solid is considered to be elastic, but very thin, a variational treatment leads us to take into account, in addition to interfacial effects, those due to elastic strain energy and (implicitly) gravity¹⁰. The approach invokes the modelling of the solid either by thin plate or membrane theory. This treatment leads to modified equilibrium conditions although in practice the effect will be very small except for very thin solids (cell walls?).

Over the last thirty years or so, the use of contact angle measurement in the study of solid-liquid interactions and problems of adhesion has become very frequent. Methods have been developed based on the concepts 1 2 of authors such as Zisman¹ (critical surface tension, γ_c) and Fowkes² (polar and apolar interactions), to name but two examples. The essence of the method of contact angle measurement is that, unless considering a very thin substrate³, the triple line where the solid, S, liquid 1 and second, immiscible fluid 2 meet can be described by Young’s equation relating the three interfacial tensions, γ_ij, and the contact angle θ:

$${\gamma _{s2}} = {\gamma _{s1}} + {\gamma _{12}}\cos \theta$$
(1)

Provided the surface characteristics of two of the phases are known, those of the third may be assessed using the theoretically unique value of θ. This contact angle will be unique provided the three phases are strictly homogeneous and smooth. Unfortunately, in practice, it is very common to obtain experimentally a whole range of contact angles for a given three- phase system. The largest angle obtainable corresponds to that observed just after a drop of liquid has advanced on the solid surface and is therefore known as the advancing angle, θ_A. Similarly the smallest angle is obtained just after the arrest of a receding liquid drop and the corresponding angle is θ_R.

We report contact angle measurements of five n-alkanes, dodecane through hexadecane, on Teflon (FEP) as a function of drop size. In all cases the contact angles decreased by approximately 5° when the drop size was increased from approximately 1 to 4 mm contact radius. A complete solution to the problem of mechanical equilibrium of a sessile drop on a solid surface indicates that the dependence of the contact angle on drop size may be explained by including the effect of line tension in the Young equation. The observed drop size dependence of the contact angle yields a line tension of (2.5 ± 0.5) × 10⁻⁶ J/m. Over the range of n-alkanes studied it was not possible to discern any dependence of the line tension on liquid surface tension.

The strength of macroscopic adhesive bonds of polymers is known to be directly proportional to the microscopic exothermic interfacial energy changes of bond formation, as measured by Dupre's 'work of adhesion'. Since the work of adhesion can be very appreciably increased by interfacial acid-base bonding with concomitant increases in adhesive bond strength, it is important to understand the acid-base character of polymers and of the surface sites of substrates or of the reinforcing fillers of polymer composites. The best known acid-base bonds are the hydrogen bonds; these are typical of acid-base bonds, with interaction energies dependent on the acidity of the hydrogen donor and on the basicity of the hydrogen acceptor. The strengths of the acidic or basic sites of polymers and of inorganic substrates can be easily determined by spectroscopic or calorimetric methods, and from this information one can start to predict the strengths of adhesive bonds. An important application of the new knowledge of interfacial acid-base bonding is the predictable enhancement of interfacial bonding accomplished by surface modification of inorganic surfaces to enhance the interfacial acid-base interactions.