A remote plasma reactor, with air as the plasma gas, has been used for in-line surface modification of linear low-density polyethylene tape (LLDPE) passing 10 cm below the main plasma zone. Line speeds of up to 0.70 m/s were tested, allowing the study of 0.014 s exposure times to the plasma. Oxygen to carbon (O/C) ratios averaging 0.11 were observed on a reproducible basis. The reactor was also used for static plasma treatment under similar experiment onditions. This allowed a comparative study of short-term (milliseconds) vs. long-term (several seconds) plasma treatment. High-resolution X-ray photoelectron spectroscopy (XPS) analysis of the treated polymer surface suggested the formation of hydroxyl (COH), carbonyl (CO) and carboxyl (OCO) groups, even after short plasma treatment. The intensities of these components were seen to increase in approximately equal quantities with increasing O/C ratio. Water washing of polyethylene surfaces with high O/C ratios showed a loss of oxygen, apparent as a decrease in OCO groups in the C 1s spectra. A smaller loss in oxygen was observed when washing samples that had been plasma-treated for milliseconds. A surface ageing study revealed that polyethylene surfaces that had been plasma-treated for short time periods showed only a negligible loss of oxygen on prolonged exposure to air. Surfaces treated for longer time periods showed a loss of up to 50% of the total oxygen on the surface within a few days of treatment. Static secondary ion mass spectrometry has provided some supporting evidence for surface damage of the treated films.

A remote plasma reactor, with air as the plasma gas, has been used for in-line surface modification of linear low-density polyethylene tape (LLDPE) passing 10 cm below the main plasma zone. Line speeds of up to 0.70 m/s were tested, allowing the study of 0.014 s exposure times to the plasma. Oxygen to carbon (O/C) ratios averaging 0.11 were observed on a reproducible basis. The reactor was also used for static plasma treatment under similar experiment onditions. This allowed a comparative study of short-term (milliseconds) vs. long-term (several seconds) plasma treatment. High-resolution X-ray photoelectron spectroscopy (XPS) analysis of the treated polymer surface suggested the formation of hydroxyl (C-OH), carbonyl (C=O) and carboxyl (O-C=O) groups, even after short plasma treatment. The intensities of these components were seen to increase in approximately equal quantities with increasing O/C ratio. Water washing of polyethylene surfaces with high O/C ratios showed a loss of oxygen, apparent as a decrease in O-C=O groups in the C 1s spectra. A smaller loss in oxygen was observed when washing samples that had been plasma-treated for milliseconds. A surface ageing study revealed that polyethylene surfaces that had been plasma-treated for short time periods showed only a negligible loss of oxygen on prolonged exposure to air. Surfaces treated for longer time periods showed a loss of up to 50% of the total oxygen on the surface within a few days of treatment. Static secondary ion mass spectrometry has provided some supporting evidence for surface damage of the treated films.

The effects of remote nitrogen plasma, remote oxygen plasma, and corona discharge treatments on linear low-density polyethylene were studied with regard to the chemical and physical surface modification, depth of modification, and surface stability. An attempt was made to correlate the type and the extent of modification with the printing and adhesion properties of the modified surfaces. Surface topography was studied using scanning electron microscopy. The relative percentages of nitrogen and oxygen on the surfaces were determined by X-ray photoelectron spectroscopy. Printing and adhesion tests were performed using standard, commercially available inks and adhesives.

The effect of a remote nitrogen plasma on polyethylene and polystyrene was studied. The gas flow rate, the dilution of reactant gas, exposure times, and reactor base pressure were all found to have a large impact on the efficiency of nitrogen incorporation. Optimum conditions caused 18 atom % nitrogen to be incorporated within 20 seconds for polyethylene and 10 seconds for polystyrene. Studying a remote nitrogen plasma treated polyethylene sample over a period of 1 month indicated that except for a drop in the % N on initial exposure to air the concentration of nitrogen on the surface remained steady within the experimental limits. Angle resolved photoelectron spectroscopy indicated that nitrogen is incorporated to a depth below the analysis depth of XPS.

New reaction products have been generated on polyethylene and polystyrene surfaces using a novel two-step process. The first stage involves exposure to a downstream nitrogen plasma, and the second to either ozone or a corona discharge. It is observed that each of the two-step reactions yields very different reaction products, with an apparent increase in the formation of CO functional groups in the former case and the formation of surface NO₂ groups in the latter case.

The reaction rates and products of remote oxygen plasma treatment, corona discharge, and ozone treatment of high and low density polyethylenes have been examined using x-ray photoelectron spectroscopy. The oxygen uptake by remote plasma treatment was faster than that of other surface treatments using excited oxygen species. A steady state concentration of 18 ± 1% oxygen can be attained within 1 s of exposure in the remote plasma.

A study has been undertaken in which both x-ray photoelectron spectroscopy (XPS) and Fast atom bombardment static secondary ion mass spectrometry (FAB-SSIMS) have been used to study the effects of remote nitrogen plasma treatment on polymers such as linear low-density polyethylene (LLDPE), poly(ethylene vinyl alcohol) (EVOH) and poly(ethylene terephthalate) (PET). For comparison, remote oxygen plasma treatment was also performed on LLDPE. A very rapid uptake of nitrogen was observed for all polymers. Negative FAB-SSIMS indicated CN⁻, CNO⁻ and C₂N fragments on each of the nitrogen plasma-treated polmers. Positive FAB-SSIMS spectra of plasma-treated LLDPE showed relatively high intensity, high mass fragments, thought to originate from additives. These were not observed for the other two polymers. Significant amounts of aromatic-type fragments were observed in the positive FAB-SSIMS spectra of all treated polymers. Surface stability studies have shown that for both nitrogen and oxygen plasma-treaed LLDPE there is a substantial decrease in the surface functionality on exposure to air. This effect was much less prevalent for EVOH and PET.

Morphological and chemical changes of the surface of low-density polyethylene (LDPE), linear middle-density polyethylene (L-MDPE), and their 80/20 blend were studied by different techniques after corona-discharge treatment in air and subsequent annealing. The surface tension was determined by wetting; the roughness was measured by atomic force microscope (AFM), and the surface chemical composition was analyzed by X-ray photoelectron spectroscopy (XPS), whereas the low-molecular-mass fraction washed off by chloroform by FTIR. The surface tension of the films increases with the electrode current. The surface roughness depends primarily on the polymer type and is less affected by the corona treatment. At the initial stage of annealing, posttreatment-type oxidation and hydrophobic recovery are competing. The former is more pronounced in L-MDPE, the latter in LDPE. After annealing at 50°C for 160 days, hydrophobic recovery becomes predominant in each film studied, which is accompanied by significant smoothening of the surface. According to XPS and FTIR results, this is due to the migration of low-molecular-mass components (oligomers, oxidized polymer fractions, and additives) to the surface. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 1529–1541, 2000
https://onlinelibrary.wiley.com/doi/abs/10.1002/%28SICI%291097-4628%2820000606%2976%3A10%3C1529%3A%3AAID-APP6%3E3.0.CO%3B2-J

One of the major disadvantages of low density polyethylene (LDPE) films is their poor adhesive properties. Therefore, LDPE films have been treated with atmospheric pressure air plasma in order to improve their surface properties. So as to simulate the possible conditions in an industrial process, the samples have been treated with two different sample distances (6 and 10 mm), and treatment rates between 100 and 1000 mm s⁻¹. The different sample distances are the distance of the sample from the plasma source. The variation of the surface properties and adhesion characteristics of the films were investigated for different aging times after plasma exposure (up to 21 days) using contact angle measurement, atomic force microscopy, weight loss measurements and shear test. Results show that the treatment increases the polar component and these changes improve adhesive properties of the material. After the twenty-first day, the ageing process causes a decrease of wettability and adhesive properties of the LDPE films (up to 60%).

The vast majority of polymers and composites have low surface energy; this is due to the low presence of functional groups on their surface which results in low adhesive properties. In order to modify this intrinsic property chemical and physical processes are commonly used. These processes present disadvantages, such as the use of products harmful to the environment. An alternative to these processes is the use of plasma technology. The main objective of this study is the improvement of the adhesive properties of the low density polyethylene (LDPE). In order to achieve the target, atmospheric plasma pretreatment has been optimized in order to promote subsequent adhesion processes, as the ones needed in the toy industry or the application of dyes or printing on surfaces. Plasma surface treatment is a clean process from the environmental viewpoint. This process does not emit any residue and it is easy to implement in an industrial process. Moreover the atmospheric plasma treatment is suitable to be applied in a large variety of materials even at high speeds when the treatment lasts less than a few seconds. In the present study it is examined the physical and chemical processes that occur in the LDPE surface as function of speed rate and distance of treatment. An increase both of the polar groups on the surface and the roughness after the treatment may increase its adhesive properties. It has been analyzed the influence of the speed rate and the nozzle distance on the final results. The adhesive properties have been evaluated using the T-peel test. The results show that at low speeds rates and low nozzle/substrate distance there is a greater inclusion of polar molecules at the surface. Consequently the adhesion properties of LDPE are improved.

Ink–cellulose interactions were evaluated using a new technique in which the adhesion properties between ink and cellulose were directly measured using a Micro-Adhesion Measurement Apparatus (MAMA). The adhesion properties determined with MAMA were used to estimate the total energy release upon separating ink from cellulose in water. The total energy release was calculated from interfacial energies determined via contact angle measurements and the Lifshitz–van der Waals/acid–base approach. Both methods indicated spontaneous ink release from model cellulose surfaces, although the absolute values differed because of differences in measuring techniques and different ways of evaluation. MAMA measured the dry adhesion between ink and cellulose, whereas the interfacial energies were determined for wet surfaces. The total energy release was linked to ink detachment from model cellulose surfaces, determined using the impinging jet cell. The influences of surface energy and surface roughness were also investigated. Increasing the surface roughness or decreasing the surface energy decreased the ink detachment due to differences in the molecular contact area and differences in the adhesiom properties.

A simple apparatus and technique are described for measuring contact angles formed by liquid drops on solid surfaces. The procedure is a modification of one described 25 years ago by Langmuir and Schaeffer. It is based on observation of the angle at which light from a point source is reflected from a liquid drop surface at its contact point with a solid. The technique gives results the same, within experimental error, as those obtained by other investigators who used the usual “drop profile” method. The apparatus has the advantages of ease of construction and operation, ruggedness, and low cost coupled with high precision and accuracy.

Adhesion between two substrates is a complex phenomenon which at present is still not well understood. The important existing adhesion models (electrical, diffusion, thermodynamic adsorption, chemical, etc.) are reviewed in order to try to explain their mechanisms. Thermodynamic adsorption is now believed to be one of the most importnat mechanisms by which adhesion is achieved. Difusion and wetting are kinetic means in attaining good adsorption of a polymer at the interface. In the case of this model (thermodynamic adsorption), the notion of surface energy is developed and the importance of this property in the understanding of adhesion phenomena is emphasized. The methods of determining the surface characteristics of low and high energy solids are presented. The role played by acid-base interactions in adhesion is also mentioned.

Equations based on a simple model of surfaces and interfaces have been found useful for relating quantitatively several previously unrelated fields of surface chemistry {10-13). These equations introduce a new term — the London dispersion force contribution to the surface free energy (7d)—and make use of this term for the accurate calculations of surface tension, interfacial tension, contact angles, heats and free energies of immersion, heats and free energies of adsorption, and the long-range van der Waals attractive forces. The accuracy of predictions of values verifiable by experiment lead one to expect that predictions of un-verifiable quantities, such as the magnitude of attractive forces at solid-solid interfaces, are to be trusted. This approach should appeal especially to those who need to use the results of surface chemistry and would prefer to calculate from existing values rather than make new experimental determinations. It should also appeal to those teaching surface chemistry in that it relates for the first time several widely separated fields of surface chemistry. Most noteworthy is the ability to calculate heats and free energies of adsorption of gases on solid surfaces directly from measurements of surface tensions and contact angles. The calculation of the long range van der Waals attractive constant, A, from values of 7d is also very attractive.

Sir: Padday and Uffindell produced a well organized and readable article, but unfortunately their mathematics is incorrect (by nearly one order of magnitude) because intermolecular potentials were integrated over molecular distances, breaking a fundamental principle of integral calculus.

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.

The growing realization of the importance of intermolecular acid-base interactions in promoting the solubility, adsorption, and adhesion of polymers to other materials has caused a demand for the quantitative characterization of the acid-base properties of the commonly used solvents, polymers, and inorganic fillers and substrates. There have been several recent advances in the measurement techniques for such determinations, especially in the fields of inverse gas chromatography, microcalorimetry, ellipsometry, FTIR, NMR, and XPS spectroscopy, all leading to the capability of determining the Drago E and C constants or the Gutmann acceptor numbers (AN) or donor numbers (DN) for the acidic or basic sites of solvents, polymers, or inorganic surfaces. In the last year, new studies have also allowed the characterization of the specific acid-base cohesive interactions in solvents and polymers, and the determination, from contact angle measurements on polymers, of the surface concentration and strength of acidic and basic surface sites. All of these techniques are discussed in this paper and it is expected that they will soon become standard laboratory practices.

In his opening remarks at the first symposium in this series Professor Robert Good pointed out that Galileo in the 17 century was quite likely the first investigator to observe contact angle behavior with his experiment of floating a thin gold leaf on top of a water surface. Since that time contact angle measurements have found wide application as a method for determining the energetics of surfaces. This, in turn, has a profound effect on the wettability and adhesion of liquids and coatings to surfaces.

Most polymers and inorganic materials have acidic or basic sites (or both) which can interact to enhance wettability, adsorption, charge-transfer, and adhesion. These interactions, termed “polar” in the past, are independent of dipole moments and occur only when one material has acid groups which can interact with basic groups of the other material. the presence of a third component (solvent, plasticizer, penetrant, etc.) can interfere with adsorption, charge-transfer, or adhesion if the third component has strong enough acid-base interactions with either or both components. Water, a weak acid and a weak base, tends to weaken adhesion of polymers to inorganic materials but not when these have strong acid-base interactions.

A quantitative approach to predicting the enthalpy ΔH^ab of acid-base interactions between polymers and inorganic surfaces is presented based on determining the E and C constants of Drago's correlation for polymers and inorganic surfaces. Preliminary E and C constants are presented for polymethylmethacrylate, for chlorinated polyvinylchloride, for silica surfaces and for iron oxide surfaces.

FUNDAMENTAL quantities associated with the wetting of solids are the free energies of the solid/vapor boundary ('Y SV) and the solid/liquid boundary ('Y SL). These have been obtained for the special case of a glassy solid fluorocarbon on which the con tact angles (8) have been measured for liquids of widely different surface tension ('Y LV). The excellent agreement between the observed angles and those calculated from the theoretical relation,

A STUDY has been made of the densities, the surface tensions and their temperature coefficients, the interfacial tensions against water, the spreading pressures, and the force-area and potential-area relations of monolayers on water of various types of linear polyorganosiloxanes. The McLeod constants and parachors have been calculated, and their application to the type analysis of the silicones is discussed. Relations have been found between the critical spreading pressure, the spreading coefficient, and the viscosity. A study of the force-area curves revealed that the polymethylsiloxanes and the related polymers containing a small proportion of phenyl substituents are able to coil reversibly into helices made up of about six monomers per turn. Conclusions relative to the molecular structures in thin films have been carried over to the three-dimensional liquid state.

1. Methods of preparing surfaces of polytetrafluoroethylene (TFE) are described which permit obtaining reproducible and reliable measurements of contact angles with liquids. Polytetrafluoroethylene is found to be an ideal low energy surface for the study of the wetting relations of a solid with a wide variety of organic and inorganic liquids.

2. It is shown that all but the lowest-boiling liquids studied do not adsorb on TFE sufficiently from the vapor to produce a significant spreading pressure. Hence the π_E term in the expression for the work of adhesion (W_A) is negligible and W_A = γ_LV(1 + cos θ). The calculated values of W_A are found to be consistent and reasonable.

3. It is found that for each homologous series of liquids cos θ is a linear function of γ_LV. The constants of the resulting relations are given. From this W_A − W_c can be expressed as a function of γ_LV alone.

4. The observations show that there is a critical surface tension (17.5–20.5 dynes/cm.) below which liquids wet TFE.

5. It is concluded that the corrected Young-Dupré relation coupled with the spreading coefficient permits a rational description of the results. The Doss and Rao relation for the surface condensation leads to such high values for high-boiling liquids as to make it unlikely that the theory is correct.

1. Data are presented on the equilibrium contact angles of a wide variety of liquids on specially prepared surfaces of halogenated derivatives of polyethylene. The free energy decrease of immersion (f_SL), the work of adhesion (W_A), and the spreading coefficients have been calculated for the liquids which do not spread. The free energy decrease on immersion of these solids in the saturated vapor of most of the liquids of this study is shown to be negligible.

2. For each solid surface the contact angle and spreading coefficient follow the relation to liquid surface tension reported previously for polytetrafluoroethylene, i.e., as the liquid surface tension increases, the contact angle increases and the spreading coefficient decreases.

3. Substitution of chlorine or hydrogen for fluorine in these polymers increases W_A, f_SL, and the spreading coefficient with respect to a given liquid, the increase depending on the proportion of nonfluorine substituent. Substitution of hydrogen for fluorine has a smaller effect in this respect than substitution of chlorine for fluorine.

4. It is shown that when f_SL for a given solid in a series of liquids is constant, the plots of W_A υs. surface tension and spreading coefficient υs. surface tension should be straight lines with positive and negative 45° slopes, respectively. The data presented for the fluorinated polymers and the liquid n-alkanes and di-n-alkyl ethers fall on such lines. The values of f_SL for the fluorinated polymers plot as a straight line against mole per cent fluorine substitution.

5. There appears to be a maximum value of W_A characteristic of the polymer, which is identical with the work of adhesion given by the liquid having a contact angle of 90° on the given solid.

6. It is shown experimentally that there are many exceptions to the “rule” that nonpolar liquids wet nonpolar solids.

7. From the data it is shown that recent attempts to measure the surface tension of solids by contact angle measurements alone involve unjustifiable assumptions.

1. Contact angle measurements have been made of a wide variety of liquids on clean, smooth surfaces of polyethylene, paraffin, and surfaces of single crystals of n-hexatriacontane (C₃₆). The calculated value of the final spreading coefficient (S_{LV^∘/SV^∘}) is given and from the data there can be calculated the values of the free energy of immersion (f_SL), and the work of adhesion (W_A). The free energy of immersion of the solid in the liquid vapor can be neglected in these calculations since it is believed to be quite small for surfaces of low adsorptivity and low free surface energy. It is shown that the methyl-rich surfaces of C₃₆ and paraffin are not wetted by a wide variety of organic liquids, including the n-alkanes, so that the rule that nonpolar solids are wetted by nonpolar liquids is again found to be erroneous.

2. Wettability is found to decrease in the order polyethylene, paraffin, C₃₆. This is attributed to the increase in the proportion of methyl to methylene groups in the surface. The C₃₆ surface, like all higher n-alkane crystals, is shown to be the least wettable of all hydrocarbon surfaces since its surface comprises only methyl groups arranged in the closest possible packing. It is shown that it should be possible to estimate the packing of adsorbed monolayers of straight-chain hydrocarbon derivatives by comparing the hydrophobic contact angle to the angle on C₃₆. Many of the variations of the hydrophobic contact angle on paraffin found in the literature are shown to be attributable to variations in the methylmethylene ratio in the surface.

3. In contrast to the fluorinated polymers, paraffin and C₃₆ are shown to have multiple curves of cos θ vs. γ. The multiplicity of these curves is attributed to differing dependencies of γ_SL on γ_LV^∘ due to variation in the constitution of the liquid. Increase in adhesion of hydrocarbon surfaces to liquids is found to be in the order: liquids containing oxygen or fluorine; aliphatic hydrocarbons, aromatic hydrocarbons.

4. It is shown that variation of the free energy of immersion of the n-alkane series of liquids on a given hydrocarbon surface is the resultant of two competing tendencies: i.e., increased adhesion due to increase in methylene to methyl ratio in the liquid, and decrease in adhesion due to increase in surface tension of the liquid. For the C₃₆ surface, the latter tendency predominates; for the paraffin surface, the former tendency predominates, f_SL is shown to be the upper bound of the solid surface tension for systems where γ_SL ⩾ 0.

5. It is shown that in general there is more than one value of the critical surface tension (below which liquids spread on a given surface) for hydrocarbon surfaces, depending on the value of γ_SL given by a liquid of given surface tension. The fluorinated polymers are shown to be a special case where the surfaces are independent of the nature of the liquid and therefore have essentially a single value of γ_C.

A study has been made of the wettability of adsorbed monolayers of branched-chain and cyclic molecules. It is shown that such monolayers are nonwetted by organic liquids with surface tensions higher than a critical value characteristic of the monolayer. Oleophobicity is therefore not restricted to monolayers whose surfaces comprise close-packed methyl groups, the latter merely representing an extreme case. It is shown, further, that adsorbed layers of nonpolar liquids behave, as regards wettability, like low-energy solids with the same atomic groups exposed in the surface. It is concluded that the free surface energy of adsorbed liquid or solid films depends only on the atomic groups in the surface and their packing.

In order to achieve a complete understanding and control of plasma processes an appropriate knowledge of the structure of the particular glow discharge utilized is necessary. This is extremely important because the electrical potential distribution inside a plasma reactor is not uniform and therefore, as a function of the reactor geometry and sample position, charged particles are accelerated from the plasma bulk to the substrate to be treated by different potential drops, ie they impinge on different surfaces with different energy.

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.

Systems and methods for improving adhesion of an insulating foam to a molded polymeric insulating structure through use of ozone gas for functionalization of molded polymeric surfaces of an internal cavity of the insulating structure.