Adhesion of glow discharge polymers

Various reaction parameters in a propylene glow discharge polymerization were investigated with the objective of synthesizing films having good adhesion to metal and glass substrate. Monomer flow rate (at a constant pumping efficiency) was found to exert a significant effect on the quality of polymer adhesion to substrates. Good adhering polymeric films were obtained only at high flow rates where the deposition rate decreases with an increase in monomer flow rate. This observation applied also to ethylene, propane, allyl bromide, and ?-caprolactam. Transmission electron microscopy of the freshly deposited polymer films indicated a defect-free structure of the polymer surface obtained at high flow rates. The polymer surface obtained at low flow rates had a bead structure superimposed by uniformly placed circular defects.     

Wettability of glow discharge polymers

Treatment of good adhering glow discharge polymerized propylene (GDPP) coatings with reactive gas plasma from oxygen, nitrogen, or water (produced in a tubular reactor operation at 27.1 MHz) results in surfaces characterized by more hydrophilic interactions. Zisman's plots indicate a change in surface energy after such treatment. Transmission electron microscopy depicts that the pronounced improvement in wettability of GDPP polymer after oxygen plasma treatment in part results from the high surface development in the polymer. However, the effect is not permanent. The wettability of plasma-treated polymer diminishes with time and reaches a limiting value in 2–3 months indicating structural rearrangements at the polymer surface. Argon, carbon monoxide, or bromotrichloromethane plasma does not change the polymer surface wettability significantly. In contrast to the O₂ plasma-treated GDPP polymer, a glow discharged polymer synthesized from ?-caprolactam maintains its inherent hydrophilicily over an extended period of time.     

Effect of electrodeless glow discharge on polymers

The effects of gas plasma generated by electrodeless (inductive coupling) glow discharge on polymers were investigated as functions of gas pressure, discharge power, exposure time, and type of plasma gas. A remarkable similarity between the plasma susceptibilities of low molecular weight organic compounds and polymers was observed; i.e., polymers which have ether, carbonyl, ester, or carboxylic acid attached to a nonaromatic structure are very susceptible to plasma. The weight loss was proportional to the exposure time and exposed area. The discharge power and type of gas were found to have a great influence on both the rate of weight loss and the morphology of the exposed surface. The predominant effect of plasma on polymers was found to be degradation (manifested by weight loss). The crosslinking effect was found to be marginal with many polymers; however, significant crosslinking was observed with double bond-containing polymers. The crosslinking was examined by swelling the treated films. With copolymers of styrene–butadiene, 4-vinylpyridine–butadiene, methacrylic acid-butadiene, and acrylic acid–butadiene, the crosslinking was greatly dependent on the discharge power, the butadiene content of the copolymers, and the exposure time. Both degradation and crosslinking by gas plasma were generally limited to the exposed surface; however, the propagation of crosslinking in the direction of thickness was observed with copolymers of styrene–butadiene. The plasma of organic vapor also cause degradation of plasma-susceptible polymers, particularly at high wattage, although the deposition of polymer occurs simultaneously.     

Preliminary experiment of surface hardening of polymers by glow discharge polymerization

Polymers including polyethylene, polycarbonate, and polytetrafluoroethylene were modified by glow discharge polymerization to enhance surface hardness. The surface hardness of the polymer substrates could be improved by glow discharge polymerizations of silicon-containing compounds. The mixture of tetramethylsilane (TMS) and oxygen was more effective than tetramethoxysilane to improve the surface hardness. The surface hardness improved by the glow discharge polymerization strongly depended on the nature of the polymer substrates to be modified. The adhesion between polymer films prepared from the TMS/O₂ mixture by glow discharge polymerization and the polymer substrates was good.     

Surface properties of polymers prepared from trimethylsilyldimethylamine and hexamethyldisilazane by glow discharge polymerization

Polymers prepared from the monomers trimethylsilyldimethylamine and hexamethyldisilazane by glow discharge polymerization were analysed by elemental analysis and infrared spectroscopy, and some surface properties were investigated. Although the polymers formed from both monomers have similar chemical structures involving CH₂, CH, Si---CH₃, Si---CH₂---CH---₂---Si, Si---O---Si, and Si---O---C groups, there is a significant difference in nitrogen residues. The nitrogen residues in polymers formed from trimethylsilyldimethylamine exist mainly as amide groups, and the residues in the polymers from hexamethyldisilazane exist as disilazanyl groups. This difference is evident in the surface energy, especially in the polar contribution. The relative ratio of polar and to dispersive contributions to the surface energy is higher for the polymers from hexamethyldisilazane (0.6) than for those from trimethylsilyldimethylamine (0.3). Adhesion between these plasma films and polymer substrates subjected to modification was also examined.     

Adhesion between polymer substrates and plasma films from tetramethylsilane and tetramethyltin by glow discharge polymerization

Adhesion between various polymer substrates and plasma films, which had been prepared from either tetramethylsilane or tetramethyltin by glow discharge polymerization and deposited on the surface of the polymer, was evaluated by the Scotch tape test and by lap-shear strength. It was found that the plasma films exhibited fairly good adhesion to the polymer substrates (with the exception of polypropylene). The position where failure occurred was determined by X-ray fluorescence analysis, scanning electron microscopy and energy diffractive X-ray analysis. This position was at an inner layer of the plasma film (cohesive failure of plasma film), within the polymer substrate (material failure of polymer) or at the interface between polymer substrate and plasma film (adhesive failure) depending upon the polymer substrate. These results indicate an important aspect of durability of surface modification by glow discharge polymerization.     

Grafting fluorocarbons to polyethylene in glow discharge

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.     

Adhesion of metals to polymers. I. Study of interfaces modified by inductive heating

The objective of the work reported in this paper is to explain theoretically the observations made in an earlier study that castings of an epoxide resin reinforced by electrically conductive inserts, after having been postcured by inductive heating, had strength properties superior to those achieved by a purely thermal postcure. A search of the literature on adhesion suggested that among the theories presented, those dealing with electrical double layer formation, interdiffusion, and mixed polymer grafting on the insert–polymer interface deserve to be considered. Experiments were then planned in such a manner as to permit a choice between these theories. Epoxide resin specimens reinforced with various metals (pure silver, silver with an oxide layer, pure copper, copper with an oxide layer, aluminum, and stainless steel) were prepared. They were then either postcured in a heated oven, or postcured in an induction furnace for various lengths of time. Bonding strength was determined by a knife-edge method, and the exposed insert and resin surfaces were tested for electrical potential and surface wettability (angle-of-contact). Most strikingly, the curves obtained with increasing induction heating times for bonding strength, electrical potential, and wettability were all sinusoidal in shape, and their maxima and minima generally were found to coincide. For oven-postcured specimens, bonding between different metal inserts and an epoxide resin is explained by double electrical layer formation (insert positive, resin negative), with bonding strength increasing as the stability of the oxide forming the surface layer of the metal decreases. Metals with no oxide surface layer thus have the highest bonding strengths. For inductively postcured specimens, bonding is effected by both electrical double layer formation and mixed polymer grafting through the oxygen atoms in the metal surfaces, with the more stable metal oxide giving the stronger bonding in grafting. Where the insert does not carry an oxygen surface layer, bonding takes place through electrical forces only. Alternating build-up and internal discharge of electrical double layers are the direct cause of the sinusoidal shape of the electrical potential curves and the indirect cause of the comparable shape of the wettability curves. Their combination results in the observed periodicity of the breaking load curves.     

Adhesion of polymers to reinforcing fibres

A single fibre pull out technique is presented which makes it possible to measure the strength in the interface between fibre and polymer with high precision even if the embedded length of the fibre is short. The method allows measurements for all kinds of fibres in thermoplastic or thermosetting polymers. The effect of fibre surface treatment can be investigated as well as the effect of morphology or internal stresses in the polymer. Examples are given. The shear strength results are compared with results of tensile shear tests performed on symmetrically notched unidirectional reinforced composite samples. The correlation is good and it is shown that the new single fibre pull out method is able to give a better discrimination between the composites than other methods. An analysis of mechanical stress induced during the pull out demonstrates the use and limitation of this method.     

Polymerisation by glow discharge electrolysis

A glow discharge passed from a metal anode to the surface of aqueous solutions of acrylamide produces radicals which initiate polymerisation. The yield of polymer together with its relative molecular mass has been examined as a function of monomer concentration and pressure in neutral solutions. The form of the variations can be explained in terms of a simplified reaction scheme. On the basis of the proposed mechanism there are two reaction zones. In the primary reaction zone no polymerisation occurs, the radical concentration being too high. About 0·2% of the radicals escape into the secondary reaction zone to initiate polymerisation, the volume of this zone being about 0·4 cm³.In alkaline solutions the yields and relative molecular masses of the polymers are very low and this is attributed to chain termination by oxygen which rises from the destruction of hydrogen peroxide in alkaline solution.     

Adhesion of polyimides to metals and metal oxides

Interface reactions between polyimides and metal/metal oxide surfaces are proposed on the basis of their acid-base strengths. These hypothetical interface reactions are discussed for the purpose of understanding adhesion at these interfaces. Some experimental data are presented to support the hypothesis as it relates to polyimide adhesion to these surfaces. The purpose of this paper, however, is not to present firm conclusions, but to initiate discussion of the concepts within the adhesion community.     

Polymerization of organic compounds in an electrodeless glow discharge. XI. Pressure in glow discharge

The pressure of a steady-state flow of monomer, p₀, changes to a new steady-state flow pressure, p_g, in glow discharge. The value of p_g is dependent on the flow rate of monomer, the pumping-out rate of the vacuum system for the product gas (which is hydrogen in many cases of plasma polymerization of hydrocarbons), and the characteristic hydrogen yield of a monomer associated with plasma polymerization. The relationships between these factors were established and examined for plasma polymerizations of acetylene, ethylene, and acrylonitrile.     

Silicone polymerization by glow discharge application

Polymerization of hexamethyldisiloxane (HMDS) on aluminum by application of capacitively coupled plasma was carried out under different polymerization conditions. The effect of applied frequency, power, and discharge duration on polymerization rate was investigated. Film adhesion was found to be directly related to applied frequency and power. Polymer deposits were examined with SEM and ATR, and the effect of the coat on wattability was sought by measuring the contact angles. The chemical structure of the coat was affected by the energy density applied during coat preparation.     

Molecular intermediates in glow discharge polymerization

Mass spectral analysis of gaseous effluents from glow discharge polymerization of propylene confirms the formation of molecular propyne as an intermediate in the polymerization reaction. Observation of oligomeric species of mass number 138 or higher at the lowest monomer feed rate of the study indicates that a certain fraction of polymerization occurs in the gas phase.     

A model for glow discharge electrolysis

Two models (a cylinder model and a cone model) are proposed to represent the inhomogeneous distribution of radicals in the liquid phase in glow discharge electrolysis. The models have been applied to polymerization by glow discharge electrolysis and the cone model has some success in accounting for the dependence of the rate of polymerization and the degree of polymerization on the monomer concentration and the pressure in the discharge.     

Polymerization of organic compounds in an electrodeless glow discharge. X. Internal stress in plasma polymers

Owing to the unique mechanisms operative in plasma polymerization, a thin layer of plasma polymer deposited on the surface of a substrate shows a tendency to expand, indicating an internal stress in the layer. This stress, σ_s, has been estimated from the observed curling of composite membranes in which the thickness of the plasma coating, d, is much smaller than the thickness of a flexible substrate, D, according to the relation where R is the radius of the roll into which the composite films curl up and E is the modulus of the substrate polymer. The stress σ_s is found to depend on the kind of monomer used and to be of the order of magnitude 10⁸–10⁹ dynes/cm² with most of the monomers here employed.     

Characterization of bis‐(phenoxy)phosphazene polymers using radio frequency glow discharge mass spectrometry

A radio frequency glow discharge mass spectrometry (rf‐GDMS) source is evaluated for future applications in the “fingerprint” characterization of polyphosphazene membranes. The rf‐GDMS spectra of a series of bis(phenoxy)phosphazene polymers contain ions that originate from both the phosphazene backbone and the phenoxy moiety, resulting in signature ions of the polymer family. “Fingerprint” ions from the substituted R‐group on the phenoxy moiety of the different derivatives allows the individual polymers to be distinguished from one another. The ability of the rf‐GDMS source to characterize these materials directly in the solid state will be useful for the continued application of these polymers as separation membranes. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 954–961, 2000     

Structure of glow discharge polysilazane thin films

Thin polysilazane films have been prepared by glow discharge polymerization of hexamethylcyclotrisilazane in an electrode static system. An increase in pressure in the reaction system observed during polymerization indicated that the monomer underwent fragmentation in the glow discharge. The structures of thin polysilazane films prepared at various current densities have been determined by elementary analysis, infra-red spectroscopy and gas chromatography. It has been found that fragmentation of hexamethylcyclotrisilazane takes place mainly through the cleavage of SiC bonds and that methyl group content in the polymer film structure decreases with increasing current density. The mechanisms of reactions leading to polysilazane film formation have been discussed.