Critical evaluation of conditions of plasma polymerization

Under conditions of plasma polymerization, we are dealing with the “reactive” or “self-exhausting” rather than the “nonreactive” or “non-self-exhausting” gas phase (plasma). Therefore, many parameters that define the gas phase, such as system pressure and monomer flow rate, which are measured in the nonplasma state (before glow discharge is initiated), do not apply to a steady state of plasma, the conditions under which most of the studies on plasma polymerization are carried out. Consequently, information based on: (1) the polymer deposition rate measured at a fixed flow rate and discharge power, (2) the dependence of deposition rate on flow rate at fixed discharge power, or (3) the dependence of deposition rate on discharge power at fixed flow rate, does not provide meaningful data that can be used to compare the characteristic nature of various organic compounds in plasma polymerization. The significance and true meaning of experimental parameters applicable to conditions of plasma polymerization are discussed. The most important feature is that plasma polymerizations of various organic compounds should be compared at comparable levels of composite discharge power parameter W/FM, where W is discharge power, F is the monomer flow rate (given in moles), and M is the molecular weight of a monomer.     

Plasma polymerization of tetrafluoroethylene. III. Reproducibility and effects of substrate and reactor materials

Plasma polylmerization occurs in plasmas surrounded by surfaces and polymer formation is one of the complicated interactions that take place between active species and molecules which constitute surfaces and gas phases. Effects of reactor wall, substrate materials, flow rate, and discharge power on polymer formation, and properties of polymer deposits were investigated by ESCA, IR (infrared) spectroscopy, and the measurement of system pressure. The effect of surface is important at the initial stage of plasma polymerization which can be easily detected by the system pressure change; however, integrated properties such as IR spectroscopy and the deposition rate show the effect in a less pronounced manner. ESCA, which reflects the properties of surface (approximately 20 A? in depth), showed the effect of surface in an even less sensitive manner. The amount and properties (including the effects of surfaces) are dependent on plasma polymerization parameter W/FM(W, wattage; F, volume flow rate; and M, molecualar weight of monomer) and the location of deposition within a reactor. IR and ESCA data clearly showed the dependence of polymer properties on W/FM; i.e., increase of W and decrease of M to be equivalent. When all these factors were kept under control, the reproducibility of plasma polymerization was found to be excellent.     

Comparisons of glow-discharge polymerizations among tetramethylsilane,dimethyldimethoxysilane, and tetramethoxysilane

Glow-discharge polymerizations among tetramethylsilane, dimethyldimethoxysilane, and tetramethoxysilane were compared by elemental analyses and infrared spectroscopy. The elemental composition of the polymers prepared from the three monomers depended strongly on the operational conditions necessary to sustain a glow discharge, i.e., the W/FM parameter, where W is the rf power, F is the flow rate of the monomer, and M is the molecular weight of the monomer. Methoxy groups were more susceptible to glow discharge than methyl groups. Differences in chemical structure among these monomers appeared in the polymer structures formed when the glow-discharge polymerization was carried out at a low value of the W/FM parameter, while the differences disappeared at high values of the W/FM parameter, for analogous polymers. Substitution with methoxy groups was favorable for the formation of polymers of low carbon content only when the glow-discharge polymerization was performed at low values of W/FM.     

Molecular adsorption and the chemistry of plasma-deposited thin organic films: Deposition of oligomers of ethylene glycol

Weakly ionized, radio-frequency, glow-discharge plasmas formed from methyl ether or the vapors of a series of dimethyl oligo(ethylene glycol) precursors (general formula: H-(CH₂OCH₂)_n-H;n=1 to 4) were used to deposit organic thin films on polytetrafluoroethylene. X-ray photoelecton spectroscopy (XPS) and static secondary ion mass spectrometry (SIMS) of the thin films were used to infer the importance of adsorption of molecular species from the plasma onto the surface of the growing, organic film during deposition. Films were prepared by plasma deposition of each precursor at similar deposition conditions (i.e., equal plasma power (W), precursor flow rate (F), and deposition duration), and at conditions such that the specific energy (energy/mass) of the discharge (assumed to be constrained byW/FM, whereM=molecular weight of the precursor) was constant. At constantW/FM conditions, two levels of plasma power (and, hence, twoFM levels) and three substrate temperatures were examined. By controlling the energy of the discharge (W/FM) and the substrate temperature, these experiments enabled the study of effects of the size and the vapor pressure of the precursor on the film chemistry. The atomic % of oxygen in the film surface, estimated by XPS, and the intensity of theC-O peak in the XPS C_ls spectra of the films, were used as indicators of the degree of incorporation of precursor moieties into the plasma-deposited films. Analysis of films by SIMS suggested that these two measures obtained from XPS were good indicators of the degree of retention in the deposited films of functional groups from the precursors. The XPS and SIMS data suggest that adsorption of intact precursor molecules or fragments of precursor molecules during deposition can have a significant effect on film chemistry. Plasma deposition of low vapor pressure precursors provides a convenient way of producing thin films with predictable chemistry and a high level of retention of functional groups from the precursor.     

Glow discharge polymerization of tetramethylsilane investigated by infrared spectroscopy and ESCA

The influence of operating conditions (W/FM parameter: W input energy of rf power; F flow rate of monomer; M molecular weight of monomer) on glow discharge polymerization of tetramethylsilane (TMS) was investigated by infrared spectroscopy and ESCA. Elemental analyses showed that the compositions of the polymers formed strongly depended on the level of the W/FM parameter, i.e., whether the W/FM value was more or less than 190 MJ/kg. The infrared spectra indicated that these polymers were composed of Si? H, Si? O, Si? C, Si? CH₃, and Si? CH₂ groups, and that there was no significant difference in structural features between polymers prepared at W/FM parameters of more or less than 190 MJ/kg. ESCA spectra (C_1s and Si_2p core-level spectra) showed that the polymers included carbonized carbon, aliphatic carbon Si? C, C? O, Si? O, and SiO₂ species, and that the sum of carbonized and aliphatic carbons reached more than 50%. Raising the W/FM value increased the formation of the carbonized carbon but did not influence the formation of Si units such as Si? C and Si? O groups. From this evidence the rupture of Si? CH₃ bonds in the polymer forming process is emphasized. The magnitude of the W/FM parameter may be related to the detachment of hydrogen from the aliphatic carbon units.     

Plasma homo- and copolymerizations of tetrafluoroethylene and chlorotrifluoroethylene

The plasma homo- and copolymerizations of tetrafluoroethylene (TFE) and chlorotrifluoroethylene (CTFE) in a capacitively coupled tubular reactor (TR) with external electrodes were studied by means of microgravimetry and FT-IR and XPS analyses. The deposition rates for CTFE/TFE plasma copolymers, as well as the ratios of IR absorbances at 1180 and 1225 cm⁻¹, and the XPS-derived Cl/C and F/C ratios, varied regularly with mol % CTFE in the feed, all of which results were dependent upon the rf power at which the plasma copolymerizations were conducted. The deposition rates for the plasma homopolymers of TFE (PPTFE) and CTFE (PPTCFE) depended markedly on rf power (W) and monomer molar flow rate (F). The F/C ratio for PPTFE was nearly independent of the composite parameter,W/FM (whereM is the monomer molecular weight), while for PPCTFE, the F/C ratio decreased significantly and the Cl/C ratio increased slightly with increase inW/FM. The percentage of carbon as CF₃ was 20–24% in PPTFE and 7–14% in PPCTFE. Plots of deposition rate versusW/FM for PPTFE and PPCTFE obtained in a TR differed considerably from corresponding plots in the literature for the same homopolymers prepared in a glass-cross or bell-jar reactor.     

Deposition behavior in a low‐temperature cascade arc torch (CAT) polymerization process

Two kinds of hydrocarbon type monomers and three kinds of organosilicons were polymerized by a low‐temperature cascade arc argon plasma torch. Their deposition behaviors were studied as a function of experimental parameters and monomer elemental compositions. It was found that the normalized deposition rate (DR), expressed as deposition yield of DR/(FM)_m, was determined by a composite operational parameter, W*(FM)_c/(FM)_m, where W is the power input, and (FM)_c and (FM)_m are the mass flow rates of carrier gases and monomers, respectively. Experimental results indicated that the deposition yield is highly dependent on the elementary compositions of monomers. Optical emission spectroscopy study on the argon plasma torch showed that the emission intensity of excited argon neutrals was proportional to the value of the parameter W*(FM)_c. These results further certified that excited argon neutrals are the main energy carriers from the cascade arc column to activate monomers in the argon plasma torch. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 967–982, 1999     

Distribution of polymer deposition in plasma polymerization. I. Acetylene,ethylene, and the effect of carrier gas

Factors which influence the distribution of polymer deposition in an electrodeless glow-discharge system were investigated for acetylene and ethylene. Under the conditions in which “full glow” is maintained, the distribution of polymer deposition from pure monomer flow systems is nearly independent of flow rate of monomer or of the system pressure in discharge, but is largely determined by the characteristic (absolute) polymerization rates (not deposition rate) of the monomers. Acetylene has a high tendency to deposit polymer near the monomer inlet, whereas ethylene deposits polymer more uniformly in wider areas in the reactor. The addition of carrier gas such as argon or partially copolymerizing gas such as N₂, H₂, and CCl₂F₂ was found to narrow the distribution of polymer deposition. The distribution of polymer deposition is also influenced by a glow characteristic which is dependent on flow rate and discharge power.     

Preparation and characterization of plasma-polymerized N-vinyl-2-pyrrolidone films

Thin films (< 1 μm) of plasma polymerized N-vinyl-2-pyrrolidone (PPNVP) have been prepared, using an inductively coupled RF glow discharge in a flow-through reactor system. PPNVP films were hydrophilic, smooth, and appeared morphologically homogeneous. The polymer deposition rate was found to increase linearly with NVP flow rate, and to decrease with the distance from the induction coil. ATR–FT–IR spectral studies suggested a highly branched polymer structure and included absorptions at 2150 and 1540 cm^?1 which were unique to the plasma polymer and derived from lactam ring opening/breaking reactions. ESCA studies demonstrated that, under a given set of plasma reaction conditions, the surface composition was consistent throughout the reactor. However, polymer composition was influenced by larger changes in the plasma energy, since the nitrogen content was found to decrease with increasing W/FM. This corresponded to concomitant increases in the advancing water contact angles and to small but reproducible changes in the IR spectrum.     

Comparisons of glow discharge polymerization between hexamethyldisilazane and trimethylsilyldimethylamine

Glow discharge polymerization between hexamethyldisilazane (HMDSZ) and trimethylsilyldimethylamine (TMSDMA) was compared by means of infrared spectroscopy and ESCA analysis. Infrared spectra pointed out differences in chemical structure between the polymers prepared from the two monomers, although the two polymers were mainly composed of resembling units such as Si? CH₃, Si? CH₂, Si? H, Si? O? Si, and Si? O? C groups: (i) The polymers prepared from TMSDMA contained N → O group, but the polymers from HMDSZ did not contain this group. (ii) Influences of the W/FM parameter (W is the input energy of rf power, F the flow rate of the monomer, and M the molecular weight of the monomer) appeared on decreasing the C? N group and increasing the C?O group in the TMSDMA system, but little influence appeared in the HMDSZ system. ESCA spectra (C_1s, Si_2p, and N_1s core levels) supported the differences between the two polymers elucidated by infrared spectroscopy, and pointed out differences in susceptibility of the Si? N bond to plasma: The N? Si sequence of TMSDMA was completely ruptured in discharge to yield polymers, and the Si? NH? Si sequence of HMDSZ remained in considerable amount.     

Plasma polymerization investigated by the substrate temperature dependence

Plasma polymerization of tetra fluoroethylene (TFE), perfluoro-2-butyl-tetrahydrofuran (PFBTHF), ethylene, and styrene were investigated in various combinations of monomer flow rates and discharge wattages for the substrate temperature range of ?50 to 80°C. The polymer deposition rates can be generally expressed by k₀ = Ae^?bt, where k₀ is the specific deposition rate given by k₀ = (deposition rate)/(mass flow rate of monomer), A is the preexponential factor representing the extrapolated value of k₀ at zero absolute temperature, and b is the temperature-dependence coefficient. It was found that the value of b is not dependent on the condensibility of monomer but depends largely on the group of monomer; that is, perfluorocarbons versus hydrocarbons. The values of A are dependent on domains of plasma polymerization. In the energy deficient region A is given by A = α(W/FM)ⁿ, where α is the proportionality constant, W is discharge wattage, FM is the mass flow rate, and n is close to unity. In the monomer deficient region A becomes a constant. The kinetic equation is discussed in view of the bicyclic rapid step-growth polymerization mechanisms.     

DC plasma polymerization of hexamethyldisiloxane

Hexamethyldisiloxane (HMDS) was polymerized onto metallic and insulating substrates in a parallel-plate DC reactor. The limits of the DC reactor with respect to pressure and power were determined for deposition of PP-HMDS films. In all conditions ranging from 5 Pa/0.3 W to 100 Pa/50 W, solid films were deposited. No powders or oily films were obtained under any condition in this operating range. The films were polymeric in nature,i.e., they were neither carbon-like nor SiO _x -like films. The structures and crosslink densities of the plasma films dependend strongly on the deposition conditions. The highest deposition rates, up to 2 μm per minute (or0.3 mg/cm² min), were obtained at high power, pressure, and flow rate conditions. An efficiency ɛ is introduced, defined as the fraction of the monomer that is retained in the form of a polymer deposited on the substrate. Efficiencies as high as 25% could be obtained in certain conditions. Pulsing the discharge power increased the conversion efficiency markedly, but the effect depended strongly on the monomer used. In addition to HMDS, plasma polymers were also deposited from pyrrole in pulsed conditions for comparison. A method is described for depositing films on insulators from a DC glow discharge using two wire meshes held at a negative potential.     

Deposition conditions influence the postdeposition oxidation of methyl methacrylate plasma polymer films

Plasma polymer films were deposited from methyl methacrylate (MMA) vapor under various plasma conditions and XPS and FTIR used to study the changes to the compositions of the films as they were stored in air for longer than 1 year. The plasma power input per monomer mass unit (W/FM) markedly affected the composition of the freshly deposited MMA plasma polymers. A low value of W/FM led to a high degree of retention of the original monomer structure, whereas a high value of W/FM resulted in substantial monomer fragmentation and the formation of a partially unsaturated material considerably different to conventional PMMA. As the MMA plasma coatings were stored in ambient air after fabrication, all showed spontaneous oxidative changes to their composition, but the extents and reaction products differed substantially. Deposition at low W/FM led to moderate oxidative changes, whereas high power led to a pronounced increase in the oxygen content over time and resulted in a wide range of carbon–oxygen functionalities in the aged material. As the initial compositions/plasma deposition conditions thus influenced the oxidative postdeposition reactions, MMA plasma polymers deposited under different conditions not only varied in their initial composition but then became even more diverse as they aged. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 985–1000, 1998     

Plasma polymerization of tetrafluoroethylene. IV. Comparison of ethylene and tetrafluoroethylene by measurement of electron temperature and density of positive ions

The double probe method was applied to plasma of tetrafluoroethylene (TFE) and ethylene and the electron temperature (T_e) and density of positive ions (n_p) were measured at various discharge wattages. The probe current-probe voltage diagrams for TFE were different from those for ethylene. The shape of its diagram indicates that a considerable number of negative ions exist in TFE plasma. The levels of n_p for TFE were also nearly six times greater than those for ethylene at the same discharge current. The dependence of TFE polymer deposition and the chemical structure of the polymer, based on ESCA data on discharge current, was related to T_e and n_p measured by the probe method. The values of T_e and n_p may not be directly related to the polymer formation in a plasma; the method provides a direct measure of plasma energy density where plasma polymerization takes place, whereas it cannot be accurately estimated by the input energy of a discharge. It was found that plasma energy density based on (n_pT_e) for TFE plasma and that for ethylene differ significantly at the same level of input parameter (W/FM), where W is the discharge wattage, F is the volume flow rate, and M is the molecular weight of the monomer.     

Some aspects of plasma polymerization of fluorine-containing organic compounds. II. Comparison of ethylene and tetrafluoroethylene

Plasma polymerizations of ethylene and tetrafluoroethylene are compared. In the plasma polymerization of ethylene and of tetrafluoroethylene, glow characteristics play an important role. Glow characteristic is dependent on a combined factor of W/F_m, where W is discharge power and F_m is monomer flow rate. At higher flow rates, higher wattages are required to maintain “full glow.” In the plasma polymerization of tetrafluoroethylene, simultaneous decomposition of the monomer competes with plasma polymerization. Above a certain value of W/F_m, decomposition becomes the predominant reaction, and the polymer deposition rate decreases with increasing discharge power. ESCA results indicate that the plasma polymer of tetrafluoroethylene that is formed in an incomplete glow region (low W/F_m) is a hybrid of polymers of plasma polymerization and of plasma-induced polymerization of the monomer. Polymers formed under conditions of high W/F_m to produce “full glow” are similar, regardless of the extent of decomposition of the monomer. They contain carbons with different numbers of F(CF₃, ? CF₂? , >CF? , >C<) and carbons bonded to other more electronegative substituents.     

Fundamental studies of grafting reactions in free radical copolymerization. II. Grafting of styrene,acrylate, and methacrylate monomers onto cis-polybutadiene using AIBN initiator in solution polymerization

Grafting can be initiated by primary and/ or polymer radical attack on the backbone polymer and it is well known that AIBN does not readily promote grafting, even when using poly-butadiene. We have studied the grafting of several different monomers onto cis-polybuta-diene using AIBN initiator and find dramatically different results among the monomers. As expected, styrene grafts at very low levels due to the inactivity of the initiator radicals and the polystyryl radicals. Methacrylate monomer grafts at a slightly higher level due to its more reactive polymer radical, while acrylate monomer readily grafts onto the poly-butadiene because polyacrylate radicals are quite reactive. The use of a kinetic model allowed the evaluation of rate coefficients for graft site initiation to be in the relative order of 0.1 : 1.0 : 10.0 (L/mol/s) for styrene:methacrylate:acrylate monomers. The model also pro-vided successful interpretations of the grafting data and its dependence upon the concen-trations of monomer, initiator, and backbone polymer. Due to the relatively higher reactivity of the polyacrylate radicals, the benzene solvent acted as a chain transfer agent in this system. This affected the molecular weight of both free and grafted acrylate polymer and also surpressed the graft level. Polyacrylate radicals attack the cis-polybutadiene backbone by abstracting an allylic hydrogen and also adding across the residual double bond. The latter mechanism is responsible for the majority of the grafting; the hydrogen abstraction leads to relatively inactive radicals which cause a retardation in the overall reaction rate. © 1995 John Wiley & Sons, Inc.     

Gamma radiation-initiated polymerization of styrene at high pressure. II. Chain termination

The pressure dependence of the termination rate constant k_t for the free radical polymerization of monomers such as styrene is a function of polymer chain length, chain stiffness, and monomer viscosity, all of which influence the rate of segmental diffusion of an active radical chain end out of the coiled polymer chain to a position in which it can react with a proximate radical. Although k_t is not sensitive to changes in chain length, the large increase in molecular weight is responsible for a significant reduction in k_t at high pressures. For most of the common vinyl polymers, which exhibit some degree of chain stiffness, k_t is inversely proportional to a fractional power of the monomer viscosity because it depends in part on the resistance of chain segments to movement and in part on the influence of viscosity in controlling diffusion of the chain ends. The fractional exponent appears to increase with pressure and this is interpreted as evidence that the polymer chains become more flexible in a more viscous solvent. Because the fractional exponent is higher for more flexible chains, the value of the activation volume for chain termination is an indication of the degree of flexibility of the polymer chains, provided that the monomer is a good solvent for the polymer and that chain transfer is negligible.     

Kinetics of destruction of diisopropyl methylphosphonate in corona discharge

The kinetics of the destruction of diisopropyl methylphosphonate (DIMP) in corona discharge has been studied using a flow tubular coaxial wire dielectric barrier corona discharge reactor. The identification and quantitative determination of DIMP, its destruction intermediates, and phosphorus‐containing destruction products were performed using molecular beam mass spectrometry and gas chromatography/mass spectrometry. Active discharge power was varied in the range 0.01–5 W. The destruction products such as isopropyl methylphosphonate, methylphosphonic acid, and orthophosphoric acid were found on the reactor walls. The dependence of the extent of the destruction, D (D = 1 ? X / X₀, where X and X₀ are DIMP mole fractions at the outlet and the inlet of the reactor), on the specific energy deposition E_x (E_x = PF^?1 X₀^?1, where F is the carrier gas flow and P is the power dissipated in discharge reactor) was measured over the DIMP mole fraction range 60–500 ppm at the pressure of 1 bar and the temperature of 340 K. Over the range of the experimental conditions studied the destruction obeys the “pseudo‐first‐order” kinetic law: ln(1 ? D) = ?KE_x. Plausible mechanisms of the destruction are discussed. It was concluded that ion mechanism is the major one responsible for the destruction process. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 331–337, 2002     

Plasma polymerization. VI. An ESCA investigation of the RF plasma polymerization of perfluoro-2-butyl tetrahydrofuran

Perfluoro-2-butyl tetrahydrofuran was polymerized by an RF glow discharge technique and detailed ESCA studies were made of the resultant films. The rate of film deposition was sensitively dependent on the W/FM parameter and the site of deposition. The ESCA data show that the molecular rearrangement accompanying plasma polymerization and the oxygen functionality is at a significantly lower level than the starting material. Under appropriate conditions plasma polymerization produces material with a C:F stoichiometry of 1:2, although the ESCA data show that the polymer is drastically different from PTFE. ESCA studies are also reported on thin films of the monomer studied at low temperature.     

Polymerization of para-xylylene derivatives (parylene polymerization). I. Deposition kinetics for parylene N and parylene C

Kinetic aspects of parylene N [unsubstituted poly(para-xylylene)] and Parylene C [monochlorosubstituted poly(para-xylylene)] were studied. The conversion of starting material (dimer of either p-xylylene or chloro-para-xylylene) to polymer is quantitative (ca. 100%). Consequently, the total polymer formed in a closed system is directly proportional to the amount of dimer charged. However, the percentage of the total amount of polymer formed which deposits on substrate surfaces, placed in the deposition chamber, as well as the polymer film growth rate are dependent on operational factors such as the temperature of the substrate, sublimation of dimer temperature, flow pattern of the reactive species, etc. Parylene C, being a heavier and more polar molecule, has the tendency to deposit easily in the deposition chamber compared to the deposition of Parylene N. Parylene C also has a higher ceiling temperature for deposition than Parylene N. This situation has been investigated from the viewpoint of excess thermal energy which hinders polymer formation (deposition) due to the exceedingly high entropy change necessary for polymer deposition to occur. The addition of a cool (i.e., room temperature) inert gas was shown to increase the deposition of Parylene N on substrate surfaces placed in the deposition chamber. The deposition increase and acceleration of deposition (film growth) rate were found to be related to the size and molecular weight of the inert gas pressure maintained in the system. The accelerating effect is explained by the increase in third-body collisions to dissipate the excess thermal energy of the reactive species.