Applications of ESCA to polymer chemistry. III. Structures and bonding in homopolymers of ethylene and the fluoroethylenes and determination of the compositions of fluoro copolymers

Molecular core binding energies have been measured by ESCA for the homopolymers of ethylene and the fluoroethylenes. The data are interpreted in terms of semiempirical all-valence electron SCF MO calculations in the CNDO/2 formalism, in conjunction with the charge potential model. The results are used as a basis for interpreting the measured core binding energies of some Viton and Kel F type polymers. The routine application of ESCA to the determination of copolymer compositions is described.     

Applications of ESCA to polymer chemistry. XII. Structure and bonding in commercially produced fluorographites

A series of fluorinated graphites, Fluorographites, of varying fluorine content has been examined by ESCA, and the chemical shifts and relative intensities of the core electron lines yield a consistent picture of the compositions and structures in the surface regions of the materials. Fluorographites of composition (C₁F_0.81)_n to (C₁F_1.00)_n are fully fluorinated in the outermost ～40 Å and consist of a bulk structure composed of tertiary →CF groups with ? CF₂? groups at the prismatic edges of the fluorinated graphite layers. This is also the case for (C₁F_1.05)_n, but, in addition, some of the C? C bonds involving edge →CF carbons are broken and replaced by C? F bonds, giving a higher concentration of ? CF₂? groups at the outermost surfaces of the particles of this material. The ESCA data for (C₁F_0.25)_n are consistent with a structure comprising six-membered aromatic rings in which each ring carbon is attached to a →CF group acting as a bridge among three similar rings, with the C? F bonds alternately pointing up and down with respect to the plane defined by the carbon atoms. At the immediate surface the valence requirements of the carbon atoms at the prismatic edges of the graphite-like layers are satisfied by bonding with oxygen. This is also the apparent structure of (C₁F_0.37)_n, but in addition there exist discrete regions of composition (C₁F₁)_n distributed uniformly throughout this material. The Fluorographites (C₁F_0.34)_n, and (C₁F_0.40)_n, and (C₁F_0.63)_n consist of blocks of C₁F₁ stoichiometry and blocks of unreacted graphite. The presence of ? CF₂? groups and the complete absence of oxygen in the surface regions of these Fluorographites suggests that the prismatic edge sites are fully fluorinated. The inhomogeneous block-like structures of (C₁F_0.37)_n, (C₁F_0.37)_n, and (C₁F_0.34)_n, (C_1F0.40)_n, and (C₁F_0.63)_n, give rise to differential sample charging, resulting in apparent shifts in the binding energy scales between the spectral components originating from different regions of the samples. Such differential sample charging is also described for a number of mixtures of the Fluorographites and of graphite with the Fluorographites. It is pointed out that in view of these results it is necessary to exercise considerable caution in using the ESCA lines of an added compound as a reference in measuring core electron-binding energies.     

Applications of ESCA to polymer chemistry: Studies of some nitroso rubbers

Molecular core binding energies of the polymers formed by copolymerization of CF₃NO with CF₂?CF₂, CF₂?CFCl, and CF₂?CFH, respectively, have been studied by means of ESCA. The results are interpreted in terms of CNDO/2 SCF MO calculations on some model systems. Some evidence for structural irregularity is found for the copolymers with CF₂?CFCl and with CF₂?CFH. The reaction mechanism for the polymerization is also discussed in terms of the experimental results and INDO SCF MO calculations.     

Applications of ESCA to polymer chemistry. Part VI. Surface fluorination of polyethylene. Application of ESCA to the examination of structure as a function of depth

Molecular core binding energies have been measured by ESCA for series of surface-fluorinated high-density polyethylene films. Detailed analysis of the results shows that the technique can be used to establish a comprehensive picture of the early stages of the surface fluorination process. Both the depth of fluorination, and stoichiometry and molecular structure as a function of depth, in the range 0–50 Å have been established. In addition, minor features such as oxidation, hydration, and hydrocarbon contamination at the surface are clearly revealed. Detailed consideration of the results allows a discussion of the dynamics of the overall fluorination process.     

Application of ESCA to polymer chemistry. VIII. Surface structures of AB block copolymers of polydimethylsiloxane and polystyrene

The surface morphology of a number of films of AB block copolymers of polydimethylsiloxane and polystyrene was examined by ESCA and contact angle measurements. In all cases the immediate surface is shown to consist of an essentially pure polydimethylsiloxane component. By comparing the intensities of elastic peaks corresponding to photoionizations from core levels without energy loss for polydimethylsiloxane and polystyrene with those for the block copolymers and by consideration of shake-up phenomena specific to the polystyrene component, an estimate of the thickness of the polydimethylsiloxane outer layer of the latter may be obtained. This is shown to vary between ～13 and 40 Å, depending on the method of formation of copolymer film.     

Applications of ESCA to polymer chemistry. XVII. Systematic investigation of the core levels of simple homopolymers

The core levels of a series of 83 homopolymers have been studied by electron spectroscopy for chemical analysis (ESCA). Comparisons of the experimentally determined core-level binding energies with theoretical calculations using the ground-state potential model in the complete neglect of differential overlap (CNDO/2) self-consistent field molecular orbital (SCF MO) formalism have been made on the C_1s and O_1s core levels for the oxygen-containing polymers in the series. A comparison of the ground-state potential model (GPM) and relaxation potential model (RPM) on a series of six model compounds representative on the series of polymers is given. Compilations are given of binding energies of C_1s, O_1s, N_1s, Cl_2p, S_2p, Si_2p, and Br_3d levels for typical structural features of common occurrence in polymer systems. These data, taken in conjunction with that previously published on fluoropolymers, provide a sound basis for the development of ESCA as a fingerprint tool in the elaboration of features of structure and bonding in polymers in general.     

Applications of ESCA to polymer chemistry. XI. Core and valence energy levels of a series of polymethacrylates

The core and valence energy levels of a series of poly(alkyl methacrylates) have been studied by ESCA. Surface compositions have been determined both from a comparison of area ratios for the O_1s and C_1s core levels and from the relative areas for the individual component peaks for these levels. The evidence presented suggests that on the ESCA depth-profiling scale the technique statistically samples the repeat unit with little or no evidence for preferential orientation of the alkyl side chain at the surface. Little evidence was found for either surface oxidation or hydrocarbon contamination at the surface of the samples studied. The valence energy levels are shown to be characteristic of the polymer system.     

Applications of ESCA to polymer chemistry. X. Core and valence energy levels of a series of polyacrylates

The core and valence levels of a series of poly(alkyl acrylates) have been studied by ESCA. From an analysis of the individual component peaks for the C_1s and O_1s core levels and from comparison of relative area ratios it is shown that ESCA may be applied to the study of surface compositions. The evidence presented strongly suggests that on the ESCA depth-profiling scale the technique statistically sample the repeat units with no evidence for preferential orientation of side chains at the surface. For some samples, ESCA provides evidence for a degree of surface oxidation and hydrocarbon contamination. The valence energy levels are shown to be characteristic of the polymer system. The measured absolute and relative binding energies of the core levels have been compared with model calculations using the charge-potential model in the CNDO/2 SCF MO formalism.     

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.     

Crystal structure of an alternating copolymer of ethylene and tetrafluoroethylene

The unit cell of an alternating copolymer of ethylene and tetrafluoroethylene was determined by x-ray diffraction. In spite of uncertainties due to irregularities in the chain structure and a low level of crystallinity, a reasonable unit cell and structure was derived which gives a calculated crystalline density of 1.9 g/cm³. The unit cell is believed to be either orthorhombic or monoclinic with the following parameters: a = 9.6 Å, b = 9.25 Å, c = 5.0 Å, (γ = 96°). The molecular conformation is that of the extended zigzag, and the molecular packing appears to be orthorhombic, each molecule having four nearest neighbors with the CH₂ groups of one chain adjacent to the CF₂ groups of the next.     

Application of ESCA to polymer chemistry. XVI. Electron mean free paths as a function of kinetic energy in polymeric films determined by means of ESCA

Electron mean free paths as a function of kinetic energy have been measured by the substrate overlayer technique for in situ-polymerized films of poly(p-xylylene) and the monochloro- and monobromo-substituted derivatives. The results are compared with previous estimates of mean free paths available in the literature for organic materials. Comparison is also drawn with corresponding experimental data for typical metals and semiconductors, and it is shown that organic polymers fit into a consistent picture which may be rationalized on the basis of existing theory. For electrons of kinetic energy ～969 eV, ～1170 eV, 1202 eV, and 1403 eV, mean free paths of ～14 Å, ～22 Å, ～23 Å, and ～29 Å, respectively, are obtained for the poly(p-xylylene) polymer films studied in this work.     

The heat of fusion of copolymers of tetrafluoroethylene and hexafluoropropylene

The Clapeyron equation has been applied to determine the heat of fusion of two copolymers of tetrafluoroethylene and hexafluoropropylene. The values obtained are within experimental uncertainty of that for the homopolymer, PTFE. This study supports the conclusion of earlier workers that the CF₃ side groups are included in the crystal lattice. In both PTFE and polyethylene, C₁ side groups are included in the crystal but have little if any effect on its density. We believe that the same relationship between the crystallinity and the heat of fusion can be used for both PTFE and copolymers of TFE and HFP.     

Effects of polymer architecture and composition on the adhesion of poly(tetrafluoroethylene).

Poly(glycidyl methacrylate), PGMA, chains in linear and arborescent structures were incorporated onto surfaces of poly(tetrafluoroethylene), PTFE, films by hydrogen plasma and ozone treatment and atom transfer radical polymerization. The epoxide groups of the PGMA chains were further reacted with acetic acid (AAc), oxalic acid (XAc), allyl amine (AA), and ethylenediamine (EDN) to introduce hydroxyl and amine groups to the surfaces of the PTFE films. Surface characterizations performed by Fourier Transform infrared attenuated total reflectance (FTIR-ATR) spectroscopy and X-ray photoelectron spectroscopy (XPS) confirmed the surface modification and the chemical structure. The PGMA chains in arborescent structures show a high effectiveness for the enhancement of the adhesion of PTFE films. The adhesion of PTFE films was also significantly enhanced by ring-opening reactions of the PGMA epoxide groups with acetic acid and amine compounds. A high value of 9.5 N cm(-1) in the optimum 180 degrees peel strength test was observed with PTFE/copper assemblies.     

Glass and melting transitions of copolymers of tetrafluoroethylene with propylene and isobutylene

Copolymers of tetrafluoroethylene and propylene were prepared that contained 30–65 mole-% of the former. Reactivity ratios of tetrafluoroethylene- and propylene-ended radicals are 0.008 and 0.06, respectively, resulting in formation of highly alternating copolymers. The glass temperatures, T_g, were determined using a differential scanning calorimeter. Values ranged from 260 to 275°K. A plot of T_g versus composition has a low maximum centered about the equimolar composition. Copolymers of tetrafluoroethylene and isobutylene were prepared that contained 30–56 mole-% of the former. Reactivity ratios of tetrafluoroethylene- and isobutylene-ended radicals are 0.005 and 0.021, respectively. The glass temperatures of these copolymers range from 257 to 313°K. A higher maximum at the equimolar composition is obtained when T_g is plotted versus composition. Isobutylene-containing copolymers having 45–54 mole-% tetrafluoroethylene are crystalline. Melting temperatures range from 416 to 476°K and have their maximum value at the equimolar composition. It is thought that long sequences of alternating units behave as a third entity in these copolymers, the other two being nonalternating units of the two monomers. Unless inhibited, ionic homopolymerization of isobutylene can be appreciable, sometimes resulting in the polymer having two T_g.     

Cocrystallization in blends of random tetrafluoroethylene fluorinated copolymers: The effect of the chain structure and crystallization conditions

The possibility of the cocrystallization of random fluorinated tetrafluoroethylene copolymers was investigated with differential scanning calorimetry and wide‐angle X‐ray scattering. In particular, mixtures composed of poly(tetrafluoroethylene)‐co‐(hexafluoropropylene) containing 8 or 1 mol % comonomer or poly(tetrafluoroethylene)‐co‐perfluoromethylvinylether (2–10 mol % comonomer) were examined. The extent of cocrystallization was determined by the difference in the comonomer content, being higher when the difference was lower, and it was favored when quenching from the melt state was adopted. Nevertheless, a key to determining the extent of cocrystallization was the behavior of counits with respect to inclusion or exclusion from the crystal lattice: when the components were different with respect to this behavior, they were not likely to be miscible in the crystal state even if the difference in the comonomer content was low. Moreover, the similarity in the crystallization rates between the components played an important role: the cocrystallization decreased as the difference in the crystallization rate increased until, when the difference became high enough, the blend became immiscible. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1477–1489, 2002     

Crystallization and structure formation in polymer blends with strong intermodular interactions: blends of poly(ethylene oxide) and styrene-hydroxystyrene copolymers

Time-resolved synchrotron wide- and small-angle X-ray scattering experiments were used to investigate crystallization behavior and microstructure development of a nearly monodisperse poly(ethylene oxide) [PEO] (M_w = 53,500), and its melt-miscible blends with two fractionated styrene - hydroxystyrene random copolymers [SHS]. PEO crystallization rates decrease significantly in the presence of the melt-miscible SHS copolymers. All low and high molecular weight SHS blends exhibit a crystallization process at relatively short times characterized by large Avrami exponents (n), followed by a dominant process with n near that of neat PEO. A model for the crystallization of these blends is proposed.