Polymerization Of Tetrafluoroethylene Research Articles

Sol–Gel Transition-Accompanied Radical Polymerization Initiated by Two Primary Radicals

When irradiation generates two radicals capable of initiating chain growth with different rate constants for chain transfer, two polymerization routes with different average chain length emerge. These pathways are not mixed in the initial part of the ballistic growth. Their mixing in chain transfer events is accompanied by transition from ballistic to diffusion growth. In the sol–gel transition region, the routes are spatially separated if the chain transfer depends on the rotational mobility of the reactants and decreases with increasing viscosity as the resulting oligomers accumulate. As an example, the polymerization of tetrafluoroethylene in solutions of silanes and oxysilanes is considered.

Study of fluoroalkylsilanes, a new class of amphiphilic oligomers: Determination of end groups and chain length

Radiation-initiated polymerization of tetrafluoroethylene (TFE) in the liquid alkylsilanes Si(CH3)4 (I), C2H5Si(CH3)3 (II), and ClSi(CH3)3 (III) produces fluoroalkylsilane oligomers with the general formula R(C2F4) n X, where X = H or Cl in I, II, and III and R is one of the radicals generated by the abstraction of H from the methyl and ethyl groups in I and II or Cl detachment from III. The nature of R has been determined using an NMR technique, and the molecular mass distribution (MMD) has been found from abundances in anion progressions in mass spectra and from DTG and GPC data. At initial TFE concentrations (C 0) changed from 0.4, 1, and 0.13 to 1, 2.7, and 0.7 mol/L in I, II and III, respectively, the average chain length 〈n〉 increases from 4–5 to 10–12 and the MMD changes from the unimodal to the bimodal pattern in which the proportion of oligomers with n < 6 decreases with increasing C 0. As 〈n〉 increases and the MMD changes, the homogeneous solution becomes colloidal and then turns into a gel. The colloidal particles are largely composed of long oligomers, and the number of solvent molecules per C2F4 unit decreases with an increase in C 0 and is 4–7 in the gel.

Differential thermogravimetric curves for a mixture of evaporable and degradable chain oligomers

Since the boiling point of oligomers increases with increasing chain length, differential thermogravimetric (DTG) curves of polymerization products are uniquely related to the molecular mass distribution of the oligomers in the chain length region in which the degradation rate is less than the rate of evaporation. Degradation is manifested by narrow, chain length-invariant peaks of the DTG curves so that they are distinguishable from broad DTG bands due to the evaporation of the mixture of oligomers. The detachment of the terminal groups at a temperature T1 and main chain scission at Td > T1 are accompanied by dimerization of macroradicals, evaporation of the dimers in the T1 < T < Td interval, and appearance of the full degradation peak at T ≈ Td. The pattern of DTG curves based on these concepts has been calculated on the assumption of free convection in the boundary layer and a spatially uniform degradation in the melt. As an example, DTG curves for the products of tetrafluoroethylene polymerization in liquid solutions have been considered.

Calorimetric study of the low-temperature postradiation polymerization of tetrafluoroethylene in the presence of reduced graphite oxide

Low-temperature postradiation polymerization of tetrafluoroethylene in the presence of carbon material obtained by explosive exfoliation of graphite oxide has been studied by the technique of calorimetry. The process results in tetrafluoroethylene grafting onto modified graphite oxide to form a new composite containing ∼40–65% polytetrafluoroethylene.

An NMR, DSC, and IR spectroscopy study of the composite formed during low-temperature postradiation polymerization of C2F4 in the presence of a 3D graphene material

By solid-state magic angle-spinning nuclear magnetic resonance spectroscopy (NMR), differential scanning calorimetry (DSC), and IR spectroscopy, polytetrafluoroethylene (PTFE) samples obtained by low-temperature (T = −196°C) postradiation polymerization of tetrafluoroethylene (C2F4) and C2F4 mixtures with a 3D graphene material formed by the microwave exfoliation of graphite oxide films (MEGO) have been investigated. It has been shown that the melting point of PTFE in the PTFE-MEGO composite is 332.5°C, which is 8.8°C above the melting point of pure PTFE obtained under the same conditions. According to the measured values of specific enthalpy of melting ΔH m (51.5 and 45.4 J/g), the degrees crystallinity (x c) of 0.63 and 0.55 have been calculated for the pure polymer and the composite, respectively. It has been also found that none of the PTFE-containing samples examined has terminal CF3 groups typical of PTFE prepared by conventional suspension polymerization.

Molecular mass distribution of oligomers in products of tetrafluoroethylene radical telomerization

The molecular mass distributions (MMD) of perfluorinated oligomers in products of tetra-fluoroethylene (TFE) radical polymerization in various solvents (telogens) were determined from an analysis of differential thermogravimetric curves and data of gel permeation chromatography and mass spectrometry. Radiolysis of the telogens generates radicals initiating polymer chain growth. The choice of the solvent and TFE concentration makes it possible to obtain oligomers with the controlled average chain length from 4 for 40 CF2 fragments and specified terminal groups. The polymerization of TFE in THF and propylene oxide affords oligomers with cyclic terminal groups capable of further polymerization due to ring opening. The appearance of two MMD maxima (low-molecular-weight at n 1 ∼6–8 and high-molecular-weight at n 2 > 10 shifting towards high n with an increase in the TFE concentration) is caused by the formation of colloidal solutions of oligomers.

Radiation-initiated telomerization of tetrafluoroethylene in acetone at a constant monomer concentration

Kinetics of radiation-initiated polymerization of tetrafluoroethylene in acetone was studied at various initial concentrations (pressures) of the monomer in the reactor, which remain constant in the course of synthesis. Concentrated solutions of tetrafluoroethylene telomers were obtained and their properties were examined.

The study of polymerization initiated by gamma-irradiated perfluoroacids and their derivatives

Polymerization of tetrafluoroethylene and methyl methacrylate has been initiated by active sites arising upon γ-radiolysis of perfluorinated organic acids and their derivatives. In the low-temperature radiolysis, two types of active sites are formed. The active sites of the first type (supposedly, -ĊF-COO−), which provide the most efficient polymerization at low temperatures, are accumulated during radiolysis of NaPFNA both at 77 and 300 K. The active sites of the second type, supposedly, of the radical type, are accumulated during radiolysis at 77 K and annealed during heating the sample to room temperature.

Enthalpies of formation of radicals and the mass spectra of the products of tetrafluoroethylene polymerization in acetone

The mass spectra of the dissociative electron-impact ionization products of telomers formed upon the radiation-chemical telomerization of tetrafluoroethylene in acetone were measured over the range of m/z from 1 to 204. The most intense bands at m/z = 43, 51, and 57 were attributed to the CH3CO+, CF2H+ and CH3COCH2+ cations—the main dissociation products of the H(C2F4)nCH2COCH3 telomers. The telomer composition was consistent with a radical telomerization mechanism, in which chain growth and chain transfer are due to the formation of the CH3COCH2· radical. Based on published data supplemented with quantum-chemical calculations, the enthalpies of formation of the radicals R(CF2)n (n = 2–8; R = H, CH3, CH3CO, and CH3COCH2) were tabulated. The formation of telomers with the same terminal groups is consistent with thermodynamic data and a polymerization mechanism in which the chain growth reaction is diffusion-limited and the chain transfer reaction is activated hydrogen-atom transfer.

Formation of colloid solutions and gels during the radiation-chemical telomerization of tetrafluoroethylene in acetone

The polymerization of tetrafluoroethylene (TFE) is initiated by the radicals generated by the radiolysis of acetone. The oligomer chain length n increases with an increase in the initial monomer concentration C. The transition from the true to the colloid solution at C ≈ 0.07 mol/l takes place at an average telomer chain length of \( \bar n \) ≈ 5. The transition of the colloid solution into gel is observed at C ≈ 0.30–0.35 mol/l. The acetone/TFE ratio in the centrifugation sediment decreases with an increase in the centrifugal acceleration and C. Its minimal value of ∼4 is attained at C > 2 mol/l when \( \bar n \) ≈ 12–15. The mean particle diameter in the phase separation region is 0.6–0.8 µm. A scaling model of radical polymerization is proposed to explain the molecular-mass distribution of telomers.

Development of methods for synthesis of surface-modified fluoropolymer-containing composite materials

The results of studies into development of methods for synthesizing new modified fluoropolymer-containing composite materials were summarized. The issues covered include: new modes and initiation methods of graft polymerization of fluoromonomers onto the surface of items of various nature and dispersity without homopolymer formation; nature of the surface active centers of the modified material and how it affects the kinetics and mechanism of both postradiation graft polymerization of tetrafluoroethylene and the subsequent chemical modification of the graft-polymer; major mechanisms and methods of blocking the terminal radicals of the grafted fluoropolymer layer, including those based on reaction with some perfluoroolefins at their double bond, yielding stable long-lived radicals, which allows controlling the physicochemical properties of composite materials; and new methods of synthesis of selective biocompatible sorbents with specific adsorption properties. Accelerating effect of oxygen on polymerization was analyzed, and the phenomenon of ozone initiation of low-temperature polymerization of tetrafluoroethylene was revealed and examined; a new class of high-performance polymerization initiators, namely, perflouroolefin ozonides, was synthesized; and new methods of initiation of polymerization and copolymerization of fluoromonomers, as well as of some other monomers with their use were developed.

Postradiation polymerization of tetrafluoroethylene on mineral supports at low temperatures

The low-temperature graft polymerization of tetrafluoroethylene (TFE) on mineral supports (SiO2, Al2O3, TiO2, and activated charcoal) has been investigated by the microcalorimetry, ESR, and IR spectroscopy techniques. It has been shown that polymerization of TFE is initiated by radical ion centers formed upon radiolysis of the support. The initiation stage includes the interaction of a radical ion particle of the support with a monomer molecule yielding a growing neutral radical. The radical growth of polymer chains has been confirmed by the appearnce of the ESR spectrum of terminal fluoroalkyl radicals ∼CF2−CF2, and the covalent C-Si bonds of the polymer with the support have been detected by means of IR spectroscopy. The efficiency of the low-temperature graft polymerization depends on the monomer sorption and the sorbent nature: the highest efficiency is observed for Al2O3 and, then, macroporous glasses; grafting proceeds to the least extent (yield below 5%) on the carbon supports, which do not the form radical ion centers during radiation activation.

Polymerization of tetrafluoroethylene initiated by fluorinated petroleum coke

The polymerization of tetrafluoroethylene initiated by the inorganic polymer fluorocarbon material CF0.95 prepared by the high-temperature fluorination of petroleum coke has been studied. The kinetics of the process has been examined, and the dependences of the rate of polymerization and the yield of polymer on monomer pressure and temperature have been considered. The free-radical mechanism of the process is established by ESR. The kinetic features of initiation and polymerization leading to an exponential growth in rate in the initial non-steady-state portion of kinetic curves are discussed.

Properties of thin poly(tetrafluoroethylene) films deposited on solid substrates by electron beam polymerization from vapor phase

The deposition of thin poly(tetrafruoroethylene) films on silicon substrates via the polymerization of tetrafluoroethylene from the vapor phase by means of an electron beam with an electron energy of 20–40 eV and the current density varied in the range 1–104 μA/cm2 has been studied. It has been shown that a variation in the current density during deposition allows one to widely vary the properties of the films being formed, in particular, their heat resistance. The crosslinked poly(tetrafluoroethylene) films prepared at a current density of 102 to 103 μA/cm2 demonstrate heat resistance up to 400–450°C. This opens wide prospects for the use of these films as dielectric layers in microelectronics.

Nanometer-scale surface modification by polymerization of tetrafluoroethylene on polymer substrates in supercritical fluoroform

Surface penetrated polymerization of tetrafluoroethylene (TFE) was carried out on a polycarbonate (PC) plate in supercritical fluoroform (scCHF3). Since the high diffusiveness is one of peculiar features of supercritical fluids, TFE monomers and initiators (perfluorinated benzoyl peroxide) could penetrate into the surface of polymer substrates and be photo-polymerized. After washing physisorbed homopolymers on the surface, polytetrafluoroethylene (PTFE) was found to penetrate into 50–800 nm depth from the surface and covered the PC surface in the proportion of 85%. The surface coverage density and the penetration depth could be controlled by adjusting of the pressure of scCHF3. The TFE-penetrated polymerization could be applied for various polymer plates such as polyethylene, polystyrene, polypropylene, poly(ethylene terephthalate), and polyimide. In addition to polymer plates, this technique could be applied to a cellulose paper, a nylon textile, and a porous PC membrane. The PTFE-penetrated nylon textile showed a high resistance for washing test with detergents, compared with the commercial fluoropolymer-sprayed nylon textile. The PTFE-penetrated porous PC membrane showed high oxygen permeability (P/P = 5.2), compared with that of the untreated PC membrane (P/P = 3.5) in gas permeation experiments of O2 and N2. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 1577–1585, 2008

Plasma polymerization of tetrafluoroethylene – towards CF2 dominated fluorocarbon films

AbstractFluorocarbon films with an exceptionally high CF2 content were obtained by plasma polymerization using a low pressure radio frequency discharge operated with a mixture of argon and tetrafluoroethylene. Substrates were placed in a remote position downstream the discharge. Gas pressure, discharge power, substrate position, gas composition and substrate temperature were changed to alter the chemical structure of the plasma polymers. The properties of the films were characterized by X-ray photoelectron spectroscopy (XPS), spectroscopic ellipsometry and contact angle goniometry. A pronounced increase of the CF2 content was obtained for elevated substrate temperatures and increased amounts of tetrafluoroethylene in the process gas. Applied as a model surface in studies of interfacial phenomena on polytetrafluoroethylene (PTFE), transparent PTFE-like thin films enable the use of numerous optical techniques not applicable to common PTFE foils.

Polymerization-filling of polytetrafluoroethylene

Polymerization of tetrafluoroethylene (TFE) in an aqueous medium using a redox system in the presence of a hard disperse filler leads to polymerization-filled polytetrafluoroethylene (PTFE). The influences of various factors on the polymer yield, the conversion of TFE and the degree of filling were studied. The kinetic dependencies concerning the polymerization-filling of PTFE with CaCO 3 were studied. The reaction order, activation energy and the ratio of rate constants for chain propagation and termination were determined.

Influences of the substrate temperature on the films plasma-polymerized at the low system pressure

Influences of the substrate temperature and the system pressure on the plasma polymerization were investigated from a viewpoint of the chemical composition of the deposited polymers. Both parameters influenced the plasma polymerization of tetrafluoroethylene (TFE). The deposition at the high substrate temperature as well as at the low system pressure led to the formation of plasma polymers being poor in fluorine moieties. This result indicates that the chemical composition of the deposited plasma polymers may be determined by interactions of activated molecules with substrate surfaces.

Role of radicals, long-lived in a liquid, in the polymerization of fluoroolefins

Using the polymerization of tetrafluoroethylene in a branched perfluoroolefin medium as an example (hexafluoropropylene, and its dimer and trimer), it is shown that hexafluoroethylene and its dimer are effective traps for growing polymer radicals. As a result of this interaction inactive, long-lived radicals not capable of mutual recombination are formed, which results in chain termination and prevents homo- and copolymerization of hexafluoropropylene. The hexafluoropropylene trimer is unable to add any radical apart from atomic fluorine and is an almost inert medium in the radical processes under consideration.

Polymérisation de monomères organiques dans les plasmas froids—4: Étude des structures de polyéthylènes et polytétrafluoroéthylènes, en particulier par résonance paramagnétique électronique

The ethylene and tetrafluoroethylene polymerization in a “cold plasma” leads to polymers with considerable deficiencies of hydrogen or fluorine atoms. With electronic spin resonance, the various kinds of radicals trapped in the macromolecular structure have been detected and in some cases identified and determined. The mechanisms could be partly explained in thermodynamic terms. Irradiation of the corresponding commercial polymers gives some information about degradation processes in the plasma.