Random Copolymer Of Tetrafluoroethylene Research Articles

Cavity size distributions in high free volume glassy polymers by molecular simulation

Two very permeable polymers, poly(1-trimethylsilyl-1-propyne) (PTMSP) and a random copolymer of tetrafluoroethylene and 2,2-bis(trifluoromethyl)-4,5-difluoro-1,3-dioxole (TFE/BDD), have very similar and large fractional free volumes (FFV), but very different permeabilities. Using atomistic models, cavity size (free volume) distributions determined by a combination of molecular dynamic and Monte Carlo methods are consistent with the observation that PTMSP is more permeable than TFE/BDD. The average spherical cavity size in PTMSP is 11.2 Å whereas it is only 8.2 Å in TFE/BDD. These cavity size distributions determined by simulation are also consistent with free volume distributions determined by positron annihilation lifetime spectroscopy.

Pulse Field Gradient NMR Study of Diffusion of Pentane in Amorphous Glassy Perfluorodioxole

Pulse field gradient diffusion measurements of pentane in a random copolymer of tetrafluoroethylene (TFE) and 2,2-bis(trifluoromethyl)-4,5-difluoro-1,3-dioxole (PDD) were made as a function of the time allowed for diffusion to occur. The initial change in echo amplitude at low values of q = γδg was used to determine the apparent diffusion constant. The apparent diffusion constant determined in this way decreases to a constant value as time increases. At a time of 12.4 ms the apparent diffusion constant was 2.2 × 10-7 cm2 s-1, and it decreases to 5.87 × 10-8 cm2 s-1 at 1 s. This change in the apparent diffusion constant was interpreted in terms of morphological structure on the micron length scale in this completely amorphous glassy polymer. This polymer has been considered to have high free volume regions which lead to the observed rapid permeation of gases and vapors. These regions are interspersed in lower free volume regions, and the low free volume regions constitute the majority component. In the pul...

Study of high permeability polymers by means of the spin probe technique

The spin probe technique was systematically applied to study a family of high permeability and high free volume glassy polymers. Rotation correlation times τ c or frequencies ν=1/τ c of 2,2,6,6-tetramethylpyperidine-1-oxyl (TEMPO) were measured in amorphous teflons — random copolymers of tetrafluoroethylene and 2,2-bistrifluoromethyle-4,5-difluoro-1,3-dioxole, polyacetylenes and polynorbornenes. Polymers distinguished by unusually high, for the glassy state, permeability and free volume exhibit large rotational mobility of TEMPO. The correlation times correspond to fast rotation of the spin probe previously observed only in rubbery polymers. Correlations between the frequency ν and gas permeability and diffusion coefficients were observed. However, in some polymers, the spin probe's rotation rate is also sensitive to side-chain local mobility, which does not affect translational diffusion coefficients of gas molecules in polymers.

Hydrocarbon and Perfluorocarbon Gas Sorption in Poly(dimethylsiloxane), Poly(1-trimethylsilyl-1-propyne), and Copolymers of Tetrafluoroethylene and 2,2-Bis(trifluoromethyl)-4,5- difluoro-1,3-dioxole

Sorption of a series of gases, perfluorocarbon vapors (CF4, C2F6, and C3F8) and their hydrocarbon analogues in poly(dimethylsiloxane) [PDMS], poly(1-trimethylsilyl-1-propyne) [PTMSP], and two random copolymers of tetrafluoroethylene [TFE] and 2,2-bis(trifluoromethyl)-4,5-difluoro-1,3-dioxole [BDD] are reported as a function of penetrant pressure at 35 °C. Sorption isotherms for all penetrants in rubbery PDMS are linear or slightly convex to the pressure axis, while those in the glassy polymers are concave and are well described by the dual mode model. Fluorocarbon sorption levels are lower than sorption levels of their hydrocarbon analogues in the hydrocarbon-based PDMS and PTMSP matrixes, while the reverse is true in the fluorinated TFE/BDD copolymers. Exceptionally low fluorocarbon solubilities in PDMS are ascribed to poor penetrant/polymer energetic interactions.

Structural variations in random copolymers of tetrafluoroethylene with kind and content of comonomer units

Random copolymers of tetrafluoroethylene with minor amounts of fluorinated comonomers (hexafluoropropene, perfluoromethylvinylether and perfluoropropylvinylether) of different compositions are characterized mainly by X-ray diffraction but also by differential scanning calorimetry and density measurements in an attempt to establish the possible inclusion of the respective side groups into the crystallites. Structural characterizations are conducted at room temperature (where all the considered copolymer samples are in the less ordered crystalline form I) as well as at − 40°C (where all the considered copolymer samples are in the more ordered crystalline form II). Increases of the lateral dimension of the unit cell and decreases of the size and amount of crystalline domains, with comonomeric unit content, are observed which are similar for different comonomer units (of largely different bulkiness) as well as for the two crystalline forms. The reported experimental results suggest a substantial exclusion from the crystallites not only of the bulkier comonomeric units (from perfluoropropylvinylether), but also, at least for high comonomer compositions, of the smaller comonomeric units (from perfluoromethylvinylether and hexafluoropropene).

Crystalline Homopolymer-Copolymer Blends: Poly(tetrafluoroethylene)-Poly(tetrafluoroethylene-co-perfluoroalkylvinyl ether)

The miscibility and crystallization behavior of mixtures of poly(tetrafluoroethylene) (PTFE) and poly(tetrafluoroethylene-co-perfluoroalkylunyl ether) (PFA) has been studied. PFA is a random copolymer of tetrafluoroethylene and a small amount of perfluoropropylvinyl ether. Although miscibility is difficult to assess directly due to the uncertain origin of the mechanical α-transition, the crystallization and melting behavior of the blends points to significant mixing. Under most crystallization conditions investigated, PTFE and PFA were found to crystallize in distinguishable crystalline regions, with PTFE acting as an effective nucleant for at least a portion of the PFA. Under very rapid crystallization conditions, DSC data indicate that PTFE and PFA cocrystallize, lending further support to the idea that the polymers are well mixed in the melt

Raman spectroscopic evidence for conformational deformation in the high pressure phase of polytetrafluoroethylene

An investigation of polytetrafluoroethylene (PTFE), nC20F42, and a random copolymer of tetrafluoroethylene and hexafluoropropylene (TFE–HFP) under pressure has been carried out using Raman spectroscopy in conjunction with a diamond anvil cell. Both nC20F42 and the TFE–HFP copolymer were found to undergo a phase transition in the 7–9 kbar range similar to PTFE as evidenced by the observation of the 625 cm−1 Raman band characteristic of phase III. With increasing pressure in the 10–52 kbar range, a continual variation in intensity of the bands at 285 and 395 cm−1 was observed and found to differ for the three fluorocarbon materials studied. By correlating a change in the I285/I395 ratio with a change in sample crystallinity (as determined by x- ray), it has been determined that an increase in transplanar content with pressure is responsible for the observed band intensity changes in the high pressure phase of PTFE.

Longitudinal acoustic modes of polytetrafluoroethylene copolymers and oligomers

Raman active longitudinal acoustic modes ( LAM) have been observed in the perfluoro n-alkanes and in a random copolymer of tetrafluoroethylene and hexafluoropropylene (TFE-HFP). Low frequency bands were found in both the melt and solid phases of the oligomers, and their comparison indicates that regular extended helical conformations exist in the melt phase of perfluoro n-alkanes below n-C 12F 26. Good agreement is found between the lamellar long spacings of the copolymer and that calculated from the LAM frequencies. Normal mode calculations, using available force fields, have been performed on the infinite homopolymer (PTFE) and n-C 16F 34. It was found that the helix reversal type of chain defect does not affect LAM frequencies and intensities.

Theory and observations of polymer crystal thickening

The thickening of polymer crystals during isothermal annealing is usually observed to be an irreversible process. Phenomenological laws that govern such processes take the form of simple proportionalities—flux being proportional to force. For polymer crystals, a thermodynamic force capable of driving the thickening phenomenon arises from the unequal free energies of the fold and lateral surfaces. By analogy with other irreversible phenomena, the rate of crystal thickening is taken to be proportional to the derivative of the surface free energy with respect to crystal thickness. After certain assumptions, integration yields an equation in which three parameters characterize the system: an initial thickness l0, an equilibrium thickness l*, and a relaxation time τ which is a function of the ``undercooling''. The theory provides a basis for considering the effects of parameters such as time, temperature, thermal history, pressure, and liquids on the thickening rate. In particular, the theory adequately describes the time and temperature dependence of crystal thickening in random copolymers of tetrafluoroethylene and hexafluoropropylene which exhibit thickening behavior completely analogous to that of homopolymers. During thickening, the unit cell dimensions of these quenched-crystallized copolymers decrease in a manner that is consistent with the concept of complete comonomer inclusion upon crystallization.

Crystallization of Random Copolymers.

A theory of crystallization is formulated for random copolymers which crystallize with the non-crystallizable co-units incorporated into the crystalline lattice as defects. The appropriate melting point equation and other associated thermodynamic properties are derived for this model as a function of crystal thickness and comonomer concentration. The formation of lamellar type morphology is assumed to be a kinetically determined phenomena and nucleation theory is utilized accordingly. The isothermal lamella thickness is predicted to increase in a definitive manner as the noncrystallizable comonomer concentration X increases, while the associated isothermal growth rate is predicted to decrease. The variation of lamella thickness with X when the copolymer is quenched or cooled at a uniform rate is also qualitatively predicted. Under these conditions lamella thickness decreases with increasing X, which is in accord with previous experimental observations on random copolymers of tetrafluoroethylene and hexafluoropropylene as well as other random copolymers. Theory also suggests how the surface free energy parameters σ e and σ can be determined from isothermal crystallization experiments for a series of random copolymers of varying composition.