William H. Tuminello

Obtaining molecular-weight distribution information from the viscosity data of linear polymer melts

Based on the model of Bersted and Slee, and later Malkin and Teishev, both differential and integral methods were developed to determine the molecular-weight distribution (MWD) from viscosity data. The more sensitive differential method can detect small inflections in the viscosity data and convert these into MWD information. The integral method is, however, capable of handling moderately incomplete viscosity data. Combining self-consistent differential and integral approaches, we are able to resolve details of a MWD and quantify reasonably broad MWDs from many sets of limited viscosity data. These methods both have very short computation times. A reported overemphasis of the high-molecular-weight end of distribution is due to the excitation of Rouse modes during the rheological measurements at high frequencies, which masks the diffusive contributions of the low-molecular-weight components to the relaxation process.

Development of a centrifuge ball viscometer for polymer melts

A centrifuge ball viscometer was developed for fluids with a wide viscosity range. The viscosity η could be obtained from an empirically derived relationship ac=b(ηV∞)c, where ac is the applied acceleration, b and c are empirically determined parameters, and V∞ is the terminal velocity. The key features of this viscometer are as follows. (1) A wide range of viscosities can be accommodated: about 10−1 to 105 Pa s with 1010 Pa s achievable, in principle, for about a one‐data point per day measurement. (2) A computer controlled motion controller allows rotation speed of the motor varying over a speed of 1 to 5000 (±0.01) RPM (revolutions per minute). Thus, measurements at different low shear rates can be accomplished. (3) About 0.5 ml of sample volume is needed for the present setup. (4) The temperature can be maintained from ambient to over 400 °C (±0.05 °C). (5) The samples are isolated by sealing them in glass tubes, a potential advantage when dealing with polymer solutions, polymer melts, and/or those sy...

Supercritical Fractionation of A Perfluorinated Copolymer

Abstract A perfluorinated copolymer, poly(tetrafluoroethylene-co-19, 3 mol % hexafluoropropylene), is fractionated using supercritical SF6. The fractionations arc performed isothermally at 163°C using an increasing pressure profile to obtain gram-sized fractions that have molecular weight distributions that are narrower than that of the parent copolymer. The fractionation pressure is increased in increments of 34 bar from 276 to 683 bar. The parent material and the fractions are characterized using SEC and FTTR spectroscopy. The first seven samples, eluting between 276 and 552 bar, show a trend of increased molecular weight of the fraction with increasing fractionation pressure. Above 552 bar, the molecular weight of the remaining fractions decreases with increasing fractionation pressure. Although these last fractions do not differ in hexafluoropropylene content, they do differ in the type and concentration of their end groups. In particular, the fractions that eluted at pressures greater than 552 bar co...

Cosolvency effect of SF6 on the solubility of poly(tetrafluoroethylene-co-19 mol % hexafluoropropylene) in supercritical CO2 and CHF3

Cloud-point data to 280°C and 2800 bar are reported for a binary mixture of poly(tetrafluoroethylene-co-19.3 mol % hexafluoropropylene) (FEP 19 ) in fluoroform (CHF 3 ) and for ternary mixtures of FEP 19 -CHF 3 -sulfur hexafluoride (SF 6 ) and FEP 19 -CO 2 -SF 6 . FEP 19 does not dissolve in CHF 3 at a temperatures less than 235oC due to strong dipolar CHF 3 -CHF 3 interactions relative to FEP 19 -CHF 3 cross interactions. However, FEP 19 dissolves in CO 2 if the temperature is greater than 185oC and the pressure is in excess of 1000 bar. When SF 6 is added to either FEP 19 -CO 2 or FEP 19 -CHF 3 mixtures, the cloud-point curve is shifted to lower pressures and temperatures due to the increase in favorable dispersion interactions with nonpolar FEP 19 . The magnitude of the shift in cloud-point pressure per amount of SF 6 added to solution decreases in a nonlinear manner with increasing amounts of SF 6 . The Sanchez-Lacombe equation of state can model the binary FEP 19 -SCF data if the FEP 19 -CO 2 and FEP 19 -CHF 3 binary interaction parameters are allowed to vary with temperature. However, a poor representation is obtained for the ternary phase behavior.

Solubility of Poly(Tetrafluoroethylene) and its Copolymers

Owing to the exceedingly small intermolecular forces in perfluoropolymers, their solubility is dominated by entropy effects. Enthalpic interactions almost always decrease solubility because they tend to favor solvent-solvent mixing. Solution melting and crystallization temperatures have been observed to decrease with lower solvent molar volume, lower undiluted polymer melting point, cyclic perfluorocarbons, low solvent polarity, low pressure, and low polymer concentration. Maximizing solvent density by increasing pressure or solvent MW favors solubility by increasing the LCST temperature. Solution stability also increases with polymer concentration.

The Solubility of Tetrafluoroethylene/Hexafluoropropylene Copolymers

Abstract The copolymers of TFE (tetrafluoroethylene) and HFP (hexafluoropropylene) form stable solutions in perfluorocarbons at much lower temperatures than TFE homopolymer (PTFE). Most of the work has emphasized the use of fused, 6-membered ring, aliphatic perfluorocarbons as solvents. Polymer concentrations as high as 10% were obtained; much higher concentrations are possible. Widely different solubility characteristics were found for several perfluorocarbon solvents suggesting the feasibility of fractionation via extraction to determine comonomer and end group distribution.