Leo J. Kasehagen

Nonlinear shear and extensional rheology of long-chain randomly branched polybutadiene

We present nonlinear shear and uniaxial extensional measurements on a series of polybutadienes with varying amounts of long-chain, random branching. Startup of steady shear experiments is used to evaluate the damping function of the melts. The damping function is found to show a trend toward decreased dependency on strain with increasing branching content. Interior chains, which are believed to be responsible for changing the damping function, are calculated to comprise less than 3 wt % of the melt. Extensional measurements are used to investigate the role of branching in strain hardening. We show that samples with increased branch contents do show larger deviations of the transient Trouton ratio from the linear viscoelastic limit of three. However, we also show that the extensional data can be fit using parameters determined solely by the shear measurements. Furthermore, we show that the changes in the damping function seen in shear have little impact on extensional behavior. The extensional behavior of the melt is found to be most affected by changes in the relaxation spectra which can result from both branching and increases in the high end of the molecular weight distribution. This statement runs contrary to the often expressed view that strain hardening behavior in extension is exclusively produced by branching.

Hydrolysis and blistering of cyanate ester networks

The hydrolysis of a cyanate ester network made from the monomer 2,2*- bis(4-cyanatophenyl)isopropylidene (bisphenol A dicyanate homopolymer) was stud- ied. Hydrolysis reactions were performed isothermally at temperatures from 150 to 1807C under conditions of excess water. The kinetics of the reaction were characterized by the decrease in Tg as measured by differential scanning calorimetry. The rate of change of Tg was found to be adequately described as first order in Tg, which is an indirect measure of the concentration of crosslink junctions. The activation energy of the reaction was found to be 115 kJ/mol. In addition, moisture-conditioned, glass- reinforced laminate samples were heated and the time to delamination or blistering was recorded as a function of temperature. The blister time at solder temperatures (T A 220-2607C) was modeled using the above kinetic results. Heat transfer to the lami- nate was considered and the criteria used for blister time was the time at T A Tg of the sample. At lower temperatures (T o 2207C), loss of water from the laminate is sufficiently fast to prevent blistering. q 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 107-113, 1997

Transient extensional viscosity from a rotational shear rheometer using fiber‐windup technique

The fiber‐windup technique, developed by Macosko and Lorntson [SPE Tech. Papers 19, 461–467 (1973)] for rotational shear rheometers, was re‐examined to obtain transient uniaxial extensional viscosity data. Macosko and Lorntson used a special force transducer to measure the tension in the filament being pulled. In contrast, in this study commercial rotational shear rheometers (Rheometrics Mechanical Spectrometer‐800 and Rheometrics Fluids Spectrometer‐II) with torque transducers were used to obtain the extensional viscosity. One end of the sample is clamped, while the other end is wound around a drum at a constant rotational speed, to achieve a given extension rate. Tests were conducted with a polyisobutylene sample at room temperature to verify reproducibility and were compared with published studies. Results for polyisobutylene and IUPAC‐X low density polyethylene were found to be in good agreement with published results. Hencky strains of up to 6 could be achieved. The technique can provide valuable ext...

Rheology of long‐chain randomly branched polybutadiene

The rheology of a series of long‐chain randomly branched polybutadienes (PBDs) has been investigated. Branched samples were made through a hydrosilation reaction between a small difunctional crosslinker and the 1,2 groups distributed on the backbone of near‐monodisperse PBD (Mw=56 000, 137 000; Mw/Mn=1.04). The resulting samples have both dispersity in molecular weight and architecture. The species distribution has been studied using size exclusion chromatography and has been found to agree with the Flory–Stockmayer distribution for the random branching of monodisperse chains. Sinusoidal oscillation and creep/creep recovery experiments were done to determine dynamic moduli, zero shear viscosity, and equilibrium creep compliance. The observation that branching is random allows structural parameters to be calculated such as weight fractions and average molecular weights of free chains, arms, and interior chains. These parameters are used in existing rheology models whose results are compared to experimental...

Structure development in cyanate ester polymerization

We present results on the product distribution obtained from the polymerization of the dicyanate monomer of bisphenol A carried out under different reaction conditions. Small quantities of side-products were observed using both size exclusion chromatography and high-pressure liquid chromatography for samples polymerized under an inert atmosphere. Despite the presence of side-products, the oligomer distribution from SEC, after correcting for differences in refractive index, was found to agree well with random branching predictions. Results from HPLC also confirm good agreement with the theoretical prediction. Furthermore, a gel conversion of approximately 0·51(-0·01, +0·03) was measured for the inert atmosphere product, which is within experimental error of the ideal value of 0·50. The monomer content of a polymerization carried out open to the atmosphere, however, showed significant disagreement with theory. ©1997 SCI