Degradation of a poly(ester urethane) elastomer. I. Absorption and diffusion of water in Estane® 5703 and related polymers

Abstract

In preparation for studying the hydrolytic degradation of Estane® 5703 and related poly(ester urethane) elastomers, the absorption (solubility) and diffusion of water in these polymers have been examined experimentally and modeled theoretically. Weight gain and loss experiments have been carried out. The amount of water absorbed per gram of sample was linear at low relative humidities (RHs) but curved upward at higher RHs. This curvature was not fit by Henry's law or the Flory–Huggins equation but was easily fit by a water‐cluster model. Diffusion coefficients were determined by fitting the time dependence of the sample weights, and the diffusion appeared Fickian to within experimental uncertainty. The similarity of related polymers was used to determine the approximate temperature dependence of the absorption. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 40: 181–191, 2002

Degradation of a poly(ester urethane) elastomer. II. Kinetic modeling of the hydrolysis of a poly(butylene adipate)

“…Consistent with the diffusion studies of the preceding article,1 given the fact that 63 PBA used by BLS2 was in the form of flakes less than 2 mm thick plus the fact that the rate at which gas‐phase water molecules impinge on the surface of the flakes depends little on the temperature, we can take k win = 4000/month independent of temperature. Then, because the equilibrium constant relates the rate coefficients via K 1 = k wout / k win , one finds that the Δ H ° for eq 5 becomes the activation energy for water evaporation from the polymer,1 and k wout has the Arrhenius form, k wout = A wout exp(− E wout / RT ), where A wout = 2.3 × 10 8 /month and E wout = 8.031 kcal/mol. Here, R is the universal gas constant in kcal/(mol‐Kelvin), and T (Kelvin) is the absolute temperature.…”

“…Water is one of the reactants in eqs 1 and 2, and its concentration must be known to model the hydrolysis and re‐esterification kinetics data of BLS. In the preceding article1 we modeled the water‐absorption data for 63 PBA and related polymers using the following equations: and where H 2 O is the monomeric water in the polymer, H 2 O(g) is the water vapor in the atmosphere, and (H 2 O) n is a cluster of n water molecules. Subsequently, these species are referenced as W , W (g), and ( W ) n .…”

“…Subsequently, these species are referenced as W , W (g), and ( W ) n . The value n = 5 was found to fit the data, and equilibrium constants K 1 ( T ) and K n ( T ) for eqs 5 and 6 were determined 1…”

“…Considering eq 6, the forward reaction is a recombination reaction, and such reactions usually have little temperature dependence. Also, the fact that the diffusion is Fickian1 implies that cluster formation is fast enough to keep up with reaction 5. Hence, we take k cf = 4000 (kg/mol)4/month independent of temperature; then, use of the equilibrium constant1 K n gives k cd Arrhenius form with A cd = 2.7 × 10 7 /month and E cd = 5.976 kcal/mol.…”

“…In this article, we consider the hydrolytic degradation of a different (but closely related) poly(ester urethane) in preparation for a study of Estane® 5703 itself. As discussed in the preceding article,1 both polymers are obtained by polymerization of 4,4′‐diphenylmethane diisocyanate (MDI) with polybutylene adipate (PBA) ester with 1,4‐butanediol (BDO) as the chain extender. They differ only in their mole fractions of these components.…”