Hydrolysis of poly(ester urethane): In-depth mechanistic pathway determination through thermal and chemical characterization

Abstract

• Below T m of PEU, hydrolysis preferentially occurs at the COO groups of soft-segments. • Above T m of PEU, hydrolysis can occur at the urethane groups of hard-segments. • PEU exhibits a structural failure point when hydrolysis attacks the urethane groups. • A critical molecular weight (M C ) is associated with this structural failure point and increases with temperature/humidity. • More structure/property relationships are established for hydrolyzed PEU. • The effect of T m on E a of PEU hydrolysis is investigated. Many structure/property relationships of hydrolyzed poly(ester urethane) (PEU) – a thermoplastic polymer have been reported. Examples include changes in molecular weight vs. elongation at break and crosslink density vs. mechanical strength. However, the effect of molecular weight reduction on some physical, thermal, and chemical properties of hydrolyzed PEU have not been reported. Therefore, a large set of hydrolyzed PEU (Estane®5703) samples were obtained from two aging experiments: 1) accelerated aging conducted under various environments (air, nitrogen, moisture) and at 64°C and below for almost three years, and 2) natural aging conducted under ambient conditions for more than four decades. The hydrolyzed samples were characterized via multi-detection gel permeation chromatography (GPC), thermogravimetric analysis (TGA), modulated differential scanning colorimetry (mDSC), UV-vis spectroscopy, nuclear magnetic resonance (NMR), and Fourier-transform infrared (FTIR) spectroscopy techniques. Hydrolysis of ester linkages in the soft-segments decreases both the molecular weight of the polymer and the melting point of Estane from ∼55°C to 39°C. Aging above this melting temperature (T m ), increased mobility of polymer chains and water diffusivity alter the PEU degradation pathway from those expected at aging temperatures below T m and have significant bearing on the critical molecular weight (M C ) at which the physical, chemical, thermal, and mechanical properties of Estane change abruptly. While a M C value of 20 kDa is found for PEU hydrolysis at mild temperatures (e.g., below 39°C), the M C increases with increasing aging temperature. To complement the existing structure/property relationships reported in the literature, more correlations are obtained, which include the effect of M w on polydispersity, intrinsic viscosity (Mark-Houwink equation), UV extinction coefficient, and dn/dc (GPC analysis) values. Furthermore, we seek to augment previously reported aging models for PEU by developing a practical model with which the extent of degradation and material performance can be predicted based on aging under different temperature ranges both above and below the melting point of the hard-segments.

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