(582a) Reaction Kinetics of Moisture-Reactive Materials for Experimental Validation of a Model for Water Vapor Reaction, Sorption, and Diffusion in Polymers

Moisture uptake or outgassing by polymeric materials may be
associated with a series of chain reactions that can cause mechanical degradation,
chemical incompatibility, and failure of materials used in applications such as
electronic devices, gas mixture membrane separation, drug delivery, and food
packaging.^1,2 The ability to model the moisture uptake, outgassing,
and reactions within materials facilitates the selection of suitable materials as
well as predictions of aging and long-term performance. We have developed a material-specific
triple-mode sorption-diffusion model of water vapor in polymeric materials^1,2,
and we are working to couple that model with experimentally-derived reaction
kinetics to arrive at a Reaction-Sorption-Transport (ReSorT)
model of water vapor in polymeric materials.

In this study, we investigate the kinetics of a
moisture-reactive material with the goal of using the rate law in our ReSorT modeling approach. Multiple experimental techniques
were used to determine the kinetic rate law of the moisture-reactive silicone
sealant RTV-734 (Dow Corning). As RTV-734 cures, the acetoxy groups react with water
to form a crosslinked silicone network and release acetic acid as a by-product.
Additionally, as the network forms and the material changes from sol to gel,
the reaction shifts from being limited by kinetics to limited by diffusion of
moisture through the cured material. Initially, low field NMR relaxometry was evaluated
as an analytical method to track the network formation in real-time to
determine the apparent kinetics of the silicone network formation. Figure 1
shows the NMR relaxometry time T2 as a function of cure time for RTV-734 cured
at four different temperatures. The relaxometry time is indicative of the
degree of cure of the polymer; the greater the T2, the more mobile the chains
are and thus the less cured. Unexpectedly, cure temperature increased chain mobility
significantly and caused an apparent increase in cure time, which contradicts
an Arrhenius-type reaction. Thus, it is necessary to evaluate various analytical
techniques such as low field NMR relaxometry, FTIR, and rheology, as well as
less complex water vapor reactions that may be used to determine the kinetic
rate law and validate the reaction component of the ReSorT
model.

Figure 1: NMR relaxometry time T2 as a function of cure time
for RTV-734 cured at four different temperatures.

This work was performed under the auspices of the U.S.
Department of Energy by Lawrence Livermore National Laboratory under Contract
DE-AC52-07NA27344. LLNL-ABS-749233

1.      Sharma, H. N.,
Harley, S. J., Sun, Y. & Glascoe, E. A. Dynamic
Triple-Mode Sorption and Outgassing in Materials. Scientific Reports 7,
(2017).

2.      Sun, Y., Harley,
S. J. & Glascoe, E. A. Modeling and Uncertainty
Quantification of Vapor Sorption and Diffusion in Heterogeneous Polymers. ChemPhysChem 16, 3072–3083 (2015).