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A silica-filled polydimethylsiloxane (PDMS) composite M9787 was investigated for potential outgassing in a vacuum/dry environment with the temperature programmed desorption/reaction method. The outgassing kinetics of 463 K vacuum heat-treated samples, vacuum heat-treated samples which were subsequently re-exposed to moisture, and untreated samples were extracted using the isoconversional and constrained iterative regression methods in a complementary fashion. Density functional theory (DFT) calculations of water interactions with a silica surface were also performed to provide insight into the structural motifs leading to the obtained kinetic parameters. Kinetic analysis/model revealed that no outgassing occurs from the vacuum heat-treated samples in subsequent vacuum/dry environment applications at room temperature (~300 K). The main effect of re-exposure of the vacuum heat-treated samples to a glove box condition (~30 ppm by volume of H2O) for even a couple of days was the formation, on the silica surface fillers, of ~60 ppm by weight of physisorbed and loosely bonded moisture, which subsequently outgasses at room temperature in a vacuum/dry environment in a time span of 10 yr. However, without any vacuum heat treatment and even after 1 h of vacuum pump down, about 300 ppm by weight of H2O would be released from the PDMS in the next few hours. Thereafter the outgassing rate slows down substantially. The presented methodology of using the isoconversional kinetic analysis results and some appropriate nature of the reaction as the constraints for more accurate iterative regression analysis/deconvolution of complex kinetic spectra, and of checking the so-obtained results with first principle calculations such as DFT can serve as a template for treating other complex physical/chemical processes as well. [ABSTRACT FROM AUTHOR]

Erratum: "The moisture outgassing kinetics of a silica reinforced polydimethylsiloxane" [J. Chem. Phys. (10)-(12) shows HT ht , where the correct expression should be HT ht . This typo (capital I T i should be used to represent temperature) has no impact on the results presented in the article. [Extracted from the article]

The statistical methodology of population balance (PB) has been applied in order to predict the effects of cross-linking and chain-scissioning induced by ionizing radiation on the distribution of molecular weight between cross-links (MWBC) of a siloxane-based elastomer. Effective molecular weight distributions were extracted from the quantification of residual dipolar couplings via multiple quantum nuclear magnetic resonance (MQ-NMR) measurements and are taken to reflect actual MWBC distributions. The PB methodology is then applied to the unirradiated MWBC distribution and considers both chain-scissioning and the possibility of the formation of three types of cross-links: random recombination of scissioned-chain ends (end-linking), random covalent bonds of free radicals on scissioned-chain ends (Y-cross-linking), and the formation of random cross-links from free radicals on side groups (H-cross-linking). The qualitative agreement between the statistical modeling approach and the NMR data confirms that it is possible to predict trends for the evolution of the distribution of MWBC of polymers under irradiation. The approach described herein can also discern heterogeneities in radiation effects in different structural motifs in the polymer network. [ABSTRACT FROM AUTHOR]

Experiments to measure the hydrogenuptake kinetics of DEB getter/Pdcatalyst/activated carbon pellets have been performed under isothermalisobaric conditions. The extracted kinetics were then used to predictthe performance of the getter pellets under different temperaturesand pressures, including nonisobaric situations. For isothermal isobaricuptake at higher H2pressure (666.6–2666.5 Pa),H2solubility in the getter matrix is responsible for theuptake observed up to a 40–60% reacted fraction. Once the hydrogenatedproduct becomes thicker, the diffusions of the reactants (atomic hydrogenand getter molecules) toward the reaction front become the rate limitingstep. However, in a dynamic but very low H2pressure, encounteredin many vacuum electronic applications, the hydrogen spillover effect,over micrometer scale, becomes the dominant reaction mechanism. Despitesuch a complex dependence of the rate limiting mechanisms on the experimentalenvironment, there is good agreement between kinetic prediction modelsand experiments. The investigation also reveals that the ultimateuptake capacity in the getter pellets scales inversely with the freevolume of the vacuum vessel in which the DEB getter pellets are used,and that DEB getter pellets’ performance greatly deterioratesduring the final 10–15% capacity (as evidenced by the sharpbend in the slopes of the reacted fraction vs time curves at 85–90%reacted fraction). [ABSTRACT FROM AUTHOR]

Modulus softening and permanent set in filled polymeric materials due to cyclic loading and unloading, commonly known as the Mullins effect, can have a significant impact on their use as support cushions. A quantitative analysis of such behavior is essential to ensure the effectiveness of such materials in long-term deployment. In this work we combine existing ideas of filler-induced modulus enhancement, strain amplification, and irreversible deformation within a simple non-Gaussian constitutive model to quantitatively interpret recent measurements on a relevant PDMS-based elastomeric cushion. We find that the experimental stress-strain data is consistent with the picture that during stretching (loading) two effects take place simultaneously: ( 1) the physical constraints (entanglements) initially present in the polymer network get disentangled, thus leading to a gradual decrease in the effective cross-link density, and (2) the effective filler volume fraction gradually decreases with increasing strain due to the irreversible pulling out of an initially occluded volume of the soft polymer domain. [ABSTRACT FROM AUTHOR]

In a recent paper we exposed a filled elastomer to controlled radiation dosages and explored changes in its cross-link density and molecular weight distribution between network junctions [ A. Maiti et al. Phys. Rev. E 83 031802 (2011)]. Here we report mechanical response measurements when the material is exposed to radiation while being under finite nonzero strain. We observe interesting hysteretic behavior and material softening representative of the Mullins effect, and materials hardening due to radiation. The net magnitude of the elastic modulus depends upon the radiation dosage, strain level, and strain-cycling history of the material. Using the framework of Tobolsky's two-stage independent network theory we develop a model that can quantitatively interpret the observed elastic modulus and its radiation and strain dependence. [ABSTRACT FROM AUTHOR]

Filled and cross-linked elastomeric rubbers are versatile network materials with a multitude of applications ranging from artificial organs and biomedical devices to cushions, coatings, adhesives, interconnects, and seismic- isolation, thermal, and electrical barriers. External factors such as mechanical stress, temperature fluctuations, or radiation are known to create chemical changes in such materials that can directly affect the molecular weight distribution (MWD) of the polymer between cross-links and alter the structural and mechanical properties. From a materials science point of view it is highly desirable to understand, affect, and manipulate such property changes in a controlled manner. Unfortunately, that has not yet been possible due to the lack of experimental characterization of such networks under controlled environments. In this work we expose a known rubber material to controlled dosages of γ radiation and utilize a newly developed multiquantum nuclear-magnetic-resonance technique to characterize the MWD as a function of radiation. We show that such data along with mechanical stress-strain measurements are amenable to accurate analysis by simple network models and yield important insights into radiation-induced molecular-level processes. [ABSTRACT FROM AUTHOR]