Constitutive Model of Radiation Aging Effects in Filled Silicone Elastomers under Strain

Abstract

Filled silicone elastomers, an essential component in many technological applications, are often subjected to controlled or unintended radiation for a variety of reasons. Radiation exposure can lead to permanent mechanical and structural changes in the material, which is manifested as altered mechanical response, and in some cases, a permanent set. For unfilled elastomers, network theories developed and refined over decades can explain these effects in terms of chain-scission and cross-link formation and a hypothesis involving independent networks formed at different strain levels of the material. Here, we expose a filled silicone rubber to gamma radiation while being under finite elongational strain and show that the observed mechanical and structural changes can be quantitatively modeled within the same theoretical framework developed for unfilled elastomers as long as nuances associated with the Mullins effect are accounted for in a consistent manner. In this work, we employ Ogden's incompressible hyperelastic model within the framework of Tobolsky's two-network scheme to describe the observed permanent set and mechanical modulus changes as a function of radiation dosage. In the process, we conclude that gamma radiation induces both direct cross-linking at chain crossings (H-links) and main-chain-scission followed by cross-linking (Y-links). We provide an estimate of the ratio of chain-scission to crosslinking rates, which is in reasonable agreement with previous experimental estimate from Charlesby−Pinner analysis. We use density functional theory (DFT)-based quantum mechanical calculations to explore the stability of −Si and −SiO radicals that form upon a radiation-induced chain-scission event, which sheds light on the relative rates of Y-linking and H-linking processes.

Polymer degradation through chemical change: a quantum-based test of inferred reactions in irradiated polydimethylsiloxane

“…The latter makes use of the so-called ''Charlesby-Pinner'' analysis, 76 which has proven to return similar results as obtained through fitting constitutive models for siloxane elastomer stress-strain responses. 77 These approaches that rely on mechanical data necessarily only account for scissioning and crosslinking that alters mechanical response. Multi-quantum NMR can measure detailed changes in molecular weight distributions, but scissioning and crosslinking are again inferred indirectly through fitting to probabilistic models.…”

Multiscale Strategy for Predicting Radiation Chemistry in Polymers

“…Irradiation of polymer materials is a common means for component sterilization, can arise during planned operations such as radiation shielding and UV curing of inks, and is a driver to accelerate aging. − Chemical reactions resulting from radiation exposure can alter polymer networks through the formation of cross-links, the scissioning of chains, and the alteration of side groups. − These atomic-scale changes to the polymer network can induce undesirable macroscale changes such as permanent set, embrittlement, breaking under load, and irreversible changes in mechanical moduli. ,− , Radical and ion chemistry is thought to be the primary cause for this damage, − but the details can be hard to elucidate from integrated experiments and often invoke mechanistic assumptions. Obtaining a clear understanding of the initial stages of radiolytic chemistry is key for developing predictive dose- or age-aware models of polymer materials and can potentially guide the development of chemically robust polymers with desired mechanical and rheological properties.…”

“…All-atom modeling using techniques such as molecular dynamics (MD) provide detailed mechanistic insights into radiolytic polymer chemistry driven by these ballistic electrons and subsequent downstream processes, including predicting radiation-induced reaction cascades, − identifying synergies between environmental factors, ,, and testing proposed chemical reaction schemes. ,, However, MD modeling relies on judicious assessments of reasonable initial conditions and, in the case of radiolytic chemistry, usually invokes one of two approximations. One approach is to preseed simulations with radicals, ,, and the other is to explicitly model a primary knock-on atom (PKA) event in which incident ionizing radiation is assumed to induce an atomic recoil. − , The latter is designed to simulate the atomic recoil resulting from a Rutherford scattering event. However, while these ad hoc approaches provide insight into the ensuing chemistry, they do not address the fundamental radiation–matter interactions that instigate these reactions, leaving considerable uncertainty in the validity of the initial conditions used for the MD.…”

“…These probabilities can be recast as a rate for Møller scattering excitation events R exc per unit volume of material for a particular radiation dose (see SI Section 2). Using typical values for PE and dose rates from previous radiolysis experiments yields a value of R exc = 168 excitations μm –3 s –1 .…”

“…Short NAMD simulations were performed with gas-phase octane molecules that were prethermalized using DFTB. Previous efforts have indicated that short-chain oligomers yield reasonable representations of the initial radiolytic chemistry in polymers due to the local nature of radiation–matter interactions on short time scales. ,,− Our NAMD simulations on octane molecules were performed using the PBE0 functional with the 6-31g* basis set. − Condensed-phase PE systems exhibit a distribution of local backbone conformations, which we bracketed by considering two limiting cases in which all the dihedral angles in octane were either all-anti or all-gauche. One hundred independent NAMD simulations were then performed for each conformation case starting from thermal configurations prepared at 300 K. The ensuing dynamics resulting from a vertical excitation to the first excited state (i.e., a HOMO → LUMO transition) were propagated for at least 80 fs, yielding a variety of excited configurations.…”