Molecular weight distributions of irradiated siloxane-based elastomers: A complementary study by statistical modeling and multiple quantum nuclear magnetic resonance

Abstract

ABSTRACT:The statistical methodology of population balance (PB) has been applied 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 by 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 formation of three types of cross-links: random recombination of scissioned-chain ends (endlinking), random covalent bonds of free radicals on scissioned-chain ends (Y-cross-linking), and formation of random cross-links from free radicals on side groups (H-cross-linking). The qualitative agreement between the statistical modeling approach and 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.--

Linking Network Microstructure to Macroscopic Properties of Siloxane Elastomers Using Combined Nuclear Magnetic Resonance and Mesoscale Computational Modeling

“…as opposed to a molecular weight dependent C n , the assumption that the end-to-end distance squared is Gaussian, and that motions about this end-to-end vector are ideal. 22,47,48 Despite these assumptions the NMR data is observed to behave roughly as expected:…”

“…Further, many of the macroscopic properties of engineering materials can be directly related to the mobility of chains comprising the elastomer network, and MQ-NMR data has been therefore used to obtain quantitative insight into network changes due to swelling, 17,19,20 thermal treatment, 16 filler content, 1 and radiation exposure. 21,22 It remains poorly understood, however, the degree to which the NMR distributions reflect actual molecular weight (MW) distributions in these soft solids and how much the distributions obtained by MQ-NMR can be relied on to predict material properties such as elastic shear modulus.…”

Polymer Degradation through Chemical Change: A Quantum-based Test of Inferred Reactions in Irradiated Polydimethylsiloxane

“…Common vectors for degradation arise from the ready uptake of environmental water [4][5][6][7] or absorption of radiation. [7][8][9][10][11][12] Understanding the degradation responses of PDMS has enormous practical applications, ranging from assessing environmental ima Physical and Life Sciences Directorate, Lawrence Livermore National Laboratory, Livermore, CA 94550, United States; E-mail: kroonblawd1@llnl.gov pacts [13][14][15] to establishing models for the aging and lifetime prediction of silicone components. [4][5][6][7][8][9][10][11][12]16,17 Ionizing radiation is a common cause for polymer degradation that can arise during component sterilization, 1,18,19 as a consequence of planned operation conditions, and as a driver in accelerated aging studies.…”

“…[7][8][9][10][11][12] Understanding the degradation responses of PDMS has enormous practical applications, ranging from assessing environmental ima Physical and Life Sciences Directorate, Lawrence Livermore National Laboratory, Livermore, CA 94550, United States; E-mail: kroonblawd1@llnl.gov pacts [13][14][15] to establishing models for the aging and lifetime prediction of silicone components. [4][5][6][7][8][9][10][11][12]16,17 Ionizing radiation is a common cause for polymer degradation that can arise during component sterilization, 1,18,19 as a consequence of planned operation conditions, and as a driver in accelerated aging studies. [7][8][9][10][11][12] In 1955, Charlesby proposed 20 a set of reaction schemes for crosslinking in irradiated PDMS that were quickly refined by others 21,22 and were based on IR vibrational spectroscopy, titration, mass spectra of off-gassing species, and chromatography (see Figure 1).…”

“…[4][5][6][7][8][9][10][11][12]16,17 Ionizing radiation is a common cause for polymer degradation that can arise during component sterilization, 1,18,19 as a consequence of planned operation conditions, and as a driver in accelerated aging studies. [7][8][9][10][11][12] In 1955, Charlesby proposed 20 a set of reaction schemes for crosslinking in irradiated PDMS that were quickly refined by others 21,22 and were based on IR vibrational spectroscopy, titration, mass spectra of off-gassing species, and chromatography (see Figure 1). These reactions respectively connect the evolution of hydrogen (H 2 ), methane (CH 4 ), and ethane (C 2 H 6 ) with the formation of silethylene (Si-C-C-Si), silmethylene (Si-C-Si), and silicon-silicon (Si-Si) crosslinks.…”