Contributions to the Total Orientation of Deformed Elastomers Arising from the Network Structure and Chain Interactions As Measured by NMR

Abstract

Deuterium NMR has been employed to determine the average orientation of chain segments in poly(butadiene) networks. It is shown that the free induction decay separates the contribution to the orientation arising from the network constraint to that from chain interactions. The NMR spectrum line shape reveals the orientational distribution of network vectors due to the cross-links, whereas the observed splitting gives information about the orientation due to segmental interactions. Both the line shape and splitting have been fitted simultaneously for a range of deformed poly(butadiene) networks. From the fitting parameters, the separate contributions to the average orientation of the chain segments arising from the network constraint and from the interactions are calculated. These in turn are used to determine the molecular weight between cross-links and the size of the segmental interactions, which we choose to express in terms of the Edwards' screening length. This work is also important to computer modeling of the stress−optical coefficient where the interchain interactions are ignored.

NMR Reveals Non‐Distributed and Uniform Character of Network Chain Dynamics

“…Traditionally, NMR methods based on transverse relaxation of proton or deuterium nuclei are the established approaches to assess the local chain order, [9,15,[17][18][19][20][21][22][23][24][25][26][27] and, as mentioned, theoretical analyses of such data were based on the assumption of a broad distribution of order parameters, related to the Gaussian distribution of chain end-to-end separations. [5][6][7][8][9][10][11][12] Applications of multiplequantum (MQ) NMR [28] recently gained momentum for the investigation of polymer chain dynamics and order, [29][30][31][32] and we found that it offers multiple advantages. [33][34][35][36] First, the quasi-static (temperature-independent) chain order phenomenon and the timescale of chain motions can be investigated reliably and separately.…”

“…[1] Specifically, the assumption that the well-documented (close-to) Gaussian end-to-end distribution of network chains and sub-chains is reflected in their conformational space and thus in the overall entropy, represents a cornerstone assumption when thermodynamic or elastic properties of networks far above T g are to be calculated on the basis of classical models of rubber elasticity. [2][3][4] The quantitative interpretation of NMR experiments on elastomers was so far also mainly based on the same assumption of a (frozen-in) Gaussian distribution of the end-to-end vectors, [5][6][7][8][9][10][11][12] and the qualitative agreement between theoretical and experimental spectra or relaxation curves appeared to support the traditional picture of rubber elasticity. In contrast, our recent results based on multiplequantum NMR are much more sensitive to the actual distribution of chain conformations, and indicate that an influence of a frozen-in Gaussian distribution on NMR observables is largely absent.…”

“…It should however be noted that a direct calculation of the entropy from a given S b is a non-trivial and model-dependent undertaking, that has only been attempted for some cases of localized dynamics in proteins. [44,45] To date, the models used to analyze NMR data are based on single chains and assume either (somewhat unphysically) no distribution of the order parameter or the residual coupling [20][21][22]24,25] or use a static average over a Gaussian end-to-end distribution, [5][6][7][8][9][10][11][12]25] PðrÞ…”

Deformation behavior of elastomeric networks studied by deuterium nuclear magnetic resonance (²H NMR)

“…Secondly there are the interactions  of the chain segments in the rubber with their many neighbors. A theoretical framework is developed [2] in order to show that the free induction decay separates the contribution to the orientation arising from the network constraint to that from chain interactions:…”

“…Nuclear magnetic resonance (nmr) allows one to relate macroscopic properties to microscopic behavior of polymeric chains via the presence of residual interaction, due to local order related to the constraints resulting from crosslink junctions. 2 H-nmr has been devoted to be a sensitive and powerful tool to study this anisotropy at a molecular level in strained elastomers. For unreformed rubbers a single resonance line is observed.…”