Chain order in filled SBR elastomers: a proton multiple-quantum NMR study

Abstract

A series of cross-linked styrene-butadiene rubbers (SBR) filled with different amounts of carbon black and silica are investigated by proton multiple-quantum nuclear magnetic resonance (NMR). The method yields reliable information on residual dipolar couplings and their distribution, which in turn are related to local chain order and the effective cross-link density in these systems. Fundamental differences between the response of a linear precursor, which undergoes reptational motion, and vulcanized SBR are discussed. For the latter, it is found that the average chain order parameter as well as its distribution does not change significantly with the amount and the type of filler. This is in surprising contrast to recent results from Hahn-echo relaxometry applied to the same samples, which indicated a significant filler effect on the cross-link density.

Hydrogel formation by photocrosslinking of dimethylmaleimide functionalized polyacrylamide

“…[83]). The reliable fitting of this second contribution, which also decays approximately exponentially, was in our case challenged by the fact that the corresponding relaxation time t B is very close to the relaxation time of the I SMQ (t DQ ) intensity of the pure network component (A) under the given conditions, even though this component is not network-like and does not contribute to the DQ intensity build-up.…”

“…The reliable fitting of this second contribution, which also decays approximately exponentially, was in our case challenged by the fact that the corresponding relaxation time t B is very close to the relaxation time of the I SMQ (t DQ ) intensity of the pure network component (A) under the given conditions, even though this component is not network-like and does not contribute to the DQ intensity build-up. This is so because the ultimately isotropic segmental motions of this component occur on a timescale that is similar to that of the network chains, as opposed to the faster and isotropically mobile sol part C. In this case, an alternative approach had to be taken that makes use of the fact that the final DQ intensity of the network part has to evolve to the same level as the corrected reference intensity, i.e., I DQ /I SMQ,corr h I nDQ ¼ 0.5 as t DQ / N [76,83,85]. This condition was met by multiplying the C-corrected sum intensity I SMQ (t DQ ) by a constant factor, 1 À f B , chosen such that the so-called normalized DQ build-up curve, I nDQ (t DQ ), approaches the theoretically expected value of 0.5 in its longtime limit.…”

“…Since true relaxation processes always act simultaneously with the dipolar dephasing, making analyses of relaxation curves modeldependent, it is advantageous to directly measure the residual dipolar couplings in an experiment where their presence leads to a quantifiable intensity build-up rather than a decay. In this respect, multiple-quantum (MQ) NMR has evolved as one of the most quantitative approaches, as not only absolute values of D res , but also its distribution can be measured [74e 76,83,84].…”

Uncertainties in the Determination of Cross-Link Density by Equilibrium Swelling Experiments in Natural Rubber

“…20 However, the most recent and quantitative approach for the measurement of weak residual dipolar coupling is the double- quantum (DQ) or, more generally, multiple-quantum (MQ) NMR technique. [21][22][23][87][88][89][90] The essential advantage of MQ spectroscopy is that, in the same experiment, two qualitatively different sets of data, a buildup curve (I DQ ) dominated by spin-pair double-quantum (DQ) coherences and a decay curve (I ref ), are measured. They can be used to independently analyze residual couplings and the chain dynamics.…”

“…9 However, the most common techniques to determine the cross-link density are mechanical measurements, [10][11][12][13][14][15] NMR spectroscopy, [16][17][18][19][20][21][22][23] and equilibrium swelling. [24][25][26][27][28] Although some of the previously mentioned techniques give some extra information about the structure of the network, the equilibrium swelling experiment is nowadays the most widely used approach in rubber science and technology.…”