Study of polymer cross-link density by time domain NMR spectroscopy

Elastomeric polymers are important materials for many applications due to their exceptional long-range elastic property obtained by cross-linking. The cross-linking is the process of creating three dimensional network structure in the polymer materials and induces elasticity. Hence, the precise characterization of the molecular weight (Mc) between cross-links or other topological constraints (i.e. entanglements, filler etc.) is essential for tuning the elastic response in polymer networks. There are several techniques available for polymer network cross-link density measurement, among them equilibrium swelling and nuclear magnetic resonance (NMR) spectroscopy are widely used. Particularly multiple–quantum (MQ) NMR spectroscopy technique is emerging as an excellent tool for network characterization in the molecular level for last three decades. The unique advantage of this technique over the equilibrium swelling is that it gives the cross-link density together with cross-link distribution, besides being a rapid and solvent free technique. MQ NMR can be applied on inexpensive low resolution instrument without compromising on the data quality, where chemical shift resolution is not necessary. Moreover, other NMR pulse sequences are available for characterizing other polymer properties such as bound rubber fraction in filled networks, polymer dynamics, segmental motions etc. In the present work, MQ proton NMR has been used to obtain the dipolar coupling constant and the cross-link density in different classes of elastomers (commercial formulation for tire industry, polyacrylate networks and shape memory ionic elastomers). The robustness of the cross-link density measurements obtained from equilibrium swelling method (both phenomenological and statistical models) is compared with MQ NMR results in formulation for tire application. Thiol probes have been applied to quantify ranking of sulfidic bridges as a function of curing time in the network. The Kraus, Lorenz and Parks correction for filler restriction on swelling has been validated from the MQ NMR results. Differently from the published papers, focused on the effect of different vulcanization conditions at the optimum curing time, here the kinetics of vulcanization is studied. In this way a detailed and comprehensive picture of the polymer network as function of curing time is provided. Particularly Baum-Pines NMR pulse sequence allows the measurement of weak dipolar coupling constants in sulfur cured natural rubber, carbon black (N234) filled polyisoprene, polybutadiene and polyisoprene/polybutadiene blend networks. The network degradation has been studied by measuring the cross-link density as a function of curing time. Furthermore the five pulse MQ sequences have been applied for measuring relatively strong dipolar coupling constants in thermally cured cross-linked polybutylacrylate networks. The cross-link density and bound rubber fraction around the ionic domains in a shape memory polymer network - (carboxylated nitrile butyl rubber (XNBR) - have been obtained by the combination of Baum-Pines and 5 pulse MQ NMR sequences. The combined approach of Baum-Pines and 5 pulse MQ NMR gives cross-link density, allowing the estimation of bound rubber fraction as well. The measured bound rubber fraction has been further validated by magic sandwich echo NMR experiments, standard approach in this field. Finally the vulcanization curves and temperature dependent mechanical properties have been studied by rubber processing analyzer (RPA), using time-temperature superposition principle. Therefore this study allows proposing time domain-NMR as an inexpensive, fast and solvent-free (green) technique readily available for quality control and day to day R&D purposes.