Study on the dependence of longitudinal relaxation time in 1H NMR method on the structure of polydimethylsiloxanes and optimization of spectra registration parameters

The results of investigations of the effects of size and shape of molecules of oligomers: hexamethyldisiloxane (HMDS) dimer, octamethylcyclotetrasiloxane (OMCTS) tetramer and polydimethylsiloxane, characterized by kinetic viscosity similar to 10 cSt (PDMS10), as well as of two linear polydimethylsiloxanes [one of viscosity about 300 cSt (PDMS300) the other of about 1000 cSt (PDMS1000)] on longitudinal relaxation time (T-1) in H-1 NMR method were presented. The samples were prepared by dissolution of the substances mentioned in the mixture of carbon tetrachloride with deuterated benzene (Table 2). It was found that T1 time did not significantly depend on the analyte concentration in the sample and was practically independent on the mutual ratio of the solvents used. It was observed the increase in the chain length of PDMS caused T, decrease while cyclization of the molecule did not influence its value (Table 2). The investigations showed that to obtain credible results of quantitative PDMS determination it was necessary to use the optimal values of spectra registration parameters, especially delay time (t(d)) and data acquisition time (t(a)) which should be 15 s and 5 s, respectively.