I suggest to read this article as it will be useful to you (Proton conducting composite membranes from polyether ether ketone and heteropolyacids for fuel cell applications)

DOI: 10.1016/S0376-7388(00)00345-8

Dear Sridhar Raj

I have no direct experience with the subject (Sulphonate PEG6000), but I do have plenty experience in various polymer degree of substitution determinations by NMR. In order to accurately determine the degree of substitution of any polymer, you need to identify unique signals in the NMR spectrum for the substituted and unsubstituted polymer, or the polymer backbone, whichever is available and most accurate. It is very important that your NMR spectra are recorded under quantitative conditions. Depending on the integrated signals, the required formula will change.

Can you be more specific on your NMR method? Which signals are you integrating and what do these signals represent?

o determine the degree of sulphonation (DS) in sulphonated PEG 6000 using NMR spectroscopy, begin by preparing a sample of approximately 10-20 mg dissolved in deuterated solvent (D₂O or DMSO-d₆, depending on solubility) with an appropriate internal reference standard such as TMS or DSS. The primary analysis relies on ¹H NMR, where the characteristic methylene protons (-CH₂-O-) of the PEG backbone typically appear between 3.5-3.8 ppm, while the sulphonated methylene protons (-CH₂-O-SO₃⁻) experience a downfield shift to approximately 4.0-4.5 ppm due to the electron-withdrawing effect of the sulphonate group. By comparing the integrated peak areas of these two regions, the degree of sulphonation can be calculated using the formula DS (%) = [Iₛₒₗₛ/(Iₛₒₗₛ + Iᵤₙₛₒₗₛ)] × 100, where Iₛₒₗₛ represents the integral of the sulphonated protons and Iᵤₙₛₒₗₛ corresponds to the unmodified PEG protons.

For more complex cases where peak overlap occurs, ¹³C NMR can provide additional confirmation, with sulphonated carbons appearing 5-10 ppm downfield of their unmodified counterparts (typically 70-75 ppm versus 60-65 ppm for PEG carbons). Two-dimensional NMR techniques like HSQC may be employed to resolve ambiguities by correlating proton and carbon signals. When performing quantitative analysis, ensure optimal NMR conditions including sufficient relaxation delays (at least 5 times the T₁ relaxation time) and proper pulse calibration to obtain accurate integrals. The calculated DS can be cross-validated through elemental analysis of sulphur content or by titration methods to quantify the sulphonic acid groups. For PEG 6000 with a molecular weight of approximately 6000 g/mol, corresponding to about 136 repeating units, a 5% degree of sulphonation would indicate roughly 1-2 sulphonate groups per polymer chain. This approach provides a reliable means to characterize the extent of PEG functionalization while avoiding the need for complex derivatization or destructive testing methods.