Microenvironmental and conformational structure of triblock copolymers in aqueous solution by 1H and 13C NMR spectroscopy

1H and 13C nuclear magnetic resonance (NMR) spectra of poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (PEO–PPO–PEO) triblock copolymers in D2O solutions have been systematically investigated. The detailed assignments of various 1H and 13C NMR signals are presented. The hyperfine structure of PO CH2 protons was clearly assigned, the arising reason of this hyperfine structure was attributed to the influence of the chiral center of CHCH3 groups and the direct coupling between the PO CH2 and CH3 protons. The external standard 2,2-dimethyl-2-silapentane-5-sulfonate sodium salt (DSS) was firstly applied in this system. Accurate chemical shift values referenced to the external standard DSS were obtained. 1H NMR chemical shift of PO CH2 and CH3 signals shows a larger decrease in ppm values than that of EO CH2 signal with the increase of PPO/PEO ratio or temperature indicating that PO segments exist in a more hydrophobic microenvironment. A new resonance signal assigned to the PO CH2 protons appeared when the temperature is above the CMT, which is attributed to the breakdown of the intra-molecular (CH)⋯O hydrogen bond between the PO CH2 protons and the ester oxygens. The breakdown of this intra-molecular hydrogen bond may result in a decrease of gauche conformers of the PPO chain. The increase of 13C NMR chemical shift of block copolymers validates this conformational change assumption. It can be inferred that the amount of gauche conformers decreases whereas that of trans conformers increases in both PO and EO chains when elevating the PPO/PEO ratio or temperature. The observed 13C NMR chemical shifts of PO segments show a bigger increase than those of EO segments, supporting the formation of a nonpolar microenvironment around PO segments.

Graphical abstract(a) One-dimensional original 1H NMR spectrum of 1% (w/v) Pluronic P105 at 20 °C in D2O solution. (b) EO CH2 and PO CH2 region (3.4–3.82 ppm3.4–3.82 ppm). (c) PO CH3 region (1.09–1.22 ppm1.09–1.22 ppm).Figure optionsDownload full-size imageDownload as PowerPoint slide