Cooperative Preassociation Stages of PEO−PPO−PEO Triblock Copolymers: NMR and Theoretical Study

Abstract

Using (1)H and (13)C 1D and 2D NMR spectra, pulsed field-gradient (PFG) diffusion measurements, and (13)C relaxations supported by density functional theory (DFT) calculations, the temperature-dependent behavior of (EO)(m)(PO)(n)(EO)(m) block copolymers (m/n = 31/14, 31/72, and 17/1) in D(2)O below and at the critical micellar temperature (CMT) was investigated in order to understand the nature of primary self-association acts and their true driving force. It was shown that a conformation change of the PO block followed by mild and reversible association with other PO blocks and eventually with the inner parts of EO blocks starts at temperatures 10-12 K below the CMT. The primary process is the entropy-driven disintegration of the PPO hydration envelope based on cooperation of hydrophobic hydration and hydrogen bonding. The partial dehydration of PPO is followed by its conformation change. Both processes are cooperative and reversible with a correlation time of the order 0.01 s and an activation energy of 51.3 kJ/mol. The PPO chain in a staggered conformation is prone to self-association starting at temperatures 5-6 K below CMT. In (EO)(m)(PO)(n)(EO)(m) block copolymers, this process is complicated by the stripping of PEO chains of a part of hydrogen-bound water and entwining them with PPO. It is shown that only inner (PPO-near) parts of PEO take part in the process, the end-groups remaining free.