Abstract:

A polymer-surfactant complex is significant in understanding the interactions between amphiphilic molecules and has great potential for use in a vast number of industries. In addition, the stimuli-responsive polymer-surfactant complex represents a hot research topic for the colloid community. However, the use of CO₂ gas to tune their interaction and the corresponding morphological change in the polymer-surfactant complex has been less documented. In this work, the commercially available triblock copolymer Pluronic F127 was used as a starting material and the macromolecular initiator Br-F127-Br was synthesized via esterification. Then, the pentablock copolymer poly(2-(diethylamino)ethyl methacrylate))-block-F127-block-poly(2-(diethylamino)ethyl methacrylate)) (PDEAEAM-b-F127-b-PDEAEMA) was prepared via atom transfer radical polymerization (ATRP) of Br-F127-Br and the monomer 2-(diethylamino)ethyl methacrylate. Both Br-F127-Br and PDEAEAM-b-F127-b-PDEAEMA were characterized by FT-IR and ¹H NMR spectroscopies as well as gel permeation chromatography (GPC). The results indicated that both Br-F127-Br and PDEAEAM-b-F127-b-PDEAEMA were synthesized successfully. The CO₂-responsive behavior of the pentablock copolymer was examined by tracking the changes in pH and electrical conductivity of the polymer solution after alternatingly bubbling CO₂ and N₂. It was found that cyclic streaming of CO₂/N₂ could alter the pH of the polymer solution between 7.2 and 5.3, leading to the protonation degree of PDEAEAM-b-F127-b-PDEAEMA varying between 0.26 and 0.96; this in turn varied the electrical conductivity of the polymer solution between 19.4 μS∙cm⁻¹ and 70.6 μS∙cm⁻¹. The reversible changes in pH and electrical conductivity of the polymer solution indicate the good CO₂-stimuli responsiveness of PDEAEAM-b-F127-b-PDEAEMA. The interaction of PDEAEAM-b-F127-b-PDEAEMA with an anionic fluorocarbon surfactant potassium nonafluoro-1-butanesulfonate (C₄F₉SO₃K) with and without CO₂ was studied by ultraviolet-visible absorption spectrometry (UV-Vis), dynamic light scattering (DLS), and transmission electron microscopy (TEM). The transmittance of the mixed solution of PDEAEAM-b-F127-b-PDEAEMA and C₄F₉SO₃K could be varied between 84% and 52% in the absence and presence of CO₂, indicating the formation of aggregates with different sizes. The DLS results showed that the size of aggregates could be modified reversibly between tens of nanometers and several micrometers by bubbling CO₂ and replacing CO₂ by N₂. The TEM image revealed the reversible morphological transition of the aggregates from spherical to wormlike micelles after bubbling CO₂. The carbonic acid formed from CO₂ and water can protonate the PDEAEMA in the pentablock copolymer to form PDEAEMA·H⁺, and thus the interaction between the pentablock copolymer and C₄F₉SO₃K becomes strong. When CO₂ is replaced by N₂, PDEAEMA·H⁺ reverts to PDEAEMA, and the interaction becomes weak once again. It can therefore be concluded that the protonation/deprotonation process of the pentablock copolymer can be controlled by bubbling CO₂/N₂. The protonation/deprotonation process can "switch" the electrostatic attraction of PDEAEAM-b-F127-b-PDEAEMA to C₄F₉SO₃K, thereby tuning the hydrophilic-lipophilic balance (HLB) of the polymer-surfactant complex reversibly, leading to the reversible morphological transition of the aggregates. The strategy of CO₂-controllable morphological alteration of a polymer–surfactant complex opens a new avenue for preparing gas-sensitive soft materials.

Key words: CO₂-induced, Polymer-surfactant complex, Self-assembly, Wormlike micelles, Pluronic