Photoreversible Order-Disorder Transition in an Ionic Liquid Solvated Block Polymer

Area of Science:

• Polymer Science
• Materials Science
• Photochemistry

Background:

• Block copolymers exhibit complex phase behavior.
• Ionic liquids offer unique solvent properties.
• Photoresponsive polymers can change properties upon light exposure.

Purpose of the Study:

• To investigate light-controlled structural ordering in a specific block copolymer solution.
• To explore the phase behavior and reversibility of this system.
• To understand the mechanism of light-induced structural changes.

Main Methods:

• Small-angle X-ray scattering (SAXS) to confirm structural ordering.
• Small amplitude oscillatory shear rheology to assess mechanical properties and transitions.
• Controlled exposure to visible and UV light at elevated temperatures.

Main Results:

• The block copolymer solution self-assembled into hexagonally closed packed spheres (HCP) upon heating under visible light.
• The system reversibly transitioned between HCP and disordered states by switching between visible and UV light at 100 °C.
• This light-induced switching was stable over six cycles, indicating robustness.

Conclusions:

• Light can effectively control the supramolecular structure of this "coil-coil" block polymer in an ionic liquid.
• The azobenzene moiety's photo-induced polarity changes are key to modulating solubility and thus structure.
• This represents the first demonstration of reversible light-triggered ordering in a "coil-coil" block polymer system, distinct from liquid crystalline behaviors.