Stabilization of bulk aggregation state in semiconducting polymer solutions

Colloidal aggregate and gel incubated by amorphous conjugated polymer in hybrid-solvent medium

A practical valuable amorphous conjugated polymer, poly(2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene (MEH-PPV), has been revealed to foster an abundance of micrometer-sized colloidal aggregates at relatively low concentration (below 1 wt %) in a hybrid-solvent medium that contains a nonsolvent, and the solution turned into gel by colloidal bridging after one-day aging at 30 °C. In contrast with typical polymer gels fostered by (anisotropic) chain cross-linking or planar packing on selective interacting sites, the MEH-PPV gel has been revealed (via dynamic light scattering, small-angle light scattering, time-sweep dynamic modulus and optical microscope) to first develop featureless aggregate clusters in solution and, as the solvent quality worsens with reduced system temperature, bridge themselves to form gel through a one-dimensional (1-D) to three-dimensional (3-D) kinetic pathway. Combined dynamic/static light scattering analyses, along with supporting scanning electron microscope image and molecular dynamics simulation, indicated a concomitant structural reorganization within the colloidal aggregates, where spontaneous chain packing was perceived to form local fiber-like materials that are elastic by nature (i.e., a q-independent decay rate). The near coincidence of the above-mentioned microscopic and macroscopic phase alterations led us to contend that similar fibrous materials have served as the exterior bridging agent to fabricate colloidal strands upon gelation. The present findings clarify previously enigmatic, much speculative, gelation phenomena of MEH-PPV, and shed light on the prospect of capitalizing on specific polymer-solvent interactions to incubate desirable colloidal aggregates and gels in room-temperature processing of practical valuable conjugated polymers.