Synthesis and Self-Assembly of Poly(diethylhexyloxy-p-phenylenevinylene)-b-poly(methyl methacrylate) Rod-Coil Block Copolymers

A series of poly(diethylhexyloxy-p-phenylenevinylene-e-methyl methacrylate) (DEH-PPV-b-PMMA) polymers with narrow polydispersity (PDI < 1.1) were synthesized using Siegrist polycondensation and anionic polymerizations followed by "click" chemistry. Alkyne-terminated-DEH-PPV and azidoterminated PMMA were synthesized first, and then the two functionalized polymers underwent 1,3-cycloaddition reaction to obtain copolymers. Both the conversion of the end-functionalization of the homopolymers and the yield of the "click" reaction were higher than 98% as determined by H-1 nuclear magnetic resonance (H-1 NMR) and gel permeation chromatography (GPC). Transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS) studies reveal the details of copolymer morphology. The DEH-PPV-b-PMMA system presented here has higher block segregation strength than many previously studied rod-coil block copolymers yet still shows experimentally accessible phase transitions with respect to temperature. As a result, this molecule offers new insight into the competition between rod-rod and rod-coil interactions that occurs in the system. The DEH-PPV rods are organized as a monolayer that is inclined with the lamellar normal (smectic C) for the copolymers containing low volume fraction of PMMA coil (< 54%). However, as the coil fraction increases, the strips containing DEH-PPV pack into hexagonal lattice. In contrast to previous work which demonstrated similar morphologies, the sequence of reversible liquid crystalline and microphase phase transitions is altered as a result of the increased block segregation. Upon heating, the low coil fraction copolymers exhibit a series of clear transitions of smectic-lamellar to amorphous-lamellar to disordered structures. In high coil fraction copolymers, the transitions between smectic-hexagonal to amorphous-hexagonal and smectic-hexagonal to disorder structures could not be clearly differentiated. The order-to-disorder temperature (ODT) decreases slowly with increasing coil fraction while the smectic-to-isotropic transition (SI) temperature stays relatively unchanged. The steady SI temperature suggests that the strong rod-rod interaction keeps the liquid crystalline rod in the nanodomain structure regardless of the amount of coil segment in the copolymers.