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

Abstract

A series of poly(diethylhexyloxy-<i>p</i>-phenylenevinylene-<i>b</i>-methyl methacrylate) (DEH-PPV-<i>b</i>-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 azido-terminated 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 ¹H nuclear magnetic resonance (¹H 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-<i>b</i>-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.