Self-Assembly of Poly(diethylhexyloxy-p-phenylenevinylene)-b- poly(4-vinylpyridine) Rod−Coil Block Copolymer Systems

Abstract

We describe the synthesis, the morphology, and self-assembly behavior of semiconducting poly-(4-vinylpyridine-b-diethylhexyloxy-p-phenylenevinylene) (P4VP-b-PPV) rod-coil block copolymer systems. Three different block copolymers with 55%, 80%, and 88% coil volume fraction were synthesized by convergent anionic polymerization in THF using lithium R-methylstyrene as initiator. The morphology of the block copolymers was studied by transmission electron microscopy, small-angle X-ray scattering, and small-angle neutron scattering as a function of the volume fraction of the rod block as well as different annealing conditions. The microphaseseparated morphologies in these block copolymers vary from lamellar, to hexagonal, and spherical, when the volume fraction of the rod is progressively reduced. By combining the lattice parameter measured by scattering techniques with the volume fraction of rod domains obtained by nuclear magnetic resonance, it was shown that the block copolymers in the lamellar structure are organized in a smectic C double layer, while in the hexagonal phase they self-organize in a homeotropic arrangement, with the rod blocks forming the dispersed phase. Furthermore, while self-assembly of rod-coil block copolymers in columnar hexagonal phase prevents close rod packing, for the lamellar phase evidence of this configuration among rods is shown by wide-angle X-ray scattering. As a consequence, the morphology and long-range order in the lamellar phase are the result of simultaneous inter-rods liquid crystalline interactions and the tendency to microphase segregation of rod and coil. As a result, depending on temperature, the lamellar phase can exist both with rods oriented in a smectic configuration and with randomly packed rods. We show that annealing the lamellar phase below its order-disorder transition temperature, T ODT , but above the maximum affordable temperature for inter-rods liquid crystalline interactions, called smectic-in-lamellar to lamellar order-disorder transition temperature, T SL (with T SL < T ODT ), leads to highly improved long-range lamellar order, which is then preserved when the system is cooled below T SL , at which temperature rod close packing is fully recovered.

Self-assembly of rod-coil block copolymers from weakly to moderately segregated regimes

“…Since ω is virtually unchanged in the two homologue block copolymer systems considered, this is consistent with an increased Flory-Huggins parameter for PPV-P4VP compared to PPV-PS, as can be expected by the increased chemical dissimilarity between PPV and P4VP. A schematics of the phase diagram theoretically expected by Landau expansion theories for systems with balanced microphase and liquid crystalline behaviors, is redrawn from references [1,28] in Figure 7. Remarkably, all the major thermodynamically stable phases predicted theoretically are observed experimentally, and their location in the phase diagram is in excellent agreement with theoretical predictions.…”

“…The synthesis of the first series, poly-(diethylhexyloxy-pphenylenevinylene)-b-(styrene) block copolymers (PPVb-PS), was achieved by atom transfer radical polymerization, starting from a PPV macroinitiator. The other block copolymer series, PPV-b-(4-vinylpyridine) (PPV-b-P4VP) was synthesized by anionic polymerization of 4-vinylpyridine and quenching with properly end functionalized PPV [28]. In what follows, the major differences in the self-assembly mechanisms and phase diagrams of these two systems are discussed, and the physics of self-assembly understood based on available Landau-expansion theories [1].…”

A New Supramolecular Route for Using Rod‐Coil Block Copolymers in Photovoltaic Applications

“…Rod-coil block copolymers are indeed well known to self-assemble through microphase separation into highly ordered nanostructures that are thermodynamically stable and exhibit spatial periodicities on the 1-10 nm length scale. [14][15][16][17][18][19] Block copolymers composed of an electron-donating and an electronaccepting block are therefore particularly interesting for PV applications and are presently studied worldwide by several research groups. [20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37] Particularly, rod-coil block copolymers using poly[(2,5-di(2-ethyl)hexyloxy)-1,4-phenylenevinylene] (DEH-PPV) as electron donor and various coil blocks (such as polystyrene or polybutylacrylate) with covalently linked fullerene moieties as electron acceptor have been investigated intensively.…”

“…P3HT-P4VP has been obtained by anionic polymerization of 4-vinylpyridine and quenching with an aldehyde endfunctionalized P3HT. This route has been adapted from the synthesis of a PPV-based block copolymer previously described [17,18] and will be discussed in detail in a separate manuscript. The synthesized diblock copolymer (Fig.…”

Self‐Assembly of Rod‐Coil Block Copolymers for Photovoltaic Applications

“…The presence of C 60 however restricts the annealing temperature and thus prevents achieving those long range order lamellar structures typical of PPV-P4VP samples annealed beyond the liquid crystalline order-disorder transition temperatures of PPV rods. [12] Due to the weak character of fullerene-P4VP bond, annealing at higher temperature causes C 60 to form macroscopic phase separated domains. Annealing above polymer T g on samples bearing larger volume fractions of C 60 , e.g.…”