Coil Diblock Copolymers Research Articles

Semiconducting Diblock Copolymers Synthesized by Means of Controlled Radical Polymerization Techniques

A donor−acceptor, rod−coil diblock copolymer has been synthesized with the objective of enhancing the photovoltaic efficiency of the PPV−C60 (PPV = poly(p-phenylenevinylene)) system by the incorporation of both components in a molecular architecture that is self-structuring through microphase separation. Diblock copolymers were obtained by using an end-functionalized rigid-rod block of poly(2,5-dioctyloxy-1,4-phenylenevinylene) as a macroinitiator for the nitroxide-mediated controlled radical polymerization of a flexible poly(styrene-stat-chloromethylstyrene) block. The latter block was subsequently functionalized with C60 through atom-transfer radical addition. In a spin-cast film of the final diblock copolymer, the luminescence from PPV is strongly quenched, indicating efficient electron transfer to C60. Under suitable conditions, solution-cast films of these diblock copolymers exhibit micrometer-scale, honeycomb-like patterns of holes.

Nitroxide‐mediated ‘living’ free radical synthesis of novel rod–coil diblock copolymers with polystyrene and mesogen‐jacketed liquid‐crystal polymer segments

The synthesis of rod-coil diblock copolymers with narrow polydispersity was achieved for the first time by TEMPO-mediated 'living' free radical polymerization of styrene and 2,5-bis((4- methoxyphenyl)oxycarbonyl)styrene. The block architecture of the two diblock copolymers thus prepared, MPCS-block-St (5400/2400) and MPCS-block-St (10 800/8700), was confirmed by GPC, 1 H and 13 C NMR and DSC studies. The liquid-crystalline behaviour of the copolymers was studied by DSC and polarized optical microscope. It was observed that both copolymers showed two distinct glass transitions, corresponding to polystyrene and poly(-2,5-bis((4-methoxyphenyl)oxycarbonyl)styrene). Above the glass transition temperature of rigid block, liquid-crystalline phase was formed. The clearing point of the phase is higher than the polymer decomposition temperature. # 2000 Society of Chemical Industry

Solubilization and Encapsulation of Fullerenes by Amphiphilic Block Copolymers

Solubilization and encapsulation of fullerenes C60 and C70 by ultralarge hollow micelles formed by rod−coil diblock copolymers and characterization of the resulting self-organized fullerene-block copolymer assemblies are reported. The solubilization capacity of each fullerene in poly(phenylquinoline)-block-polystyrene (PPQ-PS) micelles in binary solvents, trifluoroacetic acid/dichloromethane, and trifluoroacetic acid/toluene, was determined to be 200 mg of fullerene/g of PPQ-PS and found to be independent of block copolymer composition and concentration. This represents fullerene solubility enhancement by factors of 1040 and 63 compared to the solubilities in pure dichloromethane and toluene, respectively. Fullerenes C60 and C70 were found to change the micellization and self-assembly of PPQ-PS block copolymers in solution from polymorphic aggregates to only hollow spheres with encapsulated fullerenes. The solubilization and encapsulation of up to 1−10 billion fullerene molecules in PPQ-PS block copolymer...

Microphase-Stabilized Ferroelectric Liquid Crystals (MSFLC): Bistable Switching of Ferroelectric Liquid Crystal−Coil Diblock Copolymers

A side-chain liquid crystal (LC) block copolymer with a ferroelectric smectic C* LC block has been shown to exhibit bistable switching behavior in electric fields. Ferroelectric switching was demonstrated for block copolymers with volume fractions of 0.58 and 0.52 liquid crystal block. SAXS and TEM showed that the bulk morphologies of these two materials were hexagonally packed minority amorphous polystyrene (PS) cylinders and alternating lamellar amorphous PS layers, respectively. Cross-sectional TEM of a 10 μm thick film of the 0.58 volume fraction liquid crystal material used for the optical switching studies showed the cylinders to be well aligned along the shear direction and orthogonal to the applied electric field direction. It is believed that switching is possible due to an unwinding of the pitch of the smectic C* phase due to the presence of the intermaterial dividing surface of the microphase-separated cylindrical domains of the block copolymer which spatially confine the mesophase. This interaction realizes the concept of a microphase-stabilized ferroelectric liquid crystal (MSFLC) device.

Molecular Design, Synthesis, and Characterization of Liquid Crystal−Coil Diblock Copolymers with Azobenzene Side Groups

The synthesis and characterization of a family of well-defined liquid crystal−coil (LC−coil) diblock copolymers have been carried out. The block copolymers in this study have been designed to have nearly identical molecular weight azobenzene-containing LC blocks in order to eliminate possible variations in LC behavior caused by the differences in the LC block molecular weight. Quantitative hydroboration chemistry was used to convert the pendent double bonds of an isoprene block to hydroxyl groups to which the mesogenic groups were attached via acid chloride coupling. The LC homopolymer and the block copolymers (LC volume fraction from fLC = 0.82 to fLC = 0.20) all exhibited smectic mesophases with similar clearing transition temperatures. The clearing transition enthalpies strongly depend on the block composition ratio and decrease as the LC block volume fraction decreases. Solvent-casting of a lamellar LC−coil copolymer (SICN5-66/60) resulted in an oriented bulk film in which both the axes of the mesogenic groups and the lamellar interfaces lie parallel to the film surfaces. A LC cylinder morphology was observed with a fLC = 0.22 LC-containing block (SICN5-176/55) using TEM and confirmed by SAXS measurements. This is the first observation with the LC block in a cylinder microdomain. Other morphologies (bicontinuous, LC sphere) were observed by TEM while retaining the smectic order in the LC microphase.

Ordered Phases in Rod−Coil Diblock Copolymers

We study spatially ordered phases in a rod−coil diblock copolymer melt. In the weak segregation limit, we solve the self-consistent field equations by a partial numerical evaluation of the single chain partition function. In the strong segregation limit, we resort to a brushlike approximation. The structural asymmetry of the blocks has a pronounced influence on the phase diagram. We find that the only stable morphologies are those in which the coils are on the convex side of the rod−coil interface. The results are compared to experiment.