Microphase separation transition and rheology of side-chain liquid-crystalline block copolymers

Microphase separation was induced from homogeneous polystyrene-block-polyisoprene (SI diblock) copolymer by grafting of a liquid-crystalline (LC) monomer onto the polyisoprene (PI) block. The origin of the formation of microdomain structure in the side-chain LC block copolymer from a homogeneous SI diblock copolymer is explained in terms of a dramatic increase in repulsive segmental interactions between the constituent blocks. For the investigation, three homogeneous SI diblock copolymers with varying block length ratios (SI-5/6, SI-10/6, and SI-14/3) were synthesized via anionic polymerization in tetrahydrofuran as solvent. This resulted in PI blocks having the following microstructures: 34% 1,2-addition, 59% 3,4-addition, and 7% 1,4-addition. Subsequently, the PI blocks in each of the SI diblock copolymers were first hydroxylated via hydroboration/oxidation reactions to yield hydroxylated SI diblock copolymers (SI-5/6-OH, SI-10/6-OH, and SI-14/3-OH), and then a liquid-crystalline (LC) monomer, 6-[(4-cyano-4'-biphenyl)oxy]hexanoic acid (5CN-COOH), synthesized in our laboratory, was grafted onto the hydroxylated PI blocks (PI-OH), yielding side-chain LC diblock copolymers (SI-5/6-5CN, SI-10/6-5CN, and SI-14/3-5CN). The disappearance of the -COOH and -OH groups after coupling reaction was confirmed using H-1 nuclear magnetic resonance spectroscopy and infrared spectroscopy. Transmission electron microscopy revealed that at room temperature SI-5/6-5CN has spherical microdomains, SI-10/6-5CN has hexagonally packed cylindrical microdomains, and SI-14/3-5CN has lamellar microdomains. The LC phase of the side-chain block copolymers was identified using polarizing optical microscopy. The clearing temperature (T-cl), determined by differential scanning calorimetry, of the side-chain LC block copolymers is found to be ca. 84 degreesC, whereas the T-cl of LC monomer 5CN-COOH is 170 degreesC. Oscillatory shear rheometry indicates that the sphere-forming block copolymer SI-5/6-5CN undergoes a lattice disordering/ordering transition at 140 degreesC and a demicellization/micellization transition at 155 degreesC, while both the cylinder-forming block copolymer SI-10/6-5CN and lamella-forming block copolymer SI-14/3-5CN undergo microphase separation transition at temperatures much higher than 240 degreesC, the highest experimental temperature employed. Binodal curves exhibiting upper critical solution temperature were constructed, via cloud point measurement, for four pairs of polystyrene (PS) and PI grafted with 5CN-COOH (PI-5CN). An expression for the temperature dependence of the Flory-Huggins interaction parameter chi for the PS/(PI-5CN) pair was determined and compared with the chi expression for the PS/PI pair. The origin of the formation of microdomain structure in the side-chain LC block copolymers SI-5CN from a homogeneous SI diblock copolymer is explained in terms of molecular weight, block composition, and chi for the PS/(PI-5CN) pair.