Simulation Studies Of The Self-Assembly Of Diblock Copolymers And Their Blending With Homopolymers Under Confinement

Block copolymers system has been studies as an important issue experimentallyand theoretically, due to their ability to self-assemble into rich nanostructures andtheir potential applications. For the simplest diblock copolymers melts, their phasebehavior is controlled by the volume fraction of one blocks and the strength of theinteraction between the different monomers. A variety of ordered bulk phases,including lamellae, hexagonally packed cylinders, a bicontinuous gyroid structure,and a body centeredcubic array of spheres, have been observed experimentally andtheoretically. In practice, confinements induced entropy decreases, the structurefrustration and the interactions between the block copolymers and surface willstrongly affect the phase behavior of block copolymers. All these parameters cangenerate novel microstructures that may have potential novel applications and can notbe found in bulk phases. A larger number of experimental and theoretical studies haveshown that, with the increase of confinement dimensions from one-dimensionalparallel plate, to two-dimensional cylindrical nanopores, then to three-dimensionalspherical nanopores, the phase behaviors of the diblock copolymers become more andmore complex. Both the influence of confinement on the microphase separation andthe self-assembly morphologies, and the understanding of self-assembly mechanismof these novel structures are important parts of polymer science. Besides putting thecopolymers in the confinement environment, it is known the addition ofhomopolymers to block copolymer melts, blends of block copolymers andhomopolymers also exhibit complex phase behavior, including blends exhibit anumber of new ordered structure, or blends form macroscopic phase separation. Inaddition, much attention has been foucused on the self-assembly of block copolymersplaced into a nano-trough or polymers grafted on a substrate with different lateralspacings.In this thesis, we systematically investigated the self-assembly of diblockcopolymers/homopolymers blends confined in3D spherical nanopores, diblockcopolymers confined in cylindrical nano-trough, and homopolymers grafted on the substrate using simulated annealing technique. The evolutions of self-assembledstructures are predicted with the variation of various parameters, including the chainlength of the homopolymer, the degree of confinement and the interaction of differentmonomers. The self-assembly mechanisms of complex morphologies in confinedenvironment are revealed. The effects of different parameters on self-assembledmorphologies of square and linear array homopolymer brushes are investigated.In chapter two, we investigated the self-assembly phase behavior of symmetricAB diblock copolymer and homopolymer A or B blends, and AB diblock copolymerand homopolymer C confined in spherical nanopores.When the pore surface isattractive to the A-monomers, it is found that the self-assembly of AB/A blendsdepends on the content of the homopolymer, the chain length of the homopolymer,and the degree of confinement. With the variation of the content of homopolymer, theblends exhibit a variety of morphologies and morphological transitions. The observedmorphologies include onion-like structures (with both concentric lamellae andnonconcentric lamellae), cage-like structures, deformed dome-like structures, helicesor stacked toroids, a disk-like layer and spherical structures. The shorter thehomopolymers chain length is, the more likely that homopolymers are compatiblewith the diblock copolymers, penetrating into the A-rich domain region of themicro-structure formed by the latter. The longer the homopolymer chain length is, theharder for the homopolymers to be compatible with the diblock copolymer, which ledto the occurrence of macroscopic phase separation. The simulation results areconsistent with relative experimental results of similar system. When the pore surfaceis neutral to be both blocks, our simulation results show that the chain length andcontent of the homopolymer have similar influence on the diblock copolymers withthe results when the pore surface attracts A-monomers. When the homopolymers Cattractive A-monomers but repulsive B-monomers, it is found that the self-assemblyof the symmetric diblock compolymers AB/homopolymers C blends depends on thecontent of the homopolymer, the chain length of the homopolymer, the degree ofconfinement and the attractive interaction strength between A-C monomers when thepore surface attractive A and C-monomers and repulsive B-monomers. With thevariation of the content of homopolymer, the blends exhibit a variety of morphologies and morphological transitions. The observed morphologies include onion-likestructures (with both concentric lamellae and nonconcentric lamellae), cage-likestructures, helices or stacked toroids and spherical structures. When the A-C attractiveinteraction strength is relatively weak, the shorter the chain length of homopolymer,the better the compatibility of the blend, and the corresponding structures occur at aslightly smaller value for the homopolymer content. When the attractive interactionstrength between the A-C monomers is strong enough, the effects caused byhomopolymer chain length become less pronounced and eventually disappear. In thiscase, the blend has a good compatibility. Moreover, when the homopolymers Crepulsive A and B-momomers, the self-assembly phase behavior of binary blends ofa diblock copolymer (AB) and an incompatible homopolymer (C) is examined forfour typical cases, representing the different selectivity of the pore surface to the A, Band C species. The internal morphology of the spherical polymeric particles iscontrolled by the homopolymer volume fraction, the degree of confinement, and thecomposition of the copolymer. Inside each particle, the homopolymers segregate toform one or, under some conditions, two domains, thus the homopolymers may act asan additional controlling parameter of the shape and symmetry of thecopolymer-domain. A rich array of confinement-induced novel diblock copolymermorphologies is predicted. In particular, core-shell particles with the copolymers asthe shell wrapping around a homopolymer-core or a copolymer-homopolymercombined core, and Janus-like particles with the copolymers and the homopolymerson different sides are obtained.In chapter three, by using coarse-grained lattice gas model and simulatedannealing technique, we systematically investigated the self-assembly phase behaviorof asymmetric diblock copolymer A9B3and nearly symmetric diblock copolymerA7B5confined in cylinderial nano-trough. The model parameters including initialpolymer density, polymer content ratio, evaporation rate, the movement rate ofpolymer, width of mesas and annealing temperature have effect on the self-assemblybehavior of diblock copolymer. The width of mesas, the movement rate of polymerand polymer content ratio have a little effect on the self-assembly morphologies ofdiblock copolymer A9B3. Moreover, we found novel porous slice structures formed by A7B5confined in cylinder nano-trough are affects by different model parametersincluding the height of the trough and degree of confinement. All our simulationresults in this chapter are consistent with related literature experimental results.In chapter four, we systematically investigated the square or linear arrayhomopolymer brush by using a simulated annealing technique. We also studied theself-assembly morphologes of homopolymer brush are effected by any parameter. Theeffection of these parameters including the chain length of homopolymer, graftingdensity, grafting width, the interaction between homopolymer and substrate, theinteraction between homopoymer and air on the self-assembly morphologes ofhomopolymer brush are investigated. Some of our simulation results such as: thetrends of the ratio (D/d) of lateral width (D) to grafting width (d) and the height ofsquare array homopolymer brush change with the d; the trends of morphlogies heightand amplitude of the linear array homopolymer brush change with the line-to-linespacing, are qualitativly compared with relative literature experimental results.