Self-assembly Of Diblock Copolymer/Nanoparticle Using Dissipative Particle Dynamics Method

Block copolymers under some external environments, such as confinement, nanoparticles, force field, solvent, and so on, due to the induction from the various external conditions, the systems exhibit different phase behaviors as to those in the bulk. It has been show that by controlling the related parameters, we can fabricate novel, and long-range order materials in nano-scale. The studies on the self-assembly of block copolymers suffer the inductions from confinement, nanoparticles, shear flow, and selective solvent, can promote the understanding of intrinsic characters of block copolymers segregation. Especially for nanoparticles, the effects of isotropic nanospheres and high aspect-ratio nanorods on the self-assembled behaviors have significant difference. Base on the additional orientational entropy of nanorods resulting from the particles'anisotropy, a consideration of enthalpic and entropic interactions can further exploite the inherent mechanism for driving these rich phase behaviors. In this dissertation, we use dissipative particle dynamics(DPD) method studied the self-assembly of the following systems:the mixtures of lamellar/cylindrical forming diblock copolymers(DBCPs) and nanoparticles(spheres, or rods), the mixtures of DBCPs and nanorods under shear flow, and the polymer tethered nanorods under selective solvent.1. The self-assembled phase behaviors of lamellar DBCPs and nanospheres mixtures. To ensure the rigidity characteristic of nanospheres, we introduce new interactive energies. We systematically study the effects of nanospheres volume fraction, radius, and the polymer-nanosphere interaction on the DBCPs microphase separation. As more, we get a phase diagram of copolymer nanocomposites in terms of these three parameters, which reflects the system phase behaviors comprehensively. The position distribution of nanospheres plays a decisive role in the phase transition from lamellar to bicontinuous morphology because of the strong excluded volume effects among nanospheres.2. The self-assembled phase behaviors of lamellar/cylindrical DBCPs and nanorods mixtures. The weak repulsions between nanorods drives the rods to aggregate. A series of parameters, such as nanorod number, length, radius, and the polymer-nanorod interaction, are introduced to analyse the cooperative phase behavior and novel morphologies of hybrids. The final phase structures of the mixtures result from the mutual inducement between mesophase-forming copolymers and NRs. When physically or chemically distinct nanoparticles are introduced into the polymer fluids, it is useful to understand the intrinsic characteristics of the composite self-assembly by considering the enthalpic and entropic interactions, especially for the nanoparticles'phase behaviors. On the one hand, the NRs distributions reveal a degree of enthalpically driven self-assembly, due to the attractions or repulsions among species. On the other hand, the NRs' aggregates and orientations show a degree of entropically generated self-assembly, based on the competition between the inherent shape anisotropy of NRs and confinement of host phase separated domains.3. The self-assembled phase behaviors of lamellar/cylindrical DBCPs and binary nanorods mixtures. The binary NRs are identical in energy but different in lengths. The repulsions between nanorods avoid the aggregates among nanorods. We consider two cases of A-block preferential and neutral nanorods, respectively. Replacing the monodisperse NRs with an equal volume fraction of bidisperse NRs, and varying the ratio of short/long nanorod has prompted not only a series of phase transformations in the polymer microstructure but also, the creation of a uniform orientation, and a discriminative distribution of NRs. The inherent mechanism for driving such rich phase behaviors arises from the competition between enthalpic and entropic effects.4. The self-assembly of lamellar DBCPs/nanorods composites under shear flow. Both selective and nonselective nanorods are considered. To preserve lamellar morphology in the nanocomposites, the nanorods concentration is controlled to be not too high. Subjected to steady shear flow, there are not only the shear-induced the reorientations of lamellae, but also the shear-induced phase transitions. Moreover, enhancing shear rate can speed up the transition process of micophase structures. For the pure DBCPs case, the shear-induced lamellae adopt parallel alignment at low shear rates, while perpendicular at high shear rates. For the pure nanorods under shear, the nanorods trend to disperse. The final morphologies of nanocomposites depend on the interplay between DBCPs and nanorods under shear flow.5. The solvent-induced self-assembly of polymer-tethered nanorods (PTN). We focus on three types of PTN molecules(one end tethered, both ends tethered, middle tethered) under different solvent conditions:the pure rod-selective solventâ… , the pure tether-selective solventâ…¡, and theâ… /â…¡mixed solvent. The observed micellar structures include:cylinders, hexagonally cylinders, bilayer lamellae, lamellae/cylinder mixed phases, inverted hollow cylinders, nematic bundles, and ordered LC phases. These morphologies depend on the topology, rod/tether length ratio, solvent selectivity, and mixed solvent content. In pure solvent case, the morphologies and morphological transitions of PTN assemblies are affected by the rod/tether length ratio, which also can be induced by varying mixed solvent content in sequence. These self-assembled structures are formed by the competition between the stretching of tethers, liquid crystalline of rods, and interfacial energy.