Ordering of anisotropic nanoparticles in diblock copolymer lamellae : simulations with dissipative particle dynamics and a molecular theory

Local distribution and orientation of anisotropic nanoparticles in microphase-separated symmetric diblock copolymers has been simulated using dissipative particle dynamics and analyzed with a molecular theory. It has been demonstrated that nanoparticles are characterized by a non-trivial orientational ordering in the lamellar phase due to their anisotropic interactions with isotropic monomer units. In the simulations, the maximum concentration and degree of ordering are attained for non-selective nanorods near the domain boundary. In this case, the nanorods have a certain tendency to align parallel to the interface in the boundary region and perpendicular to it inside the domains. Similar orientation ordering of nanoparticles located at the lamellar interface is predicted by the molecular theory which takes into account that the nanoparticles interact with monomer units via both isotropic and anisotropic potentials. Computer simulations enable one to study the effects of the nanorod concentration, length, stiffness, and selectivity of their interactions with the copolymer components on the phase stability and orientational order of nanoparticles. If the volume fraction of the nanorods is lower than 0.1, they have no effect on the copolymer transition from the disordered state into a lamellar microstructure. Increasing nanorod concentration or nanorod length results in clustering of the nanorods and eventually leads to a macrophase separation, whereas the copolymer preserves its lamellar morphology. Segregated nanorods of length close to the width of the diblock copolymer domains are stacked side by side into smectic layers that fill the domain space. Thus, spontaneous organization and orientation of nanorods leads to a spatial modulation of anisotropic composite properties which may be important for various applications

Liquid-crystal ordering and microphase separation in the lamellar phase of rod-coil-rod triblock copolymers. Molecular theory and computer simulations

A molecular model of the orientationally ordered lamellar phase exhibited by asymmetric rod-coil-rod triblock copolymers has been developed using the density-functional approach and generalizing the molecular-statistical theory of rod-coil diblock copolymers. An approximate expression for the free energy of the lamellar phase has been obtained in terms of the direct correlation functions of the system, the Flory-Huggins parameter and the Maier-Saupe orientational interaction potential between rods. A detailed derivation of several rod-rod and rod-coil density-density correlation functions required to evaluate the free energy is presented. The orientational and translational order parameters of rod and coil segments depending on the temperature and triblock asymmetry have been calculated numerically by direct minimization of the free energy. Different structure and ordering of the lamellar phase at high and low values of the triblock asymmetry is revealed and analyzed in detail. Asymmetric rod-coil-rod triblock copolymers have been simulated using the method of dissipative particle dynamics in the broad range of the Flory-Huggins parameter and for several values of the triblock asymmetry. It has been found that the lamellar phase appears to be the most stable one at strong segregation. The density distribution of the coil segments and the segments of the two different rods have been determined for different values of the segregation strength. The simulations confirm the existence of a weakly ordered lamellar phase predicted by the density-functional theory, in which the short rods separate from the long ones and are characterized by weak positional ordering

Effect of Cross-Linking on the Structure and Growth of Polymer Films Prepared by Interfacial Polymerization

Interfacial polymerization of tri- and bifunctional monomers (A₃B₂ polymerization) is investigated by dissipative particle dynamics to reveal an effect of cross-linking on the reaction kinetics and structure of the growing polymer film. Regardless of the comonomer reactivity and miscibility, the kinetics in an initially bilayer melt passes from the reaction to diffusion control. Within the crossover period, branched macromolecules undergo gelation, which drastically changes the scenario of the polymerization process. Comparison with the previously studied linear interfacial polymerization (Berezkin, A. V.; Kudryavtsev, Y. V. Linear Interfacial Polymerization: Theory and Simulations with Dissipative Particle Dynamics <i>J. Chem. Phys.</i> <b>2014</b>, <i>141</i>, 194906) shows similar conversion rates but very different product characteristics. Cross-linked polymer films are markedly heterogeneous in density, their average polymerization degree grows with the comonomer miscibility, and end groups are mostly trapped deeply in the film core. Products of linear interfacial polymerization demonstrate opposite trends as they are spontaneously homogenized by a convective flow of macromolecules expelled from the reactive zone to the film periphery, which we call the reactive extrusion effect and which is hampered in branched polymerization. Influence of the comonomer architecture on the polymer film characteristics could be used in various practical applications of interfacial polymerization, such as fabrication of membranes, micro- and nanocapsules and 3D printing

Salt-Induced Changes in Triblock Polyampholyte Hydrogels: Computer Simulations and Rheological, Structural, and Dynamic Characterization

We investigate the influence of ionic strength on the structural properties of stimuli-responsive hydrogels from triblock polyampholytes PAA-b-P2VP-b-PAA (PAA and P2VP are negatively charged poly(acrylic acid) and positively charged poly(2-vinylpyridine)). In our previous studies, we found that the transition behavior depends on the charge asymmetry which is controlled by pH and which alters the degree of ionization of the two types of blocks [Dyakonova Macromolecules 2014, 47, 7561]. The same triblock polyampholyte, but with chemically quaternized P2VP (QP2VP) instead of P2VP as the middle block, is highly positively charged, independently of pH. In the present investigation, PAA-b-P2VP-b-PAA at pH 3 and PAA-b-PQ2VP-b-PAA at pH 5 were chosen to investigate the influence of the ionic strength on the micellar network morphology by adding NaCl at concentrations in the physiological range. Computer simulations of the latter system show that salt addition results in the formation of larger complexes due to increased hydrophobicity in the system upon screening of charges and that the distance between these complexes increases accordingly. Rheological studies reveal that the hydrogels from PAA-b-P2VP-b-PAA at pH 3 become softer when the ionic strength is above 0.10 M. Small-angle neutron scattering studies have indicated that, in salt-free solution, both systems form networks. Particularly, it was found that in PAA-b-PQ2VP-b-PAA, which has a high charge asymmetry, a variation of the ionic strength leads to significant changes in network architecture. In contrast, in PAA-b-P2VP-b-PAA at pD 3, which has a lower charge asymmetry and the morphology is less sensitive to salt, because the hydrophobic effect prevails. These findings demonstrate that the different response of the two systems to the variation of ionic strength is a consequence of the nature of the predominant interactions, namely charge screening and hydrophobic interactions

Restructuring in block copolymer thin films:In situ GISAXS investigations during solvent vapor annealing

Block copolymer (BCP) thin films have been proposed for a number of nanotechnology applications, such as nanolithography and as nanotemplates, nanoporous membranes and sensors. Solvent vapor annealing (SVA) has emerged as a powerful technique for manipulating and controlling the structure of BCP thin films, e.g., by healing defects, by altering the orientation of the microdomains and by changing the morphology. Due to high time resolution and compatibility with SVA environments, grazing-incidence small-angle X-ray scattering (GISAXS) is an indispensable technique for studying the SVA process, providing information of the BCP thin film structure both laterally and along the film normal. Especially, state-of-the-art combined GISAXS/SVA setups at synchrotron sources have facilitated in situ and real-time studies of the SVA process with a time resolution of a few seconds, giving important insight into the pathways and mechanisms of SVA induced restructuring. We give a short introduction to the GISAXS method and review recent theoretical studies, experimental techniques such as sample preparation and in situ chambers together with SVA protocols, and we review and discuss experimental results. We conclude by giving an outlook on emerging developments of the in situ real-time GISAXS scattering technique in combination with new approaches to control BCP thin film structures using SVA