Hongdong Zhang

Constructing hierarchically structured interphases for strong and tough epoxy nanocomposites by amine-rich graphene surfaces

The exquisite structure of natural materials manifests the importance of particle mobility and load transfer in developing advanced polymer nanocomposites; however, it is difficult to concurrently meet these two mutually exclusive requirements. To address this issue, we demonstrate an approach that constructs a hierarchical, flexible interphase structure in epoxy nanocomposites through a local amine-rich environment around graphene sheets (GNs) and volume exclusion effect of grafting chains. Long-chain aromatic amines, which are chemically similar to the curing agent, are covalently bonded on the surface of GNs by diazonium addition. They play multifold roles in the structure formation of epoxy composites, (1) promoting the exfoliation and molecular level dispersion of GNs in the matrix, (2) serving as a linker between GNs and epoxy networks for improved load transfer, (3) modulating the stoichiometric ratio around GNs to construct a hierarchical structure that can dissipate more strain energy during fracture. With the addition of 0.6 wt% amine-functionalized GNs, the resulting composite exhibits significant mechanical improvements, 93.8 and 91.5% increases in fracture toughness and flexural strength, respectively. This approach affords a novel design strategy for developing high-performance structural composites.

MICROPHASE SEPARATION OF DIBLOCK COPOLYMER INDUCED BY DIRECTIONAL QUENCHING

Computer simulation is carried out for studying the microphase separation of a two-dimensional diblock copolymer (DBCP) system under directional quenching. By setting the quenching boundary between the stable and the unstable phase, and shifting the boundary with a constant velocity, the time evolution of the domain morphologies is examined numerically on the basis of the time-dependent Ginzburg–Landau type equation with the free-energy functional for the DBCP. Three different types of morphologies are found for the symmetric (i.e., f=0.5) DBCP system. One is the irregular lamellar morphology and is essentially equivalent to that produced by homogeneous quenching. The other two are regular and are characteristics of directional quenching process. One of the regular lamellar morphologies is perpendicular to the quench boundary on the average, whereas the other one is parallel to the quench boundary. For the asymmetric DBCP system with f=0.4, which forms the equilibrium morphology of triangular phase, the m...

Oscillatory shear induced anisotropic domain growth and related rheological properties of binary mixtures

Numerical simulation based on the modified time-dependent Ginzburg–Landau (TDGL) model has been performed on the domain growth and related rheological properties of binary mixtures under oscillatory shear. The simulation results reveal that the domain growth is anisotropic and depends on the quench depth. It is found that, in the deep quench case, the disclike domain with the normal parallel to the velocity gradient direction is observed, while in the shallow quench case, the rodlike domain with rod axis aligned along the flow direction is observed. The scattering functions for different light incident directions are calculated and suggest that the undulated rodlike morphology is formed in the shallow quench case. This undulated rodlike morphology shows the anomalous rheological response. A plausible interpretation for the anomalous rheological property is proposed based on the deformation of the undulated rodlike morphology under oscillatory shear.

Hydrodynamic effects on phase separation of binary mixtures with reversible chemical reaction

The hydrodynamic effect on the phase separation dynamics of chemically reacting binary mixtures is investigated based on the extended model H. Our simulation results reveal that many interesting patterns are obtained under different chemical reaction rates due to the coupling of hydrodynamics and chemical reaction. For the case of high reaction rate, when the average order parameter at equilibrium is equal to zero, spiral structures appear due to the delicate coupling between the hydrodynamic flow and chemical reaction regardless of the value of initial order parameter. When the chemical reaction rate is low, the pattern observed under the critical quench seems like the result of double phase separation. On the other hand, under the off-critical quench, at the same low chemical reaction rate, phase inversion behavior is observed, and it is aggravated under the hydrodynamic flow. Moreover, the domain growth kinetics under both critical and off-critical quenches is investigated.

Polymerization-induced bimodal phase separation in a rubber-modified epoxy system

The bimodal phase separation process of a rubber-modified epoxy system, consisting of diglycidyl ether of bisphenol A (DGEBA), and a hydroxyl-terminated butadiene–acrylonitrile random copolymer (HTBN), during curing with tetrahydro-phthalic anhydride was studied by time-resolved small-angle light scattering (TRSALS), differential scanning calorimetry (DSC), and digital image analysis (DIA). The HTBN/DGEBA mixture reveals an upper critical solution temperature (UCST). At higher curing temperatures, double-peak structure from the matrix was investigated by TRSALS and confirmed by DIA. The special two characteristic size distribution behavior was explained qualitatively by nucleation growth coupled with spinodal decomposition (NGCSD) and the competition between phase separation and polymerization.

Chain stretching effect on domain growth during spinodal decomposition of binary polymer mixtures under simple shear flow

The chain stretching effect on domain growth during spinodal decomposition of binary polymer mixtures under simple shear flow is investigated by computer simulation. The simulation is based on a modified time-dependent Ginzburg–Landau equation, in which the chain stretching effect is introduced in the free energy functional. It is found that, for higher value of Rouse terminal relaxation time, the critical strain value for the burst of the domains is higher, thereby the domains are highly elongated. This may be responsible for the stringlike patterns observed experimentally under strong shear. When the chain stretching effect is introduced, the shear rate dependencies of the shear stress and first normal stress difference become stronger. The shear stress and first normal stress difference reach their maxima for the system of 1:1 mixture. The simulated results agree with the experimental observations qualitatively.

The Self-Consistent Field Study of the Adsorption of Flexible Polyelectrolytes onto Two Charged Nano-objects

The continuum self-consistent field theory (SCFT) is applied to the study of the adsorption of flexible polyelectrolyte (PE) onto the surfaces of two two-dimensional charged square objects with a constant electric field strength immersed in a weakly charged polyelectrolyte solution. The dependences of the different chain conformations, that is, bridging, loop, tail, and train, and in particular, the bridging chain conformation, on various system parameters (the charge fraction of the PE chains, the surface charge density, the object size, the salt concentration, etc.) are investigated. The efficient multigrid method is adopted to numerically solve the modified diffusion equation and the Poisson equation. It is found that the thickness L(B) of the boundary layer of the adsorbed PE chains is independent of the chain length and scales with the surface charge density σ and the fraction of charges on PE chains α(P) as L(B) ~ σ(-0.36) and L(B) ~ α(P)(-0.36), respectively. Simulation results reveal that the total amount of bridging chain conformation in the system scales linearly with respect to the size of the charge objects and scales linearly with the chain length in the long polymer chain regime. Simulation results reveal that the total amount of the bridging chain conformation in the system scales with the charge fraction of PE chains as a power law and the scaling exponent is dependent on all of the other system parameters. Simulation results show that the total amount of charges on the adsorbed chains in the system can overcompensate the surface charges for relatively long chains with high charge fractions.

Easy synthesis of dendrimer-like polymers through a divergent iterative “end-grafting” method

We report here an easy method for the synthesis of dendrimer-like polymers with high branching functionality (1 → 8). The synthetic process involves iterative grafting reactions in a divergent way. A multi-functional core containing short segments of polyisoprene (PI), either as a star-like block copolymer of isoprene and styrene or as a linear triblock copolymer of isoprene, styrene and isoprene (coded G1), is epoxidized on the double bonds and grafted with a living block copolymer, polyisoprene-b-polystyrenyllithium (PI-b-PSLi), again with a short PI segment, through the ring-opening reaction of oxirane by polymeric anions. The resulting graft polymer, G2, possesses a definite number of PI segments at the periphery. These PI segments are further epoxidized followed by the ring-opening addition of PI-b-PSLi, affording G3. Repeating the process leads to the synthesis of a dendrimer-like polystyrene up to 5th generation with a polydispersity lower than 1.21, as measured by SEC. A feature of the process is the easily accessible high chain density in the final product, although defects exist due to steric hindrance in the reactions of high generations. The solution properties of the dendritic products are investigated using viscometry and dynamic and static laser light scattering on the molecular conformation. The results support a compact globular conformation model for the dendrimer-like products. In addition, the chain density of the products from the star-like core is higher than that of products from a linear triblock core. AFM results show that the dendritic products adopt flattened conformations and tend to form lateral sphere-like aggregates on mica substrate.

Self-assembly of ABC star triblock copolymer thin films confined with a preferential surface: a self-consistent mean field theory.

The microphase separation and morphology of a nearly symmetric A(0.3)B(0.3)C(0.4) star triblock copolymer thin film confined between two parallel, homogeneous hard walls have been investigated by self-consistent mean field theory (SCMFT) with a pseudospectral method. Our simulation experiments reveal that under surface confinement, in addition to the typically parallel, perpendicular, and tilted cylinders, other phases such as lamellae, perforated lamellae, and complex hybrid phases have been found to be stable, which is attributed to block-substrate interactions, especially for those hybrid phases in which A and B blocks disperse as spheres and alternately arrange as cubic CsCl structures, with a network preferred structure of C block. The results show that these hybrid phases are also stable within a broad hybrid region (H region) under a suitable film thickness and a broad field strength of substrates because their free energies are too similar to being distinguished. Phase diagrams have been evaluated by purposefully and systematically varying the film thickness and field strength for three different cases of Flory-Huggins interaction parameters between species in the star polymer. We also compare the phase diagrams for weak and strong preferential substrates, each with a couple of opposite quality, and discuss the influence of confinement, substrate preference, and the nature of the star polymer on the stability of relatively thinner and thick film phases in this work.

Dissipative particle dynamics simulations on inversion dynamics of spherical micelles

We simulate the inversion process of a spherical micelle composed of symmetric diblock copolymers by means of dissipative particle dynamics. The evolution of micelle morphology reveals that the inversion is a two-staged process, in which a rapid agglomeration of outer lyophobic blocks occurs first, followed by a slow penetration of inner lyophilic blocks through the porous lyophobic layer. Calculation of the radius of gyration and hydrodynamic radius indicates that an intermediate with a dilute core and a dense shell emerges in the inversion. The characteristic time of inversion scales with the block copolymer chain length with the scaling exponent ranging from 1.67 to 1.89, which can be well described by a simplified chemical-potential-driven flow model. Further simulations incorporating different denaturation times for the two types of blocks indicate the inversions do not experience molecularly scattered states, but form either collapsed intermediates or loosely associated clusters of small sizes. Possible connections of the simulations to the light scattering experiments are discussed.