Guangcui Yuan

Surface depletion of the fluorine content of electrospun fibers of fluorinated polyurethane.

For materials containing fluorine, it has been generally accepted that fluorinated segments or end groups tend to aggregate in the outer surface because of the low surface energy, which endows the fluorinated materials with special surface properties such as self-cleaning, superhydrophobicity, and so forth. However, for the electrospun fibrous membranes of polyurethane elastomers containing perfluoropolyether segments (FPU), abnormal fluorine aggregations in the core of the electrospun fibers were observed. The XPS analysis indicated a rather low fluorine content at the surface of the electrospun FPU fibers. Further study with dynamic light scattering and fluorescence showed that FPU chains can form aggregates in the concentrated solution. Therefore, it can be deduced that the rapid evaporation of solvent and fast formation of fibers during the electrospinning process could result in the freeze-in of the aggregated chain conformation and the depletion of fluorine units on the surface of the electrospun FPU fibers.

Liquid-gel-liquid transition and shear thickening in mixed suspensions of silica colloid and hyperbranched polyethyleneimine.

The rheological property of mixed suspensions of silica colloid and hyperbranched polyethyleneimine (hPEI) was studied as functions of particle volume fraction, ratio of polymer to particle, and pH value. A mechanism of liquid-gel-liquid transition for this mixed system was proposed based on the amount and the conformation of polyelectrolyte bridges which were able to self-arrange with solution environments. The hPEI, which is adsorptive to the surface of silica colloid, can induce bridging or stabilizing effect between particles depending on whether the concentration of hPEI (Cp) is smaller or larger than the equilibrium adsorbed amount (Cp*) for a given volume fraction of particles. In dilute colloid suspensions, the Cp* can be determined by dynamic light scattering as the correlation function returns back to a narrow distributing single relaxation with increasing Cp. In concentrated colloid suspensions, the Cp* can be determined by rheological measurement as gel-liquid transition occurs with increasing Cp. The Cp* is an important concentration ratio of polymer to particle denoting the transition of irreversible and reversible bridging. For mixed suspensions at equilibrium adsorbed state (Cp ≈ Cp*), the adsorption-desorption of polymer bridges on the particles can reversibly take place, and shear thickening is observed under a steady shear flow as a result of rapid extension of bridges when the relaxation time scale of extension is shorter than that of desorption.

Observation of a characteristic length scale in the healing of glassy polymer interfaces

We examine the evolution (‘healing’) of the interface between two polymer films at various temperatures below the glass transition temperature, Tg. Specifically, neutron reflectometry is used to study the relaxation of the interface between both unentangled and entangled deuterated polystyrene (d-PS) and hydrogenated polystyrene (h-PS) films in the glass state where these bilayer films are supported on silicon substrates. We find that the initially sharp interface between the glassy polymer layers broadens with time (t), but the average interfacial thickness Δσ(t) between these layers then saturates after long time to a thickness (ξp) in a range between 1 nm and 3 nm after a long annealing time (≥1 h) for the range of temperatures investigated. This characteristic scale, and the temperature dependence of the interfacial relaxation time τ, were unanticipated, and we thus investigated the dependence of ξp and τ on molecular mass (M), annealing temperature (T) and the thickness (hf) of the PS films. We find that ξp increases with hf at a fixed T and increases with T at fixed hf. Our observation of a ‘healing length’ ξp, regardless of whether the polymers are entangled or not, and the dependence of ξp on T rule out an interpretation of this parameter in terms of the reptation model. On the other hand, ξp has a scale comparable to the mobile interfacial layer thickness reported in both small molecule and polymeric materials in the glass state, suggesting that ξp is a well-defined dynamical length scale characterizing the interfacial properties of glassy materials. The existence of such an interfacial layer has numerous implications for the processing and scientific understanding of thin polymer films.

Influences of hyperbranched polyethylenimine on the reactive compatibilization of polycarbonate/polyamide blends

The influences of hyperbranched polyethylenimine (hPEI), which possesses many reactive amino end-groups, on the blending properties of bisphenol-A polycarbonate (PC) and amorphous polyamide (aPA) were systematically investigated. Scanning electron microscopy (SEM) and differential scanning calorimetry (DSC) were used to observe the effect of hPEI on morphologies of PC and aPA phases in bulk blends. While the interfacial fracture toughness between planar PC and aPA layers with and without hPEI was studied by using augmented double cantilever beam (ADCB) method. Results show that the compatibility in PC/aPA blends can be significantly improved by adding a small amount of hPEI, mainly due to the interchange reactions between the polymers leading to the formation of block copolymers, cross-linked polymers and molecules with other constitutions. The augmented double cantilever beam experiments showed that the reactive process drastically reinforced the interfacial adhesion between planar layers of PC and aPA. However, degradation takes place during annealing at 180 °C, which was responsible for the production of small molar mass species of PC.

Shear-Thickening in Mixed Suspensions of Silica Colloid and Oppositely Charged Polyethyleneimine

The liquid-gel-liquid transition tuned by increasing concentration of linear and hyperbranched polyethyleneimine in suspension of silica colloids, and the accompanying shear-thickening phenomena, were investigated by rheological measurements. The influence from linear and hyperbranched polymer conformation and from different size-ratio between particle and polymer on the rheological properties of suspensions flocculated by absorbing polyelectrolyte were considered. Charge neutralization and bridging mechanism are the main reasons for the flocculation of silica colloid in this study. Because of charge reversal, the irreversible bridges are turned into flexible reversible bridges with increasing adsorption amount of oppositely charged polymer, which leads to an abrupt transition from gel to liquid. Over a narrow composition range, around the gel to liquid transition region, shear-thickening flow is observed. It is found that, for given particle volume fraction, the composition region exhibiting shear-thickening for mixed suspension with linear polyethyleneimine is broader than that for mixed suspension with hyperbranched polyethyleneimine, and the onset of shear-thickening depends only on size-ratio, regardless of the actual size of particle and polymer in the range of this study. The relationship between the gel to liquid transition and shear-thickening was discussed.

Asymmetrical phase separation and gelation in binary mixtures of oppositely charged colloids.

Two types of colloidal particles, which are nearly the same in chemical composition but carry opposite surface charges, are mixed in water. Depending on the relative proportion of the oppositely charged particles, the process of aggregation leads to the formation of discrete clusters of various sizes in dilute dispersions, and to the development of particle gel networks in more concentrated systems. Due to the significant difference in the absolute values of surface charges (negative particle: -48 mV, positive particle: +24 mV), the phase separation and the gelation behaviors are asymmetric with respect to the mixing ratio. Mixtures with excess negative particles are more stable, while mixtures with excess positive particles are easily affected by phase separation. The hetero-aggregation triggered by the addition of microscopically large macro-ions is similar to what is often observed in a mono-component charged colloidal system, i.e., phase separation occurs through addition of small electrolyte ions. Within the concentration region investigated here, it is clear that the gel line is buried inside the phase separation region. Gelation occurs only when the number and size of the clusters are large and big enough to connect up into a space-spanning network. Our results indicate that, in this binary mixture of oppositely charged colloids, although the interaction between unlike species is attractive and that between like species is repulsive, the onset of gelation is in fact governed by the equilibrium phase separation, as in the case of purely attractive systems with short-range isotropic interaction.

Controlling the magnetic properties of polymer–iron oxide nanoparticle composite thin films via spatial particle orientation

We investigated the effect of Fe3O4 nanoparticle orientation on the magnetic properties of hybrid polymer nanocomposite thin films. A multilayer thin film consisting of alternating layers of polymers and assembled iron oxide nanoparticles was prepared by spin coating and Langmuir–Blodgett techniques. Transmission electron microscopy and neutron reflectivity measurements were employed to determine structural information related to the lateral orientation of the Fe3O4 nanoparticle monolayer and the layered architecture along the depth of the multilayer, respectively. The magnetic properties of the hybrid multilayer were characterized by SQUID magnetometry and compared with the properties of a spin-coated polymer nanocomposite thin film containing homogenously dispersed Fe3O4 nanoparticles. We found that the closely-packed monolayer structure of the Fe3O4 nanoparticles changed the magnetic properties on account of the dipolar interactions between particles, whereas the homogeneously-dispersed nanoparticles embedded in the polymer matrix exhibited zero remanent magnetization and coercivity due to isolation of the nanoparticles and lack of dipolar interactions.

Glass transition: from interaction potential changes to the corresponding structural and relaxation responses along the path of reaching the final equilibrium

Glass transition involves a many-body interaction and relaxation process. The split of relaxation spectrum and the extremely slow dynamics bring into considerations of non-linearity and non-equilibrium. Some of our recent findings in two measurable colloidal systems are reviewed, one with a simple attractive interparticle potential and the other with a competitive (repulsive vs. attractive) inter-particle interaction. With an approach from interaction potential changes to the corresponding structural and relaxation responses, along the path of reaching the final equilibrium, we illustrate some interesting physics in glass formation process. Also, some reviews on the popular glass transition theories are made to remind readers to avoid artifacts and misinterpretations.

Perpendicular Orientation of Diblock Copolymers Induced by Confinement between Graphene Oxide Sheets

We have studied an orientation structure of self-assembled block copolymers (dPS-b-PMMA) of deuterated polystyrene (dPS) and poly(methyl methacrylate) (PMMA) confined between graphene oxide (GO) surfaces. The results of combination techniques, such as neutron reflectivity, time-of-flight secondary-ion mass spectrometry, grazing-incidence small-angle X-ray scattering, and scanning electron microscopy, show that self-assembled domains of the block copolymers in thin films near the GO sheets are oriented perpendicular to the surface of the GO monolayers, in contrast to the horizontal lamellar structure of the copolymer thin film in the absence of the GO monolayers. This is due to the amphiphilic nature of the GO, which leads to a nonpreferential interaction of both dPS and PMMA blocks. Double-sided confinement with the GO monolayers further extends the ordering behavior of the dPS-b-PMMA thin films. Continuous vertical orientation of the block copolymer thin films is also obtained in the presence of alternating GO layers within thick copolymer films.