In this study, mixed suspensions of large hard polystyrene microspheres and small soft poly(N-isopropylacrylamide) microgels are used as model systems to investigate the static and viscoelastic properties of suspensions which go through liquid to gel transitions. The microgels cause short-range attraction between microspheres through the bridging and depletion mechanism whose strength can be tuned by the microgel concentration. Rheological measurements are performed on suspensions with the volume fraction (Φ) of microspheres ranging from 0.02 to 0.15, and the transitions from liquid-like to solid-like behaviors triggered by the concentration of microgels are carefully identified. Two gel lines due to bridging attraction under unsaturated conditions are obtained. Ultra-small angle neutron scattering is used to probe the thermodynamic properties of suspensions approaching the liquid–solid transition boundaries. Baxter's sticky hard-sphere model is used to extract the effective inter-microsphere interaction introduced by the small soft microgels. It is found that the strength of attraction (characterized by a single stickiness parameter τ) on two gel lines formed by bridging is very close to the theoretical value for the spinodal line in the τ–Φ phase diagram predicted by Baxter's model. This indicates that the nature of the gel state may have the same thermodynamic origins, independent of the detailed mechanism of the short-range attraction. The relationship between the rheological criterion for the liquid–solid transition and the thermodynamic criterion for the equilibrium–nonequilibrium transition is also discussed.

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.

Gelation and glass transition in a mixed suspension of polystyrene (PS) microsphere and poly(N-isopropylacrylamide) (PNIPAM) microgel were studied as a function of the total colloid volume fraction and mixing ratio of these two components. The PNIPAM microgel, which is adsorbable on the PS microsphere surface, can induce bridging or stabilizing effect between microspheres depending on whether the volume fraction of microgel (ΦMG) is smaller or larger than the saturated adsorption concentration (Φ*MG) for a given volume fraction of the microsphere (ΦMS). Φ*MG is in a linear relationship with ΦMS, and the value of ΦMG/Φ*MG can be taken as an approximate measure of surface coverage. A state diagram of gelation and glass transition is constructed with the short-ranged attractive interaction, resulting from the well-defined bridging bonding. Keeping ΦMG/Φ*MG = 0.20 and increasing ΦMS from 0.25 to 0.55, the mixed suspension transforms from a bridging gel into an attractive glass; moreover, while keeping ΦMS = 0.45 and increasing ΦMG/Φ*MG from 0.20 to 1.2, the mixed suspension changes from a bridging gel into an attractive glass, and then to a repulsive glass. The bridging effect and the cage effect can be distinguished by the yielding behaviors in rheological measurements. In the nonlinear dynamic rheological experiments, one-step yielding, corresponding to the disconnecting of bridge network, is observed in the bridging gel, and one-step yielding, corresponding to the breaking of cage, is observed in the repulsive glass. However, a two-step yielding behavior is found in the bridging-induced attractive glass, which is attributed to the bridging effect of microgels and the caging effect of the dense environment.

Interfacial evolution of deuterated polycarbonate (dPC) and poly(methyl methacrylate) (PMMA) bilayer films supported on silicon substrates was examined by neutron reflectometry. The dPC was in the glassy state and the PMMA was in the melt state when the samples were annealed at temperatures (from 400 to 415 K) between the bulk glass transition temperature (Tg) of dPC (Tg ∼ 418 K) and PMMA (Tg ∼ 390 K). Asymmetric concentration profiles were observed as a result of composition dependence of the mobility. Once the slow moving dPC molecules leave the dPC-rich side (which was glassy) and enter the PMMA side, the dPC molecules become fast moving and diffuse freely in a viscous melt environment, while the fast moving PMMA molecules become slow as PMMA enter the glassy dPC matrix. Slight change in molecular mass of dPC gives rise to remarkable difference in concentration profile, indicating that mutual diffusion is very sensitive to molecular friction and entanglement of medium. With the penetration of PMMA into the dPC-rich side, the sluggish relaxation of dPC with high molecular mass results in the swelling of the dPC layer.

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.

Bisphenol-A polycarbonate (PC) and amorphous polyamide (aPA) were used as reactive system to study the interfacial interchange reaction between condensation polymers. Aminolysis is the main process during thermal annealing at 160–180 °C. The simultaneously scission of PC chains and formation of PC-aPA copolymer chains during the reaction process, can act as interfacial compatibilization agents between incompatible homopolymers. Reaction kinetics measured by in-situ Fourier transform infrared spectrum (FTIR) at interfaces of well-separated bilayer films and phase separation blends were compared. The reaction follows a first-order diffusion controlled mechanism and three time regimes were observed. First, the functionalized chains located in the vicinity of the interface participate into the reaction and the annealing time dependence of the integral area of difference IR spectra follows a power law relationship as [Area] ∼ t1/4. Second, a depletion layer of reactants in the interfacial region is formed with the progression of reaction, and center-of-mass diffusion of reactants is required for further reaction which results in a transition of the power law relationship into [Area] ∼ t1/2. Finally, the potential barrier arising from the previously formed copolymers at the interface suppressed the reaction. When the conversion reaches a critical value, thermal fluctuation can induce interface destabilization. An acceleration of reaction rate observed by FTIR for sample annealed at 180 °C is synchronous with the interfacial roughness development analyzed by atomic force microscopy.

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.

In this work, aggregation and gelation behaviour of mixed suspensions of polystyrene microspheres and poly(N-isopropylacrylamide) microgels were studied. In dilute microsphere suspensions, with increasing concentration of microgel (ϕMG), microspheres first aggregated with each other through the bridging of the microgels, then dispersed individually when saturated adsorption was achieved, and finally depletion clusters formed at even higher concentrations of microgel. The concentration of microgel for saturated adsorption (ϕ*MG) is in a linear relationship with the given concentration of microsphere (ϕMS). For given ϕMS, the largest bridging clusters formed at ϕMG ≅ 0.5 ϕ*MG. In concentrated microsphere suspensions, with increasing ϕMG, the mixed suspensions showed a liquid–“bridging gel”–liquid–“depletion gel” transition. The maximum modulus and yielding stress of the bridging gel are also obtained at ϕMG ≅ 0.5 ϕ*MG. The gelation of the microsphere can be explained by the percolation of the clusters and the elasticity of bridging gel arising from the mutual attractions between the microspheres.

Poly(N-isopropylacrylamide) (PNIPAM) microgels are adsorbable to the surface of polystyrene (PS) microspheres in the experimental temperature range (25–40 °C) and can induce aggregation of microspheres through the bridging mechanism. The bridging mechanism proceeds in two elementary steps: the first is an adsorption (negligible desorption relatively) of free microgel to one microsphere surface, and the next is a reversible connection of the adsorbed microgel to another microsphere surface. The surface coverage is found as an important factor in triggering the stabilization, aggregation, and gelation of the mixtures. And most importantly, the nonnegligible dynamic disconnection of a bridge is the essential prerequisite for structure or state transition. For mixtures with the same concentration of microsphere, depending on the concentration of microgel, various transitions can be induced by changing temperature, including the transition from a weaker gel to a stronger gel, from a fluid to a bridging gel and from a depletion gel to a bridging gel.

Poly(acrylic acid)-graft-poly(ethylene oxide) (PAA-g-PEO) in aqueous solutions shows one fast and one slow relaxation mode in dynamic light scattering (DLS), but the mixture of PAA and PEO (PAA/PEO) in aqueous solution only has a single fast mode. The effects of pH, polymer concentration, and salt concentration on these two modes have been investigated using laser light scattering (LLS), viscometry, and rheological measurements. Our results showed that the hydrogen bonding between carboxylic group and ether oxygen led to the formation of large complexes among PAA-g-PEO chains, which were absent between PAA and PEO chains in PAA/PEO aqueous solutions. The addition of formamide can break these interchain complexes because the hydrogen bonding between formamide and PAA segment is stronger than that between PEO and PAA segment. Thermodynamically speaking, the formation of hydrogen bonds among PAA-g-PEO chains leads to a less entropy loss than that between PAA and PEO chains in PAA/PEO aqueous solution, because in the former case PEO is already chemically connected to PAA backbone. Therefore, the same enthalpy gain is sufficient to compensate the entropy loss in PAA-g-PEO aqueous solution relative to that in PAA/PEO aqueous solution, resulting in large interchain PAA-g-PEO complexes.