Donghua Xu

Fabrication of stretchable, flexible conductive thermoplastic polyurethane/graphene composites via foaming

Stretchable and flexible conductive polymers have aroused great interest recently because of their applications in the fields of novel electronics, such as smart textiles, artificial electronic skin, flexible electronic displays, etc. In this work, stretchable and flexible conductive thermoplastic polyurethane (TPU)/graphene composite foams have been developed by water vapour induced phase separation. The as-prepared TPU/graphene composite foams exhibited a lower modulus, larger elongation at break, and lower hysteresis during a cycle tensile test than a TPU/graphene composite did. It is expected that the improved elasticity of the TPU/graphene composite foams was caused by the deformation of cells, which partially offset the deformation of the TPU matrix. In addition, the cell walls divided the whole composites into many small parts, which could further restrain plastic deformation of hard segment domains under deformation.

Flow-induced structure and rheological properties of multiwall carbon nanotube/polydimethylsiloxane composites

The structure and rheological properties of multiwall carbon nanotube (MWNT)/polydimethylsiloxane (PDMS) composites under shear are investigated, as the molecular weight of PDMS, aspect ratio and concentration of MWNT are systematically varied. Negative normal stress differences (ΔN) are observed at low shear rates for samples with low molecular weight (Mw) of PDMS (lower than the critical entanglement molecular weight (Mc)), whereas positive ΔN is found in samples with high molecular weight of PDMS (Mw > Mc). More interestingly, negative ΔN is also observed for some samples under confinement when the molecular weight of PDMS is higher than the critical value (Mw > Mc). Moreover, the aspect ratio and concentration of MWNT show negligible influence on the sign of ΔN. Based on the results of optical-flow experiments, a phase diagram for the structures of samples under shear is obtained. It is concluded that the vorticity banding of MWNT aggregates results in the negative ΔN under shear through relating the evolution of structure and the rheological properties of samples under shear.

The effect of particle shape on the structure and rheological properties of carbon-based particle suspensions

The structure and rheological properties of carbon-based particle suspensions, i.e., carbon black (CB), multi-wall carbon nanotube (MWNT), graphene and hollow carbon sphere (HCS) suspended in polydimethylsiloxane (PDMS), are investigated. In order to study the effect of particle shape on the structure and rheological properties of suspensions, the content of surface oxygen-containing functional groups of carbon-based particles is controlled to be similar. Original spherical-like CB (fractal filler), rod-like MWNT and sheet-like graphene form large agglomerates in PDMS, while spherical HCS particles disperse relatively well in PDMS. The dispersion state of carbon-based particles affects the critical concentration of forming a rheological percolation network. Under weak shear, negative normal stress differences (ΔN) are observed in CB, MWNT and graphene suspensions, while ΔN is nearly zero for HCS suspensions. It is concluded that the vorticity alignment of CB, MWNT and graphene agglomerates under shear results in the negative ΔN. However, no obvious structural change is observed in HCS suspension under weak shear, and accordingly, the ΔN is almost zero.

Rheological Study on the Cure Kinetics of Two-component Addition Cured Silicone Rubber

The cure kinetics for two-component silicone rubber formed by addition reaction was studied by the rheological method. The influence of reaction temperature (T) on the cure kinetics was explored in detail. It was observed that the data of gel time (tgel, i.e. the time when the reaction reaches the gel point) or a specific reaction time (tnc) (defined as the reaction time before which time the influence of confinement of network on the diffusion of reaction components can be neglected) versus T obey certain functional relationship, which was well explained by the cure kinetics model of thermoset network. The cure kinetics for the two-component silicone rubber can be well fitted by the Kamal-Sourour(autocatalyst) reaction model rather than Kissinger model. When the reaction time was before or equal to tnc, the reaction order obtained by the Kamal-Sourour reaction model was 2, which was consistent with the reaction order inferred from the two components chemical reaction when the diffusion of reaction components was not influenced by the formed cross-linked polymer network. When the reaction time was larger than tnc, such as to the end of reaction (te), the influence of confinement of network on the diffusion of reaction components cannot be neglected, and the reaction order obtained by the Kamal-Sourour reaction model was larger than 2. It was concluded that the confinement effect of network had a greater influence on the cure kinetics of the silicone rubber. The reaction rate constants (kr) under different temperatures were also determined by Kamal-Sourour reaction model. The activation energy (E) for the two-component silicone rubber was also calculated from the results of lntgel, lntnc, and lnkr versus 1/T, respectively. The three values of E were close, which indicated that above analyses were self-consistent.

Impact of particle surface chemistry on the structure and rheological properties of graphene-based particle/polydimethylsiloxane composites

The structure and rheological properties of graphene-based particle (GP-x)/polydimethylsiloxane (PDMS) composites are investigated as the surface oxygen content of graphene-based particle is varied, i.e., from 6.6% (GP-1) to 15.3% (GP-2), 25.5% (GP-3) and 43.1% (GP-4). Interestingly, the dispersion state of graphene-based particles in PDMS does not change monotonically with increasing surface oxygen content. The size of layered stacks and aggregates first decreases from GP-1 to GP-3 and then increases from GP-3 to GP-4 with increasing surface oxygen content. The larger size of layered stacks and aggregates in GP-1 and GP-4 suspensions results from strong inter-particle π–π and hydrogen bonding interactions. Under weak shear, GP-1 and GP-4 form larger aggregates in PDMS, which align along the vorticity direction, inducing negative normal stress differences (ΔN) in the composites. However, GP-2 and GP-3 do not further aggregate under weak shear and the ΔN is almost zero. It is further inferred that the strong inter-particle attractive interaction leads to the vorticity alignment of aggregates under weak shear.