Hesheng Xia

Physically crosslinked poly(vinyl alcohol) hydrogels with magnetic field controlled modulus

Physically cross-linked isotropic and anisotropic poly(vinyl alcohol) (PVA) hydrogels containing micron-sized carbonyl iron particles were prepared through a cyclic freezing–thawing process. The PVA hydrogel can respond to a magnetic field and shows a magnetorheological (MR) effect, i.e., the modulus of the PVA hydrogel can be adjusted under a magnetic field. The chain-like structures of carbonyl iron are formed in the PVA hydrogel after orientation under a magnetic field of 1.5 T. Also some magnetic field induced oriented pores with a tunable diameter are observed in the dried PVA gel. The MR effect can be adjusted by changing the carbonyl iron content, the initial concentration of PVA solution and test frequency. The formation of aligned chain-like structures of carbonyl iron in the anisotropic PVA MR hydrogel improves the compression properties and the MR effect. At a carbonyl iron content of 70 wt%, the maximum absolute and relative MR effect of anisotropic PVA MR hydrogels are ∼1.24 MPa and ∼230%, respectively. The PVA hydrogels with good MR effects and moderate mechanical strength have potential applications in artificial muscle, soft actuators and drug release.

Ultrasonically induced microemulsion polymerization of styrene

Abstract In this paper, ultrasonically induced microemulsion polymerization of styrene was successfully performed, possessing many merits such as high polymerization rate, the formation of small latex particles with a narrow size distribution, the absence of initiator and relatively low surfactant concentration. The monomer conversion reached 70% in 1 h, and the average diameter of polystyrene (PS) latex was about 30 nm which could be prepared with 3% surfactant (sodium dodecyl sulfate, SDS) concentration. The molecular weight of PS was around 10 6 and the poly-distribution index was 1.06, indicating a very narrow distribution. Several influencing factors were investigated in detail, showing that ultrasonically induced microemulsion polymerization is a new route to prepare PS nanoparticles.

Electro-activated surface micropattern tuning for microinjection molded electrically conductive shape memory polyurethane composites

Shape memory polymers with surface micropatterns have seen rising demand for high value applications such as adjustable adherence surfaces, dynamic micro-geometries for cell culture studies and switchable information carriers. Recently, microinjection molding has emerged as an efficient way to manufacture devices which contain surface micro-features using a wide range of polymers with high accuracy. In this study, shape memory polyurethane–carbon nanotube composites were prepared by twin-screw melt extrusion and subsequently processed using microinjection molding to obtain components with surface micropatterns. Then an electro-activated surface micropattern tuning system was developed which could recover the original micropatterned surface of the components after a thermal deformation by applying a current which heats the component using resistive heating. In order to optimize the technique, three key areas were investigated in this work: conductivity of the microinjection molded microparts, the retention of shape memory micropatterns on the surface of microparts during annealing treatment, and the macroscopic area shrinkage of microparts after thermal treatment. It has been found that the electrical conductivity of microinjection molded parts is relatively low due to the high shear rates prevalent in the process. An annealing treatment improves the electrical conductivity by several orders of magnitude, but can be detrimental to the dimensional stability of the micropatterns, which depends significantly on the micro-injection molding parameters, especially the mold temperature. Increasing the mold temperature, melt temperature, injection speed and injection pressure result in better retention of the micropattern and improved dimension stability during annealing treatment. This work demonstrates the potential of electro-activated surface micropattern control for microinjection molded electrically conductive shape memory polymer composites, which could be a promising technology for a range of application areas including electro-adjustable adherence, information storage, and anti-counterfeiting technology.

Smart polyurethane foam with magnetic field controlled modulus and anisotropic compression property

A new kind of polyurethane (PU) magnetorheological (MR) foam was prepared via in situ polymerization and foaming for the first time. A chain-like structure of carbonyl iron particles in the liquid polyol is formed after orientation at a magnetic field and fixed in the PU foam after the in situ curing. The anisotropic PU MR foam possesses an anisotropic compression property. The compression strength along the magnetic chain direction reaches ∼1053.5 KPa for the sample with 80 wt% carbonyl iron content, which is ∼878 times that of the blank foam and ∼57 times of that at the vertical direction of the same sample. The PU MR foam exhibits a magnetic field controlled shear modulus, i.e., MR effect, which can be adjusted by changing the orientation structure and content of the carbonyl iron, test frequency and magnetic field strength. The maximum absolute and relative MR effects for the anisotropic PU MR foam are ∼1.07 MPa and ∼27.1%, respectively. Moreover, the introduction of carbonyl iron particles can notably improve the acoustic absorption properties in the low frequency region.