Da-Ming Wang

Preparation of porous scaffolds by using freeze-extraction and freeze-gelation methods.

Freeze-fixation and freeze-gelation methods are presented in this paper which can be used to prepare highly porous scaffolds without using the time and energy consuming freeze-drying process. The porous structure was generated during the freeze of a polymer solution, following which either the solvent was extracted by a non-solvent or the polymer was gelled under the freezing condition; thus, the porous structure would not be destructed during the subsequent drying stage. Compared with the freeze-drying method, the presented methods are time and energy-saving, with less residual solvent, and easier to be scaled up. Besides, the problem of formation of surface skin can be resolved and the limitation of using solvent with low boiling point can be lifted by the presented methods. With the freeze-extraction and freeze-gelation methods, porous PLLA, PLGA, chitosan and alginate scaffolds were successfully fabricated. In addition to the presentation of the morphologies of the fabricated scaffolds, preliminary data of cell culture on them are as well included in the present work.

Preparation and applications of Nafion-functionalized multiwalled carbon nanotubes for proton exchange membrane fuel cells

Nafion-functionalized multiwalled carbon nanotubes (MWCNT-Nafion) are prepared through an ozone-mediated process. The chemical structure of MWCNT-Nafion is characterized by Raman spectroscopy, Fourier-transform infrared spectroscopy and X-ray photoelectron spectroscopy. Nafion chains covering on the outer surfaces of MWCNTs in MWCNT-Nafion is observed in high-resolution transmission electron micrographs. The Nafion fraction of MWCNT-Nafion is about 12 wt%. MWCNT-Nafion is effective to prepare Nafion/MWCNT-Nafion composite membranes for proton exchange membrane fuel cells. Compared to the pristine Nafion membrane, the composite membrane with a MWCNT-Nafion loading of 0.05 wt% (N/MN-0.05) exhibits an 1.5-fold increase of mechanical strength and a five-fold increase of proton conductivity. In single cell tests, the current density at 0.6 V and the maximum power density measured with the N/MN-0.05 composite membrane are 1556 mA cm−2 and 650 mW cm−2, respectively. Both of the values are 1.5-times the values measured with a pristine Nafion membrane.

A Review on Polymeric Membranes and Hydrogels Prepared by Vapor-Induced Phase Separation Process

In 1918, Zsigmondy and Bachmann presented a new method to induce phase separation of a homogeneous polymeric solution from a vapor phase. The so-called vapor-induced phase separation (VIPS) was born. In a century, the body of knowledge on polymer membranes and hydrogels prepared by VIPS has grown importantly, which suggests the need for a critical review. Slowness of mass transfers involved in VIPS, attributed to the resistance at the gaseous phase/liquid phase interface, permits reaching better control of polymer membrane formation than with the popular wet-immersion process. As a result, a broad variety of morphologies can be obtained and well controlled. The control of testing conditions and formulation parameters also permits tuning and tailoring morphologies, which arises in various membranes properties, and led scientists to investigate the possibility of forecasting mass transfers in VIPS. Therefore, at the end of the twentieth century, first models were developed to describe this process, and validated by comparing simulated data to experimental results. Afterwards, studies demonstrated the possibility of predicting membrane morphologies from the knowledge of operating conditions. This article aims at reviewing the work done so far reporting this process to prepare polymer membranes and hydrogels. The experimental set-ups will be introduced as well as the different polymer/solvent/nonsolvent and polymer/additive(s)/solvent/nonsolvent systems used and the morphologies obtained. The effect of testing conditions and formulation parameters on the structure of the matrices will be subsequently discussed. Close attention will be given to the fundamental theory of VIPS before moving onto the potential applications of such polymer matrices.

Effects of polymer chain length and stiffness on phase separation dynamics of semidilute polymer solution.

Three-dimensional dissipative particle dynamics (DPD) simulations were performed to investigate the phase separation dynamics of semidilute polymer solutions with different polymer chain length and stiffness. For the polymer solution composed of shorter and more flexible chains, a crossover of the domain growth exponent from 1/3 to 2/3 was observed during the course of phase separation, indicating that the growth mechanism altered from diffusion to interfacial-tension driven flow. When the chain flexibility was kept the same but the chain was lengthened to allow for the chain entanglement to occur, the growth exponent changed to 1/4 in the diffusion-dominating coarsening regime while the growth exponent remained 2/3 in the flow-dominating regime. When the chain length was kept short but the stiffness was increased, the growth exponent became 1/6 in the diffusion-dominating regime and little effect was observed in the flow-dominating coarsening regime. The slow down of the phase separation dynamics in the diffusion-dominating coarsening could be explained by that the polymer chains could only perform wormlike movement when chain entanglements occurred or when the chain motion was limited by chain stiffness during phase separation. Moreover, when both the effects of chain length and stiffness were enhanced, polymer networks composed of longer and stiffer chains appeared and imposed an energy barrier for phase separation to occur. As a result, the polymer solution with stiffer and longer chains required a larger quench depth to initiate the phase separation and caused the delay in crossover of the coarsening mechanism from diffusion to flow.

Adsorption of Bovine Serum Albumin on Poly(vinylidene fluoride) Surfaces in the Presence of Ions: A Molecular Dynamics Simulation

Adsorption of bovine serum albumin (BSA) on poly(vinylidene fluoride) (PVDF) surfaces in an aqueous environment was investigated in the presence and absence of excess ions using molecular dynamics simulations. The adsorption process involved diffusion of protein to the surface and dehydration of surface-protein interactions, followed by adsorption and denaturation. Although adsorption of BSA on PVDF surface was observed in the absence of excess ions, denaturation of BSA was not observed during the simulation (1 μs). Basic and acidic amino acids of BSA were found to be directly interacting with PVDF surface. Simulation in a 0.1 M NaCl solution showed delayed adsorption of BSA on PVDF surfaces in the presence of excess ions, with BSA not observed in close proximity to PVDF surface within 700 ns. Adsorption of Cl- on PVDF surface increased its negative charge, which repelled negatively charged BSA, thereby delaying the adsorption process. These results will be helpful for understanding membrane fouling phenomena in polymeric membranes, and fundamental advancements in these areas will lead to a new generation of membrane materials with improved antifouling properties and reduced energy demands.

Dual Superlyophobic Aliphatic Polyketone Membranes for Highly Efficient Emulsified Oil–Water Separation: Performance and Mechanism

Efficient treatment of difficult emulsified oil-water wastes is a global challenge. Membranes exhibiting unusual dual superlyophobicity (combined underwater superoleophobicity and underoil superhydrophobicity) are intriguing to realize high-efficiency separation of both oil-in-water and water-in-oil emulsions. For the first time, a robust polymeric membrane demonstrating dual superlyophobicity to common apolar oils was facilely fabricated via a simple one-step phase separation process using an aliphatic polyketone (PK) polymer, thanks to a conjunction of intermediate hydrophilicity and re-entrant fibril-like texture upon the prepared PK membrane. Further chemical modification to improve surface hydrophilicity slightly can enable dual superlyophobicity to both apolar and polar oils. It is found that a nonwetting composite state of oil against water or water against oil was obtainable on the membrane surfaces only when the probe liquids possess an equilibrium contact angle (θow or θwo) larger than the critical re-entrant angle of the textured surfaces (73°), which can explain the existences of dual superlyophobicity and also the nonwetting to fully wetting transitions. A simple design chart was developed to map out the operational windows of material hydrophilicity and re-entrant geometry, that is, a possible zone, to help in the rational design of similar interfacial systems from various materials. Switchable filtrations of oil-in-water and water-in-oil nanoemulsions were achieved readily with both high flux and high rejection. The simplicity and scalability of the membrane preparation process and the well-elucidated underlying mechanisms illuminate the great application potential of the PK-based superwetting membranes.