Zhen-Mei Liu

Microporous polypropylene and polyethylene hollow fiber membranes. Part 3. Experimental studies on membrane distillation for desalination

Three polypropylene (PP) and one polyethylene (PE) microporous hollow-fiber membranes were used in direct contact membrane distillation (DCMD) and vacuum membrane distillation (VMD) for the desalination of simulated seawater. The influence of feed temperature and feed flow on distillate pure water flux was investigated. The comparison of the PP and PE membranes in DCMD and VMD was carried out. It was found that the water flux increased with the feed temperature and feed flow in both DCMD and VMD. The data also showed that, compared with the PP membranes, higher water flux could be obtained by using PE membranes in both the DCMD and VMD processes.

Surface engineering of macroporous polypropylene membranes

The surface properties of polymer membranes are crucial to their application. This Review provides concise comments on surface engineering strategies for macroporous polypropylene membranes. The applications of surface engineering concepts in membrane-based bioreactors, bioseparation, biosensing, biosynthesis, environmental analysis, water purification, energy technology, medical devices (artificial lung and liver), and some novel separation processes (intelligent membrane separation) are summarized. The prospect of surface engineered biomimetic polypropylene membrane is also looked at.

Surface modification of microporous polypropylene membranes by the grafting of poly(γ-stearyl-l-glutamate)

Abstract A polypeptide, poly(γ-stearyl- l -glutamate) (PSLG), was grafted on the surface of hydrophobic polypropylene hollow fiber membranes through the ring opening polymerization of N-carboxyanhydride (NCA) of γ-stearyl- l -glutamate initiated by amino groups which was generated by ammonia plasma. X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR), together with water contact angle and bovium serum albumin adsorption measurements were used to characterize the modified membrane surface. The XPS and FT-IR spectra demonstrated that polypeptide was actually grafted on the membrane surface despite of the low degree of graft polymerization due to the hydroxyl groups on the membrane surface. To subject the ammonia plasma-treated membrane with γ-(aminopropyl)triethanoxysilane (γ-APS) which can react with hydroxyl groups and leave amino groups, the degree of graft polymerization could be improved. The bovium serum albumin adsorption measurement was conducted to further examine the surface properties of modified and original membranes. Potential applications of the PSLG grafted membranes are expected for enantiomer separation and/or enzyme immobilization.

Microporous polypropylene hollow fiber membranes: Part II. Pervaporation separation of water/ethanol mixtures by the poly(acrylic acid) grafted membranes

Polypropylene hollow fiber membranes grafted with poly(acrylic acid) were used for water–ethanol separation by pervaporation. The effects of grafting degree of poly(acrylic acid) (PAA), the cross-linking degree of the grafted-PAA, the sodium counter-ions as well as the operation temperature and the ethanol concentration in feed mixtures on pervaporation properties were investigated. It was found that water-permselective pervaporation membranes could be prepared by the grafting polymerization of acrylic acid on microporous polypropylene hollow fiber membrane surface. The separation factor increased with the increase of grafting degree of PAA in the range of 20–70 wt.%. Incorporating counter-ions as well as multifunctional comonomer into the grafted chains could improve the selectivity but sacrificed the permeation flux. The separation factor of the counter-ion containing membrane decreased according to the sequence Al 3+ >K + >C a 2+ >N a + >L i + and the permeation flux increased following the sequence Al 3+ < Ca 2+ < K + < Na + < Li + . Remarkable increase in permeation flux without decrease in separation factor was observed for Al 3+ as counter-ion in this membrane. These results might be ascribed to the increase of packing density and the decrease of swelling degree of grafted layer on the membrane surface.

Preparation and characterization of polyacrylonitrile-based membranes: Effects of internal coagulant on poly(acrylonitrileco-malefic acid) ultrafiltration hollow fiber membranes

Abstract Ultrafiltration hollow-fiber membranes (UHFMs) ofpoly(acrylonitrile-co-malefic acid) (PANCMA) were prepared by a dry-wet phase inversion process. The morphologies of inner surface and cross section for these hollow fibers were inspected with scanning electron microscopy. It was found that, by increasing the amount of solvent DMSO in internal coagulant, the number and size of macrovoid underneath the inner surface decreased. The water flux of the UHFMs also decreased while the bovine serum albumin rejection increased minutely. These results were interpreted based on the ternary phase diagrams for the PANCMA/DMSO/(H2O+DMSO) system, which was obtained from the experimental cloud point measurements and empirical linearized cloud point relation. It was envisaged that the membrane surface could be further modified by the reaction of acid groups with poly(ethylene glycol).

Chitosan-Modified Poly(acrylonitrile-co-acrylic acid) Nanofibrous Membranes for the Immobilization of Concanavalin A

Lectin affinity membranes have been receiving much attention for the separation and detection of various glycoconjugates. In this work, we present a simple and efficient method for the preparation of lectin affinity nanofibrous membranes. Chitosan-modified poly(acrylonitrile-co-acrylic acid) (PANCAA) nanofibrous membranes were first prepared by a coupling reaction between the primary amino groups of chitosan and the carboxyl groups of PANCAA electrospun membranes. Surface characterizations by attenuated total reflectance Fourier transform infrared spectroscopy (FT-IR/ATR), X-ray photoelectron spectroscopy (XPS) and field-emission scanning electron microscopy (FESEM) confirm the chemical and morphological changes of the studied nanofibrous membranes. Fluorescence-labeled concanavalin A (FL-Con A) was then immobilized on these membranes via noncovalent binding. Analyses by fluorescence spectrophotometer (FS) and confocal laser scanning microscopy (CLSM) reveal that the immobilization of Con A onto the modified nanofibrous membranes has been successfully achieved on the basis of the electrostatic interaction and the specific recognition between Con A and chitosan. The results show that the amount of adsorbed FL-Con A increases dramatically with the increasing coupling degree of chitosan (CDC) on the nanofibrous membrane. Moreover, Con A immobilized on the chitosan-modified nanofibrous membranes (CMNMs) can remain relatively stable at pH 5.3. Therefore, it is believed that this work may provide a new kind of material for affinity application.

Key properties to understand the performance of polycarbonate reprocessed by injection molding

The reprocessing of injection-molded polycarbonate was studied in an attempt to understand the physical background for the variation of its performance upon sequential molding cycles. The effect of reprocessing on different properties of the moldings and the regrind is presented and discussed. A quantitative analysis of the Fourier transform IR spectra indicates that phenol and carbon dioxide are released during the initial cycles, probably due to molecular scission near the chain ends. Most of the results obtained agree well with the findings reported elsewhere. Essentially, the molecular weight, the free volume, and the specific volume were found to be critical properties for understanding the global performance of the moldings. Both the free volume and the reciprocal of the molecular weight depend linearly on the number of reprocessing cycles. Based on these properties, it is possible to develop relatively simple relations to estimate the variation of the rheological, optical, and mechanical properties with the number of reprocessing operations.

Fabrication of glycosylated surfaces on microporous polypropylene membranes for protein recognition and adsorption

Synthetic membranes with glycosylated surfaces have great potential in pharmaceutics, clinically in diagnosis and protein purification. A versatile method of constructing glycosylated membrane surfaces was developed using poly(2-hydroxyethyl methacrylate) (poly(HEMA))-tethered microporous polypropylene membranes (MPPMs) as supports. Glucose moieties were bound on the membrane surfaces through the reaction between glucose pentaacetate and the hydroxyl group of poly(HEMA). The binding degree of glucose moieties on the MPPMs was controlled in a wide range by changing the grafting degree of poly(HEMA). The maximum binding degree reaches 10 µmol cm−2 when the grafting degree of poly(HEMA) on the membrane surface is 29.40 µmol cm−2. These glycosylated membrane surfaces were characterized by attenuated total reflectance Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and scanning electron microscopy. Water contact angles of the glycosylated membrane surfaces before and after removing the acetyl groups differ remarkably from each other. After deprotection, the glycosylated membrane surfaces become highly hydrophilic, which greatly inhibits the non-specific adsorption of bovine serum albumin or peanut agglutinin on these surfaces. Furthermore, the glycosylated membranes can selectively recognize concanavalin A (ConA). When the binding degree of glucose moieties is above 0.39 µmol cm−2, the “glycoside cluster effect” occurs and plays an important role in the recognition and adsorption of proteins. Due to the reversible affinity, the adsorbed ConA can be desorbed with glucose solution. The results suggest that these MPPMs with glycosylated surfaces are promising for the isolation/purification of proteins.

Recognition Mechanism of Theophylline-Imprinted Polymers: Two-Dimensional Infrared Analysis and Density Functional Theory Study

Molecular imprinting polymers (MIPs) are synthetic materials having specific cavities tailored for a target molecule. Thoroughly understanding the molecular recognition mechanism is favorable for the rational design, preparation, and application of MIPs. In this work, theophylline (THO)-imprinted poly(acrylonitrile-co-acrylic acid) (PANCAA) films with acrylic acid (AA) as the functional monomer were fabricated and a set of concentration-dependent Fourier transform infrared (FT-IR) spectra were collected. Two-dimensional (2D) correlation analysis of the spectra and density functional theory (DFT) calculation were conducted to evaluate the molecular recognition mechanism. DFT at the B3LYP/6-31+G(d,p) level is efficacious to calculate the binding energies (DeltaE) and the theoretical vibration frequencies for the possible configurations of THO_AA complexes. An optimized cyclic hydrogen-bonded configuration (complex THO_AA1) has the highest binding energy (-16.63 kcal mol(-1)) that is more stable than others. In addition, the experimental vibrations of the carbonyl groups in the FT-IR spectra were assigned on the basis of the DFT results. Moreover, methylacrylic acid (MAA) and caffeine (CAF) as compared analogues were also investigated. The DFT-based theoretical predictions are coincident with the reported results.

Polyimide/Poly(Styrene-co-4-Vinylpyridine) Nanocomposite Films as Precursors for the Preparation of Polyimide Nanofoams

ABSTRACT A method for the preparation of polyimide nanofoams with high thermal stability, which can be used as dielectric layers in microelectronics is presented. Poly[styrene-co-(4-vinylpyridine)] (PSVP) nanospheres were prepared by emulsion polymerization. The dimension of these particles is between 30 and 40 nm. PMDA-ODA nanocomposite films with various contents of PSVP were obtained by in-situ condensation polymerization in the presence of the nanospheres, casting of polyamic acid solution containing the nanospheres, and imidization. The dispersion of the nanospheres in the films and the morphology of the composite films were characterized by transmission electron microscopy (TEM) and a gas diffusion method. Relative homogenous distributions were observed for the polyimide/PSVP nanocomposite films, which could be ascribed to the formation of poly(amic acid) amine salts between the pyridinyl groups of the nanosphere and the carboxyl groups of the poly(amic acid) before the films being cured. Upon a thermal treatment, the thermally unstable nanospheres undergo thermolysis, leaving pores the size and shape of which are nearly dictated by the initial nanoparticle morphology. The resultant foams were characterized by TEM, TG, density measurements and dielectric constant measurements respectively. The results showed that foams with high thermal stability and low dielectric constant approaching 2.0 could be prepared using this polyimide nanocomposite approach.