Ultrafiltration Membranes Composed of Highly Cross‐Linked Cationic Polymer Gel: the Network Structure and Superior Separation Performance

Abstract

Hydrophilic polymer gels are attractive materials for a wide range of applications in the life sciences and bioengineering. The gels are environmentally responsive and have high potential in numerous applications such as drug delivery, sensors, actuators, absorbents, and scaffolds. The transport of ions and molecules through hydrated polymer gels has been investigated by many researchers, as has the bioaffi nity and mechanical properties of these gels. [ 1 ] Poly(4-vinylpyridine) (P4VP) forms typical hydrated polymer gels. This polymer is readily cross-linked with alkyl dihalide at room temperature via quaternization of the pyridine groups. Cross-linkers of various size and shape can be used, resulting in P4VP gels with diverse structures and properties. Thin fi lms made of cross-linked poly(vinylpyridine) gels have been widely investigated as a means of applying stimuli-evoked optical, impedance, and volume changes to sensors and biodevices. [ 2 ] P4VP has intrinsic selective permeability for gases, [ 3 ] and the network structures of these gels can be controlled by controlling the extent of cross-linking. Therefore, this polymer has also been studied for its utility in gas separation, desalination, and pervaporation membranes. [ 4 ] However, like most polymer gels, P4VP gels are mechanically weak, and the gel membranes, which are generally supported on a microfi ltration membrane, have thicknesses of a few to several tens of micrometers. In order to be practical, the separation layer needs to be as thin as possible to realize high liquid/gas fl uxes, which requires that the gels are mechanically stable. We here report ultrafi ltration membranes with thicknesses of several tens to hundreds of nanometers, prepared from P4VP gels by means of a two-step cross-linking of the polymer chains. The detailed structure of the polymer network was characterized using spectroscopic techniques and permeation and rejection experiments. Furthermore, the gel membranes were demonstrated to distinguish between water-soluble proteins of different molecular masses.The methodology for preparing ultrathin ultrafi ltration membranes of the cross-linked P4VP gel is shown in Figure 1 . First, P4VP was cross-linked with 1,3-dibromopropane (DBP) in dimethyl sulfoxide (DMSO), then the resulting weakly gelled solution was diluted with water (Figure 1 a). To form P4VP gel membranes, we used an extremely fl at sacrifi cial layer of metal hydroxide nanostrands. Without this layer, the gel readily penetrates into the submicrometer pores of the microfi ltration membrane, resulting in decreased permeability to water. The nanostrands spontaneously form in dilute aqueous solution of copper, zinc, or cadmium nitrate (or chloride) upon neutralization of the solution with base (see Supporting Information). [ 5 ] In the present study, cadmium hydroxide nanostrands with a diameter of 1.9 nm and lengths of a few micrometers were fi ltered on a polycarbonate (PC) membrane fi lter (Figure 1 b). [ 6 ] The resultant ultrathin nanofi brous layer guaranteed the...

Highly permeable and robust membranes assembled from block-copolymer-functionalized carbon nanotubes

“…The peak position of 399.0 eV N 1s was the same as the pristine sample, which corresponds to the uncharged pyridyl rings of the P4VP chains. The other peak emerging at 402.1 eV was typically found for cationized nitrogen atoms of ammonium groups, [23,24] which confirms the successful quaternization reaction.…”

“…The adhesion of composite 23 membranes was successfully improved via chemical crosslinking. Moreover, the addition of CNTs significantly enhanced the mechanical strength of the composite membranes.…”

Ultrathin free-standing membranes from metal hydroxide nanostrands

“…These nanostrands are readily dissolved in weakly acidic aqueous solutions, they exhibit poor thermal stability in water. Metal hydroxide nanostrands can, however, form very uniform nanometer-thick sheets when an aqueous dispersion is passed through a microfiltration membrane, and such sheets have been widely used as sacrificial layers to form freestanding nano-or ultra-filtration membranes [39,40]. Some of these membranes offer filtration performance three orders of magnitude higher than commercially available products [41].…”

“…However, ultrathin ultrafiltration membranes of hydrated polymer gel were successfully prepared using cadmium hydroxide nanostrands [40]. In a typical experiment, poly(4-vinylpyridine) (P4VP) was converted to its hydrated gel by crosslinking with 1,3-dibromopropane (DBP) at room temperature (in DMSO) via quaternization of the pyridine groups.…”

Functional polyelectrolyte multilayer membranes for water purification applications

“…Water flux data were also used to estimate the effective pore radius of the prepared membranes using the Hagen-Poiseuille equation [64,65]:…”