Tuning Structure and Properties of Graded Triblock Terpolymer-Based Mesoporous and Hybrid Films
Tuning Structure and Properties of Graded Triblock Terpolymer-Based Mesoporous and Hybrid Films
ABSTRACT: Despite considerable efforts toward fabricating ordered, water-permeable, mesoporous films from block copo- lymers, fine control over pore dimensions, structural character- istics, and mechanical behavior of graded structures remains a major challenge. To this end, we describe the fabrication and performance characteristics of graded mesoporous and hybrid films derived from the newly synthesized triblock terpolymer, poly(isoprene-b-styrene-b-4-vinylpyridine). A unique morphol- ogy, unachievable in diblock copolymer systems, with enhanced mechanical integrity is evidenced. The film structure comprises a thin selective layer containing vertically aligned and nearly monodisperse mesopores at a density of more than 1014 per m2 above a graded macroporous layer. Hybridization via homopolymer blending enables tuning of pore size within the range of 16 to 30 nm. Solvent flow and solute separation experiments demonstrate that the terpolymer films have permeabilities comparable to commercial membranes, are stimuli-responsive, and contain pores with a nearly monodisperse diameter. These results suggest that moving to multiblock polymers and their hybrids may open new paths to produce high-performance graded membranes for filtration, separations, nanofluidics, catalysis, and drug delivery.
Figure 1: The triblock polymer consists of isoprene-styrene-vinyl pyridine, with IP forming the outer shell, and VP forming the inner shell. The geometry of this triblock polymer is seen to be a HCP arrangement as seen in the TEM image. The strength of the triblock polymer is higher than that of the diblock, as reflected by the the stress-strain graph. This increased strength is attributed to the tensile rubber-like isoprene domain added on the end.
Figure 2: Hybrid membranes are formed by CSP-NIPS as described in the paper. It basically involves evaporating the solvent to promote self-assembly of the mesoporous membranes, followed by non-solvent induced phase separation to render a mechanically strong basal structure. 2a shows the graded assembly, with finest pores (mesoporous) on the top, with solid macroporous structures on the bottom. 2b and 2c show the zoomed-in version of the same property in parent and hybrid membrane respectively.
Figure 3: Effect of evaporation times (ET) on the pore size. Lesser ET do not give sufficient time and concentration for the solute to self-assemble and orient perpendicularly to the base, as seen by macropores rising all the way till the top. Intermediate ET work good, but provide insufficient nucleation sites, while larger ET provide dense nucleation sites to render a perfectly nanoporous (meso-) structure on the top.
Figure 4: 4a and 4b show the arrangement of isoprene and vinyl pyridine domains using selective stains respectively, and in general matches with the assembly shown in figure 1. Long running mesopores due to the VP chains are shown in figure 4c.
Figure 5: Permeability is pH dependent, with the permeability rising with the pH, for both parent and hybrid membrane. 5b shows that rejection is higher for parent than hybrid membranes, implying improved sieving characteristics at the same pH for hybrid films. Pore size estimation by back-calculating the pore diameter from the solute rejection yields almost similar porosity for both parent and hybrid membranes.