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Latex Composite Ultrafiltration Membranes: Report

Subramanian Ramakrishnan (PI)

ACS PRF# 47954-G9

This proposal seeks to overcome current difficulties in current membrane manufacture by utilizing the advantages of particle self-assembly combined with tailoring particle surfaces to fabricate membranes with a wide range of narrow pore size distributions and surface compositions.  The focus will be to synthesize ultrafiltration (UF) membranes (10 – 50 nm in pore sizes) with higher fluxes and narrower pore size distributions than commercial membranes.  By placing an array of colloidal particles on a porous support and stabilizing the array, the interstitial spaces between the particles serve as narrow dispersed pores for size separations.

Results  (since August 2009)

1)      Synthesis and Characterization of Latex Composite Membranes:  Monodisperse Particles

Particles of 200 nm, 650 nm and 900 nm were successfully synthesized using an emulsion polymerization technique.  Membranes were then fabricated by depositing these particles on commercial supports and by heat stabilizing them.

Figure 1:  Pore size distribution of Supor 450 and 4 layers of 900nm latex deposited on top of Supor 450.The peak in pore size decreases to lower pore sizes with addition of particles and also results in a narrower pore size distribution.

Figure 2: Pore size as a function of number of layers for different size particles used in this work.

Figure 3:  Pore size for 6 layers of particles as a function of particle size.  It is found that the pore diameter is approximately 14 % of the particle size which is slightly lower than a hexagonal or FCC packing.

Figure 4 Comparison of water flow rate data for Supor 200, Supor 450 with approximately 4 layers of deposited 900 nm polystyrene particles, and PCTE 0.2

Figure 4 demonstrates that when 4 layers of 900 nm particles are deposited on a base membrane of Supor 450, a water flux comparable to that of Supor 200 is achieved.  The advantage here is that the latex composite membrane has a pore size of 0.12 microns while the Supor 200 has a pore size of 0.23 microns.  Thus using this technology one has formed a membrane with a narrower pore size and higher flux.

2)      Synthesis and Characterization of Latex Composite Membranes:  Particle Mixtures

In the previous section it was shown that it was possible to narrow pore size distributions and reduce pore sizes of commercial supports by depositing latex particles on top of base supports. All of the earlier composite membranes were fabricated with particles of a single size. The purpose of this section is to determine whether the pore size distribution of latex particle membranes can be further improved by using particles of more than one size. The idea is that the smaller particles can pack in between the pores of the larger particles resulting in smaller pores and narrower distributions. Another advantage to using mixtures of particles is that a large, thus cheaper, porous support can be used as a base to produce compound membranes of small pore size. Larger size particles could first be stabilized on the porous support and then smaller size particles can be used to reduce the pore size further.

            Membranes were synthesized using mixtures of 650 nm + 200 nm particles and 900 nm + 650 nm particles by two different methods – (1) layered particle membranes: the particles were laid down on the base support sequentially in layers. For example, approximately 2 layers of 650 nm particles were laid down on Supor 200 then stabilized and then 2 layers of 200 nm particles were filtered on top of the 650 nm particles. (2) mixed particle membranes: different amounts of the two size particles were mixed together and then filtered onto the base support. For instance, 200 nm and 650 nm particles were mixed and then filtered onto Supor 200. Only mixtures from two different size particles were used in this work.

Figure 5:  Top View SEM Images of Multi Size Mixed Packed Beds on Supor 450

Figure 6:  Comparison of normalized relative flow as a function of pore size for PCTE 0.2 and mixture membrane made up of 2 layers 900 nm and 2 layers of 650 nm particles on Supor 450

Figure 6 clearly demonstrates that it is possible to take a porous substrate and modify the pore size distribution using mixtures of latex particles and to yield smaller pore sizes. Figure 7 below is a comparison of the water flow rate for the mixture membrane and commercial PCTE 0.2. Figures 6 and 7 prove that it is possible to create composite membranes with narrow pore size distributions and low resistances to water flux. The mixture membrane shown allows for higher water flux than the commercial PCTE 0.2 membrane.

Figure 7. Comparison of flow rate as a function of applied pressure for Supor 100, PCTE 0.2, and mixture membrane made up of 2 layers 900 nm and 2 layers of 650 nm particles on Supor 450

Overall Conclusions:  It is possible to have good control of pore sizes and their distribution by forming membranes from colloidal particles.  The pore sizes were ~ 14% of the particle diameter and the water fluxes could be predicted using the Carmen-Kozeny equation for packed beds.  Using mixtures of particles, it was shown that one could achieve smaller pore sizes (~ 28 nm) and that one could achieve pore size distributions similar to commercial track etched membranes but at the same time have higher fluxes. 

A paper based on current work is under preparation for submission to a peer reviewed journal.

Impact of Research

1)      This work resulted in the first Masters Student (Erin Holley) graduate through the Materials Science program at Florida State University. 

2)      It is playing a key role in the undergraduate education of Velencia Witherspoon and Djeri Harris (both African American women) who are working in my lab.  Both of them want to pursue graduate school after their undergraduate studies.