Exploring the Feasibility of Utilizing Nanopore-Based Ultrafiltration for the Purification of Graft-Onto Polymerization Products

Exploring an efficient way to purify the graft-onto polymerization products is an important but challenging problem. This work explores the feasibility of utilizing nanopore-based ultrafiltration for this purpose. First, a library of PPANb-g-PSNg-σ graft-onto products with polystyrene (PS) sidechain and poly(propargyl acrylate) (PPA) backbone has been prepared, and the effect of molecular parameters, including contour length (lc), sidechain radius (Rsidechain), and persistence length (lp), on the flow-assisted translocation and cutoff performance of PPANb-g-PSNg-σ through nanopores with diameter D = 20 nm has been studied. At a low macroscopic flow rate Q = 0.2 mL/h, it is found that for PPANb-g-PSNg-σ chains with Rsidechain > 1/2D, they are completely retained due to the size-sieving effect. More generally, it is found that the decrease of lc from ∼500 to ∼50 nm and Rsidechain from 18.0 to 3.9 nm facilitates the diffusive passage of chains through nanopores; surprisingly, the increase of lp from 5.4 to 22.2 nm also facilitates the process, which actually signifies the possibility of enhanced chain orientation. At a high macroscopic flow rate Q > 0.2 mL/h, the increase in lc and Rsidechain results in a larger critical macroscopic flow rate (Qc) for chains to squeeze into the nanopores. When Rsidechain > 1/2D, no lc-dependence is observed for the cutoff performance. Furthermore, it is found that the separation efficiency can be efficiently enhanced by increasing the solution concentration in the regime of C < C* with C* being the overlap concentration, which is demonstrated by one example of the practical purification process of the PPA780-g-PS41-60% model sample.