(173k) The Growth of Glycidyl Methacrylate on Ultrafiltration Membrane: Spatial Control on Surface Initiated Aget-ATRP with Chain End Potential Functionalities

Arijit Sengupta¹, Blaine M. Carter¹, Xianghong Qian², S. Ranil Wickramasinghe¹*

¹Ralph E Martin Department of Chemical Engineering, ²Department of Biomedical Engineering, University of Arkansas, Fayettteville, AR 72701

Corresponding author: swickram@uark.edu

Abstract

Surface modification is frequently used to tune membrane properties [1]. Atom Transfer Radical Polymerization (ATRP) was found to be one of the most convincing methods for controlled polymer growth on the outer surface and inside the pore of the membrane depending upon the associated chemistry and pore size [2]. ATRP has been shown to provide great control over the grafted nano structure [3]. Though the ATRP showed very attractive features, the most challenging step of the ATRP is to keep air free or oxidant free environment to sustain the modification [4]. The less stable Cu in +1 oxidation states is mainly responsible for the sustaining of ATRP reaction. The surface initiated Activator generated electron transfer (AGET) ATRP provides the advantages of ATRP removing the stringent condition of traces of oxidant free environment [5]. Ultrafiltration membranes are having large pore size (1-100 nm) frequently being used for size exclusion, pressure driven membrane based separation for protein concentration and buffer exchange. Though the controlled surface modification is the remedy protein fouling of UF membrane; but no spatial control on the modification was so far reported involving AGET-ATRP on UF membrane which is very important because of the large pore size provides possibilities of outside surface or inside pore modification [6].

An attempt was made to have spatial control of the modification on external versus internal pore surface of regenerated cellulose (RC) ultra-filtration (UF) membrane by glycidil methachrylate (GMA) polymers using activator generated electron transfer (AGET) atom transfer radical polymerization (ATRP). This controlled polymerization was optimized for GMA on RC UF resulting epoxy functionalized polymer brush end which on subsequent ring opening generates variety of chain end functionalities. The optimization of the experimental parameters of the surface initiated AGET ATRP revealed that the increase in relative concentration of the reductant i.e. ascorbic acid to the Cu makes a constant ratio of the two oxidation states of Cu for sustaining the AGET-ATRP, while the increased ATRP time can lead to the formation of longer polymer chain on the membrane. With increase in the initiator concentration during initiator immobilization step, its density either on the outside surface of the membrane or on the inside surface of the pore enhanced resulting decrease in the water flux. 100 mM initiator concentration with 1 min of immobilization followed by AGET ATRP for 2 hours with 2M monomer concentration with ascorbic acid to copper ratio 0.4 was found to be the optimized experimental condition. The appearance of carbonyl peak in Infra-Red spectroscopy confirmed the initiator immobilization step on the RC UF membrane while the peak due to epoxy ring confirmed the AGET-ATRP. The opening of epoxy ring by the treatment of sodium sulphonete and monoethanol amine was signature by X-ray Photoelectron Spectroscopic analysis. The performance of the membrane in terms of change in water flux and rejection of specified protein also confirmed the tuning of modification on the outer surface of the membrane vis a vis internal pore surface.

The interfacial modification using different pore filling solvents were found to bring the surface selective modification of the RC-UF membrane compared to the non-selective solution phase modification. The nature of the pore filling solvents, their viscosity coefficient, the interaction of initiator molecules with the pore filling solvents and the solvent of reaction played significant role in tuning the spatial selectivity. Depending upon the interaction of the initiator with these solvents the spatial selectivity of the plugging of initiator and subsequent polymer growth will be determined. The initiator needs to be partitioned between the pore filling solvent and the solvent of reaction and followed by diffusion to the pore to be immobilized. More the partitioning of the initiator into the pore filling solvent more will be the chance of immobilization inside the pore. Lesser the viscosity of the pore filling solvent lesser will be the probability for the initiator to diffused through it and plugged on the surface of the pore. The ratio of flux of ATRP to the flux of initiator immobilization was found to follow the trend: glycerol > L 64 > PEG 400 > ethanol > acetonitrile ~ solution. Since acetonitrile is the solvent used for initiator immobilization, using it as pore filling solvent showed the similar scenario of solution phase immobilization and hence no spatial selectivity was observed. For glycerol, the ratio was found to be 0.9 indicated the selective plugging of the initiator on the external surface of the membrane not on the surface of the pore. High immiscibility of glycerol and acetonitrile makes the partitioning of BIB into the glycerol difficult and again the higher viscosity coefficient of the glycerol makes the diffusion of the BIB even more difficult to reach to the surface of the pore. Therefore, probably, only external surface got modified in case of glycerol as pore filling solvent. In case of other solvent, depending upon their miscibility with acetonitrile and viscosity coefficient, there will be selective initiator immobilization on the external surface over internal surface of the pore as indicated by the flux ratio measurement. The maximum selectivity was found to be achieved by using glycerol as pore filling solvent due to its highly viscous nature as well as immiscibility with reaction medium.

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