Synthesis of thin film composite membrane using mixed dendritic poly(amidoamine) and void filling piperazine monomers

Highlights

• Piperazine was used as void-filler in synthesizing TFC membrane.

• Direct quantification of TMC concentration changes during reaction by using UV–vis.

• Dendritic monomer able to render membrane with higher free volume.

• Enhanced membrane permeability with increase of PAMAM content.

• Rate of interfacial polymerization is controlled by monomer׳s diffusivity.

Abstract

Ethylenediamine cored poly(amidoamine) (PAMAM) has recently emerged as an alternative monomer in synthesizing polyamide thin film composite (TFC) membranes through interfacial polymerization due to its dendritic and hydrophilic nature. Despite the various PAMAM contents used, the membrane separation performances were marginally improved which can be ascribed to the larger PAMAM free volume. In view of this, piperazine, which has a smaller molecular size, was introduced as co-polymerized monomer during interfacial polymerization. Piperazine plays its role as void-filler and hence will produce a more compact structured membrane. The membranes׳ physicochemical properties were characterized using Scanning Electron Microscopy (SEM), Attenuated Total Reflectance-Fourier Transform Infra-red (ATR-FTIR), X-ray Photoelectron Spectroscopy (XPS) and zeta potential, while their performance was evaluated in terms of pure water permeability and inorganic salt rejection. The mixed monomer TFC membranes had synergistically improved permeability and rejection up to 0.3% (w/v) PAMAM content. Top surface of the TFC membrane was mainly dominated by PAMAM–TMC when higher PAMAM concentration was used during the reaction. The reaction is diffusion controlled due to the dense selective layer as well as low molecular diffusivity.

Introduction

Thin film composite (TFC) membrane is a sandwich-like membrane, consisting of a thin and dense film mounted on a porous support. Several routes were used to synthesis TFC membrane; interfacial polymerization (IP) is one of the most convenient and most reported routes [1]. IP is a process involved in the polycondensation at the interface between polyfunctional amine and acyl chloride, which are dissolved in two immiscible solvents [2]. With the IP technique, a TFC membrane with different support and separating layer properties can be optimized separately.

Over the years, lot of work have been done on improving membrane separation performances by changing the membrane surface chemistry and morphology. Selection of appropriate monomers is the key factor in determining the resulting membrane chemical properties and performances. For instance, the tightness of an RO membrane can be regulated using a mixed amine monomers of piperazine and m-phenylene diamine (MPD) with various compositions [3]. In their work, addition of piperazine produces membrane with a looser structure due to its molecular chair shape compared to planar and compact MPD-TMC layer. Enriching the membrane surface with hydrophilic groups such as sulfonic acid [4], [5], carboxylic acid [6] and amine group [7] did enhance the membrane surface hydrophilicity and hence improve the membrane permeability. Moreover, the presence of these active functional groups will contribute to the improvement of salt rejection due to the intensified co-ion repulsion effect. Some of the reported monomers have synergistic advantages in terms of molecular shape and chemical properties. For example, the addition of hyperbranched polyethyleneimine (PEI), which consisted of multiple branch consisting of repeating unit of amine group, was able to enhance both the membrane permeability and solute rejection as the flexibility of its alkyl chain allows some charged amine groups drifting into interior of the active layer [8]. Those ‘hanged’ charged amine will form ionic bridge with incoming ions without affecting much of the permeability. Synthesis of polyester TFC membrane with the use of a torus-shaped β-cyclodextrin (β-CD) in the presence of triethanolamine (TEOH) was found to be able to enhance membrane separation performances and anti-fouling performance [9]. This is because β-CD possesses a hydrophilic external surface due to the presence of abundant hydroxyl groups and inner cavity, which allows channeling of water.

Dendrimer, which consists of multiple arms with a high density of end functional groups, is a suitable candidate to produce a high performance membrane. An ethylenediamine cored dendrimer, poly(amidoamine) (PAMAM) which is one of the dendrimers that consist of ethylene diamine as end functional group and amide as well as tertiary amino at center core [10], [11]. Both the primary and tertiary amine groups are arranged regularly along the molecule chain, which makes it a useful hydrophilic material for membrane synthesis. Meanwhile, contributed by its high molecular monodispersity, the lattice cavities of PAMAM could serve as a water channel, which may enhance the water permeability of the resulting membrane.

Due to PAMAM unique features, it can be used in surface modification of RO membrane for antifouling enhancement purpose [12]. First use of PAMAM as a monomer in IP process was demonstrated by Lianchao et al. [13] who found that the membranes produced were positively charged and exhibited pH responsive property. The increment of PAMAM concentration and generation number will introduce high amino content on the membrane surface, which will then increase the membrane׳s hydrophilicity and thus lead to flux enhancement. Meanwhile, the protonation of free amino group will intensify the repulsion between membrane surface and co-ion in the feed. An ultrafiltration membrane made from blend of polysulfone and palmitoyl chloride modified PAMAM was found able to retain cadmium ion through metal complexation at lower operating pressure [14]. Meanwhile, a hydrophobic PTFE microfiltration membrane modified with hydrophilic hyperbranched PAMAM showed great permeability improvement and could be reused for several cycles in heavy metal water treatment [15].

Despite the multiple advantages of PAMAM, using PAMAM as a monomer in the synthesis of TFC membrane shows relatively low salt rejection due to its large free volume. In this work, TFC membranes were synthesized via interfacial polymerization of mixed PAMAM and piperazine with trimesoyl chloride. The feasibility of using piperazine as void-filler and the influence of piperazine addition towards interfacial polymerization were investigated.