Published: July 08, 2011 r2011 American Chemical Society 11062 dx.doi.org/10.1021/la2018314 | Langmuir 2011, 27, 11062–11070 ARTICLE pubs.acs.org/Langmuir Multilayered Poly(vinylidene fluoride) Composite Membranes with Improved Interfacial Compatibility: Correlating Pervaporation Performance with Free Volume Properties Quanfu An, †,‡ Jung-Tsai Chen, † Manuel De Guzman, † Wei-Song Hung, † Kueir-Rarn Lee,* ,† and Juin-Yih Lai † † R&D Center for Membrane Technology, Department of Chemical Engineering, Chung Yuan University, Chung-Li 32023, Taiwan ‡ Department of Polymer Science and Engineering, Key Laboratory of Macromolecule Synthesis and Functionalization (Ministry of Education), Zhejiang University, Hangzhou 310027, China ’ INTRODUCTION Ethyl acetate is one of the most practical fatty acid esters as it finds wide-ranging use as a quick-drying solvent. Its industrial production is chiefly by the Fischer esterification reaction of ethyl alcohol and acetic acid: CH 3 CH 2 OH ethylalcohol þ CH 3 COOH aceticacid a CH 3 COOCH 2 CH 3 ethylacetate þ H 2 O water The equilibrium can shift to the right by the removal of water, resulting in higher yield of ethyl acetate. This removal of the byproduct water can be done with a membrane by a perva- porative separation technique. 1 Several advantages are pro- vided with such a technique: easy process design, high selectivity, and low energy consumption. Pervaporation offers the possibility of separating azeotropes or liquid mixtures containing components with close boiling points that are difficult to separate by distillation. Pervaporation composite membranes have therefore been used to dehydrate aqueous ethyl acetate mixtures. 1À4 However, the permeation flux obtained was low. To increase the flux without sacrificing the selectivity, composite membranes with a hydrophilic skin layer on the top of a hydrophobic porous support have been applied. 5À9 Since a hydrophilic surface is necessary for water- selective pervaporation composite membranes, this study made use of a highly hydrophilic poly(2-hydroxyethyl methacrylate) (PHEMA) polymer as a layer deposited on a substrate by a process of spin-coating. Moreover, the application of PHEMA/ substrate composites for pervaporative separation has not been extensively investigated. 10À12 Nonetheless, because of the poor compatibility between a hydrophilic skin layer and a hydro- phobic sublayer, causing a weak adhesion at the interface, peeling often occurs when drying the coating layer. To over- come this shortcoming, researchers have developed some methods to modify the interfacial adhesion: plasma treatment, 13À17 interfacial cross-linking, 5,18 introduction of an adhesion layer, 19,20 grafting polymerization, 6,8,21À25 and chemi- cal modification. 26,27 On the basis of these studies, the conclu- sion is that a strong adhesion can be achieved either with the presence of a strong physical interaction such as hydrogen bonding or with the formation of chemical bonds between the active layer and the support. It was indicated that a membrane has a greater stability if interfacial layers are mutually bound by covalent bonding. Xu et al. 28 modified the surface of micro- porous polypropylene hollow fiber membranes by graft Received: May 16, 2011 Revised: July 8, 2011 ABSTRACT: A spin-coating process integrated with an ozone-induced graft polymerization technique was applied in this study. The purpose was to improve the poor interfacial compatibility between a selective layer of poly(2-hydroxyethyl methacrylate) (PHEMA) and the surface of a poly(vinylidene fluoride) (PVDF) substrate. The composite membranes thus fabricated were tested for their pervaporation performance in dehydrating an ethyl acetate/water mixture. Furthermore, the composite membranes were characterized by field emission scanning electron microscopy (FE-SEM) for morphological change observation and by Fourier transform infrared spectroscopy equipped with attenuated total reflectance (ATR-FTIR) for surface chemical composition analysis. Effects of grafting density and spin-coating speed on pervaporation performance were examined. The composite membrane pervaporation performance was elucidated by means of free volume and depth profile data obtained with the use of a variable monoenergy slow positron beam (VMSPB). Results indicated that a smaller free volume was correlated with a higher pervaporation performance of a composite membrane consisting of a selective layer of spin-coated PHEMA on a PHEMA-grafted PVDF substrate (S-PHEMA/PHEMA-g-PVDF). The composite membrane depth profile illustrated that an S-PHEMA layer spin-coated at a higher revolutions per minute (rpm) was thinner and denser than that at a lower rpm.