Preparation of PVDF/PMMA blend hollow fiber membrane via thermally induced phase separation (TIPS) method

Abstract

Porous poly(vinylidene fluoride) (PVDF)/poly(methyl methacrylate) (PMMA) blend hollow fiber membranes were prepared via thermally induced phase separation (TIPS) process. From the measurement of X-ray diffraction, the crystalline structure of PVDF hardly changed by blending with PMMA. When total polymer concentration was fixed at 30 wt%, water permeability of the blend membrane increased by increasing PMMA concentration. However, when PVDF concentration was fixed at 25 wt%, the trend of water permeability with the increase of PMMA concentration was contrary to the former case. Treating with acetone was applied to extract the PMMA from the obtained blend membranes. The extraction of PMMA was effective to increase water permeability. In addition, the extraction of PMMA brought about the improvement for the membrane elongation. The PVDF crystallinity was found to increase with the PMMA extraction due to the secondary crystallization. This was the reason for the elongation enhancement.

Introduction

Poly(vinylidene fluoride) (PVDF) as a semi-crystalline polymer has wide applications due to its excellent properties such as good mechanical strength, stability against vigorous chemicals and good thermal stability [1], [2]. Also PVDF has been widely studied because of its relatively good piezoelectric and pyroelectric response and the affluence of its polymorphic forms that have been used in the development of electronic devices [3], [4], [5] such as lithium ion batteries [6], [7], [8]. During the ultrafiltration process, high hydrophobic property and low fouling resistance of PVDF membranes lead to the protein adsorption, and thus membrane pores are blocked. A number of studies have been done to overcome the disadvantages of the PVDF membranes. One method is the chemical treatment of the membrane surface with a strong alkaline solution [9]. This kind of modification occurs only near the membrane surface. Another efficient method to improve the hydrophilicity of PVDF is polymer blending with hydrophilic polymers. As PVDF is highly miscible with oxygen containing polymers, which is related to the interaction between the fluorine atoms and carbonyl groups of the partner polymer [5], [10], several pairs of blends have been investigated, such as PVDF/poly(vinylpyrrolidone) (PVP) [11], PVDF/poly(ethylene glycol) (PEG) [12], PVDF/poly(acrylonitrile) (PAN) [13], PVDF/poly(vinyl acetate) [14], and PVDF/poly(methyl methacrylate) (PMMA) [6], [7], [8], [10], [15], [16]. Among these polymers, PMMA is more interesting due to its good compatibility with PVDF all over the concentration range in blend polymer [15].

PVDF has different crystalline phases. There are at least four different crystalline phases (α, β, γ, and δ phases) in PVDF, which could transform from one to the other under certain conditions. Electrical properties of different PVDF phases are a direct result of its crystalline structure. Among the four crystalline phases, the most common phase is α form, which could be produced during crystallization from the melt [3], [4], [5]. Because of the good piezoelectricity and pyroelectricity property of the β phase PVDF, most attention has been paid to obtain more β phase crystalline phases in the prepared samples [5]. The β phase PVDF can be obtained from non-polar α phase PVDF by various processes such as mechanical deformation, poling under large electric fields, and crystallization from the melt under high pressure or very high cooling rates [17].

Kim et al. studied effects of PVDF/PMMA composition and quenching rate on the crystalline structure of the PVDF from both polymer melt and cast solution [5]. They found that when PVDF/PMMA was quenched from a polymer melt, reduction of the crystallization rate upon addition of PMMA favors the formation of the β crystalline phase. The critical quenching temperature, above which the formation of the β phase was reduced, increased by adding PMMA. However, when PVDF/PMMA was cast from solution, the addition of the PMMA had minor effect on crystal phases, and solvent type was predominant factor determining the crystalline structure. However, Zhang and co-workers reported that the crystallization behavior of PVDF/PMMA blends was highly dependent on the composition of the blend solutions [10]. They observed that the β phase PVDF was obtained as the PVDF weight fraction was above 30 wt% and addition of 10 wt% PMMA could favor the growth of β phase of PVDF crystals.

Jungnickel and co-workers investigated the composition distribution of PMMA within PVDF spherulites [18], [19]. The kinetic competition between crystallization of the crystalline component and the chain diffusion of the non-crystalline component played an important role. It caused the PMMA composition profiles in the internal and the surrounding of spherulite. At high crystallization rate of the PVDF and low crystallization temperature, PMMA chains was caught and included in the growing spherulites and trapped in their inter-lamellar regions or between the lamellar stacks. For low crystallization rate of the PVDF and high crystallization temperature, however, PMMA chains can diffuse fast and spread evenly into the remaining zone. For intermediate range of crystallization temperatures, diffusion rate and spherulite growth rate were balanced. PMMA was partially rejected from the spherulite growth front into the melt zone. In this case, the concentration of the PMMA at the interface of the spherulites was higher than those in other cases.

Several papers have been reported about the preparation of the blend PVDF membranes via nonsolvent-induced phase separation (NIPS) method [20], [21], [22]. However, few papers have been reported on preparation of the flat blend membrane via TIPS method [6], [10]. Upon our knowledge, this is the first report about the preparation of the PVDF blend hollow fiber membrane via TIPS method. The aim of this work is to prepare porous PVDF/PMMA blend hollow fiber membrane via TIPS method. PVDF and PMMA were used in this study as crystalline and non-crystalline polymer. The effect of the PMMA concentration on the crystallization temperature was studied and then blend hollow fiber membranes were fabricated. The membrane performance was investigated in detail for the prepared membranes. Treating with acetone was applied to extract the PMMA from the obtained membrane. Effect of PMMA extraction on the water permeability, elongation and structure of the membranes was studied.