The Functionalization Study of PVDF/TiO2 Hollow Fibre Membranes Under Vacuum Calcination Exposure

In this study, polyvinylidene fluoride (PVDF) hollow fibre membrane was modified by adding TiO2. TiO2 presence affects the membrane structure becomes more less hydrophobic which makes the membrane less fouling. Membranes were made via dry-wet spinning method and calcined under vacuum condition by furnace (100, 300, and 500 °C). Besides, PVDF-TiO2 uncalcined membrane were also prepared as comparison to investigated the effect of calcination on hollow fibre membrane’s functional groups. Fourier Transform Infrared (FT-IR) spectra indicated that all PVDF-TiO2 membranes have bands of OH in the TiO2 at ∼1600 cm−1. Peaks of α-phase PVDF crystals appeared at ∼876, ∼876, and ∼872 cm−1 for uncalcined, 100 and 300 °C, while for 500 °C the PVDF peak only shows at 874 cm−1. The peaks at ∼1200 cm−1 represent CF2 groups. Peaks at ∼1400 cm−1 assigned to CH2 groups, but it does not observed for 500 °C. Deconvolution by Fityk software that shows calcination using vacuum condition gives the compounds gradually decomposes. At high temperature calcination lead the CH2 peak extremely lost.


Introduction
Nowadays, a method for removing salt of water through a selective barrier called membrane is known as desalination technology. To save more energy consumption, desalination via pervaporation is preferred to reverse osmosis (RO) because it only requires 1 bar pressure [1-3]. Pervaporation has been applied for water desalination, alcohol dehydration, and volatile removal [2, 4-10]. Pervaporation separates the mixture by partial vaporization. Polyvinylidene fluoride (PVDF) suits for desalination because of their high salt rejection [11]. Moreover, the hydrophobicity of membrane caused on fouling.
In the last decade, polymeric membranes immobilized with TiO 2 have gained much attention due to specific benefits. Firstly, chemical modification cannot be happened because there is no covalent bond formation between polymeric and catalyst. Secondly, different chemicals show different affinities for polymeric membranes [12]. TiO 2 was also chosen because it is inexpensive, non-toxic, and

Experimental
The fabrication of PVDF-TiO 2 hollow fiber membranes followed the process on previous research [34]. It consist of three stages (1) dope solution preparation to remove moisture with drying 21 wt% PVDF and 3 wt% commercial TiO2 at 50ºC for 24 h, (2) mixing PVDF (PVDF, kynar 760 powder series), commercial TiO 2 with DMAC (DMAc, QReC) as a solvent (3) spinning membrane through dry-wet spinning technique. The obtained PVDF/TiO 2 hollow fibre membranes were calcined at varied temperature using furnace vacuum for an hour. Membranes were characterized using Fourier Transform Infra Red (FTIR). Fityk was used to deconvoluted overlapping peaks in PVDF-TiO 2 material. Schematic set up spinning hollow fibre membrane can be seen in Figure 1, as follow.

. Results and discussion
The FTIR analysis was carried out to know the crystal structure of PVDF/TiO 2 hollow fibre. As can be seen in Figure 2. Strong bands at ~1600 cm -1 and below 800 cm -1 attributed to OH area in the spectrum of immobilized TiO 2 . Bands at 876, 876, and 872 cm -1 could be indicated as α-phase PVDF [35] for uncalcined, 100 and 300 •C, respectively. Small difference of α-phase PVDF band become very weak was observed at 874 cm -1 for 500 •C. This indicates that α-phase PVDF transformation to β-phase PVDF has occurred along with increasing temperature during vacuum calcination. This transition results similar to earlier study in literature [36]. PVDF crystalline has different phases such as α, β, and γ depending on processing methods [37]. Specifically, there are strong peak of CF 2 groups at ~1200 cm -1 for uncalcined, 100 and 300 with a weak peak of CH 2 at 500 •C. While, CH 2 groups found at ~1400 cm -1 . However, the CH 2 peak does not appeared in 500 •C. (CH 2 -CF 2 )n itself is the chemical structure in the PVDF molecules. Solvent impurities almost completely disappeared at higher calcination. This condition promotes TiO 2 crystallization at 500 •C [38].The result obtained in this study is same with Dzinun, Othman, Ismail, Puteh, Rahman and Jaafar [39] which fabricated dual layer of hollow fibre membrane. There are many overlapping peaks in PVDF. This peak can be deconvoluted using fityk software.  To confirm the network structure is affected by calcination temperature, a quantitative analysis was conducted by deconvolution of the FTIR patterns using Fityk software. Figure 3 illustrated Gaussian bands of IR spectra of PVDF/TiO 2 hollow fibre membranes which uncalcined and calcined varied temperature. Deconvolution method was applied to deconvolute the FTIR spectra by fitting the peaks until the deconvolution spectra approach the experimental data [4,40]. The peak envelope in the range 1600 and 700 cm -1 is assumed to consist of peaks components arising from the CH 2 , CF 2 , PVDF, and TiO 2 group. It is found that there are large reductions on the areas under the 1500 to 700 cm -1 wavelength after calcination process over 300 °C. The results in Figure 3 prove that the stretching βcrystal transformed into α-crystal by the calcination. The peak area value of PVDF/TiO 2 hollow fibre membrane which uncalcined and calcined to high temperature was presented on Figure 4. The uncalcined of PVDF/TiO 2 hollow fibre membrane shows five main peaks which consist strong TiO 2 , CH 2 , CF 2 and PVDF groups. When the samples calcined at 100-500 °C, the PVDF and both of C group gradually decomposed. The CH 2 and CF 2 groups extremely disappear at calcined temperature of 500 °C and left over the TiO 2 (1.69 unit area) and weak PVDF groups (0.51 unit area) based on Figure 4. It is only exhibited the TiO 2 group and small PVDF peaks. The α PVDF was transformed into β-crystal as increasing calcined temperature at 300 °C. βcrystal is the most desired crystal structure in PVDF as TTTT configuration which produce the highest dipole moment [41]. Uncalcined hollow fibre membranes showed sharp and narrow curve at absorption band of 1400 cm -1 . The absorption band at 1400 cm -1 is referred to the in plane bending vibration of CH 2 bond, which belongs to the PVDF chain (Figure 3). Reduced in absorbance for absorption band 1400 cm -1 signified low bending vibration it is due to at 500 °C that CH 2 groups are already oxidized [4,42]. As observed for calcined hollow fibre membranes at 300 °C was presented the low PVDF group of 1.38 which indicating the existence of β-crystal [41]. It concluded that PVDF/TiO 2 hollow fibre membrane with calcination temperature of 300 •C was the optimized membrane in this work due to the β-crystal of PVDF was increased.

Conclusion
This work shows that calcination temperature has a considerable influence on structure properties of hollow fibre membranes derived from PVDF/TiO 2 . The FTIR spectra of all PVDF/TiO 2 membranes indicated bands of OH in the TiO 2 at ~1600 cm -1 . Peaks of α-phase PVDF crystals appeared at ~876, ~876, and=~872 cm -1 for uncalcined, 100 and 300 °C, while for 500 °C the PVDF peak only shows at 874 cm -1 . The peaks at ~1200 cm -1 represent CF 2 groups. Peaks at ~1400 cm -1 assigned to CH 2 groups, but it does not observed for 500 °C. Deconvolution by Fityk software that shows calcination using vacuum condition gives the compounds gradually decomposes. At high temperature calcination lead the CH 2 peak extremely lost due to oxidized reaction. The highest β-crystal PVDF properties is necessary obtained by calcined at temperature 300 °C. It is considered that in Gaussian peak component are related to the different calcined temperature which allow us to design the polymer structure of PVDF/TiO 2 based on its peak intensities.