Thermoresponsive Membranes from Electrospun. Mats with Switchable Wettability for Efficient

Transcription

1 Thermoresponsive Membranes from Electrospun Mats with Switchable Wettability for Efficient Oil/Water Separations Yan Liu, a,b Sinem Tas, b Kaihuan Zhang, b, Wiebe M. de Vos, c Jinghong Ma, a,* and G. Julius Vancso b,* a State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai, P. R. China. mjh68@dhu.edu.cn b Materials Science and Technology of Polymers, MESA+ Institute of Nanotechnology, University of Twente, P.O. BOX 217, 7500 AE Enschede, the Netherlands. g.j.vancso@utwente.nl c Membrane Science and Technology, University of Twente, P.O. Box 217, 7500 AE Enschede, the Netherlands. 1

2 Synthesis of macroinitiator Br-PCL-Br The macroinitiator Br-PCL-Br was synthesized as follows: OH-PCL-OH (5g) was dissolved in anhydrous dichloromethane (50 ml) with TEA (500 μl) under argon protection and cooled in an ice water bath. Subsequently, an excess of BIBB (430 μl) was added into the solution dropwise. The reaction mixture was slightly stirred for 24 hours to produce the initiator. The final product was precipitated by an excess of methanol, following by filtration and subjected to three precipitation cycles. The macroinitiators were stored in the nitrogen box for the electrospinning process. 1 H NMR spectra proved the formation of end groups with active Br for ATRP polymerization (yield: 95.9 %). The results are shown in Figure S1. Samples were dissolved in CDCl 3 and the spectra were recorded at room temperature. Chemical shifts (δ) were expressed with respect to the CDCl 3 signals. The 1 H NMR spectrum of HO-PCL-OH presents the 2

3 characteristic signals of methylene ( CH 2 ) groups in PCL (δ=1.31, 1.58, 2.24, and 3.99 ppm). After using BIBB to substitute all OH end groups, a new signal at 1.96 ppm appeared in the spectra due to the introduction of methyl ( CH 3, δ=1.86) groups in the structure of BIBB. Figure S1. 1 H NMR of HO-PCL-OH and macroinitiator Br-PCL-Br. PCL membranes featuring Br terminated macroinitiators Porous membranes decorated with initiators with two different thickness values were fabricated by electrospinning. The ratio of the substrate PCL (Mn 80,000 g. mol 1 ) and the macroinitiator Br-PCL-Br was 100:20. The concentration of PCL substrate in solvent was 10 wt% and the final ratio of the Br was 0.27 wt%. The pre-spining solution was 3

4 stirred overnight until complete dissolution. The electrospinning set-up consisted of a high voltage power supply, spinerette, and a rotating drum. The voltage of the transverse electric field between the tip and the rotating drum was 13.5 kv. The injection rate controlled by a pump (KDS 100 Legacy Syringe Pump, KD Scientific) was 1 ml/hour. The distance between the tip and the rotating drum was 13 cm and the speed of the drum was set to 400 rpm. The diameter of the drum was 15 cm as demonstrated in Figure 1. After 1 h of spinning, porous membranes decorated with Br terminated macroinitiators in two different thicknesses were prepared. These membranes were removed from the drum and transferred into an oven at 40 C to evaporate the solvent. Characterization of membranes The membranes were characterized by FTIR, DSC, swelling, and microscopy. 4

5 Figure S2. FTIR of membranes. (a) PCL membranes, (b) PCL membranes decorated with macroinitiators, and (c) PCL-PNIPAM thermoresponsive porous membranes. Figure S3. DSC of PCL membranes decorated with macroinitiators and PCL-PNIPAM thermoresponsive porous membranes.. Figure S4. Swelling ratio of PCL-PNIPAM membranes. 5

6 Estimation of the brush chain length and grafting density From the distribution of the fibers diameter, we can estimate the value of the thickness of brush layers on the curved fibers. We obtain the value of 250 nm, which is larger than the thickness of high-density PNIPAM brushes on flat surface obtained under similar polymerization conditions. 1 As the calculation of the chain length depends on the swelling ratio of PNIPAM brush layer, we estimate the chain length using the PNIPAM from flat surface by our previous work. 2 The molar mass (related to chain length): NM =0.38 M 0 (h swollen ) 3/2 /(h dry ) 1/ g/mol The grafting density: 0.21 chains/nm 2 σ = ρ 0 h dry N A /(NM 0 ) Considering the influence of the curved surface and the porous structure, the value of the grafting density at the fiber-brush interface will be larger than 0.21 chains/nm 2. In addition, based on the results of XPS added into the Supporting Information as Figure S7, the mean value of [Br]/[C] is , which can be used to estimate the 6

7 initiator density according to the reference. 3 The Br density on the surface are finally estimated to be about 0.88 chains/nm 2. Therefore, the grafting density will be in the range of chains/nm 2. Rough estimate of the pore size by Darcy s law. Considering the porosity and tortuosity, a rough estimate of the pore size can be obtained by Darcy s law. F = kδp μl, k = εr2 τ8 ε porosity, r pore radius, τtortuosity Based on flux values, and assuming a swollen thickness of 150 μm, and a porosity (fractional surface area that is pore of the membrane) of 0.1, the estimated pore size above LCST is approximately 50 nm, and the pore size below LCST is around 20 nm. The reason of the small pore size could be explained by the staggering of the pores of different nanofiber layers in the membrane. 7

8 Here, we need to mention that this estimate is relatively rough as the permeability of the membrane will be different at temperatures below LCST and above LCST. Figure S5. Distribution of nanofibers diameter in membranes. (a) PCL membranes decorated with macroinitiators and (b) PCL-PNIPAM thermoresponsive porous membranes. 8

9 Figure S6. SDS stabilized oil/water separation. (a) UV-Vis absorbance of the initial emulsion filtrates at 20 C and 50 C. (b-d) Optical images of the initial emulsion and filtrates at 20 C and 50 C. Figure S7. XPS of nanofibers featuring Br terminated macroinitiators (Five positions have been measured, the values of the Br concentration are 0.25%, 0.32%, 0.21%, 0.32%, 0.28%). The mean value of the Br concentration is 0.28 % and the mean value of C concentration is %. References 9

10 [1] Chen, Q.; Kooij, E. S.; Sui, X., Padberg, C. J.; Hempenius, M. A.; Schön, P. M.; Vancso, G. J. Collapse from the top: brushes of poly (N-isopropylacrylamide) in cononsolvent mixtures. Soft Matter 2014, 10, [2] Klein Gunnewiek, M.; Benetti, E. M.; Di Luca, A.; van Blitterswijk, C. A.; Moroni, L.; Vancso, G. J. Thin polymer brush decouples biomaterial s micro-/nanotopology and stem cell adhesion. Langmuir 2013, 29, [3] Xu, F. J.; Wang, Z. H.; Yang, W. T. Surface functionalization of polycaprolactone films via surface-initiated atom transfer radical polymerization for covalently coupling cell-adhesive biomolecules. Biomaterials 2010, 31,