Supercritical Carbon Dioxide-Treated Electrospun Poly(vinylidene ﬂuoride) Nanoﬁbrous Membranes: Morphology, Structures and Properties as an Ionic-Liquid Hosta
Wu Aik Yee, Shanxin Xiong, Guoqiang Ding, Chien Anh Nguyen, Pooi See Lee, Jan Ma, Masaya Kotaki, Ye Liu, Xuehong Lu*
A reverse-barrier technique is used to enable the treatment of electrospun poly(vinylidene ﬂuoride)nanoﬁbrous membranes with supercritical carbon dioxide. The treatment induces the formation of nanopores and extended-chain b crystallites of small lateral dimensions in the nanoﬁbers. It also creates interﬁber junctions, resulting in a remarkable improvement in mechanical properties of the membranes. The treated membranes are able to retain their shape very well after loading with an ionic liquid(IL). The ionic conductivity of the IL-loaded membrane is very close to that of the neat IL.
In the past few years, ionic liquids (ILs) have been increasingly explored as the electrolytes in various
X. Lu, W. A. Yee, S. Xiong, G. Ding, C. A. Nguyen, P. S. Lee, J. Ma School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798,Singapore E-mail: email@example.com M. Kotaki Division of Advanced Fibro Science, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan Y. Liu Institute of Materials Research & Engineering, 3 Research Link, 117602, Singapore
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electrochemical devices due to the excellent thermal, chemical, and electrochemical stabilities of the ILs as well as their high ionic conductivity over a wide temperature range. Many polymers have been investigated for hosting the ILs,[1–6] among which poly(vinylidene ﬂuoride) (PVDF) and its copolymers have been identiﬁed as one of the mostsuitable hosts because of their high polarity, low cost, and good stabilities.[5,6] It has been demonstrated that IL/PVDF gel electrolytes can render electrochemical devices excellent long-term performance. Although IL/PVDF gel electrolytes exhibit good stabilities, the ion transportation in the gels is slower than that in neat ILs due to the highly viscous nature of the gels. By hosting ILs inporous polymer matrices, higher ionic conductivities could be achieved. Different techniques,
Macromol. Rapid Commun. 2010, 31, 1779–1784 ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
W. A. Yee et al.
such as chemical foaming and immersion precipitation, have been attempted to induce porous morphology of PVDF-basedpolymers.[8–10] Recently, porous PVDF membranes consisting of nanoﬁbers have been produced via electrospinning.[5,11,12] This facile technique is suitable for large-area fabrication of porous membranes with uniform thickness and high porosity. Although the ion transport properties of the IL-loaded electrospun PVDF membranes have been demonstrated in various electrochemical devices,[5,13] aprevious research also shows that such membranes are mechanically weak. Through thermal treatment under high pressure, mechanical properties of electrospun PVDF membranes could be improved. However, this occurs at the expense of higher crystallinity, which may decrease the IL-absorbing ability of the membranes. An alternative high-pressure thermal treatment is the treatment with supercriticalcarbon dioxide (SCCO2). It was reported that the SCCO2 treatment could result in foaming,[16,17] while having no signiﬁcant impact on crystalline structures of the PVDF bulk ﬁlms. In this work, by adopting a reverse-barrier technique, electrospun nanoﬁbrous membranes were successfully treated with SCCO2 for the ﬁrst time. Herein we report the effects of the SCCO2 treatment on structures and...