INSTITUTE OF PHYSICS PUBLISHING NANOTECHNOLOGY Nanotechnology 16 (2005) 2852–2860 doi:10.1088/0957-4484/16/12/020 Ultra-fine polyelectrolyte hydrogel fibres from poly(acrylic acid)/poly(vinyl alcohol) Lei Li and You-Lo Hsieh 1 University of California at Davis, CA 95616, USA E-mail: ylhsieh@ucdavis.edu Received 27 June 2005, in final form 8 September 2005 Published 14 October 2005 Online at stacks.iop.org/Nano/16/2852 Abstract Ultra-fine fibrous membranes were prepared by electrospinning of aqueous mixtures of poly(acrylic acid) and poly(vinyl alcohol) at 17%–83% PAA or 0.14–3.5 COOH/OH molar ratios and cross-linked by heat-induced (140 ◦ C, 5 min) esterification. The fibre diameters increased from 270 to 450 nm with increasing PAA contents. The swelling ratio of the fibrous hydrogels increased up to 31 times their dry weight with increasing pH from 2 to 7, and most significantly between pH 4 and pH 5. The liquid uptake of the fibrous hydrogels was attributed to both the liquid diffused into the fibres and that held in the inter-fibre pores. About 53%, 35%, 43% and 37% of the swelling was contributed by the liquid in the inter-fibre pores when the fibrous hydrogels were swollen at pH 2, 4, 5 and 7, respectively. The fully swollen fibrous membranes could be triggered by an applied electric field to swell further. Such additional swelling was dependent upon the polymer compositions, strength of electric field and pH, and was reversible. 1. Introduction Hydrogels are three-dimensional hydrophilic polymer net- works that can swell to many times their mass and volume in aqueous environments. Polyelectrolyte hydrogels are known to exhibit additional swelling in response to environmental stimuli, such as pH, polarity of solvent, ionic strength, or elec- tric field [1–3], and have been studied for applications such as sensors and actuators [4–6], drug delivery [7–9] and entrap- ment of biomolecules [10]. Hydrogels prepared from mixtures of poly(acrylic acid) (PAA) and poly(vinyl alcohol) (PVA) are among the most investigated polyelectrolyte hydrogels. As in any polymeric solids and hydrogels, the stimulus-triggered responses are diffusion controlled and the kinetics is proportional to the specific surface. In the case of PAA/PVA hydrogel thin films [11, 12] and rods [13, 14], the magnitude and speed of responses to the applied stimuli have been found to be inversely related to their thickness and diameters [11, 13, 14]. For fully swollen PAA/PVA hydrogel rods placed in an electric field (10 V cm −1 ), no deflection was observed when the diameters exceeded 10 mm [14]. The rate in which the rods deflected increased 25-fold (from 0.04 to 1 mm s −1 ) when the diameter 1 Author to whom any correspondence should be addressed. decreased to less than an eighth (from 10 to 1.2 mm) [14]. For thinner rods, the degree of bending in an electric field (30 V in 0.1 M Na 2 SO 4 ) increased from 30% to 85% with decreasing diameters from 1.6 to 0.6 mm [13]. As the diameters of these rod-shaped hydrogels are 30 or more times the 20– 50 µm diameters of conventional fibres, hydrogel fibres should exhibit faster responses. Further reducing fibre sizes to the sub- micrometre range would significantly improve the response. The capability of making nano- and micro-fibres from water-soluble polymers including polyelectrolytes [15–18] by electrospinning offers one way to create hydrogels with nano-and micro-structures. In our laboratory, we have successfully generated fibrous hydrogels composed of ultra- fine fibres with diameters in the range of 500 nm–1.2 µm by electrospinning of PAA/β -cyclodextrin [3] and PAA/poly(N- isopropylacrylamide) (PNIPAAm) [19] mixtures followed by subsequent heat-induced cross-linking. These fibres were three orders of magnitude finer than the thinnest rods reported before. They also exhibited pH- and temperature- responsive behaviour. This article reports our attempt to generate fibrous hydrogels composed of even finer fibres by electrospinning PAA/PVA aqueous mixtures followed by cross-linking via heat-induced esterification. The effects of the polymer compositions and the cross-linking conditions on the morphology, packing and stability of the hydrogel fibres were 0957-4484/05/122852+09$30.00 © 2005 IOP Publishing Ltd Printed in the UK 2852