Abstract
Abstract: Porous polymeric films were obtained by immersing a solution of cellulose acetate and high molecular weight poly(acrylic acid) into an ice-water mixture. The films, the porosity of which can be controlled (reversibly) by immersion into hot water for varying periods of time, were characterized by IR spectroscopy and lowvacuum scanning electron microscopy.
Introduction
The use of polymeric membranes for filtration and separation processes is a field which has attracted continuous attention from the scientific community due, among other reasons, to the important applications which have already been developed for the day-to-day human life. One outstanding example is desalination via reverse osmosis. Most of the procedures developed for manufacturing membranes are based on the use of polymer mixtures. In particular, the formation of voids and large cavities in cellulose acetate desalination membranes has been reported by various groups [1, 2] . Recently, considerable interest has been attracted by the use of membranes consistent with Donnan's exclusion mechanism [2] . Early studies [3] involved the preparation of poly(acrylic acid) films cast directly onto a finely porous support membrane composed of a mixture of cellulose esters. Also, some pioneering works [4] have reported the use of several commercially available polyelectrolytes and a number of supports as materials for sewage treatment. It seems, from experiences described in the literature, that poly(acrylic acid) is one of the most promising materials for producing membranes for wastewater treatment, and some high molecular weight materials of this sort, cast onto polysulfones [5, 6] , have shown adequate transport properties. In any case, in order to be able to produce a working membrane from the films, the available technology requires the use of a proper substrate or support. Also, the porosity produced in the cast films is basically a matter of chance, according to the specialized reports. The next step, thus, would be to develop a technology able to produce free-standing films which require no support and in which the porosity could, at least to some extent, be controlled as desired for a particular application. In this work, an alternative method for directly producing cellulose acetate membranes is described, resulting in a porous film with improved mechanical properties as compared to standard cellulose films and without the need of a support.
Experimental part
The films were prepared by using cellulose acetate (Fluka Chemika) with a weightaverage molecular weight (M w ) of 37 000, substitution rate of 40% acetyl groups, and poly(acrylic acid) (Aldrich Chemical Company Inc.), with M w = 15 000 and 30 000, respectively. (M w determined by gel permeation chromatography). 2.5 g of cellulose acetate were dissolved in 45 ml of glacial acetic acid at 25°C with constant stirring for 30 min. Then, 5 ml of either of the above poly(acrylic acid) samples were added, and the solution was heated at 60°C with constant stirring for 20 min. Then the solution was allowed to cool down to room temperature under room conditions. The solution was cast into a flat glass mold, 10 cm in diameter, floating on an ice-water mixture at 4°C, and it was allowed to set for 3 min. Then, the whole mold with the solution was immersed into the water bath until the film was formed. The film was collected from the mold and washed with water at 60°C for 3 min. The films were pressed into sheets of filter paper to eliminate the remaining water and keep them flat. A Perkin Elmer FTIR 1600 was used to directly characterize the films, along with a lowvacuum scanning electron microscope (JEOL LV-5900) operating at 20 kV. Fig. 1 shows a typical IR spectrum of the cellulose acetate employed for producing the porous films. The characteristic bands at 1748, 1234 and 1046 cm -1 are clearly observed. Fig. 1 . IR spectrum of the cellulose acetate utilized to prepare the films Fig. 2 shows the IR spectrum of the solution made of cellulose acetate and glacial acetic acid from which the porous films were produced. As can be seen, the intensity of the characteristic bands of the cellulose acetate increases. This is due to the incorporation of acetate groups from the glacial acetic acid into the cellulose acetate.
Results and discussion

Fig. 2. IR spectrum of the solution of cellulose acetate and glacial acetic acid
The reaction between cellulose acetate and poly(acrylic acid) can be followed by means of IR spectroscopy of the resulting films, as shown in Fig. 3a and b. As can be noticed there, the characteristic bands reflect the progress of the reaction, through substitution of the acetate groups of cellulose acetate by the carboxyl groups of poly-(acrylic acid). The molecular weight of the poly(acrylic acid) employed has no noticeable effect on the IR spectra, since both molecular weights employed in this work (viz., 15 000 and 30 000, as described above) yield basically the same result. However, it is extremely interesting to notice that the molecular weight of the poly-(acrylic acid) used is the key for the morphology of the films, as can be appreciated in the low-vacuum SEM micrographs of Fig. 4a and b.
Fig. 2. IR spectrum of the solution of cellulose acetate and glacial acetic acid
For a molecular weight of 15 000, the SEM image shows a network with a broad size distribution, ranging from 1.32 to 9.9 µm. Within the larger grains, small pores are visible. When poly(acrylic acid) with a molecular weight of 30 000 was used, a homogeneous porous film was obtained, the pore size distribution ranging from 0.4 to 2.8 µm in diameter. It is also worth mentioning that the resulting film was very flat and had an excellent mechanical behaviour, which will be reported, along with the corresponding transport properties, separately. By changing the molecular weight of the poly(acrylic acid), a wide variety of pore ranges can be produced.
Conclusions
Poly(acrylic acid) reacted with cellulose acts as a sort of plasticizer, which enables to produce films with improved mechanical properties, as compared to other cellulose-based membranes which have been proposed as filters [7] . The method described here does not require the use of a substrate to produce self-sustained membranes, which is an improvement over the current technologies [8] . Also, other reports indicate that the concentration of cellulose acetate required to produce working membranes is around 25 wt.-%, whereas the proposed methodology reduces this concentration down to near 5.5 wt.-%. Finally, through proper choice of the molecular weight of the poly(acrylic acid) modifier, the porous structure of the resulting films can be controlled. 

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