Influence Of The Annealing Treatment

Desalination 200 (2006) 437–439

Structure and morphology of membranes prepared from polyvinyl alcohol and silver nitrate: influence of the annealing treatment and of the film thickness S. Clémenson, P. Alcouffe, L. David, E. Espuche* Ingénierie des Matériaux Polymères, IMP, UMR CNRS 5627, Laboratoire des Matériaux Polymères et des Biomatériaux, Université Claude Bernard-Lyon I, Bât. Istil, 15 Bd A. Latarjet, 69622 Villeurbanne Cedex, France Tel. +33 (0)4 72 43 27 01; Fax +33 (0)4 72 43 12 49; email: [email protected] Received 26 October 2005; accepted 2 March 2006

1. Introduction Organic–inorganic hybrid materials have attracted great attention because of their potential to combine the features of organic materials with those of inorganic materials. Particularly, organic–metal hybrid materials such as polymer–silver composites are promising functional materials in domains such as optical, antimicrobial and electrical applications. Two main methods can be used to prepare these materials: dispersion of preformed inorganic fillers [1] and in situ generation of inorganic fillers [2–4]. The second approach has become predominant these last years and a lot of work has been concerned with polyvinyl alcohol as matrix and silver nitrate as metal precursor [3–6]. It has been shown that very different morphologies could be obtained as a function of the preparation conditions leading to different properties. The aim of this work is to study the influence of three parameters, the film thickness, the precursor

*Corresponding author.

concentration and the annealing treatment on the structure and morphology of the hybrid films. 2. Results and discussion Silver nitrate (AgNO3 > 99%) was purchased from Aldrich and polyvinyl alcohol (PVA, Mw = 22,000, percentage of hydrolysis = 88) from Acros Organics, France. Metal-doped polyvinyl alcohol solutions were prepared by first dissolving the metal complex in deionised water, and then adding an aqueous solution of the PVA. The complex content was calculated in order to obtain a theoretical weight percent of Ag in the final PVA film equal to 2.5%, 5% and 7.5%, respectively. Pure PVA films and hybrid films were prepared by two methods using 15 wt.% polymer (excluding the complex) solutions. Films of 10 mm thick were obtained by casting the solutions onto an amorphous PET plate using a doctor blade. Thinner films (about 150 nm) were obtained by spin coating the polymer solutions under a rotation speed of 3000 rpm during 60 s

Presented at EUROMEMBRANE 2006, 24–28 September 2006, Giardini Naxos, Italy. 0011-9164/06/$– See front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.desal.2006.03.395

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onto UV transparent glass supports and also onto TEM grids for direct TEM observation. The coatings were allowed to dry in open air atmosphere for 12 h. After this step, the 10-mm thick films could be peeled off the PET plate to give free standing films. The films and the coatings were then annealed during 1 h at different temperatures ranging from 50° C to 160° C. The structure and morphology of the films were studied by UV spectroscopy, FTIR spectroscopy, X-ray diffraction analysis, differential scanning calorimetry, scanning electron microscopy and transmission electron microscopy. The UV visible spectra of the films containing 2.5 wt.% of silver are presented in Fig. 1. No absorption in the range 350–600 nm was observed for the untreated film and the film annealed at 50° C. A broad peak appeared at 415 nm for the films treated at a temperature higher than 90° C. The intensity of this peak increased as the annealing temperature increased and this trend was observed whatever be the silver content and the film thickness. According to the literature results, this band could be attributed to chelate formation of Ag+ coordinated with the hydroxyl group of PVA [3] or to silver particle formation [4]. The diffraction peaks of f.c.c. (facecentred cubic) crystalline silver were clearly 4 1 h at 160°C 1 h at 110°C 1 h at 90°C 1 h at 50°C Untreated

3.5 3

A

2.5 2

200 nm

Fig. 2. TEM image of the 150 nm film containing 5 wt.% of silver and annealed at 110° C.

observed on the diffraction patterns of our hybrid films annealed at temperatures higher than 90° C. The evolution of the morphology, that is the size and the dispersion of the particles, was studied as a function of the film thickness, the complex concentration and the annealing temperature. The size of the particles decreased when the film thickness decreased and when the initial silver nitrate concentration increased. As shown in Fig. 2, a homogeneous dispersion of nanoparticles in the range 7–15 nm could be achieved on thin films by annealing at temperatures below the melting temperature. The morphology developed after annealing at 160° C has not been determined yet. It could be different since the thermal treatment is made in this case just at the beginning of the melting peak thus in a medium of higher mobility. TEM and MEB analysis are under progress to determine this morphology.

1.5 1 0.5 0 200

3. Conclusions 300

400 500 600 Wavelength (nm)

700

800

Fig. 1. UV/VIS optical absorption spectra for films containing 2.5 wt.% of silver after different annealing treatments.

Hybrid films, prepared from PVA and AgNO3, were annealed at different temperatures. It was shown that a thermal treatment at temperatures higher than 90° C led to the formation of metallic crystalline silver. The size of the

S. Clémenson et al. / Desalination 200 (2006) 437–439

particles decreased as the film thickness decreased and a homogeneous dispersion of nanometer sized particles could be obtained. The influence of annealing at high temperature, a temperature located at the beginning of the melting peak, is under study and could lead to a different morphology. Future work will compare the effects of annealing treatments and of a UV irradiation using a laser. Functional properties will be studied and related to the developed morphologies and structures.

[2]

[3]

[4]

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