SYNTHESIS AND PROPERTIES OF POLYACRYLAMIDE-BISMUTH HALOGENATED HYBRIDS

When solid polyacrylamide (PAam) is added to a solution of bismuth chloride, a polymeric salt of 
composition (CH2-CH-CONH2)n BiCl3 (n > 10) is obtained. This amorphous material is transparent in 
the visible. In this hybrid, bismuth seems to be coordinated both to amine and carbonyl groups. When 
the PAam-BiCI3 hybrid is swollen with a KI solution, the corresponding iodide salt is formed with 
probably small particles of Bil3 precipitated into the polymeric matrix. Chemical, thermal, and optical 
(in the UV, visible, and infrared regions) properties of the salt and of the composite are studied.


INTRODUCTION
Semiconductors in nanoparticle form have special electronic properties compared to those of bulk materials.They are studied either from a theoretical point of view (quantum confinement) or for their potential applications in optical devices because of their non-linear optical properties.
CdS, CdSe, and Bil 3, in nanoparticle form, present a great interest.In the case of CdS and CdSe, such particles can be obtained either by precipitation in inorganic glasses [1], or as colloidal dispersions [2], or dispersed in polymer (PMMA [3], NAFION  [4]).Most of the studies devoted to bismuth iodide have been achieved on colloidal particles dispersed in various liquids (see [5] for example).
Polyacrylamide (PAam) is a water soluble polymer.It interacts with many metal cations in hydrous solutions.Gels and polymer salts can be obtained by desiccation of these .solutions[6].Optically homogeneous films can be obtained with most of the soluble inorganic chlorides.We have prepared PAam-MCIx hybrids with M Li/, Ti3+, Cr3+, Mn2/, Co2/, Ni2/, Cu2/, Zn 2/ and Cd2/; in the case of the cadmium hybrid we have also prepared a composite PAam-CdS in which cadmium sulfide is present as nanoparticles [7].Therefore, it was tempting to prepare the bismuth iodide nanoparticles dispersed in a PAam polymeric matrix.The present work is devoted to the study of hybrids obtained by reaction of polyacrylamide in hydrochloric acid solutions of trivalent bismuth and to the precipitation of bismuth iodide particles in these materials.

EXPERIMENTAL CONDITIONS
PAam used in this study was a commercial product (Aldrich, M.W. 5-6 106, Tg 165-180C).Solutions, with various concentrations of Bi3+, were prepared by dissolving Bi203 (PROLABO) in hydrochloric acid (pH 1-2).PAam was slowly added to 25 cc of this solution.After few minutes a viscous solution was obtained.
It was poured on a polyethylene or TEFLON surface and dried at 30C for 24 h.A solution of potassium iodide with a concentration corresponding to the amount of BiCl3 in the film (BiCl3 + 3K1 --Bil3 + 3 KC1) was added.The yellow-reddish film obtained was washed with water or acetone to remove the potassium chloride formed.Then it was dried using the same conditions as the PAam-BiC13 films.The films had a thickness of 10-20 lam.
The IR spectra were recorded using a Perkin-Elmer 983G spectrometer in the 4000-200 cm -1 range.PAam, PAam-BiCI3, and PAam-Bil3 films were deposited on a silicon window.X-ray diffraction patterns (Cu Ka) were obtained by superimpos- ing several films in order to obtain a thickness of about 50 lam.UV-Visible spectra were recorded from 190 up to 800 nm with a VARIAN CARY 2415 spectropho- tometer.TGA measurements were performed in the range 20-500C using a TAG24 SETARAM under argon atmosphere.A DSC, fluxmeter type, built in our laboratory was used for the determination of Tg [13].

Thermal Properties
Figure 1 shows the TGA curves of a film of composition PAam40, BiC13 and that of the corresponding iodide compound under argon.The weight loss below 120C corresponds to the evolution of water.Both hybrids are stable up to about 250-270C.At higher temperature, they decompose into NH3, H2, and CO.They seem to be slightly less stable than pure PAam which does not decompose below 300C [8].
In the case of a film of composition PAam40, BiC13, the DSC thermogravimetric graph shows an endothermic transition at 256C.It could be attributed to Tg of this hybrid.Similar transition in the same range was not observed in the corresponding iodide hybrid.
Properties in the UV-Visible Region Transparent films are obtained when the ratio [CH2-CH(CONH2)]/BiC13 is higher than 20.When the BiCI 3 concentration is higher, the films are opalescent.The transparent films remain stable provided they are stored in dry atmosphere.Under humidity, and after a few days of exposure at ambient atmosphere, the films become opalescent due to the hydrolysis of bismuth chloride.The bismuth iodide films, stored in dry atmosphere, are nearly transparent (yellow-reddish colored) when the concentration of bismuth iodide is low.
The UV-visible transmission spectra show a strong evolution from pure PAam to PAam-BiCI3 and PAam-Bil3.The cut-off is shifted from 250 nm for pure PAam to 365 nm for [PAam]2oBiC13 and to 470 nm for the corresponding iodide film (fig.2).

X ray Analysis
Pure PAam films are amorphous.
[PAam]nBiCl3 films are also non-crystalline when n is higher than 10.When n is lower, the opalescent films present few lines.These lines does not correspond to any identified phase (bismuth chloride, oxychloride or oxide hydroxide chloride).In the case of transparent [PAam]nBil3, the films are amorphous; however, a broad band appears around 3.30 ]k corresponding to the strongest line of Bil3 (hkl 113) [9].

Infrared Spectroscopy
The characteristic peaks of polyacrylamide are the two peaks maximum absorbance of v(NH2) absorption band at 3200 cmand v(NH2) at 3340 cm-1, and that of C O stretching at 1760 cmand (NH) amide at 1625 cm -(fig. 3).The IR spectra of the PAam-Bil3 show a decrease in the sharpness of the two peaks at 3200, 3340 cm -1 into a rather broader peaks.A new sharp peak was observed for PAam-BiC13 and PAam-Bil3 at 1230 cm-1.This peak is not due to the vibrations Bi-CI, Bi-I or Bi-O that have been observed at much lower frequencies in the corresponding inorganic compounds (Bi-CI, 288-242 cm-1; Bi-1 145-115 cm-; Bi-O, 645 cm -1 [10]).At the present time, it is not possible to account for this peak due to the various reactions that may occur in the polymer hybrid.Further studies are underway to understand the detailed structure of this hybrids,

DISCUSSION
X-ray diffraction patterns and IR spectra show that an amorphous polymeric salt is formed between polyacrylamide and bismuth chloride.Its formula can be written as [[CH2-CH(CONH2)]n,BiC13] m with n > 10.The PAam-BiC13 films are brittle while the pure PAam films are flexible.This could be due to the cross-linking of the polymer chains by the bismuth (Fig. 4).Bismuth could be bonded either to the carboxyl groups or to the amine groups.The simultaneous shift of the frequencies of both groups seems to show that the two types of bonding are present.On the contrary, in the case of complexes obtained between terbium chloride and a copolymer of polyacrylamide and acrylic acid, Rodrigues and Galembeck concluded that the binding involves only COO-groups [11].On the other hand, Alonoz reported the synthesis of donor ligands complexes involving sulfur and amine [12] in which a Bi-N bond is formed.
For the iodide hybrid, the situation is more complex.As bismuth iodide is insoluble in water, taking into account the formation conditions of the material, it is reasonable to think that particles of bismuth iodide are formed in the polymeric matrix.However, the infrared spectrum being very similar to that of the chloride hybrid, it is very probable that a part of bismuth is bonded to carbonyl and amide groups.Consequently, it is reasonable to assert that the hybrid is a composite material made of a polymeric salt PAamnBil3, within which particles of Bil3 are precipitated.This interpretation is in agreement with the fact that when the material is washed with acetone, a part of Bil3 is extracted without change of the IR spectrum.As X-ray diffraction lines are not observed, these particles have to be of very small size.Indeed, when bismuth iodide is formed in the same condition but without polymer, a clear diffraction pattern is observed.

FIGURE 4
FIGURE 4 Schematic representation of a possible structure of the complexes formed by PAam and Bi 3/ ions.