Effect Of Agno3 Filling And Uv-irradiation On The Structure

Polymer Testing 18 (1999) 449–461

Material Characterisation

Effect of AgNO3 filling and UV-irradiation on the structure and morphology of PVA films H.M. Zidan* Physics Department, Faculty of Science of Damietta, Mansoura University, Damietta, Egypt Received 16 April 1998; accepted 24 June 1998

Abstract Polyvinyl alcohol films, with various AgNO3 filler mass fractions ( ⱕ 5%), were prepared. The structural and morphological variations, due to filling and UV-irradiation, were investigated using the following techniques; differential scanning calorimetry (DSC), UV/VIS optical absorption spectroscopy, X-ray diffraction and scanning electron microscopy (SEM). Two different crystalline phases (one is due to the PVA matrix and the other is attributed to the PVA–Ag+ chelates) were detected besides the PVA amorphous phase, for the non-irradiated and the UV-irradiated (for 4 and 6 h) films. The PVA–Ag+ chelates disappeared at 2 h UV-irradiation. It is implied that the structural morphology changes vastly due to the changes in filling level and/or UV-irradiation time. The observed morphological patterns were discussed.  1999 Elsevier Science Ltd. All rights reserved.

1. Introduction The introduction of metal ions into a polymer, particularly when the metal is linked chemically with a polymer chain, often imparts new or improved properties to the polymer. Polymer materials are used extensively under terrestrial sunlight. Fundamental studies on photodegradation of polymers are required for various applications of polymer materials. These studies are valid for the development of photostable polymers, photoresistant or photodegradable polymers, estimation of the lifetime of polymers and so on. Although various types of semiconducting organic polymers have excellent electric properties, * Tel: ⫹ 20 50 341701; fax: ⫹ 20 50 346781; e-mail: [email protected] 0142-9418/99/$ - see front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 2 - 9 4 1 8 ( 9 8 ) 0 0 0 4 9 - X

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they are of no practical use because of difficulties in processing them into electronic materials [1]. In order to develop easily processed organic semiconductors, a method is proposed using polymers as a base which can be easily processed and modified. PVA is water-soluble polymer, that can react with metal salts in an aqueous solution to form a metal chelate. This metal chelate can be readily moulded into fibers, films, and other mouldings without detriment to the physical properties of the original polymers. Doping with iodine on the chelate film was found to obtain good electric conductivity [2–4]. Recently, Yen [5] proposed a new method for preparing conductive polymers. PVA silver chelate solutions were prepared by silver nitrate mixed with PVA and films were prepared from this solutions. These PVA metal chelate films were reduced by a photographic developer. The silver ions in the PVA/AgNO3 chelate films were reduced to silver on the surface of the films, and therefore, metallized conductive polymer films were obtained. The present study is carried out to investigate the effect of UV-irradiation on the structure of PVA filled with different amounts of silver nitrate by using UV spectroscopy, differential scanning calorimetry (DSC), X-ray diffraction and scanning electron microscope (SEM). 2. Experimental procedures The films from PVA filled with different amounts of silver nitrate were prepared by a casting method as follows [6]. The powder of commercial PVA was dissolved in distilled water and then heated gently, using a water bath, for complete dissolution. Silver nitrate was dissolved in distilled water also and added to the polymeric solution. The solutions were left to reach a suitable viscosity, after which they were cast in glass dishes and left to dry in a dry atmosphere at room temperature. Samples were transferred to an electric air oven held at 60°C for 48 h to minimize the residual solvent. The samples were stored in the dark to avoid direct exposure to light. The thickness of the obtained films was in the range 0.1 to 0.2 mm. PVA films of the following AgNO3 mass fractions were prepared : 0, 0.1, 0.5, 1.0, 2.0, 3.0 and 5%. The thermal analysis for the studied films were performed by DSC with a Stanton Redcroft DTA 673-4 apparatus which was operated at a scan speed of 10 K/min from 298 to 523K. The samples were irradiated by a UV lamp at ␭ ⫽ 254 nm at room temperature. The distance between the light source and sample was 5.0 cm. UV/VIS absorption spectra were measured in the wavelength region from 200 to 900 nm. X-ray diffraction of the samples were measured by a Philips PW 1050/80 diffractometer. Morphology of sample surface was observed by SEM, (JEOL JSM6100). The SEM specimens were prepared by evaporating gold onto the film surface after drying under vacuum. All measurements carried out in the Physics Department, University of Warwick, England. 3. Results and discussion 3.1. Differential scanning calorimetry (DSC) It is reported [7] that PVA consists of two inextricably mixed phases: crystalline solid and amorphous glass. This results in quite a complex behaviour when the polymer is heated. Ito et

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Fig. 1.

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DSC curves for PVA films filled with different fractions of AgNO3.

al. [8] observed four transitions in the relationship of volume and temperature for PVA, at about 337, 398, 433 and 483K. In the present work the thermal behaviour of PVA filled with different fractions of AgNO3 was observed by DSC in the temperature range 298 to 523K, Fig. 1. From this figure we observe that the pure PVA film displayed four transitions at: 351, 398, 443 and 465K. The transition at about 351K is preferably attributed to the glass transition (Tg) relaxational process resulting from micro-Brownian motion of the main-chain backbone. There is another exothermic peak at about 443K which is attributed to the ␣-relaxation associating the crystalline regions. Similar transitions were seen by Garrett and Gurbb [9] at 358 and 433K. The magnitude of Tg of the pure PVA is greater than those for the filled samples, but the ␣-relaxation temperature T␣ is smaller Table 1. This may be due to the greater crystallinity or formation of another crystalline phase in filled samples. The greater breadth of the relaxations of filled samples can be attributed to the wider range of crystallite sizes and morphology produced [9]. The endothermic peak at 465K in pure PVA has been attributed to the melting point of PVA [10–12]. The thermal curves of the AgNO3 filled samples exhibit double melting transitions, indicating the existence of two different phases. The first melting transition appears as a shoulder at about 473K which Table 1 The glass transition (Tg) and ␣-relaxation temperatures for films of AgNO3 filled PVA systems FL Tg T␣

0 78 170

0.5 72 173

1 70 175

2 67 180

3 63 186

5 62 190

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is referred to the pure PVA phase. The second melting transition exhibits a clear endothermic peak at 503K and it is correlated with the filled PVA phase. 3.2. Optical spectroscopy The UV/VIS optical absorption spectra of PVA filled with various mass fractions of AgNO3 are shown in Fig. 2. The observed spectra exhibit transitions characterising PVA, which can be

Fig. 2.

The UV/VIS optical absorption spectra for films of AgNO3 filled PVA system before irradiation.

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Fig. 3.

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Structure for PVA–Ag+ chelate proposed by Huang et al. [14].

assigned as follows [7,13]. The peak at 225 nm is appropriate for carbonyl groups conjugated with one, in line, ethylenic group. The shoulder at 265 nm is due to the absorption by simple carbonyl groups along the polymer chain. The peak at 280 nm is assigned to the carbonyl groups associated with ethylene unsaturation of the type –Co(CH⫽CH)2–, and the weakening (or the shoulder form) of this band is due to the presence of isolated carbonyl groups. The weak shoulder at 344 nm is attributed to –C(CH⫽CH)3Co– structure. It is remarkable that a relatively strong and broad peak appears at 443 nm for all of the AgNO3 filled PVA films. This band is characteristic of chelate formation for Ag+ coordinated with the hydroxyl group of PVA of a structure previously proposed by Huang et al. [14] and is shown in Fig. 3. Moreover, Yen et al. [15] argued that when the amount of AgNO3 in the PVA–Ag+ chelate film is greater than 0.1 wt%, some of the Ag+ ions were first coordinated with the hydroxyl group of PVA. Then, the surplus Ag+ ions are not chelated by PVA but coordinated by the NO3− ion, as shown in Fig. 4. This may account for the nonmonotonic filling level dependence of the peak height at 433 nm, observed in Fig. 2. The analysis of the UV spectra of UV-irradiated PVA revealed that exposure of PVA has no influence on their UV spectral features. Bravar et al. [13] mentioned that even prolonged exposure of PVA to UV-irradiation did not provoke an increase of the existing bands nor the appearance of a new band. Figs. 5–7 display the post irradiation spectra of the present PVA system for irradiation time IT 2,4 and 6 h, respectively. It is clear from Fig. 5 that the spectrum of the unfilled PVA is nearly unaffected by 2 h IT, while the spectra of the filled samples were changed as follows. The peak at 433 nm disappeared and strong peaks at ⬇ 280 nm are noticed. This indicates that the PVA–Ag+ chelates are destructed and the concentration of –Co(CH⫽CH)2– carbonyl group is increased. It is noticed in Figs. 6 and 7 that the peaks at 433 nm reappeared with sharper and stronger forms due to 4 and 6 h ITs. This indicates the reformation of PVA–Ag+ chelates.

Fig. 4. Structure for the PVA–Ag+ chelate proposed by Yen et al. [15].

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Fig. 5. The UV/VIS optical absorption spectra after IT 2 h for films of AgNO3 filled PVA system.

Fig. 6. The UV/VIS optical absorption spectra after IT 4 h for films of AgNO3 filled PVA system.

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Fig. 7. The UV/VIS optical absorption spectra after IT 6 h for films of AgNO3 filled PVA system.

3.3. X-ray diffraction (XRD) The XRD scans of the un-irradiated films of the present PVA system are shown in Fig. 8. A main peak at 2␪ ⫽ 20.5°, characterising the PVA crystalline phase [16], is noticed for all the studied samples. Two other peaks are clearly observed at 2␪ ⫽ 9.2° and 18.3° and the former is stronger and sharp. These two peaks neither belong to the pure PVA nor AgNO3 crystalline spectra, but they may arise from scattering atomic planes of some crystalline patterns of PVA–Ag+ complex. The appearance of these two peaks at filling level (FL) ⱖ 2% supports the assumption of complex formation. Fig. 9 indicates no significant change in the XRD spectra, due to 2 h IT, except for the scattering peaks at 2␪ ⫽ 9.2° in which there are: a sharp increase at 5% FL and a slight decrease at 3% FL. The IT dependences of the peak heights at 2␪ ⫽ 20.5° and 9.2° (characterising the PVA and PVA–Ag+ crystalline phases, respectively), for 3% FL films, are shown in Figs. 10 and 11. The insets indicate that the peak heights decay as IT increases. 3.4. SEM micrography SEM micrographs of the morphology of PVA films of various FLs of AgNO3 are shown in Fig. 12(a)–(e). The micrograph, in Fig. 12(a), of the unfilled PVA is characterised by normal crystalline uniformly shaped spherulites (of smooth boundaries and the grown ones are nearly of equal sizes, of average diameter ⬇ 8 ␮m), randomly distributed in a continuous amorphous phase. Fig. 12(b) depicts that 1% AgNO3 filler revealed circularly deformed patches (of maximum core diameter ⬇ 45 ␮m ) having shells of relatively broad thickness ( ⬇ 10 ␮m). Fig. 12(c) indicates

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Fig. 8. XRD scans for the unirradiated films of AgNO3 filled PVA system.

Fig. 9. XRD scans for films of AgNO3 filled PVA system after IT 2 h.

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Fig. 10. XRD scans from 2␪ ⫽ 10° to 35° for 3% filling level in PVA films at different ITs (a) 0; (b) 0.5; (c) 2; (d) 4; (e) 6 and (f) 8 h. The inset shows the dependence of peak height on IT.

Fig. 11. As Fig. 10 from 2␪ ⫽ 8° to 10°.

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Fig. 12. SEM micrographs for PVA films of various FL’s of AgNO3 before irradiation (a) 0; (b) 1; (c) 2; (d) 3, and (e) 5%.

that increasing FL to 2% gives rise to noncircular crystalline patches of a ciliar rim and central hub. The average patch diagonal length is ⬇ 24 ␮m. Fig. 12(d) shows the micrograph of 3% FL which contains two shapes of crystalline patches, a small circular one and a large nonuniform shape. The origin of these structures is attributed to the nucleation and growth mechanism which can be explained as follows. At the first stage, the pattern is thought to contain small circular patches due to the nucleation process. Then two growth types are assumed to proceed :(i) a normal growth of all the initial nuclei and (ii) a mergence of two (or more) adjacent circular patches to form a larger nonuniform patch which grows more due to the tendency of a cascaded mergence. A dendritic peculiar texture containing long fibrils is noticed in Fig. 12(e) for the micrograph of the 5% FL sample. These micrographs imply that there is a drastic influence of AgNO3 content on the structural morphology of PVA films. It seems important to clarify the effect of UV-irradiation on the structural morphology of PVA

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containing various FLs of AgNO3. This can be done using the micrographs of Fig. 13(a)–(e). These micrographs obtained from the samples irradiated for 8 h. Fig. 13(a) stands for 1% filler. A secondary phase separation can be noticed inside the deformed circular patch, due to UVirradiation. In the case of 3% FL, [Fig. 13(b) and (c)], a spongy texture with some fibril tracks

Fig. 13. SEM micrographs for PVA films of various FL’s of AgNO3 after irradiation (a) 1; (b,c) 2, and (d,e) 5%.

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is formed by UV-irradiation. Fig. 13(d) and (e) indicates that, for the 5% filler case, the dendritic texture is transformed to a random distribution of well formed spherulites of crystalline phase. The large magnification in Fig. 13(e) explores more details about the fine structure of these spherulites. The primitive unit of these structures is changed like a bird’s fantail. Two fantails can be connected together, through their sharp sides, forming a double arm unit, which in its turn enhances the connection of third and fourth fantails to form a complete spherulite. Such well formed spherulites of PVA had been previously prepared by Packter and Nerurkar [17] by holding glycolic PVA gels at a temperature of 353K for several hours.

4. Conclusions The DSC implied the presence of two different phases, with two different melting points, one phase was the pure PVA and the other was assumed to be a PVA–Ag+ complex phase. An exothermic peak, due to crystal destruction, was found to be more sharp and higher with increasing FL. The optical absorption spectra of the un-irradiated films indicated the existence of the isolated and conjugated carbonyl groups which are normally found in PVA. Moreover these films were characterised by a broad and strong peak at 433 nm which was assigned to PVA–Ag+ chelate. The 2 h UV-irradiation resulted in: (a) the disappearance of the peaks at 433 nm, i.e. the destruction of the PVA–Ag+ chelate, and (b) the presence of strong peaks at 280 nm, indicating an increased concentration of –CO(CH⫽CH)2– carbonyl groups. The 4 and 6 h IT lead to the reappearance of the peak at 433 nm (more strong and sharp) and a clear shoulder at 280 nm. The X-ray analysis confirmed the presence of two different crystalline phases, which agrees with the DSC and optical absorption findings. The SEM micrography explored the structural morphology of the investigated films at various states.

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