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 structureand 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 (# 5%), were prepared. The structuraland morphological variations, due to filling and UV-irradiation, were investigated using the followingtechniques; differential scanning calorimetry (DSC), UV/VIS optical absorption spectroscopy, X-ray dif-fraction and scanning electron microscopy (SEM). Two different crystalline phases (one is due to the PVAmatrix and the other is attributed to the PVA–Ag+ chelates) were detected besides the PVA amorphousphase, for the non-irradiated and the UV-irradiated (for 4 and 6 h) films. The PVA–Ag+ chelates disappearedat 2 h UV-irradiation. It is implied that the structural morphology changes vastly due to the changes infilling level and/or UV-irradiation time. The observed morphological patterns were discussed. 1999Elsevier Science Ltd. All rights reserved.

1. Introduction

The introduction of metal ions into a polymer, particularly when the metal is linked chemicallywith a polymer chain, often imparts new or improved properties to the polymer.

Polymer materials are used extensively under terrestrial sunlight. Fundamental studies on photo-degradation of polymers are required for various applications of polymer materials. These studiesare valid for the development of photostable polymers, photoresistant or photodegradable poly-mers, estimation of the lifetime of polymers and so on.

Although various types of semiconducting organic polymers have excellent electric properties,

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450 H.M. Zidan/Polymer Testing 18 (1999) 449–461

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 usingpolymers 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 chelatecan be readily moulded into fibers, films, and other mouldings without detriment to the physicalproperties of the original polymers. Doping with iodine on the chelate film was found to obtaingood electric conductivity [2–4].

Recently, Yen [5] proposed a new method for preparing conductive polymers. PVA silverchelate solutions were prepared by silver nitrate mixed with PVA and films were prepared fromthis solutions. These PVA metal chelate films were reduced by a photographic developer. Thesilver 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 ofPVA filled with different amounts of silver nitrate by using UV spectroscopy, differential scanningcalorimetry (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 castingmethod as follows [6]. The powder of commercial PVA was dissolved in distilled water and thenheated gently, using a water bath, for complete dissolution. Silver nitrate was dissolved in distilledwater also and added to the polymeric solution. The solutions were left to reach a suitable vis-cosity, after which they were cast in glass dishes and left to dry in a dry atmosphere at roomtemperature. Samples were transferred to an electric air oven held at 60°C for 48 h to minimizethe residual solvent. The samples were stored in the dark to avoid direct exposure to light. Thethickness of the obtained films was in the range 0.1 to 0.2 mm. PVA films of the followingAgNO3 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 RedcroftDTA 673-4 apparatus which was operated at a scan speed of 10 K/min from 298 to 523K. Thesamples were irradiated by a UV lamp atl 5 254 nm at room temperature. The distance betweenthe light source and sample was 5.0 cm. UV/VIS absorption spectra were measured in the wave-length region from 200 to 900 nm. X-ray diffraction of the samples were measured by a PhilipsPW 1050/80 diffractometer. Morphology of sample surface was observed by SEM, (JEOLJSM6100). The SEM specimens were prepared by evaporating gold onto the film surface afterdrying under vacuum. All measurements carried out in the Physics Department, University ofWarwick, 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 andamorphous glass. This results in quite a complex behaviour when the polymer is heated. Ito et

451H.M. Zidan/Polymer Testing 18 (1999) 449–461

Fig. 1. 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 about337, 398, 433 and 483K. In the present work the thermal behaviour of PVA filled with differentfractions of AgNO3 was observed by DSC in the temperature range 298 to 523K, Fig. 1. Fromthis figure we observe that the pure PVA film displayed four transitions at: 351, 398, 443 and465K. The transition at about 351K is preferably attributed to the glass transition (Tg) relaxationalprocess resulting from micro-Brownian motion of the main-chain backbone. There is anotherexothermic peak at about 443K which is attributed to thea-relaxation associating the crystallineregions. Similar transitions were seen by Garrett and Gurbb [9] at 358 and 433K. The magnitudeof Tg of the pure PVA is greater than those for the filled samples, but thea-relaxation temperatureTa is smaller Table 1. This may be due to the greater crystallinity or formation of another crystal-line phase in filled samples. The greater breadth of the relaxations of filled samples can be attri-buted to the wider range of crystallite sizes and morphology produced [9]. The endothermic peakat 465K in pure PVA has been attributed to the melting point of PVA [10–12]. The thermalcurves of the AgNO3 filled samples exhibit double melting transitions, indicating the existenceof two different phases. The first melting transition appears as a shoulder at about 473K which

Table 1The glass transition (Tg) anda-relaxation temperatures for films of AgNO3 filled PVA systems

FL 0 0.5 1 2 3 5Tg 78 72 70 67 63 62Ta 170 173 175 180 186 190


is referred to the pure PVA phase. The second melting transition exhibits a clear endothermicpeak 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.


Fig. 3. 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 conjugatedwith one, in line, ethylenic group. The shoulder at 265 nm is due to the absorption by simplecarbonyl groups along the polymer chain. The peak at 280 nm is assigned to the carbonyl groupsassociated with ethylene unsaturation of the type –Co(CH5CH)2–, and the weakening (or theshoulder form) of this band is due to the presence of isolated carbonyl groups. The weak shoulderat 344 nm is attributed to –C(CH5CH)3Co– structure. It is remarkable that a relatively strong andbroad peak appears at 443 nm for all of the AgNO3 filled PVA films. This band is characteristic ofchelate formation for Ag+ coordinated with the hydroxyl group of PVA of a structure previouslyproposed by Huang et al. [14] and is shown in Fig. 3. Moreover, Yen et al. [15] argued thatwhen the amount of AgNO3 in the PVA–Ag+ chelate film is greater than 0.1 wt%, some of theAg+ ions were first coordinated with the hydroxyl group of PVA. Then, the surplus Ag+ ions arenot chelated by PVA but coordinated by the NO3

− ion, as shown in Fig. 4. This may account forthe 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 noinfluence on their UV spectral features. Bravar et al. [13] mentioned that even prolonged exposureof PVA to UV-irradiation did not provoke an increase of the existing bands nor the appearanceof a new band. Figs. 5–7 display the post irradiation spectra of the present PVA system forirradiation time IT 2,4 and 6 h, respectively. It is clear from Fig. 5 that the spectrum of the unfilledPVA is nearly unaffected by 2 h IT, while the spectra of the filled samples were changed asfollows. The peak at 433 nm disappeared and strong peaks at< 280 nm are noticed. This indicatesthat the PVA–Ag+ chelates are destructed and the concentration of –Co(CH5CH)2– carbonylgroup is increased. It is noticed in Figs. 6 and 7 that the peaks at 433 nm reappeared with sharperand 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].


Fig. 5. The UV/VIS optical absorption spectra after IT 2 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. Amain peak at 2u 5 20.5°, characterising the PVA crystalline phase [16], is noticed for all thestudied samples. Two other peaks are clearly observed at 2u 5 9.2° and 18.3° and the former isstronger and sharp. These two peaks neither belong to the pure PVA nor AgNO3 crystallinespectra, 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)$ 2% supports the assumption ofcomplex formation.

Fig. 9 indicates no significant change in the XRD spectra, due to 2 h IT, except for the scatteringpeaks at 2u 5 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 2u 5 20.5° and 9.2° (characterising the PVA andPVA–Ag+ crystalline phases, respectively), for 3% FL films, are shown in Figs. 10 and 11. Theinsets 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 inFig. 12(a)–(e). The micrograph, in Fig. 12(a), of the unfilled PVA is characterised by normalcrystalline uniformly shaped spherulites (of smooth boundaries and the grown ones are nearly ofequal sizes, of average diameter< 8 mm), randomly distributed in a continuous amorphous phase.Fig. 12(b) depicts that 1% AgNO3 filler revealed circularly deformed patches (of maximum corediameter< 45 mm ) having shells of relatively broad thickness (< 10 mm). Fig. 12(c) indicates


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.


Fig. 10. XRD scans from 2u 5 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 2u 5 8° to 10°.


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 centralhub. The average patch diagonal length is< 24 mm. Fig. 12(d) shows the micrograph of 3% FLwhich contains two shapes of crystalline patches, a small circular one and a large nonuniformshape. The origin of these structures is attributed to the nucleation and growth mechanism whichcan be explained as follows. At the first stage, the pattern is thought to contain small circularpatches due to the nucleation process. Then two growth types are assumed to proceed :(i) a normalgrowth of all the initial nuclei and (ii) a mergence of two (or more) adjacent circular patches toform 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 ofthe 5% FL sample. These micrographs imply that there is a drastic influence of AgNO3 contenton the structural morphology of PVA films.

It seems important to clarify the effect of UV-irradiation on the structural morphology of PVA


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 UV-irradiation. 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%.


is formed by UV-irradiation. Fig. 13(d) and (e) indicates that, for the 5% filler case, the dendritictexture 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 spheru-lites. The primitive unit of these structures is changed like a bird’s fantail. Two fantails can beconnected together, through their sharp sides, forming a double arm unit, which in its turnenhances the connection of third and fourth fantails to form a complete spherulite. Such wellformed spherulites of PVA had been previously prepared by Packter and Nerurkar [17] by holdingglycolic 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, onephase was the pure PVA and the other was assumed to be a PVA–Ag+ complex phase. Anexothermic peak, due to crystal destruction, was found to be more sharp and higher with increasingFL. The optical absorption spectra of the un-irradiated films indicated the existence of the isolatedand conjugated carbonyl groups which are normally found in PVA. Moreover these films werecharacterised by a broad and strong peak at 433 nm which was assigned to PVA–Ag+ chelate. The2 h UV-irradiation resulted in: (a) the disappearance of the peaks at 433 nm, i.e. the destruction ofthe PVA–Ag+ chelate, and (b) the presence of strong peaks at 280 nm, indicating an increasedconcentration of –CO(CH5CH)2– carbonyl groups. The 4 and 6 h IT lead to the reappearanceof the peak at 433 nm (more strong and sharp) and a clear shoulder at 280 nm. The X-ray analysisconfirmed the presence of two different crystalline phases, which agrees with the DSC and opticalabsorption findings. The SEM micrography explored the structural morphology of the investigatedfilms at various states.

References

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Effect of AgNO3 Filling and UV-Irradiation on the Structure