Characterization and Thermally Stimulated Depolarization Current Studies of Rhodamine-Dye-Doped Poly(acrylonitrile-butadiene-styrene) Films

Polymer International 39 (1996) 105-111

Characterization and Thermally Stimulated Depolarization Current Studies of R hodami ne- Dye- Doped

Poly( acrylonit ri Ie-butad ienestyrene) Films

M . D. Migahed,* M . 1. Abdel-Hamid & N. A. Bakr

Polymer Laboratory, Physics Department, Faculty of Science, Mansoura University, Mansoura, Egypt

(Received 24 April 1995; revised version received 9 August 1995; accepted 8 September 1995)

Abstract: IR and UV-absorption spectra, and the thermally stimulated currents of pure and Rhodamine-6G-doped poly(acrylonitri1e-butadiene-styrene) (Rh- doped ABS) films were investigated. Structural characteristics could be specified from these techniques. Both IR and UV-absorption studies revealed a modification of the structure of ABS on blending with Rhodamine 6G; Rh molecules are partially dispersed in the ABS matrix and partially attached as side groups to the ABS backbone. Thermally stimulated depolarization current (TSDC) studies confirmed this result. The results revealed that incorporation of Rh 6G in ABS locks the dipole in the ABS matrix after electric poling. The TSDC spectra have been found, depending on the polarization temperature, to be characterized by three peaks. The phenomenon of the existence of these current maxima is dis- cussed and analysed in terms of dipolar and ionic relaxations.

Key words: poly(acrylonitri1e-butadiene-styrene), rhodamine doping, TSDC spectra, thermally stimulated depolarization, characterization.

I NTR 0 D U CTI 0 N ation is complicated by the fact that the nature of the relaxational behaviour of these systems with regard to

Organic dye-polymer composites have received con- poling conditions and different kinds of possible molec- siderable attention as potential electro-optic ular motions is presently not fully understood and still a

The host polymer usually contains large matter of controversy. Efforts are reported in the liter- conjugated a-electron organic (guest) molecules termin- ature which aim to understand the molecular processes ated by strong acceptor and donor groups, dissolved or of such systems after switching off the external electric covalently attached to the polymer backbone and non- Important parameters in this context are absol- centrosymmetrically ordered within the polymer ute temperature, molecular structure of both the m a t r i ~ . ~ - ~ Due to the fact that such isotropically doped polymer and the dopant, and the amount of the so- polymers lack a macroscopic second-order harmonic called free volume. activity, a common method to introduce these effects is Thermally stimulated polarization/depolarization to apply a strong DC poling field to the film, in order to current (TSPC/TSDC) are useful tools break the symmetry by aligning the dopants, above its because they commonly work in ranges of very low glass transition temperature T, and then cooling it equivalent frequencies and thus lead to well-resolved below Tg with the electric field still applied. The situ- spectra." Further, these methods are also highly sensi-

tive, thus allowing one to follow easily any variation in * To whom correspondence should be addressed. the peak properties as a result of structural modification

Polymer International 0959-8103/96/$09.00 0 1996 SCI. Printed in Great Britain 105

106 M . D . Migahed, M . I. Abdel-Hamid, N . A. Bakr

r -I

I I

I I I CN I I L

Scheme 1. ABS terpolymer

or aggregate formation." TSDCs of poly(acrylonitri1e- butadiene-styrene) (ABS) have been investigated in the high temperature range. A peak associated with the glass transition temperature was observed and has a characteristic of a dipolar relaxation process.13 On the other hand, DC conductivity and TSDC in ABS sensi- tized with leucomalachite green (LMG) have been investigated owing to possibilities of application of these materials in electrophotography. The TSDC

ofC"" Scheme 2. Rh 6G

Fig. 1. IR spectra

studies revealed that the temperature at which total decay takes place decreases as the LMG wt% increases and the increase in conductivity is due to the intro- duction of anionic states in ABS by addition of LMG.'4.'S

In this study we report the experimental results of TSDC data obtained-for thermal poling of ABS films doped with Rhodamine 6G (Rh 6G). The films have been characterized by ultraviolet/visible optical absorption (UV/VIS) spectroscopy, infrared (IR) absorption spectroscopy and polarized optical microscopy (POM). Our measurements clearly distinguish the thermally activated processes and trap levels and point toward a decoupling of the dye motion from the bulk a-process of the ABS host terpolymer.

EXPERIMENTAL PROCEDURE

Acrylonitrile-butadiene-styrene terpolymer (ABS) from Polyscience, USA, was used. ABS grafted terpolymer is a thermoplastic material and can be considered to be composed of hard component (styrene-acrylonitrile) connected with a soft rubbery component (butadiene- acrylonitrile). It consists of acrylonitrile (A)-styrene ( S ) copolymer grafted on to polybutadiene (B). The relative

4000 3500 3000 2500 2000 1500 1000 500 200 Wavenumber ( c r i i ' )

for (a) pure ABS; (b) 0.5 wt% Rh-doped ABS films; and (c) 0.5 wt% Rh-doped ABS heated at 393 K for 1 h.

POLYMER INTERNATIONAL VOL. 39, NO. 2, 1996

Rhodamine-dye-doped poly(acrylonitri1e-butadiene-styrene) films 107

Wavelength (nm.) 400 so0 600

I I 13.700 200 300 4.500 I

Wavelength (nm.)

Fig. 2. UV-visible absorption spectrum of (a) pure ABS; (b) 0.5 wt% Rh-doped ABS; and (c) 0.5 wt% Rh-doped ABS heated at 393 K for 1 h.

concentrations of A, B and S were 29.9, 9.3 and 60.8 mol% (Scheme 1). Rhodamine 6G (Rh 6G) was supplied by Radient Dyes Chemie, Germany. The chemical structure of the constituent (Rh 6G) is given in Scheme 2. A certain weight of ABS and appropriate weight of Rh were dissolved in chloroform as a common solvent. The mixture was cast in a flat glass dish, and thin films of pure and Rh-doped ABS of thickness ranging from 45 to 160pm were obtained. Due to the doping, the colour of ABS films changed from faint pink (0.1 wt% Rh) to orange (0.5 wt% Rh). Conducting surfaces of film were prepared using colloidal graphite and silver paste. Details of the experimental set-up and measuring techniques for polarization and depolarization of the polymer have been described in previous ~ o r k s . ' ~ ~ ' ~ ~ ' '

The sample was heated to the predetermined polarization temperature, T,, and then a direct field, E , , was applied for a known time, t,. After poling, the sample was cooled to room temperature with the applied field on. After removing the field, the TSDC current was measured under short-circuit condition at a constant heating rate /? (4Kmin-I). The current was measured by Keithly electrometer (610C). IR studies were carried out using a Mattson 5000 FTIR spectrometer. UV- absorption measurements were made at room temperature with a Unicam UV/VIS spectrometer (UV2) in the wavelength range 200-900 nm.

No signs of rhodamine crystallization under ambient conditions could be noted in the concentration range used in any samples by visual inspection (POM).

RESULTS AND DISCUSSION

Characterization of Rh -doped ABS films

The structural modification of ABS due to the incorporation of Rh 6G was confirmed by spectroscopic studies. Figure l(a) shows the IR spectrum of pure ABS film. The spectrum clearly shows bands at 920 and 960 cm- ' due to the double bonds of butadiene rubber," and a band at 2238cm-' caused by the nitrile group of the acrylonitrile segment. A dramatic change appeared in the IR spectrum on complexing ABS with Rh 6G. The bands at 960 and 2238cm-' nearly disappeared, as shown in Fig. l(b). On the other hand, the absorption band at 1720cm- ', characteristic for the carbonyl group on the chain, became stronger. After thermal curing in open air at 353 K for 1 h, no change in the IR spectrum of 0-5wt% Rh-doped ABS was observed. Further curing of the film at 393 K (7'' of ABS - 388 K) for l h led to the liberation of the Rh radical-the bands at 920 and 2238cm-' became stronger (see Fig. lc). These observations suggest that, on doping ABS with Rh, structural changes occur, which assumes oxi- dation or reduction of the polymer and incorporation of a counter-ion to preserve the electroneutrality. Then, doped ABS contains charged species and therefore the issue of Coulombic interaction among charge sites is highly relevant. Since doping requires local reorganiza- tions of the polymer, for rhodamine cations it is reason- able to assume that sites are defined by a locally


108 M . D. Migahed, M . I . Abdel-Hamid, N . A. Bakr

attractive coordination environment of tetrahedrally disposed nitrogen atoms of the polymer and double bonds of the polymer backbone. In conclusion, Rh is either dispersed in the ABS matrix or is attached as side groups to its backbone.

The UV absorption spectrum of pure ABS film is shown in Fig. 2(a). Two bands located at 250 and 286nm can be seen. Doping ABS with Rh 6G causes the appearance of another two new bands at 520 and 559nm, as seen in Fig. 2(b), which may be due to the dissolving of Rh 6G in the p ~ l y m e r . ' ~ * ~ ~ On the other hand, a marked decrease of these absorption bands associated with heating the film at 393 K for 1 h and a new peak appearing at 332 nm is shown in Fig. 2(c). The band gap energy, determined from a linear relationship between (ahv)' and photon energy hv," where a is an absorption coefficient of the medium, for pure and 0-5wt% Rh-doped ABS it is 4.2eV and 2.7eV, respectively. These results indicate that the apparent elec- tronic band structure of ABS is affected by the incorporation of Rh, which is in agreement with the results of the IR investigation.

Temporal effects of dopant

In order to gain a quantitative understanding of the poling process, a series of poling experiments with simultaneous current measurements were carried out. A general characteristic behaviour observed during the poling process was an increase in the polarizing current when the poling process was carried out at T > 353K. The current always started rising and reached a steady state value on a time scale of several minutes.

Figure 3(a) shows the time dependence of TSC in 0.5 wt% Rh-doped ABS. These results were obtained at Tp = 393 K and at different electric fields. Figure 3(b) represents the TSC-time relation in 0.5 wt% Rh-doped ABS at E = 3.7 x 104Vcm-' for different temperatures. A remarkable increase in the conducting current occurred by thermal dissociation for Rh 6G in ABS; hence, Rh acted as a transport centre for holes. Thus, the contribution to electrical polarization might result from the drift of ionic dopant toward electrodes of opposite charge. The conducting current-time can be simply written in the form

I = I , exp( - t /z)[ 1 - exp( - t /z)]

where z is the time corresponding to a current peak in the polarization curves at all electric fields and for temperature > 353 K.

These results may be interpreted as being due to an increase of the thermal ionization of Rh molecules and its withdrawal, both caused by the applied field (thermo-electric field effect). This effect may be used to determine the ion mobility, p, using the relation y = d/zE,16 where d is the distance between the electrodes and E is the electric field strength. From Fig. 3(a),

16' I 0 4 8 12 16 20 24 28

Time( mm I

16" 4 8 12 16 10 24 28

Tlrnc(rnin)

Fig. 3. (a) Time dependence of TSC in 0.5 wt% Rh-doped ABS at Tp = 393 K for various electric fields. Colloid graphite as electrodes. (b) Time dependence of TSC in 0.5wt% Rh- doped ABS for various temperatures and at constant electric

field, E = 3.7 x lo4 V cm- '. Colloid graphite as electrodes.

one obtains pl = 1.8 x lop9, y, = 5.1 x lo-'' and ps = 4-4 x 10-10cm2s-'V-' El = 2.5 x lo4, E , = 3.7 x lo4 and E , = 5 x 104Vcm-', respectively. These values are in good agreement with those of the Rh cation in Rh-doped polyester;" they seem to be electric field independent. Hence, the transport process in Rh-doped ABS is controlled by local charge carrier motion in the amorphous regions, as in the case for diffusion processes in many polymer systems. Therefore doped-ABS can retain electrical polarization owing to greatly diminished diffusion coefficients.

at

Thermally stimulated depolarization current

TSDCs in short-circuit configuration have been studied as a function of Rh concentration, poling field E , ,


Rhodamine-dye-doped poly(acrylonitri1e-butadiene-styrene) jilms 109

Pure A 0 5 0 1 Wt ‘CRh OSW I %Rh

, I

I

I I

/ I

/ , /

/

lo“’ !----+- 313 3.33 353 373 393 k113

Tempcralurel K )

10’0 I

323 363 LO3 Temperature( I( )

/ I

I I

I I

I I

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: : , I - -

I 16’’ I .

303 3w 383 423 Trmperotur~(K )

Fig. 4. (a) TSDC of (---) pure ABS; (-.-.-) 0.1 wt% Rh-doped ABS; (-) 0.5 wt% Rh-doped ABS, at Tp = 373 K and E , = 1 Silver paste as electrodes. (b) TSDC of 0.5 wt% Rh-doped ABS at Tp = 393 K and at various electric fields. Colloid

graphite as electrodes. (c) TSDC of 0-5 wt% Rh-doped ABS for various polarizing temperatures and at electric field 3.7 x lo4 V cm- ’. Colloid graphite as electrodes.

x lo4 V cm-


110 M . D . Migahed, M . I . Abdel-Hamid, N . A . Bakr

poling temperature Tp, and for constant poling time tp . Figure 4(a) shows the TSDC spectra of pure and Rh- doped ABS. The spectrum of pure ABS is significantly affected by the presence of Rh 6G. The peak exhibits a shift towards lower temperature with increasing dopant concentration. This is surprising since the concentration is very low and, generally speaking, impurities have little influence on the local molecular relaxation in poly- m e r ~ . ~ ~ For 0.5 wt% dye concentration there is a possi- bility of dye interaction with the polymer chains, and this may impede the motion of the molecular dipoles. Therefore, the broadening in the TSDC peak for 0.5 wt%-doped ABS films could correspond to a slight increase in distribution of relaxation times resulting from uniformly distributed localized active centres.

Figures 4(b) and 4(c) represent the dependence of the TSDCs on E , and T p , respectively. In either case, the resulting current record resembles a 'spectrum' of peaks, some of which originate from dipolar relaxation [a- relaxation located at about 393 K] l 3 and others from the relaxation of the ionic species. It is clear in Fig. 4(b) that the a-region is structured and new relaxation phe- nomena are taking place. The new relaxation peaks are located on the low-temperature side of the a-process, thus involving an enhanced mobility in the glass transition region. This could correspond to a local increase in free volume in the vicinity of ionic associations.

The TSDC spectrum of the film polarized at T p =

353K (see Fig. 4(c)) is characterized in general by two peaks. The first TSDC peak is at lower temperature (343K), and the second is located at about 368K, sug- gesting the formation of localized ionic species or induced dipoles. The existence of the two TSDC peaks in Rh-doped ABS polarized at 353K is in good agreement with the fact that there exist two types of Rh 6G molecules in the copolymer of methyl methacrylate with methacrylic acid with different photofading quantum yields and that the fraction of dye with higher quantum

yield reduces with heat-treated time.24 The TSDC peak at 393 K in the polarized film at Tp = 393 K may correspond to the a-relaxation in pure ABS. The activation energy was calculated using the initial-rise method with the peak cleaning techniques.' 7 ~ 2 3 Using this method for very weak current intensities I in comparison with the maximum I,,, of the peak, i.e. ( I < lmaJ10),25 the linear portion of the TSDC peak in Fig. 4(a) can be described by

I = I , exp( - E/K T)

where K is the Boltzmann constant and E is the activation energy. From the activation energy E of the ith current peak, the peak temperature T,, and the heating rate j, the attempt-to-escape frequency v , is determined as follows:26

v o = Ei B exp(Ei/KTm, i)/K(L, i)'

Table 1 summarizes the results for undoped, 0.1 wt% and 0.5 wt% doped ABS films respectively. The values for the TSDC peak at 393K are in general agreement with earlier measurements on ABS.I3 On the other other hand, the values for the other TSDC peaks depend on both the poling temperature and Rh concentration.

CONCLUSIONS

Introducing a few percent of Rh in ABS can modify the ABS structure and lock the dipole orientation (a-relaxation) in the polymer matrix after electric poling. The Rhodamine cations give rise to polarization effects in Rh-doped ABS and the existence of TSDC peaks in 353K polarized doped ABS film reflects the ionic or induced dipole relaxation. Further, the existence of two TSDC peaks suggests that there are two types of Rh 6G molecules incorporation in ABS terpolymer.

TABLE 1. Depolarization characteristics and relaxation characteristic parameters for undoped and doped ABS films

~ ~~ ~

Dye content Poling Poling Peak Relaxation parameters (wt%) temperature, field, E p temperature,

TP (K) ( xt04Vcm- ' ) Tln (K) E (ev) yo b-')

0.0 373 0.1 373 0.5 373 0.5 393

393 393

0.5 333 353

393

1 .o 1 .o 1 .o 2.5 3.7 5.0 3.7 3.7

3.7

393 400 368

389 393 388

338 368 343 393

1.24 0.89 0.30 0.71 0.88 0.84 0.42 0.35 0.60 0.82

5.4 x 10' 7.5 x 10' 2.3 x 10'

6.0 x 10' 9.0 x 108 3.7 x 108

5.4 x 103 1.3 x 10' 2.7 x 10' 1.4 x 10'


Rhodamine-dye-doped poly(acrylonitri1e-butadiene-styrene) Jilms 111

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Documents

Characterization and Thermally Stimulated Depolarization Current Studies of Rhodamine-Dye-Doped Poly(acrylonitrile-butadiene-styrene) Films