Polymer International 40 (1996) 295-298

Electrical p-Relaxation of Die-drawn Isotactic Polypropylene

S. Abdul Jawad* & A. Alnajjar

Department of Physics, UAE University, PO Box 17551, Al-Ah, United Arab Emirates

(Received 11 October 1995; revised version received 17 January 1996; accepted 18 March 1996)

Abstract: Measurements of a.c. impedance were carried out in the frequency range 1 Hz to lo6 Hz at room temperature on isotropic and die-drawn isotactic polypropylene. It was found that the a.c. impedance decreases with increasing draw ratio in the frequency range 1 Hz to lo4 Hz and then becomes independent of draw ratio with increasing frequency. A dramatic drop in the a.c. impedance was observed in the frequency range lo4 to 106Hz, which is associated with a 8-relaxation process. The intensity of the 8-relaxation peak decreases with increasing draw ratio and nearly disappears for a draw ratio of 16.8. The Cole- Cole plots show good semicircles for isotropic and die-drawn samples indicating that a certain polarization mechanism is dominant in polypropylene.

Key words: drawn polypropylene, relaxation, phase angle, impedance.

INTRODUCTION

The morphology of highly drawn polymers is of con- siderable interest, particularly in relation to the mecha- nical, dynamic mechanical and electrical properties of such materials. The morphology of isotactic poly- propylene (i-PP) is highly modified by deformation pro- cesses such as conventional drawing,' die-drawing2 and EXPERl M ENTAL hydrostatic extr~sion.~ As a result of deformation, the molecules in the crystalline regions are aligned along Materials the draw direction, the taut tie molecules in the amorp- hous region are oriented while the amorphous materials The polymer used was isotactic polypropylene (GSE are constrained. At high draw ratio a complete trans- 108), melt flow index = 0.8, manufactured by ICI plc, formation of the original lamellae into a microfibrillar UK, as a commercial product. Samples were prepared structure may be a~hieved.~ by a die-drawing technique.' Briefly, an isotropic rod

In a previous publication5 using dynamic shear and machined from a cast billet was drawn through a heated tensile measurements, it was shown that the /?- 4mm die at various drawing speeds to obtain a range of relaxation in die-drawn i-PP is associated with molecu- draw ratios. The draw temperature was controlled at lar motion in the amorphous region. A simplified 110°C to an accuracy of & 1°C. Further control of draw two-phase model was employed to explain the decrease ratio could be achieved by varying the diameter of the in the intensity of the /3-relaxation peak upon drawing. isotropic feed stock billets, so that the draw ratio of the The model predicts that the decrease in the loss factor drawn rod is determined by the change in cross- in dynamic shear and tension upon drawing is associ- sectional area of the billet. All drawn samples were ated with a change in continuity from a series to a nearly transparent except for the highest draw ratio

(16.8), which was opaque due to formation of micro- * To whom all correspondence should be addressed. voids during the drawing process.

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

parallel situation. In this paper we report the effect of deformation on the a.c. impedance and on the intensity of the electrical /3-relaxation in die-drawn i-PP. The a.c. impedance was measured in the frequency range 1 Hz to lo6 Hz at room temperature.

295

296 S . A. Jawad, A. Alnajjar

a s . impedance measurements logE I -1.000e8

The a.c. impedance measurements were carried out at room temperature in the frequency range 1Hz to lo6 Hz using a Solartron-1260 impedance gain-phase analyser (Schlumberger Instruments). The measure- ments were performed on a disc-shaped specimen of 3 mm diameter and 1.3 mm thickness; graphite elec- trodes were carefully applied to each surface. The sample was shielded and connected to the Solartron- 1260. Supplied software Z60w and Z View were used for data handling and analysis.

RESULTS AND DISCUSSION

Figure 1 shows the variation of the phase angle with frequency for isotropic and die-drawn samples of i-PP. The phase angle is negative over the frequency range 1Hz to 106Hz. Therefore, the material can be con- sidered to consist of capacitative and resistive networks. However, at low frequencies the material indicates more ohmic than capacitative behaviour. This suggests that a leakage current may exist in the bulk material which makes the specimen more ohmic. On the other hand, at high frequencies, i.e. > lo4 Hz, the material shows more capacitative behaviour and has low impedance, which varies almost with the inverse of frequency. In the fre- quency range 1 Hz to - 5 x lo3 Hz, the ax. impedance is nearly independent of frequency as shown in Fig. 1. The drop in the a.c. impedance at frequencies higher than 104Hz is associated with a relaxation process as shown Fig. 2, which shows a plot of the imaginary com-

1 ogl, -1 00

-75 10

N

-50 5

10 2 nl -25

10 0

1 o5 loo 10' lo2 lo3 lo4 to5 10

Frequency (Hz)

Fig. 1. ax. impedance 2 (ohm) and phase-angle (theta) versus frequency for isotropic and die-drawn isotactic polypropylene : +, isotropic; 0, draw ratio 10.3; 0, draw ratio 12.8; 0,

draw ratio 16.8.

I 0 ! -7.500e7

-5.000e7

9 -2.500e7

0.000000

10 '3 lo3 I p 0 e 7 10' 10 loo 10' 1 fmquency (Hzj

Fig. 2. The imaginary component Z and the real component Z of the complex a.c. impedance versus frequency for iso- tropic and die-drawn isotactic polypropylene : +, isotropic; 0, draw ratio 10.3; 0, draw ratio 12.8; 0, draw ratio 16.8.

ponent 2" and the real component 2' of the complex a.c. impedance. This relaxation was observed in dynamic mechanical shear and tensile ~tudies .~ It was labelled as a /?-relaxation, which is associated with the glass-rubber transition in the amorphous region and arises from the chain motion in this region. Neverthe- less, in a.c. impedance measurements a well defined peak was observed at lo3 Hz for an isotropic sample in the plot of Z versus frequency as shown in Fig. 2. The intensity of the /?-relaxation process peak decreases and moves to a higher frequency with increasing draw ratio. It nearly disappears for a draw ratio of 16.8.

The increase of om, of 2" with increasing draw ratio, as shown in Fig. 2, is in agreement with observations from dynamic mechanical measurements of oriented p~lypropylene.~-' It was found that the peak of the /?- relaxation in the plot of tan 6 versus temperature moves slightly to a lower temperature at constant frequency. This implies that the drawing process at 110°C has no appreciable effects on the degree of crystallinity for i-PP. Vittoria et al." have found that the degree of crys- tallinity for drawn i-PP remains nearly constant up to a draw ratio of 14.

Using a density column with a mixture of water and methanol as a fluid, the degree of crystallinity was determined by means of the equation of Danusso et al.," assuming the amorphous density is 0.858 g ~ m - ~ and the crystal density as 0 - 9 3 6 g ~ m - ~ . ' ~ It was found that the degree of crystallinity of isotropic i-PP is of the order of 62% and for a draw ratio 16.8 it is of the order of 60%. It was found that the reduction in density was due to the formation of microvoids in the samples. The presence of such microvoids has been confirmed in a previous p~blication'~ from small-angle X-ray scat-

POLYMER INTERNATIONAL VOL. 40. NO. 4, 1996

Electrical impedance of polypropylene

-3.000e8

-2.00068

N

-1 .oooe8

Real Center. 9.60137 Imag. CCnter: 6.4- Diameter: 1.92e8 Low lniercept -1.3Bs5 High Intercept: 1.92eB

Estimated R(ohms): 1.9308 Estimated C(farads): 1.-12

w - w : 4005

0 . a 0

1 . m e 8 2.OOoe8 3.OOoe8 7 1

-1.500e7

-1.000e7

N

L

Real Center: 5.71e6 hag. Center: 3.57e5 Diameter 1.15e7 Low Intempt: -2.2484 High Intercept 1.14e7

Estimated R(ohms): 1 .15~7 Estimated C(farads): 1.52e-12

(c)

w - w : 5.71e4

z

Imag. Ceter: 3.03e5 Diameter: 3.03a7 Low Intercepl: 3.91~34 High In- 3.04e7 w_mpx: 2.12e4 Estimated R(ohms): 3.03e7 Estimated C(faradr): 153-12

-3.000e7

t -2.OOOe7

I

z

Real Center 1 63e6 lmag Center 1 1084 Dlafmter 3 226 Low Inlsrcep( 2 2Oe4 Huh Intercept 3 2 5 4 w-max 2 1085 Eshmatcd R(ohms) 3 2206 Estmated C(farads) 1 47,12

-3.OOOe6

" t -2.000e6 t

B I I I 7

6 2.000e6 3.000e6 4.0 0 . _ _

1 .woe z

297

)e7

Oe6

Fig. 3. Plots of 2' versus Z for isotactic polypropylene, samples: (a) isotropic; (b) draw ratio 10.3; (c) drawn ratio 12.8; (d) draw ratio 16-8.

tering patterns, which show two streaks at the edges. For high draw ratio samples, the deformation occurs along the longitudinal direction resulting in a thinning of the fibds, which does not necessarily require an

TABLE 1. Bulk resistance ( R ) , equivalent capac- itance (C) and relaxation time (r) for isotropic and

die-drawn isotactic polypropylene

Isotropic 1.92 x 1 O8 1.29 x lo- ' ' 0.249 10.3" 3.03 x 1 O7 1.52 x 1 0-l' 0.0473 12.8" 1.14 x 10' 1.52 x lo-'' 0.0175 16.8" 3.2 x 10' 1.47 x 1 O-'' 0.0047

" Draw ratio

increase in crystal size.14 Therefore, we can conclude that the shift in the B-peak to a higher frequency upon drawing is mainly due to the morphological changes taking place in the amorphous regions, and the degree of crystallinity has no effect on the observed decrease in the intensity of the peak.

Isotactic polypropylene is essentially a non-polar polymer. Curtis" has estimated that the net contribu- tion of C-H dipoles is sufficient to account for the observed loss in the dielectric measurement. Since the B-relaxation occurs in the amorphous region, then the decrease in the intensity occurs as a consequence of a decrease in the number of molecules contributing to this relaxation. This is the case when a decrease in volume is due partially to the collapse of localized high volume, high entropy regions, which remain in an internal ther- modynamic equilibrium.16 By increasing the draw ratio

POLYMER I~VTERNATIONAL VOL. 40. NO. 4,1996

298 S. A . Jawad, A. Alnajjar

the degree of orientation of the amorphous chains increases steadily up to the highest obtainable draw ratio.” Hsu and Kwan’* found that the intensity of the dielectric dissipation factor (tan 6) is a minimum when the electric field is applied parallel to the draw direction. They concluded that the motion of dipoles would neces- sarily involve rocking of the chain back-bone. This implies that a comparatively large activation energy is needed. Therefore, increasing the degree of orientation in the amorphous region, causes continuous increase of the fraction of taut tie molecules’g axially connecting crystalline blocks of the same or different microfibrils, and the constraint imposed on the amorphous region will affect the molecular mobility and as a result decrease the relaxation intensity associated with this motion. On the other hand, orientation of the crys- tallites and taut tie molecules will affect the electronic polarizability and lead to a decrease of the a.c. imped- ance of die-drawn i-PP as shown in Fig. 1.

Figures 3a-d show plots of 2” versus Z for isotropic and die-drawn samples. The relation between 2” and Z at intermediate frequencies yields Cole-Cole plots with a particular relaxation time. The plots for isotropic and die-drawn samples give nearly perfect semicircles with different characteristics. These plots show some inter- esting aspects. The first is that the imaginary com- ponent 2” decreases with increasing draw ratio, indicating that the specimens become less capacitative and more conductive.

The decreasing values of the resistive component, determined from the intercepts of the semicircles with the 2’ axis, show that the Z component decreases with increasing draw ratio from 1.92 x lo8 R for an undrawn isotropic sample to 3.2 x lo6 R for a draw ratio of 16.8.

The second aspect is that the semicircular plots give different maximum frequencies ( WmaJ, implying that there is no unique relaxation time for the electrical con- duction mechanism. However, the relaxation time con- stant for certain draw ratios can be determined from the Cole-Cole plots shown in Figs 3a-d by using the formula W,,, z = 1, where W,,, is the peak frequency and z is the relaxation time. The values of z and the equivalent resistance and capacitance for isotropic and die-drawn samples of i-PP are given in Table 1. The bulk resistance of the isotropic sample is about 60 times higher than the bulk resistance of the 16.8 draw ratio sample. The decrease in the bulk resistance upon increasing draw ratio is associated with a decrease in

the relaxation time, the deformation process hmdering the motion of the chains in the amorphous region.

CONCLUSIONS

The present study of the a.c. impedance of isotropic and die-drawn samples of isotactic polypropylene reveals the following results.

1. The overall a.c. impedance decreases with increas- ing draw ratio in the frequency range - lo3 to lo6 Hz.

2. The a.c. impedance decreases with increasing draw ratio in the frequency range 1Hz to -103Hz, but is nearly independent of frequency for certain draw ratios.

3. The dramatic drop in a.c. impedance at high fre- quency is associated with the /?-relaxation process.

4. The intensity of the /?-relaxation process decreases with increasing draw ratio, and nearly disappears for a draw ratio of 16.8.

5. A plot of 2” versus Z reveals Cole-Cole semi- circles for isotropic and die-drawn samples with differ- ent characteristics.

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9 10

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19

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POLYMER INTERNATIONAL VOL. 40. NO. 4, 1996