Polymer International 40 (1996) 261-267

Dielectric Analysis of the a-Relaxation of PVC Stabilized with Cadmium Laurate

Salah Mahrous

Physics Department, Faculty of Education, Fayim 631 11, Egypt

(Received 23 November 1995; accepted 8 March 1996)

Abstract: The dielectric permittivity and losses of PVC stabilized with cadmium laurate were measured in the temperature range 300-400 K and frequency range 1 kHz-1 MHz. The relaxation behaviour was found to obey the Williams- Landel-Ferry (WLF) equation. The observed activation energy was 1.1 eV which is in fair agreement with that of the a-relaxation in the literature. The a- relaxation peak was found to be due to both C-C1 and - 0 O C groups. A plasti- cization effect of the polymer was observed.

Key words: PVC, cadmium laurate, a-relaxation, dielectric analysis, stabiliza- tion.

INTRODUCTION

In the amorphous phase of polymers, there are two main chain motions. The first is the micro-Brownian motion, which occurs at the glass transition tem- perature. The second is a more local type of motion, which occurs in the glassy state. While the detailed mechanism of the second type seems rather uncertain, a local relaxation mode has been proposed to explain it. This mode, proposed by Okada,'*2 suggests that on a time scale shorter than the relaxation times of the micro-Brownian motion, motions of the main chain can be regarded as small scale, confined to the neighbour- hood of the local equilibrium conformation.

The ideal PVC stabilizer is considered to be a multi- functional material capable of fulfilling the following requirements :3 (i) provision of a shield, protective screen, or sink, for UV radiation; (ii) absorption and neutralization of hydrogen chloride evolved; (iii) neu- tralization or inactivation of stabilizer degradation pro- ducts; (iv) chain reaction disruption; (v) neutralization or inactivation of polymer impurities; and (vi) displace- ment of active, labile substituent groups, such as the chlorine atom attached to a tertiary carbon.

The laurate soap of cadmium has been considered to be one of the most efficient materials commercially

available for PVC stabilization: though its commercial use has declined somewhat in recent years due to tox- icity issues. The stabilization mechanism appears to be as follows:

OOCR /

Cd + -CH-CH=CH-- \ I

OOCR C1 OOCR

/

\ -CH-CH=CH- + Cd (1)

c1 I

OOCR

The Cd(00CR)Cl produced could stabilize a second site, but at this point a molecule of CdC1, would be formed, which would almost immediately initiate the destruction of the polymer by a Friedel-Crafts type alkylation leading to further crosslinking -CH,-CCH-CH,-CH-

I c1

I C'

A C1- $d-C1 __j

I -CH-CH-CH-CH,-

I c1

I c1

26 1 Polymer International 0959-8103/96/$09.00 0 1996 SCI. Printed in Great Britain

262 S. Mahrous

-CHZ-CH-CHz-CH* I c1

*CH- CH-CH- CH,- I c1

I c1

+ H+(CdC13)- 1 (2)

HC1+ CdClz

In this paper we report the dielectric properties of PVC stabilized with a high concentration (10 wt%) of cadmium laurate and discuss the mechanisms associ- ated with the relaxation process.

EXPERIMENTAL

The polymer used was poly(viny1 chloride) purchased from Polymer Laboratories Ltd. The weight-average molecular weight A?, was 2 x lo5 and Mw/Mn was 1.9 (where A, is the number-average molecular weight). The polymer and 10wt.% of the stabilizer (cadmium laurate) were mixed in a small container and melted in an electric furnace. The melt filled up the space between the silver-coated brass electrodes of diameter 10 mm. The distance between the electrodes was 0-1 mm, which is the sample thickness.

The AC dielectric measurements were done using parallel plate geometry. A Hioki 3530 LCR Hi Tester was used to measure the capacitance (C), and dissi- pation factor (tan 6 = E"/E') of the sample in the fre- quency range 1 kHz-1 MHz from 300 to 400K at a heating rate of l°C/min. The real E' and imaginary 8''

components of the dielectric constant were calculated using

E' = Cd/Eo A

E" = E' tan 6

where d is the sample thickness, E~ the vacuum permit- tivity (8.854 x lo-', F/m), A the .electrode area and 6 the loss angle.

(3)

RESULTS

The variation of the dielectric constant (8') against logf for pure PVC and PVC stabilized with cadmium laurate at different fixed temperatures (300-400 K) is shown in Fig. 1. A saturation region at low frequency was observed for all temperatures in both cases. This satura- tion region was less at higher temperatures. With further increase in frequency, E' began to decrease.

Figure 2 exhibits the dependence of the dielectric loss E" on logf in the frequency range 1 kHz-1 MHz. The curves show a single relaxation peak in the temperature range studied. This relaxation peak is the a-relaxation for PVC, which is characterized as a dipolar relaxation. It is noticed that the introduction of stabilizer reduces

T g for the polymer. The peak width also increased with the addition of cadmium laurate to the polymer. The peak position shifted towards the high frequency region with increasing temperature. The peak height also increased with temperature.

The loss curves at various temperatures can be reduced to a master curve (Fig. 3) by using the peak position (f,,.J and peak value (~$3. Since a master curve exists for each case, the temperature change in the shape of the E" curve can be neglected for the present temperature range. The shape of the master loss curve is found to be broader than that of the Debye relaxation and is asymmetric.

The peak frequency, hereafter denoted as f,, , increases with increasing temperature. This behaviour allows the assignment of the peak to the a-relaxation of the stabilized PVC. As shown in Fig. 4, an Arrhenius plot of fmax(df) can be used to calculate the apparent activation energy, which is about l.leV, in fair agree- ment with that of the a-relaxation values in the liter- a t ~ r e . ~ - ~

The value off,,, can be well reproduced in terms of the Williams-Landel-Ferry equation,* which is applic- able to a free volume limited mechanism and is charac- teristic of the collective mechanisms of motion of the chain segments at T, .

(4)

where C1 = 9, Cz = 100 K, fo = 3 kHz and To = 300 K. Figure 5 shows the reduced value off,,, as a function of (T - To). All the data are on the WLF master curve.

DISCUSSION

The crystallinity of PVC has been the subject of some controversy. Studies using small-angle neutron scattering3 and X-ray diffract i~n~. '~ have shown that it ranges from 5 to 15%. It has been proposed from infra- red spectroscopy' ' that PVC has crystalline regions composed of extended syndiotactic segments. Thus, sta- bilized PVC can be considered as a three-dimensional network with crystalline crosslinks (see eqns 1 and 2). It is possible that the amorphous regions are solvated by the stabilizer, while the crystalline domains are not modified. This suggestion was reinforced by NMR mea- surements," which show that the additive molecules easily penetrate the amorphous regions. Thus it may be concluded that there are two domains present in the sample, one reached by the cadmium laurate molecules (the amorphous part) and the other not (the crystalline phase).

The introduction of cadmium laurate increases the free volume in the system and lowers the glass tran- sition temperature. Growth in the free volume also lowers the interaction between molecules. Cadmium

POLYMER INTERNATIONAL VOL. 40. NO. 4, 1996

a-Relaxation of PVC stabilized with cadmium laurate

d

5.5

5.0

4.5

4.0

3.5

3.0

2.5

2.0

263

3.0 3.5 4.0 4.5 5.0 5.5 6.0

losf

Fig. 1. Dependence of E' on the logarithm of frequency at various temperatures. (a) Pure PVC; (b) PVC stabilized with cadmium laurate.

laurate screens the polar groups of the polymer and pre- On the other hand, the stabilizer molecules may act vents the formation of polymer-polymer bonds, thus as widely spaced crosslinks between adjacent PVC lowering q. This indicates that cadmium laurate chains. The greater effectiveness of the plasticization behaves as a plasticizer for PVC. effect is shown by the lowering of T g and the increased

POLYMER INTERNATIONAL VOL. 40, NO. 4, 1996

264 S. Mahrous

s"

3.0 3.5 4.0 4.5 5 .O 5.5 6.0

losf

14

12

10

a

6

4 3.0 3 .s 4.0 4.5 5 .o 5.5 6.0

losf

Fig. 2. Variation of E" with frequency. (a) Pure PVC; (b) PVC stabilized with cadmium laurate.

broadening of the loss peaks (Fig. 2). The relaxation peaks are related to the glass-rubber

transition. At this point the C 4 1 groups become mobile, resulting in a sharp increase in the dielectric

constant and strong loss peaks. The shift of the peaks to higher frequency, compared with those for pure PVC, with increasing temperature, may be due to the rota- tional motion of the ester groups (which exist in the

POLYMER INTERNATIONAL VOL. 40. NO. 4, 1996

a-Relaxation of PVC stabilized with cadmium laurate 265

t 3 T 7 Y - 0 2 ~ - ( Y o * 9 9

laglflfux

Fig. 3. Dependence of reduced dielectric losses (&’’/&J on the logarithm of reduced frequency (flfa for stabilized PVC.

6.5

8

5.5 8

8

8

8

3 ‘ 2.4 2.5 2.6 2.7 2.8 2.9 3 3.1 3.2 3.3

1OWK

Fig. 4. Arrhenius plot of the logarithm offma versus 1000/T for PVC stabilized with cadmium laurate.

2

1.5

1

- 8 -0.5 -

-1

-1.5

8

-2

0 20 40 60 80 1 0 0

T To K I Fig. 5. Logarithm of ( versus (T - &).

POLYMER INTERNATIONAL VOL. 40. NO. 4, 1996

266

19.5

18.5

17.5

16.5

2 15.5 I-

14.5

13.5

12.5

11.5

10.5

S. Mahrous

.-

.-

.-

. -

.- a

~-

- -

. -

-. 8 - -

8

280 300 320 340 360 380 400 420

T t K )

Fig. 6. Dependence of TAc on temperature for stabilized PVC.

stabilizer) in combination with the C-C1 groups of the PVC. From that point of view, the relaxation peak may be due to the release of the frozen-in dipolar groups (i.e. C - C I and -0OC groups) and their co-operative motion with adjoining segments of the main PVC chains.

Alternatively, a-relaxation peaks show increasing magnitude with increasing temperature as a result of Brownian motion.13 The increase in the dielectric con- stant is determined by the number of orienting dipoles per unit volume, their dipole moment, and the dipole- dipole correlation factor." Thus, the increase of the peak magnitude is caused by increase in the density of dipoles contributing to the relaxation process with increasing temperature. This is due to the fact that increasing temperature releases more dip01es.I~

Figure 6 shows the temperature dependence of TAE for PVC stabilized with cadmium laurate. The dielectric strength would be proportional to 1/T, that is, TAE would be constant, if there were no correlation of move- ment among the segments. However, it is found (Fig. 6) that TAE increases with temperature, i.e. correlation of movement exists between the segments.

The 8'' curves are broader than that for Debye relax- ation, and are asymmetric. The broadness and asym- metry of E" can be evaluated by fitting the data to the

Havriliak-Negami equation :I5

c*(o) = E , + AeJC(1 + iozij)'+"jlsj (5)

where E* is the complex dielectric constant; Agj = - ej,, where ejS and tj, are the low-frequency and high-frequency limits of the dielectric constant, respec- tively; o is the angular frequency; z is the relaxation time; and a and are parameters characterizing the shape of the dielectric relaxation curves. The real and imaginary parts are given in the following equations :I6

E'(o) = E , + r-B/2A& cos(p8) (6)

E"(o) = r-B/2AE sin(j?8) (7)

+ ((wzo)l - O L cos(a./2))2 (8)

r = (1 + (wo z O ) l F a sin(a~/2))~

(9) (ozO)l-a cos (ax/2)

1 + (wz0)l - a sin(az/2) 8 = arctan

Equations (5) and (6) contain a, j?, AE and zo. In other words, if eqn (5) is used as the model for the frequency- dependent data, then four of the five parameters can be determined. These can be used to calculate ~'(o) - E ,

from eqn (5). The best fitted results were obtained to

TABLE 1. Havriliak-Negami and standard deviation parameters of dielectric relaxation of stabilized

PVC

300 0.196 0.882 0.265 3.45 -5.84 0.057 320 0.230 0.789 1.22 3.40 -4.92 0.070 340 0.230 0.787 1.85 3.34 -5.66 0.020 370 0.421 0.750 1.22 3.31 -4.41 0.012 400 0.423 0.820 1.27 3.45 -5.27 0.041

POLYMER INTERNATIONAL VOL. 40. NO. 4, 1996

a-Relaxation of PVC stabilized with cadmium laurate 267

minimize the standard deviation given by

where n denotes the number of data points. The stan- dard deviation of d was obtained in the present case with the parameters which minimize the standard deviation of E". The dielectric parameters and the standard deviations obtained by this analysis are listed in Table 1.

In conclusion, the results show that cadmium laurate, which is applied to stabilize PVC, decreases the inter- molecular interaction between the chains and conse- quently has a plasticization effect.

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