Materials Chemistry 5 (1980) 407 - 410 © CENFOR S.R.L. - Printed in Italy

SHORT COMMUNICATION

EFFECT OF ANNEALING BELOW THE GLASS TRANSITION ON THE LOSS PEAK OF GLASSY POLYCARBONATE

Received 25 July 1980; accepted 8 September 1980

Many properties of glassy polymers undergo significant changes upon anneal- ing 1. These changes which may be reversed with a brief heating to above the glass transition temperature, have,been related to enthalpy and/or free volume relaxa-

tion. As both enthalpy and free volume tend either increase or decrease depending

on their values with respect to equilibrium at the annealing conditions, polymers starting with deficient values of free volume and enthalpy should be expected to show converse property changes with respect to those starting with excess values. In fact some evidence of this is already present in the literature 2.

On the other hand densified samples prepared by cooling under very high pressure from above the glass transition temperature showed properties very close to those of samples prepared under much lower pressure 3.

The effect of pressure, during cooling from above Tg, on the annealing be- haviour of glassy polymers is further investigated in this work by means of dynam- ic mechanical and dielectric measurements performed on polycarbonate samples.

The material studied was polycarbonate Sinvet 301 manufactured by Anic. Samples 0.2 mm thick were prepared by compression moulding at 180"C un-

der two values of pressure: 5 and 150 atm. A cooling rate of about 0.5°C/sec was adopted down to room temperature. The samples were then subjected to annealing

procedures at 1300C and 1 atm. Dynamic mechanical data were collected on a Rheovibron DDV II C instru-

ment at a frequency of 110 Hz. Dielectric measurements were performed at 10 ~ Hz on disk shaped samples,

408

7 cm in diameter, by means of a three electrode cell Balsbough LD-3 and a Gen- eral Radio capacitance bridge.

For both type of measurements the temperature was varied from room tem- perature to above Tg with a heating rate of l*C/min.

The temperature Tm corresponding to the maxima of loss modulus and loss factor as obtained from dynamic mechanical and dielectric measurements have been plotted in Fig. 1 versus the annealing time. Each point in this plot corresponds to the average value of Tm evaluated on the basis of three measurements. The temper- ature Tin, which is often indicated as the glass transition temperature, corresponds to the situation when the structural relaxation times decrease to the order of recip- rocal t'requency and is certainly different from the conventional glass transition tem- perature as defined by Hutchinson and Kovaks 4 .

A constant Tm difference between the dielectric and the corresponding dynam.

155 dielectric 10 3 Hz

[ 6 _ -o-_~_

150 ~J high pressure ~ , &

~E low pressure O , •

dynamic mech.110 Hz

t, days

140 ! i 0 10 20

Fig. 1 - Temperature o f loss peak from dielectric and dynamic mechanical meas- urements vs annealing time at 130°C. Samples were prepared by compression mouM- ing under: o, • low pressure; A, • high pressure.

409

ic data may be observed in Fig. 1; this difference is very close to that expected on the basis of the frequency ratio between the two types of measurements.

The data taken on the samples prepared under low pressure show that Tm in.

creases with the annealing time t. A similar behaviour was already observed in other cases 1 and is consistent with calorimetric results ~" a, s, ~. Also the expansion coef- ficient has a peak at a temperature which increases with the annealing time*. All these phenomena have been explained relating the rate of change of free volume

departure from equilibrium to molecular mobility which in its turn is related to

free volume: longer annealing times give rise to smaller molecular mobility which requires larger times and temperatures Tm in a constant heating rate experiment before the relaxation time undergoes a sufficient decrease.

The behaviour of samples prepared under larger pressure, for which Tm de-

creases with the annealing time can also be explained within this picture. In this case, during the cooling, the equilibrium free volume at each temperature is de-

creased by effect of pressure; a smaller value of free volume is thus obtained and in fact, in similar cases 3 , a density increase was measured. If this value is smaller

than the equilibrium one in the annealing conditions, the relaxation time is expect- ed to evolve towards smaller values by effect of annealing. Indeed smaller values of Tm are shown in Fig. 1 at large annealing times. At smaller times the curve of Tm shows a moderate minimum. This is related to the kinetics of the processes towards the equilibrium status in the annealing conditions.

At large annealing times the same value of Tm is reached by samples prepar- ed under both high and low pressure; this was expected in view of the fact that, indipendently from the initial status, the same equilibrium is eventually approach-

ed.

A ckno wledgemen t

This work has been supported by C.N.R. Grant N. CT 79.01192.03.

G. Titomanlio, F.P. La Mantia

Istituto di Ingegneria Chimica, Facoltd di Ingegneria, Universitd di PALERMO - Italy.

REFERENCES

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Mechanical Behavior, E. Baer and S.V. Radcliffe Eds, American Society for Metals, Metals Park, Ohio, 1974.

2. S.E.B. PETRIE - A C S Polym. Prepr., 15(2), 336, 1974. 3. H.W. BREE, J. HEIJBOER, L.C.E, STRUIK, A.G.M. TAK - J. Polym. Sci.,

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