Physical aging of amorphous linear polyesters

IOURNA L OF

ELSEVIER

Journal of Non-Crystalline Solids 172-174 (1994) 622-627

Physical aging of amorphous linear polyesters P. Cort6s*, S. Montserrat E.T.S. Enginyers Industrials de Terrassa. Universitat Politecnica de Catalunya, E-08222, Terrassa, Spain

Abstract The physical aging of a series of linear amorphous polyesters (poly(propylene isophthalate), poly(propylene terephthalate), poly(ethylene terephthalate), poly(dipropylene terephthalate) and poly(diethylene terephthalate)) has been studied by differential scanning calorimetry. These polyesters have been submitted to comparable aging temperatures below the glass transition temperature, Tg: T8 - 27, T~ - 15 and Tg - 9(°C), for a period from 15 min to 480 h. The excess enthalpy of aged samples, the relaxation time and the activation energy were calculated. It was found that the polyesters with an ether group have broader peaks and a higher relaxation rate, that the methyl group seems to increase the relaxation rate and that ring substitution can affect the structural relaxation.

1. Introduction Physical aging in amorphous polymers below the glass transition temperature, Tg, occurs due to the non-equilibrium nature of the glassy state. The thermodynamic properties, specific volume or enthalpy, of the amorphous polymers cooled below Tg and aged in the glassy state during a period of time decrease to their equilibrium values; this relaxation toward the equilibrium state is commonly referred to as physical aging or structural relaxation. In physical aging, the free volume decreases to approach an equilibrium value; this reduces the chain segmental mobility and this in turn determines that the rate of diminution of the free volume

* Corresponding author: Tel: + 34-3 739 8100. Telefax: + 343 739 8101.

also decreases. As a consequence, the rate of the relaxation process depends on the chain segmental mobility of the polymer [1]. It is clear that any factor which controls the free volume, or mobility, of a polymer at a constant temperature can affect the process of structural relaxation. One of these factors is the chemical structure that determines the intramolecular forces, which related both to the structural parameters affecting the stiffness of the chain backbone which are determined by the barrier to internal rotation, and also by the steric hindrance introduced by the presence of substituents on the backbone chain atoms. Within the chemical structure, the role of the intermolecular forces is also important, and is related to the presence of highly polar groups along the polymer chain [2,3] The main goal of this work is to study the effect of the chemical structure on structural relaxation rate for a series of linear amorphous polyesters.

0022-3093/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0 0 2 2 - 3 0 9 3 ( 9 4 ) 0 0 0 4 5 - 0

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P. Cortbs, S. Montserrat / Journal of Non-Crystalline Solids 172-174 (1994) 622-627 Table 1 Mn and Tg values for the polyesters studied Polymer

Formula

foo Ocooc.,cH , . t PPTP

tOOC O C O O C H ( C H 3 ) C H 2 t

M.

Tg(°C)

1,

77 95

9300

tOO©OOCHC4 PDPT

tOOC O C O O C H ( C H

PDET

[OOCOCOOCHzCHzOCH2CH2 ]

77

3)CH2OCH2(CH 3)CHt

2. E x p e r i m e n t a l

2.1. Materials The polymers used in this work are a series of fractionated polymers: poly(propylene isophthalate) I-4] (PPIP); poly(propylene terephthalate) [4] (PPTP); poly(dipropylene terephthalate) [5-], (PDPT); and poly(diethylene terephthalate) 1-6-1 (PDET); and a commercial and unfractionated poly(ethylene terephthalate) (PETP). The formulae and characteristics of these polymers are listed in Table 1 2.2. Thermal analysis A Mettler Thermoanalyser TA 4000 equipped with a low temperature range DSC 30 differential scanning calorimetry module was used to perform the thermal analysis. The calorimeter was previously calibrated with indium, lead and zinc standards. The differential scanning calorimetry (DSC) curves were obtained using about 10 mg of sample with a heating rate of 10°C/min. 2.3. Thermal history and enthalpy relaxation measurements The thermal history which was used in this work is: (a) sample at T2 > Tg for 5 min in order to erase its previous history; (b) sample cooled at 20°C/min to the aging temperature, Ta, and aged for an aging

9700

25

7400

22

Table 2 Aging temperatures, Ta, aging times, ta and temperature above Tv T2, for the different polyesters used Polymer

Ta ('~C)

ta (h)

T2

PPIP PETP PPTP PDPT PDET

50; 60; 68 50; 60; 68 68; 80; 86 10; 16 7; 13

0.25 480 0.25-480 0.25-480 0.5-192 1-192

100 100 125 70 60

time, t,, in a thermostatic bath; (c) sample cooled at 20°C/min to the scan starting temperature, T~; (d) sample scanned at heating rate of 10°C/min from T~ to 7"2 collecting all data. In order to obtain a second scan, the sample was cooled at 20°C/min to T1 and reheated to T2. Table 2 shows the values of Ta, ta and T2 for the different polyesters. It can be shown that the enthalpy relaxation, AH, can be calculated by the expression AH

= HB --

Hc

=

TI

(Cp, aged -- Cp.... ged)dT

= --1 ft2 (Paged -- Punaged) dt , m tl

(1)

where Cp, aged and Cp.... sea are the specific heat capacities for the aged and unaged samples, and Paged and Punaged are the output power signal for the aged and unaged samples, respectively.

P. Cortbs, S. Montserrat / Journal of Non-Crystalline Solids 172-174 (1994) 622-627

624

2.4. Calculation o f the activation energy

between lniql and 1/Tf [7]:

In order to calculate the activation energy for the polyesters, the different polyesters have been submitted to intrinsic cycles at different cooling rates, q, of - 0 . 5 , - 1 , - 2 . 5 , - 5 , - 1 0 , - 1 5 and with the subsequent heating at lO°C/min. From these experiments, it is possible to calculate the fictive temperature, Tr, of the glass for each cooling rate, and the activation energy, Ah*, is determined from the slope of the linear relationship

dlnlql d 1/Tr

The values of the activation energy, Ah*, calculated by Eq. (2) are in Table 3.

-25°C/min

3. Results 3.1. D S C curves

The DSC scan of the aged samples, shows the presence of an endothermic peak whose position and intensity depend on the aging conditions, both temperature and time. Fig. 1 shows the DSC scan for the PETP. The response of both the PPIP and the PPTP is similar to the PETP; this behaviour is in a good agreement with other polymer results and is a general feature of the structural relaxation of glassy polymers. However, it is observed that the responses shown by the PDPT and the PDET are different from those of the other polyesters: the PDPT and the PDET have broad peaks, and this different behavi-

Table 3 Activation energy, Ah*, for the different polyesters calculated for Eq. (2) Polymer

Ah* (kJ/mol)

Ah*/R (kK)

PPIP PPTP PETP PDPT

648 880 1044 1097

78 106 126 132

_ 32 _+ 87 _ 98 ___255

___4 -t- 11 -t- 12 +_ 31

I

II

4°:

48411~

(2)

Ah*/R .

l~ff ~

4e 72 \'-~%.~68

24, /

s~

i

o.

:I

0257/IIII

I

I

I

40.

60.

80.

"

('c}

I

1-

I

o.

20.

40.

"

'--

('c)

Fig. 1. D S C scan for the polyesters PETP, P D P T and P D E T , aged at 7". = Tg - 15(°C) for the indicated times (h).

P. Cortbs, S. Montserrat / Journal of Non-Crystalline Solids 172-174 (1994) 622 627

our could be related by the relaxation time spectrum. 3.2. Enthalpy relaxation

The enthalpy relaxation values calculated by Eq. (1) are plotted against log ta in Fig. 2 for the

5

pplp /

ITa=Tg-27 (°C)] 40

•

3"1"

2-

•

PETP

o

0

i

i

i

0

1

2

3

log ta (h)

4 Ta=Tg-15 (°C) 3

625

different polyesters at the indicated Ta where it can be seen that the enthalpy relaxation increases with log ta. This behaviour appears non-linear, up to a limit value, if a wide range of aging time is considered. For T a = Tg - 27 (°C), P P I P is the polymer with highest value of AH, followed by PPTP, with P E T P having the smallest value of AH. For Ta = Tg - 15(°C), AH increases with log ta but for the P P I P AH becomes practically constant when ta is of about 10 h because the sample is close to reaching its structural equilibrium state. Over the tirnescale during which enthalpy relaxation is occurring, the highest values of AH are, again, for PPIP. For T a = T g - 9(°C), both the P P I P and the P P T P have reached equilibrium and the AH for the P E T P becomes practically constant for t~ ~> 48 h. On the other hand, the AH of both P D P T and P D E T continues to increase with log ta and in neither case is structural equilibrium achieved within the experimental times used,

cPET

PDPT

PP

4. D i s c u s s i o n "1-

o

I-1

4.1. Kinetics o f the enthalpy relaxation process

t

1

-"

i

i

i

0

1

2

3

To examine the kinetics of the enthalpy relaxation, let us introduce a relaxation function given by

log ta (h)

= l - ( A H / A H . ) = (3/AH~),

(3)

where 6, the excess enthalpy, is the difference between the enthalpy of the aged glass and the enthalpy of the glass at equilibrium at the same Ta.

4-

31 ITa--Tg'9'°°' I

6 = AH~ - A H ,

(4)

where AH~ is a limiting enthalpy relaxation. This can be obtained by assuming a linear extrapolation of liquid heat capacity, Cp.t: 0

-

o

1

2 log ta (h)

Fig. 2. Enthalpy relaxationversuslog ta for the differentpolyesters for the indicated Ta. '3, PPIP; O, PPTP; +, PETP; Z], PDPT; &, PDET. Lines are drawn as guides for the eye.

AH~ =

I

T*

(Cp. 1 -- Cp

....

ged)

dT,

(5)

Ta

where T* is a temperature above Tg, Cp. z is a linear least-squares fit to the heat capacity of the unaged

P. Cortbs, S. Montserrat / Journal of Non-Crystalline Solids 172-174 (1994) 622-627

626

a single effective relaxation time, z©ff,which is related with the relaxation function by [8]

2

Ta=Tg-27 (°C) ] 0

PPIP

==-2

t~ln~b(ta) = -

In 1

(6)

Teff

0

_c -4

•

+4-

-6

I

I

2

4

-8

I 6 ~) (J/g)

8

2

Ta=Tg-15 (°C)[ 0

PPIP

Y

-2

-6"

./

~PETP

'/o/

T + ~ /

F~DPT

/

-8

0

i 2

i 4

i 6

8 (J/g)

2 Ta=Tg-9

(°C) I

0-

-2

T

.E -4 -6 °8

0

|

i

2

4

6

8 (J/g)

where 1/'Ceff is a measure of the rate of relaxation of the system as it approaches thermodynamic equilibrium. Fig. 3 shows the relation between the calculated values of ln(1/zeff) and the excess enthalpy, 6, for the five polyesters and for the indicated aging temperatures• The rate of relaxation of the system decreases as the system approaches equilibrium, i.e., as the excess enthalpy decreases. As shown in Fig. 3, the ln(1/zeff) values decrease in the order of PPIP, PPTP, PETP and PDPT for the three aging temperatures. For a given value of 6, the value of 1/'~ef f for the PPTP is higher than that for the PETP; Ah* for the PPTP is lower than for the PETP and for Ta = Tg - 9 (°C) the PPTP has reached structural equilibrium, whereas structural relaxation continues for the PETP. All of these results suggest that the presence of the methyl group in the propylene polyester seems to accelerate the molecular relaxation. This behaviour has also been observed previously in another polymer system [9]. The value of 1/'~ef f and of AH for the PPIP are higher than those for the PPTP; the PPIP has reached structural equilibrium at Ta = Tg - 15 (°C) after an aging time shorter than that for the PPTP, and Ah* is slightly lower in the PPIP than in the PPTP. These differences suggest that the ring substitution can affect structural relaxation, but this effect seems to decrease when T~ is close to

7",. Fig. 3. In 1/'t©ff against excess enthalpy for the different polyesters for the indicated T=. ©, PPIP, e , PPTP, + , PETP; IS], PDPT. Lines are drawn as guides for the eye.

sample in the liquid state and Cp.... g~dis the experimentally determined heat capacity of the unaged sample. In order to compare the enthalpy aging data for the different polymers, we use the concept of

The effect of the ether group and the longer length between the terephthaloyl groups can be analyzed by comparing the PPTP and the PDPT. Noting that the Tg for the PPTP is 95°C and for PDPT is 25°C, one might anticipate that the chain mobility for the PDPT is higher than for the PPTP. However, the values for the relaxation times for the PDPT are longer than for the PPTP. Moreover, the PPTP at T g - 9 ( ° C ) has achieved structural equilibrium while the PDPT continues the structural relaxation process until much longer times.

P. Cortbs, S. Montserrat / Journal of Non-Crystalline Solids 172 174 (1994) 622 627

According to Diaz Calleja et al. [5] it has been shown that the conformational entropies of the PDPT and the PPTP are respectively 0.84 and 1.00cal/K g. This could involve a more marked effect of the intramolecular interactions in the PDPT which could cause a decrease in the rate of structural relaxation.

5. Conclusions Physical aging of polyesters with different chemical structures suggest that: (a) the methyl group accelerates structural relaxation; (b) ring substitution can affect structural relaxation; and (c) different behaviour between the PPTP and PDPT can be explained in terms of the different intramolecular interactions between these polyesters. Financial support for this work has been provided by CICYT (Project no. MAT 92/0707). The authors are grateful to Professor J. Guzm~in and Professor E. Riande for supplying the samples of polyesters.

Note added in proof The results of poly(diethylen terephthalate) (PDET) reported in this paper correspond to a partially crystalline polymer, not to an amorphous one. According to Guzm/m and Fatou [10], the PDET can present amorphous or semicrystalline character depending on the method of preparation

627

and thermal history. The samples of PDET used were obtained by dissolving in ethyl acetate and lowering the temperature of the solution. The calorimetric analysis of the fraction of Mn --- 7400 shows an endothermic peak corresponding to the melting process, with a temperature and enthalpy of 75°C and 40 J/g, respectively. The polymer cannot crystallize from the melt, irrespective of the crystallization temperature, and remains as an amorphous polymer with a midpoint glass transition temperature of 20°C. The samples of PDET submitted to aging were heated at 60°C, and then aged at temperatures of 7 and 13°C.

References [1] L.C.E. Struick, Physical Aging in Amorphous Polymers and other Materials (Elsevier, Amsterdam, 1978). [2] J.J. Aklonis, W.J. Macknight and M. Shen, Introduction to Polymer Viscoelasticity, (Wiley-Interscience, New York, 1972). [3] P. Meares, Polymer. Structure and Bulk Properties (Van Nostrand, London, 1967). [4] C. Perez, J. Guzm~n, E. Riande and J.G. de la Campa, Makromol. Chem. 189 (1988) 691. [5] R. Diaz Calleja, E. Riande and J. Guzm/m, Macromolecules 22 (1989) 3654. [6] E. Riande and J. Guzmfin, J. Polym. Sci, Polym. Phys. 21 (1983) 2473. [7] C.T. Moynihan, A.J. Easteal, M.A. DeBolt and J. Tucker, J. Am Ceram. Soc. 59 (1979) 12. [8] S. Montserrat, in: Trends in Non-Crystalline Solids, eds. A. Conde, C.F. Conde and M. Millan (World Scientific, Singapore, 1992) p. 305. [9] T. Hatakeyama, H. Yoshida, S. Hirose and H. Hatakeyama, Thermochim. Acta 163 (1990) 175. 1-10] J. Guzmfin and J.G. Fatou, Eur. Polym. J. 14 (1978) 943.