About the Comment from E. Marchal on “Thermally Stimulated creep: A Theoretical Understanding of the Compensation Law”

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About the Comment from E. Marchal on “Thermally Stimulated creep: A Theoretical Understanding of the

Compensation Law”

J. Perez, J. Cavaillé

To cite this version:

J. Perez, J. Cavaillé. About the Comment from E. Marchal on “Thermally Stimulated creep: A

Theoretical Understanding of the Compensation Law”. Journal de Physique III, EDP Sciences, 1995,

5 (12), pp.1925-1926. �10.1051/jp3:1995238�. �jpa-00249427�

J.

Phys.

III

France

5

(1995) 1925-1926 DECEMBER1995,

PAGE 1925

Classification Physics

Abstracts

62.40

+I

65.70 + y

83.20Di

About the Comment from E. Marchal _on "Thermally Stimulated

creep: A Theoretical Understanding of the Compensation Law"

J. Perez(~) and J-Y- Cavail14(~)

(~) Groupe

d'Etude de

M4tallurgie Physique

et

physique des

matAriaux

j* ),

20 avenue Albert

Einstein,

69621

Villeurbanne Cedex,

France

(~) CERMAV, CNRS,

Domaine

Universitaire,

BP

53X,.38041

Grenoble

Cedex,

France

(Received

and

accepted

8 November

1995)

The

comment

of E. Marchal (E.M.) contains

_some

remarks about the _paper under

_concern

and

_an

alternative proposition [I] ^{for the} calculation of

_r~

and T~ (compensation parameters).

Firstly, let

_us

consider those remarks:

the expansion of equation (7c) is made only for relating

_r~

and

_rrnoj

but, obviously, equation (7c) in its whole is

to _use

for describing the creep

near

Tg

as

far

_as

only the strain component

proper to the characteristic time

_rr

is concerned (elementary relaxation phenomenon resulting from thermal sampling);

7a~ ^is identified

to 7~

(not

_to

_7rnoj!) thanks

to

simple and usual rheological arguments (refer

to

the model of standard linear solid);

can an

is

not

neglected sinc~ it is proposed, in the P-C- _paper,

_to

attribute the origin of the a' relaxation to that strain component.

Such

_a

distinction between the role of both strain components _cup and

_can

is clearly supported by the results of Oleinik _[2] showing

two

peaks of thermostimulated creep after low temperature

deformation of amorphous polymers:

one

just above the deformation temperature, ^due

to can

(restoring force resulting from internal energy) and the second

at

T higher but

_near

Tg, ^due

to

cup (restoring force resulting from entropy I-e- rubbery effect); both peaks _merge when the temperature ^of deformation increases and approaches _Tg.

Using E-M- approach _[I] leads

to

theoretical values of

_r~

which

_are

in

_a

better agreement with experimental data than with the P-C- theory. But, such

_an

approach is based upon

a

rather arbitrary postulate: the origin of time is taken

at

the beginning of the linear heating

rate

with T

=

To

+

bt; that is questionable since the

true

origin is

at

the actual beginning of the experiment I-e- when the stress (or electric field) is applied in order

to

polarize the specimen.

Moreover,

_some

features such

_as

the relation between log(rap) and Uap (see Figure 3b of P-C-

paper)

_are not

taken into account. Nevertheless, it would be valuable

to

develop

_some

finite element method describing the whole experiment, from the preliminary poling until the final unpoling, including thermal sampling in order

to state

what is the best _way

to

describe the molecular mobility; similarly, deeper experiments

_are

required for specifying

some

aspects ^such

as

the heating

rate

dependence, the value of the compensation parameters ^of a' relaxation, (*) URA CNRS

341

@

Les Editions de

Physique

1995

1926

JOURNAL

DE

PHYSIQUE

III N°12

and the last but

not

the least, all those experiments have

to

be correlated with others such

_as

mechanical spectroscopy,

stress

relaxation [3j ^{and other} spectroscopic

measurements.

Note:

_erratum

In Table I of P-C- _paper, for amorphous P-E-T,, it

must

be read: Tg

=

350 K, experimental T~/Tg: ^{1.03 and} theoretical T~/Tg: 1.06.

References

iii

Marchal

E.,

J.

Non-Cryst.

Sol. 172-174

(1994)

902.

[2) Oleinik

E-F-, Salamantina O-B-,

Rudnev

S.N.

and

Shenogin S-V-, Poiym.

Sm. 35

(1993)

1532.

[3) Quinson R.,

Perez

J., Germain

Y.

and Murraciole J.M., Polymer

36

(1995)

743.