HAL Id: jpa-00225122
https://hal.archives-ouvertes.fr/jpa-00225122
Submitted on 1 Jan 1985
HAL is a multi-disciplinary open access
archive for the deposit and dissemination of
sci-entific research documents, whether they are
pub-lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d’enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.
A TRANSIENT GRATING STUDY OF ACOUSTIC
PROPERTIES OF POLYMER MATERIALS
Dominique Blanchard, R. Casalegno, M. Pierre, H. Trommsdorff
To cite this version:
A TRANSIENT GRATING STUDY OF ACOUSTIC PROPERTIES OF POLYMER MATERIALS
D. Blanchard, R. Casalegno, M. Pierre and H.P. Trommsdorff
Laboratoire de Spectrometrie Physique , Universite Scientifique et Medicate de Grenoble, B.P. 87, 38402 Saint-Martin d'Heres Cedex, France
Résumé - La technique des réseaux transitoires constitue une méthode très sen-sible pour l'étude de nombreux phénomènes de relaxation et de transport en phase condensée. Elle permet de produire, avec des impulsions lumineuses cour-tes , des phonons monochromatiques de haute fréquence, puis de suivre leur évo-lution. Elle offre ainsi une méthode adaptée à la mesure des propriétés acous-tiques (propagation et atténuation) de matériaux liquides et solides. Dans ce travail cette technique est appliquée à l'étude d'un polymère (PMMA.) au cours de transformations physiques ou chimiques.
Abstract - Transient grating techniques have been established as a very sensi-tive method to study numerous relaxation and transport properties in condens-ed phases. With short light pulses coherent ultrasonic phonons can be generat-ed and monitorgenerat-ed. Transient grating techniques afford thus a convenient method to measure and to follow acoustic (propagation and damping) properties of li-quid and solid materials. In this work this technique is used to study the transformation of a polymer material (PMMA) undergoing physical and chemical transformations.
INTRODUCTION
Acoustic properties of polymer materials have been widely studied by conventional me-thods using tranducers to produce and to detect the acoustic waves. These meme-thods be-come obviously inapt when the properties of the polymer make it difficult to esta-blish a good mechanical contact with the tranducer or when the acoustic attenuation becomes so high that the wave is damped over a few cycles only. These situations arise however when the material undergoes the most interesting transformations : one example is the glass-rubber transition as a function of temperature, an other is the transformation from monomer to polymer during the polymerization.
It is the purpose of this work to demonstrate the usefulness of a purely optical me-thod to induce and monitor coherent acoustic phonons. The interference of two cross-ed laser beams of equal frequency produces a spatially periodic excitation of the sample which in turn leads to an index grating of the same periodicity. The temporal evolution of this grating is probed by the diffraction of a third beam. This tran-sient grating method is well established and has been used to monitor a variety of dynamical processes in condensed phases /1/. When the duration of the exciting laser pulses is short with respect to the period of a sound wave with wavelength equal to the grating spacing the impulsional excitation of the material creates a standing acoustic wave 111. The spatial-temporal density variations linked with this wave contribute in turn to the evolution of the index grating and are probed by the time-delayed pulse. The frequency and damping of an ultrasonic wave are thus measured within a small volume inside the sample and all the problems of interfaces and sample size existing in the conventional methods mentioned above are eliminated.
In this work we apply these techniques to the study of the two aformentioned trans-formations of a specific polymer material : PMMA.
associe au C.N.R.S.
JOURNAL
DE
PHYSIQUEEXPERIMENTAL
The experimental s e t up is given i n f i g . 1. A cw pumped N ~ ~ + : Y A G l a s e r is acousto- o p t i c a l l y Q-switched and mode-locked, and d e l i v e r s a t a r e p e t i t i o n r a t e of 500
Hz
t r a i n s of l i g h t pulses of 120 ps duration. The frequency doubled 532
nm
pulse t r a i n i s used t o synchronously pump a cavity dumped dye l a s e r . The single pulses a t % 560nm
have a duration of about 25 ps and an energy of 1 5 p J ; they a r e s p l i t i n t o three beams. Two time coincident pulses El and E2 of equal amplitude a r e crossed i n t h e sample ; the t h i r d probe pulse E3 i s time delayed. One of the exciting beams is chopped a t h a l f t h e pulse r e p e t i t i o n r a t e and t h e signal detected a t t h i s frequency i s f e d i n t o an X-y recorder, t h e X of which is d i r e c t l y proportional t o t h e reflec-t o r p o s i t i o n of t h e delay l i n e .
Fig. 1
-
Experimental s e t upMATERIALS
High q u a l i t y P M 4 samples were provided by Rohm G.m.b.H. They were colored by a non-fluorescing dye which, i n the samples of 3 mm thickness, absorbed 60 % of t h e l i g h t a t the frequencies of t h e incident l a s e r beams. In order t o avoid l o c a l heat- ing by t h e l a s e r beams, the sample was r o t a t e d at 500 r.p.m. inside a temperature regulated oven. Measurements during t h e f;olymerization where done i n a r o t a t i n g c e l l . The monomer was colored with 5.10- mole/% of malachite green. The polymeriza- t i o n was i n i t i a t e d by adding 0.1 % of AIBN and maintaining t h e sample a t 40 O C .
GENERATION OF ULTRASONIC WAVES
The f r i n g e spacing i n t h e sample i s given by A = =/2 sin(@/2) where X and @ a r e t h e wavelength and t h e angle between t h e El and E2 beams respectively measured i n the a i r outside of t h e sample.
In a non-absorbing medium the l i g h t f i e l d is coupled t o the material by electro- s t r i c t i o n but the d i s t o r t i o n of t h e medium is small in t h i s case and t h e generation of t h e acoustic wave is not very e f f i c i e n t , resulting i n poor signal t o noise ra- t i o s .
where 17 is the d i f f r a c t i o n efficiency,
Y
the damping constant and f the frequency of the acoustic wave. Heat diffusion a s well a s the build-up of a permanent thermal grating due t o slower relaxation processes and t o the damping of t h e acoustic wave a r e neglected i n t h i s expression (see below).A l l experiments were performed with O % 20" resulting i n a phonon wavelength of 1.6
1-1 and frequencies between 1 and 2 GHz. The measurement of the period T gives direct- l y the corresponding speed of sound v = A/T. Expression (1) shows t h a t t h e amplitude of signal maxima do not decay exponentially even i f t h e acoustic waves are exponen- t i a l l y damped out.
VARIATION OF SOUND VEMCIn AND ATTENUATION IN PMMA AROUND
THE
GLASS-RUBBER TRANSI- TIONPMMA
V : 2 7 6 0 m r '
Fig. 2 - The time evolution of a standing ultrasonic wave i n PMMA a t 22 "C and 120 "C.
JOURNAL DE PHYSIQUE
Fig. 3
-
Variation of t h e speed of sound and of t h e acoustic attenuation in P M a s a function of temperature between 22 O C and 150 "C. The dashed v e r t i c a l l i n e marksthe onset of the glass-rubber t r a n s i t i o n .
VARIATION OF THE ACOUSTIC PROPERTIES DURING THE POLYMERIZATION OF MMA
This study demonstrates the p o s s i b i l i t y of measuring acoustic properties i n amedim which i s undergoing a polymerization. Fig. 4 shows the increase of t h e speed of sound from 1225 m / s i n t h e monomer, a t y p i c a l value f o r an organic l i q u i d , t o 2760 m / s i n t h e polymer. This increase i s monotonic and can therefore be used t o evalua-
t e t h e degree of polymerization. The corresponding v a r i a t i o n of t h e damping is more i n t e r e s t i n g i n passing through a maximum value a t intermediate degrees of polymeri- zation.
I A I
Intsrmediote 51-1.
This study shows t h a t picosecond l a s e r techniques are suitable t o measure acoustic properties of samples undergoing physical and/or chemical changes making them d i f f i - c u l t or impossible t o study by conventional methods. The technique is p a r t i c u l a r l y powerful when the damping becomes important and the acoustic wave i s damped over a few microns.
The application t o PMMA exposes the large variation of the v e l o c i t y and the damping of ultrasonic waves as a function of temperature across the glass-rubber t r a n s i t i o n and a s a function of the degree of polymerization.
Further applications of these techniques include t h e study of t h e swelling of poly- mer materials i n solvants, t h e i r degradation a t high temperatures and the study of diphasic materials such a s PW/PBA presently underway.
ACKNOWLEDGEMENT
We would l i k e t o thank D r . W. Wunderlich from R6hm G.m.b.H., Darmstadt, f o r provid- ing us with most of t h e samples used i n t h i s study and f o r helpful advice. D.B. i s grateful t o D r . Gandini f o r introducing him i n t o the a r t of polymerization.
REFERENCES
/ l / Ippen, E. P. and Shank, C. V., Ultrashort l i g h t pulses, S.L. Shapiro Editor, Springer Verlag (1 977) 1 1 1
.
/2/ Nelson, K. A., Fayer, M. D., J. &em. Phys. 72 (1980) 5202.
/3/ Miller, R. J. D., Casalegno, R., Nelson, K.
