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DYNAMIC MECHANICAL THERMAL ANALYSIS
OF MATERIALS
R. Wetton, J. Gearing, M. Stone
To cite this version:
R. Wetton, J. Gearing, M. Stone. DYNAMIC MECHANICAL THERMAL ANALYSIS OF
MATE-RIALS. Journal de Physique Colloques, 1985, 46 (C5), pp.C5-689-C5-694. �10.1051/jphyscol:1985590�.
�jpa-00224833�
JOURNAL DE PHYSIQUE
Colloque C5, suppl6ment a u n08, Tome 46, a o Q t 1985 page C5-689
DYNAMIC MECHANICAL THERMAL A N A L Y S I S OF M A T E R I A L S
R.E. Wetton, J.W.E. Gearing and M.R. S t o n e
Polymer Laboratories Ltd, The TechnoZogy Centre, EpinaZ Way, Loughborough, Leicestershire LEI1 OQE,
U.
K .RESUME
La s p e c t r o m g t r i e mgcanique e s t un moyen tres p u i s s a n t pour l ' 6 t u d e d e s p o l y - rn8res. d e s caoutchoucs, d e s c o m p o s i t e s , d e s cgramiques e t mBme d e c e r t a i n e s s u b s t a n c e s a l i m e n t a i r e s . Nous r a p p e l o n s l e s p r i n c i p e s fondarnentaux d e l a rn6- t h o d e e t p r k s e n t o n s q u e l q u e s t e c h n i q u e s expgrirnentales pour o b t e n i r d e s fonc- t i o n s r h 6 o l o g i q u e s en f o n c t i o n d e l a f r g q u e n c e e t de l a t e m p e r a t u r e . A t i t r e d ' i l l u s t r a t i o n pour l e module e t l ' a m o r t i s s e r n e n t nous p r 6 s e n t o n s d e s exemples pour q u e l q u e s polymQres s o l i d e s en f o n c t i o n d e l a t e m p g r a t u r e .
ABSTRACT
The dynamic mechanical method is a powerful tool f o r studying polymers, rubbers, composites, c e r a m i c s and food stuffs. The principles of t h e method a r e outlined together with s o m e experimental techniques f o r collecting d a t a against frequency and temperature. Examples a r e given a s modulus and damping versus t e m p e r a t u r e for a number of polymeric materials.
INTRODUCTION
Plastics, rubbers, organic coatings, advanced composites, ceramics and foodstuffs such a s s t a r c h a r e just s o m e of t h e materials currently being studied and characterized by t h e Dynamic Mechanical Thermal Analysis (DMTA) technique. A small bar o r disc of material is subjected t o a small oscillating mechanical s t r a i n and t h e resulting s t r e s s resolved into real and imaginary components. This procedure essentially d e t e c t s all changes in t h e s t a t e of molecular motion a s t e m p e r a t u r e is scanned. It is thus a most powerful technique f o r studying t h e e f f e c t of not only molecular s t r u c t u r e but also phase morphology and filler addition on t h e physical properties required for product design.
Examples a r e given of applications t o phase-separated copolymers, rubber toughening, filler reinforcement in thermoplastics, epoxy cross-linking studies, composites, t h e study of coatings and films and finally, using frequency multi- plexing, t h e rapid generation of relaxation s p e c t r a and activation energies.
THEORY
When a sinusoidal s t r e s s is applied t o a perfectly e l a s t i c solid t h e deformation, and hence t h e strain, occurs exactly in-phase with t h e stress. In extension or bending a f t e r allowance for t h e c o r r e c t geometrical f a c t o r s t h e dynamic Young's modulus (E*) is given basically a s s t r e s s amplitude/strain amplitude. In shear deformation t h e dynamic rigity modulus (G*) is obtained. However, when s o m e internal molecular motion is occurring in t h e s a m e frequency range a s t h e impressed stress, t h e material responds in a visco-elastic manner and t h e strain response lags behind t h e stress.
It is t h e n convenient t o resolve E* or G* i n t o perfectly e l a s t i c and perfectly viscous components, called t h e s t o r a g e (El, G') and loss components
(E",
Go'), a sC5-690 JOURNAL DE PHYSIQUE
shown in t h e Argand diagram in Fig. 1. A more useful parameter is t h e dimensionless ratio, t a n S = E"/E1, often referred t o a s t h e damping factor. The relaxation process may be scanned by changing frequency at constant t e m p e r a t u r e (frequency plane d a t a ) or changing t e m p e r a t u r e a t constant frequency (temperature plane data). Although t h e former a r e purer data, in t h a t structural changes may occur during a thermal scan, experimentally accessible frequency ranges a r e s o limited t h a t even a single relaxation process cannot conveniently b e encompassed. The 'dynamic mechanical thermal analysis' technique is thus t o scan t e m p e r a t u r e over a wide range, typically from -150°C secondary processes due t o limited chain o r side-group motion c a n be seen a s well a s t h e main Tg process ( oc )
in a 'thermal spectrum'.
Fig. 1. Argand diagram showing resolution of complex moduli (G*) or compliances ( I * ) into their storage and loss components.
Damping peaks shift t o higher temperatures T with higher impressed measurement frequencies
f
(Hz). The shift allows t h e activation energy f o r t h e process t o b e determined a s AE = -R[d In f/d(l/T)]. Further background theory c a n b e found in References I and 2.EXPERIMENTAL
The PL-DMTA, shown schematically in Fig 2 has been used exclusively. In this instrument, stress is proportional t o t h e level of
a.c.
c u r r e n t fed t o t h e drive coil from t h e analyser module. The frequency of oscillation is selectable from 0.01 t o 200 H z and does not depend on sample stiffness. Strain is proportional t o t h e displacement of t h e drive c l a m p and is monitored by a non-contacting eddy current transducer. The analyser unit compares t h e stress and strain signals and by using refined out-of-phase (loss) components. When t h e sample geometry constant is dialled into t h e instrument log E' and t a n 6 a r e computed. The sample t e m p e r a t u r e is controlled in t h e range -150°C t o +500°C by a t e m p e r a t u r e programmer such t h a t i t c a n be ramped up o r down at controlled r a t e s (up t o 15°C min-l for measurement) o r isothermed at any desired temperature. Use of a microcomputer and t h e IEEE i n t e r f a c e allows frequency t o b e multiplexed during a slow thermal scan and with a l l d a t a stored for subsequent manipulation.Figure 2
Dlsplacamnt transducer
Mechanical head of PL-DMTA showing the essential features of sample mounting, vibrator system and transducer.
The normal mode of deformation geometry is by bending small bars a s dual or single cantilevers a s shown in Fig. 3(a). in t h e single cantilever mode considerable thermal expansion, such a s occurs through a melting point, c a n b e accommodated because of t h e lateral compliance of t h e drive. Alternative geometries a r e available and Fig. 3(b) shows, for example, shear sandwich geometry, for measuring t h e rigidity modulus of rubbers and s o f t adhesives.
Figure 3
(a) Dual cantilever clamping of small rectangular bar sample, typically 30 x 10 x 2 mm in dimensions. (h) Shear sandwich geometry for clamping soft materi- als such as adhesives and rubbers. Horizontal and vertical orientations can be
employed if required.
RESULTS AND DISCUSSION
Phase
structure measurementsIn random copolymers t w o inherently incompatible polymer sequences a r e forced t o co-exist in a single phase. The polymer exhibits a single relaxation process intermediate between t h a t of t h e parent homopolymers. In contrast, incompatible
30URNAL
DE
PHYSIQUEsequences in block copolymers, in general, phase separate. This can b e assessed by DMTA via t h e observation of t w o s e p a r a t e processes. Figure 4 shows this for effectively a block copolymer of styrene-butadiene-styrene random sequences. However, t h e poly(styrene) peak in particular is occurring a t a lower t e m p e r a t u r e than f o r homopoly(styrene) indicating s o m e mutual solutions and phase boundary mixing. The modulus level in t h e region between t h e t w o Tg processes allows a n assessment of t h e phase structure. If t h e phase is continuous ~t will b e low. These e f f e c t s c a n b e quantified by model studies.
Rubber toughening of glassy polymers such a s poly(styrene) abd poly(methy1 meth- acrylate) is a common commercial practice. DMTA measurements allow a characterisation of t h e phases present and of t h e modulus of t h e resulting material. Fig 5 shows t h e presence of a rubber phase. relaxation a t low t e m p e r a t u r e s for i m p a c t toughened poly(styrene). T h e impact strength changes linearly with peak a r e a below 20% rubber content.
Frequency Multiplexing 032 -025 -020 (O 2 -015 -Om 005 9.5- 90- N 2-8,5- iu 8' 8 0 - 75- 7.0
R e c e n t advances in computer control have led t o new more-rapid and in s o m e ways more-acurate methods of d a t a acquisition. If during a slow t h e r m a l scan, typically a t 1°C min-I
,
frequency is continually changed (multiplexed) by a n external computer, t h e d a t a c a n b e sorted and displayed a s a series of single measurement frequencies versus temperature. Fig 6 shows t h i s for a n epoxy sample through i t s Mprocess. Here five frequencies have been multiplexed. The peak shift is plotted versus reciprocal t e m p e r a t u r e and gives a n activation energy of (-2.303R x slope) =383 k J mol-l. R a t h e r complete rheological characterisation is given by this Ternprraturo , .C
PL-DMTA scan of block copolymer of butadiene with SBR at 1 Hz and 5 "Cmin-' heating rate. The two loss peaks clearly show phase separation but with
considerable phase boundary mixing.
Figure 4
-
relaxation --
I I I I I I I I I I , -60 -40 -20 0 20 40 60 80 1 0 0 120technique and, besides t h e time-saving advantages over isothermal work, t w o main benefits accrue. Any t e m p e r a t u r e error is exactly t h e s a m e f o r every frequency because t h e t e m p e r a t u r e is increasing linearly with time. Furthermore, clamping during heating s t a y s positive a s t h e r e is a tendency for t h e sample t o expand into t h e clamps. During isothermal measurements most normal samples tend t o relax away from clamping pressure with time. Thus a c c u r a t e rheological characteris- ation can also b e achieved by this new approach.
Figure 6
7
I L
XI 80 70 80 90 (00 110 122 Temperature, *C
Frequency multiplexing with the PL-DMTA. The sample was a moderately cross-linked epoxy. This data was acquired at 1 'C min-' giving a total time of
80 min.
Frequency multiplexing with the PL-DMTA. The sample was a moderately cross-linked epoxy. This data was acquired at l0Cmin-' giving a total time of
JOURNAL
DE PHYSIQUEENGINEERING DATA
Advanced composites, filled s y s t e m s and rubbers have been studied, both in t h e multi and single frequency modes. Simple examples a r e shown in Figure 7 f o r carbon fibre composites measured a s bar samples. In one c a s e t h e fibre reinforcement is uni-directional along t h e bar length. In t h e other c a s e t h e rein- f o r c e m e n t is bi-directional and in t h e f o r m of a woven mat. The t r u e T process
B
f o r t h e s e highly cross-linked s y s t e m s is believed t o b e t h a t seen a t 130 C. The second process occurs a t higher t e m p e r a t u r e s only when shear c a n occur between t h e fibres.
I
1
romp so 1 0 0 150 2 0 0 250-c
Fig. 7. PL-DMTA scans of carbon fiber/epoxy composites with unidirectional long
fibers only I u l and a woven fiber mat with e q u a l transverse and longitudinal
fibers [wl. The resin is the same in each case. The tan 6 curves [right axis1
show maxima. U,".'&."".1U Figure 7 APPLICATION AREAS 11.0 10.5 10.0
--
The main application a r e a s in material science a r e a s follows: Engineering data: composites, rubbers, ceramics. Phase morphology/composites.
Adhesive performance.
Curing characteristics: epoxies, silicones. Physical ageing.
Structure/properties of coatings. Activation energies.
Environmental/humidity effects. Rheology of food stuffs.
L o g € [Po) V = U N I - D I R E C l I O N A L C-FIB1(EIEPOXY Ton 6
REFERENCES
-
W = W V E N MATT C-FIBREIEPOXV stram-
..
i..q- I"' 4'Cl.l" U ... .... .... ....--
... . . . . . . ._.' .. . . . ... ... . . . . " . . . W_
-.::2 . . . . . ... ... . . . ... ..".." . . . ....
...__
_.. .... .. . . . . . . . . . . . ....
.It.. . . . ..
. . .
. . .
..,. . . . '. . . . .., ... ... .. ..__ '.._. ..__,
. '_ 'b., . " . . . ....
'. -.: ... . . . ... .... . ,.. ...._."
... ... . " v ... ... -.,,, :,..:::... ...1. Ferry, J. D. (1961). Viscoelastic Properties of Polymers. New York, John Wiley & Sons Inc.
r 0.2
--
0.12. McCrum, N. G., Read, B. E. and Williams, G. (1967). Anelastic and Dielectric E f f e c t s in Polymeric Solids, New York, John Wiley & Sons Inc.
