DIELECTRIMETRY APPLICATIONS INVESTIGATION AND CHARACTERISATION OF POLYMERS
Rel c05/03
CAPAAB 22 / 32 Sentier des Torques BAT 8 92290 Chatenay Malabry Tel / Fax : 01 71 22 73 72
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DIELECTRIC ANALYSIS: The technique Dielectric Analysis measures the two fundamental electrical characteristics of a material – capacitance and resistance- as a function of time, temperature and electric field frequency. The capacitive nature of a material is its ability to store electric charges, and conductive nature (inverse of resistance) is its ability to transfer these charges.
This technique can give : Complex Permittivity Loss Factor Degree of cure Rate of cure Glass transition temperature
While these electrical properties are important in themselves, they have even more significance when correlated to molecular activity. Such correlation allows the chemistry, rheology (flow) and molecular mobility (relaxation) of viscoelastic materials to be studied.
Ionic Conductivity Secondary transition temperatures Polymer morphology
FIELD OF APPLICATION Research & Development
Assurance / Quality control
Research on new materials and processes
Vendor certification
New materials development
Outgoing/incoming product consistency
Formulation optimisation
Process optimisation
Applications development
Tracking of heat history
End-use performance prediction
Finished product performance
Competitive product evaluations
Troubleshooting
CAPAAB 22 / 32 Sentier des Torques BAT 8 92290 Chatenay Malabry Tel / Fax : 01 71 22 73 72
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DIELECTRIC MATERIAL BEHAVIOUR IN AN ELECTRIC FIELD : Copyright Evry 2000 AOIP sdm
⊕ + - +
-
- -
V= 0
+
Dipole
-
+
+ -
- + + ⊕
+
⊕
-
+ -
-
Induced dipoles are those created by the applied electric field which causes redistribution of electrons shared between bonded atoms with similar electronegativity.
⊕ - ions
- V>0
+⊕
+
⊕
- - -
-
+ + + ⊕ + ⊕ +
⊕
⊕
⊕
ε’ = permittivity due to induced dipoles + permittivity due to the alignment of dipoles
ε’’ = dipole loss factor + ionic conductance
σ
=
A dipole can be represented as a chemical bond which has an unbalanced distribution of charge in a molecule. One part is partially negative and the other partially positive. Permanent dipoles exist in the absence of an applied electric field and are due to the differences in electronegativity of the bonded atoms (e.g. carbonyl bond C = O, C-N, etc.) Induced dipoles are those created by the applied electric field which causes redistribution of electrons shared between bonded atoms with similar electronegativity
Z q 2 N 6π rη
Z : number of charges, q : charge of electron, N : ion concentration, r : ion radius, η : viscosity
The measured capacitance is proportional to material permittivity ε’, losses being proportional to the imaginary part ε’’ of the permittivity. ε’ represents the amount of alignment of the dipoles to the electric field. For polymers at low temperature, ε’ is low, because molecules are « frozen » in place and the dipoles cannot move to align themselves with the electric field. For the same reasons, a highly cross-linked resin has a low ε. ε’’ measures the amount of energy required to align dipoles or move ions. Ionic conduction is not significant until the polymer becomes fluid (e.g. the glass transition Tg or melting point). ε’’ displays a peak as the polymer passes through Tg. Above the Tg, ε’’ is used to calculate the bulk ionic conductivity σ using the relation : σ = ε’’ ω εο . (ω : angular frequency of electric field, εo : absolute permittivity of free space 8.85 pF/m) The conductivity σ measurement can be used to monitor the changes of the rheology occurring during the utilization of hydraulic binder. The conductivity is directly correlated to the viscosity, the mobility of ions being linked to their ability to move inside the media (Stokes relationship).
CAPAAB 22 / 32 Sentier des Torques BAT 8 92290 Chatenay Malabry Tel / Fax : 01 71 22 73 72
Copyright Evry 2000 AOIP sdm
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DIELECTRIC MEASUREMENT THEORY Dielectric property measurements make use of parallel plate capacitors. The sample is placed between the plates. A coplanar geometry can be used : the sample is deposited on the electrodes.
Applied voltage + Measured current Guard ring
Sample
The electrical field is created by means of an alternating voltage applied to one plate of a capacitor, the other plate being grounded. The current « flowing » through the sample is measured at the plate to which the voltage is applied (Patented technique).
Dielectric sample between parallel plates
Interdigitized coplanar probe (Top view)
V
The applied field creates a polarisation within the sample causing oscillation at the same frequency, as the field but with a phase angle shift φ.
φ
I
time
The measurement of φ is achieved by comparing the applied voltage V to the measured current.
The measured values at frequency f are related to an equivalent capacitance Cp, in parallel with a resistance Rp. We get
I cos φ
Cp = I sin φ / 2 πV f Rp = V / I cos φ
δ
I
φ I sin φ
We define Tan δ = 1/ 2πRpCp f = ε’’/ ε’ Tan δ is equal to the loss factor.
The CAPCR10/100 (*) system developed by CAPAAB is designed to display Cp, Rp, Tg δ, ε’, ε’’ vs. time and for a wide frequency range. In order to acquire and process these measurements, software, developed under LABVIEW®, allows an easy and simple operating mode with powerful analysis tools. The entire process (from frequency generation to final calculation) takes place almost instantaneously, making meaningful results available in real-time. The CAPRM 20 (*) module is a single range single frequency unit providing 2 outputs (R and C) (*) See technical data sheet
Complementary associated measurements: PRESSURE AND TEMPERATURE
The CAPCR10/100 system is able to acquire data input from temperature and pressure probes, process and display them using LABVIEW® software, similar to that used for the dielectric measurements.
CAPAAB 22 / 32 Sentier des Torques BAT 8 92290 Chatenay Malabry Tel / Fax : 01 71 22 73 72
Copyright Evry 2000 AOIP sdm
www.capaab.fr
APPLICATIONS Dielectric analysis, or dielectrimetry (DEA), allows highly valuable information about molecular and rheological behaviour of materials to be gathered. Quantitative, reproducible data generated by the CAPCR ( X) systems, designed and manufactured by CAPAAB, can help industry to identify the structure of materials or predict processing behaviour and end-use performance. DEA can characterise molecular relaxation, monitor the flow and cure of resins and calculate activation energies for molecular relaxation. The ultra sensitivity of DEA makes possible the detection of transitions which cannot be detected by other techniques. The ability to measure bulk or surface properties of materials in solid, paste or liquid form makes the DEA exceptionally versatile and useful. Sensor selection depends on both the physical form of the material being examined and on the objective of the experiment. The single-surface sensor is designed to evaluate surface properties. Parallel-plate sensors, designed primarily for evaluating bulk properties, are ideal for use with solid materials. The following examples are representative of the extensive capability and utility of this technique using the related CAPAAB system. The CAPMCRM 20 module is more specifically designed to volume humidity applications. Correlation between viscosity and dielectric losses 4
400
3,5
350 300 Log Viscosity
Losses
2,5
250
Log dielectic losses T emperature
2
200
1,5
150
1
100
0,5
TEMPERATURE
3
Log visco/Log
The graph shows compared evolution, during a non-isothermal polymerisation, between viscosity, measured by the mean of a mechanical rheometer, and the dielectric losses. The evolution indicates a strong correlation up to Tg (~130°C). Above Tg, the rheometric method becomes inefficient, while the dielectric loss method is able to track the whole process.
COMPARED EVOLUTION BETWEEN VISCOSITY AND DIELECTRIC LOSSES
50
0 0
50
100
150
200
250
0 300
Time (minutes)
PMMA molecular relaxation The very high sensitivity of the dielectric method allows separation of the α and β transitions which become increasingly distinct as the test frequency is decreased. The α transition, which involves motion of longs segments of the main polymer chain, is related to Tg. The β transition involves rotation of short chain ester side groups and therefore occurs below Tg. The frequency dependency of the β transition temperature can be used to calculate the energy of the molecular motion, which provides important information for characterising the structure and predicting the performances of polymers.
6
ε'
PMMA molecular relaxation
ε ''
Transition α
5E-01
5,5
E' a 100Hz
5E-01
5 4,5
E' a 100kHz
4E-01
E'' a 100Hz E'' a 100kHz
4E-01 Transition β
4
3E-01 3E-01
3,5
2E-01
3 2,5
2E-01
2
5E-02
1E-01
1,5 -100
CAPAAB 22 / 32 Sentier des Torques BAT 8 92290 Chatenay Malabry Tel / Fax : 01 71 22 73 72
-50
0
50
Température (°C)
www.capaab.fr
100
150
0E+00 200
Polymetacrylate characterisation Copyright Evry 2000 AOIP sdm
(PMMA, PEMA, PBMA)
Thanks to dielectric loss measurements, the α transition temperature dependence is clearly shown: the alpha transition significantly decreases with temperature as the size of nonpolar alkyl group increases. PMMA → 119.8 °C PEMA → 83.1 °C PBMA → 34.4 °C Thin layer polymer analysis A thin polysulfone film (~30 µm) is directly deposited on the probe electrodes, under vacuum to achieve the best results. Dielectric analysis (ε’ and ε’’) vs. temperature reflects the α transition at Tg, which shows a step change of ε’ and a peak in the loss factor (ε’’) at about 200 / 220°C, depending on the excitation frequency.
ε ''
Polysulfone characterisation
ε'
10 9
E' à 100 Hz
8
E' à 100 kHz
7
E' à 100 Hz
2,5 2
E' à 100 kHz
6
3
1,5
5
1
4 0,5
3
0
2 0
50
100
150 Temperature (°C)
200
250
300
Composition analysis of copolymer ethylene vinyl acetate (EVA) Loss factor ε’’ measurements show that the glass transition temperature is relatively insensitive to the level of vinyl acetate. However, the intensity of the loss factor peak at Tg, is very dependent on the vinyl acetate EVA copolymer concentration. Tg value is less dependent on the % of vinyl acetate (14% to 33%), however the amplitude of losses increases. This last point is consistent with the fact that increasing the level of vinyl acetate increases the flexibility of the polymer.
2,5E-01
ε ''
Analysis of effect of vinyl acetate on EVA 14% acétate de vinyl
2,0E-01
25% acétate de vinyl 33% acétate de vinyl
1,5E-01 1,0E-01 5,0E-02 0,0E+00 -80
Study of urethane polymerisation
-60
-40
-20 0 Temperature (°C)
20
40
60
Urethane characterisation
6,00
Minimum of viscosity 5,00 Log(conductivity) pmho/cm
The plot shows the evolution of the conductivity (rheological behaviour image) for 3 frequencies (0.1, 1 and 10KHz) during urethane polymerisation. The main phases are identified: the product glass transition before polymerisation occurs around - 30°C; at about 60°C, mobility is a maximum and the volatilisation begins, then polymerisation starts to end at about 150°C.
End of polymérisation
4,00
3,00
10kHz 2,00
1kHz 100Hz
1,00
0,00 -100
CAPAAB 22 / 32 Sentier des Torques BAT 8 92290 Chatenay Malabry Tel / Fax : 01 71 22 73 72
-50
0
50 100 Temperature (°C)
150
www.capaab.frCopyright Evry 2000 AOIP sdm
200
250
Study of urethane glass transition following polymerisation
Such studies can be combined with tests conducted under isothermal conditions in order to obtain phase transition diagrams vs. time / temperature.
Urethane characterisation
6,00
Minimum of viscosity 5,00
Log(conductivity) pmho/cm
After heating polymerised urethane, the temperature of glass transition reaches 20°C as indicated by the conductivity measurement.
End of polymérisation
4,00
3,00
10kHz 2,00
1kHz 100Hz
1,00
Combination of these results enables the collection of useful information in order to optimise storage, shipping and processing conditions.
0,00 -100
-50
0
50 100 Temperature (°C)
150
200
250
Epoxyde / amine vitrification Regarding end-use properties (high mechanical strength and solvent resistance / optimal impact resistance), the control of the degree of vitrification constitute a powerful means to achieve the best properties.
Epoxy-amine vitrification
1,8 Minimum of viscosity
1,6 1,4
Monitoring the loss value ε’’ vs. time, while maintaining isothermal curing, allows the different process phases to be distinguished: at the beginning, resin flows and its viscosity decreases, loss increases, followed by polymerisation, where ε’’ diminishes. The second maximum reflects vitrification, where the material becomes less rubbery.
Log ( '')
1,2 1
Fluidification
0,8 Polymerisation 0,6 Vitrification point 0,4 0,2 0 0
10
20
30
40
50 Temps (min)
60
70
80
90
100
A. Br Ver b11/99
MMI EXAMPLE OF THE CAPAAB CAPRC 10 /100 CAPACITIVE / RESISTIVE BRIDGE
CAPAAB 22 / 32 Sentier des Torques BAT 8 92290 Chatenay Malabry Tel / Fax : 01 71 22 73 72
www.capaab.fr Copyright Evry 2000 AOIP sdm
