Dielectric Thermal Analysis

Duncan PriceIPTME, Loughborough University© Copyright: D.M.Price@lboro.ac.uk (2006)

contents

Equipment Thermally stimulated current analysis Theory Example – multi-frequency temperature

sweep Time-temperature superposition Applications

equipment

dielectric relaxation

thermally stimulated current analysis

voltage-current relationship in a capacitor

theory

The admittance of the sample (Y) given by:

Y = <I>/<V>

Y is a vector quantity and is characterised by its magnitude |Y| and direction .

Capacitance (C) is the ability to store electrical charge and is given by:

C = |Y| sin()/

Conductance (G) is the ability to transfer electric charge and is given by:

G = |Y| cos()

theory (contd.)

Data are presented in terms of the relative permittivity (ε’) and dielectric loss factor (ε”) – these are related to capacitance and conductance by:

ε’ = C/(ε0.A/D)

&

ε” = G/(.ε0.A/D)

where ε0 is the permittivity of free space (8.86×10-12 F m-1) and A/D (in m), is the ratio of electrode area (A) to plate separation or sample thickness, D for a parallel plate capacitor. ε’ & ε” are dimensionless quantities.

The ratio ε”/ε’ is the amount of energy dissipated per cycle divided by the amount of energy stored per cycle and known as the dielectric loss tangent or dissipation factor (tan ).

poly(ethylene terephthalate)

’ vs. frequency

master curve @ 130°C

Arrhenius plot

self adhesive film

base film

in-situ photodegradation monitoring

Dielectric loss factor, ” (0.1 Hz) for adhesive with different stabiliser packages during accelerated weathering

Sample “standard” “poor” “good”

initial 3.00 2.86 3.14

after 600 hr 8.59 12.6 3.18

after 1200 hr 10.4 71.0 6.45

online cure monitoring

dielectric thermal analysis of water in pores

summary

Similar to TMA/DMTA Very sensitive for polar/ionic components Ideal for thin films Suitable for remote monitoring

recommended reading

A. Blythe and D. Bloor; “Electrical Properties of Polymers”, Cambridge University Press, Cambridge (2005)

D. Q. M. Craig; “Dielectric Analysis of Pharmaceutical Systems”, Taylor and Francis, London (1995)

P. Hedvig; “Dielectric spectroscopy of polymers” Wiley, New York (1977)

N. G. McCrum, B. E. Read and G. Williams; “Anelastic and Dielectric Effects in Polymeric Solids”, Dover, New York (1991)

D. M. Price, “Thermomechanical and Thermoelectrical Methods”, in P.J. Haines (ed.) “Principles of Thermal Analysis & Calorimetry”, Royal Society of Chemistry, Cambridge (2002)

J. P. Runt and J. J. Fitzgerald, ed., “Dielectric Spectroscopy of Polymeric Materials: Fundamentals and Applications”, American Chemical Society, Washington DC (1997)