Determination of the Glass Temperature by DMA

Introduction 

The glass transition temperature is an important criterion for the technological use of polymeric materials. The application range of elastomers is usually restricted to temperatures significantly above the glass transition temperature to ensure that deformation behavior is as far as possible entropy elastic.

The glass transition temperature of a polymer is, however, strongly dependent on frequency, and the deformation of elastomers is usually time dependent (e.g. seals, surface of tires, etc.). This means that a glass transition temperature measured under quasi static conditions (for example by DSC) is not a good criterion for the characterization of the low temperature behavior of dynamically stressed materials. The temperature limits for dynamically stressed elastomeric materials can be determined by measuring the glass transition temperature using methods in which a periodic stress is applied.  The measurement frequency should of course correspond to the frequency at which the component is in practice stressed. If such a measurement is not possible, for example at frequencies that are significantly higher or lower than the measurement range of the instrument, then the glass transition temperature can be obtained by extrapolation using the time-temperature superposition principle. A quantitative relationship between the glass transition temperature and measuring frequency can be obtained through the semi-empirical WLF or analogous Vogel-Fulcher equation.

In this article, the experimental relationship between frequency and time are determined for a number of primary elastomers (NR, BR, SBR, NBR, IIR) from temperature-dependent and frequency-dependent shear modulus measurements. The quantitative description of the time-temperature superposition principle is performed using the semi-empirical WLF or analogous Vogel-Fulcher equation.

Samples and Sample Preparation

Samples for frequency and temperature-dependent measurements were prepared by compressing the raw polymers to 2-mm thick sheets under a pressure of 2 MPa at 80 °C. Two cylindrical disks of 6-mm diameter suitable for use with the double sandwich sample holder of the DMA were punched out from the sheets for each measurement. The dynamic mechanical measurements were performed on NR (natural rubber) and BR (polybutadiene) homopolymers, and on SBR, NBR, EPDM and IIR copolymers using a METTLER TOLEDO DMA/SDTA861e dynamic mechanical analyzer.

Measurements and Results 

First of all, the complex shear modulus was measured for each sample as a function of temperature. To do this, the raw polymer was first cooled to well below its glass transition temperature. The complex modulus was then measured at five frequencies (1 Hz, 10 Hz, 100 Hz, 300 Hz and 1000 Hz) at a heating rate of 1K/min up to a temperature of 100 °C.

Summary

Both temperature-dependent modulus measurements at different frequencies and frequency-dependent measurements at different temperatures were performed on all the polymers studied. The combination of the two methods allowed the experimental relationship between the temperature and frequency location of the glass process to be determined and quantified by fitting the experimental data to the empirical VogelFulcher equation. With the exception of polyisobutylene (IIR), it was shown that all the samples studied can be described within measurement accuracy by identical Vogel-Fulcher parameters, f₀ and D_E. The glass transition temperature limits the practical use of elastomeric materials at low temperatures. Since the glass process is also frequency dependent, the behavior of these materials at low temperatures also depends on the frequency of the dynamic stress. The low-temperature behavior can be characterized through a combination of temperature- and frequency-dependent measurements of the shear modulus and a quantitative description on the basis of the time-temperature superposition principle using the empirical Vogel-Fulcher equation.

_{Determination of the Glass Temperature by Dynamic Mechanical Analysis | Thermal Analysis Application No. UC 163 | Application published in METTLER TOLEDO Thermal Analysis UserCom 16}