The Dynamic Glass Transition - METTLER TOLEDO

The Dynamic Glass Transition

Purpose

To show how information can be obtained on relaxation behavior using isothermal measurements in which the frequency is varied (frequency sweep). For example, the glass transition and the frequency dependence on the modulus is discussed in this section. 

 

Sample

Cured KU600 epoxy resin powder.

 

Conditions

Measuring cell: DMA with the shear sample holder

Sample preparation: Two cylinders, 5 mm in diameter and 0.56 mm thick were made by compressing fine uncured KU600 powder. The cylinders were mounted in the shear sample holder, heated to 250 C at 2 K/min to cure the resin, and then cooled to room temperature

DMA measurement: The measurement was performed at 115 C in the frequency range 1 kHz to 1 mHz. Maximum force amplitude 5 N; maximum displacement amplitude 20 um: offset control zero. 

Atmosphere: Static Air

 

Interpretation

In an isothermal measurement, the modulus changes with the frequency. In the relaxation range, the storage modulus changes stepwise. At high frequencies, it is larger than at low frequencies. This means that at high frequencies, the measurement frequency is higher than the frequency of the corresponding cooperative rearrangements. The sample therefore appears hard and has a large storage modulus. At low frequencies, the molecular rearrangements are able to react to the external stress. The sample appears soft and has a low storage modulus. 

In the relaxation range, a peak occurs in the loss modulus, G", In the sample measured, the frequency at the peak maximum was 10 Hz. The shape of the peak in the loss modulus corresponds to a distribution of relaxation times. A broad peak means a broad distribution of the relaxation times and therefore a larger difference in the possibilities for cooperative rearrangements. 

 

Conclusions

The measurement of the frequency dependence of the mechanical properties under isothermal conditions is a very sensitive method for the investigation of relaxation behavior. Besides the relaxation intensity and the frequency range of the relaxation, the width and curve shape are also important: information can be gained about molecular and supermolecular structures or their changes. Because the relaxation range is wide, for interpretation purposes experiments should cover the largest possible frequency range. 
Since many technological processes and material applications involve dynamic stress over a wide frequency range, such measurements can also be useful for product organization or for failure analysis. A dynamic measurement should as far as possible performed in the frequency range in which the material is actually stressed because the behavior at other frequencies can be quite different and extrapolation does not always ensure satisfactory results. 

 

The Dynamic Glass Transition | Thermal Analysis Application No. HB 26 | Application published in METTLER TOLEDO TA Application Handbook Thermosets Volume 1 

 

 Handbook   
 Page   
Keywords
Thermoplastics
  110
 thermoplastics, DSC, high temperature polymers, PEEK, PES, PTFE, blank curve, glass transition, heat capacity, semicrystalline,


 

 

 Handbook   
 Page   
Keywords
 Thermosets 1
  119
 thermoset, curing, DMA, shear mode, cooperative rearrangements, dynamic glass transition, frequency dependence, frequency sweep, relaxation process, molecular structure, KU600 epoxy resin,