PET, Enthalpy Relaxation
Sample
Polyethylene terephthalate from the neck of a soft drinks bottle
Conditions
Measuring cell: DSC821e with air cooling and sample changer
Pan: Aluminum standard 40 μl, pierced lid
Sample preparation: Flat piece cut from the neck of the bottle, 23.16 mg
DSC measurement: Heating from 30 to 300 °C at 10 K/min. At 300 °C the sample changer removes the molten sample within a few seconds and places the pan on the cold tray. This results in reproducible quench-cooling with a mean cooling rate of approx. 3000 K/min.
After quench-cooling, the sample was annealed at 65 °C for periods of 0 to 24 h for enthalpy relaxation and each time measured again from 30 to 300 °C at 10 K/min. The entire temperature program was executed fully automatically using the sample changer over a weekend.
Atmosphere: Nitrogen, 50 cm3/min
Evaluation
Note: The glass transition temperature determined by the midpoint, can be lower than by the onset if a rather large enthalpy relaxation occurs (see next diagram).
Conclusions
The enthalpy peak influences the evaluation of the glass transition and is often a result of the thermal history. To eliminate thermal history, the sample under investigation is heated to a temperature slightly above the glass transition and is often a result of the thermal history. To eliminate thermal history, the sample under investigation is heated to a temperature slightly above the glass transition, quench-cooled, and then measured a second time. Polymers that crystallize readily (such as PET) are heated to above the melting point and quench-cooled to suppress the formation of crystallites (only the mobile amorphous fraction undergoes a change in heat capacity).
The relaxation enthalpy can, however, provide important information regarding the thermal and mechanical history of a sample (e.g. storage temperature, storage time, cooling rate).
PET, Enthalpy Relaxation | Thermal Analysis Handbook No.HB236 | Application published in METTLER TOLEDO TA Application Handbook Thermoplastics