PTFE Polymorphism by DSC and TMA - METTLER TOLEDO

PTFE Polymorphism by DSC and TMA

Sample

Commercial PTFE rod of 4.2-mm diameter

 

Conditions

Measuring cellDSC821e with cryostat cooling, TMA40 with ball-point probe and low temperature accessory

Pan: DSC: aluminum standard 40 µl, pierced lid; TMA: the sample is mounted between fused silica disks of 6-mm diameter and 0.5-mm thickness

Sample preparation: DSC: Disk cut from rod, approx. 0.6-mm thick; TMA: cylindrical sample 3.35-mm high for axial measurement. For the radial measurement two parts were cut off from opposite sides so that a 2.42-mm thick center sample was obtained.

Measurements: In the first heating run to 50 °C, the sample adapts to the silica disks.  The actual measurement is performed after uncontrolled cooling to the start temperature, i.e. heating from -20 °C (TMA -30 °C) to 50 °C at 5 K/min

Load: 0.05 N

Atmosphere: Nitrogen, 50 cm3/min (DSC), Helium 200 cm3/min (TMA)

 

 

 

Evaluation

As the TMA curve and table show, the radial expansivity exhibits a maximum during the transition.  In the axial direction, there is a corresponding minimum (negative expansivity).

 

Temperature in °CExpansivity, radial in ppm/K
10151.6
15189.2
20362.9
25645.6
30265.7

 

Interpretation

The crystallites of PTFE exist below about 19 °C as the triclinic modification II.  Between 19 and 30 °C, the hexagonal form IV is stable. On heating, this transforms to the hexagonal for I.

The rods are anisotropic as a result of the manufacturing process.  This is clearly apparent from the dilatometric curves measured along the two main axes.

On the TMA curves, there is only an indistinct shoulder at 30 °C.  The IV-I transition is clearly visible on the expansivity curve.  Due to thermal inertia, the curve is broader than the DSC curve.

 

Conclusions

PTFE is the only commercial polymer with an enantiotropic solid-solid transformation with corresponding enthalpy and volume change.  DSC as well as TMA curves can therefore be used to identify PTFE.

 

PTFE Polymorphism by DSC and TMA | Thermal Analysis Application No. HB243 | Application published in METTLER TOLEDO TA Application Handbook Pharmaceuticals