Structure
On SEM images of the surface of an initial PTFE blanks, the lamellar structure is pronounced. Lamels consist of ribbons with coherent packing of fibrils oriented along the direction of the ribbons. Tapes are laid in strips up to 300nm in size. The direction of the lamellas is perpendicular to the stripes.
Images of a scanning electron microscope (SEM) of conventional (unmodified) PTFE blanks:
Image 1
Image 2
Image 3
On SEM images of the surface of the Raflon material, a complete change in the morphology of the polymer microstructure is observed. The lamellas are destroyed and reorganized into spherulites, consisting of fibrils oriented radially. The center of the spherulite are densely packed fibrils. The intersferolite region has the form of a fibrillar lattice.
To confirm the change in the structure of the PTFE blanks, an analysis was carried out by differential scanning calorimetry (DSC).
Images of a SEM of modified PTFE blanks (material Raflon):
Image 4
Image 5
Image 6
Image 7. Thermogram of PTFE blanks melting, depending on the absorbed dose.
Scan speed 5 K/min. Dose 0-200kGy
On the thermograms of unirradiated PTFE, 1 peak is responsible for the phase transition.
After the radiation modification, the thermograms show two pronounced peaks in the absorbed dose range from 10–200 kGy.
Image 8. Dependence of thermal characteristics on radiation dose for PTFE blanks.
The obtained DSC thermal data for the PTFE blanks made it possible not only to study the thermal characteristics of the initial PTFE blanks, but also to detect the phenomenon of double melting.
The phenomenon of double melting is the presence of two temperature peaks in a thermogram. Double melting is observed only during modification and only in a narrow range of absorbed doses (20 - 200 kGy). The double melting mechanism implies the presence of two types of crystalline structures. Because, each peak is responsible for a certain crystalline structure.
Then for the original PTFE blanks:
The reason for the formation of two different types of crystallites is the reorganization of the supramolecular structure from lamellar to spherulite. Spherulites are formed from fibrils under the influence of radiation on the polymer melt. Spherulite crystalline structures have a smaller crystallite size compared to lamellar crystallites.
In lamellas, an increase in crystallite size is associated with a coherent stacking of the crystalline regions of fibrils. Therefore, the transition from lamellar to spherulite packing of fibrils is accompanied by a general decrease in crystallite size, which in turn leads to a decrease in the temperature of phase transitions.
The coexistence of two morphological structures - spherulitic and lamellar, is observed at low radiation doses (20-200 kGy). An increase in the radiation dose leads to a decrease in the fraction of the high-temperature peak and an increase in the fraction of the low-temperature peak in the total melting enthalpy. So, at an absorbed dose of 200 kGy, there is no high-temperature peak in the thermograms. This indicates a complete change in the morphology of the sample. The presence of two types of crystals is confirmed by SEM images of the surface (Images 1,2,3 of unmodified PTFE blanks and Images. 4,5,6 of Raflon's material).
The physico-mechanical properties of the PTFE blanks change due to radiation effects, which are achieved due to the occurrence of the radiation-chemical reaction as a result of treatment with ionizing radiation. This effect arises and increases with increasing absorbed energy of ionizing radiation (absorbed dose of this radiation) in a unit volume. A quantitative characteristic of the radiation-chemical reaction is the radiation-chemical yield (the magnitude of the changes in the physicomechanical properties of the workpiece as a result of absorption of 100 eV of ionizing radiation).
Radiation modification of the PTFE blanks has the strongest effect on wear resistance. At an absorbed dose of 200 kGy. the specific wear rate rapidly decreases by more than 4 orders of magnitude (Image. 9) and decreases by another half order with an increase in the radiation dose to 800 kGy. As a result, the wear resistance of the PTFE blanks (tm Raflon) is (1-3)*10^–8 mm3 / (H × m) at an absorbed dose of 200 kGy.
Image 9. Dependence of the specific wear rate of PTFE blanks I on the absorbed dose (contact pressure 5 MPa, sliding speed 1 m/s).
Also, the strength characteristics of the material change