**3.1 The procedure of cross-linking and ceramization**

The samples were loaded onto the sample holders. The cross-linking was carried out under dry nitrogen atmosphere at 130°С/12h plus 200°С/3h. The samples' weights before (m0) and after (m) cross-linking were measured so that the mass retaining ratio (m/m0) can be obtained. Ceramization was carried out in a sealed alumina tube furnace in flowing argon gas at 25- 1300°С, with a heating rate 1°С/min and a dwell time of 3h. The crucible loaded with sample was weighed before and after pyrolysis so that ceramic yield may be calculated.
