There is an old analytical method called gravimetry. Since no solid residues remain except TiO2 when your polymer is burned, you can use this method. You need an analytical balance, a laboratory oven and a crucible. The measurement method is clear and simple: you weigh the polymer before burning (the mass of the crucible is tared) and after burning, the difference is the TiO2 mass.

You can measure the amount of TiO2 by TGA, provided the melting point of TiO2 is significantly higher than that of the thermoplastics.

Thank you very much for helping me Mr Vadim Verlotski and Mr Jp Wu