Molecular structure and thermal stability of dye-doped polyurethanes for laser technologies

For the first time, the influence of the xanthene-type rhodamine dyes, Rhodamine 6G (RG) or Rhodamine B (RB), on the thermal resistance of two different sorts of polyurethane resins (PUR) has been elucidated. The PURs were prepared either by direct reaction of the toluene diisocyanate and poly-oxypropylene glycol prepolymer hardened by trimethylol propane (PU), or by a radical photo-polymerization reaction of oligourethane acrylate with benzoin isobutyl ether (PUA). A correlation between the chemical structure of the polymers and their thermal properties was established using Fourier transformed infrared spectroscopy (FTIR, 400–4000 cm–1), thermogravimetric analysis (TGA, 30–800 °C), and differential scanning calorimetry (DSC). The DSC analysis shows that neither PU nor PUA display any phase transitions or melting points over the temperature interval of –100–300 °С, which is characteristic of amorphous polymers. The TGA study indicates that the PUA polymer is more thermal-resistant than the PU due to increased number of crosslinking points between the hard and soft segments. Сompared to nondoped polymers, the dye-doped PU-RB shows clearly improved thermal stability. It is found that a small presence of the RB dye improves the PU thermal stability, on account of the formed covalent and hydrogen-bonded network structures, while the effect is much smaller for the RG dye due to the presence of bulky CH3 groups and carboxyl group absence hampering the bonding formation. PUA samples prepared by radical photopolymerization are almost not sensitive to the presence of the dye molecules.