If such grafting has been done then HPLC is not recommended. Try dissolving your polymer and measure viscosity. Look for the Mark-Houwink-Sakurada equation and design some experimental. Virgin PLA molecular weight by the same method will be pretty useful. Good Luck!
I agree with allan as i also want to know that if there is covalent bonding between pla and Ti then which method is best to determine the molecular weight?
I previously have measured the viscosity and after that I have calculated the viscosity molecular weigth in PLA-Cellulose nanocrystal based nanocomposites by means of the Marke Houwink relation: