Dear Noor,

Actually, PHB in nanoparticle nor crystalline nor amorphous.

Burst release at different pH is caused by nature of active ingredient and drug load coefficient.

Also it depend on buffer you used for pH stabilization.

Moreover, if you have drug overload due to salt formation between carboxylic group of PHB and amine group of drug loaded, in pH range less than 4,5 these salts undergoes ion exchange and drug release.