What method you are going to use to obtain a standard graph? Either calorimetry or GC method. If it is a calorimetry, u can do with crotonic acid. If it is with GC, make a standard graph, PHB is the suitable candidate than crotonic acid. Becoz, when PHB is heated and it results in crotonic acid. Please choose the method accordingly to your need.
We use PHB we obtained from Brazil as the standard. The Sigma stuff isn't very good. We use acid hydrolysis and analyse by HPLC. Using a PHB standard helps adjust for inefficiencies in acid hydrolysis which would result in an underestimation of PHB content if you used crotonic acid.