Multiple poly(vinyl butyral) layer interlayers that can be used in multiple layer glass panel type applications that require a high level of impact protection, for example in hurricane protection applications or in bullet proof glass applications. This effect is achieved by forming a poly(vinyl butyral) interlayer that has a relatively stiff poly(vinyl butyral) inner layer disposed between two relatively soft outer poly(vinyl butyral) layers, where the stiffness difference is achieved by a plasticizer differential that is achieved at least in substantial part by a residual hydroxyl content difference among the poly(vinyl butyral) layers.