multiple poly 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 interlayer that has a relatively stiff poly inner layer disposed between two relatively soft outer poly 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 layers .