Mech Time-Depend Mater (2010) 14: 347–357 DOI 10.1007/s11043-010-9114-9 Physical aging behavior of the normal force and torque in polymer glasses Anny Flory · Gregory B. McKenna Received: 7 December 2009 / Accepted: 12 April 2010 / Published online: 30 April 2010 © Springer Science+Business Media, B. V. 2010 Abstract In prior work we reported results from torsion in stress relaxation experiments in which we measured simultaneously, torque and normal force for two types of amorphous polymers. In one set the materials, poly(methyl methacrylate) (PMMA) and poly(ethyl methacrylate) (PEMA), have large sub-vitreous β relaxations, while in the second set, poly- carbonate (PC) and polysulfone (PSF), have weak β relaxations. It was found that the promi- nent β process influences the magnitude of the normal force modulus of PMMA and PEMA while the shear modulus is primarily affected by the α process. In the present work we have extended our study on the effect of the β and α relaxations on the normal force and torque in amorphous polymers by examining the aging behavior of both torque and normal forces of PMMA, PEMA and PC measured at various temperatures between the β and α tran- sitions and at a given deformation (γ = 0.04). It is found that for the PMMA the normal force shows aging behavior that is different from the torque response for the PMMA but ages in the same fashion as the torque for both the PEMA and the PC. These results are only partially consistent with the hypothesis that the β process is the cause of the differing behaviors. Keywords Physical aging · Aging · Glassy polymer · Normal force · PMMA · PEMA · Torque · Relaxation · Nonlinear mechanical properties · Viscoelasticity · Nonlinear viscoelasticity 1 Introduction A common method used to obtain an amorphous solid is to quench the material from the liquid state to a temperature below the glass transition temperature (T g ). When the result- A. Flory · G.B. McKenna ( ) Department of Chemical Engineering, Texas Tech University, Lubbock, TX 79409-3121, USA e-mail: greg.mckenna@ttu.edu Present address: A. Flory Wire and Cable R&D, The Dow Chemical Company, 171 River Road, Piscataway, NJ 08854, USA e-mail: alflory@dow.com