Structural recovery and physical aging of polymeric glasses

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Amorphous polymeric materials are often used in the so-called glassy state, as is the case for polystyrene, polycarbonate, and poly(methyl methacrylate), as well as for epoxy and polycyanurate resins in composite systems. For such materials, the structural recovery and physical aging behavior is reasonably well understood and can be treated in the context of glass-forming materials as described below. On the other hand, for semicrystalline polymers, such as nylon or PEEK, both glassy amorphous domains and reinforcing crystalline domains are present. In addition, the amorphous phase in the neighborhood of the crystallites may be modified, leading to “composite” material behavior that can be quite complex. Furthermore, even “simple” composites exhibit complexity in their physical aging responses that is not fully understood. In the following sections, we first describe the important aspects of the glassy behavior as related to physical aging in amorphous materials, and then we discuss the physical aging process and its measurement in amorphous, glassy polymers. We then move on to reinforced amorphous materials and semicrystalline polymers. Finally,2.1 Introduction 232.2 Glass formation and structural recovery 242.2.1 Intrinsic isotherms 252.2.2 Asymmetry of approach 272.2.3 Memory effect 282.2.4 Signatures of structural recovery in enthalpy space 282.2.5 τeff-paradox 31 2.2.6 Models of structural recovery 332.2.7 Best practices for measuring volume and enthalpy recovery 342.3 Physical aging of polymers and composites 372.3.1 Effective time theory 392.3.2 Aging in composite materials 412.3.3 Aging in semicrystalline polymers 442.4 Influence of structural recovery on nonlinear viscoelastic properties 452.5 Impact of structural recovery and physical aging 462.6 Conclusions 46Acknowledgment 47References 47because materials are frequently used at stresses or deformations above the linear viscoelastic limit, we briefly discuss the aging behavior of materials in the nonlinear regime up to the point of yield. We end with brief conclusions.