Shape-memory polymers are mechanically active materials designed to store a temporary deformed shape and subsequently recover a pre-programmed permanent shape upon exposure to external stimuli such as temperature or light. [1][2][3] The capacity of shape-memory polymers to transition between different physical shapes has led to a myriad of proposed minimally invasive biomedical applications [4] including sutures, [5] vascular stenting and repair, [6][7][8][9][10] stroketreating devices, [11,12] and microfluidic delivery devices. [13] Almost all of the proposed biomedical devices rely on large strain deformation at elevated temperatures. Despite the importance of large strain deformation in a useful shape-memory effect, work has not been performed to systematically understand the strain limits in shape-memory polymer networks as a function of network structure or relevant deformation parameters. Embedded in the use of shape-memory polymers for biomedical applications is their large-strain deformation response at various characteristic temperatures, often spanning multiple material states. The objective of this paper is to understand the relationships between strain to failure, temperature, and polymer structure in biocompatible shape-memory polymer networks.Researchers have examined thermomechanical strain storage and recovery couplings in a variety of shape-memory polymers. [6,[14][15][16][17] The standard approach is to deform the polymer at a temperature (T d ) typically above material's transition temperature (T trans ), which may be a characteristic melting or glass transition temperature (T m or T g ). Once the appropriate level of deformation is obtained, the polymer is held at the imposed strain and cooled under constraint to a temperature adequately below T trans . The material is then stored at a temperature (T s ) below the transition temperature indefinitely. When recovery is desired, the stored polymer is heated from T s and strain recovery occurs through a time and temperature dependent recovery process. Free strain recovery is performed without any external constraint, while constrained stress recovery is performed in the presence of fixed strain constraint. Shape storage and recovery are rooted in the viscoelastic nature of polymer networks. Consequently, the free and constrained recovery behaviors have been shown to depend strongly on T d and the recovery temperature (T r ) relative to the glass transition. [6,14] In addition, the recovery behavior is strongly influenced by the applied stress state stored in the polymer (tension versus compression) due to the influence of thermal expansion. [15] These prior studies have built a foundation to understand the complex thermomechanical behavior of shape-memory polymers, including the storage and recovery of stress and strain as a function of time, temperature, and constraint.Despite the growing base of knowledge on the constitutive response of shape-memory polymers, minimal work has focused on the link between polymer structure, temperature, and deform...
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