Deformation Limits in Shape‐Memory Polymers

Abstract

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...

Synthesis of Programmable Main-chain Liquid-crystalline Elastomers Using a Two-stage Thiol-acrylate Reaction

“…The failure strain of our 15 mol% PETMP system was maximized when strained near T g , as measured by the peak of tan ∂. This is also in good agreement with previous studies that demonstrated the maximum strain in amorphous SMP networks occurred between the onset of the glass transition and T g ; 35,36 however, the LCE samples did not experience a rapid decrease in failure strain when heated above T g , as shown in most elastomers. 37 This can be attributed to the elevated tan ∂ region that exists between T g and T i (i.e., the nematic phase).…”

Effect of the Network Structure and Programming Temperature on the Shape-Memory Response of Thiol-Epoxy “Click” Systems

“…The samples were prepared in the same way as for their mechanical characterization (see Figure 2). The experiments were performed following the steps shown in Figure 4 three Tprog were chosen: Tg E′ , Tg and Tg + 20, to quantify the SMR at the optimal mechanical point (Tg E′ ) [9,10] and along the glass transition process (Tg and Tg + 20). The samples were heated up to Tprog and held during 5 min at this temperature to ensure thermal equilibrium (point 1 in Figure 4).…”

“…The results indicated that by carefully varying the network structure it was possible to increase the tensile elongation while maintaining high values of glass transition temperature (T g ) and obtaining excellent shape-recovery performances. On the other hand, another approach to enhance the ultimate strain based on the effect of the programming temperature (T prog ) was investigated by Yakacki et al [9] working with acrylates and Rousseau et al [10] using epoxy-based systems. The results demonstrated an improvement of the strain at break of three-to five-fold, when deforming at a temperature slightly below the T g , corresponding to the onset point of the network relaxation process (measured by dynamic-mechanical analyses).…”

“…In this sense, it is expected to clarify as much as possible the parameters one should consider depending on the desired applications. On the other hand, by varying the programming temperature, it is intended to link both the enhancement on the mechanical properties [9,10] with the effect on the SMR. For this purpose, in this work, (3-and 4-functional) thiol components have been used as curing agents for a commercial diglycidyl ether of bisphenol A (DGEBA) via "click" chemistry using encapsulated amines as latent initiators.…”

New understanding of the shape-memory response in thiol-epoxy click systems: towards controlling the recovery process

“…For comparison purposes, the shape-recov-366 ery ratios, R r , of the shape-memory experiments 367 carried out at the corresponding programming tem-368 peratures and up to e D = 75 % are also shown in 369 Table 3. 370 First of all, as explained in our previous work [14], 371 the optimal mechanical point was found at T g E ', in 372 which both the stress and strain at break values were 373 enhanced, a physical effect called viscoelastic tough-374 ening [34,35]. Moreover, the values obtained are 375 higher than those reported by other authors with 376 epoxy-based systems, especially for the stress at 377 break (up to 55 MPa on the 3thiol-NEAT formulation 378 with 95 % of e b at T g E' ) [ 36,37].…”