Amorphous Shape Memory Polymers Research Articles

Regulation of Glass Transition Temperature in Thermo‐Responsive Epoxy Shape Memory Polymers: The Roles of Chemical Crosslinking Density and Chain Flexibility

ABSTRACTShape memory polymers (SMPs) are novel kinds of smart materials whose shape can be changed in external conditions such as heat stimulus. Due to their excellent structural versatility and high elastic strain, they have important applications in the field of biomaterials. Here, a series of amorphous shape memory polymer materials are prepared and their shape memory properties and glass transition temperature (Tg) are also investigated. A special kind of epoxy polymer, whose shape can be changed at around physiological temperature, is synthesized by regulating the crosslinking density and introducing flexible aliphatic epoxy chains. The shape memory function of this epoxy polymer is studied by deformation test. In addition, to explore their application in the field of biomedical materials, the biocompatibility of the materials are evaluated, including the effects on the erythrocyte hemolysis rate and cell activity. The experimental results show that this kind of epoxy polymer has good shape memory function and biocompatibility.

Experimental Evaluation of Mechanical Properties, Thermal Analysis, Morphology, Printability, and Shape Memory Performance of the Novel 3D Printed PETG‐EVA Blends

AbstractPolyethylene terephthalate glycol (PETG) is a novel amorphous shape memory polymer with excellent printability for 4D printing. In this article, ethylene‐vinyl acetate (EVA) is used as a biocompatible and non‐toxic copolymer to improve plasticity and shape memory performance of PETG. PETG‐EVA blends are prepared and 3D printed using a melt mixing method and an upgraded fused deposition modeling (FDM) with a pneumatic feeding system. The results of the thermal analysis show that the blends exhibit two tan‐delta peaks, each related to their components, and morphology images confirm that they are biphasic and immiscible with good compatibility. The morphology of both EVA10 and EVA30 matrix droplets is observed, with the droplets being larger for EVA30. The use of a pneumatic feeding system, along with the ability to control the output melt flow, results in the best printing ability for EVA30, with minimal microholes between the grids and interlayer cracks. The tensile strength of PETG‐EVA blends ranged from 25.38 to 20.14 MPa, with the highest tensile strength achieved for EVA30. The shape memory performance of all three blends is similar; with shape recovery exceeding 90% in 20 s. Blends with higher EVA content exhibited faster shape recovery within the first 10 s.

A bridging model of a water-triggered shape-memory effect in an amorphous polymer undergoing multiple glass transitions

Hydrothermally-driven shape memory polymers (SMPs) have been extensively studied due to their advantage of having multiple response capabilities. In these SMPs,bound water reduces their glass transition temperatures (T g) by plasticizing the soft segments to achieve a water-triggered shape-memory effect (SME). However, the effect of bound water on hard segments, which has a synergistic effect on the T g and water-triggered SME of the soft ones, remains largely unexplored. In this study, we propose a new model to explore the working principles and hydrothermally-driven shape memory behaviors of amorphous SMPs. The bound water molecules are first divided into bridging and non-bridging bound water, and then a bridging effect is proposed to convert hard segments into soft ones, thus affecting the T g and water-triggered shape memory behavior in SMPs. An extended Gordon–Taylor model is formulated to identify the effects of bound water weight fraction and T g. Furthermore, a constitutive relationship between strain and relaxation time has been developed to describe the effects of temperature and bound water weight fraction on the hydrothermally-driven shape memory behaviors. Finally, the effectiveness of the proposed models is verified using the experimental results of amorphous SMPs reported in the literature.

Thermomechanical shape memory testing of 4D printed novel material rhombus-shape structure

4D printing of functional energy generation/absorption structures by material extrusion technique can capitalize on the exciting applications in intelligent damping devices and patterns to deform spontaneously. This paper investigated the capitalization of this acquired knowledge by studying the shape memory effects of a functional rhombus shape structure. Initially, the shape memory characteristics of energy absorption and dissipation capacity were analyzed using dynamic mechanical testing to develop databases to gather thermophysical data before expressing the material behavior and recovery performances of printed shape memory structures. Then a complete thermomechanical cycle (shape programming and recovery) on 4D printed amorphous and semi-crystalline shape memory polymers exhibited deep insights into shape memory performance. The performance factors (i.e., recovery and fixity ratios) are influenced by variable printing parameters (i.e., layer height, printing temperature, and speed) and stimuli-based testing conditions. Results reveal both materials have significant shape memory effects with a maximum recovery ratio of up to 92.30 ± 0.32% from the programmed configuration. The amorphous polymer was extremely affected by printing temperature, whilst the semi-crystalline was influenced heavily by the interaction of all three parameters. Finally, shape memory effects predicted by a high-order design model and compared with experimental results showed negligible error. The analyses and assessments presented in this paper are adequate to understand the shape memory behavior under process control parameters to establish a data-driven model of a 4D printed semi-crystalline and amorphous polymer reactive to thermal stimuli.

An extended shoving model for dynamic fluctuation of glass transition in amorphous polymer towards cooperative shape-memory effect

Phase transition theory is a powerful approach to study shape-memory effect (SME) and dynamic relaxation behaviour in amorphous shape-memory polymers (SMPs) undergoing glass transition processes. However, previous studies are mostly limited to building-up of experimental models, mainly due to poor understanding of dynamic fluctuation of glass transition. In this study, the dynamic fluctuation of glass transition in the amorphous SMPs, whose thermodynamic SME is governed by the phase transition theory, was investigated using a shoving model. A constitutive relationship between potential energy and cooperatively rearranging region (CRR) volume is firstly developed to explore the working principle of phase transition in the thermodynamic SME of amorphous SMPs, based on the shoving model and Adam–Gibbs domain size model. The effect of CRR volume on the phase transition is further formulated using the strain function, and then employed to develop a constitutive stress–strain equation based on the extended Maxwell model. The working principles of cooperative SME and dynamic relaxation behaviours have been explored and discussed. Finally, the effectiveness of the analytical results obtained using the proposed model has been verified using the experimental results of amorphous SMPs reported in literature.

Thermo-mechanical modeling of semicrystalline triple shape memory polymers

Compared with amorphous shape memory polymers (SMPs), semicrystalline SMPs have more diverse shape memory effects (SMEs) which promotes their application in smart structures. To reveal the driving mechanism of the triple SMEs of semicrystalline SMPs, our study focuses on developing the constitutive model under the condition of finite deformation. In the paper, a thermo-mechanical constitutive model under consideration of the second law of thermodynamics is developed based on the theory of thermodynamics with internal state variables. The model can be used to describe the nonequilibrium response of the amorphous and semicrystalline components in the vicinity of the glass transition, melting, and crystallization. To verify the validity of model, numerical simulation is carried out for a thermo-mechanical shape memory cycle which can be divided into a two-step programing process and a two-step recovery process. The comparison between the model results and the test data shows good agreement.

A thermoviscoelastic finite deformation constitutive model based on dual relaxation mechanisms for amorphous shape memory polymers

ABSTRACT This paper proposes a new thermoviscoelastic finite deformation model incorporating dual relaxation mechanisms to predict the complete thermo-mechanical response of amorphous shape memory polymers. The model is underpinned by the detailed microscopic molecular mechanism and effectively reflects the current understanding of the glass transition phenomenon. Novel evolution rules are obtained from the model to characterize the viscous flow, and a new theory named an internal stress model is introduced and combined with the dual relaxation mechanisms to capture the stress recovery. The rationality of the constitutive model is verified as the theoretical results agree well with the experimental data. Moreover, the constitutive model is further simplified to facilitate engineering applications, and it can roughly capture the characteristics of shape memory polymers.

Investigation of thermo-mechanical shape memory signatures of 3D printed and Injection molded polymers

This research paper investigates the shape memory signatures (SMSs) of SMPs fabricated by two different manufacturing processes, namely Fused Filament Fabricated (FFF) and Injection-molding (IM). The SMSs of each manufacturing route is first investigated by rating the thermophysical and thermodynamic properties of amorphous and semicrystalline-type shape memory polymers. Secondly, the capturing of SMSs is completed by a series of thermomechanical shape programming/recovery cycles on FFF printed specimens, and the results are correlated with the IM-produced samples of similar nature. Furthermore, the thermomechanical cycles are programmed at varying heating rates (1, 2, 3, and 5 °C min−1) to analyze their influence on shape memory behavior and cycling time. Results depict that an increase in induced pre-strain driven by small to large deformations (10% → 25%) in the elastic range leads to maximum shape memory performance and the highest performance was reported while heating at 2 and 3 °C min−1, respectively. Finally, the IM-produced samples showed slightly better shape recovery compared to the 3D-printed samples. The shape memory signatures of 3DP specimens are indeed up to 86% like the ones produced by injection molding, which is mostly governed by the distinct residual stress/strain induced during the two different manufacturing processes.

A dynamic hysteresis model for customized glass transition in amorphous polymer towards multiple shape memory effects

Coexistence of multiple and discrete segments as well as their distinctive hysteresis relaxations enables amorphous shape memory polymers (SMPs) exhibiting complex disordered dynamics, which is critical for the glass transition behavior to determine the shape memory effect (SME), but remained largely unexplored. In this study, a dynamic hysteresis model is proposed to explore the working principle and collective dynamics in discrete segments of amorphous SMPs, towards a dynamic connection between complex relaxation hysteresis and glass transition behavior, which can be applied for design and realization of multiple SMEs in the amorphous SMPs. In combination of free volume theory and Adam-Gibbs domain size model, a phase transition model is formulated to identify the working principle of dynamic relaxation hysteresis in the glass transition of amorphous SMP. Furthermore, constitutive relationships among relaxation time, strain, storage modulus, loss angle and temperature have been established to describe the dynamic connection between complex relaxation hysteresis and customized glass transition, which is then utilized to achieve multiple SMEs based on the extended Maxwell model. Finally, effectiveness of the proposed models is verified using experimental results of SMPs with multiple SMEs reported in literature.

A coupling model for the cooperative actuation mechanism of thermochemically responsive shape memory polymers

Thermochemically responsive shape memory polymers (SMPs) have attracted great interest in biomedical applications such as drug-releasing capsules and implantable medical stents, because body temperature can drive their shape recovery behaviors. However, it is difficult to determine the configurational dynamics of polymer segments due to the complexities of environmental stimuli (e.g. solute concentration, temperature change, and solvent diffusion). Besides, the cooperative actuation mechanism behind the thermochemical-driven shape memory effect (SME) is still poorly understood. In this study, we describe the effects of temperature and solvent absorption on conformational rearrangements in SMPs using the size change of cooperative rearrangement region (CRR) derived from the Adam–Gibbs model. The quasi-lattice model is further combined with Fick’s second law to characterize the kinetic diffusion behavior of solvent molecules in the CRR. The dependences of dual- and quadruple-SMEs in amorphous SMPs on immersion time, solution concentration, and programming temperature are quantitatively investigated using the proposed model. The theoretical results are also compared with experimental data and a good agreement is achieved. The proposed model is expected to provide theoretical guidance for understanding the working mechanism of thermochemically responsive SMPs and advancing their engineering applications.

Insight in thermomechanical constitutive modeling of shape memory polymers

Shape memory polymers (SMPs) are a new member of the smart materials family. SMPs have found wide applications or potential applications in almost all manmade structures and devices. In order to better design smart structures and devices using SMPs, thermomechanical constitutive modeling is essential. In this insight paper, we will focus on presenting several multi-length-scale and multi-physics modeling frameworks, including the thermodynamics consistent model, elasto-viscoplastic model, statistical mechanics model, and phase evaluation law model. The SMPs modeled will include amorphous one-way shape memory polymers, semicrystalline one-way shape memory polymers, semicrystalline two-way shape memory polymers, and functional and mechanical damage effects on SMPs. Finally, we will give some in-depth perspectives on future development in this area of study.

Dynamic equilibria with glass transition heterogeneity and tailorable mechanics in amorphous shape memory polymers

Modeling dynamic heterogeneity in amorphous shape memory polymers (SMPs) is a huge challenge due to the complex statistics of strain energy distributions during their thermodynamic relaxations. In this study, based on the dynamic heterogeneity of strain energy distribution, we have considered, for the first time, the influences of different temperature rates and strain rates on strain energy evolution as a dynamic equilibria, rather than a quasi-static problem. We propose a phase transition model incorporated with Gaussian distribution statistics to investigate the dynamic equilibria with glass transition heterogeneity and tailorable mechanics for the amorphous SMPs. The Gaussian distribution statistics is firstly applied to characterize the heterogeneity of strain energy distributions in the amorphous polymers. Phase transition theory is then developed to describe working principles of strain energy evolution, glass transition heterogeneity, thermodynamic relaxation and tailorable mechanics. Finally, the dynamic equilibria of heterogeneity about the statistics of strain energy distribution are formulated based on the one dimensional Maxwell multi-branch model. The analytical results are compared with the experimental data of epoxy, polyamide and vinylester SMPs reported in literature, and good agreements between them are demonstrated. This study provides a new insight into the dynamic heterogeneity in the mechanics of amorphous SMPs.

Controlling Morphology and Physio-Chemical Properties of Stimulus-Responsive Polyurethane Foams by Altering Chemical Blowing Agent Content.

Amorphous shape memory polymer foams are currently used as components in vascular occlusion medical devices such as the IMPEDE and IMPEDE-FX Embolization Plugs. Body temperature and moisture-driven actuation of the polymeric foam is necessary for vessel occlusion and the rate of expansion is a function of physio-chemical material properties. In this study, concentrations of the chemical blowing agent for the foam were altered and the resulting effects on morphology, thermal and chemical properties, and actuation rates were studied. Lower concentration of chemical blowing agent yielded foams with thick foam struts due to less bubble formation during the foaming process. Foams with thicker struts also had high tensile modulus and lower strain at break values compared to the foams made with higher blowing agent concentration. Additionally, less blowing agent resulted in foams with a lower glass transition temperature due to less urea formation during the foaming reaction. This exploratory study provides an approach to control thermo-mechanical foam properties and morphology by tuning concentrations of a foaming additive. This work aims to broaden the applications of shape memory polymer foams for medical use.

A 3D finite strain viscoelastic model with uncoupled structural and stress relaxations for shape memory polymers

In this paper, a 3D finite strain viscoelastic model with uncoupled structural and stress relaxations is proposed to characterize the thermomechanical behaviors of amorphous shape memory polymers (SMPs). The temperature- and rate-dependent viscoelastic behaviors and stress relaxation are described by the modified Eyring model and nonlinear Adam-Gibbs model based on molecular mechanism. A modified weight function analogous to frozen fraction in phase transition theory is proposed to further improve accuracy of the model. The developed model is implemented with MATLAB to simulate shape recovery experiments of amorphous SMPs under constraints or free in the literature. Good consistency between simulation results and experimental data is obtained.

Yielding mechanisms for mechano-chemo-thermal couplings in amorphous shape memory polymer undergoing molecular entanglement

Shape memory polymers (SMPs) have good properties of recovering their original shape in the presence of an external stimulus. However, the thermodynamic behaviors behind the yielding effects on mechano-chemo-thermal shape-memory effect (SME) are yet to be explored. In this study, molecular entanglement theory is employed to develop a thermodynamic model in terms of glass transition temperature. Based on the Arrhenius equation and Adam–Gibbs theory, an extended yield model is formulated to investigate the synergistic coordination of yield stress and stored strain energy on mechano-chemo-thermal SME and relaxation behavior in the amorphous SMPs. Furthermore, the effectiveness of this model is demonstrated by applying it to predict thermomechanical and mechano-chemo-thermal shape-recovery behaviors of the SMP, and the theoretical results are well validated by the experimental data reported in the literature. This study explores the working principle of stored strain energy in terms of molecular entanglement and yield stress, and describes a constitutive relationship between molecular entanglement and mechano-chemo-thermal coupling thermodynamics in the amorphous SMP.

A Thermodynamic-Consistent Model for the Thermo-Chemo-Mechanical Couplings in Amorphous Shape-Memory Polymers

Chemically-responsive amorphous shape-memory polymers (SMPs) can transit from the temporary shape to the permanent shape in responsive to solvents. This effect has been reported in various polymer-solvent systems. However, limited attention has been paid to the constitutive modeling of this behavior. In this work, we develop a fully thermo-chemo-mechanical coupled thermodynamic framework for the chemically-responsive amorphous SMPs. The framework shows that the entropy, the chemical potential and the stress can be directly obtained if the Helmholtz free energy density is defined. Based on the entropy inequality, the evolution equation for the viscous strain, the temperature and the number of solvent molecules are also derived. We also provide an explicit form of Helmholtz free energy density as an example. In addition, based on the free volume concept, the dependence of viscosity and diffusivity on the temperature and solvent concentration is defined. The theoretical framework can potentially advance the fundamental understanding of chemically-responsive shape-memory effect. Meanwhile, it can also be used to describe other important physical processes such as the diffusion of solvents in glassy polymers.

On the Size Effect of Additives in Amorphous Shape Memory Polymers.

Small additive molecules often enhance structural relaxation in polymers. We explore this effect in a thermoplastic shape memory polymer via molecular dynamics simulations. The additive-to-monomer size ratio is shown to play a key role here. While the effect of additive-concentration on the rate of shape recovery is found to be monotonic in the investigated range, a non-monotonic dependence on the size-ratio emerges at temperatures close to the glass transition. This work thus identifies the additives’ size to be a qualitatively novel parameter for controlling the recovery process in polymer-based shape memory materials.

A hygro-thermo-mechanical constitutive model for hygrothermally activated shape memory polymers under finite deformations

With regard to the application of shape memory polymers (SMPs) in smart structures, it is difficult to accurately predict their shape memory effects (SMEs) triggered by various external environments. Therefore, developing constitutive models for SMPs in multi-field environments is crucial. In this paper, a hygro-thermo-mechanical finite deformation constitutive model incorporated with structural relaxation and moisture diffusion is proposed for hygrothermally activated amorphous SMPs. One of the main novelties is that for the first time an internal state variable modeling approach is used in the model to describe the structural relaxation of hygrothermally activated SMPs in the vicinity of the glass transition. The other novelty is that the theory of moisture diffusion is applied to split the absorbed moisture into the mobile (free) and bound phases, and their respective effects on the hygro-thermo-mechanical behavior are also considered in the model. Comparisons between the model results and the test data for both, thermally activated and moisture-activated SMEs, present a good agreement.

Modeling the thermo-mechanical behavior and constrained recovery performance of cold-programmed amorphous shape-memory polymers

The low recovery stress has limited utilizing shape-memory polymers (SMPs) as actuators. In this work, we demonstrate that the resultant recovery stress of amorphous SMPs can be significantly increased through cold programming. A three-dimensional model is formulated to describe the thermo-mechanical behavior and shape-memory performance of amorphous polymers in large deformation. The constitutive relationship is derived based on the two-temperature thermodynamic framework employing an effective temperature as a thermodynamic state variable to describe the nonequilibrium structure of amorphous polymers. The model also incorporates the molecular orientation with a relaxation mechanism to describe strain hardening. The model is applied to simulate the stress-strain relationship and stress relaxation behaviors of an amorphous SMP in the programming process and the stress response in the constrained recovery process. The model can accurately reproduce the measured stress response at different temperatures and loading rates. Both experimental and simulation results show that polymers deformed at a larger loading rate exhibit a lower stress in the holding period, though simulation pronouncedly underestimates the magnitude of stress drop in the stress relaxation process. The model also captures the magnitude and location of the peak stress during the constrained recovery process for SMPs programmed at different temperatures and with different applied strains.

Collective and cooperative dynamics in transition domains of amorphous polymers with multi-shape memory effect

The multi-shape memory effect (multi-SME) in amorphous shape memory polymers (SMPs) linked with collective and cooperative rearrangements and accommodation of monomeric segments leads to the generation of complex thermodynamic modes. In this study, an extended domain size model is initially formulated to describe variations in temperature-dependent relaxation behavior and domain transitions in amorphous SMPs. According to the Adam–Gibbs theory, a cooperative model is employed to identify the principle role of domain size in the collective dynamics of multi-SMEs in amorphous SMPs. The phase transition theory is then combined with the multi-branch Kelvin model to describe the collective and cooperative relaxation behavior of the SMPs with multiple transition domains. It is shown that the proposed model is able to characterize the thermomechanical transitions and multiple shape recovery processes. Finally, the model is applied to predict the shape recovery behavior of SMPs with triple- and quadruple-SMEs, respectively, and the theoretical results are well validated by the experimental results.