A unified modeling approach for amorphous shape memory polymers and shape memory polymer based syntactic foam

Shape memory polymers (SMPs) and shape memory polymer composites have drawn considerable attention in recent years for their shape memory effects. A unified modeling approach is proposed to describe thermomechanical behaviors and shape memory effects of thermally activated amorphous SMPs and SMP‐based syntactic foam by using the generalized finite deformation multiple relaxation viscoelastic theory coupled with time–temperature superposition property. In this paper, the thermoviscoelastic parameters are determined from a single dynamic mechanical analysis temperature sweep at a constant frequency. The relaxation time strongly depends on the temperature and the variation follows the time–temperature superposition principle. The horizontal shift factor can be obtained by the Williams–Landel–Ferry equation at temperatures above or close to the reference temperature (T_r), and by the Arrhenius equation at temperatures below T_r. As the Arruda–Boyce eight‐chain model captures the hyperelastic behavior of the material up to large deformation, it is used here to describe partial material behaviors. The thermal expansion coefficient of the material is regarded as temperature dependent. Comparisons between the model results and the thermomechanical experiments presented in the literature show an acceptable agreement. Copyright © 2016 John Wiley & Sons, Ltd.     

Body temperature triggered shape-memory polymers with high elastic energy storage capacity

Shape-memory polymers (SMPs) that respond near body temperature are attracting broad interest, especially in the biomedical fields. In this study, the triggering temperature of poly(caprolactone) SMP networks is precisely adjusted by inclusion of non-crystallizable molecular linkers and by variation of prepolymer molecular weight. Longer, non-crystalline linkers and lower molecular weight prepolymers interfere with crystallization, lowering the transition temperature. Networks are prepared with crystallization temperatures that are beneath the human body temperature and yet are above room temperature. Upon cooling such amorphous networks to room temperature, crystallization is sluggish. There, elastomers can be easily strained by several hundred-percent to induce crystallization, thereby fixing strained states. If subsequently heated, programmed SMPs can release significant amounts of stored strain energy (∼3 MJ/m³). SMPs that combine elastic energy storage and exhibit triggering temperatures near the human body temperature could benefit emerging applications in the biomedical space. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1397–1404     

A fully coupled thermo‐viscoelastic finite element model for self‐folding shape memory polymer sheets

The thermo‐mechanical response of heat activated shape memory polymers (SMPs) has been investigated using a thermo‐viscoelastic finite element analysis that accounts for external and internal heat sources. SMPs can be thermally stimulated by external heat sources, such as temperature and surface heat flux, or from internal viscous heating. Viscous heating can significantly affect the response of SMP sheets by increasing the temperature during pre‐strain, which accelerates stress relaxation. This stress relaxation results in a slower shrinking rate when the SMP is reheated. Viscous heating also causes an increase in temperatures during unconstrained recovery. The predicted results elucidate how the coupled thermo‐mechanical loading conditions affect folding and unfolding of SMP sheets in response to localized heating in a hinged region. A parametric study of sheet thickness, hinge width, degree of pre‐strain, and hinge surface temperature is also conducted. The validated results can provide guidelines for the design of functional, self‐folding structures. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 1207–1219     

Porous shape memory polymers: Design and applications

Porous shape memory polymers (SMPs) exhibit geometric and volumetric shape change when actuated by an external stimulus and can be fabricated as foams, scaffolds, meshes, and other polymeric substrates that possess porous three-dimensional macrostructures. These materials have applications in multiple industries such as textiles, biomedical devices, tissue engineering, and aerospace. This review article examines recent developments in porous SMPs, with a focus on fabrication methods, methods of characterization, modes of actuation, and applications. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1300–1318     

A novel fractional viscoelastic constitutive model for shape memory polymers

Shape memory polymers (SMPs) are a class of smart materials which can recover from a deformed shape to their original shape by a certain external stimulus. To predict the deformation behaviors of SMPs, different constitutive models have been developed in the last few years. However, most of the constitutive models need many parameters to be determined by specific experiments and complex calibration processes. This drawback has limited their application in promoting the development of SMPs. Thus, it is imperative to develop a new constitutive model which is not only accurate, but also relatively simple. In our work, a novel fractional viscoelastic constitutive model coupling with time‐temperature superposition principle is first proposed for SMPs. Then, frequency sweep and temperature sweep experiments are conducted to determine the parameters of the model. Finally, the shape memory free recovery experiments are carried out to validate the predictive capability of the developed model. By comparing the predicted results with experimental data, we find that though our model has only eleven parameters in total, it could capture the thermomechanical behaviors of SMPs in very good agreement with experimental results. We hope the proposed new model provide researchers with guidelines in designing and optimizing of SMP applications. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 1125–1134     

Stress‐Induced Melting of Crystals in Natural Rubber: a New Way to Tailor the Transition Temperature of Shape Memory Polymers

Lightly cross‐linked natural rubber (NR, cis‐1,4‐polyisoprene) was found to be an exceptional cold programmable shape memory polymer (SMP) with strain storage of up to 1000%. These networks are stabilized by strain‐induced crystals. Here, we explore the influence of mechanical stress applied perpendicular to the elongation direction of the network on the stability of these crystals. We found that the material recovers its original shape at a critical transverse stress. It could be shown that this is due to a disruption of the strain‐stabilizing crystals, which represents a completely new trigger for SMPs. The variation of transverse stress allows tuning of the trigger temperature T_trig(σ) in a range of 45 to 0 °C, which is the first example of manipulating the transition of a crystal‐stabilized SMP after programming.     

Hydrogel‐coated polyurethane/urea shape memory polymer foams

Shape memory polymers (SMPs) are a class of responsive polymers that have attracted attention in designing biomedical devices because of their potential to improve minimally invasive surgeries. Use of porous SMPs in vascular grafts has been proposed because porosity aids in transfer of fluids through the graft and growth of vascular tissue. However, porosity also allows blood to leak through grafts so preclotting the materials is necessary. Here hydrogels have been synthesized from acrylic acid and N‐hydroxyethyl acrylamide and coated around a porous SMP produced from lactose functionalized polyurea‐urethanes. The biocompatibility of the polymers used to prepare the cross‐linked shape memory material is demonstrated using an in vitro cell assay. As expected, the hydrogel coating enhanced fluid uptake abilities without hindering the shape memory properties. These results indicate that hydrogels can be used in porous SMP materials without inhibiting the shape recovery of the material. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 1389–1395     

Thermoreversibly Crosslinked Poly(ε‐caprolactone) as Recyclable Shape‐Memory Polymer Network

A new concept to build shape memory polymers (SMP) combining outstanding fixity and recovery ratios (both above 99% after only one training cycle) typical of chemically crosslinked SMPs with reprocessability restricted to physically crosslinked SMPs is demonstrated by covalently bonding, through thermoreversible Diels–Alder (DA) adducts, star‐shaped poly(ε‐caprolactones) (PCL) end‐functionalized by furan and maleimide moieties. A PCL network is easily prepared by melt‐blending complementary end‐functional star polymers in retro DA regime, then by curing at lower temperature to favour the DA cycloaddition. Such covalent network can be reprocessed when heated again at the retro DA temperature. The resulting SMP shows still excellent shape memory properties attesting for its good recyclability.

     

Molecular mobility of poly(methyl methacrylate) glass during uniaxial tensile creep deformation

An optical photobleaching method has been used to measure the segmental dynamics of a poly(methyl methacrylate) (PMMA) glass during uniaxial creep deformation at temperatures between T_g ? 9 K and T_g ? 20 K. Up to 1000‐fold increases in mobility are observed during deformation, supporting the view that enhanced segmental mobility allows flow in polymer glasses. Although the Eyring model describes this mobility enhancement well at low stress, it fails to capture the dramatic mobility enhancement after flow onset, where in addition the shape of the relaxation time distribution narrows significantly. Regions of lower mobility accelerate their dynamics more in response to an external stress than do regions of high mobility. Thus, local environments in the sample become more dynamically homogeneous during flow. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1713–1727, 2009     

Preparation of ureido group bearing polymers and their upper critical solution temperature in water

Poly(2‐ureidoethylmethacrylate) (PUEM_n) was synthesized via reversible addition‐fragmentation chain transfer (RAFT) radical polymerization and following polymer reaction. We prepared two PUEM_n samples with different degrees of polymerization (n = 100 and 49). The polymers exhibited upper critical solution temperature (UCST) in phosphate‐buffered saline (PBS) solution. The phase separation temperature (T_p) in PBS can be controlled ranging from 17 to 55 °C by changing molecular weight of the polymer, polymer concentration, and adding NaCl concentration. The polymers in PBS formed coacervate drops by liquid–liquid phase separations below T_p. Results of the dielectric relaxation measurement, the hydration number per monomeric unit was 5 above T_p. Based on a fluorescence study, the polymer formed slightly hydrophobic environments below T_p. The liquid–liquid phase separation was occurred presumably because of weak hydrophobic interactions and intermolecularly hydrogen bonding interactions between the pendant ureido groups. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 2845–2854     

Shape memory induced structural evolution of high performance copolyimides

In this work, two kinds of high temperature shape memory copolyimides were prepared and the shape memory cycles induced structural evolution of macromolecular chains was investigated in detail. The glass transition temperature (T_g) of poly(benzoxazole‐co‐imide) (PI1) and poly(benzimidazole‐co‐imide) (PI2) are 280 °C and 355 °C, respectively. The results show that PI1 could keep stable macromolecular chain structure under shape memory cycles and exhibit outstanding shape memory performance (R_f > 98%, R_r > 97%) under different stretch condition. Whereas, shape memory cycles induced orientation with more ordered macromolecular chains packing is formed for PI2 after several thermal mechanical cycles, which strongly affect physical crosslinking points, thermal mechanical properties as well as shape memory behaviors. The study on macroscopic property and microscopic structure evolution will promote a better understanding of the shape memory effect of polyimides and accelerate development of high performance polyimides for shape memory applications. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 3858–3867     

Effects of Na‐sulfopropyl groups on the mechanical properties and morphology of polystyrene‐co‐methacrylate ionomers

The dynamic mechanical properties and morphology of poly(styrene‐co?3‐sulfopropyl sodium‐methacrylate) SSPMANa ionomers were investigated. It was found the increasing rate of ionic moduli of the SSPMANa ionomer was very low, and the cluster T_g of the ionomers remained more or less constant with increasing ion content. A well‐developed SAXS peak was seen for low ion content SSPMANa ionomers and the peak position changed slightly with ion content. Thus, it was suggested that the presence of the alkyl ester side chains made the ion pairs form multiplets more easily at their prevalent distances, and the small‐agglomerated multiplets were dispersed in the polymer matrix relatively evenly. The interpretation of ionic moduli using a number of theories implied that the multiplets and clusters acted as effective crosslinks and filler particles, respectively, and the size and shape of the clusters were irregular. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1043–1053     

PCL-based Shape Memory Polymers with Variable PDMS Soft Segment Lengths

Thermoresponsive shape memory polymers (SMPs) are stimuli-responsive materials that return to their permanent shape from a temporary shape in response to heating. The design of new SMPs which obtain a broader range of properties including mechanical behavior is critical to realize their potential in biomedical as well as industrial and aerospace applications. To tailor the properties of SMPs, "AB networks" comprised of two distinct polymer components have been investigated but are overwhelmingly limited to those in which both components are organic. In this present work, we prepared inorganic-organic SMPs comprised of inorganic polydimethyl-siloxane (PDMS) segments of varying lengths and organic poly(ε-caprolactone) (PCL) segments. PDMS has a particularly low T(g) (-125 °C) which makes it a particularly effective soft segment to tailor the mechanical properties of PCL-based SMPs. The SMPs were prepared via the rapid photocure of solutions of diacrylated PCL(40)-block-PDMS(m)-block-PCL(40) macromers (m = 20, 37, 66 and 130). The resulting inorganic-organic SMP networks exhibited excellent shape fixity and recovery. By changing the PDMS segment length, the thermal, mechanical, and surface properties were systematically altered.     

Synthesis and characterization of new polydiolcitrates with tunable properties

Polydiolcitrates are an emerging class of biocompatible polyesters with a great potential in the field of biomedicine and packaging for food and drug materials. In this work, a new type of (co‐)polydiolcitrates made of citric acid (CA) and ethylene glycol (EG) and/or poly(ethylene glycol) (PEG) is investigated. By varying both the EG/PEG and the CA/diol molar ratios, materials exhibiting very different swelling behavior, mechanical and thermal properties are obtained. In particular, the substitution of EG segments with longer and flexible PEG ones results in an increase in crosslinking density, with remarkable effects on swelling capacity, glass transition temperature, and Young modulus. Moreover, polyesters with CA/diol molar ratio equal to 1:1 exhibit shape memory properties, with full capacity of keeping the temporary shape and high capacity of recovering the original shape. This work demonstrates that the appropriate choice of polyester composition allows modulating the sample properties, that permits to these materials to cover a wide range of possible applications. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 3713–3720     

Ortho‐positronium lifetime distribution analyzed with MELT and LT and free volume in poly(ε‐caprolactone) during glass transition,melting, and crystallization

Positron annihilation lifetime spectroscopy (PALS) was used to study the free volume behavior in the temperature range between 100 and 370 K in semicrystalline poly(ε‐caprolactone) (PCL). For the analysis of the spectra we used the well‐known routine MELT as well as the new program LT8.0, which allows both discrete and log‐normal distributed annihilation rates. From experiments, confirmed by the analysis of simulated spectra, we found that MELT returns too large values for the o‐Ps lifetime τ₃ associated with too small intensities I₃. This is due to the underestimation of the width of o‐Ps lifetime distribution in MELT (the spectra analyzed contained 3 million counts). The same effects were observed in the parameters obtained from the discrete term analysis. LT, however, returns, when allowing the o‐Ps lifetime to be distributed, rather accurate values for τ₃, I₃, and the width (standard deviation σ₃) of the o‐Ps lifetime distribution. The effect of the glass transition, melting, and crystallization on the annihilation parameters was observed. These results were compared with differential scanning calorimetry (DSC) and pressure–volume–temperature (PVT) experiments. From this comparison, the number density of holes and the fractional free (hole) volume have been estimated. At a “knee” temperature T_k ≈ 1.5 T_g, a leveling off of the o‐Ps lifetime τ₃ and a distinct decrease in the width, σ₃, of its distribution was observed; the latter effect was detected for the first time. Fast motional processes and/or the disappearance of the dynamic heterogeneity of the glass and the transition to a homogeneous liquid are discussed as possible reasons for these effects. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 3077–3088, 2003     

Thermodynamic analysis of the phase separation during the polymerization of a thermoset system into a thermoplastic matrix. II. Prediction of the phase composition and the volume fraction of the dispersed phase

A thermodynamic simulation of the phase‐separation process of an off‐critical blend, based on a thermoplastic matrix with a reactive epoxy system undergoing polycondensation at a constant temperature, was performed. The model considered the composition dependence of the interaction parameter, χ(T,Φ₂) (where T is the temperature and Φ₂ is the volume fraction of polystyrene), along with the polydispersity of both polymers. For every level of conversion, the simulation provided the amount, composition, stoichiometric ratio, and conversion of each phase present. The accuracy of the model was proved by the good agreement between the experimental and predicted glass‐transition temperatures and heat capacity changes at the glass‐transition temperatures for both phases. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1361–1368, 2004     

Twinkling fractal theory of the glass transition

In this paper we propose a solution to an unsolved problem in solid state physics, namely, the nature and structure of the glass transition in amorphous materials. The development of dynamic percolating fractal structures near T_g is the main element of the Twinkling Fractal Theory (TFT) presented herein and the percolating fractal twinkles with a frequency spectrum F(ω) ∼ ω^df–1 exp −|ΔE|/kT as solid and liquid clusters interchange with frequency ω. The Orbach vibrational density of states for a fractal is g(ω) ∼ ω^df–1, where d_f = 4/3 and the temperature dependent activation energy behaves as ΔE ∼ (T² − T). The key concept of the TFT derives from the Boltzmann population of excited states in the anharmonic intermolecular potential between atoms, coupled with percolating solid fractal structures near T_g. The twinkling fractal spectrum F(ω) at T_g predicts the correct dynamic heterogeneity behavior via the spatio-temporal thermal fluctuation autocorrelation relaxation function C(t). This function behaves as C(t) ∼ t^−1/3 (short times), C(t) ∼ t^−4/3 (long times) and C(t) ∼ t⁻² (ω < ω_c), which were found to be in excellent agreement with published nanoscale AFM dielectric force fluctuation experiments on a glassy polymer near T_g. Using the Morse potential, the TFT predicts that T_g = 2D_o/9k, where D_o is the interatomic bonding energy ∼ 2–5 kcal/mol and is comparable to the heat of fusion ΔH_f. Because anharmonicity controls both the thermal expansion coefficient α_L and T_g, the TFT uniquely predicts that α_L×T_g ≈ 0.03, which is found to be universal for a broad range of glassy materials from Pyrex to polymers to glycerol. Below T_g, the glassy structure attains a frustrated nonequilibrium state by getting constrained on the fractal structure and the thermal expansion in the glass is reduced by the percolation threshold p_c as α_g ≈ p_cα_L. The change in heat capacity ΔC_p = C_pL–C_pg at T_g was found to be related to the change in dimensionality from D_f to 3 in the Debye approximation as the ratio C_pL/C_pg = 3/D_f, where D_f is the fractal dimension of the glass. For polymers, the TFT describes the molecular weight dependence of T_g, the role of crosslinks on T_g, the Flory-Fox rule of mixtures and the WLF relation for the time-temperature shift factor a_T, which are traditionally viewed in terms of Free-Volume theory. The TFT offers new insight into the behavior of nano-confined glassy materials and the dynamics of physical aging. It also predicts the relation between the melting point T_m and T_g as T_m/T_g = 1/[1−p_c] ≈ 2. The TFT is universal to all glass forming liquids. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2765–2778, 2008     

Thermal and mechanical properties of random copolymers of diisopropyl fumarate with 1‐adamantyl and bornyl acrylates with high glass transition temperatures

We report the thermal, optical, and mechanical properties of random copolymers produced by radical copolymerizations of diisopropyl fumarate (DiPF) with 1‐adamantyl acrylate (AdA) and bornyl acrylate (BoA). The effects of a methylene spacer included in the main chain and bulky ester alkyl groups in the side chain on the copolymer properties are discussed. The produced copolymers are characterized by NMR and UV–vis spectroscopies, size exclusion chromatography, thermogravimetric analysis, differential scanning calorimetry, and dynamic mechanical analysis (DMA). The copolymerization rate and the molecular weight of the copolymers increase with an increase in the acrylate content in feed during the copolymerization (M_w = 25–110 × 10³). The onset temperature of decomposition (T_d5) and the glass transition temperature (T_g) of the copolymers also increase according to the content of the acrylate units (T_d5 = 296–329 °C and 281–322 °C, T_g = 80–133 °C and 91–106 °C for the copolymers of DiPF with AdA and BoA, respectively). Transparent and flexible copolymer films are obtained by a casting method and their optical properties such as transparency and refractive indices are investigated (n_D = 1.478–1.479). The viscoelastic data of the copolymers are collected by DMA measurements under temperature control. The storage modulus decreases at a temperature region over the T_g value of the copolymers, depending on the structure and amount of the acrylate units. The sequence structure of the copolymers is analyzed based on monomer reactivity ratios and composition in order to discuss the copolymer properties related to chain rigidity and sequence length distribution. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 288–296     

Fabrication of well‐defined shape memory graft polymers derived from biomass: An insight into the effect of side chain architecture on shape memory properties

Fully sustainable shape memory polymers (SMPs) derived from ethyl cellulose (EC, derived from cellulose), tetrahydrofurfuryl methacrylate (THFMA, derived from furfural), and lauryl methacrylate (LMA, derived from fatty acids) were prepared via “grafting from” atom transfer radical polymer (ATRP). The “grafting from” ATRP strategy allows to fabricate SMPs with EC as a backbone, and LMA and THFMA copolymer as a side chain. By utilizing the one‐pot and sequential monomer addition approach, two types of SMPs with random/semi‐block side chain architectures were obtained, respectively. Random/semi‐block side chain architecture of SMPs was confirmed by DSC, DMA, SAXS, and TEM. The presence of microphase separation in the SMPs with semi‐block side chain architecture provided two distinct thermal transitions, which was needed for triple‐shape memory behavior. Shape memory study showed that SMPs with semi‐block side chain architecture exhibited excellent triple‐shape memory property, and also had higher shape recovery speed and shape recovery ratio than those with random side chain architecture. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 1711–1720     

The linear rheological responses of wedge‐type polymers

The linear rheological responses of a series of specially designed wedge‐type polymers synthesized by the polymerization of large molecular weight monomers have been measured. These wedge polymers contained large side groups which contained three flexible branch chains per polymer chain unit. The master curves for these polymers were obtained by time temperature superposition of dynamic data at different temperatures from the terminal flow regime to well below the glass transition temperature, T_g. While these polymers maintained a behavior similar to that of linear polymers, the influence of the large side group structure lead to low entanglement densities and extremely low rubbery plateau modulus values, being near to 13 kPa. The viscosity molecular weight dependence was also somewhat higher than that normally observed for linear polymers, tending toward a power law near to 4.2 rather than the typical 3.4 found in entangled linear chains. The glassy modulus of these branched polymers is also found to be extremely low, being less than 100 MPa at T_g ?60 °C. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 899–906