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     

A novel shape memory poly(ε‐caprolactone) network via UV‐triggered thiol‐ene reaction

A facile method to prepare shape memory polymers crosslinked by SiO₂ is described. A series of biodegradable shape memory networks were obtained through thiol‐ene reaction triggered by UV irradiation between surface‐thiol‐modified SiO₂ nanoparticles and end‐acrylate poly (ε‐caprolactone) (PCL). The highly selective thiol‐ene reaction ensured a uniform distribution of PCL chains between crosslinkers, contributing well‐defined network architecture with enhanced mechanical and shape‐memory properties. Thiol‐functionalized silica nanoparticle was characterized by using FTIR and XPS analysis, and ¹H NMR spectra was used to confirm the successful modification of terminal hydroxyl group of PCL diol. Surface‐modified silica particles were found well dispersible in acrylate‐capped PCL supported by SEM. Thermal and crystalline behaviors of the obtained polymers were analyzed by DSC and XRD, and DMA measurement proved good mechanical property. The shape memory behavior and tensile strength was somewhat tunable by the length of PCL. Acceptably, sample SiO₂‐SMP2k presented 99% recovery ratio and 97% shape fixity, and its relatively high tensile strength showed an attractive potential for biomedical application. Finally, a possible molecular mechanism accounting for the shape memory property was illustrated. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 692–701     

Hydrogen bond-reinforced double-network hydrogels with ultrahigh elastic modulus and shape memory property

Development of tough hydrogels has greatly expanded their applications as load-bearing materials. However, the elastic modulus of tough hydrogels is usually lower than 1 MPa. It remains a challenge to design tough hydrogels with high modulus. We report here a series of tough double-network (DN) hydrogels with ultrahigh elastic modulus (up to 200 MPa) by forming robust hydrogen bonds between the first poly(acrylic acid) network and the second poly(N-isopropyl acrylamide) network. The dense cooperative hydrogen bonds greatly reduce the segmental mobility and thus improve the rigidity of gel matrix. Owing to the dynamic nature of hydrogen bonds, the modulus of hydrogels is strongly influenced by temperature and pH, affording the gels shape memory property. The strategy by forming robust noncovalent bonds between interpenetrating networks should be applicable to other systems for designing tough and versatile hydrogels with diverse promising applications. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 1281–1286     

Water vapor permeability of shape memory polyurethane with amorphous reversible phase

Shape memory thermoplastic polyurethanes (TPUs), based on amorphous soft segment from the reaction of hexamethylene diisocyanate and 1,2‐butane diol, and the crystalline hard segment from 4,4′‐methylenediphenyl diisocyanate and 1,6‐hexanediol, were modified by hydrophilic segments, diol‐terminated poly(ethylene oxide) or dimethylol propionic acid (DMPA). Differential scanning calorimetry, dynamic mechanical testing, tensile testing, and the measurement of shape memory effect, water swell, and water vapor permeability were carried out to examine these TPUs. The hydrophilic segment increased the hysterisis in shape memory effect by reducing the crystallinity of the hard segment. The neutralized DMPA unit enhanced the sensitivity of the thermoresponsive water vapor permeability (WVP) by amplifying the increase of WVP at the temperature range above the glass transition temperature. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 3009–3017, 2000     

Optical and shape memory properties of semicrystalline poly(cyclooctene) upon cold‐drawing

Semicrystalline thermoplastic poly(cyclooctene) (PCO) shows significant improvement in transparency when cold‐drawn at room temperature, unlike other semicrystalline polymers whose fibrillated chains cause crazing upon cold‐drawing, making the polymers opaque to visible light. Upon heating, transparent cold‐drawn PCO recovers its original opacity as well as its undeformed shape. In situ wide‐ and small‐angle X‐ray diffraction and polarized Fourier transform infrared analyses show that molecular density differences between the PCO crystalline and amorphous phases were reduced due to strain‐induced crystallization and that fibrillated chains and voids, an indication of craze, were not observed due to chain entanglements concentrated in trans double‐bond regions. These two factors explain the unique optical properties of PCO. Finally, it is demonstrated that crosslinked PCO enhanced optical and shape memory recovery without deterioration of the transparency of the polymer. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 1595–1607     

Shape memory polybenzoxazines based on a siloxane‐containing diphenol

A siloxane‐containing diphenol is synthesized from 1,1,3,3‐tetramethyldisiloxane and o‐allylphenol, followed by the Mannich condensation with aniline, methylamine, and formaldehyde yielding two siloxane‐containing benzoxazines. The onset polymerization temperature of aniline‐based benzoxazine is higher than that of the methylamine counterpart. The dynamic mechanical properties of the polybenzoxazines depend on the structure of the starting primary amines. Both polybenzoxazines exhibit one‐way dual‐shape memory behavior in response to changes in temperature, and they show excellent shape fixity ratios in bending, tension, and tensile stress–strain tests, high shape recovery ratios in bending and tension tests, but relatively low shape recovery ratios in tensile stress–strain test. The network chain segments including the alkylsiloxane units serve as a thermal control switch based on the glass transition temperatures (39 and 53 °C) for the polybenzoxazines. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1255–1266     

Hydrogen‐Bonding Toughened Hydrogels and Emerging CO2‐Responsive Shape Memory Effect

A double hydrogen bonding (DHB) hydrogel is constructed by copolymerization of 2‐vinyl‐4,6‐diamino‐1,3,5‐triazine (hydrophobic hydrogen bonding monomer) and N,N‐dimethylacrylamide (hydrophilic hydrogen bonding monomer) with polyethylene glycol diacrylates. The DHB hydrogels demonstrate tunable robust mechanical properties by varying the ratio of hydrogen bonding monomer or crosslinker. Importantly, because of synergistic energy dissipating mechanism of strong diaminotriazine (DAT) hydrogen bonding and weak amide hydrogen bonding, the DHB hydrogels exhibit high toughness (up to 2.32 kJ m⁻²), meanwhile maintaining 0.7 MPa tensile strength, 130% elongation at break, and 8.3 MPa compressive strength. Moreover, rehydration can help to recover the mechanical properties of the cyclic loaded–unloaded gels. Attractively, the DHB hydrogels are responsive to CO₂ in water, and demonstrate unprecedented CO₂‐triggered shape memory behavior owing to the reversible destruction and reconstruction of DAT hydrogen bonding upon passing and degassing CO₂ without introducing external acid. The CO₂ triggering mechanism may point out a new approach to fabricate shape memory hydrogels.     

Actuation design for high‐performance shape memory polyurethanes

A series of shape memory polyurethanes were synthesized from poly(tetramethylene glycol), 4,4‐methylene diphenyl diisocyanate, and 1,3‐butanediol. The prepolymers with different molecular weights (M_c) were capped with 2‐hydroxyl ethylacrylate or 3‐aminopropyltriethoxysilane (APTES) and crosslinked by UV curing or a sol–gel reaction. Variations of the crosslinker functionality (f), subchain density (N), and hard segment content (HSC) produced systematic variations of the glass transition temperature (6–45 °C), accompanied by changes in the mechanical, dynamic mechanical and shape memory properties. More than 95% of shape fixity and 98% of shape recovery up to the fourth cycles were obtained with APTES crosslinked 3000M_c with 30% of HSC. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013 , 51, 1473–1479     

Shape‐memory behavior of segmented polyurethanes with an amorphous reversible phase: The effect of block length and content

Segmented thermoplastic polyurethanes (TPU)s with amorphous soft segments from the reaction of hexamethylene diisocyanate and 1,2‐butanediol and crystalline hard segments from 4,4′‐methylenediphenyl diisocyanate and 1,6‐hexanediol showed sharp glass‐transition temperatures that could be used as shape‐recovery temperatures. The thermal, mechanical, and shape‐memory effect of these TPUs of various block compositions and lengths were studied by differential scanning calorimetry, dynamic mechanical testing, and tensile testing. As the block lengths decreased, phase mixing increased and hysteresis in the shape‐memory behavior decreased. Too low a content of hard segments increased the hysteresis in the shape‐memory behavior. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2652–2657, 2000     

Room temperature deformable shape memory composite with fine‐tuned crystallization induced via nanoclay particles

It is known that particular types of semi‐crystalline/elastomer polymer blends exhibit shape memory effects (SME) due to the dispersion of two immiscible phases. In this study, the crystal structure of polylactic acid (PLA)/ thermoplastic polyurethane (TPU) based shape memory polymer (SMP) is altered by incorporating small amounts of montmorillonite (MMT) nanoclay. The results indicate the incorporation of MMT can improve the compatibility of the two different polymers. Moreover, the presence of MMT affects the total crystallinity of the SMP and improves mechanical properties. Lastly, uniaxial stretching deformation can be applied to the SMP at room temperature conditions while maintaining its shape memory properties. With 1 wt % MMT particles, the recovery ratio (Rr) was nearly 95%, which indicated a strong recovery effect. The shape‐fixing ratio (Rf) remained above 95% for all composites due to plastic deformation applied at room temperature. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 1197–1206     

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     

High‐Strain Shape Memory Behavior of PLA–PEG Multiblock Copolymers and Its Microstructural Origin

Shape memory properties of two thermoplastic multiblock copolymers composed of poly(lactic acid) (PLA) and poly(ethylene glycol) (PEG) having different PEG‐segment lengths of 6 and 11 kDa were studied. The performance as a shape memory polymer at high strain level (600%) and its interrelations with shape‐programming conditions, molecular orientation, and microstructural changes are elucidated. A significant contribution of strain‐induced crystallization of PLA segments to the improvement of temporary shape fixation was evidenced upon increasing draw ratio and/or shape‐holding duration as well as programming temperature (within certain range) without largely sacrificing the shape recoverability. Series of microstructural characterizations reveal the occurrence of fibrillar‐to‐lamellar transformation upon shape recovery (at 60 °C) of the samples programmed at 40 °C, generating shish–kebab crystalline morphology. Such phenomenon is responsible for the high‐strain shape memory effect of these materials. The unprecedented formation of shish–kebab structure at such relatively low temperature (instead of the melting temperature range) in solid state observed in these copolymers as well as their high‐strain shape memory functionality would bestow the promising future for their practicability in diverse areas. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 241–256     

Thermo‐/moisture‐responsive shape‐memory effect of poly(2‐ethyl‐2‐oxazoline) networks

In this work, poly(2‐ethyl‐2‐oxazoline) (PEtOx) is crosslinked to realize a moisture‐ and thermo‐responsive shape‐memory polymer. The obtained PEtOx networks exhibit excellent shape‐memory properties with storable strains of up to 650% and recovery values of 100% over at least 10 shape‐memory cycles. The trigger temperature (T_trig) of 68 °C of a PEtOx network at a relative humidity (RH) of 0% decreases with increasing moisture and equals room temperature at an RH of 40%. Thus, programmed PEtOx networks trigger sensitively on a certain temperature/moisture combination and, further, can be programmed as well as triggered at room temperature exclusively by varying humidity. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 1053–1061     

High modulus ratio shape‐memory polymers achieved by combining hydrogen bonding with controlled crosslinking

A new type of poly(methyl acrylate)‐co‐(acrylic acid) (PMA‐AA) networks obtained by combining hydrogen bonding with controlled crosslinking exhibit full and rapid shape‐memory recovery. The structure, thermal properties, dynamical mechanical properties and shape‐memory effects of these networks were presented. High modulus ratios were achieved for the series of PMA‐AA networks based on intense self‐complementary hydrogen bonding in poly(acrylic acid) (PAA) segments. This lead to excellent shape‐memory effects with strain‐recovery ratio above 99%. Meanwhile, faster recovery speed was achieved by the synergistic effect of hydrogen bonding and controlled crosslinking compared to the linear PMA‐AA copolymers. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 1241–1245, 2011     

Soft bacterial polyester‐based shape memory nanocomposites featuring reconfigurable nanostructure

In this work, a novel soft shape memory polymer nanocomposite derived from a bacterial medium‐chain‐length polyhydroxyalkanoate, poly(3‐hydroxyoctanoate‐co‐3‐hydroxyundecenoate) (PHOU), used to form a covalent network grafted with polyhedral oligomeric silsesquioxane (POSS), a crystallizable inorganic–organic hybrid nanofiller, was prepared. The PHOU–POSS nanocomposite, PHOU–POSSw‐net [w (= POSS content, wt %) = 0, 20, 25, 30, and 38], is a completely amorphous elastomer (w ≤ 20) or contains POSS nanocrystals embedded in the amorphous PHOU matrix (w ≥ 25). The hybrid nanostructure of PHOU–POSSw‐net (w ≥ 25) is featured by its reconfigurability, based on aggregation and disaggregation of POSS covalently connected to the PHOU network, which enables excellent shape fixing and recovery. Furthermore, it exhibits soft and elastomeric mechanical properties even in the fixed state. Taking advantage of the shape memory ability as well as the softness in the fixed state, we demonstrate microscale dynamic surface topography of PHOU–POSSw‐net. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Hydrogels with Crystalline or Liquid Crystalline Structure

The formation of ordered structure in hydrogels derived from copolymers of hydrophilic and hydrophobic monomers with crystalline or liquid‐crystalline moieties is reviewed. The role of water in the formation of ordered structure and its influence on the thermal and mechanical properties of hydrogels are clarified. For example, by inducing a certain amount of water, an amorphous to crystalline transition occurs in gels of acrylic acid/alkyl acrylate copolymers. On the other hand, water induces a liquid‐crystalline (SmA) to liquid‐crystalline (SmI) transition in copolymers consisting of acrylic acid and 11‐(4′‐cyanobiphenyloxy)undecyl acrylate. These specific features regarding the formation of ordered structures in hydrogels might shed some light on the formation of ordered structure in biological tissues.     

Impact of poly(ɛ‐caprolactone) architecture on the thermomechanical and shape memory properties

We report the synthesis of linear‐ and brush‐type poly(?‐caprolactone) (PCL) networks and investigate their thermal, mechanical, and shape memory behavior. Brush‐PCLs are prepared by ring‐opening metathesis polymerization (ROMP) of a norbornenyl‐functionalized ?‐caprolactone macromonomer (MM‐PCL) of different molecular weights. The linear analog, diacrylate end‐functionalized PCL (linear‐PCL), having comparable molecular weight of side chain of brush‐PCL is also synthesized. These polymers are thermally cured by a radical initiator in the presence of poly(ethylene glycol) diacrylate crosslinker. Thermal and linear viscoelastic properties as well as shape memory performance of the resulting PCL networks are investigated, and are significantly impacted by the PCL architecture. Therefore, our work highlights that tailoring macromolecular architecture is useful strategy to manipulate thermal, mechanical, and resulting shape memory properties. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55 , 3424–3433     

Shape‐memory effect in hyperbranched poly(ethyleneimine)‐modified epoxy thermosets

A series of shape‐memory epoxy thermosets were synthesized by crosslinking diglycidyl ether of bisphenol A with mixtures of commercially available hyperbranched poly(ethyleneimine) and polyetheramine. Thermal, mechanical and shape‐memory properties were studied and the effect on them of the content and structure of the hyperbranched polymer was discussed. Measurements showed that the glass transition temperature can be tailored from 60 °C to 117 °C depending on the hyperbranched polymer content, and all formulations showed an appropriate glassy/rubbery storage modulus ratio. Shape‐memory programming was carried out at TgE′ given the excellent mechanical properties of the materials, with maximum stress and failure strain up to 15 MPa and 60%, respectively. The resulting shape‐memory behavior was excellent, with maximum shape recovery and shape fixity of 98% as well as a fast shape‐recovery rate of 22%/min. The results show that hyperbranched poly(ethyleneimine) as a crosslinking agent can be used to enhance mechanical and shape‐memory properties with different effects depending on the crosslinking density. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 924–933     

Ultra low density and highly crosslinked biocompatible shape memory polyurethane foams

We report the development of highly chemically crosslinked, ultra low density (～0.015 g/cc) polyurethane shape memory foams synthesized from symmetrical, low molecular weight, and branched hydroxyl monomers. Sharp single glass transitions (T_g) customizable in the functional range of 45–70 °C were achieved. Thermomechanical testing confirmed shape memory behavior with 97–98% shape recovery over repeated cycles, a glassy storage modulus of 200–300 kPa, and recovery stresses of 5–15 kPa. Shape holding tests under constrained storage above the T_g showed stable shape memory. A high volume expansion of up to 70 times was seen on actuation of these foams from a fully compressed state. Low in vitro cell activation induced by the foam compared with controls demonstrates low acute bio‐reactivity. We believe these porous polymeric scaffolds constitute an important class of novel smart biomaterials with multiple potential applications. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Recovery stress and work output in hyperbranched poly(ethyleneimine)‐modified shape‐memory epoxy polymers

In this study a series of hyperbranched modified shape‐memory polymers were subjected to constrained shape recoveries in order to determine their potential use as thermomechanical actuators. Materials were synthesized from a diglycidyl ether of bisphenol A as base epoxy and a polyetheramine and a commercial hyperbranched poly(ethyleneimine) as crosslinker agents. Hyperbranched polymers within the structure of the shape‐memory epoxy polymers led to a more heterogeneous network that can substantially modify mechanical properties. Thermomechanical and mechanical properties were analyzed and discussed in terms of the content of hyperbranched polymer. Shape‐memory effect was analyzed under fully and partially constrained conditions. When shape recovery was carried out with fixed strain a recovery stress was obtained whereas when it was carried out with a constraining stress the material performs mechanical work. Tensile tests at Tg^E′ showed excellent values of stress and strain at break (up to 15 MPa and almost 60%, respectively). Constrained recovery performances revealed rapid recovery stress generation and unusually high recovery stresses (up to 7 MPa) and extremely high work densities (up to 750 kJ/m³). The network structure of shape‐memory polymers was found to be a key factor for actuator‐like applications. Results confirm that hyperbranched modified‐epoxy shape memory polymers are good candidates for actuator‐like shape‐memory applications. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1002–1013