Stress‐Induced Stabilization of Crystals in Shape Memory Natural Rubber

In contrast to all known shape memory polymers, the melting temperature of crystals in shape memory natural rubber (SMNR) can be greatly manipulated by the application of external mechanical stress. As shown previously, stress perpendicular to the prior programming direction decreases the melting temperature by up to 40 K. In this study, we investigated the influence of mechanical stress parallel to prior stretching direction during programming on the stability of the elongation‐stabilizing crystals. It was found that parallel stress stabilizes the crystals, which is indicated by linear increase of the trigger temperature by up to 17 K. The crystal melting temperature can be increased up to 126.5 °C under constrained conditions as shown by X‐ray diffraction measurements.     

Revealing Triple‐Shape Memory Effect by Polymer Bilayers

Bilayer polymers that consist of two epoxy dual‐shape memory polymers of well‐separated glass transition temperatures have been synthesized. These bilayer epoxy samples exhibit a triple‐shape memory effect (TSME) with shape fixities tailorable by changing the ratio between the two layers. The triple‐shape fixities of the bilayer epoxy polymers can be explained by the balance of stress between the two layers. Based on this work, it is believed that the following three molecular design criterions should be considered in designing triple‐shape memory polymers with optimum TSME: 1) well‐separated thermal transitions, 2) a strong interface, and 3) an appropriate balance of moduli and relative ratios between the layers (or microphases).

     

New Strategy to Access Dual‐Stimuli‐Responsive Triple‐Shape‐Memory Effect in a Non‐overlapping Pattern

Multistimuli‐responsive shape‐memory polymers are highly desirable in various applications, and numerous modes have been developed in recent years. However, most of them need to reprogram before they are ready to respond to another stimulus while one is triggered. Here, a new strategy is developed to achieve dual‐stimuli‐responsive triple‐shape memory with non‐overlapping effect in one programming cycle. Here, a series of poly(l ‐lactide)‐poly(tetramethylene oxide) glycol copolymers (PLA‐PTMEG‐A) is prepared by selected dangling photoresponsive anthracene moieties on the crystalline PTMEG backbone. The architectures of the copolymers are well‐controlled in order to keep a good balance between the crystallinity of the soft segment and the mobility of the anthracene moieties. Thus, PLA‐PTMEG‐A's can respond to heat and light with non‐overlapping effect. The thermally‐induced shape‐memory effect (TSME) is realized by the crystallization–melting transition of PTMEG soft segments, while the light‐induced shape‐memory effect (LSME) is achieved by the reversible photodimerization of anthracene groups. In view of the non‐overlapping effect of TSME and LSME in the copolymers, a triple‐shape‐memory effect triggered by dual‐stimuli is realized in one programming and recovery cycle.

     

A New Strategy to Prepare Polymer‐based Shape Memory Elastomers

A new strategy that utilizes the microphase separation of block copolymer and phase transition of small molecules for preparing polymer‐based shape memory elastomer has been proposed. According to this strategy, a novel kind of shape memory elastomer comprising styrene‐b‐(ethylene‐co‐butylene)‐b‐styrene (SEBS) and paraffin has been prepared. Because paraffins are midblock‐selective molecules for SEBS, they will preferentially enter and swell EB blocks supporting paraffins as an excellent switch phase for shape memory effect. Microstructures of SEBS/paraffin composites have been characterized by transmission electron microscopy, polarized light microscopy, and differential scanning calorimetry. The composites demonstrate various phase morphologies with regard to different paraffin loading. It has been found that under low paraffin loading, all the paraffins precisely embed in and swell EB‐rich domains. While under higher loading, part of the paraffins become free and a larger‐scaled phase separation has been observed. However, within wide paraffin loadings, all composites show good shape fixing, shape recovery performances, and improved tensile properties. Compared to the reported methods for shape memory elastomers preparation, this method not only simplifies the fabrication procedure from raw materials to processing but also offers a controllable approach for the optimization of shape memory properties as well as balancing the rigidity and softness of the material.

     

Redox‐ and Glucose‐Induced Shape‐Memory Polymers

A novel redox‐induced shape‐memory polymer (SMP) is prepared by crosslinking β‐cyclodextrin modified chitosan (β‐CD‐CS) and ferrocene modified branched ethylene imine polymer (Fc‐PEI). The resulting β‐CD‐CS/Fc‐PEI contains two crosslinks: reversible redox‐sensitive β‐CD‐Fc inclusion complexes serving as reversible phases, and covalent crosslinks serving as fixing phases. It is shown that this material can be processed into temporary shapes as needed in the reduced state and recovers its initial shape after oxidation. The recovery ratio and the fixity ratio are both above 70%. Furthermore, after entrapping glucose oxidase (GOD) in the system, the material shows a shape memory effect in response to glucose. The recovery ratio and the fixity ratio are also above 70%.

     

Chemical Cross‐linking of Polypropylenes Towards New Shape Memory Polymers

In this work, syndiotactic polypropylene (sPP) as well as isotactic polypropylene (iPP) are cross‐linked to gain a shape memory effect. Both prepared PP networks exhibit maximum strains of 700%, stored strains of up to 680%, and recoveries of nearly 100%. While x‐iPP is stable for many cycles, x‐sPP ruptures after the first shape‐memory cycle. It is shown by wide‐angle X‐ray scattering (WAXS) experiments that cross‐linked iPP exhibits homoepitaxy in the temporary, stretched shape but in contrast to previous reports it contains a higher amount of daughter than mother crystals.

     

Polymeric Photoresist Nanoparticles: Light‐Induced Degradation of Hydrophobic Polymers in Aqueous Dispersion

Nanoparticles consisting of a photoreactive polymer able to radically switch its hydrophobicity are successfully prepared by miniemulsion polymerization. Irradiation with UV light causes degradation of the particles whereat complete dissolution is achieved by changing the initial hydrophobic photoresist polymer into hydrophilic poly(methacrylic acid). Incorporation of the fluorescence‐sensitive Nile red serves as a solvatochromic probe to study the particle degradation. Diffusion of either Nile red out from or water into the former hard spherical nanoparticles is studied and not only renders the described material an ideal system for applications, where in situ dissolution of nanoparticles may be needed, but also bears the additional advantage of performing controlled burst release.     

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     

Shape Memory Hydrogel based on a Hydrophobically‐Modified Polyacrylamide (HMPAM)/α‐CD Mixture via a Host‐Guest Approach

A novel thermally sensitive shape memory (SM) hydrogel is prepared by block copoly­merization of a cationic surfactant monomer, dimethylhexadecyl[2‐(dimethylamino)ethylmethacrylate]ammoniumbromide (C₁₆DMAEMA), and acrylamide (AM) in the presence of α‐cyclodextrin (α‐CD) using N,N’‐methylenebisacrylamide (MBA) as a crosslinker. XRD, solid state ¹³C NMR, and DSC measurements show that the crystalline domains, induced by the hydrogen bonds between α‐CDs threaded on the hydrophobic units of the polymer chains through the host‐guest approach, can reversibly melt and crystallize at different temperatures. Rheological measurements show that both the elastic modulus G’ and viscous modulus G’’ drastically change due to the formation and dissolution of the crystalline domains. These thermo‐sensitive crystalline domains serve as reversible physical crosslinks, endowing the hydrogel with excellent SM properties. Cyclic experiments show that the hydrogel can recover to almost 100% of the deformation in each cycle and can be reused several times.

     

Modeling of Shape Memory Induction and Recovery in Heat‐Shrinkable Polymers

Summary: A mechanical model was developed to describe qualitatively and quantitatively the stress‐strain‐time behavior of a prepared shape memory crosslinked polyethylene during hot stretching, stress relaxation under 200% strain at high temperature and strain recovery of the heat shrinkable polymer. The stress‐strain, the stress relaxation and the irrecoverable strain behavior of the model were established by driving the constitutive equation, which could qualitatively represent the behavior of the real material. By choosing significant values for the parameters of the proposed model, an excellent fit was obtained between the experimental behavior of the polymer and that predicted by the model. It was also revealed that the main source responsible for the imperfect recovery of the induced strain observed was the stress relaxation occurring during the stretch holding‐cooling time step.

Stress relaxation of crosslinked polyethylene under 200% strain at 160 °C.     

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     

Mechanically Adaptive and Shape‐Memory Behaviour of Chitosan‐Modified Cellulose Whisker/Elastomer Composites in Different pH Environments

Biomimetic polymer composites with water‐active mechanically adaptive and shape‐memory behaviour in different pH environments are synthesised by using chitosan‐modified cellulose whiskers (CS‐CWs) as the stimulus‐responsive phase and thermoplastic polyurethane (TPU) as the resilient matrix. The effect of surface modification on the mechanically adaptive behaviour of CS‐CW/TPU composites is investigated by using three representative solutions with various pH values. The results show that surface modification significantly enhances the modulus contrast under wet and dry conditions with the acidic solution as the stimulus, while maintaining the high modulus contrast with the basic solution as the stimulus. CS‐CW/TPU composites also exhibit excellent shape‐memory effects in all three solutions that are comparable to those pristine CW/TPU composites. Furthermore, activation of force generation in the stretched CS‐CW/TPU composites by water absorption/desorption was observed.     

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     

Self‐Assembly and Dispersion of Chromogenic Molecules: A Versatile and General Approach for Self‐Assessing Polymers

Summary: Self‐assessing polymer blends based on poly(ethylene terephthalate glycol) or linear low‐density polyethylene and small amounts (0.5–2% w/w) of chromogenic sensor dyes are prepared and investigated. The cyano‐substituted oligo(p‐phenylene vinylene) dyes employed in the study exhibit pronounced optical absorption changes upon self‐assembly, because of charge‐transfer interactions or conformation changes. The extent of dye aggregation (and therewith the optical absorption characteristics) in these blends is significantly influenced by exposure to external stimuli. Subjecting appropriately processed samples to either temperatures above their glass transition or mechanical deformation can significantly change the extent of aggregation, which in turn leads to a color change.

Mechano‐optical response of a 1.0% w/w LLDPE/C18‐RG blend film. Pristine films are orange due to aggregated dye molecules. Deformation leads to dispersion of the dye and irreversibly changes the color to yellow.     

On the Uniaxial–Biaxial Nematic Phase Transition in Liquid‐Crystalline Polymers and Elastomers

We study how the uniaxial–biaxial nematic phase transition changes its nature when going from a low‐molecular‐weight liquid crystal to a liquid‐crystalline elastomer or polymer (the latter above the Maxwell frequency) and find a qualitative change due to the presence of a coupling to the strain field in these materials. While this phase transition can be of second‐order in low‐molecular‐weight materials, as is also experimentally observed, we show here that the order of this phase transition is changed generically to no phase transition at all or to a first‐order phase transition in mean‐field approximation. We analyze the influence of an external mechanical stress field above the uniaxial–biaxial nematic phase transition and find that either biaxial nematic order is induced, which is linear or quadratic in the stress intensity, or no response to an external stress results at all, depending on the relative orientation of the applied shear with respect to the director of the uniaxial nematic phase.     

Dextran Hydrogels Containing Poly(N‐isopropylacrylamide) as Grafts and Cross‐Linkers Exhibiting Enzymatic Regulation in a Specific Temperature Range

Summary: Specific temperature‐responsive biodegradable hydrogels were synthesized and characterized in terms of their regulation of enzymatic accessibility based on the physical properties of the temperature‐responsive polymers. The hydrogels consist of glycidyl methacrylate‐modified dextran grafted with the poly(N‐isopropylacrylamide) (PNIPAAm) homopolymer, and cross‐linked by co‐polymerization with NIPAAm and N,N‐dimethylacrylamide (DMAAm). The coil‐globule change in the grafted poly(NIPAAm) chains and only a slight dehydration of the poly(NIPAAm‐co‐DMAAm) cross‐linkers are effective in controlling the enzymatic degradation over a specific temperature range.

The thermo‐responses of the graft chains (steric hindrance) and the crosslinkers (slight deswelling of the hydrogel networks) control the enzymatic degradation of the hydrogel.     

Self‐Assembled Microspheres Driven by Dipole‐Dipole Interactions: UCST‐Type Transition in Water

A double hydrophilic block copolymer, poly(ethylene glycol)‐poly(3‐dimethyl (methacryloyloxyethyl) ammonium propane sulfonate) (PEG‐SB), is synthesized by reversible addition‐fragmentation transfer (RAFT) polymerization using PEG methyl ether (4‐cyano‐4‐pentanoate dodecyl trithiocarbonate) as a chain transfer agent. PEG‐SB forms multi‐layered microspheres with dipole‐dipole interactions of the SB side chains as the driving force. The PEG‐SB polymers show an upper critical solution temperature (UCST) and the UCST is controllable by the polymerization degree. The PEG‐SB microspheres are dissociated above the UCST and then monodispersed microspheres (∼1 μm) are obtained when the solution temperature is decreased below the UCST again. The disassociation/association of the microspheres is also controllable using the concentration of NaCl. These multi‐responsive microspheres could be a powerful tool in the field of nano‐biotechnology.

     

Physical Microfabrication of Shape‐Memory Polymer Systems via Bicomponent Fiber Spinning

As emerging technologies continue to require diverse materials capable of exhibiting tunable stimuli‐responsiveness, shape‐memory materials are of considerable significance because they can change size and/or shape in controllable fashion upon environmental stimulation. Of particular interest, shape‐memory polymers (SMPs) have secured a central role in the ongoing development of relatively lightweight and remotely deployable devices that can be further designed with specific surface properties. In the case of thermally‐activated SMPs, two functional chemical species must be present to provide (i) an elastic network capable of restoring the SMP to a previous strain state and (ii) switching elements that either lock‐in or release a temporary strain at a well‐defined thermal transition. While these species are chemically combined into a single macromolecule in most commercially available SMPs, this work establishes that, even though they are physically separated across one or more polymer/polymer interfaces, their shape‐memory properties are retained in melt‐spun bicomponent fibers. In the present study, we investigate the effects of fiber composition and cross‐sectional geometry on both conventional and cold‐draw shape memory, and report surprisingly high levels of strain fixity and recovery that generally improve upon strain cycling.

     

Design of Surfactant‐Grafted Hydrogels with Fast Response to Temperature

Summary: Here we show a new design concept of functional polymer gel for rapid deswelling by utilizing micelle‐forming ability of surfactant. A thermosensitive polymer bearing a surfactant was synthesized by using N‐isopropylacrylamide and a reactive surfactant. Above lower critical solution temperature, the grafted surfactant acts to form micelle structure. In the shrinking process, the inside water is rapidly squeezed out through hydrophilic channel between the formed micelles and consequently the gel shrinks quickly.

Shrinking mechanism of PNS gel in response to temperature increase.     

A Smart Polymer with Ion‐Induced Negative Shift of the Lower Critical Solution Temperature for Phase Transition

A novel thermo‐responsive smart copolymer that can selectively respond to specific ions, poly[(N‐isopropylacrylamide)‐co‐(benzo‐15‐crown‐5‐acrylamide)], has been synthesized and characterized. The copolymer exhibits a negative shift of the lower critical solution temperature (LCST) for phase transition that is specifically responsive to certain alkali metal ions. The order of significance of the LCST shift that is induced by ions is K⁺ > Cs⁺ > Na⁺ > Li⁺. The greater the number of crown ether units in the copolymer, or the larger the ion concentration, the higher the sensitivity and selectivity of the copolymer for cation recognition. Because of its novel ion‐responsive characteristics, the proposed smart copolymer is a promising new candidate material for sensors, actuators, switches, and so on.