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.

     

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.

     

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%.

     

Novel Shape‐Memory Polymer Based on Hydrogen Bonding

Summary: Due to a large difference in storage modulus below and above the glass transition temperature, a novel shape‐memory poly[(methyl methacrylate)‐co‐(N‐vinyl‐2‐pyrrolidone)]/poly(ethylene glycol) semi‐interpenetrating polymer networks structure was synthesized, which is stabilized by hydrogen‐bonding interactions. The recovery ratio of these polymers could reach 99%. In such a system the maximum molecular weight of PEG required for the semi‐IPNs formation reaches 1 000.

Transition from the temporary shape (chem) to the permanent shape (four rods) for a shape‐memory P(MMA‐co‐VP)/PEG1000 semi‐IPNs.     

Shape Memory Effect of Bacterial Poly[(3‐hydroxybutyrate)‐co‐(3‐hydroxyvalerate)]

Summary: A bacterial poly[(3‐hydroxybutyrate)‐co‐(3‐hydroxyvalerate)] biosynthesized by Pseudomonas sp. HJ‐2 was found to be a shape memory polymer. Permanent shapes were set by annealing at room temperature the samples that had been pre‐treated above 95 °C in specified shapes. The temporary shapes were set by stretching and holding the elongated samples. Thermal shrinkage began at 45 °C and stopped at 75 °C to recover to their permanent shapes. Apparently, the orientation induced the formation of hard segments that were responsible for setting the temporary shapes. The shape memory effect of this polymer was explained based on the DSC and XRD results at different phases.

The recovery of a coil shape upon heating a strip of HJ‐2 PHB35V, demonstrating the polymers shape memory effect.     

Chemical Dual‐Responsive Wettability of Superhydrophobic PANI‐PAN Coaxial Nanofibers

PANI‐PAN coaxial nanofibers have been prepared by electro‐spinning during polymerization. The surface of the resulting nanofibers is superhydrophobic with a water contact angle up to 164.5°. Conductivity of the PANI‐PAN nanofibers is about 4.3 × 10⁻² S · cm⁻¹. The superhydrophobic nanofibers show a chemical dual‐responsive surface wettability, which can be easily triggered by changing pH value or redox properties of the solution. A reversible conversion between superhydrophobicity and superhydrophilicity can be performed in a short time. The strategy used here may provide an easy method to control the wettability of smart surfaces by using properties of low‐cost functional polymers.

     

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.     

Resilient Amorphous Networks Prepared by Photo‐Crosslinking High‐Molecular‐Weight D,L‐Lactide and Trimethylene Carbonate Macromers: Mechanical Properties and Shape‐Memory Behavior

Tough networks are prepared by photo‐crosslinking high‐molecular‐weight DLLA and TMC macromers. These amorphous networks exhibit tunable thermal and mechanical properties and have excellent shape‐memory features. Variation of the monomer ratio allows adjustment of T_g between approximately ?13 and +51 °C. The elastic moduli at room temperature can be varied between 4.5 and 2730 MPa. The crosslinks allow the networks to return to their original shape after deformation. 60:40 DLLA:TMC networks have T_g values between room temperature and body temperature, with mechanical properties at body temperature close to soft tissues. Several medical devices are prepared from these networks.

     

Graphene Enhances the Shape Memory of Poly (acrylamide‐co‐acrylic acid) Grafted on Graphene

Acrylamide and acrylic acid are grafted on graphene by free‐radical polymerization to produce a series of graphene–poly(acrylamide‐co‐acrylic acid) hybrid materials with different contents of graphene. The materials demonstrate shape memory effect and self‐healing ability when the content of graphene is in the range of 10%–30% even though poly(acrylamide‐co‐acrylic acid) itself had poor shape memory ability. The permanent shape of the materials can be recovered well after 20 cycles of cut and self‐healing. The result is attributed to the hard–soft design that can combine nonreversible “cross‐link” by grafting copolymer on graphene and reversible “cross‐link” utilizing the “zipper effect” of poly(acrylamide‐co‐acrylic acid) to form or dissociate the hydrogen‐bond network stimulated by external heating.

     

Novel Shape‐Memory Polymer with Two Transition Temperatures

Summary: This communication describes a novel kind of PMMA‐PEG semi‐interpenetrating network (semi‐IPN) which shows excellent shape‐memory behavior at two transition temperatures, the T_m of the PEG crystal and the T_g of the semi‐IPN. Based on a reversible order‐disorder transition of the crystals below and above the T_m of PEG, and the large difference in storage modulus below and above the T_g of the semi‐IPN, the polymer has a recovery ratio of 91 and 99%, respectively.

Shape‐memory phenomena of PMMA‐PEG2000 semi‐IPN.     

Tailored (Meth)Acrylate Shape‐Memory Polymer Networks for Ophthalmic Applications

The unique features of shape‐memory polymers enables their use in minimally invasive surgical procedures with a compact starting material switching over to a voluminous structure in vivo. In this work, a series of transparent, thermoset (meth)acrylate shape‐memory polymer networks with tailored thermomechanics have been synthesized and evaluated. Fundamental trends were established for the effect of the crosslinker content and crosslinker molecular weight on glass transition temperature, rubbery modulus and shape‐recovery behavior, and the results are intended to help with future shape‐memory device design. The prepared (meth)acrylate networks with high transparency and favorable biocompatibility are presented as a promising shape‐memory ophthalmic biomaterial.

     

Investigation and Modeling of Temperature Dependence Recovery Behavior of Shape‐Memory Crosslinked Polyethylene

Shape memory was induced in crosslinked low‐density polyethylene by a heating‐stretching‐cooling cycle. The effect of crosslink content on thermal properties and temperature dependence recovery behavior was studied experimentally. The importance of stretching temperature and crosslink content on recovery behavior could be reasonably explained by the observed changes in the thermal properties which were attributed to the differences in crystalline structures and mechanism of crystal formation during the heating‐stretching‐cooling process. A mechanical model was developed to describe qualitatively and quantitatively the temperature dependence recovery behavior of the prepared shape memory crosslinked polyethylene at nonisothermal state under various conditions by driving constitutive equations using a set of model constants. These model constants were determined with the help of a set of optimization codes using a genetic algorithm method. By choosing a suitable set of model constants one can describe with high accuracy the temperature dependence recovery behavior of any shape memory polymer.

     

Silica‐Protected Micron and Sub‐Micron Capsules and Particles for Self‐Healing at the Microscale

A generalized silica coating scheme is used to functionalize and protect sub‐micron and micron size dicyclopentadiene monomer‐filled capsules and polymer‐protected Grubbs' catalyst particles. These capsules and particles are used for self‐healing of microscale damage in an epoxy‐based polymer. The silica layer both protects the capsules and particles, and limits their aggregation when added to an epoxy matrix, enabling the capsules and particles to be dispersed at high concentrations with little loss of reactivity.

     

Enthalpy Relaxation of Polymers: On the Possible Role of Liquid‐Crystalline Order

Isothermal physical ageing experiments were performed by differential scanning calorimetry to probe the enthalpy relaxation in a methacrylate copolymer carrying azobenzene mesogenic side groups. Further evidence of the ability of the configurational entropy model developed by Gomez Ribelles in describing the structural relaxation mechanism of polymers is provided. The trend of the equilibrium structural relaxation time was also determined as a function of the reduced temperature T_g/T. The comparison of the aging dynamics of the copolymer with those of previous analogous copolymers containing different amounts of azobenzene counits allowed us to highlight effects of the liquid‐crystalline nematic order on the properties of structural relaxation.

     

Viscoelastic Behavior and Force Nature of Thermo‐Reversible Epoxy Dry Adhesives

The gecko adhesion phenomenon has stimulated efforts to produce synthetic patterned dry adhesives. Besides introducing surface patterns on dry adhesives, it is also highly desirable to understand their intrinsic material properties. This communication reports the viscoelastic behavior of non‐patterned epoxy elastomers exhibiting intrinsic adhesion that is much higher than that of elastomers typically used for structure patterning. The diverse molecular origin of the adhesion is revealed through the study of adhesion against various substrates.

     

1,1‐Diphenyl Ethylene‐Mediated Radical Polymerisation: A General Non‐Metal‐Based Technique For The Synthesis Of Precise Core Cross‐Linked Star Polymers

This communication details the successful synthesis of low polydispersity core cross‐linked star (CCS) polymers via DPE‐mediated polymerisation. We demonstrate the ability to produce poly(methyl methacrylate) and poly(acrylonitrile) CCS polymers that are currently inaccessible via the two most common non‐metal‐based controlled radical polymerisation techniques (NMP and RAFT polymerisations).

     

Biodegradable Multiblock Copolymers Based on Oligodepsipeptides with Shape‐Memory Properties

Thermoplastic phase‐segregated multiblock copolymers with polydepsipeptides and PCL segments were prepared via coupling of diol and PCL‐diol using an aliphatic diisocyanate. The obtained multiblock copolymers showed good elastic properties and a shape memory. Almost complete fixation of the mechanical deformation, resulting in quantitative recovery of the permanent shape with a switching temperature around body temperature, was observed. In hydrolytic degradation experiments, a quick decrease of the molecular weight without induction period was observed, and the material changed from elastic to brittle in 21 d. These materials promise a high potential for biomedical applications such as smart implants or medical devices.

     

Poly(4‐vinylbenzoyl azide): A New Isocyanato Group Generating Polymer

4‐Vinylbenzoyl azide was synthesized from p‐vinylbenzoic acid and polymerized by free radical polymerization. The obtained polymer contained acyl azide groups which were thermally transformed to the corresponding isocyanato groups. Reactions on these polymers with ethanol, hydroxyethyl methacrylate and 1‐pyrenebutanol proceeded quantitatively. Time‐resolved FT‐IR studies of the reactions with ethanol were carried out by varying the concentration and temperature. The effect of the solvent polarity on the Curtius rearrangement was investigated.

     

Supramolecular Network Based on the Self‐Assembly of γ‐Cyclodextrin with Poly(ethylene glycol) and its Shape Memory Effect

A novel supramolecular network has been prepared based on the formation of inclusion complexes between γ‐cyclodextrin and poly(ethylene glycol), in which the PEG chains are interlocked by γ‐CD rings. This PEG/γ‐CD network exhibits good shape memory behavior because of the crosslinked structure. The crosslinked PEG/γ‐CD inclusion complexes and PEG crystallites account for the fixing phase and reversible phase, respectively. The characteristics of the materials have been investigated by ¹H NMR spectroscopy, XRD, DSC, DMA, viscosity tests, and swelling measurements.

     

Novel Metal‐Polyurethane Complexes with Enhanced Antimicrobial Activity

Summary: The silver coating of polymers has been intensively investigated in the last few decades as an effective non‐resistance‐inducing strategy to prevent medical device‐related infections. We have developed a completely new approach to incorporate silver ions in polymers by the synthesis of a carboxylated polyurethane able to coordinate Ag⁺. The obtained polymers possess mechanical properties suitable for the development of medical devices, without any risk of loss of activity. To minimize the risk of increasing antibiotic resistance, the metal ion‐containing polymers are loaded with ciprofloxacin, which possesses a different mechanism of antimicrobial action, thus a system able to inhibit Staphylococcus epidermidis and Pseudomonas aeruginosa growth for at least one month is developed.

SEM images showing a mature S. epidermidis biofilm on the surface of the carboxylated polyurethane (left) and the surface of the silver ion‐containing polyurethane free from bacterial colonization (right) after 48 h of incubation.