PCL Network Research Articles

Microfluidic Templating and Initiator‐Free Photocrosslinking of Protein‐Loaded PCL Microcapsules

AbstractPolymer network materials are interesting alternatives to thermoplastic polymers. Here, the preparation of polymer capsules is investigated, which are made from poly(ε‐caprolactone) (PCL) networks and are compartmentalized in a crosslinked PCL shell and a core that is suitable to enclose payloads of interest. Aided by microfluidic templating, PCL network capsules with a narrow size distribution (176 ± 5 µm) and thin shells (≈7.5 µm) are formed from 4‐arm star‐shaped 12 kDa PCL precursors by photoinitiator‐free UV light‐induced radical polymerization of methacrylate end‐groups. FITC‐BSA is encapsulated as a model protein. The physicochemical characterization of the capsules indicated a partial crosslinking of methacrylate endgroups into netpoints. Microscopy revealed a fraction of collapsed capsules that are discussed in the context of network stability and mechanical stress created at the capsule interfaces during solvent removal. The incubation of particles with human embryonic kidney (HEK) cells showed good cell compatibility, suggesting their potential use in biosciences and beyond.

Nature-Inspired Dual Purpose Strategy toward Cell-Adhesive PCL Networks: C(-linker-)RGD Incorporation via Thiol-ene Crosslinking.

In an attempt to mimic nature's ability to adhere cells, PCL is often coated with nature-derived polymers or its surface is functionalized with a cell-binding motif. However, said surface modifications are limited to the material's surface, include multiple steps, and are mediated by harsh conditions. Here, we introduce a single-step strategy toward cell-adhesive polymer networks where thiol-ene chemistry serves a dual purpose. First, alkene-functionalized PCL is crosslinked by means of a multifunctional thiol. Second, by means of a cysteine coupling site, the cell-binding motif C(-linker-)RGD is covalently bound throughout the PCL networks during crosslinking. Moreover, the influence of various linkers (type and length), between the cysteine coupling site and the cell-binding motif RGD, is investigated and the functionalization is assessed by means of static contact angle measurements and X-ray photoelectron spectroscopy. Finally, successful introduction of cell adhesiveness is illustrated for the networks by seeding fibroblasts onto the functionalized PCL networks.

Full-Atomistic Optimized Potentials for Liquid Simulations and Polymer Consistent Force Field Models for Biocompatible Shape-Memory Poly(ε-caprolactone).

Thermally induced shape memory poly(ε-caprolactone) (PCL)-based polymers are one of the most extensively researched families of biocompatible materials. They are degradable under physiological conditions and have high applicability in general biomedical engineering, with cross-linked PCL networks being particularly useful for tissue engineering. In this study, we used the optimized potentials for liquid simulations (OPLS) force field, which is well suited for describing intermolecular interactions in biomolecules, and the class II polymer consistent force field (PCFF) to investigate the properties of telechelic PCL with diacrylates as reactive functionalities on its end groups. PCFF has been specifically parameterized for simulating synthetic polymeric materials. We compare the findings of all-atom molecular dynamics simulations with known experimental data and theoretical assumptions to verify the applicability of both these force fields. We estimated the melt density, volume, transition temperatures, and mechanical characteristics of two-branched PCL diacrylates with a molecular weight of 2481 Da. Our findings point to the utility of the aforementioned force fields in predicting the properties of PCL-based polymers. It also opens avenues for developing PCL cross-linked polymer models and employing OPLS to investigate the interactions of synthetic polymers with biomolecules.

Development of Flexible Nanocomposites Based on Poly(ε-caprolactone) for Tissue Engineering Application: The Contributing Role of Poly(glycerol succinic acid) and Polypyrrole

Poly(ε-caprolactone) (PCL), a semi-crystalline polyester, has been widely used for tissue engineering and regenerative medicine applications due to its favorable mechanical properties, biocompatibility, and long-term biodegradation. Despite desirable physical and chemical characteristics, PCL shows low hydrophilicity, thus constraining its utility for biomedical applications. Here, we report the synthesis and characterization of elastomeric PCL/poly(glycerol succinate) (PGSu) blends and their nanocomposites with improved surface wettability and tunable mechanical properties, and controlled biodegradation. Also, the effect of the polypyrrole (PPy) coating layer on the properties of scaffolds was investigated. By tailoring the surface and bulk properties of scaffolds using a change in PCL to PGSu ratio, adding nano-hydroxyapatite (n-HA) as a filler, and deposition of PPy on the scaffolds, it is possible to engineer scaffolds with customized degradation and mechanical properties. The addition of PGSu and n-HA increased hydrophilicity of the PCL matrix, developed its mechanical performance, and enhanced in-vitro cell metabolic activity. It was shown that Young’s modulus of PCL-PGSu-based scaffolds could be tuned from about 11.5 MPa to 4.5 MPa by altering the amount of PGSu and n-HA within the PCL network. The crystallinity of PCL was also considerably influenced by PGSu and n-HA. Moreover, deposition of PPy on the scaffolds resulted in a desirable antibacterial activity and electrical conductivity of about 5*10-2 S/cm. As expected, the rate of degradation increased with an increase in PGSu concentration. At the same time, the PPy coating layer delayed the degradation process, indicating that the biodegradation rate of the PCL matrix can be regulated. PCL-PGSu-based scaffolds also supported cell viability and proliferation, where PPy coating significantly improved cell viability. Therefore, the synthesized scaffolds can be used for a range of tissue engineering applications.

Evaluating the protective role of carrier microparticles in preserving protein secondary structure within electrospun meshes

AbstractDirect incorporation of proteins into electrospun meshes using approaches such as blend electrospinning can promote adverse interactions with hydrophobic polymers, organic solvents and high voltage, potentially leading to loss of protein activity. However, pre‐encapsulation within a protective carrier phase can preserve protein conformation by avoiding exposure to harsh processing conditions. In this study, bovine serum albumin (BSA) was loaded within cellulose microparticles (MPs) and the BSA‐loaded MPs were dispersed in a solution of poly(ethylene oxide) (PEO). Particle‐mesh composites were created using a sacrificial fiber/co‐electrospinning approach in which the BSA/MP/PEO solution was simultaneously electrospun against a poly(caprolactone) (PCL) solution. Post‐fabrication, sacrificial PEO fibers were selectively dissolved by treatment with ethanol. Microscopy, weight loss analysis and FTIR spectroscopy together confirmed selective dissolution of PEO fibers and the retention of BSA‐loaded MPs within the PCL network without significant loss of either the MPs or the protein. Circular dichroism spectroscopy and intrinsic fluorescence measurements on BSA extracted from the co‐electrospun meshes indicated minimal disruption to secondary structure, although partial sheet induction was observed. In contrast, direct exposure of BSA to four commonly used electrospinning solvents resulted in a large decrease in helical content and significant induction of sheets, revealing significant changes to the secondary structure. In summary, our results demonstrate the protective role of MPs in minimizing adverse effects of electrospinning on the secondary structure of incorporated protein.

Thermo-Responsive Self-Healing Polycaprolactone Urethane Coatings from Waste Palm Cooking Oil

The evolution of responsive polymers has amplified expressively in consequence of the growth of precise polymer architecture with predetermined structure-to-property behavior. In this study, new thermal responsive self-healing polycaprolactone urethane coatings were prepared from waste palm cooking oil. Nine different formulations of coatings with various percentages of ε-caprolactone (ε-CL) were successfully developed and the effect of polycaprolactone network in the polyurethane coating on the self-healing behavior. The remendability of the coatings was studied by using thermal analysis and dynamic mechanical analysis. The PCLU coating with the highest content of PCL network was nearly completely healed after 2 hours at 90ºC. The results were confirmed by scanning electron microscopy before and after healing. The mechanism of scratch closure by thermal stimuli-responsive self-healing was also discussed.

From shape and color memory PCL network to access high security anti-counterfeit material

Shape memory polymers have been widely applied in actuators and space deployable materials due to the shape-change feature under external stimuli. Actually, it will show greater potential in smart materials if the shape memory technique creatively combined with other functions. Herein, a shape and color memory material was prepared by incorporating a photochromic molecule spiropyran into the shape memory poly(ε-caprolactone) network. Integrating shape memory effect with color variance in this network, we firstly mimic a blooming rose with color change as it warms through specific assembly and procedures. Assisted with dynamic bond exchange of urethane and ester linkages, surface patterning, and molding technology, a series of application models can execute SME accompanied with color change were demonstrated, including high security information storage material, anti-counterfeit label and encryption label.

The effects of PCL diol molecular weight on properties of shape memory poly(ε‐caprolactone) networks

ABSTRACTShape memory poly(ε‐caprolactone) (PCL) networks with different molecular weights of PCL diol were prepared via thiol–ene reaction in this work. The highly efficient thiol–ene reaction ensured a uniform distribution of PCL chains between crosslinker, contributing well‐defined network architecture with good mechanical and shape memory properties. 1H NMR spectra were used to confirm that PCL diol was completely converted into acrylate‐terminated PCL. Gel content experiment and Fourier‐transform infrared showed that almost all samples exhibited virtually the complete crosslinking network due to the highly selective thiol–ene reaction between acrylate‐terminated PCL and tetrathiol crosslinker. Differential scanning calorimetry and X‐ray diffraction tests revealed that the melting temperature and crystallinity of the prepared PCL networks by using high molecular weight of PCL diol displayed a higher result compared to ones using low molecular weight. The dynamic mechanical analyzer results revealed that the storage modulus of the network dropped evidently across the thermal transition, these characteristics of the PCL network indicated that all exhibited a good shape memory effect. The shape fixing ability was kind of being affected by the PCL diol molecular weight, while the shape recovery ratio of all samples can almost reach 99% irrespective of the length of PCL diol. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47055.

Self-recoverable semi-crystalline hydrogels with thermomechanics and shape memory performance

Stimuli-responsive hydrogels have become one of the most popular artificial soft materials due to their excellent adaption to complex environments. Thermoresponsive hydrogels triggered by temperature change can be efficiently utilized in many applications. However, these thermoresponsive hydrogels mostly cannot recover their mechanical states under large strain during the process. Herein, we utilize the heterogeneous comb-type polymer network with semicrystalline hydrophobic side chains to design self-recovery semi-crystalline hydrogels. Based on hydrophilic/hydrophobic cooperative complementary interaction and heterogeneous polymer network, hydrogels can be endowed with excellent thermosensitive properties and mechanical performance. The hydrogels exhibit high compressive strength (7.57 MPa) and compressive modulus (1.76 MPa) due to the semi-crystalline domains formed by association of the hydrophobic poly (ε-caprolactone) PCL. The melting-crystalline transition of PCL and elastic polymer network provide the hydrogels excellent thermomechanical performance and self-recovery property. Furthermore, the hydrogels exhibit shape memory behavior, which can be realized by simple process and smart surface patterning. With these excellent properties, our hydrogels can be applied in sensors, flexible devices and scaffolds for tissue engineering.

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

ABSTRACTWe report the synthesis of linear‐ and brush‐type poly(ɛ‐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 ɛ‐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

Thermogel-Coated Poly(ε-Caprolactone) Composite Scaffold for Enhanced Cartilage Tissue Engineering

A three-dimensional (3D) composite scaffold was prepared for enhanced cartilage tissue engineering, which was composed of a poly(ε-caprolactone) (PCL) backbone network and a poly(lactide-co-glycolide)-block-poly(ethylene glycol)-block-poly(lactide-co-glycolide) (PLGA–PEG–PLGA) thermogel surface. The composite scaffold not only possessed adequate mechanical strength similar to native osteochondral tissue as a benefit of the PCL backbone, but also maintained cell-friendly microenvironment of the hydrogel. The PCL network with homogeneously-controlled pore size and total pore interconnectivity was fabricated by fused deposition modeling (FDM), and was impregnated into the PLGA–PEG–PLGA solution at low temperature (e.g., 4 °C). The PCL/Gel composite scaffold was obtained after gelation induced by incubation at body temperature (i.e., 37 °C). The composite scaffold showed a greater number of cell retention and proliferation in comparison to the PCL platform. In addition, the composite scaffold promoted the encapsulated mesenchymal stromal cells (MSCs) to differentiate chondrogenically with a greater amount of cartilage-specific matrix production compared to the PCL scaffold or thermogel. Therefore, the 3D PCL/Gel composite scaffold may exhibit great potential for in vivo cartilage regeneration.

Dynamic polymer substrates with increasing stiffness for regulating smooth muscle cells

Event Abstract Back to Event Dynamic polymer substrates with increasing stiffness for regulating smooth muscle cells Xifeng Liu1, Lei Cai1 and Shanfeng Wang1, 2 1 The University of Tennessee, Knoxville, Department of Materials Science and Engineering, United States 2 The University of Tennessee, Knoxville, Institute of Biomedical Engineering, United States Cardiovascular tissues bear constant blood shear and dynamic hardening under diseased conditions, which causes the tissue stiffness varies all the time. To mimic the dynamic changing environment in the vessel tissues and investigate the influence of dynamically changing substrate mechanical properties on the cell behaviors, we fabricated a model polymer network from poly(e-caprolactone) triacrylate that can gradually stiffen in 24 h through impeded crystallization at body temperature (37ºC). Rat primary smooth muscle cells (SMCs) were cultured on both static and dynamic substrates and distinct SMC attachment, proliferation and spreading were found. Quantification of contractile gene expression and protein content showed that the dynamic substrates could facilitate the contractile conversion process of SMCs. The analysis of focal adhesions and integrin expression indicated that the cellular abilities to sensing and adhering to the substrate surface were enhanced by the dynamic stiffening stimulation. These results extend the knowledge about SMC mechanosensing to dynamic substrates with increasing stiffness, and demonstrate a new method of regulating SMC adhesion, growth, and functional conversion on substrates. We have developed biodegradable and photo-crosslinkable poly(ε-caprolactone) triacrylates (PCLTAs) that can be photo-crosslinked into networks with high gel fractions and controllable thermal and mechanical properties[1]-[3]. Elastic modulus ranging from 1 to 200 MPa was achieved by modulating the crosslinking density and crystallinity through the nominal molecular weight (Mn) of the polymer[2],[3]. When the molecular weight increases from 2000 to 20000 g/mol, the PCL network changes from an amorphous, compliant elastomer to a semi-crystalline, stiff material with a crystallinity of 42%. For the semi-crystalline PCLTA networks, the melting temperature (Tm) of the substrate increased from 13.8ºC for Mn of 5000 g/mol to 50.4ºC for Mn of 20000 g/mol. Because of the constraints of the crosslinks, polymer crystallization in a network is suppressed and impeded, as demonstrated by reduced crystallinity and slower crystallization. By controlling the Tm of the PCLTA network to be slightly above 37ºC, PCL segments in the network crystallized very slowly at 37ºC and the substrate stiffness increased gradually with the increase in crystallinity over a time period of 24 h. Based on this unique mechanism of impeded polymer crystallization in PCLTA networks, we explore to apply this spontaneously increasing substrate stiffness to regulate SMCs when they are seeded on the substrates and cell attachment occurs at the same time. To study the effect of the gradually increasing substrate stiffness on SMC adhesion, spreading, proliferation, and phenotypic conversion, we prepared a series of PCLTA networks, melted and crystallized at 37ºC for 0, 4, 8, or 24 h. The PCLTA synthesized in this study had Mn of 7985 g/mol, weight-average molecular weight of 9626 g/mol, and polymer molecular weight distribution of 1.206. Figure 1. Scheme of PCLTA photo-crosslinking and crystallization at 37°C. Figure 2. (a) The elastic modulus (G') of the PCLTA networks as monitored using a rheometer at 37ºC. (b) The strain-stress curves for four melted amorphous PCLTA networks pre-treated at 37ºC for 0, 4, 8, and 24 h, respectively. (c) Tensile modulus of these networks at 37ºC. Figure 3. (a) SMC attachment rate on the dynamic and static networks at 4 h post-seeding, as normalized to that on TCPS positive control. (b) SMC spread area calculated at 12 h and day 1 post-seeding on these substrates. (c) SMC numbers at 12 h, days 1, 2, and 4 post-seeding on the dynamic and static samples using TCPS as positive control. *: p < 0.5 to the 24 h static sample. $: p < 0.5 to the 0 h dynamic sample. Conclusions: SMCs receiving dynamic stiffening stimulation had stronger focal adhesions and higher expression of environmental sensing integrin subunits. SMC attachment, spreading, proliferation, and expression of contractile gene markers and calponin protein were all promoted upon receiving the dynamic mechanical stimulation. National Science Foundation (DMR-11-06142)

Nanoscale Order and Crystallization in POSS–PCL Shape Memory Molecular Networks

The angstrom- and nanometer-scale crystallization and long-range ordering behavior of polyhedral oligosilsesquioxane–poly(e-caprolactone) (POSS–PCL) shape memory nanocomposites under thermomechanical shape memory cycles and uniaxial stretching were studied by simultaneous wide-angle and small-angle X-ray scattering (WAXS/SAXS). POSS/PCL cross-linked molecular networks featuring a single POSS moiety centered between two PCL network chains, previously reported [Alvarado-Tenorio Macromolecules 2011, 44, 5682−5692], with molecular weight of 2600 g/mol and exhibiting shape memory behavior, were synthesized with variation of cross-linking molar ratio (POSS–PCL diacrylate/tetrathiol cross-linker, 2:1, 2:1.5, and 2:2). The photocured networks exhibited a morphology consisting of POSS crystals embedded in an amorphous PCL matrix, and the POSS crystals were ordered in a cubic nanostructure. However, under tensile stress afforded by a shape memory cycle, the networks yielded a double-induced orientation (90° and 180...

Tailoring the viscoelastic, swelling kinetics and antibacterial behavior of poly(ethylene glycol)-based hydrogels with polycaprolactone

Polyethylene glycol (PEG) diacrylate was photocured with poly(ɛ-caprolactone) (PCL) diacrylate and the thermal, viscoelastic and antibacterial properties were studied. The aim was to reinforce the hydrogels and induce shape memory behavior. The diacrylates of PEG of 3350g/mol and PCL of 10,000g/mol were photocured in the presence of a crosslinker tetrathiol and photoinitiator. The concentration of PCL macromers were varied from 10 to 50mol%. Through scanning electron microscopy, differential scanning calorimetry, and rheology the influence of PCL was investigated. The dry PEG–PCL networks exhibited only one melting endotherm due to transition overlapping. The dry samples exhibited complex spherulitic morphology due to PEG and PCL competitive crystallization. Upon wetting, the crystalline phase of PEG “melted” away leaving only the crystalline phase of PCL, which acted as nanoreinforcer thus enabling a hydrogel with robust shear modulus. Moreover, PCL tuned the swelling rate, total swell and pore size of the hydrogels and its crystals melting transition provided for an activation temperature for shape memory behavior. Furthermore, antimicrobial activity of silver loaded hydrogels was investigated through exposure to Pseudomonas aeruginosa and Staphylococcus aureus with inoculation to a controlled cell density. The results showed that there was antibacterial activity even after 168h. Therefore, PCL was effective in fine tuning the mechanical properties, swelling ratio and water uptake kinetics of PEG-based hydrogels thus opening up opportunities for intelligent substrates for antibacterial properties and drug delivery.

Gelation of microsphere dispersions using a thermally-responsive graft polymer

Dispersions of microspheres (MSs) that form self-supporting particle gels are fundamentally interesting from the viewpoints of gel formation and mechanical properties. Here, we investigate model mixed MS/thermally responsive polymer dispersions that exist as particle gels at 37°C. The MS comprised poly(caprolactone) (PCL) and was prepared by solvent evaporation. The thermally responsive polymer contained a cationic backbone and poly(2-(2-methoxyethoxy)ethyl methacrylate) side chains and is abbreviated as PMA. Mixed PCL/PMA dispersions formed weak gels due to depletion at 20°C. At higher temperatures they formed stronger gels due to a combination of bridging of PCL MS by PMA and reinforcement by a PMA network. A key parameter controlling the mechanical properties of the reinforced MS particle gels was the volume fraction of PMA with respect to total polymer present (ΦPMA). Self-healing behaviour was observed for the gels using dynamic rheology and this depended on ΦPMA. The MS particle gel mechanical properties were conceptually described in terms of isostress and isostrain blending laws. At ΦPMA less than or greater than 0.057 the gels were dominated by the PCL or PMA networks, respectively. The latter value is suggested to be analogous to a phase inversion point.

Hydrolytic and enzymatic degradation of a poly(ε-caprolactone) network

Long-term hydrolytic and enzymatic degradation profiles of poly(ε-caprolactone) (PCL) networks were obtained. The hydrolytic degradation studies were performed in water and phosphate buffer solution (PBS) for 65 weeks. In this case, the degradation rate of PCL networks was faster than previous results in the literature on linear PCL, reaching a weight loss of around 20% in 60 weeks after immersing the samples either in water or in PBS conditions. The enzymatic degradation rate in Pseudomonas Lipase for 14 weeks was also studied, with the conclusion that the degradation profile of PCL networks is lower than for linear PCL, also reaching a 20% weight loss. The weight lost, degree of swelling, and calorimetric and mechanical properties were obtained as a function of degradation time. Furthermore, the morphological changes in the samples were studied carefully through electron microscopy and crystal size through X-ray diffraction. The changes in some properties over the degradation period such as crystallinity, crystal size and Young's modulus were smaller in the case of enzymatic studies, highlighting differences in the degradation mechanism in the two studies, hydrolytic and enzymatic.

Biodegradable Photo‐Crosslinked Polymer Substrates with Concentric Microgrooves for Regulating MC3T3‐E1 Cell Behavior

Both intrinsic material properties and topographical features are critical in influencing cell-biomaterial interactions. We present a systematic investigation of regulating mouse pre-osteoblastic MC3T3-E1 cell behavior on biodegradable polymer substrates with distinct mechanical properties and concentric microgrooves. The precursors for fabricating substrates used here were two poly(ϵ-caprolactone) triacrylates (PCLTAs) synthesized from poly(ϵ-caprolactone) triols with molecular weights of ∼7000 and ∼10000 g mol(-1) . These two PCLTAs were photo-crosslinked into PCL networks with distinct thermal, rheological, and mechanical properties at physiological temperature because of their different crystallinities and melting temperatures. Microgrooved substrates with four groove widths of 7.5, 16.1, 44.2, and 91.2 μm and three groove depths of 0.2, 1, and 10 μm were prepared through replica molding, i.e., photo-crosslinking PCLTA on micro-fabricated silicon wafers with pre-designed concentric groove patterns. MC3T3-E1 cell attachment and proliferation could be better supported by the stiffer substrates while not significantly influenced by the microgrooves. Microgroove dimensions could regulate MC3T3-E1 cell alignment, nuclear shape and distribution, mineralization, and gene expression. Among the microgrooves with a fixed depth of 10 μm, the smallest width of 7.5 μm could align and elongate the cytoskeleton and nuclei most efficiently. Strikingly, higher mineral deposition and upregulation of osteocalcin gene expression were found in the narrower microgrooves when the groove depth was 10 μm.

Shape-memory polymer networks from sol–gel cross-linked alkoxysilane-terminated poly(ε-caprolactone)

A novel type of covalently cross-linked semi-crystalline polymer with shape-memory and biocompatibility properties was prepared from alkoxysilane-terminated poly(e-caprolactone) (PCL) by sol–gel process that allowed the generation of silica-like cross-linking points. A fine tuning of the cross-linking density and thermal properties (melting temperature) of the materials was obtained by controlling the molecular weight of the PCL precursor (and thus the molecular structure of the resulting network) and the curing conditions. The shape-memory behaviour was investigated with bending tests. Recovery times of less than one second were observed in water depending on the temperature, and a linear correlation of the recovery time with cross-linking density and molecular weight of PCL network precursor was observed.

Triple-shape properties of star-shaped POSS-polycaprolactone polyurethane networks

Here, we present an investigation of the triple-shape properties of star-shaped polyhedral oligomeric silsesquioxane-poly(e-caprolactone) polyurethanes (SPOSS-PUs), which have three-dimensional network structures. In a typical ‘triple-shape functionalization process’, mostly consisting of two tensile deformations at different temperatures, chain immobilization of the polymer network component poly(e-caprolactone) (PCL) was successfully realized first through crystallization and then through vitrification. Subsequently, large parts of the respective strains were released under stress-free recovery conditions. The two-fold fixed (‘programmed’) specimens responded to heating with two independent length contractions (switching steps); the first shape change was associated with PCL devitrification and the second one with the melting of hitherto crystalline PCL. It was revealed that the triple-shape properties of SPOSS-PU networks considerably depend on PCL network chain length. When applying exactly the same ‘triple-shape creation procedure’, larger strain releases were detected in the first transition for polymers with a higher PCL network chain length, whereas the second transition was more accentuated for SPOSS-PU networks with a shorter PCL chain length. In the course of thermo-mechanical cycling, the formation of a neck during the second tensile deformation was repeatedly detected for SPOSS-PUs with higher PCL network chain length; in the subsequent recovery process the specimens even exhibited the highest total strain recoverability. Finally, gradual strain release could be achieved at temperatures below the PCL melting transition through the selection of up to four temperature holding steps, at which every time stable shapes were formed.

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.