Article pubs.acs.org/Macromolecules
Triple-Shape Memory Materials via Thermoresponsive Behavior of Nanocrystalline Non-Isocyanate Polyhydroxyurethanes Vitalij Schimpf,† Barbara Heck,‡ Günter Reiter,‡ and Rolf Mülhaupt*,† †
Freiburg Materials Research Center (FMF) and Institute for Macromolecular Chemistry, University of Freiburg, Stefan-Meier-Strasse 21 and 31, 79104 Freiburg, Germany ‡ Institute of Physics, University of Freiburg, Hermann-Herder-Strasse 3, 79104 Freiburg, Germany S Supporting Information *
ABSTRACT: Crystallization of long n-alkyl side chains within the confined environment of nonisocyanate polyhydroxyurethane (PHU) networks renders PHUs thermoresponsive, enabling thermomechanical programming of temperature-induced shape changes. Key intermediates of shape memory PHUs are highly branched, semicrystalline polyamidoamine curing agents tailored by amidation of a polyamine-terminated hyperbranched polyethylenimine with semicrystalline long chain behenic acid. Both cure temperature and content of n-alkyl side chains, varied independently, govern crystallization behavior, phase separation and mechanical properties of semicrystalline PHU networks obtained by curing pentaerythritol-based polyfunctional cyclic carbonates with hyperbranched, semicrystalline polyamidoamines. As compared to conventional PHUs, the incorporation of hydrophobic, crystalline n-alkyl side chains significantly lowers hydrophilicity. Typically, the n-alkyl side chains of behenic amides in PHU networks melt at temperatures varying between 40 and 75 °C. According to analyses by means of atomic force microscopy (AFM) and differential scanning calorimetry (DSC) crystallization of the behenic amide side chains accounts for nanophase separation producing nanocrystalline PHUs with programmable shapes. Hence, controlled PHU crystallization and PHU nanostructure formation afford thermomechanical programming of PHU triple-shape memory materials memorizing two different shapes in addition to the original shape within a single shape memory cycle. Opposite to conventional polyurethanes, triple-shape memory PHUs require neither the use of isocyanates nor phosgene.
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position.2,4,6−8 Whereas remarkable progress has been made in the development of PHUs for coatings9−11 and flexible foam12−14 applications, all state-of-the-art PHUs retain their properties unchanged throughout their entire product lifetime. Inspired by natural material systems, which are interactive and adjust to changes of their environment, it is highly desirable to design multifunctional PHU materials capable of responding to external stimuli with changes of their mechanical properties and shapes. Shape memory polymers (SMPs) can be thermomechanically programmed to memorize one or more temporary shapes in addition to their original shape, enabling stimuliresponsive switching between different shapes triggered by an external stimulus such as irradiation,15−17 water,18 electromagnetic fields,19,20 or temperature.21−31 As an intriguing class of smart materials, SMPs, among them polyurethanes, hold great prospects for a variety of applications ranging from foams with shape-adaptive grip and thermal expansion32 to actuators, self-repair coatings,33,34 smart adhesives,35 self-adjusting medical sutures24 and implants for minimal invasive surgery.31 Typically, thermally triggered triple-shape memory polymers (tSMP) exhibit two different thermal phase transitions (Ttrans1,
INTRODUCTION Since the pioneering advances of Otto Bayer in 1947 polyurethanes have emerged as an extraordinarily versatile class of polymeric materials, which are readily tailored to meet the needs of diversified applications ranging from rigid and flexible foams to elastomers, fibers, coatings, adhesives, membranes, biodegradable and biocompatible plastics.1,2 Traditionally, polyurethane materials are produced by polyaddition of polyfunctional polyols with isocyanates which require special safety precaution and handling procedures owing to their toxicity in conjunction with high moisture sensitivity.2−4 During the past 2 decades, the quest for green chemistry and sustainability, aiming at improving resource-, energy-, and ecoefficiency of polyurethanes together with occupational health and safety, has fostered the development of nonisocyanate polyurethanes and the exploitation of renewable resources in polyurethane production.5,6 Among nonisocyanate polyurethanes, polyhydroxyurethanes (PHUs) are attracting increasing attention in academia and industry, especially in view of tailoring new PHU materials which are not paralleled by conventional polyurethanes.6,7 Principally PHUs are produced by polyaddition reaction of di- and polyfunctional cyclic carbonates with di- or polyfunctional amines, in which aminemediated ring-opening of the cyclic carbonates yields urethane groups bearing primary or secondary hydroxyl groups in β© XXXX American Chemical Society
Received: March 7, 2017 Revised: April 13, 2017
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DOI: 10.1021/acs.macromol.7b00500 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Ttrans2) above room temperature, required to thermomechanically program two temporary shapes (B, C) in addition to the permanent original shape A, as is illustrated in Scheme 1.
containing pendant mesogenic units. The two thermal transitions corresponding to glass transition and phase transition of the liquid crystalline phase trigger triple shape memory behavior.21 Going beyond tSMPs with covalent bonds, supramolecular tSMPs exploit H-bonding interactions between a polyurethane bearing carboxyl groups with mesogenic units such as cholesteryl isonicotinate.43 Amorphous PU thermosets were obtained from curing MDI with a mixture of poly(ethylene glycol) and a polyol and exhibited one broad glass transition temperature which was able to fix two temporary shapes. More interestingly the respective work group was able to reconfigure the permanent shape at 170 °C and explained this behavior with the transcarbamoylation of the carbamates.44 Indeed transcarbamoylation in mechanically activated PHU vitrimers allows for remolding PHU thermosets and could be an interesting feature in PHU-based shape memory polymers.45 To the best of our knowledge to date no attempts have been reported regarding polyurethane-based tSMPs prepared by means of isocyanate-free synthetic routes and by simultaneously exploiting renewable resources. In particular, this approach toward nonisocyanate tSMPs is attractive with respect to biomedical applications. Herein we report on an isocyanate-free route to thermomechanically programmable and thermoresponsive semicrystalline polyhydroxyurethanes (PHU) tailored as biobased tSMPs. Key intermediates are semicrystalline polyamine curing agents, prepared by partial amidation of hyperbranched polyethylenimine with natural behenic acid. Upon cure with pentaerythritol-based polyfunctional cyclic carbonate these curing agents incorporate semicrystalline, longchain fatty acid amide side chains into the PHU network. The controlled crystallization of such semicrystalline side chains in the confined environment of PHU networks is investigated and exploited to afford triple shape memory behavior.
Scheme 1. Strategy for Thermomechanical Programming of Two Temporary Shapes (B, C) Using a tSMP Network with Two Thermal Transitions (Ttrans1 > Ttrans2) and Subsequent Temperature-Induced Shape Changes Due to Triple Shape Memory Effect
In most strategies toward tSMP network formation linear semicrystalline segments are incorporated into the polymer backbone, exploiting their melting temperature together with network glass transition temperature to thermomechanically program two temporary shapes.36−38 In contrast, much less is known with respect to tSMPs in which the crystallization of semicrystalline side chains governs triple shape memory behavior. For instance, tough tSMP and two-way SMPs based on polyurethanes networks were prepared from polydopamine and semicrystalline PCL in a three-step process.22 In the case of the multiphase poly(ε-caprolactone) dimethacrylate/poly(cyclohexyl methacrylate) network system a PCL content varying between 35 and 60 wt % accounted for triple shape memory behavior.39 Star-shaped polyhedral oligomeric silsesquioxane−PCL cured with hexamethylene diisocyanate yielded a polyurethane compound exhibiting three-dimensional network structures, in which the PCL chain length plays an important role regarding triple shape memory behavior.40 The thermomechanical programming and triple-shape functionalization process comprise two tensile deformations at different temperatures and shape fixation by PCL crystallization and vitrification. In another approach toward tSMPs, the three-step polyaddition of isocyanates with semicrystalline PCL (Mn = 10000 g mol−1) and polytetrahydrofuran (Mn = 2900 g mol−1) affords polyurethane-based tSMPs having two different melting temperatures attributed to the presence of two kinds of polymer crystals.41 The UV-cure of polyurethane containing semicrystalline PCL segments and cinnamon groups affords polyurethane-based tSMPs having two phase transition temperatures corresponding to PCL melting and glass transition of the photocured hard segments.38 Multistimuli responsive SMPs were obtained by grafting polyurethane onto poly(vinyl alcohol), which renders SMP responsive to both temperature and moisture.42 Polyaddition of liquid-crystalline diols with diisocyanate in the presence of pentaerythritol as a cross-linker yielded liquid crystalline polyurethane tSMP networks
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EXPERIMENTAL SECTION
Materials. Lupasol FG (Mn = 800 g mol−1) was supplied by BASF (Ludwigshafen, Germany). Propylene carbonate (≥99%) was obtained from Alfa Aesar (Karlsruhe, Germany). Pentaerythritol glycidyl ether (ipox CL 16, EE: 165 g/eq) was kindly supplied by ipox chemicals (Laupheim, Germany) and TBAB (≥99%) was purchased from SigmaAldrich. Behenic acid (≥85%) was obtained from ABCR (Karlsruhe, Germany). Characterization. NMR spectra were recorded in deuterated chloroform on an ARX 300 spectrometer from Bruker at room temperature. The chemical shifts were referenced to the solvent signals. A Bruker FT-IR Vector 2200 with attached Goldengate unit was used for FT-IR spectroscopy. The spectra were recorded from the solid plaques using attenuated total reflectance (ATR) technique. The resulting spectra were averaged over 20 measurements with a resolution of 4 cm−1 in the range from 800 to 4000 cm−1. DSC measurements were performed using a PerkinElmeŕs Pyris 1 with a heating and cooling rate of 10 K min−1. Scanning electron microscopy was performed using a Quanta 250 FEG. Atomic force microscopy was carried out on a Nano Wizard from JPK Instruments. Small angle Xray scattering (SAXS) experiments were carried out with a Kratky camera attached to a conventional Cu Kα X-ray source employing a position sensitive metal wire detector and a temperature controlled sample holder. After background correction a deconvolution algorithm was applied on the slit-smeared scattering data.46 DMA measurements were performed on a Q800 from TA Instruments using a dual cantilever measuring system. After equilibration at −40 °C for 5 min, the rectangular sample (80 × 12 × 3 mm) was heated with a rate of 3 K min−1, a deformation of 0.1% and a frequency of 1 Hz. Preparation of the Pentaerythritol Glycidyl Ether-Based Carbonate (PGC). Following procedures reported in literature,47 the carbonation of pentaerythritol glycidyl ether (1004 g, 6.085 mol epoxy B
DOI: 10.1021/acs.macromol.7b00500 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
Scheme 2. (a) Semi-Crystalline Polyamidoamine Curing Agents PEI-BA Prepared by Amidation of Polyethyleneimine (PEI) with Behenic Acid (BA) and (b) Preparation of Semicrystalline PHU Thermosets with Fatty Acid Side Chains
groups) was performed in a stainless steel reactor at 135 °C for 96 h at 30 bar CO2 pressure in the presence of catalytic amounts of tetrabutylammonium bromide, TBAB (20.0 g, 62.0 mmol, 2 wt %). The product was used without further purification and was received as a high-viscosity, yellowish liquid. 1 H NMR (299.87 MHz, CDCl3, δ): 1.00 (TBAB, t, −CH3), 1.43 (TBAB, dt, 3-CH2CH3), 1.65 (TBAB, m, −NCH2CH2−), 3.27 (TBAB, m, −NCH2−), 3.37−4.00 (m, 9.72 H, −CH2O−), 4.33− 4.55 (m, 2.04 H, −CH2−OCO2−), 4.83 (m, 1.00 H, −CH−OCO2−) ppm. FTIR (ATR): νmax = 2876 (C−H, str.), 1784 (CO, str.), 1480, 1394, 1168, 1102, 1043 cm−1. Preparation of Semicrystalline, Hyperbranched Polyamidoamine Curing Agents. Polyethylenimine (PEI) with weight-average molar mass of 800 g mol−1 was mixed together with the respective amount of behenic acid. The amidation reaction was performed in bulk at 170 °C under continuous nitrogen flow using a distillation apparatus to remove water. The reaction times varied between 5.5 and 9 h depending on the amine/behenic acid molar ratio. The respective weighed portions are shown in Table S1 in the Supporting Information. The complete conversion of behenic acid was verified by means of 13C NMR spectroscopy. The resulting waxy solids were dried under reduced pressure (100 °C, < 1 mbar, 30 min). PEI-3.7BA (C). 1H NMR (299.87 MHz, CDCl3, δ): 0.86 (t, −CH3), 1.23 (m, alkyl), 1.59 (m, −NH−C(O)CH2CH2−, N−H), 2.14 (m, −NH−C(O)CH 2 −), 2.43−2.79 (m, R′ 2 NCH 2 CH 2 NR 2 , R2NCH2CH2−NH−C(O)−, R/R′ = H/PEI), 3.04−3.19 (m, −CNCH2CH2 −, imidazoline ring protons), 3.19−3.37 (m, R2NCH2CH2−NH−C(O)−, R = H/PEI), 3.64 (m, −C NCH2−, imidazoline ring protons) ppm. 13C NMR (75.41 MHz, CDCl3 , δ): 14.2, 22.8, 26.0 (−NH(CO)CH 2CH2 −), 26.6 (−Cq, imiCH2−), 28.2 (−Cq, imiCH2CH2−), 29.5−29.8, 32.0, 36.8 (−NH(CO)−CH 2 −), 39.3, 39.0−58.3 (PEI-signals), 173.4 (−NH(CO)−) ppm. FTIR (ATR): νmax = 1048 (C−N, str.), 1119 (C−N, str.), 1298, 1357, 1465, 1561, 1585 (N−H, bend.), 1641 (CO, str., amide), 2807 (C−H, str.), 2848 (C−H, str.), 2917 (C− H, str.), 3275 (N−H, str.) cm−1. PEI-1.2BA (A), PEI-1.9BA (B), and PEI-3.7BA (C) show identical spectral shifts and only vary in intensity (Supporting Information; Figure S4−S12). Preparation of Amorphous and Semicrystalline Polyhydroxyurethanes. PGC was degassed under reduced pressure (
