Letter pubs.acs.org/macroletters
Biomass Approach toward Robust, Sustainable, Multiple-ShapeMemory Materials Zhongkai Wang,† Yaqiong Zhang,‡ Liang Yuan,† Jeffery Hayat,† Nathan M. Trenor,† Meghan E. Lamm,† Laetitia Vlaminck,§ Stijn Billiet,§ Filip E. Du Prez,§ Zhigang Wang,‡ and Chuanbing Tang*,† †
Department of Chemistry and Biochemistry, University of South Carolina, Columbia, South Carolina 29208, United States CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026, China § Department of Organic and Macromolecular Chemistry, Polymer Chemistry Research Group, Ghent University, Krijgslaan 281 S4-bis, B-9000 Ghent, Belgium ‡
S Supporting Information *
ABSTRACT: We report biomass-derived, shape-memory materials prepared via simple reactions, including “grafting from” ATRP and TAD click chemistry. Although the biomass, including plant oils and cellulose nanocrystals, has heterogeneous chemical structures in nature, these materials exhibit excellent multiple shape-memory properties toward temperature, water, and organic solvents, which are comparable to petroleum counterparts. The work presented herein provides burgeoning opportunities to design the next-generation, lowcost, biomass-prevalent, green materials for niche applications.
T
exhibit a broad glass transition or a wide melting transition.6,7 In parallel, a few multiple stimuli-responsive SMPs have been developed.8 Almost all these SMPs are exclusively based on pure petrochemicals with definite chemical structures. Until now, there are a limited number of examples showing the preparation of SMPs utilizing a structurally more sophisticated biomass. The reported ones mostly make use of plant-oil-derived polyurethanes and polyesters.9 However, most of these SMPs are thermally induced, dual-shape-memory materials. Currently, there are, to the best of our knowledge, no reports on the fabrication of multistimuli-responsive or multishape-memory materials using biomass. Herein we report a robust, efficient, and scalable strategy to prepare biomass-derived SMPs with multiple responsive and multishape-memory properties using novel biomass chemistry that involves plant oils and cellulose nanocrystals (CNCs). In the preparation of SMPs, we employed surface-initiated atom transfer radical polymerization (SI-ATRP, Figure S1) to prepare plant-oil-derived, polymer-grafted CNCs.10 On the other hand, the use of additive-free triazolinedione (TAD) chemistry introduced a straightforward “click coupling” among the unsaturated double bonds from plant oils (Figure 1a).11 This approach allows two types of cross-links within the material: permanent TAD-induced cross-linked junctions and dynamic physically cross-linked by the hydrogen bonds
he academic and public demand for the sustainable use of fuels, chemicals, and materials has driven biomass research to a new level. It has been increasingly recognized that the utilization of biomass enhances sustainability, reduces carbon footprints, and curtails the undesirable impact on ecosystems. A variety of biomass products has been used for commodity chemicals, polymers, and advanced materials, as well as for niche applications.1 Compared to petrochemicals, biomass typically possesses diverse chemical entities, originating from sophisticated resources in nature. One of the most critical challenges relies on how to overcome this structural diversity and chemical heterogeneity of biomass in order to achieve comparable properties to the petrochemical counterparts. It is becoming important to explore robust biomass chemistry and to come up with innovative approaches to take this chemistry to the next level. Shape-memory polymers (SMPs) are a class of materials with the ability to deform and generate mechanical stress and strain at the macroscopic scale under specific stimuli.2 Most SMPs exhibit dual-shape memory properties (dual-SMPs) that can exchange between two different shapes, a temporary and a permanent shape, but are only responsive to one stimulus (heating, light, chemical, or magnetic).3 Multishape-memory polymers (multi-SMPs) with memorization of two, three, or even more temporary shapes have also been developed.4 MultiSMPs were initially designed by a combination of two or more distinct well-defined transitions (e.g., glass transition temperature and/or melting point).5 Multi-SMPs with advanced macromolecular architectures have been later developed to © XXXX American Chemical Society
Received: March 27, 2016 Accepted: April 23, 2016
602
DOI: 10.1021/acsmacrolett.6b00243 ACS Macro Lett. 2016, 5, 602−606
Letter
ACS Macro Letters
demonstrated the incorporation of PSBAM on the surface of CNCs. PSBAM/CNC-g-PSBAM nanocomposites were subsequently prepared via a solution-mixing method. The PSBAM polymer contains secondary amide groups that can form hydrogen bonds intra- and intermolecularly. A proposed microstructure model of the nanocomposites is shown in Figure S6. A physical network is formed due to the existence of hydrogen bonds within PSBAM and CNC-g-PSBAM. The prepared nanocomposite had a weight content of CNCs of 2.3 wt % and was labeled as NC2.3. For comparison, a blank nanocomposite containing a simple blend of PSBAM with 2.3 wt % of virgin CNCs was also prepared under exactly identical conditions, labeled BNC2.3. Typical stress−strain curves of PSBAM, NC2.3, and BNC2.3 are shown in Figure S7a. NC2.3 shows both higher tensile strength and higher elongation at break than the simple blend, indicating that the grafting of PSBAM helps CNCs to be dispersed in polymer matrix. The introduction of a permanent chemically cross-linked network was performed via the TAD chemistry, which was recently demonstrated by some of us as a click-type reaction with numerous chemical substrates.11 One of the characteristics of this reaction is its rapid addition onto internal double bonds with high fidelity under ambient conditions without the need of a catalyst or other trigger via an alder-ene pathway.14 In this respect, the obtained nanocomposite was mixed with a bis-TAD and labeled as shape-memory nanocomposite (SMNC; Figure 1a). As depicted in Figure 1b, these SMPs include two independent networks: a physical network based on supramolecular hydrogen bonding and a chemical network via TAD click coupling. The supramolecular hydrogen bonding derived from amide bonds has been shown to be reversible under different temperatures.15 When heated to 80 °C, hydrogen bonds disappear, and the SMPs become soft and elastic. A temporary stretching shape can be introduced and further locked by cooling to room temperature to recover hydrogen bonds. When reheated to 80 °C, highly strained polymer chains relax with the favorable gain of entropy, which drives SMPs to recover back to their permanent shape. One should not rule out that the hydrogen bonds of urazole adducts (formed after TAD coupling) also plays a role, though much minor due to their low content in the composites.16 To achieve a better understanding of the effect of “click coupling” density on the mechanical properties, model reactions were performed on PSBAM (without CNCs) with bis-TAD in the amount of 0 to 10 mol %, relative to the double bonds in PSBAM (Figure 2a). The tensile strength of “click coupled” PSBAM increased with the increase of bis-TAD
Figure 1. (a) Schematic illustration for the preparation of SMNC via click coupling using bis-TAD (for simplicity, the possible H-bonding involving TAD is not included). (b) Proposed mechanism of shapememory behavior of SMNCs using heat as a stimulus.
uniquely designed with the plant oil chemistry (and also from the TAD moiety). The chemical network endows SMPs with a permanent shape, while the physical network empowers them with multiresponsive and multishape-memory properties. As illustrated in Figure 1b, upon heating, the SMPs can be deformed due to decomplexation of hydrogen bonds. After cooling, hydrogen bonds recover and lock the temporary shape. Further heating would break up the hydrogen bonds again, and the SMPs return to their entropically favored permanent shape. In addition, chemicals that can break the hydrogen bonds or plasticize the plant oil polymers could trigger the shape recovery of SMPs. Due to the broad glass transition and the dynamics of hydrogen bonds, these SMPs further facilitate multishape memory properties. The nanocomposites contain plant oil polymer grafted CNCs that are dispersed in a homopolymer derived from plant oil. Specifically, we use high oleic soybean oil (HOSO) containing ∼75% oleic acid, ∼7% linoleic acid, 12% of saturates, and 6% other fatty acids. The matrix polymer (PSBAM, in Figure 1a) was prepared by radical polymerization of soybean amide methacrylate (SBAM) according to a procedure reported earlier (Mn = 49100 g/mol).12 The graft polymer (Mn,theor = 22100 g/mol) was prepared by SI-ATRP of SBAM from CNCs that were surface-anchored with alkyl bromide as ATRP initiators (Figures S1−S3).13 The successful preparation of CNC-g-PSBAM was demonstrated by atomic force microscopy imaging. As shown in Figure S4a−c, there was an obvious change in the morphology of CNCs as a result of the immobilization of initiators with subsequent graft polymerization. Both the height and diameter of CNC-g-PSBAM (38 ± 4 nm and 107 ± 22 nm) were significantly higher than those of virgin CNCs (6 ± 2 and 35 ± 8 nm), which is an indication of the presence of polymer brushes on the surface of CNCs. In addition, dynamic light scattering (Figure S5) and X-ray photoelectron spectroscopy (Figure S4d and Table S1) also
Figure 2. Monotonic stress−strain curves of (a) PSBAM cross-linked with 0−10 mol % bis-TAD; (b) Monotonic stress−strain curves of SMNC2.3 in comparison with PSBAM and PSBAM cross-linked with 5 mol % bis-TAD. 603
DOI: 10.1021/acsmacrolett.6b00243 ACS Macro Lett. 2016, 5, 602−606
Letter
ACS Macro Letters content, while the tensile strain gradually decreased. As visualized in Figure S9, chemically cross-linked gels formed within 1 h with bis-TAD contents starting from 5 mol %, indicating that such TAD content was high enough for the generation of a chemical network. Thus, NC2.3 was crosslinked with 5 mol % bis-TAD and labeled as SMNC2.3. A stress−strain curve of SMNC2.3 is also shown in Figure 2b. In comparison with PSBAM cross-linked with 5 mol % bis-TAD, the strength of SMNC2.3 showed an appreciable increase. SMNC2.3 was first submitted to thermally induced, dualshape-memory experiments and quantitatively evaluated by stress-controlled dynamic mechanical analysis (DMA). For this, the sample was first heated to 80 °C with a 0.1 MPa stress applied. A strain of 65% was reached within 5 min. The stretched state could be maintained after cooling to 25 °C and subsequent removal of the stress. Figure 3a presents the evolution of strain, stress, and temperature during the dualshape-memory programming steps.
Figure 4. (a) DMA curve of SMNC2.3; (b) Triple-shape-memory programming for SMNC2.3 with two consecutive uniaxial stretchings at 80 and 40 °C and subsequent triple-shape recoveries at 40 and 80 °C; (c) Triple-shape-memory effect for SMNC2.3: (A) original shape 1; (B) shape 2, bent at 80 °C and cooled to 40 °C; (C) shape 3, bended at 40 °C and cooled to 0 °C; (D) recovered shape 2 after heating to 40 °C; (E) recovered shape 1 after heating to 80 °C.
MPa. After cooling to 0 °C, a final strain of 39% was observed. Triple-shape-memory properties were subsequently achieved, with two recovery steps performed at staged temperatures of 40 and 80 °C, respectively. The shape fixing ratio for shape 2 is 86% after the uniaxial stretching at 80 °C and cooling down to 40 °C. On the other hand, the shape fixing ratio for shape 3 is 97%, much higher than that of shape 2. One of the reasons is that the polymer chains are partially in a viscoelastic state at 40 °C. Concerning the recovery of shape 2 at 40 °C, a progressive decrease in strain from 39% to 31% was observed, and the shape recovery ratio of shape 2 was calculated to be 57%. After the recovery of shape 1 at high temperature (80 °C), a residual strain close to 7% was observed. The shape recovery ratio of shape 1 was calculated to be higher than 100% (108%), which might be attributed to the shape recovery of shape 2 that continued simultaneously with the shape recovery of shape 1. Compared with the dual-shape memory experiments, the reduced overall shape recovery ratio (86%) for the triple-shape memory is most probably related to the much lower recovery rate at 40 °C. As shown in Figure 4c, the triple-shape-memory properties were also qualitatively characterized with pictures taken at different stages of the recovery process. The initial elongated shape was bended and fixed, respectively, at 80 and 40 °C. The nearly full recovery at 40 and 80 °C demonstrated that the initial and intermediate shapes could be effectively recovered. In order to compare the effect of Tg on shape-memory properties with/without hydrogen bonds, we designed a new system that used a monomer (SBMA) with tertiary amide, which would not induce H-bonding in the composites. As shown in Figures S13 and S14, the new composite SMNC2.3PSBMA (without H-bonding) has a Tg much lower than SMNC2.3 (with H-bonding), and only behaves as a soft elastomer. This control study demonstrated that the existence of hydrogen bonds increases the Tg and further induces shapememory properties. For the SMNC, the temporary shape is fixed by the formation of supramolecular hydrogen bonds between polymer chains (vide supra). Conceptually, any chemicals, which can break the hydrogen bonds or plasticize the SMNC (to decrease
Figure 3. (a) Dual-shape-memory programming for SMNC2.3; (b) Photos of shape recovery process of SMNC2.3 at 80 °C; the labels indicate the recovery time at 80 °C.
SMNC2.3, stretched at 80 °C, showed a high shape fixity ratio of 99% and a shape recovery ratio of 94% (For definition, see Supporting Information). Figure 3b shows representative pictures about the shape recovery process of SMNC2.3. A spiral shape was made at 80 °C and cooled to room temperature to fix the temporary shape. Then, the spiral shaped sample was reheated to 80 °C, and pictures of the sample were taken at different times (
