Article Cite This: ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX
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Shape Memory and Self-Healing Properties of Poly(acrylate amide) Elastomers Reinforced with Polyhedral Oligomeric Silsesquioxanes Sen Xu, Bingjie Zhao, Muhammad Adeel, and Sixun Zheng* Department of Polymer Science and Engineering and State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, P. R. China
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S Supporting Information *
ABSTRACT: The organic−inorganic nanocomposite elastomers were synthesized via the random copolymerization of 5acetylaminopentyl acrylate (AA) with 3-methacryloxypropyl heptaphenyl polyhedral oligomeric silsesquioxane (MAPOSS). The morphological results showed that the random copolymers were microphase-separated and the POSS structural units were aggregated into the nanodomains that were composed of tens of silsesquioxane cages. Compared to neat PAA, the organic−inorganic copolymers possessed enhanced glass transition temperatures and improved mechanical strengths. Most importantly, the organic− inorganic copolymers simultaneously had shape memory and self-healing properties. The shape memory properties can be attributed to the formation of physical cross-linking in the random copolymers; in the physically cross-linked network, the POSS nanodomains constituted the net points. The self-healing properties are responsible for the dynamic exchange of the intramolecular hydrogen bonds among acrylate amide groups in spite of the nanoreinforcement of POSS nanodomains. The formation of microphase-separated morphologies is critical for the organic−inorganic copolymers simultaneously to have the shape memory and self-healing properties. KEYWORDS: poly(amide acrylate), POSS, microphase separation, shape memory properties, self-healing properties
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INTRODUCTION Smart or intelligent materials can respond to environmental changes and then manifest their functional properties; this class of materials has attracted considerable interest because of their scientific and technological significance.1−3 Of them, shape memory polymers (SMPs) can return to their original shapes in response to external stimuli.4−12 Thermally induced shape memory behavior is more common; shape recovery is readily triggered with thermal transition temperatures (e.g., glass to rubber transition and melting temperatures). For thermally induced shape memory behavior, the polymers generally have two structural elements: (i) network structures and (ii) sufficient entropic elasticity of network chains in response to thermal transitional temperatures.13 Cross-linking of macromolecular chains can be achieved via chemical or physical approach. Although chemical cross-linking affords stable networks, it simultaneously results in insolubility and infusibility of polymers and thus prevents the materials from melt (or solution) reprocessing and recycling. In contrast, physical cross-linking is reversible, which allows remolding of the materials. However, it is a challenging task to design stable and recoverable shape memory networks. Biological tissues have the ability to heal injury to prolong the lifetime of organisms. In contrast, synthetic materials generally do not possess this feature. Designing synthetic materials that have self-healing properties to maintain © XXXX American Chemical Society
mechanical strength and structural integrity of materials is interesting.14,15 In this regard, one of the pioneering investigations is to design the microcapsules containing healing agents; these microcapsules are then embedded in epoxy thermosets. The healing agents accommodated with the microcapsules are released and dispersed in the thermosetting matrix while cracks are generated and propagated.16−21 Recently, it is realized that materials can be bestowed with intrinsic self-healing behavior upon introduction of reversible and dynamic covalent or noncovalent bonds.22−24 Under certain conditions, these dynamic bonds can reversibly undergo breaking and re-forming so that the wounds are healed.25 Dynamic and reversible chemical reactions such as Diels−Alder reaction,26,27 light-triggered reshuffling of trithiocarbonates,28 hindered urea bonds,29 and transesterification30,31 have been explored for introduction into the materials, endowing the materials with self-healing properties. As one of the important physical interactions, the supramolecular hydrogen bonds have been recently utilized to design the polymers displaying self-healing properties via the dynamic exchange behavior of hydrogen bonds.33−36 Received: November 11, 2018 Accepted: January 25, 2019
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DOI: 10.1021/acsapm.8b00116 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX
Article
ACS Applied Polymer Materials Scheme 1. Synthesis of P(AA-r-MAPOSS)s
display shape memory properties while its single end of chain was capped with a POSS cage. In this work, we attempted simultaneously to endow PAA with shape memory and self-healing properties. First, AA was copolymerized with a POSS macromer [viz., 3-methacryloxypropyl heptaphenyl POSS (MAPOSS)] to afford the organic−inorganic random copolymers. Because of the immiscibility of PAA with a POSS component, the organic− inorganic copolymers are microphase-separated and the POSS microdomains would be generated via POSS−POSS interactions. Because of the POSS microdomains, the PAA matrices would be significantly reinforced and thus the organic− inorganic copolymers displayed improved thermomechanical properties. More importantly, the POSS microdomains can behave as the net points to afford a physical cross-linking of the copolymers because there were the chemical linkages between the POSS cages and PAA main chains. With the physical crosslinking, the organic−inorganic nanocomposites are newly endued with shape memory properties. Because of the microphase-separated morphologies, the elastomeric nature of PAA was still preserved, which would endow the copolymers with self-healing properties via the intense dynamic exchange of the intermolecular hydrogen bonds among acrylate amide groups of PAA. Investigating the effect of POSS nanoreinforcement on the self-healing properties of the copolymers is interesting.
Shape memory and self-healing behaviors are two inherent characteristics of biomaterials. However, it is not easy to simultaneously implement both of them in synthetic polymer materials.32 Poly(acrylate amide) (PAA) is a new acrylic polymer;33 it displays elastomeric properties at room temperature. In addition, PAA is a highly self-associated polymer via the intramolecular hydrogen bonding interactions of acrylate amide groups. In general, linear PAA does not exhibit thermally induced shape memory properties unless it is cross-linked. From the viewpoint of structure, PAA can possess the self-healing properties through the dynamic exchange of its dense intramolecular hydrogen bonds. However, PAA cannot be directly used as the structural and functional materials due to its low mechanical strength. Therefore, the reinforcement is critical for the application of this polymer. Guan et al.33,34 have reported the reinforcement of PAA through a multiphase design via (i) introducing thermoplastic nanophases (i.e., polystyrene) with the synthesis of brushlike copolymers and (ii) preparing PAA-grafted core−shell nanoparticles.35 Chung et al. reported a modification of PAA via the preparation of PAA-grafted nanocellulose (viz., lignin) and then investigated the self-healing properties of the reinforced PAA.36 These PAA nanocomposites displayed excellent self-healing properties. However, their shape memory properties remained unexplored. More recently, Cao et al.37 reported that PAA can B
DOI: 10.1021/acsapm.8b00116 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX
Article
ACS Applied Polymer Materials
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EXPERIMENTAL SECTION
to MAPOSS, P(AA-r-MAPOSS) copolymers were obtained with variable contents of POSS (see Table 1). The 1H NMR
Materials. 5-Acetylaminopentyl acrylate (AA) was prepared by following the literature method with a slight modification33 (see the Supporting Information). 3-Methacryloxypropylheptaphenyl POSS (MAPOSS) was synthesized with a corner-capping reaction of heptaphenyltricycloheptasiloxane trisodium silanolate with 3-methacryloxypropyltrichlorosilane38 (see Scheme S1 and Figure S1). 2Methyl-2-[(dodecylsulfanylthiocarbonyl)sulfanyl]propanoic acid (MDFC) was synthesized by following the method reported by McCormick et al.39 2,2-Azobisisobutylnitrile (AIBN) was commercially available and was recrystallized from ethanol twice before use. All of the organic solvents were purchased from Shanghai Reagent Co. Synthesis of Poly(AA-r-MAPOSS) Copolymers and the PAA Homopolymer. Typically, into a flask were charged AA (8.000 g, 40.8 mmol), MAPOSS (2.000 g, 1.6 mmol), MDFC (0.072 g, 0.19 mmol), AIBN (0.010 g, 0.06 mmol), and dimethylformamide (DMF) (10 mL); then three freeze−pump−thaw cycles were performed. After the mixture was heated to 80 °C, the polymerization was carried out for 72 h. Cooled to room temperature, the viscous polymerized mixture was added dropwise to a large amount of petroleum to afford the precipitates. The precipitates were subjected to three dissolution− precipitation procedures to remove the unreacted monomers (viz., AA and MAPOSS). After the mixture was dried in vacuo at 60 °C for 48 h, the product [i.e., P(AA-r-MAPOSS17)] (9.200 g) was obtained with a total conversion of monomers of 90 wt %: 1H NMR (CDCl3) δ 7.79 (s, 1H, -CH2NHCO), 7.41−7.62, 7.67−7.79 (m, 5H, protons of phenyl groups), 4.01 (t, 2H, -COOCH2CH2-), 3.41 (m, 1H, -CHCOO-), 2.99 (m, 2H, -CH2NH-), 2.01 (m, 3H, -NHCOCH3-), 1.79 (m, 2H, -CCH2CH-), 1.49 (t, 2H, -COOCH2CH2-), 1.41 (t, 2H, -COOCH2CH2CH2CH2-), 1.30 (t, 2H, -COOCH2CH2CH2-); MAPOSS (wt %) 17.4 [this value was determined with yields of degradation by means of thermogravimetric analysis (TGA) in an air atmosphere]. The procedure described above was also used for the synthesis of the PAA homopolymer without adding MAPOSS. Gel permeation chromatography (GPC): Mn = 42700 Da with Mw/Mn = 1.27. Measurements and Characterization. Nuclear magnetic resonance (NMR) spectroscopy, dynamic light scattering (DLS), thermal analysis [viz., TGA and differential scanning calorimetry (DSC)], small-angle X-ray scattering (SAXS), transmission electron microscopy (TEM), and tensile mechanical tests were carried out on the same instruments as reported elsewhere40 and in the Supporting Information). For tensile tests, the dumbbell-shaped specimens were prepared with dimensions of the central and parallel sections of 40 mm × 8 mm × 2 mm. At room temperature, the tests were carried out at a loading rate of 100 mm min−1. For the measurement of selfhealing properties, each specimen was first cut into two sections and the as-obtained two ends were placed in contact with each other and annealed at 30 °C for variable times. The cut and then healed specimens were subjected to tensile mechanical tests to evaluate the self-healing efficacy.
Table 1. Polymerization Results of P(AA-r-MAPOSS)s sample P(AA-rMAPOSS0) P(AA-rMAPOSS4.5) P(AA-rMAPOSS8.7) P(AA-rMAPOSS13.6) P(AA-rMAPOSS17.4)
[POSS]:[AA] (wt)a
conversion (%)
Mn (Da)
POSS (wt %)d
0:100
97.2
47200b
0
5:95
94.0
47000c
4.5
10:90
93.5
46700c
8.7
15:85
92.7
46300
c
13.6
20:80
90.3
45100c
17.4
a
Feed ratios. bGPC results; the polydispersity index was Mw/Mn = 1.27. cCalculated from conversion. dDetermined in terms of TGA results.
spectra of the PAA homopolymer and P(AA-r-MAPOSS4.5) are shown in Figure 1. For PAA, the resonance of the protons
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RESULTS AND DISCUSSION Synthesis of P(AA-r-MAPOSS)s. The radical copolymerization of AA with MAPOSS was carried out to obtain the P(AA-r-MAPOSS) copolymers (see Scheme 1). For the copolymerization of the monomers with a large difference in molecular weights, Matyjaszewski and Golubev et al.41−43 found that random (or statistical) copolymerization took place while a living/controlled radical polymerization was performed from a single-batch reaction. In this case, both AA and MAPOSS monomers have similar polymerizable groups (viz., acrylate) but a large difference in molecular weights. To achieve a random copolymerization from a single-batch reaction, reversible addition−fragmentation chain transfer (RAFT) polymerizations were employed with MDFC as the chain transfer agent. Via the variation of the mass ratios of AA
Figure 1. 1H NMR spectra of PAA and P(AA-r-MAPOSS4.5).
of methyl, methylene, and methine groups occurred at 1.20, 1.30, 1.38, 1.55, 1.79, 2.99, 3.36, and 4.01 ppm; the signal at 7.79 ppm can be assigned to the protons of N−H groups. For P(AA-r-MAPOSS4.5), the new signals were detected in the range of 7.00−7.80 ppm, which can be assigned to the protons of phenyl groups of MAPOSS. The 1H NMR results indicate that the copolymer had the combined structural features of AA C
DOI: 10.1021/acsapm.8b00116 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX
Article
ACS Applied Polymer Materials
temperature (see Figure S5). This observation suggests that physical cross-linking occurred at lower temperatures. It is proposed that the POSS cages grafted onto the main chains of PAA were aggregated into the POSS microdomains composed of tens of silsesquioxane cages. The POSS microdomains behaved as the net points of PAA chains, causing physical cross-linking. The formation of POSS microdomains can be investigated by means of SAXS, TEM, and rheological measurements (the experimental details are presented in the Supporting Information). Shown in Figure 2 are the SAXS profiles of
and MAPOSS; i.e., the copolymers of AA with MAPOSS were obtained. We have ever tried directly to measure the molecular weights by using GPC (see the Supporting Information). At room temperature, unfortunately, the DMF dispersions of P(AA-r-MAPOSS) copolymers failed to pass through the protecting column of the GPC apparatus. It is suggested that in the DMF dispersions there were objects with sizes of a couple of micrometers; i.e., there could be some self-organized objects in the dispersion of DMF. This speculation was confirmed by means of DLS, and the DLS results are shown in Figure S3. From the plots of the hydrodynamic radius distribution as a function of hydrodynamic radius (Rh), it is seen that the dispersion of P(AA-r-MAPOSS8.7) in DMF at a concentration of 0.1 g mL−1 exhibited bimodal distributions at 25 °C with hydrodynamic radii (Rh) of 458 and 5112 nm, respectively. When this dispersion was heated to 60 °C, the peak at Rh = 458 nm shifted to a position at Rh = 121 nm whereas the intensity of the peak at Rh = 5112 nm was significantly reduced. The latter peak did not disappear until the dispersion was heated to 90 °C, at which the peak at Rh = 121 nm shifted to Rh = 28 nm. When the dispersion was further heated to 140 °C, the peak at Rh = 28 nm was observed to shift to Rh = 2.4 nm. The hydrodynamic radius Rh = 2.4 nm is quite close to the size of one (or two) POSS cage. The DLS results indicate that no chemical cross-linking was formed in the copolymers and that in the dispersion there were strong POSS−POSS interactions at low temperature. Because of the presence of the large objects with Rh = 5112 nm, the dispersions failed to pass across the protecting column of the GPC apparatus at room temperature. Therefore, it is not feasible to measure the molecular weight by using GPC. In this work, we alternatively estimated the molecular weights in terms of the molar ratios of MDFC (viz. CTA) to the monomers as well as the overall conversion of the monomers as listed in Table 1. Notably, the high-molecular weight products were obtained for the synthesis of the copolymers with variable compositions. Because of a large difference in the yield of degradation between PAA and POSS components, it is possible to measure the contents of POSS in the copolymers by means of TGA; the P(AA-r-MAPOSS) copolymers were subjected to TGA (see Figure S4). From Figure S4, it is seen that in an air atmosphere, neat PAA was fully decomposed at 800 °C. Under identical conditions, however, all of the copolymers preserved some residues of degradation; the yields of degradation residues increased with the feed ratios of MAPOSS to AA. It is plausible to propose that the residues are mainly composed of the silica, which was produced from the degradation and oxidation of oligomeric silsesquioxane (viz., POSS component). By using the yields of degradation residues, the contents of MAPOSS in the copolymers were calculated (see Table 1). The POSS contents calculated with TGA were slightly lower than those from the feed ratios, suggesting that the reactivity of the MAPOSS macromer was slightly lower than that of AA due possibly to the bulky size of MAPOSS. The 1H NMR and TGA results indicate that copolymers of AA with MAPOSS were successfully obtained. Formation of POSS Microdomains. Plain PAA was soluble in common solvents such as DMF. However, the P(AA-r-MAPOSS) copolymers displayed a quite different behavior. All of the P(AA-r-MAPOSS) copolymers can be dissolved in DMF only at elevated temperatures (viz., 70 °C). However, these solutions (or dispersions) were transformed into the transparent gels while they were cooled to room
Figure 2. SAXS profiles of the P(AA-r-MAPOSS) copolymers containing various contents of MAPOSS.
PAA and the random copolymers. For neat PAA, no discernible scattering was displayed. However, all of the P(AA-r-MAPOSS)s exhibited scattering peaks, the intensity of which increased with MAPOSS content. While the contents of MAPOSS increased, the scattering peaks gradually shifted to the higher scattering vector (q) positions. The SAXS results show that the P(AA-r-MAPOSS) copolymers were indeed microphase-separated; in the organic−inorganic copolymers, the inorganic component (viz., POSS) was self-organized into the microdomains that were dispersed into PAA matrices. In terms of the position of scattering peaks, the microphaseseparated morphologies of the copolymers were calculated to have long periods of 10.2, 8.8, 7.6, and 6.9 nm for P(AA-rMAPOSS4.5), P(AA-r-MAPOSS8.7), P(AA-r-MAPOSS13.6), and P(AA-r-MAPOSS17.4), respectively. The microphaseseparated morphologies are readily confirmed by TEM. The TEM micrograph of P(AA-r-MAPOSS17.4) is representatively presented in Figure 3. Notably, this copolymer displayed a heterogeneous morphology. Considering the difference in electron density between PAA and POSS components, it is judged that the dark regions can be assigned to POSS microdomains and that the light matrix to PAA. It is worth noting that the spherical POSS microdomains were 20−50 nm in diameter and thus were the aggregates of tens of D
DOI: 10.1021/acsapm.8b00116 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX
Article
ACS Applied Polymer Materials
performed at 50 °C, and the data are presented in Figure 4. Notably, the values of the dynamic storage modulus (G′) and the dynamic loss modulus (G″) were dependent on shear rate. For neat PAA (Mn = 47200 with Mw/Mn = 1.27), G″ was higher than G′ when the shear rate was
