Macromolecular NanotechnolgyCarbon nanotube–polyurethane shape memory nanocomposites with low trigger temperature
Graphical abstract
Introduction
Shape memory polymers (SMPs) are a class of stimuli responsive materials, which may recover their original shape from large deformation upon the application of an external stimulus such as heat [1], [2], [3], [4], [5], [6], electricity [7], [8], [9], [10], [11], alternating magnetic fields [12], [13], [14], [15], [16], light [17], [18], [19], water [20]. In recent years, SMPs have received increasing interest due to their low cost, low density, high shape recoverability and easy processing, compared to conventional shape memory alloys (SMAs) or shape memory ceramic materials (SMCs) [21]. SMPs can be easily processed into complex forms by conventional methods, such as injection molding, film casting, fiber spinning, profile extrusion, and foaming. Therefore, SMPs are steadily expanding their applications in various areas like smart coatings [22], [23], textiles [24], packaging materials [25], biomedical applications [1], [2], [19], [26], [27], [28], [29], [30], [31], [32], smart actuators [11], [33], [34], [35], [36], and self-healing bladders [37].
Nevertheless, there are still some scientific and technical barriers which prevent widespread applications of SMPs. For instance, SMPs have relatively low recovery stress, which is usually 1–3 MPa compared to 0.5–1 GPa for shape memory metal alloys [38]. The relatively low recovery stress becomes a limiting factor in many applications especially in cases where SMP articles should overcome a large resisting stress during shape recovery. A common method to improve mechanical and shape recovery properties is to introduce inorganic fillers, especially nanoparticles with high modulus to SMPs. Various inorganic fillers such as hydroxyapatite [39], carbon nanotubes (CNTs) [7], [10], [11], [40], Fe3O4 [12], [15], [16], SiO2 [1], [41], [42], nanoclay [5], polyhedral oligomeric silsesquioxane (POSS) [2], [3], [43], [44], TiO2 [29] have been studied about their effects on the mechanical and shape memory properties of SMPs. The most popular nanofillers for SMPs are CNTs, both to reinforce and to induce electrical and infrared light assisted heating to actuate shape recovery. Vaia et al. [45] prepared shape memory polyurethane (SMPU) filled with multiwall CNTs (MWNTs) that responded to infrared radiation and electric current. The results showed that composite with 5 wt% filler content offered higher recovery stress than pristine SMPU and SMPU filled with 20 wt% carbon black (CB). This increase was thought to be induced by the presence of MWNTs acting as nucleating agents for increased crystallinity. Sahoo et al. [46] prepared SMPU composites filled with CB and CNTs. The results showed that electrical resistance of the composites increased while elongation decreased with increasing filler loading.
Polyurethane has been extensively studied because of its shape memory properties, which arises from the phase-separated structure of its hard and soft segments. The hard segments form physical crosslinks due to polar interaction, hydrogen bonding or crystallization in the hard domains, while the soft segments form the reversible phase because of molecular motion in a rubbery state [11]. The shape memory effect of PU is usually induced by thermal stimulation by heating PU above its transition temperature, such as the glass transition temperature or melting temperature. Multi-stimuli sensitive and multi-shape memory properties can be introduced by grafting certain groups to PU chains [47], [48]. And some other stimulating sources, such as electric field, light, alternating magnetic fields, may also be used to actuate the shape recovery of SMPUs when proper functional nanoparticles are introduced to the PU matrix [7], [10], [11], [20]. CNTs were incorporated in SMPU to endow SMPU with electroactive shape memory performances [7], [11], [49]. In order to increase the interaction between CNTs and PU matrix, surface modification of CNTs (by acid treatment) was carried out. Fine dispersion of the modified CNTs in the matrix resulted in the formation of nanocomposites with excellent electrical and thermal conductivity, which in turn resulted in the significant improvement of electro active shape memory behaviors [7], [11], [49]. In situ polymerization was used to improve the dispersion and interfacial interaction between CNTs and PU matrix as well. The composites obtained by in situ polymerization showed enhanced mechanical properties as well as good electroactive shape memory performances. The original shape of the sample with 3 wt% CNTs was almost recovered when an electric field of 50 V was applied [10]. Though better dispersion is easier to achieve by solution casting and in situ polymerization, melt mixing seems to be simpler to use with better repeatability, thus detaining a greater potential technique at a larger scale [40].
In this work, a commercially available polyurethane Desmopan 385S with low Tg (about −34 °C) from Bayer Materials Science was used to develop shape memory nanocomposites with low trigger temperature, which have potential applications for controlling tags or proof marks in the area of frozen food. CNTs and oxidized CNTs (ox-CNTs) were introduced by melt mixing to improve the mechanical and shape memory properties of the PU matrix. And the dispersion of CNTs on the mechanical properties and shape memory behaviors of the nanocomposites were also investigated.
Section snippets
Materials
CNTs from catalytic vapor deposition (specific surface area ⩾200 m2/g, purity ⩾95%.) were purchased from Beijing Cnano Technology Limited, China. The CNTs (Flo Tube 9000) are commercial products with an average diameter of 11 nm and an average length of 10 μm. Because the pristine CNTs involve impurities such as amorphous carbon, metal catalysts and graphite particles, they were oxidized by mixed acid (sulphuric acid: nitric acid = 3:1 M ratio) at 50 °C for 24 h and then rinsed with deionized water to
Structures of the pristine CNTs, the ox-CNTs and the nanocomposites
As the pristine CNTs tend to agglomerate due to Van der Waals forces, chemical modification is thought to be an effective technique to improve the dispersion of CNTs in polymer matrix and the mechanical properties of carbon nanotube–reinforced composites [11]. In this work, the pristine CNTs were functionalized via mixed acid of sulphuric and nitric acids. The FTIR spectra of the pristine CNTs and the ox-CNTs are presented in Fig. 1. The FTIR spectrum of the ox-CNTs, which were modified by
Conclusions
Well dispersed carbon nanotube–reinforced PU nanocomposite were obtained via melt mixing of the ox-CNTs and the PU matrix. A slight increase of Tg of Desmopan 385S was detected by DMA, which is due to the existence of crosslinking CNTs and more hydrogen bonds formed between ester groups of the PU chains and hydroxyl groups on the ox-CNTs surfaces. A slight increase of E′ was observed for nanocomposites at temperature below Tg due to the stiffness effect of the CNTs or the ox-CNTs. Better
Acknowledgement
The work was supported by National key technology R&D program (Grant No. 2012BAI17B05).
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