Carbon nanotube–polyurethane shape memory nanocomposites with low trigger temperature

Highlights

• Shape memory polymer nanocomposites with low trigger temperature were developed.

• The carbon nanotube–reinforced nanocomposites were obtained via melt mixing.

• The nanocomposites exhibit high shape fixity and recovery ratio above 98%.

• The nanocomposites have applications for controlling tags in the area of frozen food.

Abstract

Ester-based polyurethane (PU) with low glass transition temperature was used to develop shape memory nanocomposites with low trigger temperature. Pristine carbon nanotubes (CNTs) and oxidized CNTs (ox-CNTs) were introduced by melt mixing to improve the mechanical and shape memory properties of the PU matrix. The dispersion of CNTs on the mechanical properties and shape memory behaviors of the nanocomposites were also investigated. The results show that better dispersion of ox-CNTs contributes to more stiffness effect below glass transition temperature (T_g) while lower storage modulus (E′) above T_g. The nanocomposites exhibit high shape fixity and recovery ratio above 98%. The ox-CNT/PU nanocomposite shows higher shape recovery ratio for the first cycle, faster recovery due to better dispersion of CNTs and have potential applications for controlling tags or proof marks in the area of frozen food. The trigger temperature can be tailored by controlling the T_g of the PU matrix or the content of the nanofillers.

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 T_g (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.