Shape and Temperature Memory of Nanocomposites with Broadened Glass Transition

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Science  23 Nov 2007:
Vol. 318, Issue 5854, pp. 1294-1296
DOI: 10.1126/science.1145593


Shape-memory polymers can revert to their original shape when they are reheated. The stress generated by shape recovery is a growing function of the energy absorbed during deformation at a high temperature; thus, high energy to failure is a necessary condition for strong shape-memory materials. We report on the properties of composite nanotube fibers that exhibit this particular feature. We observed that these composites can generate a stress upon shape recovery up to two orders of magnitude greater than that generated by conventional polymers. In addition, the nanoparticles induce a broadening of the glass transition and a temperature memory with a peak of recovery stress at the temperature of their initial deformation.

Shape-memory materials (1) are usually made of lightweight polymers (2) or metallic alloys (3). They have been investigated for more than 30 years and have applications in packaging, biomedical devices, heat-shrink tubing, deployable structures, microdevices, etc. Shape-memory polymers are deformed at a high temperature (Td) and then cooled down under fixed strain to trap the deformed polymer chains, thus storing mechanical energy. Upon reheating, typically in the vicinity of the glass transition temperature (Tg), the polymer chains become mobile, and the material can relax by reverting toward its original and more stable shape. The efficiency of a shape-memory polymer is empirically controlled by its composition, as defined by the polymer's chemical structure, molecular weight, degree of cross-linking, and fraction of amorphous and crystalline domains (2, 47). The energy that is restored with shape recovery is a growing function of the energy supplied during the deformation at a high temperature (4, 8). Shape-memory polymers can exhibit large strain when they revert toward their initial shape. Unfortunately, this large strain is usually associated with a low stress recovery from a few tenths of a megapascal to a few tens of megapascals (2, 811). Consequently, the energy density, which results from a combination of stress and strain, is rather low and not among the best as compared with other actuator technologies (9). For example, the recovery stress of shape-memory metallic alloys can reach 800 MPa for the best materials (3). Shape-memory effects of alloys are based on a temperature-induced martensitic phase transformation from a low- to a high-symmetry crystallographic structure (3, 12, 13). In spite of their large stress recovery, shape-memory alloys have several drawbacks when compared to polymers. They are six times heavier than polymers, and their recovery strain does not exceed 8%. Combining large stress and large strain recovery, as well as finding more controlled programming procedures, remain critical challenges for the development of smarter and stronger shape-memory materials.

The inclusion of nanoparticles has been shown to improve the behavior of shape-memory polymers. These efforts include an increase in their mechanical properties (1417), the addition of conductive nanoparticles to achieve shape-memory effects, which can be triggered by Joule's heating (18), or the inclusion of magnetic nanoparticles, which can cause heating in the presence of an alternating magnetic field (19). Other smart nanocomposites have been made by adding carbon nanotubes (CNTs) to conductive polymers (20) or elastomers (21).

We studied fibers that contain a large fraction of CNTs embedded in polyvinyl alcohol (PVA). They are obtained by a particular coagulation spinning process that allows a homogeneous distribution of the CNTs within the fibers. Those fibers are investigated because they are known to exhibit an exceptional energy to failure (22, 23), which is a necessary condition to store a large amount of mechanical energy. Elicarb CNTs (single walled nanotubes from Thomas Swan, County Durham, United Kingdom) were used (see section 1 in 24). The spinning process (25) consists of injecting a dispersion of surfactant-stabilized CNTs in the co-flowing stream of a coagulating polymer solution. PVA (molecular weight 195,000 provided by Seppic, Paris, France) was used as coagulating agent. This method leads to nanotube-PVA composite fibers with a fraction of nanotubes of ∼20 weight percent. PVAwas chosen because it exhibits strong interactions with CNTs. This is reflected in the effective coagulation spinning and high toughness of the composite fibers.

The obtained fibers are stretched at a deformation temperature Td and then cooled down to room temperature under fixed strain. Their length does not change when the load is released at room temperature, thus showing good “shape fixity.” The fibers, however, shrink substantially when they are reheated (24). Quantitative characterizations are obtained by performing thermomechanical measurements in a temperature-controlled chamber (see section 3 in 24). Figure 1 shows the stress needed to stretch the fibers up to 800% at different temperatures. A greater stress is needed to deform the fibers at low Td. At higher Td, the fibers become softer and can be more easily deformed; this softness is associated with a lower supply of mechanical energy. This can be estimated from Fig. 1, where the area under each curve corresponds to the energy supplied to the fibers at different Td, from 70° to 180°C, and upon mechanical stretching.

Fig. 1.

Stress versus strain curves of nanotube-composite fibers. The fibers are stretched up to 800% at different temperatures (Td). The area under the curves corresponds to the mechanical energy supplied to the fibers.

As shown in Fig. 2, when reheated at fixed strain the fibers generate a strong stress with a maximum at a well-defined temperature (Ts). The occurrence of a peak recovery stress in conditions of fixed strain has already been observed for other shape-memory polymers and nanocomposites (8, 26), but with no direct link between Ts and Td. In conventional materials, the peak of recovery stress occurs in the vicinity of the glass transition of the neat polymer. In fact, this is interpreted in the literature as a direct manifestation of the glass transition of the pure polymer (26). When the materials are initially deformed above the glass temperature transition, the peak disappears and the stress generated by shape recovery substantially decreases. This occurs because polymer chains can relax when deformed at temperatures well above Tg, thus decreasing the potential for stored mechanical energy.

Fig. 2.

Stress generated by a nanocomposite fiber when it is reheated. The strain is fixed and the temperature is increased from room temperature to 230°C at a rate of 5°C/min. The different colors correspond to the temperatures Td at which the fibers have been initially deformed. A peak is observed in each case for a temperature Ts roughly equal to Td.

In this case, the peak is preserved well above the Tg of the neat PVA and, more strikingly, Ts and Td are roughly equal. This near-equality means that the fibers memorize the temperature at which they have been deformed. The peak of stress generated can be observed up to 180°C, which is ∼100°C above the Tg of the neat PVA (see section 4 in 24). This distinctive feature provides an opportunity to rationally control Ts without varying the chemical structure of the material. In addition, it is observed that the maximal stress generated by the fiber is close to 150 MPa. This value is from one to two orders of magnitude greater than the stress generated by conventional shape-memory polymers. It is obtained for fibers that have been deformed at 70° and 90°C, temperatures that are in the vicinity of the Tg of the neat PVA. These temperatures correspond to the conditions for which the greatest energy is supplied during the initial deformation. The stress recovery is closer to the stress generated by shape-memory metallic alloys, which ranges between 200 and 800 MPa for NiTi alloys (3, 9, 12, 13). However, nanotube fibers, like other polymeric materials, are much lighter (1.4 versus 6.5 g/cm3 for NiTi) and exhibit strain recovery greater than that of the best metallic alloys (several tens of a percent versus 8% for NiTi).

The strain recovery rate under conditions of free load is shown in Fig. 3. The strain recovery rate is taken as Embedded Image, where ld is the length of the fiber after deformation at Td and cooling at room temperature, and l(T) is the length of the fiber upon reheating at a given temperature T. Other practical examples of large strain recovery are shown in Movies S1 and S2 (24). The curves in Fig. 3 are shifted to the right with increasing Td. This is again a consequence of the temperature memory, and fibers that have been stretched at greater Td recover their shape at higher temperatures. An inflexion region can be seen in the curves of Fig. 3. The temperature of this inflexion is close to Td. Although the temperature memory is more clearly observed through peaks of recovery stress, it is still apparent in strain recovery experiments.

Fig. 3.

Strain recovery upon reheating in conditions of free load for nanotube fibers that have been deformed at different Td. The recovery strain is taken as Embedded Image. The arrows indicate an inflexion region that can be noticed in the increase of strain recovery.

Additionally, because CNT fibers are electrically conductive, the thermal shape-memory effects can be triggered by Joule's heating when an electrical current is passed through the fiber (24). This can be beneficial for the direct use in microdevices where heating by an external source can be difficult.

The present results can be understood on the basis of previous knowledge of the structure of CNT-PVA fibers and on the main known features of nanocomposites and shape-memory polymers. Shape-memory polymers usually involve two phases (2, 4, 5, 17): (i) a fixed one, which can be made of crystallites, rigid segments, or chemical cross-links, and (ii) a mobile one, which is made of amorphous polymer. The latter drives shape-memory effects through elongation and contraction of the polymer chains during programming and shape recovery, respectively, but the fixed phase is necessary to lock deformations in the material. Shape-memory effects are more pronounced in the vicinity of Tg, because this temperature corresponds to the relaxation of the amorphous fractions of the polymer. CNTs substantially alter the thermomechanical properties of the composite fibers in several ways. First, and as shown in Fig. 4, they act as reinforcements characterized by an increase of one order of magnitude of the storage modulus. Second and as already reported (23, 27), they favor the stabilization of crystalline domains. This can contribute to the locking of mechanical constraints. In a similar way, Meng et al. reported that CNTs can interact with the rigid segments of block copolymers to improve shape-memory phenomena (17). However, the most distinctive feature arises from the alteration of the relaxations of the polymer.

Fig. 4.

Storage modulus E′ as a function of temperature for neat and dried PVA (squares) and for dried CNT-PVA fibers (circles). The Tg of the neat PVA is about 80°C. The presence of the CNTs substantially alters the thermomechanical properties of the polymer: (i) The storage modulus is greater, (ii) the relaxation at Tg is not observed anymore, and (iii) the storage modulus is much less temperature-dependent; indicating thereby a broadening of the glass transition.

Neat PVA can exhibit several thermomechanical relaxations, depending on its degree of cross-linking and humidity (28). The Tg of the material presently used is about 80°C in its dry state (24). The relaxation at Tg is characterized by a large decrease of the storage modulus (28). The main relaxation in the vicinity of Tg is not seen in the presence of CNTs. The storage modulus is less temperature-dependent and reflects a broadening of the glass transition. It has been shown that large gradients of Tg can develop at the interface of nanoparticles (29). The transition can be shifted up by more than 100°C when the polymer is confined at 1 nm from the interface and by only a few degrees Celsius at 10 nm (29). The effect becomes negligible at greater distances. The average diameter of the polymer layers around the CNTs is Embedded Image, where r is the average diameter of the CNT bundles, and ϕ is the CNT volume fraction. This yields D = 11 nm by assuming r ∼ 5 nm and an equal density for the CNTs and the polymer (24). The polymer shells around the CNTs largely overlap and percolate, such as the CNTs themselves, meaning that there is a distribution of polymer-to-CNT distances that ranges from molecular contact to several nanometers. This distribution of confinement results in a wide broadening of the relaxation-time spectrum and specifically the glass transition through a distribution of polymer fractions that exhibit different Tg. This property is responsible for peaks of stress recovery well above the Tg of the neat polymer. Indeed, when the material is stretched at Td, the polymer fractions that have lower Tg (far from the interface) can quickly relax and do not efficiently participate in the storage of mechanical energy. In contrast, polymer fractions with Tg close to Td dominate the behavior by storing and restoring mechanical energy. Composites treated in the vicinity of Tg of the neat polymer still exhibit higher toughness and generate greater stress recovery than those treated at temperatures well above Tg of the neat polymer. This indicates that the fractions of amorphous polymer with unshifted or slightly shifted Tg remain the major components of the composite. Polymer fractions with strongly shifted glass transition are confined in smaller volumes closer to the interface of the nanotubes. The effect of the polymer fractions with high Tg is thus less pronounced. The lower volume of more confined polymer results in lower recovery stress at high temperatures, as experimentally observed. The decreasing volume of polymer fractions closer to the CNT interfaces can also contribute in sharpening memory effects. Polymer fractions with Tg above Td can participate in the shape-memory behavior, but their volume becomes smaller as we consider higher Td. This lowers the recovery stress above Td and contributes in defining more precisely the temperature of maximal recovery stress.

The phenomena investigated herein, and shape-memory properties in general, are expected to depend on kinetics, such as the glass transition of conventional polymers. But relaxations in glasses become exponentially slow or fast when the temperature is shifted, respectively, below or above temperatures of the glass transition region. The present effects have been observed from 70° to 180°C in the same conditions, meaning that temperature memorization is not incidentally due to the heating rate. Preliminary experiments with a heating rate of 1°C/min confirmed this feature and yielded the same temperature-memory behavior. Furthermore, the intrinsic mechanical properties of the CNTs are not expected to play a critical role in the investigated phenomena because the levels of mechanical stresses are too low to achieve large deformations of the CNTs. The Young's modulus of CNTs is several hundred gigapascals (30), whereas the maximal stress of the composite fibers during deformation does not exceed 0.5 GPa. However, polymer nanoconfinement and the high fraction of CNTs are essential features to achieve temperature-memory and strong shape-memory effects through alterations of the thermomechanical properties of the polymer.

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