Progress in actively moving polymers

Andreas Lendlein

Actively moving polymers are stimuli-sensitive materials, which are able to shift their shape in response to suitable stimuli. In general two categories of such intelligent materials can be distinguished: shape-memory polymers and shape-changing polymers.¹ Shape-memory polymers can be deformed and temporarily fixed in a second shape. The process to create the second, temporary shape is called programming. This temporary shape, which is determined by the deformation process, is stable until exposed to a suitable stimulus. Once the original shape is recovered the material can be programmed again to exhibit another shape-memory effect. In contrast, the shape-changing effect is reversible. However, the temporary shape is only stable as long as the sample piece is exposed to the stimulus and the deformation is the same in subsequent cycles. A prominent example of shape-changing materials are liquid-crystalline elastomers.

Most shape-memory polymers studied so far are exhibiting a thermally-induced shape-memory effect. They consist of two components: (permanent) netpoints determining their permanent shape and switching segments, which can fix the temporary shape by solidification of the associated switching domains (temporary netpoints). Depending on the thermal transition related to those domains, they can vitrify or crystallize. Also thermal transitions associated with a liquid crystalline phase have been used to control a shape-memory effect in polymers.² In thermoplastic shape-memory polymers the permanent netpoints are physical crosslinks. In multiblock copolymers these crosslinks result from a phase segregation of hard segments and switching segments. For that reason the material's morphology plays a key role in enabling a shape-memory capability. Its characterization allows tailoring of the programming procedure. D'hollander et al. (DOI: 10.1039/b923734h) elucidated the microphase morphology of thermoplastic shape-memory polyurethanes by combination of solid-state proton wide-line nuclear magnetic resonance relaxometry, synchrotron small-angle and wide-angle X-ray scattering. The morphological model (Fig. 1) obtained enabled explaining the material's mechanical behavior during repeated cycles.

Fig. 1 Morphological model of a thermoplastic shape-memory polyurethane (taken from D'hollander et al. DOI: 10.1039/b923734h).

As the shape-memory effect results from the polymer network architecture in combination with the programming procedure, a wide range of material properties can be generated by variation of the chemical structure of the chain segments (e.g. type of repeating units, sequence structure) and the molecular architecture of the network. In this way families of polymers are obtained, in which macroscopic properties (e.g. elasticity and thermal properties) can be varied in a wide range by only small changes in their chemical structure. Rousseau and Xie describe a series of shape-memory epoxies as an example of such a polymer system (DOI: 10.1039/b923394f). These materials are characterized by high strength well above room temperature, durability, stability, and high thermo-mechanical endurance.

The area of shape-memory polymers attracted tremendous attention in the last few years because of its scientific and technological significance, which becomes apparent in the rapidly increasing number of scientific publications (Fig. 2) as well as published patents and patent applications.³ The reasons for this impressive development are manifold. Fundamental research aims toward different stimuli than heat and enabling more complex movements on demand. Triple-shape polymers (Fig. 3) enable two subsequent shape changes when heated (highlighted by Behl et al., DOI: 10.1039/b922992b). Even a polymer capable of a quadruple-shape effect was reported meanwhile.⁵ Major milestones in the realization of different stimuli are the indirect actuation of the thermally-induced effect by IR irradiation, electric current, humidity or alternating magnetic fields as well as direct triggering with light.^1,3 In moisture-sensitive materials water acts as a softener of glassy switching domains. The shape-memory effect in these material systems is triggered as a result of a decrease of the glass transition temperature to a value below the environmental temperature (Huang et al., DOI: 10.1039/b922943d). The incorporation of magnetic nanoparticles into triple-shape polymer networks enables the non-contact activation of the triple-shape effect in an alternating magnetic field (Narendra Kumar et al., DOI: 10.1039/b923000a).

Fig. 2 Result of a literature search for the term “shape-memory polymer” in CAPlus database performed with Scifinder on March 12th, 2009: red = publications in English, magenta = publications in Chinese or Japanese, black = other languages (taken from ref. 3 with kind permission of Springer Science + Business Media).

Fig. 3 Intelligent tube illustrating the triple-shape effect of a multiphase polymer network moving from shape (a) to (b) and finally (c) stimulated by a stepwise increase in the environmental temperature (taken from ref. 4, Copyright 2006, National Academy of Sciences, USA).

Another notable development of the last decade is the successful realization of products based on the shape-memory polymer technology platform as well as the development of a product pipeline to be realized in the near future. Besides established applications as packaging materials (e.g. heat-shrinkable films and tubes), smart textiles based on shape-memory polyurethanes were commercialized. The active movement of fibers enables fascinating effects. The aesthetic look of a garment and its tactile nature can change from static to dynamic. Protective wear can adjust the level of insulation according to environmental requirements. Furthermore, the temperature dependence of water vapor permeability through a material related to the thermal transition of the switching domains is used in smart textiles. At low temperatures, the fabric is less permeable and retains body heat. At high temperature, moisture permeability increases and heat is released (Hu and Chen, DOI: 10.1039/b922872a).

Biomedical applications are an emerging application field for shape-memory polymers, which might play an important role in minimally invasive surgery. Bulky implants could be implanted in a compressed temporary shape through a small incision. Heating to body temperature could trigger the shape-memory effect and recover the application-relevant shape. If the shape-memory polymer is designed to be degradable, a second surgery for removal of the implant could be avoided. Small et al. give an overview about biomedical applications of thermally-activated shape-memory polymers (DOI: 10.1039/b923717h).

Multimaterial approaches are followed as a strategy to achieve the capability to actively move or to enhance the mechanical properties or to add additional functionalities (e.g. electrical conductivity or magnetic properties). The incorporation of inorganic (nano)particles or (nano)fibres into shape-memory polymer matrices aims at improved mechanical properties by reinforcement and retarding relaxation processes especially in thermoplastic polymers. Fundamental investigations analyzing the non-covalent interactions between the nanoparticulate filler and the matrix material, like that reported by Sedat Gunes et al. (DOI: 10.1039/b922027e), form the basis for tailoring the design of shape-memory nanocomposites. For enhancing the shape-memory properties the nanofillers should strengthen the hard domains by physical interaction with the hard segments. On the other hand the processes leading to the solidification of the switching domains resulting in the fixation of the temporary shape such as switching segment crystallization should not be hindered by the inorganic particles.

Nanotubes could be oriented in a shape-memory polymer matrix by fibre drawing. This spatial organization is a critical issue as it greatly influences material properties. Viry et al. observed that the fibres can generate a stress one order of magnitude greater than that achieved with unaligned assemblies of nanotubes (DOI: 10.1039/b924430a). The incorporation of nanolayered graphene in epoxy-based shape-memory polymers enhances scratching resistance and the thermal heating capability of the material (DOI: 10.1039/c0jm00307g). The matrix polymers of nanocomposites can also be covalently crosslinked. Xu et al. incorporated different nanofillers, such as alumina, silica and clay, into chemically crosslinked styrene-based copolymers and investigated the influence of the nanofiller shape on the macroscopic properties of the composite (DOI: 10.1039/b923238a). Covalently binding nanofillers to the shape-memory polymer matrix is an efficient method to substantially retard relaxation processes, which becomes apparent from the low hysteresis even over several programming–recovery cycles (Jung et al., DOI: 10.1039/b922775j).

An interesting strategy of stimuli-sensitive multimaterial systems is the creation of the capability to actively move by combining two materials which do not move on demand by themselves. Examples are bilayer materials as presented by Simpson et al. (DOI: 10.1039/b922972h). A bilayer consisting of a polydimethysiloxane film with a thickness of several microns coated with a nanometer-thick gold film is able to reversibly alter its geometry from a three-dimensional tube to a two-dimensional layer upon changes in temperature. The authors propose to apply these intelligent particles as delivery vesicles.

A film consisting of a main-chain liquid crystalline elastomer with shape-memory capability was exploited to temporarily fix an embossed, micron-scale pattern on a surface and to recover the original flat surface on demand (Fig. 4). Burke and Mather envision various applications for this surface shape-memory phenomenon, including soft lithography and microfluidics (DOI: 10.1039/b924050k).

Fig. 4 Micron-scale pattern on the surface of a shape-memory liquid crystalline elastomer (taken from Burke and Mather, DOI: 10.1039/b924050k).

Although adsorbed on silicon wafers, poly[(N-isopropylacrylamide)-co-acrylic acid] microgel particles swell and shrink upon changes in temperature. The swelling/deswelling behavior of the adsorbed microgel particles was investigated by scanning force microscopy and compared to the corresponding bulk conditions by Burmistrova and von Klitzing (DOI: 10.1039/b923969c).

The field of actively moving materials spans over different scales from single molecules over polymer brushes at surfaces to the above described polymer networks. Ionov gives an overview of these stimuli-sensitive systems in a feature article (DOI: 10.1039/b922718k). Glucose responsive polymer brushes were evaluated by exposing them to different glucose concentrations for a range of pH values (DOI: 10.1039/b925583d). Chen et al. speculate that polymer brush-functionalized micromechanical cantilevers could be used as glucose detectors. Hmadeh et al. studied [c2]daisy chain molecules, which undergo reversible molecular movements upon addition of base/acid in organic solvents. If this movement on the molecular scale can be amplified to the macroscopic level, artificial muscles could be created (DOI: 10.1039/b924273b).

Finally, I thank all authors for their contributions to this themed issue of Journal of Material Chemistry about actively moving polymers. On one hand the novel mechanically active materials described in this issue might inspire the interdisciplinary readership to think about potential applications for this fascinating technology. On the other hand the presented results of fundamental research aiming at a deeper mechanistic understanding of the shape-memory effect as well as the presented stimuli-sensitive molecules (e.g. containing photolabile domains, DOI: 10.1039/b922613c) and self-assembling systems (e.g. block copolymers, DOI: 10.1039/b922956f) might form the basis for a next generation of actively moving polymers. The major challenge for future developments in this field is the creation of materials which are adaptive to environmental conditions and teachable.

References

1. M. Behl and A. Lendlein, Actively moving polymers, Soft Matter, 2007, 3(1), 58–67.
2. I. A. Rousseau and P. T. Mather, Shape Memory Effect Exhibited by Smectic-C Liquid Crystalline Elastomers, J. Am. Chem. Soc., 2003, 125(50), 15300–15301.
3. M. Behl, J. Zotzmann and A. Lendlein, Shape-Memory Polymers and Shape-Changing Polymers, Adv. Polym. Sci., 2010 DOI:10.1007/12_2009_26 (published online on November 5, 2009).
4. I. Bellin, S. Kelch, R. Langer and A. Lendlein, Polymeric Triple Shape Materials, Proc. Natl. Acad. Sci. U. S. A., 2006, 103(48), 18043–18047.
5. I. S. Kolesov and H. J. Radusch, Multiple shape-memory behavior and thermal-mechanical properties of peroxide cross-linked blends of linear and short-chain branched polyethylenes, eXPRESS Polym. Lett., 2008, 2(7), 461–473.

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