Cover Image: Issue 7-8, Materials Today.
Cover Image: Issue 7-8, Materials Today.

Shape memory polymers (SMPs) have the ability to return from an imposed deformed shape to their original shape when prompted by an external trigger such as temperature, pH, moisture, light, or magnetic field. Thermally activated SMPs usually contain two elements: a reversible transition which acts as a molecular switch and is responsible for maintaining the imposed deformed shape and a restoration mechanism which is responsible for restoring the original shape. The reversible transition could be a glass transition temperature or a melting point (Tm). The restoration mechanism could be physical or chemical crosslinking or could be a phase whose transition temperature is higher than the reversible transition temperature. Compared to fully dense SMPs, porous SMPs can achieve higher temporary deformations and exhibit higher deformations when they recover their permanent shape.

PolyHIPEs are crosslinked polymers with emulsion-templated porous structures that are synthesized within the continuous, external phase of high internal phase emulsions (HIPEs) [1] and [2]. In HIPEs, which are usually highly viscous, the dispersed, internal phase constitutes more than 74% of the volume. Mayonnaise, for example, is an oil-in-water HIPE. The HIPEs for polyHIPE synthesis are usually formed by the slow dropwise addition of the internal phase to the external phase under constant stirring. The HIPE is usually stabilized using an amphiphilic surfactant which is only soluble in the continuous, external phase. Pickering HIPEs are HIPEs that are stabilized using amphiphilic nanoparticles (NPs) which preferentially migrate to the oil-water interface, replacing the surfactant. The NPs used to stabilize Pickering HIPEs can also serve as crosslinking centers, replacing the crosslinking comonomer [3]. Typically, polyHIPEs are based upon the polymerization at elevated temperatures of hydrophobic monomers and crosslinking comonomers within the continuous phase of water-in-oil (w/o) HIPEs. A free radical polymerization (FRP) initiator is either added to the organic phase, for organic-phase initiation, or is added to the aqueous phase, for interfacial initiation. Both the structure and properties of polyHIPEs from Pickering HIPEs have been shown to be strongly dependent upon the locus of initiation [4] and [5]. Evacuating the internal phase through the multiple holes that develop in the polymer walls during polymerization leaves voids in place of the internal phase droplets and produces polymers with highly interconnected porous structures.

Recent work has shown that porous SMP polyHIPEs based on acrylates and methacrylates that bear long, crystallizable, aliphatic side-chains can be synthesized within NP-stabilized w/o Pickering HIPEs (where the NPs also serve as crosslinking centers) using organic-phase initiation [6]. PolyHIPEs based on stearyl acrylate (A18, Tm ∼ 33 °C) with crystallizable C18H37 side-chains were synthesized by heating the external phase to above the Tm before adding the internal phase in a dropwise fashion. The A18-based polyHIPEs exhibited SMP properties of interest, maintaining the new shape imposed by a compressive strain of 70% and then exhibiting recoveries of around 90% for multiple cycles. The next stage in the investigation involved: (a) synthesizing polyHIPEs based on behenyl acrylate (A22, Tm ∼ 45 °C) with crystallizable C22H45 side-chains within almost identical NP-stabilized w/o Pickering HIPEs (where the NPs also serve as crosslinking centers); (b) synthesizing both A18- and A22-based polyHIPEs using interfacial initiation. The resulting porous structures of the A18- and A22-based polyHIPEs synthesized using interfacial initiation were quite unusual.

This month's cover shows a high-resolution scanning electron microscope image of spherical polymer protuberances (∼ 40 μm in diameter) with a thread-like “ball of string” texture. The spherical protuberances seem to have been exuded from the polyHIPE walls into the internal phase droplets. This exact structure was only found for A22-based polyHIPEs synthesized within NP-stabilized w/o Pickering HIPEs (where the NPs also serve as crosslinking centers) using interfacial initiation. In A18-based polyHIPEs synthesized using interfacial initiation, spherical polymer protuberances (∼ 40 μm in diameter) with smooth surfaces seem to have been exuded from the walls into the internal phase droplets. No such protuberances were found for A18- and A22-based polyHIPEs synthesized using organic-phase initiation. No such protuberances were found for similar polyHIPEs synthesized using interfacial initiation of acrylates whose Tm was below room temperature. Interestingly, no such protuberances were found for the A18- and A22-based polyHIPEs that were synthesized using interfacial initiation from an internal phase that was heated before its addition to the heated external phase.

The formation of these spherical protuberances seems to depend upon the presence of crystallizable side-chains with a Tm above room temperature, the use of interfacial initiation, and the addition of a room temperature internal phase to a heated external phase. These factors combine to create a situation in which the HIPE begins to destabilize before the polymerization and crosslinking have “set” the wall structure. The crystallization of the monomer surrounding the room temperature internal phase droplet seems to prevent interfacial initiation and delay polymerization and crosslinking. In addition, this crystallization seems to encompass the stabilizing NPs, reducing, if not eliminating, their mobility, and allowing a fluid instability to develop at the oil-water interface. The delay in the polymerization that “sets” the wall structure and the generation of a fluid instability do not occur when a monomer with a Tm below room temperature is used or when a heated internal phase is used. Since the monomer does not crystallize in these cases, the initiator and the stabilizing NPs can perform their respective functions. The delay in the polymerization that “sets” the wall structure does not occur when organic-phase initiation is used since the initiation occurs throughout the wall and is not restricted to the interface. The tendency towards phase inversion/phase separation of a system in which the continuous phase is only around 15% of the HIPE, the high interfacial tension, and the high interfacial curvature result in the exudation of spherical protuberances from the continuous external phase into the droplets of the dispersed internal phase. When polymerization finally does occur, the partially destabilized HIPE structure becomes “set” and the spherical polymer protuberances are the result.

Further reading
[1] M.S. Silverstein, et al., Encyclopedia of Polymer Science and Technology - Online,
Wiley, 2010 ,
[2] M.S. Silverstein, Progress in Polymer Science, in press (2013). DOI:10.1016/j.prog-
[3] I. Gurevitch, et al. Macromolecules 45 (2012) 6450.
[4] S. Livshin, et al. Macromolecules 41 (2008) 3930.
[5] T. Gitli, et al. Soft Matter 4 (2008) 2475.
[6] I. Gurevitch, et al. Soft Matter 8 (2012) 10378.

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DOI: 10.1016/j.mattod.2013.07.006