Shape memory properties of polypeptide hydrogels having hydrophobic alkyl side chains
Graphical abstract
Introduction
Shape memory materials have been attracted much attention because of their applicability as functional materials. Especially in these years, many researches concerning shape memory polymers (SMPs) have been reported [1], [2], [3], [4], [5], [6], [7]. In thermo-responsive SMPs, the material remembers its original shape, and can keep the temporary deformed shape at low temperature, and recovers to the original shape spontaneously by heating. The shape memory mechanism is described as follows: in SMP, the polymer chains are chemically or physically cross-linked and the polymer network can recover to the original permanent shape after deformation because of its entropy elasticity. By cooling the deformed material below its crystallization or glass transition temperature, the material cannot recover to the original shape because the segmental motion of the polymer chain is frozen by the phase transition of the constituent polymers. So the material can be fixed at the temporary deformed shape at low temperature. By heating the material above the phase transition temperature, the segmental motion of the polymer chain becomes possible, and the material recovers to the original permanent shape spontaneously. The cross-linking point in SMP is called as a fixing phase, and the component undergoes transition between liquid and solid states is called as a reversible (switchable) phase.
Osada and coworkers reported a shape memory behavior of hydrogels [8], [9], [10], [11]. The hydrogels were synthesized by copolymerization of n-stearyl acrylate (SA) and acrylic acid (AA) in presence of a cross-linker. Copolymer of SA and AA can be regarded to consist of a water-soluble poly(AA) as main chain and hydrophobic stearyl (octadecyl) chain as side chains. When the SA mole fraction was high, crystallization of the octadecyl chains occurred in the prepared hydrogel, which played a role to fix the temporary deformed shape as the reversible phase, and the obtained gel acted as shape memory hydrogel.
Recently, we have reported association behavior of amphiphilic polypeptides based on water-soluble non-ionic polypeptide, poly[N5-(2-hydroxyethyl) l-glutamine] (PHEG), with hydrophobic modification in various manners [12], [13], [14]. In graft-type amphiphilic polymers [14], hydrophilic PHEG main chain had some long alkyl chains as hydrophobic side chains, and this structure is similar to the copolymer of SA and AA by Osada et al. Therefore, by cross-linking of the PHEG having alkyl side chains, a shape memory polypeptide hydrogel is expected to be prepared. PHEG hydrogel membrane was reported to be applicable as biomaterials [15], [16], [17], [18], therefore, the obtained shape memory polypeptide hydrogel will be possibly used as novel biomaterials and biodegradable materials. Until now, only limited shape memory polypeptide hydrogels have been reported: for example, shape memory behavior of hydrogels consisting of telechelic polypeptide was recently reported by Skrzeszewska et al. [19].
In this paper, we report preparation and shape memory properties of the polypeptide hydrogels consisting of PHEG with alkyl side chains –CnH2n+1 (PHEG-Cn). In order to investigate the effect of the alkyl chains on the shape memory property, hydrogels with various side chain lengths (n = 8, 16, and 18) and side chain mole fraction (falkyl = 0.05–0.16) were prepared. In the hydrogels prepared in this work, crystallization of the alkyl side chains was not confirmed even at 0 °C, as the result, the observed shape memory behavior was different from that in other normal SMPs. From the experimental results, shape memory mechanism of the PHEG-Cn hydrogels will be discussed.
Section snippets
Materials
Triethylamine, 2-aminoethanol, 2-hydroxypyridine, 1,4-dioxane, N,N-dimethylacetamide (DMAc), and N,N-dimethylformamide (DMF) were fractionally distilled before use. N-carboxyanhydride (NCA) of γ-benzyl l-glutamate (BLG) was prepared by reacting BLG with triphosgene, and purified by recrystallization from tetrahydrofurane/petrorium ether [12]. n-Octyl isocyanate (C8-NCO), n-hexadecyl isocyanate (C16-NCO), n-octadecyl isocyanate (C18-NCO), hexamethylene diisocyanate (HMDI), and di-n-butyltin
Characterization of polymers and hydrogels
Weight-averaged molecular weight of the starting PHEG's, evaluated from SEC chart, were 280,000, 281,000, and 418,000, and their polydispersity index (Mw/Mn) was 1.3, 1.7, and 1.3, respectively. Characteristics of the obtained samples of PHEG-Cn are listed in Table 1. In order to investigate the effect of alkyl side chains on shape memory properties, we prepared PHEG-Cn samples with different chain length (n) of the alkyl chain –CnH2n+1 and alkyl chain mole fraction (falkyl). Values of falkyl
Conclusion
Polypeptide hydrogels were prepared by cross-linking of PHEG having hydrophobic alkyl chains as side chain. The alkyl chain length n was 8, 16, and 18, its mol fraction in the polypeptide was varied in the range of 0.05–0.16, and these polymers were cross-linked with using HMDI. In the prepared hydrogels, crystallization of the side chains could not be confirmed. Instead of their non-crystalline nature, the hydrogels had the ability to fix the deformed temporary shape, although the absolute
Acknowledgments
Financial support by Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (No. 18550193) is gratefully acknowledged.
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