A new water-soluble aromatic polyamide hydrogelator with thixotropic properties

Abstract

The water-soluble aromatic polyamide poly(3-sodium sulfo-p-phenylene terephthalamide) forms a hydrogel with anisotropy, which exhibits good thixotropic behaviour, even at the critical gel concentration of the gelator (1.0 wt%).

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Organic polymer hydrogels have attracted much research interest due to their applicability in a wide range of fields, such as in electro-optical materials and medical products.¹ In contrast to inorganic materials that feature rigid and tough properties, organic polymer hydrogels exhibit flexible mechanical and/or chemical responses, and these types of flexible stimuli-responsive properties make them very interesting soft materials. As a result, both academic and industrial researchers have worked to elucidate their physical chemistry properties and to develop new advanced organic polymer gels with various applications, particularly those related to the medical and healthcare fields.

Among the methods for the preparation of organic polymer hydrogels, polymer hydrogelators^1a,2 that provide self-assembled molecular networks resulting in gels have the merits of facile synthesis, enhanced mechanical properties and reduced risk to humans compared with low-molecular-weight compounds. For example, polymer gelators based on natural polyamino acids (such as gelatine^2,3a) and polysaccharides (such as agarose^2,3b) remain a topical research area, and studies involving their application to medical materials are underway. Exploration of new synthetic polymer hydrogelators can widen the application area for molecular gels prepared using polymer gelators.⁴

There are several reports of aromatic polyamide hydrogelators with liquid crystalline properties. Okamoto studied the hydrogelation of sulfonated poly(p-phenylene terephthalamide) (critical gel concentration (CGC): 0.80 wt% (ref. 5a) and 0.60 wt% (ref. 5b)), poly(4,4′-stilbene terephthalamide 2,2′-disulfonic acid) (CGC: 0.99 wt%)^5b and partially sulfonated poly(benzoyl-1,4-phenylene) (CGC: 0.68 wt%),^5b which exhibit lyotropic liquid crystallinity (LC) in aqueous solution. Picken also studied the synthetic method and hydrogelation of sulfonated poly(p-phenylene terephthalamide) (CGC: 2.0 wt%).^5c Sulfonated poly(4,4′-phenylene benzidine amide) also exhibited lyotropic LC in an aqueous solution, however, it did not form hydrogels, at least at low concentrations (<5 wt%).^6a,6b These results revealed that not all LC polyamides can form hydrogels and indicated that additional synthetic approaches and further research are needed to identify new polymer hydrogelators.

In general, gelators such as polymer gelators and low-molecular-weight gelators^7,8 may be applied in medical applications⁹ where thixotropic properties that involve mechanical reversible transitions from the gel-to-sol and sol-to-gel states^10–12 are required for the spreadability of ointments and the injectability of parenteral drug formulations.^9,13 Thus, the exploration of polymer gelators with thixotropic characteristics is meaningful for widening their versatility and utility in medical applications.

Herein, we describe the hydrogel forming ability of the water-soluble polyaromatic amide poly(3-sodium sulfo-p-phenylene terephthalamide) (NaPPDT, Scheme 1) and the thixotropic properties of the hydrogel formed using this new polymer hydrogelator. Previously, Picken reported that the acid and salt forms of poly(3-sulfo-p-phenylene-terephthalamide) are water-soluble lyotropic LC polymers, and his group utilized them as anisotropic agents.¹⁴ However, the gelation ability of the acid and salt forms of this LC polymer have not yet been reported.

Scheme 1 Chemical structure of poly(3-sodium sulfo-p-phenylene-terephthalamide) (NaPPDT).

NaPPDT was synthesized via polycondensation of sodium 3-sulfoterephthalic acid and p-phenylene diamine in NMP using triphenyl phosphite as the activating agent for the carboxylic acid following Picken's work.¹⁴ However, whilst Picken used 3-sulfoterephthalic acid as the dicarboxylic acid, the commercially available sodium salt of 3-sulfoterephthalic acid was employed in the present study.

After polycondensation and following several reprecipitation steps in NMP/methanol and NMP/acetone, the reprecipitated polymer was filtered and dried. Next, dialysis of the polymer in water was performed to remove any remaining monomer and NMP, the water was then removed in vacuo and the resulting pale yellow solid polymer was further dried. The NaPPDT was characterized using ¹H-NMR spectroscopy (Fig. S1, ESI†), and was found to be soluble in water and NMP, but insoluble in acetone, methanol and even 0.01 M aqueous NaCl. The number-average molecular weight (M_n) was 10 000, which was in good agreement with the results for NaPPDT (M_n = 10 000) reported by Picken.¹⁴

The gel forming properties of the NaPPDT hydrogel were evaluated using the vial-inversion method (ESI†), and the critical gel concentration was determined to be 1.0 wt% (Fig. 1). Corresponding to Picken's results, NaPPDT exhibited anisotropic properties visible in optical micrographs captured under crossed-Nicols condition and via small-angle X-ray scattering (SAXS) analysis, shown in Fig. 2 and S2 (ESI†), respectively. The existence of an anisotropic structure at concentrations greater than 0.5 wt% NaPPDT in water can be seen in Fig. 2. In addition, a structure with a size on the order of one hundred nanometres is shown in Fig. S2† and is likely due to nematic LC, as reported in the literature.¹⁴ On the basis of these results, it can be concluded that aqueous NaPPDT exhibits both hydrogel and anisotropic structures (i.e. it is a lyotropic nematic LC), but that NaPPDT can function as new lyotropic LC polymer hydrogelator at concentrations similar to those for the polyamides reported by Okamoto.^5a,5b Note that in the hydrogel comprising NaPPDT, the sodium atoms may play an important role in the formation of the gel state due to the electrostatic interactions of the intermolecular sulfonic acid groups and sodium atoms, which may result in physical crosslinking of the polymer chains.

Fig. 1 Photos of NaPPDT aqueous solutions under crossed-Nicols: (a) NaPPDT aqueous solutions in vials, (b) inverted vials shown in (a).

Fig. 2 Photos of NaPPDT aqueous solutions under crossed-Nicols: (a) 2.0 wt%, (b) 1.0 wt%, (c) 0.5 wt% and (d) water.

To investigate the gel state of the NaPPDT hydrogel, rheometric measurements were obtained (Fig. S3, ESI†), and the existence of a gel state was confirmed, as seen in the case of other polymer gels.¹⁵ With increasing stress, the relative magnitudes of the storage modulus, G′ and the loss modulus, G′′, were reversed upon transition from the gel state (G′ > G′′) to the sol state (G′ < G′′). In addition, during the frequency sweep, gelation was indicated by a pseudo plateau, where the magnitude of G′ was greater than that of G′′.

With respect to mechanical properties of the NaPPDT hydrogel, good thixotropic properties were observed. Fig. 3 shows the thixotropic behaviour of the NaPPDT hydrogel, which involved mechanical collapse of the hydrogel followed by recovery after a rest period. Quantitative data for the thixotropic behaviour of the NaPPDT hydrogel were then obtained via step-shear analysis with repeated application of a large deformation force and rest period (Fig. 4). It is seen in the figure that the aqueous NaPPDT solutions in the sol state (G′ < G′′) recovered to the gel state (G′ > G′′), with the extent of the recovery dependent on the concentration of the NaPPDT. Specifically, the 1.0 wt% aqueous NaPPDT solution underwent a change from a gel-like state (G′ ≥ G′′) to a liquid-like state (G′ is approximately equal to G′′) after deformation, and the 1.0 wt% and 2.0 wt% gels recovered from the liquid state right after deformation (G′ < G′′) to the gel state (G′ > G′′), at which point the values remained stable. Furthermore, repeated recovery to the same level for each cycle was observed for the gels containing >1.0 wt% NaPPDT. These results indicate that reconstruction of the hydrogel from the broken gel after deformation was faster in this system.

Fig. 3 Thixotropic evaluation of the NaPPDT 1.0 wt% hydrogel.

Fig. 4 Periodic step-shear test results for the NaPPDT hydrogels.

To further evaluate the thixotropic properties, the thixotropic loops were examined for NaPPDT hydrogels with different concentrations of polymer gelator (Fig. S4, ESI†).^10,16 An increase in the modulus in loops was observed as a shear force that was repeatedly applied to the hydrogels. For hydrogels with good thixotropic behaviour, the shapes of the forward and backward curves are similar because of the gel state recovery. The thixotropic loops for the NaPPDT hydrogels had nearly the same shapes, indicating and that they possessed good thixotropic properties. This hydrogel therefore has potential for use in medical applications such as ointments, which require materials with thixotropic behaviour.

Next, to gain insight into the inner microstructure of the hydrogels, scanning electron microscopy (SEM) images of the dried 1.0 wt% NaPPDT hydrogel (xerogel) were obtained. A close look at the SEM images in Fig. 5 revealed that the xerogel was constructed of a micrometre-sized porous fibrous network with a wrinkled surface. In addition, this network appeared to comprise the polymer hydrogelator used to form the hydrogel. It is likely that during the hydrogel drying process, the polymer chains aggregated to form bundles in the smaller regions while maintaining the network structure in the larger regions, thus resulting in the wrinkled and bundled fibrous structure with a submicrometre diameter. On the basis of these results, it is concluded that the NaPPDT hydrogel formed a network-like structure similar to that seen for other types of hydrogelators such as gelatine and agar and that this network contributed to the gel-like behaviour of the NaPPDT hydrogel.

Fig. 5 SEM images of the NaPPDT xerogel prepared from the corresponding NaPPDT 1.0 wt% hydrogel.

The results of the SAXS analyses for the NaPPDT hydrogels with different polymer gelator concentrations are presented in Fig. 6. A value for the slope of 1.6 indicated that the hydrogels did not comprise simple rigid rods of polymer chains or simple rigid rods of bundled polymers (a slope of −1 in the double logarithmic graph means the existence of rigid rods).¹⁷

Fig. 6 SAXS data for NaPPDT aqueous solutions and their hydrogels, including an exponential approximation for the 2.0 wt% hydrogel (0.055 < q < 0.140).

Finally, to further investigate the hydrogel network, infrared (IR) spectra of precipitated NaPPDT and the 1.0 wt% NaPPDT hydrogel and xerogel in the region of the sulfonyl stretching band were obtained (Fig. S5, ESI†). It is seen in the figure that the lower absorption peak for precipitated NaPPDT (1181 cm⁻¹) shifted to a higher wavenumber (1190 cm⁻¹) in the NaPPDT xerogel. This shift may indicate that the intermolecular interactions of the sulfonyl groups were weaker in the NaPPDT xerogel state than in the precipitated NaPPDT, and that finer bundled NaPPDT polymer chains may be components of the hydrogel network.

In summary, we demonstrated that the water-soluble polyaromatic amide poly(3-sodium sulfo-p-phenylene terephthalamide), NaPPDT, forms lyotropic LC hydrogels in aqueous solutions. This newly developed polymer hydrogelator exhibited good thixotropic behaviour with a network structure on the order of submicrometres. The application of NaPPDT hydrogels as components of composites intended for use in the medical and healthcare fields is currently under investigation.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra16824d

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