Dual pH-sensitive layer-by-layer films containing amphoteric poly(diallylamine-co-maleic acid)

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Abstract

Layer-by-layer (LbL) thin films were prepared using an amphoteric copolymer consisting of alternating diallylamine and maleic acid monomer units (PDAMA), and pH-induced decomposition of the films was studied. LbL deposition of poly(styrene sulfonate) (PSS) and PDAMA on a solid surface afforded thin films at pH 3.0, while thin films did not form at pH 8.0. In contrast, PDAMA-poly(N-ethyl-4-vinylpyridine) (PEVP) films were constructed by LbL deposition of PDAMA and PEVP at pH 8.0, though films could not be obtained at pH 3.0. These results were explained based on the amphoteric nature of PDAMA, which is net positively and negatively charged at pH 3.0 and 8.0, respectively. PSS–PDAMA films were stable at pH 3.0–4.0 in a 150 mM NaCl solution and decomposed upon immersion in solutions with pH 4.3 or higher as a result of the reversal of the net PDAMA charge from positive to negative. In contrast, PDAMA–PEVP films disintegrated at pH 4.0 or lower. Thus, PDAMA-containing LbL films were found to be dual pH-sensitive: the films can be decomposed at acidic or neutral pH depending on the type of counter polymer in the film. PSS–PDAMA and PDAMA–PEVP films were also sensitive to salt concentration, where a higher concentration of NaCl in the solution in which the films were immersed facilitated decomposition of the films. The effect of salt concentration was attributed to partial breakage of the ion pairing between PDAMA and the counter polymers as a result of competitive binding with Na+ and Cl ions.

Highlights

► Dual pH-sensitive LbL films can be prepared using amphoteric copolymer. ► Amphoteric copolymer-PSS films are stable at neutral pH but decompose at acidic pH. ► Amphoteric copolymer-PEVP films are stable at neutral pH but decompose at acidic pH. ► Amphoteric copolymer-containing LbL films are sensitive to salt concentration.

Introduction

Considerable effort has been devoted to the development of stimulus-sensitive layer-by-layer (LbL) films and hollow microcapsules whose physical and chemical properties can be regulated by changing temperature [1], [2], [3], [4], [5], illumination [6], [7], magnetic field [8], [9], electric potential [10], [11], [12], [13], [14], pH [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], salt concentration [31], [32], [33], and the presence of certain biomolecules [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44]. Among stimuli-sensitive LbL assemblies, pH-sensitive films and microcapsules that decompose in response to changes in environmental pH have been most extensively studied due to their potential use for controlled release [15], [16], [17], [18], [19], [21], [22], [24], [25], [26], [27], [28], [29], [45]. Polyelectrolyte LbL films and microcapsules constructed through electrostatic interactions are usually stable over a wide pH range and decompose only in extremely acidic or alkaline media [46]. The high stability of polyelectrolyte LbL films is related to the polyvalent nature of electrostatic interaction between polymers in the films.

On the other hand, it has been reported that LbL films constructed through hydrogen bonding disintegrate in aqueous media of neutral or basic pH. For instance, LbL films composed of poly(methacrylic acid) (PMA) and poly(vinylpyrrolidone) (PVPON) are stable at pH 6.0 or lower but disintegrate at pH 7.0 or higher due to cleavage of the hydrogen bonds as a consequence of deprotonation of PMA [15]. The pH response of hydrogen bonding LbL films can be regulated by using different combinations of donor and acceptor polymers [18], [19], [21]. Zwitterionic poly(carboxybetaine) (PCB) and poly(sulfobetaine) (PSB) were also used for constructing LbL films through electrostatic bonding or hydrogen bonding, and their pH-dependent stability was studied [26], [27], [28]. The resulting films decomposed in neutral aqueous solutions as a result of increased ionization of carboxylic groups. These results show that pH-sensitive LbL films that decompose under mild pH conditions can be constructed by using polymeric materials whose electric charges are pH-dependent.

In an alternative route, hydrolytically degradable LbL films were also studied. For this purpose, cationic poly(β-amino ester) was built into LbL films with anionic polysaccharides, including heparin and chondroitin sulfate [22]. The poly(β-amino ester) LbL films decomposed about 10 times faster in phosphate buffered saline at pH 7.4 than in solution at pH 6.0. Citraconic amide-substituted PAH was also used as an anionic component for constructing pH-sensitive LbL films [24]. Cationic PAH was regenerated by acid hydrolysis of the citraconic amide-substituted PAH in the LbL films, resulting in decomposition of the film due to loss of the electrostatic binding force.

For future development of LbL film-based devices for controlled release, it is still important to study LbL films composed of different types of materials. In the present study, we have employed an amphoteric copolymer consisting of alternating diallylamine and maleic acid monomer units (PDAMA, Fig. 1) for constructing LbL films through electrostatic binding. The amphoteric nature of PDAMA may allow us to use this copolymer for constructing pH-sensitive LbL films that disintegrate in an acidic, neutral, or alkaline medium depending on the type of counter polymer in the film. To our knowledge, no report has appeared on the use of amphoteric copolymer for constructing pH-sensitive LbL films. In fact, LbL deposition of anionic poly(styrene sulfonate) (PSS) and PDAMA afforded thin films at pH 3.0, while LbL films composed of PDAMA and cationic poly(N-ethyl-4-vinylpyridine) (PEVP) were constructed at pH 8.0. The PSS–PDAMA and PDAMA–PEVP films were found to decompose at neutral and acidic pHs, respectively. Thus, the dual pH-sensitive nature of PDAMA-based films differs notably from that of hydrogen-bond-mediated or poly(betaine)-based LbL films. The present paper reports on the preparation of PDAMA-based LbL films and their pH-sensitive decomposition.

Section snippets

Materials

Poly(diallylamine-co-maleic acid) (PDAMA) hydrochloride was kindly donated by Nittobo Chemical Co. (Osaka, Japan). PDAMA is a copolymer that consists of alternating diallylamine and maleic acid monomer units. The molar ratio of diallylamine and maleic acid moieties in the copolymer was confirmed to be 1:1 based on elemental analysis. Calculated for (C10H16NO4Cl)n (HCl salt of 1:1 copolymer): C, 48.1%; N, 5.6%. Found: C, 47.8%; N, 5.8%. Poly(sodium styrene sulfonate) (PSS; MW, ∼500,000) was

Preparation of PSS–PDAMA and PDAMA–PEVP films

The sign of the net electric charge of PDAMA depends on pH of the medium because diallylamine and maleic acid residues in PDAMA are dissociable. PDAMA is net negatively charged in basic/neutral solutions due to the deprotonation of carboxylate residues, whereas the net charge of the polymer becomes positive in acidic media as a result of protonation to the carboxylate and diallylamine residues. Consequently, it may be possible to use PDAMA as a cationic component in acidic solutions and as a

Conclusion

The present study demonstrated that LbL films containing PDAMA can be successfully prepared at both acidic and neutral pH levels depending on the type of counter polymer. PSS–PDAMA films prepared at pH 3.0 decomposed in media with pH 4.3 or higher, while PDAMA–PEVP films formed at pH 8.0 disintegrated in acidic solutions. Thus, PDAMA-based LbL films exhibited dual pH sensitivity. For both films, the pH threshold for decomposition was found to be pH 4.0–5.0, which was qualitatively in accord

Acknowledgment

This work was supported in part by JSPS KAKENHI Grant Number 24390006.

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