Effect of structural isomerism and polymer end group on the pH-stability of hydrogen-bonded multilayers

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Abstract

Association of tannic acid (TA) with structurally isomeric poly(N-isopropylacrylamide) (PNIPAM) and poly(2-isopropyl-2-oxazoline) (PIPOX) has been examined at surfaces to understand the effect of different molecular arrangements in a polymer repeating unit of structural isomers on the construction and pH-stability of hydrogen-bonded multilayers. Films were fabricated using layer-by-layer (LbL) technique through hydrogen-bonding interactions primarily between carbonyl groups of neutral polymers and hydroxyl groups of TA molecules at pH 2. PIPOX and TA formed thinner and more stable films in the pH scale with a critical dissolution pH of 9 when compared to films of PNIPAM and TA with a critical pH of 8. The differences in the thickness and pH-stability were due to different conformational behavior of PNIPAM and PIPOX in water which affects the accessibility of carbonyl groups for participation in the hydrogen bonding and the number of binding sites between the polymer pairs. Addition of electrostatic interactions by introducing amino groups only at the PIPOX chain end shifted the critical dissolution pH to higher values and resulted in gradual dissolution of the films in a wide pH range of 9–12. Such films hold promise for use in biomedical field due to biocompatibility and lower critical solution temperature (LCST) behavior at near physiological temperature of PNIPAM and PIPOX together with the pH-response of the hydrogen-bonded films.

Graphical abstract

LbL films of a polyacid and PIPOX or PNIPAM, two structural isomers, showed difference in pH-stability. Addition of electrostatic interactions only at the chain end shifted the critical dissolution pH to higher values.

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Highlights

► Structurally isomeric hydrogen accepting neutral polymers behaved differently within LbL films. ► Different arrangement of atoms in isomeric polymers resulted in different film thickness and pH-stability of multilayers. ► pH-stability of hydrogen-bonded films can be enhanced by functionalizing only the chain end of the hydrogen accepting polymer.

Introduction

Layer-by-layer (LbL) self-assembled films guided by hydrogen bonding interactions has attracted great attention since it has been first demonstrated by Stockton and Rubner using aqueous polymer solutions [1] and by Wang et al. in the presence of organic solvents [2], [3]. Use of hydrogen bonding as a driving force for LbL assembly has widened the range of polymers that can be used in multilayer film fabrication. Soon after, Sukhishvili and Granick demonstrated the first example of hydrogen-bonded multilayers of water-soluble neutral polymers and polycarboxylic acids [4], [5]. Multilayers of neutral polymers and polyacids are generally prepared at acidic pH when the polyacid is protonated. Immersion of the films to solutions of gradually increasing pH results in disruption of the hydrogen bonds due to ionization of the polyacid. At some point-defined as the “critical dissolution pH” of that particular film-ionization of the polyacid exceeds a critical value and from that point on, the remaining number of binding points between the layers is not sufficient to keep the film intact, thus disintegration of the multilayers occur [6], [7]. Since multilayers can be completely removed from the surface by simply increasing the solution pH, these films are also called “erasable films” [4]. Response at mild pH values made such hydrogen-bonded films of interest for controlled delivery applications.

The pH-stability of hydrogen-bonded multilayers is mostly controlled by the strength of hydrogen bonding which is determined by the chemical nature of the hydrogen accepting and donating polymers and correlates well with the acid dissociation constant of the polyacid (pKa) [5], [6], [8]. Kharlampieva et al. reported that hydrogen-bonded films dissolve at lower pH values when multilayers are composed of weakly associating polymer pairs, such as poly(ethylene oxide) (PEO) or poly(vinyl methyl ether) (PVME) and poly(methacrylic acid) (PMAA), than multilayers of strongly associating polymer pairs, such as poly(N-vinylpyrrolidone) (PVPON) or poly(N-vinylcaprolactam) (PVCL) and PMAA [6], [9]. Significance of the contribution of hydrophobicity of film components to LbL self-assembly has been shown for electrostatically bound films [10], [11] and later was also demonstrated for the growth as well as the pH-stability of hydrogen-bonded multilayers [8], [12]. For example, it has been reported that critical dissolution pH of hydrogen-bonded multilayers can be shifted to higher pH values when PVPON is replaced with its homologue polymer, PVCL, which contains two additional methylene groups resulting in an increase in hydrophobicity [12]. Effect of temperature on the growth of hydrogen-bonded films has been demonstrated by Caruso and co-workers for multilayers of temperature-responsive PNIPAM and poly(acrylic acid) (PAA) [13]. They reported an increase in film thickness when temperature of the assembly solution was close to lower critical solution temperature (LCST) of PNIPAM. Similarly, Sukhishvili and co-workers observed an increase in film thickness with increasing temperature when polymers such as PEO and PVP were deposited even at temperatures far below their LCST [6]. Recently, Zhuk et al. has reported on the effect of temperature on the pH-stability of hydrogen-bonded multilayers. It was shown that critical dissolution pH of hydrogen-bonded films can shift to higher values with increasing temperature at the post-assembly step when polymers exhibiting LCST such as PNIPAM, PVME and PVCL are used as the hydrogen accepting film component [7].

In this study, we investigated the effect of different arrangement of atoms in a polymer repeating unit on the association of hydrogen-bonded polymers at surfaces and the pH stability of the films. Two structurally isomeric polymers, PNIPAM and PIPOX were employed as hydrogen accepting neutral polymers and a polyphenol, TA, as the hydrogen donor to drive the LbL self-assembly (see the chemical structures in Chart 1). Among these two polymers, the nature of phase transition in aqueous PNIPAM solutions and the role of hydrogen bonding have long been investigated by many research groups [14], [15], [16]. PNIPAM has possible applications in the area of biomedical engineering due to its LCST of 32 °C [17] being close to human body temperature. PIPOX, on the other hand, has been considered as a thermosensitive pseudopeptide due to its similar LCST of 36 °C [18]. There are very few reports on the comparison of differences in the behavior of PNIPAM and PIPOX in aqueous solution [19], [20] and to the best of our knowledge, this study is the first reporting the difference in the behavior of PNIPAM and PIPOX within LbL films. We show that the association between PNIPAM and TA is weaker than the association of PIPOX and TA leading to thicker but less stable films in the pH scale. The results correlate with the hydrogen bonding ability between the film components resulting from different chemical structure and conformational behavior of the neutral polymers. Moreover, for a deeper understanding of the effect of interaction strength between the layers on the association, pH-stability, and dissolution profile of the multilayers, we employed PIPOX with primary/secondary amine groups only at the chain end (see Chart 1, PIPOX-amino) and TA for the preparation of hydrogen-bonded multilayers. Increasing pH at the post-assembly step induced the electrostatic interactions between the ionized TA molecules and ammonium groups at the PIPOX-amino chain ends within the multilayers resulting in enhanced pH-stability. To the best of our knowledge, this finding is the first that demonstrates the enhancement in the pH-stability of hydrogen-bonded multilayers via only functionalizing the chain end of the hydrogen accepting polymer. The results reported in this manuscript are important in understanding the effect of molecular arrangement in structural isomers on the physical characteristics of LbL films as well as the role of polymer end-group on the pH-stability of hydrogen-bonded multilayers.

Section snippets

Materials

Branched polyethyleneimine (BPEI, Mw 25,000) was purchased from Sigma–Aldrich Chemical Co. Tannic acid (TA; Mw 1701.2 Da, total percentage of impurity <1%), hydrochloric acid, sodium hydroxide and monobasic sodium phosphate were purchased from Merck Chemicals. All chemicals were used as received. The deionized (DI) water was purified by passage through a Milli-Q system (Millipore).

Synthesis and characterization of PNIPAM

PNIPAM was synthesized by Reversible Addition-Fragmentation Chain Transfer (RAFT) polymerization of NIPAM in dioxane

Hydrogen-bonded PNIPAM/TA and PIPOX/TA multilayers

Amide groups are known to participate in strong hydrogen bonding. The greater electronegativity of the oxygen atom together with an increased electron density on oxygen due to amide resonance makes the oxygen atom more likely to act as a hydrogen acceptor [22]. PNIPAM and PIPOX are structurally isomeric neutral polymers, yet they have significant differences in chemical and physical properties. PNIPAM has secondary amide groups being located on the side chain, whereas PIPOX has tertiary amide

Conclusions

We have studied the effect of structural isomerism in the repeating unit of hydrogen bonding neutral polymers on LbL deposition and pH stability of hydrogen-bonded multilayers. We found that the extent of interpolymer association and the pH stability of the corresponding hydrogen-bonded LbL films changed depending on the arrangement of the amide groups in the repeating units of the neutral polymers. We showed that PIPOX and TA formed thinner and more stable films in the pH scale compared to

Acknowledgments

The authors thank Jiayin Yuan, Anja Gress, and Matthias Meyer for the synthesis of PNIPAM and PIPOX samples, Marlies Gräwert for SEC measurements and Huseyin Enis Karahan for AFM measurements. A.L.D. thanks Turkish Academy of Sciences for financial support.

References (28)

  • N.A. Kotov

    Nanostruct. Mater.

    (1999)
  • H.G. Schild

    Prog. Polym. Sci.

    (1992)
  • W.B. Stockton et al.

    Macromolecules

    (1997)
  • L. Wang et al.

    Macromol. Rapid Commun.

    (1997)
  • L. Wang et al.

    Langmuir

    (2000)
  • S.A. Sukhishvili et al.

    J. Am. Chem. Soc.

    (2000)
  • S.A. Sukhishvili et al.

    Macromolecules

    (2002)
  • E. Kharlampieva et al.

    J. Macromol. Sci., Part C: Polym. Rev.

    (2006)
  • A. Zhuk et al.

    Langmuir

    (2009)
  • I. Erel-Unal et al.

    Macromolecules

    (2008)
  • E. Kharlampieva et al.

    Adv. Mater.

    (2009)
  • S.L. Clark et al.

    Langmuir

    (2000)
  • E. Kharlampieva et al.

    Macromolecules

    (2005)
  • J. Quinn et al.

    Langmuir

    (2004)
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