Elsevier

Polymer

Volume 54, Issue 12, 24 May 2013, Pages 2979-2987
Polymer

Tough interpenetrating Pluronic F127/polyacrylic acid hydrogels

https://doi.org/10.1016/j.polymer.2013.03.066Get rights and content

Abstract

Tough interpenetrating polymer network (IPN) hydrogels with pH- and temperature sensitivity were prepared by crosslinking copolymerization of acrylic acid (AAc) and N,N′-methylenebis(acrylamide) in 20 w/v% aqueous solutions of F127 (PEO99-PPO65-PEO99). The presence of F127 within the gel network slightly decreases the elastic modulus while the loss factor significantly increases, revealing increasing energy dissipation in IPN hydrogels. Cyclic compression tests show large mechanical hysteresis in IPN hydrogels due to the reversible formation of ionic clusters and hydrophobic associations of F127 molecules. The dissipative mechanisms created by F127 lead to the improvement in the mechanical performance of IPN hydrogels when compared to the polyacrylic acid (PAAc) gel controls. PAAc hydrogel formed at 10% AAc fractures under a compression of 0.2 MPa at 78% strain, while the corresponding IPN hydrogel sustains up to 7 MPa compressions at 98% strain, leading to an increase of toughness from 31 to 335 kJ/m3. IPN hydrogels subjected to the heating–cooling cycles between below and above the micellization temperature of F127 show characteristic features of F127 solutions, i.e., increase of the dynamic moduli on raising the temperature, and thermal hysteresis behavior.

Introduction

Hydrophobic interactions play a dominant role in the formation of large biological systems. These interactions can be generated in synthetic polymer systems by incorporation of hydrophobic sequences within the hydrophilic polymer chains. Aqueous solutions of hydrophobically modified hydrophilic polymers constitute a class of soft materials with remarkable rheological properties [1], [2]. Above a certain polymer concentration, the hydrophobic groups in such associative polymers are involved in intermolecular associations that act as reversible breakable crosslinks creating a transient 3D polymer network. Poly(ethylene oxide) – poly(propylene oxide) – poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymers, known under the trade name Pluronics, are typical amphiphilic polymers with associative properties. These copolymers may undergo thermoreversible micellization and gelation in aqueous solutions via associations of hydrophobic poly(propylene oxide) (PPO) blocks [3], [4], [5], [6], [7], [8]. PPO center blocks form the core of Pluronic micelles while the relatively hydrophilic poly(ethylene oxide) (PEO) blocks forming the micelle shells interact with those of neighboring micelles [7]. As the temperature is increased, the number of Pluronic micelles also increases leading to the formation of thermoreversible hydrogels via intermicellar entanglements between PEO segments [9].

In the Pluronic family, the most extensively studied member is F127 (PEO99-PPO65-PEO99) due to its good solubility in water and a high capacity for hydrophobic association through the relatively long PPO block. Above a certain concentration and temperature, aqueous solutions of F127 exist as spherical micelles with an aggregation number of about 50 [9], [10]. The degree of overlap of the micelle shells depends on F127 concentration; at or above 18% F127, the spherical micelles pack onto a simple cubic lattice to form physical gels [9]. Pluronic F127 has attracted much attention as injectable drug delivery systems [11], [12], [13], [14]. The sol state of F127 in aqueous solutions at room temperature facilitates incorporation of bioactive molecules, while the gel state at physiological temperature allows F127 hydrogels to serve as a drug-delivery depot. One limitation of F127 hydrogels is that they are mechanically weak and easily dissolve in physiological environments, which limit their use in load-bearing applications [13]. Several efforts have been made recently to improve the mechanical performance of F127 hydrogels [12], [13], [14], [15]. For example, acrylate-functionalized F127 has been polymerized to obtain chemically crosslinked F127 hydrogels [16]. It was also shown that the crosslinking of F127-diacrylates in the presence of clay nanoparticles produces high-toughness nanocomposite hydrogels [17]. In ethoxysilane-capped Pluronic copolymers, the ethoxysilane groups hydrolyze over time to form silanol groups which covalently crosslink the copolymers [18]. Alternatively, amine-terminated Pluronics can be grafted with hyaluronic acid to form hydrogels with a reduced rate of dissolution due to the hyaluronic acid grafts [19].

In order to prevent dissolution of F127 hydrogels in aqueous media and to improve their mechanical properties, we describe here the preparation of interpenetrating polymer network (IPN) hydrogels composed of F127 and polyacrylic acid (PAAc) network chains. Thus, we apply the concept of double network (DN), first described by Gong and coworkers [20], [21]. Gong's DN hydrogels were synthesized via a two-step sequential free-radical polymerization process, in which a neutral loosely crosslinked second network is incorporated within a swollen, densely crosslinked, polyelectrolyte first network. In comparison to these DN's, our IPN's consist of a physical, neutral F127 first network and a chemically crosslinked, ionic second network. More recently, DN gels with similar components as to those reported here, but with no micellization behavior have been prepared [22], [23]. The design principle of the present IPN hydrogels bases on the fact that polyethers form long-lived macroradicals in the presence of radical initiators [24], [25]. It was shown that the free-radical polymerization of monomers such as acrylic acid (AAc) with the chain transfer to F127 results in grafting of PAAc chains onto the Pluronic backbone [25], [26], [27]. F127-g-PAAc copolymers have unique graft-comb like structure whereby PAAc chains were attached to PPO segments of F127 via C-C bonding [24]. The graft copolymers, as linear chains or, as intramolecularly crosslinked molecules (microgels), are capable of self-assembly in response to temperature changes in aqueous media [25], [26], [28], [29], [30].

To obtain high-toughness IPN hydrogels with pH- and temperature-sensitive properties, we performed the free-radical crosslinking copolymerization of the AAc monomer and N,N′-methylenebis(acrylamide) (BAAm) crosslinker in aqueous 20 w/v% F127 solutions. Gelation reactions were monitored by classical rheometry using oscillatory deformation tests. The complex shear modulus G* measured can be resolved into its real and imaginary components, i.e.,G*=G+iGwhere the elastic modulus G′ is a measure of the reversibly stored deformation energy, and the viscous modulus G″ represents a measure of the irreversibly dissipated energy during one cycle. As will be shown below, the presence of F127 in the gelation solution significantly increases the loss factor tan δ (=G″/G′) indicating increase of the viscous, energy dissipating properties of the chemically crosslinked PAAc hydrogels. This increase in tan δ leads to the improvement in the mechanical performance of the resulting IPN hydrogels, as determined by uniaxial elongation and compression tests. It was also of inherent interest to investigate the viscoelastic properties IPN hydrogels in response to temperature changes between below and above the micellization temperature of F127. The results show that IPN hydrogels exhibit the characteristic features of both PAAc and F127 components.

Section snippets

Materials

Pluronic F127 (PEO99-PPO65-PEO99) was purchased from Sigma–Aldrich and used without further treatment. The nominal molar mass of this copolymer is 12,600, and the weight fraction of PEO is 70%. For the rheological measurements, F127 was dissolved in water at temperatures below 10 °C for 2 h under stirring. Acrylic acid (AAc, Fluka) was freed from its inhibitor by passing through an inhibitor removal column purchased from the Aldrich Chemical Com. N,N′-methylenebis(acrylamide) (BAAm, Merck),

Formation and elasticity of IPN hydrogels

Free-radical crosslinking copolymerization of AAc and BAAm was carried out in 20 w/v% aqueous F127 solutions in the presence of APS-SMS redox initiator system. The crosslinker ratio was fixed at 1/50 while the initial monomer (AAc) concentration was varied between 5 and 30 w/v%. For comparison, hydrogels were also prepared in the absence of F127. In the following, hydrogels formed with and without F127 are called IPN and PAAc hydrogels, respectively. The gelation reactions were first monitored

Conclusions

Free-radical crosslinking copolymerization of AAc and BAAm was carried out in 20 w/v% aqueous F127 solutions in the presence of APS-SMS redox initiator system. The crosslinker ratio was fixed at 1/50 while the initial monomer (AAc) concentration was varied between 5 and 30 w/v%. The presence of F127 in the gelation solution slightly decreases the final elastic modulus of the hydrogels while the loss factor significantly increases (from 10−3 to 10−1), revealing increasing energy dissipation in

Acknowledgment

Work was supported by the Scientific and Technical Research Council of Turkey (TUBITAK), TBAG –109T646. O. O. thanks Turkish Academy of Sciences (TUBA) for the partial support.

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