Elsevier

Polymer

Volume 49, Issue 10, 13 May 2008, Pages 2595-2603
Polymer

Graft-type poly(N-isopropylacrylamide-co-acrylic acid) microgels exhibiting rapid thermo- and pH-responsive properties

https://doi.org/10.1016/j.polymer.2008.03.040Get rights and content

Abstract

A novel graft-type poly(N-isopropylacrylamide-co-acrylic acid) (poly(NIPAM-co-AAc)) microgel with linear grafted poly(NIPAM-co-AAc) side chains was prepared by incorporating the dual stimuli-responsive chains into conventional copolymerized poly(NIPAM-co-AAc) backbones as side chains through a grafted modification of molecular structure. The grafted chains with unfettered ends can unfold and shrink freely in response to environmental stimuli. Due to the mobile nature and the tractive force of the grafted poly(NIPAM-co-AAc) side chains, the prepared microgels were verified to have a synchronously rapid thermo- and pH-response property. When the environmental temperature was increased suddenly from 25 °C to about 63 °C, the graft-type microgels in ultrapure water deswelled 95% by volume at best within 40 s but the normal-type microgels deswelled only 17% by volume within the same period. The analogous trends were observed in either alkaline or acid buffered solution. The graft-type microgels may thus be very promising for use in many applications in which rapid responses to dual environmental stimuli are required.

Introduction

Because of their unique volume phase-transition properties in response to environmental changes, stimuli-sensitive hydrogels have attracted much attention in the past 20 years [1], [2], [3]. These stimuli-sensitive “intelligent” hydrogels are considered to have potential for application in numerous fields, including drug delivery, chemical separations, chemical sensors, enzyme and cell immobilization [4], [5], [6], [7], [8], [9], [10], [11]. Poly(N-isopropylacrylamide) (PNIPAM) is a widely investigated thermo-sensitive polymer that undergoes a volume phase transition around the lower critical solution temperature (LCST, approximately 32 °C) [12], [13]. Early studies have mainly been limited to the macroscopic hydrogels that need a long time to reach the swelling equilibrium. However, for a lot of potential applications, a fast response is necessary for their practical usage. Tanaka et al. have demonstrated that the responsive time is approximately in proportion to the square of a linear dimension of the gel [14], [15]. Moreover, the size of the gel needs to be small enough to exert effects in many particular regions, especially the drug delivery systems. Thereupon, microgels with much faster volume change than macroscopic hydrogels with the same chemical structure, have recently attracted increasing attention and many significant results have been obtained [16], [17], [18].

However, to broaden the functions of microgels in the aforementioned applications, it is desirable to incorporate other functional groups within the gel matrix. Along these lines, microgels with temperature- and pH-sensitivity have been commonly investigated in more recent works, because both parameters are important environmental factors in biomedical and other systems [19], [20], [21], [22], [23], [24], [25]. Usually, pH-ionizable monomer with carboxylic acid groups is incorporated into PNIPAM-based gel networks by randomly copolymerizing to provide a pH-sensitive property for the thermo-responsive gels [26], [27], [28].

Nevertheless, the randomly introduced ionic groups will also result in an adverse effect. Impregnating microgels with them would reduce or even eliminate the thermo-sensitivity due to the increase in the hydrophilicity and the break in the continuous thermo-sensitive isopropylamide pendant groups of PNIPAM [29], [30], [31]. In some cases, the strong electrostatic repulsion will even smear out the temperature-induced transition [23]. These effects greatly limit the potential application of such hydrogels. For instance, an acting actuator requires an instantaneous feedback after receiving signals.

In order to achieve improved dual stimuli-sensitive microgels with much faster response properties using conventional synthesis techniques, we evolved a novel strategy through a grafted modification of molecular structure which had been put forward by Chen and Hoffman [32] and Yoshida et al. [33]. This technique has been developed in many aspects [34], [35], [36], [37], [38], [39], [40], [41]. Based on a fixed total content of sensitive units that was same in normal copolymerizing method, this strategy modified their locations in gel structure only. Utilizing this methodology, we have synthesized a macroscopic hydrogel with rapid thermo- and pH-responsive phase-transition rate [42]. Unlike introducing the single thermo-sensitive PNIPAM grafted chains into hydrogel networks in our previous work [42], we prepared a dual thermo- and pH-sensitive macromonomer as the grafted chain with freely mobile ends in this study for the first time, and then incorporated them into the dual thermo- and pH-sensitive polymer backbone networks simply via free radical copolymerization. The novel microgel has potential for application in variety of fields, such as new drug carriers, and sensors. The effects of the novel chemical structure on the swelling and deswelling properties of microgels were investigated systematically by changing the environmental temperature and pH conditions.

Section snippets

Materials

N-Isopropylacrylamide (NIPAM; Kohjin Co.) was recrystallized from a mixture of acetone and n-hexane. Acrylic acid (AAc; Tianjin Bodi Chemical Engineering Co., Ltd.) was purified by vacuum distillation at 40 °C and 10 mm Hg. Tetrahydrofuran (THF; Chongqing Chuandong Chemical Engineering Co., Ltd.), 2-hydroxyethanethiol (HESH; Sanland-chem International Inc.), benzoyl peroxide (BPO; Tianjin Jingxing Chemical Reagents), diethyl ether (Tianjin 1st Chemical Reagents), acetone (Tianjin 1st Chemical

Synthesis and characterization of poly(NIPAM-co-AAc) macromonomer

The poly(NIPAM-co-AAc) macromonomer was prepared by radical telomerization of NIPAM and AAc monomers using HESH as a chain transfer agent. A spectrum of the macromonomer obtained with 1H NMR spectroscopy measurements exhibited peaks at 1.1 ppm (–CH3) and 3.9 ppm (–CH–), while two broad peaks at 1.6 and 2.0 ppm due to methylene proton and methyne proton on the main chains were observed. Significantly, the peaks of vinyl proton at 5.9–6.5 ppm were detected, indicating that a polymerizable end group

Conclusions

Graft-type poly(NIPAM-co-AAc) microgels with dual stimuli-responsive grafted chains were successfully prepared. The results showed that the temperature dependence of the swelling degree of the microgels in ultrapure water is dominated by the quantity of thermo-sensitive and pH-sensitive components, regardless of its location in the microgel. The grafted chains in the polymeric networks inside the microgels responding to both temperature and pH stimuli could unfold and shrink freely so as to

Acknowledgments

The National Natural Science Foundation of China (20674054), the Key Project of the Ministry of Education of China (106131) and the Specialized Research Fund for the Doctoral Program of Higher Education by the Ministry of Education of China (20040610042) are acknowledged for supporting this research. The authors are grateful to the Kohjin Co., Ltd., Japan, for kindly supplying the N-isopropylacrylamide.

References (46)

  • A.S. Hoffman

    Adv Drug Deliv Rev

    (2002)
  • H. Kasgoz et al.

    Polymer

    (2003)
  • Y. Matsumura et al.

    Polymer

    (2005)
  • B.H. Tan et al.

    Polymer

    (2007)
  • C.L. Zhang et al.

    Polymer

    (2007)
  • X.H. Zhang et al.

    Polymer

    (2007)
  • D. Peng et al.

    Polymer

    (2007)
  • D. Peng et al.

    Polymer

    (2006)
  • D. Neugebauer

    Polymer

    (2007)
  • Z.Y. Li et al.

    Polymer

    (2006)
  • N. Kutsevol et al.

    Polymer

    (2006)
  • M. Ballauff et al.

    Polymer

    (2007)
  • J. Zhang et al.

    Polymer

    (2007)
  • Y. Liu et al.

    Polymer

    (1999)
  • Y. Wang et al.

    Polymer

    (2004)
  • G. Bokias et al.

    Polymer

    (2000)
  • Y. Hirokawa et al.

    Chem Phys

    (1984)
  • T. Tanaka et al.

    Phys Rev Lett

    (1985)
  • J.H. Holtz et al.

    Nature

    (1997)
  • F. Sauzedde et al.

    Colloid Polym Sci

    (1999)
  • H. Kawaguchi et al.

    Bioseparation

    (1998)
  • H. Suzuki

    J Intel Mat Syst Str

    (2006)
  • I.C. Kwon et al.

    Nature

    (1991)
  • Cited by (0)

    View full text