Elsevier

Polymer

Volume 48, Issue 1, 5 January 2007, Pages 245-254
Polymer

Volume phase transition of “smart” microgels in bulk solution and adsorbed at an interface: A combined AFM, dynamic light, and small angle neutron scattering study

https://doi.org/10.1016/j.polymer.2006.10.026Get rights and content

Abstract

In the present article the swelling behavior of copolymer microgel particles made of poly(N-isopropylacrylamide)-co-vinylacetic acid using dynamic light scattering (DLS), neutron scattering, and in situ atomic force microscopy (AFM) for various copolymerized amounts of vinylacetic acid (VA) (up to 2.5 mol%) under slightly acidic conditions is studied. The transition temperature of these microgel particles is found to be ≈32.5 ± 1 °C, independent of the VA content. Microgel particles adsorbed onto a solid substrate display a similar volume phase transition as their dissolved counterparts. However, their swelling capacity is reduced by approximately one order of magnitude compared to the bulk value. Nevertheless, the observed effect still is sufficiently large to be exploited for the use of these particles in sensors or as nanoactuators. In addition it can be concluded that the continuous character of the transition observed in solution does not arise from the polydispersity of the particles but can be attributed to the heterogeneity inside each individual microgel particle. Finally, AFM images reveal a pattern on the surface of the collapsed particles, which we attribute to globules formed by collapsed dangling polymer chains. In solution these dangling ends form a brush contributing to the hydrodynamic dimensions of the microgels.

Introduction

Since the first report on the preparation of poly(N-isopropylacrylamide) microgels [1], numerous studies discussing different aspects of these interesting functional polymer materials have been published. The most interesting feature in the behavior of N-isopropylacrylamide (NIPAM) based systems certainly is the volume phase transition [2], [3]. This phenomenon was already investigated in some detail in macroscopic gels [4], [5], [6] and also in different microgels [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [44]. A general overview on microgels can be found in Refs. [17], [18], or in Ref. [19] with a focus on scattering methods. In macroscopic gels, the volume phase transition can be very slow and in some cases it takes several days until the equilibrium state is reached [20]. Microgels react faster upon changes in temperature, ionic strength, solvent quality, or pH. Therefore, they are well suited to investigate the volume transition. Moreover, due to their colloidal character [21] they are interesting model systems to study processes like formation of mesoscopic crystals [22], [23], [24], [25], [26]. Maybe their most visionary potential lies in the possible construction of actuators on the nanometer scale, which are driven by physico-chemical processes like swelling.

In the present study, we describe the properties of poly(N-isopropylacrylamide)-co-vinylacetic acid (PNIPAM-co-VA) microgels with different contents of vinylacetic acid. The local structure of the polymer network is studied by means of small angle neutron scattering (SANS). The swelling behavior is investigated by dynamic light scattering (DLS) and moreover by tapping mode atomic force microscopy (AFM). DLS allows to obtain the average degree of swelling in bulk solution, whereas AFM is used to investigate the transition of individual microgel particles attached to an interface. AFM is an excellent tool for this approach.

For applications of such attached particles as sensors or for actuators it is of course necessary to conserve reversibility of the transition. Linear PNIPAM-copolymer chains adsorbed at an interface and in the thin polyelectrolyte multilayers were found to collapse irreversibly [27]. The present study was motivated by the idea that microgels due to their shape stability might conserve reversibility of the transition also in the adsorbed state.

The volume transition of PNIPAM microgels as observed in bulk solution is usually found to be continuous. However, in solution usually the ensemble averaged size of the microgels is followed and therefore it is not clear whether the continuous character of the volume phase transition arises from the polydispersity of the particles (only apparently continuous) or is a property of a single PNIPAM microgel. The present work also aims to answer this question.

Section snippets

Microgel synthesis and characterization

N-isopropylacrylamide (NIPAM; synthesis grade, purity 97%), N,N′-methylene bis-acrylamide (BIS; synthesis grade, purity 99%), vinylacetic acid (VA; synthesis grade), and potassium persulfate (KPS; purity 99%) were obtained from Sigma–Aldrich. In contrast to macroscopic gels it was shown previously that for microgels re-crystallization of the chemicals does not lead to significantly different particle properties [28] and hence, all chemicals were used without further purification. The microgel

DLS

The swelling behavior of the four different microgels was investigated by means of DLS. The microgels are rather large. Therefore, the angular dependence of the decay of the correlation functions was studied first. According to Γ = DTq2 a plot of Γ vs. q2 should be linear. In Supplementary data the data for the sample VA1 are given.

The linear fit does not go through zero. This can be attributed to growing intra-particle interference at larger scattering angles. However, the effect is small and of

Conclusions

The combination of DLS, SANS, and AFM gives a detailed view at various aspects of the swelling behavior of PNIPAM-co-vinylacetic acid microgel particles.

All experiments indicate that the volume phase transition temperature is not influenced by the VA content of the particles and is located between 32 °C and 34 °C.

The AFM experiments clearly show that the volume phase transition of the PNIPAM-co-VA microgels is still reversible for adsorbed microgels, but the swelling capacity (given as α)

Acknowledgments

We are grateful to Wim Pyckhout-Hintzen for help with the SANS experiments and to the FZ Jülich for providing the beamtime. L.Z. and F.M. acknowledge funding from the Sonderforschungsbereich 569 ‘Hierarchic Structure Formation and Function of Organic–Inorganic Nano Systems’.

T.H. acknowledges funding by the EUROCORES program within the project Higher levels of self-assembly of ionic amphiphilic block-copolymers (SONS-AMPHI).

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