Volume transition and structure of triethyleneglycol dimethacrylate, ethylenglykol dimethacrylate, and N,N′-methylene bis-acrylamide cross-linked poly(N-isopropyl acrylamide) microgels: a small angle neutron and dynamic light scattering study

doi:10.1016/S0927-7757(01)00821-4

Colloids and Surfaces A: Physicochemical and Engineering Aspects

Volume 197, Issues 1–3, 4 February 2002, Pages 55-67

https://doi.org/10.1016/S0927-7757(01)00821-4 Get rights and content

Abstract

The influence of the cross-linker type on the swelling behaviour and the local structure of colloidal poly(N-isopropyl acrylamide) microgel particles is investigated by dynamic light scattering and small angle neutron scattering (SANS). The cross-linkers ethylenglykol dimethacrylate and triethyleneglycol dimethacrylate are found to form particles with larger swelling/de-swelling capacities compared to N,N′-methylene bis-acrylamide (BIS) cross-linked particles at identical cross-linker concentration. At low temperature the SANS spectra reveal similar values for the blob size, ξ, of the solution like domains in the three different networks. However, the temperature dependent scaling behaviour of ξ differs strongly between the BIS cross-linked and the other two species.

Introduction

Gels consist of a three dimensional polymer network immersed in a solvent and behave like intermediates between solids and liquids. The rather complex mixture of properties of these limiting states makes gels interesting materials, but on the other hand also difficult to understand [1].

In recent years the so-called intelligent hydrogels were intensely studied [2], [3], [4], [5], [6], [7]. They are called intelligent, since they are able to react on external stimuli like changes in temperature [4], [5], [8], [9], pH, or ionic strength [10] by dramatic changes in volume. Predominantly, systems based on poly-N-isopropyl acrylamide (PNIPAM) cross-linked with N,N′-methylene bis-acrylamide (BIS) are investigated. In some of the studies gels based on NIPAM-co-acrylic acid copolymers have been exploited also connected using BIS [8], [11], [12]. Macroscopic gels have one major disadvantage i.e. the long equilibration times needed when an external parameter is changed [3]. For large gels in some cases it may take several days until the equilibrium is reached [2], because the swelling time is proportional to the square of the dimensions of the gel [13].

Therefore, very recently several groups have focused on the investigation of colloidal microgel particles [14], [15], [16], [17], [18], [19], [20], [21], [22], [23]. In these studies a variety of experimental techniques like small angle neutron scattering (SANS), dynamic light scattering (DLS), and real space imaging techniques were used to access the overall size and the swelling behaviour as well as the internal structure and dynamics of these species (For a recent review see Ref. [24]). The use of these particles for studies of the swelling behaviour has the important advantage compared to macroscopic gels, that the equilibration times become very short [13], [23]. Additionally, they are because of the microscopic size, more suitable for applications as drug carriers. Besides their gel properties, they behave like soft-sphere colloids [25] e.g. they form colloidal crystals [18], [25]. These properties may lead to applications in nanocasting and e.g. to the production of photonic bandgap materials [26].

In the present study we address the question whether the length of the cross-linker has a significant influence on the swelling behaviour and the internal structure of microgels. We will present swelling/de-swelling curves for three differently cross-linked microgels using dynamic light scattering (DLS) (Fig. 1). For DLS experiments on microgels the radius of gyration, R_g, of the particles is still smaller then the reciprocal scattering vector q⁻¹ and hence the overall size of the particles can be monitored by this method. Moreover, the internal structure of the microgel particles has been investigated by means of SANS. The q-range which is accessible to SANS is well suited to resolve local structures inside the particles, because with respect to the wavelength of the neutrons R_g≫q⁻¹.

Section snippets

Synthesis of the microgels

The microgel preparation is based on the procedure described by Pelton et al. [27]. We have slightly modified the starter concentrations, which however has an important effect on the particles as will be discussed later. For the synthesis of the particles used here we employed a conventional stirring technique as described elsewhere [28]. One and quarter grams (10.05 mmol) N-isopropyl acrylamide (NIPAM) and the desired amounts of BIS, ethylenglykol dimethacrylate (EGDMA) or triethyleneglycol

Overall size and swelling behaviour

A good tool to get information about the size and the polydispersity of microgel particles is scanning electron microscopy. In Fig. 2 SEM images of the three different investigated particle types are shown. Already from these images the low polydispersity of all prepared samples becomes obvious. To get more quantitative information we used DLS. The size and the polydispersity of the three different microgel types were characterised using DLS. Therefore several intensity correlation functions

Conclusions

TREGDMA and EGDMA microgel particles can be obtained with an even lower polydispersity then BIS cross-linked ones, and under the same preparation conditions BIS particles rest significantly smaller. However, the transition temperature is not influenced significantly by the nature of the cross-linker (see T_c-values in Table 1).

The most striking result of this study is certainly the difference in swelling/de-swelling of the particles in dependence of the cross-linker type. Especially, for

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Volume transition and structure of triethyleneglycol dimethacrylate, ethylenglykol dimethacrylate, and N,N′-methylene bis-acrylamide cross-linked poly(N-isopropyl acrylamide) microgels: a small angle neutron and dynamic light scattering study

Abstract

Introduction

Section snippets

Synthesis of the microgels

Overall size and swelling behaviour

Conclusions

Physica A

Coll. Surf. A

Coll. Surf. A

J. Coll. Interf. Sci.

Adv. Coll. Interf. Sci.

Coll. Surf.

J. Coll. Interf. Sci.

Comput. Phys. Comm.

Computer Physics Comm.

J. Chem. Phys.

J. Chem. Phys.

Phys. Rev. Lett.

J. Chem. Phys.

J. Chem. Phys.

Responsive gels: volume transitions I

Responsive gels: volume transitions II

J. Chem. Phys.

J. Chem. Phys.

Macromolecules

Ber. Bunsenges. Phys. Chem.

J. Chem. Phys.

Polymer

J. Phys. Chem. B

J. Chem. Phys.