Using intra-microgel crosslinking to control the mechanical properties of doubly crosslinked microgels

Abstract

Microgels (MGs) are crosslinked polymer particles that swell when the pH approaches the pKa of the constituent polymer. Our earlier work showed that concentrated MG dispersions can be covalently interlinked to form macroscopic hydrogels, which are termed doubly crosslinked microgels (DX MGs). Here, we study for the first time the effects of intra-MG crosslinking on the swelling of the MGs and the mechanical properties of the DX MGs. The MGs were synthesised by emulsion copolymerisation of ethyl acrylate (EA) or methacrylic acid (MAA) and divinylbenzene (DVB). The latter was a crosslinking monomer. For comparison, MGs were prepared where DVB was replaced by either 1,4-butanediol diacrylate (BDDA) or a 1 : 1 mixture of both DVB and BDDA. The MG swelling behaviours were studied by dynamic light scattering; whereas, the DX MG mechanical properties were studied by dynamic rheology and uniaxial compression measurements. Inclusion of DVB within the MGs resulted in both highly swelling MGs and highly ductile DX MGs. The average strain-at-break value for the DVB-containing DX MGs was 76% which represents the highest value yet reported for a DX MG prepared using commercially available monomers. It was also shown that good tuneability of the DX MG properties could be obtained simply by controlling the DVB and BDDA contents within the MG particles. Analysis of the swelling and compression data enabled relationships between the volume-swelling ratio of the MGs and either the modulus or strain-at-break values for the DX MGs. These relationships also applied to a DVB-free system prepared with a low BDDA content. An interesting conclusion from this study is that the DX MGs can be thought of mechanically as macroscopic MG particles. The results of this study provide design tools for improving DX MG ductility and hence increasing the range of potential applications for this new class of hydrogel.

Synthesis of polyacid nanogels: pH-responsive sub-100 nm particles for functionalisation and fluorescent hydrogel assembly

“…The DX NGs were also more ductile than comparable MMA-or EA-based DX MGs. 37,38 These differences indicate that lower extents of intra-and/or interparticle crosslinking were present within the DX NGs.…”

Design of a Rubbery Carboxymethyl Cellulose/Polyacrylic Acid Hydrogel via Visible-Light-Triggered Polymerization

“…In addition to these two kinds of hydrogels, macromolecular cross‐linked hydrogels have received more and more attentions from researchers due to the special cross‐linking network structure. Macromolecular cross‐linking hydrogels with functional macromolecules as cross‐linker and/or initiator endow hydrogels with an effective energy dissipation mechanism, leading to excellent mechanical performance . There are two main approaches for obtaining such a structure: (i) macromolecules not only as an initiator but also as a cross‐linker, in this case, the activation of the functional macromolecule generally requires a harsh condition where the macromolecule is irradiated with 60 Co γ‐rays, and (ii) macromolecules containing a large number of double bonds act only as a cross‐linker.…”

Highly deformable hydrogels constructed by pH-triggered polyacid nanoparticle disassembly in aqueous dispersions

“…The breaking strain of this new hydrogel is much greater than any value reported for DX MGs. 24,33,34,50,56 Furthermore, Gel-MMA-10 showed excellent recovery as shown by a qualitative deformation test (Fig. 5(c)).…”

“…In our previous work on DX MGs, it was shown that the pore size increased as the gel modulus is reduced. 50 Here, larger pores were obtained at higher pH, suggesting that the gels became less stiff (lower moduli). These images suggest that increasing the preparation pH leads to more deformable gels because gel stiffness is often inversely related to deformability.…”