Jennifer M. Saunders

Doubly crosslinked microgel–polyelectrolyte complexes: three simple methods to tune and improve gel mechanical properties

Doubly crosslinked microgels (DX MGs) are hydrogels composed of covalently interlinked microgels. They are injectable and have potential application in soft tissue repair. Simple methods for tuning their mechanical properties are required. Here, we investigate the effect of added polyelectrolyte (polycations or a polyanion) as well as NaCl on the mechanical properties of the gels for the first time. Addition of polycations improved ductility and decreased the storage modulus (G′) of the DX MG–polyelectrolyte complex (DX MG–PECs) gels. The best DX MG–PEC gel had a yield strain (γc) of 109%, which is the highest reported to date for any DX MG. Furthermore, our DX MG–PEC gels were robust and maintained high G′ values as well as good ductility when swollen at pH = 7.5. The DX MG–PEC gels with improved ductility contained less than 10% polycation. We also investigated the effect of addition of NaCl solution during DX MG and DX MG–PEC preparation. This caused remarkable increases in both G′ and ductility. The best gel had a G′ of 300 kPa which is the highest elasticity reported for a DX MG to date. Addition of linear polyacrylic acid during DX MG preparation also increased the G′ values. This study has provided three new simple methods for tuning the mechanical properties of DX MGs Because the MGs used here belong to a broad class of polymer colloids the results obtained in this study should be generally applicable.

A study of hydrogel composites containing pH-responsive doubly crosslinked microgels

Recently, our group established a new approach for preparing injectable hydrogels using vinyl-functionalised pH-responsive microgel particles [Liu et al., Soft Matter, 2011, 7, 4696]. pH-responsive microgels swell when the pH approaches the pKa of the particles. Liu et al. used inter-particle crosslinking of vinyl-functionalised microgel particles to prepare hydrogels composed of doubly crosslinked microgels (D-microgels). Here, we combine vinyl-functionalised microgels with added, small-molecule, crosslinkers to prepare high modulus D-microgel/hydrogel (H-X) composites for the first time. The vinyl-functionalised microgel particles used were poly(EA/MAA/BDD)/GM; where, EA MAA, BDD and GM are ethyl acrylate and methacrylic acid, 1,4-butanediol diacrylate and glycidyl methacrylate, respectively. Two added crosslinkers were used to demonstrate the versatility of our approach. They were ethyleneglycol dimethacrylate (EGD) and N,N′-methylenebisacrylamide (BA). We compare the data to control hydrogel composites prepared using non-vinyl-functionalised singly crosslinked microgels (S-microgels). All of the composites showed pH-dependent swelling behaviours and mechanical properties. The storage modulus value for the as-made D-microgel/H-EGD composite was 0.12 MPa and is the highest reported to date for a hydrogel containing pH-responsive microgels. The as-made control S-microgel/H-X composites had high ductilities. Dynamic rheology data were used to determine the effects of vinyl functionalisation on the composite mechanical properties. All of the composites exhibited pH-dependent swelling and a “breathing in” transition occurred. The swollen D-microgel/H-X composites retained their high modulus values upon swelling; although, their ductilities decreased. Because we used two different crosslinkers and pH-responsive microgels containing carboxylic acid groups, the method introduced here for preparing high modulus hydrogel composites should be widely applicable.

A study of polypyrrole/poly(N-isopropylacrylamide-co-acrylamide) dispersions: Electrically conducting polymer dispersions stabilised by copolymers with lower critical solution temperatures

Aqueous dispersions have been studied where the particles consist of an electrically conducting polymer (polypyrrole, PPy) and anchored temperature-responsive sheaths. The sheaths consist of poly( N -isopropylacrylamide- co -acrylamide) [poly(NP- x AM) ( x =35, 50 and 65)] copolymer [where x represents the mol% of acrylamide (AM) used during synthesis]. The properties of PPy/PAM (PAM=polyacrylamide) and PPy/PVA [PVA=poly(vinyl alcohol)] dispersions were also investigated for comparison. Photon correlation spectroscopy (PCS) measurements show that the PPy/poly(NP- x AM) dispersions exhibit temperature induced contraction of the sheaths over the temperature range 30–70°C. The extent of contraction increases with decreasing value of x . Dispersion stability in pure water and aqueous NaCl solution was found to be due to electrosteric and steric stabilisation, respectively. Flocculation in the presence of electrolyte (NaCl) was studied using PCS and optical density measurements. PPy/poly(NP-35AM) dispersions in aqueous 0.10 and 1.90 M NaCl solution exhibited upper critical flocculation temperatures (UCFT) of 58 and 30°C, respectively. These values were indistinguishable from the lower critical solution temperature (LCST) for poly(NP-35AM) copolymer measured under identical conditions. Flocculation occurred due to segment–segment attraction of the sheaths under worse than ϑ-solvency conditions. Theoretical calculations indicate that synthesis of stable PPy dispersions requires a minimum sheath thickness (δ) to core radius ( a ) ratio of δ/ a ∽0.30 when ferric chloride is the oxidant. Electrical conductivity measurements of pressed pellets yielded room temperature conductivities in the range 0.0035–0.14 S cm-1; increased levels of AM incorporation within the sheaths increased the conductivity.

Temperature-triggered capture of dispersed particles using a laponite-poly(NIPAM) temperature-responsive surface

In this study a new method is investigated that enables a conductive surface to be modified so as to capture dispersed particles when the temperature is increased. Poly(NIPAM) (NIPAM is N‐isopropylacrylamide) was grafted from electrodeposited Laponite RD particles using surface‐initiated atom transfer radical polymerization (ATRP) to give a temperature‐responsive surface. This was used to capture dispersed polystyrene particles. In the first part of the study the conditions used to electrodeposit Laponite onto a carbon foam electrode were determined. The ability of the temperature‐responsive surface to capture dispersed polystyrene particles was investigated between 20 and 50°C. Temperature‐triggered particle capture was reversible or irreversible depending on the conditions used during ATRP. A high surface concentration of poly(NIPAM) on the particle electrodes is believed to increase the extent of polystyrene particle capture and also reversibility. A theoretical analysis in terms of interaction energy–distance curves is presented for the capture behavior. It is concluded that the temperature‐responsive surface has both electrostatic and steric contributions to the total interaction energy. The steric component (which originates from poly(NIPAM)) is temperature‐dependent and provides the basis for temperature‐triggered particle capture.

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

Most hydrogels are prepared using small-molecule monomers but unfortunately this approach may not be feasible for certain biomaterial applications. Consequently, alternative gel construction strategies have been established, which include using covalent inter-linking of preformed gel particles, or microgels (MGs). For example, covalently interlinking pH-responsive MGs can produce hydrogels comprising doubly crosslinked microgels (DX MGs). We hypothesised that the deformability of such DX MGs was limited by the presence of intra-MG crosslinking. Thus, in this study we designed new nanoparticle (NP)-based gels based on pH-swellable NPs that are not internally crosslinked. Two polyacid NPs were synthesised containing methacrylic acid (MAA) and either ethyl acrylate (EA) or methyl methacrylate (MMA). The PMAA-EA and PMAA-MMA NPs were subsequently vinyl-functionalised using glycidyl methacrylate (GMA) prior to gel formation via free-radical crosslinking. The NPs mostly disassembled on raising the solution pH but some self-crosslinking was nevertheless evident. The gels constructed from the EA- and MMA-based NPs had greater breaking strains than a control DX MG. The effect of varying the solution pH during curing on the morphology and mechanical properties of gels prepared using PMAA-MMA-GMA NPs was studied and both remarkable deformability and excellent recovery were observed. The gels were strongly pH-responsive and had tensile breaking strains of up to 420% with a compressive strain-at-break of more than 93%. An optimised formulation produced the most deformable and stretchable gel yet constructed using NPs or MGs as the only building block.