Zhengxing Cui

A Study of Physical and Covalent Hydrogels Containing pH-Responsive Microgel Particles and Graphene Oxide

In this study we mixed low concentrations of graphene oxide (GO) with microgel (MG) particles and formed composite doubly cross-linked microgels (DX MG/GO) gels. The MG particles comprised poly(ethyl acrylate-co-methacrylic acid-co-1,4-butanediol diacrylate) with pendant glycidyl methacrylate units. The MG/GO mixed dispersions formed physical gels of singly cross-linked MGs (termed SX MG/GO), which were subsequently heated to produce DX MG/GO gels by free-radical reaction. The influence of the GO concentration on the mechanical properties of the SX MG/GO and DX MG/GO gels was investigated using dynamic rheology and static compression measurements. The SX MG/GO physical gels were injectable and moldable. The moduli for the DX MG/GO gels increased by a factor of 4-6 when only ca. 1.0 wt % of GO was included. The isostrain model was used to describe the variation of modulus with DX MG/GO composition. Inclusion of GO dramatically altered the stress dissipation and yielding mechanisms for the gels. GO acted as a high surface area, high modulus filler and played an increasing role in load distribution as the GO concentration increased. It is proposed that MG domains were dispersed within a percolated GO network. Comparison of the modulus data with those published for GO-free DX MGs showed that inclusion of GO provided an unprecedented rate of modulus increase with network volume fraction for this family of colloid gels. Furthermore, the DX MG/GO gels were biocompatible and the results imply that there may be future applications of these new systems as injectable load supporting gels for soft tissue repair.

Pickering Emulsions Stabilized by pH-Responsive Microgels and Their Scalable Transformation to Robust Submicrometer Colloidoisomes with Selective Permeability

Colloidosomes are micrometer-sized hollow particles that have shells consisting of coagulated or fused colloid particles. While many large colloidosomes with sizes well above 1.0 μm have been prepared, there are fewer examples of submicrometer colloidosomes. Here, we establish a simple emulsion templating-based method for the preparation of robust submicrometer pH-responsive microgel colloidosomes. The colloidosomes are constructed from microgel particles based on ethyl acrylate and methacrylic acid with peripheral vinyl groups. The pH-responsive microgels acted as both a Pickering emulsion stabilizer and macro-cross-linker. The emulsion formation studies showed that the minimum droplet diameter was reached when the microgel particles were partially swollen. Microgel colloidosomes were prepared by covalently interlinking the microgels adsorbed at the oil-water interface using thermal free-radical coupling. The colloidosomes were prepared using a standard high-shear mixer with two different rotor sizes that corresponded to high shear (HS) and very high shear (VHS) mixing conditions. The latter enabled the construction of submicrometer pH-responsive microgel-colloidosomes on the gram scale. The colloidosomes swelled strongly when the pH increased to above 6.0. The colloidosomes were robust and showed no evidence of colloidosome breakup at high pH. The effect of solute size on shell permeation was studied using a range of FITC-dextran polymers, and size-selective permeation occurred. The average pore size of the VHS microgel-colloidosomes was estimated to be between 6.6 and 9.0 nm at pH 6.2. The microgel-colloidosome properties suggest that they have the potential for future applications in cosmetics, photonics, and delivery.

Hydrogel Composites Containing Sacrificial Collapsed Hollow Particles as Dual Action pH-Responsive Biomaterials

In this study hydrogel composites are investigated that contain sacrificial pH-responsive collapsed hollow particles (CHPs) entrapped within a poly(acrylamide) (PAAm) network. The CHPs were prepared using a scalable (mainly) water-based method and had a bowl-like morphology that was comparable to that of red blood cells. The CHPs were constructed from poly(methyl methacrylate-co-methacrylic acid), which is a pH-responsive copolymer. The PAAm/CHP composite morphology was probed with optical microscopy, CLSM and SEM. These data showed the CHPs were dispersed throughout the PAAm network. Inclusion of the CHPs within the gel composites increased the modulus in a tunable manner. The CHPs fragmented at pH values greater than the pKa of the particles, and this process decreased the gel modulus to values similar to that of the parent PAAm hydrogel. CHPs containing a model drug were used to demonstrate pH-triggered release from PAAm/CHP and the release kinetics obeyed Fickian diffusion. The composite gels had low cytotoxicity as evidenced by Live/Dead and MTT assays. The hydrogel composites showed dual action pH-triggered softening with simultaneous drug release which occurred without a volume increase. The hydrogel composites may have potential application as enteric gels or for intra-articular drug delivery.

Controlled aggregation of quantum dot dispersions by added amine bilinkers and effects on hybrid polymer film properties

Blends of conjugated polymers with semiconducting nanocrystals (NCs) have attracted much interest as photoactive layers for hybrid polymer solar cells. However, uncontrolled aggregation of the NCs within the hybrid films remains a major processing challenge that adversely affects hybrid film properties and device performance. Here, we deliberately triggered the aggregation of dispersed NCs prior to hybrid polymer film formation and studied aggregation effects on film morphology, thickness and absorbance. The aggregation of parent ZnO and PbS dispersions was triggered by addition of 1,2-ethylenediamine. The latter bilinker is more user-friendly than 1,2-ethanedithiol, which is widely used for NC-based solar cells. The extent of NC aggregation was controlled in this study by the bilinker concentration and mixing time prior to spin coating. The effects of ZnO aggregation on poly(3-hexylthiophene) (P3HT)/ZnO hybrid film morphologies were studied using AFM and SEM. The light absorption and emission properties were probed using UV-visible and photoluminescence spectroscopy. We found that P3HT/ZnO film thickness and absorbance both increased as ZnO aggregation became more pronounced. The results of this study imply that NC aggregation during hybrid polymer solar cell construction may increase light absorption due to increased film thickness. The implications of these effects for hybrid solar cell device optimisation studies are discussed.

Composite hydrogels of polyacrylamide and crosslinked pH-responsive micrometer-sized hollow particles

Whilst hydrogels and hollow particles both continue to attract much attention in the literature there are few examples of hydrogel composites containing hollow particles. Here, we study composite polyacrylamide (PAAm) hydrogels containing micrometer-sized pH-responsive shell-crosslinked hollow particles (abbreviated as HPXL) based on poly(methylmethacrylate-co-methacrylic acid) functionalised with glycidyl methacrylate (GMA). The HPXL particles were prepared using our scaleable emulsion template method and inclusion of GMA was found to promote spherical hollow particle formation. The pendant vinyl groups from GMA enabled shell-crosslinked hollow particles to be prepared prior to formation of the PAAm/HPXL composite gels. The morphologies of the particles and composite gels were studied by optical microscopy, confocal laser scanning microscopy and scanning electron microscopy. Dynamic rheology measurements for the composite gels showed that the modulus variation with HPXL concentration could be described by a percolation model with a HPXL percolation threshold concentration of 4.4 wt% and a scaling exponent of 2.6. The composite gels were pH-responsive and largely maintained their mechanical properties over the pH range 4.0 to 8.0. Because the composite gels had tuneable mechanical properties (with modulus values up to 530 kPa) and were pH-responsive they are potential candidates for future wound healing or membrane applications.