Nadia Adrus

Novel hydrogel pore-filled composite membranes with tunable and temperature-responsive size-selectivity

Hydrogel pore-filled composite membranes (HPFCM) based on polyethylene terephthalate (PET) track-etched membranes with pore diameters between 200 and 5000 nm and temperature-responsive poly(N-isopropylacrylamide) (PNIPAAm) hydrogels were successfully prepared. A premodification of the pore walls by grafted linear PNIPAAm led to stable anchoring of crosslinked PNIPAAm prepared in a subsequent step. Proper tuning of photopolymerization conditions resulted in a desired microstructure of the hydrogels and thus tailored barrier properties of the composite membranes. The very interesting separation performance of HPFCM was due to diversification of the hydrogel network that caused adjustable sieving properties via synthesis conditions and also largely switchable barrier properties in response to the temperature. The interplay between the immobilized hydrogel and various pore sizes of the membrane support was also investigated. The base membrane provides mechanical support and confines the hydrogel within its pores, and it thus allows using the hydrogel mesh size for size-selective solute transport. Completely stable and selective HPFCM were only obtained with base pore sizes of about 2 μm or smaller. The size-selectivity (molecular weight cut-off) of the same HPFCM was higher under diffusive than under convective flow conditions; this is presumably mainly caused by elasticity deformation of the hydrogel network. The apparent cut-off from diffusion experiments was well correlated to the mesh-size of the hydrogel determined from the Darcy model applied to permeability data obtained under convective flow conditions. Upon temperature increase beyond 32 °C, flux increased and rejection decreased very strongly; this remarkable change between macromolecule-size selective ultrafiltration and microfiltration/filtration behavior was fully reversible.

New UV LED curing approach for polyacrylamide and poly(N-isopropylacrylamide) hydrogels

When a UV LED was used, the energy generated from its light source triggered photopolymerization to directly convert acrylamide and N-isopropylacrylamide monomers to polyacrylamide and poly(N-isopropylacrylamide) hydrogels, respectively. As compared to UV mercury lamps, a UV LED has more concentrated energy at its monochromatic wavelength (i.e. 365 nm), which can offer more efficient photopolymerization. In this study, a feasible photoinitiator was synthesised in parallel with the development of a UV LED water based hydrogel curing system. This is because the commercially available water soluble photoinitiator has no overlap in emission with the absorption wavelength of the UV LED. The water soluble photoinitiator (WSPI) was obtained from complexation of 2,2-dimethoxy-2-phenyl acetophenone and methylated-β-cyclodextrin. The results presented in this work suggested that WSPI was an excellent choice of photoinitiator for the UV LED system to achieve hydrogels with high monomer conversion (>90%). These findings give a promising alternative for hydrogel curing in various applications, including contact lenses and dental materials.

Development of UV LED Hydrogel Formulation Based on Polyacrylamide Hydrogel

Recent trends show that the UV LED light source can greatly reduce environmental effect without compromising performance as compared to conventional UV mercury-based lamp (UV Hg) system. In this study, the possibility of using UV LED technology for photopolymerization of polyacrylamide (PAAm) hydrogels is presented. This has strongly drives the needs for suitable water-soluble photoinitiator formulation. Specifically, the photoinitiator must possess characteristic wavelength within UV LED range as UV LED emits monochromatic light sources only (365 nm or 385 nm). Here, the commercially available photoinitiator Oligo [2-hydroxy-2-methyl-1-[4-(1-methylvinyl) phenyl] propanone] (Chivacure 300) was chosen. However, Chivacure 300 has limited solubility in water. Hence, addition of tetrahydrofuran (THF) at various ratios was required. The results demonstrated that 9.5:0.5 ratio of water/THF was the minimum ratio needed for the solubility of Chivacure 300. After the synthesis of PAAm hydrogels, almost complete conversion of hydrogels was achieved (> 80 %). The highest conversion was achieved with formulation of Chivacure 300 in 9.5:0.5 ratio of water/THF. At this ratio, the hydrogels obtained were transparent. With increasing ratio of THF in water, the appearance of hydrogels gradually changed to cloudy. Indeed, UV LED technology can be used to polymerize PAAm hydrogels with comparably high conversion to conventional UV Hg system.

Pineapple Leaf Fibers Coated with Polyacrylamide Hydrogel

Pineapple leaf fibers (PALF) have several advantages such as low cost, eco-friendly, and high specific strength. However, the brittleness of PALF limits its application. To overcome this limitation of PALF, it is essential to synergize the advantages of PALF with elastic properties of hydrogel. In this study, PALF was coated with polyacrylamide (PAAm) hydrogel under direct UV light exposure (UVA>300nm). Prior to this coating, PALF was alkali treated to introduce more OH group on PALF fiber. The main purpose of this study was to investigate the effect of untreated/treated PALF coated PAAm hydrogel on the flexibility of the fiber using tensile measurements. From the results, treated PALF coated PAAm hydrogel showed better results in tensile properties compared to untreated PALF due to the alkali treatment which improved the interfacial adhesion between PAAm hydrogel and fiber surface. In general, this study is precursor for further development in natural fiber coating technology.

Mechanical Properties of the ‘Stretchable’ Polyacrylamide-Gelatin Double Network Hydrogel

Double network (DN) hydrogels have drawn considerable attention as innovative materials possessing both high water content as well as improved mechanical properties. In this study, DN hydrogels were formed from a combination of two hydrogel networks. The first network composed of acrylamide (AAm) and N’,N’-methylenebisacrylamide (MBAAm). AAm and MBAAm were covalently crosslinked via photopolymerization simultaneously with/without the presence of the second network pre-gel mixture; physically crosslinked gelatin-calcium carbonate (GCa). The mechanical properties characterization of the hydrogels revealed that tensile strength, Young’s modulus and elongation at break increased with the increasing amount of second network component; i.e. GCa. These data could confirmed that the polyacrylamide (PAAm)-GCa DN hydrogels possessed ‘stretchability’ character. Overall, PAAm-GCa DN hydrogels had shown better mechanical strength than the PAAm single network hydrogels. We foreseen that DN hydrogels are highly potential to be developed as artificial muscles.