David K. Wang

Synthesis and characterization of a POSS-PEG macromonomer and POSS-PEG-PLA hydrogels for periodontal applications.

A novel water-soluble macromonomer based on octavinyl silsesquioxane has been synthesized and contains vinyl-terminated PEG 400 in each of the eight arms to promote water solubility. The macromonomer was characterized by NMR and FTIR and its aqueous solution properties examined. In water it exhibits an LCST with a cloud point at 23 °C for a 10 wt % aqueous solution. It is surface active with a CMC of 1.5 × 10(-5) M in water and in 20:80 v/v acetone/water the CMC is 7.1 × 10(-5) M, and TEM images showed spherical 22 nm aggregates in aqueous solution above the CMC. The macromonomer was copolymerized in a 20:80 v/v acetone/water mixture with a vinyl-terminated, triblock copolymer of lactide-PEG-lactide to form a library of cross-linked hydrogels that were designed for use as scaffolds for alveolar bone repair. The cross-linked copolymer networks were shown to contain a range of nm-μm sized pores and their swelling properties in water and PBS at pH 7.4 were examined. At pH 7.4 the hydrogel networks undergo a slow hydrolysis with the release of principally PEG and lactic acid fragments. The hydrogels were shown to be noncytotoxic toward fibroblast cultures at pH 7.4, both initially (days 1-5) and after significant hydrolysis had taken place (days 23-28).

Micromechanics and Poroelasticity of Hydrated Cellulose Networks

The micromechanics of cellulose hydrogels have been investigated using a new rheological experimental approach, combined with simulation using a poroelastic constitutive model. A series of mechanical compression steps at different strain rates were performed as a function of cellulose hydrogel thickness, combined with small amplitude oscillatory shear after each step to monitor the viscoelasticity of the sample. During compression, bacterial cellulose hydrogels behaved as anisotropic materials with near zero Poissons ratio. The micromechanics of the hydrogels altered with each compression as water was squeezed out of the structure, and microstructural changes were strain rate-dependent, with increased densification of the cellulose network and increased cellulose fiber aggregation observed for slower compressive strain rates. A transversely isotropic poroelastic model was used to explain the observed micromechanical behavior, showing that the mechanical properties of cellulose networks in aqueous environments are mainly controlled by the rate of water movement within the structure.

Reversible Redox Effect on Gas Permeation of Cobalt Doped Ethoxy Polysiloxane (ES40) Membranes

This work reports the remarkable effect of reversible gas molecular sieving for high temperature gas separation from cobalt doped ethoxy polysiloxane (CoES40) membranes. This effect stemmed from alternating the reducing and oxidising (redox) state of the cobalt particles embedded in the ES40 matrix. The reduced membranes gave the best H2 permeances of 1 × 10−6 mol m−2 s−1 Pa−1 and H2/N2 permselectivities of 65. The reduction process tailored a molecular gap attributed to changes in the specific volume between the reduced cobalt (Co(OH)2 and CoO) particles in the ES40 structure, thus allowing for the increased diffusion of gases. Upon re-oxidation, the tailored molecular gap became constricted as the particles reversed to Co3O4 resulting a lower gas diffusion, particularly for the larger gases ie. CO2 and N2. The ES40 matrix proved to be structurally rigid enough to withstand the reversible redox effect of cobalt particles across multiple cycles.

Performance and Long Term Stability of Mesoporous Silica Membranes for Desalination

This work shows the preparation of silica membranes by a two-step sol-gel method using tetraethyl orthosilicate in ethanolic solution by employing nitric acid and ammonia as co-catalysts. The sols prepared in pH 6 resulted in the lowest concentration of silanol (Si–OH) species to improve hydrostability and the optimized conditions for film coating. The membrane was tested to desalinate 0.3–15 wt % synthetic sodium chloride (NaCl) solutions at a feed temperature of 22 °C followed by long term membrane performance of up to 250 h in 3.5 wt % NaCl solution. Results show that the water flux (and salt rejection) decrease with increasing salt concentration delivering an average value of 9.5 kg m–2 h–1 (99.6%) and 1.55 kg m–2 h–1 (89.2%) from the 0.3 and 15 wt % saline feed solutions, respectively. Furthermore, the permeate salt concentration was measured to be less than 600 ppm for testing conditions up to 5 wt % saline feed solutions, achieving below the recommended standard for potable water. Long term stability shows that the membrane performance in water flux was stable for up to 150 h, and slightly reduced from thereon, possibly due to the blockage of large hydrated ions in the micropore constrictions of the silica matrix. However, the integrity of the silica matrix was not affected by the long term testing as excellent salt rejection of >99% was maintained for over 250 h.

Generalized T distribution and its applications to process data reconciliation and process monitoring

Process data are conventionally characterized by normal distribution and techniques based on this assumption could suffer performance and efficiency losses when the assumption is violated. In this paper, the generalized T distribution is introduced and its robustness characteristics are investigated. Using this probability density function to characterize the process data, it is shown that both efficiency as well as robustness of some of the techniques currently employed in process systems engineering can be improved. Performance is illustrated by its applications to process data reconciliation and process fault detection of chemical engineering case studies.

Mixed Matrix Carbon Molecular Sieve and Alumina (CMS-Al2O3) Membranes

This work shows mixed matrix inorganic membranes prepared by the vacuum-assisted impregnation method, where phenolic resin precursors filled the pore of α-alumina substrates. Upon carbonisation, the phenolic resin decomposed into several fragments derived from the backbone of the resin matrix. The final stages of decomposition (>650 °C) led to a formation of carbon molecular sieve (CMS) structures, reaching the lowest average pore sizes of ~5 Å at carbonisation temperatures of 700 °C. The combination of vacuum-assisted impregnation and carbonisation led to the formation of mixed matrix of CMS and α-alumina particles (CMS-Al2O3) in a single membrane. These membranes were tested for pervaporative desalination and gave very high water fluxes of up to 25 kg m−2 h−1 for seawater (NaCl 3.5 wt%) at 75 °C. Salt rejection was also very high varying between 93–99% depending on temperature and feed salt concentration. Interestingly, the water fluxes remained almost constant and were not affected as feed salt concentration increased from 0.3, 1 and 3.5 wt%.

Ternary Phase-Separation Investigation of Sol-Gel Derived Silica from Ethyl Silicate 40

A ternary phase-separation investigation of the ethyl silicate 40 (ES40) sol-gel process was conducted using ethanol and water as the solvent and hydrolysing agent, respectively. This oligomeric silica precursor underwent various degrees of phase separation behaviour in solution during the sol-gel reactions as a function of temperature and H2O/Si ratios. The solution composition within the immiscible region of the ES40 phase-separated system shows that the hydrolysis and condensation reactions decreased with decreasing reaction temperature. A mesoporous structure was obtained at low temperature due to weak drying forces from slow solvent evaporation on one hand and formation of unreacted ES40 cages in the other, which reduced network shrinkage and produced larger pores. This was attributed to the concentration of the reactive sites around the phase-separated interface, which enhanced the condensation and crosslinking. Contrary to dense silica structures obtained from sol-gel reactions in the miscible region, higher microporosity was produced via a phase-separated sol-gel system by using high H2O/Si ratios. This tailoring process facilitated further condensation reactions and crosslinking of silica chains, which coupled with stiffening of the network, made it more resistant to compression and densification.

Physicochemical characterisation and hydrothermal stability investigation of cobalt-incorporated silica xerogels

The hydrothermal stability of the cobalt oxide silica xerogels was comprehensively investigated, including the effect of Co/Si molar ratio (0.00–0.50), vapour content (0–75 mol%), exposure time (0–100 h) and temperature (250–550 °C). Physicochemical properties of the xerogels were characterised by nitrogen sorption, FTIR, solid-state 29Si NMR (CP/MAS), micro-Raman, XRD and HR-TEM techniques. The structural characterisation indicated that increasing cobalt incorporation inhibited the degree of condensation in the silica network, and that the formation of tricobalt tetroxide (Co3O4) nanocrystals in the silica matrix was only observed in high cobalt loading samples (Co/Si ≥ 0.25). The hydrothermal stability of the xerogels assessed by N2 sorption was found to be strongly dependent on the cobalt loading; particularly when the presence of Co3O4 in the silica matrices was implicated. For the unstable xerogels (Co/Si < 0.25), the materials stability was significantly decreased by both vapour content and exposure time, resulting in an almost 90% surface area reduction. On the other hand, the high cobalt loading xerogels (Co/Si ≥ 0.25) were found to contain Co3O4 and were much more stable, losing less than 25% of surface area and maintaining microporous structure after exposing to a harsh condition of 75 mol% vapour at 550 °C for 40 h. A structural model is proposed whereby the cobalt oxide particles ‘shield’ the silica matrix and inhibit the hydrolysis and condensation of the silica in the pores walls. This effectively limits the structural rearrangement that hydrothermal treatment typically invokes and therefore confers improved hydrothermal stability.

Robust model predictive control design using a generalized objective function

An alternative robust model predictive control (MPC) design method is proposed. This approach is based on the use of a generalized objective function. It exploits the similarity between the parameter estimation problem and MPC. Robustness is achieved by choosing the objective function according to the error probability density function, as in robust system identification. The stability of the controller is demonstrated through the nominal asymptotic stability and that the plant cost can be located in the demanded vicinity of the non-increasing nominal cost, given a suitable objective function. One advantage of the proposed method is its relative simplicity regarding to the problem of choosing suitable model uncertainty descriptions in other robust approaches. The performance of the proposed method is illustrated by a chemical engineering example.

The role of residual Cu(II) from click-chemistry in the catalyzed hydrolysis of Boltorn polyester-based hydrogels

In recent times the Cu(I)-catalyzed click reaction between azides and alkynes has been extensively utilized in the preparation of polymers proposed for the use in biomedical applications. There is a potential for some Cu residues to remain in the polymers, and there has been discussion about whether this renders the polymers unsuitable for medical applications. It is well known that the presence of Cu(II) complexed to ester groups can accelerate their hydrolysis. The question then arises as to whether residual Cu(II) from the click reaction can catalyze the hydrolysis of polyesters. In this work the role of residual Cu(II) in the hydrolysis of copolymer hydrogels containing ester groups is elucidated. The copper(I)-catalyzed “click” reaction was used to synthesize a hyperbranched Boltorn-based polyester macromer which was then used in the preparation of a series of copolymer hydrogels formed with PEG diacrylate. Trace amounts of Cu(II) residues were found by AAS and EPR to remain in the hydrogels, even after extensive washing with EDTA solutions. The effect of these residues on the hydrolytic degradation of the hydrogels has been investigated and the presence of complexed Cu(II) was found to accelerate the hydrolysis of the hydrogels and to contribute also to the formation of pits in the pore walls of the hydrogels at intermediate degradation times.