Modeling the molecular weight distribution of block copolymer formation in a reversible addition-fragmentation chain transfer mediated living radical polymerization

Block copolymers have become an integral part of the preparation of complex architectures through self-assembly. The use of reversible addition-fragmentation chain transfer (RAFT) allows blocks ranging from functional to nonfunctional polymers to be made with predictable molecular weight distributions. This article models block formation by varying many of the kinetic parameters. The simulations provide insight into the overall polydispersities (PDIs) that will be obtained when the chain-transfer constants in the main equilibrium steps are varied from 100 to 0.5. When the first dormant block [polymer-S-C(Z)=S] has a PDI of 1 and the second propagating radical has a low reactivity to the RAFT moiety, the overall PDI will be greater than 1 and dependent on the weight fraction of each block. When the first block has a PDI of 2 and the second propagating radical has a low reactivity to the RAFT moiety, the PDI will decrease to around 1.5 because of random coupling of two broad distributions. It is also shown how we can in principle use only one RAFT agent to obtain block copolymers with any desired molecular weight distribution. We can accomplish this by maintaining the monomer concentration at a constant level in the reactor over the course of the reaction. (c) 2005 Wiley Periodicals, Inc.

Biomolecular screening with novel organosilica microspheres

Organosilica microspheres synthesised via a novel surfactant-free emulsion-based method show applicability towards optical encoding, solid-phase synthesis and high-throughput screening of bound oligonucleotide and peptide sequences.

Design strategies for controlling the molecular weight and rate using reversible addition-fragmentation chain transfer mediated living radical polymerization

Living radical polymerization has allowed complex polymer architectures to be synthesized in bulk, solution, and water. The most versatile of these techniques is reversible addition-fragmentation chain transfer (RAFT), which allows a wide range of functional and nonfunctional polymers to be made with predictable molecular weight distributions (MWDs), ranging from very narrow to quite broad. The great complexity of the RAFT mechanism and how the kinetic parameters affect the rate of polymerization and MWD are not obvious. Therefore, the aim of this article is to provide useful insights into the important kinetic parameters that control the rate of polymerization and the evolution of the MWD with conversion. We discuss how a change in the chain-transfer constant can affect the evolution of the MWD. It is shown how we can, in principle, use only one RAFT agent to obtain a poly-mer with any MWD. Retardation and inhibition are discussed in terms of (1) the leaving R group reactivity and (2) the intermediate radical termination model versus the slow fragmentation model. (c) 2005 Wiley Periodicals, Inc.

W/O microemulsions for ocular delivery: Evaluation of ocular irritation and precorneal retention

Water-in-oil microemulsions (w/o ME) capable of undergoing a phase-transition to lamellar liquid crystals (LC) or bicontinuous ME upon aqueous dilution were formulated using Crodarnol EO, Crill 1 and Crillet 4, an alkanol or alkanediol as cosurfactant and water. The hypothesis that phase-transition of ME to LC may be induced by tears and serve to prolong precomeal retention was tested. The ocular irritation potential of components and formulations was assessed using a modified hen's egg chorioallantoic membrane test (HET-CAM) and the preocular retention of selected formulations was investigated in rabbit eye using gamma scintigraphy. Results showed that Crill 1, Crillet 4 and Crodamol EO were non-irritant. However, all other cosurfactants investigated were irritant and their irritation was dependent on their carbon chain length. A w/o ME formulated without cosurfactant showed a protective effect when a strong irritant (0.1 M NaOH) was used as the aqueous phase. Precorneal clearance studies revealed that the retention of colloidal and coarse dispersed systems was significantly greater than an aqueous solution with no significant difference between ME systems (containing 5% and 10% water) as well as o/w emulsion containing 85% water. Conversely, a LC system formulated without cosurfactant displayed a significantly greater retention compared to other formulations. (c) 2005 Elsevier B.V. All rights reserved.

Methacryloxyethyl phosphate-grafted expanded polytetrafluoroethylene membranes for biomedical applications

Expanded polytetrafluoroethylene (ePTFE) membranes were modified by graft copolymerization with methacryloxyethyl phosphate (MOEP) in methanol and 2-butanone (methyl ethyl ketone (MEK)) at ambient temperature using gamma irradiation. The effect of dose rate (0.46 and 4.6 kGyh(-1)), monomer concentration (1-40 %) and solvent were studied and the modified membranes were characterized by weight increase, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). XPS was used to determine the % degree of surface coverage using the C-F (ePTFE membrane) and the C-C (MOEP graft copolymer) peaks. Grafting yield, as well as surface coverage, were found to increase with increasing monomer concentration and were significantly higher for samples grafted in MEK than in methanol solution. SEM images showed distinctly different surface morphologies for the membranes grafted in methanol (smooth) and MEK (globular), hence indicating phase separation of the homopolymer in MEK. We propose that in our system, the non-solvent properties of MEK for the homopolymer play a more important role than solvent chain transfer reactions in determining grafting outcomes. (c) 2005 Society of Chemical Industry.

Scaling laws for the critical rupture thickness or common thin films

Despite decades of experimental and theoretical investigation on thin films, considerable uncertainty exists in the prediction of their critical rupture thickness. According to the spontaneous rupture mechanism, common thin films become unstable when capillary waves. at the interfaces begin to grow. In a horizontal film with symmetry at the midplane. unstable waves from adjacent interfaces grow towards the center of the film. As the film drains and becomes thinner, unstable waves osculate and cause the film to rupture, Uncertainty sterns from a number of sources including the theories used to predict film drainage and corrugation growth dynamics. In the early studies, (lie linear stability of small amplitude waves was investigated in the Context of the quasi-static approximation in which the dynamics of wave growth and film thinning are separated. The zeroth order wave growth equation of Vrij predicts faster wave growth rates than the first order equation derived by Sharma and Ruckenstein. It has been demonstrated in an accompanying paper that film drainage rates and times measured by numerous investigations are bounded by the predictions of the Reynolds equation and the more recent theory of Manev, Tsekov, and Radoev. Solutions to combinations of these equations yield simple scaling laws which should bound the critical rupture thickness of foam and emulsion films, In this paper, critical thickness measurements reported in the literature are compared to predictions from the bounding scaling equations and it is shown that the retarded Hamaker constants derived from approximate Lifshitz theory underestimate the critical thickness of foam and emulsion films, The non-retarded Hamaker constant more adequately bounds the critical thickness measurements over the entire range of film radii reported in the literature. This result reinforces observations made by other independent researchers that interfacial interactions in flexible liquid films are not adequately represented by the retarded Hamaker constant obtained from Lifshitz theory and that the interactions become significant at much greater separations than previously thought. (c) 2005 Elsevier B.V. All rights reserved.

Bounding film drainage in common thin films

A review of thin film drainage models is presented in which the predictions of thinning velocities and drainage times are compared to reported values on foam and emulsion films found in the literature. Free standing films with tangentially immobile interfaces and suppressed electrostatic repulsion are considered, such as those studied in capillary cells. The experimental thinning velocities and drainage times of foams and emulsions are shown to be bounded by predictions from the Reynolds and the theoretical MTsR equations. The semi-empirical MTsR and the surface wave equations were the most consistently accurate with all of the films considered. These results are used in an accompanying paper to develop scaling laws that bound the critical film thickness of foam and emulsion films. (c) 2005 Elsevier B.V. All rights reserved.

Biomolecular engineering at interfaces

A broad review of technologically focused work concerning biomolecules at interfaces is presented. The emphasis is on developments in interfacial biomolecular engineering that may have a practical impact in bioanalysis, tissue engineering, emulsion processing or bioseparations. We also review methods for fabrication in an attempt to draw out those approaches that may be useful for product manufacture, and briefly review methods for analysing the resulting interfacial nanostructures. From this review we conclude that the generation of knowledge and-innovation at the nanoscale far exceeds our ability to translate this innovation into practical outcomes addressing a market need, and that significant technological challenges exist. A particular challenge in this translation is to understand how the structural properties of biomolecules control the assembled architecture, which in turn defines product performance, and how this relationship is affected by the chosen manufacturing route. This structure-architecture-process-performance (SAPP) interaction problem is the familiar laboratory scale-up challenge in disguise. A further challenge will be to interpret biomolecular self- and directed-assembly reactions using tools of chemical reaction engineering, enabling rigorous manufacturing optimization of self-assembly laboratory techniques. We conclude that many of the technological problems facing this field are addressable using tools of modem chemical and biomolecular engineering, in conjunction with knowledge and skills from the underpinning sciences. (c) 2005 Elsevier Ltd. All rights reserved.

Reversible active switching of the mechanical properties of a peptide film at a fluid-fluid interface

Designer peptides have recently been developed as building blocks for novel self-assembled materials with stimuli-responsive properties. To date, such materials have been based on self-assembly in bulk aqueous solution or at solid-fluid interfaces. We have designed a 21-residue peptide, AM1, as a stimuli-responsive surfactant that switches molecular architectures at a fluid-fluid interface in response to changes in bulk aqueous solution composition. In the presence of divalent zinc at neutral pH, the peptide forms a mechanically strong 'film state'. In the absence of metal ions or at acid pH, the peptide adsorbs to form a mobile 'detergent state'. The two interfacial states can be actively and reversibly switched. Switching between the two states by a change in pH or the addition of a chelating agent leads to rapid emulsion coalescence or foam collapse. This work introduces a new class of surfactants that offer an environmentally friendly approach to control the stability of interfaces in foams, emulsions and fluid-fluid interfaces more generally.

Surface-functionalized polymer nanoparticles for selective sequestering of heavy metals

Xanthate-mediated (reversible addition-fragmentation chain transfer) emulsion polymerization has been used to create novel polystyrene nanoparticles with functionalized surfaces (see Figure) for the selective sequestering of heavy metals from water below ppm levels. These nanoparticles show a high degree of selectivity for Hg-II over Co-II. This technology has potential for the selective remediation of heavy metals from the human blood system.

Peptide surfactants (Pepfactants) for switchable foams and emulsions

We have developed a series of sustainable peptide surfactants (Pepfactants) capable of stabilizing foams and emulsions in a stimuli-responsive manner, based on the reversible formation of a mechanically strong interfacial film. Under conditions where the interfacially adsorbed peptide forms a mechanically strong film state, foam or emulsion stabilization occurs as a direct result of the film strength. Under conditions where the interfacially adsorbed peptide forms a mobile detergent state, foam or emulsion stabilization is either reduced, or does not occur. Preformed foams or emulsions stabilized by Pepfactants undergo rapid phase coalescence when the film state is converted to the detergent state. Switching between film and detergent states is readily and reversibly achieved by a change in the bulk solution composition, such as a change in pH, or the addition or sequestering of metal ions. Copyright © 2007 Curtin University of Technology and John Wiley & Sons, Ltd.