Yung-Lung Lin

Effects of molecular architectures and solvophobic additives on the aggregative properties of polymeric surfactants

The aggregative behavior of the polymeric surfactants with various molecular architectures in dilute solutions is studied by dissipative particle dynamics. The effects of the solvophobic/solvophilic length, polymeric architecture (linear, star, dendritic, and cyclic type), chain rigidity, and solvophobic additives on the critical micelle concentration (CMC) and the aggregative patterns are systematically investigated. It is found that molecular architectures have a noteworthy impact on the aggregative properties. For linear diblock copolymers, the CMC declines with increasing solvophobic length but rises with increasing solvophilic length. Nonetheless, the solvophobic group has comparatively greater influence on the CMC. Imposition of the star, dendritic, or cyclic structures onto the solvophobic or solvophilic parts of the polymeric surfactant leads to an increase in the CMC. On the contrary, polymers imposed with the greater degree of the rigidity on the solvophobic or solvophilic block have lower CMC. The addition of solvophobic additives results in a decrease of CMC as well. The effects of the concentration and length of the additives on the aggregative behaviors of polymer surfactants were investigated. Interesting supramolecular structures such as caterpillar and worm-like micelles were observed.

Structural and mechanical properties of polymersomes formed by rod–coil diblock copolymers

Polymersomes formed by rod–coil diblock copolymers (RxCy) are fundamentally different from those of coil–coil diblock copolymers. RxCy denotes the polymer comprising of x rod-like beads and y coil-like beads. The morphological phase diagram of RxCy in selective solvents and the essential physical properties of the RxCy-polymersomes are explored by using dissipative particle dynamics. Our simulation results show that small-sized polymersomes can only take shape for short coil-block lengths. Moreover, the rod-block length cannot be too long and π–π stacking must be weak because anisotropic rod packing actually resists membrane bending and thus vesicle formation. The detailed membrane structures of polymersomes are also investigated and it is found that the rods within the membrane are highly interdigitated which is intrinsically different from the ordered bilayer of the liposomes formed by triblock copolymers (Rx)2Cy. The structural and mechanical properties of RxCy-polymersomes are studied as well. As the coil-block length is increased, both the thickness and area density of the rod domain decline. Since the coil stretching in the inner corona is more distinct than that in the outer one, significant interdigitation results due to the outward movement of the inner leaflet for relaxing overcrowded coil-blocks. The membrane tension exhibits a maximum while the stretching and bending moduli display a minimum at the intermediate coil-block length as y varies from 1 to 3. Fusion between two polymersomes of R5Cy is investigated and the outcome relates to the membrane tension. R5C2-polymersomes fuse most easily. However, R5C1-polymersomes do not proceed beyond the hemifusion stage and R5C3-polymersomes can not even move past the initial kissing stage.

Effect of grafting architecture on the surfactant-like behavior of clay-poly(NiPAAm) nanohybrids

A new class of clay-polymer nanohybrids was synthesized by grafting poly(N-isopropylacrylamide) (PNiPAAm) on the edge of nanoscale silicate platelets (NSPs) through covalently bonded linkers to form various architectures. The inherent ionic character of NSPs and the organic moieties of isopropyl amide in PNiPAAms impart surface active properties to the nanohybrids. Surface tension and particle size measurements were used to determine the critical micelle concentrations (CMCs) of the nanohybrids. It was found that PNiPAAm brushes grafted onto NSPs with the single-headed linkers are loosely packed and can expand easily in water causing inter-hybrid interactions. In contrast, PNiPAAm brushes grafted onto NSPs with the double-headed linkers may alternatively exhibit intra-hybrid interactions and the hybrids tend to exist in a dispersed state. Consequently, the latter has a higher CMC than the former. In addition, the CMC can be tailored by adjusting the grafting density of the linkers on the NSP surfaces. The densely grafted nanohybrids exhibit close inter-hybrid contact resulting in a lower CMC than that for the sparsely grafted nanohybrids. Molecular simulations were also performed to study the effects of the polymer-grafted architecture and the density of the linkers on the micellar behavior of NSP-PNiPAAm hybrids. The simulation results were found to be in good agreement with the experimental observations. Thus, it is possible to control the surface active properties and aggregation of the clay-PNiPAAm hybrids by manipulating the organic grafting architectures of the silicate platelets.

Photodissociation of CBrCl3 at 193 nm by translational spectroscopy

The photodissociation of CBrCl3 at 193 nm has been studied by translational spectroscopy. Two major dissociation channels, (1) CBrCl3→CCl3+Br and (2) CBrCl3→CBrCl2+Cl, are detected with product translational energies of 17 and 22 kcal/mol, respectively. The relative yield of (1):(2) is calculated to be 7:3. The primary product, CBrCl2, which is internally excited, undergoes unimolecular decay to form the CCl2+Br products. From the derived values of the anisotropy parameter β, we conclude that these processes take place rapidly after excitation via an A1←A1 transition, with the transition moment aligned parallel to the threefold axis. Results obtained for these two primary pathways and other minor processes are discussed in terms of a simple direct dissociation mechanism.