The Horner method was used to synthesize random copolymers of poly(2-methoxy-5-(2'-ethylhexyloxy)-p-phenylene vinylene) (MEH-PPV) that incorporated different backbone-directing monomers. Single-molecule polarization absorption studies of these new polymers demonstrate that defects that preserve the linear backbone of PPV-type polymers assume the highly anisotropic configurations found in defect-free MEH-PPV. Rigid defects that are bent lower the anisotropy of the single chain, and saturated defects that provide rotational freedom for the chain backbone allow for a wide variety of possible configurations. Molecular dynamics simulations of model defect PPV oligomers in solution demonstrate that defect-free and linearly defected oligomers remain extended while the bent and saturated defects tend toward more folded, compact structures.

This paper investigates the influence of polymer molecular weight (M(W)) on the chemical modifications of poly(methyl methacrylate), PMMA, and polystyrene, PS, films doped with iodonaphthalene (NapI) and iodophenanthrene (PhenI), following irradiation at 248 nm (KrF excimer laser, 20 ns fwhm and hybrid excimer-dye laser, 500 fs fwhm) and at 308 nm (XeCl excimer laser, 30 ns fwhm). The changes of intensity and position of the polymer Raman bands upon irradiation provide information on cleavage of the polymer bonds. Degradation of PMMA, which is a weak absorbing system at 248 nm, occurs to a higher extent in the case of a larger M(W), giving rise to the creation of unsaturation centers and to degradation products. For highly absorbing PS, no degradation is observed upon irradiation with a KrF laser. Consistently irradiating doped PS at 308 nm, where the absorption is low, induces degradation of the polymer. Results provide direct support for the bulk photothermal model, according to which ejection requires a critical number of broken bonds. In the case of irradiation of doped PMMA with pulses of 248 nm and 500 fs, neither degradation nor dependence with polymer M(W) are observed, indicating that mechanisms involved in the femtosecond laser ablation differ from those operating in the case of nanosecond laser ablation. Participation of multiphoton/avalanche processes is proposed.

hydrogen bonding interactions between adjacent GO sheets, whereas the spacing is controlled by the extent of intercalation of oxygen based molecular species. To this end, graphene oxide assemblies are considered as potential nanoporous membranes for selective separation studies. Permeability measurements of binary gaseous mixtures through ultrathin GO membranes (thickness less than 10 nm) has only been recently investigated and the results showed enhanced selectivity towards the diffusion of specifi c substances. [ 10,11 ] Concerning the permeation rates of neat vapor substances, [ 9,12 ] it has been found that even small size monoatomic molecules, such as He, hardly diffuse through the pores of thin GO membranes, whereas water vapor passes freely. [ 9 ] In order to explain the unimpeded water permeability through GO membranes (thickness from 0.1 to 10 µm), the authors suggested that a monolayer of water fi lls the network of graphene nanocapillaries and is capable of undergoing a lowfriction fl ow in between the graphene channels. [ 9 ] Chemical reduction/deoxygenation of GO thin membrane at a specifi c temperature resulted in a decrease of water permeability by a factor of 100, due to the reduction in the channel width, which is too small for a water monolayer to form and fl ow. As complementary information to the experimental fi ndings, recent atomistic models for hybrid systems composed of water and graphene oxides [ 13 ] suggest that formation of hexagonal ice bilayer in between the fl akes is crucial to realize the perfect water permeation across the whole stacked structures. The distance between adjacent layers as well as the melting transition of the ice at the fl ake edges which are assumed to be passivated by hydrophilic edge groups was shown to determine the water fl ow rate. In contradiction, a recent work based on molecular dynamics (MD) simulations has shown that the fast slip fl ow for water inside the interlayer gallery between the graphene layers weakens with the presence of chemical functionalization and relaxation of nanoconfi nement in graphene oxides. [ 14 ] It is obvious that the specifi c conditions needed for a tailored permeability of a gaseous substance through a GO membrane are far from clear and no predictions can be made on its behavior at different temperatures. In specifi c, the effect of both interlayer distance and chemical speciation of attached oxygencontaining groups to the permeation rate of water vapor needs to be clarifi ed in detail. For that reason, detailed studies on the In the present study, the infl uence of the thermal reduction on the water vapor transmission properties of thin graphene oxide (GO) membranes is evaluated. The macroscopically measured property of the Water Vapor Transmission Rate (WVTR) exhibits step like dependence contrary to the gradual microscopic structural alterations identifi ed by several techniques (XPS, FTIR and XRD) applied in situ during the thermal annealing process. Three distinct regions of WVTR-values associated with distinct interlayer...