G Hadziioannou

Influence of anchor block size on the thickness of adsorbed block copolymer layers

We present surface force data on three different polystyrene/poly(2-vinylpyridine) block copolymers (PS/P2VP) with a fixed size of the nonadsorbing PS block but widely varying sizes of the adsorbing P2VP block. With respect to the sizes of the two blocks, they range from moderately to highly asymmetric. The equilibrium force profiles are almost overlapping, which means that the variation in layer thickness is very small over a large range of anchor block size. This finding is in disagreement with the predictions of simple scaling models for polymer brushes. However it agrees with findings from neutron reflectivity data on a comparable series of PS/P2VP block copolymers. We also find agreement with recent neutron reflectivity experiments on poly(dimethylsiloxane)/polystyrene and with self-consistent field calculations on selectively adsorbed block copolymers.

Model compounds for light-emitting PPV's: Optical and structural data of substituted oligomers

Phenylene-vinylene oligomers are investigated as model compounds for the luminescent moiety of PPV-type homopolymers or copolymers having oligo(phenylenevinylene) units. Some of the rings carry substituents for improved solubility (as in PPV derivatives) and cyano groups have been substituted at various positions on the vinylene linkages. For several compounds single crystals have been obtained and their structure has been resolved, which allows the spectral response to be related to molecular conformation and packing. Thin film spectra demonstrate that cyano substitution results in a considerable red shift of the emission: a more planar conformation, π-stacking and, possibly, dipolar interactions cooperate with the electronic effect of the cyano group. The use of a cyano-substituted five-ring compound in a multilayer LED was found to strongly increase the luminescence efficiency.

LEDs based on conjugated PPV block copolymers

A way to control the bandgap in semi-conducting polymers is by preparing polymers with a partially conjugated backbone. In our laboratory, three conjugated copolymers containing PPV trimers as light emitting chromophores have been synthesized, which emit in the blue, green and orange wavelength region. The copolymers have a well defined conjugated backbone consisting of regularly alternating terphenylene and p-phenylene-vinylene blocks. The desired control of conjugation length is achieved through steric interaction induced by the side-chains in the terphenyl blocks of the copolymer. In this paper we evaluate the electrical and optical properties of LED devices based on these conjugated PPV block copolymers.

Blue Light Emitting Diodes based on a partially conjugated Si-containing PPV-copolymer in a multilayer configuration

Efficient blue Light Emitting Diodes (LEDs) based on a novel partially conjugated co-polymer (SiPPV) have been realized by a combination of techniques known to enhance the quantum efficiency of organic devices. The copolymer is homogeneously blended in a PVK-matrix to reduce the number of non-radiative decay paths. In single-layer devices the high barrier at the Al top electrode limits efficient electron-injection into the device. In double-layer structures with an evaporated PBD-film at the cathodic side, electron and hole currents are much more balanced, resulting in an internal quantum efficiency of 3.2%. The average turn-on field of a double-layer device is reduced compared to a single-layer LED.

Compatibilization of blends of low density polyethylene and poly(vinyl chloride) by segmented EB(SAN-block-EB)(n) block copolymers

Hydrogenated segmented poly[butadiene-block-((styrene-co-acrylonitrile)-block-butadiene)n] block copolymers, which were developed by use of the polymeric iniferter technique, were tested for their compatibilizing capacities for (10/90) LDPE/PVC blends. The acrylonitrile content of the SAN blocks of the block copolymers was, as expected, found to be an extremely important factor for their miscibility with the poly(vinyl chloride) phase. When the SAN blocks of the block copolymers have the proper composition, they are excellent agents for the dispersion of the polyethylene phase in the blend into smaller domains. Addition of one percent of block copolymer proved to be sufficient to give a large improvement of the mechanical properties of the blend. These compatibilized blends showed toughnesses comparable to the virgin poly(vinyl chloride). Scanning electron micrographs showed improved adhesion between the dispersed polyethylene phase and the poly(vinyl chloride) matrix.

NEUTRON-SCATTERING BY MULTIBLOCK COPOLYMERS OF STRUCTURE (A-B)(N)-A

Multiblock copolymers were prepared by anionic copolymerization of deuterated and classical styrene monomers. The use of bifunctional initiators gives copolymers with an odd number of sequences. Neutron scattering experiments were performed on these samples in order to check the existing theories which are extended in the first part to multiblock copolymers with an odd number of blocks. The agreement between theory and experiment is excellent and, surprisingly, the height of the maximum of the curves: scattered intensity versus scattering angle, does not depend on the number of blocks.

Neutron reflectivity study of adsorbed diblock copolymers

SummaryIn this paper, we summarize our cumulative work on neutron reflectivity studies of polystyrene-poly(vinyl-2-pyridine) (PS-PVP) and polystyrene-polyethylene oxide (PS-PEO) adsorbed at a quartz-solvent interface. Deuterated toluene was chosen as the solvent since it is a good solvent for PS and a poor one for either of the other two blocks. In this case, the polystyrene dangles into the solvent while the other block acts as an anchor. The neutron reflectivity studies reveal that the form of the polymer density profile normal to the substrate may be varied from an extended «brush» to a condensed «mushroom» conformation by manipulating the ratio of the molecular weights of the two blocks. In addition, we present new data on the PS-PEO system in a poor solvent, deuterated cyclohexane, under conditions of shear flow in Poiseuille geometry. We find that when the PS-PEO diblock is absorbed from cyclohexane and is allowed to relax, the PS chain takes on a «mushroom» conformation. However, when the shear is applied, the layer shear thickens due to the PS chains extending to nearly twice their original lengths.

Synthesis of segmented PB(SMA-block-PB)(n) block copolymers through a polymeric iniferter technique

Abstract A technique is described for the synthesis of segmented block copolymers of polybutadiene and poly(styrene-co-maleic anhydride) through a polymeric therma iniferter based on polybutadiene, which was used as an initiator for the thermal copolymerization of styrene and maleic anhydride. The polymeric iniferters and block copolymers were characterized by GPC, elemental analysis, 1H NMR, FT-IR spectroscopy, and differential scanning calorimetry. The compositions and SMA block lengths in the block copolymer could be regulated by varying the polymeric iniferter concentration. The maleic anhydride content of the SMA blocks was regulated by adjusting the monomer feed composition. The synthesized block copolymers are very promising materials for use as compatibilizers for nylon-based blends, in particular for blends of nylon with ABS or polyethylene.