The promoter of the Dictyostelium glycogen phosphorylase-2 (gp2) gene possesses a profound AT-bias, typical of promoters in this organism. To understand how Dictyostelium achieves specificity during ...transcriptional regulation under the constraint of this highly biased nucleotide composition, we have documented the changes in chromatin structure associated with developmental induction of gp2 gene expression DNase I hypersensitive analyses indicated the presence of several developmentally regulated nuclease-sensitive sites located upstream of the start codon: two strong sites at approximately -250 bp and -350 bp and three substantially weaker sites at -290 bp, -445 bp, and -505 bp. In vitro footprint analyses using nuclear extracts derived from several stages of development (corresponding to varying levels of gp2 expression) revealed three large regions of occupation that were developmentally regulated and corresponded to these nuclease-sensitive sites: -227 to -294 bp (domain 1) -327 to -383 bp (domain 21), and -416 to -534 bp (domain 3). The presence and the extent of the three regulatory domains was confirmed by in vivo footprint analyses spanning the same developmental time points. Southwestern analyses using probes encompassing these footprints demonstrated that probes corresponding to domains 1 and 3 both interacted with 83 and 77 kDa peptides. The domain 3 probe also interacted with a 92 kDa peptide, while only a 62 kDa peptide is recognized by the domain 2 probe. In all cases, peptides capable of binding these probes were found in nuclear extracts derived from differentiated cells and not in undifferentiated cell nuclear extract. Using nuclear extract from differentiated cells and probes corresponding to the three domains, gel mobility shift analyses detected ladders of retarded bands for both domains 1 and 3 and three major retarded bands for domain 2
We have partially purified the protein and isolated the glcS gene for glycogen synthase in Dictyostelium. glcS mRNA is present throughout development and is the product of a single gene coding for ...775 amino acids, with a predicted molecular mass of 87 kD. The sequence is highly similar to glycogen synthase from human muscle, yeast, and rat liver, diverging significantly only at the amino and carboxy termini. Phosphorylation and UDPG binding sites are conserved with Km values for UDPG being comparable to those determined for other organisms, but in vitro phosphorylation failing to convert between the G6P-dependent (D) and -independent (I) forms. Enzyme activity is relatively constant throughout the life cycle: the I form of the enzyme isolates with the soluble fraction in amoebae, switches to the D form, becomes pellet-associated during early development, and finally reverts during late development to the I form, which again localizes to the soluble fraction. Deletion analysis of the promoter reveals a GC-rich element which, when deleted abolishes expression of glcS
It will be shown that ternary blends comprising a low-density polyethylene (LDPE) matrix and dispersed poly(ether-block-amide)/poly(butylene-adipate-co-terephthalate) (PEBA/PBAT) or ...PEBA/polyvinylidene fluoride (PVDF) can result in different wetting morphologies which can significantly enhance or fully restrain the surface migration of a conductive PEBA copolymer in the blend films. In the LDPE/PEBA/PBAT blends, the minor PEBA and PBAT phases combine to form a unique highly associated dispersed phase morphology demonstrating some weak partial wetting characteristics (i.e., three-phase contact), but neither PEBA nor PBAT can be regarded as the middle phase partially wetting the other component. This unique morphology is also a result of the particularly low interfacial tension between PEBA and PBAT. The use of a low-viscosity PBAT results in an increase in the surface composition of PEBA of up to 4 times as compared to that of the binary LDPE/PEBA blends. This is due to the enhancement of the combined PEBA/PBAT continuity, the low interfacial tension between PEBA and PBAT, and the high migration velocity of PBAT in the ternary blend. On the other hand, the LDPE/PEBA/PVDF blend system generates a completely wet morphology where PEBA is confined as a layer phase at the LDPE and PVDF interface. In this latter system, PEBA/PVDF also has a low interfacial tension, and even under conditions of similar viscosity and continuity to the partially wet LDPE/PEBA/PBAT system, the PEBA surface migration in LDPE/PEBA/PVDF can be fully suppressed. With these different PEBA surface migration characteristics in LDPE/PBAT/PEBA and LDPE/PEBA/PVDF, either a hydrophilic or a hydrophobic film surface can be generated at various surface resistivities.
When a conductive polymer is blended with commodity polymers such as polyolefins and/or polystyrene (PS) as a ternary blend, it has a tendency to form the core phase due to its high interfacial ...tension with the other components. This can limit its capacity to reduce resistivity compared to situating it at the interface. In this work, starting with a ternary low-density polyethylene/polystyrene/poly(ether-block-amide) (LDPE/PS/PEBA) blend, we examine the influence of the conductive PEBA concentration on morphology and resistivity when it exists as a core phase. Then, the hierarchical structuring of the PEBA phase will be modified through two strategies: by the addition of a fourth phase (polyethylene terephthalate (PET) or polyvinylidene fluoride (PVDF)) and by the addition of a copolymer interfacial modifier to the LDPE/PS/PEBA blend. Each of these approaches is shown to be capable of allowing the conductive PEBA to form a percolated structure assembled at the interface of two other continuous phases. The completely wet layered structuring of PEBA between PS and PVDF in the quaternary LDPE/PS/PEBA/PVDF blend leads to an exceptionally low percolation threshold of 0.37% compared to 9.7% in the initial LDPE/PS/PEBA blend where PEBA is the inside or core phase. To the best of our knowledge, this is the lowest value ever reported in the literature for a conductive polymer in melt blended systems.
Two ternary blend systems of low-density polyethylene/poly(ether-block-amide)/polyethylene terephthalate (LDPE/PEBA/PET) and LDPE/PEBA/polyvinylidene fluoride (PVDF) are prepared by melt blending to ...thermodynamically assemble the ionically conductive PEBA copolymer at the continuous interface. The LDPE/PEBA/PET blend demonstrates weak partial wetting and a novel morphology transition to complete wetting was observed as the PEBA composition increases from 3% to about 10%. The phenomena can be explained by a mechanism based on the competition between dewetting and coalescence of the PEBA phase at the interface. In the completely wet LDPE/PEBA/PVDF system, a minimum concentration is required to form intact PEBA layers with a thickness of ∼100 nm. Assembling PEBA at the interface of the ternary systems results in the formation of conductive pathways of very low percolation thresholds and thus leads to a significant reduction in the resistivity for both ternary systems as compared to binary blends with PEBA. A particularly sharp drop in resistivity is observed for the complete wetting morphology of LDPE/PEBA/PVDF.
Display omitted
•Surface resistivity is determined by continuity, miscibility and constriction effect.•PS is partially miscible with PEBA (the PEO block) at low PEBA compositions (≤10%).•A “capillary ...instability morphology” is observed in the PS/PEBA blends.
Binary blends of low-density polyethylene (LDPE)/poly(ether-block-amide) (PEBA) and polystyrene (PS)/PEBA were prepared by melt blending. The interfacial tensions were measured by the breaking thread method with a value of 8.0mN/m for LDPE/PEBA and 1.6mN/m for PS/PEBA. The modulated DSC results show a partial miscibility between PS and PEBA at low PEBA concentrations. The continuity development in the LDPE/PEBA blend follows a droplet-droplet coalescence mechanism. However, in the case of the low interfacial tension PS/PEBA blend, a frozen capillary instability morphology was observed for the first time and a new continuity development mechanism was proposed. The continuity and morphology of the blends, and their effect on surface resistivity was examined. Our results indicate that although the continuity is crucial to controlling surface resistivity, other factors, such as the morphology (e.g., constriction) and interfacial properties also play an important role.
Polymer migration to the film surface during the melt processing of polymer blends is an important phenomenon, but has been limited in study. In this work, melt extruded cast films of conductive poly ...(ether-block-amide) (PEBA) with low-density polyethylene (LDPE) and polystyrene (PS) are prepared and the critical roles of phase continuity, interfacial tension and viscosity on PEBA surface migration are studied. When blended with high viscosity LDPEs, PEBA tends to migrate to the film surface, but significant surface enrichment only occurs at high PEBA continuities (typically > 50%). A possible surface migration mechanism based on the draining of PEBA through the connected networks is proposed and the migration process is facilitated by high interfacial tension. In compatibilized LDPE/PEBA and in PS/PEBA, the surface migration of PEBA is fully suppressed even at high continuity levels due to the low interfacial tension between the components. The surface resistivities of the binary blends are critically determined by the continuity of PEBA with a limited influence from surface enrichment. It appears that the continuity threshold to influence surface resistivity is lower than that for surface migration. A ternary LDPE/PS/PEBA blend with double percolation can simultaneously reduce the film surface resistivity and control PEBA surface migration by confining PEBA within a continuous low interfacial tension PS phase.
Display omitted
•Surface migration is influenced by continuity, interfacial tension and viscosity.•Surface resistivity critically depends on phase continuity.•A ternary blend can reduce surface resistivity while controlling surface migration.•A surface migration mechanism based on the draining of PEBA is proposed.
