The degree of in-plane molecular orientation of various polyimides (PIs) and their precursors, poly(amic acid)s (PAAs), were estimated by measuring the visible dichroic absorption at an incidence angle for a rod-like dye (perylenediimide, PEDI) dispersed in the matrices. The effects of PI chain structure, film thickness, heating rate, and residual solvent on a spontaneous in-plane orientation phenomenon were examined to fully understand the mechanism. All PAA films cast on a substrate showed the low degrees of in-plane orientation of the chain axis, nearly independent of the chain structure. Upon thermal imidization of the PAA films adhered on a substrate, a striking spontaneous in-plane orientation behavior was observed for some PI systems with rigid chains; in contrast to that, some flexible PI systems showed no spontaneous behavior. Cure of the PAA films adhered on a substrate induced the spontaneous orientation even if the films were considerably thick (∼50 μm); in contrast to that, the cure of the free-standing thick film did not. However, upon cure of the free-standing thin films (∼10 μm), the spontaneous orientation behavior was observed. For rigid PI systems in which interchain stacking preferentially occurs, thermal cure of the PAA films on a substrate forms liquid-crystal-like highly oriented regions, and simultaneously, apparent stretching (due to constraint of film contraction during imidization) promotes the molecular orientation further. The mechanism is closely associated with a “cooperative effect” in which the neighboring chains enhance the molecular orientation of each other during cure. In addition, structural changes (orientations of the chain axis and a molecular plane and molecular packing) upon stepwise annealing were followed. Polarized infrared absorption spectra measured at an incidence angle demonstrated that the phthalimide molecular plane in the PI film on a silicon wafer aligns somewhat parallel to the film plane, but no significant orientational and conformational changes occurred upon stepwise annealing.

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The eukaryotic GINS complex is essential for the establishment of DNA replication forks and replisome progression. We report the crystal structure of the human GINS complex. The heterotetrameric complex adopts a pseudo symmetrical layered structure comprising two heterodimers, creating four subunit-subunit interfaces. The subunit structures of the heterodimers consist of two alternating domains. The C-terminal domains of the Sld5 and Psf1 subunits are connected by linker regions to the core complex, and the C-terminal domain of Sld5 is important for core complex assembly. In contrast, the C-terminal domain of Psf1 does not contribute to the stability of the complex but is crucial for chromatin binding and replication activity. These data suggest that the core complex ensures a stable platform for the C-terminal domain of Psf1 to act as a key interaction interface for other proteins in the replication-initiation process.

Asymmetric biphenyl type polyimides (PI) were prepared by thermal imidization of polyamic acids (PAA) derived from 2,3,3‘,4‘-biphenyltetracarboxylic dianhydride (a-BPDA) and p-phenylenediamine (PDA) or 4,4‘-oxydianiline (ODA). The dynamic mechanical properties of these PIs were compared with those of the isomeric PIs derived from symmetric 3,4,3‘,4‘-biphenyltetracarboxylic dianhydride (s-BPDA). a-BPDA/PDA polyimide has a considerably bent chain structure compared to semirigid s-BPDA/PDA. Nevertheless, the a-BPDA/PDA film annealed at 350 °C showed a higher T g than the s-BPDA/PDA film prepared under the same conditions. When these PIs were annealed at 400 °C, a-BPDA/PDA exhibited an abrupt E‘ decrease at the T g (=410 °C) as well as the counterpart annealed at 350 °C, whereas s-BPDA/PDA showed no distinct T g in the E‘ curve. Similar annealing effects were also observed for the ODA systems. The unexpectedly higher T g's of a-BPDA-based PIs could be explained in terms of the more restricted conformational change through the crank shaft-like motion. The difference between the extents of the E‘ decrease at the T g for a- and s-BPDA-based PIs is attributed to the difference of the intensity of intermolecular interactions. The blends of s-BPDA/PDA with a-BPDA-based PI (80/20) and the corresponding copolyimide improved the insufficient thermal processability of homo s-BPDA/PDA without causing a decrease in T g. The results revealed that, for semirigid s-BPDA/PDA, a-BPDA-based PIs are better matrix polymers than s-BPDA/ODA.

Emission mechanism in an aromatic polyimide, PI(BPDA/PDA), derived from biphenyltetracarboxylic dianhydride and p‐phenylene diamine were studied with ultraviolet visible absorption and fluorescence spectra of a series of the model compounds. The excitation spectrum of the intermolecular charge‐transfer (CT) fluorescence peaking around 550 nm of PI(BPDA/PDA) thin film was completely consistent with the absorption spectrum, indicating that the intermolecular CT fluorescence emission of PI(BPDA/PDA) film is not caused by direct excitation of the CT absorption band, but by light absorption due to structural units in the polymer backbone. The UV‐vis. absorption spectra of the model compounds corresponding to the structural units in PI(BPDA/PDA) showed that the longest wavelength absorption band is due to the biphenylbisimide moiety. The band was assigned as π, π* transition with the polarization spectrum of the model compound. The fluorescence spectra of the model compounds changed sensitively depending on the conformation around N‐phenyl bond. The lifetime measurement for the model compounds suggested that intramolecular CT process occurs very rapidly. © 1993 John Wiley & Sons, Inc.

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The effect of exchange randomness on field-induced magnetic ordering was investigated through specific heat measurements in Tl 1Àx K x CuCl 3 with x 0:22. The isostructural parent compounds TlCuCl 3 and KCuCl 3 are coupled spin dimer systems with a gapped ground state and their field-induced antiferromagnetic ordering is described by the Bose-Einstein condensation (BEC) of spin triplets. Welldefined field-induced phase transitions were observed in Tl 1Àx K x CuCl 3 . The critical exponent of the phase boundary defined by TðHÞ / ðH À H c Þ 1= is reevaluated in TlCuCl 3 as ¼ 1:67 AE 0:07, which is close to BEC ¼ 3=2 derived from the magnon BEC theory. For x 6 ¼ 0, the exponent decreases systematically with x. The phase boundary observed below 2 K for x > 0:1 approximates a linear function of temperature T. In the low-field region for H < H c , no magnetic ordering is observed despite finite susceptibility. These properties are discussed in connection with the Bose glass phase argued by Fisher et al. [Phys. Rev. B 40 (1989) ) can hop to neighboring dimers and interact with one another due to the transverse and longitudinal components of the interdimer interaction. Hence, the system can be mapped onto the interacting boson system. [2][3][4] When the hopping term is dominant, magnons can undergo Bose-Einstein condensation (BEC) in a magnetic field higher than the critical field H c corresponding to the gap. This leads to field-induced transverse magnetic ordering.2-5) On the other hand, when the repulsive interaction due to antiferromagnetic interdimer interactions is dominant, magnons can form a superlattice accompanied by a magnetization plateau as observed in SrCu 2 (BO 3 ) 2 . 6)TlCuCl 3 and KCuCl 3 have the same monoclinic crystal structure composed of the chemical dimer Cu 2 Cl 6 , 7,8) in which Cu 2þ ions have spin 1=2; their magnetic ground states are spin singlets with excitation gaps Á=k B of 7.5 K and 31 K, 9-11) respectively. The gaps originate from the strong antiferromagnetic exchange interaction between spins in the chemical dimer.12-15) The neighboring spin dimers couple antiferromagnetically in three dimensions. Field-induced magnetic ordering in TlCuCl 3 has been extensively studied by various techniques. 10,[16][17][18][19] The results obtained were in accordance with the magnon BEC model. 4) Magnetic excitations in magnetic fields were investigated by neutron inelastic scattering, 20) and the results were clearly explained by a theory using the bond operator method. 21,22) Fisher et al.23) theoretically discussed the behavior of lattice bosons in random potential, and argued that a new Bose glass phase exists at T ¼ 0 in addition to superfluid and Mott insulating phases. In the Bose glass phase, bosons are localized due to randomness, but there is no gap, so that the compressibility is finite. Fisher et al. showed that the superfluid transition occurs only from the Bose glass phase, and that near T ¼ 0, the transition temperature T c is expressed as T c $ ½ s ð0Þx , and s ð0Þ $ ð À c Þ , where c...

The degrees of uniaxial and in-plane molecular orientation for homopolyimides (PI) and PI/PI binary blends were estimated from the absorption dichroism of perylenetetracarboxydiimide (PEDI) bound to the main chains. Thermal cure of a slightly drawn film of poly(amic acid) (PAA) derived from 3,4,3,′4′-biphenyltetracarboxylic dianhydride (BPDA) and p-phenylenediamine (PDA), BPDA/PDA, showed marked spontaneous orientation toward the stretching direction. The dependence on the chain structure revealed that in addition to the chain stiffness (linearity) the molecular packing is an important factor for the spontaneous orientation behavior. In a miscible binary blend system composed of semirigid BPDA/ PDA labeled with PEDI and flexible BPDA/ODA (ODA: 4,4′-oxydianiline), the extent of spontaneous orientation for the labeled BPDA/PDA chains decreased with increasing BPDA/ODA content in the blends. From the result, the marked spontaneous orientation behavior of homo-BPDA/PDA is interpreted as a kind of "cooperative phenomenon" where the neighboring chains promote the molecular orientation of each other in the cure process. Effects of film thickness, amount of residual solvent, cure method, and heating rate on the extent of spontaneous orientation were also discussed for the homo-BPDA/PDA system.