Polymers - Vol. 58, No. 10 (October, 2009)

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Vol. 58, No. 10, October ( 2009)

An Intelligent Switchover of Hydrogen Bonds involved in the Zipper-type Screw-Sense Inversion of α-Helical Polyaspartates
Akihiro ABE* and Hidemine FURUYA**
Nano-Science Research Center, Tokyo Polytechnic University* Department of Organic and Polymeric Materials, Tokyo Institute of Technology**

Shown in the figure is the transition behavior of poly(β-phenethyl L-aspartate) (PPLA) in the lyotropic liquid crystalline state (tetrachloroethane) observed by the 2H NMR technique. A theoretical description of the transition has been attempted by adopting the Ising concept. The parameter s represents the free energy difference between the left- and right-handed residues, and σ takes care of the instability at the R/L junction: lns=ΔHc/RTc (1-Tc/T). As illustrated in the diagram, the experimental curve is reproduced by a single moving-front model, with ΔHc/R=0.29 (per mole of residue) and σ=0.001. The unwinding and rewinding of α-helices during the transition require an intelligent switchover of the hydrogen-bond network. Polymer Preprints, Japan 2009, 58, 791.

Photoisomerization Behavior of Azobenzene Crystals Nano-dispersed in Aqueous PVA Solutions
Kunihiro ICHIMURA
Faculty of Science, Toho University
Reversible photoisomerizability of azobenzene in crystals was revealed by the preparation of aqueous nanodispersions of the crystals through the two following procedures. The first consists of the beads milling of the crystals in an aqueous PVA solution, whereas the second is based on the ultrasonic treatment of a powdery core-shell type nanohybrid of azobenzene crystals and silica nanoparticles in water in absence of PVA. Spectral changes of the former upon UV exposure resemble those in a polymer film, implying that azobenzene molecules are partially incorporated in PVA chains in water. The anomalous spectral changes of the latter dispersion are due to the consecutive processes involving the photoisomerization of disordered trans-isomer to cis-isomer, followed by the disaggregation of trans-isomer to result in further photoisomerization. Polymer Preprints, Japan 2009, 58, 1342.

Viscoelastic Properties of Associating Rouse Chains
Tsutomu INDEI* and Jun-ichi TAKIMOTO
Department of Polymer Science and Engineering, Yamagata University
We have developed a theory and a Brownian dynamics simulation model to describe the viscoelastic properties of a transient polymer network formed by associating polymer chains having a number of associative groups on the backbone. The intrachain association and excluded volume interaction are disregarded for the sake of simplicity. The mechanical relaxation behavior of chains with a few associative groups is Maxwell-like around the frequency regime ω~1/τX (τX: lifetime of association) and Rouse-like around ω~1/τR (τR: Rouse relaxation time of partial chains between two associative groups). On the other hand, for chains with many associative groups, another Rouse-like mode appears in the lower-frequency regime around ω~1/τA (see Figure) if the association rate of associative groups is much larger than their dissociation rate. Polymer Preprints, Japan 2009, 58, 655.

New Epitaxial Phase Transition between DG and HEX in PS-b-PI
Hae-Woong PARK1, Jueun JUNG1, Taihyun CHANG1, Kazuyuki MATSUNAGA2 and Hiroshi JINNAI*2,3
1 Department of Chemistry and Polymer Research Institute, Pohang University of Science and Technology, Pohang, Korea 2 Department of Macromolecular Science and Engineering, Graduate School of Science and Engineering, Kyoto Institute of Technology 3 WPI Advanced Institute for Materials Research (WPI-AIMR), Tohoku University
Block copolymers undergo order-order phase transitions that often exhibit an epitaxial relationship between the morphology of two phases. One of the extensively studied examples is the phase transition from or to double gyroid (DG) structure, which is more intriguing than the others due to the structural complexity of the DG phase. We here investigated the epitaxial phase transition from the DG to hexagonally packed cylinder (HEX) phases of a polystyrene-b-polyisoprene diblock copolymer in thin film on a Si wafer substrate. Internal structure of the thin film was investigated by grazing incidence small-angle X-ray scattering, transmission electron microscopy and transmission electron microtomography (TEMT). TEMT allows observing the 3D-image of coexisting DG and HEX directly. The good domain orientation in thin film and the 3D-imaging of TEMT made it possible to observe the coexisting phase of DG and HEX and a new epitaxial transition path from DG to HEX was found. Polymer Preprints, Japan 2009, 58, 672. J. Am. Chem. Soc. 2009, 131, 46.

Phase Behaviors of Poly(N-alkoxy(meth) acrylamide)s in Methanol/ Water Mixtures
Yasushi MAEDA and Shinji TAKAKU
Department of Applied Chemistry and Biotechnology, Graduate School of Engineering, University of Fukui
Phase behaviors of poly(N-alkoxy (meth)acrylamide)s in alcohol/water mixtures were compared with those of poly(N-alkyl (meth)acrylamide)s such as poly(N-isopropylacrylamide) (PNiPAm). Their Tp monotonously increased with increasing concentration of methanol unlike reentrant phase separation of PNiPAm. The amide I bands indicate that the number of hydrogen bonds to the amide C=O decreased with increasing concentration of methanol. The red shifts of the ν(C-H) bands also indicate that water molecules surrounding the alkyl groups are replaced by methanol. Although the solvation change is qualitatively the same as that of PNiPAm, poly(N-alkoxy (meth)acrylamide)s have lower number of H-bonds than PNiPAm at any compositions. Importance of the solvation at the amide and the alkyl groups as well as at the ether oxygen on the phase behaviors of poly(N-alkoxy (meth)acrylamide)s was suggested. Polymer Preprints, Japan 2009, 58, 707.

Glass Transition and Memory Effects in Stress Relaxation
Yoshihisa MIYAMOTO*1,2 and Yui UEHARA2
1 GSHES, 2 FIHS, Kyoto University
Relaxations that are dependent on the sample history are observed in the glass transition region of polymers. It is generally considered that the non-linearity, the non-exponentiality of the relaxation function and the aging effects are the origins of these relaxations. We have observed that the relaxation is showing a peak at a given temperature and a strain after the controlled strain history in the stress relaxation of cis-1,4-polyisoprene rubber. The results can be experimentally reproduced by decomposing the strain history. These results indicate that the non-exponentiality is the main origin of the present relaxation exhibiting a peak. Polymer Preprints, Japan 2009, 58, 592.

Thermal Molecular Motion of Poly(vinyl alcohol) Nano-Fibers in Films Prepared by an Electro-Spinning Method
Keiji TANAKA
Department of Applied Chemistry, Kyushu University
Dynamic mechanical analysis was applied to poly(vinyl alcohol) (PVA) nano-fibers in films. In temperature-tan δ curves for the PVA nano-fibers in the films, three peaks were observed at 260, 350 and 400 K. The weak absorption peak at 400 K was assignable to the crystalline relaxation (αc) process. The strong one at 350 K was ascribed to the segmental motion in the amorphous region (αa). The broad peak at 260 K should correspond to the twisted motion for hydrated PVA main chains (γ). Relaxation temperatures for the αa and αc processes decreased with decreasing fiber diameter. This can be explained in term of an increment of surface region in which segmental motion is activated, resulting from an increase of the ratio of surface to total volume with decreasing fiber diameter. Besides, X-ray diffraction and Fourier transform infrared measurements revealed that crystallinity of PVA decreased with decreasing fiber diameter. This might be related to the diameter dependence of Tαc. Polymer Preprints, Japan 2009, 58, 824. .

Birefringence Study on Molecular Orientation of Natural Rubber During Strain-Induced Crystallization
Masatoshi TOSAKA
Institute for Chemical Research, Kyoto University
Birefringence and tensile stress of cross-linked natural rubber were measured simultaneously by a specially devised instrument which was relatively insusceptible to clarity of the samples. Because the problem arising from scattering by strain-induced crystals was overcome, the data could be collected up to high extension ratio. The results revealed the significant effect of the finite extensibility of network chains on tensile stress, leading to the departure from the linear stress-birefringence relationship of the Gaussian chains. Nature of network chains which were incorporated into strain-induced crystals was found to depend on the network-chain density (ν): Poorly oriented molecular chains were converted to oriented crystals when ν was low, while highly oriented chains mostly formed the oriented crystals when ν and extension ratio were high. Polymer Preprints, Japan 2009, 58, 626.

Reorganizable Chemical Gels Based on Dynamic Covalent Chemistry
Yoshifumi AMAMOTO, Hideyuki OTSUKA* and Atsushi TAKAHARA**
Graduate School of Engineering, Institute for Materials Chemistry and Engineering, Kyushu University
Reorganizable chemical gels, whose cross-linking structures could be changed by thermal stimulus, have been developed. Dynamic covalent chemistry based on reversible covalent bonds enables to bring structure reconstruction. In the present system, dynamic covalent chemistry was applied to chemical gels whose cross-linking states are generally difficult to change. Polymer gels with alkoxyamine units as representative reversible covalent bonds in cross-linking points were prepared, and two characteristic behaviors are shown. The de-cross-linking reaction was accomplished by radical crossover reaction between alkoxyamine at cross-linking points and added excessive alkoxyamine, and as a result sol states could be obtained. Furthermore, the controlled monomer insertion into cross-linking points was achieved by heating gels with styrene, and their network sizes became larger. We believe such reorganizable gels can be applied to recyclable gels, stimuli responsive gels, and so on. Polymer Preprints, Japan 2009, 58, 352.