Summary: The stereoselective polymerization of partial rac‐lactide was attempted by using crystallization during polymerization. Polymerizations were carried out with trifluoromethanesulfonic acid in a mixed solvent of toluene and octane. The molar content of L ‐lactate in the feed was 95%. Polymerization proceeded with the elimination of D ‐lactate by means of crystallization. Poly(L ‐lactic acid) (PLLA) crystals were first precipitated and then plate‐like poly(lactic acid) (PLA) crystals were finally obtained, of which the composition of L ‐lactate was 97.6% and the was 1.68 × 104.
The polymerization time dependence of the microstructure of poly(lactic acid), i.e., the percentage of mm triads (mm%), prepared from partial rac‐lactide in toluene‐70. 相似文献
The absolute concentrations of a nitroxide radical in samples in a loop-gap resonator (LGR) were determined by using a radio-frequency (about 720 MHz) electron paramagnetic resonance (EPR) imaging system. EPR imaging of phantoms containing a nitroxide radical, 3-carbamoyl-2,2,5,5-tetramethylpyrrolidin-1-yloxy (carbamoyl-PROXYL), dissolved in various concentrations of an aqueous sodium chloride solution was made to investigate the influence of dielectric losses and sample position within the LGR. As it was found that these influences on the signal intensity were sufficiently small (less than 6%), it is possible to use identical radical solutions in which the radical is dissolved in a known concentration as an internal marker. Two phantoms containing aqueous solutions of 3 mM (as a marker) and 1, 2, 3, 4, or 5 mM (as a sample) carbamoyl-PROXYL were placed together in the LGR. From EPR images of these phantoms, the absolute concentration of the sample could be calculated by using the gray-scale value (i.e., the signal intensity) of the marker and sample within a small margin of error (about 4%). 相似文献
We study numerically the director and orientational order parameter configurations in a nematic liquid crystal around a two-dimensional
spherical particle on the basis of the tensor order parameter formalism. To properly account for the large length scale difference
between the particle and the accompanying orientational defect, we devise an adaptive grid scheme in which the lattice spacing
is automatically and locally adjusted in response to the spatial gradient of the orientational order parameter. This adaptive
grid scheme is useful in studying dynamical as well as static orientational structures. We present a simulation result which
shows how a hedgehog defect of topological charge -1 becomes unstable in two dimensions, and splits into a defect pair of
topological charge -1/2, located symmetrically around the particle.
Received 14 September 2000 and Received in final form 27 December 2000 相似文献
The magnetic behavior of a decagonal quasicrystal Al40Mn25Fe15Ge20 system has been investigated by Mössbauer spectroscopy, magnetization and AC susceptibility measurements. Magnetization measured at low fields as a function of temperature shows a cusp-like peak around 41K. AC susceptibility also shows a sharp cusp, and the cusp shifts to a higher temperature as the frequency increases. These results strongly suggested that Al40Mn25Fe15Ge20 is a spin glass system with Tg=41K. From the temperature dependence of the Mössbauer spectrum, we observed the appearance of a hyperfine field below around 50K. The temperature is a little higher than the cusp temperature, which is rather usual in spin glass systems. 相似文献
In holographic femtosecond laser processing, diffractive parallel pulses are distorted by phase discontinuities and mutual
interference between the neighborhoods in the reconstructed image of a Fourier computer-generated hologram when the interval
is smaller than the beam diameter. We investigated holographic fabrication on a glass surface using parallel pulses with different
intervals. We found the closest parallel pulses with sufficient separation to avoid mutual interference in holographic femtosecond
laser processing. The minimum interval was 2.8 times larger than the diffracted beam diameter. The experimental results were
also supported by a computer simulation. Our findings will be very useful in the design of holographic laser processing systems. 相似文献
Two series of mesogenic compounds having both a perfluorinated substituent and a hydrogen bonding active site were synthesized and their phase behavior investigated. Due to the chemical architecture of these materials exhibiting amphiphilic character, structures of nano-segregation are expected to form. We found a thermotropic cubic phase with Ia3d symmetry in one of the acid/base hydrogen-bonded complexes, which is a nano-segregated structure. Moreover materials exhibiting a first order smectic A to smectic C phase transition were found, which was ascertained by differential scanning calorimetry measuring a large latent heat, and X-ray diffraction experiments observing abrupt changes of physical properties at the phase transition, i.e. the tilt angle, the intensity and the half-width of the small angle reflection. This first order phase transition occurs due to the frustration of nano-segregated structures of lamellar phases. 相似文献