排序方式: 共有57条查询结果,搜索用时 10 毫秒
41.
M. Soccio M. Gazzano N. Lotti L. Finelli A. Munari 《Journal of Polymer Science.Polymer Physics》2009,47(14):1356-1367
Poly(butylene/diethylene naphthalate) copolymers (PBN‐PDEN) were synthesized in bulk according to the polycondensation procedure and examined by NMR, GPC, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and XRD. At room temperature they appeared as semicrystalline materials; the copolymerization caused a lowering in the Tg value, a decrement of Tm and of the crystallization rate. Pure α or β′ form was obtained at low and high DEN unit content, respectively; crystalline form transition never occurred in the solid state, analogously to PBN. After cooling from the melt, the pure α form was always evidenced in PBN‐PDEN10 and PBN‐PDEN20, whereas the pure β′ crystal phase develops in the copolymer containing 40 mol % DEN units, independently on the cooling rate. In the case of PBN‐PDEN30 a pure α or β′ form was obtained at low and high cooling rate, respectively. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1356–1367, 2009 相似文献
42.
We report on a space-time compression technique allowing for complete and independent control of the longitudinal dynamics and of the transverse pulse localization by means of spatial beam shaping. We experimentally observe both strong temporal compression and high transverse localization, of the order of a few wavelengths, along free-space propagation. 相似文献
43.
Poly(butylene oxalate) (PBO) and poly(butylene oxalate/butylene azelate) random copolymers (PBOBAz) of various compositions were synthesized in bulk and characterized in terms of chemical structure and thermal properties. The thermal behavior was examined by thermogravimetric analysis and differential scanning calorimetry. All copolymers were found to be partially crystalline and thermally stable up to about 290 °C. The main effect of copolymerization was a decrease in melting and glass transition temperatures with respect to PBO homopolymer. The pure crystalline phase characteristic of PBO was evidenced by means of X-ray measurements in all the copolymers under investigation. The fusion temperatures appeared to be well correlated to composition by Baur's equation.Amorphous samples were obtained after melt quenching and showed a monotonic decrease of glass transition temperatures as the content of the flexible butylene azelate units is increased. Fox equation described well the Tg-composition data. Lastly, the overall crystallization rate of PBO was found to decrease regularly with increasing butylene azelate unit content. 相似文献
44.
G. Cattapan P. Lotti 《The European Physical Journal B - Condensed Matter and Complex Systems》2007,60(1):51-60
We consider T–shaped, two–dimensional quantum waveguides containing attractive or repulsive impurities with a smooth, realistic
shape, and study how the resonance behavior of the total conductance depends upon the strength of the defect potential and
the geometry of the device. The resonance parameters are determined locating the relevant S–matrix poles in the Riemann energy
surface. The total scattering operator is obtained from the S–matrices of the various constituent segments of the device through
the
–product composition rule. This allows for a numerically stable evaluation of the scattering matrix and of the resonance parameters. 相似文献
45.
Lotti N. Siracusa V. Finelli L. Munari A. Manaresi P. 《Journal of Thermal Analysis and Calorimetry》2001,66(3):827-840
Poly(2-hydroxyethoxybenzoate), poly(ε-caprolactone), and random poly(2-hydroxyethoxybenzoate/e-caprolactone) copolymers were
synthesized and characterized in terms of chemical structure and molecular mass. The thermal behavior was examined by DSC.
All the samples appear as semicrystalline materials; the main effect of copolymerization was lowering in the amount of crystallinity
and a decrease of melting temperature with respect to homopolymers. Flory's equation described well the T
m-composition data. Amorphous samples (in the 20–100%2-hydroxyethoxybenzoate unit concentration range) obtained by quenching
showed amonotonic decrease of the glass transition temperature T
g as the content of caprolactone units is increased. The Wood's equation described the T
g-composition data well.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
46.
A double‐phase algorithm, based on the block recursive LU decomposition, has been recently proposed to solve block Hessenberg systems with sparsity properties. In the first phase the information related to the factorization of A and required to solve the system, is computed and stored. The solution of the system is then computed in the second phase. In the present paper the algorithm is modified: the two phases are merged into a one‐phase version having the same computational cost and allowing a saving of storage. Moreover, the corresponding non‐recursive version of the new algorithm is presented, which is especially suitable to solve infinite systems where the coefficient matrix dimension is not a priori fixed and a subsequent size enlargement technique is used. A special version of the algorithm, well suited to deal with block Hessenberg matrices having also a block band structure, is presented. Theoretical asymptotic bounds on the computational costs are proved. They indicate that, under suitable sparsity conditions, the proposed algorithms outperform Gaussian elimination. Numerical experiments have been carried out, showing the effectiveness of the algorithms when the size of the system is of practical interest. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
47.
We study two criteria to evaluate quadrature formulas when used in automatic quadrature programs. The former consists of the computation of a quantity depending on both the truncation error behavior and the geometric properties of the nodes of the rule. This measure allows estimating the asymptotical computational cost in various abstract models of automatic quadrature. The latter is a testing technique which can be used to measure the efficiency of the formulas under consideration in a real environment. The relationships between the two criteria are investigated and the two approaches seem in good agreement.Work supported by CNR, Grant No. 93.00570.CT01. 相似文献
48.
Miriam A. Martins Alho Maria I. Errea Vanesa L. Sguerra Norma B. D'Accorso Laura B. Talarico Cybele C. García Elsa B. Damonte 《Journal of heterocyclic chemistry》2005,42(5):979-983
1,2‐O‐Isopropylidene‐α‐L‐threofuranosyl heterocyclic derivatives were synthesized from 1,2‐O‐iso‐propylidene‐α‐D‐xilopentadialdo‐1,4‐furanose and tested for antiviral activity against herpes simplex virus type 1, dengue virus type 2 and Junin virus. For comparative propose, the antiviral activity of some of their pyranosyl analogues were also tested. The furanosyl derivatives showed to be moderate inhibitors of Junin virus and, in general, proved to be more effective than the pyranosyl analogues. 相似文献
49.
M. Vannini L. Finelli N. Lotti M. Colonna C. Lorenzetti A. Munari 《Journal of Polymer Science.Polymer Physics》2005,43(12):1441-1454
The thermal behavior of poly(ethylene‐co‐2,2‐bis[4‐(ethylenoxy)‐1,4‐phenylene]propane terephthalate) (PET/BHEEBT) copolymers was investigated by thermogravimetric analysis and differential scanning calorimetry. A good thermal stability was found for all the samples. The thermal analysis carried out using DSC technique showed that the Tm of the copolymers decreased with increasing BHEEBT unit content, differently from Tg, which on the contrary increased. Wide‐angle X‐ray diffraction measurements permitted identifying the kind of crystalline structure of PET in all the semicrystalline samples. The multiple endotherms similar to PET were also evidenced in the PET/BHEEBT samples, due to melting and recrystallization processes. By applying the Hoffman–Weeks' method, the Tm° of PET and its copolymers was derived. The isothermal crystallization kinetics was analyzed according to Avrami's treatment and values of the exponent n close to 3 were obtained, independently of Tc and composition. Moreover, the introduction of BHEEBT units was found to decrease PET crystallization rate. Lastly, the presence of a crystal‐amorphous interphase was evidenced. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1441–1454, 2005 相似文献
50.
M. Soccio N. Lotti L. Finelli A. Munari 《Journal of Polymer Science.Polymer Physics》2008,46(8):818-830
The melting behavior and the crystallization kinetics of random poly(propylene/neopenthyl terephthalate) copolymers (PPT‐PNT) were investigated by means of differential scanning calorimetry and hot‐stage optical microscopy. Multiple endotherms were evidenced in the PPT‐PNT samples, due to melting and recrystallization processes, similarly to PPT. By applying the Hoffman‐Weeks' method, the Tm° of the copolymers was derived. Baur's equation described well the Tm‐composition data. The isothermal crystallization kinetics was analyzed according to the Avrami's treatment. The introduction of NT units decreased the crystallization rate in comparison to pure PPT. Values of the Avrami's exponent close to three were obtained in all cases, regardless of Tc, in agreement with a crystallization process originating from predeterminated nuclei and characterized by three dimensional spherulitic growth. As a matter of fact, space‐filling spherulites were observed by optical microscopy at all Tcs. Banded spherulites were found for PPT‐PNT5 and PPT‐PNT10, the band spacing being affected by both Tc and composition. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 818–830, 2008 相似文献