Effects of stereoregularity and crystallization mode on the amorphous phase dynamics are investigated for poly(lactic acid) PLA. An isothermal crystallization from the melt and a cold crystallization are imposed. For each PLA, the cold crystallization leads to the appearance of a less perfect crystalline phase and to an important rigid amorphous fraction RAF content (35%), although only 10% of RAF is generated after crystallization from the melt. Temperature Modulated Differential Scanning Calorimetry is used to determine the Cooperative Rearranging Regions (CRR) size at the glass transition temperature in the mobile amorphous phase MAP. It is shown that the CRR size in the MAP is not modified by the appearance and the spherulite growth. For the intra-spherulite MAP, a confining effect is evidenced, causing an amorphous phase thickness decrease during crystallization, and inducing a drastic CRR size reduction. 相似文献
ABSTRACTThe aim of this study is to investigate the influence of a hydrogen- or lithium-bonding interaction on the cooperativity of chalcogen bonds in linear NCH···(OCX)2–5 and NCLi···(OCX)2-5 clusters (X?=?S, Se). The nature of interactions in the optimised structures is analysed by means of molecular electrostatic potential, quantum theory of atoms in molecules, natural bond orbital and electron density difference methods. According to our results, the formation of a lithium-bonding interaction in NCLi···(OCX)2-5 clusters induces a large increase in the strength of X···O chalcogen bonds, and hence their cooperativity. This can be mainly rationalised in terms of the electrostatic nature of chalcogen bonds as well as the fundamental orbital interaction between the interacting OCX subunits. The results of this study provide a theoretical evidence for the tuning of chalcogen bonds cooperativity by a hydrogen- or lithium-bonding. 相似文献
A model is proposed to rapidly evaluate the individual hydrogen bonding energies in linear water chains. We regarded the two O--H bonds of a water molecule as two dipoles. The magnitude of the O--H bond dipole mo- ment can be varied due to the other water molecules' presence. An analytic potential energy function, which explicitly contains the permanent dipole-dipole interactions, the polarization interactions, the van der Waals interactions and the covalent interactions, was therefore established. The individual hydrogen bonding energies in a series of linear water chains were evaluated via the analytic potential energy function and compared with those obtained from the CP-corrected MP2/aug-cc-pVTZ calculations. The results show that the analytic potential energy function not only can produce the individual hydrogen bonding energies as accurately as the CP-corrected MP2/aug-cc-pVTZ method, but is very efficient as well, demonstrating the model proposed is reasonable and useful. Based on the individual hy- drogen bonding energies obtained, the hydrogen bonding cooperativity in the linear water chains was explored and the natures of the hydrogen bonding in these water chains were discussed. 相似文献
A novel host which shows homotropic and negative allostery for alkali metal ion recognition is constructed by utilizing a biscalix[4]arene skeleton bearing biphenyls and ester moieties. As the ionic radius of the guest increases, recognition of the second guest is suppressed more effectively. A larger ion changes the structure of the first binding site more drastically to cause conformational change unfavorable for the guest binding of the second site. 相似文献
P450s have attracted tremendous attention owing to not only their involvement in the metabolism of drug molecules and endogenous
substrates but also the unusual nature of the reaction they catalyze, namely, the oxidation of unactivated C–H bonds. The
binding of substrates to P450s, which is usually viewed as the first step in the catalytic cycle, has been studied extensively
via a variety of biochemical and biophysical approaches. These studies were directed towards answering different questions
related to P450s, including mechanism of oxidation, substrate properties, unusual substrate oxidation kinetics, function,
and active-site features. Some of the substrate binding studies extending over a period of more than 40 years of dedicated
work have been summarized in this review and categorized by the techniques employed in the binding studies.
Circularly polarized photodetectors require chiral light absorption materials with high sensing efficiency and low costs. Here readily accessible point chirality has been introduced to dicyanostilbenes as the chiral source, facilitating remote chirality transfer to the π-aromatic core by cooperative supramolecular polymerization. The single-handed supramolecular polymers display powerful circularly polarized photodetection capability with a dissymmetry factor value as high as 0.83, superior to those of π-conjugated small molecules and oligomers. Strong chiral amplification occurs between the enantiopure sergeants and the achiral soldiers. The resulting supramolecular copolymers exhibit comparable photodetection efficiency to those of the homopolymeric ones, with a 90 % decrease in the enantiopure compound consumption. Therefore, cooperative supramolecular polymerization provides an effective yet economical avenue toward circularly polarized photodetection applications. 相似文献
Although fragility of glass forming liquids is traditionally related to cooperativity in molecular motion, the connection between those parameters remains unclear. In this paper we present the estimates of cooperativity (heterogeneity) length scale ξ obtained from the boson peak spectra. We demonstrate that ξ agrees well with the dynamic heterogeneity length scale for the structural relaxation estimated by 4-dimensional NMR, justifying the use of ξ. Presented analysis of large number of materials reveals no clear correlation between ξ and fragility. However, there is a strong correlation between the cooperativity volume ξ3 and the activation volume measured at Tg. This observation suggests that only the volume (pressure) dependence of structural relaxation time correlates directly with the cooperativity size. However, the pure thermal (energetic) contribution to the structural relaxation, the so-called isochoric fragility, exhibits no correlation to the heterogeneity length scale ξ, or the amount of structural units in ξ3. The presented results call for a revision of traditional view on the role of cooperativity/heterogeneity in structural relaxation of glass forming systems. 相似文献
This paper presents the results concerning anti-cooperativity effects between two H-bonds of a water molecule. The IR OH stretching band shifts Δν—as a measure of the H-bond energy—are compared for HOD with different bases B of 1:1 complexes B…HOD…Cl4 (Δν11) and 1:2 complexes B…HOD…B (Δν12). We found that Δν12 of the 1:2 complexes for different bases B are 25% smaller than Δν11 for the 1:1 complexes. Corrections for the solvent shifts are introduced. This effect is in line with different observations concerning cooperativity effects of OH H-bonds by polarization with neighbouring molecules. The reduction of Δν12 in 1:2 complexes can be understood on the assumption of a negative polarization by the first H-bond to the second OH or OD group and is called anti-cooperativity. This anti-effect has been already detected by NMR observation on NH2.
We already observe a decrease of the CCl4 solvent shift by van der Waals forces OH…CCl4 of 1:1 complexes, induced by the H-bond of the other OH or OD group. This decrease is measured by the CCl4 solvent effect for monomers. This indicates a real negative polarization by the H-bond in 1:1 complexes on the second OH/OD. This experiment establishes the real polarization and excludes the importance of repulsions of the bases as the cause. The dependence of intermolecular forces is known on the polarizability. Our method demonstrates directly the polarization by interactions.
The anti-cooperativity of symmetric complexes B1…HOD…B1 by a strong base B1 can be reduced in unsymmetric 1:2 complexes B1…HOD…B2 by weaker bases B2. This weakening of the anti-cooperativity of the stronger base could be predicted quantitatively. Similarly, the anti-cooperativity of the weaker base B2 is strengthened in unsymmetric 1:2 complexes by stronger bases B1.
It is known that H-bonds XH…B can be strengthened by cooperativity with a second H-bond XH…XH…B. They can be weakened for water by anti-cooperativity of two H-bonds B…HOH…B. The H-bond B1…HO of 1:2 complexes B1…HOH…B2 can be weakened if the base strength of B2 is stronger than of B1 or strengthened if B2 is weaker than B1. Nature may use these possibilities in biochemistry. 相似文献
The synthesis and characterization of (tBuPBP)Ni(OAc) ( 5 ) by insertion of carbon dioxide into the Ni−C bond of (tBuPBP)NiMe ( 1 ) is presented. An unexpected CO2 cleavage process involving the formation of new B−O and Ni−CO bonds leads to the generation of a butterfly-structured tetra-nickel cluster (tBuPBOP)2Ni4(μ-CO)2 ( 6 ). Mechanistic investigation of this reaction indicates a reductive scission of CO2 by O-atom transfer to the boron atom via a cooperative nickel-boron mechanism. The CO2 activation reaction produces a three-coordinate (tBuP2BO)Ni-acyl intermediate ( A ) that leads to a (tBuP2BO)−NiI complex ( B ) via a likely radical pathway. The NiI species is trapped by treatment with the radical trap (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) to give (tBuP2BO)NiII(η2-TEMPO) ( 7 ). Additionally, 13C and 1H NMR spectroscopy analysis using 13C-enriched CO2 provides information about the species involved in the CO2 activation process. 相似文献