It has been shown that the free volume fraction at T_g is not a universal parameter for linear polymers of different molecular structure. The reason is that the volume expansion at T_g is partially contributed from the change of the numbers of conformations of isolated molecular chains due to internal rotation. In this paper, glassy transformation was connected with internal rotation of isolated molecular chains, and the relationship between free volume fraction of polymers at T_g and energy e of rotational isomerization of isolated molecular chains was formulated, e=-k·T_g·In (△α·T_g/1-△α. T_g). The values of calculated from the above formula are in good agreement with those published in the literatures. Thus, the method described in this paper can be used to estimate a parameter for the flexibility of isolated molecular chains.7 相似文献
Structural changes in the local conformation of poly(N-isopropylacrylamide) (PNiPA) during the thermally and solvent-induced coil-globule transitions in an aqueous solution were studied by using attenuated total reflection / infrared (ATR/IR) spectroscopy combined with density functional theory (DFT) calculation. DFT calculation makes it possible to connect the spectral changes observed during the transitions with the structural changes of the local conformation of polymer chains. The results suggest that some of the intramolecular C=O···H-N hydrogen bonds of amide groups are broken, and the changes in local conformations occur during the coil-globule transitions of PNiPA. In this paper, an overview of our recent studies on the coil-globule transitions of PNiPA is given for introducing a new idea that may explain the stimulus sensitivities of PNiPA in solutions; the solubility of segments concerning with the local conformation of repeating monomer units is changed by an external perturbation, and then the polymer system shows the coil-globule transition. 相似文献
We have performed calculations of the glycine zwitterion surrounded by water molecules with the help of the mutually consistent
field (MCF) method and perturbation theoretical expressions. Two different models for the hydration shell have been chosen,
the glycine·6H2O and glycine·12H2O complexes, representing the most probable first and second solvation shell, respectively. To calculate the exchange and
charge transfer energy contributions we have applied approximative expressions derived from perturbation theory for weakly
overlapping subunits. For the sake of comparison we also calculated the interaction energy in the supermolecule approach for
the smaller of the two solvation complexes. Furthermore, we have investigated the part of the potential energy surface which
is determined by varying the lengths of the hydrogen bonds between glycine and water in the complex glycine·12H2O using the electrostatic approach. The exchange energy contribution to the interaction energy for different points on the
surface was approximated with the help of an analytical expression fitted to three directly calculated points. For the charge
transfer energy a polynomial expansion of second order was established on the basis of five values, computed with the aid
of the perturbation theoretical expression. To get a more detailed insight in the relatively strong hydrogen bonds between
the water molecules and the ionic hydrophilic parts of glycineab initio model studies on NH
4+
·3H2O and HCOO−·3H2O systems are reported. 相似文献
A series of hydrogen-bonded complexes between N-heterocyclic analogs of Indene and amino acid side-chain mimics have been analyzed employing second-order Møller-Plesset perturbation (MP2) theory and density functional theory with dispersion function (DFT-D) calculations with the aim of gaining greater insight in to the nature of intermolecular interactions in these systems. In this study, the hydrogen bonding ability of N-heterocyclic analogs of Indene towards amino acid side-chain mimics follows the sequence Azaindazole (AIND) > Indazole (IND) > Azaindole (AIN) > Indole (IN) whereas the hydrogen bonding ability of amino acid side-chain mimics towards N-heterocyclic analogs of Indene follows the sequence AcOH > MeNH2 > MeOH > MeSH. Bader’s theory of atoms in molecules (AIM) and natural bond orbitals (NBO) analyses are employed to elucidate the interaction characteristics in the complexes under study. The purpose of conducting these studies is to measure the relative strength of hydrogen bonding interactions such as N-H···O=C, N-H···O, N-H···S, N-H···N, and O-H···N in these complexes and their role in providing stability to the complexes. The AIM theory shows good correlation of the electron density and its Laplacian at the bond critical points (BCP) with the computed stabilization energy for all the complexes under study.
Summary: The electrochemical behaviour of four types of (phenylene ethynylene)‐alt‐(phenylene vinylene) hybrid polymers, 1 , 2 , 3 , and 4 have been investigated with respect to the influence of the grafted alkoxy side chains. In the case of the fully substituted polymers 2 , 3 , and 4 , the strong insulating nature of longer linear octadecyl or bulky branched 2‐ethylhexyl side chains lowers the HOMO levels of the polymers thereby increasing the discrepancy, ΔEg, between the electrochemical, Eequation/tex2gif-stack-1.gif, and the optical, Eequation/tex2gif-stack-2.gif, bandgap energies. Thus it is not possible to establish a direct correlation between the open circuit voltage, VOC, of bulk heterojunction solar cell devices of the configuration glass substrate/ITO/PEDOT:PSS/polymer 3 :PCBM(1:3, w/w)/LiF/Al and the HOMO energy levels of polymer 3 solely, as postulated in the literature. The photovoltaic (PV) parameters greatly depend on the grafted side chains.
Linear I–V curves of solar cell devices from polymers 3a – d , measured in the dark and under 100 mW · cm−2 solar simulator illumination. 相似文献
Heterogeneous network polymers composed of rigid polypeptide chains and flexible polyether chains were synthesized. That is, poly(L -glutamic acid) (PLGA) was crosslinked with poly(oxyethylene glycol) (PEG) at various carboxy/hydroxyl mole ratios K. The solubility tests and hydrolysis of heterogeneous network polymers suggest that the crosslinking reaction proceeds by esterification. The dynamic mechanical properties of these polymers(100 Hz, ?100–200°C) are greatly influenced by the presence of a trace of water and the weight per cent of PLGA. In addition, some of these polymers show only one maximum in the temperature dispersion of dynamic loss modulus E″ and tan δ, although their shape is rather broad. The x-ray photographs of these polymers show an amorphous halo or weak Debye-Sherrer rings. These findings suggest that these polymers are not simple adducts; neverthless PLGA and/or PEG domains exist. 相似文献
The self-consistent APW – k · p method is utilized to obtain the band structure of NaCl in the “muffin-tin” approximation. Qe have investigated the convergence of many intermediate results, e.g., crystalline potential, matrix elements of the momentum operator, and energy eigenvalues at the Γ point. The summation in reciprocal space, included in the definition of the matrix D of the theory, is performed by direct sum and also by a special points technique. For the convergence criteria used, the results converged after five iterations. 相似文献
Three coordination polymers containing Cd(II) and Co(II), connected via 4-[(3-pyridyl)methylamino]benzoate (L?), have been synthesized in hydrothermal conditions. In [Cd(L)Cl]n (1), adjacent Cd(II) cations are linked by carboxylates to give a dinuclear cluster. Pairs of L? bridge the dinuclear cluster to form double helical chains, and these chains are further linked by Cl? to produce a 4-connected net with (42?·?63?·?8) topology. [CdL2]n (2) contains 1-D ladder-like chains. The packing structure displays a 3-D supramolecular structure, with π?···?π interactions stabilizing the framework. [CoL2]n (3) has a 2-D extended supramolecular structure via π?···?π interactions of 1-D coordination polymers of 3. The crystal structures of 1–3 have been determined by single-crystal X-ray diffraction. Luminescent properties for 1 and 2 are discussed. 相似文献
The energy of an infinite, homogeneous electron gas is examined by second order perturbation theory using a Hartee-Fock rather than a noninteracting particle unperturbed state. The second order energy still diverges for small promotions k , albert than as ln|ln k| rather than as In k. 相似文献
The negative factor counting method (in its simple and matrix block form) for the determination of the density of states of disordered polymer chains and its applications to different aperiodic organic polymers are reviewed. The problems of the calculation of the correlation energy in large systems like polymers will be discussed. Different ways for the partitioning of an energy band into regions make it possible to perform Møller–Plesset perturbation theoretical calculations on polymers. Applications to hydrogen chains are presented. Finally, possible applications to disordered polymers are also discussed. 相似文献
It is important to know the rate of intra-molecular contact formation in proteins in order to understand how proteins fold clearly. Here we investigate the rate of intra-molecular contact formation in short two-dimensional compact polymer chains by calculating the probability distribution p(r) of end-to-end distance r using the enumeration calculation method and HP model on two-dimensional square lattice. The probability distribution of end-to-end distance p(r) of short two-dimensional compact polymers chains may consist of two parts, i.e. p(r) = p1(r) p2(r), where p1(r) and p2(r) are different for small r. The rate of contact formation decreases monotonically with the number of bonds N, and the rate approximately conforms to the scaling relation of k(N) ∝ N-α. Here the value of α increases with the contact radius a and it also depends on the percentage of H (hydrophobic) residues in the sequences of compact chains and the energy parameters of εHH, εHP and εPP . Some comparisons of theoretical predictions with experimental results are also made. This investigation may help us to understand the protein folding. 相似文献
We have investigated the dispersion interaction in hydrogen chain models via density functional theory-based symmetry-adapted perturbation theory using the asymptotically corrected PBE0 energy functional. The quasimetallic and the insulating prototype systems were chosen to be hydrogen chains with equally and alternately spaced H(2) units, respectively. The dependence of the dispersion energy on the chain length for quasimetallic and insulating cases has been determined for two chains arranged either in pointing or in parallel geometries. The results are compared with those previously calculated from a continuum coupled-plasmon approach [Phys. Rev. B 77, 075436 (2008)]. The interaction energy has also been modeled by pairwise summations over short fragments of the chains, demonstrating the failure of the additivity principle for the quasimetallic case, while confirming that the additivity is a qualitatively reasonable hypothesis for the insulating case. 相似文献
The separation and isolation of semiconducting and metallic single‐walled carbon nanotubes (SWNTs) on a large scale remains a barrier to many commercial applications. Selective extraction of semiconducting SWNTs by wrapping and dispersion with conjugated polymers has been demonstrated to be effective, but the structural parameters of conjugated polymers that dictate selectivity are poorly understood. Here, we report nanotube dispersions with a poly(fluorene‐co‐pyridine) copolymer and its cationic methylated derivative, and show that electron‐deficient conjugated π‐systems bias the dispersion selectivity toward metallic SWNTs. Differentiation of semiconducting and metallic SWNT populations was carried out by a combination of UV/Vis‐NIR absorption spectroscopy, Raman spectroscopy, fluorescence spectroscopy, and electrical conductivity measurements. These results provide new insight into the rational design of conjugated polymers for the selective dispersion of metallic SWNTs. 相似文献