共查询到20条相似文献,搜索用时 15 毫秒
1.
Rutger J. I. Knoop Willem Vogelzang Jacco van Haveren Daan S. van Es 《Journal of polymer science. Part A, Polymer chemistry》2013,51(19):4191-4199
Furan‐2,5‐dicarboxylic acid (FDCA) is a widely advocated renewable substitute for terephthalic acid (TA). Preparation of high molecular weight FDCA based polyesters by an industrially common combination of melt polymerization and subsequent solid state post condensation is described. Ultimately, poly(ethylene 2,5‐furanoate) (PEF) with absolute Mn = 83,000 g mol?1 is obtained, determined by triple detection Size Exclusion Chromatography. The bulk polymer properties of FDCA based polyesters, necessary to evaluate their industrial potential were determined the Young's modulus of PEF is determined to be 2450 ± 220 MPa and the maximum stress 35 ± 8 MPa. The influence of crystallinity on the mechanical properties as function of temperature was determined by dynamic mechanical thermal analysis. A detailed differential scanning calorimetry study on the crystallization behavior of high molecular weight PEF allowed to calculate the equilibrium melting temperature (Tm0) of 239.3 and 239.7 °C for the first and second melting peak, respectively. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 4191–4199 相似文献
2.
C. W. Pyun 《Journal of Polymer Science.Polymer Physics》1979,17(12):2111-2115
If Mmin and Mmax are lower and upper bounds, respectively, to the molecular weights of different molecular weight species contained in a polymer, the weight-average to number-average molecular weight ratio M w/M n cannot exceed (1 + Mmax/Mmin)2/(4Mmax/Mmin). The ratio attains this maximum possible value if the masses of the two species with molecular weights Mmin and Mmax are equal and the masses of all the other species are negligibly small, corresponding to maximum spread in the molecular weight distribution within the specified bounds. Also for a given value of M w/M n = α, the Mmax cannot be smaller than [2α ? 1 + 2α1/2(α ? 1)1/2]Mmin. The minimum possible value of Mmax/Mmin consistent with α given is obtained in the case of maximum spread described above. If only one species is predominant, then both M w/M n and Mmax/Mmin approach unity, as is well known. Similar relations hold for the ratios of higher-order average molecular weights for which the role of the mass fractions is replaced by higher-order distribution functions. 相似文献
3.
4.
Marta Worzakowska 《Journal of Thermal Analysis and Calorimetry》2012,108(2):461-466
The thermal, viscoelastic, and mechanical properties of cured dicyclopentadiene (DCPD)-containing polymers prepared from novel
DCPD-modified unsaturated epoxypolyesters and styrene were evaluated. This was accomplished using thermogravimetric analysis,
differential scanning calorimetry, dynamic mechanical analysis, three-point bending test, and Brinell’s hardness. The thermal,
viscoelastic, and mechanical properties of DCPD-containing polymers were strongly dependent on chemical structure. The cross-linking
density (υ
e) of obtained networks increased with increasing content of carbon–carbon double bonds in the poly(ester) structure. In addition,
the introduction of DCPD rings into the poly(ester) structure increased the rigidity of the molecular backbone. It resulted
in obtaining polymers which showed great improvement in mechanical properties including remarkably higher storage modulus
(
E20 °\textC¢ E_{{20\,{}^{\circ}{\text{C}}}}^{'} ), flexural modulus at bending (E
mod), hardness, lower extension at maximum force (ε-F
max), as well as higher thermal stability. These good properties make these materials highly promising as potential candidates
for structural applications. 相似文献
5.
A. V. Rogov B. L. Grigorenko A. V. Bochenkova A. A. Granovskii A. V. Nemukhin 《Moscow University Chemistry Bulletin》2007,62(3):123-127
The mechanism of hydrolysis of deprotonated methyl triphosphate (MTP) to methyl diphosphate (MDP) and inorganic phosphate (Pi) in water clusters in the presence and absence of magnesium cations has been modeled. Modeling has been performed by the effective fragment potential-based quantum mechanical/molecular mechanical method. The energies and energy derivatives in the quantum subsystem including MTP, reacting water molecules, and Mg2+ has been calculated at the density functional theory (B3LYP) level, whereas water-water interactions have been described by the TIP3P model potential. The minimum-energy path for the reaction MTP + H2O → MDP + Pi is consistent with a two-stage dissociative process in the absence of Mg2+ and with a one-stage mechanism in the presence of Mg2+. 相似文献
6.
Jean -Marie André Eric G. Derouane Joseph G. Fripiat Daniel P. Vercauteren 《Theoretical chemistry accounts》1977,43(3):239-251
Quantum mechanical theoretical calculations have been performed on the linear atomic chain
in order to simulate the interaction of molecular hydrogen with the defects present at the surface of activated MgO. The total energy of the system, the relative energy of the various molecular orbitals, and the electronic charge distribution have been computed for various lattice parameters (d
O-O = 4.0–4.8 Å) as a function of the H-H (or O-H) separation. A symmetrical motion of the hydrogen nuclei with respect to the central Mg2+ vacancy was assumed. It is shown that chemisorption of hydrogen on surface O–ions sites results in the formation of pseudo-hydroxyl groups. For a small lattice parameter (4.0 Å), no stable state of molecular hydrogen has been found while an increase in the lattice parameter results in a uniform increase of the calculated activation energy for the molecular hydrogen activation process. A mechanism is proposed which is not so different from that put forward for the hydrogen activation by transition metal complexes. Molecular hydrogen is found to act as an electron donor. 相似文献
7.
Jean‐Franois Lutz Nazeem Jahed Krzysztof Matyjaszewski 《Journal of polymer science. Part A, Polymer chemistry》2004,42(8):1939-1952
Segmented terpolymers, poly(alkyl methacrylate)‐g‐poly(D ‐lactide)/poly(dimethylsiloxane) (PLA/PDMS), were prepared with a combination of the “grafting through” technique (macromonomer method) and controlled/living radical polymerization (atom transfer radical polymerization or reversible addition–fragmentation transfer polymerization). Two synthetic pathways were used. The first was a single‐step approach in which a low‐molecular‐weight methacrylate monomer (methyl methacrylate or butyl methacrylate) was copolymerized with a PLA macromonomer and a PDMS macromonomer. The second strategy was a two‐step approach in which a graft copolymer containing one macromonomer was chain‐extended by a copolymerization of the second macromonomer and the low‐molecular‐weight methacrylate. The kinetics of both synthetic approaches were investigated, showing that the polymerizations exhibited a controlled/living behavior. Furthermore, the molecular structure of the terpolymers (composition, molecular weight distribution, and microstructure) was investigated by two‐dimensional liquid chromatography. Well‐defined terpolymers with controlled branch distribution, composition (Fw,PMMA/Fw,PLA/Fw,PDMS ~ 50/30/20) molecular weight (Mn ~ 50,000 g · mol?1), and a narrow molecular weight distribution (Mw/Mn ~ 1.3) were prepared via both pathways. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1939–1952, 2004 相似文献
8.
D. R. Wiff Matatiahu Gehatia Anthony Wereta 《Journal of Polymer Science.Polymer Physics》1975,13(2):275-284
Two different polybenzimidazole (PBI) samples have been investigated in order to correlate the differences in molecular weight distribution (MWD) with changes in the elastic modulus and strength of undrawn and drawn fibers. It has been found that within the weight-average molecular weight range (7,000 ≤ Mw ≤ 13,000) there was no obvious correlation with Mw and Mn. However, one sample had a narrow unimodal molecular weight distribution and the other a wide bimodal molecular weight distribution. The small percentage of high molecular weight present in the latter sample gave its fibers better mechanical properties. X-ray diffraction studies showed that the orientation in both drawn fiber samples was equal. This isolated the effects of the molecular weight distribution on mechanical properties. 相似文献
9.
Mitsutoshi Fukuda Makoto Fukutomi Yoshio Kato Tsutomu Hashimoto 《Journal of Polymer Science.Polymer Physics》1974,12(5):871-890
A series of polystyrenes with weight-average molecular weight M?w up to 1.3 × 107 was prepared by anionic polymerization in tetrahydrofuran (THF). Each sample was characterized by gel-permeation chromatography, light scattering, and viscometry. It was found that each sample had an almost symmetrical and very narrow molecular weight distribution (M?w/M?n < 1.07). The mean-square unperturbed radius of gyration 〈S2〉0 was determined in trans-decalin at 20.4°C as 〈S2〉0 = 7.86 × 10?18M?w (cm2). The particle scattering factor was well represented by the Debye equation irrespective of solvent in the range of M?w < 4 × 106, and only a small deviation was observed in benzene at higher molecular weights. The penetration function Ψ ≡ A2M2/4π3/2NA〈S〉23/2 was found to approach a relatively low asymptotic value of 0.21–0.23 at molecular weights above 2 × 106 in benzene at 30°C, where A2 is the second virial coefficient and NA is Avogrado's number. It was also found that the theta temperature in trans-decalin was affected by the nature of polymer samples. A difference of about 3°C in the theta temperature was observed between two series of anionic polystyrenes, one prepared in THF and the other in benzene, but there was practically no difference in unperturbed chain dimension. 相似文献
10.
Giuliano Alagona Caterina Ghio Claudio Pratesi 《Journal of computational chemistry》1991,12(8):934-942
Parameters suitable to describe the conformational behavior of α – β unsaturated aminoacids and peptides in the framework of an existing force field for nucleic acids and proteins (S.J. Weiner, P.A. Kollman, D.A. Case, U. Chandra Singh, C. Ghio, G. Alagona, S. Profeta, Jr., and P. Weiner, J. Am. Chem. Soc., 106 , 765 (1984)) are proposed and tested. Attention is primarily focused on dehydrophenylalanine and dehydroalanine containing peptides. The values of the parameters needed were obtained from experimental measurements available in the literature and from ad hoc quantum mechanical calculations. The selected values have subsequently been adapted and refined through molecular mechanical simulations on model compounds, such as Ac-ΔPhe-NMe, for which it was possible to carry out STO-3G/SCF calculations to check selected points on the ?,ψ conformational map, and Ac-ΔAla-NMe, where the comparison was carried out at the 4-31G level. The newly determined force field was then applied to Ac-ΔPhe-Ala-ΔPhe-NMe, whose minimal energy structures allowed us to explain the different circular dichroism behavior observed in CH2Cl2 and in dioxane. Starting from two minimum energy geometries of this tripeptide, the full optimization with AM1 produced an independent guess to their structure and stability in good agreement with the molecular mechanical one. 相似文献
11.
We prove by elementary geometric methods and within the Born–Oppenheimer approximation that as the nuclei of a molecule are dissociated into spatially separated clusters, the discrete molecular energies approach sums of the energies of isolated subsystems. Our methods also show that the spectral projections associated with the discrete molecular spectrum asymptotically approach direct sums of suitable spectral projections for the isolated subsystems. These results apply to any system of particles interacting by asymptotically vanishing pair potentials. We prove that the 1/R expansion for discrete molecular potential curves is asymptotic as R → ∞, and we discuss the behavior of the coefficients of the 1/R expansion for the ground state of H2+. 相似文献
12.
Carl S. Ewig Lidia Smentek-Mielczarek B. Andes Hess Jr. 《Theoretical chemistry accounts》1989,75(2):129-141
A unified summary is presented of the mathematical approach developed by McDowell for employing perturbation theory to correct for basis-set incompleteness in ab initio SCF calculations. Revised expressions for the corrections to the wavefunction both in terms of orbitals and spin-orbitals are presented with explicit incorporation of the spin variables. Employing H2O as an example, we show that this approach is considerably more powerful for computing molecular energies with standard basis sets than was indicated by previous work. In particular at the higher levels of approximation it accurately reproduces the effect of polarization functions in sets such as 6-31G* and 6-31G**. The equilibrium molecular structure of H2O was also computed by this approach and found to give good accuracy. In each case perturbing functions coupled to both occupied and virtual orbitals are required for acceptable results. 相似文献
13.
A complete cycle of chemical transformations for the serine protease prototype reaction is modeled following calculations with the flexible effective fragment quantum mechanical/molecular mechanical (QM/MM) method. The initial molecular model is based on the crystal structure of the trypsin–bovine pancreatic trypsin inhibitor complex including all atoms of the enzyme within approximately 15–18 Å of the oxygen center O of the catalytic serine residue. Several selections of the QM/MM partitioning are considered. Fractions of the side chains of the residues from the catalytic triad (serine, histidine and aspartic acid) and a central part of a model substrate around the C–N bond to be cleaved are included into the QM subsystem. The remaining part, or the MM subsystem, is represented by flexible chains of small effective fragments, whose potentials explicitly contribute to the Hamiltonian of the QM part, but the corresponding fragment–fragment interactions are described by the MM force fields. The QM/MM boundaries are extended over the C–C bonds of the peptides assigned to the QM subsystem in the enzyme, C–C and C–N bonds in model substrates. Multiple geometry optimizations have been performed by using the RHF/6-31G method in the QM part and OPLSAA or AMBER sets of MM parameters, resulting in a series of stationary points on the complex potential-energy surfaces. All structures generally accepted for the serine protease catalytic cycle have been located. Energies at the stationary points found have been recomputed at the MP2/6-31+G* level for the QM part in the protein environment. Structural changes along the reaction path are analyzed with special attention to hydrogen-bonding networks. In the case of a model substrate selected as a short peptide CH3(NHCO-CH2)2 – HN–CO–(CH2–NHCO)CH3 the computed energy profile for the acylation step shows too high activation energy barriers. The energetics of this rate-limiting step is considerably improved, if more realistic model for the substrate is considered, following the motifs of the ThrI11–GlyI12–ProI13-–CysI14–LysI15–AlaI16–ArgI17–IleI18–IleI19 sequence of the bovine pancreatic trypsin inhibitor. 相似文献
14.
A. Flores J. Aurrekoetxea R. Gensler H. H. Kausch F. J. Baltá Calleja 《Colloid and polymer science》1998,276(9):786-793
The influence of molecular weight on the mechanical properties of isotactic poly(propylene) (iPP) and iPP blended with ethylene-propylene
copolymers has been investigated by means of the microhardness technique. The hardness (H) of iPP is shown to slightly decrease with increasing molecular mass, within the range of molecular weights investigated.
The H-decrease is correlated to a loss of crystallinity as the average molecular weight increases. On annealing, the mechanical
properties are enhanced as a consequence of an increase in both, the degree of crystallinity and the crystalline lamellar
thickness. A value of H
∞
c for iPP crystals of infinite thickness in the α-form is proposed for the first time. The inclusion of EPR particles in the
iPP matrix softens the material. This result could be explained in terms of an increase in the basal surface free energy of
the iPP crystals with increasing amount of rubber content.
Received: 2 February 1998 Accepted: 11 May 1998 相似文献
15.
The flow curves of fractionated polydimethylsiloxanes of different molecular weights were obtained over a wide range of shear rates, from 3 × 10?1 to 4.3 × 106 sec?1, by use of a gas-driven capillary viscometer designed to decrease the experimental error in high shear rate region. Non-Newtonian flow can occur at molecular weights below the critical molecular weight Mc for the entanglement of polymer chain. The critical molecular weight M′c for the onset of the non-Newtonian flow is identical with that of the segment of viscous flow. For the polymer of molecular weights from M′c to Mc, the upper Newtonian viscosity increases with an increase in molecular weight. Above Mc, the upper Newtonian viscosity is almost independent of the molecular weight. 相似文献
16.
H. Lee Woodcock Milan Hodošček Paul Sherwood Yong S. Lee Henry F. Schaefer III Bernard R. Brooks 《Theoretical chemistry accounts》2003,109(3):140-148
A replica path method has been developed and extended for use in complex systems involving hybrid quantum/classical (quantum
mechanical/molecular mechanical) coupled potentials. This method involves the definition of a reaction path via replication
of a set of macromolecular atoms. An “important” subset of these replicated atoms is restrained with a penalty function based
on weighted root-mean-square rotation/translation best-fit distances between adjacent (i±1) and next adjacent (i±2) pathway steps. An independent subset of the replicated atoms may be treated quantum mechanically using the computational
engine Gamess-UK. This treatment can be performed in a highly parallel manner in which many dozens of processors can be efficiently
employed. Computed forces may be projected onto a reference pathway and integrated to yield a potential of mean force (PMF).
This PMF, which does not suffer from large errors associated with calculated potential-energy differences, is extremely advantageous.
As an example, the QM/MM replica path method is applied to the study of the Claisen rearrangement of chorismate to prephenate
which is catalyzed by the Bacillus subtilis isolated, chorismate mutase. Results of the QM/MM pathway minimizations yielded an activation enthalpy ΔH
†† of 14.9 kcal/mol and a reaction enthalpy of −19.5 kcal/mol at the B3LYP/6-31G(d) level of theory. The resultant pathway was
compared and contrasted with one obtained using a forced transition approach based on a reaction coordinate constrained repeated
walk procedure (ΔH
†† =20.1 kcal/mol, ΔH
rxn = −20.1 kcal/mol, RHF/4-31G). The optimized replica path results compare favorably to the experimental activation enthalpy
of 12.7±0.4 kcal/mol.
Received: 16 December 2001 / Accepted: 6 September 2002 / Published online: 8 April 2003
Contribution to the Proceedings of the Symposium on Combined QM/MM Methods at the 22nd National Meeting of the American Chemical
Society, 2001.
Correspondence to: H.L. Woodcock e-mail: hlwood@ccqc.uga.edu
Acknowledgements. The authors thank Eric Billings, Xiongwu Wu, and Stephen Bogusz for helpful discussions and related work. The authors also
show grateful appreciation to The National Institutes of Health and The National Science Foundation for support of the current
research. 相似文献
17.
R. G. Rinaldi A. J. Hsieh M. C. Boyce 《Journal of Polymer Science.Polymer Physics》2011,49(2):123-135
Transparent poly(urethane urea) (TPUU) materials offer an avenue to enable material designs with potential to achieve simultaneous enhancements in both physical and mechanical properties. To optimize the performance required for each application, the molecular features that influence the microstructure, the glass transition temperature (Tg), the deformation mechanisms, and the mechanical deformation behavior must be understood and exploited. In this work, a comprehensive materials characterization of select model PUUs with tunable microstructures is addressed. Increasing the hard segment (HS) content increases the stiffness and flow stress levels, whereas altering the soft segment (SS) molecular weight from 2000 to 1000 g/mol leads to an enhanced phase mixing with a SS Tg shifted ~17 °K toward higher temperatures as well as broadening of the SS relaxation closer to room temperature. As a result, the 1K TPUU materials display greater rate‐dependent stiffening and strain hardening on mechanical deformation over the broad range of strain rates covered in this work (10?3 to 104 s?1). In such case of similar urea‐based HS content, the molar content of the urethane linkages, per stoichiometric requirements, is much higher in the 1K TPUUs than the 2K TPUUs. These additional urethane moieties lead to an increase in the extent of intermolecular interactions, via hydrogen bonding between the HS and the SS, providing not only further phase mixing and stronger rate sensitivity but also provide 1K TPUUs with drastically improved barrier properties. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011 相似文献
18.
Using an ab initio method the potential energy has been calculated for the 25 lowest molecular states of symmetries 2Σ+, 2Π, 2Δ for the molecular ion KH+. The calculation is based on nonempirical pseudopotentials and parameterized ℓ-dependent polarization potentials. Gaussian basis sets have been used for both atoms. The spectroscopic constants for 18 electronic sates have been calculated by fitting the calculated energy values to a polynomial in terms of the internuclear distance R. Through the canonical functions approach the eigenvalue Ev, the abscissas of the corresponding turning points (Rmin and Rmax) and the rotational constants Bv have been calculated up to 24 vibrational levels for the considered bound states. The comparison of the present results with those available in literature shows a very good agreement. 相似文献
19.
Using an ab initio method, the potential energy has been calculated for the 29 lowest molecular states of symmetries 2Σ+, 2Π, 2Δ for the molecular ion RbH+. The calculation is based on nonempirical pseudopotentials and parameterized ?‐dependent polarization potentials. Gaussian basis sets have been used for both atoms. The spectroscopic constants for 18 electronic sates have been calculated by fitting the calculated energy values to a polynomial in terms of the internuclear distance R. Through the canonical functions approach the eigenvalue Ev, the abscissas of the corresponding turning points (Rmin and Rmax) and the rotational constants Bv have been calculated up to 24 vibrational levels for the considered bound states. The comparison of the present results with those available in literature shows a very good agreement. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010 相似文献
20.
The procedure of combined semiempirical quantum mechanical (AM1) and molecular mechanical potential7 was used to study the nucleophilic addition of hydroxide to formaldehyde in solution. The gas phase AM1 potential surface is approximately 26 kcal/mol more exothermic than the corresponding ab initio 6-31 + G* calculation results. The free energy profile for the reaction in solution was determined by means of molecular dynamic simulations. The resulting free energy of activation is approximately 5 kcal/mol. The difference of the free energy of solvation between the reactant and the product states is about 38 kcal/mol. As the reaction goes on, the number of hydrogen bonds formed by the hydroxide oxygen with the surrounding water molecules decreases, whereas the number of hydrogen bonds formed by the carbonyl oxygen increases. There is no significant change in the total number of hydrogen bonds between the solute and the solvent molecules, and the average number of these hydrogen bonds is between five and six during the entire reaction process. These results are consistent with previous studies using a model based on ad initio 6-31 + G* calculations in the gas phase. The reaction path in solution is different from the gas phase minimum energy reaction path. When the two reactants are at a large distance, the attack route of the hydroxide anion in solution is close to perpendicular to the formaldehyde plane, whereas in the gas phase the route is collinear with the carbonyl group. These results suggests that although AM1 does not yield accurate energies in the gas phase, valuable insights into the solvent effects can be obtained through computer simulations with this combined potential. This combined procedure could be applied to chemical reactions within macromolecules, in which a quantitative estimation of the effects of the environment would not be easily attainable by another technique. © 1994 by John Wiley & Sons, Inc. 相似文献