The effect of alkyl substitution on the conformation of intramolecularly hydrogen bonded β-γ (enolic) unsaturated alcohols

The equilibrium between associated (OH ⋯ π) and free OH conformations of β-γ unsaturated alcohols has been found to be highly dependent upon the position of alkyl substitution. Two types of intramolecularly hydrogen bonded conformations can be identified which are separated by a difference in OH stretching frequency of approximately 10 cm⁻¹. The occurence of either conformation is dependent upon methyl substitution at the β carbon and the conformations are mutually exclusive. The intensity ratio of the free and intramolecularly hydrogen bonded OH stretching bands are dependent upon the alkyl chain length and also the primary or secondary nature of the alcohol. In both cases, changes in relative intensity result from steric interactions. Increased alkyl chain length decreases the relative intensity of the intramolecularly hydrogen bonded band, whilst a change from primary to secondary alcohol increases the relative intensity of the intramolecularly hydrogen bonded band.     

The effects of adsorbed water on tensile strength and Young's modulus of moldings determined by means of a three-point bending method

Young's moduli (E) of three representative tableting excipients and their mix powders were measured for compressed rectangular beam specimens over a range of porosities using a three-point bending technique. We also examined the effects of the amount of water adsorbed on the tensile strength of these specimens. The maximal tensile strength (sigma(max)) decreased with increasing water vapor adsorption for microcrystalline cellulose (MCC) and mixed powders of lactose and MCC. Sigma(max) increased with increasing compression stress and specimen weight for all samples. Sigma(max) of an alpha-lactose and cornstarch mixture with a ratio of 7:3 showed a large value. Young's modulus (E) and the crushing energy (CE) of MCC were larger than those of the other samples. Young's modulus of specimens decreased as the proportion of alpha-lactose increased. Disintegration time (DT) of tablets comprised of lactose and MCC mixture was much faster than those of tablets comprised of individual powders. This appeared to demonstrate the effect of MCC swelling on the disintegration time of the tablet. The disintegration time of the lactose/cornstarch series increased only when Young's modulus increased sharply.     

Thermoelastic behaviour of polyethylene terephthalate on approaching the glass transition range

The mechanical and thermal properties of the simple linear reversible extension of a solid can be well described by means of Young's modulus and linear thermal expansion coefficient. In this paper it is shown that on stretching near the glass transition temperature the temperature dependence of Young's modulus must also be taken into account.Dedicated to Prof. Dr. H.-G. Kilian on the occasion of this 60th birthday.     

Polymer translocation through a nanopore under an applied external field

We investigate the dynamics of polymer translocation through a nanopore under an externally applied field using the two-dimensional fluctuating bond model with single-segment Monte Carlo moves. We concentrate on the influence of the field strength E, length of the chain N, and length of the pore L on forced translocation. As our main result, we find a crossover scaling for the translocation time tau with the chain length from tau approximately N2nu for relatively short polymers to tau approximately N1+nu for longer chains, where nu is the Flory exponent. We demonstrate that this crossover is due to the change in the dependence of the translocation velocity v on the chain length. For relatively short chains v approximately N-nu, which crosses over to v approximately N(-1) for long polymers. The reason for this is that with increasing N there is a high density of segments near the exit of the pore, which slows down the translocation process due to slow relaxation of the chain. For the case of a long nanopore for which R parallel, the radius of gyration Rg along the pore, is smaller than the pore length, we find no clear scaling of the translocation time with the chain length. For large N, however, the asymptotic scaling tau approximately N1+nu is recovered. In this regime, tau is almost independent of L. We have previously found that for a polymer, which is initially placed in the middle of the pore, there is a minimum in the escape time for R parallel approximately L. We show here that this minimum persists for weak fields E such that EL is less than some critical value, but vanishes for large values of EL.     

Langevin dynamics simulations of polymer translocation through nanopores

We investigate the dynamics of polymer translocation through a nanopore using two-dimensional Langevin dynamics simulations. In the absence of an external driving force, we consider a polymer which is initially placed in the middle of the pore and study the escape time tau(e) required for the polymer to completely exit the pore on either side. The distribution of the escape times is wide and has a long tail. We find that tau(e) scales with the chain length N as tau(e) approximately N(1+2nu), where nu is the Flory exponent. For driven translocation, we concentrate on the influence of the friction coefficient xi, the driving force E, and the length of the chain N on the translocation time tau, which is defined as the time duration between the first monomer entering the pore and the last monomer leaving the pore. For strong driving forces, the distribution of translocation times is symmetric and narrow without a long tail and tau approximately E(-1). The influence of xi depends on the ratio between the driving and frictional forces. For intermediate xi, we find a crossover scaling for tau with N from tau approximately N(2nu) for relatively short chains to tau approximately N(1+nu) for longer chains. However, for higher xi, only tau approximately N(1+nu) is observed even for short chains, and there is no crossover behavior. This result can be explained by the fact that increasing xi increases the Rouse relaxation time of the chain, in which case even relatively short chains have no time to relax during translocation. Our results are in good agreement with previous simulations based on the fluctuating bond lattice model of polymers at intermediate friction values, but reveal additional features of dependency on friction.     

弹性竿模型下超螺旋DNA分子的构象研究

采用弹性竿模型 (Elasticrodmodel) ,用MonteCarlo方法对DNA分子的构象进行研究 .通过计算发现 ,DNA分子的能量是由弯曲势能EB 和扭转势能ET 两部分组成 ,通常EB 比ET 大一至两个数量级 .同时给出了均方回转半径与链长之间的关系为〈R2g〉 =1 1 69 5 -3 5×n +0 0 2 5×n2 ,它体现了DNA分子结构的特点 .验证了公式Lk=Wr+Tw ,得出Lk与Wr比较接近的结论 ,考虑DNA分子的构型 ,意味着DNA分子容易被弯曲而不易被扭转 ,但随着连接系数的增加 ,DNA被扭转的几率也在增加 .这为分析DNA分子的结构特征提供了一种新方法     

Energy Elasticity of Tie Molecules in Semicrystalline Polymers

Elastic response of the disordered phase between crystal lamellae in semicrystalline polymers is modelled on the assumption that the stress is transferred by bridging (tie) molecules. The deformation characteristics of short poly(methylene) (PM) bridges were computed by using two methods: (a) the single‐molecule loading by molecular mechanics (MM) calculations and (b) the chain‐ensemble averaging by lattice simulations. The energy elastic functions ensuing from both methods differ considerably. In MM the loading of chains containing numerous gauche defects by an external force F yields the sawtooth‐like profile of the force (F)–length (R) functions brought about by the stress‐induced gauche–trans conformational transitions. The Young's moduli E of PM chains containing several gauche defects can be less than 1% of the all‐trans value E_T; by elimination of the defects the moduli steeply increase. In contrast, the ensemble‐averaging approach gives a smooth increase of the (positive) elastic force f with chain length R and a decrease of the (negative) energy component of the elastic force f_U with R. Both energy deformation mechanisms, single‐chain loading (by F) and statistical (by f_U), are complementary and can simultaneously be operative in the interlamellar (IL) phase. Their proportion in the stretching process should depend on the chain mobility and structural homogeneity (history) of the sample, particularly on the presence of the so‐called rigid amorphous fraction in the IL phase.     

Chain-length effects on molecular conformation in and chirality of self-assembled monolayers of alkoxylated benzo[c]cinnoline derivatives on highly oriented pyrolytic graphite

Self-assembled structures of alkoxylated benzo[c]cinnoline derivatives prepared on highly oriented pyrolytic graphite at room temperature from their solutions in solvents such as 1-phenyloctane, toluene, and 1-octanol were studied by scanning tunneling microscopy. The alkoxy chain length markedly affected the molecular conformations in 2-dimensional assemblies of these derivatives. Long-chain derivatives adopted the trans conformations more often than cis, whereas short-chain derivatives took exclusively the cis conformations in the self-assembled monolayers (SAMs). For the derivatives of intermediate chain lengths, polymorphism existed, with four molecular conformations identified experimentally. Experimental evidence substantiated the formation of chiral SAM structures at the surface, which can be explained by the conformations of the molecules. The chirality was also affected by the chain length of the molecules. A simple method analyzing the angles between different domains in the SAMs was used to identify the molecular conformations and to predict their relative structures.     

Bond rupture in highly oriented crystalline polymers

The strength-limiting process in the fracture of semicrystalline fibers and highly oriented films is the rupture of tie molecules connecting the folded chain lamellae in the machine direction. This view is supported by the data on stress and temperature dependence of lifetime of fibers under load and on radical formation during the fracture experiment. The observed tensile strength, however, is about 10 times smaller and the number of fractured chains between 100 and 1000 times larger than expected on the basis of the known number of tie molecules in the fracture plane. This discrepancy is a consequence of the inhomogeneity of the micromorphology of fiber structure, which causes a much larger stress concentration on the most unfavorably located tie molecules than the average value one would expect in the case of perfectly uniform stress distribution on identical tie molecules. The fluctuation of amorphous layer thickness, of number and length of tie molecules, produces such a high stress concentration on some tie molecules throughout the sample that they rupture long before the average stress concentration is sufficient for chain fracture. By accumulation of damage caused by gradual chain rupture the weakening of the sample locally proceeds so far that at the maximum damage concentration, microcracks start to form, and the fiber breaks.     

Kinetics of loop formation in polymer chains

We investigate the kinetics of loop formation in ideal flexible polymer chains (the Rouse model), and polymers in good and poor solvents. We show for the Rouse model, using a modification of the theory of Szabo, Schulten, and Schulten, that the time scale for cyclization is tau(c) approximately tau(0)N(2) (where tau(0) is a microscopic time scale and N is the number of monomers), provided the coupling between the relaxation dynamics of the end-to-end vector and the looping dynamics is taken into account. The resulting analytic expression fits the simulation results accurately when a, the capture radius for contact formation, exceeds b, the average distance between two connected beads. Simulations also show that when a < b, tau(c) approximately N(alpha)(tau), where 1.5 < alpha(tau) < or = 2 in the range 7 < N < 200 used in the simulations. By using a diffusion coefficient that is dependent on the length scales a and b (with a < b), which captures the two-stage mechanism by which looping occurs when a < b, we obtain an analytic expression for tauc that fits the simulation results well. The kinetics of contact formation between the ends of the chain are profoundly effected when interactions between monomers are taken into account. Remarkably, for N < 100, the values of tau(c) decrease by more than 2 orders of magnitude when the solvent quality changes from good to poor. Fits of the simulation data for tau(c) to a power law in N (tau(c) approximately N(alpha)(tau)) show that alpha(tau) varies from about 2.4 in a good solvent to about 1.0 in poor solvents. The effective exponent alpha(tau) decreases as the strength of the attractive monomer-monomer interactions increases. Loop formation in poor solvents, in which the polymer adopts dense, compact globular conformations, occurs by a reptation-like mechanism of the ends of the chain. The time for contact formation between beads that are interior to the chain in good solvents changes nonmonotonically as the loop length varies. In contrast, the variation in interior loop closure time is monotonic in poor solvents. The implications of our results for contact formation in polypeptide chains, RNA, and single-stranded DNA are briefly outlined.     

Ultrasonic measurements of the elastic moduli of ultradrawn polyoxymethylene

We have employed an ultrasonic method to measure from ?40 to 60°C the five independent elastic moduli C₁₁, C₁₃, C₃₃, C₄₄, and C₆₆ of polyoxymethylene with draw ratio λ from 1 to 26 prepared by continuous drawing under microwave heating. The elastic moduli are controlled by three major factors: molecular orientation in the crystalline regions, fraction of noncrystalline taut tie molecules, and void content. The steep rise in the axial extensional modulus C₃₃ and axial Young's modulus E₀ with increasing draw ratio results from the alignment of chains in the crystalline blocks and an increase in the number of disordered taut tie molecules. Below the γ relaxation (located at 0°C at our measurement frequency of 10 MHz), these two factors also give rise to a slight decrease in the transverse extensional modulus C₁₁, Young's modulus E₉₀ and shear modulus C₆₆. At high temperature where the amorphous regions have very low modulus, the stiffening effect of taut tie molecules becomes dominant, leading to an increase in all moduli as λ increases from 1 to 10. At higher λ the void fraction increases appreciably, causing small decreases in E₉₀, C₁₁, and C₆₆ at all temperatures.     

Deformation mechanisms of constrained polymer chains. I. Geometrically induced correlations in the deformation response mechanisms of tie molecules

The geometrical constraints acting on sections of tie molecules in noncrystalline regions severely limit the number and type of available polymer chain conformations. It is shown that these constraints induce explicit correlations in the rotations about the backbone bonds. These correlated rotations, in turn, specify distinct structural conversion paths which define the molecular mechanisms underlying the deformation response of tie molecules. Application of these constraining relationships to highly oriented polyethylene shows that the kink and jog structures of tie molecules can be decomposed into combinations of three primary conformational building blocks. Each of the basic conformational subunits follow an explicit set of dihedral angle correlations and, consequently, imparts specific characteristics to the composite structure of tie molecules. It is proposed that the composite response characteristics of tie molecules can be described as linear combinations of the response characteristics of these three primary conformational subunits.     

Tight-binding molecular dynamics study of the role of defects on carbon nanotube moduli and failure

We performed tight-binding molecular dynamics on single-walled carbon nanotubes with and without a variety of defects to study their effect on the nanotube modulus and failure through bond rupture. For a pristine (5,5) nanotube, Young's modulus was calculated to be approximately 1.1 TPa, and brittle rupture occurred at a strain of 17% under quasistatic loading. The predicted modulus is consistent with values from experimentally derived thermal vibration and pull test measurements. The defects studied consist of moving or removing one or two carbon atoms, and correspond to a 1.4% defect density. The occurrence of a Stone-Wales defect does not significantly affect Young's modulus, but failure occurs at 15% strain. The occurrence of a pair of separated vacancy defects lowers Young's modulus by approximately 160 GPa and the critical or rupture strain to 13%. These defects apparently act independently, since one of these defects alone was independently determined to lower Young's modulus by approximately 90 GPa, also with a critical strain of 13%. When the pair of vacancy defects adjacent, however, Young's modulus is lowered by only approximately 100 GPa, but with a lower critical strain of 11%. In all cases, there is noticeable strain softening, for instance, leading to an approximately 250 GPa drop in the apparent secant modulus at 10% strain. When a chiral (10,5) nanotube with a vacancy defect was subjected to tensile strain, failure occurred through a continuous spiral-tearing mechanism that maintained a high level of stress (2.5 GPa) even as the nanotube unraveled. Since the statistical likelihood of defects occurring near each other increases with nanotube length, these studies may have important implications for interpreting the experimental distribution of moduli and critical strains.     

Cndo/2 conformational calculation for conjugated and non-conjugated molecular systems

In order to check the effectiveness of the well known CNDO/2 method for predicting molecular conformations, twenty-six molecular systems, involving B, C, N, O, F, Si, S and Cl atoms, were studied. If these molecules are classified into three groups, according to simple rules, the CNDO/2 method is found to fail systematically in one of them. In this group the twisted bond is expected to be delocalized. It is concluded that the CNDO/2 method is not useful in predicting conformations of molecules in which the twisted bond is delocalized.     

Experimental study of molecular entanglement in polymer networks

Elastomeric molecular networks have been prepared by endlinking polydimethyl siloxane molecules having functional chain ends, both in the presence of an unreactive polymeric diluent and in the undiluted state. Values of tensile (Young) modulus were found to be in good agreement with the simple molecular theory of rubberlike elasticity for networks prepared in a highly diluted state. For concentrated systems the modulus was anomalously high, however. The discrepancy can be attributed to chain entanglements. A second interpenetrating network was introduced into networks formed in the diluted state by replacing the diluent polymer by reactive polymer, which was then gelled in situ. The modulus of these combined networks was much higher than the sum of the moduli of the constituent networks, implying a large contribution from molecular entanglements. © 1994 John Wiley & Sons, Inc.     

Isometric and Isotensional Force‐Length Profiles in Polymethylene Chains

Summary: The force‐length curves and related thermodynamic quantities of single polymethylene (PM) chains in the high‐force region were calculated by the statistical mechanics. Two statistical mechanics ensembles (isometric and isotensional) were used to represent the chain stretching under the conditions, respectively, of the fixed length L or of the fixed force F in single‐chain experiments by AFM and related techniques. The input deformation potentials of highly extended conformations of PM chains were obtained from the molecular‐mechanics calculations. Variations of the energy, entropy, and Helmholtz energy with the end‐to‐end length L of chains were computed under the condition of fixed length. The ensuing isometric profile of the mean force 〈F〉(L) is non‐monotonic, featuring a sawtooth‐like pattern of ascending peaks. In contrast, the mechanical equation under isotensional conditions, given by the variation of the mean length 〈L〉(F), shows a conventional monotonous shape with a distinct plateau region at low temperatures. The considerable difference in the shapes of 〈F〉(L) and 〈L〉(F) curves arises from the large fluctuations of mean values due to the small number of conformers present in molecules at high strains. The computed data should be relevant to the AFM stretching experiments on short polyethylenes or on other soft matter materials involving linear paraffinic chains. It is argued that the dual character of elastic response described by the conjugated force profiles is a universal feature of mechanochemistry of chain molecules whenever the chain length discontinuously increases at a transition.

Force‐length curves of short polymethylene chains at 300 K.     

Polymer translocation through a nanopore. II. Excluded volume effect

Following our previous study of a Gaussian chain translocation, we have investigated the transport of a self-avoiding chain from one sphere to another sphere through a narrow pore, using the self-consistent field theory formalism. The free energy landscape for polymer translocation is significantly modified by excluded volume interactions among monomers. The free energy barrier for the placement of one of the chain ends at the pore depends on the chain length N nonmonotonically, in contrast to the N-independence for Gaussian chains. This results in a nonmonotonic dependence of the average arrival time [tau0] on N for self-avoiding chains. When the polymer chain is partitioned between the donor and recipient spheres, a local free energy minimum develops, depending on the strength w of the excluded volume interaction and the relative sizes of the donor and recipient spheres. If the sizes of spheres are comparable, the average translocation time tau (the average time taken by the polymer, after the arrival at the pore, to convert from the donor to the recipient) increases with an increase in w for a fixed N value. On the other hand, for the highly asymmetric sizes of the donor and recipient spheres, tau decreases with an increase in w. As in the case of Gaussian chains, tau depends nonmonotonically on the pore length.     

二维HP格点模型中的紧密高分子链构象及其热力学性质的研究

采用二维HP模型用精确计数法和MonteCarlo方法研究了链长为N(≤ 2 2 )的紧密高分子链的构象和热力学性质 .发现不同HP序列的紧密高分子链的平均自由能和平均配分函数与链长N存在关系 :〈F〉=aN+b ,　ln〈Z〉=a′N +b′ .同时发现对于可折叠成基态且简并度为 1的紧密高分子链 ,其平均自由能和平均配分函数与链长N也存在相似的关系 .在HP模型中对于链长为N的紧密高分子链 ,存在着 2 N + 1 个不同的HP序列 .我们发现可以折叠成基态且简并度为 1的蛋白质分子的HP序列数目NS 为NS =a× 2 N+ 1 　 (a =0 0 2 5 ) ,对应的HP序列中 ,疏水基团 (H)数目的含量为 4 0 %～ 6 0 %的序列出现的几率最大 .同时在这些紧密高分子链中有些具有相同的结构 ,发现结构的‘简并度’为 3 3～ 4 0 (10≤N≤ 16 ) .在紧密高分子链折叠过程中 ,折叠的初期能量下降比较快 ,折叠的中期能量下降比较缓慢 ,折叠的后期能量下降也是比较快     

Comparison of one-particle basis set extrapolation to explicitly correlated methods for the calculation of accurate quartic force fields, vibrational frequencies, and spectroscopic constants: application to H2O, N2H+, NO2+, and C2H2

One-particle basis set extrapolation is compared with one of the new R12 methods for computing highly accurate quartic force fields (QFFs) and spectroscopic data, including molecular structures, rotational constants, and vibrational frequencies for the H(2)O, N(2)H(+), NO(2)(+), and C(2)H(2) molecules. In general, agreement between the spectroscopic data computed from the best R12 and basis set extrapolation methods is very good with the exception of a few parameters for N(2)H(+) where it is concluded that basis set extrapolation is still preferred. The differences for H(2)O and NO(2)(+) are small and it is concluded that the QFFs from both approaches are more or less equivalent in accuracy. For C(2)H(2), however, a known one-particle basis set deficiency for C-C multiple bonds significantly degrades the quality of results obtained from basis set extrapolation and in this case the R12 approach is clearly preferred over one-particle basis set extrapolation. The R12 approach used in the present study was modified in order to obtain high precision electronic energies, which are needed when computing a QFF. We also investigated including core-correlation explicitly in the R12 calculations, but conclude that current approaches are lacking. Hence core-correlation is computed as a correction using conventional methods. Considering the results for all four molecules, it is concluded that R12 methods will soon replace basis set extrapolation approaches for high accuracy electronic structure applications such as computing QFFs and spectroscopic data for comparison to high-resolution laboratory or astronomical observations, provided one uses a robust R12 method as we have done here. The specific R12 method used in the present study, CCSD(T)(R12), incorporated a reformulation of one intermediate matrix in order to attain machine precision in the electronic energies. Final QFFs for N(2)H(+) and NO(2)(+) were computed, including basis set extrapolation, core-correlation, scalar relativity, and higher-order correlation and then used to compute highly accurate spectroscopic data for all isotopologues. Agreement with high-resolution experiment for (14)N(2)H(+) and (14)N(2)D(+) was excellent, but for (14)N(16)O(2)(+) agreement for the two stretching fundamentals is outside the expected residual uncertainty in the theoretical values, and it is concluded that there is an error in the experimental quantities. It is hoped that the highly accurate spectroscopic data presented for the minor isotopologues of N(2)H(+) and NO(2)(+) will be useful in the interpretation of future laboratory or astronomical observations.