bluuebunny/crossref_metadata_2025_split huggingface dataset:
On the product distribution of degradable polymers
Authors: Carl Wilfried 
Published: April 2003

AbstractWe present a simplified model for calculating the product distribution of degradable polymers which is based on a grand canonical approach. The distribution depends on two physical parameters, the energy of a bond formation and the initial concentration of monomers. Due to its simplicity this model covers the two limiting cases, namely a polymerization process (no degradation) and micellar association (dominant degradation), and allows for straightforward extension to branched chain topologies or flow fields. In shear flow, a flow rate dependent length distribution leads to shear thinning.
Distribution of the shape asymmetry of linear and star‐branched random walks
Authors: Zifferer Gerhard 
Published: April 2003

AbstractLinear and star‐branched (off‐lattice) random walks with up to F = 96 arms and a total chain‐length of 961 segments have been produced by means of Monte Carlo simulation. The probability distribution of the asphericity δ*–a quantity ranging from 0 in case of perfect radical symmetry to 1 for completely elongated (rodlike) shapes–has been calculated. In addition, the correlation between δ* and other shape‐descriptors as well as between δ* and quantities characteristic of the size of configurations have been evaluated. In accordance with existing literature the distribution H(δ*) of δ* is found to be very broad in case of linear chains. With increasing number of arms. H(δ*) becomes sharper but for F = 96 still ranges within an interval [0, ≈0,05]. Of course, other shape descriptors are directly correlated with the asphericity. Actually, for all functionalities evaluated, the global size of configurations is strongly correlated with δ* as well: the larger the asymmetry the larger the dimensions of the configuration under consideration.
Grafted polymers under strong shear: Scaling and non‐equilibrium Monte Carlo studies
Authors: Lai Chee‐Yuen , Lai Pik‐Yin 
Published: April 2003

AbstractEnd‐grafted polymer chains exposed to strong shear solvent flow in the x‐direction are investigated by a non‐equilibrium Monte Carlo method using the bond‐fluctuating model. The solvent flow is modelled by an enhanced jump rate of monomers in the flow direction. Under strong enough shear flow, the shear force is non‐linear with the blob size ζ depending on the shear force and not on the monomer volume fraction ϕ. For the case of a grafted single chain, our data on the end‐to‐end distance of the polymer for a wide range of shear rate γ agree well with the proposed scaling form and compare reasonably well with the Langevin force law in the extreme strong shear case. Also the scaling for the blob size is derived with ζ = aNvK(γN2+v) for some scaling function K, and verified by our simulation data. For the case of a polymer brush, we derive the crossover force scale F* separating the weak and strong shear regime with F*a/(kT) = ϕv/(3v−1) where v is the usual self‐avoiding walk exponent. For the case of a polymer brush under strong shear, the velocity profile, v(z), inside and above the brush is calculated analytically by treating the flow as in a porous medium and solving the Brinkman equation. The solution of the velocity profile is then combined with non‐equilibrium Monte Carlo simulation data which allow a self‐consistent determination of the chain end‐to‐end length and the incline angle of the chain. Also we derive the scaling form for the positions x(n) of the nth monomer and verify it by our simulation data. We further obtain an analytical expression for x(n) in terms of the effective viscosity η of the model, given by F = ηav(z). The effective viscosity is also found from our simulation to be ηa2/(kT) ≃ 0.02 ± 0.005.
On the importance of including intermolecular correlations in the measurement of polymer interdiffusion by fluorescence nonradiative energy transfer
Authors: O'Neil Gregory A. , Torkelson John M. 
Published: April 2003

AbstractThere has been a great deal of recent interest in monitoring polymer interdiffusion with fluorescence nonradiative energy transfer (NRET), a typical experiment consisting of polymer chains labeled with a donor chromophore interdiffusing with chains labeled with an acceptor chromophore, causing an increase in energy transfer efficiency, E. A factor nearly always ignored in models of this process is the fact that chromophore interactions are screened out because of intermolecular correlation effects, leading to a lower E than would otherwise be expected. The model of Dhinojwala and Torkelson (Macromolecules 27, 4817 (1994)) has been modified to include correlations in order to explore the impact that ignoring correlations has on the diffusion coefficients calculated in such experiments. When correlations are included in the model, E is proportional to (Dpt)1/2/x for t<x2/(16Dp) (where x is the thickness of the donor‐labeled film and Dp is the polymer diffusion coefficient), similar to when correlations are ignored. Within this linear regime errors in Dp associated with ignoring correlations are typically less than 20% for logically defined systems, which may be insignificant given the uncertainty with which diffusion coefficients are normally reported. However, this study clearly demonstrates that precautions should be used in all experiments of this type to stay in this linear regime, as improperly accounting for or ignoring correlation effects at longer diffusion times can lead to the spurious result that apparent diffusion coefficients decrease with annealing time, by factors of 2 to 5 to as much as an order of magnitude or more at sufficiently long time. Systems designed to minimize correlation effects should have a large Förster radius and a sufficiently high ratio of acceptors to donors; with such a situation and the use of NRET data from the linear regime of E vs. t1/2, it is possible under many experimental circumstances to determine diffusion coefficients within small error even when the effects of correlations are ignored.
Distribution functions and dynamical properties of stiff macromolecules
Authors: Winkler Roland G. , Harnau Ludger , Reineker Peter 
Published: April 2003

AbstractAn analytically tractable model for chain molecules with bending stiffness is presented and the dynamical properties of such chains are investigated. The partition function is derived via the maximum entropy principle taking into account the chain connectivity as well as the bending restrictions in form of constraints. We demonstrate that second moments agree exactly with those known from the Kratky‐Porod wormlike chain. Moreover, various distribution functions are calculated. In particular, the static structure factor is shown to be proportional to 1/q at large scattering vectors q. The equations of motion for a chain in a melt as well as in dilute solution are presented. In the latter case the hydrodynamic interaction is taken into account via the Rotne‐Prager tensor. The dynamical equations are solved by a normal mode analysis. In the limit of a flexible chain the model reproduces the well‐known Rouse and Zimm dynamics, respectively, on large length scales, whereas in the rod limit the eigenfunctions correspond to bending motion only. In addition, the coherent and incoherent dynamic structure factor is discussed. For melts we show that at large scattering vectors the incoherent dynamic structure factor is a universal function of only the combination q8/3tp1/3, where 1/(2p) is the persistence length of the macromolecules. The comparison of the theoretical results with quasielastic neutron and light scattering experiments of various polymers in solution and melt exhibits good agreement. Our investigations show that local stiffness strongly influences the dynamics of macromolecules on small length scales even for long and flexible chains.
Distributions of cyclic oligomers formed by irreversible step‐growth polymerisation. Results from kinetic modeling
Authors: Ercolani Gianfranco , Mencarelli Paolo 
Published: April 2003

AbstractThe distribution of cyclic oligomer concentrations (examined up to the cyclic pentamer) in the process of step‐growth polymerisation decreases, in concentrated solution, with i−2.5 if the chains follow Gaussian statistics (i: degree of polymerization). This picture is analogous to that shown by cyclic oligomer distributions obtained under thermodynamic control and is in contrast with that shown by cyclic oligomer distributions obtained by irreversible chain‐growth polymerisation which is characterized by a gradient of −1.5. This result has been initially obtained by numerical integration of the differential rate equations pertinent to a kinetic model relative to the reaction of a bifunctional reactant A—B under batchwise conditions, and then confirmed by analytical integration of the differential rate equations under the condition that propagation is much faster than cyclisation. The case in which the monomeric ring is strained has been also investigated. Also in this case the obtained distribution is analogous to that shown under equilibrium conditions. From an examination of the distributions of the ring yields, the best conditions for the preparation of either the cyclic monomer or the cyclic dimer are suggested.
Chain‐length‐dependent termination rate constant: Effect on molecular weight distributions formed by pulsed laser radical polymerization
Authors: Ntkitin Anatolij N. , Evseev Alexander V. 
Published: April 2003

AbstractFor polymerization initiated by an arbitrary sequence of laser pulses a numerical technique for calculating molecular weight distributions (MWDs) is developed, which takes into consideration the chain length dependence of the termination rate constant kt. The MWDs for methyl methacrylate and styrene are calculated by use of α and k0 values (for the law k = k0(i)−α of termination of radicals with chain length i) and averages \documentclass{article}\pagestyle{empty}\begin{document}$ \overline {(i,{\rm }j)} $\end{document} (for rate constants k = k0\documentclass{article}\pagestyle{empty}\begin{document}$ \overline {(i,{\rm }j)} $\end{document} of termination of radicals with different degrees of polymerization) taken from the literature. The dependences of the overall termination constant 〈kt〉 on initiation parameters (pulse repetition rate (v) and pulse intensity for initiation by periodic laser pulses) are presented. Two methods are proposed for α and k0 determination: (a) by experiments on polymerization with periodic laser pulses where monomer‐to‐polymer conversions per pulse are determined for different v; (b) by experiments on polymerization with packets of pulses where the constant kp (the rate constant of propagation), α and k0 can be determined simultaneously from MWD. For both methods simple analytical equations are derived for evaluation of the constants. The limits of application of the methods are determined by use of the numerical technique for MWD calculation.
Theoretical study on the linking process of divinyl compounds with living precursors, 1. Uniform precursor chains
Authors: Yan Deyue , Zhou Zhiping 
Published: April 2003

AbstractThe theoretical aspect of the linking reaction between divinyl compounds and uniform living precursors has been studied by using the model of threads going through the holes of buttons. The expressions of various molecular parameters of the clusters formed during the linking process, such as the number‐ and the weight‐average molecular weights, the average degree of branching, the branch distribution function, and the molecular weight distribution function, were rigorously derived. The critical condition of gelation is given as well. The theoretical results obtained are appropriate to the core formation process for the core‐first preparation procedure of star‐shaped polymers, and to the arm‐first approach using a uniform living precursor with low or moderate molecular size.
Concentration dependence of static chain properties, 1. Monte carlo simulations in a cubic lattice
Authors: Olaj Oskar Friedrich , Petrik Thomas , Zifferer Gerhard 
Published: April 2003

AbstractMonte Carlo simulations of 5‐way cubic lattice chains have been performed over the whole concentration range starting with the isolated chain up to totally dense systems devoid of any vacancies. Athermal as well as θ‐conditions have been investigated in order to give a complete description of the changes with respect to the global size and shape characteristics of the coils on the one hand and to connect these findings to the behaviour of local bond‐structures on the other hand. Especially, we have found that the global dimensions of an isolated θ‐chain are already reached by bulk chains at a volume fraction of approximately 0,7. However, the comparable global size and shape is realized in a totally different way on a local scale which is obviously due to the different behaviour of inter‐ and intramolecular forces.
Optimal screening distance for ulnar neuropathy at the elbow
Authors: Landau Mark E. , Barner Kristen C. , Campbell William W. 
Published: April 2003

AbstractFocal demyelinating lesions typically occur within a 1‐cm segment of a nerve. In electrodiagnostic studies, measurements over longer distances decrease the chance of detecting such lesions, but measurements over shorter distances result in greater experimental error. Our objective was therefore to determine the optimal screening distance for ulnar neuropathy at the elbow (UNE) incorporating previously derived experimental errors for calculating nerve conduction velocity (NCV). Using a lesion model wherein prolongation of 0.4 ms was added to the expected latency of a 1‐cm nerve segment, new NCVs were derived for distances between 1 and 10 cm for nerves normally conducting between 40 and 65 m/s. Lesion detection, or sensitivity, was defined as the likelihood of calculating a decrease of 10 m/s from the normal NCV while including the experimental error. Specificity was related to the likelihood of an inadvertent calculation of such a decrease in NCV in a segment without a lesion. Sensitivity and specificity were derived at multiple distances with varying NCVs. The total percentage error was the sum of the false‐negative and false‐positive percentages. The least total percentage error occurred at 3–4 cm, 4–6 cm, and 6–8 cm for nerves normally conducting at 40–50 m/s, 50–60 m/s, and 60–65 m/s, respectively. We conclude that the optimal distance for screening UNE, considering both sensitivity and specificity, is significantly less than 10 cm, perhaps as low as 4–6 cm; considering in addition the likely locations of focal lesions, the best distance is 6–8 cm. Muscle Nerve 27:570–574, 2003
Surface electromyographic recording of volitional activity: A technique to detect partial motor conduction block
Authors: Chaudhry Vinay , Corse Andrea , Skolasky Richard L. 
Published: April 2003

AbstractPartial motor conduction block, an electrophysiological hallmark of demyelination, helps to identify acquired demyelinating neuropathies but its electrophysiological detection can be difficult. We report a technique that may be helpful in this regard. Twenty‐five patients with partial motor conduction block secondary to acquired demyelinating polyneuropathy (ADP), 7 with amyotrophic lateral sclerosis (ALS), 7 with stroke, and 11 control subjects, were studied. Amplitude of compound muscle action potentials was recorded after distal electrical (E) stimulation and for volitionally (V) induced responses in 82 muscles. Mean ± SD V/E ratio was 12.3 ± 6.6 for ADP patients, 58.1 ± 17 for ALS patients, 11.4 ± 9 for stroke patients, and 55.4 ± 12.3 for controls. The V/E ratios for patients with partial motor conduction block and stroke were significantly reduced compared with ALS patients and healthy controls (P < 0.05). Surface electromyographic (EMG) recording for determination of the V/E ratio may be a useful technique for detection of a proximal conduction block if a central lesion or poor effort can be excluded. Further study of this novel technique is necessary. Muscle Nerve 27: 590–594, 2003
Neuromuscular and musculoskeletal problems in instrumental musicians
Authors: Lederman Richard J. 
Published: April 2003

AbstractOver the past 20 years, there has been increasing interest in the medical problems of performing artists. In this review, the major playing‐related disorders seen in instrumental musicians are discussed. Among the 1353 instrumentalists personally evaluated, the major diagnoses included musculoskeletal disorders in 64%, peripheral nerve problems in 20%, and focal dystonia in 8%. Of these instrumentalists, 60% were women, although men were the majority in the group with focal dystonia. The average age at the time of evaluation was 37 years for men and 30 years for women. Among musculoskeletal disorders, regional muscle pain syndromes, particularly of the upper limb, upper trunk, and neck, were most common. Specific entities such as tendinitis and ligament sprain were less common. Frequent peripheral nerve disorders included thoracic outlet syndrome, ulnar neuropathy at the elbow, and carpal tunnel syndrome. Each instrument group showed a characteristic distribution of symptoms and signs that appeared to be directly related to the static and dynamic stresses inherent in the playing of the instrument. Electrodiagnostic studies are an important part of the evaluation of these disorders, particularly nerve entrapment syndromes. With carefully designed treatment, the majority of instrumental musicians can return to full and pain‐free playing. Nerve entrapment syndromes have the highest treatment success rate, followed by musculoskeletal pain syndromes. Despite some recent innovative approaches, focal dystonia remains largely resistant to therapy. Muscle Nerve 27: 549–561, 2003
Perugia urodynamic method of analysis (PUMA): A new advanced method of urodynamic analysis applied clinically and compared with other advanced methods
Authors: Porena Massimo , Biscotto Sauro , Costantini Elisabetta , Mearini Ettore , Verdini Livio 
Published: April 2003

AbstractAimsThe aim of this study is to compare PUMA curves with different pathologic conditions causing bladder dysfunction in 158 men and 83 women.MethodsPUMA results in terms of bladder outlet obstruction and detrusor contractility were compared in 92 men with benign prostatic hypertrophy (BPH) and pves≅pdet (i.e., pabd≅0) with the results of the urodynamics operator's opinion, the provisional International Continence Society method, Abrams and Griffith's diagram, urethral resistence factor (URA), Schäfer's diagram, and Watt factor. PUMA curves correlated reliably with different pathologic conditions such as obstructive BPH, orthotopic bladder, cystocele, the neurological bladder, and bladder diverticulum. Statistical analysis indicated excellent agreement between PUMA and URA; agreement with other methods was good in cases of obstruction and nonobstruction. In doubtful cases, as diagnosed by standard methods, PUMA agreed only with the Abrams and Griffith's diagram. PUMA and Wmax were in good agreement on detrusor con traction force. Agreement between PUMA and Schäfer's diagram was excellent for patients with detrusor hypercontractility and good for patients with detrusor hypocontractility and normocontractility. PUMA is the only method applicable to women. It is easy to perform. When integrated with other diagnostic tests, it provides realistic data for diagnosis, medical or surgical therapy, and outcome. Neurourol. Urodynam. 22:206–222, 2003. © 2003 Wiley‐Liss, Inc.
Bladder filling by autologous urine production during cystometry: A urodynamic pitfall!
Authors: Heesakkers John P.F.A. , Vandoninck Vera , van Balken Michael R. , Bemelmans Bart L.H. 
Published: April 2003

AbstractAimsThe rate of autologous urine production should not have a major disturbing influence on cystometric urodynamic parameters such as first filling sensation, normal desire to void, strong desire to void, and cystometric bladder capacity. Instructions to patients and drinking behavior can have considerable impact, especially if filling cystometry is preceded by free uroflowmetry. We studied the influence of autologous urine production during filling cystometry on total bladder volume.MethodsUrodynamic investigations performed between September of 2000 and February of 2001 were analyzed. Only those urodynamic investigations for which total bladder capacity could be calculated were taken into account (i.e., catheterization before and after cystometry and no urine loss during the investigations).ResultsAfter screening, 186 investigations were used for further analysis. Mean filled volume (external infusion plus autologous urine production) was 346 ± 152 mL, but mean real bladder capacity (i.e., voided volume + residual urine) was 391 ± 170 mL. In all patients, 14% extra urine was produced due to autologous urine production (mean filling rate, 6.1 mL/min). In 42% of the investigations, the real bladder capacity was more than 110% of the infused volume. In 18% of the patients, the contribution of natural bladder filling was more than 25% of the infused volume.ConclusionsNatural bladder filling plays a substantial role during filling cystometry and has a disturbing influence on calculated urodynamic parameters. Attention should be paid to patient instructions before the urodynamic investigation. The combination of free uroflowmetry followed by filling cystometry should be avoided. This avoidance is especially important if interventional studies are performed. Careful interpretation of studies depending on bladder capacity parameters is mandatory, and such parameters should be corrected for autologous bladder filling. Neurourol. Urodynam. 22:243–245, 2003. © 2003 Wiley‐Liss, Inc.
Determining the reliability of ultrasound measurements and the validity of the formulae for ultrasound estimation of postvoid residual bladder volume in postpartum women
Authors: Yip Shing‐Kai , Sahota Daljit , Chang Allan Ming‐Zing 
Published: April 2003

AbstractAimsTo determine the reliability and validity of ultrasonic assessment of the postvoid residual bladder volume (PVRBV) in postpartum women.MethodsUltrasound estimated PVRBVs were performed on 99 consecutive women with postpartum urinary retention, to validate 12 published bladder volume‐estimation formulae. This ultrasound‐predicted volume was compared with the immediately collected catheterized volume. Comparison of individual formula with the catheterized volumes was performed by using the Intraclass Correlation Coefficient (concordance), and difference plots (bias, linearity of the difference). All volumes were transformed logarithmically to ensure a normal distribution.ResultsThe postpartum bladder maintained its ellipsoid appearance. One‐way analysis of variance showed the variance of the individual formulae ranged from 83.42 to 3463.66 (SD 9.13 to 58.85). The four formulae with the least variance had an intraclass correlation coefficient ranging from 0.93 to 0.96, and a mean difference between volume estimated by the formula and catheterized volume ranging from −0.05 to −0.11 (SD 0.09 to 0.11). The error between the value predicted by the formulae and that of the catheterized volume was linear in only one formula: Volume = (π × W × Dl × H) ÷ 6.ConclusionsThe results of this study have shown that ultrasonic assessment of the PVRBV in the postpartum period is accurate, and it can be used as a guide to whether transurethral catheterization is necessary. Neurourol. Urodynam. 22:255–260, 2003. © 2003 Wiley‐Liss, Inc.