From caging to Rouse dynamics in polymer melts with intramolecular barriers: a critical test of the Mode Coupling Theory

By means of computer simulations and solution of the equations of the Mode Coupling Theory (MCT), we investigate the role of the intramolecular barriers on several dynamic aspects of non-entangled polymers. The investigated dynamic range extends from the caging regime characteristic of glass-formers to the relaxation of the chain Rouse modes. We review our recent work on this question, provide new results and critically discuss the limitations of the theory. Solutions of the MCT for the structural relaxation reproduce qualitative trends of simulations for weak and moderate barriers. However a progressive discrepancy is revealed as the limit of stiff chains is approached. This disagreement does not seem related with dynamic heterogeneities, which indeed are not enhanced by increasing barrier strength. It is not connected either with the breakdown of the convolution approximation for three-point static correlations, which retains its validity for stiff chains. These findings suggest the need of an improvement of the MCT equations for polymer melts. Concerning the relaxation of the chain degrees of freedom, MCT provides a microscopic basis for time scales from chain reorientation down to the caging regime. It rationalizes, from first principles, the observed devations from the Rouse model on increasing the barrier strength. These include anomalous scaling of relaxation times, long-time plateaux, and non-monotonous wavelength dependence of the mode correlators.Comment: 15 pages, 14 figure

Cluster glasses of ultrasoft particles

We present molecular dynamics (MD) simulations results for dense fluids of ultrasoft, fully-penetrable particles. These are a binary mixture and a polydisperse system of particles interacting via the generalized exponential model, which is known to yield cluster crystal phases for the corresponding monodisperse systems. Because of the dispersity in the particle size, the systems investigated in this work do not crystallize and form disordered cluster phases. The clustering transition appears as a smooth crossover to a regime in which particles are mostly located in clusters, isolated particles being infrequent. The analysis of the internal cluster structure reveals microsegregation of the big and small particles, with a strong homo-coordination in the binary mixture. Upon further lowering the temperature below the clustering transition, the motion of the clusters' centers-of-mass slows down dramatically, giving way to a cluster glass transition. In the cluster glass, the diffusivities remain finite and display an activated temperature dependence, indicating that relaxation in the cluster glass occurs via particle hopping in a nearly arrested matrix of clusters. Finally we discuss the influence of the microscopic dynamics on the transport properties by comparing the MD results with Monte Carlo simulations.Comment: 17 pages, 23 figure

The Role of Intramolecular Barriers on the Glass Transition of Polymers: Computer Simulations vs. Mode Coupling Theory

We present computer simulations of a simple bead-spring model for polymer melts with intramolecular barriers. By systematically tuning the strength of the barriers, we investigate their role on the glass transition. Dynamic observables are analyzed within the framework of the Mode Coupling Theory (MCT). Critical nonergodicity parameters, critical temperatures and dynamic exponents are obtained from consistent fits of simulation data to MCT asymptotic laws. The so-obtained MCT $\lambda$-exponent increases from standard values for fully-flexible chains to values close to the upper limit for stiff chains. In analogy with systems exhibiting higher-order MCT transitions, we suggest that the observed large $\lambda$-values arise form the interplay between two distinct mechanisms for dynamic arrest: general packing effects and polymer-specific intramolecular barriers. We compare simulation results with numerical solutions of the MCT equations for polymer systems, within the polymer reference interaction site model (PRISM) for static correlations. We verify that the approximations introduced by the PRISM are fulfilled by simulations, with the same quality for all the range of investigated barrier strength. The numerical solutions reproduce the qualitative trends of simulations for the dependence of the nonergodicity parameters and critical temperatures on the barrier strength. In particular, the increase of the barrier strength at fixed density increases the localization length and the critical temperature. However the qualitative agreement between theory and simulation breaks in the limit of stiff chains. We discuss the possible origin of this feature

Erratum to: Poor outcome at 7.5years after Stanisavljevic quadriceps transposition for patello-femoral instability

Introduction: Congenital dislocation of the patella and recurrent symptomatic dislocation in adolescents are difficult pathologies to treat. Stanisavljevic described an extensive release procedure essentially involving medializing the entire lateral quadriceps and medial soft tissue stabilization. There are no significant series reporting the success of this method. This procedure has been performed in our institution over several years and we report our results. Method: Retrospective case series. Between 1990 and 2007, 20 knees in 13 children and adolescents (mean age 12.8years; 4-17, 7 female) with recurrent or congenital dislocation of the patella (8 knees) underwent this procedure after failed conservative treatment (mean follow-up 7.5years; 4-16). All were immobilized in a long leg cast for 6weeks. Results: Five knees in five patients (20%, 1 congenital dislocation) reported their knees as improved without further dislocations. Out of the 15 knees with failures (80%) 12 in six patients (60%) were revised due to redislocation. Three knees in two patients (15%) still had dislocations or subluxations, but any revision was refused. Three knees in three patients caused pain and discomfort during daily activity. Redislocation first developed after a mean of 21.3months (4-72) postoperatively. Only one patient had returned to sport at the 12-month follow-up. Discussion: The Stanisavljevic procedure produces a mediocre success rate with our long-term follow-up series showing a failure rate up to 80%. We therefore recommend more specific procedures dealing with the anatomical deformity such as trochleaplasty to produce superior success rates

Transfer of plantar pressure from the medial to the central forefoot in patients with hallux valgus

Background: The aim of the study was to evaluate changes in plantar pressure distribution in feet affected by hallux valgus compared with their contralateral non-affected feet and with the feet of healthy control subjects. Methods: Thirty-six patients with unilateral hallux valgus who were indicated for surgery and 30 healthy subjects were assessed on a pedobarographic instrumented treadmill for step length and width, mean stance phase, and plantar foot pressure distribution. Plantar pressure distribution was divided into eight regions. Results: Significantly higher plantar pressures were observed in hallux valgus feet under the second and third metatarsal heads (p = .033) and the fourth and fifth toes (p < .001) than in the healthy control feet. Although decreased pressures were measured under the hallux in affected feet (197 [82–467] kPa) in contrast to the contralateral side (221 [89–514] kPa), this difference failed to reach statistical significance (p = .055). The gait parameters step width, step length, and single-limb support did not show any differences between hallux valgus and control feet. Conclusion: Although the literature on changes in plantar pressures in hallux valgus remains divided, our findings on transferring load from the painful medial to the central and lateral forefoot region are consistent with the development of transfer metatarsalgia in patients with hallux valgus

Disturbed eating at high altitude: influence of food preferences, acute mountain sickness and satiation hormones

Purpose: Hypoxia has been shown to reduce energy intake and lead to weight loss, but the underlying mechanisms are unclear. The aim was therefore to assess changes in eating after rapid ascent to 4,559m and to investigate to what extent hypoxia, acute mountain sickness (AMS), food preferences and satiation hormones influence eating behavior. Methods: Participants (n=23) were studied at near sea level (Zurich (ZH), 446m) and on two days after rapid ascent to Capanna Margherita (MG) at 4,559m (MG2 and MG4). Changes in appetite, food preferences and energy intake in an ad libitum meal were assessed. Plasma concentrations of cholecystokinin, peptide tyrosine-tyrosine, gastrin, glucagon and amylin were measured. Peripheral oxygen saturation (SpO2) was monitored, and AMS assessed using the Lake Louis score. Results: Energy intake from the ad libitum meal was reduced on MG2 compared to ZH (643±308 vs. 952±458kcal, p=0.001), but was similar to ZH on MG4 (890±298kcal). Energy intake on all test days was correlated with hunger/satiety scores prior to the meal and AMS scores on MG2 but not with SpO2 on any of the 3days. Liking for high-fat foods before a meal predicted subsequent energy intake on all days. None of the satiation hormones showed significant differences between the 3days. Conclusion: Reduced energy intake after rapid ascent to high altitude is associated with AMS severity. This effect was not directly associated with hypoxia or changes in gastrointestinal hormones. Other peripheral and central factors appear to reduce food intake at high altitud

Downregulation of duodenal SLC transporters and activation of proinflammatory signaling constitute the early response to high altitude in humans

Solute carrier (SLC) transporters mediate the uptake of biologically active compounds in the intestine. Reduced oxygenation (hypoxia) is an important factor influencing intestinal homeostasis. The aim of this study was to investigate the pathophysiological consequences of hypoxia on the expression and function of SLCs in human intestine. Hypoxia was induced in human intestinal epithelial cells (IECs) in vitro (0.2; 1% O2 or CoCl2). For human in vivo studies, duodenal biopsies and serum samples were obtained from individuals (n = 16) acutely exposed to 4,554 meters above sea levels. Expression of relevant targets was analyzed by quantitative PCR, Western blotting, or immunofluorescence. Serum levels of inflammatory mediators and nucleosides were determined by ELISA and LC/MS-MS, respectively. In the duodenum of volunteers exposed to high altitude we observed decreased mRNA levels of apical sodium-dependent bile acid transporter (ASBT), concentrative nucleoside transporters 1/2 (CNT1/2), organic anion transporting polypeptide 2B1 (OATP2B1), organic cation transporter 2 (OCTN2), peptide transporter 1 (PEPT1), serotonin transporter (SERT), and higher levels of IFN-γ, IL-6, and IL-17A. Serum levels of IL-10, IFN-γ, matrix metalloproteinase-2 (MMP-2), and serotonin were elevated, whereas the levels of uridine decreased upon exposure to hypoxia. Hypoxic IECs showed reduced levels of equilibrative nucleoside transporter 2 (ENT2), OCTN2, and SERT mRNAs in vitro, which was confirmed on the protein level and was accompanied by activation of ERK1/2, increase of hypoxia-inducible factor (HIF) proteins, and production of IL-8 mRNA. Costimulation with IFN-γ and IL-6 during hypoxia further decreased the expression of SERT, ENT2, and CNT2 in vitro. Reduced oxygen supply affects the expression pattern of duodenal SLCs that is accompanied by changes in serum levels of proinflammatory cytokines and biologically active compounds demonstrating that intestinal transport is affected during systemic exposure to hypoxia in humans