Anisotropy of the monomer random walk in a polymer melt: Local-order and connectivity effects

The random walk of a bonded monomer in a polymer melt is anisotropic due to local order and bond connectivity. We investigate both effects by molecular-dynamics simulations on melts of fully-flexible linear chains ranging from dimers (M = 2) up to entangled polymers (M = 200). The corresponding atomic liquid is also considered a reference system. To disentangle the influence of the local geometry and the bond arrangements, and to reveal their interplay, we define suitable measures of the anisotropy emphasising either the former or the latter aspect. Connectivity anisotropy, as measured by the correlation between the initial bond orientation and the direction of the subsequent monomer displacement, shows a slight enhancement due to the local order at times shorter than the structural relaxation time. At intermediate times - when the monomer displacement is comparable to the bond length - a pronounced peak and then decays slowly as t -1/2, becoming negligible when the displacement is as large as about five bond lengths, i.e. about four monomer diameters or three Kuhn lengths. Local-geometry anisotropy, as measured by the correlation between the initial orientation of a characteristic axis of the Voronoi cell and the subsequent monomer dynamics, is affected at shorter times than the structural relaxation time by the cage shape with antagonistic disturbance by the connectivity. Differently, at longer times, the connectivity favours the persistence of the local-geometry anisotropy, which vanishes when the monomer displacement exceeds the bond length. Our results strongly suggest that the sole consideration of the local order is not enough to understand the microscopic origin of the rattling amplitude of the trapped monomer in the cage of the neighbours

Ordine locale e dinamica in fasi polimeriche.

I liquidi, in condizioni di equilibrio, sono caratterizzati dalla mancanza di ordine a lungo raggio. Inoltre un liquido può continuare a rimanere tale anche a temperature inferiori a quella di cristallizzazione. In letteratura si discute molto su l'esistenza di una correlazione tra ordine locale e dinamica. In questa tesi viene caratterizzato l'ordine a corto raggio e la dinamica di rilassamento della densità per un sistema polimerico. I sistemi sono stati studiati tramite simulazioni con la tecnica di dinamica molecolare (MD). Il modello considerato è un bulk di liquido polimerico nel quale i monomeri sono soggetti ad un potenziale Lennard-Jones più un opportuno potenziale elastico. Particolare interesse è stato dedicato al confronto tra la statica e la dinamica reale, e la statica e la dinamica considerando i soli stati, detti inerenti, di equilibrio meccanico del sistema. Questi stati sono caratterizati dal fatto che la forza che agisce su ogni monomero è nulla, in quanto viene sottratto ogni contributo vibrazionale intorno alla posizione di equilibrio. Questo tipo di analisi, in precedenza utilizzato solamente per liquidi atomici, è stato esteso per la prima volta ai liquido molecolari ed ha permesso di individuare in modo estremamente chiaro, le tipologie di ordine a corto raggio presenti in essi. Per studiare l'ordine a corto raggio sono state prese in considerazione grandezze sia statiche che dinamiche. Tra le grandezze statiche sono state studiate le seguenti grandezze: • Parte intra-catena e inter-catena della funzione di distribuzione radiale di coppia. • Distribuzione degli angoli di legame tra monomeri della catena polimerica. • Fattore di struttura statico dei monomeri. • Numero di conformazioni tetraedriche dei monomeri presenti nel sistema. • Parametri d'ordine orientazionale locale del sistema. Al fine di caratterizzare la dinamica dei monomeri delle catene polimeriche sono state inoltre studiate le seguenti grandezze: • Funzione intermedia di scattering incoerente. • Spostamento quadratico medio dei monomeri. • Funzione di correlazione dello stress meccanico locale. • Funzione di Van Hove. Tutte le grandezze sono state studiate al variare di: densità, temperatura e peso molecolare

Thermodynamic scaling of relaxation: Insights from anharmonic elasticity

Using molecular dynamics simulations of a molecular liquid, we investigate the thermodynamic scaling (TS) of the structural relaxation time Tα in terms of the quantity Tp-γts, where T and p are the temperature and density, respectively. The liquid does not exhibit strong virial-energy correlations. We propose a method for evaluating both the characteristic exponent γts and the TS master curve that uses experimentally accessible quantities that characterise the anharmonic elasticity and does not use details about the microscopic interactions. In particular, we express the TS characteristic exponent γts in terms of the lattice Grneisen parameter δL and the isochoric anharmonicity δL. An analytic expression of the TS master curve of Tα with δL as the key adjustable parameter is found. The comparison with the experimental TS master curves and the isochoric fragilities of 34 glassformers is satisfying. In a few cases, where thermodynamic data are available, we test (i) the predicted characteristic exponent γts and (ii) the isochoric anharmonicity δL, as drawn by the best fit of the TS of the structural relaxation, against the available thermodynamic data. A linear relation between the isochoric fragility and the isochoric anharmonicity δL is found and compared favourably with the results of experiments with no adjustable parameters. A relation between the increase of the isochoric vibrational heat capacity due to anharmonicity and the isochoric fragility is derived

Effect of nematic ordering on the elasticity and yielding in disordered polymeric solids

The relation between elasticity and yielding is investigated in a model polymer solid by Molecular-Dynamics simulations. By changing the bending stiffness of the chain and the bond length, semicrystalline and disordered glassy polymers — both with bond disorder — as well as nematic glassy polymers with bond ordering are obtained. It is found that in systems with bond disorder the ratio tau_Y/G between the shear yield strength tau_Y and the shear modulus G is close to the universal value of the atomic metallic glasses. The increase of the local nematic order in glasses leads to the increase of the shear modulus and the decrease of the shear yield strength, as observed in experiments on nematic thermosets. A tentative explanation of the subsequent reduction of the ratio tau_Y/G in terms of the distributions of the per-monomer stress is offered

Thermodynamic scaling of vibrational dynamics and relaxation

We investigate by thorough molecular dynamics simulations the thermodynamic scaling (TS) of a polymer melt. Two distinct models, with strong and weak virial-energy correlations, are considered. Both evidence the joint TS with the same characteristic exponent γts of the fast mobility - the mean square amplitude of the picosecond rattling motion inside the cage - and the much slower structural relaxation and chain reorientation. If the cage effect is appreciable, the TS master curves of the fast mobility are nearly linear, grouping in a bundle of approximately concurrent lines for different fragilities. An expression of the TS master curve of the structural relaxation with one adjustable parameter less than the available three-parameter alternatives is derived. The novel expression fits well with the experimental TS master curves of thirty-four glassformers and, in particular, their slope at the glass transition, i.e., the isochoric fragility. For the glassformer OTP, the isochoric fragility allows to satisfactorily predict the TS master curve of the fast mobility with no adjustments

Microscopic order and mechanical response in polymeric liquids, glasses and crystals

During my Ph.D. I have worked on several topics, all connected by the same purpose to understand, from microscopic basis, the relaxation processes in glass-forming liquids. Following interesting and recent works, particular attention has been addressed to the link between fast vibrational dynamics on picosecond time scales and the slow relaxation. The first part of my work has been devoted to the study of the local order and a possible correlation with the fast dynamics. Then I focused on the extended processes involving the whole system such as the spatial-temporal displacements correlations and the elastic modulus. In the last period I have studied and characterized the geometry and the elastic properties of the polymer crystals, glasses and atomic crystals. The study of the glass-forming liquids is approached here from a numerical point of view. By performing Molecular Dynamics (MD) simulations, I was able to study the dynamics on the microscopic level and to collect informations on every observable of interest with a good precision level, while the same process in experiments would require much more effort. To study the phenomenology of the glass-forming liquids, I have chosen as prototype of viscous liquid the simple beads and springs model for polymeric chain liquids. Polymers represent indeed a central class in this kind of research because of their natural disorder: in most cases a polymer liquid, rather than crystallize in a regular lattice, reaches an amorphous glassy state. However for a few purposes, even the atomic and binary mixtures liquids have been studied

A novel homozygous mutation in CETP gene as a cause of CETP deficiency in a Caucasian kindred.

ObjectiveTo analyze the cholesteryl ester transfer protein (CETP) gene and the plasma HDL phenotype in a Caucasian subject with extremely elevated plasma high density lipoprotein-cholesterol (HDL-C).Methods and resultsThe proband, a 63-year-old male of Swedish ancestry with elevated HDL-C (208 mg/dl) and apoA-I (and 272 mg/dl), was found to be homozygous for a point mutation in exon 2 of CETP gene (c.109 C > T) resulting in a premature termination codon (R37X). Plasma CETP mass and activity were undetectable. Plasma HDL were characterized by predominance of large HDL with enhanced pre\u3b2-HDL content. The proband's sons, heterozygotes for the mutation, had reduced plasma CETP activity and moderately elevated HDL-C. Serum of CETP deficient subjects showed a normal or enhanced cholesterol efflux capacity via ABCG1/SR-BI; cholesterol efflux via ABCA1 and macrophage cholesterol removal were lower than normal. The proband was healthy and had no atherosclerotic plaques in carotid or femoral arteries.ConclusionComplete CETP deficiency caused by mutations in CETP gene is exceedingly rare in Caucasians; the description of this single case indicates that CETP deficiency does not predispose to atherosclerosis in the absence of major cardiovascular risk factors

Severe HDL deficiency due to novel defects in the ABCA1 transporter.

none15OBJECTIVES: The objective was the identification and functional characterization of mutations in the ABCA1 gene in four patients with severe HDL deficiency. SUBJECTS: Patients were referred to the clinic because of almost complete HDL deficiency. METHODS: The ABCA1 gene was sequenced directly. The analysis of the ABCA1 protein, ABCA1 mRNA and ABCA1-mediated cholesterol efflux was performed in cultured fibroblasts. Intracellular localization of ABCA1 mutants was investigated in transfected HEK293 cells. RESULTS: Two patients were homozygous for mutations in the coding region of the ABCA1 gene, resulting in an amino acid substitution (p.A1046D) and a truncated protein (p.I74YFsX76). The third patient was homozygous for a splice site mutation in intron 35 (c.4773 + 1g>a), resulting in an in-frame deletion of 25 amino acids (del p.D1567_K1591) in ABCA1. These patients had clinical manifestations of accumulation of cholesterol in the reticulo-endothelial system. The fourth patient, with preclinical atherosclerosis, was a compound heterozygote for two missense mutations (p.R587W/p.W1699C). ABCA1-mediated cholesterol efflux was abolished in fibroblasts from patients with p.A1046D and del p.D1567_K1591 mutants and in fibroblasts homozygous for p.R587W. A reduced ABCA1 protein content was observed in these cells, suggesting an increased intracellular degradation. The mutant p.W1699C was largely retained in the endoplasmic reticulum, when expressed in HEK293 cells. CONCLUSIONS: The homozygotes for mutations which abolish ABCA1 function showed overt signs of involvement of the reticulo-endothelial system. This was not the case in the compound heterozygote for missense mutations, suggesting that this patient retains some residual ABCA1 function that reduces cholesterol accumulation in the reticulo-endothelial system.Impact Factor 4.901mixedPISCIOTTA L.; BOCCHI L.; CANDINI C.; SALLO R.; ZANOTTI I.; FASANO T.; CHAKRAPANI A.; BATES T.; BONARDI R.; CANTAFORA A.; BALL S.; WATTS G.; BERNINI F.; CALANDRA S.; BERTOLINI S.Pisciotta, Livia; Bocchi, L.; Candini, C.; Sallo, R.; Zanotti, I.; Fasano, T.; Chakrapani, A.; Bates, T.; Bonardi, R.; Cantafora, A.; Ball, S.; Watts, G.; Bernini, F.; Calandra, S.; Bertolini, Stefan

Clinically guided genetic screening in a large cohort of italian patients with pheochromocytoma and/or functional or nonfunctional paragangliomas

Purpose: The aim of the study was to define the frequency of hereditary forms and the genotype/phenotype correlations in a large cohort of Italian patients with pheochromocytomas and/or functional or nonfunctional paragangliomas. Design: We examined 501 consecutive patients with pheochromocytomas and/or paragangliomas (secreting or nonsecreting). Complete medical and family histories, as well as the results of clinical, laboratory, and imaging studies, were recorded in a database. Patients were divided into different groups according to their family history, the presence of lesions outside adrenals/paraganglia considered syndromic for VHL disease, MEN2, and NF1, and the number and types of pheochromocytomas and/or paragangliomas. Germ-line mutations in known susceptibility genes were investigated by gene sequencing (VHL, RET, SDHB, SDHC, SDHD) or diagnosed according to phenotype (NF1). In 160 patients younger than 50 yr with a wild-type profile, multiplex ligation-dependent probe amplification assays were performed to detect genomic rearrangements. Results: Germline mutations were detected in 32.1% of cases, but frequencies varied widely depending on the classification criteria and ranged from 100% in patients with associated syndromic lesions to 11.6% in patients with a single tumor and a negative family history. The types and number of pheochromocytomas/paragangliomas as well as age at presentation and malignancy suggest which gene should be screened first. Genomic rearrangements were found in two of 160 patients (1.2%). Conclusions: The frequency of the hereditary forms of pheochromocytoma/paraganglioma varies depending on the family history and the clinical presentation. A positive family history and an accurate clinical evaluation of patients are strong indicators of which genes should be screened first