1,721,020 research outputs found
Modeling of Poly(3-hexylthiophene) and Its Oligomer's Structure and Thermal Behavior with Different Force Fields: Insights into the Phase Transitions of Semiconducting Polymers
Full text linkThe polymorphism of poly(3-hexylthiophene) (P3HT), one of the reference systems in fundamental studies of polymer semiconductors, is explored by molecular dynamics modeling of selected 3-hexylthiophene (3HT) oligomers, comparing structural and thermal behavior simulation results with rare monodisperse oligomer experimental data. The relative stability of the two crystalline polymorphs and the mechanism of interconversion between them, as the degree of polymerization grows in (3HT)n oligomers (n = 10, 16, 20) to the polymer, can be investigated in infinite periodic oligomer crystals without implicitly imposing infinite molecular weights, as inevitable for polymers. To evaluate the impact of different descriptions of molecular interactions, simulations were performed by using three different force fields specifically adapted to poly(3-alkylthiophenes) (P3ATs). Our results show that MD may adequately describe the key features and relative stability of the different crystal phases and suggest plausible interconversion mechanisms for very rapid solid-solid or melting transitions, albeit with complementary differences among different force fields, which become substantial modeling highly disordered crystal structures or mesophases
The in vitro evolution of resorbable brushite cements: A physico-chemical, micro-structural and mechanical study
No full textThe mechanisms by which calcium phosphate bone substitutes evolve and are resorbed in vivo are not yet fully known. In particular, the formation of intermediate phases during resorption and evolution of the mechanical properties may be of crucial interest for their clinical efficiency. The in vitro tests proposed here are the first steps toward understanding these phenomena. Microporous Dicalcium Phosphate Dihydrate (DCPD) samples were immersed in tris(hydroxymethyl)aminomethane (TRIS) and Phosphate Buffered Saline (PBS) solutions, with or without daily refresh of the medium, for time-points up to 14 days. Before and after immersion, samples were extensively characterised in terms of morphology, chemistry (XRD coupled with Rietveld analysis), microstructure (X-ray tomography, SEM observations) and local mechanical properties (instrumented micro-indentation). The composition of the immersion solutions was monitored in parallel (pH, elemental analysis). The results show the influence and importance of the experimental set-up and protocol on the formation of apatite and octacalcium phosphate concurrently to DCPD dissolution; moreover, strong inter-correlations between physico-chemistry, microstructure and mechanics are demonstrated. Statement of Significance Ideally, the resorption kinetics of biodegradable bone substitutes should be controlled to favor the healing processes of bone. Although biodegradable bone grafts are already used in surgeries, their resorption process is still partially unknown. The present work studies these resorption phenomena, their kinetics and mechanisms and their consequences on the properties of a calcium phosphate resorbable material. The original in vitro approach developed in this work couples for the first time physico-chemical, micro-structural and mechanical assessments. The dissolution of the CaP phase in body fluids and the reprecipitation of more stable phases are studied on a local scale, which has permitted to evidence and monitor the development of a gradient of properties between the surface and the core of the samples
Resorption of calcium phosphate materials: Considerations on the in vitro evaluation
No full textLiterature is quite abundant concerning the resorption of calcium phosphates. However, articles on in vitro acellular dissolution tests are scarcer, although these immersion experiments are often considered a first step in establishing a predictive dissolution test and understanding the resorption mechanisms. Also, the comparison of results obtained after immersion in model fluids reveals large discrepancies. The first part of this paper is a literature review focused on in vitro aging of apatite, dicalcium phosphate dihydrate (DCPD) and beta tricalcium phosphate (β-TCP). In the second part, experimental findings obtained by immersing DCPD and β-TCP samples in TRIS and PBS solutions for different times demonstrate the influence of the immersion conditions and possible artefacts induced by the experimental protocols. Suggestions are provided to improve the design of in vitro experiments. In addition, the interest to follow-up at the same time and at a local scale physico-chemical, microstructural and mechanical properties is discussed
Spherical instrumented indentation as a tool to characterize porous bioceramics and their resorption
No full textResorbable ceramics used as porous bone substitutes are designed to favor bone in-growth and to be gradually replaced by natural tissues after in vivo resorption. However, a lack of experimental techniques to quantitatively monitor the evolution of their mechanical properties during resorption is noted. In this paper, we propose to use spherical instrumented indentation to follow-up the resorption of microporous resorbable ceramics at a local scale. Tests were performed at the core and at the surface of samples immersed for different durations in model fluids. Instrumented indentation was found to be an efficient technique to characterize and to follow-up the resorption of microporous ceramics, in excellent agreement with microstructural changes observed with X-ray diffraction and X-ray tomography. Instrumented indentation has the ability to capture the presence of gradients in the samples, enables the direct testing of wet samples and appears as a superior technique to compression tests mostly used in the literature
The Polymorphic Behavior of Poly(2,2'-dimethyltrimethylencarbonate) and of its Cyclic Monomer. Evidences for a High Entropy Conformationally Disordered Modification
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Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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