56 research outputs found
A relatively simple look at the rather complex crystallization kinetics of PLLA
[Abstract] This work demonstrates that, despite the existence of a significant number of works on PLA crystallization, there is still a relatively simple way, different from those already described, in which its complex kinetics can be observed. The X-ray diffraction (XRD) results presented here confirm that the PLLA under study crystallizes mostly in the α and αâČ forms. An interesting observation is that at any temperature in the studied range of the patterns, the X-ray reflections stabilize with a given shape and at a given angle, different for each temperature. That means that both α and αâČ forms coexist and are stable at the same temperatures so that the shape of each pattern results from both structures. However, the patterns obtained at each temperature are different because the predominance of one crystal form over the other depends on temperature. Thus, a two-component kinetic model is proposed to account for both crystal forms. The method involves the deconvolution of the exothermic DSC peaks using two logistic derivative functions. The existence of the rigid amorphous fraction (RAF) in addition to the two crystal forms increases the complexity of the whole crystallization process. However, the results presented here show that a two-component kinetic model can reproduce the overall crystallization process fairly well over a broad range of temperatures. The method used here for PLLA may be useful for describing the isothermal crystallization processes of other polymers
Properties of Mosquito Repellent-Plasticized Poly(lactic acid) Strands
This article belongs to the Special Issue Thermal and Rheological Characterization of Polymeric Materials[Abstract] Poly(lactic acid) (PLA) is an attractive candidate for replacing petrochemical polymers because it is fully biodegradable. This study investigated the potential of PLA as a sustainable and environmentally friendly alternative material that can be developed into commercially viable wearable mosquito repellent devices with desirable characteristics. PLA strands containing DEET and IR3535 were prepared by twin screw extrusion compounding and simultaneously functioned as plasticizers for the polymer. The plasticizing effect was investigated by thermal and rheological studies. DSC studies showed that the addition of DEET and IR3535 into PLA strands reduced the glass transition temperature consistent with predictions of the Fox equation, thus proving their efficiency as plasticizers. The rheology of molten samples of neat PLA and PLA/repellents blends, evaluated at 200 °C, was consistent with shear-thinning pseudoplastic behaviour. Raman studies revealed a nonlinear concentration gradient for DEET in the PLA strand, indicating non-Fickian Type II transport controlling the desorption process. Release data obtained at 50 °C showed initial rapid release followed by a slower, near constant rate at longer times. The release rate data were fitted to a novel modification of the Peppas-Sahlin desorption modelThis work was financially supported from the Deutsche Forschungsgemeinschaft (DFG), Germany [Grant number AN 212/22-2] and University of Pretoria (UP) Postdoctoral Fellowship programme. The research has been also supported by Spanish MINECO grant MTM2017-82724-R through the ERDFDeutsche Forschungsgemeinschaft (DFG); AN 212/22-2https://www.mdpi.com/journal/molecules/special_issues/Thermal_Rheological_Polymeric_Material
Slow-DEET-release mosquito-repellent system based on poly(butylene succinate)
Bio-sourced and biodegradable poly(butylene succinate) (PBS) strands containing up to 40 m% mosquito-repellent
N,N-diethyl-3-methylbenzamide (DEET) were obtained by extrusion, for an initial evaluation of the DEET evaporation
characteristics and the possible application of such strands as
biodegradable slow-release repellent-delivery devices. For DEET
concentrations up to 20 m%, DEET is entrapped in the
semicrystalline spherulitic superstructure of PBS. In contrast, at
higher DEET concentrations, the liquid repellent, at least partially,
is not fully incorporated in the PBS spherulites rather than
segregates to form an own macrophase. Quantification of the
release of DEET to the environment by thermogravimetric analysis
at different temperatures between 60 and 100 °C allowed estimation of the evaporation rate at lower service temperatures,
suggesting an extremely low release rate with a time constant of the order of magnitude of 1â2 years at 25 °C, independent of the
initial concentration.Deutsche Forschungsgemeinschaft (DFG) and Martin Luther University Halle-Wittenberg.http://pubs.acs.org/journal/acsodfChemical Engineerin
Tribological and mechanical properties of polyamide-11/halloysite nanotube nanocomposites
This article reports some morphological, tribological, and mechanical data on polyamide-11(PA11)/ halloysite nanotube (HNT) nanocomposites prepared by melt-compounding. HNTs extracted from the Djebel Deb- bagh deposit in Algeria were incorporated into the polymer at 1, 3, and 5 wt%. For comparison, commercial HNTs were also used under the same processing conditions. Scanning electron microscopy showed that both HNTs were homogeneously dispersed in the PA11 matrix, despite the presence of few aggregates, in particular at higher filler contents. The tribological properties were significantly improved, resulting in a decrease in the friction coefficient and the wear rate characteristics due to the lubricating effect of HNTs. This is consistent with optical profilometry data, which evidenced the impact of both types of HNTs on the surface topography of the nanocomposite samples, in which the main wear process was plastic deformation. Furthermore, Youngâs modulus and tensile strength were observed to increase with the filler content, but to the detriment of elongation at break and impact strength. Regarding the whole data, the raw Algerian halloysite led to interesting results in PA11 nanocomposites, thus reveal- ing its potential in polymer engineering nanotechnology
LSST: from Science Drivers to Reference Design and Anticipated Data Products
(Abridged) We describe here the most ambitious survey currently planned in
the optical, the Large Synoptic Survey Telescope (LSST). A vast array of
science will be enabled by a single wide-deep-fast sky survey, and LSST will
have unique survey capability in the faint time domain. The LSST design is
driven by four main science themes: probing dark energy and dark matter, taking
an inventory of the Solar System, exploring the transient optical sky, and
mapping the Milky Way. LSST will be a wide-field ground-based system sited at
Cerro Pach\'{o}n in northern Chile. The telescope will have an 8.4 m (6.5 m
effective) primary mirror, a 9.6 deg field of view, and a 3.2 Gigapixel
camera. The standard observing sequence will consist of pairs of 15-second
exposures in a given field, with two such visits in each pointing in a given
night. With these repeats, the LSST system is capable of imaging about 10,000
square degrees of sky in a single filter in three nights. The typical 5
point-source depth in a single visit in will be (AB). The
project is in the construction phase and will begin regular survey operations
by 2022. The survey area will be contained within 30,000 deg with
, and will be imaged multiple times in six bands, ,
covering the wavelength range 320--1050 nm. About 90\% of the observing time
will be devoted to a deep-wide-fast survey mode which will uniformly observe a
18,000 deg region about 800 times (summed over all six bands) during the
anticipated 10 years of operations, and yield a coadded map to . The
remaining 10\% of the observing time will be allocated to projects such as a
Very Deep and Fast time domain survey. The goal is to make LSST data products,
including a relational database of about 32 trillion observations of 40 billion
objects, available to the public and scientists around the world.Comment: 57 pages, 32 color figures, version with high-resolution figures
available from https://www.lsst.org/overvie
Geometric effects control isothermal oxidation of graphite flakes
The oxidation, in a neat oxygen atmosphere, of high-purity and highly crystalline natural
graphite and synthetic Kish graphite was investigated. The physico-geometric model function
of the kinetic rate equation was experimentally determined by isothermal thermogravimetric
analysis at 650 °C. Analytic solutions for basic flake shapes indicate that this function strictly
decreases with conversion. However, for both samples the experimental data trend was a
rapid initial increase followed by the expected decrease to zero. High resolution field
emission scanning electron microscopy (FEGSEM), of partially oxidized flakes, provided
plausible explanations for this discrepancy. Rapid development of macroscopic surface
roughness during the initial stages of oxidation was evident and could be attributed to the
presence of catalytic impurities. Large fissures along the planes of the natural graphite and
the initiation, growth and coalescence of internal cavities in the Kish graphite were observed.
Flake models incorporating the latter two features are difficult to analyse analytically.
However, a facile probabilistic approach showed that reasonably good agreement with
experimental data was possible.The Skye Foundation, PBMR,The South African Research Chairs Initiative of the Department of Science and Technology and the National Research Foundation.http://www.springerlink.com/content/102948
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