112,565 research outputs found
Preparation of Poly(l-lactic acid) Scaffolds by Thermally Induced Phase Separation: Role of Thermal History
No full textPoly-L-Lactic Acid (PLLA) scaffolds for tissue engineering were prepared via thermally induced phase separation of a ternary system PLLA/dioxane/tetrahydrofurane. An extension to solution of a previously developed method for solidification from the melt was adopted, the technique being based on a Continuous Cooling Transformation (CCT) approach, consisting in recording the thermal history of rapidly cooled samples and analysing the resulting morphology. Different foams were produced by changing the thermal history, the dioxane to THF ratio (50/50, 70/30, 90/10 v/v) and the polymer concentration (2, 2.5, 4% wt) in the starting ternary solution. Pore size, porosity, melting and crystallization behavior were studied, together with a morphological and kinetic analysis of the foams produced. A large variety of morphologies was achieved, the largest pore size (20 mu m) was achieved at the highest polymer concentration (4% wt) and the lowest dioxane concentration (50/50 dioxane/THF v/v), whereas the largest porosity (90%) was attained at the highest dioxane concentration (90/10). The average pore size is related to cooling rate, with a 1/3 power law exponent at low polymer concentrations and low dioxane content for thermal histories driven by low undercoolings. At high undercoolings, the growth of the demixed domains significantly departs from the diffusive-like regime
Real-time orientation and crystallinity measurements during the iPP film casting process
No full textRadiometric impact assessments and shielding of CubeSat class satellites in interaction with orbital radiation fields and representativity of the calibration procedure and radiation damage tests in the ground facility
No full textUsing FLUKA and MCNP Monte Carlo nuclear particle transport codes, support for radiation shielding design and assessing damage to aerospace components and systems has been developed. To this aim, many comparisons between the simulation of irradiation of aerospace components and systems with accelerators, nuclear research reactors, and spontaneous decay sources with the orbital sources were obtained from the design of a CubeSat mission
Isotactic polypropylene solidification under pressure and high cooling rates. A master curve approach
No full textSolidification in industrial processes very often involves flow fields, high thermal gradients and high pressures: the development of a model able to describe the polymer behavior becomes complex. Recently a new equipment has been developed and improved to study the crystallization of polymers when quenched under pressure. An experimental apparatus based on a modified, special injection moulding machine has been employed. Polymer samples can be cooled at a known cooling rate up to 100 °C/s and under a constant pressure up to 40 MPa. Density, Micro Hardness (MH), Wide angle X-ray diffraction (WAXD), and annealing measurements were then used to characterize the obtained sample morphology. Results on one iPP sample display a lower density and a lower density dependence on cooling rate for increasing pressure. Micro hardness confirms the same trend. A deconvolution technique of WAXD patterns is used to evaluate the final phase content of samples and to assess a crystallization kinetics behavior. A master curve approach to explain iPP behavior under pressure and high cooling rates was successfully applied on density results. On the basis of this simple model it is possible to predict the final polymer density by superposition of the effect of cooling rate and the effect of pressure in a wide range of experimental conditions
An experimental methodology to study polymer crystallization under processing conditions. The influence of high cooling rates
No full textA new experimental route for investigating polymer crystallization under very high cooling rates (up to 2000°C/s) is described. A complete and exhaustive description of the apparatus employed for preparing thin quenched samples (100-200. μm thick) is reported, the cooling mechanism and the temperature distribution across sample thickness is also analysed, showing that the final structure is determined only by the thermal history imposed by the fast quench apparatus. Details concerning the characterization techniques used to probe the final structure are reported, including density measurements and wide angle X-ray diffraction patterns. Experimental results concerning isotactic polypropylene, polyethylenetherephthalate and polyamide 6 are reported, showing the reliability of this experimental route to assess not only a quantitative information but also a qualitative description of the crystallization behaviour of different classes of semi-crystalline polymers
Influence of "Controlled Processing Conditions" on the solidification of iPP, PET and PA6
No full textIn this work reliable experimental data for three semicrystalline polymers (iPP, PA6, PET) crystallised under pressure and high cooling rates are supplied. These results were achieved on the basis of a model experiment where drastic "controlled" solidification conditions are applied. The final objective was to quantify the effect of two typical operating conditions (pressure and cooling rate) on the final properties and morphology of the obtained product. The influence of processing conditions on some macroscopically relevant properties, such as density and micro hardness is stressed, together with the influence of processing conditions on the product morphology, investigated by means of Wide Angle X-Ray Scattering (WAXS). Results on the iPP samples display a decrease of density and micro hardness, due to the pressure increase, in a wide range of cooling rates (from 0.01 to 20°C/s). PET samples exhibit an opposite behaviour with density and micro hardness increasing at higher pressures in the whole range of cooling rates investigated. PA6 samples behave similarly to PET displaying a less significant increase of density and micro hardness with pressure than PET samples
Phenomenological approach to compare the crystallization kinetics of isotactic polypropylene and polyamide-6 under pressure
No full textReliable experimental data for semicrystalline polymers crystallized under pressure are supplied on the basis of a model experiment in which drastic solidification conditions are applied. The influence of the pressure and cooling rate on some properties, such as the density and microhardness, and on the product morphology, as investigated with wide-angle X-ray scattering (WAXS), is stressed. Results for isotactic polypropylene (iPP) samples display a lower density and a lower microhardness with increasing pressure over a wide range of cooling rates (from 0.01 to 20 °C/s). Polyamide-6 (PA6) samples exhibit the opposite behavior, with the density and microhardness increasing at higher pressures over the entire range of cooling rates investigated (from 1 to 200 °C/s). A deconvolution technique applied to iPP and PA6 WAXS patterns has allowed us to evaluate the final phase content and to assess the crystallization kinetics. A negative influence of pressure on the α-crystalline phase crystallization kinetics can be observed for iPP, whereas a slightly positive influence of pressure on the crystallization kinetics of PA6 can be noted. © 2001 John Wiley & Sons, Inc. J. Polym. Sci. Part B: Polym. Phys
CHARACTERIZATION OF HYDROPHOBIC POLYMERIC MEMBRANES FOR MEMBRANE DISTILLATION PROCESS
No full textHydrophobic microporous membranes are utilized in membrane distillation (MD) processes, e.g. seawater desalination at moderate temperatures. The vapour permeability of commercial hydrophobic membranes with different pore sizes (0.2-1 micron) was characterized through a simple apparatus designed-on-purpose. A cylindrical vessel had a face closed by the membrane and the other connected to a thin graduate tube. The water level variation in the tube is recorded and related to the vapour flux across the membrane.
Measurements were taken in the temperature range 20-80°C. A fan tangential to membrane surface was employed to maintain a constant driving force for vapour transport. Vapour flux did not depend on pore dimension, but the membrane and support material resulted to influence the mass transfer.
Moreover, the results showed that the main resistance for mass transfer is located in the permeate side, thus addressing future works on the set up of a MD pilot unit
Solidification of syndiotactic polystyrene by a continuous cooling transformation approach
No full textSyndiotactic polystyrene (sPS) was solidified from the melt under drastic conditions according to a continuous cooling transformation methodology developed by the authors, which covered a cooling rate range spanning from approximately 0.03 to 3000 °C/s. The samples produced, structurally homogeneous across both their thickness and surface, were analyzed by macroscopic methods, such as density, wide-angle X-ray diffraction (WAXD), and microhardness (MH) measurements. The density was strictly related to the phase content, as confirmed by WAXD deconvolution. The peculiar behavior encountered (the density first decreasing and then increasing with the cooling rate) was attributed to the singularity of the phases formed in sPS; that is, one of the crystalline phases (α) was less dense than the amorphous phase, and the latter, in turn, was less dense than the other crystalline phase (β). With an increasing cooling rate, the thermodynamically stable phase (β) disappeared first, and it was followed by the α phase. On the other hand, the MH values remarkably depended on the amount of the β phase, the α-phase content influencing the mechanical properties only to a minor extent. The behavior of the crystallization kinetics was described through a modified multiphase Kolmogoroff–Avrami–Evans model for nonisothermal crystallization
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