Search results for "thermal [radiation mechanism]"

Effect of irradiation temperature and dose rate on the mechanical tensile behaviour of low density polyethylene

1993

Abstract The effect of ageing due to gamma radiation, at various dose rates and temperatures, on the mechanical tensile behaviour of low density polyethylene is studied. In order to detect synergistic effects, tensile tests on samples subjected to thermal treatment corresponding to the same temperature for the same time as for the irradiation tests have also been performed. The results indicate a generalized decrease of the elongation at break, with brittleness of the material for the most severe ageing treatments. For modulus values, an increase is observed for particular experimental conditions. These results are related to morphological and structural modifications induced in the polymer…

Time-resolved rheology as a tool to monitor the progress of polymer degradation in the melt state - Part I: Thermal and thermo-oxidative degradation …

2015

Abstract Thermal and thermo-oxidative degradation of polyamide 11 (PA11) in the melt state (T = 215 °C) are studied by resorting to time-resolved mechanical spectroscopy. Such an approach allows to elude the changes in the rheological properties occurring while testing, thus enabling the rigorous study of polymer degradation in the melt state. Different concurrent degradation reactions in oxidative (air) and non-oxidative (N2) environment are promptly guessed by studying the time evolutions of rheological functions. In particular, changes in the zero-frequency complex viscosity reflects changes in the average molecular weight, while the appearance of a yield stress in the complex viscosity …

Rheological behavior and thermal stability of poly(phenylene sulfide)/vectra-B950 blends

1994

Blends of polyphenylene sulfide (PPS) with a commercial, wholly aromatic, liquid crystalline polymer (LCP), Vectra-B950, have been prepared by melt-blending. Their rheological behavior has been studied in order to determine if the LCP displays a processing aid ability, and under what conditions it gives rise to potentially reinforcing fibrils dispersed in the PPS matrix. The problem of the thermal stability of PPS/LCP blends, which has been considered by some authors as the main obstacle to the production of usable materials due to the evolution of gaseous substances during processing, has been discussed. © 1994 John Wiley & Sons, Inc.

Effects of filler type and mixing method on the physical properties of a reinforced semirigid liquid crystal polymer

1996

Semirigid liquid crystalline polymers (LCPs) show some advantages with respect to rigid LCP: in particular lower processing temperatures, better compatibility with flexible thermoplastics, but also some disadvantages. Thermal resistance of rigid LCPs is much better and the maximum working temperature is, in general, very high. The thermomechanical properties can be improved by adding inorganic fillers. In this work processing, mechanical and thermomechanical properties of a filled semirigid liquid crystal copolyester are reported. Several inorganic fillers have been used in order to put in evidence the influence of shape and dimensions of the particles on the properties of the filled materi…

2019

Abstract The preparation method of a polymer composite and the filler loading are amongst the factors that influence the properties of the final composites. This article studies the effect of these factors on the thermal stability and thermal degradation kinetics of poly(methyl methacrylate) (PMMA)/mesoporous silica (MCM-41) composites filled with small amounts of MCM-41. The PMMA/MCM-41 composites were prepared through in situ polymerisation and melt mixing methods, with MCM-41 loadings of 0.1, 0.3, and 0.5 wt.%. The presence of MCM-41 increased the thermal stability of PMMA/MCM-41 composites prepared by melt mixing, but in the case of the in situ polymerised samples, the MCM-41 accelerate…

Thermal and viscoelastic properties of entangled supramolecular polymer networks as a powerful tool for prediction of their microstructure

2018

Abstract Thermal and viscoelastic properties of entangled supramolecular polymer networks, SPNs, depend strongly on binary and collective assembly of associative groups. The collective assemblies can phase separate from polymer matrix chains and form domains with different sizes and shapes, which have different melting point transitions. By increasing content of associative groups along the polymer chains, their high-order association leads to formation of domains, which have higher melting temperatures than other ones. We prepared a SPN system that contains three networks. All networks have similar precursor polymer backbone, but different content of ureidopyrimidinone, UPy, moiety as stro…

Hydrothermal ageing of radiation cured epoxy resin-polyether sulfone blends as matrices for structural composites

2010

Abstract The hydrothermal ageing of epoxy–thermoplastic blends, used as matrices for carbon fibre composites, cured by electron beam, has been studied. Two different thermoplastic percentages have been adopted. A suitable choice of both curing process and formulation parameters allows to carry out irradiation at mild temperature with several advantages, coming from a “non thermal” process, for both the final properties of the materials and the environment. Nevertheless the occurring of vitrification phenomena needs the use of a short thermal treatment after irradiation on the already solid materials, in order to complete the cure reactions. Radiation cured epoxy based matrices have been sub…

Interactions between aroma and edible films. 1. Permeability Of methylcellulose and low-density polyethylene films to methyl ketones.

1998

This work contributes to the study of aroma transfers through edible and plastic packaging films. Permeability, sorption, and diffusivity of three methyl ketones (2-heptanone, 2-octanone, and 2-nonanone) in and through low-density polyethylene and methylcellulose-based edible films have been determined. Permeability was measured using a dynamic method coupled with a gas chromatograph. The methyl ketone permeability of polyethylene films mainly depends on diffusivity of the penetrant in the polymer. In the case of 2-heptanone, a saturation of the polymer network is observed at high vapor concentrations. The formation of clusters could take place when concentrations are higher in the vapor ph…

Thermal diffusivity measurements for Mg-Mn ferrites

1992

Abstract The thermal diffusivity of Mg-Mn ferrites is studied through a modified scheme of the photothermal deflection method able to derive the diffusivity of low-diffusivity samples and to avoid any influence of the vertical offset parameter. The examined samples have been realized under different preparation conditions producing different grain dimensions, and a negligible influence of the grain dimensions is found on the thermal properties.

3D Metamaterials with Negative Thermal Expansion and Negative Effective Compressibility

2018

Materials with negative thermal expansion are desired for controlling thermal stresses, but unusual in nature. With two-component metamaterials it is possible to tune the thermal expansion from positive over zero to negative values, even if both components have positive thermal expansion. Using gray-tone laser lithography we fabricate three-dimensional two-component polymer microlattices, exhibiting zero or negative thermal expansion [1].