The structural properties of PMMA, which has been melt-spun and treated using a specific cooling profile, is investigated in order to evoke desired optical and mechanical properties. Several PMMA fibres, which had been melt spun and subsequently processed with different temperature profiles, were analysed by small-angle X-ray scattering (SAXS) measurements. These results will be compared to a combination of numerical models, which consider the quenching of a filamentary PMMA polymer melt in water. This multi-scale simulation considers macroscopically the cooling process in the water and within the fiber. The spatially resolved cooling rates, which have been simulated at different locations serve as input for a 3D-Monte-Carlo polymer simulation model, which takes, among others, the Lennard-Jones, the bending and bond potentials into account in order to predict the resulting PMMA structure of the fabricated fiber
These simulated structures are then evaluated in order to analyse their macroscopic properties. These comprise for instance the polymer entanglement, which describes the interaction of neighboring polymer chains leading to stronger, but stiffer fibers. Entanglement will also affect the glass-transition temperature, which determines the maximum operation temperature. But this can also lead to increased optical scattering, which will be subject to investigations, as well.
KEYWORDS: Monte Carlo methods, Polymers, Particles, Polymer optical fibers, Computer simulations, Polymethylmethacrylate, Mechanics, Chemical species, 3D modeling, Device simulation
We present a model for a Monte-Carlo simulation of polymer optical fiber fabrication based on a novel method using heat treatment after melt spinning. The polymer is modeled using three-dimensional bond-fluctuation with the Leonard-Johns potential for non-bonded interactions of different polymers alongside the bond, bend and torsion potential for the bonded interactions within the same polymer. The studied fabrication parameters were different cooling rates, pressure and the polymer chain length. Their influence is investigated on properties such as the radius of gyration RG, physical and optical density ρ and n, and the isothermal compressibility, which will be affected e.g. by the phase-transition temperature Tg. They influence both mechanical and optical properties of the produced fiber.
We present new results regarding a novel melt spinning process for the continuous production of polymer optical fibers (POF) with a graded index profile. The fabrication process comprises a conventional melt spinning process of PMMA and a conclusive abrupt cooling of the monofilament. During the quenching, the outer part of the fiber cools down faster than the inner part, which leads to a density gradient within the fiber due to the time difference for thermal expansion along the fiber radius. This results in a radial density and by that in a refractive-index profile. The density of the POFs is determined with a pycnometer. Moreover, the structural properties of the fibers are investigated by small-angle X-ray scattering (SAXS). Lastly, the resulting optical parameters such as refractive-index profile, fiber attenuation and scattering properties, are studied and related to the manufacturing parameters. The measurements of the density and refractive index show that the refractive-index profile of a PMMA fiber can be strongly influenced in the outer sector of the fiber, but the influence decreases radially inwards. The SAXS measurements indicate different polymer chain structures within the fiber form.
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