Witold Brostow (1), Magdalena Jaklewicz (1,2), Shreefal Mehta (3), Pablo Montemartini (1,4)

(1) Department of Materials Science and Chemistry, University of North Texas, Denton, TX 76203-5310, USA; brostow@unt.edu
(2) Department of Mechanical and Biomedical Engineering, Cracow University of Technology, Cracow, Poland
(3) Southwestern Medical Center, University of North Texas at Dallas, Dallas, TX 75235, USA
(4) Consejo Naccional de Investigaciones Cientificas y Tecnicas (CONICET), Universidad Nacional de Mar del Plata, Mar del Plata, Argentina

ABSTRACT

We have placed a copolymer of poly(ethylene terephthalate + 0.6 mole fraction of p-hydroxybenzoic acid) in magnetic fields up to 1.8 Tesla, heated it to the molten state ( at several temperatures in the range 240-315 °C) and cooled to the room temperature under the field to maintain the orientation so acquired.  Then the extent of the orientation has been determined with an anisotropy parameter a based on the ultrasound critical-angle reflectometry (UCR); the parameter is calculated from the ratio of the maximum velocity to the minimum velocity of the reflected wave at critical incidence angles at which total internal reflection occurs.  A significant orientation effect is found.  As expected, anisotropy increases with time, magnetic field strength and to a lesser extent with the temperature.  Large changes in the extent of the anisotropy are connected to phase transitions determined before. The Freederickscz transition is observed.  The influence of the orientation effect on the mechanical properties of the samples has been established by flexural tests.  The strengthening observed has been explained using an extension of the statistical-mechanical theory of Flory in terms of channeling of flexible sequences in the polymer liquid crystal (PLC) and changes in the phase structures under the magnetic field.  The strengthening effect depends strongly on the presence, size and distribution of liquid crystal (LC) islands in the material.  The islands act similarly as dispersoids in other types of materials.  Maxima of the strengthening effect with respect tot eh alignment are found and related to the size of LC-rich islands in the structure.

Keywords: Polymer liquid crystals, Magnetic field, Flexural properties