THERMAL-DIFFUSIVITY OF POLYMER-FILMS BY PULSED PHOTOTHERMAL RADIOMETRY

We have developed a pulsed photothermal radiometry technique for deter mining the thermal diffusivity parallel to the surface of a polymer fi lm that involves flashing a line-shaped laser beam on the surface of t he sample at right angle to its length, and monitoring the temperature change with time at a distance from the line source using an infrared detector. Combining this with our previous laser-flash radiometry met hod for thermal diffusivity measurement perpendicular to the film surf ace, we can now measure the thermal diffusivity of a polymer film alon g all directions. These two techniques have been used to study uniaxia lly and biaxially oriented poly(ethylene terephalate) and uniaxially d rawn ultrahigh molecular weight polyethylene films. For uniaxially ori ented poly(ethylene terephalate), the thermal diffusivity along the dr aw direction is substantially higher than that in the transverse direc tion, which in turn, is slightly higher than that in the thickness dir ection. For a polyethylene him with a draw ratio of 200, the axial the rmal diffusivity is extremely high, being about five times that of sta inless steel. The anisotropy of the thermal diffusivity of this film e xceeds 90. (C) 1997 John Wiley & Sons, Inc.