A Sample Holder for Hot Stage Microscopy WE1116 PORT Standard practice for melting point determination using a hot o FLATHE- SCREW stage microscope (e.g., Mettler FP52) is t o place afew microcrystals on a 1 X 3-in. glass slide and cover them with a 1-in.square glass cover Two ST slip. For many systems this is entirely satisfactory. However, there OF KAPTo are a significant number of cases where a hetter sample holder is necessary due to complications arising from high sample volatility, thermal degradation in air, or chemical reaction with the glass holder a t elevated temperatures. We experienced the latter in our studies of tetracyanoquinodimethane (TCNQ) a highly reactive electron acceptor whieh undergoes sublimation and reaction with glass a t elevated temperature. The problem was solved using the sample holder design shown below. EXPLODED VIEW OF SAMPLE HOLDER Two strips of a transparent plastic, in this case Kapton' (E.I. DuPont & Co.), a high thermal stability (>50OoC), nonreactive, transparent (orange) polyimide, were placed between two stainless steel plates, 2 X 8 X 0.1 em and 2 X 3 X 0.05 cm, respectively, and squeezed together using #80 flathead screws. A #45 hole 1-0.2 em d~~~. i a l was drilled coneentricallv throueh , " both d a t e s corresoondine to the viewine area in the Mettler FP52 hot stage. A few crystals of TCNQ were placed between the strips and slipped between the plates so that the crystals were clearly visible in the viewing area. Tightening the screws resulted in a good pressed seal around the periphery of the hole. In this configuration the sample is always warmer a t the edge of the hole and any sublimation will occur to the center of the viewing area. On melting the sample is also constrained within the viewing area unlike the situation using only the glass slides in which the molten sample frequently flows out of the field of view. An additional health advantage of this system is that containment of the sample prevents evaporation of any toxic substances into the air around the microscope by the equilibration gas stream flowing through the system. Samples of TCNQ were studied both with the simple glass slide holder and the new sample holder. With glass slides it was necessary to heat the sample to 260-C then scan rapidly through a t 10°C/min and ahserve large erystallites melting as chemical reaction occurred. A melting range of 292-294°C was determined by this method, and i t was not possible t o resolidify the sample far a repeat determination. Using the new sample holder i t was possible to scan through a t 2"C/min, partially melt, resolidify and remelt several times. The results of three melting cycles on a high purity TCNQ sample were 291.7-293.g°C, 293.0-293.6°C, 292.7-293.2% These results indicate that TCNQ is relatively melt stable though slow reaction was observed with the Kapton which prevented determination of the equilibrium melting point. Work supported by NSF grant # DMR-72-03025. ~~~
1.aboralnry for Research on t h e Slructure of Matter University of Pennsylvania Philadelphia. 19174
Volume 53,Number 10, October 1976 / 637
