DMA——三点弯曲.pdf

Improvements in DMA Measurements Using Low-Friction 3-Point Bending Clamps Sujan E. Bin Wadud TA Instruments, 109 Lukens Drive, New Castle DE 19720, USA ABSTRACT For modulus measurements, the purest form of evaluation occurs when using 3-point bending and rectangular samples. If, however, the sample has a twist along its length, then there is only partial contact between the clamp and the sample across its width at the middle. This creates an artifact in the thermal curves of an apparent increase in the storage modulus just before the onset of the glass transition. New low-friction self- adjusting 3-point bending clamps, offered by TA instruments, eliminate this artifact. INTRODUCTION One of the most common experiments performed in dynamic mechanical analysis is heating with continuous mechanical oscillation of the test specimen at constant frequency and amplitude. This experiment provides valuable information about the glass transition temperature (Tg), the change in modulus and stiffness of the material with temperature, as well as qualitative information about cross-linking, molecular weight, etc. When using the 3-point bending clamps, it is assumed that the sample has good contact with the clamps. However, if there is a twist in the sample, no matter how subtle, the contact is less than adequate. Figure 1 illustrates this effect in an exaggerated form. Figure 1 – Poor Sample Clamping For very rigid, high modulus materials, such as high-performance composites, the inadequate contact leads to erroneous evaluations of modulus. Even very small twists in the sample cause the effects shown in Figure 2. The storage modulus seems to increase near 135 °C just before the glass transition. This occurs as the material soften