2.4 Mechanical characterization
Nanoscale dynamic mechanical analysis (Nano-DMA) tests were performed at room temperature using a Hysitron TI950 TriboIndenter device (Bruker Corporation, Billerica, USA) equipped with a Berkovich tip. Samples were glued to an epoxy holder to ensure stability during measurement. A grid with 100 measurement points (10 x 10) was created for oscillatory measurements to simultaneously obtain both the linear- and visco-elastic responses of the sample. Specimens were loaded with a sinusoidal force-time-function and a maximum load of 75μN oscillating at eight different frequencies (10, 31, 25, 115, 136, 157, 178, and 201 Hz). Loss (E”) and storage modulus (E’) were calculated from the measured force-displacement hysteresis loops using the software provided with the Bruker nanoindenter. The indents are approximately twice as small as the fiber diameter; we assumed that if an indent reached a pore, it would measure the fiber directly below it.