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.