1 Department of Micro- and Nanotechnology, Technical University of Denmark2 Amphiphilic Polymers in Biological Sensing, Department of Micro- and Nanotechnology, Technical University of Denmark3 Nanoprobes, Department of Micro- and Nanotechnology, Technical University of Denmark4 Department of Energy Conversion and Storage, Technical University of Denmark5 Functional organic materials, Department of Energy Conversion and Storage, Technical University of Denmark6 Center for Intelligent Drug Delivery and Sensing Using Microcontainers and Nanomechanics, Center, Technical University of Denmark7 Center for Nanostructured Graphene, Center, Technical University of Denmark
Resonant microstrings show promise as a new analytical tool for thermal characterization of polymers with only few nanograms of sample. The detection of the glass transition temperature (Tg) of an amorphous poly(d,l-lactide) (PDLLA) and a semicrystalline poly(l-lactide) (PLLA) is investigated. The polymers are spray coated on one side of the resonating microstrings. The resonance frequency and quality factor (Q) are measured simultaneously as a function of temperature. Change in the resonance frequency reflects a change in static tensile stress, which yields information about the Young’s modulus of the polymer, and a change in Q reflects the change in damping of the polymer-coated string. The frequency response of the microstring is validated with an analytical model. From the frequency independent tensile stress change, static Tg values of 40.6 and 57.6 °C were measured for PDLLA and PLLA, respectively. The frequency-dependent damping from Q indicates higher Tg values of 62.6 and 88.8 °C for PDLLA and PLLA, respectively, at ∼105 Hz. Resonant microstrings facilitate thermal analysis of nanogram polymer samples measuring the static and a dynamic glass transition temperature simultaneously.