Development of Nanomechanical Thermal Analysis System and Its Applications to Polymeric Materials
- Development of Nanomechanical Thermal Analysis System and Its Applications to Polymeric Materials
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- The international confederation for thermal analysis and calorimetry (ICTAC) defined thermal analysis formally as “Thermal analysis refers to a group of techniques in which a property of a sample is monitored against time or temperature while the temperature of the sample, in a specified atmosphere, is programmed.” There are many conventional thermal analysis techniques like differential scanning calorimetry (DSC), differential thermal analysis (DTA), thermomechanical analysis (TMA), dynamic mechanical analysis (DMA), rheometer, thermogravimetric analysis (TGA) and dilatometry. The major difficulty with conventional thermal analysis techniques is that they measure the response of the whole sample. If one observes a broad change in behavior on heating a specimen, this could be the result of a genuine effect in a homogeneous system or be due to a series of overlapping responses from a heterogeneous system, where there may be a gradation in properties throughout the sample. Alternatively, a weak effect seen in the entire sample could arise from a strong response from a minority component (e.g. an impurity) within in the bulk of the material. The same statements are true of other classical analytical techniques which require moderately sized amounts of material for sampling. In order to overcome those limitations in conventional systems many investigators tried to develop new or combined techniques for microthermal analysis (Micro-nano differential scanning calorimetry, AFM related technique, scanning thermal microscopy, local thermal analysis etc.). Using those microthermal analysis technique, different phase transition behavior from bulk property related to confinement effect also could be probed and those research topic is regarded as very important part in recent nanoscale research and industry area.
The objective of this thesis is development of nanomechanical thermal analysis (NTA) system using microcantilever and microresonator platform and application of this system to probe thermal property of polymeric materials. Based on this objective, we have developed silicon microcantilever based NTA system with in-situ measurement and calculation of deflection, resonant frequency and Q factor of microcantilever. By using this system, glass transition behavior of synthetic high polymers such as polystyrene (PS) and polyvinyl acetate (PVAc) was probed both static and dynamic manner with single measurement.
The difference between the volume expansion coefficients of polymer and silicon induced the cantilever to bend, and this variation was used to determine the static glass transition temperature as well as the physical properties of the polymer sample. The changes in the resonance frequency and the inverse of the Q factor of the cantilever are related to the variations in the elastic modulus and the loss tangent of polymer. The loss modulus of polymer was calculated from the resonance frequency and Q factor, and used to determine the glass transition temperature (Tg) of polymer, which was found to be higher than that obtained from the cantilever deflection in static manner. This difference is attributed to the high resonance frequency of the cantilever (or the fast rate of the applied stress). A series of measurements at various frequencies were carried out to obtain the accurate Tg and apparent activation energy for the glass transition.
Also, phase transition between rotator phases of several n-alkanes were sensitively probed with NTA system. Nanogram amounts of paraffin were coated onto a silicon cantilever, and the resonance frequency and deflection of the cantilever were measured as a function of temperature. Changes in the cantilever resonance frequency, which were related to the variations in the paraffin moduli, were used to determine the temperatures at which phase transitions between the rotator phases of tricosane, tetracosane, and pentacosane occurred. The resonance frequency was also used to determine the melting and crystallization temperatures. The thermal hysteresis in the resonance frequency of a tetracosane-coated cantilever differed from that of the tricosane- and pentacosane-coated cantilevers, which was attributed to the even-odd effect on the crystal structures of paraffin. The even-odd effect was also observed in the temperature dependent deflection measurements. Compared to conventional thermal analysis tools such as DMA and DSC which cannot probe entire rotator phase of n-alkanes, NTA can probe entire rotator phases of n-alkanes very sensitively.
In order to improve conventional NTA system, two electrical detection method (pizoresistive and piezoelectric) was adoped to NTA system. By using electrical detection scheme, miniaturization and fast data acquisition were achieved. And using commercial quartz tuning forks for piezoelectric NTA system, very handy and economical platform could be constructed.
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