“…ISO 75 standard) represents a superposition of information from all of the constituents in a specimen; therefore, it is unsuitable for examining the heat resistance of nonwoven materials [63]. In the field of scanning probe microscopy, localized thermomechanical analysis (LTMA) offers the "micro" equivalent of thermomechanical investigation on micrometre-scale areas within a 10-20-s period [64][65][66]. By exploiting the high spatial resolution of this technique, the fibres themselves play the role of TMA specimens.…”
This research concentrates on the marked discrepancies in the crystalline structure of poly(lactic acid) (PLA) nano-and microfibres, achieved by different annealing strategies. PLA nonwoven mats were produced by high-speed electrospinning. The high-speed production technique allowed the manufacturing of PLA microfibres with diameters of 0.25-8.50 µm with a relatively high yield of 40 g h −1. The crystalline content of the inherently highly amorphous microfibres was increased by two methods; thermal annealing in an oven at 85 °C was compared to immersion in absolute ethanol at 40 °C. The morphology of the fibres was examined by scanning electron microscopy; crystalline forms and thermal properties were assessed using X-ray diffractometry, Raman spectrometry, differential scanning calorimetry (DSC) as well as modulated DSC. As a consequence of 45-min heat treatment, the crystalline fraction increased up to 26%, while solution treatment resulted in 33% crystallinity. It was found that only disordered α′ crystals are formed during the conventional heat treatment; however, the ethanol-induced crystallization favours the formation of the ordered α polymorph. In connection with the different crystalline structures, noticeable changes in the macroscopic properties such as heat resistance and mechanical properties were evinced by localized thermomechanical analysis and static tensile test, respectively.
“…ISO 75 standard) represents a superposition of information from all of the constituents in a specimen; therefore, it is unsuitable for examining the heat resistance of nonwoven materials [63]. In the field of scanning probe microscopy, localized thermomechanical analysis (LTMA) offers the "micro" equivalent of thermomechanical investigation on micrometre-scale areas within a 10-20-s period [64][65][66]. By exploiting the high spatial resolution of this technique, the fibres themselves play the role of TMA specimens.…”
This research concentrates on the marked discrepancies in the crystalline structure of poly(lactic acid) (PLA) nano-and microfibres, achieved by different annealing strategies. PLA nonwoven mats were produced by high-speed electrospinning. The high-speed production technique allowed the manufacturing of PLA microfibres with diameters of 0.25-8.50 µm with a relatively high yield of 40 g h −1. The crystalline content of the inherently highly amorphous microfibres was increased by two methods; thermal annealing in an oven at 85 °C was compared to immersion in absolute ethanol at 40 °C. The morphology of the fibres was examined by scanning electron microscopy; crystalline forms and thermal properties were assessed using X-ray diffractometry, Raman spectrometry, differential scanning calorimetry (DSC) as well as modulated DSC. As a consequence of 45-min heat treatment, the crystalline fraction increased up to 26%, while solution treatment resulted in 33% crystallinity. It was found that only disordered α′ crystals are formed during the conventional heat treatment; however, the ethanol-induced crystallization favours the formation of the ordered α polymorph. In connection with the different crystalline structures, noticeable changes in the macroscopic properties such as heat resistance and mechanical properties were evinced by localized thermomechanical analysis and static tensile test, respectively.
“…Since then the technique has developed such that the AFM tip may be replaced by a V-shaped Wollaston wire, whereby the silver sheath is etched away to expose the platinum core at the apex of the tip (Figure ). This tip may then act as a resistive heat source as well as a resistance thermometer and may apply both linear and modulated heating signals to a sample. , The method has now been commercialised and has attracted considerable interest within the polymer sciences , and more recently within the pharmaceutical arena. − 1 SEM image of the microthermal analysis tip.…”
The use of microthermal analysis as a means of differentiating between amorphous and crystalline forms of a model low molecular weight solid has been investigated, with a view to establishing the strengths and limitations of the technique as a means of identifying different physical forms of the same substance within a single sample. Samples of the drug indometacin were prepared in amorphous, crystalline, and partially crystalline forms and the samples studied using microthermal analysis (MTA) and modulated temperature differential scanning calorimetry (MTDSC). MTDSC studies revealed a simple melting process for the crystalline material corresponding to the ? form of the drug, while a glass transition, recrystallization and subsequent melting was seen for the amorphous indometacin; in addition, the sample demonstrated a shift in the phase angle at approximately 65 °C which has been previously associated with flow of the sample in the DSC pan. MTA studies indicated that the thermal conductivity is dominated by the surface topology and no clear differentiation between amorphous and crystalline regions was obtained in this mode. A discontinuity was seen in the localized thermal analysis (LTA) profile for the crystalline sample at 152 °C corresponding to the melting of the material. Similarly, the amorphous material showed a shift in probe position at 64 °C, these responses being ascribed to sample softening. Topographical examination of the sample following LTA studies indicated the presence of a “crater” approximately 10-20 µm in diameter, giving an indication of the scale of scrutiny of the experiment. Studies using different scanning rates (2 to 20 °C/s) did not demonstrate significant changes to the temperatures of transition and crater size. Studies on partially crystalline samples clearly showed that the technique was able to differentiate between the two physical regions in LTA mode. The technique may therefore have considerable potential as a means of identifying distinct physical regions in a single sample. However, consideration needs to be given to the experimental and interpretative issues raised in this investigation
“…The topographic images are of particular importance as such thermal events are strongly related to the thermal expansion coefficient and elastic modulus, more so than upon the thermal conductivity [3]. However, during a phase transition, changes in measured signals are partly due to the increase in contact area and the temperature of the sample material in immediate contact with the probe can be measured provided a suitable calibration of the probe-tip temperature is previously carried out [4][5][6][7][8][9].…”
This study aims to assess the in situ spatial distribution of glass-transition temperatures (T g ) of the main lignocellulosic biopolymers of plant cell walls. Studies are conducted using scanning thermal expansion microscopy to analyze the crosssection of the cell wall of poplar. The surface topography is mapped over a range of probe-tip temperatures to capture the change of thermal expansion on the sample surface versus temperature. For different temperature values chosen between 20 • C and 250 • C, several quantitative mappings were made to show the spatial variation of the thermal expansion. As the glass transition affects the thermal expansion coefficient and elastic modulus considerably, the same data line of each topography image was extracted to identify specific thermal events in their topographic evolution as a function of temperature. In particular, it is shown that the thermal expansion of the contact surface is not uniform across the cell wall and a profile of the glass-transition temperature could thus be evidenced and quantified corresponding to the mobility of lignocellulosic polymers having a role in the organization of the cell wall structures.
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