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Thermal Analysis by Structural Characterization


T e wide range of temperatures for the transition from solid to liquid requires comment. T e dehydration step probably results in signifi cant disruption of the crystalline structure which, in turn, aff ects melting temperatures. Diff erent levels of disruption occur as a consequence of diff erences in local conditions including sample thickness (which aff ects local concentrations of the evolved gas during dehydration). T is would produce a wide range of temperatures at which particles transition from a signifi cantly disordered solid to liquid. Parallel melting and decomposition is not uncommon for organic materials and is seen in this case from the DSC curve. T ere are, therefore, two sources of structural disruption: the dehydration step and the nucleation and growth of decomposition product. Both contribute to the melting process occurring over a broad range. T e DSC data and the TASC data are highly comple- mentary. T e former shows how the energetics of the reactions in the sample change so radically that it is clear decomposition is occurring. T e TASC soſt ware shows quantitatively that melting continues well into the exotherm so the areas under the endotherm and exothermal cannot be taken in isolation. T ermomechanical Analysis . T e TMA capability is achieved by placing a tripod on the hot zone of the stage with one leg (the probe) on the sample and the other two on the heated block. Figure 5 shows this setup and a set of measurements on polycaprolactone (PCL). T e microscope is fi rst used to position where the point of the probe rests on the sample, then the focal plane is raised to the midpoint of the probe. Because it is angled, as it moves up and down, the position where it is in focus shiſt s laterally. In contrast to the TASC measurements described above, it is this movement that is tracked, not structural change, so expansion and indentation can be measured. A special feature of this measurement is that both x and y movements are tracked, something not possible with conventional TMA instruments. T e results for an experiment on a particle of polycapraloactone are given. T e x and y information could be used to characterize, for example, asymmetric frozen-in stress in a stretched polymer fi lm.


Discussion T e spatial resolution of TASC can be estimated to be about


10 µm. T e object that is being studied must exhibit suffi cient structure so that any change can be clearly detected. T e theory of the TASC method has been given elsewhere


[ 2 ]; briefl y the following equation obtains τ = υR/(λC) (1)


where τ is the time constant for the relaxation process, λ is viscosity, υ is surface tension, and R and C are constants related to the size and geometry of the indentation and the sample. Preliminary results for measuring the kinetics of the glass transition using multiple heating rates have already been given [ 2 ]. A signifi cant advantage of TASC for making these types of measurements is that both very slow and very fast heating rates can be used. Slow heating rates can be used because, unlike diff er- ential scanning calorimetry, the sensitivity of the measurement is unaff ected by the duration of the experiment. Unlike TMA, very thin samples and a small heated chamber can be used; thus, fast heating rates don’t cause large temperature gradients. T is ability to cover a very large dynamic range—almost fi ve orders of magnitude has already been demonstrated [ 2 ]—means the kinetics can be studied in more detail than with other approaches, so the deviation from Arrhenius behavior can be clearly seen [ 2 ].


22 www.microscopy-today.com • 2017 September


By using z -stacking, this type of experiment could be extended using materials with known viscosity and surface tension, and the structural constants R and C could be determined. T is opens up the possibility of new types of measurements where the surface tension is known, possibly from modeling: local viscosity could be determined, and, where the viscosity is known possibly from a separate measurement using a rheometer, local surface tensions could be measured. In this way the range of localized information hot stage microscopy can provide would be extended. Future work will explore these further.


Conclusion T e information available from hot stage microscopy has been substantially increased. T e types of transitions that can be detected have been expanded to include local glass transitions, and the range of materials that can be analyzed has been increased to include opaque samples. T ermal analysis images can now be obtained based on transition temperatures, and thermal analysis with 3D images has been demonstrated. Finally mechanical measurements can be made. T is is possible using a relatively inexpensive instrument that may also serve other functions.


References [1] X Dai et al ., Adv Drug Del Rev 64 ( 2012 ) 449 – 60 . [2] M Reading et al ., in Microscopy Advances in Scientifi c Research and Education Vol. 2 , ed. A Mendez-Villas, Formatex Research Centre , Badajoz, Spain , (2010 ) 1083 – 89 .


[3] M Alhijjaj et al ., Anal Chem 87 ( 21 ) (2015 ) 10848 – 55 . [4] M Alhijjaj et al ., Pharmaceut Res 34 ( 5 ) ( 2017 ) 971 – 89 . [5] R Riscica et al ., J Pharm Sci 99 ( 12 ) (2010 ) 4962 – 72 .


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