Micro-thermal Analysis Research Articles

Two- and Three-Dimensional Imaging of Multicomponent Systems Using Scanning Thermal Microscopy and Localized Thermomechanical Analysis

The aim of this study was to develop a novel approach to the spatial characterization of multicomponent samples, based on the emergent technique of microthermal analysis. More specifically, we present an assessment of the use of scanning thermal microscopy as a means of component mapping via thermal conductivity; we include a new statistical approach to data handling, which allows reduction of topographic effects. We also introduce a novel three-dimensional mapping technique based on localized thermomechanical analysis. Tablets of paracetamol and hyproxypropyl methylcellulose (HPMC) and 50:50 mixes of the two were prepared and the materials characterized in scanning and localized modes using a TA Instruments 2990 microthermal analyzer with a Thermomicroscopes Explorer AFM head and Wollaston wire thermal probe. L-TMA studies of the pure components indicated markedly differing thermal responses, with the paracetamol showing a sharp melting accompanied by a probe pull-in effect, while HPMC showed only thermal expansion over the temperature range studied. Thermal conductivity and topographic images indicated that two-dimensional differentiation between the components was possible in scanning mode. A means of delineating the relative contribution of the topographic and conductivity effects was developed based on a regression analysis of the thermal conductivity measurements on a set of terms representing the local surface curvature. The results of three-dimensional imaging using a grid of L-TMA measurements is presented. This technique utilized the distinct thermal responses of the two components to allow the probe to melt through the paracetamol down to the underlying HPMC. The advantages and limitations of this novel imaging method are discussed in the context of pharmaceutical and broader uses of the approach.

Preparation of monolithic matrices for oral drug delivery using a supercritical fluid assisted hot melt extrusion process

The use of supercritical fluids as plasticisers in polymer processing has been well documented. The body of work described in this research paper outlines the use of a supercritical CO 2 assisted extrusion process in the preparation of a hot melt extruded monolithic polymer matrix for oral drug delivery. Several batches of matrix material were prepared with Carvedilol used as the active pharmaceutical ingredient (API). These batches were subsequently extruded both with and without supercritical CO 2 incorporation. The resultant matrices were characterised using steady-state parallel plate rheometry, differential scanning calorimetry (DSC), atomic force microscopy (AFM), micro-thermal analysis (μTA) and dissolution testing. Dissolution analysis showed that the use of supercritical CO 2 during the extrusion process resulted in a faster dissolution of API when compared with unassisted extrusion. The supercritical CO 2 incorporation also resulted in reduced viscosity during processing, therefore allowing for quicker throughput and productivity. The results detailed within this paper indicate that supercritical fluid assisted hot melt extrusion is a viable enhancement to conventional hot melt extrusion for the production of monolithic dosage forms.

Physical and thermal characterisation of chiral omeprazole sodium salts

The physical properties of drug substances may affect stability, manufacturing, dissolution and bioavailability. Variations in the degree of crystallinity in a pharmaceutical substance may exhibit physicochemical differences that impact at therapeutic, manufacturing, commercial and legal levels, yet no reference has been found on the physical properties of micronised omeprazole. This study reports on the physical and thermal characterisation of the sodium salts of S- and R-omeprazole, using diffuse reflectance infrared Fourier transform spectroscopy (DRIFT), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), microthermal analysis (μTA) and powder X-ray diffraction (XRPD). DSC experiments were performed in order to determine not only their thermal stability, but also the thermal history of both forms. SEM results indicate similar morphology, particle size and shape of powdered drug, while, μTA of processed discs shows different topographical images for S- and R-omeprazole, exhibiting a smoother surface for the S-form, indicative of the smoother particle size not evident in the SEM results. The low level of crystallinity of both enantiomers was confirmed by DRIFT spectroscopy and XRPD. Thermal stability by DSC of S- and R-omeprazole sodium salts was superior to that of the neutral omeprazole. This study has examined the physical and thermal properties of both forms and in highlighting their differences provides an explanation for the potential differences in bioavailability and therapeutic efficacy.

Evaluation of the microstructure and melting behavior of drawn polypropylene fibers with a microthermal analyzer

The strong correlation of melting behavior with the microstructure of original and zone-drawn isotactic polypropylene fibers was evaluated by microthermal analysis (micro-TA) combined with wide-angle X-ray diffraction analysis. The crystal structure of both the original and zone-drawn fibers was a monoclinic α-form with a ∼ 0.64 nm, b ∼ 2.03 nm, and c ∼ 0.65 nm. In contrast to the absence of any oriented polymer molecules in the original fiber, the polymer molecules in the zone-drawn fiber were extended and highly oriented in at least two different states; one arranged along the drawn axis and the other at 40° to the drawn axis. The micro-TA-derived melting points corresponded to these microstructural changes caused by the zone-drawing process and differed from the melting points obtained by differential scanning calorimetery. The micro-TA melting point of 140°C corresponded to the z-e-p melting point of the original fiber, being much lower than the conventional differential scanning calorimetery melting point of 163°C. As for the zone-drawn fiber, an increase of 19°C in the melting temperature revealed its high orientation level, and the appearance of three elevated melting peaks (161, 167, and 178°C) at the highest heating rate (1500°C/min) coincided with its several oriented states. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 1306–1311, 2006

Sizing effects on main relaxation process of cyanate ester glass fiber composites using dynamic mechanical and microthermal analyses

AbstractThe influence of sizing/polymer interaction or interphase on dynamic mechanical relaxation properties of cyanate ester composites was investigated by means of dynamic mechanical analysis. The dynamic mechanical behavior of different samples (three types of composites with different sizing and neat resin) was analyzed by using two phenomenological models (TFV and WLF). Related coefficients such as C1, C2, and T∞ (Vogel temperature) were evaluated. Results have shown that these parameters were strongly dependent on sizing state, therefore, on interphase. In addition, the frequency dependence of the molecular relaxation process was well described by the Cole‐Cole plot. These results were confirmed by the sizing extract/resin blend study. Microthermal analysis has shown that partial miscibility existed between resin and sizing extract. Local thermal analyses were carried out by positioning the probe over selected regions: bulk resin and sizing/resin blend. A decrease in resin glass‐transition temperature was observed in the sizing resin blend. The different results have shown that a local plasticization of resin by sizing occurred with crosslink density modification. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 205–214, 2006

Photo Thermal Micro-Spectroscopy — A New Method for Infared Analysis of Materials

Abstract Many modern materials are composite structures with complex morphologies that play a large role in determining the material function. The ability to investigate the relationship between structure and property on a microscopic scale can play a crucial role in material development. Micro-Thermal Analysis (μTA)™ is a unique set of analytical techniques for characterizing materials on a micrometer and nanometer scale. Micro-TA combines the imaging power of atomic force microscopy with the ability to analyze physical, mechanical, thermal, and chemical characteristics at a specific point of interest on the surface of a material. At the heart of the technique is a miniaturized thermal probe. In a new technique termed Photo Thermal Micro-Spectroscopy (PTMS), this thermal probe is used to detect temperature fluctuations in samples that have been irradiated by IR radiation. A Fourier Transform Infrared (FT-IR) spectrum can be constructed from this information. This paper describes the PTMS technique and discusses recent applications.

Photochemical surface modification of PET by excimer UV lamp irradiation

UV irradiation has interesting potential for the photochemical modification of polymers. In order to study cross-linking effects and/or thin-layer deposition following a treatment in the presence of bi-functional media or in inert atmosphere, irradiation of PET in various atmospheres was performed using a KrCl excimer lamp. Surface properties were investigated by atomic force microscopy, nanoindentation, micro-thermal analysis, and X-ray photo-electron spectroscopy. The studies reveal that surface chemical composition, morphology, adhesion, thermomechanics, and stiffness/modulus are strongly affected by UV irradiation in the presence of bi-functional media. Films treated in octadiene and argon show an increase of surface modulus, much less expansion, and lower soft/melt temperatures, which is an indication of the surface cross-linking effect and a decrease of crystallinity within the near-surface layer. In the case of a diallylphthalate-treated film, depending on the local structure, either a strong decrease of melting temperature or no melting point is found, which is attributed to the irregular cross linking and thickness of the modified layer associated with a decrease of surface modulus. A significant increase of the alkali resistance is found after irradiation, as a result of both wetting and cross-linking effects on the polymer surface.

Through-thickness analysis of the skin layer thickness of multi-layered biaxially-oriented polypropylene films by micro-thermal analysis

A novel method is presented for the determination of the thickness of a polymer layer on a solid substrate by through-thickness local thermal analysis (LTA) measurements using a micro-thermal analyser (μTA). The feasibility of the method is illustrated for a poly(methyl methacrylate) film spin-cast on a silicon wafer. Subsequently the method is applied to determine the skin layer thickness of multi-layered biaxially-oriented polypropylene (BOPP) films. Although the melting temperatures of skin and core layer are only 15 °C different, it proved to be possible to determine the skin layer thickness. The film thickness obtained by μTA correlates well with the thickness observed by transmission electron microscopy (TEM) in a 0.1–1.6 μm range. The method is shown to be accurate, robust, and fast.

Surface stability of polypropylene compounds and paint adhesion

AbstractThermoplastic olefins based on polypropylene compounds are being increasingly used for the production of painted automotive parts. The poor adhesion properties of these compounds are improved with flaming, which results in good adhesion for waterborne paints. The surface stability and adhesion properties of two commercial injection‐molded compounds and their base polymer blends (polypropylene/ethylene–propylene rubber) were investigated after they were flamed with various parameters under typical vapor jet conditions. The compounds and the corresponding base blends showed mainly the same behavior. Thus, filler particles and additives seemed to have only a minor influence on the adhesion properties of these compounds. For the characterization of the surface itself and the near‐surface region, scanning and transmission electron microscopy, XPS, and microthermal analysis were used. The utmost surface layer for all specimens consisted of a skin of pure polypropylene. The oxygen concentrations at the surface after flaming were rather similar for both compounds. Differences could be found in the surface roughness, the oxygen diffusion from the interior to the surface, and, probably most importantly, the viscosity and elasticity of the impact modifier. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 797–805, 2005

Interphase investigation in glass fibre composites by micro-thermal analysis

The micro-thermal analysis combines the advantages of both microscopy (visualisation) and thermal analysis (characterisation), with high spatial resolution. The experiment was performed on glass fibre composites based on cyanate resin matrix in order to highlight the interphase in these composites. Two types of glass fibres, which differed only by the sizing, were inserted in the resin. The subsurface distribution of the glass monofilaments can be observed into the conductivity images. Localised thermal analysis were carried out by positioning the probe over selected regions: glass monofilaments, bulk resin and interphase. The glass transition decreases from the bulk resin value to a value about 40 °C lower, for composites with sizing glass monofilaments. A less dense cross-linkage can be observed in the interfacial area.

Study of Pharmaceutical Solid Dispersions by Microthermal Analysis

The main objective of the study was to determine the ability of microthermal analysis (microTA) to assess the crystallinity of the drug in two noncommercial pharmaceutical solid dispersions. Pure substances, physical mixes, and solid dispersions were analyzed by microTA. The thermal values obtained by microTA were compared with data obtained by more conventional techniques like differential scanning calorimetry. microTA was able to detect the drug in the waxy matrix. The technique was capable of showing that relatively large amorphous drug domains (up to 70 microm) are formed during the solidification of the solid dispersion. These amorphous domains are thermodynamically unstable and can crystallize upon aging. microTA was successfully applied to the physical characterization of solid dispersions. Local thermal analysis (LTA) offers a unique way to probe a selected area on the surface of the sample. It is capable of melting the drug locally without melting the lower melting point excipient. These results gave an understanding of the poor dissolution performance of these solid dispersions upon aging.

Crystalline structures and ferroelectric properties of ultrathin films of vinylidene fluoride and trifluoroethylene copolymer

Thin and ultrathin films of copolymer of 83% mole fraction vinylidene fluoride and 17% mole fraction trifluoroethylene were prepared using a spin-coating technique. Atomic force microscopy results showed that spun films formed tiny grains on the surface. A micro-thermal analyzer showed that tiny grains had a ferroelectric Curie transition at 128.8 °C and melted at 140.7 °C. The crystallinity of the film was measured to be ca. 40% by an X-ray diffraction for the film with thickness above 170 nm. Crystallinity decreased quickly with decreasing thickness. The crystallinity was at most 5% for the film with thickness below 75 nm. We found the ferroelectric polarization reversal effect strongly depended on the film thickness. Clear polarization reversal due to a switching of ferroelectric dipoles was observed for the film with thickness above 90 nm and broad ferroelectric reversal was obtained for the film with thickness below 90 nm. For the films with thickness above 100 nm, the remnant polarizations were almost constant and the coercive fields were significantly decreased with decreasing film thickness.

Novel Polymeric Thin Film Deposition System: Injector‐Apparatus/PECVD Reactor#

A monomer injection apparatus has been developed to be used with a plasma enhanced chemical vapor deposition (PECVD) reactor to deposit polymeric thin films from precursor monomers with boiling points ranging from low to very high. The system (PECVD reactor–injection apparatus) allows deposition of layered, as well as doped, polymeric thin films. Thin film deposition from a mixture of two monomers with very different boiling points—thiophene and 2,3‐dihydrothieno [3,4‐b]‐1,4‐dioxin (EDOT)—and other monomers like dicyclopentadiene (DCPD) and 2‐chloro‐p‐xylene has been performed to demonstrate the capabilities of the deposition system. Thermal analysis of the deposited thin film using microthermal analysis (µTA™) and differential scanning calorimetry performed on deposited films is also presented. The injection apparatus associated with a PECVD reactor proved to be a very versatile tool for novel polymeric thin film deposition. #Dedicated to the Memory of Professor Sukant K. Tripathy.

APPLICATION OF MICROTHERMAL ANALYSIS AND PULSED FORCE MICROSCOPY TO CHARACTERIZE NANOSTRUCTURED POLYMER

In this investigation, we present the first results of Microthermal analysis (MicroTA) and Pulsed-Force-Mode (PFM) scanning force microscopy employed for visualization and characterization of nanostructured side-chain crystalline polymeric material. PFM was employed to clearly visualize the interesting self-organizing characteristics and the intimate contact of crystalline order as well as amorphous disorder in such polymer. As the sample was heated above the melting point, the well-defined crystalline regions observed at lower temperature no longer exist, and diffused melt boundaries are clearly observed using PFM. The nanophase separated system undergoes a sharp change in its adhesion and stiffness properties with temperature below and above the crystal melting point. Local thermal analyses using MicroTA exhibit consistent rapid crystal melting curves and its lateral homogeneity.

Macro and microthermal analysis of plant wax/surfactant interactions: plasticizing effects of two alcohol ethoxylated surfactants on an isolated cuticular wax and leaf model

Synperonic A7 and A20 are surfactants commonly used in the agrochemical industry to improve the physicochemical properties of formulations. This paper describes the use of differential scanning calorimetry (DSC) and localised thermal analysis (LTA) to investigate the bulk and surface effects of these surfactants on extracted leaf wax and an intact leaf system. Both DSC and LTA analysis reveal a plasticizing effect in the presence of the A7 surfactant. Further to this, LTA analysis allows us to spatially map micron scale variations in this effect throughout the residue of individual deposits. In contrast to this no plasticizing effect is revealed following the application of the A20 surfactant in either the DSC of LTA analysis. These results are discussed in context with the current theory of surfactant/wax interaction.

Microthermal analysis of the melting behavior in zone-drawn isotactic polypropylene fibers

A microthermal analysis (μTA) method with an ultrahigh heating rate was applied to a zone-drawn isotactic polypropylene fiber to evaluate the melting behaviors inherent to the microstructural changes occurring during the drawing process. The μTA melting point decreased with a reduction in the melting temperature because of the suppression of crystal reorganization and increased with an increase in the melting temperature because of the suppression of the relief of the orientation of the molecular chains, in comparison with the conventional melting point obtained by traditional differential scanning calorimetry. This study examined a convenient and powerful method for investigating the melting behavior for polymeric materials inherent to the original microstructure with less influence from thermal-measurement heating. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3677–3681, 2004

Uniformity and physical states of troglitazone in solid dispersions determined by electron probe microanalysis and microthermal analysis

Solid dispersions of troglitazone with PVP K30 in a weight ratio of 1:2 were prepared by the closed melting method. Solid dispersions that exhibit different degrees of crystallinity (0 and 36%) were prepared by changing the charged amount of water, which functions as a plasticizer for PVP K30. Electron probe microanalysis (EPMA) and microthermal analysis (μTA) of the solid dispersions were performed to investigate the uniformity and physical state of troglitazone in the solid dispersions. The EPMA study confirmed that troglitazone was dispersed homogeneously in the sample whose apparent crystallinity was 0%. However, the sample with a 36% crystallinity was heterogeneous. The μTA study showed that the sample with a 36% crystallinity was present in two states, crystal and amorphous. EPMA and μTA would be useful tools to confirm the uniformity and physical states, respectively, of solid dispersions.

A combined imaging, microthermal and spectroscopic study of a multilayer packaging system

AbstractThe effectiveness of a packaging solution for the pharmaceutical and food industry is dependent on the integrity of the constituent layers and the interfaces formed between them. The deconvolution and analysis of the many intimate layers found in packaging is analytically challenging, requiring techniques capable of identifying sub‐micron regions. Here we have characterized the chemical and physical nature of the layers in a multilayer packaging system along with the interfaces, using a combination of high‐resolution atomic force microscopy (AFM), microthermal analysis using scanning thermal microscopy (SThM), and Fourier transform infrared (FT‐IR) spectroscopy. In particular, localized thermal analysis is shown to reveal the thermal transitions of the individual layers, but it was found that care must be exercised when melting through one layer to the next, as this can result in overestimates of melting temperatures of the underlying layer due to excess power loss from the SThM probe to the already molten top layer surrounding the probe. Copyright © 2004 John Wiley & Sons, Ltd.

Novel micro-thermal characterisation of thin film NiTi shape memory alloys

Abstract A novel approach in determining the transition temperatures of thin films of NiTi shape memory alloys was investigated and compared with conventional techniques. The technique is based on micro-thermal analysis utilising a scanning thermal microscope (SThM). This method has the potential to allow the transformation temperatures of thin films to be investigated in situ. In this study, thermal micro-analysis using an SThM was investigated on thin film samples of 47.5 at.% Ni and 49.6 at.% Ni in Ti. Thin amorphous NiTi films were deposited on silicon substrates using DC plasma assisted sputtering. The thin films were recrystallised and subsequently characterised by SThM. Results were then compared with DSC, XRD and resistivity measurements. The SThM sensor variation with temperature showed clear onset and completion of transformation from martensite to austenite during a heating cycle. The small probe size of 1 μm2 allowed variations in transformation temperatures across the NiTi thin film to be investigated. A range of transition temperatures was observed, which was consistent with the difference between AS and AF as determined by resistivity and bulk DSC measurements. The linear contraction between martensite and austenite was also measured by this technique and was found to be similar to the theoretical transformation of 0.45%. This novel method will provide a route to investigate both the shape memory transformation temperatures of NiTi thin films and their spatial variation across devices such as micro-actuators.

Variations in surface gloss on rubber‐modified thermoplastics: Relation to morphological and rheological behavior

AbstractThe parameters that promote the formation of surface defects on multi‐phase polymer systems during injection molding are not yet clearly understood. The objective of this work is to establish the influence of composition, particularly the role of rubber, on the rheological properties of such multi‐phase systems and relate that to variations in gloss that appear on the surface of ASA and PC/ASA blends. Analysis of morphology by micro‐thermal analysis (μTA™) in combination with oscillatory rheology, time‐temperature‐superposition (TTS), creep recovery and PVT testing revealed distinct trends of surface gloss with each material's viscoelastic behavior and surface finish. It has been shown that influential properties such as rubber content, particle size and distribution, molecular weight, viscosity ratios and the degree of acrylonitrile mismatching play key roles in each material's rheology, developed morphology and subsequent trends in surface gloss after injection molding.