Local Thermal Analysis Research Articles

Interphase formation in model composites studied by micro-thermal analysis

Micro-thermal analysis was used for studying interphases formed in particle-filled composites based on an epoxy resin matrix. The subsurface distribution of the particles can be observed in the conductivity images, while spatial variations in thermal properties can be determined by local thermal analyses. For an anhydride-cured epoxy system filled with porous silica particles, an interphase is observed around the particles. In the ca. 60 μm wide interphase, the glass transition temperature gradually decreases from the bulk value to a value about 10 °C lower. The less densely cross-linked interphase can be detected in the thermal conductivity images and in the local thermal analyses. The interphase formation is attributed to the effect of adsorbed water on the polymerisation mechanism.

Thermally assisted nanosampling and analysis using micro-IR spectroscopy and other analytical methods

Micro-thermal analysis is a technique in which the probe in a scanning probe microscope is equipped with an ultra-miniature electrical resistor that serves both as a source of heat and a means of measuring temperature. Thermal properties can be imaged and local thermal analysis can be performed by placing the tip on a selected point and linearly ramping its temperature. When a material is softened in this way, the tip often becomes contaminated with the sample. The tip can easily be cleaned by heating it to a temperature high enough to cause the contaminant to decompose. However, prior to this, the contaminant can be analysed by, amongst other techniques, photothermal IR spectroscopy.

Morphology of multiblock copolymers as studied by micro-thermal analysis

The goal of this research was study of the relation between morphological features of semicrystalline and multiblock polymeric materials and their thermal properties with use of microthermal analysis. Samples of semicrystalline poly(butylene terephthalate) (PBT) and its copolymer were crystallized from the melt showing a spherulitic morphology. The surface of the spherulitic shapes was subjected to localized thermal analysis (L-TA) at selected regions of different thermal conductivity (at the centre of the spherulite and at its outer surface) and revealed information which cannot be obtained from classical bulk thermal analysis. The method allowed us to record thermally activated near-surface processes like melting transition within volumes of a few μm3.

Extraordinary elevation of the glass transition temperature of thin polymer films grafted to silicon oxide substrates

We used local thermal analysis and ellipsometry to measure the glass transition temperatures (Tg) of supported thin films of poly(4-hydroxystyrene) (PHS) and hydroxy terminated polystyrene (PS-OH). The films were spuncast from solution onto silicon oxide substrates and annealed under vacuum at elevated temperatures to graft the polymer to the substrate. Grafting was verified and characterized in terms of the thickness of and the advancing contact angle of water on the residual layer after solvent extraction. For PHS, each segment of the polymer chain was capable of grafting to the substrate. The thickness of the residual layer increased with increasing annealing temperature. For this polymer the critical thickness below which the Tg of the film deviated from the bulk value was nearly 200 nm after annealing at the highest temperature (190 °C); the Tg of films 100 nm thick or less were elevated by more than 50 °C above the bulk value. For PS-OH films the polymer was only capable of grafting at one chain end, forming a brush layer at the substrate interface. The critical thicknesses for PS-OH films and the Tg elevations were substantially higher than for ungrafted PS films, but were not as large as for PHS. The film thickness dependence of Tg for PHS and PS-OH were well described as piecewise linear, consistent with a “dual-mechanism” model.

The use of micro-thermal analysis as a means of in situ characterisation of a pharmaceutical tablet coat

The technique of micro-thermal analysis (MTA) has been applied to a commercial sugar coated ibuprofen tablet in order to identify the ability of the method to differentiate between the coat and the tablet core and to characterise the thermal properties of both components using localised thermal analysis. Thermal conductivity measurements in conjunction with intensity histogram analysis indicated that differentiation across the coat/core interface was possible, with a bimodal distribution of pixel intensities corresponding to thermal conductivity noted. Localised thermal analysis studies indicated that the bulk response was dominated by the incorporated ibuprofen, with a discontinuity seen at ca. 70–80°C, corresponding to the published melting point of the drug. The coat showed a discontinuity at ca. 220°C that may be reasonably ascribed to the melting process. It was also noted that the coat showed a small discontinuity at a temperature corresponding to the melting of ibuprofen. In summary, the technique was shown to be capable of identifying the core/coat interface using thermal conductivity measurements, while localised thermal analysis experiments enable the operator to perform thermal analysis experiments on the individual components in situ.

An application of micro‐thermal analysis to polymer blends

Micro-thermal analysis (micro-TA) is a new subsurface thermal analysis technology. The average of the DC signal is a function of the thermal conductivity, and the response to the AC modulation signal is a function of the thermal diffusivity of the subsurface. Using this technique, three images based on topography, thermal conductivity, and thermal diffusivity are obtained simultaneously. Specific areas and domains in these images can then be characterized by simply positioning the probe and performing a localized thermal analysis experiment. The technique has been used to study the phase separation process in a 50:50 (by weight) polystyrene (PS)–poly(vinyl methyl ether) (PVME) blend and natural rubber–nitrile rubber blends. For these polymer blends, considerable contrast between phases is obtained, based on thermal conductivity, whereas optical and electron microscopy would show them as being very similar. For example, it is difficult to image the morphology of natural and nitrile rubber blends by means of transmission electron microscopy, because of their similar chemical structures. Micro-TA gives an excellent image of the morphology of these natural–nitrile rubber blends. This opens a new way for rubber industries to study morphologies of rubber–rubber blends in general. In the 50:50 PS–PVME blend, annealed at 125°C, spinodal decomposition occurred. With increasing time, the domain size and the glass transition temperature of PS-rich domains increased, indicating that the concentration of PVME in the PS-rich phases decreases. The results imply that micro-TA can be used to image the composition in the near-surface or surface regions in multicomponent materials, if the resolution is high enough. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 2136–2141, 2001

Dependence of the Glass Transition Temperature of Polymer Films on Interfacial Energy and Thickness

The glass transition temperatures (Tg's) of ultrathin films (thickness 80−18 nm) of polystyrene (PS) and poly(methyl methacrylate) (PMMA) were measured on surfaces with interfacial energies (γSL) ranging from 0.50 to 6.48 mJ/m2. The surfaces consisted of self-assembled films of octadecyltrichlorosilane (OTS) that were exposed to X-rays in the presence of air. Exposure to X-ray radiation systematically modified the OTS by incorporating oxygen-containing groups on the surface. The interfacial energy for PS and PMMA on the OTS surface was quantified as a function of X-ray dose using the Fowkes−van Oss−Chaudhury−Good model of surface tension. The Tg values of the films were characterized by three complementary techniques: local thermal analysis, ellipsometry, and X-ray reflectivity. Within the resolution of the techniques, the results were in agreement. At low values of γSL, the Tg values of PS and PMMA films were below the respective bulk values of the polymers. At high values of γSL, the Tg values of PS and PMMA films were higher than the bulk values and increased monotonically with increasing γSL. The deviation of the Tg values of the films compared to the bulk values increased with decreasing film thickness. For a specific film thickness of PS and PMMA, the difference between the Tg of the film and Tg of the bulk polymer (ΔTg = Tgfilm − Tgbulk) scaled linearly with γSL irrespective of the chemistry of the polymer.

Identification of Crystalline and Amorphous Regions in Low Molecular Weight Materials Using Microthermal Analysis

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.

An application of micro‐thermal analysis to polymer blends

AbstractMicro‐thermal analysis (micro‐TA) is a new subsurface thermal analysis technology. The average of the DC signal is a function of the thermal conductivity, and the response to the AC modulation signal is a function of the thermal diffusivity of the subsurface. Using this technique, three images based on topography, thermal conductivity, and thermal diffusivity are obtained simultaneously. Specific areas and domains in these images can then be characterized by simply positioning the probe and performing a localized thermal analysis experiment. The technique has been used to study the phase separation process in a 50:50 (by weight) polystyrene (PS)–poly(vinyl methyl ether) (PVME) blend and natural rubber–nitrile rubber blends. For these polymer blends, considerable contrast between phases is obtained, based on thermal conductivity, whereas optical and electron microscopy would show them as being very similar. For example, it is difficult to image the morphology of natural and nitrile rubber blends by means of transmission electron microscopy, because of their similar chemical structures. Micro‐TA gives an excellent image of the morphology of these natural–nitrile rubber blends. This opens a new way for rubber industries to study morphologies of rubber–rubber blends in general. In the 50:50 PS–PVME blend, annealed at 125°C, spinodal decomposition occurred. With increasing time, the domain size and the glass transition temperature of PS‐rich domains increased, indicating that the concentration of PVME in the PS‐rich phases decreases. The results imply that micro‐TA can be used to image the composition in the near‐surface or surface regions in multicomponent materials, if the resolution is high enough. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 2136–2141, 2001

Analysis of polymeric composite interphase regions with thermal atomic force microscopy

AbstractPrevious work on the characterization of interphase regions in thermosetting composite systems has focused on the inference of an interphase layer from effects noticed through macroscale mechanical and thermal testing. With the development of atomic force microscopy and active thermal probes for this technique, it is now possible to examine material thermal properties on a much smaller scale. Variations in microscale thermal properties of an aerospace‐grade thermosetting resin system were evaluated for carbon and glass fiber reinforcement, using the modulated local thermal analysis mode of a TA Instruments 2990 μTA. The variations observed clearly demonstrate the presence of a soft interphase layer in the glass material and underline the importance of fiber–matrix interactions during the formation of the interphase. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 1643–1649, 2001

Characterization of the surface structural, mechanical, and thermal properties of benzocyclobutene dielectric polymers using scanned probe microscopy

Scanning probe microscopy (SPM) techniques are used to characterize surfaces related to the processing of benzocyclobutene (BCB) dielectric thin films. Thermally cured resins and photodefineable resins are sold under the trade name CYLCOTENETM1) for electronic applications. TappingMode AFM (TMAFM) imaging is used to follow changes in adhesion promoter morphology upon baking to help explain adhesion performance. Power spectral density (PSD) analysis of TMAFM images of plasma treated BCB surfaces are unique and can be used to ‘fingerprint’ processes. Selective oxidation of the BCB surface can be used to fabricate a phase imaging standard for TMAFM. Near surface modulus of the BCB materials is 3.6 ± 0.2 GPa and the hardness is 0.38 ± 0.2 GPa measured by depth-sensing nanoindentation. Creep recovery of indents can be used to qualitatively distinguish between thermal and photocureable materials. A heated tip in a scanning thermal microscope can induce the thermal curing of BCB over micron sized areas. Local thermal analysis with the same probe allows the measurement of the changes in the glass transition, Tg, at the surface with cure temperature.

Micro-thermal analysis and evolved gas analysis

Micro-thermal analysis employs an active heated tip in a scanning probe microscope. This allows topographic and thermal imaging of surfaces to be carried out and permits localised thermal analysis of selected areas of material. In order to provide a means for spatially resolved chemical characterization, a method of trapping the gases that are evolved when the thermal probe is used to pyrolyze the surface has been developed. The trapped gases can then be thermally desorbed and passed into a GC–MS system for separation and identification. With this system it is possible to ablate a small (<10 μm×10 μm) area of a sample (or a domain, feature or contaminant) and elucidate its composition. This paper presents initial results for a homogeneous polymer (poly(methyl methacrylate)) — and a heterogeneous polymer blend (polystyrene/poly( α-methyl styrene)).

An investigation into the surface deposition of progesterone on poly (d,l-) lactic acid microspheres using micro-thermal analysis.

Polylactic acid microspheres are well established as delivery vehicles for a range of low molecular weight and proteinaceous drugs (1–3). However, there is arguably a paucity of information available regarding the physical characteristics of these spheres, with few studies (4) addressing the issue of the manner in which drugs are distributed through the polymeric matrix. Following earlier studies (5,6) we recently investigated the use of modulated temperature DSC (MTDSC) as a means of monitoring the distribution of progesterone within poly(d,l-lactide) spheres using a range of loadings up to 50% w/w (7). By monitoring the thermal response of the spheres we obtained strong evidence for the drug being present as a separate amorphous phase at 30% w/w loading and as a distinct crystalline phase at 50% w/w loading. Complementary SEM studies showed marked changes in the surface morphology of the spheres at these two concentrations, leading to the suggestion of surface drug deposition. However, it was not possible at that stage to definitively establish the location of the drug using the methodologies available. A recent advance within the thermal analysis field has been the introduction of micro-thermal analysis (micro-TA, 8–10). This method is based on the same principles as conventional AFM but involves the replacement of the probe tip with a Wollaston wire loop. The apex of the wire is etched away to leave the silver filament exposed, resulting in a higher resistance in this section of the tip. Application of a voltage therefore results in Joule heating, hence one may apply a thermal signal to highly specific regions of a sample. In addition, the technique allows isothermal measurement of thermal conductivity by rastering over the surface and measuring the tip resistance. The technique has attracted considerable interest, particularly within the polymer science field, as it represents a unique method of performing thermal analysis on highly specific regions of a sample without the necessity of heating the material in its entirety. Early studies on pharmaceutical materials have also yielded highly encouraging results (11,12). The ability of micro-TA to perform localised thermal analysis suggests that the method may be a means of establishing whether the progesterone is indeed present on the surface of the aforementioned microspheres. The objective of the present study was therefore to investigate the use of micro-TA as a novel means of characterising the surface of the PLA microspheres, with a particular view to obtaining further evidence for the presence or absence of progesterone on the exterior of the spheres.

Scaling of Tg and reaction rate with film thickness in photoresist: A thermal probe study

A thermal probe technique, local thermal analysis, was used to measure the glass transition temperature (Tg) and reaction rate as a function of film thickness in chemically amplified photoresists. Using this technique, heat loss into a resist film was monitored as the temperature of the probe was ramped from ambient to temperatures as high as 200 °C. The thermal events, glass transition temperature or heat evolved during reaction, were recorded as a function of the probe temperature. The Tg of the photoresists UVN 30, UV6, UV3, KRS, and KRS-XE was measured for thick films and for ultrathin films approximately 50 nm thick. The measured Tg in ultrathin resist films was 4–22 °C higher relative to that measured in thick films. We also investigated the behavior of polyhydroxystyrene films, and found that crosslinking to the substrate can increase Tg by a large amount. The photoresist films were then exposed with x-ray radiation at the same dose (950 mJ/cm2) for both thick and ultrathin films to ensure a constant photogenerated acid concentration with thickness. The exposed areas of the films were heated with the thermal probe, and an increase in heat flow into the exposure area, attributed to the heat of reaction, prior to the glass transition temperature was measured. Kinetic rate constants were estimated with data from the power supplied to the probe as a function of temperature using a first order reaction model. The results indicate that the rate of reaction in ultrathin resist films is smaller than in thicker films for resists processed at the same postexposure bake temperature. We find that Tg and reaction rate depend on film thickness in ultrathin photoresist films; the differences in these properties are expected to lead to large changes in the processing conditions used for ultrathin films relative to thick films.

Two new microscopical variants of thermomechanical modulation: scanning thermal expansion microscopy and dynamic localized thermomechanical analysis

We describe two ways in which thermomechanical modulation may be used in conjunction with scanning thermal microscopy, in order to distinguish between different components of an inhomogeneous sample. The sample is subjected to a modulated mechanical stress, and the heating is supplied locally by the probe itself. Scanning thermal expansion microscopy is an imaging mode, in which an imposed localized temperature modulation is used to generate thermal expansion, which in turn produces mechanical strain and gives thermal expansion contrast images. We present results using two types of active thermal probe. For polymer/resin samples, the depth of material contributing to the measured thermal expansion is typically a few micrometres. Under certain conditions we observe a reversal in contrast as the frequency of the temperature modulation is increased. In dynamic localized thermomechanical analysis, the modulated stress is applied directly, and accompanied by a localized temperature change, as used in other forms of localized thermal analysis. The resulting modulated lateral force signals are obtained. The glass transition of polystyrene is detected, and shows a significant variation with frequency. The amplitude or phase signal may be used to obtain image contrast for inhomogeneous samples.

Thermal Probe Measurements of the Glass Transition Temperature for Ultrathin Polymer Films as a Function of Thickness

The glass transition temperature of polystyrene and poly(methyl methacrylate) films on polar and nonpolar substrates was measured as a function of thickness using a thermal probe in contact with a polymer film. Using a technique called local thermal analysis, heat loss into the film was monitored as the temperature of the probe was ramped from ambient temperature to temperatures as high as 200 °C. The glass transition temperature was determined from a change in slope in the heat loss versus temperature plot. The Tg of polystyrene on silicon oxide decreased by as much as 25 °C below the bulk value for films 13 nm thick. The same trend in the glass transition temperature was observed for polystyrene films on silicon oxide treated with hexamethyldisilizane (HMDS). The Tg of poly(methyl methacrylate) on silicon oxide increased by up to 7 °C above the bulk value for films 18 nm thick. For poly(methyl methacrylate) on silicon oxide treated with HMDS, the Tg decreased by 10 °C below the bulk value for films 21 n...

Discrimination of polymorphic forms of a drug product by localized thermal analysis.

In chemical processing, it is important to distinguish between and identify polymorphic forms. We demonstrate the novel use of scanning thermal microscopy (SThM) and localized thermal analysis to distinguish and identify polymorphic forms of the drug cimetidine. These forms cannot be resolved by classical bulk thermal analysis. SThM reveals a sample consisting of a 50 : 50 mixture of the polymorphs contains regions of different thermal conductivity, corresponding to the different polymorphs. Localized thermal analysis of small volumes of pure polymorphic samples (approximately 50 microm3) shows that the origin of the thermal conductivity contrast lies, at least in part, with the presence of a surface water layer on the more hydrophilic polymorph.

Influence of polymer specimen structure on the reproducibility of micro-thermomechanical transitions

Glass transitions of amorphous polystyrenes with low polydispersity were evaluated using the modulated Local Thermal Analysis mode of the TA Instruments 2990 µ TA and evaluating the thermomechanical signal. Transition temperature variance and fraction of transitions measured were compared for high molecular mass thermosetting materials and the melt of Nylon 6.6. The transition reproducibility was found to decrease as the molecular size of the polymer samples increased. Reproducibility also decreased for thermosetting materials when the experimental ramp rate was decreased. Heat transfer within the specimen was evaluated using finite element analysis, allowing scaling of microscale experimental results for comparison to bulk transitions.

Peritectic decomposition of the metal oxide YBa2Cu3O x

The existing data on the mechanism of peritectic decomposition of the metal oxide YBa2Cu3O x are ambiguous. This process cannot be studied without quite sensitive methods of phase analysis and allowance for its dependence on the partial pressure of oxygenp(O2) in the gas phase. The methods of x-ray diffractometry, scanning electron microscopy, and local electron-probe and thermal analyses are used to study the transformations in heating YBa2Cu3O x above 900°C in air and in vacuum at a residual pressurep(O2)=700 Pa.

Scanning thermal microscopy: Subsurface imaging, thermal mapping of polymer blends, and localized calorimetry

We have used a platinum/10% rhodium resistance thermal probe to image variations in thermal conductivity or diffusivity at micron resolution and to perform localized calorimetry. The probe is used as an active device that acts both as a highly localized heat source and detector; by generating and detecting evanescent temperature waves, we may control the maximum depth of sample that is imaged. Earlier work has shown that subsurface images of metal particles buried in a polymer matrix are consistent with computer simulations of heat flows and temperature profiles, predicting that a 1 μm radius probe in air will give a lateral resolution of ∼200 nm near the surface, with a depth detection of a few μm. We have a special interest in polymer blends, and we present zero-frequency mode and temperature-modulation mode thermal images of some immiscible blends in which the image contrast arises from differences in thermal conductivity/diffusivity between single polymer domains. The behavior of domains is observed in real time as the blends are subjected to a slow temperature rise. We have also achieved localized differential thermal analysis of a number of polymers, and recorded events such as glass transitions, meltings, recrystallizations, and thermal decomposition within volumes of material estimated at a few μm3. This opens the way forward towards calorimetric imaging, by which it should be possible to distinguish between different regions undergoing either reversible or irreversible changes as the temperature is varied.