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We introduce the scanning photopyroelectric microscope and demonstrate its application for the investigation of delamination sites below organic coatings applied onto metallic substrates. The technique is based on a photothermal method, i.e., the photopyroelectric one, and uses a flexible polyvinylidene-difluoride sensor in contact with the sample surface. It is suitable for studying with high spatial resolution subsurface features like air gaps in multilayer systems and determining their depth below the surface. Using a one-dimensional approach, we derived an analytical expression for the signal contrast arising from subsurface thermal barriers. We also developed a three-dimensional finite element model that allows describing the thermal response of more complex systems and the lateral resolution in the actual measurement. The theoretical models are validated using both, model and real-life samples, and we show that it is possible to detect air gaps with thickness down to 1 μm below organic coatings applied onto metallic substrates.

The structure and dynamics of segmented polyurethane networks prepared from poly(butadiene) diols (PBD) of different molecularweights M n ¼ 1200; 2160, 2650, 4690 and 10,300 g mol 21 , 4,4 0 -diphenylmethane diisocyanate (MDI) and poly(oxypropylene) triol (POPT)ðM n ¼ 710 g mol 21 Þ have been investigated by dynamic mechanical analysis, dielectric relaxation spectroscopy, differential scanningcalorimetry (DSC), small- and wide-angle X-ray scattering (SAXS and WAXS). Microphase separation on nanometre scale has beenconfirmed by SAXS in all networks. Except for the highest molecular weight of PBD, the networks consist of microdomains of similar sizeand composition dispersed in the soft phase of polybutadiene segments. Dielectric spectroscopy revealed the existence of three types of non-crystalline phases, each characterised by a well-developed dynamic glass transition. Absence of crystallinity has been proven by WAXS andDSC. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Polyurethane network; Microphase separation; Dielectric spectroscopy

Several strategies for the analysis of isochronal and isothermal spectra as well as complete three-dimensional (3D) spectra are outlined and applied to data of (glass forming) liquids and polymers. Conduction-free loss spectra are calculated using a compact solution of the Kramers–Kronig transformation and an approximation based on oe 0 =o ln x. The usefulness of the dielectric modulus for data analysis is also evaluated. Apart from oe 0 =o ln x, other useful derivatives in the frequency and temperature domain are treated as well. Finally, we describe the evaluation of 3Drelaxation maps by means of 3D fitting. 2002 Elsevier Science B.V. All rights reserved.

We are pleased to announce that the June 2018 issue of the IEEE Transactions on Dielectrics and Electrical Insulation (TDEI) will be a special issue on electrets and related phenomena. This issue is open to all authors. Presenters of papers at the IEEE 16th International Symposium on Electrets (ISE16) to be held during September 4-7, 2017 in Leuven (Belgium) are especially encouraged to submit their work to this special issue. For more information on ISE16 please go to www.ise16.eu. ispartof: IEEE Transactions on Dielectrics and Electrical Insulation vol:24 issue:3 pages:1974-1974 status: published

Thermally induced phase separation in concentrated lower critical solution temperature (LCST) blends of poly[(α-methyl styrene)-co-acrylonitrile]/ poly(methyl methacrylate) (PαMSAN/PMMA) in presence of multiwall carbon nanotubes (MWNTs) with different surface characteristics (∼NH2 functionalized and polyethylene modified) was monitored by modulated differential scanning calorimetry, melt rheology, conductivity spectroscopy and microscopic techniques. At a concentration of 2 wt%, the MWNTs clearly reduce the macromolecular mobility of the blend components and induce phase separation at lower temperatures as compared to the neat blends. Electron microscopic images revealed that phase separation resulted in a selective thermodynamically driven localization of both types of MWNTs in the PαMSAN phase whereas they were randomly distributed in the mono-phasic materials. Hence, a percolative MWNT network was formed at much lower concentrations as compared to those needed for percolation in the blend components. However, the significant changes in viscosity and elasticity of the blend components, brought about by the MWNTs, can also affect morphology development. This way, effective percolation in the blends can be hindered, as was observed for the 60/40 blend with polyethylene coated MWNTs (PE-MWNTs). Finally, a dramatic transition from an insulating one-phasic material at room temperature to a highly conductive material in the melt was induced by the phase separation. © 2014 Elsevier Ltd. All rights reserved. publisher: Elsevier articletitle: Effect of multiwall carbon nanotubes on the phase separation of concentrated blends of poly[(α-methyl styrene)-co-acrylonitrile] and poly(methyl methacrylate) as studied by melt rheology and conductivity spectroscopy journaltitle: European Polymer Journal articlelink: http://dx.doi.org/10.1016/j.eurpolymj.2014.01.030 content_type: article copyright: Copyright © 2014 Elsevier Ltd. All rights reserved. ispartof: European Polymer Journal vol:53 issue:1 pages:253-269 status: published

The molecular diffusion is intrinsically bound to the viscosity of the environment via the Stokes–Einstein (SE) relation, fixing an equality between the energetic barrier for translational motion and that for rotational motion. This relation, valid for melts at equilibrium, usually breaks down upon supercooling in proximity of the glass transition. As a common feature of the glassy dynamics, diffusion is enhanced compared to the viscosity and the segmental relaxation. In this chapter, we revise recent experimental evidence on the anomalous decoupling between rotational and translation motion in thin polymer films. While the segmental relaxation is almost unperturbed down to few tens of nanometers, diffusion of small molecules and cold crystallization kinetics tremendously slow down already at thicknesses exceeding by several folds the macromolecular size. After reviewing experimental methods permitting to assess the dynamics in confinement, we propose a unifying picture on the anomaly in the SE relation based on the different impact of irreversible chain adsorption on the rotational and translation motion. In particular, we experimentally verified the validity of a relation to predict the crystallization time of thin polymer films based on finite size effects and the slowing down in the dynamics scaling according to the SE relation. Remarkably, such expression fails in correspondence of a critical size comparable to the thickness of the layer irreversibly adsorbed within the timeframe of the experiment. Similarly, we observe a severe drop in tracer diffusivity of dielectric probes into apolar matrices, not explainable in terms of perturbations in the segmental dynamics, but ascribable to the presence of adsorbed layers, limiting the Brownian movements, precursors of molecular diffusion.

Confined at the nanoscale level, polymers crystallize much slower than in bulk, and in some cases the formation of ordered structures results inhibited for extremely long experimental time scales. Here, we report on the thickness dependence of the cold crystallization of thin poly(l-lactide) (PLLA) films (, The authors gratefully acknowledge the financial support of the Spanish Ministry of Science and Innovation (MICINN) through MAT 2011-23455. D.E.M.-T. thanks CSIC for the tenure of the JAE-Pre fellowship and Fondo Social Europeo (FSE) for cofinancing the JAE Program. B.V. and M.W. acknowledge financial support from the Research Council of the KU Leuven, Project No. OT/11/065, and financial support from FWO (Fonds Wetenschappelijk Onderzoeks−Vlaanderen). S.N. acknowledges financial support from the funds FER of the UniversitéLibre de Bruxelles.

Deviations from the bulk behavior of the temperature dependence of the structural relaxation time \(\tau \)(T), typically well described by the Vogel-Fulcher-Tammann (VFT) relation, are the most obvious signature for “confinement effects,” which comprise various scenarios ranging from slight changes in the VFT parameters to cases where the VFT behavior completely breaks down to an Arrhenius law. An attractive idea that has finally stimulated many studies on nanoconfined glass formers is the concept of cooperativity with a cooperative length scale \(\xi \) that dates back to Adams Gibbs and is also a subject of modern concepts for the description of glasses and supercooled liquids. In this chapter, we discuss the question in how far experiments on glass-forming liquids, confined to nanometer-sized geometries, can prove the cooperative nature and of the dynamic glass transition and provide quantitative information about its characteristic length scale \(\xi \). Here, we have particularly focused on experimental evidence regarding deviations of \(\tau \)(T) from the bulk behavior. To classify experimental findings from very diverse systems, we introduce three major “deviation scenarios,” which range from a complete breakdown of the VFT-law (type I) via a VFT-Arrhenius cross-over scenario (type II) to a scenario (III) representing a perturbed VFT behavior in terms of an overall accelerated or retarded glass transition dynamics. Four cases are discussed in detail, all of them referring to H-bonding molecules or molecular groups. EG confined in various zeolitic hosts revealed either bulk dynamics or the scenario I, while EG mixed with amylopectine (starch) showed an evolution through the cross-over scenario (II) toward genuine Arrhenius behavior (I). A third system, representing a H-bonding “liquid” confined to a self-assembled (smectic) layer structure, yielded a clear-cut scenario II along with a linear relation between the layer thickness and the cross-over frequency. Subsequently, the dynamics of ultrathin films of glycerol having one free surface was discussed as an example for a perturbated VFT-scenario (type III). Apart from all molecular details and the diversity of confining host structures, it is evident that the strength of the confinement, expressed by the number of spatially restricted dimensions (1D \(=\) layer, 3D \(=\) cavities), should be linked to the severeness of changes in the dynamics. Inventarization of various experimental findings reported in the literature including them described above indeed confirm a correlation between the “deviation scenario” and the number of constrained axes. Finally, the effect of mechanisms other than cooperativity arguments, e.g., surface interactions, density, packing, and orientation effects, will briefly be discussed to rationalize obvious exceptions from the proposed correlation.

A curious, strong dielectric relaxation process (δ) was found in rapidly cooled poly(ethylene naphthalate). This process, which is located between two known β and β* relaxations of PEN, appears predominantly after rapid cooling and remains present even after heating above the glass transition temperature. In view of its very low activation energy of ∼10 kJ/mol, its markedly high relaxation strength of up to Δɛ=5, and its Debye-like peak shape, a collective relaxation mechanism is proposed, which involves collective crankshaft motions of the -O-CH2-CH2-O- sequences in a regular arrangement of the main chains. The analogy between this δ-relaxation and an ultra-slow relaxation recently found in the smectic E phase of a side-chain liquid crystalline polymer suggests a (close-to) hexagonal smectic ordering in PEN. The very existence of liquid-crystalline order in PEN is corroborated by the observation of a thermo-reversible discontinuity in the relaxation parameters around −90 °C, which resembles a broadened LC-LC phase transition. Re-evaluation of experimental data of the β* relaxation, which occurs in the non-crystalline fraction of PEN, suggests that this relaxation is sensitive to the local orientational order, which extends from nematic to isotropic. The shift in the temperature of the β* peak and even the splitting of this peak found by other authors can be ascribed to the lowering of the activation energy by the local ordered packing of the PEN chains in line with a lower activation energy in the nematic order. The coexistence of isotropic and nematic regions in PEN is put in the context of orientational order fluctuations during the induction period of cold crystallisation of semi-flexible polymers.

The interaction between amylopectin, a starch polysaccharide, and ethylene glycol (EG) was investigated using broad-band dielectric relaxation spectroscopy. Water-free amylopectin (AP) was mixed with 21 wt % ethylene glycol. This resulted in a continuous ethylene glycol phase, as well as a molecularly mixed AP/EG fraction. After storage at room temperature or annealing, the mixture shows dynamic properties typical of a polymer with weak intermolecular interactions, suggesting that EG binds preferentially to AP and forms intrachain H-bridges leading to increased chain stiffness and thus an increased glass transition temperature. This structure evolution is accompanied by a sharp reduction in the size of the ethylene glycol droplets to a few nanometers, as revealed by pronounced confinement effects in the alpha-relaxation of the dispersed EG.

Supramolecular polymers, which are non-covalently bonded and formed by self association of di or trifunctional monomers exhibit, by virtue of quadruple hydrogen bonds, many of the properties of normal high molecular weight polymers, e.g. a dynamic rubber plateau. We focus on the molecular and cooperative dynamics of self-assembled linear polymers and networks, studied by broadband dielectric relaxation spectroscopy (DRS) in the frequency range from 10/sup -2/ to 10/sup 6/ Hz. The DRS analysis was backed up by dynamic mechanical and rheological experiments. In the high temperature region two loss processes (/spl alpha/ and /spl alpha/*) show up, the relaxation times of which obey the Vogel-Fulcher-Tammann (VFT) law. The dielectric /spl alpha/ process is related to the dynamic glass-rubber transition and is slightly faster than the corresponding mechanical /spl alpha/ process. A slower (high-temperature) /spl alpha/* relaxation is identified as chemical, corresponding to the mean lifetime of the hydrogen bonded linkages in the supramolecular chains. Its relaxation time /spl tau//sub /spl alpha/*/ was found to be 1 to 2 decades larger than the terminal flow relaxation time, indicating that the relaxation of an entire chain is dominated by the joint dynamics of many hydrogen bonds. The /spl beta/ relaxation, observed in both DRS and dynamic mechanical analysis (DMA) at temperatures below T/sub g/, arises from the local junction dynamics of hydrogen bonded units in the glassy state. Details of the temperature dependence and the shape of the loss peaks of the /spl alpha/ and /spl alpha/* relaxations will be discussed in terms of temporary physical networks and cooperativity.

Scanning pyroelectric microscopy was applied to study the local thermal diffusivity in polar perhydrotriphenylene (PHTP) inclusion compounds. A thermal wave technique with special heat flow and pyroelectric detection principles was used to measure the longitudinal (K-parallel to) and transversal (K-perpendicular to) thermal diffusivity of needle shaped crystals. As expected, we have found a higher thermal conductivity along the channel direction: K-parallel to> K-perpendicular to. K-parallel to also showed a variation along the needle axis, the K-parallel to values near the capping faces being higher than in the center region. Based on these observations we suggest a relation between the longitudinal thermal conductivity and the local density of orientational defects of the dipolar guest molecules which reaches a maximum in the area around the seed. These findings are in agreement with previous pyroelectric studies, revealing a macrodomain structure and a spatially dependent pyroelectric activity for two different PHTP inclusion compounds. (C) 2000 American Institute of Physics. [S0021-8979(00)05416-5]. ispartof: Journal of Applied Physics vol:88 issue:4 pages:2108-2117 status: published

A new type of dumb-bell-shaped host molecule (6-8) has been synthesised, of which 1,8-bis((1)-adamantyl)-1,3,5,7-octatetrayne (8 = BAOT) forms an open porous architecture when cocrystallised with a number of typical solvent molecules. Adamantyl substituents attached to a tetraalkyne spacer build up the walls of parallel channels wherein guest molecules are aligned. Surprisingly, the tetraalkyne unit is significantly bent. Desolvation experiments provide evidence for a reversible inclusion of guests. In the case of the inclusion of 2-butanone, a partial substitution by symmetrical and asymmetrical long-chain chromophores during crystallisation was possible. Stained crystals showed optical frequency doubling. The crystal structure analysis revealed a centric space group, although considerable translational and orientational disorder was present. Application of scanning pyroelectric microscopy revealed that the growth of inclusion compounds with 2-butanone produced polar ordering of guest molecules, which were aligned in two macro-domains of opposing polarity. The resulting orientation of the carbonyl dipoles is in agreement with the theoretical prediction of a Markov model of spontaneous polarity formation based on molecular recognition processes on growing crystal faces. The present case represents a new example of a property-driven supramolecular synthesis.

The spatial polarisation distribution in inhomogeneously polar molecular crystals has been imaged by scanning pyroelectric tomography and simulated by a Markov process. The experimental technique combines scanning pyroelectric microscopy (SPEM) with layerwise thinning of crystals. The SPEM probes the local spontaneous polarisation by its temperature dependence (pyroelectric effect). A focused and intensity modulated laser beam scans the surface of a polar sample and induces temperature changes in a volume depending on the laser spot size and the thermal diffusion length λth. Since high lateral resolution of the SPEM is only available for small values of λth. depth information to a resolution of ∼ 10 μ is achieved by repeated scanning and stepwise thinning of crystals. A layer by layer technique can provide 3D imaging of the polar ordering with a lateral resolution of ∼ 20 μ at a probed layer thickness of ∼ 10 μ. Applied to perhydrotriphenylene (PHTP) co-crystallised with 1–(4-nitrophenyl)piperazi...

The polar domain structure and dynamics of host/guest materials which include polar rodlike molecules were investigated by pyroelectric and dielectric techniques. Using scanning pyroelectric microscopy (SPEM), spatially resolved information on a /spl mu/m scale was obtained of the 3-D domain structure of the polarization caused by an acentric host lattice and/or the polar arrangement of dipolar guest molecules. Two classes of pyroelectric host/guest materials were examined with SPEM: (1) an inorganic host (zeolite AlPO/sub 4/-5) which was loaded with p-nitroaniline (PNA) guest molecules by an adsorption process, and (2) an organic channel-type inclusion material, which establishes its macroscopic polarity by cocrystallization of perhydrotriphenylene (PHTP) with the polar guest 1-(4-nitrophenyl)piperazine) (NPP). Despite great differences in the chemical composition and morphology, both types of host/guest systems revealed 180/spl deg/ macro-domains, of which the particular shape and sign of the polarization were in accordance with models about crystal growth and/or directed adsorption of polar molecules. In order to assess the molecular dynamics of the included guest molecules, frequency dependent local pyroelectric measurement, based on SPEM, and broad-band dielectric relaxation spectroscopy were performed. Whereas guest molecules included in PHTP channels show no rotational dynamics, we found several relaxation processes in PNA-loaded AlPO/sub 4/-5 crystals, which were assigned to local and cooperative relaxation modes of molecular chains of the hydrogen-bonded PNA guest molecules.

Extensive investigation of polymer layers confined in nanometer-sized geometries revealed that the presence of an absorbing substrate or a free surface alters properties of polymers such as biaxial creep behavior, flow and intermolecular entanglements, diffusion of small molecules inside the matrix, crystallization kinetics, physical aging, the glass transition temperature (Tg), 7 and thus local chain (segmental) mobility. In bulk, the local chain mobility can be investigated by means of several experimental approaches. On the contrary, due to obvious instrumental difficulties, a technique probing the local chain dynamics of freely standing ultrathin polymer films was not available so far. In this Communication, we introduce a novel experimental method taking advantage of the sensitivity of dielectric spectroscopy (DS) and being able to probe the segmental dynamics of freely standing ultrathin polymer layers over a broad frequency range (1 Hz-1 MHz) without altering or eliminating their two free surfaces. The glass transition temperatures assigned by the approach described in this Communication are in excellent agreement with data from the literature. In our approach, polymer films are suspended over interdigitated comb electrodes, IDE. The electric signal is measured by applying an ac voltage to elevated metallic electrodes made up by the two interdigitated comb structures deposited on a highly insulating substrate (see Figure 1a). For films of thickness D much smaller than the separation between two neighbored electrodes (∼10 μm), the electric field lines penetrate inside the layer with a direction parallel to the surface. Consequently, the individual complex electric capacitances of all sublayers (of thickness Di) constituting the polymer membrane add up to the total capacitance according to CTOT * (ω,T,D) ) ∑iCi(ω,T,Di), an expression that holds for any angular frequency ω and temperature T. Under these conditions, the different contributions to the relaxation spectra are linearly superimposed. The comb electrode geometry, in fact, avoids ambiguities arising from spectra of ultrathin films probed by an electric field orthogonal to the surface, as in the parallel plates geometry where capacitances enter in a series model, CTOT * (ω,T,D)-1 ) ∑iCi(ω,T,Di). To prove the feasibility of our technique, we investigated the structural relaxation dynamics of atactic * To whom correspondence should be addressed. E-mail: simone.napolitano@fys.kuleuven.be (S.N.) or wubbenhorst@fys.kuleuven.be (M.W.). Figure 1. (a) Schematic representation of the IDE chip; the red and blue fingers are at opposite potential. The height of the structures is 0.8 μm, the width of each finger is 5 μm, and the mean distance between two neighbored fingers is 8 μm. The black arrows represent the direction of the E-field inside the film. (b) AFM image 20 μm × 20 μm (topography) of a 65 nm thick film of PS after annealing at 70 °C above its Tg (see text). The white dashed lines reproduce the structure of the empty chip. The peaks at the edge of the electrodes metal buildups already present in the empty chip. Volume 42, Number 5 March 10, 2009

The dynamics of molecular chains of hydrogen-bonded p-nitroaniline (pNA), confined in the one-dimensional pore system of AlPO4-5, was studied by broadband dielectric spectroscopy (10(-2)-10(6) Hz). Two relaxation processes were observed. A fast process (the alpha process) shows above 72 degrees C a Vogel-Fulcher-Tammann behavior typical of glass-forming materials, indicating cooperative dynamics of the "condensed" pNA chains. Below the glass temperature the alpha-relaxation peak broadens significantly and obeys an Arrhenius law. Both observations imply a change in length scale of cooperativity xi from a temperature-controlled to a chain-length-dependent one. The second relaxation process is tentatively assigned to structural relaxations of the AlPO4-5 host lattice itself, since it is also present in calcined (empty) crystals. (C) 1999 American Institute of Physics. [S0021- 9606(99)70437-X]. ispartof: Journal of Chemical Physics vol:111 issue:13 pages:5637-5640 status: published

Melt condensation of 1,5-bis(9-hydroxy-1,4,7-trioxanonyl)naphthalene (2) with bis-acid chlorides, adipoyl chloride (3a), terephthaloyl chloride (3b), and 3,6,9,12-tetraoxatetradecane bis-acid chloride (3c), respectively, gives amorphous linear poly(ether-ester)s 1a-c, which contain 1,4,7- trioxanonyl (triethylene glycol) units at regular intervals in their main chain. Solid polymer electrolytes were prepared by mixing THF solutions of either LiClO4 with 1a-c or NaClO4 with 1b. The polymer electrolytes containing LiClO4 are fully amorphous, whereas in the case of NaClO4 and Na + /1b ratios larger than 0.125, crystalline NaClO4 is present. Despite the fact that the 1,4,7-trioxanonyl moieties in 1a-c are shorter than the minimum required for complete solvation of Li + and Na + , dielectric relaxation spectroscopy shows that the solid polymer electrolytes Li + /1a ,L i + /1b, and Li + /1c possess ionic conductivities of U ) 3.2 10 -5 , 1.9 10 -6 , and even 1 10 -4 Sc m -1 , respectively, at 368 K. A Vogel- Tammann-Fulcher (VTF) analysis of the ionic conductivity U and the relaxation time of the R-relaxation revealed a strong relationship between U and the relaxation behavior of the chain segments. By means of a fine structure analysis of the activation energy, the dielectric R-process around the glass transition was closely studied in the absence and presence of dissolved LiClO4 (1a-c) or NaClO4 (1b). From the highest apparent activation energy the Tg was determined and found to agree very well with values from DSC. In addition, the fractional free volume at Tg was quantified. It increases with increasing amount of dissolved salt; this becomes in particular clear from the fine structure analysis. Dielectric spectroscopy at T < Tg showed the presence of three secondary relaxations (A, ‚1, ‚2), of which ‚1 and ‚2 strongly overlap. Two of them are assigned to local relaxations involving either free (A) or coordinated (‚2 )E O sequences, resulting in a decrease or increase of the relaxation strength with salt concentration, respectively. Molecular modeling supports the idea that the ‚2 process arises from a chemical relaxation by the temporary breaking up and remaking of at least one O-Li+ coordination bond within the tetrahedral polymer-cation complex. The third (‚1) relaxation is in particular active in weakly complexed samples exposed to ambient humidity, suggesting a local motion involving the ester moieties.

The temperature dependence of the deviations from bulk behavior in ultrathin polymer films is demonstrated. The validity of a model aiming to rationalize the influence of interfacial interactions on the structural relaxation of polymers has been experimentally verified. The polymer layers at the very interface with the metal have a high number of contacts with the wall, because of specific hydrogen bond interactions. The resulting changes in the chain conformation allow only restricted segmental mobility. The influence of an absorbing substrate has been treated as a small perturbation in the activation energy, without affecting the energy landscape scenario. The deviations from bulk behavior are derived from the balance of such a perturbation and the energy of the thermal bath. The model results, supported by experimental evidence, show how these effects are related to the time scale of the experiment and increase upon cooling. ispartof: Macromolecules vol:41 issue:4 pages:1061-1063 status: published

858. [3] F. S. Spano, S. Mukamel, Phys. Rev. 1989, A 40, 5783. [4] H. Ishihara, K. Cho, Phys. Rev. 1990, B42, 1724. [5] D. Avnir, D. Levy, R. Reisfeld, J. Phys. Chem. 1984, 88, 5957. [6] D. Avnir, V. R. Kaufman, R. Reisfeld, J. Non-Cryst. Solids 1985, 74, 395. [7] C. Sanchez, F. Ribot, New J. Chem. 1994, 18, 1007. [8] Sol-gel Science (Eds: C. J. Brinker, G. W. Scherer) Academic Press, New York 1990. [9] G. S. He, J. Zieba, J. T. Bradshaw, Kazmierczak, P. N. Prasad, Opt. Comm. 1993, 104, 102. [10] V. S. Gorelik, A. D. Kudryavtseva, A. I. Sokolovskaya, N. V. Chernega, Opt. Spectrosc. 1996, 81, 369. [11] J. POcaut, J. P. LOvy, R. Masse, J. Mater. Chem. 1993, 3, 999. [12] M. Bienfait, R. Kern, Bull. Soc. Franc. MinOr. Crist. 1964, 87, 604. [13] F. Lefaucheux, M. C. Robert, in Crystal Growth in Gels. Handbook of Crystal Growth, (Ed: D. T. J. Hurle), North-Holland, Amsterdam 1994, 2b, p. 1271. [14] V. K. LaMer, R. H. Dinegar, J. Am. Chem. Soc. 1950, 17, 4847.

The dielectric characterization of a thermotropic liquid crystalline copolyesteramide was extended. The influence of orientation and crystallization was studied. An effect of the draw ratio on the dielectric properties in the transverse direction was hardly found. However, the orientation of the sample in the electric field (parallel or perpendicular) showed a significant effect. The birefringence predicted from the dielectric data at high frequencies agrees well with the optical value. Annealing of extruded and compression-molded samples resulted in an increase of the α relaxation peak.

Dielectric relaxation analysis was carried out on the main chain thermotropic liquid crystalline copolyesteramide Vectra B950. The dielectric spectra of Vectra B950 show four relaxations, instead of three as cited in the literature. These correspond, from high to low temperature, to the glass transition, the rotation of hydroxynaphthoic acid moieties, the rotation of terephthalic acid moieties, and the combined rotation of terephthalic acid and aminophenol moieties. The combination of TSD and DMA analysis made it possible to distinguish between dipole and charge relaxations. Although the TSD and the dielectric results compare well for the low-temperature dipole relaxations, they are not directly comparable for the α relaxation. TSD experiments reveal that free charges have a significant influence on the α relaxation.

The amorphous polyester 1 a prepared by melt condensation of 1,5-his-(9-hydroxy-1,4,7-trioxanonyl)naphthalene (2a) and adipoylchloride (3a), which contains bis-1,4,7-trioxanonyl (triethyleneglycol) segments in its main chain, represents a matrix for ion-conducting materials. Despite the fact that only triethyleneglycol segments are present, which are shorter than the minimum required for full solvation of Li+ cations, ion-conductivities of σ = 3.3 x 10 -5 at cm -1 at 368 K are found for Li + /1a 0.25 (cations per polymer repeat unit). Hence, more than one is-1,4,7-trioxanonyl segment (either intra- or interchain) has to be involved in Li + complexation. The σ value compares favorably with previous data for more complex polymer matrices.

In this paper we discuss the use of dielectric spectroscopy as a means to study the morphology of polymer blends. We have investigated a model system of thermotropic liquid crystalline polymer (Vectra B950) fibers lined up in a polypropylene (Stamylan P 13E10) matrix. The appearance of interfacial polarization at the interface between the two polymeric phases makes it possible to calculate the shape of the inclusions. For this purpose a formula is derived, which gives the shape factor of a distorted fiber as a function of the distortion amplitude. The calculated shapes of the fibers agree very well with the actual shapes, despite the approximations made. The agreement was proven by performing dielectric measurements under an optical microscope. As expected, the growth rate of thick fibers is smaller than that of thinner ones. A comparison between experiments under the microscope and in a dielectric sample holder showed the importance of the heating rate on the breakup of the fibers.

Two side-chain polyurethanes, HMDI-PU (2a) and TDI-PU (2b), have been synthesized from 2,2'-iminodiethanol (diethanolamine) N-substituted with a nitrostilbene group on a hexamethylene spacer with, respectively, hexamethylene diisocyanate and 1-methyl-2,4-phenylene diisocyanate (2,4-toluenediisocyanate). 2a showed a smectic liquid-crystalline phase ; 2b proved to be amorphous. The phase behaviour was characterized by means of differential scanning calorimetry, polarization microscopy and wide-angle X-ray scattering. Thermally stimulated discharge current measurements (TSD) revealed at least two strong relaxation processes above the glass transition temperature T g which are related to the dynamic glass transition involving the backbone part (a) and the mesogenic units (λ). Non-linear optical analysis of 2a showed that the Pockels coefficient r 13 was low in the mesophase and high in the isotropic phase. Contrary to expectation, poling in the mesophase did not enhance the Pockels effect. The amorphous 2b gave both a high and a stable Pockels coefficient (r 33 around 6.0 pm/V). The stability could be improved by ageing and prolonged poling at the ageing temperature. However, the Pockels coefficient proved to be dependent on the temperature : high above 85°C, low below 85°C (provided that the sample had been heated once before). This was attributed to an antiparallel association of the chromophores. The role of hydrogen bonds influencing poling has been assessed by temperature-dependent infrared spectroscopy.

This paper presents a step-wise poling method of a Vinylidene Fluoride TriFluoroEthylene (VDF/TrFE) copolymer film. A 1 μm VDF/TrFE copolymer layer is deposited on a silicon substrate using a spin-coating technique. After a poling treatment the unstretched copolymer will be pyroelectric. The step-wise poling that allows fractional poling is compatible with semiconductor technology and performed at room temperature.

The inverse photopyroelectric technique (IPPE) was adapted for the measurement of the thermal effusivity of resins during isothermal curing. The pyroelectric sensor consisted of a 9 μm PVDF film heated by a modulated laser beam and coated with a thick layer of uncured epoxy. From the high frequency part of the pyroelectric spectra of this bilayer the sensitivity of the pyroelectric detector was determined, while from the low frequency part the sample's effusivity (cρk)1/2 was found. The isothermal crosslinking of an DGEBA-based epoxy with DDM was studied at cure temperatures between 38 and 60°C. The thermal effusivity shows a marked increase until it drops when the rubber to glass transition is passed, i.e. when the network formed vitrifies to its glassy state. Concurrent results from modulated differential scanning calorimetry (MDSC) showed that the increase in the effusivity by up to 30% is mainly caused by the increase in thermal conductivity due to the crosslinking, while the stepwise decrease should ...

The nature of the glass transition, the transformation of a liquid into a disordered solid, still remains one of the most intriguing unsolved problems in materials science. Recent models rationalize crucial features of vitrification with the presence of medium-range ordered regions coexisting with the isotropic liquid. Here, in line with this prediction, we report an extraordinary enhancement in bond orientational order in ultrathin films of supercooled polyols, grown by physical vapour deposition. By varying the deposition conditions and the molecular size, we could tune the kinetic stability of the liquid phase enriched in bond orientational order towards conversion into the ordinary liquid phase. We observed a strong increase in the dielectric strength with respect to the ordinary supercooled liquid and slower structural dynamics, suggesting the existence of a metastable liquid phase with improved orientational correlations., SCOPUS: ar.j, info:eu-repo/semantics/published

To observe the pyroelectric response of bio-organic materials, several innovations have been proposed and implemented to push the sensitivity limits of a Scanning PyroElectric Microscope (SPEM). These include the use of noise reducing software, interdigitated comb electrodes instead of parallel plate electrodes and the use of rotating mirrors. Using these suggested techniques, the general performance of a SPEM measurement has been improved and allows the successful study of materials with a naturally weak spontaneous polarization and thus extremely low pyroelectric coefficients. ispartof: IEEE Transactions on Dielectrics and Electrical Insulation vol:19 issue:4 pages:1186-1190 status: published

The pyroelectric effects of dipolar guest molecules in molecular sieve crystals have been studied for the first time. The measurements have been done on assemblages of aligned crystals as well as on individual single crystals of the aluminophosphate molecular sieve AlPO4-5 loaded with p-nitroaniline (PNA). The PNA molecules form unidirectionally aligned dipole chains in parallel molecular sieve channels. We found that the PNA dipoles are oppositely directed in the two halves of a single crystal, forming two macroscopic regions of opposite polarization. It is concluded that the preferred direction of the dipole chains in the channel is a result of the adsorption process of the molecules into the channels. ispartof: Journal of Physical Chemistry vol:98 issue:47 pages:12315-12319 status: published

Recently [S. Capponi, S. Napolitano, and M. Wubbenhorst, Nat. Commun. 3, 1233 (2012)], vapor deposited glasses of glycerol have been found to recover their super-cooled liquid state via a metastable, ordered liquid (MROL) state characterized by a tremendously enhanced dielectric strength along with a slow-down of the relaxation rate of the structural relaxation. To study the calorimetric signature of this phenomenon, we have implemented a chip-based, differential AC calorimeter in an organic molecular beam deposition setup, which allows the simultaneous measurement of dielectric relaxations via interdigitated comb electrodes and specific heat relaxation spectra during deposition and as function of the temperature. Heating of the as-deposited glass just above the bulk Tg and subsequent cooling/reheating revealed a step-wise increase in cp by in total 9%, indicating unambiguously that glycerol, through slow vapour deposition, forms a thermodynamically stable glass, which has a specific heat as low as that of crystalline glycerol. Moreover, these glasses were found to show excellent kinetic stability as well as evidenced by both a high onset-temperature and quasi-isothermal recovery measurements at -75 °C. The second goal of the study was to elucidate the impact of the MROL state on the specific heat and its relaxation to the super-cooled state. Conversion of "MROL glycerol" to its "normal" (ordinary liquid, OL) state revealed a second, small (∼2%) increase of the glassy cp, a little gain (

Bio-organic materials such as bones, teeth and tendon generally show pyro- and piezoelectric activity that implies the existence of macroscopic polarity. The first discoveries date from 1957 1 in the bones and tendon of both humans and animals and has led to the suggestion that the presence of spontaneous polarity might play a crucial role during morphogenesis and could facilitate other biological functions. © 2011 IEEE. ispartof: pages:75-76 ispartof: 2011 14th international symposium on electrets (ise) pages:75-76 ispartof: 14th International Symposium on Electrets (ISE) Montpellier, FRANCE, AUG 28-31, 2011 location:FRANCE, Montpellier date:28 Aug - 31 Aug 2011 status: published

The effects of thermally reduced graphene sheets (TRG) on the phase separation in poly[(α-methyl styrene)-co-(acrylonitrile)]/poly(methyl-methacrylate) blends were monitored using melt rheology, conductivity spectroscopy, and electron microscopic techniques. The TRG were incorporated in the single-phase material by solution mixing. The composite samples were then allowed to phase separate in situ. The thermodynamics of phase separation have been investigated by monitoring the evolution of the storage modulus (G') as a function of temperature as the system passes through the binodal and the spinodal lines of the phase diagram. The phase separation kinetics were probed by monitoring the evolution of G' as a function of time at a quench depth well in the spinodal region. It was observed that TRG significantly influenced the phase separation temperature, the shape of the phase diagram and the rate of phase separation. The state of dispersion of TRG in the blends was assessed using electron microscopy and conductivity spectroscopy measurements. Interestingly, the composite samples (monophasic) were virtually insulators at room temperature, whereas highly conducting materials were obtained as a result of phase separation in the biphasic materials.

Monitoring the impact of annealing on the dynamic glass transition of nanometres-thick polymer layers provides new insights into the mechanisms behind the tremendous changes in the performance of macromolecular materials in close proximity to an interface. Here we present results revealing a correlation between deviations from bulk behaviour, manifesting in changes to the glass transition temperature, the reduction of dielectric strength and the growth of an irreversibly adsorbed layer (Guiselin brushes). The non-universal behaviour of polymers under confinement could be explained in terms of a dimensionless number given by the ratio between the timescale of adsorption and the annealing time. In particular, in the case of slow adsorption kinetics, such as for polystyrene on aluminium, deviations from bulk behaviour correspond to metastable states with an extremely long lifetime. ispartof: Nature communications vol:2 issue:1 status: published

We developed an experimental method for the determination of the tracer diffusivity D(tr) in ultrathin polymer films, and the changes in the segmental mobility of tracer molecules while they diffuse through matrices of different thickness and get adsorbed onto a target substrate. D(tr) starts decreasing already at 120-150 nm and drops to 1% of its bulk value in films as thin as 7.5 nm. We discuss the results highlighting a strong decoupling between the reduction in mass transport at the nanoscale and the increase in the glass transition temperature determined via capacitive dilatometry together with a breakdown of the Stokes-Einstein relation between orientational and translational degrees of freedom.

The molecular dynamics in nanometer thin films of glycerol was investigated upon thickness reduction by combining organic molecular deposition with in situ broadband dielectric spectroscopy. Changes in the cooperative dynamics with respect to bulk glycerol were observed for films of thicknesses down to 1.6 nm (corresponding to roughly three molecular layers). Systematic investigation revealed no pure size effects addressable merely to geometrical constraints. However, an increase in the glass transition temperature by 3.5 K was observed for the thinnest film, indicating the presence of a layer with reduced mobility in close proximity to the substrate. The impact of both the upper and lower interfaces has been disentangled by measurements performed during slow desorption. Moreover, proof is given for the existence of a layer with enhanced mobility in the vicinity of the free surface enslaved to the dynamics of the rest of the film. © 2010 American Chemical Society. ispartof: Journal of Physical Chemistry C vol:114 issue:39 pages:16696-16699 status: published

The alignment and relaxation dynamics of a polar dye molecule, N,N-dimethyl-4(4-nitrophenylazo)aniline (DNAA), in zeolite L and perhydrotriphenylene (PHTP) channels were investigated by means of a combination of optical, dielectric, and quantum-chemical methods. Both the zeolite L and PHTP channels enable the dye molecules to align along the channel axis. An amplified net dipole moment of DNAA in PHTP is observed and attributed to enhanced 1D close alignment of dye molecules. In zeolite L channels, a concentration gradient is found with aggregation at the channel entrances. The dynamics of the dye in zeolite L channels reveals localized conical rotational fluctuation modes following Arrhenius-type activation with energy of 0.31 eV, which we assign to small noninteracting fluctuating polar units of the dyes being loosely aligned or isolated. Unlike zeolite L, relaxations in PHTP are characterized by cooperative wobbling motions interpreted as increased intermolecular dipole interaction due to a closely packed one-dimensional array. Temperature-dependent activation energies of 0.25 eV below 0 degrees C and 0.37 eV at ambient temperature reflect the role of the soft channel walls in the activation process.

Real-time dielectric relaxation spectroscopy for a molecular beam deposited glass forming liquids is proposed as a versatile approach for the study of the dynamic glass transition in geometric confinement. To achieve the highest sensitivity down to monomolecular organic layers in a wide frequency range (0.1 - 10^7 Hz) during simultaneous deposition and desorption, we have used μm spaced interdigitated electrodes under ultrahigh vacuum conditions. Experiments using glycerol deposited on fused silica at -40◦C revealed a dielectric glass transition process for a layer thickness as low as 0.7 nm. While its peak position hardly changes upon thickness reduction, a clear broadening of the relaxation peak is observed that implies an increasing heterogeneous mobility scenario for the thinnest films caused by molecules being part of a reduced (at the substrate) or enhanced (free surface) mobility layer. This finding is supported by desorption experiments that reveal a strong retardation of the desorption rate for films below 1 nm. ispartof: European Physical Journal. Special Topics vol:189 issue:1 pages:181-186 ispartof: location:Grenoble status: published

The temperature and thickness dependence of the thermal expansivity of ultrathin thin films of poly(tert-butylstyrene) capped between aluminum layers revealed an unusual and intriguing confinement scenario. Below 50 nm, both the glass transition temperature and the thermal expansion coefficients decreased. Such a mixed behavior implies an enhancement of the molecular mobility, without the presence of any free surface, but dead layers. On the basis of a careful analysis of averaged quantities measured by capacitive dilatometry, we deduced a profile of thermal expansivity that explains our experimental data and previous observations in polymer nanocomposites. The effect of density-conformation coupling in proximity of a nonattractive interface allows the coexistence of an immobilized fraction in direct contact with the metal and an excess of thermal expansivity, arising from the long-range effects of packing frustration penetrating inside the bulk-like core of the film.

Scanning pyroelectric microscopy (SPEM) has successfully been applied to study spatial polarisation distributions in channel-type inclusion crystals and zeolite based host-guest systems, which show macroscopic polarity as the result of polar growth or oriented sorption of hyperpolarisable molecules.

We perform molecular dynamics Simulations to study the dielectric relaxation of a head-spring model for a polymer melt in the bulk and in Supported films. By assigning dipole moments parallel and perpendicular to the backbone of all chains ill the completed simulation trajectories, we calculate the dielectric spectra of so-called type-A polymers which exhibit relaxation processes due to the local motion of chain segments ("segmental mode") and due to fluctuations of the end-to-end vector ("normal mode"). We investigate the dependence of both processes on film thickness and chain length and for the segmental mode also oil temperature T. We find that the relaxation of both modes is enhanced in the films relative to the bulk. For the segmental mode this difference between film and bulk dynamics increases oil cooling toward the glass transition. By a layer-resolved analysis of the segmental relaxation, we show that the acceleration of the average film dynamics is a consequence of a smooth gradient in relaxation, originating from both interfaces where the segmental dipoles relax much faster than in the bulk. Additionally. near the interfaces the segmental relaxation is more strongly stretched than in the center of the film where bulk behavior prevails. As the average film dynamics comprises contributions from all layers, the dielectric spectra of the films are broader than in the bulk at the same T. Finally. starting from the layer-resolved analysis which associates a dielectric function and so a capacitance with each layer. we suggest to think of a film as being a system of capacitors. The capacitors are arranged ill series, if the electric (E) field is perpendicular to the plane of the film (the usual experimental Situation), and in parallel. if the field is parallel to the plane. Because of these different arrangements of the capacitors, the resulting dielectric spectra depend on the direction of the E-field. For instance, we find that, although the segmental relaxation ill each layer is taken to be the same for both field directions. the average dielectric spectra differ because the layer-dependent dielectric strength and file limiting high-frequency permittivity enter into the average dielectric response in different ways for the E-field being perpendicular or parallel to the film plane. ispartof: Macromolecules vol:41 issue:20 pages:7729-7743 status: published

The versatility of the laser intensity modulation method (LIMM) will be discussed concerning the interpretation of polarization distributions in multilayer films. Our LIMM equipment has been used to study monolayers, a bimorphic, and a four-layer filmof PVDF. The growth of the polarization is measured in dependence on temperature for monolayer films. The poling dynamics is governed by thermally stimulated phase transitions and can be interpreted by an Arrhenius model. The LIMM capabilities and limitations are discussed for multilayer samples, including interfacial space charge effects between polarized and nonpolarized layers. ispartof: Polymeric Layers pages:23-37 ispartof: PROGRESS IN COLLOID AND POLYMER SCIENCE vol:85 pages:23-37 ispartof: 7th International Seminar on Polymer Physics location:Güntersberge 1990 status: published

Dielectric spectroscopy has been used to monitor the cold crystallization kinetics of poly(3-hydroxy butyrate), PHB, just above the glass transition temperature of the amorphous chains. Although the polymer shows a relatively complex dielectric scenario, an easy and fast analysis of the crystallization kinetics was performed by choosing an appropriate temperature range in which the structural relaxation is the only process present in the spectra of the amorphous samples. It was possible to monitor in real time the crystallization kinetics by analysing the changes in the intensity of the a-relaxation process. A recently proposed methodology to overcome some systematic deviation of the Avrami approach, was applied to treat the experimental data. (c) 2007 Elsevier B.V. All rights reserved. ispartof: Journal of non-crystalline solids vol:353 issue:47 pages:4357-4361 ispartof: location:POLAND, Poznan status: published

Extremely thin films of poly(styrene) (h

The structure of the charge-transfer complex hexakis(n-hexyloxy)triphenylene-2,4,7-trinitro-9-fluorenone (HAT6-TNF) has been characterized by neutron scattering, X-ray diffraction (XRD), optical microscopy, and dielectric relaxation spectroscopy (DRS). On the basis of these data and the 1:1 stoichiometry, a consistent structure for the complex is proposed. This structure differs markedly from structures previously proposed for similar materials, because the TNF molecules are found to be situated between the discotic columns rather than sandwiched between the discotic molecules of a given column. The addition of TNF to HAT6 is found to stiffen the structure, and quasi-elastic neutron scattering shows that the local dynamics of the discotic molecules in HAT6-TNF is slowed by the presence of the TNF molecules. This scenario is consistent with the observation of two VFT-type (VFT=Vogel-Fulcher-Tamman) dielectric relaxation processes that relate to the columnar glass transition and a polyethylene-like hindered glass transition originating from the nano-phase-separated fraction of the aliphatic tails.

The cold crystallization kinetics of ultrathin films of poly(3-hydroxybutyrate) (PHB) have been investigated by dielectric spectroscopy. Upon reduction of the film thickness, a lowering of the Avrami exponent accompanied by an increase of the crystallization time was observed. The experimental results are analysed in terms of reduction of the total number of nuclei involved in the crystallization process. © IOP Publishing Ltd. ispartof: JOURNAL OF PHYSICS-CONDENSED MATTER vol:19 issue:20 ispartof: location:ITALY, Pisa status: published

In ultrathin polymer films, the coupling between the segmental mobility, precursor of the molecular diffusion, and the crystallization rate is broken down because of interfacial interactions. In particular, in the presence of a reduced mobility layer at the interface with the substrate, the crystallization kinetics slow down at a length scale bigger than the one connected with the deviation from bulk behavior of the structural relaxation. By modeling the influence of the substrate interactions on the parameters governing the temperature evolution of the main relaxation time, it was possible to reproduce the effect of geometrical confinement on the quantities connected to the diffusion-limited crystallization rate. Upon reduction of the thickness or increasing of the substrate interaction, the films show an apparent higher glass stability in terms of an increase of the cold crystallization temperature and of the crystallization time. The deviations from bulk behavior were found to vanish above a crossover temperature as already observed for the phenomena connected to the glass transition.