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ispartof: Physica Status Solidi A-Applications And Materials Science vol:219 issue:23 status: accepted

The behavior of polymers confined in ultrathin films (thickness < 200 nm) can sensitively differ from that observed in macroscopic samples. Based on the simple arguments of finite size and interfacial effects, film thickness, and surface interactions should be sufficient to univocally determine the deviation from bulk behavior. However, recent models suggest that a third key parameter, namely, the interfacial free volume, should also be considered. We describe a novel methodology that quantifies the volume available for structural relaxation at the interface between a thin polymer layer and its supporting substrate. Experiments performed at different annealing conditions verified that the shift in glass transition temperature, measured in thin films upon confinement, is proportional to the degree of adsorption and, thus, to the interfacial free volume.

Confined at the nanoscale level, polymers crystallize much slower than in bulk, and in some cases, the formation of ordered structure is inhibited within extremely long experimental time scales. Here, we report on the thickness dependence of the conversion rate of the amorphous fraction of ultrathin films of poly(ethylene terephthalate) during isothermal cold crystallization. We present a new analytical method assessing the impact of irreversible chain adsorption and permitting to disentangle finite size and interfacial effects. From the μm range down to a few tens of nm, we observed an increase in crystallization time scaling with the inverse of the film thickness, which is a fingerprint of finite size effects. Films thinner than ∼20 nm did not crystallize, even after prolonged annealing in the temperature range where the crystallization rate reaches its maximum value. Noticing that this threshold corresponds to the total thickness of the layer irreversibly adsorbed within our investigation time, we explain these findings considering that chain adsorption increases the entropic barrier required for the formation of crystalline structures.

We introduce a method permitting remote control of the crystallization of thin polymer films, which does not require changes in the film thickness or the interfacial chemistry. The procedure is based on the assembly of hybrid multilayers, where ultrathin films of a crystallizable polymer - poly(ethylene terephthalate), PET, - are embedded in parallel with noncrystalline polymer layers - polystyrene, PS, - into metallic slabs. Variation of the size of the PS film allowed tuning the crystallization rate of PET layers of constant thickness. We demonstrate that this phenomenon is ascribable to an increase in the interfacial dispersive forces that, enhancing physisorption of PET at the polymer/metal interface, causes a shortage of material at the crystal growth front., SCOPUS: ar.j, info:eu-repo/semantics/published

The local and cooperative dynamics of atactic PS (a- PS) were studied by broadband dielectric relaxation spectroscopy (BDRS) and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR). The a-PS has been subjected to thermal rejuvenation and subsequent quenching, short-term aging (6 weeks), and long-term aging (1 year) at ambient conditions. Where for the rejuvenated sample only an α- and a γ-relaxation is observed, short-term aging results in an additional β*-relaxation that merges with the α-relaxation at longer aging times. The γ-relaxation is increasing in intensity and activation energy during aging. The α-process shows no spectral changes and shift in the relaxation time upon aging. This may be attributed to a possible erasure of history of the material during the temperature-sweep mode measurement. Fourier transform infrared spectroscopy (FTIR) results suggest that the energetically favorable trans-trans (tt) conformers are increased in population with aging. ispartof: JOURNAL OF POLYMER SCIENCE PART B-POLYMER PHYSICS vol:57 issue:20 pages:1394-1401 status: published

The effect of plastic deformation on the molecular dynamics of atactic polystyrene (a-PS) was studied by broadband dielectric relaxation spectroscopy (BDRS), Fourier-transform infrared spectroscopy (FTIR) and polarized-light microscopy. Sheets of a-PS have been subjected to cold rolling, that is, mechanical rejuvenation, followed by a quenching step and fast heating above its glass-transition temperature, resulting in thermal rejuvenation. Cold rolling revealed, in addition to the known α- and γ(I)-relaxations, four hitherto unknown relaxation processes (II, III, IV and V). Using the framework of craze formation and multiplicity of the glass transition (E. Donth, G. H. Michler, Colloid Polym. Sci. 1989, 267, 557–567), supported by an activation-enthalpy/entropy analysis (Starkweather, W. Howard, Macromolecules 1981, 14, 1277–1281), the following physical picture emerges: (a) processes I and II represent local conformation transitions γ referring to chains of two different degrees of stretching (T/G-ratio); and (ii) processes III and IV were identified as helix-inversion processes of T2G2 helices as reported earlier for syndiotactic-rich PS—an assignment supported by FTIR results. Finally, the relaxation V could be attributed to the onset of the fibrillar glass transition (within crazes), leading to stress release by collapse of the fibrils and hence dying out of process V. Polarized-light microscopy confirmed the creation of oriented structures and internal stresses upon cold rolling, and their removal upon thermal rejuvenation.

The translational diffusion of molecules dispersed into polymer matrices slows down tremendously when approaching a nonrepulsive interface. To unravel the origin of this phenomenon, we investigated the diffusion of molecular probes in the direction normal to an adsorbing wall. Using adsorbed polymer layers as matrices, we were able to decouple interfacial and finite size effects and determined the relation between the diffusion time and the area available at the polymer/solid interface. Based on the results of our investigation, we present a physical picture, suggesting that the reduction in diffusion rate is correlated to the degree of chain adsorption onto the substrate, that is, the density of surface obstacles encountered by tracer molecules. ispartof: Acs Macro Letters vol:9 issue:3 pages:318-322 ispartof: location:United States status: Published online

Herein, 1-palmitoyl-2-oleoyl-glycero-3-phosphocholine (POPC) vesicles are studied using a newly designed setup that combines dielectric and gravimetric (quartz crystal microbalance, QCM) measurements. This setup allows to monitor the molecular dynamics and phase transitions of lipid structures under controlled humidity and temperature conditions. The main relaxation process (R2), related to the intrinsic headgroups' dynamics, is found in all phases and reveals a systematic acceleration with increasing hydration level. In contrast, a faster relaxation mode (R1) is found exclusively in the ?liquid-crystalline,? high-temperature state, favoring water clusters located in the disordered aliphatic phase as origin. A third process (R3), being slower than the R2 mode, is attributed to a decoupled dynamics of hydration water in the hydrophilic part of the lipid bilayers occurring at higher hydration levels. In addition to molecular dynamics, both QCM and dielectric relaxation spectroscopy (DRS) measurements are demonstrated to yield consistent information on the phase behavior of the POPC samples which show a marked sensitivity to the hydration level. Moreover, contact angle measurements and atomic force microscopy confirm irreversible changes from the initial vesicles to a stacked bilayer structure upon repeated hydration-dehydration, which manifests in the hydrophobization of the surface and water reduced absorption kinetics. doi: 10.1002/pssa.201900986 ispartof: Phys. Status Solidi A vol:217 issue:13 status: Published online

Previous studies have shown that selective synthetic cell receptors can be produced by cell imprinting on polymer layers. However, knowledge on the fundamental detection mechanisms remains limited. In this article, while using yeast cells (Saccharomyces cerevisiae) as model cells, the factors influencing cellular recognition by surface-imprinted polymers (SIPs) are studied by means of spectroscopic and microscopy techniques and a transducer platform based on interfacial thermal transport, the so-called heat-transfer method (HTM). These analyses indicate that cell imprinting creates selective binding sites on the surface of the SIP layer in the form of binding cavities that match the cells in shape and size. Also, we show that phospholipid moieties are incorporated into the SIP cavities during imprinting, while membrane proteins do not seem to be transferred. More importantly, we demonstrate that the incorporated phospholipids significantly enhance cell adhesion to the SIP, and thus play a significant role in the cell-SIP binding mechanism. Furthermore, the hydrophobicity of the SIP layer was found to be considerably higher when compared with a non-imprinted polymer layer (NIP), an effect that could not be attributed to the presence of cavities on the surface of the SIP layer. Therefore, we suggest that the role of phospholipids in the SIP recognition mechanism is mediated by long range hydrophobic forces. ispartof: Sensors and Actuators B, Chemical vol:255 issue:1 pages:907-917 status: published

In this work, we present a new technique for the sensitive and facile detection of Saccharomyces strains with a detection limit of 30 cells/mL in both buffer solution and food samples such as beer and yoghurt. This is achieved by combining ultrathin surface-imprinted polymer (SIP) layers as receptors with impedance spectroscopy as the readout principle. Binding of cells to the imprinted polymer surface results in a concentration-dependent increase of the impedance signal. According to the equivalent-circuit modelling of the impedance spectra, this is due to an increase of the resistive component of the solid-liquid interface while capacitive effects are negligible. Regarding selectivity, we prepared imprints with three different template strains and compared the sensor response for using the same three strains as targets. The response is strongest for combinations in which templates and targets belong to the same strain. For other situations, the sensor signal is reduced by a factor of three, indicating a certain level of selectivity, which can be interpreted in terms of size and cell wall differences. In summary, the combination of SIP-type receptors with impedance spectroscopy offers a fast and sensitive technique for on-site analysis of complex food matrices outside a laboratory environment. The results for yeast should be transferable to other microorganisms, including pathogenic ones.

The mechanism governing the electrical properties for phase separating blends containing multiwall carbon nanotubes (MWNTs) is unravelled by tuning the compatibilization efficiency. Thereto, compatibilization by interfacially segregated block and random copolymers (cp) of poly(styrene-random/block-methyl methacrylate)(PS-r/b-PMMA) was achieved in phase separating blends of poly[(α-methyl styrene)-co-acrylonitrile]/poly(methylmethacrylate) (PαMSAN/PMMA) undergoing spinodal decomposition. A systematic study of the effects of copolymer architecture and molecular weight on the percolating network of selectively localized MWNTs is performed. Effective compatibilization is achieved with block cp irrespective of the ability of the blocks to entangle, whereas for random cp only long random cp having the ability to effectively entangle with the homopolymers are as efficient as long block cp. With increasing copolymer concentration, an increase and subsequent saturation of the electrical conductivity is attained. This originates from an increased connectivity and refinement of the MWNT laden PαMSAN phase, as confirmed by optical microscopy and the linear viscoelastic response of the blends. An effective compatibilizer led to an interfacial tension mediated suppression of an interfacial coarsening of the PαMSAN phase during phase separation, subsequently leading to percolation of MWNTs selectively localized in the PαMSAN phase. Furthermore, the copolymer having the ability to promote the development of a percolated network of MWNTs also allowed the network to be formed at a lower copolymer concentration as compared to that of blends with copolymers leading to restricted improvement in the connectivity of MWNTs. Our robust and simple procedure to tune the blend morphology via the efficiency of the compatibilizers can be used to achieve a synergistic increase in conducting properties.

© 2018 American Chemical Society. It is generally accepted in membrane technology that crystalline zones in polymeric membranes do not contribute to transport of liquids or gases. In the current study, poly(3-alkylthiophene)s (P3ATs), i.e., homopolymers and random copolymers, were synthesized to study the influence of the supramolecular organization on membrane gas separations. The monomers were polymerized via KCTCP, and GPC analysis shows that the polymers have a narrow dispersity. DSC analysis of the polymers reveals that the homopolymers, in contrast to the copolymers, crystallized, confirming their higher degree of supramolecular organization. This was supported by UV-vis absorption spectra of the polymer films, where a red-shift and a characteristic shoulder absorption peak around 600 nm were observed for the homopolymers, while absent for the copolymers. More surprisingly, the homopolymers were found to be 2 orders of magnitude more permeable to CO2 than the copolymers and also more selective. ispartof: MACROMOLECULES vol:51 issue:23 pages:9943-9950 status: published

The dynamics of supramolecular networks made up of partially hydrolyzed poly(n-butyl acrylate) [PnBA] is investigated. These linear entangled random copolymers [PnBA-AA] self-assemble via hydrogen bonding interactions between carboxylic acid groups. Two types of supramolecular assemblies are revealed, i.e., binary assembly of carboxylic acid dimers and collective assembly of dimers into distinct poly(acrylic acid) [PAA] domains. The latter is proved by emergence of new relaxation processes in broadband dielectric spectroscopy while the former is evident by an increase of the glass transition temperature as well as retardation of segmental mobility observed by rheology. Therefore, a “sea–island” morphology containing geometrically confined PAA nanodomains embedded in a PnBA-rich matrix is suggested for the supramolecular network. Thermodynamic theories are employed to rationalize the existence of an interlayer with restricted mobility between the two phases. A fraction of PnBA-AA segments that are trapped ...

We describe an approach to tailor the dielectric interfacial properties of polymer blends by the interplay of compatibilizer effects on blend morphology and on blocking of charge carriers. A systematic study of the effect of the concentration of the compatibilizer, a random copolymer of poly(styrene-random-methyl methacrylate) (PS-r-PMMA), on the interfacial properties of stacked polymer films and a phase separating blend of poly[(α-methylstyrene)-co-acrylonitrile]/poly(methyl methacrylate) (PαMSAN/PMMA) was performed. From dielectric spectroscopy, "conductivity free" dielectric loss spectra revealed interfacial blocking of charge carriers at low frequencies owing to the conductivity contrast of the blend components, resulting in a dielectric interfacial peak. From the dielectric response of stacked polymer films, the interfacial polarization was significantly suppressed when the thickness of the more conducting film approached the Debye length (LD), on the basis of which LD could be calculated using the Trukhan model. With increasing concentration of interfacially localized copolymer, an increase of the relaxation strength of the interfacial polarization occurred due to a pronounced decrease of LD. The characteristics of the former were further investigated in biphasic polymer blends and were governed by the intrinsic length scale of the system, i.e., the ratio of structure dimension (Dv) to Debye length (LD). Upon compatibilization, the interfacially segregated copolymer not only results in refinement of the PαMSAN phase but also provides an augmented conductivity difference leading to a slower dynamics of PαMSAN charges at the interface. This resulted in a substantial increase of the peak intensity of the interfacial polarization, which was attributed to a pronounced attenuation of LD. The latter parameter was estimated using either the impedance formalism or the interfacial relaxation time. Similar to the stacked polymer films, upon compatibilization, an increase of the relaxation strength of the interfacial polarization occurred in biphasic polymer blends, which corroborated a pronounced decrease of LD. Our results provide a methodology to characterize and tune the morphology and blocking of charge carriers with the aim of tailoring the dielectric interfacial properties of biphasic morphologies.

We describe an approach to engineer bi-continuous conductive blends of polymers and multiwall carbon nanotubes (MWNTs) by formation of a percolating network of MWNTs in one phase of the blend. Thereto, spinodal decomposition combined with compatibilization by an interfacially segregated random copolymer (rcp) is proposed. A systematic study of the effect of the concentration of the random copolymer poly(styrene-random-methyl methacrylate) (PS-r-PMMA) on the electrical conductivity of a phase separating poly[( α -methyl styrene)-co-acrylonitrile]/poly(methyl methacrylate) (P α MSAN/PMMA) blend with MWNTs was performed above the spinodal temperature (at 220 °C) and at room temperature. Compatibilization results in a huge conductivity increase, whereby blends with 0.5 wt% MWNTs and 0.25 wt% copolymer exhibit the same conductivity as percolating bi-phasic blends with 2 wt% MWNTs. In addition, the linear viscoelastic moduli show a power law increase with the concentration of copolymer. It was deduced that the observed increase in conductivity was caused by a substantial morphology refinement and increased degree of cocontinuity after copolymer addition leading to the formation of double percolated networks in the blends. These findings were corroborated with optical micrographs and scanning transmission electron microscopy (STEM) images for blends with 0.5 wt% and 2 wt% carbon nanotubes, respectively. The morphology changes can be explained by an interfacial tension reduction, which alters structure dynamics during annealing. The effectiveness of the long random copolymer in compatibilizing the blend is attributed to multiple interface crossings coupled with the ability of the copolymer blocks to anchor into the homopolymers. This simple approach can provide a pathway to develop low cost and ubiquitous high performance dielectric materials with ultra-low percolation thresholds.

While physical vapour deposition of glass forming materials below their glass transition temperature, Tg, is an exciting route towards glasses with an extremely high packing density, strong kinetic and thermodynamic stability, vapour deposition of polymer systems is less evident and has so far not been investigated systematically. Here we have successfully prepared ultrathin films of low molecular mass polystyrene (800 g/mol) by thermal evaporation from the melt into a UHV chamber at a maximum deposition rate of 1 nm/h. Samples were studied in situ and in real-time by dielectric spectroscopy, chip-based ac-calorimetry and a quartz crystal microbalance, both during deposition and after reaching the final sample thickness of 5 nm. During film growth well below the bulk-Tg, an initially retarded deposition kinetics was observed along with an accelerated dielectric relaxation dynamics compared to the bulk glass transition. Subsequent temperature cycling above the bulk-Tg revealed continuous changes in the (dielectric) glass transition dynamics and finally lead to desorption of the material at elevated temperatures without restoring the “dielectric” bulk glass transition dynamics. In contrast, simultaneous specific heat spectroscopy revealed bulk dynamics, a striking discrepancy that was discussed in terms of a dominant and accelerated response of PS end-groups in the dielectric spectra in combination with terminal sub-chain dynamics and some degree of end-group segregation.

The processes used to create synthetic spider silk greatly affect the properties of the produced fibers. This paper investigates the effect of process variations during artificial spinning on the thermal and mechanical properties of the produced silk. Property values are also compared to the ones of the natural dragline silk of the Nephila clavipes spider, and to unprocessed (as-spun) synthetic silk. Structural characterization by scanning pyroelectric microscopy is employed to provide insight into the axial orientation of the crystalline regions of the fiber and is supported by X-ray diffraction data. The results show that stretching and passage through liquid baths induce crystal formation and axial alignment in synthetic fibers, but with different structural organization than natural silks. Furthermore, an increase in thermal diffusivity and elastic modulus is observed with decreasing fiber diameter, trending toward properties of natural fiber. This effect seems to be related to silk fibers being subjected to a radial gradient during production. (Figure presented.).

The phase transition observed by various methods in poly(vinyl methyl ether)/water systems at around 18 °C has been assigned by some investigators to the pre-melting of water and by others to a glass transition of the polymer. In this study, broadband dielectric spectroscopy and temperature modulated differential scanning calorimetry were used to identify this transition in radiationally crosslinked poly(vinyl methyl ether) hydrogels, as well as to analyse sub-zero relaxation processes in such a three-phase (polymer/ice/liquid water) system. The process at 18 °C was related to the pre-melting of water induced by the segmental motions of the polymer; however, it was seen to be one transition due to the cooperative motions of both compounds. The atypical (two regimes) temperature dependence of the segmental motion process was observed and was related to confinement of the polymer chains between ice clusters below approximately −24 °C; furthermore, the main dielectric process of hexagonal ice was identified and a Maxwell-Wagner effect was observed.

The hydrogen-bonded hexagonal columnar LC (Col(hd)) phases formed by benzene-1,3,5-tricarboxamide (BTA) derivatives can be aligned uniformly by an electric field and display switching behavior with a high remnant polarization. The polar switching in three symmetrically substituted BTAs with alkyl chains varying in length between 6 and 18 carbon atoms (C6, C10, and C18) was investigated by electro-optical switching experiments, dielectric relaxation spectroscopy (DRS), and solid-state NMR The goal was to characterize ferroelectric properties of BTA-based columnar LCs, which display a macroscopic axial dipole moment due to the head-to-tail stacking of hydrogen-bonded amides. The Col(hd) phase of all three BTAs can be aligned uniformly by a dc field similar to 30 V/mu m. Moreover, C10 and C18 display extrinsic polar switching characterized by a remnant polarization and coercive field of 1-2 mu C/cm(2) and 20-30 V/mu m, respectively. In the absence of an external field, the polarization is lost in 1-1000 s, depending on device details and temperature. DRS revealed a columnar glass transition in the low-temperature region of the LC phase related to collective vibrations in the hydrogen-bonded columns that freeze out below 41-54 degrees C. At higher temperatures, a relaxation process is present originating from the collective reorientation of amide groups along the column axis (inversion of the macrodipole). Matching activation energies suggest that the molecular mechanism underlying the polar switching and the R-processes is identical. These results illustrate that LC phases based on BTAs offer the unique possibility to integrate polarization with other functionalities in a single nanostructured material.

The focus of this work is the evaluation and analysis of the state of dispersion of functionalized multiwall carbon nanotubes (CNTs), within different morphologies formed, in a model LCST blend (poly[(alpha-methylstyrene)-co-(acrylonitrile)]/poly(methyl-methacryla te), P alpha MSAN/PMMA). Blend compositions that are expected to yield droplet-matrix (85/15 P alpha MSAN/PMMA and 15/85 P alpha MSAN/PMMA, wt/wt) and co-continuous morphologies (60/40 P alpha MSAN/PMMA, wt/wt) upon phase separation have been combined with two types of CNTs; carboxylic acid functionalized (CNTCOOH) and polyethylene modified (CNTPE) up to 2 wt%. Thermally induced phase separation in the blends has been studied in-situ by rheology and dielectric (conductivity) spectroscopy in terms of morphological evolution and CNT percolation. The state of dispersion of CNTs has been evaluated by transmission electron microscopy. The experimental results indicate that the final blend morphology and the surface functionalization of CNT are the main factors that govern percolation. In presence of either of the CNTs, 60/40 P alpha MSAN/PMMA blends yield a droplet-matrix morphology rather than co-continuous and do not show any percolation. On the other hand, both 85/15 P alpha MSAN/PMMA and 15/85 P alpha MSAN/PMMA blends containing CNTPEs show percolation in the rheological and electrical properties. Interestingly, the conductivity spectroscopy measurements demonstrate that the 15/85 P alpha MSAN/PMMA blends with CNTPEs that show insulating properties at room temperature for the miscible blends reveal highly conducting properties in the phase separated blends (melt state) as a result of phase separation. By quenching this morphology, the conductivity can be retained in the blends even in the solid state. (C) 2011 Elsevier Ltd. All rights reserved.

Determination of phase diagrams of polymer solutions with conventional techniques, like differential scanning calorimetry (DSC), is limited to sufficiently high concentrations due to a low signal/noise ratio. Controlling the nanoscale morphology formation upon solution processing of dilute conjugated polymer solutions however requires this information. In this work, we introduce an experimental methodology able to detect phase transitions in solutions over a wide concentration regime, by means of dielectric spectroscopy. We tested the validity of our method for solutions of poly(3-hexylthiophene) in toluene. Apart from providing crystallization and melting temperatures consistent with DSC data for the overlapping sample concentrations, our method turned out to be able to detect phase transition down to trace level mass fractions as low as 10 ppm.

N-Isopropylpropionamide (NiPPA), which can self-associate via hydrogen bonds, was found to undergo a solid-solid transition as identified by DSC and X-ray diffraction. Below the melting temperature of 51 °C NIPPA adopts a plastic crystalline state with a tetragonal unit cell until it transforms into an ordered crystal with a monoclinic structure at temperatures ≤10 °C. Dielectric spectroscopy was used to characterize the dynamics of the system, determining the activation parameters for the plastic to crystalline phase transition. The activation enthalpy is relatively high, as expected for a system that involves hydrogen bonds. However, most of the activation energy as the plastic phase assumes a more crystalline state is due to the activation entropy, suggesting that the increased cooperativity observed in the relaxation processes is due to a steric locking of the molecules. © 2010 American Chemical Society.

We investigated by dielectric relaxation spectroscopy the distribution of glass transition temperatures and dielectric relaxation strength inside ultrathin polymer films capped between metallic layers. Measurements of the local dielectric properties were achieved by selectively placing layers of dye-labeled polystyrene at different depth inside films of neat polystyrene of different thickness. We show experimental evidence for an interfacial nature of the deviations from bulk behavior; in particular, the value of the dielectric strength and the glass transition temperature strongly depend on the distance from the solid interface. These peculiar profiles of static and dynamic dielectric properties are discussed in terms of a physical picture based on competition between chain adsorption and packing frustration at different annealing conditions. Such a picture was able to rationalize common features observed in properties of ultrathin films like reduction of the relaxation strength, broadening of the dynamic glass transition process, and finally a shift of the structural relaxation time. © 2010 American Chemical Society. ispartof: Macromolecules vol:43 issue:20 pages:8686-8691 status: published

In addition to a tremendous reduction in the glass transition temperature, dielectric spectra of freely standing films reveal two other intriguing features: a temperature dependent asymmetric broadening of the structural relaxation peak towards lower temperatures and a reduction of the dynamic fragility down to the monomer limit. We verified that this experimental evidence is a manifestation of a gradient of glass transition temperatures across the film thickness induced by an enhanced molecular mobility at the two free surfaces of the membrane. As a direct implication of the peculiar features just described, the properties of freely suspended membranes neither correspond to those in bulk nor to a simplified scenario where the structural relaxation peak is merely shifted towards lower temperatures.

Cross-linked polymers in which part of the covalent cross-links have been replaced by reversible cross-links have potential application in coatings with improved stress relaxation. Here we present a detailed study of materials based on copolymers of ε-caprolactone and l-lactic acid, containing different ratios of covalent and ureidopyrimidinone (UPy) dimer cross-links. Their thermal and mechanical properties were tested by differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and dielectric relaxation spectroscopy (DRS). By extrapolation of the α* relaxation process in DMA (arising from dynamics of the reversible UPy cross-links) to temperatures below the glass transition temperature (Tg), it was shown that the reversible cross-links contribute significantly to the relaxation of stress below Tg that was observed in creep measurements. © 2010 American Chemical Society. ispartof: Macromolecules vol:43 issue:20 pages:8664-8669 status: published

The thermotropic phase behavior of symmetric cyclohexanetriamides carrying various linear and branched alkyl chains was investigated using calorimetry, microscopy, solid-state NMR, dielectric relaxation spectroscopy, and X-ray scattering techniques. Cyclohexanetriamides carrying C(6) or longer linear alkyl chains formed columnar plastic phases with a pseudocentered rectangular lattice. Those with C(8) or longer alkyl chain also showed a nematic liquid crystalline phase. Cyclohexanetriamides carrying branched octyl chains displayed columnar phases with rectangular lattices, except for the triamide with the highly branched tetramethylbutyl group. The occurrence of less symmetrical columnar phases is ascribed to the mode of stacking of cyclohexanetriamides which leads to noncylindrical columns. Dielectric relaxation spectra also featured highly cooperative relaxation processes related to reorientation of the macrodipolar columns in the mesophase, showing the potential of these molecules as building blocks in responsive materials.

The molecular dynamics and optical switching behavior (photo alignment) of novel azobenzene containing side-group (co)polymers were studied by time-resolved optical spectroscopy and broadband dielectric spectroscopy (BDS). To elucidate the effect of the molecular structure on the photochromic properties, two series of poly(methacrylates) with different aliphatic spacer units and four different concentrations of the chromophoric monomer were investigated. For both series, an inverse relation between the switching (retardation and relaxation) times and the chromophore content was found as well as a strong correlation between switching times and alignment efficiency. Dielectric spectroscopy on all materials revealed up to three relaxation processes ( α , α *, δ) above the glass transition temperature that were assigned to the dynamic glass transition of the polymer backbone and the fast and slow fluctuation of the chromophores around their long and short molecular axes. In the glassy state, occasionally two Arrhenius-type relaxations were observed that were identified as local motions of the butyl side group and the chromophore in its anisotropic environment. Both materials series showed a monotonic increase in T g and the dynamic fragility with increasing chromophore concentration, which was explained by an increasing effect of physical crosslinking provided by the increasingly dense packed side groups.

Degraded clear and pigmented epoxy coatings were investigated by dielectric sorption analysis (DSA). Differences in dynamics of absorption, due to increased hydrophilicity, crosslinking and porosity, were found between UV degraded and undegraded epoxy coatings. Desorption was observed for longer degradation times, caused by swelling of the coating, squeezing out the excess of water. Model system measurements of pigmented coatings with various filters on top reproduced the desorption behaviour. Due to crack formation during DSA measurements, clear coatings showed fluctuating results, a result of release of tension of the UV degraded clear coating by humidified nitrogen. Pigmented coatings did not show this behaviour, resulting in an increasing water absorption trend with increasing degradation times. Larger desorption peaks were found for lower frequencies, indicating that either polarization takes place or water-hydrophilic interaction. Based on these results, DSA is suitable for non-destructive investigation of degraded coatings and paints.

Structure and molecular dynamics of α,ω-dihydroxy terminated polybutadienes of varying number averaged molecular weight (1320–10 500 g/mol) have been investigated by Fourier-transformed infrared (FTIR) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, dielectric relaxation spectroscopy (DRS), differential scanning calorimetry (DSC) and wide-angle X-ray scattering (WAXS). DSC and DRS revealed an increase in the glass transition temperature upon decrease of the molecular weight, accompanied by an increasing dynamic fragility m (or steepness index) of the dielectric α-process. This correlation between T g and m for different molecular weights indicates the presence of a physical network, where H-bonded end-group clusters act as temporary crosslinks. From the dielectric relaxation strength Δ ɛ α ( M n ), the fraction of associated hydroxy groups ( f bond ) was estimated showing a peak value for the two but shortest polymers, a behaviour that strongly resembles the molecular weight dependence of the fragility. By considering the quantity f bond ( M n ) in a modified Fox-Flory approach, the measured T g ( M n ) behaviour could be reproduced in a satisfying way. FTIR results support this general picture and show a considerable dependence of the extent of hydrogen bonding and formation of hydroxy groups associates on the molecular weight. Further, WAXS and DSC results disprove the idea of formation of pseudo-crystalline hydrophobic microdomains in these compounds as suggested by other authors.

Received 7 November 2006; accepted 1 February 2007DOI 10.1002/app.26337Published online 23 April 2007 in Wiley InterScience (www.interscience.wiley.com).ABSTRACT: The sorption of water vapor in various or-ganic coatings (polyimide, polyamide 6.6 (PA6.6), epoxy,polymethyl methacrylate, low density polyethylene) andfilters (polyvinylidene fluoride, polytetra-fluoro-ethylene,porous polyethylene, nitro cellulose, cotton linter) has beeninvestigated by a technique called Dielectric SorptionAnalysis (DSA). The technique is based on high-resolu-tion time-resolved, capacitance measurements performedduring exposure of an organic coating to humidifiednitrogen. The DSA technique could distinguish betweenall samples, and the diffusion coefficient and maximumsorption is calculated for all samples. A frequency sweepwith a dielectric analyzer showed that in all cases thewater has no interaction with the polymer films, exceptPA6.6. For PA6.6 it is assumed that electrode polarizationtakes place. For filters a frequency sweep showed in twocases electrode polarization (porous polyethylene, cottonlinter), two cases no interaction (polytetra-fluoro-ethylene,nitro cellulose) and two cases an intermediate effect (poly-vinylidene fluoride 0.22 mm, polyvinylidene fluoride0.45 mm). The filters described a desorption profile, likelydue to swelling of the filters that caused the vapor to bepressed out of the pores of the filters. Mass transportproperties derived from complementary weight measure-ments on epoxy films were in good agreement with theDSA results.

The correspondence of the state of alignment of macromolecules in biomimetic materials and natural tissues is demonstrated by investigating a mechanism of electrical polarity formation: An in vitro grown biomimetic FAp/gelatin composite is investigated for its polar properties by second harmonic (SHGM) and scanning pyroelectric microscopy (SPEM). Hexagonal prismatic seed crystals formed in gelatin gels represent a monodomain polar state, due to aligned mineralized gelatin molecules. Later growth stages, showing dumbbell morphologies, develop into a bipolar state because of surface recognition by gelatin functionality: A reversal of the polar alignment of macromolecules, thus, takes place close to that basal plane of the seed. In natural hard tissues (teeth and bone investigated by SPEM) and the biomimetic FAp/gelatin composite, we find a surprising analogy in view of growth-induced states of polarity: The development of polarity in vivo and in vitro can be explained by a Markov-type mechanism of molecular recognition during the attachment of macromolecules.

This paper describes the preparation, characterization, and the permeation properties of poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) dense polymer films containing fullerenes (C60). The C60 are either dispersed or covalently bonded to PPO at various concentrations. The gas permeability results are very different between covalently bonded and dispersed PPO−C60. The gas permeability of PPO−C60 bonded increases up to 80% with increasing fullerene concentration. The gas pair selectivity, however, stays constant. This behavior is probably due to stiffening of the polymer structure and increase of free volume. The gas permeability through the PPO−C60 dispersed decreases in comparison to pure PPO. This reduction is due to C60 clustering in the polymer. The clusters seem to induce polymer crystallinity, and they are probably aligned along the film plane, creating an extra barrier for gas permeation.

The effect of thickness reductions on the glass transition dynamics in ultrathin films of polystyrene has been studied by dielectric spectroscopy (DS) and capacitive dilatometry (CD). Upon reduction of the film thickness, a systematic decrease in the dilatometric glass transition temperatures, T g (dil), was observed via CD, while DS revealed a continuous speed-up and broadening of the α-process, accompanied by only minor reductions in the fragility index. A good agreement between ‘spectroscopic’ and the dilatometric glass transition temperatures was found for films thicker than 20 nm, while for thinner films both quantities diverge increasingly. A likely explanation for this discrepancy is the presence of another dynamic process showing Arrhenius-behavior ( E a ∼ 72 kJ/mol) with a pre-exponential factor of 10 −12 s being indicative for non-cooperative dynamics. Such a new process might be assigned to distinct surface dynamics in polystyrene films as suggested in recent papers.

Photochromic silica based organic-inorganic hybrid materials containing covalently linked cyanoazobenzene chromophores were investigated by optical and dielectric spectroscopy. These materials, obtained via sol-gel process, were deposited onto glass substrates by spin coating technique to achieve thin transparent films. To investigate photoinduced alignment, the UV-Vis absorption spectra of the sol-gel films were recorded under illumination with linearly polarized blue light. Dielectric relaxation spectroscopy revealed a variety of relaxation processes: the α-process related to the dynamic glass transition temperature located around 150°C, and an Arrhenius-type β-relaxation (activation energy 58–60 kJ/mol) that was assigned to orientational fluctuations involving the azobenzene group. The correspondence between dielectric and photochemical behavior was discussed.

Simone Napolitano* and Michael Wu 1bbenhorstKatholieke UniVersiteit LeuVen, Laboratory for Acousticsand Thermal Physics, Department of Physics and Astronomy,Celestijnenlaan 200D, B-3001 HeVerlee, BelgiumReceiVed June 12, 2006ReVised Manuscript ReceiVed July 25, 2006The crystallization kinetics in ultrathin polymer films havebeen investigated by several spectroscopic techniques in bothisothermal

The local and cooperative dynamics in atactic (a-PS) and syndiotactic (s-PS) polystyrene were studied by broadband dielectric spectroscopy. Besides the known α-relaxation, two additional relaxation...

The dynamics and phase behavior of the discotic liquid crystalline compound hexahexyloxytriphenylene (HAT6) and a derivative were studied by broad-band dielectric spectroscopy, differential scanning calorimetry, X-ray diffraction and optical microscopy. While the pristine compound HAT6 forms both a columnar mesophase (Colh) and a plastic crystal phase, no liquid crystallinity was observed for the highly asymmetric compound HAT6-C10Br. This paper focuses on the dielectric relaxations in the plastic crystal phase. For HAT6-C10Br, a 'high temperature' glass transition, manifested by a Vogel-Fulcher-Tammann (VFT) type α2-process, was found at -31 °C that was assigned to the columnar glass transition in accordance with previous literature. The main result of our study is the observation of a second, low-temperature VFT process (α1) for both compounds, which indicates co-operative liquid dynamics within the framework of the plastic crystal order at temperatures as low as -100 °C. Comparison of these fast dynamics with relaxation data from polyethylene and polymer series with long alkyl groups identifies this process as a 'hindered' polyethylene-like dynamic glass transition that originates from the nanophase-separated, spatially confined fraction of aliphatic tails. © 2005 Elsevier B.V. All rights reserved. ispartof: JOURNAL OF NON-CRYSTALLINE SOLIDS vol:351 issue:33-36 pages:2622-2628 ispartof: location:NETHERLANDS, Delft status: published

Recently we introduced a rigid-rod type chromophore, E-4,4′-N,N- dibutylamino-nitro-stilbene (DBANS), that acts as a dielectric and fluorescent probe, and enables the study of molecular dynamics in apolar polymers by dielectric relaxation spectroscopy (DRS). This work focuses on the effect of the probe size and concentration on the specific probe response. For this purpose, a series of probe molecules with core lengths (l) between 0.6 and 2 nm was used to 'dope' polystyrene at concentrations from 0.1 to 1.0 wt%. For all probes, a selective amplification of the intrinsic α-relaxation was found that was linear within the whole probe concentration range. Moreover, two clear size effects on the probe dynamics were revealed. At high temperatures, the probe relaxation time obeys τ ∝ l3, which confirmed rotational diffusion as the linking mechanism between the polymeric segmental dynamics and dielectric probe response. The shortest probe revealed an additional Arrhenius-type (βz.str;) relaxation that was attributed to large angular probe fluctuations involving discrete jumps. Its exclusive occurrence indicates a critical lower probe size (lz.str; ∼ 0.6 nm) below which the dielectric probe acts also as a 'free-volume probe' as indicated by the pronounced anormality in the τβ

Dielectric permittivity measurements were used to monitor physical aging of polycarbonate (PC) at about Tg – 100 K. The effect of film thickness and cooling rate from the melt was investigated to give new insight into the microscopic nature of the physical aging process. The results show that both the film thickness and the cooling rate play an important role in the kinetics of physical aging. Indications of a diffusive mechanism for physical aging are given, where free volume holes can disappear either at the external surface of the sample or at an internal surface created during the cooling process. Evidence for the existence of the an internal surface is provided.

Dielectric relaxation spectroscopy using dielectric probes was applied to study the (glass transition) dynamics in binary blends of isotactic PP, PS and LDPE. The blends were prepared by melt-mixing and doped with 0.5% of the dielectric probe 4,4 0 -(N,N-dibutylamino)-(E)- nitrostilbene (DBANS) (van den Berg O, Sengers WGF, Jager WF, Picken SJ, Wubbenhorst M. Macromolecules 2004;37:2460. (17)). Due to the selective amplification of the dielectric relaxation processes related to the dynamic glass transition of the polymers, accurate relaxation data were obtained, even for the minor phases. No substantial influence of the blend composition and the blend morphology on the glass transition dynamics was found, indicating that both blend constituents behave like homogeneous bulk materials. The normalised relaxation strength of glass transition processes remained constant, regardless of the blend type and blend composition. This indicates that the probe molecule, DBANS, was equally distributed over the two blend components in all three polymer combinations PE-PP, PE-PS and PP-PS. q 2005 Elsevier Ltd. All rights reserved.

A three-step procedure is proposed to investigate growth-induced polarity arising in centrosymmetric crystals of dipolar molecules. It is based on (i) calculation of molecular interaction energies by force field methods, (ii) determination of the morphology, and (iii) use of the energies in a Markov-type growth mechanism on faces (hkl). Applied to trans-4-chloro-4‘-nitrostilbene (CNS), the procedure showed that CNS crystals, although globally centric, are composed of sectors exhibiting different polar properties. The sectors related to the +b and −b directions show opposite polarity and are mainly responsible for observed second-harmonic generation and pyroelectric effects. Influence of the calculation method on the results was investigated by comparing different force fields or charges.

Investigation of glass transition dynamics in polyolefins by broadband dielectric spectroscopy (DRS) is facilitated by the addition of a novel dielectric probe, (4,4‘-(N,N-dibutylamino)-(E)-nitrostilbene (DBANS), which introduces dipoles and made the polymers dielectrically active. For probe concentrations between 0.1% and 1.0% the dielectric strength Δe associated with the dynamic glass transition increases proportionally to the probe concentration. This result indicates that the probe exhibits no intramolecular relaxations, and the probe rotational diffusion effectively senses the “microviscosity” of the probe environment on the length scale of the segmental dynamics. Temperature-dependent fluorescence spectroscopy on doped polymers shows no changes in fluorescence wavelength around the glass transition temperature. Crystallization and melting of the polyolefin matrix results in an increase or decrease of the probe concentration in the amorphous phase, which was clearly detected by real-time fluorescenc...

A novel bi-functional photorefractive (PR) methacrylate polymer with carbazolyl fragments and chromophoric azophenyl sulfonyl group were synthesized and characterized. The generation of photocurrent on illumination and the photoconductive properties of the homopolymers and copolymers were studied. The addition of 2,4,7-trinitro-9-fluorenone (TNF) resulted in a considerable increase of the photoconductivity by about one order of magnitude.

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.

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