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Smart polymers as poly-N-isopropylacrylamide (PNIPAM) are well known due to their fast response to variations of parameters like temperature, pH, and pressure. [...]

The response to pH variation of polymeric cyclodextrin-based hydrogels has been investigated by a multi-technique approach based on UV Raman and Brillouin light scattering (BLS) together with Small Angle Neutron Scattering (SANS). By exploiting the complementary information of these three investigation methods, the structural, viscoelastic and molecular modifications of the polymer brought about by the pH changes have been examined, over a spatial range going from mesoscopic to nanoscopic length-scale. The data provide a picture where an increase of pH promotes the change of the characteristic size of the hydrophilic pores when the cross-linker has the suitable structural and acid-base properties, and leads to the reinforcement of the polymer domains interconnections, providing a stiffer gel network on the length-scale probed by BLS. Raman signals are sensitive both to structural changes of the polymer network and to changes of the intermolecular ordering of water due to solvent-polymer interactions. The destructuring effect on the tetrahedral ice-like configurations of water is especially evident at high pH, and might be ascribed to an increased exposition to the solvent of the ionic portions of the polymer surface.

Brillouin spectroscopy is a powerful technique to probe the viscoelastic properties of materials. However, the phenomenon of multiple scattering makes getting information from opaque liquids quite difficult, thus limiting the use of this spectroscopy. In this paper we present a new method that greatly simplifies the problem of analyzing Brillouin spectra affected by multiple scattering from samples of moderate opacity. Our approach is based on the observation that multiple-scattered contributions broaden the spectrum acquired in external backscattering geometry, while preserving in the external side the information related to internally backscattered light. The new strategy avoids unnecessary approximations and requires minimum numerical effort to extract physical information. Here, we show the results of two Brillouin light scattering experiments performed on prototypical hard and soft colloidal systems. First, measurements on latex suspensions as a function of depth are used to validate the method and to derive new relations between the back-scattered and multiple-scattered components of the Brillouin spectrum. Second, measurements on poly-N-isopropylacrylamide (PNIPAM) microgels in water as a function of temperature are used as a testing ground to demonstrate the method's capabilities. Our analysis confirms that sound waves are extremely sensitive to the volume-phase transition of thermoresponsive particles. The presented approach, however, shows that a marked increase of attenuation is accompanied by only a moderate decrease of sound velocity. The study revises the viscoelastic properties of PNIPAM suspensions; more generally, it provides a new guideline in the characterization of moderately opaque media and fosters new theoretical investigations. © 2018 Elsevier B.V.

Brillouin and Raman scattering spectroscopy are established techniques for the nondestructive contactless and label-free readout of mechanical, chemical and structural properties of condensed matter. Brillouin-Raman investigations currently require separate measurements and a site-matching approach to obtain complementary information from a sample. Here we demonstrate a new concept of fully scanning multimodal micro-spectroscopy for simultaneous detection of Brillouin and Raman light scattering in an exceptionally wide spectral range, from fractions of GHz to hundreds of THz. It yields an unprecedented 150 dB contrast, which is especially important for the analysis of opaque or turbid media such as biomedical samples, and a spatial resolution on sub-cellular scale. We report the first applications of this new multimodal method to a range of systems, from a single cell to the fast reaction kinetics of a curing process, and the mechano-chemical mapping of highly scattering biological samples., Comment: 18 pages, 4 figures

UV Raman and Brillouin light scattering (BLS) experiments have been used in this study to explore the complex phase change behavior occurring in pH-responsive polysaccharide hydrogels as a function of temperature. Due to the different physical quantities measured by the two techniques, the joint analysis of Raman and BLS spectra has provided an unprecedented large-scale characterization of the molecular rearrangements and of the different kinds of hydrophilic and hydrophobic interactions that cooperate to determine the phase transformation observed in these hydrogels during the heating of the gel. As the main result, the analysis of the Raman and BLS spectra showed the existence of a correlation between the local (molecular) and collective properties of the gels during the phase transformation undergone by the system, which is markedly triggered by pH. The joint set of experimental results suggests a model according to which the mechanism of pH dependence in the hydrogels under investigation is dominated by the interactions involving the hydrophobic parts of the polymer skeleton, whereas the solvation process observed under heating of the gels is driven by the progressive distancing of the polymer domains among them, as monitored by the Brillouin sound velocity.

An integrated experimental approach, based on inelastic light-scattering techniques, has been here employed for a multilength scale characterization of networking properties of cyclodextrin nanosponges, a new class of cross-linked polymeric materials built up from natural oligosaccharides cyclodextrins. By using Raman and Brillouin scattering experiments, we performed a detailed inspection of the vibrational dynamics of these polymers over a wide frequency window ranging from gigahertz to terahertz, with the aim of providing physical descriptors correlated to the cross-linking degree and elastic properties of the material. The results seem to suggest that the stiffness of cross-linked polymers can be successfully tuned by acting on the type and the relative amount of the cross-linker during the synthesis of a polymer matrix, predicting and controlling their swelling and entrapment properties. The proposed experimental approach is a useful tool for investigating the structural and physicochemical properties of polymeric network systems.

We compare the specific heat and dielectric relaxation (T) data of two epoxy resins, poly[(phenyl glycidyl ether)-co-formaldehyde] and diglycidyl ether of bisphenol A, with the predictions of the entropy theory of Adam and Gibbs. Specific heat data from temperature-modulated differential scanning calorimetry are used to evaluate configurational entropy S C(T) data experimentally. The systems studied show two secondary (β and γ) relaxations inside the experimentally accessible frequency window of dielectric spectroscopy related to two crossover regions along the trace of the dynamic glass transition. The analysis, bypassing the use of the Vogel-Fulcher-Tamman equation, supports a good description of the structural relaxation time made in terms of the Adam-Gibbs model in the temperature range T g < T < Tβ (or T B), with Tβ the temperature where the structural and secondary β-relaxation times tend to merge, and T B the temperature of deviation from a high-temperature Vogel-Fulcher-Tamman τ(T) depend...

The dynamics of an epoxy resin, poly(phenyl glycidyl ether-co-formaldehyde), has been investigated in the supercooled and glassy phases by wideband dielectric spectroscopy (10/sup -2/ to 3/spl times/10/sup 9/ Hz) and compared with that of the previously investigated epoxy resin, diglycidyl ether of bisphenol-A. The temperature evolution of the dynamics of the system is monitored through the characteristic parameters of the relaxations, namely the relaxation times, the relaxation strengths and the shape parameters. Two transition regions are revealed: the glass transition and the split between structural and fast secondary relaxation, where the onset of the structural relaxation is located also.

Isothermal and isobaric dielectric measurements of a supercooled epoxy resin have been compared. A simple scaling relates isobaric and isothermal spectra corresponding to the same frequency of the main loss peak. Thus, the main and secondary processes retain a relative weight that is the same under isothermal and isobaric conditions. It is inferred that both pressure and temperature, equivalently, are able to take effect on the relaxation processes, without changing the relaxation mechanism itself. Careful analysis of the structural relaxation time behavior revealed that the traditional free volume equation, where only the macroscopic volume controls the pressure evolution of free volume, is not a suitable description of the data, as well as a Vogel-Fulcher (VF) type pressure dependent function. Based on a derivative method, a different function for describing the bidimensional surface tau(T,P) has been proposed, which accounts for the observed behavior through a nonlinear correction of the critical temperature T0 in the VF law. The function we propose predicts pressure dependencies of the glass transition temperature and fragility which are appealing in view of a comparison with experimental results in this and many other systems. Interesting hints for interpreting the phenomenological results can be obtained within the Adam-Gibbs theory.

Recent developments in dielectric monitoring of cure processes are considered. Direct current (dc) conductivity and dielectric data concerning the crosslinking of an epoxy resin are analyzed and compared with the results of late microwave experiments. The analysis of the dielectric behavior of the system carried out on a rather wide frequency interval (103-1010Hz) has provided a deeper insight into the relationships between dielectric parameters (i.e., dc conductivity, permittivity, relaxation time and shape parameters, and the physical and chemical modifications of the systems). The results confirm the possibility to utilize dielectric quantities to obtain information on relevant parameters such as conversion, viscosity change, sol-gel transition, glass transition temperature, and dynamics of cooperative relaxation phenomena of the system. The required dielectric data can be gathered by simple methods that facilitate in situ applications. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65: 17–25, 1997

Despite the differences in the molecular processes involved in chemical and physical vitrification, surprising similarities are observed in the dynamics and in the thermodynamical properties of the resulting glasses. We report on a systematic study of reactive glass-formers undergoing a process of progressive polymerization of the constituent molecules via the formation of irreversible chemical bonds. The formation of most of the materials used in engineering plastics and the hardening of natural and synthetic resins, including epoxy resins, are based on chemical vitrification. The clear analogies characterizing the dynamic evolution of physical and chemical glass-formers, on the time scale of the structural and the low-frequency vibrational dynamics, are briefly reviewed.

Raman-scattering measurements are used to follow the modification of the vibrational density of states in a reactive epoxy-amine mixture during isothermal polymerization. Combining them with Brillouin light and inelastic x-ray scattering measurements, we analyze the variations of the boson peak and of the Debye level while the system changes from liquid to glass upon increasing the number of covalent bonds among the constituent molecules. The shift and intensity variation of the boson peak are explained by the modification of the elastic properties throughout the reaction, and a master curve for the boson peak can therefore be obtained. Surprisingly, bond-induced modifications of the structure do not affect this master curve.

International audience; Synchrotron-based X-ray fluorescence (S-XRF) is a powerful technique for imaging the distribution of many biologically relevant elements, as well as of "artificial" elements deliberately introduced into tissues and cells, for example through functionalized nanoparticles. In this study we explored the potential of S-XRF for chemical nanoimaging (100 nm spatial resolution, nanoXRF) of human cells, through the use of functionalized CdSe/ZnS quantum dots (QDs). We used a commercially available QD -- secondary antibody conjugate to label the cancer marker HER2 (Human Epidermal growth factor Receptor 2) on the surface of SKOV3 cancer cells, and beta-tubulin, a protein associated with cytoskeleton microtubules. We set up samples with epoxy inclusion and intracellular labeling, and samples without epoxy inclusion and with surface labeling. Epoxy inclusion, also used in electron microscopy, has the advantage to preserve cell morphology, and to guarantee long term stability. QDs proved to be suitable probes for nanoXRF, due to the Se emission band which is not in close proximity to any other emission band, and the signal specificity which is preserved in both types of labeling. Therefore, nanoXRF using QD-based markers can be very effective to colocalize specific intracellular targets with elements naturally present in the cell, and may complement confocal fluorescence microscopy in a synergistic fashion.

We introduce an experimental method of assessing the vibrational and configurational components of the excess entropy of a liquid over crystal, based on a joined investigation of dynamic and thermodynamic properties as a function of temperature and pressure. We analyze light scattering, dielectric, calorimetric and dilatometric measurements of three prototype glass formers, orthoterphenyl, salol, and glycerol. In all cases we find that about 70% of the excess entropy is configurational in nature., Comment: RevTeX4, 4 pages, 2 EPS figures

Broadband dielectric spectroscopy, heat capacity spectroscopy (3ω method), and viscosimetry have been used to study the dynamic glass transition of two glass-forming epoxy resins, poly [(phenyl glycidyl ether)-co-formaldehyde] and diglycidyl ether of bisphenol-A. In spite of their rather simple molecular structure, the dynamics of these systems is characterized by two well-separated crossover regions where the relaxation times of main transition and the two secondary relaxations β and γ approach each other. The main transition has three parts: The a process at high temperature, the a′ process between the two crossover regions, and the α process at low temperatures. Both the γ-crossover region [around a temperature Tc(γ)∼(1.4–1.5)Tg and a relaxation time τc(γ)≈10−10 s] and the β-crossover region [around Tc(β)∼(1.1–1.2)Tg and τc(β)≈10−6 s] could be studied within the experimentally accessible frequency–temperature window. Different typical crossover properties are observed in the two regions. The γ-crossove...

The dynamics of the epoxy compound polyphenyl-glycidyl-ether-coformaldehyde was investigated by wideband dielectric spectroscopy (10/sup -2/ to 10/sup 10/ Hz) by varying either pressure or temperature. The measurements were undertaken in two different conditions: under isobaric condition (atmospheric pressure) varying temperature down to the supercooled and glassy phases and under isothermal conditions by changing the pressure from 0.1 to 270 MPa. The analysis of the structural relaxation time behavior either varying the temperature or the pressure was found to agree well with a previously derived expression for /spl tau/(T, P) based on a cooperative model. In particular it was found that with a unique set of parameters it is possible to describe the 2-dimensional surface /spl tau/(T, P) over a wide range of temperature and pressure. This result agrees well with the observed independence of the fragility in both isothermal and isobaric conditions.

A description of the pressure dependence of the structural relaxation time has been derived from the Adam-Gibbs theory by writing the configurational entropy in terms of the excess heat capacity and the molar thermal expansion. This new equation was tested successfully on dielectric relaxation data for an epoxy compound over a wide range of temperature and pressure.

The conducting polymer poly(3n-decylprrole) (P3DP) has a promising chemical stability and processability. The long alkylic chains makes P3DP soluble in common organic solvents even if they partially hinder a regular molecular arrangement. On account of structural disorder, the materials has a moderate conductivity despite the high doping level. The d.c. conductivity and the dielectric response of P3DP films with different dopants and synthesis conditions were measured at temperatures between 80 and 300 K. The d.c. conductivity of all the samples was well described by the variable range hopping model; the hopping parameters were found to be much affected by the synthesis and doping conditions. The electrical response exhibited a well-defined relaxation peak, visible only after deducting the d.c. conductivity contribution from the loss factor. The temperature behavior of the loss peak frequency parallelled that of the d.c. conductivity. The Barton-Nakajima-Namikawa equation, relating d.c. conductivity, relaxation time and relaxation strength, was verified. The relaxation strength, too large for being connected with a dipolar relaxation, was ascribed to the displacement of hoping charges. The conclusion of the analysis has been that the electrical response of the system was dominated by the hopping charge transport.

The dynamics of monoepoxide (CGE) and diepoxide (DGEBA) compounds, investigated by wideband (102–1010 Hz) dielectric spectroscopy from below to above the glass transition temperature in the range 130–360 K, shows a bimodal relaxation and the presence of a dc conductivity contribution. The fitting of the data by the Havriliak–Negami function provided the temperature behaviour of conductivity, relaxation times, dielectric strengths and shape parameters. The glass transition phenomenon was detected by the changes of both the relaxation strength and the low frequency slope of the secondary relaxation, and a transition temperature TB > Tg between two different relaxation regimes, coincident with the splitting temperature of the systems, was also recognised. The comparative analysis of the results draws an overall picture which improves our information on the dynamics of the systems and also supports some recent ideas on the splitting between main and secondary relaxations.

We discuss the behaviour of the dielectric relaxation times of an epoxy-amine system both undergoing a polymerisation reaction and cooled below the glass transition. The relaxation times versus conversion are suitably described by equations which parallel the Vogel-Fulcher and Arrhenius equations, commonly used to describe the behaviour of the relaxation time versus temperature. Under special conditions, the bilinear dependence of the glass-transition temperature on conversion, predicted by both theoretical models and experimental results, is verified and the increase with conversion of the activation energy of the secondary relaxation process is found to be linear.