Cristina Alvarez

Molecular motions in molecular glasses as studied by thermally stimulated depolarisation currents (TSDC)

Abstract The technique of thermally stimulated depolarisation currents (TSDC) was used to study the molecular dynamics in the fragile glass formers salol, glycerol and maltitol. The glass transition relaxation was carefully studied in these materials using the thermal sampling (TS) procedure. It is suggested that the amplitude of the deviation from the zero entropy prediction is related to the fragility of the glass and a definition of fragility based on the TSDC results is proposed. Moreover, a β-relaxation was detected and fully characterised in maltitol.

Glass transition relaxation and fragility in the molecular glass forming m-toluidine: A study by thermally stimulated depolarization currents

The glass transition relaxation in m-toluidine was studied by thermally stimulated depolarization currents (TSDC). The departure from the zero entropy prediction, also called compensation behavior, was analyzed and the fragility index of the glass-former m-toluidine was calculated from the TSDC data according to a recently proposed procedure. The obtained result was compared with literature values. A particular attention was focused on the physical aging process of the nonequilibrium glass, which was followed easily because of its slow evolution. The influence of the aging temperature on the aging rate was analyzed. The significance of the so-called compensation point was discussed on the basis of the results obtained in this study of structural aging.

Fragility in side-chain liquid crystalline polymers : the TSDC contribution

In the present work a procedure is proposed to obtain the fragility of a glass from Thermally Stimulated Depolarisation Currents (TSDC) data. From previous TSDC studies on wide series side-chain liquid crystalline polymers, and particularly from a careful thermal sampling analysis of their glass transition relaxation, the fragility parameter was obtained. The proposed fragility scale is based on the concept of deviation from the zero entropy prediction, and is such that zero is the limit of fragility for infinitely strong glasses. The fragility of 10 side-chain liquid crystalline polymers is discussed in terms of their molecular structure.

Influence of texture of suwari gels on kamaboko gels made from sardine (Sardina pilchardus) surimi

The textural characteristics and water holding capacity of suwari (set) and kamaboko (set and cooked) sardine surimi gels were examined in order to clarify the influence of the initial network formed in setting conditions (25, 35 and 40°C for 30 and 60 min) on the texture of the kamaboko gels. Although the texture of suwari gels set at 35°C improved with longer setting, both setting times ensured kamaboko gels with the highest gel strength. Suwari gels set at 25°C also improved with longer setting but the gel strength of both suwari and kamaboko gels was lower than at 35°C. For gels set at 40°C prolonged setting weakened the suwari networks formed, leading to kamaboko gels with poorer textural characteristics. ©1997 SCI

Cold crystallization of poly(ethylene naphthalene-2,6-dicarboxylate) by simultaneous measurements of X-ray scattering and dielectric spectroscopy

The isothermal cold crystallization of poly(ethylene naphthalene-2,6-dicarboxylate) was investigated by simultaneous small and wide angle X-ray scattering and dielectric spectroscopy (DS). By this experimental approach, simultaneously collected information was obtained about the specific changes occurring in both crystalline and amorphous phases during crystallization, namely about the chain ordering through wide angle X-ray scattering, about the lamellar crystals arrangement by means of small angle X-ray scattering, and about the amorphous phase evolution by means of DS. The results indicate that average mobility of the amorphous phase suffers a discontinuous decrease upon passing from the primary to the secondary crystallization regime. We interpret these results assuming that the restriction to the mobility of the amorphous phase occurs mainly in the amorphous regions between the lamellar stacks and not in the amorphous regions within the lamellar stacks.

Poly(Ethylene Oxide) Functionalized Polyimide-Based Microporous Films to Prevent Bacterial Adhesion

Preventing microbial adhesion onto membranes is a crucial issue that determines the durability of the membrane. In this Research Article, we prepared aromatic polyimides (extensively employed for the elaboration of ultrafiltration membranes) containing PEO branches. Four polyimide-g-PEO copolymers were prepared from 6F dianhydride and a novel aromatic diamine containing PEO-550 side groups. The copolymers were designed to have variable PEO content, and were characterized by their spectroscopic and physical properties. The Breath Figure technique was successfully applied to create an ordered surface topography, where the PEO chains were preferentially located on the surface of the micrometer size holes. These unique features were explored to reduce bacterial adhesion. It was established that surface modified polyimide membranes have a high resistance to biofouling against Staphylococcus aureus. In particular, we observed that an increase of the PEO the content in the copolymer produced a decrease in the bacterial adhesion.

Relaxation response of polymers containing highly flexible side groups monitored by broadband dielectric spectroscopy

The relaxation behavior of poly(5-acryloxymethyl-5-methyl-1,3-dioxacyclohexane), a polymer containing highly flexible side groups, is studied by broadband dielectric spectroscopy in the frequency and temperature ranges 10(-1)-10(9) Hz and 123-473 K, respectively. Above the glass transition temperature T(g) the dielectric loss in the frequency domain exhibits a prominent alpha absorption, followed in increasing order of frequencies by two secondary absorptions called beta and gamma. At temperatures slightly higher than T(g), the a relaxation is well separated from the beta, but as temperature increases overlapping between both relaxations augments forming an alphabeta absorption in the vicinity of 420 K. This latter absorption displays a shoulder on its high-frequency side corresponding to the y relaxation. The strength of the a relaxation decreases with increasing temperature, eventually vanishing at the temperature at which the alphabeta absorption is formed. The time retardation spectra of the isotherms are calculated and further used to facilitate the deconvolution of the overlapping relaxations. The fact that the temperature dependence of the beta relaxation also describes that of the alphabeta absorption suggests that both relaxations have the same nature. It seems that as temperature increases, the a relaxation feeds on the beta absorption until its complete disappearance. The gamma relaxation, in turn, seems to increase at the expense of the alphabeta process at high temperature.

Thermally stimulated depolarization current investigation of the relaxation behavior of polymers with chlorocyclohexyl side groups

The considerable dielectric activity exhibited by acrylate polymers containing chlorocyclohexyl side groups is investigated by using both global and partial thermally stimulated depolarization current (TSDC) techniques. The critical interpretation of the spectra obtained by TSDC experiments on poly(2‐chlorocyclohexyl acrylate) (P2CCA), together with previous results related to the dynamics of 2‐chlorocyclohexyl acetate, model compound of the side group of P2CCA, seem to suggest that conformational transitions about O–Cy and CH–C*O* bonds, rather than chair to reverse chair transition in the cyclohexyl group, may be involved in the absorption exhibited by these polymers in the glassy state. Attempts are also made to link the relaxation spectra obtained by TSDC techniques with the ac dielectric spectra.

Microstructure ofsuwari andkamaboko sardine surimi gels

In a previous work it was suggested that the texture of kamaboko (set and cooked) gels made from sardine surimi under varying setting conditions was predetermined by the specific matrix forming in each suwari (set) gel. This paper describes the microstructure of the networks formed in suwari and kamaboko gels set at 25, 35 and 40 °C for 30 or 60 min as examined by scanning electron microscopy (SEM). Cooking conditions for kamaboko gels were fixed at 90 °C for 30 min; other preparation conditions were invariable. At low magnification (≤×500) the gel matrixes were compact, with practically no differences among lots. At higher magnification (×20 000), the suwari gel matrixes formed at low temperature consisted of globules. At higher temperatures the globules joined up to form fibrillar structures (fibres) and zones of disordered globule aggregation (coagula); at longer setting times, lateral bonding of the fibres became apparent. Kamaboko gels produced from unstructured globular matrixes exhibited only a few fibrillar zones and large areas of coagula. Where there was already an incipient fibrous formation, these developed into individual fibres or bundles of fibres that correlated with the best texture characteristics. Suwari gels with extensive lateral bonded fibres gave rise to kamaboko gels with a highly compact appearance under SEM; this correlated with a decline in texture values. These different structures suggest that the protein–protein bonds in the suwari networks have different levels of stability to heat, and these levels determine whether or not the proteins can subsequently be reorganised when the kamaboko gel forms.

Relaxation behavior of semiflexible polymers at very low frequencies

The dielectric activity of poly(monocyclohexylmethylene itaconate) (PMCI) and poly(dicyclohexylmethylene itaconate) (PDCI) in the glassy region and in the glass–rubber transition is studied by thermostimulated discharge current (TSDC) techniques. The spectra obtained by global TSDC experiments show a prominent α-glass–rubber relaxation peak with maxima located at 97 and 55 °C for PMCI and PDCI, respectively, following in decreasing order of temperature for a well developed β absorption and a comparatively low intensity relaxation. Better resolution of the relaxation behavior of these polymers in the glassy region is obtained by calculating the components of the complex dielectric permittivity e* at extremely low frequencies from partial TSDC experiments. The ac spectra thus obtained suggest that the β absorption is composed by two relaxations, each of them presumably associated with the motions of a side group. The differences observed in these spectra with those reported for the β mechanical relaxation r...