Marta Carsí

Monitoring molecular dynamics of bacterial cellulose composites reinforced with graphene oxide by carboxymethyl cellulose addition

Broadband Dielectric Relaxation Spectroscopy was performed to study the molecular dynamics of dried Bacterial Cellulose/Carboxymethyl Cellulose-Graphene Oxide (BC/CMC-GO) composites as a function of the concentration of CMC in the culture media. At low temperature the dielectric spectra are dominated by a dipolar process labelled as a β-relaxation, whereas electrode polarization and the contribution of dc-conductivity dominate the spectra at high temperatures and low frequency. The CMC concentration affects the morphological structure of cellulose and subsequently alters its physical properties. X-ray diffractometry measurements show that increasing the concentration of CMC promotes a decrease of the Iα/Iβ ratio. This structural change in BC, that involves a variation in inter- and intramolecular interactions (hydrogen-bonding interactions), affects steeply their molecular dynamics. So, an increase of CMC concentration produces a significantly decrease of the β-relaxation strength and an increase of the dc-conductivity.

Understanding the thermal and dielectric response of organosolv and modified kraft lignin as a carbon fibre precursor

Understanding the thermal behaviour of lignin is crucial in order to realise its valorisation as an engineering polymer. Two hardwood lignins, organosolv (OSL) and chemically modified kraft lignin (ML) have been chosen to represent important classes of renewable and abundant raw materials. The relationship between ionic mobility and viscosity in OSL and ML has been studied. The rheological results have been interpreted in terms of the competitive processes of thermal plasticisation and stiffening through crosslinking. Results show that with OSL, crosslinking proceeds relatively rapidly, and this is consistent with its more reactive structure. Higher molecular weight (Mw) influenced the melt stability as cross-linking kinetics was reduced and this was attributed to the reduction of chain ends available for cross-linking reactions. Scanning calorimetry has shown that both materials are glassy and pass through the glass transition between 100 °C and 115 °C, with the higher molecular weight modified material having a slightly higher Tg. Both lignins show pronounced maxima in the Gram–Schmidt plots for methane or methanol around 400 °C. However, a significant difference between the materials is observed with the detection of a strong carbonyl peak in the evolution products of the ML, which is attributed to the scission of the hydroxypropyl substituent present in the ML structure. The differences in the degradation processes are further reflected in the dielectric properties of the partially degraded materials where loss maxima occur at different temperatures and show different degrees of frequency dependence. An important observation is the difference in conductivity, where higher values for OSL are attributed to the cross-linking between adjacent benzene rings, whereas with the ML, a lower conductivity is associated with intrinsically less conductive intermolecular linkages. These results demonstrate that the thermal decomposition of the two lignins follows significantly different paths at the molecular level. With the more reactive OSL, it appears to be the case that there is a greater tendency to form direct ring to ring crosslinks and this is very significant for the properties of the intended end product.

Molecular dynamics of carrageenan composites reinforced with Cloisite Na+ montmorillonite nanoclay

Nanocomposites comprising biodegradable carrageenan and glycerol (KCg) as the host polymer, with different contents of natural montmorillonite (MMT) as filler, were prepared by a solution casting process. Different techniques have been used to determine the interaction/behavior among the different components of the samples such as Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), Transmission electron microscope (TEM) and, mainly, Dielectric relaxation spectroscopy (DRS). FTIR indicates hydrogen interaction between carrageenan matrix and silicate that is confirmed by the XRD data indicating some kind of carrageenan intercalation between the MMT layers. A rather homogenous distribution of MMT into KCg matrix were observed using transmission electron microscopy. The MMT effect on the molecular mobility at the glass transition was studied by dielectric relaxation spectroscopy. The MMT addition resulted in a slower relaxation and a wider distribution of the relaxation times. The fragility index, m, increased upon MMT incorporation, which may be attributed to a reduction in mobility chains, due to the MMT confinement of the KCg network. In addition, the apparent activation energy associated with the relaxation dynamics of the chains at Tg increased with the MMT content. The modified films developed in this paper could be used to prepare biodegradable and edible packaging films and films for biomedical applications with improved mechanical and good dielectric response.

Molecular Dynamics of Functional Azide-Containing Acrylic Films

A report on the syntheses, thermal, mechanical and dielectric characterizations of two novel polymeric acrylic materials with azide groups in their pendant structures is presented. Having the same general structure, these polymers differ in length of oxyethylene units in the pendant chain [-CONH-CH2CH2-(O-CH2CH2)nN3], where n is 1 (poly(N-(2-(2-azidoethoxy)ethyl)methacrylamide), PAzMa1) or 2 (poly(N-2-(2-(2-azidoethoxy)ethoxy)ethyl)methacrylamide), PAzMa2), leading with changes in their dynamics. As the thermal decomposition of the azide group is observed above 100 °C, dielectric analysis was carried out in the temperature range of −120 °C to 100 °C. Dielectric spectra of both polymers exhibit in the glassy state two relaxations labelled in increasing order of temperature as γ- and β-processes, respectively. At high temperatures and low frequencies, the spectra are dominated by ohmic conductivity and interfacial polarization effects. Both, dipolar and conductive processes were characterized by using different models. Comparison of the dielectric activity obtained for PAzMa1 and PAzMa2 with those reported for crosslinked poly(2-ethoxyethylmethacrylate) (CEOEMA) was performed. The analysis of the length of oxyethylene pendant chain and the effect of the methacrylate or methacrylamide nature on the dynamic mobility was analysed.

Controlling dielectrical properties of polymer blends through defined PEDOT nanostructures

The paper reports the crucial role of the morphology of poly(3,4-ethylenedioxythiophene) (PEDOT) nanostructures on the thermal and dielectric properties of polymer blends prepared thereof. PEDOT nanostructures with two different morphologies (nanoparticles and nanowires) were synthesized. The size for the nanoparticles was in the range 10–40 nm and the diameter of the nanowires was of ca. 200 nm. These nanostructures were blended with an insulating polymer matrix, poly(methyl methacrylate) (PMMA), to evaluate the dielectrical properties of the materials. The results of broadband dielectric spectroscopy showed a strong correlation between the morphology of the nanostructure and the improvement of the electrical properties of the material.

The effect of cross-linking on the molecular dynamics of the segmental and β Johari-Goldstein processes in polyvinylpyrrolidone-based copolymers.

The effect of the cross-link density on the molecular dynamics of copolymers composed of vinylpyrrolidone (VP) and butyl acrylate (BA) was studied using differential scanning calorimetry (DSC) and dielectric relaxation spectroscopy (DRS). A single glass transition was detected by DSC measurements. The dielectric spectra exhibit conductive processes and three dipolar relaxations labeled as α, β and γ in the decreasing order of temperatures. The cross-linker content affects both α and β processes, but the fastest γ process is relatively unaffected. An increase of cross-linking produces a typical effect on the α process dynamics: (i) the glass transition temperature is increased, (ii) the dispersion is broadened, (iii) its strength is decreased and (iv) the relaxation times are increased. However, the β process, which possesses typical features of a pure Johari-Goldstein relaxation, unexpectedly loses the intermolecular character for the highest cross-linker content.