Carbon Nanotubes as Reinforcement of Cellulose Liquid Crystalline Responsive Networks.

The incorporation of small amount of highly anisotropic nanoparticles into liquid crystalline hydroxypropylcellulose (LC-HPC) matrix improves its response when is exposed to humidity gradients due to an anisotropic increment of order in the structure. Dispersed nanoparticles give rise to faster order/disorder transitions when exposed to moisture as it is qualitatively observed and quantified by stress-time measurements. The presence of carbon nanotubes derives in a improvement of the mechanical properties of LC-HPC thin films.

Biomineralization in chitosan/Bioglass® composite membranes under different dynamic mechanical conditions.

Fundamental aspects of biomineralization may be important in order to understand and improve calcification onto the surface of biomaterials. The biomineralization process is mainly followed in vitro by assessing the evolution of the apatite layer that is formed upon immersion of the material in Simulated Body Fluid (SBF). In this work we propose an innovative methodology to monitor apatite deposition by looking at the evolution of the mechanical/viscoelastic properties of the sample while immersed in SBF, using non-conventional dynamic mechanical analysis (DMA) performed under distinct displacement amplitudes (d). The biomimetic biomineralization process in composite membranes of chitosan (CTS) with Bioglass® (BG) was followed by measuring the change of the storage modulus, E', and the loss factor, tan δ, at 37 °C and in SBF, both online (d=10 µm and d=30 µm) and offline (d=0 µm). The online experiments revealed that the E' decreased continuously up in the first hours of immersion in SBF that should be related to the dissolution of BG particles. After that, an increase of the stiffness was verified due to the apatite deposition. SEM/EDS observations upon 24h of immersion in SBF showed higher development of apatite deposition with increasing displacement amplitude.

Chitosan membranes containing micro or nano-size bioactive glass particles: evolution of biomineralization followed by in situ dynamic mechanical analysis.

A new family of biodegradable polymer/bioactive glass (BG) composite materials has emerged based on the availability of nano-sized bioactive particles. Such novel biocomposites can have enhanced performance, in terms of mechanical properties and bioactivity, and they can be designed to be used in bone regeneration approaches. In this work, membranes of chitosan (CTS) and chitosan with bioactive glass (BG) both micron and nano sized particles (CTS/µBG, CTS/nBG, respectively) were prepared by solvent casting. Microstructural and mechanical properties were evaluated in order to compare the effects of the incorporation of micro (µBG) and nano (nBG) particles in the chitosan matrix. In vitro bioactivity tests were performed to characterize the apatite layer that is formed on the surface of the material after being immersed in simulated body fluid (SBF). The biomineralization process on the biomaterials was also followed using non-conventional dynamic mechanical analysis (DMA), both online and offline. In such DMA experiments, the change in the storage modulus, E', and the loss factor, tan δ, were measured as a function of the immersion time in SBF. The results demonstrated that CTS/nBG membranes possess enhanced mechanical properties and higher bioactivity in comparison with the CTS/µBG membranes. Such results suggest the potential of nBG for the development of bioactive composites for bone regeneration applications.

Investigating the influence of morphology in the dynamical behavior of semicrystalline Triton X-100: insights in the detection/nondetection of the α'-process.

The paper investigates the influence of the crystalline structure in the dynamical behavior of semicrystalline Triton X-100 allowing enlightening the reason for the detection/nondetection of the α'-process. The work was preceded by the study of the full amorphous material for which dielectric relaxation spectroscopy (DRS) identified multiple relaxations: the α-process associated with the dynamical glass transition and two secondary relaxations (ß- and γ- processes). To evaluate how crystallinity affects the detected relaxation processes, different crystallizations were induced under high and low undercooling conditions. While the secondary relaxations are unaffected by crystallization, the mobility of the cooperative bulk α-process is sensitive to the distinct morphologies. The distinct semicrystalline states were structurally characterized by X-ray diffraction and polarized optical microscopy (POM). Differential scanning calorimetry (DSC) was used as a complementary tool. Depending on the extension of undercooling, large and well-defined shperulites or grainy-like structure emerge, respectively, for low and high undercooling degrees, as monitored by POM. In the two crystalline structures, X-ray diffraction patterns detected the amorphous halo meaning that both are semicrystalline. However, no differences between the amorphous regions are indentified by this technique; the distinction was done by means of dielectric measurements probing different mobilities in each of those regions. When the large spherulites evolve, the bulk-like α-process never goes to extinction and slightly shifts to low frequencies increasing the associated glass transition by 2-3 K, as confirmed by DSC; the slight change is an indication that the dimensions of the persisting amorphous regions become comparable to the length scale inherent to the cooperative motion that determines the glass transition in the full amorphous material. For the grainy-like structure, the α-process becomes extinct and an α'-process evolves as revealed by isochronal plots of dielectric measurements, with the features of a glass transition as confirmed by temperature modulated differential scanning calorimetry; both techniques indicate a 10-12 K displacement of the associated hindered glass transition toward higher temperatures relative to the amorphous glass transition. It is concluded that the detection of the α'-process in Triton X-100 is greatly determined by the high degree of constraining of the amorphous regions imposed by the grainy crystalline structure disabling the occurrence of a bulk-like α-process. Triton X-100 can be taken as a model for understanding low molecular weight materials crystallization, allowing correlating the observed dynamical behavior with the achieved crystalline morphology.

Bioactivity and viscoelastic characterization of chitosan/bioglass® composite membranes.

Membranes of chitosan (CTS) and composite membranes of CTS with bioglass are prepared by solvent casting. The composite membranes are shown to induce the precipitation of apatite upon immersion in SBF. The biomineralization process is followed by measuring the variation of the viscoelastic properties of the membranes immersed in SBF, both online and offline. Non-conventional DMA is used to measure the change in the storage modulus, E', and the loss factor, tan δ, as a function of the immersion in SBF. A simple model is used to estimate the E' of the apatite layer formed in vitro that is about 130 MPa. This work shows that innovate mechanical tests can be useful to characterize the mechanical performance of composites under physiological conditions.

Phase transformations undergone by Triton X-100 probed by differential scanning calorimetry and dielectric relaxation spectroscopy.

The phase transformations of the surfactant Triton X-100 were investigated by differential scanning calorimetry (DSC), polarized optical microscopy (POM), and dielectric relaxation spectroscopy (DRS). In particular, crystallization was induced at different cooling rates comprised between 13 and 0.5 K min(-1). Vitrification was detected by both DSC and DRS techniques with a glass transition temperature of ∼212 K (measured on heating by DSC) allowing classifying Triton X-100 as a glass former. A fully amorphous material was obtained by cooling at a rate ≥10 K min(-1), while crystallization was observed for lower cooling rates. The temperature of the onset of melt-crystallization was found to be dependent on the cooling scan rate, being higher the lower was the scan rate. In subsequent heating scans, the material undergoes cold-crystallization except if cooled previously at a rate ≤1 K min(-1). None of the different thermal histories led to a 100% crystalline material because always the jump typical of the glass transformation in both heat flux (DSC) and real permittivity (DRS) is observed. It was also observed that the extent/morphology of the crystalline phase depends on the degree of undercooling, with higher spherulites developing for lower undercooling degree (24 K ≤ T(m) - T(cr) ≤ 44 K) in melt-crystallization and a grain-like morphology emerging for T(m) - T(cr) ≈ 57 K either in melt- or cold-crystallization. The isothermal cold- and melt-crystallizations were monitored near above the calorimetric glass transition temperature by POM (221 K) and real-time DRS (T(cr) = 219, 220, and 221 K) to evaluate the phase transformation from an amorphous to a semicrystalline material. By DRS, the α-relaxation associated with the dynamic glass transition was followed, with the observation that it depletes upon both type of crystallizations with no significant changes either in shape or in location. Kinetic parameters were obtained from the time evolution of the normalized permittivity according to a modified Avrami model taking in account the induction time. The reason the isothermal crystallization occurs to a great extent in the vicinity of the glass transition was rationalized as the simultaneous effect of (i) a high dynamic fragile behavior and (ii) the occurrence of catastrophic nucleation/crystal growth probably enabled by a preordering tendency of the surfactant molecules. This is compatible with the estimated low Avrami exponent (1.12 ≤ n ≤ 1.6), suggesting that relative short length scale motions govern the crystal growth in Triton X-100 coherent with the observation of a grainy crystallization by POM.

Crosslink effect and albumin adsorption onto chitosan/alginate multilayered systems: an in situ QCM-D study.

The adsorption of HSA onto CHI/ALG multilayer assemblies was assessed in situ using QCM-D. It was found that the behavior of HSA on biomaterials surface can be tuned by adjusting parameters of the polyelectrolyte system such as pH, layer number, crosslinker and polymer terminal layer. Our results confirmed the key role of electrostatic interactions during HSA adsorption, since oppositely charged surfaces were more effective in promoting protein adhesion. QCM-D data revealed that crosslinking (CHI/ALG)(5) CHI films allows HSA to become adsorbed in physiological conditions. Our results suggested that the biological potential of biopolymers and the mild conditions of the LbL technique turn these natural nanoassemblies into a suitable choice to be used as pH-sensitive coatings.