Veronica Ambrogi

Morphological and Thermal Properties of Cellulose−Montmorillonite Nanocomposites

Cellulose-layered montmorillonite (MMT) nanocomposites were prepared by precipitation from N-methylmorpholine- N-oxide (NMMO)/water solutions. Two hybrid samples were obtained to investigate the influence of the reaction time on the extent of clay dispersion within the matrix. It was observed that longer contact times are needed to yield nanocomposites with a partially exfoliated morphology. The thermal and thermal oxidative properties of the hybrids, which might be of interest for fire-resistant final products, were investigated by thermogravimetry and chemiluminescence (CL). The nanocomposites exhibited increased degradation temperatures compared to plain cellulose, and the partially exfoliated sample showed the maximum stability. This result was explained in terms of hindered transfer of heat, oxygen, and degraded volatiles due to the homogeneously dispersed clay filler. Kinetic analysis of the decomposition process showed that the degradation of regenerated cellulose and cellulose-based hybrids occurred through a multistep mechanism. Moreover, the presence of nanoclay led to drastic changes in the dependence of the activation energy on the degree of degradation. CL analysis showed that longer permanence in NMMO/water solutions brought about the formation of carbonyl compounds on the polymer backbone. Moreover, MMT increased the rate of dehydration and oxidation of cellulose functional moieties. As a consequence, cellulose was found to be less stable at temperatures lower than 100 degrees C. Conversely, at higher temperatures, the hindering of oxygen transfer prevailed, determining an increase in thermo-oxidative stability.

Liquid crystalline vinyl ester resins for structural adhesives

Rigid-rod vinyl ester resins represent an interesting family of materials. They can be synthesized starting from the corresponding epoxy resin. Due to their composition, they exhibit outstanding properties, such as heat and chemical resistance, high impact, and corrosion resistance. These features, combined with the ease of curing, make vinyl ester resins good candidates for several industrial applications, such as glass fiber composites, adhesives, corrosion-resistant coatings, electrical encapsulation, and radiation-curable inks. In the first part of this study, a new liquid crystalline methacrylated monomer was prepared and tested as an adhesive. The synthesis was carried out by reacting methacrylic acid with a rigid-rod epoxy monomer derived from the reaction between p-(2,3-epoxypropoxy)-α-methylstilbene (DOMS, diglycidyloxy α-methylstilbene) and aniline in a molar ratio of 3 to 2. The monomer was named md3a2 (methacrylated derivative of DOMS-aniline epoxy compound). Md3a2 is liquid crystalline in a wide range of temperatures, as shown by differential scanning calorimetry (DSC) and optical microscopy. In the second part of the work, the adhesion properties of the novel liquid crystalline resin were tested and compared with those exhibited by a conventional (non-liquid crystalline) adhesive, the methacrylated derivative of an epoxy compound synthesized from a 3 to 2 molar ratio of Epon 825® (which is a commercial epoxy) and aniline. This monomer was named mep3a2 (methacrylated derivative of Epon 825-aniline epoxy compound). The adhesion of the two resins on a pretreated aluminum substrate was evaluated using mechanical tests (single-lap joint test, according to ASTM standard D1002-72) and scanning electron microscopy (SEM).

Artificial Biomelanin: Highly Light-Absorbing Nano-Sized Eumelanin by Biomimetic Synthesis in Chicken Egg White

The spontaneous oxidative polymerization of 0.01-1% w/w 5,6-dihydroxyindole (DHI) in chicken egg white (CEW) in the absence of added solvents leads to a black, water-soluble, and processable artificial biomelanin (ABM) with robust and 1 order of magnitude stronger broadband light absorption compared to natural and synthetic eumelanin suspensions. Small angle neutron scattering (SANS) and transmission electron microscopy (TEM) analysis indicated the presence in the ABM matrix of isolated eumelanin nanoparticles (≤100 nm) differing in shape from pure DHI melanin nanoparticles (SANS evidence). Electron paramagnetic resonance (EPR) spectra showed a slightly asymmetric signal (g ∼ 2.0035) similar to that of solid DHI melanin but with a smaller amplitude (ΔB), suggesting hindered spin delocalization in biomatrix. Enhanced light absorption, altered nanoparticle morphology and decreased free radical delocalization in ABM would reflect CEW-induced inhibition of eumelanin aggregation during polymerization accompanied in part by covalent binding of growing polymer to the proteins (SDS-PAGE evidence). The technological potential of eumelanin nanosizing by biomimetic synthesis within a CEW biomatrix is demonstrated by the preparation of an ABM-based black flexible film with characteristics comparable to those of commercially available polymers typically used in electronics and biomedical applications.

Self-toughening liquid crystalline vinyl ester adhesives

Vinyl ester resins are widely used as thermosetting matrices for production of advanced composite and structural adhesives. They are obtained by end-capping unsaturated vinyl groups on a large variety of suitable oligomeric precursor, resulting in thermosetting resin with a good processability and outstanding end-use properties. In this paper we describe the shynthesis of a new methacrylated derivative of p-(2,3-epoxypropoxy)-α-methylstilbene exhibiting liquid crystalline phases, referred to as MD3A2. For the sake of comparison, a model compound has been synthesized with very similar chemical backbone. The model compound, referred to as MEP3A2, features isotropic phase and is derived form the commercially available Epon 825®epoxy resin (Shell Oil Company). Fracture resistance of samples has been tested according to ASTM D1002-72 (Strength Properties of Adhesives in Shear by Tension Loading) on aluminum supports and related to the presence of isotropic or liquid crystalline phases.

5,6-Dihydroxyindole oxidation in phosphate buffer/polyvinyl alcohol: a new model system for studies of visible chromophore development in synthetic eumelanin polymers.

The determinants of the broadband absorption spectrum of eumelanins are still largely unknown. Herein we report a novel approach to investigate eumelanin chromophore which is based on the biomimetic oxidation of the key monomer precursor, 5,6‐dihydroxyindole (DHI, 1), with peroxidase/hydrogen peroxide in phosphate buffer, pH 7, containing 1–5% polyvinylalcohol (PVA, 27u2003000u2003Da). This approach relies on the discovery that as low as 1% PVA can prevent precipitation of the growing melanin polymer thus allowing investigation of the chromophoric phases accompanying oxidation of DHI without confounding scattering effects. Spectrophotometric monitoring showed the initial development of a band around 530u2003nm persisting for about 1u2003h before gradually changing into the typical broadband spectrum of eumelanin. Reductive treatment caused a significant absorbance decrease in the visible region without affecting an absorption band around 320u2003nm. Initial product analysis indicated an altered formation ratio of 2,4′‐biindolyl (2) and 2,7′‐biindolyl (3) relative to control experiments. Overall, these results demonstrate for the first time that the development in solution of visible chromophores since the early oligomer stages is independent of strong aggregation/precipitation phenomena.

Chitosan hydrogels embedding hyper-crosslinked polymer particles as reusable broad-spectrum adsorbents for dye removal

The removal of dye and toxic ionic pollutants from water is an extremely important issue that requires systematic and efficient adsorbent preparation strategies. To address this challenge, we developed composite chitosan (CS)-based hydrogels containing hyper-crosslinked polymer (HCP) particles to be used as broad-spectrum adsorbents. The goal is to efficiently combine the dye adsorption ability of chitosan and the capacity of the porous particles of trapping pollutant molecules. The HCP particles are well distributed and firmly embedded into the chitosan matrix and the composite hydrogels exhibit improved mechanical properties. Adsorption experiments reveal a synergistic effect between CS and HCP particles, and the samples are able to remove both anionic and cationic dyes (indigo carmine, rhodamine 6G and sunset yellow) from water. The maximum dye uptake is higher than that of comparable biosorbents. Moreover, the mechanical properties of the composite hydrogels are enhanced respect to pure CS, and the samples can be regenerated and reused keeping their adsorption ability unaltered over successive cycles of adsorption, desorption, and washing.

Epoxy-Based Shape-Memory Actuators Obtained via Dual-Curing of Off-Stoichiometric “Thiol–Epoxy” Mixtures

In this work, epoxy-based shape-memory actuators have been developed by taking advantage of the sequential dual-curing of off-stoichiometric “thiol–epoxy” systems. Bent-shaped designs for flexural actuation were obtained thanks to the easy processing of these materials in the intermediate stage (after the first curing process), and successfully fixed through the second curing process. The samples were programmed into a flat temporary-shape and the recovery-process was analyzed in unconstrained, partially-constrained and fully-constrained conditions using a dynamic mechanical analyzer (DMA). Different “thiol–epoxy” systems and off-stoichiometric ratios were used to analyze the effect of the network structure on the actuation performance. The results evidenced the possibility to take advantage of the flexural recovery as a potential actuator, the operation of which can be modulated by changing the network structure and properties of the material. Under unconstrained-recovery conditions, faster and narrower recovery-processes (an average speed up to 80%/min) are attained by using materials with homogeneous network structure, while in partially- or fully-constrained conditions, a higher crosslinking density and the presence of crosslinks of higher functionality lead to a higher amount of energy released during the recovery-process, thus, increasing the work or the force released. Finally, an easy approach for the prediction of the work released by the shape-memory actuator has been proposed.

Light-Responsive Polymer Micro- and Nano-Capsules

A significant amount of academic and industrial research efforts are devoted to the encapsulation of active substances within micro- or nanocarriers. The ultimate goal of core–shell systems is the protection of the encapsulated substance from the environment, and its controlled and targeted release. This can be accomplished by employing “stimuli-responsive” materials as constituents of the capsule shell. Among a wide range of factors that induce the release of the core material, we focus herein on the light stimulus. In polymers, this feature can be achieved introducing a photo-sensitive segment, whose activation leads to either rupture or modification of the diffusive properties of the capsule shell, allowing the delivery of the encapsulated material. Micro- and nano-encapsulation techniques are constantly spreading towards wider application fields, and many different active molecules have been encapsulated, such as additives for food-packaging, pesticides, dyes, pharmaceutics, fragrances and flavors or cosmetics. Herein, a review on the latest and most challenging polymer-based micro- and nano-sized hollow carriers exhibiting a light-responsive release behavior is presented. A special focus is put on systems activated by wavelengths less harmful for living organisms (mainly in the ultraviolet, visible and infrared range), as well as on different preparation techniques, namely liposomes, self-assembly, layer-by-layer, and interfacial polymerization.

Effect of cellulose structure and morphology on the properties of poly(butylene succinate-co-butylene adipate) biocomposites.

Composites based on poly(butylene succinate-co-butylene adipate) (PBSA) containing amorphized and crystalline cellulose reinforcements have been prepared and characterized. In order to improve the polymer/filler interfacial adhesion, an efficient compatibilizing agent has been synthesized by chemical modification of PBSA and characterized by FT-IR, FT-NIR and (1)H NMR spectroscopy. Uncompatibilized and compatibilized composites have been tested through morphological, mechanical, calorimetric and thermogravimetric analysis. Moreover, water vapor permeability and biodegradation kinetics of composites have been investigated. The addition to PBSA of cellulose fillers differing from each other by crystallinity degree and morphology, and the use of a compatibilizing agent have allowed modulating tensile and thermal properties, water vapor transmission rate and biodegradation kinetic of the composites.

Synthesis and adsorption study of hyper-crosslinked styrene-based nanocomposites containing multi-walled carbon nanotubes

New nanocomposite microporous materials obtained by adding functionalized multi-walled carbon nanotubes (MWCNT) to styrene/vinylbenzyl chloride/divinylbenzene hyper-crosslinked resins were prepared and characterized. In order to promote the embedding of the MWCNT within the gel-type precursor, a suitable surface modification strategy was set up, based on the grafting of a poly(vinylbenzyl chloride) (PVBC) resin, able to participate in the hyper-crosslinking step, onto the nanotube surface. Characterization of the nanocomposites by FTIR spectroscopy and electron microscopy enabled the assessment of the effect of the nanotubes on the structure and the morphology of the resin. Moreover, gas sorption measurements indicated that by addition of nanotubes it is possible to modulate the pore size distribution, the uptake of CO2 and H2 and the CO2/N2 selectivity. Finally, modified MWCNT are also able to improve the adsorption capacity of phenol from water solutions, suggesting the possible application of the new microporous nanocomposites for water remediation.