Claudio J. Pérez

Rheological study of linear high density polyethylenes modified with organic peroxide

Abstract Four high density polyethylenes were modified using different concentrations of an organic peroxide in order to change their molecular structure. The effects of the presence of vinyl groups in the original polymer molecules and of the peroxide concentration used in the modification process were analyzed. All the concentrations of peroxide used in this study were below the critical concentration that produces a macroscopic molecular network. The weight-average molecular weight of all the polyethylenes augments and the molecular weight distribution gets wider as the concentration of peroxide increases. These results support the general belief that the chain-linking reactions dominate the modification process. Evidence of the important role played by the vinyl groups is found not only in the change of the width of the chromatograms but also in the position of their maximums. The vinyl-containing polymers display the largest molecular changes for a given peroxide content. The magnitude of the viscous and elastic moduli of the polyethylenes goes up as the concentration of peroxide used increases showing the effect of the generated large molecules. The linear viscoelastic response of the modified polymers is thermo-rheologically complex. This complexity can be associated with the generation of branched molecules. For similar molecular weights and peroxide concentration, the flow activation energy displayed by the polyethylenes with larger concentration of vinyl groups is larger. This result suggest that a much more complex molecular structure is formed in the presence of vinyl groups. The dynamic moduli of the polymers were analyzed using the generalized viscoelastic model. The spectrum of relaxation times was determined for each polymer and analyzed as a function of the peroxide concentration.

Antimicrobial Activity of Starch Hydrogel Incorporated with Copper Nanoparticles

In order to obtain an antimicrobial gel, a starch-based hydrogel reinforced with silica-coated copper nanoparticles (Cu NPs) was developed. Cu NPs were synthesized by use of a copper salt and hydrazine as a reducing agent. In order to enhance Cu NP stability over time, they were synthesized in a starch medium followed by a silica coating. The starch hydrogel was prepared by use of urea and water as plasticizers and it was treated with different concentrations of silica-coated copper nanoparticles (Si-Cu NPs). The obtained materials were characterized by Fourier transform infrared (FT-IR) spectroscopy, electron paramagnetic resonance (EPR) spectroscopy, scanning electron microscopy (SEM), and rheometry. FT-IR and EPR spectra were used for characterization of Cu NPs and Si-Cu NPs, confirming that a starch cap was formed around the Cu NP and demonstrating the stability of the copper nanoparticle after the silica coating step. SEM images showed Cu NP, Si-Cu NP, and hydrogel morphology. The particle size was polydisperse and the structure of the gels changed along with particle concentration. Increased NP content led to larger pores in starch structure. These results were in accordance with the rheological behavior, where reinforcement by the Si-Cu NP was seen. Antimicrobial activity was evaluated against Gram-negative (Escherichia coli) and Gram-positive (Staphylococcus aureus) bacterial species. The hydrogels were demonstrated to maintain antimicrobial activity for at least four cycles of use. A dermal acute toxicity test showed that the material could be scored as slightly irritant, proving its biocompatibility. With these advantages, it is believed that the designed Si-Cu NP loaded hydrogel may show high potential for applications in various clinical fields, such as wound dressings and fillers.

Water soluble nanocomposite films based on poly(vinyl alcohol) and chemically modified montmorillonites

Different montmorillonites were added to poly(vinyl alcohol) in order to improve their properties. The used montmorillonite’s were: Cloisite Na+ (Na), Nanofil (NF), and Cloisite 30B (30B). Poly(vinyl alcohol) + montmorillonite films, obtained by casting, were characterized by means of Fourier transform infrared spectroscopy, differential scanning calorimetry, thermogravimetric analysis, X-ray diffraction pattern, transmission electron microscopy, water absorption, contact angle, and mechanical properties. Sodium and organically modified montmorillonites were used. The montmorillonite basal peak shifted to a lower angle for composites, with sodium montmorillonites, and the films were exfoliated–intercalated nanocomposites. Composites with organically modified montmorillonites presented a micro-morphology and the lowest water absorption. The best mechanical properties were obtained for the composite with sodium montmorillonites.

Sustainable and smart keratin hydrogel with pH-sensitive swelling and enhanced mechanical properties

Protein based hydrogels are a very interesting type of biomaterials with many probed strengths related to their source and chemical structure. Biocompatibility and biodegradability are accompanied by affordability when it comes to low cost sources. The main keratin source is agroindustrial waste, such as feathers, horns, hooves, hair and wool. Thus, the main cost of keratin hydrogels derives from their processing. Here is presented a new strategy for the obtaining of a keratin hydrogel with enhanced mechanical properties using low cost reagents. This keratin hydrogel is stiff enough to allow handling without special cares and also presenting a reversible pH-responsive behavior. The minimum swelling is observed at low pH due to a collapsed and disordered protein network with water tightly adsorbed to the hydrophilic sites. The swelling rises significantly above pH6 and the maximum swelling appears above pH8 where an expanded network allows water to enter to the pores.

Chitin based hybrid composites reinforced with graphene derivatives: a nanoscale study

In this work, we present two novel nanostructured hybrid materials based on a chitin matrix loaded with increasing amounts of graphene oxide and reduced graphene oxide nanosheets (nGO and rGO, respectively). Both kinds of material (Chi:nGO and Chi:rGO) were studied using different spectroscopic and rheological techniques with the aim of understanding the interaction mechanism between chitin and nGO/rGO and explaining how the type of filler and its proportion affects its reinforcement. The production of these hybrids represents not only the obtention of low-cost materials with mechanical resistance but also a good opportunity for developing materials with several applications according to their composition. The nGO and rGO were characterised through FT-IR and ESR for the determination of the oxidation degree of each nanofiller. Then, the hybrids were spectroscopically analysed with FT-IR, ESR and SAXS which demonstrated that the components do not interact through covalent bonding and the nanosheets are well-dispersed among the chitin matrix. Finally, a rheological behavior assay was performed and its results were analysed in terms of G′ and η*. In short, all the results allowed us to conclude that nGO acts as a more efficient reinforcer than rGO due to the higher amount of hydrogen bonding established with chitin.

Smart release of antimicrobial ZnO nanoplates from a pH-responsive keratin hydrogel

A smart antibacterial biomaterial based on a keratin hydrogel with pH-dependent behavior and Zinc Oxide nanoplates as biocide agent has been developed. The pH of a chronic wound is basic due to bacterial metabolism. Originally shrank at acid pH, keratin hydrogels swell upon contact with a bacterial contaminated media leading to the release of the nanoparticles. The material has been thoroughly characterized by infrared spectroscopy, Raman, scanning electron microscope, swelling behavior, Differential scanning calorimetry, Small-angle X-ray scattering, rheology, antimicrobial activity and cytotoxicity. The results show that 5% of Zinc Oxide nanoparticles concentration is the optimum for wound dressing applications.