Raúl Quijada

Polypropylene with embedded copper metal or copper oxide nanoparticles as a novel plastic antimicrobial agent

Aims:  To develop novel polypropylene composite materials with antimicrobial activity by adding different types of copper nanoparticles.

Hydrogels based on modified chitosan, 1. Synthesis and swelling behavior of poly(acrylic acid) grafted chitosan

A large number of carboxylic groups were introduced onto chitosan by grafting with poly(acrylic acid) as an efficient way of modification. The reactions were carried out in a homogeneous aqueous phase by using potassium persulfate and ferrous ammonium sulfate as the combined redox initiator system. The efficiency of grafting was found to depend on monomer, initiator, and ferrous ion concentrations as well as reaction time and -temperature. It was observed that the level of grafting could be controlled to some degree by varying the amount of ferrous ion used as a co-catalyst in the reaction. Evidence of grafting was obtained by comparison of FTIR spectra of chitosan and the graft copolymer as well as solubility characteristics of the products. The swelling behavior of chitosan samples, grafted with 115% and 524% poly(acrylic acid), was studied as function of pH, in distilled water and in aqueous NaCl solution. The sample that has 115% grafting swells considerably more both in distilled water (pH 5.8) as well as higher pH values than the sample with 524% grafting. In contrast, an inverse swelling behavior at pH 2 was observed. An unusual swelling behavior at pH 2 was found for the sample with 524% grafting, which swells to a greater extent than in distilled water. This was attributed to the amphiphilic nature of the modified chitosan from which complex inter- and/or intramolecular interactions could originate.

Influence of the comonomer content on the thermal and dynamic mechanical properties of metallocene ethylene/1-octene copolymers

Studies of ethylene/1-octene copolymers prepared by a metallocene catalyst were carried out through dynamic-mechanical experiments (DMTA), differential scanning calorimetry (DSC), Raman spectroscopy and gel permeation chromatography (GPC). The influence of comonomer content on the ethylene/1-octene properties, specially those related to the dynamic-mechanical behaviour were studied. It was observed that the intensity of the β-transition increases as the comonomer content increases and decreases crystallinity.

Toward Tailor-Made Biocide Materials Based on Poly(propylene)/Copper Nanoparticles

A set of poly(propylene) composites containing different amounts of copper nanoparticles (CNP) were prepared by the melt mixed method and their antimicrobial behavior was quantitatively studied. The time needed to reduce the bacteria to 50% dropped to half with only 1 v/v % of CNP, compared to the polymer without CNP. After 4 h, this composite killed more than 99.9% of the bacteria. The biocide kinetics can be controlled by the nanofiller content; composites with CNP concentrations higher than 10 v/v % eliminated 99% of the bacteria in less than 2 h. X-ray photoelectron spectroscopy did not detect CNP at the surface, therefore the biocide behavior was attributed to copper in the bulk of the composite.

Melt functionalization of polypropylene with methyl esters of itaconic acid

Polypropylene (pp) was functionalized in the melt phase by grafting with itaconic acid derivatives. 2,5-dimethyl-2,5-bis(tert-buthylperoxy) hexane (lupersol 101) was used as radical initiator. A Brabender Plasticorder was used for all the experiments. The optimum conditions for the grafting reaction were 190°C, 75 rpm and 6 min. The itaconic acid derivatives used in this work were mono- and dimethyl esters. When dimethyl itaconate (DMI) was used as functional polar monomer, a maximum of 1.5% of grafting was reached. A similar value was found for grafting of monomethyl itaconate (MMI) where a maximum of 1.6% by weight of this monomer was incorporated in pp. The existence of grafted MMI and/or DMI in pp was confirmed by FT-IR spectroscopy. Carbonyl absorption bands from either MMI and/or DMI were observed in the FT-IR spectrum of the modified pp. The percentage of grafting attained was also followed by FT-IR analysis, where the amount of monomer incorporated in pp was estimated from a calibration curve established for the monomers used in this work. A systematic study of the reaction variables allowed the optimization of the grafting process. It was found that the percentage of grafting attained depends on the initial concentration of both monomer and initiator used in the reaction. Some properties of the modified pps were evaluated by employing dilute solution viscosity and differential scanning calorimetric (DSC) measurements. It was found that the molecular weight of the grafted polymer decreased with increasing percentage of grafting. DSC measurements indicated that functionalized pp samples have slightly higher crystallinity but show the same melting temperature as pp.

Synthesis and characterization of copolymers of ethylene and 1-octadecene using the rac-Et(Ind)2ZrCl2/MAO catalyst system

The copolymerization of ethylene and 1-octadecene using a bridged metallocene was studied in order to observe the effect of the comonomer on the catalytic activity. A noticeable increase in activity is seen as the concentration of 1-octadecene in the reaction medium increases. 13 C NMR analysis shows 6.4 mol-% incorporation of comonomer at the highest 1-octadecene concentration in the feed used here. The molecular weight of the copolymers shows a drastic decrease that may be attributed to chain termination by transfer or β-elimination of the comonomer. As to the molecular weight distribution, it remains within a narrow range, as expected with metallocene catalysts. The melting temperature and the enthalpy of melting of the copolymers show a decrease with increasing comonomer content. As usual for ethylene copolymers, the X-ray crystallinities are higher than those determined from the enthalpy of melting.

Nanoparticles prepared by the sol–gel method and their use in the formation of nanocomposites with polypropylene

Hybrid layered aluminosilicate nanoparticles (HLNP) containing octadecylamine (ODA) as the organic part, and silica nanoparticles with spherical morphology containing ODA (HSNP) or without ODA (SNP) were prepared by the sol–gel method and used for the formation of nanocomposites with polypropylene. The polypropylene matrices, of different molecular weight and polydispersity, were prepared using polymers obtained via Ziegler–Natta or metallocene catalysts. A strong influence of the morphology and the presence of ODA on the surface of the nanoparticles was found on the formation and characteristics of the nanocomposites. The mechanical properties and thermal stability of these materials were determined and compared with those of nanocomposites prepared with 2:1 phylosilicate clays such as montmorillonite and hectorite in similar polymer matrices. X-ray diffraction, transmission electron microscopy, and the study of mechanical properties showed that the use of HLNP allows nanocomposites with considerably improved mechanical properties to be obtained, compared with nanocomposites prepared with exfoliated clays. In the case of nanocomposites prepared with spherical particles functionalized with ODA (HSNP), materials with high specific strength combined with high elongation before rupture were obtained. The thermal stabilization of polypropylene matrices containing the synthesized nanoparticles (HLNP, HSNP or SNP) occurs about 50 °C higher than that attained with clays.

Effect of the comonomer content on the mechanical parameters and microhardness values in poly(ethylene-co-1-octadecene) synthesized by a metallocene catalyst

Stress–strain and microhardness measurements were carried out on a series of copolymers of ethylene and 1-octadecene with different comonomer contents in the corresponding homopolymer of ethylene, synthesized with a metallocene catalyst. The different mechanical properties, deduced from the stress–strain curves (Youngs modulus, yield stress, deformation at break, and energy to break) are interpreted in terms of the crystallinity and molecular weight of the samples because these two characteristics show considerable variations with the comonomer content. The microhardness values are explained in terms of these properties, and they are also correlated with Youngs moduli and yield stresses deduced from the stress–strain curves. Linear relations are found between microhardness and yield stress and between the logarithm of the microhardness and the logarithm of the elastic modulus. The properties deduced from these lines are compared with literature values.

Influence of the type and the comonomer contents on the mechanical behavior of ethylene/α-olefin copolymers

The influences of the type and concentration of α-olefin (1-hexene, 1-octene, 1-decene, 1-octadecene, 4-methyl-1-pentene) on the mechanical behavior and crystallinity degree of some ethylene/α-olefin copolymers obtained by metallocene catalysts were studied by means of stress/strain experiments. The crystallinity degree of these copolymers has been determined by X-ray measurements. It has been observed that the copolymers show less resistance to strain as the comonomer content increases and the crystallinity decreases. Most of the studied copolymers exhibit a significant increase in the crystallinity level after the stress/strain experiments.

Porous silica derived from chitosan-containing hybrid composites

In this paper, we report the preparation by the sol-gel technique of organic-inorganic hybrid composites containing the biopolymer chitosan incorporated in a siloxane-based inorganic network. The hybrid xerogels were transformed into porous silica particles by elimination of the organic phase. Surface characteristics of the silica samples can be easily tailored. In this way Brunauer-Emmett-Teller areas, pore volume, and pore diameter of the prepared silica can be predetermined within a wide range. Morphology of the particles at longer length scales can be designed to obtain either irregularly shaped particles with layered morphology or spherical particles. The results are explained on the basis of the cationic polyelectrolytic properties of chitosan, which allows easy association with siloxane oligomers, the precursors of silica in forming hybrid nanocomposites.