Jose Luis Alessandrini | Universidad Nacional de La Plata (original) (raw)

Papers by Jose Luis Alessandrini

The electrostatic component of the solvation free energy of polyelectrolytes in the description o... more The electrostatic component of the solvation free energy of polyelectrolytes in the description of dilute polymer solutions through the introduction of a dielectric dependent pearl-necklace model is taken into account. In this two-dielectric model, the solvent is assumed to have a high dielec-tric constant while the pearls are modeled as charged spheres with low dielectric constant (e in). Generalized Born (GB) models of electrostatic solvation give approximate solutions to the Poisson electrostatic problem through the Born radii of the pearls. Explicit calculations of the mean dimensions of the molecule, as a function of the solvation free energy, are performed on a minimal polyelectrolyte model (MPM), consisting of three identical beads with a single degree of freedom—the bond angle. Born radii calculated from the GB-Z 6 model (based on Kirkwood electrostatics) and from the GB-Z 4 model (based on the Coulomb Field Approximation) are compared with " perfect " radii (calculated from the Green's function of finite difference Poisson equation (PE)) in a wide range of molecular conformations. The best agreement is obtained with the first GB model.

A new approach to thermal decomposition of organic iron precursors is reported, which results in ... more A new approach to thermal decomposition of organic iron precursors is reported, which results in a simpler and more economical method to produce well crystallized γ -Fe 2 O 3 nanoparticles (NPs) with average sizes within the 3-17 nm range. The NPs were characterized by TEM, SAED, XRD, DLS-QELS, Mössbauer spectroscopy at different temperatures, FT-IR and magnetic measurements. The obtained γ -Fe 2 O 3 NPs are coated with oleic acid and, in a lower quantity, with oleylamine (about 1.5 nm in thickness). It was shown that changing operative variables allows us to tune the average particle diameters and obtain a very narrow or monodisperse distribution of sizes. The γ -Fe 2 O 3 NPs behave superparamagnetically at room temperature and their magnetization saturation is reduced by about 34% in comparison with bulk maghemite. The results indicate that the distance between two neighbour NPs, generated by the coating, of about 3 nm is insufficient to inhibit interparticle magnetic interactions when the average diameter is 8.8 nm. The good quality of the NPs, obtained through the present low-cost and easy-handling process, open a new perspective for future technological applications.

Magnetite (Fe 3 O 4 ) nanoparticles are proper materials for Magnetic Fluid Hyperthermia applicat... more Magnetite (Fe 3 O 4 ) nanoparticles are proper materials for Magnetic Fluid Hyperthermia applications whenever these conjugate stability at physiological (neutral pH) medium and high specific dissipation power. Here, magnetite nanoparticles 9−12 nm in size, electrostatically stabilized by citric acid coating, with hydrodynamic sizes in the range 17−30 nm, and well dispersed in aqueous solution were prepared using a chemical route. The influence of media acidity during the adsorption of citric acid (CA) on the suspension's long-term stability was systematically investigated. The highest content of nanoparticles in a stable suspension at neutral pH is obtained for coating performed at pH = 4.58, corresponding to the larger amount of CA molecules adsorbed by one carboxylate link. Specific absorption rates (SARs) of various magnetite colloids, determined calorimetrically at a radio frequency field of 265 kHz and field amplitude of 40.1 kA/m, are analyzed in terms of structural and magnetic colloid properties. Larger dipolar interactions lead to larger Neél relaxation times, in some cases larger than Brown relaxation times, which in the present case enhanced magnetic radio frequency heating. The improvement of suspension stability results in a decrease of SAR values, and this decrease is even large in comparison with uncoated magnetite nanoparticles. This fact is related to interactions between particles. a CS i (i = 1−6) labeled colloids synthesized by one-step coprecipitation at 60°C, followed by citric acid adsorption at pH ads medium acidity and finally suspended at pH susp . [X] stands for concentration as mass of magnetite per solution volume; typical error is ±0.2. Data derived from SQUID-DC magnetometry: M s stands for specific saturation magnetization, d m for magnetic core diameter, σ dm for standard deviation, and δ for twice the magnetically frustrated layer thickness. d TEM stands for nanoparticle diameter retrieved from TEM images, d H for weight (averaged nanoparticle hydrodynamic diameter), T Bi for blocking temperatures determined from ZFC curve maximum, T B for blocking temperature for same size noninteracting nanoparticles, and SAR for Specific Absorption Rate at 40.1 kA/m and 265 kHz in Watts per gram of magnetite.

Free radical polymerization of dialkyl fumarates (R:isopropyl, cyclohexyl, 2-ethylhexyl, 2-phenyl... more Free radical polymerization of dialkyl fumarates (R:isopropyl, cyclohexyl, 2-ethylhexyl, 2-phenylethyl) under microwave irradiation was investigated. The polymerizations were carried out at different powers of irradiation and initiator concentrations (benzoyl peroxide, BP) and the effect of the monomer structure on the conversion, average molecular weights and the polydispersity index (M w /M n ) was analyzed. A significant enhancement of the rates of polymerization was found, as compared with those obtained under thermal conditions. r

b-Chitin from Illex argentinus squid pens was isolated by using chemical methods and the pens com... more b-Chitin from Illex argentinus squid pens was isolated by using chemical methods and the pens composition determined. The structural characteristics of b-chitin were identified with infrared (IR), solid-state cross-polarization/magic-angle-spinning (CP-MAS) 13 C NMR spectroscopy and X-ray analysis. The viscosity average molecular weight M v , calculated from the intrinsic viscosity was above 2 Â 10 6 g mol À1 . The stiffness of the macromolecule was analyzed using the Yamakawa and Fujii theory for wormlike cylinders.

Microwave assisted free radical copolymerization of diisopropyl fumarate (DIPF) and benzyl acryla... more Microwave assisted free radical copolymerization of diisopropyl fumarate (DIPF) and benzyl acrylate (BzA) with different copolymer compositions was performed using benzoyl peroxide as initiator. The effect of the reaction conditions on the macromolecular characteristics, monomer reactivity ratio and copolymer properties were studied. The monomer conversion and average molecular weights increase with the content of BzA units in the copolymer. The copolymers were characterized by IR, 1 H and 13 C-NMR spectroscopies and the molecular weights were analyzed with size exclusion chromatography (SEC). The reactivity ratios obtained from an extended Kelen-Tüdös method under microwave irradiations are a factor which is double than those obtained by thermal copolymerization. The product r 1 r 2 = 0.152 suggests a preference of both propagating macroradicals toward consecutive homopolymerization. The hydrodynamic and polydispersity size were measured in ethylacetate, tetrahydrofuran and methylethylketone with a quasi-elastic light scattering (QELS) technique showing that the quality of the solvents increases in the order: EA < THF < MEK.

Simulated data from static light scattering produced by several particle size distributions (PSD)... more Simulated data from static light scattering produced by several particle size distributions (PSD) of spherical particles in dilute solution is analyzed with a regularized non-negative least squares method (r-NNLS). Strong fluctuations in broad PSD's obtained from direct application of NNLS are supressed through an averaging procedure, as introduced long ago in the inversion problem in dynamic light scattering.

Journal of Sound and Vibration, 1999

The classic problem of the transverse vibrations of a string is of basic and technological intere... more The classic problem of the transverse vibrations of a string is of basic and technological interest, since it constitutes an acceptable model for the dynamic behavior of oceanographic cables and musical instruments among others . One of the important points in this area is the propagation of waves in systems with interferences or in mediums with different propagation speed. However, the literature shows, in general, very little information on the main aspects of stationary waves in these types of systems. In this work we studied the behavior of the transversal stationary waves in a mechanical system composed of three strings connected to a ramification point ( . First, the spectrum of natural frequencies of the system for a completely general case is presented. Then two particularly simple situations are studied: the case of a string with three identical branches and that of a string with two equal branches and the third of different length. Results obtained for a string with branches of different chain lengths are similar to those for a linear string of fixed ends in some particular situations.

Journal of Polymer Science Part B-polymer Physics, 1999

A model to describe the dynamics of networks with linear pendant chains has been formulated based... more A model to describe the dynamics of networks with linear pendant chains has been formulated based on the properties of ensembles of micronetworks, using the Rouse model. 1 This development indicates that the terminal relaxation time of pendant chains with relatively large molecular weight scales with the square of the molecular weight of those chains. On the other hand, when the molecular weight of pendant and elastically active chains are comparable, a nearly exponential growth of the terminal relaxation time with the molecular weight is predicted. The main predictions of the model are compared with experimental results of model poly(dimethyl siloxane) (PDMS) networks, with controlled amounts of linear pendant chains of known molecular weight. The terminal relaxation time of these networks was estimated from the values of the loss modulus GЉ() measured experimentally. An exponential dependence on the molecular weight of pendant chains was derived for the terminal relaxation time. This behavior is in good agreement with the predictions of our model for micronetworks, provided that the friction coefficient scales linearly with the number of entanglements.

A model to describe the dynamics of networks with linear pendant chains has been formulated based... more A model to describe the dynamics of networks with linear pendant chains has been formulated based on the properties of ensembles of micronetworks, using the Rouse model. 1 This development indicates that the terminal relaxation time of pendant chains with relatively large molecular weight scales with the square of the molecular weight of those chains. On the other hand, when the molecular weight of pendant and elastically active chains are comparable, a nearly exponential growth of the terminal relaxation time with the molecular weight is predicted. The main predictions of the model are compared with experimental results of model poly(dimethyl siloxane) (PDMS) networks, with controlled amounts of linear pendant chains of known molecular weight. The terminal relaxation time of these networks was estimated from the values of the loss modulus GЉ() measured experimentally. An exponential dependence on the molecular weight of pendant chains was derived for the terminal relaxation time. This behavior is in good agreement with the predictions of our model for micronetworks, provided that the friction coefficient scales linearly with the number of entanglements.

Journal of Polymer Science Part B-polymer Physics, 1999

A model to describe the dynamics of networks with linear pendant chains has been formulated based... more A model to describe the dynamics of networks with linear pendant chains has been formulated based on the properties of ensembles of micronetworks, using the Rouse model. 1 This development indicates that the terminal relaxation time of pendant chains with relatively large molecular weight scales with the square of the molecular weight of those chains. On the other hand, when the molecular weight of pendant and elastically active chains are comparable, a nearly exponential growth of the terminal relaxation time with the molecular weight is predicted. The main predictions of the model are compared with experimental results of model poly(dimethyl siloxane) (PDMS) networks, with controlled amounts of linear pendant chains of known molecular weight. The terminal relaxation time of these networks was estimated from the values of the loss modulus GЉ() measured experimentally. An exponential dependence on the molecular weight of pendant chains was derived for the terminal relaxation time. This behavior is in good agreement with the predictions of our model for micronetworks, provided that the friction coefficient scales linearly with the number of entanglements.

Journal of Sound and Vibration, 1999

The classic problem of the transverse vibrations of a string is of basic and technological intere... more The classic problem of the transverse vibrations of a string is of basic and technological interest, since it constitutes an acceptable model for the dynamic behavior of oceanographic cables and musical instruments among others . One of the important points in this area is the propagation of waves in systems with interferences or in mediums with different propagation speed. However, the literature shows, in general, very little information on the main aspects of stationary waves in these types of systems. In this work we studied the behavior of the transversal stationary waves in a mechanical system composed of three strings connected to a ramification point ( . First, the spectrum of natural frequencies of the system for a completely general case is presented. Then two particularly simple situations are studied: the case of a string with three identical branches and that of a string with two equal branches and the third of different length. Results obtained for a string with branches of different chain lengths are similar to those for a linear string of fixed ends in some particular situations.

The present works attempts to explain the viscoelastic behavior of model silicone networks contai... more The present works attempts to explain the viscoelastic behavior of model silicone networks containing well defined amounts of pendant chains of uniform molecular weight. A discrete spectrum of relaxation times is calculated. It is divided in three parts, corresponding to three different mechanisms: a Rouse relaxation type at short times, a modification of the Rouse spectrum for the relaxation of the tube contour, and a reptation form for terminal times. The proposed model requires some information on the molecular structure. The spectrum of relaxation times is used to obtain an adequate constitutive equation in order to predicted the theoretical viscoelastic moduli. Those values were then compared with the experimental viscoelastic measurements obtained from model polydimethylsiloxane networks synthesized in our laboratories.

Macromolecules, 2001

ABSTRACT Stress relaxation of model poly(dimethylsiloxane) networks with pendant chains and nearl... more ABSTRACT Stress relaxation of model poly(dimethylsiloxane) networks with pendant chains and nearly constant cross-linking density was studied. The networks were obtained by end-linking a mixture of long chains bearing terminal reactive groups with a trifunctional cross-linker. Long poly(dimethylsiloxane) chains in the initial mixture contain 90 wt % of difunctional molecules with reactive groups at both ends and 10 wt % of monofunctional chains with a single reactive terminal group located in one of their ends. Difunctional chains will be mainly elastically active chains after cross-linking while monofunctional chains will remain as long pendant molecules. The fitting of the experimental stress relaxation values to the classic Chasset−Thirion equation shows a strong dependence of the exponent on the molecular mass distribution of the pendant chains. From these results, a new model for the terminal relaxation of elastomers is proposed, taking into account the molecular mass distribution of pendant chains. The dynamics of polymer networks in the terminal relaxation zone is modeled considering the reptation theory. In the terminal zone, the proposed model behaves similarly to the Chasset−Thirion equation. The model adequately describes the behavior of networks synthesized by end-linking. On the other hand, when applied to different networks, it leads to similar conclusions as previous theories developed for networks obtained by random cross-linking.

Polymer Degradation and Stability, 2004

... Viscosity ageing indexes, calculated considering the changes in viscosity at 100 °C between P... more ... Viscosity ageing indexes, calculated considering the changes in viscosity at 100 °C between PAV and unaged states, increase in the order: AM2 (2.2), AM1 (2.9 ... Hence, the modifications of the rheological properties of asphalts during their ageing processes depend ...

Journal of Biomaterials Science-polymer Edition, 2001

A delivery system for vanadium was developed using poly(beta-propiolactone) (PbetaPL) films. The ... more A delivery system for vanadium was developed using poly(beta-propiolactone) (PbetaPL) films. The release kinetics of a complex of vanadium (IV) with aspirin (VOAspi) was evaluated with films prepared from polymers of different molecular weights, as well as with variable drug load. A sustained release of vanadium over 7 days was achieved. The drug release kinetics depends on contributions from two factors: (a) diffusion of the drug; and (b) erosion of the PbetaPL film. The experimental data at an early stage of release were fitted with a diffusion model, which allowed determination of the diffusion coefficient of the drug. VOAspi does not show strong interaction with the polymer, as demonstrated by the low apparent partition coefficient (approximately 10(-2)). UMR106 osteosarcoma cells were used as a model to evaluate the anticarcinogenic effects of the VOAspi released from the PbetaPPL film. VOAspi-PbetaPL film inhibited cell proliferation in a dose-response manner and induced formation of approximately half of the thiobarbituric acid reactive substances (TBARS), an index of lipid peroxidation. compared to that with free VOAspi in solution. The unloaded PbetaPL film did not generate cytotoxicity, as evaluated by cell growth and TBARS. Thus, the polymer-embedded VOAspi retained the antiproliferative effects showing lower cytotoxicity than the free drug. Results with VOAspi-PbetaPL films suggest that this delivery system may have promising biomedical and therapeutic applications.

The electrostatic component of the solvation free energy of polyelectrolytes in the description o... more The electrostatic component of the solvation free energy of polyelectrolytes in the description of dilute polymer solutions through the introduction of a dielectric dependent pearl-necklace model is taken into account. In this two-dielectric model, the solvent is assumed to have a high dielec-tric constant while the pearls are modeled as charged spheres with low dielectric constant (e in). Generalized Born (GB) models of electrostatic solvation give approximate solutions to the Poisson electrostatic problem through the Born radii of the pearls. Explicit calculations of the mean dimensions of the molecule, as a function of the solvation free energy, are performed on a minimal polyelectrolyte model (MPM), consisting of three identical beads with a single degree of freedom—the bond angle. Born radii calculated from the GB-Z 6 model (based on Kirkwood electrostatics) and from the GB-Z 4 model (based on the Coulomb Field Approximation) are compared with " perfect " radii (calculated from the Green's function of finite difference Poisson equation (PE)) in a wide range of molecular conformations. The best agreement is obtained with the first GB model.

A new approach to thermal decomposition of organic iron precursors is reported, which results in ... more A new approach to thermal decomposition of organic iron precursors is reported, which results in a simpler and more economical method to produce well crystallized γ -Fe 2 O 3 nanoparticles (NPs) with average sizes within the 3-17 nm range. The NPs were characterized by TEM, SAED, XRD, DLS-QELS, Mössbauer spectroscopy at different temperatures, FT-IR and magnetic measurements. The obtained γ -Fe 2 O 3 NPs are coated with oleic acid and, in a lower quantity, with oleylamine (about 1.5 nm in thickness). It was shown that changing operative variables allows us to tune the average particle diameters and obtain a very narrow or monodisperse distribution of sizes. The γ -Fe 2 O 3 NPs behave superparamagnetically at room temperature and their magnetization saturation is reduced by about 34% in comparison with bulk maghemite. The results indicate that the distance between two neighbour NPs, generated by the coating, of about 3 nm is insufficient to inhibit interparticle magnetic interactions when the average diameter is 8.8 nm. The good quality of the NPs, obtained through the present low-cost and easy-handling process, open a new perspective for future technological applications.

Magnetite (Fe 3 O 4 ) nanoparticles are proper materials for Magnetic Fluid Hyperthermia applicat... more Magnetite (Fe 3 O 4 ) nanoparticles are proper materials for Magnetic Fluid Hyperthermia applications whenever these conjugate stability at physiological (neutral pH) medium and high specific dissipation power. Here, magnetite nanoparticles 9−12 nm in size, electrostatically stabilized by citric acid coating, with hydrodynamic sizes in the range 17−30 nm, and well dispersed in aqueous solution were prepared using a chemical route. The influence of media acidity during the adsorption of citric acid (CA) on the suspension's long-term stability was systematically investigated. The highest content of nanoparticles in a stable suspension at neutral pH is obtained for coating performed at pH = 4.58, corresponding to the larger amount of CA molecules adsorbed by one carboxylate link. Specific absorption rates (SARs) of various magnetite colloids, determined calorimetrically at a radio frequency field of 265 kHz and field amplitude of 40.1 kA/m, are analyzed in terms of structural and magnetic colloid properties. Larger dipolar interactions lead to larger Neél relaxation times, in some cases larger than Brown relaxation times, which in the present case enhanced magnetic radio frequency heating. The improvement of suspension stability results in a decrease of SAR values, and this decrease is even large in comparison with uncoated magnetite nanoparticles. This fact is related to interactions between particles. a CS i (i = 1−6) labeled colloids synthesized by one-step coprecipitation at 60°C, followed by citric acid adsorption at pH ads medium acidity and finally suspended at pH susp . [X] stands for concentration as mass of magnetite per solution volume; typical error is ±0.2. Data derived from SQUID-DC magnetometry: M s stands for specific saturation magnetization, d m for magnetic core diameter, σ dm for standard deviation, and δ for twice the magnetically frustrated layer thickness. d TEM stands for nanoparticle diameter retrieved from TEM images, d H for weight (averaged nanoparticle hydrodynamic diameter), T Bi for blocking temperatures determined from ZFC curve maximum, T B for blocking temperature for same size noninteracting nanoparticles, and SAR for Specific Absorption Rate at 40.1 kA/m and 265 kHz in Watts per gram of magnetite.

Free radical polymerization of dialkyl fumarates (R:isopropyl, cyclohexyl, 2-ethylhexyl, 2-phenyl... more Free radical polymerization of dialkyl fumarates (R:isopropyl, cyclohexyl, 2-ethylhexyl, 2-phenylethyl) under microwave irradiation was investigated. The polymerizations were carried out at different powers of irradiation and initiator concentrations (benzoyl peroxide, BP) and the effect of the monomer structure on the conversion, average molecular weights and the polydispersity index (M w /M n ) was analyzed. A significant enhancement of the rates of polymerization was found, as compared with those obtained under thermal conditions. r

b-Chitin from Illex argentinus squid pens was isolated by using chemical methods and the pens com... more b-Chitin from Illex argentinus squid pens was isolated by using chemical methods and the pens composition determined. The structural characteristics of b-chitin were identified with infrared (IR), solid-state cross-polarization/magic-angle-spinning (CP-MAS) 13 C NMR spectroscopy and X-ray analysis. The viscosity average molecular weight M v , calculated from the intrinsic viscosity was above 2 Â 10 6 g mol À1 . The stiffness of the macromolecule was analyzed using the Yamakawa and Fujii theory for wormlike cylinders.

Microwave assisted free radical copolymerization of diisopropyl fumarate (DIPF) and benzyl acryla... more Microwave assisted free radical copolymerization of diisopropyl fumarate (DIPF) and benzyl acrylate (BzA) with different copolymer compositions was performed using benzoyl peroxide as initiator. The effect of the reaction conditions on the macromolecular characteristics, monomer reactivity ratio and copolymer properties were studied. The monomer conversion and average molecular weights increase with the content of BzA units in the copolymer. The copolymers were characterized by IR, 1 H and 13 C-NMR spectroscopies and the molecular weights were analyzed with size exclusion chromatography (SEC). The reactivity ratios obtained from an extended Kelen-Tüdös method under microwave irradiations are a factor which is double than those obtained by thermal copolymerization. The product r 1 r 2 = 0.152 suggests a preference of both propagating macroradicals toward consecutive homopolymerization. The hydrodynamic and polydispersity size were measured in ethylacetate, tetrahydrofuran and methylethylketone with a quasi-elastic light scattering (QELS) technique showing that the quality of the solvents increases in the order: EA < THF < MEK.

Simulated data from static light scattering produced by several particle size distributions (PSD)... more Simulated data from static light scattering produced by several particle size distributions (PSD) of spherical particles in dilute solution is analyzed with a regularized non-negative least squares method (r-NNLS). Strong fluctuations in broad PSD's obtained from direct application of NNLS are supressed through an averaging procedure, as introduced long ago in the inversion problem in dynamic light scattering.

Journal of Sound and Vibration, 1999

The classic problem of the transverse vibrations of a string is of basic and technological intere... more The classic problem of the transverse vibrations of a string is of basic and technological interest, since it constitutes an acceptable model for the dynamic behavior of oceanographic cables and musical instruments among others . One of the important points in this area is the propagation of waves in systems with interferences or in mediums with different propagation speed. However, the literature shows, in general, very little information on the main aspects of stationary waves in these types of systems. In this work we studied the behavior of the transversal stationary waves in a mechanical system composed of three strings connected to a ramification point ( . First, the spectrum of natural frequencies of the system for a completely general case is presented. Then two particularly simple situations are studied: the case of a string with three identical branches and that of a string with two equal branches and the third of different length. Results obtained for a string with branches of different chain lengths are similar to those for a linear string of fixed ends in some particular situations.

Journal of Polymer Science Part B-polymer Physics, 1999

A model to describe the dynamics of networks with linear pendant chains has been formulated based... more A model to describe the dynamics of networks with linear pendant chains has been formulated based on the properties of ensembles of micronetworks, using the Rouse model. 1 This development indicates that the terminal relaxation time of pendant chains with relatively large molecular weight scales with the square of the molecular weight of those chains. On the other hand, when the molecular weight of pendant and elastically active chains are comparable, a nearly exponential growth of the terminal relaxation time with the molecular weight is predicted. The main predictions of the model are compared with experimental results of model poly(dimethyl siloxane) (PDMS) networks, with controlled amounts of linear pendant chains of known molecular weight. The terminal relaxation time of these networks was estimated from the values of the loss modulus GЉ() measured experimentally. An exponential dependence on the molecular weight of pendant chains was derived for the terminal relaxation time. This behavior is in good agreement with the predictions of our model for micronetworks, provided that the friction coefficient scales linearly with the number of entanglements.

A model to describe the dynamics of networks with linear pendant chains has been formulated based... more A model to describe the dynamics of networks with linear pendant chains has been formulated based on the properties of ensembles of micronetworks, using the Rouse model. 1 This development indicates that the terminal relaxation time of pendant chains with relatively large molecular weight scales with the square of the molecular weight of those chains. On the other hand, when the molecular weight of pendant and elastically active chains are comparable, a nearly exponential growth of the terminal relaxation time with the molecular weight is predicted. The main predictions of the model are compared with experimental results of model poly(dimethyl siloxane) (PDMS) networks, with controlled amounts of linear pendant chains of known molecular weight. The terminal relaxation time of these networks was estimated from the values of the loss modulus GЉ() measured experimentally. An exponential dependence on the molecular weight of pendant chains was derived for the terminal relaxation time. This behavior is in good agreement with the predictions of our model for micronetworks, provided that the friction coefficient scales linearly with the number of entanglements.

Journal of Polymer Science Part B-polymer Physics, 1999

A model to describe the dynamics of networks with linear pendant chains has been formulated based... more A model to describe the dynamics of networks with linear pendant chains has been formulated based on the properties of ensembles of micronetworks, using the Rouse model. 1 This development indicates that the terminal relaxation time of pendant chains with relatively large molecular weight scales with the square of the molecular weight of those chains. On the other hand, when the molecular weight of pendant and elastically active chains are comparable, a nearly exponential growth of the terminal relaxation time with the molecular weight is predicted. The main predictions of the model are compared with experimental results of model poly(dimethyl siloxane) (PDMS) networks, with controlled amounts of linear pendant chains of known molecular weight. The terminal relaxation time of these networks was estimated from the values of the loss modulus GЉ() measured experimentally. An exponential dependence on the molecular weight of pendant chains was derived for the terminal relaxation time. This behavior is in good agreement with the predictions of our model for micronetworks, provided that the friction coefficient scales linearly with the number of entanglements.

Journal of Sound and Vibration, 1999

The classic problem of the transverse vibrations of a string is of basic and technological intere... more The classic problem of the transverse vibrations of a string is of basic and technological interest, since it constitutes an acceptable model for the dynamic behavior of oceanographic cables and musical instruments among others . One of the important points in this area is the propagation of waves in systems with interferences or in mediums with different propagation speed. However, the literature shows, in general, very little information on the main aspects of stationary waves in these types of systems. In this work we studied the behavior of the transversal stationary waves in a mechanical system composed of three strings connected to a ramification point ( . First, the spectrum of natural frequencies of the system for a completely general case is presented. Then two particularly simple situations are studied: the case of a string with three identical branches and that of a string with two equal branches and the third of different length. Results obtained for a string with branches of different chain lengths are similar to those for a linear string of fixed ends in some particular situations.

The present works attempts to explain the viscoelastic behavior of model silicone networks contai... more The present works attempts to explain the viscoelastic behavior of model silicone networks containing well defined amounts of pendant chains of uniform molecular weight. A discrete spectrum of relaxation times is calculated. It is divided in three parts, corresponding to three different mechanisms: a Rouse relaxation type at short times, a modification of the Rouse spectrum for the relaxation of the tube contour, and a reptation form for terminal times. The proposed model requires some information on the molecular structure. The spectrum of relaxation times is used to obtain an adequate constitutive equation in order to predicted the theoretical viscoelastic moduli. Those values were then compared with the experimental viscoelastic measurements obtained from model polydimethylsiloxane networks synthesized in our laboratories.

Macromolecules, 2001

ABSTRACT Stress relaxation of model poly(dimethylsiloxane) networks with pendant chains and nearl... more ABSTRACT Stress relaxation of model poly(dimethylsiloxane) networks with pendant chains and nearly constant cross-linking density was studied. The networks were obtained by end-linking a mixture of long chains bearing terminal reactive groups with a trifunctional cross-linker. Long poly(dimethylsiloxane) chains in the initial mixture contain 90 wt % of difunctional molecules with reactive groups at both ends and 10 wt % of monofunctional chains with a single reactive terminal group located in one of their ends. Difunctional chains will be mainly elastically active chains after cross-linking while monofunctional chains will remain as long pendant molecules. The fitting of the experimental stress relaxation values to the classic Chasset−Thirion equation shows a strong dependence of the exponent on the molecular mass distribution of the pendant chains. From these results, a new model for the terminal relaxation of elastomers is proposed, taking into account the molecular mass distribution of pendant chains. The dynamics of polymer networks in the terminal relaxation zone is modeled considering the reptation theory. In the terminal zone, the proposed model behaves similarly to the Chasset−Thirion equation. The model adequately describes the behavior of networks synthesized by end-linking. On the other hand, when applied to different networks, it leads to similar conclusions as previous theories developed for networks obtained by random cross-linking.

Polymer Degradation and Stability, 2004

... Viscosity ageing indexes, calculated considering the changes in viscosity at 100 °C between P... more ... Viscosity ageing indexes, calculated considering the changes in viscosity at 100 °C between PAV and unaged states, increase in the order: AM2 (2.2), AM1 (2.9 ... Hence, the modifications of the rheological properties of asphalts during their ageing processes depend ...

Journal of Biomaterials Science-polymer Edition, 2001

A delivery system for vanadium was developed using poly(beta-propiolactone) (PbetaPL) films. The ... more A delivery system for vanadium was developed using poly(beta-propiolactone) (PbetaPL) films. The release kinetics of a complex of vanadium (IV) with aspirin (VOAspi) was evaluated with films prepared from polymers of different molecular weights, as well as with variable drug load. A sustained release of vanadium over 7 days was achieved. The drug release kinetics depends on contributions from two factors: (a) diffusion of the drug; and (b) erosion of the PbetaPL film. The experimental data at an early stage of release were fitted with a diffusion model, which allowed determination of the diffusion coefficient of the drug. VOAspi does not show strong interaction with the polymer, as demonstrated by the low apparent partition coefficient (approximately 10(-2)). UMR106 osteosarcoma cells were used as a model to evaluate the anticarcinogenic effects of the VOAspi released from the PbetaPPL film. VOAspi-PbetaPL film inhibited cell proliferation in a dose-response manner and induced formation of approximately half of the thiobarbituric acid reactive substances (TBARS), an index of lipid peroxidation. compared to that with free VOAspi in solution. The unloaded PbetaPL film did not generate cytotoxicity, as evaluated by cell growth and TBARS. Thus, the polymer-embedded VOAspi retained the antiproliferative effects showing lower cytotoxicity than the free drug. Results with VOAspi-PbetaPL films suggest that this delivery system may have promising biomedical and therapeutic applications.