Gestión ambiental de canteras de materiales para la construcción en la provincia de Matanzas, Cuba

Las canteras de materiales para la construcción al dejar de ser minadas se usan, a menudo, como rellenos "sanitarios" o basureros, generando un problema ambiental. En la provincia de Matanzas más del 75 % de las canteras se encuentran abandonadas. Ello condujo a una investigación en la que, a partir de la identificación y validación de 16 variables, y su evaluación in situ en 250 canteras, pudo obtenerse una base de datos que refleja las características de cada uno de estos espacios. La aplicación del análisis de agrupamiento (método de Ward) a los datos permitió obtener cuatro grupos de manejo ambiental. A cada grupo se le asignó posibles variantes de uso, así como propuestas de objetivos y programas para el manejo, lo cual facilitó la elaboración de un procedimiento para la rehabilitación de estos espacios degradado

Effect of crosslinker on swelling and thermodynamic properties of polyacrylamide gels

PAA networks have been prepared by crosslinking copolymerization of acry- lamide (AA) and N,N'-methylen-bis-acrylamide (BA) in water. The degree of swelling was measured in the range v2 =0-0.16 as a function of crosslinker proportion, C (%w/w) and total concentration, Cr (g/ 100 ml). The swelling of two networks with BA contents of 3 ,;; and 10% were measured as a function of temperature in the range 10-30°C. The interpretation of elastic and swelling data in terms of the scaling law lfh I =Av 2m, shows a transition from a good solvent regime to a collapsed state for the polymer chains, when the crosslinker proportion is around 5 to 7%. Results of elastic and swelling measurements were used to determine the x parameter. A different dependence of x with v2 was found for gels with BA contents below and above 5-7%. The enthalpic contribution to the interaction parameter XH was calculated from swelling experiments as a function of temperature. The results show that when the BA proportion is increased, the enthalpicFinancial support from CAICYT (SPAIN) under grant nc PA86/0370 is gratefully acknoweldged

Stress-strain behavior of polyacrylamide networks

The mechanical properties of a number of polyacrylamide gels have been studied by means of elastic measurements in elongation. Different gels were prepared by varying the proportions of both comonomers, acrylamide and N, N-methylene-bisacrylamide which is used as crosslinker, and a very broad range of concentrations was covered. The most important results considered were the elastic phantom modulus, the degree of (equilibrium) swelling and the elongation at rupture. The data were interpreted in terms of the mechanism of the gelation process taking into account a number of parameters that affect the final structure of the gels: comonomer reactivities, sequence length distributions and cycles formation. Consideration of these factors can explain properly all the features of the elastic results as well as some other properties related to the gelation process such as gel point data.This work was supported by a grant from the CAICYT (0528/84). The authors wish to thank Dr. A. Horta and Dr. J. Freire for many helpful discussions

Network defects in polyacrylamide gels

The weight conversion at the gel point (αGᵂ) was determined by the gas bubble method in two sets of polyacrylamide (PAA) gels crosslinked with N,N′-methylene-bisacrylamide (BA). αGᵂ remained practically constant when the crosslinker ratio was C = 10% (w/w) and the total concentration of comonomers in the feed ranged from CT = 5 to 10 g/100 ml. When CT was kept constant at 5 g/100 ml and C changed from 0 to 25% (w/w), αGᵂ showed a minimum at C = 7%. By comparing the experimental values of αGᵂ with those calculated for an ideal network, it was concluded that the crosslinking efficiency of BA is very low. The dependence on C of αGᵂ and other properties like the elastic phantom modulus, |ƒₚₕ*|, and the polymer volume fraction at swelling equilibrium, v₂ₛo, was explained in terms of the formation of network defects: the formation of BA sequences longer than unity and that of intramolecular cycles both have an increasing contribution at C > 7% but can be neglected at lower values of C.This work has been supported by a grant from CAICYT No. GG85-0013. The authors thank Dr A. Horta and Dr J. J. Freire for many helpful discussions

Elastic properties of highly crosslinked polyacrylamide gels

Mechanical properties of polyacrylamide gels covering a wide range of polymer concentrations have been studied. Gels were synthesized by using N,N'-methylenebis (acrylamide) as cross-linking agent whose weight percentage, with respect to the total weight of comonomers, ranged from 0.663% to 14.50%. The resulting gels were analyzed by means of their stress-strain isotherms in elongation at 30 °C. Mooney-Rivlin type plots of the data show a large increase of the modulus or upturn, particularly on gels with high percentage of cross-linking monomer and at high polymer concentrations, due to non-Gaussian effects arising from the very heterogeneous molecular network structure. The elastic modulus was found to increase exponentially with total comonomer concentration, keeping constant the percentage of bisacrylamide comonomer. On the other hand, the modulus passes through a maximum as the amount of cross-linking agent is increased. The ultimate properties found and the comparison of the cross-linking densities obtained from the elastic results with the theoretical ones, determined from the initial comonomer compositions, confirm the very high heterogeneity of polyacrylamide gels.We thank Dr. P. Deporcellinis for his kind cooperation. This work was supported in part by Grant 0528/84 from the Comision Asesora de Investigación Científica y Técnica. Registry No. (Acrylamide) (N,N'-methylenebisacrylamide) (copolymer), 25034-58-6

Polyacrylamide gels. Process of network formation

This paper refers to the crosslinking copolymerization of acrylamide (AA or monomer-1) and N,N′-methylene-bisacrylamide (BA or monomer-2) in aqueous solution at 22°, covering a broad range of comonomer concentrations. The extent of reaction, pT, the composition of the remaining comonomer mixture, f₂, and the copolymer composition, F₂, have been determined by high resolution 1H-NMR with polymerization in situ. The non-linear regression fit of f₂ as a function of pT yields almost the same values for the reactivity ratios for any of the studied comonomers feeds. The instant copolymer composition changes with the copolymerization time due to a shift in the residual comonomer composition and, to a smaller extent, to changes in the comonomer relative reactivities. This effect contributes to the heterogeneity of the final network.Financial support from CAICYT (Spain) under Grants No. GG85/0013 and PA86/0370 is gratefully acknowledged. We are also indebted to Dr R. Gonzalez Rubio for his computer assistance, and to Dr J. Guzman for sorne interesting comments

Polyacrylamide networks. Kinetic and structural studies by high field H-NMR with polymerization in situ

The crosslinking polymerization of acrylamide (AA) and N,N′-methylene-bis-acrylamide (BA) in aqueous solution at 22°C has been studied by H.R. H-NMR spectroscopy. The initial comonomer mixture was placed inside the NMR spectrometer probe head and the polymerization was followed for 10 hr. This procedure allows measurement of the instant composition of the residual comonomer mixture even during and after gelation and therefore to calculate the composition of the formed copolymer or network. When the reaction was finished, the structure of the most mobile part of the network was also analyzed

Polyacrylamide networks. Sequence distribution of crosslinker

The crosslinking copolymerization of acrylamide (AA) and N,N′-methylene-bis-acrylamide (BA) has been studied by high resolution ¹H-NMR with copolymerization in situ. This procedure allows calculation of the copolymer composition at zero degree of conversion and as a function of the polymerization time. Monomer reactivity ratios were calculated by the Kelen-Tüdös method in the pre-gel state and during gelation. Sequence distributions of both comonomers were then characterized.Financial support from CAICYT (Spain) under Grant No. 0528/84 is gratefully acknowledged