Summary: In this paper the chemical structure of an acrylamide‐N,N‐dihexylacrylamide copolymer was established by IR and NMR. Static and dynamic light scattering in formamide were used in order to evaluate the polymer structural parameters, such as weight‐average molecular weight (Mw), second virial coefficient (A2), radius of gyration (RG), the form factor P(q) and the hydrodynamic radius (RH). Additionally to the classical characterization, those results indicated the presence of aggregation, showing that formamide is not a very good solvent, as stated in earlier investigations. The rheological behavior in aqueous solutions was evaluated as a function of the salt concentration. The solutions presented an important viscosity increase in the presence of NaCl and did not show any sensitivity to the presence of CaCl2. This result is in favor of the oil recovery especially in high salinity reservoirs.

Recebido em 25/10/06; aceito em 24/8/07; publicado na web em 19/12/07 EVALUATION OF GELS OBTAINED FROM ACETYLATION OF CHITOSAN IN HETEROGENEOUS MEDIUM. Chitosan was acetylated during 2, 5 and 10h and physical gels were obtained at different polymer concentrations in N,N-dimethylacetamide containing 5% of LiCl. Acetylation was confirmed by infrared spectroscopy and 13 C NMR, and degrees of acetylation in the range of 0.82-0.91 were determined by NMR. The O-acetylation degree (0.12-0.15) was exclusively determined by a volumetric method. Rheological studies showed that the storage modulus values were smaller for the more acetylated samples and increased with the temperature and the polymer concentration. All the gels presented storage modulus superior to loss modulus, evidencing more elastic than viscous characteristics. The results obtained in this work suggest a gelation process based on a balance between O and N-acetylation and intermolecular bonds.Keywords: chitosan; gel; rheology. INTRODUÇÃOEm geral, os polissacarídeos são biocompatíveis, biodegradáveis e abundantes, e apresentam numerosas aplicações comerciais devido a sua ampla variedade de propriedades funcionais, que são efetivas em níveis muito baixos de concentração 1 . Além disso, a presença de vários grupos funcionais reativos ao longo da cadeia polimérica favorece interações com outras espécies moleculares, tanto dissolvidas quanto dispersas, podendo ocorrer ligação, quelação, complexação, microencapsulação, emulsificação, floculação, estabilização ou suspensão. Os glicanos também podem atuar como adsorventes, carreadores, suportes hidrofílicos, agentes trocadores de íons, formadores de filmes e de géis 2 . A quitosana 3,4 é um polissacarídeo encontrado em alguns microorganismos, mas que usualmente é obtido a partir da desacetilação da quitina em meio alcalino. Sua estrutura química é constituída de unidades de 2-amino-2-desoxi-D-glicopiranose e 2-acetamida-2-desoxi-D-glicopiranose interligadas por ligações glicosídicas β-(1→4). Em função da sua biodegradabilidade e biocompatibilidade, esse polímero tem sido estudado extensivamente como excipiente em várias formulações farmacêuticas, sob a forma de comprimidos, micro e nanopartículas, microesferas, granulados, lipossomas, géis e hidrogéis [5][6][7][8][9] . Esses produtos aplicados pelas vias parenteral, transdérmica, oral, vaginal, nasal, ocular, além da terapia gênica, tornam a quitosana um polímero versá-til no desenvolvimento de sistemas de liberação controlada de fármacos 10,11 , sendo que o resultado final depende do comprimento da cadeia, densidade de carga e distribuição dessas cargas no polissacarídeo 7 . Foi sugerido, inclusive, que grande parte dessas aplicações utilizando a quitosana se deve ao seu mecanismo de ação, que seria uma combinação de mucoadesão e um efeito sobre a zonula occludens ("tight junctions") intercelular, levando a um aumento da permeabilidade a diversos fármacos 7 , o que possibilita à quitosana atuar como cicatrizante e gastroprotetor. A zonula occludens, também conhe...

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The literature presents different methods for quantifying and characterizing partially hydrolyzed polyacrylamide (HPAM) in solution. However, only a few of these are suitable for determining the critical overlap concentration (C \ ). The evaluation of C \ is important for describing the transition from the dilute to the semidilute regime, that is, when the solution depicts a characteristic viscosity at concentrations above C \ . This article describes the determination of C \ for HPAM in solution by potentiometry. The molecular weight, degree of hydrolysis, and polymerization degree are determined by mathematical manipulation of the constant of formation of aggregates, which is based on the law of mass action. The potentiometric curves were initially linear; asymptotic behavior followed. The inflection point was determined by the intersection, and the resulting equation of mathematical development statistically satisfied the experimental data and described the number of moles of monomers, the equilibrium constant for the formation of the aggregate, C \ , and the acidity constant of the polymer. The results for C \ , the degree of copolymerization, and the molecular weight proved that this method is a good alternative for the characterization of polymers with ionizable monomers and that are soluble in water.

In this study, seed productivity and oil content of three safflower genotypes (IMAMT 1750, IMAMT 946 and IMAMT 894) cultivated in the Brazilian semi‐arid region were evaluated to identify which genotype has the best in‐field performance. The acidity and saponification index were evaluated, as well as the efficiency of lipid extraction through thermal analysis, aiming to estimate the species' potential for biofuel production in semi‐arid regions. Field cultivation under semi‐arid conditions lasted for ~75 days and no herbivory were observed on the plants. The IMAMT 894 genotype presented higher seed yield than the other genotypes, IMAMT 1750 presented 34% oil, and IMAMT 946 and IMAMT 894 presented 32% oil with efficient extraction in the Soxhlet system and high oil content and quality; higher than traditionally used crops such as soybean and cotton. The IMAMT 894 genotype showed higher seed productivity, but all three genotypes presented good oil yield and quality for biodiesel production. Oil extraction in the Soxhlet system was efficient since the thermogravimetric analysis showed no oil in the residual biomass after oil extraction.