Linear low-density polyethylenes (LLDPE) are a class of polyethylenes with linear chains containing only short chain branches due to the insertion of α-olefin units during the copolymerization with ethene. The α-olefins commonly used are 1-butene, 1-hexene and 1-octene. Depending on the α-olefin and the catalyst used for the polymerization, LLDPE presents different microstructures which determine the thermal and mechanical properties. One simple and efficient method to evaluate the microstructure of LLDPE is the fractionation by Multiple-Step Isothermal Crystallization from Melting State conducted by DSC. This method is based on several steps of isothermal crystallization of the polymer on decreasing the temperature from the melt. This process favors the separation of the crystalline material into groups having different lamellae thickness depending on the amount and distribution of the α-olefin units in the macromolecular chains and on the molar mass. The melting endotherm of a fractionated sample is made up of the same number of peaks as the isothermal crystallization steps, which inform the relative comonomers distributions between different LLDPE chains. In this work, this methodology was applied to determine the relative comonomers distribution of different LLDPE. The isothermal temperature, temperature range and the time influence the efficiency of the fractionation and these parameters must be chosen according to the LLDPE microstructure.

In this study, a series of poly(styrene-covinyl phosphonic acid) [P(S-co-VPA)] copolymers were synthesized by the free-radical copolymerization of styrene and vinyl dimethyl phosphonate followed by alkaline hydrolysis. The P(S-co-VPA) copolymers were characterized by size exclusion chromatography (gel permeation chromatography), Fourier transform infrared vibrational spectroscopy, proton nuclear magnetic resonance, thermogravimetric analysis, differential scanning calorimetry, dynamic mechanical analysis, and electrochemical impedance spectroscopy. Despite the difference between the copolymerization ratios of styrene and vinyl dimethyl phosphonate, the resulting copolymers presented single glass transitions at temperatures that depended on the acidic group amount. The glass transition shifted to a higher temperature and became broader as the amount of phosphonic acid increased. The storage modulus at temperatures higher than the glass transition also increased with increasing acidic groups because of intramolecular and intermolecular interactions. All of the acid copolymers were thermally stable to at least 300 C. A high oxidative stability was found for 3 : 1 P(S-co-VPA), which also presented conductivity values on the order of 10 À6 X À1 cm À1 at room temperature. The 1 : 1 P(S-co-VPA) membrane presented Arrhenius-type behavior at temperatures from 30 to 80 C and conductivity on the order of 10 À5 X À1 cm À1.

No abstract

Resumo: Os polietilenos lineares de baixa densidade (LLDPE) apresentam propriedades variando amplamente em função da concentração e do tipo de comonômero utilizado na copolimerização, além do tipo de catalisador. Neste trabalho diversos tipos de LLDPE obtidos com diferentes tipos de comonômeros e catalisadores foram caracterizados estruturalmente por RMN-13 C e FTIR. Os resultados obtidos por meio destas duas técnicas foram discutidos e comparados.Palavras-chave: Polietileno, caracterização, RMN 13 C, FTIR, comonômeros. Linear Low Density Polyethylene Characterization -I. Spectroscopic Methods for Comonomer Content DeterminationAbstract: Linear Low Density Polyethylenes (LLDPE) show a broad variation in their properties by changing the type and comonomer content, besides the catalyst used. LLDPE properties change depending on the comonomer and its concentration at polymer chains and also by the catalyst used in polymerization. Several types of LLDPE containing different comonomers in various concentrations, obtained by metallocenic and Ziegler-Natta catalyst, were characterized by 13 C NMR and FTIR. The results obtained from these two techniques were discussed and compared.