We study the stress relaxation of model polymer networks containing low contents of star shaped and linear dangling polymers. As compared with their melts, the behavior of star and dangling polymers leads to a dynamic response with unprecedented large relaxation times. By comparing data of star melts with those corresponding to stars and dangling chains residing in polymer networks, we were able to identify the effects of dynamic dilution clearly. Since in polymer networks the dynamic dilution effect is suppressed, we were able by the first time to experimentally test the validity of the potential for arm retraction proposed by Pearson and Helfand.

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The controlled synthesis of poly(dimethylsiloxane) homopolymers (PDMS) using hexamethyl(cyclotrisiloxane) monomer (D 3 ), a mixture of ciclohexane/tetrahydrofuran 50/50 v/v and sec-Bu -Li þ as initiator was studied using different experimental conditions, and whole-sealed glass reactors according to standards procedures in high-vacuum anionic polymerization. It was observed that polydispersity indexes (PD) and conversions strongly depend on temperature and reaction times. For PDMS homopolymers with molar masses below 100,000 g/mol, high conversion ([90%) and PD \ 1.1 can be achieved at long reaction times (24 h) and mild temperature conditions (below or up to 30 C). On the other hand, to synthesize PDMS homopolymers with molar masses higher than 100,000 g/mol and PD \ 1.1 it is necessary to increase the temperature up to 50 C and decrease the reaction time (8 h). However, under these reaction conditions, it was observed that the conversion decreases (about 65-70% conversion is achieved). Apparently, the competition between propagation and secondary reactions (redistribution, backbiting, and reshuffling) depends on the molar masses desired. According to the results obtained in this study-which were compared with others found in the scientific literature-propagation is favored when M n \ 100,000 g/mol, whereas secondary reactions seem to become important for higher molar masses. Nevertheless, model PDMS homopolymers with high molar masses can still be obtained increasing the reaction temperature and shortening the total reaction time. It seems that the combined effect of these two facts favors propagation against secondary reactions, and provides model PDMS homopolymers with molar masses quite close to the expected ones. V V C 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 4774-4783, 2009

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Two metallocene ethylene-1-octene copolymers differing in comonomer content were cross-linked either by dicumyl peroxide (DCP) or b-radiation with doses ranging from 0.5 to 4% DCP and 25 to 200 kGy, respectively. The effect of cross-linking on the crystalline morphology was analyzed by differential scanning calorimetry (DSC). Slight alterations in the crystalline structure were found, which were more severe in the case of peroxi modification. Through infrared spectroscopy (FTIR analysis), oxidation during the cross-linking process was detected on the DCP cross-linked samples, while b-irradiated samples do not exhibited significant degrees of oxidation. The state of cure was studied following the changes in the rheological properties in small-amplitude oscillatory shear mode, and the evolution of the molecular weight and molecular weight distribution through size exclusion chromatography. Irradiation doses below 200 kGy increased the molecular weight and branching of both copolymers but were not sufficient to reach gelation. All the peroxide modified samples resulted in a post-gel condition. Optimal mechanical properties were obtained with concentration of about 1% DCP. At higher doses, scission reactions diminish the tensile strength and the elongation at break. When polymers with equivalent amounts of gel fraction obtained by the two modification procedures studied in this work are compared, higher tensile strength and elongation at break are obtained with irradiation.

Nanocomposites obtained from incorporation of TiO 2 nanoparticles in different amounts, ranging from 0.5 to 5 wt.%, into an isotactic polypropylene (iPP) matrix are achieved via a straightforward and cost-effective melting process. These materials exhibit a powerful germicide capability over a wide variety of regular bacteria and other microorganisms widely present in the environment that cause infections and serious illness.The iPP-TiO 2 nanocomposites show similar or improved structural characteristics than those of the pure iPP matrix and aspects as important as processability and final mechanical performance seem to be not affected because of the incorporation of these TiO 2 nanoparticles.Validation of time-temperature superposition of the molten polymers is observed within the temperature range analyzed. On the other hand, the α polymorph is the one primarily attained for these specimens. Crystallinity and most probable crystallite size are slightly dependent on TiO 2 content, both increasing as oxide composition is enlarged.