Showing papers by "Rosario Benavente published in 2008"

Comonomer Length Influence on the Structure and Mechanical Response of Metallocenic Polypropylenic Materials

Abstract: The structure and properties have been comprehensively studied for metallocene copolymers of propylene-1-hexene (CiPH) and propylene-1-octadecene (CiPOD). The comonomer content constitutes the most important factor affecting both structure and properties in these CiPH and CiPOD copolymers, although the length of the comonomer is also very important. Thus, a considerable decrease in crystallinity and an easier obtainment of mesomorphic-like ordered entities are observed in the two kinds of copolymers as comonomer content increases. The variations in crystalline structure significantly influence the viscoelastic and mechanical behaviors of these CiPH and CiPOD copolymers. Consequently, the location and intensity of the different relaxation mechanisms as well as stiffness parameters (storage, Young's moduli, and microhardness) and deformation mechanism are strongly dependent upon composition.

Nonisothermal melt‐crystallization kinetics of isotactic polypropylene synthesized with a metallocene catalyst and compounded with different quantities of an α nucleator

Abstract: The nonisothermal crystallization kinetics of a metallocene-made isotactic polypropylene (m-iPP) and its compounds with 0.1 wt % and 0.3 wt % of a sorbitol derivative [1,3:2,4-bis(3,4-dimethylbenzylidene)sorbitol (DMDBS); an α nucleator] were investigated by differential scanning calorimetry at different cooling rates from the melt. The nucleation efficiency was proved by a significant increase in the crystallization temperatures (accompanied by a slight augmentation of the degree of crystallinity and a decrease in the crystal sizes). This increase in the crystallization temperatures led to higher amounts of fractional content in the γ polymorph, even though DMDBS was supposed to be a nucleator for the α form. The Avrami and Ozawa methods effectively described only the early stage of crystallization, whereas a combined Avrami–Ozawa method was valid for the whole crystallization process. The values of the exponent for this method decreased for nucleated samples in the later stage of crystallization, especially in the case of m-iPP with 0.3 wt % DMDBS added (m-iPP03). The activation energy of the process and the surface free energy were also estimated. The production of considerable proportions of the γ polymorph in m-iPP03 corresponded to higher values of the activation energy and lower values of the surface free energy. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008