María L. Cerrada

Effect of composition and molecular weight on the crystallization behavior of blends of iPP and a metallocenic ethylene/1-octene copolymer

Blends covering the entire range of compositions of a metallocenic ethylen/1-octene copolymer and two conventional isotactic poly(propylene)s (iPPs) of different molecular weight have been prepared, analyzing (by differential scanning calorimetry and X-ray diffraction) the effect of composition and molecular weight on the crystallization behavior. It was found that those blends rich in the iPP component show a behavior practically coincident with the weighted addition of the two components. On the contrary, significant deviations were found for the blends where the ethylene copolymer is the major component. These deviations are considerably more important in the case of the blends with iPP of higher molecular weight. In such a case, the blend with a 25% by weight of iPP presents a normalized crystallinity degree of only 0.20, in comparison with the value of 0.57 exhibited by the pure iPP. After annealing at 120°C for 10 minutes, and cooling back to room temperature, that blend increases its crystallinity degree up to only 0.40.

The effect of thermal treatment on the structure and relaxation processes of olefinic polymers synthesized with metallocene catalysts

The relationships between the structure and properties have been established in several copolymers of ethylene and 1-hexene, synthesized by a metallocene catalyst. A dependency with composition and thermal history has been found. The branches cannot be incorporated into the orthorhombic crystal lattice, and consequently, structural parameters such as crystallinity and crystal lattice constants are considerably affected as 1-hexene content increases in the copolymer. The β-relaxation does not appear in the homopolymer. In the copolymers, it is shifted to lower temperature and its intensity is increased as 1-hexene content raises. On the other hand, the α-mechanism associated to motion within the crystalline regions is also moved to lower temperatures but its intensity is diminished as 1-hexene content increases in the copolymer.

Biodegradable Polycaprolactone-Titania Nanocomposites: Preparation, Characterization and Antimicrobial Properties

Nanocomposites obtained from the incorporation of synthesized TiO2 nanoparticles (≈10 nm average primary particle size) in different amounts, ranging from 0.5 to 5 wt.%, into a biodegradable polycaprolactone matrix are achieved via a straightforward and commercial melting processing. The resulting nanocomposites have been structurally and thermally characterized by transmission electron microscopy (TEM), wide/small angle X-ray diffraction (WAXS/SAXS, respectively) and differential scanning calorimetry (DSC). TEM evaluation provides evidence of an excellent nanometric dispersion of the oxide component in the polymeric matrix, with aggregates having an average size well below 100 nm. Presence of these TiO2 nanoparticles induces a nucleant effect during polymer crystallization. Moreover, the antimicrobial activity of nanocomposites has been tested using both UV and visible light against Gram-negative Escherichia coli bacteria and Gram-positive Staphylococcus aureus. The bactericidal behavior has been explained through the analysis of the material optical properties, with a key role played by the creation of new electronic states within the polymer-based nanocomposites.

Effect of Short Glass Fiber on Structure and Mechanical Behavior of an Ethylene–1‐Octene Copolymer

Several composites with short glass fiber of a copolymer of ethylene and 1-octene, synthesized with a metallocene catalyst, have been prepared, up to a fiber content of 60 wt.-%. The effect of the fiber on the structure and mechanical behavior has been analyzed. Though the peak melting and crystallization temperatures are not altered, the incorporation of the fibers leads to a slightly higher crystallinity in the composites, and the mechanical behavior is modified. Since fibers introduce rigidity, both Youngs modulus and the microhardness increase and either elongation or stress-at-break decrease as the glass fiber content rises in the composite. However, the dependence of Youngs modulus on the fiber content is very different from that of the microhardness. Moreover, an essentially homogeneous deformation takes place along the different materials analyzed, but, depending upon reinforcement composition, one or two yield stresses are observed. Finally, all specimens show three viscoelastic mechanisms. Due to the rigidity introduced by fibers and their influence on the crystallinity, the distinct relaxations in the composites are shifted to higher temperatures and the apparent activation energies associated with them are higher. The effect of fiber is more significant in either the β or α processes than in that of the γ. The increase of the temperature of the α relaxation is particularly important and thus the incorporation of the fibers considerably enlarges the useful temperature range, which can expand the application fields of the plain copolymer.

Crystalline Structure and Viscoelastic Behavior in Composites of a Metallocenic Ethylene-1-octene Copolymer and Glass Fiber

Several composites with short glass fiber of a copolymer of ethylene and 1-octene, synthesized with a metallocene catalyst, have been prepared, up to a fiber content of 40 wt.-%. The effect of the fiber on the crystalline structure and on the viscoelastic response are analyzed. The role of glass fiber incorporation on crystallite development is discussed based on real-time variable-temperature WAXS and SAXS experiments. The viscoelastic behavior is considerably modified by incorporation of the glass fiber when compared with the plain copolymer. Thus, both the intensity and location of the γ and β relaxations are dependent on the fiber content. Moreover, the stiffness is considerably increased in the composites, allowing the analysis of the viscoelastic behavior up to temperatures well inside the melting region of the copolymer crystals, and thus enlarging substantially the service temperature of these composites, The β relaxation is assigned to the glass transition temperature, in agreement with the calorimetric results (DSC and MDSC) in these composites.

Viscoelastic and mechanical properties of poly(butyl acrylate-g-styrene) copolymers

Abstract Viscoelastic relaxations of four poly(butyl acrylate- g -styrene) copolymers were studied over a wide range of temperatures. The temperature location and apparent activation energy of the distinct relaxations found are discussed. A single relaxation associated with cooperative motion has been observed in the range of graft content analysed. In addition, the stress–strain behaviour of these graft copolymers was studied. Films of the copolymer samples were stretched at room temperature since the possible application as adhesives might take place at about room temperature. Some mechanical parameters have been estimated, such as elastic modulus, tensile strength and toughness. Moreover, microhardness has been measured and its relationship with other mechanical properties has been established.

Microhardness and thermal study of the annealing effects in vinyl alcohol—ethylene copolymers

Abstract The influence of annealing and thermal history on the microhardness, MH, of three vinyl alcohol—ethylene copolymers has been studied and the results compared with those for the corresponding homopolymers, poly(vinyl alcohol) and polyethylene. The results show that the microhardness of the copolymers increases with the vinyl alcohol content, although the degree of heterogeneity also seems to play an important role. Two thermal histories (fast quenching or slow cooling from the melt) have been imposed on the samples. The quenched (Q) specimens present lower values of MH than the corresponding slowly cooled (S) samples. Both Q and S samples were subsequently annealed at different temperatures. In the annealed samples, it has been found that the variation of MH is different for the two thermal histories. Thus, the Q samples show an increase of MH with the annealing temperature while the contrary is found for the S specimens. These results can be explained by the heterogeneous nature of these copolymers, as revealed by differential scanning calorimetry traces, which show an appreciable degree of material melting at temperatures well below the main melting peak.

Viscoelastic processes in vinyl alcohol–ethylene copolymers. Influence of composition and thermal treatment

Abstract Viscoelastic relaxations of three samples of vinyl alcohol–ethylene copolymers, richer in the former comonomer, were studied in a wide range of temperature. The temperature location, intensity and apparent activation energy of the distinct relaxations found are discussed and compared with those of the homopolymers, poly(vinyl alcohol) and polyethylene. Differential scanning calorimetry and X-ray diffraction results of the specimens are also discussed in the frame of the dynamic mechanical analysis, showing that the polymorphism exhibited in some copolymers is a result of the thermal treatment.

Light-driven novel properties of TiO2-modified polypropylene-based nanocomposite films.

Nanocomposite films obtained by TiO2 incorporation into a polypropylene (PP) polymer matrix were prepared via a straightforward melting process. The structural characteristics of the nanocomposite materials were examined by a multitechnique approach showing the outstanding morphological uniformity of the films. TiO2 incorporation into the polymer matrix renders self-sterilized and self-degradable materials which showed an impressive performance against gram-positive, gram-negative, coccus and yeast micro-organisms. Optimization of the novel properties of TiO2-PP nanocomposite films is reached with a 2 wt% of oxide content loading. The physico-chemical bases of this optimum are discussed.

Experimental evidence of the glass transition in a metallocene ethylene-1-octene copolymer and its composites with glass fibre

Abstract Experimental evidence is reached by differential scanning calorimetry (DSC), and modulated differential scanning calorimetry (MDSC), about the existence of a glass transition in a commercial ethylene-1-octene copolymer synthesized with a metallocene catalyst and in its corresponding composites with glass fibre at different content of reinforcement. The glass transition is found in the region from −50 to −45°C depending upon the empirical protocol. Its location is quite independent on the fibre content. The presence of reinforcement in the polymeric matrix has some influence on the crystalline regions, since slightly more perfect crystals seem to be obtained in the composites with not very high glass fibre content, judging from the better resolution of the (110) diffraction and from the slightly higher melting temperatures. However, the normalized crystallinity remains practically unchanged in all the samples.