Browsing by Author "Ribeiro, M. R."
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- Biodegradable polyesters reinforced with mesoporous silica particlesPublication . Campos, João M.; Ribeiro, M. R.; Deffieux, A.; Péruch, F.; Lourenço, J. P.; Cerrada, María L.In recent years, the development of mechanically reinforced nanocomposites based on biodegradable polyester matrices has gained a strong incentive due to their potential use as biomaterials and to growing worldwide environmental concerns.
- Copolymerization of ethylene and non-conjugated diene with metallocene/methylaluminoxane system supported on MCM-41 mesoporous materialPublication . Lopes, D. E. B.; Marques, M. F. V.; Dias, M. L.; Ribeiro, M. R.; Lourenço, J. P.Ethylene was copolymerized with the non-conjugated diene 5,7-dimethylocta-1,6-diene by metallocene catalysts, homogenous and supported on MCM-41 mesoporous material activated by methylaluminoxane at room temperature. The employed metallocenes were (BzCp)2ZrCl2 (1), (C18H37Cp)2ZrCl2 (2), (CH3)2Si(Ind)2ZrCl2 (3) and /2C(Cp,Flu)ZrCl2 (4). Diene incorporation and thermal characteristic of the polymers were determined by 13CNMR and DSC. For the homogeneous systems, only the unbridged metallocene (1) showed a decrease in the activity as the diene content was increased. The ability for diene incorporation in the polymer chain followed the catalyst order (1) < (3) < (2) < (4). Regarding the supported systems, it was observed that all of them showed lower activity than the corresponding homogeneous catalysts. A sharp loss of activity in relation to the homogeneous counterpart was noticed by employing the ansa-metallocenes on MCM-41, especially the complex (4) supported catalyst. The effect of the mesoporous material as support on the behavior of the studied metallocene homogeneous systems in ethylene–diene copolymerization was an increase of polymer molecular weight, maintaining the same level of diene incorporation, except for catalyst (2), when a decrease of the diene incorporation was observed.
- Decorated MCM-41/polyethylene hybrids: crystalline details and viscoelastic behaviorPublication . Cerrada, María L.; Pérez, Ernesto; Lourenço, J. P.; Bento, A.; Ribeiro, M. R.Polyethylene-based nanocomposites have been prepared by in-situ polymerization of ethylene with mesostructured MCM-41. Different approaches are used to improve interfacial adhesion between components, high density polyethylene (HDPE) matrix and MCM-41. These consist either of the incorporation of UA in the polymer chains by copolymerization in a very small content or of the modification of MCM-41 surface with the system based on undecenoic acid, UA/triisobutylaluminum, TIBA, before polymerization. The materials attained exhibit minimal changes in the crystallinity resulting from first melting process although a slight increase of long spacing at room temperature is seen as MCM-41 is incorporated in the hybrids. Moreover, a confinement effect is noticed and, thus, a delay in the crystallization process of the macrochains within pores and channels is detected. These novel self-reinforced nanocomposites show higher stiffness than other similar hybrids with poorer interfacial adhesion. Accordingly, they exhibit an enhancement in their mechanical performance without changing the final processing temperature since Tm is the same for all of the specimens.
- Ethylene polymerisation with zirconocene supported in Al-modified MCM-41: catalytic behaviour and polymer propertiesPublication . Campos, João M.; Ribeiro, M. R.; Lourenço, J. P.; Fernandes, A.Heterogeneous olefin polymerisation catalysts were prepared by combining zirconocene with a series of purely siliceous and Al-containingMCM- 41 supports with different Si/Al ratios, using simplified and improved versions of the well-known direct impregnation and “MAO pre-treatment” methods. The catalysts were tested in ethylene polymerisation. Analysis of polymerisation activities and kinetic profiles led to a rationalisation on the effect of support Si/Al ratios and methods used for catalyst preparation over the catalytic behaviour for ethylene polymerisations. Polymers were characterised by scanning electron microscopy and differential scanning calorimetry. A mechanism is proposed to associate the formation or absence of fibrous morphology with polymerisation activities.
- Gas permeability properties of decorated MCM-41/polyethylene hybrids prepared by in-situ polymerizationPublication . Bento, A.; Lourenço, J. P.; Fernandes, A.; Ribeiro, M. R.; Arranz-Andrés, J.; Lorenzo, V.; Cerrada, María L.Mixed matrix membranes, MMM, based on polyethylene,PE, and silica mesoporous material, MCM-41, have been prepared using these hybrid materials obtained by in situ polymerization in order to achieve higher permeability, selectivity or both magnitudes relative to the existing polymeric PE membranes. Different methodologies are used to enhance interfacial adhesion between components, PE matrix and MCM-41. These basically consist of promoting the modification of MCM-41 surface with undecenoic acid, UA/triisobutylaluminum, TIBA, system before the polymerization and of incorporating UA in the polymer chains by copolymerization. The influence of UA on the degradation behavior is evaluated for all of the hybrids and the transport properties, in terms of permeability and diffusivity coefficients as well as permselectivity at different gases and temperatures are checked in some of the resulting materials.
- HDPE/mesoporous silica nanocomposites by in-situ polymerisation: structural details and mechanical responsePublication . Bento, A.; Lourenço, J. P.; Fernandes, A.; Pérez, Ernesto; Cerrada, María L.; Ribeiro, M. R.Mesoporous silicas exhibit stable three-dimensional structures, made of ordered channels with well-defined geometries and dimensions at nanometer scale, able to host intercalation reactions.
- Hybrid HDPE/MCM-41 nanocomposites: crystalline structure and viscoelastic behaviourPublication . Cerrada, María L.; Pérez, Ernesto; Lourenço, J. P.; Campos, João M.; Ribeiro, M. R.Polyethylene-based nanocomposites have been prepared by in situ polymerisation of ethylene with mesostructured MCM-41 within a large range of nanofiller concentrations. The structural, thermal and viscoelastic studies have shown that the use of mesoporous MCM-41 as catalyst carrier and its further presence in the final material is an effective route for the successful attainment of nanocomposites. Data shows that minimal changes in crystallinity are observed but crystallites become thicker as MCM-41 content is raised in the nanocomposites. A confinement effect is found and a delay in the crystallisation process of the macrochains within pores and channels is observed. These novel self-reinforced nanocomposites present an enhanced rigidity, which becomes more important as temperature is raised. Accordingly, they exhibit an improved mechanical performance without varying the final processing temperature since Tm is the same for all of the specimens. In agreement with other works, it is confirmed that the MCM-41 acts as promoter for polyethylene degradation, an easier degradability being observed in these nanocomposites.
- Hybrid materials based on polyethylene and MCM-41 microparticles functionalized with silanes: catalytic aspects of in situ polymerization, crystalline features and mechanical propertiesPublication . Cerrada, M. L.; Bento, A.; Pérez, Ernesto; Lorenzo, V.; Lourenço, J. P.; Ribeiro, M. R.New nanocomposites based on polyethylene have been prepared by in situ polymerization of ethylene in presence of mesoporous MCM-41. The polymerization reactions were performed using a zirconocene catalyst either under homogenous conditions or supported onto mesoporous MCM-41 particles, which are synthesized and decorated post-synthesis with two silanes before polymerization in order to promote an enhanced interfacial adhesion. The existence of polyethylene chains able to crystallize within the mesoporous channels in the resulting nanocomposites is figured out from the small endothermic process, located at around 80 C, on heating calorimetric experiments, in addition to the main melting endotherm. These results indicate that polyethylene macrochains can grow up during polymerization either outside or inside the MCM-41 channels, these keeping their regular hexagonal arrangements. Mechanical response is observed to be dependent on the content in mesoporous MCM-41 and on the crystalline features of polyethylene. Accordingly, stiffness increases and deformability decreases in the nanocomposites as much as MCM-41 content is enlarged and polyethylene amount within channels is raised. Ultimate mechanical performance improves with MCM-41 incorporation without varying the final processing temperature.
- MCM-41 as Nanofiller in Polyethylene Hybrid MaterialsPublication . Ribeiro, M. R.; Campos, João M.; Bento, A.; Lourenço, J. P.; Pérez, Ernesto; Cerrada, María L.Mesoporous MCM-41 exhibits a stable framework structure, well-defined nanopores and a large surface area. When combined with metallocene polymerisation catalyst, these mesoporous materials, provide a unique route for preparing polyolefin-based nanocomposites by in situ polymerisation.
- Mesoporous Ga-MCM-41: a very efficient support for the heterogenisation of metallocene catalystsPublication . Campos, João M.; Lourenço, J. P.; Fernandes, A.; Ribeiro, M. R.Mesoporous Ga-containing silicates prepared by direct synthesis, Ga-MCM-41, are used in combination with metallocene Cp2ZrCl2 to provide very effective supported catalysts for ethylene polymerisation. A pre-treatment with methylalumoxane is not required in order to achieve high polymerisation activity levels with these catalysts, whose stability is improved when compared to the case where a purely siliceous support was used for the immobilisation of the metallocene.