Mecanismo de formación y cristalización del poli (óxido de trimetileno) y del 3,3-dimetilderivado

Tesis - Universidad Complutense de Madrid, 1982.Depto. de Química FísicaFac. de Ciencias QuímicasTRUEProQuestpu

Study of the Molecular Weight Influence on the Formation of the ß-polymorph in Metallocenic Isotactic Polypropylene

Isotactic polypropylene, iPP, is one of the most important thermoplastic polymers due to its low manufacturing cost and rather versatile properties. iPP exhibits a very interesting polymorphic behavior depending on the polymerization procedure, thermal history and use of different nucleating agents. Recently, studies on different aspects of the iPP β-polymorph have been published [1, 2]. This paper completes these previous works by analyzing the formation of the β-polymorph starting from samples of metallocenic iPP of different molecular weight. We also consider the effect of the cooling rate in the appearance of β-polymorph in the iPP samples that were studied

Efecto del Peso Molecular en la Formación del Polimorfo Beta en Polipropileno Isotáctico Metalocénico.

Existe un gran número de referencias relativas al efecto que la adición de agentes nucleantes, AN, en el polipropileno, iPP, supone en la generación de los distintos polimorfos, no existiendo estudios previos en el iPP metalocénico, m-iPP. Es objetivo de este trabajo llenar la laguna existente en dicha área, con el fin de mejorar la estabilidad térmica y la transparencia, acortar los ciclos de producción y optimizar las propiedades. Se han estudiado diferentes tipos de nucleantes de última generación que conllevan, en primer lugar, la formación de la celdilla monoclínica α y el aumento de la temperatura de cristalización, lo que acorta los ciclos de transformación. Para la evaluación exhaustiva5 de la forma β, objetivo de este trabajo, se ha utilizado como AN una mezcla de estearato cálcico y ácido pimélico (1:2), denominada a5. La competencia entre las formas α, β y γ en el m-iPP depende de la velocidad de cristalización y de la temperatura de cristalización isoterma5. Además, el contenido de AN β necesario para generar esta forma cristalina en el m-iPP es considerablemente superior en relación con el iPP sintetizado con catalizadores Ziegler-Natta, Z-N, debido a la coexistencia del polimorfo γ en el primero. La competencia entre las tres formas cristalinas en el m-iPP provoca que la generación de la forma β en función de la velocidad de enfriamiento sea, bajo ciertas condiciones, opuesta a la observada previamente en el iPP Z-N convencional. Estos resultados novedosos son de gran interés dado que han establecido las bases científicas sobre las que se cimenta el objetivo de la adición de AN β en los diferentes iPPs seleccionados, objeto de estudio en esta comunicación. Se han mezclado dos m-iPP con índices de fluidez, IF, de 15 y 30 g/min, suministrados por Basell, con un 0.3 % en peso de AN, en un mezclador interno a 190 ºC, 40 rpm y 10 min. Posteriormente, se han preparado filmes mediante moldeo por compresión en muestras sin y con AN. Las muestras se han enfriado desde el fundido a unos 100 ºC/min (entre platos de agua fría y presión), tratamiento Q y a 1.5 ºC/min (velocidad de enfriamiento de la prensa bajo presión), tratamiento S. La caracterización de la estructura y la competición entre las diferentes formas cristalinas se ha seguido mediante difracción de rayos X y DSC, mientras que el estudio de las propiedades mecánicas se ha realizado mediante DMTA y microdureza. En la figura se reproducen los difractogramas de RX para las muestras en estudio sin y con 0.3% de AN y tratamiento térmico S. Las muestras sin aditivar, sn, presentan polimorfismo, es decir, la coexistencia de las difracciones a 18.5 y 21.2º características de las celdillas cristalinas α y γ, respectivamente. Por otro lado, la presencia del AN a5 da lugar a una difracción adicional a 16.1º asociada a la fase trigonal β. La intensidad de dicha difracción es mayor para el polímero de menor peso molecular (mayor IF)

Caracterización del polimorfismo de un polirpopileno metalocénico por medio de espectroscopia FTIR

Se presentan resultados sobre al caracterización morfológica de PP's metalocénicos por medio de espectroscopía infrarroja

Lightweight nanocomposites based on polypropylene and aluminum nanoparticles and their shielding capability to ionizing radiation

Nanocomposites based on isotactic polypropylene and different contents of aluminum (Al) nanoparticles have been prepared in order to primarily analyze their electromagnetic interference shielding capability. Promising results have been attained in this area since values as high as 40 dB, which provide at least a 99% attenuation of the electromagnetic radiation, are reached. The good balance between shielding, sample weight, cost, and processability makes these lightweight materials a good alternative to replace neat metals for that application. Moreover, a complete structural and morphological characterization, as well as an evaluation of their dielectric behavior is also performed. © 2002-2012 IEEE.Peer Reviewe

Fast scanning calorimetry study of the structural relaxation in a random propylene-co-1-octene copolymer

The structural relaxation (or physical aging) has been analyzed by means of fast scanning calorimetry for a random propylene-co-1-octene copolymer, in a wide range of annealing temperatures and times. For annealing temperatures well below Tg only the structural relaxation is observed, and both the rate of enthalpy recovery and the peak temperatures related to the relaxation show a remarkable reduction with decreasing annealing temperatures. When the glass transition temperature is approached, although the structural relaxation is observed first, mesophase ordering is obtained at higher annealing times. The analysis of this mesophase ordering suggests homogeneous nucleation, and, in agreement with prior research in this field, the main feature is that the ordering in the glassy state requires prior completion of the enthalpy relaxation.Peer Reviewe

Morphology, thermal properties and mechanical relaxations of metallocene syndiotactic polypropylenes

The complex polymorphic behaviour of four syndiotactic polypropylene (sPP) samples have been analysed by means of DSC and WAXD techniques. Two samples (sPP1, sPP2) were synthesised via metallocene polymerization by using the ansa-zirconocene Ph2C(Cp)(9-Flu)ZrCl2 as catalyst. Finally, two additional specimens with different molar masses (sPP1-Fr1, sPP1-Fr2) were prepared from the most syndiotactic sPP1 sample by temperature gradient extraction. The WAXD analysis shows that together with the orthorhombic form I, form II can coexist in a variable but small proportion depending on both the chain features and the processing conditions. The relative contribution of the disordered and the ordered types of form I is also dictated by molar mass, configurational microstructure as well as processing conditions. The observed changes in the thermal properties as measured by DSC and in the mechanic-dynamical relaxations of the samples, on slowing the crystallisation rate down, can be rationalised in terms of two concurrent processes, namely the perfection undergone by the crystals and the segregation of the amorphous phase.Peer Reviewe

Microstructure of metallocene isotactic propylene-co-1-pentene-co-1-hexene terpolymers

This study aims at characterizing in depth the microstructure of propylene-co-1-pentene-co-1-hexene terpolymers, which have been recently reported to develop the isotactic polypropylene δ trigonal polymorph when the total comonomer content is high enough. Such a specific crystalline form had been only reported so far in the analogous copolymers containing either 1-pentene or 1-hexene. A comparative 13C NMR study in solution of the aforementioned terpolymers and copolymers allows asserting the random insertion of both comonomers during chain growth under the polymerization conditions used. The reaction parameters, mainly catalyst and temperature, have been chosen for the purpose of assuring relatively high molar mass polymers.Peer Reviewe

Synthesis, molecular characterization, evaluation of polymorphic behavior and indentation response in isotactic poly(propylene-co-1-heptene) copolymers

Different polymorphs are developed in isotactic poly(propylene-co-1-heptene) copolymers depending on 1-heptene molar content and thermal history: monoclinic and orthorhombic crystallites as well as mesomorphic entities. Indentation hardness values and deformability are found to be related to those variables as well as to crystallinity degree. Consequently, mechanical behavior can be tailored mainly by varying all of these parameters and a range from rigid to soft and more easily deformable materials is attained by synthesis of copolymers based on isotactic propylene with 1-heptene as comonomeric unit. Evaluation of the influence of molecular weight, intrinsic viscosity and microstructural details is also performed.Peer Reviewe