Influence of microstructure and semi-crystalline morphology on the β and γ mechanical relaxations of the metallocene isotactic polypropylene (original) (raw)
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Polymer, 2007
This study aims to assess the relative influence of microstructure, molar mass and morphology on the local a relaxation of isotactic polypropylene (iPP). For that purpose, three families of samples have been prepared. The ZieglereNatta type provides a wide molar mass range where both the isotactic average length (n 1 ) and the nature of the isotacticity interruptions change according to chain size. The metallocene type allows to analyse also the samples in which molar mass and n 1 are related but the nature of the defects stays the same. Finally, three propylene-like EP copolymers (containing only isolated ethylene units) have been synthesised via metallocene catalysis in order to study samples which combine both relatively high molar masses and short n 1 .
Angewandte Makromolekulare Chemie, 2017
The influence of annealing on mechanical alphac\alpha_calphac-relaxation of isotactic polypropylene (iPP) is investigated. In the sample without annealing, polymer chains in the intermediate phase are constrained by crystallites with a broad size distribution, leading to one alphac\alpha_calphac-relaxation peak with activation energy (Ea) of 53.39 kJ/mol$^{-1}$. With an annealing temperature between 60 oC~105 C$^{\circ}$ imperfect lamellae melting releases a part of the constraining force. Consequently, two alphac\alpha_calphac-relaxation peaks could be observed ($\alpha_{c1}$- and alphac2\alpha_{c2}alphac2-relaxation in the order of increasing temperature). Both relaxation peaks shift to higher temperatures as annealing temperature increases. EaE_aEa of alphac1\alpha_{c1}alphac1-relaxation decreases from 38.43 to 35.55 kJ/mol$^{-1}$ as the intermediate phase thickness increases from 2.0 nm to 2.2 nm. With an annealing temperature higher than 105 C$^{\circ}$, a new crystalline phase is formed, which enhances the constraining force on the...
Morphology, thermal properties and mechanical relaxations of metallocene syndiotactic polypropylenes
e-Polymers, 2012
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 Ph 2 C(Cp)(9-Flu)ZrCl 2 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.
Macromolecules, 2005
Highly regioregular isotactic polypropylene samples containing only rr defects of stereoregularity in a wide range of concentration have been obtained with a series of regiospecific metallocene catalysts. A study of the relationships between kinetics of melt-crystallization and the microstructure of polypropylene chains is reported. The equilibrium melting temperatures for isotactic polypropylenes of different stereoregularity have been estimated. The kinetic study has shown that the crystallization rate increases with increasing the concentration of rr defects and decreasing the molecular weight. During isothermal crystallizations from the melt, crystals of R form develop at the beginning of the crystallization, whereas the formation of crystals of γ form is observed only at longer crystallization times. According to the literature, the crystals of γ form are nucleated over the preformed crystals of R form.
European Polymer Journal, 2007
A new metallocene catalyst applied to propene polymerization provides an expansion of the properties of polypropylene (PP) by generating semi-crystalline materials having 0-60% crystallinity. The purpose of the present study was to investigate such low crystalline elastomeric polypropylene (ePP) samples in order to compile further information on their crystal structure and the related phase transitions in the temperature range of 298-423 K.
Component Relaxation Processes within Elastomeric Polypropylene
Macromolecules, 1999
The behavior of specific components of elastomeric polypropylene was directly observed in situ using a dynamic, infrared polarimetry technique. The elastomeric nature of ePP is presumed to arise from a multiblock structure of isotactic (iPP) and atactic (aPP) polypropylene blocks. Elastomeric polypropylene is a heterogeneous material in terms of tacticity that can be separated into fractions differing in tacticity. A series of samples were prepared in which one fraction was labeled with deuterium. Infrared dichroism measurements made at the C-2 H stretching vibration were used to follow the relaxation behavior of fractions of various tacticity. The samples were subjected to a series of step shear strain experiments at a variety of temperatures from the melt and into the crystalline region. By simultaneously measuring birefringence (which measures the bulk sample's local orientation) and IR dichroism (which measures the orientation dynamics of the deuterated fraction), it is possible to isolate the behavior of specific populations of chains. It was revealed that chains with the highest level of tacticity crystallize first, as is expected, and strongly influence the subsequent crystallization of the remaining chains. In addition, evidence of cocrystallization between the solvent fractions was observed.
Crystallization kinetics of metallocene type polypropylenes
Journal of Thermal Analysis, 1996
The crystallization kinetics from the melt of metallocene type isotactic poly(propylenes) having the same chain defect concentration and molecular weights ranging from 68480 to 288430 have been studied by differential scanning calorimetry. The crystallization rates and the variation of the rates with crystallization temperature follow a pattern that is basically independent of molecular weight. This result contrasts with the molecular weight dependence on the crystallization rate observed in linear polyethylene, random ethylene copolymers as well as other semierystalline systems.
Characterization of the molecular structure of two highly isotactic polypropylenes
Polymer, 2001
Two polypropylenes, PP 1 and PP 2 , produced with different heterogeneous Ziegler-Natta catalytic systems were studied in this work. Preliminary characterization of the non-fractionated materials showed that a low difference in their average tacticity (PP 2 Ͼ PP 1 ) leads to an important modification of their rigidity properties. In order to establish correlation between the molecular structure parameters and the rigidity properties of these polymers, fractionation of the materials according to crystallizability was performed by means of temperature rising elution fractionation (TREF). Analysis of the fractions of both PP 1 and PP 2 was carried out by means of 13 C NMR, size exclusion chromatography (SEC), differential scanning calorimetry (DSC) and atomic force microscopy (AFM). The results first showed that TREF does not strictly fractionate PP according to tacticity, but according to the longest crystallizable sequence in a chain. 13 C NMR, SEC and DSC analysis of the fractions demonstrated that the inter-chain tacticity distributions of the polypropylenes is affected by the change of the polymerization conditions, which, in turn, modifies the rigidity properties of the materials. Some results also seem to indicate that the intrachain tacticity distributions are different for the two PP. An AFM study of the elastic modulus was carried out for the first time on the TREF fractions. It showed that the rigidity of the fractions strongly increases as the TREF elution temperature increases in accordance with a concomitant increase of isotacticity and the crytallinity of the fractions. PP 2 TREF fractions were, moreover, found to exhibit a higher elastic modulus than PP 1 TREF fractions at all elution temperatures. This study allowed us to further identify the TREF fractions that were responsible for differences in rigidity. To summarize, it is shown how the experimentally observed increase of the average rigidity of one of these two polypropylenes can be rationalized via information collected from a TREF fractionation. ᭧
Novel features of the rheological behaviour of metallocene catalysed atactic polypropylene
Polymer, 2003
This study was designed to determine the linear viscoelastic properties of a family of metallocene-catalysed propylene homopolymers in the ‘plateau’ and terminal regions. A low value of the plateau modulus, around 4.8×105 Pa, was obtained. Our results differ from those reported for conventional amorphous polypropylene in several recent studies. As previously undertaken for metallocene-catalysed polyethylenes, the rheological behaviour observed was discussed in terms of a possible decrease in the entanglement density and/or an increase in chain flexibility with respect to other polymers of similar chemical composition. We subsequently applied an existing model, based on simplified molecular dynamics, that provided a correct description of the dynamic modulus from knowledge of the form of the molecular weight distribution and the materials' rheological constants GN0 and η0. This method has been successfully applied to several types of polymer.