Mechanical and temperature dependant properties, structure and phase transitions of elastic polypropylenes (original) (raw)
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Morphology of Thermoplastic Elastomers: Elastomeric Polypropylene
Macromolecules, 2002
The morphologies of low-density (0.86 g/cm 3 ), elastomeric polypropylene (ePP) derived from bis(2-arylindenyl) hafnium dichloride were investigated using a combination of polarized optical microscopy (OM), differential scanning calorimetry (DSC), wide angle X-ray scattering (WAXS), Fourier transform infrared (FT-IR) spectroscopy, and tapping mode atomic force microscopy (TM-AFM). These low-crystallinity polypropylenes, when crystallized isothermally from the melt, exhibit morphologies reminiscent of classical semi-crystalline polymers. The presence of lamellae, cross-hatching, hedrites, and spherulites was revealed by high resolution TM-AFM. These elastomeric polypropylenes can be fractionated into components of different average tacticities and crystallinities, but similar molecular mass. The analysis of the morphologies of all of the fractions revealed both large hierarchical structures and cross-hatching typical of the α-modification of crystalline isotactic polypropylene for all but the lowest crystalline ether soluble fraction. Evidence for high-melting crystals in all of the fractions are most consistent with a stereoblock microstructure of atactic and isotactic sequences.
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.
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.
Thermal and dynamic mechanical properties of metallocene polyethylene
Polymer, 2001
A comprehensive study has been made of a metallocene polyethylene (m-PE) characterising the isothermal crystallisation kinetics, melting and crystallisation behaviour, crystal growth and dynamic mechanical properties in order to understand the relationship between molecular structure and mechanical properties of this new class of polyethylene. The melting behaviour after step-wise crystallisation showed that m-PE consisted of molecular fractions with different molecular weight and branch distribution. Dynamic mechanical property studies showed that three transitions existed in m-PE with the a-transition increasing in intensity and shifting to higher temperatures in samples crystallised at higher temperature compared with rapidly cooled samples. q
Synthesis and properties of elastomeric poly(propylene)
Macromolecular Chemistry and Physics, 1999
Elastomeric poly(propylene) with low percentages of isotactic pentads and high molecular weight was synthesized using unbridged "oscillating" metallocenes. The polymer sample with the highest [mmmm] content (22%) shows a small amount of crystallinity, which is highly influenced by the thermal history, and differs significantly in stress-strain and dynamic mechanical properties from all other samples. Polymers with lower isotacticity index, although lacking crystallinity in thermal analysis, differ in their viscoelastic behaviour from those of atactic poly(propylene). In dynamic mechanical analysis the c relaxation at low temperature shows high sensitivity to low isotactic contents.
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. ᭧
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.
The Role of Crystals in the Elasticity of Semicrystalline Thermoplastic Elastomers
Chemistry of Materials, 2006
The role of crystals in the elasticity of semicrystalline polymers is discussed in the case of syndiotactic polypropylene, which provides an example of a thermoplastic elastomer with a degree of crystallinity that can be tailored by changing and controlling the stereoregularity. This can be achieved using metallocene catalysts with different structures and stereoselectivity. The comparison of crystallization and physical properties of samples of syndiotactic polypropylene of different stereoregularity, with rrrr pentad concentrations being variable in the wide range 26-96%, prepared with different catalysts, has shown that syndiotactic polypropylenes present different types of elastic behavior, depending on the degree of crystallinity. For the most-stereoregular and crystalline samples with high melting temperatures, crystals actively participate to the elastic response of the material and elasticity has a mainly enthalpic character attributable to the metastability of the trans-planar form III that transforms into the more-stable helical form II during elastic recovery. For less-crystalline samples, with low melting temperatures, elasticity has instead a pure entropic origin as in conventional thermoplastic elastomers, and crystals act only as knots of the physical elastomeric network.
European Polymer Journal, 2009
In this work, the characteristics of the b and c mechanical relaxations, i.e., temperature and relative intensity, of a series of metallocene iPP samples (MPP) are analysed. The hypothesis that the temperature and the intensity of the glass transition (b relaxation) and local sub-T g motions (c relaxation) are related mainly to chain parameters and morphology has been corroborated. On the one hand, it has been found a critical average isotactic length (n 1 ) around 30 propylene units, under which the b and c dynamics are promoted with respect to the a relaxation. On the other hand, it is apparent that the features which determine the degree of constraint within the inter-lamellar region, i.e., the fraction of low-T m crystals, drive the relative intensities of the a, b and c relaxation processes.