iPP Samples Research Articles

Interplay between crystallization behaviors and extensional deformation of isotactic polypropylene and its blend with poly(ethylene-co-octene)

Comparison investigation of the interaction between crystallization behaviors and extensional deformation of both isotactic polypropylene (iPP) and its blend with poly(ethylene-co-octene) (iPP/PEOc) was carried out in this study. The samples of iPP and the iPP/PEOc (80/20, wt.%) blend were prepared by changing the cooling rate during nonisothermal crystallization. Tensile testing showed that with the decrease of cooling rate, the progressive destruction of ductility of the two samples was resulted from the more perfect crystallites formed in the cooling process. The influence of cooling rate on the tensile properties is more prominent for pure iPP than for the iPP/PEOc blend. The crystalline structure was proven to be partially destroyed under the extensional deformation, and such crystalline structure destruction was in close association with the deformation of the specimens. The oriented noncrystalline molecular chains could easily be reorganized into more perfect crystals in postheating runs. The original crystalline structure has been found, to some extent, to determine the extensional deformation and the final crystallization behavior.

Fast and Slow Dynamics of Isotactic Polypropylene Melts

We report a quasi-elastic neutron scattering (QENS) study of local dynamics in isotactic polypropylene (iPP) melts. QENS measurements were carried out on the spectrometer IRIS (ISIS, UK), using a series of selectively deuterated iPP samples. Analysis of the intermediate incoherent scattering functions was conducted using two different models: (a) a two-component model involving a fast process and conformational transitions and (b) a three-component model including methyl group reorientations. We show that consistent results between the selectively deuterated samples are only obtained when methyl group reorientations are explicitly taken into account. The results presented here are consistent with the simulated intermediate scattering functions of atactic polypropylene [Ahumada, O.; Theodorou, D. N.; Triolo, A.; Arrighi, V.; Karatasos, C.; Ryckaert, J.-P. Macromolecules 2002, 35, 7110] which revealed contributions from torsional transitions of the methyl groups and segmental motion, in addition to a fast process.

A Comparison of the Effects of Calcium Glutarate and Pimelate on the Formation of β Crystalline Form in Isotactic Poly(propylene)

The effect of calcium glutarate (Cagt) and calcium pimelate (Capt) on the formation of β crystalline form in isotactic poly(propylene) in the crystallization temperature range of 110–130°C has been investigated. The content of β phase crystals increase with the addition of calcium glutarate. K (relative content of β crystalline form in the iPP sample) attains its maximum value for iPP doped with 0.3 wt.% Cagt isothermally crystallized at 110°C (26.71%) or 120°C (30.27%), and for iPP doped with 0.2 wt.% Cagt isothermally crystallized at 130°C (31.97%), respectively. Compared with the K values of iPP doped with 0.1 wt.% Capt (78.33–94.76%), the β nucleation ability of Cagt is inferior to that of Capt. The spherulite size of iPP doped with Capt is smaller than that of iPP doped with Cagt. The difference in the β nucleation ability between Cagt and Capt is explained by the difference between their crystal structure parameters and those of β‐iPP.

Effect of Temperature and Annealing on the Phase Composition, Molecular Mobility, and the Thickness of Domains in Isotactic Polypropylene Studied by Proton Solid-State NMR, SAXS, and DSC

A combination of DSC, SAXS, WAXD, 1H time-domain, and frequency domain NMR measurements was used for determining the amount of rigid/crystallinity, semirigid, and soft fractions of iPP. Changes in the rigid, semirigid, and soft fractions of isotactic polypropylene (iPP) were investigated as a function of temperature, annealing time, and annealing temperature. The most probable iPP morphology was established by TEM and by comparing 1H spin-diffusion data with data from multidimensional solutions of the spin-diffusion equations. Proton NMR spin-diffusion method, which employs double-quantum (DQ) and Goldman−Shen dipolar filters, was used in order to provide the domain thickness in iPP. The temperature dependence of spin diffusivities was taken into account, and a semiquantitative theory is presented for this dependence in the case of amorphous domains. A combination of 1H spin-diffusion NMR and SAXS was used to estimate the lamellar thicknesses for nonannealed and annealed iPP samples. Annealing at temperat...

Effect of Solubility and Nucleating Duality of N,N‘-Dicyclohexyl-2,6-naphthalenedicarboxamide on the Supermolecular Structure of Isotactic Polypropylene

N,N‘-Dicyclohexyl-2,6-naphthalenedicarboxamide (NJS) is an efficient nucleating agent to prepare isotactic polypropylene (iPP) samples rich in the β-modification (β-iPP). However, NJS is not a selective β-nucleating agent, as the related samples always contain both α and β modification of iPP. Depending on the final temperature of heating (Tf), NJS may be partially or completely dissolved in the iPP melt. The dual nucleation ability of NJS was studied by polarized light microscopy (PLM) and calorimetry (DSC). It was established that the lateral surface of the needle crystals of NJS acts as α-nucleating agent. Because of the solubility and dual nucleating ability of NJS, a wide variety of supermolecular structures may form in its presence. The feature of the structure formed during the crystallization of iPP depends on the concentration of the nucleating agent, on the end temperature of heating, and on the thermal conditions during cooling and crystallization. In this study we observed an αβ-transcrystalli...

Crystallization-Induced Fluid Flow in Polymer Melts Undergoing Solidification

The influence of processing conditions on the morphology of solidified semicrystalline polymer materials formed under dynamic melt processing conditions has been of concern for decades.1 Although there has been much recent progress in modeling the kinetics and origin of polymer spherulitic crystallization,2,3 the influence of fluid flow and the viscous properties of the undercooled polymer melt on the crystallization morphology have received relatively little attention due to the great computational problems involved in modeling and the experimental difficulties in observing this phenomenon. It is often argued that the macromolecules of the undercooled melt can diffuse to the crystal growth front during crystallization,4 and fluid flow processes are simply ignored. While this assumption may be a reasonable approximation under low undercooling conditions, one may well wonder whether this situation remains true under the conditions of rapid solidification that normally arise during polymer material processing. Flow in the undercooled polymer melts has not been studied much experimentally previously because of the lack of methods of sufficient temporal and spatial resolution to discriminate between the crystalline and amorphous phases during the course of the crystallization process. It is well-known, however, that the formation of commercial semicrystalline polymeric products by injection moldings, film extrusions, film blowing, etc, can be accompanied by the formation of unwanted defects (voids, pores, and other imperfections) and that the ultimate properties of these materials can be appreciably affected by processing conditions.5 The present study indicates that rapid fluid flow can occur in a model undercooled polymer melt (a common commercial polymer, isotactic polypropylene) during the course of crystallization under confinement conditions (simply crystallizing polymer film between cover glasses). We have also observed a similar behavior (not yet published) in crystallizing poly(ethylene oxide) (PEO) melts, indicating some degree of generality for the phenomenon. The isotactic polypropylene (iPP) polymer employed in this work was a commercial grade polymer obtained from the Αldrich Chemical Co., and we show typical results for an isothermal crystallization temperature of 138 °C. The “weight” and “number” molecular masses Mw and Mn were about 340 000 and 97 000, respectively, so this polymer was rather polydisperse, typical of commercial material. The carbon black (CB) relative mass of the probe particles to the polymer matrix was relatively low, 0.5%. The iPP and CB were mixed by meltpressing repeatedly (five times) on a hot plate at 200 °C. Samples for the crystallization and flow measurements were prepared by pressing the melt mixtures between two cover glasses into thin films having a thickness of about 30 μm, thus providing a model confinement environment for the polymer melt crystallization process. Our optical microscope (Carl Zeiss JENA, made in Germany), equipped with a CCD camera (HV1301UC, made by the Da Heng Co. in Beijing), was used to image flows in the undercooled melts by following the motion of CB particles during isothermal crystallization. The resolution of the CCD camera in the x and y directions was about 0.2 μm. A homemade dual-temperature microscope hot stage provided a temperature control, with a temperature uncertainty of ( 0.1 °C. The iPP samples were first melted at 200 °C for 10 min to melt the crystallized structures formed in the course of the previous sample history and then were rapidly transferred to a crystallization temperature below the melting temperature (160 °C). Figure 1 shows a serial of micrographs of the iPP sample during isothermal crystallization at 138 °C. The present work not only considers the standard problem of characterizing the spherulite growth, but also considers how spherulite growth induces flow in the surrounding polymer melt by observing the motion trajectories of CB particles in the undercooled iPP “melt”. The basic concept of our measurement can be understood from the following nautical analogy: The undercooled melt can be viewed as a sea and the growing spherulites as islands that grow up out of this sea. The CB particles are convected by the fluids, providing information about the prevailing local “currents” in the undercooled polymer melt.

Crystalline morphology of isotactic polypropylene (iPP) in injection molded poly(ethylene terephthalate) (PET)/iPP microfibrillar blends

The poly(ethylene terephthalate) (PET)/isotactic polypropylene (iPP) in situ microfibrillar blends have been prepared through a “slit die extrusion–hot stretch–quenching” process, in which PET assumes microfibrils with 0.5–15 μm in diameter depending on the hot stretching ratios (HSR, the area of the transverse section of the die to the area of the transverse section of the extrudate). The injection molded specimens of virgin iPP and the PET/iPP blends were prepared by conventional injection molding (CIM) and by shear controlled orientation injection molding (SCORIM), respectively. The effect of shear stress and PET phase with different shape on superstructures and their distribution of injection molded microfibrillar samples were investigated by means of small angle X-ray scattering (SAXS) and wide angle X-ray scattering (WAXS). The shear (or elongational) flow during CIM and SCORIM can induce oriented lamellae (i.e. kebabs induced by shish). The shish-kebab structure appears not only in the skin and intermediated layers of CIM samples, but also in the whole region of SCORIM samples. For the neat iPP samples, a more “stretched” shish-kebab structure with higher orientation degree can be obtained in the interior region (intermediate and core layers) by the SCORIM method; moreover, the SCORIM can result in the growth of β-form crystal both in intermediate layer and in core layer, which only appears in intermediate layer of the neat iPP samples obtained by CIM. For the PET/iPP blends, interestingly, the addition of microfibrils as well as their aspect ratios can affect the orientation degree of kebabs only in the intermediate layers, and the addition of microfibrils with a low aspect ratio can bring out a considerable increase in the orientation degree of kebabs along the flow direction. However, for the SCORIM, the addition of microfibrils seems to be a minor effect on the orientation degree of kebabs, and it tends to hamper the formation of a more “stretched” shish-kebab structure and suppresses the growth of β-form crystal distinctly. Furthermore, It appears from experiment that γ-form crystals can grow successfully in this oriented iPP melt with the synergistic effect of shear and pressure only when the growth of β crystals can be restrained by some factors, such as the PET dispersed phase and thermal conditions (cooling rate).

Crystallization kinetics of iPP: Influence of operating conditions and molecular parameters

AbstractAn analysis of the crystallization kinetics of different grades of isotactic polypropylene (iPP) is here presented. To describe the crystallization kinetics as a function of molecular and operating parameters, the methodological path followed was the preparation of quenched samples of known cooling histories, calorimetric crystallization isotherms tests, differential scanning calorimetry cooling ramps, wide angle X‐ray diffraction (WAXD) measurements, and density determination. The WAXD analysis performed on the quenched iPP samples confirmed that during the fast cooling at least a crystalline structure and a mesomorphic one form. The diffractograms were analyzed by a deconvolution procedure, to identify the relationship between the cooling history and the distribution of the crystalline phases. The whole body of results (including calorimetric ones) provides a wide basis for the identification of a crystallization model suitable to describe solidification in polymer‐processing operations, based on the Kolmogoroff–Avrami–Evans nonisothermal approach. The kinetic parameters, determined for all the materials, are discussed, highlighting the effect of molecular parameters on the crystallization kinetics: molecular mass and distribution, tacticity, nucleating agents, and ethylene units content. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 1358–1367, 2007

Synthesis of hydrophilic isotactic polypropylenes promoted by metallocene catalysts

AbstractIn this paper, the synthesis of isotactic polypropylenes (i‐PP) modified with OH groups in sides (iPP‐OH), by combination of polyinsertion ansa‐metallocene catalysis and ring opening of propene oxide (PO), is described. i‐PP sequences interrupted by isolated ethylene/p‐methylstyrene units forms the backbone. This enchainment is obtained by controlled copolymerization of propene with ethylene and p‐methylstyrene comonomers in the presence of rac‐ethylenebis(1‐indenyl)zirconiumdichloride/methylalumoxane system. The metallation reaction of the p‐methyl group with sec‐BuLi generates an anionic center that can be reacted with PO. The iPP‐OH is the result of the monoaddition reaction of PO followed by hydrolysis with acidified methanol. By changing experimental conditions in the backbone synthesis, a tuned number of the functionalizable sites as well as polypropylene sequence lengths can be obtained. As a consequence, iPP samples with a different number of OH groups for the backbone can be synthesized after the PO monoaddition reaction. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 7008–7013, 2006

Non-isothermal melt-crystallization and mechanical properties of titanium(IV) oxide nanoparticle-filled isotactic polypropylene

The present contribution reports an investigation on the effect of the addition of titanium(IV) oxide (TiO 2) with diverse surface characteristics [i.e. neat, silica (SiO 2)-coated and stearic acid-coated] and various contents (ranging from 5 to 30 wt%) on non-isothermal melt-crystallization, subsequent melting behavior and mechanical properties of filled isotactic polypropylene (iPP). The cooling rate used ranged between 5 and 30 °C min −1.The crystallization exotherm for all of the samples investigated became larger and shifted towards a lower temperature as the cooling rate increased. The crystallization peak temperature ( T p) values for all of the iPP samples filled with neat TiO 2 nanoparticles were slightly greater than those of the neat iPP and were not affected by the variation in the filler content. For iPP samples filled with stearic acid-coated TiO 2 nanoparticles, the T p values were not much different from those of the neat iPP. For iPP samples filled with 5, 20, and 30 wt% SiO 2-coated TiO 2 nanoparticles, marked differences in the T p values in comparison with those of the neat iPP were observed. Together with the Avrami analysis, the nucleation ability of the various fillers investigated can be ranked as follows: SiO 2-coated TiO 2 nanoparticles >neat TiO 2 nanoparticles >stearic acid-coated TiO 2 nanoparticles. Lastly, the presence of the nanoparticles caused the rigidity of the resulting composites to increase (especially when the filler content was greater than or equal to 20 wt%.

Stretching Isotactic Polypropylene: From “cross-β” to Crosshatches, from γ Form to α Form

A method for the detailed analysis of the possible crystalline structures and preferred orientations of crystals that develop during stretching metallocene-made isotactic polypropylene (iPP) samples is presented. The analysis is illustrated in the case of various iPP samples characterized by a different distribution of stereodefects along the chain, generated by different catalysts. The method is based on the calculation of schematic X-ray fiber diffraction patterns of model structures and the comparison between calculated and experimental diffraction patterns recorded during stretching. The analyzed samples, initially crystallized in the γ form or in disordered modifications intermediate between the α and γ forms, undergo structural and morphological transformations during stretching. At low deformations, crystals of the γ form are oriented with chain axes directed nearly perpendicular to the stretching direction. The γ form gradually transforms into the α form, with chain axes parallel to the stretching direction, with increasing deformation. At high deformations, when the sample is almost totally crystallized in the α form, the crosshatched morphology, characterized by the presence of daughter lamellae epitaxially grown on mother lamellae, develops. The origin of the nonstandard orientation of crystals of the γ form with chain axes perpendicular to the fiber axis (perpendicular chain axis orientation or “cross-β”) is investigated. This kind of preferred crystal orientation involves lamellae of the γ form frozen in strained regions of the polymer matrix. Its development may be ascribed to the peculiar architecture of the γ form, consisting in a nearly perpendicular arrangement of the chain axes in the unit cell, and to the elongated shape of the γ lamellae in the direction normal, rather than parallel, to the chain axes. At low draw ratios, the chain axes, which emerge at the crystal boundaries according to two mutually perpendicular directions, act as anchors for the crystals of the γ form in the perpendicular chain axis orientation. At high draw ratios, these crystals transform into the α form and assume the orientation with chain axes parallel to the stretching direction as in the standard fiber morphology. The relationships between structure and morphology on one hand and mechanical properties of the analyzed iPP samples on the other hand are also discussed.

Effects of high molecular weight species on shear-induced orientation and crystallization of isotactic polypropylene

In situ rheo-SAXS (small-angle X-ray scattering) and—rheo-WAXD (wide-angle X-ray diffraction) techniques were used to investigate the role of high molecular weight species on the evolution of oriented microstructure in isotactic polypropylene (iPP) melt under shear flow. The two iPP samples, designated as PP-A and PP-B, respectively, had the same number-average (Mn) but different weight-average (Mw) and Z-average (Mz) molecular weights. Molecular weight distribution (MWD) of PP-A and PP-B was such that for MW<105 the MWD curves overlapped; whereas in the high MW tail region, the amount of high molecular weight species was higher in PP-B than PP-A. Both samples were subjected to an identical shear condition (rate=60s−1, duration=5s, T=155°C). In situ 2D SAXS and WAXD images allowed the tracking of shear-induced oriented structures in the melt. It was found that the shish structures evolved much earlier, and the degree of crystal orientation and oriented crystal fractions were higher in PP-B than PP-A. Moreover, PP-B exhibited faster crystallization kinetics than PP-A. These results, along with the predictions of double reptation models of chain motion and experimental studies of chain conformation dynamics in dilute solutions under flow, suggest the following: When a polymer melt that consists of entangled chains of different lengths is deformed, the chain segments aligned with the flow eigenvector can undergo the abrupt coil-stretch-like transition, while other segments would remain in the coiled state. Since, flow-induced orientation decays much more slowly for long chains than for short chains, oriented high molecular weight species play a prominent role in formation of the stretched sections, where shish originates. Our experimental results are strong evidence of the hypothesis that even a small increase in the concentration of high molecular weight species causes a significant increase in the the formation, stability and concentration of the flow-induced oriented microstructure.

Ageing of isotactic polypropylene due to morphology evolution, experimental limitations of realtime density measurements with a gradient column

Ageing in crystalline polymers is responsible for the deterioration of physical properties leading, for example, to a decrease in toughness and to dimensional changes that are to some extent responsible for warpage and scrap production in injection molding. Since, it depends on the mutual transformation of stable and metastable phases, being always related to changes in morphological organization, it is here preferred to call it ‘Morphological ageing’.Although, one would expect the ageing regime to be determined by the complex morphology with amorphous phases of different mobility and eventually multiple crystalline phases, transformed into each other at an associated transition, existing literature always shows a more trivial linear with the logarithm of time dependence of every probe used to describe ageing.Existing literature always overlooks the initial morphology, often complex, or, adopts well equilibrated samples, not representative of processing conditions. In this work, ageing of iPP was addressed producing samples characterized by an homogeneous morphology even at the largest cooling rates adopted using a CCT approach. This paper describes few of the many attempts undertaken to quantitatively relate ageing to initial morphology in iPP melt solidified under conditions emulating polymer processing.Ageing was monitored by measuring the density time dependence offline, i.e. separately applying the ageing protocol, and online in a gradient column conditioned at the ageing temperature following samples' apparent density evolution. The offline method can give hints about the role of temperature and initial morphology but the results so obtained suffer from significant data scattering. The online method on the other hand, can provide a more accurate interpretation of ageing if due account is made of all the features of the fluid dynamic transient superimposed on the densification due to morphological ageing.Densification during ageing of the iPP samples used, solidified between 1 and 100°C/s, takes place as a superposition of two phenomena: one is the linear increase of density with the logarithm of time, which is quantitatively related to the initial morphology, i.e. the cooling rate, and the other is a density jump that takes place at times much smaller than the available resolution and thus not quantitatively accessible.

Large strain requirements for shear-induced crystallization of isotactic polypropylene

We studied strain effects on the crystallization of a series of isotactic polypropylenes (iPP) of various molecular weights using rheology, rheo-optical measurements, differential scanning calorimetry (DSC), and wide-angle X-ray diffraction (WAXD). The samples were pre-sheared and then crystallized both at the same temperature. Transmitted light intensity measurements demonstrate that the effect of pre-shear on crystallization rate keeps increasing with strain up to surprisingly large strain levels, much beyond strains that are required to reach steady shear flow (at given We). Crystal orientation sets in at a total strain of about % MathType!Translator!2!1!AMS LaTeX.tdl!TeX -- AMS-LaTeX! % MathType!MTEF!2!1!+- % feaaeaart1ev0aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbbjxAHX % garmWu51MyVXgatuuDJXwAK1uy0HwmaeHbfv3ySLgzG0uy0Hgip5wz % aebbnrfifHhDYfgasaacH8qrps0lbbf9q8WrFfeuY-Hhbbf9v8qqaq % Fr0xc9pk0xbba9q8WqFfea0-yr0RYxir-Jbba9q8aq0-yq-He9q8qq % Q8frFve9Fve9Ff0dmeaabaqaciGacaGaaeqabaWaaeWaeaaakeaacq % aHZoWzdaWgaaWcbaGaaGimaaqabaGccqGH9aqpcaaI2aGaaGimaiaa % icdaaaa!3D61! $$\gamma _{0} = 600$$ or higher. WAXD and DSC measurements corroborated the light transmission results. Total shear strains % MathType!Translator!2!1!AMS LaTeX.tdl!TeX -- AMS-LaTeX! % MathType!MTEF!2!1!+- % feaaeaart1ev0aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbbjxAHX % garmWu51MyVXgatuuDJXwAK1uy0HwmaeHbfv3ySLgzG0uy0Hgip5wz % aebbnrfifHhDYfgasaacH8qrps0lbbf9q8WrFfeuY-Hhbbf9v8qqaq % Fr0xc9pk0xbba9q8WqFfea0-yr0RYxir-Jbba9q8aq0-yq-He9q8qq % Q8frFve9Fve9Ff0dmeaabaqaciGacaGaaeqabaWaaeWaeaaakeaacq % aHZoWzdaWgaaWcbaGaaGimaaqabaGccqGH9aqpcaaIYaGaaGimaiaa % icdaaaa!3D5D! $$\gamma _{0} = 200$$ to 1000 were applied to the iPP samples at the beginning of a crystallization experiment, after the samples had reached the crystallization temperature of 145°C (under-cooled state). A constant Weissenberg number We=1 (We is defined as the product of shear rate and a relaxation time) was maintained for all pre-shearing of this study. We=1 corresponds to the onset of shear thinning in steady shear. Deborah number values were low, De<<1, indicating that steady shear flow had been reached in all pre-shearing runs (De is defined as the ratio of relaxation time to pre-shearing time). Further studies are needed at high We as there are indications that strain requirements are much reduced at high We. A fundamental understanding is still missing.

The influence of thermoelastomers on the crystallization behavior of isotactic polypropylene under shear

AbstractThe crystallization behaviors of isotactic polypropylene (iPP) and its blends with thermoelastomers have been investigated with in situ X‐ray scattering and optic microscopy. At quiescent condition, the crystallization kinetics of iPP is not affected by the presence of elastomers; while determined by the viscosity, the differences are observed on sheared samples. With a fixed shear strain, the crystallization rate increases with increasing the shear rate. The fraction of oriented lamellar crystals in blends is higher than that in pure iPP sample, while the percentage of β phase is reduced by the presence of the elastomers. On the basis of experimental results, no direct correlation among the fraction of oriented lamellae, the percentage of β phase, and growth rate can be deduced. The evolution of the fraction of oriented lamellae supports that shear field promotes nucleation rather than growth process. Shear flow induces the formation of nuclei not only with preferring orientation but also with random orientation. The total density of nuclei, which determines the crystallization kinetics, does not control the ratio between nuclei with and without preferring orientation, which determines the fraction of oriented lamellae. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1188–1198, 2006

Role of the interphase dynamics in the induction time of the thermo-oxidation of isotactic polypropylene

This work completes an earlier study on the influence of molar mass and microstructure on the thermal stability of low molar mass isotactic polypropylenes (iPP). The relative dependence of induction time ( t i) on both parameters has been assessed for new metallocene iPP samples (M-PP) with molar masses from 65,000 to 182,000. The new M-PP series includes one metallocene ethylene–propylene copolymer (M-EP) with an ethylene content of 2.6 mol%. When the new t i data are considered together with those previously reported, neither the molar mass nor the microstructure, taken as sole parameters, is able to explain the global t i evolution of the M-PP samples. On the basis of the results corresponding to the M-EP copolymer, it is proposed that local chain dynamics occurring at high free volume regions, associated to propylene segment interruptions, play a main role in the ability of the interphase to initiate the oxidation. The correlation of the characteristic interphase chain dynamics, as measured by DMTA, with t i data supports this suggestion. This hypothesis provides a more unified insight about the actual origin of the iPP thermo-oxidation, as it integrates the influence of parameters which have been found to drive partially the thermal stability of iPP, in particular, molar mass and microstructure.

New kinetic aspects on the mechanism of thermal oxidative degradation of polypropylenes with various tacticities

Thermal oxidative degradation behavior of polypropylene (PP) with different tacticities was studied based on the activation energy (Δ E) data obtained by thermogravimetric analysis (TGA). The Δ E value showed a negative proportion to the content of meso pentad fraction ( mmmm) in all of isotactic PP (iPP) samples, and that of syndiotactic PP sample considerably deviated from this negative proportion relationship. Since the value of mmmm was directly related to polymer chain conformation, the Δ E value was thought to have close connection with the concentration of 3 1 helix conformation in the iPPs. The Δ E changes would be caused by the competition between uni- and bimolecular hydroperoxide decomposition, which was controlled by concentration and character of conformations of PPs.

Effect of Clay/Diamond and Clay/Carbon Nanosystems on Structure‐Properties Relationships of iPP

AbstractThe effect of the addition of two combined fillers, smectite clay and diamond and smectite clay and carbon nanoparticles, on structure, morphology, isothermal and non isothermal crystallization behaviour, tensile and thermal properties of isotactic polypropylene (iPP) has been investigated by using several techniques: wide angle X‐ray diffraction, optical and scanning electron microscopy, thermogravimetry, differential scanning calorimetry and tensile techniques. It was found that nanoparticles of diamond and carbon favour the nucleation of the β‐form of iPP crystal, whereas the clay nanolayers do not have any influence on the crystal structure of iPP. The thermal stability of iPP/(clay+diamond) and iPP/(clay+carbon) is improved with respect to neat iPP, whereas no influence is detected when only clay is added to iPP. At the given crystallization conditions, the overall crystallization peak of iPP/(clay+diamond) almost exactly overlaps the crystallization peak of neat iPP, whereas in the case of iPP/clay and iPP/(clay+carbon) the maximum of the crystallization peaks is shifted to higher temperature. The spherulite growth rate, G values do not differ from one another. The iPP/(clay+carbon) system shows ductile behavior. The other systems show brittle behavior with failure before necking. These results were related with the very high percentage of beta phase present in the samples of iPP/(clay+carbon).

Structural deformation behavior of isotactic polypropylene with different molecular characteristics during hot drawing process

Structural evolution in hot drawing process of isotactic polypropylene (iPP) films with different molecular weight distribution (MWD) and isotacticity (IT) was investigated by in situ time-resolved measurements of synchrotron-sourced wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS). Any significant difference was not recognized among different molecular characteristics as for the changes in the WAXD patterns, indicating that the deformation behavior viewed at the crystal lattice scale was almost the same among these samples. On the other hand, the deformation behavior of lamellar stacking structure was found to be significantly dependent on the molecular characteristics of the sample used. For iPP sample with narrower MWD and higher IT, only the lamellar stacking structure with c-axis crystallites oriented along the drawing direction was detected at the deformation stage after necking, but the oriented fibrillar structure was observed in addition to the lamellar stacking structure for the iPP sample with broader MWD and IT distribution. The structural deformation models were presented for both the samples with different molecular characteristics, and these models were reasonably related with the difference in the stress–strain curve.

Effect of the polypropylene type on polymer–diluent phase diagrams and membrane structure in membranes formed via the TIPS process: Part II. Syndiotactic and isotactic polypropylenes produced using metallocene catalysts

Syndiotactic polypropylene (sPP) and isotactic polypropylene (iPP) were used to investigate the effect of the polypropylene microstructure (tacticity) on phase diagrams for PP–diphenyl ether systems and the morphology of membranes made from these systems. The syndiotactic and isotactic polypropylenes were both produced via metallocene catalysts and had similar average molecular weights. It was found experimentally that polypropylene microstructure affects the location of the binodal curve and the morphology of membranes produced via the thermally induced phase separation (TIPS) process. The influence on the binodal curve was attributed to stereochemistry effects, chain rigidity, and local chain packing. In isothermal experiments, droplet size was similar for the two types of PP, while in non-isothermal experiments, the sPP sample yielded larger droplets than the iPP sample. These results were attributed mainly to viscous effects and to differences in the growth period (resulting from a shift in the cloud point and dynamic crystallization curves). The final morphology of the sPP membrane was related to polymer chain flexibility and to shrinkage of the sample during diphenyl ether extraction and sample drying.