iPP Samples Research Articles

A reexamination of the relationship between β to α crystal transition and the surface crater formation in biaxially oriented polypropylene film

The formation mechanism of the crater-like surface of biaxially oriented polypropylene (BOPP) film remains controversial. To clarify this issue, in this work, three isotactic polypropylene (iPP) samples with different tacticities are selected to prepare BOPP films. It is shown that the relative β crystal content (kβ) in the cast sheet increases with tacticity. The β crystal transforms to the α crystal when stretched at 145 °C along the machine direction. For all the samples, rough surfaces are obtained even if the kβ value of the cast sheet is very low (kβ < 0.1). The arithmetic mean roughness (Ra) of the BOPP films increases with the increase of the kβ in the cast sheet. While higher kβ in the iPP cast sheet contributes to a higher Ra value, the β crystallites hinder the enlargement of roughening rings, and cause surface defects (cavitation), which are detrimental to applications. Our results indicate that the β crystal in the cast sheet is not essential for the crater formation and a higher β crystal content is not always beneficial for BOPP capacitor films.

Measuring Oxygen Solubility in Amorphous and Semicrystalline Polyolefins Using Test Particle Insertion: A Comparative Study of Polyethylene and Isotactic Polypropylene.

The test particle insertion method is used to study the solubility of oxygen in two commonly used polymers: polyethylene (PE) and isotactic polypropylene (iPP). Amorphous samples for both polymers were prepared by means of Monte Carlo and molecular dynamics simulations, and the oxygen solubility was measured across different temperatures. The solubility-temperature dependence for iPP proved to be nonmonotonic due to the interplay between binding and reorganizational enthalpy, while for PE, it appeared to be monotonic based on the available data in the studied temperature range. A broad comparison to experiments and simulations is included. Further oxygen insertions were also performed in semicrystalline PE and iPP samples at ambient temperature, and the obtained values were compared to a linear relationship which correlates the solubility in the purely amorphous phase with the solubility in the crystalline phase. The solubility of PE closely follows the linear relationship, while iPP exhibits some divergence. All the semicrystalline samples were previously annealed at elevated temperatures for long periods (a few μs), and a strong effect of annealing was observed on the structure and the solubility of iPP. A well-developed iPP lamellar structure emerged at longer annealing times, while PE develops that structure already in the early crystallization stages. The solubility of semicrystalline iPP samples with lamellar morphology exhibited better agreement with extrapolated solubility values of the amorphous state─the extrapolation was made using a linear relationship connecting solubility in the purely amorphous phase and solubility in mixed phases (amorphous and crystalline). Results on the correlation of the solubility with the local structural ordering are also present.

The Impact of Regiodefects on the Melt-Memory of Isotactic Polypropylene.

The memory of crystalline phase in the melt of isotactic polypropylene (iPP) in regiodefective samples of iPP characterized by different concentrations regiodefects, constituted by secondary 2,1 propene units, is studied. The self-nucleation (SN) experiments have demonstrated that the presence of 2,1 regiodefects produces a strong memory of the crystalline phase in the melt that persists up to temperatures much higher than the melting temperature. The extension of the heterogeneous melt (domain II) containing self-nuclei increases with increasing the concentration of regiodefects. The higher the concentration of regiodefects the higher the temperature at which the self-nuclei are dissolved and the homogeneous melt is achieved. This demonstrates that a strong memory of the crystalline phase of iPP in the melt exists not only in copolymers with noncrystallizable bulky comonomeric units rejected from the crystals but even when small defects are largely included in the crystals.

Effect of microstructure tuning by rapid heat cycle molding on the photodegradation behavior of isotactic polypropylene

AbstractRapid heat cycling molding (RHCM) is an injection molding technique that improves the product quality of isotactic polypropylene (iPP) by tuning the microstructure, but its effect on the photodegradation stability remains to be investigated. In this work, the effect of RHCM tuned microstructure on the photodegradation behavior of iPP was investigated and compared with conventional injection molding (CIM). Differential scanning calorimetry (DSC), x‐ray diffraction (XRD), Fourier transform infrared spectrometer (FTIR), atomic force microscopy (AFM), and shore durometer (D scale) were used to examine the photodegradation behavior of iPP samples. The results showed that RHCM increased the crystallinity, crystal size, and β‐crystal content of iPP samples. With the increase of photodegradation time, iPP samples injected with RHCM showed higher stability in terms of microstructure and surface quality. The relative changes in CI and Ra of RHCM90 were reduced by 48% and 40% after 800 h of UV irradiation, respectively. In addition, the combined results of dynamic mechanical analysis (DMA) and UV–visible absorption spectrum showed that RHCM promoted the development of crystalline structure and reduced the mobility of the surface chain segments as well as UV absorption coefficient. These combined effects contributed positively to its photodegradation stability.

Effect of Crystalline Morphology on Electrical Tree Morphology and Growth Characteristics of PP Insulation: From Mesoscopic to Macroscopic

This paper aims to investigate the effects of crystalline morphology on electrical tree morphology and growth characteristics of isotactic polypropylene (iPP). The crystalline morphology of iPP is adjusted by altering the crystallized temperature, and the mesoscopic electrical treeing in these samples with a thickness of 280 μm is studied. The polarizing microscope images show that an electrical tree is easier to grow along the spherulites boundary and form a bush tree within spherulites. Compared with samples crystallized at a cooling rate of 2.5 °C/min and the isothermal temperature of 132 °C, the spherulite size of the sample crystallized at a cooling rate of 10 °C/min significantly decreases, resulting in an obvious decrease of the accumulated damage area and length of the mesoscopic electrical tree. The macroscopic electrical treeing is also conducted in the iPP samples with different crystalline morphologies with a thickness of 2 mm. There exists a relation between the mesoscopic and macroscopic electrical growth characteristics by using the Pearson correlation coefficient. It is concluded that the reduction of the spherulite size and the spherulite boundary gap limits the partial discharge and the development of an electrical tree in the amorphous region at the spherulite interface, making it easier to form the bush tree and leading to improved resistance to electrical treeing.

Effect of Shear Stress during Processing on Structure, Morphology, and Properties of Isotactic Polypropylene Nucleated with Silsesquioxane-Based β-Nucleating Agent.

The study aimed to determine the influence of shear stress during real-life industrial processes such as compression molding and injection molding to different cavities on the crystallization of the isotactic polypropylene nucleated with a novel silsesquioxane-based β-nucleating agent. Octakis(N2,N6-dicyclohexyl-4-(3-(dimethylsiloxy)propyl)naphthalene-2,6-dicarboxamido)octasilsesquioxane (SF-B01) is a highly effective nucleating agent (NA) based on the hybrid organic-inorganic silsesquioxane cage. The samples containing various amounts of the silsesquioxane-based and commercial iPP β-nucleants (0.01-0.5 wt%) were prepared by compression molding and injection molding, including forming in the cavities with different thicknesses. The study of the thermal properties, morphology, and mechanical properties of iPP samples allows for obtaining comprehensive information about the efficiency of silsesquioxane-based NA in shearing conditions during the forming. As a reference sample, iPP nucleated by commercial β-NA (namely N2,N6-dicyclohexylnaphthalene-2,6-dicarboxamide, NU-100) was used. The static tensile test assessed the mechanical properties of pure and nucleated iPP samples formed in different shearing conditions. Variations of the β-nucleation efficiency of the silsesquioxane-based and commercial nucleating agents caused by shear forces accompanying the crystallization process during forming were evaluated by differential scanning calorimetry (DSC) and wide-angle X-ray scattering (WAXS). The investigations of changes in the mechanism of interactions between silsesquioxane and commercial nucleating agents were supplemented by rheological analysis of crystallization. It was found that despite the differences in the chemical structure and solubility of the two nucleating agents, they influence the formation of the hexagonal iPP phase in a similar way, taking into consideration the shearing and cooling conditions.

Influence of Molecular Weight on the Nucleation and Growth of Different Crystal Forms in Isotactic Polypropylene: In Situ Synchrotron Microfocus X-Ray Scattering Combined with Fast-Scanning Chip Calorimetry Investigations

The molecular weight (Mw)-dependent formation of mesophase and the α phase of isotactic polypropylene (iPP) was explored by utilizing in situ synchrotron microfocus X-ray scattering measurements combined with a portable fast-scanning chip calorimetry (FSC) technique. A phase diagram illustrating the relationship between the lamellar long spacing (dac) and crystallization as well as melting temperatures of three iPP samples with different molecular weights suggests that a common crystallization line was shared by the mesophase of all three samples, whereas three different crystallization lines were observed for these iPP samples. Furthermore, unlike in the case of the α phase where the melting lines were significantly different from their crystallization lines, the melting line of the mesophase was found to overlap with its crystallization line. The results indicate that the crystallization of the α phase satisfied the multistage crystallization model in which an intermediate layer is suggested to be generated prior to the formation of native crystallites. The Mw-independent formation of the mesophase demonstrated that the nucleus size did not rely on the Mw of iPP because the final sizes of the mesophase were close to the initial nuclei sizes. The larger difference of the lamellar long spacing presented in the α phase of iPP with different Mw indicated that the growth from nuclei to crystallites was significantly influenced by the Mw.

Competition of shearing and cavitation effects on the deformation behavior of isotactic polypropylene during stretching

Owing to the unique cross-hatched structure of α crystal, the deformation behavior of isotactic polypropylene (iPP) differs considerably with the traditional polymers. The influence of crystalline structures on the cavitation and shearing effects of α-iPP under tensile loading is still not well established. In this work, various iPP samples were prepared via crystallizing from 0 °C to 130 °C, after which the crystalline structures were characterized via differential scanning calorimetry, polarized optical microscopy and scanning electron microscopy. The lamellar thickness and crystallinity of α-iPP precursor films change slightly, while the content of tangential lamellae in α-spherulite and the spherulite size increase significantly with the decreasing supercooling. On the other hand, the morphological evolution of α-iPP during stretching was tracked by in-situ two-dimensional small angle X-ray scattering. It is found that cavitation effect become dominant instead of shearing effect when the size of α-spherulite exceeds 25 μm. Furthermore, we disclose four typical deformation behaviors of α-iPP resulting from the competition between intra-spherulitic deformation and inter-spherulitic deformation, namely the shearing without cavitation, shearing then localized cavitation, homogeneous cavitation with concomitant shearing, and cavitation with absence of shearing. Accordingly, we construct a route for the structural evolution of α-iPP with different crystalline structures during stretching, which provides an effective guidance to produce diverse iPP films with desired structures and tunable functionalities.

A combined melt-stretching and quenching setup for experimental studies of polymer crystallization under complex flow-temperature environments.

A combined melt-stretching and quenching setup is designed and developed to allow experimental investigations of polymer crystallization under the complex flow-temperature environments comparable to those encountered in the actual industrial processing. The melt-stretching proceeds by two drums rotating in the opposite directions with simultaneous recording of a stress-strain curve, where the Hencky strain and strain rate (≤233s-1) are adjustable over a large range. After stretching, liquid N2 is used as a cooling medium to quench the free-standing melt, which is sprayed directly to the deformed melt driven by an electric pump. To ensure a high cooling efficiency, a three-way solenoid valve is employed to execute a sequential control of the liquid N2 flow direction to reduce the boil-off of liquid N2 before entering the sample chamber. The melt cooling rate depends on the liquid N2 flow rate controlled by a flow valve, which is up to 221 °C/s when quenching the isotactic polypropylene (iPP) melt with a thickness of 0.28mm at 150 °C. Two independent temperature control modules are designed to meet the requirements of different stages of melt-stretching and quenching. To verify the capability of the setup, we have performed the melt-stretching and quenching experiments on iPP samples. The setup is demonstrated to be a valuable new tool to study polymer crystallization under coupled flow-cooling fields.

Mold temperature- and molar mass-dependent structural formation in micro-injection molding of isotactic polypropylene

The structural formation and development of isotactic polypropylene (iPP) upon the micro-injection molding process was investigated at different mold temperatures and molecular weights utilizing a real-time synchrotron radiation small angle X-ray scattering (SAXS) technique combined with a customized micro-injection molding apparatus. Shish-kebab structure and parent-daughter lamellae were found to be formed during micro-injection molding for all iPP samples. In the case of kebab lamellae, a considerable growth in the long period and in the average thickness of lamellar crystallites and amorphous domains is observed at initial stages of crystallization for samples molded at varying temperatures. This effect is caused by the successive formation of thin lamellae in the outer layer and thick lamellae in the inner layer during the manufacturing process as evidenced by the spatial distribution of the crystalline lamellae across the thickness. In addition, the length of the shish formation increases remarkably at the onset of crystallization, the extent of which is dependent on the mold temperature. Despite the large changes of the lamellar stacks and the shish misorientation, the final length of the shish remains essentially unchanged when varying mold temperature. Since there is a critical orientation molecular weight above which the chains are stretched and oriented to form stable shish, the iPP sample with a low molar mass exhibits an overall decrease in the scattering intensity of SAXS patterns compared to the high molecular weight polypropylene. • The structural formation of iPP is investigated utilizing a customized micro-injection molding device. • Shish-kebab and parent-daughter lamellae are formed during micro-injection molding. • A considerable increase in the long period is observed at initial stages of crystallization. • The length of the shish formation increases remarkably at the onset of crystallization. • The final shish length remains essentially unchanged when varying mold temperature.

Role of Rigid–Amorphous chains on mechanical properties of polypropylene solid using DSC, WAXD, SAXS, and Raman spectroscopy

We evaluated the rigid–amorphous (RA) fraction of isotactic polypropylene (iPP) samples by various measurement techniques and investigated the role of the RA chains on the mechanical properties of iPP. The RA fraction was increased by annealing above the α-relaxation temperature, although the RA fraction was almost independent of the molecular weight. The RA fraction obtained by differential scanning calorimetry increased linearly with the number of consecutive helical chains with 12–14 monomer units, suggesting that the RA chains are attributable to crystalline defects. Moreover, the α-relaxation temperature varied linearly with the RA fraction; the RA chains present as crystalline defects were removed from the crystalline structure by α relaxation. The yielding region of the stress–strain curve became broad by increasing the RA fraction; the fragmentation strength of the lamellar crystals was widely distributed owing to the large number of crystalline defects.

Morphology and crystallization kinetics of G‐resins and isotactic polypropylene

AbstractIsothermal crystallization and morphology of G‐resin (GPP) and isotactic polypropylene (iPP) samples were investigated and compared using synchrotron small‐ and wide‐angle X‐ray scattering/diffraction (SAXS/WAXD) and differential scanning calorimetry (DSC). Two GPP resins (MT12‐s and MT18‐s) are iPP containing a small amount of 1‐butene. All polymers have similar MW and PDI, denoted by V30G, MT12‐s, and MT18‐s with 1‐butene = 0.00, 6.43, and 5.20 wt%, respectively. DSC isothermal crystallization at 130°C showed that the crystallinity of G resins were lower, but the crystallization rate of G resins was faster in the order as MT18‐s ≥ MT12‐s &gt; V30G. Simultaneous synchrotron SAXS/WAXD was employed to determine polymer morphology crystallized from the melt. The electron density correlation function was calculated to estimate the lamellar thickness. The long periods (D) are around 19.0, 25.8, and 25.8 nm for V30G, MT12‐s, and MT18‐s, respectively. The GPP samples have a larger amorphous portion and a higher crystalline thickness (L) but a lower L/D ratio than IPP. The degree of crystallinity by WAXD is also smaller for GPP, that is, MT18‐s (31%) ≈ MT12‐s (31%) &lt; V30G (&gt;50%). In addition to the monoclinic α forms for all the PP samples, the orthorhombic γ forms were found only for MT12‐s and MT18‐s. The crystallization of G‐resins should be nucleated from the transparent filler and are not fully an intrinsic property of the copolymer chains.

3D printing-enabled self-assembling β-nucleating agent alignment: Structural evolution and mechanical performances

Achieving good alignment of fillers within the composites is critical towards developing the high performance of the final 3D printed objects. Herein, flow-induced the alignment of self-assembling β-nucleating agent (SANAβ) within the isotactic polypropylene (iPP) matrix was achieved via fused deposition modeling (FDM) process. The influence of printing speed on the alignment of SANAβ, crystalline structure, and mechanical properties of FDM printed samples were investigated. Due to the converging nozzle effect, SANAβ fibers were fragmented from 26.13 μm to 9.98 μm in length, generating more oriented templates and further preferentially aligned along the printing direction to trigger the iPP crystallization. As a result, the crystal morphology of the printed sample was transformed from α-form to β-form hybrid shish-kebab structure. Highly oriented β-crystal was observed in iPP/SANAβ (FT) samples, as evidenced by 2D-WAXD. 2D-SAXS results revealed the formation of plenty of oriented lamellae in FT samples, which were arranged perpendicular and parallel to the printing direction. The aligned SANAβ also contributed to maintaining a high β-crystal content to overcome the shortcomings of rapid decrease of β-crystal for iPP at a high printing speed. Benefiting from this, with the increasing printing speed, compared with elongation at break of iPP samples decreased from 241.36% to 61.32%, those of FT sample increased dramatically from 305.07% to 390.23% but with a slight fade of less than 1% in the tensile strength. Additionally, the as-printed honeycomb array structure of FT sample presented superior energy absorption capacity and thus could function as loading-bear objects, highlighting the advantages of the alignment of self-reinforcing β-form hybrid shish-kebab.

In-Situ Synchrotron X-ray Study on the Structure Variation of Morphology-Identified Injection-Molded β-Nucleated iPP under Tensile Deformation

The deformation behavior of semi-crystalline polymers is strongly dependent on the morphology formed during processing. In this study, in-situ synchrotron X-ray was firstly used to identify the morphological distributions of injection-molded isotactic polypropylene (iPP) with different concentrations of β-nucleating agent. It was found that under relatively high concentration of β-nucleating agent (i.e., ≥0.03 wt.%), the outer region (skin and shear region) of the iPP was dominated by mainly highly oriented α-phase as well as certain amount γ-phase, while the core region was rich in β-crystals with little if any orientation. The addition of the β-nucleating agent was beneficial for the formation of lamellae with large lamellar stacking distance in the shear layer. Then the synchrotron X-ray was applied to study the structure variation of those morphology-identified samples under tensile deformation. It was found that voids and cavities along the stretching direction existed in the deformed iPP samples and their volume increased with increasing concentration of β-nucleating agent. The increased volume of void and cavity was associated with the β to α phase transition, which mainly occurred at the core region. In addition, upon stretching crystalline fragmentation and rearrangement took place following the formation of thinner lamellae.

Assessing Fast Structure Formation Processes in Isotactic Polypropylene with a Combination of Nanofocus X-ray Diffraction and In Situ Nanocalorimetry.

A combination of in situ nanocalorimetry with simultaneous nanofocus 2D Wide-Angle X-ray Scattering (WAXS) was used to study polymorphic behaviour and structure formation in a single micro-drop of isotactic polypropylene (iPP) with defined thermal history. We were able to generate, detect, and characterize a number of different iPP morphologies using our custom-built ultrafast chip-based nanocalorimetry instrument designed for use with the European Synchrotron Radiation Facility (ESRF) high intensity nanofocus X-ray beamline facility. The detected iPP morphologies included monoclinic alpha-phase crystals, mesophase, and mixed morphologies with different mesophase/crystalline compositional ratios. Monoclinic crystals formed from the mesophase became unstable at heating rates above 40 K s−1 and showed melting temperatures as low as ~30 K below those measured for iPP crystals formed by slow cooling. We also studied the real-time melt crystallization of nanogram-sized iPP samples. Our analysis revealed a mesophase nucleation time of around 1 s and the co-existence of mesophase and growing disordered crystals at high supercooling ≤328 K. The further increase of the iPP crystallization temperature to 338 K changed nucleation from homogeneous to heterogeneous. No mesophase was detected above 348 K. Low supercooling (≥378 K) led to the continuous growth of the alpha-phase crystals. In conclusion, we have, for the first time, measured the mesophase nucleation time of supercooled iPP melted under isothermal crystallization conditions using a dedicated experimental setup designed to allow simultaneous ultrafast chip-based nanocalorimetry and nanofocus X-ray diffraction analyses. We also provided experimental evidence that upon heating, the mesophase converts directly into thermodynamically stable monoclinic alpha-phase crystals via perfection and reorganization and not via partial melting. The complex phase behaviour of iPP and its dependence on both crystallization temperature and time is presented here using a time–temperature–transformation (TTT) diagram.

Mechanical Property of Polypropylene Gels Associated with That of Molten Polypropylenes.

This study aims to understand the fundamental mechanical relationship between polypropylene (PP)-gels and solid PPs without solvent through mechanical and thermal analyses, by which the mechanical similarities between molten PPs and PP gels were found, leading to the reliable estimate of the mechanical properties of semi-crystalline gels. The gelation of syndiotactic and isotactic polypropylenes (sPP and iPP) was found when PPs were dissolved in 1,2,3,4-tetrahydronaphthalene (tetralin). Interestingly, it was found that the storage modulus of sPP-gel became higher than that of iPP-gel at low PP concentration (<~40 wt%). The result was distinctly different from the result of neat solid PPs (without solvent), where the modulus of solid sPP is generally significantly lower than that of solid iPP. Such inversion behavior in the mechanical property of semi-crystalline gels had not been reported and discussed before. By further investigation of the storage moduli of neat sPP and iPP, it was found that the storage modulus of sPP became higher than that of iPP above the melting points of PP, which was similar to the behavior of the storage moduli observed in the diluted PP-gels. Such similarity between PP-gels and PP melts was also observed within iPP samples with different molecular weights.

Markedly improved photo-oxidation stability of α form isotactic polypropylene with nodular morphology

Photo-oxidation degradation behaviors of isotactic polypropylene (iPP) films with different crystalline morphology were investigated by artificially accelerated aging test. The α form iPP with nodular morphology was obtained by quenching the melt and subsequent annealing process, and its photo-oxidation degradation behavior was compared with that of the melt-crystallized α form iPP sample. Fourier transform infrared spectroscopy (FTIR), gel permeation chromatography (GPC), differential scanning calorimetry (DSC), X-ray scattering, dynamic mechanical analysis (DMA) and atomic force microscopy (AFM) were utilized to characterize the microstructural variations of these two kinds of iPP films during photo-oxidation degradation process. The results showed that iPP film of nodular morphology exhibits improved photo-oxidation stability compared with that of lamellar and spherulitic structure. UV-Vis spectra and O2 diffusivity test of unexposed and aged samples demonstrated that the iPP film of nodular morphology has higher transparency and lower O2 diffusivity than that of lamellar morphology. Combining the microstructural analysis and surface morphology observation, the improved photo-oxidation stability of “nodular” iPP was correlated with its higher UV light transparency, lower O2 diffusion coefficient as well as the slower segmental mobility in the amorphous phase.

A Simple Way to Control Small-Strain Cavitation in Die-Drawn Isotactic Polypropylene

Increasing attention has been focused on small-strain cavitation behavior of semicrystalline materials, due to the increased need of porous films with low cost and advanced chemical stability. The control of cavitation is crucial to the properties of the final product as well as the fabrication process. In this work, we proposed a simple way to regulate small-strain cavitation of isotactic polypropylene (iPP) samples via modifying the fraction of lamellae with their normal perpendicular to the stretching direction by stretching at 125 °C. Deformation at 125 °C could effectively reduce the fraction and the long period of those perpendicular lamellae in the iPP system without inducing small-strain cavitation. When successively deformed at 50 °C, the modified samples exhibited weaker ability to initiate small-strain cavitation as determined by the USAXS techniques.

Radiation, thermo-oxidative and storage induced changes in microstructure, crystallinity and dielectric properties of (un)oriented isotactic polypropylene

Orientation and radiation processing of polymeric materials are the main steps in certain modern technologies and have extensive applications. The properties of such treated polymers can change significantly in time and under various exploitation conditions and therefore, investigation of some specific effects of aging and storage of these systems is of great importance, although the overall degradation behavior of iPP is well known. In this work, isotactic polypropylene (iPP) samples were oriented via solid-state stretching at elevated temperature (at 100°C, up to draw ratio λ=20). This was followed by gamma irradiation in the air (up to 700 kGy) and/or thermo-oxidative (TO) aging (in oxygen, at 100°C, up to 8 days) and/or storage (at room temperature, up to 3 years). As the iPP is very sensitive to orientation and degradation, microstructures were analyzed by optical (OM) and scanning electron microscopy (SEM). Wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC) were used to monitor changes in structure and recrystallization. Considering the fact that at higher irradiation doses (>200 kGy), iPP starts to gel under the ambient conditions, gel measurements were employed to determine the yield of network formation induced by irradiation of stretched structures. Irradiation, TO degradation and storage, can induce substantial alternations of (di)electric properties which can be properly connected to the changes in microstructure and crystallinity; therefore, dielectric loss analysis was performed in a wide temperature range and polar groups that were introduced in apolar iPP by oxidation were considered as tracer groups.

Time-resolving small angle X-Ray scattering analysis of melt crystallization of mixtures of regular and irregular isotactic polypropylene samples

The melting/crystallization properties of blends obtained by mixing two isotactic polypropylene (iPP) samples synthesized using single-site metallocene catalyst systems and containing a high and low concentration of rr triads as stereo-defects, are studied. The changes occurring at lamellar length scale during a heating/cooling cycle at constant scanning rate are followed in situ by performing time-resolved small angle X-ray scattering (SAXS) measurements. Data analysis demonstrates that the evolution of the SAXS intensity with increase/decrease of the temperature is controlled by the separate melting/crystallization of the two components, the differences in the thermal expansion (contraction) coefficient of the amorphous and crystalline phases and the role of thermal fluctuations in electron density. The two components give rise to different populations of intermixed lamellar stacks in the blends which originate from the good miscibility of the low and high stereoregular samples in the melt.