Isotactic Polypropylene Samples Research Articles

Solid-State NMR Characterization of the Chemical Defects and Physical Disorders in α Form of Isotactic Poly(propylene) Synthesized by Ziegler–Natta Catalysts

The order–disorder phenomenon and spatial heterogeneity of chain packing, partitions of stereodefects, and molecular dynamics of α form of isotactic polypropylene (iPP) samples, which are synthesized by Zieglar–Natta catalysts, are investigated by solid-state (SS) NMR. High-resolution 13C NMR under high-power TPPM decoupling at field strengths of 110 kHz allows observation of the order–disorder phenomenon in the chain-packing structures of α form. High isotacticity samples (isotacticity at pentad level, ⟨mmmm⟩ = 99.4%) give a maximum ordered packing (α2) fraction of 66% at crystallization temperature (Tc) of 155 °C while low stereoregularity samples (⟨mmmm⟩ = 91.0%) have only 47% at the same Tc. However, Mw (58.7–982 kg/mol) does not play a significant role in ordered packing formation. Using 13C-labeled CH3 of iPP, direct spatial correlations between the α2 and α1 structures are investigated by 13C detection of two-dimensional (2D) 1H–1H spin-diffusion (CHHC) experiments. The time dependence of the spin-diffusion polarization transferred signal intensities determines the average domain size of the α1 and α2 structures of iPP crystallized at 150 °C, which was found to be 40 nm under an assumption of 2D spin diffusion. Additionally, the 13C filter CPMAS NMR spectrum on 13C CH3-labeled iPP demonstrates that chemical defect is almost excluded from the crystalline region at Tc = 150 °C (defect free crystal) while ca. 2% is in melt quench sample. Moreover, 13C centerband-only detection of exchange experiments on α2-rich sample with highest ⟨mmmm⟩ = 99.4% indicate that crystalline dynamics follows a single Arrhenius plot with an activation energy of 116 kJ/mol across reported order–disorder transition temperatures (157–159 °C).

A Simple Model for Heterogeneous Nucleation of Isotactic Polypropylene

A model for heterogeneous nucleation of isotactic polypropylene in terms of a “cylindrical cap” with the critical size and barrier set by a contact angle is described. Quiescent crystallization data for bulk samples of isotactic polypropylene are analyzed to extract a contact angle in the range of 40° to 70° with a mean value of 56°, which corresponds to a factor of 6 decrease in the critical barrier from the homogeneous case. This value is in reasonable agreement with the contact angle of 79° ± 3° estimated from molecular scale theory (DOI 10.1021/ma400703m). This model could be combined with calculations from melt rheology of entropy reduction due to flow to frame discussions of flow-induced crystallization.

Course of the changes in thick and thin isotactic polypropylene samples subjected to natural aging

The course of degradation in natural environment of polypropylene items was studied with DSC and thermogravimetric techniques. The enthalpic patterns of the materials vary with time of exposure. Measurements of the melting enthalpies in comparison with the heats of crystallization enabled to assess the evolution of crystallinity during exposure of plates and films of iPP. The induction time before the photochemical attack is of 800 h regardless of the thickness.The plates lose all practical mechanical properties after 4800 h of exposure, the film only after 2000 h. At the end of exposure, the plate provides a brittle and cracked material that includes a volatile phase and a heat resistant share due to crosslinking reactions. Some hypotheses on the degradation mechanism are presented.

Polarization-modulated synchrotron infrared microspectroscopy for the study of crystalline morphology in some semicrystalline polyolefins

The spatially resolved dichroic behaviour of a series of isothermally crystallised samples of isotactic polypropylene and polybutene-1 was investigated using polarization-modulated synchrotron infrared microspectroscopy (PM-SIRMS). The high brightness of the synchrotron source provides a good signal-to-noise performance at high spatial resolution in spite of the significant loss in infrared intensity through the photoelastic modulator. This technique gives access to information on average polymer chain orientation and crystalline morphology at the microscale, widening the possibilities for spatially resolved dichroic mapping of polymeric materials.

Investigation of the stereodefect distribution and conformational behavior of isotactic polypropylene polymerized with different Ziegler–Natta catalysts

AbstractTwo isotactic polypropylene (iPP) samples (PP‐A and PP‐B) were obtained by utilizing two different heterogeneous Ziegler–Natta catalysts in a given polymerization system. The molecular structure and conformational behavior of the samples were studied. The results of determination of xylene soluble material (XS) and 13C NMR showed that the average isotacticity of the samples were nearly same. However, the results of high‐resolution high‐temperature 13C NMR (HRHT 13C NMR) and successive self‐nucleation and annealing (SSA) fractionation revealed that the amount of high isotacticity of PP‐B was lower than that of PP‐A, and the amount of relative medium and low isotacticity of PP‐B was higher than PP‐A, indicating that the stereodefect distribution of PP‐B was more uniform than PP‐A. The calculation of average meso sequence length from SSA was found to be in good agreement with that calculated from the results of HRHT 13C NMR. Moreover, Fourier transformation infrared was utilized to study the influence of stereodefect distribution on the conformational behavior of iPP. The result suggested that the molecular conformation of the PP‐B was more disordered than PP‐A, the regularity of molecular structure for PP‐B was lower than that of PP‐A. The related action mechanism and the influences of which on crystallization behavior were discussed. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Effect of Structural Inhomogeneity on Mechanical Behavior of Injection Molded Polypropylene Investigated with Microbeam X-ray Scattering

Relationship between the structure of injection-molded isotactic polypropylene and its tensile mechanical properties, necking and fracture behaviors in particular, was investigated in terms of micrometer-scale structural inhomogeneity of nanometer- and subnanometer-scale structures. To clarify the micrometer-scale inhomogeneity, we employed scanning microbeam wide-angle X-ray diffraction and small-angle X-ray scattering technique. Four isotactic polypropylene samples were studied, produced using different injection-condition and thermal treatments. The results of scanning microbeam X-ray scattering measurements showed the presence of two types of micrometer-scale structural inhomogeneity in addition to the orientation of molecules: the distribution of polymorphs and of crystalline ordering. The results of scanning microbeam X-ray scattering of deformed sample showed the disappearance of the β-form isotactic polypropylene crystals at the outer regions accompanied by the plastic deformation. It is indicated that the inhomogeneous distribution of crystalline ordering and the existence of different polymorphs are highly related to the tensile mechanical behavior.

Phase Transitions in Prequenched Mesomorphic Isotactic Polypropylene during Heating and Annealing Processes As Revealed by Simultaneous Synchrotron SAXS and WAXD Technique

Time-resolved simultaneous synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) technique was used to investigate the phase transitions in prequenched mesomorphic isotactic polypropylene (iPP) samples during heating and annealing processes, respectively. For the heating process, it is shown that the mesomorphic-to-monoclinic phase transition is relatively faster for the mesomorphic iPP sample obtained with the high quenching rate than that with the low quenching rate. For the former, the stability of α-monoclinic crystals formed during heating is relatively higher. As for the annealing process, WAXD and SAXS data illustrate that the higher the annealing temperature (T(a)), the earlier the mesomorphic-to-monoclinic phase transition occurs. Namely, T(a) controls the phase transition rate. Both heating and annealing processes show that the increase of content of α-monoclinic crystal phase is mainly at the expense of the mesomorphic phase, with the content of amorphous phase almost invariable. The isothermal crystallization kinetics for the prequenched mesomorphic iPP sample was analyzed through the Avrami equation, revealing a two-dimensional crystal growth under the diffusion-limited mechanism.

The effects of vibration field on crystal structures and orientation of isotactic polypropylene injection samples

In this study, isotactic polypropylene (iPP) samples were prepared by self-made pressure vibration injection device, in which a periodical shear field was imposed on the iPP melt during the filling and packing stages. The crystal structures and orientation of samples were investigated by wide-angle X-ray diffraction and polarizing light microscopy (PLM). The β form and γ form were found in vibration injection molding samples. Compared with the static samples, the orientation of iPP chains within lamellae was observed in the transition layer (at the depth of 1.5 mm from the surface) of the vibration injection molding samples, and the orientation was found even in the core layer. The PLM observation indicates the β form exists in the transition and core region of the vibration sample. Besides, two types of β-PP could be observed simultaneously in the core region under high magnification. The bright β crystallites (βIII) are fan-shaped and lie perpendicularly to the flow direction. In the other hand, the ringed β crystallites (βIV) almost remain their spherulite shape. Moreover, fiber crystal was observed in vibration sample, even in the core region. Some fiber crystals in the transition region reappeared in the cooling circle after the sample was held at 180 °C for 2 min. This could be explained in structure memory effect and its high thermal stability.

Tailoring the Mechanical Properties of Isotactic Polypropylene by Blending Samples with Different Stereoregularity

The structure and properties of some blends obtained by mixing two semicrystalline samples of isotactic polypropylene (iPP) characterized by different degree of stereoregularity have been analyzed in the whole range of composition. The two components are a highly isotactic and crystalline sample (iPP1) and a stereoirregular and less crystalline sample (iPP2), and have been synthesized using two different metallocene catalysts. The blends show separate and independent crystallization and melting of the two components, and crystallinity content decreasing almost linearly with increasing the concentration of the low stereoregular component. A continuous change of tensile properties with the composition occurs, with properties ranging from those of thermoplastic materials with high stiffness, typical of the neat iPP1, to those of high-strength elastomers, typical of neat iPP2. The mechanical properties of these blends reflect the complex lamellar morphology that develops upon crystallization, characterized by...

Shish–kebab-like cylindrulite structures resulted from periodical shear-induced crystallization of isotactic polypropylene

In this study, isotactic polypropylene (iPP) samples were prepared by conventional injection molding (CIM) and pressure vibration injection molding (PVIM), in which a periodical shear field was imposed on the iPP melt during the cooling solidification. The distribution of supermolecular structures of samples was investigated by Polarized Light Microscopy (PLM) and Scanning Electron Microscopy (SEM). Results show that the through-the thickness-morphology of sample prepared by CIM features a typical skin-core structure, as a result of general shear-induced crystallization. This structure can be divided into three layers, including a skin layer in which the shish–kebab structure was found, a transition region with deformed spherulite structure and a core layer with spherulitic structure. However, the morphology of the sample prepared by PVIM, as a result of periodical shear-induced crystallization, features a richer and fascinating supermolecular structure and can not be roughly divided into three layers. A region full of shish–kebab-like cylindrulite structures was found between the transition region and the core layer, which is rare to be seen in conventional injection molding. Based on their various core structures, two kinds of shish–kebab-like cylindrulites were defined: one is multi-fibril-core cylindrulite of which core is an assembly of multiple fibrils, and the other is single-fibril-core cylindrulite of which the core just contains a single fibril. Based on the investigated results, a schematic illustration is proposed to depict the through-the thickness-distribution of supermolecular structure of iPP sample prepared by PVIM. The mechanism of the formation of the two kinds of shish–kebab-like cylindrulite structures is also depicted by a schematic illustration, and it was discussed in terms of periodical shear-induced crystallization.

Stem Tilt in α-Form Single Crystals of Isotactic Polypropylene: A Manifestation of Conformational Constraints Set by Stereochemistry and Minimized Fold Encumbrance

Two high molecular weight (MW) isotactic polypropylene (i-PP) samples, one containing a small amount of rr stereodefects denoted R1 and one conventional, commercial i-PP denoted S1, were used to grow single crystals in thin films at high temperatures (Tx = 145 °C for R1, Tx = 155 °C for S1). Elongated α2-form lathlike single crystals could be found in these two samples, indicating that the growth of this type of single crystal is a general phenomenon in i-PP and is independent of the small amount of tacticity defects. On the basis of our selected area electron diffraction (SAED) experimental results, the stems in these lathlike single crystals were tilted at an unusual 17° angle around the b-axis (i.e., a rotation of 17° of the c-axis within the ac-plane toward the a-axis direction), while in the traditional scheme of the α-form in i-PP, the stem axis is understood to be normal to the fold surface. This 17° stem tilt in the α2-form, lathlike single crystals grown at high Tx values appears to depend upon two constraints: (a) the conformational restrictions on chirality and clinicity of isotactic polyolefin stems linked by a fold, as analyzed by Petraccone et al. [Polymer 1986, 27, 1665; Eur. Polym. J. 1989, 25, 43], and (b) a minimized encumbrance at the fold surface can be achieved by those folds which start from and end on the chemical bonds normal to the fold surface with a favorable gauche conformation of i-PP chains. This is because in i-PP (and most isotactic polyolefins) with a helical stem conformation every other chemical bond is nearly parallel (in a trans conformation) or perpendicular (in a gauche conformation) to the stem axis (and thus, nearly within or normal to the fold surface). The combination of these two constraints favors a (102̅) fold surface in the i-PP case, as is observed in this study.

Isotactic Polypropylene with (3,1) Chain-Walking Defects: Characterization, Crystallization, and Melting Behaviors

Using two living nickel α-diimine complexes (rac-1 and rac-4) activated with methylaluminoxane (MAO), isotactic polypropylene (iPP) samples containing five types of regiodefects were synthesized. Both isolated and successive groups of (2,1) and (3,1) enchainments were identified by solution-state 13C NMR spectroscopy. The extended monomer and bulky nature of these defects add some restraints to the crystallinity level that can be achieved in these polymer samples. The complex based on a cumyl-derived ligand, rac-4, produces higher molecular weight iPP with a higher content of bulky defects than rac-1. Melting temperatures and crystallinity levels of rac-4-derived iPP are accordingly lower than the polymers obtained from rac-1. Although the nature of the chain-walking defect is similar to the addition of the ethylene unit, the difference in polymer properties is profound. At equivalent point defects per total monomers (XB), iPP samples with chain-walking defects display lower melting temperatures and much lower degrees of crystallinity than random 1-alkene copolymers, including those with comonomers excluded from the crystal lattice such as the 1-hexene and 1-octene counits. Furthermore, iPP containing (3,1) regiodefects develops significantly higher contents of the γ-polymorph than any other iPP or random copolymer with a matched XB composition. The experimental evidence is consistent with shorter crystallizable sequences for iPP with chain-walking defects based on (3,1) enchainments. Taking crystallizable and noncrystallizable total units as the basis to compute point defects, the properties of (3,1) iPP adhere to the basis of exclusion equilibrium theory, indicating that the defects are random-bulky or generated in a random fashion but of a defined extended/multimonomer nature.

Theoretical Investigation of Nano-Scale Organization in Blends of Semicrystalline/Semicrystalline Polymers by Small Angle X-ray Scattering

A theoretical model has been derived for the calculation of small-angle X-ray scattering (SAXS) data from stacks of crystalline lamellae in which two or more kinds of lamellae are mixed within the same stack according to different statistical models, ranging from the fully random mode, to alternated one, up to block configurations. The model has been used to study the complex lamellar morphology which develops from melt in some blends obtained by mixing two semicrystalline samples of isotactic polypropylene (iPP) characterized by different degree of stereoreoregularity, similar molecular masses, and melting temperatures of ≈162 °C for the more stereoregular component iPP1 and ≈74 °C for the less stereoregular one iPP2. It is shown that for samples crystallized in conditions far from thermodynamic equilibrium, the observed SAXS intensity profiles may easily show absence of correlation peaks, associated with high amount of diffuse scattering, not easy to unravel. Using our theoretical approach we show that featureless SAXS intensity distribution may correspond to formation of disordered lamellar stacks with lamellae of the two components randomly mixed within the same stacks. The tendency of iPP1 and iPP2 to form mixed lamellar stacks with a random configuration provides clear evidence that the two components are miscible in the melt.

Spatial Distribution of γ-Crystals in Metallocene-Made Isotactic Polypropylene Crystallized under Combined Thermal and Flow Fields

The present Article reports the relationships between molecular orientation, formation, and spatial distribution of gamma-crystals in metallocene-made isotactic polypropylene (m-iPP) samples prepared by two industrial processes: conventional injection molding (CIM) and oscillatory shear injection molding (OSIM), in which combined thermal and flow fields typically exist. In particular, spatial distributions of crystallinity, fraction of gamma-crystal (f(gamma)) with respect to alpha-crystal, and lamella-branched shish-kebab structure in the shaped samples were characterized by synchrotron two-dimensional (2D) wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) techniques. The results showed that the crystallinity in any given region of OSIM samples was always higher than that of CIM samples. The value of f(gamma) increased monotonously from skin to core in CIM samples, whereas the corresponding f(gamma) increased nonmonotonically in OSIM samples. The spatial distribution of gamma-crystal in OSIM samples can be explained by the epitaxial arrangement between gamma- and alpha-crystal in a lamella-branched shish-kebab structure. In the proposed model, the parent lamellae of alpha-crystal provide secondary nucleation sites for daughter lamellae of alpha-crystal and gamma-crystal, and the different content of parent lamellae results in varying amounts of gamma-crystal. In OSIM samples, the smallest parent-daughter ratio ([R]) = 1.38) in the core region led to the lowest fraction of gamma-crystal (0.57), but relatively higher gamma-crystal content (0.69) at 600 and 1200 mum depth of the samples (corresponding to [R] of 4.5 and 5.8, respectively). This is consistent with the proposed model where more parent lamellae provide more nucleation sites for crystallization, thus resulting in higher content of gamma-crystal. The melting behavior of CIM and OSIM samples was studied by differential scanning calorimetery (DSC). The observed double-melting peaks could be explained by the melting of gamma- and alpha-crystal of the shaped samples. The f(gamma) distribution calculated from the relative areas of the peaks in the DSC scans was also consistent with the WAXD results.

Effect of atomic force microscope tip geometry on the evaluation of the crystal size of semicrystalline polymers

The objective of this study is the evaluation of the effect of the tip geometry on imaging the structure of semicrystalline polymers at the nanometer scale by atomic force microscopy. The size and habit of crystals in model-like samples of isotactic polypropylene were analyzed using a standard silicon tip and a high-resolution tip with approximate front curvatures of the tip apexes of 1 and 10 nm, respectively. The experimental data demonstrate that the true size of typical polymer crystals, within the investigated range between 5 and 25 nm, can accurately only be evaluated by using the high-resolution tip. The tip of a larger front curvature of 10 nm yielded apparent crystal sizes which were about 5–10 nm larger than that obtained on using the sharper tip.

Effect of N,N′‐diphenyl adipamide on the formation of the β‐crystalline form in isotactic polypropylene

AbstractThe polymorphic compositions and mechanical properties of isotactic polypropylene (iPP) samples nucleated by a selective β‐nucleating agent [N,N′‐diphenyl adipamide (DPA)] were investigated with wide‐angle X‐ray diffraction, polarized light microscopy, scanning electron microscopy, and mechanical tests. It was found that β‐phase crystals emerged with the addition of DPA, and the relative proportion of the β‐crystalline form reached the maximum value of 0.97 with the addition of 0.1 wt % DPA. The curved lamellae in the β spherulites were like flowers. The β spherulites were etched more easily than α spherulites because amorphous regions were distributed inside the β spherulites. The Izod notched impact strength increased sharply with the addition of DPA and attained the maximum value of 7.30 kJ/m2 (the value of blank iPP was 3.13 kJ/m2) with the addition of 0.1 wt % DPA. An analysis of the misfit factors between DPA and β‐iPP showed that β‐iPP could epitaxially crystallize on the DPA crystal well. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

The β - and γ -Form Distributions of Isotactic Polypropylene Injection Molding Controlled via Melt Vibration

The effect of melt vibration on the crystal form of isotactic Polypropylene (iPP) samples prepared by low-frequency Vibration-Assisted Injection Molding (VAIM) was investigated. The distribution of crystal forms through the thickness was investigated under different VAIM processing conditions, with Conventional Injection Molding (CIM) for comparison. Wide-Angle X-ray Diffraction (WAXD) showed that the content of β -form crystals decreases from the surface to the core region in the CIM and VAIM samples. The β -form distribution through the thickness is controlled by the VAIM processing conditions. The occurrence of γ -form crystals was found in the VAIM samples produced at relatively low vibration frequency and relatively large pressure vibration amplitude.

Mechanical Property, Thermal Property and Crystal Structure of Isotactic Polypropylene Samples Prepared by Vibration Injection Molding

A self-designed pressure vibration injection device was used to study the effect of vibration frequency and vibration pressure on tensile strength and impact strength of iPP F401 vibration injection molding samples. Furthermore, vicat softening temperature, WAXD measurements and polarized microscopic observation were conducted. According to the results, tensile strength and impact strength increase with increasing vibration frequency and vibration pressure. The maximum increment of tensile strength is 26.1%. Under certain process conditions, there is a transition of the impact strength, whose maximum increment is 85%. The vicat softening temperatures have a significant increase of 6∼8 °C for the samples obtained at high vibration frequencies compared with that of static samples. According to pole figures, α-PP of vibration samples orientates much stronger than that of static samples. PM micrograghs show that vibration changes the crystal structure of samples and enhances their orientation.

Crystal form and orientation of isotactic polypropylene samples prepared by vibration‐injection molding

AbstractA vibration–injection molding equipment was developed to prepare isotactic polypropylene injection samples to investigate their crystal form and orientation. Wide‐angle X‐ray scattering experiments (WAXD) were conducted in two modes: theta–theta mode and tube‐fixed mode. Through vibration–injection molding, in additional to α form, β form, or γ form can be obtained under different conditions. At high melt temperature (230), β‐PP can be induced and the core of the sample contains more β‐PP than the surface. At low melt temperature (190), γ‐PP can be induced and the core and the surface of the sample contain approximate same proportion of γ‐PP. Pole figures show that α‐PP of a static sample just orientates slightly along M direction, while that of vibration samples orientate much stronger. The orientation of the normal of (040)α plane of the sample obtained at T = 230°C, f = 0.5 Hz, and Pv = 75MPa is preferred in M direction, and the orientation of the normal of (040)α plane of the sample obtained at T = 190°C, f = 1.5 Hz, and Pv= 35 MPa is preferred in N direction. β‐PP and γ‐PP do not show obvious orientation. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007

Structural transitions in propylene-1-hexene copolymer films during low-temperature plasma treatment

The effect of a dc discharge on the structure of film samples of isotactic and syndiotactic polypropylene, as well as propylene-1-hexene copolymers with isotactic and syndiotactic sequences, was studied. It was found that the plasma treatment of homopolymer films did almost not change their phase composition. However, in the propylene copolymers with 1-hexene (0.9 and 1.7 mol % 1-hexene) synthesized in an isospecific system, the γ-to-α phase transition was observed. The structural transition from a maximally ordered crystalline modification of form I, which has a body-centered orthorhombic unit cell, to a maximally disordered form of I, which has a simple orthorhombic unit cell, also occurred in a copolymer (1.1 mol % 1-hexene) synthesized in a syndiospecific system. It was found that an electric field that resulted from the localization of plasma electrons in the surface layers of polymer samples was responsible for the structural transformations observed.