γ–α transformation in isotactic polypropylene

A γ-phase to α-phase transformation in a specimen of isotactic polypropylene crystallized under conditions of high pressure was induced by drawing at 100°C. X-ray studies showed that the unoriented component remained in the γ-phase, and that the oriented component was found only in the α-phase. This evidence supports a previous suggestion that the phase transformation is martensitic in character. The consequences of such an assumption are discussed. The role of dislocations in polymeric systems is generally believed to be not too significant, but since martensitic reactions involve cooperative movements of atoms, an exception in this case is suggested. A possible mechanism for the phase transformation is suggested.     

Blends of the β-modification of polypropylene

Preparation, crystallization and melting characteristics of blends of β-nucleated isotactic polypropylene with different elastomers and polyethylenes were studied. Blends of β-polypropylene can be prepared if the polymer additive is amorphous or has negligible α-nucleating effect during the crystallization of polypropylene.     

Oriented γ-isotactic polypropylene crystallized at atmospheric pressure

A method for crystallizing oriented samples of the γ-phase of isotactic polypropylene (iPP) under atmospheric pressure in the presence of nucleating fibers has been developed. The technique uses iPP grades of high molecular weight and high isotacticity and produces a mixture of α and γ-phase crystals within the matrix of both pitch-based carbon and Kevlar®-reinforced composites. Two-dimensional wide-angle X-ray scattering (WAXS) patterns from these samples show that the content of the γ-phase decreases as the fiber loading in the composites decreases, suggesting that the γ-phase is directly nucleated by the fibers. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 2821–2827, 1998     

Studies on the α and β forms of isotactic polypropylene by crystallization in a temperature gradient

Samples of isotactic polypropylene (PP) were zone-solidified in temperature gradients up to 300°C/cm at growth rates down to 3 μm/min. Oriented α-type spherulites were obtained only by nucleation. While β nucleation is extremely rare, the β phase is easily initiated by growth transformations along the oriented α front. Since the β phase was found to grow considerably faster than the α phase, the α-to-β transformation points diverge across the sample, interrupting growth of the oriented α fibrils. This causes subsequent nucleation to yield teardrop-shaped α spherulites. Differential scanning calorimetry (DSC) studies of zone-solidified PP show the β-phase to be favored by slow growth rates, high temperature gradients, and large degrees of superheat in the melt—all of which tend to suppress nucleation. Differential thermograms of largely β-PP obtained at a heating rate of 1°C/min show the actual melting and recrystallization of the β spherulites into the α form.     

On the crystallization of γ-isotactic polypropylene: A high pressure study

The non-parallel chain structure determined for γ-phase isotactic polypropylene (γ-iPP) is confirmed by Rietveld analysis for highly isotactic high molecular mass iPP crystallized at 200 MPa. The new refinement shows that: (i) stereoregularity or crystallization pressure do not significantly influence the lattice dimensions; (ii) defect inclusion in γ-iPP crystals is unlikely. The α and γ forms have nearly identical bulk internal energy and density, but α-iPP should be normally kinetically favored over γ-iPP which may in turn predominate due to its greater ability to host defects at the crystalline-amorphous interface.     

The molecular origin of lamellar branching in the α (monoclinic) form of isotactic polypropylene

The specific lamellar branching characteristic of isotactic polypropylene (iPP) in its α monoclinic modification, first investigated in detail by Khoury, is analyzed in molecular terms on the basis of the geometrical and structural models of Padden and Keith and of Binsbergen and de Lange, respectively. As suggested by these authors the branching corresponds, structurally, to an homoepitaxy of the daughter lamellae on the lateral (010) faces of the parent lamellae; the epitaxy is favored by a satisfactory interdigitation of the molecular subgroups (methyl side chains) of facing planes and by the near identity of a and c parameters of the α monoclinic unit cell of iPP. On a molecular basis, the branching is initiated on a lateral (010) face made up of chains of a given hand by the deposition of chains of the same hand, whereas the crystal structure of the α modification would call for chains of opposite hand. Favorable interactions of side chains of helices of the same hand is realized only when the helix axes are at a substantial angle (in the present case, 100°) to one another. Related behavior is observed for the packing of α helices in the structure of globular proteins, in which molecular interactions, but no crystallographic interactions, are involved. On the basis of this molecular picture, the widespread occurrence of lamellar branching in the crystallization of the α form of isotactic polypropylene is accounted for, especially for high growth rates. At the same time, the specificity of the branching (only observed for the α crystal modification of this polymer) is explained by stringent geometrical and structural requirements that must be fulfilled at the contact plane.     

Adsorption of polypropylene from dilute solutions on a zeolite column packing

Faujasite type zeolite CBV-780 was tested as adsorbent for isotactic polypropylene by liquid chromatography. When cyclohexane, cyclohexanol, n-decanol, n-dodecanol, diphenylmethane, or methylcyclohexane was used as mobile phase, polypropylene was fully or partially retained within the column packing. This is the first series of sorbent-solvent systems to show a pronounced retention of isotactic polypropylene. According to the hydrodynamic volumes of polypropylene in solution, macromolecules of polypropylene should be fully excluded from the pore volume of the sorbent. Sizes of polypropylene macromolecules in linear conformations, however, correlate with the pore size of the column packing used. It is presumed that the polypropylene chains partially penetrate into the pores and are retained due to the high adsorption potential in the narrow pores.     

Studies on the β-crystalline form of isotactic polypropylene

A study has been made of the mechanical, thermal, and morpholigical characteristics of melt-crystallized isotactic polypropylene containing high levels of the β or pseudohexagonal crystalline form. Different levels of β-form crystallinity were produced in the polymer by blending in low levels of quinacridone dye nucleating agent. Microscopical studies of the crystallization process revealed that both α-form (monoclinic), and β-form spherulites nucleated on the dye particles, with α-spherulite growth commencing at a higher temperature. These observations were able to qualitatively explain the dependence of β-form level on both the nucleant concentration and its state of dispersion in the polymer. Improving the dispersion of the nucleant was found to reduce the level of β-form crystallinity if the nucleant concentration exceeded an optimum level. A new procedure for quantifying the volume fraction of β spherulites in a sample was developed which utilized the technique of selective solvent extraction. From volume-fraction, x-ray, and density data, the pure α and β crystal densities were obtained. Dynamic mechanical measurements-obtained on unoriented specimens containing varying levels of β-form crystallinity showed an increase in the magnitude of the damping in the post-T_g region with increasing β content. High levels of the β form lead to lower values of the modulus and yield stress, and higher values of the elongation at break and impact strength.     

Crystal morphology of the γ (triclinic) phase of isotactic polypropylene and its relation to the α phase

γ-phase crystals of isotactic polypropylene (iPP) obtained from low-molecular-weight extracts of pyrolyzed polymers are examined by electron microscopy and electron diffraction. γ-phase crystals differ from α-phase crystals in three important respects: (i) they are elongated along the b^* rather than the a^* axis, (ii) the chain axis is inclined at 50° to the lamellar surface (indexed as 101) rather than normal to it, and (iii) they show screw dislocations, while α crystals do not. γ crystals are nucleated on the lateral (010) faces of a α crystals; the b_α and b axes are parallel. Virtually no nucleation of the α phase takes place on the γ phase, which is therefore not involved in the repetitive lamellar branching leading to iPP quadrites. Crystallization of the γ phase appears to be favored by or linked to the absence of chain folds and may be involved in the macroscopic curvature of iPP branches.     

γ–α Solid–solid transition of isotactic polypropylene

X-ray diffraction patterns have been taken as a function of time and temperature on a sample of polypropylene held under high pressure (4.14 kbar) for 180 hr. at a temperature of 248°C. and subsequently cooled to room temperature. The molded sample initially crystallizes in the triclinic γ–phase but transforms to the γ–phase at elevated temperatures. The rate of conversion from γ to α is a function of time and temperature and tends to approach a constant value with increasing time. The nature of the thermal changes occurring in the sample was also studied by differential scanning calorimetry. It appears that at low scan speeds, there is a solid–solid transformation from the α-phase to the γ–phase, but at high scan speeds, the γ–phase melts without conversion to the α-phase.     

Fracture behaviour of β‐polypropylene as a function of processing conditions

The commercial grade of isotactic polypropylene (iPP) homopolymer (Mosten 58.412) was doped with different amount of β‐nucleant (NJ Star NU‐100). The fracture behavior of the β‐iPP was examined on injection‐ and compression moulded specimens under dynamic load. The fracture toughness K_Id and J_Id, respectively, were used to describe the fracture behavior. The influence of processing conditions (the mould temperature and injection velocity) and the distance from the gate were also taken into the consideration. Structural changes during processing were characterized by DSC analysis. It was found that: (i) the amount of 0.03% of β‐nucleant gives the highest toughness (ii) the toughness of the β‐iPP depends on the distance from the moulding gate and on the processing conditions (iii) the degree of crystallinity correlates with the fracture parameters.     

Crystalline order and melting behavior of isotactic polypropylene (α form)

Samples of isotactic polypropylene in the α form may have different degrees of order in the up and down positioning of the chains. By suitable programming of the crystallization history of unoriented samples it is possible to show, from DSC and WAXS data, that a strict correlation exists between the melting behavior and the degree of ordering of the chains. The process of ordering at annealing temperatures above 150°C may lead to a “continuum” of modifications of the α form from less ordered to highly ordered ones; correspondingly we observe an increase in the melting point by as much as 15°C.     

The role of intermolecular interactions in determining the mode of packing of crystalline polymers. Energy calculations on isotactic polypropylene

Packing energy calculations for isotactic polypropylene chains have been performed with the purpose: (a) to predict the most stable kind of monoclinic lattice for the α-modification; (b) to contribute to the understanding of the structural disorder which is commonly found in the α-modification; (c) to evaluate the order of magnitude of the differences in energies among the three modifications (α, β, γ) experimentally observed. The results for the α-form show that the best packing of the chains is achievable for the ordered P2₁/c space group, while more or less disordered structures may exist, which only slightly differ in energy from the most stable structure. In connection with point (c), our results are in agreement with the possibility of polymorphism in isotactic polypropylene.     

Formation of β-crystal from nonisothermal crystallization of compression-molded isotactic polypropylene melt

The crystalline structures of isotactic polypropylene (iPP) subjected to compression-molding were studied by means of differential scanning calorimeter (DSC), optical microscope (OM) and wide-angle X-ray diffraction (WAXD). β-crystal can be formed from nonisothermal crystallization of the sample compression-molded at the molten state and the β-phase content increased with increasing pressure of molding. Thermal treatment of the molten sample at 200 °C could eliminate the effect of compression-molding on crystalline structure. It was suggested that the compression-molding of iPP melt plays an important role in improving the nucleation ability of β-crystal.     

Lattice imaging in melt crystallized polypropylene thin films

 Melt crystallized isotactic polypropylene thin films of thickness between 30 and 100 nm have been investigated by high-resolution transmission electron microscopy at room temperature. The c-axis projection of the 2^*3₁ helices and their packing in the lattice were clearly visible in flat-on lamellae of the α-phase following reconstruction from the components of the image Fourier transform corresponding to the (1 1 0) and (0 4 0) lattice planes, and the image power spectra also indicated contributions from (1 3 0) and (0 6 0) relfections, corresponding to a line resolution of about 0.35 nm. These results are discussed in terms of Bloch wave calculations based on the generally accepted structure for the α-phase. Attempts to obtain lattice images of the β-phase in isotactic polypropylene and melt crystallized syndiotactic polypropylene under similar operating conditions are also briefly discussed, although these provided relatively little structural information. Received: 27 June 1997 Accepted: 15 August 1997     

Annealing of the β-modification of polypropylene

Expressive memory effect has been proven in the course of the annealing of the β-modification of polypropylene (β-PP). The character of the structural changes taking place in β-PP during annealing strongly depends on the thermal history. In dependence on the thermal history βα or ββ-recrystallization takes place during thermal treatment. In case the annealing starts from the crystallization temperature β-PP is recrystallized into a more perfect structure with higher melting point - similarly to thermodynamically stable modifications. The samples cooled below a certain temperature before subjected to annealing transform partly or fully into the α-modification. The βα-recrystallization tendency prevalent on the effect of recooling can be reduced or eliminated with the aid of stepwise annealing with decreasing temperature steps.     

The effect of specific nucleation on molecular and supermolecular orientation in isotactic polypropylene

The molecular and supermolecular orientation, morphology and structural changes observed during cold drawing of injection moulded isotactic polypropylene modified by specific α, and β nucleating agents were studied by polarised photoacoustic FTIR spectroscopy, wide-angle X-ray diffraction and differential scanning calorimetry. Significantly lower molecular orientation was found in the core of the β-nucleated injection moulded specimens as compared to unmodified and α-nucleated materials. This has been ascribed to the fast growth of the β-crystallites which inevitably dislocates the flow-induced orientation within the crystalline regions and in their vicinity. Moreover, it was found that the presence of the developed β-crystallites distinctly diminishes the efficiency of the orientational solid-state drawing assessed on both levels of the hierarchical structure (molecular and crystalline). This structural observation is directly connected with macroscopic softening effect of the β-phase: lowering the yield stress and flattening the neck shoulder. Thus, the interrelation between the microstructural and macroscopic effects of the β-phase could be described as a feedback process.     

Molecular deformation mechanisms of isotactic polypropylene in α‐ and β‐crystal forms by FTIR spectroscopy

The orientation behavior of isotactic polypropylene (iPP) in α‐ and β‐crystal form was investigated by rheo‐optical Fourier transformed infrared (FTIR) spectroscopy. This method enabled quantification of the degree of orientation as a feature of structural changes during uniaxial elongation in not only the crystalline phase but also the amorphous one. Molecular orientation mechanisms can be successfully derived from experimental results. Generally, three mechanisms were detected for iPP: (1) interlamellar separation in the amorphous phase, (2) interlamellar slip and lamellar twisting at small elongations, and (3) intralamellar slip at high elongations. The third mechanism was favored by α‐PP, whereas β‐PP favored the second mechanism, which, in fact, was responsible for the different mechanical properties of both materials at the macroscopic level. On the other hand, crystallization conditions may have significantly affected the amorphous orientation. Nevertheless, for both iPP types the chains in the amorphous phase always oriented less than did those in the crystalline phase. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 4478–4488, 2004     

The effect of specific nucleation on tensile mechanical behaviour of isotactic polypropylene

Commercial-grade isotactic polypropylene was modified with a specific β-nucleation agent NJ-Star (N,N^′-dicyclohexylnaphthalene-2,6-dicarboxamide) in concentrations 0.03, 0.10 wt.% and with a specific α-nucleating agent Millad 3988 (1,2:3,4-bis-O-(3,4-dimethylbenzylidene)sorbitol) in a concentration of 1.0 wt.%. Specimens for mechanical studies were prepared by injection moulding. Two types of tensile mechanical testing were performed at room temperature: (1) stress-strain test encompassing the plastic behaviour well behind the yield point and (2) tensile creep in the region of non-linear viscoelasticity. The results derived from the stress-strain traces show a distinct decrease in Young's modulus and yield stress for samples containing the crystalline β-phase as compared with non-nucleated and α-nucleated samples. This decrease was more pronounced with samples containing the lower β-nucleant concentrations (0.03 wt.%). Higher compliance of specimens containing the β-phase was also manifested in their creep behaviour. However, the creep rate of the specimen with the higher nucleant content (0.10 wt.%) did not rise with time so that its creep curve intersected the creep curves of non-nucleated and α-nucleated samples. Thus, at creep times longer than 1000 min, the sample with 0.1 wt.% of the β-nucleant showed a lower compliance than non-nucleated polypropylene and at 10 000 min reached the compliance of the α-nucleated sample. The different softening effect of the β-phase in the high-strain and low-strain regions has been ascribed to a specific structure of the amorphous interlayer induced by the presence of the β-crystallites.     

Characterization of stress‐strain behavior for binary blends of isotactic polypropylene with ethylene‐α‐olefin copolymer

The characterization of the mechanical nonlinear behavior of isotactic polypropylene/ethylene‐1‐hexene copolymer blends with various kinds of morphology was carried out using a nonlinear constitutive equation in which the plastic deformation and the anharmonicity of elastic response are taken into account. It was found that the mechanical nonlinearity of the incompatible blends showing phase separation is much greater than that of the compatible blends having rubbery components in the interlamellar regions. Moreover, the mechanical behavior is governed by the plastic deformation for the incompatible blends, whereas the anharmonicity strongly affects the mechanical behavior for the compatible blends. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1513–1521, 1999