A bimodal structure of solution-grown isotactic polypropylene with orthogonally crossed lamellae

Isotactic polypropylene (iPP) was crystallized from solution on a uniaxially-oriented iPP film. Small-angle x-ray scattering patterns obtained from the sample showed two perpendicularly crossed lameliae 9.3 nm thick that overgrew flat-on and edge-on on the substrate. In the through wide-angel x-ray diffraction pattern (taken with incident x-rays normal to the iPP film surface), strong hkO reflections were arranged in an hkO net pattern indicating that the a-axis of the monoclinic α unit cell was oriented parallel to the chain direction of the substrate. From this, it was concluded that the flat-on lamellae grew with the a-axis parallel to the chain axis of the substrate and with the b-axis parallel to its surface. In the edge wide-angle x-ray diffraction pattern (X-rays incident on the edge of the film), arced, strong 110 and 220 reflections from overgrown crystals were observed on the equator of the fiber pattern of the substrate. This indicated that the edge-on lamellae epitaxially grew with the c-axis aligned parallel to the chain axis of the substrate. The homoepitaxy explains the correlated growth mode between the orthogonally crossed lamellae: they grew epitaxially, the a-axis of one lamella coinciding with the c-axis of the other and the {010} planes in contact. © 1993 John Wiley & Sons, Inc.     

Polyethylene–isotactic polypropylene epitaxy: Analysis of the diffraction patterns of oriented biphasic blends

Diffraction patterns of oriented blends of isotactic polypropylene (iPP) and polyethylene (PE) published recently by several authors are analyzed on the basis of a unique epitaxial relationship between the iPP and PE crystal lattices. The contact planes are (100)_PE and (010)_iPP, and the PE chains lie at 50° to the iPP chain axis, parallel to the helical path of the iPP helices, which is formed by rows of side-group methyls.     

Insight into understanding the evolution of the epitaxy crystallization in isotactic polypropylene and polyethylene blends

In this article, epitaxial structures have been successfully obtained in the isotactic polypropylene (iPP)/polyethylene (PE) blends by an accessible injection molding methods. By studying a series of iPP/PE blends, the evolution of the epitaxial growth of PE lamellae on the oriented iPP lamellae has been detailedly discussed via wide‐angle X‐ray diffraction, small‐angle X‐ray scattering, scanning electron microscopy and differential scanning calorimetry. Unexpectedly, the exactly epitaxial angles between peculiarly arranged PE lamellae and oriented PP lamellae are all larger than the classical epitaxy theory value of 50°, and it even increases gradually with increasing PP content. It is inferred that the special crystallization of PE is the consequence of joint construction of the oriented PP crystals and the continuous intense shear field provided by pressure vibration injection molding. The epitaxial structures play a positive role in the interfacial connection between two components; thus, the mechanical properties of the blends are improved. This work provides an insight understanding on the formation mechanism of the epitaxy crystallization under shear field. Copyright © 2017 John Wiley & Sons, Ltd.     

Interfacial adhesion mechanisms in incompatible semicrystalline polymer systems

The mechanism of adhesion at semicrystalline polymer interfaces between isotactic polypropylene (iPP) and linear low‐density polyethylene (PE) was studied with transmission electron microscopy (TEM) and an asymmetric‐double‐cantilever‐beam test. From the TEM images, both the interfacial width and the lamellar thickness of the polymers were extracted. During annealing, the interfacial width increased with the annealing temperature, and this indicated the accumulation of amorphous polymers at the interface. The interfacial strength, determined from the critical fracture energy (G_c), also increased with the annealing temperature and reached a maximum above the melting temperatures of iPP and PE, whereas the smallest G_c value was obtained below the melting temperatures of the two materials. A mechanism of interfacial strengthening was proposed accounting for the competition between the interdiffusion of PE and crystallization of iPP. As the annealing temperature increased, the rates of PE diffusion and iPP crystallization increased. Although the crystallization of iPP hindered the interdiffusion of PE, both the interfacial width and the fracture energy increased with the temperature, and this indicated that PE interdiffusion dominated iPP crystallization. Below the critical temperature, the fracture surfaces of both iPP and PE were smooth, and chain pullout dominated the fracture mechanism. Above the critical temperature, iPP crystallization still hindered the interdiffusion, and crazes could be seen on the iPP side. Above the melting temperatures of the two materials, ruptured surfaces could also be seen on the PE side, and crazing was the fracture mechanism. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2667–2679, 2004     

Structural relationships in blends of isotactic polypropylene and polymers with aliphatic sequences

Blends of isotactic polypropylene and polyethylene or polyamides often display highly unusual crystalline morphologies in which the chain axes of the helical and linear polymers are at large angles to each other. These crystalline morphologies are explained on the basis of a specific epitaxial relationship that rests on the existence, in the (010) plane of iPP, of rows of methyl groups nearly 5-Å apart. The structural basis of all the observed morphologies is the parallel alignment of the aliphatic segments of the PE or polyamides and of the iPP methyl rows (or vice versa) and matching of the near 5-Å distances between PE or PA chains and iPP methyl rows. The implications of this unusual epitaxial relationship with respect to the apparent “universality” of polymer nucleating agents are examined.     

Folded chain crystallization of poly(N-vinylcarbazole)

A study of the morphology of thermally induced crystallization in poly(N-vinylcarbazole) has shown the formation of folded chain lamellae. Diffraction analysis of multiple and single lamellae indicates a paracrystalline structure with hexagonal symmetry about the chain axis and an interchain spacing of 12.00 Å. No sharp reflections due to chain axis periodicity were observed and the chain axis period was calculated from the crystalline density extrapolated to 100% crystallinity. This gave a chain axis period/monomer unit of 2.16 Å consistent with an isotactic 3/1 screw axis and a basic trigonal structure with a = 12.00 Å and c = 6.47 Å.     

Nanostructure and crystallization phenomena in multilayered films of alternating iPP and PA6 semicrystalline polymers

The present work is concerned with the study of the crystalline morphology and the nanostructure of a multilayered system of two alternating immiscible semicrystalline polymers: isotactic polypropylene (iPP) and polyamide 6 (PA6). Films with a volume ratio of 70/30 were prepared by means of layer multiplying coextrusion. Contrary to previous experiments, performed with semicrystalline/amorphous and amorphous/amorphous nanolayered systems, the studied iPP/PA6 film does not exhibit a well defined maximum in the USAXS patterns. This result accounts for an irregular layered structure, as further confirmed by means of TEM images. Nevertheless, such a layered assembly still influences the crystallization behaviour of both constituent polymers. On the one hand, the crystallization of PA6 within the multilayered material is substantially hindered as evidenced by its weak scattering intensity. Real time studies as a function of temperature undoubtedly detect the presence of a WAXS peak and a SAXS maximum associated to PA6 above the melting temperature of iPP. Room temperature AFM studies also confirm the occurrence of crystalline structures within the PA6 layers. On the other hand, SAXS and WAXS measurements at room temperature reveal the occurrence of an oriented lamellar morphology within the iPP layers bearing uniaxial symmetry around an axis perpendicular to the layers surface. Results show that the crystalline molecular chains are placed mainly parallel to the layer surfaces forming edge-on lamellae. Moreover, X-ray scattering results are in agreement with the occurrence of two populations of lamellae, both edge-on and perpendicular to each other, in agreement with the crosshatched morphology observed by AFM.     

降温速率对高密度聚乙烯在全同立构聚丙烯上附生结晶的影响

用欠焦电子显微术和电子衍射技术研究了降温速率对高密度聚乙烯（ＨＤＰＥ）在全同立构聚丙烯（ｉＰＰ）上附生结晶的影响．ＨＤＰＥ在高取向ｉＰＰ基质膜上的附生生长仅发生在ＨＤＰＥ与ｉＰＰ的直接接触面上，存在一临界附生层厚度，超出这一厚度的ＨＤＰＥ与ｉＰＰ无取向附生关系．降温速率不影响附生层内的ＨＤＰＥ与ｉＰＰ的附生结构关系，但对ｉＰＰ基质膜上附生生长的ＨＤＰＥ的厚度，即ＨＤＰＥ的临界附生层厚度有明显影响．在缓慢降温（０．５℃／ｍｉｎ）条件下，ＨＤＰＥ在ｉＰＰ上的附生层厚度约为１００ｎｍ．而室温空气降温条件下，ＨＤＰＥ在ｉＰＰ上的附生层厚度则为２５０ｎｍ．     

Epitaxial behavior of HDPE on the boundary of highly oriented iPP substrates

The epitaxial crystallization behavior of high-density polyethylene on the boundary of highly oriented isotactic polypropylene (iPP) substrates has been investigated by means of atomic force microscopy (AFM) and transmission electron microscopy (TEM). The results obtained from AFM and TEM indicate that the epitaxial nucleation of HDPE on the highly oriented iPP substrates occurs earlier than that in the pure HDPE phase, i.e., homogeneous nucleation. Therefore the epitaxially grown HDPE lamellae can grow across the boundary of the iPP substrate into the HDPE spherulitic phase with the epitaxial orientation relationship remaining.     

聚丙烯／氯化聚乙烯共混物的形态结构

 用透射电子显微镜方法研究了聚丙烯（ｉＰＰ）和氯化聚乙烯（ＣＰＥ）共混物溶液浇铸膜的形态结构．共混物中ＣＰＥ含量≤７０％时不妨碍ｉＰＰ球晶两种结构（交叉结构和条状结构）区域的生成．在ＣＰＥ含量≥８０％时，分散相ｉＰＰ形成近乎直角（８０°）交叉的稀疏的片晶网络．在共混物的全组成范围内，ＣＰＥ结晶在ｉＰＰ片晶上附生生长，二者结晶Ｃ轴的交角为５０°     

Characterization of polyolefin melts using the polymer reference interaction site model integral equation theory with a single‐site united atom model

A general approach, based on the polymer reference interaction site model (PRISM) integral equation theory, suitable for characterizing arbitrarily complex polyolefin melts is described. We tested the method by calculating the melt structures of linear polyethylene (PE) and isotactic polypropylene (iPP) and the spinodal decomposition temperatures for PE/iPP blends. The computational expense of the PRISM calculation was reduced with a single‐site united atom model in which the polyolefin CH, CH₂, and CH₃ groups were approximated as chemically equivalent sites with spherically symmetric energetic interactions. The site–site interactions were defined by a potential function comprising a hard core with an attractive Lennard–Jones term. These energetic parameters were optimized with a central composite design strategy that enabled a simultaneous fit of experimental melt density and structure factor data. Values were obtained for PE and iPP individually and for common universal parameters that could potentially be used for all polyolefins. The rotational isomeric state–metropolis Monte Carlo (RMMC) technique was used to generate sets of conformers at specified temperatures covering the melt‐temperature range of the polymers. The characteristic ratio was used to assess the quality of the conformers and the RMMC method. Values of 9.68 for PE and 9.27 for iPP were obtained. The single‐chain structure factors calculated by the RMMC method were used to calculate the total structure factor for each melt. These were validated against published X‐ray diffraction results. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1803–1814, 2001     

Epitaxial and graphoepitaxial growth of isotactic polypropylene (iPP) from the melt on highly oriented high density polyethylene (HDPE) substrates

Although under normal conditions only the crystallization behavior of PE on oriented iPP substrates can be studied due to the higher melting point of iPP, the faster crystallization rate of a molten, oriented HDPE film compared to a nonoriented iPP layer was used to study the crystallization of iPP on the oriented HDPE film by means of transmission electron microscopy (TEM) and electron diffraction (ED). Besides the known epitaxial relationship of HDPE/iPP with their chains 50° apart, two new orientation relationships with (a) chains of both polymers parallel and (hk0)_iPP in contact with the HDPE substrate, and (b) the a‐axis of iPP crystals parallel to the chain direction of HDPE but (001)_iPP in contact with the HDPE substrate were observed. Both orientations are assumed as graphoepitaxy. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1893–1898, 1999     

Deformation and structure of doubly oriented polyethylene

Doubly oriented low-density polyethylene with parallel lamellae was compressed along the initial draw direction (i.e., at right angles to the lamellar surfaces) at 20°C. Wide- and low-angle x-ray diffraction were used to determine the changes in the molecular orientation and in the texture. During the compression, specimens previously annealed at or near 102°C were found to undergo changes in length, in long spacing, and in molecular orientation which were consistent with an (001) chain slip mechanism. In specimens annealed at higher temperatures x-ray diffraction indicated that during compression some series component of the long spacing was compressed by a much smaller amount than the remainder of the long spacing, which deformed by chain slip; in these cases it was found that the macroscopic strain along the compression axis (ε_y) was greater than the strain in the long spacing along that axis (ε_d). It is suggested that the missing strain which makes ε_y greater than ε_d is due to partial melting and the consequent development of amorphous regions between the stacks of lamellae.     

A permanganic etchant for polyolefines

A permanganic etchant has been developed which reveals lamellar and other fine detail in surfaces of at least three crystalline polyolefines, viz., polyethylene (of both high and low density), isotactic polypropylene, and isotactic poly(4-methylpentene-1). In typical treatments of high-density polyethylene ca. 2 μm of material is removed with defective regions suffering preferential attack. The etchant also discriminates between lamellar orientations, eating deeper where side surfaces of laminae are exposed than on fold surfaces, and between different polymers, attacking isotactic polypropylene more strongly than polyethylene. Comparison with other techniques authenticates the detail exposed and samples appear to be otherwise unaltered by their treatment. Besides normal imaging, it is also possible to use etched samples for transmission diffraction studies in the electron microscope. The method has very considerable application for revealing lamellar details in crystalline polyolefines (which can be chosen to be representative or selective according to the nature of the surface used). Examples are given of a wide variety of melt-crystallized morphologies for the three polymers cited and also of lamellae in a drawn polyethylene sample. It is pointed out that permanganic etching is complementary to the technique of chlorosulfonation used to stain polyethylene in a similar way as bright field microscopy is to dark field.     

Quantitative characterization of deformation in drawn polypropylene films

The submicroscopic morphology of uniaxially deformed isotactic polypropylene films has been examined by small-angle light scattering (SALS), electron microscopy, optical microscopy, small-angle x-ray scattering (SAXS), wide-angle x-ray diffraction, birefringence, sonic modulus, and density methods. Several new interpretations and extensions of existing theories are developed and verified experimentally as follows. (1) The V_v SALS pattern is shown to be a new tool for the identification of the sign of the birefringence of spherulites too small to be seen in the optical microscope. The theoretical dependence of the V_v SALS pattern is developed and verified experimentally with patterns from isotactic polypropylene, polyethylene, Penton, nylon 6,6, poly(ethylene terephthalate), and nylon 6,10. (2) Intraspherulitic lamellar behavior during deformation can be identified from the SAXS pattern. This includes quantitative evaluation of the long spacing between lamellae and their average orientation. (3) The two-phase sonic modulus theory is valid over the wide range of deformations, crystallinities, processing temperatures, and molecular weights used in this study. The deformation of isotactic polypropylene films drawn at 110 and 135°C. has been characterized quantitatively in terms of an integrated picture of mass movement on all morphological levels: the molecular, the interlamellar, and the spherulitic. At both temperatures, the spherulites deform affinely with extension, whereas the deformation mechanisms within the spherulite depend on the location of the radii with respect to the applied load. During spherulite deformation, lamellar orientation and separation processes predominate, whereas at high extensions, fibrillation occurs and crystal cleavage processes predominate. The noncrystalline region orients throughout the draw region. At 135°C. non-orienting relaxation processes appear in the noncrystalline region which retard the rate of molecular orientation with extension.     

High-resolution electron microscopy of polyethylene and paraffin crystals: Stability in the electron beam

Uncollapsed polyethylene pyramids (200–1500 Å) in length are irradiated with the electron beam of a 100-kV transmission electron microscope. Their high stability is remarkable compared to the stability of 1–10 μm crystals collapsed on the substrate, usually taken as a reference. Therefore, the maximum magnifications (300,000–750,000 X) of the microscope can be used and high-resolution images can be obtained. No lattice defects can be detected in the images of PE pyramids. Irradiation with D_c > 800 C/m² induces the orthorhombic → hexagonal transition, and slight lattice distortions appear in the high-resolution image of the hexagonal phase. For an irradiation dose D_c ≈ 2400 C/M², the diffraction pattern disappears. Normal C₃₆ orthorhombic and monoclinic paraffins have the same stability as orthorhombic PE and high-resolution patterns are obtained. These exceptional stabilities are discussed in detail. From the diffraction pattern of these uncollapsed pyramids, the fold surfaces of PE pyramids have been indexed as the {111} and paracrystalline distortions in the orthorhombic PE have been measured at low irradiation dose. Along the a and b axes g is ca. 5% and along the chain axis c it is ca. 17%; these values agree with the previous x-ray determinations of PE crystallized from the melt. The large difference between these two distortion factors may be interpreted in terms of packing.     

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.     

Advantage of Preserving Bi-orientation Structure of Isotactic Polypropylene through Die Drawing

The isotactic polypropylene(iPP) usually shows a unique parent-daughter lamellae structure in which the parent and daughter lamellae are against each other with a near perpendicular angle(80° or 100°). Inducing a high fraction of oriented cross-hatched structure in iPP during processing is desirable for designing the bi-oriented iPP products. We processed a commercial iPP via tensile-stretching and die-drawing to evaluate the structural evolution of oriented parent-daughter lamellae. It turned out that the die-drawing process had an advantage in attaining a high fraction of oriented cross-hatched structure of iPP, as compared to the free tensile stretching. Besides, the presence of α-nucleating agents affected the formation of oriented parent-daughter lamellae in the die-drawn samples whereas such influence diminished in the free stretched ones. It was found that the confined deformation inside the die led to the well-preserved oriented cross-hatched structure in the die-drawn iPP.     

Probing into the epitaxial crystallization of β form isotactic polypropylene: From experimental observations to molecular mechanics computation

Compared with the most stable crystalline form of isotactic polypropylene (α‐iPP), β‐iPP shows superior impact strength and high temperature performance, though the mechanism of how the frustrated structure of β‐iPP is formed still remains unclear. In present work, the single crystal structure of a traditional β‐iPP nucleating agent, N,N′‐dicyclohexylterephthalamide (DCHT), was obtained for the first time and correlated with the epitaxial crystallization of β‐iPP on the surface of DCHT crystal. The combination of synchrotron radiation X‐ray microdiffraction and molecular chain packing model confirmed that a two dimensional match of chain‐axis and inter‐chain direction coexists between β‐iPP (110) plane and DCHT (001) plane. It was further found that an epitaxial model is helpful to understand the formation of the frustrated structure of 3₁ helices packing in β‐iPP. The molecular mechanics computation showed that as the (001) plane of DCHT is fixed, the packing mode of β‐iPP (110) plane on the substrate surface is more stable than that of α‐iPP (010) plane. This work clarifies the epitaxial crystallization mechanism of β‐iPP on DCHT by employing both experimental and computational evidences. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 418–424