Evidence for a partially ordered component in polyethylene from wide-angle X-ray diffraction

Cryogenic 4D-STEM analysis of an amorphous-crystalline polymer blend: combined nanocrystalline and amorphous phase mapping

Understanding and visualizing the heterogeneous structure of immiscible semicrystalline polymer systems is critical for optimizing their morphology and microstructure. We demonstrate a cryogenic 4D-STEM technique using a combination of amorphous radial profile mapping and correlative crystalline growth processing methods to map both the crystalline and amorphous phase distribution in an isotactic polypropylene (iPP)/ethylene-octene copolymer (EO) multilayer film with 5-nm step size. The resulting map shows a very sharp interface between the amorphous iPP and EO with no preferential crystalline structure near or at the interface, reinforcing the expected incompatibility and immiscibility of iPP and EO, which is a short-chain branched polyethylene. This technique provides a method for direct observation of interfacial structure in an unstained semicrystalline complex multicomponent system with a single cryogenic 4D-STEM dataset.

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Cryogenic 4D-STEM can simultaneously map amorphous and crystalline structure

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Direct observation of beam-sensitive polymers reveals nanostructure near interfaces

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Cryogenic 4D-STEM enables mapping of chemically/structurally similar polymers in blend

The role of an objective function in the mathematical modelling of wide-angle X-ray diffraction curves of semi-crystalline polymers

The choice of an objective function in constructing a mathematical model of an experimental wide-angle X-ray diffraction curve of semi-crystalline polymers is discussed.

To decompose a wide-angle X-ray diffraction (WAXD) curve of a semi-crystalline polymer into crystalline peaks and amorphous halos, a theoretical best-fitted curve, i.e. a mathematical model, is constructed. In fitting the theoretical curve to the experimental one, various functions can be used to quantify and minimize the deviations between the curves. The analyses and calculations performed in this work have proved that the quality of the model, its parameters and consequently the information on the structure of the investigated polymer are considerably dependent on the shape of an objective function. It is shown that the best models are obtained employing the least-squares method in which the sum of squared absolute errors is minimized. On the other hand, the methods in which the objective functions are based on the relative errors do not give a good fit and should not be used. The comparison and evaluation were performed using WAXD curves of seven polymers: isotactic polypropyl­ene, polyvinylidene fluoride, cellulose I, cellulose II, polyethyl­ene, polyethyl­ene terephthalate and polyamide 6. The methods were compared and evaluated using statistical tests and measures of the quality of fitting.

Crystallinity of polyolefins with large side groups by low-field 1H NMR T2 relaxometry: Isotactic Polybutene-1 with form II and I crystals

Phase composition and molecular mobility were studied using ¹H NMR T₂ relaxometry in isotactic polybutene-1 (iPB-1) with two polymorphs - form I and II crystals. Several types of NMR relaxation methods and data analysis were evaluated for determining the most reliable way for studying physical phases in iPB-1. Three-phase model provided the most appropriate description of the phase composition in iPB-1, i.e., a crystal-amorphous interface separates the crystalline and the amorphous phases. Due to complex molecular mobility in iPB-1, the amount of rigid fraction should be considered as NMR crystallinity number. Two types of chain segments are present in the amorphous phase: (1) chain segments with anisotropic mobility due to chain anchoring to crystals and chain entanglements; and (2) highly mobile chain end segments. The polymorphic phase II to I transition causes significant immobilization of polymer chains in the crystalline and the amorphous phases. Molecular weight of iPB-1 largely influences phase composition and molecular mobility in crystalline and amorphous phases.

Structure Development in Melt Spinning and Cold Drawing of Poly(ethylene-co-octene) Copolymer

An experimental study of melt spinning and cold drawing of polyethylene and its Dow Chemical INSITEØ catalyst octene copolymers was carried out. The structure and morphology of both melt spun and cold drawing fibers were investigated using wide angle x-ray diffraction (WAXD), small angle x-ray scattering (SAXS), scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and birefringence. Up to 5.9 mol.% octene, the fibers exhibit only the Bunn orthorhombic crystal structure. Fibers with 9.7 mol.% and 13.3 mol.% octene, whose DSC crystallinity is 0.23 and below, have an additional reflection at about 0.45nm. The Hermans-Stein orientation factors for the melt spun fibers were determined and correlated with spinline stress. Cold drawn polyethylene fibers exhibited additional reflection at 0.45, 0.38 and 0.35nm which may be associated with the Tanaka-Seto et al. monoclinic unit cell. The copolymers contained a decreased monoclinic level which disappears at about 5.9 mol.%. However, a broad reflection at 0.45nm, reappears at 9.7 mol.% octene. This reflection is also present at the 13.3 mol.% level. It is surmised that a pseudohexagonal or mesomorphic structure is developing. The Hermans-Stein orientation factors were determined for the drawn fibers as a function of cold draw ratio. Drawing results in high crystalline orientation and fibrillation for polyethylene fibers. For copolymers, with higher octene content, the fibrillation decreases.

Stress-induced crystal-to-crystal transformations in high-density polyethylene-layered silicate nano-composites: a spectroscopic study

High-density polyethylene (HDPE)-clay nano-composites have been prepared using the melt intercalation technique. Organically modified montmorillonite at various loadings (0.5--7%) was used as a nano-additive. Fourier transform infrared spectroscopy (FT-IR) was utilized for the first time to monitor the stress-induced crystal-to-crystal transformations of the polyethylene matrix with respect to the clay loading as well as to the degree of mechanical strain. In addition, polarized infrared measurements revealed information on both the orientation and the stress-induced distortion of the crystals. It was concluded that the crystal-to-crystal transformations are hindered by the presence of the clay, which also prevented the crystals from orienting even at low clay loadings (1%). Finally, X-ray diffraction (XRD) and differential scanning calorimetry (DSC) measurements confirmed the presence of the stress-induced crystalline structures in agreement with the infrared measurements.

Formation of High-Stress Phase and Extrusion of Polyethylene due to Nanoconfinements during Ziegler−Natta Polymerization Inside Nanochannels

Polyethylene nanofibers were synthesized by heterogeneous Ziegler-Natta polymerization inside nanochannels of robust anodized aluminum oxide (AAO) membranes. The polymerization catalysts were chemisorbed at the inner wall of the nanochannels and monomers were provided through diffusion from the outside. Polyethylene is produced inside the nanochannels in the 10-20 mum region from the channel entrance. Polyethylene fibers were extruded from the nanochannels up to 3-5 mum during the polymerization. X-ray diffraction, differential scanning calorimetry, and Fourier transform infrared analyses indicated formation of a highly stressed crystalline structure although the polymerization was carried out without any external pressure or mechanical work. The highly stressed phase formation inside nanochannels and some degree of polyethylene nanofiber extrusion from nanochannels were attributed to catalytic production of excess amounts of polyethylene inside nanoconfined templates.