The heat of fusion measured with differential scanning calorimetry (DSC) is typically divided by a constant value of the heat of fusion of 100% polyethylene (PE) crystal (⌬H m o ) for the estimation of the fraction crystallinity of PE copolymers, regardless of the density [i.e., the short-chain branching (SCB) concentration]. In this work, values of ⌬H m o of about 288 J/g were determined with a combined DSC and X-ray diffraction (XRD) method for a series of PE copolymers containing SCB from 0 to 50 Br/1000 C (density ϭ 0.965-0.865 g/cc). There was no systematic change in ⌬H m o observed across this density range. This result supports the suitability of determining the fraction crystallinity of PE of any density by the simple division of the observed heat of fusion determined by DSC by a constant value of ⌬H m o . This DSC method yielded values of PE crystallinity in good agreement with corresponding values determined by XRD for a series of PE copolymers. The determination of ⌬H m o involved a small precision error for higher density (lower SCB) PEs, but the precision error increased for lower density (i.e., higher SCB) PEs. This was due to the difficulty in measuring the heat of fusion for lower density PEs, which exhibited low values of the heat of fusion and melted only slightly above room temperature, and due to the difficulty of measuring lower values of crystallinity by XRD. The crystal thickness measured by small-angle X-ray scattering for this series of PE copolymers decreased exponentially from about 280 to 6 Å.

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Thermoplastic olefin (TPO)/clay nanocomposites were made with clay loadings of 0.6 -6.7 wt %. The morphology of these TPO/clay nanocomposites was investigated with atomic force microscopy, transmission electron microscopy (TEM), and X-ray diffraction. The ethylene-propylene rubber (EPR) particle morphology in the TPO underwent progressive particle breakup and decreased in particle size as the clay loading increased from 0.6 to 5.6 wt %. TEM micrographs showed that the clay platelets preferentially segregated to the rubber-particle interface. The breakup of the EPR particles was suspected to be due to the increasing melt viscosity observed as the clay loading increased or to the accompanying chemical modifiers of the clay, acting as interfacial agents and reducing the interfacial tension with a concomitant reduction in the particle size. The flexural modulus of the injection moldings increased monotonically as the clay loading increased. The unnotched (Izod) impact strength was substantially increased or maintained, whereas the notched (Izod) impact strength decreased modestly as the clay loading increased.

ABSTRACT:We report structural factors affecting the optical properties of blown polyethylene films. Two types of blown polyethylene films of similar degrees of crystallinity were made from (1) single-site-catalyst high-density polyethylene (HDPE; STAR ␣) and (2) Ziegler-Natta-catalyst HDPE (ZN) resins. The STAR ␣ film exhibited high clarity and gloss, whereas the ZN film was turbid. Small-angle X-ray scattering (SAXS), small-angle light scattering (SALS), and optical microscopy gave quantitative and qualitative information regarding structure and orientation in the films. A new approach is described for determining the three-dimensional lamellar normal orientation from SAXS. Both the clear STAR ␣ and turbid ZN films had similar lamellar crystalline structures and long periods but displayed different degrees of orientation. It is demonstrated that optical haze is related to surface features that seem to be linked to the bulk morphology. The relationship between haze and structural orientation is described. The lamellar orientation is linked to rodlike structures seen in optical microscopy and SALS through a stacked lamellar or cylindrite morphology on a nanometer scale and through a fiberlike morphology on a micrometer scale. The micrometer-scale, rodlike structures seem directly related to surface roughness in a comparison of index-matched immersion and surface micrographs. The higher haze and lower gloss of the ZN film was caused by extensive surface roughness not observed in the STAR ␣ film.

The morphologies of a series of blown films and machine‐direction‐oriented (MDO) films, all produced from high density polyethylene, were characterized. In the blown film process, the crystalline morphology develops while the melt is under extensional stress. In the MDO process, drawing takes place in the solid state and deforms the crystalline morphology of the starting film. The films were characterized by wide‐angle X‐ray scattering (WAXS), small‐angle X‐ray scattering (SAXS) and atomic force microscopy to determine the lamellar morphology. The effect of the type of deformation on the lamellar morphology was studied and relationships were developed between the lamellar and polymer chain morphology using SAXS and WAXS. Blown and MDO films were found to have very different morphologies. However, an integrated mechanism was developed linking the sequential events in the deformation and morphology development in blown and MDO films. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1834–1844, 2007