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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.

The structure of ethylene copolymers modified by α‐olefins has become an area of intense investigation since the successful commercialization of so‐called linear low‐density polyethylene (LLDPE) resins. The molecular structure of a series of typical commercial LLDPE copolymers was investigated and compared to LDPE and HDPE. The commercial LLDPE resins studied contained about 7% by weight of butene‐1. The resins were fractionated according to short‐chain branching content by a technique called temperature rising elution fractionation. Size exclusion chromatography, x‐ray diffraction, 13C nuclear magnetic resonance, intrinsic viscosity, and differential scanning calorimetry were used to fully characterize the whole polymers as well as fractions of a selected LLDPE resin. A broad set of data was assembled in this work to investigate the short‐chain branching, long‐chain branching, and the molecular‐weight distribution of these commercial resins. The melting behavior of the LLDPE resins was found to be strikingly different from that of LDPE and HDPE. The broad and multimodal melting envelope of the LLDPE resins was found to be due to a broad and multimodal short‐chain branching distribution. No significant long‐chain branching was found in the LLDPE resins. The short‐chain branching was found to decrease with the increase of molecular weight in a typical commercial LLDPE resin. The unique physical properties of these resins are certainly strongly controlled by the expression of the distinctive heterogeneous comonomer incorporation in the solid‐state morphological structure. The physical and mechanical properties of these materials should be ultimately understandable on the basis of the unique morphology which results from the extremely heterogeneous incorporation of modifying α‐olefin in these commercial LLDPE resins.

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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.

The multimodal differential scanning calorimetry melting endotherms observed for commercial linear low-density polyethylenes are due to broad and multimodal short-chain-branching distributions. Multiple peaks, observed in melting endotherms of isothermally melt-crystallized and compositionally homogeneous polyethylene copolymers are due to intrachain heterogeneity. This intrachain heterogeneity is quantified by the distribution of ethylene sequence lengths within the chains. These compositionally homogeneous copolymers undergo a primary crystallization, which produces a population of thicker lamellae, creating a network that places severe restrictions on segment transport in subsequent secondary crystallization, which produces a population of thinner crystals. The restrictions on segment transport imposed by the initial network created by the primary crystallization of thicker lamellae severely limits the total crystallinity achieved in the random copolymers studied. The solution crystallization of such copolymers produces a continuous distribution due to more facile segment transport in a dilute solution, in contradistinction to the multimodal distribution produced in the melt crystallization.