We present an experimental investigation on the creep behavior of molten polypropylene
organically modified clay nanocomposites. The nanocomposite hybrids were prepared by melt intercalation
in an extruder in the presence or absence of a compatibilizer. They were subsequently annealed and
simultaneously characterized using high-temperature wide-angle X-ray diffraction and controlled stress
rheometry. The creep resistance of compatibilized hybrids was significantly higher than that of
uncompatibilized hybrids and also increased with annealing time. The microstructure of the nanocomposites as investigated by TEM and high-temperature WAXD showed the presence of clay crystallites
dispersed within the polymer matrix. The creep data together with the microstructural investigation are
probably indicative of a small amount of exfoliation from the edges of the clay crystallites during extrusion
and annealing. The zero shear viscosity of the compatibilized nanocomposites containing greater than 3
wt % clay was at least 3 orders of magnitude higher than that of matrix resin and the uncompatibilized
hybrids. Importantly, the large increase in zero shear viscosity was not accompanied by any increase in
the flow activation energy compared to the matrix polymer. The compatibilized hybrids also showed an
apparent “yield” behavior. We conclude that the solidlike rheological response of the molten nanocomposite
originates from large frictional interactions of the clay crystallites. Compatibilizer has a significant
influence in modifying the rheological behavior.
Front Cover: In this work, a modeling pathway and software tool for linking entangled linear polymer molecular properties to linear viscoelasticity and melt index (MI) values is presented. A reptation model links molecular properties to the flow curve, and then, an ANSYS Polyflow model calculates MI values based on the flow curve predicted. The method is thoroughly tested and validated for uni‐and bi‐modal, low‐ and high‐density polyethylene grades. An overall accuracy level in the range of 90% on average is exhibited, considering both model prediction steps: (i) MWD to flow curve and (ii) flow curve to MI. These promising results offer a valuable tool to enhance product development toward the direction of end‐use polymer bulk properties prediction. Further details can be found in the article by Vasileios Touloupidis,* Christof Wurnitsch, Alexandra Albunia and Girish Galgali on page 392.
Polyethylenes (PE) are the most commonly occurring ingredients for post-consumer recyclates (PCR). The structure–property relationships of different types of model PE-based blends are established using multiple thermo-rheological analyses. Although considered “simple”, the complex behavior of model PE-based blends is experimentally demonstrated for the first time for metallocene-catalyzed, linear, low-density polyethylenes (mLLDPE) with different microstructures that are commonly encountered in PCR. During non-isothermal crystallization, the microstructure of mLLDPE predominantly influences the interaction between mLLDPE and LDPE. Based on the mLLDPE microstructure, the molten LDPE phase acts either as a nucleating agent or as a crystallization rate promoting agent. Both rheological and thermal analyses show that higher activation energy is required for the reptation or movement of polymer chains in a highly branched microstructure with long chain branching (LCB) compared to a linear microstructure with short chain branching (SCB). The quasi-melt response, as measured by thermal analysis under non-isothermal conditions, is distinctly different and sensitive to both the SCB and LCB present in the LLDPE/LDPE blends.
The microstructure–sealing performance linkages of linear low density polyethylene (LLDPE) and low density polyethylene (LDPE) blends using thermo‐rheological tools are discussed. The effect of molecular architecture in metallocene catalyzed LLDPE (mLLDPE), Ziegler–Natta catalyzed LLDPE (ZN‐LLDPE) on the miscibility, and rheological behavior of the blends with LDPE is shown. Even though the macro parameters like density, melt flow rate of the LLDPEs are comparable, subtle differences in the microstructure manifested by comonomer type and its distribution across molecular weight affects the sealing performance of the LLDPE/LDPE blends. For LLDPE matrix, tailor‐made comonomer distribution affording thinner lamellas (viz., thickness <11.8 nm) is more critical to sealing process than having total higher comonomer content. For LLDPE/LDPE blends, the thicker lamellae of main chain polymer of LDPE co‐crystallize with homo‐polyethylene like fraction of LLDPE. The co‐crystallization of LLDPE and LDPE helps to achieve hot tack sealing at lower percentage of molten polymer and it appears to be the dominating factor over diffusion of polymer chains across the melt interface to achieve sealing.
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