We report the effect of molecular weight and comonomer content on melt crystallization of model random ethylene 1-butene copolymers. A large set of narrowly distributed linear polyethylenes (PE) was used as reference of unbranched molecules. The samples were crystallized from a melt state above the equilibrium melting temperature and cooled at a constant rate. The exothermic peaks of the melt-solid transition are reported as the crystallization temperatures (T c ). Following expectations, the T c of unbranched PE samples was constant and independent of the initial melt temperature. The same independence was observed for copolymers (2.2 mol % ethyl branches) with molar mass below 4500 g/mol. Moreover, the T c of copolymers with higher molar mass depends on the temperature of the initial melt, T c increases as the temperature of the melt decreases. We attribute the increase in T c to a strong crystallization memory in the melt above the equilibrium melting, and correlate this phenomenon with remains in the melt of the copolymer’s crystallizable sequence partitioning. Albeit molten, long crystallizable sequences remain in the copolymer’s melt at a close proximity, lowering the change in free energy barrier for nucleation. The residual sequence segregation in the melt is attributed to restrictions of the copolymer crystalline sequences to diffuse upon melting and to reach the initial random topology of the copolymer melt. Erasing memory of the prior sequence selection in copolymer melts requires much higher temperatures than the theoretical equilibrium value. The critical melt temperature to reach homogeneous copolymer melts (T onset ), and the comonomer content at which melt memory above the equilibrium melting vanishes are established. The observed correlation between melt memory, copolymer crystallinity and melt topology offers strategies to control the state of copolymer melts in ways of technological relevance for melt processing of LLDPE and other random olefin copolymers.

This study is an effort to modify conventional batch processes to be able to produce polymeric foams with high cell density and small cell size, which cannot be reached by conventional batch foaming processes. This has been attained by controlling the foaming temperature and controlled stabilization of the cellular structure. The method was tested for both with and without addition of nanosized particles in polymeric matrix. The desired morphologies were obtained using a novel apparatus with the capability of instantaneous pressure drop and controlling stabilization of the foam structure. The design of the said apparatus was based on the idea that in a foaming process, nucleation is the predominant mechanism that determines the final foam structure. The produced foam products have uniform structures without any unfoamed skin. Results show that the control of the foaming temperature and the cell stabilization are the predominant factors in adjustment of the final foam morphology. A wide range of microcellular structures with cell densities between 107 and 1012 bubbles/cm3 and average cell sizes of 500 nm–20 μm were produced. Foaming of polystyrene‐nano‐silica nano‐composites with the same method showed that nanoparticles act as nucleating agent and increase the cell density in the final foam products compared with that of neat polystyrene. POLYM. ENG. SCI., 50:1558–1570, 2010. © 2010 Society of Plastics Engineers

This work is an experimental study of the effects of nanoparticles with different characteristics and contents on foaming composites made of three different nanosilica particles with different geometrical and chemical surface properties in a polystyrene matrix. In addition to the general characteristics reported in our last study on the morphology of polymer-nanoparticle composites, this study shows that nanosilicas of larger sizes can result in foams of higher cell densities. Additionally, the cell densities of foams can be reduced if the nanoparticle surface becomes more affine to the polymer matrix chemically. These results show a correspondence with the effects of the characteristics of the nucleation agent on the nucleation of bubbles, which have been explored previously.

The mechanical properties of binary blends of propylene-1-hexene random copolymers (with 11 and 21 mol% 1-hexene) are studied in parallel with polymorphic transformations under uniaxial tensile deformation. The modulus, yield stress, and draw ratio of the pure PH copolymers decrease with increasing 1-hexene content, while for the blends the change of mechanical properties with composition is highly non-linear. The addition of just 10 wt % PH11 to PH21 doubles the elastic modulus and yield strength of the blends in reference to the value of PH21, reaching for all blends values close to the performance of pure PH11. The elongation at break and the ultimate tensile strength increase more gradually with content of PH11. On tensile deformation, pure components and blends undergo morphological and polymorphic transformations, such as a reversible lamellar to fibrillar transformation of trigonal PH21, or an irreversible α crystal to mesophase in pure PH11 and the blends. In blends and neat PH11, a fibrillar trigonal morphology that develops under deformation is stabilized by the transformation of α to mesophase, and remains after removing the load, explaining the lower elastic recovery of the blends compared to PH21. The formation of stress-induced trigonal crystallites in PH11 and blends after strains > 150% is explained as a decrease of the free energy barrier for nucleation of a phase that requires short iPP sequences.

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