Very high molecular weight (M n up to 700 000) and narrowly polydispersed (PDI = 1.1−1.4) hyperbranched polyglycerols (HPG) were synthesized by ring-opening multibranching polymerization of glycidol using dioxane as an emulsifying agent. Broader molecular weight distributions with low molecular weight fractions were obtained when diglyme was used as the emulsifying agent. But the low molecular weight fractions could be removed by dialysis. Isolated yields in both the cases were 70−90%. The different result in the case of dioxane may be due to faster cation exchange which leads to low polydispersites. HPGs of various molecular weights were characterized by a GPC system coupled with a multiangle laser light scattering detector and a triple detector array. The intrinsic viscosities were low for these polymers and did not increase with molecular weight. The dimensions of these polymers (R g, R h, R η) and their dependence on molecular weights are described. The hydrodynamic radii were very small with dimensions similar to those of dendrimers. Our results show that these polymers are very compact and have spherical conformations in water with no indications of aggregate formation. The melt viscoelastic properties were also studied. Despite their self-similar structures, depending on the type of solvent used to synthesize them (diglyme vs dioxane), topologically restricted configurations are produced that result in completely different entanglement dynamics.

No abstract

The rheological, thermal, and mechanical properties of blends consisting of a linear high melt flow rate polypropylene (PP) and two branched PPs are characterized in detail. Blends containing branched PPs display evidence of miscibility in the melt state and exhibit high melt elasticity together with significant strain hardening in extensional deformation while retaining good flow properties. Out of the two blend systems examined the blends containing linear and branched PPs with similar melt flow rates have better mechanical properties, higher crystallization temperatures, and higher crystallinities. POLYM. ENG. SCI., 47:1133–1140, 2007. © 2007 Society of Plastics Engineers

The influence of a new processing additive (particles of organically modified nanoclays) on the processibility of polyolefins in extrusion is studied. The equipment used includes an Instron capillary rheometer with two types of dies, namely capillary dies and special annular dies (Nokia Maillefer wire coating crosshead) attached to the rheometer. Ziegler Natta and metallocene polyethylenes and one polypropylene were tested using these two pieces of equipment. It was found that the nanoclay additive had a significant effect on the extrudate appearance of polyethylene. It eliminates surface melt fracture and postpones the critical shear rate for the onset of gross melt fracture to significantly higher values depending on resin type, temperature, and additive concentration (typically 0.05 to 0.5 wt%). To explain the possible mechanism for the effect of the additive on the processibility of the resins, shear and extensional rheological measurements were carried out for the pure resins as well those loaded with the nanoclay additives. It seems that the presence of nanoclays suppresses the development of extensional stresses to such high levels that can cause melt fracture phenomena. Finally, it was found that the combination of nanoclays with traditional processing aids such as fluoropolymers produce an enhanced processing aid that can increase the critical shear rates for the onset of melt fracture to levels much higher than the individual constituents when they are used independently. POLYM. ENG. SCI., 45:1098–1107, 2005. © 2005 Society of Plastics Engineers

No abstract