Shear-Induced Crystallization in Blends of Model Linear and Long-Chain Branched Hydrogenated Polybutadienes

Abstract

Quiescent and shear-induced crystallizations were performed on several well-defined linear monodisperse hydrogenated polybutadiene blends with a high-molecular-weight long-chain branched (LCB) combshaped additive. The connection between microscopic molecular motion and crystallization kinetics has been quantitatively studied with respect to the formation of either isotropic or oriented shish kebab crystal morphology using time-resolved X-ray scattering techniques. Using a constant preshear rate, the addition of small amounts of LCB combs to the sample blends, at concentrations below and just above the overlap concentration c*, has significantly increased the crystallization rates compared to quiescent conditions. However, only one blend showed the formation of an oriented shish kebab morphology. Also, for these model blends, the transition between isotropic and oriented crystals occurs quite sharply between 5 and 10%, which is around c*. We explain these data by using a shish formation mechanism in which, to form shish kebabs, the combs must be mutually overlapping and the comb Weissenberg number must be in the strong stretch regime.

Effect of processing parameters on the morphology development during extrusion of polyethylene tape: An in-line small-angle X-ray scattering (SAXS) study

“…Finally, at 1500 mm a highly oriented 'tear-drop' pattern is observed without any additional radial scattering. The SAXS patterns can be interpreted in terms of a two-phase lamellar system where the lamellae stacks are perpendicular to the tape elongation direction, as has previously been reported for polyolefins when crystallized from oriented melts during shear and melt-spinning processes [9,15,34,36,49,50]. It should be mentioned that some of the 2D SAXS patterns in Figure 3, also show evidence of scattering perpendicular to the elongation direction in the form of equatorial streaks.…”

“…The 2D SAXS data were reduced to 1D profiles of intensity (I) versus scattering vector (q), where q = (4π sin θ/λ) and θ is the scattering angle. This was achieved by performing a sector integration on the meridional scattering pattern using a fixed radius and integration angle with the CCP13 BSL software [15,42]. The integration plots were then analysed using purpose written correlation function software known as Corfunc [15,42,43].…”

“…This results in the formation of the shish structures. The critical strain rate at a fixed temperature, is a function of the molecular weight distribution; higher molecular weight chains can be stretched at lower strain rates and lower molecular weight chains at much higher strain rates [15,56]. At constant strain rate a portion of molecules above the molecular weight threshold (M*) will stretch fully, whereas chains below M* will not stretch and remain as random coils.…”

“…The route to crystallization during flow is generally accepted to be via the formation of 'shishkebab' morphology [6][7][8][9]. Proposed mechanisms and kinetics for the formation of shish-kebab morphology usually relate to data from shear flow conditions using custom made shearing devices [10][11][12][13][14][15][16]. However, during extrusion, the polymer melt exits the die head and begins to cool rapidly, and at the same time it is uniaxially elongated under constant force, between the die head and take-up device.…”

Copolymerization of 1-hexene and 1-dodecene with 1,3-butadiene by a versatate/diisobutylaluminum hydride/t-butyl chloride catalyst system

“…The literature contains findings that indicate the insertion of alpha-olefins in the polymeric matrix influences the crystallinity of samples, modifying the thermal properties [29][30][31] . In this way, it is possible to suggest that the presence of alpha-olefin in the polybutadiene chain creates crystalline segments, generated by the packing of the branches, resulting in the increase of the materials' thermal stability.…”