Morphology of Blends of Linear and Long-Chain-Branched Polyethylenes in the Solid State: A Study by SANS, SAXS, and DSC

Abstract

Differential scanning calorimetry (DSC), small-angle neutron scattering (SANS), and X-ray scattering (SAXS) have been used to investigate the solid-state morphology of blends of linear (high density) and long-chain-branched (low-density) polyethylenes (HDPE/LDPE). The blends are homogeneous in the melt, as previously demonstrated by SANS using the contrast obtained by deuterating the linear polymer. However, due to the structural and melting point differences (~20 °C) between HDPE and LDPE, the components may phase segregate on slow cooling (0.75 °C/min). For high concentrations (<p > 0.5) of HDPE, relatively high rates of crystallization of the linear component lead to the formation of separate stacks of HDPE and LDPE lamellae, as indicated by two-peak SAXS curves. For predominantly branched blends, the difference in crystallization rate of the components becomes smaller and only one SAXS peak is observed, indicating that the two species are in the same lamellar stack. Moreover, the phases no longer consist of the pure components and the HDPE lamellae contain up to 15-20% LDPE (and vice versa). Rapid quenching into dry ice/2-propanol (-78 °C) produces only one SAXS peak (and hence one lamellar stack) over the whole concentration range. The blends show extensive cocrystallization, along with a tendency for the branched material to be preferentially located in the amorphous interlamellar regions. For high concentrations ( > 0.5) of HDPE-D, the overall scattering length density (SLD) is high and the excess concentration of LDPE between the lamellae enhances the SLD contrast between the crystalline and amorphous phases. Thus, the interlamellar spacing (long period) is clearly visible in the SANS pattern. The blend morphology is a strong function of the quenching rate, and samples quenched less rapidly (e.g., into water at 23 °C) are similar to slowly cor'ed blends. The combination of SANS, SAXS, and DSC techniques allows us to interpret morphological differences in the solid state of these blends.

Crystallization kinetical and morphological peculiarities in binary crystalline/crystalline polymer blends

“…41 For a PVDF/PHB blend, it was concluded that existing spherulites of PVDF nucleate PHB crystallization and such ILC. 16 Cocrystallization may occur in blends of polymers, which contain chemically identical monomer units as in LDPE/HDPE blends 54,55 or in blends of copolymers, which are in part built up by identical monomer units (PVDF/PTrFE 57 ). Cocrystallization has also been found in blends of deuterated and protonated PE.…”

Miscibility and Crystallization Behavior of Poly(butylene succinate) and Poly(vinylidene fluoride) Blends

“…Although such binary blends of crystallizable polymers are very rare, these blends can be divided into three main categories. These are (i) polymer/low molecular weight diluents, such as PCL/ Trioxane mixture 1 3; (ii) blends of chemically similar species, such as blends of two poly(aryl ether ketones), 14 blends of two polyethylenes, 15 • 42 -44 homopolymer/segmented block copolymer, 16 • 17 copolymers of vinylidene fluoride/trifluoroethylene, 18 vinylidene fluoride-co-tetrafl uoroethy lene /vi ny lidene fluoride-co-hexafl uoroacetone19; (iii) two polymers of chemically different structures, such as PCL/poly(vinylidene chloride), 20 · 21 PHB/poly(ethylene oxide) (PEO), 22 and poly(butylene adipate) (PBA)/PVDF. 10 - 12 The blends in category (ii) form cocrystals in a certain crystallization conditions, however, the blends in category (iii) form segregated crystals.…”

Sustainable eco-composites obtained from agricultural and urban waste plastic blends and residual cellulose fibers

“…This is the usual behavior, as it is well known that LDPE and HDPE form homogeneous materials in the melt but only co-crystallize under certain conditions, depending on the composition of the mixture and the cooling rate (Wignall et al, 1995). At high cooling rates, the co-crystallization has been demonstrated (Fonseca and Harrison, 1998); however, when the melt is solidified at low or moderate cooling rates, as in our case, a blend of the two polymers, with two defined melting peaks, is obtained.…”