The role of the interface in melt linear viscoelastic properties of LLDPE/LDPE blends: Effect of the molecular architecture of the matrix

Abstract

The rheological properties of blends consisting of a long chain branched low-density polyethylene (LDPE) and two linear low-density polyethylenes (LLDPE) are studied in detail. The weight fractions of the LDPE used in the blends are 5 and 15%. The linear viscoelastic characterization is performed at different temperatures for all the blends to check thermorheological behavior and miscibility in the melt state. Blends containing metallocene LLDPE as the matrix display thermorheologically complex behavior and show evidences of immiscibility in the melt state. The linear viscoelastic response exhibits the typical additional relaxation ascribed to the form deformation mechanism of dispersed phase droplets (LDPE). The Palierne model satisfactorily describes the behavior of these blends in the whole frequency range explored. However, those blends with Ziegler-Natta LLDPE as the matrix fulfill the time-temperature superposition, but exhibit a broad linear viscoelastic response, further than the expected for an immiscible system with a sharp interface. The rheological analysis reveals that, in addition to the droplets form relaxation, another mechanism at lower frequencies exists. The broad linear response of the blends with the ZieglerNatta LLDPE can be explained by hypothesizing a strong interaction between the high molecular weight linear fraction of the LLDPE and the low molecular weight (almost linear) chains of the LDPE phase, forming a thick interface with its own viscoelastic properties.

Role of the interface in the melt‐rheology properties of linear low‐density polyethylene/low‐density polyethylene blends: Effect of the molecular architecture of the dispersed phase

“…It is documented in the literature that linear polyethylene (high‐density polyethylene) is miscible with LDPE in a whole range of compositions in blends prepared under similar conditions to those used in this study 5, 6. Then, we expected some kind of interaction between the linear species from LLDPE and the lowest molecular weight species from LDPE samples because both had very high relaxation time values 12. The morphological picture proposed for the LLDPE/LDPE blends here was similar to that proposed by Wagner et al9 These authors suggested a two‐phase system composed by one phase with the branched shortest chains of both LLDPE and LDPE polymers and another phase with the longest chains of both components.…”

“…In a previous work, we already revealed that the application of this model for LLDPE/LDPE blends did not explain the experimental shear moduli measured, as the broad relaxation response could not be described only as a consequence of the shape relaxation mechanism of the dispersed LDPE droplets. A change in the parameter α/ R did not improve the prediction 12. An additional relaxation mechanism should be postulated to explain the broadening of the melt mechanical relaxation.…”

“…Blends of low‐density polyethylene (LDPE) and linear low‐density polyethylene (LLDPE) have been extensively studied in recent decades, and a dichotomy of results supporting both miscibility and immiscibility can be found 1–11. We recently reported that LLDPE‐rich blends (with 5–15% LDPE) were immiscible, and the final properties were strongly dependent on the molecular architecture of the LLDPE matrix (Ziegler–Natta or metallocene) 12. In the case of blends of Ziegler–Natta LLDPE with LDPE, an additional contribution in the linear viscoelastic spectrum was found, which was not explained by the assumption of a simple dispersion of LDPE droplets in the LLDPE matrix.…”

Fundamentals of structure–property relationships in blown films of linear low density polyethylene/low density polyethylene blends

“…There have been many studies performed to understand rheology (especially elongational viscosity and melt strength), structure evolution during film blowing, and thermal properties of LLDPE/LDPE blends. 1–14 Studies have been performed to understand the effect of the addition of small amounts of LDPE, i.e. less than 20%, to a LLDPE to understand the impact of the LDPE addition on LLDPE film physical properties such as tensile properties, Elmendorf tear strength, dart impact strength, etc.…”

Effects of Alterations to Ziegler–Natta Catalysts on Kinetics and Comonomer (1‐Butene) Incorporation

“…Polyolefines dominate the plastics market due to their price and performance . In contrast to polypropylene (PP) and the classical polyethylene (PE) modifications (high‐density polyethylene and low‐density polyethylene), linear low‐density polyethylene (LLDPE) is the youngest member of the polyolefin family which is becoming increasingly important due to its material properties, which can be tuned within a wide range, and its flexibility in production. Its material properties are heavily influenced by the amount and type of α‐olefin used as comonomer, which further extends the versatility usability of LLDPEs.…”