Thermorheology and processing of polyethylene blends : macromolecular structure effects (original) (raw)
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Journal of Applied Polymer Science, 2012
The dynamic rheological behavior of low‐density polyethylene (LDPE)/ultra‐high‐molecular‐weight polyethylene (UHMWPE) blends and linear low‐density polyethylene (LLDPE)/UHMWPE blends was measured in a parallel‐plate rheometer at 180, 190, and 200°C. Analysis of the log–additivity rule, Cole–Cole plots, Han curves, and Van Gurp curves of the LDPE/UHMWPE blends indicated that the blends were miscible in the melt. In contrast, the rheological properties of LLDPE/UHMWPE showed that the miscibility of the blends was decided by the composition of LLDPE. The differential scanning calorimetry results and scanning electron microscopy photos of the LLDPE/UHMWPE blends were consistent with the rheological properties, whereas with regard to the thermal and morphological properties of LDPE/UHMWPE blends, the results reveal three endothermic peaks and phase separation, which indicated a liquid–solid phase separation in the LDPE/UHMWPE blends. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013
Thermal and rheological properties of polyethylene blends with bimodal molecular weight distribution
Journal of Applied Polymer Science, 2013
Polyethylene blends with bimodal molecular weight distribution were prepared by blending a high molecular weight polyethylene and a low molecular weight polyethylene in different ratios in xylene solution. The blends and their components were characterized by the high temperature gel permeation chromatograph (GPC), different scanning calorimetry (DSC), and small amplitude oscillatory shear experiments. The results showed that the dependence of zero-shear viscosity (g 0) on molecular weight followed a power law equation with an exponent of 3.3. The correlations between characteristic frequency (x 0) and polydispersity index, and between dynamic cross-point (G x) and polydispersity index were established. The complex viscosity (g *) at different frequencies followed the log-additivity rule, and the Han-plots were independent of component and temperature, which indicated that the HMW/ LMW blends were miscible in the melt state. Moreover, the thermal properties were very similar to a single component system, suggesting that the blends were miscible in the crystalline state. V
The Open Macromolecules Journal, 2011
The influences of short chain branching (SCB) and molecular weight (Mw) of low density polyethylene (LDPE) on its melt miscibility with polypropylene (PP) were investigated by rheological techniques. Rheological measurement and different data-treatment techniques as well as rheological models suggest that blends of PP with LDPE of high SCB (22 CH 3 /1000 C) are miscible at all compositions. However, blends of LDPE with low SCB (8.3 CH 3 /1000 C) are immiscible in the LDPE-rich and miscible in PP-rich regimes. It was suggested that matching the conformations of PP and LDPE and matching the Mw of LDPE and PP is a key to the melt miscibility of PP/LDPE blends. The experimental results are in agreement with theoretical predictions on the miscibility of polyolefin blends.
Quantitative analysis of melt elongational behavior of LLDPE/LDPE blends
Rheologica Acta, 2004
Blends of polymers have been used extensively in recent years for their desirable properties. Observation of improved processing and solid state properties by adding even small amounts of long-chain branched (LCB) polymers to conventional short-chain branched (SCB) or linear polymers has incited extensive research on polymer blends, specifically polyolefin blends. However, in spite of these efforts, a review of previous investigations to understand various aspects of polyolefin blends, such as miscibility and phase separation, processability and strain hardening, crystallization and mechanical behavior, reveals the complexity of the problem and highlights the need for further research.
Polymer International, 2004
In this paper, the implications of melt compatibility on thermal and solid-state properties of linear low density polyethylene/high density polyethylene (LLDPE/HDPE) blends were assessed with respect to the effect of composition distribution (CD) and branch content (BC). The effect of CD was studied by melt blending a metallocene (m-LLDPE) and a Ziegler-Natta (ZN) LLDPE with the same HDPE at 190 • C. Similarly, the effect of BC was examined. In both cases, resins were paired to study one molecular variable at a time. Thermal and solid-state properties were measured in a differential scanning calorimeter and in an Instron mechanical testing instrument, respectively. The low-BC m-LLDPE (BC = 14.5 CH 3 /1000 C) blends with HDPE were compatible at all compositions: rheological, thermal and some mechanical properties followed additivity rules. For incompatible high-BC (42.0 CH 3 /1000 C) m-LLDPE-rich blends, elongation at break and work of rupture showed synergistic effects, while modulus was lower than predictions of linear additivity. The CD of LLDPE showed no significant effect on thermal properties, elongation at break or work of rupture; however, it resulted in low moduli for ZN-LLDPE blends with HDPE. For miscible blends, no effect for BC or CD of LLDPE was observed. The BC of LLDPE has, in general, a stronger influence on melt and solid-state properties of blends than the CD.
Polymer international, 2005
The influence of M w of LLDPE on the rheological, thermal and mechanical properties of m-LLDPE/ HDPE blends of low and high branch content (BC) was studied. Melt rheology of m-LLDPE blended with linear HDPE revealed strong influence of M w on melt miscibility at both branching levels. Low M w m-LLDPE/HDPE blends are suggested to be miscible at all compositions, while viscosity of high M w m-LLDPE/HDPE blends showed negative deviation from log additivity suggesting layered morphology of these blends. The DSC results suggest that compatibility in the solid state is independent of M w and BC. For all blends studied, the HDPE-rich blends were found to contain single crystal populations suggesting high degree of cocrystallization, whereas, m-LLDPE rich phase showed separate crystallization. The melt miscibility and the crystallization of high BC m-LLDPE blends with HDPE are suggested to be controlled by different factors. Small strain mechanical properties of these blends were found to be a strong function of blend compatibility and the specific properties of the blend components.
Chemical Engineering Science, 2009
Blends of a long-chain branched polypropylene (LCB-PP) and four linear polypropylenes (L-PP) having different molecular weights were prepared using a twin screw extruder. The linear viscoelastic properties suggested the immiscibility of the high molecular weight L-PP based blends, and the miscibility of the low molecular weight L-PP based blends. In addition, the Palierne emulsion model showed good predictions of the linear viscoelastic properties for both miscible and immiscible PP blends. However, as expected, the low-frequency results showed a clear effect of the interfacial tension on the elastic modulus of the blends for the high molecular weight L-PP based blends. A successful application of time-temperature superposition (TTS) was found for the blends and neat components. Uniaxial elongational properties were obtained using a SER unit mounted on an ARES rheometer. A significant strain hardening was observed for the neat LCB-PP as well as for all the blends. The influence of adding LCB-PP on the crystallinity, crystallization temperature, melting point, and rate of crystallization were studied using differential scanning calorimetry (DSC). It was found that the melting point and degree of crystallinity of the blends first increased by adding up to 20 wt% of the branched component but decreased by further addition. Adding a small amount of LCB-PP caused significant increase of the crystallization temperature while no dramatic changes were observed for blends containing 10 wt% LCB-PP and more. Furthermore, the crystalline morphology during and after crystallization of the various samples was monitored using polarized optical microscopy (POM). Compared to the neat linear polymers, finer and numerous spherulites were observed for the blends and LCB-PP. Dynamic mechanical (DMA) data of the blends and pure components were also analyzed and positive deviations from the Fox equation for the glass transition temperature, T g , were observed for the blends.
Materials Science, 2011
Blends of high-density polyethylene (HDPE), moderate and hyper-branched LLDPEs (LLDPE and HbPE, respectively) have attained widespread commercial applications, though the understanding of the mechanical and melt-flow properties of such blends has been handicapped by the absence of a consensus concerning the degrees of mixing of the components. Moreover, usually the blends are obtained by melt blending, which may not ensure the initial homogeneity of the components. In our work the mixtures were prepared by dissolving the conventional LLDPE having branching content 7.2 wt% with HbPE with comonomer content 17.8 wt% in xylene at 130 °C and stirring for 2 hours. The same procedure was applied for the blending of HDPE with HbPE. After dissolving the mixtures were cooled in liquid nitrogen and after that freeze dried in vacuum line. The ratio of components in the blends was varied. Differential scanning calorimetry has been used to investigate the miscibility and thermal behavior of the blends. For this purpose isothermal and non-isothermal treatment of prepared blends were conducted. By preliminary study the double melting peaks in non-isothermal endotherms have been observed in all the studied blends. The presence of two peaks in DSC scan can be attributed to the formation of separated crystals from both the high density/linear low density and highly branched components. However, certain limited degree of co-crystallization
Journal of Applied Polymer Science, 2002
This article presents the tensile properties and morphological characteristics of binary blends of the high-density polyethylene (HDPE) and a linear low-density polyethylene (LLDPE). Two constituents were melt blended in a single-screw extruder. Injection-molded specimens were evaluated for their mechanical properties by employing a Universal tensile tester and the morphological characteristics evaluated by using a differential scanning calorimeter and X-ray diffractometer. It is interesting to observe that the mechanical properties remained invariant in the 10 -90% LLDPE content. More specifically, the yield and breaking stresses of these blends are around 80% of the corresponding values of HDPE. The yield elongation and elongation-at-break are around 65% to corresponding values of HDPE and the modulus is 50% away. Furthermore, the melting endotherms and the crystallization exotherms of these blends are singlet in nature. They cluster around the corresponding thermal traces of HDPE. This singlet characteristic in thermal traces entails cocrystallization between these two constituting components. The clustering of thermal traces of blends near HDPE meant HDPE-type of crystallites were formed. Being nearly similar crystallites of blends to that of HDPE indicates nearness in mechanical properties are observed. The X-ray diffraction data also corroborate these observations.
Morphology and properties of blends of polyethylene with a semiflexible liquid crystalline polymer
Journal of Applied Polymer Science, 1995
Blends of three polyethylene (PE) samples (two HDPE grades and LLDPE) with an experimental sample of a semiflexible liquid crystalline polymer (SBH 1:1:2 by Eniricerche) have been prepared in a Brabender compounder. The processing‐aid effect of the LCP has been demonstrated by the decreased energy required for extruding the blends, as compared to that needed for neat PE. The thermal properties, as studied by differential scanning calorimetry (DSC), have shown that the two components of the blends are immiscible. However, the dispersed SBH phase has been found to act as a nucleating agent for the crystallization of LLDPE, whereas no such effect was observed for HDPE. This has been taken as an indication that the phase interactions of SBH with LLDPE are more pronounced than with HDPE. The morphological study of the blends, done by scanning electron microscopy (SEM), has confirmed this conclusion. In fact, the SBH particles show a much better dispersion and a narrower size distribution...