Melt Rheological Properties

It is important to consider the types of deformation and flow conditions in which material properties are measured. This gives more insight into the meaning of these properties in 

Most melt flow investigations have been in shear (i.e., in flows in which the local velocity v in a 1,2,3 orthogonal coordinate system may be expressed)  

Various instruments shown in Fig. 1.3 have been used to determine the shear viscosity of compoimds. 

Capillary rheometers are the most widely used rheological instruments for polymer melts. They are, however, generally limited to rather high shear rates. Rotational rheometers can provide data at lower shear rates. Cone-plate and parallel disc instruments have been popular with thermoplastic melts. Pressurized instruments, such as biconical or Mooney shearing disc instruments, are used with elastomers to prevent slippage [39]. Sandwich rheometers are used at the lowest shear rates and shear stresses. 

The rheological properties of polymer melts depend upon molecular weight, molecular weight distribution, long chain branching, and copolymer microstructure, as well as temperature. At low molecular weights, the melt behaves as a Newtonian fluid and has a constant shear viscosity, i.e.. 

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Ruckdaschel H, Rausch J, Sandler JKW, Altstadt V, Schmalz H, Muller AHE (2008) Correlation of the melt rheological properties with the foaming behavior of immiscible blends of poly(2,6-dimethyl-l,4-phenylene ether) and poly (styrene-co-acrylonitrile). Polym Eng Sci 48 2111-2125... 

Figure 3a-b illustrate the LCB effect on the melt rheological properties. The response of the rheological behaviour to the copolymerisation ability and vinyl end group selectivity of the siloxy-substituted metallocenes has been investigated from their dynamic modulus curves. The frequency dependency of the dynamic modulus of the polyethenes produced with catalysts 2 is demonstrated in Fig. 3a. For comparison dynamic modulus for a linear polyethene, prepared by the catalyst -BuCp2ZrCl2, is shown in Fig. 3b. 

Previous investigations (4, 5,6,7,8) have shown that block copolymers exhibit unusual melt rheological properties such as a very high viscosity, elasticity, and non-Newtonian behavior even at very low shear rates which are all attributed to the multiphase structure resulting from the incompatibility between the two copolymer units in the melt state. 

A few rheometers are available for measurement of equi-biaxial and planar extensional properties polymer melts [62,65,66]. The additional experimental challenges associated with these more complicated flows often preclude their use. In practice, these melt rheological properties are often first estimated from decomposing a shear flow curve into a relaxation spectrum and predicting the properties with a constitutive model appropriate for the extensional flow [54-57]. Predictions may be improved at higher strains with damping factors estimated from either a simple shear or uniaxial extensional flow. The limiting tensile strain or stress at the melt break point are not well predicted by this simple approach. 

N. Robledo, J.F. Vega, J. Nieto, J. Martinez-Salazar, 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. J. Appl. Polym. Sci. 119, 3217-3226 (2011)... 

Dynamic mechanical and melt rheological properties of sulfonated poly(butylene succinate) ionomers. Polymer, 44, 7165. 

The melting is the most difficult aspect due to the very low thermal conductivity of polymers. The melting depends on the thermodynamic and melt rheological properties of the polymer, and also the processing conditions such as temperature, speed and pressure, and the size and design of the screw. 

AK Gupta, PK Kumar, BK Ratnam. Glass-fiber-reinforced polypropylene/EPDM Blend. I. Melt rheological properties. J Appl Polymer Sci 42 2595-2611, 1991. AR Gupta, KR Srinivasan, PK Kumar. Glass fiber reinforced polypropylene/EPDM Blends. II. Mechanical properties and morphology. J Appl Polymer Sci 43 451-462, 1991. 

Thermodynamically stable, bicontinuous microemulsions have recently been shown to be obtainable in symmetric ternary blends of two homopolymers and a diblock copolymer by formulating alloys with compositions near mean-field isotropic Lifshitz points. In the present paper, it is argued that practical apphcation of this design criterion could require use of homopolymers of unequal molec.wts. and block copolymers of different structure. The existence of, and explicit location of, mean-field isotropic Lifshitz points in ternary blends with homopolymer molec.wt. asymmetry and either AB diblock or ABA triblock copolymer structures were demonstrated. These calculations significantly expanded the parameter space for observing bicontinuous miCToemulsions and allowed for more flexibility in tailoring melt rheological properties and solid-state mechanical properties. 29 refs. 

Two types of NR (ADS and STR 5L) were used to prepare maleated natural rubber (MNR). Melt rheological properties of MNRs i.e. MNR-ADS and MNR-STR 5L) and their blends with various blend ratios were investigated. Two sets of rubber blends including MNR-ADS and MNR-STR 5L were prepared using a two-roll mill at a mixing temperature of 60 °C. Mastication of... 

In the case of polymer nanocomposites, measurements of melt rheological properties are not only important to understand the processability of these materials but are also helpful in determining the strength of polymer-filler interfacial interactions and the structure-property relationships in nanocomposites. This is because rheological behaviors are strongly influenced by the nanoscale structure and interfacial characteristics. 

Flow curves - Melt rheological properties of PES were evaluated on a capillary instrument attached to a Shimadzu Universal Materials Testing Machine model AG-IOTA. Viscosity curves measured at 315, 330 and 350°C and for shear rates ranging from 10 to 10000 1/s are presented in Figure 1. A typical pseudo-plastic behavior can be seen. That is, the melt viscosities of PES decrease with the increase of apparent shear rates. 

Liebscher M, Tzounis L, Pbtschke P, Heinrich G. Influence of the viscosity ratio in PC/SAN blends filled with MWCNTs on the morphological, electrical, and melt rheological properties. Polymer 2013 54 6801-8. 

Table 10.1 Melt rheology properties of poly (lactic acid) (Dorgan et aL. 1999)...

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