Viscosity polystyrene melt

Free volume approach to polystyrene melt viscosity. J. Appl. Phys- 29, 1395-1398 (1958). 

The response of melt viscosity to changes in shear rate over the range normally encountered in mixing, extrusion and injection molding is generally similar to that of thermoplastics such as polyethylene or polystyrene. Melt viscosities for a radial block polymer based on butadiene and styrene, an oil masterbatch and a compound based cn the masterbatch are shown in Figure 8. 

Figure 2,13 Influence of talc on polystyrene melt viscosity [26]...

Fig. 6. Melt viscosity dependence on shear rate for various polymers A, low density polyethylene at 210°C B, polystyrene at 200°C C, UDEL P-1700 polysulfone at 360°C D, LEXAN 104 polycarbonate at 315°C and E, RADEL A-300 polyethersulfone at 380°C.

The melt viscosity of a polymer at a given temperature is a measure of the rate at which chains can move relative to each other. This will be controlled by the ease of rotation about the backbone bonds, i.e. the chain flexibility, and on the degree of entanglement. Because of their low chain flexibility, polymers such as polytetrafluoroethylene, the aromatic polyimides, the aromatic polycarbonates and to a less extent poly(vinyl chloride) and poly(methyl methacrylate) are highly viscous in their melting range as compared with polyethylene and polystyrene. 

It should be noted that polystyrene with a number average molecular weight of 50000 has a Tg only about 2°C less than would be expected of a polystyrene of infinitely high molecular weight. Hence increasing the molecular weight beyond this point in order to raise the Tg would not be very effective and at the same time it would lead to large increases in melt viscosity. 

Polymers of a-methylstyrene have been marketed for various purposes but have not become of importance for mouldings and extrusions. On the other hand copolymers containing a-methylstyrene are currently marketed. Styrene-a -methylstyrene polymers are transparent, water-white materials with BS softening points of 104-106°C (c.f. 100°C for normal polystyrenes). These materials have melt viscosities slightly higher than that of heat-resistant polystyrene homopolymer. 

To produce mouldings from polystyrene with minimum strain it is desirable to inject a melt, homogeneous in its melt viscosity, at a high rate into a hot mould at an injection pressure such that the cavity pressure drops to zero as the melt solidifies. Limitations in the machines available or economic factors may, however, lead to less ideal conditions being employed. 

Electric discharge methods are known [31] to be very effective for nonactive polymer substrates such as polystyrene, polyethylene, polypropylene, etc. They are successfully used for cellulose-fiber modification to decrease the melt viscosity of cellulose-polyethylene composites [32] and to improve the mechanical properties of cellulose-polypropylene composites [28]. 

Modification of filler s surface by active media leads to the same strong variation in viscosity. We can point out as an example the results of work [8], in which the values of the viscosity of dispersions of CaC03 in polystyrene melt were compared. For q> = 0.3 and the diameter of particles equal to 0.07 nm a treatment of the filler s surface by stearic acid caused a decrease in viscosity in the region of low shear rates as compared to the viscosity of nontreated particles more than by ten times. This very strong result, however, should not possibly be understood only from the point of view of viscometric measurements. The point is that, as stated above, a treatment of the filler particles affects its ability to netformation. Therefore for one and the same conditions of measuring viscosity, the dispersions being compared are not in equivalent positions with respect to yield stress. Thus, their viscosities become different. 

At room temperature, atactic polystyrene is well below its glass transition temperature of approximately 100 °C. In this state, it is an amorphous glassy material that is brittle, stiff, and transparent. Due to its relatively low glass transition temperature, low heat capacity, and lack of crystallites we can readily raise its temperature until it softens. In its molten state, it is quite thermally stable so we can mold it into useful items by most of the standard conversion processes. It is particularly well suited to thermoforming due to its high melt viscosity. As it has no significant polarity, it is a good electrical insulator. 

The polyphenylenes were brittle and did not form self-standing films when cast from solution. Therefore, they were considered poor materials. The use of these polymers was instead investigated as additives in polystyrene to improve processing and mechanical properties. A mixture of polystyrene and hyperbranched polyphenylene (5%) was studied and the results showed that the melt viscosity, especially at high temperatures and shear rates, was reduced by up to 80% as compared to pure polystyrene. Also, the thermal stability of polystyrene... 

Fig. 11. Melt viscosity at 85 °C vs molar mass for hydroxy-functional hyperbranched aliphatic polyesters based on bismethylol propionic acid. Theoretical molar mass based on core bis-MPA ratio ( ) and Mn determined with SEC relative to linear polystyrene standards (O) [117]...

The lack of mechanical strength for thermoplastic hyperbranched polymers makes them more suitable as additives in thermoplast applications. Hyperbranched polyphenylenes have been shown to act successfully as rheology modifiers when processing linear thermoplastics. A small amount added to polystyrene resulted in reduced melt viscosity [31]. (Sect> 3.1). 

For the unfilled polystyrene melt at low elongational rates a constant value of Tjg is achieved given by three times the zero shear viscosity according to Trou-... 

In several cases the melt viscosity of a series of lightly-branched polymers has been determined as a function of MW, and compared with that of linear polymers, and it has been found or may be deduced from the published data that there is a cross-over molecular weight, below which the branched polymer is less viscous, but above which it more viscous, than the linear polymer of equal MW. This behaviour is observed with some comb-shaped polystyrenes (35) and poly(vinyl acetate)s (59, 89), star polybutadienes (57, 58, 123), and randomly-branched polyethylenes (56,61). Jackson has found (141) that if the ratio ZJZC of the number of chain atoms at the cross-over point, Zx, to the number at the kink in the log 0 — logM curve, Zc, [as given in Ref. (52)], is plotted against nb, the number of branches, a reasonable straight line is obtained, as in Fig. 5.1. 

Pannell (38) has studied a range of polystyrenes with comb-like branching, but with relatively long branches. He has correlated the low-shear melt viscosities with calculated values of , finding i/o°c(so)4 8, whereas the exponent for linear polymers is about 3.4. Fujimoto s results can be correlated in a similar way, but with a rather higher exponent, 5.1, though rather better correlations would be obtained if separate lines were used for each branching frequency. 

Ballman and co-workers have used carbon particles to determine flow patterns for polystyrene melts in plate-cone and capillary viscometers (70). Complex patterns, rather than the simple flow expected, were observed for high molecular weight samples. These may have been caused, however, by differences in viscosity between adjacent layers of pure melt and melt with suspended particles. 

Fig. 12.12 Comparison of the viscosity and swelling ratio dependence on shear rate for a polystyrene melt of Mw = 2.2 x 105 and Mw/Mn = 3.1. [Reprinted hy permission from W. W. Graessley, S. D. Glasscock, and R. L. Crawley, Die Swell in Molten Polymers, Trans. Soc. Rheol., 14, 519 (1970).]...

Figure 3.12 Time-temperature superposition along 45° trajectories (x=const.) to a reference temperature T0= 170°C for the viscosity function of the polystyrene melt whose viscosity functions were measured between 150 and 200°C...

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