Poly melt viscosities

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. 

Melt viscosity is a function of 7 - 7g, and a major cause of the difference between the viscosity of poly(methyl methacrylate) at its processing temperature (where 7 - 7g = 100°C approx.) and the viscosity of polyethylene at its processing temperature (where 7 - 7g = 200°C approx.) is explicable by the above relationship. 

This article is an overview of the novel technology of self-reinforced LCPs with polyesters, poly(ethylene terephthalate) (PET) and poly(ethylene naphtha-late) (PEN) [10-13, 21, 23], LCP/polyester blends in a polyester matrix form in situ fibrils which improve the mechanical properties. LCPs have an inherently low melt viscosity, and provide LCP/polyester blends that effectively lower the melt viscosity during melt spinning [24], and fast injection-molding cycles. The miscibility between the LCP and polyesters can be controlled by the degree of transesterification [25] in the reactive extrusion step, and fibril formation in LCP-reinforced polyester fibers has been studied. 

Two main types of viscometers are suitable for the determination of the viscosity of a polymer melt The rotation viscometer (Couette viscometer, cone-plate viscometer) and the capillary viscometer or capillary extrusiometer. The latter are especially suitable for laboratory use since they are relatively easy to handle and are also applicable in the case of high shear rates. With the capillary extrusiometer the measure of fluidity is not expressed in terms of the melt viscosity q but as the amount of material extruded in a given time (10 min). The amount of ex-trudate per unit of time is called the melt index or melt flow index i (MFI). It is also necessary to specify the temperature and the shearing stress or load. Thus MFI/2 (190 °C)=9.2 g/10 min means that at 190 °C and 2 kg load, 9.2 g of poly-... 

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. 

Poly(trimethylene terephthalate). Poly(trimethylene terephthal-ate) (PIT) is a crystalline polymer that is used for fibers, films, and engineering plastics. The polymer has an outstanding tensile elastic recovery, good chemical resistance, a relative low melting temperature, and a rapid crystallization rate. It combines some of the advantages of poly(ethylene terephthalate) (PET) and poly(butylene terephthalate) (PBT). Disadvantageous are the low heat distortion temperature, low melt viscosity, poor optical properties, and pronounced brittleness low temperatures. 

Gibson and coworkers also found that the melt viscosity of a polymer is also altered by the formation of a polyrotaxane [19], Poly(ester rotaxane) 60 containing 42C14 as the cyclic component had a melt viscosity equivalent to that of the parent polyester with 2.5-fold higher molecular weight. This result clearly indicates that there is less chain entanglement in the polyrotaxane than in the backbone polymer by itself. 

The usefulness of piperazine and its derivatives for the preparation of polyamides has been known for some time, and it is one of the few disecondary diamines which can be employed to prepare high-melting polycondensates. Piperazine polyamides are especially useful as hot melt adhesives (68USP3377303), since they display unusually good adhesion to vinyl-based polymers such as poly(vinyl chloride). Piperazine is also the preferred diamine for the preparation of thermoplastic polyamides which exhibit the combined properties of low melt viscosity, high softening point and impact resistance at low temperatures (80USP4218351). 

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