Polypropylene molecular weight distribution

Studies of melt flow properties of polypropylene indicate that it is more non-Newtonian than polyethylene in that the apparent viscosity declines more rapidly with increase in shear rate. The melt viscosity is also more sensitive to temperature. Van der Wegt has shown that if the log (apparent viscosity) is plotted against log (shear stress) for a number of polypropylene grades differing in molecular weight, molecular weight distribution and measured at different temperatures the curves obtained have practically the same shape and differ only in position. 

The narrow molecular weight distribution means that the melts are more Newtonian (see Section 8.2.5) and therefore have a higher melt viscosity at high shear rates than a more pseudoplastic material of similar molecular dimensions. In turn this may require more powerful extruders. They are also more subject to melt irregularities such as sharkskin and melt fracture. This is one of the factors that has led to current interest in metallocene-polymerised polypropylenes with a bimodal molecular weight distribution. 

The living nature of ethylene oxide polymerization was anticipated by Flory 3) who conceived its potential for preparation of polymers of uniform size. Unfortunately, this reaction was performed in those days in the presence of alcohols needed for solubilization of the initiators, and their presence led to proton-transfer that deprives this process of its living character. These shortcomings of oxirane polymerization were eliminated later when new soluble initiating systems were discovered. For example, a catalytic system developed by Inoue 4), allowed him to produce truly living poly-oxiranes of narrow molecular weight distribution and to prepare di- and tri-block polymers composed of uniform polyoxirane blocks (e.g. of polyethylene oxide and polypropylene oxide). 

Reactive extrusion is the chemical modification of polymer while it is being transported in an extruder. In this work, polypropylene is intentionally degraded by the addition of a free radical initiator (a peroxide) during extrusion. The product has improved flow properties because of the removal of the high molecular weight tail and the narrowing of the molecular weight distribution. 

Polypropylene grades manufactured using Ziegler-Natta catalysts are predominandy isotactic, with a comparatively broad molecular weight distribution (Afw/Mn 3.5). Ziegler-Natta catalyzed polypropylene comprises the majority of commodity grade resins. 

How do metallocene catalysts define the molecular weight distribution and tacticity of polypropylene ... 

Atactic polypropylene exhibits a greater vapor permeability relative to either syndiotactic or isotactic polypropylene of the same molecular weight distribution. Why is this so ... 

Fig Typical cumulative molecular-weight distribution curve for a sample of polypropylene gradient-elution data (O) and data from gel permeation chromatography. 

A foamable isotactic polypropylene homopolymer is obtained by metallocene catalysis and has a molecular weight distribution and density, which fall within broad ranges. It may be prepared in a multiple stage polymerisation process nsing the same metallocene component in at least two stages. 

Subsequently, Takada et al.55) studied the molecular weight distribution of syndio-tactic polypropylenes produced with the soluble VC14/A1(C2H5)2C1 catalyst in heptane... 

Doi, Ueki and Keii47,48) have found that the soluble catalyst composed of V(acac)3 (acac = acetylacetonate anion) and A1(C2H5)2C1 polymerizes propylene in toluene at —78 °C to give a syndiotactic living polypropylene having a narrow molecular weight distribution (Mw/Mn = 1.05-1.20). This low-temperature polymerization of propylene was shown to satisfy all criteria for the living polymerization 47). 

Fig. 8. GPC elution (molecular weight distribution) curves of polypropylenes obtained at different polymerization times at —78 °C with the soluble V(acac)3/A1(C2H5)2C1 catalyst (from Ref. 47))...

Figure 8 shows the molecular weight distributions (MWD) of polypropylenes obtained after different times at —78 °C. The unimodal peak of the curve shiffts toward higher molecular weights with an increase in the time, retaining a narrow MWD. The polydispersity (JVIw/JVIn) is as narrow as 1.13 0.05, independent of time. The MWD is close to a Poisson distribution. 

Figure 26 shows a typical GPC elution curve of the ethanol insoluble polymers obtained at 0 °C, (b), together with the GPC curve of the original iodine-terminated polypropylene, (a). Curve b clearly shifted toward higher molecular weights relative to curve a, but retained a narrow molecular weight distribution (Mw/Mn = 1.14), indicative of the formation of a propylene-THF diblock copolymer. The 13C NMR spectrum of the block copolymer is shown in Fig. 27. 

Nogales, A., Hsiao, B.S., Somani, R.H., Srinivas, S., Tsou, A.H., Balta-Callja, F.J. and Eqzuerra, T.A. (2001) Shear-induced crystallization of isotactic polypropylene with different molecular weight distributions in situ small- and wide-angle X-ray scattering studies, Polymer 42(12), 5247-5256... 

The differences between the different forms of polypropylene produced (assuming equal molecular-weight distribution, equal percentage of branching, etc.) are remarkable (Table II). 

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