Effects of Short-Chain Branching

Making simplifying assumptions, Stockmayer [60] derived an equation for the ratio of intrinsic viscosities in short-chain branched and linear polymers of the same molecular weight, which is shown below as Eq. 2.105. The assumptions, which are unrealistic for any real material, are that the polymer is monodisperse and that the short branches are equally spaced. 

Considering the copolymer to be branched polyethylene, the branching index, g, is given by  

Scholte et al. [61] foimd Eqs. 2.106 and 2.108 to be reasonably reliable for polypropylene and ethylene-propylene copolymers. 

For a polydisperse polymer in which the fraction of short side chains, i.e., the comonomer fraction, is defined to be / for aU molecules having the molecular weight M , Ambler [62] derived the following equation for the intrinsic viscosity  

Aerts [63] modeled the intrinsic viscosity of dendritic and hyperbranched materials. 

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Before discussing the effect of short-chain branching on the kinetics of crystallization process, it is necessary to revisit the theory of secondary nucleation and the concept of regimes as given by Lauritzen and Hoffmann... 

Kleintjens, L. A. Koningsveld, R. Gordon, M., "Liquid-Liquid Phase Separation in Multicomponent Polymer Systems. 18. Effect of Short-Chain Branching," Macromolecules, 13, 303 (1980). 

T. Hameed, LA. Hussein, Effect of short chain branching of LDPE on its miscibility with linear HOPE. Maciomol. Mater. Eng. 289(2), 198-203 (2004)... 

Figure 3.7 Effect of short-chain branches on polyethylene density and crystaUiiiity. (o) Methyl branch from ethylene/propylene copolymers ( ) Ethyl branch from ethylene/1-...

Figure 5.2. The effect of short chain branching on blocking force measured for polyethylene tihns. [Data from Vincent, O Osmont, E., Soc. Plast. Eng., Inc., Antec, Conf. Proc., paper 131, 2002.]...

The branched polymers produced by the Ni(II) and Pd(II) a-diimine catalysts shown in Fig. 3 set them apart from the common early transition metal systems. The Pd catalysts, for example, are able to afford hyperbranched polymer from a feedstock of pure ethylene, a monomer which, on its own, offers no predisposition toward branch formation. Polymer branches result from metal migration along the chain due to the facile nature of late metals to perform [3-hydride elimination and reinsertion reactions. This process is similar to the early mechanism proposed by Fink briefly mentioned above [18], and is discussed in more detail below. The chain walking mechanism obviously has dramatic effects on the microstructure, or topology, of the polymer. Since P-hydride elimination is less favored in the Ni(II) catalysts compared to the Pd(II) catalysts, the former system affords polymer with a low to moderate density of short-chain branches, mostly methyl groups. 

Willbourn, A. H. Polymethylene and the structure of polyethylene study of short-chain branching, its nature and effects. J. Polymer Sci. 34, 569—597 (1959). 

The effect of short-chain (LLDPE) and long-chain (LDPE) branching in PE alters the physical and mechanical properties and melt rheology. For the same density, LLDPE has a higher melting point, tensile strength and flexural modulus. Because film is the major outlet for LDPE, this means that thinner film (down-gauging) and hence less material can be used for comparable applications. However, because LLDPE is stiffer, heavy-duty sacks are still made of LDPE. 

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