Phase behavior, polymers polyolefins

In this chapter, the possibihty of using late transition metal catalysts to synthesize polyolefins in supercritical carbon dioxide was demonstrated [43]. The multicomponent phase behavior of polyolefin systems at supercritical conditions was studied experimentally by measuring cloud-point curves as well as by modeling polymer systems at supercritical conditions. The cloud-point measurements show that CO2 acts as a strong antisolvent for the ethylene-PEP system, which implies that the polymerization concerned will involve a precipitation reaction. The model calculations prove that SAFT is able to describe the ethylene-PEP-CO2 system accurately. Solubility measurements of the Brookhart catalyst reveal that the maximum catalyst solubility is rather low (in the order of 1x10 mol L ). However, a number of strategies are given to enhance this value. 

As the neutron scattering characterization generally requires deuterium labeled polymers, the question arises relative to the effect of deuterium labeling on the observed phase separation versus the unlabeled blends. As noted above for PS/PVME blends, it has been shown that deuterated PS raises the lower critical solution temperature [192]. Balsara et al. also demonstrated deuteration can yield shifts in the phase behavior of polyolefin blends [196]. Examples where SANS has been employed to characterize polyolefin blends are noted in Chapter 4. 

Scheffold, F., Budkowski, A., Steiner, U., Eiser, E., Klein, J., Fetters, L.J. Surface phase behavior in binary polymer mixtines. II. Surface emichment Irom polyolefin blends. J. Chem. Phys. 104, 8795-8806 (1996)... 

The usual phase equilibrium issue in polymer systems consists of determining whether phase separation occurs, and if it does, then what the phase compositions are. Although measuring is still the most accurate way at present to obtain information about the phase behavior [39-42], it is rather time-consuming. Most of the experimental work described in the literature has focused on polymer-solvent systems rather than on non-solvents. For example, binary systems of linear and branched polyethylenes in ethylene have been measured [40, 43]. In addition, the effect of carbon dioxide as a non-solvent on polyolefin-solvent systems has been studied [44-47]. 

The rest of this article is organized as follows. Section 2 discusses the structure and thermodynamics of polymer melts with emphasis on the calculation of intramolecular and inter-molecular correlations, the development of equations of state, and a comparison to experiment and simulations for model polymers and alkanes. Section 3 describes the theory of polymer blends with an emphasis on the phase behavior of symmetric blends and polyolefin blends. Some conclusions are presented in Section 4. 

When a block copolymer is blended with a homopolymer that differs in composition from either block the usual result is a three-phase structure. Miscibility of the various components is not necessarily desirable. Thus styrene-butadiene-styrene block copolymers are recommended for blending with high density polyethylene to produce mixtures that combine the relative high melting behavior of the polyolefin with the good low temperature properties of the elastomeric midsections of the block polymers. 

Concerning compatibilized blends, the interfacial behavior of the compatibilizer has an important effect upon the crystallization of the blended components as it was shown for crystaHine/crystalline polymer blends (60,65-67) and for crystalline polymer/LCP blends (32,37,38,68). For the latter blends, the enhanced phase interactions and improved interfacial adhesion could increase the above-mentioned nucleation activity of the LCP toward the crystallizable matrices. In the particular case of using polyolefin-g-LCP copolymer compatibilizer, the crystallization of the two blend phases might have a reverse effect upon the compatibilizing activity. Moreover, the miscibility (69,70) and/or cocrystallization (60) between the bulk homopolymers and corresponding segments of the copolymer could strongly influence the crystallization behavior of the blends. 

Different polyolefin structural forms react differently to UV. Highly branched LDPE tends to degrade more readily than LLDPE or HOPE, and overall, degradation takes place more easily within the amorphous phase of a polymer than in the crystalline phase. Moreover, PE and PP photo-oxidation behaviors are different enough that the same additive approach for protecting PE may not work the same in PP, even in the same applications. For example, a common... 

It was shown that for most crystalline polymers, including polypropylene and other polyolefins, the tensile drawing proceeds at a much lower stress than kinematically similar channel die compression [10,17]. Lower stress in tension was always associated with cavitation of the material. Usually a cavitating polymer is characterized by larger and more perfect lamellar crystals and cavities are formed in the amorphous phase before plastic yielding of crystals. If the lamellar crystals are thin and defected then the critical shear stress for crystal plastic deformation is resolved at a stress lower than the stress needed for cavitation. Then voiding is not activated. An example of such behavior is low density polyethylene [10]. 

An interesting example of this behavior occurs with the combination of an aliphatic hydrocarbon acid (e.g., octadecanoic acid or stearic acid) with a polyolefin and a polyamide. These molecules do not dissolve in either phase but rather locate themselves at the interface at the two polymers. The hydrocarbon section of the molecule embeds itself in the polyolefin phase and the acid end group associates with the polyamide. The carboxylic add may chemically react with the amine and groups of the polyamide forming a block copolymer of (polyamide-stearic acid) at the interface. The molecule acts as a smfactant and reduces the interfacial tension. This seems first reported by Baraboim and Rakityanskii [25]. This reduced interfacial tension prevents coalescence of the polyolefin and polyamide phases. The dispersed phase sizes are reduced. 

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