Elastomer blends, miscible

Miscible blends are most commonly formed from elastomers with similar three-dimensional (Hansen, 1967a,b Hansen and Beerbower, 1971) solubility parameters. An example of this is blends from copolymer elastomers (e.g., ethylene-propylene or styrene-butadiene copolymers) of slightly different composition, or microstructure. When the forces between the components of the polymer blend are mostly dispersive, miscibility is only achieved in neat polymers with a very close match in Hansen s three-dimensional solubility parameter (Hansen, 1967a,b Hansen and Beerbower, 1971), such that small combinatorial entropy for high molecular weight elastomers can drive miscibility. 

Miscible blends of elastomers differ from corresponding blends of thermoplastics in two important areas. First, the need for elastic properties requires elastomers to be high molecular weight. This reduces both the kinetic rate and the thermodynamic driving force for the interdiffusion and thus formation of a miscible single phase of dissimilar elastomers. Second, elastomers are plasticized in conventional compounding with process oils. The presence of plasticizers leads to both a higher free volume for the blend components and a decrease of the endothermal interactions. 

The change of glass transition temperature of an elastomer is an important characteristic of elastomer blends. Above the Tg, the kinetic forces are stronger 

The formation of miscible rubber blends slows the rate of crystallization (Runt and Martynowicz, 1985 Keith and Padden, 1964) when one of the components is crystallizable. This phenomenon accounts for data that show lower heats of fusion that correlate to the extent of phase homogeneity (Ghijsels, 1977) in elastomer blends. Additionally, the melting behavior of a polymer can be changed in a miscible blend. The stability of the liquid state by formation of a miscible blend reduces the relative thermal stability of the crystalline state and lowers the equilibrium melting point (Nishi and Wang, 1975 Rim and Runt, 1520). This depression in melting point is small for a miscible blend with only dispersive interactions between the components. 

Changes in polymer crystallinity have also been employed to study the homogeneity of elastomer blends (Morris, 1967 Sircar and Lamond, 1973). 

---

Additional examples of miscible elastomer blends are given in Table 4.1. 

Krishnamoorti, R. in Miscibility of Blends of Saturated Hydrocarbon Elastomers. Rubber Division, Proceedings of the American Chemical Society, Nashville, TN, Sept. 29-Oct. 2, 1998, Paper No. 33, 1-14. 

E-EA-GMA (see Table 14.3) and EEA are often used in combination as a toughening system. The optimum blend ratio of reactive elastomers non-reactive elastomers (e.g. Lotader Lotryl) is 30/70. Since the E-EA-GMA terpolymer and EEA copolymer are mutually miscible, when blended together with PET the mixture acts as a single elastomeric phase, which is interfacially grafted to the PET continuous phase. 

Elastomers with similar polarities and solubility characteristics can be easily combined to produce miscible polyblend (18). Miscible polymer blend is a polymer blend, which is homogeneous down to the molecular level and associated with the negative value of the free energy of mixing and the domain size is comparable to the dimensions of the macromolecular statistical segment. Complete miscibility in a mixture of two polymers requires that the following condition be fulfilled (19) ... 

These concepts for formation of miscible blend of elastomers with similar or near equivalence of solubility parameters require the components to be similar in properties. Thus a wide variation in the properties of the elastomer blends by changing the relative amounts of the two elastomers is not typical since it is unlikely that, for example, a nonpolar polyolefin elastomer and a polar elastomer like acrylate would be similar in solubility parameters. This relative invariance in the properties of the blend compared to the components is an inherent limitation on the basic, economic, and technological need for elastomer blends, which is to generate new properties by blends of existing materials. Similar or near equivalence of solubility parameters can be difficult to predict from chemical structure. For example, chemically distinct 1,4-polyisoprene and 1,2-polybutadiene are miscible, but isomeric 1,2-polybutadiene and 1,4-polybutadiene are immiscible. It is illustrative of this concept that an apolar hydrocarbon elastomer and a highly polar elastomer such as an acrylate cannot have, under any practical structural manifestation for either, a similar solubility parameter and thus be miscible. 

While true miscibility may not be required for elastomer properties, adhesion between the immiscible phases is required. Immiscible polymer blends that fulfill this criteria provide a significant opportunity to change the rheological, tensile, and wear properties of elastomer blends compared to miscible blends. 

The formulation and use of elastomer blends is technologically demanding. Miscible blends are widely used but usually not recognized since analytical separation of the vulcanized elastomer is experimentally difficult. Immiscible blends require excellent phase dispersion and interfacial adhesion typical of all polymer blends. In addition, they require control of filler distribution and crosslink density in each component. This is due to the need for mechanical integrity in vulcanized elastomers. 

Two notable reviews of elastomer blends exist. The first, by Hess et al. [3], reviews the applications, analysis, and the properties of the immiscible elastomer blends. The second, by Roland [4], has in addition a discussion of the physics of mixing immiscible polymer blends and a more recent account of the analytical methods. Other reviews by Corish [5] and McDonel et al. [6] deal with specific aspects of elastomer blends. These reviews are focused on immiscible blends of elastomers. In this review we will complement this with information on miscible blends. 

The principal effect of miscibility of elastomer blends of dissimilar elastomers is alteration of the glass-transition temperature. Since miscible blends should have negligible changes in the conformation of the polymer chains, the entanglement density of miscible blends should be a compositionally weighted average of entanglement density of the pure components. 

Nuclear magnetic resonance (NMR) has been applied to the study of homogeneity in miscible polymer blends and has been reviewed by Cheng [11a] and Roland [11b]. When the components of a blend have different Tg s, proton NMR can be used to assess the phase structure of the blend by taking advantage of the rapid decrease of proton-proton coupling with nuclear separation [lie]. For blends containing elastomers of almost identical Tg, proton MAS NMR is applied to blends where one of the components is almost completely deuterated [12], Another technique is crosspolarization MAS NMR [13], The transfer of spin polarization from protons to the atoms of... 

---