= Polymer blend
A \b[<polymer blend>] is a macroscopically homogeneous mixture of two or more different <polymer> species. The terminology does not require molecular-scale mixing: distinct microscopic phases may remain. IUPAC also treats the components as, in principle, separable by physical means.<IUPAC Compendium of Chemical Terminology, fifth edition (Gold Book)>[Polymer blend, entry P04736, definition and notes 1–3; DOI 10.1351/goldbook.P04736]{ref}
This distinguishes blending from making a <copolymer>. In a <polymer blend>, the different chains remain different molecular populations. In a <copolymer>, chemically different units occur within the same <macromolecule>. A blend can itself contain a <copolymer> as one component, so these descriptions answer different structural questions.
Composition also needs a stated denominator. For an original binary example, mix 30 g of one <polymer> with 70 g of another. The first component has <mass fraction> $w_1=0.30$. If their densities are stipulated as $\rho_1=1.2$ and $\rho_2=1.0\,\mathrm{g\,cm^{-3}}$, and component volumes are additive, its <volume fraction> is instead
$$
\phi_1=\frac{m_1/\rho_1}{m_1/\rho_1+m_2/\rho_2}
=\frac{25}{25+70}\simeq0.2632.
$$
The total modeled volume is 95 cubic centimetres. Nonadditive mixing volumes would require a different conversion. Neither 30 mass percent nor 26.32 volume percent is a chain-number fraction without information about the chain <molar masses>.
<Polymer blend miscibility> asks whether the components form one equilibrium phase at a specified <temperature>, <pressure> and composition. A macroscopically uniform specimen can still have microscopic domains: the IUPAC definition deliberately does not equate a blend with a single phase. Conversely, chemical similarity alone does not establish complete mixing at every temperature.
An informative experiment therefore combines a stated composition and thermal history with observations at a stated length scale. <Microscopy> can reveal resolved domains, while <light scattering> can reveal composition fluctuations on other scales. An image with no resolved domains establishes only the limits of that observation. It does not prove that different chains have been covalently joined, or that a specimen is a single equilibrium phase. The distinction makes the term useful for describing a material before assigning a particular microscopic structure.
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