Interphase and Compatibilization by Addition of a Compatibilizer
Polymer blends are mixtures of at least two macromolecular species, polymers and/or copolymers. For practical reasons, the name blend is given to a system only when the minor component content exceeds 2 wt%. Depending on the sign of the free energy of mix
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Abdellah Ajji
National Research Council Canada, Industrial Materials Institute, Boucherville, QC, Canada
4.1
Introduction
4.1.1
General
Polymer blends are mixtures of at least two macromolecular species, polymers and/or copolymers. For practical reasons, the name blend is given to a system only when the minor component content exceeds 2 wt%. Depending on the sign of the free energy of mixing, blends are either miscible or immiscible. In a general sense, the polymer/polymer miscibility does not exist — it is always limited to a “miscibility window”, a range of independent variables, such as composition, molecular weight, temperature, pressure, etc. More than 1600 of these “miscibility windows” have been identified for two, three or four components’ blends. The immiscibility dominates the field [Utracki, 1989]. For more details on the thermodynamics of mixing and phase diagrams, the reader is referred to Chapter 2. Thermodynamics of Polymer Blends in this volume. 4.1.2
Definition of the Surface and Interface Tension Coefficients
The surface tension is the reversible work required to create a unit surface area at constant temperature (T), pressure (P) and composition (n) [Wu, 1982]: νi = (∂G / δA)T, P, n
(4.1)
where νi is the surface tension coefficient of the substance i, G is Gibbs’ free energy of the system, and A is the surface area. In immiscible liquids, interactions between components are located at the physical boundary creating the interface. The energy required to reversibly separate the two liquids is expressed as the work of adhesion: W= ν1 + ν2 - ν12
(4.2)
where ν1 and ν2 are surface tension coefficients of neat components and ν12 is the interfacial tension coefficient between the liquids 1 and 2.
L.A. Utracki (Ed.), Polymer Blends Handbook, 295-338. © 2003 Kluwer Academic Publishers. Printed in the Netherlands.
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4.1.3
Abdellah Ajji
Importance of the Interfacial Properties in Polymer Blends
The structure and morphology of immiscible blends depends on many factors among which the flow history and the interfacial properties are the most important. At high dilution, and at low flow rates the morphology of polymer blends is controlled by three dimensionless microrheological parameters: (i) the viscosity ratio, λ = η1 / η2, where η1 is the viscosity of the dispersed liquid and η2 that of the matrix; (ii) the capillarity number, κ = σ12 d / ν12 , where σ12, and d are respectively the shear stress, and the drop diameter; and (iii) the reduced time, t* = t y / κ, where t is deformation time, and y is the rate of shear [Utracki, 1989, 1994]. Thus, the interfacial and rheological properties are the keys for the morphology development in polymer blends, which in turn is the controlling factor for their performance. To improve performance of immiscible blends, usually they need to be compatibilized. There are three aspects of compatibilization: (1) Reduction of the interfacial tension that facilitates fine dispersion, (2) Stabilization of morphology against its destructive modification during the subsequen
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