Partially miscible polymer blends

Several polymer blends that are commercial products in the industry are partially miscible. Partially miscible polymer blends are those that exhibit some shift from their pure component glass transition temperatures. Thus, a binary miscible polymer blend will exhibit one glass transition temperature and a partially miscible polymer blend will exhibit two distinct glass transition temperatures different from their pure component glass transition temperatures. Some experimental systems that have been reported as partially miscible polymer blends are PET and poly(hydroxy butyrate) (PHB) and PC/SAN. Later on, in a separate chapter, a mathematical framework 

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Partially Miscible Polymer Blends A subclass of polymer blends including those blends that exhibit a window of miscibility, i.e, they are miscible only at certain concentrations and temperatures. 

The problems and challenges Inherent to developing useful materials with optimal morphologies and properties from an Immiscible or partially miscible polymer blend are not trivial and have spawned considerable Industrial and academic research. Work on polymer miscibility, compatibilizing agents, reactive systems, and the Influence of flow on the structure and properties of blends Is described in later chapters. 

The determination of phase separation in partially miscible polymer blends by means of thermal analysis is often difficult because of the small demixing enthalpy and the slow rate of the diffusion-controlled process. Dreezen et al. (2001) smdied the phase separation of PEO/PES and PEO/Aramid blends by optical microscopy, conventional DSC and MTDSC. The onset of phase separation from optical microscopy corresponds very well to the onset of a small stepwise increase in the MTDSC heat capacity (Fig. 10.31). 

The methods of measurement of the kinetic parameters of miscible or partially miscible polymer blends crystallized isothermally from the subcooled melt, fCj, n, k, a, and o, are the same as those for homopolymers. However, the dilution effect of non-crystallizable molecules on the thermodynamics and the mobility of the constituents should be examined. 

The morphologies of the miscible, immiscible, and partially miscible polymer blends are distinct from each other. In an immiscible blend, two phases are present the discrete phase (domain), which is lower in concentration, and the continuous phase, which is higher in concentration. The miscible polymer blends exhibit singlephase morphology. Partially miscible polymer blends may form completely miscible blends at a different composition. The two phases may not have a well-dehned boundary. Each component of the blend penetrates the other phase at a molecular level. The molecular mixing that occurs at the interface of a partially miscible two-phase blend can stabilize the domains and improve the interfacial adhesion. A compatible blend that has commercial possibilities may be immiscible, and a miscible polymer blend may lack commercial applications due to other factors such as cost, source of raw materials, safety, and environmental issues such as recyclability. 

What is the form of quadratic equation to predict the two glass transition temperatures observed in partially miscible polymer blends ... 

A mathematical framework is provided to account for the multiple glass transition temperatures found in partially miscible polymer blends. 

K. R. Sharma, Mathematical Modeling of Glass Transition Temperature of Partially Miscible Polymer Blends and Copolymers, 214th ACS National Meeting, Dallas, TX, 1998. 

K. R. Sharma, Dimensionless Ratio of Heat Capacity Change to that of Entropy of Mixing for Partially Miscible Polymer Blends, 95th AIChE Annnal Meeting, San Francisco, November 2003. 

When polymers undergo phase separation in thin films, the kinetic and thermodynamic effects are expected to be pronounced. Phase morphology with a single characteristic length scale can be synthesized by quenching a partially miscible polymer blend below the critical temperature of demixing. 

A mathematical framework is provided to account for the multiple glass transition temperatures found in partially miscible polymer blends. The change in entropy of mixing at the glass transition temperature of the blend can be used to account for the observations. A quadratic expression for the mixed glass transition temperature is developed from the analysis. The chain sequence distribution information can be used in estimating the entropy of the copolymer. 

In addition, the partially miscible polymer blends are thermorheologically simple, so that all considerations available for homogeneous systems can be applied in their case as well (Utracki 1990). The composition of the two phases from the vicinity of phase separation is not the same as in the case of immiscible mixtures where it is supposed that, if droplets are present in their corresponding morphology, they are composed from one of the mixture components and that their total volume is equal to that of the blend composition. This theory is not applicable to partially... 

As a conclusion to the aforementioned observations, it can be asserted that the rheological features of complex partially miscible polymer blends are only partially elucidated in terms of their thermodynamics, based on the behavior of immiscible blends and if their phase state is well known. 

Chopra Divya. Thermodynamics and rheology of partially miscible polymer blends. A dissertation presented to the graduate faculty of the university of British Columbia, April 2002. 

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