Characteristics of immiscible polymer blends

Blend properties strongly depend on which polymer is the continuous phase. The majority of commercially important compatibilized blends of semi-crystalline polymers with amorphous polymers are prepared with compositions such that the semi-crystalline component is the matrix and the amorphous component is the dispersed phase. The blends show adequate solvent resistance since in this morphology the surface consists largely of the dominant, matrix phase,. 

The formation of optimum dispersed phase particle size and the stabilization of the resulting blend morphology are critical if the blend is to have optimum properties and in particular good mechanical properties. Eigure 5.2 shows a morphology generated by processing an uncompatibilized 

This particular strategy is limited to those cases in which an immiscible polymer blend contains two semi-crystalline polymers that can co-crystalUze. Nadkarni and Jog [1989, 1991] have reviewed examples of this type of compatibilized blend. Co-crystaUization may also occur as a secondary process in an intimately mixed blend containing a copolymer with concomitant elfects on blend properties as shown in a few of the examples of this review. 

In these cases a dispersed phase of a crosslinkable rubber is vulcanized in the presence of a matrix of a second, immiscible, non-vulcanizable polymer during the residence time of melt processing. Examples have also been reported in which a mixture of two vulcanizable polymers has been employed. Coran [1995] has summarized five key requirements for preparing optimum compositions by dynamic vulcanization  

Good match between surface energies of the dispersed phase and the matrix. 

---

Miscibility between the individual polymers is the most important factor to determine the performance characteristics of a polymer blend. Mutual solubility of the phases, the thickness and properties of the interphase formed during blending and the structure of the blend are mainly dependent on the miscibility of individual polymers within a polymer. As a result, a quantitative estimation of interactions is very much important for the prediction of blend properties. Comparison of solubility parameters of individual polymers is an effective method to predict the extent of miscibility within a blend. According to the Hildebrand solubility theory, a large difference in solubility parameters (6p) of individual matrices results in immiscibility between them in the absence of any interfacial compatibil-izer [222]. Jandas et al. have reported that PLA and PHB have Hildebrand solubility parameters (6p) of 23.5 J /cm and 19.8 J Vcm which can turn out to be partially miscibile blends in between them [35]. In case of partially miscible blends, the miscibility can be controlled by compatibili-zation using proper interactables. 

The crystallization of miscible and immiscible polymer blends can differ remarkably from that of the neat crystallizable component(s). In the case of crystallizable miscible blends (discussed in this section), important polymer characteristics with respect to crystallization are the chemical nature and molecular mass of the components, their concentration in the blend, and the intermolecular interactions between the components. 

The use of interfacial agents in immiscible polymer blends is of great importance if good mechanical properties are required. They appear to be necessary especially when soft polymers are mixed with rigid plastics in order to improve the impact resistance. At the present time, however, general correlations between blend composition and molecular characteristics of the interfacial agent on the... 

The properties of the finished articles made from immiscible blends are governed by the morphology created as a result of the interplay of processing conditions and inherent polymer characteristics, including crystallizability. Therefore, a scientific understanding of the crystallization behavior in immiscible polymer blends is necessary for the effective manipulation and control of properties by compounding and processing of these blends. 

Interfacial polarization or MaxwelVWagner/SUlars (MWS) polarization (Wagner 1914 SUlars 1937) is a phenomenon that is characteristic for phase-separated or multiphase systems like immiscible polymer blends. Precondition for the observation of a MWS polarization is that the different phases have nonidentical properties. As a result of this, for instance, accumulation of charges takes place at the interfaces of the different phases. Steeman and van Tumhout (2003) published a compilation concerning polymeric materials including polymer blends. 

Polymer blending has attracted the attention of researchers because polymers with extraordinaiy properties obtained by chemical synthesis are more expensive than existing polymers and blending operations. Furthermore, a wise choice and combination of the polymeric materials in specific amounts may lead to the fabrication of blend materials with desirable properties. There are various numbers of polymers that can be combined to form blends with different physical properties. The characteristics of the polymeric blend are influenced by the nature of the dispersed and dispersion phases, the volume ratio of the phases, the sizes and size distributions of the particles of the dispersed phase and interfacial adhesion. One of the popular questions being addressed regarding the polymer blend is the miscibility between the components. The blends formed can be miscible, partially miscible or fully immiscible. The miscible polymer blend is formed by choosing polymers with compatible chemical structures which are capable of specific interactions. ... 

Copolymers with a blocky structure have been designed as compatibilizers for immiscible polymer blends. More recently, most research effort has been devoted to reactive compatibilization, with special attention paid to the molecular characteristics of the reactive precursors of the compatibilizer, such as molecular weight, content, and distribution of the reactive groups and kinetics of the interfacial reaction. The interplay between these factors and the various processing factors are important. 

For a judicious control of the macroscopic properties of polymer blends, phase morphology constitutes a key parameter for many specific applications. The blending process of immiscible polymers in the melt state results in a heterogeneous morphology that is characterized by the shape, the size, and the distribution of the component phases. Depending on the composition, the homopolymer characteristics and the processing conditions used to mix them, two main types of morphologies are obtained, a dispersed type (a particle can be of any shape rod, platelet, flacks, disc, sphere, etc.) or a co-continuous one. 

---