Compatible polyblends

Dynamic-Mechanical Measurement. This is a very sensitive tool and has been used intensively by Nielsen (17) and by Takayanagi (18). When the damping curves from a torsion pendulum test are obtained for the parent components and for the polyblend and die results are compared, a compatible polyblend will show a damping maximum between those of the parent polymers whereas the incompatible polyblend gives two damping maxima at temperatures corresponding to those of the parent components. Dynamic mechanical measurement can also give information on the moduli of the parent polymer and the polyblend. It can be shear modulus or tensile modulus. If the modulus-temperature curve of a polyblend locates between those of the two parent polymers, the polyblend is compatible. If the modulus-temperature curve shows multiple transitions, the polyblend is incompatible. 

Compatible Polyblends. When the polymeric materials are compatible in all ratios, and/or all are soluble in each other, they are generally termed polyalloys. Very few pairs of polymers are completely compatible. The best known example is the polyblend of polyCphenylene oxide) (poly-2,6-dimethyl-l,4-phenylene oxide) with high-impact polystyrene (41). which is sold under the trade name of Noryl. It is believed that the two polymers have essentially identical solubility parameters. Other examples include blends of amorphous polycaprolactone with poly(vinyl chloride) (PVC) and butadiene/acrylonitrile rubber with PVC the compatibility is a result of the "acid-base" interaction between the polar substituents (1 ). These compatible blends exhibit physical properties that are intermediate to those of the components. 

The marginally compatible polyblend is analogous to the poor solvent case in that presence of the discrete dispersed phase results in less interaction between phases and, therefore, lower melt viscosities. Styrene-butadiene block copolymers have lower melt viscosities than random copolymers of the same composition and yield solutions of lower viscosity at the same concentration. This is because of the incompatibility but inseparability of the segments of the chain. 

Isomorphous Systems (Homogeneous Copolymers or Compatible Polyblends)... 

Thus physical processes alone can contribute toward practical compatibilization of polyblend systems. It should be remembered, however, that while sueh solely physical processes may affect the degree of compatibility, they do not generally make a qualitative differenee between incompatible and compatible polyblends. For such qualitative improvement in compatibility, additives and/or reactions are generally required. 

Evidently, the homogeneous complex phase is an equilibrium property for copolymers. Any effects resulting from the history of specimen preparation (cf., photomicrographs and dashed lines in Figures 9 and 10) are eliminated by annealing. In contrast, for polyblends the compatible phase is a metastable state. 

Elastomeric graft copolymers of methyl methacrylate upon diene and acrylic rubbers were prepared by R. G. Bauer and co-workers. These elastomers are compatible with rigid methyl methacrylate-styrene copolymers of identical refractive index, yielding transparent polyblends. 

Of course, Flory has not defined the criteria for compatibility. If compatibility is to mean single homogeneous phase and is associated with nearly complete mixing of the molecules at the molecular level, as it does for simple liquids, no polyblends are known to be truly compatible. So what does compatibility mean when applied to polyblends A search of the literature revealed that various methods have been used to determine compatibility of polyblends and each method has its own standard and sensitivity. 

Appearance of Fused Product. From the fabrication point of view, if two polymers give a smooth band on a two-roll mill, the polyblend is said to be compatible. If the fused product is cheezy, it is said to be incompatible. Frequently, the fused product is pressed into a flat sheet. Transparency of the sheet signifies compatibility, whereas an opaque appearance means incompatibility. Obviously, these criteria are arbitrary and crude. They are subject to great variation owing to difference in individual judgement. In addition, they give no information on the morphological feature of the system. 

Glass Transition Temperature. If the glass transition temperatures of the polymeric components are known and the glass transition temperature of the polyblend is determined, one of two things can happen. If the polyblend shows two distinct transitions corresponding to the parent polymers, it is incompatible. If the polyblend shows one transition only, the system is compatible. Since the glass transition temperature is a measure of the segmental mobility of a polymer, it must be sensitive... 

Matsuo, Nozaki, and Jyo (20) showed that heterogeneity at 100 A scale and under can be detected readily. Thus, microscopy can offer a measure of heterogeneity down to 0.01 p scale which is much smaller than the domain size of most polyblends. Results of microscopy have established convincingly that nearly all polyblends are heterogeneous two-phase systems. How does one describe the results Obviously, heterogeneity as revealed by microscopy is a relative property. If compatibility is used in a qualitative sense, a polyblend with a finer domain size will be more compatible than one with a larger size, provided equilibrium size distribution has been attained in both cases. 

Where the two phases are completely compatible, a homogeneous polyblend results which behaves like a plasticized resin (one phase). If two polymers are compatible, the mixture is transparent rather than opaque. If the two phases are incompatible, the product is usually opaque and rather friable. When the two phases are partially compatibilized at their interfaces, the polyblend system may then assume a hard, impact-resistant character. However, incompatible or partially compatible mixtures may be transparent if the individual components are transparent and if both components have nearly the same refractive indices. Furthermore, if the particle size of the dispersed phase is much less than the wavelength of visible light (requiring a particle size of 0.1/a or less), the blends may be transparent. 

In certain cases polyblends of two very compatible polymers also show just one T. The value of such a Tg can be predicted from either of the following equations ... 

It is well known that systems like polystyrene or polystyrene-acrylonitrile—generally considered brittle materials—have a remarkable increase in toughness and resistance to impact when polyblended with finely dispersed, crosslinked, but partly compatible, rubber particles. These particles are generally 0.1-10 fi in size and frequently consist of butadiene which has been grafted with monomers of similar composition to the matrix or continuous phase. 

Heat-resistant [218] soft foams were prepared from the blends of hdPE with E-P random copolymers. The azodicarbanamide acts as a thermal antioxidant and the crosslinking of the blend was increased by electron beam radiations and foamed at 225 °C with 2320% expansion. A blend of 35 wt.% PE-PP (8 92), 15 wt.% E-P block copolymers, and 50 wt.% EPDM showed accelerated weathering resitance [219] 1000 h probably due to crosslinking between constituents of the block copolymer, polyblend and EPDM. The effect of filler and thermodynamic compatibility on kaolin-filled PE-PP blend was studied by Lipatov and coworkers [220]. The thermodynamic interaction parameter (%) decreased and thermodynamic stability increased by filler addition, the degree of crystallinity decreased with increasing thermodynamic compatibility of the components due to sharp decrease in the phase separation rate during cooling. 

Fig, VI-7 shows the electrical conductivity vs, weight fraction of the PANI-CSA complex in polyblends with PMMA [61,282,283], The results are typical in that a similar smooth onset for conductivity is observed with a number of host polymers fabricated with appropriate compatible counterions (e,g, DBSA for polythylene, etc,) [282,283], As shown in Fig, VI-7, these PANI blends are remarkable in that electrical conductivities of order 1 S/cm can be obtained in a polyblend containing only about 2% of the conductive component, with no indication of a sharp percolation threshold [284],... 

Polyblends are made by intimately mixing two or more polymers on mill rolls, in extruders, or in Banbury or other mixing devices. The polyblends are admixtures of either two rigid polymers or two elastomeric polymers, or combinations of the two types. Their properties, and therefore their end uses, are strongly dependent on the degree of compatibility of the components. 

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