Polyblends phase separation

Some commercial, linear (thermoplastic) polymers produce blends with lignin and lignin derivatives that fail to result in phase separation on macroscopic scale. Polyblends with lignin derivatives sometimes resemble plasticized or anti-plasticized materials. The greatest contribution lignin can make to thermoplastic systems is that of modulus and this is the same as that which lignin makes to the amorphous component of wood. 

Heterogeneity, as in polyblends, has also been observed in random copolymers. F. Kollinsky and G. Markert found phase separation in binary mixtures of copolymers of methyl methacrylate and butyl acrylate. C. Kraus and K. W. Rollmann discovered heterogeneity in blends of random copolymers of butadiene and styrene if they differ by more than 20% in composition. 

Multicomponent polymers systems such as polyblends, and block copolymers often exhibit phase separation in the solid state which results in one polymer component dispersed in a continuous phase of a second component. The morphological properties of these systems depend upon a number of factors such as the molar ratios of the components, the molecular weights, the thermal history of the system and, for solvent cast films, the solvent and drying conditions. 

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. 

Homogeneous single-phase polyblends are very rare. Liquid-liquid phase separation of optically homogeneous polyblends of a styrene/acrylonitrile copolymer with poly (methyl methacrylate) has been studied by L. P. McMaster. A quantitative test method of the dynamic mechanical properties of multiphase polymer systems was developed by L. Bohn. He was able to demonstrate the correlation between shear modulus and gel volume of brittle polymers... 

Block and graft copolymers (incompatible copolymers) — For block or graft copolymers in which the component monomers are incompatible, phase separation will occur. Depending on a number of factors — for example, the method of preparation — one phase will be dispersed in a continuous matrix of the other. In this case, two separate glass transition values will be observed, each corresponding to the Tg of the homopolymer. Figure 4.6 shows this behavior for polyblends of polystyrene (100) and 30/70 butadiene-styrene copolymer (0). 

When the polymer components in a blend are less miscible, phase separation will form larger domains with weaker interfacial bonding between them. The interfaces will therefore fail under stress and properties of polyblends are thus likely to be poorer than for either of the polymers in the blend. U-shaped property curves (Figure 4.40c) thus provide a strong indication of immiscibility. In most cases they also signify practical incompatibility, and hence lack of practical utility. 

Scientists and engineers working in the fields of polyblends and block copolymers have realized for many years that phase separation of the two components takes place, and that this is indeed important to the development of the mechanical behavior characteristic of these materials. However, it was not until the development of the electron microscope that the structure of any but the coarsest mechanical blends could be discerned, and even then lack of contrast between the two phases remained serious. This problem was solved in 1965 by Kato (1966, 1968), who discovered that osmium tetroxide preferentially stains polymer molecules containing carbon-carbon double bonds, such as in polybutadiene and polyisoprene. The osmium tetroxide also hardens the rubbery phase, allowing convenient ultramicrotoming of specimens to 500 A thickness. 

Discussion of the detailed structure of the graft-type polyblend latex particle requires amplification. In the formation of the ABS type G resin, part of the AS copolymer forms a shell around the seed latex (Kato, 1968), as shown in Figure 3.6. As with other types of graft copolymers, some monomer dissolves within the seed latex. Upon polymerization, the second monomer mix phase-separates to yield the complex inner morphology observed. After coagulation, the glassy AS polymer forms the matrix, while the portion occluded within the latex particles remains within the rubber phase (Figure 3.7). 

Therefore, to minimize the effect of phase separation, the as-prepared polyblend solution is poured into an excess amount of nonsolvent such as methanol to obtain a quenched bulk powder of polymer blend. The obtained polymer blends could be molded into the desired shape by hot-pressing at 180°C for 5 min. Figure 8.43 shows the electrical conductivity of the molded blend as a function of PPy-DBSA weight fi-action [68]. The electrical conductivities of PPy-DBSA/PMMA blend exhibit a percolation threshold level at about 40%. Nevertheless, the PPy-DBSA/PMMA blends exhibit conductivities ranging from 10 to 10 S/cm with 2% PPy-DBSA, which satisfy the electrical conductivity required for static dissipative (10 to S/cm) or antistatic applications (10 to 10 S/cm). 

The Tg s of polyblends in general, and of IPNs in particular, can be studied in a variety of ways. Unfortunately, the study of IPNs shares a problem common to all of the phase-separated blends, grafts, and blocks, and indeed to virtually all science no one investigator has systematically studied the same series of samples by all of the important techniques. 

While such phase separation may benefit certain specialized properties such as melt fiuidity, easy-opening packaging, and lubricity, in most products the loss of strength, ductility, and impact strength is a serious handicap, so most polyblends are therefore labelled incompatible. ... 

Polyblending offers the possibility of combining the best properties of both polymers in the blend, particularly in a two-phase system [1-4]. Thus, when gross phase separation causes incompatibility, it is highly desirable to reduce the size and morphology of phase separation, and in particular to strengthen the interface... 

Polymer blending is a very convenient technique to produce materials of improved property/cost performances. Since most polymers are immiscible, polyblends usually have to be compatibilized in order to improve the poor mechanical performances associated with gross phase separation and low interfacial adhesion. Excellent reviews have been published on the compatibilization of multiphase polyblends [1-11]. 

Block copolymers and polyblends frequently exhibit phase separation which typically gives rise to a dispersed phase consisting of one polymer component in a continuous matrix of the second polymeric component. The detailed morphology of the domain structure in these systems depends upon such factors as the relative proportions of the two components, the molecular weights, the thermal and physical histories of the pol3nners, and for solvent cast films, upon the solvent and temperature. 

For useful polyblends, the term compatibilization refers to the absence of separation or stratification of the components of the polymeric alloy during the expected useful lifetime of the product. Optical clarity of a polyblend is related to the particle size of the dispersed phase and/or the difference in the... 

It so happens that most polymers are not miscible rather they separate into discrete phases on being mixed. Differences between miscible and immisdble polyblends are manifested in appearance (miscible blends are usually clear, immiscible blends are opaque) and in such properties as glass transition temperature (miscible blends exhibit a single Tg intermediate between those of the individual components, whereas immiscible blends exhibit separate TgS characteristic of each component). 

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