Molecular miscibility

Polymer blends can be subdivided into two kinds those of compatible and those of incompatible polymers. Real compatibility is an exception (see 9.1) an example is PS with PPE (polyphenylene ether, also called PPO, polyphenylene oxide). These two polymers can be blended with each other on such a small scale that it really looks like molecular miscibility. This blend shows, therefore, only one single glass transition. 

The majority of polymers are immiscible and, in bulk, they phase separate to form domains of varying sizes and shapes, depending on their relative volume fraction. This happens because of the very low entropy of mixing in the case of large polymeric molecules. Therefore, unless there is a large favorable enthalpic contribution, most polymers do not form molecularly miscible systems. The same is true for block copolymers, in which the length of each block exceeds a certain critical value. As mentioned earlier, block copolymers are systems wherein two (or more) different types of homopolymers are linked to each other at the chain end(s) diblock copolymers, represented as (A) -(B) , are systems in which two homopolymers are linked to each other at one end, while triblock copolymers, represented as (A)m-(B) -(C)p, are systems in which one central homopolymer block is linked at either end with two other homopolymers. The values m, n and p, represent the... 

To be useful, most commercial polymer blends are either designed or selected to have some degree of the technological compatibility between the components to resist delamination and loss in ductility. Compatibility is defined here as the ability for the polymer components to co-exist either as molecularly miscible or as morphologically distinct phases, but interfacially stabilized, without a tendency for delamination. 

R 696 K. Akasaka, NMR Technologies for Protein Structural Analysis in Aqueous Solution , Kagaku (Tokyo, Japan), 2003,73,74 R 697 H. Akutsu, Molecular Miscibility in the Two-Dimensional Solvent of Lipid Bilayer Studied by Solid-State NMR Molecular Mechanism of the... 

Crystallizing systems may be grouped into systems wMeh show a eutectio point E, systems which show molecular miscibility of the sohd phase, and systems whieh show both (Rittner and Steiner 1985). Respective phase diagrams of such systems are given in Fig. 2.3-4 for the binary case. 

Fig. 12.14 Schematic representation (relaxation map) of the temperature dependence of the rate of the a-relaxation for a binary polymeric blend (a) theoretical expectation for a molecular miscible blend, (b) A blend with a partial miscibility of the two components...

In bromobutyl/chlorobutyl rubber blends, both elastomers have the polyisobutylene backbone and halogen reactive functionality. These polymers, being molecularly miscible, constitute an ideal system for co-vulcanization. Bromobutyl and chloro-butyl can be used interchangeably without significant effect on state of cure as measured by extension modulus, tensile strength, and cure rheometer torque development. Bromobutyl will increase the cure rate of a blend with chlorobutyl. However, where bromobutyl is the major part of the blends, chlorobutyl does not reduce scorch tendencies because the more reactive halogen unit can dominate. 

At the other extreme, polymeric plasticization requires molecular miscibility and examples of such plasticized systems which have been claimed are quoted in Sect. 6. 

Molecular Miscibility in ASD Using pXRD and Computational Analysis... 

It was concluded that, as with bulk blend systems, PPO and PS are miscible at all compositions confirmation was also provided that molecular miscibility is not affected by film thickness dovm to 20 nm [43]. 

The blending of a polymer with molecularly miscible tackifiers and plasticizers allows manipulation of the Tg of the system. The Tg can be roughly predicted by the Fox equation (Eq. 1). In this case the components in the summation are simply the different components in the mixture and the values needed are the glass transition temperatures for each of the materials the polymer, the tackifier, and the plasticizer. Tackifiers generally have Tg values in the range from —30 to - -100 C. Plasticizers have lower Tg values. Again, the transition position in time scale will be related to the Tg as described above for copolymers. 

Polyolefins containing carboxylic acid groups, sometimes neutralized to form ionomers, form much stronger intermolecular hydrogen bonding and ionic attractions than simple polyolefins, and can thus contribute greatly to practical compatibility or even molecular miscibility of polyblends, particularly blends with more polar polymers. Occasionally sulfonated polyolefins offer similar benefits. Carboxylation of polyolefins has been noted occasionally throughout this survey. In the current section the emphasis is on carboxylic and sulfonic acid copolymers and their ionomers. 

The miscibility of PMMA and poly(vinylidene flu oride) (PVDF) has been established by various meth ods. Both F and F H CP and molecular miscibility in blends of PVDF and isotactic, syndio tactic and atactic PMMAs were investigated by triple resonance H, i F, 13C solid state CP MAS NMR. The fraction of nonmixed PMMA was determined for these blends, and it was found that this fraction was smaller for isotactic than for atactic and syndiotactic PMMAs. 

The glass transition temperatures of all substances that show a possible molecular miscibility with the polymer matrix were simulated. As described in Section 16.2.2, a previously reported MD method [27] was applied. 

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