Phase separation block/graft copolymers

Of course, in multiblock copolymers (and some types of graft copol)nners) the phase-separated blocks constitute the basis of physical crosslinks, and those types of bonds are included in Table I. 

Copolymerizations of benzvalene with norhornene have been used to prepare block copolymers that are more stable and more soluble than the polybenzvalene (32). Upon conversion to (CH), some phase separation of nonconverted polynorhornene occurs. Other copolymerizations of acetylene with a variety of monomers and carrier polymers have been employed in the preparation of soluble polyacetylenes. Direct copolymeriza tion of acetylene with other monomers (33—39), and various techniques for grafting polyacetylene side chains onto solubilized carrier polymers (40—43), have been studied. In most cases, the resulting copolymers exhibit poorer electrical properties as solubiUty increases. 

It is important to note that the Tg values of component polymers may be unaffected when they are present in multiphase blends (separate phases at the microlevel), and this is the basis for many multiviscosity oils. This is also true for block and graft copolymers, which have characteristic Tg values corresponding to the polymers of each of the comonomers. 

The chief reason for the interest in graft copolymers originates from the incompatibility between polymer chains of different chemical nature. Intramolecular phase separation results, because grafts and backbone repell each other, and these compounds exhibit a marked tendency to form mesomorphic phases like block copolymers and soaps do. When these species are mixed with a solvent that exhibits a preferential affinity for one of the components (grafts or backbone) the incompatibility may be enhanced. This intramolecular phase separation has led to a number of applications. If small amounts of a graft copolymer are included into a homopolymer of the same nature as the grafts (or the backbone), surface modifications can result as described below. 

Unlike the random copolymers, block and graft copolymers separate into two phases, with each phase exhibiting its own Tg (or TM).40 The modulus-temperature behavior of a series of... 

In random copolymers chain flexibility and crystallinity can be quite different from those in either of the component /zomopolymers. In consequence, the copolymer may well present an entirely new set of physical properties. On the other hand, the different chain segments in block or graft copolymers often segregate into effectively separate phases, when the properties of the composite resemble those of a mixture of the individual homopolymers. 

A random co-polymer or a blend of compatible polymers will have a single glass transition temperature intermediate between those of the two homopolymers. An example is shown in Figure 14 for nitrile-butadiene-rubber (22). The specific weight percents shown are those of commercial interest for NBR. In contrast, most polymer blends, graft and block copolymers, and interpenetrating polymer networks (IPN s) are phase separated (5) and exhibit two separate glass transitions from the two separate phases. Phase separated systems will not be considered here. 

Statistical Copolymers. The term statistical is used here to refer to copolymers in which the sequence distribution of comonomers can be inferred statistically from the simple copolymer model (Chapter 7) or alternative theory. In the present context statistical copolymers excludes block and graft structures and incorporates all other copolymers. It is useful first of all in this section to point out that statistical copolymers are not mutually miscible if the mixture involves abrupt changes in copolymer composition. Coatings chemists observe this phase separation as haze (internal reflections) in films. 

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