Polystyrene segregation

Ide and White W studied the viscoelastic effects in agitating polystyrene solutions with a turbine. At concentrations below 50% PS, flow was normal. Abovfe 35%, the viscoelastic forces caused the flow to reverse, moving away from the impeller along the axis. At 30 to 35% PS, both occurred, causing a segregated secondary flow around the turbine. 

Triblock copolymers, as shown in Fig. 5.8 d), comprise a central homopolymer block of one type, the ends of which are attached to homopolymer chains of another type. As with other block copolymers, the components of triblocks may be compatible or incompatible, which will strongly influence their properties. Of particular interest are triblocks with incompatible sequences, the middle block of which is rubbery, and the end blocks of which are glassy and form the minor phase. When such polymers phase-segregate, it is possible for the end blocks of a single molecule to be incorporated into separate domains. Thus, a number of rubbery mid-block chains connect the glassy phases to one another. These materials display rubber-like properties, with the glassy domains acting as physical crosslinks. Examples of such materials are polystyrene/isoprene/polystyrene and polystyrene/polybutadiene/polystyrene triblock copolymers. 

Block copolymers of polystyrene with rubbery polymers are made by polymerizing styrene in the presence of an unsaturated rubber such as 1,4 polybutadiene or polystyrene co-butadiene. Some of the growing polystyrene chains incorporate vinyl groups from the rubbers to create block copolymers of the type shown in Fig. 21.4. The combination of incompatible hard polystyrene blocks and soft rubber blocks creates a material in which the different molecular blocks segregate into discrete phases. The chemical composition and lengths of the block controls the phase morphology. When polystyrene dominates, the rubber particles form... 

The micelle formation is not restricted to solvents for polystyrene but also occurs in very unpolar solvents, where the fluorinated block is expected to dissolve. Comparing the data, we have to consider that the micelle structure is inverted in these cases, i.e., the unpolar polystyrene chain in the core and the very unpolar fluorinated block forming the corona. The micelle size distribution is in the range we regard as typical for block copolymer micelles in the superstrong segregation limit.2,5,6 The size and polydispersity of some of these micelles, measured by DLS, are summarized in Table 10.3. 

Owing to their amphiphilicity and a balanced molecular architecture these molecules from micelles in all solvents for polystyrene as well as in solvents for the fluorinated block. The structure parameters of these micelles have to be regarded as typical for other block copolymers in the superstrong segregation limit.5 6... 

The transmission electron microscopy results are consistent with a segregated latex particle consisting of a polystyrene rich core and a soft poly-n-butyl acrylate rich shell. 

The desirability of segregation in block copolymers can be demonstrated by considering the behaviour of SBS, which is one of the oldest types. It has about the same chain composition as SBR, but, rather than SBR, it shows two glass-rubber transitions, namely that of polybutadiene and that of polystyrene. Between these two temperatures it behaves as a rubber, in which the PS domains act as cross-links it is, therefore, a self-vulcanizing rubber (see also Figure 3.8 see Qu. 9.14). Moreover, the hard domains play the role of a reinforcing filler. 

Fig. 2. Central slice of a rotating tube 75% full, with an aspect ratio of 3, comprising mustard seeds (MR active) and polystyrene beads (MR inactive). The mustard seeds are identified as the light grey pixels. The polystyrene beads give no signal intensity. In images (a)-(c) the mustard seeds and polystyrene beads are seen to segregate rapidly but retain a non-mixing core region. Reproduced with permission from Metcalfe et at. (1999).

S)-amino acicis, mexiule C, two stereoisomers, and tethered to polystyrene via mcxlule B, two stereoisomers) was prepared by encoded split synthesis on 100 pm polystyrene synthesis beads so that different library members were segregated on different beads (i.e. one bead, one chiral SO). This library was then screened by a two-colour differential binding method amino acid SAs were labelled via a linker with red ((/ )-amino acids) and blue ((S)-amino acids) dyes and the chiral beads treated with an equimolar mixture of the labelled SA enantiomers. Enantioselective binding beads are either red or blue, whereas unselective beads are brown. 

A large polymer molecule is able to exist with different parts of the chain in different phases, and an increase in the concentration of the species will concentrate solvated parts together as well as concentrating the precipitated parts. With two different polymers, e.g., polystyrene and poly butadiene, blocks of one will form within a continuous phase of the other, with domain sizes between 10 and 100 nm usual. This segregation into phases will be enhanced by the swelling effect of the remaining polymer. 

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