Block structure, segment density

In a previous paper (15) the segment density of PVA adsorbed on PS latex in water was presented and it was noted that H Cgans was at the extremity of the s.a.n.s. profile. Calculating <5 assuming a value of a of 0.5 nm gives 13 nm in contrast to the experimental value of 18 nm. The discrepancy here is much smaller than in the case of PE0. This effect is difficult to interpret without further theoretical work but may be attributable to the fact that the PVA chain is less flexible than PEO and that the block structure (PVA is a random block copolymer of vinyl acetate. 12%, and vinyl alcohol) makes the formation of tails less likely. 

It is usually assumed that the micellar corona is a continuous phase extending from the micellar core to the micellar radius Rm. The internal structure of the micelle can be described by a density profile as shown in Fig. 8. The micellar core is a homogeneous melt or glass of insoluble polymer blocks. For hydrophobic blocks in aqueous solutions, the polymer volume fraction in the micellar core is 0C 1. The micellar shell is swollen with water or aqueous salt solution and has a polymer segment density that is expected to decrease in the radial direction as 0(r) r-a as typical for star polymers or... 

Figure 1. Segmental density fluctuation of alternating lamellae of pure polystyrene-b-polyisoprene-b-polystyrene block copolymer. The density (referenced to each pure segment composition) varies between 0 and 1 for each domain phase and in the interphase region which has finite thickness, d. The periodic pattern distance of the lamellae structure is denoted by D.

OBCs are advantaged in their eapability to retain oil when compared to Polyolefin Elastomers of similar density and Ml, and can therefore deliver lower hardness compounds ( 20 Shore A). This is hypothesized to be due to the OBC block structure wherein the blocks of amorphous segments may allow for more swelling and oil incorporation. Oil-filled OBCs also show decreased compression set at both room temperature and elevated temperature, which is again attributed to their imique block structure. These combined benefits expand Ae application range of OBC compounds to include markets currently served by high performance thermoplastic elastomers, such as styrenics block copolymers (SBCs), thermoplastic urethanes (TPUs) and thermoplastic vulcanizates (TPVs). 

The adsorption of block and graft copolymers is more complex, as the intimate structure of the chain will determine the extent of adsorption [37]. Random copolymers adsorb in an intermediate fashion compared to that of the corresponding homopolymers. Block copolymers retain the adsorption preference of the individual blocks. The hydrophilic block (e.g., PEO the buoy) extends away from the particle surface into the bulk solution, while the hydrophobic anchor block (e.g., PS or PPO) provides a firm attachment to the surface. Figure 6.14 shows the theoretical prediction of diblock copolymer adsorption according to SF theory. In this case, the surface density cr was plotted versus the fraction of anchor segments v, and adsorption was shown to depend on the anchor/buoy composition. 

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