Triblock looping

Fig. 57 Schematic comparison of chain conformations of the midblock for ABC and ABA triblocks. ABC triblock terpolymers (a) have bridge conformations only, whereas ABA triblock copolymers (b) have bridge and loop conformations. From [159]. Copyright 2002 Wiley...

Analogous equations to (3.20-3.26) can be written for triblock copolymer micelles in a homopolymeric solvent (Balsara et at. 1991 ten Brinke and Hadziioannou 1987). However, in a BAB triblock copolymer where the solvent is selective for the A block, for a single micelle the A block must be looped. Then each chain enters the core twice, and eqn 3.21 must be multiplied by two, with a similar multiplier of the analogous term in eqn 3.22. An additional contribution must be added to the free energy of the corona due to looping. Balsara et at. (1991) estimated this to be... 

GPC fractionation and ozonisis of the products, the yield of cyclic polymer was estimated to be ca. 90%. The morphological transition of the cyclic block polymer depends on composition in essentially the same manner as that of the linear triblock copolymers, whereas the domain spacing of polystyrene-block-polyisoprene cyclic block copolymers were all smaller than those of the corresponding SIS linear triblock copolymers, which is attributed to looped chain conformation. 

Triblock copolymers of ABA-type have basically the same behavior in giving a star-like morphology (Fig. 6a) when dissolving into a selective solvent for the outer A blocks. However, the micellization of ABA-type triblock copolymers in selective solvent for the middle B blocks deals with a much more complex situation [42], A relatively low concentration of the copolymer in the selective solvent and/or a low molecular weight of the A blocks can lead to isolated flower-like micelles (Fig. 6b) with the middle B block being looped - referred to as petals - and with the two outer A blocks taking part of the same micellar core. However, if the copolymer concentration or the A block molecular weight is increased, a micelle association... 

Fig. 10 Schematic depiction of bridge chain and loop chain configurations in cylindrical morphology from the ABA triblock copolymer self-assembly...

In the low 2 regime, where crazing is not active, the 290-470-290 triblock fails by chain scission. Clearly, in this case, the 460 middle block of the small triblock cannot be considered as two equivalent PS blocks of 230 since an interface reinforced by such chains would have failed by simple chain pullout. Intuitively, this result is not surprising as the geometry of pulling out a loop may require breaking a bond and thus should be more difficult than pulling out two linear chains.1... 

Cyclic block copolymers with blocks that microphase separate in bulk are expected to form only loops at both sides of the interface, while their linear triblock analogues are able to form loops and bridges. This significant difference is expected to give very interesting morphological properties to the cyclic copolymers. 

Fig. 7. Transition from a two-domain structure to a three-domain structure in an ABC triblock copolymer lamellar phase. Although state ii is thermodynamically the most stable state, transition to this state from i is hindered because of the high free energy cost in switching the orientation of the bridges and in turning the loops into bridges. Thus, a kinetically more likely process is for the A and C blocks from the bridge conformation to separate laterally, with the loops straddling the interfaces between the A and the C domains.

The triblock copolymer is more effective than the corresponding diblock copolymer in increasing the fracture toughness, because most of the triblock copolymer chains loop around to form staple structures (which are not possible with the diblock copolymer) at the interface. 

In practice, block copolymer based PSA are often formulated from base polymer blends of triblock and diblock copolymers in various proportions. Setting aside cost considerations, the reasons for using a certain blend or even a pure triblock copolymer are typically based on performance in standardized PSA tests such as loop tack, peel, or shear tests. Yet, the effects of adding diblocks to a triblock copolymer on the details of the mechanisms of debonding are not known. 

Figure 2.16 Typical nitrogen adsorption-desorption isotherms at 77K for (a) MCM-41 materials templated with alkyltrimethylammonium bromide surfactants with hydrophobic tails of different lengths as indicated (volumes adsorbed for C12, C14, C16 and C18 were incremented by 200, 400, 600 and 800 ml(STP) g, respectively) and (b) nonionic triblock copolymer templated SBA-15 silicas synthesised at different temperatures (volumes adsorbed for 353 K and 373 K were incremented by 200 and 400 ml(STP) respectively). The hysteresis loops observed in this case are typical of the larger mesopores in these materials. Desorption points are represented by closed symbols. Reprinted with permission from Morishige, K. Tateishi, M., Accurate relations between pore size and the pressure of capillary condensation and the evaporation of nitrogen in cylindrical pores, Langmuir, 22, 4165 169. Copyright (2006) American Chemical Society...

Investigations on the self-assembly in thin films of an amorphous P2VP-PS-P2VP triblock copolymer, forming cylindrical microdomains in bulk, showed that the orientation of microdomains due to the P2VP/substrate interactions persisted in the entire film in contrast to the diblock case [14], This was viewed as a result of the formation of an interconnected structure in the triblock coming from the formation of loops within the microdomains. More recently, AFM and SAKS measurements on a PEO-PBO-PEO amorphous-semicrystalline triblock thin film revealed the presence of a semicrystaUine PEO monolayer at the substrate, comprised of unfolded chains, and PBO blocks at the air/polymer siuface in a looped conformation (15]. 

Figure 15 Structural and dynamic properties underlying the low elastic modulus and fast erosion of ACiqA gels, (a) Disengaged clusters form readily due to the propensity for intramolecular association and loop formation of the triblock polymers, (b) Three possible states of tetrameric aggregates. The number of loops is given by /. Reproduced with permission from Shen, W. Zhang, K. C. Kornfield, J. A. Tirrell, D. A. Nat. Mater. 2006, 5. 153-158. ...

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