Molecular dispenser

The focus here will be on the consideration of designed copolymers exhibiting selective interactions with the surfaces and interfaces. In particular, we will consider some properties of adsorption-tuned copolymers and partly cross-Unked polymer envelopes that function as a molecular dispenser. 

To characterize the complexes formed between molecular dispenser described in Sect. 2.2.4 and colloidal particles, the probability P(a, T) of finding a complex made from the copolymer envelope and the particle of a given size, a, was calculated as a function of temperature T [57]. 

It was found that the selectivity of the complex formation strongly depends on the number of crosslinks in the envelope. Typical results for a moderately crosslinked envelope are shown in Fig. 45a. It is seen that the selectivity of the complex formation with the particle of a certain size is indeed reached, that is, the idea of a molecular dispenser works. 

The reason for the selective adsorption of a colloidal particle of parent size is explained by the typical snapshots in Figs. 45b and c. We see that the particle of parent or smaller size (a < central cavity (Fig. 45b), because the corresponding fitting was ensured by the sequence design procedure (Fig. 10b). On the other hand, a particle of larger size (a > ap) turns out to be too big for a central cavity (Fig. 45c), and thus the complex formed does not saturate all the possibilities for the attraction of red units to the surface of the particle. As to small particles, they easily penetrate inside the molecular dispenser, but the complex formed is not stable (especially at high temperature) because of the small surface of such particles. All these factors explain the peak in P(T,a) observed at a ap for moderately cross-linked copolymer envelopes (Fig. 45a). 

Velichko et al. suggested the model of a so-called molecular dispenser. This idea is a further development in the direction of CDSD. Namely, they considered the conformation of a homopolymer chain adsorbed on a spherical colloidal particle (Figure 8, bottom panel) and performed design of sequence for this state of macromolecule. The motivation behind this design procedure is that if we eliminate the parent colloidal particle after the design is completed (e.g., by etching), the resultant copolymer will be tuned to adsorb selectively another colloidal particle of a parental size crp. For instance, if such copolymer is exposed to a polydisperse colloidal solution of particles of different size, it will selectively choose to form a complex with the particle having the same radius as that in parental conditions. That is why such a macromolecular object can be called a molecular dispenser. 

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