MSAPS · Modelling the self-assembly of polymers in solution

Modelling the self-assembly of polymers in solution

Project context and objectives Polymers are chains of small molecules linked together, and form the basis of a huge variety of materials, including proteins, plastics and adhesives. The project focused on the theoretical studies of one type of polymer that is currently causing great technological interest: block copolymers. In block copolymers, monomers of a given type are grouped together in long intervals, or blocks. The utility of block copolymers arises from their ability to self-assemble into a range of structures. A striking example of this phenomenon is seen when the copolymer molecules are dispersed in a solvent such as a liquid or another polymer. If one or more of the blocks is hydrophobic, or incompatible with the solvent, the polymers will cluster together in order to minimise their contact with the solvent. A wide variety of structures can be formed in this way. The project largely considered the self-assembly of vesicles (hollow spheres), like those formed by lipids in cells. Much of the motivation for the strong interest in these copolymer vesicles arises from their potential for the encapsulation and delivery of active chemicals, for example drugs. Although the basic principle behind the self-assembly of block copolymers can be easily explained, predicting which structure will be formed by a given system is a much more difficult problem. Designing block copolymers that will self-assemble into the structure required by a particular application can thus be time-consuming and costly; there is, therefore, a clear need for theoretical modelling work to provide guidelines for the design and manufacture of block copolymers. The overall aim of the project was to use theoretical methods to predict the structures formed by a range of block copolymers in solution and therefore to understand the existing data and motivate new experiments. Three related problems were considered. Work performed and main results a. Controlling the size of vesicles There is no sure way of fixing the size of vesicles formed from simple copolymers; the distribution of vesicle shapes and sizes is determined by the details of the preparation process rather than by the properties of the molecules. We performed detailed calculations to show how more complex polymers with four blocks can be designed to form vesicles of a controlled and uniform size. To the best of our knowledge, this work is the first theoretical demonstration that molecular architecture can be used to fix vesicle size directly, without the need for more complex experimental methods, such as dialysis or the mixing of different polymer species. In addition to its basic theoretical importance in soft matter science, this result is also highly relevant to drug delivery applications, and could yield precise control over the dosage of chemicals encapsulated by vesicles. Preliminary experimental investigations on these polymers have suffered from the problem that unwanted structures often form at the same time as the target vesicles. We have also extended our calculations to show how the length of the various polymer sections must be varied to avoid this difficulty. b. Encapsulation of chemicals in the vesicle wall The systems investigated in part a) are suitable for the encapsulation and delivery of hydrophilic chemicals. However, many drugs and industrial chemicals are hydrophobic, and can be more easily encapsulated as droplets in the central hydrophobic region of the vesicle wall. A comprehensive theoretical study of this problem was performed, focusing on how the properties of the hydrophobic chemical affect the shape and stability of the droplet. The impact of this work lies in the fact that we have provided information on which chemicals are likely candidates for encapsulation. We also found that the main features of our detailed calculations could be explained by a simpler model involving the balance of the tensions in the vesicle. This work was driven by discussions with industrial researchers and has motivated experimental work in the host group. In addition, droplet formation in the membrane wall has been proposed as a possible explanation for certain results of microscopy experiments in cell biology. The stability of droplets over a wide range of oil molecule types found in our calculations is also evidence for the plausibility of this conjecture. c. Promotion of phase separation in the vesicle wall by added oil Following on from part b), and guided by current experimental work in the host group, we studied how oil can be added to a vesicle formed from a mixture of two different lipids to promote phase separation between these and hence gain greater control over the properties of the vesicle. The main result of this work was the theoretical demonstration that there is an optimum size of oil molecule that reduces the extra surface tension arising from phase separation. The likely impact of this result is in nanotechnology applications, such as the localisation of membrane proteins and nanomaterials.

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