Major factors affecting the CDSA process

1.6 Major factors affecting the CDSA process

During the self-assembly process of such crystalline-coil block copolymers, three factors are mainly considered that determine the resultant morphology; the folding of the crystalline core, the repulsion of the soluble corona, and the exposed area of the crystal surface. Reducing the folding amount of the core block is advantageous from an entropic standpoint but this could lead to an increase in entropically unfavourable stretching of the corona block (Figure 1.7). Apart from that, the active crystal area prefers to be minimised to lower the free energy of the system. The nanostructures eventually obtained are a result of a balance in terms of these interactions.⁵¹ A range of parameters are reported to affect the core crystallinity and the resultant self-assembly morphologies such as core/coronal block ratio, solvent properties, temperature, etc.

Figure 1.7 Chain-folding model of crystalline-coil diblock copolymers with low and high folding number.

Higher chain folding number Lower chain folding number

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1.6.1 Copolymer composition

Cao et al. prepared a series of poly(isoprene)-block-PFDMS (PIP-b-PFDMS) crystalline-coil block copolymers with various block ratios. They found that block copolymers with larger corona/core ratios favoured the formation of cylindrical morphologies whereas the shorter counterparts resulted in the formation of lamellae (Figure 1.8). The soluble corona is stretched to a greater degree in a lamellar structure than in the cylinders and thus polymers with long corona chains prefer to assemble into cylindrical micelles to reduce the energy cost for the corona stretching.⁵²

Figure 1.8 Transmission electron micrographs of the aggregates formed by A) PIP320 -b-PFDMS53, and B) PIP30-b-PFDMS60 in a mixed solvent (Tetrahydrofuran and hexane v/v 2:8).⁵²

Although there are many examples that support this trend,^53-55 Inam et al. remove the contradictory results in their PLLA-b-poly(N,N-dimethylacryamide) (PLLA-b-PDMA) block copolymer system in which diamond-shaped platelets were formed for large corona–core ratios, whilst more elongated and ill-defined structures were formed for smaller block ratios (Figure 1.9). This was rationalised by the polymer solubility i.e. polymers with short corona lengths become less soluble which prevent the PLLA chain from adopting a preferred crystal conformation (kinetically trapped) leading to ill-defined crystal structures (cylinders instead of platelets).⁵⁶ The overall relationship

A) B)

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between the block ratio and resultant self-assembly morphology appears to be complex, and thus further studies and insight are required to understand these systems.

Figure 1.9 Transmission electron micrographs of the aggregates formed by A) PDMA600-b-PLLA48 and B) PDMA150-b-PLLA48. Samples were self-assembled in ethanol at 90 ℃ for 8 h and cooled to room temperature. Scale bar = 1 μm.⁵⁶

1.6.2 Solvent condition

Similar to the process of culturing a single crystal, the self-assembly conditions for crystalline-coil block copolymers is chosen to be neither too soluble, in which case the polymer could stabilise as a single polymer chain, nor insoluble to prevent polymer precipitation. The intermediate solubility not only triggers aggregation of the crystalline core-forming block but also enables the polymer chain to pack into a favourable crystal lattice. Therefore, the solvent properties have a significant impact upon the CDSA process. Shen et al. assembled PFDMS-b-poly(2-vinylpyridine) (PFDMS-b-P2VP) diblock copolymers in different alcoholic solvents (methanol, ethanol, 2-propanol). The best solvent according to solubility parameter arguments was 2-propanol which triggered the nucleation and growth of the polymers within a day, whereas the worst solvent methanol showed no evidence of nucleation after aging for one week.⁵⁷ Following this work, Hsiao et.al reported the use of a mixed solvent system, tetrahydrofuran (THF) and isopropanol (i-PrOH), to self-assemble polymers

A) B)

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PFDMS-b-P2VP diblock copolymers. By increasing the solvent ratio of the non-selective good solvent (THF) over the non-selective solvent (i-PrOH), the self-assembly morphology transformed from amorphous spheres into crystalline cylinders and eventually to lenticular platelets. It was proposed that the non-selective good solvent THF acted as a “plasticiser” to increase the solubility of the crystallisable core block (PFDMS) and therefore facilitate crystallisation.⁵⁸ These results were further supported by Schmalz et al. for their polyethylene (PE) based triblock copolymers system.⁵⁹ It was found that the polymers favoured self-assembling into spherical micelles in a poor solvent (toluene) whereas in THF, a good solvent for molten PE, worm-like morphologies were formed.

1.6.3 Presence of additives

Xu et al. carried out a series of investigations to explore the morphological transformation of crystalline micelles by adding additives to the self-assembly solution. For example, PCL-b-poly((2-dimethylamino)ethyl methacrylate) (PCL-b-PDMAEMA) diblock copolymers were reported to form platelet micelles via a CDSA approach and by adding a small amount of organic solvent (0.74% v/v n-hexanol) to the self-assembly solution (water), the lamellar structure disassembled into cylinders.

It was proposed that n-hexanol was able to interrupt the PCL crystal lattice by introducing H-bonding interactions to the crystalline units.⁶⁰ Meanwhile, by adding a phenol to PCL-b-PEO,⁶⁵ or monoamine to PE-block-poly(acrylic acid) (PE-b-PAA) diblock copolymers,⁶¹ the coronal repulsion in the cylindrical micelles could be reversibly tuned which consequently facilitated the morphological transformation.

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Other than that, semicrystalline homopolymers have been found to play a significant role in the CDSA process. Compared to block copolymer chains, which experience restrictions as a consequence of their covalent attachment to an incompatible polymer block, homopolymers crystallise more effectively which normally result in single-crystalline platelets.⁶² Therefore, blending homopolymers into the CDSA of block copolymers provides an effective tool for developing the original 1D structure into 2D micelles. For instance, it was found that for a PEO-b-PCL diblock copolymer system in water, co-assembly with a PCL homopolymer can induce a series of morphological changes which mainly evolve the micelles from rods to lamellae by increasing the amount of homopolymer blended into the solution (Figure 1.10).⁶³ Based upon the same concept, there are several studies in which mixed unimers of homopolymer and block copolymers were utilised to transform crystal nuclei into 2D aggregates.^64-67 Interestingly, the added homopolymer can even act as a glue to connect already formed crystalline cylinders to form a micelle network.⁶⁸

Figure 1.10 Morphological transformations of PEO45-b-PCL24 diblock copolymers induced by incorporation of a PCL10 homopolymer with different varying homopolymer content. Scale bar = 2μm.⁶³

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