Alternative Methods of Self-Assembly

The initial self-assembly study showed that the two diblock copolymers PLA₄₆-PHEMA₇₉ and PLA₁₉₇-PHEMA₅₁ and a triblock copolymer PHEMA₁₆-PLA₁₈₁-PHEMA₁₆ were all shown to form micellar structures. These three block copolymers were selected to investigate the morphogenic effects of alternative methods for aggregate preparation in order to observe different morphologies. Aggregates were prepared via three methods:

nanoprecipitation; the addition of water to the polymer dissolved in acetone, a common solvent for both blocks; and the reverse addition approach. The order of addition with respect to nanoprecipitation is important, the addition of the polymer in a water miscible solvent to a large volume of water results in an increased rate of precipitation in comparison to the reverse addition leading to the formation of smaller particles.¹⁹⁶ Aggregates were also prepared by dissolving the polymer in an alternative common solvent, DMSO which is favourable for both segments. The solvent was removed by exchange with water via dialysis over a 24 h period. The particle size data obtained by DLS and TEM for the self-assembled structures are summarized in Table 4-3.

The common solvent determines the nature of the polymer solvent interaction which influences the coil dimensions and as a result the degree of core stretching.²⁰² The polymer solvent interaction is dependent on both the solubility parameter of the polymer and solvent in addition to the dielectric constant of the solvent (Table 4-2).²⁰² From Table 4-2 it can be concluded that the interaction between PLA and the solvent is stronger in acetone than in DMSO, the closer the solubility parameter of the polymer and solvent the greater the dissolution of the polymer. Due to the polarity of PHEMA, the polymer solvent interaction is dependent on the dielectric constant therefore the strength of the PHEMA-solvent interaction is greater in DMSO than in acetone.¹⁹⁶ At the beginning of self-assembly the core during the water addition becomes more swollen when the solvent favours the core forming block.²⁰² As the core becomes more swollen the degree of core chain stretching increases which is entropically unfavourable and the penalty for high stretching leads to changes in morphology in order to reduce the free energy of micellization.¹⁵³ The degree of core chain stretching decreases in order of spherical to cylindrical to vesicle morphologies.²⁰²

88 Table 4-2. Solvent and Polymer Physical Properties^{203, 204}

Solvent/Polymer Solubility Parameter(J/cm³)^1/2 Dielectric Constant

Acetone 20.1 20.7

DMSO 29.7 46.7

Water 47.9 80.1

PLA 19.2-21.0

PHEMA 27.5

Table 4-3. Alternative Methods for the Self-Assembly of PLA-PHEMA Block copolymers-Aggregate Morphologies and Particle Size from TEM

Block 2 10-14, 39-125 Micelles, Vesicles Opaque

3 15-23 Micelles Translucent

Blue

4 63-139 Micelles, Vesicles Translucent

Blue

aAggregates of PLA-PHEMA block copolymers were prepared via several methods; (1) dialysing the block copolymer in DMSO against water, (2) the slow addition of water to the polymer dissolved in acetone, (3) the

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slow addition of the polymer in acetone to water, (4) heating aggregates prepared by method (2) at 80^oC for 2h before cooling rapidly. The polymer concentration of each solution was 1 mg/ml.

bDiameter range determined by TEM

cMorphology observed by TEM

The self-assembly of PLA-PHEMA via nanoprecipitation is evidenced by ¹H NMR spectroscopy, Figure 4-6 shows the ¹H spectrum of the block copolymer in CDCl3/MeOD and in D2O demonstrating the loss of PLA signals. The self-assembly of PLA46-PHEMA79

by nanoprecipitation (addition of water to polymer solution) yielded vesicles the target morphology and structures similar to that observed in Figure 4-2B; large aggregates (Figure 4-7B) composed of micelles which is more apparent in the enlarged TEM image (Figure 4-7C). The aggregates identified by TEM were larger than previously measured with a diameter range of 300-900 nm. Vesicles (Figure 4-7A) were observed with a broad diameter range of 39-125 nm. DLS measured a narrowly disperse population ranging between 350-404 nm (Figure 4-9A). By reversing the addition i.e. addition of polymer solution to water, micelles were formed exclusively with a diameter range of 15-23 nm measured by TEM (Figure 4-7D) which was expected under kinetic control. The TEM images shown in Figure 4-7 show a layering which is a drying effect of the dye on the grid. DLS detected larger aggregates with a diameter range of 151-171 nm (Figure 4-9B).

Dialysis-induced self-assembly of PLA₄₆-PHEMA₇₉ was shown to have little effect on the morphology of the self-assembled structures. Similarly to those observed previously the micelles appear to be interacting, forming larger aggregates (Figure 4-8), which are detected by DLS with a particle size range of 208-275 nm (Figure 4-9C). Larger aggregates were expected due to the slower mixing rates of DMSO with water, however, the result is in keeping with the dependence on the compatibility between blocks and the solvents calculated by Hildebrand solubility parameters. The self-assembly study has demonstrated the morphogenic effect of solvent; utilising a solvent which interacts strongly with the hydrophobic block had resulted in the formation of vesicles in contrast to DMSO which yielded micelles which is attributed to the greater mobility of PLA chains and swelling of the core of the micelle in acetone.

The self-assembly of PLA197-PHEMA51 via the addition of water to the polymer solution yielded clustered micelles (Figure 4-10A) which are more visible in the enlarged TEM image (Figure 4-10B). The clusters are detected by DLS but not the individual micelles (Figure 4-11A). The diameter range of the micelle clusters formed was determined by TEM as 90-126 nm which is comparable with that determined by DLS (Figure 4-11A).

With the polymer possessing a longer hydrophobic block and using acetone as the common

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solvent; vesicles were the targeted morphology however micelles formed exclusively. The overriding factor in determining the morphology of the diblock copolymers must be the length of the hydrophobic block; longer PLA chains are prone to faster precipitation leading to smaller aggregates.¹⁹⁶ Reverse addition was analyzed by DLS alone, two particle distributions were observed with a diameter range of 105-127 nm and 224-261 nm (Figure 4-11B). The morphology observed does not differ from the previous method based on DLS results although in this case micelle structures were expected due to rapid precipitation. The self-assembly of the block copolymer by dialysis resulted in the same morphology; aggregation of micelle clusters, with a diameter range of 87-91 nm (Figure 4-10C, 4-10D) which was comparable with that determined by DLS (Figure 4-11C). The frequency of micelle clusters is lower than that observed by nanoprecipitation due to the change of concentration upon dialysis. DMSO was utilised to obtain larger aggregates, however, similarly to PLA₄₆-PHEMA₇₉ micelle structures were formed. The aggregates formed from PLA197-PHEMA51 were reproducible by each method.

The triblock copolymer PHEMA16-PLA181-PHEMA16 self-assembled via the addition of the. aqueous phase to the polymer solution, to form micelle clusters (Figure 4-12A, 4-12B) similar to that of the diblock copolymer PLA197-PHEMA51. The diameter range was determined by TEM for the individual micelles as 31-43 nm and the micelle clusters as 104-138nm which was detected by DLS (Figure 4-13A). The self-assembly of PHEMA16 -PLA181-PHEMA16 via nanoprecipitation is further demonstrated by ¹H NMR spectroscopy;

Figure 4-14 shows a ¹H NMR spectrum of the block copolymer in D2O showing the absence of PLA signals. Reversing the addition for nanoprecipitation resulted in the aggregation of micelles (Figure 4-12C, 4-12D) producing a different aggregate structure from the previous self-assembled structures observed for PHEMA16-PLA181-PHEMA16. The predominant size distribution measured by DLS was 127-221 nm (Figure 4-13B), and the width of these structure as measured by TEM as 27-45 nm. The larger particle sizes (>

400 nm) observed were a result of aggregation (Figure 4-13B). The self-assembly of the triblock by dialysis was unsuccessful as mass precipitation occurred which was not observed during the self-assembly of the diblock copolymers. As stated in section 4.2.1 ABA block copolymers tend to form micelles via a U-shape structure and the self assembly of the triblock with f_HEMA of 0.24 has been shown to be reproducible for each method used for aggregate preparation. The self-assembly of the triblock copolymer has a greater dependency on the block copolymer composition than the methods used for the preparation of aggregates. The structures of di-and triblock copolymers observed by TEM were present across the grid and the inability for the individual micelles to be detected by DLS

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shows that these micellar aggregates exist and are present extensively through the solution.

The CMC’s of the PLA-PHEMA di- and triblock copolymers have yet to be determined however based on the ability to form self-assembled structures at 1 mg/ml for analysis it can be concluded that the CMC’s of these materials must be smaller than 1 mg/ml.

Figure 4-6. ¹H NMR spectra of PLA197-PHEMA51 in CDCl3/MeOD-d4 (top) and in D2O (bottom).

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Figure 4-7. TEM images obtained for negatively stained spherical aggregates formed from PLA46 -PHEMA79 by various preparation methods; (A) nanoprecipitation (water added to polymer solution), (B) nanoprecipitation (water added to polymer soln), (C) an enlarged image of (B), (D) nanoprecipitation (polymer solution added to water).

A B

C D

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Figure 4-8. TEM images for negatively stained spherical aggregates prepared by dialysis of PLA46 -PHEMA79.

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Figure 4-10. TEM images obtained for negatively stained aggregates formed from PLA197 -PHEMA51 by various preparation methods; (A) nanoprecipitation (water added to polymer solution), (B) an enlarged image of (A), (C) dialysis (DMSO against water), (D) an enlarged image of (C).

A B

C D

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Figure 4-12. TEM images obtained for negatively stained spherical aggregates formed from PHEMA16-PLA181-PHEMA16 by various preparation methods; (A) nanoprecipitation (water added to polymer solution), (B) an enlarged image of (A), (C) nanoprecipitation (polymer solution added to water), (D) an enlarged image of (C).

A B

C D

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Figure 4-14. ¹H NMR spectrum (400 MHz, D2O) of PHEMA16-PLA181-PHEMA16.