Solubility behavior of amphiphilic block and random copolymers ...

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520 LAMBERMONT-THIJS ET AL. nonyl oxazoline is randomly distributed in the random copolymer chain resulting in a decreased hydrophilicity of the entire chain. Therefore, the effect on the cloud point temperature is much larger when compared with the block copolymer. 38,39 The long nonyl side group which is not dissolved in the aqueous rich solutions reduces the accessibility of the amide groups that form the hydrogen bonding with water, which are responsible for the LCST behavior. From Figure 3, it can be concluded that the cloud point is increasing when adding ethanol to the aqueous solution due to a better solvation of hydrophilic EtOx block of the copolymer as was observed for EtOx-PhOx copolymers. 31 The hydrophobic pNonOx domains are not significantly affected by adding small amounts of EtOH because pNonOx remains insoluble in these solvent mixtures. The block copolymer exhibits a cloud point up to 12 wt % EtOH and thereafter, the polymer stays in solution up to a temperature of 100 C. The random copolymers showed a small increase in cloud point up to 9 wt % EtOH although this increase is less prominent in comparison to the block copolymers. Further increasing the amount of ethanol above 9 wt % revealed a decrease in cloud point. This unexpected effect could be explained by the formation of hydration shells of water around ethanol molecules which is most prominent at low ethanol concentrations. 3 Therefore, at low EtOH concentrations, the ethanol prefers the water environment resulting in decreased polarity causing a small increase in cloud point temperature due to a better solvation of the copolymer. A further increase in the amount of ethanol seems to cause ‘‘water structure breaking,’’ resulting in an increased attraction between the ethanol and the hydrophobic part of the polymer. Therefore, a decrease in LCST temperature is observed due to the decreased interaction between the polymer and aqueous solution. A similar effect is described for longer n-alcohols (C 4 –C 6 ) at low concentrations causing a decrease in LCST for ethylene oxide– propylene oxide copolymers. 40,41 Micelle Formation of the Block Copolymers After each transmission measurement, the vials were visually inspected and some of the block copolymers showed bluish translucent solutions in specific solvent mixtures indicating the presence of micelles (squares Fig. 1, top). Those solutions were investigated by DLS to study the influence of binary solvent mixtures and the ratio of Figure 4. CONTIN histogram obtained by analysis of the DLS data for the EtOx 70 -b-NonOx 30 sample in 40/60 EtOH/water solution. The first peak in the histogram is attributed to single micelles while the second peak corresponds to clusters of micelles. hydrophilic–hydrophobic block lengths on the micellar size. As discussed previously, the micelles consist of a pNonOx core and a pEtOx corona. DLS revealed the presence of bimodal distributions of aggregates for several of the investigated samples. Most probably, the smaller distribution corresponds to single micelles and the larger population is due to clusters of micelles as it is commonly observed for poly(2-oxazoline) micelles. 42,43 In the following, the Rh of the micelles will be estimated as the value determined at the maximum of the first peak observed in the CONTIN histogram (Fig. 4). As expected, larger micelles are formed as the content of hydrophobic block is increased while keeping the DP constant. In this respect, micelles with a Rh of 22 nm are observed for the sample with 30 mol % NonOx in 60 wt % EtOH solutions, whereas micelles with a Rh of 35 nm are formed for the sample with 40 mol % NonOx in the same solvent mixture. However, more interesting is the observed increase in the micellar size with the increasing amount of ethanol. For example, the Rh of the micelles formed by the sample with 30 mol % NonOx increased from 18 nm to 22 nm when the EtOH content increased from 40 wt % to 60 wt %, respectively. Further increasing the amount of ethanol to 80 wt % EtOH led to a Rh of 2.4 nm, indicating single chains in solution. Jordan and coworkers investigated the inner structure of MeOx-NonOx block copolymers in aqueous solution using SANS and concluded that pNonOx blocks are stretched from Journal of Polymer Science: Part A: Polymer Chemistry DOI 10.1002/pola
AMPHIPHILIC BLOCK AND RANDOM COPOLYMERS 521 Figure 5. Schematic representation of the investigated micelles demonstrating the effect of changing the amount of ethanol. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.] the surface of the core to the center, which is most likely caused by the large nonyl side groups. 44 The presence of stretched pNonOx chains in the core implies that changing the amount of ethanol will probably have a negligible effect on the inner core because pNonOx chains are not soluble in these binary solvent mixtures. Therefore, the increase in micellar size with increasing amounts of ethanol is most likely related to the pEtOx corona. The pEtOx chains are better soluble in EtOH than in water and, therefore, it is proposed that the pEtOx will be better solvated with the increasing amounts of ethanol causing a stretching of the chains, and thus, the micellar size will grow by expansion of the corona as depicted in Figure 5. As already stated above, the solutions that were translucent were initially identified as micelle containing solution. To determine if the dispersions as marked in the solubility phase diagram are not containing large micelles they were also measured by DLS. However, these samples appeared to be too milky to be measured by DLS or were clearly suspensions (big particles visible). In addition, the clear solutions that were close to translucent samples in the phase diagram indeed did not contain the micelles as evidenced by DLS. CONCLUSIONS Journal of Polymer Science: Part A: Polymer Chemistry DOI 10.1002/pola In this contribution, we compared the solution properties of random and block pEtOx-pNonOx copolymers in binary solvent mixtures ranging from pure water to pure ethanol. In the solubility phase diagram, different regions could be identified from insoluble copolymers or copolymers that formed stable dispersions to regions where the copolymer is soluble or solubilizes at elevated temperatures. The large difference in the solubilization temperatures between the random and block copolymers could be explained by the reduced crystallinity in the random copolymers. Ethanol can, therefore, easily penetrate into the random copolymers that are dissolved at lower temperatures in comparison to the block copolymers. The copolymers containing 10 mol % pNonOx (random and block) showed a LCST behavior in aqueous solution. The significant difference in LCST between the random and block copolymer could be explained by the formation of hydrophobic microdomains in the block copolymer. The formation of these microdomains causes a smaller effect on the LCST in comparison to the random distribution of the nonyl side chains through the polymer chain. The cloud points were further investigated by increasing the amount of ethanol in small steps up to 24 wt % EtOH. We could observe an increase in LCST for the block copolymers up to a solvent mixture of 12 wt % EtOH after which the polymer stayed in solution up to 100 C. The random copolymers displayed a small increase in LCST up to a solvent mixture of 9 wt % EtOH, further increase of the ethanol content led to a decrease in LCST temperature which is probably due to the ‘‘water-breaking’’ effect causing an increased attraction between ethanol and the hydrophobic part of the copolymer resulting in a decreased LCST temperature. Furthermore, the EtOx-NonOx block copolymers revealed the formation of micelles in specific binary solvent mixtures. All micellar sizes were investigated by DLS showing that with increasing ethanol concentration the micellar size is increasing. This phenomenon is thought to be related to the enhanced solubility of EtOx in EtOH which causes expansion and less coiling of the EtOx chains.
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