To investigate the stability of the film in the transition plateau, hysteresis experiments were performed on all samples. In the left panel of figure 6.4 the data on PEG6-b-PnBA132 is used as a representative example. Three compression/expansion cycles were repeated without any delay at the smallest and highest area in the isotherm. With each compres-sion/expansion cycle the general slope of the isotherm stays the same but is being shifted to smaller areas. Using the characteristic area mmAtrans this shift can be quantified and is displayed in the right panel of figure 6.4 based on its relative shift from the position in the first cycle. The characteristic area mmAtrans shifts with respect to the initial area to 17−29 % after the first to 29−40 % after the second cycle. Furthermore, subse-quent compressions of the monolayer perfectly overlap with previous expansions. These observations indicate that a fraction of the polymer is permanently removed from the compressible film in the plateau. The removed amount depends on the absolute area be-ing compressed in the plateau and therefore reduces from the first to the second hysteresis cycle. Similar results have been reported in reference for PnBA films[30]. The results for the different polymers vary a little but it was not possible to observe a molecular weight trend or differences between diblocks and homopolymers.
With hysteresis experiments it is also possible to calculate the work performed by the system. The necessary work dWA to increase the surface area by a small quantity dAis given by dWA=γdA, with the work performed during one hysteresis cycle:
WA,i = Z
A
Πcompression,i(A)−Πexpansion,i(A)dA≈Πc·∆Ai (6.1) Here,Πcand∆Ai are the plateau pressure and the shifting area mmAtransas shown in the left panel of figure 6.5. There is almost no difference between the slope of the expansion and compression curve which is illustrated by the overlap of the second compression and 6.2 Stability and Macroscopic Structure in the Transition Plateau 71
P
A
Pc
A1 A2 A3
DA1
DA2
3.0 2.5 2.0 1.5 1.0 0.5 0.0 WA per missing monomer / kBT
300 250 200 150 100 50 0
NPnBA
first cycle second cycle
Figure 6.5:The left panel shows a sketch of the hysteresis cycles and all necessary values for equation (6.1) to calculated the work WAperformed by the system. The right panel displays the energy per missing monomer vs the number of PnBA monomers in the chain for the first and second hysteresis cycle.
the first expansion. Due to this the integral can be described very good by multiplying the plateau pressure with the area shift ∆Ai. The right panel of figure 6.5 shows the in the system deposited energy for each missing monomers for the first and second cycle. The number of missing polymer can be calculated from the data shown in figure 6.4.
The data shown in figure 6.5 illustrates that the energy per missing monomer ranges between 1−2kBT. In the plateau, the surface pressureΠc is equivalent to the spread-ing coefficient S reaching 0. As a consequence no additional energy is necessary to remove a PnBA monomer from the interface. The monomers excluded from the layer can either move towards the air-polymer or polymer-water interface. The surface ten-sionsγpolymer-water< γair-polymer suggest that the monomers move towards the water sub-phase[30]. This being the case one has to consider the hydrophobic interaction in order to understand how much energy is necessary to move a monomer into the subphase.
Data on the hydrophobic interaction of nBA was not available so that its side chain n-butyl will be used to estimate the energy necessary for one molecule to be moved into the water subphase. Due to the hydrophobic interaction, the energy necessary to move one n-butane molecule from pure butane into water is around 10kBT. However, 85 % of the contribution to the energy arise from a decrease in entropy because the water molecules need to arrange a cage around butane[21]. A thermodynamic model consid-ering the entropic penalty of confining polymer chains to a monolayer suggest that the excess polymer should form a single large domain in order to minimize the large surface at the water-polymer interface[30]. However, the mechanism is not only determined by ther-modynamics but also by hydrodynamics of the water and the film. As a consequence, it is likely that the polymer is locally trapped forming globular domains as observed in BAM imaging. For the hydrophobic interaction this also means that the decrease in entropy is dramatically reduced. Taking into account that its contribution is 85% of the 10kBT, the energy loss WAmeasured by hysteresis experiments can be explained by hydrophobic interaction between subphase and polymer.
To reveal the structure in the plateau, BAM imaging has been performed simultane-ously to the hysteresis experiments. Results from mean intensity measurements have 72 6 Structure in Densely Packed PnBA Films during Compression
6
100µm
5
100µm
1
100µm
2
100µm
3
100µm
4
100µm
Figure 6.6:BAM pictures of PEG6-b-PnBA132during a compression isotherm are displayed. The position within the isotherm is given by the black number in each frame. The setup of the microscope is optimized for visualizing purposes, which means that the polarizor and analyzor allow some s-polarized intensity to expose structures in the monolayer.
been presented in chapter 5.3 for p-polarized light. They reveal that upon crossing into the plateau a fundamental change is visible in the mean intensity reflected at the inter-face. An increasing number of bright domains appear and intensity continues to increase along the plateau. The domains are a manifestation of structural changes in the optical properties of the film. Additionally to the these measurements, high-contrast images were taken of the macroscopic film structure by adding a small amount of s-polarized intensity.
A series of images taken during compression of PEG6-b-PnBA132 are displayed in Figure 6.6. In detail, small aggregates are already visible in the semi-dilute regime (image 1), 6.2 Stability and Macroscopic Structure in the Transition Plateau 73
however, their number starts to increase dramatically when the isotherm crosses the tran-sition point from image 2 to 3. Consequently, these aggregates can be associated to the small bright domains in the experiment with the p-polarized laser. Further in the plateau two different phases can be observed (image 4, 5 and 6). One of those phases consist of closely packed aggregates which can be barely distinguished from each other due to their size and distances being close to the resolution limit of the setup (lateral resolution >
2µm). The second phase consists of flat globular structures with a radius of30−60µm.
Their thickness of1−3µm can be estimated by thin-film interference pattern. The ring pattern inside of the objects corresponds to interference of the laser being reflected at the interface and the bottom of the structure (see image 5 in figure 6.6).
These results are a visual confirmation of a controlled collapse in the film. The collapse is reversible apart from the amount of missing polymer as confirmed by the hysteresis measurements. It is initiated when the surface pressure reaches the critical value Πc, which is slightly higher for the diblock polymers due to the influence of the PEG block on the surface tension of the water subphase[84,88]. Furthermore, the surface pressure in the transition plateau is equivalent to the surface tension change associated with the removal of PnBA chains from the air-water interface. This value is given by the already mentioned spreading coefficient S (S = Πc = γair-water−(γair-PnBA+γPnBA-water)). The surface pres-sure reaching the critical value Πc means S is reaching 0. The polymer is now able to dewet from the interface. This is what most likely happens upon further compression.
The PnBA chains dewet from the water surface instead of being further compressed in a uniform monolayer. A thermodynamic model considering the entropic penalty of con-fining polymer chains to a monolayer suggests that the removed polymer forms a single large domain in order to minimize the large surface at the water-polymer interface[30]. However, details of this mechanism of dewetting are not only controlled by thermody-namics but also by the hydrodythermody-namics of subphase and film. The observed coexistence of monolayer and dewetted domains is caused by the slow kinetics of structure formation.
The results indicate that the PnBA films at the air-water interface are in an initial stage of dewetting from the water interface. However, our BAM images show that although many small domains exist, larger ones are also present that seem to be able to grow with time by integrating smaller domains. We cannot exclude the possibility that these large objects in the BAM pictures represent regions of collapsed polymer already coexisting in the monolayer. Formation of such macroscopic structures have been observed for a block copolymer MEH-PPV depending on how the films were prepared[66].