Microscopic Mechanisms of Electric Field Induced Alignment of Block Copolymer
6.2. Experimental Section 1. Synthesis
6.4.2. Mechanism of Domain Alignment
One of the most important aspects for the understanding of the reorientation behavior of block copolymer microdomains in solution is the knowledge of the underlying mechanisms contributing to the rearrangement of domains. In contrast to in-situ birefringence8,34, in-situ SANS28,35 and ex-situ SAXS36,37,38 measurements on block copolymer melts and solutions under shear, which lead to detailed insight into the respective mechanisms, so far only little is known about the microscopic processes during electric field alignment. Synchrotron-SAXS combines the advantages of birefringence (high time resolution) with the detailed and straightforward information about the microscopic order characteristic of scattering methods.
Indeed, the SAXS data indicate two distinctly different mechanisms of microdomain reorientation. At low concentrations and high temperatures (Figure 6-8A, Figure 6-11C), a
destruction of the initial peaks is followed by a built-up of scattering intensity at the final peak positions. At high concentrations and low temperatures, on the other hand, the scattering pattern merely shifts into new peak positions with only a minor temporary loss in peak intensities.
These findings point to two different underlying mechanisms responsible for microdomain reorientation in the presence of the electric field. Close to the order/disorder transition (ODT), i.e. at low concentrations and high temperatures, microdomains aligned parallel to the electric field grow at the expense of those aligned parallel to the electrodes. Intermediate orientations, however, are not observed. This behavior matches the notion of the migration of grain boundaries (Figure 6-13A), which has previously been described for microdomain alignment under shear37 and which was assumed to play a role in electric field experiments as well12,13. In this case one lamella grows at the expense of another with a significantly different orientation by motion of a tilt boundary (wall defect) between them, leading to a direct transfer of scattering intensity between widely separated azimuthal angles j. This is indeed observed in Figure 6-8A and Figure 6-11C, where we find an almost complete decrease of the peak intensities (at j = 0°, 180°) before new peaks start to evolve (at j = 90° and 270°).
A B
E E E
Figure 6-13: Schematic representation of proposed mechanisms: (A) migration of grain boundaries, (B) rotation of grains.
Further away from the ODT, i.e. for high concentrations and low temperatures, the scattering pattern seems to be preserved and merely shifts into the new orientation. This
observation points to the rotation of entire grains as an alternative orientation process. In contrast to the migration of grain boundaries, microdomain orientations intermediate between the original and the final orientations are observed. At the same time no increase in isotropic scattering is found. Nevertheless, the peak intensity decreases temporarily and is recovered only after the final orientation is reached. This decrease indicates that the grains do not rotate about the X-ray beam direction, but rather about some other axis not fulfilling the Bragg law.
We note that in contrast to mechanical shear fields, the electric field does not impose a preferred direction of domain rotation on the system. The fact that the final orientation parallel to the electric field vector is not fully reached within the experimental time frame is in agreement with the notion that the driving force for grain alignment almost vanishes as the aligned state is approached13.
0.0 0.1 0.2 0.3 0.4 0.5 0.6 10
-310
-210
-11 10
In tens ity [c ounts ]
q [nm
-1]
Figure 6-14: Scattering intensity profiles of a 37.5 wt.-% solution prior () and during () application of an electric field of E = 1 kV/mm.
We note that the observed behavior near ODT may also indicate what is typically referred to as the dissolution/reformation mechanism (“selective melting”)39,40. This mechanism would involve partial dissolution of microdomains (at the size of several microns) which are perpendicular to the external field, followed by creation of domains parallel to the electric field. We do, however, not expect electric field induced mixing of PS and PI, since we were not able to detect any shifts in the ODT induced by the electric field. In addition, no peak broadening in the q dependence is observed during the reorientation process (Figure 6-14).
We therefore tend to exclude the “melting” of entire microdomains as an important mechanism in our experiments. We note, however, that for migration of grain boundaries this
process may play a role on a molecular level, as in principle single chain motion is sufficient to stepwise change the orientation of large areas along a wall defect (“molecular scale reorientation” or “molecular scale dissolution/reformation”).
E E E
A
C D
E
0 90 180 270 360
P2 = -0.30
Intensity [a.u.]
j [°]
B
Figure 6-15: SAXS and TEM data of a bulk sample prepared from a 40 wt.-% solution of SI-80 in toluene dried under an applied electric field of 1 kV/mm. The arrows indicate the direction of the electric field vector. (A) 2D-SAXS pattern and (B) azimuthal intensity distribution at first order reflection (P2 = -0.34). The TEM pictures show a defect free domain (C) and characteristic kink band defects (D/E). The scale bar represents 400 nm.
Our mechanistic model is further corroborated by typical defects, so called “kink bands”
(Figure 6-15D) , which are characteristic of the above described mechanisms and have been identified in similar processes during shear-induced lamellar reorientation37. Figure 6-15E shows the annihilation of a kink band by rotation of the defect structure.
We note that in most of our experiments both migration of grain boundaries and grain rotation seem to contribute to microdomain reorientation. One process dominates the other depending on the degree of segregation (i.e. in a certain concentration or temperature regime).
An example for an intermediate regime is shown in Figure 6-11B, where we clearly observe the coexistence of both mechanisms. The decrease in intensity of the initial orientation is accompanied by the development of a shoulder which results in a new peak. This new signal shifts to the position of the final orientation (j = 90° and 270°) and increases at the expense of the remaining intensities at the starting orientation (j = 0° and 180°).
0 250 500 750 1000 1250 0
15 30 45 60 75
90 A
Dj
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Time [sec]
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2B
t rot
[s ec ]
h
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Figure 6-16: (A) Angular shift of rotational component at different concentrations (! = 35 wt.-%, , = 47.5 wt.-%, 7 = 50 wt.-%) at field strength of E = 1 kV/mm at T = 25 °C . The solid lines represent the least squares fits to the data yielding the rotational time constant, trot; (B) Double logarithmic plot of trot versus solution viscosity, h (trot µ h2.85 ± 0.22).
In order to separately access the kinetics of the two different microscopic reordering mechanisms, we model the azimuthal scattering intensity around j = 180° by two Gaussians, one fixed at the initial peak position and the other being allowed to shift towards the final position as a function of time. We reveal the respective peak intensities and the position Dj of the maximum of the shifting peak from least-squares fits to the experimental data. The data for the position of the shifting peak, Dj, versus time for different copolymer concentrations (not shown) can again be fitted by a single exponential yielding an effective rotational time constant, trot. With these data we can establish a relation between the (microscopic) rotational time constant, trot, and the (macroscopic) viscosity, h,of our block copolymer solutions (Table 6-1). A double logarithmic plot of trot versus h (at 1 rad/s) (Figure 6-16B) yields a straight line with a slope of 2.85 ± 0.22 indicating a power law behavior trot µ h2.85 ± 0.22.
If one considers the macroscopic viscosity dependence on the polymer concentration we find a typical behavior41 as hµ c3.8. Obviously, the microscopic viscous properties of our system differ significantly from the macroscopic viscous response. Therefore, once the challenge of modeling the relationship between trot and h has been resolved, the determination of the rotational time constant, trot, might yield valuable insight into the microscopic properties of our system, i.e. information about viscous responses on the length scale of grain sizes ranging from a few to some hundred microns.
At this point we conclude that we are able to get detailed insight into the prevailing orientation mechanisms for lamellar domain alignment in block copolymer solutions by virtue of an external electric field. With increasing segregation power (i.e. increasing concentration/decreasing temperature, cµfP, cµ 1/T) we observe a transition from orientation by migration of grain boundaries to orientation by grain rotation. Intermediate conditions indicate the simultaneous action of both processes. In addition, we are able to separate both processes by a simple fitting procedure.
The transition from grain rotation to migration of grain boundaries when approaching ODT can be explained by the fact that at high concentrations and low temperatures, i.e. in a strongly segregated system, grain boundaries are thermodynamically unfavorable. Therefore, larger grains are formed which exceed a certain critical size, so that they can be rotated effectively by the electric field, which has already been anticipated for diblock copolymer melt systems12. At low concentrations and high temperatures, i.e. in a weakly segregated system, the energetic penalty for the creation of boundary interfaces is much lower.
Furthermore, close to ODT, we also expect a high defect density and a high mobility of
defects. The vast majority of grains formed are obviously not large enough to be rotated by the electric field. Polarizing optical microscopy of block copolymer solutions of different concentration yields a broad distribution of grain sizes ranging from a few to some hundred microns (for example see Figure 6-17 which shows the evolution of birefringent domains during electric field-induced orientation of the lamellae). On the other hand, the mobility of defects such as grain boundaries (wall defects) is large, which allows the system to orient its domains parallel to the electric field by single chain based migration of grain boundaries.
A B
C D
E
Figure 6-17: Polarizing optical micrographs of a 40 wt.-% solution of SI-80 in toluene between two ITO covered glass slides (d = 2mm) at 4 kV. (A) prior to electric field, (B) after 1 min, (C) 2 min, (D) 3 min and (E) 5 min. The viewing direction is parallel to the electric field vector. The scale bar represents 200 mm.