4.3 Results
4.3.2 SGLCP-b-PS Structure and Self-Assembly
The minor radii of both end-on and side-on polymers in the nematic phase are seen to be slightly smaller than their isotropic radii, consistent with stretching of the polymer along the other axis. The scaling exponents obtained are consistent with behavior intermediate between a Gaussian chain (ν = 0.5) and an excluded volume polymer (ν = 0.6) with the exception of BB880 (ν=0.77 perpendicular to the director). This anomalously high value may be related to the segment density profile in the direction of the very narrow dimension (6.8nm) of the coil, as this phenomenon is not observed for the longer BB2000.
Figure 4.5: Two dimensional SANS patterns for SGLCP-b-PS block copolymers at 1wt% in nematic d5CB at T = Tni - 7℃ taken on beamline NG-3 at a sample-to-detector distance of 11 meters . Note that a smaller beam stop (2”) was used for BB900-PS1150 rather than the 3” used for all other samples.
Above the Tni the scattering patterns become isotropic with dramatically lower intensity, consistent with both blocks becoming solvated in isotropic d5CB [20].
Figure 4.6: Sector averages parallel and perpendicular to the director taken from 2d SANS patterns for 1wt% solutions of SGLCPs in d5CB in the nematic phase. The end-on polymers are shown on the left hand side and side-on polymers on the right with the top images showing the sector parallel to the director and the bottom images showing the sector perpendicular to it. The homopolymer data (black circles) is the same as shown in Figure 4.3.
Sector averages of the scattering patterns parallel and perpendicular to the nematic director (Figure 4.6) show the multiple length scales of anisotropy quite clearly.
In the end-on polymers (Figure 4.6 left) the scattering parallel to the director overlaps
with the scattering from a homopolymer at high q (q>0.3Å-1) and then increases in intensity rapidly at mid q, beginning to plateau at the lowest available q values. The sector average perpendicular to the director also overlaps with the scattering from a homopolymer at high q (q>0.3Å-1), but unlike Ipar, the increase of intensity with decreasing q shows two distinct features. The scattering feature that appears at mid q (0.01Å-1 ≤ q ≤ 0.03Å-1) becomes progressively more intense with increasing polystyrene block length. The side-on polymers (Figure 4.6 right) show interesting similarities and differences with respect to the end-on polymers.
Qualitatively, the shape of Iperp(q) for BB SGLCP-b-PS resembles that of Ipar(q) for their CB SGLCP-b-PS counterparts. There is a suggestion at low-q that a second feature may be present in Ipar(q) of BB SGLCP-b-PS like the “double hump” character of Iperp(q) for their CB SGLCP-b-PS counterparts. At high q, the BB SGLCP-b-PS inherit from the BB SGLCP homopolymers a much steeper q-dependence for Ipar(q) than for Iperp(q), hence much weaker high-q scattering in the parallel sector than in the perpendicular one. Indeed, at high q, Ipar(q) for BB SGLCP-b-PS overlaps with the scattering from a BB SGLCP homopolymer. However, unlike the case for CB SGLCP-b- PS, the magnitude of the high-q scattering in the sector perpendicular to the director does not overlap that of the homopolymer (compare lower right of Figures 4.6 and 4.3): in the case of the shorter PS blocks, Iperp(q) at high q is greater than expected based on the homopolymer, and in the case of the longer PS blocks it is less than Iperp(q) at high q of the homopolymer.
Above the Tni both BB and CB SGLCP-b-PS polymers have simple isotropic scattering patterns that resemble those of the corresponding homopolymer (i.e., an
excluded volume polymer) and vary weakly with polystyrene block length (Figure 4.7).
This indicates that the solution is sufficiently dilute to have the Microphase Separation Temperature (MST) very near the Tni for all of the present block lengths, consistent with earlier results that found a much greater concentration (>5wt%) is required for segregation to persist above Tni[22]. Consistent with having similar SGLCP backbone lengths (DP 1075±175, corresponding to 480kg/mol ± 40kg/mol for CB and 620kg/mol ± 60kg/mol fpr BB SGLCP-b-PS) and a small PS fraction (Table 4.1), there is only moderate variation in the isotropic-phase scattering patterns for the different molecular weights, indicating similar Porod exponents and overall coil sizes (
Table 4.4). The mid-q intensities for isotropic solutions of BB900-PS1150 and BB1100-PS800 overlap well but both are lower than those for BB1250-PS400 or BB1050-PS550; although this difference is much less than that observed in the nematic scattering perpendicular to the director, it is qualitatively similar.
Figure 4.7: 1D scattering intensity for end-on (left) and side-on (right) SGLCP-b-PS polymers at 1wt% in d5CB above the Tni. Scattering for the corresponding homopolymer is shown for comparison.
The scaling exponents for all of the block copolymers are very similar for both mesogen types and all polystyrene block lengths and are all ~0.6, which is expected of an excluded volume polymer. This is somewhat unexpected for the side-on polymers given the more stretched exponent observed for the homopolymer. The observed Rg values appear to be essentially independent of the polystyrene block length for the end-on polymers, which is consistent with the very similar overall molecular weights for CB1250-PS400,CB1050-PS550,CB1100-PS800, and CB900-PS1150 (Table 4.1) which are 480kg/mol ± 40kg/mol. For the side-on polymers the variation in size is much more pronounced, with BB1100-PS800 and BB900-PS1150 having significantly larger radii than BB1250-PS400 and BB1050-PS550, although their molecular weights are not any more varied at 620kg/mol ± 60kg/mol. It is interesting to note that the division between small and large side-on diblocks corresponds to the intensity groupings observed in the 1D scattering patterns.
Table 4.4: Sizes obtained by fitting an excluded volume polymer form factor the I(q) data for both End-On and Side-On polymers in the isotropic phase (Tni+5℃). Data for CB1100-PS800 was not available.
Transmission Electron Micrographs of unstained dilute solutions of SGLCP-b-PS in 5CB show roughly circular objects with considerable polydispersity for both end-on (Figure 4.8,left) and side-on (Figure 4.8,right) block copolymers.
Figure 4.8: Transmission Electron Microscopy images of solutions of SGLCP-b-PS polymers in 5CB. The four images on the left are of end-on polymers while the two on the right are of the corresponding side-on polymers. The top images are for polystyrene block length of 550 repeat units and the bottom are for the longest PS block length of 1150 repeat units. Stained images were obtained using RuO4 as the stain.
The focus and contrast were adequate for image analysis, simplified by the absence of any staining artifacts. The mean and standard deviation of the diameters of the objects seen in unstained TEM range from approximately 30-55nm and appear to increase with increasing PS length and are similar for end-on and side-on counterparts (Table 4.5).
Table 4.5: Diameters of the objects seen in TEM for SGLCP-b-PS in 5CB. These were obtained by using image J to quantitatively characterize a number of objects from each image to obtain average sizes.
Attempts to stain solutions of side-on polymers were unsuccessful and it was not possible to obtain usable images for the unstained samples without a listed result.
TEM of end-on polymer samples stained with ruthenium tetroxide (RuO4) show objects that are larger than those seen in the corresponding unstained images. Some of the objects seen in the stained images have lighter inner regions surrounded by a darker annulus. This is consistent with greater extent of staining in the SGLCP corona and thus the images are consistent with micelles composed of a polystyrene-rich core surrounded by a diffuse, SGLCP-rich corona. The stained TEM, for both CB1100-PS800 and CB900-PS1150 show inner diameters that agree with the diameter of the features seen in unstained TEM Table 4.5. The outer diameters are much larger, beyond the range of our SANS measurement: data were limited to q>0.005Å-1, corresponding to structures smaller than 60nm. Interestingly, for each pair of end-on and side-on block copolymers with the same polystyrene block length (available for PS550 and PS1150), the core sizes determined by TEM are indistinguishable. This is consistent with the similarity in the q position of the upturns seen in the sector averages Ipar(q) for the CB SGLCP-b-PS and Iperp(q) for its BB SGLCP-b-PS counterpart (Figure 4.6).