Figure 3-1 displays the SAXS profiles of neat PS-b-P4VP, PS-b-P4VP containing 3.3 vol% CdSe NPs, and PS-b-P4VP incorporating 5.3 vol% Au NPs. The SAXS profile of the PS-b-P4VP block copolymer reveals periodic scattering peaks located at values of Q of 0.0143, 0.0286, 0.0432, 0.0575, and 0.0712 Å–1, representing a lamellar structure. The lamellar spacing (43.9 nm) was extracted from the primary peak of the SAXS profile using Bragg law (2π/Q). For PS-b-P4VP incorporating 3.3 vol% CdSe NPs, the SAXS profile exhibits a similar lamellar ordering with the primary peak at a value of Q of 0.0172 Å–1, indicating a lamellar spacing of 36.5 nm.
The reduction in the lamellar spacing of 7.4 nm (i.e., from 43.9 to 36.5 nm) relative to that of the neat state (Figure 3-1b) may have been caused by compression of the P4VP chains (Figure 3-1c). After the nitrogen atoms of the P4VP blocks bonded to the CdSe NPs, the P4VP chains possessed constrained conformations. For PS-b-P4VP incorporating 5.3 vol% Au NPs, the SAXS profile after deconvolution reveals a lamellar spacing of 44.9 nm (calculated from scattering peaks at values of Q of 0.0140, 0.0281, and 0.0423 Å–1), slightly larger than that of pure PS-b-P4VP. This increase arose presumably because of the weak interactions between the Au NPs and the PS chains, resulting in the Au NPs being located selectively within the PS lamellae (Figure 3-1d). TEM images of the binary composites reveal clearly that the polar
CdSe-Py QDs resided in the P4VP blocks (Figure 3-2a), whereas the hydrophobic Au NPs were dispersed in the PS phase (Figure 3-2b).
Figure 3-3 displays a series of 2-D SAXS images of the copolymer/NP composites incorporated with various amounts of CdSe and Au NPs. The lamellar rings observed for the composite of a low CdSe NPs concentration of 0.2 vol%, indicate that the P4VP/CdSe domains are still dominated by P4VP chains, and prefer to phase-separate from the PS phase for the regular lamellar structure of the neat copolymer, regardless the Au NP concentration (1.3-5.3%). As the loadings of NPs increased to 0.3%-0.8% for CdSe NPs and 2.6%-5.3% for Au NPs, reflections other than that of the lamellar structure start to appear, indicating the formation of a new ordered phase; the lamellar structure still contributes to the SAXS patterns. As the CdSe NP concentration reaches 1.7 vol%, the point-like reflections of the new phase of a single-crystal-like structure replace nearly fully the lamellar powder rings, and the reflection spots are sharpened and intensified upon the increase of the Au-NP concentration from 1.3 to 5.3%. As the CdSe NP concentration is up to 2.4 or 3.3 vol%, the isotropic ring patterns present the lamellar structure, regardless of the Au-NP concentration. The phenomena can be explained in the following. The actual formation of the final morphology of PS-b-P4VP/CdSe/Au composites involves a two-stage process and depends on both the volume fraction of CdSe and Au NPs.
The first stage involves the formation of binary PS-b-P4VP/CdSe composites, in which regardless of the CdSe NP concentrations from 0.2 vol% to 3.3 vol%, the PS-b-P4VP/CdSe binary composites formed with a lamellar structure after thermal annealing. However, the density of the P4VP–CdSe NP binding increases with the loading of CdSe NPs and the edge-to-edge interparticle distance between CdSe NPs decreases from 14.1 nm at 0.2 vol% CdSe NPs to 3.5 nm at 3.3 vol% CdSe NPs as listed in Table 3-1 (see Appendix 2 for the calculating details). Then, the second stage
consists of using THF to dissolve both PS-b-P4VP/CdSe composites and dodecanethiol-coated Au NPs and subsequent CH2Cl2 solvent annealing. For a low loading with CdSe NPs, such as 0.2 and 0.8 vol%, the degree of P4VP–CdSe binding was not high enough (i.e. P4VP chains are mobile.) to prevent P4VP/CdSe lamellae from being dissolved in THF to form a micellar structure that consists of a P4VP/CdSe core and a PS corona. Consequently, the PS-b-P4VP/CdSe/Au solution with a low density of P4VP–CdSe NP binding could coalesce into P4VP/CdSe lamellae and PS/Au lamellae during CH2Cl2 solvent annealing. As the loading of CdSe NPs increases to 1.7 vol%, the density of P4VP–CdSe NP binding may reach a state that P4VP/CdSe domains have the proper density of P4VP–CdSe binding that is not too high to prevent them from being dissolved in THF. Subsequently, during CH2Cl2 solvent annealing process, owing to the presence of dodecanethiol-coated Au NPs in the PS domains, the P4VP/CdSe domains can not coalesce into lamellar.
However, at a higher loading of CdSe NPs, such as 2.4 and 3.3 vol%, the P4VP/CdSe lamellae that experienced thermal annealing, containing a higher density of P4VP–CdSe binding (a high cross-linking density), could not be redissolved in THF completely, so no further phase transition was found in the second stage. In other words, the lamellar morphology of the composites in the first stage has been retained.
Consequently, The interesting phase transition from the lamellar structure to the a single-crystal-like structure induced by the CdSe NPs and Au NPs must be crucial, as we cannot find a single-crystal-like structure in the PS-b-P4VP/Au or PS-b-P4VP/CdSe composite, nor by adding dodecanethiol molecules into the PS-b-P4VP/CdSe composite.
Figure 3-4 displays the SAXS patterns and the corresponding TEM image of the ternary PS-b-P4VP/CdSe/Au composite incorporated with 1.7 vol% CdSe NPs and 5.3 vol% Au NPs. Figures 3-4a and 3-4b reveal the single-crystal-like reflections
obtained from different sample areas of the ternary composite. Both of the reflections in Figures 3-4a and 3-4b can be grouped into three orientations of the tetragonal cell in Figure 3-4d, including reflections from the a×b plane (white indices) and reflections from the twin a×c (orange indices) and b×c (red indices) planes at a titled angle of ca. 30°. Moreover, we have extracted the large cell dimensions to be values of a and b of 48.7 nm and a value of c of 68.9 nm. Lack of diffraction patterns from other orthogonal incident directions, the tetragonal structure cannot be fully confirmed; an alternative support for the tetragonal-like cell, however, is obtained from the TEM image shown below.
Figure 3-4c displays the corresponding TEM image; the inset presents an enlarged TEM image indicating isolated P4VP/CdSe nanodomains surrounded by a continuous PS/Au phase. The sample volume (1 × 1 × 0.07 μm3) under the TEM electron beam was not sufficiently large to produce an electron diffraction pattern comparable to that of SAXS (500 × 500 × 200 μm3). A fast Fourier transform (FFT) image (Figure 3-5a) of the TEM image in Figure 3-4c revealed two pairs of diffraction patterns with a four-fold symmetry axis. Furthermore, inverse FFT images in Figure 3-5 displayed clear fourfold gridlike patterns. The EDX spectra in Figure 3-6 confirmed that the CdSe and Au NPs were located in the P4VP nanodomains and PS blocks, respectively. As a result, we observed a periodic structure for the P4VP/CdSe domains along the in-plane direction of the pristine P4VP lamellae; such in-plane ordering crossed by the neighboring PS/Au layers develops an additional ordering direction perpendicular to the in-plane direction. The two striations visible in the TEM image of the P4VP/CdSe packing (Figure 3-4c) are the [110] and [001] cell directions of the tetragonal cell, corresponding to the plane (red parallelogram) revealed in Figure 3-4d, with the characteristic spacing (34 nm) matching well with
the d-spacing of the (110) (34.4 nm) and (002) (34.4 nm) facets of the tetragonal cell suggested by SAXS.
Figure 3-7a displays an azimuthal scan of the single-crystal-like pattern in Figure 3-4b and the relative peaks of each reflection plane. The azimuthal scan also reveals relative sharp reflections having a full width at half maximum (FWHM) of ca. 2°, which corresponds to the mosaicity of the single-crystal-like structure. Moreover, Figure 3-7b displays the I(Q) profile taken along a symmetrical scattering direction (10-1) from the image in Figure 3-4b. We extracted the crystal size to be ca. 0.8 μm using the Debye–Scherrer[153] equation: D = kλ/[β cos(θ)], where D is the thickness of the crystal grain, k is the shape correction constant (normally equal to 0.95 for a spherical particle), λ is the incident X-ray wavelength, β is the FWHM of the Bragg peak, and θ is the Bragg angle. In our case, λ was 1.24 Å and β was 0.0085° (from QFWHM = 0.0015 Å–1).
Based on the SAXS and TEM results, we propose a possible route for the formation of the tetragonal cell (Scheme 3-2). During the first stage, the PS-b-P4VP/CdSe composite displays a lamellar structure of a smaller spacing than that of pure PS-b-P4VP, with CdSe-free voids at the PS/P4VP interface (corresponding to those in Figure 3-2a). The formation of these CdSe-free voids in P4VP/CdSe lamellae arose from the PS-b-P4VP/CdSe micelles combining to form a roughened PS/P4VP interface during the evaporation of pyridine and the thermal annealing. Subsequently, the PS-b-P4VP/CdSe composite and Au NPs were mixed in THF; micelles possessing a P4VP/CdSe core and a PS corona formed, but there should be no interactions between the PS-b-P4VP/CdSe micelles and the Au NPs, as evidenced from the related solution-state SAXS results which indicated no additional scattering intensity in the case of the coexistence of CdSe and Au NPs (Figure 3-8).
Finally, after removing THF and a subsequent solvent-annealing with CH2Cl2, the
PS-b-P4VP/CdSe/Au composite formed a tetragonal-like structure with P4VP/CdSe nanodomains taking the lattice cites in the PS/Au network matrix.
In general, diblock copolymers form body-centred cubic, face-centred cubic, and simple cubic ordered spherical morphologies. In contrast, the ternary PS-b-P4VP/CdSe/Au composite incorporating with appropriate CdSe NPs (~1.7 vol%) and Au NPs (1.3-5.3 vol%) can form a tetragonal-like structure, which, for a diblock copolymer, is unusual, especially in diblock copolymer/NP composites; they have been observed previously only under influences of special interactions.[154] Recent studies indicated that modification of interfacial curvature of diblock copolymers through the incorporation of liquid crystalline molecules could also lead to various novel microdomain structures, and a tetragonal packing structure could be obtained with symmetrical PS-b-P4VP upon the incorporation with dodecylbenzenesulfonic acid (DBSA).[155] In our case, a non-spherical distribution of the CdSe NPs (inset of Figure 3-2a) inside the P4VP domains might perturb the originally spherical neat P4VP domains, leading to an anisotropic curvature of the P4VP/CdSe microdomains in the composite, thus, a tetragonal micelle packing rather than the conventional BCC packing. The interactions between Au NPs and CdSe NPs might further enhance the anisotropic curvature effect for the long-ranged tetragonal-like structure.
3-4 Conclusions
We have demonstrated that pre-synthesized hydrophilic CdSe NPs and hydrophobic Au NPs can collectively self-organize in the two distinct blocks of a PS-b-P4VP diblock copolymer to form a highly ordered structure. At optimal concentrations of the CdSe NPs and Au NPs, the binding between the P4VP blocks and the CdSe NPs and the weak interactions between the PS blocks and the Au NPs led to dispersion of the two types of NPs in their respective P4VP and PS blocks and
subsequent formation of a single-crystalline-like structure comprising P4VP/CdSe nanodomains situated at the apexes of the tetragonal cell and a matrix filled with the PS/Au NP network.
Scheme 3-1. Schematic Representation of the Method of Preparation of the PS-b-P4VP/CdSe/Au Ternary Composites.
*The Au NPs dispersed throughout the continuous PS phase are not presented in the depiction of the tetragonal crystal.
Scheme 3-2. Formation of Tetragonal Crystals from PS-b-P4VP/CdSe/Au Ternary Composites.
Table 3-1. Average center-to-center (D) and edge-to-edge (d) interparticle distances. The schematic representation of the cubic lattice model for the free volume per CdSe dot in a single P4VP domain.
volume fraction
(%) D(nm) d(nm)
0.2 17.6 14.1
0.8 11.1 7.6
1.3 8.7 5.2
2.4 7.8 4.3
3.3 7.7 3.5
L PS
CdSe CdSe
d
D
Figure 3-1. (a) SAXS profiles of PS-b-P4VP, PS-b-P4VP/CdSe NPs (3.3 vol%), and PS-b-P4VP/Au NPs (5.3 vol%). (b–d) Cartoon representations of the structures of the PS-b-P4VP/NP binary composites representing the SAXS profiles in (a).
Figure 3-2. TEM images of the (a) PS-b-P4VP/3.3 vol% CdSe NPs and (b) PS-b-P4VP/5.3 vol% Au NPs binary composites. The insets to (a) and (b) display enlarged partial images.
Figure 3-3. 2D SAXS patterns in the low-Q range (q = 0.06 - 0.2 nm–1) for PS-b-P4VP/CdSe/Au ternary composites possessing various NP loadings. The contrast for each pattern is independent.
Figure 3-4. 2D SAXS patterns in low-Q range (q = 0.06 - 0.2 nm–1) for the PS-b-P4VP/CdSe/Au composite containing 1.7 vol% CdSe NPs and 5.3 vol% Au NPs. (a, b) The Miller indices are marked according to a tetragonal cell, used to index the diffraction patterns of the ab plane (white), superimposed over the tilted ac (orange) and bc (red) planes. (c) TEM images of PS-b-P4VP/1.7 vol%
CdSe NPs/5.3 vol% Au NPs ternary composites revealing the [110] and [001] cell directions of the tetragonal cell. The upper-right-hand inset displays an enlarged image. (d) Corresponding orientation for the TEM images.
Figure 3-5. (a) TEM image of the PS-b-P4VP/1.7 vol% CdSe NP/5.3 vol% Au NP composite. The inset displays the FFT of (a). (b) Reconstructed image (inverse FFT image) from the FFT image taken by the two pairs of FFT spots highlighted in the inset. (c, d) Reconstructed images taken by the single pairs of FFT spots highlighted in the respective insets.
Figure 3-6. (a) TEM image of the PS-b-P4VP/1.7 vol% CdSe NP/5.3 vol% Au NP composite. EDX spectra recorded from the (b) PS/Au and (c) P4VP/CdSe phases marked by red dashed circles in the enlarged TEM image.
0 60 120 180 240 300 360
0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0
Figure 3-7. (a) Azimuthal scanning profile of the image in Figure 3-4b (Q = 0.161 nm–1); the (100), (0-1-1), (-101), (-10-1), (0-1-1), (-100), (01-1), (01-1), (10-1), (101), and (011) reflections are found at values of Ø of 0°, 36°, 65°, 113°, 143°, 180°, 216°, 244°, 293°, and 323°, respectively. (b) SAXS profile obtained along the (101) spot in Figure 3-4b.
Q(Α-1)
0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20
Δ(Ι)
Figure 3-8. SAXS data for a control experiment employing PS-b-P4VP/CdSe/Au composites in solution. The PS-b-P4VP/NP composites incorporating 1.7 vol%
CdSe NPs and/or 5.3 vol% Au NPs.