Precise Design of Nitrogen-containing Heterocyclic Copolymers Study of their Self-assembly Behavior for the Application of Heavy Metals Adsorption Sensing
- Publication Date: 2020-04-03
| Execution Methods | In this study, synthesis of poly(styrene)-b-poly(2-vinyl pyridine) four-arm star block copolymers ((PS-b-P2VP)4 sBCPs) possessing high interaction parameter (χ) feature was investigated. To obtain well-defined sBCPs, a challenge on ATRP of pyridine-containing monomers is foreseeable. We thus scrutinized the domino effect from PS macroinitiators (MIs) having different halogen chain ends (i.e., (PS–X)4, where X = Br/Cl) to the subsequent chain extensions. As evident from the model reactions of benzyl halides (BzX, where X = Br/Cl) and 2-methylpyridine (2MP), the combination of BzCl and 2 MP can significantly suppress the substitution side reaction. Consequently, well-defined (PS-b-P2VP)4 sBCPs were obtained (Mn = ca. 58 k–82 k with PDI <1.5). From the analyses of SAXS, TEM, and AFM, self-assembly behaviors of the sBCPs were observed due to the high χ feature, including cylinder and lamellae morphologies (ca. 30 nm). |
| Performance Evaluation | We attempt to synthesize well-defined PS-b-P2VP sBCPs via ATRP technique which is still quite challenge. The synthesis of PS-b-P2VP sBCPs via ATRP techniques is still limited. Beneficial from ATRP systems, we can facilely attain multi-functional initiators. A tetra-functional initiator (i.e., 4f-BiB) was synthesized. Similar initiating sites (i.e., alkyl 2-bromopropionate) to achieve well control of ATRP of styrene (S) were reported. We thus carry out normal ATRP of S with 4f-BiB and different copper catalysts (I.e., CuBr and CuCl). We investigated the domino effect from the obtaining PS macroinitiators (MIs) having different chain ends (i.e., (PS–X)4; X = Br or Cl) to the subsequent chain extensions. We thus scrutinized the challenges on chain extensions of (PS–X)4 MIs with 2-vinyl pyridine (2VP) to attain well-defined (PS-b-P2VP)4 sBCPs. The self-assembly behaviors of (PS-b-P2VP)4 sBCPs in the solid-state were analyzed by small angle X-Ray scattering (SAXS), transmission electron microscopy (TEM), and atomic force microscope (AFM). In Fig. 1, 1H NMR (400 MHz, CDCl3) spectra of the obtaining (co) polymers were demonstrated. Typical PS signals were shown in Fig. 1A spectrum, including peaks a–d. Fig. 1B and C displayed (PS-b-P2VP) star copolymers through chain extension from (PS–Br)4 and (PS–Cl)4 MIs, respectively. Comparing the spectra, notable P2VP signals were both observed (i.e., peaks e and f), indicating insignificantly chemical structure variations of the copolymers with bi-modal and mono-modal distributions. Based on Mn of the first PS block and estimated from the area ratio of f (ca. 7.25 ppm) and c–e (ca. 6.1–7.25 ppm), we can obtain the m and n repeating units in each block (i.e., obtaining (PSm-b-P2VPn)4 sBCPs). Next, we attempt to study the formation of well-ordered nanostructures from the self-assembly of (Sm2VPn)4 sBCPs in the bulk state. The samples were thermally treated to eliminate their thermal histories before one-dimensional SAXS and TEM identification. As mentioned above, the Tgs of the (Sm2VPn)4 sBCPs based on the DSC measurements were in a range of 100–111 ◦C. For (S892VP42)4 (ΦPS = 70%), the SAXS result (Fig. 2a) shows a blurred scattering profile without definite reflection peaks, indicating a disordered structure. The microphase separation structures can be further examined in TEM images after ruthenium tetroxide (RuO4) staining as shown in Fig. 3a. Notably, the RuO4 complexed with the double bonds in the PS blocks, rendering the PS microdomains in dark regions due to the mass-thickness contrast. By contrast, the bright regions can be referred to the P2VP microdomains under TEM observation. Similar to the SAXS results, the corresponding TEM image further reveals dark spherical domains dispersed in a bright matrix without definite self-assembled morphologies. The observed disordering of self-assembled nanostructures should be attributed to the large PDI value of the (S892VP42)4, leading to insufficient self-assembly tendency.  We then examined the other sBCP samples with small PDI values (i. e., PDI < 1.5). In the (S892VP65)4 (ΦPS = 60%) sample, we acquired a well-ordered cylindrical phase with hexagonal packing reflected in the 1D SAXS profile (i.e., Fig. 2b) with the q ratios of 1: √3: √4: √7. Using the primary peak, the d-spacing of the (100) plane was determined as 34.4 nm and the interspacing of each cylinder was then calculated as 39.8 nm. Bright cylinder-forming domains of the P2VP part were well-dispersed in a dark matrix of the PS part as shown in TEM images (i.e., Fig. 3b). In the (S892VP105)4 (ΦPS = 48%) sample, the self-assembled morphology was identified as a lamellar phase according to the reflection peaks at the q ratios of 1: 2: 3 shown in Fig. 2c. The d-spacing of the (100) plane was determined as 33.2 nm from the primary peak of the reflections. The corresponding TEM image (i.e., Fig. 3c) showed alternative dark and bright sheets, confirming the lamellar nanostructure of the (S892VP105)4 (ΦPS = 0.48) from the self-assembly in the melt state. In the samples of (S1002VP41)4 (ΦPS = 0.72) and (S1002VP60)4 (ΦPS = 0.64), because of the larger PS volume fractions, well-ordered cylindrical phases were both observed. Fig. 2d and e demonstrated the corresponding 1D SAXS profiles while Fig. 3d and e represented the corresponding TEM images. Notably, obvious black and bright sheets can be observed as shown in Fig. 3c (such as the point of the white arrow). Nano-sheet structures are critical features of lamellar phase from polymer self-assembly. TEM image of (S892VP105)4 (ΦPS = 0.48) sample was thus identified as a lamellar nanostructure. We also examined the spin-coating films of the sBCPs to analyze their self-assembly behaviors. As shown in Fig. 3a’–3e′ , the tapping mode atomic force microscopy (AFM) phase images clearly displayed the corresponding nanostructures. Notably, the darker areas in the phase image are attributed to the stiff PS domains and the brighter areas belong to the soft P2VP domains. We thus attained disorder (i.e., Fig. 3a’) and cylinders (i.e., Fig. 3b’, 3d’, and 3e’) morphologies. In the sample of (S892VP105)4 thin films (Fig. 3c’), distinctively, we observed minor dark PS domains dispersed in the brighter P2VP wetting matrix. It is plausibly the affinity effect by which air exhibits preferential selectivity toward the P2VP blocks, resulting in the formation of a P2VP wetting layer.  |
| Conclusion & Suggestion | We synthesized well-defined (PS–X)4 MIs (PDI < 1.4) with various chain ends (X = Br or Cl) via normal ATRP. Then a significant substitution side reaction was revealed from the model reactions of BzX and 2MP, illustrating the importance of the chain end to suppress the subsequent chain extensions of (PS–X)4 MIs with 2VP. A s a result, well-defined four-arm (PS-b-P2VP)4 sBCPs (Mn = ca. 58 k–82 k with PDI < 1.5) can be synthesized by using AGET ATRP in proper conditions. From the analyses of SAXS, TEM, and AFM, self-assembly behaviors of the sBCPs driven from the highly incompatible of the P2VP and PS blocks were revealed. Nanostructures, mainly including cylinder and lamellae morphologies were attained (ca. 30 nm). |
| Appendix |