Polydimethylsiloxane-assisted alignment transition from perpendicular to parallel of cylindrical microdomains in block copolymer films

Abstract

The orientation transition from perpendicular to parallel alignment of PEO cylindrical microdomains within PEO-b-PMA(Az) films has been demonstrated via introducing a small amount of polydimethylsiloxane (PDMS) into block copolymers (BCPs). The introduced PDMS can change the surface energy difference of the two blocks and induce a soft shearing force during the annealing process, which affects the alignment of PEO cylindrical domains synergically. With this understanding, the PDMS-assisted method can possibly be generalized to align a wide variety of other BCPs. The BCP films with controllable alignment can be used as lithographic nanotemplates to prepare nanowire arrays, which have enormous potential application in the field of integrated circuits.

---

Introduction

The self-assembly of block copolymers (BCPs), which have two chemically dissimilar polymers and spontaneously phase-separate into periodic microdomains, are potential templates for the next generation nanolithography.^1–6 Some applications, such as integrated circuits, bit-patterned media,⁷ nanoporous membranes usually desire defined lamellar or cylindrical domains with parallel or perpendicular orientation on the substrate. Graphoepitaxy,^8–11 chemical prepatterning^12,13 and external fields^14–16 are often applied to achieve ordered alignment of cylindrical microdomains, however these processes always take much time and require a large capital cost in the fabrication apparatus.¹⁷ Considering of their applications, it is desired for simple and low-cost methods to control the alignment of self-assembled microdomains between perpendicular and parallel patterns. In general, the orientation of cylindrical microdomains within the thin films of BCP depends on the difference of interfacial energies between both blocks and the substrate, and the surface energies of the two blocks. The larger difference of interfacial energies or surface energies will bring about the perpendicular orientation of cylindrical morphology, while the lower one will induce the parallel structure.³ In most systems, some methods for orientation transition have been realized by adjusting the interfacial energies, such as modifying substrates by random copolymer brushes¹⁸ or coating a polymer layer, such as polyvinyl alcohol (PVA)¹⁹ and polydimethylsiloxane (PDMS),²⁰ on the top surface of BCP films. Polydimethylsiloxane (PDMS), well-known for ultra-low surface tension of 20.4 mN m⁻¹ at 25 °C,²¹ is always used in the top-coating methods.^22–26 Bryan D. Vogt et al. have achieved alignment transition from lamellar morphology to unidirectional alignment of the cylindrical domains of a series of BCPs via the PDMS top-coating technology, such as polystyrene-block-polyisoprene-block-polystyrene (SIS), polystyrene-block-polydimethylsiloxane (PS-b-PDMS), and polystyrene-block-poly(2-vinlypyridine) (PS-b-P2VP).^25,26 However, the method by adjusting surface energies is rarely reported.

Previously, we studied a designed amphiphilic liquid crystalline (LC) diblock copolymers PEO-b-PMA(Az),^27–30 consisting of ploy(ethylene oxide) and poly(methacrylate)-bearing an azobenzene (Az) segment in the side chain, synthesized by atom transfer radical polymerization (ATRP), as shown in Fig. 1(a). Perpendicularly aligned PEO cylindrical domains surrounded by PMA(Az) liquid crystalline phase can be obtained easily by spin-coating appropriate weight percent toluene or chloroform solutions on different substrates, for instance, silicon wafer, mica, and poly(ethylene terephthalate), followed by thermo-annealing at 140 °C in vacuum for several hours³¹ (i.e., without any processes such as surface modification or external fields treatment). In addition, PEO-b-PMA(Az) films with parallel oriented PEO cylindrical microdomains were realized by a PDMS top-coating method,²⁰ a rubbing technique,³² and a simple micropore extrusion process.³⁰

Fig. 1 (a) Chemical structure of the PEO_m-b-PMA(Az)_n block copolymer. (b and c) TEM images of narrow and wide stripes of BCP films subjected to PDMS-assisted (M_PDMS = 13 kg mol⁻¹, 0.5 wt%) process, respectively. The insets in (b and c) show the corresponding fast Fourier transform images.

In this work, we demonstrate another simple method to transfer orientation transition of cylindrical domains from perpendicular to parallel in large scale by introducing a small amount of polydimethylsiloxane (PDMS) into the block copolymers and thermo-annealing at 140 °C in vacuum sequentially. The effects of molecular weight and weight percent of PDMS have been studied in details. A plausible mechanism is proposed in this work. In addition, we have prepared SiO₂ nanowire arrays vertical and parallel to the substrates templated from PEO-b-PMA(Az) films with different alignment of PEO cylindrical domains. The BCP films can be used as lithographic nanotemplates to prepare nanowire arrays, which have enormous potential application in the field of integrated circuits.

Results and discussion

Previously, we have obtained highly ordered hexagonal arrangement of PEO cylindrical domains in the absence of PDMS after thermo-annealing at 140 °C in vacuum.^27–30 As shown in ESI Fig. S1,† the dark dots are PEO domains stained by RuO₄, and the grey regions are attributed to PMA(Az) matrix. The average diameter of PEO domains and center-to-center distance are (6.8 ± 0.4) nm and (17.5 ± 0.6) nm, respectively. Interestingly, the alignment of PEO cylinders changed to parallel to the substrate after introduced 0.5 wt% of PDMS into BCP films, as shown in Fig. 1(b) and (c). The narrow and wide stripes are parallel PEO cylinder domains viewed from two different directions, corresponding to (10) and (11) planes of hexagonal alignment, respectively, as illustrated in Fig. 2(a). The insets are the corresponding FFT image, further indicating the parallel arrangement of PEO domains. The center-to-center of PEO cylinders are (8.7 ± 0.9) nm and (15.2 ± 1.1) nm, respectively, measured from Fig. 1(b) and (c). While, the diameters of PEO domains are both around 5.7 nm, smaller than the value in Fig. S1.† The calculated periodicity of PEO cylindrical domains, P, is (17.5 ± 1.4) nm, according to the equation illustrated in Fig. 2(b), which is similar with the value of vertical alignment. It can be concluded that PDMS-assisted process had successfully transferred the alignment of PEO cylinders from perpendicular to parallel within PEO-b-PMA(Az) films.

Fig. 2 (a) Schematic representations of orientation transition from perpendicular to parallel alignment of PEO cylindrical microdomains. After introducing PDMS, perpendicular cylinders will be transited to in-plane cylinders for two viewing directions, corresponding to (10) and (11) planes of hexagonal alignment, forming narrow stripes and wide stripes, respectively. (b) Geometrical relationship of perpendicular cylinders, narrow stripes and wide stripes. P, P₁, P₂ are center-to-center distances of perpendicular cylinders, narrow stripes and wide stripes, respectively.

To investigate the mechanism of the PDMS on the alignment of PEO-b-PMA(Az) films, the effects of the molecular weight of PDMS have been discussed. Fig. 3(a)–(d) show TEM images of BCP films introduced with 0.5 wt% PDMS of different molecular weight. It is interesting to note that PEO cylinders keep vertical alignment with slightly tilt after being introduced with low molecular weight PDMS (M_PDMS = 1.25 kg mol⁻¹). The obtained d and P values are no obvious difference with those of pure PEO-b-PMA(Az) films. However, PEO cylinders become parallel to the substrate with M_PDMS increased up to 6 kg mol⁻¹ (see Fig. S2†), while the center-to-center of the stripes are not so inerratic. Fig. 1(c) and (d) shows the regular parallel aligned PEO cylindrical domains in the case of M_PDMS = 13 kg mol⁻¹. While, with M_PDMS continuously increased to 28 kg mol⁻¹, the formed stripes (Fig. 3(c) and (d)), especially wide ones, are not as standard as those shown in Fig. 1(b) and (c). In this case, the mixed toluene solution of BCP and PDMS is non-transparent and inhomogeneous, which formed visible macro-phase separation. It is apparent that the PDMS molecular weight plays an important role in the alignment of PEO cylinders within BCP films.

Fig. 3 TEM images of self-assembled PEO₁₁₄-b-PMA(Az)₃₀ after introducing process with PDMS (0.5 wt%) of different molecular weight. (a and b) 1.25 kg mol⁻¹, (c and d) 28 kg mol⁻¹.

Fig. 4 shows TEM images of BCP films after being introduced with different weight percent of PDMS. It is obvious that the alignment of PEO cylinders transferred to parallel within BCP films even if mixed with very small amount of PDMS (0.01 wt%), as shown in Fig. 4(a) and (b). The alignment of PEO cylinders has no change with increasing of PDMS content up to 1 wt% (Fig. 4(e) and (f)). From the obtained TEM images (Fig. 1(b) and (c)), it can be said that the orientation of PEO cylinders within BCP films has few defects in the case of 0.5 wt% of PDMS. The detailed diameter (d-value) and the center-to-center distance (P-value) of PEO cylindrical domains are listed in Table 1, measured from TEM images. When the PDMS content increased from 0.1 to 1 wt%, the diameter of PEO domains become slightly bigger, while the center-to-center distance has no obvious difference. The calculated periodicities of PEO cylindrical domains are (17.9 ± 1.6) nm, (17.6 ± 1.7) nm, (17.5 ± 1.4) nm, and (17.7 ± 2.5) nm, respectively, which are nearly the same within the experimental uncertainties.

Fig. 4 TEM images of self-assembled PEO₁₁₄-b-PMA(Az)₃₀ containing different weight percent PDMS (M_PDMS = 13 kg mol⁻¹). (a and b) 0.01 wt%, (c and d) 0.1 wt%, (e and f) 1 wt%.

Table 1 Stripes parameters with different weight percent of PDMS^a

a d-Value: the diameter of PEO cylindrical domains, P-value: the center-to-center distance of PEO cylindrical domains.
Weight percent of PDMS (%)	0.01	0.1	0.5	1
d-Value of narrow stripes (nm)	6.3 ± 0.8	5.7 ± 1.2	6.1 ± 0.7	6.1 ± 1.1
P1-Value of narrow stripes (nm)	8.7 ± 1.2	8.7 ± 1.1	8.7 ± 0.9	8.7 ± 1.7
d-Value of wide stripes (nm)	6.2 ± 0.7	5.8 ± 1.3	6.1 ± 1.0	6.2 ± 1.5
P2-Value of wide stripes (nm)	15.3 ± 1.0	15.2 ± 1.3	15.2 ± 1.1	15.4 ± 1.8

---

As well as the characteristics of a block copolymer, the interfacial interactions between the BCP film top surface with the air and the bottom surface with the substrate has significant effect on the self-assembly of BCP films. Numerous researches had been devoted to adjust these two interfacial interactions. Russell T. P. et al. reported that PDMS had been used to modified the silicon substrate to form hexagonally ordered PEO cylindrical domains surrounded by poly(fluorinated methyl methacrylate) matrix.³³ Moreover, as mentioned above, PDMS has been frequently employed as a top-coating layer on the BCP films to provide a soft shear force to achieve alignment transition from lamellar morphology to unidirectional alignment of the cylindrical domains of a series of BCPs.^25,26 However, most of these works have focused on the conventional amorphous–amorphous block copolymers. In this paper, PEO-b-PMA(Az), containing azobenzen mesogen side chains on the hydrophobic block, can self-assemble to form hierarchically ordered structures: micro-phase separated nanostructures and regular liquid crystalline structure. For pure PEO-b-PMA(Az), the PEO cylindrical domains perpendicularly aligned on the substrate after thermal annealing at 140 °C for a short time (such as 30 min) without any modification on the BCP film top surface or the substrate. We recently proposed that the BCP micelles' morphology on the top surface determines the final orientation of PEO cylindrical domains within PEO-b-PMA(Az) films.³⁰ The vertical alignment of PEO cylinders in PMA(Az) matrix is formed or initiated at the top surface and goes to the bottom.²⁰ To PEO-b-PMA(Az), it can be concluded that the difference in surface energies between the blocks should be very small to form normal alignment of cylindrical domains.^3,4,19 In this work, small amount of PDMS with relatively lower molecular weight was mixed with the toluene solution of PEO-b-PMA(Az) homogenously, and then the transparent mixed solution was spin-coated onto a unmodified silicon substrate. PDMS is failed to be detected by TEM mapping due to its ultra low content, making it difficult to point out the location of PDMS within BCP films. However, it is reasonable that the surface energy difference of blocks should become bigger after introduced small amount of PDMS with suitable molecular weight, which makes the parallel alignment of cylindrical domains within the PMA(Az) matrix. However, the effect of PDMS with lower molecular weight is not enough to make the orientation transition. The mechanism is still in progress in our group.

Previously, we have attained perpendicular alignment of SiO₂ nanorod arrays from PEO-b-PMA(Az) films,^28,29 as shown in Fig. 5(a). The diameter can be adjusted by block copolymers template with different volume fraction of PEO domains. Here, we demonstrate in-plane SiO₂ nanowire arrays achieved by the same technology template by this PDMS-assisted BCP film, as shown in Fig. 5(b). The hydrophilicities of SiO₂ have been investigated by automated contact angle tester, as shown in Fig. 5(c) and (d). The water contact angle of in-plane SiO₂ nanowire arrays is 33.4° compared to 74° of out-of-plane SiO₂ perpendicular cylinders, which confirms that Si wafer substrates bearing in-plane SiO₂ nanowire arrays are more hydrophilic.

Fig. 5 (a) SEM image of vertically aligned SiO₂ nanorod arrays from PEO₁₁₄-b-PMA (Az)₄₅ films. (b) SEM image of SiO₂ nanowire arrays obtained from PEO₁₁₄-b-PMA (Az)₄₅ films with PDMS-assisted process. (c, d) Water contact angles of out-of-plane SiO₂ perpendicular nanorod arrays and in-plane SiO₂ nanowire arrays, respectively.

Conclusions

In summary, PDMS-assisted orientation transition from perpendicular to parallel of PEO cylindrical microdomains surrounded by PMA(Az) segments has been demonstrated. PDMS with low surface tension changes the surface energy difference of the two domains, and induces the soft shear force during the annealing process, leading to the alignment transition of PEO cylindrical domains of BCP films. The effects of molecular weight and weight percent of PDMS have been discussed. It is important that, the BCP/PDMS solutions should be transparent and homogeneous without macro-phase separation. The method is possible to be generalized to other BCPs, which is in progress in our group. The BCP films can be used as lithographic nanotemplates to prepare nanowire arrays, which have enormous potential application in the field of integrated circuits.

Experimental section

The amphiphilic liquid crystalline diblock copolymers consisting of poly(ethylene oxide) and poly(methacrylate) bearing an azobenzene segment in the side chain, PEO_m-b-PMA(Az)_n, were synthesized via atom transfer radical polymerization (ATRP). The details in synthesis and characterization of the block copolymer were described in our previous publication.²⁷ In this work, PEO₁₁₄-b-PMA(Az)₃₀ with an average molecular weight of 19.8 kg mol⁻¹ and PEO weight fraction of 0.25 were used. PDMS (M_n = 1.25, 13 and 28 kg mol⁻¹) were purchased from Alfa Aesar. 2 wt% PEO-b-PMA(Az) block copolymers were mixed with 0.01–1 wt% PDMS in toluene and stirred for 2 hours. TEM samples were prepared via spreading 2 wt% toluene solutions onto water surface and then transferring the thin films onto a copper TEM grid. All the film samples were annealed at 140 °C in vacuum for 2 h. For TEM observation, the films on Cu grids were exposed to RuO₄ vapor at room temperature for 2 min to selectively stain the PEO block. TEM images were obtained with a JEOL 2100.

Preparation of SiO₂ arrays

A precursor solution was prepared as described in the following procedure:²⁸ a mixture of ethanol (5.5 g), tetraethylorthosilicate (TEOS, 2.08 g), water (0.5 g), and 0.4 g of aqueous hydrochloric acid solution (0.1 M) was heated at 70 °C for one hour. The BCP films were then immersed in the precursor solution and kept at room temperature for 30 minutes. The samples were then rinsed with water and dried at 60 °C overnight, before being heated to 550 °C at a rate of 1 °C min⁻¹ at which temperature they were kept for 6 h. For SEM observation, Au layer (thickness ∼ 2 nm) was deposited onto the surface of the samples. SEM images were achieved with a Hitachi S-5200 and 4800. The water surface wettability of samples was attained by a Dsa-100 (3 μL drops, 25 °C).

Acknowledgements

This work was financially supported by National Natural Science Foundation of China (No. 51272010, 51472018), Beijing Nova Program (No. XX2013009), Program for New Century Excellent Talents in University (NCET-12-0035).

Notes and references

1. M. L. Wu, D. Wang and L. J. Wan, Nat. Commun., 2016, 7, 10752.
2. K. G. A. Tavakkoli, K. W. Gotrik, A. F. Hannon, A. Alexander-Katz, C. A. Ross and K. K. Berggren, Science, 2012, 336, 1294–1298.
3. J. Bang, U. Jeong, Y. Ryu du, T. P. Russell and C. J. Hawker, Adv. Mater., 2009, 21, 4769–4792.
4. C. A. Ross, K. K. Berggren, J. Y. Cheng, Y. S. Jung and J. B. Chang, Adv. Mater., 2014, 26, 4386–4396.
5. J. Y. Cheng, C. A. Ross, H. I. Smith and E. L. Thomas, Adv. Mater., 2006, 18, 2505–2521.
6. I. Otsuka, S. Tallegas, Y. Sakai, C. Rochas, S. Halila, S. Fort, A. Bsiesy, T. Baron and R. Borsali, Nanoscale, 2013, 5, 2637–2641.
7. H. G. Yoo, M. Byun, C. K. Jeong and K. J. Lee, Adv. Mater., 2015, 27, 3982–3998.
8. R. A. Segalman, H. Yokoyama and E. J. Kramer, Adv. Mater., 2001, 13, 1152–1155.
9. S. J. Jeong, H. S. Moon, B. H. Kim, J. Y. Kim, J. Yu, S. Lee, M. G. Lee, H. Choi and S. O. Kim, ACS Nano, 2010, 4, 5181–5186.
10. H. Y. Tsai, J. W. Pitera, H. Miyazoe, S. Bangsaruntip, S. U. Engelmann, C. C. Liu, J. Y. Cheng, J. J. Bucchignano, D. P. Klaus, E. A. Joseph, D. P. Sanders, M. E. Colburn and M. A. Guillorn, ACS Nano, 2014, 8, 5227–5232.
11. C. Girardot, S. Bohme, S. Archambault, M. Salaun, E. Latu-Romain, G. Cunge, O. Joubert and M. Zelsmann, ACS Appl. Mater. Interfaces, 2014, 6, 16276–16282.
12. S. O. Kim, H. H. Solak, M. P. Stoykovich, N. J. Ferrier, J. J. De Pablo and P. F. Nealey, Nature, 2003, 424, 411–414.
13. R. Ruiz, H. Kang, F. A. Detcheverry, E. Dobisz, D. S. Kercher, T. R. Albrecht, J. J. de Pablo and P. F. Nealey, Science, 2008, 321, 936–939.
14. C. Liedel, C. W. Pester, M. Ruppel, C. Lewin, M. J. Pavan, V. S. Urban, R. Shenhar, P. Bösecke and A. Böker, ACS Macro Lett., 2013, 2, 53–58.
15. M. Ruppel, C. W. Pester, K. M. Langner, G. J. Sevink, H. G. Schoberth, K. Schmidt, V. S. Urban, J. W. Mays and A. Boker, ACS Nano, 2013, 7, 3854–3867.
16. A. Boker, H. Elbs, H. Hansel, A. Knoll, S. Ludwigs, H. Zettl, V. Urban, V. Abetz, A. H. Muller and G. Krausch, Phys. Rev. Lett., 2002, 89, 135502.
17. M. Luo and T. H. Epps, Macromolecules, 2013, 46, 7567–7579.
18. S. Ham, C. Shin, E. Kim, D. Y. Ryu, U. Jeong, T. P. Russell and C. J. Hawker, Macromolecules, 2008, 41, 6431–6437.
19. E. Kim, W. Kim, K. H. Lee, C. A. Ross and J. G. Son, Adv. Funct. Mater., 2014, 24, 6981–6988.
20. M. Komura, A. Yoshitake, H. Komiyama and T. Iyoda, Macromolecules, 2015, 48, 672–678.
21. J. Jeong, J. S. Ha, S. S. Lee and J. G. Son, Macromol. Rapid Commun., 2015, 36, 1261–1266.
22. D. E. Angelescu, J. H. Waller, D. H. Adamson, P. Deshpande, S. Y. Chou, R. A. Register and P. M. Chaikin, Adv. Mater., 2004, 16, 1736–1740.
23. G. Singh, K. G. Yager, B. Berry, H. C. Kim and A. Karim, ACS Nano, 2012, 6, 10335–10342.
24. J. W. Jeong, Y. H. Hur, H. J. Kim, J. M. Kim, W. I. Park, M. J. Kim, B. J. Kim and Y. S. Jung, ACS Nano, 2013, 7, 6747–6757.
25. Z. Qiang, Y. Zhang, J. A. Groff, K. A. Cavicchi and B. D. Vogt, Soft Matter, 2014, 10, 6068–6076.
26. Z. Qiang, L. H. Zhang, G. E. Stein, K. A. Cavicchi and B. D. Vogt, Macromolecules, 2014, 47, 1109–1116.
27. Y. Q. Tian, K. Watanabe, X. X. Kong, J. Abe and T. Iyoda, Macromolecules, 2002, 35, 3739–3747.
28. A. Chen, M. Komura, K. Kamata and T. Iyoda, Adv. Mater., 2008, 20, 763–767.
29. A. H. Chen, Q. J. Zhu, Y. B. Zhao, T. Tastumi and T. Iyoda, Part. Part. Syst. Charact., 2013, 30, 489–493.
30. T. Qu, Y. Zhao, Z. Li, P. Wang, S. Cao, Y. Xu, Y. Li and A. Chen, Nanoscale, 2016, 8, 3268–3273.
31. M. Komura and T. Iyoda, Macromolecules, 2007, 40, 4106–4108.
32. H. Yu, J. Li, T. Ikeda and T. Iyoda, Adv. Mater., 2006, 18, 2213–2215.
33. H. Li, W. Y. Gu, L. Li, Y. M. Zhang, T. P. Russell and E. B. Coughlin, Macromolecules, 2013, 46, 3737–3745.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra21165h

---