Ion-modulated flow behavior of layer-by-layer fabricated polymer thin films

Abstract

Flow behavior of polyelectrolyte thin films plays an important role in the fabrication of functional nanomaterials. Therefore, modulation of the flow behavior of polymer thin films becomes a challenge. Here we report that ions could facilely tune the flow behavior of polymer thin films. A “high flow” behavior of polymer thin films assembled in Cl-ion solutions is clearly observed under a wet mold, while such thin films assembled in Br-ion solutions exhibit nearly frozen behavior when they are in contact with a wet mold.

---

Polymer thin film fabrication through layer-by-layer (LbL) assembled deposition has been extensively studied since it was introduced and developed by Decher et al.¹ This facile and robust technique involves the simple immersion of a solid substrate into solutions containing oppositely charged polyelectrolytes.² Moreover, this technique can be applied to many substrates such as glasses, metals, and particles.³ The structures and properties of polymer thin films can be affected by many factors during the assembly or post-treatment process, including polymer compositions, salts, temperature and pH, etc.^4–6

Mechanical properties of polymer thin films have been modulated by different approaches including adjusting pH, in situ cross-linking reaction, and electrochemistry.^7–9 Recently, it has been reported that polymer thin films exhibit a flow behavior under a wet mold without applying external pressure which reveals that polymer thin films could move from contact regions to non-contact regions.¹⁰ Unlike conventional mechanical deformation of polymer thin films under a mold with applying external pressure, the flow of polymer thin films is mainly induced by capillary force due to their softness. Utilizing the flow of polymer thin films, nanoparticle arrays have been fabricated on the film surfaces. Since flow behavior of polymer thin films plays an important role in the successful fabrication of functional nanomaterials, modulation of the flow behavior of polymer thin films is crucial for understanding the mechanism, and more importantly, for developing the fabrication technique towards various functional nanomaterials. Although the effect of ions on polymer thin films assembly has been well documented,^11–13 the majority of these work usually focuses on the behavior of assembly or post-treatment of films. To date, these are still no reports on the effect of ions on the flow behavior of polymer thin films. It is well known that the mechanical properties of polymer thin films are very important in the design of biomaterials with desirable properties.^14,15 Since the flow behavior of polymer thin films is related to their stiffness, it is undoubted that the control of the flow of polymer thin films is important for the macroscopic functions of designed biomaterials. Furthermore, as mentioned above, the flow behavior of polymer thin films would have potential applications in the fabrication of functional materials. Therefore, controlling the flow of polymer thin films might be the prerequisite for the further development of various functional biomaterials and biointerface.

Herein, we demonstrate that ions could modulate the stiffness of polymer thin films which can facilely tune the flow behavior of polymer thin films. Results show the polymer thin films assembled in Cl-ion solutions can easily flow from contact regions to non-contact regions under a patterned mold, while such thin films assembled in Br-ion solutions exhibit nearly frozen behavior when they are in contact with a wet mold, which may be attributed to the different molecular conformation during assembly. Those interesting results would have potential applications in the fabrication of nanomaterials and biointerface.

Polymer thin films were assembled using poly(diallyldimethylammonium chloride) (PDAC) and poly(4-styrenesulfonic acid-co-maleic acid) sodium salt (PSMA) as the building blocks. (PSMA/PDAC)₇ thin films were prepared by the LbL self-assembly method. Those two polymers were chosen here since the weakly charged MA segments of PSMA are more sensitive to the salt while the strong PDAC polyelectrolyte can keep the assembled films stable.¹⁶ Different ion may result in significantly different assemble behavior⁵ of polymer thin films which could modulate the flow behavior of films (Scheme 1). Here we used Cl-ion and Br-ion as the model ions to investigate their effect on the flow of polymer thin films.

Scheme 1 Illustration of flow behavior of polymer thin films under a wet mold.

(PSMA/PDAC)₇ thin films were prepared by the LbL self-assembly method as the model system for investigation of the ions effects on flow behavior of polymer thin films. Chemical structures of polyelectrolytes are shown in Fig. 1a (inset). The deposition process of PSMA and PDAC was characterized by ellipsometry (Fig. 1a). The results showed that the thickness of film increased with the increase of layer numbers which revealed that the PSMA/PDAC thin film could be successfully assembled in both Cl- and Br-ion solutions. The total thickness of thin films assembled in Br-ion solutions is slightly larger than the one assembled in Cl-ion solutions.

Fig. 1 (a) Thickness of the (PSMA/PDAC) thin films as a function of layer number assembled in Cl- and Br-ion solutions, insets are chemical structures of polyelectrolytes. (b) and (c) AFM images of polymer thin films assembled in Cl- and Br-ion solutions, respectively. Insets are corresponding water contact angles.

Although the thicknesses of the polymer thin films assembled Cl- and Br-ion solutions showed no significant difference, the topographies of the films were quite different. The surface morphologies of different polymer thin films were measured by atomic force microscopy (AFM), respectively. As shown in Fig. 1b and c, the surface of polymer thin film assembled in Br-ion solutions is much rougher than the one assembled in Cl-ion solutions. For example, it can be clearly seen that there are many nano-aggregates on the surface of the polymer thin film assembled in Br-ion solutions which obviously reveals the roughness of the polymer thin film assembled in Br-ion solutions is much larger. According to AFM results, the roughness of the polymer thin films assembled in Br-ion solution is ∼4.1 nm, whereas the roughness of the polymer thin films assembled in Cl-ion solutions is ∼2.5 nm. The nano-protruded topography also suggests that the polyelectrolyte molecules of polymer thin film assembled in Br-ion solutions adopt a more coiled structures,⁵ while chain conformation of polyelectrolyte molecules of polymer thin films assembled in Cl-ion solutions is relatively flat since the topography is much smoother.⁵ Insets in Fig. 1b and c are typical water contact angles of polymer thin films assembled in Cl- and Br-ion solutions, respectively. The water contact angle of polymer thin films assembled in Cl-ion solutions is ∼54° (inset in Fig. 1b), while the water contact angle of polymer thin films assembled in Br-ion solutions can reach up to 78° (inset in Fig. 1c) which may be attributed to the larger surface roughness.

Since the size of the Br-ion is larger than that of the Cl-ion, a more coiled format of polyelectrolytes will occur when polymer thin films are assembled in Br-ion solutions.⁵ Furthermore, Br-ion has a smaller hydration shell than that of Cl-ion resulting in a higher polarizability which leads to a stronger attraction between Br-ions and polyelectrolytes.⁵ Thus, a stronger coiling of polyelectrolytes in the polymer thin films assembled in Br-ion solutions results in an increase in thickness and roughness, and a stronger attraction between Br-ions and polyelectrolytes implies the stiffness of polymer thin film is much larger.

Flow of polymer thin films plays a key role in nanomaterial fabrication.¹⁰ Thus, here we study the effect of ions on the flow behavior of films. When the assembled polymer thin films touch a wet poly(dimethylsiloxane) (PDMS) stamp, the flow of polymer thin films will occur during this process. We assume the patterns are induced by the flow of polymer thin films moving from contact regions to non-contact regions. A detailed demonstration is presented in the ESI.† Fig. S1† clearly shows that patterns are formed by the flow of the polymer thin films since the pattern height is much larger than the as-prepared film thickness. After PDMS stamps were peeled off, the surface morphologies of the polymer thin films were measured by AFM (Fig. 2). As shown in Fig. 2b, the patterns were very clear and regular, and the pattern heights were about 100 nm which are much larger than the original thickness of the polymer thin films assembled in Cl-ion solutions (about 47 nm, measured by ellipsometer). This “high flow” behavior of films assembled in Cl-ion solutions is clear since the ratio of pattern heights to film thickness can reach up to 2.13. The induced patterns can be attributed to the flow of films moving from contact regions to non-contact regions, as demonstrated before.¹⁰ The flow of film shows the polyelectrolyte molecules or complexes within the film assembled in Cl-ion solutions have a high mobility under high water content. However, when the polymer thin films assembled in Br-ion solutions, the as-prepared thin films can only generate patterns with very small heights when they touch a wet mold (Fig. 2a). The pattern heights were lower than 15 nm which were quite smaller than the thickness of the polymer thin films assembled in Br-ion solutions (55 nm, measured by ellipsometer) revealing the flow of the films is very limited. This “low flow” behavior of films assembled in Br-ion solutions is revealed by the ratio of pattern heights to film thickness which is only 0.27. This interesting result indicates that the mechanical properties of polymer thin films assembled in Cl- and Br-ion solutions are quite different although their thicknesses are almost identical. Additionally, we also investigated the effect of ion concentration on flow behavior of the films, similar results were obtained regardless of the ion concentration (Fig. S2†), and the flow behavior of films only depends on specific ion.

Fig. 2 AFM images of induced patterns by the flow of polymer thin films assembled in Br-ion solutions (a) and (b) in Cl-ion solutions. Below are corresponding line profiles.

As we know, a stronger interaction between Br-ions and polyelectrolytes would occur since the Br-ion has a higher polarizability compared to the Cl-ion because of its larger ion size. Thus, the polymer thin films assembled in Cl-ion solutions exhibit a high flow behavior whereas polymer thin films assembled in Br-ion solutions obviously show a low flow behavior which is attributed to the strong interaction between Br-ions and polyions (Fig. 3).

Fig. 3 Schematic illustration of pattern formation on the polymer thin films assembled in Cl- and Br-ion solutions by using water induced flow.

The low flow behavior of the PSMA/PDAC films assembled in Br-ion solutions with much stronger interactions between the polymer chains could generate more compact microstructure of the films compared to the films assembled in Cl-ion solutions. This conclusion can be further confirmed by the nanoindentation experiment in Fig. 4. It has been demonstrated that the modulus of PSMA/PDAC thin films assembled in Br-ion solutions is much higher than that assembled in Cl-ion solutions. Fig. 4 describes nanoindentation curves of PSMA/PDAC film assembled in Cl- and Br-ion solutions, respectively. Generally speaking, when the films were assembled in Br-ion solutions, it is clear that 8 nm deformations of the films could be generated after 0.6 nN loading forces were applied on the films. Nevertheless, when the films were assembled in Cl-ion solutions, almost 12 nm deformations of the films could be generated after 0.6 nN loading forces were applied on the films. It suggests that films assembled in Br-ion solutions need much higher loading force to induce the same deformation compared to the films assembled in Cl-ion solutions. The nanoindentation results clearly show that the films assembled in Cl-ion solutions are much softer than those assembled in Br-ion solutions. The rigid films show that the interactions between the polymer chains are much stronger, and result in the microstructure of the film is more compact. The reason of the weaker interaction between PSMA and PDAC assembled in Cl-ion solutions might be due to a larger hydration shell of Cl-ion than that of Br-ion, which results in a lower polarizability which causes a relatively flat structure during assembly. A stronger attraction between the Br-ions and polyelectrolytes greatly improves the stiffness of the films with a low flow behavior.

Fig. 4 Indentation curves of PSMA/PDAC thin films assembled in Cl- and Br-ion solutions.

Conclusions

In summary, we explored in this work controlling the flow behavior of polymer thin films by ions. It was found that the stiffness of PSMA/PDAC thin films could easily be tuned by ions which greatly affect the flow behavior of corresponding films. We demonstrated a facile and efficient approach to control the flow of polymer thin films which might be applicable in the nanomaterials and biomaterials. Further studies such as effect of film thickness and more types of ions on flow behavior are underway.

Acknowledgements

This study is financially supported by the National Natural Science Foundation of China (No. 21104069).

Notes and references

1. G. Decher and J. D. Hong, Makromol. Chem., Macromol. Symp., 1991, 46, 321–327.
2. G. Decher, Science, 1997, 277, 1232–1237.
3. Z. G. Estephan, Z. X. Qian, D. Lee, J. C. Crocker and S. J. Park, Nano Lett., 2013, 13, 4449–4455.
4. X. Gong, Phys. Chem. Chem. Phys., 2013, 15, 10459–10465.
5. R. von Klitzing, Phys. Chem. Chem. Phys., 2006, 8, 5012–5033.
6. M. Salomaki, I. A. Vinokurov and J. Kankare, Langmuir, 2005, 21, 11232–11240.
7. M. T. Thompson, M. C. Berg, I. S. Tobias, M. F. Rubner and K. J. Van Vliet, Biomaterials, 2005, 26, 6836–6845.
8. J. A. Lichter, M. T. Thompson, M. Delgadillo, T. Nishikawa, M. F. Rubner and K. J. van Vliet, Biomacromolecules, 2008, 9, 1571–1578.
9. D. J. Schmidt, F. C. Cebeci, Z. I. Kalcioglu, S. G. Wyman, C. Ortiz, K. J. van Vliet and P. T. Hammond, ACS Nano, 2009, 3, 2207–2216.
10. X. Gong, RSC Adv., 2014, 4, 54494–54499.
11. J. E. Wong, H. Zastrow, W. Jaeger and R. von Klitzing, Langmuir, 2009, 25, 14061–14070.
12. R. A. Ghostine, M. Z. Markarian and J. B. Schlenoff, J. Am. Chem. Soc., 2013, 135, 7636–7646.
13. J. B. Schlenoff, A. H. Rmaile and C. B. Bucur, J. Am. Chem. Soc., 2008, 130, 13589–13597.
14. A. A. Chen, S. R. Khetani, S. Lee, S. N. Bhatia and K. J. van Vliet, Biomaterials, 2009, 30, 1113–1120.
15. X. Gong, Nano LIFE, 2015, 05, 1542002.
16. E. Tjipto, J. F. Quinn and F. Caruso, Langmuir, 2005, 21, 8785–8792.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra11734h

---