Laser ablation of block copolymers with hydrogen-bonded azobenzene derivatives

Abstract

Supramolecular assemblies (PS-b-P4VP(AzoR)) are fabricated by hydrogen-bonding azobenzene derivatives (AzoR) to poly(4-vinyl pyridine) blocks of polystyrene-block-poly(4-vinyl pyridine) (PS-b-P4VP). PS-b-P4VP(AzoR) forms phase separated nanostructures with a period of ~75–105 nm. A second length scale structure with a period of 2 µm is fabricated on phase separated PS-b-P4VP(AzoR) by laser interference ablation. Both the concentration and the substituent of AzoR in PS-b-P4VP(AzoR) affect the laser ablation process. The laser ablation threshold of PS-b-P4VP(AzoR) decreases as the concentration of AzoR increases. In PS-b-P4VP(AzoR) with different substituents (R= CN, H, and CH ₃), ablation thresholds follow the trend: PS-b-P4VP(AzoCN)<PS-b-P4VP(AzoCH ₃)<PS-b-P4VP(AzoH). This result indicates that the electron donor group (CH ₃) and the electron acceptor group (CN) can lower the ablation threshold of PS-b-P4VP(AzoR).

Graphical abstract

Keywords

laser ablation / block copolymers / hydrogen-bond / azobenzene derivatives / supramolecular assembly

Introduction

The interaction of high energy lasers with polymers can result in ablation of polymers at irradiated areas [¹–¹¹]. Laser ablation is a quick and efficient method to fabricate microstructures and devices on polymers because structures can be generated by a single laser pulse at a time scale of nanosecond or even shorter [¹,³–⁵,¹⁰,¹¹]. Ablated structures on polymers show many applications, such as superhydrophobic surfaces [⁶], multichip modules [⁷], distributed feedback lasers [⁴], inkjet printer nozzles [⁸], and diffractive optical elements [⁵].

Although laser ablation of polymers is extensively studied and shows many applications, studies on laser ablation of block copolymers (BCPs) are rare [⁹]. BCPs can form various microphase separated nanostructures [¹²]. The self-assembly of block copolymers is a promising platform for the fabrication of nanostructured materials and devices [¹³–²⁰]. We previously reported that BCPs can be hierarchically structured by combining phase separation with laser interference ablation [¹⁰]. Shorter length scale structures of the hierarchical structures are phase separated nanostructures and longer length scale structures are interference patterns generated by laser ablation [¹⁰]. However, there is a lack of fundamental understanding of laser ablation of block copolymers. Understanding laser ablation of BCPs is helpful for the design of BCPs which can be effectively ablated and patterned. Additionally, lowering ablation thresholds of BCPs is important because low thresholds can increase the ablation rate and save energy. So, it is highly desirable to understand laser ablation of BCPs and decrease ablation thresholds of BCPs.

In this work, we studied the laser ablation of polystyrene-block-poly(4-vinyl pyridine) (PS-b-P4VP) with hydrogen-bonded azobenzene derivatives (AzoR). We found that the ablation threshold of BCP supramolecular assemblies PS-b-P4VP(AzoR) decreases as the concentration of AzoR increases. In PS-b-P4VP(AzoR) with different substituents (R= CN, H, and CH₃), ablation thresholds follow the trend: PS-b-P4VP(AzoCN)<PS-b-P4VP(AzoCH₃)<PS-b-P4VP(AzoH). We showed how chemical structures affect the laser ablation behavior of PS-b-P4VP(AzoR). According to these results, we can effectively fabricate tunable hierarchical structures on BCPs by laser ablation.

Experimental

Materials

The chemical structures of used materials are shown in Fig. 1. PS-b-P4VP (M_n = 330-b-125 kg·mol⁻¹ and PDI= 1.18) was purchased from Polymer Source. The azobenzene derivatives AzoCN and AzoCH₃ were synthesized according to our previous work [¹⁰,²¹]. AzoH was purchased from Aldrich.

Preparation of thin films

PS-b-P4VP and AzoR were dissolved in cyclopentane separately. Solutions of PS-b-P4VP(AzoR)_x were prepared by combining the above two solutions. Here, x denotes the molar ratio of AzoR versus repeat units of P4VP. The mixed solutions were stirred overnight and filtered through a 0.2 mm filter before use. Thin films of PS-b-P4VP(AzoR)_x were prepared by spin-coating. The films had a typical thickness of 200–230 nm. The spin-cast films were dried in an oven under vacuum at room temperature overnight. To induce phase separation, thin films were transferred to a glass container with saturated atmosphere of 1,4-dioxane. The container was completely closed and thin films were kept in the container for 2 days at room temperature.

Methods

Ultraviolet-visible (UV-vis) absorption spectra were measured on a Perkin-Elmer Lambda 900 UV-vis spectrometer. AFM images were obtained on a Dimension 3100 system using tapping mode. The optical setup for the three-beam interference was reported elsewhere [³,⁵,¹⁰,¹¹]. In brief, a Q-switched Nd:YAG laser at 355 nm with laser pulse duration of 35 ns was used as the light source. The laser beam with pulse energy 1.18 mJ and a diameter of ~1.4 mm was split into three beams with equal intensity to overlap on the sample. The intersecting angle between the interference beams was 5.88°. Interference patterns were fabricated by single pulse irradiation. The intensity distribution of the interference pattern was calculated according to our previous work [³,⁵,¹¹].

Results and discussion

Effects of the concentration of azobenzene derivatives on laser ablation

It is well known that small molecules with hydrogen bonding donors can be hydrogen-bonded to P4VP blocks of PS-b-P4VP [²²–²⁷]. Here, the phenolic group in AzoR is a hydrogen bonding donor (Fig. 1). AzoR can be hydrogen-bonded to P4VP [¹⁰,²⁸,²⁹]. The hydrogen bonding between AzoCN and P4VP is proved by infrared spectroscopy (Fig. S1, cf. Electronic Supplementary Material).

Figure 2 shows UV-vis absorption spectra of PS-b-P4VP(AzoCN)_x films with different concentrations of AzoCN. The band at 300–400 nm is the p-p* absorption band of azobenzene groups. The absorption coefficient (a) increases as the concentration of AzoCN increases.

PS-b-P4VP(AzoR)_x forms phase separated nanostructures (Fig. S2) [¹⁰]. The schematic model in Fig. 3(a) shows that PS forms the continuous phase and P4VP(AzoR)_x forms dispersed phases. Figure 3(c) shows the hexagonal interference pattern of three beams, which is calculated according to our previous work [³,⁵,¹¹]. When PS-b-P4VP(AzoR)_x is exposed to interference beams, the polymers at bright interference fringes (high intensity areas) will be ablated. Hierarchical structures with both phase separated nanostructures and interference patterns can be obtained by laser interference ablation (Fig. 3(b)). We will show effects of the concentration of AzoCN on laser ablation.

Figure 4 shows atomic force microscope (AFM) images of PS-b-P4VP(AzoCN)_x (x = 0.05, 0.1, 0.3, and 0.5) after single pulse interference irradiation. All samples form hierarchical structures, which resemble the schematic model in Fig. 3(b). Shorter length scale structures of the hierarchical structures are phase separated nanostructures and longer length scale structures are interference patterns. AFM phase images (insets of Figs. 4(a–d)) clearly show the phase separation between PS and P4VP(AzoCN)_x. The average center-to-center distance (period) of nearby P4VP(AzoCN)_x phases increases form ~75 nm (x = 0.05) to ~105 nm (x = 0.5). The period of the interference patterns is ~2 µm. We compared the AFM profiles of PS-b-P4VP(AzoCN)_x with different x (Fig. 4(e)). PS-b-P4VP(AzoCN)_0.05 forms volcano-like structures, indicating its surface is swelled by laser beams. In the other samples, deep ablated holes are observed. PS-b-P4VP(AzoCN)_0.1 forms very sharp holes. PS-b-P4VP(AzoCN)_0.3 and PS-b-P4VP(AzoCN)_0.5 form deep and wide holes. The depth and diameter of the holes increases as the concentration of AzoCN increases (Fig. 4(f)).

Effects of substituents of azobenzene derivatives on laser ablation

We also studied effects of substituents of azobenzene derivatives on laser interference ablation. The hydroxyl group on the para-position of azobenzene is an electron donor (D) [³⁰]. The methyl group (CH₃) of AzoCH₃ is also an electron donor. The nitrile group (CN) of AzoCN is an electron acceptor (A). So, AzoH is a D-p molecule, AzoCH₃ is a D-p-D molecule, and AzoCN is a D-p-A molecule. These substituents affect the absorption of azo chromophores. Figure 5 shows UV-vis absorption spectra of AzoR in solutions and PS-b-P4VP(AzoR)_x films. The band at 300-400 nm is the characteristic absorption band of azobenzene groups. The bands of AzoCH₃ in solution and thin film slightly red-shift compared with those of AzoH. The bands of AzoCN obviously red-shift compared with those of AzoH. These results show that the D-p-D structure slightly affects the absorption of azo chromophores, and D-p-A structure strongly affects the absorption of azo chromophores.

Figure 6 shows AFM images of PS-b-P4VP(AzoH)_0.5 and PS-b-P4VP(AzoCH₃)_0.5 after interference irradiation. They form hierarchical structures with phase separated nanostructures and laser interference patterns. We compared the profiles of PS-b-P4VP(AzoR)_0.5 (Figs. 6(c) and 6(d)). PS-b-P4VP(AzoH)_0.5 forms sharp holes. The hole of PS-b-P4VP(AzoCH₃)_0.5 is slightly broader than that of PS-b-P4VP(AzoH)_0.5. The hole of PS-b-P4VP(AzoCN)_0.5 is much broader than that of PS-b-P4VP(AzoH)_0.5. These results indicate that the substituent of AzoR plays an important role on laser ablation.

The mechanism of laser ablation is as follows: When a target is irradiated by a high energy laser, the target absorbs light. Then, the target is heated or decomposed, and subsequent vaporization occurs [¹–³,³²]. PS-b-P4VP(AzoR)_x is different from normal targets because the light absorber AzoR is in P4VP phases but not homogenously dispersed in the matrix. When PS-b-P4VP(AzoR)_x is exposed to laser beams, AzoR is heated or decomposed. The heat diffuses to hydrogen bonded P4VP chains and nearby PS chains. The heated parts are ablated. P4VP(AzoR) phases are heating centers and the PS phase that does not absorb light obtains heat by thermal diffusion. Considering that the laser pulse duration is 35 ns and thermal diffusivity of PS is in the order of ~10⁻³ cm²∙s⁻¹ [³³,³⁴], the heated range of PS during an exposure time of a single pulse is several tens of nanometers near P4VP(AzoR) phases.

The concentration of AzoCN affects the ablation process (Fig. 4). The reason is that AzoCN is the light absorber and the linear absorption coefficient of PS-b-P4VP(AzoCN)_x increases as x increases (Fig. 2 and Table 1). Ablation should be more efficient with more light absorbers. We calculated ablation thresholds of PS-b-P4VP(AzoCN)_x by AFM (Fig. S3). Ablation thresholds of PS-b-P4VP(AzoCN)_x decrease as x increases (Table 1). This result explains why different morphologies are generated on PS-b-P4VP(AzoCN)_x with different x (Fig. 4).

Substituents of AzoR also affect the interference ablation (Fig. 6). As shown in Table 1, the linear absorption coefficients of PS-b-P4VP(AzoR)_0.5 (R= H, CN, and CH₃) are nearly the same, but their ablation thresholds are quite different. This result indicates that not only the linear absorption coefficient affects laser ablation. It is well known that azo dyes show nonlinear absorption, i.e., the absorption coefficient of an azo dye is strongly dependent on the laser intensity [³⁰,³⁵,³⁶]. D-p-D and D-p-A molecules usually show better nonlinear absorption [³⁰,³⁵–³⁸]. AzoCH₃ and AzoCN are D-p-D and D-p-A molecules, respectively, which can more efficiently absorb light of high energy lasers. So, different ablation thresholds of PS-b-P4VP(AzoR)_0.5 (R= H, CN, and CH₃) are due to different chemical structures of AzoR.

Conclusions

Both the concentration and the substituent of AzoR dominate the laser ablation of PS-b-P4VP(AzoR)_x. The ablation threshold decreases as the concentration of azobenzene derivatives increases. This result is because the linear absorption coefficient increases as the concentration of azobenzene derivatives increases. The samples with different substituents (PS-b-P4VP(AzoR)_0.5) have nearly the same linear absorption coefficient, but their ablation thresholds are quite different. Ablation thresholds follow the trend: PS-b-P4VP(AzoCN)_0.5<PS-b-P4VP(AzoCH₃)_0.5<PS-b-P4VP(AzoH)_0.5. This trend is due to the D-p-A and D-p-D structures of AzoCN and AzoCH₃, respectively. These results are helpful for the design of BCP systems with low ablation thresholds, which can be effectively structured at low laser intensities.

Tunable hierarchical structures can be fabricated by laser interference ablation of BCPs. Phase separated nanostructures are tunable by changing the composition of BCPs [¹⁰] or the concentration of azobenzene derivatives (insets of Fig. 4). Interferences patterns are tunable by changing interference conditions [¹⁰], concentration of azobenzene derivatives (Fig. 4), or substituents of azobenzene derivatives (Fig. 6). We observed that hierarchical structures on PS-b-P4VP(AzoR)_x show brilliant structural colors. The hierarchical structures show potential applications as photonic materials.

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