Segmented polymer nanowires and nanorods by one-step template wetting with a hyperbranched polymer and linear polymer blend

Abstract

Although segmented polymer nanostructures have been fabricated, it is still a challenge to simplify the fabrication procedures, particularly for the construction of new segmented structures. In this work, a one-step approach is proposed to fabricate hyperbranched polymer (HBP)/linear polymer (LP) segmented nanostructures by wetting the channels of anodized aluminum oxide (AAO) templates with HBP/LP blend melts. The surface properties including wettability and droplet impact behavior of the as-prepared segmented nanostructures can be regulated by tuning the content or the number of terminal hydroxyl groups of HBP.

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Introduction

In the past two decades, one-dimensional (1D) polymer nanostructures such as nanorods, nanotubes, nanowires, and nanobelts, have attracted extensive attention due to their size-dependent properties^1–4 and their applications in photoelectric devices, microfluidics, catalysts, separation membranes, sensors, and drug delivery.^5–10 To integrate the properties/functions of different polymer nanostructures, gradient or segmented structures which feature multicomponents,¹¹ tunable structures (segment length, aspect ratio, etc.)¹² and multifunctions¹³ along their long axes have been developed. For example, polythiophene/polypyrrole segmented (heterojunction) nanowires exhibit smart responses to electrochemical redox potentials.¹⁴ They can act as either resistors in oxidized state or diodes in reduced state. Moreover, such electrical transport mode can be reversibly changed by alternately exposing the segmented nanowires to negative and positive potentials.

Electrochemical deposition has been the typical approach to fabricating segmented nanotubes and nanorods out of conducting polymers.^11,15 This approach imposes limitations on the range of processable polymers and excludes the coupling/synergy of important functional polymers other than conducting polymers. Therefore, versatile approach that combined membrane-templating with electrodeposition and layer-by-layer assembly was developed to prepare functional hybrid multisegmented nanotubes and nanowires.¹³ Recently, segmented hybrid nanofibers composed of block copolymer/homopolymer were fabricated by bidirectional template wetting, which involved two successive wetting steps starting from the opposite surfaces of porous alumina templates.¹⁶ Room-temperature face-to-face wetting with polymeric solutions of poly(4-chlorostyrene) (PClS), poly(4-bromostyrene) (PBrS), and poly(vinylchloride) (PVC) was also introduced to fabricate segmented PVC/PBrS and PClS/PBrS nanotubes with sharp interfaces.¹⁷ Although progresses have been made to extend the range of processable polymers, most of the reported procedures require multi-steps and/or are time-consuming.

To simplify the fabrication procedures of polymer segmented nanostructures, here, we propose a one-step anodized aluminum oxide (AAO) template wetting strategy with hyperbranched polymer and linear polymer blend melts to fabricate segmented nanowires/nanorods (see experimental details in ESI†). Unlike the bidirectional template wetting¹⁶ or room-temperature face-to-face wetting¹⁷ strategies in which infiltration of polymer melts individually starting from the opposite surfaces of the templates, the infiltration of melted hyperbranched polymer/linear polymer blend starts from only one surface (the upper surface) of AAO template, and the formation of segmented nanowires/nanorods structures is attributed to the phase separation of hyperbranched polymer and linear polymer. Due to the strikingly low intrinsic viscosity and high solubility of hyperbranched polymers,^18–20 phase separation occurs when hyperbranched polymer is mixed with linear polymer under appropriate conditions,^21,22 leading to the formation of various ordered patterns, such as bicontinuous structure, islands, or holes.¹⁴ By confining the phase separation of hyperbranched polymer/linear polymer in the nanochannels of AAO template, we successfully prepared segmented structures consisting of hyperbranched polyester (HBP) nanowires as the top layer and linear poly(methyl methacrylate) (L-PMMA) nanorods as the bottom layer. We also show that the surface properties (including wettability and droplet impact behavior) of the as-prepared segmented nanowires/nanorods, which have not be systemically investigated previously, can be finely adjusted by tuning the number of terminal hydroxyl groups of HBP.

Results and discussion

In the proof-of-concept experiments to fabricate segmented polymer nanowires and nanorods, HBP-64-OH and L-PMMA were selected as model polymers (see Scheme S1 for the structure of HBP-64-OH and L-PMMA in ESI†). By reversible addition-fragmentation transfer polymerization (RAFT, see details in ESI†), L-PMMA (M_w = 38 610 g mol⁻¹) with narrow distribution (M_w/M_n = 1.23) was synthesized. It should be point out that the molecular weight of polymers has influence on their nanofabrication.²³ Here we are focused on hyperbranched polymer and linear polymer blend-induced segmented nanostructures, so the influence of molecular weight distribution on AAO template wetting should be minimized. In the as-synthesized L-PMMA, HBP-64-OH was uniformly dispersed under ultrasonication to obtain a series of L-PMMA/HBP-64-OH polymer blend films (Scheme 1a). Such blend films were heated at 160 °C to melt and wet the walls of AAO nanochannels (180 ± 20 or 265 ± 24 nm in diameter, 60 μm in length), during which phase separation occurred in melted L-PMMA/HBP-64-OH blend, due to the high heat energy (Scheme 1b). Compared with L-PMMA, the nanoflow behavior of HBP-64-OH was stronger, since (1) HBP-64-OH melts with lower intrinsic viscosity could obtain a higher nanoflow rate in AAO nanochannels. According to our previous work,²⁴ within the same wetting time, polymer melts with a higher nanoflow rate can obtain a longer displacement in nanochannels, leading to the formation of longer nanofibers after removing the AAO template. Therefore, HBP-64-OH with a higher nanoflow rate in AAO nanochannels formed the top layer (long nanofibers) of segmented nanostructures. (2) Hydroxyl-terminated groups on HBP-64-OH surfaces rendered HBP-64-OH higher surface energy, which could enhance the nanoflow behaviors of HBP-64-OH in AAO nanochannels. Accordingly, under identical wetting conditions, HBP-64-OH tended to form long nanowires while L-PMMA favored the formation of short nanorods (Scheme 1c). After removing AAO templates with NaOH, segmented HBP-64-OH/L-PMMA polymer nanostructures were fabricated, as shown in Scheme 1d and Fig. 1D and E. The segmented nanostructures were confirmed by the cross-section scanning electron microscopy (SEM) image of the sample (Fig. 1F), and the average length of HBP-64-OH nanowires and L-PMMA nanorods are found to be ī_nanowires ∼ 10 μm and ī_nanorods ∼ 0.86 μm, respectively.

Scheme 1 Schematic diagram of the fabrication processes of segmented nanostructures. (a) L-PMMA/HBP-64-OH polymer blend film; (b) phase separation occurs in melted L-PMMA/HBP-64-OH blends; (c) L-PMMA and HBP-64-OH melts wet the walls of AAO nanochannels; (d) segmented L-PMMA/HBP-64-OH polymer nanostructures are fabricated after removing AAO templates.

Fig. 1 SEM images of pure L-PMMA and L-PMMA/HBP-64-OH (a mass ratio of L-PMMA to HBP-64-OH is 95/5) fabricated by AAO template wetting. (A) Top view of L-PMMA film; (D) top view of L-PMMA/HBP-64-OH film; (B) and (E) magnified images of A and D; (C) cross-section view of L-PMMA film; (F) cross-section view of L-PMMA/HBP-64-OH film.

To verify the roles of phase separation and nanoflow behavior difference in the formation of segmented polymer nanostructures, control experiments with pure L-PMMA was also conducted. Experimental results showed that nanorod arrays of similar length (ī_nanorods ∼ 0.90 μm, cf. Fig. 1C) were formed on pure L-PMMA substrate and no segmented nanostructures were found (cf. Fig. 1A or B), suggesting that pure L-PMMA melts experienced identical filtration/wetting process in the AAO nanochannels. On contrast, segmented HBP-64-OH/L-PMMA nanostructures fabricated by phase separation and nonidentical filtration/wetting distributed randomly on the surfaces of L-PMMA nanorod arrays (cf. Fig. 1D–F), leading to substantial changes in the nano-/micro-scale surface roughness and in the surface morphology, which may significantly alter the surface properties – typically, static wettability and dynamic wettability²⁵ – and be important for the applications of segmented polymer nanostructures in nanophotonic field.¹²

The surface roughness of segmented HBP-64-OH/L-PMMA nanostructures is correlated with the length of HBP-64-OH nanowires, and can be regulated by the content of HBP-64-OH in the blends and the number of terminal hydroxyl of HBP-64-OH. It should be noted that we tried to prepare polymer blends with much higher amount of HBP-64-OH, but the serious phase separation occurred as the amount of HBP-64-OH exceeded 10% of blends in the process of preparation. Thus, we considered that the amount of HBP-64-OH in blends exists a limited value. We figure out that the length of HBP-64-OH nanowires and the surface roughness of segmented HBP-64-OH/L-PMMA nanostructures increase with increasing content of HBP-64-OH in the blends. As shown in Fig. 2 and Fig. 1D and E, the average length of HBP-64-OH nanowires are estimated to be 4.6 μm (Fig. 2A–C), 10 μm (Fig. 1D–F) and 14 μm (Fig. 2C–E), respectively, as the content of HBP-64-OH in polymer blends increases from 2.5% to 5% and to 10%. Such a trend can be rationalized by the fact that as the content of HBP-64-OH increases, the phase separation in L-PMMA/HBP-64-OH blend becomes more pronounced, leading to the growth of longer HBP-64-OH nanowires. On the other hand, the hydroxyl-terminated groups also pay an important role in the growth of HBP segments. When HBP-16-OH is used instead of HBP-64-OH, short nanorods with average length of only ∼1.67 μm formed on the surfaces of L-PMMA nanorod arrays (cf. Fig. 3A and B). Compared with Fig. 1D and E, the degree and region of the segmented nanostructures become fairly unobvious as shown in Fig. 3A and B, in which the long nanowires almost disappear. This result implies that when the hydroxyl-terminated number decreases from 64 to 16, the hyperbranched topological structure becomes insignificant, resulting in the fairly unobvious segmented nanostructures.

Fig. 2 SEM images of L-PMMA/HBP-64-OH film surfaces fabricated by AAO template wetting: (A) top view of L-PMMA/HBP-64-OH (a mass ratio of L-PMMA to HBP-64-OH is 97.5/2.5); (D) top view of L-PMMA/HBP-64-OH (90/10); (B) and (E) magnified images of A and D; (C) cross-section view of L-PMMA/HBP-64-OH (97.5/2.5); (F) cross-section view of L-PMMA/HBP-64-OH (90/10).

Fig. 3 SEM images of L-PMMA/HBP-16-OH film surfaces fabricated by AAO template wetting: (A) top view of L-PMMA/HBP-16-OH (95/5); (B) magnified images of A; (C) cross-section view of L-PMMA/HBP-16-OH (95/5).

The wettability of segmented HBP-64-OH/L-PMMA nanostructures was studied by measuring static water CA values. The experimental results are presented in Fig. 4A, together with the optical images of water droplets on the respective surfaces. Compared with the CA of the pure L-PMMA (θ_c = 101°), all of the CA values of segmented L-PMMA/HBP-64-OH polymer increase significantly, from 126° to 140° as the HBP-64-OH content increases from 2.5 wt% to 10 wt%, indicating that the surfaces of segmented L-PMMA/HBP-64-OH polymer are highly hydrophobic and the hydrophobicity are dependent on the HBP-64-OH content. The improved hydrophobicity of segmented L-PMMA/HBP-64-OH polymer surfaces can be attributed to the increase in the length of HBP nanowires and the nano-/micro-scale surface roughness. Quantitatively, the relationship between a compounded rough surface, θ_c, and the equilibrium contact angle on a flat surface, θ_e, can be described by the Cassie–Baxter's equation:²⁶

cos θ_c = Φ_s cos θ_e − (1 − Φ_s)

where Φ_s is the solid–liquid contact (or solid) fraction of the surface. With the increasing of HBP-64-OH content, the peak-to-peak roughness of segmented L-PMMA/HBP-64-OH polymer surfaces increases. In this case, a smaller Φ_s value is caused by the decrease of contact area fraction of water droplet and solid polymer, leading to the increase of CAs.²⁷

Fig. 4 Static water CA values of L-PMMA, L-PMMA/HBP-64-OH film surfaces fabricated by AAO template wetting, together with the optical images of water droplet on the respective surfaces.

The dynamic wettability of segmented L-PMMA/HBP-64-OH nanostructured surfaces was also explored by analyzing the impact behavior of water droplets on the segmented nanostructured surfaces (cf. Fig. 5). Till now, only a limited number of studies on the impact effect have been reported.^28–31 Here, we investigated the impact behavior based on our previous work about impact dynamics of droplets.³² Fig. 5 shows the typical optical images of a water droplet impacting the segmented L-PMMA/HBP-64-OH nanostructured surfaces. When a water droplet encountered a solid surface, its initial spherical shape was forced into a pancake-like form that stretched out over the surface at 4 ms for all samples. The water droplet then retracted to minimize its surface tension (Fig. 5A). In contrast, the water droplet showed different impact behavior on the segmented nanostructured surfaces with different CA values (Fig. 5B–D). Although the rebounding tendency of water droplet became more and more obvious with increasing HBP content (Fig. 5B–D), the wetting adhesion was so intense that the water droplet could not bounce off the surface after impact. The whole droplet was fastened onto the surfaces although it was elongated to a maximum degree (t = 24 ms). In addition to the impact behavior, the maximum spreading of the droplet, D_max/D₀, is significantly weakened for segmented L-PMMA/HBP-64-OH (90/10) nanostructure compared with other samples (Fig. 5E), which was in agreement with the results of CA and impact behavior in Fig. 4A and 5B–D. Here, our results of the impact behavior of water droplets on these film surfaces further indicate that the surface properties of segmented nanostructure can be adjusted by tuning the number of terminal hydroxyl groups of HBP.

Fig. 5 Dynamics of water droplets impacting on the surfaces with different segmented nanostructures. The volume of droplet is 8 μL and impact velocity V_i ≈ 0.5 m s⁻¹. (A)–(D) correspond to L-PMMA, L-PMMA/HBP-64-OH (97.5/2.5), L-PMMA/HBP-64-OH (95/5), L-PMMA/HBP-64-OH (90/10) films, respectively. (E) Time evolution of nondimensional contact diameter of the droplets scaled by the initial diameter before impact.

Conclusions

In summary, we proposed a one-step strategy to fabricate segmented polymer nanostructures by wetting the nanopores of AAO templates with hyperbranched polymer/linear polymer blend melts. The key point of our approach is the phase separation of polymer blend melts within nanopores. With our approach, the segmented structures consisting of hyperbranched polyester nanowires as the top layer and linear poly(methyl methacrylate) nanorods as the bottom layer can be conveniently obtained. The as-prepared segmented nanowires/nanorods showed tunable wettability and droplet impact behavior. By changing the content or the number of terminal hydroxyl groups of hyperbranched polymer, the static and dynamic wettability can be controlled. Of particular interests in the future research is to regulate the optoelectronical property of polymer with our approach, which should be of significant importance for the fabrication of polymer-based optoelectronic devices.

Experimental section

Materials

HBP with 64 or 16 primary hydroxyl groups per molecule, denoted respectively as HBP-64-OH (M_{n,theoretical} = 7323 g mol⁻¹, M_n,SEC/MALLS = 4480 g mol⁻¹, M_w,SEC/MALLS = 5400 g mol⁻¹, M_w/M_n = 1.20, T_g = 94 °C) and HBP-16-OH (M_{n,theoretical} = 1750 g mol⁻¹, M_n,SEC/MALLS = 1830 g mol⁻¹, M_w,SEC/MALLS = 3130 g mol⁻¹, M_w/M_n = 1.71, T_g = 65 °C), were purchased from Polymer Factory Co. (Sweden). S,S′-bis(α,α′-dimethylacetic acid) trithiocarbonate (BDATTC) was synthesized according to reported procedures.³³ MMA was purchased from ACROS Chemical Industries (USA), and purified by distillation under reduced pressure. 2,2′-azobisisobutyronitrile (AIBN) was purchased from Tianjiin Kermel Chemical Reagents Development Center (China), and was recrystallized twice from ethanol before use. AAO templates were purchased from Whatman Co Ltd. (U.K.), with through-hole channel diameter of 200 nm and channel length of about 60 μm. NaOH and ethanol were purchased from Alfa Aesar China Co. Ltd. (China).

Synthesis of L-PMMA

Reversible addition-fragmentation transfer polymerization (RAFT) was adapted to synthesize L-PMMA using MMA as monomer, BDATTC as chain transfer agent, and AIBN as initiator, respectively.³³ In a flask equipped with magnetic stirring, BDATTC (141 mg, 0.5 mmol) and AIBN (2.46 mg, 0.015 mmol) were dissolved in tertbutyl alcohol (5 mL) before the addition of MMA (15 g, 0.15 mol) under vigorous stirring. The mixture was degassed under vacuum for 15 min, and purged under dry nitrogen for 30 min. The procedure described above was repeated three times. The flask was then sealed under vacuum, heated at 80 °C for 10 h and cooled to ambient temperature. The product was precipitated in cold methanol for three times and dried under vacuum at 40 °C for 5 days. FTIR (KBr): 2998 cm⁻¹, 2950 cm⁻¹ (ν C–H), 1730 cm⁻¹ (ν C=O), 1052 cm⁻¹ (ν C=S). ¹H NMR (DMSO-d₆, δ, ppm): 3.56 (–O–CH₃); 1.68–2.16 (–CH₂–); 0.82–1.04 (–CH₃). SEC/MALLS: M_n,SEC/MALLS = 31 330 g mol⁻¹, M_w,SEC/MALLS = 38 610 g mol⁻¹, M_w/M_n = 1.23, Degree of polymerization (DP_n) = 310. DSC: T_g = 106 °C.

Preparation of L-PMMA/HBP-64-OH and L-PMMA/HBP-16-OH blends

In a beaker equipped with magnetic stirring, L-PMMA (190 mg) and HBP-64-OH (10 mg) was dissolved in THF (5 mL). The mixture was treated with ultrasonication until all the solvent was completely evaporated in fume hood. The final L-PMMA/HBP-64-OH blends were dried under vacuum at 40 °C for 2 days, and then hot pressed into films with thickness of about 200 μm. Other L-PMMA/HBP-64-OH blends with different mass ratios (97.5/2.5, 90/10), and L-PMMA/HBP-16-OH (95/5) blends were prepared and treated in similar procedures.

Fabrication of segmented nanostructures

The as-prepared polymer blend films were placed on the top of an AAO template. The film/AAO template was assembled in a mould to ensure good contact between the film and AAO template while no extra pressure was applied.²⁵ The mould was heated at 160 °C for 15 min for the polymer blends to melt and to flow into the AAO nanochannels, and then cooled to ambient temperature, leading to the formation of segmented polymer nanostructures in the AAO nanochannels. After removing AAO template with a NaOH solution, the segmented polymer nanostructures were washed with deionized water and ethanol, and dried under vacuum at room temperature for 5 h.

Characterization

The structure of L-PMMA was confirmed by Fourier transform infrared spectroscopy (FTIR), ¹H nuclear magnetic resonance (¹H NMR) spectroscopy, size exclusion chromatography/multi angle laser light scattering (SEC/MALLS) and differential scanning calorimetry (DSC). The FTIR spectra of L-PMMA in thin film of KBr were recorded on a WQF-31 spectrometer (Rui Li, Beijing, China). The ¹H NMR spectrum was conducted on a Bruker Avance 300 spectrometer (Bruker BioSpin, Switzerland) operating at 300 MHz with DMSO-d₆. The molecular structure parameters and viscosity of polymers were determined on a DAWN EOS SEC/MALLS instrument equipped with viscometer (Wyatt Technology, USA), HPLC grade THF (at 25 °C) was used as eluent at a flow rate of 0.5 mL min⁻¹. DSC (TA 2910, USA) was used to obtain the T_g of L-PMMA, HBP-64-OH and HBP-16-OH. The samples were heated from −20 to 200 °C with scan rate of 20 °C min⁻¹ to remove thermal history, and then T_g was obtained in the second scanning from −20 to 200 °C with scan rate of 10 °C min⁻¹.

A series of segmented nanostructures were characterized using scanning electron microscopy (SEM, JEOL Model JSM-6490). All the samples were coated with 5.0 nm thick Au film. Static water contact angle (CA) measurements were carried out by using a Rame-hart Model 250-F1 Standard Goniometer with DROP image Advanced 2.1 at ambient temperature. CA was determined by fitting a Young–Laplace curve around the drops. Five parallel measurements were made for each sample, and the average contact angle value was taken.

Impact dynamics monitor

Droplets with volume of 9–15 mL were generated using a micro-syringe. The droplets were allowed to freely fall and impact onto the surfaces of as-prepared L-PMMA, L-PMMA/HBP-64-OH film. The impact velocity V_i was controlled by the falling height of droplets. The impact behavior of the drop was monitored using a high speed camera Trouble Shooter HR (US) at a frame rate of 2000 fps. The image data was analyzed using MaxTRAQ software.

Acknowledgements

This work was financially supported by the National Natural Science Foundation (21374088, 21303273), the Program for New Century Excellent Talents of Ministry of Education (NCET-13-0476), Fundamental Research Funds for the Central Universities (14lgpy21), Research Fund for the Doctoral Program of Higher Education (20130171120001), the Program of Youth Science and Technology Nova of Shaanxi Province of China (2013KJXX-21), Natural Science Foundation of Guangdong Province (S2013040016770), and the Program of New Staff and Research Area Project of NPU (13GH014602).

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Footnote

† Electronic supplementary information (ESI) available: Polymer structures and SEM images can be found in ESI. See DOI: 10.1039/c4ra09155h

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