Preparation of polydisperse polystyrene-block-poly(4-vinyl pyridine) synthesized by TEMPO-mediated radical polymerization and the facile nanostructure formation by self-assembly
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This article reports a successful nanostructure formation from block copolymer having broad distribution of molecular weight. The block copolymer synthesis and the nanosphere formation are facile; therefore, it is promising for fabrication of nanostructure materials in large-scale manufactory. The polydisperse diblock copolymer of polystyrene-block-poly(4-vinyl pyridine) (PSVP) was prepared by the nitroxide-mediated radical polymerization that contains the fraction of poly(4-vinyl pyridine) block of 45 mol% and the overall polydispersity index of 2.08. The phase separation of PSVP was induced by the simple evaporation of co-solvent DMF:THF (70:30 v/v) of the PSVP solution. The SEM images of the self-assembled polydisperse PSVP display the spherical morphology with the diameter of ~ 50 nm, which is larger than that of block copolymer having narrow molecular weight distribution. By simply immersing the self-assembled film into iron chloride solution, the transformation from the spherical structure to the porous structure occurred directly without sacrificing the block copolymer component indicating the advantage of stimuli-response properties of the self-assembled PSVP. The results demonstrated that the polydisperse block copolymer could be used for the nanostructure formation by simple synthesis and evaporation procedures and, therefore, it is suitable for industrial applications.
KeywordsNanosphere Nitroxide-mediated polymerization Self-assembly Polydisperse block copolymer
Microphase separation of the amphiphillic block copolymer (BCP) of PSVP, which is driven by the strong repulsive interaction between the non-polar polystyrene (PS) block and the polar poly(4-vinyl pyridine) (P4VP) block, generates the various morphologies at nanometer scale such as sphere, cylinder, lamellar, and gyroid. The diversity of the nanometer morphologies has broadened their applications  from electronics devices , (nano)lithography [3, 4], nanoporous structure , self-cleaning surface, membrane filtration [6, 7, 8] to biomedical materials [9, 10]. Therefore, the studies on the microphase separation of BCP have been attracted many attentions in academia and industry. However, in the large-scale or in the industrial synthesis, the self-assembly process for nanostructure morphologies is difficult because the molecular weight distribution (MWD) of the polymer is broader than that synthesized in the ideal laboratory conditions (polydispersity index, PDI < 1.2) . Thereby, more attention has been focused on the nanostructure formation from polydisperse BCPs, which are synthesized by controlled/”living” radical polymerization (CRP) .
Strategies to approach the polydisperse BCP are either by blending of block copolymers/homopolymer having different molecular weight [13, 14, 15] or by adjusting the polymerization conditions [16, 17, 18] of CRP. The latter is considered closer to the nature of BCP structure in the large-scale industrial process . Previous reports demonstrated that the morphologies such as disordered microstructure [19, 20, 21], ordered lamellar or cylinder [17, 19, 20, 21] were observed depending on the fraction of the individual components. The coexistence of two morphologies  and the shift of the phase boundaries [17, 18] were assigned to the broad PDI of BCPs. In addition, the sizes [13, 17] and the size distribution  of the microdomain increased as the PDI increased. However, there is a disagreement issue on the presence of macrophase domain [13, 18, 19, 22], which is due to the phase separation of low molecular weight homopolymer incorporated to polydisperse BCPs. The macrophase separation was observed in the self-assembly of polydisperse BCPs having high concentration of homopolymer  and high PDI (above 1.8) .
Materials and experimentals
Styrene (St, Acros, 99%) was freshly purified by passing through neutral aluminium oxide column under nitrogen gas before being used. 4-Vinyl Pyridine (4VP, Acros, 99%) was freshly distilled before being used. 2,2,6,6-Tetramethylpiperidin-1-yl oxy (TEMPO, Acros, 98%), benzoyl peroxide (BPO, China, 99%), FeCl2·4H2O, FeCl3·6H2O (Xilong Scientific Co. Ltd, China), and solvents (Xilong Scientific Co, Ltd., China) were used as received.
Preparation of polystyrene capped with TEMPO (PS)
The synthesis of polystyrene (PS) was carried out in a 250-mL schlenk flask under nitrogen. The reactor was charged with St (50 mL), BPO (0.36 g, 1.50 × 10−3 mol), and TEMPO (0.31 g, 1.95 × 10−3 mol), degassed by bubbling nitrogen gas for 20 min. The polymerization was carried out at 95 °C for 60 min, raised to 130 °C for 7.5 h. The viscous solution was diluted with THF, precipitated in methanol, filtered, and dried at 70 °C for 12 h. The white powder of PS having TEMPO end functional group was obtained.
Preparation of polystyrene-block-poly(4-vinyl pyridine) (PSVP)
The synthesis of polystyrene-block-poly(4-vinyl pryridine) (PSVP) was carried out in a 250-mL schlenk flask under nitrogen gas. The synthesis procedure of PSVP is the same as PS with the reactor ratio of [4VP]:[BPO]:[PS] = 290:1:1.3. The reaction was proceeded for 2 h at 130 °C under inert gas. Then, the solution was exposed to air to stop the reaction. PSVP was precipitated from the THF solution in cold hexane, filtered, washed several times, and dried at 70 °C for 12 h.
Preparation of self-assembly film
The glass substrates were cleaned from the organic contaminants by immersing in piranha solution (H2O2 30%:H2SO4 98%—1:3 v/v) at room temperature for 60 min, cleaning with distilled water, acetone and ethanol, and drying under nitrogen flow. Then, the 20 wt% solution of PSVP in co-solvents of DMF:THF (70:30) was casted on slide glass, dried at 50 °C for 20 min, immersed in distilled water overnight, and dried under reduced atmosphere for 24 h. The film was subsequently immersed in 13 wt% iron chloride solution ([FeCl2]:[FeCl3] = 1:2) for 24 h at room temperature to afford the porous structure. The porous film was dried at ambient condition.
Molecular weight and molecular weight distribution of polymer were determined by gel permeation chromatography (PL GPC 50Plus—Varian) equipped with Mesopore columns (7.5 × 300 mm), THF eluent at the flow rate 1 mL/min. Linear polystyrene standards were used for calibration. The polymer structure was characterized by Fourier transform infrared spectroscopy (FT-IR, EQUINOX 55 Bruker, Germany) and proton nuclear magnetic resonance (1H-NMR, 500 MHz, Bruker Avance, Germany). The morphology of nanostructure was observed from FE-SEM images (FE-SEM S4800, Hitachi, Japan).
Results and discussion
The chromatograms (Fig. 3) gradually shift to lower retention time with increasing St conversion, indicating the increase of PS molecular weight. In addition, the unimodal curves of PS 95-130, PS 95-140 chromatograms are the evidence for the “living” behavior of PS polymerization. On the contrary, the bimodal curve of PS 145 (St conversion ~ 0.57) indicates the loss chain-end. This result supports the deviation from linearity of the evolution of Mn as discussed above. Although PS 95–145 and PS 95–130 possess the narrow PDIs, the appearance of the tails on GPC curves may indicate that there are polymer chains that lose their functional end groups. These terminated chains are confirmed through the bimodal curves of the chain extension of PS 95–130 with 4VP (PSVP on Fig. 3c). It can be observed on GPC curve that there is the similar peak at evolution time (~ 450 s) of PSVP to the peak of PS 95–130 at conversion of ~ 3%. Hence, it can be concluded that PSVP contains a portion of PS low molecular weight ~ 3500 g/mol. The overall number-average molecular weight and PDI of PSVP are 24,000 g/mol and 2.08, respectively.
These results showed that the polydisperse BCP could be employed for microphase separation for many applications such as nano-coating layer for self-cleaning surface, as nanoporous materials for membrane fabrication. The incorporation of PSVP with metal ion also promises for the development of the hydrid inorganic–organic nanomaterials for electronic or smart devices. The interesting stimuli-responsive properties of PSVP are suitable for the application in drug-delivery system or in other biomedical applications.
In this paper, the simple self-assembly from BCP to nanostructure has been reported. BCP composed of PS and P4VP has been synthesized at high polymerization temperature in the presence of TEMPO as the capping agent. The “living” fashion of PS polymerization has been observed. However, at high temperatures, the spontaneous self-initiation of St and the termination reactions have occurred that caused the low molecular weight PS homopolymer as the observed tails in chromatogram curves. Hence, the extension of PS with 4VP monomer produced block copolymer with broad breadth (PDI 2.08) and bimodal on GPC curves. The self-organization of BCP is driven by the differences in polarity between PS block and P4VP showed the loose packing of spheres on thick film fabricated from high-concentration solution in DMF/THF co-solvents. The spherical morphology is speculated to compose of P4VP block core and the PS block corona. Low molecular weight of terminated PS interacts with these corona relaxing PS chains that lead to increase the corona domains. Additionally, the short chains of PSVP distributed among the long chains of PSVP cause the chains’ packing density decrease and change the interfacial curvature. As a result, the micelle domains is large with the average diameter of 50 nm. Furthermore, this speculated structure is indirectly proved by the immersion of the film into the iron chloride solution to perform the swelling and coordinating of metal with P4VP core. The subsequent solvent evaporation generated the opened pores with the similar pore sizes to original P4VP spherical core. Our results are the experimental evidences for the self-assemblable nanoscale structure of high polydispersity of BCP by the simple polymerization and self-organization which is expected to be able to the scale-up to industrial production for many applications.
This research is funded by the Vietnam National University Ho Chi Minh City (VNU-HCM) under Grant Number C2015-18-15.
Compliance with ethical standards
Conflict of interest
The authors declare that there are no conflicts of interest.
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