Grafting Amphiphilic Brushes onto Halloysite Nanotubes via a Living

Article pubs.acs.org/JPCB

Grafting Amphiphilic Brushes onto Halloysite Nanotubes via a Living RAFT Polymerization and Their Pickering Emulsification Behavior Yifan Hou, Junqing Jiang, Kai Li, Yanwu Zhang,* and Jindun Liu School of Chemical Engineering and Energy, Zhengzhou University, 100# Science Road, Zhengzhou 450001, People’s Republic of China ABSTRACT: Amphiphilic brushes of poly(4-vinylpyridine)-block-polystyrene (P4VPb-PS) and polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) are grafted onto halloysite nanotubes (HNTs) via a surface reversible addition−fragmentation chain transfer (RAFT) living polymerization through anchoring R group in RAFT agent S-1dodecyl-S′-(R,R′-dimethyl-R″-acetic acid) trithiocarbonates (DDMAT). The characterization of TGA, TEM, and GPC show that amphiphilic brushes are successfully grafted onto HNTs in a living manner. To verify the amphiphilicity of HNTs grafted with block copolymers, their Pickering emulsification behavior in water/soybean oil diphase mixture is studied. The results show that modified HNTs can emulsify water/soybean oil diphase mixture and the emulsification performance is dependent on microstructure of amphiphilic brushes such as hydrophilic/hydrophobic segment size and sequence.

1. INTRODUCTION Polymer brushes refer to polymeric assemblies tethered at one end to a solid substrate either through covalent attachment or physical adsorption.1 Initially, polymer brushes have played the primary and relatively limited roles as functional surface coatings exhibiting excellent long-term mechanical stability and chemical robustness. As the microstructure of polymer brushes is precisely designed and well-defined, polymer brushes can be considered as nanoscale soft building blocks capable of enabling practical nanotechnology by bestowing functions, from redox activity and photophysical properties to biocompatibility and capacity for energy storage, upon a broad range of materials with their concomitant practical benefits.2 Due to their intrinsic stability in different solvent conditions, polymer brushes via covalent anchoring are preferable to that via physical adsorption in most cases. There are primarily three methods to create polymer brushes via covalent anchoring: “graft through”, “graft to”, and “graft from”. Therein, “graft from” is preferable to other methods due to higher grafting density and thicker polymer layers. Controlled radical polymerizations (CRP) offer an attractive technique to design polymer brushes, allowing for the control over multiple molecular variables including chain composition, molecular weight, architecture, and sequence, as well as end- and side-group functionality. The main CRP includes atom transfer radical polymerization (ATRP), nitroxide-mediated polymerization (NMP), and reversible addition−fragmentation chain transfer (RAFT) polymerization.3,4 Because of versatility of monomer choice, the excellent control of polymer microstructure, lack of metal catalysts and mild reaction conditions, RAFT polymerization have been rapidly used in the past decade.5−7 Amphiphilic polymer brushes consisting of hydrophobic segments and hydrophilic segments are most attractive because © 2014 American Chemical Society

they provide more designable variables such as segment size, sequence and ratio between different segments in designing of polymer brushes. Poly(4-vinylpyridine) (P4VP) is a pHsensitive hydrophilic polymer with functional groups that can coordinate with various transition metal ions.8,9 Considering the similar chemical structure and reactivity of 4-vinylpyridine (4VP) and styrene (St), St is usually used as a hydrophobic comonomer in the synthesis of amphiphilic block copolymers of 4-vinylpyridine via a RAFT polymerization.10,11 By now, except that several reports about grafting of P4VP polymer brushes, amphiphilic block copolymers of P4VP have seldom been grafted onto nanoparticles via a surface RAFT polymerization.12−14 Halloysite nanotubes (HNTs) are one type of tubule alumosilicate clay with external diameter of 50−80 nm and are potential to be applied in controlled release, nanocomposites and catalysis.15 In our previous work, P4VP brushes of different lengths were grafted onto halloysite nanotubes (HNTs) via SI-ATRP and controlled immobilization of methyltrioxorhenium (MTO) was realized through stoichiometric coordination between MTO and N atom in pyridine rings along P4VP brushes.16 But their distribution in diphase mixtures is challenging. Herein, a living RAFT polymerization is adapted to graft amphiphilic brushes consisting of P4VP segments and polystyrene (PS) segments onto HNTs through anchoring R group of RAFT agent. The size and sequence of segments are precisely designed and well controlled. To verify the amphiphilicity of HNTs grafted with block copolymers, Received: November 26, 2013 Revised: February 2, 2014 Published: February 4, 2014 1962

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their Pickering emulsification behavior in water/soybean oil diphase mixture is studied.

Scheme 1. Anchoring of RAFT Agent onto HNTs

2. EXPERIMENTAL SECTION Materials and Methods. Halloysite nanotubes (HNTs; outer diameter: 30−50 nm, inner diameter: 10−20 nm, length: 100−400 nm, specific surface area: 62.23m2/g) originated from Henan, China. 4-Vinylpyridine (95%) and styrene were purchased from J&K and used after reduced pressure distillation. 3-Glycidoxypropyltrimethoxysilane (GTMS, 97%), N,N′-dicyclohexylcarbodiimide (DCC, 99%), 4-dimethylaminopyridine (DMAP, 99%), and 4,4′-azobis(4-cyanovaleric acid) (98%; V501) were purchased from J&K and used as received. S-1-Dodecyl-S′-(R,R′-dimethyl-R″-acetic acid) trithiocarbonates (DDMAT) was synthesized according to Lai.17 FT-IR spectra were recorded on a Biorad 3500 GX spectrophotometer in the 400−4000 cm−1 range. All the spectra were collected with a resolution of 4 cm−1 and accumulation of 64 scans. Transmission electron microscopy (TEM) was carried out on JEOL-JEM 1200 equipment operating at 100 kV. TGA data were obtained in a nitrogen atmosphere with TA-60 (Shimadzu Corporation). The number average molecular weight (Mn) and polydispersity index (PDI, Mw/Mn) were measured by PS-calibrated GPC with an Agilent 1100 series RI detector (Agilent 1100, Germany). DMF was used as the eluent at a flow rate of 1 mL/min. Samples were prepared as the following procedure. HNTs grafted with polymer brushes were soaked in the mixture of THF, HF (49 wt %), and HCl (38 wt %; 25/1/1 Vol) and stirred for 12 h. After the liquids were evaporated under an air flow, DMF was added to dissolve the polymer. Then the inorganic was filtered out through a polyamide micromembrane filter (0.45 μm) and the recovered polymer solution was subjected to GPC analysis. Anchoring of RAFT Agent. HNTs (5.0 g) were baked at 400 °C for 2 h in a muffle furnace to remove absorbed water and then were dispersed in toluene (50 mL) at 10000 rpm with high-speed emulsifier for 10 min. The colloidal suspension was transferred to a flask equipped with a condenser and a magnetic stirrer and refluxed for 48 h after GTMS (9.0 g) was added. The product was collected through centrifugation and washed three times with toluene (10 mL). After drying in vacuum at 60 °C for 12 h, the product was named as “HNTs-Epoxy”. “HNTs-Epoxy” was refluxed in a methanol solution (300 mL) of hydrochloric acid (1.6 M) for 24 h. The product was collected through centrifugation and washed three times with methanol (10 mL). After drying in vacuum at 60 °C for 12 h, the product was named as “HNTs-OH”. “HNTs-OH” (2.80 g), DDMAT (1.10 g, 3 mmol), DCC (0.62 g, 3 mmol), and DMAP (0.73 g, 0.6 mmol) were dispersed into dichloromethane (100 mL) and reacted for 48 h at room temperature under stirring. The resulting HNTs anchored with RAFT agent were centrifuged and washed three times sequentially with distilled water (10 mL), methanol (10 mL), and acetone (10 mL). After drying in vacuum at room temperature for 12 h, the product was named as “HNTs-CTA”. The whole procedure is schematically shown as Scheme 1. Growing Polymer Brushes onto HNTs via a Surface RAFT Polymerization. The procedure of grafting polymer brushes onto HNTs via a surface RAFT polymerization is schematically shown as Scheme 2. The details about polymerization are listed in Table 1. To guarantee the solubility of polymers, DMF was used as a polymerization media.

Scheme 2. Grafting of Amphiphilic Brushes onto HNTs via a Surface Living RAFT Polymerization

Table 1. Polymerization Conditionsa run 1 2 3 4

St (g)

4VP (g)

anchoring-CTA (g)

V501 (mg)

10.5

0.96b 0.96b 0.12c 0.11d

14.0 14.0 1.0 0.4

10.4 0.7 0.3

DDMAT (mg) 91.0 2.7

a

Monomer concentration: 28 wt %; polymerization temperature: 67 °C; [M]/[anchoring-CTA]/[V501] = 2000:5:1. bHNTs-CTA, CTA content: 0.26 mmol/g. cHNTs-P4VP, CTA content: 0.172 mmol/g. d HNTs-PS, CTA content: 0.067 mmol/g.

The typical procedure of grafting homopolymers is as the following. Monomers (4VP or St) and “HNTs-CTA” were dispersed in DMF by ultrasonication for 30 min. The suspension was degassed via three freeze−pump−thaw cycles under N2 at 0 °C. After the temperature increased to 67 °C, the initiator, V501, was added and the polymerization started. Parts of the suspension were taken out after a certain polymerization time and quenched with dry ice for GPC characterization. For graft polymerization of St, some of free CTA was added. The solid products were collected through centrifugation and washed three times with DMF (10 mL). After drying in vacuum at 40 °C for 12 h, HNTs grafted with polymer brushes 1963

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were named as “HNTs-P4VPn” or “HNTs-PSn”, where the subscript n represented the polymerization time. The typical procedure of grafting block copolymers is as the following. 4VP (St) and HNTs-PS19 (HNTs-P4VP6) were dispersed in DMF by ultrasonication for 30 min. The suspension was degassed via three freeze−pump−thaw cycles under N2 at 0 °C. After the temperature increased to 67 °C, the initiator, V501, was added and the polymerization started. Parts of the suspension were taken out after a certain polymerization time and quenched with dry ice for GPC characterization. For graft polymerization of St, some of free CTA was added. The solid products were collected through centrifugation and washed three times with DMF (10 mL). After drying in vacuum at 40 °C for 12 h, HNTs grafted with polymer brushes were named as “HNTs-P4VPn-b-PSn” or “HNTs-PSn-b-P4VPn”, where the subscript n identified the polymerization time. Pickering Emulsification Behavior. In a dry glass vial, modified HNTs (5 mg) was dispersed in a mixture of soybean oil and water (2 mL, 1/1 Vol) under ultrasonic for 1 h and then observe the dispersion behavior of modified HNTs after standing 24 h.

Figure 2. TGA curves of (a) HNTs, (b) HNTs-Epoxy, (c) HNTs-OH, and (d) HNTs-CTA.

weight loss difference between HNTs-OH and (d) HNTs-CTA according to eq 1. The amount of anchored CTA is 0.26 mmol/g.

3. RESULTS AND DISCUSSION 3.1. Anchoring of DDMAT. FT-IR was used to qualitatively characterize surface functional groups of HNTs and the spectra of (a) HNTs, (b) HNTs-Epoxy, and (c) HNTs-CTA were shown in Figure 1. Comparing to Figure 1a,

amount of anchored CTA(mmol/g) =

WB 100 − WB

× 100 − WA (1)

M × 100

WA and WB are the weight loss of HNTs-A and HNTs-B in the range of 100−700 °CM: the difference of the molecule weight between A and B 3.2. Grafting Polymer Brushes onto HNTs. To verify the living behavior of grafting polymer brushes, TGA and GPC were adapted to monitor the polymerization. TGA analysis was carried out under N2 and the weight losses at 700 °C for HNTs-P4VPn and HNTs-PSn are listed in Table 2. The weight Table 2. Results of TGA Characterization for HNTs-P4VPn and HNTs-PSn

Figure 1. FT-IR spectra of (a) HNTs, (b) HNTs-Epoxy, and (c) HNTs-CTA.

the new absorbance band at 625 cm−1 in Figure 1b is ascribed to Si−O−Si resulted from the reaction between surface hydroxyl groups on HNTs and GTMS. In Figure 1c, the appearance of a weak peak at 1730 cm−1 ascribing to −COO group indicates that the DDMAT was reacted with hydroxyl groups and anchored onto HNTs successfully. Because there exists strong absorption around 900−1130 cm−1 of HNTs, the characteristic peaks of epoxy groups (910 cm−1) in HNTsEpoxy and CS (1062 cm−1) cannot be observed due to the overlap.18 Based on the same reason, the spectrum of HNTsOH was not provided. To quantitatively determine the amount of anchored CTA, TGA was used to characterize weight loss of (a) HNTs, (b) HNTs-Epoxy, (c) HNTs-OH, and (d) HNTs-CTA and their TGA curves were presented in Figure 2. As the size of the anchored groups increase, the weight loss increases. The amount of anchored CTA can be calculated based on the

sample

weight loss at 700 °C (%)

sample

weight loss at 700 °C (%)

HNTs-P4VP2 HNTs-P4VP3 HNTs-P4VP4 HNTs-P4VP5 HNTs-P4VP6

34.9 35.7 38.6 43.1 46.3

HNTs-PS3 HNTs-PS6 HNTs-PS9 HNTs-PS19 HNTs-CTA

24.1 25.6 48.6 50.8 23.0

losses increase as the polymerization time increases, which indicates more organic polymers are grafted with the increase of polymerization time. The weight losses for HNTs-P4VPn are greater than that for HNTs-PSn. This indicates that P4VP is easier to be grafted onto HNTs than PS though free CTA is added for grafting of PS. Polymer brushes were cleaved from HNTs prior to GPC characterization through etching HNTs using HCl/HF acid mixture. The results show that Mn of cleaved P4VP and PS increases as the polymerization time increases (shown in Figure 3). Meanwhile, their PDI is kept below 1.5. These indicate that brushes of homopolymers have been grafted onto HNTs in a living manner. Further, HNTs grafted with homopolymers continued to act as CTA in the procedure of grafting amphiphilic brushes. The shift to left of GPC traces indicates that more comonomers incorporate into brushes as the polymerization time increases (shown in Figures 4 and 5). But when St acted as comonomers, 1964

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Figure 5. GPC traces of cleaved PS19-b-P4VP for different polymerization times: (a) 3, (b) 6, and (c) 9 h.

Figure 3. Plots of Mn and PDI vs polymerization time for polymers cleaved from (a) HNTs-P4VP and (b) HNTs-PS.

Figure 4. GPC traces of cleaved P4VP6-b-PS for different polymerization times: (a) 3, (b) 6, and (c) 9 h.

Figure 6. Plots of Mn and PDI vs polymerization time for amphiphilic brushes cleaved from (a) HNTs-P4VP-b-PS and (b) HNTs-PS-bP4VP.

the peak has obvious shoulder peaks in the GPC trace at longer polymerization time. This is attributed to poorly living behavior of St in case of solid CTA though access free CTA is added. Their corresponding Mn and PDI of different polymerization time are plotted in Figure 6. Mn of block copolymers increases linearly with polymerization time and their PDI is kept at low level. Also, the weight losses for grafted block copolymer brushes increase compared with that for grafted homopolymer brushes, as indicated in Figure 7. These indicate that CTA linked at the free end of brushes can still control the propagation of polymer chains.

The growth of polymer brushes from HNTs can be ascertained from TEM images. In Figure 7, a uniform polymer coating is observed on the surface of HNTs grafted with polymer brushes and the walls of HNTs grafted with brushes become thicker and the inner radius become smaller (Figure 7b) compared with HNTs (Figure 7a). These indicate that polymer brushes have been grafted onto the HNTs outer and inner walls via a surface RAFT polymerization. 3.3. Pickering Emulsification Performance. As known, nanoparticles can act as Pickering emulsifier to stabilize the emulsion and their surface properties play an important role in 1965

dx.doi.org/10.1021/jp411610a | J. Phys. Chem. B 2014, 118, 1962−1967

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Figure 8. Water−soybean oil emulsions stabilized by (a) HNTs, (b) HNTs-P4VP6, (c) HNTs-P4VP6-b-PS3, (d) HNTs-P4VP6-b-PS9, (e) HNTs-PS19-b-P4VP3, and (f) HNTs-PS19-b-P4VP6.

into the water-rich phase than HNTs-P4VP, and as the length of the PS segment increases, the interface between the oil-rich phase and the water-rich phase grows dim. Conversely, when HNTs-PS19-b-P4VP3 and HNTs-PS19-b-P4VP6 act as emulsifiers, more modified HNTs exist in the oil-rich phase as the size of the P4VP segment increases. These phenomena are different from the opinion about simple colloidal particles: “Hydrophilic particles tend to form oil-in-water (o/w) emulsions, whereas hydrophobic particles form water-in-oil (w/o) emulsions”.20 The reason may be ascribed to assembling behavior and special morphology of amphiphilic brushes at the interface. Among samples, the diphase mixture emulsified by HNTs-PS19-bP4VP3 tends to one phase. The above results show that modified HNTs can emulsify water/oil diphase mixture and their distribution in two phases is dependent on the hydrophilic/hydrophobic segment size and sequence.

4. CONCLUSIONS PS and P4VP brushes of different length can be grafted onto HNTs via a surface RAFT living polymerization through anchoring R group of chain transfer agent. In the subsequent block copolymerization, the homopolymer brushes grafted onto HNTs as a chain transfer agent can effectively control the polymerization. Therefore, amphiphilic block copolymer brushes of PS-b-P4VP and P4VP-b-PS were grafted onto HNTs in a living manner. The segment size in block copolymer brushes can be adjusted through changing the polymerization time. HNTs grafted with polymer brushes have certain emulsifying function to the mixture of water and oil, and their emulsification performance is dependent on the microstructure of amphiphilic brushes such as hydrophilic/hydrophobic segment size and sequence.

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Figure 7. TEM images of (a) HNTs and (b) HNTs grafted with brushes.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

Pickering emulsification.19 To characterize the effect of the brushes microstructure on amphiphilicity of HNTs grafted with block copolymers, Pickering emulsification performance of modified HNTs in a water/oil diphase mixture was observed and the effect of segment length and sequence on the emulsification performance was studied. As shown in Figure 8, all the samples have certain emulsification functions to the diphase mixture. Therein, most of HNTs existed at the interface between oil-rich and water-rich phases and the weakest emulsification effect of HNTs can be ascribed to the small size of HNTs. After P4VP brushes are grafted onto HNTs, more HNTs-P4VP diffuse into the water-rich phase. When HNTs-P4VP6-b-PS3 and HNTsP4VP6-b-PS9 act as emulsifiers, more modified HNTs diffuse

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (50903074), Key Scientific Research Project of Chinese Education Ministry (212107).

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dx.doi.org/10.1021/jp411610a | J. Phys. Chem. B 2014, 118, 1962−1967