Surface-initiated ATRP from polydopamine-modified TiO2 nanoparticles

Contents lists available atScienceDirect

European Polymer Journal

Surface-initiated ATRP from polydopamine-modi

fied TiO

2

nanoparticles

Maciej Kope

ć

, Järvi Spanjers

, Elio Scavo

, Dennis Ernens

, Joost Duvigneau

,

G. Julius Vancso

a_{Materials Science and Technology of Polymers, MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands} b_{Laboratory for Surface Technology and Tribology, Engineering Technology, University of Twente, Enschede, The Netherlands}

c_{Wells R&D, Innovation, Research & Development, Shell Global Solutions International BV, Rijswijk, The Netherlands}

A R T I C L E I N F O Keywords: SI-ATRP Titanium dioxide Grafting from Nanocomposites Polydopamine Lubrication A B S T R A C T

A robust approach for modification of TiO2nanoparticles with polymer brushes by atom transfer radical poly-merization (ATRP) is presented. TiO2surface wasfirst coated with polydopamine (PDA) followed by im-mobilization of an ATRP initiator,α-bromoisobutyryl bromide (BiBB). Poly(methyl methacrylate) (PMMA) and poly(butyl acrylate) (PBA) were then grafted from the PDA-modified TiO2of different size (25 and 300 nm) in DMF at room temperature via supplemental activator reducing agent (SARA) ATRP using only 100 ppm of the copper catalyst. Hybrid core-shell particles with high organic contents (40–88 wt%) and grafting densities (0.16–0.25 nm−2_{) were obtained. Reaction conducted in the presence of sacrificial initiator confirmed excellent} control over the polymerization and produced PMMA and PBA with narrow molecular weight distributions (Mw/ Mn< 1.25). Obtained particles were tested as lubricating additives in pipe dope compositions. Addition of polymer-grafted TiO2to the base grease resulted in a reduced coefficient of friction (COF) and wear over un-coated TiO2as revealed by reciprocating pin-on-disc tests. The model pipe dopes with PMMA-grafted particles were found to perform on par with commercial American Petroleum Institute (API) dope.

1. Introduction

Titanium dioxide (TiO2) is a widely known pigment used in paints

and sunscreens. TiO2exhibits three distinct polymorhps, namely

ana-tase, rutile and brookite; since the discovery of the semiconductive nature of anatase, TiO2nanoparticles (NPs) have attracted tremendous

interest in photovoltaics and photocatalysis[1–3]. Moreover, TiO2NPs

were shown to exhibit good lubricating properties and to reduce wear and friction when used as additive in various lubricants[4–9].

A crucial issue in many applications of NPs is the ability to control their dispersion-forming properties and to reduce the tendency to ag-glomerate, often realized by surface modification with polymer brushes [10,11]. Reversible deactivation radical polymerization (RDRP) tech-niques, such as atom transfer radical polymerization (ATRP), allow to synthesize well-defined polymers with controlled molecular weight (MW), molecular weight distribution (MWD), chain composition, to-pology and functionality[12–16]. Due to its versatility and simplicity, surface-initiated ATRP (SI-ATRP) has become a routine tool for grafting polymer brushes from various nanostructures[11,17–21].

However, reports on functionalization of TiO2NPs with polymer

brushes are relatively rare. SI-ATRP was used to graft poly(methyl

methacrylate) (PMMA) [22], polystyrene (PS) [23], poly(N-iso-propylacrylamine) (PNIPAM) [24]and poly(ethylene glycol) methyl ether (meth)acrylate (PEGMA)[25]from TiO2NPs, however without

any assessment of grafting densities or control over the polymerization. Furthermore, normal ATRP with high catalyst loading was used in previous reports. Activator regeneration techniques for ATRP such as activator regeneration by electron transfer (ARGET) ATRP, initiators for continuous activator regeneration (ICAR) ATRP or supplemental acti-vator reducing agent (SARA) ATRP allow to reduce the catalyst con-centration from 1000 to 10,000 ppm to 10–100 ppm (vs monomer), thus rendering the process more environmentally friendly and less ex-pensive [14,15,26]. SARA ATRP has been recently utilized to graft polymer brushes from various metal oxide NPs[27]or planar surfaces [28]. However, in the case of TiO2 NPs very low grafting density (б

= 0.03 nm−2) was observed, most likely due to the weak bonding be-tween the carboxyl group of the fatty-acid inspired ATRP initiator and the TiO2substrate[27].

Alternatively, catechol chemistry can efficiently immobilize in-itiators on TiO2surface. Dopamine-based ATRP initiators were used for

functionalization of TiO2 NPs with polymer brushes by both grafting

from[22]and grafting to[29]approaches. Furthermore, by spontaneous

https://doi.org/10.1016/j.eurpolymj.2018.07.033

Received 29 May 2018; Received in revised form 9 July 2018; Accepted 21 July 2018 ⁎_{Corresponding authors.}

E-mail addresses:m.m.kopec@utwente.nl(M. Kopeć),g.j.vancso@utwente.nl(G. Julius Vancso).

Available online 23 July 2018

0014-3057/ © 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/BY/4.0/).

controlled manner, producing polymer brushes with low dispersities (Mw/Mn= 1.13–1.23), and high grafting densities (0.16–0.25 nm−2).

PBA and PMMA-grafted TiO2 NPs (d = 300 nm) were then tested as

novel,‘greener’ additives to pipe dopes, i.e. greases which lubricate and seal threaded connections in pipes used for oil extraction, effectively reducing friction and wear[35–39].

2. Experimental 2.1. Materials

Dopamine hydrochloride, tris buffered saline, n-butyl acrylate (BA), methyl methacrylate (MMA), triethylamine (99%) (TEA), tris(2-pyr-idiylmethyl)amine (TPMA), α-bromoisobutyryl bromide (BiBB), ethyl α-bromoisobutyrate (EBiB), copper wire (99.999%) and TiO2

nano-powder (99.9% anatase, nominal d < 25 nm, Brunauer-Emmet-Teller specific surface area (SBET) = 65 m2/g) were purchased from

Sigma-Aldrich. TiO2 particles (mixed phase anatase/rutile, d∼ 300 nm,

SBET= 16.66 m2/g) were donated by Artecs BV. Monomers were passed

through a column of basic alumina to remove inhibitors. Cu wire was cleaned in a mixture of hydrochloric acid (HCl))/methanol (MeOH) (1:4 v/v) for 20 min, rinsed with ethanol and dried under stream of air directly before use. Other materials were used as received. Solvents were of analytical grade and supplied by Sigma-Aldrich or Biosolve (Valkenswaard, The Netherlands).

2.2. Coating of TiO2-with polydopamine (TiO2-PDA)

1 g of TiO2and dopamine hydrochloride were added to a

round-bottom flask and dispersed in a mixture of ultrapure water/ethanol (EtOH) 4:1 using sonication for 30–45 min. A tablet of tris-buffered saline was added while stirring. Sodium hydroxide (NaOH) solution (0.1 M) was added until a pH of 8.5 was reached and the mixture was stirred for 24 h at room temperature. The particles were collected with a centrifuge (10,000 rpm, 20 min) and washed with EtOH in three centrifuge-redispersion cycles.

2.3. Immobilization of the ATRP initiator (TiO2-PDA-BiB)

0.30 g PDA-modified TiO2 particles (TiO2-PDA) was dispersed in

20 mL of THF by sonication for 20 min. The flask was degassed for 20 min with nitrogen under continuous stirring. The mixture was cooled to 0 °C followed by the addition of 2 mL of TEA. 5 mL of BiBB solution in THF (1:4 v/v) was then added dropwise. The reaction was stirred for 30 min at 0 °C and for 24 h at room temperature. The par-ticles (TiO2-PDA-BiB) were washed with EtOH by three

centrifugation-sonication cycles (10000 RPM, 20 min) and dried in a vacuum oven overnight.

2.4. Surface-initiated SARA ATRP

100 mg of TiO2-PDA-BiB particles, 5 mL of monomer (35 mmol or

47 mmol for BA and MMA, respectively) and 5 mL of DMF were added to a Schlenkflask. The mixture was sonicated for 30 min to disperse the particles. After sonication, a stock solution of CuBr2/TPMA in DMF

(100 ppm CuBr2vs monomer) was added, followed by the addition of

the sacrificial initiator (EBiB) if indicated. The mixture was degassed

supplied by Shell which was composed of a mineral base oil, bentonite as thickener (5 wt%) and an undisclosed anti-oxidant (0.3 wt%). The particles were subsequently added into the base grease (20–23 wt%). Bare TiO2 was stirred into the dope whereas hybrid particles were

dispersed in THF before addition. The resulting mixture was sonicated for 30 min to disperse the particles in the dope, after which the THF was evaporated overnight under ambient conditions.

2.6. Characterization

Gel permeation chromatography (GPC) was performed on a Waters system (pump: Waters 515, USA, injector: Hewlett-Packard 1050 USA, detector: Waters 2414 and Waters Styragel HR3-6 columns) at 50 °C with DMF 50 mM LiCl as eluent (1 mL/min). Polymer molar masses were calculated using linear polystyrene standards for PBA or PMMA standards for PMMA. Scanning electron microscopy (SEM) images were recorded on a JSM6330F microscope (JEOL, Japan). Thermogravimetry (TGA) was performed on a Pyris 1 thermogravimeter (PerkinElmer, USA) in the temperature range from 40 °C to 600 °C at a rate of 15 °C/ min under nitrogen. FT-IR spectroscopy was performed on a (Alpha FTIR spectrometer, Bruker, USA). Tribological performance was de-termined using a reciprocating pin-on-disc test on a UMT Tribolab tribometer (Bruker, USA). The tests were performed with AISI 52,100 polished (Ra = 0.01μm) steel balls with a surface hardness of 700 HV and an AISI 4130 polished (Ra = 0.01μm) quenched and tempered steel disc with a surface hardness of 320 HV. The applied load was 10 N giving a maximum Herztian contact stress of 1 GPa [40]. The re-ciprocating cycle (back + forth) was set to 1 mm at a sliding velocity of 0.5 mm/s. The test comprised 1000 cycles yielding a cumulative sliding distance of 1 m. The choice for these test parameters was motivated by the connection metal-to-metal seal tribosystem[37]. The grease was applied on both the disc and ball surface in sufficient amounts. The tests were repeated three times. The friction force was measured during the test and used to determine the COF. Wear scar depth and width were measured with a VK-9700 confocal microscope and the accompanying VK analyser software (Keyence, Japan). Using this set-up, the perfor-mance of the model pipe dopes was compared to American Petroleum Institute (API) modified (ISO 13678:2010, Jet-Lube, USA) which is the industry standard pie dope and acted as a baseline performance re-ference.

3. Results and discussion

3.1. Modification of TiO2NPs with polymer brushes

The general synthetic approach is depicted inScheme 1. First, TiO2

particles (d = 300 nm, SBET= 16.66 m2/g, Fig. S1a in Supporting

In-formation) were dispersed in buffered water/EtOH mixture (4:1 v/v) and reacted with dopamine hydrochloride under basic pH (8.5) to yield PDA nanolayer on the surface. Although the exact structure of PDA is still disputed, residual amine groups enable its further functionaliza-tion. Thus, PDA-coated TiO2NPs were reacted withα-bromoisobutyryl

bromide (BiBB) to immobilize the ATRP initiator, following recently published procedures [32–34]. Both steps were monitored by FT-IR (Fig. S2) which showed characteristic bands at 1500–1700 cm−1_(C]C

stretch and NeH bending) after deposition of polydopamine, and be-tween 2800 and 3000 cm (CeH stretch) upon the attachment of

bromoisobutyrate groups. TGA measurements confirmed the formation of a thin organic layer (2.6 and 3.6 wt% for TiO2-PDA and TiO2

-PDA-BiB, respectively) on TiO2particles (Fig. 1a). Additionally, morphology

of the NPs did not visibly change after modification with a PDA-BiB layer, as evidenced by SEM (Fig. S1b).

Supplementary data associated with this article can be found, in the online version, athttps://doi.org/10.1016/j.eurpolymj.2018.07.033.

BiB-modified TiO2particles were used as hybrid initiators for

SI-ATRP in DMF. SARA SI-ATRP mechanism was selected as it employs zerovalent metal, typically Cu wire or powder, to reduce Cu(II) species formed due to irreversible termination events[14,41]. Notably, the use of heterogeneous reducing agent in SI-ATRP allows to avoid any pos-sible side reactions with modified NPs[27]. n-butyl acrylate (BA) was selected as a model acrylate monomer. 100 ppm of CuBr2/TPMA

com-plex was used as a catalyst, since it was already proven to provide good control over bulk SARA ATRP of acrylates in DMF [42]. Cu wire (d = 1 mm) was used as a reducing agent and the polymerization was conducted at room temperature. Performed SI-SARA ATRP reactions are summarized inTable 1.

Due to the presence of a polymeric PDA layer on the TiO2surface,

detachment of the brushes after grafting was not feasible. Thus, a polymerization with a sacrificial initiator, ethyl α-bromoisobutyrate (EBiB), was performed to enable GPC measurements. Reaction in the presence of sacrificial initiator yielded hybrid particles with 8 wt% of organic content in 24 h as determined by TGA (Fig. 1a,1_{red curve). Free}

poly(butyl acrylate) (PBA) was analyzed by GPC to show well-defined polymer with Mn= 15500 g/mol and low dispersity (Mw/Mn= 1.13, Fig. 1b). Based on the GPC and TGA data, the grafting density was determined from the Eq.(1):

= − σ f N f M A (1 ) TGA np a np n s (1)

whereσTGAis grafting density, fnpis mass fraction of TiO2, Nais the

Avogadro’s number, Asis specific surface area of NPs and Mnis molar

mass of the free polymer. Grafting density was calculated to be бPBA= 0.21 nm−2.

Reaction conducted under the same conditions without sacrificial initiator produced hybrid particles with 42 wt% organic content in 24 h. Successful functionalization of the surface with PBA brushes was fur-ther confirmed by observation of a strong peak in the FT-IR spectrum at 1730 cm−1(C]O stretch, Fig S2). Assuming the same grafting density бPBA= 0.21 nm−2, the Mnof the formed polymer brush was calculated

to be 116,000 g/mol.

Similar conditions were employed to graft poly(methyl methacry-late) (PMMA) from PDA-modified TiO2. Thinner Cu wire

(d = 0.25 mm) was used as a reducing agent to partially compensate for higher activity of methacrylates in ATRP. As expected, significantly faster reaction than in the case of BA was observed; 40 wt% organic content was measured by TGA after 9 h and 88 wt% after 24 h (Fig. 2a, blue and green curves, respectively). With the addition of sacrificial initiator, 12 wt% of organic content was determined. GPC showed well-defined PMMA with Mn= 20,600 g/mol and low dispersity (Mw/

Mn= 1.23,Fig. 2b). The grafting density was calculated from the Eq. (1)to be бPMMA-300nm= 0.25 nm−2. Molar masses of PMMA grafted

without the sacrificial initiator were calculated to be 100,700 g/mol and 1,108,000 g/mol for the 9 h- and 24 h- reaction, respectively. Al-though these values should be treated with caution, as they are derived from assumption-based calculations rather than by direct measure-ments of detached brushes, it is worth mentioning that such high molar masses of grafted PMMA chains were previously only reported for SI-ATRP conducted under high pressure from silica NPs[43]. However, since the viscosity of the reaction medium radically increased with 100 200 300 400 500 600 0 20 40 60 80 100 Weight Loss (%) Temperature (o_C) TiO2 TiO2-PDA TiO2-PDA-BiB TiO2-PBA-SI-24h TiO2-PBA-24h

(a)

(b)

Fig. 1. Grafting PBA from PDA-modified TiO2particles (300 nm) by SARA ATRP. (a) Thermograms showing respective mass losses for particles after each mod-ification step as well as grafting with and without sacrificial initiator (SI); (b) GPC trace of free PBA synthesized in the reaction with sacrificial initiator. Reaction conditions: [BA]:[EBiB]:[CuBr2]:[TPMA] = 400:1:0.04:0.12, mTiO2-PDA= 100 mg, V0BA= 5 mL, Cu wire = 1 cm × 1 mm; BA/DMF 1:1 (v/v); room temperature.

Scheme 1. Polydopamine-assisted synthesis of polymer-grafted TiO2particles.

1_{For interpretation of color in Figs.1, 2, the reader is referred to the web} version of this article.

time, it is likely that a partial loss of control over the polymerization occurred, leading to broader MWDs. Nevertheless, hybrid TiO2particles

with very high organic content (88 wt%) were obtained.

Furthermore, in order to demonstrate versatility of the developed approach, PMMA brushes were grafted from small particles (d < 25 nm, SBET= 65 m2/g, Fig. S1 c/d) of pure anatase phase TiO2

under the same conditions. Controlled polymerization was observed in the presence of sacrificial initiator, which yielded a well-defined PMMA with Mn= 17,400 g/mol and low dispersity (Mw/Mn= 1.18,Fig. 3).

Grafting density was calculated to beбPMMA-25nm= 0.16 nm−2. Hybrid

particles with 39 wt% of organic content were obtained in 24 h in a reaction without the sacrificial EBiB.

3.2. Tribology

A typical pipe dope composition is a mixture of a base oil (mineral or synthetic) and additives such as thickeners, anti-oxidants and solid lubricants[44,45]. The latter are primarily heavy metal particles (e.g.

7 Anatase, < 25 nm PMMA no 0.39 39,400 – –

a _{Determined by GPC.}

b _{Calculated from Eq.(1)assuming grafting density determined for the reactions with sacrificial initiator.} c _{Calculated from Eq.(1).}

100 200 300 400 500 600 0 20 40 60 80 100

(a)

(b)

Fig. 2. Grafting of PMMA from PDA-modified TiO2particles (300 nm) by SARA ATRP. (a) Thermograms showing respective mass losses for particles after each modification step as well as grafting with and without sacrificial initiator (SI); (b) GPC trace of free PMMA synthesized in the reaction with sacrificial initiator. Reaction conditions: [MMA]:[EBiB]:[CuBr2]:[TPMA] = 400:1:0.04:0.12, mTiO2-PDA= 100 mg, V0BA= 5 mL, Cu wire = 1 cm × 0.25 mm; MMA/DMF 1:1 (v/v); room temperature. 100 200 300 400 500 600 0 20 40 60 80 100

(a)

(b)

Elution Volume (mL)

24 h, Mn = 17400, Mw/Mn = 1.18

Fig. 3. Grafting of PMMA from anatase particles (25 nm) by SARA ATRP. (a) Thermograms showing respective mass losses for particles after each modification step as well as grafting with and without sacrificial initiator (SI); (b) GPC trace of free PMMA synthesized in the reaction with sacrificial initiator. Reaction conditions: [MMA]:[EBiB]:[CuBr2]:[TPMA] = 400:1:0.04:0.12, mTiO2-PDA= 100 mg, V0BA= 5 mL, Cu wire = 1 cm × 0.25 mm; MMA/DMF 1:1 (v/v); room temperature.

Cu, Zn, Pb), but due to environmental concerns alternatives such as NPs are necessary[5,46–49]. TiO2NPs are a promising choice as they

ex-hibit good lubricating properties and can be regarded as‘green’ alter-native for heavy metals[4–9].

Since grafting polymer brushes can improve dispersibility of NPs as well as reduce wear and friction by providing additional lubrication, [50,51]synthesized core-shell particles were tested as pipe dopes ad-ditives. The tribology tests were performed for the base grease, mix-tures of the base grease with synthesized particles (300 nm) and com-mercial American Petroleum Institute (API) modified dope. The samples were denoted as: TiO2-PBA-40 (40 wt% organic content), TiO2

-PMMA-40 (40 wt% organic content) and TiO2-PMMA-88 (88 wt%

or-ganic content).

The friction results are shown inFig. 4. All lubricants tested showed a distinct period of running in during thefirst 100 s of the test. Then, the friction started to rise again and for most lubricants stabilized at some point marking that a steady boundaryfilm was formed. Compared to bare TiO2(300 nm), the addition of hybrid particles improved the

lubricating properties for both PBA and PMMA-grafted TiO2, achieving

a lower COF and a stable friction condition. However, PMMA-grafted NPs provided a significantly greater reduction of COF and reached stable friction condition in a shorter time. Additionally, thicker PMMA brushes (namely TiO2-PMMA-88) resulted in a slightly lower COF

compared to TiO2-PMMA-40. This in agreement with previous studies

indicating that thicker brushes lead to lower friction due to decreased adhesion by a formation of a boundaryfilm[50].

The scar depths and widths shown inFig. 5a and b support these assertions. PMMA-grafted particles resulted in the lowest wear com-pared to bare TiO2. In addition, the TiO2-PMMA-88 performed slightly

better. For the PBA-coated particles however, whereas the scar depth decreased, the width increased. Lower wear in the presence of the polymer-grafted particles can be related to the reduction of the abrasive effect of the TiO2. In addition, polymer-coated particles have a lower

concentration of TiO2, which acts as the main abrasive bodies. The

higher wear width of PBA-coated particles might be due to strong ad-hesive interaction between the grafted particles and subsequent ag-glomeration, moving particles beyond the friction area of the pin.

Optical microscopy images of the scars (Fig. S3) reveal that the scars looked similar for all tested samples, showing formation of grooves in the sliding direction of the pin indicating abrasive wear. The boundary film is also observed coming from the bentonite thickener (Fig. 5a) and/ or the TiO2and the polymer (Fig. S3c–f) or graphite, zinc and lead

(Fig. 5b).

The best-performing dope (TiO2-PMMA-88) was also compared with

the commercial API dope. Both dopes provided similar COF, with the API dope outperforming the TiO2-PMMA-88 only slightly. The main

difference was observed in the scar depth and width, namely for API: width 148μm and depth 0.5 μm, TiO2-PMMA-88: width 183μm and

depth 0.8μm (Fig. 5). As the API dope includes softer, malleable heavy

metals in comparison to TiO2, abrasion of the surface more likely occurs

in the TiO2-based dopes. However, in a single make-up phase in which

lubrication is needed, the abrasion caused by the TiO2particles might

be acceptable. Notably, the base grease is already effective in reducing friction and wear and the addition of the particles reduced its e ffec-tiveness. Nevertheless, due to the versatility of the polydopamine chemistry, the approach reported here is not limited to TiO2. Hence,

softer particles can be chosen to further reduce the abrasiveness in the system and result in an optimal grease design.

4. Conclusion

A straightforward method to graft polymer brushes from TiO2

na-noparticles by SI-ATRP has been developed. Pre-modification of TiO2

surface with a polydopamine layer enabled facile incorporation of the ATRP initiator followed by surface-initiated polymerization of BA or MMA by SARA ATRP. Hybrid core-shell particles with high organic content and grafting densities were obtained from TiO2NPs of two

different sizes. Large particles (300 nm) were tested as lubricating ad-ditives in a bentonite base grease and showed to decrease friction and wear compared to bare TiO2 as well as performed on par with API

modified commercial dope. Such environmentally-friendly, TiO2-based

lubricants could replace heavy-metal-based lubricants traditionally used in the oil & gas industry. Furthermore, given the versatility of the developed approach and ubiquitous applications of TiO2,such designer

core-shell hybrid nanostructures can be of particular interest in pho-tocatalysis, photovoltaics, self-cleaning paints, nanocomposites etc.

Acknowledgements

This research was performed within the framework of the 4TU.High-Tech Materials research program:‘New Horizons in designer materials’ (www.4tu.nl/htm). We would like to thank Shell Global Solutions BV for giving us the opportunity to publish our work and acknowledge the support of Shell Lubricants Hamburg in preparing the base greases. 0 500 1000 1500 2000 0.10 0.12 0.14 0.16 0.18 0.20

COF (reciprocating test)

Time (s)

Base grease API modified

TiO2-PMMA-88 TiO2-PMMA-40

TiO2-PBA-40 TiO2

Fig. 4. Dependence of the coefficient of friction (COF) on time for base grease with the addition of hybrid particles.

0 100 200 TiO2-PBA-40 TiO2-PMMA-88 TiO2-PMMA-40 Scar width ( m) Base grease API modified TiO2

(a)

(b)

Fig. 5. Scar width (a) and depth (b) as calculated from optical microscopy images. Standard deviation (n = 3) was calculated and shown as error bars in the graphs.

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