UCST-Type Thermoresponsive Polymers in Synthetic Lubricating Oil

6 days ago - This Article reports a family of UCST-type thermoresponsive polymers, poly(alkyl methacrylate)s with an appropriate alkyl pendant length ...
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Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

UCST-Type Thermoresponsive Polymers in Synthetic Lubricating Oil Polyalphaolefin (PAO) Wenxin Fu, Wei Bai, Sisi Jiang, Bryan T. Seymour, and Bin Zhao* Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996, United States S Supporting Information *

ABSTRACT: This Article reports a family of UCST-type thermoresponsive polymers, poly(alkyl methacrylate)s with an appropriate alkyl pendant length in an industrially important nonvolatile organic liquid polyalphaolefin (PAO). The cloud point (CP) can be readily tuned over a wide temperature range by changing the alkyl pendant length; at a concentration of 1 wt % and similar polymer molecular weights, the CP varies linearly with the (average) number of carbon atoms in the alkyl pendant. PAO solutions of ABA triblock copolymers, composed of a PAO-philic middle block and thermoresponsive outer blocks with appropriate block lengths, undergo thermoreversible sol−gel transitions at sufficiently high concentrations. The discovery of thermoresponsive polymers in PAO makes it possible to explore new applications by utilizing PAO’s unique characteristics such as thermal stability, nonvolatility, superior lubrication properties, and so on. Two examples are presented: thermoresponsive physical gels for control of optical transmittance and injectable gel lubricants.

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INTRODUCTION

Using surface-initiated atom transfer radical polymerization and reversible addition−fragmentation chain transfer (RAFT) polymerization, we recently synthesized a series of poly(alkyl methacrylate) brush-grafted silica and titania nanoparticles (hairy NPs) as friction and wear reduction additives for PAO.21,22 The hairy NPs with a sufficiently long alkyl pendant (>8 carbon atoms, such as 12, 13, 16, and 18) exhibited superior stability in PAO at both low and high temperatures and excellent lubrication properties with significant friction and wear reductions. Interestingly, while poly(n-hexyl methacrylate) (PC6) brush-grafted, 23 nm silica NPs could not be dispersed in PAO at either ambient or elevated temperatures,22 poly(2ethylhexyl methacrylate) (B-PC8) hairy silica NPs formed a transparent, homogeneous dispersion in PAO upon heating at 80 °C but turned cloudy and precipitated out upon cooling. The latter was attributed to the presence of silica NP core because B-PC8 is soluble in PAO at room temperature; the van der Waals attractive forces between NPs cannot be overcome by the PAO solvation of the brushes at lower temperatures. Clearly, the alkyl pendant’s length is critical for the dispersibility of hairy NPs in PAO. Intrigued and prompted by these observations, we hypothesized that UCST-type thermoresponsive polymers in PAO could be obtained from poly(alkyl methacrylate) polymers with an appropriate number of carbon atoms in the alkyl pendant. Using RAFT polymerization, we synthesized a series of homopolymers and random copolymers of various

Thermoresponsive polymers exhibit a lower (LCST) or an upper critical solution temperature (UCST) transition, accompanied by a sharp, discontinuous change in solubility in response to temperature changes in a liquid medium, most notably water1−5 and more recently ionic liquids.6,7 These polymers have been intensively studied in recent years and have shown potential applications in a wide variety of areas,8−12 including injectable drug-delivery systems, tissue engineering, actuators, and electronics, among others. Although thermoresponsive polymers in organic solvents have been known,4,13−16 for example, polystyrene in cyclohexane (UCST) and poly(oligo(ethylene glycol) methyl ether methacrylate) (POEGMA) in alcohol solvents (UCST), the volatility of common organic solvents has limited their possible practical use. Polyalphaolefin (PAO), an industrially important, nonvolatile, and noncrystallizable organic liquid, is widely used as a base oil for commercial engine lubricants, gear box oils, and hydraulic fluids owing to its superior lubrication properties, excellent thermal stability, chemical inertness, and general compatibility with a wide variety of lubricant additives.17,18 These unique characteristics originate from its branched and saturated hydrocarbon molecular structure. Thus far, no thermoresponsive polymers with LCST or UCST transitions in PAO have been reported, although the function of viscosity modifiers for motor oils has been interpreted as temperature-induced reversible swelling/shrinking of polymer coils.19,20 One can envision that thermoresponsive polymers with definitive LCST or UCST transitions in PAO could further expand PAO’s applications. © XXXX American Chemical Society

Received: December 29, 2017 Revised: February 10, 2018

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DOI: 10.1021/acs.macromol.7b02755 Macromolecules XXXX, XXX, XXX−XXX

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Scheme 1. (A) Molecular Structures of Homopolymers of Alkyl Methacrylates with Linear and Branched Alkyl Pendants as well as Random Copolymers and (B) Synthesis of ABA Triblock Copolymers Composed of a PAO-philic Middle Block and Two UCST-Type Thermoresponsive Outer Blocks by Two-Step RAFT Polymerizations

(>99%, Fisher), 4,4′-azobis(4-cyanopentanoic acid) (ACVA) (98%, Alfa Aesar), iodine (pure, Acros), N-(3-(dimethylamino)propyl)-N′ethylcarbodiimide hydrochloride (EDC, 99%, Oakwood Products), and 4-(dimethylamino)pyridine (DMAP, ≥99%, Aldrich) were used without further purification. Azobis(isobutyronitrile) (AIBN, 98%, Aldrich) was recrystallized from ethanol and dried under vacuum before use. The PAO used in this work was SpectraSyn 4 PAO fluid from ExxonMobil with pour point of −87 °C. All other chemical reagents were purchased from either Aldrich or Fisher and used without further purification. Characterization. 1H and 13C NMR spectra were recorded on a Varian Mercury 500 NMR spectrometer, and the residual solvent proton signal was used as the reference peak. All spectra were collected at 25 °C. Size exclusion chromatography (SEC) of all polymers was carried out at room temperature using PL-GPC 20 system from Polymer Laboratories equipped with a Waters 510 pump, a refractive index detector, one PLgel 5 μm guard column (50 × 7.5 mm), and two PLgel 5 μm mixed-C columns (each 300 × 7.5 mm, linear range of molecular weight from 200 to 2 million Da). THF was used as the solvent with a flow rate of 1.0 mL/min for analysis. The SEC system was calibrated by using a series of narrow-dispersed polystyrene standards, and the data were processed using Cirrus GPC/SEC software (Polymer Laboratories). The CPs of thermoresponsive polymers in PAO in the range of 6.9 to 61.7 °C were measured by monitoring the transmittance at 500 nm of a polymer solution in PAO in a 3.5 mL quartz cell using a T60U UV−vis spectrophotometer equipped with a Peltier temperature controller system (PTC-2) in conjunction with UVWIN software version 6.1.0 (Persee Analytics). The polymer solution was equilibrated at each temperature for 1 min with a heating/cooling rate of ∼1 °C/min. The CP was defined as the temperature at which 50% transmittance change occurred. CPs above 61.7 °C and below 6.9 °C were determined by visual inspection using a temperature-controlled oil bath and an Isotemp water bath (Fisher Scientific, model 3006); the temperature was decreased stepwise (1 °C each time) and was recorded as the CP when the solution turned cloudy. Rheological experiments were conducted on a rheometer from TA Instruments (model TA AR 2000ex) using a cone−plate geometry with a cone diameter of 20 mm and an angle of 2° (truncation 52 μm). The temperature was controlled by the bottom Peltier plate. For each measurement, a polymer PAO gel (∼80 mg) was loaded onto the plate by a spatula. The solvent trap cover was used to minimize temperature fluctuation. Oscillatory shear experiments were performed in heating/ cooling ramps at a constant frequency of 1 Hz, a strain amplitude of

alkyl methacrylate monomers (Scheme 1A) and studied their solution behavior in PAO. It was found that poly(alkyl methacrylate)s with an appropriate (average) alkyl pendant length exhibited a UCST-type thermoresponsive behavior in PAO. The influences of polymer concentration, molecular weight, and the (average) number of carbon atoms in the alkyl pendant per repeat unit on the cloud point (CP) were systematically studied. In addition, we prepared a set of ABA linear triblock copolymers, composed of a PAO-philic middle block and thermoresponsive outer blocks, from a difunctional chain transfer agent (CTA) by RAFT polymerization (Scheme 1B). At appropriate block compositions and sufficiently high concentrations, the PAO solutions of ABA triblock copolymers undergo thermally induced reversible sol−gel transitions, and the sol−gel transition temperature can be readily tuned by varying the chemical composition and block lengths of ABA copolymers as well as the polymer concentration in PAO. By utilizing PAO’s unique characteristics such as nonvolatility, thermal stability, and superior lubrication properties, we show that the thermoresponsive PAO gels of ABA triblock copolymers can be used for control of optical transmittance and as injectable gel lubricants.

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EXPERIMENTAL SECTION

Materials. The synthesis and characterization of RAFT CTAs, nbutyl (2-cyano-2-propyl) trithiocarbonate (bCTA, Scheme S1) and difunctional CTA (DiCTA, Scheme 1B and Scheme S1), can be found in the Supporting Information.23 n-Butyl methacrylate (C4MA, 99%, Aldrich), n-hexyl methacrylate (C6MA, 97%, TCI), 2-ethylhexyl methacrylate (B-C8MA, > 99.0%, TCI), n-octyl methacrylate (LC8MA, > 99%, Scientific Polymer), and lauryl methacrylate (C12MA, 97%, Acros) were passed through a column of silica gel (bottom)/ activated basic aluminum oxide (top) (2/1 v/v) to remove the inhibitor and stored in a refrigerator prior to use. The synthesis and characterization data of n-heptyl methacrylate (L-C7MA) and 3-heptyl methacrylate (B-C7MA) monomers are included in the Supporting Information. Toluene and THF were dried with sodium/benzophenone, distilled under a nitrogen atmosphere, and used immediately. 1Heptanol (99%, Alfa Aesar), 3-heptanol (98%, Acros), methacryloyl chloride (95%, Acros), 1-butanethiol (98%, Acros), carbon disulfide B

DOI: 10.1021/acs.macromol.7b02755 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Table 1. Characterization Data for UCST-Type Thermoresponsive ABA Triblock Copolymers Synthesized by RAFT Polymerization sample

middle blocka

one outer blocka

Mn,SEC (×104 Da)b

PDIb

Tsol−gel (°C)

G′exp (at 5 °C)e

gelation efficiencyf

ABA-1 ABA-2 ABA-3 ABA-4

PC12313 P(C8183-co-C12193) P(C8240-co-C12248) P(C8240-co-C12248)

PC6163 P(C4106-co-C627) P(C4104-co-C627) P(C4166-co-C648)

8.61 10.2 10.9 11.9

1.27 1.16 1.22 1.23

∼0c 52d 57.7d 63.5d

444.0 727.6 526.4

19.5% 38.3% 32.3%

a Subscript indicates the number of each monomer in the final copolymer. bNumber-average molecular weight (Mn,SEC) and polydispersity (PDI) were determined by SEC using THF as solvent relative to polystyrene standards. cTsol−gel was estimated by visual inspection. dSol−gel transition temperature (Tsol−gel) was determined by rheological measurement for 15 wt % ABA in PAO. eG′exp was the dynamic storage modulus (G′) at 5 °C from cooling ramp in a rheological study. fGelation efficiency is defined as the ratio of G′exp to the theoretical maximum value of dynamic storage modulus (G′theo) at the same temperature calculated from G′theo = υkBT and υ = cNA/M,32−35 where kB is Boltzmann constant, T is the absolute temperature, c is the polymer concentration, NA is the Avogadro’s number, and M is the molecular weight of the ABA copolymer.

1.0%, and a heating/cooling rate of 3 °C/min, giving dynamic viscoelastic properties (dynamic storage modulus G′ and loss modulus G″) of the sample as a function of temperature. Synthesis of UCST-Type Thermoresponsive Poly(alkyl methacrylate) Homopolymers and Random Copolymers by RAFT Polymerization. UCST-type thermoresponsive poly(alkyl methacrylate) homopolymers and random copolymers were synthesized by RAFT polymerization of alkyl methacrylate monomers or monomer combinations with various molar ratios in anisole at 70 °C. Described below is the synthesis of random copolymer P(0.51C6-co0.49C8) (Table S2).23 Other polymers were prepared using the same procedure. A stock solution of bCTA in anisole with a concentration of 65.7 mg/g (0.155 g, 0.0437 mmol), n-hexyl methacrylate (C6MA, 0.631 g, 3.71 mmol), 2-ethylhexyl methacrylate (B-C8MA, 0.723 g, 3.64 mmol), a stock solution of AIBN in anisole with a concentration of 9.89 mg/g (0.145 g, 0.00873 mmol), anisole (1.051 g), and trioxane (16.0 mg, 0.18 mmol) as the internal standard were added to a 50 mL two-necked round bottom flask equipped with a magnetic stir bar. The mixture was degassed by three cycles of freeze−pump−thaw, and a 0 min sample was taken out from the reaction mixture by a degassed syringe for 1H NMR spectroscopy analysis before the flask was placed in a 70 °C oil bath. The polymerization was monitored by 1H NMR spectroscopy; the monomer conversion reached 92.7% for C6MA and 92.1% for B-C8MA after 20 h, at which point the polymerization was stopped by immersing the flask in an ice/water bath and bubbling with air. The polymer was purified by precipitation three times in methanol from dichloromethane and then dried under high vacuum at room temperature. SEC results: Mn,SEC = 2.39 × 104 Da and Mw/Mn = 1.20 relative to polystyrene standards. The numbers of C6MA and BC8MA monomer units in the copolymer were 79 and 76, respectively, calculated from their corresponding monomer conversions determined by 1H NMR spectroscopy using the integral changes of the peak located at 4.10 to 4.18 ppm (−COOCH2− of n-hexyl methacrylate) and the peak at 4.04 to 4.09 ppm (−COOCH2− of 2-ethylhexyl methacrylate) with respect to the peak of the added internal standard at 5.16 ppm (Figure S5). For the synthesis of random copolymers from monomers with linear alkyl pendant groups, the monomer conversion of each monomer was calculated directly from the integral change of the vinyl peak compared with the internal standard, which was assumed to be the same for different monomers. This is because different monomers with linear alkyl pendant groups cannot be differentiated from each other in the 1H NMR spectra; the characteristic proton signals from both vinyl and ester groups were overlapped, as shown in Figure S6. The SEC data of poly(alkyl methacrylate) polymers are shown in Figure S7. Synthesis of UCST-Type Thermoresponsive ABA Triblock Copolymers by RAFT Polymerization. All ABA triblock copolymers were synthesized by two-step RAFT polymerizations from a difunctional CTA (DiCTA) in anisole at 70 °C. Described below is the synthesis of ABA-2 in Table 1. Other ABA triblock copolymers were prepared using the same procedure. The synthesis of P(C8183-coC12193) macro difunctional CTA (macro-diCTA), where the subscripts denote the number of each monomer unit in the copolymer, is shown in the Supporting Information. The stock solution of P(C8183-

co-C12193) macro-diCTA in anisole with a concentration of 446.7 mg/ g (1.792 g, corresponding to 0.801 g macro-diCTA, 0.00931 mmol), C6MA (0.132 g, 0.775 mmol), C4MA (0.418 g, 2.94 mmol), a stock solution of AIBN in anisole with a concentration of 22.0 mg/g (0.0260 g, corresponding to 0.572 mg of AIBN, 0.00348 mmol), and anisole (0.339 g) were added to a two-necked 50 mL flask equipped with a stir bar. The mixture was degassed by three cycles of freeze−pump−thaw and placed in a 70 °C preheated oil bath. The polymerization was monitored by 1H NMR analysis and was quenched after 20 h by immersing the flask in an ice/water bath. A sample was withdrawn immediately from the mixture for 1H NMR analysis; the monomer conversion was found to be 66.5% by comparing the integrals of −COOCH2− peaks from the monomers at 4.09 to 4.21 ppm and the copolymer at 3.85 to 4.07 ppm. The triblock copolymer was precipitated in methanol from dichloromethane three times and then dried under high vacuum at ambient temperature. SEC analysis results (THF as eluent): Mn,SEC = 10.2 × 104 g/mol and PDI = 1.16, relative to polystyrene standards (Figure S11). The numbers of C6MA and C4MA monomer units in each outer block were 27 and 106, respectively, calculated from the monomer conversion and the monomer-to-macro-diCTA molar ratio.

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RESULTS AND DISCUSSION Synthesis and Thermoresponsive Properties in PAO of Poly(alkyl methacrylate) Homopolymers and Random Copolymers. In consideration of the aforementioned behaviors of PC6 and B-PC8 brush-grafted silica NPs in PAO, we first synthesized poly(n-heptyl methacrylate) (L-PC7, Scheme 1A) that contains seven carbon atoms in the pendant by RAFT polymerization using n-butyl (2-cyano-2-propyl) trithiocarbonate as CTA and 2,2′-azobis(2-methylpropionitrile) (AIBN) as initiator in anisole at 70 °C. The solution behavior of an L-PC7 with a degree of polymerization (DP) of 165 in PAO at a concentration of 1 wt % was investigated by both visual inspection and turbidimetry using a UV−vis spectrometer. Consistent with the expected UCST behavior, the sample underwent a clear-to-cloudy transition upon cooling from 50 to 20 °C, with a CP of 35.0 °C (Figure 1). The transition was reversible; heating induced a cloudy-to-clear transition, although there was a small hysteresis of ∼2 °C. The CP increased with increasing polymer concentration, from 28.2 °C at 0.5 wt % to 48.5 °C at 5 wt % (Figure S9A),23 which is in agreement with the behavior of typical UCST polymers.2,24−26 For UCST thermoresponsive polymers in water, the CP is known to be heavily influenced by the polymer molecular weight.25,26 Similarly, we observed that the CP of L-PC7 varied significantly with the DP; at a concentration of 1 wt %, the CP of L-PC7 in PAO increased from 24.7 to 45.5 °C with increasing the DP from 100 to 228 (Figure S9B).23 C

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to make random copolymers.1−4 To examine how the average number of carbon atoms in the alkyl pendant per repeat unit in the copolymer affects the CP, we prepared a series of binary random copolymers via RAFT polymerization of two monomers from 2-ethylhexyl methacrylate (B-C8MA), nhexyl methacrylate (C6MA), n-butyl methacrylate (C4MA), and lauryl methacrylate (C12MA). The copolymers were named according to the monomers used and the composition of the final copolymer. For example, P(0.50C4-co-0.50C6) represents a random copolymer of C4MA and C6MA with the molar ratio of 0.50 to 0.50 in the final copolymer (Table S2).23 The DPs of these random copolymers were around 155 with a range of 145 to 164, similar to those of aforementioned homopolymers (average of 151 with a range of 137 to 165), which were purposely obtained to minimize the effect of molecular weight on CP. The CPs of these copolymers in PAO at a concentration of 1 wt % were measured and are summarized in Figure 2 as a function of average number of carbon atoms in the alkyl pendant group per repeat unit () in the copolymer along with the CPs for homopolymers. The value of for each copolymer was calculated from the mole fractions of two monomers in the copolymer. All data points either fell right onto or were very close to the fitted straight line for the homopolymers with linear alkyl pendants, indicating that there is a linear dependence of CP on at the same or similar DPs. If a linear fitting is performed on all data points shown in Figure 2, then CP = 275.4 − 34.5 with R2 of 0.9909, which is essentially the same as that for homopolymers with linear alkyl pendants. Clearly, the CPs of these thermoresponsive polymers in PAO can be finely tuned over a large temperature range by simply controlling the (average) length of the alkyl pendant group(s). PAO Gels of UCST-Type Thermoresponsive ABA Triblock Copolymers. PAO is a nonvolatile, colorless, and transparent organic liquid with a saturated hydrocarbon molecular structure widely used in industry. The discovery of thermoresponsive polymers with definitive and tunable UCST transitions in PAO presents new opportunities for utilizing PAO’s unique characteristics and properties in new applications. In particular, it is possible to use UCST-type thermoresponsive polymers to solidify PAO in a thermoreversible fashion. Therefore, we sought to physically gel PAO with ABA triblock copolymers in the same manner as for the formation of hydrogels of thermoresponsive ABA linear triblock copolymers in water27,28 and used them in two demonstrations of possible applications: durable, nonvolatile gel systems for reversible thermo-control of optical transmittance and injectable gel lubricants for friction reduction with higher loadcarrying capacity. A difunctional trithiocarbonate-based chain transfer agent (DiCTA) was synthesized and used to prepare ABA triblock copolymers with a PAO-philic middle block and two UCST-type thermoresponsive outer blocks by two consecutive RAFT polymerizations (Scheme 1B). Similar to UCST-type ABA triblock copolymers in water, 28 the thermoresponsive poly(alkyl methacrylate)-based ABA triblock copolymers would be soluble in PAO at higher temperatures, forming clear solutions. Upon cooling, the thermoresponsive outer blocks will undergo a UCST transition and self-assemble to form micellar cores acting as cross-linking points, and the middle block will function as bridging chains, resulting in microphase-separated, physically cross-linked 3-D network PAO gels (Scheme 2).

Figure 1. Optical transmittance as a function of temperature for a 10 mg/g solution of an L-PC7 in PAO in a cooling−heating cycle measured at 500 nm using a UV−vis spectrometer.

Encouraged by this result, we then synthesized a set of homopolymers of various alkyl methacrylate monomers with similar DPs (Table S1) and examined their solution behaviors in PAO. The polymers include linear and branched PC8 (L/BPC8), branched PC7 (B-PC7), PC6, and PC4 (Scheme 1A); all of these homopolymers exhibited a UCST transition in PAO. At a concentration of 1 wt %, the CP was 0.5 °C for L-PC8, 3.8 °C for B-PC8, 36.1 °C for B-PC7, 71.5 °C for PC6, and 138 °C for PC4; the CP spanned a range of 0.5 to 138 °C with decreasing the carbon number in the alkyl pendant from 8 to 4. Note that poly(lauryl methacrylate) (PC12) is totally soluble in PAO even at −20 °C.21 Clearly, the longer the alkyl pendant, the lower the UCST in PAO (i.e., the better the solubility in PAO). Interestingly, both linear PC7 and PC8 exhibited a lower CP, by 1.1 and 3.3 °C, respectively, than their branched counterparts; this is likely because a linear alkyl pendant has slightly stronger van der Waals interactions with the solvent molecules than a branched alkyl due to the larger area of contact with PAO, similar to the observed difference in boiling points of linear and branched alkane isomers. Moreover, for four polymers with linear alkyl pendants, a linear relationship was observed between the CP and the number of carbon atoms (nC) in the alkyl pendant; a linear fitting gave CP (°C) = 276.4 − 34.4nC with R2 of 0.9995 (Figure 2), which means that the CP increases by 34.4 °C with decreasing the carbon number by 1. A common strategy to tune the CP of thermoresponsive polymers is to copolymerize two or more different monomers

Figure 2. Plot of cloud point of 1 wt % polymer in PAO versus (average) number of carbon atoms in the alkyl pendant groups per repeat unit (). The detail for each sample (data point) can be found in Figure S10. D

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overtook G″ after a crossover point, suggesting that the sample changed from a liquid into a gel. The crossover point of two curves in the temperature ramp was commonly taken as the Tsol−gel.32−35 The sol−gel transitions were reversible, with only a small hysteresis between Tsol−gel and gel−sol transition temperature (Tgel−sol). For example, for ABA-2, the Tgel−sol was 52.9 °C (Figure S12), which is only 0.9 °C higher than Tsol−gel (52.0 °C). The transition temperatures and G′ values at 5 °C from cooling ramps are summarized in Table 1. For a physically cross-linked transient gel, the modulus is governed by the number density of elastically active polymer chains (i.e., bridging chains among micellar cores), which, in turn, depends on the chemical composition and block lengths of the ABA triblock copolymer.32−35 As shown in Table 1, for ABA-2 and -3 having similar outer block lengths, with increasing the middle block length from 376 to 488, Tsol−gel, G′exp, and gelation efficiency all increased, indicating that a longer middle block led to more bridging chains. For ABA-3 and -4 having the same central block, Tsol−gel increased from 57.7 to 63.5 °C with increasing the outer block length from 131 to 214, consistent with the expectation that a longer outer block would have a higher CP and thus give a higher Tsol−gel. In addition, polymer concentration is a critical factor for gelation. Taking ABA-2 as an example, Tsol−gel increased with increasing concentration, from 44.6 °C at 10 wt %, to 51.0 °C at 12 wt %, to 52.0 °C at 15 wt %, and 60.6 °C at 20 wt %; the hysteresis between Tsol−gel and Tgel−sol was very small for all concentrations (Figure S13).23 Compared with aqueous and volatile organic gels where solvent evaporation is usually inevitable, PAO gels are unique because of the nonvolatility of PAO, which means that these gels can be very robust with a long-term stability even in an open environment. By employing the clear-to-cloudy transition of UCST thermoresponsive polymers in PAO, we showed that the PAO gels of ABA triblock copolymers in the presence of a thermoresponsive random copolymer can undergo a reversible clear-to-cloudy transition in response to temperature change, which can be used to modulate the optical transmittance, for example, for “smart” windows, without resorting to other means. A PAO solution containing 14.3 wt % ABA-2 and 5 wt % P(0.22C4-co-0.78C8) (RCP-1) was prepared. Note that P(0.22C4-co-0.78C8) had a of 7.1 (CP = 30.6 °C at 1 wt % conc.), much larger than that for the outer blocks of ABA-2 (nC = 4.4, CP = ∼124 °C at 1 wt % conc. according to Figure 2). Thus this random copolymer is unlikely to participate in the formation of 3-D network of ABA-2. As shown in the top row of Figure 4, this sample underwent clear sol-clear gel-cloudy gel transitions upon cooling, with clouding at 36 °C. For comparison, we prepared another PAO gel containing 5 wt % P(0.11C4-co-0.89C8) (RCP-2, nC = 7.6; CP = 19.5 °C at 1 wt % conc.). The clear gel-to-cloudy gel transition occurred 25 °C (the bottom row of Figure 4). Note that the cloudy gels were very stable; no any changes were observed after being kept open to air at room temperature for 4 weeks (Figure S14), indicating that the collapsed random copolymer molecules were effectively trapped in the PAO gel network without settling. The thermoresponsive physically cross-linked PAO gels of ABA triblock copolymers can be used as lubricant−gel lubricants at lower temperatures and liquid lubricants at higher temperatures. They can be applied as liquids at elevated temperatures, allowing for conformal contact with solid surfaces and the use as gel lubricants at lower temperatures. Note that gel lubricants, which under certain conditions (e.g., high load)

Scheme 2. Schematic Illustration of Reversible Formation of a 3-D Network PAO gel of an ABA Triblock Copolymer Composed of a PAO-philic Middle Block and UCST-Type Thermoresponsive Outer Blocks

We first synthesized an ABA triblock copolymer composed of a PC12 middle block with a DP of 313 and PC6 outer blocks each with a DP of 163 (ABA-1 in Table 1) and prepared a 15 wt % solution in PAO. Although the CP of PC6MA in PAO was 71.5 °C, the 15 wt % PAO solution of ABA-1 formed only a weak gel at ∼0 °C by visual examination. The UCST transition of a thermoresponsive polymer in a solvent is known to be more heavily influenced by the molecular weight, end group, and attachment to another polymer than LCST thermoresponsive polymers.2,24−26,28−31 The inefficient gelation of ABA-1 likely resulted from the excellent solubility of the PC12 middle block. In light of this observation, we changed the central block to the copolymer of B−C8MA and C12MA with a molar ratio of ∼1:1 and the outer blocks to P(C4-co-C6) with a molar ratio of ∼4:1 and prepared three ABA triblock copolymers (ABA-2, -3, and -4 in Table 1). According to Figure 2, the central block would have a CP of about −70 °C, and the outer blocks would exhibit a CP of ∼123 °C in PAO if they are not covalently linked. A PAO solution with a concentration of 15 wt % was prepared for each ABA copolymer, and, as expected, they all exhibited a characteristic UCST sol−gel transition upon cooling from elevated temperatures (see the photos for ABA-2 in Figure 3). Their sol−gel

Figure 3. Dynamic storage modulus G′ and dynamic loss modulus G″ of 15 wt % ABA-2 in PAO as a function of temperature upon cooling. The inset are photos of 15% ABA-2 in PAO at 25 (left) and 80 °C (right).

transition temperatures (Tsol−gel) were determined by oscillatory shear rheological measurements using a constant frequency of 1 Hz and a strain amplitude of 1.0% in a temperature ramp with a cooling rate of 3 °C min−1. Figure 3 shows the cooling ramp for the 15 wt % solution of ABA-2 in PAO; other rheological data can be found in the Supporting Information (Figure S12).23 At higher temperatures, the values of dynamic storage modulus (G′) and loss modulus (G″) were small and fluctuating, indicating a liquid state. Upon decreasing temperature, G′ increased faster than G″ and eventually E

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and the surface asperities from the opposing surfaces came into contact and collided with each other, resulting in a higher friction. In contrast, the PAO gel retained the liquid PAO in the network even under higher load and exhibited noticeably small μs values. Moreover, at small loads (≤50 g), the static μs values for the PAO gel were too small to be measurable.

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CONCLUSIONS A family of UCST thermoresponsive polymers in PAO was discovered, and the CPs of these polymers with similar DPs showed a linear dependence on the (average) number of carbon atoms (nC) in the alkyl pendant group. Using a difunctional CTA, ABA triblock copolymers, composed of a PAO-philic middle block and thermoresponsive outer blocks, were prepared. At appropriate block compositions and sufficiently high concentrations, these ABA triblock copolymers can thermally gel PAO in a reversible fashion. The discovery of thermoresponsive polymers opens up new avenues for applications of PAO. As a demonstration, we showed that the PAO gels containing thermoresponsive random copolymers exhibited thermoreversible clear-to-cloudy transitions with a long-term durability, which could be used for modulating optical transmittance. Furthermore, the PAO gels can be used as lubricants, exhibiting significantly lower friction with a better load-bearing property compared with PAO. By combining the unique characteristics of PAO, such as low cost, nonvolatility, optical transmittance, chemical inertness, and superior lubrication property, and the thermoresponsiveness of polymers with various architectures, a plethora of opportunities can be envisioned, including viscosity modifiers, transmittance modulator, and injectable gel lubricants, among others.

Figure 4. Optical photos of a 14.3 wt % PAO solution of ABA-2 containing 5 wt % of (A) P(0.22C4-co-0.78C8) (RCP-1, top row) or (B) P(0.11C4-co-0.89C8) (RCP-2, bottom row) at 80, 40, 36, and 25 °C showing thermoreversible clear sol-clear gel-cloudy gel transitions. The addition of different random copolymers allowed for modulating the gel’s optical transmittance at different temperatures.

are more advantageous than liquid lubricants, have received growing interest in recent years.36−38 To test the lubrication properties of the PAO gel of 10 wt % ABA-2 and PAO under different loads, a homemade setup for the measurement of static friction at glass−glass interface was constructed (Figure 5A) in the same spirit as that used in Amonton’s law and for

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b02755. Synthesis of CTAs (bCTA and DiCTA) and monomers (L-C7MA and B-C7MA); characterization and thermoresponsive properties in PAO of homopolymers and random copolymers; synthesis of P(C8183-co-C12193) macro CTA and SEC data of ABA triblock copolymers; rheology data for PAO solutions of ABA triblock copolymers; and photos of PAO solutions of 14.3% ABA-2 containing 5 wt % of a random copolymer after 4 weeks. (PDF)

Figure 5. (A) Schematic illustration of the homemade friction test setup. (B) Plots of static coefficients of friction of PAO- and 10 wt % ABA-2 PAO gel-lubricated glass−glass interface versus load.

testing the friction of solid−solid contact.39,40 Glass substrates (a) and (b) were precleaned microscope glass slides with a size of 7.5 × 5.0 × 0.1 cm; glass (a) was bonded firmly by glue to glass (c) with dimensions of 3.0 × 3.0 cm to ensure no relative motion between (a) and (c). PAO or PAO gel in the amount of 9 mg was applied on glass (b), and the bonded glasses (a) and (c) with a string were then placed on top of lubricant-covered glass (b). A load (W) was then applied, and water was gradually added into the container suspended from the pulley until the glass (c) started to move, at which point the weight that caused the motion was recorded (F). The static coefficient of friction (μs) for the lubricated glass−glass interface was calculated, μs = F/W. Note that the rotation friction of pulley was negligible compared with that between glasses. For each load, the experiment was repeated three times, and the average of μs and the standard deviation were calculated (Figure 5B). Clearly, in the studied load range of 50 to 250 g, the static coefficients of friction for the glass−glass interface lubricated by the PAO gel were significantly smaller, by ∼4 times, than those lubricated by PAO. We believe that this is because PAO was squeezed out easily from the glass−glass interface due to the liquid nature,

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Bin Zhao: 0000-0001-5505-9390 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by NSF through DMR1607076, University of Tennessee Knoxville, and DOE EERE (DE EE0006925). We thank Dr. Jun Qu of ORNL for PAO (SpectraSyn 4, ExxonMobil). F

DOI: 10.1021/acs.macromol.7b02755 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

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DOI: 10.1021/acs.macromol.7b02755 Macromolecules XXXX, XXX, XXX−XXX