Polymer Architecture: Does it Influence Shear Stability? Lelia

Subscriber access provided by AUSTRALIAN NATIONAL UNIV

Applied Chemistry

Polymer Architecture: Does it Influence Shear Stability? Lelia Cosimbescu, Joshua W Robinson, and Jordan P. Page Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b02609 • Publication Date (Web): 07 Aug 2018 Downloaded from http://pubs.acs.org on August 13, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Industrial & Engineering Chemistry Research

Polymer Architecture: Does it Influence Shear Stability? Lelia Cosimbescu†*, Joshua W. Robinson†, Jordan P. Page† †

Pacific Northwest National Laboratory, Richland, WA 99352 [email protected]

Abstract Hyperbranched and linear poly(alkyl methacrylate)s with and without polycaprolactone segments were designed and prepared via a core-first strategy, then evaluated with respect to their rheology and shear stability performance. The focus of this work was to study the effect of architecture on mechanical shear stability, as it relates to lubricant performance. The polymers were prepared from functionalized macroinitiators subsequently subjected to ATRP/ARGETATRP conditions with dodecyl methacrylate and 2-ethylhexyl methacrylate mixtures. As expected, most compounds displayed an increased viscosity index along with increasing molecular weights. The inclusion of polycaprolactone appears to have enhanced the viscosity index in select samples. Although the hyperbranched polymers studied here varied in the number of arms from about twenty to one (linear), the data presented supports the empirical understanding that shear stability is mainly influenced by molecular weight and not architecture or topology. The polymers with caprolactone blocks and shorter methacrylate pendants demonstrated a positive effect on the shear stability, as in possessing the lowest permanent shear stability index, four times lower than other compounds included in this study as well as the benchmark. Keywords: polymethacrylates, shear stability, hyperbranched polymers, star polymers, viscosity modifiers, viscosity index improvers.

1 ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Introduction Hyperbranched polymers have found utility in a variety of applications, from biomaterials and drug delivery systems1,2,3 to more applied fields such as wastewater treatment,4 cellulose functionalization, 5 coatings,6 inkjet printing,7 encapsulation technologies,8 and last but not least, lubricant additives. The latter is the focus of our work, as viscosity modifiers (VMs) or viscosity index improvers (VIIs) play a major role in the formulation and performance design of multigrade engine oils, hydraulic fluids, transmission fluids, and other oils. Throughout their lifecycles, lubricants experience a wide range of external and internal conditions such as mechanical shear and temperature variability. The resistance of the oil’s acute change in viscosity over a temperature range is indicated by its viscosity index (VI). This performance metric can readily be influenced by the type of polymer added to the lubricant formulation. An excellent review that discusses how VM functionality is directly correlated to the properties of the polymers themselves, their size, chemical composition, and structure can be found in the recent literature.9 Lubricants of high VI value (200-250) possess a better viscosity profile over a broad temperature range. Molecular weight and macromolecular architecture including chemical composition are key polymer features that influence the additive’s ability to thicken lubricants over a wide temperature range. Fuel economy of an automobile is partly determined by the fuel efficiency of the engine which is highly dependent on polymer additives’ ability to provide a minimal thickening at low temperatures, while preventing the fluid from thinning at elevated temperatures. As fuel economy goals have pushed the boundaries of oil viscosities lower and lower, well-designed VIIs that maintain lubrication and minimize friction and wear in a hot running engine are even more imperative. Therefore, providing a high VI polymer is only part of the solution; another important parameter to consider is its shear stability. In the context of this


Page 2 of 28

Page 3 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


work, the polymer’s ability to resist mechanical shear-induced macromolecular degradation under high-shear conditions was investigated. These conditions are ubiquitous in most mechanical systems.10,11 Similar to the viscosity thickening effect at high temperatures, the shear stability of a polymer is also governed by its molecular weight and architecture. Thus, polymers of higher molecular weight usually possess lower shear stability. 12,13 A linear polymer of high molecular weight may provide a high VI, however it suffers tremendously from poor shear stability due to a dramatic molecular weight decrease caused by irreversible chain breakages under these high mechanical shear conditions. As a result, even high VI performance polymers can lose their efficacy only after a few engine run cycles reducing the efficiency and lifecycle of the lubricant. It is the conventional belief that polymers with the so-called “star-shaped” architecture exhibit much higher shear stability at high molecular weights due to their compact globular structure compared to linear polymers with similar molecular weights and composition. There are several reports of star-shaped polymers (such as polystyrene, hydrogenated polyisoprene, poly (1,4-butadiene), and their copolymers) as VI improvers with significantly enhanced shear stability,14,15,16 however none were polymethacrylates specifically designed for shear stability benefits. Hyperbranched polymers and dendrimers, have seldom been studied for applications as lubricant VI improvers, although extensive research17,18 on their novel synthesis and applications in such areas as drug delivery and nanotechnology has been carried out in the past decade. Based on conventional belief, they too are poised to possess superior shear stability owing to their highly branched dendritic chain structure. Ye and coworkers demonstrated that indeed hyperbranched polyethylenes provided a shear stability advantage over linear counterparts, albeit VI values diminished alongside the shift in topology from linear to highly branched (dendritic).19 Our



group studied such structures in the past with a focus on co-polymerization and the effect of polar moieties on friction.20 Our previous studies did not investigate the relationship between these polymer structures and shear stability. Ye presented a systematic investigation of the unique effects of topology on viscosity thickening power and shear stability in regards to dendritic polyethylenes. They demonstrated that highly-branched polyethylenes with high molecular weights possess extremely high shear stability. Unfortunately, these polymers had relatively low VIs and therefore are not ideal for today’s high performance demands where VIs of around 250 are desired. Many molecular design approaches were explored21,22 in an effort to improve the aforementioned parameters. Inspired by Hawker23 et al.’s colorful reference to dendritic molecules as “molecular bearings”, we set out to explore the effect of branched architectures towards shear stability. The surface congestion of dendritic molecules, coupled with the high degree of branching, is expected to reduce and potentially prevent inter-chain entanglements. Of course, dendritic molecules are costly and laborious to produce whereas their hyper-branched analogs have become, in recent years, commercially available. Furthermore, a core-first synthetic strategy requires only a small amount of a hyper-branched macro-initiator to grow arms from, thereby making such molecules economically feasible. To the best of our knowledge, this approach has not been explored towards the preparation of viscosity index improvers. There are several reports of hyperbranched polycaprolactone syntheses2,6,24,25 and only one literature report26 that resembles our synthetic design albeit with a different application in mind. We utilized this “core-first” sequential polymerization strategy to prepare multi-arm star block copolymers with an oil insoluble hyper-branched core and a lipophilic periphery composed of linear arms to investigate viscosity behavior and shear stability of non-linear polymer additives.


Page 4 of 28

Page 5 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Methods General considerations. ഥ w = 600 g/mol, and ‫ܯ‬ ഥ w = 1800 g/mol) were purchased from Poly(ethylenimines) (PEI; ‫ܯ‬ Polysciences,

Inc.

methoxypolyethylene

Hyperbranched glycol

ഥn (‫ܯ‬

bis-MPA 350

g/mol),

polyester-16-hydroxyl copper

bromide

(MPA-OH16), Cu(I)Br)/Cu(II)Br

N,N,N’,N”,N”-pentamethyldiethylene-triamine (PMDETA), tin(II)octoate, α-Bromoisobutyryl bromide (BiBB), were purchased from Sigma Aldrich and used as received. The monomers dodecyl methacrylate (DMA) and 2-ethylhexyl methacrylate (EHMA) were purchased from Sigma Aldrich and passed through a neutral alumina or silica plug to remove inhibitors. Caprolactone (CL) was purified by heating over CaH for 2h, followed by distillation under reduced pressure. Anhydrous, inhibitor free tetrahydrofuran was used to prepare the 0.1 M CuBr:PMEDTA (1:3) solution as well as the reaction solvent in some cases. Reaction equipment was oven dried, placed under vacuum while cooling, and backfilled with argon. Regular solvents, such as

methanol (MeOH),

acetonitrile (ACN), dichloromethane (DCM),

isobutyronitrile (IBN) were purchased from Fisher. Group III base oil (4Yubase) which was kindly donated by SK Lubricants, was used to prepare lubricant mixtures with synthesized polymers. The benchmarks is a commercial viscosity modifier kindly donated by Evonik and employed here as comparative example. Characterization. Nuclear magnetic resonance (NMR) spectra were obtained using an AgilentOxford 500 MHz spectrometer at the following frequencies: 500 MHz (1H) and 125.7 MHz (13C{1H}). The chemical shifts are reported in delta (δ) units, parts per million (ppm) downfield from tetramethylsilane (TMS) and coupling constants for small molecules are reported in Hertz



(Hz). Samples were prepared in deuterated chloroform (CDCl3). Relative molar mass distributions were obtained by size exclusion chromatography (SEC) analysis. The molar masses were determined relative to the elution volumes of linear poly(methyl methacrylate) standards pushed through two columns of Jordi Gel DVB Mixed bed (250x10mm) in THF (mobile phase) and detected via a refractive index detector (λ) or a triple detector in some cases. Values reported are the average of two runs. Polymerization. The monomer and initiator (Initiator 1–7) were transferred into a two-neck reaction flask fitted with an air condenser and rubber septum. The reaction flask was degassed via vacuum-argon cycles (3x) and left under vacuum for ~30 minutes. In parallel, a fresh 0.1 M CuBr:PMDETA/THF solution was prepared in a Schlenk round bottom flask under argon. The reaction flask was backfilled with argon and heated to 70 °C. The catalyst solution was injected into the reaction mixture and the external temperature was increased to ~120 °C, over ca. 30 minutes. In general the reaction mixture changed from green to amber and started becoming viscous in ca. 60 minutes. An aliquot of the reaction mixture was periodically analyzed by 1H NMR to monitor the progression of the polymerization. Typically, the reaction reached 80% conversion overnight but in some cases was allowed to run for two days to reach a ca. 90% conversion. The reaction flask was opened to the air which effectively terminated the polymerization. The crude polymer was dissolved in THF or DCM and precipitated into warm acetonitrile or methanol, followed by decantation of the solvent. This process was repeated until a clean polymer with less than 5 mol % of monomer was obtained, typically three times. Rheology measurements.


Page 6 of 28

Page 7 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Polymers were dissolved into 4Yubase (Y4) at a concentration of 5% w/w and resulting blends were measured by a spindle viscometer to determine dynamic viscosity (centipoise, cP = mPa•s) at 40 and 100 °C. The dynamic viscosity values were converted into kinematic viscosity (KV) values (cSt = mm2•s-1) by dividing the centipoise value by the density of the blend (0.83 g•cm-3 at r.t.). The densities of the blends were roughly the same, independent of the polymer used. A Brookfield digital (LVDV-E) spindle viscometer was fitted with a cooling/heating jacket that was continuously flowing with oil supplied by an external cooling/heating bath that regulated the jacketed temperature at 40 or 100 °C. A rotating spindle (ULA code 00; 0.3−100 RPM) was submerged into the blended oil at the regulated temperatures for 30 minutes. This standard practice for calculating viscosity index (VI) from the acute viscosity at 40 and 100 °C is described in standard D2270 and was used to generate viscosity index values. Shear Stability Measurements. Shear stability is a measure of a lubricant’s resistance to viscosity loss when it is subject to high shear stress or passed through narrow passageways in components such as bearings or gears. A common test for this property is the Taper Roller Bearing Rig (KRL) test CEC-L-45-99. This test was the most convenient for us as it is fast, requires small amounts of sample (50–80 mL) and provides valuable shear stability data to establish trends. Test oil is run in a fitted tapered roller bearing for 20 hours under design load. Before and after viscosities were measure and used to generate permanent viscosity loss (PVL) and permanent shear stability index values (PSSI). The measurements were conducted at Southwest Research Institute. Results and Discussion Molecular Design and Synthesis



Page 8 of 28

The focus of the molecular design was to explore the influences of core architecture, crystalline segments and molecular weight towards viscosity and shear stability. A core-first strategy was ഥ௪ ≈ employed to prepare multi-arm stars from hyper-branched cores: poly(ethyleneimine (PEI; ‫ܯ‬ 1800 Da; –NH2 ≈ 10.5; NH ≈ 21), 2,2-bis(methylol)propionic acid hyperbranched polyester (MPA; generation 2; –OH ≈ 16), MPA-(caprolactone-CL) (MPA-CL-OH, Initiator 1), cresol, and polyethylene glycol-polycaprolactone (PEG-CL) (Initiator 5). These cores were treated with αbromoisobutyryl bromide (BiBB) and subsequently utilized as a head group for atom transfer radical polymerization (ATRP or ARGET ATRP) with DMA or a combination of DMA-EHMA. MPA-CL-OH was prepared as an extended core to promote these aforementioned effects by way utilizing the crystalline segment polycaprolactone, as an inner block. The synthesis is captured in Scheme 1. Compounds 8–11 were prepared from the same macro Initiator 2, with varying amounts of DMA to probe for the effect of PCL as well as molecular weight on the shear stability.

n


Page 9 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


n

Scheme 1: Synthesis and structure of Initiators 1 and 2, and Compounds 8–11. The compounds have the same structure, but differ in the amount of DMA. A1, A2, A3 and A4 are identical arms to the one drawn from the nearest quaternary carbon.



The direct functionalization of MPA-OH with BiBB, led to a polymer growth directly from the hyperbranched core, as illustrated in Scheme 2. In this case both DMA and a combination of DMA and EHMA were used to explore the potential effect of pendant chain length on shear stability. In a previous study EHMA showed improved performance versus DMA not only with respect to friction and wear, but also shear stability.27 The shear stability of EHMA polymers was increased by two times versus those with DMA only, which was attributed to the shorter pendant chain, hexyl versus dodecyl. The same effect was probed here, with the expectation that EHMA/DMA would produce a more shear stable polymer versus that containing DMA alone. ഥ ws. Two different copolymers were prepared, 13 and 14, with two different ‫ܯ‬


Page 10 of 28

Page 11 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


O

A4

Initiator 3

CuBr, PMDETA

O q

A4 A4

C12 O

O O

C12 O O O O

O

p

O

O O O

DMA, EHMA 100-120 °C

O

O

O

O

O

O

O O

O q O p O O O C

q

p

O p

O q

O

C12

12

Compounds 13, 14

Scheme 2. Synthesis and structure of Initiator 3 and Compounds 12–14. A1, A2, A3 and A4 are identical arms to the one drawn from the quaternary carbon.

Linear initiators were prepared to target linear polymers, as comparative models to the hyperbranched polymers. In one example, cresol was chosen as an initiator (Initiator 4), in an ഥ n estimation from 1H attempt to integrate aromatic protons versus DMA protons for a better ‫ܯ‬ NMR. This sequence is shown in Scheme 3. Although the aromatic protons could not be detected by 1HNMR in the final polymer (due to the large number of repeating units), the sequence essentially produced all DMA linear polymers of different molecular weights (15 and 16).



Scheme 3. Synthesis of linear initiator 4 and linear polymers 15, 16.

In order to have a direct comparison for the hyperbranched analog MPA-PCL-DMA, a linear initiator with a polar (PEG) and crystalline (PCL) segment was also prepared, in an effort to mimic properties of the extended MPA-PCL core. The transformation sequence is illustrated in Scheme 4. The subsequent DMA polymerization sequence underwent with low conversion due to the relatively large size of the mono-functionalized initiator; therefore the molecular weight ഥ w of 118 kDa versus ~250 kDa). Nonetheless, compound 17 obtained was lower than targeted (‫ܯ‬ provided a good comparative example for compound 11, the lowest molecular weight hyperbranched analog with similar composition.

Scheme 4: Synthesis of Initiators 5 and 6 and polymer 17

Finally, another hyperbranched core, PEI1800 was selected, with a lower molecular weight but higher polarity than MPA-OH16. This starting molecule provided yet another multi-arm handle to probe for the influence of hyperbranching on shear stability. The synthesis is illustrated in Scheme 5. This synthesis and subsequent analysis of initiator 7 proved challenging. The reaction conditions promoted unexpectedly, a substantial degree of secondary amine functionalization 12 ACS Paragon Plus Environment

Page 12 of 28

Page 13 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


with BiBB. NMR analysis indicated the incorporation of 22 BiB functional groups per molecule, which seemed very high considering unfavorable steric interactions. The intermediate was subjected to elemental analysis to confirm the finding, which indicated 36% Br (w/w), equivalent to 25 BiB units per molecule. It is important to note that the intermediate was chromatographed and all hydrolyzed BiBB, α-bromoisobutyric acid, was removed and therefore did not contribute to the high number of BiBs in the molecule. However, it is unlikely all BiBs were effective initiators towards ATRP due to steric crowding.

Scheme 5: The synthesis of Initiator 7 and polymer 18. The core and initiator selection was based on their level of hyperbranching, commercial availability, relative ease of synthesis and functionalization, comparable architectures and differing chemical compositions. The initiators employed in this work, their compositions, average molecular weights, and degree of hyperbranching, are summarized in Table 1. Table 1: Composition and characterization of macroinitiators. Compound

ഥ napp (g/mol) ‫ܯ‬

Maximum

Composition

# of arms Initiator 1

20,468a

Initiator 2

a

Initiator 3

21,600 /38,900 3,425

b

b

16

MPA-CL-OH

9.25

MPA-CL-BiBB

11.5

MPA-BiBB 13

ACS Paragon Plus Environment


Initiator 4

—

linear

Cresol-BiBB

Initiator 5

2,632a

linear

PEG-CL-OH

Initiator 6

2,782a

linear

PEG-CL-BiBB

Initiator 7

a

a

5,100

22

Page 14 of 28

PEI-BiBB 1

b

Molecular weight was estimated from H NMR; Molecular weight measured by SEC.

The post-modification and chemical composition of starting materials and intermediates, as well as polymerizations were verified by standard NMR spectroscopy. Relative molecular weights, as compared to polymethylmethacrylate (PMMA) standards, were acquired by SEC. Where indicated, the molecular weights were calculated from 1H integrations. All NMR and GPC data/spectra may be found in the supporting information, whereas the polymer characterization results are shown in Table 2. Due in part to the polydispersity of most starting materials (MPAOH16, PEG 350, PEI1800), the final polymers are rather non-uniform themselves, despite the use of a controlled polymerization technique. Other competing polymerization events such as cross-coupling between macroinitiators may have also contributed to the problem, as suggested by the multi-modal or broad polymer peaks. Chain transfer events are likely the source of polydispersity in compounds 15 and 16, where a single small molecule was the initiator. However, in the case of the multifunctional initiators it is conceivable that not all arms will grow equally; a statistical number of arms will terminate prematurely due to steric interactions, increasing heterogeneity, and early terminus deactivation events under the reaction conditions prescribed. The increased temperature required to drive conversion of some of the reactions could be in part responsible for the observed polydispersity increase. Table 2. Characterization of multi-star polymers. Analog

Composition

αc/DMA/EHMA (g)

ഥ napp ‫ܯ‬

ഥ wapp ‫ܯ‬

a

a

(kg/mol)


(kg/mol)

Đெ b

Page 15 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


8

MPA-CL-DMA

1.1/10.6/0

177

332

1.9

9

MPA-CL-DMA

0.86/16.3/0

235

534

2.3

10

MPA-CL-DMA

1.0/7.0/0

129

460

3.5

11

MPA-CL-DMA

1.0/5.0/0

50.7

249

4.9

12

MPA-DMA

0.11/19.2/0

158

465

2.9

13

MPA-DMA-EHMA

0.042/2.69/2.03

10.4

24.3

2.3

14

MPA-DMA-EHMA

0.050/8.40/6.55

201

543

2.7

15

Linear DMA

0.008/13.0/0

152

362

2.3

16

Linear DMA

0.016/21.0/0

294

1,092

3.7

17

Linear PEG-PCL-DMA

0.123/11.3/0

86

118

1.4

18

PEI-DMA

0.053/8.2/0

184

520

2.8

ഥ napp) and apparent weight-average molecular a) Apparent number-average molecular weight (‫ܯ‬ app ഥ w ) were determined via SEC against poly(methy methacrylate) standards and are weight (‫ܯ‬ ഥ ௔௣௣ ‫ܯ‬ expressed in either kg/mol or kDa; b) Molar dispersity (ĐM) was calculated from ௪ ൘ഥ ௔௣௣ = ‫ܯ‬௡ Đெ ; c) α denotes the multi-arm macroinitiator(s) described in Table 1.

Rheology performance The polymeric solutions were prepared at 5% (w/w) concentrations in 4Yubase. Their dynamic viscosities (DV) were measured by a Brookfield viscometer at 40 and 100 °C. Table 3 includes the measured (dynamic - DV) and calculated viscosities (kinematic - KV) at 40 and 100 °C for the 5% w/w blends. These numbers were utilized to generate viscosity index (VI) values using ASTM D2270. Table 3. Viscosity performance of 5% (w/w) [additive]/[4yubase] blends versus benchmark. Material

DV @ 40C

DV@100C

KV40

KV100

(cP)

(cP)

(cSt)

(cSt)

4-Yubase

15.92

3.54

19.30

4.29

132

Benchmarka

46.10

12.0

55.43

14.45

274

8

26.34

6.06

31.54

7.26

206

9

59.8

13.02

71.96

15.67

232


VI


Page 16 of 28

10

29.55

6.67

35.60

8.03

209

11

29.94

6.44

36.07

7.76

193

12

58.98

13.58

70.89

16.32

247

13

19.04

4.248

22.93

5.12

161

14

45.85

11.16

55.24

13.44

254

15

49.35

11.15

59.31

13.40

235

16

108.7

130.96

28.50

34.33

302

17

24.00

5.82

28.90

7.01

219

18

55.86

12.24

67.30

14.74

231

Viscometer recorded dynamic viscosity in centipoise (cP) at 40 and 100 °C. Kinematic viscosity measured in centistokes (KV) was derived for 40 and 100 °C by dividing DV by the blend’s density, which was 0.83 g/mL for all polymer blends. aThe concentration of the benchmark measured was 4 % (w/w); The compounds are color coded based on high (green) and low (blue) KV100s. As evident from Table 3, different polymers have different thickening efficiencies and therefore different KV100 values. Ideally, the KV100 should be normalized, by formulating all fluids to the same KV100 in order to make accurate comparisons between Mw and VIs. For a quick assessment, the data was analyzed by grouping the compounds with similar thickening efficiencies as follows: Compounds 9, 12, 14, 15 and 18 had KVs between 13.4 and as high as 16.3 cSt, while compounds 8, 10, 11, 13 and 17 had much lower VIs between 5.12 and 8.03 cSt. Compound 16 was an outlier with high Mw, very high thickening efficiency and high VI at 5% w/w loading in the base oil, so it will not be part of this comparison. To better visualize the ഥ w were plotted against VI for the two sets, as effect of molecular weight on viscosity index, ‫ܯ‬

shown in Figure 1a and 1b. Compounds 9, 12, 14, 15, and 18 show that all hyperbranched compounds with roughly same molecular weight, have similar VIs Notably, analog 12 (MPA-


Page 17 of 28

DMA) has a very high VI relative to its modest molecular weight, while analog 15, has the lowest Mw and a relatively high VI. Compound 15 demonstrates the large effect of a linear topology on viscosity. It is notable, at first glance, that increasing VI may not correspond to ഥ wapp). In particular, 12 has a VI of 247 and ‫ܯ‬ ഥ wapp of 465 increasing apparent molecular weights (‫ܯ‬ ഥ wapp is greater at 534 kDa. A likely kDa whereas 9’s VI is slightly lower at 233 albeit the ‫ܯ‬ explanation for the discrepancy is the solvophobic effect between polymers in THF (used for SEC) versus polymers in base fluid (used for VI measurement). PCL most likely is well solvated ഥ w is most likely due to in THF whereas poly-DMA is less so. Therefore, compound 9’s greater ‫ܯ‬ the expanded core’s (PCL) favorable interaction with THF over compound 12’s (PDMA) less favorable. This also implies that the expansion of 9 is not enhanced by the melting of the crystalline segments as we had envisioned, at least not in a nonpolar base oil within the studied temperature range. The opposite would be true in a polar base fluid, such as poly(alkylene) glycol, or a polyester. It is also possible that these small differences are due to differing KV100 values.

Mw versus VI 280 260

Viscosity Index

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


14

12

240

15

9

220

18

200 180 160 140 120 100 0

100

200

300

400

Mw (kDa) a.


500

600


Mw versus VI 240 220

Viscosity Index

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 28

17

10

8

200

11

180

13

160 140 120 100 0

100

200

300

400

500

Mw (kDa) b. ഥ wapp for compounds with high thickening efficiency and low Figure 1. Viscosity Index versus ‫ܯ‬

KV100 (a) and compounds with low thickening efficiency and high KV100 values (b) The Mw values are measured from SEC, and are found in Table 2, while the viscosity index values are found in Table 3.

Figure 1b compares lower thickening polymers, compounds 8, 10, 11, 13 and 17. Compounds 8– 11 that contain caprolactone semi-crystalline segments, were designed to probe the potential differential solubility of the various segments shown in other systems28,29 to provide a boost in viscosity index. The crystalline segment, i.e. the amount of PCL, was expected to augment the VI. Compound 11 has the highest amount of PCL (mole %, versus DMA), followed by 10, then 8, although the molecular weights do not follow that trend. The effects of interplay between architecture and molecular weight are somewhat difficult to decouple. Compound 13 is also an outlier, it has the lowest Mw and subsequently VI. One of the more interesting compounds is 17, which is a linear version of 8–11; it has a high VI while its molecular weight is relatively low. In this case, it appears that the semi-crystalline segment has a beneficial effect on VI. Overall, it appears that molecular weight governs the viscosity behavior of these molecules, but introduction of semicrystalline blocks/segments that are less favorably solvated in oil may have a beneficial effect on VI for linear architectures. 18 ACS Paragon Plus Environment

Page 19 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Shear Stability Because our general focus was the design of novel structures for viscosity performance, targeting a rather applied field such as lubricant additives, rheology testing and assessment of the polymers was necessary. However, the main focus of this work was more fundamental in nature, geared towards understanding and probing the conventional belief that hyperbranching increases shear stability. In that regard, multi-arm stars or hyperbranched polymers were targeted for study, alongside their linear counterparts. The idealized shape of the polymers targeted is shown in Figure 2 (a–f).

a.

b.



c.

e.

d.

f.

Figure 2: General architecture of polymers under study: a. compounds 8–11; b. compound 2; c. compounds 13 & 14; d. compound 18; e. compounnds 15 & 16; f. and compound 17.

Shear stability is a complex result of polymer molecular weight, its concentration in the base oil, polymer composition and architecture. In this study the polymer solutions were subjected to harsh conditions as they might encounter in an engine (100 °C, under mechanical shear). Mechanical stress commonly breaks down the polymers additives in lubricants thereby resulting in permanent viscosity losses. The results are reported as permanent shear stability index and ഥ w and can be found in Table 4, which summarizes other relevant polymer data such as VI, ‫ܯ‬ architecture. The formulae used to calculate the PSSI is found in the SI, page 37. It is well accepted that molecular weight has a significant influence on the shear stability, with viscosity losses increasing with increased molecular weight. The published literature suggests that hyperbranching has a positive effect on shear stability, however the trends obtained from this study point to molecular weight playing the biggest role in shear stability. In Figure 3, the compounds with high thickening efficiencies (and high KV100) are highlighted in orange, while compounds with lower thickening efficiencies (and lower KV100) are highlighted in green.


Page 20 of 28

Page 21 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Clearly the highest molecular weight analog 16 experiences the highest viscosity loss under shear, which also coincides with the fact that it is linear. The lowest molecular weight analog 13, experiences a viscosity gain of 1%, which in essence means no viscosity loss, considering the experimental error of the measurement can easily be ± 1%. That is a hyperbranched analog, but it also is a very low molecular weight one. If only the low KV100 compounds are considered, it appears that PCL is responsible for the relatively lower PSSIs. Compound 17, which is linear has only slightly higher PSSI versus the hyperbranched analogs 8, 10 and 11, which have a much higher Mw. The MPA-CL-DMA series of four compounds, at first glance suggests that increased amounts of caprolactone grafting increased the shear stability. Another evident example of the lack of influence of hyperbranching is the comparison between compounds 12 and 15. Both have ഥ w, but one is linear while the other has an average of 11 arms per molecule; and very similar ‫ܯ‬ yet the shear stability of the two is very similar, with a PSSI of ~ 70. . Among the high KV100 series illustrated in orange in Figure 3, analogs 9, 12, 14 and 18 have a hyperbranched topology and similar molecular weights, yet their respective PSSIs range from 50 to 72. Analogs 9 and 14 have the lowest PSSI, one containing PCL, the other containing a shorter methacrylate chain. Another important aspect of the shear stability of the polymethacrylates studied here is that DMA has a relatively long alkyl chain which may contribute to the increased shearing of the polymer. It is possible that caprolactone appears to have a positive effect due to the lower amount of pendant chains per molecule. In order to keep the molecules lipophilic and oil soluble, long floppy alkyl methacrylates had to be used as monomers. To probe for pendant chain length effects, compounds 13 and 14 were compared to analog 12. In analog 14, 50% of the DMA feed ratio was replaced with EHMA, which has a chain of six carbons versus twelve. Compound 12 and 14 appear to have similar molecular weights, as targeted, and the strategy of introducing a



shorter chain appears to have been beneficia, as it resulted in almost 50% decrease in PSSI (72 versus 50). The Mw versus PSSI is illustrated in Figure 3.

Permanent Shear Stability Index

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 28

Mw vs. PSSI 80 12

15

70

18

60 50

9

14

40 17

30

8 10

20

11

10 0

13 0

100

200

300

400

500

600

Mw (kDa)

Figure 3. Mw versus PSSI, at 100 °C. The benchmark is not included in the plot due to proprietary Mw, while 16 is not included to avoid overcrowding. As mentioned earlier, hyperbranched systems have been studied by others for shear stability purposes, but none of the reported literature included long alkyl chain methacrylates. Several researchers report increased shear stability for hyperbranched polymers over their linear polymer analogs at comparable concentrations. This can be partly explained by the significantly lower chain overlap due to the more compact conformation of hyperbranched polymers in solutions relative to linear polymers of comparable molecular weight. However, in those cases the study compounds were poly(methyl methacrylate)s,7 polyethylenes,19 and 6-arm poly(methyl methacrylate)s15, and styrene copolymers.28 It is conceivable that the beneficial effects of hyperbranching are offset by the long chain pendants (increased intra-molecular strains and the possibility of increased entanglements), as present in the current work. Indeed short chain methacrylates might provide a shear stability benefit due to lower chain entanglements but also 22 ACS Paragon Plus Environment

Page 23 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


suffer from insolubility in base oils, and are unlikely to provide high VIs, essential requirements of the present application. Finally, several polymers investigated in this study had improved shear stability as compared to the benchmark. However, we cannot make any structural assessments of the result, since the structure is proprietary, we can only use the benchmark as a performance gauge of the researched compounds. Table 4. Architecture and composition versus viscosity loss. Compounda

206 232 209 193 247 161

Viscosity lossb (%) 15.0 23.5 12 7 34 -1

PSSIc (%) 33.4 53.6 24 18 74 0

254

35.5

50.4

235 302 219

33 78 17.5 43 31

71 87.5 34.7 61.8 64

# arms Composition

VI

8 9 10 11 12 13

Mw (SEC) 332 534 460 249 465 24

9.25 9.25 9.25 9.25 11.5 11.5

14

543

11.5

15 16 17 18

362 1,092 118 520 —

linear linear linear 22 —

Benchmark 1.7%

MPA-CL-DMA MPA-CL-DMA MPA-CL-DMA MPA-CL-DMA MPA-DMA MPA-DMAEHMA MPA-DMAEHMA DMA DMA PEG-PCL-DMA PEI-DMA —

231 238

a

All polymers were tested as 5% w/w solutions in 4Yubase, with the exception of the benchmark which was tested at specified concentration. bThe viscosity loss (%) was calculated from viscosity loss during shear, divided by the viscosity before shear, times 100. The sear permanent shear stability index (SSI) was calculated to decouple the difference in KVs of various analogs. The formulae of both permanent viscosity loss and permanent shear stability index. Conclusions The main focus of this work was to investigate the effects of the chain topology and overall architecture on the shear stability of a variety of polymers based on poly(dodecyl)methacrylates. In addition, the influence of topology and molecular weight on viscosity index was also studied due to its relevancy to lubricant applications. Rheology and shear stability are important parameters in lubricant design and have a critical impact on its performance and lifetime in an 23 ACS Paragon Plus Environment


engine. Multiple analogs with hyperbranched architectures were prepared via the core-first strategy from hyperbranched cores. Linear analogs were prepared to afford a side-by-side shear stability performance study. Hyperbranched polyethyleneimine and hyperbranched polyester (MPA-OH16) were either first extended with polycaprolactone or directly functionalized to generate multi-arm macro ATRP initiators. Polymerizations were carried out in the presence of dodecylmethacrylate with or without 2-ethylhexyl methacrylate. Rheology and shear stability of all analogs were evaluated and analyzed via established methods. In general, viscosity index correlated well with molecular weight which is a known trend, the higher the molecular weight, the higher the VI. In several cases, subtleties in structure offset this trend, as it appeared to be the case for polycaprolactone grafted polymers, which appeared to provide a VI enhancement. Much less expected and contrasting the conventional belief, was the shear stability data. Our study did not demonstrate that a hyperbranched architecture has a positive influence on shear stability when long pendant alkylmethacrylates are part of the design. It is clear from this study, that increasing intra-arm entanglements via long lipohilic pendant chains or increased arm numbers is not beneficial towards shear stability. Therefore, in this case, shear stability appears to be mainly governed by molecular weight and not by the number of arms. It is possible that in less stressed systems (i.e. shorter pendants) increased number of arms has a positive influence on shear stability, as suggested by analog 14. Notably, as lubricant additives, several polymers studied here have a far improved shear stability over the commercially available benchmark, with two analogs displaying a PSSI as low as 18 and 24 respectively, versus the benchmark who had a PSSI of 64. Overall, strategies such as incorporation of a semi-crystalline segment and shorter methacrylate pendants, appeared beneficial towards shear stability. Future work will include a similar extended core model with caprolactone that includes variance in the arm chain design


Page 24 of 28

Page 25 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


such as shorter pendant groups of methacrylates with dodecylmethacrylate, to further explore the effects of side chain entanglements on viscosity and shear stability. In addition, poly(2ethylhexyl)methacrylate hyperbranched and linear homopolymers will be explored, as they are anticipated to possess a better shear stability, while still remaining soluble in non-polar media. Supporting Information: synthetic details, spectroscopic characterization (1HNMR,

13

CNMR)

data for each compound and intermediate; GPC chromatograms of final polymers. Acknowledgements This project was funded by the Office of Vehicle Technology (VT) of the U.S. Department of Energy (US DOE), (VT0604000-05450-1004897). PNNL is proudly operated by Battelle for the U.S. DOE (under Contract DE_AC06–76RLO 1830). The authors kindly acknowledge contributions from JoRuetta Ellington (Evonik) for providing benchmark materials. We thank Afton Chemical for generously donating base oils for screening purposes. Author contributions statement L.C. proposed the original material design, analyzed results, synthesized compounds and performed rheology measurements. J.W.R. proposed, synthesized, characterized and analyzed results. J.P.P. prepared and characterized materials. All authors reviewed this manuscript. Conflict of Interest Disclosure The authors declare no competing financial interest. References

(1) Zhao, J.; Lu, C.; He, X.; Zhang, X.; Zhang, W.; Zhang, X. (2015). Polyethylenimine-grafted cellulose nanofibril aerogels as versatile vehicles for drug delivery. ACS Appl. Mater. & Interfaces, 2015, 7, 2607-2615.



(2) Arce, E.; Nieto, P. M.; Díaz, V.; Castro, R. G.; Bernad, A.; Rojo, J.. Glycodendritic structures based on Boltorn hyperbranched polymers and their interactions with Lens culinaris lectin. Bioconjugate ch. 2003 14, 817-823. (3) Pu, K. Y.; Li, K.; Shi, J.; Liu, B. Fluorescent single-molecular core− shell nanospheres of hyperbranched conjugated polyelectrolyte for live-cell imaging. Chemistry of Materials, 2009, 21, 3816-3822. (4) Zhang, L. W.; Hua, J. R.; Zhu, W. J.; Liu, L.; Du, X. L.; Meng, R. J.; Yao, J. M. Flocculation Performance of Hyperbranched Polyethylenimine-Grafted Cellulose in Wastewater Treatment. ACS Sustainable C. Eng., 2018, 6, 1592-1601. (5) Zhu, W.; Liu, L.; Liao, Q.; Chen, X.; Qian, Z.; Shen, L, J.; Yao, J. Functionalization of cellulose with hyperbranched polyethylenimine for selective dye adsorption and separation. Cellulose, 2016, 23, 3785-3797. (6) Claesson, H.; Malmström, E.; Johansson, M.; Hult, A. Synthesis and characterisation of star branched polyesters with dendritic cores and the effect of structural variations on zero shear rate viscosity. Polymer, 2002, 43, 35113518. (7) Wheeler, J. S.; Longpré, A.; Sells, D.; McManus, D.; Lancaster, S.; Reynolds, S. W.; Yeates, S. G. Effect of polymer branching on degradation during inkjet printing. Polym. Degrad. Stab., 2016, 128, 1-7. (8) Stiriba, S. E.; Kautz, H.; Frey, H. Hyperbranched molecular nanocapsules: comparison of the hyperbranched architecture with the perfect linear analogue. J. Am. Chem. Soc., 2002, 124, 9698-9699. (9) Martini, A.; Ramasamy, U. S.; Len, M. Review of Viscosity Modifier Lubricant Additives. Tribol. Lett., 2018, 66, 58. (10) Hsu, S. M.; Gates, R. S. Boundary lubricating films: formation and lubrication mechanism. Tribol. Int., 2005, 38, 305-312. (11) Glass, J. E.; D. N. Schultz; C. F. Zukoski. Polymers as Rheology Modifiers: an overview. ACS Symp. Ser., 1991. (12) Mishra, M. K.; & Saxton, R. G. Polymer additives for engine oils. Chemtech, 1995, 25. (13) Covitch, M. J.; Weiss, J.; Kreutzer, I. M. Low-temperature rheology of engine lubricants subjected to mechanical shear: Viscosity modifier effects. Lubr. Sci., 1999, 11, 337-364. (14) Eckert, R. J. (1978). U.S. Patent No. 4,116,917. Washington, DC: U.S. Patent and Trademark Office. (15) Xue, L.; Agarwal, U. S.; Lemstra, P. J. Shear degradation resistance of star polymers during elongational flow. Macromolecules, 2005, 38, 8825-8832. (16) Wang, T. Y.; Tsiang, R. C. C.; Liou, J. S.; Wu, J.; Sheu, H. C. Preparation and characterization of a star-shaped polystyrene-b-poly (ethylene-co-propylene) block copolymer as a viscosity index improver of lubricant. J. Appl. Polym. Sci., 2001, 79, 1838-1846. (17) Gao, C.; Yan, D. Hyperbranched polymers: from synthesis to applications. Prog. Polym. Sci., 2004, 29, 183-275. (18) Bosman, D. A.; Janssen, H. M.; Meijer, E. W. About dendrimers: structure, physical properties, and applications. Chem. Rev. 1999, 99, 1665-1688. (19) Wang, J.; Ye, Z.; Zhu, S. Topology-engineered hyperbranched high-molecular-weight polyethylenes as lubricant viscosity-index improvers of high shear stability. Ind. Eng. Chem. Res., 2007, 46, 1174-1178. (20) Robinson, J. W.; Zhou, Y.; Qu, J.; Bays, J. T.; Cosimbescu, L. Highly branched polyethylenes as lubricant viscosity and friction modifiers. React. Funct. Polym., 2016, 109, 52-55. (21) Savoji, M. T.; Zhao, D.; Muisener, R. J.; Schimossek, K.; Schoeller, K.; Lodge, T. P.; Hillmyer, M. A. Poly (alkyl methacrylate)-Grafted Polyolefins as Viscosity Modifiers for Engine Oil: A New Mechanism for Improved Performance. Ind. Eng. Chem. Res., 2018, 57, 1840-1850. (22) Wang, T. Y.; Tsiang, R. C. C.; Liou, J. S.; Wu, J.; Sheu, H. C. Preparation and characterization of a star-shaped polystyrene-b-poly (ethylene-co-propylene) block copolymer as a viscosity index improver of lubricant. J. Appl. Polym. Sci., 2001, 79, 1838-1846. (23) Hawker, C. J.; Farrington, P. J.; Mackay, M. E.; Wooley, K. L.; Frechet, J. M. Molecular ball bearings: the unusual melt viscosity behavior of dendritic macromolecules. J. Am. Chem. Soc, 1995, 117, 4409-4410. (24) Trollsås, M.; Atthoff, B.; Claesson, H.; Hedrick, J. L. Hyperbranched poly (ε-caprolactone) s. Macromolecules, 1998, 31, 3439-3445. (25) Huskić, M.; Pulko, I. The synthesis and characterization of multiarm star-shaped graft copolymers of polycaprolactone and hyperbranched polyester. Eur. Polym. J., 2015, 70, 384-391.


Page 26 of 28

Page 27 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(26) Hedrick, J. L.; Trollsås, M.; Hawker, C. J.; Atthoff, B.; Claesson, H.; Heise, A.; Dubois, P. Dendrimer-like star block and amphiphilic copolymers by combination of ring opening and atom transfer radical polymerization. Macromolecules, 1998, 31, 8691-8705. (27) Cosimbescu, L.; Vellore, A.; Ramasamy, U. S.; Burgess, S. A.; Martini, A. Low molecular weight polymethacrylates as multi-functional lubricant additives. Eur. Polym. J. 2018, 104, 39. (28) Jukic, A.; Vidovic, E.; Janovic, Z. Alkyl methacrylate and styrene terpolymers as lubricating oil viscosity index improvers. Chem. Technol. Fuels and Oils, 2007, 43, 386-394.



For Table of Contents use only


Page 28 of 28

Polymer Architecture: Does it Influence Shear Stability? Lelia

Recommend Documents