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Megasupramolecules for safer, cleaner fuel by end association of long telechelic polymers

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Science  02 Oct 2015:
Vol. 350, Issue 6256, pp. 72-75
DOI: 10.1126/science.aab0642

Transient polymer links are better

Very long polymer molecules elongate in shearing flows. This ordering of the chains lowers the viscosity of small-molecule solvents. The chains also reduce the dangers of misting during explosive spreading of the solvents. However, the long polymer chains degrade during normal fuel handling and clog pumping equipment. Wei et al. used telechelic polymers—short chains with reactive end groups—to form extremely long chains in organic solvents (see the Perspective by Jaffe and Allam). These reformable polymers offer the potential for better fuel safety without the drawbacks of covalently bonded long-chain polymers.

Science, this issue p. 72; see also p. 32

Abstract

We used statistical mechanics to design polymers that defy conventional wisdom by self-assembling into “megasupramolecules” (≥5000 kg/mol) at low concentration (≤0.3 weight percent). Theoretical treatment of the distribution of individual subunits—end-functional polymers—among cyclic and linear supramolecules (ring-chain equilibrium) predicts that megasupramolecules can form at low total polymer concentration if, and only if, the backbones are long (>400 kg/mol) and end-association strength is optimal. Viscometry and scattering measurements of long telechelic polymers having polycyclooctadiene backbones and acid or amine end groups verify the formation of megasupramolecules. They control misting and reduce drag in the same manner as ultralong covalent polymers. With individual building blocks short enough to avoid hydrodynamic chain scission (weight-average molecular weights of 400 to 1000 kg/mol) and reversible linkages that protect covalent bonds, these megasupramolecules overcome the obstacles of shear degradation and engine incompatibility.

Ultralong polymers (weight-average molecular weight Mw ≥ 5000 kg/mol) exhibit striking effects on fluid dynamics even at low concentration; for example, polymer concentrations of 100 parts per million (ppm) or less can enable mist control (1, 2) and drag reduction (3). The key to both mist control and drag reduction is the ability of polymers to store energy as they stretch, such that the fluid as a whole resists elongation. The high potency of ultralong linear polymers is due to the onset of chain stretching at low elongation rates and to the chains’ high ultimate conformational elongation (4). For example, increasing Mw from 50 kg/mol to 5000 kg/mol decreases the critical elongation rate by more than three orders of magnitude and increases the ultimate molecular elongation by two orders of magnitude.

Unfortunately, ultralong backbones undergo chain scission during routine handling because hydrodynamic tension builds up along the backbone to a level that breaks covalent bonds; this “shear degradation” continues until the chains shorten to a point that their valuable effects are lost (Mw < 1000 kg/mol) (3). Self-assembly of end-associative polymers creates supramolecules that can potentially break and reassociate reversibly, but formation of megasupramolecules (Mw ≥ 5000 kg/mol) at low concentration has never been realized for two reasons: (i) End-to-end association at low concentration predominantly leads to rings of a small number of chains (5), and (ii) the size of the building blocks is limited because end association is disfavored when they are larger than 100 kg/mol (68).

The current study focuses on megasupramolecules soluble in low-polarity fluids, especially in liquid fuels. Transportation relies on such liquids, presenting the risk of explosive combustion in the event of impact, such as in the 1977 Tenerife airport disaster—an otherwise survivable runway collision that claimed 583 lives in the post-crash fireball. Subsequent tests of ultralong, associative polymers [e.g., ICI’s “FM-9,” >3000 kg/mol copolymer, 5 mole percent (mol %) carboxyl units] in fuel increased the drop diameter in post-impact mist (1, 9), resulting in a relatively cool, short-lived fire. However, these polymers interfered with engine operation (10), and their ultralong backbone—essential for mist control—degraded upon pumping (3).

Our goal is to create megasupramolecules at low concentration that behave like ultralong polymers, exhibiting expanded (“self-avoiding”) conformation at rest and capable of high elongation under flow (Fig. 1A, right). This is in contrast to the collapsed, inextensible supramolecules formed by long chains with associative groups distributed along their backbone (Fig. 1A, left) (11, 12). To mimic ultralong polymers, association must occur at chain ends and be predominantly pairwise. In contrast to multimeric association (6, 8) that leads to flower-like micelles at low concentration (Fig. 1A, center), recent studies have shown that hydrogen bonding can readily achieve pairwise association for short chains with Mw ≤ 50 kg/mol (7, 1319). At low concentration, these have no significant rheological effects, consistent with the theory of ring-chain equilibrium (5, 2023); small rings are the predominant species at low concentration (Fig. 1A, center). We realized that using very long chains as the building blocks would disfavor rings, because the entropy cost of closing a ring increases strongly with chain length.

Fig. 1 Design and synthesis of long telechelic polymers (LTPs).

(A) Proposed assembly of long telechelic polymers (LTPs) into megasupramolecules (right; only linear species shown) compared to that of randomly functionalized associative polymers (left) and prior end-associative telechelics (center) in terms of degree of polymerization and conformations at rest and in elongational flow. (B) Ring-chain equilibrium distribution of cyclic and linear supramolecules. 1 Mg/mol = 1000 kg/mol. (C) Synthesis of telechelics (nonassociative with R1 end groups; structures in fig. S12) and postpolymerization conversion to associative telechelics (R2 end groups, bottom). (1): Grubbs II, dichloromethane (DCM), 40°C, 1 hour; (2): Grubbs II, DCM, 40°C, until stir bar stops (>5 min), equivalents of COD for desired molecular weight. DA, diacid; DB, dibase; TA, tetraacid.

Despite prior reports indicating that end association becomes difficult as chain length increases (68), we ventured into the regime of long telechelic polymers (LTPs, Fig. 1A, right; see table S1 for list of polymers) at concentrations below 1%, using theory to guide the selection of molecular structures. To aid material design, we used a lattice model in which the polymer molecular weight simply maps onto the corresponding number of connected lattice sites, each site with a volume equal to that of an effectively freely jointed segment (“Kuhn segment”)—a well-established property tabulated for many polymers (table S2). We chose unsaturated hydrocarbon backbones because of their solubility (remaining in solution down to the freezing point of fuel) and strength (table S3) (24). In addition to the Kuhn segment volume, two additional attributes of the polymer backbone enter into the entropy cost of ring closure: the Kuhn segment length (how short the distance between ends must be for a ring to close) and the excluded volume parameter (how expanded the chain is in solution). The end-association strength (i.e., energy penalty for unpaired ends) enters through the chemical potential of the linear species.

With this treatment, we computed the inventory of all cyclic and linear supramolecules as a function of concentration, backbone length, and end-association strength by solving the system of equilibrium relationships in a population balance model (Fig. 1B). The resulting predictions indicate that an adequate concentration of megasupramolecules [e.g., >50 ppm of supramolecules with Mw ≥ 5000 kg/mol (2)] forms if the concentration of LTPs is 1400 ppm, their backbone has approximately 6000 Kuhn segments [Mw = 500 kg/mol for polycyclooctadiene (PCOD)], and their ends associate pairwise with an energy of 16 to 18 kT [modeling after Goldstein (25); figs. S1 to S6]. Furthermore, theory shows that the favorable window of chain lengths and association strengths is relatively narrow. If the backbone is too short (e.g., 200 kg/mol PCOD), the fraction of material that is “lost” to the formation of small cyclics increases, and consequently the concentration of telechelics must be increased. If the backbone is too long (e.g., 1000 kg/mol PCOD), the individual telechelics become susceptible to degradation in strong flows (below). If the association energy is too low (e.g., 14 kT), formation of supramolecules is inadequate. If the association energy is too high (e.g., 20 kT), dangling ends are overly penalized and too few linear species form. Two implications emerge from the computations. One is the need for an unusually strong association strength (because the concentration of end groups, 6 μM, is three orders of magnitude lower than examined previously for supramolecular polymers). The other is the need for an unusually strong backbone to avoid shear degradation of the chains themselves.

The need for end groups that favor pairwise association motivated us to seek hydrogen-bonding moieties (7, 1319) that could confer the required association strength. There are several associating supramolecular motifs that offer multidentate hydrogen bond association (7). We tested both a tridentate pair (THY/DAAP) and a hexadentate pair (HR/CA); both failed to produce significant rheological consequences (fig. S7), despite nuclear magnetic resonance (NMR) confirmation that they associate (figs. S8 and S9). In both of these pairs, repulsive secondary electrostatic interactions cut the association strength in half relative to that expected for the number of hydrogen bonds (26), consistent with the association constant of HR/CA, 1.5 × 104 M−1 (corresponding to ~10 kT) (7). Therefore, we turned to charge-assisted hydrogen bonds [CAHBs (27)] that are typically 3 times stronger than ordinary hydrogen bonds (each CAHB provides about 8 to 9 kT binding energy). Simply placing two tertiary amines at each end of the “dibase” chains (DB) and two carboxylic acids at each end of the “diacid” chains (DA) (Fig. 1C) provides an association strength of 16 to 18 kT (27), as recommended by the theoretical results.

To install functional groups at both chain ends with high fidelity (>95%; fig. S10), we use a two-step ring-opening metathesis polymerization (ROMP) protocol (Fig. 1C) (28, 29) in the presence of a chain transfer agent (CTA). Polymers conforming to the theory are synthesized using carefully purified cis,cis-1,5-cyclooctadiene (COD; fig. S11) (29, 30) and CTAs bearing functional end groups (COD/CTA ratio > 3000:1, adjusted to give the desired molecular weight). End groups with discrete numbers of hydrogen bonds (difunctional ends, denoted DA or DB; tetrafunctional ends, denoted TA) (Fig. 1C) can be installed after polymerization by conversion of ester- or chloride-ended polymers (which serve as nonassociative controls, NA), with degrees of conversion >95% (figs. S12 to S14). To test predicted effects of backbone length, we also prepared corresponding telechelics with shorter backbones (e.g., Fig. 2A, Mw ≈ 45, 140, or 300 kg/mol; see table S1).

Fig. 2 Evidence of supramolecules in solutions of equimolar mixture of α,ω-di(isophthalic acid) and α,ω-di(di(tertiary amine)) polycyclooctadienes (DA/DB).

(A) Effect of size of telechelics (k ≡ kg/mol) on specific viscosity of supramolecular solutions and nonassociative (NA) controls in cyclohexane (CH) at 2 mg/ml (0.25 wt %, 25°C). (B) Effect of solvent on specific viscosity for 2 mg/ml (0.25 wt %) solutions (25°C) of telechelics having Mw = 670k due to both polarity (dielectric constant, table S4) and solvent quality for the backbone (fig. S15A). (C) Static light scattering (35°C) shows that association of ~670k DA with DB chains in CH at 0.22 mg/ml (0.028 wt %) produces supramolecules (solid symbols) with an apparent Mw greater than 2000 kg/mol (2.2M = 2200 kg/mol), which separate into individual building blocks (x) when an excess of a small-molecule tertiary amine is added [open symbols, triethylamine (TEA), 10 μl/ml; see fig. S15B for its effect on viscosity]. Curves show predictions of the model for complementary telechelics (1000 kg/mol) in solution at 1400 ppm (solid, supramolecules; dashed, nonassociated telechelics; details in fig. S15C). (D) Concentration-normalized SANS intensities (25°C) for 50k telechelics in d12-cyclohexane at concentrations well below the overlap concentration of NA (2 mg/ml for NA and DB; 0.05 mg/ml for DA and DA/DB) show that DA/DB adopts a relatively open conformation.

The formation of megasupramolecules is evident from solution viscosity and multiangle laser light scattering (MALLS) measurements. Shear viscosities show that our longer telechelics do associate into supramolecules [e.g., at 2 mg/ml in cyclohexane, 300k (i.e., 300 kg/mol) DA/DB gives a shear viscosity comparable to 670k NA (Fig. 2A); this holds for tetralin and Jet-A as well (Fig. 2B and fig. S15A)]. Even for telechelics with Mw of 670k—for which the concentration of end groups is less than 10 μM (one-thousandth of previously studied concentrations) (7)—the ends manifestly associate: The viscosity of the 670k DA/DB solution is twice that of the nonassociative control (Fig. 2A), and multimillion–molecular weight supramolecules are confirmed by MALLS (Fig. 2C and fig. S15, C and D). At concentrations as low as 0.22 mg/ml (0.028 wt %), 670k LTPs form supramolecules with an apparent Mw of 2200 kg/mol (Fig. 2C), in accord with the model prediction that Mw corresponds to approximately a three-chain assembly for these conditions, because rings and chains from dimer to tetramer dominate (fig. S16; for 300k DA/DB, fig. S15, C and D). According to our model, more than one-third of the telechelics are in species with molecular weight greater than the Mw of the supramolecules. Because of the greater strength of CAHB, acid-base pairing dominates over acid-acid pairing (measured by 1H-NMR; fig. S17). Small-angle neutron scattering (SANS) confirms that complementary end-associative polymers avoid the problem of chain collapse. The conformation on length scales up to the radius of gyration (Rg) of the individual chains is just as open for end-associative chains as it is for the corresponding nonassociative chains: at scattering vector q > 2π/Rg ≈ 0.03 1/Å, their scattering patterns coincide (Fig. 2D). Together, MALLS, NMR, and SANS reveal the molecular basis of the rheological behavior (Fig. 2, A and B)—complementary end association into megasupramolecules with expanded conformations.

Unlike ultralong polyisobutylene (4.2M PIB, 4200 kg/mol) (Fig. 3A), LTPs survive repeated passage through a fuel pump (Fig. 3B and fig. S18) and allow fuel to be filtered easily. The acid number, density, and flash point of the fuel are not affected by megasupramolecules (table S5). Initial tests in diesel engines indicate that fuel treated with LTPs can be used without engine modification (fig. S19); in a long-haul diesel engine (360HP Detroit Diesel), power and efficiency were not measurably affected (fig. S19B). Additionally, LTPs provided a 12% reduction in diesel soot formation (Fig. 3C).

Fig. 3 Shear resistance and engine compatibility of LTPs.

(A) The decrease of specific viscosity for 4.2M PIB (1.6 mg/ml, 0.2 wt %) in Jet-A at 25°C after approximately 60 passes through a Bosch fuel pump as shown in fig. S18A (sheared) relative to as-prepared (unsheared) indicates shear degradation. (B) Specific viscosities of 2.4 mg/ml (0.3 wt %) of a 1:1 molar ratio of α,ω-di(isophthalic acid) and α,ω-di(di(tertiary amine)) polycyclooctadienes (~670k DA/DB) in Jet-A at 25°C, sheared versus unsheared. (C) Emission data using an unmodified long-haul diesel engine. Control: untreated diesel. Treated: diesel treated with 0.14 wt % 670k DA/DB (details in fig. S19).

Experiments under turbulent pipe flow compare the drag reduction effect of ultralong polymers with that of megasupramolecules: For a given pressure drop, the increase of volumetric flow rate with LTPs (670k DA/DB at 0.1 wt %) is comparable to that with ultralong linear polyisobutylene (4200 kg/mol PIB at 0.02 wt %), with the distinction that LTPs retain their efficacy after multiple passes (fig. S20). Similarly, high-speed impact experiments (fig. S21A) show that, unlike ultralong PIB, LTPs retain their efficacy in mist control after repeated passage through a fuel pump. For untreated Jet-A fuel, the impact conditions generate a fine mist through which flames rapidly propagate into a hot fireball within 60 ms (movie S1). Polymer-treated fuel samples are tested in two forms: as prepared (“unsheared”) and after approximately 60 passes through a fuel pump (“sheared”) (fig. S18). Ultralong PIB (4200 kg/mol, 0.35 wt %) is known to confer mist control that prevents flame propagation (Fig. 4A, top left, and movie S2) (2); however, “sheared” PIB loses efficacy (Fig. 4A, top right, and movie S3). LTPs (TA, properties shown in fig. S22) provide mist control both before and after severe shearing (Fig. 4A, bottom, and movies S4 and S5), confirming their resistance to shear degradation (figs. S18C and S22B). The qualitative effects seen in still images at 60 ms (Fig. 4) are quantified by computing the average brightness of each frame (3000 images in 300 ms), which shows that both “unsheared” and “sheared” TA-treated fuels control misting (fig. S21C). Moreover, the test also proves that the chain length of the telechelics plays a crucial role in mist control (Fig. 4B), consistent with the hypothesis that megasupramolecules are the active species conferring the observed effect.

Fig. 4 Impact test in the presence of ignition sources (60 ms after impact, maximal flame propagation) for Jet-A solutions treated with 4.2M PIB or α,ω-di(di-isophthalic acid) polycyclooctadienes (TA).

(A) Jet-A with 4.2M PIB (0.35 wt %) and Jet-A with 430k TA (0.3 wt %), “unsheared” and “sheared” as in Fig. 3. (B) Effect of TA molecular weight (76 to 430 kg/mol) in Jet-A at 0.5 wt % (unsheared).

In the absence of theory, it was not known whether individual chains with lengths below the threshold for shear degradation (1200 kg/mol for PCOD; fig. S18) and end-association strengths much weaker than a covalent bond (150 kT) could form megasupramolecules. Theory inspired us to test telechelics with the predicted end-association strength (16 to 18 kT) and chain lengths, which do form megasupramolecules even at low concentration. They cohere well enough to confer benefits typically associated with ultralong polymers, including mist control and drag reduction. These megasupramolecules reversibly dissociate under flow conditions that would break covalent bonds, allowing the individual LTPs to survive pumping and filtering, and allowing treated fuel to burn cleanly and efficiently in unmodified diesel engines.

Supplementary Materials

www.sciencemag.org/content/350/6256/72/suppl/DC1

Materials and Methods

Figs. S1 to S22

Tables S1 to S5

Movies S1 to S5

References (3140)

References and Notes

  1. Acknowledgments: Funding for this research was provided from U.S. Army TARDEC, FAA, and NASA (NAS7-03001) and the Gates Grubstake Fund. B.L. is grateful for support from the Schlumberger Foundation Faculty for the Future Program. We thank P. Arakelian (GALCIT) and T. Wynne (JPL) for assistance with the fuel impact tests; B. Hammouda (NIST) and L. He (ORNL) for assistance with SANS; T. Durbin, R. Russell, D. Pacocha, and K. Bumiller at UCR CE-CERT for assistance with engine tests; A. Meyer at Wyatt Technology and Caltech graduate student J. Kim for assistance with MALLS; and Caltech undergraduates S. Li and A. Guo for assistance with shear degradation tests and rheological measurements. A patent application (WO/2014/145920) based on some results reported here has been submitted.
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