Letter pubs.acs.org/macroletters
High Morphological Order in a Nearly Precise Acid-Containing Polymer and Ionomer Edward B. Trigg,† Brandon J. Tiegs,‡,§ Geoffrey W. Coates,‡ and Karen I. Winey*,† †
Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853-1301, United States
‡
S Supporting Information *
ABSTRACT: Linear polyethylenes with functional groups at precise intervals along the backbone possess a number of remarkable properties, but the current synthetic methods that produce these precise polymers are difficult to scale up beyond the laboratory setting. When evaluating alternative synthetic routes, a critical question is how precise must the polymer microstructure be to achieve the properties of interest? As a first step in answering this question, we present morphological characterization of a nearly precise polymerthat is, an acid-containing polymer wherein the acid groups are separated by either n or n + 1 methylene groups. We find that the size scale and uniformity of the amorphous morphologies of the nearly precise acid-containing polymer and its sodium-neutralized ionomer are essentially indistinguishable from the precise polymers based on X-ray scattering. Meanwhile, the nearly precise polymer is strikingly distinct from a pseudorandom copolymer with similar average composition. This result suggests that the properties of nearly precise polymers could likewise be quite similar to truly precise polymers and beckons future work to explore their properties.
P
recise functionalized polyethylenes are linear polyethylenes with functional groups located at precise intervals and have been shown to possess interesting and attractive properties not found in random ethylene copolymers. Depending on the functional group chemistry and frequency, these properties include strain hardening caused by hydrogen bonding between layers of aligned functional groups;1,2 increased melting temperature relative to pure polyethylene;3 percolated ionic aggregates in precise ionomers,4−7 which could lead to improved ion conduction8 and mechanical properties; and exceptionally narrow crystalline lamellae thickness distributions,9,10 which have been shown to increase tensile impact strength.11 These precise polyethylenes are synthesized via acyclic diene metathesis (ADMET) polymerization,12−16 a technique that provides remarkable control over polymer microstructure but remains challenging to scale up to industrially relevant levels of production. To capitalize on the attractive properties discovered in ADMET polymers, more scalable synthetic routes are desired. When evaluating alternative synthetic routes, it is critical to know whether true precision in the polymer microstructure is required or whether similar properties would be found in nearly precise polyethylenesand how close to true precision the microstructure needs to be. This work is a first step toward answering this question. We present morphological characterization of a nearly precise polymer, containing carboxylic acid or sodium carboxylate groups and ester groups located at nearly precise intervals along an alkyl backbone. This polymer, termed 11/12SAnh (Figure 1), is synthesized via a recently reported simple one-pot method: autocatalytic solvent-free self-polymerization of succinic anhy© XXXX American Chemical Society
Figure 1. Chemical structures of polymers studied. In 11/12SAnh, each repeating unit has two very similar possible structures, making it a nearly precise polymer. The syntheses of the pseudorandom polymer (r15AA) and the precise polymers (p9AA and p15AA) were reported previously.18
Received: June 19, 2017 Accepted: August 10, 2017
947
DOI: 10.1021/acsmacrolett.7b00450 ACS Macro Lett. 2017, 6, 947−951
Letter
ACS Macro Letters dride−alcohol monomers.17 Here, the amorphous morphology of 11/12SAnh is directly compared with amorphous ADMET polymers and ionomers, as well as a pseudorandom copolymer, via X-ray scattering. We show that the morphological uniformity of the nearly precise polymer is essentially indistinguishable from the precise polymers and highly distinct from the pseudorandom copolymer. The syntheses of all polymers were reported previously.17,18 The 9 and 15 in p9AA and p15AA refer to the precise number of backbone carbon atoms in each repeating unit. In 11/ 12SAnh, the repeating unit can contain either 11 or 12 backbone carbon atoms. In r15AA, the overall chemical composition is identical to p15AA, but the microstructure is pseudorandom. All chemical structures are shown in Figure 1. All polymers were partially neutralized with sodium by addition of a sodium salt. For the sodium ionomers, the percentage of acid groups neutralized with sodium (determined by elemental analysis) is included in the naming scheme. Details can be found in the Supporting Information. To confirm the neutralization process, FTIR spectroscopy was performed on 11/12SAnh before and after neutralization with sodium (Figure 2). In the protonated (unneutralized)
Figure 3. Room-temperature X-ray scattering of (a) the polymers with fully protonated carboxylic acid groups and (b) the polymers with a fraction of their carboxylic acid groups neutralized with sodium. Triangles in (a) indicate low-q peak positions. In (a), p9AA and p15AA are reproduced from ref 20, and r15AA is reproduced from ref 18. In (b), p9AA-33Na is reproduced from ref 6. All data are normalized by the amorphous halo amplitude and shifted vertically for clarity. (a) and (b) are shown with different intensity scales.
the scattering length density at a length scale of 1−2 nm, leading to the X-ray scattering peak. The microstructure of the pseudorandom copolymer does not give rise to such regular compositional periodicity, and therefore no low-q peak is observed in r15AA. X-ray scattering of 11/12SAnh is very similar to that of p9AA and p15AA (Figure 3(a)), implying that the morphology of the nearly precise polymer is closely related to that of the precise polymers. The d* corresponding to the low-q peak is plotted as a function of the all-trans length of the average repeating unit (dall‑trans) in Figure 4(a). In 11/12SAnh, a low-q peak is present at 0.51 Å−1, clearly following the trend of the precise polymers. Furthermore, the peak width as a function of dall‑trans is plotted in Figure 4(b), and the width of the low-q peak in 11/12SAnh also follows the trend of the precise polymers. Thus, not only does the nearly precise polymer show a low-q peak where the pseudorandom copolymer does not, but also this peak’s position and width follow the trends of the precise polymers very closely. Note that the greater amplitude of the low-q peak relative to the amorphous halo in 11/12SAnh is likely due to the additional oxygen atoms in the ester groups. We can thus infer that the amorphous morphology of 11/ 12SAnh is related to p9AA and p15AA. Atomistic molecular dynamics (MD) simulations of p9AA at 150 °C show that the acid groups form hydrogen-bonded aggregates, many of which are larger than dimers, and contain four or more acid groups.21 We can infer that the morphology of 11/12SAnh also consists of acid groups in hydrogen-bonded acid aggregates, and the ester groups may participate in the aggregate structure as hydrogen bond acceptors. Next, we compare the X-ray scattering of the sodium ionomers, presented in Figure 3b. The neutralization percent as determined by elemental analysis is included in the naming
Figure 2. FTIR spectroscopy of protonated 11/12SAnh and 11/ 12SAnh-43Na. The spectra were normalized by the absorbance of the CH bending mode at 1466 cm−1 and shifted vertically for clarity.
sample, two absorption bands are easily distinguished at 1707 and 1734 cm−1, corresponding to the CO stretch in carboxylic acid groups and ester groups, respectively. After neutralization, peaks appear at 1570 and 1410 cm−1 , corresponding to negatively charged carboxylate groups,19 and the peak at 1707 cm−1 decreases in intensity, showing that carboxylic acid groups have been neutralized. X-ray scattering of 11/12SAnh is compared with two precise carboxylic-acid-containing polyethylenes, p9AA and p15AA,20 and a pseudorandom copolymer, r15AA,18 in Figure 3(a). All samples show an amorphous halo centered at ∼1.35 Å−1. r15AA shows an additional weak feature at ∼1.5 Å−1 due to a small degree of crystallinity, which arises from the statistically occurring longer alkyl chain segments devoid of acid groups.18 p15AA and p9AA each show a peak at low q (0.45 Å−1 and 0.63 Å−1, respectively), while r15AA does not. In p15AA and p9AA, the precisely periodic locations of carboxyl pendant groups along the backbone result in well-defined spatial periodicity of 948
DOI: 10.1021/acsmacrolett.7b00450 ACS Macro Lett. 2017, 6, 947−951
Letter
ACS Macro Letters
these precise ionomers have been investigated extensively via atomistic MD simulations, and a variety of ionic aggregate morphologies have been observed, from compact isolated aggregates to stringy branched aggregates that can form percolating networks.4,5 Based on these simulations, p15AA35Na has a stringy but not percolating aggregate morphology, while p9AA-33Na has a fully percolating aggregate morphology.6 Without further investigation we cannot say which morphology 11/12SAnh-43Na exhibits. However, the ester groups may promote a percolating morphology in 11/12SAnh43Na because they increase the volume fraction of polar groups. The simulations have shown that un-neutralized COOH groups participate in ionic aggregates, suggesting that the polar ester groups could participate as well. The extra methylene in some of the carboxyl pendants of 11/12SAnh may also play a role, extending carboxyl groups further from their main chains and potentially changing the aggregate sizes and shapes. Note that although 11/12SAnh contains ester groups the low-q X-ray scattering peak is still comparable to the precise polymers because this peak is indicative of the aggregate structure, and the peak width is tied to the uniformity of this structure. The glass transition temperatures of the amorphous materials were measured via DSC (Table 1). Thermograms are available Table 1. Glass Transition Temperatures (DSC) of Polymers in Acid Form and Ionomer Form Tg (°C)
Figure 4. (a) Characteristic length scale (d*) and (b) full width at half-maximum (fwhm) of the low-q peak as a function of the all-trans length of the repeating unit of the polymer (dall‑trans). For 11/12SAnh and r15AA, dall‑trans is calculated using the average repeat unit. d* and fwhm values are from fits of the X-ray scattering profiles presented in Figure 3. Circles are fully protonated, and triangles are partially neutralized with sodium. Dashed lines are to guide the eye.
a
material
acid form
Na ionomer form
11/12SAnh p9AA p15AA
−14 7a 3a
29 52 46
These values were taken from ref 1.
in the Supporting Information (Figure S1). The Tg of 11/ 12SAnh is substantially lower than that of p9AA or p15AA, due to the additional chain mobility imparted by the ester groups present in the backbones. Upon neutralization with sodium, the Tg increases by 43−45 °C in all three amorphous polymers, and a single Tg is observed. r15AA contains a small degree of crystallinity, and its Tg was not observed via DSC in either acid or ionomer forms. We have shown that the morphological size scale and uniformity of a nearly precise polymer and sodium ionomer are essentially equivalent to that of analogous precise polymers and ionomers, as quantified using X-ray scattering. The low-q X-ray scattering peak of 11/12SAnh follows the trends of p15AA and p9AA with respect to peak position and peak width. The sodium ionomer form of 11/12SAnh also follows the trends of the p15AA and p9AA sodium ionomers. This is in sharp contrast to the pseudorandom polymer r15AA that fails to show a low-angle peak in the acid form and shows a much broader peak in ionomer form. Thus, the morphology of 11/ 12SAnh is much closer to that of the precise polymers than the pseudorandom copolymer. This work suggests that the special properties of precise polymers may also be found in polymers with random placement of one additional methylene group in each monomeric repeat unit. Future work will further explore the effect of small variations in spacer length on structure and properties of functionalized polyethylenes. It is desirable to systematically vary the degree of spacer heterogeneity, i.e., compare n/n + 1 spacers (shown in
scheme (e.g., 43Na means 43% of COOH groups are neutralized with sodium). Upon partial neutralization with sodium, the low-q peak of p15AA-35Na and p9AA-33Na dramatically increases in intensity and decreases in width and shifts to lower q. These changes are the result of the presence of ionic aggregates,22 wherein strong electrostatic interactions cause aggregation of ionic moieties. The unusually sharp peaks found in precise ionomers are due to highly regular periodicity of the scattering length density in the amorphous morphology and are a direct result of the precise microstructures.22 In r15AA-28Na a very broad ionomer peak emerges upon partial neutralizationthe peak width is three times greater than that of the precise ionomers (Figure 4(b)). The characteristic length scale of the ionomer peak, d*, in r15AA-28Na also does not follow the trend of the precise polymers (Figure 4(a)). The larger d* and broader peak in the random ionomer have been reported before,22 are predicted by coarse-grain simulations,23 and are attributed to larger and more heterogeneous ionic aggregates in random ionomers compared with precise ionomers. The X-ray scattering of the ionomer form of 11/12SAnh is quite similar to the precise ionomers. Like the un-neutralized polymers, the low-q peak position and width in 11/12SAnh43Na follow the trends of p15AA-35Na and p9AA-33Na, implying that the nearly precise ionomer retains the high degree of morphological order of the precise ionomers, despite the one-carbon variation of spacer length. The morphologies of 949
DOI: 10.1021/acsmacrolett.7b00450 ACS Macro Lett. 2017, 6, 947−951
Letter
ACS Macro Letters this study) with n ± 1, n ± 2, etc., and measure the resulting morphologies and properties. For instance, in which microstructures do the layer formation and strain hardening found in precise polymers1,2 exist? Coarse-grain simulations similar to Hall et al.,23 paired with X-ray scattering studies, could evaluate the effects of these microstructures on morphology. Another critical question is how semicrystalline precise polymers are affected by small variations in spacer length. ADMET chemistry has produced semicrystalline polymers with precisely spaced pendant methyl groups,24 halides,25 sulfone groups,3 and others.13 These functional groups tend to arrange in layers within the crystals, and these layers will be perturbed to an unknown degree when the functional group placement is not precise. Recently, an unusual multilayer chain-folded structure was reported in an ADMET polyethylene with pendant carboxylic acid groups every 21st carbon.26 How stable is this structure with respect to lack of true precision? Answering these questions may open the door for a wider array of synthetic routes to produce nearly precise functional polymers with morphologies and properties similar to those discovered in precise polymers.
■
Acid Chemistry on the Morphological Evolution and Strength in Precise Polyethylenes. Macromolecules 2016, 49, 8209−8218. (3) Gaines, T. W.; Trigg, E. B.; Winey, K. I.; Wagener, K. B. High Melting Precision Sulfone Polyethylenes Synthesized by ADMET Chemistry. Macromol. Chem. Phys. 2016, 217, 2351−2359. (4) Bolintineanu, D. S.; Stevens, M. J.; Frischknecht, A. L. Influence of Cation Type on Ionic Aggregates in Precise Ionomers. Macromolecules 2013, 46, 5381−5392. (5) Bolintineanu, D. S.; Stevens, M. J.; Frischknecht, A. L. Atomistic Simulations Predict a Surprising Variety of Morphologies in Precise Ionomers. ACS Macro Lett. 2013, 2, 206−210. (6) Buitrago, C. F.; Bolintineanu, D. S.; Seitz, M. E.; Opper, K. L.; Wagener, K. B.; Stevens, M. J.; Frischknecht, A. L.; Winey, K. I. Direct Comparisons of X-ray Scattering and Atomistic Molecular Dynamics Simulations for Precise Acid Copolymers and Ionomers. Macromolecules 2015, 48, 1210−1220. (7) Middleton, L. R.; Winey, K. I. Nanoscale Aggregation in Acidand Ion-Containing Polymers. Annu. Rev. Chem. Biomol. Eng. 2017, 8, 499−523. (8) Gronowski, A.; Jiang, M.; Yeager, H.; Wu, G.; Eisenberg, A. Structure and Properties of Hydrocarbon Ionomer Membranes. 1.: Poly (methyl methacrylate-co-methacrylic acid). J. Membr. Sci. 1993, 82, 83−97. (9) Hosoda, S.; Nozue, Y.; Kawashima, Y.; Utsumi, S.; Nagamatsu, T.; Wagener, K.; Berda, E.; Rojas, G.; Baughman, T.; Leonard, J. Perfectly Controlled Lamella Thickness and Thickness Distribution: A Morphological Study on ADMET Polyolefins. Macromol. Symp. 2009, 282, 50−64. (10) Hosoda, S.; Nozue, Y.; Kawashima, Y.; Suita, K.; Seno, S.; Nagamatsu, T.; Wagener, K. B.; Inci, B.; Zuluaga, F.; Rojas, G. Effect of the Sequence Length Distribution on the Lamellar Crystal Thickness and Thickness Distribution of Polyethylene: Perfectly Equisequential ADMET Polyethylene vs Ethylene/α-Olefin Copolymer. Macromolecules 2010, 44, 313−319. (11) Hosoda, S.; Uemura, A. Effect of the Structural Distribution on the Mechanical Properties of Linear Low-Density Polyethylenes. Polym. J. 1992, 24, 939−949. (12) Berda, E. B.; Wagener, K. B. Recent Advances in ADMET Polycondensation Chemistry. Mater. Sci. Technol. 2013, 587−600. (13) Atallah, P.; Wagener, K. B.; Schulz, M. D. ADMET: The Future Revealed. Macromolecules 2013, 46, 4735−4741. (14) Santonja-Blasco, L.; Zhang, X.; Alamo, R. G. Crystallization of Precision Ethylene Copolymers. In Polymer Crystallization I; Springer International Publishing: Berlin, Heidelberg, 2015; Vol. 276, pp 133− 182. (15) Ortmann, P.; Mecking, S. Long-Spaced Aliphatic Polyesters. Macromolecules 2013, 46, 7213−7218. (16) Ortmann, P.; Wimmer, F. P.; Mecking, S. Long-Spaced Polyketones from ADMET Copolymerizations as Ideal Models for Ethylene/CO Copolymers. ACS Macro Lett. 2015, 4, 704−707. (17) Tiegs, B. J. Ring-Expanding Carbonylation of Epoxides: Applications in the Synthesis of Polyesters Pharmaceuticals, and Fine Chemical Building Blocks; Ph.D. Dissertation, Cornell University: Ithaca, New York, 2016. (18) Baughman, T. W.; Chan, C. D.; Winey, K. I.; Wagener, K. B. Synthesis and Morphology of Well-Defined Poly(ethylene-co-acrylic acid) Copolymers. Macromolecules 2007, 40, 6564−6571. (19) Painter, P.; Brozoski, B.; Coleman, M. FTIR Studies of Calcium and Sodium Ionomers Derived from an Ethylene−Methacrylic Acid Copolymer. J. Polym. Sci., Polym. Phys. Ed. 1982, 20, 1069−1080. (20) Buitrago, C. F.; Jenkins, J. E.; Opper, K. L.; Aitken, B. S.; Wagener, K. B.; Alam, T. M.; Winey, K. I. Room Temperature Morphologies of Precise Acid- and Ion-Containing Polyethylenes. Macromolecules 2013, 46, 9003−9012. (21) Lueth, C. A.; Bolintineanu, D. S.; Stevens, M. J.; Frischknecht, A. L. Hydrogen-Bonded Aggregates in Precise Acid Copolymers. J. Chem. Phys. 2014, 140, 054902. (22) Seitz, M. E.; Chan, C. D.; Opper, K. L.; Baughman, T. W.; Wagener, K. B.; Winey, K. I. Nanoscale Morphology in Precisely
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.7b00450. Experimental methods pertaining to polymer neutralization, FTIR, and X-ray scattering; and DSC thermograms (PDF)
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Edward B. Trigg: 0000-0002-4723-650X Geoffrey W. Coates: 0000-0002-3400-2552 Present Address §
Boulder Scientific Company, 4161 Specialty Place, Longmont, Colorado 80504, United States. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS E.B.T. and K.I.W. acknowledge funding from the Army Research Office (W911NF-13-1-0363), from the National Science Foundation (1506726), and from the National Science Foundation PIRE REACT grant (1545884). B.J.T. and G.W.C. acknowledge funding from the National Science Foundation under the Center for Sustainable Polymers (CHE-1413862). Facility use was funded in part by the MRSEC Program of the National Science Foundation (DMR 11-20901). We thank Dr. C. Francisco Buitrago for X-ray scattering data collection and L. Robert Middleton and Eric Schwartz for DSC data collection.
■
REFERENCES
(1) Middleton, L. R.; Szewczyk, S.; Azoulay, J.; Murtagh, D.; Rojas, G.; Wagener, K. B.; Cordaro, J.; Winey, K. I. Hierarchical Acrylic Acid Aggregate Morphologies Produce Strain-Hardening in Precise Polyethylene-Based Copolymers. Macromolecules 2015, 48, 3713−3724. (2) Middleton, L. R.; Trigg, E. B.; Schwartz, E.; Opper, K. L.; Baughman, T. W.; Wagener, K. B.; Winey, K. I. Role of Periodicity and 950
DOI: 10.1021/acsmacrolett.7b00450 ACS Macro Lett. 2017, 6, 947−951
Letter
ACS Macro Letters Sequenced Poly(ethylene-co-acrylic acid) Zinc Ionomers. J. Am. Chem. Soc. 2010, 132, 8165−8174. (23) Hall, L. M.; Seitz, M. E.; Winey, K. I.; Opper, K. L.; Wagener, K. B.; Stevens, M. J.; Frischknecht, A. L. Ionic Aggregate Structure in Ionomer Melts: Effect of Molecular Architecture on Aggregates and the Ionomer Peak. J. Am. Chem. Soc. 2012, 134, 574−587. (24) Lieser, G.; Wegner, G.; Smith, J. A.; Wagener, K. B. Morphology and Packing Behavior of Model Ethylene/Propylene Copolymers with Precise Methyl Branch Placement. Colloid Polym. Sci. 2004, 282, 773− 781. (25) Boz, E.; Wagener, K. B.; Ghosal, A.; Fu, R.; Alamo, R. G. Synthesis and Crystallization of Precision ADMET Polyolefins Containing Halogens. Macromolecules 2006, 39, 4437−4447. (26) Trigg, E. B.; Stevens, M. J.; Winey, K. I. Chain Folding Produces a Multilayered Morphology in a Precise Polymer: Simulations and Experiments. J. Am. Chem. Soc. 2017, 139, 3747−3755.
951
DOI: 10.1021/acsmacrolett.7b00450 ACS Macro Lett. 2017, 6, 947−951
