Synthesis and Characterization of Segmented Polyurethanes

Letter Cite This: ACS Macro Lett. 2018, 7, 846−851

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Synthesis and Characterization of Segmented Polyurethanes Containing Trisaminocyclopropenium Carbocations Robert H. Lambeth, III,*,† MyVan H. Baranoski,† Alice M. Savage,† Brian F. Morgan,† Frederick L. Beyer,† Brent A. Mantooth,‡ and Nicole E. Zander† †

U.S. Army Research Laboratory, Aberdeen Proving Ground, Aberdeen, Maryland 21005, United States U.S. Army Edgewood Chemical Biological Center, Aberdeen Proving Ground, Aberdeen, Maryland 21010, United States

‡

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S Supporting Information *

ABSTRACT: Diol-functionalized trisaminocyclopropenium (TACP) carbocations were used as chain extenders in a twostep synthesis of a segmented polyurethane. Differential scanning calorimetry demonstrated significant differences in the crystallization behavior of the poly(tetramethylene oxide) soft segment when minor changes were made to the TACP structure and when compared to a control that was chain extended with butane diol. Fourier transform infrared spectroscopy was used to characterize the different level of hydrogen bonding in the polymers and showed that the bulky, charged TACP chain extender limited hydrogen bonding interactions when compared to the control. Dynamic mechanical analysis was used to probe the thermomechanical behavior of polymers that showed that the TACPcontaining polymers were much more resistant to flow at high temperatures when compared to the control. Small-angle X-ray scattering showed a phase separated morphology for all the polymers tested. Tensile testing of the TACP polyurethanes demonstrated an elastic response over a wide range of strain, followed by a significant strain hardening. These results suggest a morphology of ionic aggregates rather than hard segment physical cross-links.

P

mechanical properties of ion-containing PUs depend on a number of factors including the type of ionic group, in which segment the ionic group is located, and the type of counterion present.9,22,23 A type of cation that has recently gained revived interest is the trisaminocyclopropenium (TACP) carbocation.24−26 TACPs are particularly stable with pkr+ values greater than 13 and thermal decomposition temperatures >300 °C.27−29 TACPs have found recent interest for use as ionic liquids, organocatalysts, and ligands.28−32 Campos and coworkers were the first to recognize the potential utility of TACPs in polymeric materials.33−36 The unique structure of TACPs lends itself well to polyelectrolyte materials because they are readily synthesized and incorporated into polymeric backbones, the charge is dispersed across three atoms rather than one, they have weak H-bond donor capacity, and they have a highly tunable steric and electronic environment.28,33 Brucks et al. demonstrated how the versatility and modularity of TACPbased polymers enabled fine-tuning of structure−property relationships in the design of biocompatible delivery vectors.35 To date, TACPs have been attached to either styrenic or polyethylenimine polymer backbones. It was hypothesized that TACPs would also be ideal candidates for incorporation into segmented PUs because the highly modular nature of the cation could potentially enable fine-tuning of the polymer

olyurethanes (PUs) are a broadly defined class of materials where urethane linkages join together various molecular units of different sizes, lengths, polarity, and functionality. The ability to combine a wide range of often chemically dissimilar structures into a single polymer (or network) has led to use of PUs in a very wide range of applications including highly crosslinked thermosets for structural materials or rigid foams and soft, elastomeric materials for adhesives, textiles, or coatings.1 Academically, linear segmented PUs have been of great interest because of the complex interplay between hard and soft segment thermodynamic compatibility, extent of hydrogen bonding within the hard phase and between hard and soft phases and molecular structure influences morphology and mechanical behavior.2,3 Given the broad parameter space available and the diverse nature of PU chemistry, the morphology and resulting physical and mechanical properties can be tuned to meet the needs of a wide range of applications.4 Ion-containing PUs are an interesting subclass of materials where the presence of charged groups can have a strong influence over the polymer microstructure due to disruption of hydrogen bonding and aggregation of ionic groups resulting in complex morphological behavior.5−7 Both anionic and cationic groups, including carboxylate, sulfonate, phosphonate, ammonium, imidazolium, and phosphonium, have been incorporated into polyurethanes in both hard and soft segments.8−18 PU ionomers have found application in adhesives, waterborne coatings, medical implants, antimicrobial coatings, and DNA transfection.19−21 The morphology and resulting physical and © XXXX American Chemical Society

Received: May 22, 2018 Accepted: June 22, 2018

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DOI: 10.1021/acsmacrolett.8b00395 ACS Macro Lett. 2018, 7, 846−851

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ACS Macro Letters morphology and physical and mechanical properties. To test this hypothesis, two TACP-containing segmented polyurethanes were synthesized and characterized and compared to a nonionic control PU. In this initial study, small changes in the TACP structure were found to produce observable differences in physical and mechanical properties as well as very unique behavior under deformation. TACPs are readily prepared by addition of a large excess of the desired amine substituent to pentachlorocyclopropane (PCCP; 1). Depending on the size of the amine, two or three equivalents of amine are added to PCCP in quantitative fashion. Here, the sterically demanding diisopropylamine or dicyclohexylamine add twice to PCCP to form diaminochlorocyclopropeniums 2 and 3. To incorporate a diol into the TACP structure for use as a chain extender, diethanolamine is added in a subsequent step to form TACPs 4 and 5. Synthesis of the segmented polyurethanes followed the standard twostep procedure where PTMO (Mn = 2 kDa) was reacted with an excess of 1,6-hexamethylene diisocyante (HDI) to form the prepolymer followed by chain extension with 4 or 5. For comparison, a control PU was prepared using butane diol (BDO) as chain extender. The synthesis of the TACPs and PUs is illustrated in Scheme 1.

Figure 1. DSC traces for PUCPCy, PUCPiP, and PUBDO; Conditions: 5 °C/min; second heating cycle (exo up).

Between 100 and 150 °C, additional endothermic transitions were seen in the PUBDO control sample and were assigned to hard segment melting. The compact and highly regioregular structure of the HDI-BDO hard segment likely led to effective chain packing and formation of strong hydrogen bonding interactions forming hard segment crystalline domains.37 Because of the strong hydrogen bonding interactions in the hard segment, phase mixing was limited and few N−H protons were free to interact with the ether oxygens on the PTMO soft segment, leading to a high level of crystallinity in the soft segment as well. In the TACP-containing PUs, the bulky and charged carbocations likely limited effective chain packing and strong hydrogen bonding interactions and prevented formation of crystalline hard segment domains, leading to an absence of thermal features above 100 °C. Interestingly, there were significant differences in the thermal behaviors of the PTMO soft segment among the PUCP samples. The slightly bulkier dicyclohexyl groups may be limiting hard segment interactions further, leading to increased phase mixing or N−H−ether hydrogen bonding, which disrupts soft segment crystallization. Fourier transform infrared (FTIR) spectroscopy can be used to determine the level of hydrogen bonding interactions of both the carbonyl and amide proton in the urethane structure.38,39 The carbonyl stretching band is known to shift to lower wavenumbers when interacting with amide protons, and the N−H stretching band is known to become more narrow when hydrogen bonding with either carbonyl groups or ether groups.8 In this work, dramatic differences were observed between the PUBDO control and TACP-containing PUs (Figure 2). As suggested by the DSC thermograms, evidence of high levels of hydrogen bonding interactions were observed in PUBDO. There was a large shift to lower wavenumber as well as significantly higher level of absorbance for the “bound” carbonyls compared to the “free” carbonyls suggesting significant hydrogen bonding. In addition, the N−H stretch is very sharp, indicating a uniformity of N−H stretching modes, which was also attributed to significant levels of hydrogen bonding. In contrast, PUCPCy and PUCPiP only display moderate levels of hydrogen bonding as evidenced by a small and poorly defined shoulder from the bound carbonyl (1650 cm−1)relative to the free carbonyl peak (1730 cm−1). The N−H stretch of these polymers is also much more broadly defined than the PUBDO. The bulky and charged nature of TACPs likely limit hydrogen bonding in the hard segment, which is consistent with previously reported ion-containing

Scheme 1. Synthesis of TACP Chain Extenders 4 and 5 from PCCP (1) and TACP-Containing PUs PUCPCy and PUCPiP

Differential scanning calorimetry (DSC) was used to analyze this series of polymers, and thermograms (Figure 1) of the second heating cycle demonstrated differences in crystallinity of the PTMO soft segment among the materials tested. The control (PUBDO) displayed high levels of soft segment crystallinity, as evidenced by the broad melting endotherm centered at 11 °C. A crystallization exotherm was not observed on the second heating pass but was observed in the previous cooling pass (Figure S9). In the PUCPiP sample, an exothermic soft segment crystallization peak was observed followed by a sharp endothermic melting peak. In contrast, very little thermal behavior was observed for PUCPCy where only a small soft segment melting transition was detected. 847

DOI: 10.1021/acsmacrolett.8b00395 ACS Macro Lett. 2018, 7, 846−851

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ACS Macro Letters

PUCPCy and PUCPiP both displayed similar PTMO Tg values in range of −67 to −68 °C, slightly lower than that of the PUBDO control, which had a Tg of −60 °C. The Tgs are higher than that of pure PTMO (−79 °C), suggesting phase mixing is occurring to some extent for each polymer with the ion containing samples being marginally less phase mixed than the control sample. Only PUCPiP displayed an increase in the modulus, starting at −55 °C, which is indicative of soft segment crystallization, consistent with DSC thermal data. Soft segment melting was observed, as evidenced by a decrease in modulus centered around 5 °C. Above this temperature, the PUBDO maintained a significantly higher modulus than the TACP PUs, which is consistent with a greater extent of hydrogen bonding.8,40 A small thermal transition was also observed in PUCPiP and PUBDO at 55 °C likely due to some hard segment melting. At higher temperatures, PUBDO was observed to flow at 150 °C, while the TACP PUs did not flow up to 150 °C. The rubbery plateau modulus for the TACP containing PUs is consistent with previously reported ion containing PUs.8 The stress−strain behavior of the polymers was analyzed to determine the influence of the TACP groups on mechanical performance at high elongations (Figure 4). Obtaining

Figure 2. FTIR spectra of (a) carbonyl region and (b) N−H region for PUCPCy, PUCPiP, and PUBDO.

PUs.8,10 Interestingly, there was no observed difference in hydrogen bond interactions among the TACP-containing PUs. When evaluating crystallization of ether-based soft segments in segmented PUs, the difference between samples is often attributed to the competitive hydrogen bonding between hard−hard and hard−soft segments, as observed by FTIR. In this case, significant differences were observed in soft segment crystallization by DSC between the PUCPCy and PUCPiP but little difference was observed by FTIR. Dynamic mechanical analysis (DMA) was used to probe the thermomechanical behavior of the three polymers (Figure 3).

Figure 4. Representative tensile data for PUCPiP, PUCPCy, and PUBDO.

accurate strain measurements at high elongations can be challenging for soft elastomers as the entire sample deforms rather than just in the gauge region.41 To address this challenge, high speed videography was used to capture the separation of lines drawn in the gauge region of the tensile specimen to more accurately determine the strain. Further details of the test methodology can be found in the Supporting Information (SI). The PUs displayed very different tensile behavior depending on the structure. Under uniaxial deformation, the PUBDO material displayed an initial yield point followed by elongation and strain-hardening due to strain-induced soft segment crystallization. The crystallization was visually apparent from significant whitening within the gauge length and is typical for PUs containing PTMO soft segments.42,43 Upon breaking, the polymer retained its shape and did not recover, most likely due to the soft segment crystallizing at high strains. The TACP-containing PUs initially displayed typical elastomeric behavior up to about 400% strain, after which remarkable strain hardening occurs, resulting in a dramatic increase in stress from ∼2.5 to ∼35 MPa. Interestingly, the whitening of the polymer in the gauge

Figure 3. DMA for PUCPiP, PUCPCy, and PUBDO. 848

DOI: 10.1021/acsmacrolett.8b00395 ACS Macro Lett. 2018, 7, 846−851

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ACS Macro Letters

typical ion-containing polymers,51 and the same tools for understanding morphology in polyurethanes are applied to non-PU ionomers, including the use of the Percus−Yevick closure for modeling liquid like disorder of ionic domains.52−55 In the TACP PUs, where H-bonding is interrupted (FTIR), HS crystallization is eliminated (DSC), and SS crystallization is reduced (DSC), one might expect to see increased phase mixing. This is not the case, however, as DMA shows reduced Tg for the TACP PUs, indicating improved microphase separation. Furthermore, the SAXS scattering maxima are well-defined, as one would expect from a well-segregated morphology. The scattering maxima observed for the PU-CP materials are generally better defined than those observed in literature reports of ion-containing chain-extended polyurethanes, another indication of little phase mixing.8,9,15 The formation of a morphology of ionic aggregates explains these observations. Such a material would also be expected to exhibit the elastomeric behavior observed in tensile testing, as well as the strong recovery observed when the TACP PU samples were released from tension. The absence of strain induced crystallization is also consistent with a material where chain conformation is constrained by the presence of strong Coulombic interactions, and the strong upturn in stress at high strains is typical of elastomers having reached the limits of molecular mobility. Thus, while more study is needed, the available data strongly suggest a morphology of ionic aggregates rather than typical PU HS physical crosslinks. In conclusion, we have synthesized novel TACP-based ionic polyurethanes using a two-step polymerization procedure and characterized the physical and mechanical properties using a variety of techniques. A noncharged control was also prepared, using BDO as a chain extender. The TACP-containing PUs had significantly less H-bonding and soft segment crystallization compared to the noncharged control. Subtle changes in the TACP structure lead to significant differences in soft segment crystallization. Microphase separation was observed using SAXS for all samples, and tensile testing revealed significant differences in mechanical behavior between the TACP-containing PUs and the PUBDO control. The charged and uncharged PUs both displayed high strain to break and high tensile strengths at high elongation but markedly different tensile strengths at low elongation. The nature of the significant strain hardening observed in the TACP PUs is currently under investigation, but is thought to result from the formation of ionic aggregates rather than HS physical crosslinks. In addition, the TACP chain extender presents significant opportunity of additional research as the structure can be finely tuned to modify the steric and electronic environment around the cationic center.

length was not observed, suggesting soft segment crystallization did not occur. This is also supported by the full shape recovery of the tensile specimen shortly after breaking. Postmortem SAXS analysis of the strained samples also showed a complete recovery of the morphology, further supporting the elastic nature of the TACP PUs (Figure S10). Morphological behavior was characterized using small-angle X-ray scattering (Figure 5) at room temperature and at zero

Figure 5. Small-angle X-ray scattering data showing significant changes in structure resulting from TACP addition.

strain. The scattering data for PUBDO is characteristic of form factor scattering, with strong Guinier character between 0.01 and 0.15 Å−1, and depression of the form factor shape below 0.03 Å−1, indicating weak interdomain scattering. The best fit to the PUBDO SAXS data was obtained using the form factor for a cylinder in combination with an interparticle interference function,44,45 similar to cylindrical morphologies previously reported in PUs and polyurethane ureas.46−49 Substitution of either cyclopropenium cation as a chain extender produces a clear change in scattering behavior, resulting in a single welldefined scattering maximum centered about 0.065 Å−1, as well as a decrease in overall scattering intensity. These features were fit using the Percus−Yevick formulism for globular domains with liquid-like ordering, which has been shown to work generally well for polyurethanes and polyurethane ionomers.49,50 This approach produced good fits to the scattering maxima and indicated that the samples have microphaseseparated morphologies, roughly spherical domains approximately 6 nm in diameter, and a center-to-center domain separation of approximately 10 nm. AFM data were collected but were inconclusive because the samples were very soft at room temperature. Representative AFM micrographs and detailed results for the SAXS data modeling are included in the SI. Although the morphological picture is incomplete at this time, the available data do suggest an origin to the remarkable tensile behavior reported in Figure 4. The TACP PUs behave much more like ionic elastomers than traditional polyurethanes, which could occur if the dominant feature were ionic domains rather than pure HS domains. Indeed, the SAXS data for the TACP PUs in Figure 5 is indistinguishable from that of

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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/acsmacrolett.8b00395. Experimental details (PDF).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Robert H. Lambeth, III: 0000-0001-7664-015X 849

DOI: 10.1021/acsmacrolett.8b00395 ACS Macro Lett. 2018, 7, 846−851

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ACS Macro Letters

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Alice M. Savage: 0000-0003-0167-4966 Frederick L. Beyer: 0000-0003-0253-2134 Brent A. Mantooth: 0000-0001-6838-1741 Author Contributions

The manuscript was written through contributions of all authors. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.M.S. and B.F.M. were supported in part by the Postgraduate Research Participation Program at the U.S. Army Research Laboratory, administered by the Oak Ridge Institute of Science and Education through an interagency agreement between the U.S. Department of Energy and U.S. Army Research Laboratory (Contract ORISE 1120-1120-99).

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DOI: 10.1021/acsmacrolett.8b00395 ACS Macro Lett. 2018, 7, 846−851