Effect of Regioisomerism on Processability and Mechanical Properties

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Article Cite This: ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX

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Effect of Regioisomerism on Processability and Mechanical Properties of Amine/Urea Exchange Based Poly(urea-urethane) Vitrimers Ainara Erice,† Itxaso Azcune,† Alaitz Ruiz de Luzuriaga,† Fernando Ruipérez,‡ Mikel Irigoyen,‡ Jon Mattin Matxain,§ Jose María Asua,‡ Hans-Jurgen Grande,†,‡ and Alaitz Rekondo*,† †

Polymers & Composites, CIDETEC, Paseo Miramón 196, 20014 Donostia-San Sebastian, Spain University of the Basque Country UPV/EHU, POLYMAT, Joxe Mari Korta Center; Avda. Tolosa 72, 20018 Donostia-San Sebastian, Spain § Kimika Fakultatea, Euskal Herriko Unibertsitatea UPV/EHU and Donostia International Physics Center (DIPC), P.K. 1072, 20080 Donostia-San Sebastian, Spain

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‡

S Supporting Information *

ABSTRACT: In this paper, three analogous poly(urea-urethane) elastomers were thoroughly characterized to determine the effect of regioisomerism on the processability and mechanical properties of the final materials. The polymers were synthesized starting from the same isocyanate-terminated polyol and the three positional isomers of phenylenediamine hardener. The main differences on the supramolecular hydrogen-bond network, as well as on the outcome of the dynamic amine/urea exchange reaction, were determined by experimental measurements (FTIR, rheology, DMA, tensile tests) and molecular quantum chemical calculations.

KEYWORDS: dynamic covalent chemistry, reversible covalent bond, vitrimers, CAN, poly(urea-urethane) elastomer, regioisomerism

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INTRODUCTION Poly(urea-urethane) (PUU) thermosets are widely used for countless applications due to their excellent chemical resistance, toughness, and flexibility. A drawback of the classical PUU thermosets is that they cannot be reprocessed, recycled, or repaired with the associated environmental impact and processing costs. All these issues could be addressed and (at least partially) mitigated by incorporating appropriate dynamic covalent bonds in the polymer networks, which provide them with new reprocessing and growth opportunities. Such materials are known as covalent adaptive networks (CAN)s, and they can change their topology by thermally activated bond-exchange reactions.1−4 CANs are classified into two main categories depending on the exchange mechanism: dissociative CANs and associative CANs (also known as vitrimers). The dissociative pathway implies that the chemical bond is cleaved and reformed during the exchange process,5−8 whereas systems ruled by the associative mechanism retain the cross-link density during the whole process.9−15 One of the main consequences is the distinct viscoelastic behavior of the systems throughout the thermal reprocessing step. Our group has reported catalyst-free self-healable and reprocessable PUUs based on dynamic aromatic disulfide bonds.16−18 However, the industrial applications in which © XXXX American Chemical Society

these PUUs can be used are limited by the fact that the aromatic disulfides are weaker bonds than C−C bonds, limiting their feasibility for demanding sectors when designing elastomeric materials. In addition, the incorporation of a new chemistry to the PUU might represent a barrier to an immediate industrial acceptance. Thus, systems that provide higher mechanical strength and require less drastic modifications in the polymer formulation would be desirable. A possibility is to transform the chemical bonds found in the PUUs, urethane (−O−CO−NH−) and urea (−NH−CO− NH−) into dynamic moieties. The urethane exchange (or transcarbamoylation) has been promoted under various conditions in polyurethane (PU) systems, that is, catalystfree exchange in polyhydroxyurethanes involving hydroxyl groups,19 dibutyltin dilaureate (DBTDL) catalyzed exchange in structurally unaltered polyurethanes,20 and catalyst-free exchange in aromatic diisocyanate derived polymers.21 However, the associative transcarbamoylation exchange of carbamate moieties in PU systems has been reported to be sluggish, and the dissociative exchange of PU moieties at Received: June 25, 2019 Accepted: August 12, 2019

A

DOI: 10.1021/acsapm.9b00589 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX

Article

ACS Applied Polymer Materials higher temperatures can lead to nondesirable side-reactions.19 These are the main reasons why the efficient reprocessing of PU systems is still a real challenge. In this context, an interesting possibility is to act on the urea bond. To the best of our knowledge, two strategies have been reported to trigger urea exchange in PUUs. On the one hand, Zhang et al. promoted the weakening and opening of the urea group via a dissociative mechanism by increasing the bulkiness of the substituents on the nitrogen atom.22,23 On the other hand, our group has recently reported a cross-linked PUU vitrimer based on the associative exchange mechanism between aromatic urea and dangling aromatic amine groups.24 The latter showed the following remarkable advantages not only compared to classical PUU thermosets but also to the mentioned PUUs based on dissociative bulky urea exchange.22,23 (i) Cross-linked PUUs can be used as raw material avoiding the use of isocyanate-based reagents and reprocessing intermediates; (ii) the presence of dangling aromatic amines is easy-to-implement by adjusting the stoichiometry of the hardener in the formulation; (iii) the mechanical properties under operating conditions are comparable to conventional PUUs because the use of primary diamine as hardener instead of N-substituted ureas enables the formation of strong H-bonds between N−H “donor” groups and C=O “acceptor” groups. It is well-known that mechanical properties of PUU based materials are greatly affected by the nature and strength of these supramolecular interactions as well as by the packaging of the hard segments.25−29 In our previous work,24 we learned that the network rearrangement that is needed for reprocessability occurred through a series-parallel mechanism in which the H-bonds were first disrupted to allow the movement of the dangling aromatic amines to undergo exchange with the aromatic urea groups. The latter reaction caused the rearrangement of the network. Those results made us consider that the structural geometry or regioisomerism of the hardener (aromatic diamine) could lead to different molecular arrangements and H-bonds. We hypothesized that the selection of the hardener could be a way to tailor-made dynamic PUUs. Therefore, in this work, the effect of the regioisomerism of the hardener on the reprocessability and mechanical properties was investigated. First, three analogous PUU networks, that is, o-PUU, m-PUU, and p-PUU, were synthesized using the three positional isomers of phenylenediamine (PD), that is, ortho-phenylenediamine (OPD), meta-phenylenediamine (MPD), and paraphenylenediamine (PPD), respectively (Figure 1). The polymers were analyzed by FTIR by focusing on the extent and intensity of H-bonds and correlating them to the mechanical properties. The dynamic behavior was determined by rheological analyses at relevant temperatures, and reprocessing tests were carried out using a hot-plate pressing technique. Additionally, the thermal degradation of o-PUU elastomer was investigated, and a plausible mechanism was proposed using a small model molecule. To that end, nuclear magnetic resonance spectroscopy (1H NMR), mass spectrometry (MALDI-TOF), and computational modeling, both classical (MD) and quantum chemical (DFT), results are presented.

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Figure 1. General scheme depicting the synthesis and chemical structures of the dynamic PUU cross-linked elastomers. 98%), dibutyltin dilaureate (DBTDL, 95%), ortho-phenylenediamine (OPD, 99.5%), meta-phenylenediamine (MPD, 99%), para-phenylenediamine (PPD, 99%), butyl isocyanate (98%), tetrahydrofuran (THF, 99.8%), dimethyl sulfoxide (DMSO, 99%), and deuterated dimethyl sulfoxide (DMSO-d6, 99%) were purchased form SigmaAldrich and were used without further purification. Synthesis of Cross-Linked o-PUU, m-PUU, and p-PUU Elastomers. The synthesis of tri-isocyanate terminated prepolymer 2 was described elsewhere.24 Briefly, the intermediate 2 was prepared by heating trifunctional PPG (Mn 4000) 1 (900 g, 225 mmol) and IPDI (167.430 g, 754 mmol) and DBTDL (100 ppm) in a 1 L steel reactor at 70 °C for 30 min under vacuum and high shear mechanical stirring (1200 rpm). The NCO content was determined, the titration monitored by FTIR with n-butylamine, following the disappearance of the NCO signal at 2264 cm−1. The dynamic o-PUU, m-PUU, and p-PUU elastomers were prepared by reacting 2 (60 g) and OPD (2.62 g, 24 mmol), MPD (2.62 g, 24 mmol), and PPD (2.62 g, 24 mmol), respectively. In each case, the corresponding hardener was first dissolved in THF (4 mL). The molar ratio of NH2/NCO was set to 1.1:1. The mixtures were degassed under vacuum, placed in a 2 mm and 1 mm thick molds, and cured for 16 h at 70 °C. Synthesis of Model Aromatic Urea U1. The model molecule U1 was synthesized from OPD (1 g, 9.2 mmol) and butyl isocyanate (2.19 g, 22 mmol) in DMSO catalyzed by 100 ppm of DBTDL in a 250 mL round bottomed flask at 60 °C under N2 atmosphere. The obtained product was precipitated using water, and the final molecule was characterized by 1H NMR. Methods. The experimental characterization was carried out following the same procedures described in our previous work.24 A JASCO-4100 spectrometer with a diamond ATR probe was used for the Fourier transform infrared (FTIR) analysis. A differential scanning calorimetry (DSC) instrument from TA Instruments (Discovery DSC25 Auto) was used to perform the thermal analysis over a temperature range from −90 to 200 °C under nitrogen at a scan rate of 10 °C min−1. TGA measurements were carried out on TA Instruments Q500 equipment from 25 to 500 °C at a heating rate of 20 °C min−1 under air atmosphere. Additionally, isothermal experiments were carried out at 120 and 160 °C for 60 min. The macroscopic scale reprocessing experiments were carried out in a VOGT hot LABO PRESS 200T. For this purpose, grounded m-

EXPERIMENTAL SECTION

Materials. Trifunctional poly(propylene glycol) (PPG) 1 (Mn 4000) was purchased from Covestro. Isophorone diisocyanate (IPDI, B

DOI: 10.1021/acsapm.9b00589 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX

Article

ACS Applied Polymer Materials PUU and p-PUU were placed in a 2 mm thick mold and compressed at 160 °C and 100 bar for 1 h. An INSTRON 3365 long travel elastomeric extensometer controlled by Bluehill Lite software was used to perform the mechanical characterization. Tensile strength measurements were performed according to UNE-EN-ISO 527 standard using dumbbell type test specimens and an elongation rate of 500 mm min−1. Strain sweep experiments were carried out in a TA Instruments AR2000ex rheometer using 25 mm plate−plate geometry to determine the linear viscoelastic region of PUU networks. Then stress relaxation tests were performed on PUU elastomer samples with thicknesses of 1 mm by applying 2.5% strain at a constant gap. Finally, temperature sweep measurements were also carried out, heating the disk from 25 to 200 °C at a heating rate of 5 °C min−1, applying 1% strain at a 1N constant normal force and 1 Hz frequency. The model molecules study was followed by 1H NMR analyses in a Bruker AVANCE III 500 MHz spectrometer. Matrix-assisted laser desorption ionization time-of-flight (MALDITOF) analysis was carried out using an Autoflex mass spectrometer. The analyte consisted of 1:10 (wt) of sample (10 mg mL−1) and DCTB matrix (10 mg mL−1). Geometry optimizations have been carried out in gas phase within density functional theory (DFT) using the ωB97XD functional30 in combination with the 6-31G(d,p) basis set. Harmonic vibrational frequencies have been performed at the same level of theory to confirm that all the structures were minima or transition states. Singlepoint calculations were carried out using the optimized geometries with the 6-311++G(2df,2p) basis set to refine the electronic energy. All DFT calculations were carried out using the Gaussian 16 package.31 Classical molecular dynamic simulations (MD) have been performed using ff14SB32 amber force-field in the AMBER 14 package.33 Molecular models were built using the LEaP module of Ambertools, and the charges were computed using the restrained electrostatic potential (RESP) fitting procedure.34 First, the ESP was calculated using Gaussian 16 package at the Hartree−Fock level with the 6-31G(d) basis set, and then the RESP charges were obtained. All the simulations were carried out in vacuum in a canonical ensemble (NVT) with a 2 fs time step. 100 000 steps of minimization (50 000 steps steep descent minimization plus 50 000 steps of conjugate gradient minimization) were followed by heating from 80 to 300 K over 200 ps, an equilibration time of 1800 ps with a Langevin thermostat and a production of 10 ns. Covalent bond lengths involving hydrogen were constrained using the SHAKE algorithm. To analyze the hydrogen bonds, a threshold for the bond length between the donor and the acceptor (X···Y) smaller than 3.5 Å was considered, and the range of X−H···Y angles varied between 140 and 220 degrees.

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Table 1. Tensile Strength Properties of Pristine and Reprocessed o-PUU, m-PUU, and p-PUU pristine

reprocessed

PUU

tensile (MPa)

elongation at break (%)

tensile (MPa)

elongation at break (%)

o-PUU m-PUU p-PUU

1.6 ± 0.1 2.0 ± 0.1 2.2 ± 0.2

253 ± 21 486 ± 41 326 ± 41

2.0 ± 0.1 2.9 ± 0.3

393 ± 11 323 ± 35

quantitatively assessed by comparing the mechanical properties of the reprocessed materials to those of the pristine samples. To that end, grounded materials were placed in a 2 mm thick mold and compressed in a hot-plate press applying the same reprocessing cycle described in our previous work24 (160 °C, 100 bar, 1 h). Both m-PUU and p-PUU were properly compressed at 160 °C leading to good quality reprocessed flat sheets (see Figure S3 in the ESI). Surprisingly, under such reprocessing conditions (160 °C, 100 bar, 1 h) o-PUU became a nonreversible viscous liquid. Moreover, when o-PUU was tested in a hot press at lower temperatures (110 and 130 °C), the elastomer also became nonreversely sticky and lumpy suggesting a low thermal stability (see Figure S3 in the ESI). The full reprocessing efficiencies of m-PUU and p-PUU can be observed in Table 1, confirming the vitrimeric behavior of this kind of PUUs based on the aromatic amine/urea associative exchange reaction described in our previous work.21 Surprisingly, p-PUU shows higher tensile strength after reprocessing compared to the pristine material. We hypothesize that better performance in tensile strength measurements could be attributed to a higher degree of the supramolecular interactions and better network packaging favored by the applied temperature and pressure during the reprocessing cycle. This was further investigated by FTIR analysis of pristine and reprocessed samples. These results showed that the structural geometry of the hardener plays a key role in thermal, mechanical, and dynamic properties of these PUUs. However, the reasons for the different behavior of o-PUU, m-PUU, and p-PUU were far from obvious. Therefore, a deep structural analysis was carried out to shed light on this issue. Structural Analysis. As it was found that the H-bonds play a critical role on the reprocessing of these materials,24 FTIR analysis of the PUU elastomers was carried out because the stretching vibrations of carbonyl groups near 1600−1750 cm−1 and NH groups near 3200−3400 cm−1 in PUU elastomers provide valuable information regarding the nature and extent of such bonds.25,35 First, the FTIR spectra of pristine and reprocessed samples were analyzed at ambient temperature to correlate with their mechanical properties and elucidate any structural changes. Then the effect of increasing temperature on the H-bond network was analyzed to measure the strength of supramolecular interactions. The FTIR spectra of the three pristine elastomers at ambient temperature showed remarkable differences, both in the urea and urethane regions (Figure 2). The comparative analysis points out that p-PUU shows the strongest H-bond network, whereas o-PUU the weakest. In the urea region (Figure 2a), pPUU presents an intense band at the lowest wavelength (1656 cm−1), meaning that the carbonyl groups are highly involved in H-bonds. For m-PUU, this band is of lower intensity, and it is

RESULTS AND DISCUSSION

Synthesis of Materials. The o-PUU, m-PUU, and p-PUU elastomers were formulated with excess of PD hardener to ensure polymer networks with dangling aromatic amines (1.1:1 NH2/NCO ratio) as this is required to trigger associative amine/urea exchange reactions.24 The curing reactions were monitored by FTIR spectroscopy where the isocyanate stretching band at 2264 cm−1 of prepolymer 2 completely disappeared in the three formulations, and a new band corresponding to new urea moieties appeared between 1650 and 1680 cm−1 (see Figure S1 in the ESI). Mechanical Properties and Reprocessability. The mechanical properties of the pristine PUU elastomers are presented in Table 1. It can be seen that p-PUU is the stiffest of the three polymers with the highest tensile strength. m-PUU has a slightly less stiff response, which is compensated by a longer elongation at break. o-PUU is the weakest material with the lowest tensile strength and elongation at break values (see Figure S2 in the ESI). The efficiency of the reprocessing was C

DOI: 10.1021/acsapm.9b00589 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX

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ACS Applied Polymer Materials

Figure 2. Overlapped FTIR spectra of PUU elastomers at ambient temperature. (a) Amplification of C=O region. (b) Amplification of N−H region. The PUUs have two main chemical moieties capable of generating H-bonds:36 urethane (1720−1750 cm−1) and urea (1650−1680 cm−1). Typically, the signal of any carbonyl group appears as a combination of two contributions: free carbonyls and H-bond associated carbonyls. The stronger the H-bond association, the more shifted the signal of the carbonyl to lower frequencies and the higher the absorption coefficient. Thus, the wavenumber and the relative intensity of the resulting carbonyl band are closely related to the strength of H-bonds within the polymer network.

Figure 3. Molecular models of o-PUU (left) and p-PUU (right) showing the different nature of the noncovalent interactions.

slightly shifted toward a higher wavelength (1663 cm−1). This band almost disappears becoming a weak shoulder centered at 1669 cm−1 in case of o-PUU. This means that the urea carbonyl moieties in the o-PUU elastomer are mostly in a free fashion. The urethane region shows a similar trend in wavelength numbers: 1712 cm−1 in p-PUU, 1712 cm−1 in mPUU, and 1725 cm−1 in o-PUU. Moreover, the same tendency observed in the carbonyl region can be appreciated in N−H band (Figure 2b). The broader peak observed in o-PUU network as well as its shift to higher wavelengths (3356 cm−1) is associated with a lower number of H-bonds. On the other hand, the thinner signals centered at lower wavenumbers of pPUU (3335 cm−1) and m-PUU (3350 cm−1) elastomers indicate a higher number of stronger H-bonds. The FTIR results strongly indicate that the different mechanical strength of the pristine PUU thermosets was due to variations in the supramolecular interactions due to Hbonding of the urea and urethane groups. H-bonding was maximum for p-PUU and minimum for o-PUU. The linearity of the PPD molecule might enable a higher packaging of the material in the final structure, whereas the constrained shape of OPD might cause just the opposite.

To confirm this assumption, molecular dynamics (MD) simulations have been performed to study the nature of the Hbonding in the three networks. The numerical analysis of the noncovalent interactions shows that the average number of Hbonds is very similar in each network, although an increasing trend is observed, namely 82.5 for o-PUU, 83.6 for m-PUU, and 84.1 for p-PUU. Nevertheless, a striking difference is found for o-PUU, where a non-negligible percentage of the H-bonds (8.9%) are intramolecular, due to the spatial vicinity of the urea groups, while for m-PUU and p-PUU are almost entirely intermolecular ( p-PUU (Ea = 122 kJ mol−1). The higher the activation energy, the larger is the difference in relaxation time H

DOI: 10.1021/acsapm.9b00589 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX

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ACS Applied Polymer Materials for a given temperature difference. In other words, networks with higher activation energies are more temperature dependent,21 meaning that they would undergo significant structural changes even with a small temperature variation. In case of mPUU, which is the elastomer with the highest Ea, this could be explained by a weaker H-bond network that could be disrupted easier than in p-PUU at high temperature. Figure 8a shows that both m-PUU and p-PUU show similar relaxation behavior up to 160 °C. Above 160 °C, m-PUU network relaxes faster due to the lack of H-bonds and thus shows higher activation energy to flow. These results would be in good agreement with the conclusions of the FTIR analysis.

Alaitz Rekondo: 0000-0003-2195-5167 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. E. Gonzalez San Roman (Polymat) for MALDI TOF mass spectrometry measurement. Technical and human support provided by IZO-SGI, and SGIker (UPV/EHU, MICINN, GV/EJ, ERDF, and ESF) is gratefully acknowledged for assistance and generous allocation of computational resources.

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CONCLUSIONS In this work, three analogous PUU vitrimer networks, that is, o-PUU, m-PUU, and p-PUU, were synthesized using the three positional isomers of phenylenediamine (PD), that is, orthophenylenediamine (OPD), meta-phenylenediamine (MPD), and para-phenylenediamine (PPD). We demonstrate that the regioisomerism of the phenylenediamine hardener has a major influence on the thermal, mechanical, and dynamic properties of the PUU elastomers prepared thereof. This is a consequence of the differences in molecular chain alignment, packaging, and H-bond networks that are created with each of the investigated isomers. According to all the experimental evidence provided by FTIR, tensile, and rheology tests, p-PUU composed of the linear hardener leads to the 3D network with the strongest supramolecular interactions, but still presents dynamic behavior capable of being reprocessed and reshaped via an associative amine/urea exchange mechanism. In the case of o-PUU configuration, a side reaction occurs that is not compatible with the structural integrity of the elastomer under the reprocessing conditions. This is presumably due to constrained structure of OPD that triggers this intramolecular reaction leading to degradation. Therefore, we conclude that this structural isomer is not suitable for the design and development of dynamic PUU elastomers. In short, this investigation proves that the structural geometry or regioisomerism of the PUU components plays a critical role in the molecular arrangements of the network. We believe that the judicious selection of the hardener could be a straightforward approach toward tailor-made dynamic PUUs.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsapm.9b00589.

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REFERENCES

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FTIR analysis of pristine PUU elastomers, tensile strength characterization and pictures of pristine and reprocessed materials, DSC thermograms and rheological temperature sweep measurements of PUU networks, 13 C NMR spectra of initial and thermally treated U1 molecule, MALDI-TOF analysis of degradation products of U1 (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Fernando Ruipérez: 0000-0002-5585-245X I

DOI: 10.1021/acsapm.9b00589 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acsapm.9b00589 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX