A novel ionomeric polyurethane elastomer based on ionic liquid as crosslinker

Abstract

A novel ionic liquid crosslinked flexible polyurethane elastomer (PUIL) was successfully synthesized by a one-pot polymerization method. A variety of characterization methods was employed to study the properties of PUIL, and they were compared with those of a linear thermoplastic polyurethane elastomer (TPU) prepared using butanediol (BDO) as a chain extender. A reference chemically crosslinked polyurethane (PUTMP) was also synthesized by using trimethylolpropane (TMP) as a non-ionic crosslinker. FT-IR spectroscopy revealed that the hydrogen bonding interaction was significantly suppressed in the presence of ionic liquid crosslinker leading to a highly flexible but tough elastomer material. Differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) demonstrated the improved flexibility of PUIL, as the glass transition temperature (T_g) of PUIL was lower than that of TPU and PUTMP. At the same time, PUIL exhibited a significantly higher tensile strength as well as higher elongation at break, as compared to TPU and PUTMP prepared without ionic liquid crosslinker. In the SAXS profiles, both TPU and PUIL showed single and broad peak characteristics of non-uniform microphase-separated morphology, whereas no phase separation was found in PUTMP. Contact angle measurement showed that the polarity of PUIL is higher than TPU and PUTMP, but importantly, the presence of ionic liquid crosslinks improved the oil resistance property.

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Introduction

Thermoplastic polyurethanes (TPUs) are comprised of alternating hard and soft segments and offer a wide range of physical properties.¹ PUs are an important class of industrial polymers and widely used in high-performance adhesives, surface coatings, surface sealants, foams, binder resins, carpet underlay, sports devices and in various elastomeric products.^2,3 They are most commonly synthesized by reacting a di- or polyisocyanate with polyols.^4,5 The hard segments are formed by the reaction of isocyanate with a diol or amine based chain extender, and the soft segments are composed of long chain oligomeric or polymeric diols or polyols. Tunable properties can be achieved by controlling the length of hard and soft segment as well as by using different types of isocyanate, diol, polyol and chain extender.^5–11 If all the components of the polyurethane are difunctional, a linear TPU elastomer is obtained which gains its elastomeric properties by physical crosslinks through the hard segment only. Linear TPU elastomers have several weak points. They show poor resistance towards mechanical strains and high-temperature deformation.¹² In addition, linear PUs have temperature sensitive properties and display larger permanent set than crosslinked polymers.¹³

The mechanical and thermal properties of the polymeric materials are improved by the introduction of chemical crosslinking between the polymer chains.¹⁴ Various studies have examined the effects of chemical crosslinking on the material properties of polyurethane elastomers. As compared to linear polyurethane (TPU), cross-linked polyurethanes have better thermal and thermomechanical properties and a good shape memory effect.^15,16 Wang et al. studied the properties of TMP cross-linked polyurethane system and found that the tensile strength reached a maximum (18 to 28 MPa) at 1.6 wt% of cross-linker and then decreased. But the elongation at break decreased monotonously (290% to 180%) with the increase in the wt% of TMP.¹⁷ Desai et al.¹⁸ also studied the effect of concentrations of the crosslinker (TMP) at NCO/OH equivalent ratio 1.2 and 1.3. They found that increasing concentration of cross-linker improved the tensile strength (0.7 MPa to 0.8 MPa), but there is a reduction in elongation at break (800% to 400%). Chung et al.¹⁹ studied the comparative properties of linear PU with a pentaerythritol cross-linked PU connected by polyethylene glycol (PEG-200) spacers. They reported that a combination of both pentaerythritol and PEG-200 spacers in the PU resulted in the improvement of both stress and strain, unlike in the conventional cross-linking method. Nasar et al. studied the effect of amine-terminated AB₂-type hyperbranched polyamides (HBP) as a cross-linker on the properties of polyurethanes. They concluded that the highest tensile strength was achieved with 1% of hyperbranched polyamide. The glass transition temperatures (T_g) and thermal stability of polyurethanes were not affected up to 6% of HBP.²⁰ However, Chiou et al.²¹ reported that there is an increase in T_g of polyurethane with an increase in the mol% of trimethylolpropane propoxylate cross-linker. Desai et al. reported that starch-based polyurethanes exhibited better mechanical properties than TMP based polyurethanes.²²

Ionic liquids (ILs) are typically poorly coordinating salts that result from combinations of bulky organic cations with an organic or inorganic anion that melt below 100 °C.²³ Due to their low volatility, chemical stability, high thermal stability, and tunable solvent properties, ionic liquids are used as ‘green’ solvents for numerous applications.^24–28 Incorporating ionic groups into polymers is an important synthetic route to tailor the polymer properties and render them suitable for emerging technologies. In polymer science, ionic liquids are not only used as polymerization solvents but also as components of various polymeric materials, as plasticizers in many kinds of polymers and they are also used in the preparation of polymer gels.^29–32 Despite the relatively large amount of literature in the use of IL in polymers, only a limited amount of work on step-growth polymerization has been reported.³³ Mansoori et al. synthesized poly(amide-imide)s from the condensation of diimide-diacid (DIDA) and different aromatic diamines in the presence of triphenyl phosphite (TPP) as condensing agent and butyl methyl imidazolium bromide as a solvent. They found that the ionic liquid method provides higher inherent viscosity, higher thermal stability and better physical appearance than the polymers obtained via classical methods in NMP.³⁴ Lee et al. synthesized imidazolium-based polyesters from the condensation polymerization of hydroxyl end-functionalized ionic liquids with diacid chlorides.³⁵ Only a few reports are available in the field of ionic liquid containing polyurethane. Gao et al. have recently synthesized an imidazolium containing polyurethane from 4,4′-methylenebis(cyclohexyl isocyanate) (HMDI) and poly(tetramethylene oxide) (PTMO) using 1,3-di(2-hydroxyethyl) imidazolium chloride as a chain extender.³⁶ Williams et al. synthesized PTMO based polyurethane using a phosphonium diol as a chain extender. They found that despite stronger hydrogen bonding in BDO extended polyurethane, phosphonium based polyurethane was more crystalline.³⁷ Zhang et al. reported the synthesis of polyurethanes having phosphonium pendant groups in the hard segments and found that as compared to BDO extended polyurethane, phosphonium based polyurethanes displayed a significant improvement in tensile strength.³⁸

As compared to linear polyurethanes, cross-linked polyurethanes have better mechanical properties. But the major drawbacks of the crosslinked polyurethane are their low elongation at break and high glass transition temperature, which make them inferior for many applications. The introduction of crosslinks into the hard segment results in hard and soft domain intermixing and leads to higher T_g and brittleness. These drawbacks of the crosslinked polyurethanes can be overcome by the incorporation ionic moieties into the hard segment. The presence of ionic liquids in the hard segment will make the hard segment more incompatible with the soft segment and will act as driving force in microphase separation. Williams et al.³⁷ also reported that the ionic interactions of the ionic liquid act as a driving force in microphase separation in the phosphonium based polyurethane. The higher phase separation will result in the decrease in the dispersion of hard segments into the soft domain, which in turn will reduce the T_g of the soft segment. Thus it may improve the flexible nature of polyurethane elastomer. Further, the incorporation of ionic moieties into the polyurethane backbone can enhance its self-healing and shape memory properties. Das et al.³⁹ recently reported the preparation of a self-healable bromobutyl rubber (BIIR) by transforming the bromine functionalities of BIIR into ionic imidazolium bromide groups.

To the best of our knowledge, there is no report on the preparation of PU elastomers using an ionic liquid as a cross-linker. To overcome the drawbacks of non-ionic crosslinked polyurethane, in this report, we have prepared a polyurethane elastomer via a one-step process using ionic liquid based on quaternary ammonium salt as cross-linker and studied the impact of this ionic liquid on the properties of polyurethane. To study the comparative properties, a linear reference thermoplastic polyurethane (TPU) elastomer as well as a non-ionic crosslinked PU (PUTMP) were also synthesized replacing the ionic crosslinker by butanediol as a chain extender or by trimethylolpropane. The obtained PUs were characterized by FT-IR analysis and contact angle measurement. We also studied the thermal and mechanical properties of the synthesized polyurethanes. Nanoindentation experiment was carried out to study the surface hardness of the samples.

Experimental

Materials

Poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (HO–PEG–PPG–PEG–OH) with molecular weight 1100 g mol⁻¹, 4,4′-methylenebis(phenyl isocyanate) (MDI) (98%), tris(2-hydroxyethyl)methylammonium methylsulfate (IL) (95%), trimethylolpropane (TMP) (≥98%), 1,4-butanediol (BDO) (99%) and dibutyltin dilaurate (DBTL) (95%) were obtained from Sigma-Aldrich, USA. MDI was used as received, and other monomers were dried in a vacuum oven at 70 °C prior to polymerization. Tetrahydrofuran (THF) was purchased from Merck Specialities Private Limited, Mumbai, India. The trace amount of moisture present in THF was removed by distillation from sodium and benzophenone mixture under the nitrogen atmosphere.

Synthesis of polyurethanes

Ionic liquid crosslinked polyurethane (PUIL) was synthesized by a one-step polymerization in solution, using HO–PEG–PPG–PEG–OH, MDI and tris(2-hydroxyethyl) methylammonium methylsulfate (THMAMS, IL) as a cross-linker. In order to study the comparative properties, a linear polyurethane elastomer (TPU) as well as a non-ionic crosslinked PU (PUTMP) were also synthesized replacing the ionic crosslinker by butanediol as a chain extender or by trimethylolpropane as a crosslinker. In all reactions the [NCO]/[OH] molar ratio was 1.05/1.⁴⁰ After drying in an oven, all the glassware was cooled to room temperature under nitrogen atmosphere. In a typical polymerization process calculated amount of HO–PEG–PPG–PEG–OH, MDI and IL were added into a three-neck round bottom (RB) flask equipped with a thermometer and a nitrogen inlet. The third neck of the RB flask was fitted with a silicone septum through which 30 ml of freshly prepared dry THF was added using a glass syringe. The RB flask was then placed on a SCOTT magnetic stirrer (model SLR, SCHOTT Instruments GmbH, Germany), preheated at 60 °C. Stirring was set at a speed of 750 rpm throughout the reaction. When all the reaction components were soluble in THF, 0.1% DBTDL catalyst with respect to the weight of OH–PEG–PPG–PEG–OH was added to the reaction mixture. After 8 h of the reaction, the RB flask was removed from the oil bath and kept at room temperature for 2 h. The obtained polymer solution was cast into a Teflon Petri dish and then kept at room temperature for 24 h. After complete evaporation of THF, the solid film was then kept in a vacuum oven at 70 °C for 48 h. A transparent film was obtained with a thickness of around 0.5 mm.

Analytical methods

FTIR spectroscopy. The FTIR spectra were recorded on a Perkin-Elmer FTIR spectrophotometer (model Spectrum RX I) within a range of 400–4000 cm⁻¹ using a resolution of 4 cm⁻¹. The casted films were directly used for FT-IR analysis.

Dynamic mechanical analysis (DMA). Dynamic mechanical analysis was performed on an Eplexor 2000 N dynamic measurement system (Gabo Qualimeter, Ahlden, Germany) using a constant frequency of 10 Hz in a temperature range from −70 to +40 °C. The samples were analyzed in the tension mode. A static load of 0.4% pre-strain was applied, and then the samples were oscillated to a dynamic load of 0.2% strain. The measurements were done with a heating rate of 2 K min⁻¹ under liquid nitrogen flow.

Differential scanning calorimetry (DSC). Differential scanning calorimetry (DSC) analysis was carried out using a Netzsch DSC 200 F3 instrument at a heating rate of 10 K min⁻¹ under nitrogen atmosphere. In this case the samples were heated from room temperature to 100 °C followed by a cooling cycle to −100 °C. Then the samples were again heated from −100 °C to 190 °C and this final cycle has been reported.

Thermogravimetric analysis (TGA). Thermogravimetric analysis (TGA) was carried out with a Mettler Toledo AG (ME-51709822G) instrument. In this case, a sample weight of 15 mg was heated from room temperature to 600 °C at a heating rate of 20 K min⁻¹ under nitrogen atmosphere.

Small-angle X-ray scattering (SAXS). Small-angle X-ray scattering was performed on a Xenocs Xeuss 2.0 SAXS instrument at 50 kV operating voltage and 0.6 mA current within a scattering range of q ∼ 0.004 to 1.5 nm⁻¹. The scattering intensity of SAXS results was normalized by sample thickness. Cu Kα radiation (λ = 1.5418 Å) was used.

Tensile properties. Tensile testing was performed as per the ASTM D 412 method using Hounsfield H10KS universal testing machine at room temperature with a crosshead speed of 500 mm min⁻¹. Using an ASTM Die-C, dumbbell-shaped samples were punched out of cast film of 0.5 mm thickness.

Nanoindentation experiment. The surface hardness of polyurethane (TPU) and ionic liquid cross-linked polyurethane (PUIL) was measured with a TI 950 TriboIndenter, Hysitron Inc., USA. The hardness measurement was performed with a constant load of 50 μN. The nanoindentation experiments were carried out at room temperature. At least the average of 10 measurements was reported for each of the samples.

Contact angle. Water and diiodomethane contact angles of synthesized polyurethanes were obtained at ambient temperature using a Rame-Hart 260 F4 standard Goniometer. A drop of liquid was placed onto the surface of the polymer film, and the contact angles were measured within 5–10 s. At least three measurements were produced for each solvent on analyzing polymeric surface.

Oil swelling test. Oil swelling experiments of polyurethane (PU and PUIL) samples were carried out in ASTM 3 oil at two different temperatures (25 °C and 85 °C) according to ASTM D471-12a. The dry weight of the test specimens was measured after drying the samples in a vacuum oven at 70 °C for 24 h. Then the test specimens were immersed in ASTM 3 oil. After the removal of the excess surface oil by filter paper, the weights of the polyurethane samples were measured at various time intervals. For every single composition, three specimens were tested, and their average values are reported.⁴¹ The swelling % was calculated from the following equation.Where M_t and M_d are the mass of samples at time t, and in the dry state, respectively.

Results and discussion

Synthesis of polyurethane

Ionic liquid based polyurethane (PUIL) was successfully prepared using tris(2-hydroxyethyl)methylammonium methylsulfate (THMAMS) as cross-linker. Scheme 1 illustrates a one-step polymerization process between HO–PEG–PPG–PEG–OH (soft segment) as a polyol, MDI as diisocyanate and THMAMS, an ionic liquid as crosslinker. In order to study the comparative properties, a linear thermoplastic polyurethane elastomer (TPU) as well as a nonionic crosslinked polyurethane (PUTMP) were also synthesized from HO–PEG–PPG–PEG–OH and MDI, but replacing THMAMS by butanediol as a chain extender and TMP as nonionic crosslinker. We used TMP as nonionic crosslinker, because it has three hydroxyl groups like THMAMS, the ionic crosslinker. Importantly, in this case the same molar ratio of ionic as well as nonionic crosslinker was used, so that the crosslinking density may be somewhat similar to have a better comparison. The butanediol chain-extended polyurethane was referred as TPU, TMP crosslinked PU as PUTMP, and polyurethane based on THMAMS ionic liquid as PUIL. Table 1 shows the chemical compositions of butanediol extended polyurethane (TPU), non-ionic crosslinked polyurethane (PUTMP) and ionic liquid cross-linked polyurethane (PUIL).

Scheme 1 Synthesis of ionic liquid cross-linked polyurethane (PUIL).

Table 1 Polymer composition and hard segment (HS) content^a

Sample code & composition	Monomer molar ratio	NCO/OH molar ratio	Hard segment (HS) content
			Weight%	Mole%
a In the above table the –OH content of 0.66 equivalent of IL (ionic liquid) is equivalent to –OH content 1 equivalent BDO (butanediol), since one molecule of IL contains three hydroxyl groups. The hard segment (HS) content was determined by using the following equation.
TPU (MDI : PEG–PPG–PEG : BDO)	2.1 : 1 : 1	1.05	35	75
PU-TMP (MDI : PEG–PPG–PEG : TMP)	2.1 : 1 : 0.66	1.05	36	73
PUIL (MDI : PEG–PPG–PEG : IL)	2.1 : 1 : 0.66	1.05	38	73

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FT-IR analysis

Fig. 1 represents the FT-IR spectra of HO–PEG–PPG–PEG–OH, MDI, TPU, PUTMP and PUIL. The absorption band at about 2265 cm⁻¹, corresponding to the stretching vibration of the NCO group of MDI, has entirely disappeared in the polymers (Fig. 1c–e). Furthermore, the FTIR spectra of synthesized polyurethanes show an absorption band at 3300 cm⁻¹, which is due to the hydrogen-bonded –NH group of urethane linkages. The absorption band at 1725 cm⁻¹ is assigned to free urethane carbonyl while the absorption band at 1709 cm⁻¹ is due to hydrogen bonded carbonyl. The absorption band which is observed around 1538 cm⁻¹ is associated with –N–H bending vibration of the urethane linkage. The absorption bands in the region 2865–2968 cm⁻¹ are assigned to the symmetric and asymmetric stretching vibrations of the aliphatic –CH₂– groups as well as stretching vibrations of –CH₃ groups.

Fig. 1 FT-IR spectra of (a) HO–PEG–PPG–PEG–OH, (b) MDI, (c) TPU, (d) PUTMP, and (e) PUIL.

In the FT-IR spectrum of PUIL (Fig. 2), the absorption bands at 610, 578 and 553 cm⁻¹ (for methylsulfate ion of ionic liquid) indicate the presence of ionic liquid in the polyurethane backbone.^42,43 These absorption bands are absent in the BDO extended polyurethane (TPU) and TMP crosslinked polyurethane (PUTMP).

Fig. 2 Amplified FT-IR spectra (at 500 to 650 cm⁻¹) of IL (ionic liquid), TPU, PUTMP and PUIL.

Fig. 3(a) represents the FT-IR spectra of TPU, PUTMP and PUIL in [double bond splayed left]C=O region at 25 °C. In [double bond splayed left]C=O region, TPU, PUTMP and PUIL displayed two distinct peaks. It is reported that the non-hydrogen bonded carbonyls appear at higher wavenumbers than the hydrogen bonded carbonyls.^44,45 In case of TPU, PUTMP and PUIL, the non-hydrogen bonded carbonyl bands appeared at the same wavenumber (1725 cm⁻¹). But the hydrogen bonded carbonyls of TPU were displayed at lower wavenumber than that of PUTMP and PUIL (1709 cm⁻¹). In addition, in TPU the intensity of the hydrogen bonded carbonyls is higher than that of non-hydrogen bonded carbonyls, whereas in PUTMP and PUIL the hydrogen bonded carbonyls are less apparent than the non-hydrogen bonded carbonyls. These results suggest that stronger CO/NH hydrogen bonding exists in TPU than in PUTMP and PUIL. Williams et al.⁴⁶ also observed the reduction of CO/NH hydrogen bonding in phosphonium based polyurethanes. Yu et al.⁴⁷ reported the decrease in the intensity of the hydrogen bonded carbonyls absorption band due to the introduction of the glycerol crosslinker in the hard segment. Fig. 3(b) represents the FT-IR spectra of TPU, PUTMP and PUIL in [double bond splayed left]C=O region at 170 °C. In case of TPU, when the temperature was increased to 170 °C, the intensity of the free carbonyls band becomes higher than that of hydrogen bonded carbonyls, whereas the case was reverse at 25 °C. This indicates the disruption of hydrogen bonding at a higher temperature. But in PUTMP and PUIL there is no such drastic change in the intensity of the absorption bands of the free and H-bonded carbonyls.

Fig. 3 Amplified [double bond splayed left]C=O absorption band in the FTIR spectra of TPU, PUTMP and PUIL at (a) 25 °C and (b) 170 °C.

Thermal properties

Thermal properties of TPU, PUTMP and PUIL were studied using DMA, DSC and TGA analyses. Fig. 4(a) illustrates the change in the storage modulus (E′) as a function of temperature in the range −70 °C to +40 °C. The storage modulus of PUTMP and TPU is higher than that of PUIL in the high temperature region. A sharp fall in the storage modulus at around −31 °C (PUIL), −3 °C (PUTMP) and −25 °C (TPU) is observed due to transition from glassy to rubbery state. The effect of temperature on the loss modulus (E′′) of TPU, PUTMP and PUIL is represented in Fig. 4(b). PUIL exhibits loss modulus maxima at −28 °C, whereas the same of TPU and PUTMP was observed at −22 °C and 1 °C respectively. Fig. 4(c) reports the effect of temperature on tan δ value of TPU, PUTMP and PUIL. TPU, PUTMP and PUIL show a single sharp tangent peak corresponding to the T_g of PEG–PPG–PEG based soft segment. The tan δ peak of PUIL was obtained at lower temperature (−13 °C) than that of TPU (−6 °C) and PUTMP (14 °C). In case of all the samples (TPU, PUTMP and PUIL) the T_g value obtained from E′′ peak is lower than that obtained from tan δ peak.

Fig. 4 DMA analysis (a) storage modulus vs. temperature curves for TPU, PUTMP and PUIL, (b) loss modulus vs. temperature curves for TPU, PUTMP and PUIL (c) tan δ vs. temperature curves for TPU, PUTMP and PUIL.

The lower T_g of PUIL than TPU and PUTMP was again supported by the DSC analysis. Fig. 5 shows the DSC thermograms of HO–PEG–PPG–PEG–OH, TPU, PUTMP and PUIL. The DSC curve of HO–PEG–PPG–PEG–OH shows a T_g at −71 °C, whereas the DSC curve of synthesized TPU shows a T_g at −22 °C and a small endotherm at 178 °C corresponding to the melting temperature of the hard segment. The DSC thermograms of PUTMP and PUIL show a T_g at −2 °C and −32 °C respectively, but no melting endotherm was observed for the hard segment. The absence of melting endotherm for PUTMP and PUIL is due to the presence of trifunctional cross-linker, which restricts the free flow of polymer chains in the polymer matrix. Petrovic et al.¹³ also reported the absence of the melting point in 70% and 100% TMP-containing polyurethanes. The T_g's of TPU and PUIL are higher than the T_g of PEG–PPG–PEG diol, because the free rotation of polymer chains was restricted since the chain ends of the soft diol get attached to hard domains. Again the increase in molecular weight due to polymerization also enhances the T_g of the soft segment. Generally, the crosslinked network structure raises the T_m and T_g of the soft segment.¹⁹ Petrovic et al. also reported that crosslinks in hard segment reduce the flexibility of soft segment, because of hard and soft segment mixing.¹³ We observed that the T_g of ionic liquid cross-linked polyurethane (PUIL, −32 °C) is lower than that of the non-ionic TMP crosslinked polyurethane (PUTMP, −2 °C) and the linear polyurethane (TPU, −22 °C). It is reported that the soft-segment glass transition temperature is also a measure of the relative purity of the soft-segment. The higher T_g in PUTMP is due to the presence of non-ionic crosslinks, which resulted in the inter-domain mixing of soft and hard phases. Król et al.⁴⁸ reported that mixing of soft domains with the hard segments leads to increase in T_g of the soft segment. Yu et al.⁴⁷ also observed the increase in the soft segment glass transition temperature of polyurethanes with increase in the concentration of glycerol (a non-ionic crosslinker). They reported that the dispersion of hard segments in the soft domain increased with the content of glycerol crosslinker, which in turns raise the T_g of the soft segment. But, when ionic liquid was used as a crosslinker (PUIL, T_g = −32 °C), instead of increasing T_g, a lowering in T_g was observed. This indicated that the mixing of hard segments with the soft domains is less in PUIL than in PUTMP. The less inter phase mixing in PUIL may be due to the presence of polar cations and anions of the ionic liquid, which make the hard segment more incompatible with the soft domains. Williams et al.³⁷ also reported that the ionic interactions of the ionic liquid act as a driving force in microphase separation in the phosphonium based polyurethane. The inter-domain mixing of soft and hard phases in PUTMP was also supported by the SAXS patterns (Fig. 7), which indicated that there is no phase separation in PUTMP.

Fig. 5 DSC curves of PEG–PPG–PEG, TPU, PUTMP and PUIL.

Thermogravimetric analysis was carried out to study the thermal stability of butanediol chain extended polyurethane (TPU), trimethylolpropane crosslinked PU (PUTMP) and the ionic liquid crosslinked PU (PUIL). TGA of TPU and PUTMP shows that the onset temperature, T_i (temperature at 5% weight loss) is 317 °C and 323 °C respectively, whereas the value of PUIL is 274 °C (Fig. 6); thus PUIL starts earlier to decompose. But the T_max value of the PUIL is higher (412 °C) than PUTMP (405 °C) and TPU (399 °C). Petrovic et al. also observed that chemical crosslinks in the hard segment reduced the crystallinity of the hard phase but improved the heat stability of the hard domains.¹³ At high temperature, the methylsulfate anion of the ionic liquid abstracts the –CH₂– hydrogen from its cation counterpart. This elimination reaction caused the earlier weight loss in PUIL. Lemus et al.⁴⁹ also reported the earlier weight loss (242 °C) in encapsulated ionic liquids (ENILs) based on carbonaceous submicrocapsules prepared using tris(2-hydroxyethyl)methylammonium methylsulfate as an ionic liquid.

Fig. 6 TGA curves of TPU, PUTMP and PUIL.

SAXS analysis

Fig. 7 shows the SAXS profiles of TPU, PUTMP and PUIL plotted as scattering intensity versus scattering vector q (nm⁻¹). Both the TPU and PUIL showed a single and very broad peak characteristics of non-uniform microphase-separated morphology. Whereas, in TMP crosslinked polyurethane (PU-TMP) no peak was obtained in the plot of scattering intensity versus scattering vector. This indicated that the presence of non-ionic crosslinker in PUTMP induces soft and hard domain mixing, and no microphase separation was found. The somewhat higher T_g in PUTMP also supports the some intermixing of soft and hard phase. According to Bragg's law, the peak position at maximum scattering intensity (q_max) relates to the inter-domain spacing (d = 2π/q_max) of polyurethane hard segments. TPU displayed a scattering peak near q_max = 0.5 nm⁻¹, whereas PUIL exhibited a peak at around q_max = 0.69 nm⁻¹. The calculated interdomain spacing of butanediol extended polyurethane (TPU) (d = 12.5 nm) is higher than that of ionic liquid cross-linked polyurethane (PUIL) (d = 9.1 nm).

Fig. 7 SAXS patterns of TPU, PUTMP and PUIL.

Mechanical properties

The stress–strain plots of TPU, PUTMP and PUIL in Fig. 8 indicate that the mechanical property was greatly influenced by the ionic liquid crosslinker. Ionic liquid crosslinked polyurethane (PUIL) displayed more flexible and tougher characteristics than BDO chain extended polyurethane (TPU) and TMP crosslinked polyurethane (PUTMP). This result was also supported by the DMA and DSC analyses, where the T_g of PUIL was found to be lower than the T_g of TPU and PUTMP. The tensile strength of PUIL (7 ± 0.35 MPa) is much higher than the tensile strength of TPU (3.6 ± 0.17 MPa) and PUTMP (5.5 ± 0.26 MPa). The elongation at break of PUIL (538 ± 48, %) is also much higher than that of TPU (276 ± 25, %) and PUTMP (238 ± 18, %). The higher tensile strength of PUIL than TPU and PUTMP is due to the presence of inter-ionic interaction between the hard segments, as it was evidenced from the lowering in d-spacing in the SAXS patterns. Again the presence of crosslinks in the hard segment also plays an important role in the improvement of tensile strength. Wang et al. reported that the tensile strength of the polyurethane films was increased with increase in trimethylolpropane, but there was a reduction in elongation at break.¹⁷ PUIL exhibited lower tensile modulus than TPU and PUTMP. The lower modulus of PUIL was also supported by the DMA results. As it was evidenced by the SAXS, DMA and DSC analyses, the presence of nonionic crosslinker in PUTMP resulted in hard and soft phase intermixing, whereas the ionic liquid crosslinker in PUIL restricted the phase mixing. The phase mixing due to nonionic crosslinker is the cause of strain-hardening behavior in PUTMP. Hsieh et al.⁵⁰ also reported that the increased phase mixing improved the strain hardening behavior in linear thermoplastic polyurethane elastomers.

Fig. 8 Stress–strain curves of TPU, PUTMP and PUIL.

Table 2 represents the surface hardness and contact depth of TPU, PUTMP and PUIL under constant load. Usually, crosslinks in the hard segment result in increasing the hardness of the polymer.⁵¹ But in our case the hardness of ionic liquid crosslinked polyurethane (PUIL) is lower than that of BDO extended polyurethane (TPU) and the non-ionic TMP crosslinked polyurethane (PUTMP). The low hardness of PUIL is due to the presence of flexible chains (soft segment) as the T_g of PUIL is lower than the T_g of TPU and PUTMP. The low hardness of PUIL was also supported by the reduction in modulus as it was evidenced by the stress–strain and DMA experiments. The higher contact depth of the nanoindenter in PUIL also supports the tough and flexible nature of PUIL.

Table 2 Surface hardness and contact depth of TPU, PUTMP and PUIL

Sample	Hardness (GPa)	Contact depth (nm)
TPU	0.0061	648
PUTMP	0.0069	598
PUIL	0.0013	1097

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Surface properties

The effect of ionic liquid on surface properties was studied by contact angle measurement. Table 3 represents the contact angle and surface energy values of TPU, PUTMP and PUIL. The water contact angle of the TPU and PUTMP was found to be 95.9° and 98.5° respectively. The presence of ionic liquid moiety in PUIL makes the surface more hydrophilic. As a result, the water contact angle was reduced to 83.5° in PUIL, thus PUIL showed higher surface energy value than TPU and PUTMP. This decrease in contact angle is due to the presence of ionic liquid moieties in the PUIL backbone.

Table 3 Contact angle and surface energy values of TPU, PUTMP and PUIL

Sample	Contact angle (°)		Polar component (γ^ps) (mN m^−1)	Dispersive component (γ^ds) (mN m^−1)	Total surface energy (γs) (mN m^−1)
	Water	Diodomethane
TPU	95.9	46.5	0.41	36.19	36.60
PUTMP	98.5	56.4	0.52	30.63	31.16
PUIL	83.5	23.7	1.46	46.61	48.07

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Swelling properties

The oil swelling behavior of TPU, PUTMP and PUIL was studied by immersing the samples in ASTM 3 oil for 80 h at two different temperatures (25 °C and 85 °C). Fig. 9 shows the oil swelling behavior of TPU, PUTMP and PUIL. The presence of ionic liquid moieties in PUIL backbone improves the polarity of the polymer, as it is evidenced by the reduction of the water contact angle. The order of oil resistance properties is; PUIL > PUTMP > TPU. The combined effect of the higher polarity, presence of ionic crosslinks, as well as the inter-ionic interactions in the hard segment of PUIL, improved the high-temperature oil resistance properties.

Fig. 9 Oil swelling of TPU, PUTMP and PUIL.

Conclusions

A novel ionic liquid cross-linked polyurethane elastomer (PUIL) was successfully synthesized by one-pot polymerization method using tris(2-hydroxyethyl)methylammonium methylsulfate as a crosslinker. To study the comparative properties, reference polyurethanes were also prepared by using butanediol as a chain extender (TPU) as well as trimethylolpropane as a non-ionic crosslinker (PUTMP). A variety of characterization methods was employed to study the relative properties. As evidenced by FT-IR spectroscopy, TPU possessed stronger hydrogen bonding than PUIL, whereas PUIL was dominated by ionic interaction. The ionic interaction in the hard segment reduced the contamination of soft domains with the hard segment. Reduced soft segment contamination was confirmed from the lower T_g of PUIL (−32 °C) than TPU (−22 °C) and PUTMP (−2 °C), as it was seen from the DSC analysis. The low T_g of PUIL was also confirmed by the DMA analysis. TGA analysis showed earlier weight loss in PUIL, but PUIL had higher T_max (412 °C) than PU (399 °C) and PUTMP (405 °C). In the SAXS profiles, both TPU and PUIL showed a single and broad peak characteristics of non-uniform microphase-separated morphology, whereas no phase separation was found in PUTMP. PUIL exhibited significantly greater tensile strength and much higher elongation at break than the conventional TPU and PUTMP but the hardness of ionic liquid cross-linked polyurethane (PUIL) is lower than that of TPU and PUTMP. Low T_g, high elongation at break and low hardness reflected the highly flexible nature of PUIL. Contact angle measurement showed that the polarity of PUIL is higher than that of TPU and PUTMP, as it was evidenced by the reduction of the water contact angle in PUIL. The improved high-temperature oil resistance property of PUIL is due to the presence of ionic cross-links. These specialty polyurethane with ionic crosslinks can have several potential applications.

Acknowledgements

Prasanta Kumar Behera is grateful to the Council of Scientific and Industrial Research (CSIR), New Delhi, India, for a Senior Research Fellowship. We are thankful to Vikram Sarabhai Space Centre (VSSC)-ISRO, Thiruvananthapuram, India for the financial assistance.

References

1. M. A. Hood, B. Wang, J. M. Sands, J. J. La Scala, F. L. Beyer and C. Y. Li, Polymer, 2010, 51, 2191–2198.
2. F. Levine, J. Escarsega and J. La Scala, Prog. Org. Coat., 2012, 74, 572–581.
3. J. H. Wynne, P. A. Fulmer, D. M. McCluskey, N. M. Mackey and J. P. Buchanan, ACS Appl. Mater. Interfaces, 2011, 3, 2005–2011.
4. Y. Qian, C. I. Lindsay, C. Macosko and A. Stein, ACS Appl. Mater. Interfaces, 2011, 3, 3709–3717.
5. J. A. Miller, S. B. Lin, K. K. S. Hwang, K. S. Wu, P. E. Gibson and S. L. Cooper, Macromolecules, 1985, 18, 32–44.
6. Y. M. Lee, J. C. Lee and B. K. Kim, Polymer, 1994, 35, 1095–1099.
7. W. Li, A. J. Ryan and I. K. Meier, Macromolecules, 2002, 35, 6306–6312.
8. J. Blackwell, M. R. Nagarajan and T. B. Hoitink, Polymer, 1982, 23, 950–956.
9. Y. T. Park, Y. Qian, C. I. Lindsay, C. Nijs, R. E. Camargo, A. Stein and C. W. Macosko, ACS Appl. Mater. Interfaces, 2013, 5, 3054–3062.
10. C. Zhang, S. A. Madbouly and M. R. Kessler, ACS Appl. Mater. Interfaces, 2015, 7, 1226–1233.
11. P. N. Coneski and J. H. Wynne, ACS Appl. Mater. Interfaces, 2012, 4, 4465–4469.
12. D. Chattopadhyay and K. Raju, Prog. Polym. Sci., 2007, 32, 352–418.
13. Z. S. Petrovic, I. Javni and V. Divjakovic, J. Polym. Sci., Part B: Polym. Phys., 1998, 36, 221–235.
14. X. Lu, J. Huang, G. He, L. Yang, N. Zhang, Y. Zhao and J. Qu, Ind. Eng. Chem. Res., 2013, 52, 13677–13684.
15. Y. Zhuohong, H. Jinlian, L. Yeqiu and Y. Lapyan, Mater. Chem. Phys., 2006, 98, 368–372.
16. J. Hu, Z. Yang, L. Yeung, F. Ji and Y. Liu, Polym. Int., 2005, 54, 854–859.
17. L. Wang, Y. Shen, X. Lai, Z. Li and M. Liu, J. Polym. Res., 2011, 18, 469–476.
18. S. Desai, I. Thakore, B. Sarawade and S. Devi, Eur. Polym. J., 2000, 36, 711–725.
19. Y.-C. Chung, T. K. Cho and B. C. Chun, J. Appl. Polym. Sci., 2009, 112, 2800–2808.
20. A. S. Nasar, M. Jikei and M.-a. Kakimoto, Eur. Polym. J., 2003, 39, 1201–1208.
21. B. S. Chiou and P. E. Schoen, J. Appl. Polym. Sci., 2002, 83, 212–223.
22. S. Desai, I. Thakore, B. Sarawade and S. Devi, Polym. Eng. Sci., 2000, 40, 1200–1210.
23. A. E. Visser, R. P. Swatloski, W. M. Reichert, R. Mayton, S. Sheff, A. Wierzbicki, J. H. Davis and R. D. Rogers, Environ. Sci. Technol., 2002, 36, 2523–2529.
24. O. A. El Seoud, A. Koschella, L. C. Fidale, S. Dorn and T. Heinze, Biomacromolecules, 2007, 8, 2629–2647.
25. T. Ueki and M. Watanabe, Macromolecules, 2008, 41, 3739–3749.
26. A. Farrán, C. Cai, M. Sandoval, Y. Xu, J. Liu, M. J. Hernáiz and R. J. Linhardt, Chem. Rev., 2015, 115, 6811–6853.
27. K. Hong, H. Zhang, J. W. Mays, A. E. Visser, C. S. Brazel, J. D. Holbrey, W. M. Reichert and R. D. Rogers, Chem. Commun., 2002, 1368–1369.
28. N. K. Singha, N. B. Pramanik, P. K. Behera, A. Chakrabarty and J. W. Mays, Green Chem., 2016 10.1039/c6gc01677d.
29. J. Yuan, H. Schlaad, C. Giordano and M. Antonietti, Eur. Polym. J., 2011, 47, 772–781.
30. P. Kubisa, Prog. Polym. Sci., 2009, 34, 1333–1347.
31. J. Lu, F. Yan and J. Texter, Prog. Polym. Sci., 2009, 34, 431–448.
32. M. P. Scott, C. S. Brazel, M. G. Benton, J. W. Mays, J. D. Holbrey and R. D. Rogers, Chem. Commun., 2002, 1370–1371.
33. H. Zhang, P. K. Behera, N. K. Singha, K. Hong and J. W. Mays, Polymerization in Ionic Liquid, Encycl. Polym. Sci. Technol., 2005, 1–19.
34. Y. Mansoori, S. S. Sanaei, S. Atghia, M. Zamanloo and G. Imanzadeh, Chin. J. Polym. Sci., 2011, 29, 699–711.
35. M. Lee, U. H. Choi, D. Salas-de la Cruz, A. Mittal, K. I. Winey, R. H. Colby and H. W. Gibson, Adv. Funct. Mater., 2011, 21, 708–717.
36. R. Gao, M. Zhang, S. W. Wang, R. B. Moore, R. H. Colby and T. E. Long, Macromol. Chem. Phys., 2013, 214, 1027–1036.
37. S. R. Williams, W. Wang, K. I. Winey and T. E. Long, Macromolecules, 2008, 41, 9072–9079.
38. M. Zhang, S. T. Hemp, M. Zhang, M. H. Allen, R. N. Carmean, R. B. Moore and T. E. Long, Polym. Chem., 2014, 5, 3795–3803.
39. A. Das, A. Sallat, F. Böhme, M. Suckow, D. Basu, S. Wießner, K. W. Stöckelhuber, B. Voit and G. Heinrich, ACS Appl. Mater. Interfaces, 2015, 7, 20623–20630.
40. M. V. Pergal, J. V. Džunuzović, R. Poręba, M. Steinhart, M. M. Pergal, V. V. Vodnik and M. Špírková, Ind. Eng. Chem. Res., 2013, 52, 6164–6176.
41. J. Dutta and K. Naskar, RSC Adv., 2014, 4, 60831–60841.
42. G. Socrates, Infrared and Raman characteristic group frequencies: tables and charts, John Wiley & Sons, 2004.
43. A. Borba, A. Gómez-Zavaglia, P. N. Simões and R. Fausto, Spectrochim. Acta, Part A, 2005, 61, 1461–1470.
44. T. Gurunathan, S. Mohanty and S. K. Nayak, RSC Adv., 2015, 5, 11524–11533.
45. F. Cao and S. C. Jana, Polymer, 2007, 48, 3790–3800.
46. S. R. Williams, W. Wang, K. I. Winey and T. E. Long, Macromolecules, 2008, 41, 9072–9079.
47. T. Yu, T. Lin, Y. Tsai and W. Liu, J. Polym. Sci., Part B: Polym. Phys., 1999, 37, 2673–2681.
48. P. Król, Prog. Mater. Sci., 2007, 52, 915–1015.
49. J. Lemus, J. Bedia, C. Moya, N. Alonso-Morales, M. A. Gilarranz, J. Palomar and J. J. Rodriguez, RSC Adv., 2016, 6, 61650–61660.
50. A. J. Hsieh, S. S. Sarva and N. Rice, Improved Dynamic Strain Hardening in Poly(Urethane Urea) Elastomers for Transparent Armor Applications, Proceedings of the 26th Army Science Conference, Orlando, 2009.
51. W. L. Chang, T. Baranowski and T. Karalis, J. Appl. Polym. Sci., 1994, 51, 1077–1085.

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