Application of Modified Amino Acid-Derived Diols as Chain Extenders

Jun 14, 2017 - We previously reported the modification of 2-amino-1,3-propanediol (serinol) to create urea and carbamate substituted diols to create c...
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Research Article pubs.acs.org/journal/ascecg

Application of Modified Amino Acid-Derived Diols as Chain Extenders in the Synthesis of Novel Thermoplastic Polyester− Urethane Elastomers Ruairí P. Brannigan,† Anthony Walder,‡ and Andrew P. Dove*,† †

Department of Chemistry, University of Warwick, Gibbet Hill Road, Coventry, CV4 7AL, U.K. The Lubrizol Corporation, 207 Lowell Street, Wilmington, Massachusetts 0887, United States

‡

S Supporting Information *

ABSTRACT: Owing to their robust processability and mechanical dexterity, thermoplastic polyurethanes (TPUs) have been utilized in a wide variety of applications from commodity to more niche biomedical applications. Despite this, the focus on deriving monomers from sustainable resources has been relatively low; however, bioderived diisocyanates, diamine/ diol chain extenders, and polyester-based polyols have all been studied. Herein we report the application of biorenewable diol chain extenders derived from amino acids using an organocatalyzed process in bulk. To determine the effect of extender chain length on the properties of the resultant materials, TPEUs were synthesized using diol extenders derived from amino acids, 1-(1,3-dihydroxypropan-2-yl)-3-ethylurea (C3u), 1-(1,4-dihydroxybutan-2-yl)-4-ethylurea (C4u), and 1-(1,5-dihydroxypentan-2-yl)-5-ethylurea (C5u). When poly(ε-caprolactone) (PCL) and 1-isocyanato-4-[(4-isocyanatocyclohexyl) methyl]cyclohexane (H12MDI) were used as the polyol and diisocyanate, respectively, TPEUs were synthesized yielding materials with a predetermined percentage “hard segment” (%HS) and molecular weight. It was established through the selection of extender chain length and by controlling the %HS, that both the thermal and mechanical properties of the TPEUs could be controlled. Furthermore, the extender chain length was found to affect both the hydrophilicity and hydrolytic degradation profile of the resultant materials. KEYWORDS: Degradable elastomer, TPEU, Amino diol

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thesis.13−16 Furthermore, the incorporation of bioderived diisocyanates (i.e., lysine diisocyanate (LDI)) and small diamine/diols (i.e., putrescine, isosorbide, etc.) as chain extenders has allowed for more bioabsorbable “hard” segments that yield nontoxic degradation products.17−20 We previously reported the modification of 2-amino-1,3propanediol (serinol) to create urea and carbamate substituted diols to create chain extenders from natural feedstocks in which the side chain chemistry was demonstrated to lead to significant differences in physical and mechanical properties of the resultant materials on account of the altered hydrogen bonding in the hard segment.21 Herein we investigate the effect of extender chain length upon materials’ properties and describe the synthesis of two novel diol extenders derived from the natural amino acids, L-aspartic acid and L-glutamic acid. These compounds provide longer chain analogues of serinol and report their application in the synthesis of novel thermoplastic poly(ester−urethane)s (TPEUs) with bioderived extenders.

INTRODUCTION As a consequence of their low biodegradability, unsustainable nature and adverse effect on the environment, the utilization of petroleum-based resources has been increasingly on the decline in the field of biomaterials.1−5 In response, as a consequence of their degradability and biocompatibility and potential environmental benefits, bioderived materials and sustainable monomer feeds have received substantial attention.6−8 Moreover, the diverse range of bioderived monomers and prepolymers available through sustainable feedstocks allows access to a plethora of materials with varied chemical, thermal, and mechanical properties.3−5,9,10 Because of their robust processability and mechanical dexterity, thermoplastic polyurethanes (TPUs), a class of segmented mutiblock copolymer synthesized via the stepgrowth polymerization of polyols, diisocyanates, and low molecular weight diol “chain-extenders”, have seen application in a wide variety of fields from the automotive industry to tissue engineering.11,12 In recent times, in order to introduce degradability and biocompatibility, telechelic polyesters such as poly(lactide-co-glycolide) (PLGA) and poly(ε-caprolactone) (PCL) have been incorporated as polyols in TPU syn© 2017 American Chemical Society

Received: April 12, 2017 Revised: June 11, 2017 Published: June 14, 2017 6902

DOI: 10.1021/acssuschemeng.7b01110 ACS Sustainable Chem. Eng. 2017, 5, 6902−6909

Research Article

ACS Sustainable Chemistry & Engineering

accelerated degradation studies (5 M aq. NaOH). Polymer samples were molded into disks via compression molding at 200 °C using a PTFE mold and allowed to cool to ambient temperature. The disks were placed in individual vials containing 20 mL of 5 M NaOH solution and incubated at 37 °C with constant agitation at 60 rpm. The surface of the disks were “dab”-dried using KIMTECH SCIENCE precision wipes in order to remove excess surface water before the weight was measured periodically using an analytical balance. Contact Angle Measurements. Static contact angle measurements were obtained using a KRUSS DSA10 drop-shape analyzer and were processed using the software package DSA3 1.72b IEEE1394b. Each polymer was dissolved in minimal DMSO before being deposited as a thin film on a glass slide. The solvent was allowed to evaporate overnight before trace solvent was removed in vacuo. TPEU samples were allowed to anneal at 25 °C in an incubator for 5 days prior to analysis. A KRUSS DSA100 was used to deposit a 100 μL droplet of DI H2O onto the surface of the film and the measurement was taken immediately and analyzed using a sessile drop type with a polynomial (tangent 2) computational method. Synthetic Methodology. General Synthesis of 2-Amino-diol Extenders Derived from Amino Acids. Amino-diol extenders were synthesized according to the literature.21−24 In a 1 L round-bottom flask fitted with a magnetic stirrer bar, L-glutamic acid (100 g, 6.80 × 10−1 mol) was suspended in 700 mL of methanol before being cooled on an ice-bath. Thionyl chloride (100 mL, 1.40 mol) was added dropwise to the cooled suspension over 30 min with stirring after which the reaction was removed from the ice-bath and allowed to warm to ambient temperature. The reaction was allowed to proceed overnight (∼12 h) before the reaction mixture was concentrated in vacuo and diluted in 300 mL of fresh methanol. The acidic solution was neutralized to pH 7 by the addition of solid sodium hydrogen carbonate and the resultant suspension was filtered. The solvent was removed from the filtrate in vacuo before the addition of 300 mL of CH2Cl2 to precipitate any remaining salts. The suspension was filtered and the solvent was removed in vacuo to yield the dimethyl Lglutamate as a colorless oil which was used without any further purification. Analysis was consistent with previous reports.2−4 1H NMR (400 MHz, DMSO-d6) δ: 3.96 (t, 3JH−H = 6.6 Hz, 1H), 3.70 (s, 3H), 3.59 (s, 3H), 2.63−2.41 (m, 2H), 2.06 (m, 2H). 13C NMR (101 MHz, DMSO-d6) δ: 172.22 (s, CO), 169.84 (s, CO), 52.62 (s, CH3), 51.46 (s, CH3), 51.32 (CH), 28.92 (s, CH2), 25.32 (s, CH2). In a 3-necked round-bottom flask fitted with a magnetic stirrer bar and an N2 inlet, dimethyl L-glutamate (25 g, 1.40 × 10−1 mol) was dissolved in 500 mL of dry THF and cooled on an ice-bath. Under an N2 flow and vigorous stirring lithium aluminum hydride was slowly in small aliquots over a 30 min period. After complete addition, the reaction mixture was removed from the ice-bath and allowed to stir overnight (∼10 h) at ambient temperature. The reaction suspension was cooled on ice and the unreacted LiAlH4 was quenched by subsequent and slow addition of 20 mL of DI·H2O, 20 mL of 1 M NaOH and 20 mL of DI·H2O. The quenched suspension was allowed to warm to ambient temperature and stir for 30 min before the addition of MgSO4 and subsequent vacuum filtration. The solvent was removed from the filtrate in vacuo to yield the crude amino diol. Analysis was consistent with previous reports.2−4 1H NMR (400 MHz, DMSO-d6) δ: 3.37 (m, 2H), 3.29−3.06 (m, 2H), 2.55 (m, 1H), 1.54− 1.37 (m, 1H), 1.18−0.99 (m, 1H). 13C NMR (101 MHz, DMSO-d6) δ: 66.79 (s, CH2), 61.13 (s, CH2), 52.73 (s, CH), 30.50 (s, CH2), 29.45 (s, CH2). The solids filtered from the reaction were collected, and any remaining product was recovered via Soxhlet extraction using THF as the solvent. The solvent was removed in vacuo, and the fractions of amino diol were combined. This method was applied in the synthesis of 2-amino-1,4-butanediol (C4) from aspartic acid. Aspartate dimethyl ester: 1H NMR (400 MHz, DMSO-d6) δ: 7.58 (s, 1H), 4.13 (t, 3JH−H = 5.8 Hz, 1H), 3.69 (s, 1H), 3.62 (s, 1H), 2.94 (d, 3JH−H = 5.8 Hz, 1H). 13C NMR (101 MHz, DMSO-d6) δ: 170.10 (s, CO), 169.95 (s, CO), 52.62 (s, CH3), 51.89 (s, CH3), 48.99 (s, CH), 35.12 (s, CH2). 2-Amino-1,4-butanediol: 1H NMR (400 MHz, DMSO-d6) δ: 3.46 (m, 1H), 3.31−3.09 (m, 1H), 2.76−2.66 (m, 1H), 1.58−1.19 (m, 1H).

Furthermore, we compare the effect of extender chain length on the chemical, thermal, and mechanical properties of the resultant materials and show that the physical and mechanical properties of the resultant TPEUs are highly dependent on extender chain length. Most notably, however, the mode of degradation (bulk vs surface erosion) is affected by the extender design which could have a significant impact on their application in biomedical devices.

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EXPERIMENTAL SECTION

Materials and Instrumental Methods. Tetrahydrofuran (THF), ethyl acetate, methanol, diethyl ether, dimethyl sulfoxide (DMSO), sodium carbonate, sodium hydroxide, and magnesium sulfate were purchased from Fischer Scientific. 2-Amino-1,3-propanediol, L-aspartic acid, L-glutamic acid, ethyl isocyanate, thionyl chloride, lithium aluminum hydride, and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) were purchased from Sigma-Aldrich. Poly(ε-caprolactone) (PCL) was obtained from Perstorp (CAPA 2201A, Mw = 2000 g/mol, hydroxyl number = 54−58). Dicyclohexylmethane 4,4′-diisocyanate (H12MDI) was purchased from Tokyo Chemicals Industry Co. Ltd. and was distilled and stored under inert conditions. All chemicals were used as received unless otherwise stated. NMR Spectroscopy. 1H and 13C NMR spectra were obtained on a Bruker DPX-400 spectrometer (400 MHz) at 293 K. All chemical shifts were reported as δ in parts per million (ppm) and referenced to the residual solvent signal ((CD3)2SO: 1H, δ = 2.50 ppm; 13C, δ = 39.52). Size Exclusion Chromatography. Size exclusion chromatography (SEC) was used to determine the dispersities (ĐM) and molecular weights of synthesized polymers. SEC was conducted in dimethylformamide (DMF) using a Varian PL-GPC 50 system equipped with 2 × PLgel 5 μM MIXED-D columns in series and a differential refractive index (RI) detector at a flow rate of 1.0 mL min−1. The systems were calibrated against Varian Polymer Laboratories Easi-Vial linear poly(methyl methacrylate) (PMMA) and analyzed by the software package Cirrus v3.3. Tensiometric Analysis. Tensile data were obtained at ambient temperature by axially loading “dog-bones” in a Tensiometric M1001CT system with a load cell capacity of 1 kN and crosshead speed of 5 mm min−1 with a premeasured grip-to-grip separation All values reported were obtained from an average of 10 repeat specimens and the results were recorded using winTest v4.3.2 software. Molten polymer samples were molded into “dog-bones” via compression molding at 100 °C using a PTFE mold and allowed to cool to ambient temperature. All TPUs were annealed for 5 days in an incubator at 25 °C. Wide-Angle X-ray Diffraction. Wide angle X-ray diffraction data were obtained using a Panalytical X’Pert Pro MPD equipped with a Cu Kα1 hybrid monochromator (λ = 0.154 nm) as the incident beam optics, and the PiXcel detector was processed using OriginPro 8 software. Each polymer was compression molded into discs and allowed to anneal for 5 days in an incubator at 25 °C before standard “powder” 2θ−θ diffraction scans were carried out at room temperature in the angular range between 5° and 60° 2θ. Dynamic Mechanical and Thermal Analysis (DMTA). Dynamic mechanical thermal analysis (DMTA) data were obtained using a Mettler Toledo DMA 1 star system and were analyzed using the software package STARe V13.00a (build 6917). DMTA samples were analyzed by single cantilever bending, oscillating at a frequency of 5.0 and 0.5 MHz with a displacement of 10 mm between −80 and 180 °C at a heating rate of 2 °C min−1. All polymers were analyzed using a Mettler Toledo DMA 1 Star system to determine the thermomechanical properties and glass transitions of the materials. Polymer samples were molded into “bars” via compression molding using a PTFE mold and allowed to cool to ambient temperature. All TPEUs were annealed for 5 days in an incubator at 25 °C. Degradation Studies. Accelerated degradation studies were conducted under conditions previously reported by Lam et al.36 All polymers which could form “degradation disks” were subjected to 6903

DOI: 10.1021/acssuschemeng.7b01110 ACS Sustainable Chem. Eng. 2017, 5, 6902−6909

Research Article

ACS Sustainable Chemistry & Engineering C NMR (101 MHz, DMSO-d6) δ: 66.48 (s, CH2), 58.73 (s, CH2), 50.50 (s, CH), 36.05 (s, CH2). General Synthesis of Protected 2-Ethylurea Diol Extenders. The urea-protected 2-amino-1,3-propanediol extender (C3u) was synthesized according to literature.21 In a 1 L round-bottom flask fitted with a magnetic stirrer bar, 2-amino-1,3-propanediol (10 g, 1.10 × 10−1 mol) was dissolved in a mixture of methanol/THF (500 mL, 1:2, respectively). The solution was cooled on an ice-bath for 20 min before the addition of ethyl isocyanate (8.69 mL, 1.10 × 10−1 mol), and the mixture was allowed to stir for 15 min. The reaction mixture was removed from the ice bath, allowed to warm to room temperature, and was stirred for 4 h. The solvent was removed in vacuo and the white off-white solid was suspended and stirred in 400 mL of diethyl ether for 20 min before being collected by filtration. The white crystalline solid was dried in vacuo to yield the pure urea-protected extender as a fluffy white solid. (16.04 g, yield 90%). Analysis was consistent with previous reports.3 1H NMR (400 MHz, DMSO-d6) δ: 5.98 (t, 3JH−H = 5.5 Hz, 1H), 5.64 (d, 3JH−H = 7.8 Hz, 1H), 4.64 (t, 3 JH−H = 5.3 Hz, 2H), 3.55−3.45 (m, 1H), 3.44−3.27 (m, 4H), 3.05− 2.93 (m, 2H), 0.97 (t, 3JH−H = 7.2 Hz, 3H). 13C NMR (101 MHz, DMSO-d6) δ: 157.85 (s, CO), 60.47 (s, CH2), 52.64 (s, CH), 33.92 (s, CH2), 15.63 (s, CH3). This method was applied in the synthesis of 2-ethylurea-1,4butanediol (C4u) and 2-ethylurea-1,5-pentanediol (C5u) extenders. C4u: 1H NMR (400 MHz, DMSO-d6) δ: 5.88 (t, 3JH−H = 5.4 Hz, 1H), 5.67 (d, 3JH−H = 8.3 Hz, 1H), 4.71 (t, 3JH−H = 5.2 Hz, 1H), 4.50 (t, 3JH−H = 5.4 Hz, 1H), 3.63−3.51 (m, 1H), 3.43−3.21 (m, 4H), 3.03−2.93 (m, 2H), 1.68−1.32 (m, 2H), 0.96 (t, 3JH−H = 7.2 Hz, 3H). 13 C NMR (126 MHz, DMSO-d6) δ: 158.56 (s, CO), 64.07 (s, CH2), 57.98 (s, CH2), 48.39 (s, CH), 35.03 (s, CH2), 34.16 (s, CH2), 15.79 (s, CH3). Anal. Calcd for C7H16N2O3: C, 47.71; H, 9.15; N 15.90%. Found: C, 47.70, H, 9.15, N, 15.91%. C5u: 1H NMR (400 MHz, DMSO-d6) δ: 5.77 (t, 3JH−H = 5.4 Hz, 1H), 5.56 (d, 3JH−H = 8.4 Hz, 1H), 4.62 (t, 3JH−H = 5.2 Hz, 1H), 4.36 (t, 3JH−H = 5.1 Hz, 1H), 3.53−3.41 (m, 1H), 3.40−3.28 (m, 3H), 3.28−3.16 (m, 1H), 3.03−2.92 (m, 2H), 1.58−1.16 (m, 4H), 0.96 (t, 3 JH−H = 7.2 Hz, 3H). 13C NMR (101 MHz, DMSO-d6) δ: 157.91 (s, CO), 63.95 (s, CH2), 60.89 (s, CH2), 50.73 (s, CH), 33.96 (s, CH2), 29.12 (s, CH2), 28.11 (s, CH2), 15.64 (s, CH3). Anal. Calcd for C8H18N2O3: C, 50.51; H, 9.54; N, 14.73%. Found: C, 5.51, H, 9.54, N, 14.73%. General Synthesis of TPEUs from 2-Ethylurea-diol Extenders. The TPU synthesis described is based on the synthesis of a polyurethane with a 30% hard-block composition using the C3u extender (30C3u). In a clean and dry vial fitted with a magnetic stirrer bar, poly(εcaprolactone) with a MW = 2000 g mol−1 (PCL2k) (1 g, 5 × 10−4 mol) and DBU (3.73 μL, 6.9 × 10−6 mol) were heated to 100 °C under a flow N2 for 20 min to aid the reduction of water within the system. The molten mixture was cooled to 80 °C before the addition of H12MDI (402 μL, 1.64 × 10−3 mol). The reaction mixture was allowed to stir under N2 for 40 min to allow for prepolymer formation before the addition of a solution of C3u extender (185 mg, 1.14 × 10−3 mol) dissolved in 150 μL of DMSO. The polymerization was allowed to proceed for 2 h before being removed from the heat to retard further reaction. The molecular weight was analyzed by GPC and any unreacted isocyanate was seen by IR spectroscopy, and was quenched by washing the TPU in 50 mL of methanol before being dried in vacuo. 1 H NMR (400 MHz, DMSO-d6) δ: 6.04−5.67 (m, 2H), 4.11−3.80 (m, 19H), 3.53−2.89 (m, 23H), 2.27 (m, 14H), 1.91−0.69 (m, 79H). Mn = 100.9 kg·mol−1, ĐM = 2.10 (RI detection, DMF GPC). 13

Scheme 1. Conversion of Amino Acids to 2-Ethylurea Diol Chain Extendersa

Reagents and conditions: (i) MeOH, thionyl chloride, 0−25 °C, 12 h; (ii) THF, LiAlH4, 0−25 °C, 10 h; (iii) methanol/THF, ethyl isocyanate, 5 h. n = 1 or 2.

a

aspartate and glutamate dimethyl esters to the corresponding C4 and C5 amino diols respectively and was verified by FT-IR and 13C APT NMR spectroscopy (Figures S1,S2). Furthermore, the relatively large lab scale synthetic accessibility of this process was highlighted by conducting the methylation of the Asp and Glu feedstocks on a 100 g scale with reasonable yields (>75%). It must be noted, however, that the subsequent reduction of the dimethyl esters to yield the 2-amino diols, was only conducted on a 25 g scale owing to issues observed in agitating the reaction at higher quantities. Employing a method previously described by Lacôte et al., commercially available serinol (C3) and previously synthesized 2-amino-1,4-butanediol (C4) and 2-amino-1,5-pentanediol (C5) were subsequently modified via a chemoselective reaction with ethyl isocyanate to yield analogous ethylurea protected diol chain extenders; C3u, C4u, and C5u, respectively, at yields >80% with no further purification required.23,24 The successful synthesis of the C3u, C4u, and C5u extenders were verified by FT-IR and 1H NMR spectroscopy and CHN elemental analysis (Figures S3−S5). Synthesis of C3u, C4u, and C5u-Based TPEUs. To investigate the effect of extender chain length on the physicochemical and thermo-mechanical properties of the resultant materials, TPEUs were synthesized via a “two-shot” method as we have previously reported,21 in which poly(εcaprolactone) (PCL, Mw = 2000 g·mol−1 as determined by SEC analysis) and 1-isocyanato-4-[(4-isocyanatocyclohexyl)methyl] cyclohexane (H12MDI) were chosen as the polyol and diisocyanate, respectively. The weight percentages of the “hard-segment” (%HS) of the final materials were determined based on the overall urethane content of the TPEUs (eq S1).25 The mole ratio of chain extender, polyol, and diisocyanate were calculated in order to yield TPEUs with %HS of 30, 45 and 60% (Table 1). By varying the %HS of the TPEUs and the chain extender length, it is possible to control the Table 1. Composition and Molecular Weight Comparison of C3u, C4u, and C5u-Based TPEUs polymera

wt % “hardsegment” (%HS)

H12MDI/polyol ratiob (R)

Mwc (kg·mol−1)

ĐMc

30C3u 45C3u 60C3u 30C4u 45C4u 60C4u 30C5u 45C5u 60C5u

30 45 60 30 45 60 30 45 60

3.28 5.36 9.12 3.21 5.22 8.74 3.14 5.09 8.50

100.9 96.5 89.7 119.0 97.0 88.4 94.9 91.8 108.7

2.10 2.23 2.26 2.14 2.19 2.31 2.11 2.10 2.46

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RESULTS AND DISCUSSION Synthesis of Amino Acid-Derived Diol Chain Extenders and TPEUs. Amino diol derivatives of L-aspartic acid (Asp) and L-glutamic acid (Glu) were synthesized by methylation of the native amino acids and subsequent reduction with LiAlH4, as previously described in the literature (Scheme 1,(i)/(ii)).22 This approach allowed for the complete conversion of the

a

Reagents and conditions: DMSO, 5 mol % DBU, 2 h, N2. Determined using eq S1. cDetermined by SEC analysis in DMF against poly(methyl methacrylate) (PMMA) standards. b

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DOI: 10.1021/acssuschemeng.7b01110 ACS Sustainable Chem. Eng. 2017, 5, 6902−6909

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ACS Sustainable Chemistry & Engineering

Figure 1. Size-exclusion chromatograms (RI detection) of (a) C3u, (b) C4u, and (c) C5u-based TPEUs (measured in DMF against poly(methyl methacrylate) standards).

Table 2. Tensile and Thermal Mechanical Properties of C3u, C4u, and C5u-Based TPEUs polymer

Ea (MPa)

εbreaka (%)

30C3u 45C3u 60C3u 30C4u 45C4u 60C4u 30C5u 45C5u 60C5u

13.0 ± 0.39 60.5 ± 0.98 167.5 ± 1.95 12.8 ± 0.40 57.9 ± 0.77 151.8 ± 1.64 32.8 ± 0.28 94.2 ± 0.82 170.4 ± 1.79

619 ± 19 309 ± 13 85 ± 6 755 ± 20 470 ± 11 56 ± 5 600 ± 14 335 ± 8 47 ± 3

UTSa (MPa)

E′ at 37 °Cb (MPa)

Tg of PCL “soft” segmentc (°C)

Tg of PU “hard” segmentc (°C)

± ± ± ± ± ± ± ± ±

15.4 77.8 183.9 15.8 61.3 179.4 69.4 104.3 201.1

−52.3 −50.9 −47.8 −55.0 −54.7 −49.0 −48.6 −47.1 −46.6

88.1 88.0 87.8 80.0 80.2 79.9 61.1 61.0 61.8

10.4 21.7 26.5 13.0 21.6 24.8 14.3 24.7 29.9

0.33 0.29 0.15 0.34 0.19 0.08 0.45 0.35 0.20

a Determined by tensiometric analysis (average of 10 samples, Figure 2). bDetermined by DMTA. cDetermined by DMTA, value taken as the peak maximum of tan δ.

the diisocyanate at ∼2275 cm−1 and ∼2250 cm−1 confirms complete consumption of the H12MDI linker (Figure S7). These spectroscopic trends are highly indicative of the successful incorporation of the 2-ethylurea diol chain extenders into the poly(ester-urethane) materials. Tensiometric Analysis of C3u, C4u, and C5u-Based TPEUs. In order to elucidate the effect of extender chain length (e.g., C3, C4 and C5) on the mechanical properties of the resultant TPEUs, materials were analyzed via tensiometric analysis. Dumbell samples were prepared by compression molding prior to annealing at 25 °C for 5 days in order to promote complete phase separation of the polyurethane “hard” segments and PCL “soft” segments. The annealed materials were subsequently subjected to axial loading at a constant crosshead speed of 5 mm·min−1 until mechanical failure. It was found for C3u, C4u and C5u-based TPEUs, typical polyurethane tensile properties were observed e.g. a proportionate increase in the Young’s modulus (E) and decrease in the elongation at break (εbreak) with increasing %HS, (E = 30 < 45 < 60%HS, εbreak= 30 > 45 > 60%HS). This is owed to the proportional increase of the “hard” polyurethane regions dictating the overall mechanical properties of the material. Furthermore, for all extender chain lengths, it was observed that at 60%HS the materials exhibited plastic deformation at low % strains (0.99 utilizing a Gaussian peak-fit analysis method previously described.21,33 It was found that when transitioning from C3u to the C4u extenders, TPEUs exhibited an increase in number of deconvoluted peaks (C3u, two peaks at 2θ = 20° and 26°; C4u, three peaks at 2θ = 20°, 25° and 28°) (Figure S10) which were indicative of an increase in minor phases and therefore decrease in uniformity. Furthermore, it was noted that for the C5u-based materials, that the number of deconvoluted peaks decreased relative to their C4u analogues (C5u, 2 peaks at 2θ = 20° and 25°) (Figure S10), indicating a return to uniformity. It is believed that because of steric hindrances, that the C3u-based “hard” segments are locked in a uniform arrangement; however, as a consequence of increased mobility, the C4u-based TPEUs are able to actualize more complex secondary structures with imperfect regularity. In addition to this, it is hypothesized that because of a further increase in flexibility, the C5u-based TPEUs are able to permit secondary structures with a higher degree of regularity (Figure 3). Dynamic Mechanical Thermal Analysis. To confirm the synthesis of phase-separated TPEUs, the materials were subjected to dynamic mechanical thermal analysis (DMTA) with the purpose of visualizing the glass transition temperature (Tg) of both the “hard” urethane-rich segment and the “soft” PCL segments. DMTA was chosen as it offers a superior degree of sensitivity to thermal transitions than other thermal analysis techniques, namely differential scanning calorimetry (DSC). Furthermore, DMTA can provide information on the mechanical properties of a material over a range of temperatures and frequencies of stress. Analogous to the preparation

Figure 2. Exemplar stress−strain curves of (a) C3u, (b) C4u, and (c) C5u-based TPEUs (Table 2). Experiments were conducted at ambient temperature (∼25 °C) at an elongation rate of 5 mm min−1 until failure.

uniform complex secondary structures, producing more rigid materials. Interestingly, however, a decrease in UTS and E was observed when transitioning from C3u-based materials to C4ubased materials. It is postulated that this is a consequence of the irregularities in the microstructure of the “hard” segment caused by only an incremental increase in flexibility of the extender yielding phase separated materials nonuniform secondary structures. To confirm this hypothesis, TPEU materials were analyzed by wide-angle X-ray diffraction. Wide Angle X-ray Diffraction Analysis. Wide angle X-ray diffraction (WAXD) analysis of C3u, C4u, and C5u extenders and resultant TPEUs was conducted with the aim of determining

Figure 3. WAXD diffractograms for (a) C3u (b) C4u, and (c) C5u-based TPEUs. 6906

DOI: 10.1021/acssuschemeng.7b01110 ACS Sustainable Chem. Eng. 2017, 5, 6902−6909

Research Article

ACS Sustainable Chemistry & Engineering

through single cantilever bending which is not observed during the application of axial loads during tensiometric analysis. Degradation Properties; Accelerated Hydrolytic Degradation, Water Uptake, Static Contact Angle. To assess the effect of extender chain-length on the hydrolytic degradability and water uptake behavior of the materials, C3u-, C4u-, and C5u-based TPEUs were subjected to accelerated degradation conditions in triplicate.36 Each of the TPEUs were processed into degradation disks by compression molding and annealing as previously described. The degradation disks were subsequently transferred into individual vials, submerged in a basic degradation media (5 M aqueous NaOH) and incubated at ambient body temperature (37 °C) with constant agitation at 60 rpm. Periodical weight measurements were taken of the surface-dried disks, using an analytical balance, until samples were irretrievable from the degradation media. As has been previously described in literature, poly(ester-urethane)s typically undergo bulk degradation, in which materials exhibit a slower rate of hydrolysis relative to water (or degradation media) ingression and therefore undergo an “uptake” phase.37−39 Because hydrophilicity is a significant factor in the rate of water uptake, static contact angle measurements were also made on solvent-cast films of the materials, so as to determine the hydrophilicity of the TPEUs.37,40 In the case of the C3u- and C4u-based TPEUs it was found that both percentage mass increase owing to water uptake and rate of water uptake increased proportionately with %HS. Notably, it was found that although the percentage mass increase of the C5u-based TPEUs followed the same trend, the initial rate of water uptake decreased with increasing %HS. More interestingly, it was noted that the overall percentage mass increase owing to water uptake decreased significantly with increasing extender chain length (Figure 5). It is hypothesized that this trend is a consequence of the increased hydrophobicity of the TPEU “hard” segments with increasing extender chain lengths, and therefore, hydrocarbon content. The increasing hydrophobicity of the materials was verified by static contact angle (SCA) measurement. It was found that the SCA increased proportionally when moving from the C3u-based TPEUs (60C3u SCA = 71.9°) to the C4u (60C4u SCA = 74.1°) and C5u (60C5u SCA = 79.9°) based materials, indicative of an increase in surface hydrophobicity (Figure 6). The degradation of the C4u and C5u-based TPEUs was found to be considerably slower than that of the C3u-based TPEUs (Figure 7). Interestingly, it was noted that with increasing extender chain length, the degradation profile of the materials

of materials for tensiometric analysis, TPEUs were compression molded into beams and subsequently annealed at 25 °C for 5 days. The annealed materials were subjected to transverse loading through single cantilever bending at a constant displacement (10 mm). The Tg, defined by the peak maxima of tan δ, and storage modulus (E′) at physiological temperature (37 °C) of the segmented TPEUs were determined. As anticipated, for all materials, it was found that the tan δ vs temperature curves exhibited two peak maxima attributed to the Tg of the “soft” PCL segment and “hard” polyurethane segment, indicative of a phase separated morphology (Table 2, Figure 4). In agreement with the previous reports, the Tg which

Figure 4. Tan δ vs temperature curves, comparison of 45C3u, 45C4u, and 45C5u TPEUs.

corresponds to the PCL “soft” segment was detected between −45 and −55 °C while the C3u-, C4u-, and C5u-based TPEUs exhibited a second Tg at ≈88 °C, ≈80 °C and ≈61 °C, respectively, corresponding to the PU “hard” segment (Figure 4).34,35 It is hypothesized that the reduction in Tg of the “hard” segments with increasing extender chain length is owed to the increased flexibility of the extenders allowing for a more efficient occupation of theoretical “free-volume”. In agreement with the tensile analysis, for all TPEUs, the storage modulus (E′) increased proportionately with increasing %HS. Furthermore, it was noted that TPEUs exhibited a higher E′, obtained by DMTA at 37 °C, than Young’s modulus (E), obtained through tensiometric analysis. It is hypothesized that this is a consequence of the culmination of both tensile and compressive strengths observed during the transverse loading

Figure 5. Percentage mass increase owing to water uptake for (a) C3u, (b) C4u, and (c) C5u-based TPEUs; average of three comparable samples. 6907

DOI: 10.1021/acssuschemeng.7b01110 ACS Sustainable Chem. Eng. 2017, 5, 6902−6909

Research Article

ACS Sustainable Chemistry & Engineering

Figure 6. Static contact angle measurements for 60C3u, 60C4u, and 60C5u TPEUs.

Figure 7. Hydrolytic degradation of (a) C3u, (b) C4u, and (c) C5u-based TPEUs. Percentage mass loss of an average of three comparable samples. 1

became more linear. It is hypothesized, in conjunction with the water uptake and static contact angle data, that this is again attributed to the increased hydrophobicity of the TPEUs. This progression toward a more linear degradation profile, alongside the decrease in water uptake, is suggestive that the mode of degradation is shifting from bulk degradation to a more surface erosion-type mechanism (Figure 7).37,40

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

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ORCID

Andrew P. Dove: 0000-0001-8208-9309

CONCLUSIONS It has been demonstrated that hydroxyl and carboxylic acid functional amino acids, such as serine, aspartic acid, and glutamic acid, offer a facile route to modifiable extenders for the synthesis of thermoplastic polyurethanes. It was found that the composition %HS of these materials could be accurately calculated and utilized in order to determine the thermal and mechanical properties of the resultant polymers. Furthermore, the variation of extender chain length could be effectively employed to control the morphological properties, hydrophilicity (C3u > C4u > C5u) and water uptake capabilities (C3u > C4u > C5u) and in turn the mode of degradation, from bulk degradation (C3u) to surface erosion (C5u). The tunable nature of the materials in conjunction with utilization of extenders derived from natural resources offers a unique and exciting platform for the advancement of sustainable elastomeric materials.

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H NMR spectra and deconvoluted diffractograms for the TPEUs (PDF)

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Lubrizol Corporation and the University of Warwick are thanked for funding to support the studentship (R.B.). The Royal Society are acknowledged for funding an Industrial Fellowship to A.P.D.

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REFERENCES

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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/acssuschemeng.7b01110. 13 C APT NMR spectra, FT-IR spectra, WAXD diffraction patterns of C3u, C4u, and C5u extenders, and 6908

DOI: 10.1021/acssuschemeng.7b01110 ACS Sustainable Chem. Eng. 2017, 5, 6902−6909

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DOI: 10.1021/acssuschemeng.7b01110 ACS Sustainable Chem. Eng. 2017, 5, 6902−6909