Synthesis and thermomechanical characterization of polyurethane elastomers extended with...

Polymer Degradation and Stability 92 (2007) 1316e1325www.elsevier.com/locate/polydegstab

Synthesis and thermo-mechanical characterization ofhigh performance polyurethane elastomers based on

heterocyclic and aromatic diaminechain extenders

Rasha A. Azzam a, Sahar K. Mohamed b, Rob Tol b, Valja Everaert c,Harry Reynaers b, Bart Goderis b,*

a Department of Chemistry, Helwan University, Ain-Helwan, Cairo 11795, Egyptb Molecular and Nanomaterials, Chemistry Department, Catholic University of Leuven, Celestijnenlaan 200F, B-3001 Heverlee, Belgium

c Centexbel Gent, Technologiepark 7, B-9052 Zwijnaarde, Belgium

Received 6 February 2007; accepted 19 March 2007

Available online 30 March 2007

Abstract

Two series of 4,40-diphenylmethane diisocyanate (MDI) and poly(ethylene glycol adipate) (PEGA)-based polyurethane and polyurethaneureaelastomers were synthesized via a one-shot polymerization method and characterized using FTIR, 1H NMR and 13C NMR. The samples in thefirst series are extended by aliphatic diol chain extenders while in the second series mixtures of aliphatic diols and furanic or aromatic diaminechain extenders are used. TGA experiments revealed that with furanic or aromatic diamine chain extenders the polymer degradation temperatureis shifted 90 �C upwards, irrespective of the annealing time at 100 �C according to ASTM 0573-99. The values of Young’s modulus and of thetensile strength are higher and the strain at break is lower for the samples in series 2 compared to those in series 1. Increasing the annealing timeat 100 �C lowers Young’s modulus. Dynamic mechanical thermal analyses points to a progressive microphase separation with annealing time.� 2007 Elsevier Ltd. All rights reserved.

Keywords: Polyurethane; Degradation; Mechanical properties; Microphase seperation

1. Introduction

Polyurethane (PU) elastomers are produced through the re-action of diisocyanates with a mixture of macroglycols (oftenpolyethers or polyesters) and low molecular weight diols or di-amines as chain extender. PU often exhibits microphase-sepa-rated morphologies. The soft segment (SS) domains areprimarily composed of the aliphatic polyether or polyestersegments and impart elastic properties to the material. Thehard segments (HS) in the hard domains are composed of dii-socyanate chains, linked with the chain extenders and are inthe glassy or semicrystalline state at room temperature. They

* Corresponding author. Tel.: þ32 16 327806; fax: þ32 16 327990.

E-mail address: [email protected] (B. Goderis).

0141-3910/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.polymdegradstab.2007.03.017

act as thermally reversible and multifunctional physical cross-links between the soft domains and also as reinforcing fillers,providing dimensional stability [1]. The separation process isprimarily driven by the strong intermolecular interaction be-tween the HS via hydrogen bonding but is also influencedby the intrinsic chemical nature of the components, the HS/SS stoichiometric ratio, the segment length distribution andthe processing history.

Typical PUs are known to exhibit small resistance to heat.PUs with HS based on 4,40-diphenylmethane diisocyanate(MDI) and SS based on polyesters are relevant to the presentwork. The HS urethane degradation involves either a depoly-condensation reaction back to alcohol and isocyanate or cleav-age of the CeO bond of the urethane through a cyclicmechanism that generates amine, olefin and CO2. The mass

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1317R.A. Azzam et al. / Polymer Degradation and Stability 92 (2007) 1316e1325

loss of HS based on MDI urethanes starts at about 245 �C [2].However, the formation of conjugated double bonds alreadyleads to a discoloration slightly above 200 �C [3]. Water needsto be avoided in SS, based on aliphatic polyesters, to preventhydrolysis during heat treatments. Otherwise, such polyester-based SS often degrade in two steps with the release of CO2

in the range 220e450 �C and a second mass loss in the range435e530 �C [4]. The shorter the aliphatic segments in thepolyesters, the larger is the mass loss in the 220e450 �Crange. As a result, the mass loss of MDI/polyester PUs asa function of temperature under thermo-oxidative conditionsis often a three-step process. The mass loss in the first stepis related to the HS content and the loss in the second andthe third steps is proportional to the SS content. An additionalstep can occur between 500 and 600 �C, corresponding to thedecomposition of the char formed in the previous steps [5].Besides the chemical nature of the components also morpho-logical items are relevant when discussing degradation. Ithas been suggested that the higher the thermal stability andthe HS crystallinity, the higher the degradation temperatureof the soft segments is [2].

Attempts have been made to increase the thermal stabilityof PU through chemical modification of its structure via e.g.the incorporation of silicone [6], imide [7e9] or thermally sta-ble heterocyclic groups [10] like e.g. furanic diol [11]. Thecross-linking ability of hydroxy-terminated polybutadiene SShas been exploited as well [12,13]. Furthermore, diaminechain extenders have been used and include hydrazine or ali-phatic [14], aromatic [13] and heterocyclic diamines [15].

With diol chain extenders urethane linkages are formed,whereas diamine chain extenders produce urea linkages. Ithas been suggested that the higher thermal stability of ureacompared to urethane linkages results from the higher hydro-gen bonding (H-bond) capacity of urea groups [14,16]. Ure-thane linkages give raise to mutual monodentate H-bondswith a bond energy of 18.4 kJ/mol, whereas urea linkages re-sult in mutual bidentate H-bonds with a higher bond energy of21.8 kJ/mol [17]. The H-bonds in a model urethane dissociateduring melting at about 120 �C whereas the H-bonds ofa model urea do not dissociate prior to degradation, which oc-curs far above 200 �C [17]. This finding was confirmed in laterstudies where inter-urethane H-bond dissociation was found tostart above 90 �C and to disappear above 130 �C [18]. In prin-ciple the presence of H-bonds tends to reinforce the samples.Therefore, often already above 90 �C the mechanical proper-ties of regular polyurethanes are no longer acceptable, limitingtheir applicability [19,20]. In contrast, polyurethaneureas often

show higher softening temperatures and lower thermoplasticitythan polyurethanes [21].

However, as mentioned above, H-bond configurations alsoindirectly act on the mechanical properties as they influencethe degree of microphase separation (the domain formation).The H-bonds exist between the HS mutually or between theHS and SS, where the NH groups act as proton donors andthe urethane, urea carbonyl, ester carbonyl (in polyester SS)or ether oxygen (in polyether SS) as proton acceptors. Fre-quent H-bonding between HS is associated with a high degreeof phase separation, whereas H-bonding between HS and SS isan indication of phase mixing [22].

Pronounced phase separation may bring better mechanicalproperties as well as hard domains with a high degree of order-ing may do [23]. Increasing the HS content increases the de-gree of microphase separation [24] and facilitates the mutualH-bonding between the HS [25]. Diamine chain extended co-polymers often exhibit more pronounced degrees of phase sep-aration at comparable HS contents [1]. However, in a quantummechanical study for the effect of H-bonding it was stated that,based on the relative magnitudes of hydrogen bond energies,considerable phase mixing between HS and SS in urethanesand ureas can be expected [26].

In this paper we explore and discuss the use of a mixture ofaliphatic diols and heterocyclic or aromatic diamines as chainextender in the synthesis of polyurethaneurea elastomers andthe intended beneficial effects on the sample thermal stabilityand mechanical properties, as compared to when pure aliphaticdiols are used as chain extenders.

2. Experimental

2.1. Chemicals

4,40-Diphenylmethane diisocyanate (MDI), ethylene glycol(EG), butylenediol (BD), hexamethylene glycol (HD), 4,40-methylenebisbenzeneamine (MDA) and dimethylformamide(DMF) were purchased from Sigma-Aldrich Chemical Co.2,5-Furandimethanamine and 2-furanyl-1,2-ethanediaminewere supplied by Rare Chemicals GmbH, Germany. Fig. 1 rep-resents the chemical structures of the monomeric furanic andaromatic diamines used in this study.

Triethylene diamine (Dabco EG), the urethane catalyst, andDabco LK-221, a non-silicon-based surfactant used to improvethe emulsification of the various components and their subse-quent reactions were supplied by Air Products and Chemicals.Poly(ethylene glycol adipate) of molar mass 4000 g/mol

OH2NH2C CH2NH2

(1)

O

CH2 NH2H2NHC

(2)

H2N CH2 NH2

(3)

Fig. 1. Furanic and aromatic diamine chain extenders: (1) 2,5-furandimethanamine, (2) 2-furanyl-1,2-ethanediamine and (3) 4,40-methylenebisbenzeneamine.

1318 R.A. Azzam et al. / Polymer Degradation and Stability 92 (2007) 1316e1325

(PEGA) and of 500 g/mol (s-PEGA) were obtained from Coims.p.u., Italy. Prior to use, MDI was melted at 80 �C for 24 hand kept thereafter at 50 �C under a nitrogen blanket. Thepolyesters and all the diols were heated to 70 �C under vacuumfor 18 h to ensure the removal of all air bubbles and watervapor that may otherwise interfere with the isocyanatereactions. All other materials were used as-received.

2.2. Synthesis of the thermoplastic polyurethaneelastomers

A polyol blend of the poly(ester) diol, the chain extenders,and the various additives were prepared by mixing 0.0285 molof polyester (PEGA) and 0.0879 mol of the chain extender(s)with 0.0056 mol of Dabco EG, and 0.1729 g of the LK-221surfactant for 20 min using a mechanical stirrer running ata speed of 3000 rpm. In the case of two different chain ex-tenders, equal moles of the chain extenders were used. Ex-treme care was taken when operating with MDA for itswell-known toxicity. Nine different thermoplastic polyure-thane (TPU) and polyurethaneurea (TPUU) elastomers wereproduced using different chain extender combinations. Thecompositions of the various samples are given in Table 1.The one-shot elastomers were prepared by mixing the polyolblend with a total of 0.1365 mol MDI at 50 �C for a few sec-onds using a mechanical stirrer running at a speed of5000 rpm, thereby aiming at a statistical distribution of thehard domain-sequences throughout the sample. The MDIweight was predetermined to ensure the complete conversionof all the reactive isocyanate, amine, and hydroxyl-functionalgroups, i.e., index 1. A Haake mixer with two counter-rotatingroller blades was then used to extrude the samples. The extru-sion temperature was 180e200 �C. The resulting beads werethen used to mold tensile bars by injection molding using

Table 1

Composition of the different thermoplastic polyurethanes and polyuretha-

neurea samples

Sample code Diisocyanate/polyol Diol Aromatic or

heterocyclic diamine

Series 1

TPU-I MDI/PEGA s-PEGA eTPU-II MDI/PEGA EG e

TPU-III MDI/PEGA BD e

TPU-IV MDI/PEGA HD e

Series 2

TPUU-V MDI/PEGA BD 3

TPUU-VI MDI/PEGA HD 2

TPUU-VII MDI/PEGA HD 1

TPUU-VIII MDI/PEGA EG 1

TPUU-IX MDI/PEGA BD 1

MDI¼ 4,40-diphenylmethane diisocyanate.

PEGA¼ poly(ethylene glycol adipate) (M. wt.¼ 4000 g/mol).

s-PEGA¼ low molecular weight poly(ethylene glycol adipate) (M. wt.¼500 g/mol).

EG¼ ethylene glycol.

BD¼ butylenediol.

HD¼ hexamethylene glycol.

1, 2, 3 Are the heterocyclic and aromatic diamines shown in Fig. 1.

a micro-extruder, designed and manufactured by DSMResearch (Geleen, The Netherlands). The extruder tempera-ture was set between 165 and 210 C, to ensure operation belowthe polymer specific degradation temperature (see Section 3).The mold temperature varied between 50 and 70 �C. Theapplied pressure ranged from 5 to 10 MPa.

2.3. Characterization

2.3.1. Spectral analysisFourier transform infrared (FTIR) spectra of thin films were

obtained in the transmission mode using a Nicolet FTIR Ma-gna-IR 560 system. TPU films were prepared by spontaneousevaporation at room temperature of a diluted solution of thepolyurethane in N,N0-dimethylformamide (DMF) placed ona KBr window. The spectra covered the infrared region4000e400 cm�1, the number of scans per experiment was32 and the resolution was 4 cm�1. The 1H NMR and 13CNMR spectra were recorded on a Bruker AC-250 spectrometerat 250 and 62.5 MHz, respectively, in dimethyl sulfoxide(DMSO-d6) using tetramethylsilane as an internal reference.The chemical shift (d) and coupling constant (J ) data arequoted in parts per million and hertz, respectively. The follow-ing abbreviations are used to describe the peak patterns whereappropriate: s¼ singlet, d¼ doublet, t¼ triplet, q¼ quartetand m¼multiplet.

2.3.2. Accelerated thermal treatment (annealing)The accelerated thermal treatment (annealing) was per-

formed on the extruded TPU and TPUU materials using theoven method as described in ASTM 0573-99. Specimens foraging were placed in an air oven preheated at 100 �C. The an-nealing time began at the time the specimens were placed inthe oven and continued for time intervals of 2, 4, 7 and 14days at 100 �C. Specimens without an annealing step are re-ferred to as samples with 0 days of annealing. After aging,the samples were removed from the oven, cooled to room tem-perature on a flat surface and allowed to stand for at least 2weeks prior to further characterization.

2.3.3. Dynamic mechanical analyses (DMA)A dynamic mechanical analyzer DMA 2980 of TA instru-

ment was used for testing all the samples. Rectangular sampleshaving a size of about 17.9� 4.5� 1.5 mm were tested in thesingle cantilever mode to measure the storage modulus (E0),the loss modulus (E00) and the phase angle as expressed bytan d. All data were obtained using a frequency of 1 Hz. Sam-ples were cooled from room temperature to �100 �C, kept iso-thermally at this temperature for 5 min to equilibrate andsubsequently heated to 150 or 200 �C at a rate of 2 �C/min.The glass transition temperature was taken at the temperatureof the maximum in the loss modulus.

2.3.4. Thermogravimetric analysis (TGA)TGA analyses were carried out using a TGA Q500 V6.3

equipped with a standard furnace. Samples of 10 mg placedin a platinum pan, were heated over the range 30e600 �C at


10 �C/min in flowing gas (O2/N2¼ 60/40) at a flow rate of60 ml/min.

2.3.5. Tensile testingRoom temperature tensile tests were carried out on a Zwick

Z020 universal tensile machine equipped with a 100 N or20 kN load cell, the strain being equal to the relative clampdisplacement. Stressestrain curves and elongation at breakwere obtained by using the 20 kN load cell and adoptinga strain rate of 20 mm/min. The stress at break is referred toas the tensile strength. The starting clamp distance was69.57 mm. For measuring the modulus the 100 N load celland a test speed of 10 mm/min were applied. The grip-to-grip separation was 57.1 mm. Each test was repeated 3 timesand averages are reported.

3. Results and discussion

For simplicity the polymers in which only aliphatic diolchain extenders were used TPU-[IeIV], will be referred toas series 1 and the polymers having a mixture of aliphaticand aromatic or heterocyclic diamine chain extendersTPUU-[VeIX] will be referred to as series 2.

3.1. Molecular characterization

3.1.1. 1H NMR and 13C NMR of TPUs and TPUUs IeIX

3.1.1.1. TPU-I. 1H NMR (250 MHz, DMSO-d6): 7.36e7.21(m, 8H); 5.68 (broad, 4H); 4.09e4.03 (m, 8H); 3.84 (s, 2H);2.57 (t, J3¼ 7.1 Hz, 4H); 2.38 (t, J3¼ 7.1 Hz, 4H); 1.71e1.59 (m, 8H).

13C NMR (62.5 MHz, DMSO-d6): 173.6, 170.9, 149.2(CO); 138.8, 136.3 (ArC); 131.9, 117.0 (ArCH); 65.8, 61.0,39.8, 34.6, 34.2, 24.9, 23.8 (CH2).

3.1.1.2. TPU-II. 1H NMR (250 MHz, DMSO-d6): 7.42e7.10(m, 8H); 6.55 (broad, 3H); 4.42 (t, J3¼ 7.2 Hz, 4H); 4.07e4.03 (m, 2H); 3.94 (s, 2H); 2.62 (t, J3¼ 7.2 Hz, 4H); 2.32 (t,J3¼ 6.9 Hz, 4H); 1.68e1.61 (m, 4H).

13C NMR (62.5 MHz, DMSO-d6): 172.5, 169.9, 153.8,153.4, 148.4 (CO); 138.0, 136.3, 135.6, 134.1, 132.2 (ArC);131.7, 131.2, 119.5, 118.7, 116.8 (ArCH); 68.8, 65.5, 56.2,40.5, 39.8, 34.4, 34.5, 24.7, 23.9 (CH2).

3.1.1.3. TPU-III. 1H NMR (250 MHz, DMSO-d6): 7.52(broad, 1H); 7.36e7.12 (m, 8H); 6.67 (broad, 2H); 4.14e4.03 (m, 8H); 3.55 (s, 2H); 2.50e2.42 (m, 4H); 2.02e1.93(m, 2H); 1.75e1.64 (m, 4H).

13C NMR (62.5 MHz, DMSO-d6): 173.6, 169.5, 152.9,152.7, 148.9 (CO); 138.2, 136.0, 135.1, 134.0 (ArC); 133.1,131.2, 118.9, 118.3, 117.3 (ArCH); 66.7, 65.9, 62.0, 40.1,34.5, 34.2, 25.7, 24.8, 23.7 (CH2).

3.1.1.4. TPU-IV. 1H NMR (250 MHz, DMSO-d6): 7.26e7.10(m, 4H); 7.45e7.30 (m, 4H); 7.61 (broad, 3H); 4.18 (t,J3¼ 7.0 Hz, 4H); 4.05e4.00 (m, 6H); 3.75 (s, 2H); 2.75 (t,

J3¼ 7.2 Hz, 2H); 2.36 (t, J3¼ 7.2 Hz, 2H); 1.74e1.62 (m,6H); 1.41e1.38 (m, 2H).

13C NMR (62.5 MHz, DMSO-d6): 174.2, 172.1, 153.5,148.9 (CO); 138.5, 138.2, 136.3, 136.1, 135.1 (ArC); 133.0,132.2, 119.9, 117.8, 117.7 (ArCH); 65.9, 65.2, 61.0, 40.3,34.6, 34.4, 27.7, 25.1, 24.8, 23.8 (CH2).

3.1.1.5. TPUU-V. 1H NMR (250 MHz, DMSO-d6): 7.90(broad, 2H); 7.65 (broad, 8H); 7.45e7.17 (m, 32H); 4.14e4.03 (m, 8H); 3.75 (s, 8H); 2.57e2.34 (m, 8H); 1.98e1.95(m, 4H); 1.68e1.60 (m, 4H).

13C NMR (62.5 MHz, DMSO-d6): 173.5, 169.9, 153.9,152.6, 148.9 (CO); 138.7, 138.3, 138.0, 136.4, 135.2, 134.0,133.0 (ArC); 131.9, 131.0, 126.0, 120.0, 118.8, 118.5, 116.0(ArCH); 66.1, 65.0, 60.8, 40.1, 39.7, 34.9, 34.7, 26.0, 25.5,24.8, 23.8 (CH2).

3.1.1.6. TPUU-VI. 1H NMR (250 MHz, DMSO-d6): 7.56(broad, 2H); 7.40e7.25 (m, 24H); 6.88 (dd, J4¼ 0.9,J3¼ 5.1 Hz, 1H); 6.11 (ddd, J4¼ 1.1, J4¼ 1.0, J3¼ 5.2 Hz,1H); 5.72 (dd, J4¼ 1.1, J3¼ 5.2 Hz, 1H); 4.85 (dt, J4¼ 1.1,J3¼ 7.2 Hz, 1H); 4.34e4.20 (m, 8H); 4.05 (s, 6H); 3.55 (d,J3¼ 7.2 Hz, 2H); 2.67e2.30 (m, 8H); 1.75e1.55 (m, 8H).

13C NMR (62.5 MHz, DMSO-d6): 175.6, 173.1, 161.1,154.9, 153.5, 151.9, 149.5 (CO); 141.9, 138.9, 138.0, 136.9,136.6, 135.9, 134.9 (ArC); 144.1, 131.9, 131.7, 131.1, 127.0,122.5, 118.5, 118.1, 117.0, 110.2, 95.2 (ArCH); 66.2, 65.9,65.2, 61.9, 50.2, 46.0, 41.2, 39.2, 34.6, 34.2, 27.5, 25.9,24.5, 23.1 (CH2).

3.1.1.7. TPUU-VII. 1H NMR (250 MHz, DMSO-d6): 7.61(broad, 10H); 7.42e7.20 (m, 24H); 6.42 (d, J3¼ 5.0 Hz,2H); 4.30e4.22 (m, 8H); 4.01 (s, 6H); 3.80 (s, 4H); 2.70e2.38 (m, 8H); 1.68e1.38 (m, 8H).

13C NMR (62.5 MHz, DMSO-d6): 175.5, 172.3, 166.5,158.5, 154.7, 154.0, 153.9, 153.2 (CO); 149.1, 139.0, 137.4,136.9, 135.9, 134.9, 134.0, 133.3 (ArC); 132.2, 131.9, 131.0,126.2, 120.9, 120.5, 119.5, 116.7, 94.0 (ArCH); 67.5, 66.4,66.1, 62.0, 41.5, 41.2, 35.6, 35.4, 26.7, 25.3, 25.0, 23.9 (CH2).

3.1.1.8. TPUU-VIII. 1H NMR (250 MHz, DMSO-d6): 7.52(broad, 10H); 7.41e7.09 (m, 24H); 6.31 (d, J3¼ 5.2 Hz,2H); 4.45 (t, J3¼ 5.2 Hz, 4H); 4.20e4.12 (m, 8H); 4.01 (s,6H); 3.69 (s, 4H); 2.62e2.33 (m, 8H); 1.72e1.64 (m, 8H).

13C NMR (62.5 MHz, DMSO-d6): 174.5, 172.2, 166.1,158.9, 154.5, 154.2, 153.1, 152.9, 149.5 (CO); 148.5, 138.9,137.4, 136.5, 135.9, 134.5, 134.0, 133.2 (ArC); 132.2, 131.8,129.9, 126.1, 122.0, 120.5, 119.0, 115.9, 92.5 (ArCH); 69.2,66.1, 62.3, 40.9, 40.2, 39.2, 34.6, 33.9, 25.2, 24.1 (CH2).

3.1.1.9. TPUU-IX. 1H NMR (250 MHz, DMSO-d6): 7.80(broad, 10H); 7.40e7.20 (m, 24H); 6.19 (d, J3¼ 5.2 Hz,2H); 4.21e4.15 (m, 8H); 3.99 (s, 6H); 3.78 (s, 4H); 2.62e2.39 (m, 8H); 1.99e1.90 (m, 4H); 1.65e1.58 (m, 8H).

13C NMR (62.5 MHz, DMSO-d6): 174.9, 173.2, 167.1,159.1, 154.7, 154.2, 153.8, 153.1 (CO); 148.5, 139.2, 137.9,137.1, 136.2, 135.1, 133.8, 133.0 (ArC); 132.9, 131.7, 129.9,


126.9, 121.2, 120.2, 119.1, 115.9, 93.5 (ArCH); 66.2, 65.9,62.1, 41.2, 40.2, 39.8, 34.2, 33.8, 26.5, 25.1, 24.0 (CH2).

3.1.2. FTIRFor all samples, the FTIR spectra revealed sharp peaks at

1730 cm�1 corresponding to the free urethane and/or estercarbonyl groups and at 1710 cm�1 corresponding to theH-bonded urethane and/or ester carbonyl groups. TPUUsVeIX showed a peak at 1694 cm�1 corresponding to thefree urea that is often incorporated within a broad bandobserved in the range of 1690e1660 cm�1, corresponding toH-bonded urea groups. These results are relevant in terms ofmolecular characterization but are unrelated to the phase mor-phology of the samples that were tested thermo-mechanicallybecause the sample history prior to the FTIR measurementswas different.

3.2. The degree of microphase separation via DMA

In principle the degree of microphase separation (DMS)can be evaluated by inspection of the glass transition temper-atures of the different phases. A single common glass transi-tion temperature (Tg) points to complete molecular mixingwhereas in the case of two completely immiscible componentsthe Tg values of the pure components are found. In the case ofpartially mixed phases, independent of whether the phasecomposition is a thermodynamic equilibrium or the processof phase separation is kinetically trapped in an intermediatestate, one observes as many Tg’s as there are ‘phases’, withtheir values in between the Tg values of the pure componentsdepending on the phase composition. DMA is a very sensitivetechnique to probe Tg’s, provided that no other transitions areinterfering, like e.g. the melting of one of the components.PEGA is a crystallisable polymer with its Tg (based on calo-rimetry) at �47 �C [27] and its melting point at 55 �C [28].As mentioned in the introduction the PU hard segment soften-ing usually starts at about 90 �C and is accompanied by theloss of H-bonds [19,20]. Polyurethaneurea (PUUs) based HSdomains often show higher softening temperatures [21].

Since the loss moduli of the series 2 samples exhibit moreclear maxima compared to the broad tan (d) peaks, (indicative)soft phase Tg values were extracted from the present DMAmeasurements via the loss modulus peak as listed in Table 2.

Although the SS in all the samples are the same they showdifferent values for the Tg. It is obvious that series 2 has higherTg values than series 1 and that the series 2 values decreaseupon annealing in contrast to the values in series 1. All Tg

values are higher than the mentioned literature value of�47 �C. However, this difference is mainly due to the measur-ing method. We also measured the soft phase Tg via differen-tial scanning calorimetry (DSC) and obtained values close to�48 �C for all series 1 samples but higher for the samplesin series 2 [29]. Therefore, it seems that rather pure SS phasesare present when Tg values close to �40 �C are observed viathe present DMA technique (series 1). The higher Tg valuesin series 2 points to the presence of HS in the soft domains.Furthermore, it seems that the series 2 DMS increases with

annealing since the Tg values shift downwards in the directionof the pure SS Tg. Note that according to the literature diaminechain extended copolymers often exhibit more pronounced de-grees of phase separation at comparable HS contents [1]. Thecounter-intuitive fact that the DMS in series 1 is larger than inseries 2 is at least partially due to kinetic problems (see Sec-tion 3.2.2.). Whether or not some (thermodynamic) repulsionbetween the urethane and urea linked HS truly promotes mix-ing, goes beyond the scope of this paper but deserves furtherattention in the future. We speculate that short HS sequencesare preferentially present in the soft domains.

Further details can be learned by inspecting the DMA dataover the entire temperature range, as is discussed below.

3.2.1. Effect of partial replacement with diaminechain extenders

Fig. 2 compares the DMA storage modulus (E0) and tan (d)behaviour of TPU-II with that of the ‘corresponding’ PUUfrom series 2, i.e. TPUU-VIII, where the chain diol chain ex-tenders are partially replaced with diamine #1 (see Fig. 1).This comparison highlights the effects due to adding this di-amine chain extender and is representative for the other ‘cor-responding’ mixtures in this work.

The storage modulus (Fig. 2 (a)), E0, in the glassy statehardly depends on whether or not a diamine chain extenderis used. There is a significant reduction in E0 at the soft seg-ment Tg. This softening is, however, smeared over a largertemperature range for the diamine-containing sample and,more clearly than the values in Table 2, illustrates that thesesamples contain a considerable amount of HS within the softphase. The width of the transition reveals the heterogeneousnature of the soft phase, i.e. the HS concentration within thesoft phase differs from spot to spot.

Both E0 curves reach a minimum value at given temperatureafter which a small increase of the storage modulus is ob-served, which is typical for a rubbery sample probed in defor-mation. In this range of increasing E0 values the rubberybehaviour is induced by the hard domains, acting as physicalcrosslinks between the soft domains. The storage moduli ofTPUU-VIII in this temperature range are higher, which mightbe due to dissolved HS in the rubbery, soft domain phase.

Table 2

Glass transition temperatures of soft domains as detected from the loss

modulus peak

Sample Tg (�C)a Tg (�C)b

TPU-I �40 �41

TPU-II �39 �39

TPU-III �37 �36

TPU-IV �37 �38

TPUU-V �34 �36

TPUU-VI �33 �37

TPUU-VII �33 �39

TPUU-VIII �29 �30

TPUU-IX �32 �36

a Corresponds to the 0 day-annealed samples.b Corresponds to the 14 days annealed samples.


We speculate that besides the rigid nature of the included seg-ments also H-bonding between the hard and soft segmentsmay contribute to the increase of the rubber plateau storagemodulus. The final decrease of the storage modulus is associ-ated with the onset of flow, necessarily involving a loss of co-hesion within the hard domains. This onset of flow for theTPU sample occurs prior to that in the TPUU sample, whichis consistent with the idea that H-bonds between urethanegroups dissociate at lower temperatures compared to that be-tween urea moieties. The temperatures where no longer canbe measured due to pronounced flow, agrees with literaturedata about the complete H-bond dissociation in urethanes[18] (130 �C, see sample TPU-II) and in model ureas [17](close to 200 �C, see sample TPUU-VIII) [17]. Withouta doubt, with diamine chain extenders the useful rubbery re-gion is considerably extended.

The tan d peak (Fig. 2(b)) is broader for the series 2 sam-ples compared to their series 1 counterparts, just as the storagemodulus transitions are smeared-out over a larger temperaturerange. The broad double-peaked tan d curves seem to be com-posed out of (at least) two events, which we associate with theglass transition of (at least) two kinds of soft matrix. The lowtemperature peak (Tg1) is due to the devitrification of the soft

TPU-IITPUU-VIII

TPU-IITPUU-VIII

10

100

1000

(a)

Sto

rag

e m

od

ulu

s (M

Pa)

-100 -50 0 50 100 1500.0

0.2

0.4

0.6

0.8(b)

Tan

d

elta

Temperature°C

-100 -50 0 50 100 150 200

200

Temperature°C

Fig. 2. Storage modulus (a) and tan d (b) as a function of temperature for

TPU-II and TPUU-VIII (0 day annealed samples).

domain fraction that contains hardly any HS while the high-temperature second transition (Tg2) is due to the glass transi-tion of soft domains richer in HS. The evolution with timeat 100 �C (see Section 3.2.2) reveals that the microphase sep-aration process is arrested at different states in the samples ofseries 2 and that for sure it did not reach the state of the sam-ples in series 1 (i.e. there is a kinetic rather than thermody-namic problem). In the tan d evolution, occasionally a verysmall peak appears with a maximum around 70 �C and an on-set around 55 �C, which can be explained as soft segment(PEGA) crystal melting [28].

3.2.2. Effect of annealing at 100 �CThe data in Table 2 illustrate how the series 2 soft domain

Tg values shift to lower temperatures as a result of annealing.From Fig. 3, sample TPUU-IX, we can see that annealing in-flates the split of the broad tan d transition into two peaks. Tg1

shifts to lower temperatures with annealing time due to an in-creasing DMS. The amount of the low Tg fraction likely in-creases with time at the expense of the Tg2 fraction, whichbecomes progressively more concentrated in hard segments,leading to an upward shift. At first sight the samples in series1 do not show significant changes as a result of annealing aswe can see from the virtually identical tan d curves inFig. 4. However, although the devitrification behaviour andhence the DMS seems to be unaffected, this annealing at100 �C does have an effect on the final flow behaviour as re-vealed from the E0 behaviour in Fig. 5 for e.g. TPU-III. Thedecrease in E0 is postponed and the actual flow (end of themeasurement) is shifted to a higher temperature, which pointsto a progressing H-bond stability within the hard domains withtime at 100 �C.

3.2.3. Effect of diol extender lengthTo address the features associated with a progressively in-

creasing diol chain extender length the DMA behaviour ofthe samples TPU-II, II and IV after 14 days of annealing iscompared in Fig. 6 since the used extenders, respectively,have 2, 4, and 6 methylene units. They almost have the

shift to higher temperature

Tg shift to lower temperature

0.0

0.2

0.4

0.6

0.8

Tan

d

elta (a.u

)

zero annealed4 days14 days

-50 0 50 100Temperature°C

Fig. 3. Tan d as a function of temperature for differently annealed TPUU-IX

samples.


same soft domain Tg value, indicating an equal DMS. It is notclear though how the different E0 levels in the rubber plateauregions should be explained.

Also TPUU-VIII was compared to TPUU-IX and TPUU-VII after 14 days of annealing as they differ only in the diolchain extender length (2, 4, and 6 methylene groups, respec-tively). Here the soft domain Tg value of TPUU-VIII is re-markably high (Table 2) and in Fig. 7 TPUU-VIII gives thebroadest tan d peak, indicative of a low DMS with a broad dis-tribution of included HS concentrations into the soft phase.Wide angle X-ray diffraction experiments on the samples re-vealed that e as the only exception in the entire set of samples eTPUU-VIII has crystalline HS domains [29]. Likely, duringcooling the earliest formed, small HS domains crystallize athigh temperature by which the system is largely immobilizedand no longer able to modify its DMS, even not during annealingat 100 �C. The two TPUU-VIII Tg values in Table 2 are virtuallyidentical. Furthermore, due to this crystallinity the onset of flowfor this sample occurs at an exceptionally high temperature, i.e.after crystal melting.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

Tan

d

elta

zero annealed4 days7 days14 days

-50 0 50 100Temperature°C

Fig. 4. Tan d as a function of temperature for differently annealed TPU-III

samples.

zero annealed14 days

10

100

1000

Sto

rag

e m

od

ulu

s (M

Pa)

-100 -50 0 50 100 150 200Temperature°C

Fig. 5. Storage modulus as a function of temperature for differently annealed

TPU-III samples.

3.3. Thermogravimetric analyses

3.3.1. Effect of partial replacement with diamine chainextenders

The TGA data of the nine samples annealed for 0 and for14 days are presented in Figs. 8 and 9, respectively, and revealthat the onset temperature of mass loss of series 2 lies up to90 �C higher than that of series 1. This stabilizing effect wasattributed earlier to the extra H-bonding capacity of ureagroups when diamine chain extenders are used [14e16] andagrees with earlier work on model ureas [17]. Indeed this ex-planation is very plausible. A similar ‘up to 90 �C’ differenceexists between the flow onsets of the two series. Flow and deg-radation do not occur simultaneously (degradation starts afterthe onset of flow), but due to this parallel in behaviour webelieve that their onsets are both proportional to the HS andH-bond stability. This also implies that even in the flow regionsome H-bonds are preserved between the HS that act on thedegradation behaviour. Maybe, the presence of aromatic andheterocyclic moieties in the main chain or as a side groupalso contributes to the increased thermal stability but this can-not be addressed independently.

The series 1 degradation profiles seem to consist of three steps,with e following the literature quoted in the introduction e the

TPU-IITPU-IIITPU-IV

TPU-IITPU-IIITPU-IV

10

100

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(a)

Sto

rag

e m

od

ulu

s (M

Pa)

-100 -50 0 50 100 1500,0

0,2

0,4

0,6

0,8(b)

Tan

d

elta

Temperature°C

-100 -50 0 50 100 150 200Temperature°C


TPU-II, TPU-III and TPU-IV (14 days annealed samples).


two high-temperature steps being associated with the SS degrada-tion and the first step with the HS degradation. The series 2 pro-files, however, only display two steps likely because the (shifted)HS step coincides or links up with the first step of the SS

10

100

1000

(a) TPUU-VIIITPUU-IXTPUU-VII

TPUU-VIIITPUU-IXTPUU-VII

Sto

rag

e m

od

ulu

s (M

Pa)

-100 -50 0 50 100 150 2000,0

0,2

0,4(b)

Tan

d

elta

Temperature°C

-100 -50 0 50 100 150 200Temperature°C


TPUU-VII, TPUU-VIII and TPUU-IX (14 days annealed samples).

Fig. 8. TGA thermograms of the samples [TPU-I]e[TPUU-IX] after 0 day of

annealing.

degradation. Exceptionally, TPUU-VIII displays three stepswith the HS degradation step occurring at a lower temperature.Recall that this sample exceptionally displays HS crystallinity(see Section 3.2.3) and as a result the DMS was very low. Assum-ing that after the onset of flow the probability of having HScontacts, necessary for H-bonds in the melt, is proportional tothe preceding DMS, one can understand the early HS degradationin TPUU-VIII as being the result of a low concentration ofH-bonds between HS.

3.3.2. Effect of annealingAnnealing induces morphological changes as discussed

above but as the soft/hard domain morphology likely disap-pears with the onset of flow, one cannot expect a major mor-phological influence on the degradation behaviour. After all,degradation sets in after flow. In Section 3.2.2 we reportedthat the flow onset of the series 1 samples shifts to highertemperatures with time at 100 �C, likely due to a progressingH-bond stability within the hard domains. Indeed, in line withthe parallel between onset of flow and degradation, a minorshift of the degradation profiles is observed to higher temper-atures (see insets to the Figs. 8 and 9) with annealing. Sucha positive shift is quite remarkable in the case of TPUU-VII.Apparently, annealing-induced morphological changes in atleast some of the series 2 samples also involve H-bondstabilization.

3.3.3. Effect of diol chain extender lengthFrom Fig. 10 we can deduce that the number of methylene

units in the chain extender does not affect the thermal stabilityof PU samples (series 1, the samples TPU-II, TPU-III, andTPU-IV having, respectively, 2, 4, or 6 methylene groups inthe chain extender).

TPU-I shows 10% weight loss at 235 �C while the othersamples in this series 1 only show 10% weight loss at250 �C. This lower thermal stability is probably due to the sig-nificantly lower MDI mass fraction and therefore a lower

Fig. 9. TGA thermograms of the samples [TPU-I]e[TPUU-IX] after 14 days

of annealing.


inter-urethane H-bond concentration (the longer s-PEGAchain extenders dilute the urethane linkages).

Fig. 11 shows that first TPUU-VII starts to degrade fol-lowed by TPUU-VIII and finally TPUU-IX. The behaviourof TPUU-IX can be termed ‘normal’ and roughly looks likethat of TPUU-V and VI. Their high thermal stability is dueto the presence of urea moieties as explained in Section3.3.1. The exceptional behaviour of TPUU-VIII was attributedto its crystallisability and its low DMS. So, only the very earlydegradation of TPUU-VII remains to be explained. In fact thissample displays a hybrid behaviour with a degradation onsettemperature typical for the samples in series 1 and an acceler-ated degradation where the series 2 samples start to degrade.

This behaviour may well be induced by a polymerizationthat did not happen as randomly as intended, by accident ordue to a lack of mixing in the polyol/chain extender blendor to a pronounced difference in reactivity between the chainextenders where diamines react faster compared to diols[30]. Such effects may lead to more blocky structures withthe diol rich sequences exhibiting a degradation behavioursimilar to that of the series 1 samples.

3.4. Mechanical properties via tensile testing

In Tables 3e5 all Young’s moduli, tensile strengths and ul-timate elongations are collected for the different samples at

160 180 200 220 240 26090

100

Weig

ht %

Temperature°C

TPU-ITPU-IITPU-IIITPU-IV

Fig. 10. Onset of degradation seen in the TGA Thermograms of the series 1

samples after 0 day of annealing.

room temperature and after the indicated annealing times.The room temperature Young’s modulus values in Table 3agree very well with the storage modulus values obtainedvia DMA at the same temperature. For example the modulusof TPUU-VIII is exceptionally high, but inspection ofFig. 7(a) reveals that this is simply due to a smeared-out glasstransition range (extending up to room temperature) as a resultof a low DMS. The other series 2 samples are in the rubberystate at room temperature but nevertheless display highermoduli compared to those of series 1 because of a lowerDMS and hence the presence of rigid heterocyclic or aromaticmoieties into the soft phase. Not unexpectedly, the series 2samples produce higher tensile strengths and lower elonga-tions at break compared to the samples of series 1. The tough-ness (calculated as the area under the stressestrain curve) ofthe series 2 samples was found to be 50% higher than thatof series 1.

The scatter in the Young’s moduli values as a function ofannealing time is considerable (Table 3). Nevertheless it seemsthat the underlying trend is that annealing at 100 �C results indecreasing Young’s moduli (Table 3), which is the expectedtrend for a progressing DMS. Degradation is not involvedhere as the degradation onsets all occur above 100 �C (seeFigs. 8 and 9). The elongation at break and tensile strengthdo not exhibit clear trends as a result of annealing.

150 200 250 300 35090

100

Weig

ht %

Temperature °C

TPUU-VIITPUU-VIIITPUU-IX

Fig. 11. Onset of degradation seen in the TGA Thermograms after 0 day of

annealing of the series 2 samples that e besides chain extender 1 (see

Fig. 1) e contain diols of different length.

Table 3

Young’s modulus (MPa) from tensile tests at room temperature

Sample Annealing time (days)

0 2 4 7 14

TPU-I 9.68� 0.24 6.89� 1.53 6.43� 0.42 7.19� 0.69 4.6� 0.62

TPU-II 11.72� 0.88 7.89� 0.52 7.31� 0.42 6.28� 0.6 5.49� 0.57

TPU-III 11.11� 0.72 7.57� 0.07 8.06� 0.62 8.48� 0.09 7.6� 0.23

TPU-IV 9.47� 0.46 7.15� 0.42 6.79� 0.09 6.57� 0.42 6.37� 0.23

TPUU-V 18.81� 0.49 17.25� 0.8 14.61� 1.12 17.58� 1.59 17.98� 2.05

TPUU-VI 29.68� 0.71 27.97� 0.32 28.22� 2.39 27.63� 0.81 25.71� 1.42

TPUU-VII 34.27� 1.4 33.43� 2.23 34.66� 1.95 36.53� 2.78 39.13� 5

TPUU-VIII 111.81� 3.88 97.86� 3.43 106.84� 20 90.98� 0.74 91.84� 1.41

TPUU-IX 28.05� 0.78 24.58� 0.33 24.84� 0.06 25.88� 0.82 23.38� 1.76


4. Conclusions

Two series of 4,40-diphenylmethane diisocyanate (MDI)and poly(ethylene glycol adipate) (PEGA)-based polyurethaneand polyurethaneurea elastomers were synthesized via a one-shot polymerization method. Aiming at polyurethanes withincreased thermal stability and mechanical properties, chainextender mixtures of aliphatic diols and heterocyclic or aromaticdiamines are explored (series 2) and compared to when purealiphatic diols are used (series 1). DMA experiments revealthat all samples separate into soft and hard domains. The de-gree of microphase separation (DMS) in the series 2 samplesis poor compared to their counterparts in series 1. The pres-ence of rigid hard segment moieties into the soft domains eprimarily composed of the PEGA segments e shifts the softphase glass transition temperature to higher temperaturesand broadens this transition because of a rather heterogeneousspatial and concentration distribution of the included hard seg-ments. As a result, Young’s modulus and the tensile strengthare higher and the elongation at break is lower for the samplesin series 2 at room temperature. Most of the samples are in therubbery state at room temperature, except for one sample inwhich the DMS was very poor due to a reduced polymer chainmobility imparted by crystallinity in the hard domains. Flow atthe end of the rubbery plateau, induced by a breakdown of thehard segment cohesion, occurs at higher temperatures for theseries 2 samples because of the stronger inter-urea hydrogen

Table 4

Tensile strength (MPa) from tensile tests at room temperature


0 2 4 7 14

TPU-I 5.4� 0.5 5.25� 0.2 5.1� 0.26 5.2� 0.7 2.6� 0.55

TPU-II 9.7� 0.6 11a 10a 10a 10a

TPU-III 11a 14a 14a 14a 14a

TPU-IV 12a 14a 14a 12a 14a

TPUU-V 37� 7 36� 2.3 35� 3 37� 5 34.0� 3.7

TPUU-VI 37� 3 37� 1.5 36� 3 40� 5 37.5� 1.2

TPUU-VII 36.5� 1.2 34� 1.6 34� 0.6 32� 2.5 31� 0.8

TPUU-VIII 23.4� 1.1 25.5� 3.3 26.1� 2 28.2� 0.18 24� 1.5

TPUU-IX 33.5� 0.3 38� 0.2 38� 4 38� 4 42� 6

a Tensile strength (MPa) obtained in a single experiment.

Table 5

Ultimate elongation (%) from tensile tests at room temperature


0 2 4 7 14

TPU-I 1170� 150 1400� 150 1263� 100 1400� 130 610� 150

TPU-II 1600� 130 1824a 1823a 1823a 1231a

TPU-III 1824a 1608a 1603a 1776a 1823a

TPU-IV 1689a 1670a 1801a 1823a 1823a

TPUU-V 1260� 210 1070� 55 1140� 130 1285� 200 1140� 130

TPUU-VI 1230� 65 1245� 60 1215� 170 1330� 140 1360� 25

TPUU-VII 1180� 130 1080� 45 1075� 65 1080� 135 1110� 110

TPUU-VIII 1039� 21 1010� 72 1110� 29 1100� 90 1080� 50

TPUU-IX 910� 25 1014� 50 960� 45 1050� 80 1250� 240

a Maximum strain (%) in a single experiment.

bonds between the hard segments. The strength of these hy-drogen bonds is also reflected in higher degradation tempera-tures (up to 90 �C higher) for the series 2 samples compared tothose of series 1. Annealing at 100 �C does not degrade thesamples but pushes the DMS further and strengthens thehard segment hydrogen bonds by which flow e and to a lesserextent degradation e occurs at higher temperatures. The roomtemperature Young’s moduli decrease with annealing time at100 �C due to the generation of more pure soft domains.

These results prove that with the use of heterocyclic and ar-omatic diamine chain extenders one can produces polyure-thane with a better thermal stability and mechanicalproperties than with aliphatic diol chain extenders only.

Acknowledgements

BG is a postdoctoral fellow of the fund for ScientificResearch-Flanders (Belgium).

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