Journal ofMaterials Chemistry A

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Materials Division, IK4-CIDETEC Researc

Donostia-San Sebastian, Spain. E-mail: iodr

Cite this: J. Mater. Chem. A, 2014, 2,5710

Received 27th November 2013Accepted 6th February 2014

DOI: 10.1039/c3ta14927g

www.rsc.org/MaterialsA

5710 | J. Mater. Chem. A, 2014, 2, 5710

The processability of a poly(urea-urethane)elastomer reversibly crosslinked with aromaticdisulfide bridges

Roberto Martin, Alaitz Rekondo, Alaitz Ruiz de Luzuriaga, German Cabanero,Hans J. Grande and Ibon Odriozola*

Recently we have shown the self-healing ability of a poly(urea-urethane) elastomer reversibly cured with

aromatic disulfide bridges. Here we show that although being chemically crosslinked, such materials can

be easily reprocessed by applying heat and pressure to obtain any desired form. This offers a unique

paradigm towards thermoset processing and recycling, as well as for isocyanate-free manipulation of

polyurethanes.

Introduction

The introduction of dynamic covalent bonds1,2 into polymernetworks has been recently used as a powerful approachtowards the design of various intrinsically self-healing polymersystems.3–14 Very recently, our group described a poly(urea-urethane) (PUU) elastomeric system having aromatic disuldebridges as crosslinks.15 The dynamic character of such disuldemoieties, as well as the hydrogen bonds of the urea groups,endowed the material with very efficient healing properties atroom temperature. Now we show another intriguing property ofthis PUU, which is also derived from the dynamic nature of suchcrosslinks, e.g. its processability.

One of the main drawbacks of classical thermoset mate-rials16–19 is the fact that once hardened they cannot be melted totheir liquid state, which makes their reprocessing or recyclingimpossible. In this context, some pioneering work has recentlyled to the rst examples of reprocessable/recyclable thermosetpolymers based on dynamic covalent bonds.20–23 Following thiswork, we decided to study the processing behaviour of a PUUsystem having reversible aromatic disulde bridges (Scheme 1).

Several examples of polymers crosslinked with aromatic24,25

and aliphatic26,27 disuldes have been recently published.However, these studies were mainly focused on their self-heal-ing properties26,28 rather than on their reprocessability. Tsar-evsky and Matyjaszewski studied the reversibility of disulde-containing polymer systems by redox chemical reactants.29

In this work we study how this PUU system can be reproc-essed in its cured state by just applying temperature and pres-sure in a mould, to obtain elastomers with the desired shape.We demonstrate that during this reprocessing operation themechanical properties of the material remain intact, as shown

h Centre, Paseo Miramon 196, 20009

iozola@cidetec.es; Tel: +34 943309022

–5715

by tensile strength measurements. In addition, stress relaxationand infrared studies are carried out to better understand thebehaviour of the material, which shows great potential towardsnew concepts on thermoset processing and recycling, as well astowards isocyanate-free manipulation of polyurethanes.

Scheme 1 Schematic representation of aromatic disulfide metathesisin the PUU thermoset elastomer described here.

This journal is © The Royal Society of Chemistry 2014

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Experimental sectionMaterials

Poly(propylene glycol)s (PPG) of different molecular weightsand functionalities (Mn 6000, trifunctional and Mn 2000,difunctional) were purchased from Bayer Materials Science.Isophorone diisocyanate (IPDI), dibutyltin dilaurate (DBTDL),bis(4-aminophenyl) disulde (AFD) and tetrahydrofurane (THF)were purchased from Sigma-Aldrich and were used withoutfurther purication.

Methods

Milling of PUU was performed using a Fritsch Universal CuttingMill Pulverisette 19, by pre-cooling the samples in liquid N2.Reprocessing experiments were carried out using a VOGT hotpress LABO PRESS 200T. Thermal analysis was performed usinga DSC instrument from Perkin Elmer (Pyris Diamond DSC) overa temperature range from �90 �C to 0 �C and from �20 �C to200 �C under nitrogen. The DSC was calibrated for temperature,at each heating rate used, from the melting points of indium(156.6 �C) and dodecane (�9.65 �C). The glass transitiontemperature (Tg) was obtained as the inection point of the heatow step recorded at a scan rate of 20 �C min�1. Mechanicaltesting was performed using an INSTRON 3365 Long travelElastomeric Extensometer controlled by Bluehill Lite soware.Tensile strength measurements were carried out according tothe UNE-EN-ISO 527 standard, using dumbbell type test speci-mens and an elongation rate of 500 mm min�1. Fourier trans-form infrared (FTIR) spectra were recorded using a NicoletAvatar 360 spectrophotometer using KBr disks compressed to2 ton cm�2 for 2 min as supports. Stress relaxation experimentswere carried out in a TA instruments AR2000ex rheometer usinga 25 mm plate-plate geometry on PUU elastomer samples withthicknesses of 0.8 mm. A 10% strain step at a constant normalforce of 2 N was applied. Prior to the experiments, we checkedthat 10% deformation was within the linear range using strainsweep experiments.

Synthesis of isocyanate-containing prepolymers

Synthesis of a tris-isocyanate terminated prepolymer.Synthesis of a tris-isocyanate terminated prepolymer wascarried out by feeding trifunctional PPG (Mn 6000) (100 g, 16.67mmol) and IPDI (11.65 g, 52.48 mmol) into a 1 L glass reactorequipped with a mechanical stirrer and a vacuum inlet. Thereaction was catalyzed with 50 ppm of DBTDL and evolved at70 �C for 45 minutes under vacuum and high shear mechanicalstirring. The reaction was monitored by FTIR spectroscopy. Theresulting tris-isocyanate terminated prepolymer was obtainedin the form of a colourless liquid and stored in a tightly closedglass bottle. The NCO content of the obtained prepolymer wasdetermined by FT-IR monitored titration with n-butylamine,following the disappearance of the NCO signal at 2264 cm�1.The determined NCO content (1.91%) was in accordance withthe theoretical estimation (1.89%).

Synthesis of a bis-isocyanate terminated prepolymer.Synthesis of a bis-isocyanate terminated prepolymer was carried

This journal is © The Royal Society of Chemistry 2014

out by adding difunctional PPG (Mn 2000) (100 g, 50 mmol) andIPDI (22.2 g, 100 mmol) into a 1 L glass reactor equipped with amechanical stirrer and a vacuum inlet. The reaction was cata-lyzed with 50 ppm of DBTDL and evolved at 60 �C for 70minutesunder vacuum and strongmechanical stirring. The reaction wasmonitored by FTIR spectroscopy. The resulting bis-isocyanateterminated prepolymer was obtained in the form of a colourlessliquid and stored in a tightly closed glass bottle. The NCOcontent of the obtained prepolymer was determined by FT-IRmonitored titration with n-butylamine, following the disap-pearance of the NCO signal at 2264 cm�1. The determined NCOcontent (3.50%) was in accordance with the theoretical esti-mation (3.48%).

Synthesis of PUU

In a 250 mL glass reactor, the tris-isocyanate terminated pre-polymer (35 g) and the bis-isocyanate terminated prepolymer(15 g) were mixed in a ratio of 70/30. Then, a solution of AFD(5.12 g, molar NH2/NCO ¼ 1.4) in THF (3 mL) was added. Themixture was degassed under vacuum and the obtained trans-parent yellowish mixture was placed in a cylindrical openmould. The curing was allowed to proceed for 16 h at 60 �C andwas monitored by FT-IR spectroscopy. The resulting poly(urea-urethane) (PUU) was obtained as a yellowish transparent elas-tomeric material.

Results and discussionSynthesis of PUU

The PUU elastomer was synthesised starting from di- and tri-functional isocyanate-terminated prepolymers and AFD. Thepreparation method basically consisted of pouring a liquidmixture of the reactants to the correspondingmould in the formof a cylinder or in the form of a 2 mm lm. Characterisation ofthe materials was also performed, in order to ensure that thecuring process was complete.

Thermal characterisation of the PUU network was carriedout using dynamic scanning calorimetry (DSC) measurements.As observed in Fig. 1a, the PUU thermoset showed a Tg at�50.39 �C arising from the PPG block. This Tg value was slightlyhigher than the one obtained for the isocyanate terminatedprepolymer mixture (�55.94 �C), indicative of network forma-tion.30,31 The absence of any peak in the range of�10 and 200 �Crevealed a non-crystalline structure.

Processability

The medium stress value at break of pristine PUU was deter-mined to be 0.8 MPa (Fig. 2c), as described before.15 In order tostudy the processability of the cured material, the cylindricalsample was placed in a 2 mm thick mould and compressed in ahot press at 150 �C and 30 bar for 20 minutes (Fig. 2a), resultingin a very homogeneous lm, apparently the same as a pristine2 mm lm made from the starting liquid components. Tensiletest measurements were performed on specimens extractedfrom the “thermoformed” lm. As shown in Fig. 2c (red traces),the stress at break remained unaltered.

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Fig. 1 (a) Low temperature DSC curves recorded for the isocyanateterminated prepolymer mixture (red trace) and PUU (black trace). (b)High temperature DSC curve recorded for a PUU sample.

Fig. 2 (a) Reprocessing of a PUU sample synthesised in the form of acylinder (i) which was introduced in a hot press at 150 �C and 30 bar for20 minutes (ii). After that time, the material was recovered in the formof a film (iii). (b) Ground PUU elastomer in the form of powder (iv) wasplaced in a 2 mm mould and reprocessed at 150 �C and 30 bar for 20minutes (v) to give a film (vi). (c) Stress vs. strain curves of pristine(black), reshaped cylinder (red) and recycled powder (blue) (curves ofeach series have been slightly shifted on the x-axis for clarity).

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With the aim of further investigating the reprocessingcapacity of PUU, another cylindrical sample was ground topowder (Fig. 2b) and subjected to the same process at 150 �Cand 30 bar for 20 minutes, resulting in a homogeneous lm.Tensile strength measurements of specimens extracted fromthe lm revealed that, again, the mechanical properties of thematerial remained unaltered, showing a stress at break of0.8 MPa. This demonstrated that the material could not only bereprocessed to a given shape, but it could also be recycled frompowder waste.

Swelling experiments were also performed in order tocorroborate that the PUU network was fully recovered duringthe reprocessing. When a pristine dice of PUU was immersed inTHF for 3 hours, its volume increased by several orders ofmagnitude (Fig. 3a). Interestingly, a similar swelling behaviourwas observed when a reprocessed piece of PUU was subjected tothe same conditions (Fig. 3b), indicating the existence of achemically crosslinked network.

Rheology experiments

The above results encouraged us to perform rheological studies,for understanding the relaxation behaviour of our PUU networkas a function of temperature. Stress relaxation experiments were

5712 | J. Mater. Chem. A, 2014, 2, 5710–5715

carried out by placing a thin disc of PUU in a rheometer, in atemperature range between 25 and 150 �C. Fig. 4 shows shearstress relaxation curves obtained at different temperatures foran applied strain (g) of 10% and a normal force of 2 N.

As shown in the gure, at 150 �C the sample needed about 4minutes to relax until 10% of its initial modulus value (G). At130 and 110 �C, the same relaxation process needed about 15minutes and 1 hour, respectively. However, at temperaturesbelow 100 �C this relaxation process appeared to take muchlonger, needing 4 hours at 90 �C. At room temperature the PUUelastomer showed very low relaxation aer 3 days of experiment.

From these results two main conclusions could be extracted.Firstly, the elastomer did not relax easily at room temperature,although having aromatic disulde bridges that are able toundergo exchange at such a temperature. This was alsocorroborated macroscopically, as “cold ow” was not observedat room temperature. Secondly, another interaction seemed to

This journal is © The Royal Society of Chemistry 2014

Fig. 3 Photograph of: (a) a pristine PUU sample before (left) and after(right) swelling in THF for 3 h; (b) a reprocessed PUU sample before(left) and after (right) swelling in THF for 3 h. Scale bars correspond to 1cm in all cases.

Fig. 4 Stress relaxation plots as a function of time recorded at roomtemperature (red), 90 �C (blue), 110 �C (black), 130 �C (green) and150 �C (purple) for an applied strain of 10%.

Scheme 2 Schematic representation of the quadruple H-bondpresent in PUU and its disruption above 100 �C.

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exist which was eliminated above �100 �C. This interaction,which was attributed to the quadruple H-bonding of the ureagroups (see Scheme 2), would be mainly responsible for pre-venting ow or relaxation at room temperature.

Fig. 5 (a) Magnification of the carbonyl region of FTIR spectrarecorded at 25, 90, 110, 130 and 150 �C for the PUU elastomer. (b)Magnification of the N–H stretching vibration region of FTIR spectrarecorded at 25, 90, 110, 130 and 150 �C. The blue arrows indicate thedifferent shifts observed when increasing the temperature.

FT-IR study

FTIR spectroscopy experiments were also carried out in order toexplore the effect of H-bonding interactions of urea groups onthe relaxation behaviour of our PUU.32–34 As shown in Fig. 5a andb, C]O and N–H stretching vibration bands were analyzed atdifferent temperatures (25, 90, 110, 130 and 150 �C). It wasobserved that the band at approximately 1670 cm�1, attribut-able to urea carbonyl groups, did not vary until the temperaturereached 110 �C.

This journal is © The Royal Society of Chemistry 2014

This could indicate that the quadruple H-bonding interactionis disrupted at around this temperature, in accordance with otherurea systems described in the literature.35 At 110 �C and above,the band decreased and shied to higher frequencies, as

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expected for free urea carbonyl bands, which are known to appearat higher frequencies and to show a lower absorption coefficient.

This was not the case for the urethane carbonyl band(�1710 cm�1), which shied already at 90 �C, indicating aweaker H-bond, thus poorly affecting the mechanical propertiesof the material.

The amide II and the N–H stretching regions are also high-lighted in Fig. 5a and b, respectively. These bands share thecontributions from both urea and urethanemoieties in their freeand associated states, thus being impossible to distinguish thecontribution from urea and urethane bands. Here, a decrease ofintensity and a shi of the two bands could be observed startingfrom 90 �C. This could be attributed to the disruption of theurethane H-bonds, as shown in its carbonyl band.

From both the rheology and the FTIR results, one couldconclude that the relaxation observed at higher temperaturescould be attributed mainly to the disruption of H-bonds. Suchinteractions (Scheme 2) would be responsible for holding thenetwork and preventing ow and relaxation at lower tempera-tures. Of course disulde metathesis would also contribute tothe relaxation of the network at higher temperatures as such anexchange would also be accelerated by the temperature.

Conclusions

In conclusion, we have shown that the special characteristics ofthis PUU network, possessing both reversible covalent bondsand multiple H-bonds, make possible the processing of thematerial in a way classical thermoset networks could not. Wehave demonstrated that such networks can be reshaped to thedesired form by just placing them in a hot press, without losingtheir initial mechanical properties. In addition, the curedmaterial could also be reprocessed from its powder state. Allthese properties provide great industrial advantages comparedto classical polyurethane systems, such as: (i) the possibility ofusing cured resin as a raw material for the fabrication ofcomponents, thus avoiding the need of manipulating toxicisocyanates by the operator; (ii) a very easy process for therecovery and recycling of polyurethane elastomers, which iscurrently an issue for the majority of elastomeric materials. Forall these reasons, the present work could represent an impor-tant breakthrough that could be easily implemented in manyindustrial sectors where polyurethanes are currently used.

Acknowledgements

The research leading to these results has received funding fromthe European Community's Seventh Framework Programme(FP7-NMP-2012-SMALL-6) under grant agreement no. 309450and from the Basque Government under the ETORTEK ACTI-MAT 2013 project. Izaskun Azcarate-Ascasua is acknowledgedfor technical help.

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