European Polymer Journal 62 (2015) 19–30

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European Polymer Journal

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Epoxy networks and hydrogels prepared from a,x-diaminoterminated poly(oxypropylene)-b-poly(oxyethylene)-b-poly(oxypropylene) and polyoxypropylene bis(glycidyl ether)

http://dx.doi.org/10.1016/j.eurpolymj.2014.11.0050014-3057/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +420 221 912 747; fax: +420 221 912 350.E-mail address: ivank@kmf.troja.mff.cuni.cz (I. Krakovsky).

Ivan Krakovsky a,⇑, Rebeca Martínez-Haya b, Gloria Gallego Ferrer b,c, Roser Sabater i Serra b,Jagan Mohan Dodda d

a Department of Macromolecular Physics, Faculty of Mathematics and Physics, Charles University, V Holešovickách 2, 180 00 Prague 8, Czech Republicb Centro de Biomateriales e Ingeniería Tisular, Universitat Politècnica de València, Camino de Vera s/n, E-46022 Valencia, Spainc Biomedical Research Networking Center in Bioengineering, Biomaterials and Nanomedicine (CIBER-BBN), Zaragoza, Spaind University of West Bohemia, New Technologies Research Centre, Univerzitní 8, 30614 Plzen, Czech Republic

a r t i c l e i n f o a b s t r a c t

Article history:Received 1 July 2014Received in revised form 24 October 2014Accepted 2 November 2014Available online 8 November 2014

Keywords:EpoxyHydrogelPolyoxyethylenePolyoxypropyleneSwelling

A series of hydrophilic epoxy networks was prepared by reaction of a,x-diamino termi-nated polyoxypropylene-b-polyoxyethylene-b-polyoxypropylene of average molar mass2000 g mol�1 with polyoxypropylene bis(glycidyl ether) of average molar mass640 g mol�1. Absence of microphase separation in the networks at ambient and highertemperatures was proved by differential scanning calorimetry (DSC) and dynamic mechan-ical analysis (DMA). Cooling of the networks to subambient temperatures induces micro-phase separation via crystallization: the crystallites of polyoxyethylene formed act as areinforcing filler in the surrounding amorphous network phase. Hydrogels obtained byswelling of the networks in water deswell continuously with increasing temperature.The deswelling becomes less pronounced with increasing POE content and decreasingnetwork density of the epoxy network. In cooling of the hydrogels to low temperaturesthe main part of water crystallizes. Water crystallites are dispersed in the amorphousnetwork swollen with remaining no-crystallized water. The hydrogels prepared can foundapplications in sensors and actuators working in aqueous environment.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Hydrogels represent a class of polymer networks that,due to their hydrophilic nature, can absorb large amountof water. A number of ways is available for their prepara-tion, e.g., crosslinking of water-soluble polymers in aque-ous environment by c-irradiation [1] or swelling ofhydrophilic polymer networks in water [2]. A delicateinterplay between rubber elasticity of polymer network

and polymer–water interaction govern structure and prop-erties of hydrogels which response very sensitively, e.g. bychange of their volume, to changes of external parameterssuch as temperature [3], pressure [4], presence of a cosol-vent [5], surfactant [6], pH [7], and ions [8]. Hydrogels canbe assembled or integrated into functional systems ormicrosystems. This, in combination with the sensitivityof hydrogels makes them very attractive for applicationsin different fields of advanced technology, as sensors[9,10], actuators [11], microfluidic [12], microopticaldevices [13] or in tissue engineering [14], among others.

Polymer networks of a broad variety of chemical struc-ture can be found in hydrogels. Hydrogels used in sensor

Table 1Composition of the networks prepared: stoichiometric ratio, r, massfraction of sol, ws, mass fraction of POE, wPOE.

Sample r ws wPOE

E0 1.00 0.125 0.62E1 1.11 0.131 0.64E2 1.18 0.146 0.66E3 1.24 0.165 0.67E4 1.39 0.195 0.69E5 1.67 0.288 0.73E6 2.22 0.476 0.78

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and actuator applications are mainly based on derivativesof polyacrylic acid, such as, e.g., poly(N-isopropylacrylam-ide), that are prepared by free radical cross-linking poly-merization initiated by heat or UV-irradiation. However,air-sensitivity of this reaction makes its control and imple-mentation in an open environment difficult. Therefore,there is an interest to investigate alternative methodssuitable for preparation of hydrogels exploiting, e.g., poly-condensation or polyaddition reactions [15].

For a long time, polyaddition reactions are used in pro-duction of polyurethane and epoxy resins that are industri-ally very important materials. Polyaddition of di- ortriamines with diepoxides provides epoxy resins. Unlikethe reactions involved in polyurethane synthesis thisreaction is not very sensitive to presence of water, oxygenor impurities. Excellent mechanical, thermal and dielectricproperties of epoxy resins have enabled their applicationsas adhesives, coatings or castings [16].

Krakovsky et al. [17–19] prepared hydrophilic epoxynetworks using reaction of a, x-diamino terminatedpoly(oxypropylene)-b-poly(oxyethylene)-b-poly(oxyprop-ylene) (Jeffamine ED-2003) with Bisphenol A propoxylatediglycidyl ether. Thermal properties of the hydrogels aswell as aqueous solutions of Jeffamine ED-2003 wereinvestigated by Gómez Ribelles et al. [20] and Salmerón-Sánchez et al. [21]. It was demonstrated by Calvert [22]that epoxy curing can be also carried out directly in waterenvironment.

To avoid using of toxic Bisphenol A propoxylate diglyc-idyl ether in preparation of epoxy networks and hydrogels,bis(glycidyl ether) of polyoxyethylene (POE) and/or poly-oxypropylene (POP) can be used. Manandhar et al. pre-pared epoxy-based hydrogels for sensor applications byreaction of Jeffamine ED-2003 with polyoxyethylenebis(glycidyl ether) in aqueous solution of sodium chloride[23].

In our previous work, we prepared epoxy networks bythe reaction of a, x-diamino terminated POP (JeffamineD-2000) and polyoxyethylene bis(glycidyl ether) in bulk[24]. We showed that swelling of these networks in waterprovides the hydrogels with thermosensitivity growingwith increasing length of the POP chains.

The thermosensitivity of the hydrogels containing POEand POP originates from a temperature dependence ofinteraction of the two polymers with water. POE of molarmass as up to 106 g mol�1 is fully soluble in water from0 �C to about 100 �C [25], being dominantly hydrophilicin the whole temperature range. On the other hand,interaction of POP with water shows more dramatic tem-perature dependence. In water, at atmospheric pressure,POPs of molar masses 1200 and 3000 g mol�1 are solubleonly at low concentrations below ca 35 �C and 18 �C,respectively [26].

Unlike our previous study [24], in this work the locationof POE and POP in the reagents used in preparation ofepoxy networks was interchanged. Consequently, the POEand POP blocks are inbuilt into epoxy networks in a differ-ent way. Since a,x-diamino terminated POE of differentmolar masses is not available a,x-diamino terminatedPOP-b-POE-b-POP with short POP blocks (Jeffamine ED-2003) of one average molar mass and polyoxypropylene

bis(glycidyl ether) (POPBGE) were used. A series of epoxynetworks was prepared by the reaction of ED-2003 withPOPBGE mixed at different proportions in excess of aminogroups. In this study the effect of the composition andnetwork density of the epoxy networks containing POEand POP blocks on their thermal/mechanical propertiesand swelling behaviour in water will be addressed. Ther-mal behaviour of the hydrogels obtained by equilibriumswelling of the epoxy networks in water is also investigated.

2. Experimental

2.1. Materials

In the preparation of the epoxy networks a,x-diaminoterminated polyoxypropylene-b-polyoxyethylene-b-poly-oxypropylene (Jeffamine� ED-2003, Huntsman) and poly-oxypropylene bis(glycidyl ether) (Sigma–Aldrich) wereused. Molar mass of ED-2003 was ca 2000 g mol�1 withPOE content about 90 wt.%. Average molar mass of POPBGEwas ca 640 g mol�1. Concentrations of amino groups in ED-2003 and epoxy groups in POPBGE determined by titrationwere cNH2 = 0.95 � 10�3 mol g�1 and cE = 3.10 � 10�3

mol g�1, respectively.A series of epoxy networks was prepared with initial

molar ratio of reactive groups (stoichiometric ratio), r,r = 2[NH2]0/[E]0 = 1.00, 1.11, 1.18, 1.24, 1.39, 1.67 and2.22 (samples E0, E1, E2, E3, E4, E5 and E6, see Table 1),where [NH2]0, and [E]0 are initial molar concentrations ofamino and epoxy groups, respectively.

Both components were first stirred at 100 �C for about15 min and then poured into Teflon moulds. Curingreaction of all systems proceeded at 120 �C for 48 h. Allthe networks prepared were transparent. Extractablefraction – sol (12–47 wt.%, see Table 1) remaining in thenetworks after curing was removed by triple extractionof the networks in a good solvent (toluene). Finally, thenetworks were dried carefully, first in the open air thenin vacuum oven at 40 �C for 48 h. Rectangular specimenswere cut from the extracted and dry epoxy networks.Hydrogel samples were obtained by their swelling to equi-librium in distilled water at room temperature for 24 h.

2.2. Measurements

2.2.1. Phase diagramHydrogel samples were cooled down in a thermostated

bath (Labio, Czech Republic) to the initial temperature

I. Krakovsky et al. / European Polymer Journal 62 (2015) 19–30 21

(1 �C) and equilibrated for 24 h. Then, they were taken outof the water, surface-dried quickly, and weighed using aprecise analytical balance. After the mass determination,they were immersed in water again, heated to the nexttemperature and equilibrated for 24 h again. The processwas repeated until data at the highest temperature inves-tigated (80 �C) were collected.

The mass fraction of polymer network in hydrogels,wnetw, was calculated by

wnetw ¼m0

mð1Þ

where m0 and m are the masses of dry and swollen sample,respectively.

2.2.2. Dynamic mechanical analysisDynamic mechanical analysis was performed using a

Seiko DMS210 Analyzer. Rectangular strips having dimen-sions ca 25 mm � 9 mm � 1 mm were cut from the sheetsof the networks prepared. The strips were suspendedbetween two clamps and measured in uniaxial extensionat heating rate 2 �C/min from �100 to 100 �C at a constantfrequency 1 Hz. Before the measurements the sampleswere dried at 40 �C in vacuum for 24 h and then storedin a desiccator. The main relaxation temperature wasdetermined from the position of the loss tangent peak.

2.2.3. Differential scanning calorimetryThe calorimetric measurements were performed using

a DSC8500 apparatus (Perkin–Elmer). Purge gas (helium)was let through the DSC cell with a flow rate of 20 mL/min. The temperature of the equipment was calibratedwith n-hexane, mercury and indium. The melting heat ofindium was used for calibrating the heat flow.

The samples of the reagents used in the network prepara-tion were subjected to a cooling scan from 30 �C to�100 �C,held at�100 �C for 5 min, and subjected to a heating scan to50 �C. Both scans were carried out at a rate of 10 �C/min.

The dry network samples were subjected to a coolingscan from 50 �C to �100 �C, held at �100 �C for 5 min,and subjected to a heating scan to 100 �C. Both scans werecarried out at a rate of 10 �C/min.

Finally, the hydrogel samples were thermostated for24 h at 20 �C prior to DSC measurements. After that, thesamples were transferred quickly to DSC pans and sub-jected to a cooling scan from 20 �C to �100 �C, held at�100 �C for 5 min, and subjected to a heating scan to50 �C. Both scans were performed at a rate of 10 �C/min.

The values of the characteristic transition temperatures(melting and glass transition) were calculated from theheating calorigrams. Glass transition temperature was deter-mined using the half-Dcp method and melting temperaturewas determined from the onset of melting endotherms.

3. Results and discussion

3.1. Reagents

Cooling and heating calorigrams of the reagents used inthe preparation of the epoxy networks are shown in Fig. 1.

In cooling of ED-2003, the onset of the crystallization ofPOE at ca 25 �C is apparent (see Fig. 1a). The crystallizationis completed at ca �20 �C. At a temperature lower than�60 �C the reagent passes into a glassy state. In heating,the glass transition is observed around �61 �C (see insertof Fig. 1a) followed by the melting of the POE crystallitesformed during the previous cooling scan with an onset atca 13 �C. The specific melting enthalpy, Dhm, of ED-2003determined from the area below the melting peak yieldsthe value 107 J g�1. Dividing this value by specific meltingenthalpy of 100% crystalline POE, Dhm (POE) = 196 J g�1

[27], and taking into account that ED-2003 contains about90 wt.%, the degree of crystallinity of POE obtained isaround 49%.

In cooling, no crystallization is observed for POPBGE byDSC (see Fig. 1b). POPBGE becomes glassy at a sufficientlylow temperature (below �70 �C). In heating, the glasstransition is the only transition observed. The value ofthe glass transition temperature, Tg, of POPBGE determinedfrom the heating scan is �77 �C. Due to the atactic arrange-ment of oxypropylene units in the POPBGE molecules, nocrystallization is observed (see Scheme 1).

All parameters determined for the reagents from theheating DSC scans are summarized in Table 2.

3.2. Dry networks

The formation of epoxy networks is described by thewell-known kinetic scheme (Scheme 2). Reaction of aprimary amino group with an epoxy group leads to aformation of secondary amine that can further react withanother epoxy group [28]. Hydroxy groups formed in thereaction can also react with epoxy groups, however, activa-tion of this reaction requires much higher temperaturethan the reaction temperature used in this work (120 �C).Therefore, this reaction is not taken into account in epoxynetwork formation in this study [29].

Simple epoxy networks have been used as model net-works for testing of network formation and rubber elastic-ity theories [28,30–32]. In a detailed review of curing ofepoxy networks [28], Dušek concludes that ‘‘. . .there isno experimental or theoretical reason to expect a generaltendency to inhomogeneous course of curing of epoxyresins and formation of inhomogeneously crosslinkedproducts. This conclusion has been obtained mainly byanalysis of simple epoxy-amine systems in which cross-linking occurs by a single alternating reaction.’’

Fairclough et al. investigated the POE/POP interactionand microphase separation in POE-b-POP copolymers[33]. The POE-b-POP copolymers with the POE blocksconsisting of ca 50 or more oxyethylene units form semi-crystalline solids at ambient temperature and melt at ca50–65 �C. It is well known that irrespective of blockcopolymer architecture the temperature dependence ofthe Flory–Huggins interaction parameter, v, follows wellthe equation

v ¼ A=T þ B ð2Þ

where A and B are constants and T is the absolute temper-ature. For the POE/POP interaction A = 20.2 K and B = 0.022was obtained [33]. A similar dependence has been

Fig. 1. DSC curves for ED2003 (a) and POPBGE (b) obtained at 10 �C/min.

22 I. Krakovsky et al. / European Polymer Journal 62 (2015) 19–30

also found for the POE/POP interaction in blends ofa,x-dihydroxy and a,x-dimethoxy terminated POEs andPOPs by Friday et al. [34].

Booth and Pickles studied miscibility in mixtures ofa,x-dihydroxy terminated POE and POP [35]. They foundthat at 77 �C the blend of POE and POP chains consistingof 91 oxyethylene units (molar mass 4000 g mol�1) and 7oxypropylene units (molar mass 410 g mol�1), respectively,is fully miscible. At this temperature Eq. (2) yields for thePOE/POP blend the value v = 0.08, therefore the criticalvalue of the interaction parameter for phase separation(demixing), vc, is necessarily higher than 0.08. The net-works investigated in this study were prepared at 120 �C,at this temperature Eq. (2) yields value v = 0.073 < vc.

Molar mass of the POE and POP blocks used in the prep-aration of the epoxy networks is lower (ca 1700 g mol�1 forPOE) and comparable (ca 500 g mol�1 for POP), respec-tively, than in the blend discussed above. In the course of

the curing reaction the POE and POP blocks are linked intocopolymers of increasing molar mass and complexity.Critical value of the interaction parameter for microphaseseparation predicted by mean-field theories for diblock,multiblock as well as more complex copolymer architec-tures is a few times higher than the critical value fordemixing of the blend of the same blocks and composition[36,37].

Therefore, microphase separation of the POE and POPblocks during the curing reaction carried out at 120 �C isnot expected. Moreover, behind the gelation point micro-phase separation in the crosslinking system is stronglyrestricted by the formation of the polymer network. Ofcourse, cooling of the networks to a sufficiently lowtemperature may induce crystallization of POE as in thePOE-b-POP copolymers.

On the grounds of above given arguments, expectedtopology and structure of the epoxy networks at 120 �C is

Scheme 1. Chemical structure and representation of reactives used in network preparation.

Table 2Thermal characteristics of reagents: glass transition temperature, Tg,specific heat capacity difference at glass transition, Dcp, onset temperatureof melting, Tm, and specific enthalpy of melting, Dhm.

Sample Tg (�C) Dcp (J g�1 K�1) Tm (�C) Dhm (J g�1)

ED2003 �61 0.270 +13 107POPBGE �77 0.674 – –

I. Krakovsky et al. / European Polymer Journal 62 (2015) 19–30 23

illustrated schematically in Fig. 2. The POPBGE moleculesare linked to long chains to which the ED-2003 diaminechains are grafted by their terminals (Fig. 2a and Scheme 1).In the network prepared from the reaction mixture withstoichiometrically balanced amino/epoxy ratio, i.e., r = 1(the stoichiometric network) a single chain of macroscopiclength consisting of mutually linked POPBGE subchainsentangled into matrix of the ED-2003 chains would beformed under hypothetical perfect reaction conditions(see top box in Fig. 2b).1 In excess of amino groups in reac-tion mixture, i.e. at r > 1, the single chain of POPBGE mole-cules is split into many shorter chains of unequal lengths(see middle and bottom box in Fig. 2b). The higher ther-value the shorter the length and the wider the lengthdistribution of the chains of the linked POPBGE molecules.The ED-2003 chains can be divided into two groups: thosegrafted by both ends and by one end, respectively, to thechains of the linked POPBGE molecules. With increasing r,the number of the chains in the second group increases atthe expense of the chains in the first group. Finally, accord-ing to the theory of branching processes applied to thissystem [30], at r P 3, formation of the epoxy network isnot possible. In Fig. 2b, a growth of the POE content and a

1 In our previous work the location of POP and POE in reagents wasreverse: stoichiometric epoxy networks were prepared by curing ofmixtures of a,x-diamino terminated POP (Jeffamine D-2000) and polyoxy-ethylene bis(glycidyl ether) (POEBGE) [24]. Therefore, the network topol-ogy was also reverse, i.e., a single long chain of mutually linked POEBGEmolecules is entangled into matrix of D-2000 (POP) chains.

decrease of the epoxy network density (crosslinking degree)with increasing r can be also noticed.

The r-dependence of the mass fraction of sol, ws, listedin Table 1 is in agreement with the trend predicted bythe theory of branching processes [28,32]. An elevatedvalue of ws found for the stoichiometric network(ws = 0.125, see Table 1) testifies a certain imperfectnessof the curing process that can originate from the presenceof a small amount of non-functionalized chains in ED-2003, uncompleted curing reaction or cyclization [38,39].However, from the r-dependence of the mass fraction ofsol it is expected that real structure of the networksprepared does not differ substantially from the trend pre-sented in Fig. 2b.

Although inhomogeneity of the epoxy networks pre-pared is not expected, it is worthwhile to verify it for anynew epoxy system using physico-chemical methods avail-able. To this purpose, DSC and DMA were used in this work.

In the cooling calorigrams presented in Fig. 3 the crys-tallization of POE and the glass transitions in all networksare registered by DSC. Unlike the networks with reverselocation of POE and POP investigated in our previous work[24] where crystallization was not observed in the stoichi-ometric network in this study (E0) the crystallization ofPOE with onset at ca 5 �C is clearly visible. This demon-strates that contrary to the stoichiometric networksreported previously where the POE blocks were inbuilt ina long chain of the POEBGE molecules, in the present sys-tem the POE blocks are grafted by both ends to the longchain of POPBGE molecules and their mobility is highenough to allow crystallization of POE.2 A new ‘‘phase’’ –crystallites of POE is formed in the system in this way.

Mobility of the POE blocks in the non-stoichiometricnetworks increases due to decreasing network density

2 Since molar masses of the diamine and diepoxide reagents are notequal, the POE content in the stoichiometric network in this study is62 wt.% (network E0, see Table 1) unlike 34 wt.% in the correspondingstoichiometric network in the previous study (network R in Table 2, Ref.[24]).

Scheme 2. Reactions involved in epoxy network formation.

Fig. 2. Schematic representation of (a) topology and (b) structure ofepoxy networks.

Fig. 3. DSC curves for dry epoxy networks: E0 (▬), E1 (▬), E2 (▬), E3 (▬)lines are shifted for clarity.

24 I. Krakovsky et al. / European Polymer Journal 62 (2015) 19–30

and growing amount of the diamine chains grafted by oneend to the chains of the linked POPBGE molecules (seeFig. 2). Therefore, with increasing r-value the onset ofcrystallization is shifted to higher temperature andcompleted in a narrower temperature interval than in thestoichiometric network as the crystallization becomes lesshindered and formation of bigger crystallites is allowed.

In heating scans shown in Fig. 4, single glass transitionsand melting of the POE crystallites formed during the pre-vious cooling scan are registered in the networks. The glasstransitions in the networks can be seen in details of theheating calorigrams depicted in Fig. 4b. A separate glasstransition expected for the amorphous chains of POElocated in intercrystallite regions is not detected by DSCbecause it merges with the glass transition of the amor-phous phase of the networks and both processes occur inthe same temperature range. The values of the glass tran-sition temperature determined for the networks from theheating scans are listed in Table 3. The glass transitiontemperature of the networks varies only slightly with thenetwork composition obtaining values between �50 �Cand �53 �C, however, these values exceed the valuesdetermined for both, ED-2003 and POPBGE (see Table 2).Incorporation of ED-2003 and POPBGE into the networkstructure that decreases their molecular mobility is thereason for the observed increase of Tg – values above thevalues expected for long copolymers of the same POE/

, E4 (▬), E5 (▬) and E6 (▬) obtained during cooling at 10 �C/min. The

Fig. 4. DSC curves for dry epoxy networks obtained during heating at 10 �C/min: full temperature range (a), glass transition region (b). The same symbols asin Fig. 3. The lines are shifted for clarity.

Table 3Thermal characteristics of networks: glass transition temperature by DSC, Tg

DSC, specific heat capacity difference at glass transition determined from heatingscans, Dcp, specific heat capacity difference at glass transition calculated by Eq. (3), Dcav

p , onset temperature of melting, Tm, specific enthalpy of melting, Dhm,degree of crystallinity of POE, wcr, glass transition temperature from maximum of loss tangent by DMA, Tg

DMA, and storage Young modulus determined by DMAat frequency f = 1 Hz and temperature T = 50 �C, E0 .

Sample r TgDSC (�C) Dcp (J g�1 K�1) Dcav

p (J g�1 K�1) Tm (�C) Dhm (J g�1) wcr (%) TgDMA (�C) E0 (MPa)

E0 1.00 �51 0.36 0.42 �15 43 35 �33 1.05E1 1.11 �50 0.37 0.42 �10 45 36 �30 1.02E2 1.18 �51 0.33 0.41 �15 48 37 �31 0.98E3 1.24 �51 0.31 0.40 �20 51 39 �31 0.86E4 1.39 �52 0.26 0.40 �15 53 39 �30 0.52E5 1.67 �52 0.28 0.38 �10 52 37 �28 0.26E6 2.22 �53 0.26 0.36 �10 59 38 – –

I. Krakovsky et al. / European Polymer Journal 62 (2015) 19–30 25

Fig. 5. Temperature dependence of storage modulus (a) and loss tangent(b) for dry epoxy networks measured at by heating at 2 �C/min: E0 (j), E1(●), E2 (▲), E3 (▼), E4 (♦), and E5 (◄).

26 I. Krakovsky et al. / European Polymer Journal 62 (2015) 19–30

POP content using, e.g., Gordon–Taylor or Couchman equa-tion [27].

Melting of the POE crystallites in the networks startsslowly at ca �20 to �10 �C, nevertheless, the determina-tion of the precise values of the onset temperature isproblematic. With increasing r, the melting peak shiftsslightly to higher temperature and the melting endothermbecomes narrower.

Dividing the values of the specific melting enthalpydetermined for the networks by the value of the specificmelting enthalpy of 100% crystalline POE (see above) andthe POE content in the networks (see Table 2) yields thevalues of the degree of crystallinity of POE ca 35–39%. Thisis somewhat less than 49% determined in the neat ED-2003confirming hindrance of the crystallization by the polymernetwork environment.

The values of the specific heat capacity difference atglass transition, Dcp, determined for the networks (seeTable 3) are smaller than the values calculated by averag-ing the Dcp-values determined for the reagents by theirmass fractions as:

Dcavp ¼wPOEDcpðED�2003Þþð1�wPOEÞDcpðPOPBGEÞ ð3Þ

in accordance with expected restriction of molecularmotion of polymer chains in the networks caused bycross-linking [40].

Dynamic mechanical analysis reflects very sensitivelychanges in polymer chain mobility and cross-linkingdegree via measurement of the temperature dependenceof the mechanical properties of polymer networks. Fig. 5aand b shows the temperature dependence of the storageYoung modulus and loss tangent, respectively, obtainedfrom the networks during heating from �100 �C at heatingrate 2 �C/min and frequency f = 1 Hz.

Two transitions are registered in all networks investi-gated. At a temperature lower than ca �60 �C, the magni-tude of the storage Young modulus for all the networksis of the order of 1 GPa, therefore the networks are in theglassy state, in agreement with the DSC results. In heating,firstly, the networks reveal the main relaxation, shown as adecrease in the storage modulus starting at ca �60 �C. Themain relaxation process, associated with the glass transi-tion is indicated by the increase of the loss tangent (seeFig. 5b). In further heating, the storage Young modulusdrops down to the values of order of 100 MPa as can beseen in Fig. 5a. After passing the main relaxation, themobility of polymer chains in the amorphous phase isincreased, however, the POE crystallites having muchhigher storage Young modulus that are dispersed in theamorphous network play a role of the reinforcing filler.The reinforcing effect increases with increasing r becauseof the increasing POE content in the networks (see Table 1).At a further heating, the POE crystallites start to melt andtheir reinforcement effect gradually vanishes. The storageYoung modulus drops down for the second time to thevalues less than ca 1 MPa as can be expected from therubber elasticity of purely amorphous polymer network[41]. At 50 �C all the networks are again in amorphoushomogeneous rubberlike state. In the rubberlike state,magnitude of E0 increases with temperature as predicted

by rubber elasticity theory and decreases with r accompa-nied by a decrease of network density (see the values of E0

determined at 50 �C given in Table 3) confirming the trendillustrated in Fig. 2b. Melting of the POE crystallites is alsoreflected in the secondary maximum in the temperaturedependence of the loss tangent as depicted in Fig. 5b.

The main mechanical relaxation observed in all thenetworks is well-correlated with the glass transitionobserved by DSC. As often observed, the values of the mainrelaxation temperature, determined from the positions ofmaxima of loss tangent by DMA (see Fig. 5b) are shiftedto higher temperature with respect to the glass transitiontemperature determined using the half-Dcp method byDSC. This difference represents ca 18–24 �C (see Table 3)and can be explained by different experimental regimesused in (static) DSC and (dynamic) DMA [42].

Absence of microphase separation in the epoxy net-works investigated at ambient and higher temperaturesis in agreement with the results of DSC investigation ofsimilar epoxy networks by Tan et al. [43]. Space distribu-tion of diepoxide chains formed in epoxy networksdepicted in Fig. 2 represents topological inhomogeneity[44], only, and manifests itself in SAXS and SANS patternsas a scattering maximum. However, the shape of thescattering maximum observed in the epoxy networks very

Fig. 6. Phase diagram (swelling curves) of epoxy networks swollen toequilibrium in water: E0 (j), E1 (●), E2 (▲), E3 (▼), E4 (♦), E5 (◄) and E6(►). wH2O denotes the mass fraction of water in hydrogels (wH2O = 1-�wnetw). Lines are a guide to eye.

I. Krakovsky et al. / European Polymer Journal 62 (2015) 19–30 27

similar to those reported here does not correspond to amicrophase separated system [43–46].

3.3. Hydrogels

The temperature dependence of the swelling behaviourof the epoxy networks in water (hydrogels) was investi-gated in the range from 1 to 80 �C. The swelling curves(the mass fraction of epoxy network in the swollen statevs. temperature) are shown in Fig. 6. Heating the hydrogelsto temperatures higher than 80 �C for prolonged periodscauses their gradual degradation.

As depicted in Fig. 6, all the hydrogels deswell continu-ously with increasing temperature. This observation is aconsequence of the temperature dependence of the

Fig. 7. DSC curves for epoxy network swollen to equilibrium in H2O at 20 �C obtaare shifted for clarity.

interaction of both, POE and POP, with water. As men-tioned in introduction, with increasing temperature waterbecomes less favourable solvent for both polymers thechange being stronger for POP. The hydrogel obtained fromthe stoichiometric network exhibits the strongest deswell-ing, the water content decreases from ca 87 wt.% at 1 �C toca 42 wt.% at 80 �C.

It is interesting to compare the swelling behaviour ofthe stoichiometric hydrogels in the present and our previ-ous work (see network R in Fig. 5, Ref. [24]). Unlike thehydrogel in our previous work the release of water fromthe stoichiometric hydrogel in the present study (E0) byheating is smaller and more gradual. The water contentin the both hydrogels at low temperature (1 �C) is similar(87 wt.% and 80 wt.% in E0 and R, respectively) since POPbecomes more hydrophilic at this temperature. On theother hand, at 80 �C, the difference becomes large(42 wt.% and 12 wt.% in E0 and R, respectively) due tomuch higher POE content in the stoichiometric networkin the present study, as mentioned above.

With increasing r, the deswelling of the networksbecomes less temperature dependent, and finally, for thehydrogel from the network with r = 2.22 (E6) is almostindependent of temperature. With increasing r, theswelling curves are shifted to the left, i.e., they absorb morewater. This effect is caused to a small extent by growingPOE content (from 62 wt.% in E0 to 78 wt.% in E6, seeTable 1) but it is mainly due to the decreasing networkdensity in the series as illustrated by decreasing thestorage Young modulus in rubbery state (from 1.05 MPafor E0 and 0.26 MPa for E5, see Table 3). The lower thestorage Young modulus of the network the higher theamount of the water adsorbed.

Gradual temperature dependence of the swelling of thehydrogels in this work can be exploited in sensors oractuators working in aqueous environment requiringcontinuous control.

ined during cooling at 10 �C/min. The same symbols as in Fig. 3. The lines

Fig. 8. DSC curves for epoxy hydrogels swollen to equilibrium in H2O at 20 �C obtained during heating at 10 �C/min: full temperature range (a), glasstransition region (b). The same symbols as in Fig. 3. The lines are shifted for clarity.

3 In two hydrogels with the highest water content (E5 and E6) the highlyexothermic crystallization of water is very fast and the cooling scan cannotbe sufficiently controlled by the calorimeter. As a result, only approximateDhcr-values are calculated for the hydrogels obtained from these hydrogels.

28 I. Krakovsky et al. / European Polymer Journal 62 (2015) 19–30

Figs. 7 and 8 illustrate the thermal properties of thehydrogels measured by DSC. Before the measurementsthe hydrogels were first thermostatted for a sufficient timeat the starting temperature (20 �C). Since the water contentin all hydrogels at the starting temperature is high(wH2O > 0.82, see Table 4), in cooling (see Fig. 7) exothermsdue to crystallization of water in epoxy network environ-ment are observed in all hydrogels. At further cooling,broad glass transitions can be also seen with the propermagnification in these calorigrams (not shown).

If all the water in the hydrogels crystallized during thecooling scan, the enthalpy increment per unit mass ofhydrogel determined by integration of crystallization exo-therm from cooling scan, Dhcr, (see Table 4) should be

proportional to the water content in the hydrogel.3

However, this does not happen and part of water remainsnon-crystallized. Therefore, in cooling, the hydrogels areconverted into heterogeneous systems consisting of twophases: water crystallites and amorphous epoxy networkswollen in the non-crystallized water.

The mass fraction of non-crystallized water, w�H2O, can bedetermined from Dhcr and the specific melting enthalpy ofwater, DhH2O, (DhH2O = 6010 J mol�1 = 333.5 J g�1 [47]) by

Table 4Thermal characteristics of hydrogels: mass fraction of water in hydrogels at the starting temperature of cooling scan (20 �C), ~wH2O, mass fraction of non-crystallized water in hydrogels, w�H2O, enthalpy increment per unit mass of hydrogel, Dhcr, glass transition temperature, Tg, specific heat capacity difference atglass transition, Dcp, specific heat capacity difference at glass transition estimated by Eq. (5), D~cp , and onset temperature of water melting, Tm.

Sample r ~wH2O w�H2O Dhcr (J g�1) Tg (�C) Dcp (J g�1 K�1) D~cp (J g�1 K�1) Tm (�C)

E0 1.00 0.825 0.157 223 �63 0.13 0.063 �20E1 1.11 0.827 0.261 189 �64 0.22 0.064 �20E2 1.18 0.839 0.222 206 �63 0.17 0.053 �18E3 1.24 0.864 0.280 195 �64 0.18 0.042 �18E4 1.39 0.893 0.243 217 �63 0.10 0.028 �15E5 1.67 0.937 ca 0.30 ca 210 �65 0.05 0.018 �8E6 2.22 0.978 ca 0.32 ca 220 – – 0.006 �2

I. Krakovsky et al. / European Polymer Journal 62 (2015) 19–30 29

w�H2O ¼ wH2O �Dhcr

DhH2Oð4Þ

if a negligible heat of demixing of water, constancy of theheat of melting of water with temperature and identityof the specific heat of water in the phase and that of purewater can be assumed. The values of the mass fractionsof non-crystallized water are listed in Table 4. They aresomewhat lower than the values determined for thehydrogels obtained by swelling of a similar epoxy networkprepared using Jeffamine ED-2003 and diglycidyl ether ofBisphenol A propoxylate in heavy water [19].

Cooling scans also reveal differences in the crystalliza-tion process between the hydrogels. The process isbroader, with several peaks, in the samples with r = 1(E0) and r = 1.11 (E1) this behaviour is not found in sam-ples with higher stoichiometric ratio.

The heating calorigrams presented in Fig. 8 reveal weakand broad glass transitions followed by melting endo-therms of water. To see the glass transitions in thehydrogels clearly, this region of the heating calorigramsis re-plotted with magnification in Fig. 8b.

Values of the glass transition temperature and specificheat capacity difference at glass transition for the hydro-gels determined from the heating scans are listed inTable 4. As expected, the values of glass transition temper-ature in hydrogels are depressed about 10–14 �C withregard to the values determined for dry networks (seeTable 3). The difference is caused by presence of the non-crystallized water in amorphous network phase increasingits molecular mobility.

The values of the specific heat difference at glass transi-tion determined for the hydrogels are also depressed rela-tive to the values for the dry network (see Table 3) due toswelling of the polymer network and decrease of the poly-mer content in the hydrogel. However, the depression isnot so large as one might expect from the epoxy networkcontent in the hydrogels by

D~cp ¼ ~wnetwDcnetwp ¼ ð1� ~wH2OÞDcnetw

p ð5Þ

where ~wnetw and ~wH2O are the mass fractions of polymerand water in the hydrogel subjected to DSC (i.e. at thestarting temperature of the cooling scan (20 �C)4) and

4 During cooling scans the water content in the hydrogels does notchange since the hydrogel sample in the calorimeter holder is not in contactwith excess of water and cannot absorb more water to become fullyswollen at lower temperature, see Fig. 6.

Dcnetwp is the specific heat difference at the glass transition

of the dry network. The D~cp – values calculated using datalisted in Tables 3 and 4 are also given in Table 4. We cansee that the experimental Dcp-values at the glass transitionare about 2–4 times higher than the values estimated bymeans of Eq. (5). The magnitude of Dcp at the glass transi-tion reflects the height of the energy barrier to be overcomeby the molecules in the hydrogels in the transition. Asexplained in our previous work [24], the elevated Dcp –values determined experimentally for the hydrogels arecaused by the presence of hydration envelopes consistingof non-crystallized water molecules that are at low temper-ature H-bonded to the POE and POP blocks inbuilt in thenetworks. The hydration envelopes make the molecularrearrangement required by the glass transition more diffi-cult than it would be in absence of the H-bonds. Formationof the hydration envelopes in the aqueous solutions of POEas well as POE–POP block copolymers has been confirmedand investigated by a number of methods [48–52].

In heating, the water crystallites formed in the hydro-gels during cooling scan start to melt. The melting occursat a temperature lower than the melting temperature ofbulk water. With increasing water content the differencebetween the melting temperature of water in the hydro-gels and bulk water becomes smaller (the values of theonset temperatures of melting, Tm, determined for thenetworks with r = 1.00 (E0) and 2.22 (E6) are ca �20 �Cand �2 �C, respectively, see Table 4). This phenomenon isdue to a very small size of water crystallites formed inthe epoxy network environment during cooling.

Unlike the hydrogels investigated in our previous work[24], the exotherms of demixing of water from the hydro-gels in the present study are not observed. We think thatthe more gradual deswelling process observed and, conse-quently, less intensive water release during heating of thehydrogels can explain the absence of the exotherms.

4. Conclusions

A series of hydrophilic epoxy networks was prepared bythe end-linking reaction of a,x-diamino terminated POP-b-POE-b-POP of average molar mass ca 2000 g mol�1 andpolyoxypropylene bis(glycidyl ether) of average molarmass ca 640 g mol�1. The networks prepared are homoge-neous at ambient and higher temperatures.

In cooling, the networks become inhomogeneous as aresult of formation of the POE crystallites dispersed in

30 I. Krakovsky et al. / European Polymer Journal 62 (2015) 19–30

the amorphous network. At subambient temperatures, thecrystallites play a role of reinforcing filler. In heating, thecrystallites melt and at a sufficiently high temperature allthe networks are again in the amorphous homogeneousrubberlike state.

The hydrogels obtained by swelling of the networks inwater show a strongly temperature dependent swellingbehaviour. They deswell continuously with increasingtemperature, the deswelling becomes less pronouncedwith increasing POE content and decreasing networkdensity of the epoxy network.

Cooling of the hydrogels with high water content to lowtemperatures enables crystallization of the main part ofwater. The hydrogels become phase separated, water crys-tallites representing one phase are dispersed in amorphousnetwork swollen by remaining (non-crystallized) water.

The hydrogels prepared and investigated in this workcan find applications in the sensors and actuators workingin aqueous environment when a continuous control of theswelling degree in a wide range is required.

Acknowledgements

The result was developed within the CENTEM project,reg. no. CZ.1.05/2.1.00/03.0088, co-funded by the ERDF aspart of the Ministry of Education, Youth and Sports OPRDI programme. The authors appreciate a generous giftof the Jeffamines used in this study, by Huntsman Corpora-tion. RSS acknowledge the support of the Spanish Ministryof Economy and Competitiveness through the projectMAT2012-38359-C03-01 (including the FEDER financialsupport).

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