Cationic copolymerization of cycloaliphatic epoxy resin with a spirobislactone with lanthanum...

Cationic Copolymerization of Cycloaliphatic Epoxy Resinwith a Spirobislactone with Lanthanum Triflate asInitiator: I. Characterization and Shrinkage

XAVIER FERNANDEZ,1 JOSEP MARIA SALLA,1 ANGELS SERRA,2 ANA MANTECON,2 XAVIER RAMIS1

1Laboratori de Termodinamica, ETSEIB, Universitat Politecnica de Catalunya, Diagonal 647, 08028 Barcelona, Spain

2Departament de Quımica Analıtica i Quımica Organica, Universitat Rovira i Virgili, Marcel�lı Domingo s/n,43007 Tarragona, Spain

Received 1 February 2005; accepted 1 March 2005DOI: 10.1002/pola.20801Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: 3,4-Epoxycyclohexylmethyl 3,4-epoxycyclohexane carboxylate (ECH) wascured with different proportions of 1,6-dioxaspiro [4,4]nonane-2,7-dione (s(�-BL))using lanthanum triflate as a catalyst. The shrinkage undergone during curing wasmonitored by means of thermomechanical analysis (TMA) in isothermal experiments.Fourier transform infrared spectroscopy in attenuated-total-reflection mode (FTIR/ATR) was used to study the evolution of lactone, epoxide, and intermediate spiroor-thoester (SOE) groups to identify the different reactions that take place during thecuring process. DSC was used to study the thermal characteristics of the curing proc-ess and to assess the glass-transition temperature (Tg) of the cured material. Thedynamic mechanical properties of the cured material were determined based on thedata obtained by DMTA. An increase in the proportion of s(�-BL) led to a decrease inthe gelation time and the shrinkage after gelation. By combining the data obtainedby TMA and FTIR/ATR, it was also possible to identify the reactive processes respon-sible for the shrinkage. It was observed that an increase in the proportion of s(�-BL)also increases the speed of the curing process and modifies the structure of the mate-rial, thus giving rise to more flexible materials. VVC 2005 Wiley Periodicals, Inc. J Polym Sci

Part A: Polym Chem 43: 3421–3432, 2005

Keywords: epoxy resins; expanding monomers; lactones; shrinkage; thermosets

INTRODUCTION

The curing of thermosetting materials is accom-panied by some degree of shrinkage, which canlead to internal stress in the material, pooradhesion of coatings to the substrate, and theappearance of microvoids and microcracks,which reduce the durability of materials.1–3

Shrinkage during curing can be reduced tosome extent by means of using high molecular

mass resins and copolymers or using fillers,which may also improve the overall propertiesof the material. A different solution entailsusing so-called expanding monomers, which canpolymerize without shrinkage or even withexpansion. Ring-opening polymerization of sev-eral expanding monomers, such as spiroorthoest-ers (SOEs) or spiroorthocarbonates (SOCs), hasbeen proposed as an effective way of reducingshrinkage during curing.3–5

SOEs are obtained by reacting lactones withepoxides under cationic catalysis.6 However, dueto their high cost, they are not commonly used

Correspondence to: X. Ramis (E-mail: [email protected])

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 43, 3421–3432 (2005)VVC 2005 Wiley Periodicals, Inc.

3421

either alone or as comonomers in technologicalapplications. It has been proposed that thecopolymerization of epoxy resins and lactonesmay take place via SOE formation and reaction,thus reducing volume shrinkage during curing.2

In our research group, we have successfullycopolymerized DGEBA epoxy resins with �-butyrolactone using lanthanide triflates as cata-lysts and have reported a reduction in theshrinkage after gelation. We have identified thedifferent competitive reactions that take placeduring the curing process and state that the pol-ymerization of SOEs at the end of the curingprocess is responsible for this reduction inshrinkage.2 Similar studies have been performedwith 3,4-epoxycyclohexylmethyl-3,4-epoxycyclo-hexanecarboxylate (ECH) and �-butyrolactone.7

The copolymerization of epoxy resins with1,6-dioxaspiro [4,4] nonane-2,7-dione (s(�-BL))has been reported through anionic catalysis.8–10

This spirobislactone exhibits the characteristicsof an expanding monomer, as described by Bai-ley.3 Chung and coworkers10 demonstrated thatthis copolymerization leads to the formation ofan alternating copolymer when working in dis-solution, but with a possible homopolymeriza-tion of the epoxy resin in bulk polymerization.Sikes and Brady8 report a reduction in contrac-tion before gelation, due to its higher reactivitywith respect to the epoxy resin undergoinganionic catalysis, but only when working withvery reduced quantities of s(�-BL). Hao and col-leagues propose a modification of one of the spi-robislactones employed by Brady, introducingmaleimido groups, to improve this expansiveeffect during the curing process with an epoxyresin.11

This article focuses on the cationic copolymer-ization of ECH with s(�-BL) using lanthanumtriflate as a catalyst. Scheme 1 shows the struc-ture of both ECH and s(�-BL). It must be takeninto account that both monomers have a func-tionality of 4, which obviously makes the systemrather complex because both mono and bisSOEcan form and react during the curing process.

Examples of these reactions are: homopolymeri-zation of both ECH and s(�-BL) (Scheme 2), mono-SOE formation, bisSOE formation (Scheme 3),reaction of mono-SOE or bisSOE with ECH,homopolymerization of mono-SOE, and homo-polymerization of bisSOE (Scheme 3).

The homopolymerization of five-memberedlactones, such as �-butyrolactone, shown inScheme 2 for s(�-BL), is thermodynamicallyunfavorable and takes place under very extremeconditions.12 Some authors13,14 have found that�-butyrolactone can copolymerize cationicallywith other lactones, such as "-caprolactone, andcan even form small blocks of homopolymers. Inthis study, we have seen that, in the absence ofECH resin, certain quantities of s(�-BL) canhomopolymerize.

The objective of this study is to determine theeffect of s(�-BL) on the shrinkage and to identifythe different reactions that take place duringthe curing process. DSC is used to determine theheat evolved in the curing and the Tg of thefully cured material. Thermomechanical analysis(TMA) will be used to monitor the shrinkageundergone during curing, and Fourier transforminfrared spectroscopy in attenuated-total-reflec-tion mode (FTIR/ATR) will be needed to identifythe different reactive processes. DMTA will beused to determine the thermomechanical behaviorof the material, the maximum of tan �, and thedegree of crosslinking of the fully cured material.

Finally, it should be mentioned that theintroduction of ester groups by copolymerizingScheme 1

Scheme 2

3422 FERNANDEZ ET AL.

conventional epoxy resins with lactones couldbe a valuable strategy, not explored until now,to improve the degradability of thermosetsunder controlled conditions, which enables thestraightforward repair or recycling of electronicdevices assembled with such materials. More-over, the mechanical properties of epoxy resinscan be improved, because the aliphatic chainsintroduced by the chemical incorporation of lac-tone increase the flexibility of the material,reducing its fragility.

EXPERIMENTAL

Materials

ECH epoxy resin (epoxy equiv. ¼ 126 g/eq)(Araldite CY 179, Vantico) and s(�-BL) (molecu-

lar mass ¼ 156 g/mol, 98%) (Aldrich) were usedas received. Lanthanum (III) trifluoromethane-sulfonate (99.99%) (Aldrich) was used withoutpurification.

Preparation of The Curing Mixtures

Samples were prepared by adding to the corre-sponding proportions of s(�-BL) and ECH 1 phrof lanthanum triflate (1 part per 100 parts ofmixture, w/w). ECH was kept for a while atroom temperature, and s(�-BL) was crushedwith a mortar. Samples were carefully stirredand kept at �18 8C before use to prevent poly-merization.

Table 1 shows the various formulations thatwere used, their notation, and the proportions ofthe different reactants.

DSC Calorimetry

The various samples were dynamically curedusing a Mettler DSC-821e calorimeter with aTSO801RO robotic arm. Samples of approxi-mately 10 mg in weight were cured in alumi-num capsules in a nitrogen atmosphere.Dynamic experiments were performed between0 and 200 8C at heating rates from 2 to 15 8C/min to determine the reaction heat. The Tg ofthe cured materials was determined based on asecond scan at 10 8C/min after dynamic curing,with samples that weighed approximately20 mg.

FTIR Spectroscopy

Samples were cured isothermally at 100 and120 8C in an FTIR Bomem Michelson MB 100spectrophotometer, with a resolution of 4 cm�1

in the absorbance mode. An attenuated totalreflection accessory with thermal control and adiamond crystal (Golden Gate Heated Single

Table 1. Relation Between the Various Formulations Studied, Notation, andProportion of Reactants, Which Contain 1 phr of Lanthanum Triflate as Initiator

Formulation NotationMolar ratioECH:s(�-BL)

s(�-BL)(%, w/w)

Mol Catalyst/Eq Epoxy

ECH/s(�-BL) 1:0 1:0 1:0 0 0.00215ECH/s(�-BL) 4:1 4:1 4:1 13.4 0.00248ECH/s(�-BL) 2:1 2:1 2:1 23.6 0.00282ECH/s(�-BL) 1:1 1:1 1:1 38.2 0.00348

Scheme 3

CATIONIC COPOLYMERIZATION OF CYCLOALIPHATIC EPOXY RESIN 3423

Reflection Diamond ATR, Specac-Teknokroma)was used to determine the FTIR spectra.

The disappearance of the absorbance peak at795 cm�1 (oxirane ring deformation) was used tomonitor the epoxy conversion. The consumptionof the reactive carbonyl group of the s(�-BL)was evaluated by measuring the changes inabsorbance at 1795 cm�1 (carbonyl C¼¼Ostretching of cyclic ester). An increase in theabsorbance peak at 1725 cm�1 (carbonyl C¼¼Ostretching of aliphatic linear ester), which corre-sponds to the carbonyl group of the ECH,seemed to indicate that SOE polymerization didindeed occur. SOE reaction could therefore beevaluated by measuring the difference betweenthe absorbance peak at a given time and the ini-tial absorbance peak at 1725 cm�1. The increasein the absorbance peak at 1075 cm�1 (C-O-Cstretching of aliphatic linear ether), which wasnot used for further calculations, showed thatepoxy homopolymerization took place. The peakat 1450 cm�1 (methylene group) was chosen asan internal standard. Thus, the normalizedabsorbances are calculated as follows:

A�xxx ¼

Axxx

A1450ð1Þ

Conversions of the different reactive groups,epoxide, s(�-BL), and SOE, were determined bythe Lambert–Beer law from the normalizedchanges of absorbance at 795, 1725, and1795 cm�1, as

�epoxy ¼ 1� A�795;t

A�795;0

!ð2Þ

�sð��blÞ ¼ 1� A�1795;t

A�1795;0

!ð3Þ

�SOE ¼ A�1725;t � A�

1725;0

A�1725;1 � A�

1725;0

!ð4Þ

where A�795;t, A

�1795;t, and A�

1725;t are the normal-ized absorbances corresponding to the peaks at795, 1795, and 1725 cm�1 at a given time t;A�

795;0, A�1795;0, and A�

1725;0 are the absorbances atthe beginning of the curing process; and A�

1725;1is the normalized absorbance at 1725 cm�1 atthe end of the curing process.

Thermomechanical Analysis (TMA)

The contraction undergone by the samples wasstudied using a Mettler TMA40 thermomechani-cal analyzer.

The samples were supported by two small cir-cular ceramic plates and silanized glass fibers,which were impregnated with the sample.

A further series of isothermal experiments wasundertaken using TMA at 100 8C to monitor con-traction during the curing process. The degree ofcontraction �TMA can be calculated as follows:

�TMA ¼ Lt � L0

L1 � L0ð5Þ

where Lt, Lo, and L? represent, respectively, thethickness of the sample at time t, at the onset,and at the end of the reaction.

Experimentally, one can determine the gelationpoint by using TMA starting at a region in whichcontraction is not observed between two contrac-tion steps in the sample. Other studies have dem-onstrated how, from this point on, the samplesbecome partially insoluble.2 This point is impor-tant because internal stresses will only be gener-ated in the resin once gelation has occurred.3

Dynamomechanical Analysis (DMTA)

A DMTA Rheometrics PL-DMTA MKIII Analyserwas used. The samples were cured isothermally in amold at 150 8C for 5 h and were then subjected to apostcuring at 180 8C for 4 h. Single cantilever bend-ing was performed on prismatic rectangular sam-ples (1.7� 8� 5 mm3). The apparatus was operateddynamically, at 2 8C/min, from �25 to 215 8C. Thefrequency of application of the force was 1 Hz.

Measurement of Density

The density of the samples was measured beforecuring at ambient temperature by directly meas-uring the sample using a pycnometer. For thecured samples, the density was determined indi-rectly by the flotation method, starting at disso-lutions of KBr with densities lower and higherthan that of the sample.

RESULTS AND DISCUSSION

Overall Characterization

An overall characterization of the materials wasmade before and after the curing process. The


reaction heat was found by monitoring thedynamic curing of the various formulations. Asecond dynamic experiment was used to deter-mine the Tg of the completely cured material.The density was determined before and aftercuring, so as to calculate the overall contractionof the samples. Table 2 summarizes the resultsof this characterization.

One can see how the increase in the propor-tion of s(�-BL) causes the exothermicity of thereaction to decrease, in absolute terms, but takeinto account the amount of epoxy present in thesample, the value is stable, except for the 1:1 for-mulation, which is considered to reach a lowerdegree of curing such as observed for ECH/�-BLmixtures.7 Thus, there is also a decrease in theTg of the cured material because of the flexiblechains introduced into the structure by thereacted lactone. As can be seen in Table 2, s(�-BL)

has no appreciable effect on the overall contrac-tion undergone during the curing process, exceptfor the 1:1 formulation. In this formulation, onecan see that the degree of curing achieved islower, and this reduction in contraction can thusbe attributed to a lesser reaction. The s(�-BL)also presents an accelerative effect in the reac-tion, as can be seen in Figure 1, in which thesamples with an increasing proportion of s(�-BL)exhibit a higher conversion at a given tempera-ture, such as observed in �-BL mixtures.1,7

Monitoring the Reactions

IR monitoring was undertaken of the 4:1, 2:1,and 1:0 formulations at temperatures of 100 and120 8C to assess the evolution of the differentreactive groups. Figure 2 shows representationsof the significant bands for the 4:1 formulation

Figure 1. Conversion against temperature for the various formulations studied.Dynamic experiments at 5 8C/min in a nitrogen atmosphere.

Table 2. Calorimetric Data Obtained Using DSC, Densities,and Contraction of the Various Formulations

FormulationDh(J/g)

Dh(kJ/ee)

Tg

(8C)�0

(g/cm3)�?

(g/cm3)Shrinkage

(%)

ECH/ s(�-BL) 1:0 627 79.8 154 1.169 1.214 3.71ECH/ s(�-BL) 4:1 532 78.2 124 1.188 1.232 3.57ECH/ s(�-BL) 2:1 489 81.4 98 1.199 1.247 3.85ECH/ s(�-BL) 1:1 322 66.4 82 1.230 1.264 2.69

Dh is the exothermic heat of reaction, in J/g and kJ/ee (epoxy equivalents). �0 and �? are thedensities before and after the curing process, respectively.


at different moments of the curing process at100 8C. At the end of the curing process, thepeak corresponding to the epoxy resin, at 795cm�1, has completely disappeared, as a result ofwhich one can conclude that the ECH resin iscompletely cured. Furthermore, based on thesmall peak remaining at 1775 cm�1 at the endof the curing process, one can deduce that thes(�-BL) has not reacted completely.

Figure 2 also shows how the peaks at 1795and 1725 cm�1 overlap. It was thus necessary toperform a mathematical deconvolution of thespectrum in this region, by using the Gaussian–

Lorentzian function (sum of areas) of the Peak-Fit program (from Jandel Scientific Software).1

Figure 3 shows the conversion of the epoxygroup over time for the 1:0, 4:1, and 2:1 formu-lations at 100 8C. As one can see, the formula-tions richest in s(�-BL) exhibit a higher epoxyreaction speed. This acceleration was previouslyobserved for the copolymerization of �-BL andDGEBA or ECH resins.7,15

Figure 4 shows the conversion of the s(�-BL)group against time for different formulations andtemperatures. At 100 8C we can see how the dis-appearance of the lactone is faster for the 4:1 for-

Figure 3. Conversion of the epoxy group against time for the various formulationsstudied.

Figure 2. FTIR spectra for the 4:1 formulation at 100 8C, at the onset, at an inter-mediate point, and toward the end of the reaction.


mulation than for the 2:1 formulation, until onereaches 0.5 conversion, at which the reaction oflactone stops for the 4:1 formulation and slowsdown for the 2:1 formulation. Further along, onecan see how, in the 2:1 formulations, the conver-sion of s(�-BL) experiences a sharp increase andcontinues to react even at higher conversions,while the 4:1 formulation reacts more slowly.

If one compares Figure 3 and Figure 4, onecan see that when the conversion of s(�-BL) is0.5, practically all of the epoxy groups havereacted. This stepping of the conversion under-gone by s(�-BL) is not exclusive to the 2:1 formu-lation. It was tried with the 4:1 formulations at120 8C, and the same phenomenon was observed:the stop at 0.5 conversion, a subsequent increase,and a continued reaction. This stepping could be

due to the fact that, once one of the rings of thes(�-BL) reacts, the other becomes less reactivedue to effects that are both kinetic (the absence offunctional epoxy groups) and topologic (a decreasein mobility and an increase of steric hindrance),and thus takes far longer to react. The fact thatthis phenomenon is so slow for the 4:1 formula-tion at 100 8C could be explained because at thistemperature, once 0.5 conversion of s(�-BL) hasbeen achieved, the material is very close tobecoming vitrified and the reaction goes frombeing under kinetic to diffusional control. For the2:1 formulation, it can continue to react, as thecure temperature is greater than the Tg of thecompletely cured material (Table 2).

Figure 5 shows the evolution of the variousgroups against the conversion of the carbonyl

Figure 4. Conversion of s(�-BL) against time for the 4:1 and 2:1 formulations atdifferent temperatures.

Figure 5. Conversion of the various reactive groups (epoxy, SOE) and the degree ofcontraction against the conversion of s(�-BL) for the 4:1 formulation. Isothermalexperiments at 100 8C.


group of s(�-BL) for the 4:1 formulation at 1008C. As one can see, the epoxy reacts veryquickly. Indeed, the conversion of the epoxygroup is nearly complete when the conversion ofs(�-BL) is 0.5. One can also see how, at thebeginning, SOE reacts more slowly than s(�-BL), but then goes on to accelerate, until theconversion reaches 0.5–0.6 for both groups.From this moment on, both groups react at simi-lar speeds. As there is no epoxy left in the sam-ple, the s(�-BL) must homopolymerize, as theopening of the ring of the s(�-BL) due to homo-polymerization shows the same signal as theopening of SOE, at 1725 cm�1. Homopolymeriza-tion of s(�-BL) with 1 phr of lanthanum triflatewas confirmed by DSC, but very little enthalpywas released.

Figure 5 also shows how the experimentalpoints of the conversion of s(�-BL) and SOE donot exceed a value of 0.85. Therefore, one couldsay that lactone does not react completely andthat the rings that have not reacted may reducethe degree of crosslinking of the cured material.As has been stated, at these temperatures thematerial vitrifies before it is completely cured.The cured material at this temperature canreach a maximum Tg of 100 8C, not of 124 8C,which would correspond to a complete cure. Wehave observed that postcuring the sample at ahigher temperature creates the possibility thatthose groups of s(�-BL) that have not reactedwill undergo a complete reaction, which thusenables the material to reach its final thermaland mechanical properties.

If we analyze the 2:1 formulation, the resultsare slightly different. Figure 6 shows the conver-sion of the epoxy group and the SOE over theconversion of s(�-BL).

The graph is divided into three parts. In thefirst part, the epoxy reacts quickly until itreaches a conversion of 0.7; the s(�-BL) reacts ataround 0.25 and SOE at up to 0.2. In the secondpart, the epoxy continues to react more slowlyuntil a conversion greater than 0.95, while s(�-BL) reacts up to 0.8 and SOE reacts very slowlyup to 0.45. The last part occurs almost entirelywithout epoxy groups. In the first part, the ECHresin is homopolymerized and SOE forms andreacts. In the second part, the system evolvesdue to the reaction of the lactone and the epoxyto give rise to SOE, which practically does notreact. Finally, once the epoxy group is nearlyrun out, the reaction continues with the homo-polymerization of the SOE that had beenformed, the formation of SOE with the littleepoxy left over and part of the s(�-BL), and thehomopolymerization of a substantial part of theremaining s(�-BL), which can’t find epoxy toform SOE.

Clearly, there are differences in the reactiveprocesses based on the amount of s(�-BL) in thesample, as was demonstrated by comparing theformulation with a low s(�-BL) content (4:1)with the formulation with a high s(�-BL) con-tent (2:1). s(�-BL) favors the formation of SOE,which reacts very slowly, over the homopolyme-rization of ECH resin. With high s(�-BL) con-tent, part of the SOE does not completely react

Figure 6. Conversion of the various reactive groups (epoxy, SOE) and the degree ofcontraction against the conversion of s(�-BL) for the 2:1 formulation. Isothermalexperiments at 100 8C.


and thus remains incorporated in the networkas a terminal group. Therefore, it reduces cross-linking in the cured material.

All these studies confirm in some formula-tions the complete chemical incorporation in thenetwork of s(�-BL), due to its homopolymeriza-tion capability, in contrast to the previousresults obtained with �-BL in which some pro-portion of lactone remains unreacted in the finalmaterial when the proportion of lactone is high.7

Contraction and Gelation

The various formulations have been tested usingTMA at 100 8C to determine the gelation pointof the resin and the contraction that takes placebefore and after gelation.

Figure 7 shows graphs of the contractioncurve for the various formulations at 100 8C.One can see how for all the formulations, thecontraction of the resin takes place in twostages, between which occurs gelation, which isassociated with the inflection point in the con-traction curve.

By increasing the proportion of s(�-BL), gelat-ion is reached earlier, and this takes place at ahigher �TMA. This is true when one comparesthe formulations that contain s(�-BL). Thechange from the 1:0 to the 4:1 formulationcauses gelation to take place earlier, but thisdoes not entail a clear increase in contractionprior to gelation. One must bear in mind, then,

that the curing process of the 1:0 and 4:1 formu-lations is not complete at these temperatures, asthey vitrify before they are completely cured. Inany case, though, the curing process of the 1:0formulation will be more incomplete than thatof the 4:1 formulations. The 2:1 and 1:1 formula-tions do not exhibit this problem, because thecure temperature is higher than their Tg.

One can compare the evolution of contractionbetween the different reactive groups during anisothermal cure at 100 8C. Figures 5 and 6 cor-respond to the 4:1 and 2:1 formulations, respec-tively.

For the 4:1 formulation, the first stage of con-traction is characterized by the homopolymeriza-tion of the epoxy resin and the formation ofSOE, which reacts very slowly. In the secondstage, in which contraction is lower, homopoly-merization of the epoxy continues to take place,though the SOE begins to react more quickly.The last stage, in which there is no epoxy leftand, likewise, there is no contraction, is mainlycharacterized by the homopolymerization of s(�-BL).

The 2:1 formulation exhibits a slightly differ-ent behavior. For this formulation, the firststage of the contraction, before gelation, coin-cides with what is outlined above: the homopoly-merization of the epoxy groups and the forma-tion and opening of a certain amount of SOE,which probably copolymerize with the epoxygroups. It is when gelation begins that the open-

Figure 7. Degree of contraction, �TMA, against time for the various formulationsstudied. Isothermal experiments with the TMA at 100 8C.


ing reaction of SOE begins to slow down. Fromthis moment on, contraction occurs once againdue to epoxy reaction with lactone to give rise toSOE, though to a lesser degree. Finally, whenthere is practically no epoxy, the remaining lac-tone is homopolymerized and SOE opened, with-out shrinkage.

We have observed how an increase in the pro-portion of s(�-BL) causes contraction to takeplace largely before gelation. This result is veryimportant, as, due to the reduction in contrac-tion after gelation, the formation of internalstresses during the curing process is minimized.The contraction undergone during the curingprocess is fundamentally associated with thehomopolymerization of the epoxy groups and, toa lesser degree, with the reaction of epoxygroups with lactone groups to give rise to SOE.

We have also demonstrated how the opening ofthe SOEs that are formed and the homopolyme-rization of s(�-BL) together produce an actualreduction in contraction, by compensating forother, unfavorable reactions.

Dynamic Mechanical Analysis

Figures 8 and 9, respectively, show the evolutionof the E0 storage module and tan � against thetemperature obtained by DMTA. The character-istic parameters that can be drawn from theexperiment are shown in Table 3.

As was seen in the samples cured with DSC,the Tg of the material (temperature of the peakof the tan �) decreases as the s(�-BL) in thesample increases. The values of the maximum oftan � obtained by DMTA are greater than Tg

Figure 8. Storage modulus, log E0, against temperature for the various formula-tions studied.

Figure 9. Tan � against temperature for the various formulations studied.


obtained by DSC due to DMTAs having agreater equivalent operating frequency.16

Assuming that the cured material is uni-formly crosslinked, one can calculate the molec-ular mass between points of crosslinking usingthe following equation, which corresponds to therubber elasticity theory17:

Mc ¼ 3��RT

E0r

ð6Þ

where R is the gas constant; T is the absolutetemperature in K, and takes the value of Tg þ50; E0

r is the relaxed module at this tempera-ture; � is the density of the cured sample; and �is an experimental factor that takes a value of 1for most polymers.

As the content in s(�-BL) increases, one canobserve how the Mc increases as well. For thisreason, one can conclude that s(�-BL) makes theformation of more flexible structures in thecured resin, although in some cases a decreasein the Tg is due to the presence of groups thathave not reacted and reduce the degree of cross-linking.

In any case, this increase in flexibility due tothe addition of s(�-BL), alongside the fact thatthe materials exhibit an increasingly smallercontraction after gelation, could be of signifi-cance to the development of coatings thatundergo little contraction and exhibit a certainflexibility.

From the graph of tan � and E0 over tempera-ture, one can determine not only the maximumof tan � but also the dispersity of the sample interms of the amplitude of the peak of tan � orthe decrease of E0. The height of the peak of tan� is also an indicator of the significance of therelaxation of the material when it undergoes

glass transition. The differences observed in thevarious formulations are the result of the differ-ent proportions of ECH resin and s(�-BL) andhow these react during the curing process.

The most significant case is that of the 1:1formulations, in which the representation of tan� shows three peaks, which correspond toregions with different degrees of crosslinkingand structure, and a very wide glass transitiontemperature range.

In the remaining formulations, one does notfind a significant difference in the amplitude ofthe peaks, even though in the 2:1 formulationsone can more clearly see an initial shoulder,which would be indicative of the presence ofmaterial with a lower degree of crosslinking,which causes the glass transition interval to bemuch wider than in the cases of the 4:1 and 1:0formulations. This means that the dynamicmechanical behavior of the material is more cor-rectly described by the entire interval of temper-atures in which the mechanical relaxation takesplace than by just using the value assigned tothe maximum of tan �. From the representationof the storage module E0, one can also deducethat the decrease in the E0 module takes placein a much wider range for the 1:1 and 2:1 for-mulations than for the 4:1 and 1:0 formulations.

As regards the height of the peak of tan �,one can see that the 4:1 formulation is thatwhich exhibits the most significant relaxation,which contrasts with the fact that this materialhas a higher glass transition temperature thanother formulations, such as the 2:1 formulation,and should as such be more flexible.

Finally, it should be said that the materialsprepared by copolymerization were easily dis-solved in 1M ethanolic KOH, which can be due tothe introduction of a higher proportion of estergroups and its secondary character in the tridi-mensional network. Moreover, the ester groupsintroduced are easier to break down by pyrolysis.The thermal degradation of these materials willbe reported in a forthcoming article.

CONCLUSIONS

ECH epoxy resin was cationically copolymerizedwith spirobislactone s(�-BL). Thermostable net-works were obtained that had a higher contentin ester groups than pure ECH resin, whichcould facilitate their controlled degradationeither thermally or via saponification.

Table 3. Parameters Obtained Using DMTA:Maximum of tan � (Tmax), Relaxed Storage Module(E0

r), and Molecular Mass Between Pointsof Crosslinking (Mc)

FormulationTmax

(8C)Log Er

0

(Pa)Mc

(g/mol)

ECH/ s(�-BL) 1:0 164 7.25 653ECH/ s(�-BL) 4:1 146 7.21 883ECH/ s(�-BL) 2:1 138 7.17 958ECH/ s(�-BL) 1:1 98a 6.99 1336

a This value of maximum of tan � has been obtained fromthe largest peak observed for this formulation, according toFigure 9.


The introduction of s(�-BL) reduces the exo-thermicity of the process, accelerates the reac-tion, and decreases the Tg of the cured material.Nevertheless, it does not exert a significanteffect on the overall contraction that takes placeduring the curing process.

Through simultaneous monitoring of the cur-ing process of formulations containing differentproportions of ECH resin and s(�-BL), usingFTIR and TMA, we were able to identify thechemical processes that characterize the curing,and their relationship with the contraction thattakes place during the process.

Even though s(�-BL) does not reduce theoverall contraction during the curing process, itplays an important role due to the fact that itdecreases contraction after gelation. In this way,internal stresses that will form during the cur-ing process are reduced.

The reactions associated with a decrease incontraction after gelation are the opening ofSOE, whether it be due to homopolymerizationor reaction with epoxy, and the homopolymeriza-tion of s(�-BL). Furthermore, the reactive proc-esses responsible for the largest initial contrac-tion prior to gelation are the homopolymeriza-tion of the epoxy resin and the formation ofSOE with epoxy resin and s(�-BL).

An increase in the proportion of s(�-BL)causes an increase in the molecular massbetween points of crosslinking and a decrease inthe glass transition. Both of these phenomenaindicate that the copolymerization of s(�-BL)with ECH resin gives rise to more flexible mate-rials, whether it be due to incorporation in a pol-ymer chain made up of longer and more mobilesegments or due to an incomplete reaction ofs(�-BL).

The authors from the Universitat Politecnica de Cata-lunya would like to thank the CYCIT (Comision Inter-misterial de Ciencia y Tecnologıa) and FEDER (Euro-pean Regional Development Fund) (PPQ2001-2764-C03-02 and MAT2004-04,165-C02-02). The authorsfrom the Rovira i Virgili University would like to

thank CYCIT, FEDER (MAT2002-00,291), and CIRIT(Comissio Interdepartamental de Recerca i InnovacioTecnologica) (SGR 00,318).

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