Dielectric properties of cured epoxy resin + poly(ethylene oxide) blends

Journal of Non-Crystalline Solids 351 (2005) 2728–2734 www.elsevier.com/locate/jnoncrysol

Dielectric properties of cured epoxy resin + poly(ethylene oxide) blends Ioannis M. Kalogeras a,*, Michalis Roussos a, Iraklis Christakis a, Anna Spanoudaki b, Dorota Pietkiewicz c, Witold Brostow c, Aglaia Vassilikou-Dova a a

c

Department of Physics, Solid State Physics Section, University of Athens, Panepistimiopolis, 157 84 Zografos, Greece b Department of Physics, National Technical University of Athens, Zografou Campus, 157 80 Athens, Greece LAPOM, Department of Materials Science and Engineering, University of North Texas, P.O. Box 305310, Denton, TX, USA

Abstract Epoxy resin (ER) + poly(ethylene oxide) (PEO) blends cured with 4,4 0 -diaminodiphenyl-methane were studied by thermally stimulated currents and dielectric relaxation spectroscopy. The results confirm components miscibility with dielectric glass transition temperature-composition dependence obeying the empirical Gordon–Taylor G–T equation with k = 0.38. Positive departures from the G–T curve appear at high PEO loadings due to crystallization of the linear polyether. Subtle perturbations of the local-chain relaxation dynamics and the relatively low k estimate advocate for weakened intermolecular-specific interactions in the miscible blends, compared with the intramolecular self-association of hydroxyls in pure ER, suggesting structural similarity as the primary driving force for phase miscibility.
2005 Elsevier B.V. All rights reserved. PACS: 64.70.Pf; 77.22.Ej; 77.22.Gm

1. Introduction Cured epoxy resins (ERs) are thermosets exhibiting crosslinked structure with excellent electrical properties, good adhesion, low shrinkage, high tensile strength and modulus, and good chemical and corrosion resistance. The fragile nature of cured epoxy or novolac resins can be improved by introduction of elastomeric modifiers and thermoplastic polymers with low-temperature (low-T) molecular relaxations [1,2]. Generally, linear thermoplastics are inherently tough and can reduce the brittleness of ERs without affecting their advantageous properties. Miscibility, intermolecular specific interactions between polymeric modifiers and ERs, reliance of

*

Corresponding author. E-mail address: [email protected] (I.M. Kalogeras). URL: http://www.unt.edu/LAPOM/

0022-3093/$ - see front matter
2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2005.03.066

miscibility on molecular weight and control of the morphology of the cured products are under intensive study since early 1990s, in an attempt to adjust their properties towards modern technology requirements. In comparison with polymer + polymer blends, reports on physically miscible systems comprising crosslinked thermosets and amorphous or semi-crystalline linear thermoplastics are rare. Thermodynamically, phase miscibility requires a negative Gibbs function change DGm, which may develop depending on the extent of interactions between the components (directly affecting enthalpy of mixing DHm) and structural factors (e.g. chain-packing and steric hindrances) that affect the entropy of mixing DSm. Structural similarity and intermolecular interactions (e.g. hydrogen-bonding, dH [3–5]) between the components are considered to favor phase miscibility. Poly(ethylene oxide) (PEO) is a semi-crystalline polyether known to exhibit strong specific interactions

I.M. Kalogeras et al. / Journal of Non-Crystalline Solids 351 (2005) 2728–2734

and miscibility with polymers possessing functional acid groups. Reports on complete or even partial miscibility of PEO with polymers with non-acid hydroxyl groups are limited [6]. Epoxy resins in general, and especially diglycidyl ether of bisphenol-A (DGEBA)-type epoxies, have a structure similar to poly(hydroxyl ether of bisphenol-A) (phenoxy) that shows complete miscibility with PEO [7]. However, polyether-type polymers, such as polyacetal (polyoxymethylene), poly(2,6-dimethyl1,4-phenylene oxide) [8] and poly(ether imide) [9] have been incorporated with crosslinking ERs yielding phase-separated morphology. Immiscibility has also been reported for DGEBA + PEO blends cured with aliphatic amines [5,10], but not in the case where aromatic amines [5,11–13] or anhydrites [5,14] were used. The influence of potential strong dH interactions on the growth rate and spherulite morphology of PEO in miscible blends has been well established, but their potential contribution on miscibility has been questioned [5]. The above observations reveal the intriguing nature of the factors governing phase structure and the need for additional studies on the parameters affecting blendsÕ dynamics. In this report dielectric probes [15–17] are used to study the influence of the molecular structure, components polarity and their interactions, on the apparent relaxation response of the initial phases and the crosslinked structure, with the aim to acquire information on their compatibility. Hitherto, dielectric methods have been implemented to monitor phase separation, gelation and vitrification processes during curing [18] and study local-chain or segmental relaxation modes in ERs [1,2,19,20]. Bulk PEO and especially its complexes with salts have also been extensively studied dielectrically [21]. Nonetheless, the relaxation dynamics of ER + PEO systems are only poorly explored. The significance and necessity for detailed dielectric and electrical studies of the molecular dynamics of related systems is manifested by observations that the addition of phenolic resins enhances the conductivity of PEO-based fast ion conductors. For example, the enhanced conductivity of PEO + LiClO4 electrolyte systems [22] has been attributed to the reduced PEO crystallinity, the lower pseudoactivation energy in lithium transfer of the blend and complicated interactions among PEO, resin and lithium salt.

2. Experimental Poly(ethylene oxide) (PEO) (M wðPEOÞ ¼ 2 104 g mol1) was used without further purification. The epoxy resin used in the study was DGEBA with a degree of oligomerization n  0.1, a number-average molecular weight of M nðERÞ ¼ 360 g mol1 and an epoxide equivalent weight near 185 g. For curing the DGEBA + PEO mixtures, we have employed a commonly used aromatic

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amine curing agent, 4,4 0 -diaminodiphenyl-methane (DDM, chemical grade, M nðDDMÞ ¼ 198.26 g mol1). Acetone solutions of the curing agent were mixed at 90 C with acetone solutions containing DGEBA and predetermined quantities of PEO. In all cases, the amount of curing agent DDM relative to the DGEBA resin was 28 phr (i.e. 28 parts of curing agent per 100 parts of resin), in order to achieve the stoichiometric amino hydrogen/epoxy ratio [r] = 1. Parameter [r] relates to the degree of crosslinking [4] and is defined as [23] ½r ¼

4mDDM =M nðDDMÞ ; 2mER =M nðERÞ

ð1Þ

where mDDM and mER are the initial weights of amine and epoxy resin prepolymer in the mixture, respectively. Curing of the well-mixed DGEBA + PEO + DDM blends was performed at 90 C for 4 h, followed by post-curing at 120 C for 2 h to attain complete curing. The PEO concentration in each sample is expressed in wt% (e.g. 70/30 ER + PEO for samples with 30 wt% PEO and 70 wt% of ER + curing agent). To remove the residual solvent and moisture traces all samples were desiccated in vacuo at room temperature for at least one day and stored (between measurements) in P2O5 desiccators. Differential scanning calorimetry (DSC) scans were performed in nitrogen atmosphere over the range 120 C to about 50 C above the melting temperature of the samples (or to 200 C when no melting was detected) using a Perkin–Elmer Pyris 6 calorimeter. The melting peak temperature Tm and the glass transition temperature Tg at half-height were determined. Tg evaluations were based on the second heating scans (heating rate 20 C min1). Dielectric relaxation spectroscopy (DRS) measurements were made using disk-like specimens sandwiched between gold-coated brass electrodes. The complex dielectric permittivity, e*, was determined as a function of frequency (102–106 Hz) at selected temperatures between 140 and 150 C (controlled to better than ±0.1 C). A Novocontrol Alpha Dielectric Analyzer was used in combination with the Quatro Cryosystem. Thermally stimulated current (TSC) scans were performed in vacuum (102–104 Pa) in the range 243 to 77 C with polarization temperature Tp between 47 and 57 C, field Ep  2 · 106 V/m, time tp = 5 min and a heating rate of b = 5 C/min. Samples of typical dimensions (5 · 5 · 1) mm3 were used. 3. Results and discussion Fig. 1 shows the transient conductivity r(T) = J(T)/ Ep (J being the discharge current density) signals of typical relaxation modes of DDM-cured DGEBA (ER) and

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pure PEO films. Thin insulating FEP foils were interposed between the PEO film and the electrodes to separate relaxation signals from the high-T dc conductivity (dashed spectrum). The main aER glass-transition signal of ER is outside the spectral range: for aromatic aminecured DGEBA aER peaks close to the calorimetric glass transition temperature (Tg,DSC = 140175 C [5,12]). The secondary transition of ER (bER relaxation, Fig. 1) is located between 120 and 70 C [2,17,19], depending on the type of the curing agent ( 100 C in the present study). In terms of the amino-crosslinked epoxy systems, the transition is assigned to the crankshaft motion of the hydroxy ether groups {glyceryl segment, –CH2–CH(OH)–CH2–O–}, which, most of them form after the amine-epoxy curing reaction. Also, it has been reported that the diphenyl propane units contribute to the relaxation with the Ôflip-flopÕ motion of the phenyl rings that would relax near 110 C [23]. The diffuse low-TcER mode is attributed to main-chain motion, most likely of the glycidyl ether segment (epoxide group), and the corresponding TSC signal appears at temperatures below 120 C [2,17,23]. The concentration of unreacted epoxy groups decreases with increasing cross-linked density; in the present case, the low dielectric strength of the cER mode is consistent with the high crosslinking density obtained for [r] = 1. According to Zheng et al. [12] the signal around 20 C is related to unreacted hardener molecules. Several signals can be found in the r(T) spectrum of pure PEO. The bPEO transition (at Tb,PEO = 12 C) is attributed to the long-range segmental chain reorientation directly correlated with the amount of amorphous

-11

1x10

ER PEO (MISIM) PEO (MSM)

α 'PEO βPEO

-12

1x10

-13

σ (S/m)

1x10

α ER

βER

γ ER

-14

1x10

-15

unreacted hardener

1x10

γ

PEO

-16

1x10

-200

-160

-120

-80

-40

0

40

o

T ( C) Fig. 1. Transient conductivity r(T) spectra of pure cured DGEBA (ER) and PEO samples. To reduce the strong high-T dc conductivity signal of PEO and reveal the submerged relaxation signals, a spectrum recorded with the MISIM electrode configuration is also shown (M = electrode, I = thin insulating FEP foil, S = sample).

phase in the amorphous-crystalline two-phase structure of PEO [21]. Tb,PEO is comparable to the DSC estimate of Tg = 23.8 C, but both values are well above the frequently reported Tg = 57 C of amorphous PEO (M wðPEOÞ > 105 g mol1 [7]). These differences are attributed the low molecular weight of the PEO phase (i.e. the higher number of OH end-groups that impede the motion of segments through dH interactions) and the overall hindering of the segmental motions by PEO crystallites. The contribution of the signal activating at higher temperatures is also partly responsible for the additional up-shift of the TSC Tg estimate. The low-T secondary cPEO process (at 170 C) is assigned to local twisting movements (crank-shaft or kink motion) in the main chains in both defective regions within crystallites and non-crystalline regions [21,24], including amorphous segments in the fold structure on crystal surfaces. Two probable scenarios account for the interpretation of the a0PEO mode recorded above the glass-transition (bPEO) band of PEO: (a) either the signal corresponds to the a crystal-disordering mechanism [25] that generally cannot be observed in dielectric studies due to high dc conductance, or, (b) to space-charge-related effects (e.g. interfacial or electrode polarization phenomena). The observation that bandÕs strength is highly dependent on the conditions of the experiment (e.g. the type of the blocking electrode system) is more consistent with the latter scenario. Blending of the two polymers induces modifications in the local-chain and segmental dynamics of the constituting phases. Based on the spectra shown in Fig. 2, the secondary signal of ER shows a weak low-T shift with the addition of PEO (bb mode in the blends). In agreement with the above observation is the result of an earlier dynamic mechanical analysis (DMA) study of DDM-cured DGEBA + PEO blends [12], which reports a bER mode in the temperature range 100 C to 0 C, with tan d peak around 59 C and 80 C, for pure ER and 50/50 ER + PEO, respectively (measured at f = 0.1 Hz). The moderate shift and the substantial in some cases (e.g. at 30 wt% PEO) strengthening of the bER mode indicate that the linear semi-crystalline polymer facilitates the local-mode motion of the hydroxy ether group. This is probably a result of reduced interand intramolecular interactions in the imperfectly crosslinked epoxy network (plasticization). Such effects are particularly strong for low PEO contents (<50 wt%), where the polyether component is completely amorphous and the blend is in a completely miscible state [12]. In agreement with the above results is a recent FTIR study on ER + PEO blends cured with 1,3,5-trihydroxybenzene (THB) [4]. According to Hu et al. [4], the FTIR bands in pure ER in the region 3100–3800 cm1 are ascribed to hydroxyl stretching vibrations, with a very broad band (centered at 3378 cm1) attributed to

I.M. Kalogeras et al. / Journal of Non-Crystalline Solids 351 (2005) 2728–2734

-13

σ (S/m)

1x10

β b (β ER) -14

1x10

γ b (γPEO+γER)

T blend ¼ g

-15

1x10

-200

The most important feature in the spectra shown in Fig. 2 is the high-T displacement of the glass-transition TSC signal with increasing ER content (perturbed bPEO signal denoted now as ab), consonant with the occurrence of dH interactions between the –OH of ER and –O– groups in PEO [4]. The shift indicates higher Tg for the blend in agreement with differential scanning calorimetry (DSC) and earlier DMA data recorded for DDM-cured DGEBA + PEO blends (Fig. 3). An empirical Gordon–Taylor (G–T) equation has been found to describe the dielectric Tg-composition relationship:

-160

-120

-80

-40

0

40

80

o

T ( C) Fig. 2. Transient conductivity r(T) spectra of selected DDM-cured DGEBA + PEO blends recorded in the MISIM electrode configuration. The notation used for the TSC bands in the binary blends (ab, bb, cb) is given for the 30 wt% PEO blend.

the self-associated hydroxyls and bands centered at 3568 cm1 ascribed to the stretching vibration of free hydroxyls. With the addition of PEO, the FTIR spectra reveal perturbations attributed to the formation of intermolecular hydrogen bonding between the hydroxyls of ER and the ether oxygen atoms of PEO. Nevertheless, the strength of the intermolecular dH interactions in the ER + PEO blends was determined to be much weaker than that of the self-association of hydroxyls in pure ER. Based on these findings, steric shielding, the screening effect, and chain connectivity resulting from the formation of the three-dimensional crosslinked network, can be considered to reduce the intermolecular hydrogen bonding interactions among hydroxyls of cured epoxy versus ether oxygen atoms of PEO. Analogous observations have also been reported for the 4 0 -diaminodiphenyl-sulphone (DDS) cured DGEBA + PEO system [5]. In the temperature region of the c modes (cb band in the blends) the modifications in the strength of the TSC signal give a qualitative description of perturbations in local chain dynamics. The strength of the cb mode remains weak in blends with high PEO contents (wPEO P 70 wt%) and reaches a maximum for wPEO  50 wt%, despite the drastic lowering of the dipole population. This phenomenon can be explained considering the calorimetric finding of an abrupt decrease in PEOÕs crystallinity for wPEO 6 40 wt% and the mixing of amorphous PEO chains with the crosslinked ER network. According to Boyd [25], the c process in PEO and several other highly crystalline polymers must be assigned to local motions in the amorphous phase only and not the crystalline phase. Our preliminary TSC results corroborate this idea.

PEO kwER T ER g þ wPEO T g ; kwER þ wPEO

ð2Þ

where k is an adjustable parameter related to the degree of curvature. It has been suggested that k can be taken as a semi-quantitative measure of miscibility and strength of the intermolecular interaction between components of polymer blends [3–5,26–28]. For example, in blends of poly(e-caprolactone) (PCL) with poly(vinyl chloride) and chlorinated poly(vinyl chloride)s, the k values increase from 0.56 to 0.76 with an increasing degree of chlorination [27], reflecting the increased amounts of intermolecular hydrogen-bonding between the a-hydrogen of the chlorinated polymers and the carbonyl of PCL [28]. The relatively low k value (0.38 ± 0.01) can be considered as a consequence of a decreased enthalpy contribution, which in turn indicates weakened intermolecular-specific phase interactions in the crosslinked structures. Analogous assessments have been reported for miscible THB (k = 0.26 [4], k = 0.31 [26]) and DDS (k = 0.39 [5]) cured DGEBA + PEO

200 Gordon-Taylor (k = 0.38) Fox TSC (this study) DSC (this study) DMA [12] DSC [12]

160 120 80 o

1x10

α b (βPEO,b)

100 wt %PEO 90 wt % 70 wt % 50 wt % 30 wt % 20 wt % pure ER

Tg ( C)

-12

2731

40 0 -40 -80 0

10 20 30 40 50 60 70 80 90 100

wt % PEO Fig. 3. Plots of glass transition temperatures as function of ER + PEO blend composition. The dashed line is based on Eq. (3) and the solid line on Eq. (2). Open symbols refer to DSC and DMA evaluations of Tg and filled symbols to TSC evaluations. The Tg value assigned to pure PEO refers to completely amorphous material [7].

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blends, with additional verification provided by FTIR spectroscopy studies. Note that in the range between

60 and 100 wt% PEO positive deviations from the G–T curve are common in the literature [4,8,11,29], irrespective of curing agentÕs type and content. In Fig. 2 we also present results of the use of the classical Fox equation 1 wPEO wER ¼ þ . T T T blend g;PEO g;ER g

ð3Þ

Except for one aberrant point, it agrees well with DMA and DSC results, but only over a half or so of the concentration range with TSC results. Eqs. (2) and (3) give results close to one another and serve almost equally well. In fact, Eq. (3) provides slightly better results but it does not contain information on miscibility like the parameter k in Eq. (2). Representative DRS spectra for the 10/90 ER + PEO blend are shown in Fig. 4. The data are expressed in terms of the electric modulus M ¼ 1=e ¼ M 0 þ jM 00 .

ð4Þ

Eq. (4) suppresses large contributions of non-local relaxations at low frequencies (dc conductivity). The cb relaxation occurs at high frequencies, even at low measuring temperatures (e.g. fc,b 105 Hz at 80 C for the 90 wt% PEO blend), providing a small contribution to the overall relaxation response [30]. The well known Havriliak–Negami model [31] was used to describe its dielectric behavior; the Arrhenius rate law fits well the relaxation [31]. The Arrhenius plots of the cb mode recorded in four ER + PEO blends (with 20, 50, 70 and 90 wt% PEO) are shown in Fig. 5. In all blends, the

cb-related activation energy barriers remained around 36 ± 3 kJ/mol. In agreement with the TSC response, the dielectric strength (Dec,b) of this mode maximizes in the 50/50 blend (inset in Fig. 5). At frequencies below the ab loss band (fa,b 50 Hz at 50 C for the 90% PEO blend) we observe intense conductivity losses (i.e. note the bend around 101 Hz in the inset of Fig. 4). The signal decreases in strength and frequency with decreasing PEO content. The analysis of the dielectric loss e00 (f) spectra supports the TSC manifestation of compositionally dependent Tg in the blends (Fig. 3). For example, in the 10/ 90 ER + PEO blend the high-T DRS scans reveal lowfrequency conductivity losses and a Havriliak–Negami simulated loss band. The dielectric glass transition temperature (Tg,DRS), defined by the convention s(Tg,DRS) = 100 s, corresponds to the temperature at which the loss band peaks at fmax = 1.6 · 103 Hz. In the 10/90 ER + PEO blend the Tg,DRS estimate is near 0 C, and is consistent with the position of the related TSC peak (Tg,TSC = 6 ± 2 C, Fig. 2). In the cases where the ab dielectric loss band could be isolated, comparative dielectric Tg values were derived (e.g. Tg,DRS  Tg,TSC = 70 ± 3 C for the 80/20 ER + PEO blend). Morphological changes in the structure of the cured ER + PEO blend that occur near a critical PEO content (around 40 wt%) are also reflected in the hydration behavior of the blends shown in Fig. 6. For low PEO content the blend displays a homogeneous semi-interpenetrating network structure and the equilibrium water

8 0.6

7

50 wt % PEO

0.5

-1

∆ εY,b

10

6 Yb

-2

o

-3

10

10 C -2

step 10

-1

log(fmax / Hz)

10

o

0

o

50 C 1

2

3

4

Y b

5

10 10 10 10 10 10 10 10

M ''

b

10

-3

10

0

10

1

step 10

10

2

70 wt% PEO

0.2

4

0.1 -130 -120 -110 -100

90 wt% PEO

-90

-80

-70

-60

-50

o

T ( C)

3

ER + PEO blends 20 wt % PEO 50 wt % PEO 70 wt % PEO 90 wt % PEO

1

o

0.3

5

2

-140 C

0.4

0

o

o

4

-70 C

10

3

10

4

10

5

10

6

f (Hz) Fig. 4. Isothermal M00 (f) spectra for a representative ER + PEO blend (90 wt% PEO) recorded at several temperatures (between 140 and +50 C, in steps of 10 C). The main relaxation modes isolated in the blends (ab, bb, cb) are indicated.

5

6 -1 1000/T (K )

7

8

Fig. 5. Arrhenius plots of the cb relaxations recorded by means of DRS in selected ER + PEO blends. Lines from previous work for bulk PEOs are also included for comparison (- - -Mw = 4 · 105 g mol1 [21]; —Mw = 5.4 · 104 g mol1 [32]). The inset shows the temperature variation of the contribution to the dielectric permittivity (dielectric strength, Dec,b) of the cb mode.

I.M. Kalogeras et al. / Journal of Non-Crystalline Solids 351 (2005) 2728–2734

metric and hydration properties of the blends. Materials containing cured epoxies already have a wide application range [33]. Studies such as the present one should contribute to a further extension of that range.

12 0 wt % PEO(pure ER) 10 wt % 20 wt % 30 wt % 40 wt % 50 wt % 60 wt % 70 wt % 80 wt % 90 wt % 100 wt % (pure PEO)

10

h (%)

8

6

70 %

Acknowledgements

50 %

4

30 %

2

0%

0 0

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The present research has been funded from E.P.E.A.K. 2 (Operational Programme for Education and Initial Vocational Training) in the frame of PYTHAGORAS (Project 70/3/7362). Partial support by the European Union, Brussels, and by the Hellenic General Secretariat for Research and Technology, Athens (Project 01EP92) is also appreciated. Comments of referees were helpful in providing final form to the present paper.

10 20 30 40 50 60 70 80 90 100

RH (%)

References Fig. 6. Equilibrium hydration level (h = (Dmmax/mdry) · 100%) as a function of the relative humidity percentage (RH%). Dashed lines represent the linear fitting curves for the behavior of pure ER and the 30 wt% PEO blend. The dotted lines for the blends containing 50 and 70 wt% PEO are guides for the eye.

absorption is a linear function of the relative humidity of sampleÕs environment. By contrast, water absorption seems to follow different dependences in the low and high hydration levels for blend with PEO contents surpassing 40 wt% (crystallization onset as evidenced by DSC). Crystallization of PEO causes stiffening of the amorphous PEO phase by reinforcement of PEO spherulites [4,13]. According to Guo et al. [13] the amorphous fraction of PEO, the branched ER chains and the imperfect ER network are located between PEO lamellae. This structural transformation increases the Tg of the blend, relative to the Tg-blend predictions; see again Fig. 3. Moreover, the transformation modifies the hydration mechanism [9] by introducing new waterabsorption sites (and more effective proton conductivity paths).

4. Concluding remarks Combined dielectric and calorimetric studies confirm that DDM-cured ER + PEO blends are completely miscible in the low PEO content regime (<50 wt%). We have seen that the dependence of the dielectric Tg on composition obeys Eqs. (2) and (3). At higher loadings, crystallization of PEO induces positive deviations from these equations. The observed low-T shift of the bER TSC signal implies that polyetherÕs addition acts as a plasticizer for the rotation of the hydroxyls groups. Addition of no less than 30 wt% of PEO causes significant morphological changes that affect dielectric, calori-

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