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Journal of Non-Crystalline Solids 351 (2005) 14–22

Changes in molecular dynamics of the tri-ethylene glycoldimethacrylate monomer upon polymerization

M. Teresa Viciosa, Carla M. Rodrigues, Madalena Dionısio *

REQUIMTE/CQFB, Departamento de Quımica, FCT, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal

Received 30 July 2004; received in revised form 6 September 2004

Abstract

The dielectric behavior of the tri-ethylene glycol dimethacrylate monomer was investigated by dielectric relaxation spectroscopy

after isothermal polymerization at different temperatures and compared with the unreacted monomer. The relaxation process asso-

ciated with the monomer glass transition slightly deviates to lower frequencies/higher temperatures in partial polymerized samples.

Additionally, it is stronger depleted the higher the polymerization temperature, vanishing after a temperature where the polymer-

ization reached completion. A secondary relaxation process related with localized mobility with the appearance of an excess wing in

the bulk monomer previously studied in a narrower frequency range, becomes better defined after partial polymerization due to the

depletion of the main relaxation process. Its location and activation energy is independent of the polymerization temperature.

Whereas its intensity does not change significantly upon polymerization, the decrease of the intensity of the glass transition process

could be taken as a degree of conversion. A ab splitting scenario was drawn that maintains its pattern in the partial polymerized

systems relatively to the bulk monomer. The fragility parameter of the bulk monomer was recalculated (m = 85), slightly increasing

when the monomer left unreacted co-exists with the newly formed polymer.

� 2004 Elsevier B.V. All rights reserved.

1. Introduction

In a recent publication [1] the molecular mobility of

the n-ethylene glycol dimethacrylate monomers (with

2 6 n 6 4) was investigated by dielectric relaxation spec-

troscopy in the temperature region where the transitionfrom supercooled liquid to a glassy state occurs. This

glass transition process for the tri-monomer (as well

for the dimonomer) showed almost linear temperature

dependence. The relative narrow frequency range was

in the origin of this apparent linearity. In the present

work the dielectric spectra of the tri-ethylene glycol

dimethacrylate monomer (TrEGDMA) was collected

0022-3093/$ - see front matter � 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.jnoncrysol.2004.09.022

* Corresponding author. Tel.: +351 212 948 300x0974/5; fax: +351

212 948 550.

E-mail address: madalena.dionisio@dq.fct.unl.pt (M. Dionısio).

in an enlarged frequency window to cover the usual

arrhenian departure occurring in glass formers. Since

this departure is related with fragility, being more pro-

nounced in more fragile glass formers [2–5], the fragility

parameter was recalculated.

Additionally, in the previous results, a high frequencyflank was detected in the higher ethylene glycol mono-

mers with the features of an excess wing as found in

some glass formers [6–9]. Some authors [8,9] argue that

the excess wing is not a separate feature in the spectra of

glass formers. For instance Brand and co-workers [8] de-

tected this high frequency flank in ethanol, glycerol and

propylene carbonate ascribing it to a second relaxation

peak, submerged under the a peak.One of the main purposes of the present work is to

investigate the origin of the high flank previously de-

tected. The experimental strategy was the combination

M.T. Viciosa et al. / Journal of Non-Cr

of two factors: (i) the enlargement of the experimental

frequency window to lower values and (ii) the decreasing

of the influence of the main relaxation process by poly-

merizing the monomer.

The experimental procedure implies the monitoring

of the monomer molecular mobility through dielectricspectroscopy at low temperatures, prior and after its iso-

thermal polymerization at a much higher temperature,

Tpol (the temperature range where the glass transition

of the monomer occurs is more than 100K below the

temperature of polymerization). This procedure pro-

vides a convenient dielectric fingerprint that indicates

the extent of the reaction as emphasized by Smith

et al. [10] that applied it to monitor the polymerizationof diethylene glycol bis(allyl carbonate); phenyl glycidyl

ether-aniline systems are also examples of polymerizing

systems investigated by this procedure [11]. Other proce-

dures to follow polymerization comprise real time mea-

surements as well illustrate by the results of Mangion

and Johari [12,13] and of Williams [14].

The evolution of the dynamics upon polymerization

of a diepoxide [15–18] follows a general pattern wherea single Johari–Goldstein process, detected in the unpo-

lymerized liquid state in the GHz frequency range, al-

most goes to extinction and another secondary process

evolves simultaneously with an a-process. Since our

equipment does not cover the GHz frequency range

we were not able to detect any high frequency process

in the bulk monomer. Only the splitting of a main pro-

cess in two processes related, respectively with the glasstransition and localized mobility was detected, allowing

to draw a ab splitting scenario for unreacted and partial

polymerized mixtures. To our knowledge no such stud-

ies were ever performed in this monomer.

Besides the fundamental interest of this study that at-

tempts to contribute to clarify some aspects of the excess

wing phenomenon which microscopic origin stills con-

troversial, the behavior of the TrEGDMA monomerupon polymerization is of practical interest. Indeed, this

type of multifunctional methacrylate monomers are the

precursors of cross-linked polymers widely used in the

production of coatings, information storage systems,

spherical lenses and dental biomaterials [19–21]. The

polymerization of the TrEGDMA monomer is induced

by the 2,2 0-azo-bis-isobutyrylnitrile (AIBN) initiator

according a free radical polymerization mechanism.The detailed characterization of the resulting polymer

will be published elsewhere, nevertheless since TrE-

GDMA is polyfunctional the final product will have a

cross-linked structure.

2. Experimental

The tri-ethylene glycol dimethacrylate monomer with

the formula, n = 3

CH2

H3C

O

(O CH2 CH2 O)n

O

CH3

CH2

and Mw = 286.36 was supplied by Fluka, cat. nbr.

90412.The monomer was previously passed through a dis-

posable inhibitor remover column from Aldrich, ref.

306312-1EA, in order to eliminate the hydroquinone

stabilizer. The polymerization initiator was 2,2 0-azo-

bis-isobutyronitrile (AIBN) from Aldrich, cat. nbr.

11630. Fresh mixtures with 0.1% w/w of AIBN/TrE-

GDMA were used.

ystalline Solids 351 (2005) 14–22 15

2.1. Dielectric relaxation spectroscopy

The dielectric measurements were carried out using a

broadband impedance analyzer, covering a frequency

range from 10�1Hz to 2MHz and in different increasing

temperature steps from �115 up to 0 �C (in the temper-

ature range �115 �C 6 T 6 �85 �C and �55 �C 6 T 6

0 �C, the dielectric spectra were recorded every 5 �C; inthe remaining temperature region the spectra were re-

corded every 2 or 3 �C). A drop of the AIBN/monomer

mixture with two silica spacers 50lm thick was placed

between two gold plated electrodes (diameter 20mm)

of a parallel plate capacitor. The sample cell was

mounted on a cryostat and exposed to a heated gas

stream being evaporated from a liquid nitrogen dewar.

The temperature control is performed within ±0.5 �C.The polymerization essays at 70 and 80 �C were per-

formed by the following procedure:

Path 1 – heating a fresh AIBN/TrEGDMA mixture

from 25 �C up to the temperature of polymerization

(Tpol).

Path 2 – isothermally maintenance of the mixture at Tpol

during 2.5h.Path 3 – cooling of the mixture down to �115 �C.Path 4 – acquisition of the dielectric spectra of the

AIBN/TrEGDMA mixture partially polymerized in

increasing temperature steps from �115 up to 0 �C.

When Tpol = 200 �C, the heating process (path 1) was

performed at a rate of 1 �C/min and the mixture was

maintained only 1min at the final temperature (200 �C)before the acquisition of the dielectric spectra.

In the case of Tpol = 120 �C, the fresh mixture was

previously maintained 2.5h at 80 �C and further heated

to 120 �C in increasing steps of 10 �C, by maintaining

it 10min at each temperature (path 5), cooled down to

�115 �C (path 6) and re-measured (path 7 equal to path

0.1 1 10 100 1000 10000 100000 10000000.01

0.1

1

10

ε ''

f/Hz*

0.7

0.8

0.9

1.0

(a)

16 M.T. Viciosa et al. / Journal of Non-Crystalline Solids 351 (2005) 14–22

4). See further on text the reason for this type of proce-

dure; this mixture will be designated as being polymer-

ized at 80 + 120 �C.A fresh polymerizing mixture was used for each Tpol.

2.2. Differential scanning calorimetry

DSC equipment fitted with a liquid nitrogen cooling

accessory was used for differential scanning calorimetry

experiments. The sample vessels were placed in the sam-

ple holder at room temperature and cooled down to

�130 �C. Measurements were performed upon heating,

at a rate of 5 �C/min, in two different runs since the heat-

ing procedure does not allow to cover the entire temper-ature range in a single run. Therefore, a first

thermogram was collected from �130 up to +70 �C fol-

lowed by a second run covering the temperature range

25 to 400 �C. Unsealed aluminium sample vessels were

used. The vessels containing the samples were weighted

in a precision balance (±1lg) before and after the DSC

essays.

f/Hz*0.1 10 100 1000 10000 100000 1000000

0.0

0.1

0.2

0.3

0.4

0.6ε''/

ε'' MAX

1(b)

Fig. 1. (a) Dielectric loss spectra of the unreacted TrEGDMA/AIBN

0.1% w/w mixture between �115 and �45�C. (b) Comparison of the

dielectric loss at �70�C of the bulk monomer previously studied (1)

and the TrEGDMA/AIBN 0.1% w/w mixtures prior to

polymerization.

3. Results

The dielectric loss spectra, e00(f), of the unreacted

monomer (fresh AIBN/TrEGDMA mixture) is shown

in Fig. 1(a), in logarithmic scale, for temperatures be-

tween �115 and �45 �C. The shape of the spectra is very

similar to that of the bulk monomer previously studied[1], as shown in Fig. 1(b) at �70 �C (the loss factor is

represented in a normalized scale).

The main relaxation process is associated with the

glass transition as already reported [1]. At the lowest

temperatures a secondary relaxation process is coming

out that was hardly detected in the bulk monomer stud-

ied in a narrower frequency range [1].

In order to better define this secondary process byreducing the influence of the main relaxation process, the

monomer was partially polymerized by following the

procedure of scheme 1 (see Section 2). Fig. 2 presents

the dielectric spectra of the AIBN/TrEGDMA mixtures

acquired in path 4 after polymerization at Tpol = 70 �C(Fig. 2(a)), Tpol = 80 �C (Fig. 2(b)) and Tpol = 200 �C(Fig. 2(c)). The loss curve in full symbols, on each figure,

corresponds to the temperature of �75 �C and acts as aguideline to help the reader to perceive the existence of

two processes in Fig. 2(a) and (b).

The results in Fig. 2 show that the main relaxation

process is being depleted with the increase of the poly-

merization temperature, vanishing for 200 �C, while the

secondary relaxation remains almost unaffected. The

isothermal plots of the dielectric loss at �75 �C illustrate

this behavior as shown in Fig. 3, where the loss curve forthe sample polymerized at 80 + 120 �C is also included.

The unreacted monomer is also shown, in a different

scale, to illustrate the deviation of the main relaxation

process to lower frequencies upon polymerization.

Another way to visualize the effect of polymerization

on the monomer mobility is through the plot of thedielectric loss at 1kHz. Fig. 4 presents the isochronal

plot at 1kHz, taken from isothermal measurements, in

a logarithmic scale to allow the definition of the second-

ary process in the unreacted mixture whose intensity is

more than one order of magnitude lower than the main

process.

The secondary process is largely merged in the low

temperature side of the glass transition relaxation inthe unreacted mixture. With the polymerization both

processes tend to separate.

The loss spectra obtained for the mixture polymer-

ized at 80 + 120 �C (see Section 2) was found to be very

similar with the mixture polymerized at 200 �C (detailed

differences will discussed further on). The temperature

of 120 �C was chosen after monitoring tgd (= e00/e 0) at

0.01

0.1

0.1 10 100 1000 10000 100000 1000000f/Hz*

1

0.1

0.010.1 10 100 1000 10000 100000 1000000

f/Hz*1

0.1 10 100 1000 10000 100000 10000000.01

0.1

ε''

ε''

ε''

f/Hz*1

(a)

(b)

(c)

Fig. 2. Dielectric loss spectra for the TrEGDMA/AIBN mixtures

polymerized at (a) 70�C, (b) 80�C; (c) 200�C (see Section 2). In full

symbols is presented the loss curve obtained at �75�C with for each

temperature of polymerization.

0.1 1 10 100 1000 10000 100000 10000000.00

0.02

0.04

0.06

0.08

0.10

ε ''

f/Hz*

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Fig. 3. Comparison between the loss curves obtained at �75�C for

each TrEGDMA/AIBN mixture (prior to polymerization – circles;

polymerized at: 70�C – triangles, 80�C – squares, 80 + 120 �C –

diamonds, 200�C – asterisks).

-100 -80 -60 -40 -20 00.01

0.1

1

10ε ''

T (ºC)

Fig. 4. Isochronal plot at 1kHz for all TrEGDMA/AIBN mixtures

(same symbols as Fig. 3 are used).

M.T. Viciosa et al. / Journal of Non-Crystalline Solids 351 (2005) 14–22 17

a fixed frequency (1kHz) during the heating process

from 80 �C (where the mixture stayed 2.5h) up to

120 �C. At each 10 �C step, tgd was measured and no sig-

nificant changes were found until the 120 �C temperature

was attained. At this temperature, tgd showed an abrupt

decrease. After 10min at this temperature, the dielectric

spectra were collected as already explained. Both80 + 120 �C and 200 �C polymerized films thus obtained,

were analyzed by differential scanning calorimetry (Fig.5). Neither the characteristic exothermal peak originated

upon polymerization, nor the glass transition of the

monomer were detected in thermograms obtained from

�100 up to 400 �C; the inset in Fig. 5 shows the thermo-

gram obtained in the first run (see Section 2) where no

monomer�s glass transition was detected. Instead, an

endothermic variation was observed for temperatures

higher than 225 �C which is associated with degradationas confirmed by a loss in weight in both samples; for

thermograms covering a narrower temperature range

(�100 to 300 �C) the weight loss is around 40% whereas

it is higher than 90% when the final temperature is

400 �C. The calorimetric detection of the bulk mono-

mer�s glass transition is presented in Ref. [1].

All the dielectric data were fitted by a sum of the well-

known Havriliak–Negami [22] (HN) function:

0.0040 0.0044 0.0048 0.0052 0.0056 0.0060 0.00640.1

0.2

0.3

0.4

0.5

6

7

8

9

10

∆ε

1/T (K)

Fig. 6. Dielectric strength in function of the reciprocal of temperature

for all TrEGDMA/AIBN mixtures (full symbols – main process; open

symbols – secondary process; usual symbols are used).

0 50 100 150 200 250 300 350 400-8

-6

-4

-2

0

2

4

m0=7.968 mg

m0=5.728 mgexo up

-120 100 -80 -60 -40 -20 20 40 60-4

-3

-2

-1

0

1

2

3

T (ºC)

T (ºC)

Φ (m

W) u

.a.

Φ (m

W) u

.a.

0

Fig. 5. DSC thermograms obtained at 5 �C/min from 30 up to 400�Cfor both samples polymerized at 80 + 120�C and 200 �C – the abrupt

variation with a minimum at 310�C is due to degradation; the inset

shows the first run taken from �130 up to 70�C where no monomer�sglass transition is detected.

18 M.T. Viciosa et al. / Journal of Non-Crystalline Solids 351 (2005) 14–22

e� ¼ e1 þX

j

Dej

½1þ ðixsjÞaHNj bHNjð1Þ

where j is the number of relaxation processes and

x0 = 2pf0 is the characteristic HN frequency closely re-lated with fmax, the frequency of maximal loss;

De = es � e1 is the dielectric strength, i.e., the difference

between the real permittivity values at, respectively,

the low and high frequency limits, s � (2pfmax)�1 is the

characteristic relaxation time, and aHN and bHN, the

shape parameters (0 < aHN < 1, 0 < aHN ÆbHN < 1). For

temperatures higher than �70 �C the data are influenced

by low frequency conductivity contribution, and anadditional term ir/xce0 was added, where e0 is the vac-

uum permittivity, r and c are fitting parameters: r is re-

lated to the dc conductivity of the sample and c

describes the broadening of the relaxation time distribu-

tion for the dc conductivity.

The fitting procedure of the loss data of the monomer

was carrying out with 2 HN functions at intermediate

temperatures, due to the merging of both main and sec-ondary processes. However, a different profile is ob-

served at the lowest temperatures: the loss curves are

not able of being fitted with only one HN function, in-

stead a second HN function, different form the one that

defines the main relaxation process, is necessary to take

in account an extended low frequency tail. The same is

true for the polymerized systems. In Fig. 3, all the curves

show a low frequency contribution that is not originatedfrom conductivity. This process is more depleted in the

mixture polymerized at 200 �C relatively to the one poly-

merized at 80 + 120 �C.The dielectric strength, De, obtained from the fittings

for both the glass transition and secondary processes is

presented versus the reciprocal of the temperature in

Fig. 6 for all the systems studied (full symbols – mainprocess; open symbols – secondary process).

The trends of the temperature dependence of the

relaxation strengths for both main and secondary pro-

cesses can be considered almost linear for all the systems

represented.

The dielectric strength of the main process (circles)

abruptly decreases relatively to the bulk monomer in

samples submitted to partial polymerization (70 �C – tri-angles; 80 �C – squares), and decreases more accentuate

the higher the polymerization temperature, vanishing

for Tpol = 80 + 120 and 200 �C. The dielectric strength

of the main process for each system slightly decreases

with the temperature increase while the opposite is true

for the secondary process. This is the normal tempera-

ture dependence of the dielectric strength for both alpha

and beta relaxations.The aHN and aHNbHN shape parameters remain

approximately temperature independent for the main

process as can be observed in Fig. 7(a) (with aHN very

close to unity), and slightly increase with the tempera-

ture increase for the secondary process as shown in

Fig. 7(b) (aHN – open symbols, aHNbHN – full symbols).

The final parameter obtained from the HN fittings is

the relaxation time, s. In Fig. 8 we present the tempera-ture dependence of the lns values for both main (a) andsecondary (b) relaxation processes of all the systems

studied. The AIBN/TrEGDMA mixture after polymeri-

zation at 80 + 120 �C and 200 �C does not present the aprocess, therefore only data in the linear branch of the bprocess are represented in this activation plot.

The secondary process of all the systems studied re-

veals Arrhenius behavior with the linear plots superim-posed. The activation energy using all data was

determined as 39 ± 1kJmol�1 with a pre-exponential

factor of s0 = (1.7 ± 0.3) · 10�15 s.

-85 -80 -75 -70 -65 -60 -55 -50

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

α,αβ

α,αβ

T (ºC)

-120 -110 -100 -90 -80 -70 -60 -50 -40 -30 -200.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

T (ºC)

αβ

α

(a)

(b)

Fig. 7. Temperature dependence of the aHN (full symbols) aHNbHN

(open symbols) shape parameters for the (a) main relaxation (b)

secondary relaxation (prior to polymerization – circles; polymerized at:

70�C – triangles, 80�C – squares, 80 + 120�C – diamonds, 200�C –

asterisks).

0.0040 0.0044 0.0048 0.0052 0.0056 0.0060 0.00640

2

4

6

8

10

12

14

16

-lnτ

1/T (K)

Fig. 8. Activation plots for both main and secondary relaxation

processes; solid lines represent the VFT fittings of the main process (see

text for the parameters) and the Arrhenius linear fit for the secondary

relaxation using all data (same symbols as Fig. 7).

Table 1

Glass transition temperatures (for s = 100 s), activation energies and

fragility indexes estimated from the VFT fittings (see text) for the

TrEGDMA monomer prior to polymerization and in partially

polymerized at Tpol mixtures

Tpol Tg (K) Ea(Tg) (kJmol�1) m

70 187.4 317 88

80 187.1 320 89

Monomer 185.5 302 85

M.T. Viciosa et al. / Journal of Non-Crystalline Solids 351 (2005) 14–22 19

In what concerns the a process, it presents an en-

hanced departure from Arrhenius behavior by showing

the usual curvature of fragile glass formers.

The activation plots for the a process of the bulk

monomer and mixtures after partial polymerization at

70 and 80 �C, were fitted by the VFT equation [23–25]

(solid lines in Fig. 8)

s ¼ s0 eB

T�T 0 ð2Þwith the following parameters: unreacted AIBN/TrE-

GDMA, s0 = (2 ± 1) · 10�15 s, T0 = 147 ± 2K, B =

1439 ± 98K; AIBN/TrEGDMA (Tpol = 70 �C), s0 =(1.5 ± 0.7) · 10�13 s, T0 = 156 ± 1K, B = 1074 ± 60K;

AIBN/TrEGDMA (Tpol = 80 �C), s0 = (1.2 ± 0.8) ·10�13 s, T0 = 156 ± 2K, B = 1075 ± 93K.

It is possible to estimate a glass transition tempera-

ture from these parameters by replacing s in Eq. (2) by

100s, since it is common to characterize the entry in

the glassy state by a relaxation time of 100s [26]. The

Tg values thus obtained are included in Table 1 together

with the values of the activation energy at the glass tran-

sition temperature, Ea(Tg), and the fragility parameter,

m, calculated, respectively, by the following equations(see Ref. [1] for details):

EaðT Þ ¼R � B

ð1� T 0=T Þ2ð3Þ

m ¼ EaðT gÞln 10RT g

ð4Þ

Eq. (4) follows the deduction of the fragility parameter

as proposed by Angell and co-workers [4].

4. Discussion

Essentially, two relaxation processes were detected in

the bulk monomer: a secondary process due to localized

mobility and a main relaxation related with the glass

transition of this glass former. In isothermal plots, thelocal process merges at the high frequency side of the

main process. The dielectric measurements of n-ethylene

glycol dimethacrylate monomers acquired in a narrower

frequency window [1], gave rise to a less defined super-

position of both a and b processes, with the appearance

20 M.T. Viciosa et al. / Journal of Non-Crystalline Solids 351 (2005) 14–22

of an excess wing. In the present work it is clear that it

can be ascribed to a second relaxation peak, submerged

under the a peak.

A third process is felt in the fitting procedure whose

origin is not clear but that is also presented in the mix-

tures after polymerization. Since it is located at the lowfrequency side of the beta relaxation, this third process

will be designate by c, and is associated to a even more

restricted mobility relatively to the b process; probably a

very localized motion within the ethylene glycol chain.

The attribution at a molecular level can be achieved

by comparative studies between the different n-ethylene

glycol monomers.

The superposition of both the monomer and the mix-ture monomer/initiator dielectric loss spectra before

heating at temperatures well above room temperature,

indicates either that the AIBN initiator does not con-

tribute significantly to the dielectric behavior, and no

polymerization occurs during the dielectric data acquisi-

tion in the temperature range from �115 up to 0 �C.Furthermore, polymerization may develop at higher

temperatures during heating up to the polymerizationtemperature, Tpol. In fact, the temperature range where

the AIBN decomposition rate is significant, i.e., where

the initiator is effective, is reported to be between 50

and 100 �C [27], but this temperature range refers to

AIBN dissolved in benzene. Moreover, the initiator

activity depends on the initiator amount. In essays per-

formed with AIBN/TrEGDMA mixtures with 1% w/w

of AIBN, the polymerization at temperatures above50 �C occurs rapidly, but in the mixtures studied in the

present work, the amount of AIBN is low, 0.1% w/w,

decreasing the reaction rate. In fact, the reaction rate

when AIBN content is 0.1% w/w, is negligible slow for

T 6 54 �C, since the monomer dielectric loss spectrum

after 2h at 54 �C shows no significant differences from

the unreacted monomer. Therefore, during the fast heat-

ing process from the temperature range where the lossspectra is collected (�115 to 0 �C) up to the temperature

of polymerization, Tpol, no significant polymerization

occurs. Consequently, we can say that Tpol essentially

determines the extent of polymerization achieved.

The temperature dependence of the relaxation

strengths of both main and secondary processes is al-

most linear for all the systems. The dielectric strength

of the alpha process, for the systems where it is detect-able, slightly decreases with the temperature increase.

The small change of the dielectric strength of the alpha

process with temperature, does not allow a reliable

extrapolation to a null value of De in order to estimate

a temperature where the onset of the a relaxation occurs,

as it is possible in other polymerization precursors as

cresyl glycidyl ether (CGE) [28] and diglycil ether of

bisphenol-A (DGEBA) [29].For the AIBN/TrEGDMAmixtures partially or com-

pletely polymerized, the dielectric strength of the sec-

ondary process slight increases with the temperature,

as usual for the beta relaxation process, and its value

does not vary significantly upon polymerization. In the

bulk monomer the dielectric strength of the beta process

has a slightly higher value due to a strong influence of

the main relaxation process.The effectiveness of the polymerization procedure of

heating up to 120 �C in steps of 10 �C after the sample

being left 2.5h at 80 �C, seems to be close to the one

achieved by polymerizing at 200 �C, due to a great

superposition of both dielectric loss spectra. Neverthe-

less, a close inspection of the loss curves of both samples

(remember Fig. 3) reveals a greater contribution at the

lowest frequencies of the c process, already found inthe bulk monomer, in the 80 + 120 �C sample relatively

to the 200 �C. In the later, a decrease of the low fre-

quency contribution could be being compensated by

an increase of the intensity of the beta process. There-

fore, the total dipole moment would be relaxing in a

complementary way through both gamma and beta pro-

cesses. These small differences detected between the

dielectric behavior of both samples, could be an indica-tion of microscopic differences in the morphology

achieved upon polymerization relatively to which the

DSC technique is insensible. In fact, the DSC thermo-

grams are quite identical for both samples (remember

Fig. 5). The absence of an exothermic peak due to fur-

ther polymerization in the DSC thermograms of both

samples, where only degradation was detected, allowing

us to conclude that polymerization reached completion(the detailed description of these measurements will be

published elsewhere) and thus, is mainly the intensity

of the alpha process that is related to the polymerization

extent.

Both a and b processes merge with the increasing of

temperature. While the b secondary process maintains

its location independently of the polymerization degree,

the glass transition process of the left unreacted mono-mer slightly deviates to lower frequencies/longer times

in the partially polymerized mixtures relatively to the

fresh mixture (with no significant changes in location be-

tween the two mixtures partially polymerized at 70 and

80 �C). Therefore, the merging of both a and b process

of the monomer that persists unreacted, changes upon

partial polymerization. This is a different splitting sce-

nario relatively to the activation plots drawn by Parthunand Johary [30] for the isothermal cure process in ther-

mosets taken at different cure times. The a and b plots

are distinct for each cure time merging at the same fre-

quency (107Hz) and converging to the same pre-expo-

nential factor of 1012Hz corresponding to the

vibrational frequency of atoms. That scenario implies

a different location for the beta process with the evolu-

tion of cure, by maintaining the same activation energy.In the present work, the secondary relaxation process is

almost independent from the environment that is chang-

M.T. Viciosa et al. / Journal of Non-Crystalline Solids 351 (2005) 14–22 21

ing upon polymerization, preserving both location and

activation energy. In what concerns the alpha process,

we are monitoring the evolution of the monomer�s glasstransition rather than the evolution of the newly formed

polymer�s glass transition, therefore, being less sensitive

to the structural changes upon polymerization as foundfor the thermoset that is being formed in the work of

Parthun and Johary.

From the VFT fitting of the temperature dependence

of the a process of the bulk monomer, a glass transition

temperature was estimated, Tg = 186K, that it is in very

good agreement with the calorimetric glass transition

temperature previously determined for bulk TrEGDMA

[1]. The Tg estimated for the monomer left unreactedslightly increases when the monomer co-exists with the

newly formed polymer Tg = 187K. Apparently the

T = Tg values here obtained for the various systems

are very similar being within the experimental error,

but the difference is real and corresponds to the displace-

ment of the VFT curves in Fig. 8 of the partial polymer-

ized mixtures relatively to the VFT curve of the bulk

monomer.There is a significant difference, for the bulk mono-

mer, between the fragility parameter herein determined

(m = 85) and the one estimated in the previous work

by VFT data (m = 65) [1]. The narrower frequency and

temperature range of the previous measurements im-

puted an apparent linearity to the relaxation time tem-

perature dependence, which reflected in a lower

fragility parameter. Due to this relatively low m value,the TrEGDMA was classified as a quite strong liquid

(the same holds for the di- and tetra-ethyleneglycol

dimethacrylate monomers). The actual m value is char-

acteristic of more fragile materials as 5-phenyl-4-ether

(m = 85) [31,32] or poly(ethyl acrylate) (m = 83)

[32,33]. The difference form the actual m value and the

one previously obtained from differential scanning calo-

rimetry measurements (44) is even bigger which confirmsthe erroneous values predicted by DSC in more fragile

glass formers [34,35] as discussed before [1].

The fragility parameters determined from the esti-

mated activation energies at the glass transition temper-

ature, slightly increase with the Tpol increase (m = 88 at

Tpol = 70 �C and m = 89 at Tpol = 80 �C). According with

Wang and Johary [36] during a melt polymerization, as

covalent bonds form, the number of configurationsaccessible to the liquid state at a constant temperature

changes, i.e., the system continuously shifts into a differ-

ent potential energy landscape thus changing their fra-

gility. From our data we may conclude that the same

could be true for the system left unreacted. Some care

must be taken making these conclusions since the activa-

tion energy and, consequently, the fragility parameter,

are very sensitive to the s0 value obtained from theVFT fitting: for instance, a 10% variation in lns0 origi-nates an uncertainty of 1.5% in the glass transition tem-

perature estimate, while it originates a much higher

variation, 16%, in the predicted activation energy. Nev-

ertheless, the fragility increase upon polymerization

seems to be a consistent result since, repeated fittings

in other analyzed partially polymerized samples, gave al-

ways rise to higher fragility parameters.

5. Conclusions

The enlarged frequency range employed in the pres-

ent work, allowed the detection of a secondary relaxa-

tion process in the tri-ethylene glycol dimethacrylate

monomer that has the features of an excess wing in anarrower frequency window. This process becomes bet-

ter defined upon polymerization due to the depletion of

the main glass transition process. While the intensity of

the alpha relaxation process decreases with the polymer-

ization extent and deviates to lower frequencies/higher

temperatures, the secondary process almost does not

change both in intensity and location. The small differ-

ences detected in the loss spectra when polymerizationreaches completion could be taken as an indication of

morphologic differences of the molecular architecture

achieved. An ab splitting scenario was drawn where

the linear b branch does not changes upon polymeriza-

tion and the curved a branch slightly deviates to lower

frequencies (longer times). The extrapolation to

s = 100s give rise to an excellent agreement between

the dielectric and calorimetric Tg of the bulk monomerallowing a recalculation of m. The fragility index of

the monomer left unreacted was found to slightly in-

crease with the extent of polymerization in partial poly-

merized samples, which may indicate that potential

energy landscape is changing upon polymerization.

Acknowledgments

The authors acknowledge the helpful discussions with

Professor J. L. Gomez Ribelles (Universidad Politecnica

de Valencia). The authors also acknowledge the finan-

cial support through both projects POCTI/CTM/

37435/2001 and POCTI/CTM/47363/2002.

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