Polycyanurate-Organically Modified Montmorillonite Nanocomposites: Structure-Dynamics-Properties...

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Polycyanurate-Organically Modified MontmorilloniteNanocomposites: Structure-Dynamics-PropertiesRelationshipsVladimir A. Bershtein a; Alexander M. Fainleib b; Polycarpos Pissis c; Irina M. Beib; Florent Dalmas d; Larisa M. Egorova a; Yurii P. Gomza b; Sotiria Kripotou c;Panayitis Maroulos c; Pavel N. Yakushev aa Ioffe Physico-Technical Institute of the Russian Academy of Sciences, St.Petersburg, Russiab Institute of Macromolecular Chemistry of the National Academy of Sciences of

Ukraine, Kyiv, Ukrainec Department of Physics, Zografou Campus, National Technical University of Athens, Athens, Greeced Institut de Chimie et des Matériaux Paris-Est (ICMPE), Equipe Systèmes Polymères Complexes, Thiais, France

Online Publication Date: 01 May 2008To cite this Article: Bershtein, Vladimir A., Fainleib, Alexander M., Pissis, Polycarpos, Bei, Irina M., Dalmas, Florent,Egorova, Larisa M., Gomza, Yurii P., Kripotou, Sotiria, Maroulos, Panayitis and Yakushev, Pavel N. (2008)'Polycyanurate-Organically Modified Montmorillonite Nanocomposites: Structure-Dynamics-Properties Relationships',Journal of Macromolecular Science, Part B, 47:3, 555 - 575To link to this article: DOI: 10.1080/00222340801955545URL: http://dx.doi.org/10.1080/00222340801955545

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Polycyanurate–Organically ModifiedMontmorillonite Nanocomposites:

Structure–Dynamics–Properties Relationships

VLADIMIR A. BERSHTEIN,1 ALEXANDER M. FAINLEIB,2

POLYCARPOS PISSIS,3 IRINA M. BEI,2 FLORENT DALMAS,4

LARISA M. EGOROVA,1 YURII P. GOMZA,2

SOTIRIA KRIPOTOU,3 PANAYITIS MAROULOS,3

AND PAVEL N. YAKUSHEV1

1Ioffe Physico-Technical Institute of the Russian Academy of Sciences,

St. Petersburg, Russia2Institute of Macromolecular Chemistry of the National Academy of Sciences of

Ukraine, Kyiv, Ukraine3National Technical University of Athens, Department of Physics, Zografou

Campus, Athens, Greece4Institut de Chimie et des Materiaux Paris-Est (ICMPE), Equipe Systemes

Polymeres Complexes, Thiais, France

Polycyanurate-modified montmorrilonite (PCN-MMT) nanocomposites weresynthesized by polymerization of dicyanate ester of bisphenol A in the presence ofMMT dispersed by ultrasound. Techniques of IR spectroscopy, WAXD, and TEMwere applied to study polymerization kinetics and structure of the nanocompositesprepared, whereas their dynamics and thermal/mechanical properties over the 230to 4208C range were studied by using DSC, laser-interferometric creep rate spec-troscopy (CRS), and dielectric relaxation spectroscopy (DRS) techniques. It wasshown that a small amount of MMT additive acts as a catalyst of polymerization andresults in the formation of complicated intercalated/exfoliated structures, as well asstrongly modifies the dynamics in the PCN network. Pronounced dynamic heterogen-eity was observed for PCN/MMT nanocomposites. Along with the main PCN glasstransition, two new glass transitions, at much higher and much lower temperatures,were revealed as a consequence of constrained dynamics in matrix interfacial nano-layers and due to incomplete local cross-linking in the PCN matrix, respectively. Inaddition, increased sub-Tg mobility was observed in these nanocomposites. A two-fold rise of modulus of elasticity as well as increasing thermal stability and arisingmicroplasticity at low temperatures, promoting, obviously, improved crack resistancein a brittle PCN network, were found for the PCN-MMT nanocomposites.

Keywords polycyanurate–montmorrilonite nanocomposites, structure, dynamics,dynamic heterogeneity, properties

Received 3 December 2007; Accepted 12 December 2007.Address correspondence to Vladimir A. Bershtein, Ioffe Physico-Technical Institute of the

Russian Academy of Sciences, 194021, St. Petersburg, Russia. E-mail: [email protected]

Journal of Macromolecular Sciencew, Part B: Physics, 47:555–575, 2008

Copyright # Taylor & Francis Group, LLC

ISSN 0022-2348 print/1525-609X online

DOI: 10.1080/00222340801955545

555

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08 Introduction

Cyanate ester resins (CER) are a relatively new and competitive class of very promising

monomers used for synthesis of high-performance polymer matrices for composites that

are applied in the aerospace and electronics industries. They are characterized with

good processability, and polycyanurate (PCN) networks, which are synthesized from

CER, exhibit unique combinations of high thermal stability and glass transition tempera-

ture (up to 270–2908C), good adhesion to different substrates, and low dielectric loss and

water uptake.[1,2] The only drawback of PCN networks, as a typical disadvantage for

highly cross-linked polymer networks, is their brittleness,[1 – 3] and improved PCN crack

resistance is required. To solve this problem, PCN is toughened by chemical incorporation

of some flexible fragments into the network structure, and by introduction of rubber

particles or engineering thermoplastics, at the stage of CER polymerization.[3 – 8] The

former way leads to formation of semi-interpenetrating polymer networks (semi-

IPNs)[9,10] with various phase structures.

In recent years, a large series of papers relating to synthesis and characterization of poly-

cyanurate–polyurethane grafted semi-IPNs and hybrid IPNs have been published.[11–27]

These results were summarized in reviews.[3,11] In addition, polycyanurate-

poly(tetramethylene glycol) hybrid networks have recently been synthesized,[28,29]

and their dynamics and structure were characterized in detail.[30,31] Using polyurethanes,

polyethers, and polyesters for modification of polycyanurates has allowed improved mech-

anical performance of the latter, but at the expense of thermal stability. Several papers on

composites prepared from CER filled with carbon fibers have also been published.[32–35]

Recently, a few papers[36 – 41] on PCN networks modified by montmorillonite

(MMT) nanolayers were published. Some data on structure, mechanical, and thermal

properties of these nanocomposites were obtained in these studies. In particular,

increase of PCN glass transition temperature, Tg, with increasing MMT content in a

nanocomposite was observed;[39] however, this temperature was anomalously low

(�2258C) for the initial neat PCN network. Disappearance of peaks in the WAXD

patterns of these nanocomposites indicated that silicate nanolayers were either totally

separated (exfoliation), or intercalation to a distance (gallery spacing) of over 3 nm

occurred. Some transfer of polymer mass into the interlayer galleries of organically

modified layered silicates (intercalation), besides exfoliation, resulted in some

increase of fracture toughness (stress intensity factor K1c) and decrease of the co-

efficient of thermal expansion of PCN.[38] It was also found[41] that combining

intercalation/exfoliation effects resulted in a substantial increase of crack resistance

of PCN-MMT nanocomposites.

In the present work, we intended to study structure–dynamics–properties relation-

ships for PCN-MMT nanocomposites in more detail. Special attention was paid to

investigating dynamic heterogeneity in these systems, which could be manifested due to

the possible presence of nanodomains with different degrees of cross-linking in hybrid

structure; i.e., of compositional nanoheterogeneity, and due to PCN-MMT interactions

resulting from intercalation/exfoliation processes. In addition, the synthesis in this

work was fulfilled without using any catalysts since catalysts being in a polymer system

can initiate early thermal decomposition of the resulting network;[1] besides, the

catalytic effect of MMT itself on polycyclotrimerization reaction of cyanate esters was

recently observed.[36,37] We aimed to obtain within this work polycyanurates with small

MMT additives as nanocomposites with improved mechanical properties and thermal

stability.

V. A. Bershtein et al.556

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08 Experimental

Materials

The PCN matrix was synthesized from dicyanate ester of bisphenol A (DCEBA), supplied

by Novocheboksarsk, Russia. Three types of commercially available MMT products

(common trade brand is Cloisite) from Southern Clay Products, USA, were used in the

experiments, as received, namely, virgin Na-MMT (Cloisite Na) and two types of organi-

cally modified MMT–Cloisite 30B (the surface modifier includes quaternary ammonium

salt, two hydroxyl groups, and a single tallow “tail”) and Cloisite 15A (the surface

modifier includes quaternary ammonium salt, two methyl groups, and two tallow “tails”).

PCN-MMT nanocomposites were synthesized in the following way. The required

amount of DCEBA was dissolved in acetone and the MMT was dispersed at room temp-

erature in the solution obtained. To provide a better dispersion of MMT in the polymer

matrix, ultrasonic treatment of the mixture for 4–5 min at 22 kHz was performed, to be

indicated in the text as u.t. Solvent was removed then by heating at 1008C, by vigorous

stirring. All compositions were degassed under vacuum before curing. The curing

schedule for all systems were as follows: 5 h at 1508, then 3 h at 1808, and 1 h at 2108C.

Characterization

Effects of MMT on the kinetics of DCEBA polymerization were investigated by means of

IRS using a Specord M-80 apparatus. A DCEBA solution in acetone was mixed with

proper amounts of MMT for 5 h to get better dispersion of the filler. This blend was

poured onto a NaCl monocrystal surface. Samples obtained were kept for 10 h at

1508C, and IR spectra were registered during this time in the range from 4000 to

600 cm21. Conversion of cyanate groups was calculated from the decrease of the absorp-

tion band at 2270 cm21 relating to -O-C;;N stretching vibrations. The absorption band of

CH3-groups stretching vibrations at 2970 cm21 was used as an internal standard.

Sol-gel analysis of the nanocomposites obtained was performed by Soxhlet extraction

for 16 h in boiling acetone.

In order to check the nanocomposites’ structure, WAXD and TEM techniques were

used. The WAXD curves were recorded by x-ray diffractometer (DRON-2) using

Debye-Sherrer method. Cu-radiation (l ¼ 1.54 A) monochromatized by a Ni-filter was

used. Specimens for the WAXD analysis were prepared by crumbling of previously

cured nanocomposites down to particles with average size of ca. 1 mm. The spacing, d,

between two silicate nanolayers of MMT was calculated according to Bragg’s law:

d ¼ l=2 sin u ð1Þ

where l is the wavelength of the x-ray radiation, and u is the measured diffraction angle.

The samples for TEM were microtomed at room temperature using a Leica MZ6

Ultracup UCT microtome with a diamond knife at a cutting speed of 0.2 mm . s21. The

thin sections of about 60 nm thickness were floated on deionized water and collected on

a 400-mesh copper grid. The TEM measurements were performed on a JEOL 2000 FX

microscope operated at 200 kV.

We used DSC, CRS, and DRS techniques for studying dynamics in the PCN-MMT

nanocomposite films with about 0.6–1.0 mm thickness.

A Perkin-Elmer DSC-2 apparatus was used for measuring the heat capacity Cp(T )

dependence, over the temperature range from 230 to 4208C. Glass-transition temperature,

Polycyanurate–Montmorillonite Nanocomposites 557

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08 Tg, at the half-height of a heat capacity step, transition breadth DTg ¼ Tg

00 –Tg0, where Tg

0

and Tg00 are the temperatures of the glass transition onset and completion, respectively, as

well as the heat capacity step, DCp, and the activation energy, Q, for segmental motion

within the glass transition were determined. The temperature regimes of stabilizing

samples and measuring DSC curves are considered in the following. Activation energy

Q values were determined, with an accuracy of 10%, from the Tg displacements with

changing heating rate, V, between 5 and 408C/min, by:[42]

Q ¼ �RdlnV=dðT�1g Þ ð2Þ

As a high-resolution technique for relaxation spectrometry and thermal analysis,[43 – 45]

CRS was used for analysis of the heterogeneity of segmental dynamics in PCN-MMT nano-

composites. It consists in precisely measuring creep rates at a constant low stress as a

function of temperature, using a laser interferometer based on the Doppler effect. The

CRS setups and experimental technique have been described elsewhere.[43 – 45]

In this work, the creep rate (CR) spectra were measured over the temperature range

between 20 and 3308C, at tensile stresses of 10 MPa at moderate temperatures and

0.5 MPa at high temperatures. These stresses were chosen in preliminary experiments

as capable of inducing sufficient creep rates to be measured, while maintaining also a

high spectral resolution, without smoothing and distortion of the spectral contour, and

preventing a premature rupture of the sample at high temperatures.

Film samples with 1 � 5 mm2 cross-section and 5 cm length were used. The CR

spectra were measured at heating with a rate of about 18C/min. The time from the

moment of loading to the moment of measuring the creep rate was kept constant (10 s).

Loading of the sample, measuring of the interferogram, and unloading were performed

upon every 58 of heating.

The time evolution of deformation is registered as a sequence of low-frequency beats

in an interferogram whose beat frequency, n, yields a creep rate:

_1 ¼ln

2I0ð3Þ

where l ¼ 630 nm is the laser wavelength, and I0 is an initial length of the working part of

the sample. Besides the CR spectra, plots of modulus of elasticity, E, versus T were also

obtained by using the laser interferometer, from the values of 1 s deformation at 0.5 MPa:

1 ¼ ln=2Io ð4Þ

where n is the number of oscillations in the interferogram.

The correlative (equivalent) frequency of the CRS experiments was equal to

1_ ¼ (2pt)21� 1023 to 1022 Hz. The instrumental error of 1 estimation did not exceed

1%. However, with repeated measurements of identical samples, the scattering in CR

peak height and its temperature location might attain 20% and 58, respectively.

In the broadband DRS measurements the complex dielectric permittivity,

1� ¼ 102i100, was determined as a function of frequency in the range of 1021 Hz to

1 MHz at 258C. Some measurements were also performed at different temperatures in

the range from 2150 to 2308C; their results were previously published.[46] The exper-

iments were carried out by using an Alfa Novocontrol analyzer in combination with the

Novocontrol Quatro cryosystem for temperature control. The samples were placed

between circular brass electrodes of 20 mm diameter. Details of the DRS technique

used may be found elsewhere.[19]


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08 Results and Discussion

PCN Formation

Dicyanic esters can undergo thermal or catalytic polycyclotrimerization as an addition

process resulting in the formation of densely cross-linked PCN network (Fig. 1). The

process of DCEBA polymerization is recorded in the IR spectra (Fig. 2) as decreasing

(up to total disappearance in fully cured network) of the O-C;;N characteristic doublet

absorption band at 2240–2270 cm21. Simultaneously, PCN formation is accompanied

by the appearance and increase of absorption bands at 1570 and 1370 cm21 as a consequence

of transformation of DCEBA (C;;N-groups) into PCN (triazine rings).

Neat DCEBA, DCEBA/Cloisite Na (98/2), DCEBA/Cloisite 30B (98/2), and

DCEBA/Cloisite 30B (95/5) mixtures were chosen for the kinetic experiments to

reveal the impact of clay type as well as the concentration of the organically modified

MMT on the process of PCN formation. Conversion at 1508C versus time and conversion

rate versus time dependencies, obtained for the systems under investigation, are shown in

Fig. 3. Introduction of the unmodified MMT (Cloisite Na) into DCEBA resulted in simul-

taneous reduction of the reaction induction time (Fig. 3a) and increase of the conversion

rate (Fig. 3b) compared to those in the neat DCEBA curing process. The replacement of

the Na-ions situated in the galleries of the natural MMT (Cloisite Na) by the quaternary

ammonium cations (Cloisite 30B) led to additional acceleration effect on DCEBA

polymerization.

This catalytic effect of MMT on DCEBA polymerization process can be explained in

the following way. First, polycyclotrimerization of DCEBA can be affected by metal

cations such as Al3þ and Fe3þ,[1,2] situated at the MMT surface, as a result of isomorphous

substitution and/or lattice imperfections.[47,48] Besides, DCEBA can react with hydroxyls

of the MMT surface modifier or water traces forming carbamates. The latter, in turn,

accelerate the process of formation of PCN network.[1]

To determine the fraction of DCEBA monomer involved in the polycyclotrimerization

reaction, sol-gel analysis of the samples was performed after completion of the thermal

curing. Gel fractions found for all composites under investigation varied from 99.1 to

99.9%, indicating practically total incorporation of DCEBA molecules into the PCN network.

Figure 1. Scheme of PCN formation.


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Structure of PCN-MMT Nanocomposites

Table 1 shows structural WAXD parameters obtained for a few of the nanocomposites

under investigation. Figure 4 shows that the main diffraction peak of Cloisite 30B shifts

in the diffractogram of the nanocomposite toward lower angle values, indicating the inter-

calation process: the interlayer spacing increases from 1.92 to 2.10–2.26 nm (Table 1).

Maximal effect is observed for minimal MMT content of 2 wt.%. At the same time, a

regular decrease of peak intensity with MMT content, compared to the neat Cloisite

30B, was observed. This phenomenon could be treated as disordering of the MMT

stacks due to prevailing of the exfoliation process. To verify this last assumption, “mech-

anical” blends of the ground cured PCN and proper amounts of MMT were prepared, and

their x-ray diffraction patterns were registered. The “mechanical” blend is a model of the

PCN/MMT system without interactions between the components and, correspondingly,

without any changes in the structure of the MMT inside the polycyanurate matrix.

Figure 4 shows that in this case of “mechanical” blend the 2u-position of the Cloisite

30B peak is the same as for the neat MMT. The obvious difference in the peak areas of

curves 2 and 3 indicates a decrease in the contribution of well-ordered MMT fraction to

Figure 2. IR spectra of (1) the initial DCEBA and (2) PCN network with an intermediate conversion

degree.

Figure 3. Effect of MMT on polymerization kinetics of DCEBA at 1508C: (a) Conversion versus

time and (b) conversion rate versus time dependencies for (1) neat PCN; (2) nanocomposites with

2 wt.% Cloisite Na; (3) 2 wt.% Cloisite 30B, and (4) 5 wt.% Cloisite 30B.


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total WAXD pattern of the nanocomposite, owing to MMT delamination occurring along

with intercalation. These results suggest that the structure of the nanocomposites

synthesized may be considered as mixed intercalated/exfoliated one.

A direct illustration of the nanocomposite structure could be obtained by TEM

technique. Figure 5 shows a typical microphotograph of the PCN containing 5 wt.% of

Cloisite 30B. One can discern dark lines representing individual sheets of Cloisite 30B

with the thickness of about 1 nm. The statistically distributed discrete plates, as well as

well-organized stacks of the MMT nanolayers, are observed simultaneously in the poly-

cyanurate matrix. Thus, our suggestion about the complex intercalated/exfoliated

structure of the PCN/MMT composites, based on the WAXD analysis, was validated

additionally by TEM.

DSC Analysis

It was shown in preliminary DSC experiments that the nanocomposite samples under

study, prepared at temperatures between 150 to 2108C, need an additional stabilizing

treatment in order to attain a maximal degree of cross-linking (conversion XCN!PCN).

Figure 6 shows the DSC curves obtained for one of the nanocomposites with different

pre-histories. The initial sample exhibits the onset of the glass transition at 2358C, with

the subsequent intense exotherm of the postcuring process (scan 1). Curve 2, obtained

Table 1Structure parameters of PCN-MMT nanocomposites

Sample (wt.%) 2u, degr d, nm

Cloisite 30B 4.6 1.92

PCN/Cloisite 30B (98/2) 3.9 2.26

PCN/Cloisite 30B (97/3) 4.2 2.10

PCN/Cloisite 30B (96/4) 4.2 2.10

PCN/Cloisite 30B (95/5) 4.1 2.15

Ultrasonic treatment for 5 min was used.

Figure 4. WAXD patterns of (1) Cloisite 30B, (2) PCN/Cloisite 30B (95/5) mechanical blend, and

(3) PCN/Cloisite 30B (95/5) nanocomposite. Interlayer spacings are indicated.


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after treatment of this sample for 20 min at 2708C, shows increasing Tg to 2878C and slight

residual postcuring at about 3008C. Finally, curve 3, obtained after heating the same

sample to 3308C, corresponds to a totally stabilized sample. This curve exhibits the Tgtransition at 2918C, corresponding to a totally crosslinked PCN network

(XCN!PCN � 1)[49] and, in addition, a new glass transition with Tg ¼ 3758C, which

directly precedes the onset of decomposition exotherm. We found that such stabilization

procedure, 20 min holding at 2708C plus heating to 3308C followed by the immediate

cooling to room temperature, was optimal and we used that for all samples under study.

Figure 7 shows that two high-temperature glass transitions, at ca. 270–2908C and

375–3808C, are typical of all stabilized PCN-MMT nanocomposites, unlike a single

glass transition in the neat PCN. In addition, Fig. 7 shows that the exothermic decomposition

Figure 5. TEM image of PCN/Cloisite 30B (95/5) nanocomposite.

Figure 6. DSC curves of PCN/Cloisite 15A (98/2, u.t. 5 min) nanocomposite, obtained at high

temperatures and illustrating the choice of the stabilization regime: scan 1 up to 2708C followed

by 20 min treatment at this temperature; scan 2 up to 3308C followed by cooling to room tempera-

ture, and scan 3 of this stabilized sample to 4208C. Heating rate was 208C/min. Cooling rate

was 3208C/min. Tg values are indicated. Dotted line corresponds to the onset of exothermic

decomposition process.


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process starts at 3508C for neat PCN but at ca. 4008C only for the nanocomposites. This

effect of increasing PCN thermal stability in PCN-MMT nanocomposites is in accordance

with the results of TGA measurements.[38]

On the other hand, DSC curves, registered at moderate temperatures, allow us to

observe the second dynamic peculiarity in the PCN/Cloisite 30B nanocomposites: the

development of a slight but discernable, very broad glass transition with Tg of about

80–1008C and transition range extending from 40–60 to 140–1458C (Fig. 8). This tran-

sition is absent, however, not only in the neat PCN but also in the PCN/Cloisite 15A

nanocomposite.

Thus, three glass transitions may be observed by DSC in the PCN-MMT nanocompo-

sites, which are located at the same temperature, much below, and much higher than the

glass transition temperature peculiar to neat PCN, respectively. Transitions’ character-

istics, as determined by DSC for the stabilized samples, are summarized in Table 2. As

one can see from Table 2 and Figs. 7 and 8, the PCN/Cloisite 15A nanocomposite

shows, besides no glass transition 1, the highest Tg2 ¼ 2918C and the largest relaxation

strength of glass transition 3 (DCp ¼ 0.15 J/g degr).

Further, the DSC curves, registered at different heating rates, gave information on

activation barriers for high-temperature glass transitions 2 and 3, and on a special

behavior of the latter one.

Figure 9 shows that relaxation strength of glass transition 3 in the identically stabil-

ized samples, as estimated by DCP value, turned out to change with temperature of

additional thermal treatment in case the samples were treated for 20 min at different temp-

eratures T . Tg1, at 307 or 3278C. The intensity of glass transition 3 decreased and,

moreover, this transition even disappeared with increasing treatment temperature.

Figure 7. DSC curves obtained over the 200 to 4208C range for neat PCN and stabilized PCN-MMT

nanocomposites. Tg values in two glass transitions are indicated. Heating rate was 208C/min.


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We supposed that glass transition 3 is associated, in contrast to main glass transition 2,

with some irreversible noncooperative process that proceeds with relatively low activation

energy. Then, although transition 3 takes place at heating rate V ¼ 208C/min at 375–

3808C, this irreversible process could take place already during thermal treatment at 3278C.

This assumption was confirmed, indeed, by registering DSC curves for a series of

identically stabilized nanocomposite samples at different heating rates. In fact, Fig. 10

shows a much larger shift of transition 3 to lower temperatures with a decreasing

heating rate from 40 to 2.58C/min, as compared with that for transition 2. The onset of

glass transition 3, as measured at V ¼ 2.58C/min, is observed just at 3278C. Figure 11

shows some of the linear lnV versus T21 plots for glass transition temperatures 2 and 3

in the nanocomposites. From these dependencies and by using formula (2), we obtained

very high apparent (cooperative) activation energies of 700–1100 kJ/mol for the main

glass transition 2 but a low, practically noncooperative activation energy of 180 kJ/mol

for transition 3.

It is worth mentioning that we have revealed and described previously the glass

transition plurality in hybrid PCN-based polymer networks.[20,30,31] This phenomenon

was a result of the presence in such networks of nanodomains with different PCN

cross-linking degrees (conversion XCN!PCN). This approach proceeded from the Tgversus XCN!PCN dependence found experimentally for neat PCN.[49,50]

The complicated glass transition dynamics in PCN-MMT nanocomposites, including

Tg changes in two opposite directions with manifestation of three glass transitions, may

tentatively be interpreted in the following way. Introduction of 2–5 wt.% Cloisite into

the reaction system, with formation of intercalated/exfoliated structure, did not affect

the majority of the PCN nanodomains, and the “usual,” most intense glass transition 2

was characteristic of these domains. However, Cloisite nanolayers could prevent total

CN ! PCN conversion in some of the PCN nanodomains that resulted in

their decreased Tg, namely, the manifestation of the broad glass transition 1 at about

Figure 8. DSC curves obtained between 230 to 1808C for neat PCN and stabilized PCN-MMT

nanocomposites. Scan 1–thin line, scan 2–thick line. The temperatures of the low-temperature

glass transition 1 onset and completion are indicated.


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Table 2

PCN-MMT nanocomposites: glass transition characteristics as determined by DSC for the stabilized� samples

Sample

Clay

content,

wt. %

Ultrasonic

treatment,

min

Glass transition 1�� Main glass transition 2 Glass transition 3

Tg0, 8C Tg, 8C Tg

00, 8CDCP, J/g

degr Tg0, 8C Tg, 8C Tg

00, 8CDCP, J/g

degr Tg0, 8C Tg, 8C Tg

00, 8CDCP, J/g

degr

PCN 0 0 — — — — 273 284 293 0.14 — — — —

PCN/Cloisite 30B 2 0 46 84 144 0.07 272 279 291 0.21 355 377 387 0.09

PCN/Cloisite 15A 2 5 — — — — 278 291 301 0.17 362 375 389 0.15

PCN/Cloisite 30B 2 5 58 97 145 0.08 269 287 295 0.25 358 379 387 0.11

PCN/Cloisite 30B 5 4 42 92 138 0.07 253 273 284 0.25 355 377 387 0.13

�Scan III, after treatment for 20 min at 2708C followed by cooling at the rate of 320 degr/min to room temperature, then heating up to 3308C followed by immediatecooling to room temperature. Heating rate was 20 degr/min.

��Scan II after heating to 1808C and cooling at the rate of 320 degr/min. Heating rate was 20 degr/min.

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40–1408C. According to Tg vs. XCN!PCN dependence in PCN,[49,50] these Tg values

correspond to incomplete conversion, XCN!PCN � (0.65–0.80).

Finally, nonstable high-temperature glass transition 3, peculiar to PCN-MMT nano-

composites, may characterize the unfreezing dynamics in PCN nanodomains adjoining

silicate nanolayers. Strong coordinate and covalent bonds between PCN and the

modified surfaces of MMT layers may result in the effect of constrained dynamics,

which has been observed earlier for many complex polymer systems including, for

instance, hybrid materials.[51 – 55] It may be assumed that the effect of constrained

dynamics decreases due to the onset of irreversible destruction of bonds between PCN

Figure 9. DSC curves obtained for four identically stabilized PCN/Cloisite 15A (98/2, u.t. 5 min)

samples after their additional thermal treatments indicated on the plots. Tg values in the glass tran-

sitions 2 and 3 are given. Heating rate was 208C/min.

Figure 10. DSC curves obtained at different heating rates for five identically stabilized PCN/Cloi-

site 15A (98/2, u.t. 5 min) nanocomposite samples. Tg values in the glass transitions 2 and 3 are indi-

cated. Dotted lines demonstrate their different shift with heating rate.


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and MMT surfaces at temperatures over ca. 3308C, close also to the beginning of PCN

decomposition process. This results, eventually, in disappearance of glass transition 3.

It should be noted that the noncooperative nature of high-temperature transition 3

(Fig. 11) is of interest but not totally understandable so far.

Dielectric Behavior at Room Temperature

Figure 12 shows the results of DRS measurements performed in the frequency range

between 0.1 Hz and 1 MHz at room temperature. The data plotted here are the average

values of three repeat measurements. The real part of dielectric function, 10, and dielectric

loss, 100, allowed us to obtain some comparative information on mobility in PCN and

PCN-MMT nanocomposites in the glassy state, where only small-scale, sub-Tg relaxations

contribute to network mobility. It should be mentioned that DRS and TSDC data obtained

for these systems over the broad temperature range have been presented in detail

elsewhere.[46]

Comparing data within Fig. 12 permits an estimation of the effect of such factors as

addition of (a) MMT, (b) ultrasonic treatment of the polymerization mixture providing

better exfoliation of MMT nanolayers, and (c) choice of MMT surface modifier on

molecular mobility at 208C in the glassy nanocomposites investigated.

The neat PCN is characterized by a relatively low value of 10 and its slight dependence

on frequency (Fig. 12a). On the whole, PCN-MMT nanocomposites exhibit, at each

frequency, higher values of 10 at 258C as compared to those in the neat PCN matrix, as

well as higher values of 100 at the lowest and highest frequencies. Enhancement of

small-scale mobility in nanocomposites at room temperatures; i.e., close to the onset of

anomalous glass transition 1 revealed by DSC, may be also understood in terms of the

local formation of an incompletely cross-linked PCN network.

Figure 12b shows some influence of the method of mixing MMT with PCN

prepolymer (only mechanical stirring vs. stirring with simultaneous ultrasonic

treatment). The ultrasonic treatment, providing better dispersion (exfoliation) of MMT

nanolayers, results in a smaller increase of sub-Tg mobility in PCN matrix, as determined

from both 10 and 100 measurements.

Finally, a small difference in small-scale mobility is discerned for nanocomposites

with different MMT surface modifiers (Fig. 12c). By using Cloisite 30B, which has a

single tallow tail, quaternary ammonium salt and two hydroxyls in the modifier

Figure 11. Logarithm heating rate versus reciprocal temperature dependencies obtained for the

glass transition temperature Tg in stabilized PCN-MMT nanocomposites. 1–glass transition 2 in

PCN/Cloisite 30B (98/2) sample; 2,3–glass transitions 2 and 3 in PCN/Cloisite 15A (98/2, u.t.

5 min) sample. The calculated activation energies Q are given.


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molecule, both coordinate and covalent bonds between PCN and MMT are formed. On the

other hand, Cloisite 15A contains two tallow tails and no hydroxyls in the modifier

molecule. For this reason, probably, the nanocomposite with Cloisite 30B exhibits

slightly lower values of 10 and 100 than the nanocomposite with Cloisite 15A.

Creep-rate Spectroscopy Measurements

To study the heterogeneity of glass transition dynamics in the PCN-MMT nanocomposites

in more detail, CR spectra were recorded over the temperature range between 20

and 3308C. After preliminary experiments, a tensile stress of 10 MPa was chosen

for measurements at moderate temperatures of 20–2008C as capable of inducing suff-

icient creep rates to be measured under these conditions. At higher temperatures

Figure 12. DRS measurements of neat PCN and PCN-MMT nanocomposites at 258C: real part of

dielectric function 10 and dielectric loss 100 as functions of frequency. The impacts of (a) MMT,

(b) ultrasonic treatment, and (c) type of MMT surface modifier are shown separately.


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200–3208C, a low stress of 0.5 MPa was applied to maintain a high spectral resolution

without smoothing of the spectral contour and to prevent a premature rupture of the

samples.

Figure 13 shows the CR spectra obtained for neat PCN and PCN-MMT nanocompo-

sites. A single CR glass transition peak with maximum of Tg � 2808C for neat PCN

network transforms, for the nanocomposites, into complicated CR spectra consisting of

a few overlapping peaks and covering the whole temperature region under investigation;

i.e., around, much below, and higher than the Tg of the neat PCN matrix. The spectra are

broken off at ca. 3008C for PCN and 3308C for nanocomposites due to fracture of

the samples.

These discrete spectra are in accordance with the DSC data and directly confirm the

pronounced heterogeneity of glass transition dynamics in the PCN-MMT nanocomposites.

Figure 13. Creep rate spectra obtained for PCN-MMT nanocomposites (solid lines) and neat PCN

(dotted lines) under tensile stresses of 10 MPa at moderate temperatures and of 0.5 MPa at higher

temperatures. Similar to Fig. 12, the impacts of (a) MMT, (b) ultrasonic treatment, and (c) type

of MMT surface modifier are shown separately.


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08 In fact, according to Georjon et al. and Georjon and Galy,[49,50] Tg around 260–2808C

corresponds to unfreezing dynamics in PCN nanodomains with a high degree of cross-

linking, XCN!PCN ¼ 0.95–0.99; i.e., to main transition 2. At the same time, introduction

of MMT with formation of intercalated/exfoliated nanocomposite structure results again

(as shown by DSC) in two opposite effects: incomplete cross-linking in some of the matrix

nanodomains (where Tg ¼ 40–1408C characterizes glass transition 1 corresponding to

conversion XCN!PCN ¼ 0.65–0.80) and, simultaneously, development of new spectral

constituents at 290–3208C. The latter may, obviously, be assigned to the onset of unfreez-

ing constrained dynamics in the PCN nanolayers at the PCN-MMT interfaces. In fact,

these temperatures coincide with the beginning of glass transition 3, as estimated by

DSC at heating rate of 2.58C/min (Fig. 10).

Thus, it is worthy to note that the CRS data characterize, to a certain extent, not only

the dynamic heterogeneity but also the compositional nanoheterogeneity of PCN-MMT

nanocomposites.

It is of special interest, with respect to applications, that the high thermal stability of

the PCN matrix is retained in the PCN-MMT nanocomposites, whereas, simultaneously,

some microplasticity around room temperature is manifested. In fact, Fig. 13 shows

total absence of creep ability of the neat PCN network at moderate temperatures, at

least up to 1308C; at the same time, some creep is observed in the PCN-MMT nanocom-

posites in the same temperature region. Much higher creep rates were observed at higher

stresses in the latter case. This means that MMT additives caused the development of some

microplasticity at low temperatures around 258C in PCN networks as a consequence of

incomplete cross-linking in local domains of the network. Therefore, it is natural to

assume that increasing fracture toughness and crack resistance, observed for PCN-

MMT nanocomposites,[38,41] might be associated not only with direct influence of 2-D

silicate nanolayers but also with the indirect effect of increasing microplasticity in the

system, due to partial relaxation of overstresses near the tips of cracks.

Figures 13a, b, and c demonstrate, separately, the influence on dynamics of such

factors as introduction of different MMT amounts; role of ultrasonic treatment to

provide improved dispersion of MMT in the matrix, and comparative efficiency of two

types of MMT surface modifiers. The addition of 2% MMT to PCN provides better

thermal stability (creep resistance) than addition of 5% MMT, probably due to

improved dispersion (Fig. 13a). Some increase in mobility, both at moderate and high

temperatures, is attained due to using the procedure of ultrasonic treatment of the

reaction mixture (Fig. 13b). Some difference in the influence of Cloisite 30B and

Cloisite 15A on glass transition dynamics may also be seen from Fig. 13c.

Temperature Dependence of Modulus of Elasticity

Figure 14 shows modulus of elasticity, E, versus temperature dependencies measured for

neat PCN and PCN-MMT nanocomposites by using a laser interferometer at a tensile

stress of 0.5 MPa. We observe a considerable impact of introduction of small quantities

of nanofiller on the elastic properties of PCN. The effect depends, to some extent, on

material composition, and it is different for different temperature regions. Over the

range between 20 and 2308C the modulus of the nanocomposites is 1.5–2.0 times

higher than that of neat PCN. The highest E value is observed for the PCN/Cloisite

30B (98/2, u.t. 5 min) sample, whereas the PCN/Cloisite 15A (98/2, u.t. 5 min)

sample exhibits the lowest one.


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Within the temperature range of steep slope of the dependecies, between 240 and

2808C, the modulus is higher for the neat PCN than for the nanocomposites; the nanocom-

posite with 5 wt.% Cloisite shows minimal E values. This effect is in accordance with

accelerating dynamics (increased creep) just at these temperatures in the nanocomposites

(Fig. 13). Finally, at 290 to 3308C the superiority of nanocomposites to neat PCN becomes

obvious again, and the best sample, PCN/Cloisite 30B (98/2, u.t. 5 min), has higher

modulus at 3208C than neat PCN at 2808C (Fig. 14).

Conclusions

A series of PCN-MMT nanocomposites with 2 or 5 wt.% nanofiller, differing in the type of

MMT surface modifier and in processing (use of ultrasound for better MMT dispersion in

the polymer matrix), were synthesized and submitted to combined WAXD/IRS/TEM/DSC//CRS/DRS analysis for revealing structure–dynamics–properties relationships.

As shown, MMT acts as a catalyst for cyclotrimerization reaction of cyanic ester with

formation of densely cross-linked PCN networks. Along with intercalation of polymers

between MMT nanolayers, their partial exfoliation in the matrix was achieved with the

use of ultrasonic treatment. Of special interest is that introduction of layered MMT nano-

filler into the matrix resulted in development of pronounced dynamic heterogeneity and

manifestation of two opposite effects, namely, of both some suppression and enhancement

of dynamics in the PCN-MMT nanocomposites. Thus, three glass transitions were found

for the nanocomposites, as compared to a single glass transition with Tg � 280–2908C in

neat PCN.

On the one hand, a new, additional glass transition with Tg � 370–3808C arose due to

the presence of MMT, which we tentatively assigned to unfreezing dynamics in PCN

nanolayers adjoining modified MMT surfaces with covalent and/or coordinate bonding

between PCN and MMT layers; i.e., to a constrained dynamics effect.

On the other hand, however, we revealed enhancement of dynamics at moderate and

low temperatures caused by introduction of MMT into the polymer matrix, including the

development of one more glass transition, extending over the temperature range from 40–

Figure 14. Tensile modulus of elasticity vs. temperature dependencies as determined for

PCN-MMT nanocomposites and neat PCN at the stress of 0.5 MPa, by using a laser interferometer.


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08 60 to ca. 1408C, and enhancing sub-Tg mobility at room temperature. These effects are

associated, obviously, with the formation of incompletely cross-linked nanodomains in

the nanocomposite matrix.

These changes in dynamics provided some improvement of thermal and mechanical

performance of PCN-MMT nanocomposites at both moderate and high temperatures. The

development of microplasticity (creep) in the brittle PCN network (even at �208C) after

introduction of MMT additive is of significance. It may be assumed that the increase of

mobility (arising of relaxation abilities) in these nanocomposites contributed to the

effects of increased crack resistance and fracture toughness[38,41] in PCN-MMT

nanocomposites.

Acknowledgment

This work was supported by the Greek General Secretariat of Research and Technology in

the frame of the PENED Program 03ED0873 and the European Social Fund.

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Polycyanurate-Organically Modified Montmorillonite Nanocomposites: Structure-Dynamics-Properties...

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