thermostable polycyanurate-polyhedral oligomeric silsesquioxane hybrid networks: synthesis, dynamics...

Macromol. Symp. 2012, 316, 90–96 DOI: 10.1002/masy.20125061290

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Thermostable Polycyanurate-Polyhedral Oligomeric

Silsesquioxane Hybrid Networks: Synthesis,

Dynamics and Thermal Behavior

Olga Starostenko,1 Vladimir Bershtein,*2 Alexander Fainleib,1 Larisa Egorova,2

Olga Grigoryeva,1 Alfred Sinani,2 Pavel Yakushev2

Summary: A series of hybrid polycyanurate - epoxy cyclohexyl-functionalized poly-

hedral oligomeric silsesquioxane (PCN/ECH-POSS) nanocomposite networks with

ECH-POSS content varying from 0.025 to 10 wt. % were synthesized and characterized

using FTIR, DSC, DMA and CRS techniques. It was revealed that already as low as

0.025 wt. % POSS cardinally changed PCN glass transition characteristics including

the strong shift of the transition onset to higher temperatures and manifesting a

second, higher-temperature glass transition characterizing interfacial dynamics;

additionally, enhancing creep resistance and thermal stability at the earlier stage

of degradation were observed.

Keywords: glass transition; nanocomposites; polycyanurates; POSS

Introduction

Densely cross-linked polycyanurates (PCN)

synthesized from cyanate ester resins have

attracted much interest in recent years

because of their excellent thermal and good

mechanical properties, which commend

them for use in high performance technology

(e.g., as matrices for composites for high-

speed electronic circuitry and transpor-

tation).[1,2] Additionally, cyanate/epoxy

composites provide superior performance

through the co-reaction between cyanate

and epoxy groups of blend components; as a

result, fine properties of the final composite

are reached.[3] Further enhancing PCN and/

or overcoming their drawbacks could be

attained in PCN hybrids and nanocompo-

sites.[4]

Last years the great attention has been

paid to preparing and characterization of

hybrid polymer/inorganic nanocomposites

stitute of Macromolecular Chemistry, NAS, 02160

yiv, Ukraine

ffe Physical-Technical Institute, RAS, 194021

.-Petersburg, Russia

mail: [email protected]

yright � 2012 WILEY-VCH Verlag GmbH & Co. KGaA

containing 3D molecules (particles) of poly-

hedral oligomeric silsesquioxane (POSS) of

about 1 nm in size.[5] POSS compounds have

the cage structureswith the common formula

(RSiO1.5)8, 10, or 12, which are called as T8, T10

and T12 cages, respectively. Typically, an

each cage silicone atom in POSS is bonded

to three oxygen atoms and to a single R

substituent. The functional groups of POSS

may react, via grafting, copolymerization or

other reaction, with monomer or polymer,

and hence POSS cages can be covalently

incorporated into a polymer matrix. Thus,

POSS offers a chance to prepare hybrid

nanocomposites with molecularly dispersed

inorganic structural units where POSS

cages may be considered, to a certain

extent, as�1 nm silica inclusions or clusters

chemically bound with a polymer matrix.

Just the ability of POSS to be dispersed as

unassociated units covalently bound to a

matrix is the key to impact POSS on

polymer dynamics and properties. The

numerous studies (see, e.g.,[5–9]) showed

that different polymer-POSS nanocompo-

sites can exhibit dramatic improvements in

polymer matrix properties such as thermal

stability, oxidation resistance, mechanical

, Weinheim wileyonlinelibrary.com

Macromol. Symp. 2012, 316, 90–96 91

behavior, surface hardness as well as

reduction in flammability and so on.

Recently, amine- or cyan-, or hydroxyl-

functionalized POSS molecules were intro-

duced into cyanate ester resin in the

amounts from 1 to 15wt. %.[10,11] Thus,

octaaminophenyl-POSS additive provided

formation of the hybrid PCN/POSS nano-

composites with the substantially changed

properties, in particular Tg shift to both

higher and lower temperatures.

In the present study, the nanocomposites

based on PCN with different doping levels

by epoxy cyclohexyl-POSS, starting from

0.025wt. %, were studied. The chemical

structure and final properties of the nano-

composites were investigated by means of

Fourier-transform infra-red spectroscopy

(FTIR), differential scanning calorimetry

(DSC), dynamic mechanical analysis (DMA)

and laser-interferometric creep rate spec-

troscopy (CRS).

Experimental Part

1,1’-bis(4-cyanatophenyl) ethane (dicya-

nate ester of bisphenol E, DCBE), under

the trade name PRIMASET1 LECy L-10

(from Lonza Group Ltd., Switzerland), and

epoxy cyclohexyl POSS1 Cage Mixture

(ECH-POSS, from Hybrid Plastics Inc.,

Hattiesburg, MS, USA) were used as

received. The formulas for this monomer

and ECH-POSS (T8 cage) are shown

below. The polymer nanocomposites

from DCBE and ECH-POSS with ECH-

POSS content c¼ 0.025, 0.05, 0.1, 0.5, 1.0,

2.0, 5.0, and 10.0wt. % were synthesized.

The initial mixtures were first stirred

at 1708C during 2 hrs for pre-polymeriza-

tion of DCBE and chemical grafting of

ECH-

Copyright � 2012 WILEY-VCH Verlag GmbH & Co. KGaA

POSS to the growing PCN network

through the reaction between cyanate and

epoxy groups. Then the filled pre-polymer

was step-by-step cured and sequentially

post-cured at 170-3008C for 6 hrs.

At monitoring the curing process, FTIR

spectra were recorded between 4000 and

600 cm�1 using a Bruker Tensor 37 spectro-

meter. For each spectrum, 32 consecutive

scans with a resolution of 4 cm�1 were

averaged. The IR band at 2968 cm�1 was

used as an internal standard. The dynamics,

thermal behavior and elastic properties

of the PCN/ECH-POSS hybrids were

characterized using the combined DSC

(Perkin-Elmer DSC-2 apparatus), DMA

(DMS 6100 Seiko Instruments, 1Hz), and

CRS[12] approach.

Results and discussion

Figure 1 shows how decreasing the inten-

sities of the absorption bands at 2237 and

2266 cm�1 characterizing cyanate groups is

accompanied with appearing the bands at

1369 and 1564 cm�1 of the cyanurate ring

vibration in the spectra during the poly-

merization of initial DCBE/POSS mixture,

as a consequence of the basic process

of polycyclotrimerization of cyanate

groups with formation of triazine cycles.

Meantime, the slight absorption band at

1738 cm�1 appears also in the nanocompo-

site spectra which confirms the presence of

oxazolidinone rings formed owing to co-

reaction between cyanate groups of DCBE

and epoxy groups of ECH-POSS.[3] This

provides the evidence of chemical hybridi-

zation between both constituents in these

systems. A simplified scheme of molecular

structure of the hybrid network formed is

presented in Fig. 2.

, Weinheim www.ms-journal.de

Figure 1.

FTIR spectra of model initial PCN/ECH-POSS reactive blend (a), and heated at 1708C for 2 hrs (b) or 3 hrs (c). The

composition of the blend was cyanate/epoxy groups¼ 1:1.

Macromol. Symp. 2012, 316, 90–9692

It was revealed that POSS additives

substantially changed PCN glass transition

characteristics, as estimated by DSC

(Figs. 3 and 4). Unlike a single glass

transition (Tg¼ 2448C) in neat PCN, DSC

Figure 2.

A scheme of PCN/ECH-POSS molecular structure fragme


curves of the hybrids exhibit two transi-

tions, the main one with Tg1 varying,

depending on a composition, from 243 to

2758C, and the weaker transition with Tg2 �3758C followed by the hybrid degradation.

nt.


Figure 4.

Main glass transition temperatures of the PCN/ECH-

POSS nanocomposites as a function of POSS content.

Figure 3.

DSC curves of neat PCN and three PCN/ECH-POSS

nanocomposites at heating up to 4008C with the

rate 208Cmin�1 (scans I and II, cooling rate

3208Cmin�1).

Macromol. Symp. 2012, 316, 90–96 93

The latter transition may be assigned to

dynamics in the interfacial layers (a strong

‘‘constrained dynamics’’ effect [12,13]).

TGA control showed that thermal

degradation with mass loss started from

� 4208C for neat PCN and low-POSS

content composites, however, degradation

of interfacial bonds started, obviously,

already at T � 4008C since the second glass

transition disappeared in the DSC curves

obtained at scan II (Fig. 3). The largest, by

�300, increasing Tg1 was recorded for the

hybrids with c¼ 0.025 or 0.1% only;

essentially, the temperature of glass transi-

tion onset, Tg1’, increased from 209 to


2618C, and main glass transition became

more narrow in these hybrids (from 540 for

neat PCN to 20-258C for the hybrids,

Fig. 4). The opposite tendency of decreas-

ing Tg1, especially Tg1’, and broadening

glass transition was observed at high POSS

contents, obviously, due to decreasing

locally PCN cross-linking because of DCBE

expense for co-reaction with ECH-POSS.

Enhancing thermal stability of PCN-

POSS hybrids compared with neat PCN at

the earlier stage of degradation was

revealed also by DSC: after scanning to

4008C in nitrogen atmosphere, the tem-

peratures of the transition onset Tg1’ at

scanning II equaled 1878C for neat PCN,

1618C for the hybrid with c¼ 10% but

2498C for the hybrid with c¼ 0.025%.

Similarly, unlike DMA peak with

Tmax¼ 2488C in neat PCN, the main peak

with Tmax varying from 246 to 2658C(the latter at c¼ 0.025%), as well as the


Macromol. Symp. 2012, 316, 90–9694

overlapping peaks at �370–390 and 4308C,associated with the interfacial dynamics

and degradation process respectively, were

observed for the hybrids. Dynamic modulus

E’ over the 20–2008C range increased for

the hybrids regarding neat PCN, maximum

by 30–40% at c¼ 0.5% (Fig. 5).

At last, the discrete creep rate spectra,

including a few overlapping peaks and

demonstrating the pronounced dynamic

heterogeneity aroundmain Tg, were obtained

(Fig. 6). The constrained dynamics effect

manifests itself here in the displacement of

the spectra by 10-208C to higher tempera-

tures regarding the PCN spectrum, enhan-

cing creep resistance and increasing the

temperature of the sharp creep acceleration

and fracture of the nanocomposites regard-

Figure 5.

DMA data (above - Tand, below – dynamic modulus E’ vs

ECH-POSS hybrids.


ing neat PCN, e.g., from 270 to 3308C at

c¼ 0.1wt.%.

Thus, the most remarkable result in this

study is the strong impact on polymer

dynamics of very low 3D nanofiller content,

viz., as low as 0.025wt. %. Really, for linear

polymer matrix with 3D nanofiller additive,

a totally nanoscopically-confined state of a

matrix is usually suggested in case an

average inter-particle distance, L, is close

to or less than the unperturbed dimensions

of macromolecular random coil, as esti-

mated by radius of gyration Rg, typically of

an order of 10 nm in size for many

polymers.[12,13] Therefore, a few percent

loading was typically required for attaining

the substantial constraining dynamics by

3D particles of 10-20 nm size. Meantime, in

. temperature plots) obtained for PCN and three PCN/


Figure 6.

Creep rate spectra obtained at tensile stress 0.5MPa for neat PCN and two hybrids.

Macromol. Symp. 2012, 316, 90–96 95

the case of semi-interpenetrating networks

such effect was strongly pronounced at

0.25wt. % nanodiamonds only when L >>

Rg; this was explained by the double

covalent bonding (hybridization) between

the matrix components and of the matrix

with nanofiller.[14]

The unusually large impact of 0.025%

POSS on PCN glass transition dynamics

may be explained, obviously, by the

combined action of a few factors. First,

separated (unassociated) 1-nm size ECH-

POSS molecules of cage structure play the

role of nanofiller particles (silica nano-

blocks) with extraordinarily high specific

surface area of a few thousands m2g�1; that

provides the enormous surface of inter-

facial boundaries in the hybrid nanocom-

posites under study and �10 nm average

inter-particle distances at c¼ 0.025wt. %

POSS. Secondly, strong interfacial interac-

tions due to covalent bonding of POSS with

the polymer matrix are of importance.

Meantime, however, the amounts of 1-

10wt. % POSS have earlier been used

typically as the blocks at preparing poly-

mer-inorganic hybrids.[5] Therefore, we

suppose that unusually strong influence of


low POSS loading on dynamics may be

treated also as a consequence of more long-

range impact of rigid 3D nanoparticles

within the densely cross-linked PCNmatrix

than in linear or loosely cross-linked

matrices.

Thus, under the optimal conditions the

nanocomposites studied behave, to a cer-

tain extent, as ‘‘interphase controlled

materials’’ containing mainly the nanodo-

mains with exclusively strongly (directly at

interfaces, glass transition at �370–3908C)and substantially suppressed dynamics

(main glass transition at � 260–2808C).

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thermostable polycyanurate-polyhedral oligomeric silsesquioxane hybrid networks: synthesis, dynamics...

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