polyurethane networks nanoreinforced by polyhedral oligomeric silsesquioxane

Polyurethane Networks Nanoreinforced by Polyhedral

Oligomeric Silsesquioxane

Hongzhi Liu, Sixun Zheng*

Department of Polymer Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, P. R. ChinaFax: 86-21-54741297; E-mail: [email protected]

Received: October 11, 2004; Revised: November 16, 2004; Accepted: December 1, 2004; DOI: 10.1002/marc.200400465

Keywords: nanocomposites; polyhedral oligomeric silsesquioxane; polyurethanes

Introduction

Incorporating inorganic or organometallic segments into

polymers to afford improved properties continues to be a

driving force for the development of new materials.[1,2]

Polyhedral oligomeric silsesquioxane (POSS) reagents,

monomers and polymers are emerging as a new chemical

technology for nano-reinforced organic-inorganic hybrids.

Polymers incorporating POSS are becoming the focus of

many studies due to the simplicity of processing and the

excellent comprehensive properties of this class of hybrid

materials.[3–12]

Most previous studies have been concerned with thermo-

plastics or thermosets and the modification of elastomeric

polymers with POSS was relatively less common.[13–15]

Polyurethanes (PU) are an important class of elastomeric

materials and their extensive applications motivated us

to prepare materials with improved properties. Hsiao

et al.[15,16] investigated the structure and properties of

linear POSS-containing polyurethane. In their work, a

3-(allybisphenol-A) propyldimethysiloxane POSS was

used as a chain extender and the POSS was grafted onto

the macromolecular backbone through one corner group.

More recently, Devaus et al.[17] reported that the incorpora-

tion of POSS into polyurethane gave materials with

improved flame retardance. To the best of our knowledge,

there are no reports on studies of polyurethane networks

containing POSS.

In this communication, we have reported the synthesis

and characterization of polyurethane networks incorporat-

ing POSS. An octaaminophenyl polyhedral oligomeric

silsesquioxane (OapPOSS) was used to replace part of the

Summary: Octaaminophenyl polyhedral oligomeric silses-quioxane (OapPOSS) was used as a crosslinking agent to-gether with 4,4

0-methylenebis-(2-chloroaniline) to prepare

polyurethane networks containing POSS. Fourier transforminfrared spectroscopy (FT-IR), dynamic mechanical analysis(DMA) and thermogravimetric analysis (TGA) were em-ployed to characterize the POSS-reinforced polyurethane.The POSS-containing PU networks displayed enhanced glasstransition temperatures (Tgs) and the storage moduli of thenetworks of the glassy state and rubber plateaus were alsoobserved to be significantly higher than that of the controlpolyurethane although only a small amount of POSS wasincorporated into the systems. The results can be ascribed tothe significant nanoscale reinforcement effect of POSS cageson the polyurethane matrix. TGA results showed the thermalstability was also improved with incorporation of POSS intothe system.

Dynamic mechanical spectra of PU and PU nanocompositescontaining POSS.

Macromol. Rapid Commun. 2005, 26, 196–200 DOI: 10.1002/marc.200400465 � 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

196 Communication

aromatic amine crosslinking agent [viz. 4,40-methylenebis-

(2-chloroaniline), MOCA] and thus the POSS cages act as

nanostructured crosslinking sites. The nanoscale reinforce-

ment effect of POSS on the polyurethane networks and the

thermal stability of the hybrid polyurethanes were address-

ed on the basis of dynamic mechanical analysis (DMA) and

thermogravimetric analysis (TGA).

Experimental Part

Materials and Preparation of Samples

The POSS monomer used in this work was an octaaminophenylpolyhedral oligomeric silsesquioxane (denoted OapPOSS),as shown in Scheme 1, which was synthesized by followingliterature methods[18,19] with slight modification.[20] Toluene-2,4-diisocyanate (TDI) was of a chemically pure grade and wasobtained from Shanghai Reagent Co., Shanghai, China. Poly-(propylene oxide) glycol was kindly supplied by PolyurethaneDivision, Gaoqiao Petrochemical Co. Shanghai, China underthe trade name of GE-210 and had a quoted molecular weightof Mn ¼ 1 000. 4,4

0-Methylenebis-(2-chloroaniline) (MOCA)

was used as the crosslinking agent and was kindly supplied byChangshan Chemical Factory, Zhejiang Province, China. Allthe solvents used in this work were obtained from commercialsource and purified in general ways before use.

Synthesis of PU Networks Containing POSS

25.68 g (25.68 mmol) of polyether glycol and 7.15 g(41.09 mmol) of TDI were charged into a 250 ml three-neckedround bottomed flask, equipped with a mechanical stirrer. Thereaction was performed under a nitrogen atmosphere at 80 8Cfor 2 h to afford the polyurethane prepolymer, which was usedfor the next reaction. 0.21 g (0.18 mmol) of OapPOSS wasdissolved in 10 ml of N,N-dimethylformamide (DMF) and thesolution was added to the above PU prepolymer at 80 8C withvigorous stirring for 2 h and the small amount of DMF wasdistilled out under reduced pressure and the POSS-containingocta-armed star-like polyurethane prepolymer was obtained.An equal molar amount of MOCAwas added to the system withrespect to the remnant number of –NCO groups in the POSS-containing polyurethane prepolymers. This was vigorouslystirred and the mixtures were then cast into a pre-heatedstainless steel mold. The samples were sealed in the molds andcured at 100 8C for 2 h and at 120 8C for 3 h. The wholepreparation procedure for the POSS-PU is depicted inScheme 1.

Measurement and Techniques

Fourier Transform Infrared Spectroscopy (FT-IR)

The infrared measurements were conducted on a Perkin-ElmerParagon 1000 Fourier transform spectrometer at room

Scheme 1. Synthesis of polyurethane networks containing POSS.

Polyurethane Networks Nanoreinforced by Polyhedral Oligomeric Silsesquioxane 197

Macromol. Rapid Commun. 2005, 26, 196–200 www.mrc-journal.de � 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

temperature (25 8C). The attenuated total reflection (ATR)accessories were used to measure the FT-IR spectra of all thespecimens at fresh surfaces. In all cases, 64 scans at a resolutionof 2 cm�1 were used to record the spectra.

Dynamic Mechanical Analysis (DMA)

The dynamic mechanical tests were carried out on a DynamicMechanical Thermal Analyzer (DMTA) (MKIII, RheometricScientific Ltd, UK) using a temperature range from �80 to100 8C. The frequency used was 1.0 Hz at a heating rate of3.0 8C min�1. The experiments were carried out until thesample became too soft to be tested.

Thermogravimetric Analysis (TGA)

A Perkin-Elmer thermal gravimetric analyzer (TGA-7) wasused to investigate the thermal stability of the hybrids. Thesamples (about 10 mg) were heated under a nitrogen atmos-phere from ambient temperature up to 600 8C at a heating rateof 20 8C �min�1 in all cases.

Results and Discussion

In order to prepare the POSS-modified polyurethane elasto-

mer, the molar ratio of isocyanate to polyether glycol was

set at 1.6:1 and the rest of the isocyanate groups were

allowed to react with the equal molar amount of amine

groups from octaaminophenyl POSS and MOCA. The

fraction of POSS amine groups was controlled to be 0, 5, 10

and 15 mol-% with respect to the total amine groups, which

corresponded to POSS weight fractions of 0, 0.53, 1.12 and

1.68 wt.-% in the total hybrid, respectively. We report here

the synthesis of PU-POSS hybrids via the three-step

continuous reaction as shown in Scheme 1.

Figure 1 shows the FT-IR spectra of the control PU and

PU hybrids containing POSS. For the control PU, the stret-

ching vibrations of the N-H groups occur at 3 300 cm�1,

which together with the carbonyl bands at 1 726 cm�1 are

indicative of the presence of urethane moieties. For

OapPOSS, the double bands at 3 462 and 3 366 cm�1 are

typical of the stretching vibration of the primary aromatic

amine in OapPOSS whereas the band at 1 116 cm�1 is

ascribed to the stretching vibration of Si-O-Si groups in the

silsesquioxane cages. For the PU-POSS hybrids, this band

unfortunately overlapped with that of the aliphatic ether. It

is worth pointing out that the disappearance of the bands at

2 250–2 275 cm�1 which are characteristic of isocyanate

indicates the completion of the crosslinking reaction be-

tween the aromatic amine and the PU prepolymers in the

control PU and PU hybrids containing POSS.

Dynamic Mechanical Properties

Shown in Figure 2 are the dynamic mechanical spectra of

the control PU and the PU hybrids containing 0, 5, 10 and

15 mol-% of OapPOSS with respect to the total amine

groups. It is interesting to note that in the glass state

((�75)–(�25) 8C) the dynamic storage moduli of all

the POSS-containing hybrids are significantly higher than

that of the control PU. The introduction of a small amount of

POSS cages (<2 wt.-%) gave rise to a significant increase in

dynamic storage modulus. It is also worth noticing that the

storage moduli of the rubbery plateau for the POSS-

containing hybrids were close to or higher than that of the

control PU. Only at the higher concentration of POSS (viz.

�15 mol-%) was the storage modulus of the hybrid lower

than that of the control PU. There are several competitive

factors which affect the moduli of the POSS-containing PU

Figure 1. FT-IR spectra of PU and PU hybrids containing POSS.A) OapPOSS; B) 5 mol-% of amine groups of POSS; C) 10 mol-%of amine groups of POSS; D) 15 mol-% of amine groups of POSS.

Figure 2. Dynamic mechanical spectra of the control PU and thePU nanocomposites containing POSS.

198 H. Liu, S. Zheng


networks. On the one hand, the nanoreinforcement of the

POSS cages on the polymer matrices will give rise to an

increase in modulus, as seen in glassy state and the rubbery

materials with the lower POSS contents, i.e. the PU matri-

ces could be reinforced on the nanoscale by the POSS cages

which were covalently bonded onto the PU networks. On

the other hand, the inclusion of POSS in the system could

give rise to a decrease in the densities of the materials,

which will result in a decrease in the moduli of the nano-

composites. The decreased densities could be ascribed to an

increase in the porosity of the nanocomposites.[21,22] The

porosity of POSS-containing nanocomposites is composed

of two portions. One portion comes from the external poro-

sity as a result of the inclusion of POSS cages, which can be

interpreted as the increase in free volume of the nanocom-

posites caused by the interactions of POSS cages and

polymer segments.[21,22] The second portion of porosity can

be attributed to the nanoporosity of the POSS core with a

diameter of 0.54 nm in a POSS cage. The crosslinking

densities per unit volume will decrease with increasing

concentration of POSS in the hybrids. Nonetheless, it can be

seen that the storage moduli of the nanocomposites con-

taining POSS with less than 10 mol-% of amine groups with

respect to the amine groups in all the crosslinking agents are

significantly higher than or close to that of the plain PU.

This implies significant nanoreinforcement by the POSS

cages, which counteracts the effect of decreased densities

on the storage moduli of the rubber plateau for the networks.

Only at higher POSS contents could the effect of the re-

duced degree of crosslinking dominate the nanoreinforce-

ment. In fact, we indeed observed this result when the

concentration of OapPOSS was 15 mol-% with respect to

the amine in all the crosslinking agents (see Figure 2).

Figure 2 also shows plots of tan d as a function of

temperature for the control polyurethane and its hybrids

with POSS. The control polyurethane network exhibits a

well-defined relaxation peak centered at �7.2 8C, which is

ascribed to the glass transition of the crosslinked poly-

urethane. The PU hybrids containing 5 and 10 mol-% of

OapPOSS also clearly display single a transitions on the

internal friction (tan d) versus temperature curves, which

correspond to the glass transition of the materials. It can be

seen that the temperature values of the tan d peaks increase

with increasing POSS concentration. For the nanocompo-

sites containing 15 mol-% of POSS, a ramp peak appeared

at ca. 50 8C, suggesting the formation of a structure with a

higher glass transition temperature, i.e. the POSS-contain-

ing PU hybrids could possess structural inhomogeneities.

This observation is ascribed to the nanoreinforcement effect

of the POSS cages on polyurethane networks. The massive

and bulky POSS cages could restrict the motion of the

macromolecular chains and thus higher temperatures are

required to provide the requisite thermal energy for the

occurrence of a glass transition in hybrid materials.[24] In

addition to the enhancement of glass transition temper-

atures, it was noted that the tan d peaks were significantly

broadened when the content of POSS cages was increased.

The width of the tan d peaks could reflect the structural

homogeneity of the crosslinked networks. The broadening

of the glass transition region could result from the incorpo-

ration of the nanosized massive and bulky POSS cages,

which could restrict the segmental motion of molecular

chains and network junctions, i.e. the topological con-

straints provided by the presence of POSS reinforcements

could slow the motion of the network junctions. Therefore,

a higher temperature is needed to reach structural equili-

brium.[24,25] It has been proposed that the broadening of the

loss peaks could prefigure superior mechanical properties,

such as damping properties.[23]

Thermal Stability

Shown in Figure 3 are the TGA curves of the control PU and

its nanocomposites with POSS. Within the experimental

temperature range, the TGA curves of all the samples dis-

played similar degradation profiles, suggesting that the

existence of the POSS did not significantly alter the degra-

dation mechanism of the matrix polymers. The incorpora-

tion of OapPOSS into the PU networks showed a significant

effect in improving the thermal stability, resulting in a

retarded weight loss rate and an enhanced char yield in the

higher temperature region.[26–29] This effect was observed

to be increasingly significant with increasing the concen-

tration of POSS cages. The improvement in weight reten-

tion was ascribed to the POSS constituent, which

participated in the formation of a homogeneous hybrid

network. The higher char yields for PU nanocomposites

implied that there were fewer volatiles being released from

the nanocomposites during heating. The decreased rate of

Figure 3. TGA curves of PU hybrids containing POSS.

Polyurethane Networks Nanoreinforced by Polyhedral Oligomeric Silsesquioxane 199


volatile release from the materials suggested the improved

flame retardance. It is proposed that the inorganic compo-

nent provides additional heat capacity, thereby stabilizing

the bulk materials against thermal decomposition, except at

surfaces where initial decomposition would begin.[30]

Conclusion

Octaaminophenyl polyhedral oligomeric silsesquioxane

was used as a crosslinking agent to prepare polyurethane

hybrid networks. The POSS-containing PU networks

displayed enhanced glass transition temperatures (Tgs).

The storage moduli of the networks at the glassy and rubber

plateaus were observed to be significantly higher than that

of the control polyurethane. The TGA results indicated

that the thermal stabilities of the nanocomposites were

improved, as evidenced by the rate of volatile release from

the materials and the enhanced char yields together with

ceramic yields. The above results could be ascribed to the

significant nanoscale reinforcement effect of the POSS

cages on the polyurethane matrix and the formation of the

specific structure of the POSS-containing PU networks.

Acknowledgements: The financial support from ShanghaiScience and Technology Commission, China under a key project(Number 02DJ14048) is acknowledged. We thank the NaturalScience Foundation of China (Grant Number 20474038,50390090) for partial support of this work.

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polyurethane networks nanoreinforced by polyhedral oligomeric silsesquioxane

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