Structure and properties of low-dielectric-constant

poly(acetoxystyrene-co-octavinyl-polyhedral oligomeric

silsesquioxane) hybrid nanocomposite

Chao Zhang a, Hong Yao Xu a,b,*, Xian Zhao b

a State Key Laboratory for Modification of Chemical Fibers and Polymer Materials & College of Material Science and Engineering,

Donghua University, Shanghai 201620, Chinab State Key Laboratory of Crystal Material, Shandong University, Jinan 250100, China

Received 7 September 2009

Abstract

Low-dielectric-constant poly(acetoxystyrenezhi-co-octavinyl-polyhedral oligomeric silsesquioxane) (PAS–POSS) organic–

inorganic hybrid nanocomposite was successfully synthesized via one-step free radical polymerization and characterized by

FTIR, high-resolution 1H NMR, 29Si NMR, DSC, TGA, AFM, spectroscopic ellipsometry and dielectric constants measurements.

The results show Tg and Tdec were elevated dramatically due to the incorporation of inorganic POSS cores. Spectroscopic

ellipsometry and dielectric constants measurement display the incorporation of POSS into PAS homopolymer can apparently

reduce the dielectric constant of materials due to the increase of relative porosity of the hybrid nanocomposites.

# 2009 Hong Yao Xu. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.

Keywords: Low-dielectric-constant; Hybrid nanocomposite; POSS

In the field of low-dielectric materials, inorganic–organic hybrid polymers have attracted great interest due to their

prominent properties including low-dielectric constant, good film forming ability, enhanced thermal and mechanical

properties, and so on. Polyhedral oligomeric silsesquioxanes (POSS) are a class of inorganic–organic hybrid

compounds possessing nanometer-sized structures and nanopore core surrounded by eight organic corner groups

(functional or inert), which have been demonstrated to be good building blocks for the formation and architecture of

novel organic–inorganic molecular hybrid materials [1–6]. The functional corner groups enable POSS to hybridize

with organic components by chemical bonds in molecular level to form molecular hybrid, which effectively enhance

the dispersion homogeneity of inorganic particles and stability of the resulting hybrids. Simultaneously, the rigid and

characteristic porous structure of POSS benefits to reduce the dielectric constant and improve the thermal and

mechanical properties of resultant materials [7–9]. Thus, in this paper, we report the reparation and properties of a

soluble star poly(acetoxystyrene-co-octavinyl-polyhedral oligomeric silsesquioxane) (PAS–POSS) inorganic–organic

hybrid nanocomposite and investigate the influence of POSS component on properties of the hybrid nanocomposite.

www.elsevier.com/locate/cclet

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Chinese Chemical Letters 21 (2010) 488–491

* Corresponding author at: State Key Laboratory for Modification of Chemical Fibers and Polymer Materials & College of Material Science and

Engineering, Donghua University, Shanghai 201620, China.

E-mail address: hongyaoxu@163.com (H.Y. Xu).

1001-8417/$ – see front matter # 2009 Hong Yao Xu. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.

doi:10.1016/j.cclet.2009.11.038

1. Experimental

PAS–POSS was prepared by one-step free radical polymerization method, as shown in Scheme 1, which was

carried out under nitrogen atmosphere using a standard Schlenk vacuum-line system [1]. The product then was poured

into excessive cyclohexane under vigorously agitation to dissolve the unreacted monomer and precipitate the

nanocomposite. The star PAS–POSS hybrid and its film were characterized by FTIR, 1H NMR, 29Si NMR, GPC, DSC,

TGA, AFM, spectroscopic ellipsometry and dielectric constants measurements.

2. Result and discussion

The FTIR spectrum of resultant hybrid nanocomposite shows two characteristic absorptions of carbonyl stretching

vibration and strong Ph–O stretching (1763 and 1216 cm�1), two out-of-plane wagging absorption bands of single-

substituted aromatic ring (699 and 756 cm�1) and a strong symmetric peak of the characteristic Si–O–Si stretching of

silsesquioxane cages (1109 cm�1), indicating that the POSS cages are incorporated into the polymeric chains

(Fig. 1A). The 1H NMR spectrum exhibits resonance peaks of aromatic ring (7.1 and 6.6), the methyl proton adjacent

to the acyl group (2.3) and methine and methylene protons from the reacted vinyl groups of POSS and PAS backbones

C. Zhang et al. / Chinese Chemical Letters 21 (2010) 488–491 489

Scheme 1. Formation of star PAS–POSS inorganic–organic hybrid nanocomposite.

Fig. 1. (A) FTIR, (B) 1H NMR and (C) 29Si NMR spectra of PAS–POSS.

(from 0.7 to 2.0), respectively (Fig. 1B). Besides, there is a wide resonance band at nearby 6.0 belonging to the

unreacted vinyl protons of POSS molecules, indicating that not all the vinyl groups of POSS have participated in the

copolymerization. The 29Si NMR spectrum shows two resonance bands of the silicon atoms connected to the reacted

and unreacted vinyl groups on the POSS cages at�66 and �79, respectively (Fig. 1C). The peak area at�66 is much

bigger than that at �79, indicating most vinyl groups of each POSS molecule have participated in the

copolymerization to form hybrid nanocomposite. This further confirms the result of 1H NMR analysis. By calculating

the peak area ratio, an average ca. 6.3 of vinyl groups per POSS molecule have been copolymerized with acetoxy

styrene, proving that the resultant hybrid is not linear molecules. In order to determine whether the hybrids are a star or

network structure, we also test its solubility in various solvents such as THF, CHCl3, dioxane, toluene, and so on. It is

found that the hybrid can dissolve in almost all above common solvents, confirming that the resultant hybrid

nanocomposite is possibly of star structure rather than network structure since network nanocomposites are virtually

insoluble in any solvent. GPC analyses show that the molecular weight (Mw) of PAS and PAS–POSS are 43.7 � 103

and 43.5 � 103 with polydispersities of 1.38 and 1.73, respectively. XRD result exhibits that no sharp crystal peak of

POSS is observed, further indicating that POSS molecules have been bonded to acetoxy styrene components via

copolymerization method, which effectively prevent the self-aggregated effects of POSS molecules (Fig. 2). In

addition, the AFM images of the hybrid thin film also show that its surface is quite smooth and no cracks and

C. Zhang et al. / Chinese Chemical Letters 21 (2010) 488–491490

Fig. 2. XRD patterns of octavinyl-POSS and PAS–POSS.

Fig. 3. 1.0 mm � 1.0 mm tapping mode AFM (A) height and (B) phase images for PAS–POSS thin film (Z ranges for the height and phase images are

5 nm).

nanoparticles of POSS aggregation are observed (Fig. 3), hinting that the hybrid has good film formability and

structural homogeneity, which further supports the result of XRD analysis.

Densities, thermal properties, dielectric constants and refractive index of PAS and PAS–POSS are summarized in

Table 1. The results show that the Tg and Tdec are elevated dramatically due to the incorporation of inorganic POSS

cores. Simultaneously, it is also found from Table 1 that dielectric constant of PAS–POSS is obviously lowered due to

the increase of relative porosity (fr), which comes from the following two parts: the intrinsic nanoporosity of POSS

molecules, and the external porosity coming from the diluent effect of POSS cages [1].

In conclusion, a soluble star PAS–POSS inorganic–organic hybrid nanocomposite was successfully prepared via

one-step free radical copolymer approach. In the hybrid, POSS bonding to acetoxy styrene can effectively prevent the

self-aggregated effect of POSS, and no phase separation is observed in AFM images of the hybrid films. The

measurement results exhibit that the introduction of POSS endowed the hybrid with dramatic enhanced Tg and Tdec,

and lower dielectric constant. This work provides a novel path for designing new materials with low-dielectric

constant and good processibility.

Acknowledgments

This research was financially supported by the National Natural Science Foundation of China (Nos. 90606011,

50472038 and 2097401), PhD Program Foundation of Ministry of Education of China (No. 20070255012), and

Shanghai Leading Academic Discipline Project (No. B603), and Open Project of The State Key Laboratory of Crystal

Materials (KF0809), the Program of Introducing Talents of Discipline to Universities (No. 111-2-04) and Doctoral

Dissertation Innovation Project (No. BC200907).

References

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[6] H.Y. Xu, B.H. Yang, X.Y. Gao, et al. J. Appl. Polym. Sci. 101 (2006) 3730.

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C. Zhang et al. / Chinese Chemical Letters 21 (2010) 488–491 491

Table 1

Densities, thermal properties, dielectric constants and refractive index of PAS and PAS–POSS.

Samples Theoretical density

(g/cm3)a

Measured density

(g/cm3)

Rel porosity

increase (fr) (%)

Tg (8C)b Tdec (8C)c Dielectric constant

(1 MHz)

Refractive index

(650 nm)d

PAS 1.130 1.13 � 0.01 0 118.5 368.2 2.81 1.615

PAS–POSS 1.142 1.02 � 0.01 10.9 127.7 379.3 2.48 1.519

a Theoretical density: estimated from the weight percentage of POSS (5.42%) obtained from the IR standard curve [1] and the density of POSS

(1.40 g/cm3) and polyacetoxystyrene (1.13 g/cm3).b Data were gathered on the second melting at a heating rate of 10 8C/min.c Data were taken to be the temperature at 5% weight loss.d Refractive index of the hybrid nanocomposites films were characterised by ellipsometer fitted with Cauchy model.