Thermal and mechanical properties of poly(methyl methacrylate) nanocomposites containing polyhedral oligomeric silsesquioxane

Chunbao Zhao1,2,a, Xin Wang2, Xujie Yang2, Wei Zhao1 1 Faculty of Microelectric Engineering, Nanjing College of Information Technology, Nanjing, 210046,

China

2 Key Laboratory for Soft Chemistry and Functional Materials of Ministry of Education, Nanjing University of Science and Technology, Nanjing, 210094, China

azhaocblww@yahoo.com.cn

Keywords: poly(methyl methacrylate), polyhedral oligomeric silsesquioxane, nanocomposites, thermal stability.

Abstract. A series of poly(methyl methacrylate) (PMMA) composites containing polyhedral

oligomeric silsesquioxane (POSS) were produced by bulk polymerization. The morphology, thermal

and mechanical properties of the composites were characterized by X-ray diffraction (XRD),

transmission electron microscopy (TEM), thermogravimetric analyses (TGA) and dynamic

mechanical analyses (DMA). Results show that the octa(3-chloropropyl)-POSS (ocp-POSS) and

trisilanolphenyl-POSS (triol-POSS) have high compatibility with PMMA and can be uniformly

dispersed into PMMA matrix. The separate incorporation of these two types of POSS contributes to

the improvement of thermal stability of PMMA composites. When the content of POSS was 7.5 wt%,

the thermal decomposition temperatures (5% mass loss) of PMMA composites with ocp-POSS and

triol-POSS were increased by about 104 °C and 130 °C, respectively. The increase of triol-POSS

content in the PMMA matrix gave slight enhanced storage modulus before glass transition.

Introduction

Poly(methyl methacrylate) (PMMA) is a type of very useful thermoplastic polymer with many

excellent properties such as good flexibility, extraordinary optical clarity, high strength and desirable

weatherability. However, its lower thermal stability restrains it from applications in higher

temperature region. To improve the thermal stability of PMMA, many inorganic fillers such as silica,

clay and carbon nanotubes were introduced into the PMMA matrices to form corresponding

composites[1-3].Nevertheless, it is noted that the incorporation of the inorganic fillers usually lead to

the decrease of transparency, which may limit the applications of PMMA materials in some fields.

During the passed a few years, polyhedral oligomeric silsesquioxanes (POSS) have attracted

considerable attention as molecular silica for polymer nanocomposites formation. POSS molecule

has special organic-inorganic hybrid cage structure and can be easily incorporated into polymer

systems through blending, grafting or copolymerization[4,5] to obtain high performance composites.

Recent studies on POSS-containing polymer nanocomposites involving PMMA system have been

reported[6-10]. Nanocomposites of PMMA containing POSS have exhibited enhancement in thermal

and mechanical properties[8,10].However, the previous research mainly focused on the POSS

molecules which can form covalent bonds with PMMA backbone. Relatively speaking, much less is

known about the influence of POSS cages without covalent attachment on the thermal, mechanical

and morphological properties of the resulting PMMA composites.

In the present study, two different types of POSS, octa(3-chloropropyl)-POSS (ocp-POSS) and

trisilanolphenyl-POSS (triol-POSS), were used to prepare transparent PMMA nanocomposites by

bulk polymerization. The morphology, thermal and mechanical properties of the composites were

characterized by XRD, TEM, TGA and DMA. Differences of morphology and properties between the

composites with two different types of POSS were investigated and discussed.

Advanced Materials Research Vols. 557-559 (2012) pp 304-308Online available since 2012/Jul/26 at www.scientific.net© (2012) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.557-559.304

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Experimental section

Materials. Methyl methacrylate (MMA), azobis-(isobutyronitrile) (AIBN) were of analytical grade,

obtained from Shanghai Chemical Reagents Co., China. MMA monomer was distilled from calcium

hydride under reduced pressure. AIBN was refined in heated ethanol and kept in a dried box.

Ocp-POSS was prepared according to Ref.[11]. Triol-POSS was supplied by Hybrid Plastics

Company and used as received. All other solvents were used as received.

PMMA/POSS samples preparation. The PMMA/POSS nanocomposites were prepared by bulk

polymerization. A desired amount of POSS and AIBN were added in MMA monomer and

prepolymerized for about 0.5h under nitrogen atmosphere at 75 °C. Then the solution was cast into a

glass mould and polymerized at 50 °C for 48 h. The transparent castings were thermally treated at

100°C for 1 h. The POSS contents of samples were 2.5, 5.0, 7.5 wt% respectively. A reference

sample of pure PMMA was prepared using the same polymerization method.

Characterization. Dynamic mechanical analysis (DMA) was conducted using a DMA Q800 (TA)

instrument with bending mode at an oscillatory frequency of 1 Hz. The temperature scan was

performed at 3°C/min heating rate in the range from room temperature to around 200°C.

Thermogravimetric analysis (TGA) was performed with a Mettler-Toledo TGA/SDTA 851e

apparatus from 50 to 600 °C at a heating rate of 20 °C/min under nitrogen. X-ray diffraction (XRD)

was recorded on a Bruker D8 X-ray diffractometer using Cu Kα radiation (λ=0.1542 nm) with the

range of the diffraction angle of 2θ=2~60° at 40 kV and 30 mA. The TEM micrographs were taken

with a JEOL JEM-2100 transmission electron microscopy at an accelerating voltage of 200 kV.

Results and discussion

Dynamic mechanical analysis. The dynamic mechanical spectra of PMMA composites containing

ocp-POSS and triol-POSS are shown in Fig. 1 and Fig. 2, respectively. The spectra of storage

modulus Eʹ and loss factor tanδ display only one α relaxation process corresponding to the glass

transition of the composites with different POSS loading, quite similar to the behavior of the pure

PMMA. The storage modulus Eʹ values of PMMA/ocp-POSS composites (Fig. 1) slightly decrease

with the increasing of the ocp-POSS content. While, the storage modulus values of the

PMMA/triol-POSS composites before the glass transition are higher than that of the pure PMMA

(Fig. 2). The difference of the storage modulus between two types of composites is mainly attributed

to the structures of the filled POSS. Compared with triol-POSS, the ocp-POSS molecule is covered

with chloropropyl substituents, which can lead to a relatively stronger plasticization effect on PMMA

matrix.

From the Fig. 1 and Fig. 2, the glass transition temperature (Tg) values for these PMMA/POSS

composites are slightly lower than that of the pure PMMA. The factors leading to the variation of Tg

of POSS-containing composites include POSS species, loading and the composites structures[4]. In

our previous work[8], the PMMA composites containing octavinyl-POSS displayed a remarkable

improvement of Tg, which was ascribed to the severe hindering effect of POSS. In this study,

however, the hindering effect of used POSS is much weaker than octavinyl-POSS, because there is no

strong chemical covalent bond linkage between the POSS and PMMA matrix.

Advanced Materials Research Vols. 557-559 305

Fig.1 Dynamic mechanical spectra of pure

PMMA and PMMA/ocp-POSS composites.

(The inset is the zoom-in plot of Eʹ data in the

temperature range of 50~100 °C)

Fig.2 Dynamic mechanical spectra of pure

PMMA and PMMA/triol-POSS composites.

(The inset is the zoom-in plot of Eʹ data in the

temperature range of 50~100 °C)

Thermal analyses. Figure 3 shows the weight-loss curves of the PMMA composites with varying

concentrations of POSS. It can be seen that the thermal decomposition temperatures of the PMMA

composites containing POSS are higher than that of the pure PMMA, and enhance markedly with

increasing POSS content in two different systems. Compared with pure PMMA, the thermal

decomposition temperatures (5% mass loss) of PMMA composites with 7.5wt% of ocp-POSS and

triol-POSS were increased by about 104°C and 130°C, respectively. It is noted that the PMMA

composites filled with triol-POSS exhibit better thermal stability for a given POSS loading. We

proposed two reasons for this difference. One is the rigidity of triol-POSS molecules with benzene

rings. The other is the silicon hydroxyl of triol-POSS, which can form hydrogen bonds with PMMA

chains. In addition, the residual char yields of the PMMA composites increase with increasing POSS

content as expected, which may be favorable to improve the flame resistance of PMMA composites.

Fig. 3 TGA curves of pure PMMA and PMMA/POSS composites

Morphology (XRD and TEM). X-ray diffraction was used to characterize the dispersion of the

PMMA/POSS composites. Diffraction patterns for the ocp-POSS and triol-POSS systems are shown

in Fig. 4(a) and (b), respectively. The characteristics of the XRD patterns for the two composite

systems are similar at comparable loadings of POSS. In the figure, a broad amorphous peak at ~14.3o

was attributed to the amorphous PMMA matrix. In all the diffraction patterns of PMMA/POSS

composites, the peaks corresponding to ocp-POSS and triol-POSS are not observed, which is similar

to the result reported by Zhang W et al.[12] for PMMA/octacyclopentyl-POSS composites. This

observation clearly indicates that the ocp-POSS and triol-POSS are well dispersed into the PMMA

matrix leading to the suppression of crystallization of pure POSS[13]. Further more, it is noted that

the amorphous peaks intensity of PMMA matrix shows a decreasing trend with the increasing of

POSS concentration in the two types of PMMA composites. This is possibly due to that the addition

of ocp-POSS and triol-POSS has an influence on the arrangement of PMMA chains.

306 Advanced Materials and Processes II

Fig.4 The XRD patterns of POSS and PMMA/POSS composites

The above results were also confirmed by the TEM images of the composites. Figure 5(a) and (b)

show the TEM images of the PMMA composites with 7.5 wt% of ocp-POSS and triol-POSS,

respectively. To contrast with the background, the TEM images were taken at the edge of the

sectioned composites. It is seen that the dark areas (the portion of the PMMA composites) are quite

homogenous and no localized domains were detected at this scale, implying that the two types of

POSS components were homogenously dispersed in the continuous PMMA matrix at the nanoscale.

The result is also consistent with the observed phenomena that the ocp-POSS and triol-POSS were

completely resolved in PMMA prepolymer and formed transparent and viscous liquid during the

course of composites preparation.

Fig. 5 The TEM images of PMMA composites with 7.5 wt% of ocp-POSS (a) and triol-POSS (b)

Conclusions

In this paper, the results of XRD and TEM showed that the ocp-POSS and triol-POSS have high

compatibility with PMMA matrix. All of these two types of POSS, when incorporated separately, can

improve the thermal stability of PMMA significantly. The thermal decomposition temperatures

(Tdec,5% mass loss) of PMMA composites with 7.5wt% of ocp-POSS and triol-POSS were increased

by about 104 °C and 130 °C, respectively. The PMMA composites filled with triol-POSS exhibit

better thermal stability for a given POSS loading. The PMMA composites containing ocp-POSS

showed slightly reduced storage modulus values due to plasticization effects, while the incorporation

of triol-POSS slightly improved storage modulus values of PMMA composites before glass

transition.

Acknowledgment

We acknowledge the financial support of Jiangsu Planed Projects for Postdoctoral Research Funds

(1001015B), Nature Science Foundation of Jiangsu Province of China (BK2011838) and Qinglan

Project of Jiangsu Province.

Advanced Materials Research Vols. 557-559 307

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