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The effects of structure of POSS on the properties of POSS/PMMA hybrid materials.

INTRODUCTION

Poly(methyl methacrylate) (PMMA) is an important thermoplastic with excellent transparency, high modulus, relative easy processing, and excellent dimensional stability which is widely used in various fields [1]. However, the lower glass transition temperature and thermal stability have greatly limited their further applications in some severer situations [2]. An effective way to solve the problem is to develop organic/inorganic composites by incorporating an inorganic fraction, particularly nanometer particles, into the polymeric matrix by physical blending [3], The resulting composites are superior to the parent polymers in thermal and mechanical properties. However, it is difficult for physical blending to disperse the particles into polymer uniformly. For example, the introduction of Si[O.sub.2] nanoparticles that can easily aggregate in the polymer matrix owing to high surface area and surface energy [4] leads to lower glass transition temperature [5]. Moreover, the introduction of inorganic nanosized filler compromises the transparency of PMMA due to the phase separation of nanocomposites and limits its applications [6].

Recently, a new type of nanoparticles, polyhedral oligomeric silsesquioxanes (POSS) [7] with organic/inorganic hybrid structure at molecular level, which includes random structure, ladder structure, age structure and random cage structure, attracts extensive attention because of their extraordinary properties derive from the synergism between inorganic nanoparticles and organic molecules [8]. POSS compounds embody a true organic/inorganic hybrid architecture, which contains an inner inorganic framework made up of silicone and oxygen (Si[O.sub.1.5])x, that is externally covered by organic substituents (functional or inert) [9, 10], these substituents can be totally hydrocarbon in nature or they can embody a range of polar structures and functional groups, also they can be easily functionalized using a broad range of organic groups commonly employed in polymerization or grafting reactions [11]. Therefore, POSS molecules are excellent fillers for preparing organic/inorganic hybrid materials with excellent interface action and without any surface modification in comparison to other inorganic nanoparticles, and for variety of architectures such as linear, pendant, branched, star-shaped as well as network. In these hybrid materials, both inorganic and organic moieties are chemically bonded to each other and nanoscale POSS can be uniformly dispersed in the composites at molecular level to result in effective reinforcement of the thermal and mechanical properties of the materials [12-14]. However, those previous researches were mostly limited to the linear or pendent polymeric systems using POSS macromers with only monofunctional comer group [15-17]. Both monofunctional and multifunctional POSS monomers are beneficial to the thermal, mechanical, and rheological properties of thermoplastic and thermoset systems.

In recent years, the synthesis of POSS-containing hybrid materials has become a research hotspot [18]. Most researches mainly used fine-tuned POSS molecular architectures in order to prepare nanocomposites by means of POSS copolymerization. Weickmann and his coworkers [19] reported the POSS/PMMA composites which prepared with nonreactive POSS such as |[Si.sub.8][O.sub.12][(OSi[Me.sub.2]H).sub.8]] (POSS1) and [[Si.sub.7][O.sub.9][(cydohexyl).sub.7][(OSi[Me.sub.2]H).sub.3]] (POSS2) via self-assembly. Although the toughness of PMMA was enhanced without compromised stiffness, the POSS species failed to improve thermal properties. Xiao and Feher [20] reported the composites with improved thermal stability and degradation temperature which prepared by copolymerization of monofunctional methacryloyl-functionalized POSS with methyl methacrylate (MMA). In view of the specific structure of POSS with more than two functional groups, they are perfect structure-guiding agents for the synthesis of star-shape or network polymers. Therefore, some researchers are shifting their interests toward this area, and some efforts have been made so far [21-23]. The types of POSS and the preparation methods of composites can greatly influence the thermal properties of POSS/ PMMA nanocomposites. In contrast to nanocomposites prepared with nonreactive POSS and polymer matrix, copolymers containing reactive POSS often exhibit enhanced thermal properties including [T.sub.g] and [T.sub.end] temperature. Moreover, the POSS/PMMA hybrid materials in which the reactive POSS and PMMA combined with covalent bonds exhibited improvement of mechanical properties and noncompromised light-transmission was observed. The hybrid materials of POSS/PMMA containing various POSS contents can be prepared by solution blending method and the resulting materials show that the cross-linking structure coming from the reaction of POSS and MMA significantly improved the properties of pure PMMA [24].

In our present work, octa-vinyl polyhedral oligomeric silsesquioxane (V-POSS) and octa-(methacryloxy) propyl polyhedral oligomeric silsesquioxane (M-POSS) with different functional groups were employed to prepare the POSS/PMMA hybrid materials with cross-linking structure via in situ polymerization. The effects of structure of functional groups on the properties of POSS/PMMA hybrid materials were discussed. The double bonds coming from V-POSS or M-POSS had the ability to react with double bonds coming from MMA, so the covalent bonds and cross-linking structure were obtained between POSS and PMMA, which were expected to be beneficial to the outstanding improvement of mechanical and thermal properties of the hybrid materials. Moreover, the solvent resistance and solvent stress cracking resistance were improved due to the cross-linking structure. The improved interface action arising from chemical bonds between POSS and PMMA showed the ability to prevent the phase separation and to obtain the hybrid materials in molecular level. In addition, the hybrid materials demonstrated excellent transparency in visible region and UV shielding property. At the same time, the dielectric constant and dielectric loss of the two kinds of hybrid materials decreased with the increasing content of POSS.

EXPERIMENTAL

Materials

Octa-vinyl polyhedral oligomeric silsesquioxane (V-POSS [greater than or equal to]99% purity) was purchased from Liaoning AP Composite Materials octa-(methacryloxy) propyl polyhedral oligomeric silsesquioxane (M-POSS [greater than or equal to]99%) was purchased from Hybrid Plastics. The structures of the two POSS were illustrated in Scheme 1. Methyl methacrylate (MMA, 99%) was purchased from Fu Chen, Tianjin Chemical. The initiator of Azobis (isobutyronitrile) (AIBN, 99%) was purchased from Yamaura Chemical.

Preparation of POSS/PMMA Hybrid Materials

POSS-n (M-POSS or V-POSS) was added to the MMA monomer containing AIBN, where n was denoted as the molar percentage of POSS, and was equal to 0%, 0.2%, 0.4%, 0.6%. The mixture was ultrasonic dispersed at 75[degrees]C for 20 min, then was poured into a glass mold followed by a polymerization process of 45[degrees]C /20 h + 80[degrees]C/2 h + 100[degrees]C /1 h.

Characterization

FTIR spectra of liquid samples of V-POSS and M-POSS and compression molded thin film samples of polymerized PMMA and POSS/PMMA composites were carried on a IS10 Fourier transform infrared spectroscopy of American Nicolet, and the scan range was 400-4000 [cm.sup.-1].

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Scanning electron microscopy (SEM) images of hybrid materials were taken on S-2700 microscope of Hitachi Japan, before measurement, the fractural surfaces of the sample were sprayed with gold for 1 min to increase conductivity, with surface scanning spectroscopy (EDS) analysis and the acceleration voltage of 10 kV.

The transparency and haze of the composites were measured on WAW haze meter with C illuminator. The transmittance of light of samples was tested on TU-1810 ultraviolet spectrophotometer of Beijing Purkinje General Instrument with a wavelength range of 200-900 nm.

Thermal stability was provided by Thermo gravimetric analysis (TGA) using a TGA-2050 analyzer of NETZSCH Group, Germany with 10[degrees]C [min.sup.-1] heating rate under [N.sub.2] atmosphere. The testing temperature is form room temperature to 600[degrees]C. The glass transition temperature ([T.sub.g]) was tested on TAMDSC2910 Differential Scanning Calorimeter (DSC) with a heating rate of 10[degrees]C [min.sup.-1] under [N.sub.2] atmosphere.

Mechanical properties (tensile properties and flexural properties) of polymerized samples were determined at ambient temperature according to GB/T2567-2008 method using a UTM5000 united testing system of Xinsansi Shenzhen. The dog-bone-shaped specimens used in the tensile testing were 50 mm long in the narrow region, 4 mm thick, and 10 mm wide along the center of the casting. The cross head speed was 2 mm min 1 during the testing process. At least five specimens were tested to obtain the effective average value of tensile properties. Elastic moduli were derived from the initial linear part of the stress-strain curves (up to about 1% strain). The performance of impact was tested according to the GB/T1043-93 Standard using XCJ-40 of Experimental Factory, Chengde Hebei impact testing machine.

The dielectric properties of composites materials were determined on a WY2851-frequency Q of Shanghai Wuyi at 25[degrees]C, the test frequencies were 1 MHz and 60 MHz.

RESULTS AND DISCUSSION

The Structures of POSS/PMMA Hybrid Materials

Figure 1 shows the FTIR spectra of POSS/PMMA hybrid materials, as well as those of pristine POSS and pristine PMMA for comparison. In the spectra of pristine PMMA, the band at 1732 [cm.sup.-1] and 1249 [cm.sup.-1] are assigned to C=O stretching vibration and C-O stretching vibration, which indicate the existence of ester group. The methyl and methylene group exhibit absorption bands at ~3000 [cm.sup.-1]. The V-POSS shows two characteristic absorptions at 1109 [cm.sup.-1] and 585 [cm.sup.-1], which are assigned to Si-O-Si stretching and bending vibration, respectively [1]. The weaker band at 1602 [cm.sup.-1] assigns to C=C stretching vibration [25]. In the spectra of V-POSS/PMMA hybrid materials, the band of C=C almost disappear, implying the reaction of V-POSS and MMA. The band of C=0 coming from pristine PMMA shift to 1726 [cm.sup.-1]. The relative weaker band of 585 [cm.sup.-1] assigned to Si-O-Si bending vibration still existed in the spectra of V-POSS, which is not presented in the pristine PMMA, implying the chemical structure of composites prepared with V-POSS and PMMA [1].

In the spectra of M-POSS, the band at 3417 [cm.sup.-1] and 1116 [cm.sup.-1] are assigned as stretching vibration of Si-OH and stretching vibration of Si-O-Si of silsesquioxane cages, respectively. However, M-POSS exhibits the obvious band at 2958 and 1718 [cm.sup.-1] assigned as -C[H.sub.3], -C[H.sub.2], and C=O, which is different from V-POSS. Also, the band at 1635 [cm.sup.-1] is attributed to absorption peak of C=C. For M-POSS/PMMA, C=O absorption peak appears at 1726 [cm.sup.-1], a slight shift to lower wavenumber has also occurred, which is similar to the V-POSS/PMMA. The band at 1635 [cm.sup.-1] almost completely disappears, suggesting that the M-POSS structures are chemically incorporated into the hybrid materials and form a cross-linking network structure in covalent bonds. The band at 1116 [cm.sup.-1] is assigned as Si-O-Si implied the hybrid materials with cage structure of POSS. Nevertheless, the methyl, methylene, and ester groups coming from M-POSS have the potential to form flexible interface structure of M-POSS/PMMA composites in comparison to that of V-POSS/PMMA composites.

According to the structure of V-POSS and M-POSS, both POSS act as the cross-linking agent in the preparation process of POSS/PMMA hybrid materials due to eight C=C groups in POSS. The possible structure is showed in Scheme 2, in which the hybrid materials will be synthesized for the formation of cross-linking structure with chemical bonds at molecular level between V-POSS or M-POSS and MMA.

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SEM and EDS were carried out to analyze the microstructure of POSS/PMMA. The SEM and Si element EDS photos of POSS/PMMA hybrid materials and also the SEM images of pristine PMMA were presented in Fig. 2. The fracture surfaces of M-POSS/PMMA (Fig. 2a) and V-POSS/PMMA (Fig. 2b) were rougher than that of pristine PMMA (Fig. 2c), the homogeneous dispersing POSS in hybrid materials was verified by EDS images in which the Si element uniformly dispersed. The rough fractural surface suggested that the POSS with inorganic core can induce plastic deformation of matrix and a lot of micro-cracks on the interface between the silica core and organic phase due to the improved interaction coming from chemical bonds. Then the induced micro-cracks absorb much energy to prevent the crack propagation, that is to say, the mechanism can be ascribed to the energy absorption and hindering effect on crack propagation by the interphase [26], which was conducive to improve the strength and toughness of materials.

The stress-whitening was observed in all samples during the tensile process whether or not to introduce the POSS. However, the stress-whitening in POSS/PMMA hybrid materials was non-apparent in comparison to pristine PMMA. Linear PMMA exhibits brittle fracture and the stress-whitening is often observed during the break process because of the generation of craze. The cross-linking structure caused by reactive POSS restrains the movement of segment, so the generation of craze is inhibited and the stress-whitening is weakened.

Solubility of POSS/PMMA Hybrid Materials

The solubility of PMMA and POSS/PMMA hybrid materials soaked in ethyl acetate for 48 h were tested to verify the cross-linking structure in the hybrid materials. The pristine PMMA completely dissolved in ethyl acetate, implying a linear structure. The 0.6 mol% V-POSS/PMMA composites exhibited a swelled sample with 72 wt% weight increment due to the ethyl acetate in the cross-linking structure. After drying the swollen material, the dried sample exhibited about 32% weight loss, implying the part linear structure still exists in this hybrid material besides cross-linking structure. Moreover, the weight of 0.6 mol% M-POSS/PMMA composites exhibited no obvious weight change after being soaked in ethyl acetate for 48 h. Therefore, V-POSS/PMMA and M-POSS/PMMA hybrid materials both formed cross-linking network structures according to the possible structure of Scheme 2, which were consistent with the chemical structure analysis in Fig. 1. Accordingly, the cross-linking density of V-POSS/PMMA was lower than that of M-POSS/ PMMA, the number of the reacted vinyl groups on each POSS molecules was determined to be 6-8 [27]. The M-POSS showed more similar structure to MMA than V-POSS due to the ethyl methylacrylate groups in its structure, so the M-POSS was easy to react with MMA and to form a uniform cross-linking structure. In the polymerization reaction, the polymerization rate is proportional to the rate constant. The rate constant 4.69 for vinyl group and 15.3 for methacryloxy group manifested that the M-POSS had higher reactivity than V-POSS. The difference of reactive rate constant leads to homopolymerization of MMA rather than copolymerization of MMA and V-POSS in V-POSS/ PMMA materials. So, M-POSS/PMMA hybrid materials showed higher cross-linking degree and more uniform structure than V-POSS/PMMA. M-POSS is more active than V-POSS when react with MMA, in this way inducing more cross-linking structure and uniform structure. The differences of cross-linking density and structures demonstrated dramatic effect on the swelling degree and properties of POSS/PMMA hybrid materials.

Figure 3 shows the photos of the 0.6 mol% POSS/PMMA hybrid materials and dried samples which were immersed in ethyl acetate for 48 h. It was obvious that the samples of hybrid materials before being immersed in solvent exhibit more excellent transparency than those dried samples. Moreover, compared with M-POSS/PMMA composites, the deteriorate transparency which caused by the enlargement of size and cracks were more obviously in V-POSS/PMMA composites. The internal stress of V-POSS/PMMA is higher than that of M-POSS/PMMA due to the nonuniform cross-linking structure. When the samples were immersed in the solvent, a lot of microcracks will be induced and propagated under the action of solvent and internal stress. The samples dried, and these cracks cause by solvent would reserve, deteriorating the transparency and mechanical properties. In general, M-POSS/PMMA composites have more excellent solvent resistance and solvent stress cracking resistance than V-POSS/PMMA composites.

Transparency of POSS/PMMA Hybrid Materials

PMMA is a thermoplastic material with excellent light transmittance and often used as optical materials. Nanocomposites prepared with nanoparticles as reinforcement filler usually sacrifice the optical properties due to the aggregation of nanoparticles [28]. So it is necessary to maintain its excellent light transmittance while being modified. The solution to obtain the nanocomposites with excellent light transmittance is preparing the hybrid materials in molecular level. The wavelength of visible light is 390-770 nm, and the size of POSS is 1-3 nm, so the crucial factor to prepare the transparent hybrid materials is the excellent dispersion of POSS without aggregation in the matrix of PMMA. The eight double bonds in the V-POSS or MPOSS molecule structure showed the reactivity with MMA to form the cross-linking structure in covalent bonds (Fig. 1). The strong interactions between POSS and PMMA due to the covalent bonds induced the homogeneous dispersion composites at molecular level. The photos of pristine PMMA, V-POSS/ PMMA, and M-POSS/PMMA hybrid materials are presented in Fig. 4, and the light transmission and haze of pristine PMMA and POSS/PMMA hybrid materials are listed at Table 1. The light transmission and haze of pristine PMMA is 93.7% and 1.0. The light transmission and haze were about 93.3-93.4% and 1.2-1.3 for V-POSS/PMMA hybrid materials, and about 93.4-93.5% and 1.0-1.2 for M-POSS/PMMA composites when the loading of POSS increased from 0.2 to 0.6 mol%. This data manifested that the introduction of POSS into PMMA almost did not compromise the transparency.

The transparency of POSS/PMMA composites was also verified by UV-vis absorption spectra (Fig. 5). All of the samples exhibited similar transmittance in the region of visible light ([lambda] = 400-700 nm). However, there are no light transmittance in the UV region ([lambda] = 200-400 nm) for POSS/PMMA hybrid materials, which was similar to pristine PMMA. This optical transmittance can be utilized as a criterion for the formation of a homogeneous phase [29], The main reason is the formation of a homogeneous structure which agrees with the EDS analysis.

Dielectric Properties of POSS/PMMA Hybrid Materials

The dielectric constant and dielectric loss of POSS/PMMA hybrid materials at different frequencies were listed at Table 2. At 1 and 60 MHz, the dielectric constant and dielectric loss of POSS/PMMA hybrid materials decreased with the increasing content of POSS no matter the type of POSS. At 60 MHz, at the loading of 0.6 mol% POSS, the dielectric constant of V-POSS/PMMA and M-POSS/PMMA reduced to 2.77 and 2.74, and the dielectric loss reduced to 0.0039 and 0.0033, respectively. Moreover, all the values of M-POSS/PMMA were lower than V-POSS/PMMA at the same POSS content.

The remarkable dielectric property improvement of the hybrid materials might be ascribed to the introduction of lower dielectric constant medium, indicating the incorporation of POSS increased the free volume of the material, and the increasing of free volume was verified by the decrease of density of hybrid materials (Table 2). As we know, the dielectric constant of air is 1, so the cage-like POSS with lower density and larger pores resulted to a lower dielectric constant ([kappa] = 2.1-2.7). On the other hand, the severe hindering effect of POSS coming from the formation of cross-linking structures with cubic silses-quioxane core inhibited the motion of the PMMA chains in the polymeric matrix and, that is, propitious to the improvements of dielectric properties.

Mechanical Properties of POSSIPMMA Hybrid Materials

The interface of hybrid materials, which affords the stress transfer and inhibit propagation of crack in composites, is a crucial factor in the improvement of mechanical properties [30], The most effective method to improve interface action is the formation of chemical bonds in the interface [31]. So, the V-POSS and M-POSS with reactive groups were employed to enhance the mechanical properties of the hybrid material. The mechanical properties of pristine PMMA, M-POSS/PMMA, and V-POSS/PMMA were summarized at Table 3. Generally, the mechanical properties, including tensile strength, tensile modulus, elongation at break, impact strength, flexural strength, and flexural modulus, were obviously improved with the addition of V-POSS or M-POSS. In comparison to the mechanical properties of pristine PMMA, the hybrid materials containing 0.6 mol% loading of M-POSS reached the maximum value of tensile strength, tensile modulus, impact strength, flexural strength, and flexural modulus, which exhibited 48.5%, 34.7%, 50.8%, 18.3%, and 55.7% increment, respectively. Only a slightly decrease of elongation at break was observed with 0.6 mol% loading of M-POSS as the reinforcement agent. For the V-POSS/PMMA hybrid material, the maximum value of tensile strength and tensile modulus were observed when the loading of V-POSS was 0.4 mol%, and exhibited 34.6% and 53.1% increment in tensile strength and tensile modulus in comparison with pristine PMMA. The elongation at break was improved greatly as the results of the plasticizer effects that the introduction of POSS cages increased the free volume of the matrix [32], At a higher POSS content, the elongation at break decreased slightly, this is probably due to the nanoeffect weakening resulting from the aggregation POSS cages [33] and the increase of cross-linking density. The maximum values of impact strength were observed at the 0.6 mol% loading of V-POSS, the flexural strength and flexural modulus, which exhibited 26.2%, 9.7%, and 27.0% increment in comparison with pristine PMMA, respectively. These results were consistent with the achievement of Hans Weickmann [34], Overall, the above results showed that compared with V-POSS, M-POSS was more effectively to improve the mechanical properties of the POSS/PMMA hybrid materials.

Both V-POSS and M-POSS are cage-like nanoparticles with inorganic core, moreover their high degree of symmetry and high stability enable them a high tensile resistance and deformation resistance [24, 35], The whole molecule of POSS is supposed to be a rigid nanosphere, and the organic groups have the ability to form chemical bonds with the soft segments in the polymer matrix. Thus, the rigid structure of POSS may restrict the movement of the molecular chains in the material and become the anchor point of the material to improve the mechanical properties of POSS/polymer hybrid materials. In present works, enhanced cross-linking density of POSS/PMMA composites due to the reaction of eight double bonds in V-POSS or M-POSS and MMA impedes the movement of segment of PMMA. Therefore, the hybrid materials manifest reinforcement and improved modulus due to the restricted movement of segment. The improvement of toughness that characterized as impact strength may attribute to the incorporation of free volume and rigid POSS particles in the system through covalent bonds [36]. Also, the ethyl methylacrylate of M-POSS produces a flexible interface between the inorganic cage of POSS and polymer matrix than vinyl of V-POSS, and the toughness of composites prepared with M-POSS is better than that prepared with V-POSS as reinforcement fillers. The structures of reactive groups of POSS also affect the improvement of PMMA. MPOSS with ethyl methylacrylate groups has the similar structure as MMA in comparison with V-POSS with ethylene groups. It indicates above that the M-POSS has similar reactive ration with MMA in comparison with V-POSS, forming a uniform cross-linking structure in hybrid materials. It is supposed that the uniform structure arised form M-POSS decreases the stress concentration and exhibit more excellent improvement of mechanical properties than V-POSS.

Thermal Properties of POSS/PMMA Hybrid Materials

Figure 6 presents the TG and DTG curves of pristine PMMA and POSS/PMMA hybrid materials. The thermal stability of POSS/PMMA hybrid materials were improved (Table 4) when the mole fraction of POSS reached 0.6 mol%. The 50% weight loss temperature ([T.sub.50]), the fastest decomposition temperature ([T.sub.max]) and 100% end temperature ([T.sub.end]) were inclined to move to the higher temperature, except the initial decomposition temperature ([T.sub.5]) and 10% weight loss temperature ([T.sub.10]). In comparison to pristine PMMA, the [T.sub.50], [T.sub.max], and [T.sub.end] of V-POSS/ PMMA increased by 19[degrees]C, 14[degrees]C, and 26[degrees]C, respectively, these of M-POSS/PMMA were 28[degrees]C, 33[degrees]C, and 31[degrees]C, respectively. Although the initial decomposition temperature was slightly reduced, the thermal degradation in the higher temperature region had been effectively suppressed. This maybe because at a relatively higher temperature, POSS molecules can form a silicon oxide layer on the surface acted as the "fence" thereby protecting the internal organic components of the hybrid materials and delaying the degradation of the hybrid materials [37]. At the same mole fraction of POSS, the thermal decomposition temperature of M-POSS/PMMA was greater than V-POSS/ PMMA. This result verified that incorporation of inorganic POSS cores could enhance the thermal stability of hybrid materials, and provide an effective way to improve the thermal stability of PMMA.

The glass transition temperature ([T.sub.g]) of the polymer is an important physical parameter, whose value determines the end use with any structural changes in chain rigidity and rotation, side groups and intermolecular forces. The [T.sub.g] of POSS/PMMA hybrid materials were found to be not always increased although the POSS was a bulky and multifunctional cross-linker [38], The DSC curves of pristine PMMA and POSS/PMMA hybrid materials obviously indicated that the [T.sub.g] of V-POSS/PMMA and M-POSS/PMMA increased from 110.6[degrees]C to 119.5[degrees]C and 122.5[degrees]C, respectively, with the addition of 0.6 mol% POSS (Fig. 7).

The main dominant factor of [T.sub.g] is the flexibility of the molecular chain. The increased cross-linking densities of PMMA and the rigidity of POSS play the main role in improving [T.sub.g]. The introduction of POSS can generate a stronger interaction between the POSS and the carbonyl of PMMA species [39, 40], forming the cross-linking structure in the hybrid materials and a hindrance effect of nanosized POSS on the motion of the PMMA molecular chain [2] may also had played the role in the [T.sub.g] increasing of the hybrid materials, which all limit the movement of the molecular chain, thereby improving [T.sub.g] of PMMA. As indicated above, the higher cross-linking density of composites prepared with M-POSS benefited to improve the thermal resistance in comparison with those prepared with V-POSS.

CONCLUSIONS

The hybrid materials V-POSS/PMMA or M-POSS/PMMA with cross-linking structure were prepared via in-situ polymerization with two kinds of cage-like POSS. The cross-linking structure in covalent bond induced a homogeneous dispersion of POSS in PMMA at molecular level which was verified by transparency and Si element of EDS analysis of hybrid materials. The enhanced mechanical and thermal properties were ascribed to the synergetic effects of POSS and PMMA. The mechanical properties of the POSS/PMMA hybrid materials were obviously improved by addition of V-POSS or M-POSS. At 0.6 mol% loading of POSS, thermal decomposition temperatures of POSS/ PMMA hybrid materials moved to the higher temperature implying the improvement of thermal stability. In general, at the same loading of POSS, M-POSS/PMMA can generate more uniform structure and higher cross-linking density, resulting in more excellent improvement in light transmittance, mechanical properties, thermal properties as well as dielectric properties than V-POSS /PMMA hybrid materials.

ABBREVIATIONS

V-POSS        Octa-vinyl polyhedral oligomeric silsesquioxane
M-POSS        Octa-(methacryloxy) propyl polyhedral oligomeric
              silsesquioxane
PMMA          Poly(methyl methacrylate)
FT-1R         Fourier transform infrared spectra
SEM           Scanning electron microscopy
EDS           Surface scanning spectroscopy analysis
TGA           Thermo gravimetric analysis
DSC           Differential Scanning Calorimeter
[T.sub.10]    The 10% weight loss temperature
[T.sub.50]    The 50% weight loss temperature
[T.sub.max]   The fastest decomposition temperature
[T.sub.end]   The 100% end temperature
[T.sub.5]     The 5% weight loss temperature


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Jian Jiao, Panpan Lv, Lei Wang, Yu Cai, Peng Liu

Department of Applied Chemistry, School of Science, Northwestern Polytechnical University, Xi'an 710072, People's Republic of China

Correspondence to: Jian Jiao; e-mail: jjiao@nwpu.edu.cn

Contract grant sponsor: National Natural Science Foundation; contract grant number: NO. 51373135; contract grant sponsor: Northwestern Polytechnical University; contract grant number: NO. Z2014170.

DOI 10.1002/pen.23921

Published online in Wiley Online Library (wileyonlinelibrary.com).

TABLE 1. Transmittance and haze of POSS/PMMA hybrid materials
(standard deviation (SD) values are given in between brackets).

              Content of        Luminous
Samples       POSS (mol%)   transmittance (%)    Haze (%)

PMMA               0           93.7 (0.34)      1.0 (0.18)
V-POSS/PMMA       0.2          93.4 (0.S6)      1.2 (0.48)
                  0.4          93.4 (1.27)      1.3 (0.27)
                  0.6          93.3 (2.04)      1.3 (0.75)
M-POSS/PMMA       0.2          93.5 (1.45)      1.0 (0.36)
                  0.4          93.4 (0.89)      1.2 (0.56)
                  0.6          93.5 (1.26)      1.1 (0.25)

TABLE 2. Dielectric constant and dielectric loss of POSS/PMMA
hybrid materials (standard deviation (SD) values are given in
between brackets).

                 Mole                          Dielectric constant
               fraction     Density (g
Samples       of POSS(%)   [cm.sup.-3])       1 MHz          60 MHz

PMMA              0        1.187 (0.093)   2.90 (0.085)   2.91 (0.234)
V-POSS/PMMA      0.2       1.186 (0.586)   2.81 (0.103)   2.84 (0.543)
                 0.4       1.183 (0.470)   2.80 (0.096)   2.81 (0.248)
                 0.6       1.181 (0.397)   2.75 (0.118)   2.77 (0.814)
M-POSS/PMMA      0.2       1.186 (0.092)   2.86 (0.108)   2.85 (0.398)
                 0.4       1.182 (0.320)   2.79 (0.098)   2.78 (0.289)
                 0.6       1.179 (0.198)   2.73 (0.046)   2.74 (0.782)

                 Mole              Dielectric loss
               fraction
Samples       of POSS(%)        1 MHz            60 MHz

PMMA              0        0.0179 (0.0097)   0.0088 (0.0018)
V-POSS/PMMA      0.2       0.0147 (0.0054)   0.0067 (0.0012)
                 0.4       0.0101 (0.0094)   0.0026 (0.0021)
                 0.6       0.0112 (0.0083)   0.0039 (0.0029)
M-POSS/PMMA      0.2       0.0159 (0.0089)   0.0053 (0.0031)
                 0.4       0.0157 (0.0038)   0.0047 (0.0013)
                 0.6       0.0133 (0.0018)   0.0033 (0.0018)

TABLE 3. Mechanical properties of POSS/PMMA hybrid materials
(standard deviation (SD) values are given in between brackets).

              Loading     Tensile       Tensile
              of POSS    strength       modulus     Elongationat
Samples       (mol%)       (MPa)         (GPa)       break (%)

PMMA             0      50.9 (7.34)   2.56 (0.87)   2.74 (0.12)
V-POSS/PMMA     0.2     56.2 (9.89)   2.62 (0.35)   4.37 (0.45)
                0.4     68.5 (9.84)   3.92 (1.03)   4.01 (0.22)
                0.6     67.2 (9.03)   3.14 (1.20)   2.59 (0.34)
M-POSS/PMMA     0.2     61.8 (10.5)   2.96 (0.23)   3.80 (0.28)
                0.4     73.8 (6.73)   3.44 (0.83)   3.99 (0.14)
                0.6     75.6 (5.12)   3.45 (0.93)   2.39 (0.18)

              Loading                       Flexural      Flexural
              of POSS   Impack strength     strength       modulus
Samples       (mol%)    (kJ [m.sup.-2])      (MPa)          (GPa)

PMMA             0        12.2 (1.29)     109.3 (9.94)   3.07 (1.0)
V-POSS/PMMA     0.2       12.7 (2.01)     114.4 (9.12)   3.38 (0.93)
                0.4       13.7 (1.96)     117.5 (16.4)   3.58 (0.97)
                0.6       15.4 (2.25)     120.0 (14.5)   3.90 (1.12)
M-POSS/PMMA     0.2       16.6 (0.98)     116.1 (13.2)   3.49 (0.95)
                0.4       17.7 (1.33)     123.5 (25.3)   4.13 (1.02)
                0.6       18.4 (2.20)     129.0 (15.7)   4.78 (0.56)

TABLE 4. Thermal decomposition temperature of PMMA and POSS/
PMMA.

                 Mole
               fraction      [T.sub.5]      [T.sub.10]
Sample        of POSS (%)   ([degrees]C)   ([degrees]C)

PMMA               0            281            297
V-POSS/PMMA       0.6           254            280
M-POSS/PMMA       0.6           241            285

               [T.sub.50]    [T.sub.max]    [T.sub.end]
Sample        ([degrees]C)   ([degrees]C)   ([degrees]C)

PMMA              331            327            404
V-POSS/PMMA       350            353            418
M-POSS/PMMA       359            360            435
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Date:Mar 1, 2015
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