Synthesis, structural and morphological characteristics of P(MA-MPTMS)/SiO₂ hybrid nanocomposites

Qiu Fengxian^a,b, Zhou Yuming^a,^* Liu Juzheng^a, Zhang Xuping^c
(a Department of Chemistry and Chemical Engineering, Southeast University, Nanjing 210096; ^b School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013; ^c Institute of Optical Communication Engineering, Nanjing University, Nanjing 210093, China)

Abstract The alkoxysilane-containing copolymer precursor was synthesized by free-radical copolymerization of MA with an alkoxysilane-containing monomer, methacryloxypropyltri methyloxysilane (MPTMS), at several feeds. The SiO₂, obtained by the hydrolysis of tetraethoxysilane (TEOS), was condensed under the catalysis of HCl. Hybrid nanocomposite from silica(SiO₂) and copolymer have been synthesized via the sol-gel process. The silica content in the hybrid films was varied from 0 to 25wt%. These nanocomposite films exhibit fairly good optical transparency and their transmittances are high up to 80% in the visible range . FT-IR spectra were used to analyze the chemical structure. The results of solvent extraction show that all gel contents in the hybrid materials are much higher. The morphological features indicates that the SiO₂ phase is well dispersed in the polymer matrix. The result shows that organic-inorganic phase pentrated each other.
Keywords Organic-inorganic hybrid material; Sol-gel process; SEM； DSC；TGA

1. INTRODUCTION
Inorganic material can be synthesized via sol-gel process in organic media under the temperatures at which organic compounds are thermally stable. In this way, it is possible to combine inorganic and organic components into one material: an organic-inorganic ^[1]. In the past decades, the synthesis and characterization of inorganic-organic hybrid materials have received considerable attention ^[2], mainly due to its mild conditions, such as low temperature and pressure ^[3,4]. In addition, the sol-gel process is a convenient method for preparing oxide films from alkoxysilyl group containing materials via continuous reaction steps of hydrolysis and condensation. This technology has been a great achievement in making ceramic or organic modified hybrid materials in the past two decades, by which homogeneous materials with higher thermal stability, density, and hardness can be produced ^[5-9].
    An approach for making hybrid material by using covalent bonds connecting the organic and inorganic phases was carried out via synthesis of the alkoxysilyl-containing organic precursors ^[7,10-13]. With this strong bond interaction at the organic-inorganic interface, hybrid material forms homogenous phase and becomes transparent, where hydrogen bonding binds the organic phase to the inorganic matrix and prevents the occurrence of macro-phase separation. The structures of the interfacial regions of composites have been studied with Ultraviolet-Visible spectroscope (UV-Vis) and Fourier-transform infrared spectroscope (FT-IR). Moreover, the effect of chemical structure and morphology on the physical characteristics has been investigated on the basis of thermal analysis with scanning electron microscope (SEM).
    In this study, a hybrid copolymer was prepared first. Copolymer precursors with trimethoxysilyl-functionality were prepared via free radical copolymerization of methyl acrylate (MA) with methacryloxypropyltri methyloxysilane (MPTMS). The copolymer precursor was then hydrolyzed and condensed in the presence of tetraethoxysilane (TEOS) and aqueous HCl catalyst to generate polymethyl acrylate/SiO₂ hybrid material. After being treated by a lay aside heat and drying process, the hybrid materials were obtained.

2. EXPERIMENTAL
2.1 Materials
Methyl acrylate (MA), TEOS, 2,2'-azobisisobutyronitrile(AIBN) were purchased from Shanghai Chemical Reagent Company. Initiating agent AIBN was purified via alcoholic recrystalization and then dried in a vacuum oven. MPTMS was purchased from Nanjing Shuguang chemical factory. Tetrahydrofuran(THF) was purified by distillation while other reagents and solvents were obtained commercially and were used as received.
2.2 Synthesis of copolymer precursor
The copolymer precursor was obtained via coploymerization and sol-gel process according to the literature ^[4,8]. A solution of MA(A) and MPTMS (B) in the mole ratio 1:1 was put in a 500mL three-neck round-bottomed flask. Then AIBN and THF were added. The copolymerization was carried out with a free radical initiator under nitrogen for about 3h. In this process was the copolymer precursor obtained. The synthetic route is shown in Fig.1.

Fig. 1 Synthesis of the copolymer precursor

2.3 Preparation of the hybrid materials
The copolymer(5mL) was dissolved in tetrahydrofuran (THF, 15mL) at room temperature and was stirred constantly to ensure precursor homogeneity. A homogeneous TEOS mixture was also prepared using deionized water (5mL) , hydrochloric acid (20mL 0.15mol·L^-1), THF(20mL) in a conical flask and was stirred for 1h. Then the TEOS hydrolysate was added into copolymer precursor solution. The sol-gel process was carried out at room temperature for 4h. In this way the homogeneous transparent sol can be obtained. Then the resulting homogeneous solution was transferred to a conical flask and was sealed by plastic film. The theoretically calculated content of SiO₂ in the hybrid films is 5,10,15,20,25wt%. The weight percent was calculated on the basis of an assumption that all the Si(OEt)₄ and P(MA-MPTMS) precursors would be converted to SiO₂ and P(MA-MPTMS) after baking. After drying for 5 days, several small holes were opened to evaporate the solvent slowly. The homogeneous transparent gel was formed. Then the sample was heated at 110^oC in vacuum for 2h to remove residual solvent and by-products (water and alcohol etc.). The thermally stable sol-gel hybrid material was obtained. The synthetic route is shown in Fig. 2.

Hydrolysis


Condensation

And/or

Net Reaction

Fig.2 Preparation of organic-inorganic hybrid film by sol-gel process

2.4 Instrumentation and characterization
The chemical structures were identified by UV-Vis, FT-IR that were recorded on a Shimadzu UV-240 spectrometer and a Nicolet AVATAR 360 spectrometer respectively. The samples for FT-IR analyses, mixed with KBr powder, were pressed into pellets. The morphology of the fractured surfaces of the hybrid materials was observed with SEM photographs (HITACHI X-650). DSC and TGA with the characterization of the thermal properties were performed on NETZSCH STA449C. The DSC and TGA measurements were performed at a heating rate of 10^oC/min under nitrogen. The temperature range for DSC and TGA measurements are from 0 to 600^oC and from 0 to 800^oC respectively. The process for the determination of sol content was: first put the obtained hybrid materials in the soxhlet extraction set and used acetone circulation reflux for 36h. Then it was dried to constant weight in the vacuum drying oven. The calculating equation is:
A=[(m₁-m₂) / m₁] ×100%
    A is defined as the sol content. m₁ and m₂ is defined as mass before extraction and mass after extraction respectively.

3 RESULTS AND DISCUSSION
UV-visible spectra of hybrid films with different SiO₂ content are shown in Fig.3. The spectrum of P (MA-MPTMS)/SiO₂-5wt% hybrid material is almost the same as pure P(MA-MPTMS). The transmittance of composite films of various samples was at least above 80% over a range of 350-650nm, and had better optical transparency. This optical transmittance may be used as an initial criterion for the formation of a homogeneous phase of both inorganic and organic components.
    The chemical structure of hybrid materials is characterized with a FT-IR spectroscopy as shown in Fig. 4. It can be easily found that the absorption peaks of Si-O-Si asymmetric stretching range was from 1090 to 1150cm^-1, and symmetric stretching was 723 cm^-1, which were due to the formation of silica structure via sol-gel process with added TEOS. The peaks were increased with TEOS content. Simultaneously absorption peaks of Si-O-C at 625 and 852 cm^-1 were also found. The symmetric C=O stretching is clearly visible at 1725 cm^-1.
    The absorption bands 3300-3500cm^-1 observed were attributed to stretching mode of water and hydroxyl. The Si-O-Si and Si-O-C demonstrate that the hybrid materials contain inorganic network. The Si-O-C may result from the interaction between the Si-O network and the Si-C in MPTMS. So stable bonding exists between the organic and inorganic components.
    As is well known, good solvent will extract the part and do not cross link with chemical bond. After the synthetic hybrid materials were extracted with acetone, the extraction sol is the coplymer, which do not combine to inorganic network with the covalent bond. The results of extraction experiment were 5.2% (P(MA-MPTMS)/SiO₂ - 5wt%), 4.8% (P(MA-MPTMS)/SiO₂ - 10wt%), 4.5% (P(MA-MPTMS)/SiO₂ - 15wt%) , 4.2% (P(MA-MPTMS)/SiO₂ - 20wt%) and 4.2% ( P (MA-MPTMS)/SiO₂ - 25wt%) respectively. It shows that they combine with chemical bond between organic and inorganic parts.
    Coupling agent MPTMS drawn into could integrate between the organic unit of the copolymer and inorganic network with covalent bond and thus restrains the hybrid material, which was extracted.

Fig. 4 FT-IR spectra of (a): MPTMS; (b): TEOS; (c): P(MA-MPTMS); (d):P(MA-MPTMS)/SiO₂ -5wt%; (e):P(MA-MPTMS)/ SiO₂-10wt%; (f):P(MA-MPTMS)/ SiO₂-15wt%; (g):P(MA-MPTMS)/SiO₂-20wt%; (h): P(MA-MPTMS)/ SiO₂-25wt%

    Fig.5 show the SEM images of P(MA-MPTMS)/SiO₂-5wt% and P(MA-MPTMS)/ SiO₂-25wt% respectively. The particle sizes of SiO₂ in P(MA-MPTMS)/SiO₂-5wt% and P(MA-MPTMS)/ SiO₂-25wt% are 15 and 60nm. This indicates that the particle size increases with the increasing SiO₂ content. The spatial distribution of P(MA-MPTMS)/ SiO₂-25wt% is denser than that of P(MA-MPTMS)/SiO₂-5wt% ^[14].These results show that silica networks are restrained under molecular level in the hybrid materials.
(a) (b)
Fig.5 SEM photographs of (a): P(MA-MPTMS)/SiO₂-5wt%; (b): P(MA-MPTMS)/ SiO₂-25wt%

    Thermal stability of hybrid materials was measured with differential scanning calorimetry (DSC) and thermal gravimetric analysis(TGA). TGA and DSC curves are shown in Fig.6 and Fig .7. Fig.6 shows that the thermal decomposition of hybrid materials occurred in a single step starting at 320 to 330^oC respectively. In the TGA curve of the hybrid material, the small weight loss at lower temperature below 150^oC is probably due to evaporation of residual alcohol and physically absorbed water. The calculated results are listed in Table 2.
    Fig.7 shows the DSC endothermic curves for the hybrid materials. The corresponding thermal data are listed in Table 2.

Fig.6 TGA curves of hybrid materials with the heating rate 10^oC/min under nitrogen
(a):P (MA-MPTMS)/SiO₂ -5wt%; (b):P (MA-MPTMS)/ SiO₂-10wt%; (c):P(MA-MPTMS)/SiO₂ -15 wt %; (d):P(MA-MPTMS)/SiO₂-20wt%; (e): P(MA-MPTMS)/ SiO₂-25wt%

Fig.7 DSC of curves of hybrid materials with the heating rate 10^oC/min under nitrogen
(a):P(MA-MPTMS)/SiO₂ -5wt%; (b):P(MA-MPTMS)/ SiO₂-10wt%; (c):P(MA-MPTMS)/SiO₂ -15 wt %; (d):P(MA-MPTMS)/SiO₂-20wt%; (e): P(MA-MPTMS)/ SiO₂-25wt%

Table 2 Thermal properties and residual SiO₂ content of hybrid materials

a. The glass transition temperature(Tg) was obtained from DSC curves, which were performed at heating rate 10^oC/min under nitrogen.
b. The thermal degradation temperatures(Td) were defined as weight loss of TGA thermogram at 5wt%.
c. Residual SiO₂ contents were available from TGA thermograms, which were performed at heating rate 10^oC/min under nitrogen.

4. CONCLUSION
P(MA-MPTMS)/SiO₂ hybrid materials were prepared successfully by the sol-gel method. The new P(MA-MPTMS)/SiO₂ hybrid films have fairly good optical transparency even with SiO₂ content (up to 25wt%). The SiO₂ nanocrystallines is well dispersed in the P(MA-MPTMS) with 15-60nm in diameter when the SiO₂ content is from 5 to 25wt%. The surface SiO₂ content increases with bulk SiO₂ content in all compositions. These hybrid materials have network structure. The hybrids were nanocomposites. Covalent bonding between the organic and inorganic components enhanced the miscibility between the silica and the copolymer. FT- IR, SEM and the determination of sol content further confirmed this. These hybrid materials have excellent thermal stability.

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