Preparation, characterization and thermal properties of organic–inorganic composites involving epoxy and polyhedral oligomeric silsesquioxane (POSS)

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  • ORIGINAL PAPER

    Preparation, characterization and thermal propertiesof organicinorganic composites involving epoxyand polyhedral oligomeric silsesquioxane (POSS)

    Chung-Hwei Su & Yi-Pang Chiu & Chih-Chun Teng &Chin-Lung Chiang

    Received: 24 June 2009 /Accepted: 16 October 2009 /Published online: 7 November 2009# Springer Science + Business Media B.V. 2009

    Abstract Organicinorganic hybrids comprising epoxyresin and polyhedral oligomeric silsesquioxanes (POSSs)were prepared via in situ polymerization of the diglycidylether of bisphenol A (DGEBA) and 4,4-diaminodiphenyl-methane (DDM). The POSSs have an active functionalgroup that takes part in the ring-opening reaction with theoxirane group. The organic and inorganic moieties arejoined by covalent bonds. These covalent bonds enhancethe compatibility of the inorganic and organic phases.Scanning electron microscope (SEM) analytical resultsindicate that there was no obvious phase separationbetween the inorganic and organic phases. The UV/VISspectrum of the epoxy hybrid demonstrates the excellentoptical transparency of the hybridsthe most importantcharacteristic for their application as protective coatings.Thermogravimetric analysis (TGA), X-ray photoelectronspectra (XPS), and nuclear magnetic resonance spectrosco-py (NMR) of the char showed that the incorporation of thePOSSs into epoxy resin improves the thermal stability ofthe hybrids.

    Keywords Hybrid . Thermal property . Epoxy . Polyhedraloligomeric silsesquioxanes (POSSs)

    Introduction

    Organicinorganic composites are typically considered anew generation of high-performance materials, as theycombine the advantages of inorganic materials (e.g., rigidityand high stability) with those of organic polymers (e.g.,flexibility, dielectric properties, ductility and processability)[17]. Polyhedral oligomeric silsesquioxanes (POSSs) arean interesting class of three-dimensional silsesquioxanesthat serve as soluble silica models. These POSSs arestructurally well-defined compounds that consist of asiliconoxygen framework that has the general formulaRSiO3/2, and which can be easily functionalized using abroad range of organic groups commonly employed inpolymerization or grafting reactions [8].

    Epoxies are a class of important thermosetting resins thatare widely used as matrices of composite materials,adhesives, and electronic encapsulating materials due totheir high mechanical strength, excellent chemical resis-tance, and processing simplicity [911]. Their extensiveapplication motivates the preparation of organicinorganichybrid composites of epoxy resins with enhanced thermaland flame retardant properties. Modifying epoxy resin byadding POSSs can provide materials with superior charac-teristics such as improved thermomechanical properties,thermal and oxidative stability, and dielectric properties. ThePOSS silanols have a hybrid inorganicorganic three-dimensional structure that contains 14 stable silanol (SiOH)groups. These POSS silanols can be incorporated into apolymer by copolymerization or grafting [1214].

    C.-H. SuDepartment of Fire Science, Wu-Feng Institute of Technology,117, Sec. 2, Jianguo Rd., Minsyong,Chiayi 621, Taiwan

    Y.-P. Chiu : C.-L. Chiang (*)Department of Safety, Health and Environmental Engineering,Hung-Kuang University,34, Chungchi Rd., Sha-Lu,Taichung 433, Taiwane-mail: dragon@sunrise.hk.edu.tw

    C.-C. TengDepartment of Chemical Engineering,National Tsing Hua University,Hsin-Chu 30043, Taiwan

    J Polym Res (2010) 17:673681DOI 10.1007/s10965-009-9355-y

  • This study describes the preparation of epoxy/tetrasila-nolphenyl POSSs (TSP-POSSs) hybrid copolymers withvarious proportions of POSSs synthesized via a ring-opening reaction between the hydroxyl group of the TSP-POSSs and the oxirane group of DGEBA epoxy monomers.The primary goal of this study was to describe thesynthesis, structure and properties of these hybrids byFourier transform infrared spectroscopy (FTIR), nuclearmagnetic resonance spectroscopy (NMR), scanning elec-tron microscopy (SEM), differential scanning calorimetry(DSC), UV/vis spectroscopy, thermogravimetric analysis(TGA), and X-ray photoelectron spectroscopy (XPS).

    Experimental

    Materials

    The epoxy resin used was the diglycidyl ether of bisphenol A(DEGBA, NPEL-128) with an epoxide equivalent weight of180, which was generously provided by Nan-Ya PlasticsCorporation, Taiwan. The tetrasilanolphenyl POSSs (TSP-POSSs, C48H44O14Si8) were purchased from Hybrid PlasticsCo., Hattiesburg, MS, USA. 4,4-Diaminodiphenylmethane(DDM) and triethylamine (TEA) were of analytical grade, andwere obtained from Acros Organics Co. Janssens Pharmaceu-ticalaan, Geel, Belgium. Tetrahydrofuran (THF) was reagentgrade and supplied by Echo Chemical Co. Ltd., Taiwan.

    Preparation of epoxy/TSP-POSS/DDM hybrids

    To prepare the nanocomposites containing the POSSs, aspecific amount of the TSP-POSSs was dissolved in aminimal amount of THF. The resulting solution was addedto a pre-weighed amount of DGEBA via vigorous stirringto create a homogeneous solution. The contents of the TSP-POSSs in the nanocomposites were adjusted to be 10, 20,30, and 40 wt%. The curing agent, DDM, was then addedrelative to the amount of DGEBA. The equivalent weightratio of DGEBA to DDM was 1:1. TEA (0.2 wt% DGEBA)was then added to the mixture. The TEA was used as acatalyst for the DGEBA and TSP-POSS reaction. Themixtures were stirred to obtain transparent solutions andthen poured into aluminum discs, and most of the solventwas evaporated at 60 C for 24 h. To remove any residualsolvent, all of the samples were dried in vacuo at 60 C fora minimum of 4 h. The systems were then cured at 80 Cfor 2 h and 180 C for 2 h to achieve complete curing.

    Reaction schemes

    Scheme 1 shows the reaction scheme for the preparation oforganicinorganic DGEBA involving TSP-POSSs.

    Measurements

    FTIR

    The FTIR spectra of the materials were recorded between4,000 and 400 cm1 on a Nicolet Avatar 320 FT-IRspectrometer, Madison, WI, USA. Thin films were preparedby the solution-casting method. A minimum of 32 scanswere signal-averaged with a resolution of 2 cm1 in the4,000400 cm1 range.

    29Si NMR

    29Si NMR was performed using a Bruker DSX-400WB,Karlsruhe, Germany. The samples were treated at 180 Cfor 2 h and then ground into fine powder.

    Differential scanning calorimetry (DSC)

    The glass transition temperatures (Tgs) of the sampleswere measured using a differential scanning calorimeter(DSC) (DuPont, Wilmington, DE, USA; model 10). Theheating rate was 10 C min1 within a temperature rangeof 50~200 C. The measurements were made with 3~4 mgsample on a DSC plate after the specimens had beenquickly cooled to room temperature following the firstscan. Tgs were determined at the midpoint of thetransition point of the heat capacity (Cp), and thereproducibility of the Tg value was estimated to be within2 C.

    Thermogravimetric analysis (TGA)

    The thermal degradation of the composite was examined by athermogravimetric analyzer (PerkinElmer, Wellesley, MA,USA; TGA 7) from room temperature to 800 C at 10 C/minin an atmosphere of nitrogen. Measurements were made using610 mg samples. Weight loss/temperature curves wererecorded.

    Scanning electron microscopy (SEM)

    The morphology of the fractured surface of the polysilses-quioxanes was examined using a scanning electron micro-scope (SEM) (JSM 840A, JEOL, Tokyo, Japan). Thedistribution of Si atoms in the hybrid was obtained bySEM EDX mapping (JSM 840A, JEOL).

    UV/vis

    UV/vis spectra were tested on a Hitachi (Tokyo, Japan) U-3300 spectrophotometer, and the sample was prepared as athin film on a glass substrate by spin coating.

    674 C.-H. Su et al.

  • Transmission electron microscopy (TEM)

    The TEM study was carried out using a Hitachi H-7500 withan accelerating voltage of 100 kV during measurements. Thesamples were microtomed with a Leica (Solms, Germany)Ultracut and cut into 60 nm-thick slices. These slices wereplaced on mesh 200 copper nets for TEM observation.

    Dynamic mechanical analysis (DMA)

    The dynamic mechanical tests were carried out on adynamic mechanical thermal analyzer (DMTA MK III,Polymer Laboratories, Church Stretton, UK) between 30 Cand 200 C with a heating rate of 2 C/min and a frequencyof 2 Hz. The rectangular bending mode was chosen and thedimensions of the specimen were 4073 mm3.

    Results and discussion

    FTIR of the reaction processes

    Infrared spectroscopy was performed to characterize thestructures of the composites via the ring-opening reaction

    between the oxirane functional group of epoxy resin andthe OH functional group of the POSSs. Scheme 1 showsthe reaction process and the structure of the epoxy/POSScomposite. Figure 1 presents FTIR results for the reaction

    Scheme 1 Preparation of organicinorganic DGEBA involving TSP-POSSs

    Fig. 1 FTIR of the reaction process for: a, the epoxy prepolymer; b,pure POSS monomer; c, epoxy/POSS composites after the curingreaction

    Preparation, characterization and thermal properties of organicinorganic composites 675

  • process of the epoxy monomer (a), the pure POSSsmonomer (b), and the epoxy/POSS composites after curing(c). The characteristic peak of the oxirane group of theepoxy resin is at 910 cm1 in curve (a). The SiOSi groupwas detected in the hybrid at 1,112 cm1, which corre-sponds to the cage structure of POSSs in curve (b). In curve(c), the oxirane group at 910 cm1 has disappeared, and theintensity of the hydroxyl group at 3,450 cm1 hasincreased, meaning that the epoxy resin has reacted withthe POSSs. Connections are formed between the organicand inorganic phases. Covalent bonds exist between theorganic and inorganic moieties. These covalent bondsenhance the compatibility of the two phases. The hybridshave networks that enhance their thermal properties.

    Morphological properties of composites

    The compatibility of organic polymers and silica markedlyaffects the thermal, mechanical and optical properties of the

    composites. The morphology of the fractured compositesurface was observed by SEM. A mapping technique wasused to elucidate the silica distribution and the separation ofthe microphases in the hybrid matrix. Figure 2a shows anSEM micrograph of the morphology of a composite. Thefracture surface was very dense and no obvious phaseseparation or gaps existed between the organic andinorganic phases. Figure 2b shows an EDX Si map of anepoxy nanocomposite. The particles were uniformly dis-persed throughout the polymer matrix. Aggregation ofinorganic particles was not observed in the SEM micro-graphs. This analytical result demonstrates that the nano-composites exhibit good miscibility between the organicand inorganic phases. The compatibility of POSS moleculesand polymers is key to achieving a well-dispersed polymernanocomposite. Generally, POSS molecules show betterdispersion than conventional additives. This enhanceddispersion can be attributed to the existence of covalentbonds between POSS particles and the epoxy resin. SomePOSS molecules can easily disperse into the polymermatrix at the molecular level, which may result incomposite properties that are very different from those ofconventional polymer composites.

    TEM was employed to study the internal morphologiesof the composites at a fine scale. Sections about 75 nmthick were found to contain entire small-sized domains andportions of larger-sized domains. Figure 3 presents a TEMmicrophotograph of a fractured composite surface. Basedon this figure, the size of the particles in the composites isabout 100 nm. Since each POSS molecule has a three-dimensional inorganic core covered with four organic sidegroups and four hydroxyl groups, it is believed that thebetter dispersion of these composites may result from thechemical bonding of the hydroxyl groups and increasedinteraction between compatible side groups and thenetwork of the epoxy resin.

    Fig. 3 TEM microphotograph of a fractured surface of a compositecontaining 2 wt% of POSS

    Fig. 2 a SEM micrograph of an epoxy/POSS composite. b EDX Simap of an epoxy/POSS composite

    676 C.-H. Su et al.

  • UV/vis spectroscopy of the hybrids

    Figure 4 shows UV/vis spectra of epoxy/POSS compositeswith different inorganic contents. No obvious absorbanceoccurred in the range 400800 nm (the visible light region),meaning that visible light can pass through the composites,and there was no obvious phase separation between theorganic and inorganic phases. These results show thatthe hybrids have excellent optical transparency and that theinorganic particles are smaller than the wavelength ofvisible light. Hybrids with different inorganic contentsshow similar transparencies. This optical transmittance canbe utilized as a criterion for the formation of a homoge-neous phase. Figure 5 presents photographs of epoxy/POSScomposites with different inorganic contents. Analyticalresults demonstrate that the hybrids exhibit excellent opticaltransparency, which is the most important characteristic fortheir application as protective coatings.

    Thermal properties of the hybrids

    The epoxy/POSSs hybrids were subjected to thermalanalysis. Figure 6 shows DSC curves for the pure epoxy

    and the hybrids. Pure epoxy had a glass transitiontemperature of 152 C. All of the DSC curves for thecomposites containing POSSs had single glass transitiontemperatures in the experimental temperature range (roomtemperature to 200 C). Notably, the presence of a singleglass transition temperature indicates that the hybrids arehomogeneous [15]. The epoxy/POSS hybrids had slightlylower Tg values than pure epoxy (Fig. 6). It is proposedthat the decreased Tg values may be responsible for theincrease in the free volume of the system through theinclusion of some of the bulky POSS cages at the nanoscalelevel. In other words, there is an effect comparable to aplasticization effect of the low molecular weight com-pounds on the polymer matrix. Nevertheless, the hybridscontaining 10, 20, 30 and 40 wt% POSSs had very similarglass transition temperatures. No clear trend in glasstransition temperatures is discernible in Fig. 6. Thus, wepropose that two competing factors determine the glasstransition temperatures of POSS-modified polymers. Clear-ly, the POSS cages hinder polymer chain motion, increasingthe glass transition temperature. Conversely, the inclusion

    Fig. 6 DSC of epoxy/POSSs composites with various POSS contents

    Fig. 5 Photographs of epoxy/POSS composites with differentinorganic contents

    Fig. 4 UV/vis spectra of epoxy/POSS composites with differentinorganic contents

    Fig. 7 TGA of epoxy/POSS composites with different inorganiccontents at 10 C/min in a nitrogen atmosphere

    Preparation, characterization and thermal properties of organicinorganic composites 677

  • of the bulky POSS groups may increase the free volume ofthe system, resulting in a decrease in Tg. Therefore, thebehavior of the glass transition in hybrids containingPOSSs may reflect these two competing factors, dependingon the characteristics of the composite system. Although allof the glass transition temperatures of the POSS hybridswere lower than tha...

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