Epoxy/polyhedral oligomeric silsesquioxane nanocomposites from octakis(glycidyldimethylsiloxy)octasilsesquioxane and small-molecule curing agents

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<ul><li><p>Epoxy/Polyhedral Oligomeric SilsesquioxaneNanocomposites fromOctakis(glycidyldimethylsiloxy)octasilsesquioxaneand Small-Molecule Curing Agents</p><p>YING-LING LIU,1,2 GUNG-PEI CHANG,1,2 KEH-YING HSU,1 FENG-CHIH CHANG3</p><p>1Department of Chemical Engineering, Chung Yuan Christian University, Chungli, Taoyuan 320, Taiwan</p><p>2R&amp;D Center for Membrane Technology, Chung Yuan Christian University, Chungli, Taoyuan 320, Taiwan</p><p>3Department of Applied Chemistry, National Chiao Tung University, Hsinchu 300, Taiwan</p><p>Received 24 February 2006; accepted 31 March 2006DOI: 10.1002/pola.21484Published online in Wiley InterScience (www.interscience.wiley.com).</p><p>ABSTRACT: Epoxy/polyhedral oligomeric silsesquioxane (POSS) nanocomposites wereobtained from octakis(glycidyldimethylsiloxy)octasilsesquioxane (OG) and diglycidylether of bisphenol A cured with small-molecule curing agents of diethylphosphite(DEP) and dicyandiamide (DICY). An increase in the POSS contents of the nanocom-posites and an improvement in the nanocomposite homogeneity were observed withthe use of the small-molecule curing agents. Phosphorus in DEP and nitrogen inDICY also performed synergism with POSS for thermal stability enhancement andammability improvement in the nanocomposites. The nanocomposites possessinghigh OG contents exhibited good thermal stability, improved ammability, and highstorage moduli. VVC 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 38253835, 2006</p><p>Keywords: curing of polymers; nanocomposites; silicons; crosslinking</p><p>INTRODUCTION</p><p>Polyhedral oligomeric silsesquioxanes (POSSs)are useful molecular nanobuilding blocks in theformation of organicinorganic nanocomposites.19</p><p>In contrast to conventional inorganic llers,POSSs have the advantages of monodispersedsize, low density, high thermal stability, andcontrolled functionalities. The functional groupsin the organic substituents bring chemical ver-satility to POSSs and make POSSs widely com-patible with various organic polymers.1016 Ep-oxy resin/POSS nanocomposites are the most</p><p>studied of polymerPOSS nanocomposites, asepoxy resins are the most commercially success-ful materials known as composite matrices.1736</p><p>Laine and coworkers2024 reported a series ofworks on the preparation of epoxyPOSS mono-mers and their polymers. Octakis(glycidyldime-thylsiloxy)octasilsesquioxane (OG) is the moststudied epoxyPOSS compound. EpoxyPOSScomposites prepared from OG and 4,4-diamino-diphenylmethane (DDM) exhibit tensile modulicomparable to and thermal stability better thanthose of conventional DDM-cured diglycidylether of bisphenol A (DGEBA) resins.20,21 Simi-lar properties have also been observed with 4,4-diaminodiphenylsulfone-cured OG composites.22</p><p>Recent studies used phenylenediamines as cur-</p><p>Correspondence to: Y.-L. Liu (E-mail: ylliu@cycu.edu.tw)</p><p>Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 44, 38253835 (2006)VVC 2006 Wiley Periodicals, Inc.</p><p>3825</p></li><li><p>ing agents in the preparation of epoxyPOSSnanocomposites, aiming to reduce the steric hin-drance in curing reactions and increase thecrosslinking densities of cured resins.31,34 Be-sides being used as a multifunctional monomer,OG is also a reactive additive used to modify ep-oxy resins. POSS-modied DGEBA epoxy resinshave been reported.17,2427 The presence of POSScages in nanocomposites reduces their dielectricconstant, broadens the glass-transition behavior,and increases the storage modulus. However,enhancements of the glass-transition tempera-tures (Tgs) and thermal stabilities are not alwaysobserved with POSS-modied epoxy resins.</p><p>Curing agents strongly affect the structureand thermomechanical properties of crosslinkedepoxy resins. In this work, two small-moleculecuring agents of dicyandiamide (DICY) and di-ethylphosphite (DEP) were used in the prepara-tion of epoxyPOSS nanocomposites. DICY iswidely used in the manufacture of FR-4 printedcircuit boards. DEP is a thermally latent curingagent.37 Phosphorus in DEP also potentiallyenhances the ame-retardant properties of ep-oxy resins.3740 In addition, DICY and DEPpotentially reduce the steric hindrance in curingreactions, increase the reaction conversions, andimprove the homogeneity of the resulting nano-composites.36 The morphology and thermal prop-erties of the prepared nanocomposites are exam-ined and discussed.</p><p>EXPERIMENTAL</p><p>Materials</p><p>OG, purchased from Hybrid Plastics, Inc. (Foun-tain Valley, CA), was used as received. DGEBA(BE188; epoxy equivalent weight 188 g/mol)was received from Chang Chun Plastics Co.(Taiwan). DICY and DEP were purchased fromAldrich Co. and used as curing agents in thiswork.</p><p>Equipment</p><p>Fourier transform infrared (FTIR) spectra wererecorded with a PerkinElmer Spectrum OneFTIR apparatus equipped with an attenuated-total-reectance accessory. The fracture surfacemorphology of the cured hybrid resins wasobserved with a scanning electron microscope(JSM-6500F, JEOL, Japan). Nanocomposite sam-</p><p>ples were put into liquid nitrogen. The quenchedsamples were taken out and broken down withpliers. The fractured surface was then applied toscanning electron microscopy (SEM) observation.Differential scanning calorimetry (DSC) thermo-grams were recorded with a Thermal AnalysisDSC-Q10 differential scanning calorimeter. Thetesting range was 30300 8C at a heating rate of10 8C/min under a nitrogen gas ow of 40 mL/min. Thermogravimetric analysis (TGA) was per-formed with a Thermal Analysis TGA-2050 ther-mogravimetric analyzer at a heating rate of10 8C/min under a nitrogen or air atmosphere.The gas ow rate was 100 mL/min. Dynamic me-chanical analysis (DMA) was performed with aPerkinElmer DMA 7e dynamic mechanical ana-lyzer at a frequency of 1 Hz and a heating rateof 3 8C/min. Limited oxygen index (LOI) valueswere measured on a Stanton Redcraft ame me-ter. The percentage in the O2N2 mixture de-emed sufcient to sustain the ame was takenas the LOI.</p><p>Preparation of the OG/DEP and OG/DICYNanocomposites</p><p>Nanocomposites were obtained from the curingof OG with DEP and DICY with various valuesof the number ratio of epoxide groups in OGover active hydrogen in the curing agent (i.e., N 2, 1, or 0.7). Predetermined amounts of OGand the curing agent (DEP or DICY) were dis-solved in a solvent (tetrahydrofuran for DEPand N,N-dimethylformamide for DICY) to obtaina homogeneous solution. The solvent was evapo-rated out at 25 8C in vacuo. The residue wascured with a heating process of 150 8C for 1 h,180 8C for 1 h, and 230 8C for 3 h. For a compar-ison of the properties, a DDM-cured OG resinwith N 1 (OG/DDM-1) was also prepared withthe same procedure used for the OG/DEP prepa-ration.</p><p>Preparation of the OG-Modied Epoxy ResinsCured with DICY and DEP</p><p>Predicted amounts of OG and BE188 were dis-solved in THF. The solution was then mixedwith solutions of the curing agent (DEP in THFor DICY in DMF) in stoichiometric ratios (N 1). The solvent was evaporated out at 25 8Cin vacuo. The residue was cured with a heatingprocess of 150 8C for 1 h, 180 8C for 1 h, and230 8C for 3 h.</p><p>3826 LIU ET AL.</p><p>Journal of Polymer Science: Part A: Polymer ChemistryDOI 10.1002/pola</p></li><li><p>Integrated Procedure Decomposition Temperature</p><p>The integral procedure decomposition tempera-ture (IPDT)41 was calculated as follows:</p><p>IPDTC AKTf Ti Ti 1</p><p>where A* is the area ratio of the total experi-mental curve divided by the total TGA thermo-gram, K* is the coefcient of A*, Ti is the initialexperimental temperature, and Tf is the nalexperimental temperature. A representation ofS1, S2, and S3 for calculating A* [A* (S1 S2)/(S1 S2 S3)] and K* [K* (S1 S2)/S1] is shown in Figure 1.</p><p>RESULTS AND DISCUSSION</p><p>Preparation of Epoxy Resins from OG Curedwith DICY and DEP</p><p>The chemical structures of the agents used inthe preparation of the epoxyPOSS nanocompo-sites are shown in Figure 2. OG was cured withDICY or DEP with various values of N. Sampleswere obtained with the three N values of 2.0,1.0, and 0.7. Figure 3 shows the FTIR spectra ofthe OG/DEP nanocomposites. The absorptionpeak of the OG epoxide groups (909 cm1) wasobserved with OG/DEP-2 (N 2.0) but not withOG/DEP-0.7 (N 0.7). In the composition ofOG/DEP-2, there were excess epoxide groups.Some epoxide groups did not participate in thecuring reaction and remained in the nanocompo-sites. An increase in the amount of DEP con-sumed all the epoxide groups, and the epoxide</p><p>absorption peak in the FTIR spectra of OG/DEP-1 and OG/DEP-0.7 disappeared. Hydroxylgroups, which were generated from the curingreaction, were observed with the absorptionpeak at 3471 cm1. An increase in the amountof DEP in the curing compositions increased thispeak intensity and brought a redshift to thispeak to 3417 cm1, which indicated the genera-tion of more hydroxyl groups and the formationof hydrogen bonding between the hydroxylgroups. Similar results were also observed forthe OG/DICY nanocomposites.</p><p>All the prepared OG/DEP and OG/DICYnanocomposites exhibited good transparency,which indicated that no macrophase separationoccurred in the curing period. The glass-transi-tion behavior was not observed below 220 8C inthe DSC measurements. Unfortunately, the sam-ples were too brittle to be used in DMA meas-urements. The high brittleness of the nanocom-posites was attributed to the use of small-mole-cule curing agents of DEP and DICY, whichsimultaneously shortened the chain lengthsbetween the crosslinks and promoted the curingreaction conversion. In addition, the nanocompo-sites possessed very high POSS fractions. Forthe N 1 cases, the inorganic SiOx fractions inthe OG/DDM and OG/DICY nanocomposites,calculated from the curing compositions, wereabout 35.5 and 40.6 wt %, respectively. An in-crease in the inorganic SiOx fractions of thenanocomposites also was attributed to the in-crease in the brittleness.</p><p>The thermal stabilities of the OG/DEP, OG/DICY, and OG/DDM nanocomposites were char-acterized with TGA (Fig. 4), and the analyticaldata are collected in Table 1. All the samplesexhibited similar degradation thermograms in</p><p>Figure 1. Representation of S1, S2, and S3.</p><p>Figure 2. Chemical structures of the epoxide mono-mers and curing agents.</p><p>EPOXYPOSS NANOCOMPOSITES 3827</p><p>Journal of Polymer Science: Part A: Polymer ChemistryDOI 10.1002/pola</p></li><li><p>an atmosphere of N2. However, the relativelylow degradation temperatures (Tds; i.e., thetemperatures at 5% weight loss) of OG/DEP-1came from their thermally unstable phospho-nate groups. Besides, the char yields (residueamount at 800 8C) of the nanocomposites werenoteworthy. OG/DDM-1 showed the smallestchar yield, which mainly was formed from itsinorganic SiOx fraction. However, OG/DEP-1and OG/DICY-1, which possessed similar SiOxfractions, showed different char yields. As thephosphonate group is known as a char-forma-tion promoter, the relatively high char yield ofOG/DEP-1 is understood with the phosphonategroup action. The char enhancement was alsoobserved with the DICY-cured nanocompositesbecause of nitrogensilica synergism.42 Thethermal degradation behaviors of the nanocom-posites made with various N values are shownas Figure 5. The weight loss started at very lowtemperatures (100250 8C) for all samplesexcept OG/DICY-2 and OG/DEP-2, which pos-sessed excess epoxide groups in the curing for-mulations. The low-temperature weight losscame from the residue curing agent after thecuring reaction. The char yields decreased withincreasing DICY amounts in the curing composi-tions because more DICY residue remained inOG/DICY-0.7. Moreover, though possessing manyphosphonate groups, OG/DEP-0.7 did not exhibitthe highest char yields among the nanocompo-sites. DEP was present in a large excess in theOG/DEP-0.7 formulation, but some DEP did not</p><p>participate in the curing reaction, and so it wasnot incorporated into the crosslinked structure.The free DEP residue volatized at low temper-atures before nanocomposite degradation andcould not promote the char formation.</p><p>Figure 6 shows the SEM micrographs of thefracture surfaces of OG/DEP and OG/DICYnanocomposites frozen under cryogenic condi-tions with liquid nitrogen. No macrophase sepa-ration appeared for the samples. The fracturesurface of OG/DICY-1, compared with othersample surfaces, was quite smooth. A smoothsurface was also observed for OG/DEP-0.7. Na-nocomposites with stoichiometric curing compo-sitions exhibited good homogeneity. The use ofsmall molecules as curing agents did not excludethe POSS cages during the curing reactions and</p><p>Figure 3. FTIR spectra of the OG/DEP nanocomposites. [Color gure can be viewedin the online issue, which is available at www.interscience.wiley.com.]</p><p>Figure 4. TGA thermograms of the OG nanocompo-sites in nitrogen.</p><p>3828 LIU ET AL.</p><p>Journal of Polymer Science: Part A: Polymer ChemistryDOI 10.1002/pola</p></li><li><p>prevented the occurrence of macrophase separa-tion.</p><p>OG-Modied Epoxy Resins Cured with DICYand DEP: Preparation and Morphology</p><p>These results suggest that the use of small mol-ecules to cure OG can enhance the curing reac-tion conversion and the nanocomposite homoge-neity. However, the nanocomposites are highlybrittle because of the short chain lengths in thecrosslinked structure. The rigid structure ofPOSS also contributes to the brittleness of thenanocomposites as they possess high POSS con-tents. For the purpose of developing high-per-formance epoxy resins, OG was used as a reac-tive modier to cocure with the conventionalDGEBA epoxy resin (BE188). OG/BE188 mix-tures with various OG contents were cured withDEP and DICY for the preparation of OGBEnanocomposites. The epoxy and curing agentswere in stoichiometric ratios (N 1) in the for-mulations. The preparation compositions of theOG-modied epoxy resins are listed in Table 2.The samples are coded as OGBE/DEP-M andOGBE/DICY-M, where M is the OG weight per-centage in the curing mixtures.</p><p>None of the OGBE nanocomposite samplesshowed epoxy group absorption (ca. 905910cm1) in their FTIR spectra (gures not shown),and this indicated the high curing reaction con-versions. The OGBE nanocomposite homogene-ity was examined with SEM (Fig. 7). Smoothsurfaces were observed for the OGBE nanocom-posites. The fracture surface morphologiesobserved for the pristine BE188-cured resin andOGBE nanocomposites were similar, indicating</p><p>the good homogeneity of the OGBE nanocom-posites. OG addition in the BE188/DEP andBE188/DICY curing compositions did not inducephase separation during the mixing and curingreaction periods. DEP and DICY are proper cur-ing agents for the preparation of OGBE nano-composites having high OG contents.</p><p>OG-Modied Epoxy Resins Cured with DICYand DEP: Thermal Properties</p><p>Figure 8 shows the DSC thermograms of theOGBE nanocomposites. No exothermic peaksappeared in the thermograms, and this indi-cated that no reactive epoxide residues werepresent in the nanocomposites and that the cur-ing reaction conversions were high. Glass-tran-sition behaviors were observed with the sam-ples. Neat DGEBA/DEP resin (OGBE/DEP-0)showed its Tg at 90 8C. The addition of OG in</p><p>Table 1. Thermal Stability Data for OG-Based Nanocomposites fr...</p></li></ul>