nanocomposites through copolymerization of a polyhedral oligomeric silsesquioxane and methyl...
Post on 06-Jul-2016
Embed Size (px)
Nanocomposites Through Copolymerizationof a Polyhedral Oligomeric Silsesquioxane andMethyl Methacrylate
NOA AMIR, ANASTASIA LEVINA, MICHAEL S. SILVERSTEIN
Department of Materials Engineering, Technion Israel Institute of Technology, Haifa 32000, Israel
Received 13 February 2007; accepted 13 April 2007DOI: 10.1002/pola.22168Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: The mechanical properties and thermal stability of polymers can beenhanced through the formation of nanocomposites. Nanocomposites consisting ofhybrid copolymers of methacrylcyclohexyl polyhedral oligomeric silsesquioxane(POSS-1) and methyl methacrylate (MMA) with up to 92 wt % (51 mol %) POSS-1and with superior thermal properties were synthesized using solution polymerization.The POSS-1 contents of the copolymers were similar to or slightly higher than thosein the feeds, the polydispersity indices were relatively low, and the degree of polymer-ization decreased with increasing POSS-1 content. POSS-1 enhanced the thermal sta-bility, increasing the degradation temperature, reducing the mass loss, and prevent-ing PMMA-like degradation from propagating along the chain. The mass loss wasreduced in a high POSS-1 content copolymer since the polymerization of POSS-1with itself reduced sublimation. Exposure to 450 8C produced cyclohexyl-POSS-likeremnants in the POSS-1 monomer and in all the copolymers. The degradation ofthese remnants, for the copolymers and for the POSS-1 monomer, yielded 75% SiO2and an oxidized carbonaceous residue. VVC 2007 Wiley Periodicals, Inc. J Polym Sci Part A:Polym Chem 45: 42644275, 2007
Keywords: copolymerization; hybrid; nanocomposites; POSS; thermal properties
Nanocomposites consisting of hybrid materialsthat combine organic and inorganic groups oftenexhibit synergistic properties (e.g., mechanical,thermal, optical, electrical).14 Silsesquioxanes(SSQ) are siliconoxygen frame-works with theempirical formula (RSiO1.5)n.
5 R can be hydrogenor an organic substituent, the simplest of whichis CH3. Oligomeric SSQ are obtained through thehydrolytic condensation of trifunctional silanessuch as RSiCl3 or RSi(OCH3)3. Polyhedral oligo-meric silsesquioxanes (POSS) are a relatively
new family of organicinorganic hybrids andhave been described in recent reviews.6,7 Themost common POSS unit is the cubic T8, com-posed of eight silicon atoms at each corner of acube and oxygen bridges between the siliconatoms.
POSS can be incorporated into polymer matri-ces through blending or through chemical reac-tion. POSS can be more easily mixed into a poly-mer matrix if the silicon atoms bear organicgroups that enhance compatibility with the poly-mer.8,9 POSS has been blended with poly(methylmethacrylate) (PMMA) to yield tougher materi-als.10 POSS becomes tethered to the polymerchain as a pendent group if one silicon atombears a group that either reacts with the poly-mer or copolymerizes with the monomer (theremaining seven silicon atoms bear nonreactive
Correspondence to: M. S. Silverstein (E-mail: email@example.com)
Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 45, 42644275 (2007)VVC 2007 Wiley Periodicals, Inc.
groups).11 POSS can crosslink the polymer ifmore than one silicon atom bears groups that ei-ther react with the polymer or copolymerize withthe monomer. POSS bearing different reactivegroups have been copolymerized with organicmonomers using diverse polymerization proc-esses (radical, condensation, ring opening me-tathesis, and living).6,7,1216
Nanocomposites of PMMA containing silicananoparticles have exhibited enhanced thermalstability.17,18 Copolymers containing tetheredPOSS often exhibit enhanced thermal proper-ties.1923 The temperature at which degradationbegins increases on POSS incorporation. A silicalayer, formed on the surface during degradationin the presence of oxygen, serves as a barrier,preventing the degradation of the underlyingpolymer.21,24 The objectives of this research wereto synthesize nanocomposite copolymers of POSSand MMA and to describe the copolymers molec-ular structures and their behavior on exposure toelevated temperatures. Such a combination ofmolecular characterization and thermal analysishas proved useful in providing insight into vari-ous organicinorganic hybrid systems.2527
POSS bearing seven nonreactive cyclohexylgroups and a reactive propylmethacrylate group((C6H11)7Si8O12C7H11O2, 1125.9 g/mol), hereafterreferred to as POSS-1, was supplied by HybridPlastics. The MMA was supplied by Merck Schu-chardt. The MMA was washed to remove the in-hibitor (once with an aqueous 5 wt % sodium hy-droxide (NaOH) solution and then three timeswith deionized water). Toluene, tetrahydrofuran(THF), benzoyl peroxide (BPO), NaOH, deuter-ated chloroform (CDCl3), polystyrene (PS) cali-bration standards for gel permeation chromatog-raphy (GPC), and potassium bromide (KBr) weresupplied by Aldrich and used as received.
Synthesis of POSS-1/MMA Copolymers
The POSS-1/MMA copolymers were prepared byfree radical solution polymerization, as illus-trated in Scheme 1. The polymerization solutionscontained a total of 10 g monomers. The propor-tions of POSS-1 and MMA in the monomer feedsare listed in Table 1. The mass of toluene used,
10 g, was equal to the mass of the monomer mix-ture. The moles of BPO used was 1% of the totalnumber of moles of POSS-1 and MMA. Themonomer mixture was added to toluene in around-bottomed three-necked ask under nitro-gen, and heated to 75 8C with constant stirring.The initiator was added after the solutionreached 75 8C and the solution was held at thattemperature for 5 h. The resulting copolymerwas precipitated in methanol, ltered, dried over-night under vacuum, and then ground using amortar and pestle.
Molecular Composition and Molecular Weight
Nuclear magnetic resonance (1H NMR) was usedto determine the amount of POSS-1 incorporatedinto the copolymers (Bruker 500-MHz NMR spec-trometer). The chemical structures were identi-ed and the comonomer compositions were calcu-lated. Measurements were made in solutions ofCDCl3 at room temperature. Chemical shifts (d)are reported in parts per million (ppm) withCDCl3 as an internal standard.
Fourier transform infrared (FTIR) measure-ments were made in transmission through KBrpellets containing 1% by weight of the sample(Brucker Equinox 55 spectrophotometer). Thepellets were made by mixing the sample withKBr, grinding until a homogenous powder res-ulted, and then pressing the powder in a mold.The spectra were taken from 500 to 4000 cm1 ata resolution of 2 cm1. The contributions of vari-ous groups to the FTIR spectra between 1000and 1300 cm1 were calculated using curve t-ting. The background was removed using theShirley method.28 The individual peaks weredescribed using a symmetric Gaussian/Lorent-zian product function whose variables are thepeak position, peak height, full width at halfmaximum, and Gaussian/Lorentzian fraction.28
The t of the sum of the peaks to the experimen-tal data was optimized and the area fraction ofeach peak, A, was calculated. X-ray photoelectronspectroscopy (XPS) was used to determine ele-mental compositions following pyrolysis (ThermoSigma Probe, VG Scientic).
The molecular weight, molecular weight distri-bution, and copolymer homogeneity were deter-mined using GPC (Waters Breeze GPC with aWaters Styragel HR 4E column). PS was used asa calibration standard, the mobile phase wasTHF, the ow rate was 1 mL/min, and the tem-perature was 30 8C.
NANOCOMPOSITE COPOLYMERS OF POSS AND MMA 4265
Journal of Polymer Science: Part A: Polymer ChemistryDOI 10.1002/pola
Glass-Transition Temperature and Exposure toElevated Temperatures
Differential scanning calorimetry (DSC) wasused to determine the glass-transition tempera-tures (Tg) and to characterize the behavior on ex-posure to elevated temperatures (Mettler DSC-821 calorimeter). The DSC samples were drypowders (810 mg) in an open pan, with an
empty aluminum pan as the reference, and meas-urements were made from 25 to 450 8C at 10 8C/min in owing nitrogen. Thermogravimetricanalysis (TGA) was used to characterize thebehavior on exposure to elevated temperatures(Thermogravimetric Analyzer, TA). The TGAsamples were dry powders (2025 mg) and meas-urements were made from 25 to 900 8C at 20 8C/min in owing nitrogen. The differential ther-
Table 1. Compositions, Molecular Weights, and Transition Temperatures
Mn(kg/mol) PDI N Tg (8C) Td (8C)
PMMA 0 0 0 0 316 1.21 3160 122 233C-11 8 11 50 58 93 1.87 437 113 280C-21 21 21 75 75 116 1.48 349 325C-51 44 51 90 92 189 1.31 310 370
Scheme 1. Copolymerization reaction.
4266 AMIR, LEVINA, AND SILVERSTEIN
Journal of Polymer Science: Part A: Polymer ChemistryDOI 10.1002/pola
mogravimetry (DTG) curves are derivatives ofthe TGA thermograms, calculated using the sup-plied software.
RESULTS AND DISCUSSION
The methoxy group in PMMA resonates at 3.58ppm and is marked M1 in Scheme 1.29,30 Thecyclohexyl hydrogens in POSS-1 resonate atthree different places because of different ringconformations (axial and equatorial) and differ-ent chemical environments. The single hydrogenon the carbon that is attached to silicon resonatesat 0.76 ppm and is marked P1 in Scheme 1.31 Thehydrogen marked P2 in Scheme 1 resonates at1.23 ppm. The 1H NMR spectra of the copolymersin Figure 1 were used to determine their compo-sitions. The relative areas of the P1 and P2 reso-nances (the areas normalized by the area of theM1 resonance, which is only found in the MMAunits) are listed in Table 2. The copolymer com-po