Morphology and Thermomechanical Properties of Melt-Mixed Polyoxymethylene/Polyhedral Oligomeric Silsesquioxane Nanocomposites

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<ul><li><p>Morphology and Thermoof Melt-Mixed PolyoxymOligomeric Silsesquioxan</p><p>Miguel Sanchez-Soto, Silvia Illescas, He</p><p>Introduction</p><p>Polyhedral oligomeric silsesquioxanes (POSS) based mate-</p><p>rials have received great attention in recent years both</p><p>as polymer nanocomposites and as organic/inorganic</p><p>hybrids.[1,2] POSS molecules are characterized by a 3D</p><p>cage structure, i.e., by a polyhedral SiO nanostructured</p><p>The functionality, solubility, polarity, and reactivity of</p><p>these molecules can be easily changed through modifying</p><p>the organic groups with a variety of functional groups. The</p><p>interactions and/or reactions of these organic functional</p><p>groups with a polymer may result in the nanometric</p><p>dispersion of POSS into the matrix.</p><p>In order to form stable polymer/nanoparticle systems,</p><p>]</p><p>direct copolymerization or grafting, via suitably reactive[813]</p><p>]</p><p>-</p><p>Full Paper</p><p>M. Sanchez-Soto, S. IllescasCentre Catala del Plastic, Universitat Politecnica de Catalunya,</p><p>condl)]t</p><p>to an increase on the thermal decompositiontemperature under nitrogen atmosphere up to50 8C. However, mm-size aggregates were observedfor other nanocomposites. There is no significantchange in other thermal properties of the nano-composites. The relationships among these effectsand the morphological characteristics of the sys-tems were analyzed.</p><p>846side groups of POSS. POSS and its derivatives have been</p><p>successfully incorporated in various commodity,[3,6,14,15</p><p>engineering[4,5,1623] and high-performance[24,25] thermo</p><p>plastic polymers, and some thermoset systems.[2628] The</p><p>incorporation of POSS or its derivates can lead to dramatic</p><p>improvement of several properties such as increases in use</p><p>temperature, oxidation resistance, as well as reductions in</p><p>flammability, and viscosity during processing.</p><p>Colom 114, 08222 Terrassa, SpainH. Milliman, D. A. SchiraldiDepartment of Macromolecular Science and Engineering, CaseWestern Reserve University, Cleveland, OH 44106-7202, USAA. ArosteguiMechanical and Industrial Production Department, MondragonUnibertsitatea, Loramendi, 4, 20500 Arrasate-Mondragon, SpainFax: 34 943 79 1536; E-mail: aarostegui@eps.mondragon.eduskeleton [general formula (SiO3/2)n], surrounded by several</p><p>organic groups linked to silicon atoms by covalent bonds.</p><p>different routes have been proposed. Thus, POSS molecules</p><p>can be either physically dispersed in polymer matrices</p><p>using traditional melt-blending processing techniques,[35</p><p>reactive blending,[6,7] or linked to the polymer chains byaddition of amino functionalized POSS, leadingAsier Arostegui*</p><p>The influence of the functionalization of fullybased nanocomposites is studied. POSS withaminopropylisobutyl, and poly(ethylene glycoPOM. Good dispersion was achieved uponMacromol. Mater. Eng. 2010, 295, 846858</p><p> 2010 WILEY-VCH Verlag GmbH &amp; Co. KGaA, Weinheim wileyonlinelmechanical Propertiesethylene/Polyhedrale Nanocomposites</p><p>nry Milliman, David A. Schiraldi,</p><p>densed POSS cages on the properties of POM-ifferent organic substituents [glycidylethyl,are taken into account and melt mixed DOI: 10.1002/mame.201000064</p></li><li><p>dynamics and conformations in polystyrene (PS)/POSS</p><p>nanocomposites solutions and films. They proved that</p><p>and it was used as received. POM pellets were characterized by</p><p>a melt volume flow rate giving a value of 12 cm3 (10 min)1 at190 8C and 2.16 kg. Prior to compounding the polymer pelletswere dried for at least 4 h under vacuum at 80 8C, and maintained indesiccators before mixing.</p><p>Glycidylethyl-, aminopropylisobutyl, and poly(ethylene glycol)-</p><p>POSS (referred herein to as ge-, apib-, and peg-POSS, respectively)</p><p>were kindly supplied from Hybrid Plastics, and used as received.</p><p>The chemical structures of the different POSS used in this work are</p><p>shown in Figure 1. The molar mass of the PEG substituents is close to</p><p>650 g mol1.</p><p>POM/POSS Nanocomposite Preparation</p><p>Nanocomposites were obtained by mixing POM and different</p><p>contents of POSS (2.5, 5, and 10 wt.-%) in a DACA model 2000 twin-</p><p>screw co-rotating microcompounding extruder (13.75 mm screw</p><p>diameter and 108 mm length). The blending process was carried out</p><p>at 190 8C and a screw speed of 100 rpm for 5 min.</p><p>Morphology and Thermomechanical Properties of Melt-Mixed . . .calculated solubility parameter was a useful tool for</p><p>predicting dispersion and segregation of POSS molecules</p><p>into polymer matrices. Thus, they showed using different</p><p>microscopic techniques that nanometric dispersion level</p><p>was achieved when the solubility parameters were very</p><p>similar, meanwhile surface segregation and POSS aggre-</p><p>gation took place with a significantly larger solubility</p><p>parameter difference. Similarly, Brus et al.[28] described</p><p>quantitatively and qualitatively the tendency to aggrega-</p><p>tion and self-organization of individual building blocks</p><p>using high-speed magic-angle spinning 1H/1H spin-diffu-</p><p>sion measurements.</p><p>Polyoxymethylene (POM) is one of the major engineering</p><p>thermoplastics commonly used to replace metal or alloy</p><p>products, owing to its high stiffness, dimensional stability,</p><p>and corrosion resistance. However, low impact toughness,</p><p>notch sensitivity, and especially low heat-resistance limit</p><p>its range of applications. The development of modified</p><p>POM by various strategies, such as copolymerization with</p><p>oxyethylene chain[29,30] and blending with elastomers[31,32]</p><p>are proposed in the literature. The hybridization with</p><p>inorganic fillers, especially organoclay, to form nanocom-</p><p>posites is another approach to improve the properties of</p><p>POM from the nanoscale structure.[33,34] The incorporation</p><p>of POSS molecules into POM matrix, by means of melt-</p><p>blending techniques, has recently been demonstrated as a</p><p>suitable way to improve the thermal stability of the</p><p>material.[23]</p><p>The current work focuses on the compatibility of three</p><p>different POSS molecules with a POM matrix and the effect</p><p>of POSS concentration on the structure, morphology, and</p><p>thermomechanical properties of POM/POSS composites.</p><p>Differences between composites containing fully con-</p><p>densed POSS molecules with different organic groups will</p><p>be examined. The structural changes of the composites</p><p>were assessed by infrared (IR) spectroscopy, the morphol-</p><p>ogy by scanning electron microscopy (SEM), energy-</p><p>dispersive X-ray analysis and atomic force microscopy</p><p>(AFM), and the thermomechanical behavior by differential</p><p>scanning calorimetry (DSC), thermogravimetric analysis</p><p>(TGA), and dynamic-mechanical analysis (DMA).</p><p>Experimental Part</p><p>Materials</p><p>The ethylene/POM copolymer type used in this work as a matrixThe majority of the studies reported to date have focused</p><p>on the morphology and thermomechanical properties of</p><p>the achieved nanocomposites. However, Misra et al.[13]</p><p>reported recently detailed understanding and of chainwas a Hostaform C13021 supplied by TICONA (Barcelona, Spain),</p><p>Macromol. Mater. Eng. 2010, 295, 846858</p><p> 2010 WILEY-VCH Verlag GmbH &amp; Co. KGaA, WeinheimFigure 1. Chemical structures of ge-POSS (a), apib-POSS (b), and</p><p>peg-POSS (c) cage molecules.</p><p> 847</p></li><li><p>conductive.</p><p>Samples were also analyzed by energy-dispersive X-ray analysis</p><p>and different POSS molecules, and therefore, chemical</p><p>at 1 084 cm1 that corresponds to the stretching of the</p><p>stretching) are attributed to peg-POSS molecules.</p><p>M. Sanchez-Soto, S. Illescas, H. Milliman, D. A. Schiraldi, A. Arostegui</p><p>848Differential Scanning Calorimetry (DSC)</p><p>The thermal behavior of the composites, and that of reference</p><p>materials, was analyzed using a Mettler Toledo DSC822e/700</p><p>instrument in hermetically sealed aluminum pans, under nitrogen</p><p>flow (60 mL min1). The apparatus was calibrated with a highpurity indium standard. Samples of approximately 5 mg were</p><p>tested at a heating rate of 10 8C min1 from 20 or 30 8C depend-ing on POSS molecules to200 8C performing three successive runs:(EDAX) to identify the presence of POSS in the bulk. In such</p><p>case samples were sputtered with carbon to avoid interferences</p><p>between gold and silicon signals.</p><p>Surface morphology studies were conducted on MultiMode</p><p>scanning probe microscope with an electronic NanoScope IV cont-</p><p>roller from Veeco Instruments, Inc. A silicon probe with a 125mm</p><p>long silicon cantilever, nominal force constant of 40 N m1, andresonance frequency of 330 kHz was used for tapping mode surface</p><p>topography studies. Surface topographies of film samples were</p><p>studied on 11mm2 scan size areas at an image resolution256256 pixels and a scan rate of 0.75 Hz. Multiple areas wereimaged and figures show representative morphology.Fourier-Transform Infrared (FTIR) Spectroscopy</p><p>The chemical structures of the composites, and that of the reference</p><p>materials, were analyzed using a Nicolet 6700 spectrophotometer.</p><p>The spectral resolution was 1 cm1, and the wavenumber intervalanalyzed was between 4 000 and 400 cm1. For each measurement,50 scans were performed. A Smart Orbit high-performance dia-</p><p>mond single bounce attenuation total reflection (ATR) accessory</p><p>was used. The depth of penetration of the equipment is 2.03 mm at</p><p>1 000 cm1. The resulting samples from the melt-mixing processwere exposed to the ATR by its direct placement upon the ATR</p><p>crystal.</p><p>Microscopic Analysis</p><p>SEM was used to analyze the microstructure of the composites and</p><p>the degree of dispersion of POSS molecules into the matrix. The</p><p>surfaces of cryogenically fractured specimens were observed in a</p><p>Jeol 5610 electron microscope at an accelerating voltage of 10 kV.</p><p>Prior to its observation the fractured surfaces were sputter-coated</p><p>with a thin gold layer of approximately 10 nm to make the surfaceThe extruded materials were compression molded into films</p><p>(constrained inside of a mold) using a Carver model C press. The</p><p>samples were first heated at 190 8C for approximately 10 min,and then a rapid compression was applied followed by release of</p><p>pressure to remove any trapped gas bubbles. The samples were</p><p>then molded at 190 8C under 45 t pressure for approximately 2 minand cooled rapidly between two water-chilled aluminum plates.</p><p>POM/POSS composites films with thickness between 0.2 and</p><p>0.3 mm were finally obtained.heating-cooling-heating. The melting temperatures and enthalpies</p><p>Macromol. Mater. Eng. 2010, 295, 846858</p><p> 2010 WILEY-VCH Verlag GmbH &amp; Co. KGaA, WeinheimFigure 2 shows the FTIR spectra of POM/POSS composites</p><p>with 10 wt.-% POSS, and that of neat POM and different</p><p>POSS molecules as reference. The additional spectra for</p><p>other composites with different POSS contents are not</p><p>included herein because of their similarity to that shown inSiOSi group. Thus, the absorption bands at 1 260 (epoxystretching), 1 020 (epoxy stretching), and 760 cm1</p><p>(deformation of CH of epoxy) are attributed to the func-tional groups of ge-POSS molecule; the absorption bands</p><p>at 1 620 (NH in-plane deformation), 840 (NH out-of-plane deformation), and 1 230 cm1 (CN stretching) areattributed to apib-POSS molecule; whereas the absorption</p><p>bands at 1 470 (CH2 deformation) and 2 920 (CHstructural changes, were studied by means of IR. The</p><p>different characteristic absorption bands corresponding</p><p>to neat POM and different POSS functional groups are</p><p>collected in Table 1. The most characteristic absorption</p><p>band of POM is located at 1 090 cm1, corresponding to thesymmetric stretching vibration of COC ether groups. Asthe functional groups of the POSS molecules were different,</p><p>they had only a common intense and wide absorption bandwere determined from the maxima and the areas of the</p><p>corresponding peaks, respectively. The degree of crystallinity of</p><p>the composites was determined by means of the ratio between the</p><p>measured melting enthalpy and the melting enthalpy of a</p><p>completely crystalline POM (taken to be 251.8 J g1[35]).</p><p>Thermogravimetric Analysis (TGA)</p><p>The thermal stability of the composites was measured by</p><p>thermogravimetric analysis using a Mettler Toledo TGA/SDTA851e</p><p>instrument. Samples sizes of about 10 mg were loaded in</p><p>aluminum pans and heated at a rate of 10 8C min1. Weightloss curve was traced as samples were heated from room tem-</p><p>perature to 600 8C under a dry nitrogen purge of 60 mL min1.</p><p>Dynamic Mechanical Analysis (DMA)</p><p>A Thermal Analysis Instruments Q800 DMA was used in tensile</p><p>mode at an oscillatory frequency of 1 Hz with applied 1% strain for</p><p>all samples. The temperature scan was performed at 2 8C min1heating rate in the range from 100 to around 150 8C. Sampledimensions were typically 4.9 mm long, 9.8 mm wide, and 0.2</p><p>0.3 mm thick.</p><p>Results and Discussion</p><p>Infrared SpectroscopyThe possible reactions and/or interactions between POMFigure 2. As it can be observed, the most characteristic band</p><p>DOI: 10.1002/mame.201000064</p></li><li><p>Morphology and Thermomechanical Properties of Melt-Mixed . . .</p><p>Table 1. Assignment of FTIR spectra characteristic absorption bands</p><p>Material Wavenumber</p><p>cm1</p><p>POM 1 090 COC symmetric stretching vibrationPOSS 1 084</p><p>ge-POSS 1 260</p><p>1 020</p><p>760</p><p>apib-POSS 1 620</p><p>840</p><p>1 230</p><p>peg-POSS 1 455</p><p>2 870of POM at 1 090 cm1 remained at the same wavenumber,whatever the POSS added. This constancy can be attributed</p><p>to the coincidence of COC and SiOSi (1 084 cm1)absorption bands of POM and POSS, respectively. Moreover,</p><p>as the amount of POSS added to POM was small, any</p><p>absorption band corresponding to POSS molecules can be</p><p>masked by POM absorption bands.[23]</p><p>As it can be also observed in Figure 2, other absorption</p><p>bands of POM remained at the same wavenumbers</p><p>and with similar peak height ratios upon the addition of</p><p>10 wt.-% ge-POSS or peg-POSS. This fact indicates that</p><p>ge-POSS or peg-POSS molecules did not interact substan-</p><p>tially with POM matrix, as they are not detectable by FTIR;</p><p>if functionalized POSS molecules linked to the polymeric</p><p>chain of POM, changes or displacements of the absorptionbands of POM should have been observed.</p><p>Figure 2. FTIR spectra of POM/POSS composites with 10 wt.-%POSS, and that of neat POM and different POSS molecules as areference. To aid clarity, the curves are shifted on the vertical axis.</p><p>Macromol. Mater. Eng. 2010, 295, 846858</p><p> 2010 WILEY-VCH Verlag GmbH &amp; Co. KGaA, WeinheimIn view of the structures of POM and apib-POSS, it is</p><p>expected that there could be intermolecular hydrogen</p><p>bonding between the ether oxygen atoms of POM and</p><p>hydrogen atoms of apib-POSS. These interactions can</p><p>readily be detected by means of FTIR in the range of 3</p><p>0003 800 cm1, corresponding to the amine and hydroxylstretching vibrations that are hydrogen bonded. No new</p><p>absorption bands were detected for the 10 wt.-% apib-POSS</p><p>composite system (Figure 2), indicating that hydrogen</p><p>bonding interactions would be very small if they exist at</p><p>all. It should be pointed out that the presence of these</p><p>intermolecular interactions could affect the degree of</p><p>association between the POM matrix and apib-POSS</p><p>molecules by affecting the molecular dynamics in the</p><p>composite system.[36,37] However, it has been proved that</p><p>SiOSi symmetric stretching vibrationepoxy symmetric stretching vibration</p><p>epoxy asymmetric stretching vibration</p><p>CH in epoxy deformation vibrationNH in-plane deformation vibrationNH out-of-plane deformation vibrationCN stretching...</p></li></ul>


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