The influence of POSS substituent on synthesis and properties of hybrid materials based on urethane dimethacrylate (UDMA) and various polyhedral oligomeric silsesquioxane (POSS)

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<ul><li><p>The Influence of POSS Substituent on Synthesis andProperties of Hybrid Materials Based on UrethaneDimethacrylate (UDMA) and Various PolyhedralOligomeric Silsesquioxane (POSS)</p><p>A. Lungu, N. M. Sulca, E. Vasile, N. Badea, C. Parvu, H. Iovu</p><p>Department of Polymer Science and Technology, University Politehnica of Bucharest, Bucharest, 010072, Romania</p><p>Received 16 August 2010; accepted 14 October 2010DOI 10.1002/app.33930Published online 30 March 2011 in Wiley Online Library (wileyonlinelibrary.com).</p><p>ABSTRACT: DSC was used to follow the degree of con-version (DC) for the methacrylic groups in the case ofsome hybrid systems based on urethane dimethacrylate(UDMA) monomer and various polyhedral oligomeric sil-sesquioxane (POSS) structures (HISO-POSS, CPENTYL-POSS, and MA-POSS). The chemical structure of the POSScompound directly influences the DC value. The conver-sion of the methacrylic groups from the hybrid compositesafter thermal curing was determined by measuring the re-sidual heat of reaction. Both SEM and AFM techniqueswere additionally used to performed an advanced mor-</p><p>phological characterization for the obtained POSS-nano-composites. The DSC data have been corroborated withthose resulted from the NIR spectroscopy. The introduc-tion of POSS compound within the UDMA matrix leadsto a decrease of the hybrid material transparency due tothe formation of agglomerates also pointed out by SEManalysis. VC 2011 Wiley Periodicals, Inc. J Appl Polym Sci 121:29192926, 2011</p><p>Key words: polyhedral oligomeric silsesquioxane;dimethacrylates; DSC; SEM-EDX; AFM; UV-VIS-NIR</p><p>INTRODUCTION</p><p>Among different nanomaterials, the structures basedon POSS (polyhedral oligomeric silsesquioxane) andvarious polymers have attracted much interest in thelast years. Being a relatively new category of nano-composite, the polymer-POSS hybrid is on its way tofind novel application areas.16</p><p>POSS molecule itself exhibits a molecular hybrid (or-ganic-inorganic) architecture, which includes an innerinorganic silicon-oxygen (SiO1.5)n cage, that is externallycovered by organic substituents. These substituentsmay be alkylic radicals or polar structures with func-tional groups. Both monofunctional and multifunctionalPOSS are considered to be suitable candidates for thenext generation of hybrid materials, which combine theadvantages of conventional inorganic (e.g., rigidity andhigh stability) and organic materials (e.g., flexibility,ductility and processability) and to some degree, supe-rior to each of them.714 POSS nanostructured moleculesexhibit diameters from 1 to 3 nm, similar to the smallestpossible particles of silica.15</p><p>Based on our previous experience in POSS struc-tures,16 we tried to synthesize urethane dimethacry-late (UDMA)/POSS nanocomposites by partiallysubstitute the UDMA with POSS. Thus POSS cagesare incorporated into polymeric matrix via copoly-merization of functional methacrylic groups. Dime-thacrylate-based polymers are products forminghighly crosslinked three-dimensional networks,which have found wide applications as biomaterialsin dentistry.17</p><p>The aim of this work was to study the copolymer-ization reaction between UDMA and different typesof POSS, mono or octafunctional, initiated by benzoylperoxide (BP) at 80C, using differential scanning cal-orimetry (DSC).18 DSC is a common experimentalmethod, which helps to understand the influence ofPOSS type (monofunctional or octafunctional) againstthe polymerization process of UDMA monomerand the final conversion degree. Previously, we stud-ied the degree of conversion (DC) by RAMAN Spec-trometry using three types of initiators at differenttemperatures, to compute the activation energy of thecuring process.16</p><p>The obtained hybrid materials were morphologi-cally analyzed by scanning electron microscopy (SEM)coupled with EDX. Atomic force microscopy (AFM)was used to study and correlate the macro-propertiesof the obtained POSS based nano-composite as wellas the nano-structure. Optical properties were also</p><p>Correspondence to: H. Iovu (iovu@tsocm.pub.ro).Contract grant sponsor: CNCSIS-UEFISCSU; contract</p><p>grant numbers: PNII-IDEI PCCE, 248/2010.</p><p>Journal ofAppliedPolymerScience,Vol. 121, 29192926 (2011)VC 2011 Wiley Periodicals, Inc.</p></li><li><p>determined by UV-vis-NIR, because the informationobtained from SEM is restricted to a narrow field, incontrast with the optical methods, which offer infor-mation about the whole specimen.</p><p>EXPERIMENTAL</p><p>Materials and methods</p><p>Three different POSS compounds were selected: twotypes of monofunctional POSS, HISO-POSS ((1-Pro-pylmethacrylate)-Heptaisobutyl substituted), andCPENTYL-POSS (((heptacyclopentyloctasiloxan-1-yloxy) dimethylsilyl) propyl methacrylate) and oneoctafunctional MA-POSS (Methacryl substitutedPOSS cage mixture, n 8, 10, 12) (Fig. 1). BothHISO-POSS and CPENTYL-POSS are available aswhite powder, in contrast with the octa-POSS (MA-POSS), which is an oily liquid at room temperature.UDMA monomer and the POSS compounds werepurchased from Sigma-Aldrich and used withoutfurther purification.BP, used as free radical initiator, was supplied by</p><p>Sigma-Aldrich and was recrystallized from methanoland dried in a dessicator.</p><p>Differential scanning calorimetry (DSC)</p><p>The DSC curves were nonisothermally recorded onDSC 204 F1 Phoenix (Netzsch) equipment at 10 K/minheating rate in 20200C temperature range undernitrogen atmosphere.</p><p>SEM and Si mapping of the composite sampleswere carried out using a Quanta Inspect F scanningelectron microscope equipped with an energy-dis-persive X-ray detector (EDX) at an operating voltageof 30 kV. Before analysis, cryogenic fracture surfaceswere covered with a thin gold layer using a metal-izer sputtering system.The surface analysis and roughness evaluation</p><p>were performed using an electrochemical atomicforce microscope (AFM) from APE Research. Areas of10 10 lm2, for UDMA homopolymer and UDMAcontaining POSS were analyzed in the contact mode.</p><p>Ultraviolet-visible-near infraredspectrometry (UV-VIS-NIR)</p><p>UVVIS-NIR spectroscopy was used for structuralanalysis of solid samples, using a Jasco doublebeamV570 Spectrophotometer (diffuse transmitance analysisperformed with the device ILN472 endowed withan integrating sphere). The UV-VIS spectra wererecorded on the samples with 2 mm thickness.</p><p>Nanocomposites synthesis</p><p>Several types of nanocomposites based on UDMAmonomer reinforced with 25% POSS compoundswere synthesized. The POSS compound is actingboth as reinforcing agent and comonomer. TheUDMA monomer and the POSS compound weremixed until the mixture was homogeneous. A gooddispersion was achieved by using an ultrasonic</p><p>Figure 1 The chemical structure of raw materials: (a) UDMA, (b) HISO-POSS, (c) CPENTYL-POSS, and (d) MA-POSS.</p><p>2920 LUNGU ET AL.</p><p>Journal of Applied Polymer Science DOI 10.1002/app</p></li><li><p>device for 10 min at room temperature. The BP ini-tiator was added to the mixture (1% w/w) and thenultrasonicated until it was dissolved. After all thecomponents were added, the mixed composite pastewas placed in split Teflon molds covered at bothsides with polyester strip and thin glass slides andsubsequently pressure was applied. Polyester stripsact as a barrier against oxygen from air, which mayinhibit the polymerization reaction.The system wasthen placed in an oven for 180 min at 80C to cure.Immediately after curing, the polyester strips andglass slides were removed, then the specimen withboth sides smooth was taken from its mold andcharacterized through SEM and AFM.To follow the polymerization process DSC tests</p><p>were done at different times over 180 min. Thussamples of 10 mg were collected every 10 min forthe first 60 min of the polymerization reaction. Fur-ther on, additional samples were taken at 30 minintervals up to 180 min, because the polymerizationrate decreased.</p><p>RESULTS AND DISCUSSION</p><p>The polymerization process followed by DSC</p><p>Figure 2 (ad) shows the DSC curves for the poly-merization of classical UDMA and for hybrid sys-tems based on 75%w UDMA with 25%w POSS(mono- and octafunctional).Regardless of the reactants (pure UDMA or</p><p>UDMA-POSS mixtures), an exothermic peak at 85C</p><p>was noticed for all the DSC curves at time 0,assigned to the maximum polymerization enthalpyof the methacrylate groups from UDMA. As thepolymerization reaction goes on (higher reactiontimes) this peak significantly decreases. This was nor-mally expected as the polymerization reaction alreadyoccurred to some extent outside the DSC equipment.Up to 200C, the hybrid systems based on HISO-</p><p>POSS exhibit a different thermal behavior comparedto MA-POSS and CPENTYL-POSS based nanocom-posites. Thus, besides the exothermic peak (85C),an endothermic one at 112C may be observed corre-sponding to the melting process of HISO-POSS andalso an exothermic peak (130C) assigned to thehomopolymerization of the CC bonds from thesubstituent of the POSS cages.19</p><p>From Figure 3 one may notice that the presence ofPOSS compounds leads to a decrease of the initialenthalpy of reaction.Among the POSS-based systems, lower enthalpy</p><p>is obtained for mono-POSS systems (HISO-POSS andCPENTYL-POSS), which tend to aggregate due tothe lower dispersion degree.The enthalpy value for the MA-POSS hybrid sys-</p><p>tem is almost similar with the enthalpy value forUDMA homopolymer. This behavior is due to theeight reactive methacrylic groups from the POSScages, which lead to a higher compatibility with thepolymer matrix.From Figure 3(b), it may be observed that the tem-</p><p>perature at which the polymerization enthalpy ismaximum increases when POSS compound is added</p><p>Figure 2 DSC curves for the polymerization of: (a) 100%UDMA, (b) UDMA_HISO-POSS (75 : 25), (c) UDMA_CPENTYL-POSS (75 : 25), (d) UDMA_MA-POSS (75:25); (1) 0 min; (2) 30 min; (3) 60 min; (4) 120 min; (5) 180 min.</p><p>HYBRIDS BASED ON UDMA AND POSS 2921</p><p>Journal of Applied Polymer Science DOI 10.1002/app</p></li><li><p>probably due to the restricted mobility of theUDMA chains in the vicinity of the POSS cages thusthese chains being less available for the polymeriza-tion process.20</p><p>The influence of POSS type on the DC</p><p>The DC was evaluated from the DSC by computingthe enthalpy values at different reaction times.The progress of polymerization reaction was cal-</p><p>culated using the following equation:</p><p>a DHT DHtDHT</p><p> 100 (1)</p><p>where: a = conversion, [%];DHT = overall reaction enthalpy, [J/g];DHt = enthalpy value at time t of reaction, [J/g].All the hybrid materials based on POSS exhibit</p><p>lower conversion than UDMA homopolymer (Fig. 4)because of the lower reactivity of the methacrylicgroups from POSS substituents in comparison withthe methacrylic group from UDMA.Among the nanocomposites, higher conversions</p><p>are obtained for HISO-POSS and CPENTYL-POSShybrids, which include only one reactive methacrylicgroup and may behave similar to UDMA matrix.The conversion results for MA-POSS show the low-est value of final conversion from all the POSS stud-ied, which is in good correlation with the highesttemperature for the maximum enthalpy of polymer-ization [Fig. 3(b)] indicating that MA-POSS exhibitsa higher compatibility with the UDMA monomerdue to the eight methacrylic groups but a stericalhindrance, which does not allow these groups to beeasier available for copolymerization with UDMA.</p><p>Morphological properties of the hybrid materials</p><p>The compatibility degree between the polymeric ma-trix and the POSS compounds exhibits a significantinfluence on the final properties of the obtainedhybrid materials. Due to the poor dispersability of</p><p>the POSS within the UDMA matrix it is predictablean increase of the POSS aggregation especially forPOSS structures with one methacrylic group. To con-firm this hypothesis, SEM with EDX Si map distribu-tion was performed to determine the extent of thePOSS dispersion.The SEM images for fracture surfaces of the pure</p><p>resin matrix (100%UDMA) and for POSS basedhybrids are shown in Figure 5.The pure UDMA homopolymer reveals a smooth</p><p>area [Fig. 5(a)].Studying the morphology of mono-POSS contain-</p><p>ing networks, it is obviously that mono-POSS givesrise to phase segregation in UDMA matrix. TheHISO-POSS and CPENTYL-POSS crystals practicallycover the surface, making the morphology deeplyinhomogeneous. The shape of the aggregates isangular and there is a random orientation of thesestructures [Fig. 5(b,c)].SEM and EDX Si map analysis reveal that the octa-</p><p>POSS used (MA-POSS) seems to be well dispersed inthe network, and no aggregates were observed [Fig.5(d)]. This good dispersion of MA-POSS hybrid objectscan be attributed to the miscibility of the POSS withinthe resin mass and to the covalent bonds formed</p><p>Figure 3 The influence of poss type on (a) initial enthalpy and (b) temperature of the first exothermic peak. [Color figurecan be viewed in the online issue, which is available at wileyonlinelibrary.com.]</p><p>Figure 4 The dependence of methacrylic groups conver-sion against time for the synthesis of nanocompositesbased on 25% POSS.</p><p>2922 LUNGU ET AL.</p><p>Journal of Applied Polymer Science DOI 10.1002/app</p></li><li><p>between the POSS particles and the methacrylate resin.These results were also reported by Galy and Gerardin their studies on the organic/inorganic hybrids mate-rials containing various POSS methacrylates.21,22</p><p>To fully understand the morphology of the POSS-based materials, AFM analysis was performed on allthe synthesized nanocomposites. Figures 6 and 7 showthe 2D and 3D images of the studied surfaces. TheRMS (root mean square) roughness (Table I) is thestandard deviation of the heights within the selectedheight profile. The roughness can be affected by thetype of POSS used.From the AFM results [Fig. 6(a), it can be seen</p><p>that the finest topography is obtained in the case ofpure UDMA. The surface of the neat UDMA-basedmaterial is smooth with no apparent features.</p><p>From Figure 6(bd), it may be noticed that incor-poration of POSS compounds in the UDMA matrixleads to modification of the polymer surface. Asshown in the AFM images (2D or 3D images), thePOSS molecules tend to aggregate due to the self-as-sembly behavior of POSS. Regarding the roughnessvalues (Table I), no major differences could be identi-fied between the two mono-POSS used. In the caseof octa-POSS systems, the roughness significantlyincreases to more than 500 nm, probably because ofhigh concentration of MA-POSS agglomerates to thehybrid material surface so that even if a good disper-sion of MA-POSS molecules within the UDMA matrixis achieved [SEM image, Fig. 5(d)], the methacrylicgroups from MA-POSS will cause the formation ofbig aggregates at the surface (Fig. 7).</p><p>Figure 5 SEM micrographs of the fractured samples at various magnifications (a) 100UDMA, (b) 75UDMA_25HISO-POSS, (c) 75UDMA_25CPENTYL-POSS, (d) 75UDMA_25MA-POSS.</p><p>HYBRIDS BASED ON UDMA AND POSS 2923</p><p>Journal of Applied Polymer Science DOI 10.1002/app</p></li><li><p>This structure may also explain the low values ofconversion obtained for UDMA_MA-POSS hybrids(Fig. 4). The MA-POSS molecules from the bulk willreact with the MA groups from UDMA but the MA-POSS molecules concentrated at the surface in agglom-erates have no chance to reac...</p></li></ul>