facile and quick synthesis of poly(n-methylolacrylamide)/polyhedral oligomeric silsesquioxane graft...

Facile and Quick Synthesis of Poly(N-Methylolacrylamide)/Polyhedral Oligomeric Silsesquioxane Graft CopolymerHybrids via Frontal Polymerization

YUAN FANG, LI CHEN, SU CHEN

State Key Laboratory of Material-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering,Nanjing University of Technology, No. 5 Xin Mofan Road, Nanjing 210009, People’s Republic of China

Received 12 September 2008; accepted 6 November 2008DOI: 10.1002/pola.23201Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: We report on a new strategy for fabricating well-defined POSS-based poly-meric materials with and without solvent by frontal polymerization (FP) at ambientpressure. First, we functionalize polyhedral oligomeric silsesquioxane (POSS) with iso-phorone diisocyanate (IPDI). With these functionalized POSS-containing isocyanategroups, POSS can be easily incorporated into a poly(N-methylolacrylamide) (PNMA)matrix via FP in situ. Constant velocity FP is observed without significant bulk poly-merization. The morphology and thermal properties of POSS-based hybrid polymersprepared via FP are comparatively investigated on the basis of scanning electronic mi-croscopy (SEM) and thermogravimetric analysis (TGA). Results show that the as-pre-pared POSS-based polymeric materials exhibit a higher glass transition temperaturethan that of pure PNMA, ascribing to modified POSS well-dispersed in these hybridpolymers. Also, the products with different microstructures display different thermalproperties. The pure PNMA exhibits a featureless morphology, whereas a hierarchicalmorphology is obtained for the POSS-based polymeric materials. VVC 2009 Wiley Periodi-

cals, Inc. J Polym Sci Part A: Polym Chem 47: 1136–1147, 2009

Keywords: frontal polymerization (FP); graft copolymers; morphology; nanocomposites;polyhedral oligomeric silsesquioxane (POSS); poly(N-methylolacrylamide) (PNMA); radicalpolymerization; synthesis

INTRODUCTION

Organic-inorganic hybrids have drawn consider-able interest because of their extraordinary prop-erties, which originate from the synergismbetween the inorganic nanoparticles and organicmolecules.1,2 However, because the entropicchanges of large molecular weight organic compo-nents are small, even small unfavorable enthalpicinteractions can cause phase segregation,3 whichleads to poor thermal and mechanical properties.

Hence, a great deal of research has concentratedon reducing the interfacial tension betweenphases to improve the thermal and mechanicalproperties.4,5 Recently, polyhedral oligomeric sil-sesquioxane (POSS) reagents offer a unique op-portunity for preparing truly molecularly dis-persed organic-inorganic nanocomposites, result-ing in promoting compatibility and adhesion incomponents. A typical POSS molecule, firstreported in 1946,6 has a well-defined cluster withan inorganic silica-like core (Si8O12) surroundedby eight organic corner groups, and has a generalformula R8Si8O12. POSS has been shown toenhance the mechanical properties, increase theirglass transition temperatures (Tgs), and improve

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 47, 1136–1147 (2009)VVC 2009 Wiley Periodicals, Inc.

Correspondence to: S. Chen (E-mail: [email protected])

1136

the thermal stabilities of polymers.7–10 POSS maybe incorporated into a polymer matrix via theclassic synthetic methods, copolymerization andgrafting, or by blending. For instance, POSS mol-ecules have been successfully incorporated intoacrylics,11,12 epoxies,13–15 styryls,9,16 poly-urethanes,17–19 and others.20–23

In the previous studies, most POSS moleculesused in POSS-containing composites were com-pletely condensed POSS (e.g., R8Si8O12) for thedesign of linear polymeric POSS systems. An effi-cient method for preparing such moleculesinvolves the corner-capping of incompletely con-densed POSS trisilanols with silane couplingagents containing functional groups for polymer-izations or grafting reactions.24,25 To the best ofour knowledge, there has been no precedent reporton the reaction of incompletely condensed POSStrisilanols with isophorone diisocyanate (IPDI). Inthis work, we functionalized phenyltrisilanolPOSS [Ph7Si7O9(OH)3, POSS-triol] with IPDI,and then incorporated into a poly(N-methylolacry-lamide) (PNMA) matrix (Seen in Scheme 1), andinvestigated the thermal properties of the POSS-based polymeric materials, along with the controlsample of PNMA/POSS polymer for comparison.

This work also introduces a novel route for thepreparation of POSS-based polymeric materialsvia frontal polymerization (FP). Typically, therates of polymerization for the conventional batchpolymerization (BP) are lower, which producespolymeric materials with time-consuming. UnlikeBP, herein, we present FP synthesis of POSS-based polymeric materials in fast fashion, alongwith well-defined microstructures. This chemical

conversion only occurs in a narrow localized reac-tion zone, known as the reaction front, allowingthe polymerization reaction can be accomplishedwith a rapid polymerization rate (reaction time\ 10 min). The fronts propagate through the cou-pling of thermal transport with the Arrhenius de-pendence of the kinetics of an exothermic polymer-ization. FP was first discovered by Chechilo andEnikolopyan in 1972 who studied methyl meth-acrylate (MMA) polymerization in adiabatic condi-tions under high pressure ([3000 atm).26 Subse-quently, this method was extended by Pojman andcoworkers to include numerous polymers.27–32

They demonstrated the achievability of FP for anumber of acrylic monomers and epoxy resinsat ambient pressure. Most of the FP work hasbeen performed on free-radical polymerizationsystem because they are usually highly exother-mic, and have a low rate of reaction at roomtemperature.

Mariani et al.33–36 demonstrated FP with ring-opening metathesis polymerization, diurethanediacrylates, epoxy resin-montmorillonite, and ep-oxy resin-POSS nanocomposites, and Fiori et al.37

produced polyacrylate/poly(dicyclopentadiene)networks frontally. Polyurethanes38 have recentlybeen prepared frontally and frontal atom transferradical polymerization has been achieved aswell.39

FP was also used for the preparation of tempera-ture-sensitive hydrogels,40 polyacrylamide,41

epoxy-acrylate IPNS,42 and other numerous poly-mer materials.43–45 Pojman et al. demonstrated FPwith thiol-ene systems46 and with a microencapsu-lated initiator.47 Vicini et al.48 developed a FPmethod for the consolidation of stone. Recently,

Scheme 1. Schematic synthesis of IPDI-POSS and PNMA-g-POSS hybrids. [Color fig-ure can be viewed in the online issue, which is available at www.interscience.wiley.com.]

FACILE AND QUICK SYNTHESIS OF PNMA/POSS 1137

Journal of Polymer Science: Part A: Polymer ChemistryDOI 10.1002/pola

Chen et al.49–57 reported that segmented PU,polyurethane-nanosilica hybrids, epoxy resin-poly-urethane hybrid networks, urethane-acrylatecopolymers, poly(hydroxyethyl acrylate), poly(N-methylolacrylamide), poly(N-methylolacrylamide)/polymethylacrylamide hybrids, and poly(N-vinyl-pyrrolidone) were synthesized by FP.

In the work described here, we report the syn-thesis of a series of new POSS-based polymers viaFP. The morphology and thermal properties ofPOSS-based hybrid polymers prepared via FP arecomparatively investigated on the basis of scan-ning electronic microscopy (SEM), differentialscanning calorimetry (DSC), and thermogravi-metric analysis (TGA).

EXPERIMENTAL

Materials

Isophorone diisocyanate, N-methylolacrylamide(NMA), the thermal initiator, ammonium persul-fate and dimethyl sulfoxide (DMSO), stannouscaprylate were used as received. Toluene wasstored over dry molecular sieves before use. Allthese reagents and solvents were supplied byAldrich. Phenyltrisilanol POSS (denoted POSS-triol) was obtained from Hybrid Plastics.

Synthesis of IPDI-POSS

A mixture of dried POSS-triol (1.0 g, 3.2 mmolequivalent to AOH group) and toluene (30.0 g)was charged to a four-necked flask, equipped witha stirrer, a thermometer, a reflux condenser, and anitrogen gas-introducing tube, and then furtherstirred until the mixture was completely dis-solved. Subsequently, IPDI (0.6 g, 4.8 mmol equiv-alent to ANCO group), containing 1.5 wt % of cat-alyst (stannous caprylate) based on the weight ofthe IPDI, was added slowly via syringe over a20-min period into a flask containing the POSS-triol. The reaction mixture was vigorouslystirred at 80 �C under a nitrogen atmospherefor about 6 h. Then, the reaction mixture wasprecipitated in an excess amount of toluene andseparated by centrifugation. The precipitatewas washed with toluene several times toremove the unreacted IPDI and catalyst, andthen the precipitated IPDI-POSS was placed ina vacuum oven at 50 �C for 2 days at a pressureof 10 kPa to remove the solvent.

Frontal Polymerization of PNMA-g-POSS Hybridswith Solvent

In all cases, monomer (NMA) concentration wasfixed, and all other reactants were based on mono-mer (wt/wt) expected where specified. The appro-priate amounts of NMA, IPDI-POSS, and ammo-nium persulfate were mixed together at ambienttemperature in DMSO in a flask. A typical compo-sition was IPDI-POSS/NMA ¼ 10–40% (wt/wt),ammonium persulfate/NMA ¼ 0.25–1.5% (wt/wt),and DMSO ¼ 50 wt %. The flask was stirred vigo-rously to obtain a homogeneous mixture. Then,the mixture was poured into a 10 mL (D ¼ 15mm) test tube. The reaction mixture was kept atabout 18–23 �C to slow bulk polymerization. Thefilled vessel was clamped into a holder � 1 cmfrom the top of the tube. The upper side of themixture was then heated by a soldering iron untilthe hot propagating front commenced.

Frontal Polymerization of PNMA-g-POSS Hybridswithout Solvent

NMA and IPDI-POSS were ground in a mill untila uniform powdered mixture was obtained, andthen the initiator was mixed in with an agatemortar and pestle. A typical composition wasIPDI-POSS/NMA ¼ 10% (wt/wt) and ammoniumpersulfate/NMA ¼ 1.5% (wt/wt). The powderswere packed into 10 mL (D ¼ 15 mm) glass tubesand fronts were initiated with a soldering iron. Aregion of solid polymer could be observed to prop-agate through the powdered monomer.

Velocity and Temperature Measurements

The velocity of the propagating front was deter-mined by measuring the distance that the fronttraveled as a function of time. When pure free-radical FP occurred, a constant velocity of thefront was attained. Temperature profiles of thepropagating front were measured by using a K-typethermocouple, the top of which was inserted intothe mixture at a fixed point from the free surface.After the completion of reaction, the samples wereremoved from the tube for further investigation.

Characterization

To further identify the chemical structures of theproducts, fourier transform infrared (FTIR) analy-sis was done using a Nicolet-6700 spectrometerfrom Thermo Electron at room temperature. Thepowders were ground into a dry KBr disk. In all

1138 FANG, CHEN, AND CHEN


cases, 32 scans at a resolution of 4 cm�1 wereused to record the spectra. Moreover, FT-Ramanspectra were also obtained by Nicolet-6700 FTIRspectrometer equipped with a NXR FT-Ramanmodule and the research-grade 2.0 w nd: yvo4laser with wavelength of 1064 nm. The spectrawere collected at a resolution of 4 cm�1.

Polymer morphologies were investigated with aQUANTA 200 SEM. Samples were cut to exposetheir inner structure and then coated with a layerof gold.

The Tgs of the samples were determined usinga NETZSCH 204F1 DSC at heating rate of 10 �C/min in the temperature range between 30 and300 �C in a nitrogen atmosphere. In all the meas-urements, the Tgs were taken from the secondscans.

The thermal property of products was deter-mined with thermogravimetric apparatus (Shi-madzu-TGA 50) in a nitrogen atmosphere with aheating rate of 10 �C/min from 30 to 800 �C. Sam-ples produced by FP with solvent were preparedby drying in a vacuum oven at 120 �C for 2 daysat a pressure of 10 kPa for solvent removal.

RESULTS AND DISCUSSION

Synthesis of IPDI-POSS

The synthesized IPDI-POSS was characterized byFTIR and FT-Raman spectroscopy. Shown in Fig-ure 1 are the FTIR spectra of pure POSS-triol andIPDI-POSS. It can be seen that the peak at 2265

cm�1 characteristic of the stretching vibration ofthe isocyanate group appeared for IPDI-POSS(seen in curve 1b), whereas the peak was almostnot observable for pure POSS-triol (seen in curve1a). The bending vibration absorption peak of theNAH group at 1556 cm�1 and the stretchingvibration absorption peak of the C¼¼O group at1640 cm�1 can be observed for IPDI-POSS. It isnoted that the intensity of the peak at 1027 cm�1

assigned to the stretching vibration of SiAOH forIPDI-POSS was significantly decreased. More-over, the stretching vibration peak of SiAOASi at1132 cm�1 for IPDI-POSS was observed. Figure 2shows FT-Raman spectra of (a) pure POSS-trioland (b) IPDI-POSS in the range of 3200–1300cm�1, respectively. By comparing Figure 2(a) with2(b), we found that the new peaks noticed at 2942and 1448 cm�1, attributing to a typical stretchingpeak of CAH group in IPDI six-membered ringsand C¼¼N group of IPDI, respectively.58 The FTIRand FT-Raman results indicate that the graftreaction between silanol hydroxyl and isocyanategroups was achieved.

Preliminary Experiments

Several preliminary experiments focused on howto prepare PNMA-g-POSS hybrids with solventby FP with a stable front but without the occur-rence of spontaneous polymerization (SP). Weassessed the pot life by preparing tubes with thereactants, leaving them at ambient temperature

Figure 1. FTIR spectra in the regions from 2000 to700 cm�1 recorded at room temperature of (a) purePOSS-triol and (b) IPDI-POSS. [Color figure can beviewed in the online issue, which is available atwww.interscience.wiley.com.]

Figure 2. FT-Raman spectra in the regions from3200 to 1300 cm�1 recorded at room temperature of(a) pure POSS-triol and (b) IPDI-POSS. [Color figurecan be viewed in the online issue, which is availableat www.interscience.wiley.com.]



and visually determining at what time they spon-taneously polymerized. We found that the mixtureof NMA, IPDI-POSS, and 1.5% (wt/wt) (based onthe amount of NMA) ammonium persulfate isinert at the ambient temperature (18–23 �C) formore than 2 h, but very reactive after beingheated for several seconds with a soldering iron.

Figure 3 shows a representative time series ofFP of polymers with DMSO ¼ 50 wt % and ammo-nium persulfate/NMA ¼ 1.5% (wt/wt) at differentconditions: (a) PNMA-g-POSS hybrid (IPDI-POSS/NMA ¼ 10% (wt/wt)), (b) PNMA/POSSpolymer (POSS/NMA ¼ 6.25% (wt/wt)), (c)PNMA/IPDI polymer (IPDI/NMA ¼ 3.75% (wt/wt)), and (d) PNMA/POSS/IPDI polymer (POSS/NMA ¼ 6.25% (wt/wt), and IPDI/NMA ¼ 3.75%(wt/wt)). The stable, self-sustaining fronts propa-gate at constant velocities for all cases. As seen inFigure 3, PNMA-g-POSS hybrid [Fig. 3(a)] showsa slow propagating front comparing with PNMA/POSS [Fig. 3(b)] and PNMA/IPDI hybrid [Fig.3(c)]. To compare with the chemical reaction ofNMA and IPDI-POSS, we also run a control sam-ple containing the mixture of NMA, POSS, andIPDI via FP for comparison. As seen in Figure3(c,d), the mixture of PNMA/POSS/IPDI polymerexhibits a yellow color, whereas the PNMA-g-POSS hybrid appears ivory-white. Moreover, bothsamples of the mixture of PNMA/POSS/IPDI poly-mer and PNMA-g-POSS hybrid are opaque.

Images of fronts with ammonium persulfate/NMA ¼ 1.5% (wt/wt) at different conditions: (a)PNMA, (b) PNMA/POSS polymer (POSS/NMA ¼10% (wt/wt)), and (c) PNMA-g-POSS hybrid(IPDI-POSS/NMA ¼ 10% (wt/wt)) without solventare given in Figure 4. Notice that ‘‘fingering’’occurs in pure PNMA [Fig. 4(a)]. The polymer isliquid at the front temperature and more dense,so it sinks. This causes the formation of ‘‘finger-ing’’ in the tube. Because of the ‘‘fingering’’ in sol-vent-free FP, no stable flat front exists and wecannot get the Vfront data.

To suppress the ‘‘fingering’’ of the monomer,POSS or IPDI-POSS was added. Figure 4(b)

shows the stable front of PNMA/POSS polymer.The interface between the polymer and unreactedmonomer can be barely seen. With the addition ofIPDI-POSS, the interface between the polymerand unreacted monomer can be clearly seen [seenin Fig. 4(c)]. The upper layer of mixture is PNMA-g-POSS hybrid and the lower layer is unreactedmixture.

The position of the front as a function of timewith DMSO ¼ 50 wt %, ammonium persulfate/NMA ¼ 1.5% (wt/wt), and IPDI-POSS/NMA ¼10% (wt/wt) is given in Figure 5. As can be seen,the experimental data are well fit by a straightline, meaning that a self-sustaining, constant-ve-locity front was obtained. Fronts were always per-formed in the descending mode to avoid buoy-ancy-driven convection.33

Another convenient way to verify the occur-rence of pure FP is given by the analysis of tem-perature profiles. Figure 6 shows the typical tem-perature profile of PNMA-g-POSS hybrid with FP.This experiment was done with DMSO ¼ 50 wt%, ammonium persulfate/NMA ¼ 1.5% (wt/wt),and IPDI-POSS/NMA ¼ 10% (wt/wt). In less than80 s, the temperature increases more than 100�C, and the Tmax is 126

�C. The constant tempera-ture in the region far from the incoming hot frontindicates that bulk polymerization is not occur-ring to a significant degree during front propaga-tion.

Effect of Initiator Concentration

Initiator concentration is an important factor inFP. To find the optimal ammonium persulfate con-centration for obtaining PNMA-g-POSS hybrid byFP, several runs were performed at different am-monium persulfate concentrations, varying from0.5 to 3.0% (wt/wt) (based on monomer), whileDMSO concentration was maintained at 50 wt %,and IPDI-POSS/NMA at a specific ratio 10% (wt/wt). For ammonium persulfate concentration lessthan 0.5% (wt/wt), no front propagated. Con-versely, for ammonium persulfate concentration

Figure 3. Sequence of images illustrating the constant-speed propagation of the po-lymerization front of polymers with DMSO ¼ 50 wt % and ammonium persulfate/NMA ¼ 1.5% (wt/wt) at different conditions: (a) PNMA-g-POSS hybrid (IPDI-POSS/NMA ¼ 10% (wt/wt)), (b) PNMA/POSS polymer (POSS/NMA ¼ 6.25% (wt/wt)), (c)PNMA/IPDI polymer (IPDI/NMA ¼ 3.75% (wt/wt)), and (d) PNMA/POSS/IPDI poly-mer (POSS/NMA ¼ 6.25% (wt/wt) and IPDI/NMA ¼ 3.75% (wt/wt)). (Inner diameterof the tube is 15 mm).



[3.0% (wt/wt), no front propagated. Therefore,the data were obtained for ammonium persulfateconcentrations between 0.5 and 3.0% (wt/wt).

Figure 7 shows the observed Tmax and front ve-locity of different ammonium persulfate concen-trations for the mixture. This experiment wascompleted by a constant concentration of DMSO(DMSO ¼ 50 wt %) and a specific ratio of IPDI-POSS/NMA (IPDI-POSS/NMA ¼ 10% (wt/wt)). Asalways has been seen in free-radical chain growthFP,46,59 the velocity monotonically increases withthe initiator concentration. Over the entire rangeof mass ratios, the velocity changes from 1.7 to 2.1cm/min with a curve approaching a maximumvalue for the initiator concentration of 3.0% (wt/

Figure 3.

Figure 4. Visual image of propagating front of (a)PNMA, (b) PNMA/POSS polymer (POSS/NMA ¼ 10%(wt/wt)), and (c) PNMA-g-POSS hybrid (IPDI-POSS/NMA ¼ 10% (wt/wt)) with solid monomers (inner di-ameter of the tube is 15 mm).



wt). It was observed that an increase of initiatorconcentration from 0.5 to 3.0% (wt/wt) led to anincrease of Tmax from 120 to 135 �C. And the Tmax

dependence on the initiator concentration levelsin a way very similar to the corresponding veloc-ity trend. We emphasize that our experimentswere performed under nonadiabatic conditionsand, for this reason, the increased velocityreduced the time for heat loss.

Effect of IPDI-POSS Concentration

We also investigated the effect of the IPDI-POSSconcentration on both Tmax and front velocity. Fig-ure 8 shows the behaviors of these parameters as afunction of IPDI-POSS concentration. As might beexpected, an increase of IPDI-POSS concentrationresulted in a decrease of Tmax from 138 to 98 �C forIPDI-POSS concentration ranging from 0 to 40%(wt/wt). The effect of IPDI-POSS concentration onfront velocity was more evident, going from 2.9 to0.9 cm/min. The decrease of Tmax was related tothe reduction in the heat-producing species and tothe decreased velocity that increases the time forheat loss under nonadiabatic conditions.

FTIR Spectra of PNMA-g-POSS Hybrids

The synthesized PNMA-g-POSS hybrids werecharacterized by FTIR spectroscopy. Shown inFigure 9 are the FTIR spectra of pure PNMA andPNMA-g-POSS hybrids (IPDI-POSS/NMA ¼ 10%(wt/wt)). It can be seen that the band at 1132cm�1 characteristic of stretching vibration of

SiAOASi group appeared for PNMA-g-POSShybrids (seen in curve 9b), whereas the band wasnot be present for pure PNMA (seen in curve 9a).Moreover, PNMA and PNMA-g-POSS hybrids ex-hibit both sharp bands at 1652, 1540, and 1020cm�1 because of C¼¼O, CAN, and CAH groups,respectively.60 This all indicates that PNMA-g-POSS hybrids by FP have different chemicalstructures than pure PNMA obtained by FP.

Morphology of Hybrids

The morphology of the organic-inorganic hybridswas investigated by means of SEM. Shown in Fig-ure 10 are the SEM micrographs of pure PNMA,PNMA-g-POSS hybrid, and PNMA/POSS poly-mer. In marked contrast to the PNMA-g-POSShybrid, the pure PNMA exhibits a featurelessmorphology [seen in Fig. 10(a)]. The PNMA-g-POSS hybrid shows an interesting hierarchicalstructure with dispersed particles on a microscale[seen in Fig. 10(b)]. The size of the microparticlesrange from about 500 nm to 1 lm can concludethat IPDI-POSS seems to be uniformly distrib-uted, and FP does not cause the particles to ag-gregate during the reaction. On the contrary, asseen in Figure 10(c), the SEM image of PNMA/POSS polymer shows serious aggregation andirregular structures than that of PNMA-g-POSShybrid, which might be attributed to phase sepa-ration. Figure 11 further indicates the SEMmicrographs of PNMA-g-POSS hybrids with dif-ferent concentrations of IPDI-POSS. As shown inFigure 11, with the concentration of IPDI-POSSof 20% (wt/wt), the well-dispersed functionalized

Figure 6. Typical temperature profile of PNMA-g-POSS hybrid prepared via FP at DMSO ¼ 50 wt %,ammonium persulfate/NMA ¼ 1.5% (wt/wt), andIPDI-POSS/NMA ¼ 10% (wt/wt).

Figure 5. Front position as a function of time ofPNMA-g-POSS hybrid prepared by FP at DMSO ¼ 50wt %, ammonium persulfate/NMA ¼ 1.5% (wt/wt),and IPDI-POSS/NMA ¼ 10% (wt/wt).



IPDI-POSS particles are still dominated withoutany aggregation in PNMA-g-POSS hybrids. How-ever, with the concentration of IPDI-POSS of 40%(wt/wt), the functionalized IPDI-POSS particlesbecome much more compact than that of the sam-ple (IPDI-POSS ¼ 20 wt %) because of higher den-sity functionalized POSS in the polymer hybrids.

Thermal Behavior of Hybrids

The POSS-containing polymeric materials aresubjected to thermal analysis. For each sample,two heating ramps were carried out ranging from30 to 300 �C (seen in Fig. 12). Here, the secondscans were done to determine Tg. As indicated inFigure 12, the Tg of the pure PNMA occurs at 239�C, whereas the Tg of PNMA-g-POSS hybrid withthe concentration of IPDI-POSS/NMA ¼ 10% (wt/wt) occurs at 260 �C, noticing that there is 21 �Cincrease in Tg after the addition of IPDI-POSS inpolymer hybrids. For comparison, the Tg of thecontrol sample of PNMA/POSS mixture (POSS/NMA ¼ 10% (wt/wt)) is 254 �C, which is less thanthat of PNMA-g-POSS hybrid. With increasingconcentration of IPDI-POSS in the polymerhybrids, a sharp Tg increase of PNMA-g-POSShybrids changing from 260 to 281 �C, which corre-sponds with IPDI-POSS concentration varyingfrom 10 to 40% (wt/wt), respectively, is obviouslyobserved. The result could be ascribed to the for-mation of well-defined microstructure dominatingin polymer hybrids, along with the occurrence ofcovalent bond between IPDI-POSS and PNMA,leading to the enhancement of the Tg.

The thermal stability of the hybrids was inves-tigated with TGA. Figures 13 and 14 show theTGA scans of the materials prepared via FP withsolvent, recorded in a nitrogen atmosphere. Theimprovement in thermal stability for PNMA byincorporating IPDI-POSS or POSS was observed.The initial thermal decomposition temperature(Td) is defined as the temperature at which amass loss of 5 wt % occurs. It is worth noticingthat the PNMA-g-POSS hybrids exhibited ahigher initial Td than the pure PNMA. In

Figure 8. Frontal velocity and Tmax of PNMA-g-POSS hybrids as a function of IPDI-POSS concentra-tion at DMSO ¼ 50 wt % and ammonium persulfate/NMA ¼ 1.5% (wt/wt) prepared by FP. [Color figurecan be viewed in the online issue, which is availableat www.interscience.wiley.com.]

Figure 9. FTIR spectra in the regions from 2000 to700 cm�1 recorded at room temperature of (a) purePNMA and (b) PNMA-g-POSS hybrids (IPDI-POSS/NMA ¼ 10% (wt/wt)). [Color figure can be viewedin the online issue, which is available at www.interscience.wiley.com.]

Figure 7. Frontal velocity and Tmax of PNMA-g-POSS hybrids prepared by FP versus concentrationsof initiator at DMSO ¼ 50 wt % and IPDI-POSS/NMA ¼ 10% (wt/wt). [Color figure can be viewedin the online issue, which is available at www.interscience.wiley.com.]



Figure 10. SEM micrographs of (a) pure PNMA, (b) PNMA-g-POSS hybrid(IPDI-POSS/NMA ¼ 20% (wt/wt)), and (c) PNMA/POSS polymer (POSS/NMA ¼ 20%(wt/wt)) prepared by FP at DMSO ¼ 50 wt %, ammonium persulfate/NMA ¼ 1.5%(wt/wt).

Figure 11. SEM micrographs of (a) PNMA-g-POSS hybrid (IPDI-POSS/NMA ¼20% (wt/wt)) and (b) PNMA-g-POSS hybrid (IPDI-POSS/NMA ¼ 40% (wt/wt)) pre-pared by FP at DMSO ¼ 50 wt %, ammonium persulfate/NMA ¼ 1.5% (wt/wt).



addition, the thermal stability of PNMA-g-POSShybrids above 250 �C was enhanced. Figure 14shows the TGA curves of hybrids with differentconcentration of POSS-triol, whereas at a con-stant concentration of IPDI (IPDI/monomer ¼3.75% (wt/wt)). As seen in Figures 13 and 14, the

increased concentration of IPDI-POSS or POSS-triol enhanced the thermal stability. Typical TGAresults for PNMA-POSS hybrids prepared by sol-vent-free FP at DMSO ¼ 50 wt %, ammoniumpersulfate/NMA ¼ 1.5% (wt/wt), and IPDI-POSS/NMA ¼ 20% (wt/wt) and by FP without solvent atammonium persulfate ¼ 1.5 wt %, IPDI-POSS/NMA ¼ 20% (wt/wt) are shown in Figure 15. By

Figure 12. DSC second scans of polymers preparedby FP with DMSO ¼ 50 wt % and ammonium persul-fate/NMA ¼ 1.5% (wt/wt) at different conditions (a)pure PNMA, (b) PNMA-g-POSS hybrid (IPDI-POSS/NMA ¼ 10% (wt/wt)), (c) PNMA/POSS polymer(POSS/NMA ¼ 10% (wt/wt)), (d) PNMA-g-POSShybrid (IPDI-POSS/NMA ¼ 20% (wt/wt)), and (e)PNMA-g-POSS hybrid (IPDI-POSS/NMA ¼ 40% (wt/wt)). [Color figure can be viewed in the online issue,which is available at www.interscience.wiley.com.]

Figure 13. TGA curves of (a) pure PNMA, (b)PNMA-g-POSS hybrids (IPDI-POSS/NMA ¼ 20% (wt/wt)), and (c) PNMA-g-POSS hybrids (IPDI-POSS/NMA ¼ 40% (wt/wt)) prepared by FP at DMSO ¼ 50wt %, ammonium persulfate/NMA ¼ 1.5% (wt/wt).[Color figure can be viewed in the online issue, whichis available at www.interscience.wiley.com.]

Figure 14. TGA curves of hybrids prepared by FPat DMSO ¼ 50 wt %, ammonium persulfate/NMA ¼1.5% (wt/wt), IPDI/NMA ¼ 3.75% (wt/wt) with differ-ent conditions: (a) POSS-triol/NMA ¼ 6.25% (wt/wt),(b) POSS-triol/NMA ¼ 12.5% (wt/wt), (c) POSS-triol/NMA ¼ 25% (wt/wt). [Color figure can be viewed inthe online issue, which is available at www.inter-science.wiley.com.]

Figure 15. TGA curves of hybrids prepared by (a)FP at DMSO ¼ 50 wt %, ammonium persulfate/NMA¼ 1.5% (wt/wt), IPDI-POSS/NMA ¼ 20% (wt/wt) and(b) solvent-free FP at ammonium persulfate/NMA ¼1.5% (wt/wt), IPDI-POSS/NMA ¼ 20% (wt/wt). [Colorfigure can be viewed in the online issue, which isavailable at www.interscience.wiley.com.]



comparison, PNMA-g-POSS hybrids prepared bysolvent-free FP show higher thermal stability inthe temperature range of the degradation step(between above 120 and 480 �C at a heating rateof 10 �C/min) than those prepared by FP with sol-vent, which might be attributed to the presence ofDMSO. As a heat sink, DMSO decreases the fronttemperature of the system and intermolecularcrosslinking in FP with solvent. In this case, weobtained PNMA-g-POSS hybrids with a differentdegree of polymerization, although PNMA-g-POSS hybrids by solvent-free FP have a uniformstructure with the same degree of polymerization.

CONCLUSIONS

An incompletely condensed POSS has been modi-fied by IPDI based on the reaction between silanolhydroxyl and isocyanate groups. The POSS-basedpolymeric materials have been prepared via FP ofN-methylolacrylamide (NMA) monomers in thepresence of modified phenyltrisilanol POSS[Ph7Si7O9(OH)3, POSS-triol]. We synthesized thePOSS-based polymeric materials via FP both withand without solvent. Constant velocity FP wasobserved without significant bulk polymerization.

The pure PNMA exhibits a featureless mor-phology, whereas a hierarchical morphology isobtained for the POSS-based polymeric materials.The products with different microstructures dis-play different thermal properties. It is noted thatall the POSS-containing polymeric materials dis-played the increase of Tg in comparison with thatof the pure PNMA. Moreover, higher IPDI-POSSconcentration resulted in an increase of Tg from239 to 281 �C with IPDI-POSS concentrationvarying from 0 to 40 wt %. The improvement inthermal properties has been ascribed to the micro-scaled dispersion of modified POSS in thesehybrid materials. Moreover, TGA characterizationindicates that POSS-based polymeric materialsprepared by solvent-free FP have higher thermalstability than those prepared by FP with solvent.Future work will focus on achieving furtherimprovement of thermal stabilities of PNMA andother polymers by tuning the relative amounts ofreaction components and the front polymerizationconditions.

This work was supported by Natural Science Founda-tions (NSFs) of China (Grant Nos. 20576053,20606016), NSF (NASA) of China (Grant No.10676013), and the NSF of the Jiangsu Higher Educa-

tion Institutions of China (Grant No. 07KJA53009). Theauthors are grateful for kind supply of phenyltrisilanolpolyhedral oligomeric silsesquioxane by Hybrid PlasticInc.

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facile and quick synthesis of poly(n-methylolacrylamide)/polyhedral oligomeric silsesquioxane graft...

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