preparation, tg improvement, and thermal stability enhancement mechanism of soluble poly(methyl...

Preparation, Tg Improvement, and Thermal StabilityEnhancement Mechanism of Soluble Poly(methylmethacrylate) Nanocomposites by Incorporating OctavinylPolyhedral Oligomeric Silsesquioxanes

HONGYAO XU,1,2 BENHONG YANG,2,3 JIAFENG WANG,2 SHANYI GUANG,1 CUN LI2

1College of Material Science and Engineering & State Key Laboratory of Chemical Fibers and PolymericMaterials Modification, Donghua University, Shanghai 200051, China

2School of Chemistry and Chemical Engineering, Anhui University, Hefei 230039, China

3Department of Chemical and Material Engineering, Hefei University, Hefei 230022, China

Received 28 March 2007; accepted 25 June 2007DOI: 10.1002/pola.22275Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: The soluble poly(methyl methacrylate-co-octavinyl-polyhedral oligomericsilsesquioxane) (PMMA–POSS) hybrid nanocomposites with improved Tg and highthermal stability were synthesized by common free radical polymerization and char-acterized using FTIR, high-resolution 1H NMR, 29Si NMR, GPC, DSC, and TGA. ThePOSS contents in the nanocomposites were determined based on FTIR spectrum,revealing that it can be effectively adjusted by varying the feed ratio of POSS in thehybrid composites. On the basis of the 1H NMR analysis, the number of the reactedvinyl groups on each POSS molecules was determined to be about 6–8. The DSC andTGA measurements indicated that the hybrid nanocomposites had higher Tg and bet-ter thermal properties than the pure PMMA homopolymer. The Tg increase mecha-nism was investigated using FTIR, displaying that the dipole–dipole interactionbetween PMMA and POSS also plays very important role to the Tg improvementbesides the molecular motion hindrance from the hybrid structure. The thermal sta-bility enhances with increase of POSS content, which is mainly attributed to theincorporation of nanoscale inorganic POSS uniformly dispersed at molecular level.VVC 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5308–5317, 2007

Keywords: nanocomposites; poly(methyl methacrylate); polysilsesquioxanes; syn-thesis; thermal properties

INTRODUCTION

Polymeric materials like poly(methyl methacry-late) (PMMA), polycarbonate and polystyreneare attractive for many routine applications

because of their excellent transparency, high mod-ulus, and relative ease of processing. However,their low glass transition temperatures (Tg) andrelatively poor thermal stabilities have limitedtheir further application in severer situations. Agood way to solve this problem is to develop or-ganic/inorganic composites by incorporating aninorganic fraction, particularly nanometer par-ticles, into the polymeric matrix by physical blend-ing. The resulting composites often show some

Correspondence to: H. Xu (E-mail: [email protected])

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 45, 5308–5317 (2007)VVC 2007 Wiley Periodicals, Inc.

5308

improvement in thermal and mechanical proper-ties compared with the parent polymers.1–8 How-ever, it is difficult for physical blending to dispersethe particles into polymer uniformly.

Polyhedral oligomeric silsesquioxane (POSS)is a special inorganic compound that has a well-defined cube-like core (Si8O12) with its eight ver-tices connected with organic functional groups9

and the type and number of the functionalgroup can be varied by molecular design. There-fore, POSS molecules are excellent monomersfor preparing organic/inorganic hybrid nanocom-posites with variety of architectures such as lin-ear, pendant, branched, star-shaped as well asnetwork. In these hybrid nanocomposites, bothinorganic and organic moieties are chemicallybonded to each other and nanoscale POSS canbe uniformly dispersed in the composites at mo-lecular level to result in effectively reinforce-ment of the thermal and mechanical propertiesof the materials. Many studies on POSS-contain-ing nanocomposite have been made and mostworks are focused on the linear or pendent poly-mers, such as poly(styrene-co-POSS),10 poly-(acrylate-co-POSS),11,12 poly(ethylene-co-POSS),13

poly(norbornene-co-POSS),14 poly(urethane-co-POSS),15,16 poly(epoxy-co-POSS),17,18 poly(vinyl-pyrrolidone-co-POSS),19,20 poly(acetoxystyrene-co-POSS),21 and so on. In view of the specific struc-ture of POSS with more than two functionalgroups, they are perfect structure-guidingagents for the synthesis of star-shaped or net-work polymers. Therefore, some researchers areshifting their interests toward this area, andsome efforts have been made so far.22–27 How-ever, these works mainly focused on the prepa-ration of hybrid nanocomposites and their prop-erties were little discussed. In this paper, wereport a novel and easy method of preparingsoluble PMMA hybrid nanocomposites by incor-porating octavinyl-POSSs using conventionalradical polymerization technique. Their thermalproperties were measured using DSC and TGA,and the Tg improvement and thermal stabilityenhancement mechanism was specially investi-gated and discussed.

EXPERIMENTAL

Materials

The vinyltriethoxysilane was purchased fromNanjing Shuguang, Nanjing, China. MMA

(Shanghai Reagent, Shanghai, China) was dis-tilled from hydroquinone and calcium hydrideunder reduced pressure, and stored in sealedampoules in a refrigerator. Azobis(isobutyroni-trile) (AIBN), also purchased from Shanghai Re-agent, was recrystallized from ethanol and keptin a dry box. Dry solvents THF and 1,4-dioxanewere predried over 4 A molecular sieves and dis-tilled from sodium/benzophenone immediatelyprior to use. Other reagents were used withoutfurther purification.

Synthesis of Octavinyl-POSS

Octavinyl-POSS was synthesized in our labora-tory according to the procedures described inRef. 28. Briefly, vinyltriethoxysilane (10.5 mL,0.05 mol) was dissolved in 24.5 mL of anhydrousalcohol with stirring and then some amount ofhydrochloric acid and 3 mL of water were addedto adjust solution to pH 3. The system wasallowed to react for 10 h at 60 8C under N2. Themixture was cooled down to room temperature.The white crystalline powder was filtered,washed with cyclohexane, and recrystallizedfrom tetrahydrofuran/methanol (1:3) to give 0.72 gwhite crystal powder.

FTIR (cm�1) with KBr powder: 1605 (CH¼¼CH), 1414, 1273(C��H), 1109(Si��O��Si); 1HNMR: (CDCl3, ppm) 5.58 (HHC¼¼CHb, 8H, dd,J ¼ 12.4 and J ¼ 15.0 Hz), 6.01(HHa0C¼¼CH,8H, dd, J ¼ 2.4 and J ¼ 12.4 Hz), 6.10 (Ha

HC¼¼CH, 8H, dd, J ¼ 2.4 and J ¼ 15.0 Hz).

Synthesis of PMMA–POSS Nanocomposites

The poly(MMA-co-octavinyl-POSS)s were pre-pared via conventional free radical polymer-ization technique as shown in Scheme 1. Allpolymerization reactions were carried out undernitrogen atmosphere using a standard Schlenkvacuum-line system. For comparison, purePMMA was also synthesized. In a typical reac-tion, 9.92 mmol of MMA and 0.08 mmol ofPOSS monomer in 5 mL dried 1,4-dioxane werestirred for 8 h at 70 8C under a nitrogen atmos-phere, using the AIBN initiator (1 wt % on thebasis of monomer). The product was then addeddropwise into heated ethanol under vigorouslyagitation to dissolve the unreacted POSS andMMA monomers and precipitate the nanocompo-site, since POSS and MMA could dissolve inheated ethanol but PMMA–POSS could not. Theprecipitated nanocomposite was filtrated out,

ENHANCEMENT MECHANISM OF PMMA BY POSS 5309

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

redissolved in 1,4-dioxane and reprecipitated inethanol. This purification procedure was re-peated three times to ensure thorough removalof the POSS and MMA in PMMA–POSS nano-composite. The product was dried in a vacuumoven. A 57.1 wt % yield was obtained throughthis procedure.

Characterization

FTIR spectra were measured with a spectral re-solution of 1 cm�1 on a Nicolet NEXUS 870FTIR spectrophotometer using KBr powder atroom temperature. 1H NMR spectra were re-corded on a Bruker AVANCE/DMX 300 spec-trometer using chloroform-d as the solvent. 29SiNMR spectra were measured on the BrukerDRX-400 spectrometer operating at resonancefrequency of 79.49 MHz. The weight-average(Mw) and number-average (Mn) molecularweights and polydispersity indices (PDI, Mw/Mn) were determined by a Waters 515 gel per-meation chromatograph (GPC). Differentialscanning calorimetry (DSC) analyses were per-formed on a TA Instruments DSC 9000 equippedwith a liquid nitrogen cooling accessory (LNCA)unit under a continuous nitrogen purge (50 mL/min). The scan rate was 20 8C /min within thetemperature range of 20–250 8C. The samplewas quickly cooled to 0 8C from the melt for thefirst scan and then scanned from 20 8C to250 8C at 10 8C /min. The glass transition tem-perature (Tg) was taken as the midpoint of the

specific heat increment. Thermogravimetricanalysis was carried out using a TA InstrumentsTGA 2050 thermogravimetric analyzer at aheating rate of 10 8C /min from 25 to 550 8Cunder a continuous nitrogen purge (100 mL/min). The thermal degradation temperature(Tdec) was defined as the temperature of 5 %weight lost.

Determination of POSS contents inPMMA–POSS Nanocomposites

The FTIR analysis was used to determine thePOSS contents in the PMMA–POSS nanocompo-sites. To establish a calibration curve, a series ofPOSS/PMMA mixtures with known amounts ofPOSS were prepared. To ensure intimate mix-ing, POSS and PMMA were first dissolved inTHF and then cast into thin films for FTIR test-ing. The carbonyl characteristic absorption peakof PMMA is at �1730 cm�1, while POSS has noabsorption band in the region. The characteristicabsorption peak of POSS is at �1109 cm�1

(Si��O��Si stretching absorption), which is par-tially overlapped with the band centered at 1145cm�1 (C��O��C stretching vibration). In orderto calculate the absorption peak area of POSS at1109 cm�1 (A1109), the Fit Gaussian program ofthe Origin Graphing and Data Analysis Soft-ware was performed. The calibration curve ofPOSS molar contents in the mixtures againstA1109/A1730 is plotted in Figure 1, showing a linearrelationship between POSS molar percentage

Scheme 1. Formation of PMMA–POSSnanocomposites via free radical polymerization.

5310 YANG ET AL.


(YPOSS) and A1109/A1730. Therefore, the POSS con-tents could be estimated by the calibration curvemethod using eq 1.

YPOSS ¼ 0:086ðA1109=A1730Þ � 0:0043 ð1Þ

HF-Treated Samples for GPC Analysis

The organic chains connected on or betweenPOSS cages were prepared by dissolving thesilica core (POSS) with HF and extracted forGPC analyses. In a typical procedure, 0.05 g ofthe PMMA–POSS nanocomposite was dissolvedin 5 mL of THF in a polytetrafluoroethylene bot-tle. 0.5 mL of 50% HF was dropped into the so-lution, and the mixture was stirred at room tem-perature for 72 h. Then, THF was removed byrotary evaporation. The residual material wasmixed with freshly dried dichloromethane toextract the organic components. The extractedsolution was washed twice with water and driedusing solid MgSO4 for 4 h, then was filtered andthe solvent in the filtrated solution was re-moved. A viscous liquid was obtained and dis-solved in 1 mL of THF for GPC analysis.

RESULTS AND DISCUSSION

Synthesis and Characterization ofPMMA–POSS Nanocomposites

Figure 2 shows the FTIR spectra of PMMA–POSS hybrid nanocomposite, as well as those ofpure POSS and PMMA for comparison. The

pure POSS possesses a strong absorption bandat �1109 cm�1, which is attributed to the char-acteristic Si��O��Si stretching vibration peak ofsilsesquioxane cages. The PMMA shows twocharacteristic absorption bands at 1730 and1145 cm�1, which are assigned to the carbonylstretching vibration and the strong C��O��Cstretching vibration absorption, respectively. Thestretching absorption bands of methylene andmethine groups are located at �2900 cm�1. Thespectra of the PMMA–POSS is rather similar tothat of the PMMA except that a double-peakbroadened band from 1150 to 1100 cm�1 appearsin all the spectra of PMMA–POSS nanocompo-sites, which may be from the overlap betweenthe characteristic Si��O��Si stretching peak ofPOSS cage and the C��O��C stretching peak ofthe PMMA. Meanwhile, we also found that theintensity of this broadened band increases withthe POSS feed ratio, indicating that the POSScage may have been incorporated into the poly-meric chains.

To further confirm that the POSS was indeedincorporated into the PMMA rather than as amixture, the following procedures were carriedout. The prepared PMMA–POSS hybrid polymerwas dissolved in THF to form a homogeneousand transparent solution. This solution wasthen added dropwise into cyclohexane, which isa poor solvent for the PMMA–POSS hybrid poly-mer but a good solvent for POSS. The pre-cipitated product was collected and dried invacuum. Such dissolution and precipitation pro-cedure was repeated more than three times. Thepurified product, characterized by FTIR, still

Figure 1. IR calibration curve for determining POSScontents in PMMA–POSS hybrid nanocomposites.

Figure 2. FTIR spectra of pure POSS, PMMA, andPMMA–POSS nanocomposites.



showed a broadened double-peak characteristicspectral absorption band in the region from1150 to 1100 cm�1 with almost the same absorp-tion intensity as before. Similar dissolution andprecipitation procedures were also carried outon a mixture of the pure PMMA and POSS. Thefinally obtained product does not display thebroadened double-peak absorption band but onlyshows a strong C��O��C stretching vibrationpeak, further confirming that the POSS macro-mer is indeed incorporated into the PMMApolymer to yield hybrid PMMA–POSS nanocom-posites. The result also implies that the band in-tensity at 1109 cm�1 can be utilized to deter-mine the POSS content of the hybrid polymersby using the PMMA carbonyl absorption band at1730 cm�1 as an internal reference. A calibra-tion curve (see Fig. 1) was plotted with A1109/A1730 ratio against the POSS mole content. Thedetermined POSS contents in the PMMA–POSSpolymers were listed in Table 1. It is clearlyshowed in Table 1 that the POSS content ofthese hybrid polymers increases with theincrease of POSS feed ratio, indicating that thePOSS contents in the hybrid nanocompositescan be effectively adjusted by changing thePOSS feed ratio.

The good solubility of the resultant PMMA–POSS nanocomposites in common solvents, suchas chloroform, 1,4-dioxane, and tetrahydrofuran,enables the characterization of molecular struc-tures with solution spectroscopic methods. Theproduct molecular weights (Mw and Mn), poly-dispersivity, and yields were determined andsummarized in Table 1. It can be seen in Table1 that the product yield decreases slightly from69.7 to 57.1 wt %, when the POSS was added

into the polymerization system with 0.81 mol %POSS feed ratio, while the polydispersity indexreduces drastically from 2.59 to 1.58. The prod-uct yield further decreases with increase of thePOSS feed ratio, such as yield 44.4% (Table 1,No. 6) for POSS feed ratio 3.18 mol %, implyingthat the incorporation of POSS reduces the yieldof hybrid polymer, which may be due to thesteric hindrance from the bulky POSS macro-mers. The similar phenomenon was previouslyobserved in the copolymerization of POSS withethylene,29 vinylpyrrolidone (VP) and acetoxyl-styrene (AS).13–15,30 However, differing from thepolymerization of AS with POSS with singlefunctional group,14 the molecular weight (Mn) ofthe product here (see Table 1) increases withthe POSS feed ratio in the polymerization ofMMA with POSS with eight vinyl functionalgroups, implying that most vinyl functionalgroups of POSS have participated in the poly-merization reaction to form star-shaped or net-work polymers.

Figure 3 shows 1H NMR spectra of POSS,PMMA, and PMMA–POSS (Table 1, No. 5) inchloroform-d solvent. For pure POSS macromer,the resonance absorption band of vinyl protonsis observed at �6.0 ppm as multiple peaksbecause of the coupling of hydrogen protons. Forpure PMMA, the resonance band at �3.6 ppm isattributed to the methyl proton connected to theester group. The proton resonance absorptionsof methylene and the substituted methyl groupsare at 1.4–2.0 ppm and 0.7–1.1 ppm, respec-tively. The PMMA–POSS hybrid nanocompositesdisplay a rather similar spectrum to that of thepure PMMA except that there is a wide reso-nance band nearby 6.0 ppm, which belongs to

Table 1. Effect of POSS Feed Ratio on the Properties of PMMA–POSS Nanocomposites

No

POSS (mol %)

Yield(wt %)

Mwb

(3 103 g/mol)Mn

(3 103 g/mol) PDI x Tgc Tdec

dChar Yield

(%)Feed

Mole RatioProduct

Mole Ratioa

1 0.00 0.00 69.7 29.5 11.4 2.59 / 109.5 192.4 02 0.17 0.05 63.4 25.3 12.1 2.09 8.0 114.7 185.2 13 0.81 0.31 57.1 24.1 15.3 1.58 7.3 124.6 209.7 24 1.46 0.84 52.3 24.7 17.3 1.43 6.9 130.3 256.1 65 2.41 0.98 49.5 34.2 21.5 1.59 6.7 133.6 276.8 96 3.18 1.36 44.4 35.3 20.5 1.72 5.8 138.5 284.6 13

a Data were obtained based on the IR standard curve.b Data were determined by GPC using the PS standard curve.c Data were gathered on the second melt using a heating rate of 10 8C/min.d Data were taken to be the temperature at 5% weight loss.

5312 YANG ET AL.


the unreacted vinyl groups of POSS cages. Theresonance bands of methine and methyleneprotons from the reacted vinyl groups of POSSsegments overlap from 0.7 to 2.0 ppm with thoseof substituted methyl and methylene protons inthe PMMA chains. Simultaneously, a new ab-sorption shoulder peak at 1.88 ppm was foundin the hybrid, which may be contributed frommethine proton substituted by POSS, and therelative intensity of the resonance absorptionband increases with increasing POSS feed ratioin all the spectra of PMMA–POSS hybrid nano-composites, further confirming that POSS mole-cules were indeed incorporated into the PMMAmatrix.

The proton resonance absorption area of eachcharacteristic group in the PMMA–POSS mole-cules holds a linear relationship with protonnumber of the group. Therefore, we can estimatethe number of the reacted vinyl groups (x) fromPOSS segments based on the 1H NMR spectraby the following formula.

x ¼ S2

S2 þ S138 ð2Þ

In formula 2, S1 and S2 represent the reso-nance absorption peak areas of hydrogen pro-tons of the unreacted (at �6.0 ppm) and thereacted vinyl groups (at �0.7–2.1 ppm) on thePOSS segment, respectively. The x is defined asthe number of reacted vinyl groups on POSScage and summarized in Table 1. From Table 1,

we see that about 6–8 vinyl groups from POSShave participated in the polymerization reaction,implying that the resulting PMMA–POSShybrids are possibly star-shaped or networkmolecules.

Figure 4 shows the 29Si NMR spectra of bothpure POSS macromer and PMMA–POSS nano-composite (0.98 mol % POSS). For pure octa-vinyl-POSS, there is only one resonance peak at�79.5 ppm, because all silicon atoms have thesame chemical environment in the POSS cages.For PMMA–POSS nanocomposite, there are tworesonance peaks at �78.9 and �65.8 ppm, whichare respectively attributed to the silicon atomsconnected to the unreacted and the reacted vinylgroups on the POSS segment. The characteristicband of unreacted vinyl group of POSS shifts tolow field by about 0.6 ppm when POSS is incor-porated into the PMMA. This phenomenon canpossibly be interpreted as a result of the interac-tion between POSS and PMMA. The relative in-tensity of the resonance peaks at �78.9 and�65.8 ppm implies that most of the vinyl groupshave participated in the polymerization reactionand only small amount reserved because thepeak area at �65.8 ppm is much bigger thanthat at �78.9 ppm. By calculating the peak arearatio, we estimate that about 6.7 vinyl groupson each POSS cage have been consumed in thepolymerization. This is consistent with whatwas calculated from 1H NMR spectra.

Generally, network structure is readilyformed during the copolymerization of MMA

Figure 3. 1H NMR spectra of pure POSS, PMMA,and PMMA–POSS with POSS content 0.98% (Table 1,No. 5).

Figure 4. High-resolution solid-state 29Si NMRspectra of pure POSS and PMMA–POSS with POSScontent 0.98% (Table 1, No. 5).



with multifunctional octavinyl-POSS. However,when comparing the reactivity and stability ofMMA with POSS free radicals initiated by theAIBN, we think the latter is less active andless stable than the former simply because ofthe strong steric hindrance from the bulkyPOSS, which has been observed in our previ-ous works.19,20 As a result, during the poly-merization, the MMA would first be initiatedand begin to form PMMA free radicals. Whenthese propagating PMMA radicals come intocontact with POSS free radicals, the termina-tion reactions occur, generating star-shapedpolymers rather than network ones. Theremay be a few cross-linkings occurring betweentwo propagating POSS free radicals or POSSfree radicals to form low cross-linking poly-mers. To support our presumption, we testedthe solubility of the resulting nanocompositesin several solvents such as CHCl3, THF, di-oxane, and so on. It turned out that thesenanocomposites could dissolve in some of thecommon solvents. This fact convinces us thatthe synthesized hybrid nanocomposites maymainly consist of star-shaped or low cross-link-ing structures rather than of large networkstructure since network polymers are hardlysoluble in any solvents.

To further study the polymer structure, wetreated our products with HF acid to dissolvethe POSS cores and leave behind PMMA chains,and then employed GPC to determine the molec-ular weight distribution of the PMMA chains.We found that most PMMA chains were of lowpolymerization degrees with molecular weightsaround 2000–3000, and only 10–25% of thechains were about 4000–5000 in molecularweight. It may indicate that most of the PMMAchains were the side chains of the POSS cages,and only small amount of them were the linkingchains between POSS cages. Based on the calcu-lation of the POSS contents in PMMA–POSScopolymers, we found that when POSS contentwas low (less than 0.31 mol %), there were only1–2 POSS in one PMMA–POSS copolymer, hint-ing that these PMMA–POSS copolymers mayhave star or linear structure. When POSS con-tent was higher (1.36 mol %), there are 2–4POSS in one PMMA–POSS, indicating thatPMMA–POSS copolymers may have linear orlow cross-linking structures, which is the mainreason that these hybrid PMMA composites con-taining octavinyl-POSS possess good solubilityin common solvent.

Thermal Properties and TgEnhancement Mechanism

Figure 5 gives the DSC thermograms of thepure PMMA and various PMMA–POSS hybridnanocomposites. The PMMA possesses a Tg at109.5 8C. With an incorporation of 0.05 mol %POSS content, the Tg of PMMA–POSS nanocom-posite increases to 114.7 8C, which is 5.2 8Chigher than that of the control PMMA. The Tg

of PMMA–POSS nanocomposite further in-creases with the increase of POSS content, forexample, the Tg becomes as high as 138.5 8Cwhen the POSS content reaches 1.36 mol %.

The thermal property of polymers is primarilyassociated with their structures. In star or lowcross-linking PMMA–POSS hybrid nanocompo-sites, PMMA chains are chemically bonded tothe POSS cores, which serve as the joint pointsfor all the PMMA chains, and therefore, largelyhinder the motion of PMMA chains in the poly-meric matrix. This may be the main reason forthe remarkable Tg improvement of the nanocom-posites. It is evident that more POSS coresmean more hindrance to the PMMA chains,accounting for the Tg increase with the increaseof POSS content.

Based on our previous study,29 however, wethink that the dipole–dipole interaction betweenPMMA chains and POSS cores may be anotherfactor that affects the Tg temperature. To fur-ther investigate Tg increase mechanism, theFTIR characterization of these PMMA–POSSnanocomposites was carried out. Figure 6 showsthe expanded FTIR spectra of pure PMMA andvarious PMMA–POSS nanocomposites rangingfrom 1780 to 1680 cm�1. The pure PMMA shows

Figure 5. DSC thermograms of pure PMMA andPMMA–POSS nanocomposites.

5314 YANG ET AL.


its carbonyl absorption at �1730 cm�1. When0.05 mol % POSS is incorporated into thePMMA, the carbonyl absorption slightly shiftsto lower wave number at �1728 cm�1. This isbecause the presence of POSS at a lower contentreduces the dipole–dipole interaction betweenPMMA chains, here POSS actually plays a roleof diluent spacer in the polymeric matrix. How-ever, compared with the effect from the chainmotion hindrance from POSS cores, this diluenteffect is insignificant; thus, we still observe a Tg

increase in PMMA–POSS0.05 hybrid. This isdifferent from our previous study in PAS–POSSsystem containing single functional POSS,30

where Tg decreases when POSS content is rela-tively low.

When the POSS content increases to 0.31mol %,the characteristic peak of carbonyl group begins toshift toward higher wave number and furthershifts with the increase of POSS content, indicat-ing that the dipole–dipole interaction betweenPMMA and POSS molecules exists in hybrid com-posite and becomes more remarkable in hybridwith higher POSS content. The interaction effecthas surpassed the diluting effect of POSS andyields positive contribution to the Tg increase ofthe hybrid. We suggest that the Tg increase ofthese hybrid nanocomposites containing octa-vinyl-POSS are attributed to the dual contribu-tions of dipole–dipole interaction between PMMAand POSS, and PMMA chain motion hindranceresulting from nanoscale POSS cores.

Figure 7 shows the TGA thermograms of purePMMA and various PMMA–POSS hybrid nano-

composites. The temperatures of 5 % weight loss(Tdec) and the char yield are listed in Table 1.From Figure 7, it is seen that both pure PMMAand PMMA–POSS0.05 show four-step degrada-tions at 191, 230, 292, and 364 8C, respectively.It could be explained as follows.

Similar to the termination reactions occurredin the synthesis of PMMA, when the free radicalpolymerization is employed to prepare PMMA–POSS polymer, the polymerization reactioncould be terminated either by the coupling reac-tion between two chain radicals, in whichPMMA–POSS-1 would be generated (see Scheme2), or through the disproportionate reaction, inwhich PMMA–POSS-2 and PMMA–POSS-3would be obtained. In PMMA–POSS-1, the C��Cbond formed through the coupling of two terti-ary carbon radicals is very weak because of thesteric hindrance to induce effect from the estergroups. It begins to break at about 190 8C.31

The PMMA–POSS-2 shows a two-step thermaldecomposition.32 One happens at �220 8C, dueto the depolymerization initiated by the endC¼¼C bonds. The other begins at �280 8C, re-sulting from the random degradation of themain chains. The PMMA–POSS-3 with an offsetdecomposition temperature at �360 8C, has muchhigher thermal stability than both PMMA–POSS-1 and PMMA–POSS-2 because it pos-sesses a more stable saturated C��C bond in thechain end.

From Figure 7, we also see that the thermalproperties of the PMMA–POSS nanocompositeswere improved with the increase of POSS con-tents. When the POSS content reaches 0.31 mol %,

Figure 7. TGA thermograms of pure PMMA andPMMA–POSS nanocomposites.

Figure 6. Expanded FTIR spectra in the regionfrom 1780 to 1680 cm�1 of pure PMMA and PMMA–POSS nanocomposites.



the PMMA–POSS nanocomposite decomposes intwo steps at higher temperatures, correspondingto the C¼¼C-initiated depolymerization and thedecomposition of the main chains, displayinghigher thermal stability than pure PMMA andPMMA–POSS0.05. This may be due to incorpora-tion of inorganic POSS cores which enhance thethermal stability of hybrid composites. With thefurther increase of POSS content, the nanocompo-sites become more difficult to decompose (higherTdec). For example, nanocomposites, PMMA–POSS0.84, PMMA–POSS0.98 and PMMA–POSS1.36, show only one turning point in theTGA thermograms, and their Tdec increase to264.1, 276.8, 284.6 8C, respectively. This resultfurther confirms that incorporation of uniformlydispersed inorganic POSS at molecular level effec-tively enhances the thermal stability of PMMAnanocomposites, which provides a good way toimprove the thermal stability of PMMA. This isconsistent with the result of DSC analysis.

CONCLUSIONS

The soluble poly(MMA-co-octavinyl-POSS) hybridwere synthesized by common free radical poly-

merization. The structure and properties of thenanocomposites were characterized using FTIR,high-resolution 1H NMR, 29Si NMR, GPC, DSC,and TGA. The results show that the hybridnanocomposites have higher Tg and better ther-mal property than the parent PMMA homopoly-mer, and the Tg increases and thermal stabilityenhances with the increase of POSS content inthe nanocomposites. The Tg improvementresults from dual contributions of the motionhindrance of PMMA chain imposed by the nano-meter POSS cores and the dipole–dipole interac-tion between PMMA chains and POSS cores.The thermal stability enhancement is mainlyattributed to incorporation of nanoscale inor-ganic POSS uniformly dispersed at molecularlevel.

This research was financially supported by theNational Natural Science Fund of China (grant nos.90606011 and 50472038), the Excellent Youth Fundof Anhui Province (grant no. 04044060), and theProgram for New Century Excellent Talents in Uni-versities (NCET-04-0588) and the Award for High-level Intellectuals (grant no. 2004Z027) from AnhuiProvince.

Scheme 2. Three types of PMMA chain structure in PMMA–POSS nanocomposites.

5316 YANG ET AL.


REFERENCES AND NOTES

1. Loy, D. A. Hybrid Org-Inorg Mater 2001, 26, 364.2. Kojima, Y.; Usuki, A.; Kawasumi, M. J Mater Res

1993, 8, 185.3. Sanchez, C.; Lebeau, B. MRS Bull 2000, 26, 77.4. Pyun, J.; Matyjaszewski, K. Chem Mater 2001,

13, 3436.5. Matejka, L, Dukh, O. Macromol Symp 2001, 171,

181.6. Scott, B. J.; Wirnsberger, G.; Stucky, G. D. Chem

Mater 2001, 13, 3140.7. Rajeshwar, K.; de Tacconi, N. R.; Chenthamarak-

shan, C. R. Chem Mater 2001, 13, 2765.8. Gomez-Romero, P. Adv Mater 2001, 13, 163.9. Baney, R. H.; Itoh, M.; Sakakibara, A.; Suzuki, T.

Chem Rev 1995, 95, 1409.10. Haddad, T. S.; Lichtenhan, J. D. Macromolecules

1996, 29, 7302.11. Lichtenhan, J. D.; Otonari, Y. A.; Carri, M. J.

Macromolecules 1995, 28, 8435.12. Pyun, J.; Matyjaszewski, K. Macromolecules

2000, 33, 217.13. Zheng, L.; Farris, R. J.; Coughlin, E. B. Macro-

molecules 2001, 34, 8034.14. Mather, P. T.; Jeon, H. G.; Haddad, T. S.; Lichten-

han, J. D. Macromolecules 1999, 32, 1194.15. Fu, B. X.; Zhang, W.; Hsiao, B. S.; Johansson, G.;

Sauer, B. B.; Phillips, S.; Balnski, R.; Rafailovich,M.; Sokolov, J. Polym Prepr 2000, 41, 587.

16. Fu, B. X.; Hsiao, B. S.; Pagola, S.; Stephens, P.;White, H.; Rafailovich, M.; Sokolov, J.; Mather, P.T.; Jeon, H. G.; Phillips, S.; Lichtenhan, J.;Schwab, J. Polymer 2001, 42, 599.

17. Lee, A.; Lichtenhan, J. D. Macromolecules 1998,31, 4970.

18. Liu, H. Z.; Zheng, S. X.; Nie, K. M. Macromole-cules 2005, 38, 5088.

19. Xu, H. Y.; Kuo, S. W.; Huang, C. F.; Chang, F. C.J Appl Polym Sci 2004, 91, 2208.

20. Xu, H. Y.; Kuo, S. W.; Lee, J. S.; Chang, F. C.Polymer 2002, 43, 5117.

21. Xu, H. Y.; Kuo, S. W.; Huang, C. F.; Chang, F. C.J Polym Res 2002, 9, 239.

22. Pyun, J.; Matyjaszewski, K. Macromolecules 2000,33, 217.

23. Choi, J.; Harcup, J.; Yee, A. F.; Zhu, Q.; Laine, R.M. J Am Chem Soc 2001, 123, 11420.

24. Lucke, S.; Stoppek-Langner, K.; Kuchinke, J.;Krebs, B. J. Organo Chem 1999, 584, 11.

25. Fasce, D. P.; Williams, R. J. J.; Ishikawa, Y.; Non-ami, H. Macromolecules 2001, 34, 3534.

26. Zhang, C. X.; Babonneau, F.; Bonhomme, C.;Laine, R. M.; Soles, C. L.; Hristov, H. A.; Yee, A.F. J Am Chem Soc 1998, 120, 8380.

27. Tamaki, R.; Tanaka, Y.; Asuncion, M. Z.; Choi,J.; Laine, R. M. J Am Chem Soc 2001, 123,12416.

28. Harrison, P. G.; Hall, C.; Kannengiesser, R. MainGroup Met Chem 1997, 20, 515.

29. Xu, H. Y.; Kuo, S. W.; Lee, J. S.; Chang, F. C.Macromolecules 2002, 35, 8788.

30. Xu, H. Y.; Yang, B. H.; Wang, J F.; Guang, S. Y.;Li C. Macromolecules 2005, 38, 10455.

31. Holland, B. J.; Hay, J. N. Polym Degrad Stab2002, 77, 435.

32. Peterson, J. D.; Vyazovkin, S.; Wight, A. J PhysChem B 1999, 103, 8087.



preparation, tg improvement, and thermal stability enhancement mechanism of soluble poly(methyl...

Documents