Influence of Polyhedral Oligomeric Silsesquioxanes onThermal and Mechanical Properties of Polycarbonate/POSS Hybrid Composites

H.L. Cai,1,2 K. Xu,1 H. Liu,1,2 X. Liu,1,2 Z.E. Fu,1,2 M.C. Chen11Guangdong Provincial Key Laboratory of Organic Polymer Materials for Electronics,Guangzhou Institute of Chemistry, Chinese Academy of Sciences, Guangzhou 510650, China

2Graduate School of the Chinese Academy of Sciences, Beijing 100049, China

Two types of silsesquioxanes were synthesized byhydrolytic condensation reaction, and then were incor-porated into polycarbonate (PC) matrix by melt blend-ing to prepare PC/POSS hybrid composites. The studyof morphology of the composites showed that octa-phenylsilsesquioxane (PH-POSS) exhibited partial com-patibility with PC matrix, while 3-glycidyloxypropylsil-sesquioxane (EP-POSS) could react with phenolichydroxyl groups of matrix. Thermal and mechanicalproperties were studied by DSC, TGA, and DMA. Theresult showed that the incorporation of POSS not onlyimproved thermal stabilities of PC composites, butalso retarded their thermal degradation. Si��O fractionsleft during POSS degradations were the key factor gov-erning the formation of a gel network layer on the exte-rior surface. This layer possessed more compact struc-tures, higher thermal stabilities, and some thermalinsulation. In addition, percentage residues at 7008C(C700) significantly increased from 10.8 to 15.5–22.8% inair. The storage modulus of two series of compositeswas slightly improved up to 908C; furthermore, thetemperature range of the rubbery state of them shiftedto high temperature. POLYM. COMPOS., 32:1343–1351,2011. ª 2011 Society of Plastics Engineers

INTRODUCTION

Recently, preparation of organic/inorganic hybrid com-

posites has drawn considerable interests because of their

extraordinary properties, which were derived from the

synergism between inorganic nanoparticles and organic

molecules [1, 2]. This class of materials has potential to

be used in a number of fields, including OLED, biomate-

rials, coating, sensors, medicine, etc [3, 4].

Polyhedral oligomeric silsesquioxanes (POSS) are a

class of organic/inorganic molecules with the general for-

mula (RSiO3/2)n where n is commonly 8, 10, or 12, etc;

and has a cage-like structure with 1–3 nm in size, which

contains a thermally stable inorganic Si��O��Si core sur-

rounded by substituent R where R can be hydrogen, alkyl,

alkylene, aryl, arylene groups, or organic functional deriv-

atives thereof [4–7].

In literatures, POSS have often been referred to as

nanoparticles, similar with well known nanofillers such as

silica, layered silicates, metal oxides, and hydroxides.

However, it is certainly more suitable to refer to POSS as

hybrids at a molecular level, which are great different

from organic-functionalized nanofillers. Because of their

perfectly defined chemical structures and peculiar proper-

ties, POSS have been widely used in a field of hybrid

nanocomposites [4, 6, 8–10].

Polymer/POSS hybrid nanocomposites can be preparedthrough grafting, copolymerization, and melt blending [6,8–10]; and properties of resulting materials greatly dependon methods of incorporation, chemical and physical prop-erties of substituents of POSS, loading contents of POSS.Methods of melt blending is very appealing, economic,and environmentally friendly processes for the industrialdevelopment.

Bisphenol A polycarbonate (PC) is one of the most

widely used engineering thermoplastics owing to its

excellent properties, such as transparency, high impact

strength, good thermal stability, and flame retardancy

[11]. However, attempt of preparation of PC with better

performance has drawn great interests of scientists and

engineers. Zhao and Schiraldi [12] first used melt blend-

ing to prepare various PC/POSS hybrid composites, which

showed differences in compatibility depending on the fil-

ler specific structure; trisilanolphenyl-POSS (TPOSS) pro-

vided better compatibility with PC than fully saturated

cage structures, and phenyl-substituted POSS was more

compatible with PC than the corresponding isoctyl-substi-

Correspondence to: K. Xu; e-mail: xk@gic.ac.cn

Contract grant sponsor: Natural Science Foundation of Guangdong Prov-

ince, China; contract grant number: 10151065004000006.

DOI 10.1002/pc.21156

Published online in Wiley Online Library (wileyonlinelibrary.com).

VVC 2011 Society of Plastics Engineers

POLYMER COMPOSITES—-2011

tuted molecule. PC/TPOSS composites possessed the best

overall performance among the POSS materials tested;

slightly enhanced mechanical properties including tensile

and dynamic mechanical modulus were observed with the

increase of TPOSS loading at the cost of decreasing duc-

tility of the nanocomposites. Song and coworkers [13, 14]

studied thermal and flame retardant properties of PC/

TPOSS hybrids, PC/TPOSS/oligomeric bisphenyl A bis

(dipenyl phosphate) (BDP) hybrids. It was found that

TPOSS significantly affected the thermal degradation pro-

cess of PC matrix; and the value of peak heat release rate

of the hybrids decreased; in addition, combination of

TPOSS with BDP in appropriate ratio enhanced thermal

stability and flame retardant properties of PC matrix.

These results were probably due to enhancement of the

thermal stability of matrix and fire-resistance of the char

layer. Liu et al. [15] reported that the combination of the

flame retardancy containing potassium diphenylsulfone

sulfonate, poly(aminopropyl/phenylsilsesquioxane), and

poly(vinylidenefluoride) decreased the activation energy

of PC degradation and elevated the thermal degradation

rate of PC to ensure the formation of an insulating carbon

layer.

However, it has been still ambiguous that how POSS

influence PC degradation behavior and few literatures

have focused on the research about PC hybrids with

POSS containing reactive substituents. In this work, octa-

phenylsilsesquioxane (PH-POSS), and 3-glycidyloxypro-

pylsilsesquioxane (EP-POSS) was synthesized and then

was incorporated into PC matrix by melt blending. Ther-

mal and mechanical properties of hybrid composites were

studied. Moreover, thermal degradation behavior and ther-

mal oxidative degradation behavior of PC/POSS hybrid

composites were also discussed for further understanding

corresponding mechanism.

EXPERIMENTAL

Materials

All reagents and solvents were purchased as reagent

grade, and were purified or dried by standard methods

before use. Tetramethylammonium hydroxide (TMA) pen-

tahydrate, ethanol, acetone, potassium hydroxide, benzene

and toluene were purchased from Aladdin-reagent, Shang-

hai, China. Petroleum ether (60–908C), magnesium sulfate

dehydrate and isopropanol were purchased from Sino-

pharm Chemical Reagent, China. The 3-glycidyloxypro-

pyltrimethoxysilane (EPSi(OCH3)3) was purchased from

Alfa Aesar1, China. Phenyltrichlorosilane (98%)

(PHSiCl3) was purchased from Shanghai Nuotai Chem.,

and used without further purification. Polycarbonate (S-

2000VR) made in Thailand Mitsubishi was purchased

from Dongguan LiAoSuJiao materials, Guangdong, China.

Prior to compounding, PC pellets were dried for at least 6

h under vacuum at 1008C.

Preparations of Two Kinds of POSS and CorrespondingComposites

Figure 1 shows different structures of silsesquioxanes

and synthetic scheme of two POSS. PH-POSS was pre-

pared by following Brown’s method [16] with PHSiCl3. It

is white crystal powder with melt point 4928C.The synthesis of EP-POSS was referred to the litera-

ture [17]. 0.9 L isopropanol, 190.4 g EPSi(OCH3)3 and 47

g 5% TMA solution in water were added into a 2-L

three-necked round bottom flask equipped with a mechan-

ical stirrer. Then the mixture was stirred rapidly at room

temperature for 3 h. After removing solvents by rotary

evaporator, 1.5 L toluene was added to dissolve hydroly-

sis products. The solution was washed with water until

neutral and dried over magnesium sulfate dehydrate. The

resulting solution and 16 g 10% TMA solution in water

were mixed and refluxed under nitrogen in a three-necked

round bottom flask equipped with a water segregator and

a condenser for another 8 h. After washing, drying, and

evaporating solution, 129.6 g (94%) colorless viscous liq-

uid was obtained.

Polycarbonate was first blended with different contents

of POSS using a twin-screw co-rotating microcompound-

ing extruder (TE-35, made in Nanking, China). The

FIG. 1. Synthesis scheme of POSS and the silsesquioxanes with differ-

ent structures.

1344 POLYMER COMPOSITES—-2011 DOI 10.1002/pc

extruded material was pelletized and then dried under

vacuum at 1008C for 8 h. Finally, the dried pellets were

injection molded into different sizes for mechanical test-

ing using an injection molding machine (HTF86 3 1,

made in Ningbo, China). Different PC composites denoted

as PH0.5, PH1, PH2, PH5, EP0.5, EP1, EP2, and EP5,

where PH, EP and the numbers represented PC/PH-POSS

composite, PC/EP-POSS composite and the weight per-

centage of corresponding POSS loading, respectively.

Characterizations and Measurements

Structure Characterization. Fourier transform infrared

(FT-IR) spectrums were recorded with an Analect RFX-

65A infrared spectrophotometer using KBr discs (t:stretching vibration, d: deformation vibration, tas: anti-

symmetric stretching vibration, tsy: symmetric stretching

vibration). 1H and 29Si nuclear magnetic resonance

(NMR) spectrums were measured on Bruker AM-400S

(400 MHz) spectrometer using tetramethylsilane (TMS) as

internal standard and chloroform-d (at 7.26 ppm) as sol-

vent. Gel permeation chromatography (GPC) was carried

out at room temperature on a Waters 1515 Isocratic

HPLC apparatus equipped with two Styragel1

HR3, HR4

columns (the line arrange of molecular weight: 500–

600,000) and a Waters 2414 refractive index detector.

THF was used as an eluent at a flow rate of 1 mL min21.

Polystyrene standards with a narrow distribution of mo-

lecular weight (Mw: 1,200–660,000) were used for molec-

ular weight calibration.

Morphology Analysis. Transmission electron micros-

copy (TEM) images were obtained with a JEOL JEM-

100CX II microscope at an acceleration voltage of 100

kV. The samples for TEM were cut at room temperature

using a Leica ULTRACUT-1 ultramicrotome with a dia-

mond knife. Polarized optical microscopy (POM) images

were obtained with a Nikon E600POL Polarizing Micro-

scope. Injection composites were redissolved in CH2Cl2and the solutions were stirred for 5 h. The composite

films with 0.5 mm thickness were prepared using a draw-

down bar on a clean glass slide. The films were dried at

room temperature for 1 day, followed by additional dry-

ing under vacuum at 808C for another 8 h to remove any

residual solvent. Surface contact angle (SCA) was meas-

ured with a Dataphysics OCA 40 Micro contact angle me-

ter at room temperature. Three points of each film were

tested, and then averaged results were recorded using

water and diiodomethane as probe liquids. The surface

energy of composite films was calculated by Owens-

Wendt-Rabel-Kaelble (OWRK) method [18].

Characterizations of Thermal Properties. Differential

scanning calorimetry (DSC) was recorded with a Dia-

mond/Pyris DSC analyzer under nitrogen from 20 to

3008C at a heating rate of 208C min21. The thermogravi-

metric analysis (TGA) measurement was performed with

a Perkin-Elmer TGA-6 thermogravimetric analyzer over a

temperature range of between 50 and 7508C at a heating

rate of 108C min21 under nitrogen or air. T5%, T10%, and

Tmax represent the temperature of the 5%, 10% and maxi-

mum mass loss rate of various samples, respectively. C700

represents the percentage residue of various samples at

7008C.

Dynamic Mechanical Thermal Analysis. The thermo-

mechanical properties of composites were measured using

dynamic mechanical analysis (DMA). Rectangular sam-

ples with approximate dimensions of 60 mm 310 mm

34 mm were tested using a NETZSCH DMA 242C in

three-point bending mode at 1 Hz with a deflection of 7.5

lm while ramping the temperature from 25 to 1858C at a

rate of 38C min21. The temperature at which the peak in

the loss angle tangent occurred was considered to be the

glass transition temperature (Tg-DMA).

RESULTS AND DISCUSSION

Characterization of EP-POSS

Generally, POSS were derived from hydrolytic conden-

sation of corresponding trifunctional monomers RSiX3

with X ¼ Hal, OH, OR, etc, and thus usually contain

products of partial condensation like TPOSS and products

of complete condensation with T8, T10, or T12 structure

(see Fig. 1) [19, 20]. As shown in Fig. 2, EP-POSS pre-

pared from EPSi(OCH3)3 had no characteristic peaks at

around 3600 cm21 assigned to Si��OH; in addition, there

was also no signals of H atom corresponding to Si��OH

in 1H NMR spectrum (see Fig. 3). These mean that

EPSi(OCH3)3 underwent the reaction of complete conden-

sation and mostly was transformed into the products of

complete condensation. In FT-IR spectrum, characteristic

absorptions at 3,000–2,800, 1,479–1,342, 1,255, 1,199,

and 908 cm21 were assigned to tC��H, dC��H, tSi��C,

tC��O��C and tepoxy, respectively; and there ware two

FIG. 2. FT-IR spectrum of EP-POSS.

DOI 10.1002/pc POLYMER COMPOSITES—-2011 1345

new absorption peaks at 1,103 and 1,039 cm21, which

were assigned to tSi��O��Si. According to article [16],

the absorption peak at around 1,040–1,050 cm21 would

disappear when random silsesquioxanes completely

changed into cage-like silsesquioxanes. This shows that

EP-POSS was a mixture of various silsesquioxanes. In

Fig. 3, the assignment and integral value of the peaks

were collected as follow: 29Si NMR: 266.57, 268.36,

270.73 ppm, 1H NMR: Ha 0.58 ppm (2H), Hb 1.61 ppm

(2H), Hc 3.41 ppm (2H), Hd 3.30 ppm (1H), He 3.66 ppm

(1H), Hf 3.09 ppm (1H), Hg 2.73 ppm (1H), Hh 2.54 ppm

(1H). The results of 1H NMR spectrum fit well with the

theoretical value, which implies that epoxy groups did not

take part in a reaction which usually occurred in acidic

conditions; moreover, 29Si NMR spectrum was a multi-

peak at around 268.36 ppm, also indicating that EP-

POSS was a mixture of silsesquioxanes with different

structures. From GPC chart of EP-POSS (see Fig. 4),

there were three peaks with polydispersity index 1.01,

1.01, and 1.14, respectively. The number average molecu-

lar weights (Mn) of three peaks correspond to 1484, 2420,

and 4561 Da. The first two of them were very close to

the theoretical molecular weight of fully condensed prod-

ucts with T8 (1,337) or T14 (2,431) structure. However,

27% of EP-POSS assigned to third peak was random sil-

sesquioxanes. Accordingly, EP-POSS was a mixture of

T8, T14, and random silsesquioxanes, and they occupied

54, 19, and 27% of the total mass, respectively.

Morphologic Analysis

After the injection moulding, the specimens were visu-

ally examined looking for qualitative differences in opti-

cal clarity of PC composites (shown in Fig. 5). PH0.5 and

PH1 were transparent as PC, while PH2 and PH5 became

translucent. POM (see Fig. 6) shown that PH-POSS was

FIG. 3. 1H NMR spectrum and the 29Si NMR spectrum (the insert) of

EP-POSS.

FIG. 4. GPC chart of EP-POSS.

FIG. 5. Photographs of composites of PH-POSS/PC and EP-POSS/PC.

1346 POLYMER COMPOSITES—-2011 DOI 10.1002/pc

distributed in crystal in a range of 1–10 lm in PH2 and

PH5; but little crystal was observed in PH1, even no crys-

tal in PH0.5. Therefore, it is probable that PH-POSS was

partially compatible with PC. When loading content under

0.5%, PH-POSS dispersed with clusters of the molecules;

however, it would form crystal grains with the increase of

loading. All PC/EP-POSS composites were all white opa-

que. This means that the sizes of EP-POSS domains

extend into visible wavelength ranges.

As shown in Fig. 7, POSS domains were distributed in

matrix with a size range of 0.1–0.5 lm. EP-POSS domains

were ellipsoidal particles with a smooth boundary in EP5.

FIG. 6. POM images of PH-POSS/PC composites. PH0.5: (a), PH1: (b), PH2: (c), PH5: (d).

FIG. 7. TEM images taken at low resolution (a,c). TEM images taken at high resolution (b,d). PH5: (a,b), EP5: (c,d).

DOI 10.1002/pc POLYMER COMPOSITES—-2011 1347

From Fig. 8, the characteristic absorption peak (917 cm21)

of epoxy stretching vibration indicates that the epoxy

groups still existed after processing of PC. However, some

of epoxy groups would participate in chemical reactions.

The film of PC/EP-POSS composites prepared by evapora-

tion of solvents of PC and EP-POSS solution in CH2Cl2had viscous liquids on the surface, but what was prepared

from PC/EP-POSS composite solutions in CH2Cl2 no

liquids. Therefore, EP-POSS may react with phenolic

groups, which originated from degradations of PC [21].

PH-POSS domains were fortuitous particles with a veining

boundary in PH5, which implies that PH-POSS existed as a

form of crystal.

Surface morphology was preliminary studied by SCA

and results were presented in Fig. 9. Surface energies of

PC, PH2, and EP2 were 43.2, 44.5, and 43.6 mN m21,

respectively. Generally, the incorporation of organic sili-

con compounds would increase hydrophobicity of matrix

surface. From Fig. 8, the contact angle of water increased

� 108–128 because of POSS incorporations, and thus the

surfaces were transformed from hydrophilicity to hydro-

phobicity. In addition, it also leaded to the increase of lip-

ophilicity of the surfaces. These results fitted well with

that of organic silicon compounds, and usually were

ascribed to surface aggregation of fillers. It is well known

that the surface aggregation was often attributed to driv-

ing to minimize surface energy, which was calculated

from surface contact angle. From surface energies of

composites, it was true that both of two POSS domains

congregated on the surfaces at different levels.

Glass-Transition Temperature (Tg)

As shown in Fig. 10, Tgs of PC/PH-POSS composites

originating from the data of DSC, decreased from 145 to

1388C with the increase of loading contents. This result,

which was similar with that literature [12] reported, could

be ascribed to the characteristic structure of PH-POSS.

Phenyl groups of PH-POSS molecules were difficult to

stack owing to tether of eight corners, so it had a rela-

tively large free volume, which caused that mobility of

PC chains happened at a relatively low temperature. Tgsof PC/EP-POSS composites slightly increased first and

then decreased with increase of loading. In such case, EP-

POSS would play a double role. On one hand, EP-POSS

would increase gel points by the reaction with phenolic

group and thus caused the improvement of Tg; on the

other hand, part of it scattered in PC matrix acting as a

plasticizer and thus caused a depression of Tg. Therefore,Tgs of PC/EP-POSS composites depended on the loading

FIG. 8. Images of surface contact angle. SCA of H2O: (a) PC, (c) PH2,

(e) EP2; SCA of CH2I2: (b) PC, (d) PH2, (f) EP2. SCA listed blow the

corresponding image is average of three measurements.

FIG. 9. FT-IR spectrums of PC and composites.

FIG. 10. DSC curves of composites of PH-POSS/PC and MA-POSS/

PC.

1348 POLYMER COMPOSITES—-2011 DOI 10.1002/pc

contents of EP-POSS, and were decided by the balance of

two effects.

Thermal Degradation Stability and Thermal OxidativeStability

TGA curves of PC, POSS and composites were shown

in Fig. 11, and the results were collected in Table 1. PH-

POSS underwent two thermal degradation stages in ranges

of 400–600 and 600–7008C under N2. C700 was 11.4%,

far below the theoretical content of Si��O (40.3%). In

air, a similar degradation behavior was observed that it

also underwent two degradation stages in ranges of 400–

550 and 550–7008C with 35.5% residues. Fina et al. [22]

reported that PH-POSS degraded in a two-step weight

loss with C700 [ 70% under nitrogen. The biggest differ-

ence of them was C700 at first glance, but it concealed dif-

ferences of degradation mode. It was well known that

pure PH-POSS upon rapid heating melt and boiled mo-

mentarily at 495–5008C before resolidifying to a colorless

polymer [16]. From the TGA curve of PH-POSS in Fig.

11b and d, PH-POSS nearly lost 80% (in nitrogen) and

30% (in air) of the weight at around 5008C, respectively.Therefore, mass loss at around 5008C mainly owed to

PH-POSS sublimation, which caused lower residues;

moreover, the presence of oxygen could retard the subli-

mation, and consequently the chemical degradation domi-

nated in weight loss. It was concluded that sublimation of

PH-POSS was minor in Fina’s work, which was probably

due to the impurity of PH-POSS, but was major in our

work, especially in nitrogen.

EP-POSS with relative lower T5% (2748C) underwent

one thermal degradation stage in nitrogen; its temperature

of the maximum mass loss rate (Tmax ¼ 3548C) was far

below those of PH-POSS and PC; and C700 was 38.3%

higher than the theoretical content of Si��O (29.32%). In

air, a similar behavior was observed that one degradation

FIG. 11. TGA curves of composites, POSS and PC in N2 (a, b) or air (c, d).

TABLE 1. TGA data of composites and POSS.

PC/POSS

samples

N2 Air

T5% T10% Tmax C700 T5% T10% Tmax C700

PC 472 491 534 21.8 436 467 525 10.8

EP0.5 472 499 547 21.2 471 499 554 20.8

EP1 443 482 544 19.8 458 492 558 20.2

EP2 409 467 556 20.9 433 474 567 18.1

EP5 440 476 539 21.9 398 452 546 15.6

PH0.5 374 455 536 17.3 473 496 538 15.5

PH1 438 477 535 20.7 472 491 536 19.2

PH2 421 459 535 21.8 460 489 542 17.7

PH5 416 467 538 24.6 370 405 532 6.2

PH-POSS 438 452 492 11.4 461 481 508 35.5

EP-POSS 274 354 414 38.3 357 392 446 37.9

DOI 10.1002/pc POLYMER COMPOSITES—-2011 1349

stage with T5% ¼ 3878C, Tmax ¼ 4238C and C700 ¼39.3%. These results show that it was little possibility

that EP-POSS lost weight by the way of sublimation, but

more probable by chain breaking releasing organic moi-

eties. In addition, residues were higher than silica (33.3%)

formed by oxidative transformation of EP-POSS, indicat-

ing entrapment of carbon in residue structures.

In nitrogen, pristine PC underwent one thermal degra-

dation stage with a high degradation temperature (T5% ¼4728C) and C700 ¼ 21.8%. T5%s of all composites were

lower than that of PC, which was probably due to lower

Tds of PH-POSS and EP-POSS. From TGA curves, it was

interesting to note that the degradation rate of composites

would be slower than that of PC after a specific tempera-

ture. T10%s of EP0.5 would be higher than that of PC

(4918C); and then Tmaxs of PC/EP-POSS composites were

all higher than Tmax of PC. But Tmaxs of PC/PH-POSS

composites at around 5358C were no significant changes.

As discussion above, PH-POSS possessed higher thermal

stability than EP-POSS, but it was found that PH-POSS

had less positive effects on PC thermal stability. Iji [23]

thought that siloxanes with branch structures, containing

aromatic groups, would form a high stability layer with

PC degradation products on the surfaces; but we think

that left Si��O fractions in degradation was the key factor

governing the thermal degradation rate. It was known that

PH-POSS left less Si��O fractions; but EP-POSS almost

leave all Si��O fractions, which formed a gel network

layer with PC degradation products on surfaces. This

layer would have compact structures, higher thermal sta-

bility, and some thermal insulation, and acted as a barrier

to inhibit the further degradation of interior PC. Further-

more, due to lower loading content, differences between

C700s were not obvious.

In air, pristine PC also underwent one stage of thermal

degradation as observed in nitrogen. However, T5%s of

composites all increased except PH5, which indicates the

thermal oxidative stability was improved with the incor-

poration of POSS at different levels. The increment of

T5%s of PC/PH-POSS composites was obvious in a range

of 24–378C, but T5%s of PC/PH-POSS composites

increased first and then decreased with the increase of

PH-POSS loading. The increase in PC molecular space,

which was caused by the incorporation of PH-POSS with

the large free volume, may lead to a decrease in the ther-

mal conductivity of the composites and thus to a small

increase in thermal oxidative stability; but the enrichment

of PH-POSS on composite surface would accelerate

releasing of the degradation products and thus lead to a

decrease in thermal oxidative stability. Therefore, the final

performance of composites would be balance of the two

effects. Due to the low degradation temperature of EP-

POSS with respect to PC, T5%s of PC/EP-POSS compo-

sites decreased from 471 to 3988C with the increase of

EP-POSS loading. Tmaxs of PC/PH-POSS composites at

around 5368C had little relationship with PH-POSS load-

ing because of the sublimation of it. Tmaxs of PC/EP-

POSS composites increased from 554 to 5678C firstly and

then decrease with the increase of the loading, which was

ascribed to that left Si��O fractions formed a more ther-

mal oxidative stability layer with PC degradation products

on composite surfaces. C700s increased in a range of 5–

10% with respect to pristine PC except PH5, which indi-

cates that the incorporations of POSS could improve the

carbon entrapment in PC degradation.

Thermal and Dynamic Mechanical Analysis

Researches on dynamic mechanical thermal analysis of

PC/POSS composites were reported in literatures [12, 24,

25]. As shown in Fig. 12, DMA spectrums of PC and

composites displayed only one a relaxation process, corre-

sponding to the glass transition of composites and PC. Tg-DMAs at which the peak in tan d occurred were 155,

158, and 1598C, which corresponded to PH2, EP2, and

EP0.5, respectively. The tendency of Tg-DMA of PC/EP-

POSS composite was consistent with the DSC results.

The storage modulus of PC/EP-POSS composites were

slightly greater than that of PC below 908C; and the tem-

perature range of the rubbery state of them shifted about

108C to high temperature. These results could were

ascribed to the reaction between PC and EP-POSS. In the

processing of PC, it would produce small amount of phe-

nolic groups because of trace moisture. These phenolic

groups would easily react with epoxy groups under nearly

3008C, which caused the fast increase of gel points in PC

matrix and the more hard mobility of PC chains. Hence,

the storage modulus and Tg-DMA were improved. Fur-

thermore, E0 of EP2 was higher than that of EP0.5 at the

temperature range of 30–1408C, but at the temperature

range of 140–1808C, curves of E0 of them nearly over-

lapped. These were probably due to the limited increment

of gel points by the reaction between limited phenolic

groups and enough epoxy groups. The storage modulus of

PH2 was also slightly higher than E0 of PC and the tem-

perature range of the rubbery state of it shifted about 58C

FIG. 12. DMA spectrums depicting the storage modulus (E0), the loss

modulus (E00) and loss factor (tan d) of PC, PH2, EP0.5, and EP2.

1350 POLYMER COMPOSITES—-2011 DOI 10.1002/pc

to high temperature. As mention above, PH-POSS was

partial compatible with PC matrix, and would form clus-

ters of molecule at low loading content. Therefore, PH-

POSS could act as a physical gel point like the literature

[26] reported and thus E0 was improved.

CONCLUSIONS

EP-POSS were synthesized and characterized, and cor-

responding hybrid composites were prepared by melt

blending. The findings are as follows:

1. PH-POSS was partial compatible with PC matrix, and

formed clusters of molecules at low loading and acted as

a physical gel point leading to the improvement of the

storage modulus. When its loading content increasing,

PH-POSS existed in PC matrix as form of crystal. EP-

POSS would react with the phenolic groups originating

from processing, and dispersed in PC matrix as ellipsoi-

dal 50–400 nm particles with a smooth boundary.

2. Because of the plasticization effect, Tgs of PC/PH-

POSS composites decreased from 145 to 1388C with

the increase of loading content. However, there was a

different observation that Tgs of PC/EP-POSS compo-

sites slightly increased first and then decreased with

the increase of loading. Tgs of PC/EP-POSS compo-

sites were the result of the balance of the gel effect

and the plasticization effect.

3. The incorporation of PH-POSS increased thermal oxida-

tive stability and percentage residues of composites. It

was not significant that PH-POSS retarded thermal deg-

radation in nitrogen due to PH-POSS sublimation. The

incorporation of EP-POSS increased thermal and thermal

oxidative stability at different levels, and C700s of com-

posites increased from 10.8 to around 20% in air.

4. Si��O fraction left by POSS degradation was the key

factor, which governed the formation of gel network

layers with PC degradation products. This layer pos-

sessed more compact structures, higher thermal stabil-

ity and some thermal insulation, and could retard the

further degradation of interior PC.

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DOI 10.1002/pc POLYMER COMPOSITES—-2011 1351