crystallization behavior and morphological development of isotactic polypropylene blended with...

Crystallization Behavior and Morphological Developmentof Isotactic Polypropylene Blended with NanostructuredPolyhedral Oligomeric Silsesquioxane Molecules

JEAN-HONG CHEN, YOUNG-DI CHIOU

Department of Polymer Materials, Kun Shan University, Tainan 710, Taiwan, Republic of China

Received 30 December 2005; revised 17 April 2006; accepted 16 May 2006DOI: 10.1002/polb.20878Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: The thermal properties and morphological development of isothermallycrystallized isotactic polypropylene (iPP) blended with nanostructured polyhedral oli-gomeric silsesquioxane (POSS) molecules at very small loading of POSS were studiedwith differential scanning calorimeter (DSC), thermal gravimetric analysis, dynamicmechanical analysis, polarized optical microscopy (POM), and wide-angle X-ray dif-fraction (WAXD). The result of DSC indicated that the crystallization rate of iPPincreases with the increase in POSS contents during crystallization; moreover, themelting temperature of iPP/POSS nanocomposites slightly decreases, while the heatof fusion increases with the addition of POSS molecules at melting and remeltingtraces. The storage modulus and thermal stability, respectively, remarkably decrease,while the glass transition temperature of isothermally crystallized iPP/POSS nano-composites increases slightly with the increase in POSS contents. The morphologiesresults of WAXD and POM show that the POSS molecules form about 35 nm sizednanocrystals and aggregate to form thread-like and network structure morphologies,respectively, in the molten state even when the POSS content is very small. Theseresults, therefore, suggest that the interaction force between the POSS moleculesshould be larger than the force between POSS molecules and iPP matrix; however,those interactions depend on the chain length of functionalized substituents on thePOSS cage. Therefore, the POSS molecules aggregate forming nanocrystals and actas an effective nucleating agent for iPP and influence the thermal properties of iPP/POSS nanocomposites due to the shorter chain length of functionalized substituents,methyl, on the POSS cage. VVC 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys

44: 2122–2134, 2006

Keywords: iPP/POSS nanocomposites; morphology; nanocrystal; thermal properties

INTRODUCTION

It is well known that isotactic polypropylene(iPP) exhibits several crystalline forms at differ-ent process conditions. In all these, crystal formswere affected not only by molecular mass andmolecular mass distribution of iPP but also by

different blending compounds and preparationconditions, i.e., isothermal temperature andnucleating agents used. The use of nucleatingagents in polymers is extensive and finds impor-tance because the control of the crystallizationbehavior allows the modification of the micro-structure and retardation or enhancement ofthe physical properties of polymers, such as ther-mal, mechanical, and optical properties. How-ever, the physical properties of polymers essen-tially controlled by the adjustment of type anddispersion of the nucleate agent blended with

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

Journal of Polymer Science: Part B: Polymer Physics, Vol. 44, 2122–2134 (2006)VVC 2006 Wiley Periodicals, Inc.

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polymer matrix. Therefore, the nucleatingagents blended with polymer attract much inter-est because the relationship between the struc-ture of nucleating agents and the physical prop-erties of polymer matrix is very complicated.1–6

In recent years, the organic–inorganic hybridpolymer nanocomposites have attracted muchattention from researchers in the polymer science.The cage-shaped polyhedral oligomeric silsesqui-oxane (POSS) molecules are new classes of nano-particles in the polymer science.7–17 The POSSmolecules contain a polyhedral siliconoxygen-nanostructured skeleton that was first introducedin 1946.18 The variety of functionalized substi-tutes can be attached to each corner silicon atomto give desirable functionality, and the POSSmole-cules can be dispersed in many polymer matricesby the adjustment of the functionalized substitu-ents on the POSS cage by synthetic routes such ascopolymerization and chemical grafting or byphysical mixing. However, the synthetic tech-niques for the POSS molecules allow variety offunctionalized substituents on the POSS cage andtherefore this route creates an opportunity forfine-tuning the interactions between POSS mole-cules and polymer matrix, and provides ways tocontrol the mechanical and physical properties ofnanocomposites.7,8,12 Recently, the relationshipsbetween the structures of POSS molecules andpolymer matrix, and the resulting physical andmechanical property changes in POSS nanocom-posites have been investigated.11,14 However,POSS molecules can be introduced into polymermatrix via copolymerization and physical blend-ing. Copolymerization is an efficient approach toPOSS nanocomposites because of the formation ofchemical bonds or chemical link between POSSmolecules and polymer materials, and enhancedmechanical performance, higher glass transitiontemperature, and higher thermal decompositiontemperature.19 Chang et al. recently reported thatthe poly(vinyl phenol-co-vinylpyrrolidone-co-poly-hedral oligomeric silsesquioxane) copolymers hadglass transition temperatures (Tg) significantlygreater than those of their corresponding POSS-lacking poly(vinyl phenol-co-vinylpyrrolidone)copolymers because of the strong hydrogen-bond-ing interactions existing between poly(vinyl phe-nol) and POSS.20,21

Compared to the chemical modification ofPOSS nanocomposites, relatively few researchershave studied physically blended POSS nanocom-posites possibly because of the unfavorable misci-bility of POSS with polymer.4–6 However, Hsiao

et al. first study the crystallization behavior atquiescent and shear states of iPP/POSS compo-sites. The octamethyl-POSS was added by meltblending with iPP at quite large concentrationsand the crystallization behavior was studied bythe means of differential scanning calorimetry(DSC) and in situ SAXS measurements.4 Theyreported that the POSS, acting as a nucleatingagent, was found to influence the quiescent meltcrystallization by enhancing or retarding the crys-tallization process, depending on POSS concentra-tion at very large loading of POSS. Then, theyhave also investigated the physical gelation inethylene–propylene (EP) copolymer melts inducedby POSS molecules. EP/POSS composites wereprepared by melt-mixing in a twinscrew micro-compounder with EP copolymers characterized bydifferent ethylene contents and varying the POSSloadings from 10 to 30 wt %.5 The results of wide-angle X-ray diffraction (WAXD) indicated that nomolecular dispersion of POSS cages could beachieved as POSS X-ray pattern was maintainedin the composites. Small-amplitude oscillatoryshear experiments showed that the EP/POSS com-posites exhibited a solid-like rheological behaviorabove melting temperature when compared withthe liquid-like behavior of the neat resin. More-over, the addition of 10 wt % of POSS was foundto increase considerably the Young’s modulus andthe Tg value when compared to neat EP. Morerecently, the influence of functionalization ofPOSS cages on the iPP/POSS was reported byFina et al.6 They indicated that a good dispersionof POSS was obtained particularly at low loadingsof POSS with longer organic chains. Thus, theseconclusions described that the POSS moleculeschanged crystallization behavior and physicalproperties of physically blended POSS composites,depending on POSS concentration at very largeloading of POSS that is above 10 wt % of POSS.However, in this work, we investigated the effectof POSS content on the crystallization behavior,which shows that the microcrystal morphologyand physical properties of iPP/POSS nanocompo-sites were affected by various POSS contents evenat very low POSS content, even below 3 wt % ofPOSS contents. Therefore, the effect of POSS con-tents on the crystallization behavior and physicalproperties of physically mixed POSS nanocompo-sites, especially at the low loading of POSS, is stilla question unresolved.

In this study, the physically blended iPP/POSS nanocomposites prepared by melt-blend-ing of isotactic propylene with POSS molecules

POLYPROPYLENE BLENDED WITH POSS MOLECULES 2123

Journal of Polymer Science: Part B: Polymer PhysicsDOI 10.1002/polb

at very small loading POSS was investigatedwith DSC, dynamic mechanical analysis (DMA),thermogravimetric analysis (TGA), WAXD, andpolarized optical microscopy (POM) techniques.This work emphasizes in two areas. The first isthe relationship between the various POSS con-tents and the thermal property of iPP/POSSnanocomposites. The second is the possible mor-phology and microstructure of POSS moleculesin iPP/POSS composites even when the POSScontent is very small. Therefore, the purpose ofthis study was to obtain a fundamental under-standing of relationship of interaction betweenPOSS molecules and iPP matrix on the thermaland morphological properties.

EXPERIMENTAL

Materials and Specimen Preparation

The iPP with a weight–average molecular weightof Mw ¼ 3.4 � 105 g mol�1 and POSS-nanorein-forced polypropylene contains 10 wt % octa-methyl-POSS, respectively, were purchased fromAldrich Chemical. In this study, the chosen POSSmolecules has an empirical formula (RSiO1.5)8,

with R groups being substituted with methyl; aschematic drawing of this octamethyl-POSS isshown in Figure 1. Melt-blended specimens ofthese materials with various compositions wereprepared by a twinscrew apparatus (MP2015APV Chemical Machinery, USA) at 230 8C for10 s. The mixing ratios of iPP/POSS nanocompo-sites are as follows (wt/wt %): 100/0, 99/1, 97/3,95/5, 93/7, 90/10, and the pure POSS were pre-pared and defined as pure iPP, POSS-1, POSS-3,POSS-5, POSS-7, POSS-10, and pure POSS.Although the decomposition temperature of purePOSS is about 230 8C, the mixing time in twin-screw apparatus is very short, and so these iPP/POSS nanocomposites may be without anydecomposition occur in this mixing process. Onthe other hand, the decomposition temperatureincreases remarkably when POSS blends withiPP forming iPP/POSS nanocomposites. This faceindicates that the thermal stability of POSSmolecules increases remarkably when POSSblends with iPP matrix because of the decreasein the thermal vibration motion of POSS mole-cules or increase in the interaction between ofPOSS molecules and iPP matrix with increasingiPP contents. The composition and thermal pro-perties of iPP/POSS nanocomposites that havebeen used in this study are compiled in Table 1.

The compression-molded films were preparedby melt-pressing of different iPP/POSS nanocom-posites for a molding of 120 � 120 � 1 mm3,placed between a pair of steel platens, at tem-perature 230 8C and at 10 min holding time.The iPP/POSS nanocomposite film was takenout and immediately submerged in a tempera-ture-controlled compression molding machine atTc under a pressure of 50 kg cm�2, where it wasbetween the two steel platens for a holding timeof 120 min. This treatment assumes that previ-ous thermal and mechanical histories wereessentially erased and provides a controlled con-dition for the film.

Figure 1. Schematic diagram of octamethyl-POSSmolecules.

Table 1. The Composition and Thermal Properties of iPP/POSS Nanocomposites

Materials Tg (8C) Tc (8C) DHc (J/g) Tmm (8C) DH f

m (J/g) Tmrm (8C) DH f

rm (J/g) Tdec (8C)

Pure iPP 4.7 104.1 �81.4 167.5 87.7 163.7 81.8 383.7POSS-1 7.1 112.2 �82.5 165.4 89.8 163.5 83.4 348.2POSS-3 7.8 112.7 �84.6 165.1 91.4 163.4 85.7 344.6POSS-5 7.6 113.5 �85.4 165.3 88.8 163.5 88.4 326.5POSS-7 8.1 115.1 �81.6 165.0 95.8 162.8 87.6 322.4POSS-10 8.5 117.0 �74.8 164.8 92.6 163.1 89.2 319.8Pure POSS 230.8

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Measurements

Differential Scanning Calorimetry

The thermal behavior of isothermally crystal-lized iPP/POSS nanocomposites was analyzedwith PYRIS Diamond DSC. About 5 mg samplesealed in aluminum pans was melted in the fur-nace in a nitrogen atmosphere from 30 to 230 8Cat a rate of 10 C min�1 and the melting thermo-gram was measured. Then the sample was keptat 210 8C for 10 min to allow complete melting,followed by cooling at a rate of 10 C min�1 andthe crystallization thermogram was measured.As soon as the temperature reached �50 8C, itwas remelting again at a rate of 10 C min�1 andthe remelting thermogram was also measured.These measurement results are shown in Table 1.The crystallization temperatures were selectedso as to compare our data with those of otherexperiments.

Wide-Angle X-Ray Diffraction

WAXD (Rigaku D/max-2500 diffractometer) in-tensity curves of iPP/POSS nanocomposites weremeasured with a graphite-monochromatized Cu-Ka radiation generated at 40 kV and 180 mA.WAXD intensities were recorded from 2h ¼ 58 to358 with a continuous scanning speed of 2h ¼ 18min�1, with data collection at each 0.058 of 2hwas performed.

Dynamic Mechanical Analysis

The dynamic mechanical tests (Perkin–ElmerDMA-7) operated in a tensile mode was used tostudy the thermal mechanical properties of iPP/POSS nanocomposites. Samples were made inthe form of rectangular strips having dimen-sions of 40 � 10 � 1.0 mm3. After clamping thesample to finger tightness, a constant tensileforce was applied to the sample. The experimentwas performed at 3 Hz, with a stretching ratioof 0.1%.

Thermogravimetric Analysis

TGA (Perkin–Elmer TGA-7) was used to investi-gate the thermal stability of the iPP/POSS nano-composites. About 10 mg sample was heatedunder nitrogen atmosphere from ambient tem-perature up to 600 8C at a heating rate of 10 8Cmin�1 in all the cases.

Polarized Optical Microscopy

Spherulite morphologies of iPP/POSS nanocom-posites were investigated using Zeiss Axioskop-40 with a Linkam TH600 hot stage. The sam-ples, inserted between two microscope coverglasses, was melted at 230 8C and squeezed toobtain thin films. The thickness of the squeezedsamples was about 10 lm. The thin film sampleswere inserted in hot stage. Each sample waskept at 230 8C for 10 min to allow completemelting, followed by cooling to isothermal crys-tallization temperature at rate of 90 8C min�1

and was maintained at that temperature duringthe time necessary for isothermal crystalliza-tion. The temperature of hot stage can be keptconstant within 0.1 8C. Dry nitrogen gas waspurged through the hot stage.

RESULTS AND DISCUSSION

Thermal Behavior of iPP/POSS Nanocomposites

The melting and remelting behaviors, respec-tively, of isothermally crystallized iPP/POSSnanocomposites as a function of POSS contentwere investigated by DSC as shown in Figure2(a,b). The results of Figure 2(a,b) show thatthese thermograms of iPP/POSS nanocompositesare characterized by single melting endotherm atmelting and remelting traces. Recently, wereported the thermal properties and morphologi-cal development of iPP homopolymer (Mw ¼ 5.8� 105 g mol�1) blended with aPP at different iso-thermal crystallization temperatures.22 We indi-cated that the aPP was locally miscible with iPPin the amorphous region and presented a phasetransition temperature at Tc ¼ 120 8C. However,below this transition temperature, imperfect a-form crystal were obtained, leading to two endo-therms. Above this transition temperature, moreperfect a- and c-form crystals were formed andonly a single endotherm was observed. Therefore,in this study, a general feature of these thermo-gram curves was the appearance of single meltingendotherms at melting and remelting traces.However, it is observed that both the narrow heatof fusion of the melting trace endotherm (DH f

m)and the broad heat of fusion of the remeltingtrace endotherm (DH f

rm) slightly increased withthe increasing POSS contents as shown in Figure2. The peak of DH f

m was higher than that of DH frm

attributable to the isothermal crystallizationeffects; therefore, the heat of fusion and morphol-



ogy of the endotherms were found to be depend-ent on the POSS contents and isothermal crystal-lization process, and so the crystallinity and mor-phology of iPP/POSS nanocomposites maybeaffected by POSS and isothermal crystallization.The melting temperature of melting trace (Tm

m)and the melting temperature of remelting trace(Tm

rm) of iPP/POSS nanocomposites, respectively,decrease slightly with the increase in POSS mole-cules. Generally, increasing the melting tempera-ture and the overall heat of fusion attribute to thepromotion of a more perfect crystal and higherdegree of crystallinity of iPP during isothermalcrystallization; however, the obtained crystals foriPP/POSS nanocomposites are more imperfectbut higher degree of crystallinity than thatformed for pure iPP was observed. The isother-mal temperature causes the distributions of twomelting endotherms of iPP have been extensivelydiscussed.22–26

Figure 3 shows the crystallization behavior ofiPP/POSS nanocomposites as a function of POSScontents during the cooling trace. The crystalli-zation temperature (Tc) and latent heat of crys-tallization (DHc) of all iPP/POSS nanocompositesincrease evidently with the increase in POSSmolecules, respectively. The result of Figure 3shows that the pure iPP has a higher tempera-ture shoulder in front of the main exothermic

peak. In contrast, all iPP/POSS nanocompositesshow only a single exothermic peak and increasein the Tc of iPP/POSS nanocomposites with theincrease in POSS contents at very small loading

Figure 2. Influence of POSS contents on the melting endotherms of iPP/POSSnanocomposites: (a) melting trace after isothermal crystallization and (b) remeltingtrace.

Figure 3. Influence of POSS contents on the crys-tallization exotherm of iPP/POSS nanocomposites af-ter melting.

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of POSS. Moreover, the time needed to reachthe exothermic maximum (tem) during the crys-tallization shows remarkable decrease with theincrease in POSS contents. This result clearlyimplies that the POSS accelerates the crystalli-zation process or nucleation rate of iPP duringcrystallization even when the POSS content isvery small. This result indicates that the POSSmolecules may act as a nucleating agent thataffects the iPP matrix. However, it appears thata minimum tem value can be achieved for POSS-10 with about 10 wt % octamethyl-POSS in thiswork. It is interesting to note that the nuclea-tion rate is faster and the DHc of POSS-10 issmaller than other iPP/POSS nanocomposites.These results indicate that the POSS moleculesessentially act as a nucleating agent to promotethe nucleation rate of iPP chains during crystal-lization. From the results of Figures 2 and 3, itwas observed that the thermal properties, suchas Tm

m and Tmrm, of iPP/POSS nanocomposites

slightly decrease, whereas the DH fm, DHf

rm, andcrystallization ability of iPP/POSS nanocompo-sites strongly depend on the POSS molecules.

Dynamic Mechanical Properties of iPP/POSSNanocomposites

Figure 4(a,b) shows the storage modulus (E0)and damping factor (tan d) of isothermally crys-tallized iPP/POSS nanocomposites as a functionof POSS contents at heating rate of 3 8C min�1

from the DMA study. It is interesting to note

that, in the glass state, the E0 of all the iPP/POSSnanocomposites are significantly lower than thatof the pure iPP as shown in Figure 4(a). In thetemperature region ranging from �100 to 0 8C,the E0 of the pure iPP was about 1.5–1.7 timesgreater than that of iPP/POSS nanocomposites.However, at a small amount of POSS contents(POSS-1), a significant decrease in E0 wasobserved. It is also worth noticing that the E0 ofthe iPP/POSS nanocomposites was close to thepure iPP in the rubbery state. However, Hsiaoet al. showed that the Young’s modulus was con-siderably increased with addition of POSS. Thismaybe due to the fact that POSS molecules havean imported reinforcement effect on the lowercrystallite of EP copolymer.18 In this study, how-ever, there are two competitive factors affectingthe E0 of the iPP/POSS nanocomposites. Thefirst factor the presence of POSS molecules pro-motes the increase in the amount of nuclei andthe reduction in the size of spherulitic of iPP,resulting in lowering of E0 of iPP/POSS nano-composites with the increase in POSS moleculesin the glass state. On the other hand, the addi-tive of POSS molecules in the polymer matrixcould give rise to hindrance of the molecular mo-bility of iPP chain in the amorphous region bythe network structure of POSS, as the interac-tion between iPP in the rubbery state is weak.However, at temperature lower the Tg, the firstfactor may play an important role than the sec-ond factor, resulting in the decrease of E0 withthe increase in POSS contents. On the other

Figure 4. Influence of POSS contents on the (a) storage modulus (E0) and (b) damp-ing factor (tan d) of iPP/POSS nanocomposites in a DMA temperature scan.



hand, at temperature higher than the Tg, thesecond factor plays a major role than the firstfactor, and so the E0 of iPP/POSS nanocompo-sites was close to that of pure iPP.

Figure 4(b) shows that the tan d of iPP/POSSnanocomposites shows a single transition or singlepeak behavior and the transition increases slightlywith the increase in POSS contents. The positionof the peak maximum in the tan d versus tempera-ture curve can be related to the Tg of the iPP ma-trix. It was seen that the Tg of iPP shifts to ahigher temperature by about 10 8C with the addi-tion of POSS molecules as shown in Figure 4(b).This result suggested that the majority of POSSmolecules in iPP/POSS composites became nano-crystals and aggregate forming a network struc-ture and exhibited some weak interactions withthe iPP chains, resulting in the hinder of the mo-lecular mobility of iPP chains at temperaturehigher than the Tg as shown in Figures 7–9. Fig-ure 4(b) also indicated that the intensity of the tand peaks was significantly increased when thePOSS was increased. The intensity of the tan dpeaks could reflect the internal friction betweenmolecules on the amorphous region. The higher Tg

and intensity of the tan d peak could result fromthe hindrance of the molecular mobility on theamorphous region of iPP by the network structureof POSS that was observed.

Thermal Stability of iPP/POSS Nanocomposites

Figure 5 shows the TGA curves of iPP/POSS nano-composites as a function of POSS contents at heat-ing rate of 10 8C min�1 recorded in nitrogenatmosphere. Within the experimental tempera-ture range, the TGA curves of all iPP/POSS nano-composites showed a similar degradation profiles,and this result suggests that the dispersion ofPOSS nanocrystals was homogenous in iPP/POSSnanocomposites, and so it did not significantly al-ter the degradation mechanism of the iPP matrix.It is interesting to note that the incorporation ofPOSS into the iPP matrix showed a significanteffect in reducing the thermal stability. For pureiPP, the initial decomposition temperature, whichwas defined as 5%mass loss temperature, is about395 8C. The ceramic yield of pure iPP at 600 8C isabout 2.3 wt %. For the iPP/POSS nanocompo-sites, the initial decomposition occurred at 320–348 8C, which was much lower than that of thepure iPP. However, the initial decomposition ofthe pure POSS is about 230.8 8C and the ceramicyield at 600 8C is about 5.7 wt %. Although the

decomposition temperature of pure POSS isabout 230.8 8C, the decomposition temperatureincreases remarkably when POSS blended withiPP forms iPP/POSS nanocomposites. This faceindicates that the thermal stability of POSS mole-cules increases remarkably when POSS is blendedwith iPP matrix because of the decrease in thethermal vibration motion of POSS molecules orincrease in the interaction between POSS mole-cules and iPP matrix with increasing iPP con-tents. Comparison of the thermal stability of pureiPP with that of iPP/POSS nanocomposites indi-cates that there were no chemical bonds or stronginteraction between POSS molecules and iPP ma-trix. The initial decomposition temperatures andthe thermal stability of iPP/POSS nanocompositeswere significantly expressed as POSS moleculeswere added at very small loading of POSS. Thisascribes to the fact that the homogeneity and thedispersion of POSS molecules promote theincrease in decomposition rate in iPP/POSS nano-composites because of the lower thermal stabilityand absence strong interaction force between thelowmolecular weight POSS and iPPmatrix.

Microstructure of iPP/POSS Nanocomposites

Figure 6 shows the WAXD intensity curves ofiPP/POSS nanocomposites after isothermal crys-tallization at 130 8C. The X-ray diffractograms

Figure 5. Influence of POSS molecules on the ther-mal stability of iPP/POSS nanocomposites in a TGAtemperature scan.

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of iPP/POSS nanocomposites show nearly the a-form diffractograms of iPP after isothermal crys-tallization. For the isothermally crystallized iPP,the characteristic peaks of the a-form crystal ofiPP can be found at 2h angles of 14.088 (110),16.958 (040), 18.58 (130), 21.28 (111), and 21.858(�131 and 041) as diffractogram marked of pureiPP shown in Figure 6.27–29 The intensity of a-form crystal of iPP increases with increase inPOSS content. This result indicates that thePOSS molecules essentially act as a nucleatingagent to promote crystallization rate of iPPchains during the crystallization, as discussed inDSC. However, the pure POSS crystals showthat a distinct colloid crystal reflection peakswere observed. As can be seen, the X-ray diffrac-

tion profile shows that the crystallinity of POSSin the iPP/POSS nanocomposites is very high,especially at the 1 wt % of POSS molecules(POSS-1) that can also be clearly seen at (011)refection plane of POSS crystal at 2h ¼ 11.08.The intensity of (011) the refection plane of POSScrystal increases, while the breadth of (011) therefection plane of POSS crystal do not evidentlychange with the increase in POSS contents. Itindicates that the amount of POSS nanocrystalsincreases, while the size of POSS nanocrystalsdo not evidently change with the increase inPOSS contents as shown in Table 2. The crystalstructures of POSS molecules have beenreported to be rhombohedra crystals accordingto the work of Barry et al.30 The X-ray results ofPOSS peak positions are in good agreementwith that reported by Barry et al. and by Hsiaoand coworkers,5 where the characteristic peaksof the POSS crystal can be found at 2h angles of11.08 (011), 13.98 (110), 21.68 (003), 23.088 (022),25.248 (300 and 113), and 26.08 (122).30 This factindicated that the majority of POSS moleculesin iPP/POSS nanocomposites formed a rhombicnanocrystals. However, the widths of thesepeaks were relatively narrow; therefore, thewidth of the strongest reflection of POSS mole-cules at 2h ¼ 118 was generally used to estimatethe average crystal size, t, using the Scherrer’sequation,31 which is based on the Laue’s diffrac-tion function, for the broadening of diffractions,as given in eq 1

t ¼ Kkb cos h

ð1Þ

where k is the wavelength of the X-ray, h is theBragg’s angle, K is the Scherrer’s constant, andwhen b is the half-width of the diffraction the Kvalue is generally 0.94. The estimation for theaverage nanocrystal size of POSS was in therange of 35 6 3 nm as shown in Table 2 and thenanocrystal size slightly, while the amount of

Figure 6. WAXD profiles of iPP/POSS nanocompo-sites as a function of POSS molecules after isothermalcrystallization for 120 min.

Table 2. Estimation of Crystallite Size of POSS Nanocrystals in iPP/POSS Nanocomposites

Materials hkl 2h Bo (degree)a b (degree) b/Bo b (rad) cos h t (nm)

POSS-1 110 10.95 0.54 0.248 0.46 0.00433 0.995 33.62POSS-3 110 10.95 0.65 0.241 0.37 0.00421 0.995 34.58POSS-5 110 11.00 0.65 0.221 0.34 0.00386 0.995 37.71POSS-7 110 11.05 0.75 0.233 0.31 0.00407 0.995 35.77POSS-10 110 11.00 0.75 0.218 0.29 0.00380 0.995 38.31

a The integral breadth of the diffraction.



nanocrystal remarkable increases with increas-ing POSS molecules. This result indicates thatthe POSS molecules were formed in nanocrys-tals in the iPP/POSS nanocomposites. Therefore,these results demonstrate that the miscibilitybetween POSS molecules and iPP matrix is poordue to the interaction forces between POSSmolecules (dipole–dipole interaction) that areremarkably larger than that between POSSmolecules and iPP matrix (dispersion force).

The dependence of the morphological develop-ment of pure iPP and POSS-10 on different iso-thermal temperatures are discussed in Figures7 and 8, respectively. However, the typicalWAXD intensity pattern of the a-form crystal ofiPP at temperature above 130 8C was observed.However, the WAXD intensity curves showed anunusual a-form pattern at temperature below130 8C, i.e., other peak appeared at 2h ¼ 16.68as shown in Figure 7. The characteristic peak ofthe presence in 2h ¼ 16.68 is the (300) crystal ofb-form of iPP; the b-form of iPP usually can befound at 2h angles of 16.18, (210) 16.68, (300)21.38, (310) 24.78, (130) and 28.078.27–29 There-

fore, the b-form crystal of iPP was observedat isothermal crystallization temperature below130 8C. It is interesting to note that increase inisothermal temperature leads to reduction of theb-form crystallization, that is the intensity ofb(300) peak at 2h ¼ 16.68 decreases with theincrease in temperature and this disappears at130 8C. These results indicate a lower degree ofperfection and instability of b-form coexistingwith the a-form crystals of iPP during crystalli-zation at a temperature below 130 8C. Rela-tively, Figure 8 shows the WAXD intensitycurves of POSS-10 at various isothermal crystal-lization temperatures. As the isothermal crystal-lization temperature increases, the intensities ofboth a-form crystals of iPP and nanocrystals ofPOSS ((011) refection plane of POSS crystal at2h is 11.08) do not evidently change with theincrease in isothermal temperature. The resultsof WAXD intensity curves indicate that athigher isothermal temperature, i.e., lower super-cooling, residual POSS molecules do not aggre-gate together and the induction at a higher

Figure 7. WAXD profiles of pure iPP after isother-mal crystallization for 120 min at different isothermaltemperatures.

Figure 8. WAXD profiles of POSS-10 after isother-mal crystallization for 120 min at different isothermaltemperatures.

2130 CHEN AND CHIOU


degree of POSS nanocrystals was observed. Themicrostructures suggested that the interassocia-tion between iPP and POSS is lower than thestrong self-association between POSS moleculesas discussed.

Morphological Development of iPP/POSSNanocomposites

The morphological development and spheruliticsize of isothermally crystallization procession foriPP/POSS nanocomposites at very small loadingof POSS were also investigated with POM at130 8C as shown in Figures 9–11. Figure 9 dis-plays the morphology obtained by POM for pureiPP. From the spherulitic morphology andgrowth process, it is indicated that the pure iPPmay hint a sporadic nucleation process, followedby a three-dimensional crystal growth as shownin Figure 9. When compared with pure iPP, thespherulitic morphologies and growth processesof POSS-3 and POSS-7 indicate an instantane-ous/predetermined nucleation process, followedby a three-dimensional crystal growth as shownin Figures 10 and 11. It is interesting that themorphologies of instantaneous/predeterminednucleation of POSS-3 and POSS-7 show a thread-like and network structure, respectively, as

POSS was added. These results indicate thatindividual POSS nanocrystal was more aggre-gate, forming thread or network structure withPOSS contents even when the POSS content isvery small. Recently, Fina et al. reported that aregular POSS aggregate was found in the centerof spherulite, acting as a sort of nucleatingagent with SEM analysis, and the thread-likePOSS crystals present at grain boundaries werealso measured by TEM analysis.6 These factsindicate that the POSS molecules must formnanocrystals first in the molten state of iPP ma-trix and the stable nuclei of POSS nanocrystalattracts the iPP chains on the surface of POSSnanocrystal because the crystallization tempera-ture of POSS molecules is higher than that ofiPP. This indicates that these nanocrystals leadto microphase separation for POSS moleculeswith iPP. Moreover, the macrocrystal of POSSmolecules appear in the molten state of POSS-7as shown in Figure 11(a). The domain sizes ofthe POSS macrocrystals in the molten state ofPOSS-7 are in the range of 10–20 lm. This factindicates that the macrophase separation ofPOSS occurs evidently with POSS concentra-tion, implying that the interaction betweenPOSS nanocrystals is evidently larger than thatbetween POSS nanocrystal and iPP. It implies

Figure 9. Spherulite morphologies of pure iPP with various crystallization times at130 8C, magnification �800.



that POSS molecular is only partially miscibleand appears microphase separation inducingthese POSS molecules aggregate together form-

ing nanocrystals and then these POSS nanocrys-tals aggregate together forming thread or net-work nanocrystal structure in molten state dur-

Figure 10. Spherulite morphologies of POSS-3 with various crystallization times at130 8C, magnification �800.

Figure 11. Spherulite morphologies of POSS-7 with various crystallization times at130 8C, magnification �800.

2132 CHEN AND CHIOU


ing crystallization as confirmed by the WAXDand POM profiles. Therefore, these facts demon-strate that the interaction force between iPPand POSS is lower than that between POSSmolecules, inducing a strong self-association forPOSS nanocrystals that occurs and forms athread and network structure in the moltenstate of iPP/POSS nanocomposites. On the otherhand, the results from Figures 9–11 also showthat the spherulitic size of iPP remarkably de-creases when compared with that of POSS. Thisresult indicates that the POSS molecules act asa nucleating agent, at very small loading ofPOSS, on iPP matrix because of the increase inthe nucleation rate of iPP matrix, which agreeswith the result of the DSC.

CONCLUSIONS

In this study, we demonstrated that the POSSmolecules exhibit nanocrystals and aggregate toform thread or network structure in iPP/POSSnanocomposites that promotes the nucleationrate of iPP during crystallization. However,Hsiao et al. reported that the POSS moleculeswere found to influence quiescent melt crystalli-zation by enhancing or retarding the crystalliza-tion process, depending on the POSS concentra-tion at POSS content above 10 wt %.4 On theother hand, Joshi et al. also reported the noniso-thermal crystallization of HDPE/POSS nano-composites. They indicated that the POSS mole-cules exhibit nucleation activity only at 10 wt %loading in HDPE and are not effective nuclei atlower loadings.32 Therefore, the effect of POSScontents on the crystallization behavior andmorphological development of the physicallyblended iPP/POSS nanocomposites, especially atvery low loading of POSS, is still a questionunresolved. In this work, the extraordinarythermal properties for physical blend of the iso-thermally crystallized iPP/POSS nanocompositewere observed at very small loading of POSS.The results of DMA displayed the higher Tg andlower E0 values, respectively, of iPP/POSS nano-composites when compared with that of the pureiPP. Moreover, DSC results showed that thethermal properties, such as Tm

m and Tmrm, of iPP/

POSS nanocomposites slightly decrease, whilethe DHf

m, DHfrm, and crystallization ability of

iPP/POSS nanocomposites increase with POSScontents. The result of TGA indicates that thethermal stability of POSS molecules is remark-

ably lower than that of pure iPP because of theabsence of strong bounding force between POSSand iPP and very lower molecular weight ofPOSS. Therefore, the thermal stability of iPP/POSS nanocomposites was lower than that ofpure iPP. The morphologies’ results of WAXDand POM show that the POSS molecules formabout 35 nm nanocrystals and aggregate to formthread-like and network structure morphologies,respectively, in molten state of iPP/POSS nano-composites at very small loading of POSS.

On the basis of result of Barry et al., theyreported that the chain length of functionalizedsubstituents on the POSS cage, such as methyl,ethyl, propyl, and butyl, products the semicrys-tal samples. The melting temperature of thesecrystals decreases, whereas the unit cell of thesecrystals increases with chain length of function-alized substituents because the crystallographyof octamethyl-POSS showed that it was isomor-phous with and structurally similar to the otherchain length of functionalized substituents POSSmolecules.30 Those results imply that the inter-action forces between POSS molecules remark-ably affected the chain length of functionalizedsubstituents on the POSS cage. Therefore, inthis work, we further postulated that the inter-action between POSS molecules dominates thecrystallization/nucleation behavior and aggre-gate POSS nanocrystals to form thread-like andnetwork structure morphologies, respectively, inmolten state of iPP/POSS nanocomposites, andthis interaction should decrease with theincrease in chain length of functionalized sub-stituents on the POSS cage. The interactionbetween POSS and iPP matrix may be more im-portant in the thermal properties of iPP/POSSnanocomposites, and thus the thermal proper-ties of iPP/POSS nanocomposites increase withthe increase in the chain length of functional-ized substituents on the POSS cage due to theincrease of miscibility between the POSS mole-cules and polymers. The possible dispersion ofPOSS nanocrystals in the iPP matrix may in-clude the following two scenarios: (1) The POSSmolecules dispersed in iPP/POSS compositesform about 35 nm nanocrystals, which are de-tectable by WAXD. (2) These POSS nanocrystalscan aggregate and form larger POSS crystalaggregates, i.e., thread-like and network struc-ture morphologies during isothermal crystalliza-tion, which are detectable by POM even whenthe POSS content is very small. However, we con-clude that the strong interaction forces between



POSS molecules and the weak interactionbetween POSS molecules and iPP matrix aredue to short chain length of functionalized sub-stituents on the POSS cage.

The authors gratefully acknowledge the financial sup-ported provided by National Science Council of theRepublic of China through project NSC 93-2216-E-168-004.

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