Journal of Industrial and Engineering Chemistry 16 (2010) 189–192

Short communication

Polyhedral oligomeric silsesquioxane and polyethylene nanocomposites andtheir physical characteristics

Sang-Kyun Lim, Eun-Pyo Hong, Hyoung Jin Choi *, In-Joo Chin *

Department of Polymer Science and Engineering, Inha University, Incheon 402-751, Republic of Korea

A R T I C L E I N F O

Article history:

Received 1 September 2009

Accepted 1 December 2009

Keywords:

POSS

Nanocomposite

Nanohybrid

Polyethylene

Thermodynamic solubility parameter

A B S T R A C T

Polyethylene (PE) nanohybrids with polyhedral oligomeric silsesquioxane (POSS) functionalized by

octamethyl-, octaisobutyl- and octaphenyl-groups were prepared by a melt mixing method. Especially,

we present a structure of POSS suitable for the PE system via a theoretical approach, in which a

thermodynamic solubility parameter was used to study specific interaction between PE and

functionalized POSS. The thermodynamic solubility parameter was calculated by Hoftyzer and van

Krevelen method which was developed based on the classical Flory–Huggins theory. Thermal and

mechanical properties of the nanohybrids were further analyzed to be correlated with those obtained

from the theoretical results.

� 2010 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights

reserved.

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1. Introduction

Several types of inorganic-polymer nanocomposites, mainlypolymers filled with nanoinorganic nanofillers, at least one ofwhose dimensions is in the nanometer ranged from 1 to 100 nm,have invoked much interest in recent years [1,2]. Variousnanofillers such as nanoclays, carbon nanotubes, and nanosilicahave been found to improve the thermal, mechanical, electrical,and barrier properties of base polymer matrix significantly at verylow filler concentration compared to conventional composites [3–8]. This has opened up possibilities for producing high-perfor-mance lightweight composites without compromising otherproperties such as optical behavior or weight. Concurrently, anew class of nanofiller, polyhedral oligomeric silsesquioxane(POSS), has been emerging for use in nanostructured materials. ThePOSS possesses a compact hybrid structure with an inorganic coremade up of silicon and oxygen (SiO1.5)n (with n = 8, 10, and 12)externally surrounded by nonreactive or reactive polymerizableligands [9]. Interaction between the organic ligand and the matrixis known to control the degree of dispersion of POSS in the polymermatrix, i.e., its compatibility, and thus the final properties of thePOSS-modifed polymers. Therefore, the POSS can be dispersed on amolecular level (1–3 nm) or as aggregates, which could be on theorder of micrometers in size [10].

Regarding both its intrinsic chemical structure and controllablecharacteristics of the interaction with polymer, the incorporation

* Corresponding authors. Tel.: +82 32 860 7486; fax: +82 32 865 5178.

E-mail addresses: hjchoi@inha.ac.kr (H.J. Choi), ichin@inha.ac.kr (I.-J. Chin).

1226-086X/$ – see front matter � 2010 The Korean Society of Industrial and Engineer

doi:10.1016/j.jiec.2010.01.049

of POSS into polymeric materials may result in dramaticimprovements in polymer properties including increase in usetemperature, oxidative resistance, and surface hardening resultingin improved mechanical properties as well as reduction inflammability and heat evolution [11–14]. These enhancementshave been shown to apply to a wide range of thermoplastics and afew of thermoset systems. Among them, Zheng et al. [15] reportedthat POSS units incorporated as pendent groups to PE backboneaggregated as nanocrystals with an anisotropic crystalline shape,lowering the crystallinity of PE. Rheological and isothermalcrystallization behaviors of HDPE/octamethyl–POSS nanocompo-sites prepared by the melt mixing route were also reported [16,17].It was found that the nanocomposites with low POSS contentranged from 0.25 to 0.5 wt% showed significantly high storagemodulus and enhanced thermomechanical properties than HDPE,however, the modulus was decreased with an increase in POSSconcentrations due to the agglomeration of POSS particles [16]. Inaddition, Joshi and Butola [17] observed that the isothermalcrystallization kinetics of HDPE and HDPE/POSS nanohybridsfollowed the Avrami model, and only the POSS dispersed atmolecular level acts as a nucleating agent while the POSSnanocrystals did not affect the crystallization process.

While much effort has been focused on the development of newPOSS-containing nanohybrids, thermodynamic interaction be-tween the POSS and polymer matrix has never been studied.Therefore, in this study, we report the thermodynamic interactionof binary components in nanohybrids, where a few percent of POSSnanoparticles, which were functionalized with octamethyl-,octaisobutyl-, or octaphenyl-groups, were incorporated in the PEmatrix using the thermodynamic solubility parameter [18]. There

ing Chemistry. Published by Elsevier B.V. All rights reserved.

S.-K. Lim et al. / Journal of Industrial and Engineering Chemistry 16 (2010) 189–192190

are several different methods showing molar attraction constantsthat can be used for the calculation of the solubility parameters.Among them, the Small and Hoy method is generally used topredict the solubility parameter due to their simplicity. However,they were assumed that no specific forces such as hydrogenbonding are active between the structural units of the chemicalspecies involved, so the Small and Hoy method is applied to theamorphous polymers mostly. The thermodynamic solubilityparameters of PE and the functionalized POSS were evaluated toderive the Flory–Huggins interaction parameter based on themethod of Hoftyzer and van Krevelen. We also characterized theinteraction between the PE and POSS by measuring the thermaland mechanical properties.

2. Experimental

PE (Mw = 35,000 g/mol) was purchased from the AldrichChemical Co. The POSS nanoparticles, octamethyl–POSS(C8H24O12Si8, Fw = 536.96, denoted as PM), octaisobutyl–POSS(C32H72O12Si8, Fw = 873.60, denoted as PB), and octaphenyl–POSS(C48H40O12Si8, Fw = 1033.53, denoted as PP), were purchased fromthe Hybrid Plastics Inc. Nanohybrids of PE and POSS-derivativeswere prepared by the melt mixing method. Initially, PE wasintroduced into the torque rheometer (Plastograph EC, Brabender)and melted at 100 8C for 10 min with a rotary speed of 60 rpm.POSS was then added to the melted PE and compounded for 15 minto concentrations of 0.5 wt%, 1 wt% and 2 wt%.

FE-SEM (S-4300, Hitachi, Japan) using an attached X-ray energydispersive spectrometer (EDX) (E-1030, Horiba, UK) was applied toexamine the microstructure and to obtain the elemental composi-tions of the dispersive POSS nanoparticles in the PE matrix. Thespot size of electron beam used in the EDX was about 5 nm indiameter. Thermal stability was measured using a TGA (Q50, TAInstruments), by heating to 700 8C at the heating rate of 20 8C/minwith air purge. In order to measure the mechanical properties, thePE/POSS nanohybrid films were subjected to uniaxial elongation atroom temperature using UTM (Hounsfiled Test Equipment, UK),with a typical sample dimension of 10 mm (wide) � 50 mm(length) � 0.1 mm (thickness).

3. Results and discussion

FE-SEM investigation, coupled to EDX analysis, allowed to studythe morphology of the PE/POSS nanohybrids, paying specificattention to Si dispersion, i.e., POSS distribution, in the polymermatrix. Fig. 1 shows the FE-SEM micrographs of the PE nanohybridcontaining 1 wt% of PB–POSS. As shown in Fig. 1(a), no POSSaggregates are observed on the sample surface, and the elementalanalysis has shown a uniform Si distribution without any visible

Fig. 1. (a) FE-SEM micrographs and (b) X-ray map

aggregates in Fig. 1(b), where the filled circles indicate the Si-richzone, thus evidencing a submicrometer POSS dispersion.

The thermodynamic interaction parameter is an importantmeasure of the solubility of polymers in solvents and of thecompatibility between pairs of chemical species such as polymer–polymer and polymer–nanoparticle combinations. The thermody-namic interaction parameter is obtained from several experimen-tal methods such as the comparison of solubility parameters [19],the analysis of melting-point depressions [20], and light scatteringdata [21]. Among them, the solubility parameter, d, which is thesquare root of the cohesive energy density (the energy ofvaporization per unit volume), was used to predict the thermody-namic interaction between polymers and POSS-derivatives in thisstudy. Since it is not possible to obtain the molar vaporizationenergies for polymers, calculations based on group contributionsare used for the solubility parameters of the polymers. There areseveral different tables showing molar attraction constants thatcan be used for the calculation of the solubility parameters [19].Generally, the Small and Hoy method is used to calculate thesolubility parameter due to its simplicity. However, it was assumedthat no specific forces such as dispersion force, polar force andhydrogen bonding are active between the structural units of thesubstances involved. Therefore, the Small and Hoy method was notsuitable for crystalline polymers. In this study, we calculated thesolubility parameters of polymers and POSS-derivatives on thebasis of the following Eqs. (1) and (2), using the Hoftyzer and VanKrevelen method [19].

dd ¼P

Fdi

V; d p ¼

ffiffiffiffiffiffiffiffiffiffiffiffiPF2

pi

qV

; dh ¼ffiffiffiffiffiffiffiffiffiffiffiffiP

Ehi

V

r(1)

d2t ¼ d2

d þ d2p þ d2

h (2)

where dd, dp and dh are the dispersion, polar and hydrogen bondingcomponents of the solubility parameter, respectively. Fdi and Fpi arethe dispersion and polar portions of the molar attraction constant.The F-method is not applicable to the calculation of dh. It hasalready been stated by Hansen that the hydrogen bonding energyEhi per structural group is approximately constant. This leads to theform of Eq. (1). For molecules with several planes of symmetry,dh = 0. To calculate the solubility parameters of polymers and POSSthrough the Hoftyzer and Van Krevelen method, two assumptionswere made. POSS is inherently an organic/inorganic hybrid, and itis commonly felt that the inorganic part of POSS is not reactedcompletely in organic material. Thus, it is assumed that thefunctional group in the outer part of POSS dominates the solubilityparameter, and the inorganic part having a siloxane bonding ofPOSS is excluded in the calculation of the solubility parameter.

ping analysis of the PE/PB–POSS nanohybrid.

Table 1Thermodynamic solubility parameters of PE and POSS-derivatives.

Fda Fp

a Eh

(J mol�1)

V

(cm3 mol�1)

dd dp dh d(J1/2 cm�3/2)

PE 270 0 0 15.55 17.36 0 0 17.36

PM 420 0 0 21.55 19.49 0 0 19.49

PB 1190 0 0 68.21 17.45 0 0 17.45

PP 1430 110 0 74.52 19.19 1.48 0 19.25

a J1/2 cm3/2 mol�1.

Table 2TGA results for the neat PE and PE nanohybrids.

Td of PE = 277.6 8C

PE/PM–POSS PE/PB–POSS PE/PP–POSS

Content

(wt%)

0.5 1 2 0.5 1 2 0.5 1 2

Td (8C) 283.1 280.4 278.5 306.4 301.8 297.8 294.3 293.1 291.7

Fig. 3. Tensile strength of the neat PE and PE nanohybrids.

S.-K. Lim et al. / Journal of Industrial and Engineering Chemistry 16 (2010) 189–192 191

The calculated solubility parameters for the polymers and POSSare shown in Table 1. It can be seen in Table 1 that for all polymersand POSS-derivatives in this study, the maximum difference ofsolubility parameter showed the lowest value in POSS functiona-lized with the isobutyl group (PB–POSS). The meaning of themaximum difference of solubility parameter can be explained inEq. (3), which interrelates thermodynamic terms [19,22].

xAB ¼Vr

RTðdA � dBÞ2 (3)

where xAB is the Flory–Huggins interaction parameter of Polymer Aand POSS B, R and T are the gas constant and temperature, and Vr isa reference volume which is the molar volume of the smallestrepeat unit. The maximum difference of solubility parameterbetween PE and PB–POSS was calculated 0.09 J1/2 cm�3/2, whichwas much smaller than that for PE/PM–POSS (2.13 J1/2 cm�3/2) andPE/PP–POSS (1.89 J1/2 cm�3/2). Hence, it is expected that theinteraction between PE and PB–POSS was more thermodynami-cally favorable than the others.

Note that even though the solubility parameter is not a directmethod (experimental approach) but an indirect method (theo-retical approach) to determine the interaction characteristicsbetween the two components, it is possible to know the interactionof two chemical components using the solubility parameter. Ishidaet al. [23] found that the dispersion of the clay layers depends onthe maximum difference of solubility parameter between polymer

Fig. 2. TGA thermograms of the neat PE and PE nano

matrix and clay particle. Jang et al. [22] also reported theinteraction relationships between the solubility parameter ofpolymers and clay using the group molar attraction constant ofHoy’s table, finding that the square of the maximum difference ofsolubility parameter was directly proportional to the Flory–Huggins interaction parameter. Thermodymic compatibility ofclay in SAN/PVC matrix based on the calculated values of thesolubility parameters has been also examined [4]. Recently, wereported the interaction characteristics of exfoliated ABS nano-composites containing three different clays [24], finding that theinteraction parameter calculations between ABS and clayscorresponded well with the results based on the morphological,thermal, mechanical and rheological properties.

POSS has been proved to be effective in improving the thermalstability of polymers [25]. Thermal degradation of the neat PE andPE nanohybrids containing 0.5 wt% of POSS-derivatives is illus-trated in the TGA curves reported on Fig. 2. As shown in Fig. 2,thermograms for the all samples show only one sharp distinctdecomposition event, and the decomposition temperatures (Td)based on the 5 wt% loss of the PE nanohybrids can be found inTable 2. Both Fig. 2 and Table 2 clearly indicate that as the chainlength of the functional group of POSS is increased, there is adistinct shift in the onset of weight loss to higher temperature.Interestingly, the TGA trace recorded on the PE/PB–POSS nanohy-brids shows thyat the onset of degradation is higher about 30 8C. Itmay be due to that the oxidation of the long alkyl-substituted POSSin air takes place on the organic chains and leads to a cagecrosslinking, producing a ceramic silica-like phase [26]. We also

hybrids. The POSS content was fixed at 0.5 wt%.

Fig. 4. Elongation at break of the neat PE and PE nanohybrids.

S.-K. Lim et al. / Journal of Industrial and Engineering Chemistry 16 (2010) 189–192192

measured the mechanical properties such as tensile strength andelongation at break of the PE/POSS nanohybrids. Being indepen-dent of the type of POSS, the mechanical properties of thenanohybrids were higher than that of the neat PE, as shown inFigs. 3 and 4. These results show that the proper incorporation ofPOSS can effectively enhance the mechanical properties of PE resin[27]. In addition, we observed that maximum tensile strength andelongation at break were obtained in the PE/PB–POSS nanohybrid,indicating that the interaction between PE and long alkyl-substituted POSS is more favorable than that of PE/PM–POSSand PE/PP–POSS. These results corresponded well with the resultsbased on the thermodynamic solubility parameters. Finally it canbe also noted that there are many factors that influence on thecalculation of the solubility parameter in the case of crystallinepolymer. Therefore, we calculated the solubility parameter of PEand POSS-derivatives on the basis of the Hoftyzer and van Krevelenmethod. The Hoftyzer and van Krevelen method is known to be thebest method for calculating the solubility parameter of thecrystalline polymer because it contains the dispersion, polarityand hydrogen bonding components of the chemical component.For example, the solubility parameter of PE is ranged from 16.9 to17.6 J1/2 cm�3/2 experimentally. This value is very similar with ourcalculation (17.4 J1/2 cm�3/2) based on the Hoftyzer and vanKrevelen method.

4. Conclusions

PE nanohybrids containing three different POSS (PM–POSS, PB–POSS and PP–POSS) formed by the melt mixing method wereprepared, and their physical properties were investigated. Especial-ly, we used the maximum difference of solubility parameter usingthe method of Hoftyzer and van Krevelen to study the specificinteraction between PE and the functionalized POSS for the

theoretical considerations. The solubility parameter deferencebetween PE and PB–POSS was calculated 0.09 J1/2 cm�3/2, whichwas much smaller than that for PE/PM–POSS (2.13 J1/2 cm�3/2) andPE/PP–POSS (1.89 J1/2 cm�3/2). Hence, we expected that the interac-tion between PE and PB–POSS was more thermodynamicallyfavorable than the others. Thermal and mechanical properties ofthe PE/POSS nanohybrids also supported the predictions based onthe thermodynamic solubility parameters.

Acknowledgements

The financial support of this work by the Brain Korea 21 Projectin 2009 and Inha University is greatly acknowledged.

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