ORIGINAL PAPER

On Interaction Characteristics of PolyhedralOligomeric Silsesquioxane Containing PolymerNanohybrids

Sang-Kyun Lim1• Jae Yun Lee1 • Hyoung Jin Choi1 •

In-Joo Chin1

Received: 1 November 2014 / Revised: 20 April 2015 /Accepted: 24 May 2015 /

Published online: 3 June 2015

� Springer-Verlag Berlin Heidelberg 2015

Abstracts A generalized functional group of polyhedral oligomeric silsesquiox-

ane (POSS) suitable for various polymer systems, e.g., polyolefins, polyesters and

polyamides, is presented using theoretical and experimental approaches to examine

thermodynamic interaction between a polymer and POSS. Both Flory–Huggins

interaction parameter and maximum difference of the solubility parameter are uti-

lized to study theoretically specific interaction between polymers and POSS

nanoparticles. Flory–Huggins interaction parameter was estimated by the melting

point depression method determined by DSC, while maximum difference of the

solubility parameter was predicted using the method of Hoftyzer and van Krevelen.

The interaction characteristics of the polymer/POSS nanohybrids are further tested

by measuring the activation energy with the Kissinger method, in which the acti-

vation energy was calculated using the temperature at the maximum degradation

rate observed TGA. Viscoelastic, dynamic mechanical, thermal and mechanical

properties of the polymer/POSS nanohybrids were also examined to correlate the

theoretical and experimental results, finding that the isobutyl group was the most

suitable functional group of POSS for polyethylene, poly(ethylene terephthalate),

and Nylon 6.

Keywords Polyhedral oligomeric silsesquioxane � Thermodynamic interaction �Interaction parameter � Solubility parameter � Activation energy

Electronic supplementary material The online version of this article (doi:10.1007/s00289-015-1405-5)

contains supplementary material, which is available to authorized users.

& Hyoung Jin Choi

hjchoi@inha.ac.kr

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

123

Polym. Bull. (2015) 72:2331–2352

DOI 10.1007/s00289-015-1405-5

Introduction

For the last decade, the number of investigations on polymer-based nanohybrids,

derived from the hybridization of inorganic materials and organic polymers at

molecular scale, has been increased dramatically with the rapid growth of nanoscale

technologies [1, 2]. Nanohybrids, combining the important properties of inorganic

materials and organic polymers in general, can show improved properties, such as

high gas barrier characteristics, solvent resistance, and reduced flammability [3–6].

Among the various nanoreinforcements, polyhedral oligomeric silsesquioxane

(POSS) is particularly interesting. The properties of POSS are unique, since one or

more of the organic groups can be functionalized for polymerization, while the

remaining unreactive groups can solubilize the inorganic core and at the same time

control the interfacial interactions occurring between POSS and the polymer matrix

[7, 8]. Generally, POSS can be incorporated into all types of polymers either by

chemical tethering to the polymer chains (e.g., grafting and copolymerization

reactions) [9–16] or physical blending (e.g., solution blending and melt mixing)

[17–27], which would result in the enhancement of the polymer properties including

the increase of thermal stability and reduction in flammability and dielectric

constant [28–32]. Because of their advantageous performance relative to their

nonhybrid counterparts, POSS-containing nanohybrids can be made with the many

of the representative polymers such as polyolefins [33–35], polyesters [36–38],

polyamides [39, 40], styrenics [41–43], acrylates [44–47], polyurethanes [48–51],

thermosetting polymers [52] and others [53–56].

While considerable effort has been focused on the thermal properties [2, 26, 29,

57–61], morphology [13, 23, 62], mechanical properties [63–66] and self-assembly

of new POSS-containing nanohybrids [67, 68], there were a few studies that dealt

with the interrelationship between the polymer and POSS by analyzing the

miscibility and physical properties. However, there have been no studies which

directly examined the thermodynamic interaction between the polymer and POSS.

Huang et al. [2] investigated the miscibility and specific interaction behavior of

the poly(methyl methacrylate) (PMMA)–POSS systems by differential scanning

calorimetry (DSC) and fourier transform infrared spectroscopy (FT-IR) where

POSS was substituted with the isobutyltrisilanol group and PMMA with phenolic

resin. They found that the phenolic/PMMA–POSS blends with a positive q value

had single glass transition temperatures (Tgs), which were higher than those of the

phenolic/PMMA blends with a negative q value. The positive deviation of the

phenolic/PMMA–POSS blends revealed that a strong interassociation interaction

existed between the POSS siloxane and phenolic hydroxyl groups. FT-IR analysis

indicated that the PMMA chain of the low molecular weight PMMA (LPMMA,

Mn = 9800 g/mol)–POSS could not form entanglements with the lower hydrogen

bonding interaction between the LPMMA and phenolic resin. Furthermore, they

found a ‘‘screening effect’’ in these phenolic/LPMMA–POSS blends caused by the

POSS chain end tethered, which has the greater interassociation equilibrium

constant between hydroxyl and POSS than the interassociations equilibrium

constant between hydroxyl and carbonyl. On the contrary, the molecular weight

2332 Polym. Bull. (2015) 72:2331–2352

123

of the high molecular weight PMMA (HPMMA, Mn = 28,900 g/mol)–POSS is

above its entanglement molecular weight, that is, the hydrogen bonding between

POSS and the hydroxyl groups becomes less than that between PMMA and the

hydroxyl groups.

The structure–property relationships in organic–inorganic nanomaterials based

on methacryl-POSS and dimethacrylate networks were reported by Bizet et al. [14]

POSS molecules, acting as the pendant unit on the network backbone, showed

strong tendency toward aggregation and crystallization, depending on the nature of

the organic ligands. The POSS–POSS interaction was found to be the main

parameter governing the network morphology. However, the dynamic mechanical

properties remained nearly at the same level as those of the neat matrix.

Multifunctional POSS showed higher miscibility with the dimethacrylate monomer

and they were dispersed very well in the cured network. As expected, the rubbery

modulus increased with increasing amount of POSS due to the high functionality of

these additional cross-links, whereas Tg remained constant. Finally, methacrylate-

functionalized POSS can be expected to improve the properties of dimethacrylate-

based networks in the field of surface properties, optical properties and shrinkage,

rather than in the field of mechanical properties.

The miscibility behavior and interaction mechanism of PMMA, poly(vinyl

pyrrolidone) (PVP), and PMMA-co-PVP blends with octa(phenol)octa-silsesquiox-

ane (OP-POSS) were investigated using the DSC and FT-IR spectroscopy

techniques [69]. For the OP-POSS/PMMA blends, the value of the association

constant (KA = 29) was smaller than that in the poly(vinyl phenol) (PVPh)/PMMA

(KA = 37.4) and in the ethyl phenol (EPh)/PMMA (KA = 101) blend system,

implying that the spacing between the phenol groups attached to the POSS

nanoparticles was smaller than those of the other two blend systems, resulting in a

decrease in the ratio of the interassociation and self-association equilibrium

constants. In addition, the intermolecular hydrogen bonding became stronger than

the intramolecular hydrogen bonding after copolymerization with vinyl pyrrolidone

(VP), because the OH groups preferred to interact with the VP segments.

Liu and coworkers [70] studied the hydrogen bonding interaction in three

different types of incompletely condensed silsesquioxanes (POSS-mono-ol, POSS-

diol and POSS-triol) [70], finding that POSS-diol and POSS-mono-ol could not

form a dimer in solution and there existed a dynamic equilibrium between the single

molecule and hydrogen-bonded dimer for POSS-triol.

Thermal and rheological behavior of polystyrene (PS)-based random copolymers

were studied, in which POSS was incorporated with three kinds of vertex groups,

viz. isobutyl (iBu), cyclopentyl (Cp) and cyclohexyl (Cy) [71]. The weak iBuPOSS-

PS segment interaction resulted in the Tg that monotonically decreased with

increasing iBuPOSS content. Conversely, the strong CpPOSS/CyPOSS-PS segment

interaction resulted in the increase of the Tg, through with complex dependence in

the case of CyPOSS. Wu et al. [71] asserted that the dependence of the Tg of the

copolymers on the vertex group results from the competing effects between the

addition of free volume and intermolecular interactions. They also found through

dynamic mechanical analysis that the effect of the vertex group played only a minor

role in the rubbery plateau. More specifically, the rubbery plateau modulus

Polym. Bull. (2015) 72:2331–2352 2333

123

decreased with increasing POSS content and in proportion to the size of the POSS,

in the order of iBuPOSS[CpPOSS[CyPOSS.

On the other hand, Tanaka and coworkers [72, 73] studied the structure–property

relationship of octa-substituted aliphatic and aromatic POSS with the conventional

polymer regarding asymmetry effect, finding that the longer alkyl chains and

unsaturated bond in the side chain of POSS are efficient to improve the thermal

stability and the elasticity of the polymer matrix. In addition, phenyl-POSS also

showed superior ability to improve the thermal and mechanical properties of

polymers.

Furthermore, the influence of symmetry/asymmetry of POSS structure on the

thermal stability of POSS/PS nanocomposites of formula R8(SiO1.5)8 POSS/PS and

R01R7(SiO1.5)8POSS/PS (where R0 = phenyl and R = cyclopentyl) was investi-

gated [74]. The initial decomposition temperature (Ti), the temperature at 5 % mass

loss (T5 %), and the activation energy of degradation of nanocomposites measured

using thermal gravimetric analyzer (TGA) were higher than those of neat PS. Thus,

they indicated that the nanocomposites have a better heat resistance and lower

degradation rate, and then a better overall thermal stability as compared to the neat

polymer. The use of POSS with a symmetric structure, in the nanocomposite,

showed a decrease of Tg not only in respect to asymmetric POSS/PS nanocomposite

but also in respect to neat polymer, thus suggesting an influence of filler structure in

the thermal properties of the materials.

Recently, we reported the Flory–Huggins interaction parameter of exfoliated

poly(acrylonitrile-co-butadiene-styrene) (ABS) nanohybrids with three different

organoclays, Cloisite10A� (C10A), Cloisite25A� (C25A) and Cloisite30B� (C30B)

using the solubility parameter [5]. The Flory–Huggins interaction parameter

between ABS and C30B was calculated to be 0.54, which was smaller than that for

ABS/C10A (0.74) and ABS/C25A (0.67), suggesting that the interaction between

ABS and C30B would be more thermodynamically favorable than that between

ABS and C10A or C25A. We also reported the interaction characteristics of

organically modified montmorillonite (OMMT) nanohybrids with a miscible

polymer blend of poly(ethylene oxide) (PEO) and poly(methyl methacrylate)

(PMMA) [75]. The interaction parameter value for the PMMA/OMMT pair was

estimated to be smaller than that measured for the PEO/OMMT pair, showing that

PMMA had better affinity for OMMT than PEO.

In this study, we attempted to establish a generalized functional group for POSS

suitable for the representative polymer systems, i.e., polyethylene (PE) for

polyolefins, poly(ethylene terephthalate) (PET) for polyesters and Nylon 6 for

polyamides, using theoretical and experimental approaches for the first time. For the

theoretical considerations, the Flory–Huggins interaction parameter, the maximum

difference of the solubility parameter, and the activation energy were used to study

the specific thermodynamic interaction between the polymers and functionalized

POSS nanoparticles. The Flory–Huggins interaction parameter was determined by

the melting point depression method using DSC, based on the classical Flory–

Huggins theory. The solubility parameters of the polymers and functionalized POSS

nanoparticles were calculated using the method of Hoftyzer and van Krevelen. The

thermodynamic interaction characteristics of the nanohybrids were further

2334 Polym. Bull. (2015) 72:2331–2352

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characterized by measuring the activation energy with the Kissinger method.

Thermal, mechanical, rheological and morphological analyses of the nanohybrids

were also performed to correlate the theoretical and experimental results. The

polymer nanohybrids containing the various functionalized POSS were prepared by

the melt mixing method.

Experimental section

Reactants and preparation method

PE (Mw = 35,000 g/mol), poly(ethylene terephthalate) (PET,Mw = 50,000 g/mol),

and Nylon 6 (Mw = 65,000 g/mol), used as the polymer matrix, were purchased from

Aldrich Chemical Co. The POSS nanoparticles employed in this study are octaphenyl-

POSS (OP-POSS, C48H40O12Si8, Fw = 1033.53), aminopropylphenyl-POSS (AP-

POSS, C45H43NO12Si8, Fw = 1014.52)), octamethyl-POSS (OM-POSS, C8H24O12-

Si8, Fw = 536.96), octaisobutyl-POSS (OB-POSS, C32H72O12Si8, Fw = 873.60),

aminopropylisobutyl-POSS (AB-POSS, C31H71NO12Si8, Fw = 874.58), and amino-

propylisooctyl-POSS (AO-POSS, C59H127NO12Si8, Fw = 1267.32). All the POSS

nanoparticles were purchased from Hybrid Plastics Inc.

Nanohybrids of the polymer and POSS nanoparticles were prepared by the melt

mixing method. Initially, the polymer was introduced into a torque rheometer

(Plastograph EC, Brabender, Germany) and melted at its melting point for 10 min

with a rotary speed of 60 rpm. POSS was then added to the melted polymer and

compounded for 15 min to concentrations of 0.5, 1 and 2 wt%. OM-POSS, OB-

POSS and OP-POSS were used for the preparation of PE and PET nanohybrids. In

the case of the Nylon 6 nanohybrids, AB-POSS, AO-POSS and AP-POSS were used

as the POSS nanoparticles.

Characterization

The nanostructures of the dispersed POSS nanoparticles in the polymer matrix were

examined by field-emission transmission electron microscopy (FE-TEM, JEM

2100F, JEOL, Japan) at an accelerated voltage of 100 kV. All ultrathin sections

(less than 3 lm) were microtomed using an ultramicrotome (RMC-MTX, USA)

with a diamond knife and observed by FE-TEM without staining. To measure the

mechanical properties, the nanohybrid films were subjected to uniaxial elongation

using a universal test machine (UTM, Houndfield Test Equipment, UK), with a

typical sample dimension of 10 mm width 9 50 mm length 9 0.1 mm thickness.

Mechanical tests were performed at room temperature and a crosshead speed of

5 mm/min. The rheological characteristics were also measured using a rotational

rheometer (HCR 300, Physica, Germany) equipped with a parallel plate geometry

(25 mm diameter) and TEK 350 temperature controller. All samples were measured

in the melt state with a gap distance of 1 mm.

The thermal stability of the samples was measured with thermogravimetric

analysis (TGA, Q50, TA Instruments, USA) by heating them from room

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temperature up to 750 �C at a heating rate of 10 �C min-1 with air flowing. The

samples were also heated to 750 �C at heating rates of 20 �C min-1 and 40 �Cmin-1 with air flowing to calculate the activation energy. Differential scanning

calorimetry (DSC, Pyris Diamond DSC, Perkin-Elmer, USA) was employed to

determine the Flory–Huggins interaction parameter between the polymer and POSS

by monitoring the change in the melting point. To determine the equilibrium

melting temperature (Tm0 ), samples were heated 10 �C above their melting

temperatures (Tm) and maintained for 5 min for the complete melting of crystals.

Subsequently samples were cooled down to their crystallization temperatures (Tc)

and kept 30 min before heating to Tm ? 10 �C with a rate of 20 �C min21.

Results and discussion

Flory–Huggins interaction and solubility parameters

The Flory–Huggins interaction parameter is an important measure that can

determine the solubility of polymers in solvents or the compatibility between pairs

of chemical species such as polymer/polymer and polymer/nanoparticle combina-

tions, and can be obtained experimentally using several methods such as the melting

point depression [75, 76], solubility parameter [77, 78], vapor sorption [79, 80],

inverse-phase gas chromatography [81, 82], small-angle neutron scattering [83, 84],

and small-angle X-ray scattering [85, 86].

Nishi and Wang [76], in their analysis of the reduction of the melting temperature

of a crystalline polymer in the presence of an amorphous one, derived a simple

equation which related the melting point depression directly to the Flory–Huggins

interaction parameter. The relation between the melting point depression and the

Flory–Huggins interaction parameter of the mixture can be described by the

following equation.

T0m � T0

mix ¼ �BV

DHT0mð1� /iÞ2 ð1Þ

where Tm0 and Tmix

0 are the equilibrium melting temperature of the crystalline

polymer and the mixture, respectively, DH/V is the heat of fusion of the crystalline

polymer per unit volume, /i is the weight fraction of the crystalline polymer, and

B is the Flory–Huggins interaction parameter between the two components. The

Flory–Huggins interaction parameter B can be obtained from the slope of the plot of

(Tm0 - Tmix

0 ) vs (1 - /i)2. A negative value of B for the binary system indicates that

the two chemical species form a thermodynamically stable and compatible mixture.

As shown in Fig. 1, the equilibrium melting temperatures of the pure PE and the PE/

POSS nanohybrids were obtained using Hoffman–Weeks plots.

Figure 2 represents the melting point depression of PE in the PE/POSS

nanohybrids as a function of the PE content. According to Eq. (1), the B values

for the PE/POSS nanohybrids were determined from the slope of the straight line of

Fig. 2. When values of 30.9 cm3 mol-1 and 970.1 cal mol-1 were used for Viu and

DHiu, respectively, the estimated Flory–Huggins interaction parameters were

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-1.95 cal cm-3, -3.39 cal cm-3 and -0.93 cal cm-3 for BPE/OM-POSS, BPE/OB-POSSand BPE/OP-POSS, respectively, indicating that the PE with OB-POSS is thermodynam-

ically most favorable hybrid.

Fig. 1 Hoffman–Weeks plotsof a PE, b PE/OM-POSS, c PE/OB-POSS, and d PE/OP-POSSnanohybrids

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By following the same experimental procedures, the Flory–Huggins interaction

parameter between PET and OB-POSS was determined to be -1.11 cal cm-3,

which was smaller than that for PET/OM-POSS (-0.29 cal cm-3) and PET/OP-

POSS (-0.10 cal cm-3) (Fig. S1 and S2 in the Supporting Information). Likewise,

the Flory–Huggins interaction parameters of Nylon 6/AB-POSS, Nylon 6/AO-POSS

and Nylon 6/AP-POSS were -0.42 cal cm-3, -0.35 cal cm-3 and

-0.23 cal cm-3, respectively. Therefore, the interaction of the PET/OB-POSS

and Nylon 6/AB-POSS nanohybrids were found to be thermodynamically more

favorable than those of the other combinations.

The solubility parameter, d, which is the square root of the cohesive energy

density (the energy of vaporization per unit volume), was also used to predict the

thermodynamic interaction between the polymer and POSS nanoparticles. Since it is

not possible to obtain the molar vaporization energies for polymers, calculations

Fig. 2 Plots of the equilibriummelting points of PE in the a PE/OM-POSS, b PE/OB-POSS, andc PE/OP-POSS nanohybrids

2338 Polym. Bull. (2015) 72:2331–2352

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based on the group contributions are used to determine the solubility parameters of

the polymers. The Small and Hoy method is generally used to compute the

solubility parameter, due to its simplicity. However, no specific forces, such as the

dispersion force, polar force and hydrogen bonding, are assumed to be active

between the structural units of the substances involved. Therefore, the Small and

Hoy method is considered to be unsuitable for crystalline polymers. In this study,

the solubility parameters of the polymers and POSS nanoparticles were calculated

according to the Hoftyzer and van Krevelen method using the following equations

[78].

dd ¼P

Fdi

V; dp ¼

ffiffiffiffiffiffiffiffiffiffiffiffiPF2pi

q

V; dh ¼

ffiffiffiffiffiffiffiffiffiffiffiffiPEhi

V

r

ð2Þ

d2t ¼ d2d þ d2p þ d2h ð3Þ

where dd, dp and dh are the dispersion, polar and hydrogen bonding components of

the solubility parameter, respectively. Fdi and Fpi are the dispersion and polar

portions of the molar attraction constant, respectively. The F-method is not appli-

cable to the calculation of dh. Hansen previously stated that the hydrogen bonding

energy, Ehi, per structural group is relatively constant, which led to Eq. (2). For

molecules with several planes of symmetry, dh = 0. A assumption was made to

calculate the solubility parameters of the polymers and POSS using the Hoftyzer

and van Krevelen method.

POSS is inherently an organic/inorganic hybrid, and the inorganic part of POSS,

mainly –Si–O–Si–, does not react completely in an organic material. The distance

between Si and O of the –Si–O–Si– is 1.64 A, and it is shorter than the sum of the

covalent radii of POSS, 1.76 A, meaning that there exists a partial double bond

character of –Si–O– [87]. Nonetheless, the barrier of rotation around the –Si–O–

axis, ca. 2.5 kJ mol-1 as well as the barrier of linearization of the –Si–O–Si– angle,

ca. 1.3 kJ mol-1, is very low. Thus, the siloxane chain is rigid, so much that the

–Si–O–Si– angle, 140�–180�, is much wider than the tetrahedral angle, the silicon

atom has a relatively large size and the substituents appear only at every second

atom in the chain [87]. These features also account for the relatively high steric

hindrance effect, which indicates the siloxane group is highly stable. Therefore, it

was assumed that the functional groups in the outer surface of POSS dominate the

solubility parameter, and the inorganic part with the siloxane bonding of POSS was

excluded from the calculation of the solubility parameter.

Table 1 lists the calculated solubility parameters for all the polymers and POSS

nanoparticles. Themaximum difference in the solubility parameter showed the lowest

value in the POSS functionalized with the isobutyl group. Themaximum difference of

the solubility parameter between PE andOB-POSSwas calculated to be 0.09 J1/2 cm-3/2,

which was significantly smaller than those for PE/OM-POSS (2.13 J1/2 cm-3/2)

and PE/OP-POSS (1.89 J1/2 cm-3/2). The maximum difference in the solubility

parameters of PET/OM-POSS, PET/OB-POSS and PET/OP-POSS were calculated

to be 1.58 , 0.46 and 1.34 J1/2 cm-3/2, respectively. Also, the maximum difference

in the solubility parameter of Nylon 6/AB-POSS (0.56 J1/2 cm-3/2) was the lowest

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among the Nylon 6/POSS nanohybrids. The variation of the maximum difference in

the solubility parameter can be explained by Eq. (4), which interrelates the

thermodynamic terms [77, 78].

vAB¼Vr

RTðdA � dBÞ2 ð4Þ

where vAB is the Flory–Huggins interaction parameter of polymer A and POSS B,

R and T are the gas constant and temperature, respectively, and Vr is a reference

volume which is the molar volume of the smallest repeat unit. Therefore, it was

expected that the interactions between the three polymers, viz., PE, PET and Nylon

6, and POSS functionalized with the isobutyl group, would be more thermody-

namically favorable than those of the others. These results correspond well with the

result obtained based on the melting point depression method.

Activation energy

The activation energy of the polymer/POSS nanohybrids can be determined by

employing the Kissinger’s equation shown below [88].

lnb

T2max

¼ lnAR

Eþ ln nð1� amaxÞn�1

h i� �

� E

RTmax

ð5Þ

where b is the heating rate (K min-1), Tmax is the temperature at the maximum

degradation rate, A is the pre-exponential factor, amax is the maximum conversion,

and n is the reaction order. Tmax was determined from the differential TGA curves.

Figure 3 shows the plots of ln(bTmax-2 ) versus Tmax

-1 according to the Kissinger’s

method for the PE/POSS and PET/POSS nanohybrids containing 0.5 wt% of POSS.

Tables 2 and 3 list the values of ln(bTmax-2 ), Tmax

-1 and calculated activation energy for

all of the neat polymers and polymer/POSS nanohybrids. The activation energies of

the PE, PE/OM-POSS, PE/OB-POSS and PE/OP-POSS nanohybrids, which were

Table 1 Solubility parameters of polymers and POSS derivatives

Fda Fp

a Eh

(J mol-1)

V

(cm3 mol-1)

dd (J1/2

cm-3/2)

dp (J1/2

cm-3/2)

dh (J1/2

cm-3/2)

d (J1/2

cm-3/2)

Polymers

–CH2– 270 0 0 15.55 17.36 0 0 17.36

–COO– 390 490 7000 23.70 16.46 20.68 17.19 17.91

–CONH– 450 980 5100 28.30 15.90 34.63 13.42 18.01

POSS derivatives

–CH3 420 0 0 21.55 19.49 0 0 19.49

–C(CH3)3 1190 0 0 68.21 17.45 0 0 17.45

–C(CH3)7 2870 0 0 154.41 18.59 0 0 18.59

–Phenyl 1430 110 0 74.52 19.19 1.48 0 19.25

a J1/2 cm-3/2 mol-1

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calculated from the slope of the straight line in Fig. 3a, were 17.3, 27.1, 33.3 and

24.4 kJ mol-1, respectively. In the case of the PET/POSS nanohybrids, the acti-

vation energies of PET, PET/OM-POSS, PET/OB-POSS and PET/OP-POSS were

9.53, 11.43, 24.54 and 7.12, respectively (Fig. 3b). Likewise, the highest energy was

obtained for the Nylon 6/AB-POSS nanohybrids. The polymer nanohybrids with the

POSS functionalized with isobutyl group showed the highest energy, which means

that isobutyl-POSS was dispersed most uniformly throughout the polymer matrix

due to the increased interaction between the polymer and isobutyl-POSS.

Fig. 3 Determination of theactivation energies for the a PE/POSS and b PET/POSSnanohybrids. The POSS contentwas fixed at 0.5 wt%

Table 2 Activation energy data for the neat PE and PE nanohybrids

PE PE/OM-POSS

b (K min-1) 283.13 293.13 313.13 283.13 293.13 313.13

1000 Tmax-1 (K-1) 1.558 1.483 1.435 1.519 1.463 1.408

ln(b Tmax-2 ) -7.431 -7.344 -7.182 -7.724 -7.533 -7.375

Ea (kJ mol-1) 17.3 27.1

PE/OB-POSS PE/OP-POSS

b (K min-1) 283.13 293.13 313.13 283.13 293.13 313.13

1000 Tmax-1 (K-1) 1.512 1.453 1.388 1.531 1.473 1.423

ln(b Tmax-2 ) -7.962 -7.677 -7.451 -7.583 -7.454 -7.275

Ea (kJ mol-1) 33.3 24.4

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POSS dispersion in the nanohybrids

The nanostructure formation of the polymer/POSS nanohybrids was examined by

FE-TEM. The FE-TEM micrographs of the POSS-filled nanohybrids show dark

zones that correspond to POSS-rich zones, because of their higher electron density

due to the presence of the silicon atoms [14]. Figure 4 shows the FE-TEM

micrographs of the PE nanohybrids containing 0.5 wt% of OM-POSS, OB-POSS

and OP-POSS. In Fig. 4b, the PE nanohybrid containing 0.5 wt% of OB-POSS

shows relatively uniform dispersion of OB-POSS, with sizes ranging from 20 to

25 nm. In contrast to the results obtained for the PE/OB-POSS nanohybrids, the

dispersion of the OM-POSS and OP-POSS domains is not uniform and somewhat

aggregated, as shown in Fig. 4a and c. Most of the aggregates have a size of about

100 nm and their structure is not well defined. Two factors may explain this

behavior; (1) the low solubility of OM-POSS and OP-POSS in the PE matrix and (2)

the strong tendency of these POSS molecules to aggregate because of the very

favorable OM-POSS/OM-POSS and OP-POSS/OP-POSS interactions [14]. The FE-

TEM results obtained for the PET/POSS and Nylon 6/POSS nanohybrids were very

similar to those obtained for the PE/POSS nanohybrids (Fig. S3 and S4 in the

Supporting Information). It is clearly shown that the OB-POSS nanoparticles were

more well dispersed in the PET matrix than the OM-POSS and OP-POSS

nanoparticles. Also, the Nylon 6/0.5 wt% AB-POSS nanohybrid contains individual

POSS particles that are well dispersed in the Nylon 6 matrix, while aggregations of

AO-POSS and AP-POSS could be seen in the FE-TEM images.

Viscoelastic and dynamic mechanical properties

Viscoelastic properties of the PE/POSS nanohybrids were characterized by

measuring their rheological properties under oscillatory shear. Figure 5 shows the

dynamic moduli (storage modulus, G0 in Fig. 5a and loss moduli, G00 in Fig. 5b) of

the PE/POSS nanohybrids as a function of the angular frequency by keeping the

Table 3 Activation energy data for the neat PET and PET nanohybrids

PET PET/OM-POSS

b (K min-1) 283.13 293.13 313.13 283.13 293.13 313.13

1000 Tmax-1 (K-1) 1.412 1.39 1.374 1.418 1.397 1.384

ln(b Tmax-2 ) -7.479 -7.476 -7.433 -7.47 -7.466 -7.419

Ea (kJ mol-1) 9.53 11.43

PET/OB-POSS PET/OP-POSS

b (K min-1) 283.13 293.13 313.13 283.13 293.13 313.13

1000 Tmax-1 (K-1) 1.415 1.401 1.391 1.415 1.39 1.373

ln(b Tmax-2 ) -7.479 -7.437 -7.408 -7.475 -7.476 -7.436

Ea (kJ mol-1) 24.54 7.12

2342 Polym. Bull. (2015) 72:2331–2352

123

oscillation within the linear region of 1 % strain amplitude. The storage and loss

moduli increased with increasing POSS content compared to those of the neat PE

throughout the applied frequency range. The slope of the change in the storage

modulus decreased with increasing POSS content, exhibiting a monotonic increase

of G0 for all frequencies, as shown in Fig. 5a. The fact that G0 became less

dependent on the frequency may be due to the formation of a network structure of

the POSS, indicating that the PE/POSS nanohybrids possessed solid-like charac-

teristics [88, 89]. In fact, the PE/OB-POSS nanohybrid shows more solid-like

behavior than the PE/OM-POSS and PE/OP-POSS nanohybrids. Moreover, the PE/

OB-POSS nanohybrids exhibited a greater increase of their storage modulus than

Fig. 4 FE-TEM images ofa PE/OM-POSS, b PE/OB-POSS, and c PE/OP-POSSnanohybrids. The POSS contentwas fixed at 0.5 wt%

Polym. Bull. (2015) 72:2331–2352 2343

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the PE/OM-POSS and PE/OP-POSS nanohybrids throughout the frequency range,

which means that the interaction between PE and OB-POSS was more favorable.

Solid-like behavior can also be observed in the complex viscosity curve in Fig. 5c.

As shown in Fig. 5c, the complex viscosity of the PE/POSS nanohybrids was higher

than that of the neat PE in the whole frequency range. The PE/OB-POSS

nanohybrid, in particular, showed more rapid shear thinning behavior, suggesting

that the interaction between PE and OB-POSS, which exhibited solid-like

characteristics, was higher than that of the other samples. These results

corresponded well with those obtained based on the Flory–Huggins interaction

parameter and the solubility parameter.

The storage modulus describes the stiffness of a material. The storage moduli for

the PE/POSS nanohybrids, measured at 1 Hz using DMA, are shown in Fig. 6 as a

function of temperature. Below Tg, the G0 is high because the polymer is in the

glassy state. However, above Tg, the G0 decreases because the polymer chains

Fig. 5 Rheological propertiesof the PE/POSS nanohybrids:a storage modulus, b lossmodulus, and c complexviscosity. The POSS content wasfixed at 0.5 wt%

2344 Polym. Bull. (2015) 72:2331–2352

123

become mobile and the polymer is in the rubbery state. The G0 of the PE/OB-POSSnanohybrids increases with increasing OB-POSS content below and above their Tg(Fig. 6a). The same trend is observed for the PE/OP-POSS nanohybrids, as shown in

Fig. 6b. It is because POSS particles are rigid and, thus, the addition of POSS would

increase the rigidity of the nanohybrid system [90]. In Fig. 6a, the a-transitiontemperature decreases monotonically for all the PE nanohybrids, suggesting that the

free volume in the PE increases upon the addition of the POSS nanoparticles. The b-transition temperature also shifts to a lower temperature, and the b-peak lowers and

broadens when POSS is added. Therefore, POSS has a plasticizing effect on the b-transition as well as on the a-transition due to the free volume on the local motions

[63–66]. The viscoelastic and dynamic mechanical behavior of the PET/POSS

nanohybrids were very similar to those of the PE/POSS nanohybrids (Fig. S5 and S6

in the Supporting Information).

Fig. 6 Dynamic mechanicalproperties of the PE/POSSnanohybrids

Polym. Bull. (2015) 72:2331–2352 2345

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However, the Nylon 6/POSS nanohybrids exhibit a significantly higher Tg than

pure Nylon 6, as shown in Fig. 7. The reason for the increase of Tg upon the

incorporation of POSS could be twofold. First, the incorporation of POSS increases

the cross-linking density of the resulting nanohybrids. The increase in the cross-

linking density leads to a higher Tg, broader tan d peak and higher storage modulus.

Second, POSS is a rigid body, that is, the addition of POSS would increase the

rigidity of the nanohybrid system [91, 92]. Although the incorporation of POSS

increases the free volume in the Nylon 6/POSS nanohybrids, this effect can be

compensated by the increase in the cross-linking density.

Thermal and mechanical properties

POSS has been proved to be effective in improving the thermal stability of polymers

[2, 26, 28, 29, 57, 60, 61]. The decomposition temperatures (Td) based on the 5 wt%

loss of the neat polymers and polymer/POSS nanohybrids are listed in Table 4.

Table 4 indicates that as the chain length of the functional group of POSS is

increased, there is a distinct shift in the onset of weight loss to a higher temperature.

Interestingly, the Td recorded for the PE/OB-POSS nanohybrid (99.5/0.5 by weight)

shows that the onset of degradation is higher by about 30 �C. The Tds of the PET/

OB-POSS (99.5/0.5) and Nylon 6/AB-POSS (99.5/0.5) nanohybrids are about 10 �Chigher than those of the neat PET and Nylon 6, respectively. This may be due to the

fact that the oxidation of the alkyl-substituted POSS in air takes place on the organic

chains and leads to the cross-linking of the cage, producing a ceramic silica-like

phase [93, 94]. On the other hand, the long alkyl-substituted POSS, that is, AO-

Fig. 7 Tan d of a Nylon 6/AB-POSS and b Nylon 6/AP-POSSnanohybrids

2346 Polym. Bull. (2015) 72:2331–2352

123

POSS (composed of eight hydrocarbon chains), induces a reduction in the thermal

stability in comparison with AB-POSS.

The highest values of the tensile strength and elongation at break were observed

for the PE/OB-POSS nanohybrid (99.5/0.5), which were 92 and 36 % above those

of the neat PE, respectively. However, the mechanical properties decrease as the

amount of the POSS nanoparticles increases to 1 and 2 wt%. With respect to the

corresponding value of the neat PET, the addition of 0.5 wt% OB-POSS to PET

causes an increase in the tensile strength of about 30 %, while a more pronounced

increase in the elongation at break (300 %) is observed for the PET/OB-POSS

nanohybrid (99.5/0.5). In addition, the maximum tensile strength and elongation at

break are obtained in the Nylon 6/AB-POSS nanohybrid (95.5/0.5). These results

suggest that the interaction between the polymer and isobutyl-substituted POSS is

more favorable than that of the other samples. The thermal and mechanical results

are also in good agreement with the theoretical considerations based on the Flory–

Huggins interaction parameter, solubility parameter, and activation energy.

Conclusions

The Flory–Huggins interaction parameters between the polymers and POSS

nanoparticles were determined using the melting point depression method. In the

case of the PE nanohybrids, PE with OB-POSS was thermodynamically most

favorable. The thermodynamic interactions of both the PET/OB-POSS and Nylon

6/AB-POSS pairs were also found to be highly favorable according to the Flory–

Huggins interaction parameters. The maximum difference of the solubility

parameter between PE and OB-POSS was much smaller than those for PE/OM-

POSS and PE/OP-POSS, indicating that the interaction between PE and OB-POSS

was more favorable than that of the others. In the PET nanohybrids, the maximum

solubility parameter difference of PET/OB-POSS pairs was less than the others,

suggesting that the thermodynamic interaction of the PET/OB-POSS pair is most

Table 4 TGA results for the neat polymers and polymer nanohybrids

PE PE/OM-POSS PE/OB-POSS PE/OP-POSS

Content (wt%) 100 0.5 1 2 0.5 1 2 0.5 1 2

Td (�C)a 277.6 283.1 280.4 278.5 306.4 301.8 297.8 294.3 293.1 291.7

PET PET/OM-POSS PET/OB-POSS PET/OP-POSS

Content (wt%) 100 0.5 1 2 0.5 1 2 0.5 1 2

Td (�C) 377.5 378.3 370.0 359.7 384.4 383.6 381.9 376.8 377.9 375.7

Nylon 6 Nylon 6/AB-POSS Nylon 6/AO-POSS Nylon 6/AP-POSS

Content (wt%) 100 0.5 1 2 0.5 1 2 0.5 1 2

Td (�C) 374.8 385.4 382.1 380.8 381.2 380.4 379.5 380.9 380.4 378.4

a 5 wt% loss

Polym. Bull. (2015) 72:2331–2352 2347

123

favorable. Nylon 6 and AB-POSS were highly compatible based on the calculated

values of the solubility parameters, whereas the Nylon 6/AO-POSS and Nylon

6/AP-POSS pairs were less compatible.

The activation energies of the PE, PE/OM-POSS, PE/OB-POSS and PE/OP-

POSS pairs were predicted by the Kissinger method. The highest activation energies

were also obtained for the PET/OB-POSS and Nylon 6/AB-POSS pairs. The fact

that the highest activation energy was obtained for the polymer nanohybrids with

isobutyl-substituted POSS means that the nanoparticles were dispersed most

uniformly throughout the polymer matrix due to their increased interaction with the

polymer. It was concluded from the theoretical approaches that the isobutyl group

was the most suitable functional group for POSS for PE, PET, and Nylon 6. The

viscoelastic, morphological, thermal and mechanical results also supported the

predictions based on the theoretical studies, while thermal and mechanical

properties of the polymer nanohybrids with isobutyl-substituted POSS were

superior to those of the other polymer nanohybrids.

Acknowledgments One of the authors (S. K. Lim) thanks Service Engineer Se-Hoon Byun at Perkin-

Elmer Ltd., Korea for generously providing the usage of the DSC.

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