Synthesis and characterization of amorphous octakis-functionalized polyhedral oligomeric silsesquioxanes for polymer nanocomposites

Synthesis and characterization of amorphous octakis-functionalized polyhedral

oligomeric silsesquioxanes for polymer nanocomposites

Yuung-Ching Sheen

, Chu-Hua Lu

, Chih-Feng Huang

, Shiao-Wei Kuo

, Feng-Chih Chang

a_{Institute of Applied Chemistry, National Chiao Tung University, 30010 Hsinchu, Taiwan, ROC}

b_{Department of Materials Science and Engineering, National Sun Yat-Sen University, 804 Kaohsiung, Taiwan, ROC}

a r t i c l e

i n f o

Article history: Received 10 May 2008

Received in revised form 24 June 2008 Accepted 27 June 2008

Available online 6 July 2008 Keywords:

Amorphous Nanoparticles Nanocomposites

a b s t r a c t

We have synthesized the polyhedral oligomeric silsesquioxane (POSS) derivatives OS-POSS and OA-POSS through the hydrosilylation of styrene and 4-acetoxystyrene, respectively, with octakis(dimethylsiloxy)-silsesquioxane (Q8M8H). We then prepared OP-POSS through acetoxyl hydrazinolysis of OA-POSS with

hydrazine monohydrate. The chemical structures of these POSS derivatives were characterized using FTIR and1_{H NMR spectroscopy and MALDI-TOF mass spectrometry. Unlike Q}

8M8H, which is crystalline, these

three hydrosilylated POSS derivatives are liquids at 25_{C. Through DSC and XRD analyses, we found that} they exhibit polymer-like glass transitions and amorphous halos. These octakis-functionalized POSS derivatives can be regarded as amorphous glasses possessing low glass transition temperatures, the values of which depend on the intermolecular interactions of their outer organic groups. To investigate the dispersion of these POSS derivatives in polymer nanocomposites, we blended OS-POSS, OA-POSS, and OP-POSS with polystyrene, poly(4-acetoxystyrene), and poly(4-vinylpyridine), respectively, and investigated the effects of the resulting intermolecular aromatic hydrophobic, dipole–dipole, and hy-drogen-bonding interactions, respectively. Dipole–dipole interactions provided the best dispersion of OA-POSS in poly(4-acetoxystyrene), in which the POSS–polymer intermolecular interactions were of similar strength to the POSS–POSS interactions.

Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction

When developing polymer nanocomposites, the attachment of organic groups to nanosized materials can have wide-ranging im-plications on the interactions occurring at the interfaces between the inorganic composite particles and the organic polymer matrices [1–5]. Polyhedral oligomeric silsesquioxane (POSS) derivatives possess well-defined chemical structures; they have the general

formula of (RSiO1.5)8, where the R units are organic groups located

at the corners of an octahedral siloxane cube (SiO1.5)8(Fig. 1a)[7–

12]. In addition to their high compatibility with polymers, POSS

derivatives impart excellent thermal and mechanical properties to polymer nanocomposites. The presence of thermally stable POSS siloxane cores can prevent surface thermal degradation of poly-meric matrices; microscopic POSS aggregation can result in hard domains that restrict polymer deformation during the application

of an external force[6,8,9,13].

Although the molar masses of POSS derivatives can reach sev-eral thousands of Daltons (g/mol), depending on the size of the attached organic groups, most POSS derivatives can be theoretically

O Si O O O Si O O β-silyl R: α-silyl R: O Si OH O Si OH β-silyl R: α-silyl R: (c) OA-POSS O Si O Si β-silyl R: α-silyl R: O Si O Si O Si O Si O Si O Si O Si O Si O _O O O R R R R R R R R O Si H b R: (d) OP-POSS a a b c d a' b' c' d a b c d e a' b' c' d e' a b c d e a' b' c' d e (a) Q8M8 H (b) OS-POSS

Fig. 1. Chemical structures of (a) Q8M8H, (b) OS-POSS, (c) OA-POSS, and (d) OP-POSS and

their assigned protons for analyzing the1_{H NMR spectra inFig. 4.}

*Corresponding author. Tel./fax: þ886 3 5131512. E-mail address:[email protected](F.-C. Chang).

Contents lists available atScienceDirect

Polymer

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / p o l y m e r

0032-3861/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2008.06.055

treated as spheres that pack hexagonally into ordered ABCA

four-layer crystals[13–17]. For norbornyl cyclopentylPOSS, the lattice

constants a and c of the hexagonal array are 1.61 and 1.71 nm,

re-spectively[17]; i.e., the lattice constant a is slightly larger than the

diameter of norbornyl cyclopentylPOSS (ca. 1.28 nm), indicating that the side chains do not interdigitate in the solid state. When POSS molecules are covalently bonded to polymeric backbones, crystalline POSS microdomains exhibiting similar crystal reflections

have been observed through XRD analysis[13,18–21].

The hydrosilylation of unsaturated monomers using

octa-kis(dimethylsiloxy)silsesquioxane Q8M8Hand Karstedt’s catalyst (a

platinum divinylsiloxane complex) has been used to yield various POSS derivatives possessing distinct functionality, including

hy-droxyl[22,23], polyethyleneglycol[24], liquid crystal mesogen[25],

acrylate[26], and phenol[27]units. Although much literature is

available highlighting the excellent performance of hydrosilylated POSS-based polymer nanocomposites, few reports have focused on why hydrosilylated POSS products are liquids or amorphous glasses, rather than crystalline materials, at room temperature. In

previous studies of POSS-based nanocomposites [27,28], we

ob-served that amorphous octakis-functionalized POSS derivatives exhibit polymer-like behavior, such as glass transition tempera-tures and film-formation properties. Therefore, we felt the need to investigate the properties of amorphous and crystalline POSS derivatives prior to performing further studies on POSS-based nanocomposites. In this present study, we synthesized and characterized three amorphous POSS derivatives: octakis[di-methyl(phenethyl)siloxy]silsesquioxane (OS-POSS),

octakis[dime-thyl(4-acetoxyphenethyl)siloxy]silsesquioxane (OA-POSS), and

octakis[dimethyl(4-hydroxyphenethyl)siloxy]silsesquioxane

(OP-POSS) (Fig. 1). We investigated the structures, thermal motion, and

packing of these POSS derivatives using FTIR and 1H NMR

spec-troscopy, MALDI-TOF mass spectrometry, DSC, and XRD analyses. We blended OS-POSS, OA-POSS, and OP-POSS with polystyrene (PS), poly(4-acetoxystyrene) (PAS), and poly(4-vinylpyridine) (P4VP), respectively, through solution blending in tetrahydrofuran or methanol to investigate the effects of intermolecular interactions [hydrophobic interactions between aromatic rings (OS-POSS/PS), dipole–dipole interactions between ester groups (OA-POSS/PAS), and hydrogen bonding between phenol and pyridine units (OP-POSS/P4VP)] on the dispersion of these POSS derivatives in the polymer matrices. We employed TEM and XRD analyses to determine the sizes and distributions of the POSS aggregates in these polymer nanocomposites, and TGA and DSC analyses to re-veal the thermal decomposition temperatures, char yields, and glass transition temperatures of the amorphous POSS-based nanocomposites.

2. Experimental section 2.1. Materials

Q8M8HPOSS was obtained from Hybrid Plastics.

4-Acetoxystyr-ene (96%), hydrazine monohydrate (98%), and platinum(0)-1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex solution [Pt(dvs)] in xylene (Pt w 2%) were obtained from Aldrich Chemical. HPLC sol-vents (toluene, ethyl acetate) were used as received from TEDIA Chemical. PS, PAS, and P4VP were prepared through nitroxide-mediated radical polymerization from the unimolecular initiator (seeSupplementary datafor details).

2.2. Measurements

A Varian Unity Inova 500 FT NMR spectrometer was employed

to obtain1H NMR spectra of samples dissolved in CDCl3or CD3OD. A

Nicolet Avatar 320 FTIR spectrometer (32 scans; resolution:

1 cm1_{; nitrogen purge) was used to record FTIR spectra of samples}

on a KBr disk. A Biflex III (Bruker Daltonics) time-of-flight mass spectrometer equipped with a 337-nm nitrogen laser was used to record MALDI-TOF mass spectra of the samples. A DuPont

DSC-9000 calorimeter (scan rate: 20_{C/min; range: from 100 to}

þ180_{C) was used to record DSC thermograms of the samples (ca.}

5–10 mg) sealed in aluminum pans. The temperature and energy were indium-calibrated. The glass transition temperature was recorded as the midpoint of the specific heat increment. An M18XHF-SPA XRD spectrometer was used to collect wide-angle

X-ray diffraction spectra (XRD) of samples using Co Karadiation.

The d-spacing distance (d) was transformed using Bragg’s law

(l¼ 2d sin

q), where

l

Hitachi H-7500 transmission electron microscope (100 kV) was used to record TEM images of the POSS-based polymer nano-composites. The samples were ultramicrotomed at room temper-ature using a diamond knife on a Leica Ultracut UCT microtome apparatus to provide 70-nm-thick sections that were mounted on carbon-coated Cu TEM grids. A TA Instruments thermogravimetric

analyzer (scan rate: 20_{C/min; from 30 to 800}_{C; nitrogen purge:}

40 mL/min) was used to record TGA thermograms of the samples positioned on a platinum holder.

2.3. Syntheses of OS-POSS and OA-POSS

Scheme 1depicts the hydrosilylation approach used to prepare

OS-POSS and OA-POSS [27,28]. A solution of Q8M8H (3.00 g,

2.95 mmol) and styrene (2.46 g, 23.57 mmol) or 4-acetoxylstyrene (3.98 g, 23.57 mmol) in toluene (30 mL) in a 100-mL Schlenk flask equipped with a reflux condenser and a magnetic stirrer was

heated at 60_{C under argon and then Pt(dvs) (0.2 mL, 0.4}

_mmol)

was added via syringe. The reaction, which was monitored by measuring the decrease in intensity of the FTIR spectral signal at

2134 cm1_{for the Si–H bonds, was complete after 4 h. The}

yel-lowish, transparent reaction mixture became clear after removal of

the Pt(dvs) catalyst through flash chromatography (neutral Al2O3;

toluene). The solvent was evaporated under reduced pressure and then the residual styrene or 4-aceoxystyrene was vacuum distilled

(80_{C, 0.2 torr), to yield OS-POSS (3.45 g, 63%) or OA-POSS (3.93 g,}

56%). OS-POSS is a fluid liquid at 25_{C; OA-POSS is a viscous liquid}

at 25_{C. Both are soluble in common organic solvents, such as}

tetrahydrofuran, chloroform, and acetone. 2.4. Synthesis of OP-POSS

Scheme 2depicts the acetoxyl hydrazinolysis of OA-POSS with

hydrazine monohydrate used to prepare OP-POSS[30]. OA-POSS

(5.00 g, 2.16 mmol) was dissolved in 1,4-dioxane and then hydra-zine monohydrate (1.73 g, 34.6 mmol, 16.0 equiv.) was added. The acetoxyl hydrazinolysis, which was monitored by measuring the decrease in intensity of the FTIR spectral signal for the C]O bond at

1762 cm1, was complete after 2 h. The solution was added

drop-wise into excess deionized water. The viscous OP-POSS was col-lected in a beaker, dissolved in ethyl acetate, dried (anhydrous

MgSO4), and filtered. The solvents were evaporated under vacuum

at 80_{C for 4 h to yield OP-POSS (3.72 g, 87%), which is highly}

viscous (as a result of hydrogen bonding between the phenol units) and soluble in polar organic solvents (such as methanol).

3. Results and discussion

3.1. Syntheses of amorphous POSS derivatives

At 1:8 molar ratios of Q8M8H to the vinyl monomers, the

hydrosilylations (Scheme 1) yielding OS-POSS or OA-POSS were

the signals of the Si–H groups (FTIR: 2134 cm1; 1H NMR:

4.7 ppm) in the reaction mixtures.1H NMR spectra of the crude

products revealed the presence of only a small amount of vinyl groups (<1 mol%). OS-POSS and OA-POSS were purified through evaporation of the residual volatile styrene and 4-acetoxystyrene, respectively, under vacuum. In a previous report, we prepared OP-POSS through alkaline hydrolysis of OA-OP-POSS with sodium

hy-droxide at 25_{C for 2 days[27]. Unfortunately, rearrangement of}

the POSS siloxane cage[27,29]and gelation reactions of the phenol

groups [30] occurs when OA-POSS is hydrolyzed by sodium

hydroxide over long periods of time. To overcome these

problems, we noted that acetoxyl hydrazinolysis (Scheme 2) using

hydrazine monohydrate could be used to selectively deprotect the acetyl groups from poly(3-caprolactone)-b-poly(4-acetoxystyrene) diblock copolymers without cleaving the ester bonds of the

poly(3-caprolactone) blocks [31]. Gratifyingly, applying acetoxyl

hydrazinolysis to OA-POSS at 25_{C for 2 h provided OP-POSS in}

high yield.

Fig. 2displays FTIR spectra of (a) Q8M8H, (b) OS-POSS, (c)

OA-POSS, and (d) OP-OA-POSS, obtained through solution casting onto KBr disks. The main stretching modes of these POSS derivatives appear

as signals at 2134 cm1_{(Si–H) for Q}

8M8H; at 1762 cm1(C]O) for

OA-POSS; and at 3540 and 3340 cm1(free and hydrogen-bonded

OH groups, respectively) for OP-POSS. A signal at 1090 cm1_{for Si–}

O–Si stretching of the POSS core is present in each spectrum, sug-gesting that the complete cubic structure of the siloxane cage

remained unchanged during the synthesis.Fig. 3displays

MALDI-TOF mass spectra of OS-POSS, OA-POSS, and OP-POSS, obtained using 2,5-dihydroxybenzoic acid as the matrix. We observe monodisperse mass distributions of the sodiated molecular ions at

1873 g/mol for [OS-POSS þ Na]þ_{, 2337 g/mol for [OA-POSS þ Na]}þ_,

and 2001 g/mol for [OP-POSS þ Na]þ; the good agreement between

the experimental and calculated molecular masses confirms the

well-defined structures of OS-POSS, OA-POSS, and OP-POSS.Fig. 4

displays1H NMR spectra of Q8M8H, OS-POSS, OA-POSS, and OP-POSS.

The hydrosilylations resulted in

b

a

organic functional group. The molar ratios of

b

a

2.22:1 and 1.82:1 for OS-POSS and OA-POSS, respectively, according to integration of the signals for the protons on the benzylic carbon

atoms marked b (2H,

b-side groups) and b

_{a-side groups) in}

Fig. 1. Thus, MALDI-TOF mass spectrometry and FTIR and1H NMR spectrometry confirmed that the POSS products were pure, but comprised several structural isomers.

O O Pt(dvs), Toluene, 60 °C O Si O O O Si O O β-silyl R: α-silyl R: Q8M8 H OA-POSS Pt(dvs), Toluene, 60 °C O Si O Si β-silyl R: α-silyl R: OS-POSS R R R R R R R R R R R R R R R R O Si O Si O Si O Si O Si O Si O Si O Si O _O O O R R R R R R R R O Si H R:

Scheme 1. Hydrosilylation of styrenic monomers with Q8M8Hto give OS-POSS and OA-POSS.

1,4-dioxane, 25 °C N2H4/H2O N H NH2 O + acetohydrazide O Si OH O Si OH β-silyl R: α-silyl R: OA-POSS OP-POSS R R R R R R R R

3.2. Characterization of amorphous POSS derivatives

Fig. 5 displays DSC thermograms of Q8M8H during the first

heating process and of OS-POSS, OA-POSS, and OP-POSS during the

second heating process. The transition of Q8M8Hfrom crystalline to

glass occurred at 104.3_{C. With its absence of interdigitated side}

groups in the crystal (lattice constant a > molecular size)[17,36],

we attribute this low transition temperature of Q8M8H to

intra-molecular rotation resulting in the isotropic glass (see Fig. S5 in Supplementary data, for a polarized optical microscopy image). After hydrosilylation, the amorphous POSS derivatives OS-POSS,

OA-POSS, and OP-POSS revealed glass transition temperatures (Tg)

of 51.8, 14.2, and þ18.7_{C, respectively, in their DSC}

thermo-grams. We attribute the increments in Tgto the increased strength

of the intermolecular interaction upon proceeding from OS-POSS (van der Waals forces) to OA-POSS (dipole–dipole interactions) to OP-POSS (phenol–phenol hydrogen bonding). For amorphous polymers, the glass transition typically occurs from a disordered glassy state to a viscous liquid as a result of disorderly thermal motions of polymer segments during thermal treatment. In this similar case, the coexistence of several isomers of these hydro-silylated POSS derivatives is responsible for the disordered

aggregation (glassy state) and has similar glass transition observed by DSC thermograms.

Fig. 6displays XRD patterns of Q8M8H, OA-POSS, and OP-POSS at

25_{C (note that OS-POSS was not a suitable subject for XRD}

anal-ysis because of its high fluidity). The major diffraction peaks (and planes) expected for ABCA rhombohedral crystal structures appear

for the crystalline Q8M8Hat 8.13(101), 10.67 (110), 11.86(102),

and 18.54 _{(113), similar to those of most POSS crystals[12–17].}

After hydrosilylation, the POSS products, which are liquids at 25_C,

display three amorphous halos for three d-spacing distances (d1, d2,

and d3;Fig. 7a). Both d1and d2have been observed previously for

amorphous poly(silsesquioxane); they are assigned to the width of

each ladder-like double chain (Si–O–Si; d1inFig. 7b) and the

dis-tance between two adjacent ladder-like double chains (Si–R/R–Si; d2in Fig. 7b)[33,35]. Because POSS can be regarded as a cyclic

tetramer of poly(silsesquioxane), as indicated inFig. 7, we assign

the two distinct amorphous halos of OA-POSS and OP-POSS at

24.10 _{(d-spacing of ca. 0.37 nm) and 17.51} _{(d-spacing of ca.}

0.51 nm) to the Si–O–Si distance (d1¼ ca. 0.33 nm) and the

di-ameter of the POSS siloxane cage (d3¼ ca. 0.53 nm; Figs.7a and8a),

4000 3500 3000 2500 2000 1500 1000 500 Si-O-Si C=O Si-H O-H (d) OP-POSS (c) OA-POSS (b) OS-POSS (a) Q₈M₈H Wavenumber (cm-1₎

Fig. 2. FTIR spectra of (a) Q8M8H, (b) OS-POSS, (c) OA-POSS, and (d) OP-POSS.

(a) OS-POSS+Na+ (b) OA-POSS+Na+ (c) OP-POSS+Na+ 1500 1600 1700 1800 1900 2000 2100 2200 2300 2400 2500 m/z 2001 2337 1873

Fig. 3. MALDI-TOF mass spectra of (a) OS-POSS, (b) OA-POSS, and (c) OP-POSS.

10 9 8 7 6 5 4 3 2 1 0 d c b' c' b a a' d e c c' b' b a a' d _c e e' _c' b' b a a' b a H₂O CD2HOD H2O CHCl3 (d) OP-POSS (c) OA-POSS (b) OS-POSS (a) Q8M8H 1_{H Chemical Shift (ppm)}

Fig. 4.1_{H NMR spectra of (a) Q}

8M8H, (b) OS-POSS, and (c) OA-POSS in CDCl3and of (d)

OP-POSS in CD3OD. -75 -50 -25 0 25 50 75 100 125 150 175 18.7 °C 104.3 °C -14.2 °C -51.8 °C (d) OP-POSS (c) OA-POSS (b) OS-POSS (a) Q8M8H

Heat flow (exothermal )

Temperature ( °C)

respectively[32–34]. The peak at 17.51_{once again confirms the}

intact cage structures of the amorphous POSS derivatives. We used CS Chem3D software (MM2 calculations) for molecular modeling to

calculate the cube size (ca. 0.53 nm,Fig. 8a) and the lengths of the

hydrosilylated ethylene groups (band

a

lengths (Fig. 8b–e) are 1.30 nm for the

b-side chain of OA-POSS,

0.97 nm for the

a-side chain of OA-POSS, 1.09 nm for the

b-side

chain of OP-POSS, and 0.76 nm for the

a-side chains of OP-POSS. For

OA-POSS, the value of 7.62 _{inFig. 6b corresponds to a distance}

between two adjacent POSS cores (d3) of ca. 1.16 nm. This distance

approaches the average of the end-to-end lengths of the

b- and

a-side chains on OA-POSS (d-spacings of ca. 1.30 and 0.97 nm,

respectively;Fig. 8b and c). This distance of 1.16 nm indicates that

attractive interactions between POSS cores promotes POSS aggre-gation with interdigitation of the coexisting linear (b) and branched (a) side chains on OA-POSS, in stark contrast to the lack of in-terdigitation of the side groups of POSS crystals. Although the

cal-culated length of the

b

the OP-POSS peak at 7.62_{is relatively sharp inFig. 6c, reflecting the}

more uniform and non-interpenetrating intermolecular d-space caused by phenol–phenol hydrogen bonding. We attribute the high glass transition temperature of OP-POSS mostly to the formation of strong intermolecular hydrogen bonds, even though their in-termolecular distances are also increased. The hydrogen bonding of OP-POSS can be further evidenced in temperature-dependent FTIR

spectra (see Fig. S6 inSupplementary data).

3.3. POSS-based polymer nanocomposites

We prepared three POSS-based nanocomposites incorporating 20 wt% POSS through solution blending of OS-POSS and poly-styrene (OS-POSS/PS) in THF, OA-POSS and poly(4-acetoxypoly-styrene) (OA-POSS/PAS) in THF, and OP-POSS and poly(4-vinylpyridine)

(OP-POSS/P4VP) in MeOH (see Figs. S1–S4 ofSupplementary datafor

the synthesis and characterization of these polymers). THF and MeOH served as good solvents for premixing the POSS derivatives and polymers because of their strong solvent–POSS and solvent– polymer interactions. Thermodynamically, the dispersion or ag-gregation of POSS molecules is determined by the mixture entropy and mixture enthalpy: the former represents the extent of POSS dispersion (high molar mass polymers can be neglected) and the

latter represents the balance of the strengths of the POSS–POSS, POSS–polymer, and polymer–polymer interactions. To elucidate the microstructures of OS-POSS/PS, OA-POSS/PAS, and OP-POSS/ P4VP, we directly observed images of their microtomed POSS

composites. Fig. 9 displays TEM images and schematic

micro-structures of the POSS-based polymer nanocomposites. Image contrast in a TEM micrograph originates from the different degrees of electron scattering from light and heavy atoms. The aggregated POSS siloxane cages appear as dark spots in the gray polymer medium comprised solely of carbon and hydrogen atoms. Wu et al.

[32] reported TEM images displaying individual POSS molecules

(sizes: 1.5–3.0 nm) covalently bonded onto PS chains. In contrast, our octakis-functionalized amorphous POSS derivatives are dis-persed through physical intermolecular interactions between their outer organic units and the organic polymer matrices. Thus, the size and distribution of the POSS aggregates depend strongly on the type and strength of the intermolecular interactions. As is evident in Fig. 9a–c, OS-POSS aggregates having rough sizes of 100-nm-diameters were well dispersed in the PS matrix. Clearly, the weak aromatic hydrophobic interactions between OS-POSS and PS can-not overcome the attraction of the POSS cores, even though pre-mixing (solution blending) promoted uniform dispersion of the POSS derivatives. With their stronger intermolecular dipole–dipole interactions, the dark OA-POSS aggregates are absent (i.e., the units

are well dispersed) in the PAS matrix (Fig. 9d–f). The improved

homogeneity of the OA-POSS/PAS system is driven thermody-namically by the mixture entropy, namely through the similar strengths of the OA-POSS–PAS and POSS–POSS intermolecular in-teractions. For the mixture of OP-POSS and P4VP, which features strong phenol–pyridine hydrogen bonds, we observed both dis-persed and aggregated OP-POSS units in the matrix, resulting presumably from competition between hydrogen bonds of similar strength (phenol–pyridine and phenol–phenol). The polar P4VP can be dissolved in methanol based on the hydrogen bond between hydroxyl and pyridine. We supposed that the hydrogen bonds be-tween high concentration OP-POSS and P4VP result in the relatively

low solubility [37]. Thus, OP-POSS aggregation was observed by

TEM images (Fig. 9g–i). After N2-purged thermal annealing at

180_{C for 1 h, OP-POSS aggregation in the P4VP medium becomes}

smaller than that before treatment (see Supplementary data).

Consequently, it appears that the dipole–dipole interaction is the optimal noncovalent interaction with which polymer nano-composites can be obtained incorporating well-dispersed

amor-phous POSS units. Fig. 10 presents XRD patterns of OA-POSS,

PAS, OA-POSS/PAS, OS-POSS/PS, and OP-POSS/P4VP. The signal for

3 6 9 12 15 18 21 24 27 30 33 36 39 42 * * d=1.16nm d=0.51nm d=0.37nm (113) (102) (101) (110) (c) OP-POSS (b) OA-POSS (a) Q8M8H 2θ [degree]

Fig. 6. XRD patterns of (a) Q8M8H, (b) OA-POSS, and (c) OP-POSS. Asterisks mark the

signals from the holder.

Fig. 7. Geometric drawings of (a) two POSS cages and (b) two poly(silsesquioxane) chains. Arrows mark the intra-chain distance (d1), the inter-chain distance (d2), and the

OA-POSS aggregation at 7.62 _{is absent for OA-POSS/PAS,}

con-firming the high dispersion of OA-POSS in the PAS matrix. For OS-POSS/PS and OP-POSS/P4VP, however, the signal for POSS

aggregations is present at 7.62_{(Fig. 10d and e).}

Table 1summarizes the molar masses and thermal properties of

the POSS derivatives, the polymers, and their composites.Fig. 11

displays TGA thermograms of OS-POSS, PS, OS-POSS/PS, OA-POSS, PAS, OA-POSS/PAS, OP-POSS, P4VP, and OP-POSS/P4VP. For P4VP

and OP-POSS/P4VP, residual polar methanol was removed by

keeping in vacuum oven at 100_{C for 1 h. Among the three}

amor-phous POSS derivatives (Fig. 11a, d and g;Table 1), OS-POSS

pos-sesses the highest thermal stability with its onset point at 431.7_C;

its char yield of only 10.3 wt% is less than its 42 wt% siloxane content. Thus, it appears that decomposition of the organic com-ponents of OS-POSS can promote further siloxane degradation, resulting in the low char yield. In contrast, OA-POSS and OP-POSS

Fig. 9. TEM images and schematic microstructures of POSS-based polymer nanocomposites (a–c) OS-POSS/PS, (d–f) OA-POSS/PAS, and (g–i) OP-POSS/P4VP. Three samples were treated by N2-purged thermal annealing at 180C for 1 h.

Fig. 8. Chem3D MM2 calculations and 3D structures of (a) a POSS core, (b) thebside chain of OA-POSS, (c) theaside chain of OA-POSS, (d) thebside chain of OP-POSS, and (e) the

have higher char yields (60.6 and 70.1 wt%, respectively) because their phenol units tend to form aromatic char structures, rather than small molecular vapors, at temperatures above their

de-composition temperatures (384.1 and 358.5_{C, respectively).}

Depolymerization of the common polymers PS, PAS, and P4VP (Fig. 11b, e and h; Table 1) occurs from the chain ends at their decomposition temperatures, resulting in low char yields. After

incorporating the POSS derivatives into these polymers (Fig. 11c, f

and i;Table 1), the char yields of OS-POSS/PS and OP-POSS/P4VP

(4.1 and 12.6 wt%, respectively) were slightly higher than the theoretical values (2.7 and 12.4 wt%, respectively) calculated for a weight average of 20 wt% POSS derivative and 80 wt% polymer, but, interestingly, in comparison with the theoretical char yield

by weight-average method as listed inTable 1, the char yield of

OA-POSS/PAS increased by ca. 5.9 wt% (¼19.8–13.9) which is more than three times the char yield increase of OS-POSS/PS (1.4 wt%) or OP-POSS/P4VP (0.2 wt%). This result can be inter-preted as the well-dispersed OA-POSS forming a larger contact interface with PAS, thereby incorporating more PAS segments in

the aromatic char structure.Fig. 12displays DSC thermograms of

PS, OS-POSS/PS, PAS, OA-POSS/PAS, P4VP, and OP-POSS/P4VP. Because a glass transition reflects local thermal motion within

several repeat units, a new glass transition temperature after incorporation of 20 wt% POSS into a polymer matrix suggests the formation of a composite microstructure. Thus, competition among multiple intermolecular interactions for POSS, the poly-mer, and the solvent directly affects the dispersion and aggre-gation of the composites after evaporation of the solvent, as was revealed in the TEM images. Therefore, the disappearance of glass transitions for the pure POSS derivatives indicates that the dark

aggregation domains (Fig. 9) were composed of POSS and

poly-mer composites. The presence of the POSS derivatives expanded the interspaces (increased the free volume) between the polymer chains, resulting in lower glass transition temperatures. Thus, we observed a single glass transition of OS-POSS/PS nanocomposite

at 72.1_{C. For the OP-POSS/P4VP nanocomposite, the strong}

phenol–pyridine hydrogen bonds compensate for the high mo-bility in the interspaces between the OP-POSS units and the P4VP

chains, resulting in a high glass transition temperature (121.9_C)

approaching that of the pristine P4VP (129.0_{C). Interestingly, the}

medium-strength dipole–dipole interactions in the OA-POSS/PAS nanocomposite resulted in a decrease of the glass transition

temperature by 36_{C, a considerably larger drop than those for}

6 12 18 24 30 36 42 POSS aggregates (e) OP-POSS/P4VP (d) OS-POSS/PS (c) OA-POSS/PAS (b) PAS (a) OA-POSS * * * 2 [degree]

Fig. 10. XRD patterns of (a) OA-POSS, (b) PAS, (c) OA-POSS/PAS, (d) OS-POSS/PS, and (e) OP-POSS/P4VP. Asterisks mark the signals of the holder.

Table 1

Molar masses and thermal properties of POSS derivatives, polymers, and their composites

Entry Molar mass Decompositiond _{Glass transition}h

Mn(g/mol) Mw(g/mol) Onset (C) Chare(wt%) Initial (C) Median (C)

OS-POSS 1850b ₁₈₅₀b _{431.7 10.3 55.5 51.8} OA-POSS 2314b ₂₃₁₄b _{384.1 60.6 18.2 14.2} OP-POSS 1978b ₁₉₇₈b _358.5f _{70.1 10.2 18.7} PS 7522c ₉₉₃₂c _{388.9 0.8 85.3 90.8} PAS 24 532c _{38 436}c _{395.4 2.2 116.4 124.8} P4VP 4693c ₇₀₀₂c _{377.2 0.3 117.9 129.0} OS-POSS/PSa _{– – 414.6 4.1 (2.7}g_{) 66.8 72.1} OA-POSS/PASa _{– – 386.2 19.8 (13.9}g_{) 80.3 93.5} OP-POSS/P4VPa _{– – 392.5 12.6 (12.4}g_{) 112.9 121.9} a_{Containing 20 wt% POSS derivatives.}

b _{Obtained from MALDI-TOF mass spectra.}

c _{Obtained from RI-GPC with DMF elution (0.6 mL/min) and PS calibration (seeSupplementary data).} d _{Obtained from TGA thermograms (heating rate: 20}_C/min).

e _{Recorded at 700}_C. f _{Recorded at 5 wt% loss.}

g _{Calculated from a weight average of 20 wt% POSS derivative and 80 wt% polymer.} h_{Obtained from the second run of the DSC thermogram (heating rate: 20}_C/min).

100 200 300 400 500 600 700 800 0 20 40 60 80 100 (a) OS-POSS (i) OP-POSS/P4VP (f) OA-POSS/PAS (d) OA-POSS (g) OP-POSS (c) OS-POSS/PS (e) PAS (b) PS (h) P4VP Weight (wt-%) Temperature (°C)

Fig. 11. TGA thermograms of (a) OS-POSS, (b) PS, (c) OS-POSS/PS, (d) OA-POSS, (e) PAS, (f) OA-POSS/PAS, (g) OP-POSS, (h) P4VP, and (i) OP-POSS/P4VP. The residual methanol in samples of P4VP and OP-POSS/P4VP was removed by vacuum distillation at 100_C

the OS-POSS/PS and OP-POSS/P4VP nanocomposites. The well-dispersed OA-POSS units result in much greater contact in the interspaces of PAS, thereby expanding the free volume between polymer segments.

4. Conclusion

The polymer-like glass transition of amorphous POSS de-rivatives, such as OS-POSS, OA-POSS, or OP-POSS, has rarely been mentioned in the literature; indeed, they are usually regarded as

pure liquids possessing melting points below 25_{C. Thus,}

aggre-gation of such amorphous POSS derivatives in polymer matrices is not expected to result in crystal-like lamellar microstructures. In this study, we improved upon our previous methods for preparing

monodisperse OS-POSS, OA-POSS, and OP-POSS, but

b

a

and R0_{is an organic functional group) resulted in multiple}

confor-mational isomers of amorphous POSS products. The distinct

ge-ometries of these isomers inhibit the POSS derivatives’

crystallization; thus, these nanosized POSS molecules without re-striction in the crystal lattice exhibit high conformational flexibility. As a result, intermolecular interactions have a great effect on their polymer-like glass transition temperatures. When incorporated into polymer blends, the distribution of the POSS derivatives and their effects on the blends’ properties are also dependent upon the specific intermolecular interactions that are present. Among three types of noncovalent interaction – aromatic hydrophobic in-teractions of OS-POSS/PS, dipole–dipole inin-teractions of OA-POSS/ PAS, and phenol–pyridine hydrogen bonds of OP-POSS/P4VP – the dipole–dipole interaction between the acetyl groups is the best selection to disperse 20 wt% amorphous POSS in the polymer ma-trices. As a consequence, amorphous POSS cages can be well dis-persed in the polymeric medium through the intermolecular

interaction, which have similar strength to the POSS–POSS attrac-tive interaction.

Acknowledgment

We thank Dr. Y.-C. Chen and Dr. C.-S. Lee of the Institute of Applied Chemistry, National Chiao Tung University, for assistance in recording the MALDI-TOF mass spectra and the XRD patterns. This study was supported financially by the National Science Council, Taiwan, Republic of China, under contract no. NSC-95-2216-E-009-018 and by the Ministry of Education’s ‘‘Aim for the Top University’’ program.

Appendix. Supplementary data

Supplementary data associated with this article can be found in

the online version, atdoi:10.1016/j.polymer.2008.06.055.

References

[1] Giannelis EP. Adv Mater 1996;8:29–35.

[2] Vo LT, Giannelis EP. Macromolecules 2007;40:8271–6.

[3] Bansal A, Yang H, Li C, Benicewicz BC, Kumar SM, Schadler LS. J Polym Sci Part B Polym Phys 2006;44:2944–50.

[4] Lu X, Chao D, Chen J, Zhang W, Wei Y. Mater Lett 2006;60:2851–4. [5] Lu X, Yu Y, Chen L, Mao H, Gao H, Wang J, et al. Nanotechnology 2005;16:

1660–5.

[6] Huang JC, He CB, Xiao Y, Mya KY, Dai J, Siow YP. Polymer 2003;44:4491–9. [7] Baney RH, Itoh M, Sakakibara A, Suzuki T. Chem Rev 1995;95:1409–30. [8] Laine RM. J Mater Chem 2005;15:3725–44.

[9] Schwab JJ, Lichtenhan JD. Appl Organometal Chem 1998;12:707–13. [10] Voronkov MG, Lavrent’ev V. Top Curr Chem 1982;102:199–236. [11] Feher FJ, Budzichowski TA. J Organometal Chem 1989;373:153–63. [12] Sheng YJ, Lin WJ, Chen WC. J Chem Phys 2004;121:9693–701.

[13] Fu BX, Hsiao BS, Pagola S, Stephens P, White H, Rafailovich M, et al. Polymer 2001;42:599–611.

[14] Cui L, Collet JP, Xu G, Zhu L. Chem Mater 2006;18:3503–12. [15] Larsson K. Ark Kemi 1960;16:203–8.

[16] Larsson K. Ark Kemi 1960;16:209–14.

[17] Waddon AJ, Coughlin EB. Chem Mater 2003;15:4555–61.

[18] Zheng L, Waddon AJ, Farris RJ, Coughlin EB. Macromolecules 2002;35:2375–9. [19] Waddon AJ, Zheng L, Farris RJ, Coughlin EB. Nano Lett 2002;2:1149–55. [20] Carroll JB, Waddon AJ, Nakade H, Rotello VM. Macromolecules 2003;36:

6289–91.

[21] Zheng L, Hong S, Cardoen G, Burgaz E, Gido SP, Coughlin EB. Macromolecules 2004;37:8606–11.

[22] Zhang C, Laine RM. J Am Chem Soc 2000;122:6979–88. [23] Kim KM, Inakura T, Chujo Y. Polym Bull 2001;46:351–6. [24] Maitra P, Wunder SL. Chem Mater 2002;14:4494–7.

[25] Zhang C, Bunning TJ, Laine RM. Chem Mater 2001;13:3653–62. [26] Sellinger A, Laine RM. Macromolecules 1996;29:2327–30.

[27] Lin HC, Kuo SW, Huang CF, Chang FC. Macromol Rapid Commun 2006;27: 537–41.

[28] Kuo SW, Lin HC, Huang WJ, Huang CF, Chang FC. J Polym Sci Part B Polym Phys 2006;44:673–86.

[29] Lichtenhan JD, Haddad TS, Feher FJ, Soulivong D. U.S. Patent 6,770,724; Aug 3 2004.

[30] Chen X, Jankova K, Kops J, Batsberg W. J Polym Sci Part A Polym Chem 1999; 37:627–33.

[31] Kuo SW, Huang CF, Lu CH, Jeong KU, Chang FC. Macromol Chem Phys 2006; 207:2006–16.

[32] Wu J, Haddad TS, Kim G-M, Mather PT. Macromolecules 2007;40:544–54. [33] Imae I, Kawakmi Y. J Mater Chem 2005;15:4581–3.

[34] Zhang J, Xu R, Yu D. Eur Polym J 2007;43:743–52.

[35] Liu C, Liu Y, Shen Z, Xie P, Dai D, Zhang R, et al. Macromol Chem Phys 2001; 202:1576–80.

[36] Liu L, Tian M, Zhang W, Zhang L, Mark JE. Polymer 2007;48:3201–12. [37] Kuo SW, Tung PH, Chang FC. Macromolecules 2006;39:9388–95.

-40 -20 0 20 40 60 80 100 120 140 160 121.9 (°C) 90.8 (°C) 72.1 (°C) 129.0 (°C) 93.5 (°C) 124.8 (°C) (f) OP-POSS/P4VP (e) P4VP (d) OA-POSS/PAS (c) PAS (b) OS-POSS/PS (a) PS

Heat flow (exothermal

)

Temperature (°C)

Fig. 12. DSC thermograms of (a) PS, (b) OS-POSS/PS, (c) PAS, (d) OA-POSS/PAS, (e) P4VP, and (f) OP-POSS/P4VP.