Effect of LiClO4 on the thermal and morphological properties of organic/inorganic polymer hybrids

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Polymer Communication

Effect of LiClO

₄

on the thermal and morphological properties

of organic/inorganic polymer hybrids

Ying-Chieh Yen

a

, Yun-Sheng Ye

a

, Chih-Chia Cheng

a

, Hsiu-Mei Chen

b

, Hwo-Shuenn Sheu

c

,

Feng-Chih Chang

a,*

a_{Institute of Applied Chemistry, National Chiao-Tung University, Hsin-Chu, Taiwan, ROC}

b_{Material and Chemical Research Laboratories, Industrial Technology Research Institute, Chutung, Taiwan, ROC} c_{National Synchrotron Radiation Research Center, Hsinchu Science Park, Taiwan, ROC}

a r t i c l e

i n f o

Article history: Received 9 April 2008

Received in revised form 27 May 2008 Accepted 10 June 2008

Available online 19 June 2008

Keywords: Hydrogen bonding

Polyhedral oligomeric silsesquoixane Ion–dipole interaction

a b s t r a c t

This paper describes the thermal properties, morphologies, and interactions within the binary and ternary blends of poly(methyl methacrylate) (PMMA), octa(phenol)octasilsesquioxane (OP-POSS), and LiClO4. In the binary PMMA/OP-POSS blends, the OP-POSS molecules tend to aggregate and result in

a decrease (19_{C) in the glass transition temperature. In the ternary PMMA/LiClO}

4/OP-POSS blends,

however, the OP-POSS molecules form small sphere-like domains (20 nm) leading to the composite’s glass transition temperature increasing by up to 30_{C. Based on these FT-IR spectra, the addition of}

LiClO4inﬂuenced the probability of hydrogen bonds formed between PMMA and OP-POSS and these SEM

micrographs, DSC, and XRD data indicated that the addition of LiClO4is a convenient and simple

ap-proach toward dispersing the OP-POSS nanoparticles within PMMA, where the presence of LiClO4

changes the physical effect of OP-POSS from that of a diluent role to a cross-linker role.

1. Introduction

Composite materials fabricated from organic polymers and inorganic materials are currently attracting great attention for both their fundamental scientiﬁc behavior and industrial applications. Polyhedral oligomeric silsesquoixanes (POSSs), which have the general formula (RSiO1.5)nare prototypical organic/inorganic

sys-tems because they are composed of inorganic cores with external organic substituents. Through appropriate control over the func-tionality of these organic substituents, both mono- and octa-func-tional macromonomers can be blended or attached covalently to linear thermoplastics or thermosetting networks to form high-performance hybrid materials [1–17]. Other interesting organic/ inorganic blend systems are the polymer electrolytes formed via the dissolution of salts into polar and high-weight macromolecules where strong noncovalent interactions between the macromole-cules and the cations of the salts result in changes in the polymers’ properties, e.g., their miscibilities[18,19]. To our knowledge, the reports which described organic/inorganic composite system fea-turing both hydrogen bonding interactions, ion–dipole interactions by incorporating the POSS nanoparticles and LiClO4into polymers

are still rare[20,21]. In addition, the interaction and conductivity

behaviors of poly(vinyl pyrrolidone-co-methyl mechacrylate) (PVP-co-PMMA) with LiClO4were reported in our previous work[22]and

we also indicated that the ionic conductivity of a LiClO4

/PMMA-co-PVP polymer electrolyte was enhanced after blending with OP-POSS [23]. To further realize the interaction mechanism within these polymer electrolytes incorporated by OP-POSS which is functionalized to behave as a strong proton donor and exhibits improved miscibility with host polymers containing proton ac-ceptors[24–27], these blends comprising PMMA, LiClO4, and POSS

derivatives were prepared and their properties were described at various compositions.

2. Experimental part 2.1. Materials

Toluene, tetrahydrofuran (THF), platinum divinyl tetrame-thyldisiloxane complex [Pt(dvs)], 4-acetoxystyrene (AS), lithium perchlorate (LiClO4), and poly(methyl methacrylate) (PMMA) were

purchased from Aldrich Chemical Co. Q8M8H, C16H56O20Si16, was

obtained from Hybrid Plastics Co. Toluene was fractionally distilled from calcium hydride under a nitrogen atmosphere. Q8M8H, Pt(dvs)

and AS were used as received. The oligomers octa(acetoxystyr-yl)octasilsequioxane (AS-POSS) and

octa(phenol)octasilsesquiox-ane (OP-POSS) were synthesized (Scheme 1) according to

a procedure described previously (seeSupplementary data)[25].

*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

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Several PMMA/LiClO4/OP-POSS blends were dissolved in THF

(15 wt%) and stirred continuously for 8 h at 25_{C. To prepare the}

sample for XRD, SEM, and DSC measurements, the solutions were cast into Teﬂon dishes and dried under a nitrogen atmosphere at 25_{C for 24 h, under vacuum at about 0.2 torr at 60}_{C for 12 h, and}

then at 120_{C for 8 h to completely remove any residual solvent or}

water. The compositions of the PMMA/LiClO4/OP-POSS blends are

summarized inTable 1. The unit phr is a concentration represen-tation: e.g., 100/25/1 refers to a system formed from 100 g of PMMA, 25 g of LiClO4and 1 g of OP-POSS.

2.2. Characterization

Thermal analyses were performed using a DuPont TA 2010 DSC instrument operated at a scan rate of 20_{C/min from 50 to 200}_C.

FT-IR spectra were recorded over the range 4000–400 cm1using a Nicolet Avatar 320 FT-IR spectrometer (32 scans; 1 cm1 resolu-tion) operated at 120_{C. To prepare the samples for the FT-IR}

ob-servation, these solutions were cast onto KBr disks and dried as mentioned above. FE-SEM and SEM-DEX images were recorded using a Hitachi-S4200I microscope operated at acceleration voltage of 5–15 kV and ambient temperature. All samples were fractured under cryogenic conditions using liquid nitrogen. X-ray diffraction (XRD) experiments were performed at ambient temperature using the wiggler beamline BL17A1 of the National Synchrotron Radiation Research Center (NSRRC), Taiwan employing a monochromated

beam [wavelength (

l

): 1.3329 Å]; the XRD pattern was collected by a curved imaging plate (IP: Fuji BAS III: area: 20 40 cm2) having a radius equivalent to the sample-to-detector distance (280 nm). 3. Results and discussion

3.1. Morphologies

Fig. 1 presents cross-sectional SEM images, which were employed previously to investigate the distribution of various functional POSS derivatives, of the (a) 100/0/5 and (b) 100/25/5 blends[5,28–34]. The POSS molecules in the 100/0/5 blend appear to aggregate into large domains (bright regions) within the PMMA matrix. In contrast, the OP-POSS molecules in the 100/25/5 blend have formed sphere-like domains having diameters of ca. 20 nm. In our previous studies[22,35], we examined blends featuring a range of competitive noncovalent interactions, including carbonyl (PMMA)/hydroxyl POSS), hydroxyl POSS)/hydroxyl (OP-POSS), carbonyl (PMMA)/Liþ, and Liþ/ClO4interactions[36–38].

From the knowledge gained from those studies, we suspected that the morphological differences between the 100/0/5 and 100/25/5 blends resulted from the different types of noncovalent interactions within them.Fig. 2(a) presents IR spectra displaying the

n

(ClO4)

internal vibration modes of these blends. The bands centered at 626 and 636 cm1correspond to the signals of the free anion and the contact ion pair, respectively[39,40]. The intensity of the blend for the contact ion pair increased upon increasing the content of OP-POSS, revealing that OP-POSS molecules tend to increase the

for-mation of contact ion pairs. Fig. 2(b) displays the carbonyl

stretching region of IR spectra of the PMMA/LiClO4/OP-POSS

blends. The bands at 1730, 1709, and 1700 cm1_{correspond to the}

free C]O groups of PMMA, and those involved in hydrogen bonds (with the OH groups of OP-POSS) and ion–dipole interactions (with Liþ_{), respectively. In the spectra of the 100/25/0 and 100/25/5}

blends, the intensity of the shoulder at 1700 cm1decreases dra-matically in the latter, indicating that the presence of OP-POSS led to a decrease in the number of PMMA C]O groups involved in ion– dipole interactions. The intensities of the hydrogen-bonded C]O of PMMA were identical for the 100/25/5 and 100/0/20 blends, in-dicating that the fractions of such groups of PMMA were identical

[35]. The presence of LiClO4 salts in the ternary blend led to an

increase of the probability of forming C]O (PMMA)/OH (OP-POSS) hydrogen bonds. Thus, there must have been a speciﬁc in-teraction occurring between LiClO4and OP-POSS.Fig. 2(c) displays

the region from 2700 to 3700 cm1in the IR spectra of the OP-POSS/LiClO4(phr) binary blends. LiClO4appears to interact with the

OH groups of OP-POSS shifting their broad band to higher fre-quency. These IR spectra reveal that the interactions between

OP-POSS and LiClO4 coexist with the C]O (PMMA)/OH (OP-POSS),

OH (OP-POSS)/OH (OP-POSS), C]O (PMMA)/Liþ_{, and Li}þ

/ClO4

interactions, with competition between these species resulting in the change in morphology[41–43].Table 1lists the values of 2

q

obtained from XRD analysis of the various blends. The signal cen-tered at 10.2 _{corresponding to the intermolecular distance}

be-tween PMMA chains [44] remained unchanged after the

incorporation of OP-POSS. In contrast, the addition of LiClO4

(25 phr) to the PMMA matrix shifted the 2

q

angle from 10.2 _to

12.6 _{which suggests that Li}þ

/O]C coordination resulted in contraction of the polymer chains. The presence of OP-POSS in the 100/25/5 blend resulted in a shift of the 2

q

angle to 12.6_{from that}

of 10.5_{for the 100/25/0 blend. This ﬁnding implies that the}

ad-dition of OP-POSS increases the contracted intermolecular distance between PMMA polymers. For the 100/25/5 blend, the 2

q

angle was almost identical to that of 100/0/5, i.e., for the association of PMMA, LiClO4, and OP-POSS. In the 100/25/5 blend, the OP-POSS molecules

were dispersed well in the presence of LiClO4, as the SEM image

O Si O Si O Si O Si O Si O Si _O Si O Si R R R R R R R R O O O O ₊ Toluene / Pt (dvs) N2 hydrolysis SiH O O Si O Si O Si O Si O Si O Si O Si O Si R1 R1 R1 R1 R1 R1 R1 R1 O O O O O O 4-acetoxystyrene Si O O O R1 = O Si O Si O Si O Si O Si O Si O Si O Si R2 R₂ R2 R2 R2 R2 R2 R2 O _O O O O _Si OH R2 = R = Q8M8H AS-POSS OP-POSS

Scheme 1. Synthesis of OP-POSS.

Table 1

Compositions, 2q(degrees), and Tgof the binary and ternary blends

PMMA/LiClO4/OP-POSS (phr) 2q(degrees) Tga(C)

100/0/0 10.2 116 100/0/1 10.1 97 100/25/0 12.6 115 100/25/1 10.5 124 100/0/5 10.2 97 100/25/5 10.5 127

a_{The glass transition temperature (T}

g) was obtained as the inﬂection point of the

heat capacity jump.

Y.-C. Yen et al. / Polymer 49 (2008) 3625–3628 3626

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revealed, which has the inﬂuence of increasing the contracted in-termolecular distance between PMMA chains. As a result of these competing inﬂuences, the 2

q

angle for the 100/0/5 and 100/25/5 blends remained virtually unchanged.

Fig. 1 illustrates the proposed mechanism leading to de-velopment of the morphologies of the 100/0/5 and 100/25/5 blends. During the initial stage, all of the components in the 100/0/ 5 and 100/25/5 blends were dissolved and distributed uniformly within the solvent. During the second stage, as the solvent evapo-rates partially, the LiClO4in the 100/25/5 blend interacts with both

OP-POSS and PMMA and forms sphere-like OP-POSS/LiClO4

do-mains. During the ﬁnal stage, as the solvent and water molecules

are removed completely, the degree of phase separation in the PMMA/LiClO4/OP-POSS is relatively lower – and, thus, the OP-POSS

domains are relatively smaller – than in the PMMA/OP-POSS blends as a result of the competitive interactions.

3.2. Thermal properties

Table 1lists DSC data of the PMMA/LiClO4/OP-POSS blends. In

the blends lacking the salt, the presence of OP-POSS decreases the glass transition temperature (Tg) of PMMA as a result of aggregation

of OP-POSS. In contrast, for the 100/25/1 blend, the addition of 25 phr LiClO4salts to the 100/0/1 blend resulted in a 27C increase Fig. 1. FE-SEM micrographs of (a) PMMA/LiClO4/OP-POSS (100/0/5) and (b) PMMA/LiClO4/OP-POSS (100/25/5); (c and d) schematic representations of the proposed mechanisms of

formation of the various OP-POSS domains.

3600 3400 3200 3000 2800 100/0 10/100

c

OP-POSS/LiClO4 Absorbance (a.u.) Wavenumber (cm-1₎ 650 640 630 620 610 Absorbance (a.u.) Wavenumber (cm-1₎ 100/25/0 100/25/5 PMMA/LiClO4/OP-POSS

a

1760 1740 1720 1700 1680 100/0/0 100/0/20 100/25/5 100/25/0

b

Wavenumber (cm-1₎ PMMA/LiClO4/OP-POSS

Fig. 2. IR spectra of various PMMA/LiClO4/OP-POSS blends recorded at 120C.

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in Tg. The change was much greater (30C) for the 100/0/5 and 100/

25/5 systems. In the 100/25/0, 100/0/1, and 100/0/5 blends, the

weak associations, i.e., Liþ/O]C (PMMA) and C]O (PMMA)/OH

(OP-POSS) had no effect or a slight decrease in the value of Tg

of the composite [22,33,45,46]. In the 100/25/1 and 100/25/5

blends, the combination of the various noncovalent interactions resulted in the formation of OP-POSS/LiClO4aggregated domains,

improving the probability of forming C]O (PMMA)/OH (OP-POSS) hydrogen bonds, smaller and better-distributed OP-POSS molecules, and an increase in the value of Tgof the composites[47– 50]. In previous studies[5,51], POSSs were found to affect the glass transition temperatures of nanocomposites through two different effects: one was a restricted effect that enhanced Tg, the other

increased the free volume of the system, which reduced Tg. Based

on our FT-IR, XRD, DSC data, and SEM micrographs, the presence of

LiClO4 in the 100/25/5 blend inﬂuenced the hydrogen bonds

formed between PMMA and OP-POSS, dispersion of OP-POSS, and the intermolecular distance between PMMA chains; thus, the addition of LiClO4shifted the physical role of OP-POSS from that of

a diuent to a restricted role (physical crosslinking). Additionally, the

interactions between PMMA, LiClO4, and OP-POSS enhanced the

restricted effect of the OP-POSS toward PMMA. Therefore, the glass transition temperature of the composites was enhanced through the simultaneous incorporation of LiClO4and OP-POSS.

4. Conclusions

In the 100/0/1 and 100/0/5 blends, the OP-POSS molecules tended to aggregate into large domains because of the tendency of their OH groups to self-associate rather than form OH/C]O hy-drogen bonds[35,52], resulting in a decrease in the value of Tgof

the PMMA/OP-POSS composites. In the 100/25/1 and 100/25/5 blends, LiClO4and OP-POSS formed a better dispersion of

sphere-like OP-POSS/LiClO4domains as a result of competition among the

various interaction pairs. Based on these FT-IR spectra, LiClO4

played critical roles through its ion–dipole interaction with PMMA and its speciﬁc interactions with OP-POSS, inﬂuencing hydrogen bonds formed between PMMA and POSS and dispersion of OP-POSS; the result was an enhancement of the glass transition tem-perature of these PMMA/OP-POSS composites. Thus, the addition of LiClO4is a convenient and simple approach toward dispersing the

OP-POSS nanoparticles within PMMA polymers, where the pres-ence of LiClO4changes the physical effect of OP-POSS from that of

a diluent to a cross-linker role. Appendix. Supplementary data

Supplementary data associated with this article can be found in the online version, atdoi:10.1016/j.polymer.2008.06.025.

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