Structural control of silane-grafted polymethylsilsesquioxane

European Polymer Journal 49 (2013) 646–651

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Macromolecular Nanotechnology

Structural control of silane-grafted polymethylsilsesquioxane Wen-Pin Chuang a,b, Yuung-Ching Sheen a, Su-Mei Wei a, Ming-Yu Yen b, Chen-Chi M. Ma b,⇑ a b

Division of Applied Chemistry, Material and Chemical Research Laboratories, Industrial Technology Research Institute, Hsinchu, Taiwan Department of Chemical Engineering, National Tsing Hua University, Hsinchu, Taiwan

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a r t i c l e

i n f o

Article history: Received 20 August 2012 Received in revised form 8 November 2012 Accepted 15 November 2012 Available online 29 November 2012 Keywords: Polymethylsilsesquioxane Refractive index tunable Low temperature Sol–gel

a b s t r a c t This study presents the preparation and fabrication of a series of silane-grafted polymethylsilsesquioxane (SGPMSQ) with a controllable structure and molecular weight. The results of FTIR analysis show that synthesizing polymethylsilsesquioxane (PMSQ) with methyltriethoxysilane (MTES) formed a regular structure such as the cage or ladder structure. The results of XRD and FTIR analysis show that the molecular structure started to change from regular to an irregular random network structure after grafting with tetraethoxysilane (TEOS) and 1H,1H,2H,2H-perfluorodecyltriethoxysilane (PFDTS) on the PMSQ. The changes in the molecular structure of SGPMSQ created by grafting silane are similar to the changes in the molecular structure of PMSQ film heated from 150 to over 250 °C, reducing the refractive index (RI). The RI of PMSQ at 583 nm decreased from 1.50 to 1.42 merely by heating it to 80 °C, introducing silane, and controlling the molecular structure. When the weight ratio of MTES:TEOS:PFDTS was 1:0.5:1, the decomposition temperature (Td) of SGPMSQ was 340 °C, which is much higher than that of pure PMSQ (Td = 267 °C). Although the glass transition temperature of SGPMSQ was 20 °C lower than that of pure PMSQ, its decomposition temperature was 73 °C higher. This shows that SGPMSQ is more flexible and has greater thermal stability than pure PMSQ. Ó 2012 Published by Elsevier Ltd.

1. Introduction Polysilsesquioxane (formula RSiO1.5)n has attracted considerable interest by both scientific and industrial research communities, and has been the subject of a number of recent studies [1–20]. It possesses many advantageous properties compared to conventional organic macromolecules, including thermal and weather resistance, flexibility, electrical insulation, and low surface energy. It is therefore widely used as a material in many industries, particularly for semiconductors and integrated circuits (ICs). Because of their low dielectric constants, polysilsesquioxanes are promising materials for use in silicon ICs. For example, polymethylsilsesquioxane (PMSQ) has a relatively low dielectric constant of approximately 2.7, ⇑ Corresponding author. Address: No. 101, Section 2, Kuang-Fu Road, Hsinchu, Taiwan. Tel.: +886 3 5713058. E-mail address: [email protected] (C.M. Ma). 0014-3057/$ - see front matter Ó 2012 Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.eurpolymj.2012.11.003

good thermal stability, and high crack resistance. Because PMSQ contains methyl groups, which are more hydrophobic than silica, it also has a low moisture absorbance. Organic methyl groups attached to the silicon of PMSQ lend a degree of flexibility to the inorganic polymer matrix. Low moisture absorption and flexibility of this polymer matrix are highly desirable properties for interlayer dielectrics. Materials that have a low dielectric constant also have a low refractive index (RI), thereby enabling their use in optical devices such as antireflection films for liquid crystal displays (LCDs) or the cover glass of solar cells. Previous studies have reported the synthesis of PMSQ with a low RI [12–14]. The RI of PMSQ can be reduced by controlling the molecular weight or molecular structure. Seok et al. [12] synthesized PMSQ with different molecular weights by varying the reaction time from 1 to 30 h. Gunji et al. [13] controlled the molecular weight of PMSQ by varying the reaction conditions, including the molar ratio of water to monomer, temperature, and reaction time. In

W.-P. Chuang et al. / European Polymer Journal 49 (2013) 646–651

2 h at 60 °C. Grafting of the PFDTS was achieved by adding 3.75 g (3 mL) PFDTS dissolved in 5 g (6 mL) MPK to the solution and heating for another 3 h at 60 °C. After synthesizing SGPMSQ using the methods described, the precursor solution was analyzed to determine its structure. Table 1 presents a summary of its formulation. The parameters of the weight ratios of MTES to TEOS and PFDTS to TEOS were varied to determine the effects of this variation on the molecular structure, molecular weight, RI, and thermal properties of the SGPMSQ. The weight ratio of MTES to TEOS was set at 2.0, 3.0, and 4.0:1. The weight ratio of PFDTS to TEOS was set at 0.5, 1.0, 1.5, and 2.0 for a fixed weight ratio of MTES to TEOS. 2.3. Preparation of powder and films for analysis The untreated PMSQ (i.e., the ‘‘precursor’’) was filtered with a 0.25 lm filter and then dried under vacuum at 80 °C for 4 h. The dried cake was ground to powder for use in X-ray diffraction (XRD) analysis, differential scanning calorimetry (DSC) analysis, and thermogravimetric analysis (TGA). To prepare a set of PMSQ thin films, the precursor (20 wt.% PMSQ in MPK) was filtrated with a 0.25 lm filter and spin-coated on silicon wafers or glass substrates at room temperature at 1500 rpm for 30 s (PM490, Swien Co., Taiwan). The film was then maintained at 80 °C on a hot plate for 30 min to remove the solvent.

2. Experimental 2.4. Characterization 2.1. Materials The materials used in this study were methyltriethoxysilane (MTES, 99%; Aldrich), hydrochloric acid (35%; Yakuri), tetraethoxysilane (TEOS, 99%; Aldrich), 1H,1H,2H,2H-perfluorodecyltriethoxysilane (PFDTS, 97%; Aldrich), tetrahydrofuran (THF, 99%, stabilized and anhydrous; Acros), and methyl n-propyl ketone (MPK, 99%; Acros). 2.2. Synthesis of silane-grafted polymethylsilsesquioxane (SGPMSQ) Scheme 1 shows the formulation and synthesis of PMSQ, indicating that PMSQ was synthesized first, and different amounts of TEOS and PFDTS were then grafted to PMSQ. The procedures are as follows. A weight ratio of MTES:TEOS:PFDTS of 2:1:0.5 was used as an example. In this case, the synthesis of PMSQ was achieved by placing 34 g (42 mL) MPK and 15 g (17 mL) MTES in a 200 mL four-necked flask. The mixture was cooled for 10 min in an ice bath. Then, 3 g of deionized water combined with 0.05 g hydrochloric acid was dissolved in 19 g (21 mL) of THF and added to the flask dropwise over 30 min, accompanied by vigorous magnetic stirring. The reaction flask was then warmed to room temperature and stirred continuously for 30 min at room temperature. Hydrolysis and condensation were achieved by maintaining the flask at 60 °C for 4 h in an oil bath under reflux conditions in a nitrogen environment. Grafting of the TEOS on the PMSQ was achieved by dissolving 7.5 g (8 mL) of TEOS in 10 g (12 mL) MPK in the solution and allowing it to react for

The Fourier transform infrared (FT-IR) spectra of the PMSQ were recorded between 400 and 4000 cm 1 using a Nicolet Avatar 320 FT-IR spectrometer (Nicolet Instrument Corporation, Madison, WI, USA). The PMSQ sample was coated on a KBr plate. The average of at least 32 scans was obtained using a resolution of 2 cm 1 within the range 400–4000 cm 1 to identify the characteristic absorption peaks of the functional group. THF was used as the solvent, and a Waters 510 GPC (RI detector) was used to determine the molecular weights of the PMSQ at a flow rate of 1 mL/ min. The GPC calibration curves were obtained using polystyrene standards. The XRD patterns were obtained by scanning at a rate of 2°/min using a Shimadzu XD-5 X-ray diffractometer (XRD; 30 kV, 30 mA) with a copper target. The material was scanned from 2° to 40° at intervals of 0.02°. Thermal degradation was measured by TGA (DU-Pont-951) from room temperature to 800 °C at a heating rate of 10 °C/min in a N2 atmosphere. The glass transition temperature was measured by DSC (DU-Pont951) from 60 to 250 °C at a heating rate of 10 °C/min in a N2 atmosphere. The RIs of the PMSQ films were recorded by a variable-angle spectroscopic ellipsometer (VESA, J.A. Woollam Co.) equipped with a He–Ne laser using an angle of incidence ranging from 70° to 80°. 3. Results and discussion 3.1. Preparation and characterization of the SGPMSQ Structural identification was confirmed by FT-IR. Fig. 1 shows the FT-IR spectra of the PMSQ grafted with different

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these reports [12,13], the RI of PMSQ decreased as the molecular weight increased. Lee et al. [14] investigated the effects of the pH value and the molar ratio of water to methyltrimethoxysilane (MTMS) on the molecular structure of PMSQ. Gunji and Lee both indicated that structural rearrangements occurred when PMSQ was heated from 150 to above 250 °C. This treatment transforms part of the PMSQ regular/cage structure into an irregular/ network structure [13,14]. This rearrangement produces a significant reduction in the RI. However, it is difficult to control the molecular weight and the polydispersity index (PDI) of PMSQ under these reaction conditions. Conversely, reducing the RI of PMSQ with a high baking temperature (exceeding 250 °C) restricts its application on substances with a low heat resistance, such as a plastic matrix. Previous research [15] prepared a phase-separated fluorinated PMSQ and used it as antireflective coating. However, that study did not investigate the changes in the molecular structure of the PMSQ films. The current study reports the preparation of a silane-grafted PMSQ (SGPMSQ) by grafting TEOS and PFDTS on PMSQ with a low RI, and investigates the RI values and thermal properties of the SGPMSQ affected by the molecular structure. The SGPMSQ film presented in this study may be formed at 80 °C, which makes the material suitable for substrates with low heat resistance.

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O

O

Si

HO

HCl / H2O

Si O

O

60 oC, 4 hr

HO

Si

O

Si

O O

Si

O

O O

Si

Si

O

O O

O

Si

Si

MTES

Si

O O

Si

O

Si

O O

Si

O

O O

Si

Si

O

O O

Si OH

Si OH

(a)

O OH OH OH OH O O O O O O O O O O O OH Si Si Si Si Si Si Si Si Si Si Si Si HO HO O O O O O O O O HO HO O O O O O O O HO Si Si Si Si Si Si Si Si OH

Si O

TEOS

HO

O

O

60 oC, 2 hr

(b) F

F

F

F

F

F

F

F

F

F

F

F

F

F

F

F

F

F

F

F

F

O O Si O O

PFDTS

60 oC, 3 hr

F F

F F

F

F

F F

F

F F

F F

F

F

F

F

F

F

F

F

F

F

F

F

F F

F

F

F F

F

F

F F

F

F

O HO Si OH

O HO Si OH F

F

F

OH O OH OH OH O O O O O O O O O O O O O O O Si Si Si Si Si Si Si Si Si Si Si Si Si Si HO HO HO O O O O O O O O HO HO HO O O O O O O O HO Si Si Si Si Si Si Si Si OH O

F F

F

F

F

F

F

F

F F

F F

F

F F

F

F

(c) Scheme 1. Synthesis of silane grafted polymethylsilsesquioxane (SGPMSQ).

Table 1 Effect of various TEOS and PFDTS contents on molecular weight. Weight ratio MTES

TEOS

PFDTS

1 4 4 4 4 4 3 3 3 3 3 2 2 2 2 2

0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

0 0 0.5 1.0 1.5 2.0 0 0.5 1.0 1.5 2.0 0 0.5 1.0 1.5 2.0

MWa

M na

Mw/Mna

1200 1600 2000 2200 2600 2900 1800 2200 2600 3000 3400 2100 2700 3300 4000 4600

800 1000 1100 1200 1400 1500 1100 1300 1400 1500 1700 1200 1500 1700 2000 2300

1.5 1.6 1.8 1.8 1.9 1.9 1.6 1.7 1.9 2.0 2.0 1.8 1.8 1.9 2.0 2.0

a

Determined by GPC using polystyrene as standards and THF as solvent.

contents of TEOS, indicating Si–CH3 absorption bands at 1273 and 768 cm 1. The corresponding CH3 absorption bands appear at approximately 2973 and 2844 cm 1. The absorption band at 3200–3600 cm 1, which corresponds to the hydroxyl functional group, appeared after the hydrolysis of MTES. The corresponding Si–OH absorption band appeared at 910 cm 1. This figure presents the Si–O–Si backbone as a broad band of absorption in the range of 1000–1250 cm 1. Previous research reported an interesting

pure MTES MTES/TEOS = 4/1

Transmittance (a. u.)

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F

F

F

F

F

F

F

MTES/TEOS = 3/1

MTES/TEOS = 2/1

νO-H νC-H

νSi-C

νSi-OH

νSi-O regularνSi-O random 3500

3000

2500

2000

1500

1000

-1

wavelength (cm ) Fig. 1. FTIR spectra of PMSQ grafted with different amounts of TEOS.

characteristic of the FT-IR spectra of PMSQ [3–5] in the splitting of the Si–O–Si absorption into two bands at approximately 1130 and 1070 cm 1. These two bands correspond to regular and irregular structures, respectively. When the PMSQ film was heated from 150 °C to more than 250 °C, the absorption band at 1130 cm 1 decreased and the band at 1070 cm 1 increased. This indicates that the PMSQ structure transformed from a regular to an irregular structure [13,14]. This study shows a Si–O–Si absorption band of pure PMSQ at approximately 1120 cm 1. The hydroxyl group was still present after condensation,

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d2 3.87Å (a)

3.90Å

(b)

4.25Å (c)

4.85Å (d)

5

10

15

2θ

30

35

1.58 Pure MTES MTES/TEOS=2/1 MTES/TEOS/PFDTS=2/1/0.5 MTES/TEOS/PFDTS=2/1/1.0 MTES/TEOS/PFDTS=2/1/1.5 MTES/TEOS/PFDTS=2/1/2.0

1.56 MTES/TEOS/PFDTS = 4/1/1.0

MTES/TEOS/PFDTS = 4/1/2.0

νC-F νSi-O regular νSi-O random 1600

25

1200

1.54 1.52 1.50 1.48 1.46 1.44 1.42

800 -1

wavelength (cm )

1.40 400

600

800

1000

1200

1400

wavelength (nm) Fig. 2. FTIR spectra of PMSQ grafted with TEOS and different amounts of PFDTS.

Fig. 4. Refractive indices of PMSQ grafted with TEOS and PFDTS.

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to control when R1 and R2 increased. In this study, the molecular weight was controlled by varying the TEOS and PFDTS contents, facilitating control of the molecular structures and the stabilization of the resulting structure. The molecular weight of the PMSQ synthesized using MTES was 1200, and the PDI was 1.5. When the weight ratio of MTES:TEOS:PFDTS was 2:1:2, the molecular weight increased to 4600 and the PDI was maintained at 2.0, confirming that the molecular weight was under control. Grafting with TEOS and PFDTS increased both the molecular weight and the PDI, providing evidence of the successful grafting of TEOS and PFDTS on the PMSQ. Fig. 4 shows the variation in the RI of the PMSQ grafted with TEOS and PFDTS, as summarized in Table 2. The RI of pure PMSQ at 583 nm is approximately 1.50. As reported elsewhere [12–14], the RI of PMSQ can be reduced by increasing its molecular weight or baking at high temperature (above 250 °C). In this study, the introduction of TEOS and PFDTS into PMSQ increased the molecular weight and changed the molecular structure. Consequently, the RI was lowered even after heated at 80 °C. The resulting material is suitable for substrates, such as plastics, with low heat resistance. The RI decreased slightly when grafted with TEOS (RI = 1.46), and decreased

MTES/TEOS/PFDTS = 4/1/0.5

2000

20

Fig. 3. X-ray diffraction patterns of PMSQ grafted with TEOS and PFDTS.

Refractive Index

Transmittance (a. u.)

MTES/TEOS= 4/1

Pure MTES MTES/TEOS=2/1 MTES/TEOS/PFDTS=2/1/1 MTES/TEOS/PFDTS=2/1/2

d1

Intensity (a. u.)

indicating that the synthesized PMSQ has a partial cage or ladder structure. Random network structures started to appear at 1070 cm 1 after grafting with 33 wt.% TEOS. This indicates that the addition of TEOS changed the main structure of the PMSQ. Fig. 2 shows the FT-IR spectra of the PMSQ films grafted with TEOS and various amounts of PFDTS. An additional C–F absorption peak appeared at 1240 cm 1 after grafting with PFDTS [2]. Similarly, random network structures began to appear at 1070 cm 1 after grafting with 12.5 wt.% PFDTS. Unlike grafting PFDTS on PMSQ, grafting TEOS on the PMSQ did not significantly change the main structure of PMSQ. This may be because the TEOS molecules are not as large as PFDTS, and therefore, do not affect the main structure of PMSQ as much. Randomness of the network structure increased as the TEOS and PFDTS contents increased, indicating that the addition of TEOS and PFDTS changed the main structure of the PMSQ. Fig. 3(a) shows the XRD pattern of PMSQ [16], showing two distinct diffraction halos. The first halo (d1), appearing at 10.58°, indicates an intramolecular chain-to-chain distance of approximately 8.36 Å (i.e., the width of each double chain). The second diffuse halo (d2), which covers a wider range of diffraction angles, appears at approximately 22.94° and indicates that the average intermolecular chain-to-chain distance in the PMSQ macromolecules is approximately 3.87 Å. As shown in Fig. 3(b)–(d), the intermolecular chain-to-chain distance increased after grafting TEOS and PFDTS. When the weight ratio of MTES:TEOS:PFDTS was 2:1:2, the intermolecular chain-to-chain distance increased to 4.85 Å. This provides further evidence that the presence of TEOS and PFDTS changed the main structure of the PMSQ. Table 1 presents a summary of the results of the GPC analyses of PMSQ grafted with various amounts of TEOS and PFDTS. As reported previously [2,12], the molecular weight of PMSQ can be controlled by the molar ratios of water to monomer (R1) or catalyst to monomer (R2). Both the molecular weight and the polydispersity index (PDI) increased with increasing R1 and R2. However, the molecular structures and stability of PMSQ were difficult

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Table 2 The optical and thermal properties of PMSQ grafted with different amounts of TEOS and PFDTS. Code

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 a

MTES content

TEOS content

PFDTS content

1 4 4 4 4 4 3 3 3 3 3 2 2 2 2 2

0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

0 0 0.5 1.0 1.5 2.0 0 0.5 1.0 1.5 2.0 0 0.5 1.0 1.5 2.0

RIa

Tg (°C)

Tdb (°C)

1.50 1.49 1.48 1.46 1.45 1.44 1.48 1.46 1.45 1.44 1.43 1.48 1.46 1.44 1.43 1.42

58 64 62 57 53 50 61 59 54 50 46 55 50 46 42 38

267 256 261 283 309 318 265 274 295 321 330 293 307 315 334 340

The refractive index at 583 nm. The decomposition temperature (Td) for 5% weight loss.

100

Weight retention (%)

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b

Weight ratio

90 80 70 60

Pure MTES MTES/TEOS=2/1 MTES/TEOS/PFDTS=2/1/0.5 MTES/TEOS/PFDTS=2/1/1.0 MTES/TEOS/PFDTS=2/1/1.5 MTES/TEOS/PFDTS=2/1/2.0

50 40 200

400

600

800

Temperature (°C) Fig. 5. TGA curves of PMSQ grafted with TEOS and PFDTS.

significantly after further grafting with PFDTS (obtaining an RI of 1.34). The grafting of increasing quantities of TEOS and PFDTS on the PMSQ produced even lower RI values. When the weight ratio of MTES:TEOS:PFDTS was 2:1:2 (labeled as Code 16 in Table 2), the RI at 583 nm was 1.42, which is even lower than the theoretical value of 1.44 predicted by the Arago Biot equation [21]. This result could have been caused by the molecular arrangement of the PMSQ that was grafted with TEOS and PFDTS. The original structure of PMSQ was regular, as Scheme 1(a) shows, allowing for a more compact arrangement of the molecules and leading to a higher RI. However, grafting with TEOS and PFDTS changed the molecular structure of the PMSQ from regular to irregular, as shown by the FT-IR and XRD results. The irregular structure shown in Scheme 1(c) produced a larger average intermolecular chain-to-chain distance, which in turn caused the molecular structure to be more widely spaced. Therefore, the RI may be lower than the theoretical value. Consequently, the RI of the SGPMSQ can be reduced both by introducing silanes and by controlling the molecular structure.

Table 2 presents a summary of the thermal properties of PMSQ grafted with TEOS and PFDTS. The glass transition temperature (Tg) of pure PMSQ is 58 °C. The GPC results indicate that grafting with TEOS and PFDTS increased the molecular weight of PMSQ. This should reduce the free volume of SGPMSQ and raise the Tg. However, the DSC results show that the Tg of SGPMSQ decreased, indicating that the free volume of SGPMSQ increased as the TEOS and PFDTS contents increased. This increase may be because the random structure raised the free volume of SGPMSQ more than the free volume decreased by increasing molecular weight. The DSC results in this study provide further evidence that the addition of TEOS and PFDTS changed the main structure of the PMSQ. When the weight ratio of MTES:TEOS:PFDTS was 2:1:2, the Tg was 20 °C lower than that of pure PMSQ. These results demonstrate that SGPMSQ is more flexible than PMSQ, and therefore, more suitable for use in flexible devices. Fig. 5 shows the pyrolysis behavior of PMSQ grafted with TEOS and PFDTS. The first step in the weight loss started at approximately 200 °C, corresponding to the pyrolysis of a low molecular weight PMSQ and changing the molecular structure. Several decomposition reactions occurred at higher temperatures. The weight reduction in this conversion centered at 540–650 °C (i.e., the evolution of CO) and at 700–750 °C (i.e., the evolution of CH4) [5]. The decomposition temperature of PMSQ increased significantly after grafting with PFDTS because of the increase in PMSQ molecular weight and the fluorine atoms, which have higher thermal stability. The 5% weight loss temperature increased by more than 73 °C (from 267 to 340 °C) when the weight ratio of MTES:TEOS:PFDTS was 2:1:2.

4. Conclusion This study reports the preparation and analysis of a series of SGPMSQs. The molecular weight, PDI, and molecular structure can be controlled by grafting TEOS and PFDTS on

PMSQ. Introducing silane and controlling the molecular structure reduced the RI at 583 nm, which was reduced from 1.50 to 1.42. The glass transition temperature of SGPMSQ was 20 °C lower than that of pure PMSQ. The decomposition temperature (Td) for 5% weight loss of SGPMSQ was 73 °C higher than that of pure PMSQ when the weight ratio of MTES:TEOS:PFDTS was 2:1:2. These results demonstrate that SGPMSQ is more flexible and has greater thermal stability than pure PMSQ. References [1] Yamada N, Takahashi T. Methylsiloxane spin-on-glass films for low dielectric constant interlayer dielectrics. J Electrochem Soc 2000;147:1477–80. [2] Lee JH, Kim WC, Min SK. Synthesis of poly(methyl-cotrifluoropropyl) silsesquioxanes and their thin films for low dielectric application. Macromol Mater Eng 2003;288:455–61. [3] Park ES, Ro HW, Nguyen CV, Jaffe RL, Yoon DY. Infrared spectroscopy study of microstructures of poly(silsesquioxane)s. Chem Mater 2008;20:1548–54. [4] Baney RH, Itoh M, Sakakibara A, Suzuki T. Silsesquioxanes. Chem Rev 1995;95:1409–30. [5] Jun M, Lianghe S, Yongyxi S. Pyrolysis of polymethylsilsesquioxane. J Appl Polym Sci 2002;85:1077–86. [6] Liu WC, Yang CC, Chen WC, Dai BT, Tsai MS. The structural transformation and properties of spin-on poly(silsesquioxane) films by thermal curing. J Non-Cryst Solids 2002;311:233–40. [7] Lee JK, Char K, Rhee HW. Synthetic control of molecular weight and microstructure of processible poly(methylsilsesquioxane)s for lowdielectric thin film applications. Polymer 2001;42:9085–9. [8] Cho HJ, Hwang DH, Lee JI, Jung YK, Park JH, Lee J, et al. Electroluminescent polyhedral oligomeric silsesquioxane-based nanoparticle. Chem Mater 2006;18:3780–7. [9] Sosaf JM. Surface characterization of methylsilsesquioxane– phenylsilsesquioxane copolymers. Macromolecules 1980;13: 1260–4.

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W.-P. Chuang et al. / European Polymer Journal 49 (2013) 646–651