Synthesis of amorphous copoly(thioether sulfone)s with high refractive indices and high Abbe numbers

European Polymer Journal 46 (2010) 34–41

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Synthesis of amorphous copoly(thioether sulfone)s with high refractive indices and high Abbe numbers Yasuo Suzuki, Tomoya Higashihara, Shinji Ando, Mitsuru Ueda * Department of Organic and Polymeric Materials, Tokyo Institute of Technology, 2-12-1-H120, O-okayama, Meguro-ku, Tokyo 152-8552, Japan

a r t i c l e

i n f o

Article history: Received 12 August 2009 Received in revised form 5 September 2009 Accepted 8 September 2009 Available online 12 September 2009 Keywords: Thermoplastics Refractive index Abbe number Poly(thioether sulfone) Random copolymer

a b s t r a c t New thermoplastic random copoly(thioether sulfone)s with high refractive indices and high Abbe numbers have been developed by the simultaneous introduction of sulfide, sulfone, and alicyclic units in the polymer chains. They were prepared by random copolymerization of 2,5-bis(sulfanylmethyl)-1,4-dithiane (BMMD) and cyclohexane-1,4-dithiol (CHDT) with divinyl sulfone (DVS) based on the Michael polyaddition. Tough, flexible, colorless and transparent copoly(thioether sulfone) (poly(BMMD/CHDT–DVS)) films were obtained, and they showed good thermal stability with the 5% weight-loss at temperatures over 300 °C in nitrogen, and the glass transition temperatures of 42–50 °C. The inherent amorphous nature of films was confirmed by differential scanning calorimetry. Poly(BMMD/CHDT–DVS) exhibited quite high level of refractive indices and Abbe numbers as thermoplastics, in the range of 1.6512–1.6022 and 42.6–50.6, respectively. The experimental refractive indices and their wavelength dispersions were well reproduced by the DFT calculations with the aid of empirical density prediction. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction The development of new functional materials has been essential for continuous progress of advanced optical devices [1–3]. Recently, optical polymers with both high refractive indices at the sodium D line wavelength (nD) and high Abbe numbers (mD) have been widely proposed for micro-optic and optoelectronic applications, such as lenses, prisms, waveguides, and diffractive gratings [4,5]. The inherent advantages of polymers are their processability, good impact resistance, and light weight compared to inorganic glasses. Indeed, poly(methyl methacrylate), polycarbonate, and cycloolefin polymers [6] are widely used as thermoplastics in camera, pickup, and projector lenses where injection molding is applied. On the other hand, thermosetting polymers, such as poly[ethylene glycol bis(allylcarbonate)] (CR-39) [7] and resins from episulfides, [8] polythiols, [9] and polyisocyanates [10], have

* Corresponding author. Tel./fax: +81 3 57342127. E-mail address: [email protected] (M. Ueda). 0014-3057/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2009.09.006

been applied for consumer use such as eyeglasses, which are manufactured by cast molding. According to Lorentz–Lorenz theory, the introduction of molecules with high molar refractions and low molar volumes effectively increase the refractive indices of polymers [11]. A variety of sulfur-containing high nD polymers has been developed for thermosets by exploiting high molar refraction of sulfur atom. In addition, the mD, which is a key parameter for the refractive index dispersion, is also of great importance for optical materials used in the visible region. The Abbe number is given by following equation;

mD ¼ ðnD  1Þ=ðnF  nC Þ

ð1Þ

where nD, nF, and nC are the refractive indices of the material at the wavelengths of sodium D (589.3 nm), hydrogen F (486.1 nm), and hydrogen C (656.3 nm) lines, respectively [12]. A high Abbe number indicates a lower dispersion in the refractive index [13]. Highly refractive materials empirically exhibit small Abbe numbers [14]. Therefore, it is very important to balance the refractive index and Abbe number. Thermoplastics with high nD and high mD have so far not been

Y. Suzuki et al. / European Polymer Journal 46 (2010) 34–41

the subject of detailed investigation compared to thermosets. Therefore, we have been interested in developing thermoplastics with high nD and high mD. To improve the refractive index dispersion, an alicyclic group is known to be effective, and shows a refractive index higher than that of an aliphatic moiety. Based on these findings, we have recently reported the synthesis of poly(thioether sulfone)s from 2,5-disulfanyl-1,4-dithiane (DSDT), 2,5-bis(sulfanylmethyl)-1,4-dithiane (BMMD), and divinyl sulfone (DVS) with very high nD (1.6859, 1.6512) and mD values (48.6, 42.6) as thermoplastics by simultaneous introduction of sulfide, sulfone, and alicyclic units in the polymer chains (see Scheme 1) [15,16]. However, poly(DSDT–DVS) became colored beyond 150 °C in air, which is probably due to the thermally unstable thioacetal groups and such coloration at elevated temperatures may cause problems in injection molding. Furthermore, poly(BMMD–DVS) exhibited a semi-crystalline nature which may cause another problem in optical device application despite the very slow crystallization rate under a normal cooling process. Copolymerization generally results in a marked decrease in the tendency toward crystallization because of the loss of structural symmetry. Based on this idea, cyclohexane-1,4-dithiol (CHDT) with a high sulfur content (43.25 wt.%) and alicyclic structure was selected as a co-monomer. This article reports the synthesis and properties of novel poly(thioether sulfone)s synthesized from BMMD, CHDT, and DVS. These polymers exhibited high optical transparency (>400 nm), high refractive indices in the range of 1.6512–1.6022 (at 589 nm) and large Abbe numbers in the range of 42.6–50.6. Completely amorphous polymers were successfully obtained by incorporating below 80 mol.% of BMMD against CHDT. 2. Experimental 2.1. Materials Cyclohexane-1,4-diol (TCI, Japan), thiourea (Wako, Japan), hydrobromic acid (47%, Wako, Japan), divinyl sulfone (DVS, TCI, Japan), and the other chemicals were used as received. 2.2. Synthesis of CHDT A flask equipped with a magnetic stirrer, a nitrogen inlet, and a condenser was charged with cyclohexane-

S HS

S BMMD

SH

+

O S O DVS

S S

35

1,4-diol (11.62 g, 0.1 mol), thiourea (30.4 g, 0.4 mol), hydrobromic acid (47%, 80 mL). The solution was refluxed for 36 h and then cooled to room temperature. To the solution, NaOH (30 g) was added carefully in a cold water bath. After addition, the mixture was heated at 90 °C for 2 h under nitrogen. Then the solution was cooled to room temperature, and acidified by adding 6N-HCl(aq.) until its pH value dropped to 2–3. The separated oil was extracted with CH2Cl2. The extract was washed with water and dried over anhydrous magnesium sulfate. The solvent was removed at a reduced pressure and the residue was subjected to vacuum distillation (b.p. = 43 °C at 0.55 torr) to give colorless CHDT (3.46 g, 25.0% yield). FT-IR (KBr, cm1): 2931.27 (–CH2–), 2854.13 (–CH2–), 2545.58 (–SH), 1442.49 (–CH2–). 1H NMR (300 MHz, CDCl3, 25 °C, ppm): d = 3.41, 3.09, 2.74, 2.38 (2H, –CH–), 2.16–1.17 (m, 10H), 1.60– 1.50 (m, 2H, –SH). 2.3. Isolation of trans CHDT To a solution of CHDT (10 mmol, 1.483 g) and p-nitrobenzoic chloride (24 mmol, 4.45 g) in dehydrated THF was added pyridine (24 mmol, 1.92 mL) dropwise in an ice bath under a nitrogen atmosphere. After stirring at room temperature overnight, the resulting reaction mixture was poured into aqueous NaHCO3. Then the mixture was extracted with CH2Cl2. The organic layer was washed with aqueous NaHCO3 and water, dried over anhydrous magnesium sulfate, filtered, and concentrated. The crude product was recrystallized by excess amount of THF (approximately 400 mL) to give trans 1,4-cyclohexylene bis(p-nitrothiobenzoate) (0.587 g, 13.1% yield). 1H NMR (300 MHz, CDCl3, 25 °C, ppm): d = 8.34–8.24 (d, 4H), 8.13–8.07 (d, 4H), 3.81–3.68 (2H), 2.30–2.18 (d, 8H), 1.83–1.67 (t, 8H). To a mixture of 1,4-cyclohexylene bis(p-nitrothiobenzoate) (1 mmol, 0.448 g) and THF (10 mL) was added hydrazine monohydrate (10 mmol, 0.505 mL) dropwise under a nitrogen atmosphere. The mixture was stirred for 6 h at room temperature. Then the mixture was poured into CH2Cl2 and water, and acidified by adding 6N-HCl(aq.) until its pH value dropped to 2–3. The organic layer was separated, washed with HCl(aq.) and water, dried over anhydrous magnesium sulfate, filtered, and concentrated. The crude product was purified by column chromatography on silica gel (an eluent, hexane/CH2Cl2 = 3:1) to give trans CHDT (0.0171 g, 11.6% yield). 1H NMR (300 MHz, CDCl3, 25 °C, ppm): d = 2.82–2.68 (2H), 2.14– 2.02 (d, 4H), 1.53–1.49 (d, 2H), 1.47–1.38 (t, 4H).

Michael polyaddition

2.4. Synthesis of polymer (poly(CHDT–DVS))

S

S

O S O n

BMMD/DVS Scheme 1. Synthesis of poly(thioether sufone).

To a solution of CHDT (2 mmol, 0.2964 g) and DVS (96%, 2 mmol, 0.2463 g) in N-methyl-2-pyrrolidone (NMP, 1 mL) was added a catalytic amount of triethylamine. The solution was stirred for 3 h at room temperature. The resulting white paste was poured into methanol. The precipitate was collected to give white powder (0.4996 g, yield: 92.1%). 1H NMR (300 MHz, DMSO-d6, 40 °C, ppm): d = 3.40 (t, 4H), 3.02, 2.78, 2.33 (m, 2H), 2.87 (m, 4H), 2.10–1.18 (m, 8H).

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Y. Suzuki et al. / European Polymer Journal 46 (2010) 34–41

Elem. Anal.Calcd for (C10H18O2S3)n: C, 45.08; H, 6.81. Found: C, 45.15; H, 6.60. 2.5. Synthesis of random copolymers (poly(BMMD/CHDT– DVS)) The general polycondensation procedure for the synthesis of poly(BMMD/CHDT–DVS) ([BMMD]/[CHDT] = 50:50) is as follows. To a solution of BMMD (1 mmol, 0.2124 g), CHDT and DVS (96%, 2 mmol, 0.2463 g) in dimethylsulfoxide (DMSO, 1 mL) was added a catalytic amount of triethylamine. The solution was stirred for 3 h at room temperature. The resulting mixture was poured into methanol. The precipitate was collected to give white powder. The other copolymers with different feed ratios were also prepared by the same procedure except changing a solvent depending on the solubility of a resulting polymer. When the feed ratio of [BMMD]/[CHDT] was 100:0, 80:20, or 50:50, DMSO was used as a solvent; whereas NMP was used in the other cases. 2.6. Measurements FT-IR spectra were obtained on a Horiba FT-720 spectrometer. Solution state 1H NMR spectra were recorded with a Bruker DPX300S spectrometer using CDCl3 or DMSO-d6 as a solvent and trimethylsilane as the reference (0 ppm). Number- and weight-average molecular weights (Mn and Mw) were evaluated by gel permeation chromatography (GPC) on a JASCO PU-2080 Plus with two polystyrene gel columns (TSK GELS GMHHR-M). DMF containing 0.01 M LiBr was used as a solvent at a flow rate of 1.0 mL min1 calibrated by standard polystyrene samples. UV–vis transmittance spectra were recorded on a JASCO V-560 UV/Vis spectrometer in the range 250–800 nm. Thermal analysis was performed on a Seiko EXSTAR 6000 TG/DTA 6300 thermal analyzer at a heating rate of 10 °C/ min for thermogravimetry (TG) and a Seiko EXSTAR 6000 DSC 6200 at a heating rate of 10 °C/min for differential scanning calorimetry (DSC) under nitrogen. The refractive indices of polymer films were measured at the wavelengths of 486, 589, 656 nm by changing monochromatic filters with an Abbe refractometer (Atago, DR-M4). A halogen lamp with high brightness was used as a white light source. 2.7. Calculations The DFT level of theory with the three-parameter Beckestyle hybrid functional (B3LYP) was adopted for calculations of wavelength-dependent molecular polarizabilities in conjunction with the Gaussian basis sets. The 6–311G (d) basis set was used for geometry optimizations under no constraints, and the 6–311++G(d,p) was used for calculations of wavelength-dependent linear polarizabilities. All the calculations were performed using the software package of Gaussian 03 (Rev.C02 and D01) [17]. The wavelengthdependent refractive indices of polymers were calculated using the Lorentz–Lorenz equation:

n2k  1 4p q  NA ak ¼ 3 Mw n2k þ 2

ð2Þ

where nk is the refractive index at a wavelength of k, q the density, NA the Abogadoro number, Mw the molecular weight, and ak the linear molecular polarizability at k. In this study, the densities of polymers were calculated according to the empirical Bicerano’s method [18].

3. Results and discussion 3.1. Synthesis of monomer CHDT was selected as a new co-monomer with BMMD. CHDT was prepared by acid-catalyzed nucleophilic substitution reaction of cyclohexane-1,4-diol with thiourea, followed by hydrolysis under an alkaline condition in onepot, as shown in Scheme 2a. As shown in Fig. 1a, the 1H NMR spectrum of CHDT shows complicated, coupled signals due to the existence of trans–cis structural isomers. In order to assign those signals, the trans-isomer was isolated by recrystallization in THF after the modification with p-nitrobenzoic chloride, followed by hydrolysis of thioester groups (Scheme 2b), and analyzed by 1H NMR as shown in Fig. 1b with all assignments for each signal. Based on these data, signals in the range of 2.67–2.82 ppm correspond to the methine d protons of the trans-isomer; whereas the signals in the range of 3.02–3.13 and 3.35–3.46 ppm are assignable to the methine b protons of the cis isomer. The signals in the range of 1.37–1.47 and 2.02–2.15 ppm are assigned to the axial methylene g protons and the equatorial methylene f protons of the trans-isomer, respectively. The sharp peaks in the range of 1.47–1.57 ppm are assigned to the thiol c protons of trans–cis isomers. All residual protons correspond to methylene a protons of the cis isomer. A reasonable signal intensity ratio between the methine b protons and methylene a protons is confirmed to be 2:8. Thus, CHDT, comprised of trans–cis isomers, could be prepared without any problems and the comprising ratio is estimated to 1:1 from the integral ratio of b protons, which should be effective for exhibiting an amorphous nature of poly(BMMD/CHDT–DVS), synthesized afterwards.

3.2. Preparation and characterization of polymers The Michael polyaddition reaction is one of the useful ways to prepare high-molecular-weight polymers [19]. The mixture of BMMD and CHDT was randomly copolymerized with DVS (Scheme 3) at varied feed ratios of [BMMD]/[CHDT] = 100:0, 80:20, 50:50, 20:80, and 0:100. The polymerization was carried out at room temperature for 3 h in the presence of a catalytic amount of triethylamine (TEA) as the basic catalyst. NMP or DMSO was chosen as a polymerization solvent depending on the feed ratio (see Section 2). The polymers thus obtained were white solids and soluble in NMP and other dipolar aprotic solvents and insoluble in chloroform, cyclohexanone, and tetrahydrofuran. The molecular weights and polydispersity indices were measured by GPC. The moderate number average molecular weights in the range of 7200– 11,600 g/mol and polydispersity indices in the range of

37

Y. Suzuki et al. / European Polymer Journal 46 (2010) 34–41

(a)

S OH

H2N

NH2

HO

S

NH

HBr aq., reflux, 36h

H2N

NH 2 2HBr

NH

S SH

1, NaOH, 90 o C, 2h 2, HCl

HS CHDT (cis and trans ) Yield:25.0%

(b)

NO2 SH

NO2 Pyridine +

HS

Cl

THF, r.t., overnight

O

(cis and trans)

S

O

O

S O 2N

( cis and trans )

NO2 Recrystallization by THF

S

O

O

S O2 N

SH

H2 NNH2 THF, r.t., 6h

HS (trans )

(trans )

Scheme 2. (a) Synthesis of CHDT and (b) Isolation of trans CHDT.

(a) a HS

b

SH

c c 5.93

b 0.99

3.5

2.5

2.0

d HSH H

2.05

1.5

1.0

PPM

1.0

PPM

e

H HS

a

1.00

3.0

(b)

a

b

e

f

4.04

g

3.99 2.19

2.00

g

f

d 3.5

3.0

2.5

2.0

1.5

1

Fig. 1. H NMR spectra of CHDT. (a) Mixture of cis and trans-isomer, (b) trans-isomer.

1.62–1.82 could be determined by using polystyrene standards. These results are summarized in Table 1.

The structures of polymers were characterized by 1H NMR and IR spectroscopy and elemental analysis. The IR

38

Y. Suzuki et al. / European Polymer Journal 46 (2010) 34–41

S HS

SH

S BMMD x

S S

+

SH HS

O S O DVS 1

+

CHDT 1-x

S

S

O S O

S xn

S

Triethylamine DMSO or NMP, r.t., 3h

O S O

DVS). In addition, no coloration occurred up to around 200 °C for all samples. Unfortunately, the Tg values of the polymers are still low, despite the rigid structure of the cyclohexyl unit was introduced in the main chain. The low Tg is mainly attributable to the flexible –(CH2)2–S–CH2– structure in the main chain. 3.4. Optical properties

(1-x)n

Poly(BMMD/CHDT-DVS) Scheme 3. Synthesis of homopolymers and copolymers.

spectra of polymers exhibited absorption at 1319 and 1111 cm1 which are characteristic of the sulfone group. Fig. 2 shows the 1H NMR spectra of poly(BMMD/CHDT– DVS) at different compositions. The intensities of signals for the terminal units which resonated at 6.28 and 6.99 ppm are very small compared with those of vinyl group, and the signal of methylene protons adjacent to sulfonyl group is clearly observed at 3.43 ppm. The compositions determined by 1H NMR are in good agreement with those estimated from the feed ratio. The complete assignments of other protons are summarized in Fig. 2. All the results clearly indicate the successful synthesis of new random copoly(thioether sulfone)s.

3.3. Thermal properties High thermal stabilities are of great importance for manufacturing optical components by injection molding. Thermally stable poly(BMMD–DVS) was employed in a previous paper [16]. However, poly(BMMD–DVS) showed a crystallization temperature (Tc) at 119 °C and a melting point (Tm) at 178 °C, which are not suitable for optical device applications. The thermal properties of the poly(BMMD/CHDT–DVS) series are summarized in Table 2. The 5 wt.% degradation temperatures (T5%) of the poly(BMMD/CHDT–DVS) series exceed 300 °C in all cases. These thermal properties adequately meet the requirements for injection molding. Fig. 3 shows the DSC thermograms of polymers. When the content of BMMD is below 80 mol.%, the thermograms indicate the inherently amorphous nature, with glass transition temperatures (Tgs) at 42–50 °C. Even a 20 mol.% addition of CHDT leads to a small heat flow for Tm and the absence of Tc, which should improve the optical properties of polymers compared to semi-crystalline poly(BMMD–

The UV–vis absorption spectra of poly(CHDT–DVS) and poly(BMMD–DVS) films with the thickness of 13 lm (poly(CHDT–DVS)) and 26 lm (poly(BMMD–DVS)) are shown in Fig. 4. Poly(CHDT–DVS) exhibits higher transparency compared to poly(BMMD–DVS) in the visible region (k = 400–800 nm). The transmittance of these polymers is over 99% at 400 nm (without Fresnel reflection). The alicyclic cyclohexyl units without p-electrons and the electronwithdrawing sulfone group endow these polymers with colorlessness and transparency. The experimental and calculated refractive indices of the films are summarized in Table 3 with the experimental Abbe numbers and their semi-crystalline/amorphous nature. The wavelength dispersion of the refractive indices (nk) thus obtained is plotted in Fig. 5 with a fitted curve using the simplified Cauchy’s formula:

nk ¼ n1 þ D=k2 :

ð3Þ

The respective nD and mD values of poly(CHDT–DVS) are found to be 1.6022 and 50.6, which is the best combination among all thermoplastics. All other polymers also exhibit high refractive indices and high Abbe numbers at the same time. The refractive index of polymers can be increased by introducing a sulfide group (–S–) with a high polarizability. However, sulfur-containing polymers generally show relatively low Abbe numbers due to the longer cut-off wavelengths appearing in the near-UV region. In contrast, sulfonyl group (–SO2–) effectively increases the Abbe number because of the low molecular dispersion, as described in the previous paper [15]. Therefore, the combination of one –SO2– group and more than one –S– group should be effective in increasing both the nD and mD values simultaneously. Fig. 6 shows the plots for nD and mD values against BMMD/CHDT molar ratios. As can be seen, the nD value increases with an increase in the BMMD fraction due to the increasing –S– content. In sharp contrast, the mD value decreases with an increase in the BMMD fraction due to the decreasing –SO2– content. It is thus possible to tune both the refractive indices and the Abbe numbers to a high de-

Table 1 Polymerization of BMMD and CHDT with DVS. [BMMD]:[CHDT] (mol.%) 100:0 80:20 50:50 20:80 0:100 a b

Solvent DMSO DMSO DMSO NMP NMP

Determined by GPC (DMF, PSt standard). Insoluble in DMF.

Yield (%) 96.5 89.3 91.3 86.3 92.1

Mna

Mwa

b

b

–

8500 8400 7200 11,600

– 15,500 15,200 13,000 18,800

Mw/Mna –b 1.82 1.81 1.80 1.62

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Y. Suzuki et al. / European Polymer Journal 46 (2010) 34–41

a S S

S

b

S

c dS f O e OS

O S O n

S

m

H2O

DMSO

[BMMD]/[CHDT] 100:0

a, d, e 80:20

b, f c

50:50

20:80

0:100 7

6

5

4

3

2

1

0

PPM 1

Fig. 2. H NMR spectra of poly(BMMD/CHDT–DVS) in DMSO-d6.

Table 2 Thermal properties of the copolymers.

a b c d

[BMMD]:[CHDT] (mol.%)

T5%a

Tcb

Tmc

Tgd

100:0 80:20 50:50 20:80 0:100

303 299 301 301 300

119 – – – –

174 152 – – –

47 45 47 42 46

Five percent of weight-loss temperature. Crystallization temperature. Melting temperature. Glass transition temperature.

gree, simply by changing the feed ratio in the random copolymer of BMMD and CHDT with DVS. Fig. 7 shows the wavelength dispersions of calculated and experimental refractive indices of the three kinds of homopolysulfones. The predicted densities based on Bicerano’s method are 1.3081 for poly(BMMD–DVS), 1.2488 for

poly(CHDT-DVS), and 1.3862 for poly(DSDT–DVS). As seen in the figure, the experimental nD values of poly(BMMD– DVS) and poly(DSDT–DVS) were well reproduced by the DFT calculations, though that of poly(CHDT-DVS) was over-estimated. This result suggests that the 1,4-dithiane structures in BMMD and DSDT do not affect the molecular packing of polymer chains, whereas the cyclohexyl ring structure in the trans-isomer of CHDT promotes the formation of loose inter-molecular packing. However, the significant effect of preventing the crystallization of copolymers by incorporating a small amount of CHDT could be ascribed to the loose molecular packing. Furthermore, the calculated refractive indices at shorter wavelengths (i.e., 486 nm) are slightly higher than the experimental values. At this moment, we cannot offer a straightforward explanation for the over-estimation of the refractive indices near the absorption edges, but the overall trends in the wavelength dispersions of homopolymers were well predicted by the calculations.

40

Y. Suzuki et al. / European Polymer Journal 46 (2010) 34–41

[BMMD]/[CHDT]

1.68

100:0

[BMMD]/[CHDT]

Exo)

1.66 Refractive Index

80:20

Heat Flow (

50:50 20:80

100:0 1.64

80:20

1.62

50:50

0:100

20:80 1.6

0:100

400 0

50

100 o Temperature ( C)

150

450

500

550

200

Fig. 3. DSC curves of homopolymers and copolymers (second heating).

600

650

700

750

800

Wavelength (nm) Fig. 5. Wavelength dispersion of the experimental refractive indices of poly(BMMD–DVS), poly(CHDT–DVS) and copolymers. The dispersion is fitted by the simplified Cauchy’s formula.

100 1.66

52

1.65

50

Refractive Index

60

40

Poly(CHDT-DVS) Poly(BMMD-DVS)

20

0

300

400

500

600

1.64

48

1.63 46 1.62 44

1.61

700

1.6

800

0

20

Wavelength (nm) Fig. 4. UV–vis spectra of the films (thickness) of poly(CHDT–DVS) (13 lm) and poly(BMMD–DVS) (26 lm).

Abbe's Number

Transmittance (%)

80

40 60 80 BMMD Content (mol %)

42 100

Fig. 6. Refractive indices and Abbe numbers of homo- and copolymers with BMMD content.

Table 3 Optical properties of the polymers.

a b c d e

[BMMD]:[CHDT] (mol.%)

Sulfur content (wt.%)

nDa

nFb

nCc

mDd

nD,calce

Nature

100:0 80:20 50:50 20:80 0:100 Ref-poly (DSDT–DVS)

48.5 46.4 43.0 39.0 36.1 53.0

1.6512 1.6440 1.6256 1.6113 1.6022 1.6859

1.6627 1.6547 1.6348 1.6202 1.6107 1.6961

1.6474 1.6401 1.6215 1.6078 1.5988 1.6820

42.6 44.1 47.0 49.3 50.6 48.6

1.656 – – – 1.621 1.687

Semi-crystalline Semi-crystalline Amorphous Amorphous Amorphous Amorphous

Measured at 589 nm. Measured at 486 nm. Measured at 656 nm. Calculated using Eq. (1). Calculated for a wavelength of 589 nm by the DFT method.

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Y. Suzuki et al. / European Polymer Journal 46 (2010) 34–41

1.78 poly(BMMD/DVS) (Calc) poly(CHDT/DVS) (Calc) poly(DSDT/DVS) (Calc) poly(BMMD/DVS) (Exp) poly(CHDT/DVS) (Exp) poly(DSDT/DVS) (Exp)

1.76

Calculated Refractive Index

1.74 1.72 1.70 1.68 1.66 1.64 1.62 1.60 1.58 1.56 400

486 nm 500

589 nm

656 nm

600 Wavelength (nm)

700

800

Fig. 7. Wavelength dispersion of the calculated and experimental refractive indices of homopolymers listed in Table 3. The filled and open circles denote the calculated and experimental refractive indices, respectively.

4. Conclusion The strategy involving the simultaneous introduction of sulfide, sulfone, and alicyclic units in the polymer chains provided new thermoplastics, random copoly(thioether sulfone)s and poly(BMMD/CHDT–DVS), with high refractive indices and high Abbe numbers. These copolymers were successfully synthesized simply by the random copolymerization of the mixture of BMMD and CHDT with DVS based on the Michael polyaddition. Inherently amorphous poly(BMMD/CHDT–DVS) films were obtained when the content of BMMD was below 80 mol.%. Both TGA and DSC analyses confirmed good thermal stability with T5% values exceeding 300 °C in nitrogen and Tg values of 42–50 °C, respectively. In order to improve such low Tg values, rigid alicyclic dithiols, i.e., adamantane dithiols, may need to be used as dithiols in place of CHDT. Poly(BMMD/CHDT–DVS) exhibited excellent combinations of high refractive indices and high Abbe numbers as thermoplastics, in the range of 1.6512–1.6022 and 42.6–50.6, respectively. The experimental refractive indices and their wavelength dispersions were well reproduced by DFT calculations with the aid of empirical density prediction.

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