Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
Low Birefringent Properties of Poly(phosphonate) Derivatives Ryoyu Hifumi†,‡ and Ikuyoshi Tomita*,† †
Department of Chemical Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology, Nagatsuta-cho 4259-G1-9, Midori-ku, Yokohama 226-8502, Japan ‡ JSR Corporation, Kawajiri-cho 100, Yokkaichi, Mie 510-8552, Japan
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S Supporting Information *
ABSTRACT: The birefringent properties of bisphenol Abased poly(phosphonate) and poly(thiophosphonate) were investigated. The orientational birefringences (CR values) of the poly(phosphonate) derivatives are in the range from +1.2 × 10−9 to +1.5 × 10−9 Pa−1, which are less than half of those of the conventional bisphenol A-based polymers. In addition, their photoelastic birefringences (CD values) are in the range from +5.7 × 10−11 to +6.1 × 10−11 Pa−1, which are also lower than those of the conventional bisphenol A-based polymers. From the DFT calculations of the polarizability anisotropies of the model compounds, the low birefringent properties of the poly(phosphonate) derivatives are most probably due to the presence of the PO and PS groups and the phenyl substituents perpendicularly oriented to the main chain direction.
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INTRODUCTION Attention has been paid to the birefringence phenomenon of polymer materials as recent progress in optical devices. For example, plastic lenses require low birefringence to obtain high-resolution images. In display devices, the birefringent properties of polymer thin films make it possible to control the polarization state of light. The birefringence (Δn) is defined as follows: Δn = n|| − n⊥
Osaki and co-workers expressed the coefficient CD on the basis of their dynamic birefringent studies as follows:3,4 C D = CG + C R E R ′(∞)/EG′(∞)
where CG is the stress-optical coefficient for the glassy state, which originates from the deformation and rotation of the local structure of the main chain and the pendant groups of the polymers.5 The values ER′(∞) and EG′(∞) are the limiting moduli of the rubbery and glassy components at high frequencies, respectively. Eq 4, which is based on the additivity of the stresses, is useful to separate the coefficient CD into the rubbery and glassy components, although some controversy may still exist for specific samples.6−8 In order to obtain low birefringent polymers, the structures of the polymers should be designed so that the difference between n|| and n⊥ becomes small, as is clear from eq 1. One rational approach of the macromolecular design is to increase the n⊥ values of the polymers, because the n|| of the repeating units of polymers are generally larger than n⊥ (i.e., positive birefringence). For example, aromatic building blocks are often introduced into the perpendicular direction of the polymer backbone: a poly(carbonate) derived from 1,1-bis(4-hydroxyphenly)-1,1-diphenlymethane has been reported to achieve a low CR value of 0.6 × 10−9 Pa−1, which is less than one-seventh of that of a 2,2-bis(4-hydroxyphenyl)propane (bisphenol A)based poly(carbonate).9 More effective macromolecular design for low birefringent polymers is to introduce cardo structures into the polymers.10
(1)
where n|| and n⊥ are the refractive indices for parallel and perpendicular directions to the reference axes, respectively. The birefringence of polymers can be classified into two types, orientational and photoelastic birefringences, depending on their origins.1 The orientational birefringence derives from the orientation of the polymers with optically anisotropic molecular structures. The orientational birefringence can be related to the stress through the stress-optical rule (SOR),2 which holds well in the rubbery state and the flow zone: Δn = C R σ
(2)
where CR and σ are the stress-optical coefficient for the rubbery state and the stress, respectively. By comparing the CR values, the orientational birefringence can be quantitatively evaluated within a wide variety of polymers. The photoelastic birefringence is observed when stress is applied to glassy polymers and can be related to the stress with the photoelastic coefficient C D through the following equation,2 similar to the SOR: Δn = C Dσ
Received: March 31, 2018 Revised: May 20, 2018
(3) © XXXX American Chemical Society
(4)
A
DOI: 10.1021/acs.macromol.8b00684 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Since the cardo structures can fix the perpendicular orientation of the orthogonal building blocks with respect to the main chain, they are expected to exhibit larger n⊥ values. For example, Sakurai and co-workers have reported the synthesis of 9,9-bis[4-(2-hydroxyethoxy)phenyl]fluorene-containing poly(ester)s, in which their high refractive indices above 1.6 and low CR values in the range from 0.3 × 10−9 to 1.8 × 10−9 Pa−1 have been described.11,12 Takata and co-workers have synthesized 9,9-diarylfluorene-based poly(ether)s with alkylene spacers, in which a polymer having a CR value of −0.15 × 10−9 Pa−1 has been described.13 As new candidates of low birefringent building blocks of polymers, phosphonate moieties are expected to exhibit high n⊥ values due to the perpendicularly aligned PX units (X = O, S) and high refractive pendant groups (Z) on the phosphorus atoms (Figure 1).
evaluated with respect to those of the conventional bisphenol A-based polymers having carbonate, ester, and ether linkers in the main chains. The origin of their low birefringent properties is also discussed on the basis of the results obtained from the DFT calculations.
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EXPERIMENTAL SECTION
Materials. Bisphenol A-based poly(carbonate) (P1), poly(arylate) (P2), and poly(ether sulfone) (P3) were obtained from Teijin Limited (Panlite L-1250Y), Unitika LTD (U-100), and BASF SE (Ultrason S3010NA), respectively (Figure 2). The poly(ether)s (P4 and P5) were synthesized by the polycondensation of bisphenol A with 2,6-difluorobenzonitrile and 4,6-dichloropyrimidine, respectively, in the presence of potassium carbonate as a base (detailed procedures are shown in the Supporting Information). The synthesis of bisphenol A-based poly(phosphonate) (P6) and poly(thiophosphonate) (P7) was described in our previous paper.14 The structures of P1−P7 are shown in Figure 2, and their molecular weights are listed in Table S1. Preparation of Films. The films of P1−P5 were prepared by hot pressing (25 MPa) under vacuum at temperatures higher than their glass transition temperatures (Tg) by 100−120 °C. The preparation of P6−P7 films was described in our previous paper.14 The thicknesses of the obtained films were in the range of 70−160 μm. Birefringence Measurements. The orientational birefringences (CR values) of the polymers were measured according to the method reported by Osaki and co-workers.17 The polymer films cut into a rectangular shape (1 × 8 cm) were uniaxially stretched with various weights in the range from 0.5 to 10 g, which correspond to the stresses of approximately 5 to 100 kPa, at the temperatures higher than their Tg values by 20 °C. The films were kept for 30 min at that temperature and then were cooled gradually over 1 h. The retardations of the films thus obtained were measured with a RETS-100 instrument (Otsuka Electronics) by a rotating analyzer method at 589 nm.18 The birefringences (i.e., the retardations/the film thicknesses) were plotted against the stretching stresses, and the CR values were determined from the slopes (eq 2). The photoelastic birefringences (CD values) were evaluated based on eq 3. The retardations of the polymer films cut into a rectangular shape (1 × 11 cm) were measured under the tensile load in the range from 0.5 to 14 N, which correspond to the stresses of approximately 0.5 to 20 MPa, at ambient temperature with the RETS-100 instrument. The birefringences (i.e., the retardations/the film thicknesses) were plotted against the tensile stresses, and the CD values were determined from the slopes (eq 3). In order to determine the signs (i.e., positive or negative) of the CR values of P2−P7, the retardations of the piled film samples composed of P2−P7 films and a P1 film, both of which were stretched in advance to the parallel direction, were evaluated. The signs of the CD values of P2−P7 were determined by use of the piled film samples composed of P2−P7 films, which are under tensile stress, and the P1 film, which was stretched in advance to the parallel direction of the tensile stress for P2−P7 films. Consequently, the retardations turned
Figure 1. Chemical structures of poly(phosphonate) derivatives.
We have recently described the synthesis of poly(thiophosphonate)s (X = S, Z = Ph) with various main chain structures (R), and as expected, they were found to exhibit high refractive indices of 1.626 to 1.687, depending on the main chain structures (R).14 Concerning the refractive indices of poly(phosphonate)s (X = O), they are higher than those of poly(carbonate)s, as described by McGrath and co-workers: the refractive indices of a bisphenol A-based poly(phosphonate) and the corresponding poly(carbonate) are 1.60 and 1.58, respectively.15 Shaver and co-workers have also synthesized poly(phosphonate)s with various structures, including a 9,9-bis(4-hydroxyphenyl)fluorene-based poly(phosphonate) with a refractive index of 1.66.16 The Abbe numbers of poly(phosphonate) derivatives (X = O, S) were found to be relatively high compared to those of the polymers with a similar range of refractive indices.14,16 In spite of these attractive optical features, the birefringent properties of the poly(phosphonate) derivatives (X = O, S) have scarcely been studied. In this paper, we wish to describe the low birefringent properties of the bisphenol A-based poly(phosphonate) derivatives (X = O, S). The orientational birefringences (CR values) and the photoelastic birefringences (CD values) were
Figure 2. Structures of the bisphenol A-based polymers (P1−P7). B
DOI: 10.1021/acs.macromol.8b00684 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Table 1. Thermal and Optical Properties of the Bisphenol A-Based Polymers (P1−P7) polymer
P1
P2
P3
P4
P5
P6
P7
Td5 (°C)a Tg (°C)a CR (10−9 Pa−1) CD (10−11 Pa−1)
471 150 +5.6 +8.9
494 191 +8.8 +12.6
514 189 +4.0 +8.4
517 170 +5.5 +7.9
452 152 +6.8 +8.3
484b 107b +1.5 +6.1
446b 120b +1.2 +5.7
a
Measured under nitrogen. bThese values have been reported in our previous paper.14
out to be equal to the sum of those of the polymer films and the P1 film, which indicated that the birefringences of all the polymers are positive as is the case for P1 having positive orientational birefringence.3 DFT Calculations. The density functional theory (DFT) calculations with the Becke-three-parameter-Lee−Yang−Parr hybrid (B3LYP) were performed using the Gaussian 09 program package.19 The 6-31G(d,p) basis set and the 6-311+G(2d,p) basis set were used for the geometry optimizations and the calculations of frequencydependent polarizabilities at a wavelength of 589.3 nm, respectively.20 The obtained polarizability tensors were transformed into diagonal matrices. Average polarizabilities (α) and polarizability anisotropies (Δα) were calculated as α = (α1 + α2 + α3)/3 and Δα = α|| − α⊥ = α1 − (α2 + α3)/2, respectively, where αi are the diagonal components of the polarizability tensor and subscripts (1−3) represent the coordinate axes (α1 corresponds to the polarizability toward the main chain direction).21
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RESULTS AND DISCUSSION Thermal Properties. The thermal properties of the bisphenol A-based polymers (P1−P7) were investigated by the thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) techniques (Table 1; for detailed profiles, see the Supporting Information). In the TGA measurements, the 5% weight loss temperatures (Td5) of P1−P7 were observed to be greater than 440 °C under nitrogen, which are sufficiently high to prepare polymer films by the hot pressing and to stretch the films above their Tg values for the CR measurements. The Tg values of poly(phosphonate) derivatives (107 and 120 °C for P6 and P7, respectively) are a little lower than those of P1−P5 (Tg = 150−191 °C).14 Temperatures higher than their Tg values by 20 °C were employed for the stretching conditions of the polymer films for the CR measurements. Birefringent Properties. The orientational birefringences (CR values) were evaluated based on eq 2. As shown in Figure 3, the birefringences (Δn) were found to be in proportion to the stretching stresses. The CR values of the polymers (P1− P7) were determined from the slopes of the solid lines (Table 1). The CR value of P1 is +5.6 × 10−9 Pa−1, which is consistent with the previously reported values.4,9,17,22−24 The CR values of P2−P5 are in the range from +4.0 × 10−9 to +8.8 × 10−9 Pa−1, and these values are in the typical range for amorphous aromatic polymers.4,25,26 It is of note that the CR values of poly(phosphonate) derivatives (P6 and P7) are +1.5 × 10−9 and +1.2 × 10−9 Pa−1, respectively, which are less than half of those of the other bisphenol A-containing polymers (P1−P5). The photoelastic birefringences (CD values) were likewise evaluated based on eq 3. As shown in Figure 4, the birefringences (Δn) were found to be in proportion to the tensile stresses. The CD values of the polymers (P1−P7) were determined from the slopes of the solid lines (Table 1). The CD value of P1 is +8.9 × 10−11 Pa−1, which is consistent with the previously reported values.4,17,22,27,28 The CD values of
Figure 3. Orientational birefringences of P1−P7. Closed circles represent measured values. Solid lines were fitted by the least-squares method.
Figure 4. Photoelastic birefringences of P1−P7. Closed circles represent measured values. Solid lines were fitted by the least-squares method.
P2−P5 are in the range from +7.9 × 10−11 to +12.6 × 10−11 Pa−1. On the other hand, the CD values of P6 and P7 are +6.1 × 10−11 and +5.7 × 10−11 Pa−1, respectively, which are lower than those of the other bisphenol A-containing polymers (P1− P5). From the correlationship between CD and CR values (Figure S5), the smaller CD values of poly(phosphonate) derivatives (P6 and P7) is due to their smaller CR values, which might also indicate that the CG and ER′(∞)/EG′(∞) values in eq 4 are not influenced significantly by the structures of the aromatic polymers examined here.4,25 C
DOI: 10.1021/acs.macromol.8b00684 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
Figure 5. Structures of the model compounds (M1−M7) for the DFT calculations.
Theoretical Discussion Based on the DFT Calculations. The CR values of polymers can be expressed with the polarizability anisotropies of their repeating units (Δα):2,29 CR =
2π (n2 + 2)2 ρNA PR 1 Δα 9 n M 0 ε E R ′(∞)
(5)
where n is the refractive index, ρ is the density of the material, NA is Avogadro’s number, M0 is the molecular weight per a repeating unit, PR is the orientation function, and ε is the strain. On the basis of the Lorentz−Lorenz equation, n2 − 1 4π ρNA = α 2 3 M0 n +2
(6)
the CR value can be expressed as CR =
Δα 1 (n2 + 2)(n2 − 1) PR 1 ε E R ′(∞) α 6 n
Figure 6. Correlationship between the Δαr and CR values.
(7)
In eq 7, PR/ε can be regarded as constant according to the affine or the quasi-affine deformation model.2 On assuming that ER′(∞) of the polymers (P1−P7) are almost identical as are the cases of P1−P3 [ER′(∞) = 26−27 MPa],4,25,26 the CR values would be in proportion to the reduced polarizability anisotropy (Δαr, that corresponds to six times the intrinsic birefringence) defined as follows: Δα r =
(n2 + 2)(n2 − 1) Δα n α
(8)
To evaluate the Δα values, the Δα/α values for compounds (M1−M7, Figure 5) as the model structures of the repeating units of the polymers (P1−P7) were estimated by DFT calculations (Table S2). The refractive indices of the polymers (P1−P7) at 589 nm (Table S1) were employed for n values. The linear relationship between the Δαr and CR values obtained in Figure 6 may also suggest that ER′(∞) are approximately constant within the samples examined in the present study, although the reason for the larger Δαr values and the deviation of the line from the origin remain unclear (see, footnote d in Table S2).26 In addition, since the Δαr values are in proportion to the Δα/α values within the series of the polymers having similar refractive indices (Figure S9), the theoretical calculations were performed to estimate their Δα/α values. As shown in Figure 7, the Δα/α values of M6 and M7 were smaller than that of M1 most probably due to the presence of the PX (X = O, S) groups and the phenyl substituents perpendicularly oriented to the main chain direction of the corresponding polymers. The phenyl substituents were found to be effective to reduce the r
Figure 7. Δα/α values of M1 and M6−M10.
Δα/α value by comparing the calculated results for the model compounds having methyl (M9 and M10) and phenyl substituents (M6 and M7). It was also found that the PS group is more effective than the PO group to reduce the Δα/α value.
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CONCLUSIONS We have investigated the birefringent properties of the bisphenol A-based poly(phosphonate) and poly(thiophosphonate). The orientational birefringences (CR values) of the poly(phosphonate) derivatives are in the range D
DOI: 10.1021/acs.macromol.8b00684 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules from +1.2 × 10−9 to +1.5 × 10−9 Pa−1, which are less than half of those of the conventional bisphenol A-based polymers. In addition, their photoelastic birefringences (CD values) are in the range from +5.7 × 10−11 to +6.1 × 10−11 Pa−1, which are also lower than those of the conventional bisphenol A-based polymers. From the DFT calculations of the polarizability anisotropies of the model compounds, the low birefringent properties of the poly(phosphonate) derivatives are most probably due to the presence of the PX (X = O, S) groups and the phenyl substituents perpendicularly oriented to the main chain direction. Further studies on the detailed birefringent properties of poly(phosphonate) derivatives are in progress.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b00684. Synthetic procedures and characterizations of polymers, optical and thermal properties of polymers, and DFT calculation results of model compounds (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Ikuyoshi Tomita: 0000-0003-3995-5528 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors appreciate Nobuyuki Miyaki and Takuma Ebata (JSR corporation) for valuable discussion and Sayuri Satoh (JSR corporation) for TGA and DSC measurements.
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REFERENCES
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