Eun Hee
Min
*a,
Kok Hou
Wong
a,
Eki
Setijadi
a,
François
Ladouceur
a,
Mark
Straton
b and
Alexander
Argyros
*b
aPhotonics Group, School of Electrical Engineering and Telecommunications, The University of New South Wales, Sydney, NSW 2052, Australia. E-mail: eunhee.min@unsw.edu.au; Fax: +61 2 93855388; Tel: +61 2 93854892
bInstitute of Photonics and Optical Science, School of Physics A28, The University of Sydney, NSW 2006, Australia. E-mail: alexander.argyros@sydney.edu.au; Fax: +61 2 93517726; Tel: +61 2 91140872
First published on 5th July 2011
This study investigates the fabrication of menthol-based copolymers for use in polymer optical fibres (POF). A chiral monomer, (−)-menthyl methacrylate ((−)-MnMA) derived from (−)-menthol, was synthesized that displays a specific rotation of [α]20D (neat) = −90. It was further copolymerized with methyl methacrylate (MMA) either in bulk or in solution viafree radical polymerization. PMMA-co-P(−)-MnMA copolymers synthesized in bulk yielded turbid polymers, whereas copolymer synthesized in solution with 50 mol% (−)-MnMA feed was transparent with some optical rotation, and compatible for drawing into fibres. Hence, PMMA-co-P(−)-MnMA in the current study is a potential chiral material for use in POF.
Circular birefringence in optical waveguides can be introduced through a break in the longitudinal symmetry of the waveguides, for example by spinning the optical fibres to produce a helical structure. This is, however, a difficult and complex technique; therefore, circular birefringence in optical fibres has not been well studied to the present.2 Another technique to introduce circular birefringence in optical fibres is via the use of chiral materials.3 The majority of chiral materials are organic and incompatible with the processing temperatures of glass optical fibres (i.e. 1800 °C). In contrast, POF, which is generally processed at 180 °C, offers unique opportunity and potential to study the effects of circular birefringence, using chiral materials in optical fibres.
To the best of our knowledge, besides theoretical studies,4–8 there is only one experimental demonstration of chiral materials incorporated into optical fibres, where a hollow core fibre was filled with a solution of chiral molecules, liquid (−)-fructose.9 Nevertheless, besides evidence of optical rotation in this fibre, the use of (−)-fructose solution is expected to be impractical as liquids in fibres are not robust and can result in leakages. Therefore, we considered using chiral polymers to generate circular birefringence in POF.10 Herein, we develop a solid and robust chiral polymer for the fabrication of optically active POF, which offers an alternative approach to the investigation of circular birefringence in optical fibres.
There are three avenues for introducing chirality into polymers. The first approach is doping PMMA, the most common POF polymeric material, with chiral compounds by swelling the polymer with solvent to allow infusion of chiral compounds (dopants)11 into the polymer. In this case, only a low loading of dopants would be expected resulting in only small optical rotation being imparted to the PMMA. The presence of residual solvent, which evaporates at elevated temperatures, may also be problematic during fibre fabrication.
The second approach is to identify a chiral monomer and homo-polymerize to produce unadulterated chiral homopolymers for use in POF (i.e. as above but without PMMA). This approach is likely to produce chiral polymers with the highest optical activities, but their suitabilities for fibre fabrication and use in POF are of concern and challenging.
The third approach is to covalently bind chiral compounds to MMA monomer viafree radical polymerization to produce PMMA-co-chiral polymers. In contrast to doping, it is expected to produce copolymers containing higher concentrations of chiral compounds with elevated optical activities. This approach also retains some of the PMMA properties that are favorable for POF fabrication.
In this study, the second and third approaches were investigated. Menthol was selected from various chiral compounds, as menthol is readily available as both left-handed (−)-menthol and right-handed (+)-menthol enantiomers, which display optical rotations of c = [α]20D = −50 in 10% ethanol, and c = [α]23D = +48 in 10% ethanol, respectively. Choosing a material for which both enantiomers are readily available means both (−) and (+) forms of the resulting chiral polymer can be realised. Compared to chiral materials with one enantiomer, they can potentially maximise optical properties (optical rotation) when incorporated into optical fibres.8,9 Our study here focuses on the use of (−)-menthol, only as a testimony of concept and potential. Our approach is shown in Scheme 1, the (−)-menthol is firstly functionalized with a methacrylate group to produce a chiral monomer, (−)-menthyl methacrylate ((−)-MnMA), and subsequently co-polymerized with MMA to produce PMMA-co-P(−)-MnMA polymers, and also homo-polymerized to produce P(−)-MnMAviafree radical polymerization. The properties of the resulting polymers and their suitability for use in optical fibres were assessed.
![]() | ||
Scheme 1 Functionalization of (−)-menthol to produce (−)-MnMA, co-polymerization of MMA and (−)-MnMA, and homo-polymerization of (−)-MnMA. |
However, this co-polymerization in solution approach produced undesirable air bubbles in the resulting copolymers unlike the ones copolymerized in bulk. The presence of air bubbles is unacceptable in terms of optical fibre fabrication. Several approaches were employed to remove the air bubbles from the polymers synthesized. Firstly, co-polymerization was conducted under vacuum to aid the escape of air bubbles from the reaction mixture solutions. Fewer air bubbles were observed in the resulting copolymers however, complete elimination of bubbles was not possible. Another approach was to increase the reaction temperature very slowly (i.e. 5 °C per hour), from room temperature to the desired co-polymerization temperature of 60 °C. The initiator in the reaction solution will decompose gradually, leading to a more steady propagation of monomers with minimum gases given off (air bubbles) compared to a sudden thermal shock co-polymerization. At the same time, inevitable air bubbles generated will have enough time to escape from the reaction mixture as the viscosity increases slowly during the process of co-polymerization. As a result, gradually increasing the reaction temperature was the most efficient way to reduce air bubbles in the current study.
In fact, when high vacuum was applied simultaneously with the slow increase in reaction temperature approach during co-polymerization of MMA and (−)-MnMA in solution, transparent and bubble free PMMA-co-P(−)-MnMA copolymers were obtained even up to (MMA/(−)-MnMA, 50/50, mol%/mol%).
![]() | ||
Fig. 1 Optical rotation of (−)-menthol, and (−)-MnMA as (a) a function of wavelength and (b) a function of (wavelength)−2. |
The rotation of the plane of linear polarization is most commonly described through the optical rotation, α (in degrees per decimetre), however other methods of expressing it become more intuitive in the context of optics and optical fibres.
From Fig. 1, it can be seen that α depends on the wavelength λ, with an α ∝ λ−2 dependence. The effect of chirality on polarization can also be expressed in terms of the chirality parameter γ, which has units of metres and is independent of wavelength. This chirality parameter can be extracted from the slope of the graph in Fig. 1(b), and is related to α through,
![]() | (1) |
![]() | (2) |
A third quantity of interest is the length of propagation required to achieve a significant change in the polarization of light. This ‘characteristic length’, which describes the strength of the optical rotation, is the beat length LB, which is defined as the length of propagation required to bring the light back to its original polarization, i.e. a rotation of 180°. This depends on the other parameters as
![]() | (3) |
Hence, the measurement of α and γ in Fig. 2 allows these parameters to be determined. The value of these parameters for the resulting polymers can be used to assess the feasibility of their use in optical fibres.
![]() | ||
Fig. 2 Optical rotation measurement of (−)-MnMA and the chiral co-polymers at a wavelength of 589 nm (D line) at room temperature. (A) (−)-MnMA (neat); (B) the copolymer (10 mol% (−)-MnMA in feed); (C) the copolymer (30 mol% (−)-MnMA in feed); (D) the copolymer (50 mol% (−)-MnMA in feed). |
Measurements of the optical rotation of different samples containing different (−)-MnMA feeds indicated that the optical rotation simply scaled linearly with the (−)-MnMA content. Extrapolating this to 100% feed (i.e.P(−)-MnMA only) reproduced the optical rotation of the (−)-MnMA monomer. Thus it was concluded that the optical rotation is not appreciably changed by the polymerization. It should be noted that the optical rotation of these copolymers was measured using solid samples of polymer, therefore it is not possible to compare directly with values of optical rotation in the literature, which were generally derived from measurements using solutions. Furthermore, optical rotation depends on the temperature, wavelength, concentration, and solvent used.
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Fig. 3 Copolymer vs. feed composition of MMA and (−)-MnMA. M1, mole fraction of (−)-MnMA in feed; m1, mole fraction of (−)-MnMA in copolymer. |
The co-polymerization reactivity ratios were calculated using the F–R method, which is governed by the fundamental equation:
![]() | (4) |
Entry | x = M1/M2 | y = m1/m2 | G = x(y − 1)/y | F = x2/y | M 1((−)-MnMA) | M 2(MMA) | m 1((−)-MnMA) | m 2(MMA) |
---|---|---|---|---|---|---|---|---|
1 | 0.11 | 0.05 | −1.98 | 0.23 | 0.1 | 0.9 | 0.05 | 0.94 |
2 | 0.43 | 0.16 | −2.20 | 1.13 | 0.3 | 0.7 | 0.14 | 0.86 |
3 | 1.00 | 0.43 | −1.33 | 2.33 | 0.5 | 0.5 | 0.3 | 0.7 |
4 | 2.33 | 0.92 | −0.19 | 5.90 | 0.7 | 0.3 | 0.48 | 0.52 |
5 | 9.00 | 3.55 | 6.46 | 22.85 | 0.9 | 0.1 | 0.78 | 0.22 |
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Fig. 4 F–R plot for co-polymerization of (−)-MnMA and MMA in solution. |
P(−)-MnMA is a thoroughly white brittle polymer as seen in the ESI†. Therefore, the cloudiness of the copolymer is attributed to the P(−)-MnMA block, which is influenced by the feed of (−)-MnMA.
Table 2 lists molecular weights and PDI of P(−)-MnMA, and PMMA-co-P(−)-MnMA regarding initial monomer feed in solution.
Entry | (−)-MnMA in feed (mol%) | MMA in feed (mol%) | M n/g mol−1 | M w/g mol−1 | M n/Mw (PDI) |
---|---|---|---|---|---|
M1 | 0 | 100 | 21![]() |
46![]() |
2.1 |
M2 | 5 | 95 | 21![]() |
47![]() |
2.2 |
M3 | 10 | 90 | 26![]() |
52![]() |
2.1 |
M4 | 30 | 70 | 41![]() |
218![]() |
5.3 |
M5 | 50 | 50 | 87![]() |
404![]() |
4.6 |
M6 | 100 | 0 | 124![]() |
337![]() |
3.1 |
As shown in Table 2, the homo-polymerizations of MMA (M1) and (−)-MnMA (M6) have a PDI of approximately 2, while an upsurge of PDI was noticed with increasing concentrations of (−)-MnMA in the copolymers. Upon further investigation, the copolymer GPC spectra in Fig. 5 exhibited multi-modal distributions, beginning with the copolymer with 10 mol% of (−)-MnMA. The magnitude of the peaks also increased and intensified as the percentage of (−)-MnMA increased in the copolymers.
![]() | ||
Fig. 5 GPC spectra of PMMA-co-P(−)-MnMA. M1, PMMA; M2, PMMA-co-P(−)-MnMA (5 mol% (−)-MnMA); M3, PMMA-co-P(−)-MnMA (10 mol% (−)-MnMA); M4, PMMA-co-P(−)-MnMA (30 mol% (−)-MnMA); M5, PMMA-co-P(−)-MnMA (50 mol% (−)-MnMA); M6, P(−)-MnMA. |
A multi-modal distributed GPC spectrum of polymers is indicative either of several populations of PMMA-co-P(−)-MnMA copolymers or an assortment of PMMA, P(−)-MnMA homo-polymers, and PMMA-co-P(−)-MnMA copolymers in the samples or even due to no purification step after polymerization. It is believed that the latter is prevalent in this study because bulk homo-polymerization of (−)-MnMA resulted in opaque polymers and bulk copolymerization of MMA and (−)-MnMA with 30 or more mol% of (−)-MnMA, resulted in less opaque polymers. It is suggested that the systems in this study are preferential to homo-polymerization of similar monomers instead of the desired co-polymerization of various monomers. This can be assigned to a significant difference in reactivity ratios of the various types of monomers in the systems as discussed above.
Table 3 is a summary of Tg values of the synthesized PMMA-co-PMnMA copolymers in this study, determined using DSC. (−)-Menthol does not have a Tg but three melting points were prominent from its compositions, isomer (∼33 °C), racemic (∼38 °C) and form- (∼44 °C). After functionalizing (−)-MnMA with a methacrylate group into (−)-MnMA and polymerization into P(−)-MnMA (M6), the (−)-menthol-based polymer exhibited an average Tg of approximately 83 °C. This temperature is lower than that of PMMA (M1), commonly used for POF. The Tg values of PMMA-co-P(−)-MnMA copolymers synthesized were determined to be among these temperatures as anticipated. In fact, reductions in Tg values were evident in copolymers with elevating concentrations of P(−)-MnMA such as copolymers M5 (∼95 °C), M4 (∼96 °C), M3 (∼100 °C) and M2 (∼104 °C). The copolymer with 5 mol% of (−)-MnMA has Tg values adjacent to that of PMMA (M1, ∼112.5 °C). These results suggest that current drawing temperatures (about 170–190 °C) for drawing PMMA POF can be applied with minimum alterations to draw chiral-based fibres from copolymers.
Entry | (−)-MnMA in feed (mol%) | MMA in feed (mol%) | T g/°C |
---|---|---|---|
M1 | 0 | 100 | 110–115 |
M2 | 5 | 95 | 102–106 |
M3 | 10 | 90 | 99–101 |
M4 | 30 | 70 | 95–97 |
M5 | 50 | 50 | 93–97 |
M6 | 100 | 0 | 81–85 |
Taking into account the value of the chiral parameter measured above, the amount of circular birefringence will be given by
![]() | (5) |
![]() | (6) |
An estimate of beat length for a fibre with a large 50 mol% (−)-MnMA copolymer core ranges from 18 to 64 cm for wavelengths across the visible. Hence, the strength of the optical rotation is sufficient for the polarization of light to be manipulated over lengths of 10s of cm, meaning the use of this polymer in optical fibres is practical and feasible in this sense.
To produce optical fibres however, there are additional criteria that the polymer must satisfy as discussed above. It must be able to be drawn into the fibre upon heating, and must not contain any volatile impurities (e.g.solvents) that will evaporate and cause bubbling, etc. To test these criteria, a 4 mm diameter × 2 cm length sample of PMMA-co-P(−)-MnMA with a 50 mol% (−)-MnMA feed was heated at 90 °C, under vacuum for several days, to ensure that any remaining solvent had been evaporated. This was placed inside a PMMA tube with an inner diameter of 6 mm and an outer diameter of 12 mm. Vacuum was applied to the PMMA tube to remove air, and the tube was heated and drawn to a fibre of diameter <1 mm. The fibre was cleaved and inspected to show no evidence of bubbling and good adhesion between the two polymers, as expected given the PMMA content of the copolymer (Fig. 6).
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Fig. 6 Optical microscope image of a preliminary chiral optical fibre consisting of PMMA-co-P(−)-MnMA core and pure PMMA cladding. |
Although this produced a fibre with a PMMA-co-P(−)-MnMA core and PMMA cladding, the difference in reflective index between the two polymers (i.e. core and cladding) was insufficient to produce strong guidance of light. Nevertheless, this result proves that the chiral polymer developed is suitable for use in POF and compatible with established PMMA–POF approaches.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c1py00056j |
This journal is © The Royal Society of Chemistry 2011 |