CRYSTAL COMPOSITION

FIELD OF THE INVENTION

The invention relates to a liquid crystal composition, its manufacturing method, and an optical filter incorporating said liquid crystal composition.

TECHNICAL BACKGROUND

Known optical filters are based on cholesteric liquid crystals technology. Cholesteric liquid crystals are mixtures comprising a liquid crystal host and a chiral dopant which, by a helical twisting power (HTP), provides a helical structure to the liquid crystal host. The helical structure is defined by its pitch and axis, and is able to reflect light by Bragg reflection. The wavelength that is reflected by the filter (also known as Bragg wavelength) depends on the helical pitch of the liquid crystals, and can thus be adjusted by tuning the pitch of the cholesteric liquid crystals.

A cholesteric liquid crystal filter device exhibits two states, respectively ON and OFF, which can be switched from one to the other on selective application of a voltage. When switching from ON state to OFF state (relaxation), cholesteric liquid crystals exhibit intermediate states which can cause undesired scattering.

In view of this, Dual Frequency Cholesteric Liquid Crystals or DFCLC are known which are responsive to voltages of two distinct frequencies. This type of cholesteric liquid crystal are of interest because they can be designed to avoid any scattering of light during transition between states, and can therefore be used in a wider range of applications.

For instance optical filter devices based on DFCLC or CLC can be used as therapeutic filters on ophthalmic lenses or in microscopy.

However the selective reflectance of Bragg tunable filters based on DFCLC can be affected by a number of external factors, the most critical of which being the variation of the cholesteric pitch with temperature. Indeed, a variation in temperature results in a variation of the helical pitch of the liquid crystals, which in turns results in a variation of the Bragg wavelength. This temperature dependency is undesired for a number of applications in which the optical filter may be used in a wide range of temperatures. The article by Shim et al., "Temperature-independent pitch invariance in cholesteric liquid crystals", in Optics Express 2014, vol. 22, n. 13, reports that pitch- temperature invariance can be achieved for cholesteric liquid crystals on a temperature range from 35°C to 60°C by mixing two chiral dopants having opposed behaviors with temperature. In particular, the mixture disclosed in this document comprises MLC-0643 (by Merck) as host nematic liquid crystal, mixed with chiral dopants being S81 1 and S501 1 (also by Merck), in the following proportions: MLC- 0643:S81 1 :S501 1 = 100:16.2:1 .7.

This result is promising. Yet, not any couple of chiral dopants added to a cholesteric liquid crystal allows obtaining temperature independence of the helical pitch. Indeed, it is known that the pitch values induced by chiral dopants can increase or decrease with temperature depending on the liquid crystal mixture itself and more specifically on its phase (smectic/nematic/isotropic), whether it is a single compound or made of a mixture of different compounds, and the chosen chiral dopant(s) (chemical structure, concentration), making the tendency difficult to predict.

Moreover, even when mixing dopants having opposite temperature behaviors, issues of solubility may arise preventing the resulting mixture from exhibiting temperature invariance. Hence it is not possible to foresee, from the temperature behaviors of chiral dopants considered alone, if their mixing will result in temperature invariance of any liquid crystal composition.

In particular, the couple of chiral dopants used in this article is not proved to provide the same temperature independence on a dual frequency cholesteric liquid crystal. Moreover, an increased range of temperature in which the helical pitch is constant in a DFCLC is desirable.

PRESENTATION OF THE INVENTION

In view of the above, it is an aim of the invention to provide a liquid crystal composition based on dual frequency cholesteric liquid crystal which Bragg wavelength is constant with temperature.

Another aim of the invention is to provide a liquid crystal composition which exhibits temperature independency regarding the Bragg wavelength on a wider range of temperatures than the composition known in the prior art. To this end, a liquid crystal composition is disclosed, comprising:

a dual frequency cholesteric liquid crystal, DFCLC, mixture, and a first chiral dopant and a second chiral dopant,

wherein the Bragg reflection wavelength of the composition is constant, within a tolerance range of 30 nm, within a temperature range comprised within 35 and

70°C.

In embodiments, the Bragg reflection wavelength of the composition is constant, within a tolerance range of 30 nm, within a temperature range comprised within 10°C and 85°C.

Preferably, a mass proportion of the first chiral dopant within the composition and a mass proportion of the second chiral dopant within the composition are such that variations of the Bragg wavelength with temperature of a mix of the DFCLC mixture with the first chiral dopant are compensated by variations of the Bragg wavelength with temperature of a mix of the DFCLC mixture with the second chiral dopant, within a tolerance range of 30 nm within the 35°C-70°C temperature range.

Advantageously, the mass proportion Ci of the first chiral dopant in the composition and the mass proportion C ₂ of the second chiral dopant are linked by the following equations

where:

D is a modulo factor smaller or equal to 1/3 nm/°C within the 35°C-70°C temperature range

Coi is the mass proportion of the first chiral dopant in a mix of said DFCLC mixture and said first chiral dopant to obtain a given pitch p of the liquid crystals at a predetermined temperature,

Co2 is the mass proportion of the second chiral dopant in a mix of said DFCLC and said second chiral dopant to obtain the same pitch p of the liquid crystals at a predetermined temperature, and Δ- _L and Δ ₂ are the respective variations of the Bragg wavelength with temperature of the mix of the DFCLC mixture and respectively said first chiral dopant or said second chiral dopant.

In a particular embodiment, the first chiral dopant is C03203 and the second chiral dopant is either R101 1 or R501 1.

R501 1 has the cas number 944537-61 -5.

In embodiments, the DFCLC mixture is selected among the group consisting of:

- MLC-2182,

- MLC-2177

- W-1978C, and

- W1952H.

In particular embodiments, the liquid crystal composition comprises the following components:

MLC-2182: MLC-2177: W-1978C: W1952H: C03203: R101 1 : R501 1 with the respective mass proportions of the components chosen within the list consisting of :

[(100:0:0:0:0.721 :7.65:0) ; (0:100:0:0:1 .22:0:2.2) ; (0:100:0:0:1 .99:5.96:0) ; (0:0:0:100:0.91 :0:2.26) ; (0:0:0:100:1.36:5.52:0) ; (0:0:100:0:2.3:0:1.13) ; (0:0:100:0:3.02:2.54:0)].

An optical filter is also disclosed, characterized in that it comprises a layer of a liquid crystal composition according to the preceding description. An ophthalmic device is also disclosed, comprising a said optical filter.

A process for manufacturing a composition is also disclosed, mixing:

a dual frequency cholesteric liquid crystal mixture, and

a first chiral dopant and a second chiral dopant,

wherein the first and second chiral dopants are selected such that the Bragg reflection wavelength of the composition is constant, within a tolerance range of 30 nm, within the temperature range comprised within 35 and 70°C.

In a preferred embodiment of the process, the first and second chiral dopants are selected such that the Bragg reflection wavelength of the composition is constant, within a tolerance range of 30 nm, within the temperature range comprised within 10 and 85°C.

Preferably, a mass proportion of the first chiral dopant within the composition and a mass proportion of the second chiral dopant within the composition are chosen during the process such that variations of the Bragg wavelength with temperature of a mix of the DFCLC mixture with the first chiral dopant are compensated by variations of the Bragg wavelength with temperature of a mix of the DFCLC mixture with the second chiral dopant, within a tolerance range of 30 nm within the 35°C-70°C temperature range.

Advantageously, the mass proportion Ci of the first chiral dopant in the composition and the mass proportion C ₂ of the second chiral dopant are linked by the following equations

where:

D is a modulo factor smaller or equal to 1 /3 nm/°C within the 35°C-70°C temperature range

Coi is the mass proportion of the first chiral dopant in a mix of said DFCLC mixture and said first chiral dopant to obtain a given pitch p of the liquid crystals at a predetermined temperature,

Co2 is the mass proportion of the second chiral dopant in a mix of said DFCLC mixture and said second chiral dopant to obtain the same pitch p of the liquid crystals at a predetermined temperature, and

Δ-L and Δ ₂ are the respective variations of the Bragg wavelength with temperature of the mix of the DFCLC mixture and respectively said first chiral dopant or said second chiral dopant.

embodiment, the process comprises the preliminary steps of:

determining the respective variations of the Bragg wavelength with temperature of a mixture of the DFCLC mixture and respectively the first chiral dopant and the second chiral dopant, and of determining the mass proportion of the first and second chiral dopant in a mixture of said DFCLC mixture and respectively said first or second chiral dopant to obtain a given pitch p of the liquid crystals at a predetermined temperature.

According to the invention, a liquid crystal composition exhibiting temperature invariance within a temperature range comprised between 35 and 70°C, and even between 10°C and 85°C is disclosed, based on a dual frequency cholesteric liquid crystal mixture.

For instance, the inventors have found out that the mixing of dopant C03203 with either R101 1 or R501 1 in proportions defined according to their temperature behaviors, provides temperature invariance to a dual frequency cholesteric liquid crystal host.

What's more, the inventors have found out that surprisingly the range of temperature invariance which has been obtained exceeds that known in the prior art for cholesteric liquid crystal. In some embodiments, temperature invariance can be achieved from 5°C to 90°C.

This broader range of temperature invariance thus opens the way to using DFCLC filter devices in new applications that are especially demanding on the stability of the reflected wavelengths.

DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention will be apparent from the following detailed description given by way of non-limiting example, with reference to the accompanying drawings, in which:

Figure 1 a represents Bragg wavelength peak position as a function of temperature for a green cell filled with MLC-2177 and R101 1 , Figure 1 b represents Bragg wavelength peak position as a function of temperature for a blue cell filled with MIC-2177 and R501 1 ,

- Figure 1 c represents Bragg wavelength peak position as a function of temperature for a green cell filled with MLC2177 and C03203, Figure 2a represents the peak reflectance variation with temperature of a DFCLC mixture composed of MLC-2177, R501 1 and C03203, Figure 2b represents the reflectance spectra of the same mixture as figure 1 a at different temperatures,

Figure 3 represents the peak reflectance variation with temperature of a DFCLC mixture composed of W-1952H, R501 1 and C03203, - Figure 4 represents the main steps of a process for manufacturing a composition according to an exemplary embodiment of the invention.

DETAILED DESCRIPTION OF AT LEAST ONE EMBODIMENT OF THE INVENTION

The present disclosure concerns a liquid crystal composition comprising a dual frequency cholesteric liquid crystal (or later named DFCLC) mixture, a first chiral dopant and a second chiral dopant.

It also concerns an optical filter comprising a liquid crystal cell comprising a first plate, a second plate, and the liquid crystal composition filled in between the plates. The plates can be glass substrates covered with an Indium Tin Oxide (ITO) layer, a polyimide layer rubbed in antiparallel condition for homogeneous alignment, and assembled with a specific uniform thickness and connected to electrodes for external driving. The cell gap can be of 3 m or 4 μηη, and is filled with the composition.

It also concerns an optical device comprising said optical filter.

In all that follows, a liquid crystal mixture is understood as a material comprising one or several types of liquid crystal molecules, whereas a liquid crystal composition is understood as comprising a liquid crystal mixture and at least one chiral dopant.

Commonly, a composition comprising a DFCLC mixture and a chiral dopant comprises liquid crystal molecules which, when no voltage is applied on the mixture, are arranged in helices having a helical pitch P. The helical pitch P depends on the chiral dopant which is added to the DFCLC mixture.

Due to this helical arrangement, a light beam impinging on a cell comprising this mixture is at least partially reflected by Bragg reflection, at a Bragg reflection wavelength λ _Β which is defined by:

λτ> =< n >. P Where <n> is the mean refractive index of the DFCLC mixture, computed based on n _e and n ₀ which are respectively the extraordinary and ordinary refractive indices of the mixture.

A cell comprising this mixture thus defines an optical filter which reflected bandwidth is centered on the Bragg wavelength.

The reflected spectral range of the filter is defined by Δλ=Ρ(η _β -η ₀ ). Thus the reflection bandwidth, as well as the Bragg wavelength, can be tuned by changing the pitch P, which in turn is done by adapting the nature and amount of chiral dopant.

However, each chiral dopant has a specific behavior with temperature, therefore the pitch of a mixture of a DFCLC mixture and a chiral dopant exhibits a

Bragg wavelength which varies according to the temperature.

In the liquid crystal composition according to the invention, the first and second chiral dopants, and their respective amounts, are selected in order to provide the composition with a temperature invariant behavior, over a temperature range which is comprised between 35 and 70°C, and more preferably comprised between 10°C and 85°C.

By "temperature invariant behavior", it is meant that the Bragg reflection wavelength of the composition is constant, within a tolerance range which width is under the full width at half maximum of the reflectance spectrum, and preferably equal 30 nm, over said temperature range.

In embodiments, it has been found out that the behavior with temperature of this composition is such that the wavelength corresponding to the reflectance peak oscillates around a mean wavelength which is constant over the considered temperature range, the amplitude of the oscillations being below 30 nm.

To this end, first and second chiral dopants are selected such that the temperature behavior of the first chiral dopant in the composition is compensated by the temperature behavior of the second chiral dopant in the composition.

In particular, the first and second chiral dopants are chosen such that the variations of the Bragg wavelength with temperature of a mix of the DFCLC mixture and respectively each chiral dopant are of opposite signs.

Moreover, the mass proportion of each chiral dopant in the composition is adapted so that the variation with temperature of the Bragg wavelength of the composition due to the first chiral dopant is compensated by the variation with temperature of the Bragg wavelength of the composition with the second chiral dopant.

With reference to figure 4, the determination of each chiral dopant and of its mass proportion is determined 100 as follows.

During a first step 100, the temperature behavior of at least two chiral dopants in a mix comprising only a DFCLC mixture and a respective chiral dopant is studied.

To perform such study, a mix comprising only a DFCLC mixture and one chiral dopant is prepared, and the behavior of the reflectance spectrum of the mix with temperature is studied in order to infer a linear trend of the Bragg wavelength variation with temperature. The Bragg wavelength is the peak wavelength of the reflectance spectrum.

An example is shown in figure 1 a, where the DFCLC mixture is the liquid crystal mixture marketed by Merck under the tradename "MLC2177", and the chiral dopant is 1 ,2-bis[4-(4-pentyl cyclohexyl) benzoate]-1 -(R)-phenyl ethane, known under the tra

It appears that the behavior of the chiral dopant R101 1 with temperature is to lower the pitch of the helices, or said otherwise the Bragg wavelength diminishes while temperature increases.

The linear trend of the Bragg wavelength with temperature can be evaluated with a slope of -1 .29 in this example.

This step is repeated for at least two chiral dopants for a same DFCLC mixture until two chiral dopants exhibiting slopes of opposite signs of the Bragg wavelength with temperature are identified.

As another example, figure 1 b shows the temperature behavior of a mix comprising the MLC2177 mixture and another chiral dopant which is marketed by Alphamicron Chemistry under the tradename C03203. Figure 1 b shows that the temperature behavior of C03203 with temperature is to increase the pitch of the helices as temperature increases. The linear trend of the Bragg wavelength with temperature can be evaluated with a slope of + 2 in this example.

In still another example, shown in figure 1 c, the mix which temperature behavior is studied comprises MLC2177 and a chiral dopant formed of (13bR)-5,6- Dihydro-5-(trans-4-propylcyclohexyl)-4H-dinaphtho[2,1 -f:1 ',2'-h][1 ,5]dioxonin, known under the tradename "R501 1 ", and which chemical formula is as follows:

This example shows that the chiral dopant R501 1 diminishes the pitch of the helices as the temperature decreases, with a slope of the Bragg wavelength variation with temperature that can be evaluated at -0.63.

In Table 1 are summed up the evaluated slopes of the evaluation of the Bragg wavelength with temperature of mixes of DFCLC mixtures with one chiral dopant, the chiral dopant being R-501 1 , C03203 or R101 1.

R-5011 CO3203 R-1011

Amount Evaluated Amount CD Evaluated Amount CD Evaluated CD (mg) slope P =f(T) (mg) slope P =f(T) (mg) slope P =f(T)

MLC-2177 2.9 -0.63 5.1 2 «9.8 -1.29

W-1978C 2.3 -0.91 4.50 0.89 7.7 -1.83

W-1952H 2.83 -0.88 4.53 3.57 8.0 -1.57

W-1831A 2.3 -0.29 4.71 -2 9.8 -1.42 During a second step 120, two chiral dopants are selected such that the Bragg wavelength variation with temperature of a mixture comprising a common DFCLC mixture and each chiral dopant are of opposite signs. For instance, among the examples given above, either the couple formed of R101 1 and C03203 or the couple formed of R501 1 and C03203 can be selected.

During a third step 130, the mass proportions of each dopant in a composition comprising said DFCLC mixture and both dopants are computed in order for one dopant to compensate the effect on the Bragg wavelength with temperature of the other dopant.

First, for each of the two dopants, an initial mass proportion C ₀ of the dopant in a mix of said dopant and said DFCLC mixture is determined to obtain a given pitch po of the liquid crystals at a predetermined temperature. The pitch p ₀ is preferably chosen to correspond to a Bragg wavelength in the range of visible wavelengths, and is equal for the two dopants.

The initial mass proportion C ₀ is determined according to the following equation:

1

Po =

C ₀ * HTP

Where HTP is the Helical Twisting Power of the chiral dopant.

The initial mass proportion is noted C ₀ i for the first chiral dopant and C ₀ 2 for the second chiral dopant.

Then, the mass proportions Ci and C ₂ of respectively the first and second dopants in the composition comprising the DFCLC mixture and both chiral dopants are selected to fulfill the following equations:

and,

where:

D is a modulo factor, which is smaller or equal to 1/3 nm/°C within the 35-70°C temperature range, and preferably smaller or equal to 1/3 nm/°C within the 10°C-85°C temperature range, and Δ- _L and Δ ₂ are the respective variations of the Bragg wavelength with temperature of the mixture of DFCLC and respectively said first chiral dopant or said second chiral dopant.

The D factor ensures that the behavior of the first dopant compensates that of the second dopant within a tolerance margin of 1/3 nm/°C within the considered temperature range.

Once the mass proportions of the chiral dopants are determined, a composition, comprising the DFCLC mixture and the chiral dopants according to these mass proportions, is prepared during a step 200 by mixing said mixture with said chiral dopants.

The reflection peak of the mixture can be tuned by adjusting the proportion of DFCLC mixture, while maintaining the relative mass proportions of the chiral dopants according to the above equations.

The behavior with temperature of the Bragg wavelength of a composition comprising said DFCLC mixture and both dopants can be studied during a stem 300 to check that the mixture is temperature invariant.

In the composition according to the invention, the DFCLC mixture is preferably chosen among the list consisting of:

- MLC-2182,

- MLC-2177

- W-1978C, and

- W1952H.

Moreover, the couple of chiral dopants is preferably either C03203 and R501 1 , or C03203 and R101 1 .

In particular, the inventors have found out that the composition comprising MLC-2177 as DFCLC mixture and R501 1 and C03203 as chiral dopants exhibits invariant Bragg wavelength with temperature. Figure 2a represents the peak reflectance variation with temperature of this composition, and Figure 2b represents the reflectance spectra of the same mixture as figure 2a at different temperatures.

The respective mass proportions of the chiral dopants are as follows:

R501 1 :C03203 = 2.2:1 .22 In the example of Figure 2a, the mass proportions of the components of the composition are as follows:

MLC2177:R501 1 :C03203 = 103.4:2.2:1 .22

According to this example, the Bragg wavelength remains comprised between 500 and 530nm over temperatures ranging between 0 and 90°C.

It can also be noted that the Bragg wavelength of the composition oscillates around an average wavelength which is constant, and equal to about 515 nm. The amplitude of the oscillations is below 30 nm. With reference to figure 3, the behavior with temperature of another composition is shown, said composition comprising W1952H as DFCLC mixture and R501 1 and C03203 as chiral dopants.

The respective mass proportions of the chiral dopants are as follows:

R501 1 :C03203=2.26:0.91

In the example of Figure 3, the mass proportions of the components of the composition are as follows:

W1952H:R501 1 :C03203=103.17:2.26:0.91

According to this example, the Bragg wavelength remains comprised between 525 nm and 508 nm over temperatures ranging between 35 and 70°C, and between 540 nm and 508 nm over temperatures ranging between 15 and 70°C.

Other compositions can be defined with the following mass proportions of the components:

MLC2182:C03203:R101 1 = 100:0.721 :7.65,

MLC2177:C03203:R101 1 = 100:1.99:5.96,

W1952H:C03203:R101 1 = 100:1.36:5.52,

W1978C:C03203:R501 1 = 100:2.3:1.13,

W1978C:C03203:R101 1 = 100:3.02:2.54.