TUNABLE MICROLENSES

Cross-reference to related application

This application claims priority to European patent application No. 18180049.1 , filed on 27 June 2018, the whole content of this application being incorporated herein by reference for all purposes.

Technical Field

The present invention relates to the field of micro-optics. More specifically the invention relates to tunable, fluid-filled membrane microlenses.

Background Art

In recent years, micro-optical components have been developed that have opened the way to a number of important applications. Crucial for most of these applications is the tunability of the micro-optical components, that means a controlled variation in their optical characteristics. The more precise and reproducible is the tunability, the higher is the functionality of the micro-optic components.

Amongst the tunable micro-optics, one of the most promising technology is that of tunable microlenses, available as individual lenses or arranged in arrays and used in many applications where a very precise and efficient focus of the light is required, or other peculiar optical features such as in the 3D imaging systems.

Tunable microlenses are generally manufactured by using a distensible elastomer membrane leaning over a sealed, micromachined cavity filled with a liquid or fluid component. The highly elastic membranes of poly(dimethylsiloxane) (PDMS) are widely used in these microlenses. These membranes define the surface shape of the lens, while the filled-in liquid does constitute the lens’ body and allows the optical properties of the lens, such as its focal length, to be tuned by variation in the pressure of the liquid through microfluidic channels, or by variation of other physical parameters in the liquid.

It is known that in tunable microlenses the filled-in liquid or fluid must have a refractive index as close as possible to that of the membrane’s material, in order to reduce scattering of light at their interface. The liquid or fluid material has to be compatible with the membrane’s material too, and have for instance a low permeability through the membrane. For instance, it is known that silicone oils cannot be used to fill a microlens with a PDMS membrane because they tend to leak from the surface of the lens through the membrane (see Friese C. et al., Transactions on electrical and electronic engineering, IEEJ Trans 2007; 2: 232-248).

EP 2465816 (Samsung Electronics Co., Ltd) discloses a varifocal lens structure including a frame having a fluid chamber filled with an optical fluid and formed of polymethylsiloxane (PDMS) containing a predetermined fluid; a transparent cover disposed on a top surface of the frame so as to cover the fluid chamber; a transparent elastic membrane disposed on a bottom surface of the frame so as to form a lower wall of the fluid chamber; and an actuator disposed on the elastic membrane.

US 7,256,943 (Teledyne Licensing, LLC) discloses a variable focus liquid-filled lens or microlens array, which is formed with an elastomer membrane and filled with a polyphenyl ether (PPE) liquid.

US 2003/181633 discloses an energy curable composition comprising a compound having an aromatic or heteroaromatic moiety, at least two fluorinated alkylenen, arylene or polyether moieties, at least one ethylenically unsaturated moiety.

EP 1057849 (Ausimont S.p.A.) discloses the use of a composition for preparing by radical route polymeric films having refractive index lower than 1.400.

(Per)fluoropolyethers (also indicated in the following by the acronym PFPEs) are fluids with high transparency in the visible and UV regions of the light spectrum, also having an extremely high compatibility with all materials. On the other hand, PFPEs have a low refractive index, which is lower than that of PDMS or of other possible materials used as elastic membrane in microlenses.

Their use as optical fluid in microlenses would be therefore not practical.

A high refraction index for an optical fluid of a microlens is also a desirable property in order to maximize the tuning range of the focal length of a microlens for a given ability to vary the lens radius of curvature. This also reduces the amount of spherical aberration for a given focal length.

Based on these grounds, in many optical applications, such as that of tunable microlenses, a higher refractive index than that of PFPEs is desirable, so that PFPEs, even if useful in many other optical applications and in photolithographic processes, are not a valuable alternative as fluids for tunable microlenses. In view of the range of different applications of the tunable microlenses and of the industrial potential of some of them, there is a strong-felt need in the art for having available improved fluids for filling the microlenses, having the above said properties of transparency, high compatibility with the elastic materials commonly used for the membranes, also having a high refractive index.

Summary of the Invention

The Applicant has now surprisingly found that by properly functionalizing the PFPEs with aromatic moieties as detailed below, the refractive index of PFPEs is increased, while maintaining a good compatibility with PDMS and other similar materials used as membranes for microlenses, also maintaining other beneficial optical properties of the PFPEs such as high transparency.

A subject of the present invention is therefore a microlens comprising an elastomer membrane filled with a (per)fluoropolyether polymer [polymer (PFPE)] comprising a (per)fluoropolyether backbone chain [chain (R _f )], optionally comprising pendant groups, and comprising two chain ends, wherein at least one aromatic group [group (Ar ^F )] is comprised in the polymer (PFPE) in at least one chain end and/or in pendant groups of chain (R _f ).

A further subject of the invention is a process for the manufacture of the microlens as defined above.

In a still further aspect, the invention relates to a microlenses array comprising a plurality of microlenses as defined above arranged on a supporting substrate.

Detailed Description of the Invention

The term“tunable” as used herein with reference to microlenses means that they are able to be adjusted in their focal length by variation of one or more parameters.

In the present invention, by the acronym“PFPE” is meant“(per)fluoropolyether", i.e. fully or partially fluorinated polyether, i.e. wherein all or only a part of the hydrogen atoms of the hydrocarbon structure have been replaced by fluorine atoms so that a higher proportion of fluorine atoms than hydrogen atoms is contained in the structure. When this acronym is used as substantive in the plural form, it is referred to as “PFPEs”.

The term“perfluorinated” denotes herein a fully fluorinated straight or branched alkyl group.

The use of parentheses before and after names of compounds, symbols or numbers identifying formulae or parts of formulae like, for example“group (Ar ^F )”,“chain (R _f )”, etc..., has the mere purpose of better distinguishing those names, symbols or numbers from the rest of the text; thus, said parentheses could also be omitted.

The elastomer membrane component of the present microlenses can be selected from among silicon elastomer membranes, and preferably is a polydimethylsiloxane (PDMS) membrane. Other elastomer components having good compatibility with the polymer PFPE of this invention can be selected by any person of ordinary skills in the art.

The choice of the aromatic group [group (Ar ^F )], comprised in the polymer (PFPE), is not particularly limited, provided that this group is aromatic.

For the avoidance of doubt, the term "aromatic group" is hereby intended to denote a cyclic substituent having a delocalized conjugated p system with a number of p delocalized electrons fulfilling the Hiickel's rule (number of p electrons equal to

(4n+2), with n being an integer).

The group (Ar ^F ) can be monocyclic or polycyclic. It can comprise one or more than one aromatic ring. Should it comprise more than one aromatic ring, these aromatic rings can be condensed or not condensed. The group (Ar ^F ) can be a heteroaromatic compound, comprising one or more heteroatoms (e.g. O, S, N) in the ring. It can be substituted or not substituted. It may be fully or partially fluorinated or fully hydrogenated.

In one embodiment of the invention, the group (Ar ^F ) is perfluorinated, that is to say, all its free valences are saturated with fluorine atoms. Non-limitative examples of group (Ar ^F ) which are suitable to the purposes of the invention are notably perfluorobenzene group, perfluorobiphenyl groups, perfluoronaphthalene groups, perfluoroanthracene groups, perfluoropyridine groups, perfluorotoluene groups and derivatives thereof comprising one or more perfluorinated substituent(s). In a particular embodiment of this invention, the group (Ar ^F ) is a perfluorobenzene group.

According to a particular embodiment of this invention, group (Ar ^F ) is an aromatic group substituted by one or more substituents comprising an aromatic moiety. The group (Ar ^F ), either when it is comprised in at least one chain end or in a pendant group of said chain (R _f ), is connected to the (per)fluoropolyether chain [chain (R _f )] by any suitable linking groups.

The (per)fluoropolyether chain [chain (R _f )] of polymer (PFPE) is preferably a chain comprising a plurality of recurring units (Ri), said recurring units having general formula: -(CF ₂ ) _k -CFZ-0-, wherein k is an integer of from 0 to 3; Z is selected from a fluorine atom, a C1-C6 peril uoro(oxy)alkyl group or a group W-(Ar ^F ), wherein W is a divalent linking group that provides a link between the recurring unit (Ri) and the group (Ar ^F ).

The divalent linking group W may comprise a (per)fluoroalkylene chain, an ether group, a thioether group, an amide group, a carbonyl group, a carboxylic group, a phosphate group or combinations thereof.

In one embodiment of the invention, the chain (R _f ) of the polymer (PFPE) complies with formula:

-(CF ₂ CFZO)a (CFZO)b-,

wherein the recurring units are statistically distributed along the chain (R _f ), wherein:

- Z is as above defined;

- a’ and b’ are integers > 0.

In a preferred embodiment of the invention, Z is a fluorine atom.

The chain (R _f ) is preferably selected so as to possess a number averaged molecular weight ranging from 400 to 2,000 Daltons.

The chain ends of polymer (PFPE), equal to or different from each other, are selected from:

- a Ci-C ₆ peril uoroalkyl group, optionally substituted with at least one halogen atom, -CFZ H2OH, -CFZ ^* COOR _h and -CFZ ^* -CH ₂ (OCH ₂ CH ₂ ) _p -OH, wherein p is an integer ranging from 0 to 10; Z ^* is F or CF3; R _h is a hydrocarbon chain; and

- a group -Y-(Ar ^F ), wherein (Ar ^F ) is as above defined; Y is a divalent linking group suitable to link (Ar ^F ) with chain (R _f ) to provide a chain end.

The divalent linking group Y may comprise a (per)fluoroalkylene chain, a a polyalkylene oxide chain, a carboxylic group an amide group, an alkylene group (Ak), a ether group, a thioether group or combinations thereof. Suitable divalent linking groups Y are, notably, groups of formula: -CFZ FhO-, - CFZ H2OCH2CH2O-, -CFZ ^* CH ₂ 0C(0)0CH ₂ -, -CFZ ^* C(0)NH- and CFZ ^* CH ₂ 0C(0)NH-, wherein Z ^* is as above defined.

Preferably, the linking group Y is a group of formula -CFZ ^* -CH ₂ (0CH ₂ CH ₂ ) _k -0-, wherein k’ is an integer ranging from 0 to 10 and Z ^* is F or CF3.

Generally, the present polymer (PFPE) complies with the following formula:

T-O-R _f -T

wherein:

R _f is a chain (R _f ), as above detailed;

each of T and T, equal to or different from each other, are chain ends as above defined,

wherein at least one among R _f , T and T’ bears a group (Ar ^F ).

In a preferred embodiment of the invention T and T’, equal to each other, are -Y- (Ar ^F ) groups.

The polymer (PFPE) of this invention preferably complies with the following formula:

T-(CF ₂ CF ₂ 0) _a' (CF ₂ 0) _b' -T’,

wherein:

- a’ and b’ are as above defined

- each of T and T’, equal to or different from each other, are selected from:

- a C1-C6 peril uoroalkyl group, optionally substituted with at least one halogen atom, -CFZ H2OH, -CFZ ^* COOR _h and -CFZ ^* -CH2(OCH ₂ CH ₂ )p-OH, wherein p is an integer ranging from 0 to 10; Z ^* is F or CF3; R _h is a hydrocarbon chain; and

- a group -Y-(Ar ^F ), wherein (Ar ^F ) and Y are as above defined.

The polymer (PFPE) of this invention more preferably complies with the following formula:

T-(CF ₂ CF ₂ 0) _a' (CF ₂ 0) _b' -T’,

wherein:

- a’ and b’ are as above defined

- T and T’, equal to each other, are a group -Y-(Ar ^F ), wherein (Ar ^F ) is as above defined; Y is as above defined. The Applicant has found that, in the polymer (PFPE) of this invention, the number of aromatic groups (Ar ^F ) as well as the length of the (per)fluoropolyether chain (R _f ) influence the refractive index of the fluid, thus allowing tuning the refractive index parameter of the polymer according to specific needs of the microlenses applications by simply varying the number of aromatic moieties and the length of the PFPE chain. As shown in the following experimental part, the Applicant has found that it is possible to modulate the PFPEs with aromatic moieties of the invention by increasing this parameter from the 1.30 value of the non-derivatized PFPEs to 1.40 and above.

At the same time, the present PFPEs with pendant and/or end aromatic groups show very good compatibility with elastomer membranes, such as PDMS membranes, commonly used for manufacturing microlenses. In particular, it was proved by the Applicant that no significant swelling occurs when the membrane contacts the fluid, or it is even immerged into it.

The microlenses of the present invention can be manufactured with the components as defined above, the elastomer membrane and the PFPE with aromatic moieties, by means of one of the known microfabrication technologies for this kind of optical components. In an aspect, the present microlenses can be manufactured by using the elastomer membranes suspended over a sealed, micromachined microfluidic cavity, which is then filled with the PFPE fluid as defined above as the optical fluid. Once enclosed in the elastomer inner chamber formed by the membrane, the PFPE fluid will act as the lens body, while the membrane will provide the shape and the curvature to the lens.

In a particular aspect of this invention, the membrane of the present microlens may be structured on a silicon fluidic chip, mounted on a glass substrate, thus forming a fluidic cavity where the polymer PFPE is collected. A tuning of the so-formed microlens and a variation of the lens’ curvature and thus of its focal length can be for instance achieved by varying the pressure of the liquid using the microfluidic channel, or by other known systems.

According to a particular embodiment of this invention, the present tunable microlenses filled with PFPEs having aromatic moieties can be used as individual lenses or they can be arranged in arrays. Should the disclosure of any patents, patent applications and publications which are incorporated herein by reference conflict with the description of the present application to the extent that it may render a term unclear, the present description shall take precedence.

The invention is described in greater detail in the following experimental section by means of non-limiting examples.

EXPERIMENTAL SECTION

Materials and methods

Materials

PFPE a,w-diol (I) having the following structure:

H0-CH ₂ CF ₂ 0(CF ₂ CF ₂ 0) _n (CF ₂ 0)mCF ₂ CH ₂ -0H

where the ratio m/n=1 and the average molecular weight Mn measured by ¹⁹ F- NMR is 980 amu.

Hexafluorobenzene (O _Q R _Q ), benzyl alcohol and isobutanol were purchased from Sigma-Aldrich and used as such.

Hexafluoroxylene (HFX) solvent was purchased from Miteni S.p.A. (Italy).

Methods

Measurement of refractive index: the measurements were done using an ABBE B (Zeiss) instrument, at room temperature, following the standard procedure ASTM D 1747/62.

Compatibility tests with PDMS:

The compatibility tests were done using a cross-linked PDMS membrane with thickness of about 200 microns. A weighed piece of material (about 100 mg) was immersed in about 50 ml of the present polymer PFPE and let stand at room temperature for 15 days. Afterwards, the material was removed from the fluid, gently dried on the surface with a piece of paper and weighed. The swelling data were expressed as weight % variation compared with the starting weight of the material.

EXAMPLES

Example 1: Synthesis of the a,w-di-pentafiuorophenyi derivative of the PFPE a,w-diol (!) identified above In a 500 ml round bottom flask 150 g (305 meq) of the PFPE diol (I) were loaded together with 62.5 g of O _Q R _Q (336 mmol) and 150 g of HFX solvent. Under stirring, 22.1 g of KOH were added and the mixture was heated at 85°C for 5 hours; afterwards further 2.2 g of KOH were added and the mixture was heated for additional 5 hours at 80°C.

After cooling at room temperature, the mixture was washed with water/isobutanol and the lower organic phase was washed twice with an acidic (HCI) water/isobutanol solution. The separated lower phase was distilled in order to remove the solvents and submitted to thin film distillation.

36.3 g of clear product were thus recovered; the ¹⁹ F-NMR analysis confirmed the following structure:

C ₆ F ₅ -0-CH ₂ CF ₂ 0(CF ₂ CF ₂ 0) _n (CF ₂ 0) _m CF ₂ CH ₂ -0-C ₆ F ₅

with ratio m/n=1 and Mn=1290.

Example 2: Synthesis of the a,oj-di-(p-phenoxytetrafluorophenyl) derivative of the PFPE a,w-dioi (I) identified above

In a 250 ml round bottom flask 60 g of the pentafluorophenyl derivative prepared according to Example 1 were charged together with 25 g of benzyl alcohol and 60 g of HFX solvent. Under stirring, 15 g of KOH were added to the mixture, which was then heated to 78-80°C for 5 hours.

After cooling at room temperature, the mixture was treated with water/isobutanol and the separated lower organic phase was then washed with an acidic (HCI) water/isobutanol solution. The lower phase was distilled in order to completely remove solvents (HFX and isobutanol) and then treated with 1 % charcoal under stirring for 2 hours. After filtration on a 0.2 micron PTFE membrane, the product was submitted to thin film distillation, recovering a distilled fraction of 10.2 g of pure and clear derivative, which submitted to ¹⁹ F-NMR analysis was found to be the target product with following structure:

C ₆ H _5- CH ₂ 0-C ₆ F ₄ -0-CH ₂ CF ₂ 0(CF ₂ CF ₂ 0) _n (CF ₂ 0) _m CF ₂ CH ₂ -0-C ₆ F ₄ -0CH ₂ -C ₆ H ₅

Example 3: Products characterization

Refractive index measurements and swelling tests on the crosslinked PDMS membrane immersed in the present products were carried out according to the methods’ description reported above, for the products obtained in Example 1 and Example 2. The data thus obtained are illustrated in the following Table 1 :

Table 1

The results above show how the introduction of aromatic moieties in the PFPE molecule does increase the value of the refractive index measured for these products, from the 1.30 value of the non-derivatized PTFEs marketed under the trade name FOMBLIN ^® , to a value of 1.341 of the product prepared in Example 1 with 2 aromatic end groups, to a further increased value of 1.409 measured for the product prepared in Example 2, including 2 further aromatic moieties on the aromatic end groups

A swelling expressed as weight percentage variation lower than 1 % in the compatibility test described above does prove how the very good compatibility of the non-derivatized PFPEs with PDMS is maintained in the present PFPEs with aromatic moieties.