Initiator molecule for a non linear absorption reaction, photopolymerisable composition that can be activated by biphotonic absorption and associated 3D printing method

Abstract

A polymerization initiator molecule, excitable by two photons and capable of generating polymerization-initiating free radicals, includes two branches grafted onto a central phenyl nucleus. Each branch includes an oligomer of oligophenyleneethynylenyl type or oligo-2,5-dihalogenphenyleneethynylenyl type. A photopolymerizable composition, activatable by two-photon absorption, includes a radically polymerizable resin and a photochemically effective amount of a radical photoinitiator system. The photoinitiator system includes at least one initiator molecule as described above. Moreover, a method and an associated device for two-photon three-dimensional printing are disclosed. The method includes transforming a volume of a photopolymerizable composition including at least one initiator molecule. The transformation includes irradiating the volume of composition with an irradiation light source emitting an irradiation signal having a wavelength L.sub.irr of between 1 and 1.5 times, and preferably between 1.1 and 1.25 times, a cut-off wavelength L.sub.CutOff of the initiator molecule. Embodiments may apply to submicron-resolution two-photon 3D printing.

Claims

1. A polymerization initiator molecule, capable of being excited by two photons and of generating polymerization-initiating free radicals, the molecule comprising two branches and a halogen atom grafted onto a central phenyl nucleus respectively at positions 1, 3, and 5, each of the branches comprising an oligomer of oligophenyleneethynylenyl type or oligo-2,5-dihalogenphenyleneethynylenyl type.

2. The polymerization initiator molecule of claim 1, wherein each of the branches comprises the oligomer of the oligo-2,5-dihalogenphenyleneethynylenyl type, the dihalogen corresponding to two bromine atoms.

3. The polymerization initiator molecule of claim 1, wherein a free end of each of the branches is terminated by a terminal phenylamine.

4. The polymerization initiator molecule of claim 1, wherein each of the branches is a branch of R=-[p-N,N-(dialkyl)aminophenylethynyl](oligophenyleneethynylenyl) or R=-[p-N,N-(dialkyl)aminophenylethynyl](oligo-2,5-dihalophenyleneethynylenyl) type.

5. The polymerization initiator molecule of claim 1, wherein the halogen atom is a bromine atom.

6. A photopolymerizable composition activatable by two-photon absorption, the composition comprising: a radically polymerizable resin; and a photochemically effective amount of a radical photoinitiator system, wherein the radical photoinitiator system comprises at least one polymerization initiator molecule of claim 1.

7. The photopolymerizable composition of claim 6, wherein the radically polymerizable resin comprises a main monomer of vinyl monomer type.

8. The photopolymerizable composition of claim 7, wherein the main monomer is a multifunctional acrylate monomer.

9. The photopolymerizable composition of claim 6, wherein the radically polymerizable resin comprises a solubilizing component comprising one or more of: a monoacrylate with at least one alkyl chain, a diacrylate with at least one alkyl chain, and a dithiol.

10. The photopolymerizable composition of claim 6, wherein the photopolymerizable composition comprises 0.1% to 10% by weight of the radical photoinitiator system.

11. The photopolymerizable composition of claim 9, wherein the photopolymerizable composition comprises: 0.1% to 10% by weight of the radical photoinitiator system; and C5% to 60% of the solubilizing component.

12. A method for two-photon three-dimensional printing, the method comprising: transforming a volume of the photopolymerizable composition of claim 6, the transforming comprising irradiating the volume of the photopolymerizable composition with an irradiation source emitting an irradiation signal having a wavelength Lin of between 1 and 1.5 times a cut-off wavelength L.sub.CutOff of the at least one initiator molecule, beyond which a molar extinction coefficient of the at least one initiator molecule is less than 1% of a maximum value of the molar extinction coefficient over an absorption band of the at least one initiator molecule.

13. The method of claim 12, wherein the irradiation signal results from a laser with a wavelength of between 508 and 578 nm.

14. The method of claim 12, wherein irradiating the volume of the photopolymerizable composition with the irradiation source comprises direct writing by a laser-type irradiation source, the direct writing being performed at a speed greater than 50 mm/s.

15. The method of claim 12, wherein irradiating the volume of the photopolymerizable composition with the irradiation source comprises parallel projection, of photolithographic type or of holographic projection type, with a number of irradiation sources exceeding ten thousand.

16. A device for three-dimensional-printing, the device comprising means arranged to implement the method of claim 12.

17. The polymerization initiator molecule of claim 3, wherein the terminal phenylamine is of a dialkylphenylamine type.

18. The photopolymerizable composition of claim 7, wherein the main monomer of vinyl monomer type comprises an acrylate monomer or a methacrylate monomer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other features and advantages of embodiments of the disclosure will become apparent on reading the detailed description of exemplary embodiments of the disclosure, given solely by way of example, and with reference to the accompanying drawings, in which:

(2) FIG. 1 (i.e., FIG. 1a, FIG. 1b, and FIG. 1c) shows examples of known initiator molecules,

(3) FIG. 2 shows a first example of initiator molecules according to embodiments of the disclosure,

(4) FIG. 3 shows a variant of FIG. 2, according to embodiments of the disclosure,

(5) FIG. 4 shows a variant of FIG. 2, according to embodiments of the disclosure,

(6) FIG. 5 shows a variant of FIG. 2, according to embodiments of the disclosure,

(7) FIG. 6 shows an example of initiator molecules similar to the example of FIG. 2,

(8) FIG. 7 shows the variation in the one-photon absorption (OPA) and two-photon absorption of the molecule of FIG. 2, as a function of the wavelength of an excitation signal,

(9) FIG. 8 (i.e., FIG. 8a, FIG. 8b, FIG. 8c, and FIG. 8d) shows results of implementing the method according to embodiments of the disclosure,

(10) FIG. 9 (i.e., FIG. 9a, FIG. 9b, FIG. 9c, and FIG. 9d) shows other results of implementing the method according to embodiments of the disclosure, and

(11) FIG. 10 shows other results of implementing the method according to embodiments of the disclosure.

DETAILED DESCRIPTION

(12) As stated above, the disclosure relates to a photopolymerizable composition activatable by two-photon absorption, the composition comprising: a radically polymerizable resin and a photochemically effective amount of a radical photoinitiator system, capable of being excited by multiple photons and of generating free radicals.

(13) The composition according to embodiments of the disclosure is characterized in that the photoinitiator system comprises at least one initiator molecule comprising two branches grafted onto a central phenyl nucleus at positions 1 and 3, each branch comprising an oligomer of oligophenyleneethynylenyl type or oligo-2,5-dihalogenphenyleneethynylenyl type.

(14) FIG. 2 shows an example of an initiator molecule with two branches, according to embodiments of the disclosure, which will be referred to hereinafter for simplification as PA2.

(15) FIGS. 3 to 5 show variants of the molecule of FIG. 2.

(16) FIG. 6 shows an example of an initiator molecule with three branches (which will be referred to hereinafter as PA3), similar to that of FIG. 2.

(17) In these examples, each branch of a molecule according to embodiments of the disclosure comprises (or starts with) a phenyleneethynylenyl such as: a phenyleneethynylenyl (FIG. 2), and more generally an oligo-2,5-phenyleneethynylenyl (FIG. 3 with n between 0 and 4) an oligo-2,5-dibromophenyleneethynylenyl (FIGS. 4 and 5 with n between 0 and 4), and more generally, an oligo-2,5-dihalogenphenyleneethynylenyl with n between 0 and 4, and two halogens per phenyleneethynylenyl group, the halogens being, for example, bromine, chlorine, iodine, fluorine or astatine.

(18) The elongation of the phenyleneethynylenyl structure facilitates the achievement of non-linear absorption, but is detrimental to the dissolution of the initiator molecules in the composition. Thus, experience has shown that, beyond n=4, the dissolution of the molecules is no longer sufficient for the overall reactivity of the composition to be advantageous.

(19) The presence of halogens makes it possible to increase the spin-orbit coupling within the molecule and optimize the photoinduced formation firstly of the photoinitiator in its triplet state and secondly of radicals (consecutively to a photoinduced transfer of electrons); the initiator molecules with halogens are therefore more efficient than similar molecules without halogens. In particular, the best results have been obtained with bromine: specifically, tests with bromine have shown that the photopolymerization threshold could be reduced, compared to their unsubstituted analogues, by a factor of 1.5 to 2.

(20) However, the presence of halogens limits the solubility of the molecule in the resin of interest. A compromise must therefore be made between the efficiency of the initiator molecule and the presence (number) of halogens.

(21) According to one embodiment of the disclosure, the free end of each branch of the initiator molecule is terminated by a terminal phenylamine (aniline), preferably a terminal phenylamine of dialkylaminophenyl type; in the examples of molecules in FIG. 2 (molecule PA2) and FIGS. 3 to 5, the phenylamine end group is a terminal dihexylaminophenylethynyl group. The presence of these carbon-based fatty chains (two alkyl groups) enables the direct dissolution of the initiator molecules. During the tests, hexyl groups (cf. the examples shown with two hexyl groups) gave the best results.

(22) The branches of the initiator molecules according to embodiments of the disclosure are thus of the type: R=-[p-N,N-(dialkyl)aminophenylethynyl](oligophenylene-ethynylenyl) or R-[p-N,N-(dialkyl)aminophenylethynyl](oligo-2,5-dihalogenphenyleneethynylenyl).

(23) The initiator molecules of FIGS. 2, 3 and 5 have two branches and comprise two branches of R type and a halogen atom (bromine here) that are grafted onto the central phenyl nucleus, respectively at positions 1, 3 and 5. They may be synthesized, for example, by the substitution, by branches of R type, of two halogen atoms present on a phenyl nucleus comprising three halogen atoms at positions 1, 3 and 5.

(24) The initiator molecule of FIG. 4 has two branches of R type grafted onto the central phenyl nucleus, respectively at positions 1 and 3, position 5 being occupied by a hydrogen. It may be synthesized, for example, by the substitution, by branches of R type, of two hydrogen atoms present on a phenyl nucleus comprising three hydrogen atoms at positions 1, 3 and 5.

(25) The initiator molecule PA3 of FIG. 6, for its part, has three branches of R type grafted onto the central phenyl nucleus, respectively at positions 1, 3 and 5. It may be synthesized, for example, by the substitution, by branches of R type, of three hydrogen atoms present on a phenyl nucleus at positions 1, 3 and 5.

(26) Tests have been carried out on the initiator molecules according to embodiments of the disclosure and compositions incorporating initiator molecules according to embodiments of the disclosure and, more particularly, on the molecule PA2 of FIG. 2. Similar tests have also been carried out on a similar initiator molecule, molecule PA3, and also on known molecules marketed and commonly used in photosensitive compositions, in the present case the molecules: Molecule 4,4-bis(N,N-diethylamino)benzophenone of NORRISH type II, represented in FIG. 1a, which will be referred to hereinafter as BDEBP, Molecule 2-isopropylthioxanthone, of NORRISH type II, represented in FIG. 1b, which will be referred to hereinafter as ITX, Molecule [1-[9-ethyl-6-(2-methylbenzoyl)carbazol-3-yl]ethylideneamino] acetate, of NORRISH type I, represented in FIG. 1c, which will be referred to hereinafter as OXE2.

(27) The tests, and the comparisons thereof, made it possible to demonstrate the following points.

(28) The absorption spectrum of the molecule PA2 was measured according to conventional techniques, for a sample of molecules dissolved in dichloromethane. FIG. 7 shows the molar extinction coefficient of the molecule PA2. The molar extinction coefficient is an intrinsic parameter of a molecule, directly proportional to the absorbance of a composition containing the molecule; more precisely, the absorbance of the molecule is obtained by multiplying the molar extinction coefficient by the concentration of the molecule in the composition used for performing the measurement and by the length of the cuvette containing the composition used for the measurement. For the molecule PA2, the following is noted: an absorption peak (or a single-photon absorption resonance) at the wavelength L.sub.absR=386 nm, a photon absorption cut-off wavelength L.sub.CutOff=462 nm, the wavelength starting from which the single-photon absorption can be considered to be negligible, since the molar extinction coefficient of the molecule is less than 1% of the maximum value of the molar extinction coefficient (absorption peak) over the absorption band, the absorption band corresponding to the range of wavelengths of signals that the molecule can absorb.

(29) By comparison, the measurement of the absorption spectrum of PA3 dissolved in dichloromethane reveals results similar to those obtained for the molecule PA2, and the single-photon absorption resonance wavelength measured under the same conditions for the molecules BDEBP, ITX and OXE2 is equal, respectively, to L.sub.absR=362 nm, L.sub.absR=386 nm and L.sub.absR=340 nm.

(30) Two-photon absorption was also studied for the initiator molecule PA2, using a technique known to the person skilled in the art, the Z-scan measurement technique, enabling a measurement of the effective two-photon absorption cross sections of the initiator molecules, including non-luminescent molecules, over broad wavelength ranges. This technique is notably described in the publication Measurements of Third-Order Optical Nonlinearity using Z-Scan Technique: A Review, Vijender Singh* et al., AIP Conference Proceedings 2142, 140035 (2019).

(31) To study the molecule PA2, a single measurement was taken at each wavelength at a given power (WL scan measurement), which makes it possible to quantify what is absorbed as a function of the wavelength. In addition, a plurality of measurements are taken for a given wavelength while varying the power (Power scan measurement) in order to verify that a quadratic law is really being followed and thus that the Z-scan is really measuring two-photon absorption. The measurement is carried out over a wavelength range extending from 490 to 960 nm (FIG. 7 for the molecule PA2, right-hand scale, effective two-photon absorption cross section), i.e. the broadest possible range while avoiding one-photon absorption (hence remaining above L.sub.CutOff=462 nm-cf. FIG. 7, left-hand scale, the curve corresponding to the molar extinction coefficient F). The analysis thus shows a two-photon absorption peak in the vicinity of L=770 nm, which corresponds to twice the wavelength L.sub.absR=386 nm of the single-photon absorption resonance. This result is consistent with conventional practice, which consists in using irradiation sources having a wavelength approximately equal to double the one-photon absorption resonance wavelength to instigate two-photon reactions involving an electronic transition to this same excited state. However, the analysis also shows (FIG. 7) that the effective two-photon absorption cross section increases greatly when the wavelength is reduced, and approaches the one-photon absorption limit. For example, for a wavelength L=770 nm, the effective two-photon absorption cross section for a molecule PA2 is of the order of GM (1 GM=10-50 cm4.Math.s photon-1), while for a wavelength of 532 nm, the effective two-photon absorption cross section for a molecule PA2 reaches 1500+/200 GM. These effective cross sections exceed 2000 GM at 500 nm for the molecule PA2 while remaining strictly two-photon in nature.

(32) By comparison, for the molecule BDEBP, the effective two-photon absorption cross section for a wavelength of 532 nm is of the order of 77+/11 GM. The sensitivity of the molecule PA2 is thus much greater than that of the molecule BDEBP commonly used today for two-photon absorption reactions, and a fortiori than those of the molecules specifically used at 532 nm (OXE2 type), the estimated values for the effective cross sections of which generally vary between a few GM and a few tens of GM.

(33) These results are all the more advantageous since they make it possible to envisage the irradiation of the molecules with irradiation sources in the visible domain (wavelengths of less than 750 nm), including molecules generally used in the near-infrared (conventionally at a wavelength double that of their single-photon resonance (L.sub.absR)). Irradiation sources in the visible domain generally offer better resolution than infrared sources and, moreover, as the disclosure shows, they provide greater two-photon absorption efficiency, contrary to the current paradigm, on account of the increase in the effective cross section near to the resonance.

(34) The disclosure also relates to a polymerizable resin composition incorporating an initiator molecule as described above. The composition comprises a main monomer of vinyl monomer type, preferably an acrylate monomer or a methacrylate monomer. These monomers are known for their high reactivity in radical polymerization. Among the compositions tested, monomers of triacrylate type such as pentaerythritol triacrylate (PETA) gave good results and monomers of dipentaerythritol penta/hexaacrylate (DPPHA) type gave the best results in combination with PA2-type initiator molecules.

(35) As has been seen above, the initiator molecules according to embodiments of the disclosure comprise, at the end of their branches, a terminal alkylphenylamine of which one of the functions is to facilitate the dissolution of the molecule in the composition. To further improve this dissolution, a solubilizing comonomer may be added, such as: a monoacrylate with at least one alkyl chain, for example a hexyl acrylate, a diacrylate with at least one alkyl chain, for example a poly(ethylene glycol) diacrylate (PEGDA), a 1,6-hexanediol diacrylate (HDODA), or a 1,10-decanediol diacrylate (DDA), a dithiol, for example a 1,10-decanedithiol (DDT).

(36) Various compositions were produced and tested, with the following components and proportions by weight (wt %): 0.1% to 10% by weight (wt %) of a photoinitiator system comprising an initiator molecule, 5% to 60% by weight of a diluent component, and the remainder to 100% by weight of main monomers.

(37) Among the compositions produced and tested, the most advantageous compositions comprise 0.2% to 5% by weight of photoinitiator system and/or 10% to 25% by weight of solubilizing component. Some examples from among the most notable ones are given below.

(38) Example 1: compositions comprising dipentaerythritol penta/hexaacrylate (DPPHA) and 1,10-decanediol diacrylate (DDA) monomers and the molecule PA2, with various proportions by mass (wt % or % by weight): composition 1a. DPPHA/DDA/PA2, proportions by mass: 79.6/19.1/0.5 composition 1b. DPPHA/DDA/PA2, proportions by mass: 59.7/39.8/0.5 composition 1c. DPPHA/DDA/PA2, proportions by mass: 89.55/9.95/0.5 composition 1e. DPPHA/DDA/PA2, proportions by mass: 79.2/19.8/1 composition if DPPHA/DDA/PA2, proportions by mass: 58.9/36.1/5

(39) DDA makes it possible to improve the dissolution of the initiator molecule PA2 in the composition and also makes the composition less viscous, but DDA is less reactive than DPPHA during 3D printing. While all of compositions 1a to if give good results, composition 1e gives the best compromise.

(40) Example 2: composition comprising dipentaerythritol penta/hexaacrylate (DPPHA) monomers, 1,10-decanedithiol (DDT) molecules and PA2 molecules, with the following proportions by mass (wt %): composition 2. DPPHA/DDT/PA2, proportions by mass: 67.04/32.11/0.85

(41) The use of DDT also gives good results in terms of microfabrication (threshold comparable to similar compositions with DDA instead of DDT), but more DDT needs to be used compared to DDA in order to achieve an equivalent solubility; composition 1e thus remains the best compromise.

(42) Example 3: composition comprising dipentaerythritol penta/hexaacrylate (DPPHA) monomers, poly(ethylene glycol) diacrylate (PEGDA) monomers and PA2 molecules, with the following proportions by mass (wt %): composition 3. DPPHA/PEGDA/PA2, proportions by mass: 49.9/49.9/0.2

(43) Example 4: compositions comprising pentaerythritol triacrylate (PETA) and 1,10-decanediol diacrylate (DDA) monomers and the molecule PA2, with various proportions by mass (wt %): composition 4a. PETA/DDA/PA2, proportions by mass: 59.7/39.8/0.5 composition 4b. PETA/DDA/PA2, proportions by mass: 60/37/3 composition 4c. PETA/DDA/PA2, proportions by mass: 60/35/5

(44) Example 5: composition comprising dipentaerythritol penta/hexaacrylate (DPPHA) monomers, 1,10-decanediol diacrylate (DDA) monomers and PA3 molecules, with the following proportions by mass (wt %): composition 5. DPPHA/DDA/PA3 proportions by mass: 79.47/19.87/0.66

(45) Compared with example 1e, it is noted that the molecule PA3 is more difficult to dissolve in the composition, an amount of 20 wt % of DDA allowing only 0.66 wt % of PA3 to be dissolved, whereas it allows at least 1 wt % of PA2 to be dissolved. Despite slightly superior characteristics of the photoinitiator, the compositions with the molecules PA3 are thus less efficient than those containing the molecules PA2 with the same proportions of DPPHA and DDA monomers.

(46) From various tests, and among the most advantageous compositions, the most reactive compositions comprise 0.5% to 1.5% by weight of photoinitiator system and/or 10% to 25% by weight of solubilizing component.

(47) In addition to an initiator molecule as described above, the photoinitiator system may also comprise a co-initiator suitable for improving the formation of radicals. A co-initiator, electron acceptor, of diphenyliodonium type for example, decomposes by generating an aryl radical. Other co-initiators such as amines of aliphatic amine type (generation of the radical by abstraction of hydrogen on the aliphatic carbon in alpha position with respect to the nitrogen) or triarylamines (electron transfer leading to the formation of a cation radical on the lone pair of the nitrogen) may also be envisaged. However, they are not indispensable on account of the alkylphenylamine end groups at the ends of each branch of the initiator molecules according to embodiments of the disclosure, which play the same role.

(48) The disclosure lastly relates to a method for two-photon three-dimensional printing, comprising a step of transformation of a volume of a photopolymerizable composition comprising a radically polymerizable resin and a photochemically effective amount of a photoinitiator system capable of being excited by multiple photons and capable of generating free radicals, the photoinitiator system comprising at least one initiator molecule, the transformation step consisting in irradiating the volume of composition with an irradiation source emitting an irradiation signal having a wavelength L.sub.irr of between 1 and 1.5 times, and preferably between 1.1 and 1.25 times, a cut-off wavelength L.sub.CutOff of the initiator molecule, beyond which the molar extinction coefficient of the initiator molecule is less than 1% of the maximum value of the molar extinction coefficient of the initiator molecule.

(49) By choosing a wavelength L.sub.irr greater than the wavelength L.sub.CutOff, the risk of a prevalence of one-photon absorption compared to two-photon absorption during the irradiation of the composition is limited. Better still, by choosing a wavelength L.sub.irr greater than 1.1 times the wavelength L.sub.CutOff, the risk of the appearance of a one-photon absorption reaction is even zero, as demonstrated by variable-power Z-scan tests (power scan, FIG. 7).

(50) By choosing a wavelength L.sub.irr less than 1.5 times the wavelength L.sub.CutOff, two-photon excitation is deliberately positioned close to the one-photon absorption resonance wavelength L.sub.absR of the initiator molecule, that is to say within the zone in which the sensitivity of the initiator molecule is at least just as great as for a wavelength L.sub.irr close to 2 times the one-photon absorption resonance wavelength L.sub.absR (FIG. 7), as shown by the analysis of the effective two-photon absorption cross sections. Better still, by choosing a wavelength L.sub.irr less than 1.25 times the wavelength L.sub.CutOff of the initiator molecule, the reaction is deliberately positioned within the zone in which the sensitivity of the initiator molecule is much greater than for a wavelength L.sub.irr close to 2 times the one-photon absorption resonance wavelength L.sub.absR (FIG. 7).

(51) According to a preferred embodiment, the initiator molecule is a molecule according to embodiments of the disclosure as described above. It is thus possible to choose an irradiation light source of a wavelength L.sub.irr=532 nm, of between 508 nm (=1.1*L.sub.CutOff, with L.sub.CutOff=462 nm, cf. FIG. 7), and 578 nm (=1.5*L.sub.CutOff).

(52) In particular, in the example of the molecule PA2, the effective absorption cross section for wavelengths L.sub.irr of less than 578 nm (=1.25*L.sub.CutOff) is greater than around 1000 GM (cf. FIG. 7), i.e. greater than the effective absorption cross section for wavelengths L.sub.irr in the vicinity of twice the one-photon absorption resonance wavelength L.sub.absR of the initiator molecule (around 650 GM at the wavelength 770 nm-cf. FIG. 7). Better still, the effective absorption cross section for wavelengths L.sub.irr of less than 532 nm is greater than around 1500 GM, and increases further when L.sub.irr decreases. For the molecule PA3, the results are less advantageous than for PA2; specifically, the effective absorption cross section for a given wavelength is substantially less for PA3 compared to PA2; for example, the effective absorption cross section for PA3 only reaches approximately 740 GM at 532 nm.

(53) Printing tests, that were more than conclusive, were carried out with a laser of wavelength L.sub.irr=532 nm. More precisely, the laser used for the tests described below is a laser pulsed at a frequency of 11.7 kHz, producing irradiation pulses with a duration of 560 ps (picoseconds). Additional tests showed the applicability of the system to pulsed lasers producing irradiation pulses with durations of the order of nanoseconds (ns) and of femtoseconds (fs), at wavelengths of between 515 and 532 nm, with similar conclusions. More precisely, the additional lasers used have the following characteristics: wavelength 515 nm, frequency from 1 Hz to 2 MHz, 280 fs pulse wavelength 522 nm, frequency of 63 MHz, 250 fs pulse wavelength 532 nm, frequency of 500 Hz, 400 ps pulse.

(54) In order to determine the efficiency of the molecules and of the compositions according to embodiments of the disclosure, the polymerization threshold (minimum power of the irradiation signal required for the polymerization) and the minimum size of objects that could be produced by the method according to embodiments of the disclosure were notably analyzed. The polymerization threshold is considered to be reached when the structures photogenerated by the irradiation step can tolerate a step of final rinsing of the monomer residues without undergoing significant distortion.

(55) For this analysis, two tests were carried out. A first test (FIGS. 8c and 8d) consists in printing a series of 20 m lines spaced apart by 4 m on a substrate; the laser is focused exactly on or very slightly above the substrate, for a precise measurement of the line width while ensuring adhesion of the lines to the substrate. A second test (FIGS. 8a and 8b) consists in printing 13 m lines suspended between two blocks 5 m apart, to ensure good mechanical stability of the line; the lines are positioned 1.5 m above the substrate. This second test makes it possible to determine the height of the lines, a parameter not accessible with the first test.

(56) For both tests, the lines are produced with decreasing irradiation powers, down to the limit of the polymerization threshold, and the printing speed is the same, 40 m/s.

(57) By comparing the two tests (line on substrate and suspended line), it is found that, for a given resin composition and at a given power, the differences between the line widths are not significant, which shows the reproducibility of the printing method and the robustness of the measurement protocols.

(58) FIGS. 8a to 8d more precisely show, for the composition 1e, DPPHA/DDA/PA2 (79.2%/19.8%/1%): FIG. 8a: a view of an entire suspended line, obtained from the composition 1e, irradiated by an irradiation signal with a power of 103 W, FIG. 8b: a view from above of suspended lines, obtained from the composition 1e, irradiated by an irradiation signal with a power decreasing, from top to bottom, from 257 W to 82 W, FIG. 8c: a view from above of a line written on a substrate, from the composition 1e, irradiated by an irradiation signal with a power of 85 W, FIG. 8d: a view from above of a series of lines written on the surface of a substrate from the composition 1e, irradiated by an irradiation signal with a power increasing, from left to right, from 75 W to 303 W.

(59) The two tests (lines on substrate and suspended lines) were also carried out for compositions comprising one of the three initiator molecules BDEBP, ITX and OXE2, with compositions comprising: DPPHA/DDA, in a proportion by mass of 80/20, to which is added 5.4 mol of an initiator molecule, BDEBP, ITX or OXE2, per gram of composition.

(60) The tests carried out with decreasing powers (FIGS. 8b and 8d for PA2) made it possible to determine the minimum polymerization threshold for each of the compositions based on the 80/20 proportion by mass DPPHA/DDA mixture and the addition of 5.4 mol of initiator molecule per gram of composition: PA2: 120 W BDEBP: 550 W ITX: 690 W OXE2: 480 W

(61) The choice to add 5.4 mol of initiator molecule to the 80/20 proportion by mass DPPHA/DDA mixture was done so as to dissolve the same amount of initiator molecules PA2, BDEBP, ITX or OXE2 in the mixture. Other tests more specifically with the molecule PA2 showed that it is possible to further lower the threshold to 85 W, by adding 10.9 mol of initiator molecule PA2.

(62) The tests thus confirm the substantially lower polymerization thresholds, by a factor of 4 to 6, for the PA2-type initiator molecules with two branches compared to the known initiator molecules (BDEBP, ITX, OXE2). This is essentially explained by the very great improvement in the capacity of the PA2-type molecules with two branches to simultaneously absorb two photons, as shown by the high value for their effective absorption cross section for a wavelength L.sub.irr=532 nm.

(63) Lastly, to analyze the printing resolution, at an irradiation power close to the polymerization threshold, periodic networks such as a grid (FIGS. 9a and 9b) with lines spaced apart by 250 nm, and three-dimensional structures of photonic crystal type, in woodpile form (FIGS. 9c and 9d), having at least one dimension less than 100 nm, were produced. More precisely, FIGS. 9a to 9d show structures obtained by irradiation of the composition 1e, at a speed of 40 m/s, by an irradiation light source of wavelength L.sub.irr=532 nm: FIG. 9a and FIG. 9b: view from above and enlarged view from above of the periodic grid, produced by irradiation of the composition 1e by an irradiation signal with a power of 103 W, FIG. 9c and FIG. 9d: perspective view and view of an entire structure of photonic crystal type, in cubic woodpile form, produced by irradiation of the composition 1e with an irradiation signal with a power of 125 W.

(64) During these tests, a lateral resolution of the order of 80 nm, an axial resolution (line height) of the order of 190 nm, and a lateral spacing of the order of 250 nm could be obtained.

(65) In summary, the disclosure proposes a family of photoinitiating molecules for a reaction for the two-photon photoinduced generation of radicals, a composition comprising such molecules and a method for three-dimensional printing by irradiation of such photosensitive compositions, which notably provide the following technical and economic benefits: initiator molecules with a very high sensitivity, notably for wavelengths in the visible domain, initiator molecules that are easier to dissolve in a photosensitive resin, making it possible to obtain highly reactive compositions since they incorporate a significant amount of initiator molecules, the use of an irradiation source in the visible domain, and at an irradiation power level that is substantially lower than in the prior art, a printing quality (resolution) of less than 100 nm and a printing speed that is at least as good as with known initiator molecules.

(66) As specified in the prior art, it is possible to use the resins according to embodiments of the disclosure for parallel writing, that is to say for simultaneously printing a plurality of structures by way of a plurality of laser beams. For this, the initial laser beam of the 3D printer can be separated into a plurality of beams by means of a diffractive optical element (DOE) placed in the optical path. Tests with parallelization of the writing by using an 1111 diffractive optical element dividing the incident beam into 121 beams of lower power thus made it possible to simultaneously print (cf. FIG. 10) 121 identical structures spaced by 1.85 m with a DOE separating the laser beam into a grid of 1111 beams using the formulation of composition 1e (DPPHA/DDA/PA2, proportions by mass: 79.2/19.8/1).