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Two photon polymerization (TPP) is a precise, reliable, and increasingly popular technique for rapid prototyping of micro-scale parts with sub-micron resolution. The materials of choice underlying this process are predominately acrylic resins cross-linked via free-radical polymerization. Due to the nature of the printing process, the derived parts are only partially cured and the corresponding mechanical properties, i.e. modulus and ultimate strength, are lower than if the material were cross-linked to the maximum extent. Herein, post-print curing via UV-driven radical generation, is demonstrated to increase the overall degree of cross-linking of low density, TPP-derived structures.
© 2016 Optical Society of America
1. Introduction
Direct laser writing (DLW) based on multip hoton polymerization (MPP), otherwise referred to as two photon lithography, is an additive manufacturing technique capable of precisely fabricating complex, 3D structures with sub-micron resolution [1]. At the heart of this process is the simultaneous absorption of two or more photons (TPA) in a single event by a photosensitive molecule, which in turn initiates local polymerization chemistries. In contrast to one photon absorption (OPA), TPA is a non-linear process in which the probability of TPA occurring is related to the square of the photon flux density. For lithography purposes, sufficiently high photon intensities are obtained by focusing a mode-locked (pulsed) laser through a high numerical aperture objective (NA > 1) into the interior of a droplet of photosensitive material. Complex structures can thus be obtained by arbitrarily scanning the laser focus point in three dimensions.
DLW-MPP has been applied to numerous applications including the production of photonic- [2–4] and mechanical metamaterials [5–8], electrically conductive devices [9], stimulus responsive micro-machines [10–13], plasmonic surfaces [14], biomimetic scaffolds [15–17] and filters [18]. The materials, or resins, employed in TPP as well as in conventional, UV-driven OPA lithography, are typically either radical mediated polyfunctional acrylate and/or thiol-ene monomers, cationic-initiated polyfunctional epoxides or combinations thereof [19–21]. While an attractive feature of DLW-MPP is the ability to fabricate feature sizes in the sub-200 nm range in a truly 3D fashion, this requires the printing process to be carried out under carefully controlled conditions near the polymerization threshold of the resin [22, 23]. The size of an individual voxel (volume-pixel) and the corresponding mechanical properties of the cured material, are directly related to the laser-pulse energy and duration of exposure [24]. Lu et al. have demonstrated that increased average laser powers correlate to greater hardness and higher Young’s modulus for series of solid micro-cubes [25]. Moreover, Lu [25] and others [23, 26, 27], have shown a direct correlation between laser intensity and the overall degree of conversion DC, i.e. the extent of cross-linking. Unsurprisingly, cross-link density is well-known to be directly related to the elastic modulus and ultimate tensile strength of cured acrylic materials [28–31]. Therein lies an important consideration during the fabrication process, although minimization of features is often desired, such features may lack the degree of mechanical stability required to support more elaborate 3D structures, see herein for a case in point.
Prototypes produced through UV-driven OPA stereolithography are routinely subjected to post-print processing, either additional UV-flood treatment or thermal bakes. These post-print processes serve two related purposes; the first is to polymerize uncured resin trapped within the prototype, and second, to further the polymerization of already cured, ‘green-state’ material, thereby increasing cross-link density [32, 33]. We have noticed a general absence of UV-post curing steps within the TPP literature, which predominately utilizes acrylate-based resins. A notable exception occurs for prototype production through profile/contour scanning, in which only the outer surface is printed leaving behind an interior of uncured resin, thus necessitating post-curing [1,34]. The same benefits derived from post-print UV-curing in OPA stereolithography, should be equally applicable to materials obtained under DLW-MPP. We demonstrate here an easy to implement, post-print UV-curing step for furthering the degree of cross-linking and thus improving the mechanical properties of TPP compatible, acrylic-based resins without degrading the writing resolution, as would be the case by simply increasing the laser power.
2. Experimental methods
All TPP fabrication was carried out with a Nanoscribe GmbH Photonic Professional GT laser lithography system. Nanoscribe is powered by a FemtoFiber pro NIR laser supplied by TOPTICA operating with a pulse duration (τp) ~100 fs at 780 nm (λ) and a repetition rate (frep) of 80 MHz. Focusing of the laser was accomplished with either a Zeiss plan-apochromat 63x1.4NA Oil DIC M27 - Figs. 1, 2 and 3 or a Zeiss LCI Plan-Neofluar 25x0.8NA DI Imm - Fig. 5 objective. Transmittance (T) at 780 nm for the 63x1.4NA and 25x0.8NA objectives is 0.75 and 0.8, respectively, as reported by the manufacturer. Average laser power (P, mW) was measured before the microscope objective and was varied to control light exposure. Laser peak intensity (I, TW/cm2) was recalculated from the following equation: I = (2PT)/(τp frepπω2), where ω (beam waist) = 0.61λ/NA. Octet-truss structures fabricated in Fig. 1 utilized Nanoscribe’s PerfectShape settings intermediate in piezo scan mode. PerfectShape settings are Nanoscribe’s proprietary path optimization algorithm, which adapts the laser intensity and scan speed to the submitted coordinates and desired trajectory. Wood-piles in Figs. 2 and 5 were printed in galvo scan mode.
Fig. 1 Power-test: 50 µm3, 10 µm unit-cell, octet-truss cubes were printed within increasing laser peak intensity from 0.2 to 0.6 TW/cm2 utilizing piezo scan mode and a 63x1.4NA objective. (a) Scanning electron micrograph of the green-state power-test. Magnified image of a collapsed octet structure fabricated at 0.26 TW/cm2. (b) Post-print, UV-cured power-test. Magnified imaged of a standing octet structure fabricated at 0.26 TW/cm2. (c) Line width/feature size impact on density as a function of laser peak intensity. Square (green) data points represent the ‘green-state’, as printed sample of octet-truss structures shown in 1a. Circle (blue) data points represent the UV post-cured sample shown in 1b.
Fig. 2 Analysis of the degree of conversion (DC) of TPP resin IP-DIP before and after UV-post curing, Woodpile structures were fabricated using 0.88-1.19 TW/cm2 laser peak intensities with galvo scan mode and a 63x1.4NA objective. (a) Scanning electron micrograph of a 50x50x100 µm3 woodpile pillar with 2 µm XY spacing and 1 µm spacing. (b) Raman spectra of IP-DIP before and after UV-curing. (c) DC dependence on UV-post curing, where the green (square) data points represent the as-printed material and the blue (circle) data points represent the UV post-cured material.
Fig. 3 Young’s modulus and yield strength vs density and the effect of post UV-curing. Both Young’s modulus and yield strength follow quadratic relationships with density, E ∝ ρ2. Dash lines are fitting results using power functions. Green (square) data points represent the as-printed, ‘green-state’ material and the blue (circle) data points are of UV post-cured material.
Nanoscribe’s proprietary resin, IP-Dip (lot #1-600-0079) was used in Figs. 1, 2 and 3, while an ‘in-house’ prepared resin consisting of pentaerythritol triacrylate (PETA) and 0.1 wt% 4,4'-((1E,1'E)-(2-((2-ethylhexyl)oxy)-5-methoxy-1,4-phenylene)bis(ethene-2,1-diyl))bis(N,N-dibutylaniline) (BPAS-PI) [35, 36] was used in Fig. 5.
The printing/development protocol is as follows: A drop of resist was applied to a 25x25x0.7 mm3 ITO coated glass slide supplied by nanoscribe. Structures were printed at the glass surface up-side-down in ‘dip-in laser lithography’ mode [37]. The glass slide was submerged in a developing bath of propylene glycol monomethyl ether acetate (PGMEA) for a period of 45 min. Next, the slides was gentle dipped into ispropanol before being placed right-side-up in swallow glass (1.5 cm deep) petri dish containing ~12 mL isopropanol and 50 mg 2,2-dimethoxy-2-phenylacetophenone (DMPA, otherwise referred to as irgacure 651). The sample was then illuminated with an 8 W, 0.32 Amps UVP UVL-28 EL Series UV lamp at a wavelength of 365 nm for a period of 10 min. The petri dish was placed against the light such that the glass slide was ~1.5 cm away from the light source. At this distance, the relative intensity was found to be 3.5 mW/cm2, as determined by a Thorlabs optical power (PM100D) and photodiode sensor (S120VC). Finally, the glass slide was once more gentle dipped into clean isopropanol before being allowed to either a) air-dry at room temperature as was the case for the structures printed in Fig. 1 and Fig. 5, or b) in the case of woodpile structures in Fig. 2, placed into acetone followed by super critical CO2 drying [38]. Critical point drying was accomplished using a EMS (electron microscopy sciences) 3100 dryer.
Scanning electron micrographs in Figs. 1 and 2 were obtained with a JEOL7401-F SEM at a 2kV accelerating voltage. The image in Fig. 5 was obtained with a Phenom Pro desktop SEM at 5kV accelerating voltage under low vacuum settings. A Nicolet Almega XR dispersive micro-Raman spectrometer (Thermo Scientific) was used to determine the overall degree of conversion. Spectra were collected at a wavelength of λ = 633 nm with a 50x objective, averaging over 25 five second scans. Compression experiments were conducted using an MTS Nanoindenter XP system equipped with 1-mm-in-radius stainless steel flat punch. Thermal drift rate was limited to be less than 5 nm/s. Quasi-static strain rate was set at 5 x 10−4 s−1. The flat punch tip was cleaned using methanol after each test to avoid contaminations. Young’s modulus was extracted from the slope of loading curves. Yield strength was measured as the stress at 0.2% of plastic strain.
DMPA, isopropanol, acetone and PGMEA were obtained from Sigma Aldrich. PETA was obtained from Alfa Aesar. 9,9-Bis(4-(2-acryloyloxyethyloxy)phenyl)fluorene - Fig. 2, and BPAS-PI - Fig. 5, were synthesized from 4,4’-(9-fluorenylidene)bis(2-phenoxyethanol) and 4-methoxyphenol, respectively, following known literature procedures.
3. Results and discussions
3.1. TPP power-test with IP-DIP
TPP printing is carried out within the interior of a drop of resin, residual resist is then gently dissolved away into a larger bath of developing solvent, followed thereafter by drying. All materials used for stereolithography undergo volumetric shrinkage upon curing and drying, which can lead to cracking, warpage or other distortions [39–41]. This is especially true of acrylics, less so for epoxies [29]. In the case of micro-scale TPP parts, capillary forces cause additional compressive stress during the drying process. One solution is to rinse the part in a low surface tension solvent prior to drying. Yet structures with higher porosity and/or finer features may necessitate even greater precautions in the form of critical point CO2 drying [38].
To overcome these process constraints, prototype fabrication typically relies on some form of a power-test strategy, in which the prototype of interest is printed repeatedly while varying the incident light intensity. The objective of this strategy is to determine the minimum feature size and corresponding printing conditions under which structurally-sound parts can be obtained following development. At the Lawrence Livermore National Laboratory, our goal is to develop mm-sized, flaw-less (without cracks or warping) low density (~50 mg/cc) structures with sub-micron features as targets for high energy density physics experiments. To do so requires micron-scale porosity and structurally sound architectures in order to reach our desired densities under specific design requirements. We have found that adding a simple 10 min post-print UV cure procedure after the resin wash step, but prior to drying, is a necessary and straightforward means to strengthen the green-state part without increasing the size of the individual voxel.
To illustrate the benefits of a post-print UV cure step, we carried out a power-test on a 10 µm unit cell octet-truss structure, Fig. 1. Ten octets were printed with increasing intensity, 0.2-0.6 TW/cm2, using a Nanoscribe Professional GT printer. Fabrication was carried out up-side-down in dip-in laser lithography (DiLL) mode [37], in which the laser is focused through a drop of resin (IP-DIP, DiLL compatible resin, Nanoscribe) using a 63x1.4NA objective. Scanning was accomplished through precise piezo-stage movements at an average rate of 46 µm/s using Nanoscribe’s PerfectShape path optimization algorithm. Two batches of octet-truss structures were printed, while only one batch was subjected to UV post-curing [Fig. 1(a) vs (b)]. During the UV post-curing step, the batch of octets shown in [Fig. 1(b)] were transferred to an isopropanol bath containing 0.5 wt% 2,2-dimethoxy-2-phenylacetophenone (DMPA, otherwise referred to as irgacure 651) followed by illumination with an 8W hand-held UV-A 365 nm light for 10 min. Both batches were then air-dried from isopropanol. Stresses experienced during drying caused structures fabricated near the polymerization threshold to warp and collapse.
From the results shown in Fig. 1, it is clear that adding the simple UV-post-curing step described above allows one to write stable structures at significantly lower laser powers (smaller voxel size). Nearly twice the peak laser intensity was required to arrive at an up-right octet-truss, 0.41 vs 0.23 TW/cm2 for the un-cured vs cured batches, suggesting an increase of mechanical stability after post-curing. Additional analysis confirmed that the UV cure step had no effect on the as-printed feature sizes, [Fig. 1(c)]. Increasing the laser intensity from 0.23 to 0.41 TW/cm2 however, causes an increase in lines width by >200 nm, which in effect nearly quadruples the estimated density.
3.2. Effects of UV-post curing on the degree of conversion and corresponding mechanical properties
The effect of the post-print UV-curing step on the cross-link density and the mechanical properties was further probed through the combination of micro-raman spectroscopy and nano-indentation. Woodpile structures was chosen for this task for their simplicity in both fabrication and deformation mode. 50x50 µm2, 100 µm tall woodpile pillars with increasing XY-spacing between lines within a layer, and a fixed 1 µm period between each layer, were printed by scanning the laser with galvo mirrors at a rate of 10 mm/s, [Fig. 2(a)]. Laser intensities were adjusted over a range of 0.88-1.19 TW/cm2 in order to account for proximity effects [42] and to arrive at a consistent ~350 nm line width across all woodpile pillars. Due to the 1.4NA objective employed during printing, the resulting 3.5:1 ratio elliptical voxels give rise to woodpile lines that sit tangential to one another between layers, thereby maximizing porosity. The size of the pillar samples were chosen based on the estimation of maximum load they could sustain and the load limit (600 mN) of our nanoindentation system. A 2:1 height/width aspect ratio was chosen for compression to avoid buckling or densification induced artifacts. Finally, each pillar was fabricated on top a full-density, printed 100x100x5 µm3 platform which served to provide additional adhesion between the glass slide substrate and the porous pillars. As before, two batches of woodpile structures were printed, of which only one batch was subjected to UV post-curing before super-critical CO2 drying.
Raman spectroscopy is a straightforward, non-destructive means for deteriming the overall degree of conversion (DC) for many polymeric materials. In the case of acrylic polymers, DC is obtained by monitoring the change of the ~1640 cm−1 C = C signal relative to the unchanging ~1730 cm−1 C = O signal of the polymerizable acrylate groups [23, 25–28]. While IP-Dip resin consists primarily of pentaerythritol triacrylate (PETA), the corresponding raman spectrum contains an additional signal in the area of interest, [Fig. 2(b)]. The 1610 cm−1 flourene peak has been identified as likely belonging to 9,9-bis(4-(2-acryloyloxyethyloxy)phenyl)fluorene, which is one of several chemicals listed on IP-Dip’s accompanying material data safety sheet (MSDS). DC was determined by fitting each signal to a Lorentzian distribution and measuring the change in area of the 1640 cm−1 signal relative to the area of the constant 1730 cm−1 signal according to the following equation: DC = (1-((Ac = c/Ac = o)/(Aoc = c/Aoc = o))*100, where Ao represents the area derived from the uncured resin. Pillars printed under the conditions described above, had an average DC of 23% ± 3, [Fig. 2(c)]. The additional UV-curing step nearly doubled DC to between 40 and 60%, depending on XY spacing. It should be noted that DC values of 100% are not physical possible with the polyfunctional monomers used in most TPP resins. This is due to the formation of highly cross-linked networks that severely restrict functional group mobility. The 44% ± 7 conversion obtained in this work, agrees with the maximum degree of conversion reported for related nanoscribe resin, IP-L 780 [25]. Similar studies of TPP resins yielded maximum DC values between 45 and 75% [23, 26, 27, 43].
Subsequent compression of both sets of pillars using a MTS nanoindenter XP system equipped with a 1-mm radius stainless flat punch revealed that both Young’s modulus (E) and yield strength (σ) increased by more than 50% after UV-curing, Fig. 3. The stiffer, stronger material that results from adding the post UV-cure process step accounts for the observations made in the power-test experiment, Fig. 1. Both E and σ follow the expected quadratic relationship with density, which in the case of log-pile architectures is varied by increasing the XY spacing between printed lines. This relationship is consistent with the bending dominated deformation mode of the designed wood-pile structure, and was not significantly influenced by UV-curing. Noticeably, a more significant decline in mechanical properties occurs at densities near or below 100 mg/cm3, which demonstrates the need of increased precautions during the development steps, in terms of both UV-curing and gentle drying techniques.
3.3. Photoinitiator consumption during printing
The post-print UV-curing procedure described here is applied in the wet state, thus strengthening the structure before it is exposed to compressive capillary forces during drying. A common OPA photoinitiator, DMPA (irgacure 651), is employed to produce radicals and further cross-link density, schematic shown in Fig. 4. Because residual resin is removed prior to the UV-curing step, the externally generated radicals only increase DC within the interior of a voxel, thereby strengthening the material without affecting the as-printed feature sizes. We have found that simply treating IP-Dip derived parts with a hand-held 8W UV light in the absence of DMPA, did not significantly increase DC. It is possible that the photoinitiator in IP-Dip is either consumed during the TPP-induced curing process or it does not undergo OPA efficiently enough to produce a sufficient quantity of radicals under 365 nm light.
Fig. 4 Chemical level depiction illustrating radical formation leading to increased cross-linking resulting from a post-print UV exposure.
To probe this question further, we formulated our own TPP resin consisting of only two components, PETA and 0.1 wt% of a 4,4-bis(diphenylamino)stilbene photoinitiator (BPAS-PI) with a large TPA cross sectional area [35, 36], [Fig. 5(a)]. Similar to before, a series of woodpile pillars were printed at 0.11 TW/cm2 in DiLL mode with 25x0.8NA objective, and subjected to several curing procedures before being allowed to air dry, including, post-print UV-cure with and without DMPA, and no additional cure step, Fig. 5. Subsequent Raman analysis revealed an avg. DC value of 22% ± 2 for the as-printed samples. UV flood treatment slightly improved the DC from 22% to 25%, while the addition of DMPA significantly increased the DC to 38% ± 2 [Fig. 5(d)]. The strong raman signal of BPAS-PI, 1590 cm−1, is noticeably absent following printing/development [Fig. 5(c)]. This result suggests that the printing process consumes all of the local photoinitiator concentration.
Fig. 5 (a) TPP compatible resin formulation consisting of a trifunctional acrylate monomer PETA, and a TPP-photoinitiator BPAS-PI. Woodpile structures were fabricated with 0.11 TW/cm2 laser peak intensity under galvo scan mode with a 25x0.8NA objective (b) Raman spectra of the TPP resin components and accompanying curing conditions. (c) Scanning electron micrograph of a 100x100x50 µm3, 2 µm XY spacing log-pile pillar. (d) Degree of conversions obtained under different curing conditions; 1) green state (as printed), i.e. no post-curing, 2) post UV-curing without DMPA, 3) post UV-curing with DMPA.
We would like to highlight that this UV post-curing procedure utilizes a relatively low intensity, commonly available UV-light source at a wavelength of 365 nm. Although this procedure employs DMPA (irgacure 651), many other photoinitiators are active at this wavelength and would perform as described. It is possible to cure acrylic materials without radical-initiators, however these processes generally require more stringent conditions, such as with either shorter irradiation wavelengths [44] or laser-driven ionization [45, 46]. Additionally, because this process utilizes an excess of radicals produced in a bath of isopropanol, radical quenching via exogenous oxygen is largely overcome as air is only partially soluble in alcoholic solvents [47, 48]. This is important for ensuring thorough curing as capillary stress induced damage is generally irreversible. Finally, we have also had success with thermally-induced radical-initiators, such as azobisisobutyronitrile (AIBN), however room temperature UV-initiated post-curing has proven to be more convenient than heating TPP samples.
4. Conclusions
We have described a quick and simple UV-curing step that can be easily implemented into post-print TPP-lithography development protocols wherever acrylic-based resins are employed. This procedure polymerizes un-reacted acrylate groups thereby furthering the degree of conversion and in turn, increasing cross-link density. The net-effect results in stiffer, stronger materials that maintain the as-printed, green-state, feature sizes.
Funding
US Department of Energy (DOE) by Lawrence Livermore National Laboratory (LLNL) under contract No. DE-AC52-07NA27344; Laboratory Directed Research and Development (LDRD) programs of Lawrence Livermore National Laboratory (LLNL) (15-ERD-019).
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