Letter Cite This: ACS Macro Lett. 2018, 7, 493−497
pubs.acs.org/macroletters
3D Printing All-Aromatic Polyimides Using Stereolithographic 3D Printing of Polyamic Acid Salts Jana Herzberger,† Viswanath Meenakshisundaram,‡ Christopher B. Williams,‡ and Timothy E. Long*,† †
Department of Chemistry and Macromolecules Innovation Institute (MII), Virginia Tech, Blacksburg, Virginia 24061, United States Department of Mechanical Engineering and Macromolecules Innovation Institute (MII), Virginia Tech, Blacksburg, Virginia 24061, United States
‡
S Supporting Information *
ABSTRACT: Polyamic acid (PAA) salts are amenable to photocuring additive manufacturing processes of all-aromatic polyimides. Due to an all-aromatic structure, these high-performance polymers are exceptionally chemically and thermally stable but are not conventionally processable in their imidized form. The facile addition of 2-(dimethylamino)ethyl methacrylate (DMAEMA) to commercially available poly(4,4′-oxydiphenylene pyromellitamic acid) (PMDAODA PAA) afforded ultraviolet curable PAA salt solutions. These readily prepared solutions do not require multistep synthesis, exhibited fast gel times (5 Pa·s. As depicted in Scheme 1, the molar ratio of DMAEMA to PAA is critically important for fast photo-cross-linking and the formation of a free-standing organogel. At the same time, the introduced DMAEMA groups only act as sacrificial cross-links to enable selective photocuring via VP and must be released during thermal conversion of PAA to PMDA-ODA PI. Consequently, the number of transient cross-links should be kept at a minimum to facilitate their release upon imidization but must be high enough to enable UV-cross-linking and to provide mechanical integrity to the organogel. Photorheological experiments of PAA salts with varying DMAEMA equivalents indicated the optimal cross-linker amount. As illustrated in Figure 1, increasing the amount of DMAEMA promoted faster curing and resulted in networks with higher G′ gel-state moduli. From this data, PAA salts with 0.5 equiv of DMAEMA per carboxyl group were optimal for VP, providing fast gel times, high plateau moduli (G′ ∼ 5 × 105 Pa), and crossover moduli of ∼3 × 103 Pa (Figure S5). Lower amounts of DMAEMA resulted in low G′ gel-state moduli after UV curing and did not provide the necessary mechanical integrity for 3D printing a suitable green part. Our previous strategy using PAA esters required two sacrificial cross-links per polymer repeat unit to yield printable organogels.12 In contrast, the PAA salts presented here already afforded strong organogels utilizing only one transient group per repeat unit, which is equivalent to 0.5 equiv of DMAEMA per carboxyl group in the polymer backbone. This reduced the number of sacrificial cross-links by 50% compared to the polyamic diacrylate esters and highlighted the advantage of PAA DMAEMA salts over PAA esters for selective photocuring via VP. In addition to the DMAEMA concentration, the amount of photoinitiator is a key factor that influenced gel time. Variation of TPO concentration from 0.5 to 5 wt % facilitated the determination of PAA DMAEMA solution gel times using photorheology. These measurements revealed an optimized concentration of 1.5 wt % TPO (Figure S6), which was the minimum amount needed to afford fast gel times. Consequently, all further studies and VP occurred with PAA salt solution containing 0.5 equiv of DMAEMA and 1.5 wt % TPO. For AM, good shelf life and stability of the polymer solution is important to facilitate handling. Storing the PAA DMAEMA
subsequently converted into PMDA-ODA PIs having a predetermined three-dimensional shape. The facile addition of an amino-functional methacrylate (2-(dimethylamino)ethyl methacrylate, (DMAEMA)) to dissolved PAA in NMP yielded UV-curable precursor solutions (Scheme 1). This approach Scheme 1. Preparation of PAA DMAEMA Salt Solutions and Subsequent Photo-Crosslinking to Yield an Organogela
a
Conventional thermal treatment resulted in PMDA-ODA PI and released all aliphatic crosslinks.
avoided tedious synthetic procedures and required only commercially available starting materials. A custom-built scanning-mask VP apparatus including a recoating blade enabled the 3D printing of such PAA salts. Subsequent thermal treatment generated the desired all-aromatic PMDA-ODA PI and released all aliphatic cross-links (Scheme 1). Processing of PAA salts using aliphatic amines was reported repeatedly in earlier literature,13−20 but to the best of our 494
DOI: 10.1021/acsmacrolett.8b00126 ACS Macro Lett. 2018, 7, 493−497
Letter
ACS Macro Letters
Figure 1. Data from photorheology. Left: Storage modulus (G′, Pa) vs time (s) during photopolymerization of PAA DMAEMA salts with varied DMAEMA amount. Irradiation starts at 30 s. G′ increases with increasing DMAEMA equivalents. Right: Gel time (s) vs DMAEMA (equiv) amount.
solutions in the dark at −20 °C for 4 weeks showed no significant impact on their solution viscosity or their UV-curing properties, indicating high solution stability and a shelf life of at least one month (Figure S8). PAA salts are known to be hydrolytically more stable than their neat PAA counterparts,21,22 providing an attractive option for use in AM and potential commercialization. A custom-built scanning-mask VP apparatus,12 complete with photopolymer recoating blade, enabled AM of PAA DMAEMA salt solutions (Figure S1, SI). The recoating blade ensured reliable refreshing of the viscous solution after each curing step. This enabled the selective photocuring of solutions with viscosities of 18 Pa·s, which is an improvement to conventional VP systems. An UV source with a wavelength near 400 nm is necessary to compensate for the strong UV absorbance of PAAs in solution.12 Creating a resin working curve elucidated the required printing parameters and indicated a critical exposure Ec of 843.76 J·m−2 and a depth of penetration Dp of 705.73 μm (Figure S9, SI).23 Figure 2A and 2B depict a 3D-printed lattice structure and additively manufactured block letters “V” and “T”. Significantly, PAA DMAEMA salts enabled printing of larger
objects compared to our previously reported method, due to a greater organogel modulus and increased self-supporting capability after removing the organogel from the vat. This is especially important to enable AM of complex geometries, typical for automotive and aerospace components. Subsequent drying under air, followed by thermal treatment under vacuum and nitrogen atmosphere, rendered the crosslinked PAA into all-aromatic PMDA-ODA PI. The heating process fully removed NMP, induced thermal imidization of the PAA, and released the aliphatic poly(DMAEMA) cross-links. In accordance with previous PAA salt literature in the 2D lithography field, thermal treatment occurred at 400 °C.13,24 This postbake step induced an isotropic, dimensional shrinkage of 48%, as indicated by the 3D objects in Figures 2C and 2D. This shrinkage value is similar to the previously reported polyamic diacrylate esters (52% shrinkage).12 For most 3D printed parts using VP, the surfaces show undulations due to the approximation of a slanted surface with discrete layers. The printed block letters “V” and “T” in Figure 2D showed the typical “staircase” surface roughness, which was confirmed with zoomed SEM images of the object surface (Figure S10). SEM images of the freeze-fractured cross section of a 3D-printed part revealed the absence of independent layers (Figure 3), which suggested that the layers are homogeneously cured to the next during each step.
Figure 2. 3D-printed lattice structure (A) and block letters “V” and “T” (B) organogel after printing. Lattice structure (C) and block letters “V” and “T” (D) after heating to 400 °C and conversion of PAA DMAEMA salt to PMDA-ODA PI.
Figure 3. SEM images of a cross-section of a 3D-printed object consisting of polyimide (heated to 400 °C, freeze fractured). Printing layers from the layer-by-layer approach are not visible. 495
DOI: 10.1021/acsmacrolett.8b00126 ACS Macro Lett. 2018, 7, 493−497
ACS Macro Letters
■
After thermal treatment, FTIR spectra exhibited the characteristic polyimide vibrational bands. The carbonyl stretches of the imide ring appeared at 1774 and 1712 cm−1, and the C−N stretch of the imide was visible at 1367 cm−1. Simultaneously, the amide bands of the polyamic acid (1655 cm−1, CO stretch, amide I and 1540 cm−1 C−NH, amide II) disappeared (Figure S11, right). This is in accordance with reported FTIR spectra for PMDA-ODA PAA and PI25 and confirmed that our heating process converted the organogel to the desired polyimide. Furthermore, TGA measurements revealed the exceptional thermal stability of PMDA-ODA PI, which makes these 3D-printed objects valuable for hightemperature applications. Measurements indicated a 5% weight loss at Td,5% = 498 °C, which occurred at marginally lower temperatures than a PI film prepared from neat PAA (Td,5% = 555 °C) (Figure S11, left). Heating the 3D-printed PMDAODA PI part to 1000 °C resulted in a char yield of 70%, while the PMDA-ODA polyimide film prepared from commercial PAA produced 56 wt % char (Figure S11, left). A higher char yield of the photocured parts suggested the presence of residual carbon, which is formed during the pyrolysis of the transient PDMAEMA cross-links. This residual carbon might be responsible for the dark color of the additive-manufactured parts. DMA measurements of a 3D printed and imidized specimen revealed a storage modulus above 1 GPa up to 330 °C (Figure S12). Thus, the additive-manufactured PMDAODA possessed similar thermomechanical properties to a PI film prepared from neat PAA, which is important for hightemperature applications. In conclusion, a facile route to 3D-printed polyimides is presented that employed UV-curable PAA DMAEMA salt solution precursors. This method required only commercially available starting materials, avoided tedious synthesis, and rendered storage-stable photopolymer solutions. In this system, DMAEMA interacted electrostatically with the PAA backbone, yielding UV-sensitive PAA DMAEMA salts. Upon UV irradiation, TPO initiated cross-linking of DMAEMA, which generated an organogel. Photorheology studies revealed the optimal composition being PAA with 0.5 equiv of DMAEMA and 1.5 wt % photoinitiator. This composition provided fast gel times (
