Highly Crystalline, Nanostructured Polyimide Microparticles via Green

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

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Highly Crystalline, Nanostructured Polyimide Microparticles via Green and Tunable Solvothermal Polymerization M. Josef Taublaender,†,‡ Manuel Reiter,†,‡ and Miriam M. Unterlass*,†,‡,§ †

Institute of Applied Synthetic Chemistry, TU Wien, Getreidemarkt 9/163, 1060 Vienna, Austria Institute of Materials Chemistry, TU Wien, Getreidemarkt 9/165, 1060 Vienna, Austria § CeMM-Research Center for Molecular Medicine of the Austrian Academy of Sciences, Lazarettgasse 144, 1090 Vienna, Austria ‡

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S Supporting Information *

ABSTRACT: Herewith, we report a straightforward, experimentally simple, and environmentally benign synthetic strategy toward cyclocondensation polymers. Using a fully aromatic polyimide as model system, we demonstrate that products of extraordinary crystallinity can be generated in various protic, polar solvents (ethanol, iso-propyl alcohol, and glycerine) as well as in their mixtures with H2O via solvothermal polymerization. Depending on the type of solvent and the employed solvent composition, respectively, several physicochemical solvent properties (density, viscosity, polarity, and ionic product) can be intentionally adjusted to generate a plethora of morphologically different microparticlespartly with highly ordered structures down to the nanorangewhile maintaining full crystallinity. The method developed here is a highly valuable addition to the to date rather limited number of synthetic approaches toward high-performance polyimides and, as we believe, for cyclocondensation polymers in general.

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INTRODUCTION High-performance polymers (HPPs) are defined as fully organic macromolecular materials showing outstanding thermal stability of >500 °C under N2 atmosphere.1 In fact, HPPs can compete with certain metals in terms of maximum service temperature (MST)only that they are much lighter (see Figure 1A). This exceptional thermal resistance of HPPs has its origin in their molecular buildup comprising exclusively strong functions. The highest possible bond energies accessible in an all-organic material can be obtained through aromatic, heteroaromatic, and heterocyclic moieties. HPPs that exclusively contain aromatic and heterocyclic functions are so-called polyheterocyclics. Examples range from polybenzoxazoles, polybenzthiazoles, and polyquinoxalines to the industrially most important representatives, polybenzimidazoles and polyimides (PIs). A multitude of advanced devices for high-end technologies uses PIs. These applications are enabled through PIs having several intriguing material properties in addition to their high thermal stability. PIs are (i) excellent electrical insulators,2 (ii) resistant to harsh chemical environments (e.g., corrosive electrolytes),3,4 (iii) resistant to several types of radiation (e.g., cosmic radiation, Xrays),5,6 and (iv) dimensionally stable at increased temperatures.7 Moreover, PIs (v) exhibit tensile strengths and Young’s moduli that are at the upper end of what is possible for fully organic materials.8 These material properties result in PIs being used in any modern communication device (e.g., cell phones, tablet computers, and laptops) in both chip components and the wiring, to name but the most prominent examples of classical PI applications. In addition to this, in © XXXX American Chemical Society

recent years, PIs also became of significant interest for energy storage applications.9,10 The synthesis of PIsand also of all other polyheterocyclicstypically relies on methods where the heterocyclic moieties are formed during polymerization, that is, by cyclocondensation reactions. To form a (linear) PI, the required monomers are aromatic diamines and aromatic bis anhydrides or derivatives thereof (see Figure 1B). During PI synthesis, these monomers undergo polycyclocondensations, which are step-growth polymerizations accompanied by the release of the corresponding low-molecular weight condensation by-products (e.g., H2O or HCl). In the vast majority of cases (and industrially exclusively), PIs are generated by the poly(amic acid) (PAA) route, where the monomers are reacted to a chain-flexible and processable PAA in an aprotic polar solvent (such as dimethyl formamide, DMF; N-methyl pyrrolidone, NMP; or m-cresol), often in the presence of a condensation promotor (a non-nucleophilic base) such as isoquinoline (see Figure 1B). The resulting PAA solution is then shaped, for example, cast into films, and further cured by applying multistep heating protocols. To date, there is only a few reported environmentally-friendly alternatives to the PAA route: (i) polymerization in ionic liquids,11 (ii) solid-state polycondensation without the need of any solvents,12,13 and (iii) hydrothermal polymerization (HTP) in H2O as the sole solvent.14,15 HTP enjoys a prominent position within all PI synthesis techniques, as it generates full crystallinity and as it Received: May 29, 2019 Revised: July 17, 2019

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DOI: 10.1021/acs.macromol.9b00985 Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. (A): Comparison of MSTs for certain representatives of metals and polymers, respectively. MSTs of HPPs as well as of the high market value polymers, polyethylene (PE), polyethylene terephthalate (PET), and polytetrafluoroethylene (PTFE), are indicated for comparison. (B): Classical synthesis of PIs via the PAA route, relying on toxic and harmful solvents and condensation promotors.

which is beneficial for diffusion-controlled reactions;24 (ii) the polarity (as reflected by the static dielectric constant ϵ) continuously decreases,25 allowing for a better dissolution of apolar organic compounds that are virtually insoluble in ambient H2O;26−28 (iii) the ionic product KIP of H2O [c(H+)· c(OH−)] increases.29 At 250 ° C, KIP is 3 orders of magnitude higher than at room temperature (rt), which is why in hydrothermal (HT) reactions H2O is not only an ordinary solvent but can additionally act as an acidic, basic, or even acido-basic bicatalyst. This is extremely beneficial for various condensation reactions and makes the use of condensation promotors or catalysts virtually obsolete. Clearly, the absence of toxic solvents and promotors, and instead the use of solely H2O, makes HTP inherently environmentally benign and significantly cheaper and safer than classical PI syntheses. Most interestingly, HTP generates fully crystalline materials. Hydrothermally obtained crystallinities are yet unmatched by products from any other route. This is an explicit advantage, as crystallinity significantly enhances several material properties such as thermal, mechanical, and chemical stabilities. This increase in performance is related to the fact that for degrading a crystalline material, in addition to the sum of bonding energies, the lattice energy also has to be furnished.30 These enhanced material properties imparted by the outstanding crystallinity are most important for the future perspective of the synthetic approach, as only such novel and green processes have realistic chances to replace the existing methods if they yield products of at least equal or even improved quality. Hence, we have set out to develop a new route that also gives property enhancement through outstanding crystallinity, and we herewith report the environmentally benign solvothermal polymerization (STP) of PIs. Note that the term “solvothermal” refers to using a solvent above its boiling point as reaction medium.31 While others used solvents such as mesitylene, NMP, or diphenylsulfone at high temperatures, yet below the solvents’ boiling points, for PI synthesis,32,33 we herein report the first example of an actual solvothermal PI synthesis. For ensuring to establish a process that is as environmentally friendly as possible, we decided to only deal with cosolvents

has already been demonstrated to be applicable for other material classes also. Clearly, the number of different synthetic strategies toward PIs is still very limited, i.e., there are not much preparation techniques to choose from. However, a large synthetic toolbox for a material class of such striking technological relevance as PIs is of utmost importance. First, the type of synthetic route has an impact on material properties such as the obtained molecular weight or morphology. Second, when tuning a material synthesis, one often wants to employ additives such as stabilizers, plasticizers, or porogens. For the various conceivable additives in any polymerization, it is advantageous to have an extended set of different syntheses for selecting one that is compatible with the additives one desires to use. Third and of utmost relevance nowadays, environmental and health concerns arewith all due rightpushing toward more benign syntheses. Hence, an in the best case, “green” expansion of the synthetic toolbox toward PIs is clearly desirable. With this contribution, we have set out to develop another environmentally benign synthetic pathway toward PIs. As starting point, we decided to take inspiration from HTP, as the technique (i) is environmentally friendly by using H2O as the only solvent, (ii) generates superior material properties through PI products that are fully crystalline, and (iii) is not limited to generating imide functions. Aside PIs,14,16 one can hydrothermally also synthesize polyamides,17 dyes, and pigments,18,19 and even organic−inorganic hybrid materials,20 all at superior crystallinity. As we briefly summarize in the following, the fact that HTP generates such impressive crystallinities is strongly related to the protic nature of H2O. Therefore, we have chosen to investigate the feasibility of PI synthesis in other solvents that are both protic and have little impact on environment and health. In a typical HTP experiment, H2O and the monomers are enclosed in an autoclave and heated to elevated temperatures (T > 100 °C). Because of the isochoric conditions, increased pressure (p > 1 bar) arises autogeneously.21 With elevating T, the hydrogen-bonding network in H2O(l) steadily breaks down, which leads to significant changes of several physicochemical properties of H2O:22,23 (i) viscosity η and density ρ decrease, B


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Figure 2. STP of PPPI: A MS precursor is formed in the first reaction step from the comonomers PMDA and PDA in H2O. In the second reaction step, the MS is dispersed in the solvent of choice and subjected to STP in an autoclave to give PPPI as the product.

Figure 3. HTP of benchmark PPPI without a cosolvent. Sample polymerized at TR = 200 °C and tR = 4 h: (A) schematic of a product mixture containing glass liner after HTP. SEM micrographs of a- (B) and b- (C) phase for benchmark PPPI as well as of MS (D).

(SEM) was performed to investigate the obtained morphologies. Synthetic Aspects and the Benchmark System. For synthesizing PPPI, the synthetic scheme depicted in Figure 2 was employed. In an initial reaction step, the monomers pyromellitic dianhydride (PMDA) and para-phenylene diamine (PDA) were brought into contact in H2O (no other solvents or solvent mixtures were used for this step) at 80 °C under ambient conditions to form a monomer salt (MS) of ideal 1:1 stoichiometry via an acid−base reaction. MS and its characterization data are well known and hence all necessary reference spectra can be found in the literature.13 After isolating and drying the MS, the actual STP was carried out in a subsequent reaction step. Therefore, the ground MS was dispersedtypically at a concentration c(MS) of 0.03 mol/L unless mentioned otherwiseby magnetically stirring it in the solvent or solvent mixture of choice. The resulting dispersion was then placed in a glass liner, which was put into a teflonlined nonstirred batch autoclave, and transferred to an oven preheated at the desired reaction temperature TR of 200 °C. After an appropriate reaction time tR (tR = 4 h for reactions in pure H2O), the autoclave was quenched in cold tap H2O to stop the reaction. For the benchmark system (solely H2O as solvent; no cosolvent), the reaction always yielded three different, distinct phases inside the glass liner, as illustrated in Figure 3A (see Supporting Information for photograph): an orange, rather dense a-phase, which stuck to the bottom; a slightly suspended brown b-phase above; and a clear supernatant of red-to-purple hue (c-phase). The macroscopic aspect of the PPPI crude products was in perfect agreement with previous reports.14,41 The phases were separated via pipetting, washed, and dried. For a regular HTP in solely H2O, the a-phase typically accounted for approximately 90−95 wt % of PPPI and the b-phase for around 10−5 wt %. The supernatant’s color can be attributed to a very small amount of dissolved, strongly colored oligomeric species, which are formed via the previously reported oxidative autopolymerization of PDA during HTP.14 The ATR-FT-IR and PXRD

that can currently be obtained from renewable resources: ethanol, iso-propyl alcohol, and glycerine.34,35 It should be noted that the majority of literature employing the term STP deals with the radical polymerization of different vinyl monomers in various solvents ranging from acetone and ethyl acetate to DMF. 36−39 In addition to this, the solvothermal ring-opening polymerization of poly(L-lactide) in toluene and chloroform has also been reported.40 However, none of the latter reports allows for generating cyclocondensation polymers and even less under environmentally friendly conditions. In stark contrast, we herewith show that by just using the benign solvents, ethanol, iso-propyl alcohol, and glycerinealone or in mixtures with H2Othe beforementioned physicochemical properties η, ρ, ϵ, and KIP can be tuned with impressive effects on the product.

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RESULTS AND DISCUSSION

For this study, we decided to use a well-known and characterized model system: the fully aromatic PI poly(paraphenylene pyromellitimide) (PPPI) (Figure 1B). The subsequent sections provide an overview of several sets of experiments employing different protic polar, nonaqueous solvents as both cosolvent (together with H2O) and pure solvent for STP. The following media were tested: ethanol (EtOH), iso-propyl alcohol (iPrOH), as well as glycerine (1,2,3-propanetriol). Protic polar systems are required for the STP of PIs, as they promote imide formation and are expected to support crystallinity.30 For EtOH and iPrOH, solutions of the following composition were prepared: H2O/solvent (v/v) = 1:9, 3:7, 1:1, 7:3, 9:1. In addition to this, experiments in pure EtOH, pure iPrOH, and pure glycerine were carried out as well as a reference experiment in pure H2O (hereafter termed as “benchmark system”). All samples were thoroughly characterized: total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy was used to monitor the progress of the reaction as well as the presence of undesired compounds. Powder X-ray diffraction (PXRD) was employed to study the samples’ crystallinity, and scanning electron microscopy C


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Figure 4. ATR-FT-IR spectra and PXRD patterns of PPPI synthesized in EtOH and EtOH/H2O = 1:1, respectively, as well as in H2O at TR = 200 °C and tR = 4 h. Only the ATR-FT-IR spectra and PXRD patterns of a-phases are shown (those of all b-phases are identical but are for reasons of better clarity not depicted). Gray boxes highlight signals that correspond to MS. (A) Typical MS modes in ATR-FT-IR spectra: △: ν̃as(Ar-NH3+), ○: ν̃s(Ar-NH3+), ◊: ν̃as(CO, Ar−COOH+), □: ν̃s(CO, Ar−COO−), and ▷: ν̃as(CO, Ar−COO−). The characteristic PPPI modes in ATRFT-IR spectra are highlighted by green dots: ν̃as(CO) ≈ 1775 cm−1, ν̃s(CO) ≈ 1720 cm−1, and ν̃a(C−N) ≈ 1365 cm−1. (B) PXRD patterns of fully converted samples (synthesized in EtOH/H2O = 1:1 and H2O) are in complete accordance with the pattern simulated from the crystal structure from the literature.13

that stuck to the bottom of the glass liner, (ii) a brownish, solid b-phase on top of the a-phase, and (iii) a liquid supernatant (cphase) of dark reddish color. In contrast, the reaction performed in pure EtOH gave only one isolable solid phase (a-phase) and an intensively colored liquid phase (c-phase). Compared to the H2O-based benchmark system, both liquid cphases were significantly darker in color in the presence of EtOH (for both pure EtOH and EtOH/H2O = 1:1). We attribute this to the higher solubility of O2 in EtOH compared to H2O43 and infer that the oxidative autopolymerization of PDA seems to occur to a higher extent than in pure H2O. Moreover, in the performed STP experiments also, the volume ratio of the two solid phases seems to be different from PPPI obtained via benchmark HTP: the more EtOH was used (comparison of pure H2O, EtOH/H2O = 1:1, and pure EtOH), the less amount of b-phase was observed. According to ATR-FT-IR analysis (Figure 4A), both solid phases (a- and b-phase) obtained in the EtOH/H2O = 1:1 mixture show full conversion of MS to PPPI. The corresponding spectra revealed the modes of the cyclic imides, most prominently: ν̃as(CO) ≈ 1775 cm−1, ν̃s(CO) ≈ 1720 cm−1, and ν̃a(C−N) ≈ 1365 cm−1 (highlighted by green dots in Figure 4A). For reasons of comparison, Figure 4A also depicts the ATR-FT-IR spectra of pure MS and PPPI obtained via HTP (in solely H2O). In contrast to the sample prepared in EtOH/H2O = 1:1, the a- and b-phase obtained in pure EtOH still showed residual MS modes, among others ν̃as(CO, Ar− COO−) ≈ 1650 cm−1 (highlighted by a gray box in Figure 4A), in addition to PPPI’s characteristic imide modes. After analyzing the ATR-FT-IR spectra, we can thus conclude that the presence of EtOH has a significant influence on the rate of PPPI formation: EtOH seems to slow down the polymerization of PPPI. However, so far, it is not clear how PPPI is formed solvothermally and whether the obtained product is crystalline or not. To address the latter question, PXRD measurements of all solid phases were carried out (Figure 4B). For reasons of comparison, the PXRD patterns of

measurements revealed that both, a- and b-phase, are composed of highly crystalline PPPI (for a detailed explanation, see subsequent section). However, in terms of morphology, the two phases differ significantly from each other. As evinced by SEM, the a-phase (Figure 3B) is composed of platelets (5−20 μm) that are decorated with smaller platelets to a certain extent. The b-phase (Figure 3C) contains microflowers of around 5 μm coexisting with rather blank platelets. The microflowers are built up of thin petals with angular edges. Note that both solid PPPI phases are morphology-wise significantly different from the MS precursor which is composed of dendritically grown, elongated particles of 50−150 μm (cf. Figure 3D). On the basis of a morphological study over time, a dissolution−polymerization−crystallization mechanism is assumed for PI formation under HT conditions:14,41 at first, a small amount of MS dissolves because of the altered physicochemical properties of H2O at elevated T, polymerizes in solution, and crystallizes at various nucleation sites with the consumption of MS for polymerization. Then, more MS can dissolve, polymerize, and crystallize. These cycles repeat until all MS is consumed. Note that it was reported that if a suitable, H2O-soluble additive is present during HTP, it can adhere to the growing PI crystal surface and consequently induce split growth.41 Initial Screening Experiments for STP. For testing the general feasibility of using alcohols as reaction media for PI formation, initial EtOH-based screening experiments were carried out using (i) pure EtOH and (ii) a 1:1 (v/v) EtOH/ H2O mixture as solvents. Because EtOH is approximately as acidic as H2O, a significant shift in the equilibrium between H+ and OH− toward one side is not expected to occur by using EtOH/H2O mixtures.42 All other reaction parameters were kept identical as in the HTP of benchmark PPPI [c(MS) = 0.03 mol/L, TR = 200 °C, and tR = 4 h]. Similar to the benchmark system, the STP in EtOH/H2O = 1:1 also yielded three different phases: (i) a rather dense dark yellowish a-phase D


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to PPPI, tR was significantly increased to 12 h, whereas TR was kept constant at 200 °C and c(MS) at 0.03 mol/L. For the performed EtOH-based experiments, STP usually yielded a liquid c-phase of red-to-purple hue, as already observed in previous screening experiments. The more EtOH was used, the darker the liquid phase became. Moreover, what could only be assumed beforehand was confirmed in this series of experiments with different solvent compositions: by increasing the EtOH content, the weight ratio of the two solid phases formed during STP is shifted toward the a-phase. While for the benchmark HTP the b-phase accounted for around 5−10 wt %, in case of the STP in EtOH/H2O = 9:1, the amount of b-phase decreased to around 1−2 wt %. When employing only EtOH as solvent, proper isolation of a pure bphase was not possible anymore. In fact, a discernible b-phase has never been observed. From the ATR-FT-IR analysis (see Figure 5A for representative spectra; for spectra of PPPI synthesized using other solvent compositions, see Supporting Information), it can be concluded that elevating tR to 12 h is even for pure EtOHsufficient to achieve full conversion to PPPI, as all a- and b-phases obtained only show characteristic imide modes (see subsection) and MS modes are not present at all. Moreover, it should be noted thateven for the sample prepared in solely EtOHno residual solvent traces are found in the corresponding ATR-FT-IR spectra of crude products. In a recent study of additive-assisted HTP of PIs,41 we found that when the additive cetyltrimethylammonium bromide (CTAB) was used, −CH2− modes of CTAB alkyl chains were visible in the ATR-FT-IR spectra even after thorough washing cycles, allowing for the conclusion that CTAB was strongly adsorbed or even enclosed in the PI. Therefore, it is known that lowmolecular-weight compounds bearing −CH2− groups present in PPPI, such as EtOH used here, would actually be visible in the corresponding ATR-FT-IR spectra and would be discernible from PPPI. From the absence of such modes in the recorded spectra, we can conclude that in the STP reported here, EtOH is not adsorbed/included in PPPIeven without the need for excessive washing steps. As to be expected, PXRD measurements clearly evince high crystallinity for all phases of all samples (see Figure 5B for representative patterns; for patterns of PPPI synthesized using other solvent compositions, see Supporting Information). In accordance with ATR-FT-IR experiments, no reflections corresponding to the MS can be found in the PXRD patterns. Hence, the optimum reaction conditions for STP in EtOH/ H2O mixtures of any tested composition at c(MS) = 0.03 mol/ L are found to be TR = 200 °C and tR = 12 h, as under these conditions conversion to PPPI is complete and all obtained products are highly crystalline. The SEM analyses of the isolated a- and b-phases partly revealed significant morphological differences compared to benchmark PPPI. Figure 6 shows the corresponding micrographs of the samples prepared in EtOH/H2O mixtures, whereas the a- and b-phase of benchmark PPPI are depicted in Figure 3. The benchmark a-phase (Figure 3B) mainly contains platelets (5−20 μm) often decorated with smaller platelets, whereas the corresponding b-phase (Figure 3C) shows microflowers of around 5 μm with angular edges that coexist with blank platelets. In contrast to the H2O-based benchmark system, for the solvent mixture with the lowest amount of EtOH (EtOH/H2O = 1:9), the a-phase is composed of partially intergrown microsheets of around 5−20 μm, as depicted in Figure 6A.

pure MS and benchmark PPPI are shown in Figure 4B as well. As can be clearly seen, samples obtained in EtOH/H2O = 1:1 are highly crystalline, as only sharp reflections occur and broad halos are completely absent. Moreover, the PXRD pattern of PPPI prepared in EtOH/H2O = 1:1 is identical to the one of benchmark PPPI. In contrast to this, samples generated in pure EtOH revealed additional reflections which are highlighted by gray boxes in Figure 4B. These reflections, namely at 13.7° (110), 14.0° (111̅), 15.7° (111), 16.5° (112̅), 20.3° (113̅), 23.1° (020), 24.7° (312̅), 25.9° (203), 26.5° (313̅), 28.4° (402̅), and 31.6° (222) (2θ, Cu-Kα), are in accordance with the literature13 and can be attributed to the presence of residual MS. The PXRD results agree well with the performed ATR-FT-IR analysis, which also revealed residual MS in STP carried out in pure EtOH. These initial screening experiments demonstrate the general feasibility of alcohol-based STP of PIs. Most importantly, the generated products are as highly crystalline as PIs synthesized in pure H2O. However, it also becomes evident that with decreasing H2O and increasing EtOH content, the rates of PI formation are lowered, which in further consequence makes prolonged tRs necessary. One possible explanation for understanding this reduction of formation rates is related to the extent of autoprotolysis, that is, self-ionization, HA ⇌ H+ + A−, of the used solvents. Similar to H2O, other protic solvents that additionally contain lone pairs of electrons such as alcohols can also undergo self-ionization. As already mentioned for the example of H2O (see Introduction), the extent of selfionization typically increases with T. However, the corresponding ionic products KIPs of pure alcohols are known to be Table 1. Values for η, ρ, and ϵ at 25 and 200 °C and for KIP at 25 °C η [mPa·s] at 25 °C η [mPa·s] at 200 °C ρ [kg/m3] at 25 °C ρ [kg/m3] at 200 °C ϵ at 25 °C ϵ at 200 °C −log KIP at 25 °C

H2O 0.895 0.136 999.0 859.6 78.5 32.8 14.0

glycerine 905.7 2.131 1258 1151 40.3 19.4 15.5

EtOH 1.064 0.099 784.1 547.4 24.9 6.50 19.7

iPrOH 2.066 0.064 781.7 544.7 20.1 5.45 20.1

several orders of magnitude lower than KIP(H2O) (see Table 1 and Supporting Information for details).44,45 Therefore, it is clear that with increasing EtOH concentration, the total ionic product of the medium (i.e., the EtOH/H2O mixture) must decrease. We now hypothesize that because of the reduced amount of ions, which act as condensation promotors, the reaction rates of PI formation also decline. However, there is also another major contributing factor that should be taken into consideration for understanding the slower reaction rates: the solubilities of MS, oligomeric, and polymeric PPPI species in EtOH under the prevalent reaction conditions. For addressing this issue further, a more detailed study investigating the effect of EtOH content on PPPI conversion, crystallinity, and morphology was performed. STP in Water-Ethanol Mixtures and Pure Ethanol. In the next set of experiments, we studied a broad range of solvent compositions. Specifically, EtOH/H2O mixtures of the following ratios (v/v) were used as solvents: 9:1, 7:3, 1:1, 3:7, 1:9, as well as pure EtOH. For ensuring full conversion of MS E


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Figure 5. ATR-FT-IR spectra and PXRD patterns of representative a-phases of PPPI synthesized in EtOH/H2O = 1:9 and pure EtOH at TR = 200 °C and tR = 12 h. Note that the spectra and patterns of b-phases, which are always identical to the corresponding a-phases, are not depicted here for better clarity. (A) ATR-FT-IR spectra. (B) PXRD patterns.

Figure 6. SEM images of PPPI from EtOH-based STP. Reactions were performed with different amounts of EtOH at TR = 200 °C and tR = 12 h. a(A) and b- (B) phase for EtOH/H2O = 1:9; a- (C) and b- (D) phase for EtOH/H2O = 3:7; a- (E) and b- (F) phase for EtOH/H2O = 7:3; a- (G) and b- (H) phase for EtOH/H2O = 9:1; a-phase (I) and zoom in surface structure (J) for pure EtOH.

the majority of the a-phase platelets (Figure 6C). Only a few blank platelets of approximately 5−20 μm remain. In contrast to this, the corresponding b-phase is dominated by the novel type of microflowers with curved petals. It seems that these petals are thinner and also more densely packed (Figure 6D) compared to the EtOH/H2O = 1:9 sample (Figure 6B). Moreover, it can be seen that these b-phase microflowers have an almost spherical shape and are often heavily agglomerated to form bigger structures. Upon elevating the EtOH content even further to EtOH/H2O = 1:1 and 7:3, respectively, the near-spherical b-phase microflowers get even denser, which translates into an overall decrease in the surface area per particle. Starting from the EtOH/H2O = 1:1 sample, single microflower petals are often not even visible anymore. Moreover, these particles resemble spheres with a rough surface and are with around 10−40 μm in diameter,

These microsheets are to a certain extent decorated with small, rather roundish microflowers of 1−3 μm. These types of microflowers in the a-phase have never been observed beforeneither for benchmark PPPI nor in case of additiveassisted HTPand are built up of well-defined interstacked petals with sharp edges. In general, this results in a rather inhomogeneous phase compared to PPPI obtained via regular HTP (see Figure 3A). The corresponding b-phase (Figure 6B) mainly contains microflowers that are rather similar to the ones found in the benchmark system. In addition to this, a new sort of microflowers starts to appear. These particles are not only slightly bigger (around 7−15 μm) compared to the classical microflowers but also their petals are more curved. When increasing the EtOH content to EtOH/H2O = 3:7, the a-phase microflower-like structures become significantly more dominant, are heavily intergrown, and decorate almost completely F


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EtOH concentrations, entire fragments of these hollow structures remain clearly visible. In the presence of EtOH as solvent, these particles are furthermore decorated with very small, blade-like crystallites (approximately 150 nm in length). This lies in contrast to HTP, for which fragments of hollow MS copies occurring in the early phases of HTP (i.e., at tR = 2 h) surface decoration with nanorange-oriented crystallites have never been observed. In the b-phase, the presence of EtOH as cosolvent leads to a significant change not only with regard to the relative amount of the b-phase but also in terms of the morphology of the obtained microflowers. While in the benchmark system the b-phase is composed of microflowers with angular petals, the microflowers synthesized in solvent mixtures of low EtOH content exhibit curved petals. Upon increasing the amount of EtOH, the microflowers become more dense and finally the morphology gradually changes toward spherical objects with rough surfaces. It is just found that for the two highest tested EtOH concentrations a small amount of flower-like structures is formed again. However, in the case of pure EtOH, the amount of b-phase becomes so small that it is virtually impossible to separate the two phases from each other. From these morphological observations, we were attempting to gain a deeper understanding of the effect of EtOH during STP reactions. The significant morphological differences between MS and all obtained product phases clearly imply that PPPI formation still occurs via a dissolution−polymerization−crystallization mechanism (see Synthetic Aspects and the Benchmark System). The transformation of MS to polymer via the well-known solid-state polymerization (which could technically occur in dispersion) can be excluded, as shapecopying of dendritic, dense MS particles is not observed.12,13 However, despite the fact that ρ and η of EtOH (see Table 1 and the Supporting Information) are lower compared to those of H2O, the diffusion lengths of dissolved MS, oligomeric, and polymeric PPPI species seem to decrease upon raising the concentration of EtOH. From the SEM images of samples prepared in solvent mixtures of high EtOH content, we infer that the majority of PPPI polymerization and crystal growth must occur close to the site of initial MS dissolution (see Figure 8D for a schematic illustration). We assume that at the beginning of the reaction, typically residual, undissolved MS must act as primary heterogeneous nuclei for PPPI crystallization. Only a very small amount of PPPI can crystallize homogeneously in solution and form microflowers (at a low EtOH content and for pure EtOH) or spherical particles (in EtOH/H2O mixtures of medium-to-high EtOH content). Also, further crystal growth mainly occurs on the initial PPPI layers which are formed on top of the MS particles. The observed perpendicular orientation of petals with respect to subjacent PPPI sheets stems from a geometrical selection process that is not only well known to occur during the crystallization of natural minerals46 but also for PPPI formed under HT conditions.14 When small PPPI crystallites start to nucleate on bigger ones, they grow faster in the perpendicular direction than in the parallel one. This can be ascribed to the fact that in close proximity to the parent crystallite the concentration of PPPI has already been lowered because of crystallization in the early stages of PPPI formation, whereas at a higher distance to the parent crystallite the PPPI concentration remains higher until a later stage of PPPI formation. As the diffusion lengths of several involved species seem to be lowered with increasing EtOH content, the formed

significantly bigger than the corresponding b-phase microflower particles at a lower EtOH content (Figure 6F). Surprisingly, when performing STP in these solvent mixtures, in the a-phase, neither isolated nor undecorated sheets can be observed anymore. Instead, we find comparatively big particles (15−30 μm) which exhibit surfaces that are heavily decorated with smaller petals (0.2−1 μm) oriented rather perpendicular to the subjacent surface. Furthermore, these petals start to show domains with a high degree of parallel orientation (Figure 6E, zoom). These oriented petals slightly resemble the morphologies that were observed using the additive PEG8000 (polyethylene glycol of M n = 8000 g/mol) at a high concentration during HTPwith the only exception that particles synthesized via additive-assisted HTP are generally smaller in size and more spherical.41 At the highest tested EtOH concentrations (EtOH/H2O = 9:1 and pure EtOH), the morphology of the a-phase changes significantly: angular, dendritic-like microparticles are basically found exclusively (Figure 6G,I). These microparticles strongly resemble fragments of the corresponding MS crystals (see Figure 3D) and appear to be hollow (cf. Figure 6J). Fascinatingly, their surface is covered with small, sharp blade-like crystallites of ca. 150 nm in size that seem to assume a common direction. It should be noted that these angular particles have a broad size distribution, whereas their surface features (i.e., the blade-like crystallites) have a narrow size distribution. Most likely, the broad microparticle size distribution can be attributed to (i) the broad size distribution of initial MS crystals and (ii) inhomogeneous fracturing of particles during STP. Moreover, it can be seen in Figure 6I that besides angular particles a very small amount of flower-like structures also occurs. As already mentioned before, it was found that with increasing EtOH content the amount of b-phase significantly decreases. Although in case of EtOH/H2O = 9:1 it was still possible to separate the two phases rather well via pipetting and rinsing, in case of the sample generated in pure EtOH, proper phase separation could not be achieved. As can be seen in Figure 6H for the EtOH/H2O = 9:1 sample, the b-phase mainly contains dense, spherical structures similar to the ones obtained in EtOH/H2O = 1:1 and 7:3. In addition to this, surprisingly some microflowers (5−10 μm) with well-defined curved petals also start to reoccur at this solvent composition. However, it should also be noted that the appearance of some isolated angular particles in Figure 6H can be attributed to difficulties in separating the two phases when STP was performed in EtOH/H2O = 9:1 as reaction medium. From this extended concentration study, it becomes evident that the employed amount of EtOH has a significant influence on both phases during PPPI formation in STP. It can be seen that with increasing EtOH concentration, at first the microhsheets in the a-phase become more and more decorated with smaller petals. While at low EtOH concentrations these petals are arranged into flower-like structures, they are oriented more in a perpendicular way with respect to the microsheets and fully cover them at medium EtOH concentrations. At a certain point, the petals even start to show domains with a high degree of common orientation. Upon further elevating the EtOH content, a gradual transition mainly toward hollow particles that very much resemble the MS morphology is observed. Similar structures have already been reported to occur intermediately during the early stages of regular HTP of PPPI.14 However, with increasing tR, they successfully vanished in HTP. In contrast to this, for the STPs performed at high G


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Figure 7. SEM images of different STP experiments. Samples prepared at TR = 200 °C and tR = 12 h. (A) a-phase of PPPI synthesized at a lowered c(MS) = 0.01 mol/L in EtOH; (B,C) a-phase of PPPI synthesized at regular c(MS) = 0.03 mol/L in iPrOH; (D,E) a-phase of PPPI synthesized at regular c(MS) = 0.03 mol/L in glycerine.

for H2O, exact values for the T range of interest (25−200 °C) are reported in the literature:52 while at 25 °C, KIP(H2O) = 10−14, at 200 °C, the value increases to 5.0 × 10−12. In addition to this, from Table 1 it becomes evident that at 25 °C, KIPs of EtOH and iPrOH are significantly lower [around 6 orders of magnitude lower than KIP(H2O)], whereas KIP(glycerine) is comparatively high (KIP(glycerine) = 3.2 × 10−16). Furthermore, it can be clearly seen in Table 1 that for alcohols with decreasing number of −CH2−/−CH3 groups per hydroxyl moiety, ϵwhich can be considered as a measure of solvent polarityincreases. This trend can be observed over the entire T range of interest, although the relative reduction of ϵ with increasing T is significantly more pronounced for EtOH and iPrOH compared to glycerine and H2O. For our purposes, it is most important that at 200 °C iPrOH has a slightly lower ϵ than EtOH, whereas glycerine has a drastically higher one. Furthermore, EtOH and iPrOH exhibit ρ values that are significantly lower than the ones of H2O, whereas glycerine is drastically higher in ρ over the entire T regime of interest. For η, the differences are even more pronounced. While iPrOH and EtOH just have slightly lower η values than H2O, glycerine is significantly more viscous in the T range from 25 to 200 °C. It can be expected that all of these differences affect PPPI polymerization and crystallization, which will be elucidated in the following. In further consequence, a similar set of experiments was performed using iPrOH under typical conditions [c(MS) = 0.03 mol/L, tR = 12 h, and TR = 200 °C]. Interestingly, the liquid phase was completely colorless for all experiments. Because of the even even higher solubility of O2 in iPrOH compared to EtOH,43 a complete absence of oxidative autopolymerization seems rather unlikely. Therefore, we assume that here instead of oligomeric dissolved species insoluble products of higher molecular weight (in the order of a few ppm) form upon polymerizing oxidatively which consequently precipitate out of solution. However, their concentration is expected to be so low that we cannot confirm this hypothesis. For iPrOH-based experiments, the amount of b-phase is even lower than that in the corresponding EtOHbased experiments. PPPI synthesized in pure iPrOH as well as in iPrOH/H2O = 9:1 does not show an isolable b-phase. Again, for these solvent compositions of high alcohol content, only one solid a-phase is obtained. According to ATR-FT-IR analysis and PXRD measurements (see Supporting Information for the spectra and diffractograms), all solid phases are fully converted to PPPI and are highly crystalline. The ATRFT-IR spectra and PXRD patterns do not show traces of unreacted MS or any other impurities (e.g., residual solvent). The SEM analysis (see Supporting Information for micro-

petals also get smaller in size (cf. SEM in Figure 6C,J). In addition to this, the rates of dissolution, polymerization, and crystallization are also reduced by the presence of EtOH. Consequently, one could expect that with increasing EtOH content, the original shape of MS particles is preserved for a longer time, as a result of which entire fragments of MS microparticles can act as solid templates for PPPI crystallization. If at a certain, high enough EtOH concentration the MS remains undissolved long enough, one can indeed find hollow microparticles of identical shape in the product phase. To further confirm the hypotheses of (i) reduced solubilities and (ii) templating effect, we decided to run an additional STP experiment in pure EtOH. The usual conditions (TR = 200 °C and tR = 12 h) were applied, whereas c(MS) was reduced from 0.03 to 0.01 mol/L. The SEM analysis of the obtained solid phase indeed revealed that for PPPI generated under these conditions less fully preserved microparticles resembling MS are found (see Figure 7A), whereas the nanosized surface features are still prevalent. At this point, the control that can be obtained over PPPI morphology in terms of crystallite size and orientation while keeping the overall crystallinity by simply adjusting the solvent composition is quite impressive. For investigating how far the approach of STP with its morphological control could be pushed, we performed a last set of experiments moving to other alcohols. Specifically, we employed iPrOH as cosolvent with H2O and pure solvent as well as glycerine as pure solvent, respectively. Investigating the Influence of Solvent Properties on STP. After demonstrating the general feasibility of alcoholassisted STP of PPPI, we were aiming for a deeper understanding of the fundamental underpinnings while simultaneously exploring further possibilities of morphological tuning. However, it would not only be interesting to obtain microparticles with novel types of structures and morphologies but also to get morphologically pure phases (unlike the product obtained in solely EtOH, where microflowers are present next to hollow, dentritic particles). To achieve the latter, it is either necessary to obtain two phases with significantly different densities that separate well or to tune the STP process in a way that only one homogeneous phase is created. Therefore, we decided to keep our experimental setup as simple as possible and just utilize alcohols of different solvent properties as (co)solvents. Table 1 lists the η, ρ, and ϵ values at 25 and 200 °C for H2O, EtOH, iPrOH, and glycerine as well as KIPs of the just mentioned solvents at 25 °C. All values are taken or calculated from the literature (see Supporting Information for more details).47−51 As already discussed earlier, an elevation of T generally leads to an increase of KIP. However, to the best of our knowledge, only H


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Figure 8. Illustration of the mechanistic ongoings during STP in all four pure solvents. The blue arrow indicates MS dissolution, whereas the orange ones represent polymerization. Crystallization of PPPI can either occur on top of undissolved MS particles or homogeneously in solution. The arrow thicknesses indicate the degree of solubility and the ratio of the two different crystallization pathways. The circles represent the diffusion lengths of MS (red) and oligomeric/polymeric PPPI species. Note that the radii of diffusion circles and arrow sizes are relative and are only to be compared with the same process in another solvent. (A) General overview, introduction of the schematic as well as comparison of the four crucial parameters η, ρ, ϵ, and KIP for the used solvents. (B−E) Illustration of the different scenarios for H2O, glycerine, EtOH, and iPrOH.

other reaction parameters constant (c(MS) = 0.03 mol/L, TR = 200 °C). Note that the used TR of 200 °C lies significantly below glycerine’s boiling point of 290 °C. In contrast to the previously tested alcohols, in case of glycerine, intense washing was necessary prior to analysis for completely removing traces of the solvent. Surprisingly, it turns out that even a tR of 4 h is sufficient to achieve full conversion of MS to highly crystalline PPPI. Furthermore, in contrast to the previously investigated pure alcohols EtOH and iPrOH, for glycerine, two phases were obtained after STP that can easily be separated. The b-phase exclusively contains microflowers rather similar to the benchmark system, whereas the a-phase shows intriguing, novel morphological features. Interestingly, contrary to the previously tested pure alcohols, no well-defined, hollow features can be observed in the a-phase generated in glycerine. Instead, we find microparticles of different size and shape (20− 100 μm) that are heavily decorated with intergrown, roundish microflowers (Figure 7D,E). On the basis of these observations, we assume that in glycerine all involved species have higher solubility and increased reaction rates compared to EtOH and iPrOH. MS is still expected to be the primary nucleation site for PPPI crystallization. The comparatively high ρ (around 35% higher than the one of H2O according to Volk)48 and η (around 17 times higher than the one of H2O according to Cheng)47 of glycerine at 200 °C might hinder the effective diffusion of the dissolved species, which consequently leads to crystallization in the form of intergrown microflowers on top of MS particles. However, under the existing conditions of rapid dissolution, shape-copying of entire MS particles cannot occur. After performing experiments in various alcohols, it is fair to say that by adjusting solvent properties not only the rate of polymerization but also the obtainable morphologies can be intentionally tuned. Because of the protic nature of the investigated solvents, STP always proceeds via a dissolution− polymerization−crystallization mechanism, and the wellknown solid-state polymerization of MS has never been observed to occur. In general, it can be said that with decreasing solvent polarity (decreasing ϵ) and decreasing promotor concentration (decreasing KIP) the reaction and

graphs) revealed similar trends as found in the EtOH-assisted STP of PPPI for both phases. In the comparatively weakly pronounced b-phase, we observed different types of microflowers. For low iPrOH concentrations, the microflowers are rather similar to benchmark PPPI, whereas with an increasing amount of iPrOH, they become bigger and their petals appear more curved. However, completely dense spherical structures as found in EtOH-based STP for medium EtOH concentrations (EtOH/H2O = 1:1 and 7:3) do not occur. In the aphase, the extent of decoration of microsheets increases from low to medium iPrOH content. At high iPrOH content, mainly hollow particles, the shape of which again resembles the one of MS, are found. These hollow particles (Figure 7B,C) have a drastically different surface morphology compared to the ones obtained in EtOH. Whereas EtOH gives rise to sharp, bladelike crystallites, here we obtain small knob-like particles for an iPrOH/H2O = 9:1 mixture and rather pyramidal nanostructures for pure iPrOH (Figure 7C). Furthermore, the a-phase synthesized in solely iPrOH shows a significantly lower content of microflowers compared to the one generated in EtOH. On the basis of this observation, we assume that by lowering solvent polarity [ϵ(iPrOH) < ϵ(EtOH)] and KIP, the solubility of MS, oligomeric, and polymeric PPPI species is further reduced. Because of the accompanying decrease in diffusion length, even less amount of PPPI can polymerize and crystallize homogeneously in solution, which is why less microflowers are observed in the a-phase prepared in pure iPrOH. Instead, the vast majority of PPPI crystallizes close to the site of initial MS dissolution, after it reacted toward PPPI in solution (see Figure 8 for a schematic illustration). Despite the assumed lowered solubility values, polymerization in iPrOH does still occur within a reasonable time frame of 12 h. Following the just introduced logic, if employing an alcohol of higher polarity (higher ϵ) and higher KIP, such as glycerine, we would expect a better solubility of the involved species, which might result in significantly different morphologies compared to EtOH and iPrOH. Therefore, we performed more STP experiments in pure glycerine under the usual experimental conditions and additionally investigated the rate of product formation by varying tR (4, 12 h), while keeping all I


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macromolecules), solvent density and viscosity, diffusion lengths, as well as solvent autoprotolysis into account. All used solvents can be considered green because of their nontoxicity as well as the fact that they can be generated from renewable resources. STP is inspired by HTP, which uses solely H2O as solvent. HTP is not limited to the generation of crystalline PIs but was shown to successfully and with striking experimental simplicity lead to dyes and pigments (even comprising fused heterocycles),18,19 polymers,17 and even inorganic−organic hybrid materials.20 Hence, we expect STP also to be applicable for a vast array of T-stable, organic materials. Moreover, HTP can be optimized for a certain morphological outcome by employing additives.41 We expect STP to be in an at least equal measure tunable by using additives. Overall, we herewith have laid the basis for the synthesis and morphological control of a wide range of crystalline materials.

crystallization rates as well as solubilities are reduced. This makes longer tRs necessary and simultaneously leads to an impressive extent of templating to finally yield hollow particles with intriguing, highly structured morphological surface features. Generally, one would expect that diffusion lengths mainly depend on η. Despite EtOH and iPrOH having the lowest η of all media investigated here, we observe morphologies that indicate shorter diffusion lengths. All used solvents are highly hydrogen-bonded systems thatat rt even present long-range order in the liquid state.53−57 For H2O, it is known that diffusion is significantly affected by the presence of ions both ways: some electrolytes increase and others decrease diffusion lengths in H2O. Although these observations are supported by both experiments and simulations, the reasons (e.g., structure-making vs structurebreaking, or spatiotemporal heterogeneities) are yet subject of intense studies.58 As EtOH, iPrOH, and glycerine are also hydrogen-bonded liquids, we believe that for them diffusion of various dissolved species will also be influenced by the presence of electrolytes, and the trends are expected to be equally complex and depending on the precise electrolyte. Therefore, we can only note that especially for EtOH and iPrOH the morphologies we observe point at diffusion lengths that are significantly smaller than those in H2O. In contrast to this, glycerine (which has a comparatively high KIP and ϵ) facilitates the formation of densely packed microflowers decorating the parent microparticles. These mechanistic ongoings including the crucial parameters of solubilities and diffusion lengths of all involved species are graphically summarized in Figure 8. In H2O, the solubilities of MS and the involved PPPI species as well as their diffusion radii are expected to be relatively high. Glycerine also features relatively high solubilities, but species seem to diffuse less than in H2O. In EtOH and iPrOH, both solubilities and diffusion lengths are even further reduced, generating fascinating nanostructured surfaces.

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ASSOCIATED CONTENT

MS Synthesis. In a three-neck flask equipped with a reflux condenser, 39.26 g of PMDA (180 mmol, 1 equiv) was suspended in 600 mL of deionized H2O, and the mixture was degassed by bubbling Ar for approximately 10 min. Subsequently, the suspension was heated up to 80 °C under Ar atmosphere. After approximately 1 h, the hydrolysis of PMDA to pyromellitic acid was complete which gave rise to a clear solution. The addition of 19.47 g of PDA (180 mmol, 1 equiv) immediately led to the formation and precipitation of an offwhite solid. The suspension was stirred at 80 °C for 4 h to achieve full conversion. Isolation via vacuum filtration, washing with deionized H2O, and drying gave the final MS as off-white solid. Yield: quantitative. 1 H NMR (250.13 MHz, DMSO-d6): δ [ppm] = 8.42 (s, 2H, ArPMAH), 6.79 (s, 4 H, ArPDA-H). 13 C NMR (250.13 MHz, DMSO-d6): δ [ppm] = 167.32, 135.29, 133.92, 132.95, 118.89. HTP and STPs. In a typical experiment, 163 mg of ground MS (4.5 mmol, c = 0.30 mol/L) was suspended in 15 mL of deionized H2O by magnetically stirring for approximately 10 min at rt. Subsequently, the liner was put into a teflon-lined autoclave (V = 45 mL) and placed in a preheated oven at the desired reaction temperature TR (typically 200 °C) without stirring. To stop the reaction after a certain reaction time tR, the autoclave was quenched in cold tap H2O. The obtained product phases were separated via pipetting, thoroughly washed with deionized H2O and subsequently EtOH, and finally dried in vacuo at 80 °C overnight. All other hydrothermal and solvothermal syntheses were carried out accordingly. To investigate the effect of solvent composition on PPPI morphology, the solvent H2O was replaced by solvent mixtures of different compositions. For preparing these solvent mixtures, the two protic, polar solvents, EtOH and iPrOH, were combined with H2O at different ratios. Typically, solutions of the following composition were prepared: H2O/solvent = 1:9, 3:7, 1:1, 7:3, and 9:1. In addition to this, experiments in pure EtOH, iPrOH, and glycerine were performed without adding H2O. Yield: quantitative.

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CONCLUSIONS With this contribution, we have shown that the fully aromatic polyimide PPPI can be generated in several protic polar solvents, that is, EtOH, iPrOH, and glycerine, as well as their mixtures with H2O, in the respective solvothermal regimes. For all presented solvent compositions, the reactions give quantitative yields. In all cases, PPPI is obtained at full crystallinity giving rise to an outstandingly high thermal stability (T95% = 610 °C; see Supporting Information). By adjusting nothing but the solvent composition, impressive morphological control can be exerted. PPPI microflower particles can be generated with either angular (high amount of H2O as cosolvent) or roundish (low amount of H2O or absence of H2O) microflower petals. Moreover, the density of microflower petals can be tuned. Most intriguingly, our study shows that the presented approachSTPis able to generate organized surface structures down to the nanorange. For instance, the use of pure EtOH as reaction medium generates surface structuring with nanosized platelets that are highly parallelly oriented and of impressively narrow size distribution (Figure 6J). Even more intriguingly, STP performed in pure iPrOH generates pyramid-shaped surface features in the nanorange, again of narrow size distribution. We additionally provide a comprehensive hypothesis for the formation of these morphologies, taking various aspects such as solvent polarity, solubility of involved species (monomers, oligomers, and

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.9b00985. Methods, chemicals, synthetic procedures, characterization (ATR-FT-IR, PXRD, and SEM), and a discussion of solvent properties (PDF) J


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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +43-(0)158801-165206. ORCID

Miriam M. Unterlass: 0000-0003-0494-7384 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge TU Wien, the Austrian Science Fund (FWF), and the Christian Doppler Research Association (CDG) for funding this project under grant no. PIR 10-N28. PXRD measurements were carried out at the X-ray Center of TU Wien (XRC) and SEM was performed at the interfaculty electron microscopy facility of TU Wien (USTEM).

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Highly Crystalline, Nanostructured Polyimide Microparticles via Green

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