Morphology-Controlled Self-Assembly and Synthesis of Biopolyimide Particles from 4-Amino-l-phenylalanine.

Self-assembling polyimides (PIs) having diketopiperazine (DKP) components were synthesized by polycondensation of a 4-amino-l-phenylalanine (4APhe) dimer, an aromatic diamine newly designed in this study. The amino acid-derived PIs showed high thermal resistance, with a 10% weight loss temperature (T d10) of 432 °C at the maximum, and did not show any glass transition below the thermal decomposition temperature. The poly(amic acid) (PAA) precursors formed nanospheres upon reprecipitation over dimethylacetamide into water. The nanospheres were then added to solvents with different polarities and sonicated to induce deformation of the spherical forms into spiky balls, flakes, or rods. The PAA particle morphologies were retained in the PIs after the two-step imidization. Finally, the PI particles with self-assembling DKP moieties were formed, and their morphologies were fine-tuned using different mixed solvents.

Introduction

The development of biobased polymers is essential for establishing a sustainable green society. Conventional biobased aliphatic polymers such as polyesters[1] and polycarbonates[2] have unsatisfactory thermal and mechanical properties, although several attempts have been made to improve these properties.[3−9] One of the most effective strategies in this regard is to incorporate an aromatic component into the polymer backbone.[6,7,10−13]

Polyimides (PIs), a class of super-high-performance plastics, are widely used in electronic devices and in aerospace applications because of their outstanding mechanical durability and high thermal and chemical stabilities, which enable them to tolerate harsh environments.[14−18] PIs were initially synthesized from petrochemical-based monomers, but there have been a few recent attempts to synthesize them from biobased monomers.[19−25] In particular, attempts have been made to prepare partial or completely biobased PIs using a biobased aromatic diamine, 4,4′-diaminotruxillic acid (4ATA), and various dianhydrides. 4ATA was synthesized by photodimerization of the biobased 4-aminocinnamic acid (4ACA), an aromatic amino acid obtained by the bioconversion of 4-aminophenylalanine (4APhe) using phenylalanine ammonia lyase (PAL), and a genetically engineered microorganism.[19] With regard to molecular design, we found that the alicyclic structure sandwiched between two aromatic rings imparted rigidity to produce thermally resistant PIs with a Td10 value of 425 °C, while the glass transition temperature, Tg, was found to be 350 °C, while retaining important functions, such as optical transparency, in the case of 4ATA. Despite the outstanding thermal stability, the low efficiency of the bioconversion step using PAL reduced the yield of 4ACA significantly. This motivated us to use 4APhe in the present study in order to eliminate the inefficient bioconversion step involving PAL. If 4APhe is dimerized, another aromatic diamine biomonomer can be synthesized.

In order to synthesize a structure in which heterocyclic compounds are sandwiched by two aromatic rings, we carried out the dimerization of 4APhe and obtained a 2,5-diketopiperazine (2,5-DKP) derivative. The core structure of several drugs contains central 2,5-DKP rings,[26−28] but 2,5-DKP has not been extensively used as a building block to synthesize functional polymeric materials.[29−32] In the present study, we have designed a monomer containing a centrosymmetric amide functionality in the 2,5-DKP ring, apart from two aromatic rings that can induce self-assembly of the corresponding polymer chains through hydrogen bonding and π–π interactions.[33−36] Particle formation can be expected as a result of the chain self-assembly, rendering these PIs suitable for applications such as fillers, heat resistant superhydrophobic coatings,[37] and ultralow-dielectric-constant films.[38]

Here, we report the synthesis of the biobased aromatic diamine 3,6-di(4-aminophenylmethyl)-2,5-diketopiperazine (DKP-4APhe) from 4APhe through a simple coupling of stepwise protection and deprotection. The corresponding PIs having the 2,5-DKP heterocyclic structure in the backbone are prepared by polycondensation with commercially available aromatic dianhydrides. The present study investigates the particle formation ability of the PIs and attempts to control their morphology starting from a spherical form into various nonspherical shapes.

Results and Discussion

Monomer Syntheses

A biobased aromatic diamine including a 2,5-DKP central core, DKP-4APhe, was synthesized by stepwise 4APhe dimerization (see Scheme ). First, the aromatic amine group in 4APhe was protected by benzyl chloroformate (z-) to obtain 4APhe-z. Successive protections of the carboxylic acid and α-amine groups were separately carried out by methanol (methyl-) in the presence of trimethylsilyl chloride (TMSCl) and by di-tert-butyl dicarbonate (Boc-) to obtain methyl-4APhe-z and Boc-4APhe-z, respectively. The linear dipeptide was then synthesized by amidation coupling of methyl-4APhe-z and Boc-4APhe-z in the presence of condensation reagents. In the last step, Boc was deprotected from the α-amine group, which was successively cyclized by a reaction with methyl ester to form the central 2,5-DKP ring (total yield; 29 wt %). It should be noted here that two other methods for cyclodimerization had been tried. Cyclodimerization of 4APhe with z-protection at aromatic amine was tried by heating at 170 °C in ethylene glycol, but the reaction did not proceed. By cyclodimerization using PCl5, 4APhe with z-protection at aromatic amine was refluxed for 4 h in tetrahydrofuran (THF) and PCl5. Although the reaction was carried out successfully, the purity of the obtained product was not satisfactory enough to be used for polymerization. As the polymerization requires high purity of monomers, an alternative approach was applied for the synthesis of the DKP monomer.

Scheme 1

Synthesis of a Biobased Aromatic Diamine Having a DKP Core, 3,6-Di(4-aminophenylmethyl)-2,5-DKP (DKP-4APhe) from 4-Amino-l-phenylalanine (4APhe)

The structure of the aromatic diamine was confirmed by 1H nuclear magnetic resonance (NMR), 13C NMR, Fourier-transform infrared (FTIR), and electrospray ionization-mass spectrometry (ESI-MS) analyses and was identical to the expected structure.

As a result of cyclization, the 1H NMR signals of the β-carbons of the 4APhe moieties (see Figure S1) shifted from 2.76–3.08 to 2.08 ppm, which coupled with the proton signals of 2,5-DKP at 7.63 (−NH−) and 3.80 ppm (−CH−). The proton signal at 4.89 ppm was attributed to the aromatic amine groups. The FTIR spectrum (Figure S2) showed N–H stretching, C=O stretching, and N–H bending bands of the amide group at 3570–3310, 1656, and 1459 cm–1, respectively. C=C and C–H stretching peaks of the aromatic ring manifested at 1516 and 3030–2870 cm–1, respectively. The formation of DKP-4APhe was also confirmed by 13C NMR (see Figure S3) and FT-ion cyclotron resonance (ICR)-MS (ESI) (see Figure S4). The observed signal at m/z 347.14764 was consistent with the calculated value 347.147642 for the species [M + Na]+.

Polymer Syntheses

Poly(amic acids) (PAAs), the precursors of PIs, were prepared by polycondensation of the prepared diamine, DKP-4APhe, with stoichiometric amounts of the dianhydrides, pyromellitic dianhydride (PMDA), 3,3′,4,4′-benzophenonetetracarboxylic dianhydride (BTDA), 4,4′-oxidiphthalic anhydride (OPDA), 3,3′,4,4′-diphenylsulfonetetracarboxylic dianhydride (DSDA), 3,4,3′,4′-biphenyltetracarboxylic dianhydride (BPDA), 1,2,3,4-cyclobutanetetracarboxylic dianhydride (CBDA), and 5-(2,5-dioxotetrahydrofuryl)-3-methyl-3-cyclohexene-1,2-dicarboxylic anhydride (DHCDA) (see Scheme ). The resulting PAAs are abbreviated as PAA–PMDA, PAA–BTDA, PAA–OPDA, PAA–DSDA, PAA–BPDA, PAA–CBDA, and PAA–DHCDA, respectively, as shown in Table . The 1H NMR and FTIR spectra of all the PAAs are shown in Figures S5 and S6, respectively. In the 1H NMR spectra of the PAAs, the main chain proton signals for the carboxylic acid, amide, cyclic amide groups of DKP, and the aromatic diamine group appeared at δ 12.2–13.2, 10.4–9.8, 8.1–7.9, and 7.5–7.0 ppm, respectively. PAAs derived from the aromatic dianhydrides, PMDA, BTDA, OPDA, DSDA, and BPDA, showed aromatic proton signals at δ 8.3–7.1 ppm in addition to the abovementioned peaks. On the other hand, PAAs derived from the aliphatic dianhydrides, CBDA and DHCDA, showed signals corresponding to cyclobutane and methylcyclohexene at δ 3.9–3.4 and 3.0–1.7 ppm, respectively. The proton signal at 5.5 ppm was assigned to the double bond in DHCDA.

Scheme 2

Syntheses of Biobased Aromatic PAAs and PIs from DKP-4APhe

Table 1

Molecular Weights of PAAs Derived from Biobased Aromatic Diamine, DKP-4APhe

dianhydrides	PMDA	BTDA	CBDA	DSDA	OPDA	BPDA	DHCDA
Mn (kDa)a	20.3	24.9	21.2	27.0	19.1	18.8	14.7
Mw (kDa)a	24.5	33.2	29.4	22.5	23.5	23.4	16.8
PDIa	1.2	1.3	1.4	1.2	1.3	1.2	1.1

Weight-average molecular weight, Mw, number-average molecular weight, Mn, and PDI of the dissolved PAA fraction in LiBr/DMF were measured by gel permeation chromatography (GPC).

The dimethylacetamide (DMAc) solution of the PAAs was used to prepare PAAs films by casting on silicon wafers and evaporating the solvent at 60–70 °C. The PIs were obtained via stepwise thermal imidization by maintaining the temperature at 100, 150, and 200 °C for 1 h each and at 250 °C for 3 h at each step in a vacuum oven. The color of the PIs changed from yellow to dark brown, making them darker than the PAAs. Figures S6 and S7 show the FTIR spectra of the PAAs and PIs, respectively. Figure S7 clearly indicates the imide ring formation, as confirmed by FTIR spectroscopy. In all the samples, the following peaks were observed: a broad band in the range 2500–3500 cm–1 (O–H stretching of carboxylate), two different carbonyl peaks at 1714 cm–1 (C=O stretching of carboxylate) and 1663 cm–1 (C=O stretching of amide), respectively, apart from two peaks at 1514 and 1437 cm–1 (aromatic C–H). After stepwise heating, all the annealed samples showed two carbonyl absorption peaks at 1712 cm–1 (C=O symmetric stretching) and at 1776 cm–1 (C=O asymmetric stretching), which were attributed to imide rings. The appearance of new peaks at 1373 cm–1 (C–N stretching of imide) and 1150 cm–1 (imide ring deformation), coupled with the disappearance of the characteristic amide peak, clearly indicates an imidization.

The solubilities of different polymers were investigated using 1 mg of samples in 1 mL of a specific solvent at two different temperatures: 20 and 60 °C. The PAAs were completely soluble in polar solvents such as concentrated sulfuric acid, N-methylpyrrolidone, DMAc, and dimethylsulfoxide (DMSO) at room temperature and partially in dimethylformamide (DMF). However, all the PIs were soluble only in acidic solvents such as trifluoroacetic acid and concentrated sulfuric acid.

The weight-average molecular weight (Mw), number-average molecular weight (Mn), and polydispersity index (PDI) were determined using the PAAs (see Table ). The PAAs had Mw and Mn values in the ranges of 16.8–33.2 and 24.9–14.7 × 103 kDa, respectively, while the PDI values ranged from 1.1 to 1.4. Note that only the dissolved fraction in LiBr/DMF was measured, in order to decrease the molecular weight and make the distribution narrower.

Thermal Properties

Thermogravimetric analysis (TGA) was used in order to study the thermal degradation of the PIs in a nitrogen atmosphere, using a heating rate of 10 °C/min. The 5 and 10% weight-loss temperatures, Td5 and Td10, were evaluated in each case. As shown in Table , all of the PIs exhibited Td10 values in the range 388–432 °C and Td5 values in the range 365–420 °C, which indicated a high degree of resistance toward thermal degradation. The PI derived from PMDA showed the highest Td10 value of 432 °C. In contrast, PI–DHCDA showed the lowest Td10 value, presumably because of the presence of fewer aromatic rings than the others, leading to more susceptible chain scission at elevated temperatures. From our previous research, the biopolyimides derived from 4ATA and PMDA, both showed a Td10 value of 425 °C,[19] which is lower compared to the values for the PIs in the present study. The higher thermal resistance observed for the PIs in this study can be attributed to the intermolecular forces from the DKP moieties.

Table 2

Thermal Degradation Temperatures of Biopolyimides Derived from DKP-4APhe

dianhydrides	PMDA	BTDA	CBDA	DSDA	OPDA	BPDA	DHCDA
Td5 (°C)a	420	411	392	383	398	401	365
Td10 (°C)a	432	427	415	397	414	414	388

5 and 10% weight loss temperatures, Td5 and Td10, were obtained from TGA curves scanned at a heating rate of 10 °C/min under a nitrogen atmosphere.

The thermal transition behavior of the PIs was investigated by differential scanning calorimetry (DSC) under a nitrogen atmosphere. There were no distinct peaks or points of inflections below the thermal degradation temperatures for all the PIs investigated here, presumably because of high glass transition temperatures. The hydrogen bonding interactions between the imide group and DKP ring or between DKP moieties could contribute to the high thermal stability of these compounds.

Particulation

The self-assembly behavior of the synthesized polymers was investigated. The polymers synthesized in this study have a DKP six-membered ring with two amide groups and other sites such as the amide and acid groups and the aromatic ring, all of which can trigger various noncovalent interchain interactions.

Here, the PAA nanoparticles with narrow size distribution (see Table S1 and Figure S8) were formulated according to the solvent displacement technique with surfactant-assisted cooperative self-assembly.[39,40] The PAAs derived from DKP-4APhe diamine with a series of dianhydrides, PMDA, BTDA, DSDA, and CBDA were taken for self-assembly (PAA–PMDA, PAA–BTDA, PAA–DSDA, and PAA–CBDA, respectively).

Typically, a preformed PAA polymer solved in DMAc, a water-miscible solvent, was slowly introduced drop by drop to an aqueous phase containing Triton X-100 as the emulsifier under vigorous magnetic stirring. After adding the organic phase into the watery phase, a slight turbidity could be observed. The PAA surfactant in water formed aggregated structures with a hydrophilic exterior and a hydrophobic interior. Subsequent self-assembly was initiated by the fast diffusion of the organic solvent (DMAc) into the aqueous medium, which progressively enriched the concentration of the polymers in emulsion droplets. To counteract the loss of the configurational entropy, cooperative noncovalent interactions of PAA chains and polymer self-assembly were induced within surfactant aggregates giving rise to uniform nanoparticles.[41] The zeta potentials of the prepared PAA particles, as measured by dynamic light scattering (DLS), indicate that the surfaces of all particles were negatively charged, irrespective of the nature of the dianhydride (see Table S2). This result suggests that the carboxyl groups in each PAA are self-arranged on the exterior surface of particles, which would help stabilize the dispersion in the aqueous colloidal system.

Figure shows changes in the hydrodynamic diameter, Dh, of PAA–BTDA particles as a function of PAA concentration and Triton X-100 concentration. The PAA particles with the smallest hydrodynamic size of 110 nm were formed at a PAA concentration of 0.5 wt %. A monotonous increase in Dh with an increase in PAA concentration can be clearly seen in Figure a. When a dilute solution of the PAA was dropped into stirred surfactant solution, nucleation and growth was initiated. The increased concentration resulted in the formation of large particles by enhanced growth. In contrast, when a very dilute PAA solution (lower than 0.5 wt %) was used, it was impossible to evaluate the particle size by either DLS or scanning electron microscopy (SEM) because the particle yield was very low. Conversely, when the PAA concentration was higher than 6 wt %, precipitation occurred macroscopically immediately after dropping the PAA solution. To summarize, the concentration range between 0.5 and 6 wt % was found to be suitable in order to obtain well-dispersed and size-controlled PAA particles.

Figure 1

Hydrodynamic diameter, Dh, of PAA–BTDA particles measured by DLS. (a) Dh dependence on the polymer concentration under a constant Triton X-100 concentration of 1 wt %. (b) Dh dependence on the Triton X-100 concentration under a constant polymer concentration of 4 wt %.

Figure b shows a decrease in Dh with an increase in the concentration of Triton X-100, from 0.1 to 3.0 wt %. The surfactant plays an important role because it can retard the aggregation of the droplets by lowering the surface tension. A higher amount of the surfactant made the droplet smaller because a large surface area could be stabilized. In solutions with very low concentrations of Triton X-100 (lower than 0.1 wt %), DLS was unable to supply the particle size data, indicating that there was very little surfactant to stabilize the polymer droplets, which resulted in aggregation. The size distribution of the PAA particles was the lowest when the concentration of the PAA and Triton X-100 were 4 and 1 wt %, respectively, suggesting that this range of concentrations should be used for investigating factors that could control particle morphologies.

To convert the PAAs to PI particles, we employed a two-step imidization, chemical imidization using a pyridine/acetic anhydride mixture (1:1 mole ratio), followed by stepwise thermal imidization. The characteristic peaks of the imide ring which appeared at 1781 and 1713 cm–1 in the FTIR spectrum (see Figure S9) confirmed the conversion. In addition, the peak at 1373 cm–1 corresponding to the C–N imide stretching and the peak at 712 cm–1 corresponding to the bending of C–N also appeared. After the two-step imidization, spherical nanosized PIs were formed with diameters of 309 ± 38, 471 ± 110, and 499 ± 171 nm for PI–BTDA, PI–CBDA, and PI–DSDA, respectively, which are in good agreement with the hydrodynamic diameters measured by DLS (see Figure , Table S1). The morphology of PI–PMDA in the SEM image was not as clear as that of the other samples because of particle agglomeration and the formation of irregular particle clusters [but DLS indicates a Dh of 780 nm (Table S1)]. Because of imide formation, no net repulsion is expected between PI particles, allowing for an increased particle size, as confirmed by DLS (see Table S1).

Figure 2

SEM images of PI particles derived from DKP-4APhe with various dianhydrides (a) BTDA, (b) CBDA, (c) DSDA, and (d) PMDA.

Figure shows the results of TGA measurements for the PI nanoparticles. The data demonstrate that the thermal behavior of the two-step imidized PI nanospheres is comparable to that of PIs with a 5% weight loss approximately at 400 °C in most cases (see Table ), indicating a high thermal resistance.

Figure 3

Thermogravimetric curves of PI particles prepared from DKP-4APhe with various dianhydrides (a) BTDA, (b) CBDA, (c) DSDA, and (d) PMDA, recorded under a nitrogen atmosphere.

Morphology Control

Particle morphologies of DKP-based PIs were controlled by external stimuli, namely, variation of the solvent polarity. After collecting the PAA microspheres from aqueous medium by centrifugation, they were redispersed in three solvent systems, acetone/water mixture, methanol/water mixture, and cyclohexane, followed by sonication for 5 h. Figure shows SEM images of the PAA–BTDA particles after treatment by different solvents, clearly revealing morphology changes. When 20% acetone/80% water mixture was used, microparticles such as spiky balls were formed (8.6 ± 1.0 μm) (see Figure a). The spiky ball can be regarded as consisting of secondary aggregates, with needles on their surface. We propose that the needles could be formed as a result of interchain self-assembly via DKP interactions. When the proportion of acetone was changed in the mixed solvent, the secondary aggregates were formed but needles were formed to some extent only at two compositions, with 10 and 50% acetone (see Figure S10a2,a3, respectively). The introduction of PAA nanospheres into a less polar solvent, such as acetone/water mixed solvent, seemed to induce polymer aggregation, with a consequent increase in the particle size. Under sonication, the polymers could rearrange themselves into more highly ordered structures. It is possible that the solvent polarity may be fine-tuned to cause the formation of needle-like structures on the secondary aggregate surfaces. When a 40% methanol/60% water mixture was used, rod-like microparticles, whose lengths ranged from 10 to 60 μm (with aspect ratios ranging from 1 to 6), were formed. The volumes of the rods were much higher than those of the pretreated spheres, and this is attributed to efficient self-assembly by using the mixed solvent (methanol/water) of the right polarity. The rod content in a SEM image seemed to be related to the proportion of methanol in the mixed solvent (see Figure S10b1,b2). This suggests that when PAA spheres were plasticized by an appropriate composition of methanol/water, the PAA chains were able to self-assemble efficiently to form the rods.

Figure 4

SEM images of PAA–BTDA obtained by redispersion of PAA spheres into (a1) 20% acetone/water, (b1) 40% methanol/water, and (c1) cyclohexane, following sonication and conversion to PI by two-step imidization as given in (a2,b2,c2), respectively.

When cyclohexane was used as a solvent, flake-like microparticles were formed. If the PAA was partially dissolved in cyclohexane under ultrasonication, the brittle and thin film formed was cast over cyclohexane solution and appeared to be flake-like particles. After the two-step imidization, the PAA particle morphologies were still maintained in the corresponding PIs (see Figure a2,b2,c2). The morphology of the PI particles was then fine-tuned by the use of mixed solvents of different polarities.

The self-assembly was investigated in terms of hydrogen bonding by FTIR techniques. Figure b shows the FTIR spectrum of the PI–BTDA spherical particles obtained by solvent displacement and subsequent imidization, showing five specific vibrations at 1780, 1718, 1667, 1516, and 1375 cm–1, which were assigned to C=O of benzophenone, C=O of the imide ring (imide I), C=O stretching of DKP (amide I), N–H bending of DKP (amide II), and C–N of imide (imide II), respectively. The FTIR peaks of the spiky balls were not very different from those of the spheres (see Figure c), although the C=O of the imide ring showed a slight shift toward higher wavenumbers. The absence of any significant change of the FTIR spectrum suggests that there is no distinct change in the hydrogen bonding patterns during the morphology conversion from spheres to spiky balls. Although the transformation from a smooth to a spiky surface could have resulted from an enhancement in hydrogen bonding, the percentage change was too small. On the other hand, rods formed by the methanol/water stimulus showed the broadening of amide I, as well as the red shifts of amide II, C=O of benzophenone, and imide II peaks, clearly indicating the enhanced hydrogen bonding in stacked polymer chains[36,42] via DKP interactions, even after imidization (see Figure d). Regarding the flakes formed by the cyclohexane stimulus, the blue shifts of amide I and imide I were observed (see Figure a), which implied that the hydrogen bonds became weaker after drying over cyclohexane. The IR analyses support the notion of solvent casting of particles to be film-like structures.

Figure 5

FTIR spectra of PI–BTDA in various forms: (a) flakes, (b) spheres, (c) spiky balls, and (d) rods.

The solvent effects for the previously reported[19] PAA–BTDA particles, derived from 4ATA dimethyl ester, were investigated for comparison. 4ATA dimethyl ester is a diamine monomer having a central cyclobutane ring sandwiched by aromatic rings similar to DKP-4APhe monomers, but having no amide linkage which makes it different from DKP. The particle morphologies were observed by SEM (see Figure S11). Although spherical particles (605 ± 157 nm) were obtained by the solvent displacement method (Figure S11a), particles were broken into diffuse shapes after redispersion into solvent mixtures of 20% acetone/80% water, 40% methanol/60% water, and cyclohexane, following sonication (Figure S11b–d, respectively). This suggests that DKP units could play a key role in the solvent-assisted morphological change in the present PAA and PI systems. Thus, the thermoresistant biopolyamide particles have the ability to transform into different shapes by the external stimuli of solvent exchange, and the morphology change can help to add to their functionality. This could in turn increase the number of possible applications for such molecules, for example, as fillers reinforcing a polymer matrix.

Conclusions

We synthesized the aromatic diamine, 3,6-di(4-aminophenylmethyl)-2,5-DKP (DKP-4APhe), by cyclic dipeptide formation of a functionalized α-amino acid, 4APhe, where the 2,5-DKP ring can induce self-assembly because of its centrosymmetric cyclic amide groups. DKP-4APhe was polymerized with various dianhydrides to obtain biobased PAAs, which were converted into PIs with high thermal resistance. Among all the PI molecules in the present study, the PI from PMDA showed the highest Td10 of 432 °C. In addition, its Tg value could not be estimated, presumably because it is higher than the temperature at which the PI decomposes. Using the simple solvent displacement method from DMAc into water, PAA and PI spheres having smooth surfaces were formed. The PAA spheres underwent morphological changes into spiky ball-, rod-, or flake-like particles upon varying the solvent polarity using an acetone/water mixture, methanol/water mixture, or cyclohexane, respectively. The morphology observed at the PAA stage was retained in the PI particles, even after the two-step imidization. FTIR analyses indicated that hydrogen bonding was enhanced in the rod-like particles, presumably because of the self-assembly of the DKP moiety. To summarize, biobased PI particles with high thermal resistance were prepared, which can be potentially used as fillers for reinforcing aromatic polymer matrixes. Indeed, nonspherical particle morphologies may enhance filler–matrix interactions.

Experimental Section

Materials

4-Amino-l-phenylalanine (4APhe: from Watanabe chemical), benzyl chloroformate (CbzCl: >96.0% from TCI), TMSCl (TMSCl: from Shin-Etsu Chemical), methanol (>99.8% from Kanto Chemical), di-tert-butyl dicarbonate (Boc2O: >95.0% from TCI), sodium hydroxide (NaOH, >97.0% from Kanto Chemical), N,N′-dicyclohexylcarbodiimide (DCC: >98.0% from TCI), hydroxybenzotriazole (HOBt: anhydrous from Dojindo), formic acid (>98.0%, from Kanto Chemical), trimethylamine (>99.5% from Aldrich), hydrogen bromide solution, 33 wt % in acetic acid (33% HBr/AcOH: from Sigma-Aldrich), and NH3 solution (concn 28.0–30.0% from Kanto Chemical) were used for monomer syntheses as received. Dianhydrides used as counter monomers such as OPDA (>98.0%), PMDA (>98.0%), BPDA (>98.0%), and DHCDA (>98.0%) were purchased from Tokyo Chemical Industry Co., Ltd (TCI) and were purified by sublimation at 160 °C under reduced pressure and annealing at 110 °C under vacuum just before use. Other dianhydrides such as CBDA (purified by sublimation >98.0% from TCI), BTDA (purified by sublimation >98.0% from TCI), and DSDA (purified by sublimation >98.0% from TCI) were used as received. Acetic anhydride (>95.0% from Kanto Chemical) and pyridine (anhydrous >99.5% from Kanto Chemical) were used for chemical imidization as received. A surfactant used for particle formation, Triton X-100 (from Acros), was used as received. Solvents such as 2-butanol (>99.0% from Kanto Chemical), acetic acid (AcOH: >99.7% from Kanto Chemical), N,N-DMAc (DMAc: >99.8% anhydrous from Kanto Chemical), 1,4-dioxane (>99.5% from Kanto Chemical), THF (>99.5% from Kanto Chemical), and toluene (>99.0% from Kanto Chemical) were used without further purification after received.

Measurements

1H NMR and 13C NMR were performed on a Bruker BioSpin AG 400 MHz spectrometer using DMSO-d6 as a solvent at 23.1 °C with 16 accumulation scans, using the proton resonance of residual nondeuterated DMSO as an internal standard (2.55 ppm). The FTIR spectra were recorded with a PerkinElmer Spectrum One spectrometer between 4000 and 400 cm–1 using a diamond-attenuated total reflection (ATR) accessory. The number-average molecular weight (Mn), weight-average molecular weight (Mw) and the molecular weight distribution (PDI, Mw/Mn) were determined by gel permeation chromatography (GPC; Shodex GPC-101, concentration 0.7 g/L, 10 mM LiBr/DMF eluent) equipped with a JASCO RI-2031 Plus Intelligent RI detector and JASCO UV-2075 Plus Intelligent UV/Vis detector after calibration with polystyrene standards. The mass spectra were measured using a FT-ICR MS (Solarix) equipped with a Nanospray source operating in the nebulizer-assisted ESI mode used in the positive ion mode and scanned from m/z 50 to m/z 1000. TGA and DSC were carried out by Seiko Instruments SII, SSC/5200 and Seiko Instruments SII, X-DSC7000T, respectively, at a heating rate of 5 °C/min under a nitrogen atmosphere. The remaining solvent and absorbed moisture in polymer samples were removed at 250 °C for 1 h before TGA and DSC measurement. PAA and PI particle morphologies were characterized with a scanning electron microscope (JCM-6000Plus Versatile Benchtop SEM). To prepare a sample, a droplet of the dispersion liquid (5 μL) was casted on a glass slide and air-dried at room temperature; the glass slide was fixed on the sample holder using double-faced carbon tape and coated with a thin layer of gold with a sputter coater (Magnetron sputter MSP-1S). The SEM instrument was operated at an acceleration voltage of 15 kV and an emission current of 10 μA. ImageJ software was used to analyze average particle diameters from the SEM images. The hydrodynamic size and zeta potential were measured with DLS (Zetasizer Nano ZS90). The calculation of size distribution from light scattering measurements is based on the assumptions that the particles are spherical. Ultrasonic sonication bath (AS ONE Corporation, AS12GTU) with an oscillation frequency at 35 kHz, 60 W was used in the particulation control study.

Monomer Synthesis

The schematic representation of the synthesis of the biobased aromatic DKP diamine from 4APhe is shown in Scheme .

Synthesis of 4APhe-z: a solution of 4APhe (10.0 g, 0.040 mol) dissolved in 10% AcOH (340 mL) was added drop by drop with 5 M NaOH to raise pH to 3. A solution of CbzCl (6 mL, 0.040 mol) in 1,4-dioxane (340 mL) was then slowly added, and the mixture was stirred overnight at room temperature. The mixture was brought to pH 7 by 5 M NaOH before filtration and washed with water. The expected product was obtained as white shiny powder with 82% yield. The specification was as follows. 1H NMR (400 MHz, DMSO-d6, δ, ppm): 2.76 (dd, 1H, J = 8.8, 14.0 Hz), 3.08 (dd, 1H, J = 3.4, 14.0 Hz), 3.60 (t, 1H, J = 3.4 Hz), 5.15 (s, 2H), 7.18 (d, 2H, J = 8.4 Hz), 7.40 (m, 7H), 9.72 (s, 1H).

Synthesis of methyl-4APhe-z: the milky mixture of 4APhe-z (5.0 g, 0.016 mol) in MeOH (80 mL) was added with TMSCl (8.5 mL, 0.067 mol). The mixture was stirred overnight at room temperature. The solvent was evaporated, and the crude sample was further recrystallized from MeOH and diethyl ether to obtain the expected product with 90% yield. The specification was as follows. 1H NMR (400 MHz, DMSO-d6, δ, ppm): 3.15 (dd, 1H, J = 5.2, 14.0 Hz), 3.65 (s, 3H), 4.17 (t, 1H, J = 5.2 Hz), 5.14 (s, 2H), 7.14 (d, 2H, J = 8.8 Hz), 7.40 (m, 7H), 8.47 (s, 3H), 9.82 (s, 1H).

Synthesis of Boc-4APhe-z: a stirred solution of 4APhe-z (4.5 g, 0.014 mol) in THF/H2O (1:1, 62 mL) was added with NaOH (1.3 g, 0.033 mol) at room temperature followed by the addition of Boc2O (3.5 g, 0.016 mol). The reaction mixture was stirred at room temperature overnight. THF was then removed by evaporation, and 1 M HCl was added to bring pH to 4 and followed by filtration to obtain the product with 98% yield. The specification was as follows. 1H NMR (400 MHz, DMSO-d6, δ, ppm): 1.35 (s, 9H), 2.92 (d, 2H, J = 7.0 Hz), 3.59 (dd, 1H, J = 7.0, 14.0 Hz), 5.13 (s, 2H), 5.71 (d, 1H, J = 6.1 Hz), 6.99 (d, 2H, J = 12.0 Hz), 7.25 (d, 2H, J = 8.0 Hz), 7.41 (m, 5H).

Synthesis of linear dipeptide-4APhe-z: to a 0 °C solution of Boc-4APhe-z (8.5 g, 0.021 mol) in dichloromethane (DCM) (120 mL), HOBt (3.05 g, 0.022 mol) was added followed by the addition of DCC (5.08 g, 0.025 mol). The reaction mixture was stirred for 1 h and allowed it to cool to room temperature. Then, a solution of methyl-4APhe-z (8.25 g, 0.023 mol) in DMF (23 mL) was added and followed by the addition of trimethylamine (3.4 mL, 0.024 mol). The reaction was stirred further at room temperature overnight. To work up, the precipitated dicyclohexylurea was filtered off and washed with little DCM. The filtrate was concentrated by evaporation and its pH was adjusted to 2−3 by adding 1H NCl under ice condition. The crude product was obtained by filtration with 70% yield. The specification was as follows. 1H NMR (400 MHz, DMSO-d6, δ, ppm): 1.29 (s, 9H), 1.68 (d, 2H, J = 22.0 Hz), 2.89 (d, 2H, J = 21.0 Hz), 3.58 (s, 3H), 4.12 (dd, 1H, J = 7.0, 14.0 Hz), 4.46 (dd, 1H, J = 7.2, 14.0 Hz), 5.14 (s, 4H), 6.81 (d, 1H, J = 9.0 Hz), 7.12 (d, 4H, J = 8.0 Hz), 7.38 (m, 14H), 8.20 (d, 1H, J = 8.1 Hz), 9.68 (s, 1H), 9.70 (s, 1H).

Synthesis of cyclic dipeptide 4APhe-z: to remove the Boc group, linear dipeptide-4APhe-z (7.77 g, 0.011 mol) was charged with 98% formic acid (466 mL), followed by stirring for 5 h at room temperature. Excess formic acid was then removed in vacuum (temperature less than 30 °C was maintained). The obtained crude was refluxed in the mixture of 310 mL of 2-butanol and 155 mL of toluene at 110 °C for 5 h, followed by the filtration and drying under vacuum to obtain the expected product with 58% yield. The specification was as follows. 1H NMR (400 MHz, DMSO-d6, δ, ppm): 2.14 (dd, 2H, J = 6.4, 13.6 Hz), 3.92 (dd, 2H, J = 4.4, 13.6 Hz), 5.12 (s, 4H), 6.94 (d, 4H, J = 8.4 Hz), 7.35 (m, 14H), 7.89 (s, 2H), 9.75 (s, 2H).

Synthesis of DKP-4APhe: 33% HBr/AcOH solution (38.5 mL) was added to cyclic dipeptide 4APhe-z (3.85 g, 0.0065 mol). The mixture was stirred at room temperature for 3.5 h. Diethyl ether was added and decanted several times. The procedure was repeated several times to remove excess acid. The crude compound was dried under vacuum. After drying, the compound was dissolved in water followed by the addition of NH3 solution drop by drop till the pH became 12 while stirring. The precipitate was collected by filtration to obtain the expected product with 98% yield. The specifications were as follows. 1H NMR (400 MHz, DMSO-d6, δ, ppm): 2.08 (dd, 4H, J = 6.8, 13.6 Hz), 3.80 (t, 2H, J = 4.0 Hz), 4.89 (s, 4H), 6.47 (d, 4H, J = 8.0 Hz), 6.70 (d, 4H, J = 8.0 Hz), 7.63 (s, 2H). ATR–FTIR: 3400–3250, 1650, 1505 cm–1. 13C NMR (100 MHz, DMSO-d6, δ, ppm): 56.35, 114.34, 123.71, 130.87, 147.78, 167.01. FT-ICR MS (ESI): calcd for [M + Na, C18H20N4NaO2]+, 347.14837; found, 347.14764.

PAA and PI Syntheses

A typical procedure for the synthesis of PAA is shown in Scheme . A diamine of DKP-4APhe (0.20 g, 0.62 mmol) mixed with an equimolar of dianhydrides such as CBDA (0.12 g, 0.62 mmol), BTDA (0.20 g, 0.62 mmol), PMDA (0.14 g, 0.62 mmol), DSDA (0.22 g, 0.62 mmol), OPDA (0.19 g, 0.62 mmol), BPDA (0.18 g, 0.62 mmol), and DHCDA (0.16 g, 0.62 mmol) was dissolved in DMAc (1 mL, 0.6 M) under a nitrogen atmosphere. The reaction mixture was stirred at room temperature for 48 h to yield a viscous PAA solution. The PAA solution was added into a 1:1 mixture of water and methanol and precipitated to obtain the respective PAA polymers in quantitative yields (PAA–BTDA: yield 93%, PAA–CBDA: yield 91%, PAA–PMDA: yield 90%, PAA–DSDA: yield 93%, PAA–OPDA: yield 90%, PAA–BPDA: yield 92% and PAA–DHCDA: yield 89%). PI was obtained by thermal imidization of the PAA in an oven under vacuum by stepwise heating at 100, 150, and 200 °C for 1 h and 250 °C for 3 h at each step.

Preparation of PAA and PI Particles

In a typical solvent displacement method, a PAA solution in DMAc (4% w/v, 100 μL) was dropped into an aqueous solution of Triton X-100 (1% w/v, 10 mL) and magnetically stirred vigorously as a poor solvent at room temperature to obtain PAA particles. Collecting PAA by centrifugation, the two-step imidization was subsequently performed to convert PAA to PI. First, PAA was chemically imidized using a mixture of pyridine and acetic anhydride (1:1 M ratio, 100 μL). After 3 h, the chemicals were removed by centrifugation, and thermal imidization was performed at 250 °C for 3 h, resulting in yellowish powder of PI particles.

To study the particulation control of PIs, PAA particles collected by centrifugation were redispersed in different solvent systems such as acetone/water mixture, methanol/water mixture, and cyclohexane and further sonicated for 5 h. After that, subsequent imidization to convert PAA to PI was carried out by the chemical procedure and thermal treatment. The morphology of redispersed particles of PAA and PI was determined using SEM, as illustrated in Figure . Here, PI–BTDA was chosen as an example for study.

Figure 6

Schematic illustration of particle formation and deformation of PAA and PI by the solvent-assisted approach.

Particle Morphology Control

Various factors during particle formation could govern the properties of fabricated particles (e.g., particle size). The effect of formulation variables (polymer concentration: 0.5, 1, 2, 4, and 6% (w/v); surfactant concentration: 0.1, 0.5, 1, and 3% (w/v); polymer structure with various dianhydrides: BTDA, CBDA, DSDA, and PMDA) on the particle size was studied.