Article pubs.acs.org/Macromolecules
Shape-Anisotropic Polyimide Particles by Solid-State Polycondensation of Monomer Salt Single Crystals Konstantin Kriechbaum,†,§ D. Alonso Cerrón-Infantes,†,§ Berthold Stöger,‡,⊥ and Miriam M. Unterlass*,† †
Institute of Materials Chemistry, ‡Institute of Chemical Technologies and Analytics, and ⊥X-ray Center Technische Universität Wien, 1060 Vienna, Austria S Supporting Information *
ABSTRACT: Shape-anisotropic particles are of broad interest, e.g., for colloidal crystals or applications at interfaces such as particle-stabilized emulsions. Despite the wealth of accessible shapes of inorganic particles, anisotropic homopolymer particles are to date mostly limited to objects derived from spheres (e.g., ellipsoidal or disk-shaped particles). Here, we report the synthesis of shape-anisotropic, angular polyimide particles by thermal solid-state polycondensation (SSP) of monomer salts. We prepare monomer salt single crystals of relatively narrow size and shape distribution by growth inside hydrogels, and solve their crystal structure. Polyimide particles are obtained by simple heating and retain the shape of the initial salt crystals. Using high-temperature X-ray diffraction, thermal analyses and microscopy techniques, we investigate the mechanism of the transformation. The obtained polyimide particles are temperature-stable up to 640 °C and virtually insoluble in any solvent. This work sheds more light on the mechanism of SSP of monomer salts and reports a new methodology for accessing nonspherical homopolymer particles, which are due to their outstanding stability potentially of interest for applications under extreme conditions.
■
INTRODUCTION Shape-anisotropic particles currently receive utmost attention in materials science.1 In contrast to spherical particles, they generate different flow profiles at interfaces,2,3 and thus hold great promise in technological applications for instance as dispersed phases in composites or in paints and varnishes.4 Moreover, their assembly into dense phases often leads to intriguing superstructures, such as quasi-crystalline or plastic crystalline colloidal crystals.1,5−8 A plethora of inorganic particles, e.g., metal nanoparticles,9,10 display nonspherical, angular form. This is a direct consequence of their crystallinity. In contrast, amorphous inorganic particles, such as silica nanoparticles, are typically obtained as spherical objects. Organic homopolymers are also obtained as spherical particles by the vast majority of polymerization techniques. In solution polymerization, for instance, a so-called polymer latex forms, because the freshly formed polymer is physicochemically different from the solution it originates from and thus will try to minimize its surface free energy by reducing its interface with the solution. Consequently, the object with the smallest surface area to volume ratio results, i.e., a sphere. Note that the situation lies differently for block copolymers. Here, one can use the fact that the two blocks are physicochemically different for generating nonspherical particles by, e.g., selectively swelling one block but not the other.11,12 Since block copolymer synthesis is tedious and costly, it is much preferable to prepare shape-anisotropic particles from homopolymers. If one can easily prepare angular inorganic particles, why the interest in © XXXX American Chemical Society
angular organic polymer particles? First, one can access a different density range (lower specific volume) than for inorganic substances. Second, organic polymers are built up of abundant elements (C, H, N, O, S); And third they are much cheaper than, e.g., heavy metal nanoparticles. To date, only a handful of examples to synthesize shape-anisotropic homopolymer particles exist. As homopolymers are typically obtained as spherical particles, the majority of techniques toward nonspherical particles relies on deforming spherical latex particles. Spherical polystyrene (PS) particles have for instance been mechanically deformed into shapes derived from the sphere (and therefore containing curved faces), such as ellipsoidal, disk-, or barrel-shaped particles.13,14 Recently, dodecahedral and octahedral homopolymer particles could be synthesized by assembly of spherical poly(methyl methacrylate) into closepackings, followed by swelling.15 Another approach is employing photopolymerization through either photolithographic masks or at the end of microfluidic devices.16,17 These combinations of photopolymerization and microfluidics can be used to access both roundish and angular nonspherical particles, with the polymer material being highly cross-linked acrylate networks in both cases.16,17 In this contribution, we report a novel technique to prepare shape-anisotropic homopolymer particles: solid-state transReceived: July 14, 2015 Revised: November 21, 2015
A
DOI: 10.1021/acs.macromol.5b01545 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules Scheme 1. Synthesis of PPPI by Solid-State Polycondensationa
a
Pyromellitic acid (PMA, 1) and p-phenylene diamine (PDA, 2) are reacting to the monomer salt [H2PDA2+PMA2−] (3), which can be converted to PPPI (4) by simple heating.
but also prone to form salts by acid−base reaction (Scheme 1).22 Typically, they are synthesized by simply mixing an aqueous solution of the diamine with an aqueous solution of the tetracarboxylic acid. While the comonomers 1 and 2 are each well soluble in protic solvents, the monomer salt [H2PDA2+PMA2−], 3, is essentially insoluble, and thus nucleates at very high supersaturation, which inevitably leads to its immediate precipitation as a polycrystalline powder.22 It has therefore been previously impossible to obtain either monomer salt single crystals of 3,22 orto our knowledge any other monomer salt. Thus, we performed both synthesis and solubility tests (Supporting Information) of 3 in various solvents in order to obtain high purity monocrystals. These attempts lead to two conclusions: First, 3 can only be formed in water and alcohols, from which we conclude that the protic environment is necessary for the acid−base reaction to take place. Second, 3 is virtually insoluble in any solvent other than in reactive solvents, where it either reacts back to the starting monomers or to the corresponding poly(amic acid) (Supporting Information). As the acid−base reaction between 1 and 2 toward 3 can only proceed in a polar and protic environment, but the solubility of 3 is extremely small in these solvents, the obtainment of high-purity single crystals of 3 via classical solution-based techniques (such as recrystallization) is extremely hard to achieve. We therefore turned our attention toward an alternative technique for the growth of singlecrystals: gel crystallization.24−26 Gel crystallization allows for obtaining high-purity single crystals of impressive size, which is mainly due to altered transport in viscous gel media as compared to fluids (convective transport phenomena are avoided in favor of purely diffusive processes). In addition, both turbulences and mechanical strain on the growing crystals are minimized in a gel environment.24 The process is very well suited for sparingly soluble substances, which is indeed the case for 3. Gel crystallizations of 3 were prepared as illustrated in Scheme 2: PMA (1) was dissolved in an agarose gel, covered with an aqueous solution of PDA (2), and further topped with a layer of petroleum ether to prevent evaporation of water and thus a change in concentration (see Supporting Information for experimental details). Upon contact of 1 and 2, an acid−base reaction takes place inside the agarose gel and 3 forms. PMA was supplied in the agarose gel, as its acidity decreases the stiffness of the hydrogel. Instead, attempts to supply PDA in the hydrogel led to very stiff gels that exclusively generated dendritic crystals of 3 instead of the desired single crystals. Crystallization of 3 was allowed to proceed for 2−24 h at temperatures between 25 and 60 °C. Crystallization at 60 °C yielded rather soft hydrogels and lead exclusively to the formation of single crystals. Monomer salt crystals were harvested by heating the agarose gels above 85 °C (leading
formations of monomer salt single-crystals. Monomer salts are salts of two comonomers, formed by acid−base reaction of an organic base comonomer with an organic acid comonomer. The most common monomer salts are obtained from (i) diamines and dicarboxylic acids (often referred to as “AHsalts”) thus precursors for polyamides, or from (ii) diamines and tetracarboxylic acids thus precursors for polyimides. For polyimides, monomer salts are obtained as crystalline solids that can be polymerized in the solid-state.18−20 We recently prepared CO2-selective microporous polyimides by solid-state polymerizations (SSP) of monomer salts, and observed that the solid-state transformation occurred without any melting or premelting phenomena.21 The used monomer salt particles were however intergrown and polycrystalline, and the mechanism of SSP could thus not be investigated in detail. With this manuscript, we have set out to go further than the mere observation of a lack of melting in SSP. We provide evidence that SSP presents a promising avenue toward shapeanisotropic, angular homopolymer particles by conservation of the monomer salt crystal shape. Here, we report the synthesis of monomer salt single-crystals of the two comonomers pphenylenediamine (PDA) and pyromellitic acid (PMA) of impressive size of several hundreds of μm, via a gel crystallization approach. By simply heating, these crystals can be transformed to polyimide particles while retaining the initial shape. While the particle sizes are yet to big too be applicable for, e.g., colloidal self-assembly, the single-crystallinity and size allowed for solving their crystal structure, and for mechanistically studying the SSP process. In combination with our knowledge of the resulting polymer’s crystal structure, which we recently refined from powder X-ray diffraction (PXRD) data,22 and using in situ high-temperature powder X-ray diffraction, in situ high-temperature optical microscopy and thermal analyses, we are now able to present a detailed analysis of the ongoings during SSP.
■
RESULTS AND DISCUSSION The scope of this contribution was to synthesize homopolymer particles of anisotropic shape of the fully aromatic polymer poly(p-phenylenepyromellite imide) (PPPI) by SSP, and to shed more light on the SSP of monomer salts. As most fully aromatic polyimides (PIs), PPPI is insoluble in virtually any solvent, has no melting point and is temperature stable up to 640 °C. PPPI is the mechanically toughest organic polymer ever reported, with a theoretical Young’s modulus of over 500 GPa.23 The starting compounds are the two comonomers pyromellitic acid (1,2,4,5-benzenetetracarboxylic acid, PMA, 1) and p-phenylenediamine (PDA, 2). Aromatic diamines and α,β,α′,β′-tetracarboxylic acids (or dianhydrides) in 1:1 molar ratio are not only the two classical comonomers forming PIs, B
DOI: 10.1021/acs.macromol.5b01545 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules Scheme 2. Reactive gel crystallization of monomer salt 3a
crystals have inclusions (cf. Figure 1G). From the overview optical micrograph Figure 1E, it becomes clear that the size distribution is relatively narrow, with crystals of 3 typically having a length of ca. 200−300 μm (length of the diagonal of the major facet, i.e. {001} and {001}̅ , respectively). Note that a certain amount of crystals are intergrown into aggregates cf. Figure 1B,F. The form of single crystals of 3 is schematically depicted in Figure 1A, viewed from two different perspectives. The major facets {001} and {001̅} (Figure 1A) are parallelogram-shaped and are found as biggest facets in all crystals. We also find the next biggest facets, i.e. {111̅}, {1̅1̅1}, {11̅1̅}, {1̅11}, {11̅0}, {1̅10}, {110}, and {1̅1̅0} (Figure 1A, black, blue, yellow and red, respectively) in all analyzed crystals of 3 (cf. Supporting Information). These facets are found to different extent, depending on the individual growth of each crystal. Four minor facets are extremely small in some crystals, but hardly visible in most, namely {111}, {1̅1̅1̅}, {11̅1} and {1̅11̅} (Figure 1A, gray). Consequently, gel-grown single crystals of 3 depict up to 14 facets, of which only 10 are visible in most cases: overall the crystal shape can be described as nonuniform polyhedral. The high quality and size of the single-crystals of 3 allowed for solving the crystal structure. 3 crystallizes in the monoclinic space group C2/c (No.15), with lattice parameters a = 12.6313(18) Å, b = 7.6720(12) Å, c = 16.025(3) Å, and β = 107.020(4)°. The crystal structure of [H2PDA2+PMA2−] is depicted in Figure 2 (see Supporting Information for crystal structure data and determination details). PMA2− and H2PDA2+ are populating the unit cell in an alternating fashion in the direction of a (Figure 2A). This alternating organization between the comonomers is ensured via intermolecular interactions, specifically by hydrogen-bonding. H-bonding between carboxylate/carboxylic acid and amminium functions has indeed already been observed for monomer salts.21 Note
a
(A) starting compounds PDA (2) and PMA (1) are dissolved in supernatant aqueous phase and agarose hydrogel, respectively. By diffusion 1 and 2 meet and react via acid−base reaction; single crystals of the salt (3) form inside the hydrogel. (B) Photograph of an ongoing crystallization of 3 after 20 h of crystallization at 60 °C.
to dissolution of the gel), and subsequent isolation of the crystals of 3 by filtration. [H2PDA2+PMA2−] was obtained as single crystals of thick tabular habit, as becomes clear from optical and scanning electron microscopy (Figure 1). The crystals depict self-similarity features, which are typical for highquality single-crystals (see Figure 1D). Optical microscopy shows fully transparent single crystals (Figure 1E−G) that are birefringent (Supporting Information), and a certain amount of
Figure 1. Form and appearance of monomer salt crystals obtained by gel crystallization. (A) Schematic of crystal shape of crystals of 3 with indexed crystal facets viewed from two different perspectives. (B−D) Scanning-electron micrographs of 3. (E−G) Optical micrographs of 3. C
DOI: 10.1021/acs.macromol.5b01545 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Figure 2. Crystal structure of [H2PDA2+PMA2−]. Element color code: gray = carbon, red = oxygen, white = hydrogen, blue = nitrogen; (A) Unit cell of 3 viewed along the b-axis. (B,C) Detailed view of the H-bonding at PMA2− (B) and H2PDA2+ (C), respectively. Note that only half of the Hbonds are displayed for visibilitythe other H-bonds are defined through the inversion center in both comonomers, located at the center of the phenyl rings. (D−F) views of the crystal within single crystals of 3, i.e. onto {001̅}, i.e., along c* (D), onto {010}, i.e., along b (E) and onto {100}, i.e., along a (F).
Figure 3. Thermal studies of the SSP of monomer salt 3 upon heating. (A) TGA of 3: The first mass loss of 20% at 210 °C, corresponding to 2 equiv of H2O liberated per cyclic imide; degradation onset of PPPI at 640 °C. (B) DSC of 3, two heating and one cooling profiles: an endothermic bimodal peak with an onset at 205 °C is observed during the first heating. (C) HT in situ powder XRD of 3. (D) Comparison of the diffractograms of monomer salt 3 (blue), PPPI 4 from SSP of single crystals of 3 (red), PPPI synthesized via classical polycondensation in m-cresol (black), and the simulated PPPI pattern (black, top) with indexing of all major reflections.
D
DOI: 10.1021/acs.macromol.5b01545 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
is formed in the first step and cyclization of to the imide occurs in a second step. However, SSP of monomer salts have been reported to occur in a concerted fashion.20 On the basis of this report of SSP of monomer salts not occurring via poly(amic acid) intermediates, two scenarios that might explain the bimodal DSC curve are in principle thinkable. (i) It is possible that the monomer salt 3 directly condensates to some degree, and that part of the formed H2O molecules are retained by the freshly formed oligo-/polyimide chains. Such retention in the form of partially hydrated chains would be conceivable as H2O molecules might interact with the carbonyl moieties of the imide functions via H-bonding. (ii) Also conceivable is the dehydration of the monomer salt by two equivalents of H2O per formula unit of [H2PDA2+PMA2−], generating p-phenylenediamine 2 and pyromellitic dianhydride (PMDA), in a first step. Corresponding to the second broader endothermic peak would then be the direct condensation to the polyimide from PMDA and 2. Currently, one can only speculate upon the precise reaction sequence. In order to follow the structural transformation, we performed in situ high-temperature powder X-ray diffraction (HT-XRD, Figure 3D). HT-XRD reveals an onset of polymerization between 180 and 190 °C, i.e. slightly lower than the Tp observed from TGA (210 °C) and DSC (205 °C). This indicates that Tp is not an absolute temperature, which seems reasonable since the transformation from monomer salt to PI is not a phase transition, but a reaction. Indeed, different heating profiles of thermal analyses (DSC and TGA) and HTK XRD were applied: While 3 was heated with rates of 10 min in
that the H-bonding in single-crystals of 3 is particularly interesting: (i) The amount of H-bonds per molecule is quite important; i.e., each PMA2− participates in 10, and each H2PDA2+ in 6 H-bonds (Figure 2B,C). (ii) The H-bonds in the crystal structure of 3 all lie in distance and angle range of strong H-bonds (see Supporting Information, Table S2) according to the Desiraju-Steiner classification, i.e. the H···acceptor distance range of 1.5−2.2 Å, and the donor-H···acceptor angular range of 130−180°.27 Strong H-bonds are known to have a distinctive effect on the crystal structure.27 Overall, this leads to an organization of the comonomers in a highly hydrogen-bonded network (see Supporting Information, Figure S3). (iii) With g 1.62 3 , the crystal density is surprisingly high for a purely cm
g
organic amminium salt (typical range ca. 1.2 − 1.4 3 ), which cm is most likely a consequence of the very effective molecular packing arising from the high number of strong H-bonds. (iv) We suppose that the strong H-bonding and thus arising efficient packing (evinced by the impressively high crystal density) is generating the high driving force for the formation of 3 from aqueous solutions, i.e. the major reason for 3 nucleating at very high supersaturation. Note that the Hbonding in 3 is already connecting the functional groups (i.e., carboxylate, carboxylic acid, and amminium groups) that will be reacting to the cyclic imide moieties in PPPI (cf. Scheme 1). Monomer salt single crystals of 3 undergo SSP to PPPI 4 with conservation of the crystal shape. Consequently, the positioning of the comonomers H2PDA2+ and PMA2− within the single crystals is of interest for discussing the transformation on a molecular level. The orientation of the comonomers within single crystals of 3 viewed along c*, b, and a is shown in Figure 2, parts D, E, and F, respectively. It becomes clear that H2PDA2+ and PMA2− are not only arranged in an alternating fashion along b, but also along c and a (Figure 2D,F). We will further consider the arrangement of the comonomers within single crystals of 3 when discussing its transformation to PPPI 4. The SSP of monomer salts is a thermally triggered phenomenon, i.e., is happening upon heating monomer salts above an elevated temperature referred to as their polymerization point (Tp). At this point, the salts transform irreversibly to polyimides, without traces of melting or premelting.21 From thermogravimetric analysis (TGA), we find Tp(3) at 210 °C (Figure 3A), and the mass loss (20 wt %) is in good agreement with the liberation of 2 equiv of H2O per cyclic imide formed, i.e., 4 equiv of H2O per repeating unit (theoretical mass loss: 19.9 wt %). Differential scanning calorimetry (DSC, Figure 3B) of 3 revealed a Tp(3) of 205 °C. Moreover, DSC analysis clearly confirms that the temperature at which the mass loss starts in TGA is indeed a polymerization and not a melting point: the phenomenon is not reoccurring upon further DSC cycles (Figure 3B). This is due to the monomer salt being polymerized after the first heating ramp, and since PPPI 4 is temperature-stable up to over 640 °C and lacks a glass transition in the measured temperature range, no thermal phenomena are visible upon cycling. From DSC it becomes clear that the SSP of 3 is endothermic and thermally irreversible. The endothermic peak (onset at 205 °C) has a bimodal shape, with a first sharp local minimum at 225 °C, and a second broader peak, centered around 240 °C. This peak shape is indeed peculiarto our knowledge, all polycondensation peaks of diamminium dicarboxylate dicarboxylic acid salts observed to date were monomodal.20,21 One might suppose that the shape is due to a stepwise imidization, where an amide
K
TGA and 5 min in DSC, the sample was kept at each measurement temperature for 240 min in HT-XRD. In HT-XRD experiments, we find that reflections associated with the monomer salt are still present at 180 and 190 °C, and completely disappear from 200 °C on. The diffractograms measured at 200 °C and higher temperatures (not depicted as pattern does not change with increasing temperature) are corresponding well to semicrystalline PPPI. They exhibit (0 0 l) reflections of the known structure (indicated in Figure 3E) of PPPI, which crystallizes in the monoclinic spacegroup Pbam (No. 55).22 However, the HT-diffractograms show the presence of broad halos, indicating that the PPPI chains are globally aligned and thus have a common director, comparable to nematic liquid crystalline phases. For in situ HT powder XRD, single crystals of 3 were finely ground prior to the measurements. This is necessary for generating a sample with randomly oriented crystallites, which is required for powder XRD. Hence, the final diffractogram in Figure 3C (200 °C, red curve) reflects the crystallinity one would obtain from a polycrystalline sample of 3. For assessing the order in PPPI, one obtains from polymerizing single crystals of 3 in the solidstate, we performed SSP of single crystals of the monomer salt. The resulting PPPI particles were finely ground and analyzed by powder XRD at ambient conditions (shown in Figure 3D, red curve). Comparing the diffractogram obtained by SSP of a powder of 3 (Figure 3C, red curve) with the diffractogram of PPPI obtained by SSP of single crystals of 3, it becomes clear that there is no significant difference in crystallinity. Both samples exhibit all major reflections of PPPI, that are however rather broad and thus correspond to orientational disorder of the PPPI chains comparable to nematic phases. Although PPPI obtained via SSP of both bulk monomer salt single crystals and of a powder of 3 shows solely global chain alignment, its E
DOI: 10.1021/acs.macromol.5b01545 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Figure 4. SSP of single crystals of 3 to PPPI 4. (A) Comparison of monomer salt 3 crystals (left) and PPPI 4 (right) particles via optical microscopy. (B−D) Optical micrographs of different PPPI particles after SSP. (E) SEM image of 4. (F−K) real-time optical microscopy of the SSP of a monomer salt crystal, at T = 80, 150, 190, 205 (5 min), 205 (10 min), and 205 °C (15 min), respectively. (L−M) Unit cell of 4. Color code: gray = carbon, red = oxygen, white = hydrogen, blue = nitrogen, with a view along the b- axis (L) and along the c - axis (M). (N) Schematic of the T- profile within a monomer salt crystal in the early stages of SSP. (O, P) Views of the crystal structure of 3 combined with the crystals shape and {3¯01} and {301̅} planes. (Q) direction of polymerization through 3.
salts of 3 are mixed crystals of the two comonomers 1 and 2, and the crystal structure of 3 shows that the comonomers are already preorganized in the alternate fashion that will be found in PPPI (cf. Figure 2), with the reacting functions already in spatial proximity of less than 2 Å, via strong H-bonding. Note that the transformation from 3 to 4 is considerably different from topochemical reactions, which are monocrystal-tomonocrystal transformations.28−30 Topochemical transformations have been shown to be feasible for polymers, however exclusively for polyadditions with the reacting functions of the monomers being either double or triple bonds.31−34 The SSP from 3 to 4 is however a polycondensation with the liberation of H2O as low-molecular weight byproduct. The considerable molecular movement of the condensate H2O at the high reaction temperatures of SSP, and its striving to leave the particles, is hampering the possibility of a topochemical transformation.
diffractogram reflects higher ordering than the diffractogram of PPPI synthesized via classical polycondensation in m-cresol (carried out for comparison, see Figure 3D). The superior crystallinity of PPPI obtained from SSPno matter if from bulk single crystals or powder of single crystals of 3as compared to classically synthesized PPPI, manifests itself by two points: (i) The reflections present in both classical and PPPI from SSP are sharper in PPPI from SSP. The relevant reflections found in both are (002), (110), (003), (122), and (202), see Figure 3C,D. (ii) There are four reflections that are nicely defined and assignable in PPPI from SSP, of which three cannot be clearly assigned in classically synthesized PPPI (red boxes in Figure 3C,D): (120) at ∼27.0°(2θ), (121) at ∼28.0°(2θ), (004) ∼ 28.8°(2θ), and (122) at ∼30.7°(2θ). As for the underpinnings of SSP of 3 leading to higher global chain alignment than classical polycondensation, we believe that the fact that 3 is highly crystalline is of key importance. Monomer F
DOI: 10.1021/acs.macromol.5b01545 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
We believe that there are three reasons for the crack formation. (i) The conformation of a PPPI chain within the crystal structure viewed along the b-axis of the unit cell (Figure 4M) shows that the imide bearing phenyl rings are twisted with respect to the initial diamine phenyl rings with a torsional angle of 109.8°. In the initial monomer salt crystal, the monomers are however packed in a near-parallel fashion (Figure 2A,D−F). Thus, the molecular movement required for the PPPI formation and conformation accounts in part for mechanical strain, leading to cracks. (ii) Two equiv. of water are liberated per imide ring formed; i.e., 20% of the initial mass is lost upon polymerization (TGA, Figure 3A). Even if one assumes, that part of the forming H2O molecules are first retained in the form of partially hydrated PPPI chains, the condensate will have entirely left the particles upon complete polymerization. At these elevated temperatures, the H2O molecules move considerably within the particles and eventually coalesce to (H2O)n clusters. Both their movement and striving outward surely creates mechanical strain, and we can only speculate that H2O even leaves the particles through these cracks. (iii) When heating crystals of 3, it is first the surface of the crystallites that becomes heated to the external temperature, while the interior of the particles is still colder. Therefore, a radial temperature gradient exists through the particles (Figure 4N). While the particles’ surface already starts polymerizing, the inside is still monomer salt, and mechanical strain arises due to two different materials being confined in the same particle. From a mechanistic point of view, these cracks are however extremely helpful, as they give an indication for the direction of polymerization within single crystals of 3: From powder XRD of PPPI from SSP, we know that the polyimide chains are globally aligned, comparable to liquid crystalline order. Since it is hardly imaginable that neither mechanical strain nor released condensation H2O would break covalent bonds, we think that both stress and water are released inbetween the chains, and consequently that the cracks form in parallel to the chains. If the cracks were perfectly perpendicular to {001} and {001̅}, they would align with {3̅01} and {301̅}. These planes align roughlybut not exactlywith the planes in which the aromatic rings of PMA2− and H2PDA2+ are found (Figure 4O,P). Hence, it seems plausible that the direction of polymerization aligns roughly with alternating comonomers in the direction of c*, as illustrated in Figure 4Q. However, as every reacting function is connected to several opposite monomers via H-bonds that vary extremely little in length (see Supporting Information) and thus in energy, it is not possible from the present study, to tell with which pair of αCOOH, β-CO2− a given amminium function reacts to form the cyclic imide moiety.
In order to subsequently study the shape retention of single crystals of 3 that we had previously observed for polycrystalline monomer salts,21 we performed SSP of single crystals of 3 obtained from gel crystallization. Having determined the onset of polymerization from DSC, TGA, and HT-XRD, we consequently performed solid-state polycondensations of 3 at a slightly higher temperature (210 °C) for 2 h under inert atmosphere. In order to drive the polycondensation to the product side, the condensate H2O typically has to be removed from the reaction mixture. In classical PI synthesis, this is ensured by employing Dean−Stark traps. In SSP however, the removal of H2O is not necessary, as the condensate phase separates effectively from the PI products by evaporation.21 As judged from FT-IR-ATR, 4 is fully condensed (no observable end-groups) after 2 h at 210 °C (see Supporting Information). Unfortunately we could not determine the average molecular weight of the PIs due to the insolubility of PPPI in virtually any solvent.22 Particles of 4 obtained from SSP retain the initial crystal shape of 3, as evinced from optical microscopy (Figure 4A). The retention of the shape of the initial single crystals was monitored by in situ HT-optical microscopy (see Supporting Information for video of the transformation). Interestingly, there are are no observable changes in dimension of the particles. As for the from TGA observed mass loss of 20% (Figure 3A), and the change in crystal density g g ( ρ(3) = 1.62 3 , and ρ(4) = 1.73 3 ), one would expect a cm cm calculated change of volume of 25% (see Supporting Information for calulation). Consequently, the expected changes of edge lengths of single crystals of 3 lie in the range of ca. 9% (see Supporting Information). Changes of ca. 9%, ≈ 20−30 μm are hard to detect with conventional optical microscopes. In addition, the calculation is based on crystal densities of fully crystalline materials. We expect the actual density of 4 to be smaller than 1.73 g 3 , as macromolecules of 4 cm
are not fully but solely globally aligned, which implies less efficient packing. Moreover, the elimination of the condensate H2O has been previously shown to generate additional porosity in polyimides obtained from SSP of monomer salts,21 which might lead to a smaller increase in apparent density than anticipated from the crystal densities of 3 and 4. Hence, small changes in volume are expected to arise from SSP, but we could not detect any such changes by optical microscopy. While we do not observe changes in dimension and shape, the aspect changes significantly: Upon SSP, the particles become darker and turn from translucent to turbid. The loss of translucence is a consequence of 4 not being single-crystalline, and the darkening toward orange-brown shades is typical for fully aromatic PIs. Most interestingly, the particles show nearlinear cracks (Figure 4B−E), which are situated roughly perpendicularly to the longer diagonal of the biggest crystal facet ({001} and {001̅}, cf. Figure 1A). When following the SSP of a monomer salt single crystal in real-time in optical microscopy, we observe that the particles remain entirely transparent until ca. 190 °C, and only darken slightly (Figure 4F−H). At 205 °C the particles start to display cracks ca. perpendicular to the longer diagonal of {001} and {001̅} (Figure 4I). When keeping the particles at 205 °C for the polymerization to complete, further cracks appear over the course of time (Figure 4J−K). The video of the thermal SSP of single crystals of 3 (Supporting Information), further illustrates that the crystals are subjected to considerable mechanical strain, which even generates movement of the particles during SSP.
■
CONCLUSION Solid-state polymerization of monomer salt crystals opens up the possibility of conceiving nonspherical homopolymer particles by simple heating. By using gel crystallization we were able to synthesize the first example of monomer salt single-crystals. This allowed for solving the crystal structure of the monomer salt, which revealed a highly dense structure with a high number of intermolecular H-bonds. SSP of the monomer salt crystals gave access to complex polyhedral homopolymer particles that retain the initial crystal shape. Using several analytical techniques, i.e., thermal analysis (TGA and DSC), microscopy (optical and SEM) and real-time measurements (in situ high-temperature optical microscopy and in situ highG
DOI: 10.1021/acs.macromol.5b01545 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
(6) Damasceno, P. F.; Engel, M.; Glotzer, S. C. Science 2012, 337, 453−457. (7) Forster, J. D.; Park, J.-G.; Mittal, M.; Noh, H.; Schreck, C. F.; O'Hern, C. S.; Cao, H.; Furst, E. M.; Dufresne, E. R. ACS Nano 2011, 5, 6695−6700. (8) Glotzer, S. C. Science 2004, 306, 419−420. (9) Grzelczak, M.; Pérez-Juste, J.; Mulvaney, P.; Liz-Marzán, L. M. Chem. Soc. Rev. 2008, 37, 1783−1791. (10) Sun, Y.; Xia, Y. Science 2002, 298, 2176−2179. (11) Lee, K. J.; Yoon, J.; Rahmani, S.; Hwang, S.; Bhaskar, S.; Mitragotri, S.; Lahann, J. Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 16057−16062. (12) Walther, A.; Müller, A. H. Soft Matter 2008, 4, 663−668. (13) Champion, J. A.; Katare, Y. K.; Mitragotri, S. Proc. Natl. Acad. Sci. U. S. A. 2007, 104, 11901−11904. (14) Ho, C.; Keller, A.; Odell, J.; Ottewill, R. Colloid Polym. Sci. 1993, 271, 469−479. (15) Vutukuri, H. R.; Imhof, A.; van Blaaderen, A. Angew. Chem., Int. Ed. 2014, 53, 13830−13834. (16) Dendukuri, D.; Pregibon, D. C.; Collins, J.; Hatton, T. A.; Doyle, P. S. Nat. Mater. 2006, 5, 365−369. (17) Xu, S.; Nie, Z.; Seo, M.; Lewis, P.; Kumacheva, E.; Stone, H. A.; Garstecki, P.; Weibel, D. B.; Gitlin, I.; Whitesides, G. M. Angew. Chem. 2005, 117, 734−738. (18) Robinson, I. M.; Edwards, W. M. Diamine salts of pyromellitic acid diester. US Patent 2,880,230, 1959. (19) Bell, V. L. J. Polym. Sci., Part B: Polym. Lett. 1967, 5, 941−946. (20) Imai, Y. J. Photopolym. Sci. Technol. 1994, 7, 251−256. (21) Unterlass, M. M.; Emmerling, F.; Antonietti, M.; Weber, J. Chem. Commun. 2014, 50, 430−432. (22) Baumgartner, B.; Bojdys, M. J.; Unterlass, M. M. Polym. Chem. 2014, 5, 3771−3776. (23) Tashiro, K. Prog. Polym. Sci. 1993, 18, 377−435. (24) Henisch, H. K. Crystals in gels and Liesegang rings; Cambridge University Press: 2005. (25) Arora, S. Prog. Cryst. Growth Charact. 1981, 4, 345−378. (26) Foster, J. A.; Piepenbrock, M.-O. M.; Lloyd, G. O.; Clarke, N.; Howard, J. A.; Steed, J. W. Nat. Chem. 2010, 2, 1037−1043. (27) Desiraju, G. R.; Steiner, T. The weak hydrogen bond in structural chemistry and biology - IUCr Monographs on Crystallography; Oxford University Press/International Union of Crystallography: 2001. (28) Kohlschütter, V.; Nägeli, A. Helv. Chim. Acta 1921, 4, 45−76. (29) Morawetz, H.; Jakabhazy, S.; Lando, J.; Shafer, J. Proc. Natl. Acad. Sci. U. S. A. 1963, 49, 789. (30) Lotgering, F. J. Inorg. Nucl. Chem. 1959, 9, 113−123. (31) Schermann, W.; Wegner, G.; Williams, J. O.; Thomas, J. M. J. Polym. Sci., Polym. Phys. Ed. 1975, 13, 753−763. (32) Colson, J. W.; Dichtel, W. R. Nat. Chem. 2013, 5, 453−465. (33) Jin, H.; Plonka, A. M.; Parise, J. B.; Goroff, N. S. CrystEngComm 2013, 15, 3106−3110. (34) Itoh, T.; Suzuki, T.; Uno, T.; Kubo, M.; Tohnai, N.; Miyata, M. Angew. Chem. 2011, 123, 2301−2304.
temperature PXRD) in combination with the knowledge of the monomer salt and the polymer crystal structure allowed for developing a mechanistic picture of the transformation. Especially the formation of cracks during SSP enabled to determine the direction of polymerization inside the crystals. The fact that PPPI is a highly solvent-resistant high-performance polymer showing temperature-stability up to 640 °C, opens the possibility to use these particles under extreme conditions and to keep them nondissolved in virtually any solvent. We believe that SSP of monomer salts will open the door to the synthesis of a wide variety of anisotropic homopolymer particles. With respect to using the reported angular polyimide particles for e.g. self-assembly or as building blocks in colloidal crystals, the major open challenges for future work are to decrease particle sizes to the lower μm range, and to develop methods for improved control over size distribution and shape.
■
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b01545. Experimental procedures, analytical procedures, calculation of expected volume changes and additional characterization data (FT-IR-ATR, single-crystal data, optical microscopy, crystal facet indexing) (PDF) Cif file for 3 (CIF) Video of in situ HT-optical microscopy of the transformation of 3 (ZIP)
■
AUTHOR INFORMATION
Corresponding Author
*(M.M.U.) Telephone: "+43(0)58801165206. E-mail: miriam.
[email protected]. Author Contributions §
Contributed equally to this work
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS The authors acknowledge TU Wien for funding this project, and are grateful to Werner Artner, Michael J. Bojdys, Emanuela Bianchi, Klaudia Hradil, Jö rg Menche, Ronald MiletichPawliczek, and Peter D. J. van Oostrum for helpful discussions. Bettina Baumgartner is acknowledged for preparing PPPI by classical polycondensation. Powder X-ray diffraction measurements, single crystal diffraction experiments, and optical microscopy were carried out at the X-ray Center of TU Wien (XRC), and SEM was carried out at the interfaculty electron microscopy facility of TU Wien (USTEM).
■
REFERENCES
(1) Agarwal, U.; Escobedo, F. A. Nat. Mater. 2011, 10, 230−235. (2) Yunker, P. J.; Still, T.; Lohr, M. A.; Yodh, A. Nature 2011, 476, 308−311. (3) Cavallaro, M.; Botto, L.; Lewandowski, E. P.; Wang, M.; Stebe, K. J. Proc. Natl. Acad. Sci. U. S. A. 2011, 108, 20923−20928. (4) Vermant, J. Nature 2011, 476, 286−287. (5) Haji-Akbari, A.; Engel, M.; Keys, A. S.; Zheng, X.; Petschek, R. G.; Palffy-Muhoray, P.; Glotzer, S. C. Nature 2009, 462, 773−777. H
DOI: 10.1021/acs.macromol.5b01545 Macromolecules XXXX, XXX, XXX−XXX
