Structural Aspects of 1,3,5-Benzenetrisamides-A New Family of Nucleating Agents Magnus Kristiansen,†,‡ Paul Smith,†,* Henri Chanzy,§ Christian Baerlocher,† Volker Gramlich,† Lynne McCusker,† Thomas Weber,† Philip Pattison,#,¶ Markus Blomenhofer,⊥ and Hans-Werner Schmidt⊥,*
CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 6 2556–2558
Department of Materials, Eidgeno¨ssische Technische Hochschule (ETH) Zu¨rich, CH-8093 Zu¨rich, Switzerland, Centre de Recherches sur les Macromole´cules Ve´ge´tales CNRS, BP 53, F-38041 Grenoble, Cedex 9, France, Swiss-Norwegian Beamline, European Synchrotron Radiation Facility, F-38043 Grenoble, France, Laboratoire de Crystallographie, EPFL, CH-1015 Lausanne, Switzerland, and Macromolecular Chemistry, and Bayreuther Institut fu¨r Makromoleku¨lforschung (BIMF), UniVersita¨t Bayreuth, D-95440 Bayreuth, Germany ReceiVed February 6, 2009; ReVised Manuscript ReceiVed April 16, 2009
ABSTRACT: Structural studies of substituted 1,3,5-benzenetrisamides, a new family of highly versatile and efficient nucleating and clarifying agents for polymeric materials, are reported. Subtle details of the molecular structure of these species are shown to have a major influence on their solid-state order, morphology, and ability to nucleate specific polymorphs of isotactic polypropylene and induce a broad spectrum of useful mechanical and optical properties in the latter material. In recent years, certain 1,3,5-benzenetricarboxamides with n-alkyl moieties have received attention because of their aggregation behavior in fluids and ability to induce thermoreversible physical gelation of a variety of organic liquids.1,2 In addition, it has been shown that trialkyl-1,3,5-benzenetricarboxamides with alkyl moieties > C6 display interesting liquidcrystalline behavior.3-5 Futhermore, the chirality transfer of chiral 1,3,5-benzenetricarboxamides to their helical stack in solution and to the helicity of achiral representatives in the liquid crystalline phase was recentlyinvestigated.6 More recently, we reported that substituted 1,3,5-benzenetrisamides represent a highly versatile family of novel nucleating and/or clarifying agents for isotactic polypropylene (i-PP).7 Most notably, the compounds were found to be capable of efficiently inducing predominantly the R- or the β-polymorph8,9 of the polymer, as well as controlled mixtures of them, depending on subtle details of the substituents. The induction of the R-phase in i-PP as well as reduction of spherulite size improve mechanical properties such as strength and stiffness, whereas β-phase nucleated i-PP products have increased toughness. Furthermore, certain species were found to render this typically translucent polymer highly transparent at unprecedented low concentrations (as low as a weight fraction of 0.00015), whereas others caused i-PP to be fully opaque. Additional studies on the crystallization behavior, the morphology development and the optical properties of nucleated i-PP with selected 1,3,5benzenetrisamide derivatives were recently published by Wang et al.10 Here, we present transmission electron microscopy and electron and X-ray diffraction studies of the solid-state structure of selected 1,3,5-benzenetrisamides that were conducted to gain a better understanding of the origin of their outstanding, highly diverse performance, which until now remained obscure. The compounds examined are listed in Table 1, together with some of their relevant characteristics and their influence on the * Corresponding author. Tel.: 0049/921/55-3200 (H.-W.S.); 0041/1/632-2637 (P.S.). Fax 0049 /921/55-3206 (H.-W.S.); 0041 /1/632-1178 (P.S.). E-mail:
[email protected] (H.-W.S.);
[email protected] (P.S.). † ETH Zu¨rich. ‡ Present address: Ciba Specialty Chemicals, CH-4002, Basel, Switzerland. § Centre de Recherches sur les Macromole´cules Ve´ge´tales CNRS. # European Synchrotron Radiation Facility. ¶ EPFL. ⊥ Universita¨t Bayreuth.
crystallization behavior of isotactic polypropylene (recaptured for convenient reference7). Because of the presence of 3 amide moieties per molecule, yielding strong hydrogen bonds, the species exhibited high melting points, most often well in excess of 300 °C, or were found to undergo sublimation at those elevated temperatures, prior to melting. To mimic the nonpolar environment of i-PP, the compounds were recystallized from the high-boiling hydrocarbon solvent squalane C30H62 at temperatures around 330 °C (under nitrogen), an approach previously used by Binsbergen.11 The species were obtained in the form of fibrillar products, indicative of their preferential 1-dimensional aggregation behavior, also observed for N,N′,N′′-tris-(2-methoxyethyl)-1,3,5-benzenetricarboxamide.12 However, remarkably different structural features and solid-state order were observed, depending on subtle structural details of the side groups R, as illustrated by representative images and electron diffraction patterns in Table 1. More specifically, compounds with relatively bulky t-butyl, 1 or cycloaliphatic (e.g., cyclohexyl, 2 substituents were found to form large needle-shaped crystals of high aspect ratios (>50), indicative of the presence of directed hydrogen bonds. In mixtures with i-PP, the crystallization temperature of the polymer was raised from about 110 °C to 125-130 °C for the branched alkyls, and to 115-125 °C for the cycloaliphatic substituents; the R-crystal polymer polymorph was predominant.7 Although compound 2, with unsubstituted cyclohexyl groups, crystallized as rather perfect needles of high aspect ratios, introduction of methyl substituents into the side groups yielded more complex structures (3, 4, Table 1). Crystalline entities formed by the compound with 2-methylcyclohexyl groups (3) displayed kinks, a feature that became more pronounced for 2,3-dimethylcyclohexyl (4). Each methyl substitution on the cyclohexyl ring induces in principle one chiral center, and thus allows for two stereochemical configurations of the substituent. As a consequence, the stacking ability is hampered and increasingly so with further substitution on the cyclohexyl ring. A concomitant conspicuous change from efficient R-nucleation and clarification of i-PP (compounds 2 and 3) to extremely effective nucleation of the mechanically distinct β-phase of the polymer, expressed in the factor k13 and a total absence of haze reduction, was observed for the bisubstituted compound 4. Species featuring side groups with more bulky substituents R (e.g., 1-adamantyl, 5), were found to form needlelike crystals of relatively low aspect ratios (∼10, cf. Table 1). The latter exhibited
10.1021/cg900139d CCC: $40.75 2009 American Chemical Society Published on Web 05/07/2009
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Table 1. Chemical Structures of the Substituted 1,3,5-Benzenetricarboxamides Investigated, Their Peak Melting Temperatures (Tm), and Crystallization Temperatures (Tc,p) of i-PP, β-Polymorph Content (k),13 and Values of Haze and Clarity of 1.1 nm Thick Injection-Molded i-PP/Additive Plaques Comprising 0.15 wt % Additivea
a subl. ) sublimation and dec. ) decomposition, typically at temperatures in excess of 300 °C. The structures of R are drawn with their bond to nitrogen to the left. Also shown are typical transmission electron micrographs of the compounds crystallized from squalane (all scale bars 1 µm) and selected area electron diffraction patterns; where applicable, the direction of the whiskers was vertical. The pattern of 4 was taken at a kink.
Figure 1. (a) Wide-angle X-ray diffraction pattern of compound 1, c* is along the vertical direction; note the streaklike features perpendicular to c*. (b) Top view of the hexagonal unit cell (a ) 14.100(2) Å, c ) 6.930(1) Å, γ ) 120°). (c, d) Planes in reciprocal space reconstructed from synchrotron X-ray diffraction data showing pronounced diffuse scattering in both the hk1 (c) and hk2 (d) planes due to structural disorder within the crystal structure. (e) Schematic representation of the two possible configurations within the columnar aggregates of 1. The direction of the helical hydrogen bonding patterns (i.e., the direction of the oxygen atoms in the amide bonds) “up” (left) or “down” (right).
relatively well-defined diffraction patterns, indicative of a rather perfect crystalline structure. This particular species was among the most thermally stable of the class of compounds based on
benzenetricarboxamides. Also, 5, as well as certain derivatives of it, was found to induce extraordinarily high crystallization temperatures of i-PP (125-131 °C).7
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The phenyl-substituted compounds exhibited relatively low melting temperatures, suggestive of much weaker hydrogen bonding in comparison with compounds 1-5 and different molecular packing. Clearly, the chemical structure of 6 exhibits a potential for electron delocalization, which is known to occur in its linear polymeric version, the aramid poly-(p-phenylene terephthalate).14 The aggregation behavior of such resonance-stabilized molecules, like those in certain discotic liquid crystals,15,16 might be increasingly dictated by π-π interactions or less by the formation of directional hydrogen bonds. Substitution with benzyl (7) instead of phenyl (6), which eliminates the electron delocalization discussed above, was found to further lower the melting point and alter the solid-state morphology of the compound. Unlike the relatively large whiskers observed for 6, species 7 formed highly flexible, nanosized fibrils (cf. Table 1), reminiscent of the twisted fibrillar structures of the well-known nucleating/clarifying agent 1,3:2,4-dibenzylidene sorbitol.17 Most remarkably, however, neither of these species promoted nucleation of i-PP as much as most of the above compounds with saturated substituents, or improved its optical properties. A detailed analysis was carried out of the crystallographic structure of compound 1, the member of this series of novel compounds found to be the most efficient R-nucleator and clarifier for i-PP. The results revealed that its structure consists of columnar aggregates that form a hexagonal lattice (panels a and b in Figure 1). Initial refinement in the space group P63/m (a ) 14.100(2) Å, c ) 6.930(1) Å) indicated that the molecules, with point symmetry C3, follow a 63 screw axis along the column direction. However, the molecules were found to be disordered across the mirror plane. When the mirror plane was removed, both orientations of the molecules were still found to be present in equal numbers (i.e., each half occupied), and no ordered structure in any subgroup of P63 could be derived. Examination of the atomic details of the columnar structure, taking all molecules to be similarly oriented within the column, revealed the presence of triple-helical hydrogenbonding patterns, indicated by dotted lines in Figure 1b. A similar feature was reported previously by Lightfoot et al.12 for N,N′,N′′tris-(2-methoxyethyl)-1,3,5-benzenetricarboxamide, but its directional orientation was not discussed. One important consequence of the presence of directional hydrogen-bonding patterns is that the linear aggregates (“strands”) can possess two different helical hands, depending on the precise arrangement of the amide moieties (see Figure 1e; the terminology “up” and “down”, taken as the direction of the oxygen atoms in the amide bonds, will be used hereafter). In addition to sharp Bragg reflections, streaklike scattering can be observed in the diffraction pattern of the h0l plane in reciprocal space (Figure 1a). The streaks are actually cross sections perpendicular to a two-dimensional system of honeycomb-like diffuse intensities in the hkl layers (l ) integer) (panels c and d in Figure 1), indicating rather perfect order within the one-dimensional homochiral columnar stacks of this compound, but disorder in the lateral directions. Because of the presence of different directional orientations of the columnar aggregates, lateral aggregation of the species at hand can proceed in different ways. If a pair of columns favors an up-down configuration, the lateral packing in a hexagonal fashion will remain frustrated, simply because the preferred packing cannot be fulfilled for three closely packed strands. A detailed discussion of the diffuse scattering intensities and of the disordered structure is beyond the scope of this communication and will be published elsewhere; but in short, that analysis showed that a frustrated “updown” configuration is, indeed, the origin of the disorder in the lateral directions.
Communications Regardless of the two possible helical hands, the exposed side groups of one single supramolecular column form a one-dimensional lattice, and thereby a regular pattern on the compound’s crystal surfaces. The repeat distance in the c-direction was found to be 6.93 Å, which is relatively close to the spacing between two methyl groups exposed in the 31-helix of i-PP of around 6.5 Å;18 with a mismatch of less than 15%, epitaxial growth is favored.19 Moreover, because of the crystal symmetry of 1, six approximately equivalent, epitaxially active surfaces are presented to the supercooled polymer melt, unlike many conventional nucleating agents that nucleate the polymer only at some of their exposed crystal facets (see, for example, ref 20). In light of these features, the radically enhanced nucleation rate of i-PP, as reflected in the very substantial increase in the peak crystallization temperature (to 128.5 °C) of the polymer in the presence of compound 1, is readily understood.
Acknowledgment. The authors are indebted to Doris Hanft and Sandra Ganzleben (Universita¨t Bayreuth) for invaluable assistance with the synthesis of the compounds, Karin Bernland (ETH Zu¨rich) with sample preparation, and to Dr. Natalie Stingelin-Stutzmann (Imperial College London, and ETH Zürich) for critical comments. Supporting Information Available: Materials, experimental conditions, and characterization (PDF). This information is available free of charge via the Internet at http://pubs.acs.org.
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CG900139D
