Introduction

Comb polymers are useful materials because of the different solid-state natures of main chains and side chains, which generally afford long-range phase-segregated structures.1, 2 Crystalline main chain with amorphous side chain3, 4, 5, 6, 7, 8, 9, 10, 11 comb polymers and amorphous main chain with crystalline side chain12, 13, 14, 15, 16, 17, 18 ones have been studied extensively and reviewed in terms of their chemistry19, 20 and physical properties.21 The movement of crystalline segments is variably inhibited by the amorphous portion; thus, formation of various segregated structures is often observed from similar materials. The ‘crystalline–crystalline’ comb polymers,22, 23, 24, 25 especially those with aromatic main chain polymers,26, 27, 28, 29 are of interest because packing of the bulky aromatic polymer main chain can be changed by varying the crystallinity of side chains. As the first example, Kevlar, poly(p-phenylene terephthalamide), was functionalized with linear hydrocarbons of 3, 4, 7, 12 and 18 carbon atoms through metalation of poly(p-phenylene terephthalamide) using sodium hydride (degree of substitution (DS) was 86–99%, inherent viscosity (ηinh) 0.06–0.14), and the packing mode was investigated for the resulting comb polymers, which were all amorphous except for the polymer with C18H37 side chains.30 Poly(3-n-alkyl-4-oxybenzoate)s (PAOBA)s with alkyl side chains varying from 1 to 18 carbon atoms were prepared and their phase properties fully investigated.31 The structure and thermal transition from crystalline to mesophase of PAOBA with short alkyl chains, such as propyl groups, was controlled by the rigid-rod main chain, whereas those of PAOBA having longer alkyl chains were controlled by the nature of side chains. N-Alkylated poly(p-benzamide) (PABAn, DS >96%) (from 10 to 18 carbon atoms) was also prepared by the metalation method, and their packing structure was investigated.32 In contrast to the high crystallinity of PAOBA, all PABAn polymers, except for the one with the 18-carbon alkyl chain, were amorphous because of bulky aromatic rings and relatively sparse semicrystalline alkyl chains. We expected that well-defined PABAn should pack easily in a different form on polymerization of a completely alkylated monomer with a side chain containing a number of carbon atoms ranging from 1 to 17. Therefore, this study focused on the preparation of high-molecular-weight PABAn from N-alkylated p-aminobenzoic acid and investigation of the packing that occurred on varying the length of alkyl side chains.

Experimental procedure

Measurements

Fourier transform infrared (IR) spectra were measured with a Jasco IR-5500 (Jasco, Tokyo, Japan) by transmittance absorption spectroscopy (KBr tablet method). The molecular mass of polyamides (weight average (Mw)) was determined using a Viscotek T60A gel permeation chromatograph (Viscotek, Houston, TX, USA) equipped with a three detector system: refractive index, light scattering detector and viscometer detector. The scattering angles, dn/dc (refractive index increment) and sample concentrations were set to be 90.0, 0.185 and 1.0 mg ml−1, respectively. Separation was carried out with Tosoh HLC-8120 GPC (Tosoh, Tokyo, Japan) using a consecutive polystyrene gel column (TSK-GEL GMHHR-M and GMHHR-N) at 40 °C and eluted with tetrahydrofuran (THF) at a flow rate of 1.0 ml min−1. Nuclear magnetic resonance (NMR) was performed on a Bruker AC-400P spectrometer (Bruker BioSpin K.K., Yokohama, Japan) at 400 MHz for 1H and at 100 MHz for 13C measurements. Deuterated chloroform (CDCl3) was used as a solvent with tetramethylsilane as an internal reference. Thermal analyses were carried out on a Seiko thermal analyzer (Seiko Instruments Inc., Chiba, Japan) (SCC 5200 system) at a heating rate of 10 °C min −1 for thermogravimetric analysis (by TG/DTA 320) under air or nitrogen. Differential scanning calorimetry (DSC) was performed on a Shimadzu DSC-60 (Shimadzu Corporation, Kyoto, Japan) at a heating rate of 20 °C min−1 under nitrogen. The packing modes of several polymers in the crystalline phase were examined by wide-angle X-ray powder diffraction (WAXD) measurements using a Rigaku R-axis Rapid diffractometer (Rigaku Corporation, Tokyo, Japan) equipped with a graphite monochromator, with CuKα radiation at 45 kV and 200 mA. The layer structure of comb polymers was characterized with a small-angle X-ray scattering (SAXS) instrument (M18XHF, MAC Science, Bruker AXS K.K., Yokohama, Japan) consisting of an 18-kW rotating-anode X-ray generator with a Cu target (wavelength, λ=0.154 nm) operated at 50 kV and 300 mA. This instrument contained a pyrographite monochromator, pinhole collimation system (φ=0.3, 0.3, 1.1 mm), vacuum chamber for the scattered beam path and a two-dimensional imaging plate detector (DIP-220). The sample-to-detector distance was adjusted to 710 mm. The exposure time for each sample was 30 min. For SAXS measurements, a sample of 1-mm thickness was placed in the sample holder so that its position remained unchanged.

Reagents

p-Aminobenzoic acid ethyl ester and p-(N-methylamino)benzoic acid were purchased from Wako Pure Chemical Industries (Osaka, Japan) and TCI Chemical Industry (Tokyo, Japan), respectively, and used as received. Dry pyridine was purchased from Kanto Chemical (Tokyo, Japan) and used under nitrogen. Hexamethylphosphorous triamide was dried over calcium hydride and distilled under reduced pressure. The other reagents and solvents were used as received.

Synthesis of p-(N-propylamino)benzoic acid

Into a three-necked flask equipped with a reflux condenser, a thermometer and a three-way stopcock was added p-aminobenzoic acid ethyl ester (79.3 g, 0.480 mol) under nitrogen. Hexamethylphosphorous triamide (222 ml) was then added and the mixture was stirred until the solid was completely dissolved at 20 °C. 1-Iodopropane (42.9 g, 0.252 mol) was then added to this solution, and the mixture was stirred at 130 °C for 4 h. The resultant mixture was poured into water (1 l), and the precipitate was washed with methanol/water (1:5 v/v)-mixed solvent to remove the residual solvent and salt. The solid was collected by filtration, purified by recrystallization from methanol and dried in vacuo at 80 °C for 12 h to give p-(N-propylamino)benzoic acid ethyl ester as yellow needles. Yield: 46.5 g (88.9%); m.p.: 70–73 °C; IR (KBr): ν 3373 (N-H), 3063 (aromatic C-H), 2964 (aliphatic C-H), 1676 (ester C=O), 1343 (Ar-N), 1173 (C-O) cm−1; 1H NMR (CDCl3, 400 MHz): δ 1.00 (t, 3H, CH3), 1.36 (t, 3H, CH3), 1.65 (m, 2H, CH2), 3.13 (t, 2H, CH2), 4.11 (s, 1H, NH), 4.31 (q, 2H, CH2), 6.53 (d, 2H, ArH), 7.86 (d, 2H, ArH) p.p.m.; 13C NMR (CDCl3, 100 MHz): δ 11.6, 14.5, 22.5, 45.2, 60.1, 111.3, 118.4, 131.5, 152.1, 166.9 p.p.m. The resulting p-(N-propylamino)benzoic acid ethyl ester (38.3 g, 0.185 mol) was placed in a 500-ml two-necked flask equipped with a reflux condenser. Potassium hydroxide (56.8 g, 0.923 mol) and ethanol (450 ml) were added to this flask and heated to 90 °C. After 7 h, the reaction mixture was poured into water, and the pH of the solution was adjusted to be around 4 with hydrochloric acid. The precipitate was collected, washed with water, recrystallized with methanol and dried in vacuo at 80 °C for 12 h to give the title compound as yellow needles. Yield: 21.2 g (64.1%); m.p.: 164–169 °C; IR (KBr): ν 3377 (N-H), 3058 (aromatic C-H), 2961 (aliphatic C-H), 1663 (C=O), 1313 (Ar-N), 1171 (C-O) cm−1; 1H NMR (CDCl3, 400 MHz): δ 1.01 (t, 3H, CH3), 1.66 (q, 2H, CH2), 3.15 (t, 2H, CH2), 6.55 (d, 2H, ArH), 7.91 (d, 2H, ArH); 13C NMR (CDCl3, 100 MHz): δ 11.5, 22.5, 45.1, 111.3, 117.0, 132.3, 152.7, 172.2 p.p.m.; anal. calcd for C10H13NO2: C, 67.02; H, 7.31; N, 7.82%; found: C, 66.92; H, 7.26; N, 7.88%.

Synthesis of p-(N-butylamino)benzoic acid

The title compound was prepared in a manner similar to that described above. The solid was collected by filtration, purified by recrystallization from methanol and dried in vacuo at 80 °C for 12 h to give pale-yellow needles. Yield: 1.36 g (74.0%); m.p.: 149–155 °C.

Synthesis of p-(N-pentylamino)benzoic acid

The title compound was prepared in a manner similar to that described above. The solid was collected by filtration, purified by recrystallization from methanol and dried in vacuo at 80 °C for 12 h to give colorless needles. Yield: 18.6 g (68.0%). m.p.: 132–133 °C.

Synthesis of p-(N-heptylamino)benzoic acid

The title compound was prepared in a manner similar to that described above. The solid was collected by filtration, purified by recrystallization from methanol and dried in vacuo at 80 °C for 12 h to give colorless needles. Yield: 11.3 g (75.0%). m.p.: 107–122 °C.

Synthesis of p-(N-octylamino)benzoic acid

The title compound was prepared in a manner similar to that described above. The solid was collected by filtration, purified by recrystallization from methanol and dried in vacuo at 80 °C for 12 h to result in yellow needles. Yield: 13.6 g (84.0%); m.p.: 117–125 °C.

Synthesis of p-(N-heptadecylamino)benzoic acid

The title compound was prepared in a manner similar to that described above. The solid was collected by filtration, purified by recrystallization from methanol and dried in vacuo at 80 °C for 12 h to result in white needles. Yield: 9.60 g (64.0%); m.p.: 126–127 °C.

Synthesis of poly(p-benzamide) (PABA0)

Into a three-necked flask equipped with a reflux condenser, a three-way stopcock and a thermometer were added lithium chloride (10.5 g, 0.247 mol, 4 wt% to the solution), N-methyl-2-pyrrolidone (200 ml) and pyridine (50 ml). The mixture was stirred until the solid was dissolved. p-Aminobenzoic acid (13.7 g, 0.100 mol) was then added to the flask, followed by triphenylphosphine oxide (31.3 g, 0.100 mol) in one portion. The flask was heated to 100 °C, and the mixture was stirred for another 6 h. The resulting heterogeneous solution was poured into methanol (1 l) and stirred for 5 h. The precipitate was collected, washed with hot water and dried in vacuo at 220 °C for 24 h to give the title compound. Yield: 11.4 g (95.0%); ηinh=1.18 per 100 ml g−1 (0.5 g per 100 ml in conc. H2SO4, 30 °C); IR (KBr): ν 3446 (N-H), 1669 (C=O), 1315 (Ar-N), 1177 (amide C-N) cm−1; anal. calcd for (C7H5NO)n: C, 70.58; H, 4.23; N, 11.76%; found: C, 69.33; H, 4.75; N, 10.96%.

Typical procedure of the direct polymerization of N-alkylated monomers: synthesis of N-methylated poly(p-benzamide) (PABA1)

Into a three-necked flask equipped with a reflux condenser, a three-way stopcock and a thermometer, were added p-(N-methylamino)benzoic acid (4.55 g, 30.1 mol), triphenylphosphine (9.44 g, 36.0 mmol) and pyridine (30 ml). The mixture was stirred until the solid was dissolved. Hexachloroethane (8.52 g, 36.0 mmol) was then added to this solution and the mixture was refluxed for 24 h. After the solution was cooled to room temperature, it was poured into methanol/hydrochloric acid and stirred for 5 h. The precipitates were collected, stirred in hot methanol for 12 h and dried at 220 °C for 24 h to give the title compound. Yield: 2.22 g (55.6%); ηinh=0.61 per 100 ml g−1 (0.5 g per 100 ml in H2SO4, 30 °C); IR (KBr): ν 3051 (N-H), 2923 (aliphatic C-H), 1645 (C=O), 1368 (Ar-N), 1175 (amide C-N) cm−1; anal. calcd for (C8H7NO)n: C, 72.16; H, 5.30; N, 10.52%; found: C, 71.49; H, 5.29; N, 10.39%.

Results and discussion

Monomer synthesis and polymerization

The crystalline structure of comb polymers can be formed by the interaction between polymer main chains or polymer side chains. To carefully evaluate the crystalline phase of PABAn, we selected the carbon number of alkyl side chain to be 0, 1, 3, 4, 5, 7, 8 and 17. The crystalline structure of PABAn, where n is 0, 1, 3, 4, 5, 7 and 8, would have arisen from the influence of the polymer main chain, and that of PABA17 could have been from the polymer side chain. p-(N-Alkylamino)benzoic acids (alkyl groups: propyl, butyl, pentyl, heptyl, octyl and heptadecyl) were prepared from p-aminobenzoic acid ethyl ester according to a previously published method33 (Scheme 1). The monomers were characterized by IR, NMR and elemental analyses. The polycondensation of p-aminobenzoic acid was performed using the method of Yamazaki et al.,34 involving a triphenyl phosphine oxide/pyridine/lithium chloride condensation agent system, to give the corresponding polymer (PABA0) with an inherent viscosity of 1.18 per 100 ml g−1 (0.5 g per 100 ml in conc. H2SO4, 30 °C). However, this condensation system was not effective for the polycondensation of p-(N-alkylamino)benzoic acid monomers, probably because of the very low nucleophilicity of secondary amine moieties that are further deactivated by the inductive effect of a p-carbonyl group.35 A hexachloroethane/triphenylphosphine condensation agent system was developed for the preparation of peptides,36 polyesters37 and polyamides.38 Heitz and his coworker39 applied this system for the polycondensation of p-(N-methylamino)benzoic acid to give the corresponding PABA1 in 47% yield with an inherent viscosity of 0.37 per 100 ml g−1 (0.5 g per 100 ml in conc. H2SO4, 30 °C) (Mw=27 300). Thus, we applied this method for the preparation of PABAn (Scheme 2). Table 1 summarizes the results of polycondensation of p-(N-alkylamino)benzoic acids. The inherent viscosity of the polymer decreased from 0.65 to 0.06 per 100 ml g−1 (0.5 g per 100 ml in conc. H2SO4, 30 °C) with the length of the alkyl side chain. PABAn (n7) is soluble in THF; therefore the weight-average molecular weight (Mw) of the polymer was determined by the light scattering method in THF. The Mw values of PABA7, PABA8 and PABA17 were 47 800, 20 600 and 93 400, respectively. In general, introduction of bulky substituents into the reactive functional group or its neighbors reduces reactivity, resulting in lower-molecular-weight polymers. Introduction of a sufficiently long alkyl group also hampers the reaction, but increases the solubility of the resulting polyamide, resulting in a relatively high-molecular-weight product as seen for PABA17. Because all polymers have a molecular weight distribution around 2.0, the degree of polymerization was calculated to be 45 for PABA8 and >100 for other PABAn polymers. These molecular weights were high enough to evaluate the differences in packing based on the characteristics of the polymer main chain and polymer side chain. Table 2 shows the solubility of PABAn in typical organic solvents. PABA0 is soluble only in conc. H2SO4, but PABA1 is soluble in m-cresol on heating. An increase in the length of the PABAn alkyl chain resulted in an improvement in the solubility of the polymer in solvents such as N-methyl-2-pyrrolidone, chloroform and THF. This indicates that the dipolar–dipolar interaction between polymer chains is effectively weakened by bulky hydrocarbon substituents. The IR of PABAn spectra supports this assumption (Figure 1 and Supplementary Figure 1); the amide carbonyl absorption of PABAn with n=1, 3, 4 or 5 was 1640 cm−1, whereas those of PABAn with n=7, 8 and 17 shifted to longer wavenumbers. Thus, the amide groups of PABAn with n=7, 8 or 17 are isolated by bulky hydrocarbon substituents, whereas those of PABAn with n=1, 3, 4 and 5 can interact with each other, which also prevents the intrusion of organic solvents.

Scheme 1
figure 1

Monomer synthesis.

Scheme 2
figure 2

Direct polycondensation of monomer I.

Table 1 Analyzed data for the polymerization products of direct polycondensation of II
Table 2 Solubility of polymers
Figure 1
figure 3

Infrared spectra of N-alkylated poly(p-benzamide) (PABAn) (n=0, 1, 3, 4, 5, 7, 8, 17). The dotted lines a, b and c indicate wavelengths at 1640, 1464 and 720 cm−1, respectively.

Thermal properties of PABAs

To determine the thermal transition and themostability of PABAn, DSC and thermogravimetric analyses were carried out under air and nitrogen (Table 3). Figure 2 depicts the thermogravimetric analysis profiles of PABA0, PABA1 and PABA17 under N2. These three polymers showed a similar 10% weight loss near 440 °C under nitrogen; however, PABA17 rapidly decomposed and lost 95% of its weight by 500 °C thereafter. In the presence of oxygen, PABA17 had the lowest decomposition temperature compared with other polymers, probably because of the presence of the alkyl side chain (Figure 2b). This is a general characteristic of comb polymers.30 Figure 3 shows the second heating DSC curves of PABAn. PABA0 shows no transition or endothermic peak of melting, whereas PABA1 shows a glass transition temperature (Tg) at 197 °C. Increasing the length of the alkyl side chain causes the Tg of polymers to gradually decrease and eventually become constant (Figure 4), because of the independence of the relaxation of the polymer main chain when the alkyl chain length is regarded as infinity. PABA3, PABA4 and PABA5 show exothermic and endothermic peaks after Tg, which were attributed to the crystallization (Tc) and melting temperatures (Tm), respectively. A further increase in the alkyl side chain length (n7) diminishes the crystalline nature of the polymer, until PABA17, which possesses crystallinity (Figure 3). This phenomenon suggests that the packing mode of PABAn with n=3, 4 and 5 is determined by the polymer main chain, and that of PABA17 is determined by the polymer side chain. PABAn polymers with n=3, 4 and 5, which form the packing mode between main chains, also rearranged in the crystallization temperature of the heating process because this type of polymer has a relatively slow crystallization rate. On the other hand, the octadecyl side chain itself in PABA17 has high crystallization ability. Note that the reported Tm of PABA17 (with a 97% degree of substituents), prepared by alkylation of poly(p-benzamide) with sodium hydride, was 3 °C, which was unexpectedly lower than that of n-hexadecane (Tm=22 °C).40 The authors attributed this reason to the difficult packing of PABA17 because of the bulky phenylene groups. However, in our case, PABA17 (with a 100% degree of substituents) melts at 31 °C, which indicates that only a 3% deficit in the introduction of alkyl side chains significantly decreases side-chain crystallinity for PABA17. The heat of fusion per monomeric unit of PABA17 determined by DSC was −36.8 J g−1, which is comparable with the reported value of comb polyamides.40, 41

Table 3 Thermal properties and the crystallinity of polymers
Figure 2
figure 4

Thermogravimetric analysis profiles of N-alkylated poly(p-benzamide) (PABAn) (n=0, 1, 17) at a heating rate of 10 °C min−1 under (a) nitrogen and (b) air.

Figure 3
figure 5

Differential scanning calorimetry profiles of N-alkylated poly(p-benzamide) (PABAn) (n=0, 1, 3, 4, 5, 7, 8, 17) at a heating rate of 20 °C min−1 under nitrogen.

Figure 4
figure 6

Relationship between the alkyl chain length and Tg of N-alkylated poly(p-benzamide) (PABAn).

X-ray scattering study of PABAs

The packing mode of PABAn was investigated using WAXD measurements. Figure 5 depicts the WAXD patterns and profiles with a reciprocal lattice of PABAn polymers. High-resolution WAXD profiles can be found in Supplementary Figures 2–9 in the Supplementary material. All polymers, except PABA7 and PABA8, produced several sharp diffraction peaks, indicating crystalline structures. Several studies have reported a crystalline structure for PABA0, in which polymers are tightly packed through hydrogen bonding of the amide group (trans conformation) to form orthorhombic systems with order parameters of a=7.74, b=5.27 and c=12.82 Å.42, 43 However, the polymer we prepared shows only two peaks at 20.3 and 23.3° in 2θ (4.4 and 3.8 Å, respectively), indicating that the polymer is loosely packed and contained some crystalline structure. Generally, the crystallization degree of crystalline polymers remarkably depended on how to crystallize. The crystallization degree of PABA0 reported in literature42 is relatively high. However, we merely discuss the crystal structure at polymerization crystallization. Essentially, aromatic polyamides having a rigid main chain are hard to crystallize with a high degree. From our experimental findings, it is found that the PABA0 crystal formed by polymerization crystallization shows low crystallinity. Interestingly, the X-ray diffraction pattern of PABA1 shows two clear peaks at 14.9 and 16.8°, along with smaller ones at 8.4, 23.8, 25.8 and 29.2° in 2θ. From the results of analysis for the reciprocal lattice to diffraction peak and previously reported crystal system of similar polymers, it is found that PABA1 are packed into a monoclinic lattice with order parameters of α=γ=90°, β=67.5°, a=7.7 Å, b=5.7 Å and c=10.8 Å (Figure 5b). The reported crystal systems of similar polymers in literature, such as poly(p-phenylene terephthalamide)30 and poly-(p-benzamide),32 are monoclinic (a=7.8 Å, b=5.19 Å, c=12.9 Å, γ=90 Å) and orthorhombic (a=7.71 Å, b=5.14 Å, c=12.8 Å) systems, respectively. In contrast, PABAn polymers with n=1, 3, 4 or 5 are packed into an orthorhombic lattice with α=β=γ=90°, a=7.7 Å, b=5.6 Å and c=13.0 Å, because all WAXD peaks of these polyamides can be located on the reciprocal lattice of an orthorhombic system (Figure 5b).

Figure 5
figure 7

(a) Two-dimensional X-ray patterns and X-ray intensity with diffraction angle θ for N-alkylated poly(p-benzamide) (PABAn) (n=0, 1, 3, 4, 5, 7, 8, 17). (b) Analysis of wide-angle X-ray powder diffraction (WAXD) peaks on the reciprocal lattice of PABA1 and PABAn (n=3, 4 and 5). a*, b*, and c* in a coordinate axis mean a vector of reciprocal lattice. (c) Small-angle X-ray scattering pattern and profile of PABA17.

Figure 6 shows the molecular arrangement and packing models of PABA1 and PABAn (n=3, 4 and 5). The small methyl moiety of PABA1 could distort the crystal lattice to afford a type of packing different from that of others. In addition, the periodic structure of 10.8 Å for PABA1 and 13.0 Å for PABAn at n=3, 4 and 5 is the pitch of the helix induced by the structural propensities of the cis conformation of N-alkylated amide bonds and syn arrangement of the benzene ring.37 Orthorhombic packing has an extremely high symmetry, similar to that of crystalline polymers. The polyethylene also packs the orthorhombic system. The polymer chains of PABA3, PABA4 and PABA5 are orderly packed in crystal lattice without the interference of side chains. In addition, the development of length along the c-axis is characteristic of crystalline polymers with a helical conformation (for example, PTFE, PLLA, PMMA and so on). A subtitle distortion of the lattice of the PABA1 crystal exhibits the contribution of side chains to lattice formation; that is, this is a contribution for the enhancement of van der Waals interaction between neighboring chains to lattice formation and symmetry. PABA7 and PABA8 showed disordered structures from WAXD profiles, which support the DSC results. It seems that existence of too long side chain disturbs the lattice formation because of steric hindrance.

Figure 6
figure 8

Proposed arrangement of main and side chains in N-alkylated poly(p-benzamide) (PABAn) (n=0, 1, 3, 4, 5).

In the case of PABA17, we can see the typical WAXD profile of ‘side-chain crystalline’ polymers. PABA17, which contains a longer alkyl chain, shows sharp peaks at 2.9°, 5.8° and 21.3° in 2θ (d001=30.5, d002=15.2, and d100=4.2 Å),44 which indicates that the side-chain crystalline structure of PABA17 is in hexagonal packing form, similar to that observed in a comb polymer. The typical trans conformation of the CH2 group appeared at 1466 cm−1 in the IR spectrum, and in our case, PABA17 also shows a strong and clear absorption at 1464 cm−1, which supports the hexagonal packing structure as suggested by WAXD measurements. According to the extended structure of polyethylene, an increase in CH2 resulted in a spacing increase of 1.25 Å.38 Thus, the double-layered space of side-chain crystal domains is calculated to be 1.25 Å × 2 × 17=42.5 Å and that estimated on the basis of the WAXD of PABA17 was 30.5 Å, which is much smaller than the calculated value. These results are also supported by SAXS measurements as shown in Figure 5c and Supplementary Figure 10. In the low-angle region below 2θ=2°, further scattering peaks and a highly ordered structure could not be confirmed. Side chain crystal is hard to form in the only single layer by van der Waals interaction because of a distance between each side chain based on the existence of a rigid aromatic ring in the main-chain. Formation of an interdigitated structure in the double-layer structure shortens both long spacing along the c-axis and the distance between side chains along the main-chain direction. Therefore, it seems that formation of an interdigitated arrangement in the layer structure is suitable. Therefore, the packing structure of the PABA17 side chain could be interdigitated as shown in Figure 7, and the formation of a solid-state structure for N-alkylated polyamides depends on side chain length, systematically. PABA1–5 polymers crystallize under the control of rigid-rod main chains. Specifically, PABA3–5 polymers pack orthorhombically, whereas PABA1 forms a distorted monoclinic system. In contrast, PABA7–8 polymers cannot form a crystalline structure and remain amorphous. The steric hindrance of longer alkyl side chains must obstruct the formation of a crystalline structure. PABA17, owing to its long side chains, no longer packs on the basis of rigid main chains. Instead, this polymer packs hexagonally by van der Waals interactions between heptadecyl side-chain substituents. These experimental findings indicate that control of the first-order structure during polymer synthesis is directly related to the structural control of crystalline morphology (third-order structure) in this system.

Figure 7
figure 9

Proposed arrangement of main and side chains in PABA17.

Summary

A route for the direct condensation polymerization of p-(N-alkylamino)benzoic acids was demonstrated using hexachloroethane, triphenylphosphine and pyridine as condensation reagents, and using methyl, propyl, butyl, pentyl, heptyl, octyl and heptadecyl substituents as alkyl side groups. Pale-yellow PABAn polymers were obtained in 46–87% yields with inherent viscosity values of 0.06–0.65 per 100 ml g−1 (conc. H2SO4, 30 °C). The structures and packing of PABAn polymers were elucidated using IR spectroscopy, elemental analysis, DSC, WAXD and SAXS, which showed that PABA1, PABAn, where n=3, 4 and 5, and PABA17 possess crystalline structures in monoclinic, orthorhombic and hexagonal packing lattices, respectively. The variable packing modes of comb polymers with aromatic main chains will be useful for the development of materials that require thermal and mechanical stability in engineering applications.