Conformational and Structural Changes with Increasing Methylene

Article pubs.acs.org/Macromolecules

Conformational and Structural Changes with Increasing Methylene Segment Length in Aromatic−Aliphatic Polyamides Yogesh. S. Deshmukh,*,†,‡ Carolus H. R. M. Wilsens,†,‡ René Verhoef,§ Michael Ryan Hansen,‡,∥,# Dmytro Dudenko,∥,% Robert Graf,‡,∥ Enno A. Klop,§ and Sanjay Rastogi*,†,‡,§,⊥ †

Bio-Based Materials, Faculty of Humanities and Sciences, Maastricht University, P.O. Box 616, 6200 MD Maastricht, The Netherlands ‡ Dutch Polymer Institute (DPI), P.O. Box 902, 5600 AX Eindhoven, The Netherlands § Teijin Aramid Research Institute, P.O. Box 5153, Arnhem, The Netherlands ∥ Max Plank Institute for Polymer Science, Ackermannweg 10, D-55128 Mainz, Germany ⊥ Department of Materials, Loughborough University, Loughborough LE11 3TU, U.K. # Institute of Physical Chemistry, Westfälische Wilhelms-Universität Münster, Corrensstr. 28/30, D-48149 Münster, Germany % Department of Physics, University of Warwick, Coventry CV4 7AL, U.K. S Supporting Information *

ABSTRACT: The synthesis and structural characterization of various aromatic−aliphatic polyamides are reported in this study. The polymers are obtained by solution polymerization of p-phenylenediamine with various aliphatic diacid chlorides. The resulting polyamides are labeled PA P-X, where X varies between 5 and 10 and corresponds to the number of carbon atoms of the dicarboxylic acid monomers used in the synthesis. The polyamides are obtained with Mn values of 10 kg/mol or higher, as determined by solution NMR spectroscopy and gel permeation chromatography (GPC). The polymers PA P-5 to PA P-8 degrade prior to melting, whereas only PA P-10 shows melting on heating. The structural changes in the polymers, with increasing methylene segments, are investigated by X-ray diffraction and molecular modeling. Conformational changes as a function of temperature have been studied by solid-state NMR spectroscopy. These studies have been illustrative in following the phase transformations in the aromatic−aliphatic polymers. For the crystal packing of the polymer based on the odd acid (PA P-5) a sheetlike structure, similar to that of the aromatic polyamide PPTA, is observed. Despite the presence of the odd spacer, PA P-5 exhibits a hydrogen bonding length very similar to that of PPTA, whereas the intersheet distance increases and the interchain distance decreases. As a result, the crystal structure of PA P-5 is distinctively different from that of the aliphatic polyamides having the same odd diacid, e.g. PA 65. In contrast, the crystal packing of PA P-6 with even diacid is similar to that of the α form of PA 46. The change of the chemical shift of the carbonyl groups with increasing number of methylene units suggests a weakening in the hydrogen bonding with respect to PPTA. For PA P-10 this weakening ultimately translates to melting of the polymer prior to degradation.

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INTRODUCTION

polymer is spun from a lyotropic liquid-crystalline solution, where the applied solvent is concentrated H2SO4. Solution spinning and heat treatment results in the formation of yarns with extended chain crystals having tensile modulus and tensile strength greater than 100 and 2.3 GPa, respectively.3,4 However, from an environmental and economic viewpoint, the usage of H2SO4 in the spinning process of PPTA is disadvantageous. In contrast to aramids, the presence of flexible methylene segments between successive amide groups in aliphatic polyamides results in melt-processable polymers, with melting temperatures in the range of 200−320 °C.5

Hydrogen-bonded polymers based on aromatic and/or aliphatic motifs are widely used in engineering applications because of their excellent combination of properties. Examples are aromatic polyamides (aramids) such as Twaron from Teijin Aramid or Kevlar from DuPont, particularly known for their high strength, high modulus, and excellent thermal stability. Aliphatic polyamides (PA), such as PA 46, have a favorable combination of mechanical and melt processing properties.1,2 The good mechanical and thermal properties of aliphatic- and aromatic-based polymers originate from their structural organization induced by strong intermolecular hydrogen bonds, which govern chain packing, melting behavior, and processing possibilities of these polymers. For example, in aramids such as PPTA the presence of a stiff aromatic backbone leads to thermal degradation prior to melting. Therefore, the © XXXX American Chemical Society

Received: August 6, 2015 Revised: January 3, 2016

A

DOI: 10.1021/acs.macromol.5b01747 Macromolecules XXXX, XXX, XXX−XXX

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mmol, 1 equiv) was added. To the diamine, 40 mL of solvent (CaCl2/ NMP, 10.6 wt %) was poured under nitrogen flow (1 mL/min), and the mixture was stirred until complete dissolution of the diamine was achieved (∼30 min). After mixing, dry Et3N (8.10 g, 80 mmol, 1 equiv) was added to the reaction mixture at room temperature, and the reaction mixture was stirred until the Et3N was dissolved. During the mixing process, the reaction mixture was cooled with an ice bath. The diacyl chloride (40.0 mmol, 1 equiv) was added directly to the mixture under vigorous stirring. After an hour of polymerization under continuous mechanical stirring, the reaction mixture was coagulated in water. The coagulant was filtered with a Büchner filter. On washing the coagulant with water and acetone, yellowish to white powders were obtained. The powders were dried overnight in vacuo at 80 °C for 24 h. Thermal Analysis. The thermal stability of the aromatic−aliphatic polyamides was evaluated using thermogravimetric analysis (TGA) on a TA Instruments Q500 machine. For measurement, the standard sampling procedure was followed. 5 mg of the polymer was heated at 10 °C/min from 20 to 700 °C in a nitrogen atmosphere. Thermal transitions in the synthesized aromatic−aliphatic polyamides were investigated using a TA Instruments DSC Q1000. The samples were subjected to heating and cooling cycles between 20 °C and the respective melting temperature of the polymers, at a rate of 10 °C/ min. The samples were left under isothermal conditions at the limiting temperatures for 5 min. The cycles were repeated three times to determine melting and crystallization temperatures of the synthesized polymers. Solution State Nuclear Magnetic Resonance (Solution State NMR). 1H NMR spectra were recorded on a Bruker Avance III 400 MHz NMR spectrometer operating at room temperature. 1H chemical shifts were expressed relative to the residual D2SO4 solvent peak set to 10.2 ppm. Solution Viscometry and Gel Permeation Chromatography. Sulfuric acid (96%) was used for both Ubbelohde viscometry and gel permeation chromatography (GPC) analysis. The molecular weights were obtained from an in-house built GPC where sulfuric acid was used as an eluent. The GPC was calibrated against homemade PPTA standards. Considering the flexible methylene units the molar mass of the copolymers reported in this publication are likely to be on the lower side of the true value. Solid-State Nuclear Magnetic Resonance (ss-NMR). Solidstate 13C{1H} cross-polarization/magic-angle spinning (CP/MAS) NMR experiments and the corresponding variable-temperature (VT) experiments were performed on a Bruker ASX 500 spectrometer (500.11 MHz for 1H and 125.77 MHz for 13C) using a doubleresonance probe for rotors with 4.0 mm outside diameter and a MAS spinning frequency of 10.0 kHz. All 13C{1H} CP/MAS NMR employed with νRF = 62.5 kHz for 1H excitation followed by a CP contact time of 3.0 ms and two phase pulse modulated (TPPM) decoupling during acquisition.15,16 For the VT 13C{1H} CP/MAS NMR experiments, the temperature was controlled using a Bruker temperature control unit in the range from 30 to 200 °C, and the spectra were recorded under isothermal conditions at intervals of 10 °C, employing a heating rate of 2 °C/min between temperatures. Reported temperatures are corrected for friction-induced heating caused by MAS using 207Pb MAS NMR of Pb(NO3)2 as a NMR thermometer.17 All VT 13C{1H} CP/MAS NMR spectra were processed identically using a Lorentzian window function of 10 Hz. The 2D 1H−1H double quantum−single quantum (DQ-SQ) correlation experiments were recorded on a Bruker AVANCE-III 850 spectrometer (850.27 MHz for 1H) using a double-resonance probe for rotors with 2.5 mm outside diameter. These experiments were performed under rotor-synchronized conditions using a spinning frequency of 30 kHz. The BaBa sequence was used for excitation and reconversion of DQ coherences.18,19 All 2D spectra were recorded using two rotor periods (67.2 μs) of BaBa DQ recoupling. Chemical shifts for 1H and 13C are reported in ppm relative to TMS using solid adamantane as an external reference.20,21 Geometry Optimization and NMR Chemical Shift Calculations. All calculations were performed with the Gaussian09 program

Clearly it is of interest to develop polyamides that can be processed directly from the melt state with properties approaching those of aramids. With this objective, the rigidity of aromatic moieties may be combined with the flexibility of aliphatic moieties. Although the introduction of flexible aliphatic moieties will cause a reduction in the thermal and mechanical properties, this class of aromatic−aliphatic polyamides can bridge the gap between the aramids and the aliphatic polyamides in terms of processing and properties. Following this approach, Morgan and Kwolek and Gaymans synthesized polyamides containing various aromatic and aliphatic components.6,7 Morgan and Kwolek have investigated polyamides based on phenylenediamines and aliphatic diacids. They reported that melting temperatures decrease with increasing chain length of the aliphatic diacid.6 Takayanagi and co-workers8 modified the synthesis route described by Morgan and Kwolek and reported the synthesis of segmented block-co-polyamides having varying aromatic segments combined with PA6 and PA66 units. Using the phosphorylation reaction route, Krigbaum and co-workers synthesized fully aromatic block copolyamides.9 The phase behavior of these block copolymers in solution with dimethylacetamide (DMAc) was investigated in the presence of LiCl (3 wt %).10 Recently, Picken and co-workers reported the synthesis of lyotropic rigidcoil poly(amide-block-aramid)s, where segmented block copolymers were prepared using PPTA and PA66 in NMP/ CaCl2.11−13 The polyamide poly(nonamethylene terephthalamide), abbreviated as PA 9-T, has been commercialized by Kuraray as an engineering thermoplastic named GENESTAR, which clearly shows the potential of semi-aromatic polymers.14 In view of the interest in semi-aromatic polyamides we have investigated polyamides based on p-phenylenediamine and various aliphatic diacid chlorides. The resulting polyamides are labeled PA P-X, where X is 5, 6, 8, and 10, corresponding to the number of carbon atoms of the dicarboxylic acid monomers. The synthesis of these polymers was already described by Morgan and Kwolek,6 who examined the polymers in terms of melting temperatures and solubility in various solvents. These authors also reported wide-angle X-ray diffraction results, indicating that the polymer chains adopt an extended chain conformation in crystalline domains. However, detailed studies on the crystal structure and polymer conformations have not been performed until now, despite the potential commercial relevance of some members of the semi-aromatic polymer family. Here we use WAXD combined with molecular modeling, solid-state NMR spectroscopy to reveal the origin of melting and the influence of methylene segments on crystal packing and chain conformation.

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MATERIALS AND EXPERIMENTS

Materials. Terephthaloyl dichloride (TDC), having 99% purity, was obtained from Teijin Aramid and was used as received. Triethylamine (Et3N) was obtained from Fluka. Prior to polymerization, Et3N was dried over activated 0.4 nm molecular sieves. A dried mixture of ∼10.6% (w/w) CaCl2 in N-methyl-2-pyrrolidone (NMP) received from Teijin Aramid, having a water content less than ∼150 ppm, as determined by Karl Fischer titration, was used as the polymerization medium. To ensure a low moisture content in the reaction medium, the NMP/CaCl2 mixture was stored in a nitrogenrich environment. p-Phenylenediamine (PPD), glutaryl chloride, suberoyl chloride, adipoyl chloride, and sebacoyl chloride were obtained from Sigma-Aldrich and were used as received. General Procedure for Polymer Synthesis. In a 60 mL custommade glass reactor, equipped with a mechanical stirrer, diamine (40.0 B


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Figure 1. Synthesis of the aromatic−aliphatic polyamides (PA P-X) and their chemical structures, where X is the number of carbon atoms between the amide groups. The 13C assignment, used for the interpretation of the NMR data, is also indicated.

Table 1. Melting and Dissolution Behavior and Molar Mass of the Synthesized Aromatic−Aliphatic Polyamides number-average molar mass (Mn) polymer

NMR (g/mol)

PPTA PA P-5 PA P-6 PA P-8 PA P-10

15000 10750 12850 11260 12570

b

dissolution in solvent

GPC (g/mol)

d

a

yield (%)

[η] (dL/g)

NMP/CaCl2

H2SO4

Tm (°C)

9500 11300 13400 14300

97 80 81 78 72

6.48 1.18 1.09 1.25 1.15

no yes yes yes yes

yes yes yes yes yes

c c c c ∼343

a

Using H2SO4 as solvent and up to 0.25 g/dL of polymer concentration. bAccording to H2SO4 GPC analysis, against PPTA standards. cNot observed. dAfter purification procedure. package.22 The isolated PPTA fragment was fully optimized at the B97-D/6-311G** level of theory using the Grimme dispersion correction.23 The optimized structure was further used to calculate NMR chemical shifts via B97-D/6-311G**. Wide-Angle X-ray Diffraction (WAXD). X-ray diffraction (XRD) measurements were carried out using a Bruker D8 Advance diffractometer in θ/2θ geometry, equipped with parallel beam optics, point detector (scintillation counter), and autochanger. The optics consists of a primary 60 mm Göbel mirror (a parabolic Ni/C multilayer device) providing Cu Kα radiation (Kα1/Kα2 doublet, Kα wavelength = 0.154 nm) and 0.12° Soller slits. The measurements were carried out in reflection mode. Crystal structure model building was carried out using the Cerius2 software package24 employing the Compass force field. Using the Cerius2 diffraction module, for comparison, simulated XRD data were calculated. The Lorentz and polarization factors were included in the calculated reflection intensities. The crystallite sizes and temperature factors were chosen to match the observed diffraction patterns.

acid (HCl) scavenger. The Et3N binds the HCl formed during polymerization and thereby prevents deactivation of the amine groups of the p-PPD monomer. The physical characteristics of the synthesized polymers are summarized in Table 1. As is reported in Table 1, the molecular weight information obtained from NMR and GPC analysis are in good agreement, and molecular weights close to 10 kg/mol are found. Relative to PPTA, the intrinsic viscosity strongly decreases with the incorporation of methylene units. The decrease in the viscosity is in accordance with the reduction in chain stiffness.25 The solubility of the aromatic−aliphatic polyamides has been further investigated in 100% H2SO4 and in a NMP/CaCl2 (10% w/w) solution. All aromatic−aliphatic polyamides show solubility in H2SO4 up to ∼35% (w/w) and in NMP/CaCl2 up to ∼20% (w/w). It should be noted that although the PA PX polymers only dissolve in NMP/CaCl2 at elevated temperatures (>110 °C), they do not precipitate upon cooling. The solutions were evaluated in between cross-polars by optical microscopy. The absence of birefringence indicates that these aromatic−aliphatic polyamides do not exhibit lyotropic behavior in these solvents. TGA of the polymers indicates that the aromatic−aliphatic polyamides are thermally stable up to 400 °C. As anticipated, PPTA did not exhibit any melting prior to degradation above 545 °C.26 Similar to PPTA, no melting is observed in PA P-5 to P-8, prior to degradation. In contrast, PA P-10 exhibits a melting peak at ∼343 °C in DSC analysis (Figure 2).

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RESULTS AND DISCUSSION Synthesis and Thermal Analysis of Aromatic−Aliphatic Polyamides. The aromatic−aliphatic polyamides are obtained after reaction of p-phenylenediamine (p-PPD) with an aliphatic dichloride. Figure 1A depicts the synthesis scheme of the polyamides. For simplification the nomenclature of PA P-X will be used for the synthesized polyamides, where X represents the number of carbon atoms in the carboxylic motifs (Figure 1B). During polymerization, triethylamine (Et3N) is used as C


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polyamides and/or that of PPTA. First, we will give a brief overview on the reported crystal structures of aliphatic polyamides. The crystal structures of aliphatic polyamides can be subdivided into three basic schemes depending on the methylene content of the repeat units and in particular on the torsion angles involving amide groups. In the first scheme the structure consists of hydrogen-bonded sheets made up of fully extended all-trans polymer chains. Such sheets are energetically favorable when NH and CO groups of adjacent polymer chains face each other so that hydrogen bonds with favorable geometry can be formed (donor−acceptor distance close to 0.280 nm, N−H---O angles close to 180°). The sheet packing can be either progressively sheared (α form) or alternatingly sheared (β form). An example of the former structure is that of PA66, first reported by Bunn and Garner in 1947.29 The latter structure was reported for polyamides 2N, 2(N + 1), e.g. by Gaymans7 and Jones et al.30 The second scheme corresponds to a pseudohexagonal structure, called the γ phase. In this structure the amide groups are tilted by 60° out of the methylene carbon plane, which leads to a characteristic shortening of the repeat unit compared to a fully extended molecular conformation. This structure is observed in aliphatic polyamides based on odd diamine or odd dicarboxylic acid monomers.31 Reports by the group of Puigali show that several even−odd or odd−even polyamides turn out to crystallize in a third structural scheme.32−34 In this scheme the amide groups of the odd monomers rotate by approximately 30° in opposite directions from the plane formed by the methylene carbon atoms, leading to a structure somewhat similar to the α phase, i.e. with lower lattice symmetry than the γ phase, but with hydrogen bonds in two different directions. Figure 3A shows the XRD patterns of the synthesized polymers after drying the samples at 80 °C for 24 h, whereas Figure 3B shows the same samples after annealing at 300 °C for 30 min. These diffraction patterns are recorded in reflection mode. For comparison, the diffraction pattern of PA46 is

Figure 2. DSC thermograms of melt-crystallized PA P-10 recorded during the second heating (red, upper) and cooling (black, lower) cycles. The asterisk marks the cold crystallization peak or the superimposed melting and crystallization behavior during the second heating cycle.

Prior to melting of PA P-10, a small peak is observed at 323 °C. This peak is likely to originate from partial melting and recrystallization of the polymer. The presence of such a small endothermic peak prior to the main melting peak is well-known in aliphatic polyamides and is associated with the reorganization processes of the amorphous component 27 in the semicrystalline polymer. On cooling from the melt state, crystallization of PA P-10 is observed at 323 °C. The manifestation of melting in PA P-10 suggests a higher thermal motion and a temperature-induced mobility of the polymer backbone at elevated temperatures compared to the PA P-5 to P-8 polymers.28 Crystal Structure of Aromatic−Aliphatic Polyamides. The composition of the aromatic−aliphatic polyamides is such that they share the aliphatic dicarboxylic acid monomer with polyamides and the aromatic diamine monomer with PPTA. It is therefore anticipated that the crystal structures of the PA P-5 to P-10 polymers will be related to the crystal structures of the

Figure 3. WAXD patterns of aromatic−aliphatic polyamides PA P-X recorded at room temperature in reflection mode: (A) before annealing after drying the polymers at 80 °C for 24 h; (B) after heat treatment at 300 °C for 30 min. The WAXD patterns of a commercial PA46 sample and of PPTA are also shown. The dashed line is drawn through the maximum of the intersheet diffraction peak of PA P-6 to guide the eye. D


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Unlike PA P-5 and PA P-10, the WAXD pattern of PA P-6 does not show a low angle reflection, similar to the pattern of PPTA. This is due to the fact that the c-axis projection of the 6carbon dicarboxylic acid is closer in length to that of the aromatic diamine than the 5- and 10-carbon dicarboxylic acids. Caution should be exercised in deriving structural models for the PA P-X polymers based on WAXD powder patterns with relatively broad peaks. However, since hydrogen bonding plays a key role in the polymers investigated, the polymers will most likely be composed of hydrogen-bonded sheets similar to the hydrogen-bonded sheets in PPTA or polyamides. Such sheets are in fact structural building blocks that show relatively little structural variability. The task then is to find out how these sheets pack together into crystal structures. For this purpose a combination of force field based energy minimization and diffraction pattern matching is used. In this procedure a suitable starting model is adapted until the simulated WAXD pattern matches the observed WAXD pattern. PA P-5. The group of Puigalli has investigated the hydrogen bond geometry of even−odd and odd−even polyamides.32−34 They pointed out that polyamides derived from odd diamine or odd dicarboxylic acid monomers cannot adopt a conventional sheet structure when molecular chains have an all-trans conformation. This is illustrated in Figure 4A, which shows

included. The measurements show that the annealing hardly influences the crystal structures of the PPTA and PA P-5 samples. However, for the PA P-6 and PA P-10 samples, the annealing process leads to peak sharpening and for PA P-10 even to the development of diffraction peaks around 6°(2θ) and 25°(2θ) (n.b.: the weak presence of these peaks is already apparent prior to annealing). This indicates that crystal perfectioning resulting from annealing becomes stronger with increasing diacid spacer length. This observation is in agreement with solid-state NMR findings on PA P-10 (Figure 13) where an increase in molecular mobility on increasing temperature results in the disappearance of the conformers associated with the amorphous component, as will be discussed in the NMR section of the paper. In the following section we will focus attention on the XRD patterns of the annealed samples. In the WAXD pattern of PPTA the intersheet 200 reflection and the 110 reflection overlap. However, the presence of two peak maxima is evident (Figure 3B). Although the WAXD pattern of PA P-5 resembles that of PPTA, the 200 and 110 diffraction peaks are no longer resolved; they seem to have merged into one single asymmetric peak. Hence, on the substitution of terephthalic acid by the dicarboxylic acid having five carbons, the crystal lattice changes from the pseudoorthorhombic PPTA lattice to a lattice that has a more hexagonal character. From these findings it is not apparent if the sheetlike hydrogen-bonding structure is retained. This point will be addressed by performing molecular modeling later in the paper. The low angle reflection in the XRD pattern of PA P-5 has a Bragg spacing of 1.24 nm, which is close to the length of the PA P-5 repeat unit consisting of the diacid monomer linked to the diamine monomer. In PPTA a similar low angle reflection is absent because of the pseudotranslational symmetry of the PPTA unit cell, more specifically due to the similarity of the aromatic diamine and diacid. The WAXD patterns of PA P-6 and PA P-10, displayed in Figure 3B, show the familiar two strong reflections that are often observed in the WAXD patterns of both aliphatic and aromatic polyamides, characteristic of the packing of hydrogen bonding sheets. The reflection around 23° (2θ) hardly shifts with increasing number of methylene units. The associated dspacing corresponds to the distance between the hydrogenbonded sheets. For the PA P-6 and PA P-10 polymers this intersheet distance is 0.399 and 0.395 nm, respectively, which is very close to that in PPTA (0.392 nm) and somewhat larger than that in PA46 (0.384 nm). This indicates that in the PA P-6 and PA P-10 polymers the diamine phenyl rings are rotated out of the plane of the hydrogen-bonded sheets, which is typical for PPTA and PPTA-like structures (see Figure 5). In Figure 3B the position of the left-hand member of the two strongest reflections changes with increasing number of methylene units. The associated d-spacing gives information on the interchain distance in hydrogen bonded sheets, but the d-spacing is also influenced by the structural details of the projection unit cell (e.g. the reciprocal angle γ*, centering, etc.) and hence by the details of the packing of the hydrogen-bonded sheets. Clearly this packing shows more variation than the intersheet distance, which explains the larger changes in the position of the lefthand reflection. Figure 3 shows that the d-spacing of the lefthand reflection of PA P-6 and PA P-10 is larger than that of the other polymers, for example 0.470 nm for PA P-10 compared to 0.434 nm for PPTA.

Figure 4. (A) Unfavorable hydrogen bond geometry between odd diamide units of PA65 when molecular chains have an all-trans conformation. Reproduced with permission from ref . Copyright 2010 Institute of Physics. (B) and (C) show PPTA crystal structures as reported in the literature: modification I and modification II, respectively.35,36

potential unfavorable hydrogen-bonding geometry in PA65 reproduced from the report by Ricart and co-workers.34 The authors presented a new polyamide structure characterized by hydrogen bonding in two different directions. In this scheme the amide groups of the odd monomer rotate about 30° in opposite directions from the plane formed by the methylene carbon atoms. PA P-5 provides another test case for unconventional hydrogen-bonding schemes due to the presence of the same odd diacid as in PA65. Clearly, the WAXD pattern of PA P-5 is markedly different from that of PA P-6 and PA P-10, with their even-numbered diacid monomers. However, since the WAXD pattern of PA P-5 resembles that of PPTA, as can be verified in Figure 3, a similar hydrogenbonding scheme as that proposed for PA65 is unlikely. In view of the similarity of the WAXD pattern of PA P-5 and PPTA, we will set up a structural model for PA P-5 starting from the structure of PPTA. The crystal structure of PPTA has been extensively discussed by Northolt et al. and Haraguchi et al.35,36 Furthermore, single crystal diffraction studies of PPTA were reported by Jackson et al. and Liu et al.37,38 The crystal structure discussed by Haraguchi et al. was observed in PPTA film spun from isotropic or anisotropic solutions. This structure E


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Macromolecules Table 2. Symmetry and Unit Cell Parameters of the PA P-X Structural Models, PPTA,35 and PA-6,1039 unit cell parameters polymer

symmetry

a (nm)

b (nm)

c (nm)

α (deg)

β (deg)

γ (deg)

PPTA (modification I) PA P-5 PA P-6 PA P-10 PA 6,10 (α form)

P11n P1 (pseudo P1a1) P1 P1 P1

0.787 0.850 0.540 0.504 0.495

0.518 0.470 0.700 0.549 0.540

1.290 2.480 2.800 3.807 2.240

90 38.5 47.1 49.0

85 77 81.9 76.5

90 90 63.5 93.6 63.5

shift the symmetry would be described by the monoclinic space group P1a1 (Pc in short notation); hence, the symmetry is pseudomonoclinic. Because of the stagger, the symmetry reduces to the triclinic space group P1. The c-axis projection of the PA P-5 model is very similar to that of PPTA, as is evident from Figure 5. The model is sheetlike, but compared to PPTA the intersheet distance is found to be expanded and the intrasheet distance contracted.

is known as modification II. After a heat treatment modification II transforms into modification I, reported by Northolt. The schematic of crystalline polymorphs reported for PPTA is illustrated in Figure 4B,C. The Northolt and Haraguchi structures have a pseudo-orthorhombic lattice with two chains per unit cell and similar cell dimensions. However, the positions of the polymer chains differ in modifications I and II, as shown in Figures 4B and 4C, respectively. For the type I polymorphs different monoclinic space groups were proposed by Northolt (Pn and P21/n), Tadokoro (P21/n), and Liu et al. (Pc).35,38 Figure 5b,d presents a view of the Pn crystal structure of Northolt. The center chain has a c-axis shift of 0.064 nm compared to the structure with the higher space group symmetry P21/n. Lowering the symmetry of the Pn structure to space group P1 followed by energy minimization produces a structure with cell parameters 0.765 nm, 0.514 nm, 1.298 nm, 90°, 90°, and 90°. This structure is very close to the Pn structure reported by Northolt, which was based on fiber XRD data (experimental unit cell parameters 0.787 nm, 0.518 nm, 1.290 nm, γ = 90°). This indicates that the Pn structure is very close to a minimum-energy structure using the Compass force field. At the same time it shows that the Compass force field is well suited to model this type of structure. A model for the crystal structure of PA P-5 was built using the Cerius2 software, starting from Northolt’s Pn structure. The terephthalic acid (TPA) monomer was replaced by a diacid monomer with three methylene groups to model the PA P-5 structure. The polymer chain axis was assumed to be all-trans, leading to an extended chain structure. The PA P-5 model structure was energy minimized using the Compass force field. Note that the odd number of methylene groups leads to a structure where the carbonyl groups of each aliphatic diacid monomer point in the same direction, unlike the carbonyl groups of the aromatic diacid in PPTA, which point in opposite directions. This also implies that PA P-5 has four monomers in its c-axis repeat unit, whereas PPTA has only two monomers in its c-axis repeat unit. Figure 5 displays the final model structure of PA P-5, together with the structure of PPTA. The unit cell parameters are listed in Table 2. Simulation of the powder XRD pattern using Cerius2 on the basis of the energy-minimized PA P-5 model produces the XRD pattern shown in Figure 6. The simulated XRD pattern matches the experimental pattern very well, thus lending support to the PA P-5 structural model. The low angle peak observed at 2θ = 7.16° (d = 1.230 nm) is indexed as 002. This peak is reproduced well in the simulated pattern. In the observed XRD pattern the left-hand side of the asymmetric main peak shows some extra intensity that may be due to amorphous material or to a type-II like polymorph. A similar asymmetry is observed in PPTA (see Figure 3B(f)). In the model the central polymer chain is staggered, just like in the Pn structure of PPTA. The size of the c-axis shift in the model is larger: 0.250 nm versus 0.064 nm. Without this c-axis

Figure 5. Structural model of PA P-5 versus the crystal structure of PPTA: (b) and (d) represent the side and top view of the PPTA crystal structure according to Northolt (modification I), showing two unit cells; (a) and (c) show the side and top view of the PA P-5 model, showing one unit cell. Color coding is as follows, red: oxygen atoms; blue: nitrogen; gray: carbon; white: hydrogen; yellow dashed lines: hydrogen bonds.

The decrease in intrasheet distance allows a hydrogen bond length that is very similar to that in PPTA: 0.290 nm versus 0.300 nm. The hydrogen bond angles are somewhat less favorable than in PPTA or in aliphatic polyamides like PA46. The hydrogen-bonded sheet structure of the PA P-5 model is shown in Figure 7. This sheet structure resembles somewhat that of the γ-structure of e.g. PA77, but it differs from the γstructure in that the polymer chains in the PA P-5 model are F


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Figure 6. WAXD pattern of the PA P-5 polymer: (a) experimental and (b) calculated based on the PA P-5 model.

The calculated WAXD pattern shows a good match with the observed WAXD pattern. Note that the low angle region is reproduced correctly; i.e. the low angle peak is absent. In the PA P-6 model, the hydrogen bonds are along the a-direction following the Bunn and Garner choice of unit cell axes (therefore the indexing of the XRD pattern follows the familiar polyamide indexing). The hydrogen bond length in the model is 0.320 nm. PA P-10. The PA P-10 structure was modeled along the same lines as the PA P-6 structure using an aliphatic diacid with 10 carbon atoms in its backbone. The resulting structure is displayed in Figure 10, and its simulated diffraction pattern is shown in Figure 11. The hydrogen-bonded sheet structure of PA P-10 (not shown) is very similar to that of PA P-6. To follow conformational changes with the increasing number of methylene units and their implications on crystal structure, solid-state NMR studies have been performed at room and elevated temperatures. Chain Conformation and Chain Mobility in Aromatic− Aliphatic Polyamides. The 13C{1H} CP/MAS NMR spectra of aromatic−aliphatic polyamides are depicted in Figure 12, where the peak assignment is performed according to the schemes shown in Figure 1B. For comparison, Figure 12 also includes a 13C{1H} CP/MAS NMR spectrum of PPTA. The spectrum is included for identification of the structural changes caused by the incorporation of methylene segments with increasing length. In PPTA, the carbonyl (13CO) resonates at a 13C chemical shift of 166.5 ppm. This is a very low 13C chemical shift value for an amide moiety and indicates the strong intermolecular hydrogen bonding of the carbonyl groups, confirming the rigid polymer backbone in PPTA.40

extended and its amide bonds are in the planes of the hydrogen-bonded sheets. The PA P-5 hydrogen-bonded sheet is very similar to a hypothetical sheet structure shown by Navarro et al.32 presenting work on nylon-65 (see Figure 3b of their paper). PA P-6. The XRD pattern of PA P-6, displayed in Figure 3B, is similar to that of aliphatic polyamide PA46, although the dspacings of the two strong reflections are somewhat larger for PA P-6 as compared to those of PA46. In view of this similarity we will use a polyamide-like structure as starting model for the PA P-6 crystal structure. Figure 8a,b displays a schematic view of the two possible sheet structures for PA46, as reported by Gaymans et al.:7 a progressively sheared intrasheet structure (α form) and an alternatingly sheared sheet structure (β-form). In the β form the diamine monomers are not adjacent to diacid monomers, whereas in the α form diamine monomers and diacid monomers are placed side-by-side. For PA P-6 the β form seems much less likely, since it would require the side-byside packing of aliphatic diacid monomers and aromatic diamine monomers side-by-side. Moreover, this is unlikely in view of the length difference between the aromatic and aliphatic monomers. Taking the above considerations into account, the PA P-6 structure was modeled starting from the classic α structure of PA66, reported by Bunn and Garner in 1947.29 In this structure the diacid and diamine monomers were replaced by a diacid monomer with four methylene groups and the aromatic diamine, respectively. The model was energy minimized, and the unit cell parameters were adapted to match the diffraction pattern. The resulting sheet structure is shown in Figure 8c and its simulated WAXD pattern in Figure 9. G


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current data for PPTA recorded at a higher magnetic field strength (11.75 T vs 7.05 T) and MAS frequency (10.0 kHz vs 4.3 kHz) compare well, including the number of observable 13C signals and their spectral resolution. These observations point toward a highly ordered crystal packing in PPTA. To assign the 13 C resonances to the specific carbon sites in PPTA, we have performed ab initio geometry optimization and subsequent NMR chemical shift calculations on an isolated PPTA fragment. This geometry optimization of the PPTA fragment results in dihedral angles similar to those of the earlier reported crystal structure35,36 of PPTA. However, deviation by ∼2−3° in the CO-centered phenylene ring occurs. Further details about these calculations are given in the Experiments section. On the basis of the geometrically optimized structure, a calculated isotropic 13C NMR spectrum of PPTA shown in Figure 12e is obtained, which shows good agreement with the experimental 13 C{1H} CP/MAS NMR spectrum in Figure 12d. The small differences between the calculated and the experimental 13C chemical shifts are most likely a result of packing effects, such as ring currents effects and conformational constraints in the solid state, which are not accounted for in the gas-phase calculations. However, clear differences between the two phenylene moieties in PPTA can be observed. For example, the 13C resonances from the NH-centered phenylene ring (AN, BN, and CN) are high field shifted compared to the corresponding resonances (AC, BC, and CC) of the CO-centered phenylene ring (see Figure 12e). The origin of this shift is caused by the unequal electron density distribution of the CO- and the NH-centered phenylene rings, due to the fact that CO and the NH act as electronic acceptor and donor, respectively. It is noteworthy to mention that the NH groups do not affect the 13C chemical shifts of the NH-centered phenylene ring significantly, whereas the CO groups do. This is evident from the calculated spectrum in Figure 12e where a large splitting between the positions of BC and CC of ∼6 ppm is observed. For BN and CN this splitting is only ∼1 ppm, making them overlap and undistinguishable in the experimental spectrum (Figure 12d). Based on these results, it is apparent that only minor differences in the 13C chemical shifts of the aromatic region are expected for the PA P-X samples because all CO-centered phenylene rings are replaced by extended methylene segments, and therefore only NH-centered phenylene rings are present in these polymers. For all aromatic−aliphatic polyamides shown in Figure 12, only four aromatic 13C resonances are observed: two narrow signals which are flanked by two broad signals. Such a reduced number of 13C resonances, compared to PPTA, is not unexpected: In principle, the solid-state packing in a crystal of the aromatic moieties in the PA P-X polymers can result in the three 13C signals, where one is associated with the quaternary carbons in the para position (AN) and two with the proton bearing carbons on each side of the phenylene ring (BN and CN, see Figure 12e) between the NH groups. However, according to the calculations performed on PPTA as presented

Figure 7. Hydrogen-bonded sheet of (a) PA P-5 and (b) PPTA.

Furthermore, the 13C{1H} CP/MAS NMR spectra in Figure 12 and Table 3 illustrate that an increasing number of methylene segments between the amide groups (see Figure 1B) leads to a gradual increase of the 13C chemical shift value and narrowing of the resonance assigned to the carbonyl group. For example, the carbonyl in PA P-5 resonates at 171.6 ppm, which increases to 172.3 ppm for PA P-10. This shows that an increasing number of methylene groups leads to a decreased magnetic shielding of the carbonyl group; i.e. the methylene groups act as a spacer between the hydrogen-bonded amide groups, offering an increase in conformational freedom for the polymer chains. Thus, the shift in the carbonyl peak with the increasing number of the methylene units, between the carbonyl motifs, suggests some weakening in the hydrogen bonding with respect to PPTA. The region from 110 to 150 ppm includes the 13C resonances corresponding to aromatic carbons of PPTA and the aromatic−aliphatic polyamides. For PPTA, five different 13 C signals are observed (Figure 12). A fully symmetric configuration of the two different aromatic moieties in PPTA, where one phenylene group is centered between NH and the other between CO groups, would lead to only four 13C resonances. These observations compare well to those reported by English and co-workers, who performed a detailed NMR study of PPTA in dilute solutions and in the solid state at ambient and elevated temperatures.41 Although the NMR studies by English were performed more than 25 years ago, the

Table 3. 13C Chemical Shifts (in ppm) Determined for the Synthesized Aromatic−Aliphatic Polyamides for the Nomenclature and Peak Assignment (See Figure 1B) polymer PPTA PA P-5 PA P-6 PA P-10

CO 166.7 171.6 172.3 172.3

AN 137.5 133.1 133.9 133.9

αC

BN 127.7 123.3 123.6 123.6

134.2 36.8 36.8 37.6 H

βC Ac

122.3 22.3 25.0 31.8

γC

ωC

29.7

26.2

Bc


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Figure 8. Hydrogen-bonded sheet structure of PA46 (a) α-form triclinic unit cell, (b) β-form monoclinic unit cell,7 and PA P-6. Color code: see Figure 5.

above, the 13C chemical shift for the BN and CN carbons were rather insensitive to the amide conformation. Thus, taking into account the large difference in the line widths between the narrow and the broad 13C resonances (Figure 12a−c), the four resonances in PA P-5 to P-10 can be assigned to signals that arise from the presence of the phenylene rings between the NH groups residing in the crystalline and the amorphous domains of the samples, respectively. This illustrates that all aromatic− aliphatic polyamides synthesized in this study are of a semicrystalline nature and have a similar degree of crystallinitiy. We note that an exact determination of amorphous fraction requires the application of single-pulse 13C MAS NMR recorded with long recycle delays to allow for full spin relaxation (5 times the longest T1 for 13C). Another feature observed for the 13C{1H} CP/MAS NMR spectra in Figure 12 is that the line width of the aromatic 13C signals decreases with the increasing number of methylene segments. This decrease in the line width of the phenyl rings with the increasing number of the methylene groups is in agreement with the decrease observed in the line width of the 13 C carbonyl groups, and it demonstrates that the extended methylene segments in the aromatic−aliphatic polyamide samples lead to dynamically averaged phenylene groups as a result of the increased conformational freedom of the polymer chains.42 The spectral range from 10 to 50 ppm includes 13C signal from the methylene units between the amine or carbonyl groups of the aromatic−aliphatic polyamides. The 13C observed signals have a systematic appearance as a result of the increasing number of methylene groups. For PA P-5, two 13C carbon signals are observed at a chemical shift of 36.8 and 23.5 ppm.

These can be assigned to the methylene groups directly bound to the carbonyl groups and methylene groups in the chain. The 13 C resonance located at 23.5 ppm also includes a shoulder at its high-field side at 22.9 ppm characteristic of methylene groups that are in gauche conformation; i.e. the 13C signal at 23.5 ppm can be assigned to the Cβ methylene unit in the trans conformation. The trans and the gauche conformers are related to crystalline and amorphous regions of the sample, respectively, since a Cβ methylene unit in gauche conformation does allow for a planar hydrogen-bonding geometry of the amide group in PA P-5.43−46 The ratio between the trans and the gauche conformers is similar to the ratio that can be derived from the resonances of the phenylene groups (see above). For the PA P-6 and P-10, the assignment of 13C resonances to gauche conformers is not feasible due to either spectral overlap or fast exchange between the two conformers. Thus, for these samples, the crystallinity can only be judged on the basis of the signals from the phenylene groups. To determine the dynamical behavior of the phenylene groups in the methylene-extended aromatic−aliphatic polyamides, we have recorded variable-temperature (VT) 13C{1H} CP/MAS NMR experiments for melt-crystallized PA P-10 as shown in Figure 13. From these spectra the dynamics of the phenylene rings in PA P-10 can be directly followed. This relies on the fact that a decrease in signal intensity for the protonated βN resonance is observed, reaching a minimum at ∼100 °C, followed by an increase in intensity at higher temperatures.47,48 Such an intensity behavior as a function of temperature is characteristic of 13C moieties that undergo mobility on the time scale of the NMR experiment; i.e. a loss of signal occurs when the frequency of the associated motion is matched by the I


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Figure 9. XRD pattern of the PA P-6 polymer: (a) experimental and (b) calculated based on the PA P-6 model.

frequency of either (i) the involved 1H−13C dipole−dipole couplings, (ii) the proton decoupling field, or (iii) the magicangle spinning.49 In the current experiments these frequencies fall in the range 10−60 kHz, illustrating that motion of the phenylene groups in the crystalline domains of melt-crystallized PA P-10 reaches this frequency scale at approximately 100 °C. The amorphous fraction, located at ∼120 ppm as a broad shoulder to the crystalline resonance, decreases rapidly with increasing temperature and is hardly visible above a temperature of ∼150 °C. Previous studies of the phenylene dynamics in crystalline PPTA using static 2H solid-state NMR methods have shown that the underlying dynamics is quite heterogeneous.50,51 The heterogeneity comes as a result of two different types of phenylene motions: a rigid motion and a motion consisting of 180° flips, where both type of motions include small-angle excursions.52,53 Such complex dynamics has also been reported recently on the basis of 1H−13C dipole−dipole couplings and 13C chemical shift anisotropy (CSA) NMR experiments for studies of molecular dynamics in shapepersistent polyphenylene dendrimers, polycarbonate, and discshaped aromatic molecules forming discotic liquid crystals.54−57 The first type of phenylene motion has been assigned to the interior of the crystallites and the second to the crystallite surfaces, which illustrates that the difference in molecular order can induce the two types of motions. For the current PA P-10, the observations made above for the crystalline and the amorphous regions suggest that these phases include similar phenylene dynamics as in PPTA. Thus, in the amorphous regions of PA P-10 where limited order is expected, the phenylene rings undergo 180° flips, while the crystalline regions

with higher order include phenylene rings that perform smallangle excursion, reaching the kilohertz regime at ∼100 °C. Both types of motions are consistent with the 13C intensity behavior observed in Figure 13. The increasing phenylene dynamics at higher temperatures influence both the carbonyl and the methylene groups, and it leads to a decrease in the intensity of signal at higher temperatures as expected. This is most pronounced for the carbonyl resonance, which also displays a shift toward higher ppm values, while only minor changes in position and intensity for the methylene groups are observed. The most significant changes for the methylene groups are related to those present in the amorphous phase (see Figure 13). These appear as a broad signal with a faster decay than those from the crystalline fraction in agreement with the conclusions drawn from the phenylene rings. Similar to the 13 C{1H} CP/MAS measurements presented above, the effect of varying aliphatic segment lengths on the molecular packing, chain mobility, and hydrogen bonding of aromatic−aliphatic polyamides has been investigated by 2D 1H−1H double quantum-single-quantum (DQ-SQ) cross-correlation spectroscopy (Figure 14). For PPTA two broad resonances are observed. These can be assigned to the hydrogen-bonded amine protons (∼9.8 ppm) and to the protons associated with the phenylene rings (∼7.0 ppm). A clear and intense DQ cross signal between these two resonances located at ∼16.8 ppm confirms that the structure of PPTA is indeed quite rigid. For the PA P-X samples, the introduction of methylene segments results in additional signals and correlations as indicated in Figure 14. These are related to both DQ cross- and autocorrelations between the different chemical groups of the J


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Figure 11. XRD pattern of the PA P-10 polymer: (a) experimental and (b) calculated based on the PA P-10 model.

Figure 10. Structural model of PA P-10: (a) side view and (b) top view.

Figure 12. Selected regions of the 13C{1H} CP/MAS NMR spectra recorded at 11.75 T (500 MHz for 1H) for (a) PA P-10, (b) PA P-6, (c) PA P-5, (d) PPTA, and (e) calculated 13C chemical shifts for PPTA (see text). Carbon assignments used in (a−c) are given in Figure 1B. All spectra employed a spinning frequency of 10.0 kHz and a CP time of 3.0 ms. The asterisks in the region 40−50 ppm are spinning side bands from the aromatic groups.

PA P-X polymers. For the amine groups a DQ signal at 10−11 ppm characteristic of methylene segments build in to the polymer backbone is observed. Likewise, the phenylene groups display a DQ cross peak to the methylene groups at 8−9 ppm. The DQ autocorrelation signals located at the diagonal suggest well-defined interactions between the aliphatic/aliphatic and aromatic/aromatic moieties. However, the most striking feature of Figure 14 is the substantial decrease in 1H line width with increasing number of methylene segments, leading to quite well resolved resonances for PA P-10. This particular sample also includes residual water molecules (likely to be adsorbed during the coagulation process after polymerization) in its structure as also observed recently in a water-crystallized sample of polyamide 46.55 The decrease in 1H line width can in principle have different origins: (i) high crystallinity or (ii) increased molecular dynamics as a result of polymer chain flexibility.56,57 The latter leads to efficient averaging of the homonuclear 1 H−1H dipolar couplings between the different groups consistent with the results discussed above.

This further leads to weakening of the hydrogen bonding, which is observed in the 2D 1H−1H DQ-SQ experiments and as a shift of the amine resonance to lower frequencies.

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CONCLUSIONS Using the well-known PPTA synthesis route, aromatic− aliphatic polyamides with varying methylene segments were synthesized. The obtained polymers were found to be soluble in their NMP/CaCl2 polymerization medium up to 20 wt %. To follow the structural changes WAXD studies combined with molecular modeling have been performed. These studies conclusively demonstrate the influence of methylene units in K


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dicarboxylic acid spacers, PA P-6 and PA P-10, the intersheet distance is 0.399 and 0.395 nm respectively, which is close to that of PPTA (0.392 nm). For PA P-6 the diffraction data indicate that the crystal structure is related to the α form of polyamide. An adapted α form structure is found to fit well with the WAXD data. The influence of flexible methylene segments on molecular conformation have been pursued by solid-state NMR. The measurements show that all aromatic−aliphatic polyamides synthesized in this study are of a semicrystalline nature and have a similar degree of crystallinitiy. The shift in the carbonyl peak with increasing number of methylene units suggests a weakening in the hydrogen bonding with respect to PPTA. For PA P-10 this ultimately leads to melting of the polymer prior to degradation.

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Figure 13. Selected regions of variable temperature (VT) 13C {1H} CP/MAS spectra for melt-crystallized PA P-10 recorded at 11.75 T (500 MHz for 1H). Dashed lines indicate 13C resonances related to the amorphous (A) fraction of the sample, whereas the gray highlighted areas are signals influenced by chain dynamics in the crystalline (Cr) phase. The assignment of the peaks follows that given in Figure 12, and the asterisk indicates a spinning sideband. The PA P-10 used for the measurement was crystallized from the melt state, cooled from 353 °C to room temperature at 10 °C/min.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b01747. Experimental details; Figures S1−S4 and Tables S1 and S2 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (S.R.). *E-mail [email protected] (Y.S.D.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been performed within the framework of the Dutch Polymer Institute (DPI, project no. 685). The authors thank DPI for financial support. Part of this work was carried out at the Teijin Aramid Research Institute, Arnhem. The authors thank Dr. Katrien Bernaerts, Dr. Jules Harings, and Dr. Martijn Veld for fruitful discussions on polymer synthesis. We also acknowledge Angelique Radier and Ir. Petra oude Lohuisde Vries for performing solution viscometery and gel permeation chromatography (GPC) measurements. We acknowledge Prof. Hans Wolfgang Spiess, MPIP, Mainz, for his continuous support.

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Figure 14. 2D rotor-synchronized 1H−1H DQ-SQ correlation spectra of (a) PPTA, (b) PA P-5, (c) PA P-6, and (d) PA P-10 recorded at 20.0 T (υL = 850.27 MHz for 1H) using a MAS frequency of 30 kHz and two rotor periods of BaBa recoupling.54,57 The labels NH, Ar, and Me refer to proton resonances that can be assigned to amide, aromatic, and methylene segments, respectively. All polymers were characterized after drying at 80 °C for 24 h.

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Conformational and Structural Changes with Increasing Methylene

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