Structure and Molecular Dynamics in Renewable Polyamides from

Article pubs.acs.org/Macromolecules

Structure and Molecular Dynamics in Renewable Polyamides from Dideoxy−Diamino Isohexide Lidia Jasinska-Walc,*,†,‡,§ Dmytro Dudenko,∥ Artur Rozanski,⊥ Shanmugam Thiyagarajan,#,‡ Paweł Sowinski,& Daan van Es,#,‡ Jie Shu,∥ Michael Ryan Hansen,*,∥ and Cor E. Koning†,‡,% †

Laboratory of Polymer Chemistry, Eindhoven University of Technology, Den Dolech 2, P.O. Box 513, 5600 MB Eindhoven, The Netherlands ‡ Dutch Polymer Institute (DPI), PO Box 902, 5600 AX Eindhoven, The Netherlands § Department of Polymer Technology, Chemical Faculty, Gdansk University of Technology, G. Narutowicza Str. 11/12, 80-952 Gdansk, Poland ∥ Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany ⊥ Centre of Molecular and Macromolecular Studies, Polish Academy of Sciences, Sienkiewicza 112, 90-363 Lodz, Poland # Food & Biobased Research, Wageningen University and Research Center, Bornse Weilanden 9, 6708 WG Wageningen, The Netherlands & Nuclear Magnetic Resonance Laboratory, Chemical Faculty, Gdansk University of Technology, G. Narutowicza Str. 11/12, 80-952 Gdansk, Poland % DSM Coating Resins, Ceintuurbaan 5, Zwolle, The Netherlands S Supporting Information *

ABSTRACT: The chemical structure, the conformation, and the flexibility of the polymer chain fragments present in the polyamides synthesized from 2,5-diamino-2,5-dideoxy-1,4;3,6dianhydrosorbitol, 1,4-diaminobutane, and either sebacic or brassylic acid have been studied by liquid-state 2D NMR spectroscopy viz. correlation spectra (COSY) and heteronuclear multiple-bond correlation spectra (gHMBC), by 13C cross-polarization/magic-angle spinning (CP/MAS) NMR, by X-ray scattering, and by FT-IR spectroscopy. The presence of 2,5-diamino-2,5-dideoxy-1,4;3,6-dianhydrosorbitol in the crystal phase of the polyamides was probed by wide-angle X-ray diffraction (WAXD), FT-IR, and solid-state 13C NMR. The incorporation of dideoxy−diamino isohexide into the backbone of PA 4.10 or PA 4.13 induces formation of gauche type conformers and gives rise to pseudohexagonal packing of the polymer chains in these semicrystalline copolymers. The experimental determination of the polymer chain structure combined with ab initio calculations revealed the presence of three most abundant diaminoisosorbide (DAIS) conformers. The combination of the 13C chemical shifts of these three conformers could explain all experimental resonances in the region of 50−90 ppm. WAXD and DSC analysis show that the crystallinity, and hence the physical properties of the investigated compositions, can be tailored by the content of the bicyclic diamine in the backbone of the polyamides.

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INTRODUCTION The number and types of applications utilizing polyamides (PA) generate new trends in the selection of monomers used in their synthesis. Nowadays, biomass-derived chemicals offer an enormous potential to replace the depleting fossil feedstock and are considered as an environmentally friendly alternative. Despite the obvious benefits offered by renewable resources, bio-based polymers are often believed to be unsuitable for hightemperature industrial chemical processes and applications. However, our recent results1,2 have shown that, based on thorough understanding of the structure−property relationships of different polyamides, it is now possible to prepare fully © XXXX American Chemical Society

bio-based polyamides having equally good properties as the ones from petrochemical origin. Widely reported renewable monomers in this field are sebacic acid, brassylic acid, 1,4diaminobutane, or isohexides. 3−13 As pointed out by Fenouillot,3 the use of starch-based 1,4;3,6-dianhydrohexitols (isosorbide, isomannide, and isoidide) or their diamino derivatives with D-manno or L-ido configuration affords entirely bio-based materials with a wide variety of applications. Received: May 29, 2012 Revised: June 28, 2012

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dx.doi.org/10.1021/ma301091a | Macromolecules XXXX, XXX, XXX−XXX

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Although Thiem et al.14 explored a successful strategy toward polyamides synthesized from these chiral diamines, the extensive application of such monomers in polymerization processes suffered from the limited isolated yield or inconvenient synthetic routes of the isohexide-based diamines via explosive intermediates. 15−19 To successfully apply dideoxy−diamino isohexides, having either exo−exo or exo− endo configurations of both amine groups, in the synthesis of polyamides, Thiyagarajan20,21 and Beller22,23 developed a new and highly efficient route toward bicyclic diamines affording appropriate stereoisomers with a high yield and purity. These findings enable an excellent possibility to use these type of diamines in the synthesis of fully renewable materials. Our recent investigations1,2 revealed that diaminoisoidide (DAII), the exo−exo diamine derivative of dianhydrohexitols, incorporated into the PA 4.10 backbone reduces the melting point and hydrogen bond density, thereby improving the flow properties and processability of the copolyamides. By the introduction of different amounts of the exo−exo bicyclic diamines, which according to our previous study were partly incorporated into the crystal phase, one can tailor the crystal structure and thus the properties of the synthesized polyamides toward desired applications. Moreover, the presence of diaminoisoidide units in the copolymer backbone proved to facilitate the formation of gauche type chain conformers and a controlled change of the crystal unit cell. Usually, the polyamides having irregular structures do not readily crystallize from the melt. However, because of similar distances between amine groups in DAII and 1,4-diaminobutane, the synthesized copolymers of DAII and 1,4-diaminobutane in combination with sebacic acid are semicrystalline in the entire studied copolymer composition range and have shown tunable thermal properties. Polymer chain motions and crystallization phenomena proved to be interesting aspects as well. To follow the conformational changes of polyamides, several spectroscopic techniques together with XRD methods were successfully employed.24−27 English et al.26 proved that the combination of such methods with quantum chemical calculations is particularly suitable for the quantitative determination of the packing efficiency and local segmental dynamics of the polymers. Recently, 13C{1H} cross-polarization/magic-angle spinning (CP/MAS) NMR spectroscopy revealed its versatility toward polypeptides and synthetic polymers with respect to the investigation of their local and supramolecular structure.24,25,28 As pointed out by Spiess,29 CP/MAS NMR can also be successfully used to follow the gamma−gauche transition of the macromolecules, since the chemical shift of the 13C nuclei in chain fragments in an extended trans conformation is reduced by about 5 ppm for every γ-neighbor in a gauche position. The combination of advanced solid-state NMR and FT-IR techniques provides also detailed insight into the differences between the conformation of the polymer chain fragments in the crystalline and noncrystalline regions or at the interface between them. Nevertheless, to elucidate the structural complexity of the semicrystalline materials the support of additional XRD techniques plays a crucial role in their characterization. Our recent results obtained for the polyamides synthesized from diaminoisoidide1,2 with the amine in the exo−exo configuration convinced us that the preparation and in-depth study of isohexide-derived materials is scientifically significant and that the products can be of commercial interest. Following these studies, a new family of diaminoisosorbide-based

polyamides, this time having an exo−endo configuration of the amine groups in the isohexide motifs, was explored (Scheme 1). The chemical structure of these polyamides was Scheme 1. Chemical Structure of the Polyamide Chain Fragments Synthesized from Diaminoisosorbide (DAIS), 1,4-Diaminobutane (DAB), and Sebacic (SA) or Brassylic Acid (BrA)

confirmed using liquid-state NMR spectroscopy including 2D NMR techniques viz. correlation spectra (1H−1H COSY) and gradient heteronuclear multiple-bond correlation spectra (1H−13C gHMBC). The characteristics of the synthesized polyamides were also correlated to their hydrogen-bonding efficiency, which was elucidated by Fourier transformed infrared spectroscopy (FT-IR) and by the CP/MAS NMR technique. To display the crystal structure, the mobility of the chain fragments and the balance of the population of trans− gauche conformers in the polymer chain fragments the spectroscopic methods were further supported by X-ray scattering. The FT-IR and solid-state NMR techniques were performed as a function of temperature to characterize the different chain conformations present in the samples close to their melting points. An important aspect is also the investigation of the stability of the crystal structure of these fully bio-based polyamides. The effect of exo−endo oriented DAIS on the conformational changes and flexibility of polymer chain fragments as well as their thermal properties were analyzed with reference to the DAII-derived polyamides.2

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EXPERIMENTAL SECTION

Materials. 2,5-Diamino-2,5-dideoxy-1,4;3,6-dianhydrosorbitol (diaminoisosorbide) was synthesized and purified according to the procedure reported by Thiyagarajan et al.21 The purity and stereochemistry of the monomer were determined by 1H NMR, 13C NMR, FT-IR, and ESI-HRMS. 1,8-Octanedicarboxylic acid (sebacic acid, 99%), 1,11-undecanedicarboxylic acid (brassylic acid, 94%, purified by recrystallization from toluene), sebacoyl chloride (99%), 1,4-diaminobutane (99%), and benzene-d6 (99 atom % D) were purchased from Sigma-Aldrich. D2O (99.8 atom % D) was purchased from Merck. Trifluoroacetic acid-d (99.5 atom % D) was available from Cambridge Isotope Laboratories, Inc. Irganox 1330 was purchased from Ciba Specialty Chemicals. 1,1,1,3,3,3-Hexafluoro-2propanol, ethanol, dry chloroform, and toluene were purchased from Biosolve. All the chemicals were used as received, unless denoted otherwise. Synthesis of 1,4-Diaminobutane−Sebacic Acid/Brassylic Acid Salts. The nylon salts were prepared according to the procedure presented in our previous paper.1 To a solution of sebacic acid (3.0 g, 0.015 mol) or brassylic acid (3.66 g, 0.015 mol) in ethanol (10 mL) at 50 °C a solution of 1,4-diaminobutane (1.3 g, 0.015 mol) in ethanol (3 mL) was added dropwise. During the addition, a precipitate was formed. The mixture was stirred at 80 °C for 1 h and then at 50 °C for B


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processed in a 1K × 1K matrix. The spectra were recorded with an acquisition time of 0.211 s, a relaxation delay of 1.4 s, and the number of scans equal to 144 × 210 increments. DSC measurements were performed using a DSC Q100 from TA Instruments. The measurements were carried out in a nitrogen atmosphere with a heating rate of 10 °C/min. The transitions were deduced from the second heating and cooling curves. Thermogravimetric analyses (TGA) were performed on a TA Instruments Q500 TGA in a nitrogen atmosphere. Samples were heated from 50 to 600 °C with a heating rate of 10 °C/min. Fourier transform infrared spectra (FT-IR) were obtained using a Varian 610-IR spectrometer equipped with a FT-IR microscope. The spectra were recorded in a transmission mode with a resolution of 2 cm−1. PA films obtained from 1,1,1,3,3,3-hexafluoroisopropanol were analyzed on a zinc selenium disk and heated from 30 °C to slightly above the melting points of the polyamides. For this purpose a Linkam TMS94 hotstage and controller were used. The samples were cooled in 10 °C steps and reheated with the same heating steps. For the study the spectra from the second heating run were collected. Varian Resolution Pro software version 4.0.5.009 was used for the analysis of the spectra. Variable-temperature (VT) 13C{1H} cross-polarization/magic-angle spinning (CP/MAS) NMR experiments were carried out on a Bruker ASX-500 spectrometer employing a double-resonance probe for rotors with 4.0 mm outside diameter. These experiments used 10.0 kHz MAS and a 4 μs π/2 pulse for 1H. All VT 13C{1H} CP/MAS NMR spectra were recorded using a CP contact time of 3.0 ms and TPPM decoupling30 during acquisition. The temperature was controlled using a Bruker temperature control unit in the range from 30 to 180 °C. The VT 13C{1H} CP/MAS NMR 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 due to spinning using 207Pb MAS NMR of Pb(NO3)2 as a NMR thermometer.31 The 2D 1H−1H doublequantum single-quantum (DQ-SQ) correlation and 13C{1H} frequency-switched Lee−Goldburg heteronuclear correlation (FSLGHETCOR) experiments32,33 were performed on a Bruker AVANCEIII 850 spectrometer using a double-resonance probe for rotors with 2.5 mm outside diameter. These experiments used spinning frequencies of 29 762 and 15 000 Hz, respectively. DQ excitation was performed using the BaBa sequence, and the FSLG-HETCOR experiment used a CP time of 2.0 ms. Chemical shifts for 1H and 13C MAS NMR are reported relative to TMS using solid adamantane as an external reference.34,35 CP conditions were optimized using L-alanine. All samples were annealed just below their melting temperature for 5 min under a flow of nitrogen gas before NMR analysis to remove precipitation-induced structural conformations. Geometry Optimization and 13C NMR Chemical Shift Calculations. All calculations were performed with a Gaussian03 program package. The geometries of the conformations obtained by merging results from a set of 1D potential energy surface (1D-PES) scanning procedure (Figure S3) via B97-D/6-311G**36 were further fully optimized at the MP2/6-311G** and the computationally less demanding B97-D/6-311G** level of theory. Harmonic vibrational frequencies and the Gibbs free energy differences were evaluated at the B97-D/6-311G** levels of theory. The most favorable conformers with respect to energy (for both DFT and MP2 methods) have been proceeded for further NMR chemical shifts calculations at the B97-D/ 6-311G** level of theory. Further detailed information on the 1D-PES procedure, comparison of B97-D and MP2 results, and the 13C NMR chemical shift calculations are given in the Supporting Information (see Figures S3 and S4 and Table S1). Note that some of the calculated 13C chemical shifts are identical for the different conformers and that Figure 5 does not take into account the differences in abundance. Analysis of the crystalline structure of the materials was performed using wide-angle X-ray scattering measurements by means of a computer-controlled goniometer coupled to a sealed-tube source of Cu Kα radiation (Philips), operating at 50 kV and 30 mA. The Cu Kα line was filtered using electronic filtering and the usual thin Ni filter.

1 h. The crude products were filtered and recrystallized from an ethanol/water mixture (10/1, v/v) to afford the salts as white crystals. 1,4-Diaminobutane-sebacic acid salt; (4.1 g, 95%), 1H NMR(D2O): δ = 2.86 (m, 4H), 1.97 (m, 4H), 1.58 (m, 4H), 1.36 (m, 4H), 1.13 (m, 8H). 1,4-Diaminobutane-brassylic acid salt; (4.6 g, 93%), 1H NMR(D2O): δ = 2.90 (m, 4H), 2.03 (m, 4H), 1.62 (m, 4H), 1.40 (m, 4H), 1.15 (m, 14H). Synthesis of Diaminoisosorbide-Sebacic Acid/Brassylic Acid Salts. The salts were prepared in an analogous way as described in our previous paper.1 To a solution of sebacic acid (3.0 g, 0.015 mol) or brassylic acid (3.66 g, 0.015 mol) in ethanol (10 mL) at 50 °C, a solution of diaminoisosorbide (2.2 g, 0.015 mol) in an ethanol/water mixture (20 mL, 10/1, v/v) was added dropwise. During the addition, a precipitate was formed. The mixture was stirred at 80 °C for 1 h and then at 50 °C for 1 h. The crude product was filtered and recrystallized from ethanol/water mixture (10/1, v/v) to afford the salt as white crystals. Diaminoisosorbide−sebacic acid salt; (4.5 g, 88%). 1H NMR (D2O): δ = 4.82 (t, 1H), 4.72 (d, 1H), 4.08 (m, 2H), 3.69−4.00 (m, 3H), 3.69 (dd, 1H), 2.04 (m, 4H), 1.40 (m, 4H), 1.16 (m, 8H). Diaminoisosorbide−brassylic acid salt (4.8 g, 83%). 1H NMR (D2O): δ = 4.80 (t, 1H), 4.69 (d, 1H), 4.05 (m, 2H), 3.78−3.96 (m, 3H), 3.67 (dd, 1H), 2.02 (m, 4H), 1.40 (m, 4H), 1.14 (m, 14H). Typical Procedure for the Bulk Polymerization. A roundbottom flask equipped with a mechanical stirrer, a Vigreux column, and a Dean−Stark type condenser was charged with 1,4diaminobutane−sebacic acid salt (0.500 g), diaminoisosorbide− sebacic acid salt (0.500 g), 1,4-diaminobutane (0.57 mmol, 0.05 g), diaminoisosorbide (0.34 mmol, 0.05 g), and Irganox 1330 (0.01 g), and the mixture was stirred at 170−175 °C for 30 min under an argon atmosphere. Then the temperature was raised to 190 °C, and the polycondensation process was continued for 2 h. The synthesized low molecular weight prepolymer was ground into powder, washed with demineralized water at 80 °C, filtered, and dried under reduced pressure at 80 °C. The resulting product was subsequently submitted to solid-state polymerization carried out at 210 °C for 24 h, as described earlier.1 Typical Procedure for the Interfacial Polymerization. Diaminoisosorbide (2.0 mmol, 0.290 g) and potassium carbonate (4.0 mmol, 0.553 g) were dissolved in water (10 mL), and a solution of sebacoyl chloride (2.0 mmol, 0.478 g) in dry chloroform (4 mL) was then added dropwise. The mixture was stirred at room temperature for 2 h under an argon atmosphere. The reaction product was filtered, washed with water, and ethanol and dried under reduced pressure at 80 °C. Measurements. Size exclusion chromatography analyses (SEC) in 1,1,1,3,3,3-hexafluoro-2-propanol were carried out using a setup equipped with a Shimadzu LC-10AD pump and a waters 2414 differential refractive index detector. PSS (2 × PFG-lin-XL, 7 μm, 8 × 300 mm) columns were used. The eluent flow rate was 1.0 mL/min. Calibration of the measurements was carried out with PMMA standards. The data acquisitions were performed using Viscotek OmniSec 4.0 and Waters Empower 2.0 software. Liquid-state 1H NMR and 13C NMR spectra were recorded using a Varian Mercury Vx spectrometer with a 400 MHz frequency. For 1H NMR experiments the spectral width was 6402.0 Hz, the acquisition time was 1.998 s, and the number of recorded scans was 64. 13C NMR spectra were recorded with a spectral width of 24154.6 Hz, an acquisition time of 1.300 s, and the number of recorded scans equal to 256. 2D NMR spectra were recorded on a Varian Unity 500 plus spectrometer at room temperature. Chemical shifts were referenced to residual signals of C6D6 or trifluoroacetic acid-d. The NMR experiments were carried out in trifluoroacetic acid/C6D6 mixture or trifluoroacetic acid-d. Correlation spectra (COSY) were acquired using standard programs provided by a Varian spectrometer library with the following parameters: spectral width SW1 = SW2 = 6075.3 Hz, acquisition time 0.221 s, relaxation delay 1.4 s, number of scans 8 × 300 increments. Heteronuclear multiple-bond correlation spectra (gHMBC) were recorded with pulse field gradients. The spectral windows for 1H and 13C axes were 6075.3 and 21 367.4 Hz, respectively. The data were collected in a 2560 × 210 matrix and C


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Table 1. Properties of the Homo- and Copolyamides Synthesized from 1,4-Diaminobutane (DAB), Diaminoisosorbide (DAIS), Sebacic Acid (SA), and Brassylic Acid (BrA)

a

entry

monomer feed, mole ratio (DAB/DAIS)a

PA1 PA2 PA3 PA4d

0.78/0.22 0.68/0.31 0.54/0.46 0/1.0

PA5 PA6 PA7

1.0/0 0.78/0.22 0.69/0.31

built-in composition (DAB/DAIS)b

Mnc (g/mol) before SSP

SA/DAB/DAIS-based polyamides 0.72/0.28 0.56/0.44 0.50/0.50 0/1.0 BrA/DAB/DAIS-based polyamides 1.0/1 0.66/0.34 0.56/0.44

PDIc

Mnc (g/mol) after SSP

PDIc

6200 8100 7960 5600

2.1 2.2 2.1 5.0

17 700 17 200 19 800

3.7 2.9 2.6

4900 7200 4900

2.5 2.1 2.6

15 500 20 300 21 300

3.4 3.5 4.1

Ratio determined by weighed-in monomers. bRatio determined by NMR. cValues determined using SEC against PMMA standards in HFIP solvent. Polyamide synthesized via interfacial polymerization.

d

The data were collected at room temperature. The 1D profiles were subsequently background-corrected and normalized.

carbon and proton resonances of the investigated polymers (see Scheme 2).

RESULTS AND DISCUSSION Chemical Structure of Diaminoisosorbide-Based Polyamides. Using the synthesis pathway consisting of bulk polycondensation of the nylon salts followed by solid-state polymerization (SSP) or using the interfacial polymerization technique, a series of polyamides based on diaminoisosorbide (DAIS, exo−endo oriented dideoxy−diamino isohexide), 1,4diaminobutane (DAB), sebacic acid (SA), or brassylic acid (BrA) were prepared. Proper synthesis routes were established based on the experience obtained from the SA, DAB, and diaminoisoidide-derived polyamides.1 As determined, this procedure leads to sufficiently high molecular weight polymers and allows incorporating of the bicyclic building blocks into the backbone of the PA 4.10 or PA 4.13. Different homo- and copolyamides with varying mole ratios of DAB/DAIS were synthesized (Table 1). Despite the possibility of the formation of intramolecular hydrogen bonds, resulting in a relatively low reactivity of the endo-oriented amine group, the applied mild polymerization conditions resulted in polymeric materials with number-average molecular weights above 15 000 g/mol, which exceeds the critical Mn value of polyamides (∼12 000 g/mol) above which no significant influence of Mn on their thermal and mechanical properties is expected. Furthermore, in line with our results, obtained for the compositions based on the exo− exo oriented diaminoisoidide,1 the solvent-free route afforded higher molecular weight polyamides than interfacial polymerization, which can be seen by comparing entries 1−3 and 5−7 with entry 4 (Table 1). As illustrated by SEC analysis, the interfacial polymerization results in relatively low molecular weight polyamides, which is most probably due to the high solubility of DAIS in water, preventing the accumulation of the monomer around the interface as well as to some spontaneous hydrolysis of diacid chloride.1,37 Note that solid-state polymerization of the homopolyamide entry 4 did not result in a further enhancement of Mn due to the low melting point of this PA4 (114−151 °C), which limits the SSP temperature to very low values. To investigate the chemical composition of the copolyamides and to prove the presence of diaminoisosorbide residues in their backbones, liquid-state 2D NMR spectroscopy was applied. The analysis of the long-range 1H/13C correlations in the gHMBC spectrum supported by the COSY technique was found to be the most suitable for proper assignments of the

Scheme 2. Chemical Structure of Diaminoisosorbide/ Sebacic Acid and 1,4-Diaminobutane/Sebacic Acid Repeat Units in the Polyamidesa

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a

Proton and carbon labels were used both for the analysis of liquidstate and solid-state NMR spectra.

The group of signals observed in the gHMBC spectrum at δH 3.40−4.80 ppm (Figure 1) provided detailed information about the configuration of the bicyclic diamine. Examination of the correlation signals H5a/C6a, H5a/C6, H5/C5a, H5a/C5, H5/C6a, and H5/C6 at 4.72/69.0, 4.72/72.0, 4.68/81.0, 4.72/86.6, 4.68/ 69.0, and 4.68/72.0 ppm proved that the NH units, after polymerization taking part in the formation of amide bonds, preserved exo−endo configuration. These findings were further supported by the cross-peaks H5a/H5, H5a/H4a, and H5/H4 at 4.72 and 4.68 ppm in the COSY spectrum (see Supporting Information Figure S1) showing that the protons H4a and H4 adopt an exo and endo configuration, respectively. Furthermore, the correlation signals of H6/C5, H6′/C4, H6a/C5a, H6′/ C5, H6a′/C4a, H6a′/C5a, and H6a′/C5 of diaminoisosorbide units in the gHMBC spectrum were found at 4.10/86.6, 3.95/58.0, 3.95/81.0, 3.95/86.6, 3.54/54.0, 3.54/81.0, and 3.54/86.6 ppm, respectively. The H7a/C14a and H7/C14 of the amide motifs were observed at 7.75/180.2 and 7.68/180.4 ppm while the cross-peaks of the protons H7 and H7a in the COSY spectrum were visible at 7.68 and 7.75 ppm. From an inspection of the COSY spectrum it becomes apparent that the resonances of H8, D


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Figure 1. 500 MHz heteronuclear multiple-bond correlation spectrum (gHMBC) of the co(polyamide) PA3 synthesized from sebacic acid, 1,4diaminobutane, and diaminoisosorbide with the DAB/DAIS mol/mol ratio equal to 0.50/0.50.

which once again proved the exo−endo configuration of the diaminoisosorbide residues incorporated into the backbone of the polyamide macromolecules. In the COSY spectrum the resonance of H1 revealed further cross-signals to H2 at 2.30− 2.35 ppm while the cross-peaks of H2/H3 were distinguished at 1.58 ppm (Figure S1 in Supporting Information). Moreover, the multiple-bond C−H correlations of H1/C2, H1/C3, H2/C3, H2/C1, and H3/C2 were found in the gHMBC spectrum at 2.30−2.35/25.8, 2.30−2.35/28.0, 1.56/28.0, 1.56/33.8, and 1.20/25.8 ppm, respectively. Thermal Properties. The examination of the melting and crystallization temperatures of diaminoisosorbide-derived polyamides was attempted using DSC and TGA analysis (Table 2). Given that the polyamides denoted as entries 1−3 and 5−7 reveal comparable molecular weights, a significant impact of the presence of randomly distributed bicyclic diamine residues in the polymer chain on their thermal properties was proven. As evidenced by DSC thermograms (Figure 2), the presence of DAIS residues in the backbone of the sebacic acid-based

originating from DAB and contributing to the formation of amide groups, are shifted to higher frequency. On this basis the cross-signals H8/H9 and H9/H10 of 1,4-diaminobutane were found at 8.39 and 3.33 ppm together with the correlation signals H8/C15, H9/C10, and H9/C15 at 8.39/180.0, 3.33/24.2, and 3.33/180.0 ppm in the gHMBC. By an in-depth analysis of the signals at δC 180.0 and 183.5 ppm, further insight into the sequence of sebacic acid−based units was obtained. The signals at 2.42/180.0 and 1.52/180.0 ppm can be assigned to H11/C15 and H12/C15 while the correlation signals H17/C16, originating from nonpolymerized carboxylic acid end groups, were found at 2.26/183.5 ppm, as also found for DAII-derived polyamides and described in our previous paper.1 Furthermore, the 1H resonances of H18 and H19 at 1.52 and 1.20 ppm, distinguished in the COSY and gHMBC spectra, proved the expected proton sequences of the polyamide chain fragments. Interesting information was provided by the HMBC cross-signals at 2.35/180.4 and 2.30/ 180.2 ppm, attributed to H1/C14 and H1/C14a, respectively, E


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Table 2. Melt and Crystallization Temperatures (Tm, Tc), Enthalpy of the Transitions upon Heating (m) and Cooling (c), and Thermal Stability of the Homo- and Copolyamidesa entry

Tm (°C)

ΔHm (J/g)

Tc (°C)

ΔHc (J/g)

T5% (°C)

Tmax (°C)

PA1 PA2 PA3 PA4b PA5 PA6 PA7

238 235 225 114, 151 228 218 217

63.1 54.3 43.1 18.3, 5.3 71.1 62.1 55.3

212 203 186 71 203 196 193

41.1 46.3 40.2 2.6 58.2 46.9 46.8

396 410 411 202 396 415 389

454 450 453 414 456 464 455

of the DAIS-based copolymers in comparison to the DAIIderived products, which were discussed in our previous paper,1 suggest that due to the steric hindrance and less symmetric monomer structure, the exo−endo oriented conformer is predominantly incorporated into the amorphous phase and therefore affects the DAB/SA-based crystalline domains in comparison to diaminoisoidide-based materials to a lesser degree (see Figure 2). One should note that the exo−exo diamine was indeed partly cocrystallizing with the DAB/SA repeat units.1 To prove our hypothesis, the copolymers were also submitted to a WAXD analysis, as presented below. On the basis of the TGA data presented in Table 2, it can be concluded that the incorporation of DAIS into the backbone of PA 4.10 or PA 4.13 via solid-state polymerization is a successful route toward thermally stable copolymers up to 390 °C. As mentioned before, the interfacial polymerization leads to low molecular weight polyamides (entry 4), which significantly affected the thermal stability. Local Polymer Chain Conformation and Mobility Probed by Solid-State NMR. Detailed information concerning distribution of diaminoisosorbide (DAIS) over the crystalline and amorphous phases in the investigated polyamides can be obtained by the analysis of the structural changes and mobility of the polymer chain fragments using variable-temperature (VT) solid-state NMR spectroscopy. In our previous study of the homo- and copolyamides based on diaminoisoidide,2 we were able to show that the bicyclic diamine adopts a range of different conformations and participates in both the formation of the crystalline and amorphous phases. Thus, to confirm the formation of the hydrogen-bonded networks and to analyze the conformation of the DAIS-derived polymer chain fragments, we have employed VT 13C{1H} CP/MAS NMR spectroscopy for the analysis of the crystalline and noncrystalline domains present in the semicrystalline polymers.25,27 The solid-state 13 C{ 1 H} CP NMR spectra of the homopolymer PA4 depicted in Figure 3a as a function of temperature display several interesting changes in the aliphatic region 20−90 ppm. The changes in position and intensities of the NMR signals are especially visible for the DAIS resonances viz. C5, C5a, C6, C6a, C4, and C4a located at 87.5, 81.6−80.1, 75.3−68.7, 58.7, and 53.4 ppm, respectively (see Table 3 and Scheme 2 for assignments). The presence of well-resolved 13C resonances in the VT 13C{1H} CP/MAS NMR spectra originating from C4 and C4a nuclei attached to the exo and endo oriented amide groups illustrates that the polymerization process does not affect the configuration of the DAIS groups (see also Figure 1). Furthermore, the resonances from C5, C5a, C6, and C6a reveal that significant changes in the intensities close to the melting point of the homopolymer occur. This complex behavior further shows that the signal C5, C5a and the amide CO resonance at 174.6 ppm exhibit lower intensities and shift to higher frequencies upon heating. The coexistence of these signals, albeit with different intensities close to the melting point, reflects the distribution of DAIS residues over the crystalline and amorphous domains of the PA4 sample. As reported by Gitsas et al.,24 the dynamic changes of the local chain mobility of the polymer result in the reduction of the effective dipole−dipole couplings and thereby lower crosspolarization (CP) efficiency at higher temperatures. Hence, by affecting the hydrogen bonding of the well-arranged polyamide chain fragments, the intensity of the signals originating from the crystalline phase will be affected more rapidly as a function of

Tm = melting point, Tc = crystallization temperature, ΔH = enthalpy of the transition during melting (m) and crystallization (c), T5% = temperature of 5% mass loss, Tmax = temperature of maximal rate of decomposition. bTemperature and enthalpy of the transition derived from the first heating run. a

Figure 2. DSC thermograms of SA/DAB/DAIS-derived copolyamides (1−3), BrA/DAB/DAIS-derived copolyamides (6, 7), and PA 4.13 (5) compared with the DSC traces of SA/DAB/DAII-based copolymers (1a−3a). The contents of DAIS and DAII are comparable in entries 1−3 and 1a−3a, respectively. The traces represent the second heating runs, employing a heating rate of 10 °C/min.

macromolecules significantly determined their Tm, Tc, and the enthalpy of the transitions. With an increasing content of isohexide diamine from 28 to 50 mol % (entries 1−3) their melting temperature decreased from 238 to 225 °C while the Tm values of the SA/DAIS-based homopolyamide were recorded at 114 and 151 °C. This melting point reduction was even more pronounced when compared with the reference PA 4.10, synthesized from SA and DAB (TmPA4.10 = 246 °C).1 A similar phenomenon was also observed for the compositions based on BrA, DAB, and DAIS (entries 5−7), where the introduction of 34 and 44 mol % of DAIS decreased the melting point of PA 4.13 from 228 to 218 °C and 217 °C, respectively. It is clear from the evidence presented above that the presence of endo−exo oriented isohexide diamine residues in the main chain of the copolyamides strongly affects the thermal properties of the investigated polymers. The comparison of the melting and crystallization temperatures of these copolymers with their analogues synthesized from diaminoisoidide (exo−exo oriented isohexide diamine), having comparable DAB/DAII and DAB/DAIS mole ratios, provides interesting information. Higher thermal transition temperatures F


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temperature compared to the signals from the amorphous domain. Interestingly, from an inspection of the C5, C6, and C4 signals it becomes apparent that part of the methylene groups in the cyclic moieties (DAIS) appears as broad shoulders at slightly higher and lower frequencies, respectively. These 13C resonances indicate the coexistence of several different DAIS conformers with different stabilities at elevated temperatures. More significant changes in these signal intensities are observed above 100 °C, emphasizing the distribution of DAIS residues in both phases of the semicrystalline homopolymer PA4. The signals C1, C2, and C3, resonating between 20 and 40 ppm and related to sebacic acid units, also show a significant decrease in their intensities above 100 °C. However, this effect is particularly visible for C2 and C3 resonances, demonstrating that the reorganization of the aliphatic units in the homopolyamide is propagated from the inner methylene groups of sebacic acid, while the C1 signals follow the trend of the carbonyl groups involved in the formation of the amide moieties. Further information about the molecular packing and hydrogen bonding for PA4 can be derived from the 2D 13 C{1H} FSLG-HETCOR and 1H−1H DQ-SQ correlation spectra shown in Figures 3b,c. These spectra show that the homopolyamide sample includes two different types of amide groups which, based on the chemical shift difference and their position, are related to hydrogen-bonded (8.3 ppm) and nonhydrogen-bonded groups (6.0 ppm). The existence of two different hydrogen-bonding environments is also visible in Figure 3a where splitting of the carbonyl resonance is observed. These two different hydrogen-bonding environments are most likely related to the noncrystalline and crystalline regions of the PA4 sample; however, no clear evidence for this behavior can be derived from the VT 13C{1H} CP/MAS NMR in Figure 3a, since both carbonyl signals display the same intensity decay as a function of temperature. Moreover, the 2D 1H−1H DQ-SQ correlation spectrum reveals that the hydrogen-bonding environments are not spatially close (no cross-peak detected) and that the missing autocorrelation between the hydrogenbonded amide groups points toward a chain-folded structure where these groups are not in close contact. We note that the results obtained here for PA4 are comparable to our recent study on DAII-based homopolymer where two different hydrogen-bonding environments were observed in addition to that related to carboxylic acid end groups.2,38 In Figure 4a, VT 13C{1H} CP/MAS NMR spectra of the copolyamide PA3 synthesized from SA, DAB, and DAIS with a molar ratio of DAB/DAIS equal to 0.50/0.50 (Table 1, entry 3) are presented. The distinct and broad signals centered at 173.8 ppm, corresponding to carbonyl groups involved in the formation of the amide groups, clearly proves the presence of DAIS build in into the PA backbone. If the DAIS units were not

Figure 3. (a) Variable-temperature solid-state 13C{1H} CP/MAS NMR spectra recorded at 11.75 T (500 MHz for 1H) of homopolyamide PA4 synthesized from sebacic acid and diaminoisosorbide. (b) 2D 13C{1H} FSLG-HETCOR spectrum acquired using a 2.0 ms CP step and five FLSG blocks per t1 increment to improve the 1 H resolution. (c) 2D 1H−1H DQ-SQ spectrum recorded using a BaBa recoupling period of 67.2 μs. Both 2D spectra were recorded at 20.0 T corresponding to 850.27 MHz for 1H. The dashed lines in (b) and (c) illustrate selected cross-peaks and autocorrelation peaks, including hydrogen-bonded (HB) and non-hydrogen-bonded (non-HB) amide fragments. Assignment is performed according to Scheme 2, and the asterisk indicates the position of a spinning sideband (ssb) from the carbonyl resonance.

Table 3. Chemical Shifts of Specific 13C CP/MAS NMR Signals for Copolyamide PA3 and Homopolyamide PA4 Recorded at Different Temperatures 13

C chemical shift/ppm

PA3 41.0 °C 173.6 °C PA4 41.0 °C 164.1 °C

1

11

2

12

3

13

CO

C /C

173.8 174.0

39.8 40.8

33.2−24.2 33.2−24.2

36.6 37.4

174.6 175.0

36.5 36.7

33.6−24.6 33.6−24.6

33.6−24.6 33.6−24.6

C /C

C /C

C

9

42.9 41.5

G

C10

C4

C4a

C5

C5a

C6

C6a

33.2−24.2 33.2−24.2

59.0 58.5

53.2 53.9

88.2 88.7

81.4 82.1

75.8−65.1 74.1−71.2

58.7 59.2

53.4 53.7

87.5 88.5

81.6−80.1 82.2−80.6

75.3−68.7 75.3−70.2


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toward higher frequencies, which is characteristic for polyamides. Furthermore, from an inspection of the 13C{1H} CP/MAS NMR spectra in the vicinity 50−90 ppm, it becomes apparent that the DAIS residues adopt slightly different organization compared to the DAIS-based homopolymer PA4. The 13C resonances of the DAIS moieties in the copolymer PA3 are relatively broad. As shown also for the DAII-based copolyamides,2 such significant spread of the 13C resonances reflects a number of different DAIS conformations in the polymer backbone and/or slightly different domains with the same conformation. The dynamic changes of the DAIS conformation can be further characterized by the inspection of the 13C{1H} CP/MAS NMR spectra recorded above 150 °C. Here, the presence of well-resolved C6 and C6a resonances (see Scheme 2 for assignments) positioned at 74.1 and 71.2 ppm, respectively, and a comparison with these signals recorded at 41.0 °C show that significant differences in the abundance of the conformers exists at different temperatures. The dynamic balance between the different conformations is further strengthened by the changes in line width for the C5, C5a, C4, and C4a signals, which are accompanied by an enhanced thermal motion of the copolyamide chain fragments and the corresponding decrease of their hydrogen bond density. Because of an enhanced flexibility of the polymer chain and the presence of a different set of conformers, the signals become sharper and sudden distinctions arise in the spectra. Furthermore, the 13C{1H} CP/MAS NMR spectra of the copolyamide PA3 display two distinct signals of C9 and C1/C11 nuclei resonating at 42.9 and 39.8 ppm (see Scheme 2 for assignment). These signals undergo similar intensity and chemical shift changes as also observed for PA 4.10 and PA 4.10/DAII.10 copolymers containing less than 40 mol % of diaminoisoidide.2 For the PA3 sample studied in this work, however, the signal at 42.9 ppm (C9) shows rather fast intensity decay on heating accompanied by a shift to lower frequency. At temperatures above 150 °C the signal from C9 is hardly visible. A direct comparison of DAIS-based copolyamides with their DAII-derived analogues indicates that the exo−exo isohexide configuration more readily facilitates the trans−gauche conformation changes of the methylene segments than the exo−endo configuration. Furthermore, the DAB resonances show changes of their 13C chemical shift position and intensities above 110 °C, whereas the sebacic acid C1 and C11 NMR signals remain almost unaffected at this temperature. On the basis of this observation, it is evident that the molecular motion of the linear chain fragments is promoted in the diamine segments. Combined these findings suggest that the structural changes of the investigated copolymers on heating originate not only from the presence of the cyclic isohexide (DAIS) in the structures but also from the fact that gauche conformers are more easily induced in the linear moieties. VT 13C{1H} CP/ MAS NMR spectra of the copolyamide PA2 (Table 1) are provided in the Supporting Information (Figure S2). The copolymer PA2 undergoes similar conformational changes as observed for the copolyamide PA3; however, the 13C resonances for DAIS units reveal a significantly reduced intensity as a consequence of their lower content in the backbone of the copolyamide. Figures 4b,c display the 2D 13 C{1H} FSLG-HETCOR and 1H−1H DQ-SQ correlation spectra for PA 3. These spectra show similar characteristics as those observed for homopolymer PA 4 (Figures 3b,c), however, with the important difference that the DAIS groups only display very weak correlations to the carbonyl groups and no visible

Figure 4. (a) Variable-temperature solid-state 13C{1H} CP/MAS NMR spectra recorded at 11.75 T (500 MHz for 1H) of copolyamide PA3 synthesized from sebacic acid, 1,4-diaminobutane, and diaminoisosorbide with a DAB/DAIS molar ratio 50/50. (b) 2D 13 C{1H} FSLG-HETCOR spectrum acquired using a 2.0 ms CP step and five FLSG blocks per t1 increment to improve the 1H resolution. (c) 2D 1H−1H DQ-SQ spectrum recorded using a BaBa recoupling period of 67.2 μs. Both 2D spectra were recorded at 20.0 T corresponding to 850.27 MHz for 1H. The dashed lines in (b) and (c) illustrate selected cross-peaks and autocorrelation peaks, including hydrogen-bonded (HB) and non-hydrogen-bonded (non-HB) amide fragments. Assignment is performed according to Scheme 2, and the asterisk indicates the position of a spinning sideband (ssb) from the carbonyl resonance.

fully incorporated in PA3, and thereby bonded to SA to form amide groups, an additional carbonyl resonance would be observed at around ∼180 ppm, corresponding to the acid groups of SA.2 As a consequence of enhanced system mobility and the changed balance in hydrogen bonding upon heating, these resonances display decreasing intensity and a gradual shift H


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experimental resonances in the region of 50−90 ppm (Figure 5). We have employed two different line widths (0.25 and 1.50 ppm) for the calculated 13C chemical shifts shown in Figure 5b, illustrating the different 13C resonances of the DAIS group and that a broadening of these signals leads to a calculated 13C NMR spectrum that is in good agreement with the experimental spectra in Figures 3 and 4. Even though the third conformer (iii) is calculated to be only responsible for 3% of the total population, according to the Boltzmann distribution in the gas phase under normal conditions, it turns out that in the solid state it gains an extra stabilization due to other factors and thus becomes experimentally visible at roughly 66 ppm (Figure 3, vertical dashed line). These stabilization factors vanish upon melting, and therefore conformer iii is not observed for both PA3 and PA4 at 173.6 and 164.1 °C, respectively. Furthermore, the 13C resonances of the two most abundant conformers (i and ii), with a difference of 1.4 kJ/mol only, are in excellent agreement with corresponding experimental melt and HMBC spectra. Relying on 1H and 13C calculated chemical shifts for these conformers, the corresponding HETCOR spectrum of the DAIS fragment was successfully reproduced and compared with the experimental one in the Supporting Information (Figure S5). FT-IR Analysis of the Polyamides. To elucidate the conformational changes arising due to the presence of DAIS in the backbone of the polyamides and to follow its influence on hydrogen bonding, a temperature-dependent FT-IR analysis was carried out (Figures 6 and 7; for the bands assignment see Table 4). Upon heating, during which the mobility of the polymer chain fragments increased significantly, several changes of the FT-IR signals intensities were observed. As reported by Skrovanek et al.39,40 and Schroeder et al.,41 a decrease in the absorptivity coefficient of the N−H stretching mode upon heating, which affects their intensity, reflects the strength of the hydrogen bonding of the polymers. Following these observations and analyzing the FT-IR spectra of homopolyamide PA4 and copolyamide PA3 recorded at 30 °C, two different infrared bands at around 3300 and 3430 cm−1 originating from hydrogen-bonded and non-hydrogen-bonded N−H stretching modes were found. The decreased intensities of the resonances at 3300 cm−1 on heating, accompanied by an enhanced absorbance of the non-hydrogen-bonded N−H and CO bands, proved the VT 13C{1H} CP/MAS NMR results exhibiting a changed balance in hydrogen bonding of the polyamides. A similar effect of loose N−H vibrations was also observed with an increasing content of DAIS in the backbone of the macromolecules. Interesting information about the population of trans and gauche conformers in the backbone of the polyamides is provided by signals of CH2 vibrational modes with trans conformation next to NH and CO groups at 1475 and 1417−1419 cm−1, respectively.42,43 The CH2 scissoring band at 1475 cm−1, present in the spectra recorded for the SA/DAIS/ DAB-based copolyamides and originating from DAB residues, shows a lower intensity above 110 °C, which arises from an increasing population of gauche conformers (Figure 7) and gives the evidence that the methylene units undergo enhanced motion upon heating. These observations are in a good agreement with the results reported by Yoshioka et al.43 proving that due to the lower energy barrier of the torsion around CH2−NH motifs, the conformational changes of these methylene groups are significantly facilitated in comparison to the CH2 attached to the carbonyl group. The sequence of

correlations to the amide groups. Furthermore, the correlations are much broader and of low intensity as shown in the VT 13 C{ 1H} CP/MAS NMR spectra in Figure 4a. These observations indicate that the DAIS groups are primarily located in the noncrystalline regions of the sample, which is further supported by the changes in signal intensity and line width for 13C resonances of the DAIS groups as a function of temperature as shown in Figure 4a. DAIS Conformational Diversity and NMR Chemical Shifts from ab Initio Calculations. Since the DAIS moiety is composed of sp3-hybridized carbon atoms, it is expected to exhibit a large set of possible conformations, which were further submitted to a comprehensive conformational analysis. In order to control the stability of the found conformers, two different methods (DFT and MP2) were used in parallel. Hence, among 36 possible structures only 17 conformations have been proven to be stable by using the B97-D/6-311G** level of theory. However, two of them were ruled out after MP2/6-311G** analysis. An extensive conformational analysis is presented in the Supporting Information. Please read this for more details. Admittedly, because of the computational cost, the current conformational study could not account for the spatial organization of the alkyl chains in the bulk, and thus all calculations were performed for the gas phase on a simplified DAIS fragment with propanoic acid side chains as terminating groups. Nevertheless, we believe that these calculations will help us to understand the real situation.

Figure 5. (a) The three most abundant conformers i, ii, and iii of DAIS and (b) the calculated 13C NMR spectrum, assuming an equal Boltzmann distribution. The calculated 13C NMR spectra in (b) have employed a broad (1.50 ppm, upper) and a narrow (0.25 ppm, lower) line width to illustrate the different 13C signals and for a comparison with the experimental results presented in Figures 3 and 4.

Because of its asymmetry, the DAIS fragment in both PA3 and PA4 displays broad resonances with hints of multiple signals from all of its three different carbon moieties (Figures 3 and 4) and shows more 13C resonances than was found in the previous study of similar DAII-based polymers.2 This fact is fully supported by the three most abundant conformers, whose combination of 13C chemical shifts explained all observed I


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Figure 6. FT-IR spectra of the homopolyamide PA4 synthesized from diaminoisosorbide and sebacic acid presenting the conformational changes of the polymer chain fragments upon heating. The spectra were normalized according to the area of the methylene bands in the interval 2800−3000 cm−1.

Figure 7. FT-IR spectra of the copolyamide PA3 synthesized from diaminoisosorbide, 1,4-diaminobutane, and sebacic acid with DAB/DAIS molar ratio 50/50 presenting the conformational changes of the copolymer chain fragments upon heating. The spectra were normalized according to the area of the methylene bands in the interval 2800−3000 cm−1.

more restricted mobility of the diaminoisosorbide-based polyamides on heating in comparison to diaminoisoididederived analogues where the C−CO stretching mode shows a noticeable decrease in their intensity close to the melting point of the DAII-based polymers. In view of the recent FT-IR results,2 obtained for diaminoisoidide-based copolyamides, it is important to emphasize that the presence of exo−exo oriented diamino isohexide in the backbone of the polyamides more readily enhances the molecular motion and the induction of gauche conformers in the linear DAB and SA-based chain fragments than the DAIS with exo−endo configuration. These observations follow from the FT-IR spectra recorded for the diaminoisoidide-derived copolymers containing around 50 mol % of the bicyclic exo−exo diamine. The spectra of the diaminoisoidide-based copolyamides do not exhibit the presence of CH 2 scissoring next to NH with trans conformation, while their analogues synthesized from diaminoisosorbide show the evidence of the rigid trans conformations in the aliphatic moieties. These observations suggest that the transition between trans and gauche conformers in the linear moieties of the DAII-based copolymers is increased and significantly facilitated. Wide-Angle X-ray Analysis. Wide-angle X-ray scattering diffractograms are presented in Figures 8 and 9. All analyzed

conformational changes of copolyamide 3 was also proven by the analysis of the CH2 mode next to the CO group with trans conformation, which revealed more restricted mobility as evidenced by the narrow bandwidth and the more preserved intensity above 110 °C. The trans−gauche conformational changes were further verified by the grow in the intensity of the signals at 1464−1466 cm−1, arising from the gradual increase in molecular motion of the polymer chain fragments and formation of the pseudohexagonal phase. A more in-depth analysis of the polyamides PA 3 and PA 4 clearly shows signals originating from isohexide units at 1070− 1100 cm−1 assigned to the typical asymmetric C−O−C stretching mode usually observed in ethers.44 The signals positioned at 1099 and 1100 cm−1 do not change their intensity upon heating while the vibrations at 1072 and 1074 cm−1 became broader and finally disappear close to the melting point of the polyamides, giving the evidence that the DAIS residues are distributed over both the crystalline and the amorphous phase of the polyamides. Furthermore, the spectra recorded for the co- and homopolyamide show symmetric CCO stretch vibrations at 949 and 960 cm−1, respectively. These bands, together with the C−CO stretching vibrations at 886−887 cm−1, exhibit gradual changes in their width and intensities; however, they are visible up to the melting point of the polyamides. The origin of this phenomenon is caused by the J


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Table 4. Frequencies and Assignment of FT-IR Bands Recorded of the Homopolyamide PA4 and Copolyamide PA3 at 30 °C band frequencies (cm−1) PA4

PA3

band assignment

3430 3298 3196 3064 2930 2854 1717 1650 1540

3431 3300 3198 3070 2928 2851

N−H stretch vibration; non-hydrogen-bonded N−H stretch vibration; hydrogen-bonded N−H stretch vibration and amide (I + II) overtone N−H stretch vibration and amide (II) overtone CH2 antisym stretch vibration CH2 sym stretch vibration amide I; CO stretch vibration; non-hydrogen-bonded amide I; CO stretching vibration amide II in-plane N−H deformation with CN and CO stretch vibration CH2 scissoring next to NH group with trans conformation CH2 scissoring not adjacent to the amide group CH2 scissoring next to CO group with trans conformation amide III; CN stretch and in-plane NH deformation amide III; CN stretch and in-plane NH deformation CH2 wagging or twisting skeletal C−C stretch skeletal C−C stretch skeletal C−C stretch amide II + III coupled with hydrocarbon skeleton asym C−O−C stretching mode asym C−O−C stretching mode skeletal C−C stretch vibrations skeletal C−C stretch vibrations CCO sym stretch vibration CCO sym stretch vibration C−CO stretching vibrations

1464 1417 1377 1286 1240 1216 1180 1100 1074 1054 1017 960 886

1642 1549 1475 1466 1419 1382 1360 1303 1254 1233 1195 1099 1072 1056 1016 949 887

Figure 8. X-ray powder diffraction profiles of 1,4-diaminobutane-, diaminoisosorbide-, and sebacic acid-based copolyamides.

Figure 9. X-ray powder diffraction profiles of 1,4-diaminobutane-, diaminoisosorbide-, and brassylic acid-based polyamides.

polyamides are semicrystalline, regardless the type of dicarboxylic acid or the content of DAIS used. Because of the relatively low content of DAIS in PA 1 and PA 2 (lower than 44%), the diffraction profiles of the DAB-, DAIS-, and SA-based polyamides exhibit strong analogueies to the diffraction profile of the homopolymer PA 4.10 synthesized from 1,4diaminobutane and sebacic acid.1,2,10,45 Therefore, one can observe four characteristic reflections in the 2Θ ranges 3°−7°, 10°−12°, and 18°−25° corresponding to the following crystallographic planes 001, 002, 100, and 010/110 (Figure 8 and Table 5). An increasing content of DAIS in the polyamides resulted in a change of packing of the macromolecules in the analyzed crystals. A single reflection in the 2Θ range 18°−25°

of the diffraction profiles of the polyamides containing 50 mol % or more of DAIS (PA3, PA4) indicates that the materials crystallized into a close to hexagonal form. In our previous paper,1 the 100 reflection (the 100 peak corresponding to the interchain distance) for the homopolymer PA 4.10 was located at 2Θ equal to 20.00°. The presence and a relative content of exo−endo oriented isohexide diamine in the copolymer clearly affects the location of said reflection. From the obtained results one can observe shifting of the 100 signal toward higher 2Θ values accompanied by the increasing DAIS content in the copolymer, which revealed a decreasing interchain distance in the crystals. The observed change in the interchain distance K


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Table 5. X-ray Diffraction Spacings of 1,4-Diaminobutane-, Diaminoisosorbide-, and Sebacic Acid-Based Polyamides 2Θ [deg] PA1 PA2 PA3 PA4

X-ray diffraction [nm]

001

002

100

010/110

001

002

100

010/110

5.82 5.76 5.58 5.53

11.64 11.52

20.48 20.48 20.80 20.88

23.95 23.82

1.518 1.534 1.584 1.598

0.760 0.768

0.434 0.434 0.427 0.425

0.371 0.373

contains three additional methylene groups in the methylene chain fragments. The presence of three additional methylene units should result in an enhancement of the length of the basic unit of the macromolecules by 0.375 nm. According to the data provided in our previous paper,1 the interplanar spacing for the 001 plane of PA 4.10 was 1.561 nm. The presence of three additional methylene units should increase the interplanar spacing up to 1.936 nm. The signal coming from the 001 plane for polyamide 4.13 should therefore be located at 2Θ equal to 4.56°. The performed experiments show that the signal of PA 4.13 at low values of 2Θ angle (Figure 9, Table 6) is located at 2Θ equal to 4.61°. Therefore, one can conclude that the signal in the 2Θ range 3°−7° of PA 4.13, similar to PA 4.10, comes from the crystallographic 001 plane. Furthermore, there are two reflections observed in the diffraction profile of PA 4.13 in the 2Θ range 18°−25°. Their location and shape may suggest a similar indexing as for PA 4.10, namely 100 and 010/110. However, to prove that these discussed reflections come from the same crystallographic planes, an additional research surpassing the framework of this paper seems to be necessary. For the sake of further discussions, the signals in the 2Θ range 18°−25° have been indexed as follows: 100* and 010/110*. The presence of DAIS in the copolymers based on brassylic acid substantially affects the crystallographic structure of the material. For DAIS contents higher than 34 mol % in the polymer backbone the packing of the macromolecules in the crystal lattice was changed (for the sebacic acid-based polyamides such change was noted at a DAIS content between 44 and 50 mol %). Moreover, as for DAB-, DAIS-, and SAbased polyamides one can observe that the 100* signal is shifted toward higher 2Θ values, which is accompanied by an increasing concentration of DAIS residues in the polymer backbone, while the reflection in the 2Θ range 3°−7° is shifted toward lower values. Additionally, for the copolyamide PA6 (Figure 9), it was possible to observe two signals, which may indicate the presence of two fractions of crystals with different interplane distances along the c-axis. It seems that a further increase of DAIS content in the copolymer favors the cocrystallization phenomenon, a symptom of which is the loss of the doublet structure of the signal for PA 7 (Figure 9).

may in turn affect the length and simultaneously the stability of hydrogen bonds located in the area of the crystalline phase (as described before,1 for the polyamides containing exo−exo oriented isohexide diamine we did not observe any change of location of the 100 reflection). The change of the way of packing of the macromolecules in the crystals of DAII-based copolymers was already observed at a DAII content between 20 and 43 mol %,1 while for their DAIS-based analogues the changes of packing of the polymer chain fragments were observed at higher DAIS contents equal to 44−50 mol %. The observed phenomena may indicate larger contribution of DAIS within the amorphous phase than DAII. The copolymer crystals of PA1 and PA2 prove to be packed in the same way as the proper homopolymer PA 4.10 and with an enhanced content of DAIS (over 44%) a change of the packing of the macromolecules in the crystals is forced. The above-mentioned phenomena (the change of interchain distance, “pushing” the chain fragments containing DAIS out of the crystalline domains into the amorphous phase area) may explain the observed higher melting temperature of the polyamides obtained from exo−endo oriented isohexide diamine in comparison to the polyamides synthesized from their exo−exo analogues, since the presence of high amounts of isohexide residues in the crystals destabilizes these and causes a lowering of the melting point. Furthermore, together with an increasing DAIS content in the copolymers the location of the 001 reflection is shifted toward lower 2Θ values. This shift reflects an increase of the caxis dimension of the analyzed copolymers resulting from a higher content of DAIS, which in turn is caused by the cocrystallization of the comonomers in the same crystallographic lattice. This proves that, although significant amounts of DAIS residues may be pushed out of the crystals, still measurable amounts of DAIS/SA repeat units are able to cocrystallize with the DAB/SA repeat units. In case of the homopolymer obtained from DAB and BrA (PA 4.13, entry 1 in Table 6), one can observe three reflections Table 6. X-ray Diffraction Spacings of 1,4-Diaminobutane-, Diaminoisosorbide-, and Brassylic Acid-Based Polyamides 2Θ [deg] PA5 PA6 PA7

100*

010/110*

001

100*

010/110*

4.61 4.01, 4.55 3.96

20.83 21.19

23.30 23.18

1.917 2.203, 1.942

0.426 0.419

0.381 0.383

2.231

0.416

21.33

■

X-ray diffraction [nm]

001

CONCLUSIONS The investigation of 2,5-diamino-2,5-dideoxy-1,4;3,6-dianhydrosorbitol-derived homo- and copolyamides revealed that the bicyclic diamine incorporated into the backbone of PA 4.10 and PA 4.13 yielded copolyamides with close to expected DAIS monomer residue contents and preserved configuration. The comparison of the exo−endo and exo−exo oriented isohexide diamines illustrates that they both affect the crystal phase of these semicrystalline materials and thus reduce their melting point. Nevertheless, because of the steric hindrance and lower degree of symmetry, the exo−endo diastereoisomer is incorporated to a lower extent than its exo−exo isomer (see also further) and therefore slightly less influences the crystal

in the 2Θ ranges 3°−7° and 18°−25°. The literature does not provide the information about the crystallographic structure of this polyamide. However, because of the similarities in the chemical structure of PA 4.10 and PA 4.13, certain information about the crystallographic structure of such homopolymer may be obtained based on the available information concerning PA 4.10. The basic unit of PA 4.13, in comparison with PA 4.10, L


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domains in comparison to similar incorporated amounts of diaminoisoidide, as evidenced by WAXD and DSC measurements. VT 13C{1H} CP/MAS NMR and FT-IR spectroscopy, used for the analysis of the dynamic of the polymer chain fragments as well as the distribution of DAIS residues over the crystalline and amorphous domains, revealed that the motion of the macromolecules upon heating first occurs in the methylene units while the hydrogen-bonded amide motifs undergo rotation at higher temperature. It was also found that the incorporated DAII residues more readily facilitate the trans− gauche conformation changes of the macromolecules than DAIS residues. VT 13C{1H} CP/MAS NMR spectra supported by ab initio calculations show that all observed experimental resonances in the region of 50−90 ppm are mainly related to three most abundant conformers of DAIS units incorporated into the backbone of the homo- and copolyamides, contributing to the total population by 62%, 35%, and 3%, according to the Boltzmann distribution in the gas phase. Wide-angle X-ray scattering proved that an increasing content of DAIS in the copolyamides resulted in a change of the packing of the macromolecules in the crystals, leading to materials crystallized into a close to hexagonal form. As observed from WAXD diffractograms, the change of the way of packing of the macromolecules in the crystals of SA-, DAB-, and DAIS-based copolymers is already observed at a DAIS content between 44 and 50 mol %, while for the DAII-based analogues these changes were noted at a DAII content 20−43 mol %. This suggests that DAIS more readily contributes to the formation of the amorphous phase in comparison to DAII, and it seems that DAIS units are pushed out from the PA 4.10 crystals to a higher extent than the better fitting DAII residues. A similar phenomenon was observed for BrA-, DAB-, and DAIS-based copolyamides where the changes in their crystal lattice were observed for dideoxy−diamino isohexide contents in the polymer backbone higher than 34 mol %.

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ASSOCIATED CONTENT

S Supporting Information *

Liquid and solid-state NMR spectra and conformational analysis. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (L.J.-W.), [email protected] (M.R.H.); Ph: +31-40-2472527; Fax: +31-40-2463966. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge Prof. J. Namiesnik and Prof. H. W. Spiess for help and fruitful discussions. This work forms part of the research program of Dutch Polymer Institute (DPI, project # 656 and #685). The financial support from DPI is gratefully acknowledged.

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REFERENCES

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Structure and Molecular Dynamics in Renewable Polyamides from

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