Article pubs.acs.org/Macromolecules
Processing of Polyamides in the Presence of Water via Hydrophobic Hydration and Ionic Interactions Jules A. W. Harings,†,‡,§ Yogesh S. Deshmukh,‡,§ Michael Ryan Hansen,⊥ Robert Graf,⊥,* and Sanjay Rastogi‡,§,⊥,∥,* †
Polymer Technology Group Eindhoven BV, P.O. Box 6284, 5600HG Eindhoven, The Netherlands Laboratory of Polymer Technology, Department of Chemical Engineering, Eindhoven University of Technology, P.O. Box 513, 5600MB Eindhoven, The Netherlands § Dutch Polymer Institute (DPI), P.O. Box 902, 5600AX Eindhoven, The Netherlands ⊥ Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany ∥ Department of Materials, Loughborough University, Loughborough, LE11 3TU, U.K. ‡
ABSTRACT: In synthetic as well as natural polyamides, hydrogen bonding and conformations of amide motifs are strongly influenced by the presence of ions and their concentration, water molecules, and their structure, as well as the pH of the solution. This concept combined with solubility of synthetic aliphatic polyamides, in particular nylons, in water at elevated temperature and corresponding vapor pressure is evaluated as a new reversible shielding route in the processing of these polymers. So far, reversible shielding has not been feasible due to a lack in controlling desired activation and deactivation of hydrogen bonding at the judicious moments. Here we show that in the presence of large halogen anions, crystallization from the random coil state is suppressed by hydrophobic hydration, where the amorphous state of the fast crystallizing nylons can be maintained even at 20 °C. Small hydrating lithium cations are favored since they strengthen the hydrophobic nature of the anions. Complete deshielding of hydrogen bonding, after processing, is facilitated by simple migration of ions in water that allows recovery of the desired conformation and structure.
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INTRODUCTION Polymers, either being of synthetic or natural origin, are long chain molecules comprising continuously repeating monomers. The properties are influenced by the primary chemical structure, secondary interactions and conformation of the molecules in a specific environment. Especially in nature, a delicate balance between primary chemical structure and intraand intermolecular interactions is essential for the function of biopolymers, for example in a range of proteins and peptides. In these biopolymers unique conformations and structures are controlled by the primary sequence of amino acids, and secondary interactions between them. In such self-organizing processes temperature, pH, ionic, and polarity-induced interactions influence specific moieties, for example hydrogen bonding motifs, and trigger the formation of myriads of mesoand macroscopic architectures. Hydrogen bonding is a noncovalent interaction specifically involving hydrogen atoms. In the classical definition of the Coulomb interaction, hydrogen bonding is established between a donor and an acceptor.1 Under specific conditions, when a proton is unshielded it becomes a donor and an electronegative atom such as O, N, F having free electron pairs act as an acceptor. This results in the accumulation of electron density © 2012 American Chemical Society
between the donor and the acceptor. The presence of lone pairs of electrons (or polarizable π electrons) enables the partially unshielded protons (the electron deficient atom) to fulfill their continuous quest for electrons.2 Further studies suggest hydrogen bonding to be a quantum mechanical effect.2 The presence of hydrogen bonding as a secondary interaction, and its implications in crystallographic packing, is evident in natural and synthetic polymers. In extreme cases hydrogen bonding between perfectly aligned extended polymer chains, as for example in poly(p-phenylene terephthalamide) fibers (Kevlar and Twaron), induce ultimate properties and performance. Less sophisticated are commodity-engineering plastics, e.g., aliphatic polyamide (PA), in which hydrogen bonding between amide moieties provides dimensional stability and excellent mechanical properties. Although hydrogen bonded polymers are desired for demanding applications, processing faces challenges in terms of high temperatures and harsh solvents. Remarkably silk peptides, which in essence are decorated polyamides, are Received: March 20, 2012 Revised: May 30, 2012 Published: June 29, 2012 5789
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processed from aqueous solutions at ambient temperature and pressure. In the spinneret of spiders hydrogen bonded moieties and peptide conformations are mediated by water molecules, ions and pH to tailor the desired functional structures.3 Generally, during evolution water and ions have adopted a vital role in self-assembly processes, such as protein folding, protein−protein recognition and protein−DNA binding by controlling van der Waals interactions, hydrogen bonding, electrostatic and hydrophobic interactions.4 These phenomena are also exploited for synthetic polymers, especially polyamides. Here, some of the findings reported in literature are recalled. It has been conclusively shown by Kim and Harget that crystallization of PA6 can be suppressed in the presence of inorganic salts and formic acid as common solvent, where the metal ions were found to coordinate with the amide motifs.5 The binding of ions to the main chain increased the chain stiffness, so did the glass transition temperature. Murthy and co-workers showed that iodide ions intercalate between the hydrogen-bonding planes of PA6, influencing the crystal packing.6−8 The authors showed that the intercalated ions form a regular structure influencing the transportation of charges in lithium iodide batteries.8 These findings were further supported by Kawaguchi and co-workers, who demonstrated the possible orientation of an ionic array (polyiodide) between the hydrogen-bonding planes in the desired directions.9 For such studies these authors have often made use of common organic solvents for polymer and salts, such as formic acid. Recently it was demonstrated that water can also act as a solvent for polyamides, where the temperature chosen for dissolution may range between 200 and 240 °C.10,11 To recall, by making judicious choice of dissolution time and temperature it has been possible to inhibit hydrolysis of polyamide during dissolution, and single crystals of polyamide can be obtained from aqueous polyamide solution.11,12 Considering water to be environmentally favorable, and a common solvent for polyamides and salts, this work describes the use of water as a medium for homogeneous dispersion of ions in a polymer matrix. Following this route we show that hydrogen bonding can be fully suppressed and amorphous polyamide can be obtained at room temperature by appropriate choice of amidesalt concentration. The amorphous polyamide can be drawn below the melting temperature and hydrogen bonding can be restored by removal of ions at a temperature just above the glass-transition temperature. To study the mechanism of shielding and deshielding of interchain hydrogen bonding in PA46, PA6, and PA66 by Fourier transfer infrared (FTIR) spectroscopy, solid-state nuclear magnetic resonance (NMR) spectroscopy and time-resolved wide-angle X-ray diffraction (WAXD), “partially and fully” solubilized samples were prepared using an in-house designed pressure cell11 following the temperature profiles of the DSC experiments. As witnessed by the DSC experiments, dependent on the ionic strength the PA samples either crystallized or remained solubilized. Regardless the physical state of the samples (crystallized or solubilized), rinsing with excess of water induces instant crystallization of the polyamides. In broad generality, the process of shielding and deshielding of polyamides in the presence of ions is similar to the spinning of natural spider silk fibers, where ions and pH changes influence the conformation and assembling of specific amino acids sequences.
Article
EXPERIMENTAL SECTION
Materials. On the basis of the Hofmeister series, a classification that describes to which extend proteins are solubilized or precipitated by the presence of ions in an aqueous environment, solubilizing salts were selected comprising kosmotropic cations like Li+, Na+, and Ca2+ and chaotropic anions such as Cl−, Br−, and I−. By definition kosmotropic ions order water molecules whereas chaotropic ions reduce the structuring of water. Stock solutions that cover the nine possible combinations of ions were prepared with ionic strengths ranging between 0 and 10 mol/L. Extruded Filaments. To generate monofilament extrudates, a premix of 84% w/w PA46 and 16% w/w 9 M LiI was fed to a Haake Rheomex OS PTW 16 corotating twin-screw extruder. Feeding was torque controlled at 80% of the maximum, 130 N m, at a screw speed of 100 rpm. The temperature profile ranged from 325 °C in the first zone to 240 °C in the last zone, resulting in an extrudate temperature of 240 °C at the die. It should be stressed that dissolution in the superheated state of water and the presence of Li+ and I− ions facilitate extrusion at temperatures well below conventional PA46 processing temperatures. The transparent extrudate was collected carefully to minimize preorientation induced by the extrusion process. As a reference polyamide 46 (PA46) extrudates were prepared identically resulting in semicrystalline samples (PA46sc). However, the temperature zones were set from 325 °C in the first zone to 300 °C in the last zone, resulting in an extrudate temperature of 310 °C at the die. Samples with 9 M LiI (PA46LiI) and the melt route (PA46sc) were drawn successively by hand in air and water at 20, 55, and 95 °C. Differential Scanning Calorimetry (DSC). (DSC) was used to study the dissolution and precipitation of commercial polyamide (PA) 46 (Stanyl, DSM), 6 (Ultramid A, BASF) and 66 (Ultramid B, BASF)13 in superheated water, whether or not in the presence of ions. Perkin-Elmer high volume DSC pans that sustain the desired pressures were used. The polymeric samples (23−38 wt %) were immersed in the aqueous solutions and exposed to a temperature program ranging between 30 and 240 °C at a rate of 10 °C/min in nitrogen atmosphere. To ensure an equilibrium state, an isothermal period of 3 min was applied at the temperature limits. The dissolution and crystallization temperature were determined by the peak maxima and minima of the DSC thermograms, respectively. Fourier Transform Infrared Spectroscopy (FTIR). FTIR spectra were recorded on a Bio-Rad FTS6000 spectrometer in attenuated total reflection (ATR) mode using a silicon crystal. The average of 100 spectra was recorded in a range of 4000 to 650 cm−1 and at a resolution of 2 cm−1. Solid-State Nuclear Magnetic Resonance (NMR). Solid-state 13 C magic-angle spinning (MAS) and 13C{1H} CP/MAS NMR experiments were performed on a Bruker Avance I spectrometer operating at a 700.28 MHz 1H Larmor frequency. The samples were packed in 2.5 mm outer diameter zirconia rotors, spun at a MAS frequency of 25.0 kHz using a 3.0 ms CP contact pulse for nonshielded samples and direct 13C excitation for the shielded samples. Both types of experiments used the SPINAL-64 scheme for high power 1H decoupling during acquisition of the free-induction decay. The 1D 7Li MAS NMR experiments of LiI and LiBr salts and PA46 shielded with 9 M LiI were recorded on a Bruker DSX spectrometer (νLarmor(7Li) = 194.36 MHz and νLarmor(1H) = 500.11 MHz) using high power 1H TPPM decoupling during acquisition. The 2D solid state 7Li{1H} CP/MAS NMR experiment was recorded on a Bruker Avance III spectrometer (νLarmor(7Li) = 330.44 MHz and νLarmor(1H) = 850.27 MHz) using a 1.3 mm MAS probe spinning at 50.0 kHz with low-power CP and low-power decoupling during acquisition.14,15 The 13C{1H} CP/MAS and 1H MAS NMR experiments of the filament samples were performed on a Bruker Avance III spectrometer (νLarmor(13C) = 213.80 MHz and νLarmor(1H) = 850.27 MHz) with a 2.5 mm double resonance probe at a MAS spinning frequency of 15 kHz or 30 kHz, respectively. The 13C{1H} CP/MAS experiment used high power 1H decoupling (SPINAL-64) during acquisition. The 1D 7 Li MAS NMR spectra of the PA46 filament samples have been recorded on a Bruker DSX spectrometer (νLarmor(7Li) = 194.36 MHz 5790
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and νLarmor(1H) = 500.11 MHz) at 15 kHz MAS using high power 1H TPPM decoupling during acquisition. Wide Angle X-ray Diffraction (WAXD). Migration of ions from the oriented samples was evaluated at the high resolution Materials Science Beamline ID11 located at the European Synchrotron Radiation Facility (ESRF), Grenoble, France. A Frelon 4 M CCD camera was used to collect two-dimensional diffraction patterns acquired using a 29.8 keV (λ = 0.417 nm) X-ray beam of 50 × 200 μm2 size and 10 s exposure time. The drawn PA46-sc and PA46-LiI filaments were placed in glass capillaries and immersed in distilled water. The filled capillaries were then placed in the in-house design pressure cell11 and exposed to a temperature cycle ranging between 30 and 150 °C at a rate of 5 °C/min, using a Linkam TMS 94 temperature controller. As a calibration standard lanthanum hexaboride was used. The diffraction patterns were subtracted for background scattering and detector response. Azimuthal integration over the arc range of individual diffraction arcs lead to the intensity against the scattering vector q. The relation d = 2π/q was used to convert the scattering angle into d-spacing. Dynamic Mechanical Thermal Analyses (DMTA). PA46 extrudates obtained from the melt and LiI route were measured at on a TA DMA Q800 with a tension setup. A temperature sweep from −50 to +350 °C was applied with a heating rate of 3 °C/min at a frequency of 1 Hz. A preload force of 0.01 N, amplitude of 10 μm and a force track of 110% were used. Tensile Testing. The drawing behavior of PA46-sc and PA46-LiI extrudates, being less than 0.5 mm diameter, was monitored at 5 mm/ min using a ZwickZ100 tensile tester equipped with a 100 N load cell and 0.1 N preload. As the initially transparent PA46-LiI filaments became white on tensile deformation, suggesting strain induced crystallization or microcrazing, the influence of drawing was evaluated by a second identical tensile test, immediately after breakage in the first tensile test.
Figure 1. Suppression of the dissolution temperature of PA46 in LiI and LiBr solutions (closed and open symbols respectively) at various ionic strengths; (●) 23, (⧫) 29, (▲) 33, and (■) 38% w/w PA46. At identical ionic strengths the dissolution temperature is suppressed more efficiently by the presence of I− ions compared to Br− ions.
concentration of lithium salts the dissolution temperature of PA46 in water decreases significantly. Besides, at equal molarity or ionic strength, which is identical due to the monovalence of the respective ions, iodide suppresses the dissolution temperature more effectively than bromide. A possible explanation for this difference is that iodide is the largest chaotropic anion of the halogens, resulting in the lowest ion−H2O hydrogen bonding energy, −10.3 ± 0.3 kcal/mol, compared to −11.7 ± 0.4 and −14.7 ± 0.6 for Br−−H2O and Cl−−H2O bond energies, respectively.20 As large chaotropic anions are nonpolar, water molecules form a surrounding cage, like a solvation shell to maintain maximum H2O−H2O hydrogen bonding that in fact is stronger than I−−H2O hydrogen bonding.21 Above a critical ionic radius of the chaotropic ion, maximum hydrogen bonding efficiency in the first shell is perturbed by geometrical limitations as depicted in Figure 2. The first solvation shell of iodide exhibits only 2.5 hydrogen bonds between water molecules on average, while bulk water possesses 3.4 water−water hydrogen bonds.22 Since in the presence of chaotropic ions not all hydrogen-bonded vacancies in water can be used optimally, the overall hydrogen bonding efficiency in water diminishes, especially in solutions with high ionic strength. These features, depicted in Figure 2 at room temperature, are monitored by changes in the proton isotropic chemical shift, δiso, for H2O using 1H NMR spectroscopy. For the same ionic strength in the aqueous solution, the figure shows a decrease in the chemical shift for water with increasing size of the cations (Li+, Na+) and anions (Br−, I−). A decrease in chemical shift value for water is also observed with an increase in the ionic strength of the aqueous solution. The top illustration in Figure 2 shows that small anions do not perturb the water−water hydrogen bonding, while large anions do disrupt the organization of water molecules.23 This is exemplified in the case of iodide and bromide where the atomic radius of iodine is larger than the radius of bromide and hence, iodide disrupts the organization of water molecules to a larger extent. For both of these chaotropic anions the protons of the water molecules are positioned toward the anions. On the other hand, cations of kosmotropic nature interact strongly with the water molecules. Compared to kosmotropic ions with larger
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RESULTS AND DISCUSSIONS Shielding and Mediation of Hydrogen Bonding by Hofmeister Ions. The dissolution of polyamides in water at elevated temperature and pressure (180 °C, 10 bar) depends on a delicate balance between weakening of hydrogen bonding in the crystalline component of semicrystalline polyamides, and the decrease in hydrogen bonding efficiency between water molecules. Generally upon heating polyamides, the rotational motion of thermally induced gauche conformers in the aliphatic segments is transferred to the amide motifs, weakening the hydrogen bonding considerably though the interactions prevail partially. With the induction of gauche conformers the lattice distance between the hydrogen bonded chains (interchain/ intrasheet) decreases with increasing temperature.16 Simultaneously, the expected thermal expansion in the direction of the van der Waals interactions (interchain/intersheet) occurs. The temperature where the interchain/intersheet and interchain/ intrasheet distances become equal is referred to as the Brill transition temperature.17 The presence of relatively mobile water molecules, which can perturb the weakened amide− amide hydrogen bonding, facilitates the solubilization of polyamides in water at temperatures near the Brill transition temperature. It is well-known that the hydrogen bonding between water molecules is strongly influenced in the presence of Hofmeister salts.18,19 Since the dissolution of polyamides is affected by the weakening of the hydrogen bonding between the water molecules, it is anticipated that the dissolution of the polyamides will be strongly influenced in the presence of ions. From the DSC data summarized in Figure 1, the influence of monovalent ions, LiBr and LiI, on the dissolution temperature of PA46 is evident. With the increasing 5791
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Figure 2. Influence of chaotropic anions and kosmotropic cations on the water−water hydrogen bonding explained by the so-called Mercedes Benz model of water. The model is a 2-dimensional model of water in which water molecules are represented by disks, having three radial wings representing a 2D projection of the 3D tetrahedral water structure. Taking the dipole moment of water into account gives a negatively charged center and a positively charged corona.23
radii, small kosmotropic ions having smaller radii interact more efficiently, such as lithium and sodium ions. These changes in the hydrogen bonding efficiency between the water molecules become apparent with the observed 1H chemical shift, Figure 2. The fact that the effect of LiBr on the dissolution temperature of polyamides is weaker, in comparison to iodide ions at identical ionic strength, is attributed to (a) the smaller ionic radius of Br− compared to I− and thus the solvation shell of Br− contains less water molecules, and (b) the fact that the Br−− H2O binding efficiency is more effective compared to I−−H2O. Thus, it could be concluded that I− ions perturb the hydrogen bonding of water molecules more effectively than Br− ions. Combined with the anions, the perturbation of hydrogen bonding in water becomes more pronounced with the decrease in binding efficiency of water molecules with increasing size of cations, for example Li+ to Na+. Therefore, larger chemical shift, δiso, is observed in the presence of NaI compared to LiI, Figure 2. In summary, the chemical shift of water protons can be directly correlated to the hydrogen bonding efficiency among water molecules. Considering that the chemical shift for water decreases with increasing salt concentration a greater perturbation in the hydrogen bonding of the water molecules is anticipated, which ultimately leads to a disappearance of “bulk” water since the water molecules primarily resides in ionic shells. From the above, it is clear that the dissolution temperature of PA46 will be strongly affected by the chosen salt. Figure 3 shows the influence of different salts, LiI and LiBr, in the dissolution of polyamides PA46, PA66, and PA6. It is apparent that compared to LiI, LiBr solutions demand more thermal energy to pursue the dissolution of PA46. The fact that the enthalpy of dissolution for the same crystallinity of polyamides, as observed using DSC, also decreases with increasing ionic strength supports the change in solvent characteristics from bulk to the superheated or expanded state of water in the presence of ions
Figure 3. (a) Suppression of the dissolution (closed symbols) and crystallization temperature (open symbols) of (⧫) PA46, (▲) PA66, and (■) PA6 in aqueous LiI solution at a polymer concentration of 29 w/w%. The same effects are presented for PA46 in aqueous LiBr solution as well (●). The amount of ions per amide moiety, expressed as the molar salt:amide ratio, to solubilize the polyamides in the respective solutions is shown in part b. The exponential trends distinguish two regimes at 30 °C, where either sufficient or insufficient ions are present to suppress the crystallization completely.
Upon cooling PA46 in the presence of ionic solutions, the crystallization temperatures and corresponding enthalpies are identically influenced by presence of ions, Figure 3a. However, dependent on the polymer concentration, crystallization of the polyamides at 30 °C is completely suppressed at relatively high ionic strength. The molar ratios, salt to amide, required to suppress crystallization at 30 °C as a function of polymer concentration, follow exponential relationships presented in Figure 3b. In comparison to PA46 and PA66, PA6 remains solubilized at relatively low ionic strengths. The exponential relationships in Figure 3b define two regimes: regime I above the curves, where sufficient ions are present to completely suppress crystallization, and regime II below the curves where the amount of ions is insufficient to suppress crystallization, though the crystallization temperature is lowered considerably. Moreover, it is to be noted that gel permeation chromatography (GPC) and FTIR measurements, as discussed later, show no significant hydrolysis within the experimental time scale. 5792
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Figure 4. Shielding and deshielding of hydrogen bonding in PA46 in the presence of Li+ and I− ions observed from (a) FTIR and (b) 13C and 13 C{1H} CP/MAS NMR. The assignment of IR bands and NMR resonances follow the color code given on top for a single PA46 repeat unit. The 13 C signals in the vicinity of 175 and 40 ppm are assigned to the carbonyl and αNCH2 (green) moieties, respectively. The sharp signals in the regions, 38 to 45 ppm and 170 to 180 ppm, arise due to the presence of mobile αNCH2 and carbonyl motifs at the fold surface of the solution crystallized PA46.11
PA46 without ions, and PA46 crystallized from regime II show a strong resemblance, see Figure 4a. The presence of the CH2 scissoring band at 1416 cm−1 is indicative of the crystalline state for PA46. In the solubilized state (9 M LiI, regime I), characterized by an overall low resolution, the gauche CH2 scissoring band at 1442 cm−1 is observed. The presence of the gauche scissoring band and the absence of the crystalline CH2 scissoring band confirm the solubilized state of the polymer. The unresolved NH stretch around 3300 cm−1 indicates dissolution of the polymer in a 9 M LiI solution. This is further supported by the shift in amide I (CO stretch) and amide II (C−N stretch and C−N−H in-plane bending) vibrations. These findings suggest perturbation of the amide−amide hydrogen bonding. The absence of a carboxylic carbonyl vibration in the range from 1800 to 1700 cm−1 reveals that no hydrolysis occurred within the experimental time scale. Changes in the local conformation and conformational dynamics of the polymer chains due to the shielding of hydrogen bonding can be probed by 13C and 13C{1H} CP/ MAS NMR experiments as illustrated in Figure 4b. In solidstate NMR, polymers show remarkably broad 13C NMR signals, which do not originate from experimental limitations, but result from the broad distribution of possible dihedral angles with slightly different electronic shielding for chemically equivalent positions along the polymer chain due to the given conformation of the macromolecule. Thus, 13C chemical shift and NMR line width are good indicators for the local organization or conformation of the polymer. In solution or highly mobile polymer systems, the different dihedral angles as well as the resulting 13C chemical shift distributions are averaged by rapid conformational exchange, which leads in the fast motional limit to very sharp 13C NMR lines known from high-resolution 13C NMR in solution state.24,25 In order to illustrate the potential to identify preferred conformations in
Because of the extremely high solubility of lithium salts in water, though sodium and calcium salts show similar results, Li+ ions were chosen as kosmotropic cation to reveal the role of the chaotropic anions. The anions, halogenic in nature, were tested with increasing atomic radii. The combination of Li+ ions with Br− ions showed the least effect on the dissolution and (re)crystallization temperature. Among the three polyamides, PA46, 6, and 66, suppression of crystallization in LiBr solutions was only observed for PA46. All three polyamides were dissolved in LiI solutions and crystallization was suppressed. Besides the fact that lithium ions are expected to exchange electrons with the electronegative carbonyl, based on their highly electron deficient character, the above findings suggest a correlation between the atomic radii of the chaotropic anions and the length of the nonpolar methylene segments of the polyamides. The extent to which the hydrogen bonding and chain conformations are mediated in both regimes, introduced in Figure 3b, is investigated by FTIR and 13C{1H} CP/MAS NMR spectroscopy, Figure 4, parts a and b. Spectra representative of regime I are obtained from a polyamide sample prepared using 9 M LiI solution, while spectra for regime II are obtained when the sample is made using a 3 M LiI solution. Spectra of water crystallized PA46 serve as reference. Labeling of the peak assignment is given in color. The following section addresses implications on hydrogen bonding and chain conformation. Figure 3 also shows respective changes on the reestablishment of hydrogen bonding after removal of ions, (washing), stressing the unique aspects of the presented concept for controlled shielding and (de)shielding of hydrogen bonding. In regime II (Figure 3b) the crystallization temperature of PA46 is significantly suppressed due to ionic interactions with the PA46 polymer chains. FTIR spectra of the water crystallized 5793
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polymers based on the 13 C chemical shift, synthetic polypeptides provide a prominent example, where the 13C chemical shift of the carbonyl and the Cα position are used to probe the presence of structural elements like α-helices or βsheets in the conformation.26−28 In the 13C NMR spectra of the studied PA46 sample (see Figure 4b) the signals assigned to the carbonyl position around 175 ppm (yellow color label) and that of the αNCH2 position at 42 ppm (green color label) are most sensitive to the presence of hydrogen bonding and the local conformation. The carbonyl signal at 173.5 ppm and αNCH2 signal at 42.8 ppm have been reported in literature for the crystalline γ form of different nylon samples, while the sharp signals at 176.6 ppm for the carbonyl sites and at 39.7 ppm for the αNCH2 position are characteristic for highly mobile polyamide chains in random coil conformation as also observed in solution NMR.29,30 The spectrum of the water crystallized PA46 sample includes signals from both crystalline and mobile random coil contributions for the αNCH2 position reflecting the semicrystalline nature of the material, where the mobile amorphous contributions most likely result from chain folds at the surface of the crystallites.11 The fact that a corresponding signal for the carbonyl sites is missing results from the CP/ MAS method used to record this spectrum, which suppresses NMR signals of mobile 13C sites without directly bonded protons. In the case of the 3 M LiI PA46 sample (regime II), the broad signals from the crystalline regions are strongly suppressed, and sharp 13C NMR signals on top of weak broad features from remaining yet undissolved crystalline regions are observed. Since this spectrum has been recorded with direct excitation due to the failure of the CP/MAS method in very mobile gel-like systems, a sharp signal at ∼176 ppm is observed from mobile carbonyl sites in a random coil conformation. At higher salt concentrations, like PA46 dissolved a 9 M LiI solution in Figure 4b, the material is completely dissolved and the broad features underneath the sharp signals in the 13C NMR spectrum vanish. After washing the PA46 samples with water in order to reduce the ion concentration in the samples and to, recrystallize the sample from salt solutions, the initial 13 C NMR spectrum of water crystallized PA46 is re-established. This demonstrates the effective shielding of hydrogen bonding at higher ionic concentrations. To reveal whether shielding arises due to interactions of electron deficient lithium ions and electron rich carbonyl moiety, which one might anticipate at first, the local environment of Li+ ions is studied by 7Li MAS NMR experiments, Figure 5. Such experiments rely on the fact that the 7Li chemical shift (δiso) reflects local coordination environments and is sensitive to diffusion and molecular dynamics.31 In Figure 5, 7Li MAS NMR spectra of LiBr and LiI salts are compared to the spectrum of a solubilized PA46 sample from a 9 M LiI solution. These spectra clearly show that Li+ ions in the solubilized state do not exist as the original salt but rather as free ions in a PA46 structure. Furthermore, the location of 7Li chemical shift at ∼0.0 ppm is characteristic of mobile Li+ ions. The mobility can occur as diffusion between the PA46 layers and/or fast discrete jumps in between two PA46 hydrogen bonded segments as sketched in Figure 5c. The proximity of Li+ ions and PA46 can be observed in 2D 7Li{1H} HETCOR experiments, Figure 5d. In this kind of experiment 1 H−7Li dipole−dipole couplings, resulting from close spatial proximity between different 1H bearing groups of PA46 and free 7Li ions, are used to correlate different species by the CP technique.32,33 Clearly, this spectrum confirms the presence of
Figure 5. Solid state 7Li MAS NMR spectra of (a) LiBr, (b) LiI and (c) Li+ ions incorporated in the localized region of PA46 (superheated with 9 M LiI). These spectra show the δiso difference between 7Li located in a well-defined ionic lattice and in PA46 where free diffusion is favored. The asterisks mark impurities. (d) 2D 7Li{1H} HETCOR spectrum illustrating the close proximity between Li+ ions and the aliphatic and amide motifs in PA46. For comparison a single-pulse 1H MAS NMR spectrum of the sample is included (dashed spectrum).
Li+ ions in the direct vicinity of PA46 polymer chains, though Li+ ions do not solely reside at the amide moieties but also close to aliphatic segments as the correlations peaks reveal. Despite that the location of lithium ions has been identified correlation of the anionic radius and the aliphatic segment length remains ambiguous. Upon cooling, H2O−H2O hydrogen bonding becomes more prominent than the van der Waals interactions between the water molecules.34 To minimize the penalty for accommodating large nonpolar species such as anions, as discussed above, water enforces clustering of nonpolar components to decrease the exposed surface area. The higher the penalty in terms of intermolecular water−water hydrogen bonding, the more pronounced is the clustering of the nonpolar components especially at lower temperatures. This hydrophobic hydration seems to be correlated with the length of the aliphatic polyamide segments as stated earlier. For good solubility a small cation is preferred. Small cations keep their hydration shell of water molecules, while larger cations interact weaker with water (Figure 2) and allows for more interactions of both cations and water with iodide, thereby weakening the partially hydrophobic nature of iodide. We are currently investigating the aspects discussed above using both experimental and theoretical methods to give a more detailed description of the water−ion and the water−ion-polyamide 5794
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amorphous component.35 At large strain deformation strain hardening is observed in both samples. Since the PA46 samples are of relatively low molecular weight, ∼30 kg/mol, the number of entanglements is relatively small, thus the strain hardening cannot be simply explained by trapped chain entanglements in the amorphous region acting as physical constrains/crosslinks.36 Instead, strain induced crystallization is likely to result in tie molecules bridging the crystallites, effectively contributing to the network. In the second tensile experiment performed on the samples drawn in the first tensile experiment, the effect of strain-induced crystallization is clearly demonstrated. Not only does the elastic modulus of the PA46-sc and PA46-LiI samples increase significantly, from 1.30 and 0.45 MPa to 2.60 and 1.10 GPa respectively, but also the maximum stress increases drastically. Typical WAXD patterns representing the evolved structures after tensile deformation are embedded in Figure 6. A detailed assignment of the diffraction signals will be discussed in a later section, addressing the structural development upon drawing in different environments. Next to the hydrogen bonding efficiency and orientation, chain alignment along the draw direction, crystal stability and chain packing are crucial parameters to achieve ultimate properties in fibers. Figure 7 shows WAXD patterns of the two semicrystalline polymers, PA46-sc (without ions) and amorphous PA46-LiI (with Li and I ions), drawn in air at 55 °C. The diffraction signals 200 and 020 are assigned to hydrogen and nonhydrogen bonding planes.37 Chain orientation can be assessed by the azimuthal distribution of the equatorial diffraction peaks, 200 and 020, in the WAXD patterns. These diffraction peaks correspond to the interchain/intrasheet (hydrogen bonding planes) and intersheet (non-hydrogen bonding planes) spacings, respectively. Figure 7 shows the monoclinic unit cell of PA46. Generally it is observed that with increasing draw ratio interchain and intersheet distances shift to values close to those observed for single crystals of polyamide. In the presence of ions, for example drawing in air (left top quarter, Figure 7) independent of the temperature, only small changes in the interchain and the intersheet distances are observed. On the contrary, when the drawing is performed in the presence of water, above the glass transition temperature, the interchain and intersheet distances adopt spacings equivalent to single crystals.37 The presence of ions in the lattice and/or at the crystal surface decreases the hydrogen bonding efficiency. On the removal of ions, the amorphous halo in the PA46-LiI samples shifts to higher d values and tends to vanish with an increase in crystallinity. It is known that the incomplete removal of ions influences the plasticization of crystals.38,39 Strain-induced crystallization in the presence of water does not result in the complete removal of ions either. The ions influence the crystallinity as well as the crystal perfectioning, where the latter has implications on the melting temperature. To achieve maximum removal of ions the concept of superheated water, earlier used for the dissolution of polyamides, can be applied. For this purpose, the drawn PA46-LiI sample is immersed in superheated water at 150 °C (below the Brill transition temperature of PA46) for 5, 10, and 15 min. WAXD patterns, DSC curves and solid-state NMR spectra before and after the superheated water treatment are compared. Structural changes of the polymer before and after immersion are clearly observed in the WAXD patterns and NMR spectra, Figures 7a and 8, respectively. The strong intensity of the isotropic amorphous halo in the PA46-LiI sample, prior to its treatment in water, is
interactions. These results will be presented in subsequent publications. Strain-Induced Crystallization of PA46 from Aqueous Lithium Iodide Solution. To make samples in larger quantities twin-screw extrusion was used where the dissolution was achieved in the presence of water and ions. Extrusion of PA46 in 9 M LiI solution (15%w/w) resulted in amorphous extrudates, possessing a glass transition temperature in the proximity of 20 °C as depicted by the peak maxima of the loss modulus in the DMTA traces, Figure 6a. Strain-induced crystallization obtained on solid-state deformation of the amorphous extrudate led to oriented chains.
Figure 6. (a) Loss and storage modulus of the PA46sc (black) and PA46LiI sample (gray) as a function of temperature, indicating that the glass transition temperature (peak maxima of the loss modulus) of both samples is approximately 20 °C. (b) Stress−strain curves of the same samples in two successive tensile tests performed just above Tg, where σy is the yield stress. Strain-induced crystallization, responsible for the strain hardening, entails crystalline structures of which the wide-angle X-ray diffraction patterns are presented. Also in this picture the black and gray curves represent the PA46sc and PA46LiI sample, respectively.
The drawability of the semicrystalline and amorphous PA46 samples was monitored upon tensile deformation. Figure 6b shows the engineering stress−strain curves of two successive tensile tests. The low yield stress of the PA46-LiI sample, 20 MPa, arises due to the absence of crystalline domains that are known to contribute more to the yield stress than the 5795
dx.doi.org/10.1021/ma300459q | Macromolecules 2012, 45, 5789−5797
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the samples are treated with superheated water. The dramatic drop in the intensity after the treatment of the sample for only 5 min suggests successful removal of ions. Since the WAXD results above suggests efficient ion migration, resulting in an increase in crystallinity with the dramatic decrease in intensity of the amorphous halo, solidstate 1H and 7Li MAS NMR and 13C{1H} CP/MAS NMR spectroscopy were performed (Figure 8). Prior to the treatment with superheated water the filament sample with ions drawn in water shows only a minor reduction of the pronounced Li signal in the 7Li MAS NMR spectrum, Figure 8c second row. When the same sample is subjected to the superheated water a dramatic intensity drop of the Li signal is observed, which confirms the successful removal of Li ions from the PA46 sample. Comparing the 7Li MAS NMR signal intensities, we can estimate that 97% of the Li ions present in the extruded PA46 filament could be removed with the treatment of superheated water. With the removal of ions the sample crystallizes and hydrogen bonding is restored. This becomes further apparent from the 1H and 13C MAS NMR spectra. For example, sharp well resolved peaks in the 1H spectra have strong resemblance with the earlier reported findings on the growth of single crystals from water.11 To recall, a broad and asymmetric signal around 8 ppm, indicates amide protons in two different chemical environment (blue color), originating from non-hydrogen bonded (7.6 ppm) and hydrogen-bonded protons (8.1 ppm). A sharp and well-resolved peak at 4.5 ppm indicates the presence of mobile water that mostly resides outside the crystalline phase of the polyamide. Moreover, the chemical shifts and well-resolved signals of the different aliphatic protons, suggests a regular packing of PA46 chains or an increased molecular mobility. With the reduction of the lithium ions and increasing water content of the extruded filament sample the carbonyl signal from amorphous regions, observed as a shoulder at higher chemical shift values (176.5 ppm, see Figure 8, third row), and the amorphous αN methylene signal at 39.6 ppm disappear. These findings indicate an increasing molecular mobility in the noncrystalline region of the washed sample, where the remaining ions and the water molecules are preferentially located and still screening the hydrogen bonding interactions, which are restored only in the crystalline regions of the sample.
Figure 7. Wide angle X-ray diffraction patterns of PA46-LiI (top pictures) and PA46-sc (bottom pictures) at 55 °C recorded before, left, and after 5 min immersion in superheated water at 150 °C, right. Arrows in part a indicate the drawing direction, while part b illustrates the unit cell of PA46 crystal having dimensions: a = 0.960 nm, b = 0.826 nm, c = 1.47 nm, α = β = 90°, and γ = 115°.
assigned to scattering arising from the electron rich iodide ions. The amorphous halo diminishes in intensity considerably once
Figure 8. Solid-state (a) 13C{1H} CP/MAS, (b) 1H MAS, and (c) 7Li MAS NMR spectra of PA46-LiI filaments as extruded, drawn in water at 55 °C, and after immersion in superheated water for 5 min. While only slight changes are observed after drawing in water at 55 °C, the immersion in superheated water entails the diffusion of the Li ions, as depicted by the loss in intensity of the 7Li MAS NMR signal in part c, and the restoration of crystalline hydrogen bonding as witnessed by the disappearance of the shielded/amorphous in part a for the carbonyl and αNCH2 signal that exist as shoulders at 176.5 and 39.6 ppm, respectively. 5796
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CONCLUSIONS This work expands on our previous studies and further strengthens the observation that in the vicinity of the Brill transition temperature polyamides can be dissolved in the superheated state of water. The addition of large nonhydrating halogen anions, in particular Br− and I−, entails increasing perturbation of the hydrogen bonding network of water molecules with increasing ionic strength. The suppressed hydrogen bonding efficiency between the water molecules has a strong influence on the dissolution and crystallization of PA46. For example, on making the judicious choice of ion− water concentration polyamides can be dissolved at much lower temperatures and crystallization at room temperature can be suppressed, leading to amorphous polyamides. The amorphous polyamide in aqueous state arises because of shielding of the amide motifs in the polymer and interaction of the anions with the aliphatic segments. The DSC, WAXD, solid-state NMR, and FTIR studies conclusively confirm the presence of amorphous PA46 that can be drawn uniaxially. In the presence of superheated water, above the glass transition temperature of the amorphous oriented nylon, the ions can be successfully removed. With the removal of the ions the hydrogen bonding is restored and the oriented structure is retained. The migration of ions combined with the restoration of hydrogen bonding has been confirmed by 1H, 13C, and 7Li MAS NMR experiments in combination with WAXD studies. A unique and versatile reversible shielding process of amide−amide hydrogen bonding in polyamide crystallites has been demonstrated, enabling structural control during processing. These studies are applicable to synthetic polyamides in general and can easily be used to develop oriented structures just above the glass transition temperature of a polymer material.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: (R.G.)
[email protected]. *E-mail: (S.R.)
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work has been performed under the framework of the Dutch Polymer Institute (DPI, Project Nos. 603 and 685). The authors gratefully thank DPI for financial support. Furthermore, the authors acknowledge the ESRF for beam time on the Materials Science Beamline ID11, especially Dr. Jonathan Wright for his expertise. We acknowledge Prof. Hans Wolfgang Spiess for his continuous support.
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REFERENCES
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