Article pubs.acs.org/Macromolecules
Dissolution and Crystallization of Polyamides in Superheated Water and Concentrated Ionic Solutions Yogesh S. Deshmukh,†,∥ Robert Graf,‡,∥ Michael Ryan Hansen,*,‡,∥ and Sanjay Rastogi*,§,∥ †
Department of Chemistry and Chemical Engineering, Eindhoven University of Technology, Eindhoven, The Netherlands Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany § Department of Materials, Loughborough University, Loughborough, United Kingdom ∥ Dutch Polymer Institute (DPI), P.O. Box 902, 5600AX Eindhoven, The Netherlands ‡
S Supporting Information *
ABSTRACT: The dissolution and recrystallization of Polyamide 46 (PA46) from “superheated” (i) water and (ii) concentrated ionic solutions of strong solubilizing mono- and divalent Hofmeister ions are studied, utilizing in situ highresolution magic-angle spinning (HR-MAS) nuclear magnetic resonance (NMR) supported by wide-angle X-ray diffraction (WAXD), attenuated total reflectance−Fourier transform infrared spectroscopy (ATR-FTIR), and gel permeation chromatography (GPC). The samples are sealed in glass capillaries, and employing variable-temperature 1H HR-MAS NMR spectroscopy, the dissolution process of PA46 as a function of temperature and pressure can be followed in situ. The purpose of such a study is to obtain molecular insight into the dissolution process of these hydrogen-bonded synthetic polymers in the different aqueous solutions. In pure water, at temperatures close to the dissolution of PA46, two distinct 1H resonances from water are observed. These resonances are associated with water molecules in the vicinity of PA46 and water in the bulk state. On further heating, the signal from water molecules in the vicinity of PA46 dominates. This sudden change in the environment suggests that water molecules, which have escaped the dense hydrogen-bonded network of bulk water, can diffuse into the structure of PA46, triggering dissolution of the polymer. This happens at a temperature that is more than 100 °C below the melting temperature of the polymer, notably without hydrolysis as verified by GPC performed prior to and after the dissolution experiments. On cooling, recrystallization of PA46 from aqueous solution is observed where water molecules are incorporated in the crystal structure. In the presence of salts, such as LiI and Cal2, weakening of the hydrogen-bonding network of the water molecules occurs. However, above room temperature, independent of the choice of salt, depopulation of the hydrogen bonding between the water molecules occurs, observed as a decrease in the 1H chemical shift value. The reduced hydrogen bonding in the presence of ions facilitates the dissolution of PA46 at much lower temperatures compared to pure water and ultimately results in the complete suppression of crystallization from solution even at room temperature. Depending on the valency of the cation a more mobile or frozen amorphous state of PA46 is obtained at low temperatures as verified by 13C HRMAS NMR, ATR-FTIR, and WAXD.
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INTRODUCTION
can be strongly influenced, resulting in ordering or disordering of the molecular structure.6 Ions that stabilize the secondary structure of molecules based on hydrogen bonding are called kosmotropic ions, whereas ions that perturb the molecular structure are called chaotropic ions.4 It is well accepted that the presence of ions in water influences structural reorganization of proteins.7 For example during the spinning process of silk, spiders make use of different ions for shielding of amide groups in the polypeptides to obtain an amorphous peptide solution.7 Once the amorphous peptide is spun out of the duct with the
One of the most ubiquitous solvents on our planet is water solution having salts. It has been documented that water can be a solvent for nearly all substances depending upon pressure and temperature, which makes water a unique solvent.1−4 Water also plays an essential role in the evolution and development of living organisms; for example, more than two-thirds of the human body consists of water with various dissolved salts or ions influencing the characteristics of cells and proteins. In 1888, Hofmeister proposed a classification of anions and cations into a series, recognizing the ionic efficiency, now commonly known as Hofmeister series.5 Depending on the ionic radius and the associated charge, molecular structures based on hydrogen-bonding groups such as proteins or water © 2013 American Chemical Society
Received: June 14, 2013 Revised: August 4, 2013 Published: August 21, 2013 7086
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studies on PA46 have revealed that the Brill transition is caused by a crystal-to-crystal transformation in the solid state induced by weakening of intrachain hydrogen bonding and intersheet van der Waals forces at elevated temperatures.9,13 Recently, pure water or ionic solutions in their “superheated” state have been used for the successful dissolution of hydrogenbonded synthetic polymers even below the Brill transition temperature.16 In the “superheated” state of water, T ∼ 200 °C and P ∼ 20 bar, the hydrogen bonding between the PA chains is likely to be further reduced due to increased diffusivity of water at higher temperatures, thereby facilitating the dissolution process. Upon cooling from solution, PA recrystallizes with improved crystal packing, illustrating the good solvent characteristics of water for dissolution and recrystallization.17 Dissolution and crystallization of polyamides in ionic solutions such as lithium chloride, lithium bromide, and lithium iodide have been studied by us recently.9 It was found that PAs dissolve at the lower temperature with increasing ionic strength of salt solutions, from the same monovalent ions. Furthermore, the size of ions selected from the Hofmeister series can increase or reduce the dissolution temperature of PAs. Differences in the dissolution temperatures for PAs at varying ionic strength and ionic radii are attributed to changes in the physical state of water in the presence of ions at a given temperature. However, the mechanism behind the dissolution process, such as (i) the interaction between water molecules and/or water−ions and (ii) and solvent interaction with the polymer at higher temperatures prior to dissolution, is not well characterized and understood. In this study, we employed in situ nuclear magnetic resonance (NMR) under high-resolution magic-angle spinning (HR-MAS) conditions to address changes in the physical state of water at ambient and high temperatures with and without ions. These studies aim to reveal the effect of temperature and pressure on the local organization of water molecules. The structural changes of water molecules in the presence of ions and their influence on dissolution and crystallization of the polyamides are then investigated using variable-temperature 1H HR-MAS NMR. These NMR studies have been performed using sealed glass capillaries fitted to the size of the MAS rotor (see Materials and Experiments for details). While 1H HR-MAS NMR mainly follows the dissolution process in the presence of water, time-resolved X-ray diffraction is applied to characterize the structural changes in terms of interchain and intersheet distances as a function of temperature. This combined approach of NMR spectroscopy and X-ray diffraction enables us to address the molecular mechanism involved in the dissolution process of PAs below the Brill transition temperature and to identify the origin of suppressed crystallization.9,16,17
removal of the ions, hydrogen bonding is recovered and the desired mechanical properties such as high tensile strength and high modulus are established.8 Considerable efforts have been made to investigate molecular interactions between water molecules at different temperatures, in the presence of monovalent as well as divalent ions.1−4 It is accepted that the local order between water molecules persists in the liquid state of water.4 It is beyond the scope of this work to discuss in depth the vast knowledge that exists in this area because, even today, strong debates persist on the structure of water at different length scales. However, with the advancements of experimental techniques and theoretical studies insight into the complexity of the structural organization of water molecules has become clearer. These findings have been summarized in two excellent reviews by Bakker et al. and Marcus.1,4 Both reviews explicitly address the influence of ions on the local organization of water molecules, which depends upon the ionic size and valency. An agreement persists on the presence of the first solvation shell around the ions, whereas a persistence of ordering beyond the first solvation shell remains a matter of debate that mainly arises on the time scale to which the ordering is perceived.1,6 In our previous work, we showed the influence of ionic concentration on the 1H chemical shift of water with increasing ionic radii.9 However, since the 1H chemical shift of bulk water results from a very broad chemical shift distribution of individual water molecules being in rapid exchange on the NMR time scale, it is impossible to draw more than a qualitative conclusion based on the NMR results alone.10,11 Therefore, we do not want to focus on local structural changes of water molecules, depending on ionic concentration, but rather want to study the dissolution process of polyamides, utilizing the changing solvation properties of water at different ionic concentrations and temperatures. In analogy to natural polypeptides, simple synthetic hydrogen-bonded polymers such as polyamides (PAs) can be dissolved in ionic solutions, i.e., monovalent and divalent ions in aqueous solution at different concentrations.9 PAs with decreasing number of hydrogen bonding amide groups, including PA46, PA66, PA6, PA12, etc.,12 are semicrystalline materials with a crystal structure that is based on intermolecular hydrogen bonds between amide groups of neighboring chains and van der Waals interactions between the hydrogen-bonded sheets. This results in chain-folded crystals that can be described by a triclinic or monoclinic unit cell. By means of X-ray diffraction, interchain and intersheet characteristics of PAs can easily be identified, since they give rise to two strong characteristic diffraction peaks at 0.37 and 0.44 nm, respectively.13 Upon heating of PAs, prior to melting, the two peaks merge into a single peak, and the structure can be described as a pseudohexagonal packing. The temperature at which this phenomenon occurs is known as the Brill transition temperature (TB), and it has been observed for many polyamides, including PA6, PA66, PA46, etc.9,13−16 However, the Brill transition temperature depends on the number of methylene units between the amine and carbonyl units and specific crystallization conditions of the polymer. PAs with higher number of methylene groups between the amine and the carbonyl group, like PA12, do not show this transition.14 Ramesh and co-workers observed a relatively high TB for a solution grown PA66 crystal,15 revealing that TB in PA66 is directly related to the crystallization conditions. In the case of PA46, depending on the crystallization conditions, TB can be observed in the temperature range of 190−245 °C. Further
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MATERIALS AND EXPERIMENTS
Materials. Monovalent (lithium and sodium halides) and divalent (calcium and magnesium halides) salts used in this work were obtained from Sigma-Aldrich and were used as received. Different salt solutions ranging from 0 to 12.0 M were prepared using demineralized water. The polyamide (PA46 Stanyl) sample was obtained from DSM research and used as received. Nuclear Magnetic Resonance (NMR). In situ variable-temperature (VT) 1H and 13C high-resolution magic-angle spinning (HRMAS) NMR experiments were performed on a Bruker DSX spectrometer operating at 500 MHz 1H Larmor frequency using a commercial MAS double-resonance probe (1H-X) for rotors with 4.0 mm outside diameter. A MAS spinning frequency of 5.0 kHz and a 4.0 7087
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μs π/2 pulse, corresponding to 62.5 kHz rf nutation frequency, were chosen for 1H and 13C. All samples were prepared by placing PA46 powder and solvent (water or concentrated ionic solutions) in glass capillaries (from Wilmad Glass) with a filling of ∼90% by volume (to ensure the coexistence of liquid and gas phase) and tightly sealed using a LPG gas flame. During the sample preparation care was taken to avoid any polymer degradation or water evaporation. The temperature was controlled using a Bruker temperature control unit, in the range from 30 to 220 °C. Reported temperatures are corrected for sample rotation induced temperature changes. The calibration was performed using 207Pb MAS NMR where the chemical shift of Pb(NO3) as a function of temperature was known. 1H HR-MAS NMR spectra were recorded for every 2 min during the temperature ramp of 2.0 °C/min. 1 H chemical shifts are reported relative to tetramethylsilane (TMS) using adamantane as an external reference.18 Calibration of 1H rf-field strength and shimming were performed at ambient conditions. For the 1 H and 13C HR-MAS study of the low-temperature gel state, the samples were heated ex situ to 200 °C and subsequently cooled to room temperature in order to obtain the dissolved polymer solution. 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 4000−650 cm−1 and at a resolution of 2 cm−1. Time-Resolved Wide-Angle X-ray Diffraction (WAXD). Timeresolved wide-angle X-ray diffraction (WAXD) was performed at the high-resolution material science beamline ID11of the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. Twodimensional diffraction patterns were recorded using a 4 M CCD camera at 29.8 keV (λ = 0.417 nm) with an X-ray beam size of 50 × 200 μm2 and an exposure time of 10−12 s. The sample-to-detector distance was calibrated with lanthanum hexabromide. From the obtained diffraction patterns, background scattering and detector response were subtracted, and azimuthal integration was performed on the individual arcs to calculate the intensity against the scattering vector q. To convert scattering vector information into the Bragg’s dspacing, the d = 2π/q relationship is used, where q = |q⃗| = (4π/λ) sin(θ). A special experimental setup for obtaining the influence of temperature and pressure was used as discussed elsewhere. The dried powder material was placed in the capillary with ionic solution, and the capillary was placed in the pressure cell.17 The temperature was controlled using a Linkam hot stage TMS94 controller. The sample was heated/cooled with 10 °C/min until the dissolution/crystallization temperature, and X-ray data were collected during heating process of the sample. Gel Permeation Chromatography (GPC). To follow the molecular weight before and after experiments, GPC measurements were carried out using MIDAS GPC instrument. The polymer sample was dissolved in 1,1,1,3,3,3-hexafluoroisopropanol (HFIP). The sample for GPC measurements was prepared by dissolving 3.0 mg of the polymer in 1.0 mL of the solvent. For the GPC measurements, the detector of the GPC apparatus was calibrated with PMMA standards. The polymer samples obtained from NMR measurements were thoroughly washed using water to remove the ions and were then used.
been performed on the structural organization of water molecules.19 In the crystalline state (obtained on cooling under normal conditions below freezing point), hydrogen bonding between water molecules is optimum and the water molecules are separated from each other with largest possible distance. Compared to the crystalline state, in the liquid state with weakening of the hydrogen bonding van der Waals forces between the water molecules become more prominent, resulting in shorter distance between the water molecules. Water in a closed vessel, between the boiling point (100 °C) and the supercritical temperature (374 °C),20 is defined as the “superheated” state of water. In this high-temperature state of water, the average intermolecular distance between water molecules increases with temperature, and continuous exchange of water molecules between liquid and/or gas phase provides high molecular mobility. Temperature-dependent changes in the electronic structure of water molecules reflecting the local hydrogen bonding of the molecules in water, with and without ions, can be monitored by 1H HR-MAS NMR spectroscopy at different temperatures as depicted in Figures 1a and 1b for pure water and 7 M LiI, respectively.
RESULTS AND DISCUSSION To follow the structural changes of PA46 in the presence of water and ionic solutions of monovalent and divalent ions at molecular, conformational, and crystallographic length scales, HR-MAS NMR techniques combined with X-ray diffraction are employed. The 1H HR-MAS NMR studies demonstrate changes in the hydrogen bonding of polymer as well as water at different temperatures and ionic concentration of the monovalent and divalent ions. Influence of Temperature on the Hydrogen Bonding of Water in the Presence of Monovalent Ions. Several molecular dynamic and Monte Carlo simulation studies have
In Figure 1a, an average 1H chemical shift of 4.6 ppm has been determined at 38 °C for the protons of water. The narrow line width observed in the NMR experiment results from more than 10 ppm wide distribution of chemical shifts predicted by Carr−Parinello molecular dynamics (CPMD) simulations,21,22 which is averaged by rapid molecular exchange between different molecular environments in water. Thus, the chemical shift reflects the averaged hydrogen bonding of water molecules in the bulk. On increasing temperature from 38 to 210 °C, a gradual change in the 1H chemical shift toward lower values is observed, which may result from a depopulation of strongly hydrogen bonded states accompanied by a decrease in the
Figure 1. Temperature-dependent 1H HR-MAS NMR spectra of (a) pure water and (b) 7 M LiI sealed in glass capillaries, illustrating the changes in 1H chemical shift and thereby the changes in hydrogen bonding between water molecules during heating and cooling.
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hydrogen bonding between water molecules. At ∼210 °C, water has a 1H chemical shift of 2.7 ppm, and on cooling from the “superheated” state of water at 210 °C down to 40 °C a gradual increase in the 1H chemical shift suggests reestablishment of hydrogen bonding between water molecules. Conradi et al. have with the help of 1H NMR investigated the changes in 1H chemical shift of water at supercritical conditions (∼400 °C and ∼400 bar).23 By combining experimental and modeling studies, they further demonstrated that 29% hydrogen bonds still exist at 400 °C and 400 bar compared to water at room temperature. In this work, temperatures up to ∼200 °C and only moderate pressures are reached (
