Nuclear Magnetic Resonance Characterization of Nylon 6 - Research Paper Example

NMR Characterization of Nylon 6 Nuclear Magnetic Resonance (NMR) spectroscopy is one of the most important methods for the solid characterization of the polymers. NMR is used to measure the molecular features, polymer chain conformation, dynamics and morphology of the molecules. NMR are able to provide information about the structure of the polymers of about 2 – 200 Aº length, which includes Nylon-6. This level of magnification is not possible with the other methods. They are also able to provide molecular level explanation for the transition states of the molecules including the ones in Nylon-6. So, this paper focusing on Nylon-6 will discuss the characterization of structure and morphology in Nylon-6 by solid state carbon-13 and nitrogen-15 NMR. Introduction Solid state NMR is one among the powerful tools for studying the polymer structure, morphology and the dynamics. (Mirau 2001). Nylon – 6 polymeric materials is an important polyamide that has wide applications ranging from carpets and automotive parts to intimate apparel. Nylon – 6 is also found to store large amounts of iodine making them a good solid-state electrolyte. Nylon 6 has the [-NH (CH2)5(CO)-] repeating group. The nylon -6 contains the crystalline lamella in an amorphous matrix. Figure 1: Structure of Nylon 6. Though it is crystalline the presence of this amorphous matrix makes the crystallography difficult. As nylon – 6 has various applications in many fields, the study about the structure and the conversion level among has become essential. As the hydrogen bonding in the polyamides makes them an important plastic because of the strength it provides. Nylon is found to exist in two different crystalline forms. The most stable form at high temperature is the alpha form which consists of anti parallel chain in an extended from with the hydrogen bonds. The second phase is known as the gamma phase. This phase is found to contain hydrogen bonds as pleated sheets between the parallel chains. In this form, the methylene next to the amide is found to adopt a skew conformation which is similar to the secondary structure confirmation. The important features of the gamma form are (i) the length between repeat units is shortened (0.35 Aº). (ii) Methylene groups in the amide groups are found tilted towards the plane. The crystal structure of nylon 6 was studied using the 13Cand 15NCross Polarization/Magic Angle Spinning (CP/MAS) NMR spectroscopy. (Casanovas & Aleman 2002). Figure 2: Charge scheme for the monomer fragment of the nylon 6. A large number of polymers are used commercially and biologically based on the nitrogen contained in the polymer. N15 NMR is used for the polymer characterization. This usage is limited because of the low abundance in the nature and the ratio between the signals to noise. The main advantage of N15 is that the nitrogen chemical shifts are much sensitive to the conformations. This is well understood in the NMR spectra for nylon-6 at 20- MHz. 13C and 15N NMR are used to characterize the structure, dynamics and morphology of the nylon 6 materials. The first solid state NMR studies of the 15N for the enriched nylon 6 polymer studies were presented by Hatfield etal, in the year 1990. Figure 3: The solid state N15 spectra for nylon-6. a) Predominantly Gamma form b) Predominantly Alpha form and c) Alpha form. The nylon-6 can be prepared in many crystal forms. The figure clearly depicts the difference between the alpha and gamma forms. The NMR helps to find out the difference in their chemical shifts in the various forms. The NMR spectra of solids are limited by the presence of near by chains in the structure and thus it limits the atomic motions. The chemical shift aniso trophy and Magic-angle spinning helps to obtain a high resolution spectrum. The spectral densities and the auto correlation facilities are the Fourier inverses of them in the time and frequency domains. Figure 4: Transition mechanism and meta stable forms of the alpha and gamma forms. N15 NMR is more advantageous than the C13 NMR in distinguishing between the two forms of nylon-6 as carbon spectra is not able to do so. Thus it concludes that the NMR spectra are much sensitive to the chain conformation. The larger differences between the crystalline and amorphous material conformations are easily differentiated by using the differences in their chemical shifts. (Bovey and Mirau 1996) To determine the factors that control the structure and properties and function of nylon 6, MSXX force field which was developed by Dasgupta et al was used. MSXX force field will give the quantum mechanics, molecular mechanics and molecular dynamics of the packing and folding structures of nylon 6.After performing the MSXX force field, the quantum mechanics simulations were carried out to validate the results of the force field. It was found that the electrostatic and Vander Waals interactions used the Accuracy Bounded Convergence Acceleration (ABCA) technique for computing the non bound energies. (Li and Goddard 2002). Figure 5: The alpha, gamma forms of the Nylon-6 crystals was examined and it was found that the linear H bonds are more in the alpha form than the other forms. This was the same in all the H-bond schemes in the crystal. It was also found from the NMR that the gamma form is less stable than the other types. But according to Hatfield etal, the H-bond in the gamma form is stronger than the alpha form. Because of these conclusions, the idea of measuring the bond strength was dropped and the rate of shielding of the amide proton which relates to the linearity of the hydrogen bond was practiced. The linear Hydrogen bonds have biggest effect on the charge distribution in the amide units, which have a direct effect on the chemical shifts as concluded by the solid-state NMR. The solid state 13C and 15N NMR spectra were used at the frequency of 75.3 and 30.3 MHz respectively. The spectra were obtained using the 15N labeled nylon 6 samples. The datas were obtained from using the Magic angle spinning (MAS) conditions and cross polarization (CP) and single pulse techniques. The versality of the NMR techniques enables us to design the probe molecular structure, orientation and dynamics. The most commonly used experiment is the single pulse experiment.(Hatfield 1990). The stretched nylon -6 fibers were treated with iodine at different time intervals at a temperature of 50 degree Celsius. The optical and structural parameters were investigated after the treatment. It was found that the refractive indices and the bi refringences values are proportional to the iodine evaporation time. It was also found that the iodine treatment has increased the surface reflectivity, isotropic reflective index, orientation function and mean polarizability per unit volume and has decreased the orientation angle. Orientation angle is the angle between the dipole moment and the chain axis. The orientation angle is found to be constant. Thus it was concluded from the experiments that the iodine treatment modifies the structure and the optical properties of the nylon 6 and it was confirmed using the NMR. This can be used for in many industrial purposes. (Shabana 2006). Solid state NMR spectroscopy of the 13C indicates that the concentration of the intercalate controls the molecular weight of the nylon 6. 13C NMR is used to investigate the morphology of the crystal structures. When Nylon -6 was mixed with the clay particles, the rates of morphological stratification of the surfaces were found starting with the distance of approximately 2 nm from the clay surfaces. Here two NMR spectrometers were used. The first spectrophotometer was used for 13C observation with the magic angle sample spinning (MAS) at a frequency of 100 MHz for protons and the second spectrometer operating at 300 MHz for protons. ( Vanderhart, 2001). Conclusion From the above analysis, it is clear that, as they use two spectrometers to monitor the alpha and gamma phases indirect measurements were practiced. It was also found that the gamma form crystallites were found to be stronger than that of the alpha form crystallites when mixed with the clay particles. Thus it concluded that the clay surfaces have the greater ability to strengthen the gamma crystallites than other particles. References: Bovey, FA and Mirau, PA, 1996, NMR of polymers, Academic Press Casanovas, J and Aleman, C 2002, “Calculated NMR chemical shifts of nylon 6: a comparison of the α and γ forms” Journal of Material Science, vol. 37, pp: 3589 – 3594. Davis, RD, Jarrett, WL and Mathias, LJ, 2001, Solution and Solid- state NMR Spectroscopy of Nylon 6 –Montmorillonite clay nanocomposites, ACS Symposium series, vol. 84, pp.117-126. Hatfield, GR, Glans, JF and Hammond, WB 1990, “Characterisation of structure and morphology in Nylon 6 by solid state 13C and 15N NMR, Macromolecules, vol. 23, pp.1654-1658. Mirau.P 2001, Solid-State NMR of Polymers, Academic press, Shabana HM 2006, “The role of iodine treatment in enhancing the optical and structural properties of stretched nylon 6 fibres”, Journal of Physics D: Applied Physics, vol. 39, Number 13, pp. 2843-2850 VanderHart. D. L, A. Asano, and J. W. Gilman 2001, “Solid-State NMR Investigation of Paramagnetic Nylon-6 Clay Nanocomposites. 2. Measurement of Clay Dispersion, Crystal Stratification, and Stability of Organic Modifiers”, Chemistry of Materials, vol.13, 3796-3809 Li, Y and Goddard, WA 2002, “Nylon 6 Crystal structures, folds and Lamellae from Theory”, Macromolecules , vol.35, pp. 8440-8455 Read More