Nadarajah Vasanthan

FTIR spectroscopic characterization of structural changes in polyamide-6 fibers during annealing and drawing

First, we report the development of Fourier transform infrared (FTIR) spectroscopic methods to determine the α/γ-crystalline phase ratio of polyamide-6 fibers and, in combination with density measurements, the total crystallinity. Using density determinations of the crystallinity of pure α and pure γ samples, we found the absorption coefficient ratio for the 930 (α) and 973 cm−1 (γ) bands to be 4.4, from which we could obtain the α/γ ratio for any polyamide-6 sample. The application of this FTIR method to the quantitative analysis of phase changes during thermal treatment and the drawing of polyamide-6 was then made. We confirmed that crystallization during thermal treatments involved increases in both phases and did not involve crystal-to-crystal transformation, whereas drawing involved both crystallization of the amorphous phase in the α form and γ→α transformation. Finally, we revisited the band assignments for the amorphous phase of polyamide-6 and found that the band at 1170 cm−1 was not an amorphous band but, because its absorbance was independent of crystallinity, could be used as an internal reference band. The band at 1124 cm−1 was reliably attributed to the amorphous phase.

Infrared spectroscopic characterization of oriented polyamide 66: Band assignment and crystallinity measurement

Having found much ambiguity in the infrared band assignments for polyamide 66 (PA66), we revisited some of these assignments before using infrared spectroscopy to assess microstructure changes resulting from multiple thermal treatments. We discovered that earlier assignments of the 1144 and 1180 cm -1 bands to the amorphous (noncrystalline) phase were incorrect, whereas the bands at 924 and 1136 cm -1 can be attributed unambiguously to the noncrystalline phase. We also confirmed that PA66 bands at 936 and 1200 cm -1 are crystalline bands. The normalized absorbance of the 1224-cm 1 fold band increases in proportion to crystallinity, indicating that chain folding is the predominant mechanism of thermal crystallization in PA66. We demonstrated that infrared spectroscopy can be used to estimate the degree of crystallinity of PA66, and two methods were explored. One is a calibration method in which the band ratio of 1200 and 1630 cm -1 is plotted against crystallinity measured by density. The other is an independent infrared method based on the assumption that PA66 satisfies a two-phase structure model. The crystallinity determined by the independent infrared method showed good agreement with the crystallinity obtained from density measurements.

Impact of Nanoclay on Isothermal Cold Crystallization Kinetics and Polymorphism of Poly(l-Lactic Acid) Nanocomposites

Poly(L-lactic acid) (PLLA) intercalated nanocomposite films containing 1, 2, 5, and 10% organically modified montmorillonite (OMMT) have been synthesized by the solvent casting approach. The thermal characteristics, isothermal cold crystallization kinetics, and structural changes of neat PLLA and its nanocomposites during annealing were studied by using differential scanning calorimetry (DSC) and Fourier transform infrared (FTIR) spectroscopy. DSC observation showed that melting temperature and final crystallinity were not affected significantly with OMMT loading. PLLA films with increasing OMMT content exhibited higher crystallization rates than neat PLLA during annealing and suggested that the silicate platelets act as a nucleation agent during annealing. The effect of OMMT content on the isothermal crystallization kinetics of PLLA was analyzed using the Avrami equation. An Avrami constant of 1-2 was observed, suggesting that crystallization proceeds through one-dimensional growth with heterogeneous nucleation. FTIR investigation showed a band at 922 cm(-1) at all T(a), and no band at 908 cm(-1) suggested that all samples form α crystal regardless of OMMT content or T(a).

Effect of Heat Setting Temperatures on Tensile Mechanical Properties of Polyamide Fibers

In this study, PA 66 and PA 6 fibers are heat set at temperatures between 25 and 200°C. Fractional recovery (f) depends on heat setting temperatures and crystallinity, which provides partial crosslinking that delays the onset of the flow regime and increases the time constant of relaxation at a given temperature. Mechanical properties are examined with an Instron tensile tester, and the effect of heat setting temperatures on mechanical properties is correlated with crystallinity changes. The terminal modulus and extension at break show a direct correlation with crystallinity, while there is no noticeable effect on initial modulus with crystallinity changes.

Novel Methods for Obtaining High Modulus Aliphatic Polyamide Fibers

Super high modulus polyethylene fibers can be created by converting high molecular weight flexible PE chains into highly oriented and extended chain conformations. However, unlike polyethylene, aliphatic polyamides have very high cohesive energy and therefore cannot be easily drawn and highly oriented. This review addresses this fundamental problem by analyzing various novel approaches that can be used to suppress hydrogen bonding in these types of polyamides. Plasticization of such polymers with ammonia, iodine, salts, and Lewis acids, as well as dry spinning, wet spinning, and gel spinning, are discussed. Specialized techniques that involve vibrational zone drawing and annealing as well as laser heating zone drawing and annealing are also reviewed. Some of these methods definitely lead to remarkable improvements in initial modulus and other mechanical properties. The development of recombinant spider silk proteins as well progress in spinning these materials is also reported. The advantages and disadvantages of all of these processes are then summarized.

Determination of Molecular Orientation of Uniaxially Stretched Polyamide Fibers by Polarized Infrared Spectroscopy: Comparison of X-Ray Diffraction and Birefringence Methods

Polarized infrared (IR) spectroscopy has been used to determine the crystalline and amorphous orientation of polyamide fibers. The transition moment angle of the band at 936 cm−1 of PA66 was determined to be 48° using IR spectroscopy and birefringence measurement. The crystalline orientation of PA66 fibers was estimated from the band at 936 cm−1 while the amorphous orientation of PA66 fibers was obtained by an indirect method. The α crystalline orientation of PA6 has been obtained using the band at 930 cm−1 and the amorphous orientation of PA6 has been determined using the band at 1124 cm−1. Crystalline orientation increased rapidly at low draw ratios (DR < 3) and increased slowly at higher draw ratios (DR > 3) for both PA66 and PA6 fibers, while the amorphous orientation increased slowly throughout the whole extension range for PA66 fibers. A good correlation was found between the crystalline orientation values obtained by infrared spectroscopy and other methods such as X-ray diffraction for PA66 and PA6 fibers. On the basis of this observation, it has been concluded that polarized infrared spectroscopy can be used reliably to measure the orientation of polyamide fibers without combining with other techniques.

Polymer-polymer composites fabricated by the in situ release and coalescence of polymer chains from their inclusion compounds with urea into a carrier polymer phase

Inclusion compounds (ICs) can be formed between small-molecule hosts and guest polymers, where the crystalline host lattice confines the guest polymers to occupy narrow cylindrical channels. The included polymers are highly extended by the narrow channel diameters and are separated from neighboring polymer chains by the walls of the small-molecule host lattice. It is possible to coalesce the polymer chains from their ICs by exposure to a solvent for the small-molecule host which is not a solvent for the included polymer chains. When crystallizable polymers are coalesced from their ICs by solvent treatment, they are observed to crystallize in an extended-chain morphology accompanied by much less chain-folding than occurs when crystallization of the same polymers take place from their disordered melt or solution environments. In this report we outline our initial efforts to create polymer-polymer molecular composites based on the coalescence of polymer chains from their IC crystals with urea, which were previously embedded in a carrier polymer phase. Both film and fiber composites made with chemically identical or distinct IC-included and carrier polymers are described. Water vapor permeation, differential scanning calorimetry (DSC) and microscopic observations are used to probe these composites; and several applications are suggested.

Morphological and conformational changes of poly(trimethylene terephthalate) during isothermal melt crystallization.

The isothermal melt crystallization of poly(trimethylene terephthalate) (PTT) has been investigated using differential scanning calorimetry (DSC), optical microscopy, and Fourier transform infrared (FTIR) spectroscopy. Triple-melting endotherms were dominant at crystallization temperatures (T(c)) below 195 °C, while double-melting endotherms were dominant at T(c) above 195 °C. These multiple-melting behaviors were attributed to melting and recrystallization as well as thermal stability of the crystallites. The difference in crystallinity observed between melt-crystallized PTT films before and after cooling to room temperature was attributed to lamellar thickening. The kinetics of isothermal melt crystallization of PTT was analyzed using an Avrami equation, and the rate constant (k) and t(1/2) increased with increasing T(c). An Avrami exponent n = 2-3 was obtained for the PTT melt crystallized from 150 to 205 °C, suggesting two-dimensional to three-dimensional growth with heterogeneous nucleation and was confirmed by polarized optical microscopy. The bands at 1358 and 976 cm(-1) associated with gauche and trans conformations of -OCH(2)CH(2)CH(2)O- were used to determine gauche and trans conformations assuming a two-phase conformational model. The crystalline and the amorphous gauche conformations were separated by combining them with DSC crystallinity. It was shown that the crystalline gauche conformation increased at the expense of the amorphous trans conformation. No significant change in the amorphous gauche conformation was found. On the other hand, the crystalline and amorphous gauche conformations were found to increase with the annealing temperature (T(a)) at the expense of the trans conformations during the annealing of PTT. The difference between melt crystallization and annealing was also discussed.

Crystallization, Crystal Structure, and Isothermal Melt Crystallization Kinetics of Novel Polyamide 6/SiO2 Nanocomposites Prepared Using the Sol–Gel Technique

Polyamide 6/SiO2 (PA6/SiO2) nanocomposites with varying amounts of SiO2 were prepared by using a novel sol-gel technique. These nanocomposites were formed in situ by hydrolysis and through the condensation of tetraethoxysilane (TEOS) using formic acid with a small amount of water as the solvent for PA6. Observations of TGA showed that the thermal stability of PA6 nanocomposite was significantly improved compared to that of neat PA6. Microstructure development during the thermally induced crystallization of PA6/SiO2 nanocomposites was investigated with a combination of differential scanning calorimetry (DSC), FTIR spectroscopy, scanning electron microscopy (SEM), and AFM. FTIR spectroscopy was used to determine the crystal form of these nanocomposites, and it was concluded that SiO2 nanoparticles have the γ-nucleating effect. The crystallinity of nanocomposites decreased with increasing TEOS loading as compared to that for neat PA6. SEM showed a very fine dispersion of nanoscale silica whereas SEM and Zetasizer proved the silica particle size was about 100-200 nm. The isothermal crystallization kinetics of these nanocomposites with increasing SiO2 content were investigated, and it was shown that the amount of SiO2 plays a significant role in crystallization kinetics.

Structure characterization of heat set and drawn polyamide 66 fibers by FTIR spectroscopy

Abstract Structural changes that occur during thermally induced and strain induced crystallization of polyamide 66 fibers were studied by infrared spectroscopy, density measurement and optical microscopy. Two bands at 924 and 1136 cm–1 were shown to arise from the amorphous phase and assignment of the bands at 936 and 1200 cm–1 to the crystalline phase were confirmed. We demonstrated that two different infrared spectroscopic methods could be used to determine the total crystallinity of polyamide 66 fibers. One is a calibration method in which the band ratio of 1200 and 1630 cm–1 is plotted against the crystallinity measured by density measurements. The other one is an independent infrared method. Crystallinity obtained by the independent infrared spectroscopic method showed good agreement with crystallinity observed by density measurement. Infrared dichroism was used to obtain the crystalline orientation using the band at 936 cm–1. The transition moment angle of 48° was found for the band at 936 cm–1 with respect to chain axis. Amorphous orientation was obtained using Stein’s equation.