Solid State NMR Applied to Evaluate Fibers Cotton

The evaluation of cotton fibers from Nuclear Magnetic Resonance (NMR) in solid state using high and low-field showed to be a good alternative to evaluate these materials. The main objective of this work was to use the sol-id-state NMR techniques to accompany the fibers nature, size and morphol-ogy to have a direct method to evaluate cotton fibers maturity. We could conclude that the solid-state NMR techniques are good source to evaluate the molecular behavior of cotton short and long fibers and give important information on sample’s morphology, indicating that this technique can be used to distinguish the size of cotton fibers.

the mass of the fiber per unit length (title), usually expressed in militex (mtex).
Maturity is the degree of maturity of a cotton fiber, which is indicated by the thickness of the cell wall in relation to the diameter of the fiber [5]. The price of cotton fiber is priced on the stock exchange on the world market. Many times these batches are priced with the low value, since the rounds have the satisfactory properties for processing. Maturity, which is largely determined by growing conditions, can be defined as the relative wall thickness (the area of the cell wall to that of a circle with the same perimeter as the fiber, or the ratio of the cell wall thickness to the overall "diameter" of the fiber).
The solid-state NMR techniques show to be good tool to analyze and identify the fibers types such as cotton samples. Knowing that solid-state NMR basic techniques are good tool to evaluate the materials in their own state, these techniques can be applied to the fibers study. Magic angle spinning (MAS) and cross-polarization magic angle spinning (CPMAS) techniques give response on samples chemical structure and domain structure [6]. The variable contact-time experiment (VCT) informs on sample molecular dynamic evaluation and the proton spin-lattice relaxation time in the rotating frame (T 1 Hρ) permits to characterize the intermolecular interactions forces and classify the samples according to their molecular dynamics, according to their molecular movements since this parameter are sensitive to the movements in the tens of kilohertz with domain size in the range of 1 to 4 nm [6] [7]. Thus, in this work, we used carbon-13 ( 13 C) VCT to determine the T 1 Hρ values from each resolved carbon to analyze the cotton fibers types according to their molecular structure and dynamics. Knowing that this parameter informs on the molecular movements in the MHz range and domain size varied from 25 to 50 nm, this study determined also the values of proton spin-lattice (with a time constant T 1 H) of the samples using a low-field NMR spectrometer [8]. According to this statement, it was analyzed carbon-13 ( 13 C) employing MAS, CPMAS, VCT and the determination of T 1 Hρ to deeply analyze the cotton fibers molecular structure and dynamics.
To complement the analyses, it was also determined the values of proton spin-lattice (T 1 H) of the samples using a low-field NMR spectrometer [9].
Thus, according to the explanation described before, the main purpose of this work was to apply the solid-state NMR techniques using high and low-field (relaxometry) NMR to evaluate the short and long cotton fibers, and obtain information on the correlation of this parameter to the fibers structural organization.

Materials
Cotton fibers standard were supplied by SENAI CETIQT, which have same specifications that are listed in Table 1. The fibers were used as received.

NMR Experiments
High-field NMR All solid-state NMR spectra were obtained on a Bruker spectrometer 7.04 Tesla, operating at 75.4 MHz for 13 C nuclei. The samples were conditioned be-

Results and Discussion
Analyzing the solid-state 13 C NMR CPMAS spectra of the cotton containing short and long fibers, it was observed that (Figure 2) there was a small difference It can be observed that the decay of long fiber sample is deeper than for the short fiber sample; which is a consequence of fiber chains organization and domain size. to cotton long fibers.
The measurements of T 1 H (Table 3) was used to evaluate the behavior of the fibers in relation to all type of hydrogen spin-nuclear population, and the domains that constitute the fibers.
From Table 4, it was not possible to distinguish the collective movements of fibers chains, according to their size and specifications. Thus, to obtain more and important information from cotton fibers the T 1 H parameter was measured in different conditions. Since the difference of each fiber type was not so sensible at the measurements at room temperature, due to the fact that for the NMR sensibility is low. Thus, the relaxation data was measured with an increase in the temperature. Table 3 shows the values of relaxation parameter after the measurements with the increase of temperature and is presented in Table 5.        It was noted that from T 1 H parameter the long cotton fiber presents higher value of this parameter for the fibers domains, due to the stronger intermolecular interaction, which comes from the fibers size and molecular organization and also the molecular collective movements are slower than the one from derived from short cotton fibers. The short cotton fibers the opposite behavior was found; confirming that these fibers are weaker comparing to long fiber cottons, because of physical characteristics being different in both fibers, which is in accordance with the Micronaire index.