The Types of Water in Cellulosic Fibers: DSC and NMR Spectroscopy

1. The Types of Water in Cellulosic Fibers: DSC and NMR Spectroscopy

2. Outline • Types of water • Differential Scanning Calorimetry • Nuclear Magnetic Resonance Spectroscopy • Analytical determination of types of water • Summary

3. Purpose • Types of water are removed in different ways • Water removal huge component of papermaking costs • Trend towards use of non-wood fibers • Has become increasingly important to understand how cellulosic materials behave in paper drying process • Behavior should be known for wood fibers for effective comparisons • Differential Scanning Calorimetry (DSC) and Nuclear Magnetic Resonance (NMR) spectroscopy

4. Types of water • Free/Bulk water • Found in large pores • Makes up inter-fiber free water in pores and intra-fiber water in lumen • Removed by centrifugation, melting point similar to bulk water • Freezing bound water • Melting and freezing temperature depressed by about 2°C due to small size of micropores and presence of polymers like hemicellulose • Loss causes most detrimental irreversible pore closure • Hubbe, M. A. WPS 527 Coursepack. North Carolina State University. • Heikkinen, S. et al. “NMR Imaging and Differential Scanning Calorimetry Study on Drying Pine, Birch, and Reed Pulps and Their Mixtures.” Journal of Applied Polymer Science. 2006. vol. 100. p. 937-945.

5. Types of water • Non-freezing bound water • “Water of hydration” associated with various surfaces • Hydrogen-bonded to hydroxyl and carboxylic acid groups in micropores • Amount calculated by subtracting total freezing water from the moisture ratio of the sample • Hubbe, M. A. WPS 527 Coursepack. North Carolina State University. • Heikkinen, S. et al. “NMR Imaging and Differential Scanning Calorimetry Study on Drying Pine, Birch, and Reed Pulps and Their Mixtures.” Journal of Applied Polymer Science. 2006. vol. 100. p. 937-945.

6. Differential Scanning Calorimetry (DSC) • Most common analytical method for bulk surface measurements • Measure difference in heat flow rate between a sample and an inert reference material as a function of time and temperature • Two types • Exothermic • Endothermic

7. Temperature range about -100°C to 900°C Major applications Determination of drug purity, reaction time for enzyme degradation, degree of crystallization at a particular temperature Two modes: Heat flux Power compensated DSC TA Instruments Q100 http://www.tainstruments.com/product.asp?n=1&id=16 Lucia, L. “DSC: A bulk analytical technique.” Lecture notes: WPS 595b Biomaterials Characterization. North Carolina State University. 9 February 2006.

8. Sample and reference heated or cooled by separate heating units Furnaces keep temperatures isothermal to one another throughout the test. Objective is to monitor electrical power used by heaters as temperatures are either increased or decreased linearly Power being sent to heaters is adjusted so that the same temperature is maintained for both sample and reference Difference in power to keep temperatures the same is used to generate curve Heat flux DSC Skoog, D. A., Holler, F. J., & Nieman, T. A. (1998). Principles of Instrumental Analysis. 5th edition. Thomson Learning Inc. Bhadeshia, H. K. D. H. “Differential Scanning Calorimetry.” University of Cambridge, Materials Science & Metallurgy. 2002.

9. Power-compensation DSC • Same heat energy transferred to sample and reference • Transported heat carefully controlled • Temperature difference is monitored Skoog, D. A., Holler, F. J., & Nieman, T. A. (1998). Principles of Instrumental Analysis. 5th edition. Thomson Learning Inc. Bhadeshia, H. K. D. H. “Differential Scanning Calorimetry.” University of Cambridge, Materials Science & Metallurgy. 2002.

10. Nuclear Magnetic Resonance (NMR) Spectroscopy • Determination of molecular structure for organic and inorganic compound • Based on measurement of adsorption of electro-magnetic radiation in the radio frequency range 60 to 800 MHz • Concept that certain atomic nuclei have magnetic and spin moments • 1H, 13C, 19F, and 31P

11. NMR • Spin and charge of the nuclei cause behavior similar to bar magnets • Energy levels split causing nuclei to possess one of two magnetic forces • Each nucleus is able to switch between energy states through the absorption of a photon.

12. NMR • Placing nuclei in strong magnetic field, can monitor energy transition when photon is absorbed • The excess of lower energy spin is essential for monitoring of the energy adsorption later • When nuclei then subjected to radio waves some absorb radiation and are raised to higher energy • Difference in energy between low and high energy states provides signal • Signal sensitivity is directly proportional to magnetic field Skoog, D. A., Holler, F. J., & Nieman, T. A. (1998). Principles of Instrumental Analysis. 5th edition. Thomson Learning Inc. Argyropoulos, D.S. “NMR Spectroscopy.” Lecture notes: WPS 595b Biomaterials Characterization. North Carolina State University. 24 January 2006.

13. NMR • Distinguish between nuclei of different elements because of chemical shifts in peaks • Fourier transform • Two-dimensional • Two different pulse frequencies bombard sample • Pulsed • Radiofrequency (RF) radiation 90-degrees to magnetic field causes nuclei to jump into higher-energy alignment • Pulse simultaneously excites nuclei in all local environments • Nuclei re-emit RF radiation and create interference pattern known as a free-induction decay (FID) http://www.chemistry.adelaide.edu.au/external/soc-rel/content/ftnmr.htm

14. Analytical determination • DSC: Detect different types of water using procedures analogous to paper drying • Temperature fluctuations help determine which type of water is present and in what quantity • NMR: Observe effects of drying because protons in each type of water give off specific signal • Proton-NMR well-suited for study of water/cellulose interactions, relaxation times, and for distribution of moisture within paper sheet

15. Level of bound water strongly dependent on type and condition of fibers Temperature-dependence of experimental readings Amount of detectable bound water was more accurate as experimental temperature decreased Ogiwara et al- NMR Ogiwara, Y., Kubota, H., & Hayashi, S. “Temperature Dependency of Bound Water of Cellulose Studied by a High-Resolution NMR Spectrometer.” Journal of Applied Polymer Science. 1970. vol. 14 p.303-309.

16. Ogiwara et al- NMR • Determined boundary temperature, Tc, where water molecules become bound to cellulose • The lower the water content became, the greater the changes in Tc • Similar DSC studies agreed that glass transition temperature was lower for moist cellulose than for an air-dried sample • These results showed that Tc represents the glass transition temperature for a given compound

17. Heikkinen- DSC and NMR NMR • Drying setup for NMR imaging probe used to measure water contents of pulps gravimetrically as a function of drying time • Drying rates used during the imaging process • Water content decreased rapidly to an inflection point around 61-68% water by weight • Decreased again to a point just below 37-45% weight • Remaining moisture tightly bound to fibers and was hard to remove • Narrowest lines and highest intensities matched with highest water content • Increases in line widths with time correspond to decreases in water content and matched two inflection points • Water distribution measured using two-dimensional NMR imaging before and after drying

18. Heikkinen- DSC and NMR DSC • Used to determine fraction of each type of water and rate at which it disappeared from the sample • Explain method briefly • Graphs

19. Summary • Three types of water • Each treated differently to maximize efficiency • Analyze using • DSC • NMR • Combination • Movement towards utilization of non-wood fibers for papermaking