Pulsed NMR study of thermal hysteresis in the swollen macroreticular and gel-type resins

Abstract

In the study of gel-water systems with the stationary and pulsed NMR method a thermal hysteresis in the proton signals and relaxation times were observed by a few authors. This phenomenon draws an attention, as it seems, to the accompanying sol-gel transition in polymer solution or it may counterpart the phase transition of water in gels below 0 °C.Woessner and Co.(1) studied the agar-water system and stated a pronounced hysteresis in spin-spin relaxation time at the temperature interval from 30 °C up to 100 °C. The same system and phenomenon were discussed by Palrna (2) on the ground of the hysteresis in signal line width and T1/T 2 ratio. The suggestion of Palma is that thermal hysteresis indicates the cooperative behaviour of polymer and water molecules. "The sol-gel transition is macromolecule-controlled but the structural information is carried by water molecules".Another mechanism is responsible for the hysteretic phenomena observed at the temperature range below 0 °C. Until now this phenomenon was observed on porous silica-gels and also on gels of polyethylene glycol (3) and collagen (4).Barnes (5), Morariu, and Mills (6), Pearson and Derbyshire (7) studied silica-gels and stressed the following experimental results: -hysteresis is not observed when the water coverage corresponds to one layer, -hysteresis appeares only if the diameters of pores in silica are in the range Of hundred of angstroms and it is absent when the size of pores corresponds to few diameters of a water molecule.The authors find the possible sources for the hysteresis in: movement of the solid-liquid boundary closer to silica surface as water progressively freezes (Morariu and Mills), in the disturbance of adsorption equilibrium by freezing of water on surface causing unfrozen water molecules move from pores to surface where pressure is lower and whereupon they freeze (Barnes) or in supercooling and superheating occurring with water of enhanced structural ordering inside the filled pores (Pearson and Derbyshire). The results of Mank (8) and Bie~kiewicz (4) add more information concerning the thermal hysteresis, observed in natural or synthetic polymers, however, there is no agreement as to the mechanism involved.In this publication we present data on low temperature hysteresis of T 2 time and FID (free induction decay) found in macropormls ion exchange resins of different porous structures. For comparison, the measurements performed with the gel-type (nonporous) ion exchange resins have also been presented.The spin-echo technique was applied. The macroporous resins have also been investigated by Doskocilova (10) and Frankel (11, 12) with the high resolution NMR method. Experimental MaterialsThree macroreticular (also called macroporous) cation exchange resins: Amberlyst 15, Amberlite 200, Amberlite 252 (R6hm and Haas Co., Philadelphia, USA) and four geltype cation exchange resins: Zerolit 225 X1, X4-5, X20 M67

H NMR Characterization of Swelling in Cross-Linked Polymer Systems

“…Highresolution 1 H NMR for the characterization of cationexchange resins was described as early as the early 1970s. [14][15][16][17][18][19][20][21][22]25 In these experiments two lines are observed for water-swollen cation-exchange resins (sulfonated S-DVB copolymers). One of the lines was due to water outside the bead and the other due to water inside.…”

“…Solid state NMR was also utilized but the level of cross-linking did not change the density of the copolymer in the nonswollen state, and therefore, this method was not sensitive to the level of cross-linking. High-resolution 1 H NMR for the characterization of cation-exchange resins was described as early as the early 1970s. − , In these experiments two lines are observed for water-swollen cation-exchange resins (sulfonated S−DVB copolymers). One of the lines was due to water outside the bead and the other due to water inside.…”