Drying and water uptake of poly(acrylic acid)lithium salts with different degrees of neutralization were studied. The slowing down of drying speed in solid state was not due to Tg but due to the difficulty of the release of water molecule bound to two metal carboxylates or carboxyls. Though the equilibrium water uptake increased with increasing neutralization %, the slope of the plot above ca.3396 neutralization was greater than that below this neutralization %. The enhanced water uptake was explained by the appearance of vacant sites in octet coordination of lithium ion

Hiraoka Kyoko

Hiraoka, Kyoko

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Hydration and Dehydration of Poly (Acrylic Acid) Lithium Salts

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REPORTS OF THE FACULTY OF ENGINEERING, NAGASAKI UNIVERSITY,No. 17, July 198199          Hydration and Dehydration of Poly (Acrylic Acid)           Lithium Salts                                        by                               Kyoko Hiraoka*           '                                         tt                                      '                    ' Drying and water uptake of poly(acrylic acid)lithium salts with different degrees of neutralizationwere studied.The slowing down of drying speed in solid state was not due to Tg but due to thedifficulty of the release of water.molecule bound to two metal carboxylates or carboxyls. Thoughthe equilibrium water uptake increased with increasing neutralization %, the slope of the plot aboveca.3396 neutralization was greater than that below this neutralization %. The enhanced water uptakewas explained by the appearance of vacant sites in octet coordination of lithium ion.                                       '                    'INTRODUCTION   As a part of the investigation of the properties ofion-containing polymers, it became desirable to knowthe hydration of ionic sites which will stronglyinfluence the electrostatic interaction between them.The preceding papers in this seriesi2) have shownthe drying process and equilibrium water uptake forpoly(acrylic acid) sodium salts(PANa) and potassiumsalts (PAK). In the present paper we wish to callfurther attention to the hydration behavior of poly(ac-rylic acid) lithium salts(PALi).EXPERIMENTAL   PALi with degrees of neutralization varying fromO% to 100% were prepared from poly(acrylic acid)(PAA, MW=100,OOO) according to the proceduredescribed beforei) .   The drying process of PALi at 100℃ underatmospheric pressure was followed by weighing themat appropriate tinie intervals.   Water uptake of PALi at 25℃ in an atmosphereof 73% relative humidity was followed by weighingthem at appropriate time intervals. A sensitive spring                               'balance made of thin phosphor bronze wire was used.The method and the apparatus used here werevirtually identical with those described for PANai) .Received April 28, 1981.'Department of Materials Science and EngineeringRESULTS AND DISCUSSION1.Drying process    Drying process of PALi is shown in Fig.1. At thebeginning of drying experiment, samples were aque-ous solution. At the stage of the second curvaturesin drying curves, all samples were in solid statealready. The rate of drying in solution state was notaffected by the extent of neutralization, indicatingthat the evaporation of free water is not influenced bythe existence of ion. In contrast, the rate of drying insolid state decreased remarkably with increasingneutralization %. These behaviors are quite similarwith those of PANa and PAK. However, residual                       'water levels are different among ion species and donot depend simply upon ionic radius. By consultingthe data presented beforei･ 9) it is noticed that theresidual water level was the highest in PALi, fol-lowed by PAK and PANa, in order of dec;easing level..Fig.2 illustrates the drying curves of PALi 60 and 15(the numbers added refer to neutralization %). Thearrow on each curve indicates the water content whichyields Tg' of 100℃ (drying temperature). Although the   'slowing down of drying appeared to occur when Tgreached drying temperature for high level of neutrali-zation in the case of PANa and PAK, this relationdoes not hold for PALi. As seen from Fig.2, the   '                                   ttt              ''                          ttt                         '100 K. Hiraoka2rloENopes3Hosp･Hqppesost,optbDorl1o-1-2o1OO 60 45 3051 2 3 4 5log t (min)Fig. 1. Drying process curves at 100℃acid) lithium salts with degrees100 percent, as indicated.and 1 atm for poly(acrylicof neutralization from O toHosNopdi3riO 1.0H.'".sts O.5gsoo 5   10time    15     -3x 10   20(min)25Fig. 2. Drying process curves at 100℃ and 1 atm for poly(acrylic acid)  salts. 60%neutralization o ; 15%neutralization e The arrows  the points where Tg reaches drying temperture.lithiumindicaterloHNopdi3Hydration and Dehydration of Poly (Acrylic Acid) Lithslowing down of drying speed occured before thesamples reached glassy state. This observation con-firms the idea that drying process of PAA salts insolid state is not governed by Tg. The level of watercontent where slowing down occurs is around O.5 ofH,,O mol/repeat unit mol as in the case of PANa and -                                           'PAK. The value of O.5 means that dehydration ofwater molecule which is bound to two carboxylates orcarboxyls by means of two electron pairs is difficult.The situation of water molecules and the possiblestructure have been discussed alreadyi) . Also, thetrend that the slowing down of drying speed becomesindistinct with decreasing neutralization % can beexplained as in the case of PANa. Thus, it isconcluded that the drying process of PALi is notgoverned by Tg (diffusion-controlled), but is gov-erned by the release of water moleculefromhydrated                                          'state (dehydration-controlled).''rloHP-rd{qDpesoQoosFig. 3.42o         Neutralization %Equilibrium water uptake at 25℃ and73% relative humidity for poly(acrylicacid) lithium salts with different de-grees of neutralization.2.Water uptake   Water uptake proceeds very rapidly in the initialstage, but becomes slow and reaches absorptionequilibrium. Fig.3 shows the equilibrium water up-take as a function of the degree of neutraliztion.               ium Salts 101                  'Though the equilibrium water content increases withincreasing neutralization %, the slope of the plot   'above ca. 33% neutralization is greater than thatbelow ca. 33% neutralization. This neutralization %where,hydration is enhanced is identical with thatencountered for PANa and is reasonably explained byassuming the octet coordination structure (Fig.4) as inthe case of PANai) .If the coordination sites areoccupied by carboxyl oxygen atoms, one metal ion for6 oxygen atoms, that is 3 carboxyl groups, therefore1/3 neutralization corresponds to equimolar situation.Hence, carboxyl oxgen atoms are in excess when thedegree of neutralization is less thep 33%. Conversely,                        '   ttwhen neutralization is higher than 33%, vacant sitesappear which may be available for coordination of          /,;i<?l/sx.C            tSN  tt lNCtt:6.ii?･..-:..s,-M-s,6･iiO       NN×IN,(stt.;......//XcFig.4.A model for the octet coordination      structure of metal atom.(the state      of 1/3 neutralization is illustrated).oxygen atoms of water molecules. Thus we canunderstand the linear increase in eguilibrium wateruptake above 33% neutralization.   On the other hand, the number of water moleculesper repeat unit for PALi 100 is 4 as seen in Fig.3. At aglace this number seems to agree with the foregoingdiscussion because the number of vacant sites permetal atom of PALi 100 is 4. However there remainssome question. The reported values of the hydrationnumbers of aikaline metal ions are given table 1.According to the results obtained from X-ray dif-fraction, it has been concluded that lithium'ion in aconcentrated aqueous solution has tetrahedral hydra-102                            K. HTable 1 Hydration numbers of         concentratq'd aqueousiraokaalkaline metalsolutions.ions in dilute and       LIonsDilute ConcentratedCompressibility3'`' Entropy3i Mobility 3･4} Cg?.a.r.e.(n,t,M.g,IEIsl)3' NMR3)X-ray5)Li+Na'K+Rb+Cs+4. 5, 5-64. 5, 6-73. 8, 6-73. 02. 554.333. 5,2. 0,3. 5-72. 42. 54.81. 043. 12. 11.61. 044466tion structure with hydration number of 45) . To obtainthe more detailed informations, the X-ray study ofradial distribution function and/or pulsed NMR studymay be promissing.   The slope of the plot of equilibrium water uptakein the low neutralization range acquires additionalinterest For PAK the slope in problem was negative,whereas it "Tas positive for PANa. The slope for PALiis positive and much greater than that for PANa. This                                   'is in line with the consideration that the equilibriumwater uptake in the low neutralization percent isdetermined by the competing effects of the increasingdielectric constant and the decreasing amount of freecarboxyls. The detail of the discussion on this slopehas been described in the preceding paper2) .                                         'ACKNOWLEDGMENT   The author expresses her deep gratitude to Prof.T.Yokoyama for his unfailing guidance throughoutthe course of this work.REFERENCES              '1) HIRAOKA, K. and YOKOYAMA, T. : Polym.                                  '  Buil., 2, 183 (1980).2) HIRAOKA, K. GOTANDA, M., and YOKOYAMA,  T.: Polym. BulL,2, 631 (1980).3) NAKAGAKI,M. ed. "Structure and Properties of  Water" (Kagakunoryoiki Zokan 106, in Japanese),  Nankgdo (1974) p.90.4) Chem. Soc. Japan ed. "Ions and Solvents"･(Kagaku  Sosetsu No.11, in Japanese), Gakkaishuppan center  (1976) p.84.5) NARTEN, A. H., VASLOW, F., and LEVYA :  J.Chem. Phys., 58, 5017 (1973).

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Hydration and Dehydration of Poly (Acrylic Acid) Lithium Salts

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