Gel-water relationships in hydrophilic polymers

(.," 1i Y O 1~ 1 ( ) I O G g qi,("

VoI, 5, N o . 6, t,..~,~

BRIEF C O M M U N I C A T I O N S

GEL-WATER

RELATIONSttlPS

X-ray

Diffraction

IN

of

tlYI)ROPIIlL1C

Fresh

and

Frozen

POLYMERS:

~_,ms"" " ~*"

CL, AR EN C.E ~ I. I!,RLIN G "~

,,

L)~?)('.;'tm(:'nt o.f I"ood S~ ~(mcc a w l "J echr.)to~z~, U,~icer.sitj/ oj (..(dzJorr, m, 1)(~ vi",, (7(~lij(n'~ ia 95~;t1(; w

•

¢

~

])owdI; .-Molim~', ,':~,:l ]{b:~freff repor,,ed t h a t i!..?. ,..!~, ~ • ..... ~¢'*, u .,~ e of ice f o r m e d i ~ l ~e' ~~.Ix " frozen g,ls of gda~.i,~, was rd:~te(! p r i m a r i l y to ,he cohere> tratio~t of ~.1~(> l)olSn~er: hex:~m~a!,. "eo~ .... alone ,,rvst:~llizrd in a 1(i% ,wl .-\1 incr(,asin,_, e(:,ncen~l'tt,l()ll. (ql]}iu, i('f ,..'.ll(] ViIFtOI.1.~. ic0 FtlOf'('F,~iV:']V r 'l:)l:t('e~t hf'x:~(.,]rd i('¢::. In ~,ls of -i0 .tnd 50(,;:` !"OI|(~ql I'::Iti~ll, v(?ry s~l~;.t][ :itltOtlntS o f }l(~2(;tgOll~l[ ia:. u'~r', l;r('~eni...~,t.at.ill t~iglwr (:oncentralions, e,~lv xirrec, t~s it,'.. \\.:~s f o r ~ e d . l're\'i:,l~s s~ulio.t i~ l}~i,~ :..s:cric'8 h:~\(~ })POll (:(.)I1- , , \rarer _ ;~nd corn~'(:/ wiIt~ 1.] m, rcle~i,.~.ship b,,'.xv ,.e.~, l.,,iy~uer a~td ~ilc. st;l~ /-)f w n l e r i.ll ..l(tll0011S c£C]S of "w'u" e:~rI.~c,.',:vn~(!llt\l ootli~lo.~e ((_;.NI(;) \'el\tire. ::~mi :tarc'/~. These,. l'(,kttio~l.~hip~ ]rove be(:n invc.s~w.t~e~l }~v :~tt:tlxsi~ of sc, r p~ion l.~rope,',ies. ~: .~ l:qear ]lltt~ll(,tl t'{..SOll,tl]c(~ °~' and (tielc,(:Iric ~
. . ' a

"

~i[ET~IODS Ea:perbr~.ent(~t m a t e r i a l s . T h e p o l y m e r ,,na~erials a n d the m e t h o d of p r e p a r a t i o n of the gels were d e s c r i b e d eaylier, u' ~ Suffice it ~.o remark tha, t the following concerti r a t i o n s were m a d e : agar (10, 20, 30, 4 0 % ) ; C M C ( 1 0 % ) ; gelatin (t0, 20, 30, 40, 50, 6 0 % ) ; s t a r c h (10, 20, 30, 4 0 % ) . T h e gets were p o u r e d into m o l d s w i t h c y l i n d r i c a l holes, 0.2 e m in d i a m e t e r bv 1.0 em R.eeeived N o v e m b e r 19, 1968. * This study was aided financially by the N a tional Science Foundation ( G B ,t782) and the Generai Foods Corporation. T h e autlmr is grateful to these organizations for this help. 393

,

-

1r ]ong ::t~t(.l athm'ed ~o t~and ]. ~tay u n d e r a dfiek l:,v:,r, of the ~,.~m(,. ._,ml. i m m e d i a t e l y . , before use, lhe ]~t,~tcetl ' ~ ' " v ,¢ ta\,:t . ai~ove ~]1(.. m o l d u".aa._t r i m n l e d off, ,tim the eylindrie:~l sp(~ch-nen was carefilllv l m s h c [ out of the lnold, w r a p p e d twice w i t h p o l y e s t v r ,.l~Ivlar, . , ])tl]hmt). iilm 1 !~, thick, whic, h was rlmn sc:tl,(t :at b o t h en(l~, a n d plaer:l ilt tt ,..~tl~rap]o ] l o ] d e r lnolli~tc'(l hi a g(mio~ne~er head. X - r a •, !., l e c h n i q , e . _~ o.,~ ", -'" • ~l . [~.t,_ulcn~ents of dlffractt, i m e n s i t v were m a d e wit]t .t S r t ) p e r d i f f r a e t o m {~ -..ira, wh(~se 20 a r m heh! ihe I)robe of u H a m n e r scintillatiot~ c o u n t e r . . u - K a : radiation was p r o duc.e~l "ox" reflecuon of c o p p e r r a d i a t i o n f r o m " ~he {002~ l~i:me of -~. et\..t,.t of t)ent:wrv~hritot." D i f f r a c i e d :lil,.t.l'l,_ltV , , ,: .. recorded frolI1 20 \VLtS 7.5 to 20 ] 3 5 . 0 ° a t a r a , e o f '2 ° (20) per rain, wi~l~ :t rate c o n s t a n t of 4: see. ( F o r repr()dueticm of fi~m'c,s the rqte e0llSlg:l.nt w,3s i / I c r o e t s o d 1~() 24 see, J(.:: l l l , n g in a slig'ht shift (~i"1Im diffractogram. !OW,'.Ir([s ]'1 rger ~~nglet.) C o n t r o l readings f r o m rolled M y l t u " fihn w i t h out ,.,..lo,~ ~ave the b a c k g r o u n d correction. , for t e a t tering b y air a n d b y the film. P i p e t t i n g of w a t e r into the. l m n e n o f the rolled fihn permitte(1 a recording of the diffrae~ogram of w a t e r u n d e r the t a m e conditions. A f t e r corrections were m a d e for absorptior~ (at nffeeted b y s p e c i m e n geometr3 # a n d comp e t i t i o n ) an.t, ' r for polarization, radial ' distribution f u n c t i o n s were e a l e u m t e d ,o on the basis of a c o m p o s i t e n o n h y d r o g e n a t o m (i.e., a t o m of a v e r a g e a t o m i c weight) for w a t e r a n d for the 10% gel of each p o l y m e r . ~ "tm e o m p o t i t e atom for wa.ter "done i n c l u d e d the o x y g e n a n d h y d r o gen a t o m s a n d hence h a d a n a t o m i c w e i g h t of 18.016, while the c o m p o s i t e a t o m in a gel w a s a s i m i l a r n o n h y d r o g e n ai.oln w i t h :m a v e r a g e a t o m i c w e i g h t which d e p e n d e d on the compost,

.

.

394

C. S'I E R L I N G

,ion of tt~e t m l y m e r (about; 1T.7). Each calculation was based on the ave.rag'ed imensities for t'ou r diff.raeto~,'ams. 3h.:thod o f f r e e z i n g ( ~ . After .the diffr:,{:togram of a ~e.,_h gel was recorded, t.hc eel was frozen in situ bv eooting 1o --4.7%; i.u a su{.,m of ~fitro~en ,,"is Tim _.as stream wn~ produced bv e,)utinu-fi boiliw~ of liquid ni~r{}~en with .'~n immersed, heater. [h.: v'~por t}asse(t through two D e w a r tubes, in the second of which n h e a t e r {.{mtr{}lhT,d ',:he l e l l l l ) e r t l l l l r { . ; {Jr Ill{?, {:merginff {xhn t~,{al strea~m. An ou {i b.~flt'{., forll~otl [}V ;f. {,oncentrie a n n u l a r s l r e q m of ht.,tl{d dry nit.rogon rots. helped l)rev(.,nt eond{.i~sati{m {;I' moisture - • ,',,e on the-p{.e~,~._n. The .1.';tt.t~ {}f eg,.>lin~ was :d}{)~l --30~C per Iniil for [he 1..-.:.i nlill aIld decro'lsed ~,-} - - 4 ° C t}er miu ,'it the.-tilt rain. when dm l e l l l ] } { ~ r ; i t i r e_ h a d reache(l n D. ] } r o x i l I ~ : : ~ t{,tv " ' , --:;B ° C .*

e

~-~

• ~l t

e

, .

]{ ~.:su L'rs

Figure. 1 shows part of a n X - r a y ,.liffr~'w.tc)eran~ of ~lm 10% get of gelatin ,,rich ihe wide a.l~ffl{~ peak.s. T h r e e broad p{,aks o(:.eur :~t tlm same positions in the fresh gels and in w n ~ e r 20 (Cu-Kee) values of 11 , _ ~ . o , and 39.5 ° T h e e, - is quile l o w , and the second is ranch firs~.. p..,~k hi~her thall the t h i r d . " Fhe,,e. ~-" peaks are essentinily noncrystalline, and the distribution of inte]]sit v in ihe diffraetograrn is most reasonably i~}*erl}rete{t as representin~ a radial distribut::i{m function t)ase.d on qn atomic (ele{trcmie) :~rr a n g e m e n t whic'h is statistically similar in the VICIlllEV •

"

"

.

of

{'.';tell a l , O l l l .

A

S l i ! ] T l l l l ( | r V O f l.}l .. c , . d b _ l .

'

0

"

"1

lated radial distribution function is given in I a b l e 1. The most distinctive peak in ,hi.-,. timedon for each s a m p l e is in tim r a n g e of 6 atoms ar 3.0 ,0 o.oA a n d the nex~ most m,trked one of 25 a i o m s at a b o u t 5el. (These v,'flu{:s are sliglltty larger than those obtained e'n'lier ~""' :'~ for water and m a y be due to the relatively Ial ~{1 movem e a t of t h e s c a n n i n g arm. A. calculation of d]eo-

retieal intermoleeular distances for w£~ter at o ~ives 3:49:* for. hexagonal dose peeking and -oO- o ka . . . o.11.~ for a. eube side in simple cubic paeking. All evidence agrees that the coordination number is greater than 4.) Very much smaller peaks *a

oc m~ rather a.t 0.7 a n d 1.5A or a t 1 . 1 i .

When t h e concentration of the gel is inereased, file indicated peaks gradually beeome less prominent, and the X-ray pattern eventually appears quite amorphous {~!g-"-)-2k shift, of the major peak of 3 "o =° (20) toward 8. lower angle aeLJ ",J ~o

.

()'

companies the increased eormentr:nion, b u t beca.u..c " ~, the ,,.t~mfieatu..c. ~ " "-. ,, of tt,e~o ........... tR,.~k,, ,. "~ as sucll is unknown, the meanii],~ of lhe shift, is obscure. T h e X - r a y diffraction l.mtt, ern of a 1.3I.u.al frozen 10% gel is s l m w n , in F i g u r e 3. Tall, aeuleat.e peaks ocm~r, in llw positions -ind with .o

the intensities_ of hexagonal. . . . . . ice/' and iho. ain,~}rphm~s b a c k g r o u n d is greatly dirninishod. Tho identifiable refleetions of hexa.,,z{mal ice are given in T a b l e '2. I n frozen .zels of other ool~centrathm& the refleelion peaks, of ice ,~.~,'o. lower m~d 1 Io.~l. 2, d m h i g h e r tim eovlcentrnlion of the ~_I. , ] n ,10% gelatin t r o d ~t,t:c] . . . . . . . ] mid in P,0':; a~ar (l:iv. 4), l he •~enera] I}o.. ,~elaHn .,.z,'ol (Fig. !i} shows no exidc~t crxsie~llino poat,:s. s , th;.t! it:-: res{,mt}It, ncre "i.o ltle fr,,~sh g{'] of ~{mi c(meen~ra:i{m (Fi,z.. 2} .is slriking, f l o w , v e t , l.}m ~iwreqs,. in opacity cry this gel {m freezi~g in(liea~{.s ltla~ ice is indee,t l)roduc{'d, ai~d twrh:tl)S it nifty l}e a{'eor{ted ~,]m r.lesi,~lm~iol~ of :t~m,r1}hous, i.e., vilrem~s, I.Le .. }

"

'

" D

"

.

O'D

.

I2} [ S C

t.raSU-3N

It: is tme,{,.tuL., l h a t there is a posilioi]:d dif)',,:renee in the radial d i s t r i b u t i o n r'un(:~ions o( ,.'~pproxinmt(ly 0.3 ,-~ between Ihe m.tjor peak of C' and that, of 1he other "" , , ,-..aln-" a~ar and t-~"\ ..,. T,.._, ~h r ~-,e ,' "-~ nei¢lfl:)or distanee in the fi~,..t. ])lOS.T h e D_{.tl(._.t, . '~ o-el~ EWO .~ ..... is thus 10;,~: closer i h a n in ge]atin, star{.h, "rod wa,rer. If this smaller distance be i n t e r p r e t e d as an indication of a stronger m e ]ocular at~lac, " "' ' tion [.mtween p o l .y m e r an.t , c w:~ter (and between w'tter and w a l e r ) in the first tamrod gels, it; wou.ld agree, closely wiih similar evidence f r o m wate.r v a p o r sorl}timl," nuclear m a g n e t i e resonance, '= and dielectric dispersionY-' Although it, is t e m p t i n g to regard the p e a k at, t.t

1.5 ~'\ as representing the averaged C C (1.54 }~.), C - - N (1.47 /\), mad C ~ O (1.43 2~) bond lengths in the p o l y m e r molecules, tl~e existence of

a small pe'~k at 0:7 ~ suggests an alternate interpretation. The peaks at 0.7 and 1.5 :-'.k may be ripples in the radial distribution function, due to errors in the reF.~tively early tennination of the Fourier series. The same interpretation may •t p p l y to tim 1.1 e~ p e a k in C M C a n d t h a t found

by Danford

and Levy ~ in water. This

sug-

(;I:L-WAT]'~tl

IIELATION~HII

~

,,,1,~

/P\

/ /'/

t

/

/

\ '\

\

/

\.

J

1

IO

2t'

5O

30

\

t l

. . . . . . .

\

l

_

\,\

/

~_i/. - .~ . . . . .

~

,

,

~

. . . .

~

.

.

.

.

. ~ .

.

.

e

:

I:ias. 1 a'o 6. X-ray di~'~ractograrns of various gels. Abscissa is graduated in degrees~20 (Cu-I,2a radiation), wllh le]t ~nargin ai~ about, 2.5°-20. I,'m. 1. Gelatin, 10cA., fm,sh. Fro. 2. Gelatin, 60%. fresh. FIG. 3. Gelatin, 10%, frozen.

I,'rr:. 4. Agsr, 30%, flozen. FIc. 5. Gelatin, 50%, frozen. l~ic,. 6. Gelat,in, 60%, frozen.

gestion is strengthened by two facts: the radlai chst,ribution function here deehnes slightly be]ow zero at about 2 e~, as does t,ha~, of Danford and Levy' at about 2.4 7X; and there is a second small

peak at 2.2 7k in Danford and Levy's curve. There is no X-ray evidence for 1Lhe existence of crystalline or quasicrystalline st ructures in water above freezing temperatures.

396

C. S T E R L I N G

Oil the whole, the X - r a y observations of Dowell et, al? are confirmed. Hc)wever, (he existenee of cubic ice 'is -t real feature of '•slowly" frozen (1.e., eoohng rate less thim 50°C per see) gels of intermediate to high eoimentration does 1 nob seem re:lsonabte. I f i(, be asmimed (lint the 1)olymer molecules are essentially elongated TABLG 1 I~.~d)IAL DtS'l'~{ll~trrloN ]'UNCTION o P ( "e t LXi p o s l ' NONIIrDmJGJ,~N ATO:,a iX 10% (IELS

cylinders w i t h a st)celtic votunie of al)out J.0 on;' per ~,° i, h c r i 111 It O"'~<' t l / c ,gel, .'30/<' ' <" Of the vohllne is (wcupled t,3" lheso eyllnders Allowing iho, cylinder, aJI givol'[l~o,_ cross-~eoi lonal ,llr.O" ' • of -O ' ) " ~.'-'~ Clio Cglll calculate that, the nmnedi:tlely. a%om1)
r E,';

ANI) IIVAJPI']It Distance to N e a r e s t Density P e a k

Sample I

I

1[

i

IV

III*

i

o.7

Agar gel CMC gel Gelatin gel 8ta,'ch gel W'uer

9

t l..i

]

3.0

1.1 0.7 1 1.5 J5 < 07 I O.7 1,5

33 3.3

t

l

5.2 49 5,3 5.1 i 5.0 i

* III Is major peak, involving about 6 atoms. PABL 15 2 ~x(-1-LKY I ) t E ~ L E O F I O N S

lle.xagonal Ice

i I

1

t

d (hkI)

I*

1*

I

d 22 8 2.t 2 26.2 30.7 39.8 4,l.0 47,3

0oo)] w (oo2) I vvs

3.90 3.67 3.40 2. (14 2.26 2.0G 1.92'

! i

Oct)! m I

VS

(102) IIt (ll0) w (103) i vs

l

(1 2) II w

l (al )

(220)

S

I11

t

* I is estimated intensity: vvs = very, very strong; vs = very strong; s = strong; m = medimn, w ---- weak.

it

(//

" i -

i

L2

b(' ~llrllll>od

I}1al, lh('

fcw

ol'~dlllZ('
A

..®

L___ ,

Jib .' h - "

">7

36"8

t L'.

.

.

.

.

.

.

.

1('o

c'r\st'[ls will be lnnsil\ raih(,r sin.ill alld lhat, the h:llf-IMghl, lm,,'ldlh_-" of the or);lalliile ln,.l..~, which vary ltlVOlSely wllb eryst:illttv .-lze, u l]l be r<
(2ubic 1,;c (hkl)

,,-

It l l l 3 y

OF ]]EX.YIIONA.1, ANI)

Cw,:c 1c~: (Cu-Ko< [~aDi.~Tio.',') ~°

4

d

lqa. 7. Tetrahedral assemblage of 5 water molecules, showing various interplanar spacings greater than t.80/~.

G EL-W A T E R R ELA TIO NSttI PS this is weakened by the reflection occurring a,t ihe n(/, quite exact half-spacillg of 1.!i)5 :~, similarly 1o the total extinction of tlio 4.52 ~ spacing in lhe eentrttl grouping. N o t e tha, t these 'chi-ee spacings are equivaleni~ ~o tJie n m j o r mlbie ,..pacings (Table 2). ]it is evident, then, /;hat the X - r a y reflections found in frozen gets ::K intermediate to high concentration need not-- and apparel~tly should not ---be iuterpretcd in terms of th(~ structure of cubic ice. These relleetions are more reasonably oxpla.i)ried on. the. b~ sis.. of what lriav., t:,e cal.lc';,(.i a "~",..,.ll3(i\.,,s-t,lllinc" " '" ' ' groul:ling." T h i s is i.lie 1,""),t,~.ic tetra!mdral ::tsseli).l.ilago that is C.OIIIlIIOI] tO both hexa
.

.

.

.

.

~

¢

a

X - r a y diffraclim~ Imtte.rns were obtained fr,,m~ fres]l alid f~t")"zcli ' iiqllOOtl£ ,,'(,l~ of "lKfil', (::LFI)oxv!lie!lly] eelilllose, gelatin, arid sl.areh, lXa(:tial dislri}n.ltioli i'tirloliolis, based ol} 'tli
397

erence to hydrogen and hydroxyl ions. J. Chem. Phys., 1 : 515~54S, 1933. 2. Bradley, A. d. The absorption factor for the' powder and rotating-crystal m,,thods of Xra.y cwstal analysis. Prec. Phys. See. (London), a~, '" • 879-899, 1935. 3. Burton, E. F., and 0liver, W. F. The crystal strucl~ure of ice al; low t,emperaturcs. Prec. tio3'. Soc. [A.mer.], Ida: 166-t72, 1935. 4. Danford, M. 11., and Levy, H. A. Tim structm'e of wai,er at room Ceml:,eraturc. J. ,4rner. Chem. See., &l-: 3!)65-3966, 1962. 5..Dowell, L. G., Moline, S. W.. and Ilinfi'et, A. Y. A low-ternper':tture X-ray diffraction study of ice structures formed in aqueous gelatin gels. Bloc}din. Biophys. Act-i, .50: 158--167, 196_° .

6. Kaizoff, S. X-ray" studies of the moh::cular arrangement in liquids. J. Chem. Phys., 9: 841-850, 1934. 7. ](avanau, . l . L . Water "rod sohli:e-waier interactions. Hohlen-Day, Inc., San l:ranciseo, 1964.

S. /olymers : Thermodyn:m~ies of water vapor so,'ption, a. AppI. Polymer 8ci., 12: 2023-2032, 1968. 12. Masuzawa, M., and Sterling. (.". (,el-water rel:ttionships: t)ielee.trie dispersion. Biopotymers. a: 1453-1459, 196S. 13. Morgan, ,J., and Warren, B. E. X-my anttlysis of water. J. Chem. Phys.. (;: 666-67a. t938. 14. Sterling, C. llapid production of large crystals of pentaerythritot. Noreleo ]{eporter, 8: 52, 195.(/.

15. Sterling. C[. and Masuzawa, M. Gel-water relationships in hydrol3hilic.: polymers: Nuclear magnetic resonance. Makromol. Chem.. 116": I-t0--145, 1968.