Sorption of water in hydrophilic polymers—I Sorption isotherms in copolymers of hydroxyethyl methacrylate and hydroxyethoxyethyl methacrylate

European Polymer .Iourna[. Vol. 12, pp. 123 to 127. Pergamon Press 1976. Printed in Great Britain.

SORPTION OF WATER IN HYDROPHILIC POLYMERS~I SORPTION ISOTHERMS IN COPOLYMERS OF HYDROXYETHYL METHACRYLATE AND HYDROXYETHOXYETHYL

METHACRYLATE

J. SV~TLiK a n d J. POUCtlL'~' Institute of Macromolecular Chemistry, Czechoslovak Academy of Sciences. 162 06 Prague 6, Czechoslovakia

(Received 26 April 1975) Abstrac~ Sorption isotherms of water vapour were determined for crosslinked poly-2-hydroxyethyl

methacrylate (PHEMA), poly-2-(2'-hydroxyethoxy)ethyl methacrylate (PHEOEMA) and statistical copolymers at 35'. In the case of PHEMA the amount sorbed does not depend on the porosity of structure: sorption is influenced by the crosslinking parameters only at higher activities. The isotherm of PHEMA is S-shaped, while that of PHEOEMA is convex; at lower activities, the sorption in mol/mol is higher in PHEMA than in PHEOEMA, although the former has a lower content of polar groups per monomer unit. It seems that the differences between isotherms could be explained by the fact (in principle identical with Kargin's hypothesis) that PHEMA is in the glassy state at the temperature of measurement, while the state of PHEOEMA is viscoelastic. The sorption data were used to calculate the parameters of the B.E.T. equation modilicd by Anderson; the concentrations of the sorption sites thus determined do not oppose the view that strongly bound water molecules are sorbed between two hydroxyl groups. The dependence of the Flory Huggins interaction parameter 7, on the volume fraction of the polymer exhibits a marked change of slope at a concentration roughly corresponding to the water content needed to transform thc polymer from the glassy into the viscoelastic state at 35L The Zimm clustering function indicates, at a higher water content, a considerable tendency towards clustering; however, for samples in the glassy state and at low amounts sorbed, this function assumes negative values, suggesting mutual isolation of the molecules of the sorbate.

INTRODL'CTION

EXPERIMENTAL

Most of the useful properties of the hydrophilic polymers derived from methacrylic acid glycol esters are directly or indirectly due to their capacity of water sorption [1]. The interaction of these materials with water has been studied by various methods [2-6]. T h e r m o d y n a m i c measurements of equilibrium water content depending on water v a p o u r pressure can provide data of direct practical interest a n d also contribute to the elucidation of the character of the waterpolymer interaction. We now present results on the equilibrium sorption of water vapour in copolymers prepared from 2-hydroxyethyl methacrylate (HEMA) a n d 2-(2'-hydroxyethoxy)ethyl methacrylate ( H E O E M A ) with additions of ethylene-l,2-dimethacrylate ( E D M A ) a n d 2ethoxyethylene-l,2'-dimethacrylate ( E O E D M A ) as the crosslinking agents. The results are evaluated on the basis of three unrelated theories. The Flory-Huggins equation is a product of a typical quasi-lattice theory of solutions in zero a p p r o x i m a t i o n ; the B.E.T. equation is based on the view of a localized multilayer sorption; consequently, b o t h theories are simplified complementary views on the water sorption in hydrophilic polymers. The clustering function originates in the general statistical theory of binary liquid mixtures, and is therefore independent of any model views,

Preparation oJ' polymers Copolymerization was initiated with azobisisobutyronitrile (0.2 0.6 wt 5o) and performed in sealed ampoules with nitrogen at 60 ~ for 8 hr. Feed compositions are given in Table 1. The polymers were extracted for 2 days with boiling distilled water until the analysis of a concentraled extract by means of a flame ionization detector proved the absence of dissolved organic compounds. The sample M S was prepared by polymecization bctween plane-parallel glass plates, with aqueous ammonium persulphate as initiator. Determination of swellim,t in liquid ~ater Samples in the form of flat plates. 2 mm thick, were dried to constant weight and equilibrated at 35.00 + 0.(35 with distilled water in a thermostat. Then the surface of the sample was dried, and the equilibrium degree of swelling was determined by weighing. Determination o[ sorption isotherms The sorption isotherms wcrc measured with a gravimetric sorption apparatus using six quartz spirals (manufactured by Precious Turnov. sensitivity 2-5 mg/mm and maximum load 1 g) and a mercury manometer. The system of tubes with spirals and the storage cell of the liquid sorbate could be separated from the rest of the apparatus by mercury valves. In the space between the spirals and .the mercury valve, a bundle of copper wires was placed 123

J. SV~TLiK and J. POUCHL'L

124

Table 1. Characteristic of samples Sample

wla

w2a

Wl a

w2 a

Sb

Tg

c

Md

Md

cd

kd

(~/a) (=o1/=ol) M

99.6

O

0.4

O

0.55

1OO

e 28

71.5

27.4

0.7

0.4

o.75

75,

c 50

49.2

49.6

0.5

0.7

1.Ol

55

0.043

O.31

4.9

0.86

0.036

0.29

4.3

o.91

o.o41

o.35

2.4

o.91

C 80

19.5

79.Z

0.2

1.2

1.51

28

0.049

0.45

1.5

0.89

C i00

0

98.5

0

1.5

1.97

ii

0.048

0.47

1.35

0.90

M-T e

99.6

0

0.4

0

0.71

0.042

0.28

4.9

0,87

M-S f

98.5

0

1.5

0

1.70

0.042

0.30

5.4

0,85

-

aMonomer content in the polymerization mixture in wt%: w 1 HEMA, w2 HEOEMA, w; EDMA, w~ EOEDMA. bEquilibrium swelling at 35 ° (g of water per 1 g of dry sample). CCalculated by interpolating dilatometrically determined [14] dependence of T9 on copolymer composition. dConstants of the B.E.T. equation modified by Anderson [12] °Monomer diluted in a ratio 1:3 with triethyleneglycol monoethyl ether. fMonomer diluted in the ratio 1:3 with 10% aqueous solution of ammonium persulphate. as a protection against penetration of mercury to the spirals. The elongation of the spiral was measured with a cathetometer with an accuracy of +0.02mm; the same device was used to measure the mercury level in the manometer, to __.0.03 mm. The tube space with the spirals was equilibrated with an air thermostat at 35.2 _+ 0-2°. The lower narrower part of each tube containing the hanging vessels with the samples was immersed in a water thermostat at 35.0 + 0.02 °. Sorption was measured with thin polymer cuts prepared from the polymers on a slide microtome with vertical knife displacement 0.02 mm. A polymer sample weighing 0.1 0.2 g was placed in a glass basket about 0.1 g in weight and hung on a spiral. On sealing the tubes, the samples were additionally dried at 50° in a vacuum of 10 _4 10-5 torr for 5-7 days to constant wcight. The vapour pressure was then adjusted to the required value by connecting with the storage cell containing the sorbate. The sorption value thus measured was regarded as the equilibrium value if repeated readings of the spiral did not differ by more than 0'04 mm during 3 hr and did not present a continuous run. Except for sample M, the measurements were started at the saturated vapour pressure by the desorption procedure.

points of the isotherm M is shown in all three diagrams. The sorption b r a n c h of the first measuring cycle was obtained only for the sample M ; it differs considerably from the following branches. The desorption b r a n c h of the first cycle has the same shape for the samples M and M - S as the sorption a n d desorption branches of further cycles. O n the

%

RESULTS AND DISCUSSION

Sorption isotherms in PHEMA samples The samples of H E M A E D M A copolymer differ not only in content of the tetrafunctional m o n o m e r , but also by the conditions of their formation. The sample M was prepared in the absence of diluent; the sample M - T was prepared by polymerization in the presence of an excess of triethyleneglycol m o n o e t h y l ether, a diluent miscible with b o t h the m o n o m e r a n d the polymer. The polymerization of the sample M S proceeded with a higher content of the crosslinking agent in the presence of such an a m o u n t of water that the microseparation of phases took place (supported by a high concentration of persulphate), giving rise to a heterogeneous spongy gel. Figures 1a~z show that the sorption isotherms of water differ only slightly for the three samples; to facilitate comparison, a curve plotted graphically t h r o u g h the

0"2

0,4

0"6

0-8

0"2

0 4.

0.6

0"8

Fig. 1. Sorption isotherms of water in samples of PHEMA at 35°; a--sample M, b~sample M-T, c--sample M-S. N = amount sorbed (g water/g polymer), a = activity of water, 1st cycle: O- sorption branch, -© desorption branch, 2nd cycle: O= sorption branch, =© desorption branch, 3rd cycle: O - sorption branch, =O desorption branch. Fig. ld. Sorption isotherms of water at 35 °. Full line--sample M, @--sample C 28, (3--sample C 100. In Figs. l a s t the full line represents the data for sample M, ignoring the data for the sorption branch of the first cycle.

Sorption of water in hydrophilic polymers 1 other hand, sample M T exhibits a small but distinct difference between the courses of the sorption and desorption branch also in the second cycle. Probably no real internal equilibrium has been achieved during measurement owing to long-term relaxations [7]. At lower activities of water (up to a = 0"65), the isotherms of the samples M, M S and M--T are virtually identical. Thus, in this region of activities, the structure parameters of the network do not become operative. In the region of higher activities, the sorption exhibited by sample M T is somewhat higher and that exhibited by sample M S is somewhat lower than the sorption by sample M. These differences can be explained by the effect of the different structures of the polymer network, such as crosslinking density of network and dilution at the moment of formation. The assumed influence of the surface adsorption and capillary condensation of water vapour in the pores of the heterogeneous sample M S within the range measured, i.e. up to a = 0.95, was not reflected either by an increased sorption or by a hysteresis of the sorption isotherm.

Comparison of the sorption isotherms of PHEMA and PHEOEMA The P H E O E M A has a higher content of polar groups per monomeric unit than PHEMA. Unlike PHEMA it is therefore soluble in water and. if crosslinked, swells to a larger extent. On the other hand, its sorption capacity for water vapour is higher only at high vapour activities (Fig. ld); at lower activity values. PHEMA sorbs more vapour. The latter also has an S-shaped isotherm (i.e. concave at lower activities) while the isotherm of P H E O E M A is convex over the whole activity range. The sorption isotherms in the HEMA and H E O E M A copolymers form a continuous transition between the homopolymers. With decreasing P H E M A content, the position of the inflexion point is shifted towards lower activities: for the sample C 80, no inflexion can be seen. According to the commonly accepted views, water sorbed in polar polymers is bonded by at least two mechanisms: (1) strongly bonded water probably forms hydrogen bonds with polar groups (sorption sites) of the polymer; (2) weakly bonded molecules of water are frequently regarded as more mobile; according to some authors, they are bonded by means of strongly bonded molecules. In our polymers the sorption sites are undoubtedly represented by hydroxyl groups, as indicated by the fact that the sorption of water in poly(alkyl methacrylates) is lower by an order of magnitude [8]. It is therefore desirable to compare also the sorption isotherms expressed in moles of water per mole of O H groups. But even then the mutual proportion of both isotherms remains qualitatively the same. It seems that the higher hydrophilicity of P H E O E M A is reflected in its stronger affinity towards loosely bonded water, but not in a higher sorption ability of the strongly bonding sites. This fact can hardly be explained on the basis of chemical structure; everything seems to suggest that one has to consider the difference in the physical states of polymers [9]. It follows from Table 1 that, at the temperature of measurement, P H E O E M A and the copoh m c r with 202g HEMA are in the viscoelastic state

125

while the other polymers arc in the glassy state. Consequently, during isothermal desorption at 35', the latter pass from the viscoelastic into the glassy state. The consequent freezing of the thermal motion of segments means that the polymer cannot assume the equilibrium configuration and equilibrium volume during further desorption. In this way, sorption sites can arise having a higher energy and a higher sorption affinity than those of a viscoelastic polymer. For sample M (PHEMA) the sorption branch of the first cycle was also measured: this branch is convex and similar to the isotherm of PHEOEMA. In this case the measurement was immediately preceded by prolonged drying at an elevated temperature, and the possibility of an equilibrium re-ordcring of the chains after loss of water was better, so that probably no stronger sorption sites were formed. The suggested explanation of the difference in the behaviour of the polymers and the hysteresis of sorption in PHEMA coincide in their main features with the views expressed by Kargin and Tager I-9, 10] concerning the relationship between the packing density of the polymer chains and the physical state of the polymer on the one hand and the sorption c:~pacity on the other.

Interpretation i~ terms Of multilayeJ so~/,tio~ o,7 sites Sorption of water on polar sites is most frequently described by the Brunauer Emmett Teller (B.E.T.I equation tor multilayer sorption. We have used Anderson's modification [12].

cka N = M {1 - ka)[1 + (c - ilka]'

{1)

Here, N gives the amount sorbed per unit amount of polymer, a is the activity of water (relative humidity), M indicates the concentration of the sorption sites, c is the ratio of the binding constants of a molecule directly bonded to the site and of that bonded indirectly, k expresses similarly the ratio of the affinity of indirect bonding to that of condensation into liquid water. On putting k = 1 one obtains the B.E.T. equation for the infinite amount of sorbate layers. &s the latter predicts an infinite value of N for a ~ 1, Anderson's modification is preferable. The value of k smaller than unity reflects the unfavourable interaction of water with the hydrophobic medium offered by the nonpolar parts of the polymer molecules. Equation (1) is also simpler than the B.E.T. equation for a finite number of layers and describes our data more adequately. To evaluate the parameters M,c,k from the sorption data, equation (1) can be linearized as follows: el N(I - ka)

....

1

ckM

c+

eM

1

a.

(2)

The proper value of k can be found by optimization. We used our data from a = 0"2 to a = 09, i.e. over a much wider range than the use of the original B.E.T. equation would have allowed. The values of the constants thus determined are given in Table I. Equation (1) with these sets of constants describes experimental data for a ~< 0"9 fairly well. and for 0"9 < a <: 1 approximately. However, the sorption values for unit

126

J. SV~TLiKand J. POUCHL~'

activity calculated from this equation are two to five times lower than the amount of water absorbed at the equilibrium swelling in liquid. The simplicity of Anderson's equation depends on a number of assumptions involved in its derivation. The invalidity of these assumptions in real sorption systems may mean that the parameters e,k,M determined from experimental data by means of Eqn. (2) do not possess their original simple meaning arid can best be considered approximations. Nevertheless, in a series of similar systems, for instance in the series of copolymers investigated in this work, they yield valuable material lor an approximate comparison and for the estimation of trends. The value of M = 0.47mol/mol found for PHEOEMA (cf. Table 1) roughly corresponds to the occurrence of one sorption site per two monomer units. The conclusion that one molecule of water is bonded to two monomer units of the polymer was also reached by studying the effect of water on the low-temperature relaxation spectrum of various polymers of the glycol methacrylate type [5]. The same stoichiometry is assumed for the sorption of water in polyamides at low activities [11]. However, the value of M found by us for PHEMA is much lower than 0'5 mol/mol; it is possible that not all hydroxyl groups participate in the sorption. A similar effect (with a similar explanation) has been described to a much larger extent for the sorption of water in other hydrophilic polymers, e.g. in partly crystalline poly(vinyl alcohol) [ 13]. While the M(mol/mol) values exhibit a slight but distinct decrease in going from PHEOEMA to PHEMA, the constant c considerably increases. This increase reflects the already discussed transition from the convex to the S-shaped isotherm, k is for PHEMA distinctly lower than for PHEOEMA and for the copolymers; this difference can be assigned to the higher hydrophobicity of the medium in PHEMA.

The Flor~Huggins interaction parameter The Flory Huggins equation is based on the assumption of random mixing and thus it does not account for the sorption of the solvent molecules to binding sites and for the clustering of the sorbate molecules. If g is so adjusted that experimental data satisfy the Flor~Huggins equation, we obtain a quantity which is markedly concentration-dependent, and is a result of a rather involved assembly of various factors. On the other hand, calculation of the concentration dependence of Z from the activity data is a useful mathematical transformation which can be employed for an analysis of the operative effects and for the detection of anomalies. The densities needed for the calculation of the dependence Z ((02) from our isothermal sorption data were obtained from published experimental data for dry copolymers [14] (cf. Table 1). The contribution of the elastic deformation did not greatly affect the calculated value of Z and was neglected. As can be seen from Fig. 2, for PHEOEMA and for the copolymer with 80~o HEOEMA, the parameter X increases monotonically with polymer concentration over the whole range of experimental data; on the other hand, for other samples, there is a pronounced maximum,

1"6

1"5

1,4

1.5

1.2

I'1

0-85

0-90

0"95

÷2 Fig. 2, Concentration dependence of interaction parameter. Samples • M, ~ C 28, O C 50, ® C 80, © C 100. so that in the region of high (02 the dependence Z ((02) has a negative slope. The maximum appears for PHEMA at ~02 = 0'9; by interpolating the data in ref. [15] we can see that at 35 ° the glass transition takes place just at the above (02. At approximately the same concentration, the course of the sorption branch of the first sorption cycle ceases to be different from that of further cycles. It seems therefore that the maximum on the ;~ ((02) curve corresponds at least roughly to the glass transition. With increasing content of HEOEMA in copolymers, the maximum is shifted toward higher (02 until it disappears completely. Such a shift can be expected from known To values of dry copolymers (Table I). A similar maximum appears in the concentration dependence of the parameter Z in the system semicrystalline poly(vinyl alcohol)-water [16, 17]. Chuang and Morawetz[18] studied the sorption of water and benzene in copolymers of acrylamide and styrene at 25 °. At the beginning, the parameter 7~ (calculated from the sorption data of water) increases with (01 for all copolymers; for most samples, a bend is found followed by a horizontal course.

Clustering .fimction An interesting analysis of the sorption data is based on the Zimm clustering function Gll [19,20]. The expression nlGII/V= (01G~i/vl (where nl is the number of molecules of the solvent, v1 is its molar volume) indicates the quantity by which the average number of molecules of the solvent in the surroundings of a given molecule of the same kind exceeds the value expected for a completely random distribution of molecules. The G 1Jv I values higher than - 1 indicate a tendency towards clustering of molecules

Sorption of water in hydrophilic polymers

J5C o ~

-

IOC-® >~ ~'~

5o

~D

o

-5c _ _ ~.00

0.95

0.9o

085

0 80

cpa

Fig. 3. Clustering function

G~~/'vv Samples •

M, ® C 28.

e C 50,® C 80, O C 100.

of the sorbate; on the other hand, if GI1/FI < - 1 . the sorbate molecules are isolated from each other by the polymer medium. Employing the Kirkwood Buff general theory of solutions, Zimm [19] derived an equation which correlates the clustering function with the concentration dependence of the activity of the sorbate:

G"/~"

=

_F /01n,p,\ ~ (1/'P')/'P2~ ~lna)r,p -- 11 .

(3)

By substitution, from the Flory Huggins equation we obtain Gll/Vl

=

1

-

2 Z 4- if q)2 2X~P1 - Z'{pl¢P2

.

(4)

In the case of PHEMA and similarly behaving copolymers, it is obviously the value of the derivative 7 / = 6Z/&o2 that has a conclusive effect upon the sign of, G~/v~. Indeed, Fig. 3 shows that at low % the clustering function indicates mutual isolation of the s o l bate molecules; at a higher amount sorbed (in the region of the elastic behaviour of the polymer) cLustering sets in. The same is true for copolymers having a high HEMA content; for PHEOEMA and a copolymer with 80% HEOEMA, molecules of water tend to clustering over the whole concentration range. CONCLLDING REMARKS Comparison of the sorption isotherms of both polymers, PHEMA and PHEOEMA, and of their statistical copolymers provided evidence for the effect of the glassy state on the sorption capacity of hydrophilic polymers. Our data suggest that the bend in the Z (~P2) curve is related to the position of the isothermal glass transition on the concentration axis. The presence of a bend is in accord with the accepted statement that the second derivatives of the Gibbs function exhibit

I

127

discontinuity at the glass transition point. A sudden change of heat capacity, thermal expansivit? and compressibility with temperature or pressure arc known characteristics of the glass transition (cf. lbr example[21]). In the polymer diluent svstcm in which the polymer is below Tq. a suddcn change of partial enthalpic changes AH ~. and AH, with composition has been observed [22]. along with a s t d d e n bend in the isosteric dependence of thc logarithnl of vapour pressure (or activity) on rcciprocal temperature [23]. corresponding to the discontinuitx on the temperature dependence of the heat of dilution Alt t. One is therelore quite justified to cxpect (of. Rehagc's theoretical considerations [24]) also a sudden changc of the dcrivalive (Fin a/&p,)~ as a function of ~?:. and thus a sudden break in thc curve )%0~}. 1-he bend in the sorption isotherm. N - / ( a ) , thc cxistcncc of which is a logical consequence, is naturally much more difficult to discern without measured data being d e f n e d with more precision (owing to lhc larger curvature of the isotherm). Acknowh,d#ement.s Wc are indcbted to I)r J. Pi=ibilov(i (Mrs) for the design of the measuring l.lpp~lr~lltlS 011d to Dr A. Zi','n5' for assistancc in computing.

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