Anomalous ‘Freezing’ of water in hydrophilic polymeric structures

Applied Energy 34 (1989) 243-259

Anomalous 'Freezing' of Water in Hydrophilic Polymeric Structures

D. Highgate, C. K n i g h t & S. D. P r o b e r t Department of Applied Energy,Cranfield Institute of Technology, Bedford MK430AL, UK

ABSTRACT Experiments, intended to establish the use of hydrophilic polymeric materials as vehiclesfor water in energy-storage devices, have revealed two anomalous effects during the freezing~melting phase-changes: (1)

the presence of a large difference between the temperatures at which latent heat is taken in, and at which the energy is evolved, i.e. a hysteresis effect in the phase changes exceeding 20°C under some circumstances; and (2) a wide variation in the magnitude of the thermal energy stored per unit mass of the hydrated sample (i.e, of the apparent latent heat of the absorbed water), which varies with the type of hydrophilic polymeric material used, and depends in particular upon the 'free moisture' content of the structure. These effects suggest that the hydrophilic polymeric structure inhibits the normal freezing process, either by mechanically interfering with the formation of a conventional ice-crystal structure, or by chemically immobilising some of the water molecules present in the system by transiently bonding them to hydrophilic sites within the polymer.

INHIBITION OF F R E E Z I N G The inhibition of freezing is of considerable interest with respect to freezing in biological systems, foodstuffs and medical products. In such natural systems, the interaction of water and ice crystals with the surrounding 243 Applied Energy 0306-2619/89/$03"50 © Elsevier Science Publishers Ltd, England. Printed in Great Britain

244

D. Highgate, C. Knight, S. D. Probert

matrix dictates the resistance of the material to freezing and thawing, and thus affects the commercial potential of freezing as a preservation process. However, its study in these complex natural systems is difficult because of the nature of the hydrophilic sites, and the degree of polymerisation of the structure cannot be controlled. The use of hydrophilic materials is being studied as part of a programme to develop energy-storage devices using fluidised-bed technology. In particular, the aim is to develop a heat sink which would work at, or near, zero degrees Centigrade and which would remain operational for several days, without any further external stimulus. It would have applications in air conditioning and industrial chilling. For instance, the design of such a heat sink is urgently required for the refrigerated cargo holds of lorries conveying melons or bananas: because of a workers' strike in 1987, delays of a few days at the blockaded French channel ports resulted in the rotting of thousands of tonnes of food, which was being transported, and it having to be discarded. Thus, high-water-content hydrophilic materials have been considered as a means of achieving the storage of latent 'cold'. Because these materials have not, until recently, found applications outside biomedical engineering, it has been deemed necessary to examine their basic thermal properties.

PRESENT EXPERIMENTAL PROGRAMME Samples of the hydrophilic materials to be tested were prepared as circular cylinders, of hydrated dimensions - 15 m m diameter and 10 mm length. The materials were specified to be of biomedical grade in order to ensure purity and freedom from 'wash-out' of water-soluble residues, which might be expected to modify their thermal behaviours. The specimens were hydrated in water and rinsed several times in fresh water. The water used was clean tap water: distilled (or ultra-pure) water was not employed. Two such hydrated specimens were mounted with a thermojunction between them, and wrapped in a thin, close-fitting polythene membrane (as shown in Fig. 1) to prevent moisture loss during prolonged testing. The resulting samples were then placed successively in still air at - 2 0 ° C and + 20°C, and the heating and cooling characteristics recorded with a data logger. This essentially simple experimental technique has been checked by substituting water for the hydrogel, in an otherwise similar arrangement, with the resulting cooling and warming characteristics shown in Fig. 2. The data presentation follows the same form throughout, i.e. as a temperature/time plot, in which the time increments are in the positive

'Freezing" of water in hydrophilic polymeric structures

TEMPERATURE- CONTROLLED ENVR I ONMENT ( +25°C ~

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HYDROPHILIESAMPLES""~-.-...I F / / / ~ - ' ~ ~

TO DATALOGGER

Fig. 1. Schematic view of the experimental rig.

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30 30

20 40 - -

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Fig. 2.

Freezing-thawing characteristic for a water sample.

245

246

D. Highgate, C. Knight, S. D. Probert

direction for the cooling cycle and in what was previously the negative direction for the warming curve. This shows clearly the hysteresis effects found for the hydrophilic-material samples.

ASSESSED MATERIALS Four types of hydrophilic material were used in the initial tests. These were selected so as to provide a wide range of equilibrium water-contents exceeding 50%, because these are of particular interest for energy storage. However, in view of the early observations, some relatively low wateruptake materials were also tested, including one of high mechanical strength. These were chosen in order to extend the ranges of moisture content and ratio of free :bound water contents achievable. Testing the one material of enhanced mechanical strength and elasticity was intended to indicate the structural effects of repeated freezing and thawing upon the hydrophilic structure. The materials were specified and manufactured with care in order to (i) minimise any chemical washout, which might be expected to depress the freezing point, and (ii) ensure well-characterised materials of good long-term chemical and mechanical stabilities. The materials used were: (1) a high water-uptake material (containing 78% by weight of water); (2) a medium water-uptake material (containing 60% by weight of water); (3) a medium/low water-uptake material (containing 36% by weight of water); and (4) a medium/low water-uptake material (containing 38% by weight of water). Materials (1), (2) and (3) contained vinyl pyrrolidone as the principal hydrophilic component, whereas material (4) was a homopolymer based on a hydroxyethyl acrylate.

E X P E R I M E N T A L OBSERVATIONS For materials (1)-(4), the experimental data for the initial complete cooling/warming cycle, to which the samples were subjected after being fully hydrated, are shown in Figs 3-6, respectively. The results demonstrate the hysteresis effect, which increases in magnitude as the equilibrium water-uptake of the hydrophilic material decreases. Also, the effective latent heat of the system (as calculated from a

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~ -10

-15

-

Fig. 3.

20 L

~I

Initial freezing-thawing cycle for Sample (1), i.e. a 78% water-uptake specimen.

TABLE 1 Experimental Data for the Initial Temperature Cycle

Sample

Mean temperature-difference between the plateaux regions of the cooling and warming characteristics

Apparent latent heat of the

water content (kJ/kg)

(°c) 1

5

2 3 4

> 12 >20 >18

251 126 ~0 21

D. Highgate, C. Knight, S. D. Probert

248

.

40

30

20

10

20

30

~

1 0 ~ WARMING0

\

~-10

50COOLING~ 6 0 TIME,Cmins)

-

-15

/ Fig. 4.

Initial

freezing thawing cycle for Sample (2), i.e. a 60% water-uptake specimen.

knowledge of the mass of water absorbed, and the slopes of the heating and cooling characteristics) is less when the equilibrium water-uptake of the hydrophilic material is lower (see Table 1). No latent heat temperature plateaux for the low equilibrium water-uptake materials are apparent.

EFFECTS OF SUCCESSIVE F R E E Z I N G / T H A W I N G CYCLING Samples (1) and (2) (see Table 1) have been subjected to further freezing/thawing cycles in order to examine the thermal effects, and to seek

'Freezing' of water in hydrophilic polymeric structures

249

)

~ -5

COOLING

WARNING

~ -10

-15

0L

Fig. 5.

x

I

Initial freezing-thawing cycle for Sample (3), i.e. a 36"/. water-uptake specimen.

to establish if there are any macroscopic consequences of thermal cycling (see Figs 7 and 8). It was not anticipated that the hydrophilic structure would suffer any large-scale disruption due to ice-crystal formation, because it is both reasonably strong and elastic when hydrated. It was therefore expected to be able to sustain the relatively small volume changes associated with normal ice-formation without suffering major damage. However, the unusual nature of the early observations indicated that the pore structure was having a profound effect upon the freezing process and thus the performance and stability of the microscopic structure could not be anticipated reliably from previous experience.

250

D. Highgate, C. Knight, S. D. Probert

20l ~ 1 ~ 10 ~ ~ 2 0

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_20L Fig. 6.

Initial freezing-thawing cycle for Sample (4), i.e. a 38% water-uptake specimen.

The samples were not in any way modified during the cycling process. No water was added to the 78% material, but minor losses occurring by evaporation were compensated for during the test on the 60% material. Consistent trends occurred, namely: (a)

the magnitude of the hysteresis effect decreased with cycling (see Tables 1 and 2); and (b) the apparent latent heat increased towards the normal value for water as the cycling process proceeded. The macroscopic structures of the samples were not affected in any

'Freezing' of water in hydrophilic polymeric structures

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I

Thermal cycling test for Sample (1), i.e. a 78% water-uptake material. TABLE 2

Results After 20 Thermal Cycles

Sample

Mean temperature-difference between the plateaux regions of the cooling and warming characteristics

Apparent latent heat of the water content (kJ/kg)

1 <5

293 230

(oc)

1 2

251

D. Highgate, C. Knight, S. D. Probert

252

(21)

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(7)(4)(2)(1)

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Fig. 8. Thermal cycling test for Sample (2), i.e. a 60% water-uptake material. obvious way, e.g. there were no significant volume or shape changes, nor did they exhibit cracks or other signs of physical breakdown as a result of temperature cycling. There was, however, some tendency for the high wateruptake materials to lose optical clarity, i.e. from their initial condition which was transparent when wet, but when unfrozen the materials became somewhat translucent (at normal ambient temperatures) after 10-15 cycles. For Sample 3 (Table 1), neither any change in its mechanical properties nor any measurable increase in the apparent latent heat was observed, even after 11 cooling/warming cycles had been completed.

'Freezing' of water in hydrophilic polymeric structures

253

DISCUSSION Hydrophilic materials as used here consist of cross-linked chemical structures, in which water may be held in two forms: either as 'bound' water more-or-less loosely associated with available chemical sites in, or on, the molecular chains, or as 'free' water in the interstices between the molecules. The chemical 'bonding' of the water molecules to the matrix is clearly weak, because the materials can be dried readily after which they return to their original dehydrated shape and size, and exhibit their original mechanical properties. Thus, the water is not strongly bound, although it cannot simply be assumed to be present in macroscopic pores as in a sponge. The 'free' water can take part in diffusion processes such as osmotic interactions with the surrounding liquid, and provide a continuum for the penetration of dye or other low-molecular-weight solutes. The concept of a measurable 'pore size' in the hydrated material has been examined by Refojo, 1 who studied the penetration into the hydrated material of a range of labelled solute molecules of various molecular weights. Refojo reported that molecules, having molecular weights of less than -~ 600, were free to penetrate the structure of polyhydroxyethylmethacrylate (HEMA), a 38% homopolymer similar to the present Sample 4, whereas larger molecules were not in general able to penetrate significantly into the structure. From this, he was able to deduce that the effective pore size was about 0.8/~m. While similar direct measurements are not available for the higher-water-content materials used here, it is reasonable to assume that the effective pore size in these higher-moisture-content systems is likely to be greater. The ratio of free-to-bound water in a fully hydrated hydrophilic material is related to the water content at full hydration. 2 A study has been carried out in which the free:bound water ratio was estimated from a measurement of the hydraulic potential of a range of chemically similar hydrophilic materials of differing equilibrium water-uptakes (covering the range 4 0 - 8 5 % ) . 3 This indirect method provides a relationship between the ratio of free:bound water content and the equilibrium water-uptake of the material.as shown in Fig. 9: the proportion of free water rises as the equilibrium water content is increased. The results reported here suggest that either (i) the bound water is excluded from the freezing process by virtue of its weak chemical attachment to the hydrophilic sites on the molecular chains, or (ii) the physical proximity of the molecular structure (i.e. the effective 'pore' size) inhibits the normal process of ice-crystal formation. Both hypotheses present difficulties of interpretation at this stage.

D. Highgate, C. Knight, S. D. Probert

254

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EQUILIBRIUM WATER- CONTENT AT FULL HYDRATION,(°/o)

Fig. 9.

Free-to-bound water characteristic of a range of methy]methacrylate vinyl pyrrolidone co-polymers.

The fact that repeated freezing/thawing cycles appear to overcome the initial inhibition without major physical disruption of the polymer structure strongly suggests that the chemical bonding cannot be responsible for the effect. For,/fit were to be responsible, and (as shown) the magnitude of the effect declined significantly with thermal cycling, then the essential structure of the polymer would be expected to become unstable, resulting in the onset of dissolving, rupturing or substantially changing its equilibrium moisture content. However, these effects were not observed. A simple pore size effect as in (ii) above appears unlikely in view of the known ability of relatively large solute molecules to penetrate the hydrated structure. This suggests that the 'pores' are very large compared with the dimensions of a water molecule, and would not therefore be expected to have so profound an effect as to eliminate freezing. Such mechanical interference is assumed to be the basis for the clathrate effects referred to in Refs 4 and 5. However, this phenomenon has totally different results, in that (a) it apparently always acts to increase the freezing point (rather than to reduce it as reported here), (b) no appreciable hysteresis effect has been reported, and (c) the latent heat contributed by the water in the structures has not been reported to be anomalous. Nevertheless, the tendency of the high water-uptake materials to become optically 'hazy' after a number of freezing/thawing cycles does suggest that there has been some modification of the pore structure, if not of the

'Freezing' of water in hydrophilic polymeric structures

255

macroscopic polymer structure. Thus, the hypothesis presented in (ii) above must be considered as the more likely to apply.

CONCLUSIONS The present experimental tests revealed interesting and unexpected phenomena, which may be summarised as follows: (i)

a large hysteresis effect in the initial freezing/thawing temperature profile of the system occurs; and (ii) a significant reduction in the effective latent heat available from the water contained in the hydrophilic material ensues. In addition, both these effects are dependent on the thermal history of the sample, although the macroscopic properties of the hydrophilic structure, and its structural integrity, do not appear to be significantly affected by previous thermal cycling.

ACKNOWLEDGEMENT The authors are grateful to the Science and Engineering Research Council for financial support of this project.

REFERENCES 1. Refojo, M. F., Chemistry and permeability of hydrogel materials. In Hydrogels for Medical and Related Applications, ed. J. D. Andrade. Am. Chem. Soc. Symp., Ser. 31, 1976, pp. 37-51. 2. Highgate, D. J., Relevant properties of hydrophilic polymers for lens performance. The Optician, 167 (1974) 10-19. 3. Highgate, D. J., Mechanical and hydraulic properties of hydrophilic materials, Internal Symposium, King's College Hospital Medical School, London, ll January 1988. 4. EPRI, Cool-storage assessment study. Electric Power Research Institute, Technical Planning Study 76/650, Report EM 468, May 1977, pp. 2-15. 5. Hagen, M., Clathrate Inclusion Compounds. Reinhold Publishing Corporation, New York, 1962. APPENDIX: H Y D R O P H I L I C MATERIALS Hydrophilic materials are defined as that general class of materials which take in water, some of which is then bound into the chemical structure of the

256

D. Highgate, C. Knight, S. D. Probert

material. Thus, hydrophilic materials are essentially different from those which are plasticised by water, the bound water in particular contributing to a change in the physical characteristics of the material. Also, a clear distinction should be made between hydrophilic (i.e. water-swellable) polymers and water-soluble polymers on the one hand, compared with hydrophobic polymers on the other. A chemical will be a solvent for another material if the attraction between it and the material considered is greater than the mutual attraction (i.e. solvent-solvent and materialmaterial interactions). Therefore, a polymer will be soluble in water if the polymer-water attractions are stronger than the polymer-polymer or water-water attractions. A polymer will be insoluble if the polymer-polymer and the water-water interaction strengths exceed the polymer-water ones. The intermediate case is where there are polymerwater or water-water attractions and polymer-polymer attractions, and it is these types of polymer which swell when immersed in water. When different atoms make up a covalent bond, polarity can occur due to the different electro-negatives of the atoms involved. The electro-negatives of carbon and hydrogen are low and similar, and therefore bonds between these atoms would have no polar character. However, the bonds between carbon and oxygen, oxygen and hydrogen, nitrogen and carbon, and nitrogen and hydrogen, are all polar. This is due to the uneven distribution of the electrons over the bond length. The oxygen and nitrogen atoms would take a partial negative-charge, and carbon or hydrogen a partial positivecharge. The water molecule is regarded as having a partial positive-charge along its dipole axis at one end and a partial negative-charge at the other. The nature of the polymer-polymer interaction can take three forms: (1) Attractions between partial negative- and positive-charges associated with different atoms in the polymer system; (2) Chemical cross-links between the polymer chains themselves, the length and density of the cross-links being of prime importance; and (3) Physical entanglements of the polymer chains due to their high molecular weights. If there are partial positive- and negative-charges within the polymer molecules, then the density of these, together with the nature of the polymer-polymer interactions mentioned above, will control whether the polymer will be soluble, or whether it will swell in water and also control the extent of the swelling. Polymers which have chemical cross-links between their chains will not be soluble, although they could swell in water: the number and lengths of the cross-links then control the water-uptake of the system. There are two general classes of hydrophilic polymers which differ by virtue of the type of polar bonds existing, e.g. with respect to water-

257

'Freezing' of water in hydrophilic polymeric structures

attracting centres. The first group depends primarily upon the electronegativity of oxygen to provide the water-attracting sites. Such a system is polyhydroxyethylmethacrylate (HEMA) and the most significant part of the repeating polymer unit for this material is represented in Fig. Al(a). The water- and polymer-attracting centres are the partial positive- and negativecharges. These centres are those which will attract opposite partial charges on the water molecule. The oxygen and hydrogen atoms are expecially H

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~+/N~,~. ~+ 0 ~ H2C C

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H2C~CH (e)

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Molecular structure for the common hydrophilicpolymers.

2

258

D. Highgate, C. Knight, S. D. Probert

important because of their exposed nature and the attraction between opposite partial charges in different parts of the polymer molecule which controls the water attraction (see Figs Al(b) and Al(c). The dotted lines in Figs Al(b) and Al(c) represent hydrogen bonds. Although only one bond is indicated for each atom, in practice many will be present due to the bonding occurring in three dimensions. Although the oxygen-carbon double-bond or carbonyl function is polar in nature, its effects on the water-polymer attractions and polymer-polymer attractions are limited and can be considered to be insignificant compared with the effect of the hydroxyl (OH) function. The stability of these polymers is controlled by cross-linking the H E M A chains rather than relying solely upon the hydrogen bond between the polymer chains. A further form of this material is shown in Fig. Al(d). The second group of materials consists of those which depend upon the polarity of carbon-nitrogen bonds in N-vinyl, 2, pyrrolidone as the main water-attracting centre. As before, the carbonyl function contributes little to the polymer-polymer and polymer-water attractions and can be neglected. Figure Al(e) shows the main repeating hydrophilic element in a polymer chain of this class of material. In polymers containing this repeating unit, the relatively shielded position of the partial negative-charge on the nitrogen atom, and the non-flexible and bulky nature of the pyrrolidone group, reduce the polymer-polymer interactions but not the polymer-water interactions. Indeed, this polymer is soluble unless the molecular weight is extremely high or chemical cross-links are introduced. Alternatively, the monomer may be polymerised with a hydrophobic monomer, thus reducing the density of the pyrrolidone rings in the polymer system. A combination of cross-linking and co-polymerisation is normally used to control the final water content and physical properties of these materials when wet. There is a third and final group of materials which contain both the pyrrolidone and hydroxyethyl functions. The hydration characteristics of these materials, that is, their wateruptake and consequent linear expansion, are related to the chemical structure as follows: (a)

The water-attracting centres, within the polymer system, give rise to what is described as 'bound' water. This is that part of the total water content which is absorbed most readily: it enters the material at an early stage in hydration and is the most difficult to remove subsequently. (b) A second important factor is the extent of the polymer-polymer interactions (both polar attractions and chemical cross-links), and

'Freezing' of water in hydrophilic polymeric structures

259

also the size and flexibility of any side groups attached to the polymer backbone. Strong polar interactions and/or the high density of short cross-links and small flexible side groups give rise to small polymerfree zones. These limit the volume available for the simple absorption of water, as opposed to moisture which is chemically bound in the structure. Such unattached moisture is generally known as 'free' water and occupies the polymer-free zones around the polymer molecules. It is this part of the total moisture content which partakes in osmotic reactions and most easily moves into, or out of, the hydrated hydrophilic material. Hydrophilics were discovered in the early 1960s at the Institute for Macro Molecular Research in Czechoslovakia and subsequently have been developed primarily for use as soft contact-lens materials. Chemically, the original material consisted of polyhydroxyethylmethacrylate. This may be produced by bulk polymerisation resulting in an optically clear, mechanically brittle solid when dry. On hydration, the material absorbs 30-50% of water by weight or, more relevantly, it can be made with an expansion ratio of 1.11 to 1"40. The mechanical properties of this material are relatively poor in the dry state--it is very brittle--and only acceptable if hydrated, when it exhibits a low tear-propagation strength, and a relatively small extension to break (less than 80%) when subjected to tensile tests. This material has had the longest period of service of any hydrophilic material and has been accepted for opthalmic use by the Food and Drug Administration of the USA. However, the material for this application is accepted only after extensive pre-use extraction processes, because, due to the manufacturing process, the material initially contains considerable quantities of unpolymerised monomer and residual peroxide catalyst, typically totally 2-3% by weight of the dry polymer. Because both residual materials are water-soluble, the first hydration results in the release of these contaminants. More recent developments have concentrated on co-polymers of vinyl pyrrolidone and other structural units, including methylmethacrylate, styrene and nylon, which have resulted in a range of materials exhibiting controllable mechanical and physical properties. Thus, it is now possible to produce high water content systems, which are strong and elastic or stiffand semi-rigid.