ISSN 0012-5016, Doklady Physical Chemistry, 2006, Vol. 411, Part 1, pp. 305–308. © Pleiades Publishing, Inc., 2006.Original Russian Text © A.V. Pastukhov, T.A. Babushkina, V.A. Davankov, T.P. Klimova, V.P. Shantarovich, 2006, published in Doklady Akademii Nauk, 2006, Vol. 411, No. 2,pp. 216–219.

305

Water as aerosols, emulsions, and disperse inclu-sions in polymers is known [1] to consist of micron-sizedrops differing significantly from bulk water. However,even in this finely dispersed state, the liquid phase ofwater is not preserved below –39 to –41

°

C [2–5]. Adielectric relaxation spectroscopy study of a systemcomposed of a microporous carbon material and waterhas shown that melting of ice in pores with a size ofabout 1.8 nm gives liquid water at –38

°

C [6]. It wassuggested [7] that, in smaller pores with dimensions ofseveral angstroms formed by the protein collagen(or methylcellulose), water transforms from the liquidto a glassy state on cooling to –100 to –140

°

C. How-ever, it is appropriate to discuss the possibility of suchpronounced supercooling of the aqueous phase onlytaking into account the strong specific interactions ofthe functional groups of hydrophilic macromoleculeswith water molecules in the matrix microcavities.

In this work, we found that, in hydrophobic poly-mers (hypercrosslinked polystyrene), liquid waterstarts to be detected at about –80

°

C.

We tested polymers with rigid microporous struc-tures that had great internal specific surface areas andpronounced hydrophobic properties, as they did notcontain any polar groups. Therefore, water was intro-duced into the polymer matrix by displacing acetone.

Hypercrosslinked polystyrenes (HCPs) wereobtained by crosslinking of gel styrene copolymerswith 0.17 or 1.4 wt % divinylbenzene (DVB) or linearpolystyrene (

M

= 300000), completely swollen indichloroethane, with bifunctional crosslinking agents,monochlorodimethyl ether (MCE) or

p

-xylylenedichloride (XDC), 0.5 or 1.0 mol per unit mole of poly-styrene [8]. In the latter case, every phenyl unit of poly-styrene was involved in the formation of two bridges

(on average) and a formal crosslinking degree of 200%was assigned to the product.

The designations of samples used below are as fol-lows: CPS(0.17)200E is a hypercrosslinked productbased on a styrene copolymer with 0.17% DVB pre-pared using 1.0 mol of MCE and, hence, having acrosslinking degree equal to 200%; LPS200X is thecrosslinking product of linear polystyrene with 1.0 molof XDC. According to a large number of physicochem-ical methods, the pore size distribution in the HCPmatrix has a peak at 2–3 nm.

After displacement of acetone by water, the HCPnetworks contained a large amount of water andremained substantially swollen (had a greater volume)compared to dry samples (table). In addition to thehypercrosslinked polystyrenes, we also tested themacroporous polydivinylbenzene sorbent AmberliteXAD-4 (Rohm and Haas), which contains a substantialportion of small-size pores.

The water-filled HCPs were studied by three inde-pendent structural methods: pulse broadband

1

H NMRof water protons, positron annihilation spectroscopy,and thermomechanical analysis (TMA).

For NMR study, polymer grains swollen in waterwere placed in a Petri dish, excess water was removedwith filter paper, and the polymer was dried in an airflow until the grains were free to move over the glasssurface. The sample was placed in an NMR tube andsealed. The

1

H NMR spectra of water protons (thechemical shift of the broad signal is

4.7

±

0.3

ppm atroom temperature) were recorded using deuteratedmethanol as the external standard. The distancebetween the

1

H NMR signals of methanol was used todetermine the sample temperature at the instant of mea-surement. Cooling and subsequent heating of the sam-ple were accomplished by means of a standard temper-ature-control unit of the spectrometer. The accuracy oftemperature maintenance was

±

1 K

. The integrated sig-nal intensity (proportional to the number of protonsresponsible for the given signal without saturation) atroom temperature (

I

20

) was taken to be 1000 arbitrary

Water in Nanopores of Hypercrosslinked Hydrophobic Polystyrene at Low Temperatures

A. V. Pastukhov

a

, T. A. Babushkina

a

, V. A. Davankov

a

, T. P. Klimova

a

, and V. P. Shantarovich

b

Presented by Academician A.R. Khokhlov June 30, 2006

Received July 3, 2006

DOI:

10.1134/S0012501606110042

a

Nesmeyanov Institute of Organoelement Compounds,Russian Academy of Sciences,ul. Vavilova 28, Moscow, 119991 Russia

b

Semenov Institute of Chemical Physics,Russian Academy of Sciences,ul. Kosygina 4, Moscow, 119991 Russia

PHYSICALCHEMISTRY

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PASTUKHOV et al.

units. A change in this value on cooling (heating) indi-cated freezing out (thawing) of a portion of water.

The size distribution of micropores was studied bythe positron annihilation method using anEGG&ORTEC positron lifetime spectrometer with atemporal resolution of 300 ps. The use of a

22

NaCl

radioactive source (

1.8

×

10

6

Bq) provided an integralstatistics with more than

10

6

coincidences. Correctionsfor the random coincidence background and for thecontribution of annihilation in the source material wereapplied by processing the spectra with the modifiedPATFIT program.

The deformation behavior on uniaxial compressionof water-swollen polymers was investigated using aUIP-70 instrument for thermomechanical analysis of

polymers (Central Design Bureau of the Russian Acad-emy of Sciences). A sample that was a single bead of aregular spherical shape with a diameter (0.6–0.8 mm)measured precisely (

±

0.001

mm) under a microscopewas placed into a specially turned polished sphericalhollow (with a radius of 1.0 mm and a depth of 0.2 mm)in a quartz plate. The variation of the grain dimensionalong the compression axis was recorded by a movingquartz rod connected to the capacitance sensor of theinstrument. The plate with the water-swollen sample(water from the surface was preliminarily removedwith filter paper) was placed into the instrument cham-ber at –95

°

C. After 10 min, the sample was loaded andheating was started at a rate of 5 K/min. The loads of0.4 and 0.6 kg used in the experiment did not induceany destruction of the polymer grains.

The positron annihilation data suggested that thehypercrosslinked polystyrenes and the XAD-4 sorbenthave a bimodal microporous structure with size distri-bution maxima at 0.6–1.2 and 1.6–4.0 nm (diameter)[9]. The micropore sizes are different and depend on theamount of divinylbenzene in the initial styrene copoly-mer before its intensive crosslinking by the bifunctionalcompound, while the ratio of the two micropore groupsis determined by the amount of the crosslinking agent.It was found [10] that filling of pores with an effectiveradius of down to 1.5 nm with nitrogen vapor occurs bythe capillary condensation mechanism, whereasadsorption in micropores (below 1.5 nm) is character-ized by bulk filling. By analogy, we suggest that waterfills micropores (II) in HCPs and in XAD-4 as a usualliquid phase, while in the nanopores (I) with a radius ofless than 1.5 nm, bulk filling of the limited space bywater molecules gives rise to a special cluster phase.The clusters are formed from a few water moleculesand, similar to water clusters on a hydrophobic metalsurface [11], may form hexamer structures.

In a

1

H NMR study of the HCP–water systems, thesignals of water protons were recorded and their char-

Properties of crosslinked polystyrenes

Type ofstructure Polymer

S

ap

, m

2

/g

d

ap

, g/cm

3

Water absorp-tion, g/g

Volumetric swell-ing in water, v/v

E

sw

, MPa

I CPS(0.17)100E 770 0.83 0.8 1.3 430

LPS100X 1000 0.74 1.0 1.4 90

CPS(1.4)200E 1600 0.76 1.0 1.3 770

XAD-4 860 0.52 1.0 1.04 440

II CPS(0.17)200E 1200 0.74 1.3 1.6 380

LPS200X 1400 0.57 1.3 1.3 100

III CPS(1.4)100E 440 0.96 0.4 1.03 1500

Note:

S

ap

is the internal specific area (determined by low-temperature nitrogen sorption);

d

ap

is the apparent density;

E

sw

is the elastic mod-ulus for compression of the water-swollen sample.

0

I

/

I

20

, arb. units

–80

0.4

0.8

1.2

–60 –40 –20 0 20

* * ** *** * * * *

**

*

** *

1 2

3

T

,

°C

*

Fig. 1.

Variation of the relative integrated intensity

I

/

I

20

(normalized to the intensity at 20

°

C) of

1

H NMR signals ofwater during heating of water-swollen hypercrosslinkedpolystyrenes: (

1

) CPS(0.17)100E, (

2

) CPS(0.17)200E, and(

3

) XAD-4 macroporous sorbent filled with water.

DOKLADY PHYSICAL CHEMISTRY

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WATER IN NANOPORES OF HYPERCROSSLINKED HYDROPHOBIC POLYSTYRENE 307

acteristics during polymer cooling to –80

°

C and grad-ual heating to +10

°

C were measured. Two regions of“thawing” of water proton mobility in the polymerpores were found, one from –80 to –40

°

C and the otherabove –20

°

C (Figs. 1, 2). This result indicates thatHCPs contain water of two types, cluster water in nano-pores I and usual water in micropores II. On raising thetemperature, in nanopores I, liquid appears at

−

80 to

−

40

°

C, whereas in larger micropores II usual water isformed much later, after ice thawing. The clearly seendifferences in the

I

/

I

20

temperature dependences for dif-ferent HCPs are caused by the morphological featuresof the porous structure of these polymer samples,depending first of all on the conditions of synthesis.Different pore size distributions are evidently responsi-ble for the size distribution of water clusters. In thepolymers CPS(0.17)100E, CPS(0.17)200E, andCPS(1.4)200E, clusters seem to be more nonuniform insize; therefore, the temperature range of increasingmobility of water protons is much broader for thesesamples than for CPS(1.4)100E and especially forXAD-4 (Figs. 1, 2).

On the basis of the differential thermomechanicalanalysis (DTMA) method (Fig. 3), the HCP–water sys-tems can be subdivided, in terms of their thermodefor-mation behavior, into three groups with clearly differ-ent structures of the polymer matrix: for group I sys-tems, the deformation rate (

v

def

) sharply increases inthe region from –70 to –40

°

C and then in the regionfrom –20 to 20

°

C; for systems II, an increase in

v

def

takes place only from –25 to +20

°

C; for systems

III thisis observed only from –80 to –40

°

C. In all systems onlyslight elastic compression deformations are observedbelow –90

°

C.

Since nanopores of 0.6–1.2 nm can accommodateonly a few molecules, the properties of the cluster water

are expected to differ sharply from the properties ofusual water in larger micropores (1.6–4.0 nm). There-fore, the abnormal increase in the mobility of watermolecules detected by

1

H NMR and the parallelincrease in the structural mobility of the samples upondeformation impact on the HCP–water system in theregion from

−

80 to –40

°

C are attributable to the trans-formation of the cluster water from the glassy state tothe liquid state. The further increase in the water protonmobility (

1

H NMR) and structural mobility (DTMA) athigher temperatures is caused by melting of the poly-morphic ice in micropores of 1.6–4.0 nm.

Study of nanometer-size pores is faced with difficul-ties. The results of our work show that detection ofthawing/freezing of water nanoclusters by the muchmore common

1

H NMR and TMA methods can be suc-cessfully used for this purpose in addition to the uncon-ventional positron annihilation technique.

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–80 –60 –40 –20 0

0.4

0.8

1.2

1

2

I

/

I

20

, arb. units

T

,

°C

0.03

0

v

def

, %/s

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0.06

I

II

III

T

,

°C

Fig. 2.

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1

H NMR signals of water during heating of water-swollenhypercrosslinked polystyrenes: (

1

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Fig. 3.

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308

DOKLADY PHYSICAL CHEMISTRY Vol. 411 Part 1 2006

PASTUKHOV et al.

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