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ISSN 0965-545X, Polymer Science, Ser. A, 2008, Vol. 50, No. 7, pp. 725–732. © Pleiades Publishing, Ltd., 2008.Original Russian Text © S.A. Vshivkov, E.V. Rusinova, 2008, published in Vysokomolekulyarnye Soedineniya, Ser. A, 2008, Vol. 50, No. 7, pp. 1141–1149.

1

Phase diagrams of multicomponent systems providefull information on the mutual solubility of componentsin wide concentration and temperature ranges. One ofthe first phase diagrams of polymer–solvent systemswas published by Kargin, Papkov, and Rogovin as earlyas 1937–1939 [1–4]. In 1941, the works of Kargin andTager [5, 6] devoted to the thermodynamics of polymersolutions were published. From the end of the 1940s,systematic research into the thermodynamic and rheo-logical properties and construction of phase diagramsof polymer solutions have been performed at the Labo-ratory of Colloid Chemistry, Ural State University,established by A.A. Tager. In 1958, this laboratory wasreorganized into the Chair of Macromolecular Com-pounds. Over the course of fifty years, phase diagramshave been constructed for hundreds of polymer sys-tems, such as polymer–solvent, polymer-polymer–sol-vent, and polymer–polymer, with amorphous and crys-talline phase separations. Many of these data wereincluded in textbooks, monographs, and reviews [7–17].

In the 1980s, studies of LC transitions in solutionsof polyamides and polyimides were launched at theChair of Macromolecular Compounds, Ural State Uni-versity [18]. The LC state of solutions and melts for anumber of cellulose derivatives was described in [19].

1

This paper is devoted to the 50th anniversary of the Chair of Mac-romolecular Compounds, Ural State University, founded by AnnaAleksandrovna Tager.

Structural studies of molecules of cellulose and itsderivatives showed that they assume rigid helical con-formations in ordered regions which are stabilized byintramolecular hydrogen bonds. If intramolecular Hbonds are preserved during dissolution of the abovepolymers, molecules remain rigid-chain and are hencecapable of ordering and forming mesophases. Ifintramolecular H bonds are destroyed in the course ofdissolution, chains become flexible and, as a conse-quence, cease to form ordered phases. Therefore, sol-vents that break down only intermolecular H bonds(DMF, DMAA, 1,4-dioxane, and chlorinated hydrocar-bons) are used to attain the LC state in solutions of cel-lulose and its derivatives. For solutions of cellulosederivatives, phase diagrams were considered in [19,20]. However, data on the phase diagrams of such sys-tems in the applied magnetic field are lacking. The goalof this work is to study phase transitions of cyanoethylcellulose (CEC)–DMAA, CEC–DMF, hydroxypropylcellulose (HPC)–ethanol, HPC–DMAA, and HPC–water systems in the presence and absence of magneticfield.

EXPERIMENTAL

CEC samples with a degree of substitution of 2.6and

M

w

= 1.9

×

10

5

and HPC samples (Klucel-JF, Her-cules) with a degree of substitution of 3.4 and

M

w

=9.5

×

10

4

(HPC-1),

1.4

×

10

5

(HPC-2), and

1.15

×

10

6

(HPC-3) were used. According to X-ray studies, the

Effect of Magnetic Field on Phase Transitionsin Solutions of Cellulose Derivatives

1

S. A. Vshivkov and E. V. Rusinova

Ural State University, pr. Lenina 51, Yekaterinburg, 620083 Russiae-mail: sergey.vshivkov@usu.ru

Received May 24, 2007; Revised Manuscript Received January 28, 2008

Abstract

—LC transitions occurring in mixtures of cyanoethyl cellulose with DMAA or DMF and hydroxypro-pyl cellulose with ethanol, DMAA, or water in the presence and absence of magnetic field have been studied.With an increase in the polarity of solvent molecules and a decrease in the molecular mass of the polymer, theLC phase develops at higher concentrations and lower temperatures. Under application of magnetic field, thedomain structure is formed in solutions and the temperature–concentration region of the LC phase widens. Cya-noethyl cellulose and hydroxypropyl cellulose solutions are found to possess memory: after the magnetic fieldis switched off, the orientation of macromolecules and the increased temperature of phase transitions are pre-served for many hours. As the molecular mass of the polymer is increased, the ability of macromolecules toorient themselves in the magnetic field declines. The threshold mechanism governing the effect of magneticfield on LC transitions in polymer solutions has been discovered. The critical value of magnetic intensity thatbrings about a shift in boundary curves is consistent with the value of

H

cr

necessary for the cholesteric liquidcrystal–nematic liquid crystal phase transition.

DOI:

10.1134/S0965545X08070018

ANNIVERSARY OF THE CHAIROF URAL STATE UNIVERSITY

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VSHIVKOV, RUSINOVA

degree of crystallinity of HPC samples did not exceed15%. The degree of crystallinity for the CEC sampleswas 35%. X-ray measurements were performed on aDRON-4-13 diffractometer using

Cu

K

α

radiation.Twice-distilled water, ethanol, DMAA, and DMF

(reagent grade) and a mixture of trifluoroacetic acid(TFAA) with methylene chloride (1 : 1) were used assolvents. The purity of the solvents was estimated fromtheir refractive indexes [21, 22]. Polymer solutionswere prepared in sealed ampoules for several weeks at298 (water), 340 (in ethanol), and 350 K (DMF,DMAA, and TFAA–methylene chloride mixed sol-vent).

Phase-transition temperatures

T

ph

were estimated bythe cloud-point method [16]. The solution temperatures

were varied at a rate of 12 K/h. The structure of solu-tions was examined with the help of an OlympusBX-51 polarization microscope. A polarization photo-electric setup was used to determine the type of phasetransition in solutions [23]. A sealed ampoule contain-ing the transparent polymer solution was placed in thegap between the crossed polaroids (a polarizer and ananalyzer), and the temperature of the ampoule wasdecreased with the use of a thermostating jacket. Thepolarized light of an LGN-015 He–Ne laser was trans-mitted through the polaroids in the direction normal tothe ampoule containing the solution (its layer thicknesswas

~5

mm). When the solution was transparent (iso-tropic), the intensity of the transmitted light was zero.As the system became turbid upon variation in temper-ature or increase in the concentration of solution, thetransmitted light intensity increased, as measured by aphotoresistor. This indicated formation of the anisotro-pic phase, that is, the LC phase transition.

Experiments in the magnetic field were performedusing a setup generating a constant magnetic field withan intensity of up to 15000 Oe [23]. The sealedampoule containing a transparent polymer solution wasplaced between the magnet poles. The magnetic fieldvector was directed normal to the solution layer(

~5

mm thick) in the sealed ampoule. The temperatureof solution was varied with the thermostating jacket,and the onset temperature of opalescence developmentwas measured. This temperature was related to theappearance of the LC state.

The coefficients of magnetic susceptibility weredetermined by means of a vibration magnetometer [24].The table lists the coefficients of magnetic susceptibil-ity

χ

for the polymers and their solutions. In order ofmagnitude, the values of

χ

obtained for the polymersare in satisfactory agreement with published data [25].The energy of the magnetic field

E

stored by the solu-tion volume unit was calculated via equation [26]

E

=

χ

H

2

, where

H

is the magnetic intensity.

RESULTS AND DISCUSSION

Solutions of HPC and CEC in Organic Solvents

Figure 1 shows the boundary curves delimitingtransparent isotropic and opalescent anisotropic solu-tions for HPC-1–DMAA, HPC-2–DMAA, HPC-1–eth-anol, HPC-2–ethanol, CEC–DMAA, CEC–DMF, andCEC–(methylene chloride/TFAA) systems. Thesecurves satisfactorily agree with the data from [19].Under conventional light, the concentrated solutions ofHPC and CEC are opalescent. This is consistent withthe published data and suggests formation of choles-teric liquid crystals [19]. For the high-molecular-massHPC samples, the boundary curves are shifted towardthe region of more diluted solutions, in agreement withthe Flory theory [27].

The dipole moments

µ

for molecules of the test sol-vents at 293 K are [28] 1.69 (ethanol), 1.59 (methylene

280

30 40

T

, K

(a)

24050 60

320

360

400

1

280

20 40

(b)

50 60

320

360

1

23

4

30

c

, wt %

3

2

Fig. 1.

Boundary curves for various systems (a):(

1

) HPC-1–DMAA, (

2

) HPC-1–ethanol, (

3

) HPC-3–DMAA, and (

4

) HPC-2–ethanol and (b): (

1

) CEC–DMF, (

2

) CEC–DMAA, and (

3

) CEC–(methylenechloride–TFAA).

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EFFECT OF MAGNETIC FIELD ON PHASE TRANSITIONS 727

chloride), 2.28 (TFAA), 3.81 (DMAA), and 3.86 D(DMFA). The value of

µ

calculated for the TFAA–methylene chloride mixture via the additivity scheme is1.94 D. A comparison of the values of

µ

and of the posi-tions of boundary curves for HPC and CEC solutionstestifies to the fact that as the solvent polarity isincreased, the LC phase appears at higher concentra-tions and lower temperatures since with an increase inpolarity, the solvent ruptures bonds between macromol-ecules to a higher extent. The values of

µ

for DMF andDMAA are very similar. Therefore, the positions of theboundary curves for CEC solutions in these solventsare nearly coincident.

Application of the magnetic field raises the temper-ature of LC phase formation

T

ph

in HPC and CE solu-tions; that is, it widens the temperature–concentrationregion of the existence of anisotropic solutions. Mole-cules of liquid crystals orient themselves in the mag-netic field so that their long chains are oriented parallelto the magnetic field lines [20]. This orientation is asso-ciated with the molecular anisotropy of macromole-cules rather than the existence of permanent magneticmoments. According to published data [20, 29, 30], thecholesteric liquid crystal–nematic liquid crystal phasetransition occurs in magnetic field. From a certain crit-ical intensity, magnetic field causes untwisting of thecholesteric helix. Eventually, nematic liquid crystalsare formed which occur at higher temperatures thancholesteric liquid crystals.

Polarization microscopy studies revealed a stripedtexture of HPC and CEC solutions treated in magneticfield, thus suggesting formation of large supramolecu-lar structures—domains. A similar phenomenon wasreported for other polymer–solvent systems [20]. It wasdiscovered that after the magnetic field was switched

off, an increased

T

ph

was preserved in solutions formany hours. This is clearly seen from Fig. 2, whichdemonstrates the time dependences of

∆

T

(

∆

T

is the dif-ference in LC phase transition temperatures in the pres-ence and absence of magnetic field). This fact providesevidence that structures induced by the magnetic fieldare preserved in solutions. Thus, the systems understudy possess memory [31]. A similar phenomenon wasobserved in [29].

On the basis of the above data, the times of relax-ation

τ

were calculated for the nematic liquid crystal–cholesteric liquid crystal reverse transition in solutionsafter switching off the magnetic field. Calculationswere performed according to the common exponentialequation. The values of

τ

were found to be 18 h(HPC-3–DMAA);

τ

1

= 11 h at 298 K and

τ

2

= 8 h at370 K (CEC–DMAA). The calculation results made itpossible to estimate the order of the enthalpy of activa-tion

∆

H

for the nematic liquid crystal–cholesteric liq-uid crystal transition in solutions after switching off themagnetic field. The value of

∆

H

was estimated via theequation

ln(

τ

1

/

τ

2

) = (

∆

H

/

R

)(1/

T

1

– 1/

T

2

)

as

~4

kJ/mol,in qualitative agreement with rather low enthalpies ofLC phase transitions [25, 30].

Figures 3 and 4 show boundary curves measured forthe HPC-3-DMAA and CEC–DMAA systems at vari-ous magnetic field intensities

H

. As is seen, with anincrease in

H

, the temperature–concentration region ofthe existence of LC solutions widens. The higher thevalue of

H

, the more pronounced the shift of the bound-ary curves. A similar behavior was observed for solu-tions of HPC in ethanol and CEC in DMF.

4

0 20

∆

T

, K

40

8

12

1

2 3

Time, h

Fig. 2.

Time dependence of

∆

T

for solutions: HCP-3in DMAA (

c

= 51.3%) at (

1

) 370, (

2

) CEC in DMAA(

c

= 48.3%) at (

2

) 370 and (

3

) 298 K treated withmagnetic field with intensity

H

= 7 kOe.

36 40 44 48 52

c

, wt %

280

320

360

T

, K

123

4

Fig. 3.

Boundary curves for the HPC-3–DMAA sys-tem.

H

= (

1

) 0, (

2

) 3, (

3

) 5, and (

4

) 9 kOe.

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HPC Solutions in Water

Aqueous solutions of HPC belong to systems withstrong electron-donor (hydrogen) bonds [32]. Becauseof the presence of two mobile protons and two unsharedelectron pairs at the oxygen atom, a water moleculemay function both as an electron donor and an electronacceptor and form four hydrogen bonds with an energyof 20 kJ/mol. Therefore, a loose structure with a largefree volume is formed in water. In the case of water, thefraction of nonspecific interaction is as low as 7% [32].Intermolecular interactions of HPC with water may bedetermined by both the hydrophilic hydration givingrise to hydrogen bonding between a polymer and a sol-vent and the hydrophobic hydration of water [33],which consists in densification of water structurearound nonpolar methyl and methylene groups of HPCmolecules during formation of solutions.

Many studies were devoted to phase equilibrium inthe HPC–water system [33–46], and the LCST valueswere reported in a number of papers. Figure 5 displaysthe phase diagram measured for the HPC-1–water sys-tem. This diagram is largely consistent with the phasediagrams described for this system in [19] and [33–46].Four regions can be distinguished in the diagram:(I) the region of isotropic transparent solutions; (II) theregion of anisotropic transparent solutions; (III) theregion of heat-induced phase separation giving rise toformation of a white anisotropic precipitate; and(IV) the region of anisotropic solutions opalescent overthe entire volume (1 refers to colorless solutions and2 refers to blue solutions, which is typical of choles-terol LC solutions [19, 47]). According to [34], crystalsolvates are formed in solutions at an HPC concentra-tion of ~80% or above.

The boundary curve that characterized the heated-induced phase transition has a binodal shape. It appearsthat the breakdown of hydrophilic and hydrophobichydration of HPC initially leads to the amorphousphase separation of solutions and formation of twocoexistent dilute and concentrated phases (the LCST is298 K). Simultaneously, anisotropic crystal solvatesprecipitate in the concentrated phase.

Application of magnetic field causes an increase inthe phase transition temperature under heating, whichis likely associated with a change in the orientation ofmacromolecules in solution (Fig. 6). Like the HPC andCEC solutions in organic solvents, the HPC–water sys-tem possesses memory: an increased Tph is preservedfor many hours after the magnetic field is switched off(Fig. 7). The calculated time of relaxation necessary toachieve the initial orientation of macromolecules is τ =260 h (c = 53.5%) and 103 h (c = 49.6%). With anincrease in the concentration of the polymer, the time ofrelaxation grows, since the viscosity of the systemincreases.

315

43 45

T, K

c, wt %47 49

1234

325

335

Fig. 4. Boundary curves for the CEC–DMAA system. H = (1) 0, (2) 3, (3)5, and (4) 9 kOe.

304

0 20

T, K

c, wt %40 60

12

320

80296

312

I

IIIII

IV

Fig. 5. Phase diagram for the HPC-1–water system.See text for explanations.

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EFFECT OF MAGNETIC FIELD ON PHASE TRANSITIONS 729

CONCLUSIONS

The results of this study demonstrate that magneticfield widens the temperature–concentration region ofthe existence of the LC phase. This effect is apparentlyrelated to the nematic liquid crystal–cholesteric liquidcrystal phase transition and the orientation of macro-molecules in the direction parallel to the magnetic fieldlines [20, 29]. In this case, large supramolecular struc-tures—domains—develop in solutions.

The effect of magnetic field on the variation in LCtransitions with the polymer concentration in solutionshows an extremal pattern. Figures 8 and 9 demonstrate

the concentration dependences of ∆T for HPC-1–DMAA, HPC-3–DMAA, and HPC-1–water systemsmeasured at various magnetic field intensities. In ana-lyzing the effect of the concentration of HPC solutionson magnetic field-induced changes in phase transitiontemperatures, two factors should be taken into account.First, as concentration increases, the number of macro-molecules capable of orientation in the magnetic fieldgrows; as a consequence, Tph should increase. Second,a rise in the polymer concentration in solution facili-tates densification of the fluctuation network of entan-glements. This impedes the occurrence of orientation

304

0 20

Tph, K

c, wt %40 60

2

316

80296

312

300

308

1

3

Fig. 6. Boundary curves for the HPC-1–water system.H = (1) 0, (2) 5, and (3) 9 kOe.

4

0100

∆T, K8

6

1

2

Time, h300 500

2

Fig. 7. Time dependence of ∆T for HPC-1 solutions inwater. c = (1) 53.5 and (2) 49.6%.

20

48

∆T, K

c, wt %52

2

560

40

10

30

1

3

4

Fig. 8. Concentration dependence of ∆T for solutionsof (1, 2) HPC-3 and (3, 4) HPC-1 in DMAA. H =(1, 3) 5 and (2, 4) 9 kOe.

4

20

∆T, K

c, wt %40

2

800

8

2

6 1

3

60

Fig. 9. Concentration dependence of ∆T for HPC-1solutions in water. H = (1) 5, (2) 9, and (3) 13 kOe.

730

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processes and weakens the effect of the magnetic field.On the whole, the concentration dependence of ∆T isapparently described by a curve with a maximum. Itshould be noted that for solutions of an HPC-1 samplewith a lower molecular mass, the value of ∆T is much

higher (Fig. 8). This observation indicates a more dis-tinct orientation of smaller molecules in the magneticfield, in agreement with the data from [48].

Figures 10 and 11 plot ∆T as a function of lnE forHPC-1–DMAA, HPC-3–DMAA, and CEC–DMAA

20

–1

∆T, K

lnE [J/m3]0

2

10

40

10

30

1

3

4

5

6

2

Fig. 10. Plot of ∆T vs. lnE for solutions of (1–4) HPC-3 and (5, 6) HPC-1 in DMAA. c = (1) 46.1, (2) 48.3, (3) 49.6,(4) 51.3, (5) 52.0, and (6) 49.0%.

3

–1

∆T, K

lnE [J/m3]0

2

1

7

1

5

1

2

Fig. 11. Plot of ∆T vs. lnE for CEC solutions in DMAA. c = (1) 46.0 and (2) 48.8%.

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EFFECT OF MAGNETIC FIELD ON PHASE TRANSITIONS 731

systems. It is seen that these dependences are describedby straight lines. With an increase in the magnetic fieldenergy stored by solutions, the value of ∆T increases.The effect of field on the phase transitions shows athreshold character: a change in Tph begins from a cer-tain critical intensity of the field Hcr. These values are2.3 and 2.0 kOe for the HPC–DMAA and CEC–DMAA systems, respectively. In order of magnitude,these values are consistent with Hcr necessary for thenematic liquid crystal–cholesteric liquid crystal phasetransition [20, 30]. In this case, ∆T = Kln(E/E0) orTph (H > Hcr) = Tph(H = 0) + Kln(E/E0). Coefficient Kdepends on the molecular mass of the polymer and itsconcentration in solution.

ACKNOWLEDGMENTS

This work was supported by the Russian Foundationfor Basic Research (project nos. 05-03-32888 and 05-08-17948) and the US Civilian Research and Develop-ment Foundation (CRDF) (project no. PG07-005-02).

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