From the Institute of Physical Chemistry, University of Uppsala, Uppsala, Sweden

Studies on Cellulose Derivatives

Part 111. Unperturbed Dimensions of Hydroxyethyl Cellulose and other Derivatives in Aqueous Solvents

By W. BROWN and D. HENLEY

(Eingegangen am 6. November 1963)

SUMMARY: Values of K for the equation [?]a = K zv have been calculated for hydroxyethyl

cellulose, ethyl hydroxyethyl cellulose, sodium carboxymethyl cellulose and cellulose using the treatment of KURATA and STOCKMAYER.

Expansion factors, u,,, were calculated from the K-values for hydroxyethyl cellulose in water and cadoxen. In these solvents the measured molecular dimensions differ greatly, but application of the corresponding a,,-values leads to the same unperturbed dimensions. In contrast, different unperturbed dimensions result when employing expansion factors calculated from second virial coefficients.

The origin of dZn[q]/dT in water solution is discussed. It appears that increasing the monomer molecular weight in cellulose derivatives

initially results in decreased unperturbed molecular extensions; the steric effect ultimately predominates when very large substituents are introduced. For the polyelectrolyte sodium carboxymethyl cellulose the unperturbed extension increases markedly with increasing substitution.

From the magnitudes of (R,2/R:)1/2 and the persistence lengths calculated from the K-values, i t is concluded that cellulose and these derivatives may be considered as normal, flexible polymers in the unperturbed state.

_ _

Z U S AMM E N F -4 S S U N G : Die GroBe K in der Gleichung [ q ] ~ = K z!$ wurde fiir Hydroxylthylcellulose, Athyl-

hydroxyathylcellulose, Na-Carboxymethylcellulose und Cellulose nach der Theorie von KURATA und STOCKMAYER berechnet.

Aus den K-Werten wurden fiir Hydroxyathylcellulose in Wasser und Cadoxen die Ex- pansionskoeffizienten a,,, berechnet. Die molekularen Dimensionen in diesen beiden Lo- sungsmitteln weichen stark voneinander ab, doch fiihrt die Anwendung der entsprechenden a,,,-Werte zu ubereinstimmenden ungestorten Dimensionen. Im Gegensatz hierzu erhiilt man verschiedene ungestorte Dimensionen, wenn man u-Werte verwendet, die aus den 2. Virialkoeffizienten berechnet werden.

Die Ursache des Temperaturkoeffizienten dln[q] /dT in wariger Losung wird disku- tiert.

Es ergibt sich, daB mit zunehmendem Molekulargewicht des Monomeren die ungestor- ten molekularen Dimensionen der Makromolekiile zunachst abnehmen. Bei sehr goBen Substituenten iiberwiegen jedoch schliefilich die sterischen Effekte. Fiir den Polyelektro- lyten Na-Carboxymethylcellulose nehmen die ungestorten Dimensionen bei zunehmender Substitution markant zu.

179

W. BROWN and. D. HENLEY

_ _ Aus der GroBe des Ausdruckes (R#/R2,)1/* und der Persistenaliinge, berechnet aus den

K-Werten, wird geschlossen, daD Cellulose und ihre in der vorliegenden Arbeit untersuch. ten Derivate im ungestorten Zustand als normale flexible Polymere zu betrachten sind.

Introduction

Part 111) of this series described the influence of solvent and temperature on the configuration of hydroxyethyl cellulose, the data having been interpreted in terms of the FLORY theory. A recent review of KURATA and STOCKMAYER~) discusses the present status of theories leading to un- perturbed dimensions of long chain molecules, and these authors intro- duce a treatment whereby expansion factors can be obtained, principally from viscosity measurements in good solvents. This approach provides a means of obtaining additional information on the system hydroxyethyl cellulose in water.

Further, recent data are a t hand for cellulose3), hydroxyethyl cellulose (M.S. = 1.67)1), ethyl hydroxyethyl cellulose (M.S. = 1.40)4-S) and sodium carboxymethyl cellulose (M.S. = 0.21,0.44,0.94 and 1.06)’**) in a common solvent (cadoxen). Application of the above treatment can give an ap- praisal of the influence of the substituent on unperturbed dimensions.

Discussion

Expansion factors, u, have been calculated for hydroxyethyl cellulosel) from the second virial coefficients, employing the OROFINO-FLORY st

relation A, - ( 1 6 1 ~ / 3 ~ / ~ ) NA (p)3/a/xa Zn[l + (x1’*/2)(a2-l)] (1)

For the data in water solution, Eq. (1) results in a-values which approxi- mate unity, and correspondingly large unperturbed dimensions. Treating the cadoxen data similarly, markedly distinct unperturbed dimensions were obtained, and it could be shown that hydroxyethyl cellulose, in this solvent, behaves as a normal flexible polymer, characterized by small “stiffness” parameters and a rather large degree of swelling. It was sug- gested that in water a substantial part of the molecular expansion originates from a stable packing of solvent molecules around the chain; however, this elspamion is not manifested in u resulting from Eq. (1). The question arises as to how we are to understand this solvent effect. Previously it was put forward that this solvent effect is of a “local” interaction type, akin to short-range interaction forces in that it hinders internal rotations in the chain1). Alternatively, it may be considered as

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Studies on Cellulose Derivatives. Part I11

0.3

0.2

0.1

0.0

a long-range interaction as it is a function of the polymer and its milieu. In an attempt to further separate the factors leading to the (experimen- tally evident) high molecular extension in water we have used the KU- RATA-STOCKMAY ER treat men t.

With the FLORY excluded volume theory and the FLORY-FOX~O,~~) viscosity theory as starting points, KURATA and STOCKMAYER~) have introduced a refined expression from which K (and consequently E) may be graphically estimated. Thus,

[TJ]”~/M’’~ = K2l3 + 0.363 (Do B[g(~c,)M~/~/[q]~’’] (2)

(3)

in which p represents the binary cluster integral and mo is the monomer unit molecular weight.

Fig. 1 includes data plotted according to Eq. (2) (employing Z instead of M) for hydroxyethyl cellulose in water and cadoxen. The lines pass

where g(cc,) = 8a3 (3a2 + l)-3’z Q0 = 2.87.102’ and B = p/mi

o HEC-Water

0.0 0.4 0.8 1.2 1.6

Fig. 1. Viscosity plot according to KURATA and STOCKMAYER for the determination of K in [q]@ = K.Z’$. Experimental data for hydroxyethyl cellulose in water and cadoxen, and

cellulose and sodium carboxymethyl cellulose in cadoxen. Measurements at 25 “C.

181

W. BROWN and D. HENLEY

C 2 10.62 C4 10.00 C8 8.95 C10 3.81 C 1 1 1.70 C 2 6.30 C4 6.00 C8 5.55 C11 1.24

_______

without strain through a common intercept and the intrinsic viscosity under theta conditions may be expressed as

(4 )

A common intercept in polar systems is not necessarily to be expected2). Nevertheless, the common intercept suggests the absence of a preferred configuration depending on the solvent medium. Thus, the presence of intramolecular hydrogen-bonding of the type suggested by HERMANS l3),

stabilizing the structure in water solution, may be assumed to be absent. Here the assumption is made that such bonds do not exist in the highly alkaline solvent cadoxen. The different slopes for hydroxyethyl cellulose in water and cadoxen are a reflection of differing coil expansions. Although substantially different cc?-values result in the two solvents the introduction of these expansion factors to measured dimensions from light scattering leads to unperturbed dimensions which are the same within experimental error (Table 1). Further, in contrast to

[ q ] ~ = 3.85 .10-zz%2.

2650 2420 2180 805 340

2560 2420 2230 330

Table 1. Hydrodynamic and configurational data for hydroxyethyl cellulose in water and cadoxen

Solvent

Water

Cadoxen

(pa)lf

(4

1830 1790 1750 1110 745

1520 1460 1405 625

%I KURATA- STOCK- MAYER

1.75 1.74 1.71 1.52 1.34 1.48 1.47 1.45 1.21

KURATA- STOCK- MAYER

1045 1030 965 730 555

1025 1000 970 515

0:

OROFINO- FLORY

1.08 1.07 1.06 1.06 1.04 1.42 1.29 1.38 1.09

(W1LZ(-Q OROFINO-

FLORY

1690 1670 1650 1045 715

1070 1140 1020 575

*) ID.,, = 236.

the large unperturbed dimensions resulting from the second virial co- efficient treatment, the a?-values (KURATA-STOCKMAYER) in water are large and lead to correspondingly small unperturbed dimensions. More- over, the magnitudes of the ccq-values resemble those of normal flexible polymers. This substantiates previous observations that the unusual extensions of cellulose derivatives in polar solvents result from local

182

Studies on Cellulose Derivatives. Part I11

solvent interactions12. *) with the chain, which are apparently not mani- fested in the virial coefficient, rather than conventional shortrange in- teractions. It appears that in such polar systems the virial coefficient can be an unsatisfactory index of the magnitude of the polymer-solvent interactions and consequently of the extension of the polymer chain.

I n cadoxen, the unperturbed dimensions are approximately the same regardless of the approach taken in calculating the expansion factor (Eqs. (1) or (2)). The coil expansion in this solvent thus originates in the normal manner from interactions characterized by the second virial treatment.

Some information can be derived concerning thermodynamic influences on the chain conformation. Fig. 2 presents viscosity data a t different temperatures for hydroxyethyl cellulose in water 14). The data suggest a

0.4

0.3

0.2

0.1

0.0 0.0 0.4 0.8 1.2

Fig. 2. Viscosity plot according to KURATA and STOCKMAYER for hydroxyethyl cellulose in water at different temperatures

common intercept, coincident with that for this polymer in Fig. 1. The solvent interaction parameter B may be evaluated from Eq. (2) for each temperature and is plotted versus 1/T in Fig. 3. The temperature de-

183

W. BROWN and D. HENLEY

I I

\ \ \

\\ 0.7 *

0.5 - 0:3

\

0 100

-

pendence of B may be accordingly described by

B = -8.5*10-27 [ [l-(+)]. (5)

The consolute temperature for hydroxyethyl cellulose in water (B = 0) is thus approximately 300 "C. From Eq. (4) we find [q10 ~ 2 . 0 for fraction C 2. It is interesting to note that extIapolation of log [q] ws. T"C.

10.0

8.0

6 .O

4.0

2 .o

0.0 0.0 1.0 2 .o 3 .O 4.0 1

T ' Fig. 3. Main figure: iqteraction parameter B for hydroxyethyl cellulose in water as a function of the, reciprocal of absolute temperature. Insert: the dependence of log [q] on

temperature for hydroxyethyl cellulose in water. All data for Fraction C 2

(insert to Fig. 3) t o this viscosity is in agreement with a consolute temperature of approximately 300 "C. As stated by KURATA and STOCK- MAYER, an equation of the form of Eq. ( 5 ) corresponds t o a negative heat of dilution causing the polymer to deswell upon heating; it also yields a negative value of the entropy parameter, qI1l), which may be related to orientation effects involving solvent molecules around polymer segments. It is possible in this way to account for the rapid decrease of intrinsic viscosity with rising temperature (insert, Fig. 3) without requiring an

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Studies on Cellulose Derivatives. Part I11

11.98 10.62 9.94 9.28 8.91

abnormal temperature coefficient of the unperturbed dimensions. As light scattering data over a range of temperature are available for hydroxy- ethyl cellulose in water it is possible to examine this question. uq-values and corresponding unperturbed dimensions have been calculated a t different temperatures (Table 2). Bearing in mind the experimental errors

775 730 690 670 655

Table 2. Temperature dependence of light scattering and viscosity data for hydroxyethyl cellulose in water (Fraction C4)*)

1.83 1.76 1.72 1.68 1.66

Temp. ("C.)

425 415 400 400 395

3 25 38 51 58

*) The intrinsic viscosities are values interpolated from the dependence of log [q] on T"C. for fraction C 2 of similar molecular weight to &action C4 (see Table 1). While the absolute magnitudes of the viscosities differ for these fractions it is assumed that 9 will differ insignificantly for the present purposes.

inherent in (p)'L2, the approximate constancy of the unperturbed di- mensions supports a common intercept in Fig. 2, which would lead to the conclusion that the temperature dependence of viscosity is prim- arily caused by deswelling (decrease in %). However, the magnitude of the temperature dependence of the intrinsic viscosity is approximately that of the uncertainty in the estimation of the cube of the unperturbed radius of gyration occurring in the FLORY expression for viscosity. Thus the present data preclude any definite conclusions as to the specific origin of dWnl .

Data for cellulose3) and sodium carboxymethyl cellulose7) in cadoxen

( 6 )

(7)

Data similarly obtained for ethyl hydroxyethyl cellulose and other samples of sodium carboxymethyl cellulose are included in Table 3.

From our data we have calculated the steric factor (@/R",ll2 which provides a measure of the hindrance to internal rotation about the single

dT

are included in Fig. 1. The intercepts lead to the equations

[-I& = 6.3 .lowz z1Lz (cellulose)

[?lo = 5.0.10-2 zl$.. (sodium carboxymethyl cellulose) and

185

W. BROWN and D. HENLEY

bonds of the chain. (G)1/2 is the R.M.S. end-to-end distance assuming free rotation about the intermonomer C-0 bonds and is given by15)

(Iq)112 = 7.75 Zl/Z ( 8 )

(%)ll2 may be calculated from K according to KURATA and STOCKMAYER,

where @,, = 2.87.1021 and mo is the monomer unit molecular weight. Heterogeneity corrections 2, have been applied by means of the relation- ship (@o)w = qw.@o. In the case of the most probable distribution, as- sumed for the present samples, qw = 0.94. Values of (Rg/Rf2)1/2 are given in Table 3. It is apparent that the glucose unit of the cellulose chain can accomodate substitution, in the present range, without pronounced alter- ation of the chain extension. F L O R Y ~ ~ ) has noted that an inspection of

--

3 .O

2.5

2 .o

1.5 0

0

2.2

2.0

1.8

I 160 200 240 260 I I I I

100 200 300 400 500 600 mO

Fig. 4. Steric factors (EE/i?:)l/z for cellulosic polymers plotted against molar weight, m,,, of the repeating unit. The filled points are, left t o right, cellulose, ethyl hydroxyethyl cellu- lose, and hydroxyethyl cellulose (see Table 3). The open points are data tabulated by KURATA and STOCKMAYER~) and data questioned by these authors are omitted. The insert depicts data for sodium carboxymethyl cellulose (Table 3). It may be noted that KURATA and STOCKMAYER employ the constant 7.90 in Eq. (8). This difference is inconsequential

for the present graphical comparison

186

Studies on Cellulose Derivatives. Part I11

236

models shows that the butyrate units in cellulose tributyrate (m, = 372) can be accomodated about the cellulose ring with less obstruction than for the phenyl groups of polystyrene.

However, in order t o examine the effect of substituents over a sub- stantial size interval we have combined our data with those listed by KURATA and STOCKMAYER. The latter authors suggest from the accumu- lated data, that values of (R8/R$)ll2 vary more or less symbatically with m,, and reflect the relative sizes of the substituent groups. On intro- ducing our data it is felt that a curve passing through a minimum at a substitution corresponding approximately to the trinitrate or tributyrate is more warranted, Fig. 4. A similar tendency has been noted for a series of polymethacrylates16). Substitution up to a certain level thus results in decreased unperturbed dimensions, but when the side groups are very large the steric effect predominates through increased hindrance to in- ternal rotation. As shown in the insert to Fig. 4 sodium carboxymethyl cellulose shows a pronounced increase in the ratio (R$/Rf)1/2 as the sub- stitution is increased over a comparatively small interval, which is pre- sumably a result of electrostatic interactions between the groups.

-. -

_ -

, 3.85

Table 3. Values of K in [ q ] ~ = K*Z1L2 according to the KURATA-STOCKMAYER theory, together with some extension parameters

Polymer

Cellulose ...................... Sodium carboxymethyl cellulose

Ethyl hydroxyethyl cellulose . . . . Hydroxyethyl cellulose . . . . . . . . .

162 178 197 237 247 210

K.10-2

6.3

5.0

6.60 *)

4.85 **)

2.0 1.9 2.0 2.1 2.4 2.0 1.9

Persist. length (4

24 22 23 26 36 24 22

- Ref.

*) The value K = 6.6-10-* for sodium carboxymethyl cellulose has been calculated from a common intercept resulting from molecular weight and viscosity data in the follow- ing solvents, 0.2 M NaCl, 0.05 M NaCl, 0.01 M NaCl, 0.005 M NaCl, for four samples comprising a 10-fold range of molecular weight.

**) The value K = 4.85.10-2 for ethyl hydroxyethyl cellulose has been calculated from a common intercept resulting from viscosity data in the solvents, water4), dimethyl sulphoxide5) and cadoxen6) and molecular weights determined by light scattering measurements in cadoxenel. Mw = 350,000, 255,000, 165,000, and 115,000, and [qIz5 =

5.40, 4.40, 3.10, and 2.15 for the fractions M12, M7, M5, and M3, respectively, in the latter solvent.

187

W. BROWN and D. HENLEY

Values of the persistance length, calculated from (@/Z)-values de- duced from Eq. (9), are given in Table 3. The magnitudes of this para- meter and (R+j/&2)1/2 indicate that in the unperturbed state cellulose and these derivatives are typical flexible polymers. A similar conclusion has been reached by KURATA and STOCKMAYER for cellulose trinitrate and finds support from small angle X-ray scattering studies on this com- pound by HEINE, KRATKY, POROD, and SCHMITZ1'). Thus these cellulose derivatives derive their high extensions in solution from polymer-solvent interactions, rather than hindrance to internal rotation arising from typical short-range interactions such as steric repulsions between side groups. Use of the KURATA-STOCKMAYER treatment thus leads to a clearer understanding of the factors contribu ting to the molecular expansion in such systems.

--

This work has been financially supported by the SWEDISH NATURAL SCIENCE RESEARCH COUNCIL and the SWEDISH TECHNICAL RESEARCH COUNCIL. This support is gratefully acknowledged.

The authors wish to thank Professor STIG CLAESSON for stimulating discussions and for placing the excellent facilities of the Institute a t their disposal.

1, W. BROWN, D. HENLEY, and J. OWUN, Makromolekulare Chem. 64 (1963) 49. 2, M. KURATA and W. H. STOCKMAYER, Fortschr. Hochpolymeren-Forsch. 3 (1963) 196. a) D. HENLEY, Ark. Kemi 18 (1961) 327. *) R. ST. JOHN MANLEY, Ark. Kemi 9 (1956) 519. 5, R. ST. JOHN MANLEY, Svensk Papperstidn. 61 (1958) 96. e, W. BROWN and D. HENLEY, unpublished data. ') W. BROWN, D. HENLEY, and J. OHMAN, Makromolekulare Chem. 62 (1963) 164. *) W. BROWN, D. HENLEY, and J. OHMAN, to be published in Ark. Krmi (1964).

lo) P. J. FLORY and T. G. FOX, Jr., J. Amer. chem. SOC. 73 (1951) 1904, 1909, 1915. 11) P. J. FLORY, Principles of Polymer Chemistry, Cornell Univ. Press, Ithaca, New York.

12) P. J. FLORY, 0. K. SPURR, Jr., and D. K. CARPENTER, J. Polymer Sci. 27 (1958) 231. la) P. H. HERMANS,, Physics and Chemistry of Cellulose Fibres, Elsevier, New York and

la) W. BROWN, Ark. Kemi 18 (1961) 227. 15) I. ELIEZER and H. J. G. HAYMAN, J. Polymer Sci. 23 (1957) 387. 16) S. N. CHINAI, J. Polymer Sci. 25 (1957) 413; S. N. CHINAI and R. J. VALLES, ibid. 39

17) S. HEINE, 0. KRATKY, G. POROD, and P. J. SCHMITZ, Makromolekulare Chem. 44-46

T. A. OROFINO and P. J. FLORY, J. chem. Physics 26 (1957) 1067.

1953.

Amsterdam, 1949.

(1959) 363.

(1961) 682.

188