Studies on cellulose derivatives. Part I. The dimensions and configuration of sodium carboxymethyl cellulose in cadoxen and the influence of the degree of substitution

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

Studies on Cellulose Derivatives

Part I. The Dimensions and Configuration of Sodium Carboxymethyl Cellulose in CADOXEN and the Influence of the Degree of Substitution

By W. BROWN, D. HENLEY, and J. OHMAN

(Eingegangen am 8. Oktober 1962)

SUMMARY: Triethylenediamine-cadtviuni h: droxide (cadoxen) has been found to be an excellent

solvent for several cellulose derivatives, making possible comparative studies of cellulose and cellulose derivatives. This paper describes an investigation of the system sodium carboxymethyl cellulose - cadoxen, using viscosity, osmometry and light scattering measurements.

Using three series of samples in the substitution range 0.2 to 1, the effect of the degree of substitution has been studied with regard to its influence on the molecular configuration and dilute solution properties.

It is found that sodium carboxymethyl cellulose in cadoxen possesses a configuration independent of the degree of substitution. Furthermore, the configuration is similar to that of cellulose in this solvent. As indicated by the low values of extension parameters such as the effective bond length, b, the KUIIN-KUHN equivalent chain segment length, Am, and the ratio of the unperturbed dimension to the dimension assuming free rotation of the chain units, (Ri/Ri)lI2, it is concluded that sodium carboxymethyl cellulose in cadoxen displays characteristics of a randomly coiled polymer in a good solvent.

- _

ZUSAMMENFASSUNG: Triathylendiamin-Cadmiumhydroxyd (Cadoxen) ist ein hervorragendes Losungsmittel

sowohl fur Cellulose als auch fur einige ihrer Derivate. Dadurch werden vergleichende Studien dieser Stoffe moglich.

Die vorliegende Arbeit beschreibt eine Untersuchung des Systems Natrium-Carboxy- methylcellulose durch Messungen von Viskositat, Osmose und Lichtstreuung.

An drei Serien von Proben wurde die Abhangigkeit der Konfiguration*) der Makromole- kiile sowie der Eigenschaften verdiinnter Losungen vom Substitutionsgrad im Bereich 0,2 bis 1 studiert.

Die Konfiguration von Natrium-Carboxymethylcellulose in Cadoxen wurde vom Sub- stitntionsgrad unabhangig und ahnlich der von Cellulose im genannten Losungsmittel gefunden.

Eine Betrachtung der gefundenen niedrigen Werte fur die Streckungsparameter, wie die effektive Bindungslange b, die Lange Am des statistischen Fadenelementes nach KURN- KUHN sowie das Verhaltnis (R$/Ri)1’2 der Dimensionen des ungestorten Molekiils zu den Dimensionen unter der Annahme freier Rotation der Fadenelemente, fiihrt zu dem SchluB, daB Natrium-Carboxymethylcellulose in Cadoxen den Charakter eines statistischen Knauel- molekuls in einem guten Losungsmittel zeigt.

--

*) Im Sinne der Formenmannigfaltigkeit.

164

Studies on Cellulose Derivatives. I.

I. Introduction

As part of a general programme a t this Institute dealing with the behaviour of macromolecules in solution, with emphasis on cellulose and its derivatives, a study of sodium carboxymethyl cellulose in aqueous solutions is now in progress.

It is well known that in thermodynamically good solvents the cellulose derivatives are generally molecules of the inflexible, highly extended type. A recent paper has described measurements on the dilute solution properties of cellulose in the triethylenediamine cadmium hydroxide complex solvent, cadoxenl). In this solvent, however, i t may be concluded that cellulose should not be considered a highly extended molecule and that its behaviour is more similar t o tha t of a flexible molecule in a good solvent, for example polystyrene, than the classical example of a stiff molecule, cellulose trinitrate. To examine the influence of different substituent groups on the configuration of the cellulose derivatives in relation to the properties of the cellulose backbone a single solvent is re- quired, and the finding that several cellulose derivatives are also soluble in cadoxen presented the opportunity of making these comparative studies. This paper (Part I) deals with measurements on the polyelectro- lyte, sodium carboxymethyl cellulose, while Parts IIz) and 1113) describe investigations on hydroxyethyl cellulose and ethyl hydroxyethyl cellulose, respectively.

It was decided to investigate the influence of the degree of substitution of the sodium carboxymethyl cellulose molecule in the range 0 t o 1. This substitution region has received little attention owing to the difficulty of finding a suitable solvent. Investigations in water solution have been necessarily restricted to substitutions greater than 0.5, although even the use of a substitution less than 1 may lead to serious problems with measurements in dilute solution4). Sodium hydroxide is a good solvent over a more extended substitution range than water but has a restricted use for precision measurements mainly because i t causes a high rate of oxidative degradation with this cellulose derivative. The low degradative action of cadoxen (vi.), combined with its ability t o dissolve sodium carboxymethyl cellulose down to a substitution approaching zero, makes i t a suitable solvent for the present study.

Par t I describes the study of a series of sodium carboxymethyl cellulose samples of different degrees of substitution using the techniques of viscosity, osmosis, and light scattering. The aim was to establish the molecular configuration of the molecule with regard to the effect of the substituent and the amount thereof.

165

W. BROWN, D. HENLEY, and J. OHMAN

I I . Experimental

1 . Samples and Solutions The samples were obtained from the Research Laboratory of SVENSKA CELLULOSA A.B.,

and were prepared as follows*). Four batches (labelled 10, 11, 12, and 13), each of 1500 g cotton linters (m ca. 5000) were mercerized in 19.5% NaOH for 1 hr. at 20%. Using the present procedure, the NaOH concentration of 19.5% has been found to give the most even substituent distribution in the etherification reaction with sodium monochloroacetate. The alkali cellulose was then pressed to a press factor of 2.55 which gave the composition 37.5% cellulose and 15.0y0 NaOH. Each batch of alkali cellulose was then divided into five parts (13a, b, c, d, and e, etc.) and these allowed to degrade in air under the following conditions:

a 0 hrs. b 2 hrs. at 20°C. c 30hrs. at 2OOC. d 82 hrs. a t 2OOC. e 1OOhrs. a t 3OOC.

To produce the desired degree of substitution, the following quantities of sodium monochloroacetate were added to the batches

10 11 0.77 mole/mole cellulose 12 1.26 mole/mole cellulose 13 1.73 mole/mole cellulose

0.40 mole/mole cellulose (162 g.)

These amounts of sodium monochloroacetate were mixed with the alkali cellulose under nitrogen in a mechanical mixer for 1.5 hrs. at 2OOC. The reaction was then allowed to continue under nitrogen in a rotating glass flask at 30'C. for 20 hrs**).

On completion of the reaction the samples were slurried in 70 yo ethanol and neutralized with HCI. After filtration they were washed with 70% ethanol, dried at 80°C. and finally powdered.

Samples 13 (a-e) were dissolved in water and precipitated with acetone. They were then redissolved in water, centrifuged and freeze dried. It was thought that after this treatment the lowest molecular weight part of the samples and also insoluble aggregates would have been removed.

All samples were stored in the dark in a desiccator. Batches 10, 11, and 13 (degree of substitution: 0.2, 0.4, and 0.9 respectively) were

selected for the measurements described below. The degrees of substitution were determined using a method similar to that of WILSON~) involving ashing and subsequent titration of the formed sodium carbonate.

The cadozen was prepared as previously described1). The concentrations of the components of the solvent were: CCd - 5.2 yo, C N ~ O H = 0.32M,

cen = 25% (yo by weight).

*) Prepared under the supervision of Dr. A. WENNERBLOM. **) Degradation continues under etherification and will be greatest for those batches

containing the most excess alkali after the reaction, i.e. those in which the lowest amounts of monochloroacetate have been used. Thus the degradation will be greatest for batch 10 and least for batch 13 (see Table 2).

166


Bulb volumes (ml.) ........................ 2.65 Flow times (1:l diluted cadoren) a t 25°C. (sec.) 105.7 Capillary diameter (mm.) ................... 0.56

The solutions were prepared as weight sodium carboxymethyl cellulose per volume cadoren and subsequently diluted 1 :1 by volume with water. Such a diluted cadoren has been found to be particularly advantageous as it no longer dissolves cellulose and can be used in contact with cellulosic membranes, for example, in osmosis and dialysis. The solutions were finally cleaned by filtration through a G-3 sintered glass filter. In the special case of light scattering measurements the solutions were further clarified by ultracentrifu- gation. New solutions were prepared for each measurement.

The experimental details are essentially those described in reference 1 except when specifically stated otherwise.

2.21 1.53 0.88 105.9 118.8 143.7

2. Viscosity The oxidative degradation rate of sodium carboxymethyl cellulose in cadoren was in-

vestigated in preliminary experiments. These measurements were made at 25OC. on solutions continually saturated with air. The degradation (measured as yo decrease of qlsp/c per hr.) was 0.1% for the sample of highest molecular weight (sample 13a). This was considered sufficiently small to be neglected for the present measurements.

The primary viscosity measurements were made in an UBBELOHDE type capillary visco- meter having four bulbs a t different heights to allow investigation of the non-NEWTONian behaviour of the solutions. The dimensions were:

All measurements were made a t 25°C. Kinetic energy corrections were applied where necessary.

The relative viscosities were linearly extrapolated to zero velocity gradient, the range of the latter being approximately 100-500 sec-l. The extrapolated values were used for the calculation of (qsp/c)~-o which was plotted versus c to determine [q].

3. Osmotic pressure Although measurements may be made directly in cadoxen (diluted 1 :1 with water), the

membranes used for this solvent are quite slow and thus this work was carried out in 0.2 M NaCl. The osmometer was of the FUOSS-MEAD block type6) and the membranes selected were Ultrafein filter “fein” from MEMBRANFILTER-GESELLSCEAFT, Gottingen. The measurements were made a t 25OC. in the concentration range 0.1 to 0.5 g.dl-l.

4. Refractive Index Increment To eliminate fluctuations due to salt concentration differences in the system, the so-

lutions were dialyzed against solvent prior to the measurements of dn/dc. The dialysis experiments were made in a plexiglass cell using ordinary cellulosic membranes (“sausage- casing“ from VISKING Co., Division of Union Carbide and Carbon Corp., Chicago, USA.). The electrolyte concentration of the system was sufficiently high to minimize possible charge effects. The membranes were equilibrated with solvent before use and the dialyses carried out for two-day periods.

167


5 . Light Scattering It was found that any foreign material could be satisfactorily removed by a combination

of a precentrifugation, and a subsequent direct centrifugation of the light scattering cell using the technique described by DANDLIKER and KRAUT '). Thus the solutions were pre- cleaned by centrifugation for 2 hrs. a t 35,000 r.p.m. followed by a further 2 hrs. a t 20,000 r.p.m. in the DANDLIEER cell (SPINCO preparative ultracentrifuge, rotors 40 and SW-25 respectively). Solutions centrifuged insufficiently showed downward curvature in both the high and low angle regions of plots of the angular dependence of Kc/Ra.

The light scattering measurements were made with a new, highly sensitive light scattering apparatus designed and constructed a t this Institute by CLAESSON and OHMAN*). This instrument is coupled to a recorder that automatically registers the intensity of the scattered light as a function of scattering angle. The absolute calibration was made by reflectivity measurements using a magnesium oxide surface having a reflectivity of 0.97 at 436 mp. A further calibration was made with LUDOX colloidal silica. The cell is mounted in a thermostat making it possible to measure over the temperature range 0"-150°C. The present measurements were made at 25°C.

The mercury blue line (436 mp) was isolated by means of a ZEISS interference 6Iter and used throughout after it had been ascertained that the green mercury line (546 mp) gave the same values of Kc/Ra indicating the absence of fluorescence. The scattered intensities were recorded on a t least six concentrations of each sample, chosen in the range 0.05 to 1 g.dl-l. The intensities were read off a t 10' intervals from 45-135" and treated in the standard way to obtain values of Kc/Ra. The scattering of the solvent was treated similarly. In these measurements benzene was used as the standard (Rs0o = 4 6 . 5 ~ 1 0 - ~ em-' a t 436 mp and 25OC.) instead of the intensity of the transmitted light a t 0" as used previously.

'

111. Results

1. viscosity

The viscosity data of Table l a were obtained from plots similar t o those in Fig. 1 where data for samples 13 (a-e) are shown. HUGGINS' constant, k', is seen t o be essentially constant over the whole molecular weight range and lies in the expected range of 0.3-0.4. However, there is a striking deviation with samples 13b, l l b , and 10b which may be the result of some peculiarity in the preparation procedure. Generally, k' is of the same order as tha t for cellulose in cadoxen.

The dependence of intrinsic viscosity on velocity gradient is depicted in Fig. 2 for samples 13a and 13b. There is a change in [q] of approximately lo%, over the gradient range 0-600 sec-l, for both samples. Both the slopes and intercepts are seen to vary with 6. Table 1 b sum- marizes the data.

It is well known tha t in solutions of high salt concentration and in sodium hydroxide the charge effects of the sodium carboxy methyl cellu-

168


Table 1 a. Intrinsic Viscosities and the HUGGINS Constant for Sodium Carboxymethyl Cellulose in Cadoxen a t 25°C

40

Z o -

Sample

13 a 13b 13c 13d 13e 11 a l l b 11 c l l d l l e 10 a 10b 1oc 10 d 10e

.

U d

005 OM OK QZQ 025

13 c

Degree of subst.

0.96 0.96 0.91 0.94 0.93 0.46 0.45 0.44 0.44 0.43 0.21 0.21 0.21 0.21 0.22

10.30 8.10 4.85 2.55 1.20 8.15 6.95 4.65 2.45 1.45 6.50 5.45 3.80 2.35 1.25

k’

0.42 0.90 0.33 0.43 0.35 0.41 0.72 0.29 0.33 0.33 0.37 0.79 0.41 0.41 0.42

169


k‘

0.42 0.40 0.38 0.35 0.90 0.89 0.88 0.86

140

120

1M1

110

YO

120

m

bo

, m po2 ca a s ms am 012cg.di’

Fig. 2. Extrapolation of reduced viscosity a t 25OC. to zero concentration a t different fixed shear rates

- “ l / A ~ .102[dl.g-l sec.]

0.09

0.07

lose molecule are essentially swamped and it possesses the characteristics of a neutral polymer. This is also found to be the case in cadoxen, where the solution is approximately 0.5 M in relation to cadmium ions, and 0.3 M with respect to NaOH.

13a

13b

Table 1 b. Intrinsic Viscosities and Related Coefficients for Sodium Carboxymethyl Cellulose in Ca&zen at 25°C.

0 200 400 600

0 200 400 600

10.30 10.15 9.95 9.75 8.10 7.95 7.85 7.70

2. Osmotic Pressure The osmotic pressure cdta were plottei-- as x/c us.’ c, of which an ex-

ample is shown in Fig. 3. The xn values for samples 13c, 13d, and 13e are given in Table 2. Samples 13a and 13b possess molecular weights too large to obtain .reliable measurements.

170


13a 13b 13c 13d 13e l l a l l b l l c l l d l l e 10a 10b 10 c 10 d 10 e

a w c m.-' 1 am

nu

m I a 02 m 01 c a . I a'*I OlZ m a1 OL as wee M ol5JbsllNl"m UI

Fig. 3 Fig. 4

Fig. 3. Reduced osmotic pressure as a function of concentration for sodium carboxymethyl cellulose (sample 13c) in 0.2 M NaCl a t 25OC.

Fig. 4. The refractive index increment, dn/dc, plotted us. degree of substitution for sodium carboxymethyl cellulose in cadoxen. The filled-in point at zero degree of substitution is

for cellulose in cadoxen (ref. l ) , dn/dc = 0.186 ml-g-l. All measurements a t 25OC.

0.96 0.96 0.91 0.94 0.93 0.46 0.45 0.44 0.44 0.43 0.21 0.21 0.21 0.21 0.22

Table 2. Summary of Light Scattering Data for Sodium Carboxymethyl Cellulose in Cadoxen at 25°C.

Degree of subst.

Sample

239 1055 4420 835 3500

2000

285

239 235 470 305 237 190 93 236 67 40 198 660 3340 197 625 3175 197 325 1650 197 155 785 196 68 345 178 470 2640 580 178 385 2160 540 178 245 1375 425 178 97 545 210 179 49 275

~ ~ - 1 0 4 [ml mole. g-2]

8

9.8 9.9 9.4

11.9 14.6 12.5 13.2 13.8 16.0 16.9 13.3 12.6 12.9 12.9 14.4

*) gn determined by osmotic pressure measurements in 0.2 M NaCl.

3. Refractive Index Increment

The results for the wavelengths 436mp and 546 mp are given in Table 3. Fig. 4 shows the refractive index increments for the samples of

171


- Degree

of subst.

0.94

0.44

0.21

different degree of substitution plotted against degree of substitution. The points were found to lie on a smooth curve, which passed through the value previously found for cellulose (Le. D.S. = 0 ) in cadoxen.

C

[g.dl-']

0.561 0.390 0.548 0.565 0.517 0.522

Table 3. Refractive Index Increment Data for Sodium Carboxymethyl Cellulose in Cudoxen a t 25°C.

Sample

13d

l l d

10d

dn/dc[ml.g-'] 546 mw

0.145 0.145 0.162 0.161 0.171 0.168

436 mw

0.147 0.148 0.164 0.162 0.175 0.171

It was noted that when the solutions were exposed to either wave length for an extended period (12 hrs.) while in the differential refracto- meter cell, there was a continuous increase in the refractive index differ- ence between the solution and solvent, which could lead to dn/dc values as high as 0.23 ml. g-l. This increase was not observed if the solutions were allowed to stand in the dark for the same period. This could indicate that there is some form of photodecomposition in cadoxen solutions of sodium carboxymethyl cellulose, even if a more trivial explanation cannot be excluded at the present time.

12.0

XD

8.0

61)

4 0

20

320

260

199

146. I 24

am

ow.

OD a2 a4 06 RE 10 Y v2

Fig. 5. Extrapolation of Kc/Ro to zero angle for fixed concentrations of sodium carboxymethyl cellulose (sample 11 b) in cadoxen at 25°C.

172


4. Light Scattering

Values of Kc/R, are plotted versus sin2 0 / 2 as shown in Fig. 5 for a high molecular weight sample (11 b). Fig. 6 shows the corresponding plot for a low molecular weight sample ( l l e ) and although the points in the latter are more scattered than in the former, compl.-tely satisfactory extrapolations could be made even in this low molecular weight region.

m

250

2ca

150

2

Fig. 6. Extrapolation of Kc/R, to zero angle for fixed concentrations of sodium carboxymethyl cellulose (sample l l e ) in cadoxen at 25OC.

Figs. 7 and 8 show the zero angle intercepts from Figs. 5 and 6 respectively plotted against c expressed in g'ml-l. Values of Nw and A, are recorded in Table 2. Data for selected constant angles were plotted us. c in the same way as the zero angle data in Figs. 7 and 8 and the intercepts a t zero concentration plotted against sin20/2 as shown by the filled circles in Figs. 5 and 6. The z-average root mean square radius of gyration, (g)i/2, was calculated for each sample from the ratio of slope to intercept of such a line using the relationship

Initial slope Intercept 3

where h/n is the wavelength of the light in the solution of refractive index n. (p):/2 values are given in Table 2.

173

W. BROWN, D. HENLEY, and J. O ~ A N

no

M

6D

l.0

20

Po

Fig. 7

).mi‘

Fig. 8

Fig. 7. Concentration dependence of (Kc/RQ)Q,o for sodium carboxymethyl cellulose (sample l l b ) in cadolcen at 25OC.

Fig. 8. Concentration dependence of (Kc/RQ)Q,o for sodium carboxymethyl cellulose (sample l l e ) in eadolcem at 25OC.

IV. Discussion

The modified STAUDINGER equation, empirically relating a polymer’s intrinsic viscosity to its molecular weight, is

[?I - K m (2)

Ideally, M should be the viscosity average molecular weight. However, for a sample having a Mw/EIn ratio of approximately 2, Nw is a close approximation to M v S ) . As shown in Table 2, the nw/Rn ratios for samples 13c, 13d, and 13e are close to 2, and it is assumed that this is so for the other samples.

A plot of log [q]26 vs. log zw (where z is the average degree of polymerization) is shown in Fig. 9 and the line may be represented by

[?Iz5 = 2.0.10-2 ZwO.73 (3)

[?Iz5 = 1.7.10-2 zW0.77 (4)

The corresponding equation for cellulose in cadoxenl) is

174


60

5 s .

52

u -

u .

The cellulose samples are cotton linters which is a similar material t o that used in the preparation of the sodium carboxymethyl cellulose for this study. Fig. 9 shows tha t all samples of sodium carboxymethyl cellulose,

.

.

l 2 t

Fig. 9. Log intrinsic viscosity us. log weight average degree of polymerization for sodium carboxymethyl cellulose in codoxen at 25OC.

independent of degree of substitution within the range 0.2 to 1, lie on a single line. Further, Equ. 3 must be considered identical within experimental error t o Equ. 4. It is interesting t o see tha t the side groups of sodium carboxymethyl cellulose in cadoxen have a negligible influence on the molecular weight dependence of intrinsic viscosity in this range of substitution.

It may be noted here tha t SCHURZ e t a1.10) have determined constants for Equ. 2 in 2 yo NaCl and 6 yo NaOH using number average molecular weights.

I&& 4

W ZY 31 I5 39 (0

W Z

Fig. 10. Log mean square z-average radius of gyration plotted against log weight average degree of polymerization for sodium carboxymethyl cellulose at 25°C. Slope = 1.25

175


I n Fig. 10 log (s"), is plotted versus log z,. The scattering of the points is more pronounced towards low molecular weight as may be expected. Taking into account the greater uncertainty in the measurement of (s), in comparison with [q], we find the same independence of the substitution degree as noted in Fig. 9, showing that the degree of substitution has no influence on the configuration.

The equation of the line in Fig. 10 may be expressed in the form

= 2 1 . zw1.z5 (AS) (5)

According to the FLORY theory9), for flexible molecules in good solvents, we have the following relationship

(6) (Fz) = K, Z(1ca). 213

where a is the exponent in Equ. 2. Thus from Equ. 3 we find an exponent of 1.15 for Equ. 5. This value agrees reasonably with that found experimentally.

- I Fig. 11. Log second virial coefficient us. log weight average .degree of polymerization for

sodium carboxymethyl cellulose in cadoxen. at 25 'C.

Fig. 11 shows log A, plotted us. log 2,. Lines drawn through the points for samples 13 (a-e) and 11 (a-e) have negative and approximately equal slopes. The points for samples 10 (a-e) are scattered although they are of the same magnitude. The equation for samples 11 (a-e) is

A 2 - - 37 . zW-O.13 (7 1 Considering samples 13 and 11, A, apparently decreases with increasing degree of substitution. A, is also qualitatively lower for these samples than for cellulose in cadoxenl). This could indicate a higher degree of solvent interaction with the lesser or unsubstituted molecules which is reasonable if the solvent mechanism of cadoxen is operative by some interaction with the hydroxyls of the anhydroglucose unit ( u . ~ ) .

176


13a 13b 13c 13d 13e

Table 4. Configurational and Hydrodynamic Parameters for Sodium Carboxymethyl Cellulose in Cadozen

6300 1.46 2040 1400 31 4980 1.41 1800 1280 33 2850 1.29 1350 1040 38 1140 1.22 810 660 38 405 1.14 455 400 40

615 545 415 260 155

2.3 2.3 2.3 2.1 2.5 1.7 2.5 1.6 2.6 1.5

As no dependence of molecular parameters on the degree of substitution has been noted (Figs. 9 and 10) for the sodium carboxymethyl cellulose fractions in cadoxen, we have chosen fractions 13 (a-e) for the purpose of estimating some configurational and hydrodynamic parameters. Number-average molecular weights have been determined for fractions 13c, 13d, and 13e. A summary of these data is given in Tables 4 and 5. According to the FLORY-FOX theory, in a good sol r-ent, we have

[q] = K . M1/2. a3

whereas in an ideal solvent

Thus, [ q ] ~ = K . M112

h l = [qlo

KURATA and STOCKMAYER~~) have recently taken into account the excluded volume of the polymer coil and introduced a refined expression from which K and M may be calculated

[q]2’3/M’f3 = K2’3 + 0.363 . qo . B [ g ( ~ ) M ” ~ / [ ~ j l ~ ’ ~ l (11)

g(cc) = 8 . c c 3 . (3 . az + 1)-3/2 (12) where

‘p,, = 2.87.1OZ1 and B = P/mi in which p represents the binary cluster integral and m,, is the monomer unit molecular weight. Using this expression, according to reference 12)

(but employing ‘i instead of M), the data were plotted as shown in Fig. 12. All points, independent of degree of substitution, lie about a single Line as in Fig. 9. K was obtained from the intercept and has the value K =

5.0 leading to the equation [?lo = 5.0.10-’ - zwl’z (13)

177


Table 5. Extension Parameters for Sodium Carboxymethyl Cellulose in Cadoxen, and for other systems

System

Sodium Carboxymethyl Cellulose in Cadoxen (sample 13a)

Sodium Carboxymethyl Cellulose in NaCl (infinite ionic strength)

Cellulose in Cadoxen

Cellulose trinitrate in ethyl acetate

Polystyrene

18

37.4 19

35

8.6

60 (12 monomer units)

- 70

(14 monomer units) 234

(45 monomer units) 33

(13 monomer units)

2.3

- 2.7

4.5

2.2

- Ref.

Employing Eqs. 13 and 10, [q10 and hence u could be calculated. The U-values are given in column 3 of Table 4. As is evident from Fig. 12, a t a given molecular weight the expansion factors for samples 10, 11, and 13 will be identical within experimental error. The magnitude of u for these samples indicates a degree of molecular flexibility comparable t o the synthetic polymersQ).

2:

OD P I 2.0 sc)

Fig. 12. Plot according to KURATA and S T O C ~ M A Y E R ~ ~ ) for sodium carboxymethyl cellulose in cadoxen

The Z,-values (Table 4) were calculated assuming fractions 13 (a-e) to have distributions of the single-peaked type, f(M) = (yh+l/h!)*Mhe-yM, where h is a parameter characterizing the polydispersity. Employing the average XTm/En ratio for samples 13 (c-e) we have for this weight distribution

m z : a w : % n = 2.46:1.73:1 (14)

178


It is probable that there are distribution differences in the samples, originating from the mode of alkaline degradation used in their preparation. Thus, the use of Equ. 14 to obtain zz may introduce discrepancies over this wide range of molecular weight.

(R")i/z values were obtained from (g)i/z using the relation for randomly coiling polymers

(15) (K2);/2 = 6112 . (gZ);/Z The FLORY parameter @ was calculated from the relationship

where q corrects the measured averages t o number averages of (@)I12

and w and is calculated in the usual way. For the assumed distribution (Equ. 14) q = 1.79. @ lies in the region 1.5 t o 2.3 (Table 4). This is a small variation in @ when one considers the fifteen-fold change in molecular weight; i t could originate from a systematic increase in polydispersity towards higher molecular weights. On comparison with the value @ = 2.1021, characteristic of randomly coiling polymers, i t is concluded that sodium carboxymethyl cellulose in cadoxen displays the hydro- dynamics of a randomly coiling polymer in a good solvent.

The configurational parameter (R2,/Z), (Table 4) shows similar behaviour to tha t of @, exhibiting a small variation. Employing the value for the highest molecular weight sample (13a) we calculate the effective bond length as

(17)

_ _

b = ($/z);I2 = 1 8 A

A, = (E/z. 5.15), = 60A

The KUHN-KUHN equivalent chain segment length is

(18)

The segment length of the KUHN chain thus comprises approximately 12 monomer units. For comparison purposes, values of b and A, are given in Table 5 for the flexible polystyrene molecule and the stiff cellulosic polymer cellulose trinitrate as well as for cellulose in cadoxen. Sodium carboxj methyl cellulose and cellulose in cadoxen have values of b and A, experimentally indistinguishable from each other. Both, however, are substantially more flexible than the cellulose trinitrate molecule and similar t o the synthetic polymers.

The seventh column of Table 4 shows the R.M.S. end-to-end distances (@):Iz calculated on the assumption of free rotation about all chain bonds, according to the relationship for trans 1-42' polysaccharides


The ratio of the unperturbed coil dimension t o that determined with free rotation of the chain units (q/w)1/2 is accepted as characteristic of the flexibility of polymer chains. In Table 5 we see that this ratio for sodium carboxymethyl cellulose is ,close to that of cellulose in cadoxen and polystyrene. It is considerably greater for cellulose trinitrate. Thus, in keeping with the expansion factor and stiffness parameters b and A, we conclude that sodium carboxymethyl cellulose has a flexibility similar to that of t,he synthetic polymers.

In linear polymers we can obtain a measure of the steric hindrance to rotation in the chain using the expression

- 1-cos 0 l+G l+cos 0 1-cos 'p

R,Z= Z . b i . ( ) ( - ) where b, is the length of the anhydroglucose unit, 5.15 A, and 0 is the oxygen valence angle (110") for the cellulosics. The mean value of Kcp for fractions 13 (a-e) is 0.71. This value is approximately that found for cellulose in cadoxen (0.78).

The various factors contributing t o the exponent a of the modified STAUDINGER equation may be considered as follow 815)

a = 0.5 + 3a1 + 1.5a, + a,

where d In a d In (E/Z) d In @ dInZ ' d In Z d In Z

al = - a2 = , a , = -

The theory of FLORY considers the magnitude of a above 0.5 to be due t o 3al, while a2 and a3 are assumed to be zero. However, from columns 6 and 9 of Table 4 we find that 1.5 a2 = -0.2 and a3 = +0.2 for the present system. al = 0.1 and thus from the cancellation of 1.5 a2 and a3 we should expect a = 0.5 + 3(0.1) = 0.8. Experimentally we find a = 0.73. It is interesting to note that the same cancellation of 1.5 a2 and a3 was found for cellulose in cadoxen.

Thus, from consideration of these hydrodynamic and configurational parameters, it is seen that sodium carboxymethyl cellulose (degree of substitution 0.2 to 0.95) in cadoxen possesses a configuration indistinguishable from that of ccllulose in this solvent. Comparison of the parameters b, A,, and (R$w)tl2 for sodium carboxymethyl cellulose and polystyrene (Table 5) shows their similarities in molecular extension. It is also noteworthy that the exponent of the modified STAUDINGER equation, a, is 0.74 for polystyrene (benzene) and that the a-values over a similar molecular weight region are close to those of sodium carboxymethyl

180


celluloses). Thus, within the molecular weight region investigated, sodium carboxymethyl cellulose exhibits the behaviour of a randomly coiling polymer in a good solvent and displays a degree of flexibility greater than most other cellulosic polymers in aqueous and organic solvents.

It appears tha t we are studying sodium carboxymethyl cellulose in the asymptotic region where hydrodynamic and configurational vaiiations with molecular weight are not found and the value of the exponent a finds its origin in the change of the expansion factor with molecular weight. It is of course probable tha t the approaches to this asymptotic behaviour with sodium carboxymethyl cellulose and polystyrene do not take place over the same molecular weight interval, but this cannot be assessed from the present data.

The behaviour of most cellulose derivatives is different in that this asymptotic behaviour is not reached until very high molecular weights. Furthermore, (Ri/Z) attains an asymptotic value which is dependent on the solvent and this leads to values of the extension parameters which are generally much higher than with the flexible polymers.

It could have been expected tha t the configuration would be signifi- cantly influenced by added steric restrictions to rotation about the chain bonds with increasing degree of substitution. However, it appears that this is not the case, the configuration being primarily controlled by polymer-solvent interactions in this solvent. Similar conclusions have been reached by CORNELL and SWENSON 18) on diethylacetamide cellulose xaiithate (DAX) in 90% dimethyl sulfoxide. These authors found that a low degree of substitution fraction was considerably more extended in solution than a high degree of substitution fraction of the same molecular weight (one would expect the opposite from steric considerations) and i t was concluded here tha t the increasing polymer-solvent interaction with decreasing degree of substitution was mainly responsible, this effect overshadowing the added steric hindrance with increasing degree of substitution.

The solvent mechanism of cadoxen is open t o speculation. However, i t has ljeen shownlQ), tha t the solvent mechanism of the cellulose solvent cuprammonium hydroxide involves some form of cyclic complex formation between the copper ion and the oxygen atoms a t C-2 and C-3 of the glucose residue in cellulose. A similar complex has been postulated in sodium cupricellulose2°). It is possible tha t a similar mechanism is operative in cadoxen. With sodium carboxymethyl cellulose of low degrees of substitution complex formation could occur between the cadmium ion and the oxygen atoms a t C-2 and C-3. It would then not be surprising

_ -

181

W. BROWN, D. HENLEY, and J. OEMAN

t o find similar configurations for both sodium carboxymethyl cellulose and cellulose in cadoxen21).

This work has been carried out a t the Institute of Physical Chemistry, University of Uppsala, Sweden, as part of a general programme on the behaviour of macromolecules in solution. It has been financially supported b y SVENSKA CELLULOSA A.B. and also by the SWEDISH NATURAL SCIENCE RESEARCH COUNCIL and the SWEDISH TECHNICAL RESEARCH COUNCIL. This support is gratefully acknowledged.

The authors wish to thank Professor S. CLAESSON for stimulating dis- cussions and for placing the excellent facilities of the Institute a t their disposal.

We wish to express our indebtness to SVENSKA CELLULOSA A.B. for providing the samples and t o Dr. A. WENNERBLOM for his interest and valuable cooperation.

l) D. HENLEY, Arkiv Kemi 18 (1961) 327. 2, W. BROWN, D. HENLEY, and J. OEMAN. To be published. a) W. BROWN, D. HENLEY, J. OHMAN, and R. ST. JOHN MANLEY. To be published. *) G. SITARAMAIAH and D. A. I. GORING, J. Polymer Sci. 58 (1962) 1107. 6, K. WILSON, Svensk. Papperstidn. 59 (1956) 218. e, S. CLAESSON and G. PALM, Institute of Physical Chemistry, Uppsala, Sweden. To be

?) W. B. DANDLIKER and J. KRAUT, J. Amer. chem. SOC. 78 (1956) 2380. published.

S. CLAESSON and J. OHMAN, Institute of Physical Chemistry, Uppsala, Sweden. To be published.

') P. J. FLORY, Principles of Polymer Chemistry, Cornell University Press, Ithaca, N.Y. (1953).

lo) J. SCHURZ, H. STREITZIG, and E. WURZ, Mh. Chem. 87 (1956) 520. 'l) M. KURATA and W. H. STOCKMAYER, JUPAC-Symposium on Macromoelcular Chem-

12) J. M. G. COWIE, Makromolekulare Chem. 53 (1962) 13. 13) I. ELIEZER and H. J. G. HAYMAN, J. Polymer Sci. 23 (1957) 287. 14) N. H. SCHNEIDER and P. DOTY, J. physic. Chem. 58 (1954) 762. 15) M. L. HUNT, S. NEWMAN, H. A. SCHERAGA, and P. J. FLORY, J. physic. Chem. 60

16) W. R. KRIGBAUM, J. Polymer Sci. 28 (1958) 213. 17) S. NEWMAN, J. RISEMAN, and F. EIRICE, Proceedings of the International Colloquium

18) R. H. CORNELL and H. A. SWENSON, J. appl. Polymer Sci. 5 (1961) 641. l') R. E. REEVES, Advances Carbohydrate Chem. 6 (1951) 107. 20) I. CROON, B. LINDBERG, and A. Ros, Svensk Papperstidn. 61 (1958) 35. 21) B. LINDBERG, Cellulose Colloquium, Institute of Physical Chemistry, Uppsala, Sweden,

istry, Montreal (1961).

(1956) 1278.

on Macromolecules, Holland (1949).

June 1962.

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Documents

Studies on cellulose derivatives. Part I. The dimensions and configuration of sodium carboxymethyl cellulose in cadoxen and the influence of the degree of substitution