Reload Index (ZRiChK UMCS)

 

 

STUDIES OF THE INFLUENCE OF ACETATE GROUPS FROM POLYVINYL ALCOHOL (PVA) CHAIN ON ADSORPTION AND ELECTROCHEMICAL PROPERTIES OF THE TiO2/POLYMER SOLUTION INTERFACE

 

 

S. CHIBOWSKI, M. PASZKIEWICZ

DEPARTMENT OF RADIOCHEMISTRY & COLLOID CHEMISTRY

 

 

An introduction of macromolecules to dispersed system may essentially change its stability, which may be increased in steric stabilization process or decreased in flocculation [1].

Presented studies were done on polyvinyl alcohol/titania system (specific surface 8.3m2/g). After fractionation the samples of PVA of the following molecular weights and hydrolysis degree, determined by viscometry, were chosen: 10,000 (80%), 72,000 (98%), 44,000 (87%) and (98%), 16.000 (87%) and (98%). Presented isotherms (Fig. 1) concern to both, polydispersed and fractionated polymer samples. Only for the latter a characteristic plateau which evidence adsorption equilibrium may be observed. For non-fractionated samples the magnitude of the adsorption depends on the absolute number of the long polymer chains which adsorbing preferentially remove smaller particles from the surface. As can be seen from Fig. 1 and electrokinetic measurements the PVA chains with many acetate groups (‑CH2COO-) tend to attain flat conformation with more train type segments than polymers with smaller number of these groups. Polymer chains with 2% acetate groups attain mainly loop and train structures. Such conformation favors the increase of the adsorption (Fig. 1).

Fig. 1. Adsorption isotherms for fractionated and non-fractionated samples of PVA on the surface of TiO2

The meaning of acetate groups for conformation of the polymer chain at the interface was confirmed by measured thickness of PVA adsorption layers on TiO2. (Tab. 1) The conformation of polymer chain at studied interface and its change for macromolecules which pass from bulk of the solution to the interface was determined from Scheutjens‑Fleer theory for adsorbing polymers and concerning parameters[, Rh; Rs] Obtained data are presented in Tab. 2 and 3.

 

Table 1.Thickness of PVA adsorption layers on TiO2 for various molecular weights and various degree of PVA hydrolysis. CePVA = 500 ppm.

Mw

degr. hyd. [%]

G [mg/m2]

d [nm]

10 000

80

0,85

6,30

16 000

87

1,70

12,10

44 000

87

1,91

12,50

16 000

98

2,00

16,80

44 000

98

2,20

17,04

72 000

98

3,30

24,60

Table 2.Characteristic of adsorbed polymer chains at TiO2/PVA solution interface for volume fraction of PVA (f = 5x10-4).

M w
PVA

Number of segments

Length (expressed as number of segments)

 

 

train

loops

tails

Train

loops

tails

 

 

10 000

23

22

1,25

4,94

4,0

18,5

 

16 000

35

34

1,33

4,91

4,5

27,0

 

44 000

87

86

1,50

4,89

5,6

63,0

 

72 000

136

135

1,50

4,88

6,0

95,0

From a comparison of the data of Tab. 1 and 2, one may judge that tails but not loops it is that form which is responsible for the increase of value of /d/ with the increase of the molecular weight of the adsorbing polymer.

Table 3. Parameters of the polymer fractions.

Mw

h [dl/g]

 [nm]

[nm]

Rh [nm]

Rs [nm]

 

 

10 000

0,1835

 9,57

3,90

2,30

3,09

 

16 000

0,2587

12,55

5,12

3,02

3,56

 

44 000

0,5429

22,51

9,19

5,42

4,85

 

72 000

0,7159

29,09

11,87

7,00

5,97

For higher molecular weights of PVA an effect of interpenetration of the adsorbed macromolecule chain at TiO2 surface may occur, as was proved by data from Tab. 3. This effect should not be observed for fractions of lower molecular weights.

 

References

[1] B. Siffert and J.F. Li , Colloids and Surfaces, 40, 1989, 207‑217.