Column fractionation of poly(oxymethylene diacetate)

$Page 1: Column fractionation of poly(oxymethylene diacetate)$
Die Makromolekulare Chemie 114, 137-147 (1973)

Societk Italiana Resine, Sesto San Giovanni (Milano), Societa Italiana Resine, Solbiate Olona (Varese), and Istituto di Chimica Industriale, Universita di Genova, Via Pastore, 3,

16132 Genova. Italia

Column Fractionation of Poly(oxymethy1ene diacetate)

TITO OLCESE*), J. ACKERMANN**), P. RADICI **), and UMBERTO BIANCHI***)

(Date of receipt: May 2, 1973)

~ S U M M A R Y : A column fractionation technique for poly(oxymethy1ene diacetate) is described. Frac-

tionation is achieved by using a single solvent, N,N-dimethylformamide, at a gradually increasing temperature.

The mode of introduction of the crystallizable polymer into the column is a decisive factor for the reproducibility and efficiency of fractionation. Best results are obtained by introducing the polymer as a gel phase.

Z U S A M M E N F A S S U N G : Eine Saulen-Fraktioniertechnik fur Poly(oxymethy1endiacetat) wird beschrieben. Die

Fraktionierung wird erreicht durch stufenweise Temperaturerhohung unter Benutzung von N,N-Dimethylformamid als einzigem Solvens.

Die Art der Einbringung des kristallisierbaren Polymeren in die Kolonne ist ein entscheiden. der Faktor fur die Reproduzierbarkeit und Wirksamkeit der Trennung. Die besten Ergebniss- werden beim Einbringen des Polymeren als Gel-Phase erzielt.

Introduction

The characterization of poly(oxymethy1ene diacetate) (POM) is normally considered a difficult task. The main reasons are the low solubility in normal solvents at room temperature and the increasing risk of polymer degradation at higher temperatures.

As a matter of fact, the number of solvents which have been practically used is quite small: N,N-dimethylformamide (DMF) in the temperature range 1 1O"-15O0C, p-chlorophenol at 60"C, phenol at 1 1O"C, a mixture phenol/ tetrachloroethane (1 : 3) at 130"C, and a mixture of hexafluoroacetone hydrate/ water') at room temperature. In any case, it has been found necessary to add an antioxidant (like diphenylamine or cr-pinene) to prevent degradation of the polymer.

*) Societa Italiana Resine, Sesto San Giovanni (Milano), Italia. **) Societa Italiana Resine, Solbiate Olona (Varese), Italia.

***) Istituto di Chimica Industriale, Universita di Genova, Via Pastore, 3, 16132 Genova, Italia.

137

T. OLCESE, J. ACKERMANN, P. RADICI, and U. BIANCHI

Several procedures have been adopted to achieve polymer fractionation; without pretending to consider all papers appeared in the literature, the following attempts will be recalled : (1) Stepwise polymer extraction, stepwise polymer precipitation; (2) Column extraction.

Stepwise polymer precipitation and extraction

Fractionation of POM based on its crystallization from diluted phenol solutions has been studied by STEINY~). The range of crystallization temperature was 65--82,7OC, the yield of fractionation 89,5-94,4 %. Every fraction needed to be re-fractionated 2-6 times. The efficiency of extraction was followed by viscosity measurements in phenol/tetrachloroethane (1 : 3 in weight) at 90°C, and no correlation between viscosity and molecular weights was made. DICK, SACK, and BE NO IT^) have recently made another effort to improve refractionation of POM stabilized by esterification with butyric anhydride by fractional precipitation and dissolution in DMF. The initial foncentration in both techniques was 1,5 % and degradation was prevented by adding diphenylamine as antioxidant. After dissolution of the polymer at 145°C for an hour, fractional precipitation was obtained by lowering the temperature until the beginning of precipitation takes place. The temperature range for precipitation was 110-127°C whereas for extraction it was 1 10-124°C; the yield of fractionation was in both cases around 77 %.

Number average molecular weights M. were calculated by the equation

[q] = 20,0.10-4.M,0*57 ( [ q ] in 100 cm3/g)

[ q ] was measured in aniline at 116,5"C and M. was based on chemical analysis of end-groups.

Column extraction

KAKIUCHI and FUKUDA4), the pioneers of column extraction of POM diacetates used Celite as stationary phase working at llO"C, extracting the polymer with a phenol/ethyl cellosolve solvent/non solvent mixture. The yield of fractionation was about 60 %. The characterization of the fraction was done by measuring two values of [ q ] in two solvents, phenol at 9OoC andp-chlorophenol at 60°C. M. have been measured by osmometry, so that they have been able to evaluate two [q] /M. equations:

[q] = 1,13.10-4 M,0*76 in phenol at 9OoC

[q] = 5,43.10-4 M:v66 in p-chlorophenol at 60°C

GROHN and FRIEDRICH ') have also worked on column extraction technique. Glass beads of 0,1-0,3 mm in diameter have been used in a column kept at 130OC; a phenol/tetrachloroethane mixture (1 : 3 in weight) and hexanol were used as solvent/non solvent system. The yield was very good: 96,3 %; characterization has been done by [q]-measurements in the phenol/tetrachloroethane mixture at 9OoC and M derived from the equation6):

[ q ] = 12,16.10-4 M0*64

THUMMLER and HANTZSCH 7 , have summarized in their paper applications of column extraction to POM. They have also compared the behaviour of different stationary phases, like grounded NaCl and seasand. The column temperature was 145°C and DMF/hexanol or

138

Column Fractionation of Poly(oxymethylene diacetate)

DMF/decalin were used as solvent/non solvent systems. Yield of fractionation was about 95 %. They measured [q] values for each fraction in phenol/tetrachloroethane (1 : 3 by weight) a t 90°C and derived M. values from the equation:

Fig. 1. Column top piece for preheating of the solvent; ar- rows indicate the solvent way. The top piece can be tightly joined to the

column

[ q ] = 2,75.10T4 M:'80

which was based on IR-determination of M, by M A J E R ~ ) .

Experimental Part

We intended to apply a simple column extraction technique for POM by using a single solvent, DMF, at gradually increasing extraction temperature. DMF, in fact, begins the dissolution of samples of low molecular weights at about 110°C and dissolves samples of high molecular weights at 150°C.

We therefore designed a jacketed stainless steel column (i. d. = 21 mm, length = 1000 mm), the temp. of which can be regulated by circulation of silicon oil. The rate of temp. increase can be regulated at choice, a normal working value being about 0,S"Cjh.

D M F is fed at the column top by a volumetric pump, at a rate of 1,7 cm3/min, and needs to be heated at the column temp. This was achieved by using the top 20 cm of the column as a heat exchanger; D M F is forced, as shown in Fig. 1, to follow a labyrinthic way before actuallyentering the column. At the exit of the column, an LKB automatic fraction collector can provide 200 cuts of 20 cm3 each in v o I u m e .

The Column Fractionation Technique

Two problems are of great importance in column extraction procedure : the nature of the stationary phase and the way in which the polymer sample is inserted.

Stationary phase

Chemical inertness toward the polymer, especially working at high temperature, must be looked upon before choosing a filling material.

In order to test these effects, we put in contact a solution of POM in DMF with a particular filling material and stirred continuously for 3 h at 150°C. Then, the polymer was recovered and its intrinsic viscosity [q] measured. The comparison with [q] of the original polymer can show the presence of degradative effects.

139


Working with glass beads of 150 mesh, powdered NaCl and powdered silica gel, a large decrease of [q] showed indeed a polymer degradation, whereas these effects were much smaller with Celite (Johns-Manville) of 80 mesh, which was therefore chosen as the stationary phase.

Polymer insertion

We soon found that the way in which the POM sample is placed into the column plays a fundamental r81e on the efficiency of the following fractionation. The crystalkinity of the polymer is a complicating factor which can influence the actual structure of the polymer in the column. We tried at first the traditional methods, like stirring a polymer solution with a given amount of Celite and slowly evaporating the solvent under vacuum.

Then we deposited the polymer on Celite by stirring a solution with Celite and gradually decreasing the temperature below the limit of polymer solubility.

In both cases repeated fractionations of these samples showed not repro- ducible results and, what is worst, spectacular inversions in the order of extraction of different molecular weights.

Much better results were obtained with a third technique, which avoids polymer crystallization, as much as possible. The polymer sample (2g) is dissolved in a given volume of DMF (25 cm3). By heating up to 150°C for complete dissolution and quenching the solution at 80°C a gel is obtained which can be mixed mechanically with a given amount of Celite and charged immediately at the top of the column.

Fraction characterization

For each fractionation, of about 2g of polymer, we may obtain 200 cuts, which, of course, must be grouped in a smaller number (ca. 30) in order to have nearly the same amount of polymer in each fraction, and enough material for the subsequent characterization.

Since the cuts in the fraction collector are at room temperature, the polymer will soon separate in a fluffy form, thus making possible a visualization of the amount of polymer present in each cut.

After the grouping of the cuts, each fraction is filtered at room temperature on a fritted glass G4 filter, washed with acetone to remove DMF, and dried under vacuum for 48 h at 80°C. Fractions have been characterized by measuring number average molecular weights M,, by IR spectroscopy.

For some fractions we have also measured the intrinsic viscosity [q] and calculated values of M. through the equation4) :

[q] = 5,43.10-4 M:*66 ( [ q ] in 100 cm3/g)

140


Viscosity measurements have been performed at 6OoC in BISCHOFF suspended level visco- meters, dissolving the polymer in p-chlorophenol with 0,1 % in volume of a-pinene as antioxidant.

FRANK, JAACKS, and KERN’) have described a method to calculate M. from the IR absorption of terminal groups. In our case, the original powdered polyner is dissolved and a film cast from its solution with a thickness of about 40 pm; the film is then compressed a t about 400 kg/cm*. The thickness has to be kept quite constant and small since deviations from the LAMBERT-BEER-law have been observed for larger thicknesses.

Extinction values of absorption bands at 5,70 pm for carbonyl groups (Eco) and at 6,80 pm for methylene groups (ECH2) are mzasured and M,, calculated from the equation:

E C H ~ M, = 4300 - Eco

Since Eco values are strongly decreasing with increasing molecular weights, we have limited our analysis up to M,,- 1,5.105 values beyond this limit showing an increasing uncertainty.

Fractionation The reproducibility of our method has been tested comparing the results

obtained on fractionating a sample of POM P-300, provided by S.I.R., which had a large molecular weight distribution; IR determination of M,, on the whole sample gave M,, = 29.103.

As already noted, the measurement of the heights of polymer precipitate in each cut gives a first indication of the polymer fractionation as a function of the extraction temperature.

Fig. 2 shows the results obtained for the two runs of POM P-300 fractionation, the height h is plotted against the extraction temperature which goes from 120°C to 150°C where all macromolecules have been dissolved.

Tertr I O C

Fig. 2. Heights h of the polymer precipitate in the fraction collector as a function of extraction temperature Textr. Comparison between two fractionations of POM P-300

141

T. OLCESE, J. ACKERMANN, P. RADICI, and U. BIANCHf

..-. - 2 - 70 L,

6 0 -

50

40

30

20

0

After the grouping of consecutive cuts in about 30 fractions, each polymer

Fig. 3 shows the integral distribution functions by plotting percentages fraction has been characterized by IR analysis.

C(M,), calculated according to SCHULZ O), against M,,.

- 0'

$0' / /' ..> 00 ,+.

- &.-' - /c

- ?' Fig. 3. Integral distribution functions C(M,) for POM

-

- 100 2 -

g 90-

p 8 0 -

70

60

50

40-

30

-

8' / -

s' -

-

Fig. 4. Integral distribution function C(M,) for DELRIN 500; the narrow-

rp - f

142


Tab. 1, 2, and 3 show all important data on the three fractionations.

Tab. 1. Pertinent data on the fractionation and characterization of POM P-300; open circles in Fig. 3

c m 3 1 g-

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

3,30

5,31

7.78

10,58

13,78

16,8 1

19,54

22,09

24,57

27,20

30,18

33,61

36,53

39,09

41,89

45,14

47,78

50,36

52,74

54,934

59,34

64,26

69,36

72,82

76,30

79,37

83,05

87,35

1,88

4,95

7,52

10,50

13,93

17,49

20.78

23.81

26,68

29,61

32,82

36.49

40,12

43,25

46,32

49.78

53,15

56.13

58,97

61,59

65.36

70,69

76,43

81,33

85,30

89,05

92,90

95.01

124,8

131,l

132,9

134,O

135,l

135,9

136,7

137,5

138,l

138,7

139.3

139,8

140,2

140,6

141,O

141,4

141,7

142,O

142,3

142,5

142,9

143,4

144,O

144,3

144,7

145,3

146,O

148,9

7,35

8,85

935

12

15,5

18

20

21

21,5

30

36,5

44 0,92

71 1,27

75.5

76,5

80

99

110

113

117 1,64

129 1,88

157

78

127

189

230,5

143


Tab. 2. Pertinent data on the fractionation and characterization of POM P-300; closed circles in Fig. 3

c m 3 1 g-

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

144

2,64

5,90

8,98

11,62

14,18

115~92

19,63

22,04

243 I

26,89

29,04

3 1,03

32,98

35,35

37,48

39,84

42,53

45,78

49,74

53,05

57,07

59,39

63,46

66,91

70,34

73,34

76,34

79,12

82,55

84.13

86,12

133

4,96

8,64

11,96

14,98

18,06

21,22

24,19

27,02

29,83

32,47

34,87

37,15

39,66

42,27

44,87

47,80

51,24

55,43

59,65

63,9 1

67,59

71,30

75,67

79,66

83,39

86,88

90,23

93,83

96,74

97,66

126,6

133,O

134,6

135,4

136,l

136,s

137,4

137,9

138,4

138,9

139,4

139,8

140,2

140,5

140,8

141,l

1413

141,9

142,l

142,4

142,7

142,s

143,l

143,3

143,6

143,s

144,2

144,7

145,5

146,5

147,9

7,45

9,05

10,8

13,6

14,6

17,2

20

25,5

29 0,58 38,8

32,5

373

43 0,83 66,8

47

60,5

65

78 1,32 135

92

116

130

148


Tab. 3. Pertinent data on the fractionation and characterization of DELRIN 500

Fraction No. % Cum. 102. C(MJ T e x t r I ° C 10- 3. M.

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

1,27

2,46

6,19

9,75

13,49

17,57

20,57

23,34

26,79

30,20

32,97

36,15

40,40

43,72

47,15

50,8 1

55,Ol

59,31

64,08

67,ll

69,91

73,76

76,75

78,92

8 1,02

83,05

85,06

87,OO

88,91

90,83

92,04

0,69

2,02

4,68

8,64

12,61

16,85

20,07

23,83

27,21

30,98

34,33

37,52

41,56

45,67

49,34

53,19

57,46

62,07

67,OO

71,24

74,41

78,02

81,73

84,53

86,85

89,lO

91,30

93,44

95,53

97,61

98,65

118

130,4

134,6

135,6

135,8

136,l

136,3

136,5

136,8

137,l

137,5

137,9

138,2

138,5

138,8

139,2

139,5

139,7

140,O

140,3

140,5

140.7

141,O

141.3

142,O

142,9

144,l

145,4

146,4

147,6

148,l

597

12.0

21

41

44,5

46,5

50,5

56

59

63,5

70.5

83

95

102

107

111

119

126

134

142

146

152

145


As a further check, we refractionated some fractions obtained from the two POM P-300 fractionations. In particular, we grouped together all fractions with M,, I 2.104, also, we grouped all fractions with M,, 2 10’. These two samples were separately inserted into the column and refractionated.

Fig. 5 shows the distribution curves C(Mi) uersm M,; the first curve on the left refers to the sample with M,, I 2-104, the second to the sample with M, 2 10’.

M,

Fig. 5. Refractionation of two groups of fractions, one with M. 5 2.104 and one with M. 2 lo5. The first curve (0 ) shows that 80 % of the material has a molecular weight < 2.104. The second curve ( 0 ) shows that molecular weights > lo5 represent about 80 %

of the total polymer

The last two points on the second curve must be considered uncertain due to the rather high molecular weights (1,8.105 and 2,4.10’) difficult to be assessed by IR.

Discussion

From Tab. 1,2, and 3 one can see that the yield of fractionation is ranging from 86 to 92 % of the polymer introduced into the column. Our further work, more recently, has given 95% as the average yield of our column. Polymer can be mechanically lost during all the manipulation of the fraction and is also

146


lost during the actual fractionation due to the unavoidable degradation at high temperature.

As far as the reproducibility is concerned, Fig. 3 shows a very good agree- ment of the two sets of low molecular weight fractions; deviations appear in the region of higher molecular weights which make necessary an averaging procedure on several runs.

We believe that the gel method of introducing the polymer sample is working fairly well for low and average molecular weight fractions, i.e. during the first period of gel extraction; fractions of low molecular weights are continuously extracted from the gel structure according to their solubility in DMF. How- ever, when the percentage of extracted material is large enough, further extraction of the components with higher molecular weight wil1 eventually cause the collapse of the gel phase, this causing irregularities in the final part of fractionation. From our experience with POM, it seems that this limit is around a molecular weight of lo5. Irregularities at high molecular weights can also be expected since it is known that DMF becomes at high temperatures less selec- tive toward high POM molecular weights.

The efficiency of our method has been tested in Fig. 5 . Here one can see that the refractionation of a group of fractions all with M, I 2.104 has shown that 80 % of the material has indeed a molecular weight below that limit. Corre- spondingly, the refractionation of a group of fractions with M,, 2 10' showed that only 22% of the whole sample contained molecular weights below that limit.

In summary the method proposed can be used to fractionate, on a routine basis, samples of POM, a crystalline polymer which is so difficult to handle with traditional procedures.

') W. H. STOCKMAYER, L. L. CHAN, J. Polymer Sci., Part A-2, 4, 437 (1966).

') R. DICK, H. SACK, H. BENOIT, J. Polymer Sci. C 16, 4597 (1969). 4, H. KAKIUCHI, W. FUKUDA, Kogyo Kagaku Zasshi 66, 964 (1963). 5, H. GROHN, H. FRIEDRICH, J. Polymer Sci. C 16, 3737 (1968). ') K. DOERFFEL, H. FRIEDRICH, H. GROHN, D. WIMMERS, Plaste Kautschuk 12, 524 (1965). 7, W. THUMMLER, G. HANTZSCH, Plaste Kautschuk 14, 881 (1967).

9, H. FRANK, V. JAACKS, W. KERN, Makromol. Chem. 114, 92 (1968).

YA. STEINY, Vysokomol. Soedin. 10, 1883 (1968).

J. MEJER, Makromol. Chem. 86, 253 (1965).

lo) G. V. SCHULZ, Z. Physik. Chem. B 47, 155 (1940); L. H. TUNG, Polymer Fractionation, edited by M. J. R. CANTOW, Academic Press, New York, London 1967, p. 388.

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Documents

Column fractionation of poly(oxymethylene diacetate)