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Polymer-polymer miscibility during copolymerization
Fur. Polym. J. Vol. 27, No. 10, pp. 1023-1027,1991 Printed in Great Britain.All rights reserved
0014-3057/91 $3.00+ 0.00 Copyright © 1991PergamonPress plc
POLYMER-POLYMER MISCIBILITY DURING COPOLYM ERIZATION SALAHE. M. EL-BEGAWYand MALCOLMB. HUGLIN* Department of Chemistry and Applied Chemistry, University of Salford, Salford M5 4WT, England (Received 7 March 1991)
Abstract--Curves of instantaneous copolymer composition vs fractional conversion (0) have been calculated for the copolymerization of N-vinyl-2-pyrrolidone (VP) with n-butyl acrylate (BA). These systems display compositional heterogeneity, the onset of which occurs at values of 0 that are lower the higher the content of VP in the feed. Low conversion samples of poly(VP-co-BA) have been mixed in proportions so as to simulate the overall composition of material present cumulatively at different stages of an actual copolymerization. These blends have been examined with regard to their light transmission and glass transition behaviour. The findings accord with the compositional heterogeneity predicted from the calculated curves. The curves also account satisfactorily for the visually assessed clarity, translucency or opacity of copolymers prepared in bulk by y-irradiation over the interval 0 = 0.016-0.998.
INTRODUCTION The conditions whereby polymers can be blended, the thermodynamic criteria of miscibility and the properties of blends have been reviewed [1] as has also the case in which the polymer-polymer interactions are specifically ionic leading to complex formation [2]. Studies have encompassed polymers, oligomers [3-5] and copolymers [6-8]. For the last group, miscibility has been shown to be affected greatly by copolymer composition [9-12] and by the precise experimental procedure adopted for the blend preparation [13-16]. The theory of miscibility in blends of copolymers, developed initially by Scott [17], has been extended by Balazs et al. [18] to account for the role played by sequence length distribution in determining the strength of inter- and intramolecular interactions. The present work is somewhat different in the respect that attention is focused on a situation in which the blending of polymers occurs simultaneously with their chemical formation, viz. the copolymerization of two monomers M~ and M2 to various levels of conversion. Depending on the feed composition and reactivity ratios, such copolymerizations can exhibit compositional homogeneity or heterogeneity according to the stage of conversion. Previous studies [19, 20] have indicated the suitability of N-vinyl-2-pyrrolidone (VP) and n-butyl acrylate (BA) as monomers M~ and M2 respectively, since there is a disparity in the reactivity ratios. Moreover, this system has an azeotropic composition at which there is no compositional heterogeneity. The overall strategy is as follows: (a) feed compositions are considered theoretically for which the compositional drift with conversion is (i) zero, (ii) moderate and (iii) very considerable. (b) Copolymerizations corresponding to categories (i), (ii) and (iii) are effected practi*To whom all correspondence should be addressed.
(c)
cally from low to high conversion and compositional heterogeneity is assessed by visual inspection of optical clarity. By means of low conversion copolymers, blends are prepared to simulate the cumulative compositions of copolymers at various stages of copolymerization in category (a) (ii) and category (a) (iii). Physical methods are used to investigate the level of miscibility within the blends and the findings are compared with the theoretically expected extent of compositional heterogeneity in copolymerizations. EXPERIMENTAL PROCEDURES
Materials N-Vinyl-2-pyrrolidone (monomer-l) and BA (monomer2) (both Aldrich Chem Co.) were purified as described elsewhere [19, 20]. The initiator, 4,4'-azobis-4-cyanopentanoic acid (AZPA) (Fluka AG), was used as received. Preparation of low conversion copolymers In all preparations, the overall concentration of AZPA was 2 × 10-3 mol dm -3 and the solvent was cyclohexanone, the ratio of total monomers/solvent being 50/50 (v/v). In order to obtain instantaneous copolymer compositions within the designed range referred to in the Introduction, the 12 gravimetrically prepared monomer mixtures in the feed had mole fractions of VP (f~) within the range 0.840-1.00. Mixtures were outgassed with gaseous N2 in stoppered Teflon ampoules and copolymerization was effected at 338 K for times ranging from 10 min (at low ft) to 2 hr (at high VP) in order to obtain low conversions, which were generally below 7%. The reaction mixtures were then cooled and the copolymers isolated by precipitation in an excess of petroleum ether. After washing, dissolution again in cyclohexanone and reprecipitation, the samples were dried in vacuo for I week at 313 K. Analysis of copolymers The VP content of copolymers was determined by analysis for nitrogen using a Tecator Kjeltech Auto Analyser. During each set of analysis, the nitrogen content was also
1023
SALAH E. M. EL-BEGAWYand MALCOLMB. HUGLIN
1024
determined for a sample of poly-VP homopolymer. This ratio of the theoretical to experimental percentage of nitrogen in poly-VP was used as a correction factor, the value of which generally lay within the range 1.02-1.06.
Medium-high conversion copolymers Two systems were examined, the first comprising 80 wt% VP in the feed (Jl = 0.821) and the second having 95 wt% VP in the feed (fl = 0.956). In neither case was initiator or solvent added. For each system, several ampoules of feed mixtures were outgassed and subjected to y-irradiation from a 9000 Ci 6°Co source for various times at 293 K. The dose rate, as determined by Fricke dosimetry, was 21 krad hr-L Ampoules were removed after different irradiation times and the contents were assessed visually for optical homogeneity as clear (c), clear-translucent (c/t), translucent (t), translucent-opaque (t/o) or opaque (o). The fractional conversion (0) was determined from the sum of the weights of linear and crosslinked polymer, the former being determined by extraction with cyclohexanone and precipitation in petroleum ether. Precipitated linear polymer and washed insoluble (crosslinked) polymer were dried in vacuo for 1 week at 313K. Blends of copolymers To simulate the cumulative composition of the material present at a particular stage of copolymerization, appropriate mixtures of low conversion copolymers were made up as solutions containing 5% (w/v) of total copolymer in chloroform. Details of the composition of the blends are given later in the Results section. The solutions were placed in Teflon moulds at 293 K and the chloroform was allowed to evaporate slowly. The resultant films were then dried in vacuo for 3 days at 313 K. Films were also cast similarly for the unmixed low conversion copolymers and poly-VP homopolymer. Film thicknesses were measured with an uncertainty of + 10 #m, using a micrometer. Light transmission The light transmission of films of low conversion copolymers and blends was measured with a Unicam SP-1750 u.v.-spectrophotometer coupled with a Unicam AR55 linear recorder. Two metallic holders (with an aperture 15 × 6 mm, thickness 2 mm) were constructed. On one of these holders, the film was mounted using black adhesive tape and the measurements were run against the other holder as blank. Optical densities were scanned over the wavelength range 500 700 nm at a slit width of 0.22 mm, a band width of 0.7 mm and a scan speed of 2 nm see -~. Measurements were made on films of several different thicknesses. Glass transition temperature (Ts) Glass transition temperatures were determined for low conversion copolymers, medium-high conversion copolymers prepared by ~,-irradiation and blends, For the last group, in order to avoid phase separation which may occur during solvent evaporation, films were dissolved in cyclohexanone and the solution of each blend was added dropwise to an excess of petroleum ether, causing rapid co-precipitation. The precipitates were washed with precipitant and dried in vacuo for several days at 313 K. Measurements were made on samples (6-14mg) by means of a Mettler DSC-30 instrument fitted with a thermal analysis data station. A heating rate of 10 K rain -~ under gaseous N2 was used, the temperature range scanned being 243-473 K. RESULTS AND DISCUSSION
Low conversion copolymers The feed compositions, reaction times, conversions as well as the compositions (mole fraction F2 o f BA)
Table 1. Characteristics of low conversion copolymers Reaction time Light Sample fl (min) 0 F2 Ts(K) transmission LI 0.840 10 0.06 0.512 305 0.84 L2 0.867 15 0.07 0.482 309 0.88 L3 0.938 20 0.05 0.448 311 0.86 L4 0.965 25 0.04 0.407 315 0.83 L5 0.978 35 0.06 0.351 323 0.80 L6 0.986 40 0.05 0.310 338 0.85 L7 0.989 45 0.02 0.257 345 0.82 L8 0.993 60 0.02 0.216 362 0.78 L9 0.995 70 0.01 0.136 376 0.85 LI0 0.997 90 0.01 0.111 410 0.86 LI 1 0.999 100 0.01 0.031 438 0.79 L12 1.000 120 0.06 0.000 447 0.87
a n d Tg values o f the resultant low conversion copolymers, designated as L1-L11 are listed in Table 1. The values o f the mole fractions of V P in the feed ( f j ) a n d copolymer (F1 = 1 - F 2 ) were used in the F i n e m a n Ross plot [21] to yield reactivity ratios rl = 0.02 a n d r2 = 0.80. These values accord well with those reported previously from experiments employing a smaller n u m b e r of d a t a points [19, 22]. The values of Tg for the two h o m o p o l y m e r s and those of the copolymers afforded excellent linearity w h e n represented according to the Fox e q u a t i o n [23]. Moreover, the Tg for poly-BA (229 K) agrees well with other reported values [24, 25]. F o r poly-VP, values o f T s ranging from 327 to 448 K have been reported [26, 27], the lower values being explained by T a n a n d Challa [26] as being due to the large influence o f a relatively small a m o u n t of sorbed water, which itself has a very low T 8. The present value o f Tg = 447 K for poly-VP accords well with the u p p e r limit a n d is indicative o f efficient drying o f the sample. The recorded optical density (OD) for each film in its holder minus the value o b t a i n e d in the absence of the film yielded the optical density due to the sample a n d hence the transmittance. If It a n d Io are the intensities of transmitted a n d incident light respectively, then t r a n s m i t t a n c e = It/Io. The optical density is defined as O D = log(Io/I,) a n d hence transmittance = 10 - ° ° = It/I o. A t any particular wavelength, transmission results from the t r a n s m i t t a n c e modified by a small a m o u n t of surface reflection. The extent of the latter can be evidenced a n d allowed for by plotting O D vs film thickness. The small b u t finite departure from zero (generally 0.02-0,06) afforded at zero thickness was then added to the actual measured values o f the t r a n s m i t t a n c e to yield the corrected values o f transmission listed in Table l which are all specific to the wavelength 600 n m a n d a film thickness of 200 # m . The average transmission for the low conversion copolymers was 0.86.
Low-medium-high conversion copolymers Two series of copolymers p r e p a r e d by radiation to different levels o f fractional conversion 0 were obtained from feed mixtures containing 80 w t % V P a n d 95 w t % VP, the I I samples in these series being
Polymer polymer miscibility during copolymerization Table 2. Optical clarity and Tg values for copolymers produced by 7-irradiation to different conversions from a VP/BA feed mixture containing 80 wt% VP
1025
Table 3. Opticalclarityand T~values for copolymersproducedby 7-irradiation to different conversionsfrom a VP/BA feed mixture containing95% VP
Sample
Dose/Mrad
Visual appearance
0
Ts (K)
Sample
Dose/Mrad
Visual appearance
0
T~ (K)
80R 1 80R2 80R3 80R4 80R5 80R6 80R7 80R8 80R9 80RI0 80RI1
0.003 0.005 0.010 0.021 0.042 0.084 0.168 0.315 0.504 1.208 1.660
c c c c/t t o o o o o o
0.118 0.155 0.234 0.260 0.294 0.334 0.422 0.845 0.880 0.972 0.998
303 306 311 312 315 317 309, 438 316, 396, 443 310, 423, 442 313,420, 445 318,413,446
95R1 95R2 95R3 95R4 95R5 95R6 95R7 95R8 95R9 95R10 95R11
0.003 0.005 0.015 0.020 0.041 0.082 0.184 0.347 0.510 1.500 1.800
c c/t c/t t t/o t/o o o o o o
0.016 0.027 0.041 0.056 0.118 0. 146 0.202 0.425 0.815 0.965 0.999
323 421 323,427 418, 439 299, 422 311,428 395,422 372,397, 426 388,425 397, 411 425,442
denoted as 80R1-80Rll and 9 5 R 1 - 9 5 R l l respectively. The characteristics of these groups of copolymers are given in Tables 2 and 3. The bulk copolymerization of the azeotropic feed was also effected by y-irradiation. The appearance of the group of copolymers 9 5 R 1 - 9 5 R l l is illustrated in Fig. 1. The copolymer prepared from the azeotropic feed, which is also illustrated in Fig. 1 for one particular value of 0 --- 0.90, is of the same appearance (i.e. high optical clarity) at all the other values of 0. Tables 2 and 3 and Fig. 1 show that, whereas optical appearance as c or c/t is evident up to 0 = 0.26 in the series 80R, such an appearance in the series 95R extends only up to a conversion of 0.04. The values of Tg for the two groups of copolymers are listed in Tables 2 and 3. The onset of significant compositional heterogeneity at relatively low conversion in the series 95R is also demonstrated by the fact that a single T~ is given only by samples prepared up to 0 = 0.027 in contrast to the situation for the series 80R in which a single Tg is obtained up to 0 = 0.334.
Blends of copolymers The compositional drift of copolymer with increasing 0, originally treated by Skeist [28] and implemented in an integral form by Meyer and Lowry [29],
has been quantified conveniently in previous publications by means of a computer program developed by Yip [22]. The program is written in Fortran 77 for use on the Prime 750, the resultant diagrams being produced on a Calcomp 1051 Plotter. Calculated curves of instantaneous copolymer composition (expressed as mole fraction F2 of BA) at different conversions are given in Fig. 2 for two systems, viz. copolymerizations 80C and 95C. For the former f~ = 0.821 (corresponding to 80 wt% VP in the feed) and for the latter fl =0.956 (corresponding to 95 wt% VP in the feed). In system 80C the compositional drift is only gradual up to 0 ~ 0.30 and thereafter becomes pronounced within the interval 0.30-0.40. At 0 > ca 0.40, the instantaneous copolymer composition is close to that of pure poly-VP. This is, of course, due to the low reactivity ratio r I of VP compared to the relatively high value of r 2. In the system 95C the onset of compositional drift becomes evident at a relatively low stage of conversion (0 ~ 0.06) and is continuous up to 0 ~ 0.16. From 0 ~ 0.20 onwards the instantaneous copolymer composition is close to that of poly-VP. To simulate these copolymerizations, blends were made up from samples within the group L1-L12. Thus for a curve in Fig. 2 at any particular value of 0, the instantaneous composition is indicated on the
Fig. 1. Appearance of copolymers prepared at different conversions by y-irradiation. Samples 95RI-95R11 are denoted in the figure by 1-11. The sample denoted by 12 in the figure is produced from the azeotropic feed mixture (fl = 0.017) at a conversion 0 = 0.90. EPJ 27/1~B
1026
SALAH E. M. EL-BEoAwY and MALCOLM B. HUGLIN
1.0j~
r0
~,~ 0.4 0
>
cl
0
u.. 0.2
0
0.2
I //I 0.6 0.8
0.4
1.0
F2 Fig. 2. Calculated curves of instantaneous copolymer composition vs fractional conversion (a) system 80C, (b) system 95C, (c) azeotropic system.
Table 4. Compositions and properties of blends 80BI-80B5 and blends 95BI-951M
Blend
Number of component copolymers
Nature of component copolymers
2 4 6 10
L1, L2 L1-L4 LI-L6 LI-LI0 L 1 -L 12
0.18 0.32 0.36 0.39 0.42
0.50 0.42 0.28 0.18 0.15
307 311 326 363, 399 309, 442
L5, L6 LS---L8 LS-LI0 L5-L12
0.08 0.10 0.135 0.22
0.52 0.43 0.40 0.21
334 345 338, 402 389, 438
80BI 80B2 80B3 80B4 80B5
12
95B1 95B2 95B3 95B4
2 4 6 8
curve. The total polymer present at this conversion comprises the copolymer of the indicated instantaneous composition in additon to copolymers produced at earlier stages of conversion. The weight fraction Wx of BA in a copolymer of instantaneous composition Fx is calculated as
Wx=hx/Eh FiLm thickness o "~T.,I -0.2
-
4o
eo I
~x.--.. ~
-
12o I
~
~8o 2oo t
~.
I
24o
2so 32o
I
I
I
Corresponding simulated Light conversion transmission
Ts (K)
where hx is the height of the integral step corresponding to the composition Fx and I~ h is the sum of the heights of the integral step between zero conversion and the particular conversion 0. To simulate the copolymerization 80C, five values of 0 were selected and for each of these a blend was prepared. The compositions of these blends 80BI-80B5 are indicated in Table 4. Similarly to simulate the material present at four different conversions in copolymerization 95C, low conversion copolymers were mixed to yield blends 95BI-95B4 of compositions also given in Table 4.
FiLm thickness (pro)
" " ~ " ~ .
0
0
40
80
120
160
200
240
280
S~)1
0,4
o
-0,6
~ -0,8 -
1.0
-
1.2
% i n ~
-
0.6
~,,~
- 08
Fig. 3. Variation of transmittance at 600 n m with thickness of film for blends (a) 80B1, (b) 80B2, (c) 80B3, (d) 801M and (e) 80B5.
-
1.o
Fig. 4. Variation of transmittance at 600 n m with thickness o f f d m for blends (a) 95B1, (b) 95B2, (c) 95B3 and (d) 95B4.
Polymer-polymer miscibility during copolymerization
1027
REFERENCES
J
c
f
S
d
"1o
233
t
I
315
39:5
Temperature
473
(K)
Fig. 5. DSC traces for blends (a) 95B1, (b) 95B2, (c) 95B3 and (d) 95B4. The corrected plots of log (It/Io) at 600 nm vs film thickness for blends 80B1-80B5 and for blends 95B1-95B4 are shown in Figs 3 and 4 respectively. From these plots the transmissions at a thickness 200/~m were read off. The values are listed in Table 4, which also gives the values of Tg for the blends. Figure 5 illustrates DSC traces for blends exhibiting a single Tg and two Tss. Comparison of the data in Table 4 with the calculated curves in Fig. 2 indicates that the compositional drift in the latter is consonant with both the blend incompatibility evidenced by two Ts s and the reduced light transmission which obtain for the former. In conclusion we note that the use of blends of low conversion copolymers serves as a useful model for the copolymer produced at various stages of copolymerization. The procedure is, of course, only semiquantitative in nature, because the number of components of different composition which are used for each blend is of necessity extremely small cf the n u m b e r existing in a copolymer produced at any stage of conversion (except for the azeotropic case). Moreover, in an actual copolymerization the quantity and composition of unreacted, liquid monomer mixture changes with conversion. It is probable that this consideration may account, in part, for the fact that the value of 0 at or above which there is departure from optical clarity and the occurrence of two Tgs in the radiation-induced copolymerizations (Tables 2 and 3) does not coincide exactly with the value of 0 for corresponding effects to prevail in the blends (Table 4). Work is about to commence to assess the thermodynamic affinity of the solvent (i.e. unreacted monomer mixtures) for the blends.
Acknowledgement---One of us (S.E.M. E-B) thanks the Egyptian government for financial support.
1. D. R. Paul and S. Newman (Eds). Polymer Blends, Vol. I. Academic Press, New York (1978). 2. B. Philipp, H. Dautzcnberg, K.-J. Linow, J. Kotz and W. Dawydoff. Progr. Polym. Sci. 14, 91 (1989). 3. M. B. Huglin, I. M. Idris and M. B. Sokro. Makromolek. Chem. Rapid Commun. 2, 17 (1981). 4. M. B. Huglin and I. Mohd Idris. Polymer 24, 1434 (1983). 5. M. B. Huglin and I. Mohd Idris. Eur. Polym. J. 21, 9 (1985). 6. J. A. Schroeder, F. E. Karasz and W. J. MacKnight. Polymer 26, 1795 (1985). 7. R. P. Kambour, J. T. Blendler and R. C. Bopp. Macromolecules 16, 753 (1983). 8. G. ter Brinke, F. E. Karasz and W. J. MacKnight. Macromolecules 16, 1827 (1983). 9. M. F. Fowler, J. W. Barlow and D. R. Paul. Polymer 28, 1177 (1987). 10. G. A. Zakrzewski. Polymer 14, 348 (1973). 11. A. C. Fernandes, J. W. Barlow and D. R. Paul. Polymer 27, 1788 (1986). 12. A. C. Fernandes, J. W. Barlow and D. R. Paul. J. appl. Polym. Sci. 32, 5357 (1987). 13. J. S. Chiou, J. W. Barlow and D. R. Paul. J. Polym. Sci. Polym. Phys. Edn 25, 1459 (1987). 14. A. C. Fernandes, J. W. Barlow and D. R. Paul. Polymer 27, 1799 (1986). 15. O. Olabisi, L. M. Robeson and M. T. Shaw. PolymerPolymer Miscibility, Chap. 3. Academic Press, New York (1979). 16. R. Vukovic, F. E. Karasz and W. J. MacKnight. Polymer 24, 529 (1983). 17. R. L. Scott. J. Polym. Sci. 9, 423 (1952). 18. A. C. Balazs, F. E. Karasz, W. J. MacKnight, H. Ueda and I. C. Sanchez. Macromolecules 18, 2784 (1985). 19. M. A. Al-Issa, T. P. Davis, M. B. Huglin and D. C. F. Yip. Polymer 26, 1869 (1985). 20. M. B. Huglin and M. B. Zakaria. Polymer 25, 797 (1984). 21. M. Fineman and S. D. Ross. J. Polym. Sci. 5, 259 (1950). 22. D. C. F. Yip. BSc Dissertation, University of Salford (1984). 23. T. G Fox. Bull. Am. Phys. Soc. 1, 123 (1956). 24. F. P. Reding, J. A. Faucher and R. D. Whitman. J. Polym. Sci. 57, 483 (1962). 25. P. Peyser. In Polymer Handbook, 3rd edn (edited by J. Brandrup and E. H. Immergut), p. VI/215. John Wiley & Sons, New York (1989). 26. Y. Y. Tan and G. Challa. Polymer 17, 739 (1976). 27. P. Molyneux. Water Soluble Polymers: Properties and Behaviour, Vol. I, Chap. 4, p. 152. CRC Press, Boca Raton, FL, U.S.A. (1983). 28. I. Skeist. J. Am. Chem. Soc. 68, 1781 (1946). 29. V. E. Meyer and G. G. Lowry. J. Polym. Sci. A 3, 284 (1965).
0014-3057/91 $3.00+ 0.00 Copyright © 1991PergamonPress plc
POLYMER-POLYMER MISCIBILITY DURING COPOLYM ERIZATION SALAHE. M. EL-BEGAWYand MALCOLMB. HUGLIN* Department of Chemistry and Applied Chemistry, University of Salford, Salford M5 4WT, England (Received 7 March 1991)
Abstract--Curves of instantaneous copolymer composition vs fractional conversion (0) have been calculated for the copolymerization of N-vinyl-2-pyrrolidone (VP) with n-butyl acrylate (BA). These systems display compositional heterogeneity, the onset of which occurs at values of 0 that are lower the higher the content of VP in the feed. Low conversion samples of poly(VP-co-BA) have been mixed in proportions so as to simulate the overall composition of material present cumulatively at different stages of an actual copolymerization. These blends have been examined with regard to their light transmission and glass transition behaviour. The findings accord with the compositional heterogeneity predicted from the calculated curves. The curves also account satisfactorily for the visually assessed clarity, translucency or opacity of copolymers prepared in bulk by y-irradiation over the interval 0 = 0.016-0.998.
INTRODUCTION The conditions whereby polymers can be blended, the thermodynamic criteria of miscibility and the properties of blends have been reviewed [1] as has also the case in which the polymer-polymer interactions are specifically ionic leading to complex formation [2]. Studies have encompassed polymers, oligomers [3-5] and copolymers [6-8]. For the last group, miscibility has been shown to be affected greatly by copolymer composition [9-12] and by the precise experimental procedure adopted for the blend preparation [13-16]. The theory of miscibility in blends of copolymers, developed initially by Scott [17], has been extended by Balazs et al. [18] to account for the role played by sequence length distribution in determining the strength of inter- and intramolecular interactions. The present work is somewhat different in the respect that attention is focused on a situation in which the blending of polymers occurs simultaneously with their chemical formation, viz. the copolymerization of two monomers M~ and M2 to various levels of conversion. Depending on the feed composition and reactivity ratios, such copolymerizations can exhibit compositional homogeneity or heterogeneity according to the stage of conversion. Previous studies [19, 20] have indicated the suitability of N-vinyl-2-pyrrolidone (VP) and n-butyl acrylate (BA) as monomers M~ and M2 respectively, since there is a disparity in the reactivity ratios. Moreover, this system has an azeotropic composition at which there is no compositional heterogeneity. The overall strategy is as follows: (a) feed compositions are considered theoretically for which the compositional drift with conversion is (i) zero, (ii) moderate and (iii) very considerable. (b) Copolymerizations corresponding to categories (i), (ii) and (iii) are effected practi*To whom all correspondence should be addressed.
(c)
cally from low to high conversion and compositional heterogeneity is assessed by visual inspection of optical clarity. By means of low conversion copolymers, blends are prepared to simulate the cumulative compositions of copolymers at various stages of copolymerization in category (a) (ii) and category (a) (iii). Physical methods are used to investigate the level of miscibility within the blends and the findings are compared with the theoretically expected extent of compositional heterogeneity in copolymerizations. EXPERIMENTAL PROCEDURES
Materials N-Vinyl-2-pyrrolidone (monomer-l) and BA (monomer2) (both Aldrich Chem Co.) were purified as described elsewhere [19, 20]. The initiator, 4,4'-azobis-4-cyanopentanoic acid (AZPA) (Fluka AG), was used as received. Preparation of low conversion copolymers In all preparations, the overall concentration of AZPA was 2 × 10-3 mol dm -3 and the solvent was cyclohexanone, the ratio of total monomers/solvent being 50/50 (v/v). In order to obtain instantaneous copolymer compositions within the designed range referred to in the Introduction, the 12 gravimetrically prepared monomer mixtures in the feed had mole fractions of VP (f~) within the range 0.840-1.00. Mixtures were outgassed with gaseous N2 in stoppered Teflon ampoules and copolymerization was effected at 338 K for times ranging from 10 min (at low ft) to 2 hr (at high VP) in order to obtain low conversions, which were generally below 7%. The reaction mixtures were then cooled and the copolymers isolated by precipitation in an excess of petroleum ether. After washing, dissolution again in cyclohexanone and reprecipitation, the samples were dried in vacuo for I week at 313 K. Analysis of copolymers The VP content of copolymers was determined by analysis for nitrogen using a Tecator Kjeltech Auto Analyser. During each set of analysis, the nitrogen content was also
1023
SALAH E. M. EL-BEGAWYand MALCOLMB. HUGLIN
1024
determined for a sample of poly-VP homopolymer. This ratio of the theoretical to experimental percentage of nitrogen in poly-VP was used as a correction factor, the value of which generally lay within the range 1.02-1.06.
Medium-high conversion copolymers Two systems were examined, the first comprising 80 wt% VP in the feed (Jl = 0.821) and the second having 95 wt% VP in the feed (fl = 0.956). In neither case was initiator or solvent added. For each system, several ampoules of feed mixtures were outgassed and subjected to y-irradiation from a 9000 Ci 6°Co source for various times at 293 K. The dose rate, as determined by Fricke dosimetry, was 21 krad hr-L Ampoules were removed after different irradiation times and the contents were assessed visually for optical homogeneity as clear (c), clear-translucent (c/t), translucent (t), translucent-opaque (t/o) or opaque (o). The fractional conversion (0) was determined from the sum of the weights of linear and crosslinked polymer, the former being determined by extraction with cyclohexanone and precipitation in petroleum ether. Precipitated linear polymer and washed insoluble (crosslinked) polymer were dried in vacuo for 1 week at 313K. Blends of copolymers To simulate the cumulative composition of the material present at a particular stage of copolymerization, appropriate mixtures of low conversion copolymers were made up as solutions containing 5% (w/v) of total copolymer in chloroform. Details of the composition of the blends are given later in the Results section. The solutions were placed in Teflon moulds at 293 K and the chloroform was allowed to evaporate slowly. The resultant films were then dried in vacuo for 3 days at 313 K. Films were also cast similarly for the unmixed low conversion copolymers and poly-VP homopolymer. Film thicknesses were measured with an uncertainty of + 10 #m, using a micrometer. Light transmission The light transmission of films of low conversion copolymers and blends was measured with a Unicam SP-1750 u.v.-spectrophotometer coupled with a Unicam AR55 linear recorder. Two metallic holders (with an aperture 15 × 6 mm, thickness 2 mm) were constructed. On one of these holders, the film was mounted using black adhesive tape and the measurements were run against the other holder as blank. Optical densities were scanned over the wavelength range 500 700 nm at a slit width of 0.22 mm, a band width of 0.7 mm and a scan speed of 2 nm see -~. Measurements were made on films of several different thicknesses. Glass transition temperature (Ts) Glass transition temperatures were determined for low conversion copolymers, medium-high conversion copolymers prepared by ~,-irradiation and blends, For the last group, in order to avoid phase separation which may occur during solvent evaporation, films were dissolved in cyclohexanone and the solution of each blend was added dropwise to an excess of petroleum ether, causing rapid co-precipitation. The precipitates were washed with precipitant and dried in vacuo for several days at 313 K. Measurements were made on samples (6-14mg) by means of a Mettler DSC-30 instrument fitted with a thermal analysis data station. A heating rate of 10 K rain -~ under gaseous N2 was used, the temperature range scanned being 243-473 K. RESULTS AND DISCUSSION
Low conversion copolymers The feed compositions, reaction times, conversions as well as the compositions (mole fraction F2 o f BA)
Table 1. Characteristics of low conversion copolymers Reaction time Light Sample fl (min) 0 F2 Ts(K) transmission LI 0.840 10 0.06 0.512 305 0.84 L2 0.867 15 0.07 0.482 309 0.88 L3 0.938 20 0.05 0.448 311 0.86 L4 0.965 25 0.04 0.407 315 0.83 L5 0.978 35 0.06 0.351 323 0.80 L6 0.986 40 0.05 0.310 338 0.85 L7 0.989 45 0.02 0.257 345 0.82 L8 0.993 60 0.02 0.216 362 0.78 L9 0.995 70 0.01 0.136 376 0.85 LI0 0.997 90 0.01 0.111 410 0.86 LI 1 0.999 100 0.01 0.031 438 0.79 L12 1.000 120 0.06 0.000 447 0.87
a n d Tg values o f the resultant low conversion copolymers, designated as L1-L11 are listed in Table 1. The values o f the mole fractions of V P in the feed ( f j ) a n d copolymer (F1 = 1 - F 2 ) were used in the F i n e m a n Ross plot [21] to yield reactivity ratios rl = 0.02 a n d r2 = 0.80. These values accord well with those reported previously from experiments employing a smaller n u m b e r of d a t a points [19, 22]. The values of Tg for the two h o m o p o l y m e r s and those of the copolymers afforded excellent linearity w h e n represented according to the Fox e q u a t i o n [23]. Moreover, the Tg for poly-BA (229 K) agrees well with other reported values [24, 25]. F o r poly-VP, values o f T s ranging from 327 to 448 K have been reported [26, 27], the lower values being explained by T a n a n d Challa [26] as being due to the large influence o f a relatively small a m o u n t of sorbed water, which itself has a very low T 8. The present value o f Tg = 447 K for poly-VP accords well with the u p p e r limit a n d is indicative o f efficient drying o f the sample. The recorded optical density (OD) for each film in its holder minus the value o b t a i n e d in the absence of the film yielded the optical density due to the sample a n d hence the transmittance. If It a n d Io are the intensities of transmitted a n d incident light respectively, then t r a n s m i t t a n c e = It/Io. The optical density is defined as O D = log(Io/I,) a n d hence transmittance = 10 - ° ° = It/I o. A t any particular wavelength, transmission results from the t r a n s m i t t a n c e modified by a small a m o u n t of surface reflection. The extent of the latter can be evidenced a n d allowed for by plotting O D vs film thickness. The small b u t finite departure from zero (generally 0.02-0,06) afforded at zero thickness was then added to the actual measured values o f the t r a n s m i t t a n c e to yield the corrected values o f transmission listed in Table l which are all specific to the wavelength 600 n m a n d a film thickness of 200 # m . The average transmission for the low conversion copolymers was 0.86.
Low-medium-high conversion copolymers Two series of copolymers p r e p a r e d by radiation to different levels o f fractional conversion 0 were obtained from feed mixtures containing 80 w t % V P a n d 95 w t % VP, the I I samples in these series being
Polymer polymer miscibility during copolymerization Table 2. Optical clarity and Tg values for copolymers produced by 7-irradiation to different conversions from a VP/BA feed mixture containing 80 wt% VP
1025
Table 3. Opticalclarityand T~values for copolymersproducedby 7-irradiation to different conversionsfrom a VP/BA feed mixture containing95% VP
Sample
Dose/Mrad
Visual appearance
0
Ts (K)
Sample
Dose/Mrad
Visual appearance
0
T~ (K)
80R 1 80R2 80R3 80R4 80R5 80R6 80R7 80R8 80R9 80RI0 80RI1
0.003 0.005 0.010 0.021 0.042 0.084 0.168 0.315 0.504 1.208 1.660
c c c c/t t o o o o o o
0.118 0.155 0.234 0.260 0.294 0.334 0.422 0.845 0.880 0.972 0.998
303 306 311 312 315 317 309, 438 316, 396, 443 310, 423, 442 313,420, 445 318,413,446
95R1 95R2 95R3 95R4 95R5 95R6 95R7 95R8 95R9 95R10 95R11
0.003 0.005 0.015 0.020 0.041 0.082 0.184 0.347 0.510 1.500 1.800
c c/t c/t t t/o t/o o o o o o
0.016 0.027 0.041 0.056 0.118 0. 146 0.202 0.425 0.815 0.965 0.999
323 421 323,427 418, 439 299, 422 311,428 395,422 372,397, 426 388,425 397, 411 425,442
denoted as 80R1-80Rll and 9 5 R 1 - 9 5 R l l respectively. The characteristics of these groups of copolymers are given in Tables 2 and 3. The bulk copolymerization of the azeotropic feed was also effected by y-irradiation. The appearance of the group of copolymers 9 5 R 1 - 9 5 R l l is illustrated in Fig. 1. The copolymer prepared from the azeotropic feed, which is also illustrated in Fig. 1 for one particular value of 0 --- 0.90, is of the same appearance (i.e. high optical clarity) at all the other values of 0. Tables 2 and 3 and Fig. 1 show that, whereas optical appearance as c or c/t is evident up to 0 = 0.26 in the series 80R, such an appearance in the series 95R extends only up to a conversion of 0.04. The values of Tg for the two groups of copolymers are listed in Tables 2 and 3. The onset of significant compositional heterogeneity at relatively low conversion in the series 95R is also demonstrated by the fact that a single T~ is given only by samples prepared up to 0 = 0.027 in contrast to the situation for the series 80R in which a single Tg is obtained up to 0 = 0.334.
Blends of copolymers The compositional drift of copolymer with increasing 0, originally treated by Skeist [28] and implemented in an integral form by Meyer and Lowry [29],
has been quantified conveniently in previous publications by means of a computer program developed by Yip [22]. The program is written in Fortran 77 for use on the Prime 750, the resultant diagrams being produced on a Calcomp 1051 Plotter. Calculated curves of instantaneous copolymer composition (expressed as mole fraction F2 of BA) at different conversions are given in Fig. 2 for two systems, viz. copolymerizations 80C and 95C. For the former f~ = 0.821 (corresponding to 80 wt% VP in the feed) and for the latter fl =0.956 (corresponding to 95 wt% VP in the feed). In system 80C the compositional drift is only gradual up to 0 ~ 0.30 and thereafter becomes pronounced within the interval 0.30-0.40. At 0 > ca 0.40, the instantaneous copolymer composition is close to that of pure poly-VP. This is, of course, due to the low reactivity ratio r I of VP compared to the relatively high value of r 2. In the system 95C the onset of compositional drift becomes evident at a relatively low stage of conversion (0 ~ 0.06) and is continuous up to 0 ~ 0.16. From 0 ~ 0.20 onwards the instantaneous copolymer composition is close to that of poly-VP. To simulate these copolymerizations, blends were made up from samples within the group L1-L12. Thus for a curve in Fig. 2 at any particular value of 0, the instantaneous composition is indicated on the
Fig. 1. Appearance of copolymers prepared at different conversions by y-irradiation. Samples 95RI-95R11 are denoted in the figure by 1-11. The sample denoted by 12 in the figure is produced from the azeotropic feed mixture (fl = 0.017) at a conversion 0 = 0.90. EPJ 27/1~B
1026
SALAH E. M. EL-BEoAwY and MALCOLM B. HUGLIN
1.0j~
r0
~,~ 0.4 0
>
cl
0
u.. 0.2
0
0.2
I //I 0.6 0.8
0.4
1.0
F2 Fig. 2. Calculated curves of instantaneous copolymer composition vs fractional conversion (a) system 80C, (b) system 95C, (c) azeotropic system.
Table 4. Compositions and properties of blends 80BI-80B5 and blends 95BI-951M
Blend
Number of component copolymers
Nature of component copolymers
2 4 6 10
L1, L2 L1-L4 LI-L6 LI-LI0 L 1 -L 12
0.18 0.32 0.36 0.39 0.42
0.50 0.42 0.28 0.18 0.15
307 311 326 363, 399 309, 442
L5, L6 LS---L8 LS-LI0 L5-L12
0.08 0.10 0.135 0.22
0.52 0.43 0.40 0.21
334 345 338, 402 389, 438
80BI 80B2 80B3 80B4 80B5
12
95B1 95B2 95B3 95B4
2 4 6 8
curve. The total polymer present at this conversion comprises the copolymer of the indicated instantaneous composition in additon to copolymers produced at earlier stages of conversion. The weight fraction Wx of BA in a copolymer of instantaneous composition Fx is calculated as
Wx=hx/Eh FiLm thickness o "~T.,I -0.2
-
4o
eo I
~x.--.. ~
-
12o I
~
~8o 2oo t
~.
I
24o
2so 32o
I
I
I
Corresponding simulated Light conversion transmission
Ts (K)
where hx is the height of the integral step corresponding to the composition Fx and I~ h is the sum of the heights of the integral step between zero conversion and the particular conversion 0. To simulate the copolymerization 80C, five values of 0 were selected and for each of these a blend was prepared. The compositions of these blends 80BI-80B5 are indicated in Table 4. Similarly to simulate the material present at four different conversions in copolymerization 95C, low conversion copolymers were mixed to yield blends 95BI-95B4 of compositions also given in Table 4.
FiLm thickness (pro)
" " ~ " ~ .
0
0
40
80
120
160
200
240
280
S~)1
0,4
o
-0,6
~ -0,8 -
1.0
-
1.2
% i n ~
-
0.6
~,,~
- 08
Fig. 3. Variation of transmittance at 600 n m with thickness of film for blends (a) 80B1, (b) 80B2, (c) 80B3, (d) 801M and (e) 80B5.
-
1.o
Fig. 4. Variation of transmittance at 600 n m with thickness o f f d m for blends (a) 95B1, (b) 95B2, (c) 95B3 and (d) 95B4.
Polymer-polymer miscibility during copolymerization
1027
REFERENCES
J
c
f
S
d
"1o
233
t
I
315
39:5
Temperature
473
(K)
Fig. 5. DSC traces for blends (a) 95B1, (b) 95B2, (c) 95B3 and (d) 95B4. The corrected plots of log (It/Io) at 600 nm vs film thickness for blends 80B1-80B5 and for blends 95B1-95B4 are shown in Figs 3 and 4 respectively. From these plots the transmissions at a thickness 200/~m were read off. The values are listed in Table 4, which also gives the values of Tg for the blends. Figure 5 illustrates DSC traces for blends exhibiting a single Tg and two Tss. Comparison of the data in Table 4 with the calculated curves in Fig. 2 indicates that the compositional drift in the latter is consonant with both the blend incompatibility evidenced by two Ts s and the reduced light transmission which obtain for the former. In conclusion we note that the use of blends of low conversion copolymers serves as a useful model for the copolymer produced at various stages of copolymerization. The procedure is, of course, only semiquantitative in nature, because the number of components of different composition which are used for each blend is of necessity extremely small cf the n u m b e r existing in a copolymer produced at any stage of conversion (except for the azeotropic case). Moreover, in an actual copolymerization the quantity and composition of unreacted, liquid monomer mixture changes with conversion. It is probable that this consideration may account, in part, for the fact that the value of 0 at or above which there is departure from optical clarity and the occurrence of two Tgs in the radiation-induced copolymerizations (Tables 2 and 3) does not coincide exactly with the value of 0 for corresponding effects to prevail in the blends (Table 4). Work is about to commence to assess the thermodynamic affinity of the solvent (i.e. unreacted monomer mixtures) for the blends.
Acknowledgement---One of us (S.E.M. E-B) thanks the Egyptian government for financial support.
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