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Crystallization of isotactic poly(methyl methacrylate) from the melt
]
Notes to the Editor
Crystallization of isotactic poly(methyl methacrylate) from the melt A. de Boer, G. O. R. Alberda van Ekenstein and G. Challa Department of Polymer Chemistry, State University of Groningen, Groningen, The Netherlands (Received 19 May 1975; revised 20June 1975)
INTRODUCTION In 1958 Fox et al, I reported the possibility of crystallizing isotactic poly(methyl methacrylate) (i-PMMA). Stroupe and Hughes 2 determined the unit cell of crystalline i-PMMA: a pseudo orthorhombic cell with the lattice distances a = 21.08, b = 12.17 and c = 10.55 A. Coiro et aL 3 concluded from X-ray fibre diagrams that i-PMMA crystallizes in a (51) helix conformation. Tadokoro et al,4refined this picture by X-ray diffraction and far infra-red measurements and suggested that the ester side groups of i-PMMA in the (51) helix conformation are rotated somewhat inwards. Also Tanaka et al, s and K1ement and Geil 6 investigated crystalline i-PMMA. In all cases the crystalline i-PMMA was obtained by means of borderline solvents or by annealing stretched thin films. In this note we present some results on the growth rate, shape and melting temperature of crystals of i-PMMA crystallized from the melt.
EXPERIMENTAL The sample of i-PMMA was prepared according to a known procedure 7. Some data of the polymer (sample no. 109) are: isotactic/heterotactic/syndiotactic triads 94:5:1, -My = , , 213 × 10 3 , Tg = 40 O C. The tactlclt~, was measured on 5 wt% o-dichlorobenzene solutions at 150vC by 60 MHz n.m.r. spectroscopy with a Varian A 60 instrument. [77] was measured in chloroform at 25°C. For the calculation of • tv, the relation a [r/] = 4.8 x 10 -5 ~tv0/~ was used. The glass transition temperature was measured with a differential scanning calorimeter (Perkin-Elmer DSC IB) at a heating rate of 8°C/min. Films with a thickness of about 10/am were obtained by evaporation of a 3 wt % chloroform solution of i-PMMA on a glass slide. To remove all solvent, the films were dried afterwards at 50°C in vacuo for 30 min. For crystallization temperatures (Tc) below 110°C the films were first heated at 180°C for 5 min and then cooled down to T c. This was done because otherwise there exist too many nuclei for detecting separate crys'als. Temperature control of the crystallization oven was within 0.2°C. A Zeiss polarization microscope with phase-contrast condenser and objective was used for light microscopic measurements on crystals grown in the films. Because of the very low growth rate (G) of i-PMMA crystals phase-contrast photographs were taken over seve-
930
POLYMER, 1975, Vol 16, December
ral days. After enlarging the photographs it was possible to measure the diagonal of the crystals with an accuracy of about 0.6/am. The magnifications were calibrated with a micrometer. Melting temperatures, Tin, were determined both with d.s.c, and a hot-stage light microscope. We used a hot stage plate (Mettler FP 5) with a heating rate of 0.2°C/min.
RESULTS AND DISCUSSION In Figure 1 the average values of the longest diagonals of the crystals are plotted against the crystallization time for various T c. From these lines the average growth rates G were calculated and plotted against crystallization temperature in Figure 2. This Figure shows that i-PMMA has a maximum value Groan of about 1 × 10 -3/am/min at about 120°C. For isotactic polystyrene (i-PS) maximum values of about 300 × 10 -3/am/min were reported 9. In the literature ~° i-PS is considered a slowly crystallizing polymer. Comparing the maximum values of G it is clear that i-PMMA should be called a very slowly crystallizing polymer. However, it should be kept in mind that the given maximum value for i-PMMA is not a general one. It will be influenced by molecular weight and tacticity of the sample. Figure 3 shows some typical photographs of crystals of
"& 30
~ 2o ~5 x .,~,, X ~
£C
Mo
0
8
16 TimcxlO-4(min)
24
Figure I Longest diagonal (~m) of crystals of 0 i-PMMA 0against /~ 0 crystallization time (rain) for various Tc: ©, 90 ; X, 100 ; ,110 ; u C]t v e;120 ; 130 C
No tes to the E d i t o r
-E E E
±
6
0
'
'
r c (°c) Figure 2 Average growth rate G (#m/min) of i-PMMA crystals against crystallization temperature Tc
i-PMMA taken at different stages of growth at 120°C. Initially only hexagonal structures were observed, which showed only little birefringence (Figures 3a and 3d), while in later stages skeletation to starlike structures occurred and finally the crystals became rounded. Hexagonal structures have also been reported for polyoxymethylene ll, isotactic polypropylene n and isotactic polystyrene 12, Besides the above mentioned hexagonal structures spherical structures were also observed, especially at lower To, e.g. 90 and 100°C. Tile spherical crystals had about the same value of G as the hexagons. In accordance with the literature x1'~3 these spherical structures showed more birefringence than the hexagonal structures. At 130°C no spherical structures could be observed at all. The above results are in agreement with those of Keith 13 for i-PS. However, we observed hexagons of i-PMMA at crystallization temperatures 100-120°C below the melting temperature, whereas Keith could detect hexagons of i-PS only at 30-40°C below the melting temperature. Moreover, the hexagons of i-PMMA could become much larger than those of i-PS, e.g. 65/am in Figure 3a versus 5 nm reported for pure i-PS. According to Keith 14 6 can be considered as a characteristic length for the growing crystal. (6 = D/G; D = diffusion coefficient for 'impurity' in the unsolidified medium; G = growth rate of the crystal face). At crystal sizes smaller than 8, impurities can easily diffuse out of the way of oncoming growth fronts. In this stage single crystals grow from the melt, producing hexagons which are multilayers of single crystals, also called hedrites. In the literature no melt viscosity data were available for i-PMMA to calculate D and it seems troublesome to use the
0 Figure 3 Various stages of crystal growth of i-PMMA at 120°C. (a), (b) and (c) are phase-contrast photographs. (d), (e) and (f) are bright field photographs between crossed Nicols. Crystallization times: (a) 10 days; (b) 14 days; (c) 18 days; (d) 10 days; (e) 14 days; (f) 18 days (magnification 640 X)
P O L Y M E R , 1975, Vol 16, December
931
Notes to the Editor
22C
180 0 v
14C
Figure 4 Phasec(~ntrast photograph of i-PMMA crystallized at 130 C during 25 days (magnification 640 ×)
available data of atactic PMMA. For 6 = 65 gm, the limiting value of the diagonal (Figure 3a) and a G value of I x 10 - 3 ~tm/sec (Figure 2) we calculate a D value of about 10 - 9 cm2/sec, which is a quite reasonable value. So, it may be assumed that the differences in supercooling and sizes of hexagons for i-PMMA and i-PS are predominantly caused by the much lower growth rate of i-PMMA compared with i-PS. Figure 4 shows a photograph of a sample crystallized at 130°C. At this temperature 6 has a larger value than at 120°C because D increases and G decreases with temperature (Figure 2). So, we expected to observe still larger hexagons, but even in very early stages no purely hexagonal structures could be observed, only structures with strong skeletation. Maybe, more faceted growth takes place at 130°C. Finally, Figure 5 shows the Tm - Tc diagram of i-PMMA. Melting temperatures Tm were recorded with a d.s.c, at various heating rates. To correct for super heating (maximum effects of 8 and 20°C for crystals grown at 90 and 130°C, respectively), Tm values were extrapolated to zero scan speed. Tm values were also estimated on the hot stage light microscope (Figure 5). The middle of the melting range ( 4 - 8 ° C ) of the crystals was chosen as Tin. The difference with d.s.c, values can be explained by the fact that especially for samples crystallized at 90°C and 100°C the beginning of the melting process is not exactly detected as a result of which too high T m values are found. Since it is very time consuming to crystallize i-PMMA at temperatures above 130°C, the line in the T m - Tc diagram had to be extrapolated over a large distance. This means that the extrapolated melting temperature Tm 0 can only be found approximately. Figure 5 shows that Tm 0 of i-PMMA is about 220°C.
932
P O L Y M E R , 1975, Vol 16, December
I00
I00
140
I 0
2 0
(°C)
Melting temperatures T m of i-PMMA as a function of crystallization temperature T c. Q, Recorded by d.s.c, extrapolated values with zero scan speed; ©, estimated by light microscopy with a heating rate of 0.2 C/rain Figure 5
REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Fox, T. G., Garret, B. S., Goode, W. E., Gratch, S., Kincaid, J. F., Spell, A. and Stroupe, J. D. J. Am. Chem. Soc. 1958, 80, 1768 Stroupe,J. D. and ttughes, R. E. J. Am. Chem. Soe. 1958, 80, 2314 Coiro, V. M., De Santis, P., Liquori, A. M. and Mazzarella, L. J. Polym. Sci. (C) 1969, 16,4591 Tadokoro, H., Chatani, Y., Kusanagi, H. and Yokoyama, M. Macromolecules 1970, 3,441 Tanaka, A. and Ishida, Y. J. Polym. Sci. (Polym. Phys. Edn) 1974, 12, 335 Klement, J.J.and GeiI, P.H.J. MacromoI. Sci. (B) 1972,6, 31 Goode, W. E., Owens, F. H., Fellmann, R. P., Snijder, W. H. and Moore, J. H. J. Polym. Sci. 1960, 46, 317 Bischof, J. and Desreux, V. Bull, Soc. Chim. Belg. 1952,61, 10 Lemstra, P. J., Postma, J. and Challa, G. Polymer 1974, 15, 757 Boon, J. Thesis, Delft (1966) Geil, P. H. 'Polymer Single Crystals', Interscience, New York, 1963, section 111/2 Danusso, F. and Sabbioni, F. Rend. Inst. Lomb. SeL Lett. (,4) 1958, 92,435 Keith, H. D. J. Polym. Sci. (A) 1964,2,4339 Keith, H. D. and Padden, Jr.,F. J. J. AppI. Phys. 1963,34, 2409
Notes to the Editor
Crystallization of isotactic poly(methyl methacrylate) from the melt A. de Boer, G. O. R. Alberda van Ekenstein and G. Challa Department of Polymer Chemistry, State University of Groningen, Groningen, The Netherlands (Received 19 May 1975; revised 20June 1975)
INTRODUCTION In 1958 Fox et al, I reported the possibility of crystallizing isotactic poly(methyl methacrylate) (i-PMMA). Stroupe and Hughes 2 determined the unit cell of crystalline i-PMMA: a pseudo orthorhombic cell with the lattice distances a = 21.08, b = 12.17 and c = 10.55 A. Coiro et aL 3 concluded from X-ray fibre diagrams that i-PMMA crystallizes in a (51) helix conformation. Tadokoro et al,4refined this picture by X-ray diffraction and far infra-red measurements and suggested that the ester side groups of i-PMMA in the (51) helix conformation are rotated somewhat inwards. Also Tanaka et al, s and K1ement and Geil 6 investigated crystalline i-PMMA. In all cases the crystalline i-PMMA was obtained by means of borderline solvents or by annealing stretched thin films. In this note we present some results on the growth rate, shape and melting temperature of crystals of i-PMMA crystallized from the melt.
EXPERIMENTAL The sample of i-PMMA was prepared according to a known procedure 7. Some data of the polymer (sample no. 109) are: isotactic/heterotactic/syndiotactic triads 94:5:1, -My = , , 213 × 10 3 , Tg = 40 O C. The tactlclt~, was measured on 5 wt% o-dichlorobenzene solutions at 150vC by 60 MHz n.m.r. spectroscopy with a Varian A 60 instrument. [77] was measured in chloroform at 25°C. For the calculation of • tv, the relation a [r/] = 4.8 x 10 -5 ~tv0/~ was used. The glass transition temperature was measured with a differential scanning calorimeter (Perkin-Elmer DSC IB) at a heating rate of 8°C/min. Films with a thickness of about 10/am were obtained by evaporation of a 3 wt % chloroform solution of i-PMMA on a glass slide. To remove all solvent, the films were dried afterwards at 50°C in vacuo for 30 min. For crystallization temperatures (Tc) below 110°C the films were first heated at 180°C for 5 min and then cooled down to T c. This was done because otherwise there exist too many nuclei for detecting separate crys'als. Temperature control of the crystallization oven was within 0.2°C. A Zeiss polarization microscope with phase-contrast condenser and objective was used for light microscopic measurements on crystals grown in the films. Because of the very low growth rate (G) of i-PMMA crystals phase-contrast photographs were taken over seve-
930
POLYMER, 1975, Vol 16, December
ral days. After enlarging the photographs it was possible to measure the diagonal of the crystals with an accuracy of about 0.6/am. The magnifications were calibrated with a micrometer. Melting temperatures, Tin, were determined both with d.s.c, and a hot-stage light microscope. We used a hot stage plate (Mettler FP 5) with a heating rate of 0.2°C/min.
RESULTS AND DISCUSSION In Figure 1 the average values of the longest diagonals of the crystals are plotted against the crystallization time for various T c. From these lines the average growth rates G were calculated and plotted against crystallization temperature in Figure 2. This Figure shows that i-PMMA has a maximum value Groan of about 1 × 10 -3/am/min at about 120°C. For isotactic polystyrene (i-PS) maximum values of about 300 × 10 -3/am/min were reported 9. In the literature ~° i-PS is considered a slowly crystallizing polymer. Comparing the maximum values of G it is clear that i-PMMA should be called a very slowly crystallizing polymer. However, it should be kept in mind that the given maximum value for i-PMMA is not a general one. It will be influenced by molecular weight and tacticity of the sample. Figure 3 shows some typical photographs of crystals of
"& 30
~ 2o ~5 x .,~,, X ~
£C
Mo
0
8
16 TimcxlO-4(min)
24
Figure I Longest diagonal (~m) of crystals of 0 i-PMMA 0against /~ 0 crystallization time (rain) for various Tc: ©, 90 ; X, 100 ; ,110 ; u C]t v e;120 ; 130 C
No tes to the E d i t o r
-E E E
±
6
0
'
'
r c (°c) Figure 2 Average growth rate G (#m/min) of i-PMMA crystals against crystallization temperature Tc
i-PMMA taken at different stages of growth at 120°C. Initially only hexagonal structures were observed, which showed only little birefringence (Figures 3a and 3d), while in later stages skeletation to starlike structures occurred and finally the crystals became rounded. Hexagonal structures have also been reported for polyoxymethylene ll, isotactic polypropylene n and isotactic polystyrene 12, Besides the above mentioned hexagonal structures spherical structures were also observed, especially at lower To, e.g. 90 and 100°C. Tile spherical crystals had about the same value of G as the hexagons. In accordance with the literature x1'~3 these spherical structures showed more birefringence than the hexagonal structures. At 130°C no spherical structures could be observed at all. The above results are in agreement with those of Keith 13 for i-PS. However, we observed hexagons of i-PMMA at crystallization temperatures 100-120°C below the melting temperature, whereas Keith could detect hexagons of i-PS only at 30-40°C below the melting temperature. Moreover, the hexagons of i-PMMA could become much larger than those of i-PS, e.g. 65/am in Figure 3a versus 5 nm reported for pure i-PS. According to Keith 14 6 can be considered as a characteristic length for the growing crystal. (6 = D/G; D = diffusion coefficient for 'impurity' in the unsolidified medium; G = growth rate of the crystal face). At crystal sizes smaller than 8, impurities can easily diffuse out of the way of oncoming growth fronts. In this stage single crystals grow from the melt, producing hexagons which are multilayers of single crystals, also called hedrites. In the literature no melt viscosity data were available for i-PMMA to calculate D and it seems troublesome to use the
0 Figure 3 Various stages of crystal growth of i-PMMA at 120°C. (a), (b) and (c) are phase-contrast photographs. (d), (e) and (f) are bright field photographs between crossed Nicols. Crystallization times: (a) 10 days; (b) 14 days; (c) 18 days; (d) 10 days; (e) 14 days; (f) 18 days (magnification 640 X)
P O L Y M E R , 1975, Vol 16, December
931
Notes to the Editor
22C
180 0 v
14C
Figure 4 Phasec(~ntrast photograph of i-PMMA crystallized at 130 C during 25 days (magnification 640 ×)
available data of atactic PMMA. For 6 = 65 gm, the limiting value of the diagonal (Figure 3a) and a G value of I x 10 - 3 ~tm/sec (Figure 2) we calculate a D value of about 10 - 9 cm2/sec, which is a quite reasonable value. So, it may be assumed that the differences in supercooling and sizes of hexagons for i-PMMA and i-PS are predominantly caused by the much lower growth rate of i-PMMA compared with i-PS. Figure 4 shows a photograph of a sample crystallized at 130°C. At this temperature 6 has a larger value than at 120°C because D increases and G decreases with temperature (Figure 2). So, we expected to observe still larger hexagons, but even in very early stages no purely hexagonal structures could be observed, only structures with strong skeletation. Maybe, more faceted growth takes place at 130°C. Finally, Figure 5 shows the Tm - Tc diagram of i-PMMA. Melting temperatures Tm were recorded with a d.s.c, at various heating rates. To correct for super heating (maximum effects of 8 and 20°C for crystals grown at 90 and 130°C, respectively), Tm values were extrapolated to zero scan speed. Tm values were also estimated on the hot stage light microscope (Figure 5). The middle of the melting range ( 4 - 8 ° C ) of the crystals was chosen as Tin. The difference with d.s.c, values can be explained by the fact that especially for samples crystallized at 90°C and 100°C the beginning of the melting process is not exactly detected as a result of which too high T m values are found. Since it is very time consuming to crystallize i-PMMA at temperatures above 130°C, the line in the T m - Tc diagram had to be extrapolated over a large distance. This means that the extrapolated melting temperature Tm 0 can only be found approximately. Figure 5 shows that Tm 0 of i-PMMA is about 220°C.
932
P O L Y M E R , 1975, Vol 16, December
I00
I00
140
I 0
2 0
(°C)
Melting temperatures T m of i-PMMA as a function of crystallization temperature T c. Q, Recorded by d.s.c, extrapolated values with zero scan speed; ©, estimated by light microscopy with a heating rate of 0.2 C/rain Figure 5
REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Fox, T. G., Garret, B. S., Goode, W. E., Gratch, S., Kincaid, J. F., Spell, A. and Stroupe, J. D. J. Am. Chem. Soc. 1958, 80, 1768 Stroupe,J. D. and ttughes, R. E. J. Am. Chem. Soe. 1958, 80, 2314 Coiro, V. M., De Santis, P., Liquori, A. M. and Mazzarella, L. J. Polym. Sci. (C) 1969, 16,4591 Tadokoro, H., Chatani, Y., Kusanagi, H. and Yokoyama, M. Macromolecules 1970, 3,441 Tanaka, A. and Ishida, Y. J. Polym. Sci. (Polym. Phys. Edn) 1974, 12, 335 Klement, J.J.and GeiI, P.H.J. MacromoI. Sci. (B) 1972,6, 31 Goode, W. E., Owens, F. H., Fellmann, R. P., Snijder, W. H. and Moore, J. H. J. Polym. Sci. 1960, 46, 317 Bischof, J. and Desreux, V. Bull, Soc. Chim. Belg. 1952,61, 10 Lemstra, P. J., Postma, J. and Challa, G. Polymer 1974, 15, 757 Boon, J. Thesis, Delft (1966) Geil, P. H. 'Polymer Single Crystals', Interscience, New York, 1963, section 111/2 Danusso, F. and Sabbioni, F. Rend. Inst. Lomb. SeL Lett. (,4) 1958, 92,435 Keith, H. D. J. Polym. Sci. (A) 1964,2,4339 Keith, H. D. and Padden, Jr.,F. J. J. AppI. Phys. 1963,34, 2409