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Solvent-induced crystallisation of compatible poly(styrene-co-acrylonitrile) and poly(vinyl chloride-co-vinyl acetate) blends
Eur. Polym. J. Vol. 32, No. 8, pp. 973-977. 1996 Copyright 0 1996 Published by ElsevierScience Ltd
PII: SOOl4-3057(%)00023-7
Printed in Great Britain. All rights reserved OOW3057/96 SIS.00 + 0.00
SOLVENT-INDUCED CRYSTALLISATION OF COMPATIBLE POLY(STYRENE-CO-ACRYLONITRILE) AND POLY(VINYL CHLORIDE-CO-VINYL ACETATE) BLENDS GAURAB
DAS and A. N. BANERJEE*
Department of Plastics and Rubber Technology, University College of Science and Technology, Calcutta University, 92 A.P.C. Road, Calcutta 700 009, India (Received 22 May 1995; accepted in final form I1 September 1995)
Ahats&-The inIluence of solvent on the development of crystalhnity in a compatible poly(styrene-coacrylonitrile) (SAN) and poly(vinyl chloride-co-vinyl acetate) (VYHH) blend has been studied by differential scanning calorimetry (DSC), optical microscopy, Fourier transform infrared spectroscopy (FTIR), and a wide angle X-ray diffraction method (WAXD), conSrming the presence of crystalhnity in the blend. The low temperature casting of the blends developed a considerable amount of solvent-induced crystalhnity in one component of the blend, whereas the higher temperature casting of the blends and also the coprecipitated blends showed no sign of development of crystallinity in the blend. So, solvent retention during film formation was responsible for the observed crystallinity in one component of the blend. Copyright 0 1996 Published by Elsevier Science Ltd
INTRODUCTION
Most compatible polymer blends are amorphous but several have at least one component that can be crystallised in the blend by an appropriate thermal or solvent treatment [l]. Such blends exhibit both a single compositionally dependent glass transition temperature (Z’& corresponding to a mixed amorphous phase and a crystalline melting temperature (T,), also compositionally dependent, which corresponds to a crystalline phase. Examples include blends of poly(vinylidene fluoride) (PV F2) with poly(methy1 methacryiate) (PMMA) [2, 31, poly(ethy1 methacrylate) (PEMA) [2,4, 51, and poly(viny1 methyl ketone) (PVMK) [6]; ternary blends of poly(viny1 chloride) (PVC), PMMA and PEMA [7]; blends of PVC with poly(c-caprolactone) (PCL) [8,9]; blends of PVC with a terpolymer of ethylene, vinyl acetate and carbon monoxide [lo]; blends of poly(2,bdimethyl-1,4-phenylene oxide) (PMMPO) with atactic and isotactic polystyrene [l 1, 121 and blends of poly(N-vinyl-2-pyrrolidone) (PVP) with a copolyamide (COPA) randomly composed of 1: 1: 1 (by weight) nylon 6, nylon 66 and nylon 610 [ 131. Under special conditions, completely amorphous polymers may be partially crystalline and be transformed into semi-crystalline polymers [14]. It is known that the properties of semi-crystalline polymers are largely dependent on the form and extent of crystallinity. It is therefore advantageous to real& the factors that control the development of crystallinity in a crystalline/compatible blend system. This paper presents the influence of solvent on the crystallisation of one component of the blend. Studies by DSC, optical microscopy, FTIR spectral analysis *To whom all correspondence
should be addressed.
and also wide angle X-ray diffraction have been performed to detect the crystalline nature of the blend.
EXPERIMENTALPROCEDURES Poly(styrene-co-acrylonitrile), with an acrylonitrile content of 22 wt% from elemental analysis, was obtained from Polychem Ltd (India) (Polylan 1000 IM-1). Poly(viny1 chloride-co-vinylacetate) (VYHH), supplied by Union Carbide International (USA), contained 87 wt% vinyl chloride, 13 wt% vinyl acetate, and had an intrinsic viscosity value (cyclohexanone at 20°C) of 0.53. Cktyl tin mercaptide (M/S ALA Chemicals, India) was used as a stabiliser for VYHH which was added as 3 wt% of the total polymer. Cyclohexanone and methanol were of reagent grade. Films of SAN, VYHH and their blends were cast from 4 wt% solutions in cyclohexanone on a mercury surface at ambient temperature to obtain uniform film thickness. Evaporation of the solvent was performed slowly with a covered glass under a stream of nitrogen in a dust free chamber, and the resulting films were dried under reduced pressure at 100°C until the films reached constant weight. Glass transition temperatures (T,s) of the blends were determined with a DSC 20 Mettler TA 3000 system with a TC 10 A microprocessor using a heating rate of lOC/min. Each sample was IIrst heated from room temperature to 130°C. The reported r, values were the average value-s based on the second and subsequent runs. IR spectra were obtained on a Perkin-Elmer 1600 series FTIR instrument; 64 scans at a resolution of 4cm-I were signal average. Optical microscopy studies of the films were done with a Leitz Laborlux 12 Po1.S underplane polarised light. Wide angle X-ray scattering was performed by means of a Jeol XRD 8020. RESULTS AND DISCUSSION
The blend films cast from cyclohexanone solution showed only one glass transition temperature Ts over 973
974
G Das and A. N Banerjee
Wt./.
VYHH
0^o 2
%AN
Fig. I. Change m Tr as a function of weight fracuon of SAN for SANlVYHH blends.
the whole composition range (Fig. I), Indicating the compatible nature of the blend. Crystalline melting endotherms of a blend were observed in the first run of several mixtures (80120, 60140, 50150. 40/60 of VYHH/SAN) as shown in Fig. 2. They were not observed in the second and subsequent runs (Fig. 3) because, in the case of the first run the crystallisation was conducted in a solution which gave high chain mobility, where it was favoured, but in the case of the second and subsequent runs crystallisation could not occur owing to the high viscosity of the bulk polymer medium in the DSC pan. This interpretation is in agreement with observations made on high molecular weight polyethylene, where dramatic changes in morphology were seen between samples prepared from melt and from solution [15]; only the latter presented a well-defined morphology. It IS generally
Wt%
VYHH
110
I
I
I
150
I
I
190
TEMPERATURE
1
.I
230 (“Cl
Fig. 2. First run of DSC traces of SAN/VYHH blends showing the crystalline melting endotherm of VYHH.
190
230
TEMPERATURE(*C)
Fig. 3 Second run of DSC traces of SAN/VYHH blends showmg no crystalhne melting endotherm of VYHH.
recogmsed [16] that in the presence of certain interactive solvents, crystallisation of an amorphous polymer can take place at a temperature well below the r, of the polymer. From the analysis of the thermograms of the solution cast samples (Fig. 2) tt is apparent that solvent (i.e. cyclohexanone) has induced a considerable amount of crystallinity in the polymer blends. Two possible causes of this behaviour [17] are the decreased solvent evaporation rate and lower solvent diffusion rate through the swollen films that would be expected at lower temperatures. Decreasing these factors would increase the time period over which the solvent remained dissolved in the blend, allowing VYHH more time to crystallise. Another factor to be considered IS the decreased solubility that occurs at lower temperature. As solubility decreases, the ratio of solvent to polymer at the gel point increases. This increased solvent/polymer ratio would result in a system (i.e. VYHH/SAN/cyclohexanone blend system) with T, lower than that normally obtained when preparing films at room temperature. If the decrease in r, obtained by casting the film at lower temperatures than would be expected, then a greater tendency for crystallisation at lower temperatures may result, because with lower temperature casting the solvent will have more time in the blend due to the slow rate of solvent evaporation. At low temperatures, due to the longer duration of solvent retention in the blend, the viscosity of the medium remams mobility
710
150
low for a longer
time, giving higher chain
of the molecules and allowing VYHH molecules more time to crystallise. When the rate of solvent evaporation was very fast, the solvent-induced crystallisation was prohibited due to the lack of time available to solubilise the blend by the solvent which was responsible for the crystallisation of one of the components of the blend. The solvent retention time during film formation was responsible for the observed behaviour. If the blend (SAN/VYHH/cyclohexanone) was cast at elevated
Solvent-induced crystallisation of compatible polymer blends
Fig. 4. Optical mxrographs
of VYHHlSAN blends. (A) 80:20 blend. (B) 70:30 blend; (C) 50:50 blend Magmficatlon x 200.
975
976
G
730
700
650
Das
and
600
WAVENUMBER (Cd’, Fig. 5 FTIR absorption spectra in the region 73&600 cm ’ (a) 80:20 (VYHH:SAN) blend-SAN; (b) 50.50 (VYHH. SAN ) blend-SAN temperatures (above 100 C) then there was no sign of the development of crystals as there was no detectable crystalline melting endotherm in the first DSC scan (observed when the same blend was cast at ambient temperature under a controlled rate of solvent evaporation). DSC thermograms are similar to those of Fig. 3. To further support the influence of solvent on the crystallisation of the blend. the blend solution was poured into a large volume of non-solvent such as methanol. In such cases, again no melting endotherm was observed because there was a lack of time available for the solubilisation of the blend. From Fig. 2 it is observed that there is no detectable crystalline melting endotherm below 20 wt% of VYHH content. The absence of crystallinity m blends with high SAN content (i.e. more than 80 wt?/o) 1s probably due to kinetics and not to thermodynamic requirements; it is due to the increase in & of the film as the solvent continues to evaporate from the film. leading to an increase in the viscosity
20
Fig 6. Wide angle X-ray diffraction curve of VYHHSAN blends: (a) 60:40 VYHH/SAN blends; (b) 50:50 VYHH SAN blends.
A. N Baneqee
of the blend and to a slowing down of the crystalhsation rate. or to the very low degree of crystallinity of VYHH in the blend when its concentration is below 20%, which is not easily detectable by the DSC scan. Microscopic examination shows the crystalline nature of the blends. as depicted in Fig. 4. From FTIR spectroscopic studies (Fig. 5). peaks at 6 I5 cm-’ (crystalline syndiotactic C-Cl peak) and 694cm-’ (amorphous isotactic C-Cl peaks) [18] are both observed. The C-Cl peak at 613 cm-’ arises mainly from the sequence of four or more trans configurations in a syndiotactic repeat unit. With a syndiotactic sequence of four or more repeat units, crystallinity can form [19]. This ease of crystallisation can be ascribed to the strong dipoledipole interactions between the C-Cl bonds. It is well known that a syndiotactic (i.e. trans sequences) structure is responsible for the crystallinity of vinyl chloride polymers [20]. The question of crystallinity may arise in the case of a copolymer. mainly due to the irregular structure [VYHH being a copolymer of vinyl chloride (87 wt%) and vinyl acetate (13 wt%)]. It has been found from the literature [21] that copolymers and structurally irregular polymers also show spheruhtic structure but in general they are of the non-banded type. Regular copolymers such as a copolymer of ethylene and tetrafluoroethylene (an alternating copolymer) and an irregular polymer such as a copolymer of ethylene and vinyl alcohol, ethylene and carbon dioxide. etc., are also crystalline [22]. If the comonomer units are comparable in size and polarity. isomorphic replacement of the minor component in the crystal lattice of the major component may occur with some distortion of the lattice parameters. A continuous change in lattice dimensions with composition may be observed if the two unit cells are similar. When different crystal structures are involved, that of the major component is usually found at the ends of the composition range, with both crystals appearing side by side at intermediate concentrations. Comonomer units that are not capable of crystallising or are too large to fit into the prevailing crystal lattice may be accommodated as defect structures at low concentrations and be rejected from the crystals at higher concentrations. In some cases very low levels of crystallinity are observed over the composition midrange, while appreciable crystallisation of the individual components occurs when their respective concentrations are high [23]. Figure 6 shows the X-ray intensity versus Bragg angle for crystalline VYHH/SAN blends cast from cyclohexanone at ambient temperature. The crystalline diffraction peaks are quite broad and ill-defined, probably due to the fact that solvent removal introduces lattice defects which result in a very imperfect crystal structure [24]. VYHH has no inherent crystallinity. In this system, its crystallinity is induced only by the solvent and due to the formation of compatible blends with a completely amorphous polymer (i.e. with SAN). Its crystal structure may be distorted, so the crystal imperfection may result as reflected in their X-ray diffraction curve.
Solvent-induced crystallisation of compatible polymer blends CONCLUSION
The solvent (i.e. cyclohexanone) has induced a considerable amount of crystallinity in one component (i.e. in VYHH which has no inherent crystallinity) of the VYHH/SAN blend system. The low temperature casting of the blends has developed crystallinity in the blend, whereas high temperature
casting of the blends and also coprecipitated blends show no sign of developed crystallinity. It may be concluded that the duration of the solvent retention time in the blend system is responsible for the development of crystallinity in one component of the blend. CSIR grant to Gaurab Das (SRF. Indian Fellowship) is acknowledged.
Acknon&=dgement-A
REFERENCES I. J. R. Fried. In Derelopmetzt in Polymer Characterisairon (edited by J. V. Dawkins), Vol. 4. p. 39. Applied Science Publishers, London (1983). 2. C. F. Hammer. Mucronto/eru~es 4, 69 (1971). 3. T. Nishi and T. T. Wang. Macromoiecules 8,909 ( 1975). 4. R. L. Imken, D. R. Paul and J. W. Barlow. PO&#. Eng. Sci. 16, 593 (1976). T. K. Kwei, G. D. Patterson and T. T. Wang. Macromolecules
9. M. Aubin and R. E. Prud’homme. J. Polvm Sri.. Polwn. Ph_w. 19, 1245 (1981). IO. E. W. Anderson, H. E. Bair. G. E. Johnson, T. T. Kwei. F. J. Padden, Jr and D. Williams. Adr. Chenr. Ser. 176,
413 (1979). 1I. R. A. Neira-Lemos. Ph.D. dissertation. University of Massachusetts. Amherst (1974). 12. H. Berghmans and N. J. Overbergh. J. Pa/w. Sci.. Polwn. Phjx 15, 1757 (1977). 13. Guo Qlpeng, Yu Min and Feng Zhihu. Po/wner 33, 893 (1992). 14. Anton Peterhn. Encyclopedia of Polwwr Science and Engineering. 2nd edn. Vol. IO, p. 26. John Wiley and Sons, New York (1987). 15. L. Mandelkern, S. Go. D. Pelffer and R. S. Stein. J. Polwn. Sri.. Polvm. Ph,l. 15, 1189 (1977). 16. Sung Soon Im and Hyung Sick Lee. J. Appl. Po!vn. Sci. 37, 1801 (1989). I7 James Runt and Peter B. Rim. Macrotnolecufes 15. 1018 (1982). 18. M. Theodorou and B. Jasse. J. Polwl. Sci., Po/wt. Phys. 21, 2263 (1983).
19. Douglas
S. Hubbell and Stuart L. Cooper. In Polwners (edited by Stuart L. Cooper and Gerald M. Esters), p_ 517. American Chemical Society. Washington. DC (1979). J. A. Brydson. Plastic Marerials. p. 281. Butterworth Scientific, London ( 1982). Leo Mandelkern. Crwrallisarion of Po(wner. p. 326. McGraw-Hill, New York (1964). Fred W. Billmeyer. Jr. TesestBook of Polymer Science. p. 267. John Wdey and Son. New York (1984). S. Y Hobbs. In Plastics Polwnet Science and Technolog! (edited by Mahendra D. Baijal), Chap. 6, p 239 John Wdey and Sons, New York (1982). W. J. MacKnight, F. E. Karasz and J. R. Fried. In Polwer Blends (edIted by D. R. Paul and S. Newman). Vol. I, p. 214. Academic Press, New York (1978). Multiphase
20. 21. 22.
8, 780 (1976).
R. E. Bernstein, D. C. Wahrmund. J. W. Barlow and D. R. Paul. Polvn. Eng. SC;. 18, 1220 (1978). T. K. Kwei, H. L. Frisch, W. Radlgan and S. Vogel. Macromolecules 10, I57 (1977). M. M. Coleman and J. Zanan. J. Po/Fnr. Sri. Po!,~nr. Phvs. 17, 837 (1979).
977
23.
24
PII: SOOl4-3057(%)00023-7
Printed in Great Britain. All rights reserved OOW3057/96 SIS.00 + 0.00
SOLVENT-INDUCED CRYSTALLISATION OF COMPATIBLE POLY(STYRENE-CO-ACRYLONITRILE) AND POLY(VINYL CHLORIDE-CO-VINYL ACETATE) BLENDS GAURAB
DAS and A. N. BANERJEE*
Department of Plastics and Rubber Technology, University College of Science and Technology, Calcutta University, 92 A.P.C. Road, Calcutta 700 009, India (Received 22 May 1995; accepted in final form I1 September 1995)
Ahats&-The inIluence of solvent on the development of crystalhnity in a compatible poly(styrene-coacrylonitrile) (SAN) and poly(vinyl chloride-co-vinyl acetate) (VYHH) blend has been studied by differential scanning calorimetry (DSC), optical microscopy, Fourier transform infrared spectroscopy (FTIR), and a wide angle X-ray diffraction method (WAXD), conSrming the presence of crystalhnity in the blend. The low temperature casting of the blends developed a considerable amount of solvent-induced crystalhnity in one component of the blend, whereas the higher temperature casting of the blends and also the coprecipitated blends showed no sign of development of crystallinity in the blend. So, solvent retention during film formation was responsible for the observed crystallinity in one component of the blend. Copyright 0 1996 Published by Elsevier Science Ltd
INTRODUCTION
Most compatible polymer blends are amorphous but several have at least one component that can be crystallised in the blend by an appropriate thermal or solvent treatment [l]. Such blends exhibit both a single compositionally dependent glass transition temperature (Z’& corresponding to a mixed amorphous phase and a crystalline melting temperature (T,), also compositionally dependent, which corresponds to a crystalline phase. Examples include blends of poly(vinylidene fluoride) (PV F2) with poly(methy1 methacryiate) (PMMA) [2, 31, poly(ethy1 methacrylate) (PEMA) [2,4, 51, and poly(viny1 methyl ketone) (PVMK) [6]; ternary blends of poly(viny1 chloride) (PVC), PMMA and PEMA [7]; blends of PVC with poly(c-caprolactone) (PCL) [8,9]; blends of PVC with a terpolymer of ethylene, vinyl acetate and carbon monoxide [lo]; blends of poly(2,bdimethyl-1,4-phenylene oxide) (PMMPO) with atactic and isotactic polystyrene [l 1, 121 and blends of poly(N-vinyl-2-pyrrolidone) (PVP) with a copolyamide (COPA) randomly composed of 1: 1: 1 (by weight) nylon 6, nylon 66 and nylon 610 [ 131. Under special conditions, completely amorphous polymers may be partially crystalline and be transformed into semi-crystalline polymers [14]. It is known that the properties of semi-crystalline polymers are largely dependent on the form and extent of crystallinity. It is therefore advantageous to real& the factors that control the development of crystallinity in a crystalline/compatible blend system. This paper presents the influence of solvent on the crystallisation of one component of the blend. Studies by DSC, optical microscopy, FTIR spectral analysis *To whom all correspondence
should be addressed.
and also wide angle X-ray diffraction have been performed to detect the crystalline nature of the blend.
EXPERIMENTALPROCEDURES Poly(styrene-co-acrylonitrile), with an acrylonitrile content of 22 wt% from elemental analysis, was obtained from Polychem Ltd (India) (Polylan 1000 IM-1). Poly(viny1 chloride-co-vinylacetate) (VYHH), supplied by Union Carbide International (USA), contained 87 wt% vinyl chloride, 13 wt% vinyl acetate, and had an intrinsic viscosity value (cyclohexanone at 20°C) of 0.53. Cktyl tin mercaptide (M/S ALA Chemicals, India) was used as a stabiliser for VYHH which was added as 3 wt% of the total polymer. Cyclohexanone and methanol were of reagent grade. Films of SAN, VYHH and their blends were cast from 4 wt% solutions in cyclohexanone on a mercury surface at ambient temperature to obtain uniform film thickness. Evaporation of the solvent was performed slowly with a covered glass under a stream of nitrogen in a dust free chamber, and the resulting films were dried under reduced pressure at 100°C until the films reached constant weight. Glass transition temperatures (T,s) of the blends were determined with a DSC 20 Mettler TA 3000 system with a TC 10 A microprocessor using a heating rate of lOC/min. Each sample was IIrst heated from room temperature to 130°C. The reported r, values were the average value-s based on the second and subsequent runs. IR spectra were obtained on a Perkin-Elmer 1600 series FTIR instrument; 64 scans at a resolution of 4cm-I were signal average. Optical microscopy studies of the films were done with a Leitz Laborlux 12 Po1.S underplane polarised light. Wide angle X-ray scattering was performed by means of a Jeol XRD 8020. RESULTS AND DISCUSSION
The blend films cast from cyclohexanone solution showed only one glass transition temperature Ts over 973
974
G Das and A. N Banerjee
Wt./.
VYHH
0^o 2
%AN
Fig. I. Change m Tr as a function of weight fracuon of SAN for SANlVYHH blends.
the whole composition range (Fig. I), Indicating the compatible nature of the blend. Crystalline melting endotherms of a blend were observed in the first run of several mixtures (80120, 60140, 50150. 40/60 of VYHH/SAN) as shown in Fig. 2. They were not observed in the second and subsequent runs (Fig. 3) because, in the case of the first run the crystallisation was conducted in a solution which gave high chain mobility, where it was favoured, but in the case of the second and subsequent runs crystallisation could not occur owing to the high viscosity of the bulk polymer medium in the DSC pan. This interpretation is in agreement with observations made on high molecular weight polyethylene, where dramatic changes in morphology were seen between samples prepared from melt and from solution [15]; only the latter presented a well-defined morphology. It IS generally
Wt%
VYHH
110
I
I
I
150
I
I
190
TEMPERATURE
1
.I
230 (“Cl
Fig. 2. First run of DSC traces of SAN/VYHH blends showing the crystalline melting endotherm of VYHH.
190
230
TEMPERATURE(*C)
Fig. 3 Second run of DSC traces of SAN/VYHH blends showmg no crystalhne melting endotherm of VYHH.
recogmsed [16] that in the presence of certain interactive solvents, crystallisation of an amorphous polymer can take place at a temperature well below the r, of the polymer. From the analysis of the thermograms of the solution cast samples (Fig. 2) tt is apparent that solvent (i.e. cyclohexanone) has induced a considerable amount of crystallinity in the polymer blends. Two possible causes of this behaviour [17] are the decreased solvent evaporation rate and lower solvent diffusion rate through the swollen films that would be expected at lower temperatures. Decreasing these factors would increase the time period over which the solvent remained dissolved in the blend, allowing VYHH more time to crystallise. Another factor to be considered IS the decreased solubility that occurs at lower temperature. As solubility decreases, the ratio of solvent to polymer at the gel point increases. This increased solvent/polymer ratio would result in a system (i.e. VYHH/SAN/cyclohexanone blend system) with T, lower than that normally obtained when preparing films at room temperature. If the decrease in r, obtained by casting the film at lower temperatures than would be expected, then a greater tendency for crystallisation at lower temperatures may result, because with lower temperature casting the solvent will have more time in the blend due to the slow rate of solvent evaporation. At low temperatures, due to the longer duration of solvent retention in the blend, the viscosity of the medium remams mobility
710
150
low for a longer
time, giving higher chain
of the molecules and allowing VYHH molecules more time to crystallise. When the rate of solvent evaporation was very fast, the solvent-induced crystallisation was prohibited due to the lack of time available to solubilise the blend by the solvent which was responsible for the crystallisation of one of the components of the blend. The solvent retention time during film formation was responsible for the observed behaviour. If the blend (SAN/VYHH/cyclohexanone) was cast at elevated
Solvent-induced crystallisation of compatible polymer blends
Fig. 4. Optical mxrographs
of VYHHlSAN blends. (A) 80:20 blend. (B) 70:30 blend; (C) 50:50 blend Magmficatlon x 200.
975
976
G
730
700
650
Das
and
600
WAVENUMBER (Cd’, Fig. 5 FTIR absorption spectra in the region 73&600 cm ’ (a) 80:20 (VYHH:SAN) blend-SAN; (b) 50.50 (VYHH. SAN ) blend-SAN temperatures (above 100 C) then there was no sign of the development of crystals as there was no detectable crystalline melting endotherm in the first DSC scan (observed when the same blend was cast at ambient temperature under a controlled rate of solvent evaporation). DSC thermograms are similar to those of Fig. 3. To further support the influence of solvent on the crystallisation of the blend. the blend solution was poured into a large volume of non-solvent such as methanol. In such cases, again no melting endotherm was observed because there was a lack of time available for the solubilisation of the blend. From Fig. 2 it is observed that there is no detectable crystalline melting endotherm below 20 wt% of VYHH content. The absence of crystallinity m blends with high SAN content (i.e. more than 80 wt?/o) 1s probably due to kinetics and not to thermodynamic requirements; it is due to the increase in & of the film as the solvent continues to evaporate from the film. leading to an increase in the viscosity
20
Fig 6. Wide angle X-ray diffraction curve of VYHHSAN blends: (a) 60:40 VYHH/SAN blends; (b) 50:50 VYHH SAN blends.
A. N Baneqee
of the blend and to a slowing down of the crystalhsation rate. or to the very low degree of crystallinity of VYHH in the blend when its concentration is below 20%, which is not easily detectable by the DSC scan. Microscopic examination shows the crystalline nature of the blends. as depicted in Fig. 4. From FTIR spectroscopic studies (Fig. 5). peaks at 6 I5 cm-’ (crystalline syndiotactic C-Cl peak) and 694cm-’ (amorphous isotactic C-Cl peaks) [18] are both observed. The C-Cl peak at 613 cm-’ arises mainly from the sequence of four or more trans configurations in a syndiotactic repeat unit. With a syndiotactic sequence of four or more repeat units, crystallinity can form [19]. This ease of crystallisation can be ascribed to the strong dipoledipole interactions between the C-Cl bonds. It is well known that a syndiotactic (i.e. trans sequences) structure is responsible for the crystallinity of vinyl chloride polymers [20]. The question of crystallinity may arise in the case of a copolymer. mainly due to the irregular structure [VYHH being a copolymer of vinyl chloride (87 wt%) and vinyl acetate (13 wt%)]. It has been found from the literature [21] that copolymers and structurally irregular polymers also show spheruhtic structure but in general they are of the non-banded type. Regular copolymers such as a copolymer of ethylene and tetrafluoroethylene (an alternating copolymer) and an irregular polymer such as a copolymer of ethylene and vinyl alcohol, ethylene and carbon dioxide. etc., are also crystalline [22]. If the comonomer units are comparable in size and polarity. isomorphic replacement of the minor component in the crystal lattice of the major component may occur with some distortion of the lattice parameters. A continuous change in lattice dimensions with composition may be observed if the two unit cells are similar. When different crystal structures are involved, that of the major component is usually found at the ends of the composition range, with both crystals appearing side by side at intermediate concentrations. Comonomer units that are not capable of crystallising or are too large to fit into the prevailing crystal lattice may be accommodated as defect structures at low concentrations and be rejected from the crystals at higher concentrations. In some cases very low levels of crystallinity are observed over the composition midrange, while appreciable crystallisation of the individual components occurs when their respective concentrations are high [23]. Figure 6 shows the X-ray intensity versus Bragg angle for crystalline VYHH/SAN blends cast from cyclohexanone at ambient temperature. The crystalline diffraction peaks are quite broad and ill-defined, probably due to the fact that solvent removal introduces lattice defects which result in a very imperfect crystal structure [24]. VYHH has no inherent crystallinity. In this system, its crystallinity is induced only by the solvent and due to the formation of compatible blends with a completely amorphous polymer (i.e. with SAN). Its crystal structure may be distorted, so the crystal imperfection may result as reflected in their X-ray diffraction curve.
Solvent-induced crystallisation of compatible polymer blends CONCLUSION
The solvent (i.e. cyclohexanone) has induced a considerable amount of crystallinity in one component (i.e. in VYHH which has no inherent crystallinity) of the VYHH/SAN blend system. The low temperature casting of the blends has developed crystallinity in the blend, whereas high temperature
casting of the blends and also coprecipitated blends show no sign of developed crystallinity. It may be concluded that the duration of the solvent retention time in the blend system is responsible for the development of crystallinity in one component of the blend. CSIR grant to Gaurab Das (SRF. Indian Fellowship) is acknowledged.
Acknon&=dgement-A
REFERENCES I. J. R. Fried. In Derelopmetzt in Polymer Characterisairon (edited by J. V. Dawkins), Vol. 4. p. 39. Applied Science Publishers, London (1983). 2. C. F. Hammer. Mucronto/eru~es 4, 69 (1971). 3. T. Nishi and T. T. Wang. Macromoiecules 8,909 ( 1975). 4. R. L. Imken, D. R. Paul and J. W. Barlow. PO&#. Eng. Sci. 16, 593 (1976). T. K. Kwei, G. D. Patterson and T. T. Wang. Macromolecules
9. M. Aubin and R. E. Prud’homme. J. Polvm Sri.. Polwn. Ph_w. 19, 1245 (1981). IO. E. W. Anderson, H. E. Bair. G. E. Johnson, T. T. Kwei. F. J. Padden, Jr and D. Williams. Adr. Chenr. Ser. 176,
413 (1979). 1I. R. A. Neira-Lemos. Ph.D. dissertation. University of Massachusetts. Amherst (1974). 12. H. Berghmans and N. J. Overbergh. J. Pa/w. Sci.. Polwn. Phjx 15, 1757 (1977). 13. Guo Qlpeng, Yu Min and Feng Zhihu. Po/wner 33, 893 (1992). 14. Anton Peterhn. Encyclopedia of Polwwr Science and Engineering. 2nd edn. Vol. IO, p. 26. John Wiley and Sons, New York (1987). 15. L. Mandelkern, S. Go. D. Pelffer and R. S. Stein. J. Polwn. Sri.. Polvm. Ph,l. 15, 1189 (1977). 16. Sung Soon Im and Hyung Sick Lee. J. Appl. Po!vn. Sci. 37, 1801 (1989). I7 James Runt and Peter B. Rim. Macrotnolecufes 15. 1018 (1982). 18. M. Theodorou and B. Jasse. J. Polwl. Sci., Po/wt. Phys. 21, 2263 (1983).
19. Douglas
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