A differential scanning calorimetry study of polyvinyl alcohol

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4. A. B. KHARENKO, E. A. STARIKOVA, V. V. LUTSENKO and A. B. ZEZIN, Vysokomol. soyed. A18: 1604, 1976 (translated in Polymer Sci. U.S.S.R. 18: 7, 1837, 1976). 5. Zh. G. GULYAEVA, M. F. ZANSOKHOVA, E. F. RAZVADOVSKII, V. S. EFIMOV, A. B. ZEZIN and V. A. KABANOV, Vysokomol. soyed. A25: 1238, 1983 (translated in Polymer Sci. U.S.S.R. 25: 6, 1436, 1983). 6. V. B. ROGACHEVA, S. V. RYZHIKOV, A. B. ZEZIN and V. A. KABANOV, Vysokomol. soyed. A26: 1674, 1984 (translated in Polymer Sci. U.S.S.R. 26: 8, 1872, 1984). 7. H. G. BUNGENBERG de JONG, Colloid Science V, chapter 10 (Edited by H. Kruyt), New York, 1949. 8. I. MICHAELI, I. Th. OVEREEK and M. I. VOORN, J. Polymer Sci. 23: 443, 1957. 9. K. N. BAKEEV, V. A. IZUMRUDOV, A. B. ZEZIN and V. A. KABANOV, Vysokomol. soyed. 29: 424, 1987 (translated in Polymer Sci. U.S.S.R. 29: 2, 476, 1987). 10. H. I. BIXER and A. S. MICHAELS, Encyclopedia of Polymer Science and Technology, Vol. 10, New York, 765, 1969. 11. V. V. LUTSENKO, A. B. ZEZIN and R. I. KALYUZHNAYA, Vysokomol. soyed. A16: 2411, 1974 (translated in Polymer Sci. U.S.S.R. 16: 11, 2797, 1974).

Polymer Science U.S.S.R. Vol.32, No. 4, pp. 722-726, 1990

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A DIFFERENTIAL SCANNING CALORIMETRY STUDY OF POLYVINYL ALCOHOL* V. S. PSHEZHETSKII, A. A. RAKHNYANSKAYA, I. M. GAPONENKO and Yu. E. NALBANDYAN M. V. Lomonosov State University, Moscow; Erevan Branch of "Plastpolimer" Scientific Production Association (Received 27 December 1988)

Polyvinyi alcohol samples prepared by radical polymerization of vinyl acetate in methanol solution and having different MM and different acetate group contents are studied by a DSC method. With increase in the number of acetate groups the proportion of crystalline phase and the melting point, and also the quantity of water bound in the polyvinyl alcohol, which attains 2 wt% under 80% humidity conditions, are significantly decreased. Moreover, half the water is in the strongly bonded state, and enters preferentially into the polymer crystal lattice. THERE has been increasing interest in recent years in studies on polyvinyl alcohol (PVA), since the unique set of physicochemical properties of this polymer has led to its greatly increased practical application [1]. The high density of the reactive hydroxyl groups determines both the chemical activity of the P V A , and to a significant extent determines its crystalline structure, because of the formation of a network of hydrogen bonds in the polymer. The degree of crystallinity of P V A can attain 70% even under radical polymerization conditions of vinyl acetate, and the mechanical properties of crystalline P V A are better than those of PE [2]. This provides the prerequisites for

*Vysokomoi. soyed. A32: No. 4, 784-787, 1990.

Calorimetry study of polyvinyl alcohol

723

obtaining high strength fibres from gelatinous PVA, since high strength fibres have been obtained from gelatinous PE fairly recently [3, 4]. One of the most popular industrial methods for the synthesis of PVA is based on the radical polymerization of vinyl acetate in methanol and subsequent alcoholysis of the PVA, in methanol in the presence of alkali [5]. This method enables both the MM and the content of acetate groups to be controlled. These two characteristics are extremely important for establishing fibres and films based on PVA, and also compositions, and provide a means of controlling their properties and the processing conditions, depending on the degree of crystallinity Z and the melting point /'me of the PVA. The ability of the PVA to retain water, which is especially important for ion-conducting membranes based on PVA [6] is associated with an increase in Z and the content of acetate groups x. The object of this work was to determine, by means of DSC, the characteristics of the crystalline phase: Tme, Z, the content and degree of bonding of the absorbed water in industrial PVA samples, having different MM and different values of x. Grade 7/1, 11/1, 16/1, 40/1, 11/4, 11/9, 11/12, 11/18, and 11/26 PVA samples were used, where the numerator indicates the viscosity of a 4% aqueous solution of PVA (kPa s), and the denominator the content of acetate groups x (wt%). The PVA was synthesized by radical polymerization of vinyl acetate in methanol [5]. The MM of the polymer was controlled by the methanol content in the reaction mixture, since it acts as a chain transfer agent. The polyvinyl acetate was subjected to alcoholysis in an alkaline solution of methanol at 50°C. To study the water bound to the polymer during synthesis, after drying the samples were placed in a sealed PE packing. In another series of samples, after drying, the specimens were held for a considerable time in an atmosphere of 80% humidity. A DuPont 1090 thermoanalyser with a DSC-10 micro-calorimeter was used for the calorimetric measurements and for calculating the heat of melting of the PVA and evaporation of the bound H20. A weighed quantity of PVA (10o25 mg) was placed in a sealed cell. The temperature was scanned at a rate of 10020 K/min, and the values of Z were determined from data on the heat of melting from the equation .~

Mo'AH

Z --- I O O ~ e

,

where M0' and M0 are the MM values for the monomer unit in the PVA and PVA without acetate groups respectively; Anme and AH are the heats of melting (J/g) of a single In crystal (1550) [7] and the PVA test sample, respectively. The H 2 0 content of the PVA samples was determined from the equation H 2 0 ( % ) = 100 AHevae AHeva t '

where AHevae and AHevat are the experimental and theoretical values of the heats of melting of H 2 0 over the temperature range 1000148°C [6]. Figure 1 shows DSC curves for PVA samples of different MM (Fig. la) and different values of x (Fig. lb). In both series three endothermic peaks can be seen on the DSC curves. The low temperature peak in the region of 50060°C obviously indicates a PVA transition from the glass-like to the high elastic state [7]. The endothermic peak within the temperature range 900170°C is due to the evaporation of water bound to the polymer. It must be noted that there is no peak within the temperature range 80-150°C (Fig. la, curve 4) on the DSC curve for PVA grade 16/1 previously

724

V.S. PSHEZHETSKIIet al.

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FzG. 1. DSC curves for PVA samples of different MM (a) or different x (b); (A-B) water evaporation range; (B--C) PVA melting range; (a) samples 7/1 (1), 40/1 (2), 16/1 (3); (4) sample 16/1, vacuum dried at 115°C over 100h; (b) x = 1 (1), 9 (2), 12 (3), and 26% (4).

vacuum dried at 115°C over 100 h, which confirms the given assignment of this peak to water bound to the PVA. The third peak indicates melting of the polymeric crystalline phase. The maxima of peaks 1 and 3 were assigned to Tg and Tree, and at the maximum of peak 2 the water content of the PVA was calculated from its heat of evaporation. Table 1 gives the results of determining Tme and AHme, and the calculated content of crystalline phase in PVA samples differing in the degree of acylation or MM. As follows from these data, in the case of PVA of different MM and small values of x the ratio of the amorphous to the crystalline phase is approximately the same. With these polymers, the values of Tme are close, which indicates that the end groups do not provide an appreciable contribution to the supermolecular organization of these polymers. With increase in x the values of Z are significantly decreased, and Tree is also decreased. It is important that melting of the PVA always precedes its degradation. Degradation begins at T > 250°C, which is not in agreement with the data provided by Rozenberg [2]. The quantity of water remaining in the PVA after methanolysis of the polyvinyl acetate is ~1 wt% for grades 11/1 and 11/2 PVA, and is inversely related to the value o f x in the macromolecules. Thus, with PVA for which x = 26% the quantity of bound water is decreased to 0.04 wt%, which is associated with an increase in hydrophobic character of the macromolecule (Table 1). Consideration of the temperature ranges of water evaporation (Fig. 2) provides information on the nature of the bonding between the water in the PVA and its possible location in it (Fig. 2). In both series of PVA samples water evaporation takes place over a narrow temperature range, i.e. 70-80°C. This indicates that the energies of bonding the water in the PVA are changed over wide limits. Similar changes in the temperature of melting of the water in PVA gels were observed by Tehnu et al. [8]. The low temperature boundary of H20 evaporation is characteristic of water which is the least strongly bonded to the polymer. With grades 7/1, 16/1, and 40/1 PVA samples, which have sorbed an additional 0.5-1 wt% H 2 0 during holding at 80% humidity up to 30% of the water is evaporated at T < 100°C (Fig. 1). This indicates that the water adsorbed by the polymer is practically in the free state. On the other hand, with a series of PVA samples which have adsorbed water during PVA alcoholysis, the water evaporates at T > 100°C. The maximum water evaporation in these samples is

Calorimetry study of polyvinyl alcohol

TABLE 1.

725

PVA MM

MELTING POINT, DEGREE OF CRYSTALLINITY, QUANTITY OF BOUND WATER IN NUMBERS OF ACETATE GROUPS AND HAVING DIFFERENT

AND CONTAINING DIFFERENT

Grade of PVA

M X 10 4

Trne 0

AHme, J/g

Z, %

AHeva, Jig

[H20], wt%

11/1 11/4 11/9 11/12 11/19 11/26 7/1" 11/1 16/1" 16/17 40/1"

3.5 3.6 3.7 3.9 4.1 4.3 2.7 3.5 4.6 4.6 8.1

226 214 205 199 192 182 227 226 225 225 230

73.4 68.0 56.0 39.9 61.4 32.9 79.8 73.4 63.4 69.3 86.2

47.5 46.2 39.2 28.6 33.7 26.3 51.6 47.5 41.0 45.0 55.5

15.4 24.0 9.1 2.3 2.8 0.9 45.5 15.4 44.8

0.73 1.10 0.41 0.11 0.13 0.04 2.0 0.73 2.0

-32.5

1.5

*The samples were held under conditions of 80% humidity over 100 h. ?Sample vacuum dried at 115"C.

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FIG. 2. Temperature ranges for the evaporation of water from PVA samples differing in x (a) or MM (b). The maximum water evaporation temperatures are denoted by the crosses. FIG. 3. Melting point (1) and upper water evaporation temperature (2) as a function of x for PVA samples of grade 1l/x. within the range 115-140°C. The energy of bonding of this part of the water remains at 1-1.5 k J/mole higher than with weakly bound water, evidently because of the inclusion of water molecules in the network of hydrogen bonds and polymer hydroxyl groups. Finally, it is possible to isolate water which is evaporated under conditions of pre-melting of the P V A , i.e. o v e r the t e m p e r a t u r e range 160-180°C. Figure 3 shows plots of Tme (curve 1) against Tev a

(curve 2) for a series of P V A samples having different values of x. The curves are similar, and show a 50"C difference in temperature, i.e. there is an interconnection between melting of the crystalline phase and evaporation of this part of the water, which is evidently bonded to the P V A crystals.

Translated by N. STANDEN PS 3 2 : 4 - H

726

Yu. K. GODOVSKII et al.

REFERENCES 1. J. SAKURADA, Polyvinyl Alcohol Fibres, Leningrad, 1985. 2. M. E. ROZENBERG, Polimery na osnove vinilatsetata (Polymers Based on Vinyl Acetate), Leningrad, 1983. 3. P. SMITH and P. S. LEMSTRA, Colloid Polymer Sci. A258: 899, 1980. 4. P. SMITH, P. S. LEMSTRA and H. C. BOOIJ, J. Polimer Sci. Polymer Phys. Ed. 19: 877, 1981. 5. S. N. USHAKOV, Polivinilovyi spirt (Polyvinyl alcohol), 1, Leningrad, 1960. 6. Handbook of Chemistry and Physics. Chem. Cleveland, 2079. 1953. 7. C. A. FINCH, Polyvinyl Alcohol. Properties and Applications, Leningrad, New York, 1973. 8. H. TEHNU, J. NUORTILA-JOKINEN and F. SUNDHOLM, Europ. Polymer J. 24: 505, 1988.

PolymerScience U.S.S.R. Vol.32, No. 4, pp. 726-732,1990

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MESOMORPHIC BLOCK COPOLYMERS. FORMATION OF ORIENTED MESOPHASE IN A POLYDIETHYLSILOXANE BLOCK ON UNIAXIAL STRETCHING OF BLOCK COPOLYMERS* Yu. K.

GODOVSKII,I. A. VOLEGOVA,A. V. REBROV,L. A. NOVINSKAYA, S. B. DOLGOPLOSK and I. G. KOLOKOLTSEVA L. Ya. Karpov Physical Chemistry Research Institute (Received 28 December 1988)

Block copolymers containing polydiethyisiloxane as one of the components are studied by scanning and deformational calorimetry and also by X-ray analysis. The critical value of the MM of the polydiethylsiloxane block corresponding to the point where the oriented mesophase starts to be formed under uniaxial stretching is found. A NEW class of block-copolymers of the polycondensation type, in which one of the blocks has mesomorphic properties has been synthesized and intensively studied in recent years. Special interest attaches to the use of polydiethylsiloxane oligomers (PDES) as mesomorphic blocks, this siloxane being a flexible chain polymer, which, in spite of the absence of classical meso groups in its macromolecules, can exist in the m e s o m o r p h i c state after melting of the crystalline phase. Since the M M values of the blocks in polycondensation block-copolymers are generally at the oligomer level, in the case of m e s o m o r p h i c block-copolymers the problem naturally arises of the effect of the M M of the m e s o m o r p h i c block on the formation of a mesophase. Furthermore, in the case of block-copolymers with m e s o m o r p h i c blocks it is interesting to study their behaviour under uniaxial *Vysokomol. soyed. A32: No. 4, 788-793, 1990.