Polisülfonların dielektrik davranışlarının incelenmesi

Abstract

ÖZET POLÎSÜLFONLARIN DİELEKTRİK DAVRANIŞLARININ İNCELENMESİ Poli(olefin sülfon) 'ların çözeltide, alışılmışın dışında dinamik özellikler göstermeleri, bu sınıf polimerlerin dielektrik davranışlarının l960'lı yıllardan bu yana ayrıntılı olarak incelenmesine neden olmuştur. Ana-zincir yapısında bulunup, zincire dikey bir dipol moment bileşeni sağlayan SO2 grubunun, nasıl olupta zincire paralel bir dipol moment bileşeni oluşturduğu inceleme konusudur. 01efin/S02 alternatif kopolimerlerinde gözlenen, diğer polimer sistemlerinde yaygın olmayan bu davranışlar, zincirdeki heliks yapıların varlığı ile açıklanabilmektedir. Bu çalışmada, aşağıdaki olefin- sülfon kopolimerlerinin benzen, dioksan, toluen ve karbontetraklorür içerisinde, 20-90 C sıcaklık bölgesinde ve 2 MHz frekanstaki dielektrik sabitleri ölçüldü : Poli (Siklohekzen Sülfon); poli(l-Buten Sülfon); poli(l-Hekzen Sülfon); poli (1-Dode sin Sülfon); poli (1-Eiko sin Sülfon); poli (Alil siklopentan Sülfon). Bu kopolimerlerin yinelenen birim başına dipol momentlerinin kareleri ortalaması, <y >/x, Guggenheim -Smith bağıntısı ile hesaplandı. Bu değerlerden yararlanarak, polimerlerin D, dipol moment oranları (D = </r>/x/ı 2) ve sıcaklık katsayıları, dln<y2>/dT belirlendi. İncelenen polimerlerin yan-zincir uzunluğu arttıkça </p->/x değerleri nin azaldığı ve bu değerlerin çözücüye de bağımlı olduğu gözlendi. Bazı araştırıcılar, l-Olefin/SO^ kopolimerlerinde ki heliks yapı varsayımını irdelemek için, l-01efin/2-01efin(veya halkalı olefinJ/SCL yapısında terpolimerler hazırlayarak, bu polimer zincirlerinin dielektriksel davranışlarını incelediler. Bu çalışmada, yukarıda bildirilen olefin-sülfon kopolimerleri nin, düşük ve yüksek- frekans (200 Hz-2MHz) bölgelerinde yapılan dielek trik ölçümlerinden yararlanılarak, konformasyon hesapları yapıldı. Ayrıca daha önce hazırlanan l-Eikosin/Siklohekzen/SC^ terpoli- merlerindeki 1-Eikosin: Siklohekzen oranının değişmesinin, <^2>/x değerine nasıl yansıdığı incelendi.

SUMMARY INVESTIGATION OF DIELECTRIC BEHAVIOURS OF POLYSULFONES The formation of poly sulphones from sulfur dioxide and olefins is a typical free-radical polymerization and can be conducted in bulk, in solution or in aqueous emulsion. The copolimerization reaction is catalyzed by such initiators as peroxides, oxygen, azo compounds and light. The dielectric properties of polar chain polymers have long been of great interest to many workers. The study of dielectric constant and loss in dilute solutions has fewer technical applications, but offers information about molecular conformation underboth equilibrium and dynamic conditions, and thus can be very useful in the characterization of macromolecular structure. The measurement of dielectric constant (permittivity) of a polymer solution can be readily achieved over an extended frequency range by using a.c. bridge methods -2 7 (from 10 to ca. 10 Hz), resonance and heterodyne beat / 8 methods ( ca. 10 to 10 Hz), transmission-line techniques (109 to 7.5 x 109 Hz), microwaves (1010 to 1011 Hz), far 11 12 infra-red spectroscopy (10 to 10 Hz) and, if required, higher frequencies (i.r., visible, u.v., etc.). It is this extended frequency range which provides versatility in dielectric measurements. Two main types of dielectric behaviour has been observed for polar polymers in solution. One group, exemplified by poly (ethylene oxide) and poly(methyl methacrylate ), show a single dispersion region with a high, molecular weight-independent, maximum frequency fm (107-10n Hz) (Type B, Figure 3-1). For flexible chains, the vector sum of a sequence of such dipoles does not correlate with the displacement lengt, so that the relevant of chain diffusion are usually short range or local modes:. A second group, exemplified by polypeptides, polyisocyanates and cellulose ethers, show a single dispersion region, but with a low, strongly molecular weight-dependent, maximum frequency fm(10 -10 Hz) (Type A, Figure 3-1). For Type A, the total dipole moment is correlated with the end-to-end vector. Thesetwo types of behaviour reflect two relaxation vimechanism, the first by segmental rotation, the second by rotation of the molecule as a whole. It might be thought that for molecules with a side-chain dipole, as in poly(methyl methacrylate ), a third relaxation mechanism would exist, namely by rotation of the side group (Type C, Figure 3-1). The relaxation time for such a mechanism would be independent of molecular weight. The alternating copolymers of alkenes with sulfur dioxide, the poly(alkene sulfones), is divided into three classes : one, ( -SO^CHRChL-) -, with a single side group R per repeat unit; two, ( -SO~CR, R^CrL ), with two side groups on the same carbon; and three, ( -SO^CHR-, CHR`-), with one side group on each of the two carbons. Groups one and two are dielectrically active in the radio-frequency region 3 6 (10 -10 Hz), with relaxation times t, strongly dependent on molecular weight. Groups one and two also exhibit a weaker relaxation in the high-frequency region (>107 Hz). These high-frequency relaxations result from local conformational rearrangements, while the low-frequency ones probe global changes in the shape. Fawcett, Ivin, and Co-workers present evidence that the group three polymers show a strong preference for trans state at the CHR, - CHR` bond. Such an arrangement leads to a net dipole moment of zero for the polymer, as may be seen by decomposing the dipole moments along the C-S bonds. They argue that this accounts for the lack of a low- frequency relaxation. The presence of the low-frequency relaxation in classes one and two indicates substantial conformational differences between these and the class three polymers. Particularly difficult to explain is how polymers of groups one and two develop a large longitudinal component of the dipole moment when the SO2 unit dipole is perpendicular to the backbone. In the accompanying communication, Fawcett and Fee propose that the unusual conformational and dynamical behaviour of l-olefin/SOo alternating copolymers in dilute solution is due to the presence of helical structures. In this work, the dipole moment measurements of the various olefin/S02 alternating copolymers and terpolymers of SO2, 1-eicosene, and cyclohexene were carried out in various solvents and temperatures. The polysulphones of cyclohexene and 1-hexene were prepared by standard free- radical recopies. Viscometric measurements of these samples were made in Ubbelohde suspended-level viscometer. The [r)] values were interpreted using the Mark-Houwink equation for poly( 1-hexene sulfone) in acetone at 20°C; [T1] = 5.9 x 10`3 M0'74 (ml/g), (1) viiand for poly ( cyclohexene sulfone) in dioxane at 25 C ; [n] = 5.7 x 10`3 M0*72 (ml/g) (2) Best grade solvents were purified by refluxing over sodium and fractionally distilled over sodium immediately before preparation of solutions for dielectric measurements. Solutions were made up by weight. The dielectric constants of these copolymers and terpolymers solutions, were measured by a WTW DM 01 Model dipolmeter working at a constant 2 MHz. frequency. 2 The total mean-square dipole moment <y > of the chain were calculated by means of the appropriate form of the Guggenheim-Smith equation :, 27 kT M de/dw9 dn2/dw, <y2>/x = ? ±[ K - - * 1] (3) 47TN Pl (e1+2)/ (n^+2)z where x is the number of repeat units, de/dw2 is the rate of change of the dielectric constant (permittivity) e of the solution with the polymer weight fraction w 9 / ni aftd Pn are the refractive index and the density of the solvent, e-i is the dielectric constant of the solvent. N, k, T and M0 refer to Avogadro's number, Boltzmann constant, absolute temperature and molecular weight of the repeat unit, respectively. The mean-temperature coefficients of the dipole mpments dln<y2>/dT of PCHS and PHS in the various solvents were calculated. The dipole moment ratio Dx is an important quantity characterizing the polymer conformation. Dx, is defined as x 2 xy Dv = *Y- (4) o In the present work, the dipole moment ratios of the Vllldifferent poly( 1-olef in sulfones) in benzene, dioxane, toluene and carbontetrachloride were determined over a wide range of temperature and compared with the existing literature data. It was seen that the numerical value of the ratio increased from less than unity for the shorter side chains to about 2 for long side chains. The paralel and perpendicular contributions to the mean-square dipole moment per repeat unit were conveniently calculated for the terpolymers and copolymers by appropriate adaptations of the relation given by Guggenheim and Smith: 9 27 kT M Ae'/w Ae'/w < 2>/x = _o_ _o _ _y (5) P 4ttN Pl (£l+2)2 (£l+2)2 and 0 27 kT M Ae'/w 2nTAn/w0 *. I OrU ± L i i/ <y >/x = [ 5` 9 T~ ] { > s 4ttN Pl (£l+2)Z (nJ+2)Z where Ae'/w and Ae'/w are the low-frequency limits for the paralel and perpendicular components, respectively. The other symbols have the meanings given in equation 1. Furthermore, 8 and v^were calculated for the same copolymer and terpolymer samples by using the dielectric increment, Ae'/w at the low-frequency limit : 6 (Ae'/w)lf 1-8 (Ae'/w)hf (7) <y2>/x = 8vwuih + (l-8)Dc yjc (.8) where 8 is the fraction helix content, (1-8) is the fraction random-coil content of the same chain, (AeVw)ir. and (Ae'/w) are the change of the dielectric constant e at the low-frequency and high-frequency of the solution with the polymer weight fraction w2, <y^>/x is the total mean-square dipole moment at the low-frequency and high- frequency of the chain, U.is the weight-average number of repeat units in a helical section, y,, is the dipole moment ixof a single paralel component (0.7 D), Dcis the dipole moment ratio within a random-coil sequence (0.6) and ^ic- is the dipole moment of a single sulfone group (^4.5 D, based on model compounds, e.g. dialkyl sulfones). These data show that the fraction helix content 0 and the weight-average number of repeat units in a helical section Vyy, in the 1-olefin series increase with increasing length of the alkyl side group. The solubility in the solvent increases with increasing side-group size. For longer side groups, steric forces may produce stereo- specificity during polymerization, which would lead to further preference of the helix. This helix is stabilized by short-range electrostatic interactions (between adjacent SC>2 groups), by side-group interference and solvation, and ( apparently, by longer range electrostatic interactions between seperate turns of the helix. As the helix becomes more important, the SO2 dipoles become more strongly correlated and the net dipole moment of the molecule grows. When the short side groups are present on adjacent carbons, the C-C bonds of the bockbone favor trans highly, and the net dipole moment is much smaller. The dependence upon composition x, of the magnitude of the reduced dipole moment relaxed at low frequencies in the terpolymers is given by (<U_>/x) R = 1 2 (9) (<y >/x), v ^p 'x=l where x is the mole fraction of 1-olefin units. It was found that the magnitude of the dipole relaxed at low frequencies in solutions of the terpolymers of SO2, 1-eicosene and cyclohexene increased with increasing the mole fraction of 1-eicosene residues. Our data were in agreement with the previously published results.