Poli (etil metakrilat)`ın dipol momentleri ve konformasyon karakteristiklerinin belirlenmesi

Abstract

ÖZET Polimer zincirleri, seyreltik çözeltilerde, segmentlerinin (tekrarlanan birim) hareketliliği nedeni ile çeşitli konfigürasyonlar alabilir. Segmentlerdeki hareketlilik, kullanılan çözücüye, polimer zincirinin uzunluğuna, bağlı bulunan yan grupların dönme açısına, sıcaklığa, taktisiteye g.b. bağlıdır. Bu özellikler, polimer çözeltisinin dieiektrik davranışı, dipol momenti, viskozitesi, karekteristik oranı gibi fiziksel büyüklüklerinde değişmelere neden olmak tadır. Bu büyük İliklerdeki değişmeler her polimer-çözücü sistemi için farklı sonuçlar göstermektedir. Bu çalışmada, poli (etil metakrilat) (PEMA) zincirlerinin benzen, karbon tetraklorür, dioksan ve toluen içindeki seyreltik çözeltilerinin 20-90 C aralığını kapsayan geniş bir sıcaklık bölgesinde dipol momentleri belirlendi. Bu çözücülerdeki, dipol moment sıcaklık katsayısı, d£n<u2>/dT, hesaplandı. Bulunan değerlerin benzende küçük negatif, karbon tetraklorür, dioksan ve toluende ise pozitif olduğu görüldü. PEMA zincirleri için, dipol moment oranları, <1^ > D = - = -, hesaplandı. D sıcaklıkla benzende negatif, karbon. tetraklorür ve dioksanda ise pozitif olmak üzere doğrusal bir değişim göstermektedir. Toluende ise bir eğri bulunmuştur. Dönme izomerleri kuramından (Rotational Isomeric State, RIS) yararlanılarak, bilgisayar yardımı ile PEMA zincirinin konformasyon parametreleri hesaplandı. RIS kuramına göre hesaplanan D değerlerinin sıcaklıkla değişimi benzende ölçülen deneysel sonuçlarla uyuşma göstermiştir. Karbon tetraklorür dioksan ve toluen çözücülerinde ise D 'un sıcaklıkla değişi mi için deneysel ve teorik olarak bulunan sonuçlar arasında uyum sağlanamamıştır. Bu uyumsuz sonucun nedeni, teorik hesaplarda, çözücü-segment karşılıklı etkileşimlerinin dikkate alınmamış olması gösterilebilir. Ayrıca PEMA'nm DOT değerlerinin taktisite ile değişimi RIS yöntemi ile hesaplanarak isotaktik zincirden sindiyotaktik zincire gidildikçe D değerinin arttığı bulundu. PEMA zincirlerinin Flory (theta) sıcaklığı dolaylarında viskoziteleri ölçüldü. Bu amaçla PEMA'nm theta çözücüsü olan isopropil alkol kullanıldı, ölçülen viskozite değerlerinden yararlanılarak intrinsik viskozite sıcaklık katsayıları, x parametreleri ve karakteristik oran hesaplandı. Bulunan sonuçlar yapılan diğer çalışmalarla karşılaştırıldı. (v)

DIPOLE MOMENT AND CONFORMATIONAL CHARACTERISTICS OF POLY (ETHYL METHACRYLATE ) SUMMARY The electrical and especially the dielectric behaviour of macromolecules makes it possible to obtain an important characterization method to investigate their structure, Dipole moment measurement of the macromolecules is one of the most important experimental methods to determine the conformational changes in dilute solution. Due to the mobility of the segments, polymer chains can show several configurations in dilute solutions. The mobility in the segments depends on several factors such as the nature of the solvent, the temparature, the molecular weight, the conformational barrier opposing rotameric transitions and composition of the polymeric system. For systems in equilibrium, the average conformational properties of polymeric chains such as the mean-square end-to-end distance <r2>, average dipole moments </iz> determines the intrinsic viscosity [n] and the dielectric behaviour of the solution. Those quantities exhibit different values depending on the solvent-polymer system. The aim of the present work is to elucidate the dependence of the dipole moment of the poly (ethyl methacrylate) (PEMA) on the temparature and the type of the solvent. Furthermore, the intrinsic viscosity [nj in the theta conditions was also measured and the configurationai characteristics of PEMA were theoretically investigated, using the rotational isomeric state (RIS) model. A sample of PEMA has been synthesized by free-radical polymerization and the dielectric measurements of dilute solutions in each of benzene, carbon tetrachloride, dioxane, toluene were performed in a temparature range of about 2Qö<T<90°C. The monomer and all the solvents were pure and freshly distilled. The molecular weight in methy ethyl ketone at 35 C, using the empirical viscosity equation, (vi)[n]35o (d£/g) = 3.46 x 105 x M0,8 5 was found to beM= 4.0 x 10. The dielectric constants of those PEMA solutions were measured by a WTW DM 01 model dipolmeter working at a constant 2 MHz frequency. The mean-square dipole moment of the chain was calculated from the Guggenheim- Smith equation <U2> 27kTM x 2 4tt(£1i-2) d± dW` 2 dW` 2 where x is the number of repeating units, k the Boltzmann constant, T the temperature, M the molecular weight of the repeating unit, N Avagadro's number, ei and di respectively, the dielectric constant and density of solvent. (de/dW2) is the rate of change of the dielectric constants (e, permittivity) of the solution with the polymer weight fraction W` and n the refractive index of the solution. The average dipole moment <u2>, the temperature coefficient dl <y2>/dT, and dipole ratio D^, of PEMA in the above solvents were calculated by using the Guggenheim- Smith formula in which the experimentally measured (ds/dW2)w 0 values were inserted. Dipole moment ratio is defined as, D = <y2> xu2 where /x is the magnitute of the dipole moment of a repeating unit. (vii)Furthermore, the viscosity of PEMA chains was measured in isopropyl alcohol (theta solvent for PEMA at 36.9°C) as a function of the temperature in the 32-42°C range. The intrinsic viscosity [n], the characteristic ratio Cm and the polymer-solvent interaction parameter x were calculated for the PEMA chain using the measured viscosity values. Intrinsic viscosities of the solutions were calculated according to the Huggins equation. c [n]+k» [n]2c where n is the specific viscosity, sp sp 'solution solvent solvent c is the polymer concentration in g/dl, k the Huggins constant. According to this equation the intrinsic viscosity can be evaluated from the intercept by plotting 1sp/c ^s. C. The intrinsic viscosity is proportional to the mean-square end-to-end distance, <rz>, according to Flory- Fox equation [`] - 4><r2>3/2 where is a constant of proportionality and M the molecular weight of polymer. Characteristic ratio is given by <r2> n£2 where <r2> is the unperturbed mean-square end-to-end distance of the chain and n the number of skeletal bond. (viii)The polymer-solvent interaction parameter x varies with the polymer volume fraction v, t the variation is given by x - x/ x2v2+x3v*... where x anc^ X are a coefficient depend on the temparature and molecular structural properties. The following results have been obtained in the present study (1) It was found that the dipole moment ratio (D^) of the PEMA in benzene decreasses with increasing the temperature, but the opposite is found with carbon tetrachloride, dioxane and toluene solvents. These variations are linear for benzene, carbon tetrachloride and dioxane and nonlinear for toluene. Depending on the chain structure, the dipole moment ratio of the dilute polymer displays a reversible variation. The solvent-polymer interactions play an important role in this variation. (2) The dipole moment of PEMA was calculated theoratically by using the rotational isomeric state, RIS, namely trans (t), gauche (g and ğ). It was observed that the Doo values calculated with computer by using the RIS theory agree with the experimental values determined with benzene. (3) For carbon tetrachloride dioxane and toluene solvents there is no agreement between the experimental and theoretical results variation of the dipole moment ratio with temperature. This disagreement seems to stem from the fact that the interaction between the solvent folymer segments (repeating unit) was not accounted for in the computations. (4) The dipole moment temperature coefficient of PEMA diluted in benzene, carbon tetrachloride and dioxane was found to be -4.09 x 10-/ 2.33 x 10-3, 2.03 x 10~3 (deg-1), respectively. Temperature coefficient of toluene was determined as 6.99 x 10-3 deg-1 in the temperature range of 20- 350C and 1.08 x 10` 3 deg-1 in the range of 35-90 oc. (ix)(5) On the other hand using the RIS model the dipole moment temperature coefficient of PEMA was calculated as -0.5 x 10 `* deg * and the result was found to be in good agreement with the experimental results obtained with the benzene solution. It was observed that the experimental and theoretical results for the dipole moment temperature coefficient differ- significantly from each other. (6) Furthermore, the variation of D» value of PEMA with the stereoregularty was calculated using the RIS method. (7) The increase of the intrinsic viscosity with the temperature was more rapid in the region below the theta- temperature compared to that above the theta- temper a ture. In this work the characteristic ratio of PEMA was measured with the isopropyl alcohol solution and was found to be «! C - 9.31. oo The characteristic ratio for PEMA calculated from the experimental result is found to be in good agreement with the result obtained by using the RIS method. (8) The variations of the x parameter with the temperature were calculated from the results for the intrinsic viscosity of PEMA in isopropyl alcohol from the obtained results, it has been determined that the x~ parameters are primarily influenced by the term Xi. (x)