Komposizyon kaymasının kopolielektrolitin yapısına ve özelliklerine etkileri

The effect of composition drift on the structure and properties of the copolyelectrolyte

Özet

Polielektrolitler yüklü birim içeren polimerlerdir. Biri yüklü ve diğeri yüksüz iki monomerin polimerleşmesi ile oluşan malzemeye kopolielektrolit denir. Gıda ve kimya sanayinde kıvam arttırıcı olarak kullanılırlar. Monomerlerin reaktiflikleri çok farklı olduğunda tepkime sırasında komonomer karışımının bileşimi sürekli değişir. Bu koşullarda oluşan kopolimerin bileşimi de reaksiyon süresince değişmektedir. Kompozisyon kayması dediğimiz bu etki reaksiyon süresince üretilen kopolimerin fiziksel özelliklerinin de farklılık içermesine yol açar. Yüklü, az yüklü ve yüksüz zincirlerin karışımı şeklinde oluşan malzemenin sadece bir kısmı viskoziteye katkı sağlar. Sodyum stiren sülfonat (NaSS) ve akrilamit (Aam) kopolimeri de polielektrolit yapıdadır. Çalışmamızda bu monomerlerin Tulane üniversitesinde sürekli izleme altında yapılan kopolimerizasyon deney verileri değerlendirilmiştir. Monomer karışımındaki ani NaSS kesri $f_a$ ve bu kesirdeki dönüşüme göre türevi $df_a/ dx$ ’den elde edilen ani olarak polimere katılan malzemedeki yüklü malzemenin kesri $F_a , F_a =f_a (1-x) (df_a / dx)$ formülü ile verilmektedir. Polimere giren NaSS kesri her iki deneyde de, deneyin karakter değiştirdiği noktada aniden artmakta, daha sonra da yüklü malzemenin oranı monomer karışımında azaldıkça polimere katılan malzemede azalmaktadır. %50 NaSS deneyinin, ani kompozisyonun dönüşüm üzerinden sayısal olarak integre edilmesi ile elde edilen malzemenin kompozisyon dağılım grafiğinde, üretilen malzeme üç farklı kompozisyonda yığılma yapar. Reaksiyon karakter değiştirdikten sonra, komposizyon kayması sonucunda, üretilen malzemede yüklü birimlerin oranı %80’lerden 0’a doğru sürekli olarak düştüğü gözlenir. Ürünün daha homojen yapıda olabilmesi için kopolimer sentezinin fazla kompozisyon kaymasına yol açmayan koşullarda gerçekleştirilmesi gerekmektedir.

Abstract

The viscosity of polymer is a function of its their hydrodynamic volume. Polyelectrolytes which are in very extended conformations due to the electrostatic mutual repulsion of charged units have very large hydroodynamic volumes. This is behind their widespread use as thickening agents for many foodstuffs. Size of a polymer chain depends on its molecular weight and persistance length. The latter depends on the linear charge density of the chain, which is limited by counterion condensation to one electronic charge per Bjerrum length. The persistence length is also limited by the Debye screening length of the solution, which depends on its total ionic strength. The composition of the copolymer changes as it forms, if one monomer is depleted faster. This effect, known as the composition drift, is especially important in copolymerization of an ionic and a nonionic monomer. As the composition of the monomer mixture changes the composition of the polymer being produced changes according to the formula, $F_a , F_a =f_a (1-x) (df_a / dx)$ , where, $F_a$ is the fraction of monomer a instantaneously entering the copolymer, $f_a$ is the monomer a fraction in the monomer mixture at that moment and x is overall conversion. Thus decreasing a f indicates that it is entering the reaction at a rate faster than its content in the feed mixture, due to its higher reactivity. Sodium styrene sulphonate - acrylamide copolymer is a copolyelectrolyte. Here, data obtained in Tulane University by continuous online monitoring of this system are evaluated, the composition drift in these reactions and its consequences are discussed. It is noted that the reactions performed with 25% and 50% initial NaSS fraction show a sharp corner in the composition versus conversion plots. This corner indicates that the nature of the reaction changes at this point. The reactivity of NaSS suddenly increases, and it begins to enter the reaction at a very high rate and is rapidly depleted. This sudden change of character of the reaction may be indicative of reaching the c* concentration where coils come into contact with each other. To find the instantaneous copolymer composition, the above formula must be applied on the measured monomer composition data. However as the inevitable experimental noise is amplified while taking the derivative, the monomer composition data is first fitted to a fitting function, and the formula is applied to the fitted values. Because of this corner, the data, were fitted in two separate stages. First stage data were fitted to a line and the second stage data were fitted to a quadratic function. The results show that in both experiments the material produced before the corner contained a slightly higher NaSS fraction than in the feed mixture. After the corner, copolymer extremely rich in NaSS was produced. However, as this production rapidly depleted it, the fraction of NaSS in polymerizing material decreased rapidly after the peak value. As a result of changing reaction conditions, about 60 – 70% of the copolymer had NaSS content above the Bjerrum limit, (approximately 30% ionic units), the rest having a distribution between 0 to 30% NaSS. The composition distribution graphic of the 50% NaSS experiment shows that production material is accumulated at three different compositions. Early production material with 50 - 60% charged units, material produced immediately after the change of character of the reaction containing 80 - 85% charged units and neutral homopolymer produced at the end of the reaction after complete depletion of the charged monomer. Thus a part of the copolyelectrolyte consists of chains with effective charge densities equal to the Bjerrum limit which are in rigid rod conformation. While other chains have lower charge densities and are in open coil conformations and chains produced at the end of the reaction are uncharged homopolymers with compact coil conformations. Under these conditions main contribution to the solution viscosity comes from the chains in the first category. Copolymers with low charge densities and uncharged homopolymeric material has negligible contribution to the viscosity of the solution.In the literature it has been noted that the reactivity ratios depend on the reaction conditions such as the pH of the medium. If the synthesis were performed under conditions that does not cause significcant drift then all of the copolymeric material would contribute maximally to the viscosity.