Akrilik asit – akrilamit kopolimerizasyon kinetiğinin bilgisayarla sürekli izlenmesi

Online monitoring of acrylic acid - acrylamide copolymerization reaction kinetics

Özet

Suda çözünen doğal ve sentetik polimerler gündelik hayatımızda geniş yer tutar. Polielektrolit denilen ve suda çözündüğünde yüklü gruplar veren polimerler de bu grubun önemli bir üyesidir. Polielektrolitler ya yüklü monomerin polimerizasyonu yada yüklü ve yüksüz monomerlerin kopolimerizasyonu ile elde edilebilir. Bu çalışmada akrilik asit (Aac) – akrilamit (Aam) kopolimerizasyonu, pH 5 ve pH 2’de gerçekleştirildi ve tüm reaksiyonlar ACOMP (Automatic Continuous Online Monitoring of Polymerizatian – Polimerizasyon Reaksiyonlarının Bilgisayarla Sürekli İzlenmesi) sistemi ile sürekli olarak izlendi. Kopolimerizasyon süresince her iki monomerin polimere giren miktarları ve monomer haldeki konsantrasyonları ultraviyole spektrofotometre (UV) dedektörü ile belirlendi. Yapılan kinetik incelemeler her iki pH’taki reaksiyonların monomere göre 1’inci dereceden sapma gösterdiğini fakat 1.25’inci ve 1.5’uncu derece kinetiğe uyduğunu gösterdi. Sürekli izleme metodunun sağladığı geniş çaplı veri alma imkanı sayesinde pH 5’te gerçekleştirilen tüm reaksiyonlarda, akrilamidin daha aktif olduğu dolayısıyla daha hızlı tükendiği ve akrilik asitin aktivitesinin ise iyonlaşma ve elektrostatik etkileşimlerden dolayı daha az olduğu belirlendi. pH 2’deki reaksiyonlarda ise tam tersine akrilamit protonlanma nedeniyle aktivitesini kaybederken akrilik asitin bu pH’da daha aktif olduğu gözlendi. Reaksiyon kinetiğindeki bu farklılaşmanın sebebi kafes etkisi açısından tartışıldı. Aynı zamanda ACOMP sisteminde yer alan dedektörlerden biri olan ışık saçılması dedektörü vasıtasıyla pH 5 ve pH 2’de yapılan tüm deneylerdeki molekül ağırlığı değişimi reaksiyon süresince izlendi.

Abstract

Water soluble macromolecules include a numerous class of polymers ranging from biopolymers essential to life process to synthetic resins of many commercial uses. Solution properties and behaviour of water soluble polymers depend on the structural characteristics of polymer chain solvated in the solution. Polymers having charged groups when dissolved in water are called polyelectrolytes. Most biopolymers are polyelectrolytes, and that natural and synthetic polyelectrolytes have a variety of uses in diverse industries. The monomers of polyelectrolytes are usually expensive and difficult to polymerize. Hence, polyelectrolytes are commonly used in copolymer form with cheaper non-polyelectrolytic comonomers. Acrylic acid (Aac) - acrylamide (Aam) copolymers form polyelectrolytes in water. Due to their charged nature, their properties closely depend on the ionization of the acid groups which is a function of the system pH. PKa of the acrylic acid is 4.56. In this study working pHs of the system are chosen as to be one above and one below the pKa value which are 5 and 2, respectively. Automatic Continuous Online Monitoring of Polymerization (ACOMP) technique was used to follow the copolymerization of Aam and Aac at pH 5 and pH 2. In the ACOMP application, a small amount of reactor material was continuously removed from the reactor by an isocratic pump and mixed at high pressure with a much larger volume of solvent drawn by another similar pump. The diluted reaction solution was then passed through a train of detectors comprising a multi-angle light scattering detector, a single capillary viscometer detector and an ultraviolet spectrophotometer detector. At pH 5 and at pH 2 copolymerization reactions were performed at different monomer compositions. The reaction kinetics was extensively discussed and reaction order with respect to monomers was determined at these pH values. Evolution of molecular weight (Mw) also was determined through ACOMP system. The reaction rate depends on the initiator and comonomer concentrations and the propagation, termination and initiator decomposition rate constants. Evolution of the logarithm of monomer concentration versus time indicated that the reactions showed a marked slowing down as compared to 1st order kinetics. The “slowing down” of the polymerization reaction can be due to a combination of a) decrease of the initiator concentration, b) composition drift and c) higher order effects. All three factors probably have a role. However, If the reaction is 1st order in monomer and the depletion of the initiator during the reaction is taken into account then it was seen that initiator decomposition was still inadequate by itself to account for the reaction kinetics. At pH 5 the reaction system contains Aam, Aac in sodium acrylate form and negatively charged initiator ACV. Due to the ionic nature of ACV, its decomposition rate will also depend on the amount of acidic comonomer at the reaction medium. If they play such a role through cage effect, then the reaction is no longer first order in monomer but 1.25th order according to Noyes, or 1.50th order according to Hamielec. Although it is possible to fit for both initiator life-time and reaction order from the curve of the reaction rate, such a fit procedure involves too many fit parameters and is thus not reliable. Instead, the kinetic data was fitted to 1.25th order kinetics and 1.5th order kinetics with initiator decay. It was seen that equations for 1.25th order and 1.50th order kinetics both fitted the data. The results showed that the first order kinetics failed at pH 2 as well; on the other hand both 1.25th and 1.5th order kinetics satisfactorily fitted the data. The Mw results obtained from measurements at pH 5 showed that higher Aam content led to higher molecular weight which was consistent with higher reactivity of Aam at this pH. The decrease of the molecular weight with increasing Aac content and hence with decreasing reaction rates originates from the propagation step. Also, Mw and the reaction rate were higher at pH 2 than pH 5 for the reaction carried out at the same feed composition. This arises from the propagation step, not initiation. If initiation step were responsible, then the increase in reaction rate would result in the decrease in Mw.