Zadnjih nekoliko desetljeća interes znanstvenih institucija, industrije i institucija zaduženih za kontrolu hrane vezano uz prisutnost,
nastajanje i potencijalni rizik od kemijskih spojeva koji ...nastaju za vrijeme kuhanja hrane odnosno termičke obrade različitih vrsta
hrane u stalnom je porastu. Otkriće akrilamida u hrani te nađene količine izazvalo je dodatnu zabrinutost vezano uz prisutnost
toksičnih tvari koje nastaju tijekom termičke obrade hrane. Grijanje hrane uzrokuje kemijske promjene koje kao rezultat mogu imati
nastajanje kemijskih spojeva sa karcinogenim i mutagenim svojstvima. Neki od spojeva koji nastaju termičkom obradom hrane su:
heterociklički aromatski amini, policiklički aromatski ugljikovodici, nitrozamini, klorpropanoli i akrilamid.
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.
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.
