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Keywords:
Hydrophobically Modified Polyacyrlamide; Suspension Polymerization; Associating
Polyarylamide
.
1.
Introduction
Hydrophobically modified water soluble polymers are hydrophilic
macromolecules containing a small fraction of hydrophobic groups which are distributed
along the polymer chain or presented as end groups of the side chain.[1] The main
hydrophilic domain renders solubility of the polymer chain in aqueous phase. For the
lipophilic groups distributed along the polymer chain, the amount of
them is small yet,
they prefer to associate therefore to form physical linkages due to the unfavorable
aqueous solvent conditions. These physical linkages exist both inter and intra molecular
hence rendering a kind of dynamic transient network. Different from high molecular
polymer, which will lose their thickening ability due to easily degradation under high
shearing. For hydrophobically modified polymer, the self-assembling is reversible in
nature as the hydrophobic interactions can be disrupted when subjected to high shearing,
but re-form when shear is reduced. [2-6] These polymer are also regarded as self-healing
assemblies in solution and play a major role in a variety of waterborne technologies
including paints, inks, drill fluids,
pharmaceutical, paper-making and cosmetics. [7-13]
Copolymer based on polyacrylamide is one of the most important
hydrophobically modified polymers which has been intensively investigated due to its
low price, bio-compatibility and widespread applications.[14] Some difficulties in
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preparing hydrophobically associative polyacrylamide (HAPAM) arise from the
insolubility of the hydrophobic monomers in water. To overcome this problem, several
methods have been developed. The two major of them are: (1) Polymerization in a polar
organic solvent or organic solvent/water mixture, in which both hydrophilic and
hydrophobic monomers are soluble.
Generally, the resulted copolymers are not soluble in
such a reaction medium and precipitate out. In conventional solution polymerization,
different monomers randomly distribute within the reaction medium and products.[15,
16] However, in a number of developed systems of solvent or solvent/water mixture,
chain transfer reaction exerted negative effects on copolymerization and molecular
weight.[17] (2). Micellar polymerization is the most widely used method for the synthesis
of HAPAM, in which hydrophobic monomers are solubilized in a synthetic system by
surfactants, generally sodium dodecyl sulphate (SDS).[6, 18-20] It was found the
obtained copolymers have an essentially block-like structure, the hydrophobe-rich
regions distributed along polyacrylamide backbone. However, sometimes the surfactants
served in conventional emulsion/suspension polymerization
system have some
drawbacks, such as poor stability of the latex since the emulsifier attaches on the latex
particle only in a physical manner, and some negative effects of the residual surfactants
for the properties of the final products.[21-23] Polymerizable surfactants, or surfmers,
had been developed to stabilize the emulsion/suspension system of synthesizing
HAPAM.[24, 25] Surfmers could be not only copolymerized into polymer chain, but also
strongly attached on to the latex particles
with covalent linkage, resulting in better
stability of latex than normal surfactant.[23, 26] Yujun Feng et al. reported HAPAMs
prepared by post-modification, which was claimed a more effective way to control the
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composition of HAPAMs compared against micellar process.[27, 28] In summary, the
compatibility of hydrophilic and lipophilic comonomers in system is the key point in
preparing hydrophobically modified polyacrylamides. In practical terms, to seek out
proper solvents is crucial for solution (homogeneous) copolymerization and for micellar
(heterogeneous) copolymerization, the importance is to find out an effective surfactant.
In this study, preparation of HAPAM had been
conducted in a water-free
suspension system, above the melting point of monomers in non-polar solvents. A
number of surfactants had been studied in stabilizing this water-free system. The
relationship between the consumption of surfactants and the loading of monomers in such
a system had been investigated as well. Crosslinked polyacrylamide microgel had been
prepared successfully through this method also after introducing crosslinker in reaction.
Furthermore, polymerizable surfactant had been employed not only as co-polymer in
polyacrylamide molecular chain, but also as an emulsifier with a covalent bonding to the
latex in system. In a water-free medium, hydrophilic monomers or co-monomers per-se
can work as the medium to solubilize hydrophobic monomers, increasing the
compatibility. Therefore, the feed ratio of the hydrophobe will not
be limited as it is in a
traditional aqueous polymerization system. Additionally, Products are dry powder and
ready to use, compared to traditional synthesis that require post-treatments, such as
energy intensive drying process. Furthermore, dry powders are stable and easier to store
and transport than wet polymer solution.
