2-Hydroxyethyl methacrylate radical Inverse
Miniemulsion Polymerization
Wang Guoxian, Wu Hu
(College of Chemistry and chemical engineering, Hunan Institute of Science and Technology, Yue yang
414000, Hunan province, P.R. China)
Abstract: In miniemulsion process, Surfactants are used to stabilize the latexes
and control the particle size. They play a very important role during the
polymerization process and also during the subsequent storage. However,
conventional surfactants have some drawbacks: the first is that the water
sensitivity of the final coating increases and affects gloss and adhesive, the
second is that the surfactants are physically adsorbed on the polymer particles
and easily desorbed from the particle surface. To overcome these drawbacks, the
polymeric surfactants have drawn increasing interest. The use of polymeric
surfactants for stabilisation of cosmetic and personal care formulations is
described in terms of their adsorption and conformation at the solid/liquid and
liquid/liquid interface. The mechanism by which these polymeric surfactants
stabilise suspensions and emulsions is briefly discussed in terms of their
interaction on approach of particles or droplets. This provides very strong
repulsion, that is referred to as steric stabilisation. This polymeric surfactant was
used for stabilisation or oil-in-water (O/W) emulsions both in aqueous media
and in the presence of high electrolyte concentrations. The amount of polymeric
surfactant required for maintenance of stability (for more than one year at
ambient temperature) was relatively low (of the order of 1 w/w% based on the
oil phase). In addition, the polymeric surfactant shows no skin irritation, no
stickiness or greasiness and it gave an excellent skin feel.
Alkali-soluble random copolymers are a kind of polymeric surfactant (ASR).
Alkali-soluble resins are random copolymers that contain both hydrophobic
groups and a large number of carboxylic acid groups, which are widely used as
the sole stabilizers in the production of waterborne dispersed polymers because
they remarkably improve the end-use properties of the latexes. However, they
reduce the radical entry rate.
Emulsion polymerization stabilized with ASRs has not been deeply investigated.
The behaviour and the role of polymeric surfactant aggregates in emulsion
polymerization may be expected to be somewhat different from those of
conventional emulsifier micelles.
Hydroxyethyl methacrylate (HEMA) is perhaps the most widely studied and
used neutral hydrophilic monomer. The monomer is soluble, its homopolymer is
water-insoluble but plasticized and swollen in water. This monomer is the basis
for many hydrogel products such as soft contact lenses, as well as polymer
binders for controlled drug release, absorbants for body fluids and lubricious
coatings. As a comonomer with other ester monomers, HEMA can be used to
control hydrophobicity or introduce reactive sites.
Hydrogels based on poly (2-hydroxyethyl methacrylate) (PHEMA) are widely
studied for biomedical applications due to their swellability, oxygen
permeability, and biocompatibility.
Poly (2-hydroxyethyl methacrylate) is a kind of biocompatible macromolecule
with good transparence and flexibility, which would be a promising material
used as artificial cornea.
No effect has been devoted to investigate systematically the polymerization
kinetics and mechanism of sterically stabilized miniemulsion systems of HEMA.
In this work, the mechanisms for radical entry in ASR-stabilized systems have
been investigated. It has been found that the mechanism responsible for the
reduction of the rate of radical entry depends on the type of ASR used, the type
of radical produced from the initiator, the phase where the radicals are produced.
To some extent the results are helpful to the understanding of the nucleation
mechanism for the miniemulsion polymerization of soluble monomers.
The miniemulsion polymerization of 2-hydroxyethyl methacrylate (HEMA) was
conducted at 70℃, using either an alkali-soluble random copolymer (ASR) or
sodium dodecyl sulfate (SDS) as the emulsifier. The kinetics of the
miniemulsion polymerization was investigated by the effects of initiator
(including 2,2’-azobisisbutyronitrile (AIBN), Potassium persulfate (KPS)), pH
of the aqueous phase, and surfactant type. ASR could form the aggregates like
micelles in the polymerization. The polymerization rate using ASR was lower
than that using SDS. This is due to a hairy ASR layer around the particles
surface to decrease the entry rate of radical through the region. This factor
results in reducing the efficiency of these radicals to initiate the polymerization.
There is significant aqueous phase polymerization, and frequent particle
nucleation and coagulation during the polymerization. Increasing pH, the
polymerization rate does decrease proportionally to the particle number.
E-mail address: wgxwzl@163.com