Symposium
Editor, I. Capek
Wiley-VCH, 2002, pp. 1-358
ISBN 3-527-30469-x
Preface
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Contents
The 15th Bratislava
International Conference on Polymers continues the series of Bratislava
meetings organized by Polymer Institute of the Slovak Academy of Sciences
(SAS). The International Symposium on Non-Conventional Polymer Dispersions
is thus the second one, following the successful Symposium on Polymer
Dispersions in Smolenice (Slovakia) 1990 both organized by Department
of Polymerization Reactions" of Polymer Institute, SAS. The Symposium
was sponsored by the International Union of Pure and Applied Chemistry
and supported by the Slovak Grand Agency (VEGA) of SAS and participants
themselves. The meeting took place at the fairy-tale-looking castle
of Smolenice located about 65 km north of Slovakian capital, Bratislava.
The conference
honoured the forty-year scientific contribution of Dr Jaroslav Barton,
the former Director of the Polymer Institute, to the development of
free radical polymerization in homogeneous and heterogeneous media.
Active participants from different countries have generated a friendly
working atmosphere and presented main and contributed lectures and
posters. The speakers were recognized experts in the field.
The main topics
of the Symposium included.
- Microemulsion,
miniemulsion and multiple emulsion polymerizations
- Association
and polymerization behavior of amphiphilic macromonomers, graft
copolymers and macroinitiators in polar media
- Novel and
non-conventional polymer products and polymer dispersions, prepared
by radical polymerization
The Symposium
included invited plenary lectures, contributed lectures and poster
presentations. This meeting was a international one: lectures and
posters were presented in English by scientists from the Czech Republic,
France, Germany, Hungary, Italy, Japan, Russia, Turkey, Taiwan and
Slovakia. The plenary and contributed lectures were presented by eminent
scientist and by well-known active researchers of younger generations.
The Symposium featured a dynamic poster section which included presentations
by many participants, especially young scientists, and was accompanied
by very lively discussions. A short account on the materials presented
at the Symposium was published in the Book of Abstracts. Regrettably,
this volume does not cover all the new and interesting results presented
at the Symposium since not all the authors were able to present their
lectures for publication in this volume (some of the material had
been published earlier or submitted elsewhere). However, it provides
a good representation of the scope of the meeting and the main topics
of the discussion.
The microsymposium
aimed to cover the study of the kinetics and mechanism of free-radical
polymerization of conventional and non-conventional (macro)monomers
in disperse systems (nanostructured media, micellar solutions, microemulsions,
miniemulsions, dispersions, double emulsions, etc). The scope of the
conference covered the preparation of polymer particles ranging from
a few tens of nanometers to the millimetre size, kinetics of particle
formation and the advanced design of particle structure. The synthetic
approaches using microemulsions, mini-emulsions and classical (multi-)
emulsions were extensively discussed. Specialty products included
magnetic, conducting and coloured colloids, thermosensitive microparticles,
submicron sized polymer microgels, polymerizable surfactants, waterborne
coatings, photo-initiators for emulsion polymerization, amphiphilic
block, graft and comb-like copolymers, hollow microspheres and other
objects with unique properties.
Polymerization
in nanostructured media (normal micelles, micelle-forming monomers
and microemulsions) allowed the preparation of functionalized polymers
carrying hydrophobic and/or ionic sites. The nature, number and size
of these sites were modulated so that it was possible to prepare polymers
with widely differing properties and therefore able to be used in
a number of applications. Different processes of polymerisation in
nanostructured media allowed the preparation of a wide variety of
stimuli-responsive polymers whose properties can be varied by tuning
external parameters such as temperature, pH, ionic strength and shear
rate. Microemulsions are deemed to be novel chemical nanoreactors
for producing nanostructured materials such as polymers, ceramics
and inorganic/polymer nano- composites. Polymerization in microemulsions
is a relatively new but attractive field, because it can produce nanoscale
polymer particles with extraordinarily high molecular weights. The
enormous number of nanodroplets in both o/w and w/o microemulsions
or water channels in bicontinuous microemulsions is the potential
loci for fast polymerization to produce non-traditional polymer nanopartides.
Although latex particles are small, polymers of molar mass exceeding
one million can readily be obtained from these systems. The present
kinetic models, however, deviate strongly from the experimental kinetic
and colloidal data, and this deviation increases with conversion.
One of the possible reasons for such a deviation is the competition
between the active and non-active polymer particles for monomer and
emulsifier. However, a consistent kinetic picture of this polymerization
system is still missing.
In contrast with
the conventional microemulsion techniques, latexes with particles
from 10 to 100 nm are efficiently obtained with Ausimont techniques
of polymerization using perfluoropolyether microemulsions. In comparison
with other cases of polytetrafluoroethylene (PTFE) microemulsion polymerization,
where low conversions and high surfactant concentration are used,
with Ausimont technology PTFE particles of small size are produced
at high conversion and with a relatively low surfactant content. Moreover,
this technique allows one to control not only the size of the PTFE
particles, but also to change their morphology in order to get rod-like
particles or spherical ones.
Polymerization
in miniemulsions is a new polymerization technique which allows the
preparation of submicron-sized latex particles within the range 100
nm < 500 nm. The growth of the nanodroplets can
effectively be suppressed by using a strong hydrophobe and an effective
emulsifier. The hydrophobe acts as an osmotic agent which stabilizes
the system against Ostwald ripening. Based on the quantitative understanding
of miniemulsions, the process allows the creation of new particle
structures. These stable monomer droplets have an extremely large
surface area and can compete effectively with the monomer-swollen
micelles for oligomeric radicals. The accumulation of hydrophobe (polystyrene,
PSt) in the monomer phase during the classical emulsion polymerisation
can increase the stability of monomer droplets. The increased stability
of monomer droplets is accompanied with the depressed transfer of
monomer to the reaction loci and the rate of polymerization. Under
such conditions, the emulsion polymerization system can preserve monomer
droplets up to high conversion.
Emulsifier-free
emulsion (dispersion in the absence of stabilizer) polymerization
and copolymerization of hydrophobic monomers with hydrophilic or amphiphilic
macromonomers in polar media led to the formation of the monodisperse
polymer particles. The macromonomer technique was used to prepare
a variety of core-corona polymeric particles and then used for different
particle-surface modifications. Amphiphilic (macro)monomers and/or
the amphiphilic graft copolymers obtained by the copolymerization
of hydrophilic (macro)monomer and hydrophobic conventional comonomer
present all the typical properties of conventional emulsifiers. In
addition, these surface-active compounds contain reactive groups due
to which the surface-active group is chemically bound to the polymer
(particle) matrix. In this manner, these reactive emulsifiers are
bound to the particle surface and therefore they are prevented from
subsequent migration. Polymerization of amphipilic PEO-type macromonomers
in water was very fast and afforded the preparation of a regularly,
highly branched comb polymers. This was discussed in terms of the
organized aggregation of macromonomer into micelles. The polymerization
increased with increasing hydrophobicity of the polymerizing alkyl-styryl
(methacryloyl) group. Furthermore, the PEO macroinimers are used for
the synthesis of various kinds of functional graft copolymers. The
ability of macromonomer and macroinimers to get involved in a copolymerisation
is controlled by their molecular weight, the type of reaction media
and reactivity ratios of (co)monomers.
Emulsifier mixtures
used for industrial application often contain non-ionic emulsifiers.
The excellent ability of non-ionics to solubilize and disperse hydrophobic
soils such as fats, mineral oils, etc. in water leads to their extensive
use. The electrostatic stabilization provided by anionic emulsifiers
improves latex stability at high temperatures, while the steric stabilization
provided by non-ionic emulsifiers enhances the chemical and freeze-thaw
stability of latex products.
Polymerizable
surfactants have been developed for use as reactive emulsifiers in
emulsion polymerizations. Although they are exclusively utilized in
emulsion polymerizations, they also have potential application in
other disperse systems. Surfactant containing both the anionic groups
and poly(oxyethylene) chain can make electrostatic and steric contributions
to particle stabilization. Polymerizable surfactants adsorb onto solid
materials from their aqueous solutions to form mono- layers at the
surface of organic solids at saturation. When polymer particles capable
of absorbing vinyl monomers such as styrene and acrylates are used
as the solid material, the emulsifier monolayers can be fixed by the
copolymerization with the monomers within the particle. The immobilization
of the surfactants adsorbed onto organic pigments can be applied to
the preparation of waterborne inks with good storage stability.
The amphiphilic
PSt/polyethylene oxide (PEO) block copolymers were reported to be
efficient stabilizers in the emulsion polymerization of vinyl acetate.
An increase in PEO content increased the polymerization rate and the
particle size. The rate of polymerization increased strongly with
increasing both the initiator and emulsifier concentration. The increased
content of PEO chains in the copolymer decreased the mentioned reaction
orders but increased the activation energy. This is attributed to
the thick interfacial layer formed by PEO chains which makes barrier
for entering radicals. The barrier varies with the chain length of
PEO units.
Heterogeneous
aqueous-phase polymerization initiated by a redox initiator offers
some unique features with respect to the preparation of amphiphilic
block copolymers. The redox system generates radicals at polymer chain
ends that subsequently form during the polymerization polymeric micelles
or polymer particles as isolated reaction loci where the block copolymer
formation takes place. Thus, the radical heterogeneous polymerisation
is suitable for the synthesis of specialty polymers such as amphiphilic
block copolymers. The blocks are built sequentially in a way that
the hydrophobic blocks are formed during the heterophase polymerization.
The hydrophilic blocks acts as polymeric stabilizers and the hydrophobic
blocks form the cores of the particles or micelles. For example, an
interesting class of block copolymers contains a poly(N-isopropylacrylamide)
block causing the thermo-reversible change in the properties of the
particles. Another interesting class are double hydrophilic block
copolymers consisting of two different hydrophilic blocks.
A non-seeded
semibatch emulsion copolymerization was used for the preparation of
different colloid copolymers containing hydroxyl, carboxyl, amide
and alkoxy- methylamide functional groups. Any application of synthetic
latexes requires the specific properties of a latex in its colloidal
state, during its application, as well as properties of a polymer
latex after application. Very specific properties are required for
latex application as a binder in waterborne paints - good colloidal
stability in the process of a paint application, good film-forming
properties in the process of film formation from the discrete particles
and, usually a crosslinking ability for further treatment of the coating
film. A need exists for waterborne polymer compositions which undergo
crosslinking upon film formation to impart one or more desired properties
to the resulting coatings. Functional groups help to increase colloidal
stability of polymer dispersions, increase polarity and improve adhesion
properties of polymer films.
In the synthesis
of latexes for use in waterborne coatings, the benefits of using reactive
surfactants are now well-known. Improvements are obtained in the stability
of the latexes, due to the fact that they are not desorbed from the
particle surface. The film properties are also better, particularly
when the films are exposed to humidity, where the water rebound has
decreased significantly. One group contains a series of anionic surfmers
(polymeric surfactants) prepared upon reacting a polymerizable alcohol
with either maleic or succinic sulfosuccinic anhydride. The other
contains a series of non-ionic block copolymer surfmers. Transurfs,
with an addition - fragmentation mechanism, is the subject of the
next group, etc.
Water-soluble
synthetic polyelectrolytes are polymers of continuously growing interest
due to their manifold applications in industrial processes. An important
part of the polyelectrolyte is the block copolymers consisting of
charged blocks with cationic and betaine structures as well as of
uncharged blocks of different hydrophilicity/hydrophobicity. The synthesis
of these block copolymers was carried out by free-radical polymerization
as well as by function alization of reactive precursor polymers. By
employing surface-active monomers, a new type of micellar polymers
bearing unusual properties form.
Hollow microspheres
show many attractive characteristics, for example, thermal resistance,
low density, thermal insulation, and optical opacity due to their
small air-void. They can be used in various fields such as paint,
ink, paper coating, face-foundation, etc. Submicron-sized hollow particles
can be prepared by utilizing phase-separation between polystyrene
(PSt) and poly(methyl methacrylate) (PMMA) at the second stage in
seeded-emulsion polymerization. The hollowed particles were also prepared
with seeded-dispersion polymerization using a dynamic swelling method.
The uniform PMMA hollow particles of around 10 µm were prepared
by one-step polymerization, a combining membrane-emulsification technique
and a subsequent swelling process.
Submicron-sized
polymer microgels have been prepared by emulsion polymerization of
or copolymerization of multifunctional monomers. The particle size
was controlled to some extent by varying the initiator, emulsifier,
and/or monomer concentrations. However, the particle-size control
is very complex and remains an area of interest. In this system, the
micellar nucleation, the homogeneous nucleation, and the coagulative
nucleation may, in principle, be operating simultaneously. Thus, information
on the locus of initiation and particle formation is a necessary prerequisite
for understanding the polymerization process in the emulsion system
containing multifunctional monomers. The network-formation process
in the free-radical monovinyl/divinyl emulsion copolymerization involves
four reactions of a growing polymer radical: I) intermolecular propagation
with two types of monomer, 2) intramolecular cyclization leading to
the formation of small ring or large loop structures, 3) intermolecular
crosslinking with the prepolymer to form an effective crosslink, eventually
leading to the gel, and 4) intramolecular cross- linking leading to
the formation of multiple crosslinks leading to microgelation.
The interest
in hybrid polymer-inorganic materials as well as that in the dispersion
of inorganics in fluids led to an increased number of studies combining
these fields. It emerged in getting hybrid polymer-inorganics as particles
dispersed in organic solvents, in polar solvents-water mixtures or
even water. The hybrids can be generated by forming silica, in the
presence of polymers or by performing the polymerization of monomers
in the presence of performed silica. The bonds between polymer and
inorganic compound are thought to be formed by the copolymerization
of unsaturated monomers on the surface of silica particles. Hybrid
materials were obtained by sol-gel reaction of silane derivatives
combined with free-radical polymerization of Vac. The change of the
glass-transition temperature and thermal stability of the polymers
in the presence of the inorganic core proved the existence of polymer-inorganic
hybrids.
The feasibility
of efficient particle growth during the seed polymerization by controlling
electrostatic charges between the seed particles and the secondary
monomer/initiator was demonstrated within a range of experimental
conditions: agitation, seed particle size, amount of emulsifier and
secondary monomer to seed polymer weight ratio.
The symposium
focused on current developments in the radical polymerization in the
micellar media and synthesis of novel (surface-active) polymer products.
It attracted an excellent attendance, reflecting the strong and wide-spread
interest in the field of preparation of non-conventional polymer dispersions
and stimulated mutual interactions between researchers working on
similar display technologies. Both an interesting venue of the conference
and well-organized scientific and social program added to the success
of the event as quoted by participants.
This brief review
of the materials of the Symposium including those presented in this
volume shows that the main interest in the investigation of the preparation
of non-traditional polymer dispersions and polymer products lies in
the nature of the reaction loci, particle nucleation and particle-growth
events.
We believe that
the 15th Bratislava International Conference on Polymers or the 2nd
on Radical
Polymerization in Disperse Systems (Non-Conventional Polymer Dispersions)
contributed significantly to the understanding of the particle nucleation
and polymerization mechanisms in the radical polymerization in disperse
systems used for the preparation of non-conventional polymer dispersions
and novel polymer products.
I. Capek
Chairman Polymer Institute
Slovak Academy of Sciences
Dubravska cesta 9,
84236 Bratislava, Slovakia
