Honors Thesis Proposal
Synthesis and Characterization of Multifunctional Polymer Nanospheres
By
Halimatu Sadiya Mohammed
Advisor: Dr Devon A. Shipp
Research Goal:
To synthesize well-defined PGMA nanospheres with uniform particle size distribution, and
understand the structure of the nanospheres produced.
Introduction:
The design and synthesis of composite nanoparticles have recently been the area of great
attention due to their diverse applicability.1,2 One of such area where the application of
nanoparticles may find significant use is in composites of organic polymers that may host metal
or semiconductor nanoparticle. Nanoparticles, as the name suggest, are particles that are less than
100nm in size. They allow flexibility in finely tuning optical, magnetic and mechanical
properties of material for their appropriate usage. An increase in the demand of miniaturized
devices has also driven one of the greatest potential applications of nanoparticles that can be
found in the pharmaceutical industry, where they can be used in drug delivery. Polymeric
nanoparticles are being developed to inactivate and kill breast cancer cell.3
Another developing area is the use of nanoparticle polymer supports that can act as
building blocks for generating regular large-scale patterned structures.2,4,5 Various applications
of polymer supported metal and semiconductor nanoparticles are envisaged for example in areas
of catalysis where they are used in the production of cosmetics, and paint,6 nonlinear optical
materials, data storage, microelectronics and photonics. Although polymer particles have over
the years been used as such supports, the dimensions of support often used are micron or nearmicron.2,4,7 not truly nanometer (ie not < 100nm) in size. This is due in part to the difficulties
encountered in making these particles smaller with uniform size distributions. The advantages of
uniformly coated and stable nanoparticles have been recognized for some years now, but the
controlled coating of polymeric composite nanospheres or nanoparticles with organized layers
still remains a technical challenge. Some of the difficulties encountered are the polymer particle
host are often prepared via emulsion or microemulsion polymerizations, the presence of
surfactant both at the particle surface and free in solution makes it difficult to precisely deposit
other species like metals. However, at the same time, colloidal stability of the product would be
significantly reduced if one attempts to remove surfactants or modify the particle surface. The
presence of surfactant at the particle surface and free in solution as mentioned earlier makes it
difficult to deposit metals and any subsequent material precisely. Thus, it will be extremely
advantageous to not only develop a method of producing nanospheres that are sufficiently
functionalized and stable to allow deposition of polymer, metal or semiconductor particles but to
control the size, morphology and chemical functionality of these nanoparticle. This is what my
work aims to address.
Previous Work:
A number of methods of polymerization have been develop for the production of particles that
consist of solid cores coated with a shell of polymeric materials8 including monomer absorption
onto particles which are then followed by subsequent polymerization,5,9,10 layer-by-layer
deposition of polyelectrolytes onto charged particles, emulsion-polymerization, bulk
polymerization, and suspension polymerization. The first approach, is one of the often-used
methods employed in order to achieve polymer coatings on solid particles. The polymerization
reaction can either be initiated by the colloidal particles or catalyzed by an initiator to promote
the process. Current and over the last decades research effort have led to lots of new strategies
developed for the modifications of particle surfaces that focus on achieving a single-component
coatings on particles through conventional polymerization and chemical modification. However,
recent techniques that primarily focus on solution self-assembly based strategies have been
proven to be highly effective in producing multicomposite, nanostructured coatings.
Previous work in the Shipp lab has shown that the synthesis of polyethylenimine (PEI)modified crossliked poly(glycidyl methacrylate) (PGMA) nanospheres and the subsequent
placement of CdS and Ag nanoparticles to the surface of these nanospheres, as shown in Scheme
1, can be achieved.2 To prepare the crosslinked PGMA nanospheres with desired particle size, an
emulsion polymerization was used. After making the PGMA nanosphere they coated it with the
surfactant (SDS). PEI was then reacted with the PGMA nanoshperes. As the PEI is deposited
onto the PGMA, the surfactants are removed and replaced by PEI. Zeta potential measurements
at pH 7 on the particles before and after PEI modification were used to ascertain the attachment
of PEI. In order to prepare the CdS and Ag nanoparticles on the surface of PEI-modified PGMA
nanosphere, Cd2+ or Ag+ complex as were formed with PEI on the polymer particle surface and
subsequent reduction yield the silver metal and precipitation gave CdS nanoparticle.
Scheme 1: Preparation of CdS, Ag nanoparticles on the surface of PEI-modified PGMA
nanospheres.
KPS, SDS
O
O
O
Glycidyl
methacrylate
H2O, 60oC
Divinyl benzene
The PGMA provided a highly functionalized nanoparticle that can be modified via various
simple ring-opening reactions involving the glycidyl group. Attachment of PEI, gives the
resulting nanoparticle/colloid both stability and functionality that allows for complexation with
metal ions of interests.1 As shown by the TEM image below, the sizes are not well-defined and
not uniform, however they were all in the nanometer range (<100nm).
Fig 1. TEM Image of PEI mdified PGMA Nanosphere.
This approach provides a means of overcoming the problems associated with colloidal and other
polymeric particle stability encountered in previous systems, which are typically carboxylated
latex particles. This concept of nanosphere synthesis and modification could easily be extended
to other metals, alloys and semiconductor, which mean creating colloidal nanoparticles with
well-defined structures and sizes while controlling the functionality. In my thesis, I intend to
make a more uniform particle sizes and to understanding the process of PEI modification and
final nanosphere structure.
Proposed Research:
In the preliminary work in the Shipp lab, PEI modified crosslinked poly (glycidyl methacrylate)
(PGMA) nanospheres were synthesized. However, questions that remained unanswered or not
fully clear relate to the control of the size distribution, morphology and chemical functionality of
these nanoparticles.
The technique of emulsion polymerization will be employed in the synthesis of PGMA
nanosphere. This will hopefully help answer questions like, how to make smaller and welldefined PGMA nanospheres. Emulsion polymerization is a technique used in the synthesis of
polymer particles. This technique uses water as a heat-transfer agent. Water and a monomer are
the basic starting material but for this type of polymerization surfactants, chain-transfer agent
and a water-soluble initiator are also needed for the polymerization to occur. In the proposed
work, the monomer, glycidyl methacrylate is first mixed with the divinyl benzene a crosslinking
agent, and hydrophobic molecules form large droplets. These monomer droplet are then
stabilized by the surfactant, SDS molecules that have their hydrophilic side towards the outside
and the hydrophobic end facing the monomer droplet. The size of the monomer droplets depends
on the agitation, concentration of surfactants and amount of the monomer used and the surfactant
above certain concentration align themselves to form micelles. Polymerization occurs in the
presence of the water at 600C when the initiator potassium persulfate (KPS) decomposes into
radicals.
In order to minimize the size of the particles, I will firstly increase the concentration of
the surfactant because typically, the more surfactant used in the polymerization the smaller the
polymer particle will be. Also thorough sonication on the reaction mixture before polymerization
and increased mechanically mixing the reaction will reduce the particle size.
Secondly, I intend investigating the morphology of these nanoparticles. In the
preliminary work, it was shown that, the PEI replaces the surfactant but it is not clear as to the
distribution of the PEI on the PGMA. That is, whether the particle is core-shell in terms of PEI
distribution. If the PEI remains on the surface of the PGMA or some penetrates the PGMA
particle. The zeta potential and particle size of these synthesized nanoparticles will be closely
monitored at each stage of the reaction. Changing the percentage of divinyl benzene content will
also change the crosslinking density of the nanosphere. It is expected that increasing the DVB
content will increase the crosslinking density of the nanosphere thereby decreasing the amount of
PEI that goes inside the particle. After making particles of various percentage of DVB, an
elemental analysis will be done on the particle to for instance determine the total content of
nitrogen atoms. If the nitrogen atom content decrease as the percentage of DVB is increased,
then it means there is less PEI in the inside of the particle compared to the surface. Nitrogen
content will then be determined using elemental analysis.
Lastly, I intend monitoring the chemical functionality of the PEI-PGMA nanospheres.
This includes investigating the amount or percentage of the PEI that is actually deposited on the
PGMA, and whether the PEI removes all the surfactants or if some of the surfactants still remain
on the particles after deposition of the PEI. I will test the effect of using PEI of different
molecular weights and conduct elemental analysis on the resulting PEI-modified nanospheres.
Analytical Techniques & Data Analysis:
The particle size and ζ-potential will be measured using an ALV particle sizer and Zeta-PALS
respectively. The particle sizer is a light scattering instrument. The amount of light and the angle
at which the light is scattered by the sample gives information about the size of the particle. So
far, previous work done have shown that the particle size of the modified particles remain close
to that of the unmodified PGMA as expected because the cationic layer around particles ensures
colloidal stability. The ζ-potential measurements of these particles give information about the
surface charge of these particles. Conductivity measurements will be used to determine if there
still remain some PEI in the solution which will tell us if the PEI has attach to the PGMA or not,
and finally electron microscopy will be used to get the image of the particle to see if they are
uniformly distributed or randomly distributed.
Preliminary Work:
In my two semesters experience in Dr Shipp’s lab, I used the Layer-by-Layer approach technique
to modify silica particles by alternating the deposition of the polyelectrolytes, PEI, a positively
charged polymer and a negatively charged polymer, polystyrenesulfonate (PSS) onto silica
particles. To do this, I prepare separate silica PEI and PSS solutions. I then alternate the
deposition of these two polymers whiles carefully removing excess PEI or PSS using
centrifugation. The zeta potential and particle size measurement of each layer at pH 7 was
measure. I have been able to build up several layers of polyelctrolytes, alternating the PEI and
then PSS on silica particles of 80nm in diameter. The technique of zeta potential, and particle
size measure and polymer modifications will be applied in this proposed research.
Schedule:
Below is the tentative schedule for the progress of my proposed work:
Month
Main Project
9/04 - 11/04
Emulsion Polymerization
12/04 & 01/05
PEI - Reactions
02/05
Morphology studies
03/05
04/05
05/05
First draft of thesis
Start final draft
Final draft of thesis completed
Sub/ break down
i. Prepare polymers
Investigation/control of the ff:
ii. Stirring rate
iii. Initiator
iv.
Uniform particle size
v.
surfactants
Investigation of the following:
i. PEI of different MW
ii. PEI Concentration
iii. PEI Removal
TEM
NMR
Reference:
(1) Fendler, J. H., Ed Nanoparticles and Nanostructured Films; Preparation, Characterization
and Applications; Wiley-VCH; Weinhein, 1998.
(2) Z., Hanying ; Li, Yuzhuo; Shipp, D. A. Submitted to Adv. Fund. Mater. 2004
(3) http://www.atip.or.jp/
(4) Zhang, J.; Coombs, N; Kumacheva, E. J. Am. Chem. Soc. 2002, 124, 14512
(5) Zhang, J; Coombs, N; Kumacheva, E; Lin, Y; Sargent, E. H Adv. Mater. 2002, 14, 1756.
(6) For a review, see; C.H. M Hofman-Caris, New J. Chem. 1994, 18, 1087
(7) Mayer, A.B. R.; Grebner, W.; Wannemacher, R. J. Phys. Chem. B. 2000, 104, 7278, and
references therein.
(8) W.D. Hergeth, U. J. Steinau, H.J. Bitrich, K. Schmutzler,. Wartewig, Prog. Colloid
(9) A. M. van Herk, NATO ASI Ser, Ser. E. 1997, 335,435.
(10)
L. Quaroni, G. Chumanov, J. Am. Chem. Soc. 1999, 121, 10642.