Hydrogel or aquagel
is a complex network of cross-linked hydrophilic polymer chains found as a
colloidal gel with water as the dispersion medium. Polyacrylamide hydrogels
formed by the hydrophobic interaction between stearyl groups existing in a
solution of sodium dodecyl sulfate(SDS) exhibits two different faces that are
dependent on the their state. Noteworthy, in the presence of SDS, the gels
assume a slow relaxation mode when subjected to dynamic light scattering (DLS)
effects and elastic moduli. This concludes that the gels with SDS have a
temporary nature estimated at a rage of seconds to milliseconds.
On removal of SDS,
the gel exhibits characteristics similar to those of the chemically cross-linked
gels. Their time-depend elastic moduli are similar alongside with the high
degree of inhomogeinity they present. They too show a single relaxation mode in
dynamic light scattering (DLS). The
structural changes experienced on gels incorporated with SDS made the physical
gels be insoluble in water that had a gel fraction of close to one. The gels
that had the surfactant had a large proportion of its physical cross-links dissociating
under force but irreversibly reforming on removal of the force. The reforming
of the hydrophobic units resulted in a self-healing efficiency of close to 100%,
which is unattainable on removal of SDS.
The aim of this experiment
is to explain what gives toughness and reforming characteristics to the gels
containing surfactants. The experiment also aims at explaining the
contradiction outcome that a critical gel could remain stable in water. The
importance of the explanation is to give future scientists essential ideas
required in the designing of self-healing soft materials.
Pure solutions of hydrophobically
modified hydrophilic polymers have soft materials with exceptional rheological
properties. The hydrophilic group is responsible for the breakable and
reversible cross-links within such a gel. A simple, free radical micellar
technique makes such a gel. A monomer dissolved in the micelle undergoes copolymerization
with acrylamide. The high level of concentration of the hyrophobe in the micelle
makes the hydrophobic monomers to be distributed along the hydrophilic polymer
backbone as randomly positioned blocks. However, the inefficiency of this
method is the fact that larger hydrphobes with more than 18 carbon groups are
insoluble in the micelle group because of their low level of solubility in
water.
The problem of the
insolubility of hdrophobes wit larger chains of carbon atoms finds a solution
on incorporation of SDS. The method works on addition of a substance that
promotes the growth of the micelle, such as a salt solution of NaCl. The gels
incorporated with surfactants using C18 blocks showed unique characteristics
such as insolubility in water and solubility in SDS solutions. They showed
nonergodicity, high degree of toughness and self-healing characteristics.
Removal of SDS shows a fragile gel.
To show this trend
of characteristics, this experiment incorporates two series of physical gels.
The first series is a gel with SDS, and the second series is a control
experiment of a gel where the SDS micelles are removed after their preparation.
Methods used to test the parameters include the DLS, rheometry and uniaxial
elongation.
The experiment
required, among others, acrylamide, sodium dodecyl sulfate, ammonium persulfate
and sodium chloride. Stearyl methacrylate acted as the C18 chain. The
copolymerization of acrylamide with stearyl methacrylate took place at a
temperature of 25 degrees for 24 hours in the presence of 0.25% v/v of ammonium
persulfate. The concentrations of NaCl solution and stearyl methacrylate were
set at 0.5M and 0.24M respectively. The stearyl methacrylate content of the
monomer mixture was 2 mol % but ranged from 5% to 15% in the total monomer
concentration. The copolymerization took place in plastic syringes of 4mm while
determining their swelling and mechanical measurements. The samples were
immersed in water set at 24 degrees for 15 days. Water was replaced daily to
extract all soluble species. The DLS measurements were conducted in light
scattering vials after filtration of the gel with a 0.2µm nylon membrane. The
test was carried out at 25 degrees using a using ALV/CGS-3 compact goniometer
instrument. A UV-spectrophotometer was used to quantify the solubilzation of
the C18-NaCl solution. This was achieved by running of sample of different
concentrations and drawing a transmittance versus concentration graph. Rheological
tests took place using a rheometer system with a cone angle of 4 degrees and
40mm diameter.
The results showed
a large swelling-ratio of the cells of the gel containing SDS at short swelling
times. This is because of the osmotic pressure of the SDS counterions within
the gel. Removal of SDS showed reduced swelling ratio as the gel converted to a
nonionic form. The dynamics of the gelatin and physical gels were conducted
before and after the addition of NaCl and C18
as well as before and after the removal of SDS. The ICF of the SDS
showed fast and slow relaxation modes. NaCl merges the relaxation into one
mode. Removal of SDS micelles leads to the disappearance of the slow mode.
Rheological results showed a time dependent dynamic moduli for a SDS gel indicating
a temporary nature of the hydrophobic association. Extraction of SDS resulted
in a time independent, dynamic moduli. The mechanical results showed an unchanged
modulus of gels containing SDS on subjection to strain. The modulus decrease
rapidly with strain upon extraction of SDS.
In conclusion,
numerous, essential characteristics of self-healing hydrogels disappear upon
removal of SDS. Among the important characteristics achieved on introduction of
SDS, include a structural relaxation and temporary nature of hydrophilic
association and reversibility of its cross-links.
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