healthmedicinet

The composition of the different gels investigated is reported in Table 1. All formulations prepared with 3% of PVA showed optical transparence and mechanical
properties suitable to be easily “peeled” after application (Figure 1a), whereas dispersions with 2% of PVA are too fluids to be handled (Figure 1b).

Figure 1. Comparison between mechanical behavior of hydrogel with 3% of PVA and 0.6% of borax
(a) and hydrogel with 2% of PVA and 0.4% of borax (b).

PVA and PEO are not completely miscible 28], 29], and to obtain a stable and transparent gel formulation the critical point is the
perfect dissolution of the polymers in the common solvent. When dissolution is not
complete the gels are opaque and undergo premature syneresis. Application and removal
tests were done on microscope glass slides. Preliminary evaluations of the mechanical
behavior of the gels have been made by simple visual observation of their plasticity
and removal characteristics. Formulations with water alone and with water/2-propanol
demonstrate handling properties that do not vary with changing the PEO concentration
in the range of 0–3%. Instead, the addition of PEO makes easier the workability and
application on the slide surface of the formulations containing acetone. This is a
positive effect, as the dispersions with water/acetone mixture are more rigid and
less easily spreadable in thin layers than the same formulations containing water
only. These observations have been corroborated by the rheological results, as described
below.

Liquid phase evaporation

The ability of the formulation to retain the solvent during the cleaning process is
an important requirement. Measurements of the liquid phase evaporation from the gels,
opportunely layered on a glass slide to simulate the application on a flat surface,
were done by gravimetry. The weight loss of the material is assumed to be due only
to the evaporation of the liquid phase and was monitored up to 30 min of exposure.

As shown in Figure 2 in the formulations with water and water/2-propanol mixtures the retention of liquid
phase is not affected by the PEO content. The high polarity of the liquid phase in
these formulations is probably responsible of this behavior: because of the liquid
phase good compatibility with PVA, the introduction of a less polar additive (PEO)
does not interfere with the interactions controlling the liquid retention. A different
situation appears to develop when the polarity of the cosolvent is lower, as it happens
in the case of acetone. In Figure 2 it may be seen that the addition of PEO is advantageous and gels with this additive
show better retention features than formulations with PVA only. The effect is not
very strong and is not function of the PEO concentration as the best performance in
terms of solvent retention is achieved with the PEO content of 1%. An increment in
solvent retention is important for a better control of the liquid composition which
comes in contact with the surface to be cleaned.

Figure 2. Liquid loss percentage of gels with different amount of PEO and different solvents
after 30 min to air exposure.

Characterization of the water state in the gel networks

The water state in the HVPDs is expected to depend from the internal microstructure
of the polymer network and the extent of the interactions which can develop with the
gellant polymer molecules in particular. It is well established that in the hydrogel
polymeric networks the water is present in three different states: free water, intermediate
water and bound water 25]. Free and intermediate water are considered freezable, whereas bound water is considered
nonfreezable and they may be differentiated based on the different freezing temperature
and melting enthalpy. The state of water in polymeric dispersion may be determined
through the use of the Free Water Index. The melting enthalpy of water is derived
from DSC thermograms, by integration of the melting endothermic peak and is used to
calculate the FWI from the following equation 18]:

(1)

where ?H
_m
is the measured enthalpy of water melting, W
_f
the water weight fraction in the gel and ?H
_t
the theoretical value of the enthalpy of fusion of water at 0°C. Low values of FWI
indicate large amounts of bound structured water in the gel.

In Figure 3 the FWI for systems containing the same amount of PVA and two different compositions
of liquid phase, water only and water/acetone, are plotted as a function of the PEO
added concentration. The most evident difference is the drastic decrease of FWI in
the formulations including the organic solvent, in strict agreement with results reported
in the literature 14], 18], 19], 25]. In the concentration range here investigated, the addition of PEO does not have
much effect on FWI. Suggestions have been advanced about the decrease of the FWI induced
by the presence of an organic content in the liquid, as being the result of a different
(increased) structuring of the water in the system 14], 19]. Different types of dynamic polar interactions have been shown to develop between
the molecules in mixtures of acetone and water 30] and it appears that as a result of these interactions in the gel environment more
water becomes bound to the polymer strands. The addition of PEO, on the other hand,
practically does not affect significantly the amount of bonded water.

Figure 3. Free Water Index as a function of PEO content. Standard deviations are reported as
error bars.

Rheological characterization

The main purpose of the rheological measurements was to find the correlation between
the mechanical properties and the addition of PEO in the formulations. The data obtained
from the measurements also allow to make some considerations about the structural
properties, which are closely related to the composition of the formulations in terms
of polymers and organic solvent concentrations.

All acetone containing formulations, that are of major interest for the cleaning purposes,
and some formulation with water only were submitted to oscillatory rheological measurements.
As expected, a dependence of storage and loss modulus from frequency is observed (see
mechanical spectra in Figures 4, 5). It is also evident that the mechanical curves cannot be fitted by a simple model
with a unique relaxation time, which is not surprising in complex structured fluids
like those considered in the present study where the relaxation mechanism is influenced
by the interaction of different components. For such reason the terminal relaxation
time could not be extracted from the oscillatory measurements and to characterize
the mechanical spectra the crossover coordinates were therefore used.

Figure 4. Effect of the solvent on the mechanical spectra of formulations with 3% of PVA, 1%
of PEO and 0.6% of borax.

Figure 5. Effect of PEO in formulations with 3% of PVA, 0.6% of borax and 30% of acetone.

The crossover coordinates (crossover frequency ?
_c
and crossover modulus G
_c
) and the asintotic value of storage modulus (), as described in the introduction, are useful to characterize the viscoelasticity
of the HVPDs. From the crossover frequency (?
_c
) it is possible to calculate an apparent relaxation time (?
_c
), related to the crosslink dynamics, with the following expression 31]:

(2)

The longer the gel relaxation time the more pronounced are the elastic features and
the shape-stability.

In complex fluids the trend of storage modulus G? versus frequency is asymptotic and
G? becomes independent from frequency in correspondence of the intrinsic elastic modulus
32]. value is directly related to the gel peelability and the limit suggested in the literature
for obtaining peeling is about 400 Pa, at the typical perturbation frequencies induced
during the gel removal in cleaning operations (around few hertz) 20]. Gels prepared with 2% PVA concentration, with and without acetone, show values of 100 and 370 Pa respectively (see Table 2) and both are not peelable, in accordance to the above observations. At this concentration,
the overlap between polymer chains is not sufficient to obtain an extensive interchain
crosslinking, responsible of the elastic character of the material.

Table 2. Mechanical characteristics of the gels as obtained from the rheological measurements

If from the macroscopic point of view is directly related to the gels peelability, from the structural aspect it is connected
to the entanglement/crosslinking density ?
_e
according to the relation 33], 34]:

(3)

where k
_b
is the Boltzmann’s constant and T the temperature.

Raising the concentration of gellant polymer from 2 to 3% a drastic increment in and in relaxation time is observed, as reported in Table 2. In the formulations with 3% of PVA, with values ranging from 1,000 to 3,000 Pa, easily peelable materials are obtained and
no macroscopic residues were observed after their removal from the glass slides. When
cleaning operations of artistic surfaces are involved, it is also important to consider
the ease of application and the thinning behavior of the gel, together with its ability
to preserve the shape during the time necessary for the cleaning action. To understand
this feature it is necessary to take into account the rheological behavior below the
crossover point, in the region where formulations show their viscous behavior. If
apparent relaxation times are long, the formulations display strong elastic behavior
also in the range of low frequencies, therefore their applicability may be problematic.
Most of the formulations containing acetone, in Table 2, have apparent relaxation time near or above 1 min and their structure is particularly
stable, thus resulting in a difficult application to surfaces. The formulations without
acetone have apparent relaxation time in the order of 10 s and their thinning is easily
obtainable. However in these cases the gels may have too strong tendency to flow and
when the PVA concentration is lower than 3% the elastic moduli are not enough high
to obtain peelability.

Acetone as cosolvent strengthens the gel network, and this effect may be evaluated
from the increase of the storage and loss moduli and the relaxation times in comparison
to the gels with only water. In Figure 4 the mechanical spectra of two formulations with different composition of liquid phase
are shown and it is evident that dispersions containing acetone display much stronger
elastic behavior than formulations with water only. This effect has been interpreted
14], 20] by means of
¹¹
B-NMR measurements showing that in the presence of organic solvent the concentration
of boron species bounded with the vinyl alcohol groups is enhanced. The environment
created by the organic liquid is less attractive to the borate ions, forcing them
to increase the complexation with PVA chains. The organic molecules in the liquid
environment strongly affect the gel structures inducing a migration of borate ions
towards PVA chains, with subsequent increase in the crossover modulus. The rheological
results are in agreement with the FWI data obtained by DSC and reported in the previous
section (Figure 3). The FWI decrement observed in the presence of acetone indicates an increment in
the amount of structured water, directly related with the enhanced elastic character.

We may therefore conclude that the increased stiffness of the gels containing acetone,
with constant PVA/borax ratio, is due to a larger amount of cross-links in comparison
with the gels containing only water.

When PEO is added to the formulations the rheological properties of the gels undergo
large changes, as shown in Figure 5 by the mechanical spectra of the gels containing different amounts of the polymeric
additive. The relevant data from the mechanical spectra of Figure 5 are listed in Table 2, and a progressive decrement in is observed in these gels when the PEO concentration increases up to 2% (Figure 6a). The concentrations of cosolvent and of borax, considered the major responsible
of the crosslinking formation, are unchanged in these formulations and, on the other
hand, it was checked that PEO alone is not able to react with borate ions to form
a network. We can therefore exclude that PEO can compete with the vinyl alcohol units
for reacting with borate ions. From literature studies it is known that PEO acts as
a porogen agent in PVA/borax membranes 23] and in PVA gels obtained with freezing/thawing method 24]. In the latter work is reported that addition of PEO in a PVA/water system causes
the decrease of PVA degree of crystallinity resulting in the increase of network mesh
size and decrease of the storage modulus. Our results are in agreement with the above
findings suggesting that dissolution of PEO molecules in the PVA hydrogels up to a
given concentration produces an increase of the network mesh size through relaxation
and partial disentanglement of the PVA chains, with a substantial change of the three-dimensional
structure and an effective reduction of the total crosslink concentration considered
as the sum of borate ions/hydroxyls complexation and polymer chain entanglements.
This process may be considered the responsible for the observed reduction of the gels
intrinsic elastic moduli.

Figure 6. (a) and relaxation time (b) as a function of PEO content. Standard deviations are reported as error bars.

The softening effect induced by the presence of the PEO molecules is at the same time
accompanied by an almost linear increase of the gel relaxation times which are calculated
from the mechanical spectra and plotted in Figure 6b. This behavior may appear contradictory, but it is necessary to consider that the
fluid composition of the gel is now changing with the PEO addition, and the relaxation
mechanisms are affected not only by the crosslink concentration but also by the surrounding
fluid viscosity. In order to clarify the effect of PEO in the PVA/borax system it
may be useful to compare the mechanical spectra not only from the quantitative point
of view by considering the crosslinking parameters, but also the overall appearance
of the curves. One method for comparing mechanical spectra is to normalize the spectra
35] but a simple way is to look at the tan(?) values, obtained from the ratio G?/G?.
Tan(?) describes the capability of the materials to dampen the applied strain (or
stress), that is the equilibrium result between conservation (G?) and dissipation
(G?) of the energy. When tan(?) is high (G?  G?) a material has stronger viscous
character, on the contrary when tan(?) is low (G?  G?) in a materials the elastic
behavior is dominant. The analysis of tan(?) values as a function of frequency can
therefore give the same information obtainable from the comparison of G? and G? curves.

If the tan(?) curves overlap it means that the relaxation mechanism of the different
formulations is almost the same, even if the time scale is different. Tan(?) as a
function of frequency for the formulations with different amounts of PEO are reported
in Figure 7. The dotted line in the figure represents the value tan(?) = 1, occurring at the
crossover frequency. The data show that no complete overlap of the tan(?) curves is
achieved, indicating that the relaxation mechanism of the different formulations changes
for the presence of PEO. In particular it may be seen in the Figure that in the low
frequency region tan(?) decreases with increasing PEO concentration. This means that
PEO enhances the ability of the material to dampen the applied stress, causing an
augmentation in the time relaxation. This may be attributed to the simultaneous presence
of sol (PEO solution) and gel (PVA/borate): the sol relaxes more quickly than the
gel network but at the same time the presence of PEO enhances the viscosity of the
fluid fraction in the formulation, slowing down the gel relaxation. On the contrary,
in the high frequency region associated to the elastic character, the tan(?) becomes
an increasing function with PEO concentration. Therefore PEO is responsible of a decrement
in the elastic character, as also indicated by the decrement of the intrinsic elastic
modulus .

Figure 7. Tan(?) as a function of frequency for formulation with 3% of PVA, 0.6% of borax, 30%
of acetone and variable amount of PEO.

The rheological characteristics which have been here determined indicate that in the
formulations containing both the organic solvent and PEO it is possible to control
at the same time the stiffness of the gel, which has relevance for the easiness of
application, and the response to external solicitation, which affects the shape stability
and the removability of the gels.

Cleaning tests

Cleaning tests were performed to check the ability of the different gel formulations
to remove Paraloid
^®
B72 from limestone specimens. Limestone was chosen as a substrate being particularly
porous and suitable for testing the “peelability” of the gel and the ability of the
gel to retain the liquid phase. The two tested formulations (with and without PEO)
show adequate mechanical properties that allow easy removal of the material after
cleaning application, as it is shown in Figure 8. It is possible to see that, notwithstanding the roughness and porosity of the surface,
the removal of the gel is complete and no evidence of residuals is visible. After
use the gel keeps its mechanical properties and is still perfectly peelable.

Figure 8. Example of gel removal from a limestone specimen.

The gel loses water (and solvent) during cleaning, as shown by the measurements in
Figure 2, but during typical application times (2–5 min) the liquid evaporation is not sufficient
to generate dramatic changes in the rheological properties. Also, the glass slide
used to cover the gel during the cleaning tests further prevents liquid phase evaporation.

For such tests a gel with 1% of PEO was preferred on the basis of the evaporation
measurements which showed for this formulation the highest capability to retain liquid
phase.

Different times of application were checked, and the best results have been obtained
with application of 4 min.

The curves in Figure 9a, b report the capillary absorption of the raw stone, of the stone after the Paraloid
B72 treatment and after the gels application. In order to verify the cleaning efficiency
the measurements have been repeated on several stone specimens and in the Figure the
experimental curves relative to a single experiment are shown for illustration. When
a gel without PEO is applied (Figure 9a) the absorption curve is nearly superimposable to the absorption curve of the coated
surfaces prior to the gel treatment. This means that the gel without PEO is not particularly
efficient in the removal of the Paraloid B72. In the first part of the curve the rate
of absorption is faster than in treated stone. Our hypothesis is that the acetone
at the gel-stone interface easily solubilizes the Paraloid film transporting the solubilized
material in the stone porosities. The Paraloid migrates from the surface to the bulk
of the stone, allowing the water to be absorbed. A different situation is achieved
when cleaning test are performed with formulation containing 1% of PEO. As observed
in the liquid phase evaporation measurements, PEO can improve to a certain extent
the capability of the gel to retain the organic solvent. The liquid capillary absorption
curves from the stone treated with Paraloid and from the same specimen after application
of gel with PEO (Figure 9b) proves that PEO is really able to retain acetone in an extent useful to prevent
the fast penetration trough the stone surface. In fact the curve, even if not coincident
with the absorption curve of the raw stone, show that Paraloid was partially removed
by the gel action: initial absorption rate is slower than in the raw stone, but faster
than in the Paraloid coated surface. Moreover, after the initial absorption step,
the absorption rate in the gel treated specimen is very similar to that in the raw
stone, proving that the acrylic protective was indeed removed and not transported
in the bulk porosity from acetone. Acetone is a good solvent for the Paraloid resin
which was employed here and we may therefore assume that the removal of the protective
film occurs by solubilization of the polymer and its incorporation into the gel matrix.

Figure 9. Water absorption curves of raw stone (diamonds), stone treated with Paraloid B72 (circles) and after cleaning test (triangles). Gels tested are composed by 3% of PVA, 0.6% of borax, 30% of acetone and 0% PEO
(a) or 1% PEO (b).