Materials Sciences and Applicatio n, 2011, 2, 1058-1069
doi:10.4236/msa.2011.28143 Published Online August 2011 (http://www.SciRP.org/journal/msa)
Hydrogels from Chitosan and a Novel Copolymer
Poly(N-Vinyl-2-Pyrrolidone-Co-Acrolein)
Janaina Aline Galvão Barros*, Antonio Jedson Caldeira Brant, Luiz Henrique Catalani
Institute of Chemistry, University of São Paulo, São Paulo, Brazil
Received August 6th, 2011; revised December 1st, 2010; accepted June 2nd, 2011
ABSTRACT
PVP, a synthetic polymer and chitosan, a natural polymer are biocompatible and have presented a great variety of in-
teresting properties for cosmetic, biomedical, pharmaceutical and biotechnological applications, Many alternatives to
improve the polymer propeties has been made as bleeding polymers, for example. In this work, two different techniques
of hydrogel attainment were used: one from mixtures of acid aqueous co-solutions of chitosan and PVP, whose resul-
tant films and solutions were irradiated, afterwards, by ultraviolet radiation (λ = 254 nm), another one from the reac-
tion of poly (N-vinyl-2-pyrrolidone-co-acrolein)-a novel copolymer synthesized in our biomaterials laboratory-and
chitosan. In the first one alternatives it was possible produce hydrogels directly from mixtures of aqueous acidic
co-solutions of both polymers. In the second one, the attainment of hydrogels from Schiff base has proved to be an ef-
fective methodology for the production of hydrogels, showing good values of gel content and swelling.
Keywords: Chitosan, Hydrogel, Copolymer, Crosslinking, Schiff Base, Poly(Vinyl-Pyrrolidone-Co-Acrolein)
1. Introduction
The search for novel biomaterials, mainly polymer hy-
drogels, is increasing throughout the world. In 1960,
Wichterle and Lim developed the first synthetic polymer
hydrogel based on 2-hydroxyethyl methacrylate (2-HEM
A) as the hydrophilic monomer [1]. Throughout the last
five decades numerous publications on this research topic
have been released. Hydrogels play an important role in
tissue engineering [2-5], dressings for burn wounds and
other kinds of injuries [6,7], “intelligent” hidrogels for
drug controlled release [8,9], and other biomedical ap-
plications [10].
Hydrogels are three-dimensional hydrophilic polymer
networks capable of swelling in water or biological fluids
and retaining a great amount of fluid in the swollen state
without dissolution of their structure [11,12]. They can
be obtained through chemical and physical processes
[13]. Dressings based on polymeric hydrogels show
many advantages, such as exudate absorption, immediate
pain relief, barrier to microorganisms, permeability to
oxygen, adjustable transparency and mechanical proper-
ties [14].
Poly(N-vinyl-2-pyrrolidone) (PVP) is a synthetic lin-
ear non-toxic, biocompatible polymer, frequently used in
food and cosmetics industries as well as in pharmaceuti-
cal formulations [15,16]. It is very soluble in water and
many organic solvents. Its use as a biomaterial in artifi-
cial blood plasma was prevalent in World War II [17-19].
Nevertheless, over the last five decades, PVP has found
other applications of greater scientific-biotechnological
interest, such as hydrogels for drug controlled release
[20,21], tissue regeneration and implants [22], wound
and burn dressings [23], and others applications. Hy-
drogel dressings have attracted much attention among
researchers for their use in the medicinal field, chiefly in
the healing of burn wounds. It must be emphasized here
that there are already numerous natural and synthetic
polymers being studied and/or applied as medicinal hy-
drogels, and PVP is among them [24,25]. But, beyond
PVP as a unique polymer, other polymer systems in-
volving VP-monomers are also used in this application
area: chemically modified PVP [26-28], copolymers
containing units of N-vinyl-2-pyrrolidone in their chains
for example, poly(methacrylamide-co-N-vinyl-2-
pyrro-lidone-co-itaconic acid) [29]; poly(N-vinyl-2-pyrr-
olidone- co-styrene) [30]; poly(N-vinyl-2-pyrro lidone-co-
acrylic acid), [31] and blends of PVP with other bio-
compatible polymers (for example, PVP-CMC [32];
PVP-PVA [33]; PVP-k-carrageenan [34]; PVP-chitosan
[35].

Hydrogels from Chitosan and a Novel Copolymer Poly(N-Vinyl-2-Pyrrolidone-Co-Acrolein) 1059
Chitosan is another polymer that has found use in
biomedical applications over the last three decades [36].
Chemically, it is constituted of units of 2-amine-2-deoxy-
D-glycopyranose and 2-acetamide-2-deoxy-D-glycopy-
ranose linked by β- (1→ 4) glycoside linkages [37] along
a linear chain. Commercially, it is obtained from alkaline
deacetylation of chitin through a hydrolysis reaction us-
ing an aqueous solution with ca. 50% KOH at tempera-
tures near to 100˚C. Chitin is a linear polysaccharide
present mainly in exoskeletons of crustaceans (shrimps,
crabs, lobsters, etc.) [38]. After cellulose, it is the most
abundant natural biopolymer on the earth [39]. Thus,
chitosan is a renewable and sustainable material. Also,
due to its biocompatibility, non-toxic properties and it
having inhibitory effects on the growth of fungi and bac-
teria, there is a growing interest in its potential applica-
tion in the biomaterials field.
Chitosan is practically insoluble in water and organic
solvents. Although it is a hydrophilic polymer, its disso-
lution in water only takes place in dilute acidic solutions,
in which organic acids (for example, formic acid, acetic
acid) or mineral acids (for example, HCl, HNO3) are
utilized. These protonate its available amine groups,
leading to the formation of a hydrosoluble polycation at
pHs < 6. Solubilization of chitosan constitutes, therefore,
an important step for its use in diverse types of biomate-
rials.
Chitin derivatives containing more than 50% of free
amine in their structure are usually denominated chitosan
[40]. The majority of commercial chitosans have degrees
of acetylation between 15% and 30%. The degree of
acetylation (DA) and the molecular weight of chitosan
define practically all its physicochemical properties and
determine its use for applications in medicine, biotech-
nology and other fields involving biomaterials [41].
Average degree of acetylation (DA ) is defined as the
average percentage of remaining acetyl groups along the
chain of chitin after its deacetylation. There are several
methods to determine it, from simple to very sophisti-
cated ones: elemental analysis (EA), infrared spectrome-
try (IR), hydrogen nuclear magnetic resonance (1H-NM-
R), UV- spectrophotometry, linear potenciometric titra-
tion, ninhydrin test, circular dichroism measurements,
others [42-43]. Among these, IR is widely used because
it is relatively simple, non-destructive, cheap and gener-
ally gives reliable results. It mainly makes use of base-
line (a) expressed by:
1665
3450
(%) 100
1.33
A
A
DA







where DA is the degree of acetylation, i.e. the percent-
age of amide groups acetylated; 1655
A
and 3450
A
are
the absorbance at 1655 cm–1 of the amide-I band as a
measure of the N-acetyl group content and the hydroxyl
absorbance band at 3450 cm–1 as an internal standard to
correct film thickness or differences in chitosan concen-
tration in powder form. This ratio of 1655
A
/ 3450
A
is
used as an internal standard peak; the factor “1,33” is the
value of the ratio of 1655
A
/ 3450
A
for fully N-acetylated
chitosan [43,44] The degree of deacetylation, DD , is
then obtained by equation:
(%) 100DD DA
Evidently, a higher DD of chitosan indicates more
free amine groups in its structure. It may contribute not
only to a lowering of chitosan crystallinity up to a certain
extent - crystallinity is maximum for both chitin (i.e. 0%
deacetylated) and fully deacetylated chitosan (i.e. 100%)
[45], but also to an improvement of its solubility in water,
what is achieved by the formation of salts soluble in wa-
ter after the protonation of amine groups.
Average molecular weight of chitosan is another pa-
rameter that plays an important role in determining ap-
plications for this polymer. Chen and Hwa [46] have
demonstrated the influence of molecular weight of di-
verse chitosan samples with the same DA on its ther-
mal, mechanical and permeability properties. Other
properties which depend upon the chitosan molecular
weight are antioxidant activity [47], antimicrobial activ-
ity [48], film water absorption [49], etc. Determination of
chitosan molecular weight can be performed by several
methods [50] usually employed in polymer chemistry:
capillary viscometry [51,52], membrane osmometry [53],
laser light scattering [54] exclusion size chromatography
(SEC) [55] others.
Additionally, chitosan is a non-toxic, biocompatible
and biodegradable linear polymer [56]. It has presented a
great variety of interesting properties for cosmetic, bio-
medical, pharmaceutical and biotechnological applica-
tions [57]. Chemical modification of chitosan mainly
aims at an improvement of its solubility in water or in
other types of solvent, however it can also aim at the
incorporation of special physicochemical properties to
the polymer for determined applications [58]. The most
common reactions of chitosan chemical modification
encompass N-deacetylation, N-acetylation and O-acetyla-
tion [37], N-acylation and O-acylation [59-60], and
N-phtalation [61]. Chitosan, chemically modified chito-
san, and chitosan in blends with hydrophilic polymers
such as poly(vinyl alcohol), poly(N-vinyl-2-pyrrolidone),
poly( ethylene oxide), starch, cellulose [62] have raised
interest in the research and development of biomate-
rials with promising results. However, in the last four




— baseline (a),

Hydrogels from Chitosan and a Novel Copolymer Poly(N-Vinyl-2-Pyrrolidone-Co-Acrolein) 1060
decades, much attention has been given to alternative
methods in blending polymers to create new materials
with improved physicochemical and mechanical proper-
ties for determined applications. Such methods generally
are simpler and cheaper than the synthesis of new poly-
mers. In this case, miscibility of blends is crucial in their
stability and performance. Miscibility depends on poly-
mer-polymer interactions at molecular level, mainly
through H-bonds [63-64], and can be evaluated by sev-
eral methods: infrared spectroscopy, differential scanning
calorimetry (DSC), dynamical mechanical analysis (DMA),
scanning electron microscopy (SEM), etc. [65-67,32].
Blends of chitosan and PVP can become good alterna-
tives for producing new biomaterials, once the inherent
properties of both polymers herein discussed as well as
their high miscibility show very favorable performance
properties for this application area.
In this work, two different techniques of hydrogel at-
tainment were used: one from mixtures of acid aqueous
co-solutions of chitosan and PVP, of which the resultant
films and solutions were afterwards irradiated by ultra-
violet radiation (λ = 254 nm), that is capable of inducing
cross-linking of PVP by recombination of free macro-
radicals and subsequent hydrogel formation [25,17]; an-
other from the reaction of poly (N-vinyl-2-pyrrolidone-
co-acrolein) a novel copolymer synthesized in our bio-
materials laboratory and chitosan. Cross-linking, in this
case, is supposed to occur through Schiff base formation
by the aldehyde group of the acrolein units of the co-
polymer and available amine groups of chitosan [68]. In
both techniques, stable hydrogels were produced at low
polymer concentrations (2% m/v, polymer/water, respec-
tively). Some physical properties of films resulting from
chitosan and PVP blends and the hydrogels from the
co-solutions of both polymers as well as the hydrogels
from the reaction of chitosan with the novel copolymer
were characterized in order to previously evaluate their
potential use as biomaterials in the near future.
2. Experimental
2.1. Materials
For the experiments, the following materials were used:
two commercial chitosan samples referred to as high-
molecular weight and medium-molecular weight from
Aldrich (Brookfield viscosity 800.000 cP and 200.000 cP,
respectively, data from the supplier), poly(N-vinyl-
2-pyrrolidone) known as Luviskol® K-90 from BASF
(n
M
= 360,000; w
M
= 1.200,000), acetic acid and so-
dium hydroxide (Vetec, São Paulo), anhydrous sodium
acetate (Sigma), sodium chloride (Synth), monomers of
N-vinyl-2-pyrrolidone and acrolein (Aldrich). Except for
the polymers and monomers here cited, all reagents used
in the experiments were of analytical grade.
2.2. Methods
2.2.1. Chitosan Characterization
1) Chitosan purification
Chitosan samples were dissolved in 2% aqueous acetic
acid solution (v/v) at polymer concentration of 2% (w/v)
under constant stirring overnight at room temperature.
The obtained solutions were filtered through cellulose
acetate MILLIPORE® membrane of 0.45 m pore size
and neutralized with 10% aqueous NaOH solution (w/v)
for up to three cycles. The precipitated polymer was then
washed extensively with distilled water, brought to pH
about 7.0 and lyophilized up to constant mass.
2) Chitosan average degree of dacetylation (DD )
3) Elemental analysis
Around 20 mg of each purified chitosan in duplicate
were taken for percentage determination of C, H and N in
elemental analysis test carried out using a Perkin Elmer
CHN analyzer. Samples were taken in duplicate.
a) Infrared spectrometry (FT-IR)
Samples of chitosan were dispersed in KBr (ca. 2%
polymer, w/w) and pressed into transparent discs for
analysis by FT-IR spectroscopy. Transmittance spectra
were obtained through a FT-IR Bomem MB 100 spec-
trometer operating in a range of 350 - 4000 cm–1, 2 cm–1
resolution, 32 runs. For DA/DD calculation, the two
standard absorption bands [69] were taken into account:
amide- I (1655 cm–1) as a measure of the N-acetyl group
content and hydroxyl (3450 cm–1 as an internal standard
to correct film thickness or differences in chitosan con-
centration. The factor ‘1.33’ denoted the value of the ratio
of A1655/A3450 for fully N-acetylated chitosan.
b) 1H nuclear magnetic resonance (1H-NMR)
1H-NMR spectra were obtained on a Bruker DRX 500
spectrometer at 70˚C from chitosan solutions prepared
with 10 mg of chitosan suspended in a solution com-
posed of 1.96 mL of D2O and 0.04 mL of DCl under
constant stirring at room temperature, having 3-(trimeth-
ylsilyl)-1-propanesulfonic-d4 acid (TSPA, Aldrich) as an
external reference. The calculations of DA were per-
formed from the ratios of areas of peaks in accordance
with equation
3
2
(%) 100,
3
CH
H
A
DA A




where: 3
CH
A
= area of the peak at 2 ppm, attributed to
the nuclei of the hydrogens of methyl group; AH2= area
of the peak at 3.2 ppm, attributed to the nucleus of hy-
drogen at position 2 of the glycosamine ring. By deduct-
ing from 100 the value of DA, one obtains the value of
DD, that is, DD (%) = 100 – DA (%).

Hydrogels from Chitosan and a Novel Copolymer Poly(N-Vinyl-2-Pyrrolidone-Co-Acrolein) 1061
2.2.2. Hydrogel Attainment from Chitosan and
PVPAC
1) Preparation and characterization of poly(vinylpy-
rrolidone-co-acrolein) (PVPAC)
Mixtures containing 90% of N-vinyl-2-pyrrolidone and
10% of acrolein (w/w) were prepared. The copolymeri-
zation was carried out in 10 mL test tubes flushed with
nitrogen and duly sealed, using as an initiator –0.05% of
azoisobutyronitrile (AIBN) by weight, based on the mo-
lar ratio of N-vinyl pyrrolidone/acrolein = 9. The mix-
tures were kept for one day at 65˚C. The obtained co-
polymer was purified by dissolving it in absolute ethanol
and, next, precipitating it in ethyl ether. Its separation
was performed by filtration. This procedure was per-
formed up to three cycles. Afterwards, the copolymer
was dried under vacuum at 60˚C to constant weight and
washed with ethyl ether to remove unreacted monomer.
Then, it was lyophilized for withdrawal of all remaining
solvent. The percentage of the incorporated co-monomer
was gotten by elemental analysis of C, H, and N on a
Perkin Elmer CHN analyzer.
2) Characterization of PVPAC
a) Size exclusion chromatography (SEC)
The molar mass of PVPAC was evaluated in a liquid
chromatograph, SHIMADZU LC-10AD equipped with
refractive index and UV (254 nm) detectors (Class- LC10),
utilizing Ultrahydrogel® columns (120, 250, 500 and 2000)
(300 × 7,8 mm) from Waters, mobile phase of distilled
water and acetonitrile (0.03 M) at a 1.0 mL/min flow. The
calibration curves have been achieved with standards of
poly (ethylene oxide) (PEO) with molar masses of 860,000;
348,000; 88,200; 24,800 and 1200 g·mol–1.
b) Membrane osmometry
The number average molecular weight of PVPAC was
obtained from 4 dilute aqueous solutions at different
polymer concentrations in water. Osmotic pressures of
diluted solutions were measured at 30˚C in a OSMO-
MAT osmometer, model 090, from GONOTEC (Ger-
many), equipped with cellulose acetate membranes
(cut-off 5 kDa e 10 kDa). Number average molecular
weight, n
M
, was calculated according to equation
2
0
1
nC
RTA c
cM







where Ris the universal gas constant, T temperature
(K), A2 is the second virial coefficient,

0c
c


is the
reduced osmotic pressure extrapolated to concentration c
= 0 by linear regression.
3) Hydrogels from PVPAC and chitosan
a) Attainment of chitosan-PVPAC hydrogels
The hydrogels were obtained from solutions of 80 and
120 g·L–1 of PVPAC copolymer and solution of 20 g·L–1
of chitosan dissolved in dilute aqueous acetic acid (2%,
v/v). Polymer solutions were then mixed at different chi-
tosan: PVPAC weight ratios, at pH ca. 2.5 under mag-
netic stirring until the attainment of a gel.
4) Characterization of PVPAC-chitosan hydrogels
a) Cross-linking kinetics and mechanical properties of
Schiff base hydrogel
Cross-linking kinetics of imine hydrogels, prepared on
the rheometer, between the parallel geometry plates, was
performed on a cone-plate rheometer—Physica MCR
300—Paar Physica. All rheological measurements were
carried out at 25˚C. The samples used in these experi-
ments were 0.5 mL (80 and 120 g·L–1) of PVPAC solu-
tion and 0.5 mL (20 g·L–1) of chitosan solution.
b) Gel fraction and equilibrium swelling ratio
The hydrogel samples obtained from chitosan and
PVPAC was weighed and immersed in deionized water
for 48 h at room temperature. Afterwards, they were re-
moved from the solvent, dried quickly in filter paper for
withdrawal of excess water, weighed and let to dry into a
ventilated oven at 70˚C during 48 h to a constant weight
(wd). The gel fraction (GF) was determined by equation:
(%) 100
d
i
m
GF m
where % GF is the percentage of gel fraction, md , the
mass of dry hydrogel after extraction, mi, initial polymer
mass before the extraction in deionized water.
The equilibrium swelling ratio was calculated from
equation.
id
i
d
mm
Sw m


where Swi is the equilibrium swelling ratio, mi , the mass
of the swollen gel, md, the mass of the dry gel. For these
experiments, the samples were taken in triplicate.
3. Results and Discussion
3.1. Average Degree of Deacetylation of Chitosan
The average degrees of deacetylation of both original and
purified chitosans are listed in Table 1. They were ob-
tained from the three different methods described herein.
The three techniques utilized in determination of DD of
the chitosans in our experiments generally showed results
with good correlations for the original and purified sam-
ples, except for elemental analysis of the original CM,
whose results were rather discrepant. The high discrepan-
cies probably are due to presence of excess of water in its
samples evaluated by this method. Chitosan is a high hy-
groscopic polymer in nature, and it is practically impossi-
ble to totally eliminate water contained in it by conven-
tional drying techniques or even utilizing lyophilization. In-
frared spectrometry showed to be a very efficient technique

Hydrogels from Chitosan and a Novel Copolymer Poly(N-Vinyl-2-Pyrrolidone-Co-Acrolein) 1062
Table 1. Average degrees of deacetylation of chitosans ob-
tained from three different methods: elemental analysis
(EA), infrared spectrometry (FTIR), and hydrogen nuclear
magnetic resonance (1H NMR).
Average degree of deacetylation,
D
D (%)
Chitosan EA FT-IR
1H -NMR
CH (original) 60,8 63,3 63,0
CM (original) 46,4 71,0 71,0
CH (purified) 74,7 78,4 78,0
CM (purified) 76,4 77,7 78,0
in these experiments since it exhibited results very close
to those from 1H-NMR, considered the most reliable
technique to ascertain DD of chitosan [70] for all
original and purified samples. However, studies on di-
verse analytical methods for determination of chitosan DD
[71] have demonstrated that the DD values may differ
greatly when computed utilizing different baselines in the
IR technique for the polymer in the forms of KBr disk and
film. The IR baseline proposed by Domszy & Roberts [69]
was the chosen one to compute the chitosan DDs in our
work, for it has given quite satisfactory results according to
literature concerned with this subject in the last three dec-
ades. Furthermore, the sample preparation and the circum-
stances at which it is tested may also have an influence on
the results [72,73]. Despite being hygroscopic, chitosan has
its water adsorption potentially diminished as its DD in-
creases. This suggests that chitosan samples of higher DD
may adsorb less moisture than those of lower DD [74].
Therefore, it is crucial that the polymer must be completely
dried prior to the IR test to avoid great discrepancies in the
results because even low-moisture content in samples
would also contribute to the hydroxyl band that affects the
DD values [75].
The 1H-NMR technique for determination of chitosan
DD is considered the safest one among some others intro-
duced in literature, nevertheless it may also present dubious
results concerning the peak area at the 2 ppm region attrib-
uted to the Hs of the acetamide group. For example, when
DCl-D2O is used for the polymer solubilization along the
tests at 70˚C - 80˚C, effects of chitosan acid hydrolysis
should be considered [76].
3.2. Copolymerization of NVP and Acrolein
The results of molar mass of the copolymer PVPAC, cal-
culated by two different methods, are shown in Table 2.
In spite of the molar ratio of N-vinyl pyrrolidone/
acrolein = 9, this co-monomer showed to be more reac-
tive than N-vinyl-pyrrolidone because its molar fraction
incorporation was larger than 10 percent as expected.
As a new methodology to cross-link natural and syn-
thetic polymers like chitosan and polymers based on
N-vinyl-pyrrolidone, respectively, a hydrogel of chitosan
Table 2. Molar mass of copolymer PVPAC (g.mol–1).
Osmometry SEC
EA
n
M
n
M
w
M
w
n
M
M
26.7 % 290,000 240,000 260,000 1.06
EA = Elemental Analysis (molar fraction incorporation)
and PVPAC was produced by the reaction of both poly-
mers. PVP is a very hydrophilic polymer with good
initial tack, transparency, chemical and biological inert-
ness, very low toxicity as well as has high compatibility
with diverse media, cross-linkable and flexible. Chitosan,
in turn, is soluble only in aqueous acidic solution. As a
randomly coiled cationic polyelectrolyte, it usually can
be cross-linked with some dialdehydes, mainly glu-
tarald-ehyde. The synthesized polymer, poly(N-vinyl-2-
pyrro- lidone-co-acrolein) - PVPAC, reacted with chitosan,
and the cross-linking occurred by Schiff base formation as
shown in (Figure 1).
As a new methodology to cross-link natural and syn-
thetic polymers like chitosan and polymers based on
N-vinyl-pyrrolidone, respectively, a hydrogel of chitosan
Figure 1. Schiff base formation and reduction. R = PVAC
as cross-linker.

Hydrogels from Chitosan and a Novel Copolymer Poly(N-Vinyl-2-Pyrrolidone-Co-Acrolein) 1063
and PVPAC was produced by the reaction of both poly-
mers. PVP is a very hydrophilic polymer with good initial
tack, transparency, chemical and biological inertness, very
low toxicity as well as has high compatibility with diverse
media, cross-linkable and flexible. Chitosan, in turn, is
soluble only in aqueous acidic solution. As a randomly
coiled cationic polyelectrolyte, it usually can be cross-
linked with some dialdehydes, mainly glutaraldehyde. The
synthesized polymer, poly(N-vinyl-2-pyrro-lidone-co-
acrolein)—PVPAC, reacted with chitosan, and the cross-
linking occurred by Schiff base formation as shown in
(Figure 1).
Throughout the experiments, PVPAC solutions were
added to chitosan solutions aiming to form homogeneous
hydrogels by Schiff base at ratios of 4:1, 6:1, 8:1, 12:1
(PVPAC:chitosan, w/w, respectively). The Schiff base re-
action occurs between amine groups of chitosan and al-
dehyde functions of PVPAC, therefore contributing to
formation of imine covalent linkages inter-molecularly.
The polymers showed a fast cross-linking (~600 s), and
their gelation kinetics were analyzed in situ and investi-
gated rheologically. Correlation between gelation kinet-
ics and hydrogel properties with PVPAC/chitosan con-
centration, their feed ratio, and temperature influence
were also evaluated (Figure 2).
The chitosan–PVPAC hydrogels were then dialyzed
against double-deionized water. After dialysis, transpar-
ent and elastic gels were obtained. The final products
were dried in a lyophilizer until attaining constant weight.
The obtained hydrogels showed a very unstable structure,
i.e. they are very unstable mechanically, probably due to
the reaction reversibility with trend to reach a balance of
amine/imine in them. Cyanoborohydride is a hydride
able to reduce imines to amines, and aldehydes to alco-
hols. We therefore proposed to produce imine hydrogels
in the presence of sodium cyanoborohydride to prevent
the regeneration of a non-cross-linked product. Low ac-
tivity cyanoborohydride serves to narrow the competition
for imine, which is formed rapidly, versus the reduction
of its par-precursor. Figure 1 shows the complete chemi-
cal reaction for achieving amine hydrogels.
3.3. Infrared Spectrometry (IR)
Infrared spectra of chitosan and chitosan hydrogels
(Figure 3) show that imine group were reduced in the
presence of sodium cyanoborohydride. Most acquisitions
related to chitosan are present in the imine hydrogel. The
for imine, which is formed rapidly, versus the reduction
of its par-precursor. Figure 1 shows the complete chemi-
cal reaction for achieving amine hydrogels. region at
~ 3500 cm–1 is related to the absorption of axial stretch-
ing and OH stretching bands, NH overlapping; regions
from ~ 1600 to ~ 900 cm–1 are related to the absorption
Figure 2. Kinetic curves from chitosan solution 20 g·L–1:
▲CH and PVPAC 120 g·L–1; ● CH and PVPAC 80 g·L–1; ■
CM and PVPAC 120 g·L–1; ▼ CM and PVPAC 80 g·L–1.
owing to deflection angle of NH, the deformation of
symmetrical angular CH3 of amide, C-N stretching of
amide and C-N stretching of amine. Those with acqui-
sition number of frequencies smaller than 900 cm–1 in-
dicate bands related to polysaccharide structures. An
absorption band very close to 1575 cm–1 in the spectrum
suggests the stretching band of C = N linkage present in
the hydrogel.
The spectrum of the amine hydrogel (imine reduced by
cyanoborohydride) shows a decrease in the C = N inten-
sity of the stretching of the linkage C = N. Thus, there is
an increase in the intensity of NH deflection angle of the
band produced by amine.
According to Figure 5(a), imine hydrogels lose vis-
cosity as shear rate increases. Such hydrogel sets flow by
shear (“shear-thinning”). This behavior is typical of
pseudoplastic materials. This is an interesting property of
materials known as "smart materials", which can be ex-
ploited in various biomedical applications. It has been
presumed that the rheological behavior of these hy-
drogels might be attributed to the disruption of the imine
balance in crosslinked hydrogel/aldehyde (non-crosslink-
ed) by mechanical forces. However, even in the presence
of NaCNBH3 (Figure 5(b)), this behavior remains, which
suggests that such behavior is not provoked by the bal-
ance shift of imine/aldehyde in imine hydrogels, but
more probably by irreversible breakages of their three-
dimensional networks.
Due to the fast kinetics and some interesting physical
properties of these hydrogels, their gel fractions and
swelling ratios were evaluated. Table 2 exhibits the re-
sults obtained.

Hydrogels from Chitosan and a Novel Copolymer Poly(N-Vinyl-2-Pyrrolidone-Co-Acrolein) 1064
3.4. Gel Fraction and Equilibrium Swelling
Ratio of Chitosan-PVPAC
Table 3 shows that gel fractions obtained from imine
hydrogels have high values, and the concentration of
copolymer PVPAC does not interfere in gel fraction re-
sults. These high gel content values show a good portion
of polymer that did not dissolve in solvent due to
cross-linking between PVPAC and chitosan, i.e. high
content of chitosan and PVPAC reacted.
Swelling ratio is highly dependent upon the concentra-
tion of copolymer PVPAC. It seems that PVPAC acts as
a chitosan cross-linker because, at higher concentrations,
there is more capacity of water absorption in the hy-
Figure 3. IR spectra of dry chitosan sample (_ _), Schiff
base hydrogel (.....) and reduced Schiff base hydrogel (___).
(a)
(a)
(b)
(b)
Figure 5. Shear rate (a) chitosan solution 20 g·L–1 ▲ CH
and PVPAC 120 g·L–1; ■ CH and PVPAC 80 g·L–1; ● CM
and PVPAC 120 g·L–1; ▼ CH and PVPAC 80 g·L–1. (b)
[PVPAC] = 80 g·L–1, [CM] = 20 g·L–1 and [NaCNBH3] = 20
g·L–1.
Figure 4. Viscometric analysis at different temperatures (a)
chitosan solution 20 g·L–1: ■ CH and PVPAC 120 g·L–1; □
CH and PVPAC 80 g·L–1; ● CM and PVPAC 120 g·L–1; ○
CM and PVPAC 80 g·L–1 and (b) [PVPAC] = 80 g·L–1, [CM]
= 20 g·L–1 and [NaCNBH3] = 20 g·L–1.

Hydrogels from Chitosan and a Novel Copolymer Poly(N-Vinyl-2-Pyrrolidone-Co-Acrolein) 1065
Table 3. Average gel fractions and equilibrium swelling ratios of hydrogels obtained from mixtures of chitosan and PVPAC
solutions in diverse proportions. Samples taken in triplicate.
CM CH
Samples Average gel fraction (%) Average swelling ratioSamples Average gel fraction (%) Average swelling ratio
4:1 86 ± 15 31 ± 9 4:1 82 ± 2 76 ± 9
6:1 87 ± 10 25 ± 9 6:1 93 ± 4 49 ± 15
8:1 85 ± 9 32 ± 11 8:1 50 ± 6 44 ± 9
12:1 94 ± 3 30 ± 6 12:1 92 ± 3 31 ± 10
Table 4. Average gel fractions and equilibrium swelling ratios of hydrogels obtained directly from chitosan-PVP blend films
and mixtures of chitosan and PVP co-solutions after submitted to UV254 nm irradiation for 1.5 h (films) and 4 h (solutions) at
ca. 30˚C, under N2 atmosphere. Samples in triplicate.
Films Hydrogel*
Samples Average gel fraction
(%)*
Average equilibrium
swelling ratio*
Samples Average gel fraction
(%)**
Average equilibrium
swelling ratio ***
PVP 64 ± 6 97 ± 21 PVPac 63 ± 8 250 ± 50
CM5 74 ± 16 45 ± 8 CM5 76 ± 4 240 ± 30
CH5 80 ± 16 79 ± 4 CH5 88 ± 3 160 ± 30
CM50 79 ± 2 27 ± 5 CM20 70 ± 5 280 ± 30
CH50 88 ± 2 30 ± 3 CH20 51 ± 7 430 ± 120
CM95 93 ± 7 28 ± 8 CM30 26 ± 8 740 ± 210
CH95 94 ± 5 30 ± 2 CH30 41 ± 7 490 ± 120
PVPac: PVP dissolved in aqueous acid solution : 2 g of PVP : 98 g of 2% acetic acid-H2O (v/v) Numbers on the right of CM and CH
represents weight % chitosan in dry hydrogel. * Films immersed in deionized water for 48 h at room conditions; ** After 24 h of
Sohxlet extraction in deionized water, dried for 72 h at 70˚C in ventilated oven; *** After Sohxlet extraction, hydrogels kept im-
mersed in deionized water for 48 h at room conditions.
drogel probably influenced by the formed networks allied
to the high hydrophilicity of the former. On the other side,
the hydrogel produced with chitosan of medium molecu-
lar weight (CM), differently of that with chitosan of high
molecular weight (CH), didn’t show dependence on
PVPAC concentration in the swelling test. That can be
related to the fact that the swelling limit had already
reached the maximum with CM.
The results obtained for the Schiff base hydrogels were
compared with those of hydrogels attained from films of
chitosan–PVP blends prepared by casting on plastic
molds as well as mixtures of 2% polymer co-solutions at
different polymer ratios on a dry basis, packed and sealed
in quartz tubes. The samples in triplicate were submitted
to the same UV radiation dose and reaction time in a UV
chamber with an N2 atmosphere. Table 3 gives a sum-
mary of the effect of the formulation compositions on
average gel fractions and swelling ratios. These hydrogels
are likely semi-interpenetrating networks (SIPNs), capa-
ble of keeping chitosan and PVP non-cross-linked mole-
cules entrapped within the networks, increasing hence the
gel fractions. Their values also corroborate the assump-
tion of the existence of a strong interaction between both
polymers, which form miscible blends [77,78], as well as
the formation of probable SIPNs induced by UV
cross-linking, in which the cross-linked part is probably
constituted of PVP. Attempts to cross-link films or solu-
tions made of pure chitosan without any photoinitiator or
chemical modification of this polymer haven’t afforded
convincing results with this process in our experiments
under the conditions described here.
It has been verified that, in attainment of hydrogels
from mixtures of 2% polymer co-solutions, the gel frac-
tions increase as PVP content increases in the mixtures,
confirming, therefore, that this polymer effectively is
more prone to cross-link than chitosan in these experi-
ments. Nevertheless, there is a limitation on directly ob-
taining hydrogels from mixtures of diluted acid aqueous
solutions of chitosan and PVP after being irradiated by
UV light, when such mixtures contain amounts above
30% of chitosan (w/w) on a dry basis. This fact may be
owing to a similar reactivity observed by Zhao et al. [79]
in attainment of hydrogels from blend of carboxy-
methyl-chitosan and PVP using EB-radiation. Hence,
chitosan oligomers and other likely products derived
from its degradation by UV irradiation would be acting
as scavengers of free radicals and would interfere with
the cross-linking of PVP, leading to products insuffi-
ciently cross-linked to form hydrogels.
3.5. Gel Fractions and Swelling Ratios of
Chitosan-PVP Mixtures
See Table 4
4. Conclusions
4.1. Hydrogels from Poly(N-vinyl-2-co-Acrolein)
and Chitosan Reaction
The novel copolymer of N-vinyl-2-pyrrolidone and ac-

Hydrogels from Chitosan and a Novel Copolymer Poly(N-Vinyl-2-Pyrrolidone-Co-Acrolein) 1066
rolein reacted with chitosan and produced a hydrogel
through imine bonds. A reduction of imine to amines
with sodium cyanoborohydride has shown to be a good
alternative to stabilize the structure of the hydrogel. It
was demonstrated that the dynamic equilibrium linkage
between the polymers can be a controllable parameter as
well as an interesting property that can be exploited in
further experiments. Moreover, the hydrogel has a poten-
tial uses as wound dressings and systems for controlled
release of drugs, because of the compatibility of PVP
with the body fluids, added to the properties of the chi-
tosan as linker and binder [80-82], fungicide, and it has a
good permeability to gas, immunogenic compatibility,
low toxicity and good bioabsorption. Attainment of hy-
drogels from Schiff base by reaction of poly(N-vinyl-2-co-
acrolein) and chitosan proved to be an effective method-
ology for the production of hydrogels. The resultant re-
action products exhibit high values of gel content and
swelling ratio, and provide a material that maintains
some characteristics of both hydrogels from PVP and
chitosan. Another advantage of this process is that they
may also be produced from more concentrated solutions
of initial polymers.
4.2. Hydrogels from Mixtures of Aqueous Acidic
Co-Solutions of Chitosan and PVP
In relation to the hydrogels obtained from chitosan-PVP
films or directly from mixtures of aqueous acidic
co-solutions of both polymers in diverse proportions un-
der UV radiation for inducing cross-linking, their resul-
tant gel fractions and swelling ratios have also shown
that these hydrogels are promising materials for several
applications, for example, as biomaterials.
5. Acknowledgements
The authors would like to thank the Fundação de Amparo
a Pesquisa do Estado de São Paulo e FAPESP and the
Conselho Nacional de Desenvolvimento Científico e
Tecnológico e CNPq, for financial support
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