The peculiarities of phase transitions in thermosensitive hydrogels based on N-vinylcaprolactam and its copolymers with sodium itaconate synthesized in the presence of cross-linking agent in water-ethanol mixture were investigated. It has been found that water-alcohol solutions affects an antagonistic action on swelling behavior of poly(N-vinylcaprolactam) gels which depend on alcohol concentration in water-alcohol mixture. The complexation mechanism of poly(N-vinylcaprolactam) and poly(N-vinylcaprolactam-co-sodium itaconate)copolymer gels with phenol and picric acid was supposed. It has to be noted that phase transition temperature of N-vinylcaprolactam based hydrogels strongly depend on environment of the networks.
Keywords: hydrogels, swelling, contraction, phase transition, complexation
It is well known that responsibilities of polymeric hydrogels to various stimuli such as solvent composition, temperature, electric field, pH and so on are determined by their internal morphological structure [1-3]. For thermosensitive networks the presence of hydrophobic groups is necessity condition, those can be aggregated in larger domains with propagation of temperature. The temperature phase transition is typical of polymers with lower critical solution temperature (LCST). The macromolecules of poly(N-vinylcaprolactam) (PVCL) can be related to systems described by sensitivity to solvent composition and to temperature. Both in first, and in the second cases, the change of environmental conditions results in structural changes in volume of hydrogels. Besides linear PVCL exhibits unique physical and chemical properties including low toxicity, high complexing ability with low and high molecular weight substances and film forming characteristics due to which this polymer has found wide application in many fields of technology, medicine and agriculture [4-6].
Recently [7,8] the swelling behavior of slightly cross-linked PVCL gels was studied under changing of temperature in water and also in the presence of low molecular weight electrolytes and surfactants. It was shown that PVCL gel has two LCST at 304,5 K and 310,5 K, respectively. First LCST is attributed to microsegregations of the neutral networks whereas second phase transition temperature is associated with the gel collapse itself.
We have investigated [9,10] the influence of a charge concentration in PVCL gels on its phase transition temperature in aqueous and water-salted solutions. The increase of concentration of charged groups in the networks results in propagation of phase transition temperature of the PVCL based gels.
The investigations of such systems represents significant interest because it allows proceed influence of a microenvironment of macromolecules on their conformational changes from friable statistical coil to compact globule.
Earlier Kirsh and co-workers [11-13] have studied thermoprecipitation behavior of linear PVCL in water-organic solutions. It was assumed that thermoprecipitation of PVCL in water is caused by hydrophobic interactions between lactame rings which amplifies at increase of temperature.
The aim of this work is to demonstrate the influence of composition of water-organic solvent mixtures on the temperature dependence of swelling behavior of poly(N-vinylcaprolactam) and poly(N-vinylcaprolactam-co-sodium itaconate) (PVCL-ITNa) hydrogels.
The hydrogels of PVCL and its copolymers with sodium itaconate (xs =0,03-0,1 mole %, xs – is the concentration of charged groups) were prepared by ?-radiation (co) polymerization at the presence of various amounts crosslinking agent (xcr =0,01-0,03 mole %) - 3,3 ‘-(ethane-1.1-diyl)bis(1-vinyl-2-pyrrolidone) in water-ethanol mixture (0,3/0,7 by vol.). The volume fraction of mixed monomers was vm=0,2. The synthesis was carried out in glass ampoules 1 cm in diameter. Then the gels were removed from ampoules and washed in the large water volume to remove of unreacted monomers and ethanol.
The concentration of elastically active network chains was determined from its shear modulus (more detail see refs [9,10]).
The swelling coefficient (Ks) of the hydrogels was determined gravimetrically according to following equatio
where ms and md are the mass of the swollen and dry gel, respectively.
The degree of swelling characterized by the volume fraction of the polymer in the swollen gel, (?2) was determined from the weight of the dry sample, (m2), density of the dry sample, (?2), weight of the solvent in the gel swollen to equilibrium, (m1), and solvent density (?1) assuming additivity of the polymer and solvent volumes according to formula
Results and their discussion
Previously we have established that PVCL hydrogels subjected to contraction in the mixture of water and alcohol. The composition of a binary mixture at which the greatest compression of hydrogels takes place is indirect depends on the length of alkyl radical of alcohols. In the presence of any alcohol the swelling degree of the gels gradually decreased with increase in alcohol concentration. The minimum of alcohol concentration at which hydrogels have lowest swelling degree depends on length of alkyl radical of alcohol molecules. After passing point of contraction shrunken gels reswelled in different alcohol concentration. Such changes of swelling degree depend on the volume fraction of alcohol molecules evidence for conformational transformations of chains of PVCL gels.
Earlier we have shown  possibility of application of Dondos-Patterson principle for the description of swelling behavior of linear and slightly cross-linked polymers in binary mixtures with various mixing heats for prediction their synergetic or antagonistic effect on conformational characteristics of hydro- and organogels. Dondos and Patterson  proceeding from the analysis of thermodynamic parameters of mixing of the individual solvents and the changes of viscosity of the linear polymers dissolved in them came to the conclusion, that for mixes of the solvents with positive mixing enthalpy ?H > 0 changes of viscosity of solutions should be positive (synergism) whereas for ?H < 0 - negative (antagonism).
As expected according to Dondos-Patterson principle in the mixtures of solvents with exothermic effect of mixing (water - aliphatic alcohol) the strong decrease of swelling degree for neutral gels is observed. The maximal heat effects of mixing in systems H2O-CH3OH, H2O-C2H5OH, H2O-C3H7OH corresponds to the volume fraction of organic solvents in water 43, 35 and 30 respectively . At these ratios the gel shows minimum value of swelling degree.
Thus the systems of the mixtures of the solvents with exothermic effects of mixing render antagonistic effect on macromolecular sizes of slightly cross-linked gels.
Authors  believe that the decrease in entropy of mixing is ascribed to the increase of interaction between water and alcohol molecules which results in the cluster formation of alcohol and water. It means that water should prefer cluster formation with alcohols to hydrophobic hydration around the polymer, as resulting in the shrinking of the gels. Besides the presence of a three-dimensional structure of water at moderate content of alcohol excludes indirect contact of polar molecules of water and hydrophobic sites of polymers which occupy the space in the ice-like structure of water. The increase of alcohol concentration in water results in destruction of three-dimensional structure of water promoting replacement of the hydrated shell of polymeric segments to solvated one. The destruction of the structure of water excludes an opportunity of formation of hydrophobic domains of macromolecules of PVCL gels and, as a consequence, results in disappearance of LCST at high alcohol concentration.
It is well known  that using polymer-solvent interaction parameter (?) the swelling behavior of the hydrogel can be discussed from a thermodynamic point of view.
Fig.1. Dependence if interaction parameter (?) of PVCL gel on alcohol concentration in water-alcohol mixtures. 1 – H2O-MeOH; 2 - H2O-EtOH; 3 - H2O-n-PrOH
Figure 1 shows the dependence of ? parameter of PVCL hydrogels on alcohol contents in various water-alcohol mixtures at room temperature. One can see that ? increased steeply when the alcohol concentration increased up to 43, 35 and 30 vol.% and decreased again with further increase of alcohol contents to more than mentioned values in water-methanol, water-ethanol and water-propanol mixtures respectively. It is necessary to note that both water and methanol do not a good solvents (? ? 0,5) while ethanol and propanol are good ones (?< 0,5) for PVCL gels. The high value of ? implies that the affinity of the polymer segments to solvent is very weak and PVCL gels collapses. Furthermore it is thought that ? reaches a minimum in ethanol and propanol because the PVCL gels consist of hydrophobic lactam rings. It is interesting that maximum of ? corresponds to the minimum of mixing ehthalpy of water-alcohol system.
To study of temperature effect on swelling behavior of PVCL gels we have chosen several ratio of water – alcohol mixtures: before and after contraction of the networks, when they are in swollen state at room temperature and have various local microenvironment. The strongest affect on conformational state in the set of methanol – ethanol –propanol was observed for propanol.
In Figure 2 the relationship between temperature and swelling degree of GPVCL in water - methanol (a), water-ethanol (b) and water-propanol (c) at various contents of alcohol is represented.
Fig. 2. Dependence of the swelling degree of PVCL gel on temperature in water-alcohol mixtures. 1 – H2O-MeOH; 2 - H2O-EtOH; 3 - H2O-n-PrOH
The phase transition temperature (Ttr) of the networks in water corresponds to 310 K. The increase of the contents propanol contents in water-alcohol mixture up to 15 vol.% results in decreasing of Ttr up to 293 K (at room temperature gel was swollen). However after passing the point of contraction (? ? 25 vol.%) the tendency of increasing of Ttr (at ? = 50 vol.% Ttr = 303 K) is observed. The further increase of the propanol content in water does not specify the presence of temperature phase transition up to 363 K. Hence complete destruction of ice-like structure of water including hydrophobic domains of PVCL results in disappearing of LCST. It confirms an aspect that for thermoresponsibility of hydrogels necessary and sufficient condition is the existence of the hydrophobic groups formed only in a structured liquid with high polarity as water is.
The similar swelling profiles were observed for alcohols with smaller length alkyl radical (methanol and ethanol) only with distinction that Ttr reduced not so stronger as in case of propanol.
It has been shown elsewhere  that linear PVCL owing to high-density of amide groups and hydrophobic rings forms a new local vicinity in environments with low dielectric permittivity, This process is accompanied by replacement of water molecules from the second hydrated core and worsens its solubility. Besides hydrocarbon radicals of molecules of organic compounds screens side chains of PVCL from water bring about own contribution to non-polar interactions between them and hydrocarbon fragments of lactam rings.
In Figure 3 the dependence of swelling coefficient of PVCL and its charged copolymer P(VCL-ITNa) (xs = 0,1) gels on their mole ratio with phenol (Ph) (1) and picric acid (PA) (2, 3) is shown. It is seen that addition of phenol to neutral PVCL gels results in contraction whereas picric acid causes collapse both neutral and charged networks. Driving force of complexation between PVCL hydrogel and phenol molecules seems to be H-bond arising from interaction of carbonyl group of lactam rings and hydroxyl group of phenol through water molecules . Under this the water molecules are replaced from hydrated core of lactam cycle. For GPVCL-PA system one can assume similar mechanism. The additional reason for collapse of neutral network under complexation with picric acid might be connected with formation of ionic pairs as a result of transition of proton of hydroxylic group of PA to oxygen atom of carbonyl group of lactam cycle according to the scheme:
Fig.3. Dependence of the swelling coefficient of PVCL and P(VCL-ITNa) (xITNa = 0,03 and 0,1 mol.%) hydrogels on the ratio of [Ph or PC]:[gel]
Fig.4. Dependence of the swelling coefficient of PVCL and P(VCL-ITNa) in the presence of Ph (a) and PA (b) on temperature at various concentration of charged groups (xITNa), mol.%: [gel]:[Ph] = 5:1; [gel]:[PA] = 10:1
This conclusion was made on the basis of comparison of ionization constants of phenol and PA (KaPh = 1,3 10-10 and KaPA = 1,6 10-1). Due to high acidity the proton of hydroxyl group of PA is movable and capable easily to bind to electron-donating oxygen atom of carbonyl group of lactam cycle of PVCL network. These results in redistribution of electronic density in a benzene ring and signify that the nitrogen atom of amide group acquires positive charge. The character of changing of swelling coefficient of the negatively charged PVCL-ITNa gels also evidence for the electrostatic mechanism of complexation. In this case addition even of insignificant amount of the PA results in sharp compression of the charged network, that is typical for the systems in which complexation occurs as a result of electrostatic interaction between oppositely charged groups which is stabilized by hydrophobic interactions between hydrophobic lactam cycles of hydrogels and benzene rings of phenol or picric acid.
One can see that mole ratio [Ph (PA)]/[gel] at which collapse takes place shifts to smaller value with increasing both of concentration of charged groups of networks and acidity of organic components.
Specific complexation phenol and picric acid molecules with PVCL based gels also reduce Тtr of the networks (Fig. 4) however as against aliphatic alcohol do not shifts phase transition temperature reversibly with increasing of concentration of organic molecules. The reason of such behavior can be the fact that for them is not characteristic effect of resolvation of hydrophobic groups of polymeric chains.
The compression of hydrogels with increasing of temperature is caused by strengthening of hydrophobic interactions between lactam cycles of PVCL gels and aromatic rings of phenol and PA.
One can note that in the system of charged hydrogel - phenol the displacement of Тtr occurs not so noticeably in comparison with the system charged hydrogel – picric acid. In the latter case the concentration of charged groups in networks does not influence strongly on Ttr. The phase transition temperature occurs approximately in close temperature interval (310-320 K) despite on the concentration of charged groups of hydrogels that testifies to more complete neutralization of charged groups of the PVCL-ITNA hydrogels.
For removing of sorbed molecules of phenol and picric acid the swelling coefficients of dried complxed with them gels in various solutions of electrolytes were studied. We have found that with increasing of concentration sodium hydroxide up to 0,1 M all previously sorbed phenol and picric acid desorbes again in the surrounding gel solution. Obtained data should be used to utilize for the decision the problems of purification of waste water from the high contents of phenol and its derivatives. The convenience of the method is the fast regeneration of the PVCL based sorbents and possibility multiple their use without essential material and power inputs.
Thus, the obtained results develop performances about mechanisms of resolvation of hydrated cores of hydrophobic macromolecules and specific interaction between functional groups of monomer units of polymers with low molecular weight substances. These data also are useful for explanation of effects of structuring and principles of self-organizing in the limited space of elements having affinity to each other due to the specific hypothetical centers of interaction.
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