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MACROMOLECULAR COMPLEXES OF HYDROGELS

УДК 541.64+648.744

Обсуждены результаты исследований интер- и интраполимерных комплексов гидрогелей.

Гидрогельдердің интер- және интерполимер жиынтығының зерттеу нәтижелері талқыланды.

Abstract

Results of investigation of inter- and intrapolymer complexes of hydrogels are considered.

Keywords: Hydrogels, polymers, complexes, swelling

1. Introduction

As well known hydrogels are capable for strong swelling of several tens times in aqueous solutions. Swelled gels may undergo a collapse-like sharp decrease of their volume to reply on some non-essential changes of external conditions, such as temperature, pH, ionic strength, chemical additives, light irradiation, pressure, either electric or magnetic fields [1,2]. Such interest to hydrogel behavior is not simply academic, because of intensively high level of their application in industry, medicine, pharmacy, agricultures and so on [3].

Polymer hydrogels also can drastically change their volume through a complex formation with different additives including another high-molecular substance. The so-called macromolecular complexes of hydrogels may be roughly divided on four major types:

1. Complexes of hydrogels with linear macromolecules or semi-interpenetrating networks;

2. Interpenetrating networks;

3. Intrapolymer complexes of hydrogels on the basis of either graft or block copolymers;

4. The so-called intergel or gel-gel complexes of contact and distant types.

If the first three types of gel complexes have scrupulously studied and descried in various literature, the last type of complexes is just on starting way of investigation. However possible practical uses of such systems could be extrapolated now on their further industrial realization. This paper is generally focused on intergel systems, moreover other types of gel complexes are described here.

2. Results and discussion

2.1 Complexes of hydrogels with linear macromolecules

An interaction of linear poly(acrylic acid) (PAA) under 1x105 MM with polyvinylpirrolidone (PVP) gels leads to drastic decrease of swelling coefficient Ks due mainly to a complex formation between linear parts of the gel and PAA stabilized by a system of cooperative hydrogen bonds and additionally by hydrophobic interactions as shown in Figure 1 [4]. In case of PAA with high MM within 2.5 ÷4.5 x105 a decrease of Ks occurs hardly. Perhaps longer molecules of PAA can not penetrate easily into gel volume and complex formation occurs only on the surface of the gel due to steric difficulties.

Figure 1. Dependence of swelling coefficient Ks of PVP gel on PAA concentration with various molecular weight: 1 –4.5x105 , 2 – 2.5x105 , 3 –1.0x105

A slight pH change impacts on the gel-PAA complex with its final destabilization. Indeed, the gel-PAA complex exists only at low acidic level below pH 3.6 (Figure 2). Increasing pH provokes Ks increase and gel reswelling is clearly observed due mainly to ionization of carboxylic groups and destruction of hydrogen bonds. The Ks value reaches the initial ratio and manifests destruction of the complex. Typical destruction of polymer-gel complexes is possible also in the presence of some additives such as an organic solvent dimethylsulfoxide (DMSO), which is responsible for both hydrogen bond break-up and decreasing hydrophobic interactions (Figure 3). As shown in the figure a drastic increase of Ks for either the gel-PAA or polymer-polymer complexes occurs in a mixture enriched with DMSO upper 50 vol.% due basically to complex weakening.

Figure 2. рН effect on swelling coefficient of PVP gel (1) and intermacromolecular complexof PAA- PVP gel (2)

The same behavior is observed for other systems, such as linear PAA in a gel of polyacrylamide (PAAm) [5]. Moreover, it has been shown a practical use of such formation to transform chemical energy to mechanic one [6]. Polyionic complexes formation stabilized by electrostatic and hydrophobic interaction is brightly demonstrated in systems – either linear sodium polyacrylate and gel of poly-2-methyl-5-vinylpyridine hydrochloride (PMVP); or linear sodium polystyrene sulfonate and gel of PMVP [7,8]. Stability of those complexes is strictly depended on pH or ionic strength range, either dimethylformaldehyde additive concentration.

Figure 3. Dependence of swelling coefficient of PVP-PAA gel complex (1) and intrinsic viscosity of its complex (2)

Interpolymer reactions between linear or rare cross-linked PAA with either cross-linked (linear) polyethyleneimine (PEI) or polydimethylaminoethylmethacrylate respectively show strong sense to any pH changes [9]. In order to describe a transportation of linear polyions into oppositely charged hydrogel the so-called relay-race mechanism is appropriately proposed.

2.2 Interpenetrating networks

Interpolymer complex formation may be brightly realized in the so-called interpenetrating networks (IPN) including at least two entangled gels of different nature, linear parts of those enable for cooperative interactions. A stark example of IPN contains PMAA and polyethyleneoxide (PEO) gel networks [10]. Formation of a complex occurs at pH below 4 owing to cooperative hydrogen bonding through the chains with final collapse of such IPN schematically depicted in Figure 4.

Figure 4. Schematic example of IPN formation, for instance PAA-PEO gels at pH ≤ 4

Complex formation – destruction occurs for an IPN system of PAA and PAAm gels at temperature below and above 20°C due to wavering hydrophilic-hydrophobic balance of hydrogen bonding [11]. However, IPN of PAA and polydimethylacrylamide (PDMA) keeps balance around 70°C due mainly to higher complementary of its hydrogen bonds [12].

2.3 Intrapolymer complexes of hydrogels

Intrapolymer complexes could be formed to hydrogels through either graft or block copolymerization when a main backbone chain on the one hand and graft-branches or blocks on the other hand are different nature. One of the example is a hydrogel of poly(methylacrylic acid) (PMAA) - graft-PEO [13,14]. In the Figure 5 a scheme of such complex formation at pH below 4 is represented due to arising of hydrogen bonds through cooperative mechanism leading finally to collapse of the gels. With pH increasing an ionization of PMAA takes place with drastic increase of Ks. That reflects on destruction of intrapolymer complexes.

Figure 5. Schematic example of complex formation based on PMAA-graft-PEO at рН ≤ 4

The same phenomenon is typical for a hydrogel system based on PEO-graft-polyisopropylacrylamide (PIPAA) where pH and temperature are affecting parameters [15]. A formation of intrapolymer complexes occurs also at the isoelectric points of some amphoteric hydrogels on the basis of vinyl-2-aminoethyl ether with sodium polyacrylate [16]. With some deviation of pH around isoelectric point the hydrogel complex undergoes destruction due basically to ionization of either acidic or aminogroups.

2.4 Intergel complexes

In our work has been shown a possibility of complex formation by interaction of two gels of different nature at their contact sides [17-19]. In Figure 6 a dependence of swelling coefficient Ks on a composition of sodium polyacrylate and polyallylamine hydrochloride is shown. The figure shows a negative deviation of Ks in the presence of some additives. It could be explained as some partial complex formation due to electrostatic interactions of oppositely charged functional groups on the surfaces of hydrogels. Moreover, it is highly possible a surfacial penetration of linear chains of the hydrogels into each other. Because of small volume of the hydrogels involve in the interaction there is no remarkable contraction in the whole system.

Figure 6. Dependence of swelling coefficient of a sodium polyacrylate – polyallylamine hydrochloride gels on their composition and its additive curve

Such behavior between the surfaces of two gels can be demonstrated as shown in Figure 7. Similar results are also obtained for the following systems: sodium polyacrylate – PEI hydrochloride, PAA – PEI, PAA – poly-4-vinylpyrridine (PVPy). The last systems shows remarkable electrostatic interaction between negatively charged carboxylic groups and positively one aminogroups with proton transportation from polyacid to polybase.

Figure 7. Schematic illustration of contact interaction of two oppositely charged gels

An effect of additives is clearly manifested as its dependence on Ks ratio for a system of PAA-PAAm gels (see Figure 8). A main reason of the deviation is formation of interpolymer complexes at the boards of hydrogel particles stabilized by cooperative hydrogen bonds as well as additional hydrophobic interactions. Unremarkable deviation of Ks may be due to short linear chains of surfacial area involved in the process. This effect could be increased using longer branch chains on the surface of gels. Recently some works dedicated to intergel interaction are considered behavior in a system between anionic and cationic gels [20,21]. A slight decrease of Ks is observed in such system comparatively to the initial one with estimated degree conversion within 15-20% range [21]. Thus, some gels of different nature at contact interaction are formed complexes through either electrostatic forces or hydrogen bonding.

Figure 8. Dependence of swelling coefficient of PAA – PAAm gels on their composition and its additive curve

Affecting between two gels located at some distance from each other takes also place in aqueous solution with some additives [22,23]. A first gel of PAA or PMAA is located at the bottom of a cylinder; however a second gel of PEI, PVP or PAAm is placed on a glass filter moveable from top to bottom of the cylinder. That allows varying distance between the gels located at the same volume in order to estimate distance effect on interaction ratio.

Effect of ratio components at 8 cm distance between gels as shown in Figure 9 is visible hardly, however a remarkable increase of Ks is observed in the distant presence of the second gel in the cylinder. For instance the initial Ks of PAA is about 22 increasing up to 50 point in the distant presence of PEI (Ks=5). Effect of weak polybase PAAm appearance is lower with Ks increasing up to 33. A reason increasing PAA swelling degree may be linked to additional polyacid dissociation in the presence of proton acceptors of polybase. This effect may be called either “distance interaction” or “distance affect” through proton transportation between polyiones of opposite charges.

The same type of proton transportation has been observed between linear PAA and linear PVPy with their complex formation [24]. It could be verified example for some systems consisting of fully charged components such as sodium polyacrylate (Ks=180) via poly-2-methyl-5vinylpyrridine hydrochloride (Ks=42), the Ks of intergels being equal 168 does not change at all. A distance effect on Ks shown in Figure 10 does not appear all distance long. However, pH influence on a system of PAA gel (Ks=22) via PEI one (Ks=5) appears with drastic swelling ratio decrease from 50 to 5 at pH below 1.15 due generally to suppressing of PAA gel ionization. As well as a drastic Ks increase occurs at pH above 12.75 up to 100 because of remarkable ionization degree of PAA.

Figure 9. Dependence of swelling coefficient of gels on ratio of the initial gel components at their distant interaction, where [Х] – PEI or PAAm; [Y] – PAA or PMAA; ◊ – PAA (68) ÷ PEI (5); □ – PAA (45) ÷ PEI (5); Δ – PAA (22) ÷ PEI (5); ○ – PMAA (22) ÷ PEI (5); ● – PAA (22) ÷ PAAm (16), where Ks inside the round brackets.

Figure 10. Dependence of swelling coefficient on distance between two opposite gels: ◊ - PAA (22) ÷ PEI (5), □ – PMAA (22) ÷ PEI (5), Δ – PAA (22) ÷ PAAm (16), ○ – PAA (22) ÷ PMAA (22), where Ks inside the round brackets

A mechanism of such distant interaction between two opposite charged gels could be considered as the formation of non-compensated electrostatic charges owing to chemical bonding of proton on polybase. As results both gels become extra swelled those occur because of either repulsing of the same charges of a gel or mutual attraction of opposite charged gels. In this case the first assumption is more plausible because the mutual interaction of the gels disappears with growing distant while Figure 10 does not show any distant effect on swelling.

An internal repulse of gel due to the same charges formation in the network might be reason of non-compensated charge or monoelectric layer of the gel surfaces

Thus, a presence of two gels of different nature at some distance may anyway effect on swelling behavior of each other using additives of a solution as mediator due to the formation of MEL on the surfaces of gels.

Commonly, consideration of macromolecular complexes of hydrogels demonstrates wide diversity of their behavior those can be exploited in industry and medicine.

3. Conclusion

Results of investigation of macromolecular complexes of hydrogels including semi-interpenetrating networks, interpenetrating ones, intrapolymer complexes of hydrogels on the basis of either graft or block copolymers were briefly summarized.

A special attention was paid for lack studied types of gel complexes such as gel-gel or intergel interactions. It was shown that complex formation takes place on boards between two unlike gels at their direct contact with partial collapse intertwining of their surfacial side chains. Such complexes might be stabilized through either electrostatic contact or hydrogen bonding leading to hydrophobization of peripheral areas of both gels those could form contracted layers.

A new phenomenon of mutual influence of hydrogels located at distance indirectly through a media was described as the so-called distance affect. A system of PAA gel via PEI one manifested remarkable additional swelling due to mutual motivation of gels through ionization of both carboxylic groups of the polyacid and aminogroup of the polybase by proton transportation between them. This fact could be interesting because of possible presence of polyions those loosed partially their co-ions. Thus, consideration of different types of hydrogel macromolecular complexes demonstrated brightly their wide diversity in behavior those could be a subject of interesting practical application.

 

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