Gel formation in a centrifugal field

Transcription

1 Plasticheskie Massy, No. 4, 2004, pp Gel formation in a centrifugal field L. R. Guseva, K. G. Kostarev, and T. M. Yudina Institute of Continuum Mechanics of the Urals Section of the Russian Academy of Sciences, Perm, and the N. N. Semenov Institute of Chemical Physics of the Russian Academy of Sciences, Moscow Selected from International Polymer Science and Technology, 31, No. 7, 2004, reference PM 04/04/38; transl. serial no Translation submitted by P. Curtis INTRODUCTION In nature there are a fairly large number of physical factors capable of influencing the progress of chemical reactions and, ultimately, the structure and properties of the substance being created. Of these factors, only two have been widely used to date: temperature and pressure. However, there is one other factor that takes a constant part in the development of the reaction gravitation. Its action is manifested in the emergence of macro- and microinhomogeneities of density in liquid and gaseous media. The present-day development of technology makes it possible to vary the level of gravitation from zero (weightlessness) to overloads of the order of thousands of g 0, where g 0 is the acceleration due to gravity on the earth s surface (~9.8 m/s 2 ). In the latter case, on the earth s surface, centrifugal force, which is also a mass force, most often acts as gravitation. The synthesis of polymeric materials may be the most promising area of application of mass forces, which is due to the considerable non-isothermic nature of the reaction and to the change in density of the reaction mixture during its polymerisation. In particular, a high gravitational sensitivity under the normal force of gravity was demonstrated by experiments on the photoinitiated production of polymers based on oligomers, butylglycidyl ether, and aqueous solutions of acrylamide [1 3]. It was found that the process of polymerisation was accompanied with a gravitation mechanism such as freely conective motion of the liquid reaction medium, which not only influenced the development of the reaction within the reactor and its characteristic times but also the structure and properties of the polymer [4]. An investigation of the influence of convection, however, left open the question of the existence of other gravitation mechanisms, the action of which it may mask. The causes of the emergence of such a question are many. Thus, the sedimentation of polyacrylamide gel during sluggishly flowing reactions is known its precipitation in a buffer solution under the influence of the force of gravity several days after the start of polymerisation [5]. Cases of the physical breakdown of polymer sponge in the course of swelling of weakly crosslinked gels with an initial monomer concentration C ~ 1 2% have also been described, and here the breakdown is ascribed to the action of gravitational forces. It must be pointed out that the gravitation mechanisms given in both examples act on the gel practically after the reaction has ended. It remains unclear whether these mechanisms can take part in the formation of the polymer in the course of the reaction. Obtaining an answer to the questions posed is made possible by experiments in inhomogeneous force fields, an example of which is a field of centrifugal forces. EXPERIMENTAL The aim of the investigations was to search for different mechanisms of a gravitational nature with maximum suppression of convection of the reaction mixture [6]. The selection of centrifuging as a method was dictated by the fact that, in contrast to homogeneous force fields, in a centrifugal field the magnitude of the force increases linearly with distance from the axis of rotation, accelerating the action of the gravitational effects. A vessel (reactor) in the form of a hollow horizontal disc rotating about its axis T/54 International Polymer Science and Technology, Vol. 31, No. 12, 2004

2 was used as the rotor of the centrifuge in the experiments. The diameter of the cavity was 50 mm, and the height was 4 mm. The speed of rotation was 3000 r/min, which made it possible to create an overload of the order of 300g 0 on the periphery of the vessel. The study was conducted on the thermally initiated polymerisation of a reaction mixture consisting of a monomer (acrylamide, 15%), a crosslinking agent (N,N - methylenebisacrylamide, 0.5%), a thermal initiator (ammonium persulphate, 0.02%), and a catalyst (N,N,N,N -tetramethylethylenediamine, 0.01%) [1]. The reaction was carried out in an aqueous buffer solution based on trismethoxymethylamine (4.6%) and hydrochloric acid (ph 8.3). The initial mixture was a Newtonian liquid with a density of 1.03 g/cm 3 and a kinematic viscosity of 0.01 Pa, and the final reaction product threedimensionally crosslinked polyacryamide gel (PAG) had a density of 1.06 g/cm 3 and possessed elastic properties. The experiments were carried out by the following scheme. Without an initiator, the reaction mixture was bubbled with argon for 30 min with the aim of forcing out oxygen, which was an inhibitor of the reaction. Then, a solution of ammonium persulphate was introduced into the monomer solution, and the mixture was poured into a vessel that was rotated at selected instants of time. For their determination, use was made of the time dependence of heat evolution during polymerisation in a stationary vessel (Figure 1, curve 2). For comparison, Figure 1 gives the dependence of heat evolution during the photoinitiated reaction (curve 1, initiator riboflavin, %). From the behaviour of the curve, the induction period of the reaction (interval 0A), the instants of gel formation throughout the reactor (the so-called gel point B), and the maximum heat evolution (C) were determined successively, and also the period of completion of the reaction (CD). If it is assumed that, at different stages of the reaction, Figure 1 different gravitation mechanisms are active, then the inclusion of a centrifugal field for the corresponding intervals of time should strengthen the action of one of the mechanisms by comparison with the others. The duration of the experiment was 1 h. When the experiment had been completed, the vessel was removed from the centrifuge and the top face was removed from it, which made it possible to study the radial distribution of elastic properties of the obtained PAG specimen without taking it out of the vessel. To determine the given distribution, use was made of a unit for investigating the dilatometric, thermomechanical, and dynamic characteristics of polymeric materials (DTMD), which made it possible to measure, in particular, the local Young s modulus without cutting the specimen into samples. The errors arising in this case on account of boundary-value effects and the action of the substrate were allowed for using the procedure set out in [7]. The diameter of the area of measurement of the Young s modulus was 1 mm, and the distance between the points of measurements with linear movement of the specimen was 2 mm. (The choice of the Young s modulus as the quantity to be measured was dictated by its fairly straightforward connection with one of the structural characteristics of the gel the average intercrosslink distance of the polymer sponge [8].) RESULTS Owing to the use of a thermal initiator, the reaction developed at the same speed throughout the monomer. The low thermal conductivity of the boundaries of the cavity (organic glass) made it possible to avoid large temperature gradients in the course of gel formation, and therefore there was no thermal convection, which was proved by additional experiments with light-scattering particles added to the monomer solution. As a result, the PAG specimen obtained without rotation had a homogeneous distribution of the elastic modulus E (curve 1, Figure 2), with the exception of two areas: the peripheral area, where the increase in the modulus was due to instrumental error, and the central area, where an air pocket arose during shrinkage of the gel. The situation changed when the vessel was rotated during the liquid-phase stage of polymerisation but was stopped immediately after the reaction mixture had passed the gel point (in this case, the gel retains the distribution of monomer conversion that has developed). As can be seen from the behaviour of curve 2 in Figure 2, the elastic modulus of the PAG specimen obtained increases along the radius. Since there is no convective motion, it is most probable that the inhomogeneous distribution of properties that has arisen is due to the action of a gravitation mechanism sedimentation in the centrifugal field. Note that, in contrast to [5], sedimentation takes a few minutes International Polymer Science and Technology, Vol. 31, No. 12, 2004 T/55

3 Figure 2 Figure 3 in total. It seems that, at an early stage of the reaction, gel macroparticles are formed that are of greater size and density than the monomer molecules. These particles (globules) are not interconnected and may move in the stationary monomer solution in the direction of action of the mass forces, which leads to an increase in their concentration on the periphery of the vessel. When the particles become sufficiently large in number, they begin to form a three-dimensional polymer sponge. Figure 3 gives the dependences of the elastic modulus on radius for cases where rotation was begun after the reaction mixture had passed through the gel point (curve 1 t = 5 min; curve 2 t = 15 min). It is known that, with the monomer concentration used, it is sufficient for the reaction mixture to achieve a total conversion of a few per cent for the formation of a three-dimensional sponge during polymerisation. The remaining chemical bonds are formed at the gel stage. If the forming polymer gel (Figure 4a) is loaded by being placed in a field of mass forces, then it is deformed (Figure 4b), forcing out the buffer solution towards the centre of the polymer disc. The deformed gel continues to be polymerised (Figure 4c), and here the newly developed bonds are already free of strains. When the load is removed, these bonds should partially retain the strains of the gel (Figure 4d). A foam rubber sponge that has been squeezed, stitched at several points, and then loosened may serve as the simplest analogue of the sponge occurring. Its new shape will differ both from the original shape and from that under compression. The earlier the polymeric specimen is rotated in relation to the gel point, the greater will be the number of free bonds retained, and the greater will be the influence of strain on the forming structure of the polymer. This is the second mechanism of a gravitational nature whose action in a normal gravitational field is usually masked by convection. The elastic modulus distribution corresponding to curve 3 in Figure 3 is the result of the successive action of Figure 4 sedimentation and strain (the specimen was rotated before the start of the reaction). As can be seen from Figure 1, the two mechanisms reinforced each other. Besides the elastic properties, the distribution of the degree of swelling was investigated in a number of PAG specimens obtained by rotation. With the availability of a model allowing for structural features of a specific gel, the given distribution can be used to determine the concentration of crosslinks of the polymer network. In the present experiment an investigation was made of free swelling of the gel in excess water. Vertical cylindrical samples of 4 mm diameter were cut out along the radius of the PAG specimen obtained. Each sample was placed in a fine-celled mesh which did not prevent the increase in size of the specimen during swelling but made it possible to avoid the random breakup of samples during repeated weighing. The investigation was carried out with disregard for the sol fraction in view of its smallness. T/56 International Polymer Science and Technology, Vol. 31, No. 12, 2004

4 Figure 5 Figure 6 The degree of swelling was calculated by means of the formula 0 W = m m m 0 where m 0 is the initial weight of the specimen and m is the mass of the specimen during swelling. The achievement of equilibrium during swelling was judged from the absence of any change in weight of the sample over a period of 3 days. Figure 5 presents the dependences of the degree of swelling on the time for which the samples cut out at different distances from the centre of the PAG specimen are in water. Curves 2 and 3 were obtained for samples from a stationary specimen, and curves 1 and 4 from a specimen in motion for the entire duration of polymerisation. Curves 1 and 2 correspond to swelling of samples positioned at the centre of the radii of polymer discs, and curves 3 and 4 to the swelling of samples positioned close to the boundary. As can be seen, the dependences have an identical nature for all specimens. Figure 6 presents the distribution of the equilibrium degree of swelling along the radius. An analysis of the distribution of the degree of swelling indicates that it is dependent on the distance to the centre of the disc. This indicates that, under the PAG structure formation conditions examined, a concentration gradient of the components of the reaction mixture is observed and, accordingly, gel formation occurs with a different degree of crosslinking as the distance from the centre of the disc increases. CONCLUSIONS 1. An analysis of the results obtained makes it possible to speak of the gel formation of PAG as a process with a stepped nature of development, which begins with the emergence of separate gel microparticles, continues with the creation of a macrostructure (sponge) by their joining, and ends with the creation of a polymer based on the original macrostructure. 2. The existence of two more gravitation-sensitive mechanisms, besides convection, that are capable of taking part in the formation of the structure and properties of gels has been demonstrated: (a) (b) sedimentation of primary polymer microparticles in a stationary buffer solution; strains of the forming polymer sponge in a force field. The sedimentation of individual microparticles, as with the convection of the entire reaction mixture, takes effect only during the liquid-phase stage of the reaction, which occupies a small portion of time of polymerisation. Strain may take part in the creation of a gel structure for a much greater time, and here its action is more effective the sooner it begins to act. 3. The use of a centrifugal field creates conditions for a significant increase in the role of gravitational mechanisms of polymerisation, making it possible to produce specimens of polymeric materials with a radial distribution of properties. This work was supported by the Russian Foundation for Basic Research (Grant No ) and was in part funded by a grant (PE-009-0) from the US Foundation for Civic Research and Development. International Polymer Science and Technology, Vol. 31, No. 12, 2004 T/57

5 REFERENCES 1. V. P. Begishev et al., Inhomogeneity of curing of oligomers, governed by convective effects. Vys. Soed., A36, No. 5, 1994, pp T. P. Lyubimova et al., Polymerisation under different gravity conditions. Acta Astronautica, 39, No. 5, 1996, pp K. G. Kostarev et al., Gravity sensitivity of polymerisation processes. Int. Polymer Sci. and Technology, 25, No. 4, 1998, pp K. G. Kostarev et al., Effect of free convection on formation of structure and properties of polyacrylamide gel. Vys. Soed., A42, No. 11, 2000, pp P. G. Righetti et al., Is gravity on our way? The case of polyacrylamide gel polymerisation. Electrophoresis, 15, 1994, pp V. A. Briskman et al., Gel polymerisation under high gravity conditions. In: Materials processing in high gravity (Ed. L. L. Regel), Plenum Press, New York, 1994, pp V. P. Begishev et al., Modelling of thermodynamic processes in crystallising polymer. Mekhanika Tverdogo Tela, No. 4, 1997, p P. De Zhen, Ideas of scaling in polymer physics, Mir, Moscow, (No date given) T/58 International Polymer Science and Technology, Vol. 31, No. 12, 2004