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Equilibrium and dynamic swelling of polyacrylates.

INTRODUCTION

Polyacrylates are polymers synthesized from a wide variety of acrylic and methacrylic ester monomers (1). In recent years, polyacrylates have become increasingly significant and beneficial materials that play a role in a number of commercial applications. Polyacrylates have proven useful in many areas including membranes in hemodialysis and ultrafiltration (2), textile fibers, ion exchange resins, and optical fibers for transmitting light. An area where polyacrylates have come to play a key role is that of absorbency. Absorbent materials are crucial in many personal care products and thus, contribute to an important and profitable industry. Typically, good absorbent materials are anionic polyelectrolytes and contain a large number of hydrophilic moieties such as hydroxyl or carboxyl groups. As a result, the most common commercial absorbent has become crosslinked, partially neutralized poly(acrylic acid) (3). Several studies have been reported on various methods of preparation of absorbent poly(acrylic acid) (310). This material is usually produced through free radical polymerization and the most commonly used crosslinking agent is trimethylolpropane triacrylate (TMPTA) (4).

One of the main properties of interest in characterizing poly(acrylic acid) in terms of its use as an absorbent material is sorption capacity both at equilibrium and under dynamic conditions (11). Certain parameters are used to describe the sorption capacity of an absorbent polymer. For example, the weight ratio of the swollen polymer to the dry polymer (g swollen polymer/g dry polymer) is called the swelling ratio, q. The ratio of the volume of the swollen polymer to the volume of the dry polymer is termed the volume degree of swelling, Q. Another experimental parameter often used to describe absorbent polymers is the volume fraction of polymer in the swollen gel, [v.sub.2,s]. This value is the reciprocal of the volume degree of swelling, Q. These parameters depend on various structural aspects within the gel network as well as the nature of the swelling agent (3).

In a swelling medium, a polymer will absorb the swelling agent until the chemical potential in the polymer gel becomes equal to that of the free solution. Equation 1 shows this in terms of the osmotic swelling pressure, which is zero at equilibrium, assuming all contributions to the swelling pressure are independent of one another:

[[Pi].sub.mix] + [[Pi].sub.elas] + [[Pi].sub.ion] + [[Pi].sub.elec] = 0 (1)

In this equation, [[Pi].sub.mix] is the swelling pressure because of the tendency of the polymer to dissolve in the solvent and, [[Pi].sub.elas] represents the elastic response of the network because of the chemical crosslinks. The terms [[Pi].sub.ion] and [[Pi].sub.elec] are terms that account for effects of ionizable groups in the gel; [[Pi].sub.ion] is the osmotic pressure arising from an ion concentration difference between gel and solution, whereas [[Pi].sub.elec] is the effect of electrostatic interactions on the polymer chains. Several models based on Eq 1 and similar theoretical relations are presented in the literature (12-17).

Another factor that contributes to the swelling of an absorbent polymer such as poly(acrylic acid) is the amount of crosslinking agent incorporated into the network. Yin et al. (4) have studied the equilibrium swelling of partially neutralized poly(acrylic acid) crosslinked with TMPTA. These researchers found that the swelling ratio monotonically decreased as crosslinker concentration was increased. The same trend was seen in gels crosslinked with bisacrylamide. Neppel et al. (18) correlated the increase in peak broadening in PAA gel 13C NMR spectra with increased concentration of crosslinking agent. This work may allow for the rapid estimation of crosslink density and thus, degree of swelling, of industrial poly(acrylic acid) absorbents.

The concentration of crosslinking agent in the polymer network is directly related to the molecular weight between crosslinks, [M.sub.c]. The molecular weight between crosslinks, which translates to the distance between two crosslink junctions, determines how far the network chains can expand to accommodate solvent molecules. As alluded to above, a polymer network with a large value of [M.sub.c] is able to swell to a higher degree than that of a network having a lower value of [M.sub.c]. The molecular weight between crosslinks can be determined experimentally from [v.sub.2,s]. The well-known Flory-Rehner (19) equation relates the molecular weight between crosslinks to the volume fraction of polymer in the equilibrium swollen gel,

[Mathematical Expression Omitted]

Here [Mathematical Expression Omitted] is the number average molecular weight of the polymer before crosslinking. [Mathematical Expression Omitted] is the specific volume of the polymer, [V.sub.1] is the molar volume of the swelling agent, [v.sub.2,s] is the polymer volume fraction in the equilibrium swollen polymer, and [Chi] is the Flory-Huggins interaction parameter. Modifications to the Flory-Rehner equation have been made by investigators such as Peppas and Merrill (20) who accounted for the presence of the swelling agent during crosslinking.

In addition to equilibrium swelling behavior, dynamic swelling behavior and mechanical properties are important characteristics of absorbent polymers (11). Previous work in these areas include that of Ogawa et al. (21) who found that the dynamic swelling of PAA microparticles seemed to follow first order kinetics and that of Schossler et al. (9) who studied the effects of degree of crosslinking and ionization on the shear modulus of poly(acrylic acid) gels. Schossler and coworkers found that the shear modulus of these gels increased with increasing concentration of crosslinking agent and decreasing degree of ionization.

Thus, the swelling and mechanical properties of absorbent polyacrylates have been shown to depend on factors such as network structure and the nature of the swelling agent. The purpose of this work is to characterize the equilibrium and dynamic swelling behavior as well as the swelling behavior under load of crosslinked poly(acrylic acid) as a function of network structure.

EXPERIMENTAL

Crosslinked, partially neutralized poly(acrylic acid) samples were prepared by solution polymerization at 37 [degrees] C for 24 h. These samples were prepared in two ratios of neutralization and in three ratios of crosslinking agent. The acrylic acid monomer (Aldrich Chemical Company, Inc.) was first vacuum distilled for purification and then diluted to a 35% aqueous solution to minimize the risk of autoacceleration during polymerization. The acrylic acid was then neutralized in ratios of 40 and 60% using a 0.2 M sodium hydroxide solution (NaOH - Mallinkrodt, Inc.). The crosslinking agent used was trimethylolpropane tri-acrylate (Sigma Chemical Company, Inc.), and it was added in nominal crosslinking ratios of 0.001, 0.005, and 0.01 mol TMPTA/mol AA. Finally, a redox initiator system of sodium metabisulfite (Fisher Scientific, Inc.) and ammonium persulfate (Polysciences, Inc.) was added. The amount of initiator used was 0.6 wt% of monomer. The reaction mixture was stirred to dissolve the solid initiator and poured into polypropylene vials. These vials were sealed tightly with duct tape. The reaction vials were then placed in a water bath at 37 [degrees] C and polymerization occurred over a 24 h period. After polymerization was complete, the vials were cut open. The crosslinked polyacrylate samples were removed and allowed to dry in a vacuum oven at 40 [degrees] C for three to four days.

To characterize and compare the poly(acrylic acid) samples, equilibrium swelling studies were performed at 37 [degrees] C. These studies were done using 0.9% saline solution (NaCl, Fisher Scientific, Inc.) and deionized water as the swelling agents. The dried polymer samples of various neutralization and crosslinking ratios were first cut into disks of 0.3 mm thickness using a diamond blade low speed saw and then weighted to determine the dry sample weight. These disks were then immersed in glass jars containing either 30 ml of saline solution or 30 ml of deionized water. The disks were weighed weekly until they reached an equilibrium swelling weight.

The short term swelling of the polyacrylate samples in a situation simulating a superabsorbent application was investigates using "tea bag" swelling experiments. In these experiments, the samples were ground into microparticles of 30 to 50 mesh using a coffee grinder. Particles of the desired size were isolated using wire mesh sieves. Next, 0.2 g of the micro-particles were placed in fabric tea bags that were sealed with staples. The tea bags were rectangular with dimensions of 40 x 50 mm. The initial weights of the tea bag/polymer sample units were determined, and then the units were immersed in 50 ml of 0.9% saline solution at room temperature. The units were removed, blotted with a paper towel, and weighed after 3, 5, 10, and 15 min in saline solution. Tea bags that did not contain polyacrylate samples were used as control samples for these experiments.

The swelling behavior of samples was also measured under a load. A diagram of the device used to perform these experiments is shown in Fig. 1. First, the samples were ground and sieved as in the tea bag swelling experiments. Next, 0.5 g of the microparticles were placed on the wire mesh in the bottom of the sample holder. The particles were then spread out as uniformly as possible on the wire mesh. The Teflon cover was placed on top of the sample. The sample holder was then suspended in a beaker as shown and placed under the measuring device such that the plunger of the device rested on the sample cover and the measuring dial read zero. A weight of 76 g was then added on the measuring device as shown in the diagram. The Teflon cover weighed 27 g. The beaker was then filled with 300 ml of 0.9% saline solution. The saline was added through the lip of the beaker using a syringe, and the wire mesh portion of the sample was, as a result, immersed in the saline solution. When the saline came into contact with the wire mesh, a stop watch was started. As the polymer microparticles swelled against the load, the height on the measuring dial was taken at various time increments until there was little height change over 5 min. At this point, the experiment was considered complete.

RESULTS AND DISCUSSION

The reference polyacrylate samples produced from the solution polymerization procedure described previously were colorless and clear. Previous polymerizations using acrylic acid neutralized with sodium carbonate, rather than sodium hydroxide, produced cloudy samples containing bubbles suspended in the polymer matrix. This was because of the carbon dioxide produced as the sodium carbonate dissolved in the reaction mixture.

As mentioned previously, when an absorbent polymer is placed in contact with a swelling agent, it will swell to an equilibrium value. The equilibrium swelling of the polyacrylate samples was first investigated using deionized water as the swelling agent. The equilibrium swelling values were used not only to determine the swelling ratios of the polyacrylate samples but also to characterize the structure of the polymeric networks. The molecular weight between crosslinks, [Mathematical Expression Omitted], was determined using Eq 3,

[Mathematical Expression Omitted]

In this analysis, the following experimental values were used: [Mathematical Expression Omitted] was assumed to be 75,000. For acrylates and methacrylates, this is a typical value. The specific volume of the polymer, v = 1/[Rho], was equal to 0.7042 [cm.sup.3]/g. This value was determined experimentally using the classical buoyancy method discussed by Barr-Howell and Peppas (22). In these calculations, it was assumed that the density of the dry, amorphous polymer did not change appreciably with nominal crosslinking. This assumption may be debatable but slight changes in density do not affect the value of [Mathematical Expression Omitted]. Additionally, the molar volume of the swelling agent was 18 [cm.sup.3]/mol. The determination of [v.sub.2,s] was done by relating the weights of the swollen and dry polymers by the following equation,

[v.sub.2,s] = [V.sub.d]/[V.sub.s] = [W.sub.d]/[[Rho].sub.p]/([W.sub.d]/[[Rho].sub.p]) + (([W.sub.s] + [W.sub.d])/[[Rho].sub.w]) (4)

Here [V.sub.s] and [V.sub.d] are the swollen and dry polymer volumes, respectively, [W.sub.s] and [W.sub.d] are the swollen and dry polymer weights, respectively, and [[Rho].sub.p] and [[Rho].sub.w] are the polymer and water densities, respectively. Finally, the Flory interaction parameter, [Chi], was assumed constant at 0.495 from previous studies (3).

These experimental [Mathematical Expression Omitted] values, henceforth designated as [Mathematical Expression Omitted], were then compared to the [Mathematical Expression Omitted] values expected if all of the crosslinking agent, (TMPTA) were to be incorporated into the polymer network during the polymerization/crosslinking reaction. The latter parameter will henceforth be termed [Mathematical Expression Omitted]. The theoretical values, [Mathematical Expression Omitted], were determined using the nominal value of the crosslinking ratio, [X.sub.nom], in other words, the crosslinking ratio based on the assumption that all of the TMPTA in the reaction mixture was incorporated into the polymer network as chemical crosslinks and that the resulting network contains no dangling chain ends. These values were related by Eq 5, where the term 3 indicates that in TMPTA there are three functional sites for crosslinking.

[Mathematical Expression Omitted]

Here, [M.sub.r] is the molecular weight of the repeating unit (acrylic acid). The actual crosslinking ratio, [X.sub.exp], expressing the true amount of crosslinking agent reacted, was also determined using the above equation and the experimental values of the molecular weight between crosslinks. Table 1 shows the results of these calculations.

As seen, for an initial crosslinking ratio of [X.sub.nom] = 0.001, the values of [X.sub.exp] and [X.sub.nom] are very similar. In [TABULAR DATA FOR TABLE 1 OMITTED] fact, on the average, 96% of the crosslinking agent added in the reaction mixture became incorporated into the matrix to yield chemical crosslinks. However, at the higher values of nominal crosslinking ratio, the experimental and theoretical results do not match as well. This is because of the limited solubility of the TMPTA in the aqueous reaction mixture. Because of the limited solubility of the TMPTA in the reaction mixture there is a limit on the amount of TMPTA that will be incorporated into the polymer network. In fact, at these higher nominal crosslinking ratios, only 12 to 27% of the crosslinking agent was incorporated into the polymer network in the form of chemical crosslinks. An additional possible reason for this discrepancy may be intramolecular cyclization.

The equilibrium swelling studies in deionized water also illustrate some of the trends expected from crosslinked polymer networks. Figure 2 is a typical example showing the relationship of swelling ratio with molecular weight between crosslinks. As seen, the polyacrylate sample with a higher molecular weight between crosslinks swells more than one with a lower value of [M.sub.c,exp]. As mentioned, this occurs because a lower molecular weight between crosslinks results in a more tightly crosslinked network that is unable to accommodate the swelling agent molecules as well as a more loosely crosslinked network (higher molecular weight between crosslinks). Thus, a [M.sub.c,exp] decreases, the swelling ratio decreases. The opposite is true for the relationship between swelling ratio and crosslinking ratio. As the crosslinking ratio decreases, the swelling ratio increases.

Short term saline swelling experiments were performed on microparticles of the polyacrylate samples. These experiments were performed by containing the microparticles in the tea bags mentioned earlier to simulate the use of these materials in disposable diapers. Again, the usual trends between swelling ratio and crosslinking ratio were expected. Figure 3 shows the swelling ratio of 40% neutralized polyacrylate microparticles as a function of time. As seen, the polyacrylate samples exhibit decreasing swelling ratio with increasing crosslinking ratio. Also, as seen by the slope of the curve, all of the polyacrylate samples exhibit a high swelling rate during the first three minutes of use than at longer times. This is a desirable behavior for absorbent polymers as the microparticles must absorb large amounts of fluid in very short times.

Swelling of microparticles of the polyacrylate samples was investigated in saline under a load of [approximately]100 g. This was done by measuring the height change of the sample as swelling occurred under load using the device described earlier [ILLUSTRATION FOR FIGURE 1 OMITTED]. The height change was then translated into a volume change using the following relation

[V.sub.s] = [h.sub.s][[Pi][D.sup.2]/4] (6)

where [V.sub.s] is the volume of the swollen gel, [h.sub.s] is the height of the swollen gel as measured by the swelling under load device, and D is the approximate diameter of the sample. The diameter was measured experimentally and taken as an average of 4.45 cm. The volume of the dry sample, [V.sub.d], was determined from Eq 6 using the initial height of the sample in place of [h.sub.s]. The initial height of the microparticles was determined experimentally and found to be an average of 0.0297 cm.

From these data, the volume degree of swelling, Q, was determined by the equation,

Q = [V.sub.s]/[V.sub.d] (7)

Figure 4 shows the swelling under load Q values as a function of time for 60% neutralized polyacrylate samples. As seen, the microparticles swell rapidly at short times, but the swelling rate eventually levels off. It is also interesting to note the differences between samples of different nominal crosslinking ratios. Samples of higher crosslinking ratios are not able to push the load as far up as the more loosely crosslinked samples because the networks are held together more tightly by the chemical crosslinks. For the same reason, the swelling rate of the samples of higher crosslinking ratio levels off in a shorter amount of time than that of the samples with lower crosslinking ratios.

The data obtained from the swelling under load experiments was also translated into mechanical property results using creep analysis (23). The stress applied to the sample was constant and determined by the relation,

[[Sigma].sub.o] = [W.sub.load]/[Pi]([D.sup.2]/4) (8)

where [W.sub.load] is the weight of the load that was 103 g, and D is again the diameter of the sample mentioned previously. The dynamic strain of the sample was evaluated by the following equation,

[Gamma](t) = [Delta]h/[h.sub.o] = h(t) - [h.sub.o]/[h.sub.o] (9)

where h(t) is the height of the sample at time t, and [h.sub.o] is the initial height of the sample. Using these results the creep compliance may be calculated using the expression given below,

J(t) = [Gamma](t)/[[Sigma].sub.o] (10)

Figure 5 shows the results of this analysis for 60% neutralized polyacrylate samples. These figures show that the rate of creep, as seen in the behavior of the compliance, is high at short times and gradually decreases to near zero. This is logical because of the fast initial swelling, or deformation, rate of these polyacrylates. The compliance may be thought of as a time dependent modulus. As seen in these figures, the compliance increases rapidly at first as the microparticles swell rapidly pushing the load upward and then levels off as the samples reach the point at which they are unable to push the load any further. At this point, the compliance reaches an equilibrium value.

Another way of analyzing this creep data is to apply the Nutting theory to the results in order to evaluate the elasticity of the polyacrylate networks. However the Nutting theory only applies to approximately the first 60% of the creep data, that is in the region where the creep rate is linear. The Nutting equation is as shown below,

J(t) = [Psi][t.sup.n] (11)

where [Psi] and n are constants. The constant n varies between zero and one and is a measure of the relative viscous and elastic contributions of the polymer to creep. If n equals zero, the material is considered perfectly elastic while if n is one, the polymer acts as a viscous liquid. Obviously, a plot of the logarithm of compliance vs. the logarithm of time in the region where the Nutting equation is valid yields the constant n in the slope. This analysis was done for the polyacrylate samples. All of the polyacrylates were found to exhibit elastic properties during the swelling under load experiments with all the values of n being [approximately]0.001. This is logical behavior for polymeric networks that swell to a high degree.

CONCLUSIONS

In this work, absorbent polyacrylates were synthesized and characterized. Crosslinked, partially neutralized poly(acrylic acid) was prepared by solution polymerization of partially neutralized acrylic acid at 37 [degrees] C. Polyacrylate samples with three different nominal crosslinking ratios and two different degrees of neutralization were synthesized. Dynamic and equilibrium swelling studies were performed on the polyacrylate samples. From the equilibrium swelling studies in deionized water, the molecular weight between crosslinks was determined. It was found that at nominal crosslinking ratios of X = 0.001, 96% of the TMPTA was incorporated into the polymer network as chemical crosslinks. However, because of the limited solubility of the TMPTA in the reaction mixture only 12 to 27% of the TMPTA was incorporated as crosslinks for the samples of higher nominal crosslinking ratios. Dynamic saline swelling studies were done on microparticles in order to investigate the swelling rate of the polyacrylates. In these studies, increased swelling with decreased crosslinking ratio was also observed as well as high initial swelling rates. The swelling of the polyacrylates under a load was also investigated. The change in height was measured as the samples swelled under a weight of 103 g. This data was used to determine creep compliance, degree of swelling under load, and elasticity. In these experiments samples of lower crosslinking ratio exhibited higher compliance and swelling under load than those of higher crosslinking ratios. Also, by applying Nutting theory to the swelling under load data, the polyacrylate samples were found to be elastic.

ACKNOWLEDGMENTS

This research was supported by a grant from Dow Chemical Co.

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Author:Bell, Cristi L.; Peppas, Nikolaos A.
Publication:Polymer Engineering and Science
Date:Jul 1, 1996
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