Estimation of distribution state of carboxyl groups within submicron-sized, carboxylated polymer particle with isothermal titration calorimeter

Figure 1 shows ITC charts at 25 °C for 1 ml of P(BMA–MAA) (a) and PS (b) emulsions, and distilled water (c) with 25 μl of 0.05 N KOH aqueous solution in which the amount of KOH was equivalent to two times that of carboxyl groups in the P(BMA–MAA) emulsions. Heat evolved in the chart (b) for the PS emulsion included the heats of mixing and dilution, which were estimated from the heats evolved in the chart (c) for the distilled water, in addition to that of the neutralization reaction. In comparison with the charts of (b) and (c), the area of the first peak was much larger than that of the distilled water, and their second and third peaks were the same. These indicate that in the first addition of KOH aqueous solution into the PS emulsion that had sulfate groups as initiator end groups at the particle surfaces, the neutralization reaction of surface groups was completely over within 100 s. On the other hand, in the chart (a) of the “batch” P(BMA–MAA) emulsion, which was prepared by batch emulsion copolymerization, the heat evolved still beyond 200 s, and it decreased with the number of the KOH addition. These results suggest that the titration of carboxyl groups by KOH proceeds gradually from the surface to the inside of the P(BMA–MAA) particle.

Figure 2 shows the relationships between the measurement temperature of ITC and the accumulated percentage of the carboxyl groups titrated with the addition frequency of KOH. With an increase in the frequency of KOH addition, the percentage of the neutralized carboxyl groups was almost the same below 25 °C, but it was clearly increased above 25 °C which corresponds to Tg of the base polymer. These results indicate that at the temperature below the Tg, carboxyl groups at the particle surface layer are only titrated because KOH does not diffuse into the inside of the particles, and above Tg, the neutralization of carboxyl groups proceeds in the inside because excess amount of KOH seems to be necessary to neutralize carboxyl groups inside the particle.

KOH and the sample emulsion were, respectively, replaced each other to ITC cell and injection syringe, where 1 ml of 0.1 N KOH in the cell was equivalent to about 90 times amount of carboxyl groups in 25 μl of P(BMA–MAA) emulsion, of which solid content was around 8 wt%, in the syringe. In this titration system, the ratio of the total heat of mixing and dilution to all the evolved heat was decreased below 0.1. A good liner relationship was established between the concentration of MAA aqueous solution and the heat of neutralization reaction as shown in Fig. 3.

P(BMA–MAA) emulsion was also prepared by emulsion copolymerization with uniform-feed monomer addition method, which were named “feed” P(BMA–MAA) emulsion, under the same recipe as the “batch” P(BMA–MAA) emulsions. The batch and the feed P(BMA–MAA) emulsions were separately mixed with H⁺ form cation-exchange resin for 5 h to change from –COO⁻ to –COOH after ultracentrifugal washing with distilled water.

Figure 4 shows the relationships between the titration time and the percentages of carboxyl groups titrated with 0.1 N KOH (1 ml) at 35 °C for the batch (a) and the feed (b) P(BMA–MAA) emulsions (25 μl). In the feed emulsion, only 17% of the total carboxyl groups were titrated in 80 s and then the percentage was increased linearly by 0.5%/min. Because the feed emulsion copolymerization proceeded under a monomer-starved state, it seems that the polymer composition had a sharp distribution, which should result in homogeneous distribution of the carboxyl groups within the particle. On the other hand, in the batch emulsion, 56% of the total carboxyl groups reacted in 80 s, and then 40 min was needed to increase the percentage by 10%. These results suggest that the batch emulsion had many carboxyl groups near the particle surface and the amount decreased toward the center of the particle.

Figure 5 shows variation of the MAA content in residual monomers throughout the batch emulsion copolymerization. In the conversion range of 25 to 65%, the MAA content in the batch emulsion was almost steady at 7 mol% because the monomers supplied constantly from monomer droplets, and then increased markedly, around the conversion of 70% at which the monomer droplets disappeared. P(BMA–MAA) formed at a final stage above 90%, had high MAA contents over 30 mol% as shown in Table 2. This result indicates that in this copolymerization, BMA was predominantly copolymerized and MAA-rich copolymer was prepared in the final stage.

The reactivity ratio between BMA (monomer 1) and MAA (monomer 2) are as follows: r ₁=1.15 and r ₂=0.73 [14]. These values indicate that BMA is predominantly polymerized, resulting in MAA-rich copolymer at the latter stage of the batch emulsion polymerization. That is, these are good agreement with the above ITC data that many carboxyl groups existed near the particles’ surface.

In addition, although 75% (based on total MAA polymerized) of carboxyl groups in the batch P(BMA–MAA) particles were titrated for 40 min at 35 °C, it was almost the same as that titrated at 50 °C. The total amount of carboxyl groups included PMAA-rich copolymers and undecomposed V-501 dissolved in the aqueous medium, which was also measured with ITC, corresponded to 15.3% of the carboxyl groups. These results indicate that about 90% of the carboxyl groups within the batch P(BMA–MAA) particle were titrated under the conditions. The copolymer composition was analyzed by ¹H NMR, but there was no repeatability in the integration because the amount of fed MAA was small. The composition analysis needs further consideration.

From the above results, it is concluded that the estimation of carboxyl groups of carboxylated particles with ITC gives a useful information about the distribution state of carboxyl groups within the particle even with an extremely small amount of sample.