Effect of disperse dye on the preparation of poly(vinyl acetate)/poly(vinyl alcohol)/disperse dye composite microspheres

Materials

VAc purchased from Aldrich Co. was washed with an aqueous solution of NaHSO₃ and water and dried with anhydrous CaCl₂, followed by distillation in nitrogen atmosphere under a reduced pressure. The initiator, 2,2′-azobis(2,4-dimethylvaleronitrile) (ADMVN) (Wako Co.), was recrystallized twice in methanol before use. PVA with number–average molecular weight of 127,000 and degree of saponification (DS) of 88% (Aldrich Co.) was used as a suspending agent. Arlacel P135 (a polyethylene glycol (30) dipolyhydroxystearate, Uniquema Americas) was used as a surfactant. Aqueous disperse dye red 60 dispersion was supplied from CID Co., Ltd., Korea. The structure of disperse dye is shown in Figure 1. All polymerizations and experiments were performed using distilled and deionized water.

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Figure 1.

Chemical structure of disperse dye used in this study.

Preparation of PVAc/disperse dye microspheres

To prepare PVAc/disperse dye microspheres, suspension polymerization of VAc was conducted. Suspending agent was dissolved in water under a nitrogen atmosphere with constant stirring in a 250-mL reactor fitted with a condenser. Arlacel P135 was dissolved in 60-ml VAc monomer. After the surfactant Arlacel P135 was dissolved, aqueous disperse dye solution was added in the monomer phase prior to suspension polymerization. The mixture was then ultrasonificated at room temperatures for 5 min and poured into the reactor. The ADMVN was added at a desired polymerization temperature. After predetermined time, the reaction mixture was cooled and kept for one day to allow the precipitation and separation of the PVAc/disperse dye microspheres. The collected PVAc/disperse dye microspheres were further washed with warm water to remove residual monomer. The detailed polymerization conditions are listed in Table 1.

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Table 1.

Conditions of suspension polymerization of VAc.

Condition	Value
Type of initiator	ADMVN
Type of suspending agent	PVA
Initiator concentration	0.0001 mol/mol of VAc
Suspending agent concentration	1.5 g/dl of water
VAc/water	0.5 l/l
Rpm	300, 400, and 500
Temperature	30, 40, and 50℃
Aqueous disperse dye solution content	0, 1, and 3 wt%

Conversion of PVAc/disperse dye microspheres

The washed PVAc/disperse dye microspheres were dried in a vacuum oven at 40℃ for 48 h before weighing. The conversion of microspheres was determined by the following expression:

Heterogeneous saponification of PVAc/disperse dye microspheres

To prepare PVA/disperse dye microspheres, heterogeneous saponification of PVAc/disperse dye microspheres was conducted in a flask equipped with a reflux condenser, a thermocouple, a dropping funnel, and a stirring device. The alkali solution used for saponification contains 10 g of sodium hydroxide, 10 g of sodium sulfate, 10 g of methanol, and 100 g of water. The pre-prepared PVAc/disperse dye microspheres (1 g) were slowly added into the alkali solution at 50℃ with gentle stirring. The saponification was stopped at required time, and a PVA shell was formed on the surface of PVAc/disperses dye microspheres. After the required reaction time, the mixture was poured into cold water and kept for one day to allow the precipitation of the spherical core/shell PVAc/PVA/disperse dye microspheres. Finally, the solid saponification product was filtered and washed several times with water and dried in a vacuum at 40℃ for 48 h.

Characterization

The core/shell structure of PVAc/PVA/disperse dye microspheres was examined using optical microscope (Leica DC 100). DS of PVAc/PVA/disperse dye microspheres was determined by the ratio of methyl and methylene proton peaks in the ¹H-NMR spectrometer (Varian, Sun Unity 300).

Preparation of PVA/disperse dye microspheres

The formation of PVAc/disperse dye microspheres that were used as a precursor for preparing PVA/disperse dye microspheres, was studied at different disperse dye contents in the aqueous solution. The initiator, ADMVN, which can lower the polymerization temperature to room temperature and increase molecular weight, was used for the suspension polymerization in this study.^22–24

Figure 2(a) presents the conversion-time plots of the polymerization at different disperse dye contents using an initiator concentration of 0.0001 mol/mol of VAc. The results indicate that the rate of the polymerization decreased as the increase in the concentration of disperse dye in the solution. The rate of the polymerization was also slightly increased by adding surfactant Arlacel P135. The reduction in the polymerization rate is probably due to the reduction of the diffusion rate of both monomer molecules and polymer chains in the presence of disperse dye molecules. For the polymerization without disperse dye, Figure 2(a) indicates that the conversion of the polymerization continually increased up to ∼40 h and reached a conversion of ∼90% in spite of low-polymerization temperature (30℃), which suggests that ADMVN is an effective initiator for low-temperature polymerization.

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Figure 2.

Conversions of suspension polymerization of vinyl acetate in the presence of difference concentrations of disperse dye. ADMVN (0.0001 mol/mol of VAc) was used as initiator (a) polymerization at 30℃ and (b) effect of temperatures on the polymerization conversion of VAc with 1 wt% disperse dye.

In a free-radical polymerization, the rate of polymerization (R_p) could be expressed by equation (2). ²⁵

Heterogeneous saponification of PVA/disperse dye microspheres

It has been well known that particle size is one of the most important factors affecting the saponification rate. In order to conduct saponification study under the same conditions, the microspheres with a similar size but different chemistries (pure PVAc, PVAc/disperse dye composites with or without surfactant) were prepared by controlling the agitation speed. To maintain spherical shapes of PVAc/disperse dye particles, the saponification was carried out in alkali aqueous solution with very gentle agitation.

The effect of disperse dye on the PVAc saponification rate was examined by optical microscope observation. In this study, the heterogeneous saponifications of pure PVAc and PVAc/disperse dye microspheres were conducted under the same conditions. Figure 3 shows the optical micrographs of PVAc/PVA (a–c), PVAc/PVA/disperse dye without surfactant (d–f), and PVAc/PVA/disperse dye with surfactant (g–i) prepared by heterogeneous saponification at different reaction times. As shown in Figure 3, partially saponified microspheres (disperse dye solution content 1 wt% of monomer) with a PVAc core and PVA shell structure could be obtained by controlling the saponification degrees. It can also be seen that the disperse dye presented in the PVAc microspheres decreases the DS, and more significantly decrease of DS was observed when both disperse dye and surfactant were co-presented. Detachment phenomena of disperse dye were observed when PVAc (-OCOCH₃) changed to PVA (-OH) in the optical micrograph study. The reason for the detachment of disperse dye from the microspheres is mainly due to the decrease in the hydrophobicity of the copolymer when more PVAc is converted to PVA because disperse dye has less affinity with hydrophilic polymer. It is reasonably assumed that the addition of surfactant will increase the dye content in the microspheres, so further decreasing the saponification rate. This assumption was confirmed by ¹H-NMR measurements as shown in Figure 4, which indicates that more dye is presented in the microspheres when surfactant was added. The decrease in the saponification rate in the presence of disperse dye is also clearly shown in Figure 3, which shows a little bit different colors between partially saponificated microspheres with and without surfactant. Figure 5 shows the effect of disperse dye on the DS of PVAc microspheres. Clearly, the saponification rate decreased remarkably with increasing disperse dye content in the polymer matrix and decreased more when the surfactant was added. However, in the case of using 3 wt% disperse dye, similar DS value was obtained for the PVAc/disperse dye microspheres made in the presence and absence of surfactant.

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Figure 3.

Optical micrographs of PVAc/PVA microspheres (a–c), PVAc/PVA/disperse dye microspheres without surfactant (d–f), and PVAc/PVA/disperse dye with surfactant (g–i). The concentration of disperse dye is 1 wt% of monomer. The agitation speeds of (a–c), (d–f), and (g–i) are 500, 400, and 300 rpm, respectively. The saponification times and DS values are (a) 16 h and 72.77%, (b) 49 h and 92.82%, (c) 91 h and 98.73%, (d) 16 h and 53.93%, (e) 49 h and 89.58%, (f) 91 h and 95.59%, (g) 16 h and 31.48%, (h) 49 h and 75.17%, and (i) 91 h and 91.31%, respectively.

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Figure 4.

¹H-NMR spectra and DS values of PVAc/PVA/disperse dye microspheres without surfactant and PVAc/PVA/disperse dye with surfactant at saponification time (a)16, (b) 49, and (c) 91 h. The concentration of disperse dye is 1 wt% of monomer.

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Figure 5.

Effect of disperse dye and surfactant content on the DS of PVAc/PVA at different saponification times. The concentration of disperse dye is 1 wt% of monomer.