Tuning Pre-Solution of an Amphiphilic Polymeric Dispersant with Low Acid-Value toward Colored-Ink Preparation

Abstract

Recently, a variety of amphiphilic block copolymers for water-based color inks as pigment dispersants have been developed. Although such dispersants require both high adsorption of pigments and dispersion-stability in water, the most crucial issue is the difficulty in controlling the affinity balance of the polymeric dispersants between the pigments and aqueous media. Therefore, it is important to increase the solubility of the hydrophobic polymers with low acid-value for ink design. Amphiphilic block copolymers containing styrene-based blocks as hydrophobic segments and methacrylic acid blocks as hydrophilic segments were prepared. The polymers with low acid-value could not dissolve in the alkaline solution directly. They could dissolve in methyl-ethyl-ketone (MEK) at room temperature and diethylene glycol (DEG), with heating. Polymer aqueous solutions were successfully prepared using polymer solutions in DEG as the pre-solutions. Because they were also unable to dissolve directly even in an alkaline solution containing DEG, the existence of DEG is not important, but the process employing the pre-solution is important. The influence of pre-solution viscosity on solubility in water was evaluated. The result suggests that the high viscosity of the DEG solution would work to slowly disperse the polymers in the alkaline solution, efficiently converting polymers into an aqueous soluble state, owing to there being enough time for the neutralization of the carboxylic acids of the polymers. Note that in the pre-solution of a lower concentration, the aqueous solution did not become clear, and the larger particle sizes were detected. These results showed that the viscosity of the pre-solution is an essential factor in solubilization in water. Using this method, the polymeric dispersants with low hydrophilicity were well dissolved in water, up to a high concentration.

1. Introduction

Due to the growing awareness of environmental issues in recent years, water-based inks are increasing their market share. In particular, digital printing such as inkjet printing is growing steadily, due to the growing demand from mass production to on-demand production. Since organic pigments, which are typical colorants, are hydrophobic, they cannot be dispersed in water; therefore, the use of dispersants is essential. Polymeric dispersants are generally used under various requirements such as maintaining stability with the long-term use of ink and fixing it after printing on paper. It has been proposed that block polymers and comb-type polymers are advantageous for developing dispersion functions [1,2,3,4,5,6,7,8,9,10].

Research on dispersion stabilization using block polymers comprising cationic and non-ionic hydrophilic domains has been actively conducted [2,3,4], where adsorption by ionic interaction with the particle surface, including inorganic nanoparticles, has been carried out. When using hydrophobic and organic pigments, particularly, amphiphilic block copolymers, which have a segregated hydrophilic and hydrophobic domain, inherently form a distinct core-shell structure, and thus they are more beneficial as ink dispersants. For example, Otake et al., reported the dispersion effect on hydrophobic dye of block co-polymers comprising a cationic 4-vinyl pyridines domain and a hydrophobic acrylates domain [5], showing the dispersion performances were indeed influenced by the polymer properties. The variation in block copolymers has increased, owing to the remarkable progress of synthesis technology as shown in the above.

As described above, organic pigments are hydrophobic. The adsorption of block copolymers on dispersed pigments under an aqueous milieu is basically determined based on the hydrophobic interaction; the larger hydrophobicity would give the better loading capacity for pigments. On the other hand, the ideal is for the block copolymer to have good water solubility as a dispersant to create inks homogeneously. Thus, there is a dilemma in designing the amphiphilic block copolymer between increasing hydrophilicity and hydrophobicity. Despite the dilemma, the amphiphilic block co-polymers with increased hydrophobicity are preferably selected, considering the advantage of the high loading-capacity of the pigments.

When the water solubility of the block copolymer is too low, the water-soluble organic solvents are feasibly applied to dissolve the polymer. Although the organic-solvent amount should be within the range where mixing with the ink is acceptable, it can be effective for ink design. Using this method, polymeric dispersants were dissolved in organic solvents and transferred into water. For example, a method of preparing a dispersed polymer in methyl-ethyl-ketone (MEK), i.e., a pre-solution, and then suspending it in a large amount of water is well known [11,12,13]. However, in this method the polymers are not necessarily dissolved in water, but dispersed as particles, appearing as a cloudy solution, of a sub-micron scale in size. Although the adsorption of polymers on pigments would occur at the molecular level by mechanical methods of the inking process, including forcibly stirring, it is assumed that the homogeneity of ink, which is strongly related to the property and application of ink, can be further improved by promoting the molecular dispersion of the polymer to self-assemble before and during the ink-manufacturing process. Note that the behavior of the amphiphilic block copolymers in water would often have hysteresis of state before dispersing, i.e., the kinetics for dispersing the polymers in water is important for determining the behavior. Thus, in this study, we focused on the process of transferring pre-solutions from the solvent to the water phase, and estimated the parameters to obtain the unimodal polymer solution in water.

Herein, we designed comparisons among a series of amphiphilic block copolymers having varying amounts of styrene (St), α-methyl styrene (α-MeSt) or diphenyl ethylene (DPE), n-butyl acrylate (n-BMA), and methacrylic acid (MAA). Note that polymer domains having a monomer structure with carboxyl groups are used and regarded as having a hydrophilic structure after neutralization by a base in the ink-production procedure. An acid value of a polymer, which is generally the weight of KOH required to neutralize 1 g of a polymer, is correlated with the hydrophilicity of the amphiphilic polymers. We mainly evaluated the effect of the kind of organic solvent used as pre-solutions and their concentrations on the behaviors of the finally dispersed polymers with low acid-values under aqueous milieu. As for the organic solvents, MEK and DEG were selected because they are both water-soluble organic solvents but differ in the viscosities as pre-solutions, which would vary the dispersion kinetics of the block copolymers in the aqueous phase.

2. Experimental Section

2.1. Materials

Styrene (St, >99.0%), diphenyl ethyrene (DPE, >98.0%), tetrahydrofuran (THF, >97.9%), methyl alcohol (MeOH, 99.7+%), phenolphthalein (98%) and methyl-ethyl-ketone (MEK, >99.0%) were purchased from Fujifilm WAKO (Tokyo, Japan). A total of 2.6 M n-Butyllithium (n-BuLi), in n-hexane solution was purchased from KANTO Chemical (Tokyo, Japan). n-butyl acrylate (n-BMA, 98%), t-butyl methacrylate (t-BMA, 99%) and α-methyl styrene (α-MeSt, 99%) were purchased from Merck (Darmstadt, Germany). Diethylene Glycol (DEG, >99.5%), KOH and pyrene (98.0%), were purchased from Tokyo Chemical Industry Co. (Tokyo, Japan). Amberlite IR120B Na was purchased from Organo (Tokyo, Japan).

2.2. Synthesis and Characterization

A micro-reactor which was used for polymerization is described in detail in the Supporting Information (Figure S1, Table S1). For synthesizing, BL-01: poly(St-DPE-MAA), the micro-reactor was operated as follows: five solutions were prepared in 100 mL eggplant flasks (a)~(e) under argon atmosphere (Figure S1).

A total of 6 M St solution: St (37.5 g, 41.5 mL) was dissolved in 18.5 mL of tetrahydrofuran (THF).

A total of 1.2 M n-BuLi solution: n-BuLi (13.8 mL, 2.6 M) was added to 16.2 mL of n-hexane on ice.

A total of 1.2 M DPE solution, DPE (6.50 g, 6.5 mL) was dissolved in 23.5 mL of THF.

A total of 3.6 M t-BMA solution, t-BMA (25.6 g, 29.0 mL) was dissolved in 21.0 mL of THF.

A total of 1.5 M methanol (MeOH) solution, MeOH (2.48 g), and THF 46.9 mL were mixed.

Four flasks (a), (b), (c) and (d) were connected to the micro-reactor through 4 plunger pumps. The reactor temperature was set to 25 °C.

Firstly, St solution in flask (a) and n-BuLi solution in flask (b) were fed into the reactor at a rate of 3.9 mL/min and 1.5 mL/min, respectively, and they were mixed in M1 to initiate the living anionic polymerization of the St. Secondly, the DPE solution in flask (c) was fed into the reactor at a rate of 1.5 mL/min to terminate the St polymerization in M2. Then, the t-BMA solution in flask (d) was fed into the reactor at a rate of 3.81 mL/min, to sequentially start the polymerization from the DPE-terminated St polymer-solution in the mixture solution in M3. Finally, the reaction mixture was discharged into the MeOH solution in flask (e) to terminate the reaction.

The obtained block copolymer was treated with a cation-exchange resin to hydrolyze the tert-butoxycarbonyl group (t-Boc) of the t-BMA structures to the carboxyl group, i.e., MAA structures.

We prepared the other 4 polymers, operating the micro-reactor in the same manner. For BL-03, poly(St-α-MeSt-MAA), BL-04: poly(St-α-MeSt-MAA), and BL-05: poly(St-α-MeSt -MAA), α-MeSt was used in flask (c) instead of DPE for terminating the St polymerizations. For BL-02, poly(St-DPE-n-BMA-MAA), n-BMA were mixed with t-BMA solutions in flask (d). The synthesis scheme and parameters for operating the micro-reactor are summarized in Figure 1 and Table S1.

The obtained polymers were dissolved in Diethyl ether-d10 (99.5+%, Fujifilm Wako, Tokyo, Japan) and characterized by ¹H NMR (JNM-ECA500, JEOL, Tokyo, Japan). Number and weight average-molecular-weights were measured by size-exclusion chromatography using the HLC-8220 (TOSOH, Tokyo, Japan) system equipped with TSKgel G5000, G4000, G3000 and G2000 columns (TOSOH, Tokyo, Japan) and eluted by THF at 1.0 mL/min. Polystyrene standards were used for calibration.

2.3. Measurement of Acid Value

A hundred mg of a polymer was dissolved in 100 mL of THF with the addition of a small amount of phenolphthalein, followed by titration with 1N KOH aqueous solution. Acid value is defined as the weight of KOH (mg) needed to neutralize 1 g of the polymers.

2.4. Direct Preparation of Aqueous Polymer Solutions

Twenty-five mg of the block co-polymers were put into the 30 mL screw vials, and then 25 mL of KOH aqueous solution was added into the vials to make 0.1 wt% polymer solutions. The mixtures were treated with ultrasonic wave for 1 h. The concentrations of the KOH aqueous solutions were determined based on the acid values of the polymers, where KOH molars were adjusted to be 1.2 times in excess of the COOH molars in the polymer in the final solutions.

2.5. Preparation of Polymer Pre-Solutions in Organic Solvents

One gram of block co-polymers was dissolved in 2.57 g of MEK and DEG, which are organic solvents with water solubility. In the case of dissolving in DEG, the solutions were heated at 50~110 °C using water or oil bath. It was stirred with a magnetic stirrer.

2.6. Solubilization of Polymer in Alkaline Water from the Organic-Solvent Solution

The above prepared pre-solutions, 0.089 g of the polymer solution (0.025 g of polymer) were put in the bottom of a 30 mL screw bottle, and then 25 mL of alkaline water solutions were poured (final polymer concentration: 0.1 wt%), where the molars of KOH were adjusted to be 1.2 times in excess of the COOH molars in the polymer. Then, the sample solutions were irradiated with an ultrasonic wave for 1 h.

2.7. Measurements of Size Distribution

The obtained aqueous solutions were evaluated by dynamic light-scattering measurements using a 632 nm laser from a scattering angle of 173° (Zetasizer Nano ZS, Malvern Panalyrtical, Worcestershire, UK) to estimate a size distribution. The rate of decay in the photon correlation function was analyzed using the cumulant method and by applying the Stokes–Einstein equation to obtain a cumulant diameter.

2.8. Measurement of Critical Micelle Concentration (CMC) of Block Copolymer

A solution of 1 mM pyrene in EtOH was prepared. Separately, 10 mL of 0.1, 0.05, 0.02, 0.01, 0.005, 0.002, 0.001, 0.0001, and 0.00001 wt% polymer solutions were prepared, followed by adding 0.1 mL of the prepared pyrene solution. The mixtures were irradiated by ultrasonic wave for 15 min. Fluorescence spectra derived from pyrene in the solutions were measured by spectrofluorometer (FP-8050, Jasco Co., Tokyo, Japan) at 25 °C, where the excited wavelength was at 335 nm.

2.9. Measurement of Viscosity

DEG solutions containing various concentrations of the polymers were prepared using BL-01, BL-03, and BL-05. Viscosity was measured at 25 °C by EMS viscometer EMS-1000S (Kyoto Electronics, Kyoto, Japan).

3. Result and Discussion

3.1. Synthesis and Characterization of Block Co-Polymers

Reactions using microreactors are attracting attention as highly efficient production methods because they can regulate reactant supplies, mix the solution rapidly, and control the reaction temperature precisely. Thus, microreactor systems are expected to improve the reaction rate and selectivity compared to conventional batch reactions [14,15,16,17,18]. In the case of synthesizing polymers, they will be beneficial for obtaining narrow molecular-weight distributions, achieving high monomer-conversion rates, and producing block copolymers by a continuous process without using a special cooling device [19,20]. Thus, the block co-polymers were prepared using the microreactor system (Sanko Seiki Kogyo Co., Tokyo, Japan), where the optimized protocols were developed for synthesizing the block copolymer. The ¹H NMR spectrum of the products after the polymerization reaction showed the signal derived from monomers containing the reactant, suggesting the polymerization had successfully proceeded. The ¹H NMR spectrum of the representative spectra BL-01 before deprotection is shown in Figure 2, which is consistent with results from a previous paper [21]. The GPC measurements of the products showed a unimodal peak with narrow distributions, further supporting the successful preparation of the block copolymers in the controlled manner. Note that the acid values, which indicate the number of carboxyl groups and hydrophilicity of the polymers, well reflected well the monomer feed ratios of the t-BMA, thus suggesting that the deprotection of the tertial-butoxy groups had successfully proceeded. The molecular weights, dispersity (Đ) and acid values are shown in

Table 1. Properties of polymers.

Sample	Monomers	Mol Ratio	Mn g/mol−1	Mw g/mol−1	Mw/Mn	Acid Value
BL-01	St, DPE, MAA,	13/1/8	2679	3107	1.16	129
BL-02	St, DPE, n-BMA, MAA	3.9/1/9.1/8	2445	4077	1.67	146
BL-03	St, α-MeSt, MAA	6.8/2/5	1509	1737	1.15	143
BL-04	St, α-MeSt, MAA	8/2/13	2685	3165	1.18	271
BL-05	St, α-MeSt, MAA	24/2/14	3950	4752	1.20	152

, where block copolymers with sharp molecular-weight distributions were obtained from synthesis by microreactors.

3.2. Dissolution of Polymers in Water

Amphiphilic polymeric dispersants with carboxyl groups are usually dissolved in alkaline aqueous solutions because neutralization of the carboxylate group would facilitate the dissolution of the polymer [22]. When the alkaline solutions, which contain 1.2 times the excess molar of KOH compared to that of the carboxyl groups in the polymers, were added to the polymers, BL-02 and BL-04 were completely dissolved after 15 min of ultrasonic irradiation. However, the other polymers, BL-01, BL-03 and BL-05 were not dissolved at all. They were observed to be cloudy even after irradiation for more than 1 h, as summarized in

Table 2. Polymer solubility in alkaline aqueous solution.

Sample	Monomers	Solubility in the KOH Solution *
BL-01	St, DPE, MAA,	Insoluble **
BL-02	St, DPE, n-BMA, MAA	Soluble
BL-03	St, α-MeSt, MAA	Insoluble **
BL-04	St, α-MeSt, MAA	Soluble
BL-05	St, α-MeSt, MAA	Insoluble **

* Polymer concentration: 0.1 wt%. ** Insoluble: the solution is not transparent after ultrasonic irradiation for 1 h.

. These three polymers had low acid-values of around 150 or less, which would be low acid-values compared to a commercially available amphiphilic-polymer-dispersant, and thus, neutralization may not be enough to dissolve the polymers.

3.3. Dissolution Method with a Water-Soluble Organic Solvent

Although the three types of polymers, BL-01, BL-03, and BL-05 cannot be directly dissolved into aqueous solutions at all, the BL-02 can be dissolved in aqueous solution in spite of having a lower acid-value of 146, almost same as the three polymers. This is because BL-02 contains not only styrene but also the comparable number of butyl groups, possibly suppressing the fast agglomeration of the styrene block. This would suggest that those three polymers could be also dissolved in aqueous solutions when also efficiently eliminating or avoiding the fast agglomeration of styrene groups. Thus, we assumed that the three polymers could also be dissolved in the aqueous phase through alkaline neutralization, being slowly transferred from the dissolved state of the polymer in a water-soluble organic solvent. In this investigation, MEK and DEG were selected as the organic solvents and the solubilities of the polymers in them were evaluated at 28 wt% of the polymer concentration, which would be enough concentrated solution to finally prepare ~0.1 wt% aqueous solutions of polymers, regardless of the aqueous solvent-polarity change. The results are summarized in

Table 3. Polymer solubility in solvent.

Sample	Monomer	Solubility * in the Water-Soluble Solvent
		MEK	DEG
BL-01	St, DPE, MAA,	Soluble	Soluble(110 °C)
BL-02	St, DPE, n-BMA, MAA	Soluble	Soluble(50 °C)
BL-03	St, α-MeSt, MAA	Soluble	Soluble(50 °C)
BL-04	St, α-MeSt, MAA	Insoluble	Insoluble
BL-05	St, α-MeSt, MAA	Soluble	Soluble(90 °C)

*: 28 wt%.

. Polymers can be dissolved in MEK at room temperature and in DEG with heating, except for BL-04, which may be due to high acid-value which increases polarity and decreases the solubility in organic solvents.

Regarding the solubility in DEG, BL-03 (soluble; 50 °C) would be higher than BL-05 (soluble; 90 °C), BL-05 would be higher than BL-01 (soluble; 110 °C) (Table 3). BL-03 would have higher solubility because its molecular weight is smaller than BL-05. The α-MeSt in BL-03 and BL-05, which terminated the styrene polymerization, may be advantageous for solubility in DEG compared to DPE in BL-01. These results suggest the chemical structure between hydrophilic and hydrophobic domains and molecular weight would affect solubility in DEG.

Subsequently, the small amounts of 28 wt% polymer solutions (BL-01, BL-02, BL-03, BL-05) in MEK and DEG were mixed with large amount of KOH aqueous solutions, where KOH molars were adjusted to be 1.2 times that of the carboxyl group in the polymers, and the final polymer concentrations were adjusted to be 0.1 wt%. As summarized in

Table 4. Polymer solubility in the KOH solution from the solvent solution.

Sample	Monomer	Solubility * in		MEK Solution	DEG Solution
		MEK	DEG	→KOHaq **	→KOHaq **
BL-01	St, DPE, MAA,	Soluble	Soluble (110 °C)	Insoluble	Soluble
BL-02	St, DPE, n-BMA, MAA	Soluble	Soluble (50 °C)	Insoluble	Soluble
BL-03	St, α-MeSt, MAA	Soluble	Soluble (50 °C)	Insoluble	Soluble
BL-04	St, α-MeSt, MAA	Insoluble	Insoluble	-
BL-05	St, α-MeSt, MAA	Soluble	Soluble (90 °C)	Insoluble	Soluble

* 28 wt%. ** 0.1 wt%, 1.2 times the excess molar of KOH, compared to that of the carboxyl groups in polymers, were added to the polymers.

, in the case of the MEK solution, the alkaline aqueous solution became cloudy instantly, after mixing with the polymer solution; the polymers were not dissolved at all, even after treatment with ultrasonic waves for more than 1 h thereafter. In contrast, aqueous polymer solutions with transparency can be obtained from the polymer solutions in DEG. It should be noted that the polymers cannot be directly dissolved in the KOH solution containing 0.3 wt% DEG (Figure 3), which is the same solvent composition finally obtained when preparing 0.1 wt% polymer aqueous solution from the 28 wt% polymer solutions in DEG. Solubilities of BL-03 and BL-05 in alkaline aqueous solution were still poor, despite increasing the DEG compositions to 0.5 and 0.9 wt%, respectively, with the addition of ultrasonic wave for 1 h (Figure 3). From these results, DEG did not necessarily work, as changing the aqueous-solution nature, including the polarity, to dissolve polymers containing DEG in the aqueous solution is not important, but the process of dissolving polymers in DEG at first is important for preparing the aqueous polymer solutions; this well validates our assumed strategy for utilizing a pre-solution of polymers in a water-soluble organic solvent.

3.4. Property of Polymer Micelles

The polymers in the above prepared aqueous-solutions were observed using dynamic-light-scattering (DLS) measurements. BL-02, BL-04, and BL-05 showed small sizes of around 10 nm, and BL-01 and BL-03 showed sizes of around 20–30 nm, as summarized in Figure 4 and

Table 5. Micelle size (Number mean size) of polymers and Đ by DLS.

Sample	From Polymer Powder		From DEG Solution
	Number Means	Đ	Number Means	Đ
BL-01	-	-	21.0	0.2
BL-02	7.7	0.37	9.5	0.23
BL-03	-	-	26.9	0.37
BL-04	12.6	0.32	-	-
BL-05	-	-	12.8	0.24

. This showed that all polymers were well dispersed in water. Because BL-02 can be directly dissolved in aqueous solution, BL-02 polymer-states in aqueous solution can be compared with and without utilizing pre-solutions of DEG. As a result, the sizes were almost equal. However, from the viewpoint of Đ, the scheme of using the DEG solution yielded a smaller value, emphasizing this scheme’s benefit too. Note that these sizes would be larger than the assumed sizes of single polymers having a molecular weight of around 2000–4000 Da. It is expected that they are molecularly dispersed and self-assembled as polymer micelles in aqueous milieu. This is further suggested by the following critical-micellization-concentration (CMC) measurement.

For the investigation of the polymer dispersant behavior in the aqueous solutions, the diluted polymer solutions in the varied concentrations were evaluated with the addition of pyrene as a fluorescence probe. The amphiphilic polymers generally have CMC [23,24,25,26,27], and polymer micelles are formed at concentrations over CMC. Formation of the micelles can be detected from the change in the fluorescence-intensity ratio of the first and third vibronic-emission-bands of the added pyrenes, I₁(374 nm)/I₃(385 nm), because the pyrene-fluorescence fine structure is markedly dependent on the solvent [28], such that the intensity ratio of I₁/I₃ is sensitive to the solvent polarity: the I₁/I₃ values decrease with decreasing solvent polarity. When the polymer micelles are formed in aqueous solution, the pyrene molecules enter the hydrophobic core of the micelle, eventually declining in the polarity of the pyrene microenvironment, to observe a decrease in the I₁/I₃ values. As shown in Figure 5, all series of polymers showed the decrease in I₁/I₃ values, indicating they all have CMC, and agreeing with the DLS results by detecting the polymer micelles at 0.1 wt% polymer concentration. The polymer series, except for BL-04, showed a decrease in I₁/I₃ values at concentrations over 0.0001 wt%, and the BL-04, where the acid values were as high as 271, showed a decrease in that at 0.001 wt%. These results suggest that the polymers in the solution would behave as they are widely acknowledged, i.e., CMC become low, owing to a low acid-value, thus verifying the fact that the polymers would be dissolved in the aqueous phase, preserving their amphiphilic nature. It should be further noted that the I₁/I₃ values of BL-01, BL-03, and BL-05, which cannot be directly dissolved in aqueous solutions, seemed to plateau at polymer concentrations over 0.005 wt%, whereas those of BL-02 and BL-04 continued to decrease until the concentration was over 0.01 wt%. An increase in polymer concentration above CMC would increase the number of micelles in the solutions. Considering that the pyrene concentrations were the same in all samples, pyrene molecules would be effectively distributed inside the micelle of BL-01, BL-03, and BL-05, compared to BL-02 and BL04.This suggests that loading capacities of hydrophobic molecules such as pyrene are higher per polymer weight in BL-01, BL-03, and BL-05 than in BL-02 and BL-04. From these results, the dispersant polymer which did not easily dissolve in water would have a high loading-capacity for hydrophobic coloring materials, validating our proposed process of preparing a pre-solution of the polymers in DEG as a water-soluble organic solvent.

3.5. Characterization of Transferring from Pre-Solution

It is the central issue of this study to consider why the DEG can work for obtaining aqueous solutions of BL-01, BL03, and BL-05 but MEK cannot. The evident difference between the DEG and MEK solutions was the speed of dispersing into the aqueous phase, based on the viscosity of the solutions; probably, the MEK solutions are too rapidly dispersed into the aqueous phase to fully neutralize the polymer by KOH at the molecular level, thus dispersing as aggregation, due to the shortage of hydrophilicity. On the other hand, DEG solutions of BL-01, BL-03, and BL-05 were slowly dispersed into aqueous solutions, eventually converting into aqueous polymer solutions, probably owing to enough time for neutralization of the polymers at the molecular level. To verify this consideration, we changed the polymer state in the aqueous phase by controlling the speed of dispersing the pre-solution of the polymers into the aqueous solution, which would be modulated by the viscosity of the pre-solutions.

Then, the DEG pre-solutions of BL-01, 03, and 05 at varied concentrations were prepared, to change the viscosity, and were all clear and viscous solutions. As shown in Figure 6, we successfully obtained the DEG pre-solutions having a viscosity in the range from below 100 mPa⋅sec to above 1000 mPa⋅sec.

Subsequently, the DEG solutions were settled on the bottom of the sample bottles, followed by pouring large amounts of KOH aqueous solution and irradiating the ultrasonic wave for 1 h, whereby the polymer concentration finally became 0.1 wt% aqueous solution and KOH molars were 1.2 times in excess of those of the carboxyl groups contained on the polymers. When using the pre-solutions of 10 wt% BL-01 and BL-03, transparencies of the finally obtained aqueous-solutions were low, whereas the pre-solutions with the higher polymer concentrations made transparent aqueous solutions of the polymers (Figure 7 and Figure 8a). Furthermore, larger particle sizes were detected in the aqueous solutions prepared from the pre-solution with the lower concentration of the BL-01 and BL-03, i.e., with lower viscosity (Figure 8b). These results indicate that the viscosity of the pre-solution would be a crucial parameter to determine the polymer state in the aqueous solution, as we expected. On the other hand, all of the BL-05 solutions among those investigated were transparent (Figure 7c), showing that the particle size of BL-05 did not change at any concentration (Figure 8b). There might be a lower concentration range which BL-05 would not solve. This phenomenon is probably due to the slower diffusion of BL-05 from the DEG solution to the aqueous solution, expected by its larger molecular weight compared to the other two (BL-01, BL-03).

3.6. Preparation of Concentrated Aqueous Solutions from Pre-Solutions

In the end, the practically useful polymer-dispersant solutions should preferably have a much higher concentration than the solution we have prepared above. To further verify the feasibility of the DEG pre-solution methods for application on coloration materials, we prepared 4.4 and 8.0 wt% BL-05 aqueous solutions, where 0.893 g and 1.786 g of DEG solution of 28 wt% BL-05 were weighed and mixed with 5 mL alkaline solutions in a screw vial under the irradiation of an ultrasonic wave. As a result, although the solution containing 8.0 wt% of BL-05 seemed to be somewhat cloudy, we successfully obtained the molecularly dissolved aqueous solution containing 4.4 and 8.0 wt% of BL-05. It should be further noted that these solutions did not make sediments, and the dominant size-distribution of the dispersed polymers were observed at 10–20 nm by DLS measurements, even for the 8.0 wt% polymer solutions. To the best of our knowledge, this would be the first report of obtaining a solution with as high a concentration as 4.4 and 8 wt% of polymer solutions, where the polymer has quite a low acid-value of 150, further emphasizing a benefit of our proposed methods using the DEG pre-solution. Because studied polymers with low acid-value would have an advantage regarding the loading capacity of hydrophobic molecules such as the pyrene as studied above, these polymers are used as a dispersant even at high pigment concentrations of water-based ink of around 10 wt%. Our proposed methods are beneficial for a polymer dispersant of coloration materials such as those usually used as concentrated pigments. If the dissolved polymer aqueous solution is in equilibrium with the pigment surface, the polymer molecules would be efficiently supplied to the pigment surface even in a concentrated-ink system. A water-based ink is an ink that has pigments in a colloidal suspension in a solvent. Although the main solvent in water-based inks is water, there can also be other co-solvents present. These co-solvents typically are volatile organic compounds (VOCs). The goal of using water-based inks is just to reduce the VOCs that are present. Concentrated aqueous dispersant-solutions prepared from concentrated dispersants dissolved in water-soluble organic solvents meet this goal well due to their low VOC-content. Although there is a need for continuing research into improvement of this, including the optimization of polymers’ hydrophobic structures and further control of the dispersion kinetics of DEG in the aqueous phase, this method would extend the variety of polymers applicable for use as a dispersant.

4. Conclusions

The series of amphiphilic block copolymers for use in colored-ink dispersant application were synthesized with varying molecular weights and acid-values by changing the monomer composition of St, DPE, α-MeSt, n-BMA and MAA. Polymers with low acid-values cannot be dissolved in KOH aqueous solution directly, but by utilizing DEG pre-solutions of these polymers we can successfully obtain the aqueous solution of these polymers. The obtained polymer solutions showed the unimodal size-distribution of around 10 to 30 nm, and the low CMC and high loading-capacity of hydrophobic molecules of pyrene, which is the expected behavior of polymers with low acid-values, suggesting the polymers have well-dissolved state at molecular level in aqueous solutions. Of note is the fact that the well-dissolved water solution cannot be obtained from the MEK pre-solutions of these polymers, which are often the dominant choice for obtaining a water solution of polymers with low water-solubility. The high viscosity of the DEG solutions would probably work to slowly disperse the polymers in the KOH aqueous solutions, efficiently converting polymers into an aqueous soluble state, owing to enough time for the neutralization of the carboxylic acids of the polymers. This hypothesis was verified by the results showing that these polymers cannot be dissolved directly in KOH aqueous solutions containing a comparable amount of DEG and that the viscosity of the polymer pre-solution in DEG had an influence on the polymer dissolved-state in the final aqueous solution.

These results show the potency of utilizing pre-solutions of polymers in DEG to control or change the polymers’ behavior in aqueous solution at molecular level, even though the polymers have low solubility in water, due to low acid-values. It should be noted that we confirmed that utilizing the pre-solution of DEG is beneficial for obtaining well-dispersed polymer aqueous solution of a concentration at 4.4 and 8 wt%, suggesting this method would have the potency for transfer to practical use in the manufacturing stage of colored ink containing pigments. Well-solubilized polymer molecules would penetrate the aggregates of the pigment particles and load the pigments efficiently, and therefore, the proposed scheme is promising for the extension of ink design and also for the properties of the ink.