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Confocal Raman Spectroscopy Studies on the Mutual Diffusion Behavior at the Interface Between Two Different Polyesters.

INTRODUCTION

Toner for electrophotography comprises binder polymers, wax, pigments, and a charge control agent, and its particle size is approximately 4-7 [micro]m. The binder polymer is polyester or styrene acrylic polymer with a number-average molecular weight ([M.sub.n]) of approximately 2,000-4,000 g/mol and a weight-average molecular weight ([M.sub.w]) of 10,000-30,000 g/mol. From the viewpoint of low-temperature fusion onto paper, polyester has been widely used as a toner binder polymer. In the current electrophotographic printer market, demand exists for toners with lower fusing temperatures that can fuse well to paper with less heat input from the fuser. Although the low-temperature fusing ability of a toner can be greatly improved through the use of lower glass-transition temperature ([T.sub.g]) polyester or lower-molecular-weight polyester as a toner component, other performance attributes of the toner, such as storage stability and durability, can be adversely affected. Thus, toners are required to exhibit both good heat resistance in storage and good low-temperature fusing performance; these two traits are contradictory.

To satisfy both requirements, researchers have proposed toners with a core-shell structure [1], where a low-[T.sub.g] polyester is used as the core component to enhance low-temperature fusion and a high-[T.sub.g] polyester is used as the shell component to enhance heat resistance during storage. Figure 1 shows the schematic of a core-shell-structured toner. Achieving good adhesion between the core and shell components requires treatment at a temperature greater than the [T.sub.g] of the high-[T.sub.g] polyester. During the annealing process, mutual diffusion of the two polyesters occurs, and precise control of interfacial thickness between the core and shell is necessary within a 20-300 nm range because the size of toner particles is approximately 4-7 [micro]m. Therefore, studies on the annealing-induced mutual diffusion of different polyesters are important. Toward this end, studies on the extent to which polymers' physical properties, such as [T.sub.g], molecular weight, and chemical structure, influence the process of interface formation and interfacial thickness are necessary. That is, the interfacial behavior in polymer blends has been a topic of intensive research. Various techniques, including neutron reflectometry [2, 3], ellipsometry [4-6], secondary-ion mass spectrometry [7], Raman spectroscopy [8], infrared spectroscopy [9], X-ray reflectometry [10], and energy-filtering transmission electron microscopy [11, 12], have been used to analyze the interfacial behavior in polymer blends. In the case of core-shell-structured toner for electrophotography, two polyesters with different chemical structures, different [T.sub.g]s, or different molecular weights are often used to fabricate the core-shell structures.

In this study, to obtain the interfacial concentration profiles of different polyesters, we introduce a confocal Raman spectroscopy method and investigate the mutual diffusion behavior at the interface between two different polyesters. Furthermore, the formation process of the interfacial layer during annealing is reported.

EXPERIMENTAL

Materials

Two types of polyesters were synthesized. Poly(propylene isophthalate) (PG-PES) was synthesized via the following procedure. Propylene glycol (PG) (5.0 mol) and isophthalic acid (IPA) (4.0 mol) were mixed together in a flask with 0.3 wt% dibutyl tin oxide as a polymerization catalyst. The reaction was first performed at 180[degrees]C for 4 h; the temperature was gradually raised to 220[degrees]C for 8 h, and the reaction was continued at 8.3 kPa for 1 h to obtain the isophthalate diester, named PG-PES. Poly([alpha],[alpha]'-[(l-methylethylidene)di-4,l- phenylene]bis [oj-hydroxypoly[oxy(methyl-l,2-ethanediyl)]]isophthalate) (BPA-PES) was synthesized via the following procedure. [alpha],[alpha]'- [(lMethylethylidene) di-4,1 -phenylene]bis[a>-hydroxypoly[oxy(methyl1,2-ethanediyl)]] (5.0 mol) and IPA (4.0 mol) were mixed in a flask with 0.2 wt% dibutyl tin oxide as a catalyst. The reaction was performed at 235[degrees]C for 6 h and then at 8.3 kPa for 1 h to obtain the isophthalate diester, named BPA-PES. The chemical structures of the two polyesters are shown in Figure 2.

Measurement of Molecular Weight Distribution

Each polyester was dissolved in chloroform at a concentration of 0.5 g/100 mL. The resultant solution was then filtered through a fluororesin filter (FP-200, Sumitomo Electric Industries, Ltd., Japan) with a 2.0 [micro]m pore size to remove insoluble components, resulting in a sample solution. Chloroform as an eluent was allowed to flow through [GMH.sub.XL] + [G3000.sub.XL] tandem chromatography columns (Tosoh Corp., Japan) at a flow rate of 1.0 mL/min, and the columns were stabilized using a CO-8010 column heater (Tosoh Corp., Japan) thermostated at 40[degrees]C. One hundred microliters of the sample solution were injected into the columns to measure the sample's molecular weight distribution. The molecular weight of each sample was calculated on the basis of a calibration curve of previously prepared polystyrene standards. The detector used in this measurement was Refractive Index Detector. The molecular weights of PG-PES and BPA-PES are listed in Table 1.

Preparation of Narrow-Dispersed Polyesters

After 8.0 wt% polymer (PG-PES or BPA-PES) dissolved in chloroform was prepared, polymer fractionation was conducted on the basis of the elution time; polyesters with a narrow molecular-weight dispersion were obtained using an LC-9110II high-performance liquid Chromatograph (Japan Analytical Industry Co., Japan). Chloroform as an eluent was allowed to flow through the aforementioned tandem column at a flow rate of 10 mL/min, and the column was stabilized under thermostatic control at 40[degrees]C. The [M.sub.n] and [M.sub.w] of the obtained polyesters were calculated from the molecular weight distribution measured using the aforementioned procedure. The molecular weights of narrow-dispersed PG-PES (ndPG-PES) and BPA-PES (ndBPA-PES) are listed in Table 1, and their molecular weight distributions are shown in Figure 3. We confirmed that the low- and highmolecular-weight components of both PG-PES and BPA-PES were completely removed and that the peak of the [M.sup.n] and [M.sub.w] for each polyester was approximately the same after fractionation.

Measurement of Glass-Transition Temperature ([T.sub.g])

The Tg was measured by differential scanning calorimetry (DSC) (DSC Q200, TA Instruments Japan, Japan) at a heating rate of 10[degrees]C/min under nitrogen. To maintain the same thermal history, each sample was preheated from room temperature to 180[degrees]C and cooled to -20[degrees]C at a cooling rate of 10[degrees]C/min. The DSC data were then recorded as the sample was heated from -20 to 100[degrees]C. The [T.sub.g] values for the two polyesters are listed in Table 1.

Measurement of Phase Behavior by Optical Microscopy

Solutions with a concentration of 5.0 wt% were prepared using tetrahydrofuran (THF) for PG-PES/BPA-PES and ndPGPES/ndBPA-PES with different compositions (30/70, 50/50, and 70/30). Each solution was cast onto a cover glass, and the solvent was evaporated quickly at 40[degrees]C to prepare the transparent specimens. The specimens were dried in a vacuum oven at 80[degrees]C for 24 h and then isothermally annealed for 48 h under flowing nitrogen at different temperatures ranging from 60[degrees]C to 160[degrees]C. The phase behavior was checked on the basis of the appearance of a phase-separated morphology, as observed under an optical microscope.

Measurement of Interfacial Thickness by Confocal Raman Spectroscopy

Bilayer films were prepared for measurement of the interfacial thickness between the PG-PES and BPA-PES specimens. First, to ensure that the thermal histories of the specimens were identical, we annealed PG-PES and BPA-PES plates at 100[degrees]C for 5 min. Both plates were then stacked and annealed at different temperatures (70, 80, 100, 120, and 160[degrees]C) for various times on a hotplate. After cooling to room temperature, the bilayer films were sliced by an ultramicrotome into thin sections, exposing the polyester interfacial region. A confocal Raman microspectrometer (Nanofinder 30, Tokyo Instrument Inc., Japan) was used to analyze the PG-PES/BPA-PES interfaces. A He-Ne laser (05-LHP-928, Melles Griot, Japan) with a 632.8 nm wavelength was focused on a sample; the laser power at the sample was approximately 10 mW. A 40X objective lens (Plan Fluor, Nikon, Japan) with an NA of 0.6 was used. The backward Raman scattering was collected using the same objective. After passing through a 100 [micro]m pinhole, the scattered light was introduced into the spectrometer and detected by a charge-coupled device (CCD) detector (DU401A BR-DD, Andor, UK). All the spectra were background subtracted and intensity-corrected. In the line-scanning measurements, the excitation laser spot was scanned across a sample. The laser spot was horizontally translated by galvano-mirrors with a 0.2 [micro]n step in the lateral direction. The spot size of the laser beam on the sample was approximately below 1.0 [micro]m. The lateral resolution of the apparatus was confirmed to be approximately below 1.0 pm based on the measurement of PS bead with 1.0 pm as calibration sample. The exposure time for each spectrum was 10 s. Figure 4 shows the Raman spectra of PG-PES and BPA-PES.

Mutual diffusion behavior across PG-PES/BPA-PES interfaces and across ndPG-PES/ndBPA-PES interfaces was investigated in the case of different annealing temperatures or the use of polymers with different molecular weights. The relative concentration profile derived from BPA-PES ([[phi].sub.BPA]) was obtained using the following Eq. 1:

[mathematical expression not reproducible] (1)

where [mathematical expression not reproducible] arrowed in Figure 4 is the experimental peak assigned to molecular vibration resulting from the change of bond angle of CCC in aromatic ring attribute to BPA-PES [13].; [mathematical expression not reproducible] is the experimental peak assigned to ring breathing vibration of 1,3-displacement of isophthalate in both-PES. The peak intensity should be same in all regions, [mathematical expression not reproducible] and [mathematical expression not reproducible] are not attributed to specific chemicals, which means just base points; and d indicates distance. Interfacial thickness ([lambda]) is defined as the distance between 0% of [[phi].sub.BPA] and 100% of [[phi].sub.BPA] obtained using Eq. 1.

RESULTS AND DISCUSSION

Phase Behavior

To confirm the miscibility of the PG-PES/BPA-PES blends and ndPG-PES/ndB PA-PES blends, the phase behaviors at different compositions (30/70, 50/50, and 70/30) were checked by optical microscopy for the appearance of a phase-separated morphology at different temperatures from 60[degrees]C to 160CC. No phase-separated morphology was observed in any of the samples in the temperature range from 60[degrees]C to 160[degrees]C, thus, confirming that both the PG-PES/BPA-PES and ndPG-PES/ndBPA-PES blends were miscible in this temperature range.

Interfacial Thickness (k)

To analyze the [lambda] between the PG-PES and BPA-PES, Raman spectroscopy measurements were carried out after the bilayer specimens were annealed at 100[degrees]C for 10, 30, 60, and 120 min. Figure 5 shows the PG-PES and BPA-PES concentration profiles across the PG-PES/BPA-PES interfaces. The [lambda] increased with increasing annealing time. We presumed that mutual diffusion occurred by annealing. Moreover, as shown in Fig. 5c and d, the concentration profiles indicate asymmetric behavior between the PG-PES and BPA-PES regions. This asymmetry indicates that the PG-PES components diffuse deeply into the BPA-PES regions one-sidedly. That is, the BPA-PES components diffuse into the PG-PES region but the diffusion speed is much slower than that of PG-PES. In a previous study, we observed asymmetric concentration profiles in poly(methyl methacrylate) and styrene-acrylonitrile random copolymer (SAN) interfaces, and the diffusion behavior was affected by the AN content in SAN [11]. As shown in Figure 3, PG-PES has low-molecular-weight component; we speculate that this component can easily diffuse into the BPA-PES region.

Figure 6 shows the variation of [T.sub.g] as a function of the molecular weight of PG-PES and BPA-PES. In the Mw region below 2,000 g/mol, the [T.sub.g] of PG-PES is lower than that of BPA-PES; this result indicates that the low-molecular-weight component of PG-PES exhibits greater mobility than that of BPA-PES. The asymmetric diffusion appears to be due to the existence of a low-molecular-weight component with a lower [T.sub.g] in PG-PES.

To confirm the influence of the low-molecular-weight components of both polyesters, interfaces across the ndPG-PES and ndBPA-PES blends were analyzed. Figure 7 shows the PG-PES and BPA-PES concentration profiles across the ndPG-PES/ ndBPA-PES interfaces. In the case of annealing times of (b) 60 min and (c) 120 min, less asymmetric behavior was observed than in the case of the PG-PES/BPA-PES interfaces. Moreover, the [lambda] values of the ndPG-PES/ndBPA-PES were 7.5 and 10.0 [micro]m for the interfaces annealed for 60 and 120 min, respectively; these [lambda] values are smaller than those of the PG-PES/BPA-PES interfaces annealed for the same times (in Figure 5c and d). With respect to the molecular weight distribution of narrow-dispersed samples, not only the concentrations of components with a molecular weight less than 3,000 g/mol but also the concentrations of components with a molecular weight greater than 10,000 g/mol decreased. When the molecular weight distribution is considered, the results show that the decrease of [lambda] of ndPGPES/ndBPA- PES is due to the decrease in concentration of low-molecular-weight components rather than the decrease in the concentration of high-molecular-weight components. We also observed that a decrease in the amount of low-molecular-weight components could suppress one-sided diffusion, thereby potentially enabling precise nano-order control of [lambda].

Apparent Mutual Diffusion Coefficient

In the next step, to compare the difference in diffusion behavior between PG-PES/BPA-PES and ndPG-PES/ndBPAPES, we plotted the relation between [lambda] and annealing time. Figure 8 shows time variations of [lambda] of PG-PES/BPA-PES and ndPG-PES/ndBPA-PES at 100[degrees]C. As shown in Figures 7 and 8, the [lambda] of ndPG-PES/ndBPA-PES was smaller.

According to the polymer-polymer interdiffusion theory proposed by Brochard et al. [14], the time variation of [lambda] is given by the following equation:

[lambda] = 2y/[square root of [D.sub.M]t], (2)

where [D.sub.M] is the mutual diffusion coefficient and t is the diffusion time. This equation and theory are not generally applicable in the case of asymmetric diffusion behavior. However, in the present study, to compare the [D.sub.M] of PG-PES/BPA-PES and [D.sub.M] of ndPG-PES/ndBPA-PES, we tentatively applied this equation in asymmetric systems. The [D.sub.M] is regarded as the apparent [D.sub.M], and it was calculated on the basis of Eq. 2. The apparent [D.sub.M] values for PG-PES/BPA-PES and ndPG-PES/ndBPA-PES were 5.9 X [10.sup.-10] [cm.sup.2]/s and 2.2 X [10.sup.-11] [cm.sup.2]/s, respectively. The results show that the apparent [D.sub.M] is dependent on the molecular weight distribution; specifically, its value changes by one order of magnitude with decreasing concentration of the low-molecular-component.

Influence of Annealing Temperature and Molecular Weight on the Apparent [D.sub.M]

To clarify how the apparent [D.sub.M] of PG-PES/BPA-PES depends on the annealing temperature, we annealed bilayers of PG-PES/BPA-PES or ndPG-PES/ndBPA-PES at 70, 80, 100, 120, and 160[degrees]C for various times. The apparent [D.sub.M] at different temperatures was calculated using Eq. 2. Figure 9a shows the temperature variation of the apparent [D.sub.M] of PG-PES/BPA-PES and ndPG-PES/ndBPA-PES. The apparent [D.sub.M] value of PG-PES/ BPA-PES changed from [10.sup.-13] to [10.sup.-18] [cm.sup.2]/s (five orders of magnitude) and the apparent Z?M value of ndPG-PES/ndBPAPES changed from [10.sup.-15] to [10.sup.-18] [cm.sup.2]/s (seven orders of magnitude) when the specimens were annealed in the temperature range from 70[degrees]C to 160[degrees]C. These results suggest that the X between the core and shell polyesters could be controlled through changes in the annealing temperature. Furthermore, from the Arrhenius plots of Figure 9b, obtained activation energy of PG-PES/BPA-PES and ndPG-PES/ndBPA-PES was 685.3 kJ/mol and 899.1 kJ/mol, respectively. It was also found that the activation energy could be decreased by the existence of low molecular weight component.

Also, to clarify how the apparent [D.sub.M] depends on the molecular weight, PG-PES, BPA-PES, ndPG-PES, and ndBPA-PES with different molecular weights were obtained via synthesis and subsequent polymer fractionation using the LC-9110II high-performance liquid Chromatograph. The molecular weights of the obtained polyesters are listed in Table 2. Bilayer specimens of PG-PES/BPA-PES and ndPG-PES/ndBPA-PES pairs with similar [M.sub.w] of approximately 5,500 g/mol (former pair), 9,000 g/mol, 12,000 g/mol, and 17,000 g/mol were then annealed at 100[degrees]C for various times. The apparent [D.sub.M] with different molecular weights was then estimated using Eq. 2. Figure 10 shows the average molecular weight (Mw) variation of the apparent [D.sub.M] of PG-PES/ BPA-PES and ndPG-PES/ndBPA-PES. In the case of both PGPES/BPA-PES and ndPG-PES/ndBPA-PES, the apparent [D.sub.M] decreased with increasing [M.sub.w]. Specifically, the apparent [D.sub.M] of ndPG-PES/ndBPA-PES was smaller by two orders of magnitude compared to that of BPA-PES/PG-PES in similar Mw regions. Although both samples have similar [M.sub.w]s, their molecular weight distributions differ and the narrow-dispersed samples do not include a low-molecular-weight component with a low [T.sub.g]. The low-molecular-weight component strongly influences the apparent [D.sub.M], which varies by two orders of magnitude. Furthermore, the change in the behavior of the apparent Z)M shows a clear inflection point at approximately [M.sub.w] = 9,000 g/mol in both cases. In a previous paper, the molecular weight between entanglement points of PG-PES was reported to be 5,000-9,000 g/mol [15]. Given these previous results, we assumed that the polymer chain of PG-PES and BPA-PES may be entangled with each other in the approximately 5,000-9,000 g/mol region, which would result in the dramatically slow movement of polymers and a lower apparent [D.sub.m]. We observed that the value of the apparent [D.sub.M] decreased from [10.sup.-9] to [10.sup.-12] [cm.sup.2]/s when [M.sub.w] was increased from ~5,000 g/mol to ~ 18,000 g/mol, when the molecular weight distribution was changed, or when the molecular weight was controlled to be within the entanglement points of polymers. The results showed that the A between core-shell interfaces could be controlled not only through manipulation of the annealing temperature but also through variation of the molecular weight distribution.

CONCLUSIONS

To control the core-shell structure of toner, we studied mutual diffusion at the polyester-polyester interface using a confocal Raman spectroscopy technique. The [lambda] between PGPES/BPA-PES for toner binder resin increased with increasing annealing time. As the annealing time was increased, the interfacial concentration profiles showed asymmetric behavior. By contrast, less asymmetric behavior was observed in the case of ndPG-PES/ndBPA-PES. This result indicates that the low-molecular-weight component of PG-PES diffuses one-sidedly into the BPA-PES region. We observed that, by decreasing the amount of the low-molecular-weight component, one-sided diffusion could be suppressed, potentially enabling precise nano-order control of the [lambda].

Furthermore, we observed that the value of the apparent [D.sub.M] obtained via Raman spectroscopic analysis increased from [10.sup.-15] to [10.sup.-8] [cm.sup.2]/s when the annealing temperature was increased from [T.sub.g] + 10[degrees]C to [T.sub.g] + 90[degrees]C or when the molecular weight distribution was changed. In addition, the apparent [D.sub.M] decreased from [10.sup.-9] to [10.sup.-12] [cm.sup.2]/s when the changing molecular weight was increased from approximately 5,000 g/mol to approximately 18,000 g/mol. To fabricate toner with a core-shell structure, the precise control of [lambda] between the core part and shell part is necessary within a 20-300 nm range. The results related to the apparent [D.sub.M] determined via Raman spectroscopy indicated that [lambda] could be effectively controlled within the range from 20 to 300 nm via (1) decreasing the amount of the low molecular weight component, (2) annealing at [T.sub.g] + 10[degrees]C, and (3) using a polyester with an [M.sub.w] greater than 9,000 g/mol; thus, the variation of factors other than molecular weight between entanglement points of polymers was effective in controlling A. With the combination of these polyesters, a [lambda] within the range from 20 to 300 nm could be possible with an annealing time on the order of a few minutes even though the polyesters are miscible. We observed that toner with a core-shell structure could be improved through controlling the diffusion behavior at the interface of the aforementioned polyesters.

ABBREVIATIONS
PG-PES      Poly(propylene isophthalate)
BPA-PES     poly([alpha], [alpha]'-[(1-methylethylidene)di-4,
            1-phenylene]bis
            [[omega]-hydroxypoly[oxy(methyl-l,2-ethanediy1)]]
            isophthalate)
[M.sub.n]   number-average molecular weight
[M.sub.w]   weight-average molecular weight
[D.sub.M]    mutual diffusion coefficient
[T.sub.g]   glass transition temperature


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Norihiro Fukuri, (1,2) Toshiaki Ougizawa [(ID).sup.2]

(1) Kao Corporation, 1334, Minato, Wakayama-city, Wakayama, 640-8580, Japan

(2) School of Materials and Chemical Technology, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo, 152-8552, Japan

Correspondence to: T. Ougizawa; e-mail: tougizawa@op.titech.ac.jp DOI 10.1002/pen.24656

Caption: FIG. 1. Schematic of a core-shell-structured toner particle.

Caption: FIG. 2. Chemical structures of the polyesters used in this study; (a) PG-PES, (b) BPA-PES.

Caption: FIG. 3. Molecular weight distribution of polyesters: PG-PES (black, thin), BPA-PES (black, bold), narrow-dispersed PG-PES (ndPG-PES) (gray, thin), and narrow-dispersed BPA-PES (ndBPA-PES) (gray, bold).

Caption: FIG. 4. Raman spectra of PG-PES (solid) and BPA-PES (dotted).

Caption: FIG. 5. PG-PES and BPA-PES concentration profiles across the PG- PES/BPA-PES interfaces (specimens annealed at 100[degrees]C for (a) 10, (b) 30, (c) 60, and (d) 120 min).

Caption: FIG. 6. Molecular weight variation of the T% of PG-PES (diamonds and dotted line) and BPA-PES (squares and dashed line).

Caption: FIG. 7. PG-PES and BPA-PES concentration profiles across the ndPG- PES/ndBPA-PES interfaces (specimens annealed at 100[degrees]C for (a) 10, (b) 60, and (d) 120 min).

Caption: FIG. 8. Time variations of X of PG-PES/BPA-PES (triangles) and ndPGPES/ndBPA-PES (circles) (specimens annealed at 100[degrees]C).

Caption: FIG. 9. (a): Time variations of apparent [D.sub.M] of PG-PES/BPA-PES (triangles) and ndPG-PES/ndBPA-PES (circles) (b): inverse temperature variation of apparent [D.sub.M] of PG-PES/BPA-PES (triangles) and ndPG-PES/ndBPA-PES (circles).

Caption: FIG. 10. Average molecular weight variations of apparent [D.sub.M] of PG-PES/ BPA-PES (triangles) and ndPG-PES/ndBPA-PES (circles) (annealed at 100[degrees]C).
TABLE 1. Characteristics of polyesters.

                            [T.sub.g]         [M.sub.a]      [M.sub.w]
                           ([degrees]C)        (g/mol)        (g/mol)

PG-PES                          57              3,300          5,700
BPA-PES                         60              3,500          5,400
narrow-dispensed PG-PES         64              4,700          5,300
(ndPG-PES)
narrow-dispensed BPA-PES        68              4,500          5,200
(ndBPA-PES)

TABLE 2. Molecular weight of each polyester.

[M.sub.w] (g/mol)

PG-PES         9,000    12,000    17,000
BPA-PES        9,400    13,000    17,800
ndPG-PES       8,500    11,800    16,300
ndBPA-PES      9,200    12,500    17,500
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Author:Fukuri, Norihiro; Ougizawa, Toshiaki
Publication:Polymer Engineering and Science
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Date:Jun 1, 2018
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