Segregation behavior of dibutyl phthalate plasticizer in polystyrene subjected to a temperature gradient

ThiKimThoaHuynh1

QuocVietDo1,2

MasayukiYamaguchi1

1Graduate School of Advanced Science and TechnologyJapan Advanced Institute of Science and Technology1-1 Asahidai923- 1292NomiIshikawaJapan

2School of Material Science and EngineeringHanoi University of Science and Technology1000HanoiVietnam

Thi Kim Thoa Huynh,^a Quoc Viet Do,^a.b Masayuki Yamaguchi^a*

^a Graduate School of Advanced Science and Technology, Japan Advanced Institute of Science and Technology, 1–1 Asahidai, Nomi, Ishikawa 923–1292, Japan

^b School of Material Science and Engineering, Hanoi University of Science and Technology, 1000 Hanoi, Vietnam

Acknowledgments

We thank Frank Kitching, MSc., from Edanz (https://jp.edanz.com/ac) for editing a draft of this manuscript

m_yama@jaist.ac.jp (Masayuki Yamaguchi)

Phone: +81-761-51-1621; Fax: +81-761-51-1149

ORCID: 0000-0002-7640-3768

ABSTRACT

In the present study, we investigated the segregation behavior of a plasticizer, i.e., dibutyl phthalate (DBP) in a polymer, i.e., polystyrene, subjected to a temperature gradient. We confirmed homogeneous distribution without phase separation when the DBP content was ≤ 15%. A sample film containing 10% DBP was annealed in a compression-molding machine, in which the top and bottom plates were held 3 mm apart at 200 and 120°C, respectively. After exposure to the temperature gradient, a DBP gradient distribution was confirmed in the thickness direction: there was a high DBP content at the high-temperature side and a low DBP content at the low-temperature side. The result indicates that a plasticized polymer adopts a plasticizer concentration gradient, i.e., a graded structure, when subjected to a temperature gradient.

Keywords:

segregation

temperature gradient

plasticizer

graded structure

Introduction

In general, each component of a miscible blend is distributed homogeneously. However, a particular region of the blend, such as its surface or an interface may be rich in a specific component [1–6]. This segregation behavior often occurs after exposure to a flow field or a temperature gradient.

In the case of a flow field, the centrifugal force produces a graded structure owing to the difference in the density of each component [7,8]. Furthermore, a low-molecular-weight fraction in a single polymer melt with a broad molecular weight distribution tends to segregate in the high-shear-rate region [9,10], and in a miscible polymer blend, the high-shear-rate region tends to be rich in low-viscosity fractions [11–13]. During injection molding, the shear rate is highest at the wall of the mold. Therefore, the surface of the product is rich in low-viscosity components [11,12]. Moreover, the surface of a product extruded by pressure-driven flow has a high content of low-viscosity components [13].

Under a temperature gradient, segregation without phase separation has also been reported [14–17]. The high-temperature region of a single polymer melt with a broad molecular weight distribution is rich in low-molecular-weight fractions, and the low-temperature region is rich in high-molecular-weight fractions [15]. Similarly, the high-temperature region of a miscible polymer blend tends to be rich in low-viscosity components. Sako et al. have reported that low-molecular-weight poly(methyl methacrylate) (PMMA) migrates towards the high-temperature region when a miscible blend comprising bisphenol-A polycarbonate and PMMA is exposed to a temperature gradient during annealing [16]. Recently, Iida et al. reported that a gradient structure forms in miscible blends of PMMA and poly(vinylidene fluoride) (PVDF) under a temperature gradient, in which the PVDF content is greatest at the high-temperature side [17]. In their experiments, the molecular weight of PVDF was much lower than that of PMMA with a lower shear viscosity of PVDF. Therefore, their results indicate that the low-molecular-weight fraction preferred the high-temperature region.

Such segregation-induced redistribution of one component in a miscible polymer blend presents an opportunity for modifying the surface properties of a product. For example, the surface localization of PMMA in a miscible bisphenol-A polycarbonate/PMMA blend improves the surface hardness and anti-scratch property of the product [11]. Various other advantageous properties such as anti-fogging and anti-static properties, stain-resistance, and self-healing property may be provided by this technique. In fact, self-healing phenomenon has been detected in the graded structure of a glassy polymer [18].

Although the mechanism by which segregation in a temperature gradient occurs is still unknown, the difference in the free volume fraction may be the driving force, as discussed previously [15]. Because low-molecular-weight components require a large free volume [19–22], they prefer to stay in a high-temperature region. Consequently, any plasticized system will demonstrate segregation under a temperature gradient, i.e., high concentration of a plasticizer at the high-temperature side. However, to the best of our knowledge the segregation behavior in a plasticized polymer has never been reported.

Herein, we investigated a blend comprising atactic polystyrene (PS) with dibutyl phthalate (DBP) as a plasticizer. It is well known that a small amount of DBP is miscible with PS without strong specific interaction [23–25].

Experimental

Materials

We used a commercially available PS (CR-2500; DIC, Tokyo, Japan) in the present study. Its number- and weight-average molecular weights were 9.9 × 10⁴ and 2.4 × 10⁵ Da, respectively, and its density at 23°C was 1040 kg m^− 3.

DBP, which was kindly provided by New Japan Chemical (Osaka, Japan), was used without further purification. Details of its properties are available in the literature [26]. Its melting point is − 35°C, its boiling point is 340°C, and its density at 20°C is 1050 kg m^− 3.

Sample preparation

Various amounts (5, 10, 15, and 20 wt.%) of DBP were added to PS using an internal mixer (Labo Plastomill 10 M100; Toyo Seiki Seisakusho, Tokyo, Japan) at 180°C. The components were mixed for 2 min, and the mixer blade rotation speed was 50 rpm.

The blend samples were compressed into a flat sheet at a pressure of 10 MPa using a compression-molding machine (SA303IS; Tester Sangyo, Saitama, Japan). The samples were heated at 180°C for 2 min, then cooled at 25°C. The dimensions of the sheet for the segregation experiments were 40 × 40 × 3 mm. Other sheets with various thicknesses were also prepared using the same procedure to characterize the samples.

The sheet was annealed under a temperature gradient using the compression-molding machine with a slight pressure to contact the sample. The top and bottom plates were kept 3 mm apart and they were held at 200 and 120°C, respectively. Therefore, a linear temperature gradient of ∂T/∂x = 27 × 10^− 3 °C m^− 1 was applied in the thickness direction of the sample sheet. After annealing, the samples were cooled at 25°C and immediately used for the measurements.

Measurements

The temperature dependence of the dynamic tensile modulus of the sample sheet was determined using a dynamic mechanical analyzer (Rheogel E400; UBM, Muko, Japan) at 10 Hz in the temperature range 25 to 200°C at a heating rate of 2°C min^− 1. Rectangular specimens (5 × 2 × 0.9 mm) were used for the measurements.

The frequency dependence of the oscillatory shear modulus was determined in the molten state at 200°C using a cone-and-plate rheometer (AR2000ex; TA Instruments, New Castle, DE, USA). The cone diameter and cone angle were 25 mm and 4°, respectively. The angular frequency sweep was 628 to 0.01 rad s^− 1 under a nitrogen atmosphere.

The DBP content at the surface of each sample sheets was evaluated by attenuated total reflectance–infrared (ATR-IR) spectroscopy after exposure to the temperature gradient. The measurements were performed at 25°C using an IR spectrometer (Spectrum100 FT-IR spectrometer; Perkin-Elmer, Waltham, MA, USA) with KRS-5 (thallium bromo iodide) as an ATR crystal. The incidence angle of the infrared beam was 45°.

The DBP distribution of each sheet in the thickness direction was determined using an IR microscope system (Spotlight 200i Microscopy; Perkin-Elmer). The annealed PS/DBP samples were cut into thin films (10 µm thick) using a microtome (RX860 Rotary microtome; Yamato, Niiza, Japan). The measurements were obtained at various points across the thickness direction of a 600 × 200 µm² area.

Results and discussion

Miscibility of PS and DBP

The transparencies of sheets with various DBP contents were evaluated. Each sheet was 3 mm thick. As shown in Fig. 1, the sheets containing 5, 10, or 15 wt.% DBP were transparent, suggesting that the DBP was miscible with the PS when the DBP content was at or less than 15 wt.%. However, phase separation occurred when the sheet contained 20 wt.% DBP.

Fig. 1

Photographs of the sheets (each 3 mm thick). The DBP contents were 5, 10, 15, or 20 wt.%

We characterized the dynamic mechanical properties of the samples in the solid state. Figure 2 shows the temperature dependence of the tensile storage modulus E′ and loss modulus E″ of each of the pure PS and plasticized PS films containing 5, 10, 15, or 20 wt.% DBP. There was a single peak in the glass-to-rubber transition region of the E″ curve in the present study, which we attributed to the glass transition temperature T_g. The T_g of PS decreased significantly as the DBP content increased. The T_g values for pure PS, PS/DBP (95/5), PS/DBP (90/10), and PS/DBP (85/15) were approximately 100.0, 83.0, 68.0, and 57.9°C, respectively, attesting the pronounced plasticizing effect of DBP. These blends each produced a sharp peak, confirming that DBP was miscible with PS when the DBP content was ≤ 15 wt.%. However, the 20 wt.% DBP sample had a T_g that was almost the same as that of the PS/DBP (85/15) sample, i.e., 52.9°C, with a broad peak. Considering the transparency, the result was as expected, i.e., PS/DBP (80/20) underwent phase separation. The T_g values and the full-width-at-half-maximum, FWHM, of the E″ peak are plotted against the DBP content in Fig. 3. The broader peak at 20 wt.% indicates a loss of miscibility, supporting the conclusion that 20 wt.% DBP is immiscible with PS.

Fig. 2

Temperature dependence of the tensile storage modulus E′ (closed symbols) and loss modulus E″ (open symbols) of sheets with various DBP contents (circles = 0 wt.%, diamonds = 5 wt.%, triangles = 10 wt.%, inverted triangles = 15 wt.%, and squares = 20 wt.%)

Fig. 3

T_g and full-width-at-half-maximum, FWHM, of the E″ peak as a function of DBP content.

The angular frequency ω dependence of the oscillatory shear moduli, i.e., the storage modulus G′ and the loss modulus G″, are shown in Fig. 4. The samples contained 0–15 wt.% DBP. The addition of DBP decreased both the G′ and G″ values, and there was no shoulder in the G′ curve. In the low-frequency region, the G′ and G″ values were proportional to ω² and ω, respectively. This is a typical behavior of the rheological terminal region of a simple polymer melt without phase separation. The result also confirms that DBP is miscible with PS in the molten state when the content is ≤ 15 wt.%.

Fig. 4

Angular frequency dependence of the shear storage modulus G′ (closed symbols) and the loss modulus G″ (open symbols) of pure PS (circles) and plasticized PS blends containing 5 wt.% (diamonds), 10 wt.% (triangles), and 15 wt.% (inverted triangles) DBP at 200°C.

Segregation behavior of PS/DBP

The infrared spectra at the surfaces of the samples were produced using ATR-IR measurements, as shown in Fig. 5. The carbonyl stretching vibration associated with DBP was detected at 1725 cm^− 1 [26]. The penetration depth d_p of the IR beam, which was calculated using Eq. (1) [27,28], was approximately 1.08 µm when KRS-5 was used as an ATR crystal:

$\:{d}_{p}=\frac{\lambda\:}{2\pi\:{n}_{1}\sqrt{{sin}^{2}\theta\:-{({n}_{2}/{n}_{1})}^{2}}}$

where λ is the wavelength, n₁ and n₂ are the refractive indices of the internal reflection element and the sample, respectively, and θ is the angle of incidence. Here, n_PS and n_KRS−5 are 1.55 and 2.5, respectively.

The out-of-plane mode of the phenyl ring in PS was detected at 538 cm^− 1 [29,30]. The absorbance at 1725 cm^− 1 was normalized by the peak at 538 cm^− 1 to quantify the DBP content.

Fig. 5

ATR-IR spectra of PS/DBP (90/10), DBP, and pure PS samples.

The PS/DBP (90/10) sample was annealed in the temperature gradient, i.e., between the two plates of the compression-molding machine, which were 3 mm apart and were held at 200 (top plate) and 120°C (bottom plate). We confirmed that the transparency of the sheet was not affected by the annealing procedure, i.e., phase separation did not occur. It was clear that the absorbance of the 200°C surface was higher than that of the 120°C surface, as shown in Fig. 6. The results demonstrate that DBP segregation occurs on the PS surface during exposure to the temperature gradient.

Fig. 6

ATR-IR spectra of both surfaces of the PS/DBP (90/10) sheet before and after exposure to a temperature gradient for 100 min (plates 3 mm apart; 200/120°C). The absorbance at 1725 cm^− 1 was normalized by that at 538 cm^− 1.

The growth of DBP content at the surfaces over the annealing period was evaluated to estimate the segregation rate in PS/DBP (90/10). As shown in Fig. 7, the DBP content at the low-temperature surface decreased with the annealing period, and eventually attained a constant value after 30 min. In contrast, the DBP content increased at the high-temperature surface. This proves that segregation is saturated after a long period of annealing. Ultimately, the difference in the DBP content at the two surfaces was approximately 2 wt.%.

Fig. 7

Growth curves of the DBP content at surfaces as a function of the annealing period in the temperature gradient.

The distribution of the DBP content in the thickness direction was evaluated by microscopic Fourier-transform infrared spectroscopy in mapping mode. Figure 8 presents a visual image of a thin film cut from the PS/DBP (90/10) sheet after exposure to the temperature gradient for 100 min. The measurement points are represented in the left figure, labeled from 1 to 4, and correspond to positions from the high- to the low-temperature regions. At each point, the measurements were obtained to generate an IR spectrum that was used to evaluate the DBP content, as shown in the right image in Fig. 8. In Fig. 9, the DBP contents at both surfaces, measured in ATR-IR mode, are also plotted.

Fig. 8

A visual image of the PS/DBP (90/10) thin film (left) and the distribution of DBP in the thickness direction (right).

Fig. 9

Distribution of the DBP content in the thickness direction of the PS/DBP (90/10) sheet after annealing in a 3 mm-thick temperature gradient of 120/200°C for 100 min.

The results indicate that the DBP content increased gradually from the low- to the high-temperature regions.

As mentioned in the introduction, this segregation may be attributed to the effect of the free volume. As temperature increases, the free volume fraction of a polymer also increases. Therefore, a low-molecular-weight compound with a high density of chain ends that require a large free volume fraction is localized. Consequently, the high-temperature side will be rich in plasticizers. In contrast, a plasticizer concentration gradient results in diffusion to homogenize the system. Therefore, the competition between temperature-gradient segregation and diffusion due to the concentration gradient decides the distribution state of a plasticizer.

Conclusion

In the present study, we investigated the segregation behavior of a PS/DBP plasticized polymer in a temperature gradient. The DBP was highly miscible with the PS in both the solid and molten states. We found that the DBP in miscible PS blends became segregated when the blends were subjected to a temperature gradient of 120/200°C between two compression-molding plates that were 3 mm apart, leading to a higher DBP content on the 200°C sample surface without phase separation. The DBP content increased monotonically in the thickness direction from the low- to the high-temperature regions. Considering that the free volume fraction increases with temperature, a high-temperature region will have a high concentration of low-molecular-weight components with large free volumes. Such phenomena offer new material design strategies to provide certain surface functions. Furthermore, our results provide improved knowledge about the segregation behavior of plasticized polymers in a temperature gradient.

Author contributions

Thi Kim Thoa Huynh: Writing– original draft, Methodology, Formal analysis, Data curation, Conceptualization.

Quoc Viet Do

Methodology, Conceptualization

Masayuki Yamaguchi

Writing– review & editing, Conceptualization, Validation, Project Administration.

Funding

Declaration: No funding.

Ethical approval:

Not applicable.

Competing interest:

There are no conflicts to declare.

Data Availability:

Data are available upon request.

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