An experimental study on the micro- and nanocellular foaming of polystyrene/poly(methyl methacrylate) blend composites

2.1 Materials

PMMA and PS were supplied by Tabriz petrochemical company (TPC), Iran. Table 1 shows the number-averaged (M_n) and weight-averaged (M_w) molecular weights and polydispersity index (PDI) of the used PMMA and PS in this study. n-Pentane (purity 99%, Merck, Germany) was used as the blowing agent, and tetrahydrofuran (THF; Merck) was used as the solvent for PS and PMMA. Calcium carbonate was obtained from Shell, India.

2.2 Preparation of PS/PMMA blends

Three different compositions of PS/PMMA blends with 80:20, 50:50, and 20:80 ratios were prepared. As mentioned, THF was used as the solvent for both PS and PMMA. Both materials (i.e. PS and PMMA), considering their weight percent in the composite, were weighed and dissolved in the solvent to prepare PS/PMMA blends. In the case of composites, calcium carbonate nanoparticles were dissolved separately in THF, and then both solutions were combined. A vacuum pump and an oven were used to dry the samples. Dispersion of the nanoparticles in the polymer matrix was studied using transmission electron microscopy (TEM).

2.3 Foaming of the samples

First, 0.5 mm layers of the samples were made using a hot-press. In this process, the samples (i.e. PS, PS80/PMMA20, PS50/PMMA50, PS20/PMMA80, and PMMA) were held under pressure for about 10 min at 90, 95, 100, 105, and 110°C, respectively. Because of the variation in the compositions of the samples, to obtain almost the same thickness for the samples, the process was carried out at different temperatures. The sheets of samples produced were cut into 2 mm×4 mm size and weighed. An autoclave was designed and used for foaming (Figure 1). The autoclave had a cavity for holding the samples. Two holes were provided in the autoclave: one for locating the samples in the cavity, and the other for connecting a pressure gage. The volume of the cavity in the chamber was measured, and the amount n-pentane needed, considering it as an ideal gas, to reach the required pressures in each test was calculated. The prepared samples and the calculated volume of n-pentane as the blowing agent were placed inside the autoclave and the holes were plugged. The autoclave was then heated to the desired processing temperature. During the test, the amount of used n-pentane in the chamber was adjusted to maintain the expected pressure, as read by the connected pressure gage.

Figure 1:

Designed autoclave for foaming.

To ensure complete dissolution of the gas in the polymer composites, the heated samples in the autoclave were held in the processing temperature for 7 h under the desired pressures. The temperature of the samples was controlled by using a temperature controller that was connected to the wall of the autoclave. Also, the chamber pressure was measured by the connected pressure gage. Finally, the heated autoclave was placed inside a water tank for 20–30 s to get the samples solidified. Then, the autoclave chamber was opened, and the foamed samples were taken out. Then, further morphological tests were carried out on the foamed samples [18], [19].

3.1 Dynamic mechanical analysis (DMA)

An experiment was carried out to analyze the variation of storage modulus for the samples when temperature was varied from 40 to 220°C. As shown in Figure 2, the storage modulus for PS and PS80/PMMA20 samples shows a huge drop when the temperate reaches 100°C. Some drop in the value of storage modulus is also seen at this temperature for PS50/PMMA50 and PS20/PMMA80 samples.

Figure 2:

Storage modulus for PMMA, PS20/PMMA80, PS50/PMMA50, PS80/PMMA20, and PS.

3.2 Solubility measurements

The properties of a polymer foam produced in a batch process have a close connection with the amount of gas dissolved in the polymer matrix [24]. There are other parameters, such as temperature and pressure, that significantly affect the amount of saturated gas and consequently the properties of the produced foams. In this study, the effect of pressure and temperature on the solubility of n-pentane gas in the produced composites was investigated. Figure 3 shows the effect of pressure on the solubility of n-pentane gas in different composites.

Figure 3:

Solubility of n-pentane in the produced samples at a temperature of 100°C, at different pressures.

As Figure 3 shows, the solubility of n-pentane in all the samples increased as a function of the saturation pressure. This is consistent with the Henry’s solubility law. With the increase in pressure, the driving force of the translating gas into the polymer matrix increased [6], [25]. PS samples show a much higher capability to dissolve the gas compared to PMMA samples. This is because of the polarity of n-pentane and its affinity to PS.

Figure 4 shows the gas solubility in the produced samples at different temperatures. As can be seen, with the increase in temperature, the solubility decreased as a result of the increase in the molecular movement of the matrix. Also, when the amount of the PMMA is decreased, the solubility increases, which is related to the higher molecular weight of the PMMA compared to that of PS.

Figure 4:

Temperature dependence of n-pentane solubility under 100 bar pressure.

3.3 Distribution of nanoparticles

Figure 5 shows the particle size and distribution of calcium carbonate nanoparticles in the composites. The TEM image, Figure 5, shows that calcium carbonate nanoparticles are well dispersed and stabilized in the matrix. The stabilization of calcium carbonate particles in PMMA is due to the presence of PMMA’s functional groups. The steric group is strongly associated with the calcium cation, which reduces their mobility. As a result, the calcium carbonate nanoparticles in the PMMA domain become stable. Because of the presence of a benzene ring in PS’s functional group, PS is nonpolar and does not have the tendency to bond with calcium carbonate nanoparticles. By increasing the percentage of nanoparticles, agglomeration was seen in the matrix.

Figure 5:

TEM images from PS80/PMMA20 with 5% calcium carbonate nanoparticles.

3.4 Foaming results

Figure 6A,B shows the SEM images of pure PS and PMMA foams processed at 90 and 110°C, respectively, under a pressure of 100 bar. The scale bar is 20 μm in both cases. The size of the cells in PS foam is much larger than that of the cells in PMMA foams. This can be due to their differences in viscosity during cell growth [26], [27]. Also, as shown in Figure 3, PS has a much higher gas solubility compared to PMMA, and this can be the other reason for the differences in cell morphology. For better understanding of these foam morphologies, the graph of their cell size distributions was extracted, as shown in Figure 6C. While 78% of the cells in PS foams have a cell size of 1–2 μm, only 5% of the cells in PMMA foams have the same range of cell sizes. On the other hand, while in PMMA foams 85% of the cells have size of 0.5–1 μm, in PS foams only 10% of the cell population has the same size. The variation in their viscosity is shown in Figure 2.

Figure 6:

SEM images for pure (A) PS and (B) PMMA foams. (C) Cell size distribution for pure PS and PMMA foams.

Three compositions of PS/PMMA blends: PS80/PMMA20, PS50/PMMA50, and PS20/PMMA80, were foamed, and their SEM images are shown in Figure 7A–C. The processing temperatures for PS80/PMMA20, PS50/PMMA50, and PS20/PMMA80 were 95, 100, and 105°C, respectively, while their processing pressure was 100 bar. While the SEM image in the first row has a scale bar of 50 μm, those in second row have a scale bar of 20 μm for better clarity. The presented SEM images in these foams were quantified for obtaining the cell size distribution. Figure 7D shows the cell size distributions for PS80/PMMA20, PS50/PMMA50, and PS20/PMMA80, respectively. Below is the description of these produced samples.

Figure 7:

SEM images of the PS/PMMA blend foams (A) PS80/PMMA20, (B) PS50/PMMA50, and (C) PS20/PMMA80. (D) Cell size distributions of the foams.

In Figure 7, the majority of the cells (95%) in PS80/PMMA20 foam samples are in the range 1–10 μm and cells in the range 2–3 and 3–4 μm are due the two highest populations. In PS50/PMMA50 foam samples, all the cells are in the range 1–10 μm. Almost half of the cells (49%) lie in the range 2–3 μm, and cells in the range 1–2 and 3–4 μm are due to the two second highest populations. It seems that PS50/PMMA50 foam samples have the narrowest range of cell size distribution. Contribution of the cells in the range 1–4 μm is 91%. On other hand, in PS20/PMMA80 foams samples, all the cells are in the range 1–30 μm. In these samples, the majority of the cells (55%) are in the range 5–15 μm; the cells in the range 5–10 μm constitute 34% of the cell populations, and cells in the range 10–15 μm form 21% of the cell populations. Also, cells up to 5 μm contribute 19% of the population. The low solubility of n-pentane in PMMA and its high viscosity, compared to PS, are the two main reasons for cell morphology variations in PS80/PMMA20, PS50/PMMA50, and PS20/PMMA80 samples.

From the SEM images in Figure 7A–C, and from the results of image analysis, shown in Figure 7D, a very narrow cell size distribution is obtained for PS80/PMMA20 samples. Also, the cells in this composition are the smallest among all the compositions. It seems that during cell growth PS and PMMA balance each other, resulting in a relatively narrow cell size range and small cell size. PS80/PMMA20 samples were studied further in the presence of nanoparticles for obtaining samples with a smaller cell size and a higher cell population [26], [28].

Figure 8A shows the SEM image for PS80/PMMA20 foam sample when 5 wt% nanoparticles of calcium carbonate was added to the composite. The processing temperate and pressure for this sample were 100°C and 100 bar, respectively. Similar to the foam samples prepared without nanoparticles, these images also show some diversity, but it is significantly less than when processed without the nanoparticles. Figure 8B shows the cell size distribution for these samples. In PS80/PMMA20 foam samples with 5% nanoparticles of calcium carbonate, the majority of the cells (61%) are in the range 400–600 nm.

Figure 8:

(A) SEM and (B) cell size distribution for PS50/PMMA50 foam samples when 5 wt% nanoparticles was added to the composite.