Effect of Growing Crystalline Phase on Bubble Nucleation in Poly(L

Feb 16, 2011 - By depressurizing the system from 11 MPa to atmospheric pressure, the dissolved CO2 in the PLLA matrix became supersaturated, and CO2 b...
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Effect of Growing Crystalline Phase on Bubble Nucleation in Poly(L-Lactide)/CO2 Batch Foaming Kentaro Taki,* Daisaku Kitano, and Masahiro Ohshima Department of Chemical Engineering, Kyoto University, Katsura Campus, Kyoto, 615-8510, Japan ABSTRACT: Creating a high-efficiency bubble nucleation agent for a polymer foaming process is essential for increasing bubble density as well as decreasing bubble size. Spherulites (crystalline phase) grown in a semicrystalline poly(L-lactide) (PLLA) matrix enhanced carbon dioxide (CO2) bubble nucleation in a batch foaming process. Several sites for PLLA spherulite formation were observed at a hold-temperature of 110 C after CO2 saturation in molten PLLA, which was heated to 180 C at 11 MPa. By depressurizing the system from 11 MPa to atmospheric pressure, the dissolved CO2 in the PLLA matrix became supersaturated, and CO2 bubbles nucleated around growing PLLA spherulites. The number of nucleating bubbles increased as a function of increasing spherulite quantity and area. A faster linear spherulite growth rate and a lower hold-temperature created more bubbles around the spherulites. From these observations, it was concluded that the growing spherulites expelled CO2 from the advancing spheruliteamorphous phase interface and that CO2 accumulated at the interface. Then, the increase in the concentration of CO2 led to an increase in the nucleation of bubbles around the spherulites.

’ INTRODUCTION Polymer foaming is a simple polymer processing technique that can be used to create voids in plastic materials. Polymeric foams exhibit excellent properties, including low weight to volume ratios, electric resistivity, shock absorbance, heat insulation, and sound absorption.1 Polymer foaming processes have historically involved using chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) as blowing agents for creating voids in plastics.2 However, due to the potential for these compounds to deplete atmospheric ozone, the use of CFCs and HCFCs was restricted by the Montreal Protocol on Substances that Deplete the Ozone Layer in 1987, and these agents are now monitored by the Rigid and Flexible Foams Technical Options Committee of the United Nations Environmental Program.3 Currently, environmentally benign blowing agents are being researched by manufacturing companies. Carbon dioxide (CO2) has been identified as one possible candidate for replacing CFCs and HCFCs. Whereas CO2 does not deplete the ozone layer, its solubility in polymers is low compared to CFCs and HCFCs. More specifically, the amount of pressure is required to dissolve CO2 in polystyrene (PS) at 100 C is four times the pressure needed for the traditional organofluorine compound HFC-152a.4 To achieve a high number density of CO2 bubbles in the polymer matrix, bubble nucleation agents were used to facilitate the bubble nucleation and obtain a high-number density of bubbles in the final foamed product. Several bubble nucleation agents are commercially available and have been investigated in regard to their ability to enhance bubble nucleation.5,6 Recently, studies investigating the impact of spherulites or polymer crystallites on bubble nucleation have been reported for several polymers. These compounds include polypropylene,7,8 poly(lactide) (PLA),9 and polyester amide.10 Reignier et al. used an ultrasound sensor to detect bubble nucleation in the CO2 foaming of poly(ε-caprolactone), which is a semicrystalline biodegradable r 2011 American Chemical Society

polymer, and demonstrated that the presence of crystals decreased the degree of supersaturation by 5-10 fold with respect to the amorphous system.11 Koga and Saito studied CO2 bubble nucleation between the lamella of polymer crystallites.12 They investigated the morphology of high-density polyethylene (HDPE) crystallized under high-pressure CO2 by using light scattering measurements and microscope observations. A fine-layered porous structure with a thickness of 500 nm was obtained in the HDPE matrix. This porous structure was attributed to CO2 being excluded from the crystal growth front and pushed into the intercrystalline amorphous region. From these observations, it was concluded that the crystalline phase of polymers facilitated and induced the bubble nucleation in polymer foaming. Namely, the crystalline phase of polymers may be used as an alternative bubble nucleation agent in the polymer foaming process. However, there has been little discussion about the mechanism whereby the crystalline phase could facilitate and induce bubble nucleation. Durning et al. developed a mathematical model for diffusion with solvent-induced crystallization to describe the macrovoid formation in a swollen polymer.13,14 They showed that macrovoid formation was related to the linear growth rate of the crystalline phase and the diffusion rate of solvent. To clarify the effect of the crystalline phase on bubble nucleation in polymer foaming, a discussion on the crystalline phase’s linear growth rate is essential. In this study, we used a visual observation apparatus to measure the linear growth rate of the poly(L-lactide)(PLLA) spherulites and to investigate the impact of the PLLA spherulite growth rate on the frequency of CO2 bubble nucleation around growing spherulites.

Received: August 1, 2010 Accepted: January 15, 2011 Revised: January 15, 2011 Published: February 16, 2011 3247

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’ EXPERIMENTAL SECTION Semicrystalline PLLA (L-type content 99%, Tg = 54 C, Tm = 172.5 C, Tc = 130 C, Mw 205,000, Mw/Mn 2.37, Uz-B2; Toyota Motor Corporation, Toyota City, Japan) was used without additional modifications. CO2 (purity 99.9%, Showa Tansan Co. Ltd., Tokyo, Japan) was used as a physical blowing agent. A visual observation apparatus, which consisted of a stainless steel body, two sapphire windows, four cartridge heaters, a temperature controller, and a CO2 pump, was used to visualize bubble nucleation and spherulite growth. This apparatus was originally developed by Otake et al.15 The details concerning the visual observation apparatus can be found elsewhere.16-18 A disk-shaped PLLA sample, which had a 0.6 mm thickness and a 5 mm diameter, was prepared at 180 C with a hot-press (Imoto Machinery, Japan). The visual observation apparatus was purged with CO2 following the insertion of the PLLA sample. The sample was heated to 180 C. Carbon dioxide at 11 MPa was loaded into the visual observation apparatus and dissolved in the sample for 1 h. The PLLA sample became transparent and molten. The sample was cooled to a hold-temperature of 110 C in the visual observation apparatus at a rate of approximately 2 C/min under a CO2 pressure of 11 MPa. The CO2 pump was running while the temperature was changed to 110 C. Because the CO2 pump had enough capacity, the CO2 pressure was kept at 11 MPa in the cell. When the temperature reached to 110 C after 10 min, several spherulites were observed. Then the CO2 pressure was released at 0.35 MPa/s while several spherulites were nucleating and growing. The CO2 dissolved in the polymer phase became supersaturated when the CO2 pressure decreased in the visual observation apparatus. At a specific pressure and degree of supersaturation, CO2 bubbles began to nucleate. It is important to note that at the moment of depressurization, the system was far from the crystallization equilibrium, which led to spherulite growth in the amorphous phase. The growth of spherulites and bubble nucleation were observed using a high-speed digital camera (HAS-200, DITECT, Tokyo, Japan) and a microscope (VQ-Z50, KEYENCE, Osaka, Japan). The volume of the camera viewing area was 1.2  1.6  0.6 (thickness) mm3. The sampling times for visual observation of spherulite growth and foaming were 2 frames per minute and 100 frames per second, respectively. The microscope was capable of identifying objects larger than 10 μm. A K-type thermocouple temperature sensor was embedded in the visual observation apparatus approximately 1 cm from the sample. During the cooling operation, the sample temperature was approximately 3 C higher than the thermocouple. The percentage of spherulites area (crystalline phase), a ratio of bubbles nucleated around the spherulites to the total number of bubbles, and number density of bubbles was introduced to quantitatively evaluate the impact that the spherulites had on bubble nucleation. Percentage of spherulite area (χc): χc ½% ¼

Total area of spherulites Viewing area

ð1Þ

As the spherulites grew, χc changed along with the crystallization time at a certain hold-temperature. The ratio of bubbles nucleated around the spherulites to the total number of bubbles (nc): nc ½% ¼

The number of bubbles around spherulites Total number of bubbles in viewing area

ð2Þ

Figure 1. Optical micrograph of foaming in a PLLA/CO2 system. The saturation pressure and temperature were 11 MPa and 180 C, respectively. The hold-temperature was 110 C. The pressure in the visual observation cell is indicated in the parentheses.

Whether a bubble was nucleated around a spherulite or in the amorphous phase was defined according to the following characteristics: bubbles that were located within a four-pixel length of 10 μm from a spherulite were included in the number of bubbles around the spherulites, bubbles that were superimposed on the spherulites were included in the number of bubbles around the spherulites. Number density of bubbles (N): N ¼

Total number of bubbles in viewing area Volume of viewing area

ð3Þ

The spherulite linear growth rate at several hold-temperatures was measured to explain the correlation between hold-temperature and the facilitation of bubble nucleation. The diameters of individual spherulites were calculated based on the assumption that the spherulites were spherical. The cross sectional area of the optically viewed spherulites was used for the calculations. The average linear growth rate was calculated from the change in diameter over time for several spherulites.19

’ RESULTS AND DISCUSSION Part a of Figure 1 shows a micrograph of a PLLA sample containing several spherulites. The gray-colored spheres in the micrograph represent growing spherulites of PLLA. The spherulites were formed after the visual observation apparatus reached a hold-temperature of 110 C. The CO2 pressure in the visual observation apparatus was released when the micrograph in part a of Figure 1 was taken. Part b of Figure 1 shows a micrograph taken at 5 s where the pressure in the visual observation cell was 7.2 MPa after CO2 depressurization from 11 MPa. The black spheres observed in part b of Figure 1 are nucleated bubbles. Many bubbles were nucleated around the spherulites. Parts c and d of Figure 1 show a series of micrographs at 6 and 7 s, respectively. The pressures in the visual observation cell changed from 3248

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Figure 2. Change in the number of bubbles as a function of time after pressure release. 9: number of bubbles in the amorphous phase, O: number of bubbles around spherulites. Solid line: pressure in the visual observation cell. The volume of the viewing area was 1.2  1.6  0.6 mm3. The saturation pressure, temperature, and hold-temperature had the same values as Figure 1.

6.8 to 6.5 MPa. The number of bubbles around the spherulites, as well as the number of bubbles in the amorphous phase, increased. A comparison of parts a-d of Figure 1 revealed that most of the bubbles were nucleated around the spherulites. The bubble nucleation occurred around the spherulites at higher pressure in the visual observation cell. A smaller supersaturation pressure was required to facilitate bubble nucleation than the pressure that was needed in the amorphous phase. Figure 2 shows the number of bubbles in the viewing area over time. The change in the number of bubbles around the spherulites and in the amorphous phase increased in a sigmoidal fashion. As shown in the figure, the number of bubbles that nucleated around the spherulites in the crystallization phase was larger than the number of bubbles in the amorphous phase. Specifically, the number of bubbles observed around the surface of the spherulites at 6 s was approximately 4.6 times greater than the number of bubbles observed in the amorphous phase. Eventually, once bubble nucleation was completed, there were approximately 1.6 times the number of bubbles near the spherulites. The super saturation pressure that was required for inducing bubble nucleation around the spherulites was the pressure difference between the saturation pressure (11 MPa) and the onset pressure for bubble nucleation (8.7 MPa), which was 2.3 MPa. The super saturation pressure in the amorphous phase was 3.6 MPa. The smaller super saturation pressure was sufficient to induce bubble nucleation around the spherulites. The results shown in Figure 2 were based on the area of the spherulites that filled the viewing area. The percentage of spherulite area, χc, the ratio of bubbles nucleated around the spherulites to the total number of bubbles, nc, and the number density of bubbles, N, were introduced to quantitatively evaluate the impact that the spherulites had on bubble nucleation (Experimental section). Parts a and b of Figure 3 depict nc and N as a function of χc at a hold-temperature of 110 C, respectively. The data point for 0% χc was obtained when the temperature reached 110 C and the CO2 pressure was simultaneously released. When the bubble nucleation was finished, no spherulites existed in the view area. The solid line was drawn with the following empirical equation: nc ¼ nc, ¥ ð1 - e-χc =b Þ where nc,¥ and b are fitting parameters.

ð4Þ

Figure 3. (a) Effect of the percentage of spherulite area (crystalline phase, χc) on the ratio of bubbles nucleated around the spherulites to the total number of bubbles (nc); (b) effect of the percentage of spherulite area (χc) on the number density of bubbles (N). The saturation pressure and temperature were 11 MPa and 180 C, respectively. The holdtemperature was 110 C.

The plot shows that nc increased as a function of increasing χc; the proportion of bubbles nucleated around spherulites increased as a function of the increasing spherulite area. However, the fact that nc did not reach 100% indicated that bubbles continued to nucleate in the amorphous phase far away from spherulites. To clarify how spherulites enhanced bubble nucleation, the total number of bubbles is shown in part b of Figure 3. As shown in the figure, the total number of bubbles also increased as a function of increasing χc. Specifically, the number density of the bubbles in the range of 15 to 80% of χc was approximately 1.5 times larger than the number density at 0% χc at which no spherulites existed. Thus, the growing spherulites enhanced bubble nucleation and facilitated the creation of additional bubbles. In summary, the growing spherulites function as bubble nucleation agents. Parts a and b of Figure 4 show the effect of hold-temperature on nc and N as a function of χc, respectively. The number of bubbles present depends on the location and age of individual spherulites. Even when the same value of χc is obtained, if two or more spherulites are aggregated, then the surface area of the spherulites at a particular χc would be different from condition with isolated spherulites. However, the temperature dependence of nc was clearly shown. Specifically, as the hold-temperature decreased at a given χc, the magnitude of nc increased. When χc exceeded 20%, the nc at 105 C exceeded 95%, which indicated that the vast majority of bubbles were nucleated around spherulites. The χc at 115 C never exceeded 30%, but the total number of bubbles at each hold-temperature increased as a function of increasing χc in 3249

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Figure 5. Linear growth rate of PLLA spherulites under 11 MPa of CO2.

Figure 4. Effect of hold-temperature on (a) nc and (b) the density of bubbles (N) as a function of χc.

part b of Figure 4. At higher temperatures, the bubble nucleation in the amorphous phase was higher than the bubble nucleation around the crystalline phase. The crystalline phase did not selectively facilitate bubble nucleation. The performance of the crystalline phase as bubble nucleation agent was poor at higher temperatures. Its effect became indistinct. The accuracy of regression at 105 C in part a of Figure 4 seemed to be better than the regression at 110 C, although the regression at 110 C had the most data points. At 110 C, the bubbles were nucleated either around spherulites or were in the amorphous phase. The nc at 105 C was over 90%, whereas the nc at 110 C was 50%. The bubble nucleation depends on the location and type of aggregation of the spherulites because the total surface area of the isolated spherulites is larger than the surface area of the aggregated spherulites even though the percentage of spherulites area is the same. At 110 C, the bubbles that were nucleated were either in the amorphous phase or around spherulites. So the location of bubble nucleation was amenable to the morphology of the spherulites. The surface area of the spherulites was not always larger when the percentage of spherulite area was larger. Therefore, the data points for the 110 C regression resemble scattered-data-points. A clear correlation was not observed between the number density of bubbles and the hold-temperature, as shown in part b of Figure 4. However, the presence of spherulites enhanced bubble nucleation over temperatures ranging from 105 to 115 C. The spherulite linear growth rate at several hold-temperatures was measured to explain the correlation between the hold-temperature and the facilitation of bubble nucleation. Figure 5 shows

the average linear growth rate of individual spherulites as a function of hold-temperature. The linear spherulite growth rate at 100 C was less than the rate at 105 C, but at temperatures over 105 C, the linear spherulite growth rate decreased as temperature increased. A hold-temperature of 105 C provided the greatest linear growth rate. Figure 6 depicts the impact of the linear spherulite growth rate (Gc) on nc. The micrographs on the right depict the distribution of bubbles near the crystalline (spherulite) region and in the amorphous region. Three ncs data points for each hold-temperature were selected in the range of 10% to 20% χc to demonstrate the effect of the linear growth rate of spherulites on nc. In the range of 10 to 20%, there is a point for 115 C, are four points for 110 C, and are two points for 105 C. The data points of 110 and 105 C, which have the closest χc of 115 C, were selected, respectively. As shown in the plot in Figure 6, nc increased as a function of the increasing linear growth rate, which indicated that a faster linear growth rate enhanced bubble nucleation around individual spherulites. A faster linear growth rate for the spherulites, and not just the presence of spherulites (or χc), is important for enhancing bubble nucleation. Figure 6 also includes several labeled optical micrographs of visual observations, in which filled circle and open circles were superimposed on the micrographs. The filled and open circles indicate the locations of bubble nucleation around spherulites and away from spherulites, respectively. As shown in Figure 6, more filled circles than open circles were present when the holdtemperature was 105 C. At 115 C, the number of open circles exceeded the number of filled circles, which indicated that most of the bubbles were nucleated in the amorphous phase. These figures demonstrate that a faster linear growth rate resulted in more bubbles around the spherulites. Gc was observed to increase as a function of decreasing holdtemperature between 105 to 115 C. CO2 was expelled faster when the sample had a higher Gc and remained at the interface longer, due to the slower diffusion rate at lower temperatures (see the temperature dependence of the CO2 diffusion coefficient from 140 to 180 C in the Appendix). The diffusion rate of CO2 in the amorphous phase and Gc influenced the accumulation of CO2. If the diffusion of CO2 was sufficiently slow, the excluded CO2 tended to accumulate around the spherulite-amorphous phase interface, and the increase in CO2 concentration led to an increase in the frequency of bubble nucleation. Thus, bubble nucleation around the growing spherulites was enhanced compared to the amorphous phase. 3250

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With a slower spherulite growth rate, it was easier to observe spherulite growth and conduct visual observation of the foaming. Growing spherulites in a polypropylene melt with a faster linear growth rate may further enhance the bubble nucleation around the spherulite.

Figure 6. Plot of spherulite linear growth rate versus nc. The linear growth rate of the spherulites was tabulated by measuring spherulites diameters over a period of time at different temperatures.

’ APPENDIX The diffusion coefficient of CO2 in the amorphous phase of PLLA was measured with a magnetic suspension balance (MSB) technique for a temperature range from 140 to 185 C and a pressure range from 10 to 11 MPa. The detail of the protocol can be found elsewhere.20 Figure 7 shows the diffusion coefficient of CO2 in PLLA. The diffusion coefficient decreased along with a decrease in the temperature between 185 to 140 C. The diffusion coefficients of CO2 at 105, 110, and 115 C, which were calculated by Arrhenius type extrapolation, were 8.5, 9.7, and 11.0  10-10 m2/s, respectively. ’ AUTHOR INFORMATION Corresponding Author

*Tel.: þ81-75-383-2696. Fax: þ81-75-383-2646. E-mail: taki@ cheme.kyoto-u.ac.jp.

’ ACKNOWLEDGMENT This study was supported by the Grant-in-Aid for Young Scientists from the Japan Society for the Promotion of Science (JSPS, 20760513 and 22760587). ’ REFERENCES

Figure 7. Diffusion coefficients of CO2 in PLLA from 140 to 180 C. The weight gain of the PLLA sample was monitored from 10 to 11 MPa. The solid line was calculated by using Arrhenius type extrapolation.

’ CONCLUSIONS Visual observation was conducted to clarify the effect of growing spherulites on bubble nucleation in a PLLA/CO2 system. The number of bubbles increased as a function of the spherulites area (χc). The experimental results indicated that increases in χc resulted in an increase in the number of bubbles that were present. The ratio of bubbles nucleated around the spherulites to the total number of bubbles (nc) correlated strongly with the hold-temperature. The lower hold-temperatures increased the linear growth rate of the spherulites and decreased the diffusion of CO2, which enhanced bubble nucleation around the spherulites. A faster linear spherulites growth rate and a lower hold-temperature created more bubbles around the spherulites. From these observations, we concluded that the growing spherulites expelled CO2 from the advancing spherulite-amorphous phase interface and that CO2 accumulated at the interface. Furthermore, increasing the concentration of CO2 at the interface of the spherulites and amorphous phase led to an increase in bubbles nucleating around spherulites. We used PLLA in this study because PLLA had a relatively slow linear spherulite growth rate compared to polypropylene.

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