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Generation of biodegradable polycaprolactone foams in supercritical carbon dioxide
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Generation of biodegradable polycaprolactone foams in supercritical carbon dioxide L Y U and K D E A N , CSIRO ± Manufacturing and Infrastructure Technology, Australia and Q X U , Zhenghou University, China
18.1 Introduction The importance of developing biodegradable polymers has been well documented in the current literature (journals, books and government discussion papers), including this book, thus it has not been repeated in this chapter. However, it should be noted today that the major barrier for growth in applications for biodegradable polymers is price. Hence, developing some high value applications (for example in the medical field) and reducing price are the key drivers in the research and development of biodegradable polymers. Foaming using supercritical CO2 offers a potential avenue to achieve these two objectives. Foamed polymeric materials are produced in a wide range of bulk densities that mainly determine their mechanical properties. A high-density foam that has an improved tensile strength and modulus can be used for load-bearing applications, such as structural parts, while a low-density foam can be used in thermal insulation and packaging applications. In addition to the foam density, the size and distribution of cells also affects the final properties of the foam. Conventional plastic foams have relatively poor mechanical properties because the cell size is typically larger than 100 m and the cell size distribution is very non-uniform.1,2 In general, foams with very fine cell size exhibit better mechanical properties. Supercritical fluids, above their critical temperature and pressure conditions, exhibit unique behavior by combining the properties of conventional liquids and gases.3 In particular, they exhibit liquid-like densities allowing for solvent power of orders of magnitude higher than gases, while gas-like viscosities lead to high rates of diffusion. A novel technique for the use of supercritical fluids as blowing agents for the generation of microcellular foams has recently been developed. It has been shown that this method can produce microcellular foam with a cell density up to 1010 per cm3 and an average cell size varying from less than 10 microns to 50 microns depending on the conditions. Several authors4±14 have reported the method using CO2 and N2 as blowing agents. Initially, the
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polymeric pellets are saturated with gas at moderate pressure (5±6 MPa) followed by heating to a temperature above the glass transition temperature of the polymer. Beckman et al.9,10 reported an analogous, yet different scheme by which to generate microcellular foams in polymers using CO2 as blowing agent; a constant-temperature variable-pressure method based on saturating the polymer with CO2 at much higher pressure (25±30 MPa), in the supercritical region of CO2, followed by a rapid pressure quench. This method takes advantage of the depression of the transition temperature of the polymers by the presence of CO2. The growth of cells is allowed by the suppression of glass transition temperature resulting from the diluent effect rather than heating the polymer to a temperature above its normal glass transition temperature. Microcellular foams are characterized by a cell size between 10 to 50 m, a cell population density greater than 109 cells/cm3, and very narrow cell size distribution. Because of these unique structures, microcellular foamed plastics offer superior mechanical properties, such as impact strength, toughness, and fatigue life when compared to an un-foamed polymer. The enhanced properties of microcellular polymers make them highly competitive in many applications such as packaging, automotive parts, aircraft parts, sporting equipment, insulation, etc., when a cost-effective, continuous manufacturing process for these materials is developed. In packaging applications, where cost is a critical issue, lowering density and improving mechanical properties are two of the best ways to reduce the price of a product. This is an important consideration for the synthetic biodegradable polymers which are currently marketed for packaging, as they are still quite expensive in comparison to more traditional polyolefins. Figure 18.1 shows a schematic representation of a supercritical fluid phase diagram. In this pressure-temperature diagram there are three lines describing the sublimation, melting and boiling process. These lines also define the regions corresponding to the gas, liquid and solid states. Points along the lines (between the phases) define the equilibrium of two of the phases. The vapor pressure (boiling) starts at the triple point and ends at the critical point. The critical region has its origin at the critical point. At this point a supercritical fluid can be defined as substance that is above its critical temperature (Tc) and critical pressure (Pc). The critical temperature is therefore the highest temperature at which a gas can be converted to a liquid by an increase in pressure. The critical pressure is the highest pressure at which a liquid can be converted to a standard gas by an increase in the liquid temperature. In the so-called critical region, there is only one phase and it possesses some of the properties of both gas and liquid. Carbon dioxide, a nontoxic fluid with a relatively low critical point (Tc 31 ëC, Pc 7.376 MPa) is the most widely used in the supercritical fluid field. Supercritical liquid CO2 is found in the triangular region formed by the melting curve, the boiling curve and the line that defines the critical pressure.15 Carbon dioxide is known to swell and significantly plasticize many amorphous polymers, such as poly(methyl methacrylate), polystyrene, polycarbonate and
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18.1 Schematic representation of a supercritical fluid phase diagram.
poly(ethylene terephthalate).2±10 The driving force for growth of bubbles using supercritical CO2 is the temperature difference, which can be controlled by manipulating glass transition temperature via changes in the CO2 pressure, instead of changing temperature by directly heating the polymer. Nucleation is induced via supersaturation caused by a sudden pressure drop from the equilibrium solution state, and the nuclei grow until the polymer vitrifies at a lower pressure. The classical homogeneous nucleation theory is not able to fully describe the nucleation activities in the temperature variation processing.6,9,16,17 Poly(-caprolactone) (PCL) is one of the representative examples for ringopening polymerization of lactones to produce synthetic biodegradable polyesters. Because of its unique combination of biocompatibility, permeability and biodegradability, PCL and some of its copolymers with lactides and glycolides have been widely applied in medicine as artificial skin, artificial bone and containers for sustained drug release.18±20 An example of microcellular foamed PCL in the medical field is guided tissue regeneration and cell transplantation. As far as guided tissue regeneration is concerned, porous implants are used as size selective membranes to promote the growth of a special tissue in a healing site. Ideally, the implant should be inherently biocompatible, have well-defined cell size and be resorbable with appropriate biodegradation rates.21 PCL is a material that meets these demands well. PCL is a biocompatible and biodegradable aliphatic polyester that is bioresorbable and non-toxic for living organisms. Packaging is one of the largest potential markets for all biodegradable polymers, but in this market it is generally price that has been the limiting factor thus far. The use of supercritical CO2 foamed PCL as a biodegradable packaging material has the potential to reduce the cost (by using less material for each packaging unit) and also to expand the number of packaging applications for PCL. Since the glass transition temperature of PCL is very low (Tg ÿ60 ëC)
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which is far below the ice point, the experimental temperature used in this work is much higher than the Tg, and it is this temperature difference that distinguishes this work from previous studies reported by other researchers. As it has been outlined, PCL foaming is of importance both commercially and scientifically. In this chapter, microcellular foaming of low-Tg biodegradable and biocompatible polycaprolactone (PCL) in supercritical CO2 will be described. The effects of a series of variable factors, such as saturation temperature, saturation pressure, saturation time and depressurization time on the foam structures and density were studied through measurement of density and SEM observation. The experimental results show that higher saturation temperatures lead to a reduction in bulk densities; and that different saturation pressures result in different nucleation processes. In addition, saturation time has a profound effect on the structure of the product. Both X-ray diffraction (XRD) and differential scanning calorimetry (DSC) results show that the foaming treatment with supercritical CO2 increased the crystallinity of PCL.
18.2 Generation of polycaprolactone foams The polycaprolactone (PCL) described in this chapter is CAPA 640 from Solvay in the form of ivory-white granules (Mn 85000, Tg ÿ60 ëC, Tm 60 ëC). Carbon dioxide with purity of 99.9% was used as blowing agent. Reactions were run in a 50.0 ml high-pressure variable-volume stainless steel reactor with two glass-viewing windows. A high-pressure syringe pump (Beijing Satellite Manufacturing Factory, DB-80) was used to charge CO2 into the reaction vessel and attached to the reactor via a coupling and high-pressure tubing. A pressure gauge consisting of a transducer (IC Sensors Co., Model 93) and an indicator (Beijing Tianchen Automatic Instrument Factory, XS/A-1) with the accuracy of 0.01 MPa was also connected to the reactor to observe in situ the pressure change of the system. Schematic representation of the set-up is shown in Fig. 18.2. In the experiments, the reactor was placed in a constant-temperature circulator, which consists of a temperature control module (Thermo Haake, C10) and a bath vessel (Thermo Haake, P5). The temperature of the bath was controlled to 0.1 ëC. Foams were prepared in a glass tube (15 mm 50 mm) inside the reactor to facilitate removal of the foamed samples. PCL was placed into the tube and the PCL and tube were then placed in the reactor together. The closed reactor was preheated in a bath to a set temperature, and flushed for a few minutes with CO2. The cell was then filled to the desired pressure. At this pressure the resin was exposed to supercritical CO2 for a prescribed period of time. Finally, the valve of the reactor was opened and the pressure was released to the atmosphere. The depressurization time was recorded in order to quantify its influence on the final product. The system was maintained at zero pressure for approximately half an hour so that the bubbles could mature completely.
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18.2 Apparatus description diagram. 1. Gas cylinder. 2. Valve. 3. Syringe pump. 4. Vent. 5. Variable-volume reactor with view windows. 6. Sample. 7. Temperature circulator. 8. Pressure gauge.
The foams were characterized to determine their densities, cell sizes and cell shapes. Density was measured using a gravity bottle with a capillary tube in its lid. The weight of the bottle filled with distilled water was measured with an analytical balance (accuracy 0.001 g) at a preset temperature. Following this, the sample was put into the bottle, water of the same volume as the sample overflowed along the capillary tube. The bottle containing both water and sample was re-weighed. The density of the sample was calculated using the following equation: w1 0 18:1 w1 w2 ÿ w3 where density of sample;0 density of water; w1 weight of the sample; w2 weight of bottle filled with water; w3 weight of bottle containing both water and the sample. The cell structures of foamed samples were also studied using an AMRAY1000B scanning electron microscope (SEM). The samples were prepared by freezing in liquid nitrogen, fracturing the surface, mounting the fracture on stubs with carbon paint and sputter coated with gold, forming a layer of approximately Ê in thickness. The cell size for each sample was calculated through eight 100 A diameter measurements and the average value was regarded as cell diameter. The desorption kinetics of CO2 in PCL under a particular processing condition (40 ëC, 10 MPa) was studied using Berens' method.22±24 Through these measurements, average mass gain was calculated, and the results were plotted versus the square root of desorption time. In Fig. 18.3 the non-linear relationship between mass gain and the square root of desorption time illustrates the inability of Fickian's theory to explain the desorption data. When this desorption data was plotted versus the natural logarithm of desorption time (see Fig. 18.4), a better linear dependence of mass change on the logarithm of time was obtained.
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18.3 Mass uptake of CO 2 versus the square root of desorption time. (Temperature 40 ëC, pressure 10 MPa, saturation time 3 h.)
Since the growth of the cells is allowed by the suppression of glass transition (Tg) resulting from the diluent effect rather than heating the polymer to a temperature T above its normal glass transition temperature, the driving force for growth of bubbles is still temperature difference `T ÿ Tg', which is controlled by manipulating Tg through changing the CO2 pressure. Nucleation is induced by supersaturation caused by a sudden pressure drop from the equilibrium solution state and nuclei growth until the polymer vitrifies at a lower pressure.
18.4 Mass uptake of CO2 versus the natural logarithm of desorption time (seconds). (Temperature 40 ëC, pressure 10 MPa, saturation time 3 h.)
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18.5 SEM photographs of cross- and longitudinal sections of PCL foams. (Temperature 40 ëC, saturation time 3 h, magnification 150.) (a) crosssection, pressure 16 MPa; (b) longitudinal section, pressure 16 MPa; (c) cross-section, pressure 15 MPa; (d) longitudinal section, pressure 15 MPa.
Observation via SEM shows that the PCL foam has the typical skin-core structure. The size of the average cell is larger than 30 m. It may be caused by the fact that the barrier force encountered during the nucleation process is quite low, since PCL is in the rubbery state at these experimental conditions. Another possible explanation is that since the PCL samples are placed into a glass tube, the bubbles are prevented from escaping by the aspect ratio of the glass tube. Figure 18.5 shows the SEM micrograph of the longitudinal section and cross-section of the foamed PCL. We can easily infer that during the nucleation process bubbles cannot grow up freely in the diameter direction because of the space limitation of the tube, in contrast to the complete growth of the cells in the length direction.
18.3 Effect of processing conditions on the foaming cell 18.3.1 Effect of temperature on foam structure In addition to advantages of the supercritical fluid (SCF), such as liquid-like densities which allow its solubility to be orders of magnitude higher than gases,
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18.6 Bulk foam density as a function of saturation temperature. (Pressure 10 MPa, saturation time 2 h.)
and gas-like viscosities which lead to high rates of diffusion, the plasticization effect of CO2 causing depression of the substrate's Tg is also important to the polymeric materials in the previous studies. In this work, however, due to the low Tg of the PCL (ÿ60 ëC), plasticization appears not to be a key factor. At the lower temperature of 35 ëC, foaming materials can also be obtained after long periods of time (up to 24 h), however, the overall shape change between raw and foamed PCL is not significant. When the temperature is higher (above 40 ëC) with a constant saturation pressure of 10 MPa, samples melt when saturated with CO2 and a larger expansion of the foamed material is obtained. Figure 18.6 shows the effect of temperature on the bulk density under 10 MPa pressure. It is observed that the bulk density is increased with increasing temperature in the region between 40±50 ëC. The mechanism of this phenomenon can be explained by solubility and viscosity of PLC at different temperatures. With the increase of temperature, the solubility of CO2 will be decreased according to Henry's Law, however the viscosity of the polymer will be reduced simultaneously. The intramolecular and intermolecular forces will be decreased as the material softens, so the sample can be saturated at 40 ëC and a lower pressure of 8 MPa for a short time. If the experimental temperature is lowered to 35 ëC a higher pressure of 16 MPa is required for a longer period of time. If the system is heated to a temperature of 45 ëC or higher, the diameter of the bubbles can grow up to one centimeter in size during depressurization, however the cells will collapse due to the weight of their walls. Figure 18.7 shows the effect of temperature on the size of the cells. It is seen that the cell size increases with increasing saturation temperature in the region of 40 to 50 ëC. When the temperature is higher than 55 ëC, the cell size starts to decrease with increasing temperature.
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18.7 Effect of temperature on cell size (10 MPa pressure; 3 h).
This unique property gives the supercritical CO2-assisted foaming a promising direction for new applications: when the exterior or interior sizes of the product are closely restricted (such as artificial skins or bones, etc.), rough casts can be shaped first and then foamed at low temperatures; when the product has no strict restriction on geometrical sizes (such as drug-release systems, drug containers, etc.), the supercritical CO2-assisted foaming can be carried out at relatively high temperatures.
18.3.2 Effect of pressure on foam structure The effect of saturation pressure on the foaming structure at 40 ëC was studied in detail and the experimental results are shown in Figs 18.8 and 18.9. Figure 18.8 shows the SEM photographs of cross- and longitudinal sections of PCL foams at different pressures. Equivalently, the foam density is plotted in Fig. 18.9. It is seen that the cell size first increases with saturation pressure from 8 MPa until 14 MPa, then decreases sharply from 14 MPa to 16 MPa. As shown by the homogeneous nucleation theory, when the magnitude of the pressure drop increases, the energy barrier to nucleation decreases, leading to more cells nucleated in a given volume. Therefore the average cell size decreases with the increase of saturation pressure from 14 to 16 MPa. This homogeneous nucleation theory was also verified by Beckman.9 In the lower pressure range, the pressure release gradient is less than that of high saturation pressure range (14~16 MPa), and the effect of this release gradient is minimal when compared with the energy barrier to nucleation. At the same time, fluid density as well as the amount of CO2 in the system is crucial, larger CO2 bubbles were formed in the saturated PCL melt at higher pressure and not at the lower pressure.
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18.8 SEM photographs of cross-sections of PCL foams formed under different saturated pressure. (Temperature 40 ëC, saturation time 3 h, magnification 150/50.) Top: 14 MPa; center: 15 MPa; bottom: 16 MPa.
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18.9 Average cell size as a function of saturation pressure. (Temperature 40 ëC, saturation time 3 h.)
18.3.3 Effect of saturation time on foam structure The effect of saturation time on foaming structure was studied at a fixed pressure (10 MPa) and temperature (40 ëC). The experimental results are shown in Fig. 18.10, showing that the average cell size changes slightly at the beginning from 1.5 h to 3 h. Cell size then increases sharply from 3 h to 4 h, and decreases sharply from 4 h to 8 h. It should be noted that the time of saturation represents the time that the PCL sample is exposed to the high-pressure CO2 prior to the pressure quench, and has no relation to the rate of pressure quench, which controls the time period for nucleation and growth. Although pressure quench is a key factor in the resulting foam structure, our study indicates that saturation time also has a contribution to the foam structure. A similar phenomenon was also reported by Beckman9 in which longer exposure to the high-pressure CO2 caused greater absorption of the CO2 by the polymer. In the work presented here, the average cell sizes (see Fig. 18.10), measured from the SEM micrographs were taken in the central region of the samples, i.e., the influence of concentration gradient across the sample thickness was excluded. So only the influence of saturation time on the foam structure was considered in cell size calculation. It should be noted that there is a peak of cell size for both saturation pressure (see Fig. 18.9) and saturation time (see Fig. 18.10) at a particular temperature. When the pressure is lower or saturation time is shorter, increasing pressure or time will increase the amount of CO2 diffused into PCL, which results in larger
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18.10 Density of the bulk foams as a function of depressurization time. (Temperature 40 ëC, pressure 10 MPa, saturation time 2 h.)
cell sizes. However, since there is a limitation to the melt strength of all materials, there is a maximum bubble size that materials can produce. Above this maximum size the bubble will collapse. The maximum size depends upon many factors, such as temperature, pressure and the physical and chemical variations in the materials during processing (crosslinking or decomposition, etc.). The experimental results indicate that there exists an optimum temperature to achieve a maximum expansion ratio for each temperature of polymer melt. Since high pressure and longer saturation time both decrease the Tg, which has a similar effect to controlling temperature, a study of the effect of temperature on the expansion and cell structure is important. Figure 18.11 is a schematic representation of the effect of temperature on the bulk density and cell size. There is a maximum point for the expansion. When the temperature is very low (below 35 ëC), the diffusion rate of the CO2 into the polymer is also very low so the polymer has little chance of foaming. Even under long saturation times when there is enough absorbed CO2, the foamed products are uniform because of melt fracture. The melt fracture phenomenon has also been observed during extrusion foaming.25 When the temperature is too high (above 55 ëC), the bubbles start to escape through the hot skin layer of the foam during expansion. The bulk density and cell size have a close relationship even though they do not correspond directly. There is a maximum point of bulk density and cell size. The volume expansion was strong as a function of both pressure and temperature at certain saturation times. When the temperature was lower than 30 ëC the cell size was small and the bulk density was higher, thus it may be concluded that the cell size
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18.11 Schematic representation of foaming at various temperatures.
will increase and bulk density will decrease with increasing temperature and saturation time.
18.3.4 Effect of depressurization rate on foam structure A series of experiments were carried out at 40 ëC and 10 MPa using a number of different depressurization rates. The experimental results are given in Fig. 18.12 and show that the bulk foam density increases with increasing depressurization time. It may be deduced that prolonging the depressurization time ensured the cells more time to contract at fixed sites, thus decreasing the bulk volume. Hence the bulk density increases with depressurization time.
18.12 Average cell size as a function of saturation time. (Temperature 40 ëC, pressure 10 MPa.)
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Since microstructure influences the performance of materials it is necessary to control the nucleation step that determines the cell size and size distribution of the foam. The models used for studying nucleation are classified as homogeneous nucleation and heterogeneous nucleation. The homogeneous nucleation models17,26,27 were built around the chemical nucleation theory. In short, the theory looks at the relative rate of obtaining a non-stable cluster of foaming phases over an activation barrier that is defined by the phase equilibrium and surface tension. The homogeneous nucleation models were used to interpret the negligible nucleation rate up to a gas saturation pressure of about 5,000 psi (34.5 MPa). However, true homogeneous nucleation is a difficult phenomenon to effect even in a laboratory utilizing temperature variation processing methods. Furthermore, homogeneous nucleation is an inherently random process and it may not be considered a foaming mechanism. Kumar and Suh28 have concluded that the cell nucleation phenomenon in polystyrene is not well described by the homogeneous chemical nucleation theory. The inadequacies of simple nucleation theory have been addressed by Kweeder et al.,8 by hypothesizing the existence of a population of preformed microvoids in the system around which nucleation takes place. Some heterogeneous nucleation methods8,9,29 have been developed to obtain a controlled, predictable nucleation mechanism resulting in the desired microstructure. However, the use and modeling of this principle relies on a good understanding of the heterogeneities introduced. Agreement with experimental data depends largely on how the physical system agrees with the model's assumptions. Campbell et al.8,29 developed a model to predict the heterogeneous nucleation through studying the effect of thermal and pressure history on the microdamage in polycarbonate and polystyrene. It was shown that samples cooled quickly under low pressure exhibited markedly more microscopic cracks. Under a constant-temperature variable-pressure processing, Beckman et al.9 have shown that the classical nucleation theory can be used to describe the nucleation under higher pressure conditions. In this constant-temperature variable-pressure processing, the polymer samples were swollen by supercritical CO2 over a sufficiently long period of time to ensure that the amount of liquid absorbed by the polymer was at equilibrium. The amount of supercritical CO2 absorbed is sufficient to reduce the Tg of the polymer to below the ambient temperature, generating a liquid, albeit concentrated, polymer solution. Quick reduction of the pressures at constant temperature generates both the pores and drives the system towards vitrification, freezing ± in the microstructure. As processing is carried out at a CO2 pressure much higher than needed to plasticize the polymer at the operation temperature, the system is in a homogeneous liquid state. Therefore, it has been shown that the classical nucleation theory can be used to describe the nucleation activity under these conditions. The homogeneous nucleation theory has also been used to study the nucleation of PMMA.
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18.4 Crystallinity of foamed polycaprolactone 18.4.1 Crystallinity of the foamed PCL One of the main purposes for developing the microcellular foams is their potential for applications in medicine, for example the containers for sustained drug delivery and as the raw material for artificial skin and bones. Apart from its biocompatibility with the body, its stability and biodegradability during the period of application are also important. Previous studies have shown that molecular weight and crystallinity are the dominant factors affecting the biodegradability of PCL30,31 and that the amorphous part of PCL degrades prior to the crystalline part in a biotic environment.32,33 XRD and DSC can be used to study the crystallinity of virgin and microcellular PCL. The X-ray diffraction (XRD) measurements were performed using a Bruker D8 Diffractometer operating at 40 kV, 40 mA, Cu K radiation monochromatized with a graphite sample monochromator. A diffractogram was recorded between 2 angles of 12ë and 45ë. The amorphous region of the sample was approximated using a Gaussian fit as illustrated in Fig. 18.13. The area of the amorphous region was subtracted from the total area of the diffractogram to give percentage crystallinity in the materials that were investigated. The XRD results for the virgin and foamed PCL are shown in Fig. 18.14 and summarized in Table 18.1. The XRD results indicate a significant increase in crystallinity (compared to the virgin PCL) for both the foamed systems, with an increase of 42.00% and 42.86% for the foamed PCL-1 (treated under 10 MPa for 2 h) and foamed PCL-2 (treated under 15 MPa for 3 h), respectively.
18.13 XRD diffractogram of foamed PCL and the Gaussian fit used to approximate the amount of the amorphous region in the sample.
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18.14 XRD results of: A, original PCL; B, original PCL melted and quenched rapidly; C, foamed PCL (pressure 10 MPa); D, foamed PCL (pressure 15 MPa); E, foamed PCL (pressure 15 MPa), re-melted and quenched.
The crystallinity of these materials, as measured by XRD, is in good correspondence to the data obtained via DSC. A Perkin-Elmer Pyris-1 DSC with internal coolant (Intracooler 1P) and nitrogen purge gas was used. Melting point and enthalpies of indium and zinc were used for temperature and heat capacity calibration. Samples were heated from 20 ëC to 100 ëC at 10 ëC/min to measure the heat capacity used for evaluation of crystallinity. The foamed samples were cooled down from 100 ëC then heated again under the same conditions to confirm the effect of foaming on crystallinity. The variation of relative crystallinity was calculated by: X HX =Hv %
18:2
in which HX is the heat capacity of formed samples and Hv is the heat capacity of virgin PCL materials. Table 18.1 XRD results showing the effect of foaming on the crystallinity in the various PCL systems Sample Virgin PCL (as received) Virgin PCL (remelted and quenched) Foamed PCL-1 Foamed PCL-2 Re-melting-2
Crystallinity (%)
Variation of crystallinity (%)
42.0 35.0 49.7 50.0 30.7
ö ö 42.00 42.86 ö
The PCL was foamed at 40 ëC. Foamed PCL-1 was treated for 2 h at 10 MPa and foamed PCL-2 was treated for 3 h at 15 MPa.
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18.15 DSC curves of virgin and foamed PCL. (Temperature 40 ëC, pressure 10 MPa, saturation time 2 h.)
Figure 18.15 shows DSC curves of virgin and a foamed PCL (foamed PCL-1, treated at 40 ëC and 10 MPa for 2 h). It is seen that both onset temperature and melting are increased after supercritical CO2 foaming. This gives a strong indication that the crystal structure has been improved or that the thickness of crystalline lamellae has been increased. Some detailed results of crystallinity are listed in Table 18.2. The data in this table shows clearly that relative crystallinity is increased significantly after foaming. This increase of crystallinity can be explained by the orientation of polymer chains during foaming.34,35
18.5 Conclusion PCL foam can be produced in supercritical CO2. Various factors affecting the foaming structure have been studied in detail. The experimental results indicate that there exists an optimum temperature to achieve a maximum expansion ratio of each polymer melt. Since high pressure and longer saturation time both decrease the Tg, and in effect are similar to controlling the temperature, quantification of Table 18.2 Effect of SC CO2 treatment on thermal behaviors of PCL Sample
Virgin PCL Foamed PCL-1 Re-melting-1 Foamed PCL-2 Re-melting-2
Onset temperature (ëC)
Melting temperature (ëC)
Heat capacity (J/g)
Variation of crystallinity (%)
52.19 54.36 52.36 55.43 52.23
54.28 57.80 55.10 59.30 54.60
41.68 60.80 43.65 61.60 43.73
ö 45.87% 47.98%
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the effect of temperature on the expansion and cell structure is important. A maximum point of bulk density and cell size was found. The volume expansion was a function of both pressure and temperature under certain saturation times. When the temperature was lower than 30 ëC the cell size was small and bulk density was higher, thus generally the cell size could be increased and bulk density decreased by increasing both the temperature and saturation time. A peak of cell size was also found for both saturation pressure and saturation time at particular temperatures. When the pressure was lower or the saturation time was shorter, increasing pressure or time increased the amount of diffused CO2 into PCL, which resulted in larger cell sizes. However, since there is a limitation to the melt strength of all materials, there is a maximum bubble size that materials can produce. Above this maximum size the bubble will collapse. Under constant-temperature, variable-pressure processing it has been shown that the classical nucleation theory can be used to describe the nucleation process under higher pressure conditions. In this constant-temperature, variablepressure processing, PCL was swollen by supercritical CO2 over a sufficiently long period of time to ensure that an equilibrium amount of liquid was absorbed by the polymer. The amount of supercritical CO2 absorbed was sufficient to reduce the Tg of the polymer to below ambient temperature, generating a concentrated liquid polymer solution. Quick reduction of the pressures at constant temperature generates both the pores and also drives the system towards vitrification, freezing ± in the microstructure. Since the processing is undertaken at a CO2 pressure much higher than that needed to plasticize the polymer at the operating temperature, the system is in a homogeneous liquid state. Therefore, it has been shown that classical nucleation theory can be used to describe the nucleation activity under these conditions. The crystallinity of PCL was increased significantly after foaming as shown by both XRD and DSC. This was promising for improving biodegradability, as previous studies have shown that crystallinity was one of the dominant factors affecting the degradation of PCL.30,31 Furthermore, understanding the complex nature of the structure/property relationships of microcellular PCL foams produced using supercritical CO2, is essential in controlling their formation and unlocking their potential for applications in both medicine and other high performance areas.
18.6 References 1. Klempner D and Frisch K C, Handbook of Polymeric Foams and foam Technology, Hanser, New York, 1991. 2. Throne J, Science and Technology of Polymer Process, Suh N P and Sung N eds, pp. 77±131, MIT Press, Cambridge, Mass, 1979. 3. McHugh M A and Krrukonis V J, Supercritical Fluid Extraction, Butterworths, Stoneham, Mass. 1986.
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4. Aubert J H and Clough R L, `Low density microcellular polystyrene foams', Polymer, 1985, 26, 2047. 5. Young A T, `Polymer-Solvetn Phase Separation as a Route to Low Density, Microcellular Plastic Foams', J. Cell. Plast., 1987, 23, 55. 6. Kumar V and Weller J E, `Microcellular polycarbonate ± Part I: experiments on bubble nucleation and growth', SPE ANTEC Tech. Papers 1991, 37, 1401. 7. Kumar V and Van Der Wel M M, `Microcellular polycarbonate ± Part I: characterization of tensile modulus', SPE ANTEC Tech. Papers 1991, 37, 1406. 8. Kweeder J A, Ramesh N S, Campbell G A and Rasmussen D H, `The Nucleation of microcellular polystyrene foam', SPE ANTEC Tech. Papers 1991, 37, 1398. 9. Goel S K and Beckman E J, `Generation of Microcellular Polymeric Foams Using Supercritical Carbon Dioxide. I: Effect of Pressure and Temperature on Nucleation', Polym. Eng. Sci., 1994, 34, 1137. 10. Goel S K and Beckman E J, `Generation of Microcellular Polymeric Foams Using Supercritical Carbon Dioxide. II: Cell Growth and Skin Formation', Polym. Eng. Sci., 1994, 34, 1148. 11. Baldwin D F, Park C B and Suh N P, `A Microcellular Processing Study of Poly(ethylene Terephthalate) in the Amorphous and Semi-Crystalline States: Part I Microcell Nucleation', Polym Eng. Sci., 1996, 36, 1437. 12. Baldwin D F, Park C B and Suh N P, `A Microcellular Processing Study of Poly(ethylene Terephthalate) in the Amorphous and Semi-Crystalline States: Part II Cell Growth and Process Design', Polym Eng. Sci. 1996 36, 1446. 13. Arora K A, Lesser A J and McCarthy T J, `Compressive Behaviour of Microcellular Polystyrene Foams Processed in Supercritical Carbon Dioxide', Polym. Eng. Sci., 1998, 38, 2055). 14. Taylor L T, Supercritical Fluid Extraction, John Wiley & Sons, New York, 1995. 15. Ramesh N S, Rasmussen D H and Campbell G A, `Nucleation and Experimental Studies of Bubble Growth during the Microcellular Foaming Process', Polym. Eng. Sci., 1991, 31, 1657. 16. Colton J S and Suh N P, `The nucleation of Microcellular Thermoplastic Foam with Additives: Part I: Theoretical Considerations', Polym. Eng. Sci., 1987, 27, 485. 17. Pitt C G, Marks T A and Schindler A, `Biodegradable Drug Delivery Systems Based upon Aliphatic Polyesters: Application to Contraceptives and Narcotic Antagonists', in Controlled Release of Bioactive Materials, R Baker (ed.) Academic Press, New York, 1980. 18. Vert M., `Biomedical polymers from chiral lactides and functional lactones: properties and applications', Makromol.Chem., Macromol. Symp., 1986, 6, 109. 19. Gogolewski S and Pennings A., "Biodegradable Materials of Polyactides, Porous Biomedical Materials Based on Mixtures of Polyactides and Polyurethanes", Macromol. Chem., Rapid. Commun., 1982, 3, 839. 20. Popove V K, Mandel F S and Howdle S M, `Supercritical fluid assisted production of synthetic bone composites', in Proceedings of the 5th meeting on supercritical fluids, Nice, France, 1998. 21. Sparacio D and Beckman E J, `Generation of microcellular biodegradable polymers in supercritical carbon dioxide' Polym. Preprint, 1997, 2, 422. 22. Berens A R, Huvard G S, Korsmeyer E W and Kunig F W, `Application of compressed carbon dioxide in the incorporation of additives in to polymers', J. App. Polym. Sci., 1992, 46, 231±242.
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23. Liang M T and Wang C M, `Production of Engineering Plastics Foams by Supercritical CO2', Ind. Eng. Chem. Res., 2000, 39, 4622. 24. Liang M T and Wang C M, `Production of very low density microcellular polypropylene by supercritical carbon dioxide', in Proceedings of the 6th meeting on supercritical fluids, Nottingham, United Kingdom, 1999. 25. Park C B, Behravest A H and Venter R D, `Low density microcellular foam processing in extrusion using CO2' Polym. Eng. Sci., 1989, 38 (11), 1812±1823. 26. Martini J E, Waldman F and Suh N P, `The Production and Analysis of Mirocellular Thermoplastic Foams', Processing of SPE ANTEC-82, 1982, 28, 674. 27. Youn J R and Suh N P, `Processing of Microcellular Polyester Composites', Polymer Composites, 1985, 6, 175. 28. Kumar V and Suh N P, Proceedings of ANTEC-88, 1988, 715-718. 29. Adams M E, Campbell G A and Cohen A, `Thermal-stress induced damage in thermoplastic matrix materials for advanced composites', Polym. Eng. Sci., 1991, 31 (18): 1337±1343. 30. Berens A R, Huvard G S, Korsmeyer R W and Kunig F W, `Application of compressed carbon dioxide in the incorporation of additives into polymers', J. Appl. Polym. Sci., 1992, 46, 231. 31. Berens A R and Huvard G S, `Supercritical Fluid Science and Technology', Johnston K P, Penniger J M L (eds). ACS Symposium Series 406, American Chemical Society, Washington, DC (1989). 32. Benedict C V, Cameron J A and Huang S J, `Polycaprolactone degradation by mixed and pure cultures of bacteria and a yeast' J. Appl. Polym. Sci. 1983, 28, 335. 33. Field R D, Rodriguez F and Finn R K, `Microbial degradation of polyesters: Polycaprolactone degraded by P. pullulans' J. Appl. Polym. Sci. 1974, 18, 3571. 34. Nadella H P, Spruiell J E and White J L, J. Appl. Polym. Sci., 1975, 18, 2539. 35. Yu L, Shanks R A and Starchurski Z H, `Kinetics of polymer crystallization;, Polym. in Progress, 1995, 20, 651±702.
Generation of biodegradable polycaprolactone foams in supercritical carbon dioxide L Y U and K D E A N , CSIRO ± Manufacturing and Infrastructure Technology, Australia and Q X U , Zhenghou University, China
18.1 Introduction The importance of developing biodegradable polymers has been well documented in the current literature (journals, books and government discussion papers), including this book, thus it has not been repeated in this chapter. However, it should be noted today that the major barrier for growth in applications for biodegradable polymers is price. Hence, developing some high value applications (for example in the medical field) and reducing price are the key drivers in the research and development of biodegradable polymers. Foaming using supercritical CO2 offers a potential avenue to achieve these two objectives. Foamed polymeric materials are produced in a wide range of bulk densities that mainly determine their mechanical properties. A high-density foam that has an improved tensile strength and modulus can be used for load-bearing applications, such as structural parts, while a low-density foam can be used in thermal insulation and packaging applications. In addition to the foam density, the size and distribution of cells also affects the final properties of the foam. Conventional plastic foams have relatively poor mechanical properties because the cell size is typically larger than 100 m and the cell size distribution is very non-uniform.1,2 In general, foams with very fine cell size exhibit better mechanical properties. Supercritical fluids, above their critical temperature and pressure conditions, exhibit unique behavior by combining the properties of conventional liquids and gases.3 In particular, they exhibit liquid-like densities allowing for solvent power of orders of magnitude higher than gases, while gas-like viscosities lead to high rates of diffusion. A novel technique for the use of supercritical fluids as blowing agents for the generation of microcellular foams has recently been developed. It has been shown that this method can produce microcellular foam with a cell density up to 1010 per cm3 and an average cell size varying from less than 10 microns to 50 microns depending on the conditions. Several authors4±14 have reported the method using CO2 and N2 as blowing agents. Initially, the
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polymeric pellets are saturated with gas at moderate pressure (5±6 MPa) followed by heating to a temperature above the glass transition temperature of the polymer. Beckman et al.9,10 reported an analogous, yet different scheme by which to generate microcellular foams in polymers using CO2 as blowing agent; a constant-temperature variable-pressure method based on saturating the polymer with CO2 at much higher pressure (25±30 MPa), in the supercritical region of CO2, followed by a rapid pressure quench. This method takes advantage of the depression of the transition temperature of the polymers by the presence of CO2. The growth of cells is allowed by the suppression of glass transition temperature resulting from the diluent effect rather than heating the polymer to a temperature above its normal glass transition temperature. Microcellular foams are characterized by a cell size between 10 to 50 m, a cell population density greater than 109 cells/cm3, and very narrow cell size distribution. Because of these unique structures, microcellular foamed plastics offer superior mechanical properties, such as impact strength, toughness, and fatigue life when compared to an un-foamed polymer. The enhanced properties of microcellular polymers make them highly competitive in many applications such as packaging, automotive parts, aircraft parts, sporting equipment, insulation, etc., when a cost-effective, continuous manufacturing process for these materials is developed. In packaging applications, where cost is a critical issue, lowering density and improving mechanical properties are two of the best ways to reduce the price of a product. This is an important consideration for the synthetic biodegradable polymers which are currently marketed for packaging, as they are still quite expensive in comparison to more traditional polyolefins. Figure 18.1 shows a schematic representation of a supercritical fluid phase diagram. In this pressure-temperature diagram there are three lines describing the sublimation, melting and boiling process. These lines also define the regions corresponding to the gas, liquid and solid states. Points along the lines (between the phases) define the equilibrium of two of the phases. The vapor pressure (boiling) starts at the triple point and ends at the critical point. The critical region has its origin at the critical point. At this point a supercritical fluid can be defined as substance that is above its critical temperature (Tc) and critical pressure (Pc). The critical temperature is therefore the highest temperature at which a gas can be converted to a liquid by an increase in pressure. The critical pressure is the highest pressure at which a liquid can be converted to a standard gas by an increase in the liquid temperature. In the so-called critical region, there is only one phase and it possesses some of the properties of both gas and liquid. Carbon dioxide, a nontoxic fluid with a relatively low critical point (Tc 31 ëC, Pc 7.376 MPa) is the most widely used in the supercritical fluid field. Supercritical liquid CO2 is found in the triangular region formed by the melting curve, the boiling curve and the line that defines the critical pressure.15 Carbon dioxide is known to swell and significantly plasticize many amorphous polymers, such as poly(methyl methacrylate), polystyrene, polycarbonate and
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18.1 Schematic representation of a supercritical fluid phase diagram.
poly(ethylene terephthalate).2±10 The driving force for growth of bubbles using supercritical CO2 is the temperature difference, which can be controlled by manipulating glass transition temperature via changes in the CO2 pressure, instead of changing temperature by directly heating the polymer. Nucleation is induced via supersaturation caused by a sudden pressure drop from the equilibrium solution state, and the nuclei grow until the polymer vitrifies at a lower pressure. The classical homogeneous nucleation theory is not able to fully describe the nucleation activities in the temperature variation processing.6,9,16,17 Poly(-caprolactone) (PCL) is one of the representative examples for ringopening polymerization of lactones to produce synthetic biodegradable polyesters. Because of its unique combination of biocompatibility, permeability and biodegradability, PCL and some of its copolymers with lactides and glycolides have been widely applied in medicine as artificial skin, artificial bone and containers for sustained drug release.18±20 An example of microcellular foamed PCL in the medical field is guided tissue regeneration and cell transplantation. As far as guided tissue regeneration is concerned, porous implants are used as size selective membranes to promote the growth of a special tissue in a healing site. Ideally, the implant should be inherently biocompatible, have well-defined cell size and be resorbable with appropriate biodegradation rates.21 PCL is a material that meets these demands well. PCL is a biocompatible and biodegradable aliphatic polyester that is bioresorbable and non-toxic for living organisms. Packaging is one of the largest potential markets for all biodegradable polymers, but in this market it is generally price that has been the limiting factor thus far. The use of supercritical CO2 foamed PCL as a biodegradable packaging material has the potential to reduce the cost (by using less material for each packaging unit) and also to expand the number of packaging applications for PCL. Since the glass transition temperature of PCL is very low (Tg ÿ60 ëC)
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which is far below the ice point, the experimental temperature used in this work is much higher than the Tg, and it is this temperature difference that distinguishes this work from previous studies reported by other researchers. As it has been outlined, PCL foaming is of importance both commercially and scientifically. In this chapter, microcellular foaming of low-Tg biodegradable and biocompatible polycaprolactone (PCL) in supercritical CO2 will be described. The effects of a series of variable factors, such as saturation temperature, saturation pressure, saturation time and depressurization time on the foam structures and density were studied through measurement of density and SEM observation. The experimental results show that higher saturation temperatures lead to a reduction in bulk densities; and that different saturation pressures result in different nucleation processes. In addition, saturation time has a profound effect on the structure of the product. Both X-ray diffraction (XRD) and differential scanning calorimetry (DSC) results show that the foaming treatment with supercritical CO2 increased the crystallinity of PCL.
18.2 Generation of polycaprolactone foams The polycaprolactone (PCL) described in this chapter is CAPA 640 from Solvay in the form of ivory-white granules (Mn 85000, Tg ÿ60 ëC, Tm 60 ëC). Carbon dioxide with purity of 99.9% was used as blowing agent. Reactions were run in a 50.0 ml high-pressure variable-volume stainless steel reactor with two glass-viewing windows. A high-pressure syringe pump (Beijing Satellite Manufacturing Factory, DB-80) was used to charge CO2 into the reaction vessel and attached to the reactor via a coupling and high-pressure tubing. A pressure gauge consisting of a transducer (IC Sensors Co., Model 93) and an indicator (Beijing Tianchen Automatic Instrument Factory, XS/A-1) with the accuracy of 0.01 MPa was also connected to the reactor to observe in situ the pressure change of the system. Schematic representation of the set-up is shown in Fig. 18.2. In the experiments, the reactor was placed in a constant-temperature circulator, which consists of a temperature control module (Thermo Haake, C10) and a bath vessel (Thermo Haake, P5). The temperature of the bath was controlled to 0.1 ëC. Foams were prepared in a glass tube (15 mm 50 mm) inside the reactor to facilitate removal of the foamed samples. PCL was placed into the tube and the PCL and tube were then placed in the reactor together. The closed reactor was preheated in a bath to a set temperature, and flushed for a few minutes with CO2. The cell was then filled to the desired pressure. At this pressure the resin was exposed to supercritical CO2 for a prescribed period of time. Finally, the valve of the reactor was opened and the pressure was released to the atmosphere. The depressurization time was recorded in order to quantify its influence on the final product. The system was maintained at zero pressure for approximately half an hour so that the bubbles could mature completely.
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18.2 Apparatus description diagram. 1. Gas cylinder. 2. Valve. 3. Syringe pump. 4. Vent. 5. Variable-volume reactor with view windows. 6. Sample. 7. Temperature circulator. 8. Pressure gauge.
The foams were characterized to determine their densities, cell sizes and cell shapes. Density was measured using a gravity bottle with a capillary tube in its lid. The weight of the bottle filled with distilled water was measured with an analytical balance (accuracy 0.001 g) at a preset temperature. Following this, the sample was put into the bottle, water of the same volume as the sample overflowed along the capillary tube. The bottle containing both water and sample was re-weighed. The density of the sample was calculated using the following equation: w1 0 18:1 w1 w2 ÿ w3 where density of sample;0 density of water; w1 weight of the sample; w2 weight of bottle filled with water; w3 weight of bottle containing both water and the sample. The cell structures of foamed samples were also studied using an AMRAY1000B scanning electron microscope (SEM). The samples were prepared by freezing in liquid nitrogen, fracturing the surface, mounting the fracture on stubs with carbon paint and sputter coated with gold, forming a layer of approximately Ê in thickness. The cell size for each sample was calculated through eight 100 A diameter measurements and the average value was regarded as cell diameter. The desorption kinetics of CO2 in PCL under a particular processing condition (40 ëC, 10 MPa) was studied using Berens' method.22±24 Through these measurements, average mass gain was calculated, and the results were plotted versus the square root of desorption time. In Fig. 18.3 the non-linear relationship between mass gain and the square root of desorption time illustrates the inability of Fickian's theory to explain the desorption data. When this desorption data was plotted versus the natural logarithm of desorption time (see Fig. 18.4), a better linear dependence of mass change on the logarithm of time was obtained.
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18.3 Mass uptake of CO 2 versus the square root of desorption time. (Temperature 40 ëC, pressure 10 MPa, saturation time 3 h.)
Since the growth of the cells is allowed by the suppression of glass transition (Tg) resulting from the diluent effect rather than heating the polymer to a temperature T above its normal glass transition temperature, the driving force for growth of bubbles is still temperature difference `T ÿ Tg', which is controlled by manipulating Tg through changing the CO2 pressure. Nucleation is induced by supersaturation caused by a sudden pressure drop from the equilibrium solution state and nuclei growth until the polymer vitrifies at a lower pressure.
18.4 Mass uptake of CO2 versus the natural logarithm of desorption time (seconds). (Temperature 40 ëC, pressure 10 MPa, saturation time 3 h.)
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18.5 SEM photographs of cross- and longitudinal sections of PCL foams. (Temperature 40 ëC, saturation time 3 h, magnification 150.) (a) crosssection, pressure 16 MPa; (b) longitudinal section, pressure 16 MPa; (c) cross-section, pressure 15 MPa; (d) longitudinal section, pressure 15 MPa.
Observation via SEM shows that the PCL foam has the typical skin-core structure. The size of the average cell is larger than 30 m. It may be caused by the fact that the barrier force encountered during the nucleation process is quite low, since PCL is in the rubbery state at these experimental conditions. Another possible explanation is that since the PCL samples are placed into a glass tube, the bubbles are prevented from escaping by the aspect ratio of the glass tube. Figure 18.5 shows the SEM micrograph of the longitudinal section and cross-section of the foamed PCL. We can easily infer that during the nucleation process bubbles cannot grow up freely in the diameter direction because of the space limitation of the tube, in contrast to the complete growth of the cells in the length direction.
18.3 Effect of processing conditions on the foaming cell 18.3.1 Effect of temperature on foam structure In addition to advantages of the supercritical fluid (SCF), such as liquid-like densities which allow its solubility to be orders of magnitude higher than gases,
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18.6 Bulk foam density as a function of saturation temperature. (Pressure 10 MPa, saturation time 2 h.)
and gas-like viscosities which lead to high rates of diffusion, the plasticization effect of CO2 causing depression of the substrate's Tg is also important to the polymeric materials in the previous studies. In this work, however, due to the low Tg of the PCL (ÿ60 ëC), plasticization appears not to be a key factor. At the lower temperature of 35 ëC, foaming materials can also be obtained after long periods of time (up to 24 h), however, the overall shape change between raw and foamed PCL is not significant. When the temperature is higher (above 40 ëC) with a constant saturation pressure of 10 MPa, samples melt when saturated with CO2 and a larger expansion of the foamed material is obtained. Figure 18.6 shows the effect of temperature on the bulk density under 10 MPa pressure. It is observed that the bulk density is increased with increasing temperature in the region between 40±50 ëC. The mechanism of this phenomenon can be explained by solubility and viscosity of PLC at different temperatures. With the increase of temperature, the solubility of CO2 will be decreased according to Henry's Law, however the viscosity of the polymer will be reduced simultaneously. The intramolecular and intermolecular forces will be decreased as the material softens, so the sample can be saturated at 40 ëC and a lower pressure of 8 MPa for a short time. If the experimental temperature is lowered to 35 ëC a higher pressure of 16 MPa is required for a longer period of time. If the system is heated to a temperature of 45 ëC or higher, the diameter of the bubbles can grow up to one centimeter in size during depressurization, however the cells will collapse due to the weight of their walls. Figure 18.7 shows the effect of temperature on the size of the cells. It is seen that the cell size increases with increasing saturation temperature in the region of 40 to 50 ëC. When the temperature is higher than 55 ëC, the cell size starts to decrease with increasing temperature.
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18.7 Effect of temperature on cell size (10 MPa pressure; 3 h).
This unique property gives the supercritical CO2-assisted foaming a promising direction for new applications: when the exterior or interior sizes of the product are closely restricted (such as artificial skins or bones, etc.), rough casts can be shaped first and then foamed at low temperatures; when the product has no strict restriction on geometrical sizes (such as drug-release systems, drug containers, etc.), the supercritical CO2-assisted foaming can be carried out at relatively high temperatures.
18.3.2 Effect of pressure on foam structure The effect of saturation pressure on the foaming structure at 40 ëC was studied in detail and the experimental results are shown in Figs 18.8 and 18.9. Figure 18.8 shows the SEM photographs of cross- and longitudinal sections of PCL foams at different pressures. Equivalently, the foam density is plotted in Fig. 18.9. It is seen that the cell size first increases with saturation pressure from 8 MPa until 14 MPa, then decreases sharply from 14 MPa to 16 MPa. As shown by the homogeneous nucleation theory, when the magnitude of the pressure drop increases, the energy barrier to nucleation decreases, leading to more cells nucleated in a given volume. Therefore the average cell size decreases with the increase of saturation pressure from 14 to 16 MPa. This homogeneous nucleation theory was also verified by Beckman.9 In the lower pressure range, the pressure release gradient is less than that of high saturation pressure range (14~16 MPa), and the effect of this release gradient is minimal when compared with the energy barrier to nucleation. At the same time, fluid density as well as the amount of CO2 in the system is crucial, larger CO2 bubbles were formed in the saturated PCL melt at higher pressure and not at the lower pressure.
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18.8 SEM photographs of cross-sections of PCL foams formed under different saturated pressure. (Temperature 40 ëC, saturation time 3 h, magnification 150/50.) Top: 14 MPa; center: 15 MPa; bottom: 16 MPa.
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18.9 Average cell size as a function of saturation pressure. (Temperature 40 ëC, saturation time 3 h.)
18.3.3 Effect of saturation time on foam structure The effect of saturation time on foaming structure was studied at a fixed pressure (10 MPa) and temperature (40 ëC). The experimental results are shown in Fig. 18.10, showing that the average cell size changes slightly at the beginning from 1.5 h to 3 h. Cell size then increases sharply from 3 h to 4 h, and decreases sharply from 4 h to 8 h. It should be noted that the time of saturation represents the time that the PCL sample is exposed to the high-pressure CO2 prior to the pressure quench, and has no relation to the rate of pressure quench, which controls the time period for nucleation and growth. Although pressure quench is a key factor in the resulting foam structure, our study indicates that saturation time also has a contribution to the foam structure. A similar phenomenon was also reported by Beckman9 in which longer exposure to the high-pressure CO2 caused greater absorption of the CO2 by the polymer. In the work presented here, the average cell sizes (see Fig. 18.10), measured from the SEM micrographs were taken in the central region of the samples, i.e., the influence of concentration gradient across the sample thickness was excluded. So only the influence of saturation time on the foam structure was considered in cell size calculation. It should be noted that there is a peak of cell size for both saturation pressure (see Fig. 18.9) and saturation time (see Fig. 18.10) at a particular temperature. When the pressure is lower or saturation time is shorter, increasing pressure or time will increase the amount of CO2 diffused into PCL, which results in larger
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18.10 Density of the bulk foams as a function of depressurization time. (Temperature 40 ëC, pressure 10 MPa, saturation time 2 h.)
cell sizes. However, since there is a limitation to the melt strength of all materials, there is a maximum bubble size that materials can produce. Above this maximum size the bubble will collapse. The maximum size depends upon many factors, such as temperature, pressure and the physical and chemical variations in the materials during processing (crosslinking or decomposition, etc.). The experimental results indicate that there exists an optimum temperature to achieve a maximum expansion ratio for each temperature of polymer melt. Since high pressure and longer saturation time both decrease the Tg, which has a similar effect to controlling temperature, a study of the effect of temperature on the expansion and cell structure is important. Figure 18.11 is a schematic representation of the effect of temperature on the bulk density and cell size. There is a maximum point for the expansion. When the temperature is very low (below 35 ëC), the diffusion rate of the CO2 into the polymer is also very low so the polymer has little chance of foaming. Even under long saturation times when there is enough absorbed CO2, the foamed products are uniform because of melt fracture. The melt fracture phenomenon has also been observed during extrusion foaming.25 When the temperature is too high (above 55 ëC), the bubbles start to escape through the hot skin layer of the foam during expansion. The bulk density and cell size have a close relationship even though they do not correspond directly. There is a maximum point of bulk density and cell size. The volume expansion was strong as a function of both pressure and temperature at certain saturation times. When the temperature was lower than 30 ëC the cell size was small and the bulk density was higher, thus it may be concluded that the cell size
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18.11 Schematic representation of foaming at various temperatures.
will increase and bulk density will decrease with increasing temperature and saturation time.
18.3.4 Effect of depressurization rate on foam structure A series of experiments were carried out at 40 ëC and 10 MPa using a number of different depressurization rates. The experimental results are given in Fig. 18.12 and show that the bulk foam density increases with increasing depressurization time. It may be deduced that prolonging the depressurization time ensured the cells more time to contract at fixed sites, thus decreasing the bulk volume. Hence the bulk density increases with depressurization time.
18.12 Average cell size as a function of saturation time. (Temperature 40 ëC, pressure 10 MPa.)
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Since microstructure influences the performance of materials it is necessary to control the nucleation step that determines the cell size and size distribution of the foam. The models used for studying nucleation are classified as homogeneous nucleation and heterogeneous nucleation. The homogeneous nucleation models17,26,27 were built around the chemical nucleation theory. In short, the theory looks at the relative rate of obtaining a non-stable cluster of foaming phases over an activation barrier that is defined by the phase equilibrium and surface tension. The homogeneous nucleation models were used to interpret the negligible nucleation rate up to a gas saturation pressure of about 5,000 psi (34.5 MPa). However, true homogeneous nucleation is a difficult phenomenon to effect even in a laboratory utilizing temperature variation processing methods. Furthermore, homogeneous nucleation is an inherently random process and it may not be considered a foaming mechanism. Kumar and Suh28 have concluded that the cell nucleation phenomenon in polystyrene is not well described by the homogeneous chemical nucleation theory. The inadequacies of simple nucleation theory have been addressed by Kweeder et al.,8 by hypothesizing the existence of a population of preformed microvoids in the system around which nucleation takes place. Some heterogeneous nucleation methods8,9,29 have been developed to obtain a controlled, predictable nucleation mechanism resulting in the desired microstructure. However, the use and modeling of this principle relies on a good understanding of the heterogeneities introduced. Agreement with experimental data depends largely on how the physical system agrees with the model's assumptions. Campbell et al.8,29 developed a model to predict the heterogeneous nucleation through studying the effect of thermal and pressure history on the microdamage in polycarbonate and polystyrene. It was shown that samples cooled quickly under low pressure exhibited markedly more microscopic cracks. Under a constant-temperature variable-pressure processing, Beckman et al.9 have shown that the classical nucleation theory can be used to describe the nucleation under higher pressure conditions. In this constant-temperature variable-pressure processing, the polymer samples were swollen by supercritical CO2 over a sufficiently long period of time to ensure that the amount of liquid absorbed by the polymer was at equilibrium. The amount of supercritical CO2 absorbed is sufficient to reduce the Tg of the polymer to below the ambient temperature, generating a liquid, albeit concentrated, polymer solution. Quick reduction of the pressures at constant temperature generates both the pores and drives the system towards vitrification, freezing ± in the microstructure. As processing is carried out at a CO2 pressure much higher than needed to plasticize the polymer at the operation temperature, the system is in a homogeneous liquid state. Therefore, it has been shown that the classical nucleation theory can be used to describe the nucleation activity under these conditions. The homogeneous nucleation theory has also been used to study the nucleation of PMMA.
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18.4 Crystallinity of foamed polycaprolactone 18.4.1 Crystallinity of the foamed PCL One of the main purposes for developing the microcellular foams is their potential for applications in medicine, for example the containers for sustained drug delivery and as the raw material for artificial skin and bones. Apart from its biocompatibility with the body, its stability and biodegradability during the period of application are also important. Previous studies have shown that molecular weight and crystallinity are the dominant factors affecting the biodegradability of PCL30,31 and that the amorphous part of PCL degrades prior to the crystalline part in a biotic environment.32,33 XRD and DSC can be used to study the crystallinity of virgin and microcellular PCL. The X-ray diffraction (XRD) measurements were performed using a Bruker D8 Diffractometer operating at 40 kV, 40 mA, Cu K radiation monochromatized with a graphite sample monochromator. A diffractogram was recorded between 2 angles of 12ë and 45ë. The amorphous region of the sample was approximated using a Gaussian fit as illustrated in Fig. 18.13. The area of the amorphous region was subtracted from the total area of the diffractogram to give percentage crystallinity in the materials that were investigated. The XRD results for the virgin and foamed PCL are shown in Fig. 18.14 and summarized in Table 18.1. The XRD results indicate a significant increase in crystallinity (compared to the virgin PCL) for both the foamed systems, with an increase of 42.00% and 42.86% for the foamed PCL-1 (treated under 10 MPa for 2 h) and foamed PCL-2 (treated under 15 MPa for 3 h), respectively.
18.13 XRD diffractogram of foamed PCL and the Gaussian fit used to approximate the amount of the amorphous region in the sample.
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18.14 XRD results of: A, original PCL; B, original PCL melted and quenched rapidly; C, foamed PCL (pressure 10 MPa); D, foamed PCL (pressure 15 MPa); E, foamed PCL (pressure 15 MPa), re-melted and quenched.
The crystallinity of these materials, as measured by XRD, is in good correspondence to the data obtained via DSC. A Perkin-Elmer Pyris-1 DSC with internal coolant (Intracooler 1P) and nitrogen purge gas was used. Melting point and enthalpies of indium and zinc were used for temperature and heat capacity calibration. Samples were heated from 20 ëC to 100 ëC at 10 ëC/min to measure the heat capacity used for evaluation of crystallinity. The foamed samples were cooled down from 100 ëC then heated again under the same conditions to confirm the effect of foaming on crystallinity. The variation of relative crystallinity was calculated by: X HX =Hv %
18:2
in which HX is the heat capacity of formed samples and Hv is the heat capacity of virgin PCL materials. Table 18.1 XRD results showing the effect of foaming on the crystallinity in the various PCL systems Sample Virgin PCL (as received) Virgin PCL (remelted and quenched) Foamed PCL-1 Foamed PCL-2 Re-melting-2
Crystallinity (%)
Variation of crystallinity (%)
42.0 35.0 49.7 50.0 30.7
ö ö 42.00 42.86 ö
The PCL was foamed at 40 ëC. Foamed PCL-1 was treated for 2 h at 10 MPa and foamed PCL-2 was treated for 3 h at 15 MPa.
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18.15 DSC curves of virgin and foamed PCL. (Temperature 40 ëC, pressure 10 MPa, saturation time 2 h.)
Figure 18.15 shows DSC curves of virgin and a foamed PCL (foamed PCL-1, treated at 40 ëC and 10 MPa for 2 h). It is seen that both onset temperature and melting are increased after supercritical CO2 foaming. This gives a strong indication that the crystal structure has been improved or that the thickness of crystalline lamellae has been increased. Some detailed results of crystallinity are listed in Table 18.2. The data in this table shows clearly that relative crystallinity is increased significantly after foaming. This increase of crystallinity can be explained by the orientation of polymer chains during foaming.34,35
18.5 Conclusion PCL foam can be produced in supercritical CO2. Various factors affecting the foaming structure have been studied in detail. The experimental results indicate that there exists an optimum temperature to achieve a maximum expansion ratio of each polymer melt. Since high pressure and longer saturation time both decrease the Tg, and in effect are similar to controlling the temperature, quantification of Table 18.2 Effect of SC CO2 treatment on thermal behaviors of PCL Sample
Virgin PCL Foamed PCL-1 Re-melting-1 Foamed PCL-2 Re-melting-2
Onset temperature (ëC)
Melting temperature (ëC)
Heat capacity (J/g)
Variation of crystallinity (%)
52.19 54.36 52.36 55.43 52.23
54.28 57.80 55.10 59.30 54.60
41.68 60.80 43.65 61.60 43.73
ö 45.87% 47.98%
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the effect of temperature on the expansion and cell structure is important. A maximum point of bulk density and cell size was found. The volume expansion was a function of both pressure and temperature under certain saturation times. When the temperature was lower than 30 ëC the cell size was small and bulk density was higher, thus generally the cell size could be increased and bulk density decreased by increasing both the temperature and saturation time. A peak of cell size was also found for both saturation pressure and saturation time at particular temperatures. When the pressure was lower or the saturation time was shorter, increasing pressure or time increased the amount of diffused CO2 into PCL, which resulted in larger cell sizes. However, since there is a limitation to the melt strength of all materials, there is a maximum bubble size that materials can produce. Above this maximum size the bubble will collapse. Under constant-temperature, variable-pressure processing it has been shown that the classical nucleation theory can be used to describe the nucleation process under higher pressure conditions. In this constant-temperature, variablepressure processing, PCL was swollen by supercritical CO2 over a sufficiently long period of time to ensure that an equilibrium amount of liquid was absorbed by the polymer. The amount of supercritical CO2 absorbed was sufficient to reduce the Tg of the polymer to below ambient temperature, generating a concentrated liquid polymer solution. Quick reduction of the pressures at constant temperature generates both the pores and also drives the system towards vitrification, freezing ± in the microstructure. Since the processing is undertaken at a CO2 pressure much higher than that needed to plasticize the polymer at the operating temperature, the system is in a homogeneous liquid state. Therefore, it has been shown that classical nucleation theory can be used to describe the nucleation activity under these conditions. The crystallinity of PCL was increased significantly after foaming as shown by both XRD and DSC. This was promising for improving biodegradability, as previous studies have shown that crystallinity was one of the dominant factors affecting the degradation of PCL.30,31 Furthermore, understanding the complex nature of the structure/property relationships of microcellular PCL foams produced using supercritical CO2, is essential in controlling their formation and unlocking their potential for applications in both medicine and other high performance areas.
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