s Phase Transition on the High-Pressure Differential Scanning

Research Article pubs.acs.org/journal/ascecg

Effect of Unexpected CO2’s Phase Transition on the High-Pressure Differential Scanning Calorimetry Performance of Various Polymers Erbo Huang,† Xia Liao,*,† Chongxiang Zhao,†,‡ Chul B. Park,†,‡ Qi Yang,† and Guangxian Li*,† †

College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu, Sichuan 610065, China ‡ Microcellular Plastics Manufacturing Laboratory, Department of Mechanical and Industrial Engineering, University of Toronto, 5 King’s College Road, Toronto, Ontario, Canada M5S 3G8 ABSTRACT: We used a high-pressure differential scanning calorimeter (HPDSC) to study polymer plasticization by compressed gases at pressures of up to 30 MPa for polylactide (PLA), polycarbonate (PC), isotactic polypropylene (iPP), and polystyrene (PS). The pressure reached values twice as high as the previously published data. We found that the polymer/carbon dioxide (CO2) system’s heating curves have an unidentified endothermic peak above 5 MPa, which turns out to be from CO2’s phase transition. The HP-DSC could accurately determine the depression of the glass transition temperature (Tg), crystallization temperature (Tc), and melting temperature (Tm) of various polymers at low pressures by simply starting at a higher temperature to avoid CO2’s phase transition; however, the increased plasticization effect of the dissolved CO2 lowered the Tg to the level of overlapping with CO2’s phase transition phenomena at elevated pressures, and therefore, the depressed Tg could not be measured above 6 MPa for PLA, PC, or PS. On the other hand, the Tc of iPP decreased with an increase in pressure, whereas Tm values of PLA and iPP decreased slightly with an increase in pressure and then remained almost unchanged above a certain pressure, which may indicate an increased hydrostatic pressure effect at elevated pressures. KEYWORDS: High-pressure differential scanning calorimeter (HP-DSC), Glass transition temperature, Melting temperature, Supercritical carbon dioxide, Phase transition

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differential scanning calorimetry (HP-DSC)10,11,37,38 have all been used to infer plasticization of polymers by compressed gases. Of these methods, thermal and calorimetric analyses have long been established as being suitable for the investigation of a polymer’s thermal behavior. Therefore, DSC is often used because it is simple, and it provides fast and accurate information about the glass to rubber transition. Ambient DSC has been used to scan polymer samples presaturated with gas to obtain the plasticized Tg, Tc, and Tm. Chiou et al.35 reported on the use of ambient DSC to evaluate the effect of absorbed gas molecules on the Tg of polymers. The polymer specimen was first enclosed in a high-pressure chamber for a period of time, and then it was taken out for the DSC measurement after the pressure had been released. However, this ambient DSC cannot accurately measure the changes in Tg, Tc, and Tm with the dissolved gas because gas will be escaping from the sample within the ambient DSC instrument. If we heat the sample quickly, some gas may remain in the sample and, therefore, may play a role in depressing these temperatures. However, because the amount of lost gas cannot

INTRODUCTION Supercritical carbon dioxide (scCO2) can dissolve considerably in a glassy polymer and, thus, can induce crystallization1−3 and depress the glass transition temperature (Tg),4−8 the crystallization temperature (Tc),9−14 and the melting temperature (Tm).6,15,16 The extent of the depression in Tg, Tc, and Tm depends on the gas’s pressure or its concentration in the polymer matrix.4,10,17−19 Knowledge of a given polymer/CO2 system’s thermal behavior is an important parameter in the development of a polymer foam fabrication process.10,20−25 It gives the information about the conditions under which the morphology of the growing cells can be arrested, as with macrocellular foams, and, additionally, about the conditions under which cell nucleation and growth can take place, as with microcellular foams.26 Such Tg−pressure, Tc−pressure, and Tm−pressure data are also required to characterize the optimal temperature−pressure window within which a gas separation membrane should be used27 and for the extraction of an unreacted species28 or impurities.29 A variety of techniques such as nuclear magnetic resonance spectroscopy (NMR),30 X-ray diffraction (XRD),31 dielectric relaxation,32 creep compliance,5 dynamic thermomechanical analysis (DMA),33 gas solubility and permeation,34 ambient differential scanning calorimetry (DSC),35,36 and high-pressure © XXXX American Chemical Society

Received: January 2, 2016 Revised: February 2, 2016

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DOI: 10.1021/acssuschemeng.6b00008 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

CO2’s phase transition curve overlaps with the lowered Tg signal. However, the observation of the Tm depression for PLA and iPP at an elevated pressure of up to 30 MPa CO2 was possible because the CO2’s phase transition curve did not overlap with the lowered Tm signal. On the other hand, the Tc− pressure profile for PLA could not be obtained because the cooling curves had overly strong noise with high-pressure CO2, most likely because of its low enthalpy of melting (93 J/g) and the small volume and pressure variations in the HP-DSC cell induced by heating or cooling processes. However, we could observe clearly the crystallization peaks of iPP with a higher enthalpy of melting (209 J/g) in the cooling curves in the range of 0.1−30 MPa. Our results contribute toward a better understanding of the polymer’s thermal behaviors under higher pressures of up to 30 MPa, which has never been disclosed before.

be measured, the observed changes in Tg, Tc, and Tm cannot be quantitatively related to the gas content. The decrease in Tm is also caused by the less closely packed crystals, especially from the larger number of crystals.10,39−41 Because the plasticizing gas effect may not be expected, the observed depression of Tm from the ambient DSC must be all from the poor degree of close packing in crystals. To overcome the shortcomings of this ex situ measurement using ambient DSC, a HP-DSC for in situ measurement has been widely used in polymer research. It is a powerful device that offers additional insight into polymer systems. Hachisuka et al.42 used a DSC instrument with high-pressure pans containing a poly(2,6-dimethyl-phenylene oxide) (PPO)/CO2 system under pressure. Handa et al.42 and O’Neill and Handa38 used a high-pressure heat-flow calorimeter to take in situ measurements of the change in the Tg values of PPO and poly(methyl methacrylate) (PMMA) as a function of gas pressure. That HP-DSC was rated for use at pressures of up to ∼7 MPa. However, even with low-pressure CO2, it was found that the baseline showed a bit of noise and slop. Those baseline characteristics became worse with pressure. At higher pressures, the baseline had extensive noise, and the curvature also became quite amplified. Zhang and Handa36,41 investigated the effect of CO2 on the glass transition behavior of polystyrene (PS). Mi and Zheng43,44 reported the Tg−pressure profiles of polycarbonate (PC)/CO2 and polyethylene terephthalate (PET)/ CO2 systems with a HP-DSC. Park et al.10,39,40,45 investigated the effects of CO2 on the Tg, Tc, and Tm changes of polylactide (PLA) in CO2 at pressures of up to 6 MPa. The in situ measurements of the Tg, Tc, or Tm values of PLA9 and the polypropylene (PP)46−50 in high-pressure CO2 have also been investigated, with the assistance of a HP-DSC. Overall, the temperature scanning technique is a dynamic thermal scan, and it is not possible to keep the polymer/gas system in thermodynamic equilibrium during the heating or cooling runs. Furthermore, the baseline stability deteriorates at elevated pressures, and the available pressure range is somewhat limited. Only a few previous studies have examined the thermal behaviors of CO2 at pressures above its critical point. However, in situ studies of the CO2-induced shift of the Tg values using a HP-DSC have been limited to within the critical pressure of CO2. For the Tm values, the investigations in CO2 using a HPDSC have been limited to within 6 and 14 MPa for PLA45 and PP,48 respectively. It has been discovered in this study that the phase transition of CO2 was one of the reasons for these unidentified noises during the high-pressure experiments. Therefore, we focused our study on the effect of the CO2 phase transition on the HPDSC traces. On the basis of these analyses, we conducted in situ investigations of polymer/CO2 systems at elevated pressures by optimizing the experimental conditions and adjusting the parameter settings. In this work, a HP-DSC was used to establish the Tg−pressure, Tc−pressure, and Tm−pressure profiles in polymer/CO2 systems at pressures higher than those previously reported. No literature has shown data available above 14 MPa. In this study, the thermal behaviors of several polymer/gas systems at a very high pressure of up to 30 MPa were disclosed. PLA, PC, and PS were used for the Tg measurements, and PLA and iPP were used for Tc and Tm measurements. We noted that despite the high-pressure capability of the HP-DSC up to 30 MPa, the observation of the Tg depression at an elevated pressure above 6 MPa CO2 for PLA, PC, and PS materials was not straightforward because the

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EXPERIMENTAL SECTION

Materials. Semicrystalline PLA pellets were supplied by NatureWorks LLC (Grade 2002D; Mn = 1.1 × 105), and its reported Disomer content is 4.3 mol %. PC was provided by Teijin Chemicals (Grade K-1300; Mn = 2.7 × 104). PS was purchased from Taiwan Polystyene (Ningbo, China; Grade GP5250) with a melt-flow index of 7.0 g/10 min (2.16 kg at 230 °C). Commercial grade iPP homopolymer was supplied by Lanzhou Petrochemical Corp. (Mn = 8.7 × 104) with a melt-flow index of 2.9 g/10 min (2.16 kg at 230 °C). To decrease their moisture level, the PLA pellets were dried in vacuum ovens at 65 °C, and other three polymer pellets were dried at 80 °C for 24 h before being used. PC, PS, and PLA were used for glass transition measurements. iPP was used for crystallization measurement. PLA and iPP were used for melting behavior measurements. The materials were used as received without any further purification. CO2 and N2 (>99.5% purity) were obtained from Qiaoyuan Industry Co. (Chengdu, China). Procedure. The glass transitions and melting behaviors of polymer/gas systems were characterized using a HP-DSC (Sensys Evo, Setaram) for in situ measurements with high-pressure vessels. The schematic of this system is shown in Figure 1. The gas pressure was

Figure 1. Schematic of the HP-DSC system. increased to the required level using an ISCO 260D syringe pump. The HP-DSC was calibrated with In, Sn, Pb, and Zn under ambient and high pressures. The following procedure was applied for every measurement. After the sample had been installed, the system was evacuated for 5 min. Then, the sample was scanned and the gas pressure applied during the whole process. The measurement sample of ∼20 mg was heated to a molten state at a rate of 10 °C/min and equilibrated at this temperature for 10 min to remove all previous thermal and stress histories, as well as to dissolve the CO2 into it. It was then cooled. Following this, the sample was reheated to a molten state at a rate of 10 °C/min. The Tg and Tm were recorded during the second heating process. The Tg measurement of the PLA in N2 was also taken using the same procedure. To B


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ACS Sustainable Chemistry & Engineering investigate the nonisothermal melt crystallization of iPP in CO2, the samples were heated and equilibrated at 200 °C for 10 min. Then, the samples were cooled to room temperature at a cooling rate of 5 °C/ min at various CO2 pressures. The nonisothermal melt crystallization was recorded during the cooling scan.

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RESULTS AND DISCUSSION Analysis of the CO2’s Phase Transition in the HP-DSC. Figure 2 shows the HP-DSC traces from the PLA tests that

Figure 5. HP-DSC traces of PLA at various N2 pressures.

Figure 2. HP-DSC scans for the PLA/CO2 system.

Figure 6. PS tests in 4 MPa CO2 started from 0 and 40 °C.

not only the sample’s property but the nature of the gas should be considered. The scan’s starting temperature and the system’s pressure were critical factors in determining the phase state and the heat transfer performance of the CO2 encountered during the scan. Thus, the changing phase of the CO2 during the scan must have produced the final unusual curve. To gain insight into the nature of the endothermic peak in this rather unexpected phenomenon, a detailed thermodynamic analysis is conducted. On the basis of the supposition mentioned above, blank tests were conducted to examine CO2’s thermodynamic properties. A blank test means there is only gas but no polymer sample in the sample cell, while the reference cell is disconnected from the gas reservoir. The sample (CO2) was then scanned while still in contact with the gas cylinder at 10 °C/min from the starting temperature to 120 °C. A series of experiments were performed with the same scan procedure in 0.1−20 MPa CO2.

Figure 3. HP-DSC traces of blank tests in CO2 at various pressures.

start from 20 °C in the CO2 at various pressures. In the tests from 0.1 to 5 MPa, the glass transitions can be clearly observed from the HP-DSC traces. However, on the vertical scale, an unidentified endothermic peak appeared in the curve of the tests at 6, 7, and 8 MPa CO2. This was unusual. These peaks should not have been caused by the PLA’s glass transitions because of their abnormal enthalpies of fusion. The HP-DSC traces reflect a PLA/CO2 system’s thermal property. Therefore,

Figure 4. (a) CO2’s phase transition diagram. (b) Pressure−Ttransition (onset) plot extracted from Figure 3. C


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Figure 7. (a) HP-DSC traces of PLA at various CO2 pressures. (b) Dependence of the ΔTg of PLA on CO2 pressure.

Figure 8. (a) HP-DSC traces of PS scanned at various CO2 pressures. (b) Depression in ΔTg of PS as a function of CO2 pressure.

appeared on the heating curves for the tests at 1−7 MPa. Moreover, the peaks were barely distinguishable from the baseline, when the experimental pressure was increased to >10 MPa. Figure 4 shows a simple sketch of the CO2’s phase diagram51,52 and the CO2 pressure versus Ttransition (onset) profile extracted from Figures 3 and 4a, where Ttransition (onset), in our study, is expressed as the initial point of the endothermic peak in the HP-DSC curves. The Ttransition (onset) obtained from the liquid−vapor phase boundary is consistent with the values reported in the literature.53 An accurate description of the CO2 phase equilibria by equations52 was developed to determine the CO2’s wide-range state, and all of the available experimental information about the triple-point temperature, the critical point, the vapor pressure, and the caloric properties of the liquid−vapor phase boundary have been previously reviewed.52 The characterization information of the data sets has been summarized for the corresponding properties, including heat capacity, which varies significantly in the vicinity of the liquid−vapor phase

Figure 9. Dependence of the ΔTg of PLA on N2 pressure.

Herein, the HP-DSC experimental findings and the discussion of the CO2’s phase transition are reported. Figure 3 shows the results of the blank tests. For the test at 0.1 MPa, the curve was smooth and noiseless, while abrupt transitions

Figure 10. (a) HP-DSC traces of PC scanned at various CO2 pressures. (b) Depression in the ΔTg of PC as a function of CO2 pressure. D


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Figure 11. (a) Cooling curves of iPP at various CO2 pressures. (b) Depression in the ΔTc of iPP as a function of CO2 pressure.

Figure 12. (a) HP-DSC diagrams of iPP at various CO2 pressures. (b) Dependence of the ΔTm of iPP on CO2 pressure.

Figure 13. (a) HP-DSC traces of PLA at various CO2 pressures. (b) Dependence of the ΔTm of PLA on CO2 pressure.

scan from −50 to 120 °C at 0.1 MPa. Therefore, the heat enthalpy remained unchanged, and no endothermic peak would have appeared in the curve. From the CO2 phase diagram and the calculation from an equation of state,52 it is clear that all of the scans from 1 to 7 MPa began with conditions below which CO2 exists as a liquid. However, CO2 changed into a gas or a supercritical fluid with an increase in temperature during the scans. Unfortunately, these transitions were dominant in the temperature ranges of interest to the present application. This complicated the HP-DSC measurements in that the results became quite erratic. Hence, the positions of the phase transition endothermic peaks and the shapes of curves in the blank tests in Figure 3 corresponded with those in the HP-DSC

Table 1. Effect of Compressed CO2 on the Tg Values (°C) of PLA, PS, and PC 0.1 MPa

1 MPa

2 MPa

3 MPa

4 MPa

5 MPa

6 MPa

56.8 101.0 149.1

53.1 96.0 144.1

50.8 88.2 134.6

47.1 78.7 129.5

43.9 75.7 120.5

39.7 66.6 107.2

36.6 94.5

PLA PS PC

boundary and in the near-critical region of the CO2. The results clearly show that the CO2’s phase transitions occurred in all the blank tests under experimental pressures, except at 0.1 MPa. From the triple-point temperature and pressure in Figure 4a, the CO2 remained in a gas state during the whole experimental Table 2. Effect of Compressed N2 on the Tg Values (°C) of PLA

PLA

0.1 MPa

1 MPa

5 MPa

10 MPa

15 MPa

20 MPa

25 MPa

30 MPa

57.0

56.5

54.1

51.5

49.9

48.7

47.7

47.8

E


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ACS Sustainable Chemistry & Engineering Table 3. Effect of Compressed CO2 on the Thermal Transition Temperatures (°C) of PLA and iPP Tm of PLA Tc of iPP Tm of iPP

0.1 MPa

1 MPa

5 MPa

10 MPa

15 MPa

20 MPa

25 MPa

30 MPa

161.6 113.0 165.4

158.3 111.8 162.8

150.8 106.8 158.7

140.9 100.4 154.7

129.8 95.4 150.3

− 93.6 146.8

− 92.2 144.4

− 92.4 144.1

function of CO2 pressure for the PLA/CO2 system at pressures of up to 6 MPa. The Tg was depressed from the initial value of 57 °C because of the increased flexibility and free volume introduced by the CO2 diluent. Takada et al.9 investigated the CO2 pressure dependence of the Tg of PLA using a HP-DSC and found that CO2 induced Tg depression at a rate of approximately −3.6 °C/MPa, which is in fairly good agreement with our result. CO2 diffuses into the amorphous regions and swells the PLA matrix, which results in an increased free volume; therefore, the Tg shifts to a lower temperature. PLA’s Tg at 6 MPa was successfully measured when the test was started from 25 °C; however, when the pressure was increased above 6 MPa, the glass transition could not be seen in the HPDSC traces. On one hand, the dissolution of the CO2 resulted in the Tg’s sharp decrease to a low temperature in the range of the CO2’s phase transition temperature. On the other hand, this phenomenon is not surprising because most polymer/gas systems follow linear trends across considerable gas pressure ranges;36,41 one such system is PMMA/CO2,62 in which the retrograde vitrification was due to very strong interactions between the polymer and the CO2.63 This retrograde vitrification behavior was found by Condo et al.;64−66 there was another glass-to-rubber transition upon cooling because of the increased solubility of CO2 at a lower temperature as well as a glass-to-rubber transition when the polymer was heated in CO2. Figure 8 shows the HP-DSC traces of PS in CO2 and the ΔTg−pressure profile. Dissolved CO2 has a drastic plasticization effect, and the decreasing rate of Tg was −7.0 °C/MPa in CO2 at pressures in the range of 0.1−5 MPa. A comparison of the effect induced by the CO2 and N2 on the PLA’s Tg suggests that the gas solubility and the polymer−gas interaction67,68 were responsible for the depressed Tg. Figure 9, extracted from Figure 5, shows the ΔTg−pressure profiles of PLA in N2. The dissolved gas reduced the number of interchain interactions. N2 was found to have a similar effect on the glass transition of the polymer in CO2, and the depression of Tg followed a trend similar to that observed in CO2, although the effect was not as large because of its low solubility. For example, the Tg’s depression rates of PLA in N2 and CO2 in the range of 0.1−5 MPa were approximately −0.60 and −3.6 °C/MPa, respectively. Researchers have demonstrated that the solubility of CO2 in PLA was many times greater than that of N2.58,59 Figure 10 shows the HP-DSC traces of PC in CO2 and the ΔTg−pressure profile. The decreasing rate of Tg was −9.1 °C/ MPa in CO2 at pressures ranging from 0.1 to 6 MPa. Zhang et al.36 investigated the effect of CO2 on the Tg of PC, and the results showed a similar decreasing rate with the data in this study. The results are rather similar to those observed for PLA with CO2, and the glass transition of the PC could not be observed because the pressure was above 6 MPa. PC is a polymer with a relatively high Tg at ambient pressure, and the drastic decrease in the Tg is due to the preferred interactions of the CO2 with the PC’s ester moieties; it was thought that polar groups of the PC weaken the tendency to be plasticized due to the dipolar interaction with the polarizable CO2 molecules.69

scans in Figure 2 at 6, 7, and 8 MPa. PLA is a polymer with a relatively low Tg, and dissolved CO2 further decreases its Tg to the temperature range of CO2’s phase transition. As a result, for the tests at 6, 7, and 8 MPa in Figure 2, CO2’s phase transition endothermic peaks overlapped with the glass transition of the PLA and the Tg of PLA in CO2 at pressures above 5 MPa could not be observed in the HP-DSC curves. In addition, CO2 easily exhibits achievable critical parameters (Pc = 7.38 MPa; Tc = 31.1 °C).52 Above this critical pressure and temperature, CO2 reaches a supercritical state, exhibiting gaslike and liquidlike properties. From the shape of the curves in Figure 3, the phase transitions from a liquid to a gas state were sharp at pressures below the critical pressure (1−7 MPa) while they became smoother when moving from a liquid to a supercritical state (≥8 MPa). This indicated that the phase transition from a liquid to a gas state was abrupt while it became a more complicated process when the transformation was from a liquid to a supercritical fluid. For comparison, the result of the Tg tests of PLA in N2 at various pressures is shown in Figure 5. The Tg can be clearly observed across the range of test pressures. Unlike CO2, N2 has a very low solubility in polymers,54−59 and because of its very low critical temperature (Tc = −147 °C),60,61 there was no phase transition during the whole scan. To avoid the influence of the endothermic peak associated with the CO2’s phase transition, experiments have been conducted in this research with a higher starting temperature, above the range of the CO2’s phase transition temperature. To begin, PS is a polymer with a relatively high Tg of ∼100 °C at ambient pressure. The Tg up to certain CO2 pressures may be beyond the temperature range of CO2’s phase transition. Thus, the Tg measurements should not be influenced. Figure 6 shows the PS test results in 4 MPa CO2, starting from 0 and 40 °C. There was no CO2 phase transition endothermic peak when the test started from 40 °C. Also, a significant PS glass transition was clearly observable from the curve. Meanwhile, in the sample, no thermal transition could be seen, even on the magnified graft, of the test’s curve, which started at 0 °C. When the experiment was begun from a temperature above CO2’s phase transition temperature, the phase transition was not involved when the temperature continued to rise. Therefore, we took extreme care in taking the measurements to avoid cooling the high-pressure cell to a temperature below the condensation temperature of the CO2. In other words, we maintained the gas over the temperature range of the CO2 phase transition, attempting to avoid any phase transition of the gas. Tg Measurements of PLA, PC, and PS under a HighPressure Gas. Employing the approach described above, the Tg values of PLA, PC, and PS were measured in CO2 at various pressures. The Tg tests of PLA in CO2 were started from 25 °C. In our study, Tg is obtained at the inflection point of the HPDSC curve. Tc and Tm are expressed as the maximal exothermic and minimal endothermic temperatures reached during the crystallization and melting processes, respectively. Figure 7 shows the HP-DSC traces and the Tg depression (ΔTg) as a F


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ACS Sustainable Chemistry & Engineering Tc Measurement of iPP in High-Pressure CO2. Figure 11a shows the crystallization exothermgrams of iPP at various CO2 pressures as it was cooled at 5 °C/min from its melt. The Tc decreases in the range of 0.1−20 MPa and decreases linearly with CO2 pressure, as shown in Figure 11b. From the slope of the fitting line, a dTc/dp value of −1.03 °C/MPa can be calculated. Previous investigations11,45,46,50,70 showed that the Tc decreased roughly linearly with increasing pressure, and the same tendency was obtained in our experiments at CO2 pressures in the range of 0.1−20 MPa. However, no literature has shown data available beyond 14 MPa in CO2 with a HPDSC. Figure 11 clearly shows that the Tc remained almost unchanged when the CO2 pressure was above 25 MPa. This may be from the hydrostatic pressure or from unknown changes in the crystalline morphology, which will be a future subject of our research. Polymer Melting Behaviors in CO2. The dissolved CO2 not only affects the glass transition and crystallization behavior but also can change the Tm of a polymer6,15,16 as well as the resultant cell morphology of a polymer foam material. To further investigate the effect of CO2 on the polymer melting behavior, iPP and PLA were used in this study. To avoid the appearance of the phase transition endothermic peak of CO2, the iPP’s Tm tests were started from 70 °C. Figure 12 shows the iPP’s melting behavior at various CO2 pressures. The melting point, Tm, as a function of CO2 pressure is shown in Figure 12b. The initial slope found for iPP was −0.83 °C/ MPa for dTm/dp in the range of 0.1−25 MPa. However, it leveled off at higher gas pressures beyond 25 MPa. Figure 13 shows the PLA’s melting behavior in high-pressure CO2. Tables 1, 2, and 3 summarize the pressure dependence of Tg, Tc, and Tm for the studied polymers in N2 and CO2. For PLA, the results showed a value of −2.1 °C/MPa for dTm/dp in the range of 0.1−15 MPa, which is in good agreement with the data reported by Park et al.45 With the PLA sample exposed to the compressed CO2, the free volume must have increased because of the plasticization effect of the dissolved CO2 on the molecular movement. Tm decreased with an increased CO2 pressure due to the CO2’s increased plasticization effect. Moreover, at higher CO2 pressures, a large number of less closely packed crystals were formed,8 and consequently, these less perfect crystals result in a Tm depression. However, when the pressure continued to rise, the position of the endothermic melting peaks remained almost unchanged from 15 to 18 MPa. This leveling off indicated that an effect opposite to plasticization may have come increasingly into play, such as the hydrostatic pressure exerted by the gas at elevated pressures.41,71 Researchers41,71,72 claimed that the gas affects the polymer’s Tm in two ways: the dissolved gas tends to lower the Tm, and the hydrostatic pressure exerted by the gas tends to increase the Tm. The cancellation of the plasticization effect by the hydrostatic effect has been observed previously.71,72 Further, the results shown in Figure 13 are very similar to those observed for iPP’s melting behavior with CO2. However, the CO2 induced a much stronger plasticization effect in PLA with a dTm/dp of −2.1 °C/MPa. The trend in the Tm depression was closely related to the gas’s solubility in the two polymers. The increased solubility of the CO2 in PLA54,73,74 over that in the iPP58,75 has been attributed to the gas’s preferred interactions with the PLA’s ester moieties.68,76,77 Moreover, the peaks were barely distinguishable from the baseline when the experimental pressure was increased above 17 MPa. In these measurements (≥17 MPa),

not only the sample but the entire high-pressure cell was subjected to pressurized gas, and the result would inevitably be influenced by the heat transformation and phase transition of the CO2. However, the baseline had a slight curvature at 10 MPa, and the baseline characteristics became worse with increased pressure and the curvature quite amplified.

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CONCLUSIONS We conducted a detailed study of the melting behavior and glass transition of polymer/CO2 systems using a HP-DSC, which is a powerful tool that provides increased and valuable polymer/gas system information. The baseline grew significantly noisy, and it worsened with an increase in pressure. Thus, the baseline stability deteriorated at elevated pressures, and the experimental pressure was limited to below the CO2’s critical pressure. We found that the PLA/CO2 system’s melting curve had an unidentified endothermic peak, which resulted from the CO2’s phase transition. The occurrence of baseline drifting was attributed to the phase transition and to CO2’s constant change in the diffusivity of CO2 in the polymer during scanning. We effectively avoided the phase transition of CO2 by increasing the starting temperature. This paper is the first to report the Tm measurements of PLA and iPP in CO2 at pressures of up to 15 and 30 MPa. We clearly observed the glass transitions of PS and PC in CO2 at pressures of up to 5 and 6 MPa in the HP-DSC traces. Because of the dissolved CO2, the Tg and Tm values of the polymers decreased; however, the gas’s increasing antiplasticization effect played a role at elevated pressures, and the Tm values of PLA and iPP remained almost constant above a certain pressure. The Tc of iPP in CO2 followed the same trend. These very high-pressure DSC data will be effectively used for various applications such as foam and supercritical technology.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Telephone: +86-28-8540-8361. *E-mail: [email protected]. Telephone: +86-28- 85469011. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51373103 and 51421061) and the Science and Technology Department of Sichuan Province, China (2015HH0026 and 2013GZ0152).

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REFERENCES

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s Phase Transition on the High-Pressure Differential Scanning

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