Systematic study of alcohols based co-blowing agents for polystyrene foaming in supercritical CO2: Toward the high efficiency of foaming process and foam structure optimization

J. of Supercritical Fluids 158 (2020) 104718

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Systematic study of alcohols based co-blowing agents for polystyrene foaming in supercritical CO2 : Toward the high efficiency of foaming process and foam structure optimization Wei Qiang a , Ling Zhao a,b , Tao Liu a , Zhen Liu c , Xiulu Gao a , Dongdong Hu a,d,∗ a

State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai, 200237, China College of Chemistry and Chemical Engineering, Xinjiang University, Urumqi, 830046, China c Department of Physics and Engineering, Frostburg State University, Frostburg, MD, 21532, USA d Department of Chemical Engineering, The University of Texas at Austin, Austin, TX, 78712, USA b

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• Alcohols based co-blowing agents for supercritical CO2 foaming were studied. • The medium-chain length alcohol was a strong plasticizer on PS. • The long-chain length alcohol increased the cell density of foams. • The structure of PS foamed was optimized with the blend of alcohols.

a r t i c l e

i n f o

Article history: Received 21 July 2019 Received in revised form 24 November 2019 Accepted 8 December 2019 Available online 13 December 2019 Keywords: Supercritical carbon dioxide Polystyrene Microcellular foaming Co-blowing agent Alcohols

a b s t r a c t A new method is presented to improve the efficiency of the supercritical CO2 foaming process and optimize the foam structure after evaluating the effect of alcohols as co-blowing agents on supercritical CO2 foaming polystyrene (PS). The simulations revealed that the solubility parameter of the blowing agents and the diffusion coefficient of CO2 for PS increased with the growth of chain length of alcohol. The addition of decanol decelerated the desorption rate of the blowing agents to maintain a higher content of blowing agents in PS, which resulted in the high cell density. Butanol improved the volume expansion ratio (VER) of the foams due to having the largest plasticization effect on PS. By combining the advantages of these two alcohols as a co-blowing agent, an optimized foam structure was obtained with smaller cell size (5.82 ␮m) and larger VER (5.83) than that obtained when foaming with pure CO2 . © 2019 Elsevier B.V. All rights reserved.

1. Introduction

∗ Corresponding author at: State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai, 200237, China. E-mail address: [email protected] (D. Hu). https://doi.org/10.1016/j.supflu.2019.104718 0896-8446/© 2019 Elsevier B.V. All rights reserved.

The importance of thermoplastic foams has been demonstrated over the years in various applications such as heat and sound insulation, food packaging, automotive parts, sporting goods, footwear, cushions, and support materials [1–3]. Nevertheless, considering

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the material costs and superior performances, researchers have attempted to fabricate microcellular foams with smaller cell size and larger VER in the polymer foaming area. [4,5]. Usually, the structure and VER of foams can be tailored via three methods: 1) modification of the polymer matrix; 2) the use of suitable blowing agents; 3) the optimization of the foaming conditions. Compared with other methods, the selection of suitable blowing agents is more convenient and easily controlled. In industrial production, there are two kinds of blowing agents used to obtain foams: chemical blowing agents and physical blowing agents. The chemical blowing agents are the chemicals that release gases under foaming manufacturing conditions. Common physical blowing agents include inert gases (such as nitrogen, carbon dioxide, and argon [6]), volatile hydrocarbons (propane, butane, and pentane), hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs) [7]. Currently, the main physical blowing agents used by the polymer foaming industry are HCFCs and HFCs [8]. However, the polymer foam industry is facing the challenge of developing physical blowing agents with zero ozone-depletion potential and low global warming potential [9,10]. Due to environmental concerns, CO2 is a potential candidate to replace HCFCs and HFCs, as it is nontoxic, chemically inert and nonflammable [11]. Compared to HCFCs and HFCs, CO2 in polymers has a much smaller solubility and much faster diffusion. Since the low solubility of CO2 in polymers causes poor cell density, and because the high diffusivity of CO2 can easily make the cells collapse, it is quite difficult to obtain foams with high cell density and a large VER. To improve the foaming performance of CO2 during the foaming process, mixing the main blowing agent with a co-blowing agent has been suggested [12]. Previous efforts extensively investigated the plasticization effect of co-blowing agents on polymers. Gendron et al. [13] studied the effects of CO2 and ethanol as foaming agents on extruded PS foam and found that ethanol significantly increased the plasticization of the system, and PS foams with uniform cell morphology in the VER (approximately 40) were prepared. The synergistic effect of CO2 and isopropanol on the plasticization effect of PMMA were found when foaming PMMA with CO2 and isopropanol [14]; the presence of 5 wt% isopropanol in CO2 enhanced the nucleation of PMMA foam. Similarly, PS foams with large VERs (approximately 36.2) were obtained by foaming with 2-ethyl hexanol (2-EH) and CO2 due to the decrease in the glass transition temperature (Tg ) of PS caused by 2-EH. For the cell density, the determining factor is the CO2 solubility in the polymer matrix. The diffusivity coefficients of CO2 and 2-EH [15] were measured to evaluate the effect of 2-EH on the diffusivity of CO2 in PS. Salerno et al. [16] foamed PCL, PLA, and PLC with CO2 and ethyl lactate (EL). The addition of 0.2 wt% EL enhanced the plasticization effect of the blowing agent on polymers and finally improved the foam structure. Zhang et al. [17] extruded PS foams with water as a co-blowing agent and found that water had no effect on the Tg of the system. The effects of the solubility and diffusion of the co-blowing agents on the foaming results were also studied. Nistor et al. [18] studied the effect of the concentration of the residua of toluene on PS foaming. The residua of toluene increased the solubility of CO2 in PS, which increased the cell density. In their other work, the effects of n-pentane and cyclopentane on PS foaming results [19] were investigated; the porosity and cell size increased along with the concentrations of the co-blowing agents. Furthermore, pre-impregnating PS with pentanes would decrease the Tg of the system, increase the CO2 solubility, accelerate the diffusion of CO2 , and enhance the cell nucleation and foam porosity. Water [20,21] acts as an efficient plasticizer in the system in the extrusion foaming of polymer with CO2 and water. Since water reduces the crystallinity of PVOH, CO2 can easily diffuse throughout the polymer matrix to form nucleation. When foaming PPSU and PSU with CO2 and ethanol [22], the introduction of 10 wt%

ethanol evidently increased the whole solubility of the blowing agents, which increased the VER of the foams. In these studies, the plasticization effect of the co-blowing agents was fully studied. However, most studies have yielded results in terms of the increase of the foam VER, the corresponding cell size was too large, and the cell density was too low. On one hand, studies [23,24] about the solubility of blowing agents in polymer have been scarce, and they have not contained any results regarding the diffusion and desorption of blowing agents in the polymer. Such results are vital for improving the efficiency of the foaming process since the diffusion of the blowing agents determines the saturation time [25], and the desorption of the blowing agents affects the nucleation efficiency [26]. On the other hand, previous scholars usually selected a single alcohol as the co-blowing agent due to the hydrogen bond between alcohol and CO2 [22,27]. The effect of the difference [28,29] in the solubility and diffusion of alcohols with different chain lengths in the polymer was neglected. Moreover, alcohols with different chain lengths were widely used as co-solvents [30,31] to increase the polarity of supercritical CO2 for the extraction [32,33] and the surfactants [34]. In this work, alcohols with different chain lengths were selected as co-blowing agents to prepare microcellular PS foams. Molecular dynamics (MD) was applied to calculate the solubility parameters of the CO2 /co-blowing agent system and the interaction energy between the blowing agents and the polymer. Then, dissipative particle dynamics (DPD) was conducted to calculate the diffusion coefficients of CO2 and the co-blowing agent in polymer. The plasticization effect of the co-blowing agents on PS was thoroughly investigated using Chow’s model and DSC measurements. Furthermore, desorption tests were performed to measure the uptake and desorption rate of the blowing agents in the polymer. Then, batch foaming was performed to evaluate the effect of the co-blowing agents on the foaming results. Finally, the structures of the foams were optimized using a mixture of alcohols as the co-blowing agent to prepare PS foams with small cell size and large VER. 2. Simulation 2.1. Molecular dynamic simulation MD simulations were used to calculate the solubility parameters of CO2 /co-blowing agents and the interaction energy between PS and the blowing agents by using Material Studio 6.0. The Material Visualizer was used to construct PS with 10 repeating units and the co-blowing agents (methanol, ethanol, 1-propanol, 1-butanol, 1-pentanol, 1-hexanol, 1-heptanol, 1-octanol, 1-nonanol, and 1decanol). For the polymer-solvent calculations, the COMPASS force field was used. Cubic boxes containing 20 frames with periodic boundary conditions were constructed using the Amorphous Cell Module. The composition of the polymer and blowing agents placed in the box is shown in Table 1. After the geometry optimization and 5 annealing cycles, 500 ps of NPT MD simulations (90 ◦ C, 15 MPa) were performed, and the final 20 configurations were applied to calculate the interaction energy and solubility parameters. 2.2. DPD simulations of the diffusion of CO2 and the co-blowing agents MD simulations were performed to obtain the solubility parameters for further DPD simulations. The detailed simulation equation was presented in previous work [35]. AB was calculated from the solubility parameters using Eq. 1: AB =

VAB 2 (ıA − ıB ) kT

(1)

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Table 1 Composition of the simulation system. Entry

Number of PS chains

Composition

Number of molecules of alcohol

Number of molecules of CO2

1 2 3 4 5 6 7 8 9 10

1 1 1 1 1 1 1 1 1 1

CO2 +5% methanol CO2 +5% ethanol CO2 +5% propanol CO2 +5% butanol CO2 +5% pentanol CO2 +5% hexanol CO2 +5% heptanol CO2 +5% octanol CO2 +5% nonanol CO2 +5% decanol

75 50 35 30 25 21 19 18 15 16

950 950 950 950 950 950 950 950 950 950

where ıA and ıB are the solubility parameters of systems A and B, respectively. VAB is calculated by using the average molar volume of beads A and B. In the DPD simulations, the repeating monomer of PS was considered as a single bead. The number of beads was determined by the molecular weight (Mw ) of the polymer, while the blowing agent molecule was also considered as a single bead. All simulations were performed in the Mesocite module. For the calculations of the PS/co-blowing agents system, a reduced DPD density was considered as in 3. For the system of PS/CO2 /co-blowing agents, the reduced density was considered to be 5 due to the high pressure of the system. A cubic box with a size of 100 × 100 × 100 in the DPD unit was established to conduct the simulations. The interaction parameters of different beads were obtained using the following equations. When the reduced density was 3: aij − aii = ai j = ij /0.286

(2)

When the reduced density was 5: aij − aii = ai j = ij /0.689

(3)

where aii is the interaction parameter for similar beads [36]; the interaction parameters ij were calculated in previous calculations. In total, 3 × 105 steps with a time step of 0.05 were performed for all simulations, and the final 300 ps was used for further analysis. The diffusion coefficient of CO2 or the co-blowing agents was determined using the Einstein relation [37]:

 1 d  lim [ri (t) − ri (0)]2 6Nt t→∞ dt Nt

D=

After being dried in vacuum at 90 ◦ C for 48 h, PS granules were compressed into sheets (20 × 10 mm) with 1-mm thickness at 180 ◦ C using hot pressing equipment. For the solubility measurement, the samples were immersed in the solvent at a certain temperature in a high-pressure vessel. The vessel was immersed in a temperature-controlled oil bath. After saturation was achieved, the samples were quickly removed from the vessel, wiped with absorbent paper to remove the excess solvent and weighted on a precise balance (ASTM D792-00). All solubility measurements were repeated 3 times. 3.3. Foaming process The sample and co-blowing agents (the mass of the co-blowing was is equal to 5 wt% of the mass of CO2 , which was calculated according to the density data [38] at a given temperature and pressure) were placed together into a 60-mL vessel. The foaming conditions were selected based on previous works on the foaming of PS [39,40]. After gas replacement was performed 3 times, the vessel was filled with CO2 using an ISCO pump. Then, the vessel was saturated at a certain temperature and pressure. The vessel was equipped with temperature and pressure controllers to maintain the temperature and pressure. The saturation time was calculated according to the sorption equilibrium of CO2 at certain temperatures and pressures. After saturation, the pressure was immediately released. Finally, the foamed samples were removed from the vessel and wiped to remove the excess solvent. 3.4. Desorption of the blowing agents in PS

(4)

i=1

where ri is the position vector of penetrant bead i; N is the number of blowing agent beads; t is the time. 3. Experimental 3.1. Materials PS (158 k) was purchased from BASF-YPC Company. Ethanol (99 %, Adams-beta), 1-butanol (99 %, Adams-beta), 1-hexanol (99 %, Adams-beta) and 1-decanol (99 %, Adams-beta) were used as received. CO2 (99.99 wt%) was purchased by Air Liquide. 3.2. Solubility measurement of alcohols in PS In this work, the solubility of the solvent XL in PS is described as follows: mL XL = ( ) × 100 (5) mL + mPS where mL is the uptake in mass of the solvent in PS, and mPS is the mass of PS.

The desorption behaviors of the blowing agents were measured using gravimetric methods. The saturation conditions were identical to those used in the foaming process. All samples were saturated with CO2 and the co-blowing agent at a certain temperature and pressure. After saturation, the vessel was immersed in ice water, and the pressure was slowly released to prevent foaming. Then, the samples were quickly wiped and placed on a balance to record their weights during the desorption process. There was a short time interval between depressurization and weight measurement. The desorption diffusion coefficient Dd of the blowing agents was calculated as follows [41]: Mt 4 =1− M0 d

 D t 1/2 d d 

(6)

where M0 was extracted from the gas loss curves by linear extrapolation; Mt was the total uptake in mass of the blowing agents at time t; d was the thickness of the sample. 3.5. Characterization A differential scanning calorimeter (DSC, NETZSCH DSC204HP) was used to measure Tg for the polymer-solvent system. The plas-

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ticization effect of alcohols on PS was mainly evaluated during the saturation time of CO2 foaming. Therefore, the saturation time used to prepare the samples for DSC measurements was determined according to the saturation time of CO2 in PS. The calculation of the saturation time is described in the Supporting Information. The measurements were performed as follows: the PS granules were saturated with solvent at 90 ◦ C for 6 h. Then, the samples were removed from the solvent as quickly as possible. Afterward, the samples were heated to 48 ◦ C and held for 5 min to remove the excess solvent from the sample surface, since the pre-experimental results showed that the mass of the samples would be stable after this process. Finally, the temperature was increased from 48 ◦ C to 150 ◦ C at a heating rate of 10 ◦ C/ min in a nitrogen atmosphere. Scanning electron microscopy (SEM) (6360 L V, Hitachi, Japan) was used to observe the morphology of the foams. The foamed samples were frozen and fractured in liquid nitrogen; then, the fracture surface was coated with gold. The density of the foamed samples was measured using the water-displacement method [41]. Eq. 7 determines the VER of the foamed samples, RV : RV = 0 /f

(7)

where f is the density of the foamed samples, and 0 is the density of the polymer. The cell density N0 was determined from Eq. 8 [42]:



N0 =

nM 2 A

3/2

Rv

(8)

where n is the number of cells observed in the SEM image, M is the magnification rate, and A is the area of the image (cm2 ). 4. Results and discussion 4.1. Simulation studies of the blowing agents To improve the efficiency of the foaming process, the effect of alcohols on the enhancement of the foamability of the blowing agents was studied. From a practical viewpoint, it is laborious to experimentally study the supercritical CO2 foaming process using all alcohols ranging from methanol to 1-decanol as the lowing agents. Simulations are more efficient because they have been proven to be reliable and time-saving for solvent and polymer calculations [43,44]. MD simulations were performed to evaluate the miscibility of the blowing agents and polymer. Meanwhile, DPD simulations were conducted to calculate the diffusion coefficient of the blowing agents in the polymer. 4.1.1. Interaction of the blowing agents with PS and the solubility parameters of CO2 and the co-blowing agents The solubility of the blowing agents in PS depends on the gas physical properties and interactions with the polymer matrix [45]. Thus, the effect of the co-blowing agents on the interaction of the polymer with the blowing agents was calculated. The interaction energy Einter between the blowing agents and polymer was calculated using the following equation [22]: Einter = Echain/solvent − (Echain + Esolvent )

(9)

where Echain/solvent is the energy of the polymer/blowing agent system; Echain and Esolvent are the energy of the polymer and blowing agents, respectively. The interaction energy between the blowing agents and PS is shown in Table 2, and some of the simulation boxes are shown in Fig. S1. The results show that the interaction energy is increased in the presence of co-blowing agents, which indicates that the introduction of co-blowing agents will improve the interaction between the blowing agents and PS. Moreover, the interaction energy increased along with the chain length of the co-blowing agents. Thus, the

Fig. 1. Solubility parameter (␦) of blowing agents and PS (90 ◦ C, 15 MPa). ( ) Solubility parameter of PS; ( ) Solubility parameters of CO2 and co-blowing agents; ) Solubility parameter of CO2 . (

incorporation of long-chain alcohols is helpful to enhance the interaction energy between the blowing agent and PS. A favorable interaction between blowing agents and polymer could slow the desorption of the blowing agents from the polymer [46]. Therefore, a stronger interaction is beneficial to maintain an increased concentration of blowing agent in the polymer matrix to aid foaming. To further investigate the effects of the alcohols on the miscibility between the polymer and the blowing agents, MD simulations were performed to calculate the solubility parameters of the blowing agents and PS, since the solubility parameter could be used to predict the miscibility between the two materials. The simulation results are shown in Fig. 1. The results reveal that the blowing agents had better solubility than pure CO2 . Moreover, the solubility parameter of the blowing agent approaches that of PS (13.58 MPa1/2 ) when the chain length increased. According to the similarity rule, blowing agents with longer chain lengths will prefer to dissolve more into the polymer matrix. 4.1.2. Diffusion of the blowing agents in PS The diffusion coefficient of CO2 in PS determined the saturation time of batch foaming because the main foaming agent used in this work was CO2 . In addition, it has been shown that the diffusion coefficients in a multicomponent system vary significantly with the addition of a second solvent [47,48]. Thus, DPD simulations were performed to study the diffusion of CO2 and alcohols into PS, and the DPD simulation is shown in Fig. S2. As shown in Fig. 2, the simulated diffusion coefficient of CO2 in PS (2.08 × 10−10 m2 /s) was close to the experimentally determined value (1.77 × 10−10 m2 /s), which indicates the reliability of the simulation results (the measurement of the CO2 diffusion coefficient in PS was described in our previous works [35,49]). The diffusion coefficients of both CO2 and the co-blowing agents were evidently increased in the ternary system (CO2 /coblowing agent/PS), e.g., the pure CO2 diffusion coefficient was 2.08 × 10−8 cm2 /s in PS and 3.28 × 10−8 cm2 /s in the ternary system, and the diffusion coefficient of decanol was 1.68 × 10−8 cm2 /s in PS and 2.97 × 10−8 cm2 /s in the ternary system. Moreover, the diffusion coefficient of CO2 increased along with the chain length of the co-blowing agents. However, the diffusion coefficients of the co-blowing agents in PS were similar among the medium chain-length alcohols (slightly increased from C2 to C5), and they decreased with the increase in chain length from C6 to C10. This

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Table 2 Interaction energy between blowing agents and PS. Blowing agents

Echain/solvent (kJ/mol)

Echainagents (kJ/mol)

Esolvent (kJ/mol)

Einter (kJ/mol)

CO2 CO2 /methanol CO2 /ethanol CO2 /propanol CO2 /butanol CO2 /pentanol CO2 /hexanol CO2 /heptanol CO2 /octanol CO2 /nonanol CO2 /decanol

16715.1 17504.2 17408.0 17513.8 17570.7 17608.8 17495.4 17454.8 17804.6 17451.0 17480.8

15705.1 1107.5 1078.2 1074.9 1119.6 1121.7 1096.2 1114.2 1146.0 1123.4 1099.1

1079.0 16547.3 16496.7 16619.7 16645.6 16683.3 16619.7 16607.1 16938.9 16630.6 16704.6

−68.5 −150.7 −167.3 −180.8 −194.7 −197.5 −220.7 −266.6 −280.4 −302.8 −323.0

Fig. 2. Diffusion coefficients (D) of the blowing agents in PS at 90 ◦ C. ( ) Diffusion coefficient of CO2 in the PS-CO2 -alcohol system under 15 MPa; (䊐) Diffusion coefficients of alcohols in the PS-CO2 -alcohol system under 15 MPa; ( ) Diffusion coefficient of CO2 in the PS-CO2 system under 15 MPa; ( ) Diffusion coefficients of alcohols in the PS-alcohol system under atmospheric pressure.

Fig. 3. Radial distribution functions (g(r)) for CO2 -alcohols and PS-CO2 ; ( ) PS) methanol-CO2 ; ( ) ethanol-CO2 ; ( ) propanol-CO2 ; ( ) butanolCO2 ; ( ) pentanol-CO2 ; ( ) hexanol-CO2 ; ( ) heptanol-CO2 ; ( ) octanolCO2 ; ( ) nonanol-CO2 ; ( ) decanol-CO2 . CO2 ; (

4.2. Experimental investigation of the effect of the co-blowing agent on the foaming process variation trend is identical to the experimental result in Bernardo et al. [29]. The radial distribution function (RDF) was calculated to explain the fact that the incorporation of long-chain alcohols increased the diffusion coefficients of CO2 in PS, as it describes the chance of finding CO2 at distance r from the co-blowing agent or polymer. The calculation is described by Eq. 10 [50]:

1 gAB (r) = AB 4r 2 · r

NAB j=1

NAB (r → r + r) NAB

(10)

where gAB (r) is the intensity of the RDF; A and B are a set of two beads; r is the distance between bead A and bead B; NAB is the number of beads; AB is the density of the system. The results of the calculations are shown in Fig. 3. For the CO2 -alcohol system, the peaks appeared at approximately 1.13 Å, which shows the strong interactions between CO2 and alcohols. The height of the peaks increased when the chain lengths of the alcohols increased, and the highest peak appeared for the CO2 -decanol system. Meanwhile, no peaks were observed for CO2 -PS. The results of the radial distribution function g(r) implied that the alcohols could attract CO2 due to the strong interactions between CO2 and alcohols. In addition, the long-chain alcohols could attract more CO2 due to the stronger interaction. In the presence of alcohols, the diffusivity of CO2 was accelerated due to the favorable absorption interactions between CO2 and alcohols.

According to the previous discussions in Chapter 4.1, a longchain alcohol could function as a strong CO2 reservoir due to the large interaction energy between CO2 and the long-chain co-blowing agent. Meanwhile, the introduction of a long-chain blowing agent increases the solubility parameter of the solvent system. However, the low diffusion coefficients of long-chain alcohols lengthen the time required to saturate the co-blowing agents. In addition, long-chain alcohols are less soluble in PS than mediumchain alcohols [28]. Considering these competing factors, ethanol, butanol, hexanol, and decanol were selected to further investigate the effect of alcohol co-blowing agents on the supercritical CO2 foaming process and cell morphology. 4.2.1. Plasticization effect of co-blowing agents on PS The co-blowing agents have plasticization effects on the polymer, which are closely related to the foaming window and porosity of the foams [19]. To investigate the plasticization effects of the co-blowing agents on PS, Chow’s model [51] and DSC measurements were performed to investigate the decline in the Tg of PS after immersion with co-blowing agents. Moreover, the solubility of alcohols in PS was measured, and the results are shown in Table S1. According to Chow’s model, the plasticizing efficiency of the co-blowing agents decreases when the chain length increases. Meanwhile, the plasticization is enhanced when the concentration of the co-blowing agent increases. However, the solubility measurement results show that hexanol had the largest solubility

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Fig. 4. Dependence of Tg for PS/co-blowing agents on the concentration of co) Methanol-CO2; ( ) Ethanol-CO2 ; ( ) propanol-CO2 ; blowing agents. ( ) butanol-CO2 ; ( ( ) pentanol-CO2 ; ( ) hexanol-CO2 ; ( ) heptanol-CO2 ; ) octanol-CO2 ; ( ) nonanol-CO2 ; ( ) decanol-CO2 ; 䊐values of Tg predicted ( by Chow’s model [51] and the solubility measurement of alcohols in PS at 90 ◦ C.

Fig. 5. DSC curves for PS after being saturated with co-blowing agents: ( ) Butanol: Tg = 60.3 ◦ C; ( ) Hexanol: Tg = 62.8 ◦ C; ( Ethanol: Tg = 67.8 ◦ C; ( Decanol: Tg = 79.1 ◦ C.

) )

in PS (12.56 g/100 g PS), while ethanol had the smallest solubility (5.39 g/100 g PS). The low solubility of the short-chain length alcohol in PS weakens the plasticization effect (See Figs. 4 and 5). To verify the results predicted by Chow’s model, the DSC measurement was performed after the PS samples were saturated with the co-blowing agents at 90 ◦ C for 6 h. As observed, the smallest Tg for PS occurred at 60.3 ◦ C via saturation with butanol instead of ethanol, whereas the Tg of PS was 79.1 ◦ C after saturating with decanol. Therefore, the highest plasticizing effect was achieved by adding the medium-chain-length co-blowing agent. 4.2.2. Solubility and desorption of CO2 /co-blowing agents in PS The solubility of blowing agents determines the nucleation density [52,53] of the foam. In addition, the desorption rate of the blowing agents affects the morphology and VER of foams [54]. Thus, desorption measurements were performed to investigate the effect of co-blowing agents on the initial uptake and desorption rate of blowing agents in PS. To ensure the reliability of the results, all desorption experiments were repeated 3 times (Ur(M0) ≤ 0.02, and Ur(D) ≤ 0.03). The desorption curves of the blowing agents in PS at 90 ◦ C and 15 MPa are shown in Fig. 6, and the desorption curves at

Fig. 6. Desorption curves of the blowing agents in PS (15 MPa, 90 ◦ C). ( CO2 /decanol; ( ) CO2 /hexanol; ( ) CO2 /butanol; ( ) CO2 /ethanol; ( ) CO2.

)

70 ◦ C and 15 MPa are shown in Supporting Information Fig. S1. The solubility and desorption coefficients of the blowing agents are shown in Tables 3 and S3. The solubility of the co-blowing agent M0(co-blowingagent) was obtained by saturating PS for the same amounts of time used in the desorption tests (90 ◦ C for 3 h; 70 ◦ C for 6 h). Then, the solubility of CO2 in PS was calculated according to equation S1, and the results are shown in Tables S3 and S4. The result shows that the addition of the co-blowing agents increased the solubility of CO2 in PS, which is consistent with the simulated results, in which the solubility parameter of the blowing agent was enhanced by incorporating the co-blowing agent. Moreover, both the total solubility of the blowing agent and the solubility of CO2 in PS increased along with the chain length of the co-blowing agent. This phenomenon is consistent with the results of previous works [55–57], in which the CO2 solubility in alcohols increased with the increase in the chain lengths of the alcohols. Meanwhile, the solubility variation trend of the blowing agents in PS is consistent with the simulation results for the increase in the solubility parameters with the chain length of the co-blowing agents. In addition, the solubility of the co-blowing agents was increased at higher temperatures. The enhanced CO2 solubility was more obvious at higher temperatures due to the increased solubility of the co-blowing agent; when decanol was used as the co-blowing agent, CO2 solubility in PS was 13.54 g CO2 / 100 g polymer at 90 ◦ C, which was larger than the solubility of 11.59 g CO2 / 100 g polymer at 70 ◦ C. Since the simulated interactions between blowing agents and PS showed an increasing trend with the chain length of the coblowing agent, the desorption tests also showed that the addition of long-chain alcohols decreased the desorption rate of the blowing agents in PS due to favorable interactions. At 90 ◦ C, the desorption coefficient of the blowing agents decreased from 6.20 × 10−11 to 1.77 × 10−11 m2 /s; meanwhile, at 70 ◦ C, the desorption coefficient of the blowing agents slightly decreased from 5.28 × 10−11 to 4.786 × 10−11 m2 /s. Since the solubility of the co-blowing agents in PS was higher at 90 ◦ C, the desorption rate of the blowing agents was predominated by the co-blowing agent at higher temperatures because the co-blowing agents had higher solubility and the desorption rate of the co-blowing agent decreased with the increase in the chain length. Thus, the decrease in the desorption rate of the blowing agents was evident at 90 ◦ C.

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Table 3 Solubility and diffusion coefficients of the blowing agents in PS at 90 ◦ C.

pure CO2 CO2 /ethanol CO2 /butanol CO2 /hexanol CO2 /decanol

M0 (blowing agent) (g total blowing agent/100 g polymer)

M0 (CO2) (g CO2 /100 g polymer)

Desorption coefficient(blowing agent) (10−11 m2 /s)

\ 11.43 14.45 14.89 15.34

9.70 11.23 11.55 11.01 13.54

4.74 5.90 4.41 3.93 1.77

Fig. 7. SEM of the foam generated with PS/blowing agents at 15 MPa and different temperatures with different co-blowing agents. (a) 90 ◦ C; (b) 90 ◦ C, ethanol; (c) 90 ◦ C, butanol; (d) 90 ◦ C, hexanol; (e) 90 ◦ C, decanol; (f) 70 ◦ C; (g) 70 ◦ C, ethanol; (h) 70 ◦ C, butanol; (i) 70 ◦ C, hexanol; (j) 70 ◦ C, decanol.

Fig. 8. Characterizations of the foam morphology at 90 ◦ C and 15 MPa.

the cell size reduced from 6.41 to 4.88 ␮m, and the cell density increased from 6.12 × 109 to 2.45 × 1010 cells/cm3 . The higher solubility of the blowing agents in the polymer matrix determined the nucleation density [58], which caused a larger cell density in PS foamed with hexanol/CO2 and decanol/CO2 . Moreover, the slow desorption rate of CO2 /decanol maintains a high amount of blowing agent in the polymer matrix, which further increases the cell density of foams. Since CO2 was more quickly desorbed than the coblowing agent was, the main plasticizer was the co-blowing agent during the bubble growth stage. In this circumstance, the use of a long-chain alcohol as the co-blowing agent was not conducive for preparing foams with a large VER due to the weak plasticization effect of the long-chain alcohol. When the co-blowing agent was ethanol, the cell morphology of PS was not improved because the interaction of the blowing agent (CO2 /ethanol) with PS was the lowest, while the desorption rate was the largest, which caused a low concentration of the blowing agent in PS.

4.2.3. Foam characterization of PS foams blown with co-blowing agents and CO2 Batch foaming was conducted to thoroughly evaluate the efficiency of the co-blowing agents in enhancing the supercritical CO2 foaming of PS. Fig. 7 shows the morphology of the foams with different co-blowing agents, which were prepared at 15 MPa and operating temperatures of 70 or 90 ◦ C. The characterization of the foams is shown in Figs. 8 and S4. For the cell size, Ur (D)≤ 0.03. Since the co-blowing agents had a stronger plasticization effect on PS at higher temperatures, the increase in the VER was more significant at 90 ◦ C; the maximum VER was 2 times larger than that of PS foamed only with CO2 . Specifically, since butanol had the largest plasticization effect on PS, the PS foamed with butanol as the coblowing agent had the largest VER. At 70 ◦ C, the smaller solubility of the co-blowing agents weakened the plasticization effect, which caused an insignificant difference in the VER. However, butanol was a good plasticizer, since the VER of the foam was increased from 1.94 to 3.91. For the cell density and cell size, the addition of longchain alcohols decreased the average cell size and increased the cell density, e.g., when decanol was used as the co-blowing agent,

4.2.4. Foam characterization of PS foams blown with a mixture of co-blowing agents and CO2 Using the co-blowing agents to obtain foams with a large VER is the main concern in most studies, but the cell density and cell size of the foams are ignored. In the above discussion, butanol had a strong plasticization effect on PS foaming, while decanol enhanced cell nucleation. To achieve the synergistic effects of the co-blowing agents to prepare PS foams with a large VER without sacrificing the cell size, a mixture of butanol and decanol was used as the coblowing agent. The foam morphologies and characterizations are shown in Figs. 9, 10 and S5. For the cell size, Ur (D)≤ 0.04. As observed, compared with the PS foamed with CO2 /butanol at 90 ◦ C, the addition of a small amount (10 wt%) of decanol in butanol increased the cell density from 6.49 × 109 to 8.64 × 109 cells/cm3 . With the increase in the amount of decanol in the co-blowing agent, the cell density evidently increased, while the cell size and VER decreased. When the mass ratio of butanol to decanol was 5:5, the cell size decreased from 6.71 to 6.12 ␮m, and the VER increased from 2.41 to 5.83. For the PS foams foamed at 70 ◦ C, the foaming results showed the same variation trend. However, due to the lower solubility of the

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Fig. 9. SEM of the foam generated with PS/blowing agents at 15 MPa and different temperatures with different mass ratios of butanol and decanol as co-blowing agents. (a) 90 ◦ C, butanol/decanol (9:1); (b) 90 ◦ C, butanol/decanol (7:3); (c) 90 ◦ C, butanol/decanol (5:5); (d) 70 ◦ C, butanol/decanol (9:1); (e) 70 ◦ C, butanol/decanol (7:3); (f) 70 ◦ C, butanol/decanol (5:5).

positions of butanol/decanol as the co-blowing agent is an approach to fabricate PS foams with different cell sizes and densities.

5. Conclusions

Fig. 10. Characterizations of the foam morphology at 90 ◦ C and 15 MPa. (䊉) Cell size; ( ) Cell density; ( ) Volume expansion ratio; Blowing agents of abscissa axis from left to right were (mass ratio of butanol to decanol): CO2 ; CO2 /butanol; CO2 /butanol/decanol (9:1); CO2 /butanol/decanol (7:3); CO2 /butanol/decanol (5:5); CO2 /decanol.

co-blowing agent in PS and the weaker plasticization effect of the co-blowing agents on the polymer, the PS foams prepared with different blowing agents showed a similar VER. Nonetheless, PS with smaller cell size and larger VER could be obtained using a mixture of decanol and butanol. In most studies of the supercritical CO2 foaming of PS, the foaming temperature varied from 80 ◦ C to 120 ◦ C. The cell size of the PS foams was usually 5−15 ␮m, and the VER was 1–8 [40,59–63]. Andra Nistor et al. [19] prepared foams with n-pentane and cyclopentane as the co-blowing agents at 70 ◦ C and 28 MPa. The cell size of those foams was 2−50 ␮m, and the VER was 1.72.5. In this article, an optimized foam structure was obtained when foaming at 70 ◦ C with a narrow cell size range (2.59–6.93 ␮m) and a larger volume expansion ratio (2.51–3.92). The above results indicate that the cellular structures can be tailored using a mixture of butanol and decanol as the co-blowing agent. In the foaming process, butanol acted as a plasticizer, while decanol acted as the nucleation agent and finally improved the morphology in terms of cell size and density. Using different com-

In this work, the effect of alcohols as the co-blowing agent on the CO2 foaming process was systematically investigated thorough simulations and experiments. The introduction of the co-blowing agent enhanced the interaction between the blowing agent and PS, and the interaction energy increased with the chain length of the co-blowing agent. Moreover, the solubility parameter of the blowing agent increased with the increase in the chain length of the alcohols. The DPD simulations show that the diffusion of CO2 into PS was accelerated in the presence of alcohols, and the saturation time of the foaming process was shortened. Since the interaction between CO2 and the co-blowing agent increased with the chain length, the diffusion coefficient of CO2 into PS increased with the length of the alcohol chain. Chow’s model and the experiments revealed that butanol had the greatest plasticization effect on PS. The total solubility of the blowing agents in PS increased (from 8.215 to 13.54 g blowing agents/ 100 g polymer), and the desorption rate of the co-blowing agent in PS decreased when the chain length of the co-blowing agent increased. As a result of the strong plasticization effect of butanol on PS, using butanol as the co-blowing agent could increase the VER of foams from 2.41 to 7.17. Meanwhile, a high nucleation rate was achieved by using decanol as the co-blowing agent due to the high content of the blowing agent maintained in PS. Due to the high diffusion rate and the low desorption rate of the foaming agents, the efficiency of the foaming process was largely improved. Furthermore, a synergistic effect of the alcohols was achieved to optimize the structure of the PS foams when using a selected mixture of alcohols as the co-blowing agent.

Declaration of Competing Interest The authors declare that there are no conflicts of interest.

W. Qiang, L. Zhao, T. Liu et al. / J. of Supercritical Fluids 158 (2020) 104718

Acknowledgements The authors are grateful to National Key Research and Development Program of China (2016YFB0302200), National Natural Science Foundation of China (21706063, 21676092). D. Hu also gratefully acknowledges the financial support from China Scholarship Council and the Fundamental Research Funds for the Central Universities. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.supflu.2019. 104718. References [1] M. Antunes, J.E.I. Velasco, Multifunctional polymer foams with carbon nanoparticles, Prog. Polym. Sci. 39 (2014) 486–509. [2] Y. Sun, Y. Ueda, H. Suganaga, M. Haruki, S.-i. Kihara, S. Takishima, Experimental and simulation study of the physical foaming process using high-pressure CO2 , J. Supercrit. Fluids 107 (2016) 733–745. [3] H. Zhang, G. Zhang, M. Tang, L. Zhou, J. Li, X. Fan, X. Shi, J. Qin, Synergistic effect of carbon nanotube and graphene nanoplates on the mechanical, electrical and electromagnetic interference shielding properties of polymer composites and polymer composite foams, Chem. Eng. J. 353 (2018) 381–393. [4] C. Okolieocha, D. Raps, K. Subramaniam, V. Altstädt, Microcellular to nanocellular polymer foams: progress (2004–2015) and future directions – a review, Eur. Polym. J. 73 (2015) 500–519. [5] X. Fan, G. Zhang, Q. Gao, J. Li, Z. Shang, H. Zhang, Y. Zhang, X. Shi, J. Qin, Highly expansive, thermally insulating epoxy/Ag nanosheet composite foam for electromagnetic interference shielding, Chem. Eng. J. 372 (2019) 191–202. [6] G. Wang, G. Zhao, L. Zhang, Y. Mu, C.B. Park, Lightweight and tough nanocellular PP/PTFE nanocomposite foams with defect-free surfaces obtained using in situ nanofibrillation and nanocellular injection molding, Chem. Eng. J. 350 (2018) 1–11. [7] D.V. Rosato, D.V. Rosato, M. Rosato, Plastic Product Material & Process Selection Handbook, 2019. [8] X. Han, Y. Delaviz, C.J. Boudreaux, M.Z. Weekley, Methods of manufacturing extruded polystyrene foams using carbon dioxide as a major blowing agent, in, Google Patents, 2019. [9] S.B. Park, S.W. Choi, J.H. Kim, C.S. Bang, J.M. Lee, Effect of the blowing agent on the low-temperature mechanical properties of CO2 - and HFC-245fa-blown glass-fiber-reinforced polyurethane foams, Compos. Part B-Eng. 93 (2016) 317–327. [10] M.K. Vollmer, R. Stefan, H. Matthias, B. Dominik, First observations of the fourth generation synthetic halocarbons HFC-1234yf, HFC-1234ze(E), and HCFC-1233zd(E) in the atmosphere, Environ. Sci. Technol. 49 (2015) 2703–2708. [11] E.J. Beckman, Green chemical processing using CO2 , Ind. Eng. Chem. Res. 42 (2003) 1598–1602. [12] K.W. Suh, A.N. Paquet, Rigid polystyrene foams and alternative blowing agents, Modern Styrenic Polym.: Polystyrenes Styrenic copolymers 6 (2003) 203. [13] R. Gendron, M.F. Champagne, Y. Delaviz, M.E. Polasky, Foaming polystyrene with a mixture of CO2 and ethanol, J. Cell. Plast. 42 (2006) 127–138. [14] R. Gendron, P. Moulinie, Foaming poly(methyl methacrylate) with an equilibrium mixture of carbon dioxide and isopropanol, J. Cell. Plast. 40 (2004) 111–130. [15] L.E. Daigneault, R. Gendron, Blends of CO2 and 2-Ethyl hexanol as replacement foaming agents for extruded polystyrene, J. Cell. Plast. 37 (2001) 262–272. [16] A. Salerno, C. Domingo, Effect of blowing agent composition and processing parameters on the low temperature foaming of poly(l -lactide/caprolactone) co-polymer by means of supercritical CO2 /ethyl lactate binary mixtures, J. Supercrit. Fluids 84 (2013) 195–204. [17] C. Zhang, B. Zhu, L.J. Lee, Extrusion foaming of polystyrene/carbon particles using carbon dioxide and water as co-blowing agents, Polymer 52 (2011) 1847–1855. [18] A. Nistor, A. Rygl, M. Bobak, M. Sajfrtova, J. Kosek, Micro-cellular polystyrene foam preparation using high pressure CO2 : the influence of solvent residua, Macromol. Symp. 333 (2013) 266–272. [19] A. Nistor, M. Topiar, H. Sovova, J. Kosek, Effect of organic co-blowing agents on the morphology of CO2 blown microcellular polystyrene foams, J. Supercrit. Fluids 130 (2017) 30–39. [20] N. Zhao, L.H. Mark, C. Zhu, C.B. Park, Q. Li, R. Glenn, T.R. Thompson, Foaming poly (vinyl alcohol)/microfibrillated cellulose composites with CO2 and water as co-blowing agents, Ind. Eng. Chem. Res. 53 (2014) 11962–11972. [21] Y. Jia, S. Bai, C.B. Park, Q. Wang, Effect of boric acid on the foaming properties and cell structure of poly (vinyl alcohol) foam prepared by supercritical-CO2 thermoplastic extrusion foaming, Ind. Eng. Chem. Res. 56 (2017) 6655–6663.

9

[22] D.-d. Hu, Y. Gu, T. Liu, L. Zhao, Microcellular foaming of polysulfones in supercritical CO2 and the effect of co-blowing agent, J. Supercrit. Fluids 140 (2018) 21–31. [23] S. Mahmood, C. Xin, P. Gong, J. Lee, G. Li, C. Park, Dimethyl ether’s plasticizing effect on carbon dioxide solubility in polystyrene, Polymer 97 (2016) 95–103. [24] H. Wu, Z. Du, H. Zhou, X. Wang, J. Mi, Evaluation of solubility enhancement of carbon dioxide in polystyrene via introduction of water, Fluid Phase Equilib. 449 (2017) 83–90. [25] D. Li, T. Liu, L. Zhao, W. Yuan, Controlling sandwich-structure of PET microcellular foams using coupling of CO2 diffusion and induced crystallization, AIChE J. 58 (2012) 2512–2523. [26] M. Antunes, V. Realinho, G. Gedler, D. Arencón, J.I. Velasco, Diffusion of CO2 in polymer nanocomposites containing different types of carbon nanoparticles for solid-state microcellular foaming applications, J. Nano Res. Trans. Tech. Publ. (2014) 63–74. [27] P. Lalanne, T. Tassaing, Y. Danten, F. Cansell, S. Tucker, M. Besnard, CO2 −ethanol interaction studied by vibrational spectroscopy in supercritical CO2 , J. Phys. Chem. A 108 (2004) 2617–2624. [28] G. Bernardo, D. Vesely, Equilibrium solubility of alcohols in polystyrene attained by controlled diffusion, Eur. Polym. J. 43 (2007) 938–948. [29] G. Bernardo, Diffusivity of alcohols in amorphous polystyrene, J. Appl. Polym. Sci. 127 (2013) 1803–1811. [30] H. Taghvaei, M.A. Amooie, A. Hemmati-Sarapardeh, H. Taghvaei, A comprehensive study of phase equilibria in binary mixtures of carbon dioxide+ alcohols: application of a hybrid intelligent model (CSA-LSSVM), J. Mol. Liq. 224 (2016) 745–756. [31] Z. Huang, Y.C. Chiew, W.-D. Lu, S. Kawi, Solubility of aspirin in supercritical carbon dioxide/alcohol mixtures, Fluid Phase Equilib. 237 (2005) 9–15. [32] R. Murga, R. Ruiz, S. Beltrán, J.L. Cabezas, Extraction of natural complex phenols and tannins from grape seeds by using supercritical mixtures of carbon dioxide and alcohol, J. Agric. Food Chem. 48 (2000) 3408–3412. [33] D. Pasquini, M.T. Pimenta, L.H. Ferreira, A.A. Curvelo, Sugar cane bagasse pulping using supercritical CO2 associated with co-solvent 1-butanol/water, J. Supercrit. Fluids 34 (2005) 125–131. [34] J. Liu, B. Han, J. Zhang, T. Mu, G. Li, W. Wu, G. Yang, Effect of cosolvent on the phase behavior of non-fluorous Ls-54 surfactant in supercritical CO2 , Fluid Phase Equilib. 211 (2003) 265–271. [35] W. Qiang, D.-d. Hu, T. Liu, L. Zhao, Strategy to control CO2 diffusion in polystyrene microcellular foaming via CO2 -philic additives, J. Supercrit. Fluids (2019). [36] A. Maiti, S. McGrother, Bead–bead interaction parameters in dissipative particle dynamics: relation to bead-size, solubility parameter, and surface tension, J. Chem. Phys. 120 (2004) 1594–1601. [37] F. Mozaffari, H. Eslami, J. Moghadasi, Molecular dynamics simulation of diffusion and permeation of gases in polystyrene, Polymer 51 (2010) 300–307. [38] S. Anwar, J. Carroll, Density (kg/m3 ) of carbon dioxide as a function of temperature and pressure, in: Carbon Dioxide Thermodynamic Properties Handbook, John Wiley & Sons, Inc, 2010, pp. 9–148. [39] W. Strauss, N.A. D’Souza, Supercritical CO2 processed polystyrene nanocomposite foams, J. Cell. Plast. 40 (2004) 229–241. [40] I. Tsivintzelis, A.G. Angelopoulou, C. Panayiotou, Foaming of polymers with supercritical CO2 : an experimental and theoretical study, Polymer 48 (2007) 5928–5939. [41] J. Pinto, J.A. Reglero-Ruiz, M. Dumon, M.A. Rodriguez-Perez, Temperature influence and CO2 transport in foaming processes of poly (methyl methacrylate)–block copolymer nanocellular and microcellular foams, J. Supercrit. Fluids 94 (2014) 198–205. [42] W. Xiao, X. Liao, S. Li, J. Xiong, Q. Yang, G. Li, The distinctive nucleation of polystyrene composites with differently shaped carbon-based nanoparticles as nucleating agent in the supercritical CO2 foaming process, Polym. Int. 67 (2018) 1488–1501. [43] H. Eslami, F. Müller-Plathe, Molecular dynamics simulation of sorption of gases in polystyrene, Macromolecules 40 (2007) 6413–6421. [44] L. Bao, S. Fang, D. Hu, L. Zhao, W. Yuan, T. Liu, Enhancement of the CO2 -philicity of poly (vinyl ester) s by end-group modification with branched chains, J. Supercrit. Fluids 127 (2017) 129–136. [45] H. Lin, B.D. Freeman, Materials selection guidelines for membranes that remove CO2 from gas mixtures, J. Mol. Struct. 739 (2005) 57–74. [46] A. Thran, G. Kroll, F. Faupel, Correlation between fractional free volume and diffusivity of gas molecules in glassy polymers, J. Polym. Sci. Part B: Polym. Phys. 37 (1999) 3344–3358. [47] W. Schabel, P. Scharfer, M. Kind, I. Mamaliga, Sorption and diffusion measurements in ternary polymer–solvent–solvent systems by means of a magnetic suspension balance—experimental methods and correlations with a modified Flory–huggins and free-volume theory, Chem. Eng. Sci. 62 (2007) 2254–2266. [48] R.K. Surana, R.P. Danner, J.L. Duda, Diffusion and equilibrium measurements in ternary polymer− solvent− solvent systems using inverse gas chromatography, Ind. Eng. Chem. Res. 37 (1998) 3203–3207. [49] J. Chen, T. Liu, L. Zhao, W.-k. Yuan, Experimental measurements and modeling of solubility and diffusivity of CO2 in polypropylene/micro-and nanocalcium carbonate composites, Ind. Eng. Chem. Res. 52 (2013) 5100–5110. [50] Y. Zhu, X. Yang, Morphology and structure transition of surfactant/scCO2 system: a dissipative particle dynamics simulation, Fluid Phase Equilib. 318 (2012) 102–109.

10

W. Qiang, L. Zhao, T. Liu et al. / J. of Supercritical Fluids 158 (2020) 104718

[51] T. Chow, Molecular interpretation of the glass transition temperature of polymer-diluent systems, Macromolecules 13 (1980) 362–364. [52] R. Li, N. Ye, V. Shaayegan, T. Fang, Experimental measurement of CO2 diffusion in PMMA and its effect on microcellular foaming, J. Supercrit. Fluids 135 (2018) 180–187. [53] X. Lan, W. Zhai, W. Zheng, Critical effects of polyethylene addition on the autoclave foaming behavior of polypropylene and the melting behavior of polypropylene foams blown with n-pentane and CO2 , Ind. Eng. Chem. Res. 52 (2013) 5655–5665. [54] Y. Li, Z. Yao, Z.-h. Chen, K. Cao, S.-l. Qiu, F.-j. Zhu, C. Zeng, Z.-m. Huang, Numerical simulation of polypropylene foaming process assisted by carbon dioxide: bubble growth dynamics and stability, Chem. Eng. Sci. 66 (2011) 3656–3665. [55] X. Gui, Z. Tang, W. Fei, Solubility of CO2 in alcohols, glycols, ethers, and ketones at high pressures from (288.15 to 318.15) K, J. Chem. Eng. Data 56 (2011) 2420–2429. [56] T.Z. Li, Z.G. Tang, H. Hu, D. Guo, Measurement of the solubility for CO2 in alcohols and its correlation with the molecular connectivity index, J. Chem. Eng. Chin. Univ. 26 (2012) 1–6. [57] T. Aida, T. Aizawa, M. Kanakubo, H. Nanjo, Relation between volume expansion and hydrogen bond networks for CO2 −Alcohol mixtures at 40 ◦ C, J. Phys. Chem. B 114 (2010) 13628–13636.

[58] Q. Ren, J. Wang, W. Zhai, S. Su, Solid state foaming of poly (lactic acid) blown with compressed CO2 : influences of long chain branching and induced crystallization on foam expansion and cell morphology, Ind. Eng. Chem. Res. 52 (2013) 13411–13421. [59] K.A. Arora, A.J. Lesser, T.J. McCarthy, Preparation and characterization of microcellular polystyrene foams processed in supercritical carbon dioxide, Macromolecules 31 (1998) 4614–4620. [60] J.A.R. Ruiz, J. Marc-Tallon, M. Pedros, M. Dumon, Two-step micro cellular foaming of amorphous polymers in supercritical CO2, J. Supercrit. Fluids 57 (2011) 87–94. [61] J.-B. Bao, T. Liu, L. Zhao, G.-H. Hu, X. Miao, X. Li, Oriented foaming of polystyrene with supercritical carbon dioxide for toughening, Polymer 53 (2012) 5982–5993. [62] H. Janani, M.H.N. Famili, Investigation of a strategy for well controlled inducement of microcellular and nanocellular morphologies in polymers, Polym. Eng. Sci 50 (2010) 1558–1570. [63] J. Yang, L. Huang, Y. Zhang, F. Chen, M. Zhong, Mesoporous silica particles grafted with polystyrene brushes as a nucleation agent for polystyrene supercritical carbon dioxide foaming, J. Appl. Polym. Sci. 130 (2013) 4308–4317.