Preparation of graphene/PMMA composites with assistance of ultrasonic wave under supercritical CO₂ conditions

Highlights

• •Ultrasound can initiate polymerization above the threshold.
• •Ultrasonic time for ultrasonic initiation polymerization also has an optimal value.
• •Ultrasound-induced methdod is better than in-situ polymerization.

Abstract

The two-dimensional material graphene has many excellent physicochemical properties such as large specific surface area, high electron migration rate, good chemical properties, good thermal conductivity, high elastic modulus and mechanical strength that make it very valuable for theoretical research and application in the preparation of graphene/polymer composites. In this paper, the effects of ultrasonic intensity and reaction time on the molecular weight and yield of PMMA under supercritical CO₂ conditions were investigated. It was found that there are threshold and optimal values of ultrasonic intensity for initiating the reaction in supercritical CO₂ system. The threshold value is 150 W/cm² and the optimal ultrasonic intensity value is 225 W/cm². There is also an optimal value of ultrasonic initiation time for ultrasonic initiation polymerization. Combining the reaction yield and the molecular weight of the product, 2 h of ultrasonic initiation is a suitable initiation reaction time. Based on the synthesis of PMMA by ultrasonic excitation, the preparation of Graphene/PMMA composites by ultrasound assistance was also investigated. The TG and DTG characterization of PMMA and complex materials prepared by ultrasonic excitation showed that the radicals generated by ultrasonic excitation were uniformly distributed and did not generate unsaturated double bonded polymers. The electrical conductivity of the Graphene/PMMA composites prepared by ultrasonic excitation at a graphene content of 1 wt% increased to 1.13 × 10^-1 S/cm, which is better than that of the Graphene/PMMA prepared by in situ polymerization. This may be attributed to the ultrasound-assisted supercritical CO₂ fluid action that resulted in a more uniform distribution of Graphene mixed with PMMA in the prepared composites. Therefore, it is of important practical significance for the preparation of Graphene/PMMA composites by ultrasound-induced polymerization.

1. Introduction

A supercritical fluid is defined as an element or compound at a pressure and temperature higher than its critical pressure and temperature. Since then, supercritical fluids have been widely used in petrochemical, extraction and separation, materials science, chemical engineering, environmental engineering, and bioengineering industries due to their special solubility, sensitivity to solubility as conditions change, selectivity, high speed and efficiency, and high safety [1]. The relationship between temperature and pressure for a pure material system is shown in Fig. 1. The upper right part is the supercritical fluid state of the fluid, the AO line represents the sublimation curve for gas–solid equilibrium, the BO line represents the melting curve for liquid–solid equilibrium, the CO line represents the saturation vapor pressure curve for gas–liquid equilibrium, and the point O is the three-phase point where the gas–liquid–solid phases coexist. Point C in the graph is called the critical point, and the corresponding temperature and pressure are called the critical temperature and critical pressure, respectively. Above the critical temperature and pressure is a supercritical fluid. Its physical and chemical properties are between those of liquid and gas, with variable density, gas viscosity, compressibility, etc..

Fig. 1.

Temperature and pressure phase equilibrium of a pure material system.

Supercritical fluids Supercritical fluids possess properties that are intermediate between gas and liquid, having both gas-like compressibility, viscosity and diffusion rate[2]. It also has liquid-like fluidity, which makes supercritical fluids suitable for replacing many solvents in reactions due to their extreme diffusivity, high density and solubility, low viscosity, and relatively high surface tension and dielectric constant. This, coupled with low cost, wide availability, environmental friendliness and non-toxicity, allows supercritical fluids to be used in a wide range of industrial applications [3].

There are many advantages in the use of supercritical CO₂ for the polymerization of polymers [4]. The viscosity of supercritical CO₂ is similar to that of a gas, one order of magnitude smaller than that of a liquid, and the diffusion coefficient is two orders of magnitude larger than that of a liquid, resulting in mild conditions, easy operation, and energy savings[5]. Since the critical pressure of CO₂ is not very high, the safety index of equipment processing is relatively high, and the processing is easy under comparable conditions. In practice, supercritical fluids can significantly reduce the reactor volume and facilitate the transfer of energy and nanoparticles in the medium[6]. These properties give supercritical fluids great potential for use in nanocomposites [7].

Among the properties that have been investigated are the ability of supercritical CO₂ to reduce the caging effect caused by excess radicals in the solvent-facing initiator content, and to increase the polymerization rate and degree of polymerization [8]. Supercritical CO₂ has the ability to change the solubility of the fluid to the polymer by varying the pressure and temperature of the system and to affect the selectivity of the nanoparticles or enzymes in it, as well as the enzymatic activity and selectivity [9]. The dielectric constant increases with the increase of pressure. This allows the solubility of the solute to be greatly increased, and for some volatile substances, the reaction can be better in supercritical fluids. The selectivity of supercritical fluids for solids is reflected in the regular change in the solubility of solids as the temperature increases[10]. After exploring the pattern, the product can be obtained by precipitating the solid particles out of the system by changing the operating conditions, thus increasing the conversion rate [11]. From the rheological theory, the solubility of CO₂ can be used to disperse inorganic nanoparticles well in the mechanism, and the optical properties of polymers can be affected by changing the conditions[12].

The CO₂ molecular structure is stable, and no side reactions such as chain transfer caused by CO₂ were found in all types of polymerization reactions in SC-CO₂ medium [13]. In the polymerization reaction of SC-CO₂, the polymerization rate, polymerization degree and the upper limit of polymerization temperature have a linear model with the pressure of the supercritical system as the pressure increases, and the structure and behavior of the polymer can be predicted by changing the reaction conditions, which is beneficial to the polymerization and industrialization[14]. SC-CO₂ can be discharged as a gas after decompression and is thus completely separated from the product without the need for subsequent treatment as in the case of organic solvents[15]. After the reaction, it is also possible to take advantage of the good solubility and swelling effect of supercritical CO₂ with polymers, and the pure polymer can be obtained by continuously passing in the exhausted CO₂ to get rid of unreacted monomers in the system while maintaining supercritical conditions[16].

CO₂ is non-toxic, non-polluting, non-residual, environmentally riendly, and widely available at low cost. As a result, SC-CO₂, which is inexpensive, readily available and non-toxic, has been widely used as a reaction medium in extraction and bioengineering[17]. The reaction medium or extraction medium has received much attention and has been extensively researched by the scientific community in various aspects such as solution polymerization, suspension polymerization, emulsion polymerization, dispersion polymerization and precipitation polymerization. In practice, SC-CO₂ has been used to replace various solvents in production[18].

Ultrasound is an elastic mechanical vibration wave with a frequency of 2 × 10⁴ ~ 10⁷ kHz [34]. Ultrasound has been of great interest since the late 1920 s due to its high efficiency, environmental friendliness, and green effect, especially in chemical synthesis, because of its great potential and many features [35]. The cavitation effect of ultrasound has made its application more theoretically and practically relevant, which has led to more interest and research [19]. The cavitation process generates a very large amount of energy in the instantaneous collapse of cavities, and the comparison of synthetic observation spectra reveals a high instantaneous temperature of 4802 ± 156 °C as well as special properties such as instantaneous high pressure and high pressure microjets [20]. Thus, the acoustic cavitation effect provides a new and very special physicochemical environment for chemical reactions that are difficult or impossible to achieve under normal conditions[21]. Compared with photochemistry and electrochemistry, ultrasound can directly transmit energy efficiently in the process of propagation through the medium[22]. When ultrasound propagates, with the vibration of ultrasound, the molecules in the medium move at the same frequency as ultrasound, and the energy brought by ultrasound is transferred by gaining kinetic energy[36], thus producing a cavitation effect on the medium, and the instantaneous generation of various high intensity and special properties can degrade monomer molecules and generate free radicals to initiate polymerization reactions[23], [37].

Polymethyl methacrylate (PMMA) has the advantages of low price, good transparency, easy processing, good chemical stability, chemical corrosion resistance and weather resistance[24]. PMMA has good water resistance in acid and alkali environments, and is resistant to many inorganic reagents, organic solvents, polar solvents, non-polar solvents, dilute acids and alkalis[25]. These advantages make PMMA more and more widely used in light boxes, lamps, safety masks, aircraft windshields, lenses, LCD light guides, cell phone screens and as medical polymer materials for joint and skull repair [26]. However, PMMA itself has some disadvantages that cannot be ignored, such as low surface hardness, easy scratching, poor toughness, poor impact resistance, poor electrical properties, and high electrostatic effects, so scholars are trying to improve the various properties of PMMA while maintaining its transparency. The addition of various inorganic particles with good properties to PMMA to improve various aspects of PMMA properties has been widely used. Among them, the preparation of composites using nanoparticles and PMMA can not only improve the properties such as toughness, heat resistance and friction resistance, but also give more characteristics to PMMA. The products obtained from PMMA nanocomposites prepared by in situ polymerization have the advantages of relatively high purity, high molecular weight and good mechanical strength.

Luche et al. [27] used a rational approach to propose a mechanism for the specific effects of acoustochemical reactions, suggesting that ultrasonic irradiation can degrade the reactants and products to produce radical intermediates. Kruus and Price [28] successfully polymerized poly(methyl methacrylate) with a number average molecular weight of 400,000 to demonstrate that the interaction between vinyl monomer and ultrasound can initiate polymerization, and demonstrated that the polymerization rate is affected by the ultrasound time, ultrasound intensity, and volume ratio of the reaction system. The reaction mechanism was demonstrated by the action of the free radical scavenger DPPH. The cavitation collapse effect of ultrasound generates free radicals and the temperature of the evidence affects the color and other properties of the composite products. Kruus et al. [29], [30] successfully initiated the polymerization of polymethyl methacrylate–styrene by ultrasound. The results are consistent with the mechanism of initiating polymerization by decomposition of monomers into free radicals brought about by ultrasonic cavitation, but the experimental results yielded lower molecular weights and conversions. It is believed that the reaction rate of the polymerization initiated by high intensity ultrasound is related to the temperature and the fluid medium in which the reaction occurs. Price et al. [31] concluded that the degradation of methyl methacrylate to produce radical-initiated polymerization reactions could be successfully carried out under high intensity ultrasonic irradiation, although the process was accompanied by degradation reactions. They also investigated the effects of some process parameters on the products and concluded that the size of the molecular weight, dispersion, and the stand-up regularity of the products prepared from the polymers could be adjusted by varying the temperature and ultrasound intensity. The rate of polymerization can also be increased when 5% PMMA is added to pure MMA. Gu et al. [32] used ultra-high intensity ultrasonic irradiation to initiate the polymerization reaction of methyl methacrylate monomer, concluded that ultrasound can be effectively applied to the preparation of polymers, and investigated the kinetics of the polymerization reaction. By establishing the model and parameters, it was demonstrated that in the process of ultrasound-initiated polymerization, the ultrasound time and ultrasound intensity are important factors to control the polymerization rate, and the ultrasound with sufficient intensity is able to initiate polymerization. On the basis of sufficient sonication intensity, the longer sonication time favors the generation of free radicals and thus the polymerization, while the addition of a certain amount of PMMA facilitates the polymerization. For the first time, electron spin resonance (ESR) spectroscopy studies were used to further validate the kinetics and mechanism of ultrasound-induced polymerization. Cheung et al. [33] showed that free radicals can be generated in liquid carbon dioxide by ultrasonic induction, and PMMA/PS was successfully prepared in liquid carbon dioxide. The polymerization of monomers to polymers in general systems requires the dispersion of monomers in fluids and the addition of stabilizers, and finally the polymers are obtained by an anti-solvent method. In the supercritical system, the polymers were produced after 2 ~ 6 h of ultrasonic irradiation without any stability. It was also verified that ultrasonic irradiation time and CO₂ and MMA ratio significantly affect the molecular weight and molecular weight dispersion of the polymer.

In this paper, the effects of ultrasonic intensity and reaction time on the molecular weight and yield of PMMA under supercritical CO₂ conditions were investigated. Based on the synthesis of PMMA by ultrasonic excitation, the preparation of Graphene/PMMA composites by ultrasound assistance was also investigated.

2. Materials and methods used in the experiments

2.1. Raw materials and reagents

The materials and reagents are as follows:

Carbon dioxide, content ≥ 99.5%, Exterminating Four Pests Environmental Protection Technology (Shenzhen) Co.; Graphite powder(CP), chemically pure, Zhengzhou Yino Chemical Co.; Benzoyl Peroxide (BPO), chemically pure, Jiangsu Peixing Chemical Co.; Methyl Methacrylate(MMA), Chemically pure, Zibo Xiechuang Chemical Co.; Ethanol, Analytical purity, Shandong Mantang Hong International Trade Co.; Deionized water, analytically pure, Suzhou Jiazhou Purification Equipment Co..

2.2. Instrumentation and characterization

The experimental instruments are as follows:

Electronic analytical balance, ESJ208-S, Shenyang Longteng Electronics Co.; Supercritical injection device, laboratory assembly; Ultrasonic instrument, JJ28-36TJR, Jiangsu Jiujiayi Ultrasonic Technology Co.; Fourier transform infrared spectrometer, iCAN 9, Tianjin Energy Spectrum Technology Co.; Flat plate press, RYJ-600Z2, Shanghai Xinuo Instrument Group Co.; Supercritical device, laboratory assembly; Scanning electron microscope, ZEISS, Suzhou Senworth Industrial Equipment Co.; Transmission electron microscope, JEM-2100Plus, Shaanxi Aopson Inspection Technology Co.; Atomic Force Microscope, Park NX-Hivac, Shanghai Luofeng Precision Testing Instruments Co.; Thermogravimetric analyzer, New STA & STA7000, Hitachi Analytical Instruments (Shanghai) Co.; X-ray diffraction analysis, AL-Y3000, Dandong Aolong Ray Instruments Group Co.; Gel permeation chromatography, PE Series 200; Universal Electronic Pulling Machine, Instron Model 4465, Instron Crop Company, USA; Four-probe metal/semiconductor resistance meter, SB100A/21A, Shanghai Qianfeng Electronic Instruments Co..

2.3. Experimental method

The device for the polymerization of polymethyl methacrylate by ultrasonic excitation is a homemade device, and the experimental equipment includes a stainless steel reactor and an ultrasonic probe. The reactor is a high-pressure vessel with a visible window that can withstand a pressure of 35 MPa, and the experimental setup is shown in Fig. 2, with a volume of 200 ml. A certain amount of monomer (120 ml) was placed in the reactor in advance. First, CO₂ was charged into the device using a high-pressure syringe pump, and the pressure relief valve VR was adjusted to ensure that the air is completely removed from the device. At the same time, the system was heated to a preset temperature using a thermostat and was maintained at a steady state. When the preset pressure and temperature were reached, the CO₂ in the system existed in a supercritical state. From the pre-experiment, it is known that holding pressure and temperature for one hour is sufficient to make the system fully infiltrated with MMA and CO₂. The ultrasonic power of the ultrasonic generator (200 W/cm²) was set, and the ultrasonic time of the ultrasonic generator was set to 1 h, 2 h and 3 h. After the sonication was stopped, the pressure was held for 24 h to allow the polymerization to continue. After the reaction, the product was removed by pressure relief and put into a surface dish and continued to be heated for 12 h in an oven set at 60 ℃. When preparing graphene/PMMA complexes: Place a certain amount of graphene into the reactor. After heating the equipment to 45 °C, carbon dioxide is added to the reactor using a hand pump and the pressure is brought to 10 MPa. When the pressure and temperature of the device reached the preset values, the graphene was ultrasonically exfoliated again for 20 min using the ultrasonic probe. The pressure and temperature were kept at 10 MPa and 45 ℃ during the exfoliation process to confirm the coupling effect of ultrasonic exfoliation and supercritical CO₂ to exfoliate into graphene. After ultrasonic exfoliation for 30 min, the ultrasound was turned back on and the monomer MMA was pumped into the reactor with a high pressure constant current pump, and the ultrasonic intensity and reaction time were adjusted to stimulate the polymerization of the mixture. At the end of the reaction of polymer preparation by ultrasonic excitation, the valve was opened to drain the carbon dioxide from the reactor and the product was removed by opening the reactor. The Graphene/PMMA composite product was prepared by heating in an oven at a temperature of 60 °C for 15 h.

Fig. 2.

Experimental Instrument Diagram. 1 - CO₂ cylinder; 2 - manual pump; 3 - ultrasonic probe; 4 - stainless steel reactor; 5 - high pressure constant flow pump.

2.4. Characterization method

2.4.1. Monomer conversion rate determination

Based on the weight method, the monomer conversion was determined according to Eq. (1):

$$Conversion\left(\%\right)=\frac{w_p}{w_m}\times 100\%$$ (1)

where w_p is the weight of PMMA obtained, and w_m is the amount of MMA monomer added.

2.4.2. GPC molecular weight test.

The molecular weights of PMMA were determined by gel permeation chromatography of the synthesized samples. Polystyrene PS was used as the standard sample, and the solvent N,N-dimethylformamide DMF was used as the mobile phase. The conditions were: the column temperature was 50 ℃, the system pressure was 43 MPa, the injection volume was 200 ml, the elution volume was from 0 to 2500 μL, and the accuracy was ± 0.5% for the test.

2.4.3. Scanning electron microscopy

Scanning electron microscopy allows visualization of the surface morphology, the presence of agglomerated particles, and the morphology of the cross-section of the prepared composites. The general scale can be down to the nanometer level, and the image is a three-dimensional image, reflecting the surface structure of the specimen.

The samples made by the hot-pressing film method were placed in liquid nitrogen, and after 20 min, they were removed and knocked into brittle fractures, and the surface was sprayed with gold on the fracture surface and then could be tested.

3. Results and discussion

3.1. Effect of ultrasonic intensity on the molecular weight of products and their distribution

In order to investigate the effect of ultrasound intensity on the molecular weight of the polymerization reaction products, the ultrasound time (90 min) was maintained, the supercritical conditions (temperature: 60 ℃, pressure: 14 MPa) were fixed, and the MMA monomer was treated with ultrasound irradiation at different ultrasound intensities and then held for 36 h. Finally, the product was obtained by heating in the oven for 14 h, and then the molecular weight and homogeneity of the product were obtained by GPC test. As shown in Fig. 3, it can be seen that with the increase of ultrasound intensity, the molecular weight of heavy average molecular weight and number average molecular weight have the same trend, and the molecular weight increases first and then decreases. When the ultrasound intensity is relatively low, it is difficult to provide enough energy to degrade the MMA monomer molecules and produce free radicals with sufficient concentration to stimulate polymerization. When the ultrasonic intensity is at 150 W/cm², the ultrasonic intensity is sufficient to degrade the monomer to produce free radicals that can stimulate the polymerization reaction. When the sonication intensity continued to increase, the molecular weight of the product was found to decrease instead. It may be due to the cavitation effect when the ultrasonic intensity is higher, on the one hand, the instantaneous high temperature and high pressure not only degrade the MMA monomer to produce free radicals, but also degrade the PMMA polymer chain, which makes it difficult to form a long-chain polymer, and thus the molecular weight of the product is difficult to increase; On the other hand, when ultrasonic cavitation generates too many cavities, the energy transfer in the reaction system is not utilized, which results in the failure to form products with higher molecular weight. As the ultrasonic intensity increases, the molecular weight dispersion Mn/Mw, which characterizes the molecular weight homogeneity, tends to decrease, i.e., the homogeneity becomes better. This is because as the ultrasonic intensity increases, the stirring effect of ultrasound is also improved, and the products are fully dispersed in the supercritical carbon dioxide fluid, so that more homogeneous products can be obtained.

Fig. 3.

Effect of ultrasonic intensity on the molecular weight of products and their distribution.

In summary, in the supercritical carbon dioxide system, it is difficult to stimulate the polymerization of MMA monomer when the ultrasonic intensity is lower than 150 W/cm², and the degradation of MMA and PMMA will change with different ultrasonic intensities, which will have a greater impact on the molecular weight of the products. The molecular weight will increase rapidly and then decrease, and it is judged that there is an optimal sound intensity for ultrasound-induced polymerization. The experimental results show that there is a threshold and an optimum value of ultrasonic intensity to stimulate the ultrasonic polymerization. Above the threshold value, the ultrasound can stimulate polymerization, and at the point of the optimum value, the energy saving effect can be achieved with guaranteed yield. When the ultrasonic intensity is greater than the optimal sound intensity, the molecular weight decreases with the increase of ultrasonic intensity due to the degradation of macromolecular polymers by cavitation. With the increase of ultrasonic intensity, the molecular weight dispersion of PMMA products becomes better and better, and the molecular weight of the products becomes more and more homogeneous.

3.2. Effect of ultrasound time on the molecular weight of products and their distribution

In order to investigate the effect of ultrasound duration on the polymerization reaction rate, the MMA monomer was subjected to ultrasonic irradiation for 36 h at different ultrasound durations, keeping the ultrasonic intensity (300 W/cm²) and supercritical conditions (temperature of 60 °C and pressure of 14 MPa) unchanged. Finally, the product was obtained by heating in an oven for 12 h.

Without ultrasonic irradiation, the conversion of the monomer was found to be 0 after 48 h. This indicates that ultrasonic irradiation is a necessary factor to stimulate polymerization. Fig. 4 shows the effect of different sonication times on the conversion rate. When the sonication time was less than 30 min the reaction could not be stimulated, presumably because the concentration of radicals generated was too low to polymerize the reactants. After time greater than 30 min, the molecular weight of the products became larger with increasing sonication time. It was inferred that the free radicals were generated by the degradation of MMA monomer under ultrasonic irradiation, and the concentration of free radicals became larger with the increase of sonication time, and the concentration of free radicals was sufficient for the polymerization reaction to occur smoothly. If the sonication time continues to increase, the number of free radicals continues to increase and the reaction rate is accelerated. When there are too many free radicals, the polymerization reaction is terminated prematurely and the polymer chain length is reduced, thus leading to a decrease in the molecular weight of the product. The molecular weight dispersion Mn/Mw, which is an indicator of molecular weight homogeneity, tends to decrease with the increase of sonication time. The molecular weight dispersion of the product is not good because the free radical concentration is insufficient at the beginning. When the concentration of free radicals was sufficient, the uniformity was improved, and the improvement of uniformity leveled off.

Fig. 4.

Effect of ultrasound time on the molecular weight of products and their distribution.

In summary, under the supercritical carbon dioxide conditions (temperature is 60 °C and pressure is14 MPa), the concentration of free radicals increased with the increase of sonication time when the ultrasonic intensity was 225 W/cm². The polymer could be generated when the sonication time was greater than 30 min. When the sonication time is 2 h, the generated polymer has high molecular weight and good homogeneity of PMMA. When the sonication time continues to increase, the molecular weight decreases due to early chain termination.

3.3. Effect of ultrasound time on product yield

The conversion rate of the product tended to accelerate with the increase of irradiation time, and the results are shown in Fig. 5. When the sonication time was less than 30 min, almost no polymer was generated for the MMA monomer. When the sonication time reached 30 min, the polymer production could be observed with a yield of about 19.6%. As the sonication time increased, the yield gradually increased to about 70%. It is inferred that the conversion rate was increased due to the increase in the number of free radicals at longer sonication time. From the figure, it can be observed that when the sonication time exceeds 2 h, the yield has a plateau and slightly decreases as the time increases. It is inferred that this is due to the increase in the number of carbon dioxide replenishments in the reactor as the sonication time increases, thus causing an increase in losses and a decrease in yield. Based on the experimental results, it can be concluded that there is an optimal value of the ultrasonic time to stimulate the ultrasonic polymerization, and the ultrasonic can stimulate the polymerization when the ultrasonic time is greater than this time. However, the point where the ultrasonic time is at the optimum value can guarantee the yield and achieve the effect of environmental protection and energy saving. In this experiment, the optimal value of ultrasonic time is around 2 h.

Fig. 5.

Effect of ultrasound time on conversion rate.

3.4. Effect of graphene content on the conductivity of complexes

The relationship between graphene content and electrical conductivity of Graphene/PMMA composites is shown in Fig. 6. As with most acrylate polymers, pure PMMA conductivity is below 10–14 S/cm. However, when graphene was added, the electrical conductivity of PMMA composites increased significantly. When the graphene content was increased from zero to 1 wt%, it rapidly increased to 8.89 × 10^-2 S/cm.It is noteworthy that a narrow range of abrupt changes in the conductivity of the composites was observed. A small increase in graphene content within this range will bring about an order of magnitude increase in conductivity, while outside this region, the rate of conductivity increases with graphene content and returns to a flat rate. From the above phenomenon, it is inferred that the conductive pathway is formed inside the composite by the contact between the graphene layers and sheets. When the graphene content is below 0.6 wt%, the probability of contact between graphene sheets is small and it is difficult to form a conductive network. And with the increase of graphene content in the composites, the very large sheet layer of graphene and the very large specific surface area lead to the formation of conductive networks very easily. Thus, the conductivity of the composite material is rapidly improved with the addition of a small amount of graphene, and excellent electrical conductivity is obtained. After the formation of the conductive network, it is difficult to show a jump in conductivity with the increase of graphene content, i.e., a smooth increase. This phenomenon is consistent with the existence of jump conductivity mechanism in composites. This content is called the conductive bleed threshold of the composite. The lower the conductivity threshold, the less graphene needs to be added in order to achieve a specific conductivity, and the better it is to maintain the mechanical properties of PMMA. Compared to the conductivity threshold of 1.5 wt% for carbon black, the threshold for graphene is reduced by 50%. This phenomenon can prove that in situ polymerization can disperse graphene uniformly into PMMA and thus can improve the electrical conductivity rapidly at a lower graphene content. Also, to verify the uniform distribution of graphene in PMMA, the conductivity was tested at several points of the sample. The test results show that the conductivity of each point is consistent, indicating that graphene is uniformly distributed in PMMA.

Fig. 6.

Effect of graphene content on the conductivity of complexes.

4. Graphene/PMMA nanocomposite

Graphene/PMMA nanocomposites were successfully prepared under the following experimental conditions: the graphene content was 1 wt%, the ultrasonic power was 500 W/cm², the ultrasonic time was 6 h, and the supercritical conditions of the system were 60 °C and 14 MPa pressure. The Graphene/PMMA nanocomposites prepared by ultrasonic excitation showed no significant changes in terms of appearance, scanning electron microscopy(SEM), infrared spectrogram(IR), and X-ray diffraction(XRD), while the Gel permeation chromatography (GPC) and Thermo Gravimetric Analyzer, (TAG) test results and electrical conductivity were different.

The TG and differential thermal gravity (DTG) curves of PMMA and Graphene/PMMA nanocomposites are shown in Fig. 7. It can be observed from the curves in Fig. 7 that the thermal stability of PMMA and Graphene/PMMA nanocomposites prepared by ultrasonic excitation is poorer. However, it can be observed from the DTG curves that there is only one more obvious peak for the products prepared by ultrasonic excitation. The results showed that the chain termination formation of double bonds was avoided due to the more uniform distribution of free radicals without the addition of excitant. This indicates that the molecular weight of the products prepared by ultrasonic excitation is more homogeneous and the products are more uniform. The thermal stability of Graphene/ PMMA nanocomposites prepared by ultrasonic excitation is better than that of PMMA prepared by ultrasonic excitation, indicating that the addition of graphene helps to improve the thermal stability of PMMA. When the graphene content was 1 wt%, it improved to 0.12 S/cm, indicating that the Graphene/PMMA nanocomposites prepared by ultrasonic excitation were more homogeneous.

Fig. 7.

TG and DTG curves of PMMA and Graphene/PMMA nanocomposites.

In terms of GPC test results, the difficulty of polymerization increases with the addition of graphene, and the molecular weight of the products tends to decrease. The instantaneous high temperature (greater than3000 k) caused by the instantaneous rupture of the bubble generated by the ultrasonic cavitation effect can cause the molecular chain breakage of the monomer molecules in the liquid phase on the cavitation surface to generate free radicals.

5. Analysis of the polymerization mechanism

Due to the high diffusivity, low surface tension, and low viscosity of the supercritical CO₂ molecules, MMA can receive more fully the energy transferred by the ultrasonic action, which leads to the degradation of MMA by ultrasound into free radicals to stimulate polymerization. During the reaction, the effect on the reaction system can be observed when ultrasonic action is added to the device, i.e., the cavitation effect generated by ultrasound can be observed. The field of view is very clear when the ultrasound is not turned on, and when the ultrasound is turned on, the field of view is blurred due to the generation of a very large number of very fine cavities. When ultrasound acts on supercritical fluids, the ultrasound can rapidly transfer energy and form microjets with strong (1.5 kg_˙cm⁻¹) impact. This can stir the solution system at high speed and accelerate the rate of mass and heat transfer between the interfaces, which can lead to “ultrasonic cavitation” in the fluid. This produces instantaneous localized high temperature and high pressure, where high temperature can reach 5000 K and high pressure can reach 500 atm, with a temperature change rate of 10⁹ K/s. The strong changes provide a new physicochemical environment for chemical reactions that are difficult or impossible to achieve under normal conditions. It is sufficient to degrade the MMA monomer into free radicals and thus stimulate free radical polymerization reactions.

The coupling effect of carbon dioxide is also very important in this process. The coupling effect of carbon dioxide can provide MMA with a space where it can be fully dispersed and with a very small viscosity and great mobility, so that it can better absorb the energy generated by the ultrasonic cavitation and break the chemical bonds of the monomer molecules[38].

After the generation of free radicals by ultrasound excitation

(i)Chain initiation

(ii)Chain growth

(iii)Chain termination

Saturated polymer product obtained after coupling termination

6. Conclusion

The preparation of PMMA by ultrasonic irradiation-excited MMA polymerization under supercritical CO₂ conditions has the most potential to produce PMMA with good performance in a green way due to the multiple effects of no exciter addition, no solvent involvement, and ultrasound with excitation and stirring during the process. PMMA was successfully prepared under supercritical carbon dioxide conditions using ultrasonic irradiation to stimulate MMA polymerization. And the effects of ultrasonic irradiation intensity and time on the molecular weight and yield of the products under supercritical conditions were analyzed to investigate the effects of ultrasonic excitation conditions on the prepared PMMA and the mechanism, and the results showed that:

• (i)In the supercritical carbon dioxide system, there is a threshold and an optimum value of ultrasonic intensity to stimulate the ultrasonic polymerization. Above the threshold value, the ultrasound can stimulate polymerization, and at the point of the optimum value, the energy saving effect can be achieved with guaranteed yield;
• (ii)There is an optimal value of the ultrasonic time to stimulate the ultrasonic polymerization. Ultrasound can stimulate polymerization when the ultrasound time is greater than this optimal time, but when the ultrasound time is at the point of the optimal value, it is possible to achieve the effect of environmental protection and energy saving while ensuring the yield. Studies have shown that the optimal value of ultrasonic time is around 2 h.
• (iii)The TG and DTG curves from the ultrasonically Graphene/PMMA nanocomposites illustrate that the free radicals generated by ultrasonic excitation are uniformly distributed due to the absence of the added exciter. The Graphene/PMMA nanocomposites prepared by ultrasonic excitation are more homogeneous.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.