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Abstract
The use of CO2 for substituting volatile organic compounds in polymer processing, i.e., supercritical CO2-assisted polymer processing, has attracted increasing attention in recent years. Dissolution of CO2 in polymer could swell, plasticize and deliver small molecules into the polymer matrixes. Consequently, the structure and morphology of the polymer would change, as well as the fundamental properties, including polymer crystallization, interfacial tension between polymer and gas, and rheology of CO2/polymers melt. CO2-induced changes in these properties could be used to realize the supercritical CO2-assisted polymer processing, e.g., CO2-assisted polymer grafting, CO2-assisted penetrating of small molecules into polymer and CO2-assisted polymer foaming. Several cases from the authors' laboratory are presented for elucidating how to use the changes to manipulate the CO2-assisted polymer processing. The cases include CO2-induced crystallization of isotactic polypropylene and syndiotactic polypropylene, CO2-induced crystal phase transition of isotactic poly-1-butene, and how to use CO2-induced crystallization to manipulate melt strength of three linear polymers, i.e., isotactic polypropylene, poly(lactic acid)and poly(ethylene terephthalate), to prepare the polymer foams with different structures.
By using supercritical carbon dioxide as physical foaming agent, microcellular poly (lactic acid)/wood fiber (PLA/WF) composite foams were prepared in a batch process via depressurization. The cellular structures of the foamed PLA/WF composites were observed by means of scanning electron microscope. The effect of WF content on cellular structure was investigated by analyzing the microstructure and rheological properties of the PLA/WF composites. The addition of WF induced higher storage modulus and complex viscosity of the PLA/WF composites in the low frequency range, resulting in much more uniform cellular structure. Cell density increased and cell size decreased gradually with the increase of WF content. The uniformity of cellular shape and structure was high for the foamed composite sample with 10% (mass) WF, whereas the uniformity decreased for those with 20% and 30% WF.
The crystallization morphology and lamellae thickness of poly(ethylene oxide)-b-poly(ε-caprolactone) (PEO-b-PCL) under high pressure CO2 was studied by means of wide-angel X-ray diffraction (WAXD) and small-angel X-ray scattering (SAXS). For the sample of PEO0.5k-b-PCL24.5k, only PCL blocks crystallized under 1-3 MPa CO2 and the lamellae thickness of PCL segment decreased with increasing CO2 pressure. The melting behavior and isothermal crystallization kinetics were investigated by using high pressure differential scanning calorimetry (HP DSC). The results indicated that only the melting peak of PCL segments can be observed and the melting peaks widened with increasing pressure. The crystallinity degree (Xc) and melting temperature (Tm) of PEO-b-PCL decreased with increasing CO2 pressure. Xc of PCL in the sample decreased from 46.9% to 36.6% and Tm decreased from 54.5°C to 47.2°C under 0 MPa and 3 MPa respectively. The Avrami analysis was performed to obtain the kinetics parameters. The overall crystallization rate decreased with increasing pressure and the crystallization process was controlled by nucleation rate. The half crystallization time (t1/2) of the sample increased from 2.23 min at 0 MPa to 5.83 min at 3 MPa. The value of Avrami index n, is between 3.7 and 4.7, demonstrating a three-dimensional spherulitic growth in the process of isothermal crystallization. The decreasing crystallinity degree and crystallization rate illustrated that pressurized CO2 hindered the crystallization of PEO-b-PCL.
Controlling sandwich-structure of poly(ethylene terephthalate) (PET) microcellular foams using coupling of CO2 diffusion and CO2-induced crystallization is presented in this article. The intrinsic kinetics of CO2-induced crystallization of amorphous PET at 25°C and different CO2 pressures were detected using in situ high-pressure Fourier transform infrared spectroscopy and correlated by Avrami equation. Sorption of CO2 in PET was measured using magnetic suspension balance and the diffusivity determined by Fick's second law. A model coupling CO2 diffusion in and CO2-induced crystallization of PET was proposed to calculate the CO2 concentration as well as crystallinity distributions in PET sheet at different saturation times. It was revealed that a sandwich crystallization structure could be built in PET sheet, based on which a solid-state foaming process was used to manipulate the sandwich-structure of PET microcellular foams with two microcellular or even ultra-microcellular foamed crystalline layers outside and a microcellular foamed amorphous layer inside. © 2011 American Institute of Chemical Engineers AIChE J, 58: 2512–2523, 2012
The work reported in this paper was aimed at exploring the advantages of using supercritical carbon dioxide (scCO2) as an environmentally benign solvent and swelling agent for carrying out the free radical grafting process of vinyl monomers onto isotactic polypropylene (PP) in the solid state. Methyl methacrylate (MMA) was chosen as a grafting monomer. Results showed several scientifically interesting and industrially relevant advantages of the scCO2-assisted solid-state grafting process over a classical solid-state or melt process. First, compared to a classical solid-state grafting process the overall reaction rate of the scCO2 assisted one became less diffusion-controlled and more reaction-controlled because of enhanced diffusion of MMA and the initiator in the PP. Second, the CO2 pressure itself constituted an additional and sensitive process parameter capable of significantly modifying the monomer grafting yield and the product quality. Third, the scCO2-assisted solid-state process produced much higher MMA grafting contents and longer MMA grafts than the classical solid-state and melt processes did.
This work is aimed at studying the effect of CO2 on the phase transition of isotactic poly-1-butene (iPB-1) with form III upon heating. The melting behaviors of form III under atmospheric N2 and compressed CO2 at different heating rates ranging from 1 to 20 °C/min were investigated using high-pressure differential scanning calorimetry (DSC). The results showed that the plasticization effect of CO2 promoted melting of form III and inhibited the phase transition of form III to II as a whole. By analyzing the melting parameters obtained from the DSC measurements, we deduced that the phase transition of form III to II might comprise another transition process besides the melt-recrystallization mechanism. In-situ wide-angle X-ray diffraction (WAXD) measurement on form III under atmospheric N2 at a heating rate of 0.25 °C/min verified that the phase transition of form III to II passed through the solid–solid phase transition before melt-recrystallization. In-situ high-pressure Fourier transform infrared (FTIR) was then used to detect the phase transition of form III under atmospheric N2 and compressed CO2 at the heating rate of 1 °C/min. It was also shown that the phase transition of form III to II passed through the solid–solid phase transition and melt-recrystallization under atmospheric N2, 1 and 2 MPa CO2. However, form II formed completely through the melt-recrystallization under 3 MPa CO2 and could not generate with further increasing CO2 pressure to 4 MPa. Moreover, more form I′ generated during heating through the solid–solid phase transition with increasing CO2 pressure. Besides carbon tetrachloride solution prepared form III, the other two solutions, i.e., dilute toluene and o-xylene, cast form III also exhibited the similar generation processes of form II upon heating under atmospheric N2 and compressed CO2 as measured by in-situ high-pressure FTIR.
The reactive extrusion process of isotactic polypropylene (iPP) grafting maleic anhydride (PP-g-MAH) initiated by dicumyl peroxide (DCP) in the presence of supercritical carbon dioxide (scCO2) is investigated. Because of its moderate supercritical conditions and well swollen performance in iPP melts, scCO2 is selected to be imported into the extruder system to reduce process temperature and melt viscosity as well as strengthen the mass transport. It has been found that the process temperature of reactive sections of co-rotating twin screw extruder can be successfully reduced from conventional 190 to 160°C when assisted with the addition of scCO2. Consequently, effective suppression of main chain degradation is observed, which leads to the products with relative higher molecular weight and narrower molecular weight distribution. The experimental results also indicate a significant increase in both the grafting degree of resultant PP-g-MAH and the grafting efficiency of MAH under certain operation conditions. Especially, the grafting efficiency is close to 90% when low concentration of both MAH and DCP are employed. A potential mechanism has been proposed to explain the effects of scCO2 in the reactive extrusion. In comparison with traditional molten grafting process, the work presents a novel approach to increase the grafting efficiency of MAH and control the molecular weight of resultant PP-g-MAH simultaneously.
In-situ high-pressure FTIR was used to investigate the polymorphous phase transition of isotactic poly-1-butene (iPB-1) with form III upon annealing at temperatures ranging from 75 to 100 °C and CO2 pressures ranging from 2 to 12 MPa. It was shown that the phase transition of form III changed from form III to II not through form III to I′ with increasing temperature and application of CO2 increased the content of generated form I′. Wide-angle X-ray diffraction (WAXD) measurement on the annealed iPB-1 with form III verified the phase transition of form III. The crystalline morphology of the annealed iPB-1 films was investigated using polarized optical microscopy (POM). The results implied that the phase transition of form III to I′ might process via a solid–solid transition, which did not affect the orientation of the lamellar stacks. The orientation of form II lamellar stacks depended strongly on the formation process. To obtain strong orientation, the formation process displayed the following order: melt crystallization at ambient condition > melt recrystallization under CO2 > phase transition upon annealing at ambient condition. Avrami equation could be well established to describe the phase transition of form III to I′ through a solid–solid phase transition.
The free volume and crystallization behaviour of poly(ethylene naphthalate) (PEN) treated in pressurized carbon dioxide was studied, using Positron Annihilation Lifetime Spectroscopy (PALS) and Differential Scanning Calorimetry (DSC). PALS probes the values of free volume cavity sizes in materials, thus making it possible to investigate the effect of pressurized carbon dioxide treatment on free volume hole sizes in PEN. The crystallinity and melting behaviour of PEN was analyzed with temperature modulated DSC. We found that the carbon dioxide pressure during treatment of PEN was the prime parameter affecting the value of free volume after the treatment. The free volume sizes were observed to be insensitive to the other two parameters, temperature and time. Increasing the time of the treatment however, increased the crystallinity of PEN. Interestingly, this was not coupled to a decrease in ortho-Positronium intensity as was expected, indicating that positronium may form in the crystalline fraction of the polymer as well as in the amorphous fraction. (C) 2009 Elsevier Ltd. All rights reserved.
The formation of crystal forms of syndiotactic polystyrene having different stability was investigated in different media having different solubility parameters with the help of wide-angle X-ray diffraction, Fourier-transform infrared spectroscopy and differential scanning calorimetry. The starting sample was the gamma form of syndiotactic polystyrene, and the solubility parameter of the media was adjusted by changing the temperature, pressure, cosolvent of supercritical carbon dioxide. With increasing solubility parameter, the stability of the resultant crystal forms was increased in the order from gamma through alpha" and beta' to beta" form. (c) 2005 Elsevier Ltd. All rights reserved.
The crystallization behavior of thin bisphenol A polycarbonate (PC) films after treatment in supercritical CO(2) (ScCO(2)) was investigated by using polarized optical microscopy (POM) and atomic force microscopy (AFM). Experimental results indicated that the crystallization ability in thin PC film of 259 nm thick was higher than that in the bulk in a much wider temperature range, and the crystallization window was further broadened when the thickness of samples decreased. The 15 nm film crystallized under 20 MPa CO(2) at 60 degrees C, i.e., more than 90 K below the glass transition temperature of the bulk at ambient pressure, while the 259 nm film remained amorphous under the same treatment condition. The results further revealed that crystalline morphology was affected by the CO(2) treatment condition and film thickness. And the 7 nm film dewetted the substrate in the treatment at 20 MPa/60 degrees C instead of crystallization. It was indicated that chain mobility of the polymer was strongly increased in ScCO(2) when the film thickness was decreased to the scale of radius of gyration (ca. 6 nm) of the polymer. A modified three-layer model was proposed to explain these findings by introducing the effect of CO(2) adsorption. The excess CO(2) adsorbed at the free surface and polymer/substrate interface enlarged portions of these two layers and enhanced the polymer mobility therein, which took effect in thin films with thickness from hundreds down to several nanometers.
The solubility and diffusivity of carbon dioxide (CO2) in the solid-state isotactic polypropylene (iPP) were studied by using the pressure−decay method at temperatures of 373.15, 398.15, and 423.15 K and pressures ranging from 1 to 15 MPa. The solubilities of CO2 in the solid-state and amorphous regions of iPP were both obtained. They increased almost linearly with increasing pressure and decreased with increasing temperature. The Sanchez−Lacombe equation of state (S-L EOS) correlated the solubility in the amorphous regions of the solid-state iPP within 3% average relative deviation in conjunction with a temperature-dependent interaction parameter. The diffusion coefficient of CO2 in the solid-state iPP showed a weak concentration dependence and had an order of magnitude of 10−10−10−9 m2/s in the solid-state iPP.
A highly oriented isotactic polypropylene (iPP) with a shish-kebab crystalline structure is used as a template to achieve nanocellular foams by CO2 foaming in the solid state. The shish-kebab crystalline structure is obtained by tuning injection-molding conditions. Nanocells are generated in amorphous domains confined by shish-kebab crystalline domains which cannot foam. The highly oriented iPP consists of both shish-kebab crystalline structure and spherulites structure and is used to investigate the effect of the crystalline morphology on the cell formation at a given CO2 saturation pressure (15 MPa) and various foaming temperatures (135, 140, 143, and 146 °C). When the foaming temperature is 143 °C, a uniform open nanocellular morphology is obtained. The cell size and cell density both increase with foaming temperature from 135 to 143 °C due to the fact that more crystals are molten, providing more space for cell nucleation and growth. However, when the foaming temperature is increased to 146 °C, the cell size abruptly increases from dozens of nanometers to dozens of micrometers, and the corresponding cell density decreases by several orders of magnitude.
This study proposes a novel process for significantly toughening isotactic polypropylene (iPP) by finely tuning and controlling the structure and morphology of iPP. The toughness of injection-molded iPP specimens can be significantly improved by controlled shearing, CO2-induced recrystallization, and adequate cooling without loss of strength. The distribution and structure of the iPP samples before and after toughening were characterized by polarized optical microscopy, Fourier transform infrared spectroscopy, wide-angle X-ray diffraction, small-angle X-ray scattering, scanning electron microscopy, and differential scanning calorimetry, to investigate the structure–property relationship. Under shear, a high degree of orientation can be obtained with “shish-kebab” crystals formed in the shear zone. During the subsequent CO2 treatment, a crystal network morphology can be formed as a result of an increase in the numbers of primary lamellae and crosshatched subsidiary lamellae, which leads to an increase in toughness. Wide-angle X-ray diffraction patterns indicate that quenching in ice–water of scCO2-treated iPP promotes the formation of nanosized mesomorphic phase domains in the shear zone, which further toughens the iPP. The impact strength of the best toughened iPP sample was found to be over 12 times that of the original sample without loss in tensile strength and modulus.
This work was aimed at studying supercritical carbon dioxide (scCO2)-induced melting temperature depression and crystallization of a syndiotactic polypropylene (sPP). Under scCO2, the melting temperature of the sPP could be significantly reduced depending on the CO2 pressure. The scCO2-induced crystallization of sPP was investigated using differential scanning calorimeter (DSC) and Fourier transform infrared spectroscopy. Two melting peaks were observed in DSC. The one at lower melting temperature referred to the melting of the sPP crystals induced by scCO2 in its amorphous phase. Its location was shifted to higher temperature, and its area increased with increasing scCO2 treatment time, temperature, and pressure. The melting peak at higher temperature corresponded to the melting of the sPP crystals that already existed before scCO2 treatment. Its location and area remained almost unaffected by the scCO2 treatment. The scCO2-induced crystallization was related to scCO2-promoted transformation of the mesophase form III of the sPP to the stable form I. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers