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Crystallization of bisphenol a polycarbonate induced by supercritical carbon dioxide
Abstract
Supercritical carbon dioxide readily induces crystallization in bisphenol A polycarbonate. Crystallization begins within one h of exposure to the CO2 at temperatures and pressures as low as 75°C and 100 atm. The degree of crystallinity increases sharply as the CO2 pressure is raised from 100 to 300 atm but levels off thereafter. This behavior is likely due to a minimum in the Tg of the polycarbonate/CO2 mixture owing to the opposite effects of the pressure on the Tg of the polymer and on the equilibrium weight fraction of CO2 absorbed. Percent crystallinities of over 20%, comparable to that achieved using acetone or other organic liquids, have been obtained after 2 h exposure to supercritical CO2. Since polycarbonate degasses quickly and quantitatively at ambient temperature and pressure, the high Tg of bisphenol A polycarbonate can be regained in the crystallized material without further vacuum treatment.
... The first polymer about which such phenomena have been reported is poly(ethylene terephthalate) (PET) [3], while extensive studies on CO 2 -induced crystallinity in PET and crystalline kinetics have been conducted [3,[7][8][9][10][11][12]. CO 2 -induced crystallization has also been observed in other polymers, such as polycarbonate (PC) [13][14][15][16][17], polylactic acid [18], poly(ether ketone) [19][20][21], poly(p-phenylene sulfide) [22], polybutylene [23], polypropylene [24][25][26][27][28][29], syndiotactic polystyrene [10,30], poly(vinylidene fluoride) [31] and TPU [32][33][34]. In addition to CO 2 , butane also induces crystallization and plasticization of the polymers [35]. ...
... Although early studies in PET and PC showed that dissolving high pressure CO 2 increased the crystallinity of polymers [3,13], it is not always true. Nofar et al. demonstrated that various factors, including isothermal and non-isothermal crystallization and CO 2 pressures, affect the crystallization kinetics and thus the final crystallinity [18]. ...
Dissolving gas in polymer caused them to plasticize and induced crystallization. As a result, the glass transition temperatures (Tg) and melting points (Tm) decreased and crystallinity changed. This study investigated the effect of N2 dissolution on the thermal behavior of polyether-based thermoplastic polyurethanes (TPUs). Although the affinity between the polymers and N2 is not strong, the solubility of N2 in TPU is below 3 wt% at pressures below 10 MPa and temperatures in the range of 190 °C~210 °C. A unique phenomenon is reported. Dissolving N2 in TPU reduced the melting point but decreased crystallinity.
A weight-loss-based test showed that the N2 sorption increased with increased soft segment (SS) content. Annealing with N2 showed an apparent plasticizing effect on TPU with high SS content. The melting peak of imperfect ordered crystals, around 100 °C, completely disappeared. This research contributes to understanding the impact of N2 plasticization on TPU crystallization and the development of nitrogen foaming technology.
... During saturation, the CO 2 induces plasticization, decreasing the glass transition temperature (T g ) and allowing the foaming [20]. Besides, under some conditions, it also induces crystallization in PC [21]. Mascia et al. [22] studied the conditions leading to cells' formation and the onset of crystallization of bisphenol-A polycarbonate in a one-step process, varying the saturation conditions. ...
... However, during the saturation process with CO 2 , the polymer suffers a recrystallization process, resulting in a melting peak that can be observed in the first heating of the DSC curve around 235°C. This melting temperature of the EPC bead foams is similar to those obtained by previous authors [21][22][23]. Fig. 5a shows the first heating step of the DSC curve in which the melting peak appears and a zoom of the melting peak region. Fig. 5c presents the evolution of the melting point temperature and the crystallinity of the EPC beads as a function of the saturation temperature. ...
Polycarbonate (PC) is a widely used engineering thermoplastic. Also, bead foaming technology is a method to produce high-performance, low-density foams with complex geometries. In this work, expanded polycarbonate (EPC) with microcellullar structure and low density have been produced for the first time using the autoclave bead foaming technique. The effect of the processing parameters, such as the saturation temperature and time, on the characteristics of the EPC bead foams, is analyzed. Contrary to the total amorphous structure of the solid material, the obtained EPC foams present a crystalline phase characterized by a melting peak around 235 °C. The crystallization process might be taking place during the saturation of the samples with CO2. Results show that the saturation temperature allows controlling density, whereas foaming time can be used as a tool to control crystallinity degree in the range from 7.5 to 45 min, without a clear influence on the cellular structure. EPC with densities between 116-592 kg/m³, cell sizes of approximately 4-5 µm, and crystallinities ranging 0-11% were produced by a proper modification of the foaming parameters. Then, the use of different saturation parameters allows controlling the density and thermal properties of the EPC, thanks to the autoclave bead foaming process.
... The sample treatments were performed in the high P, T reactor at 148 atm and 120 • C for a maximum reaction time of 5 h. These conditions were selected based upon prior studies of the crystallization behavior of amorphous polycarbonate treated with scCO 2 alone (Liao et al., 2003;Sun et al., 2014;Beckman and Porter, 1987). It has been reported that supercritical CO 2 enables the crystallization of initially amorphous PC at relatively low temperatures and that the effect depends on the temperature and pressure of the scCO 2 contact. ...
... It is evident that hydrogen peroxide (H 2 O 2 ) is not necessary to induce the physical changes in the model PCB samples. DSC measurements show that these changes are accompanied by the crystallization of the initially amorphous PC on much shorter timescales than typical for scCO 2 exposure alone (on the order of 4-6 h) (Liao et al., 2003;Beckman and Porter, 1987). This study demonstrates that the exposure of model PCB, PC and Cu laminates, to scCO 2 and 2 M H 2 SO 4 at elevated temperature and pressure for just 30 min results in significant morphological changes, including delamination. ...
Electronic waste (e-waste) is one of the fastest growing waste segments in the world. This study investigates the use of supercritical CO2 (scCO2) and aqueous acid as co-solvent for the treatment of e-waste, specifically for the extraction of copper. Printed circuit board (PCB) was selected as the e-waste of study. In order to perform controlled experiments, melt-pressed Cu foil and polycarbonate sheets were prepared as surrogates for PCBs. It was found that a scCO2 and acid pre-treatment induced drastic morphological changes in the polymer, creating pores, cracks, and delamination. This finding was translated to the actual waste PCB system. This unique process involved the pre-treatment of the PCB with scCO2 and aqueous sulfuric acid at 120 °C and 148 atm for 30 min followed by leaching of the treated PCB in a solvent containing 2 M sulfuric acid and 0.2 M hydrogen peroxide at ambient conditions. Experimental results showed that 82% of the copper contained in the PCB was extracted in under 4 h. The characterization of the PCB demonstrated that the pre-treatment with scCO2 and acid induced the crystallization of the plastics (polymer component), creating pores and weakening the structure of the PCB, thereby enhancing the transport of the solvent to the buried metal interfaces. This novel process using scCO2 could reduce physical processing (e.g. grinding of the PCB) and acid usage during the extraction of Cu from e-waste, providing a greener alternative for current methods of recycling of metals, which are energy intensive with large environmental footprints.
... Supercritical carbon dioxide (Sc-CO 2 ) is usually used as non-toxic solvents in extractions [18,19], separations [20,21], chemical reactions [22], and various other applications [23][24][25][26]. At present, more and more experts use Sc-CO 2 to induce the crystallization of polymers and fibers [27][28][29]. Owing to the characteristics of the Sc-CO 2 fluid, it can be applied to the hot-drawing process of PAN fibers at low temperatures. In this paper, we tried to find a new way to prepare the PAN-based CFs with higher performance, and Sc-CO 2 was used to treat the PAN fibers during the hot-drawing process. ...
... Supercritical carbon dioxide (Sc-CO2) is usually used as nontoxic solvents in extractions [18,19], separations [20,21], chemical reactions [22], and various other applications [23][24][25][26]. At present, more and more experts use Sc-CO2 to induce the crystallization of polymers and fibers [27][28][29]. Owing to the characteristics of the Sc-CO2 fluid, it can be applied to the hot-drawing process of PAN fibers at low temperatures. In this paper, we tried to find a new way to prepare the PAN-based CFs with higher performance, and Sc-CO2 was used to treat the PAN fibers during the hot-drawing process. ...
The hot-drawing process of polyacrylonitrile (PAN) fibers is an important step during the production of PAN-based carbon fibers. In this study, supercritical carbon dioxide (Sc-CO2) was used as one kind of media for thermal stretching of PAN fibers to study the effect of different pressures of Sc-CO2 on crystallinity, degree of orientation and mechanical property of PAN fibers during the hot-drawing process. The changes of microstructure and mechanical properties in the PAN fibers were investigated by wide-angle X-ray diffraction, small angle X-ray scattering and monofilament strength analysis. The results showed that as the pressure increased, the crystallinity and degree of orientation of PAN fibers increased. Furthermore, when the pressure was 10 MPa, the crystallinity increased from 69.78% to 79.99%, which was the maximum crystallinity among the different pressures. However, when the pressure was further increased, the crystallinity and degree of orientation of the fibers were reduced. The test results of the mechanical properties were consistent with the trends of crystallinity and degree of orientation, showing that when the pressure was 10 MPa, the tensile strength of the fibers increased from 4.59 cN·dtex−1 to 7.06 cN·dtex−1 and the modulus increased from 101.54 cN·dtex−1 to 129.55 cN·dtex−1.
... This behavior of PC has already been observed in the literature and was attributed to the scCO 2 induced crystallization phenomenon [17,18,21]. ...
... In our experiments performed at 50 °C, in particular the outer part and the corners of the pellets turned in some cases opaque after sorption. Based on literature data reported elsewhere [17,18,21], we also attribute this phenomenon to the scCO 2 induced crystallization of PC. After 3 hours of impregnation time at 200 and 300 bar at 50 °C samples remain transparent, but somewhat opaque pellets were obtained after 6 hours of sorption time. ...
In this work, the applicability of the supercritical CO2 dyeing process on polycarbonate pellets was investigated by the use of two azo-disperse dyes; disperse red 1 (DR1) and disperse red 13 (DR13). Experiments were performed in the range of 100–300 bar and 40 °C to 60 °C with 3–24 h of impregnation time. Dyeing took place in a high pressure vessel and kinetics was studied and explained. Impregnation efficiency on the polymer pellets was measured by UV–vis spectroscopy. The process was successfully applied and resulted in an entirely, equally deep-dyed polymer with excellent dyeing fixation. Arising from the different solubility and chemical structure of the dyes, their sorption kinetics was found to be different. Maximal dye uptake obtained with DR1 and DR13 were 0.010 wt% and 0.055 wt% respectively, with respect to the mass of the polymer. New solubility data for DR13 in supercritical CO2 has also been measured and partition coefficients (Kc) for the dyes between the fluid and the polymer phase were calculated.
... CO 2 can increase PC chain mobility, which is favorable for CO 2 sorption, but it has also been reported to enhance PC crystallization and thus be detrimental to CO 2 sorption [67]. Crystallization of PC after scCO 2 treatment was observed in the literature [67][68][69]. However, in the range of temperatures investigated, no crystallization was expected [45,67] which was further confirmed by DSC curves (Section 3.5). ...
The development of active packaging for food storage containers is possible through impregnation of natural extracts by supercritical CO2-assisted impregnation processes. The challenge of scCO2-impregnation of natural extracts is to control the total loading and to ensure that the composition of the loaded extract may preserve the properties of the crude extract. This study aimed at investigating the scCO2-impregnation of clove extract (CE) in polycarbonate (PC) to develop antibacterial packaging. A design of experiments was applied to evaluate the influences of temperature (35–60 °C) and pressure (10–30 MPa) on the clove loading (CL%) and on the composition of the loaded extract. The CL% ranged from 6.8 to 18.5%, and the highest CL% was reached at 60 °C and 10 MPa. The composition of the impregnated extract was dependent on the impregnation conditions, and it differed from the crude extract, being richer in eugenol (81.31–86.28% compared to 70.06 in the crude extract). Differential scanning calorimetry showed a high plasticizing effect of CE on PC, and high CL% led to the cracking of the PC surface. Due to the high loading of eugenol, which is responsible for the antibacterial properties of the CE, the impregnated PC is promising for producing antibacterial food containers.
... There are just a few examples of crystallization in aromatic polycarbonates such as poly(BPA carbonate) by solvent vapors 33,34 or supercritical CO 2 . 35 Polycarbonates from tyrosol are less rotationally constrained than poly(BPA carbonate) and thus may exhibit the crystallization behavior of aliphatic polycarbonates such as PTMC. Low molecular weight PTMC (<12 kDa) is reported to spontaneously crystallize under ambient conditions with a low melting point of 36°C. ...
One of the challenges in materials science is to design functional, synthetic polymers that match the desired properties of a specific application. Achieving high stiffness and strength under physiological conditions while maintaining the proper erosion profile is particularly difficult. This challenge is addressed here by synthesizing polymers with sequence control followed by optimal processing of the polymer. A new class of biodegradable, aromatic-aliphatic polycarbonates based on tyrosol, a naturally derived (hydroxy)alkyl phenol with an alternating (altTyPC) or scrambled (scrTyPC) sequence, is reported. AltTyPC contained strictly alternating sequences of diaryl (head-to-head, HH) and dialkyl (tail-to-tail, TT) carbonate backbone isomers; this was accomplished by using preprogrammed diaryl carbonate diol and triphosgene in polycondensation. The scrambled sequence (scrTyPC) of HH, TT, and the aryl-alkyl (head-to-tail, HT) carbonate isomers was obtained by direct polymerization of tyrosol. The isomer sequence had a large effect on the thermal behavior: altTyPC rapidly transitioned from the amorphous phase into a 1D mesophase at 90 °C and then into a 3D crystalline phase at 150 °C before melting at 204 °C; in contrast, scrTyPC remained in a 1D mesophase after extensive annealing at 100 °C for 20 h and melted at 149 °C. The modulus of the oriented, semicrystalline altTyPC films was higher than that of the similarly processed scrTyPC films (5.4 ± 0.3 vs 3.8 ± 0.2 GPa). The erosion behavior was also different: AltTyPC showed more rapid mass and thickness loss over time than scrTyPC. Thus, a combination of sequence control and processing optimization in aromatic-aliphatic polycarbonates gives rise to a new platform of tunable polymers for numerous applications.
... In order to facilitate the crystallization of PC, some researchers have been done through various methods. Exposure to organic solvent vapor (acetone, butyl acetate, carbon tetrachloride et al.) or supercritical carbon dioxide can induce considerable crystallization for PC molecules [9][10][11]. Long-time annealing or keeping under high-pressure can also promote the crystallization of PC [12][13][14]. ...
This work provides a facile strategy to induce crystallization of polycarbonate (PC) through solvent casting method. Pristine fullerene (C60) and PC-modified C60 nanoparticles are applied as nucleating agents for PC, and the crystallization behavior of PC nanocomposite films is controlled by solvent casting preparation conditions. To investigate the relationship between crystallization behavior and mechanical properties of PC nanocomposite films, X-ray diffraction, differential scanning calorimetry, polarizing microscope and tensile tests are carried out. Pristine C60 can only induce the formation of bulk PC spherocrystals which have no help or even deteriorates the mechanical properties of PC/C60 nanocomposite films. By contrast, the PC-modified C60 achieves uniformly disperse in the matrix and induces the formation of microcrystalline structures. It is found that appropriate degree of crystallinity for the PC/modified C60 nanocomposite films can effectively improve the mechanical properties. Typically, the PC/0.50 wt%-modified C60 nanocomposite films prepared under 40 °C by tetrahydrofuran exhibits up to a 33% increment in tensile strength and 45% increment in Young’s modulus compared to neat PC films.
... Since CO 2 can plasticize the glassy state of PLA into its rubbery state and provide enough mobility to the molecules to rearrange and form crystals, one would expect that such enhanced mobility due to the presence of CO 2 would tremendously affect the crystallization kinetics as measured under isothermal conditions. In fact, it is well recognized that carbon dioxide can even contribute to forming a crystalline phase in polymers that are commonly reported as amorphous ones, such as polycarbonate [49]. ...
... The plasticization effect of the dissolved CO 2 in polymer matrix also affected the crystallization behavior of the polymers. Relative publications showed that the plasticization effect of CO 2 would depress the glass transition temperature (T g ) [23][24][25], crystallization temperature (T c ) [26] and melting temperature (T m ) [27,28], and even affect the crystallization rate [29] and crystallinity [30] of the polymers. Several researchers also investigated the effect of CO 2 on the isothermal crystallization kinetic in neat polymers. ...
Both the melting and non-isothermal crystallization behaviors of polypropylene (PP) and PP/montmorillonite (MMT) nanocomposites under atmospheric N2 and pressurized CO2 were carefully studied by using high-pressure differential scanning calorimeter (DSC). Jeziorny modified Avrami and Mo's methods were applied to analyze the non-isothermal crystallization kinetics, respectively. The Avrami exponent decreased with CO2 pressure while increased with the addition of nano-MMT. The Mo's methods demonstrated a success for the systems investigated. The cooling rate needed to reach a certain crystallinity (FT) showed that the crystallization rates fluctuated with increasing CO2. This trend was flattened after introducing nano-MMT because of its heterogeneous effects. The activation energy, Ea, decreased with increasing CO2 pressure while increased in the presence of nano-MMT. The overall crystallization rate characterized by the crystallization half-time, t1/2, showed an decrease in PP/MMT nanocomposites, indicating that MMT had significant heterogeneous nucleation effect on enhancing the crystallization rate despite hindering the polymer chain movement.
... Supercritical processing has been applied to the extraction of residual solvents, unreacted starting material, and unwanted side products; 8,9 separation, crystallization, 10 and blending of polymers; 11,12 for the swelling and sorption, 13−16 which is relevant to the impregnation of new materials into the polymer; 17,18 as well as in organic synthesis, catalysis, and coordination chemistry. 7,19,20 Polymer processing has benefitted from the use of scCO 2 , where the longer term objectives are for this approach to effectively replace harmful organic solvents currently used in such applications. ...
ATR-FTIR spectroscopy was used in situ to study nine unsaturated polyketones derived from cis-1,4-polybutadiene rubber, each containing a different concentration of carbonyl groups, under high-pressure CO2 conditions (up to 100 bar). The study was aimed to systematically determine the relationship between the concentration of carbonyl groups in the polyketones and their ability to absorb CO2 and swell. A linear relationship between increasing carbonyl concentration and the overall degree of swelling and CO2 sorption was observed for polyketones with a concentration of carbonyl groups below a specific value based on quantitative analysis from the ATR-FTIR spectra. However, polyketones, which had the highest concentration of carbonyl groups, did not follow this correlation. Instead, there was evidence of intermolecular interactions between the carbonyl groups in the polymer chains, which decreases the total CO2 sorption capacity and inhibited swelling. The effect of the different molecular weights of polymer was also studied with respect to polyketone swelling and CO2 sorption. No correlation was observed when comparing polymers with different molecular weights but contained a similar concentration of carbonyl groups. Hence, the main physical properties that affect the overall swelling and CO2 sorption into polyketone samples were quantitatively determined using ATR-FTIR spectroscopy.
... Formation of the spherulites with concentric rings under 5 MPa reflected the competition between the growth of the crystals and the diffusion of polymer chains. It is generally believed that the dissolved CO 2 in polymers acts as a plasticizer and eases the movement of the polymer molecules [18][19][20] ; with the adsorption of CO 2 the polymer chains can diffuse to the growth front more easily. When the diffusing polymer chains became insufficient to continue growth as a ridge ring, the valley rings formed; when the diffusion rate was approximately the same as the crystal growth rate, spherulites with alternating ridge and valley rings formed at 5 MPa. ...
Melt crystallization of PEO-b-PCL thin films was conducted under compressed CO2 and the morphology of the spherulites obtained at various pressures was investigated by polarizing optical microscopy (POM) and atomic force microscopy (AFM). At 3 MPa CO2 classical ring-banded spherulites with periodical twisting lamellae formed. However, in the spherulites with concentric ring-banded structures formed at 5 MPa CO2, the rings were formed due to periodical variations in thickness along the radial direction.
Due to abundant disposable insulation materials causing serious environmental problem, degradable poly (butylene adipate-co-terephthalate) (PBAT) foam is a solution. To enhance the degradation rate and improve supercritical foaming for PBAT, poly (butylene succinate) (PBS) was selected in this work to blend with PBAT. PBAT/PBS foam with foam density lower than 0.1 g/cm³ and cell size smaller than 40 μm was successfully obtained, whose thermal conductivity was only 34 mW/m·K and specific compressive strength was over 3000 N·m/kg with an extremely fast degradation rate (97% material degraded within 10 days). The designation strategy is as follows: PBS’s fully aliphatic hydrocarbon structure ensures faster degradation rate than PBAT; PBS’s better molecular chain regularity accelerates crystallization for PBAT/PBS blend which ensures larger strength for blend foams; crosslinking structure and micro-phase separation structure in PBAT/PBS blend ensures better cell growth and hence larger expansion ratio, that is better thermal insulation performance.
In this chapter, we review the developments in the field of polymer research using Fourier transform infrared (FTIR) spectroscopic approaches, which can reveal valuable information about a range of polymer samples. The first section of this chapter will summarize some of the most important discoveries associated with the application of FTIR spectroscopy and FTIR spectroscopic imaging for polymer research, such as miscibility in polymer blends, intermolecular interactions between polymers, and polymer crystallization. The second section will focus on high‐pressure CO 2 ‐induced phase separation, crystallization, and swelling in polymers and multicomponent polymer system. The interaction between polymers and high‐pressure CO 2 has a great effect on the physical and mechanical properties of polymers. Understanding the behavior of polymers and multicomponent polymer systems with or without high‐pressure CO 2 is integral to understand and develop more efficient polymer processing routes. Finally, a summary of the most recent developments using FTIR spectroscopy and spectroscopic imaging will be provided with an outlook to future opportunities in this research field.
Crystallization and foaming behaviors of a semi-crystalline polymer in conditions comparable to those found in polymer processing, where the polymer melt experiences shear under elevated pressures, are key for modeling polymer processes and predicting the final structure and mechanical properties of polymer products. We investigate the crystallization behaviors of two different high-melt-strength polypropylene (PP) resins using a novel high-pressure visualization system that was developed to capture the crystallization process of a plastic specimen under high pressure and controlled shear. In situ visualization results under isothermal conditions demonstrate that both linear and long chain branched (LCB) PP exhibit crystal growth rates and nucleation densities that increase with higher applied shear stress but reach an optimum value with increasing CO2 pressure. By selectively choosing processing conditions, we can enhance the formation of observable critical size nuclei and the exclusion of CO2 from the crystal growth front. Therefore, overall crystallization kinetics can be easily controlled through the effect of induced-shear stress and the presence of pressurized CO2.
The achievements of polycondensation in supercritical carbon dioxide over the past 20 years are considered. The contribution of the A. N. Nesmeyanov Institute of Organoelement Compounds (Russian Academy of Sciences) to polycondensation in supercritical media was shown using the synthesis of polyimides, polyimidosiloxanes, polyarylates, polyphenylfluorenes, ferrocene-containing polyphenylenes, polyphenylquinoxalines, and polynaphthoylenbenzimidazoles.
HCl (0.31 mol%) and CaCl2 (2.3 mol%) ionized polyamide (PA)‐66's, respectively, are successfully prepared by room‐temperature PA‐66/HCl and PA‐66/CaCl2 solution reactions. The PA‐66 ionenes, against PA‐66, as a nucleator are used along with cholesteryl nonanoate (CN) as an active plasticizer to advance poly(bisphenol‐A carbonate) (PC) crystallization. Relative to the PC/PA‐66/CN, the PC/PA‐66‐HCl/CN crystallizes far more whereas the PC/PA‐66‐CaCl2/CN almost equally, indicating that the PA‐66‐HCl heterogeneously nucleates PC crystallization much more effectively than the PA‐66‐CaCl2. The rationale may be that the strong ion‐dipole interactions (IDIs) of the PA‐66‐HCl's HCl‐coordinating amides and PC's carbonates remarkably boost the PC/PA‐66‐HCl interfacial compatibility, which causes considerably finer, denser PA‐66‐HCl crystals with significantly enhanced nucleation efficiency. Nevertheless, owing to its greatly larger ion content, the PA‐66‐CaCl2 seems to have such stronger IDIs with PC that the resulting PC/PA‐66‐CaCl2 immense compatibility (ie, vastly more, thicker amorphous interlayers) turns to dramatically inhibit PC crystallization despite the nucleation of extremely refined, densified PA‐66‐CaCl2 crystals. Upon its slight ionization with HCl, polyamide (PA)‐66 nucleator causes exceptionally boosted heterogeneous nucleation and hence melt crystallinity of poly(bisphenol‐A carbonate) (PC) matrix due to the occurrence of strong ion–dipole interactions between the PA‐66 ionene HCl‐coordinating amide and PC carbonate groups to give their controlled, greatly improved interfacial compatibility, by which the PA‐66 ionene's refined, densified dispersion dramatically enhances its nucleation efficiency without significant compromisation by the amorphous, enlarged interfacial area and thickened interlayers.
Supercritical fluid dyeing is a promising technology that was first proposed in the 1980s to overcome the high energy demand and water consumption of conventional textile coloration. This review covers its advances from 2014 to the present, from the successful industrial implementation of supercritical fluid dyeing of polyethylene terephthalate to the most recent results obtained for the dyeing of other synthetic and natural textiles. Specific attention is also dedicated to the most innovative applications of supercritical fluid dyeing such as the functionalisation of textile and non‐textile substrates, which may give rise to the development of other sustainable processes or novel advanced materials in the near future.
This review considers the recent advancements of using supercritical carbon dioxide (scCO2) in the synthesis and activation of metal-organic frameworks (MOFs). The first section outlines MOF synthesis by scCO2-based methods; pure scCO2 system, and also in conjunction with a co-solvent, auxiliary ligand, expandable solvent or an ionic liquid. This is followed by elaborating on MOF activation techniques that employed scCO2, be it independently or together with thermal activation. The third section considers the effects of scCO2 on MOF and polymer structures. To conclude, projections are made for current researchers in this area to better exploit scCO2 utilization.
A low-reflectance film was used to reduce the reflection of light from the displays of electronic devices (personal computers, televisions, smartphones, etc.). In this study, an ultraviolet (UV)-curable resin was used to form a nano/microstructure by organic-solvent-induced crystallization and to fill the cracks in the polycarbonate plate. Two different processes were examined: Process 1: the surface of the polycarbonate plate was crystallized at 40, 60, 80, or 100 oC for 10 min and thoroughly rinsed with isopropyl alcohol; and Process 2: the surface of the polycarbonate plate was crystallized at 40 oC for a desired time. The excess UV-curable resin was removed by UV-induced curing and CO2 foaming during UV exposure using high-pressure CO2 equipment. CO2 pressures of 4.5, 5.5, and 6.5 MPa and CO2 impregnation times of 80, 160, 240 s were investigated to optimize the microstructure. It was clarified that spherulites of polycarbonate existed on the surface by differential scanning calorimetry and scanning electron microscopy. The surface prepared with Process 2 had a relative reflectance that was about 600 and 30 times lower than those of the untreated polycarbonate plate and a commercially available low-reflectance film, respectively.
SnCl2·2H2O-modified polycarbonate (PC)/poly(methyl methacrylate) (PMMA) (80/20 w/w) blends were prepared by melt blending using a torque rheometer. Supercritical carbon dioxide (scCO2) has been used to induce the crystallisation of PC/PMMA blends while the crystallisation behaviour and morphology of these blend systems under different saturation temperatures, saturation pressures, saturation times and co-solvent contents were characterised by differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). The gravimetric experiment shows that the solubility of scCO2 in PC/PMMA blends increases with the increase of saturation temperature, pressure and time. The DSC curves demonstrate that in these blend systems, the induced crystallisation of PC has been found and two kinds of crystals with different thicknesses exist in it. From the SEM images, it can be found that the structure of PC crystal becomes more perfect and its crystallinity also increases with the increase of saturation temperature, pressure, time and co-solvent content.
The sorption of CO2 in two commercial grades of multiblock copolymers (PolyActive™) is investigated using a magnetic suspension balance in the temperature range 30-70 °C. The isotherms follow Henry's law when the poly(ethylene glycol) (PEG) blocks of PolyActive™ are in a molten state. A significant deviation is observed when the PEG blocks are still in a semicrystalline state. Differential scanning calorimetry (DSC) is used to study the thermal transitions of the polymers. The melting and crystallization of the PEG blocks is studied using high pressure differential scanning calorimetry in both CO2 and N2 gas environment. A substantial decrease of melting and crystallization temperatures occurs with increasing CO2 pressure. But no change occurs under N2 pressure. The observed experimental results are discussed on the basis of established thermodynamic equations reported in the literature which provide an adequate guideline to correlate the sorption isotherms with the thermal transition of the PEG blocks.
An experimental study of the rheological behaviour of aluminium silicate filled ethylene-octene copolymer vulcanizates in extrusion containing blowing agent has been carried out. The cell morphology development has been studied through scanning electron microscope. Rheological properties of unfilled and aluminium silicate filled systems with variation of blowing agent, extrusion temperature and shear rate have been studied using Monsanto processability tester (MPT). The total extrusion pressure (APT), apparent shear stress apparent viscosity (eta(a)), and die swell (%) of the unfilled and aluminium silicate filled compounds have been determined by using MPT. It is found that there Is a reduction in stress and viscosity with blowing agent loading. It is observed that incorporation of blowing agent led to decreased shear thinning behaviour resulting in an increase in power law index. The viscosity reduction factor (VRF) of unfilled vulcanizates is found to be dependent on concentration of blowing agent, shear rate and temperature, whereas VRF of aluminium silicate filled vulcanizates is found to be independent of shear rate and temperature but dependent on blowing agent concentration.
Essential characteristics of chain molecules, their formation, properties and processing aspects are presented with a special focus on solvent dependent features. How supercritical fluids with their tunable characteristics can be put into use as the process or processing solvents in various stages of polymer formation, purification, modification, fabrication, recovery and recycling is discussed. Thermodynamic aspects of polymer solubility and the effects of various factors such as the polymer type, molecular weight and molecular weight distribution, concentration, solvent type, temperature and pressure on the solubility of polymers in supercritical fluids are presented. Modeling of polymer solutions at high pressures with lattice fluid (Sanchez-Lacombe) and perturbation (SAFT) methods is reviewed and their effectiveness is compared. The question of kinetics of phase separation is addressed and a new technique which permits repetitive penetration into the metastable and unstable regions of a polymer-solvent system by multiple slow- or rapid (at rates approaching 1000 MPa/s) pressure drops is described.
The definition of supercritical fluids is given; the main representatives of these substances, primarily supercritical carbon dioxide, are listed; and their physical and chemical properties, advantages, and drawbacks are considered. The use of supercritical CO2 as a reaction medium, monomer, and catalyst for the synthesis of mostly polycondensation polymers is analyzed.
Rheology is the science that deals with the way materials deform when forces are applied to them. There are two principal aspects of rheology. One involves the development of quantitative relationships between deformation and force for a material of interest. Another aspect the development of relationships that show how rheological behavior is influenced by the structure and composition of the material and the temperature and pressure. This concept of rheology is very useful and powerful tool in development of industrial materials, so it has wide application to R & D area of industrial company. In this work, it holds two examples which are applied in industrial material development. One is the investigation of the effect of dissolved supercritical carbon dioxide on the viscosity and morphological properties for PE/PS blends in a twin screw extruder, the other is the fabrication of PVDF hollow fiber membrane and elucidation of the membrane morphology as functions of dope and external coagulant rheology in the phase inversion process.
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.
Poly(bisphenol A carbonate) (BPA-PC) was post-polymerized by solid-state polymerization (SSP) after supercritical CO2-induced crystallization in low molecular weight particles prepolymerized via melt transesterification reaction. The effects of the crystallization conditions on melting behavior and SSP of BPA-PC were investigated with differential scanning calorimetry (DSC), Ubbelohde viscosity method and gel permeation chromatography (GPC). The reaction kinetics of the SSP of crystallized prepolymers was studied as a function of reaction temperatures for various reaction periods. As a result, the viscosity average molecular weight of BPA-PC particles (2 mm) increased from 1.9 × 104 g/mol to 2.8 × 104 g/mol after SSP. More importantly, the significantly enhanced thermal stability and mechanical properties of solid-state polymerized BPA-PC, compared with those of melt transesterification polymerized BPA-PC with the same molecular weight, can be ascribed to the substantial avoidance of undergoing high temperature during polymerization. Our work provides a useful method to obtain practical product of BPA-PC with high quality and high molecular weight.
The effect of CO 2 ‐induced crystallization on the mechanical properties, in particular the yield and the ultimate stresses, of polyolefins is studied. PP and SEBS copolymer blends are used as examples and foamed after sorption of CO 2 at temperatures below T m . CO 2 sorption thickens the crystalline lamellae and consequently increases T m from 160 to 178 °C for both pure PP and PP/SEBS blend systems. Foams with an average cell size smaller than 250 nm retain the ultimate stress at the level of the polymer before foaming, even without the effect of CO 2 ‐induced crystallization. Including CO 2 ‐induced crystallization, the yield and the ultimate stresses of the foam can be improved by 30 and 50% over solid PP and by 22 and 40%, for solid PP/SEBS blends, respectively.
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The solubility and diffusion coefficient of supercritical CO2 in polycarbonate (PC) were measured using a magnetic suspension balance at sorption temperatures that ranged from 75 to 175 °C and at sorption pressures as high as 20 MPa. Above certain threshold pressures, the solubility of CO2 decreased with time after showing a maximum value at a constant sorption temperature and pressure. This phenomenon indicated the crystallization of PC due to the plasticization effect of dissolved CO2. A thorough investigation into the dependence of sorption temperature and pressure on the crystallinity of PC showed that the crystallization of PC occurred when the difference between the sorption temperature and the depressed glass transition temperature exceeded 40 °C (T − Tg ≥ 40 °C). Furthermore, the crystallization rate of PC was determined according to Avrami's equation. The crystallization rate increased with the sorption pressure and was at its maximum at a certain temperature under a constant pressure.
This article reports the transitions of morphological patterns of polycarbonate crystals in thin films by solvent-induced crystallization (SINC). As a substrate (silica glass) deposited with an amorphous and micron-thick bisphenol A polycarbonate polymer film is partially dipped into a liquid acetone bath, acetone penetrated rapidly through the polymer film. The rate of acetone penetration is significantly higher than the predicted by Fickian diffusion or anomalous diffusion model, indicating that the capillary force through stress-induced cracks may have played a major role in the upward transport of acetone through the polymer films. The morphologies of polycarbonate at different vertical positions on a substrate surface were analyzed by scanning electron microscopy and atomic force microscopy. It was observed that depending on the local acetone concentration the polymer morphologies showed quite diverse patterns ranging from stress-induced cracks to fully developed three-dimensional spherulites. The diverse morphologies developed during the thin film SINC may serve as a useful platform for further detailed mechanistic analysis of structures and crystallization kinetics. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012
As high-pressure processing is used increasingly for the treatment of packed products, different packaging has been investigated with respect to their structural behaviour and stability under high-pressure processing. Often, failures and changes of the polymeric structure occur. Common damage symptoms of high-pressure-treated packaging materials are defined and classified in this review. These damage symptoms are allocated to the physico-chemical effects that created them. The effects may be separated into direct effects induced by the action of the high-pressure alone and indirect effects that are mediated via compressed contents of the package, i.e. filled product and gaseous headspace. The direct effects split up again in reversible and irreversible structural changes. The indirect effects are generated by compressed headspace gases, other compressed substances and the consequences of increased amounts of gases dissolved in the polymers. If applicable, current theoretical approaches have been allocated to the different categories of damage. Copyright © 2013 John Wiley & Sons, Ltd.
A single screw extrusion process in which CO2 is injected into the polymer melt has been utilized to produce low density biodegradable poly(DL-lactic acid) (PDLLA) foams. High pressure differential scanning calorimetry (DSC) and rheology are used to observe the melting point and viscosity depression of PDLLA 3051D in the presence of CO2. Foams of PDLLA 3051D are produced in a static environment under a range of conditions, and these data are utilized to extrude PDLLA 3051D into foamed sheets for packaging applications. The density, crystallinity, mechanical properties and microstructure of these foams are evaluated.
Foams generated via carbon dioxide (CO2) processing typically exhibit a solid skin layer on the exterior surface and a closed-pore structure with limited interconnectivity in the core section thus limiting its application for biomedical intent. By controlling the properties of poly(l-lactic acid)/poly(d,l-lactic acid) (PLLA/PDLLA) blends and using CO2 with specific processing parameters, skinless foams with interconnected porous structure were prepared in this work using only CO2 as a physical foaming agent, which overcome the necessity to use organic solvents and solid porogens. The crystallization behaviors and sorption kinetics of PLLA and its blends were studied. Addition of PDLLA reduces the crystallinity of PLLA/PDLLA blends while treated with CO2 as compared to neat PLLA. The solubility and diffusion coefficients of CO2 in PLLA and its blends were found to be similar. Furthermore, the effect of PLLA/PDLLA blend ratio and CO2 treatment conditions on the foam morphologies was investigated. Through fine parameter control, well interconnected pore structures with a porous surface were generated. Results indicated that by controlling the physical properties of samples combined with optimizing CO2 foaming process, it is indeed possible to create biodegradable interconnected porous structures for potential biomedical applications.
Supercritical carbon dioxide (SC-CO2) provides a highly tunable technique to induce changes in morphology and crystallization kinetics of various polymers. In this study, the effect of SC-CO2 treatment on the crystallinity of isotactic polypropylene (i-PP) was analyzed by differential scanning calorimetry (DSC). The Avrami equation is suitable for describing crystallization kinetics of various solids. However, the assumption that the crystallinity approaches 1.0 as the time approaches infinity is not valid for semicrystalline polymers, especially in the atmosphere of SC-CO2. An improved kinetic model for the CO2-induced crystallization of i-PP was purposed by introducing equilibrium crystallinity, as well as temperature- and pressure-dependent terms into the Avrami equation. The parameters of the crystallization kinetics model were obtained by least-squares fitting of the DSC data. The results show that the improved kinetic model provides a reasonable description of the crystallization behavior of i-PP induced by SC-CO2. The successful application of the improved kinetic model to the CO2-induced crystallization of i-PP suggests that this model may be adopted to other SC-CO2-semicrystalline polymer systems.
We investigated the crystallization growth of isotactic polypropylene (PP)/clay phase-separated microcomposites under carbon dioxide (CO2) by in situ observation with a specially designed high-pressure visualized cell. The number density of the spherulite was larger and the size of the spherulite filled in the whole space was smaller in the PP/clay microcomposites than those in the neat PP, indicating nucleation effect of clay on the crystallization of PP. The increase of the number density of the spherulite by addition of clay was much larger under CO2 than that under air at ambient pressure, suggesting that the nucleation effect of clay is enhanced under CO2. Microscopic observation and small angle X-ray scattering measurement revealed that lamellae grow from the surface of the clay tactoids, and the lamellae and lamellar stacks arrange irregularly due to the growth of lamellae from the irregular shaped clay in different directions. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers
The use of supercritical CO2 to facilitate the processing of polymers is becoming increasingly popular. In light of this, it is important to understand the effect supercritical CO2 has on these polymers especially in terms of glass transition temperature and melting point depression and the induction of crystallization. The aim of the current study is to use infrared spectroscopy to probe these properties. Plaques of polycaprolactone were exposed to supercritical CO2 at a range of temperatures (41–60 °C) and pressures (0–200 bar). Examination of the carbonyl peak at 1720 cm−1 at 41 °C while increasing the pressure shows a change in morphology at 100 bar, indicative of a melting point. This is a 22°C reduction in the melting point compared with atmospheric pressure. Repetition of this experiment over a range of temperatures shows that the pressure required to induce melting reduces as the temperature is increased. Infrared spectroscopy is also used to observe the crystallization of polycaprolactone during the depressurization of CO2.
Atomic force microscopy (AFM), wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry are used to analyze the crystallization morphology and melting behavior of 4-arm PEO-b-PCL under high-pressure CO2. It is demonstrated that CO2 has certain effect on the melting and crystallization behavior of the samples. After crystallization under CO2 at 4 MPa, spherulites with concentric ring-banded structure are formed which are composed of crystals with periodic thickness variation, and the band distance decreases with increasing treatment pressure. Due to the plasticization effect of CO2, depression of the melting temperature is observed with sorption of CO2 in polymers.
The thermal transitions of a poly(ethylene terephthalate)/carbon dioxide (PET/CO2) system were investigated by using a differential scanning calorimeter accessorized with a high-pressure DSC cell. It was found that the glass transition temperature of PET decreases with an increase in the CO2 pressure due to the plasticization effect, which is quite noticeable even at rather low CO2 pressures. The sorbed CO2 enhances the mobility of the chain segments and depresses the crystallization temperature of the PET. The CO2-induced crystallization of PET at high pressure is attributed mainly to the plasticization effect, which causes a lower Tg than room temperature for PET, and hence crystallization of PET can occur at room temperature. The sorbed CO2 was also found to be able to induce the crystallization of PET at temperatures lower than the glass transition temperature of PET. The results of high-pressure DSC were supported by measurements of wide-angle X-ray diffraction (WAXD).
In this study, the crystal polymorph of poly(l-lactide) (PLLA) formed under high-pressure CO2 and its transition behavior with CO2 desorption were examined using mainly wide-angle X-ray diffraction and Fourier transform infrared spectroscopy. We demonstrated that PLLA forms the complex crystal with CO2 under high-pressure CO2 below room temperature, and the crystal transition to the α-form, which is accompanied with gradual changes in the packing and conformation of PLLA chains, occurs with CO2 desorption and subsequent annealing in air (annealing was needed only for films with a draw ratio smaller than three). Compared with the α-form, the oriented CO2 complex film showed shorter a-, longer b-, and shorter c-axis lengths, resulting in a slight increase in the unit cell volume. The hexagonal packing (a/b ≈ 1.73), which is seen in the α-form, no longer exists for the oriented CO2 complex film (a/b ≈ 1.33). It was indicated that the chain helical conformation of PLLA in the CO2 complex is similar but different to that in the α-form (10/7 helix for both forms), because of the interactions between PLLA and CO2. It seems likely that CO2 molecules are encapsulated in the cavity surrounded by four PLLA chains. With CO2 desorption, the a-axis length increased and the b-axis one decreased, so that a/b increased to 31/2 (hexagonal packing), keeping the orthorhombic system. It was proposed that the formation of α″-crystals results from PLLA being trapped in the quasi-stable state during the CO2 complex-to-α-form transition, and the energy barrier between the α″- and α-forms can be overcome by only CO2 desorption in the case of a draw ratio higher than two.
The crystallization behavior of bisphenol-A polycarbonate (PC) and PC/clay nanocomposites were studied in the presence of supercritical carbon dioxide (SCCO2) using DSC, WAXD and AFM. In the absence of SCCO2, nano-scale clay itself does not change the crystallization behavior of PC under our experimental conditions. In the presence of SCCO2, clay appears to be an efficient nucleating agent and enhances the crystallization of PC. The addition of clay reduces the induction time of crystallization and increases the crystallization rate. The increase in crystallinity with clay depends on the crystallization time. When the crystallization time is sufficient, PC and PC/clay composites tend to have similar crystallinity in the range of 26%. Two melting temperatures are observed during the DSC heating scan, and are mainly associated with the melting of both secondary and primary crystals. Results show that the clay influences the primary crystallization process more than the secondary crystallization process.
The solid phase transition mechanism of α- to β-form crystal upon specific treating with supercritical CO2 + cosolvent on original pure α and mixed (α+β) form syndiotactic polystyrene (sPS) was investigated, using wide angle X-ray diffraction and differential scanning calorimetry measurements as a function of temperature, pressure, and cosolvent content. As in the supercritical CO2, sPS in supercritical CO2 + cosolvent underwent solid phase transitions from α- to β-form, and higher temperature or higher pressure favored this transformation. Due to the higher dipole moment of acetone, small amounts of acetone used as cosolvent with CO2 made the transition of α- to β-form occur at lower temperature and pressure than in supercritical CO2, and made the α-form crystal completely transform to β-form in the original mixed (α+β) form, whereas ethanol did not. The original β-form crystal in the original mixed (α+β) form sample acted as the nucleus of new β-form crystal in the presence of cosolvent as it did in supercritical CO2, when compared with the original pure α-form sample. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1625–1636, 2007
CO2-induced crystallization of isotactic polypropylene (iPP) by annealing had been studied using differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS). The iPP before annealed was in α-form and amorphous states. At lower temperatures by CO2 isothermal treatments, iPP chains crystallized from the amorphous phase and only one crystal form, i.e., α-form, was observed. At higher temperatures by CO2 isothermal treatments, both crystallization from the amorphous phase and thickening of existing crystal lamellae were observed. Moreover, light γ-form crystal appeared in the treated iPP. The crystalline lamellar thickness of iPP annealed at different CO2 pressures had been determined. Using the Gibbs–Thomson plot method, the equilibrium melting temperature was found to be 187.6°C.
An experimental study of the rheological behaviour of ethylene/octene copolymer compounds in extrusion containing blowing agent has been carried out. The cell morphology development was studied using scanning electron microscopy. Rheological properties of unfilled and precipitated CaCO3 filled systems with various blowing agents, extrusion temperatures, and shear rates were studied using a capillary rheometer. The total extrusion pressure, apparent shear stress, apparent viscosity, and die swell of the unfilled and CaCO3 filled compounds were also determined and the effect of blowing agent on the rheological properties of the compounds studied. It was observed that there is reduction of stress and viscosity with blowing agent loading. Incorporation of blowing agent led to decreased shear thinning behaviour resulting in an increase in the power law index. The viscosity reduction factor of the unfilled compound was found to be dependent on the concentration of blowing agent, the shear rate, and the temperature.
The thermodynamic effect of free volume changes in mixing is treated with the use of the corresponding states theory of Prigogine and collaborators. The interaction parameters χ1, κ1, ψ1 and their concentration dependences are derived. Each parameter contains two terms. The first is due to differences in cohesive energy and sizes between segments of polymer and solvent: the second or “structural” contribution is due principally to the difference in chain lengths between polymer and solvent. The structural effect gives a large value of χ1 ∼ 0.4 for a polymer with a chemically identical short oligomer as solvent. A minimum but nonzero value of χ1 occurs for a solvent with higher cohesive energy than between polymer segments. At high temperatures a lower critical solution temperature must occur due to the increase of the solvent configurational heat capacity to infinity as the solvent vapor-liquid critical point is approached, the heat and entropy of dilution becoming negative. At ordinary temperatures χ and κ increase with polymer concentration. The solubility parameter approach is compared; particular polymer liquid models used by Prigogine and by Flory are briefly discussed. χ1 as calculated for natural rubber and a variety of solvents and fair agreement is obtained with published values.
The melting behavior of isotactic polystyrene, crystallized from the melt and from dilute solutions in trans-decalin, has been studied by differential scanning calorimetry and solubility measurements. The melting curves show 1, 2, or 3 melting endotherms. At large supercooling, crystallization from the melt produces a small melting endotherm just above the crystallization temperature Tc. This peak originates from secondary crystallization of melt trapped within the spherulites. The next melting endotherm is related to the normal primary crystallization process. Its peak temperature increases linearly with Tc, yielding an extrapolated value for the equilibrium melting temperature Tc° of 242 ± 1°C as found before. By self-seeding, crystallization from the melt could be performed at much higher temperature to obtain melting temperatures as high as 243°C, giving rise to doubt about the value of Tc° found by extrapolation. For normal values of Tc and heating rate, an extra endotherm appears on the melting curve. Its peak temperature is the same for both melt-crystallized and solution-crystallized samples, and independent of Tc, but rises with decreasing heating rate. From the effects of heating rate and partial scanning on the ratio of peak areas and of previous heat treatment on dissolution temperature, it is concluded that this peak arises from the second one by continuous melting and recrystallization during the scan.
The differential scanning calorimetry (DSC) melting curves of drawn nylon 6 were studied from the standpoint of reorganization of the crystals during the heating process. A new method was presented to obtain the DSC curve associated with the growth and melting of the original crystals, and that with the recrystallization and final melting process, separately. The results obtained show that, in the case of a heating rate of 10°C/min, the original crystals in the sample start perfecting themselves at temperatures far below their initial melting temperature and melt out below 222°C, recrystallization starts at about 210°C, and the newly emerged crystals melt out at 228°C. The superposition of two such constructed DSC curves reproduces the observed DSC curve well. Therefore, the double melting peaks of the sample are considered to be the result of superposition of three processes which occur successively during heating; perfection of the original crystals, melting of the perfected crystals concurrently with recrystallization, and melting of the recrystallized crystals.
Previous work has shown that sorption of CO2 at relatively high pressures by glassy polymers reduces their glass transition temperatures and may convert the glass into a rubber under certain conditions. It is shown here that this plasticization by a gas can induce crystallization just as sorption of vapors or liquids is known to do. This point is extensively explored for miscible blends of poly(vinylidene fluoride) and poly(methyl methacrylate) and to a lesser extent for poly(ethylene terephthalate). In some cases, this secondary crystallization process results in small crystals whose melting endotherms are just above the glass transition and are very similar to peaks resulting from heat capacity overshoots, or enthalpic relaxation, caused by sub-Tg annealing; however, by appropriate techniques peaks arising from these two separate mechanisms can be distinguished. For oriented materials, evidence is shown which demonstrates that the additional crystals formed on CO2 sorption have the same preferential orientation as the original material.
Amorphous films of Lexan polycarbonate have been exposed to acetone vapor at controlled temperatures and partial pressures in order to study sorption kinetics and thermodynamics and polymer crystallization behavior. Sorption isotherms show a discontinuity is slope at or near the depressed glass transition, which itself was identified by torsion pendulum measurements. Crystallization abruptly begins to occur at partial pressures equal to or slightly above that of the solubility transition and is manifested by delayed desorption and whitening phenomena. In this process 20% crystallinity is usually developed, as measured by calorimetry which, however, produces a 40% drop in acetone solubility. Although the depressed glass temperature is near 0°C. in saturated atmospheres—a drop of 145°C.—the melting point is only depressed 60 or 70°C. Such disparity probably accounts for the enhanced polycarbonate crystallization rate in acetone over that in the dry bulk polymer above the normal Tg.
The two endotherms found during differential scanning calorimetry (DSC) analysis of annealed or drawn polyethylene terephthalate (PET) are discussed in greater detail. The authors explain the reasons for recrystallization, and present data showing that samples cooled at various rates from the melt also exhibit recrystallization in the DSC, in much the same manner as samples annealed for different lengths of time. By prolonged annealing before analysis, part of the recrystallization exotherm can be observed in the DSC scan. The data indicate a likelihood of at least partially extended morphologies in cold-drawn PET; these observations do not apply to PET drawn at high temperatures or to polyethylene.
The pressure-volume-temperature (PVT) relationships of bisphenol-A polycarbonate, polyarylate, and phenoxy were studied at pressures to 1800 kg/cm2 and in both the glassy and melt states. Earlier data on polysulfone are included in the analysis and discussion of the results. All four polymers contain the bisphenol-A residue in their repeat unit, together with a moiety of varying complexity, and are therefore somewhat related. At the glass transition, equations of the Ehrenfest type hold, provided the pressure dependence of the glass transition temperature is defined from the line obtained by intersecting the quasiequilibrium PVT relationship of the glass with the equilibrium PVT surface of the melt. The Prigogine-Defay ratio r = ΔκΔCp/TgVg(Δα)2 at P = O is unity within experimental error for all four polymers. The melt data were fitted successfully to the Simha-Somcynsky theory. Molecular parameters deduced from the reducing parameters vary in a reasonable manner among these four related polymers, lending support to the foundations of the theory.
A technique is described which uses differential scanning calorimetry to estimate the glass transition of polymers containing a dissolved gas. The technique is simple and appears to give reliable results. The effects of CO2 sorption at pressures up to 25 atm were examined in detail for poly(methyl methacrylate) and its blends with poly(vinylidene fluoride). Less extensive results for polystyrene, polycarbonate, poly(vinyl chloride), and poly(ethylene terephthalate) are also given. Reductions in Tg of up to 50°C are observed. A theoretical relation by Chow predicts results in reasonable agreement with the experimental data. These findings are relevant to various applications such as membrane separation processes for gases.
Environmental effects of high pressure carbon dioxide gas on polystyrene Ph
- C-W Wang