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Abstract
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.
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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.
The physical and degradative properties of polyhydroxybutyrate-hydroxyvalerate copolymer blends with polycaprolactone were investigated. Blends containing low levels of polycaprolactone (< 20%) were found to possess a considerable degree of compatibility, whilst those with higher levels of polycaprolactone were incompatible and showed phase separation behaviour. This incompatibility was most marked in blends containing approximately 50% of each component. In blends containing low levels of polycaprolactone, processing conditions governed the ease of crystallization of polycaprolactone in the polyhydroxybutyrate-hydroxyvalerate matrix and thus the mechanical property of the blend. The degradation rate of these blends was found to be influenced by a complex set of factors, including temperature, pH and polycaprolactone content of the blend. Although crystallinity affected the mechanical properties of the blends, its influence on the hydrolytic degradation rate was masked by the large difference in the molecular weight of the polyhydroxybutyrate-hydroxyvalerate copolymers (Mw ∼ 300 000) and polycaprolactone (Mw ∼ 50 000). The polyhydroxybutyrate-hydroxyvalerate/polycaprolactone blends were found to be much more stable to hydrolytic degradation than polyhydroxybutyrate-hydroxyvalerate/ polysaccharide blends previously studied. Here the combined techniques of goniophotometry and surface energy measurements proved extremely valuable in monitoring the early stages of degradation, during which surface, rather than bulk degradation, processes predominate.
The solubility of carbon dioxide in polycaprolactone (CAPA 6800) was measured at (313 and 333) K between pressures of (80 and 200) bar. Data were obtained using a static method that allowed saturation of the polymer with CO2 under the experimental conditions.
Block copolymers consisting of poly(ethylene glycol) (PEG) and poly(ε-caprolactone) (PCL) were synthesized by adding ε-caprolactone to the PEG ends, and the morphology and melting behavior of these copolymers were investigated by small-angle X-ray scattering (SAXS), wide-angle X-ray diffraction (WAXD), and differential scanning calorimetry (DSC). A binary blend of PEG and PCL oligomers was also prepared to compare the morphology with that of the copolymers. The long spacing L, an alternate period of the lamella and amorphous layer, evaluated from the angular position of the SAXS maximum dramatically decreased by adding a short PCL block to the PEG ends, and then increased linearly with connected PCL block length. The melting temperature Tm showed maximum depression of ca. 20°C for the copolymer with an equal proportion of the PEG and PCL blocks. The WAXD results revealed that the crystals of the PEG and PCL blocks independently existed and there was no diffraction from an eutectic crystal composed of the two blocks. In the PEG/PCL blend, L and Tm did not change with changing the PCL fraction and exactly corresponded to those of the constituent homopolymers. These facts suggest that the covalent bond between the PEG and PCL blocks restricts the formation of the favorable crystalline morphology which appears in the PEG/PCL blend, and eventually a characteristic morphology is formed in these copolymer systems.
In the course of structural studies of aliphatic polyesters, the crystal structure of poly-ε-caprolactone, [–(CH2)5–CO–]n, was determined by interpretation of X-ray diffraction patterns. The crystallographic data are: a=7.47Å, b=4.98Å, c(fiber axis) =17.05Å, the orthorhombic space group P212121-D24, two molecular chains in the unit cell. The chain conformation of poly-ε-caprolactone is almost planar zigzag, but evidently deviates from the fully extended form, i.e., the CH2 sequences are planar but the plane of atoms of the ester group tilts slightly to the fiber axis. The molecular chains are arranged side by side in the same way as in polyethylene, but the carbonyl groups of the two chains in the unit cell are separated by an amount of 3/14c along the fiber axis.
Lamellar crystals of diblock, triblock and four-arm poly(ethylene glycol)-b-poly(ɛ-caprolactone) (PEG-b-PCL) crystalline-crystalline copolymers were successfully obtained from their solution. Morphology and structure of lamellar crystals of crystalline-crystalline copolymers were investigated using tapping-mode atomic force microscopy (AFM) and selected area electron diffraction (SAED). All of these samples showed the truncated-lozenge multilayer basal shapes with central screw dislocation or central stack, which were all obtained simultaneously from the oil bath. The diffraction pattern of PEG block lamellar crystal is attributed to the (120) diffracting planes and the pattern of PCL block lamellar crystal is attributed to the (110) diffracting planes and (200) diffracting planes according to the SAED results. Four (110) crystal growth planes and two (200) crystal growth planes are discovered for the PCL blocks, but the (120) crystal growth planes of PEG blocks are hided in the figure of AFM. The crystalline structure of the four-arm copolymers (FA) is more disorder and confused than that of the diblock (DI) copolymer and the striated fold surface structures of lamellar crystals of four-arm copolymers (FA) are smoother than these of linear analogues, owing to the confused crystallization of blocks caused by the mutual restriction of blocks and the hindrance of the dendritic cores. In addition, the aspect ratio of FA is greater than that of the others. It is hypothesized that there are two reasons for the change of aspect ratios. First, the (200) diffracting planes of PCL crystals grew slowly compared to their (110) diffracting planes because of difference in the energy barrier. Secondly, edge dislocations on the (200) diffracting planes are also responsible for the variation of the aspect ratio. Consequently, the crystalline defects are augmented by the competing blocks crystallized simultaneously and the hindrance of the dendritic cores.
Powdery coating particles mingled polymethyl methacrylate (PMMA) and calcium carbonate inorganic pigment were obtained under water-free conditions in supercritical carbon dioxide (sc-CO 2) . The formation was confirmed by FTIR and GPC method. The effects of monomer, initiator, stabilizer concentration and the reaction temperature and time on the polymerization conversion rate and molecular weight of the polymer were discussed and the experimental results indicated that the molecular weight and the molecular weight distribution of the PMMA could be controlled within a suitable range when the reaction pressure was 10 MPa, reaction temperature was 75°C, reaction time was 8 h, monomer concentration was 0. 10 g/mL, initiator concentration was 0.10 × 10 -2 g/mL and stabilizer concentration was 0.06 × 10 -2 g/mL. From SEM observation, it was found that the pigment granules were able to evenly disperse and embedded in the PMMA matrix and didn't show conglomeration.
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.
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 results obtained in a study of the heat and entropy of fusion, and a few data on the crystallization kinetics, of poly-β-propiolactone and poly-ϵ-caprolactone are reported. Attention is focused on the entropy data for the two polymers. A comparative, theoretical analysis of the constantvolume entropy of melting of the polylactones and of three aliphatic polyesters is made. The remainder of the entropy of melting of the polymers considered is qualitatively correlated with chemical constitution and crystal structures.
Crystallization behavior, structural development and morphology evolution of a series of poly (ethylene glycol)-poly(ε-caprolactone) diblock copolymers (PEG-b-PCL) were investigated via differential scanning calorimetry (DSC), X-ray diffraction
(XRD) and atomic force microscopy (AFM). In these copolymers, both blocks were crystallizable and biocompatible. The mutual
effects between the PEG and PCL blocks were significant, leading to the obvious block composition dependence of the crystallization
behavior and morphology of the PEG-b-PCL copolymers. The relative block length determined which block crystallized first.
The temperature-dependent XRD measurements confirmed which block crystallized first from the copolymer during the cooling
procedure. Single crystals of the PCL and PEG homopolymers and the PEG-b-PCL copolymers were obtained and observed by AFM.
The block (PCL or PEG) crystallized first would determine the crystal morphology. The block crystallized later acted as a
solvent, which was advantageous to forming perfect single crystals of the whole block copolymers.
The crystallization behavior and morphology of the crystalline-crystalline poly(ethylene oxide)-poly(epsilon-caprolactone) diblock copolymer (PEO-b-PCL) was studied by differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), Fourier transform infrared spectroscopy (FTIR), small-angle X-ray scattering (SAXS), and hot-stage polarized optical microscope (POM). The mutual effects between the PEO and PCL blocks were significant, leading to the obvious composition dependence of the crystallization behavior and morphology of PEO-b-PCL. In this study, the PEO block length was fixed (Mn = 5000) and the weight ratio of PCL/PEO was tailored by changing the PCL block length. Both blocks could crystallize in PEO-b-PCL with the PCL weight fraction (WFPCL) of 0.23-0.87. For the sample with the WFPCL of 0.36 or less, the PEO block crystallized first, resulting in the obvious confinement of the PCL block and vice versa for the sample with WFPCL of 0.43 or more. With increasing WFPCL, the crystallinity of PEO reduced continuously while the variation of the PCL crystallinity exhibited a maximum. The long period of PEO-b-PCL increased with increasing WFPCL from 0.16 to 0.50 but then decreased with the further increase of WFPCL due to the interaction of the respective variation of the thicknesses of the PEO and PCL crystalline lamellae. Only the PEO spherulites were observed in samples with WFPCL of 0.16-0.36 by POM, in contrast to only the PCL spherulites in samples with WFPCL of 0.56-0.87. For samples with WFPCL of 0.43 and 0.50, a unique concentric spherulite was observed. The morphology of the inner and outer portions of the concentric spherulites was determined by the PCL and PEO spherulites, respectively. The growth rate of the PEO spherulites reduced rapidly with increasing WFPCL from 0 to 0.50. However, when increasing WFPCL from 0.43 to 0.87, the variation of the growth rate of the PCL spherulites exhibited a maximum rather than a monotonic change.
The use of supercritical carbon dioxide as a processing solvent for the physical processing of polymeric materials is reviewed. Fundamental properties of CO2/polymer systems are discussed with an emphasis on available data and measurement techniques, the development of theory or models for a particular property, and an evaluation of the current state of understanding for that property. Applications such as impregnation, particle formation, foaming, blending, and injection molding are described in detail including practical operating information for selected topics. The review concludes with some forward-looking discussion on the future of CO2 in polymer processing.
The effect of self-seeding nucleation on the crystallization behavior of poly(trimethylene terephthalate) (PTT) was studied. Differential scanning calorimetry (DSC) indicated that the crystallization temperature of PTT notably increased after self-seeding nucleation. Avrami equation was applied in the analysis of the isothermal crystallization process of PTT. The resulting average value of the Avrami exponent at n = 3.34 suggests that primary crystallization may correspond to a three-dimensional spherulitic growth. Self-seeding nucleation, leading to a decrease in active energy for crystallization and chain folding work, promotes the overall crystallization process of PTT.
Non-isothermal crystallization kinetics were characterized by using differential scanning calorimetry (DSC) analysis on neat semicrystalline syndiotactic polystyrene (sPS) and its nanocomposites with polystyrene (PS) functionalized full-length single walled carbon nanotubes (SWNT-PS), which was prepared by copper (I) catalyzed click coupling of alkyne-decorated SWNTs with well-defined, azide-terminated PS. The crystallization behavior of neat sPS polymer was compared to its SWNT based nanocomposites. The results suggested that the non-isothermal crystallization behavior of sPS/SWNT-PS nanocomposites depended significantly on the SWNT-PS content and cooling rate. The incorporation of SWNT-PS caused a change in the mechanism of nucleation and the crystal growth of sPS crystallites, this effect being more significant at lower SWNT-PS content. Combined Avrami and Ozawa analysis was found to be effective in describing the non-isothermal crystallization of the neat sPS and its nanocomposites. The activation energy of sPS determined from non-isothermal data decreased with the presence of small quantity of SWNT-PS in the nanocomposites and then increased with increasing SWNT-PS content.
Morphological changes of semicrystalline polycaprolactone (PCL), induced by melting under high-pressure CO2 and recrystallization during depressurization, were investigated by DSC and SAXS. Isothermal CO2 condition at 35°C and nonisothermal condition at a cooling rate of 0.5°C/min from 90 to 30°C were both studied at pressures of 36, 84, and 304atm. At 35°C, the semicrystalline PCL having melting temperature of about 60°C was found to melt under CO2 at 84 and 304atm, except at 36atm. The CO2-assisted melting of PCL recrystallized during depressurization of CO2, resulting in a varied thickness of crystal layers. The thickness of the formed crystal layers decreased with increasing CO2 pressures. Moreover, heterogeneity, with a size larger than the thickness of the crystalline and amorphous layers, was found to form in the PCL sample after CO2 treatments as observed by SAXS and supported by DSC data. This heterogeneous morphology of PCL formed during CO2 depressurization might arise from the segregated amorphous domains that were located between bundles of the lamellar stacks and/or arise from lamellar stacks that had considerably different sizes, possibly as a result of the molecular dragging and/or molecular fractionation on PCL during depressurization of the PCL-interacted CO2.
Phase morphology, crystal orientation, and overall crystallization kinetics as determined by self-organization of the diblock copolymer, vitrification of the amorphous block, and crystallization of the crystallizable blocks have been investigated for a lamellar-forming poly(ethylene oxide)-b-polystyrene (PEO-b-PS) diblock copolymer. The diblock copolymer has number-average molecular weights of 8700g/mol for the PEO blocks and 9200g/mol for the PS blocks. Based on wide angle X-ray diffraction (WAXD) observations, the PEO crystals possess the same crystal structure as pure PEO. When the crystallization temperature (Tc) is below 40°C, the PEO crystals melt below 55°C, which is much lower than the melting point of their homopolymer analogs. The glass transition temperature of the PS blocks is 62°C, determined by the 50% heat capacity change during vitrification observed in differential scanning calorimetry (DSC). The order–disorder transition temperature is determined to be 160°C using a one-dimensional (1D) small angle X-ray scattering (SAXS) experiment. The mean-field segmental interaction parameter is determined to be χEO/St=−7.05×10−3+21.3/T, based on the SAXS scattering intensity analysis for T>TMF (crossover temperature from the concentration-fluctuation region to the mean-field region). Using real-time resolved simultaneous SAXS and WAXD techniques, the PEO block is observed to crystallize within the confined lamellae provided by the glassy PS layers. The crystal (the ĉ−axis) orientation within the confined lamellae is determined by combined two-dimensional (2D) SAXS and WAXD experiments. The PEO crystals are observed to tilt away from the lamellar surface normal (n̂) at Tc
The morphology and melting behaviour of ε-caprolactone-styrene diblock copolymers (PCL-b-PS) quenched from the melt were investigated by small-angle X-ray scattering and differential scanning calorimetry as a function of glass transition temperature of the PS block, Tg,PS. A plasticizer miscible only with the PS block, such as polystyrene oligomer or dioctyl phthalate, was added to PCL-b-PS in order to decrease Tg,PS successively (plasticizer-added PCL-b-PS). The crystallizability of the PCL block depended strongly on Tg,PS; the PCL block did not crystallize when Tg,PS was higher than the crystallization temperature Tc, while it crystallized partially when Tg,PS was lower than Tc. However, details of the morphology formed (i.e. the long spacing and lamellar thickness) were independent of Tg,PS. These results suggest that the mobility of PCL-b-PS influences extremely the crystallizability of the PCL block, but not the resulting morphology. The morphologies formed in the plasticizer-added PCL-b-PS were compared with those of PCL-b-PS cast from toluene or cyclohexane solution, and the characteristics of these morphologies were qualitatively discussed.
Compressed gases such as CO2 above their critical temperatures provide a highly tunable technique that has been shown to induce changes in phase behavior, crystallization kinetics and morphology of the polymers. Gas induced plasticization of the polymer matrix has been studied in a large number of polymers such as polystyrene, and poly(ethylene terephathalate). The knowledge of polymer–gas interactions is fundamental to the study of phenomena such as solubility and diffusivity of gases in polymers, dilation of polymers and in the development of applications such as foams and barrier materials.
Different kinetic models like the Avrami, Tobin and Urbanovici-Segal models have been applied for determining the isothermal crystallization kinetics of virgin poly(ethylene terephthalate) (PET) and PET/poly(methyl methacrylate) (PMMA) blends. The different compositions investigated were PET90/PMMA10, PET75/PMMA25 and PET50/PMMA50 [wt/wt%]. The experimental data was fitted using Solver, a non-linear multi-variable regression program and linearization method. The effect of composition variation of PET/PMMA on parameters like crystallization rate constant and crystallization exponent were investigated. Urbanovici-Segal and Avrami models gave the best fit to the experimental data. Tobin model does not seem to fit the experimental data for the systems under investigation. Experimental results indicated that the crystallization rate constant values increased with decreasing temperatures.
The electron diffraction (ED), morphology and structure of single crystals of poly(ethylene glycol) (PEG)-poly(ε-caprolactone) (PCL) diblock copolymers were examined. The ED patterns of both PEG and PCL were observed by choosing proper operating parameters of electron diffraction. The single crystals of PEG, PCL, and PEG-b-PCL were grown from their dilute solutions in hexanol and the solution was heated to 100 °C to eliminate its heat history and was isothermally crystallized at 40 °C. The concurrence of the ED patterns of PEG and PCL dingle crystals and the large lamellar thickness in PEG-b-PCL provide evidence for the co-existence of the two single-crystal layers formed from the two blocks. The results show that that the block copolymers assume the apparent morphology of PCL but can form much thicker and larger single crystals.
The glass transition temperature of MePEEK, a monosubstituted methyl derivative of poly(aryl ether ether ketone), PEEK, is 145-degrees-C, the same as that of PEEK. However, the onset of melting of MePEEK, crystallized by thermal annealing, occurs at 210-degrees-C, much lower than the onset at 335-degrees-C observed for PEEK. MePEEK, crystallized by successively annealing for 100 min each at 175, 190, and 200-degrees-C, melts in a two-step process spread over a broad temperature range of about 50 deg. This double melting behavior is observed in PEEK also. When MePEEK is annealed in a similar manner in the presence of supercritical CO2 at 100 atm, the melting event is still broad but the two peaks are now overlapped. Similar conditioning in the presence of supercritical 0.9CO2 + 0.1CH2Cl2, and 0.9CO2 + 0.1CH3OH at 100 atm gives a single and sharp melting peak with onset at 229 and 260-degrees-C, respectively. The melting event is now spread over about 15 deg only. The density of amorphous MePEEK is 1.233 g cm-3. The density and heat of melting of 100% crystalline MePEEK are estimated to be 1.348 g cm-3 and 74 J g-1, respectively, on the basis of density, thermal, and X-ray diffraction measurements on semicrystalline samples. The corresponding values for PEEK are 1.40 g cm-3 and 130 J g-1.
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 nonisothermal crystallization behavior and melting process of the poly(ϵ-caprolactone) (PCL)/poly(ethylene oxide) (PEO) diblock copolymer in which the weight fraction of the PCL block is 0.80 has been studied by using differential scanning calorimetry (DSC). Only the PCL block is crystallizable, the PEO block with 0.20 weight fraction cannot crystallize. The kinetics of the PCL/PEO diblock copolymer under nonisothermal crystallization conditions has been analyzed by Ozawa's equation. The experimental data shows no agreement with Ozawa's theoretical predictions in the whole crystallization process, especially in the later stage. A parameter, kinetic crystallinity, is used to characterize the crystallizability of the PCL/PEO diblock copolymer. The amorphous and microphase separating PEO block has a great influence on the crystallization of the PCL block. It bonds chemically with the PCL block, reduces crystallization entropy, and provides nucleating sites for the PCL block crystallization. The existence of the PEO block leads to the occurrence of the two melting peaks of the PCL/PEO diblock copolymer during melting process after nonisothermal crystallization. The comparison of nonisothermal crystallization of the PCL/PEO diblock copolymer, PCL/PEO blend, and PCL and PEO homopolymers has been made. It showed a lower crystallinity of the PCL/PEO diblock copolymer than that of others and a faster crystallization rate of the PCL/PEO diblock copolymer than that of the PCL homopolymer, but a slower crystallization rate than that of the PCL/PEO blend. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 63: 1793–1804, 1997
The effect of CO2 on the isothermal crystallization kinetics of poly(ethylene terephthalate), PET, was investigated using a high-pressure differential scanning calorimeter (DSC), which performed calorimetric measurements while keeping the polymer in contact with presurized CO2. It was found that the crystallization rate followed the Avrami equation with values of the crystallization kinetic constant dependent on the crystallization temperature and concentration of CO2 in PET. The presence of CO2 in the PET increased its overall crystallization rate. CO2 also decreased the glass transition temperature, Tg, and the melting temperature, Tm. As a result, the observed changes in crystallization rate caused by CO2 can be qualitatively predicted from the magnitude of Tg depression and that of the equilibrium melting temperature, Tm0.
The kinetic theory of the rate of growth G and the initial lamellar thickness l<sub>g</sub><sup>*</sup> of chain-folded crystals is extended so that it is applicable at high undercoolings. Attention is centered on the details of how the first step element and the first fold are put down on the substrate. A parameter φ that varies between zero and unity, which apportions the free energy of attachment of the step element between the forward and backward reactions, is used to denote variations in this process. Expressions for G are derived from flux equations for two limiting cases: regime I, single surface nucleation act with rapid substrate completion and regime II, numerous surface nucleation acts with very slow substrate completion. Data from the literature on G for isotactic polystyrene (regime II) and polyethylene single crystals (regime I) are analyzed to obtain surface free energies, and these are used with the revised theory for l<sub>g</sub><sup>*</sup> to predict the lamellar thickness of these polymers. Good agreement between l<sub>g</sub><sup>*</sup> and published data is found for 1 ≫ φ ≫ 0. Values of φ below unity imply that the molecules are physically adsorbed onto the substrate prior to actual crystallographic attachment. A discussion is given of the segmental transport effects that dominate the behavior of G at high undercoolings in bulk polymers.
tert-Butyl-substituted poly(ether ether ketone) (tBuPEEK), which does not undergo crystallization with thermal annealing, crystallizes readily when treated with compressed CO2. The dissolved CO2 causes a reduction in the glass-transition temperature of the polymer–gas system and enhances the chain mobility of the macromolecules, thereby bringing about crystallization. In the presence of CO2, crystallization is increasingly favored with increasing CO2 pressure and treatment temperature. The melting point of tBuPEEK crystals increases linearly with the CO2 pressure applied in the treatment, indicating an increase in the order and/or size of the crystals. The extent of crystallinity increases when small amounts of methanol or dichloromethane are used as a cosolute with CO2. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1505–1512, 2001
When CO2 dissolves into a polypropylene (PP), its crystallization kinetics changes These changes were studied, in the expectation that the information would reflect on the behavior of other semicrystalline polyolefins. The isothermal crystallizatior rate of the PP-CO2 solutions was measured using a high-pressure differential scanning calorimeter (DSC), which performed calorlmetric measurements while keeping the polymer in contact with pressurized CO2. Although the measured crystallizatior rate followed the Avrami equation, the value of the crystallization kinetic constant was different from that measured for PP crystallized in air under atmospheric pressure. The dissolved CO2 decreased the overall crystallization rate of PP within the nucleation dominated temperature region. This suggests that the dissolved CO2 decreases the melting and the glass transition temperatures and prevents formation of critical size nuclei.
The effect of CO2 on the isothermal crystallization kinetics of poly(L-lactide), PLLA, was investigated using a high-pressure differential scanning calorimeter (DSC), which can perform calorimetric measurements while keeping the sample polymer in contact with pressurized CO2. It was found that the crystallization rate followed the Avrami equation. However, the crystallization kinetic constant was changed depending upon the crystallization temperature and concentration of CO2 dissolved in the PLLA. The crystallization rate was accelerated by CO2 at the temperature in the crystal-growth rate controlled region (self-diffusion controlled region), and depressed in the nucleation-controlled region. CO2 has also decreased the glass transition temperature, Tg, and the melting temperature, Tm. As a result, the CO2-induced change in the crystallization rate can be predicted from the magnitudes of depression of both Tg and the equilibrium melting temperature. The crystalline structure and crystallinity of polymers crystallized in contact with pressurized CO2 were also investigated using a wide angle X-ray diffractometer (WAXD). The resulting crystallinity of the sample was increased with the pressure level of CO2, although the presence of CO2 did not change the crystalline structure.
The physical and degradative properties of polyhydroxybutyrate-hydroxyvalerate copolymer blends with polycaprolactone were investigated. Blends containing low levels of polycaprolactone (less than 20%) were found to possess a considerable degree of compatibility, whilst those with higher levels of polycaprolactone were incompatible and showed phase separation behaviour. This incompatibility was most marked in blends containing approximately 50% of each component. In blends containing low levels of polycaprolactone, processing conditions governed the ease of crystallization of polycaprolactone in the polyhydroxybutyrate-hydroxyvalerate matrix and thus the mechanical property of the blend. The degradation rate of these blends was found to be influenced by a complex set of factors, including temperature, pH and polycaprolactone content of the blend. Although crystallinity affected the mechanical properties of the blends, its influence on the hydrolytic degradation rate was masked by the large difference in the molecular weight of the polyhydroxybutyrate-hydroxyvalerate copolymers (MW approximately 300,000) and polycaprolactone. (MW approximately 50,000). The polyhydroxybutyrate-hydroxyvalerate/polycaprolactone blends were found to be much more stable to hydrolytic degradation than polyhydroxybutyrate-hydroxyvalerate/polysaccharide blends previously studied. Here the combined techniques of goniophotometry and surface energy measurements proved extremely valuable in monitoring the early stages of degradation, during which surface, rather than bulk degradation, processes predominate.
Poly(ethylene glycol)-poly(epsilon-caprolactone) diblock copolymers PEG-PCL were synthesized by ring-opening polymerization of epsilon-caprolactone using monomethoxy poly(ethylene glycol) as the macroinitiator and calcium ammoniate as the catalyst. Obvious mutual influence between PEG and PCL crystallization was studied by altering the relative block length. Fixing the length of the PEG block (Mn = 5000) and increasing the length of the PCL block, the crystallization temperature of the PCL block rose gradually from 1 to about 35 degrees C while that of the PEG block dropped from 36 to -6.6 degrees C. Meanwhile, the melting temperature of the PCL block went up from 30 to 60 degrees C, while that of the PEG block declined from 60 to 41 degrees C. If the PCL block was longer than the PEG block, the former would crystallize first when cooling from a molten state and led to obviously imperfect crystallization of PEG and vice versa. And they both crystallized at the same temperature, if their weight fractions were equal. We found that the PEG block could still crystallize at -6.6 degrees C even when its weight fraction is only 14%. A unique morphology of concentric spherulites was observed for PEG5000-PCL5000. According to their morphology and real-time growth rates, it is concluded that the central and outer sections in the concentric spherulites were PCL and PEG, respectively, and during the formation of the concentric spherulite, the PEG crystallized quickly from the free space of the PCL crystal at the earlier stage, followed by outgrowing from the PCL spherulites in the direction of right angles to the circle boundaries until the entire area was occupied.
The crystallization behaviors of the poly(ethylene glycol)-poly(epsilon-caprolactone) diblock copolymer with the PEG weight fraction of 0.50 (PEG(50)-PCL(50)) was studied by DSC, WAXD, SAXS, and FTIR. A superposed melting point at 58.5 degrees C and a superposed crystallization temperature at 35.4 degrees C were obtained from the DSC profiles running at 10 degrees C/min, whereas the temperature-dependent FTIR measurements during cooling from the melt at 0.2 degrees C/min showed that the PCL crystals formed starting at 48 degrees C while the PEG crystals started at 45 degrees C. The PEG and PCL blocks of the copolymer crystallized separately and formed alternating lamella regions according to the WAXD and SAXS results. The crystal growth of the diblock copolymer was observed by polarized optical microscope (POM). An interesting morphology of the concentric spherulites developed through a unique crystallization behavior. The concentric spherulites were analyzed by in situ microbeam FTIR, and it was determined that the morphologies of the inner and outer portions were mainly determined by the PCL and PEG spherulites, respectively. However, the compositions of the inner and outer portions were equal in the analysis by microbeam FTIR.