THE EFFECTS OF IRRADIATION ON THE THERMAL PROPERTIES OF SEMI-CRYSTALLINE POLYAMIDES

Irradiation crosslinking of semi-crystalline polyamides was performed by high energy electronswith various dosages. It is known that the melting behavior of the polymers after irradiation is acomplex phenomenon. In company with the wide angle X-ray diffraction and DSC data of irradiatedand unirradiated polyamides it is possible to develop the local order and perfection of the crystallinitiesslightly which resulted from introduction of intermolecular crosslinking in amorphous region, incl-uding in amorphous-crystalline interface and crystalline defect regions due to irradiation. It canbe explained that slight increase of melting temperature (T_m) and heat of fusion (△H_f) with increasingdosage for both of higher crystallinity nylon 4 and nylon 6. For irradiated lower crystallinity nylons,in contrast, the T_m and △H_f decrease obviously with increasing dosage. In this case, radiation cross-linking "freeze in" the pre-existing morphology, and then the prevention for reorganization duringheating is a dominant effect. The T_m from the second melting for all of the samples were depressed,corresponding with Flory theory. Therefore the crosslinks imposed on the molecules restrainedthe molecular mobility, and that not only depresses the crystallinity but also increases the imperfec-tion of crystallites when the radiated polymer melted and then recrystallized. These are also reflectedin the depression of heat and entropy of fusion as well as the appearance of double melting peakson the DSC thermograms.     

Nylon 6 9 can crystallize with hydrogen bonding in two and in three interchain directions

Nylon 6 9 has been shown to have structures with interchain hydrogen bonds in both two and in three directions. Chain-folded lamellar crystals were studied using transmission electron microscopy and sedimented crystal mats and uniaxially oriented fibers studied by X-ray diffraction. The principal room-temperature structure shows the two characteristic (interchain) diffraction signals at spacings of 0.43 and 0.38 nm, typical of α-phase nylons; however, nylon 6 9 is unable to form the α-phase hydrogen-bonded sheets without serious distortion of the all-trans polymeric backbone. Our structure has c and c^* noncoincident and two directions of hydrogen bonding. Optimum hydrogen bonding can only occur if consecutive pairs of amide units alternate between two crystallographic planes. The salient features of our model offer a possible universal solution for the crystalline state of all odd–even nylons. The nylon 6 9 room-temperature structure has a C-centered monoclinic unit cell (β = 108°) with the hydrogen bonds along the C-face diagonals; this structure bears a similarity to that recently proposed for nylons 6 5 and X3. On heating nylon 6 9 lamellar crystals and fibers, the two characteristic diffraction signals converge and meet at 0.42 nm at the Brill temperature, T_B · T_B for nylon 6 9 lamellar crystals is slightly below the melting point (T_m), whereas T_B for nylon 6 9 fibers is ≅ 100°C below T_m. Above T_B, nylon 6 9 has a hexagonal unit cell; the alkane segments exist in a mobile phase and equivalent hydrogen bonds populate the three principal (hexagonal) directions. A structure with perturbed hexagonal symmetry, which bears a resemblance to the reported γ-phase for nylons, can be obtained by quenching from the crystalline growth phase (above T_B) to room temperature. We propose that this structure is a “quenched-in” perturbed form of the nylon 6 9 high-temperature hexagonal phase and has interchain hydrogen bonds in all three principal crystallographic directions. In this respect it differs importantly from the γ-phase models. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1153–1165, 1998     

Nonisothermal cold crystallization behavior and kinetics of polylactide/clay nanocomposites

The nonisothermal cold crystallization behavior of intercalated polylactide (PLA)/clay nanocomposites (PLACNs) was studied using differential scanning calorimetry, polarized optical microscope, X‐ray diffractometer, dynamic mechanical thermal analysis, and Fourier transform infrared spectrometer. The results show that both the cold crystallization temperature (T_cc) and melting point (T_m) of PLA matrix decreases monotonously with increasing of clay loadings, accompanied by the decreasing degree of crystallinity (X_c%) at the low heating rates (≤5 °C/min). However, the X_c% of PLACNs presents a remarkable increase at the high heating rate of 10 °C/min in contrast to that of neat PLA. The crystallization kinetics was then analyzed by the Avrami, Jezioney, Ozawa, Mo, Kissinger and Lauritzen–Hoffman kinetic models. It can be concluded that at the low heating rate, the cold crystallization of both the neat PLA and nanocomposites proceeds by regime III kinetics. The nucleation effect of clay promote the crystallization to some extent, while the impeding effect of clay results in the decrease of crystallization rate with increasing of clay loadings. At the high heating rate of 10 °C/min, crystallization proceeds mainly by regime II kinetics. Thus, the formation of much more incomplete crystals in the PLACNs with high clay loadings due to the dominant multiple nucleations mechanism in regime II, may have primary contribution to the lower crystallization kinetics, also as a result to the higher degree of crystallinity and lower melting point in contrast to that of neat PLA. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1100–1113, 2007     

Poly(p‐phenylene sulfide) nonisothermal cold‐crystallization

The poly(p‐phenylene sulfide) (PPS) nonisothermal cold‐crystallization behavior was investigated in a wide heating rate range. The techniques employed were the usual Differential Scanning Calorimetry (DSC), and the less conventional FT‐IR spectroscopy and Energy Dispersive X‐ray Diffraction (EDXD). The low heating rates (Φ) explored by EDXD (0.1 K min^?1) and FT‐IR (0.5–10 K min^?1) are contiguous and complementary to the DSC ones (5–30 K min^?1). The crystallization temperature changes from 95 °C at Φ = 0.05 K min^?1 to 130 °C at Φ = 30 K min^?1. In such a wide temperature range the Kissinger model failed. The model is based on an Arrhenius temperature dependence of the crystallization rate and is widely employed to evaluate the activation energy of the crystallization process. The experimental results were satisfactorily fit by replacing in the Kissinger model the Arrhenius equation with the Vogel–Fulcher–Tamann function and fixing U* = 6.28 k J mol^?1, the activation energy needed for the chains movements, according to Hoffmann. The temperature at which the polymer chains are motionless (T_∞ = 42 °C) was found by fitting the experimental data. It appears to be reasonable in the light of our previously reported isothermal crystallization results, which indicated T_∞ = 48 °C. Moreover, at the lower heating rate, mostly explored by FT‐IR, a secondary stepwise crystallization process was well evidenced. In first approximation, it contributes to about 17% of the crystallinity reached by the sample. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2725–2736, 2005     

Thermal behavior of polytriazole films: A thermal analysis study

The thermal behavior of poly(1,3-phenyl-1,4-phenyl)-4-phenyl-1,2,4-triazole has been investigated using different scanning calorimetry (DSC) and thermogravimetry (TG). Processes are studied for this thermally stable polymer that take place between 200 and 500°C. While the polycondensation reaction product in powder from appeared to be partially crystalline, films prepared by casting from a formic acid solution appeared to be completely amorphous. A thermal treatment between T_g(～ 270°C) and T_m(～ 430°C) can introduce crystallinity in the films because of the polymer's ability to cold crystallize. The cold crystallization temperature T_c seems to be dependent on the preparation history of the solid polymer phase. Thermal annealing of the films just below T_g does not introduce crystallinity but inhibits subsequent cold crystallization at higher temperatures. Crystallization upon cooling from the crystalline melt has not been observed either. At temperatures just above the crystalline melting point the polymer starts to decompose in an exothermic reaction.     

Crystalline transition behaviors of nylons 12 20 and 10 20

The crystalline transition behaviors under different crystalline conditions of newly synthesized long alkane nylon 12 20 and nylon 10 20 are studied by wide-angle X-ray diffraction (WAXD) and real time Fourier transform infrared spectroscopy (FT-IR). The results show that their crystalline transition behaviors under WAXD were, to a large extend, related to the condition under which the crystals were prepared. The dilute solution-grown lamellar crystals of nylons 12 20 and 10 20 did not show distinct Brill transition behaviors before melting. Unlike the lamellar crystals of many other even-even nylons which display two crystal signals until melting temperature (T_M), they presented a broad amorphous-like signal when the temperature increased to around 10 °C below T_M. However, the post-annealing samples of nylons 12 20 and 10 20 displayed Brill transition at 155 and 157 °C, respectively, and the solution casting samples of nylons 12 20 and 10 20 at 110 and 135 °C, respectively. Furthermore, the IR spectra of nylons 12 20 and 10 20 displayed an interesting phenomenon: the intensity of the peak at 942 cm⁻¹ declined on heating and finally disappeared around Brill temperature (T_B), instead of T_M as is in usual nylons. This suggests that the long alkane segments, introduced by 18-octadecanedicarboxylic acid, may undergo a local melting at T_B.     

Nylon–selected topics

Elements of the technology of nylon fibers and nylon plastics are reviewed. These include the development of equations to quantify the effects of end‐group imbalance and chain‐ending impurities, the kinetics of the polymerization of caprolactam, the solid‐state polymerization of nylon‐66, and the dependence of fiber tensile strength on molecular weight. This is followed by remarks that include comments on the significance of the amidation equilibrium for melt behavior and estimates of the activation energy of viscous flow, the pyrolysis of nylon, the use of the glass‐transition temperature (T_g) to predict the effect of moisture on properties, and the prospect for new nylons. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2565–2570, 2001     

New insights into the multiple melting behaviors of the semicrystalline ethylene‐hexene copolymer: Origins of quintuple melting peaks

A semicrystalline ethylene‐hexene copolymer (PEH) was subjected to a simple thermal treatment procedure as follows: the sample was isothermally crystallized at a certain isothermal crystallization temperature from melt, and then was quenched in liquid nitrogen. Quintuple melting peaks could be observed in heating scan of the sample by using differential scanning calorimeter (DSC). Particularly, an intriguing endothermic peak (termed as Peak 0) was found to locate at about 45 °C. The multiple melting behaviors for this semicrystalline ethylene‐hexene copolymer were investigated in details by using DSC. Wide‐angle X‐ray diffraction (WAXD) technique was applied to examine the crystal forms to provide complementary information for interpreting the multiple melting behaviors. Convincing results indicated that Peak 0 was due to the melting of crystals formed at room temperature from the much highly branched ethylene sequences. Direct heating scans from isothermal crystallization temperature (T_c, 104–118 °C) were examined for comparison, which indicated that the multiple melting behaviors depended on isothermal crystallization temperature and time. A triple melting behavior could be observed after a relatively short isothermal crystallization time at a low T_c (104–112 °C), which could be attributed to a combination of melting of two coexistent lamellar stack populations with different lamellar thicknesses and the melting‐recrystallization‐remelting (mrr) event. A dual melting behavior could be observed for isothermal crystallization with both a long enough time at a low T_c and a short or long time at an intermediate T_c (114 °C), which was ascribed to two different crystal populations. At a high T_c (116–118 °C), crystallizable ethylene sequences were so few that only one single broad melting peak could be observed. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2100–2115, 2008     

Miscibility in crystalline/amorphous blends of poly(3-hydroxybutyrate)/DGEBA

A differential scanning calorimetry (DSC) and small-angle X-ray scattering (SAXS) study of miscibility in blends of the semicrystalline polyester poly(3-hydroxybutyrate) (PHB) and amorphous monomer epoxy DGEBA (diglycidyl ether of bisphenol A) was performed. Evidence of the miscibility of PHB/DGEBA in the molten state was found from a DSC study of the dependence of glass transition temperature (T_g) as a function of the blend composition and isothermal crystallization, analyzing the melting point (T_m) as a function of blend composition. A negative value of Flory–Huggins interaction parameter χ_PD was obtained. Furthermore, the lamellar crystallinity in the blend was studied by SAXS as a function of the PHB content. Evidence of the segregation of the amorphous material out of the lamellar structure was obtained. © 2013 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013     

Crystallization,morphology, and thermal behavior of poly(p‐phenylene sulfide)

This article deals with the structure, crystallization, morphology, and thermal behavior of poly(p‐phenylene sulfide) (PPS) with low‐molecular mass, probed by DSC, optical, and electron microscopy. The growth rates of spherulites were measured over the temperature range 235–275°C. A regime II–III transition was found at T = 250°C. The regime transition was accompanied by a morphological change from sheaflike structure to classical spherulites. The Avrami equation poorly described the isothermal crystallization of PPS, for the occurrence of mixed growth mechanisms and secondary crystallization, in agreement with the morphology and the thermal behavior. Two melting peaks were detected on DSC curves and attributed to the melting of crystals formed isothermally at T_c by primary and secondary crystallization. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 415–424, 2001     

Thermal analysis of the double-melting behavior of poly(L-lactic acid)

The melting and crystallization behavior of poly(L -lactic acid) (PLLA; weight-average molecular weight = 3 × 10⁵) was studied with differential scanning calorimetry (DSC). DSC curves for PLLA samples were obtained at various cooling rates (CRs) from the melt (210 °C). The peak crystallization temperature and the exothermic heat of crystallization determined from the DSC curve decreased almost linearly with increasing log(CR). DSC melting curves for the melt-crystallized samples were obtained at various heating rates (HRs). The double-melting behavior was confirmed by the double endothermic peaks, a high-temperature peak (H) and a low-temperature peak (L), that appeared in the DSC curves at slow HRs for the samples prepared with a slow CR. Peak L increased with increasing HR, whereas peak H decreased. The peak melting temperatures of L and H [T_m(L) and T_m(H)] decreased linearly with log(HR). The appearance region of the double-melting peaks (L and H) was illustrated in a CR–HR map. Peak L decreased with increasing CR, whereas peak H increased. T_m(L) and T_m(H) decreased almost linearly with log(CR). The characteristics of the crystallization and double-melting behavior were explained by the slow rates of crystallization and recrystallization, respectively. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 25–32, 2004     

Effect of microstructure on hydrolytic degradation studies of poly (l-lactic acid) by FTIR spectroscopy and differential scanning calorimetry

Structural changes during thermally induced crystallization and alkaline hydrolysis of Poly(l-lactic acid) (PLLA) films were investigated using differential scanning calorimetry (DSC), FTIR spectroscopy, weight loss, HPLC and optical microscopy. It was shown that crystallinity (χ_c), glass transition temperature (T_g) and melting temperature (T_m) were found to be strongly annealing temperature (T_a) dependent. The FTIR study of PLLA films suggested that the bands at 921 and 956 cm⁻¹ could be used to monitor the structural changes of PLLA. An independent infrared spectroscopic method was developed for the first time to determine crystallinity of PLLA before degradation and it showed good qualitative correlation with DSC crystallinity. The higher crystallinity values determined by FTIR were attributed to the intermediate phase included in the IR crystallinity. Both the weight loss data and the percentage of lactic acid obtained by HPLC showed that the alkaline hydrolysis of PLLA films increased with increasing crystallinity. The DSC observation showed an increase in T_g and no significant change in T_m and heat of fusion, while IR showed an increase in IR crystallinity with increasing hydrolysis time. The increase in IR crystallinity and T_g with hydrolysis time suggested that degradation progressed from the edges of the crystalline lamellas without decreasing lamellar thickness, but increased the intermediate phase and the short-range order.     

Investigation on nonisothermal crystallization kinetics of the high‐flow nylon 6 by differential scanning calorimetry

The crystallization kinetics of the high‐flow nylon 6 containing polyamidoamine (PAMAM) dendrimers units in nylon 6 matrix was investigated by differential scanning calorimetry. The Ozawa and Mo equations were used to describe the crystallization kinetics under nonisothermal condition. The values of Avrami exponent m and the cooling crystallization function F(T) were determined from the Ozawa plots, which showed bad linearity, and were divided into three sections depending on different cooling rates. The plots of the m and log F(T) values versus crystallization temperatures were obtained, which indicated that the actual crystallization mechanisms might change with the crystallization temperatures. The high‐flow nylon 6 has higher values of m and log F(T) than those of pure nylon 6, which implied that the high‐flow nylon 6 had more complicated crystallization mechanisms and slower crystallization rate than those of pure nylon 6. The good linearity of the Mo plots verified the success of this combined approach. The activation energies of the high‐flow nylon 6 ranged from 157 to 174 kJ/mol, which were determined by the Kissinger method. The ΔE values were lower than those of pure nylon 6, and the ΔE values were affected by both the generation and the content of PAMAM units in the nylon 6 matrix. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2201–2211, 2008     

Cold crystallization and thermal shrinkage of uniaxially drawn poly(ethylene 2,6-naphthalate) by solid-state coextrusion

Solid-state coextrusion has been used to prepare uniaxially drawn films from isotropic poly(ethylene 2,6-naphthalate) (PEN) of a minimum degree of crystallinity (ca. 5%) both below and above its glass transition temperature T_g. The onset of cold crystallization (T_c) of the drawn films has been studied as a function of the extrusion temperature (ET) and the draw ratio (EDR). It has been shown that T_c decreases markedly on draw, as much as 95°C, and, at constant draw ratio T_c goes through a minimum in the T_g region. For undrawn PEN, annealing below 153°C has no significant effect on T_c. To evaluate the crystallization rate constant (k) and the activation energy (E_a) of the drawn specimens, a nonisothermal DSC procedure has been used. With increasing EDR, k increases markedly and E_a goes down over threefold compared with the undrawn polymer. At high ET, strain-induced crystallization has also been shown to play an important role in lowering E_a for cold crystallization. Thermal shrinkage above T_m indicates a high elastic recovery, underlining the efficiency of deformation, ca. 93%, achieved by solid-state coextrusion.     

Study of the Crystallization of an Aromatic Poly-ether-ketone (PK99) by Calorimetric and X-ray Analysis

An analysis of the crystallization behaviour of a new poly(aryl-ether-ether-ketone-ketone), PK99, by differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD) is presented. Isothermal crystallization TG were obtained in the whole range between the glass transition temperature (T _g) and the melting temperature (T _m) as a consequence of the slow crystallization kinetics stemming from the closeness of these transitions. The calorimetric results, compared with WAXD data, were applied to determine the theoretical melting temperature and crystallization enthalpy. The DSC and WAXD data were combined in order to calculate the total amount of the crystallizable fraction of the polymer, and a model was proposed to explain the difference between the fractions of crystallinity observed with these techniques. The thermal and X-ray data were also correlated with different lamellar morphologies arising from the crystallization conditions. Finally, DSC experiments on the crystallized sample were used to detect the presence of a rigid amorphous phase which does not relax at T _g. This revised version was published online in July 2006 with corrections to the Cover Date.     

Self‐decelerated crystallization in poly(butylene terephthalate)/poly(ε‐caprolactone) blends

Isothermal crystallization of poly(butylene terephthalate) (PBT) blended with oligomeric poly(ε‐caprolactone) (PCL) is investigated by polarized optical microscopy and differential scanning calorimetry at various temperatures (T_c). The growth rate of PBT spherulites is found to depend on time (t), as the spherulite radius (r) linearly increases with t at the early stages of crystallization (r ∝ t), then, with the progress of phase transition, the spherulite radius becomes dependent on the square root of the time (r ∝ t^1/2) until termination of crystal growth. The nonlinear advance of the crystal growth front is caused by a varied composition of the melt phase in contact with the growing crystals, due to diffusion of mobile PCL chains away from the spherulite surface. The melt phase becomes spatially inhomogeneous, causing self‐deceleration of PBT crystallization until a limit composition that prevents further crystallization is reached in the melt. The maximum crystallinity achievable during isothermal crystallization decreases with T_c. The lowering of the temperature after termination of the isothermal crystallization allows to complete the crystal growth, but the final developed crystallinity still depends on T_c, being lower at higher T_cs. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 3148–3155, 2007     

Hydrophilic segmented block copolymers based on poly(ethylene oxide) and monodisperse amide segments

Segmented block copolymers based on poly(ethylene oxide) (PEO) flexible segments and monodisperse crystallizable bisester tetra‐amide segments were made via a polycondensation reaction. The molecular weight of the PEO segments varied from 600 to 4600 g/mol and a bisester tetra‐amide segment (T6T6T) based on dimethyl terephthalate (T) and hexamethylenediamine (6) was used. The resulting copolymers were melt‐processable and transparent. The crystallinity of the copolymers was investigated by differential scanning calorimetry (DSC) and Fourier Transform infrared (FTIR). The thermal properties were studied by DSC, temperature modulated synchrotron small angle X‐ray scattering (SAXS), and dynamic mechanical analysis (DMA). The elastic properties were evaluated by compression set (CS) test. The crystallinity of the T6T6T segments in the copolymers was high (>84%) and the crystallization fast due to the use of monodisperse tetra‐amide segments. DMA experiments showed that the materials had a low T_g, a broad and almost temperature independent rubbery plateau and a sharp flow temperature. With increasing PEO length both the PEO melting temperature and the PEO crystallinity increased. When the PEO segment length was longer than 2000 g/mol the PEO melting temperature was above room temperature and this resulted in a higher modulus and in higher compression set values at room temperature. The properties of PEO‐T6T6T copolymers were compared with similar poly(propylene oxide) and poly(tetramethylene oxide) copolymers. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4522–4535, 2007     

X-ray scattering and thermal analysis study of the effects of molecular weight on phase structure in blends of poly(butylene terephthalate) with polycarbonate

Blends of Poly(butylene terephthalate), PBT, with Polycarbonate, PC, were studied for a range of molecular weights and blend compositions. Blends were available in PBT/PC compositions 80/20 and 40/60, and with M_w designated by H (high) or L (low). Samples were prepared by melt crystallization, or by cold crystallization following a rapid quench from the melt. Addition of PC reduces the crystallization kinetics of PBT so that the resulting crystals are more perfect than those which form in the homopolymer. Degree of crystallinity of the blends followed the rank ordering: L/L > L/H > H/L = H/H. The glass transition behavior was investigated using dynamic mechanical analysis (DMA) and modulated differential scanning calorimetry (MDSC). All blends exhibited two glass transitions at intermediate temperatures between the T_gs of the homopolymers, indicating existence of a PBT-rich phase and a PC-rich phase. Blends L/L were most, and H/H the least, miscible. Small-angle X-ray scattering was performed at room temperature on cold crystallized blends, or at elevated temperature during melt crystallization. The long period was consistently larger, and the linear stack crystallinity was consistently smaller, in blends L/L or H/L. These results indicate that in blends containing low M_w PC, there is more PC located within the PBT-rich phase. The long period was consistently smaller in cold crystallized samples, while the linear stack crystallinity was nearly the same, regardless of melt or cold crystallization treatment. Reduction of the average long period in cold crystallized samples could result from crystallization of PBT within the PC-rich phase. This is consistent with thermal analysis results, which indicate that cold crystallized samples have greater overall crystallinity than melt crystallized samples. A hypothetical liquid phase diagram is presented to explain the differences between melt and cold crystallized blends. © 1996 John Wiley & Sons, Inc.     

Application of zone-drawing and zone-annealing method to poly(p-phenylene sulfide) fibers

A zone-drawing and zone-annealing treatment was applied to poly(p-phenylene sulfide) fibers in order to improve their mechanical properties. The zone-drawing (ZD) was carried out at a drawing temperature of 90°C under an applied tension of 5.5 MPa, and the zone-annealing (ZA) was carried out at an annealing temperature of 220°C under 138.0 MPa. The differential scanning calorimetry (DSC) thermogram of the ZD fiber had a broad exothermic transition (T_c = 110°C) attributed to cold-crystallization and a melting endotherm peaking at 286°C. The T_c of the ZD fiber was lower than that (T_c = 128°C) of the undrawn fiber. In the temperature dependence of storage modulus (E′) for the ZD fiber, the E′ values decreased with increasing temperature, but increased slightly in the temperature range of 90–100°C, and decreased again. The slight increase in E′ was attributable to the additional increase in the crosslink density of the network, which was caused by strain-induced crystallization during measurement. The resulting ZA fiber had a draw ratio of 6.0, a degree of crystallinity of 38%, a tensile modulus of 8 GPa, and a tensile strength of 0.7 GPa. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1731–1738, 1998     

Cold crystallization and postmelting crystallization of PLA plasticized by compressed carbon dioxide

The cold crystallization at temperature T_cc (melting > T_cc > glass transition) and the postmelting crystallization of polylactic acid plasticized by compressed carbon dioxide (CO₂) were studied using a high-pressure differential scanning calorimeter. The kinetics of the two kinds of crystallization were evaluated by the Avrami equation as a function of pressure at certain temperatures. The effects of using talc as a nucleation agent on the two types of crystallization under pressure were also investigated. The results show that compressed CO₂ increased the mobility of the polymer chains in solid state, resulting in an increased rate of cold crystallization. The decreased rate of postmelting crystallization was mainly in the nucleation-controlled region, which indicates that the number of nuclei was decreased by the compressed CO₂. The growth rate of the two crystallization types followed the Avrami equation, but the kinetics of each depended upon temperature and pressure. The inclusion of talc accelerated postmelting crystallization but had little effect on cold crystallization. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2630–2636, 2008