Isothermal and non-isothermal crystallization behavior of poly(l-lactic acid): Effects of stereocomplex as nucleating agent

The effects of incorporated poly(d-lactic acid) (PDLA) as poly(lactic acid) (PLA) stereocomplex crystallites on the isothermal and non-isothermal crystallization behavior of poly(l-lactic acid) (PLLA) from the melt were investigated for a wide PDLA contents from 0.1 to 10 wt%. In isothermal crystallization from the melt, the radius growth rate of PLLA spherulites (crystallization temperature (T_c)≥125 °C), the induction period for PLLA spherulite formation (t_i) (T_c≥125 °C), the growth mechanism of PLLA crystallites (90 °C≤T_c≤150 °C), and the mechanical properties of the PLLA films were not affected by the incorporation of PDLA or the presence of stereocomplex crystallites as a nucleating agent. In contrast, the presence of stereocomplex crystallites significantly increased the number of PLLA spherulites per unit area or volume. In isothermal crystallization from the melt, at PDLA content of 10 wt%, the starting, half, and ending times for overall PLLA crystallization (t_c(S), t_c(1/2), and t_c(E), respectively) were much shorter than those at PDLA content of 0 wt%, due to the increased number of PLLA spherulites. Reversely, at PDLA content of 0.1 wt%, the t_c(S), t_c(1/2), and t_c(E) were longer than or similar to those at PDLA content of 0 wt%, probably due to the long t_i and the decreased number of spherulites. This seems to have been caused by free PDLA chains, which did not form stereocomplex crystallites. On the other hand, at PDLA contents of 0.3-3 wt%, the t_c(S), t_c(1/2), and t_c(E) were shorter than or similar to those at PDLA content of 0 wt% for the T_c range below 95 °C and above 125 °C, whereas this inclination was reversed for the T_c range of 100-120 °C. In the non-isothermal crystallization of as-cast or amorphous-made PLLA films during cooling from the melt, the addition of PDLA above 1 wt% was effective to accelerate overall PLLA crystallization. The X-ray diffractometry could trace the formation of stereocomplex crystallites in the melt-quenched PLLA films at PDLA contents above 1 wt%. This study revealed that the addition of small amounts of PDLA is effective to accelerate overall PLLA crystallization when the PDLA content and crystallization conditions are scrupulously selected.     

Crystallization, spherulite growth, and structure of blends of crystalline and amorphous poly(lactide)s

The effects of incorporated amorphous poly(dl-lactide) (PDLLA) on the isothermal crystallization and spherulite growth of crystalline poly(l-lactide) (PLLA) and the structure of the PLLA/PDLLA blends were investigated in the crystallization temperature (T_c) range of 90-150 °C. The differential scanning calorimetry results indicated that PLLA and PDLLA were phase-separated during crystallization. The small-angle X-ray scattering results revealed that for T_c of 130 °C, the long period associated with the lamellae stacks and the mean lamellar thickness values of pure PLLA and PLLA/PDLLA blend films did not depend on the PDLLA content. This finding is indicative of the fact that the coexisting PDLLA should have been excluded from the PLLA lamellae and inter-lamella regions during crystallization. The decrease in the spherulite growth rate and the increase in the disorder of spherulite morphology with an increase in PDLLA content strongly suggest that the presence of a very small amount of PDLLA chains in PLLA-rich phase disturbed the diffusion of PLLA chains to the growth sites of crystallites and the lamella orientation. However, the wide-angle X-ray scattering analysis indicated that the crystalline form of PLLA remained unvaried in the presence of PDLLA.     

Melting behavior of poly(l-lactic acid): X-ray and DSC analyses of the melting process

To clarify the melting behavior of poly(l-lactic acid) (PLLA), the wide-angle X-ray diffraction patterns of the isothermally crystallized PLLA samples (ICSs) were successively obtained during heating. We have already suggested the discrete change in the crystallization behavior of PLLA at a crystallization temperature (T_c) of 113 °C (= T_b) and formation of two crystal modifications for the ICSs obtained in the temperature range T_c ≤ T_b and T_c ≥ T_b. It was elucidated from the change in the X-ray diffraction pattern that the phase transition from the low-temperature crystal modification (α′-form) to the high-temperature one (α-form) occurred in a range 155-165 °C for the ICSs(T_c ≤ T_b), and that the crystal structure for the ICSs(T_c ≥ T_b) did not change. Recrystallization during heating, which is the origin of the multiple melting behavior, was proved by the increase in the diffraction intensity before steep decrease due to the final melting. A temperature derivative curve of the X-ray diffraction intensity almost coincided with the DSC melting curve.     

Morphology of melt-quenched poly(ε-caprolactone)-block-polyethylene copolymers

The morphology of a melt-quenched crystalline-crystalline diblock copolymer, poly(ε-caprolactone)-block-polyethylene (PCL-b-PE), was studied by small-angle X-ray scattering and transmission electron microscopy. The melting behavior of PCL-b-PE was also investigated by differential scanning calorimetry. The melting temperature of PCL blocks, T_m,PCL, was ca. 55 °C and that of PE blocks was ca. 96 °C. Therefore, the PE block always crystallized first during quenching from the microphase-separated melt into various temperatures T_c below T_m,PCL to yield an alternating structure composed of PE lamellae and amorphous layers (PE lamellar morphology), and subsequently the crystallization of PCL blocks started at T_c after some induction period. The PE lamellar morphology was preserved after the crystallization of PCL blocks at low crystallization temperatures (T_c<30 °C), that is, the PCL block crystallized within the PE lamellar morphology. At high crystallization temperatures (45 °C>T_c>30 °C), on the other hand, the crystallization of PCL blocks destroyed the PE lamellar morphology to result in a new lamellar morphology mainly consisting of PCL lamellae and amorphous layers (PCL lamellar morphology). The PE crystals were fragmentarily dispersed in the PCL lamellar morphology.     

Formation of retinyl palmitate-loaded poly(l-lactide) nanoparticles using rapid expansion of supercritical solutions into liquid solvents (RESOLV)

Poly(l-lactide) (PLLA) nanoparticles loaded with retinyl palmitate (RP) were successfully prepared by rapid expansion of a supercritical carbon dioxide (CO₂) solution into an aqueous receiving solution containing a stabilizing agent (RESOLV). Three stabilizing agents, Pluronic F127, Pluronic F68, and sodium dodecyl sulfate (SDS) have been employed and the Pluronic F127 was found to be more effective for stabilizing PLLA/RP nanoparticles than Pluronic F68 and SDS, as RESOLV into a 0.1 wt% Pluronic F127 solution produced a stable nanosuspension consisting mainly of well-dispersed, individual nanoparticles. The effect of rapid expansion processing conditions (i.e., degree of saturation (S), pre-expansion temperature (T_pre), and concentrations of PLLA and RP (C_PLLA, C_RP)) on the particle size, form, and RP loading was systematically investigated. It was found that spherical PLLA/RP nanoparticles with an average size range of ∼40-110 nm and RP loadings of 0.9-6.2 wt% were consistently produced by RESOLV. The size of PLLA/RP nanoparticles increased from ∼30-80 to ∼30-160 nm as the solution degree of saturation changed from S < 1 to S > 1, independent of T_pre, C_PLLA, and C_RP. The entrapment capacity of RP in PLLA nanoparticles was predominantly determined by T_pre and C_RP. Increasing the T_pre from 70 to 100 °C and the C_RP from 0.05 to 0.15 wt% increased the encapsulated RP content at least twofold. Our results show that the technique with benign supercritical CO₂ should be generally applicable to nanoparticle fabrications of other important active ingredients, especially in liquid form, in polymeric nanoparticles.     

Improvement of heat stability of poly(l-lactic acid) by radiation-induced crosslinking

Poly(l-lactic acid) (PLLA) has poor heat stability above its glass transition temperature (T_g∼60 °C). To improve its softing above T_g, PLLA was mixed with small amount of crosslinking agents and irradiated with various irradiation doses to introduce crosslinking between polymer chains. The most effective agent for radiation crosslinking was triallyl isocyanurate (TAIC). For melt-quenched PLLA, it was found that the most optimal conditions to introduce crosslinking were around 3% of TAIC and the irradiation dose of 50 kGy. The typically crosslinked PLLA showed very low crystallinity because of wide formation of molecular chain network that inhibited molecular motion for crystallization. Notable heat stability above T_g was given by annealing of PLLA samples. Enzymatic degradation of PLLA was retarded with introduction of crosslinks.     

Spherulite growth of l-lactide copolymers: Effects of tacticity and comonomers

The spherulite growth behavior and mechanism of l-lactide copolymers, poly(l-lactide-co-d-lactide) [P(LLA-DLA)], poly(l-lactide-co-glycolide) [P(LLA-GA)], and poly(l-lactide-co-ε-caprolactone) [P(LLA-CL)] have been studied using polarization optical microscopy in comparison with poly(l-lactide) (PLLA) having different molecular weights to elucidate the effects of incorporated comonomer units. The incorporation of comonomer units reduced the radius growth rate of spherulites (G) and increased the induction period of spherulite formation (t_i), irrespective of the kind of comonomer unit. Such effects became remarkable with the content of comonomers. At a crystallization temperature (T_c) of 130 °C, the disturbance effects of comonomers on the spherulite growth decreased in the following order: d-lactide>glycolide>ε-caprolactone, when compared at the same comonomer unit or reciprocal of averaged l-lactyl unit sequence length (l_l). The t_i estimation indicated that the glycolide units have the lowest disturbance effects on the formation of spherulite (crystallite) nuclei. The PLLA having the number-average molecular weight (M_n) exceeding 3.1×10⁴ g mol⁻¹ showed the transition from regime II to regime III at T_c=120 °C, whereas PLLA with the lowest M_n of 9.2×10³ g mol⁻¹ crystallized solely in regime III kinetics and the copolymers excluding P(LLA-DLA) with 3% of d-lactide units crystallized solely according to regime II kinetics. The nucleation and front constant for regime II and III [K_g(II), K_g(III), G₀(II), and G₀(III), respectively] estimated with each (not with a fixed for high-molecular-weight PLLA) decreased with increasing the amount of defects per unit mass of the polymer for crystallization, i.e. with increasing the comonomer content and the density of terminal group through decreasing the molecular weight.     

Synchronous and separate homo-crystallization of enantiomeric poly(l-lactic acid)/poly(d-lactic acid) blends

High-molecular-weight poly(l-lactic acid) (PLLA) and poly(d-lactic acid) (PDLA) are blended at different ratios and their crystallization behavior was investigated. Solely homo-crystallites mixtures of PLLA and PDLA were synchronously and separately formed during isothermal crystallization in the temperature (T_c) range of 90–130 °C, irrespective of blending ratio, whereas in addition to homo-crystallites, stereocomplex crystallites were formed in the equimolar blends at T_c above 150 and 160 °C. Interestingly, in isothermal crystallization at T_c = 130 °C, the spherulite morphology of blends became disordered, the periodical extinction (periodical twisting of lamellae) in spherulites disappeared, and the radial growth rate of spherulite (G) of the blends was reduced by the synchronous and separate crystallization of PLLA and PDLA and the coexistence of PLLA and PDLA homo-crystallites. However, the interplane distance (d), the crystallinity (X_c), the transition crystallization temperature (T_c) from α′-form to α-form, the alternately stacked structure of the crystalline and amorphous layers, and the nucleation mechanism were not altered by the synchronous and separate crystallization of PLLA and PDLA and the coexistence of PLLA and PDLA homo-crystallites. The unchanged d, X_c, transition T_c, long period of stacked lamellae, and nucleation mechanism strongly suggest that the chiral selection of PLLA or PDLA segments on the growth sites of PLLA or PDLA homo-crystallites to some extent was performed during solvent evaporation and this effect remained even after melting.     

Melting behavior of poly(l-lactic acid): Effects of crystallization temperature and time

Effects of crystallization temperature and time on the melting behavior of poly(l-lactic acid) were studied with differential scanning calorimetry (DSC). The isothermal crystallization was performed at various temperatures (T_cs), and DSC melting curves for the isothermally crystallized samples were obtained at 10 K min⁻¹. When T_c was lower than T_d (∼135 °C), the double melting peaks appeared. The melting behavior, especially T_c dependence of the melting temperature (T_m), discretely changed at T_b (=113 °C), in accordance with the discrete change of the crystallization behavior at T_b, which was previously reported. When T_c was higher than T_d, a single melting peak appeared. In addition, T_c dependence of dT_m/dT_c discretely changed at T_d. That is, the melting behavior, especially T_c dependence of T_m and dT_m/dT_c, are different in three temperature regions of T_c divided by T_b and T_d: Regions I (T_c ≤ T_b), II (T_b ≤ T_c ≤ T_d), and III (T_d ≤ T_c). The effect of crystallization time on the melting behavior, melting temperature and heat of fusion in each temperature region of T_c is also discussed.     

Glass transition behavior and dynamic fragility in polylactides containing mobile and rigid amorphous fractions

The segmental dynamics of polylactide chains covering the T_g − 30 °C to T_g + 30 °C range was studied in absence and presence of a crystalline phase by dynamic mechanical analysis (DMA) using the framework provided by the WLF theory and the Angell's dynamic fragility concept. An appropriate selection of stereoisomers combined with a thermal conditioning strategy to promote crystallization (above T_g) or relaxation of chains (below T_g) was revealed as an efficient method to tune the ratio of the rigid and mobile amorphous phases in polylactides. A single bulklike mobile amorphous phase was taken for poly(d,l-lactide) (PDLLA). In turn three phases, comprising a mobile amorphous fraction (MAF, X_MA), a rigid amorphous fraction (RAF, X_RA) and a crystalline fraction (X_c) were determined in poly(l-lactide) (PLLA) by modulated differential scanning calorimetry (MDSC) according to a three-phase model. The analysis of results confirms that crystallinity and RAF not only elevate the T_g and the breadth of the glass transition region but also yields an increase in dynamic fragility parameter (m) which entails the existence of a smaller length-scale of cooperativity of polylactide chains in confined environments. Consequently it is proposed that crystallinity is acting in polymeric systems as a topological constraint that, preventing longer range dynamics, provides a faster segmental dynamics by the temperature dependence of relaxation times according to the strong-fragile scheme.     

AFM study of crystallization and melting of a poly(ethylene oxide) diblock copolymer containing a tablet-like block of poly{2,5-bis[(4-methoxyphenyl)oxycarbonyl]styrene} in ultrathin films

Crystallization and melting of a poly(ethylene oxide) (PEO) diblock copolymer containing a tablet-like block of poly{2,5-bis[(4-methoxyphenyl)oxycarbonyl]styrene} (PMPCS) in ultrathin films have been studied using atomic force microscopy (AFM) coupled with a hot stage. The PEO and PMPCS block possess the number-average molecular weights (M_n) of 5300 and 2100 g/mol, respectively. The ultrathin films on the mica and glow-discharged carbon surfaces were obtained by static dilute solution casting at room temperature. Isothermal melt crystallization from ultrathin films always leads to flat-on lamellae. Selective area electron diffraction (SAED) experiments have demonstrated that the PEO blocks crystallize with a monoclinic structure identical to that of homo-PEO and the chain direction is perpendicular to the substrate. At T_c<44 °C, the monolayer crystals are dendrites. At T_c>48 °C, square-shaped crystals are formed with the (100) and (020) planes as the crystal edges. At 44 °C≤T_c≤48 °C, an intermediate monolayer morphology is observed. The monolayer thickness increases monotonically with increasing T_c. At the same T_c, the monolayer lamellae with the top and bottom amorphous layers contacting with the atmosphere and the substrate possess a significantly larger overall thickness than the long period of the crystals in bulk. For the spiral terraces induced by screw dislocation, the thickness of each terrace is close to that of the monolayer formed at the same T_c, and their melting is mainly determined by the terrace thickness.     

Thermal, spectroscopy, and morphological studies on polymorphic crystals in poly(heptamethylene terephthalate)

Polymorphism and its influential factors in poly(heptamethylene terephthalate) (PHepT) were probed using differential scanning calorimetry (DSC), Fourier-transform infrared (FTIR) spectroscopy, and wide angle X-ray diffraction (WAXD). PHepT exhibits two crystal types (α and β) upon crystallization at various isothermal melt-crystallization temperatures (T_cs) by quenching from different T_maxs (maximum temperature above T_m for melting the original crystals). Melt-crystallized PHepT with either initial α- or β-crystal by quenching from T_max lower than 110 °C leads to higher fractions of α-crystal, but crystallization from T_max higher than 140 °C leads to higher fractions of β-crystal. In addition to T_max, polymorphism in PHepT is also influenced by crystallization temperature (T_c = 25-75 °C). When PHepT is melt-crystallized from a high T_max = 150 °C (completely isotropic melt), it shows solely β crystal for higher T_c, and solely the α-crystal for T_c < 25 °C; in-between T_c = 25 and 35 °C, mixed fractions of both α- and β-crystals. However, by contrast, when PHepT is melt-crystallized from a lower T_max = 110 °C, it shows α-crystal only at all T_cs, high or low.     

Composition dependence of crystallized lamellar morphology formed in crystalline-crystalline diblock copolymers

We have investigated the crystallized morphology formed at each temperature T_c (20 °C ≤ T_c ≤ 45 °C) in double crystalline poly(?-caprolactone)-block-polyethylene (PCL-b-PE) copolymers as a function of composition (or volume fraction of PE blocks ?_PE) by employing small-angle X-ray scattering (SAXS) and differential scanning calorimetry (DSC) techniques. When PCL-b-PE with ?_PE ≤ 0.58 was quenched from a microphase-separated melt into T_c, the crystallization of PE blocks occurred first to yield an alternating structure consisting of thin PE crystals and amorphous PE + PCL layers (PE lamellar morphology) followed by the crystallization of PCL blocks, where we can expect a competition between the stability of the PE lamellar morphology (depending on ?_PE) and PCL crystallization (on T_c). Two different morphologies were formed in the system judging from a long period. That is, the PCL block crystallized within the existing PE lamellar morphology at lower T_c (<30 °C) to yield a double crystallized alternating structure while it crystallized by deforming or partially destroying the PE lamellar morphology at higher T_c (>35 °C) to result in a significant increase of the long period. However, the temperature at which the morphology changed was almost independent of ?_PE. For PCL-b-PE with ?_PE ≥ 0.73, on the other hand, the morphology after the crystallization of PE blocks was preserved at every T_c investigated.     

Thermally- and solvent-induced crystallization kinetics of syndiotactic polystyrene viewed from time-resolved measurements of infrared spectra at the various temperatures (1) estimation of glass transition temperature shifted by solvent absorption

syndiotactic Polystyrene (sPS) glass crystallizes into the α form when it is heated above the glass transition temperature (T_g, about 100 °C). sPS can be crystallized also into the δ form in the solvent atmosphere at room temperature. In order to trace the structural evolution process, the time-resolved infrared spectral measurements have been performed in the isothermal crystallization from the glass to α form and in the solvent-induced crystallization from the glass to δ form at the various temperatures. Absorbance of crystallization-sensitive infrared bands was plotted against time, from which the crystallization kinetics were analyzed on the basis of Avrami equation: X(t)=1−exp[−(kt)ⁿ] where X is a normalized crystallinity, n is an index, k is a rate constant, and t is a time. The isothermal crystallization was investigated also by carrying out the temperature jump experiment of DSC thermograms, giving almost the same results as the infrared spectral measurements. The Avrami index n was 2-5 depending on the crystallization temperature (T_c). The k was also dependent on the T_c, about 10⁻¹-10⁻⁴ s⁻¹ and could be fitted reasonably by the equation of crystallization kinetics. An extrapolation of the k vs T_c plot to the negligibly small k value allowed us to predict the temperature at which no crystallization should occur, ca. 100 °C, in good agreement with the observed T_g value. On the other hand, the solvent-induced crystallization was investigated for the first time at the various temperatures from 50 to 9 °C by the time-resolved measurement of infrared spectra. Compared with the experiment at room temperature, the crystallization was highly accelerated at 40-50 °C, while the crystallization rate was reduced remarkably at such a low temperature as 9 °C. The time dependence of infrared absorbance was analyzed for the crystallization-sensitive bands on the basis of Avrami equation as the first approximation, although the crystallization mechanism was more complicated than the isothermal crystallization case. The logarithm of the k value was found to change almost linearly with temperature and an extrapolation to infinitesimally small k value gave a T_g of about −15 °C. That is to say, the glass transition temperature was estimated to shift remarkably from 100 to −15 °C by absorbing solvent molecules or by a plasticizing effect.     

A comparison of crystallization behavior for melt and cold crystallized poly (l-Lactide) using rapid scanning rate calorimetry

A unique rapid scanning rate differential scanning calorimeter is used to examine the differences in melt and cold crystallized poly (l-Lactide) (PLLA), a biodegradable semi-crystalline polymer. After isothermal melt and cold crystallization at various temperatures, both melt and cold crystallized PLLA are characterized by similar melting temperatures (T_m) and exhibit multiple melting behavior on heating at 500 °C/min. However, cold crystallization results in a higher degree of crystallinity (w_c) compared to melt crystallization. While the overall amorphous fraction is higher for melt crystallization, the mobile amorphous fraction (w_a) is found to be higher for cold crystallization. The rigid amorphous fraction (w_raf) in PLLA is determined to be higher for melt crystallization than for cold crystallization at almost all temperatures. The higher values of w_raf also appear to result in higher values of the glass transition temperature (T_g) for melt crystallized samples due to a reduction in mobility of amorphous phase. These dramatic differences depending on whether the material is brought to the crystallization temperature from the melt or the glassy state, could have profound implications for processing and optimizing the properties of PLLA.     

Crystallization behavior of poly(l-lactic acid)

The crystallization behavior of poly(l-lactic acid) was studied in the range of 80-160 °C. The peak crystallization time (τ_p) was defined and obtained from the crystallization isotherm measured with a differential scanning calorimeter (DSC). Isothermal crystallization temperature (T_c) dependence of log(τ_p) discretely changed at 113 °C (= T_b). The linear growth rate of spherulite, G, was measured with a polarizing microscope. The T_c dependence of G and the size of the spherulite also discretely changed at T_b. Crystal structures for samples isothermally crystallized at temperatures which were higher and lower than T_b were orthorhombic (α-form) and trigonal (β-form), respectively. The discrete change of the crystallization behavior was explained by the formation of different crystal.     

Poly(trimethylene terephthalate) copolyester containing ethylene glycol moieties. Part 1: Crystallization kinetics and melting behavior

A copolyester was characterized to have 91 mol% trimethylene terephthalate unit and 9 mol% ethylene terephthalate unit in a random sequence by using ¹³C NMR. Differential scanning calorimeter (DSC) was used to investigate the isothermal crystallization kinetics in the temperature range (T_c) from 180 to 207 °C. The melting behavior after isothermal crystallization was studied by using DSC and temperature modulated DSC (TMDSC). The exothermic behavior in the DSC and TMDSC curves gives a direct evidence of recrystallization. No exothermic flow and fused double melting peaks at T_c = 204 °C support the mechanism of different morphologies. The Hoffman-Weeks linear plot gave an equilibrium melting temperature of 236.3 °C. The kinetic analysis of the growth rates of spherulites and the morphology change from regular to banded spherulites indicated that there existed a regime II → III transition at 196 °C.     

Crystallization, melting, and morphology of a poly(ethylene oxide) diblock copolymer containing a tablet-like block of poly{2,5-bis[(4-methoxyphenyl)oxycarbonyl]styrene}

A poly(ethylene oxide) diblock copolymer containing a short block of poly{2,5-bis[(4-methoxyphenyl)oxycarbonyl]styrene} (PEO-b-PMPCS) has been successfully synthesized via atom transfer radical polymerization (ATRP) method. The number average molecular weights (M_n) of the PEO and PMPCS blocks are 5300 and 2100 g/mol, respectively. Combining the techniques of differential scanning calorimetry (DSC), optical microscopy (OM), wide angle X-ray diffraction (WAXD), and small angle X-ray scattering (SAXS), we have found that the PMPCS blocks, which are tablet-like, can significantly affect the crystallization and melting of the diblock copolymer. The sample studied can form the crystals with a monoclinic crystal structure identical to that of the homo-PEO. The melting temperature (T_m) of the diblock copolymer increases monotonically with crystallization temperature (T_c), which is remarkably similar to the behavior of long period. On the basis of Gibbs-Thomson relationship, the equilibrium T_m of the diblock copolymer is estimated to be 65.4 °C. In a wide undercooling (ΔT) range (14 °C<ΔT<30 °C), the isothermal crystallization leads to square-shaped crystals. The PEO-b-PMPCS crystallization exhibits a regime I→II transition at ΔT of 19 °C. The PEO blocks are non-integral folded (NIF) in the crystals, and the PMPCS blocks rejected to lamellar fold surfaces prevent the NIF PEO crystals from transforming to integral folded (IF) ones. Furthermore, the PMPCS tablets may adjust their neighboring positions up or down with respect to the lamellar surface normal, forming more than one PMPCS layer to accompany the increase in the PEO fold length with increasing T_c.     

The nucleation and crystallization of MgO-B2O3-SiO2 glass

The nucleation and crystallization of MgO-B₂O₃-SiO₂ (MBS) glass were studied by means of a non-isothermal, thermal analysis technique, X-ray diffraction and scanning electron microscopy. The temperature range of the nucleation and the temperature of the maximum nucleation rate for MBS glass were determined from the dependences of the inverse temperature at the DSC peak (1/T_p) and the maximum intensity of the exothermic DSC crystallization peak ((δT)_p) on the nucleation temperature (T_n). For MBS glass the nucleation occurred at 600-750 °C, with the maximum nucleation rate at 700 °C, whereas the nucleation and crystal growth processes overlapped at 700 °C < T ≤ 750 °C. The analyses of the non-isothermal data for the bulk MBS glass using the most common models (Ozawa, Kissinger, modified Kissinger, Ozawa-Chen, etc.) revealed that the crystallization of Mg₂B₂O₅ was three-dimensional bulk with a diffusion-controlled crystal growth rate, that n = m = 1.5 and that the activation energy for the crystallization was 410-440 kJ/mol.     

Blends of isotactic and atactic poly(lactide). I. Effects of mixing ratio of isomers on crystallization of blends from melt

Poly(L -lactide) (PLLA) and poly(DL -lactide) (PDLLA) blends were crystallized from the melt and their crystallization behaviors and morphologies were investigated using differential scanning calorimetry and polarizing microscopy. PLLA could crystallize from the melt in the presence of PDLLA when the PLLA content in the blend (X_PLLA) was higher than 0.2. Spherulites were formed when X_PLLA was between 0.6 and 1. The radius of the spherulites formed in the presence of PDLLA was larger than that of nonblended PLLA, probably because the coexisting PDLLA reduced the density of nuclei for the spherulites. Small crystallite assemblies were formed when X_PLLA was between 0.2 and 0.5, and grew spherically from the nuclei when X_PLLA was 0.5. Isotactic PLLA and atactic PDLLA seem to be miscible prior to crystallization, but PDLLA will be trapped between the lamellae of the spherulites or spherulitic assemblies during crystallization. PDLLA delayed the induction for crystallization of the blends. In spite of different morphology, the melting temperature and crystallinity of PLLA remained virtually constant when annealing was conducted for 600 min at X_PLLA between 0.5 and 1. © 1995 John Wiley & Sons, Inc.