Crystallization and phase separation kinetics in blends of linear low-density polyethylene copolymers

We have systematically studied the crystallization and liquid-liquid phase separation (LLPS) kinetics in statistical copolymer blends of poly(ethylene-co-hexene) (PEH) and poly(ethylene-co-butene) (PEB) using primarily optical microscopy. The PEH/PEB blends exhibit upper critical solution temperature (UCST) in the melt and crystallization temperature below the UCST. The time evolution of the characteristic morphology for both crystallization and LLPS is recorded for blends at various compositions and following a quench from initial homogenous melts at high temperature to various lower temperatures. The crystallization kinetics is measured as the linear growth rate of the super structural crystals, whereas the LLPS kinetics is measured as the linear growth rate of the characteristic length of the late-stage spinodal decomposition. The composition dependence crystallization kinetics, G, shows very different characteristics at low and high isothermal crystallization temperature. Below 116 °C, G decreases with increasing PEB content in the blend, implying primarily the composition effect on materials transport; whereas at above 116 °C, G shows a minimum at about the critical composition for LLPS, implying the influence of the LLPS. On the other hand, LLPS kinetics at 130 °C is relatively invariant at different compositions in the two-phase regime. The length scale at which domains are kinetically pinned, however, depends strongly on the composition. In a blend near critical composition, a kinetics crossover is shown to separate the crystallization dominant and phase separation dominant morphology as isothermal temperature increases.     

Structural and morphological development in poly(ethylene-co-hexene) and poly(ethylene-co-butylene) blends due to the competition between liquid-liquid phase separation and crystallization

Isothermal crystallization behavior of poly(ethylene-co-hexene) (PEH) and the 50/50 blend (H50) of PEH with amorphous poly(ethylene-co-butylene) (PEB) was studied by time-resolved synchrotron simultaneous small-angle X-ray scattering/wide-angle X-ray diffraction (SAXS/WAXD) techniques and optical microscopy (OM). The X-ray study revealed the changes of structural and morphological variables such as the scattering invariant, crystallinity and lamellar long period, et al. In H50, the lamellar morphology was found to be dependent on competition between liquid-liquid phase separation (LLPS) and crystallization. At high temperature, LLPS becomes dominating, resulted in crystallization of PEH with minimal influence of PEB. At low temperature, LLPS is suppressed, PEB component shows obvious influence on PEH crystallization, PEB is thought to be partially included into PEH lamellar stacks and PEH-PEB co-crystallization is unlikely, however, possible. Optical microscopy was used to monitor crystal nucleation and growth rates in PEH and H50, providing complementary information about the effect of temperature on LLPS and crystallization. Real-space lamellar morphologies in PEH and H50 were characterized by atomic force microscopy (AFM), PEH exhibited sheaf-like spherulites while H50 exhibited hedrites. Overall, the competition between LLPS and crystallization in H50 blend influences the structural and morphological development. Controlling the interplay between LLPS and crystallization of PEH/PEB blends, it is possible to control the structure and morphology as practically needed.     

Coupling effects of spinodal decomposition and crystallization on mechanical properties of polyolefin blends

The influences of preferentially occurred liquid-liquid phase separation (LLPS) and following crystallization processes on the mechanical properties of statistical copolymer blends of poly(ethylene-co-hexene) (PEH) and poly(ethylene-co-butene) (PEB) have been investigated in detail through tensile deformation tests with a relatively high extension rate to avoid the effect of interfacial properties of the blends. Crystallinity and lamellar thickness of the samples are estimated by using the wide-angle X-ray diffraction and small-angle X-ray scattering techniques, respectively. The tensile modulus and yield stress are found to increase with LLPS time up to 6 h, but decrease afterwards, under the conditions of temperature of 120 °C and isothermal crystallization time of 10 min. It is considered that the instantaneous tensile properties are substantially largely affected by the much perfect lamellar structures formed during crystallization with a long time prior LLPS step. This finding is further experimentally substantiated by the scanning electron microscope observation. Whereas the strain-hardening modulus described by a simple neo-Hookean relation increases with LLPS time and reaches a plateau after 6 h, which can be accounted for by the cooperation effect between amorphous entanglement density, insensitive to LLPS time, and crystallinity redistribution. The similarity of the results observed on the blends experiencing the spinodal decomposition (SD) process supports that the redistribution of crystallizable components contributes to the tensile stress increase, which is primarily controlled by the development of LLPS process. This simple relationship gives us a new insight of what controls the mechanical properties of the phase separated polymer blends and of how we might be able to predict the mechanical properties of as yet unmixed polymer pairs.     

Effects of spinodal decomposition on mechanical properties of a polyolefin blend from high to low strain rates

The effects of spinodal decomposition, a typical type of liquid-liquid phase separation (LLPS), on the mechanical properties of a pretreated statistical copolymer blend of poly(ethylene-co-hexene) (PEH) and poly(ethylene-co-butene) (PEB) were characterized by tensile testing under different strain rates. An important finding was that the strain rate and the crystallization temperature had to be considered as independent variables in analyzing the effects of spinodal decomposition on the tensile behaviors. At the high strain rate, the stress-strain curves kept irrespective of LLPS time, in which the interfacial relaxation between phase domains could not be detected, except the case crystallizing at 120 °C for 10 min. This was explained in terms of the distribution of the crystals elaborated by differential scanning calorimetry (DSC) results. However, when a relatively low strain rate was employed, a clear deterioration of tensile properties with LLPS proceeding was observed for the cases with low crystallization temperature because of its detection ability for large scale structural information, such as the phase boundary; unexpectedly, the effect of LLPS on the tensile properties was found to disappear in the high crystallization temperature cases which was due to the cooperative functions of the phase boundary and the internal structures of the phase domains. These abundant results provided a novel and indispensable instruction for the processing of polymer blends from the theoretical viewpoint.     

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.     

Ductile-brittle transition controlled by isothermal crystallization of isotactic polypropylene and its blend with poly(ethylene-co-octene)

The correlation between crystalline morphology development and tensile properties of isotactic polypropylene (iPP) and its blend with poly(ethylene-co-octene) (PEOc) was investigated to study the ductile-brittle transition (DBT) in fracture modes. The sample processing strategy and the scientific observations have never been reported previously. The samples were first isothermally crystallized at 130 °C, 123 °C or 115 °C for a wide range of crystallization times, and then quenched to 35 °C for characterization. It was found that the crystallization conditions including crystallization temperature and time governed the crystalline morphology and even the tensile properties of iPP and the iPP/PEOc (80/20) blend. The lower the crystallization temperature, the shorter the crystallization time was needed for the occurrence of DBT, and the sharper the transition would be. The addition of the elastomer component delayed the DBT occurrence for the iPP/PEOc blend in terms of the crystallization time, owing to the fact that the existence of PEOc domains between the iPP lamellar stack regions or at the iPP spherulitic boundaries enhanced the ductility of the blend. The X-ray diffraction results displayed the oriented and destroyed crystalline structure characterizing the ductile fracture, while unoriented structure describing the brittle failure. The DBT is closely related to the crystal perfection, and factors such as the crystallization temperature and time and the compositions have been proven to be significant variables in determining the DBT occurrence.     

Liquid-liquid phase separation and crystallization behavior of poly(ethylene terephthalate)/poly(ether imide) blend

The liquid-liquid (L-L) phase separation and crystallization behavior of poly(ethylene terephthalate) (PET)/poly(ether imide) (PEI) blend were investigated with optical microscopy, light scattering, and small angle X-ray scattering (SAXS). The thermal analysis showed that the concentration fluctuation between separated phases was controllable by changing the time spent for demixing before crystallization. The L-L phase-separated specimens at 130 °C for various time periods were subjected to a temperature-jump of 180 °C for the isothermal crystallization and then effects of L-L phase separation on crystallization were investigated. The crystal growth rate decreased with increasing L-L phase-separated time (t_s). The slow crystallization for a long t_s implied that the growth path of crystals was highly distorted by the rearrangement of the spinodal domains associated with coarsening. The characteristic morphological parameters at the lamellar level were determined by the correlation function analysis on the SAXS data. The blend had a larger amorphous layer thickness than the pure PET, indicating that PEI molecules in the PET-rich phase were incorporated into the interlamellar regions during crystallization.     

Interplay of crystallization and liquid-liquid phase separation in polyolefin blends: A thermal history dependence study

The kinetic interplay between crystallization and liquid-liquid phase separation (LLPS) in random copolymer blends of poly(ethylene-ran-hexene) (PEH) and poly(ethylene-ran-butene) (PEB) has been studied using optical microscopy. Morphologies of blends gone through three different thermal histories are compared: (1) single-quench (SQ), a homogeneous melt quickly cooled to isothermal crystallization temperatures (T_cry), (2) double-quench (DQ), a homogeneous melt quickly cooled to an intermediate temperature (T_lps) between binodal and equilibrium melting temperature (T_m⁰) and stored for a period of time and then cooled to T_cry, and (3) cyclic-quench (CQ), a homogeneous melt quickly cooled to T_lps and stored for a period of time, then gone through four cycles of crystallization and remelting. Comparing DQ morphologies to SQ ones, both crystal growth rate and nucleation density in the former are affected by prior LLPS. A scaling argument has been provided to partially account for the observed phenomena. In CQ, characteristic lengths of secondary features induced by crystallization depend strongly on the overall PEH composition, whereas are insensitive to temperature cycling. The contrast of large domains becomes more prominent upon cyclic crystallization and remelting. On the other hand, primary LLPS domains coarsen with CQ while loosing the contrast.     

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.     

Dynamic interplay between phase separation and crystallization in a poly(?-caprolactone)/poly(ethylene glycol) oligomer blend

We have investigated the crystallization effect on the phase separation of a poly(?-caprolactone) and poly(ethylene glycol) oligomer (PCL/PEGo) blending system using simultaneous small-angle light scattering and differential scanning calorimetry (SALS/DSC) as well as simultaneous small-angle X-ray scattering (SAXS), wide-angle X-ray scattering (WAXS), and DSC (SAXS/WAXS/DSC). When the PCL/PEGo system, of a weight ratio of 7/3, is quenched from a melt state (160 °C) to temperatures below the spinodal point and the melting temperature of PCL (63 °C), the structural evolution observed exhibits characteristics of (I) early stage of spinodal decomposition (SD), (II) transient pinning, (III) crystallization-induced depinning, and (IV) diffusion-limited crystallization. The time-dependent scattering data of SALS, SAXS and WAXS, covering a wide range of length scale, clearly show that the crystallization of PCL intervenes significantly in the ongoing viscoelastic phase separation of the system, only after the early stage of SD. The effect of preordering before crystallization revives the structural evolution pinned by the viscoelastic phase separation. The growth of SAXS intensity during the preordering period conforms to the Cahn-Hilliard theory. In the later stage of the phase separation, the PCL-rich matrix, of spherulite crystalline domains developed due to the faster crystallization kinetics, traps the isolated PEGo-rich domains of a slower viscoelastic separation.     

Structure formation in poly(ethylene terephthalate) upon annealing as revealed by microindentation hardness and X-ray scattering

The micromechanical properties (microindentation hardness, H, elastic modulus, E) of poly(ethylene terephthalate) (PET), isothermally crystallized at various temperatures (T_a) from the glassy state are determined to establish correlations with thermal properties and nanostructure. Analysis of melting temperature and crystal thickness derived from the interface distribution function analysis of SAXS data reveals that for T_a<190 °C the occurrence of two lamellar stack populations prevails whereas for samples annealed at T_a>190 °C a population of lamellar stacks with a unimodal thickness distribution emerges. The H and E-values exhibit a tendency to increase with the degree of crystallinity. The results support a correlation E/H∼20 in accordance with other previously reported data. The changes of microhardness with annealing temperature are discussed in terms of the crystallinity and crystalline lamellar thickness variation. Unusually high hardness values obtained for PET samples crystallized at T_a=190 °C are discussed in terms of the role of the rigid amorphous phase which offers for the hardness of amorphous layers constrained between lamellar stacks a value of H_a∼150 MPa. On the other hand, for T_a=240 °C the decreasing H-tendency could be connected with the chemical degradation of the material at high temperature.     

Linear low-density polyethylene/poly(ethylene-ran-butene) elastomer blends: Miscibility and crystallization behavior

The phase and crystallization behavior of the blends consisting of LLDPE (0.7 mol% hexene copolymer) and PEB (26 mol% butene copolymer) have been investigated using optical microscopy (OM), differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD). The blends exhibited an upper critical solution temperature of 162°C. The solubility parameter analysis showed that the solubility parameter of LLDPE decreased more rapidly than that of PEB with temperature. However, due to the slow kinetics of phase separation, at lower crystallization temperatures, the crystallization and melting behavior of LLDPE mainly reflected the miscibility between LLDPE and PEB. Crystallization from the two-phase state could present two crystallization peaks. PEB didnt change the crystal cell unit and crystallinity of LLDPE, but changed its distribution of lamellar thickness or crystal perfection. The dilute effect of PEB also changed the overall nature of the nucleation and growth process of LLDPE. The equilibrium melting temperature in this blend could be obtained by the Hoffman-Weeks method, and comparing with that of the pure LLDPE, it was reduced and kept relatively constant in the bi-phase state. The phase diagram made up of the LLPS boundary, equilibrium melting temperatures and melting temperatures observed may be better to indicate the phase and crystallization behavior of LLDPE/PEB blends.     

Nanocomposites of poly(vinylidene fluoride) with organically modified silicate

We report a study of the impact of cold crystallization on the structure of nanocomposites comprising poly(vinylidene fluoride) (PVDF) and Lucentite STN™ organically modified silicate (OMS). Nanocomposites were prepared from solution over a very wide composition range, from 0.01 to 20% OMS by weight. Thermal preparation involved cold crystallization at 145 °C of quenched, compression-molded plaques. Static and real-time wide and small angle X-ray scattering (WAXS, SAXS), Fourier transform infrared spectroscopy (FTIR), and differential scanning calorimetry (DSC) were used to investigate the crystalline phase of PVDF. For OMS content greater than 0.50 wt%, WAXS studies show that that the silicate gallery spacing increases modestly in the nanocomposites compared to neat OMS film, indicating a level of polymer intercalation.Using Gaussian peak fitting of WAXS profiles, we determine that the composition range can be divided into three parts. First, for OMS greater than 0.5 wt%, alpha phase fraction, ?_alpha, is insignificant (?_alpha∼0-0.01). Second, at the intermediate range, for OMS between 0.5 wt% down to 0.025 wt%, beta phase dominates and the beta fraction, ?_beta, is related to alpha by ?_beta>?_alpha. Third, below 0.025 wt% OMS, alpha dominates and ?_alpha>?_beta. The ability of small amounts of OMS (≥0.025 wt%) to cause beta crystal domination is remarkable. Overall, crystallinity index (from the ratio of WAXS crystal peak area to total area) ranges from about 0.36 to 0.51 after cold crystallization. Real-time WAXS studies during heating of initially cold crystallized nanocomposites show that there is no inter-conversion between the alpha and beta phase PVDF crystals, where these crystals coexist at room temperature. While all samples showed a strong SAXS Bragg peak, indicating existence of two-phase lamellar stacks, the sample containing predominantly beta phase had poorly correlated lamellar stacks, compared to samples containing predominantly alpha phase.     

Structural changes in non-isothermal crystallization process of melt-cooled polyoxymethylene. [I] Detection of infrared bands characteristic of folded and extended chain crystal morphologies and extraction of a lamellar stacking model

Structural change in the crystallization process of polyoxymethylene (POM) cooled from the molten state has been investigated by the measurements of infrared spectra and small-angle (SAXS) and wide-angle X-ray scatterings (WAXS). When the melt was cooled slowly, the infrared bands characteristic of a folded chain crystal (FCC) were observed to appear around 156 °C. Below 140 °C, the infrared bands intrinsic of an extended chain crystal (ECC) were detected to increase in intensity. In the SAXS measurement, the peak (L₁) corresponding to a stacked lamellar structure with the long period of ca. 14 nm was found to grow in parallel to the growth of infrared FCC bands. In the temperature region of the observation of infrared ECC bands, the new peak (L₂) of long period of ca. 7 nm was found to appear and the intensity exchange occurred between the L₁ and L₂ peaks, that is, with decreasing temperature the L₂ peak increased the intensity and its height became comparable to the L₁ peak height. By combining all these experimental data, a model to illustrate the formation process of lamellar stacking structure has been presented. After the appearance of stacked lamellar structure of 14 nm long period from the melt, new lamellae are created in between the already existing lamellae and the long period changes to the half value, 7 nm. Some of molecular chain stems in a lamella are speculated to pass through the adjacent lamellae to form a bundle of fully extended taut tie chains, which are considered to be observed as the infrared bands characteristic of ECC morphology. Although the POM samples used in this experiment may contain small amount of low-molecular-weight macrocyclic component, it was not plausible judging from the various experimental data to assign the secondarily observed 7 nm SAXS peak to the repeating period originating from the stacked structure of macrocyclic compounds.     

Spherulite morphology and crystallization behavior of poly(trimethylene terephthalate)/poly(ether imide) blends

The spherulite morphology and crystallization behavior of poly(trimethylene terephthalate) (PTT)/poly(ether imide) (PEI) blends were investigated with optical microscopy (OM), small-angle light scattering (SALS), and small-angle X-ray scattering (SAXS). Thermal analysis showed that PTT and PEI were miscible in the melt over the entire composition range. The addition of PEI depressed the overall crystallization rate of PTT and affected the texture of spherulites but did not alter the mechanism of crystal growth. When a 50/50 blend was melt-crystallized at 180 °C, the highly birefringent spherulite appeared at the early stage of crystallization (t < 20 min). After longer times, the spherulite of a second form was developed, which exhibited lower birefringence. The SALS results suggested that the observed birefringence change along the radial direction of the spherulite was mainly due to an increase in the orientation fluctuation of the growing crystals as the radius of spherulite increased. The lamellar morphological parameters were evaluated by a one-dimensional correlation function analysis. The amorphous layer thickness showed little dependence on the PEI concentration, indicating that the noncrystallizable PEI component resided primarily in the interfibrillar regions of the growing spherulites.     

Lamellar morphologies and crystal stability of syndiotactic polystyrene in α-crystalline form

Syndiotactic polystyrene (sPS) samples melt-crystallized into neat α″ hexagonal modifications were prepared at various temperatures thoroughly for the extensive morphological studies. Lamellar morphologies of the as-prepared sPS samples were investigated using small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). Absence of a discernible scattering peak was found for SAXS conducted at room temperature, resulting from a negligible difference in the electron density between the lamellar and amorphous layers. To enhance the scattering contrast and strength, SAXS was carried out at 180 °C to obtain more reliable morphological parameters. Due to the broad thickness distribution of morphological features as revealed from the TEM observations, a pronounced variation is found for the long periods derived from the Bragg's law, one-dimensional correlation function, and interface distribution function of the SAXS data. In addition, relatively irregular packing of lamellar stacks with short lateral dimensions was detected in the as-prepared α″-form sPS, leading to the absence of spherulitic birefringence under polarized optical microscopy. Based on the interface distribution function analysis of the SAXS intensity profiles, the lamellar thicknesses were estimated. Using the Gibbs-Thomson relation, the ratio of fold surface free energy (σ_e) to the fusion enthalpy for α″-form sPS was successfully deduced to be ca. 0.057 nm, which is lower than that of β′-form sPS, ca. 0.12 nm. On this basis, a comparison of critical lamellar thicknesses for α″- and β′-form sPS at various crystallization temperatures is provided and the crystal stability associated with the lamellar thickness is discussed as well.     

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.     

Amorphous phase and crystalline morphology in blend of two polymorphic polyesters: Poly(hexamethylene terephthalate) and poly(heptamethylene terephthalate)

Crystalline/crystalline blends of two polymorphic aryl-polyesters, poly(hexamethylene terephthalate) (PHT) and poly(heptamethylene terephthalate) (PHepT), were prepared and the crystallization kinetics, polymorphism behavior, spherulite morphology, and miscibility in this blend system were probed using polarized-light optical microscopy (POM), differential scanning calorimetry (DSC), temperature-resolved wide-angle X-ray diffraction (WAXD), and small angle X-ray scattering (SAXS). The PHT/PHepT blends of all compositions were proven to be miscible in the melt state or quenched amorphous glassy phase. Miscibility in PHT/PHepT blend leads to the retardation in the crystallization rate of PHT; however, that of PHepT increases, being attributed to the nucleation effects of PHT crystals which are produced before the growth of PHepT crystals. In the miscible blend of polymorphic PHT with polymorphic PHepT, the polymorphism states of both PHT and PHepT in the blend are influenced by the other component. The fraction of the thermodynamically stable β-crystal of PHT in the blend increases with increasing PHepT content when melt-crystallized at 100 °C. In addition, when blended with PHT, the crystal stability of PHepT is altered and leads to that the originally polymorphic PHepT exhibits only the β-crystal when melt-crystallized at all T_c's. Apart from the noted polymorphism behavior, miscibility in the blend also shows great influence on the spherulite morphology of PHT crystallized at 100 °C, in which the dendritic morphology corresponding to the β-crystal of PHT changes to the ring-banded in the blend with higher than 50 wt% PHepT. In blends of PHT/PHepT one-step crystallized at 60 °C, PHepT is located in both PHT interlamellar and interfibrillar region analyzed using SAXS, which further manifests the miscibility between PHT and PHepT.     

Single crystals morphology of biodegradable double crystalline PLLA-b-PCL diblock copolymers

The morphology of solution grown single crystals of a series of double crystalline diblock copolymers derived from l-lactide and ?-caprolactone has been investigated by means of transmission electron microscopy. The copolymers had a variable composition with a poly(l-lactide) weight percentage that ranged between 81 and 10%. All samples had a low polydispersity index (1.4-1.1) and a similar number average molecular weight (20,000-35,000 g/mol).Bulk crystallization and melting behaviour of diblock copolymers were evaluated by DSC and the results demonstrated the double crystalline nature of the samples. Fractionated crystallization clearly occurred in copolymers having an intermediate composition.Isothermal crystallizations were performed in dilute n-hexanol solutions at temperatures that ranged between 80 and 50 °C. Crystal morphologies were dependent on the crystallization temperature and even on the composition. Thus, the inability of poly(?-caprolactone) (PCL) blocks to crystallize between 80 and 70 °C rendered lozenge, truncated and spindle-shaped crystals associated to the poly(l-lactide) (PLLA) block. These usually had thicker edges due to PLLA overgrowths that mainly took place in their periphery. However, an overgrowth of irregular PCL crystals during subsequent cooling and crystallization at room temperature was also detected. Complex morphologies constituted by lamellar crystals of both PCL and PLLA blocks were developed at intermediate temperatures (70-65 °C), whereas elongated hexagonal morphologies mainly associated to the PCL block were detected at the lowest crystallization temperature. In general, electron diffraction patterns showed for all samples’ reflections associated to both poly(?-caprolactone) and poly(l-lactide) (α-form) crystals. The relative intensity between the two types of reflections varied according to the copolymer composition.     

Characterization, crystallization kinetics and melting behavior of poly(ethylene succinate) copolyester containing 5 mol% trimethylene succinate

Copolyester was synthesized and characterized as having 94.4 mol% ethylene succinate units and 5.6 mol% trimethylene succinate units in a random sequence as revealed by NMR. Differential scanning calorimeter (DSC) was used to investigate the isothermal crystallization kinetics of this copolyester in the temperature range (T_c) from 30 to 80 °C. The melting behavior after isothermal crystallization was studied by using DSC and temperature modulated DSC (TMDSC) by varying the T_c, the heating rate and the crystallization time. DSC and TMDSC curves showed triple melting peaks. The melting behavior indicates that the upper melting peaks are primarily due to the melting of lamellar crystals with different stabilities. A small exothermic curve between the main melting peaks gives a direct evidence of recrystallization. As the T_c increases, the contribution of recrystallization gradually decreases and finally disappears. The Hoffman-Weeks linear plot gave an equilibrium melting temperature of 108.3 °C. The kinetic analysis of the spherulitic growth rates indicated that a regime II → III transition occurred at ∼65 °C.