Liquid phase behavior and its effect on crystalline morphology in the extruded poly(ethylene terephthalate)/poly(ethylene-2,6-naphthalate) blend

Morphology in an extruded poly(ethylene terephthalate)/poly(ethylene-2,6-naphthalate) was investigated using time-resolved light scattering, optical microscope and small-angle X-ray scattering. During annealing at 280 °C, the domain structure via spinodal decomposition preceded, the transesterification followed, and then the transesterification between the two polyesters induced the dissolution of the liquid-liquid (L-L) phase separation, i.e. the homogenization. The annealed specimen for various time periods (t_s) at 280 °C was subjected to a temperature-drop to 120 °C for the isothermal crystallization and then the effects of liquid phase morphology on crystallization was investigated. With t_s, the H_ν (cross-polarization) light scattering patterns exhibited the dramatic change from a four-leaf clover pattern with maximum intensity at azimuthal angle 45° (×-type scattering pattern) to a diffuse pattern of circular symmetry and then a four-leaf clover pattern with maximum intensity at azimuthal angles 0 and 90° (+-type scattering pattern). This suggests that the crystalline structure depends on the level of the block and/or random copolymer produced by the transesterification during annealing. The H_ν scattering patterns reflected differences in the principle polarizability of the crystalline lamellae with respect to the spherulitic radius. On the other hand, the long period L_B, an average distance between two adjacent crystalline lamellae, increased with t_s at 280 °C. The dependence of L_B on t_s was explained by the change in the crystallization rate G.     

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.     

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.     

Lamellar morphology of poly(trimethylene terephthalate)/poly(ether imide) blends studied via small‐angle X‐ray scattering

The lamellar morphology of a melt‐miscible blend consisting of poly(trimethylene terephthalate) (PTT) and poly(ether imide) (PEI) prepared by solution precipitation has been investigated by means of optical polarized microscopy (POM) and small angle X‐ray scattering (SAXS). From the observation under POM, it was suggested that PEI was predominantly segregated into the interlamellar and/or interfibrillar regions upon PTT crystallization since the PTT spherulitic morphologies of blends were volume‐filling. From results of SAXS data analysis, a larger amorphous layer thickness was identified in the blends, showing that some PEI was incorporated inside the interlamellar regions after crystallization. Despite the swelling of the amorphous layer, the amorphous layer thickness was relatively independent of the blend composition. It was concluded that amorphous PEI was located in the interlamellar regions of PTT as the weight fraction of PEI (w_PEI) [≤] 0.1, while amorphous PEI was predominantly segregated into the interfibrillar regions of PTT as w_PEI > 0.1, and the extent of interfibrillar segregation increased with increasing w_PEI. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci, 2006     

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.     

Viscoelastic effects on the phase separation in thermoplastics modified cyanate ester resin

A high temperature thermosetting bisphenol-A dicyanate, BADCy was blended with a thermoplastic poly(ether imide) (PEI). The phase separation behavior of the blend was investigated by scanning electron microscopy (SEM) and time resolved light scattering (TRLS). It was found by SEM that the blend with 20 and 25 wt% PEI had a phase inversion structure. The results of TRLS displayed clearly that the phase separation took place according to a spinodal decomposition (SD) mechanism and the evolution of both scattering vector q_m and the maximum scattering intensity I_m followed Maxwell-type relaxation equation. The temperature-dependent relaxation time τ for the blends can be described by the Williams-Landel-Ferry equation. It demonstrated experimentally that the phase separation behaviors in PEI/BADCy blends were affected by viscoelastic effect.     

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.     

Crystallization kinetics and morphology of poly(butylene succinate)/poly(vinyl phenol) blend

Biodegradable crystalline poly(butylene succinate) (PBSU) can form miscible polymer blends with amorphous poly(vinyl phenol) (PVPh). The isothermal crystallization kinetics and morphology of neat and blended PBSU with PVPh were studied by differential scanning calorimetry (DSC), optical microscopy (OM), wide angle X-ray diffraction (WAXD), and small angle X-ray scattering (SAXS) in this work. The overall isothermal crystallization kinetics of neat and blended PBSU was studied with DSC in the crystallization temperature range of 80-88 °C and analyzed by applying the Avrami equation. It was found that blending with PVPh did not change the crystallization mechanism of PBSU, but reduced the crystallization rate compared with that of neat PBSU at the same crystallization temperature. The crystallization rate decreased with increasing crystallization temperature, while the crystallization mechanism did not change for both neat and blended PBSU irrespective of the crystallization temperature. The spherulitic morphology and growth were observed with hot stage OM in a wide crystallization temperature range of 75-100 °C. The spherulitic morphology of PBSU was influenced apparently by the crystallization temperature and the addition of PVPh. The linear spherulitic growth rate was measured and analyzed by the secondary nucleation theory. Through the Lauritzen-Hoffman equation, some parameters of neat and blended PBSU were derived and compared with each other including the nucleation parameter (K_g), the lateral surface free energy (σ), the end-surface free energy (σ_e), and the work of chain folding (q). Blending with PVPh decreased all the aforementioned parameters compared with those of neat PBSU; however, the decrease extent was limited. WAXD result showed that the crystal structure of PBSU was not modified after blending with PVPh. SAXS result showed that the long period of blended PBSU increased, possibly indicating that the amorphous PVPh might reside mainly in the interlamellar region of PBSU.     

Simultaneous liquid–liquid demixing and crystallization and its effect on the spherulite growth in poly(ethylene terephthalate)/poly(ether imide) blends

Poly(ethylene terephthalate) (PET)/poly(ether imide) (PEI) blends were miscible in the melt, but exhibited simultaneous liquid–liquid phase separation and crystallization over a wide range of temperature and composition. The interplay between these two processes is expected to dominate the morphological formation in the blends. In this study, the phase diagram of PET/PEI blend was determined to evaluate the envelop within which liquid–liquid phase separation was operative with crystallization. A UCST phase diagram below 240°C was identified for this system. The effect of liquid–liquid phase separation on the growth of PET spherulites was studied by small-angle light scattering (SALS). Nonlinear spherulite growths were observed for the blends at higher crystallization temperatures of 210°C and 220°C, while the growths were basically linear below 210°C. The nonlinear growth behaviour was discussed based on the competition between spherulite growth and spinodal decomposition.     

Liquid-liquid phase separation in a polyethylene blend monitored by crystallization kinetics and crystal-decorated phase morphologies

A series of polyethylene (PE) blends consisting of a linear high density polyethylene (HDPE) and a linear low density polyethylene (LLDPE) with an octane-chain branch density of 120/1000 carbon was prepared at different concentrations. The two components of this set of blends possessed isorefractive indices, thus, making it difficult to detect their liquid-liquid phase separation via scattering techniques. Above the experimentally observed melting temperature of HDPE, T_m = 133 °C, this series of blends can be considered to be in the liquid state. The LLDPE crystallization temperature was below 50 °C; therefore, above 80 °C and below the melting temperature of HDPE, a series of crystalline-amorphous PE blends could be created. A specifically designed two-step isothermal experimental procedure was utilized to monitor the liquid-liquid phase separation of this set of blends. The first step was to quench the system from temperatures of known miscibility and isothermally anneal them at a temperature higher than the equilibrium melting temperature of the HDPE for the purpose of allowing the phase morphology to develop from liquid-liquid phase separation. The second step was to quench the system to a temperature at which the HDPE could rapidly crystallize. The time for developing 50% of the total crystallinity (t_1/2) was used to monitor the crystallization kinetics. Because phase separation results in HDPE-rich domains where the crystallization rates are increased, this technique provided an experimental measure to identify the binodal curve of the liquid-liquid phase separation for the system indicated by faster t_1/2. The annealing temperature in the first step that exhibits an onset of the decrease in t_1/2 is the temperature of the binodal point for that blend composition. In addition, the HDPE-rich domains crystallized to form spherulites which decorate the phase-separated morphology. Therefore, the crystal dispersion indicates whether the phase separation followed a nucleation-and-growth process or a spinodal decomposition process. These crystal-decorated morphologies enabled the spinodal curve to be experimentally determined for the first time in this set of blends.     

Finite strain behavior of poly(ethylene terephthalate) (PET) and poly(ethylene terephthalate)-glycol (PETG)

Uniaxial and plane strain compression experiments are conducted on amorphous poly(ethylene terephthalate) (PET) and poly(ethylene terephthalate)-glycol (PETG) over a wide range of temperatures (25-110 °C) and strain rates (.005-1.0 s⁻¹). The stress-strain behavior of each material is presented and the results for the two materials are found to be remarkably similar over the investigated range of rates, temperatures, and strain levels. Below the glass transition temperature (θ_g=80 °C), the materials exhibit a distinct yield stress, followed by strain softening then moderate strain hardening at moderate strain levels and dramatic strain hardening at large strains. Above the glass transition temperature, the stress-strain curves exhibit the classic trends of a rubbery material during loading, albeit with a strong temperature and time dependence. Instead of a distinct yield stress, the curve transitions gradually, or rolls over, to flow. As in the sub-θ_g range, this is followed by moderate strain hardening and stiffening, and subsequent dramatic hardening. The exhibition of dramatic hardening in PETG, a copolymer of PET which does not undergo strain-induced crystallization, indicates that crystallization may not be the source of the dramatic hardening and stiffening in PET and, instead molecular orientation is the primary hardening and stiffening mechanism in both PET and PETG. Indeed, it is only in cases of deformation which result in highly uniaxial network orientation that the stress-strain behavior of PET differs significantly from that of PETG, suggesting the influence of a meso-ordered structure or crystallization in these instances. During unloading, PETG exhibits extensive elastic recovery, whereas PET exhibits relatively little recovery, suggesting that crystallization occurs (or continues to develop) after active loading ceases and unloading has commenced, locking in much of the deformation in PET.     

In situ poly(ethylene terephthalate) microfibers- and shear-induced non-isothermal crystallization of isotactic polypropylene by on-line small angle X-ray scattering

In situ microfibrillar reinforced blend (MRB) based on poly(ethylene terephthalate) (PET) and isotactic polypropylene (iPP) was elaborated by a slit die extrusion, hot stretching, and quenching process. The scanning electronic microscopic images show well-developed PET microfibers in the blends. The on-line small angle X-ray scattering (SAXS) test shows that PET microfibers have high nucleation for iPP crystallization. At the same time, after shear, neat iPP and microfibrillar blend both can faster crystallization rate. Three nucleation origins are proposed in microfibrillar reinforced blends under shear flow field: (a) the classical row nuclei model, (b) fiber nuclei and (c) nuclei induced by fiber assistant alignment. The polarized optical microscopic images indicate that, during the non-isothermal crystallization at a cooling rate of 10 °C/min from 200 °C to room temperature, the neat iPP forms common spherulites, while the diluted microfibrillar blend with 1 wt% of PET has a typical transcrystalline structure.     

Crystallization and vitrification effect in a poly(styrene)-g-poly(ethylene oxide) graft copolymer

Crystallization and solid state structure of a poly(styrene)-graft-poly(ethylene oxide) (PS-g-PEO) graft copolymer with crystallizable side chains were studied using simultaneous small angle X-ray scattering/wide angle X-ray scattering/differential scanning calorimetry (SAXS/WAXS/DSC). It is found that the glass transition temperature (T_g) of PS main chain is remarkably higher than that of PS homopolymer. The start cooling temperature (T_o) has a great influence on crystallization of the PEO side-chain. When the graft copolymer is cooled from the temperature above T_g, phase separation is suppressed due to the low mobility of the PS main chain and the homogeneous melt is vitrified. The unfavorable conformation of the rigid main chain results in a single crystallization peak and lower crystallinity. When PS-g-PEO is only heated to a temperature lower than the T_g and then cooled, phase separation is retained. Both the PEO side chains with high and low crystallizability can crystallize in the phase-separated state, leading to double crystallization peaks and higher crystallinity. The effect of solvent on crystallization of the graft copolymer was also examined. It is observed that addition of toluene reduces the T_g of the PS main chain and leads to the disappearance of the vitrification effect.     

Uniaxial orientation and tensile properties of poly(ethylene terephthalate)/poly(ethylene-2,6-naphthalate) blends

Amorphous films of poly(ethylene terephthalate)/poly(ethylene-2,6-naphthalate) (PET/PEN) blends with different blend ratios were uniaxially drawn by solid-state coextrusion and the structure development during solid state deformation was studied. As-prepared blends showed two T_gs. The lower T_g was ∼72 °C, independent of the blend ratio. In contrast, the higher T_g increased with increasing PEN content. Thus, the coextrusion was carried out around the higher T_g of the sample. At a given draw ratio of 5, which was close to the achievable maximum draw ratio, the tensile strength of the drawn samples from the initially amorphous state increased gradually with increasing PEN content. On the other hand, the tensile modulus was found to decrease initially, reaching a minimum at 40-60 wt% PEN, and then increased as the PEN content increased. The results indicate that we can get the drawn films with a moderate tensile modulus and a high tensile strength. The drawn samples from the blends containing 40-60 wt% of PEN showed a maximum elongation at break, and a maximum thermal shrinkage around 100 °C. Also, the degree of stress-induced crystallinity showed a broad minimum around the blend ratio of 50% of PEN. These morphological characteristics explained well the effects of blend ratio on the tensile modulus and strength of drawn PET/PEN blend films.     

Morphology development and crystallization behavior of a poly(ethylene terephthalate)/polycarbonate blend

The morphology development and crystallization behavior of an extruded poly(ethylene terephthalate)/polycarbonate blend were studied with optical microscopy, light scattering, and differential scanning calorimetry (DSC). During annealing at 280°C, liquid–liquid phase separation via spinodal decomposition proceeded in a melt‐extruded specimen. After the formation of the domain structure, the blend slowly underwent phase homogenization by transesterification between the two polymers. The specimen, annealed for various times (t_s's) at 280°C, was subjected to a temperature drop to 180°C for the isothermal crystallization, and then the effects of liquid‐phase changes on crystallization were investigated. The crystal growth rate decreased with t_s. The slow crystallization with a large t_s value was associated with the composition change of the separated phases and the change of the sequence distribution in the polymer chains during annealing. The influence of t_s on the endothermic behavior of the samples was examined. As t_s increased, the recrystallization rate was retarded during the DSC scan, displaying multiendothermic behavior. The DSC data also suggested that the increased level of transesterification would give rise to a higher number of species being rejected from the primary crystals, leading to enhanced secondary crystallization. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci, 2006     

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.     

Acceleration or retardation to crystallization if liquid-liquid phase separation occurs: Studies on a polyolefin blend by SAXS/WAXD, DSC and TEM

Blends of statistical copolymers containing ethylene/hexene (PEH) and ethylene/butene (PEB) exhibited the behavior of upper critical solution temperature (UCST). The interplay between the early and intermediate stage liquid-liquid phase separation (LLPS) and crystallization of the PEH/PEB 50/50 blend was studied by time-resolved simultaneous small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) techniques. Samples were treated by two different quench procedures: in single quench, the sample was directly quenched from 160 °C to isothermal crystallization temperature of 114 °C; while in double quench, the sample was firstly quenched to 130 °C for 20 min annealing, where LLPS occurred, and then to 114 °C. It was found that in the early stage of crystallization, the integrated values of Iq² and crystallinity, X_c, in the double quench procedure were consistently higher than those in the single quench procedure, which could be attributed to accelerated nucleation induced by enhanced concentration fluctuations and interfacial tension. In the late stage of crystallization, some morphological parameters were found to crossover and then reverse, which could be explained by retardation of lamellar growth due to phase separation formed during the double quench procedure. This phenomenon was also confirmed by DSC measurements in blends of different compositions at varying isothermal crystallization temperatures. The crystal lamellar thickness determined by SAXS showed a good agreement with TEM observation. Results indicated that the early stage LLPS in the PEH/PEB blend prior to crystallization indeed dictated the resulting lamellar structures, including the average size of lamellar stack and the stack distribution. There seemed to be little variation of lamellar thickness and long period between the two quenching procedures (i.e., single quench versus double quench).     

Plasticization of semicrystalline poly(l-lactide) with poly(propylene glycol)

Plasticization of semicrystalline poly(l-lactide) (PLA) with a new plasticizer - poly(propylene glycol) (PPG) is described. PLA was plasticized with PPG with nominal M_w of 425 g/mol (PPG4) and 1000 g/mol (PPG1) and crystallized. The plasticization decreased T_g, which was reflected in a lower yield stress and improved elongation at break. The crystallization in the blends was accompanied by a phase separation facilitated by an increase of plasticizer concentration in the amorphous phase and by annealing of blends at crystallization temperature. The ultimate properties of the blends with high plasticizer contents correlated with the acceleration of spherulite growth rate that reflected accumulation of plasticizer in front of growing spherulites causing weakness of interspherulitic boundaries. In PLA/PPG1 blends the phase separation was the most intense leading to the formation of PPG1 droplets, which facilitated plastic deformation of the blends that enabled to achieve the elongation at break of about 90-100% for 10 and 12.5 wt% PPG1 content in spite of relatively high T_g of PLA rich phase of the respective blends, 46.1-47.6 °C. Poly(ethylene glycol) (PEG), long known as a plasticizer for PLA, with nominal M_w of 600 g/mol, was also used to plasticize PLA for comparison.     

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.     

Aging of poly(lactide)/poly(ethylene glycol) blends. Part 2. Poly(lactide) with high stereoregularity

Blending poly(ethylene glycol) (PEG) with poly(lactide) (PLA) decreases the T_g and improves the mechanical properties. The blends have lower modulus and increased fracture strain compared to PLA. However, the blends become increasingly rigid over time at ambient conditions. Previously, it was demonstrated that a PLA of lower stereoregularity was miscible with up to 30 wt% PEG. Aging was due to slow crystallization of PEG from the homogeneous amorphous blend. Crystallization of PEG depleted the amorphous phase of PEG and gradually increased the T_g until aging essentially ceased when T_g of the amorphous phase reached the aging temperature. In the present study, this aging mechanism was tested with a crystallizable PLA of higher stereoregularity. Changes in thermal transitions, solid state structure, and mechanical properties were examined over time. Blends with up to 20 wt% PEG were miscible. Blends with 30 wt% PEG could be quenched from the melt to the homogenous amorphous glass. However, this composition phase separated at ambient temperature with little or no crystallization. Changes in mechanical properties during phase separation reflected increasing rigidity of the continuous PLA-rich phase as it became richer in PLA. Construction of a phase diagram for blends of higher stereoregular PLA with PEG was attempted.