Miscibility and phase structure of binary blends of polylactide and poly(methyl methacrylate)

Blends of amorphous poly(DL‐lactide) (DL‐PLA) and crystalline poly(L‐lactide) (PLLA) with poly(methyl methacrylate) (PMMA) were prepared by both solution/precipitation and solution‐casting film methods. The miscibility, crystallization behavior, and component interaction of these blends were examined by differential scanning calorimetry. Only one glass‐transition temperature (T_g) was found in the DL‐PLA/PMMA solution/precipitation blends, indicating miscibility in this system. Two isolated T_g's appeared in the DL‐PLA/PMMA solution‐casting film blends, suggesting two segregated phases in the blend system, but evidence showed that two components were partially miscible. In the PLLA/PMMA blend, the crystallization of PLLA was greatly restricted by amorphous PMMA. Once the thermal history of the blend was destroyed, PLLA and PMMA were miscible. The T_g composition relationship for both DL‐PLA/PMMA and PLLA/PMMA miscible systems obeyed the Gordon–Taylor equation. Experiment results indicated that there is no more favorable trend of DL‐PLA to form miscible blends with PMMA than PLLA when PLLA is in the amorphous state. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 23–30, 2003     

Miscibility,crystallization kinetics,and morphology of poly(β‐hydroxybutyrate) and poly(methyl acrylate) blends

The miscibility, spherulite growth kinetics, and morphology of binary blends of poly(β‐hydroxybutyrate) (PHB) and poly(methyl acrylate) (PMA) were studied with differential scanning calorimetry, optical microscopy, and small‐angle X‐ray scattering (SAXS). As the PMA content increases in the blends, the glass‐transition temperature and cold‐crystallization temperature increase, but the melting point decreases. The interaction parameter between PHB and PMA, obtained from an analysis of the equilibrium‐melting‐point depression, is −0.074. The presence of an amorphous PMA component results in a reduction in the rate of spherulite growth of PHB. The radial growth rates of spherulites were analyzed with the Lauritzen–Hoffman model. The spherulites of PHB were volume‐filled, indicating the inclusion of PMA within the spherulites. The long period obtained from SAXS increases with increased PMA content, implying that the amorphous PMA is entrapped in the interlamellar region of PHB during the crystallization process of PHB. All the results presented show that PHB and PMA are miscible in the melt. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1860–1867, 2000     

Polyethylene‐poly(L‐lactide) diblock copolymers: Synthesis and compatibilization of poly(L‐lactide)/polyethylene blends

A model polyethylene‐poly(L ‐lactide) diblock copolymer (PE‐b‐PLLA) was synthesized using hydroxyl‐terminated PE (PE‐OH) as a macroinitiator for the ring‐opening polymerization of L ‐lactide. Binary blends, which contained poly(L ‐lactide) (PLLA) and very low‐density polyethylene (LDPE), and ternary blends, which contained PLLA, LDPE, and PE‐b‐PLLA, were prepared by solution blending followed by precipitation and compression molding. Particle size analysis and scanning electron microscopy results showed that the particle size and distribution of the LDPE dispersed in the PLLA matrix was sharply decreased upon the addition of PE‐b‐PLLA. The tensile and Izod impact testing results on the ternary blends showed significantly improved toughness as compared to the PLLA homopolymer or the corresponding PLLA/LDPE binary blends. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2755–2766, 2001     

Influence of graphene nanosheets on stereocomplex crystallization behaviors of star‐shaped poly (D(L)‐lactide) stereoblock copolymer

The nanocompsites of star‐shaped poly(D‐lactide)‐co‐poly(L‐lactide) stereoblock copolymers (s‐PDLA‐PLLA) with two‐dimensional graphene nanosheets (GNSs) were prepared by solution mixing method. Crystallization behaviors were investigated using differential scanning calorimetry, polarized optical microscopy, and wide angle X‐ray diffraction. The results of isothermal crystallization behaviors of the nanocompsites clearly indicated that the GNS could remarkably accelerate the overall crystallization rate of s‐PDLA‐PLLA copolymer. Unique stereocomplex crystallites with melting temperature about 207.0°C formed in isothermal crystallization for all samples. The crystallization temperatures of s‐PDLA‐PLLAs shifted to higher temperatures, and the crystallization peak shapes became sharper with increasing GNS contents. The maximum crystallization temperature of the sample with 3 wt% GNS was about 128.2°C, ie, 15°C higher than pure s‐PDLA‐PLLA. At isothermal crystallization processes, the halftime of crystallization (t_0.5) of the sample with 3 wt% GNS decreased to 6.4 minutes from 12.9 minutes of pure s‐PDLA‐PLLA at 160°C.The Avrami exponent n values for the nanocomposites samples were 2.6 to 3.0 indicating the crystallization mechanism with three‐dimensional heterogeneous nucleation and spherulites growth. The morphology and average diameter of spherulites of s‐PDLA‐PLLA with various GNS contents were observed in isothermal crystallization processes by polarized optical microscopy. Spherulite growth rates of samples were evaluated by using combined isothermal and nonisothermal procedures and analyzed by the secondary nucleation theory. The results evidenced that the GNS has acceleration effects on the crystallization of s‐PDLA‐PLLA with good nucleation ability in the s‐PDLA‐PLLA material.     

Crystallization kinetics study of poly(L‐lactic acid)/carbon nanotubes nanocomposites

Effects of carbon nanotubes (CNT) on the isothermal crystallization kinetics of poly(L ‐lactic acid) (PLLA) were quantitatively investigated using the Avrami equation and the secondary nucleation theory of Lauritzen and Hoffman. CNT via grafting modification with PLLA could well disperse in the PLLA matrix and give significantly enhanced crystallization rate and crystallinity of PLLA as analyzed by differential scanning calorimetry and polarized optical microscopy. Analysis of isothermal crystallization kinetics using the Avrami equation demonstrated that CNT significantly enhanced the bulk crystallization of PLLA. Analysis of spherulite growth kinetics using the secondary nucleation theory of Lauritzen and Hoffman found that CNT could expand the temperature range of the crystallization regime III of PLLA. Values of the nucleation constant (K_g) in crystallization regimes III and II of PLLA both increased with increasing CNT contents. The K_g III/K_g II ratios were found to be close to the theoretical value 2 but were not clearly found to depend on the CNT contents. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 983–989, 2010     

Melt-crystallization behavior and isothermal crystallization kinetics of crystalline/crystalline blends of poly(ethylene terephthalate)/poly(trimethylene terephthalate)

The melt-crystallization and isothermal melt-crystallization kinetics of poly(ethylene terephthalate)/poly(trimethylene terephthalate) blends (PET/PTT) were investigated by differential scanning calorimetry (DSC) and polarized optical microscopy. Although PET and PTT in the binary blends are miscible at amorphous state, they will crystallize individually when cooled from the melt. In the DSC measurements, PET component with higher supercooling degree will crystallize first, and then the crystallite of PET will be the nucleating agent for PTT, which induce the crystallization of PTT at higher temperature. On the other hand, in both blends of PET80/PTT20 and PET60/PTT40, the PET component will crystallize at higher temperature with faster crystallization rate due to the dilute effect of PTT. So the commingled minor addition of one component to another helps to improve the crystallization of the blends. For blends of PET20/PTT80 and PET40/PTT60, isothermal crystallization kinetics evaluated in terms of the Avrami equation suggest different crystallization mechanisms occurred. The more PET content in blends, the fast crystallization rate is. The Avrami exponent, n = 3, suggests a three-dimensional growth of the crystals in both blends, which is further demonstrated by the spherulites formed in all blends. The crystalline blends show multiple-melting peaks during heating process.     

Immiscibility–miscibility phase transformation in blends of poly(ethylene succinate) with poly(L‐lactic acid)s of different molecular weights

Using differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and Fourier transformed infrared spectroscopy (FTIR), upper critical solution temperature (UCST) phase behavior with immiscibility–miscibility transformation in blends of poly(ethylene succinate) (PESu) with poly(lactic acid)s (PLAs), such as poly(D ,L ‐lactic acid) (PDLLA), poly(L ‐lactic acid) (PLLA), poly(D ‐lactic acid) (PDLA), differing in D/L configurations and molecular weights were investigated. All three binary blends of PDLLA/PESu, PLLA/PESu, and PESu/PDLA exhibit UCST behavior, which means they are immiscible at ambient temperature but can become miscible upon heating to higher temperatures at 240–268 °C depending on molecular weights. The PLLAs/PESu blends at UCST could be reverted back to the original phase‐separated morphology, as proven by solvent redissolution. The blends upon quenching from above UCST could be frozen into a quasi‐miscible state, where the Flory‐Huggins interaction parameter (χ₁₂) was determined to be a negative value (by melting point depression technique). The interaction between PDLLA and PESu in blend resulted in significant reduction in spherulite growth rate of PESu. Furthermore, blends of PESu with lower molecular weight PLLA or PDLA (M_w of PLLA and PDLA are 152,000 and 124,000 g/mol, respectively), instead of the higher M_w of PDLLA (M_w of PDLLA = 157,000 g/mol), are immiscible with UCST phase behavior, which are affected by molecular weights rather than the ratio of L/D monomer in the chemical structure of PLAs. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1135–1147, 2010     

Effects of carbon nanotubes on crystallization and melting behavior of poly(L‐lactide) via DSC and TMDSC studies

In this work, multiwalled carbon nanotubes (MWNTs) were surface‐modified and grafted with poly(L ‐lactide) to obtain poly(L ‐lactide)‐grafted MWNTs (i.e. MWNTs‐g‐PLLA). Films of the PLLA/MWNTs‐g‐PLLA nanocomposites were then prepared by a solution casting method to investigate the effects of the MWNTs‐g‐PLLA on nonisothermal and isothermal melt‐crystallizations of the PLLA matrix using DSC and TMDSC. DSC data found that MWNTs significantly enhanced the nonisothermal melt‐crystallization from the melt and the cold‐crystallization rates of PLLA on the subsequent heating. Temperature‐modulated differential scanning calorimetry (TMDSC) analysis on the quenched PLLA nanocomposites found that, in addition to an exothermic cold‐crystallization peak in the range of 80–120 °C, an exothermic peak in the range of 150–165 °C, attributed to recrystallization, appeared before the main melting peak in the total and nonreversing heat flow curves. The presence of the recrystallization peak signified the ongoing process of crystal perfection and, if any, the formation of secondary crystals during the heating scan. Double melting endotherms appeared for the isothermally melt‐crystallized PLLA samples at 110 °C. TMDSC analysis found that the double lamellar thickness model, other than the melting‐recrystallization model, was responsible for the double melting peaks in PLLA nanocomposites. Polarized optical microscopy images found that the nucleation rate of PLLA was enhanced by MWNTs. TMDSC analysis found that the incorporation of MWNTs caused PLLA to decrease the heat‐capacity increase (namely, ΔC_p) and the C_p at glass transition temperature. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1870–1881, 2007     

Miktoarm star‐shaped poly(lactic acid) copolymer: Synthesis and stereocomplex crystallization behavior

The miktoarm star‐shaped poly(lactic acid) (PLA) copolymer, (PLLA)₂‐core‐(PDLA)₂, was synthesized via stepwise ring‐opening polymerization of lactide with dibromoneopentyl glycol as the starting material. ¹H NMR and FTIR spectroscopy proved the feasibility of synthetic route and the successful preparation of star‐shaped PLA copolymers. The results of FTIR spectroscopy and XRD showed that the stereocomplex structure of the copolymer could be more perfect after solvent dissolution treatment. Effect of chain architectures on crystallization was investigated by studying the nonisothermal and isothermal crystallization of the miktoarm star‐shaped PLA copolymer and other stereocomplexes. Nonisothermal differential scanning calorimetry and polarizing optical microscopy tests indicated that (PLLA)₂‐core‐(PDLA)₂ exhibited the fastest formation of a stereocomplex in a dynamic test due to its special structure. In isothermal crystallization tests, the copolymer exhibited the fast crystal growth rate and the most perfect crystal morphology. The results reveal that the unique molecular structure has an important influence on the crystallization of the miktoarm star‐shaped PLA copolymer. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, 814–826     

Crystallization and morphology of poly(vinylidene fluoride)/poly(3‐hydroxybutyrate) blends. II. Morphology and crystallization kinetics by time resolved X‐ray scattering

The development of the morphology in poly(vinylidene fluoride)/poly(3‐hydroxybutyrate) (PVDF/PHB) blends upon isothermal and anisothermal crystallization is investigated by time‐resolved small‐ and wide‐angle X‐ray scattering. The components are completely miscible in the melt but crystallize separately; they crystallize stepwise at different temperatures or sequentially with isothermal or anisothermal conditions, respectively. The PVDF crystallizes undisturbed whereas PHB crystallizes in a confined space that is determined by the existing supermolecular structure of the PVDF. The investigations reveal that composition inhomogeneities may initially develop in the remaining melt or in the amorphous phases of the PVDF upon crystallization of that component. The subsequent crystallization of the PHB depends on these heterogeneities and the supermolecular structure of PVDF (dendritically or globularly spherulitic). PHB may form separate spherulites that start to grow from the melt, or it may develop “interlocking spherulites” that start to grow from inside a PVDF spherulite. Occasionally, a large number of PVDF spherulites may be incorporated into PHB interlocking spherulites. The separate PHB spherulites may intrude into the PVDF spherulites upon further growth, which results in “interpenetrating spherulites.” Interlocking and interpenetrating are realized by the growth of separate lamellar stacks (“fibrils”) of the blend components. There is no interlamellar growth. The growth direction of the PHB fibrils follows that of the existing PVDF fibrils. Depending on the distribution of the PHB molecules on the interlamellar and interfibrillar PVDF regions, the lamellar arrangement of the PVDF may contract or expand upon PHB crystallization and the adjacent fibrils of the two components are linked or clearly separated. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 974–985, 2004     

Isothermal crystallization behavior of poly(L‐lactide) in poly(L‐lactide)‐block‐poly(ethylene glycol) diblock copolymers

The crystal unit‐cell structures and the isothermal crystallization kinetics of poly(L ‐lactide) in biodegradable poly(L ‐lactide)‐block‐methoxy poly(ethylene glycol) (PLLA‐b‐MePEG) diblock copolymers have been analyzed by wide‐angle X‐ray diffraction and differential scanning calorimetry. In particular, the effects due to the presence of MePEG that is chemically connected to PLLA as well as the PLLA crystallization temperature T_C are examined. Though we observe no variation of both the PLLA and MePEG crystal unit‐cell structures with the block ratio between PLLA and MePEG and T_C, the isothermal crystallization kinetics of PLLA is greatly influenced by the presence of MePEG that is connected to it. In particular, the equilibrium melting temperature of PLLA, T, significantly decreases in the diblock copolymers. When the T_C is high so that the crystallization is controlled by nucleation, because of the decreasing T and thereafter the nucleation density with decreasing PLLA molecular weight, the crystallinity of PLLA also decreases with a decrease in the PLLA molecular weight. While, for the lower crystallization temperature regime controlled by the growth mechanism, the crystallizability of PLLA in copolymers is greater than that of pure PLLA. This suggests that the activation energy for the PLLA segment diffusing to the crystallization site decreases in the diblocks. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2438–2448, 2006     

Crystallization behaviors of poly(3-hydroxybutyrate) and poly(l-lactic acid) in their immiscible and miscible blends

By adjusting the molecular weight of the poly(l-lactic acid) (PLLA) component in poly(3-hydroxybutyrate) (PHB)/PLLA blends, we investigated the crystallization behaviors of the two components in their immiscible and miscible 50:50 blends by real time infrared (IR) spectroscopy. In the immiscible PHB/PLLA blend, the stepwise crystallization of PHB and PLLA was realized at different crystallization temperatures. PLLA crystallizes first at a higher temperature (120 degrees C). Its crystallization mechanism from the immiscible PHB/PLLA melt is not affected by the presence of the PHB component, while its crystallization rate is substantially depressed. Subsequently, in the presence of crystallized PLLA, the isothermal melt-crystallization of PHB takes place at a lower temperature (90 degrees C). It is interesting to find that there are two growth stages for PHB. At the early stage of the growth period, the Avrami exponent is 5.0, which is unusually high, while in the late stage, it is 2.5, which is very close to the reported value (n approximately 2.5) for the neat PHB system. In contrast to the stepwise crystallization of PHB and PLLA in the immiscible blends, the almost simultaneous crystallization of PHB and PLLA in the miscible 50:50 blend was observed at the same crystallization temperature (110 degrees C). Detailed dynamic analysis by IR spectroscopy has disclosed that, even in such apparently simultaneous crystallization, the crystallization of PLLA actually occurs faster than that of PHB. It has been found that, both in the immiscible and miscible blends, the crystallization dynamics of PHB are heavily affected by the presence of crystallized PLLA.     

Crystallization and melting behavior of poly(ethylene oxide)/poly(n-butyl methacrylate) blends

The phase diagram, crystallization and melting behavior of poly(ethylene oxide) (PEO)/poly(n-butyl methacrylate) (PnBMA) blends have been investigated using differential scanning calorimetry and optical microscopy. The results show that the blends are miscible up to 85 °C and show an lower critical solution temperature-type demixing at a higher temperature. The isothermal crystallization studies of the blends indicate a reduction in the overall rate of crystallization. Analysis of isothermal crystallization data by means of Avrami equation leads to average values of the Avrami index of 2.5 for pure PEO and 3.0 for the different blend compositions. The melting behavior of the blends reveals double endotherms, which is ascribed to both secondary crystallization and recrystallization. The melting point depression study yielded χ₁₂=0, indicating a relatively low interaction strength.     

Morphology of poly(ethylene oxide)/poly(vinyl acetate) blends

The morphology of poly(ethylene oxide)/poly(vinyl acetate) (PEO/PVAc) blends was examined using small angle X-ray scattering (SAXS) and optical microscopy. The morphological and structural parameters of the blends are dependent on both composition and crystallization conditions. Optical microscopy revealed that blend samples prepared by solution casting crystallized with volume-filling crystals up to a composition of 30/70 wt% PEO/PVAc; at higher PVAc content there was no evidence of crystallization in the temperature range studied. Pure PEO always crystallized with a spherulite-hedrite morphology. The formation of spherulites was relatively favoured at lower crystallization temperatures and by addition of PVAc to PEO. Small angle X-ray intensity profiles were analyzed using a recently developed methodology and it was found that, for a given crystallization temperature, the amorphous and interphase thicknesses increased with increasing PVAc content but that the average crystalline thickness was independent of composition. The morphological and structural properties of the PEO/PVAc blends were attributed to the presence of non-crystallizable material in both the interlamellar and interfibrillar regions.     

Oriented crystallization of poly(L‐lactic acid) in uniaxially oriented blends with poly(vinylidene fluoride)

This article describes the oriented crystallization of poly(L ‐lactic acid) (PLLA) in uniaxially oriented blends with poly(vinylidene fluoride) (PVDF). Uniaxially drawn films of PLLA/PVDF blend with fixed ends were heat‐treated in two ways to crystallize PLLA in oriented blend films. The crystal orientation of PLLA depended upon the heat‐treatment process. The crystal c‐axis of the α form crystal of PLLA was highly oriented in the drawing direction in a sample cold‐crystallized at T_c = 120 °C, whereas the tilt‐orientation of the [200]/ [110] axes of PLLA was induced in the sample crystallized at T_c = 120 °C after preheating at T_p = 164.5–168.5 °C. Detailed analysis of the wide‐angle X‐ray diffraction (WAXD) indicated that the [020]/ [310] crystal axes were oriented parallel to the drawing direction, which causes the tilt‐orientation of the [200]/ [110] axes and other crystal axes. Scanning electron microscopy (SEM) suggested that oriented crystallization occurs in the stretched domains of PLLA with diameters of 0.5–2.0 μm in the uniaxially drawn films of PVDF/PLLA = 90/10 blend. Although the mechanism for the oriented crystallization of PLLA was not clear, a possibility was heteroepitaxy of the [200]/[110] axes of the α form crystal of PLLA along the [201]/[111] axes of the β form crystal of PVDF that is induced by lattice matching of d₁₀₀(PLLA) ≈ 5d₂₀₁(PVDF). © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1376–1389, 2008     

Isothermal crystallization behavior and unique banded spherulites of hydroxyapatite/poly(L-lactide) nanocomposites

<正>Hydroxyapatite/poly(L-lactide)(HA/PLLA) nanocomposites were prepared by the solvent mixing method.The isothermal crystallization behavior was studied by differential scanning calorimetry(DSC) and polarized optical microscopy (POM).The results show that the crystallization behavior of HA/PLLA composites was strongly affected by the content of HA and crystallization temperature,and the addition of HA could promote nucleation and enhance the crystallization rate. When isothermal crystallization was carried out at 110℃,the HA/PLLA nanocomposite with 1%HA content crystallized most rapidly among all the composites and the half crystallization time was only 1.0 min.Banded spherulites were observed for the HA/PLLA composites,but no banded spherulites were seen in the crystals of PLLA under the same condition.     

Poly(L‐lactide)/layered double hydroxides nanocomposites: Preparation and crystallization behavior

Novel nanocomposites from poly(L ‐lactide) (PLLA) and an organically modified layered double hydroxide (LDH) were prepared using the melt‐mixing technique. The structure and crystallization behavior of these nanocomposites were investigated by means of wide‐angle X‐ray diffraction (WAXD), transmission electron microscopy (TEM), Fourier transform infrared (FTIR) spectroscopy, differential scanning calorimetry (DSC), and polarized optical microscopy (POM). WAXD results indicate that the layer distance of dodecyl sulfate‐modified LDH (LDH‐DS) is increased in the PLLA/LDH composites, compared with the organically modified LDH. TEM analysis suggests that the most LDH‐DS layers disperse homogenously in the PLLA matrix in the nanometer scale with the intercalated or exfoliated structures. It was found that the incorporation of LDH‐DS has little or no discernable effect on the crystalline structure as well as the melting behavior of PLLA. However, the crystallization rate of PLLA increases with the addition of LDH‐DS. With the incorporation of 2.5 wt % LDH‐DS, the PLLA crystallization can be finished during the cooling process at 5 °C/min. With the addition of 5 wt % LDH‐DS, the half‐times of isothermal melt‐crystallization of PLLA at 100 and 120 °C reduce to 44.4% and 57.0% of those of the neat PLLA, respectively. POM observation shows that the nucleation density increases and the spherulite size of PLLA reduces distinctly with the presence of LDH‐DS. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2222–2233, 2008     

Influence of hyperbranched against linear architecture on crystallization behavior of poly(ε‐caprolactone)s in binary blends with poly(vinyl chloride)

Nonisothermal and isothermal crystallization behaviors of the hyperbranched poly(ε‐caprolactone) (HPCL)/poly(vinyl chloride) (PVC) and linear poly(ε‐caprolactone) (LPCL)/(PVC) blends were characterized with various blend composition such as 100/0, 95/5, 90/10, and 80/20, respectively. HPCL was synthesized through polycondensation of AB₂ macromonomer while LPCL and PVC were commercially purchased. The architectural characterization performed on ¹H NMR spectra revealed that HPCL consisted of about 3 AB₂ units and the linear segments consisted of 25 ε‐CL units. Through the nonisothermal crystallization analyses by modified Avrami approach with DSC crystallization exotherms, it was found that the crystallization rate was retarded by the increase in the noncrystallizable component (PVC) in the blends. This is in good agreement with the results of the isothermal crystallization analyses where time resolved small angle light scattering (SALS) and polarized optical microscopy (POM) were used. The effect of molecular architectural difference between HPCL and LPCL on the crystallization of their binary blends with PVC was elucidated by comparing the crystallization kinetic parameters. Both the nonisothermal and isothermal crystallization analyses showed that the crystallization rates of HPCL/PVC blends was faster than LPCL/PVC blends at given blend compositions. The faster crystallization of the HPCL/PVC blends is ascribed to the two specific architectural characteristics of HPCL; the branched structure and the incorporated long linear segments. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 577–589, 2007     

Effects of divinylbenzene‐maleic anhydride copolymer hollow microspheres on crystallization behaviors,mechanical properties and heat resistance of poly(l‐lactide acid)

Divinylbenzene‐maleic anhydride copolymer hollow microspheres (DMs) were used as novel organic nucleating agents to promote crystallization of poly(l‐lactide acid) (PLLA). The effects of these DMs on crystal behaviors of the PLLA were investigated by differential scanning calorimeter (DSC), polarizing optical microscopy (POM), and wide angle X‐ray diffraction (WAXD). Both isothermal and non‐isothermal processes in DSC demonstrated that the DMs significantly altered the crystal behaviors of PLLA as both crystallization velocity and degree of crystallinity increased with increasing DM loadings from 0 to 3%. Our POM results also indicated that as nucleating agents, the DMs promoted nucleating densities and decreased spherulitic sizes. In addition, WAXD suggeted that the addition of DMs did not induce new types of crystals. Finally, our results showed that the ductility of the PLLA was enhanced by a small amount of DMs during the PLLA crystallization process since 0.5% DMs added to the PLLA resulted in 1.4‐fold increase in the elongation at break in comparision with the neat PLLA.     

Linear and four‐armed poly(l‐lactide)‐block‐poly(d‐lactide) copolymers and their stereocomplexation with poly(lactide)s

Linear and four‐armed poly(l ‐lactide)‐block‐poly(d ‐lactide) (PLLA‐b‐PDLA) block copolymers are synthesized by ring‐opening polymerization of d ‐lactide on the end hydroxyl of linear and four‐armed PLLA prepolymers. DSC results indicate that the melting temperature and melting enthalpies of poly (lactide) stereocomplex in the copolymers are obviously lower than corresponding linear and four‐armed PLLA/PDLA blends. Compared with the four‐armed PLLA‐b‐PDLA copolymer, the similar linear PLLA‐b‐PDLA shows higher melting temperature (212.3 °C) and larger melting enthalpy (70.6 J g^?1). After these copolymers blend with additional neat PLAs, DSC, and WAXD results show that the stereocomplex formation between free PLA molecular chain and enantiomeric PLA block is the major stereocomplex formation. In the linear copolymer/linear PLA blends, the stereocomplex crystallites (sc) as well as homochiral crystallites (hc) form in the copolymer/PLA cast films. However, in the four‐armed copolymer/linear PLA blends, both sc and hc develop in the four‐armed PLLA‐b‐PDLA/PDLA specimen, which means that the stereocomplexation mainly forms between free PDLA molecule and the inside PLLA block, and the outside PDLA block could form some microcrystallites. Although the melting enthalpies of stereocomplexes in the blends are smaller than that of neat copolymers, only two‐thirds of the molecular chains participate in the stereocomplex formation, and the crystallization efficiency strengthens. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 1560–1567