Stereocomplex Crystallization Behavior and Physical Properties of Linear 1‐Arm, 2‐Arm,and Branched 4‐Arm Poly(L‐lactide)/Poly(D‐lactide) Blends: Effects of Chain Directional Change and Branching

For number‐average molecular weight (M _n) below 1 × 10⁴ g mol^?1, the comparison of cold crystallization temperature and spherulite growth rate and crystallinity of linear 1‐arm, 2‐arm, and branched 4‐arm poly(L ‐lactide)/poly(D ‐lactide) blends exhibits that the effects of chain directional change and branching significantly disturb stereocomplex crystallization. In contrast, the comparison of glass transition and melting temperatures of linear 1‐arm, 2‐arm, and branched 4‐arm poly(L ‐lactide)/poly(D ‐lactide) blends indicates that the effects of chain directional change and branching insignificantly alter and largely increase the segmental mobility of the blends, respectively, and the crystalline thickness of the blends is determined by M _n per one arm not by M _n and is not affected by the molecular architecture.

     

Separate Crystallization and Cocrystallization of Poly(L‐lactide) in the Presence of L‐Lactide‐Based Copolymers With Low Crystallizability,Poly(L‐lactide‐co‐glycolide) and Poly(L‐lactide‐co‐D‐lactide)

Poly(L ‐lactide) (PLLA) and poly(L ‐lactide‐co‐glycolide) (78/22)0 [P(LLA‐GA)] were phase‐separated in PLLA/P(LLA‐GA) blends, forming independent domains and thence crystallizing separately. The crystallization of PLLA was not disturbed or delayed by the presence of P(LLA‐GA) and vice versa. PLLA and poly(L‐lactide‐co‐D ‐lactide) (77/23) [P(LLA‐DLA)] were miscible in the PLLA/P(LLA‐DLA) blends, overall crystallization was delayed, and the growth of crystallites was disturbed in the presence of P(LLA‐DLA). In isothermal crystallization, the originally noncrystallizable P(LLA‐DLA) became crystallizable in the presence of PLLA, with which it cocrystallized. The disturbance effect on periodical lamella twisting of PLLA was larger for P(LLA‐GA) than for P(LLA‐DLA).     

Heterostereocomplex Crystallization and Homocrystallization From the Melt in Blends of Substituted and Unsubstituted Poly(lactide)s

Poly(L ‐2‐hydroxybutyrate) [P(L ‐2HB)] and poly(D ‐lactide) (PDLA) in their blends were phase separated to form P(L ‐2HB)‐ and PDLA‐enriched domains, and heterostereocomplex (HTSC) crystallization occurred at the interface between these domains. In the blends, HTSC crystallites were formed at crystallization temperature (T_c) = 80–160 °C, and their exclusive formation without the formation of other crystalline species was observed at T_c of 150 or 160 °C. The effects of polymer blend ratio on the types of formed crystalline species were very small due to the phase separation of the blends. The presence of a small amount of P(L ‐2HB) decreased the low limit of crystallizable T_c of PDLA homocrystallites from 80 to 60 °C. The equilibrium melting temperature of HTSC crystallites was 257.6 °C.     

Stereocomplex Crystallization of Star‐Shaped 4‐Armed Equimolar Stereo Diblock Poly(lactide)s with Different Molecular Weights: Isothermal Crystallization from the Melt

The isothermal crystallization of star‐shaped four‐armed equimolar stereo diblock poly(lactide) (4‐LD) polymers with different molecular weights is investigated. Solely stereocomplex (SC) crystallites are formed in all equimolar 4‐LD polymers, irrespective of molecular weight and crystallization temperature (T_c). The wide‐angle X‐ray diffractometry, differential scanning calorimetry, and polarized optical microscopy results for crystalline species, crystallinity, and maximum radial growth rate of spherulites values indicate that both branching and diblock architectures disturb the SC crystallization and spherulites growth of equimolar 4‐LD polymers, and the disturbance effect is larger for branching architecture than for diblock architecture. The equilibrium melting temperature (T_m⁰) values are 181.9–266.0 °C, which are comparable with or lower than the value reported earlier (279 °C). The crystallite growth geometries of equimolar of 4‐LD polymers are independent of molecular weight and T_c.

     

Thermal Properties and Degradation Behavior of Linear and Branched Poly(L‐lactide)s and Poly(L‐lactide‐co‐glycolide)s

Poly(lactide)s and poly(lactide‐co‐glycolide)s with different number of arms are synthesized from L ‐lactide and glycolide monomers using stannous(II) 2‐ethylhexanoate and alcohols containing 1, 2, 25, and 51 hydroxyl groups. 1‐dodecanol is used to produce the 1‐arm polymer, poly(ethylene glycol) for the 2‐arm polymer, and polyglycidols of appropriate molecular weight are used to initiate the 25‐ and 51‐arm branched polyesters. The polymers are characterized by ¹H NMR and GPC. The thermal properties of the polymers are studied using DSC. Their degradation behavior is indestigated using a combination of thermogravimetry, FTIR spectroscopy, and isoconversional kinetic analysis.     

Hetero‐Stereocomplex Crystallization between Star‐Shaped 4‐Arm Poly(l‐2‐hydroxybutanoic acid) and Poly(d‐lactic acid) from the Melt

Four‐arm poly(l ‐2‐hydroxybutanoic acid) [P(L‐2HB)]/poly(d ‐lactic acid) (PDLA) blends are phase‐separated to form P(L‐2HB)‐rich and PDLA‐rich domains. Hetero‐stereocomplex (HTSC) crystallization should occur at the interface of two types of domains. The crystallization temperature ranges, wherein HTSC crystallites, P(L‐2HB), and PDLA homocrystallites are crystallizable in the star‐shaped 4‐arm P(L‐2HB)/PDLA blends, are much narrower than those reported for linear 1‐arm P(L‐2HB)/PDLA blends, indicating that the star‐shaped architecture disturbs the isothermal crystallization of the blends. Exclusive HTSC crystallization is not observed for the star‐shaped 4‐arm blends during isothermal crystallization in marked contrast with the result reported for the linear 1‐arm blends.

     

Ring–Ring Equilibration in Solid,Even‐Numbered Cyclic Poly(l‐lactide)s and their Stereocomplexes

Even‐numbered cyclic poly(d ‐lactide) and poly(l ‐lactide) are prepared by ring‐expansion polymerization. The cyclic pol(l ‐lactide) is annealed either at 120 or at 160 °C for several days. The progress of transesterification in the solid state is monitored by the formation of odd‐numbered cycles via matrix‐assisted laser desorption/ionization‐time of flight mass spectrometry. The changes of the crystallinity are monitored by differential scanning calorimetry, wide‐ and small‐angle x‐ray scattering (WAXS and SAXS) measurements. Despite total even‐odd equilibration at 160 °C, the crystallinity of poly(l ‐lactide) is not reduced. Furthermore, the crystallinity of the stereocomplexes of both cyclic polylactides do not decrease or vanish, as expected, when a blocky or random stereosequence is formed by transesterification. This conclusion is confirmed by ¹³C NMR spectroscopy. These measurements demonstrate that transesterification is a ring–ring equilibration involving the loops on the surfaces of the lamellar crystallites thereby improving crystallinity and 3D packing of crystallites without significant broadening of the molecular weight distribution.     

Stereocomplex Crystallization and Spherulite Growth of Low Molecular Weight Poly(L‐lactide) and Poly(D‐lactide) from the Melt

In isothermal crystallization from the melt, only stereocomplex crystallites as a crystalline species were formed in all the blends at crystallization temperature above 130 °C. The spherulite growth rate and crystallinity values decreased monotonically with deviation of the PDLA content from 50%. Surprisingly, regime analysis revealed that the crystallization mechanism of the blends was independent of PDLA content. In non‐isothermal crystallization of melt‐quenched specimens during heating, the cold crystallization of blends takes place rapidly at a lower temperature compared to that of pure PLLA and PDLA. This is attributable to the rapid stereocomplex crystallization or the nucleating effect of stereocomplex crystallites formed during quenching from the melt or second heating.

     

Poly(L‐lactide) Nanoparticles via Ring‐Opening Polymerization in Non‐aqueous Emulsion

The preparation of poly(L ‐lactide) nanoparticles via ring‐opening polymerization (ROP) of L ‐lactide is conducted in non‐aqueous emulsion. In this process, acetonitrile is dispersed in either cyclohexane or n‐hexane as the continuous phase and stabilized by a PI‐b‐PEO, respectively, a PI‐b‐PS copolymer as emulsifier. The air and moisture sensitive N‐heterocyclic carbene 1,3‐bis(2,4,6‐trimethylphenyl)‐2‐ididazolidinylidene (SIMes) catalyzes the polymerization of L ‐lactide at ambient temperatures. Spherical poly(L ‐lactide) nanoparticles with an average diameter of 70 nm and a tunable molecular weight are generated. Hence, the non‐aqueous emulsion technique demonstrates its good applicability toward the generation of well‐defined poly(L ‐lactide) nanoparticles under very mild conditions.     

Effects of Solvents on Stereocomplex Crystallization of High‐Molecular‐Weight Polylactic Acid Racemic Blends in the Presence of Carbon Nanotubes

Stereocomplex (SC) crystallization of solution‐casted high‐molecular‐weight polylactic acid/carbon nanotube (CNT) composites is investigated under the effects of solvents, including dichloromethane (CH₂Cl₂), chloroform (CHCl₃), N ,N‐dimethylformamide (DMF), and 1,4‐dioxane (DIOX). It is found that addition of 0.1 wt% CNTs can promote the SC formation in the equimolar poly(l ‐lactic acid) (PLLA)/poly(d ‐lactic acid) (PDLA) blends. More interestingly, the final content of SC crystallites is mediated by solvent species to a large extent. Results of wide‐angle X‐ray diffraction and differential scanning calorimetry reveal that the capability of solvents for enhancing the SC content in PLLA/PDLA/CNT composites follows the sequence: DMF > DIOX > CHCl₃ > CH₂Cl₂. Especially, exclusive SC crystallites are formed in the DMF. Such a disparity is explained in terms of solubility parameter and vapor pressure of the solvents. The above results also provide the possible solution to regulate the crystalline composition of the PLLA/PDLA/CNT blends, by which the expected performance may be obtained.     

Organobase‐Catalyzed Synthesis of Multiarm Star Polylactide With Hyperbranched Poly(ethylene glycol) as the Core

Multiarm star copolymers consisting of the polyether‐polyol hyperbranched poly(ethylene glycol) (hbPEG) as core and poly(L ‐lactide) (PLLA) arms are synthesized via the organobase‐ catalyzed ring‐opening polymerization of lactide using hbPEG as a multifunctional macroinitiator. Star copolymers with high molecular weights up to 792 000 g mol^?1 are prepared. Detailed 2D NMR analysis provides evidence for the attachment of the PLLA arms to the core and reveals that the adjustment of the monomer/initiator ratio enables control of the arm length. Size exclusion chromatography measurements show narrow molecular weight distributions. Thermal analysis reveals a lower glass transition temperature, melting point, and degree of crystallization for the star‐shaped polylactides compared to linear polylactide.     

In Situ Forming Poly(ethylene glycol)‐ Poly(L‐lactide) Hydrogels via Michael Addition: Mechanical Properties,Degradation, and Protein Release

Chemically crosslinked hydrogels are prepared at remarkably low macromonomer concentrations from 8‐arm poly(ethylene glycol)‐poly(L ‐lactide) star block copolymers bearing acrylate end groups (PEG‐(PLLA_n)₈‐AC, n = 4 or 12) and multifunctional PEG thiols (PEG‐(SH)_n, n = 2, 4, or 8) through a Michael‐type addition reaction. Hydrogels are obtained within 1 min after mixing PEG‐(PLLA₄)₈ ‐AC and PEG‐(SH)₈ in phosphate buffered saline, quickly reaching a high storage modulus of 17 kPa. Lysozyme and albumin are released for 4 weeks from PEG‐(PLLA₁₂)₈‐AC/PEG‐(SH)₈ hydrogels. Lysozyme release from PEG‐(PLLA₁₂)₈‐AC/PEG‐(SH)₂ and PEG‐(PLLA₁₂)₈‐AC/PEG‐(SH)₄ hydrogels is significantly faster with complete release in 3 and 12 d, respectively, as a result of a combination of degradation and diffusion.     

Long‐Chain Branched Poly(Lactide)s Based on Polycondensation of AB2‐type Macromonomers

A series of long‐chain branched poly(d‐/l ‐lactide)s is synthesized in a two‐step protocol by (1) ring‐opening polymerization of lactide and (2) subsequent condensation of the preformed AB₂ macromonomers promoted by different coupling reagents. The linear AB₂ macromonomers are prepared by Sn(Oct)₂‐catalyzed ROP of D ‐ and L ‐lactide with 2,2‐bis(hydroxymethyl)butyric acid (BHB) as an initiator. Optimization of the polymerization conditions allows for the preparation of well‐defined macromonomers (M _w/M _n = 1.09–1.30) with adjustable molecular weights (760–7200 g mol^?1). The two‐step approach of the synthesis comprises as well the coupling of these AB₂ macromonomers and hence allows precise control over the lactide chain length between the branching units in contrast to a random polycondensation.     

Formation of Crystallosolvates Comprising Nano‐Crystals of Stereocomplex in a Ternary Mixture of Poly(L‐lactide)/Poly(D‐lactide)/Diphenyl Ether

Equal amounts of poly(L ‐lactide) (PLLA) and poly(D ‐lactide) (PDLA) were mixed with diphenyl ether (DE) at high temperature in DE‐to‐polymer composition from 0 to 80 wt.‐%. Cooling of the ternary mixtures produced white crystallosolvates without deposition of DE. Various analyses revealed that the crystallosolvates consisted of both homo‐chiral (hc) and stereocomplex (sc) crystals of PLLA and PDLA as well as amorphous domains involving DE. The crystallosolvate obtained at a DE content of 80 wt.‐%, comprised only the sc crystals. Atomic force microscopy (AFM) revealed that many granules having a size of 20 nm in diameter were formed from rod‐like hc crystallites and that the DE was slightly phase‐separated to form nano‐sized reservoirs. The stronger sc interaction between the PLLA and PDLA chains excluded DE from the polymer matrix.

     

Synthesis,Sequential Crystallization and Morphological Evolution of Well‐Defined Star‐Shaped Poly(ε‐caprolactone)‐b‐poly(L‐lactide) Block Copolymer

Summary: Well‐defined star‐shaped poly(ε‐caprolactone)‐b‐poly(L ‐lactide) copolymers (PCL‐b‐PLLA) were synthesized via sequential block copolymerization, and their molecular weights and arm length ratio could be accurately controlled. Both differential scanning calorimetry and wide angle X‐ray diffraction analysis indicated that the crystallization of both the PLLA and PCL blocks within the star‐shaped PCL‐b‐PLLA copolymer could be adjusted from the arm length of each block, and both blocks mutually influenced each other. The sequential isothermal crystallization process of both the PLLA and PCL blocks within the PCL‐b‐PLLA copolymers was directly observed with a polarized optical microscope, and the isothermal crystallization of the PCL segments was mainly templated by the existing spherulites of PLLA. Moreover, the PLLA blocks within the star‐shaped PCL‐b‐PLLA copolymer progressively changed from ordinary spherulites to banded spherulites when the arm length ratio of PCL to PLLA was increased while concentric spherulites were observed for the linear analog. Significantly, these novel spherulites with concentric or banded textures and the morphological evolution of the spherulites have been observed for the first time in the PCL‐b‐PLLA block copolymers.

     

Nanostructures of Stereocomplex Polylactide in Poly(l‐lactide) Doped with Poly(d‐lactide)

Nanostructures of stereocomplex polylactide (sc‐PLA) are obtained and studied in poly(l ‐lactide) (PLLA) doped with a low amount of poly(d ‐lactide) (PDLA) during successive melt‐quenching, extrusion, spinning, and drawing processes corresponding to quiescent, shear flow, elongational flow, and tensioned annealing conditions, respectively. Nanogranules of predominantly sc‐PLA are initially formed with rapid quenching in quiescent and shear flow, which developed into microspheres with slow quenching and uniform nanofibrils in elongational flow. While only amorphous or the α′‐form PLLA is formed with the quenched melts and macroscopic fibers, the embedded nanogranules and nanofibrils are highly crystallized with the coexistence of sc‐PLA and the α‐crystals. A 1D coalescence of nascent sc‐nuclei into nanofibrils in elongational flow is preliminarily proposed to explain the structure evolution and the minor reinforcement of the nanofibrils on the macroscopic fibers.

     

Melt Structure and its Transformation by Sequential Crystallization of the Two Blocks within Poly(L‐lactide)‐block‐Poly(ε‐caprolactone) Double Crystalline Diblock Copolymers

Summary: Sequential crystallization of poly(L ‐lactide) (PLLA) followed by poly(ε‐caprolactone) (PCL) in double crystalline PLLA‐b‐PCL diblock copolymers is studied by differential scanning calorimetry (DSC), polarized optical microscopy (POM), wide‐angle X‐ray scattering (WAXS) and small‐angle X‐ray scattering (SAXS). Three samples with different compositions are studied. The sample with the shortest PLLA block (32 wt.‐% PLLA) crystallizes from a homogeneous melt, the other two (with 44 and 60% PLLA) from microphase separated structures. The microphase structure of the melt is changed as PLLA crystallizes at 122 °C (a temperature at which the PCL block is molten) forming spherulites regardless of composition, even with 32% PLLA. SAXS indicates that a lamellar structure with a different periodicity than that obtained in the melt forms (for melt segregated samples). Where PCL is the majority block, PCL crystallization at 42 °C following PLLA crystallization leads to rearrangement of the lamellar structure, as observed by SAXS, possibly due to local melting at the interphases between domains. POM results showed that PCL crystallizes within previously formed PLLA spherulites. WAXS data indicate that the PLLA unit cell is modified by crystallization of PCL, at least for the two majority PCL samples. The PCL minority sample did not crystallize at 42 °C (well below the PCL homopolymer crystallization temperature), pointing to the influence of pre‐crystallization of PLLA on PCL crystallization, although it did crystallize at lower temperature. Crystallization kinetics were examined by DSC and WAXS, with good agreement in general. The crystallization rate of PLLA decreased with increase in PCL content in the copolymers. The crystallization rate of PCL decreased with increasing PLLA content. The Avrami exponents were in general depressed for both components in the block copolymers compared to the parent homopolymers.

Polarized optical micrographs during isothermal crystallization of (a) homo‐PLLA, (b) homo‐PCL, (c) and (d) block copolymer after 30 min at 122 °C and after 15 min at 42 °C.     

Transesterification in the Solid State of Cyclic and Linear Poly(l‐lactide)s

Poly(l ‐lactide)s are synthesized and annealed at 120 °C and changes of the molecular weight distribution (MWD) are monitored by matrix‐assited laser desorption/ionization time‐of‐flight (MALDI‐TOF) mass spectrometry. For example, benzyl alcohol+SnOct₂ causes equilibration of odd‐ and even‐numbered chains and the final goal of the transesterification is the most probable distribution. The underlying intermolecular transesterification is even observed at 100 and 80 °C in the solid state. However, cyclic tin mercaptide catalysts transform the initial most probable distribution into a MWD with maxima, which display a conspicuous fine structure due to a preferential crystallization of certain ring sizes. The optimum ring sizes for the crystallization are provided by ring‐ring equilibration. The gradual formation of a special morphology shifts the melting temperature to values up to 187 °C. Annealing of commercial poly(l ‐lactide) with a cyclic tin catalyst also yields a distribution of mass peaks with a maximum showing the characteristic fine structure.

     

Heterostereocomplex‐ and Homocrystallization and Thermal Properties and Degradation of Substituted Poly(lactic acid)s,Poly(l‐2‐hydroxybutanoic acid) and Poly(d‐2‐hydroxy‐3‐methylbutanoic acid)

Heterostereocomplex‐ and homocrystallization behavior, thermal properties and degradation of neat poly(l ‐2‐hydroxybutanoic acid) [P(l ‐2HB)], poly(d ‐2‐hydroxy‐3‐methylbutanoic acid) [P(d ‐2H3MB)], and their equimolar blend are first investigated. Regime I and II kinetics are observed for neat P(l ‐2HB), whereas regime II and III kinetics are seen for the blend. The growth geometry of the neat P(l ‐2HB) is linear and circular while that of the blend is spherical, whereas that of the neat P(d ‐2H3MB) changes from linear to spherical, depending on crystallization temperature (T _c). The main crystalline species is heterostereocomplex (HTSC) in the blend for a wide T _c range of 0–180 °C and a very small amount of P(d ‐2H3MB) homocrystallites form for melt‐crystallization at T _c below 70 °C and solution‐crystallization. The equilibrium melting temperature of P(l ‐2HB)/P(d ‐2H3MB) HTSC crystallites (234.5 °C) is higher than those of P(l ‐2HB) and P(d ‐2H3MB) homocrystallites (114.9 and 208.6 °C, respectively). The activation energy values for thermal degradation of the P(l ‐2HB)/P(d ‐2H3MB) blend (190–219 kJ mol^?1) are between those of neat P(l ‐2HB) and P(d ‐2H3MB) (164–180 and 210–380 kJ mol^?1, respectively), reflecting that the interaction between the polymers with opposite configurations is similar to or lower than that between the polymers with the same configurations at a high temperature in the melt.

     

Immiscible Poly(L‐lactide)/Poly(ε‐caprolactone) Blends: Influence of the Addition of a Poly(L‐lactide)‐Poly(oxyethylene) Block Copolymer on Thermal Behavior and Morphology

Summary: A binary blend of poly (L ‐lactide) (PLLA) and poly(ε‐caprolactone) (PCL) of composition 70:30 by weight was prepared using a twin screw miniextruder and investigated by differential scanning calorimetry (DSC), optical microscopy and scanning electron microscopy (SEM). Ternary 70:30:2 blends were also obtained by adding either a diblock copolymer of PLLA and poly(oxyethylene) (PEO) or a triblock PLLA‐PCL‐PLLA copolymer as a third component. Optical microscopy revealed that the domain size of dispersed PCL domains is reduced by one order of magnitude in the presence of both copolymers. SEM confirmed the strong reduction in particle size upon the addition of the copolymers, with an indication of an enhanced emulsifying effect in the case of the PLLA‐PEO copolymer. These results are analyzed on the basis of solubility parameters of the blend components.

Optical micrograph of M3EG2 blend melt quenched at 125 °C.