Melting behavior of hydrolyzable poly(oxyethylene)/poly(L-lactide) copolymers

The melting behavior of poly(oxyethylene) (PEG)/poly(L-lactide) (PLLA) triblock copolymers with PEG contents ≦18,3 wt.-% was studied by differential scanning calorimetry and polarizing optical microscopy. The influence of the PEG segment contents and the hydrolysis of the PEG/PLLA copolymers on the melting process was investigated. The melting temperatures of the PLLA homopolymers decrease with increasing hydrolysis time up to 600 h, and the degree of crystallinity increases from 46 to 56%. This causes the arrangement of the chains more ordered. Therefore, the entropies of fusion (ΔS_m) of the PLLA homopolymers increase during the hydrolysis time up to 600 h. The melting temperatures of the PEG/PLLA copolymers decrease, while the recrystallization temperatures increase with increasing hydrolysis time up to 600 h. In addition, both the increases of PEG content and hydrolysis time of the PEG/PLLA copolymers cause a decrease of ΔS_m. The PEG segments and the hydrolysis are responsible for the disordered arrangement of the chains.     

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.

     

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.     

Crystallization kinetics and melting behavior of poly(butylene adipate), poly(butylene isophthalate) and their copolymers

The melting behavior and the isothermal crystallization kinetics of poly(butylene adipate), poly(butylene isophthalate) and their random copolymers were investigated by means of differential scanning calorimetry. Multiple endotherms, commonly observed on the melting polyesters, were found to be influenced by crystallization temperature and composition. By applying the Hoffman-Weeks method to the isothermally crystallized samples, the equilibrium melting temperatures of the homopolymers were obtained. From calorimetric results on samples with different degree of crystallinity, the equilibrium melting enthalpy of poly(butylene isophthalate) was calculated and the presence of a crystal-amorphous interphase in copolymers was evidenced. Isothermal melt crystallization kinetics was analyzed according to the Avrami equation. As expected, the introduction of a comonomer was found to decrease the overall crystallization rate of the polymers. Values of Avrami exponent close to three were obtained for all the samples, independently of composition and crystallization temperature, in agreement with a crystallization process originating from predetermined nuclei and characterized by three-dimensional spherulitic growth. In the case of poly(butylene isophthalate), the dependence of the rate of crystallization on T_c shows a maximum at an undercooling of approximately 60°C.     

Stereocomplex formation in ABA triblock copolymers of poly(lactide) (A) and poly(ethylene glycol) (B)

Two series of triblock copolymers of poly(ethylene glycol) (PEG, number-average molecular weight M _n = 6000) and poly(L -lactide) (PLLA) or poly(D -lactide) (PDLA) were prepared by ring-opening polymerization of lactide initiated by PEG end groups using stannous octoate as a catalyst, either in refluxing toluene or in the melt at 175°C. The weight percentage of PLA in the polymers varied between 15 and 75 wt.-%. Blends of polymers containing blocks of opposite chirality were prepared by co-precipitation from homogeneous solutions. The melting temperatures of the crystalline PEG and PLA phases strongly depended on the composition of the polymers. The melting temperature of the PLA phase in the blends was approximately 40°C higher than that of the single block copolymers. Stereocomplex formation between blocks of enantiomeric poly(lactides) in PEG/PLA block copolymers was established for the first time. Water uptake of polymeric films prepared by solution casting was solely determined by the PEG content of the film.     

Thermal behavior and spherulitic superstructures of SBC triblock copolymers based on polystyrene (S), polybutadiene (B) and a crystallizable poly(ε-caprolactone) (C) block

The dynamic crystallization and the melting behavior of polystyrene-block-poly(ε-caprolactone) (PS-b-PCL, short notation SC), polybutadiene-block-poly(ε-caprolactone) (PB-b-PCL, BC) and polystyrene-block-polybutadiene-block-poly(ε-caprolactone) (PS-b-PB-b-PCL, SBC) have been studied using differential scanning calorimetry. The copolymers with high molecular weight exhibit microphase separation into microphases consisting of polystyrene, polybutadiene and poly(ε-caprolactone) and partial crystallization of the corresponding PCL block. The crystallization occurs at temperatures below the PS glass transition. Depending on the block copolymer composition, crystallization takes place through a combination of heterogeneous and homogeneous nucleation. Isothermal crystallization was studied in order to determine the equilibrium melting point, T_m⁰, of the PCL block, which depends on the weight ratio w_PB: w_PCL. The crystallization kinetics was analyzed in terms of Avrami equation. A general decrease in the overall crystallization rate in the block copolymers relative to the equivalent PCL homopolymer was found. Additionally, the growth rate of the spherulites was followed using polarized optical microscopy. Depending on the composition and flexibility of the block connected to PCL, some copolymers showed ring-banded spherulites.     

Gelation Behavior of Bioabsorbable Hydrogels Consisting of Enantiomeric Mixtures of A–B–A Tri‐block Copolymers of Polylactides (A) and Poly(ethylene glycol) (B)

A–B–A tri‐block copolymers of poly(L ‐lactide) (PLLA: A) and poly(ethylene glycol) (PEG: B) and those of poly(D ‐lactide) (PDLA: A) and PEG (B) were prepared and suspended in saline. Mixing suspensions consisting of the enantiomeric copolymers with identical block compositions induced a temperature‐dependent sol‐to‐gel transition. It was found that the composition window of the copolymers that allowed the spontaneous sol–gel transition around body temperature was considerably narrow, being affected by how easily the PLLA and PDLA blocks of the copolymers can form the stereocomplex in the mixed suspensions. The gelation rate and gel strength also depended on the copolymer composition and concentration at a constant gelation temperature of 37 °C.     

Thermal properties and crystallization behavior of poly(ether ether ketone ketone)/poly(ether biphenyl ether ketone ketone) copolymers

Thermal properties of poly(ether ether ketone ketone) (PEEKK)/poly(ether biphenyl ether ketone ketone) (PEDEKK) copolymers were investigated by means of differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The glass transition temperature (T_g) increases from 154°C to 183°C as the content of PEDEKK units increases. The melting point (T_m) of the copolymers varied in the range between 314°C and 409°C and showed the behavior of eutectic type copolymer. From the investigation of the crystallization behavior of the copolymers, it was found that the cold-crystallization temperature (T_c′) of the amorphous copolymers assumes a maximum value for the copolymer with a mole fraction of the PEDEKK segment (n_B) of about 0.6, isothermally crystallized PEEKK and the PEEKK/PEDEKK copolymer exhibit double-melting behavior.     

Synthesis and water-swelling of thermo-responsive poly(ester urethane)s containing poly(epsilon-caprolactone), poly(ethylene glycol) and poly(propylene glycol)

Thermo-responsive multiblock poly(ester urethane)s comprising poly(epsilon-caprolactone) (PCL), poly(ethylene glycol) (PEG), and poly(propylene glycol) (PPG) segments were synthesized. The copolymers were characterized by GPC, NMR, FTIR, XRD, DSC and TGA. Water-swelling analysis carried out at different temperatures revealed that the bulk hydrophilicity of the copolymers could be controlled either by adjusting the composition of the copolymer or by changing the temperature of the environment. These thermo-responsive copolymer films formed highly swollen hydrogel-like materials when soaked in cold water and shrank when soaked in warm water. The changes are reversible. The mechanical properties of the copolymer films were assessed by tensile strength measurement. These copolymers were ductile when compared to PCL homopolymers. Young's modulus and the stress at break increased with increasing PCL content, whereas the strain at break increased with increasing PEG content. The results of the cytotoxicity tests based on the ISO 10993-5 protocol demonstrated that the copolymers were non-cytotoxic and could be potentially used in biomedical applications.     

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.     

Multiblock poly(ether-ester-amide)s based on polyamide-6 and poly(ethylene glycol), 1. Effect of polyether segment length on the properties of poly(ether-ester-amide)s with various polyamide/polyether ratios

A series of multiblock poly(ether-ester-amide)s (PEEA) based on polyamide-6 (PA6) and poly(ethylene glycol) (PEG) is synthesized in a two-step process. The first step represents the hydrolytic polymerization of ε-caprolactam (CL) in the presence of adipic acid (AA) to carboxyl-terminated PA6 oligomers of different chain lengths and the second is their polycondensation with PEG of molecular weights 420 (PEG 400) and 1150 (PEG 1000) in the presence of catalyst and thermostabilizer. The formation of the new ester bond between PA6 and PEG is proved by infrared spectroscopy. The content of the hard (PA6) and the soft (PEG) segments in the copolymer is determined by ¹H NMR and elemental analysis. The shift in glass transition temperature T_g above that of pure PEG and the depression of the melting temperature T_m assigned to PA6 are characteristic for phase-separated block copolymers. The degree of crystallinity of the PA6 component does not seem to be affected by the actual weight composition but depends on the different segment lengths. For the copolymers containing PEG-1000 it is close to that of the respective homopolymer PA6. The tensile parameters of the copolymers studied are similar to those of available PEEA products.     

The morphology of semicrystalline segmented poly(ether ester) thermoplastic elastomers

The supermolecular structure of melt-crystallized segmented copoly(ether ester)s based on poly(tetramethylene terephthalate) and poly(tetramethylene oxide) is investigated by polarizing microscopy and electron microscopic techniques. Spherulites are formed on isothermal crystallization at supercoolings ΔT_u < 30°C. The spherulites recrystallize to form dendritic structures on further annealing at higher temperatures. The spherulites and dendrites are builtup from lamellae which exhibit little variation concerning the thickness of their crystalline core. The internal phase boundaries in these systems are made visible with a newly developed staining method. A network of interlocked individual lamellae is formed on rapid quenching of the melt films. Row crystallization (shish-kebabs) is observed, if the melt is sheared during the crystallization. It is concluded from these observations that the chemical structure of the segmented copolymers bears little relevance for the supermolecular structure of these materials except for the thickness of the crystalline core of the lamellae which seems to be related to the molecular length of the hard segment sequences occurring with the highest concentration in the sample.     

Synthesis and Non‐Isothermal Crystallization Characteristics of Poly[(ethylene)‐co‐(trimethylene terephthalate)]s

A series of random copolyesters were synthesized by the bulk polycondensation of dimethyl terephthalate with ethylene glycol (EG) and trimethylene glycol (TMG) in various compositions. Their composition, molecular weight, and thermal properties were determined. The copolymers containing 22.2 mol‐% ≤ TMG ≥ 46.0 mol‐% are crystallizable while the other copolymers containing 32.8 mol‐% ≤ TMG ≤ 39.3 mol‐% are amorphous. Non‐isothermal crystallization exotherms were measured over the cooling rate of 2.5–20.0 K/min by calorimetry and analyzed by the Avrami method. For the crystallization at 2.5 K/min, the Avrami exponent n was estimated to be in the range of 2.91–3.81, depending on the composition. The results suggest that the primary crystallization follows a heterogeneous nucleation and spherulitic growth mechanism. However, when the cooling rate increases and the content of the minor component unit (EG or TMG) increases, the crystallization behavior becomes deviated from the prediction of the Avrami analysis, which is due to the involvement of secondary crystallization and the achievement of relatively low crystallinity. The crystallization rate is accelerated by increasing cooling rate but retarded by incorporating the minor component into the polymer backbone. In addition, the activation energy in the crystallization was estimated.     

Synthesis of Star‐Comb Double Crystalline Diblock Copolymer of Poly(ε‐caprolactone)‐block‐poly(l‐lactide): Effect of Chain Topology on Crystallization Behavior

A series of highly branched star‐comb poly(ε‐caprolactone)‐block‐poly(l ‐lactide) (scPCL‐b‐PLLA) are successfully achieved using star‐shaped hydroxylated polybutadiene as the macroinitiator by a simple “grafting from” strategy. The ration of each segment can be controlled by the feed ratio of comonomers. These star‐comb double crystalline copolymers are well‐defined and expected to illustrate the influences of the polymer chain topology by comparing with their counterparts in linear‐shaped, star‐shaped, and linear‐comb shape. The crystallization behaviors of PCL‐b‐PLLA copolymers with different architectures are investigated systematically by means of wide‐angle X‐ray diffraction, differential scanning calorimetry, and polarized optical microscopy analysis. It is shown that the comb branched architectures promote the crystallization behavior of each constituent significantly. Both crystallinity and melting temperature greatly raise from linear to comb‐shaped copolymers. Compared to linear‐comb topology, the star‐comb shape presents some steric hindrance of the graft points, which decrease the crystallinity of scPCL‐b‐PLLA. Effects of copolymer composition and chain topology on the crystallization are studied and discussed.

     

Crystallization kinetics and morphology of poly(ferrocenyldimethylsilane)

A series of poly(ferrocenyldimethylsilanes) were prepared via anionic ring-opening polymerization. The isothermal crystallization kinetics were investigated by means of differential scanning calorimetry and analyzed in terms of the Avrami equation. The value of the Avrami exponent is approximately 3 for the majority of samples and crystallization temperatures. This suggests a three-dimensional spherulitic growth and an instantaneous nucleation mechanism. The morphology of melt-crystallized samples was studied by atomic force microscopy. Hedrites (immature spherulites) were formed at low undercoolings whereas at high levels of undercooling mature, well developed spherulites were observed with small hedritic features in their center. The melting enthalpy for a 100% crystalline polymer was estimated to be 36 J/g from differential scanning calorimetry and X-ray diffraction data obtained by using specimens with different degrees of crystallinity.     

Synthesis and characterization of new ABA triblock copolymers with poly[3(S)-isobutylmorpholine-2,5-dione] and poly(ethylene oxide) blocks

ABA type block copolymers with poly[3(S)-isobutylmorpholine-2,5-dione] (PIBMD, A) and poly(ethylene oxide) (M_n = 6 000, PEO, B) blocks, PIBMD-b-PEO-b-PIBMD, were synthesized via ring-opening polymerization of 3(S)-isobutylmorpholine-2,5-dione in the presence of hydroxytelechelic poly-(ethylene oxide) with stannous octoate as a catalyst. M_n of the resulting copolymers increases with increasing 3(S)-isobutylmorpholine-2,5-dione content in the feed at constant mole ratio of monomer (M) to catalyst (C) (M/C = 125). No racemization of the leucine residue takes place during both homopolymerization of IBMD and polymerization of IBMD in the presence of PEO and Sn(Oct)₂. The melting temperature of the PIBMD segments in the block copolymers depends on the length of the PIBMD blocks. The melting temperature of the PEO blocks is lower than that of the homopolymer, and the crystallinity of the PEO block decreases with increasing length of the PIBMD blocks. The PIBMD block crystallizes first upon cooling from the melt. This leads to only imperfect crystallization or no crystallization of the PEO blocks.     

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.     

Poly(Ether/Ester)s based on poly(tetramethylene terephthalate) and poly(ethylene glycol), 3. Effect of thermal treatment and drawing on the structure of the poly(ether/ester)s

Annealed drawn and undrawn bristles of poly(ether/ester)s based on poly(tetramethylene terephthalate) (PTMT) and poly(ethylene glycol) PEG 1000 (in various ratios) are studied by means of small-angle X-ray scatering and differential scanning calorimetry (DSC). The samples with the lowest PTMT content (49 wt.-% PTMT) show on abrupt increase of the scattering intensity and the long spacing with increasing annealing temperature T_a, this increase becoming less pronounced with increasing fraction of hard segment (PTMT). The DSC heating scans are characterized by a low-temperature glass transition due to the soft polyether segments and a multiple melting behaviour at about 200°C due to the crystalline hard PTMT segments. The data suggest a dependence of the thermal behaviour on the chemical composition, annealing temperature, and orientation. The characteristics of the undrawn samples are similar to those of the homopolyester PTMT and the copolymers based on PTMT and poly(tetramethylene oxide). Evidence is found for an increase of phase separation with an increase of the annealing temperature. The formation of larger amorphous regions of PEG with increasing T_a determines the thermal and scattering behaviour observed.     

Effects of solvent treatment on cold crystallization behavior and morphology of poly(ether ether ketone)

Isothermal crystallization kinetics at temperatures between 160–194°C and the morphology of virgin and solvent-treated poly(ether ether ketone) (PEEK) were investigated by using differential scanning calorimetry, scanning electron microscopy (SEM), and wide-angle X-ray scattering analysis. Kinetic analysis showed that the cold crystallization of virgin (untreated) PEEK proceeds with an Avrami exponent of n = 2,5–2,6, and the SEM results revealed a grainy-texture morphology with no regular spherulites, suggesting a planar growth. The X-ray characterization revealed low to medium crystallinity, which is consistent with the SEM results. This is quite different from the typical impinged spherulitic morphology of high crystallinity in melt-crystallized PEEK. Post cold-crystallization annealing at temperatures near the melting point (300–320°C), however, induced a spherulitic morphology resembling that produced from melt crystallization. Furthermore, methylene dichloride treatment on PEEK was found to significantly affect the cold-crystallization kinetics and the resulting morphology. The cold crystallization of solvent-desorbed PEEK samples proceeds with n = 1,0–1,2, reflecting probably an irregular line growth with heterogeneous nucleation. Additionally, SEM examination revealed a highly porous texture in the desorbed/cold-crystallized PEEK that hindered direct observation of the actual crystalline morphology; nevertheless, neither spherulites nor planar lamellar crystals were observed.