Preparation and characterization of poly(?-caprolactone)-b-polyacrylonitrile (PCL-b-PAN) and poly(l-lactide)-b-polyacrylonitrile (PLLA-b-PAN) copolymers by aluminum and lithium alkoxides containing double-headed initiators

Three new metal alkoxides, [(MMPEP)Al(μ-OCH₂C₆H₄CH₂Cl)]₂ (1), [(MMPEP-H)Li·(BnOH)]₂ (2) and [(MMPEP-H)Li·(HOCH₂C₆H₄CH₂Cl)]₂ (3) (MMPEP-H₂: 2,2′-methylene-bis{4,6-di(1-methyl-1-phenylethyl)phenol}) have been synthesized and characterized. Complex 1 was prepared by the reaction of [(MMPEP)Al(CH₃)(Et₂O)] with p-(chloromethyl)benzyl alcohol. Followed by the reaction of MMPEP-H₂ with ⁿBuLi, BnOH or p-(chloromethyl)benzyl alcohol was added to give complexes 2 and 3, respectively. Complex 1 has shown excellent catalytic activity towards ring-opening polymerization (ROP) of ?-caprolactone. Both complexes 2 and 3 are active for ROP of l-lactide. Block copolymers of poly(?-caprolactone)-b-polyacrylonitrile (PCL-b-PAN) and poly(l-lactide)-b-polyacrylonitrile can be synthesized by combining a technique of atom transfer radical polymerization (ATRP) and ROP using a double-headed initiator. Microphase-separated morphology of PCL-b-PAN has been observed by transmission electron microscopy, indicating the formation of self-assembled nanostructure.     

Thermal properties of di- and triblock copolymers of poly(l-lactide) with poly(oxyethylene) or poly(ε-caprolactone)

High molecular weight di- and triblock copolymers of poly(l-lactide), PLLA, (80 wt%) with a crystallizable flexible second component such as poly(ε-caprolactone), PCL, or poly(oxyethylene), PEO, (20 wt%) were obtained in nearly quantitative yields by ring opening of l-lactide initiated by PCL or PEO hydroxy terminated macromers. The copolymers were characterized by ¹H NMR and FTIR spectroscopy and size exclusion chromatography and showed unimodal and narrow molecular weight distributions. X-ray diffraction measurements revealed high crystallinity (38-56%) of the PLLA blocks and gave no clear evidences of PCL or PEO crystallinity. DMTA and DSC techniques showed a melting behaviour of the copolymers (T_m=174-175 °C; ΔH_m=19-37 J/g) quite similar to that of PLLA. PCL and PLLA segments are immiscible, while PLLA and PEO segments are partially miscible in the amorphous phase. Stress-strain measurements indicated a ductile behaviour of the copolymers, characterized by lower tensile moduli (225-961 Pa) and higher elongations at break (25-134%) with respect to PLLA.     

Semi-interpenetrating polymer networks composed of poly(l-lactide) and diisocyanate-bridged 4-arm star-shaped ɛ-caprolactone oligomers

Semi-interpenetrating polymer networks (semi-IPNs) were prepared by reactions of methylenediphenyl 4,4’-diisocyanate (MDI) and hydroxy-terminated 4-arm star-shaped ε-caprolactone oligomers (H4CLO_n's) with the degrees of polymerization per one arm, n = 3, 5 and 10 in the presence of poly(l-lactide) (PLA). Morphologies, thermal and mechanical properties of the MDI-bridged H4CLO_n (MH4CLO_n)/PLA semi-IPNs were evaluated by comparing with those of poly(?-caprolactone) (PCL)/PLA blends. Two tan δ peaks related to MH4CLO_n and PLA were observed in a dynamic mechanical curve of the semi-IPN. Although all the semi-IPNs and blends had micro-phase separated morphologies, the phase-separated droplets of MH4CLO₅/PLA 50/50 were much finer than those of PCL/PLA 50/50. Differential scanning calorimetry (DSC) analyses revealed that MH4CLO₃ and MH4CLO₅ are substantially amorphous, while MH4CLO₁₀ is semi-crystalline, and that cold crystallization of the PLA component of MH4CLO_n/PLA is more strongly disturbed for the semi-IPN with a smaller n value and more MH4CLO_n content. Tensile modulus, toughness and elongation at break of MH4CLO₅/PLA 50/50 semi-IPN were much higher than those of PCL/PLA 50/50 blend.     

Toughening of poly(l-lactide) with poly(ε-caprolactone): Combined effects of matrix crystallization and impact modifier particle size

Enhancing matrix crystallization has been demonstrated to be an effective method to simultaneously improve the impact toughness and heat resistance of poly(l-lactide) (PLLA) modified with flexible polymers, such as poly(ε-caprolactone) (PCL). Unfortunately, increasing PLLA matrix crystallinity alone cannot guarantee the enhancement of impact toughness in most cases, so other structural parameters should be considered. In this work, taking PLLA/PCL (80/20) blend as an example, the combined roles of matrix crystallization and impact modifier particle size in the toughening have been investigated. PLLA matrix crystallinity was controlled by adding a highly effective nucleating agent and PCL particle size was tailored by varying processing conditions while maintaining constant interfacial adhesion. It is interesting to find that toughening is efficient only if matrix crystallinity and particle size are well matched. With the significant increase of matrix crystallinity, an evident decrease of optimum particle size for toughening PLLA has been identified for the first time. Therefore, suitable particle size is the precondition for highly crystalline matrix to work effectively in the toughening because only small particles (0.3–0.5 μm) are effective in trigger shear yielding mechanism of the matrix needed for good toughness, whereas relatively large particles (0.7–1.1 μm) are only capable of toughening amorphous matrix effectively by initiating multiple crazing of the matrix. Importantly, our findings can be used to well explain the reason for the different roles of matrix crystallization in the toughening of different PLLA blends reported in the literature. Furthermore, the heat resistance of the blend with a highly crystalline matrix is much better than that of the blend with an amorphous one as expected. This work could not only provide a new insight into the synergistic roles of matrix crystallization and modifier particle size in the toughening of PLLA but also set up a universal framework for designing high-performance PLLA products with both good impact toughness and high heat resistance.     

Ring-opening polymerization of ε-caprolactone and l-lactide using organic amino calcium catalyst

Poly (ε-caprolactone) (PCL) and poly (l-lactide) (PLA) were prepared by ring-opening polymerization catalyzed by organic amino calcium catalysts (Ca/PO and Ca/EO) which were prepared by reacting calcium ammoniate Ca(NH₃)₆ with propylene oxide and ethylene oxide, respectively. The catalysts exhibited high activity and the ring-opening polymerization behaved a quasi-living characteristic. Based on the FT-IR spectra and the calcium contents of the catalysts, and based on the ¹H NMR end-group analysis of the low molecular weight PCL prepared using catalysts Ca/PO and Ca/EO, it was proposed that the catalysts have the structure of NH₂-Ca-O-CH(CH₃)₂ and NH₂-Ca-O-CH₂CH₃ for Ca/PO and Ca/EO, respectively. The ring-opening polymerization of CL and LA follows a coordination-insertion mechanism and the active site is the Ca-O bond.     

Catalysts for the ring-opening polymerization of ε-caprolactone and l-lactide and the mechanistic study

Two novel magnesium aryloxides have been prepared and their catalytic activities toward ring-opening polymerization (ROP) of ε-caprolactone and l-lactide have been investigated. The reaction of 2,2′-(2-methoxybenzylidene)-bis(4,6-di(1-methyl-1-phenylethyl)phenol) (MEMPEP-H₂) (1) and 2,2′-methylene-bis(4,6-di(1-methyl-1-phenylethyl)phenol) (MMPEP-H₂) with ⁿBu₂Mg yield dimeric magnesium complexes [Mg(μ-MEMPEP)(THF)]₂ (2) and [Mg(μ-MMPEP)(THF)]₂ (3), respectively. Catalytic studies of complexes 2 and 3 illustrate that both 2 and 3 are good catalysts in ε-caprolactone and l-lactide polymerization. Theoretical study of the ROP mechanism of ε-caprolactone catalyzed by 2 demonstrates that the initiator, benzyl alcohol, is activated by the formation of a hydrogen bond with the phenoxy oxygen of MEMPEP²⁻ ligand.     

Microstructure analysis and thermal properties of l-lactide/?-caprolactone copolymers obtained with magnesium octoate

A series of copolymers with various compositions were prepared by the ring opening copolymerization of l-lactide (l-LA) and ?-caprolactone (?-CL) using nontoxic magnesium octoate as a catalyst in bulk. The copolymerization process and the influence of transesterification on the chain microstructure were examined by ¹H and ¹³C NMR. A tapered block or gradient copolymer is expected to be formed on the basis of the reactivity difference between l-LA and ?-CL. Two modes of transesterification occurred and played an important role in the redistribution of comonomer sequences but not a completely random distribution. The CLC sequence formed by the second mode of transesterification was observed at the end of reaction. The coefficient of the second mode of transesterification (T_II) increased as the feed mole fraction of ?-CL increased. In terms of the overall feed compositions, the L_LL^r values of lactidyl sequences calculated from the reactivity ratio exceeded the L_LL^e values determined from the product, however, the L_C^r values of caproyl sequences were identical or shorter than the L_C^e values. The thermal properties and crystallinities of the obtained copolymers were investigated by DSC and WAXD. The thermal properties and crystallinities depend on both the composition and the chain microstructure. The l-LA/?-CL copolymer with intermediate composition exhibited some blocky character by DSC. Only single T_g was observed for each copolymer and in agreement with the calculated value from Fox equation, indicating that the amorphous region of the copolymers is miscible. The obtained copolymers can best be described as random copolymers with more or less blocky chain structure.     

NMR determination of crystallinity for poly(?-l-lysine)

Crystallinity of poly(?-l-lysine) (?-PL) was discussed by analyzing the differences in the ¹H spin-spin relaxation times (T₂^H), the ¹³C spin-lattice relaxation times (T₁^C), and the ¹³C NMR signal shapes between the crystalline and the non-crystalline phases. The observed ¹H relaxation curve (free induction decay followed by solid-echo method) showed the sum of Gaussian and exponential decays. Similarly, the observed ¹³C relaxation curves obtained from the Torchia method were double-exponential. The ¹³C NMR spectrum of ?-PL was divided into the narrow and the broad lines by utilizing the intrinsic differences in the ¹H spin-lattice relaxation times in the rotating-frame between them, which are attributed to the crystalline and the non-crystalline phases, respectively. Even though the crystallinity is obtained from the identical NMR measurements, the estimated values are different with each other. The crystallinity estimated from the T₂^H differences was 75.8 ± 0.1% at 333 K and 60.7 ± 0.4% at 353 K. From the T₁^C differences, the value was estimated to be 62 ± 11%. Furthermore, the value estimated from the NMR signal separation was 54 ± 5%. In this study we have explained these discrepancies by the difference in susceptibility among the experiments for the inter-phase, which exists in-between the crystalline and the amorphous phases. Furthermore, the estimated crystallinity was ascertained by the X-ray diffraction experiment.     

Molecular ordering and α′-form formation of poly(l-lactide) during the hydrolytic degradation

Hydrolytic degradation ability is an intriguing characteristic of poly(l-lactide) (PLLA) and it has been intensively investigated recently. However, the microstructure evolution of PLLA during the early stage of the hydrolytic degradation process is neglected. In this work, amorphous PLLA was hydrolyzed in alkaline media at temperatures from 40 to 60 °C. The variations of weight loss and molecular weight were measured to study the degree of the hydrolytic degradation of PLLA. The effect of hydrolytic degradation on microstructure of the amorphous PLLA was investigated by differential scanning calorimetry (DSC), Fourier transform infrared spectroscope (FTIR), wide angle X-ray diffraction (WAXD) and scanning electron microscope (SEM). The results clearly proves the occurrence of molecular ordering and the formation of α′-form PLLA, which is greatly related to the hydrolytic degradation conditions. At relatively low hydrolytic degradation temperature (40 and 50 °C), locally ordered structure is provoked, while a large number of α′-form crystallites are induced at relatively high temperature (60 °C). This work is very significant in understanding the microstructure evolution of PLLA during the hydrolytic degradation process.     

Porous microspheres of methoxy poly(ethylene glycol)-b-poly(?-caprolactone-co-d,l-lactide) prepared by a melt dispersion method

Surfactant-free biodegradable porous microspheres of methoxy poly(ethylene glycol)-b-poly(?-caprolactone-co-d,l-lactide) (MPEG-b-PCLDLL) diblock copolymers were prepared by a simple melt dispersion method in water at 80 °C with magnetic stirring. Any organic solvents and surfactants can be neglected for this method. Different CL/DLL ratios in the MPEG-b-PCLDLL were investigated for preparation of the porous microspheres. It was found that microsphere sizes decreased and surface pore sizes increased as the increasing DLL ratio. The pores were well interconnected throughout the microsphere matrices for all MPEG-b-PCLDLLs. The larger pore sizes can be obtained when the PEG was blended with diblock copolymer before preparation of porous blended microspheres. Possible mechanisms for formation of the porous microspheres with and without PEG blending were also proposed.     

Well-defined sterically hindered zinc aryloxides: Excellent catalysts for ring-opening polymerization of ?-caprolactone and l-lactide

Three novel sterically hindered zinc aryloxides have been prepared and well characterized. Their catalytic activities toward ring-opening polymerization (ROP) of ?-caprolactone and l-lactide have been investigated. The reaction of 2,2′-ethylidene-bis(4,6-di-tert-butylphenol) (EDBP-H₂), 2,2′-(2-methoxybenzylidene)bis(4-methyl-6-tert-butylphenol) (MEBBP-H₂) and 2,2′-(2-methoxybenzylidene)bis(4,6-di(1-methyl-1-phenylethyl)phenol) (MEMPEP-H₂) with ZnEt₂ in THF yields dimeric zinc complexes [(μ-EDBP)Zn(THF)]₂ (1), [(μ-MEBBP)Zn(THF)]₂ (2) and [(μ-MEMPEP)Zn(THF)]₂ (3), respectively. Experimental results show that all three compounds are good catalysts for ROP of ?-caprolactone and l-lactide yielding polymer in a controlled fashion with low polydispersity indexes.     

Synthesis and characterization of a functionalized biodegradable copolymer: poly(l-lactide-co-RS-β-malic acid)

The ring-opening copolymerization of l-lactide (l-LA) and RS-β-benzyl malate (MA) was performed in the presence of stannous octoate. Copolymers were successfully synthesized using different feeding doses. ¹H NMR analysis revealed that the compositions of the copolymers with high MA contents were similar to the feeding doses. GPC measurements showed that the molecular weight of the copolymers decreased as the MA content increased. The hydrophilicity of these copolymers was remarkably improved after they were hydrogenolyzed over palladium on charcoal, which removed the pendant benzyl groups. These de-protected copolymers had higher glass transition temperatures (T_g) than both the corresponding protected copolymers and the PLLA homopolymer due to the formation of intermolecular hydrogen bonds between the pendant carboxyl groups. The morphology of the copolymers changed from crystalline to amorphous with increasing RS-β-malic acid content.     

A-B-A-Triblock and multiblock copolyesters prepared from ε-caprolactone, glycolide and l-lactide by means of bismuth subsalicylate

Bismuth (III) subsalicylate, a commercial drug, was used as catalyst for 1,4-butanediol-initiated copolymerizations of ε-caprolactone (εCL) and glycolide (GL). Telechelic copolyesters having two OH-endgroups and predominantly alternating sequences were obtained. These copolyesters are amorphous with glass transition temperatures (T_gs) below −30 °C. In a second series of polymerizations, in situ chain extension with l-lactide (LLA) was performed, whereby A-B-A triblock copolymers were obtained without significant transesterification between A- and B-blocks. Finally, these A-B-A triblock copolymers were transformed into multiblock copolymers by chain extension with 1,6-hexamethylene diisocyanate. The block copolymers were characterized by ¹H and ¹³C NMR spectroscopy, by viscosity, SEC and DSC measurements.     

Poly(l-lactide-co-?-caprolactone) electrospun nanofibers for encapsulating and sustained releasing proteins

The aim of this study was to investigate coaxial electrospun poly(l-lactide-co-?-caprolactone) [PLLACL] nanofibers for the application in nerve tissue engineering. The hypothesis was that the nanofibrous mats fabricated by coaxial electrospun PLLACL could be effective scaffolds for releasing proteins, such as Bovine Serum Albumin (BSA) or/and Nerve Growth Factor (NGF), in a sustained manner. To test the hypothesis, the coaxial electrospun nanofibers with PLLACL as the shell and BSA/NGF as the core were characterized. Morphologies and mechanical properties of nanofibrous mats were examined. BSA released behavior was studied. The results demonstrated that BSA could be sustainedly released from coaxial electrospun PLLACL nanofibers, however, BSA released from mix electrospun nanofibers present the burst release behavior. Bioactivity of released NGF from coaxial electrospun nanofibers was verified by testing the differentiation of rat pheochromocytoma cells (PC12).     

Thermal degradation and theoretical interpretation of vibrational spectra of poly (β,l-malic acid)

Poly (β,l-malic acid) (PMLA) was obtained by Physarum polycephalum. The FT-IR and FT-Raman spectra were recorded experimentally and used for normal mode analysis using Wilson GF matrix method and phonon dispersion of PMLA. The non redundant set of internal coordinate are simplified. Urey-Bradley force field approximation was employed in normal mode analysis and to calculate the potential energy distribution (PED) of each fundamental vibration. Apart from detailed assignment, various characteristic features of dispersion curve have also been explained, arising due to internal symmetry in energy momentum space. Predictive values of the intramolecular contribution to the heat capacity of this polymer calculated by the density of states, are also been reported. The thermal degradation of PMLA was investigated in nitrogen atmosphere by thermogravimetric analysis in combination with DTA and FT-IR. The spectra of gases found in the thermal degradation of PMLA are very much similar to the spectra of gases released during decomposition of fumaric acid.     

Spectroscopic and thermal analyses of α′ and α crystalline forms of poly(l-lactic acid)

Poly(lactic acid) samples rich in α′ or α crystals have been characterized using spectroscopic and thermal methods. Cryogenic infrared and Raman spectroscopy were used to probe the differences in chain conformation and packing. Compared to the α crystal, the α′ crystal has weakened specific carbonyl and methyl interactions. Experimental spectroscopic analysis in conjunction with simulation studies have shown that the α′ crystal has uniform conformational disorder in the C_α-C torsion angle. This disorder in chain conformation and packing leads to different crystalline forms with different stabilities. The difference in thermal stability was quantified by measuring enthalpic change at melting for both crystalline forms. Significantly different values for the two crystalline forms were obtained ( and ). The transformation from the less stable α′ to the more stable α phase has been characterized. This analysis provides an explanation to the double melting peaks usually found in PLLA samples.     

Solid-state reorganization,melting and melt-recrystallization of conformationally disordered crystals (α′-phase) of poly (l-lactic acid)

Conformationally disordered α′-crystals of poly (l-lactic acid) were formed by crystallization of the melt at high supercooling at 95 °C. Analysis of their melting temperature as a function of the crystallinity revealed absence of crystal thickening during isothermal crystallization. Annealing of α′-crystals between the crystallization temperature of 95 °C and their zero-entropy production melting temperature of 150 °C leads to their stabilization, mainly by solid-state reorganization. Heating faster than 30 K s⁻¹ suppresses reorganization and permits superheating of the α′-phase. Consequently, isothermal melting followed by melt-recrystallization becomes accessible. Melting is completed within few hundreds of milliseconds, and melt-recrystallization is about two orders of magnitude faster than crystallization of the isotropic melt at identical temperature. The time required for melting decreases with superheating and increases with the lateral dimension of the lamellar crystals. Laterally extended lamellae require long time for melting of the outer crystal layers, which allows stabilization of the central crystal part. These crystal remnants then serve as seed for immediate recrystallization. In case of complete melting of smaller lamellae, melt-recrystallization is retarded but still distinctly faster than cold- and melt-crystallization, due to incomplete isotropization of the melt.     

Preparation and characterization of poly(ɛ-caprolactone) nonwoven mats via melt electrospinning

Laser melt electrospinning is a novel technology to produce nonwoven scaffolds for tissue engineering (TE) applications. This solvent-free process is far safer than common solution electrospinning. In this paper, we demonstrated the poly(?-caprolactone) (PCL) fibers diameters could be governed from 3 to 12 μm with changing electrospinning parameters. The various diameters can meet the needs of scaffold properties such as porosity, pore size, etc. Our experiential results also showed that the fibers diameter tended to decrease as laser current increased. The degradation of PCL molecular chains often occurs in the melt electrospinning process due to mechanical scission and thermal degradation. The crystallinity of as-spun PCL fibers was approximately equal to that of the annealing fibers by X-ray diffraction (XRD) and differential scanning calorimetry (DSC). In our experiential, the collected PCL electrospun fibers often fused together to form a three-dimension network structure, which is favorable to mechanical properties.     

Self-organized thermosets involving epoxy and poly(?-caprolactone)-block-poly(ethylene-co-ethylethylene)-block-poly(?-caprolactone) amphiphilic triblock copolymer

In this work, we investigated the self-assembly behavior of poly(?-caprolactone)-block-poly(ethylene-co-ethylethylene)-block-poly(?-caprolactone) (PCL-b-PEEE-b-PCL) triblock copolymer in epoxy thermosets. The PCL-b-PEEE-b-PCL triblock copolymer was synthesized via the ring-opening polymerization of ?-caprolactone with a hydroxyl-terminated poly(ethylene-co-ethylethylene) as the macromolecular initiator. The hydroxyl-terminated poly(ethylene-co-ethylethylene) was prepared with the hydrogenation reaction of a hydroxyl-terminated polybutadiene. The triblock copolymer was incorporated into the precursors of epoxy to obtain the nanostructured thermosets. It was found that the self-organized nanophases were formed in the mixture before curing reaction and the nanostructures can be further fixed via curing reaction. The self-assembly behavior of the triblock copolymer in epoxy thermosets was investigated by means of atomic force microscopy (AFM), small-angle X-ray scattering (SAXS) and dynamic mechanical thermal analysis (DMTA). Differential scanning calorimetry (DSC) shows that the formation of the self-organized nanophase in the thermosets caused that a part of poly(?-caprolactone) subchains were demixed from epoxy matrix with the occurrence of curing reaction; the fractions of demixed PCL blocks were estimated according to the T_g-composition relation of the model binary blends of epoxy and PCL.     

Crystal transformation from the α- to the β-form upon tensile drawing of poly(l-lactic acid)

A film of poly(l-lactic acid) (PLLA) consisting of highly oriented α crystals was uniaxially drawn by tensile force. The effects of the draw ratio (DR), draw temperature (T_d), and draw stress on the crystal/crystal transformation from the α- to the β-form crystals were studied. At the initial stage of drawing, the highly oriented α crystals of the starting film transformed into a broader orientation distribution, and significant crystal disorder was introduced. Upon further drawing, the α crystals steadily transformed into β crystals with increasing the DR. For the drawing at a constant T_d, the crystal transformation proceeded more efficiently at a higher draw rate and, hence, at a higher draw stress. Furthermore, for the drawing at a constant draw rate, the transformation proceeded with DR most efficiently for the tensile draw at a T_d around 140 °C, although the draw stress increased with decreasing the T_d. The present result combined with the previous finding in the drawing of PLLA by solid-state extrusion [Macromolecules 36 (2003) 3601] suggests that there is a T_d of around 140 °C at which the crystal transformation proceeds most efficiently with DR, suggesting that there are two factors that have opposite effects on the efficiency of the crystal transformation with increasing the T_d. However, as a result of the combined effects of the T_d and DR on the crystal transformation and the ductility increase with the T_d, an oriented film consisting predominantly of β crystals was obtained by tensile drawing at a T_d in the range of 140-170 °C to the highest DR achieved at each T_d.