Novel biodegradable aliphatic poly(butylene succinate-co-cyclic carbonate)s with functional carbonate building blocks. 1. Chemical synthesis and their structural and physical characterization

This study presents chemical synthesis, structural, and physical characterization of novel biodegradable aliphatic poly(butylene succinate-co-cyclic carbonate)s P(BS-co-CC) bearing functional carbonate building blocks. First, five kinds of six-membered cyclic carbonate monomers, namely, trimethylene carbonate (TMC), 1-methyl-1,3-trimethylene carbonate (MTMC), 2,2-dimethyl-1,3-trimethylene carbonate (DMTMC), 5-benzyloxytrimethylene carbonate (BTMC), and 5-ethyl-5-benzyloxymethyl trimethylene carbonate (EBTMC), were well prepared from ethyl chloroformate and corresponding diols at 0 degrees C in THF solution with our modified synthetic strategies. Then, a series of new P(BS-co-CC)s were synthesized at 210 degrees C through a simple combination of poly-condensation and ring-opening-polymerization (ROP) of hydroxyl capped PBS macromers and the prepared carbonate monomers, and titanium tetra-isopropoxide Ti(i-OPr)4 was used as a more suitable catalyst of 5 candidate catalysts which could concurrently catalyze poly-condensation and ROP. By means of NMR, GPC, FTIR, and thermal analytical instruments, macromolecular structures and physical properties have been characterized for these aliphatic poly(ester carbonate)s. The experimental results indicated that novel biodegradable P(BS-co-CC)s were successfully synthesized with number average molecular weight Mn ranging from 24.3 to 99.6 KDa and various CC molar contents without any detectable decarboxylation and that the more bulky side group was attached to a cyclic carbonate monomer, the lower reactivity for its copolymerization would be observed. The occurrences of 13C NMR signal splitting of succinyl carbonyl attributed to the BS building blocks could be proposed due to the randomized sequences of BS and CC building blocks. FTIR characterization indicated two distinct absorption bands at 1716 and 1733 approximately 1735 cm(-1), respectively, stemming from carbonyl stretching modes for corresponding BS and CC units. With regard to their thermal properties, it is seen that the synthesized P(BS-co-CC)s exhibited thermal degradation temperatures 10 approximately 20 degrees C higher than that of PBS. On the basis of the synthesized P(BS-co-BTMC)s, new aliphatic poly(butylene succinate-co-5-hydroxy trimethylene carbonate)s were further synthesized, bearing hydrophilic hydroxyl pendant functional groups through an optimized Pd/C catalyzed hydrogenation. These semi-crystalline new biodegradable aliphatic copolymers with tunable physical properties and functional carbonate building blocks might be expected as potential new biomaterials.     

Molecular mobility and phase structure of biodegradable poly(butylene succinate) and poly(butylene succinate-co-butylene adipate)

Molecular mobility and phase structure of biodegradable poly(butylene succinate) (PBS) and poly(butylene succinate-co-20 mol % butylene adipate) [P(BS-co-20 mol % BA)] have been investigated by high-resolution solid-state (13)C NMR. For both samples, two components with different (13)C spin-lattice relaxation time (T(1C)) values have been observed in the crystalline region. The crystalline component with shorter T(1C) value is assignable to the interface near amorphous phase. The crystalline component with longer T(1C) value is ascribed to the inside of the crystalline region. On the basis of T(1C), it has been concluded that the BA units are not included in the crystalline region of P(BS-co-20 mol % BA). Molecular mobility and higher-ordered structure of amorphous phase have been also compared between the melt and solid state. Variable-temperature high-resolution (13)C NMR measurements for the amorphous phase have revealed the remarkable difference in dynamics and structure between the melt and solid state.     

Novel lipase-catalyzed ring-opening copolymerization of lactide and trimethylene carbonate forming poly(ester carbonate)s.

Poly(lactide-co-trimethylene carbonate)s were prepared by the lipase-catalyzed ring-opening copolymerization of lactide and trimethylene carbonate having carbonate content from 0 to 100%. Their thermal properties and enzymatic degradability were measured. The L,L-, D,D- and D,L-lactides were copolymerized with trimethylene carbonate by porcine pancreatic lipase to produce random copolymers having molecular weights of up to 21000. The glass transition temperature (Tg of the copolymer was dependent on the carbonate content and the Tg values linearly decreased from 35 degrees C (polylactide) to -8 degrees C [poly(trimethylene carbonate)]. Among the lipases tested, the porcine pancreatic lipase and proteinase K showed biodegradability towards poly(ester-carbonate)s.     

Synthesis and characterization of poly(butylene succinate-co-butylene malate): a new biodegradable copolyester bearing hydroxyl pendant groups

A new biodegradable copolyester, poly(butylene succinate-co-butylene malate) P(BS-co-BM), has been preliminarily prepared with optically active centers and lateral hydroxyl functional groups via a four-step synthetic strategy. First, an optically active benzyl-protected dimethyl malate was synthesized from a starting material of (S)-dimethyl malate and purified with good yield. Then, copolyester poly(butylene succinate-co-benzyl-protected butylene malate), P(BS-co-BBM), was prepared through a skilled condensation copolymerization of the benzyl-protected dimethyl malate, dimethyl succinate, and 1,4-butanediol in the presence of titanium tetraisopropoxide as the catalyst. Finally, a Pd/C catalyzed hydrogenation was applied to eliminate the benzyl protection group in a mixed solution of THF and methanol; thus the target copolyester P(BS-co-BM) was attained. On the other hand, physical properties of the synthesized copolyesters were systematically characterized by means of nuclear magnetic resonance spectrometer, Fourier transformed infrared spectrometer, gel permeation chromatography, optical polarimeter, quantitative hydroxyl titration, and thermal analytical instruments. The experimental evidence demonstrated a successful construction of the product P(BS-co-BM) bearing lateral hydroxyl functional groups. It was also revealed that the lower BBM unit content was in the benzyl-protected optically active P(BS-co-BBM) copolyester, the higher melting point T(m), crystallinity, the broader molecular distribution, and the lower glass transition temperature T(g) would be detected, and these results can be accounted for the presence of bulky lateral benzyl moieties. In contrast, the deprotected product P(BS-co-55 mol % BM) showed a higher T(m), crystallinity and lower T(g) than its counterpart P(BS-co-55 mol % BBM). Interestingly, a thermal stability as high as that of the linear PBS was observed for P(BS-co-55 mol % BM) while a strong BBM unit content dependence of thermal stability was detected for the benzyl-protected copolyester P(BS-co-BBM)s. Therefore, these results may be beneficial for the new optically active P(BS-co-BM) bearing hydrophilic hydroxyl functional groups as a potential biomaterial.     

Structural effects of terminal groups on nonenzymatic and enzymatic degradations of end-capped poly(L-lactide)

Poly(L-lactide) (PLLA) with various alkyl ester chain end groups were synthesized by ring-opening polymerization of L-lactide in the presence of zinc alkoxide as a catalyst. The structural effect of chain end groups on the rate of enzymatic and nonenzymatic degradations for amorphous films of PLLA were investigated at 37 degrees C in a Tris-HCl buffer solution (pH 8.6) with proteinase K and at 60 degrees C in a phosphate buffer solution (pH 7.4), respectively. The rate of enzymatic degradation for PLLA films was dependent on the carbon numbers of alkyl ester chain end groups, and the rates of PLLA samples with dodecyl (C12), tridecyl (C13), and tetracocyl (C14) ester end groups were much lower than those of the other samples. The surface morphologies of PLLA films after enzymatic degradation were characterized by scanning electron microscopy. After the enzymatic degradation, non-end-capped PLLA, PLLA with methyl (C1) and hexyl (C6) ester chain ends, were degraded homogeneously by proteinase K and the film surface was very smooth. In contrast, the PLLA with alkyl ester chain ends of carbon numbers over 12 were degraded heterogeneously by the enzyme, and the sponge-like network structure was formed on the film surface. These results indicated that the long alkyl ester groups at the chain ends of PLLA molecules aggregated in the amorphous films and the erosion rate was depressed due to the coverage of the aggregated terminal groups on the film surface. For the nonenzymatic degradation, the molecular weight of non-end-capped PLLA was remarkably decreased with progress of degradation. In contrast, the molecular weight of the end-capped PLLA gradually reduced at the initial stage of degradation and then the rate of degradation was accelerated. The decreases of molecular weight of PLLA by autocatalyzed degradation were retarded by the capping of carboxyl chain ends.     

Evidence for selective hydrolysis of aliphatic copolyesters induced by lipase catalysis

High molar mass random poly(butylene succinate-co-butylene sebacate), P(BS-co-BSe), and poly(butylene succinate-co-butylene adipate), P(BS-co-BA), with different composition, were synthesized and subjected to enzymatic hydrolysis by Lipase from Mucor miehei or from Rhizopus arrhizus. The enzymatic hydrolysis of P(BS-co-BSe)s and P(BS-co-BA)s films produced a mixture of water-soluble monomers and co-oligomers that were separated and identified by on-line high performance liquid chromatography/electrospray ionization mass spectrometry (HPLC/ESI-MS). Optimization of the HPLC analysis allowed the separation of isobar co-oligomers, differing only for the co-monomers sequence. Oligomers with the same monomer composition and molar mass but different sequence were identified by HPLC/ESI-MS-MS on-line analysis. The results obtained show a preferential hydrolytic cleavage induced by the lipases used. In particular, these enzymes prefer cleaving sebacic ester bonds in P(BS-co-BSe) copolymers, whereas succinic ester bonds appear to be hydrolyzed faster than adipic ester bonds in P(BS-co-BA) copolyesters. 1H NMR analysis further substantiates these findings. The primary products generated by lipase hydrolysis of polyester films underwent further degradation at longer reaction times. The HPLC/ESI-MS analysis of these mixtures at various times provided the first evidence that lipase catalysis is active also in water solution, a hydrophobic effect induced by the aliphatic units of these polyesters.     

In vivo study on the histocompatibility and degradation behavior of biodegradable poly(trimethylene carbonate-co-D,L-lactide)

The aim of this study was to explore the in vivo behavior and histocompatibility of poly(trimethylene carbonate-co-D,L-lactide) (PDLLA/TMC) and its feasibility of manufacturing cardiovascular stents. Copolymers with 50/50 molar ratio were synthesized by ring-opening polymerization with TMC and D, L-LA, or TMC and L-LA. Poly(L-lactide) (PLLA) was synthesized as a control. The films of the three polymers were implanted into 144 Wistar rats. At different time points of implantation, polymer films were explanted for the evaluation of degradation characteristics and histocompatibility using size exclusion chromatography , nuclear magnetic resonance , environmental scanning electron microscope , and optical microscope. Results showed that there were differences in the percentage of mass loss, molecular weight, shape and appearance changes, and inflammation cell counts between different polymers. With the time extended, the film's superficial structure transformed variously, which was rather obvious in the polymer of PDLLA/TMC. In addition, there were relatively lower inflammation cell counts in the PDLLA/TMC and poly(trimethylene carbonate-co-L-lactide) (PLLA/TMC) groups at different time points in comparison with those in the PLLA group. The differences were of statistical significance (P< 0.05) in the group of PDLLA/TMC vs. PLLA, and the group of PLLA/TMC vs. PLLA, but not within the PDLLA/TMC and PLLA/TMC groups (P> 0.05). These results suggested that the polymer of PDLLA/TMC (50/50) with favorable degradation performance and histocompatibility is fully biodegradable and suitable for manufacturing implanted cardiovascular stents.     

Resilient bioresorbable copolymers based on trimethylene carbonate, L-lactide, and 1,5-dioxepan-2-one

The new combinations of monomers presented in this work were evaluated in order to create an elastic material for potential application in soft tissue engineering. Thermoplastic elastomers (TPE) of trimethylene carbonate (TMC) with L-lactide (LLA) and 1,5-dioxepan-2-one (DXO) have been synthesized using a cyclic five-membered tin alkoxide initiator. The block copolymers were designed in such a way that poly(trimethylene carbonate-co-1,5-dioxepan-2-one) formed an amorphous middle block and the poly(L-lactide) (PLLA) formed semicrystalline terminal blocks. The amorphous middle block consisted of relatively randomly distributed TMC and DXO monomer units, and the defined block structure of the PLLA terminal segments was confirmed by 13C NMR. The properties of the TMC-DXO-LLA copolymers were compared with those of triblock copolymers based either on LLA-TMC or on LLA-DXO. Differential scanning calorimetry and dynamic mechanical analysis data confirmed the micro-phase separation in the copolymers. The mechanical properties of the copolymers were evaluated using tensile testing and cycling loading. All of the copolymers synthesized showed a highly elastic behavior. The properties of copolymers could be tailored by altering the proportions of the different monomers.     

Blends of aliphatic polyesters. VI. Lipase-catalyzed hydrolysis and visualized phase structure of biodegradable blends from poly(epsilon-caprolactone) and poly(L-lactide)

Phase-separated biodegradable polymer blends were prepared from poly(epsilon-caprolactone) (PCL) and poly(L-lactide) (PLLA), and Rhizopus arrhizus lipase-catalyzed hydrolysis and phase structure of the blend films were investigated. Gravimetry revealed that the lipase-catalyzed hydrolysis of PCL in PCL- and PLLA-rich phases is disturbed by the presence of PLLA. Polarimetry confirmed the occurrence of a predominant hydrolysis of PCL and subsequent removal of the hydrolyzed water-soluble PCL oligomers in the blend films. Gravimetry and gel permeation chromatography of the non-blended PLLA film indicated that R. arrhizus lipase has no catalytic effect on the hydrolysis of PLLA. The phase structure of the blend films could be visualized by selective enzymatic removal of one component and subsequent scanning electron microscopic observation.     

Enzymatic, alkaline, and autocatalytic degradation of poly(L-lactic acid): effects of biaxial orientation

The hydrolytic degradation of biaxially oriented and de-oriented (melt-crystallized) poly(l-lactic acid) (PLLA) films was investigated in Tris-HCl-buffered solution (pH 8.6) with proteinase K, alkaline solution, and phosphate-buffered solution (pH 7.4) by the use of gravimetry, gel permeation chromatography, differential scanning calorimetry, and scanning electron microscopy. Biaxial orientation disturbed the proteinase K-catalyzed enzymatic degradation of PLLA films and the effects of biaxial orientation overcame those of crystallinity. The former may be due to the fact the enzyme cannot attach to the extended (strained) chains in the amorphous regions of the biaxially oriented PLLA film or cannot catalyze the cleavage of the strained chains. Another probable cause is that the enzyme can act only at the film surface of the biaxially oriented PLLA film, in marked contrast with the case of the de-oriented PLLA films where enzymatic degradation can proceed beneath the spherulitic crystalline residues. The effects of biaxial orientation on the alkaline and autocatalytic degradation of the PLLA films were insignificant for the periods studied here. The crystallinity rather than the biaxial orientation seems to determine the alkaline and autocatalytic degradation rates of the PLLA films. The accumulation of crystalline residues formed as a result of selective cleavage and removal of the amorphous chains was observed for the de-oriented PLLA films, but not for the biaxially oriented PLLA film, when degraded in the presence of proteinase K. This means the facile release of formed crystalline residues from the surface of the biaxially oriented PLLA film during enzymatic degradation, due to the fact that the crystalline regions of the biaxially oriented PLLA film were oriented with their c axis parallel to the film surface.     

Cocrystallization model for synthetic biodegradable poly(butylene adipate-co-butylene terephthalate)

Synthetic biodegradable poly(butylene adipate-co-butylene terephthalate), P(BA-co-BT), with 56 mol % butylene adipate, BA, was characterized by solid-state NMR spectroscopy, thermal analysis, X-ray diffraction, computer modeling, and polarization microscopy. The NMR study showed the presence of BA and butylene terephthalate, BT. T(1C) NMR measurements proved that some BA and BT units were in crystalline regions. Thermal analysis showed one glass-transition temperature and a single diffuse melting endotherm corresponding to a large melting-point depression of about 100 degrees C compared with poly(butylene terephthalate), PBT. These results suggest that there is only one crystalline phase. An X-ray fiber diagram of a stretched film could be indexed with the same unit cell as that for PBT. Computer modeling showed that the adipate unit fits into the crystal structure of PBT by adopting a TTGTG dihedral angle sequence in the crystalline conformation proposed for the cocrystallization model. The predicted fiber diagram from the proposed model qualitatively agrees with the experimental one. Polarization microscopy revealed that the spherulite growth rate of P(BA-co-BT) was similar to that for poly(butylene adipate), PBA.     

Liquid,phenylazide-end-capped copolymers of epsilon-caprolactone and trimethylene carbonate: preparation,photocuring characteristics,and surface layering

Photoreactive phenylazide-end-capped liquid copolymers were prepared by ring-opening copolymerization of epsilon-caprolactone (CL) and trimethylene carbonate (TMC) at an equimolar monomer feed ratio in the presence of a polyol, namely, a low-molecular-weight alcohol (di-, tri-, and tetraol) or poly(ethylene glycol) (PEG) as an initiator and tin(II) 2-ethylhexanoate as a catalyst, followed subsequently by phenylazide derivatization at their hydroxyl terminus. These tri- and tetrabranched liquid copolymers (precursors) with a molecular weight from approximately 2500 to 7000 g/mol were cross-linked to yield insoluble solids by ultraviolet (UV) light irradiation. The photocuring rate increased with increasing functionality of phenylazide and UV intensity and decreasing thickness of the liquid film of precursors. The photo-cross-linkability of phenylazide-derivatized liquid copolymers was found to be higher than that of the corresponding coumarin-derivatized liquid copolymers. Poly(lactide) (PLA) films surface-layered with photocured copolymers were prepared by coating surfaces with phenylazide-derivatized copolymers and their subsequent photoirradiation. Endothelial cells adhered well on the nontreated PLA and low-molecular-weight alcohol-based copolymer-layered and photocured films. Little cell adhesion was observed on the hydrolytically surface-eroded PLA film and the PEG-based copolymer-layered film. When a phenylazide-derivatized hexapeptide with the cell-adhesion tripeptidyl sequence, Arg-Gly-Asp (RGD), common to cell adhesive proteins, was photoimmobilized on these surfaces, the surfaces became cell adhesive. Microarchitectured surfaces, which were prepared by sequential procedures of surface coating and photocuring using a photomask with lattice windows, produced regionally differentiated cell adhesiveness.     

In vitro biocompatibility of schwann cells on surfaces of biocompatible polymeric electrospun fibrous and solution-cast film scaffolds

The in vitro responses of Schwann cells (RT4-D6P2T, a schwannoma cell line derived from a chemically induced rat peripheral neurotumor) on various types of electrospun fibrous scaffolds of some commercially available biocompatible and biodegradable polymers, i.e., poly(3-hydroxybutyrate) (PHB), poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), polycaprolactone (PCL), poly(l-lactic acid) (PLLA), and chitosan (CS), were reported in comparison with those of the cells on corresponding solution-cast film scaffolds as well as on a tissue-culture polystyrene plate (TCPS), used as the positive control. At 24 h after cell seeding, the viability of the attached cells on the various substrates could be ranked as follows: PCL film > TCPS > PCL fibrous > PLLA fibrous > PHBV film > CS fibrous approximately CS film approximately PLLA film > PHB film > PHBV fibrous > PHB fibrous. At day 3 of cell culture, the viability of the proliferated cells on the various substrates could be ranked as follows: TCPS > PHBV film > PLLA film > PCL film > PLLA fibrous > PHB film approximately PCL fibrous > CS fibrous > CS film > PHB fibrous > PHBV fibrous. At approximately 8 h after cell seeding, the cells on the flat surfaces of all of the film scaffolds and that of the PCL nanofibrous scaffold appeared in their characteristic spindle shape, while those on the surfaces of the PHB, PHBV, and PLLA macrofibrous scaffolds also appeared in their characteristic spindle shape, but with the cells being able to penetrate to the inner side of the scaffolds.     

Syntheses and physical characterization of new aliphatic triblock poly(L-lactide-b-butylene succinate-b-L-lactide)s bearing soft and hard biodegradable building blocks

This study presents chemical syntheses and physical characterization of a new aliphatic poly(L-lactide-b-butylene succinate-b-L-lactide) triblock copolyester with soft and hard biodegradable building blocks. First, poly(butylene succinate) (PBS) prepolymers terminated with hydroxyl functional groups were synthesized through melt polycondensation from succinic acid and 1,4-butanediol. Further, a series of new PLLA-b-PBS-b-PLLA triblock copolyesters bearing various average PLLA block lengths were prepared via ring opening polymerization of L-lactide with the synthesized hydroxyl capped PBS prepolymer (Mn = 4.9 KDa) and stannous octanoate as the macroinitiator and catalyst, respectively. By means of GPC, NMR, FTIR, DSC, TGA, and wide-angle X-ray diffractometer (WAXD), the macromolecular structures and physical properties were intensively studied for these synthesized PBS prepolymer and PLLA-b-PBS-b-PLLA triblock copolyesters. 13C NMR and GPC experimental results confirmed the formation of sequential block structures without any detectable transesterification under the present experimental conditions, and the molecular weights of triblock copolyesters could be readily regulated by adjusting the feeding molar ratio of L-lactide monomer to the PBS macroinitiator. DSC measurements showed all single glass transitions, and their glass transition temperatures were found to be between those of PLLA and PBS, depending on the lengths of PLLA blocks. It was noteworthy that the segmental flexibilities of the hard PLLA blocks were found to be remarkably enhanced by the more flexible PBS block partner, and the PBS and PLLA building blocks were well mixed in the amorphous regions. Results of TGA analyses indicated that thermal degradation and stabilities of the PLLA blocks strongly depended on the average PLLA block lengths of triblock copolyesters. In addition, FTIR and WAXD results showed the coexistence of the assembled PLLA and PBS crystal structures when the average PLLA block length became larger than 7.8. These results may be beneficial for this new biodegradable aliphatic triblock copolyester to be applied as a potential biomaterial.     

Enzymatic degradation of poly(L-lactide) film by proteinase K: quartz crystal microbalance and atomic force microscopy study

Enzymatic degradation of the poly(L-lactide) (PLLA) amorphous film by proteinase K has been investigated by combination of the complementary techniques of quartz crystal microbalance and atomic force microscopy (AFM). The erosion rate increased with increasing enzyme concentrations and attained to be constant under the condition of [proteinase K] > 100 microg/mL. The amount of the enzyme molecules adsorbed to the film was quantitatively evaluated at various concentrations by AFM, and it revealed that the erosion rate is determined by the amount of adsorbed enzyme. Adsorption of proteinase K was irreversible despite lack of the binding domain, so that the enzyme molecules on the film surface could be observed directly by AFM. Transformation of the enzyme molecule caused by packing in high density on the surface was observed at higher enzyme concentrations. The "footprint" of the individual proteinase K molecule on the PLLA film after enzymatic degradation suggests that the enzyme moves on the surface to hydrolyze the film around it.     

Synthesis and hemocompatibility of biomembrane mimicing poly(carbonate urethane)s containing fluorinated alkyl phosphatidylcholine side groups

In this article, we designed and synthesized biomembrane mimicing segmented poly(carbonate urethane)s containing fluorinated alkyl phosphatidylcholine (PC) side groups. To obtain these novel poly(carbonate urethane)s, a new diol with a long side chain fluorinated alkyl phosphatidylcholine polar headgroup (2-[2-2,2,3,3,4,4,5,5,6,6,7,7,8,8,9,9-hexadecafluoro-10-ethoxy-decyloxy-N-(2-hydroxy-1-hydroxymethyl-1-methyl-ethyl)-acetamide] phosphatidylcholine, HFDAPC) was first synthesized and characterized. Then a series of poly(carbonate urethane)s containing fluorinated alkyl phosphatidylcholine side groups were synthesized using methylenebis(phenylene isocyanate) (MDI), poly(1,6-hexyl-1,5-pentyl carbonate) diol (PHPCD), 1,4-butandiol (BDO), and HFDAPC. The obtained fluorinated phosphatidylcholine poly(carbonate urethane)s (FPCPCU) possessed high molecular weight, narrower molecular weight distribution, and good mechanical properties as characterized by GPC and Instron, showing an increased hydrophilicity and a possible arrangement of surface structure as characterized by water contact angle. XPS results indicated that the phosphatidylcholine polar headgroups have been indeed pulled out to the surface with the help of the migration of the fluorinated side chain that was directly connected with the phosphatidylcholine polar headgroup. A preliminary result by protein adsorption and platelet adhesion experiments suggested that only 5 approximately 12.5 mol % phosphatidylcholine could be enough for good hemocompatibility. The current work demonstrates a new synthetic approach that can be used to bring the bioactive PC groups to the surface of the PC-containing polyurethanes more effectively.     

Phase structure and enzymatic degradation of poly(L-lactide)/atactic poly(3-hydroxybutyrate) blends: an atomic force microscopy study

Phase structures and enzymatic degradation of poly(l-lactide) (PLLA)/atactic poly(3-hydroxybutyrate) (ata-PHB) blends with different compositions were characterized by using atomic force microscopy (AFM). Differential scanning calorimetry (DSC) thermograms of PLLA/ata-PHB blends with different compositions showed two glass transition temperatures, indicating that the PLLA/ata-PHB blends are immiscible in the melt. Surface morphologies of the thin films for PLLA/ata-PHB blends were determined by AFM. Phase separated morphology was recognized from the AFM topography and phase images. The domain size of the components was dependent on the blend ratio. Enzymatic degradation of the PLLA/ata-PHB blends was performed by using both PHB depolymerase and proteinase K. Either PLLA or ata-PHB domains were eroded depending on the kinds of enzyme. Surface morphologies after enzymatic degradation have revealed the phase structure along the depth direction. Enzymatic adsorption of PHB depolymerase was examined on the surface of PLLA/ata-PHB blends. The enzyme molecules were found on both domains of the binary blends. The larger number of enzyme molecules was found on the PLLA domains relative to those on the ata-PHB domains, suggesting the higher affinity of the enzyme against PLLA domain.     

Branched poly(lactide) synthesized by enzymatic polymerization: effects of molecular branches and stereochemistry on enzymatic degradation and alkaline hydrolysis

In this article the effects of the number of molecular branches (chain ends) and the stereochemistry of poly(lactide)s (PLAs) on the enzymatic degradation and alkaline hydrolysis are studied. Various linear and branched PLAs were synthesized using lipase PS (Pseudomonas fluorescens)-catalyzed ring-opening polymerization (ROP) of lactide monomers having different stereochemistries (L-lactide, D-lactide, and D,L-lactide). Five different alcohols were used as initiators for the ROP, and the monomer-to-initiator molar feed ratio was varied from 10 to 100 and 1000 for each branch in the polymer architecture. The properties of branched PLAs that would affect the enzymatic and alkaline degradations, i.e., the glass transition temperature, the melting temperature, the melting enthalpy, and the advancing contact angle, were determined. The PLA films were degraded using proteinase K or 1.0 M NaOH solution, and the weight loss and changes in the number average molecular weight (Mn) of the polymer were studied during 12 h of degradation. The results suggest that an increase in the number of molecular branches of branched PLAs enhances its enzymatic degradability and alkali hydrolyzability. Moreover, the change in Mn of the branched poly(L-lactide) (PLLA) by alkaline hydrolysis indicated that the decrease in Mn was in the first place dependent on the number of molecular branches and thereafter on the length of the molecular branch of branched PLA. The branched PLLA, poly(D-lactide) (PDLA), and poly(D,L-lactide) (PDLLA) differed in weight loss and change in Mn of the PLA segment during the enzymatic degradation. It is suggested that the branched PDLLA was degraded preferentially by proteinase K.     

Trimethylene carbonate and epsilon-caprolactone based (co)polymer networks: mechanical properties and enzymatic degradation

High molecular weight trimethylene carbonate (TMC) and epsilon-caprolactone (CL) (co)polymers were synthesized. Melt pressed (co)polymer films were cross-linked by gamma irradiation (25 kGy or 50 kGy) in vacuum, yielding gel fractions of up to 70%. The effects of copolymer composition and irradiation dose on the cytotoxicity, surface properties, degradation behavior, and mechanical and thermal properties of these (co)polymers and networks were investigated. Upon incubation with cell culture medium containing extracts of (co)polymers and networks, human foreskin fibroblasts remained viable. For all (co)polymers and networks, cell viabilities were determined to be higher than 94%. The formed networks were flexible, with elastic moduli ranging from 2.7 to 5.8 MPa. Moreover, these form-stable networks were creep resistant under dynamic conditions. The permanent deformation after 2 h relaxation was as low as 1% after elongating to 50% strain for 20 times. The in vitro enzymatic erosion behavior of these hydrophobic (co)polymers and networks was investigated using aqueous lipase solutions. The erosion rates in lipase solution could be tuned linearly from 0.8 to 45 mg/(cm (2) x day) by varying the TMC to CL ratio and the irradiation dose. The copolymers and networks degraded essentially by a surface erosion mechanism.     

Microbial degradation of poly(d-3-hydroxybutyrate) by a new thermophilic Streptomyces isolate

A new thermophilic microorganism capable of degrading poly(D-3-hydroxybutyrate) (PHB) was isolated from soil. A phylogenetic analysis based on 16S rDNA sequences indicated that the new isolate belongs to genus Streptomyces. PHB film and powder were completely degraded after 6 and 3 d cultivation, respectively at 50 degrees C. Scanning micrographs showed adherence of the microbial cells to the entire film surface, indicating that biodegradation occurs by colonization of the PHB surface. The film was degraded both by microbial attack and by the action of an extracellular enzyme secreted by the microorganism. The strain can also degrade poly(ethylene succinate), poly(ester carbonate), polycaprolactone and poly(butylene succinate), but to a lesser extent.