A series of copolymers with various compositions were synthesized by one‐step and two‐step bulk ring‐opening polymerizations of L‐lactide (LA) and ε‐caprolactone (CL) using stannous octoate [Sn(Oct)2] and 1‐hexanol as the initiating system. For the sequential two‐step polymerization, a poly(ε‐caprolactone) (PCL) prepolymer was polymerized first to a percent conversion of approximately 70% and LA then added in order to produce a copolymer with a chain microstructure different from that obtained from the corresponding one‐step reaction. The resulting copolymers were characterized using a combination of nuclear magnetic resonance spectroscopy (1H‐ and 13C‐NMR), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and gel permeation chromatography (GPC). The average sequence lengths of the lactidyl ($l_{{\rm LL}}^{\rm e}$) and caproyl ($l_{{\rm cap}}^{\rm e}$) units, the degree of randomness (R) and the transesterification coefficient (T(II)) were calculated from the 13C‐NMR spectra. The appearance of a signal due to CapLCap sequences was directly attributable to transesterification of lactidyl (LL) units. It was found that both $l_{{\rm LL}}^{\rm e}$ and $l_{{\rm cap}}^{\rm e}$ values from the two‐step syntheses were significantly longer than from the corresponding one‐step syntheses, leading to different semi‐crystalline morphologies and chain microstructures. The copolymers all showed at least some blocky chain structure as a result of the significant difference in monomer reactivity (LA > CL) between LA and CL. Thermal properties including stability depended on both composition and chain microstructure which could be controlled by the method of synthesis. From their DSC curves, the two‐step copolymers were seen to be semi‐crystalline whereas the one‐step copolymers were mainly amorphous. A more blocky microstructure, as obtained from the two‐step method, appeared to result in a lower thermal stability. Copyright © 2007 John Wiley & Sons, Ltd.

No abstract

Electrospinning can produce nanofibrous scaffolds that mimic the architecture of the extracellular matrix and support cell attachment for tissue engineering applications. In this study, fibrous membranes of polyhydroxybutyrate (PHB) with various loadings of poly(L-lactide-co-ε-caprolactone) (PLCL) were successfully prepared by electrospinning. In comparison to PLCL scaffolds, PLCL blends with PHB exhibited more irregular fibre diameter distributions and higher average fibre diameters but there were no significant differences in pore size. PLCL/PHB scaffolds were more hydrophilic (<120°) with significantly reduced tensile strength (ca. 1 MPa) compared to PLCL scaffolds (150.9 ± 2.8° and 5.8 ± 0.5 MPa). Increasing PLCL loading in PHB/PLCL scaffolds significantly increased the extension at break, (4–6-fold). PLCL/PHB scaffolds supported greater adhesion and proliferation of olfactory ensheathing cells (OECs) than those exhibiting asynchronous growth on culture plates. Mitochondrial activity of cells cultivated on the electrospun blended membranes was enhanced compared to those grown on PLCL and PHB scaffolds (212, 179, and 153%, resp.). Analysis showed that PLCL/PHB nanofibrous membranes promoted cell cycle progression and reduced the onset of necrosis. Thus, electrospun PLCL/PHB composites promoted adhesion and proliferation of OECs when compared to their individual PLCL and PHB components suggesting potential in the repair and engineering of nerve tissue.

This paper describes the synthesis and characterization of a block copolymer of L-lactide (LL) and epsilon -caprolactone (CL) and its subsequent melt spinning into a monofilament fiber. The synthesis reaction was a two-step process. In the first step, an approximately 50:50 mol% random copolymer, P(LL-co-CL), was synthesized via bulk copolymerization of LL and CL. This first-step prepolymer then became the macroinitiator in the second-step reaction in which more LL monomer was added to form a block copolymer, PLL-b-P(LL-co-CL)-b-PLL. Both the prepolymer and block copolymer were characterized by a combination of analytical techniques comprising dilute-solution viscometry, GPC, 1H and 13C NMR, DSC and TG. The block copolymer was then processed into a monofilament fiber using a small-scale melt spinning apparatus. The fiber was spun with a minimum amount of chain orientation and crystallinity so that its semi-crystalline morphology could be constructed under more controlled conditions in subsequent off-line hot-drawing and annealing steps. In this way, the fiber's tensile properties and dimensional stability were developed, as indicated by the changes in its stress-strain curve. The final drawn and annealed fiber had a tensile strength (>400 MPa) approaching that of a commercial PDS II suture of similar size. It is considered that this type of block copolymer has the potential to be developed further as a lower-cost alternative to the current commercial monofilament surgical sutures.

No abstract

Four titanium(IV) alkoxides, namely: Ti(IV) npropoxide (1), Ti(IV) n-butoxide (2), Ti(IV) tert-butoxide (3), and Ti(IV) 2-ethylhexoxide (4), have been used as initiators in the bulk ring-opening polymerization (ROP) of ε-caprolactone (ε-CL). The influence of the alkoxide group on the course of the ROP of ε-CL was investigated by means of 1 H-NMR and differential scanning calorimetry (DSC). The 1 H-NMR spectra confirmed that the ROP reaction of ε-CL proceeded via the widely accepted coordinationinsertion mechanism for each of the four initiators. Isoconversional methods have been used to evaluate non-isothermal DSC data via the equations of Friedman, Kissinger-AkahiraSunose (KAS) and Ozawa-Flynn-Wall (OFW). The kinetic studies showed that the polymerization rate for the four initiators (1-4) was in the order of 1>2≈4>3. The lowest activation energies (40-47, 42-44, and 49-52 kJ/mol for the Friedman, KAS and OFW methods respectively) were found in the polymerizations using Ti(IV) n-propoxide (1), while the highest activation energies (84-107, 77-87, and 80-91 kJ/mol for the Friedman, KAS and OFW methods respectively) were obtained using Ti(IV) tert-butoxide (3). Differences in the rates of polymerization and the activation energies amongst the four initiators appeared to be governed mainly by the different degrees of steric hindrance in the initiator structure. These results represent important findings regarding the steric influence of the alkoxide groups on the kinetics of the ROP of ε-CL initiated by titanium(IV) alkoxides.

Kaolinite layers were made apart by silver particle intercalation through chemical reduction. The modified clay then allowed formation of intercalated‐exfoliated composites based on poly(lactic acid) with properties notably enhanced.

No abstract