Surface-modified hydroxyapatite linked byL-lactic acid oligomer in the absence of catalyst
ABSTRACT A new surface modification method of hydroxyapatite nanoparticles (n-HA) by surface grafting reaction of L-lactic acid oligomer with carboxyl terminal (LAc oligomer) in the absence of any catalyst was developed. The LAc oligomer with a certain molecular weight was directly synthesized by condensation of L-lactic acid. Surface-modified HA nanoparticles (p-HA) were attested by Fourier transformation infrared spectroscopy, 31P MAS-NMR, and thermal gravimetric analysis (TGA). The results showed that LAc oligomer could be grafted onto the n-HA surface by forming a Ca carboxylate bond. The grafting amount of LAc oligomer was about 13.3 wt %. The p-HA/PLLA composites showed good mechanical properties and uniform microstructure. The tensile strength and modulus of the p-HA/PLLA composite containing 15 wt % of p-HA were 68.7 MPa and 2.1 GPa, respectively, while those of the n-HA/PLLA composites were 43 MPa and 1.6 GPa, respectively. The p-HA/PLLA composites had better thermal stability than n-HA/PLLA composites and neat PLLA had, as determined by isothermal TGA. The hydrolytic degradation behavior of the composites in phosphate buffered saline (PBS, pH 7.4) was investigated. The p-HA/PLLA composites lost their mechanical properties more slowly than did n-HA/PLLA composites in PBS because of their reinforced adhesion between the HA filler and PLLA matrix. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 5177–5185, 2005
- SourceAvailable from: Pieter J Dijkstra[Show abstract] [Hide abstract]
ABSTRACT: Hetero-telechelic, low-molecular-weight polylactides (PLAs) were prepared by the zinc-catalyzed ring-opening polymerization of l-lactide or d-lactide using functional initiators and subsequent reaction with termination reagents, yielding –OH, –COOH, –NH2 and –SH as functional chain ends. Structural characterization was performed by molecular weight analysis, NMR spectroscopy and MALDI-TOF mass spectrometry. The thermal behavior of the species was investigated by DSC, which revealed that the modification of the hydroxyl terminus lowered the number of lactic acid units within a PLA chain that can effectively participate in crystallization. Both the Tm and ΔHm values for these polymers were lower compared to those of PLAs of comparable chain length with no modification of the hydroxyl end-group. In stereocomplexes prepared from equimolar amounts of the hetero-telechelic PLLAs and PDLAs, this suppressive effect on Tm and ΔHm was also observed. Modification of the hydroxyl ends to produce –COOH, –NH2 and –SH end groups reduced the stereocomplex Tm values to ∼10–20 °C. The lower limit for the crystallization of the stereocomplexes was found at a DP of 5.Reactive and Functional Polymers 01/2013; 73(1):30–38. DOI:10.1016/j.reactfunctpolym.2012.10.003 · 2.82 Impact Factor
- [Show abstract] [Hide abstract]
ABSTRACT: Biomaterial science has become an important aspect in medical developments, and many so-called ‘biomaterials’ have emerged. These materials should be biocompatible, i.e. perform with an appropriate host response on a specific application without causing e.g. inflammatory or toxicological responses, and preferably biodegradable, i.e. removed from the body due to natural, biological processes. Important examples are poly(lactic acid)s (PLA) and poly(ε-caprolactone)s (PCL), which are conveniently prepared by ring-opening polymerization (ROP). For these chiral monomers, stereoselectivity of the ROP catalyst has become an important topic, since enrichment in stereoregular sequences in the resulting polymers can significantly affect the material properties. Seven ROP catalysts have been decribed in the thesis, that is: three phenolate-ligated zinc-based catalysts of the type [LnZnEt]2 (Chapter 3) and three thiophenolate-ligated zinc-based catalysts of the type [LnZnEt]n (Chapter 4) for the ROP of lactide, and R,R’-(salen) aluminum isopropoxide for the ROP of 4-methyl-ε-caprolactone and 6-methyl-ε-caprolactone (Chapter 5). Polymerizations of the zinc phenolates were fast and well-controlled. The stereoselectivity of 2,6-bis[(dimethylamino)methyl]-4-methylphenoxy ethylzinc was shown to depend on the polymerization medium, giving atactic polymers in dichloromethane and isotactic enrichments in THF and toluene (Pm ≈ 0.60). The polymerizations using the zinc thiophenolates resulted in heterotactic enrichment (Pr ≈ 0.60). The catalyst R,R’-(salen) aluminum isopropoxide (previously shown to preferably polymerize S,S-lactide over R,R-lactide) showed a preference for the R-enantiomer in the ROP of 6-methyl-ε-caprolactone, but no stereoselectivity upon polymerizing 4-methyl-ε-caprolactone. Chapter 6 describes the synthesis of γ-Boc-amino-ε-caprolactone and ring-opening reactions thereof. Rather than giving the functional amine-protected poly(ε-caprolactone), γ-Boc-amino-ε-caprolactone rearranged into an oxopyrrolidine carboxylate. Chapter 7 decribes the synthesis of several heterotelechelic polylactides. It was shown that PLA crystallization as well as stereocomplexation is hampered by modification of the chain ends. Chapter 8 describes the synthesis of PLA-dextran copolymers, with grafting based upon carbamate and secondary amine linkages, and the formation of hydrogels thereof. Although the hydrolytic stability of the copolymers was higher when compared to grafting by ester or carbonate linkages, their relatively poor water solubility resulted in rather weak hydrogels (200-500 Pa), which cannot bear heavy loads but may serve as vehicles for the release of e.g. medicine.
- [Show abstract] [Hide abstract]
ABSTRACT: Hydroxyapatite (HAp)/poly(L-lactic acid) (PLLA) interconnected macroporous materials are facilely and effectively fabricated by templating water-in-dichloromethane with PLLA (W/O) Pickering emulsions. The HAp nanoparticles modified with PLLA (g-HAp nanoparticles) are used as effective particulate emulsifiers to produce stable W/O Pickering emulsions. Evaporation of the W/O Pickering emulsions directly leads to interconnected porous nanocomposite materials in absence of any chemical reactions. This simple and effective method can be used to prepare a variety of functionalized porous materials, which are suitable for biomedical applications.Macromolecular Materials and Engineering 03/2014; 299(9). DOI:10.1002/mame.201300449 · 2.78 Impact Factor