The doctoral dissertations of the former Helsinki University of Technology (TKK) and Aalto University Schools of Technology (CHEM, ELEC, ENG, SCI) published in electronic format are available in the electronic publications archive of Aalto University - Aaltodoc.
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Branched and Crosslinked Resorbable Polymers Based on Lactic Acid, Lactide and ε-Caprolactone

Antti Helminen

Dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the Department of Chemical Technology for public examination and debate in Auditorium KE 2 (Komppa Auditorium) at Helsinki University of Technology (Espoo, Finland) on the 19th of December, 2003, at 12 o'clock noon.

Overview in PDF format (ISBN 951-22-6841-8)   [968 KB]
Dissertation is also available in print (ISBN 951-22-6840-X)

Abstract

Branched and crosslinked degradable polyesters based on lactic acid, lactide and ε-caprolactone were prepared by utilizing different polymerization methods. Chain linking of hydroxyl telechelic lactic acid oligomers with 1,6-hexamethylene diisocyanate (HMDI) as a chain extender, yielded lactic acid based poly(ester-urethanes). When an excess of HMDI was used, polymers with broader molecular weight distribution and long-chain branching were obtained. Long-chain branching was confirmed by NMR and was seen in high shear thinning and melt strength in rheological measurements. Ring-opening polymerization of D,L-lactide, L-lactide and ε-caprolactone in the presence of stannous octoate and a co-initiator was employed as a method to prepare polyesters with controlled molecular structure. The initiation activity of different hydroxyl containing compounds was evaluated and new co-initiator compounds, polyglycerines, were shown to yield polylactides with up to 8 arms. Furthermore, by increasing the number of hydroxyls in the co-initiator, the polymerization rate increased and polylactides with higher molecular weight were obtained.

Crosslinked polymers with controlled molecular structure were prepared from end functionalized linear and branched polyester precursors. The effect of molecular architecture, i.e. branching, molecular weight and different monomer units in the precursors, on the curing and properties of the final networks was studied. Triethoxysilane and methacrylic double bond functionalization and polymerization of these moieties were utilized in the preparation of hydrolytically degradable networks with different properties. High strength was obtained with DLLA based networks, whereas the use of flexible CL/DLLA copolymers as network precursors yielded highly elastic crosslinked polymers with creep resistance.

In addition to the preparation and characterization, the degradability of the crosslinked polymers was evaluated. The polyester networks exhibited degradation similar to the corresponding thermoplastic polyesters. In general, the degradation occurred through bulk degradation where the mechanical properties of the polyesters deteriorated first and the mass loss followed. The degradation of the networks was enhanced by introducing labile anhydride bonds as weak linkages into the polyester network structure. This approach led to very rapidly degrading, crosslinked poly(ester-anhydrides) based on PCL, PDLLA, and PLLA. When a short PCL based precursor was used, the network showed characteristics of surface-erosion: the poly(ester-anhydride) network lost its mass and the dimensions of the specimen decreased at a steady rate over two days.

This thesis consists of an overview and of the following 6 publications:

  1. Helminen A., Kylmä J., Tuominen J. and Seppälä J. V., 2000. Effect of structure modification on rheological properties of biodegradable poly(ester-urethane). Polymer Engineering and Science 40, pages 1655-1662.
  2. Korhonen H., Helminen A. and Seppälä J. V., 2001. Synthesis of polylactides in the presence of co-initiators with different numbers of hydroxyl groups. Polymer 42, No. 18, pages 7541-7549.
  3. Helminen A., Korhonen H. and Seppälä J. V., 2001. Biodegradable crosslinked polymers based on triethoxysilane terminated polylactide oligomers. Polymer 42, No. 8, pages 3345-3353.
  4. Helminen A., Korhonen H. and Seppälä J. V., 2002. Structure modification and crosslinking of methacrylated polylactide oligomers. Journal of Applied Polymer Science 86, No. 14, pages 3616-3624.
  5. Helminen A., Korhonen H. and Seppälä J. V., 2002. Cross-linked poly(ε-caprolactone/D,L-lactide) copolymers with elastic properties. Macromolecular Chemistry and Physics 203, No. 18, pages 2630-2639.
  6. Helminen A., Korhonen H. and Seppälä J. V., 2003. Crosslinked poly(ester anhydride)s based on poly(ε-caprolactone) and polylactide oligomers. Journal of Polymer Science Part A: Polymer Chemistry 41, No. 23, pages 3788-3797.

Keywords: ring-opening polymerization, characterization, biodegradable, molecular architecture

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© 2003 Helsinki University of Technology

Last update 2011-05-26