RHEOLOGY OF NOVEL POLY(L,D-LACTIC ACID) (PLDLA) NANOCOMPOSITE FORMED FROM IN SITU BULK POLYMERIZATION OF L,D-LACTIDE BY STEARATE-INTERCALATED LAYERED DOUBLE HYDROXIDE
Edward D. McCarthy1, Douglas M. Fox2, Gale A. Holmes1, Mauro Zammarano1, Paul H. Maupin3, Jeffrey W. Gilman1
1Polymers Division, National Institute of Standards and Technology, Gaithersburg, MD, USA.
2Dept. of Chemistry, American University, Washington, D.C., USA.
3Office Of Science, US Department of Energy, Germantown, MD, USA.
Previously we demonstrated the spontaneous formation of an insoluble network in a poly(L,D-lactide) melt polymerized in situ from L,D-lactide dimer. At 5 % by mass layered double hydroxide, the insoluble network comprised 24 % by mass of the final reaction product and had a content of 84 % by mass poly(lactic acid) and 16 % by mass of inorganic material. The network was insoluble in methylene chloride, indicating comprehensive integration of the remaining polymer within an organic-inorganic structure. The polymerization was accompanied by the disintegration of the essential layered double hydroxide structure. This was demonstrated by loss of characteristic d-spacing and metal-metal peaks in the x-ray scattering data, as well as a dramatic fall in the ratio of magnesium to aluminium from 2.2:1 in the original layered double hydroxide to 11:1 in the insoluble residue.
Here, we present differential scanning calorimetry (DSC) and parallel-plate rheology which exhibits an unusual, positive relationship between temperature and shear modulus in the fully-annealed poly(L,D-lactic acid), PLDLA-LDH product at constant shear strain and frequency, over a high temperature interval at least 60 °C above glass temperature. Two possible causes are suspected: a) strain-induced crystallization, (SIC), and b) the reversible formation of ionomer complexes or clusters with increasing temperature. Any role of SIC has yet to be investigated. The positive temperature-modulus relationship described here has not previously been reported for conventional ionomers as far as the authors are aware. Apart from crystallization and ionomer phenomena, it is not clear what other mechanism might be responsible for this behavior, except the formation of covalent crosslinks within the residue. However, the latter would not normally be associated with reversible modulus increases with temperature as they are persistent rather than transient in nature.
Acknowledgements. This material is based upon work supported by the Air Force Office of Scientific Research under Award No. F1ATA00236G002.
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*This work was carried out by the National Institute of Standards and Technology (NIST), an agency of the U. S. Government and by statute is not subject to copyright in the United States. Certain commercial equipment, instruments, materials, services, or companies are identified in this paper in order to specify adequately the experimental procedure. This in no way implies endorsement or recommendation by NIST.