Covalent and non-covalently coupled polyester–inorganic composite materials

Two types of organic/inorganic materials were synthesized via sol–gel reactions for tetraethylorthosilicate (TEOS) and organo-alkoxsilane monomers in the presence of poly(ε-caprolactone) (PCL): (1) non-covalent hybrids, in which PCL and silicate, and PCL and organically modified silicate (ORMOSIL), phases are coupled by non-bonded interactions; (2) covalent hybrids in which triethoxysilane–telechelic PCL molecules form chemical bonds with a sol–gel-derived silicate phase. Chemical structures were verified using FTIR and 13C NMR spectroscopies and MALDI-TOF mass spectrometry. The constant PCL phase Tg for the silicate–PCL hybrids of (1) implies poor organic/inorganic mixing, but dual-melting endotherms varied with silicate content. TGA revealed significant elevation of degradation onset temperature (Td) of (1) and suppression of the low temperature chain scission process. No PCL glass transition is seen for ORMOSIL–PCL hybrids where diethoxydimethylsilane is the co-monomer and melting occurs in one step, and there are significant increases in Td. Likewise, no glass transition is seen when acetoxypropyltrimethoxylsilane is the semi-organic co-monomer, but there are dual-melting endotherms. Triethoxysilane-endcapped PCL was synthesized and its microstructure verified by GPC, MALDI-TOF mass spectrometry, FTIR and NMR spectroscopies. Tg for this telechelic PCL that was reacted with a small fraction of TEOS increases relative to hydroxy-telechelic PCL due to formation of phase-linking Si–O–Si bonds through end groups. The temperature/magnitude of the melting transition decreased upon inorganic modification. TGA showed appreciable increase in Td relative to PCL and both the high and low temperature degradation processes were hindered. Silane–telechelic PCL films have oxygen permeability values less than that of pure PCL, which is totally attributed to a decrease in diffusion coefficient.