Abundant literature exists on starch or modified starch blended with biodegradable polyesters to achieve good performance with cheap compost plastics. The level of miscibility in these blends is one of the most relevant parameters. In the present study, solid-state 1H and 13C NMR spectra, as well as carbon spin-lattice relaxation times T1(C) and proton spin-lattice relaxation times T1(H) and proton spin-lattice relaxation times in the rotating frame T1ρ(H) of biodegradable starch (or starch formate)/polycaprolactone (PCL) (or polyester (PE) oligomers) blends and samples of the neat components were measured. From the T1ρ(H) and T1(H) relaxation times it follows that blends starch/PCL, starch/PE-oligomers and starch formate/PE-oligomers are phase separated even on the scale of 20–110nm. On the contrary starch formate/PCL blend is phase separated on the scale 2.5–12nm but homogeneously mixed on the scale 20–90nm. Moreover, shorter T1(C) and especially T1ρ(H) values found for the starch or starch formate component in all these blends in comparison with neat samples show that molecular mobility of starch and starch formate segments is affected by blending. This indicates some miscibility also in phase separated blends which can happen in amorphous channels of starch.

There is a great challenge in finding treatments that may degrade the initial grain structure of native starch in order to get better mixing properties of the resulting degradation product with other polymers. Among the possible treatments, O-formylation has already been proposed but only little structural information on the resulting degradation products is available, certainly due to its heterogeneity. The present study aims to complement our knowledge on the structure of macromolecules following the chemical reaction by means of different techniques. At first, polarimetry reveals, on amylose and maltodextrin samples, that the helix content remains unchanged after esterification. Then, high resolution 1H NMR measurements allow to determine the association parameter, p, and it was found that the fraction of associated helix, although strongly influenced by the nature of the solvent, is similar for native and starch formate. It was deduced that formic acid is not reactive on the less accessible macromolecules such as the aggregates preserved from the initial destructuration of starch during gelanitisation and following the thermal treatment, i.e. the larger molar weight samples. Finally, AFM observation corresponded well with the results obtained by the other techniques, since large aggregates, such as helix assemblies or amorphous and globular aggregates, were found to be unaffected by the formic acid treatment.

A starch-based biodegradable material was prepared in two steps. Firstly, starch was chemically modified by using formic acid at 20 degrees C to obtained degrees of substitution of about 1.2. The level of destructuration was also assessed using dynamic rheological measurements. Native starch or starch ester were then mixed with poly(caprolactone) and different polyester oligomers were added as compatibilisers and plasticizing agents. PCL oligomers were found to be the most efficient ones. A significant improvement of the elongation at break of starch formate/PCL/oligo PCL blends was achieved.

The mixing of dry starch with 40 or 99% (v/v) formic acid (FA) produces an O-formylation reaction which causes a combination of acid hydrolysis and starch destructuration. Moreover, this esterification reaction is highly exothermic in the presence of pure FA. When O-formylation is performed at temperatures higher than 20degreesC, starch formate content is high (degree of substitution, DS, of 2.15 after 60 min at 105degreesC) but then molecular weight decreases (eta(red) < 10 mL/g). Under thermally-controlled conditions at 20 degrees C in formic acid, degrees of substitution reach 1.5-1.6 after 6 h reaction times and polymer degradation seems to be limited (eta(red) = 110 to 140 mL/g). The degrees of substitution obtained in water/formic acid mixtures are below those in formic acid alone. The level of destructuration of starch in formic acid and water/formic acid mixtures was also evidenced by dynamic rheological measurements and optical microscopy. Plots of storage modulus (G') versus frequency (omega) was used to characterize both the gelatinization and the gel destruction processes as a function of reaction temperature (T-r) and FA concentration.