Structure and Mechanical Properties of Hydroxypropylated Starch Films David Lafargue, Bruno Pontoire, Alain Buléon, Jean Louis Doublier, and Denis Lourdin* INRA, UR1268 Unité Biopolymères Interactions et Assemblage, rue de la Géraudière, F-44000 Nantes, France Received August 29, 2007; Revised Manuscript Received October 9, 2007 Films of acid-hydrolyzed hydroxypropylated pea starch with average molecular weight M w ranging from 3.3 × 10 4 g/mol to 1.6 × 10 6 g/mol were prepared from 25% (w/w) solution by casting. The structure of the ﬁlms was investigated by means X-ray diffraction and calorimetry, evidencing a B-type crystalline structure. In similar drying conditions, 25 °C and 40% of relative humidity, the crystallinity varied from 24% for the low molecular weight (A5) to almost none for the highest molecular weight (A160). The inﬂuence of the drying temperature was also investigated. A reduction of the crystallinity from 16% to almost none was found when increasing temperature from 25 to 65 °C. The glass transition temperature (T g ) at different water contents was determined. The difference of T g between the ﬁrst and the second scan was interpreted by changes in the water distribution between phases into the B-type crystalline structure. Mechanical properties of the ﬁlms determined by tensile tests and by DMTA in the glassy state showed no effect of the average molecular weight or of crystallinity. In contrast, thermomechanical experiments by DMTA showed that the average molecular weight of the sample inﬂuenced the mechanical relaxation and the moduli in the rubbery state. Introduction Among all polysaccharides investigated as potential alterna- tives to materials from oil, such as conventional plastics, starch has attracted a large amount of attention. 1 This is one of the most abundant natural plant polysaccharides and, thanks to its cheapness, renewability, and biodegradability, it can be used as a raw material for the elaboration of biologically degradable materials and, thus, promises an environmental solution to the plastic waste issue. Starch is composed of amylose, an almost linear macromol- ecule with a molecular weight ranging from 10 5 to 10 6 g/mol, and amylopectin, a highly branched macromolecule with a higher molecular weight of around 10 7 -10 9 g/mol. These two high molecular weight and polydisperse polymers of D-glucose are organized at the native state in semicrystalline granules. Different techniques have been used to produce starch ﬁlms. Casting, which consists of drying solutions, is an attractive method to prepare free-standing ﬁlms. From the pioneering work of Wolf et al. (1951) on mechanical and barrier properties of amylose ﬁlms, 2 many studies have been focused on starch-based ﬁlms cast from solution or gel state. 3–10 Inﬂuence of amylose content on the mechanical properties of starch ﬁlms has also been investigated. 4,11 It has been found that linear amylose chains confer better mechanical properties compared to branched amylopectin molecules. It is known that crystallization of polymers strongly depends on molecular weight, branching, concentration, temperature, and the solvent used. Crystallinity of starch ﬁlms prepared by a casting procedure has been widely studied. A B-type crystalline structure is expected when starch ﬁlms are cast from aqueous solutions. 12 The effect of the preparation conditions (tempera- ture, relative humidity (RH)) on the structure of a water-cast amylose ﬁlm has been investigated. 13 All ﬁlms showed a B-type X-ray diffraction pattern. The effect of temperature, RH, and time of drying have also been investigated on potato starch ﬁlms. 5,14 It was shown that crystallinity depends on the drying conditions. Films formed at elevated temperature were found to be amorphous, while those obtained after long drying at ambient temperature showed relatively high crystallinity. The crystalline structure has a strong impact on the thermal and mechanical properties of starch ﬁlms in the rubbery state. 15–17 The increase of their crystallinity has been reported to yield a stiffening of ﬁlms by increasing elastic modulus, tensile stress, and decreasing the tensile strain. Crystallinity also affects the glass transition temperature (T g ) of amorphous regions of semicrystalline polymers. 18 According to Slade and Levine (1989), the increase of the T g of starch with increasing the crystallinity has been interpreted by the stiffening effect of crystalline cross-links, which decrease the mobility of chain in amorphous phases. 19,20 T g , which can be also affected by the presence of plasticiz- ers, 21 has a major impact on the properties of starch ﬁlms. Depending on the plasticizer content, the material is in the glassy state or in the rubbery state at ambient temperature, thus determining the material properties and the ﬁeld of application. Water greatly affects the T g of starch 22,23 but other small molecules like glycerol, 24 sorbitol 6 are commonly used to plasticize and stabilize starch ﬁlms. The T g of starch also depends on the molecular weight. 25 The T g dependence on the molecular weight has been well described for linear synthetic polymers. 26 T g usually decreases linearly with increasing the inverse of the molecular weight, which was ascribed to the free volume of polymers. 25 The inﬂuence of molecular weight on the mechanical properties of extruded thermoplastic potato starch has been demonstrated in the rubbery state. 27 The lower strain and tearing energy of the low molecular mass materials were attributed to the reduced amylose chain length as well as the degree of branching of the amylopectin molecules, resulting in less effective entangled macromolecules. * Corresponding author. E-mail:lourdin@nantes.inra.fr. Biomacromolecules 2007, 8, 3950–3958 3950 10.1021/bm7009637 CCC: $37.00  2007 American Chemical Society Published on Web 11/27/2007