Deformation mechanisms of plasticized starch materials.

The aim of this paper is to understand the influence of plasticizer and plasticizer amount on the mechanical and deformation behaviors of plasticized starch. Glycerol, sorbitol and mannitol have been used as plasticizers. After extrusion of the various samples, dynamic mechanical analyses and video-controlled tensile tests have been performed. It was found that the nature of plasticizer, its amount as well as the aging of the material has an impact on the involved deformation mechanism. The variations of volume deformation could be explained by an antiplasticization effect (low plasticizer amount), a phase-separation phenomenon (excess of plasticizer) and/or by the retrogradation of starch.

Lignin incorporation combined with electron-beam irradiation improves the surface water resistance of starch films.

We investigated the potential of an electron-beam post-treatment to tailor the properties of 70/30 and 80/20 wt. extruded starch-lignin films. The effect of a 400 kGy radiation on films differing essentially by the kind of lignins incorporated (lignosulfonates/alkali lignins) was assessed both at the macroscopic and the molecular levels. Changes in the polymer molecular structure were studied by IR spectroscopy, by thioacidolysis as well as by model compound experiments. Electron beam-irradiation at 400 kGy, a rather high dose for processing natural polymers, alters to some extent the mechanical resistance of the starch-based materials. However this treatment substantially reduces the hydrophilic surface properties of the films, while not harming their biodegradability. Involved in radical cross-coupling reactions, lignin phenolic compounds are likely to play a primary role in the formation of a hydrophobic condensed network. This study suggests that lower irradiation doses might yield biomaterials with improved usage properties.

Compatibilization of starch-allylurea blends by electron beam irradiation: spectroscopic monitoring and assessment of grafting efficiency.

The chemical changes induced by electron-beam irradiation of mixtures of N-allylurea (AU) and amorphized starch were studied by spectroscopic methods for identifying and monitoring the reactions providing the blend with stabilized physical properties. Spectral modifications essentially concerned the AU constituent in the irradiated mixtures. FTIR and NMR analyses were used to quantify the progress of AU conversion upon irradiation and to gain information on the structure of the products. The influence of sample temperature and moisture on AU conversion rate was examined. The kinetic treatment of conversion vs dose data, from blends with different contents in AU, suggested that the phenomenological order for the reaction rate was zero, relative to the concentration in AU. The grafting yield was determined from combined (1)H NMR data recorded after selective solubilization of the constituents allowing for extraction of AU monomer and homopolymer from the grafted polysaccharide. Graft polymerization was more efficient than homopolymerization in samples containing AU in amounts less than its limiting solubility and relatively less efficient in thermodynamically unstable blends.

Phase diagrams and morphology of polymer dispersed liquid crystals based on nematic-liquid-crystal-monofunctional-acrylate mixtures

Phase diagrams of unpolymerized and UV-polymerized 2-ethyl hexyl acrylate (EHA) mixtures with the liquid crystal E7 are established using optical microscopy and differential scanning calorimetry. Both diagrams show upper critical solution temperature behavior. From 50 to 90 wt % liquid crystal (LC), the (I+I) phase located between the (N+I) and (I) phases was clearly shown. The nematic phase inside the droplets exhibits a twisted radial structure indicating that homeotropic anchoring occurs at the polymer interface. The experimental phase diagrams were successfully analyzed using a model based on the Flory-Huggins theory of isotropic mixing supplemented with the Maier-Saupe theory of nematic order. The LC solubility limit in the polymer matrix and the fractional amount of LC contained in the droplets were deduced from the calorimetric measurements. For the specific composition EHA/E7 (50:50), the scattering and morphological properties of the films were studied as a function of time elapsed after UV exposure. Drastic changes in the size, shape, spatial distribution, and number density of nematic droplets were observed and analyzed in terms of coalescence/diffusion phenomena.

Physical stabilization of starch-allylurea blends by EB-grafting: a compositional and structural study.

A structural and compositional study of thermoplastic blends prepared from native potato starch (NPS) and various amounts of allylurea (AU) was performed to gain a better understanding of possible radiochemical routes to physically stable materials. The blends, mixed at ca. 130 degrees C, were studied in the form of 150 microns-thick films. Upon aging at room temperature, the samples obtained from these blends exhibit macroscopic phase separation under the form of allylurea blooming at their surface. The maximal compatibility of allylurea in amorphized potato starch was assessed by gravimetry and compared to that of urea in mixtures with NPS prepared in similar conditions. Physical aging of the unstable blends was monitored for various initial AU contents. Electron beam (EB) processing of fresh films with AU content above the solubility limit was shown to prevent phase separation, essentially as a consequence of radiation grafting onto starch of the unsaturated additive. X-ray diffractometry was performed to control (i) the effective amorphization of starch upon mixing, (ii) the recrystallization of the incompatible AU fraction from untreated blends in the tetragonal form, and (iii) the retardation or the suppression of this phenomenon after EB processing. The physical stability of the blends treated with a sufficient radiation dose (400-800 kGy) was confirmed by dynamic thermomechanical analysis of samples submitted to various hygrometric conditioning.

Phase Behavior of Blends of Polymers and Smectic-A Liquid Crystals.

The phase behavior of blends of polymers and smectic-A liquid crystals (LCs) is investigated using Flory-Huggins and Maier-Saupe-McMillan theories. Various examples are considered to depict the effects of the architecture and the size of the polymer together with the nature of anisotropic ordering forces on the phase diagram. The strength of these forces is characterized by a parameter alpha which is directly related to the temperature of the smectic-nematic transition. Three cases are considered depending on the value of alpha, and the corresponding phase diagrams are constructed. Substantial differences are observed in these diagrams, and the reasons for these differences are discussed. A comparative study is performed between mixtures of polymers and LCs, where the polymer is made of linear and crosslinked chains. The LC consists either of molecules with nematic ordering only or of molecules presenting both nematic and smectic-A ordering. Blends where polymer matrices are cross-linked networks are also examined. Remarkable properties are found in the nature of the phase diagrams for such mixtures. In general, it is observed that the ordering forces favor unmixing with a stronger effect for the higher smectic-A ordering. Spinodal curves are also reported for these mixtures. The effects of fluctuations near the transition temperatures are briefly discussed.