Moisture desorption isotherms and glass transition temperatures of osmo-dehydrated apple and pear

The aim of this work was to evaluate osmo-dehydrated pear and apple stability on the basis of combined data of moisture desorption isotherms and glass transition temperatures (Tg) investigated in the temperature range of 30–60 °C. Parallelipedic pieces of pears and apples were osmo-dehydrated at 30 °C in sucrose syrups at 70% during 65 min. To study their hygroscopic properties behavior, the osmo-dehydrated fruits were stored in static desiccators at 30, 45 and 60 °C, and relative humidities ranging from 6 to 80%. The GAB model satisfactory described the relationship between the water activity (aw) and water content (X) of osmo-dehydrated fruits (r² ≥ 0.994). Differential scanning calorimetry was used to measure Tg of osmo-dehydrated fruits conditioned at various water activities. A strong plasticizing effect of water on Tg was found with a large reduction of Tg when the water content increased. The dependence of Tg on water activity data was satisfactory correlated by the Roos model (0.993 ≤ r² ≤ 0.999). Whereas the Tg ～ X_w was adequately described by Gordon and Taylor model (0.898 ≤ r ≤ 0.990). The changes of Tg ～ aw and Tg ～ X_w were established and the critical values of water contents were determined for the investigated range of temperature.     

Rheological studies on PNIPAAm hydrogel synthesis via in situ polymerization and on resulting viscoelastic properties

Bulk poly(N-isopropylacrylamide) hydrogels were prepared via free radical polymerization. Two different initiation methods were studied: redox- and photoinitiation. It was demonstrated that the desired final properties of resulting hydrogels, i.e., high monomer conversion (>95%) and adjustable swelling were only obtained by selecting best suited initiation conditions. For redox polymerization, this was achieved by tuning the ratio of accelerator N,N,N′,N′-tetramethylethylenediamine to initiator ammonium persulfate. The key parameters for achieving optimum photopolymerization conditions were photoinitiator concentration and UV irradiation time. With help of in situ rheological measurements, optimum conditions could be further verified and quantified by monitoring the liquid-to-gel transition. Overall, photoiniated crosslinking copolymerization was postulated to offer better options for in situ preparation of tailored functional hydrogels, in particular for the integration of smart soft matrices within membrane pores or other microsystems via a rapid reaction. Rheology was also used to investigate the hydrogel after ex situ preparation, revealing “perfect” soft-rubbery behavior. A good correlation between the mesh sizes determined from swelling and rheology was also found. In conclusion, rheology has been found to be a powerful tool because it provides valuable data on polymerization and gelation kinetics as well as information about the hydrogels microstructure based on their viscoelastic character.     

Poly(ethylene terephthalate), modified with bisphenol S units,with increased glass transition temperature

Novel copolyesters have been prepared by melt mixing poly(ethylene terephthalate) (PET) with an ethoxylated bisphenol S, with the aim to prepare new polyesters with increased T_g, to be used in a wider range of temperatures with respect to neat PET. No side reactions occur during the synthesis of the samples, as proved by NMR analysis. The insertion of the bisphenol S (sulfonyldiphenol) groups does not significantly alter the thermal stability of PET. The thermal analysis showed that T_m and crystallization rate of the copolymers decreased with incasing co‐unit content. The T_g of the copolyesters can be increased by bisphenol S insertion, up to 40°C higher with respect to neat PET, that allows the use of amorphous PET in a wider range of applications. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Synthesis and characterization of novel poly(butylene succinate)‐based copolyesters designed as potential candidates for soft tissue engineering

Poly(butylene succinate/diglycolate) copolymers by reactive blending were subjected to molecular and solid‐state characterization to study the effects of transesterification reactions. ¹³C‐nuclear magnetic resonance analysis evidenced the formation of block copolymers. Thermal measurements showed that all the samples were semicrystalline, with a soft amorphous phase and a rigid crystal phase. Wide‐angle x‐ray diffraction measurements indicated that the copolymers are characterized by cocrystallization. The mechanical properties were found related to crystallinity degree. The random copolymer, characterized by the lowest crystallinity degree, exhibits the lowest elastic modulus and the highest deformation at break. Therefore, solid‐state properties of poly(butylene succinate/diglycolate) copolymers can be tailored by acting on the molecular architecture. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers     

Miscibility of bacterial poly(3-hydroxybutyrate-co-3-hydroxyvalerate) with ester substituted celluloses

Summary Blends of bacterial Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) with cellulose acetate butyrate (CAB) and cellulose acetate propionate (CAP) were prepared by melt blending. When CAB or CAP content in the blends is 50%, the blends are stable, homogeneous mixed glasses, characterized by a glass transition that follows the Fox equation. When the cellulose ester content is lower than 50%, PHBV crystallizes upon room storage and the blends are partially crystalline. Besides the strongly composition dependent glass transition, another slightly composition dependent relaxation is observed by both DMTA and DSC measurements in the vicinity of the glass transition of PHBV. It is suggested that both blend components partake in both mobilization processes.