Green chemistry for the synthesis of methacrylate-based hydrogels crosslinked through Diels–Alder reaction

This study describes an environmentally friendly and green synthetic approach for the preparation of poly(aminoethylmethacrylate)-based hydrogels crosslinked through Diels–Alder (DA) reaction in water. This “click” reaction offers the possibility of preparing chemically crosslinked hydrogels in the absence of any catalyst, initiator or coupling agent, thus preserving the biocompatibility of the material. The suitable furan diene was obtained by modifying a methacrylate polymer by its reaction with furfural, a first generation compound derived from renewable resources. Methacrylate-based complementary polymeric dienophiles were also prepared by introducing maleimide groups into the structure. The products obtained at different steps were characterized by FTIR, NMR and TGA techniques. The study of the rheological properties of the hydrogels proved the success of this green “click” synthetic strategy confirming the formation of chemically crosslinked networks by the use of the Diels–Alder reaction. Furthermore, SEM studies revealed promising morphological properties of the ensuing hydrogels in terms of biomedical applications.     

Curing kinetics of amine and sodium hydroxide catalyzed phenol-formaldehyde resins

The effect of both formaldehyde content and catalyst type used in the synthesis of several resole type phenolic resins has been studied by using differential scanning calorimetry. In this study Kissinger-Akahira-Sunose (KAS), Ozawa-Flynn-Wall (OFW) and Friedman model-free kinetics are applied in order to correlate the dynamic cure behaviour with the mentioned synthesis variables. Strong upward dependency of activation energy on conversion has been detected in all cases up to a maximum value. Lower the formaldehyde content fewer changes in activation energy have been detected, revealing a more homogeneous polymerization. As formaldehyde content increases, stronger variations of energy values have been observed and the maximum value is shifted to lower conversions. By comparing triethylamine and sodium hydroxide catalysts similar behaviour has been observed, with higher energy values and shifting of the maximum in the latter. Friedman approach has been resulted in more convenient and accurate for the energy values determination and KAS method seems useful for the dynamic cure prediction of that type of thermoset.     

A review of bacterial cellulose: sustainable production from agricultural waste and applications in various fields

Bacterial cellulose (BC) is a polymer with interesting conformation and properties. BC can be obtained in different shapes and is easily modified by chemical and physical means, so its applications in the production of new materials and nanocomposites for different purposes have been in the focus of many research projects. However, one of the major challenges to address in bacterium-derived polymer technology is to find suitable carbon sources as substrates that are cheap and do not compete with food production for achieving large scale industrial applications. Agricultural wastes are defined as the residues from the growing and processing of raw agricultural products such as crops, fruits, vegetables and dairy products. Their composition can vary depending on the type of agricultural activity and harvesting conditions, but these residues are suitable for the production of BC. The aim of this review is to give insight into the production of BC using agro-wastes and an overview of the most interesting and novel applications of this biopolymer in different areas i.e. environmental applications, optoelectronic and conductive devices, food ingredients and packaging, biomedicine, and 3D printing technology.

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Bacterial cellulose films with controlled microstructure–mechanical property relationships

Bacterial cellulose (BC) films with different porosities have been developed in order to obtain improved mechanical properties. After 13 days of incubation of Gluconobacter xylinum bacteria in static culture, BC pellicles have been set. BC films have been compression molded after water dispersion of BC pellicles and filtration by applying different pressures (10, 50, and 100 MPa) to obtain films with different porosities. Tensile behavior has been analyzed in order to discuss the microstructure–property relationships. Compression pressure has been found as an important parameter to control the final mechanical properties of BC films where slightly enhanced tensile strength and deformation at break are obtained increasing mold compression pressure, while modulus also increases following a nearly linear dependence upon film porosity. This behavior is related to the higher densification by increasing mold compression pressure that reduces the interfibrillar space, thus increasing the possibility of interfibrillar bonding zones. Network theories have been applied to relate film elastic properties with individual nanofiber properties.     

Replacing the Rhamnose-Xylose Moiety of QS-21 with Simpler Terminal Disaccharide Units Attenuates Adjuvant Activity in Truncated Saponin Variants

Adjuvants are key immunostimulatory components in vaccine formulations, which improve the immune response to the co-administered antigen. The saponin natural product QS-21 is one of the most promising immunoadjuvants in the development of vaccines against cancer and infectious diseases but suffers from limitations that have hampered its widespread human use. Previous structure–activity relationship studies have identified simplified saponin variants with truncated carbohydrate chains, but have not focused on the influence of the linear oligosaccharide domain of QS-21 in adjuvant activity. Herein, an expeditious 15-step synthesis of new linear trisaccharide variants of simplified QS-21-derived adjuvants is reported, in which the complex terminal xylose-rhamnose moiety has been replaced with commercially available, simpler lactose and cellobiose disaccharides in a β-anomeric configuration. In vivo immunological evaluation of the synthetic saponins showed attenuated antibody responses, highlighting the negative impact of such carbohydrate modifications on adjuvant activity, which could be associated with higher saponin conformational flexibility.     

By-products of the cider production: an alternative source of nutrients to produce bacterial cellulose

In the present work a culture process to produce bacterial cellulose (BC) using by-products of the cider production from the Basque Country was investigated. The apple pomace was mixed with sugar cane (AR/SC medium) and the mixture was found to be a potential carbon source for Gluconacetobacter medellinensis strain ID13488 since higher cellulose production was observed with respect to the commercial Hestrin and Shramm medium (H–S). The culture media were characterized in terms of pH, oxygen and sugars consumption. The expression level of the operon bcs (genes involved in BC biosynthesis) in apple residue containing medium respect to standard H–S medium was determined. It was found that in AR/SC medium the expression levels of bcsA gene, wich is the first gene of the bcs operon, was increased in 1.5-fold respect to the H–S media which correlates with the fact that BC production in AR/SC media is higher than in H–S media. The physico-chemical and mechanical properties, microstructure, crystallinity and water holding capacity of the biosynthesized BC membranes were analyzed and it was found that, in general, the BC obtained from AR/SC medium presented superior properties than that obtained from H–S medium. In this study an economic method for BC production is proposed with suitable properties for many applications.     

Crystallinity and Crystalline Confinement of the Amorphous Phase in Polylactides

Summary: Stereorregular polylactides such as poly (L-lactide) (PLLA) or poly (D-lactide) result from polymerization of optically pure lactides and are semicrystalline. Optically non-active poly(D,L-lactide) (PDLLA) can be regarded as random or atactic copolymers, show a random moiety distribution, and are completely amorphous. In this work three phases, comprising mobile amorphous fraction (MAF, χ_MA), rigid amorphous fraction (RAF, χ_RA) and crystalline fraction (χ_c) were determined in PLLA. It will be shown that RAP fraction not only elevates Tg but also increases the dynamic fragility (m) of polylactide chains around the Tg. These results agree with reported cases in which topologycal constraints inhibit longer range dynamics and suggest a smaller length scale of cooperativity of chains in confined environments.