Colorless and transparent polyimide nanocomposites: Thermo-optical properties, morphology, and gas permeation

Nanotechnology application in food packaging: A plethora of opportunities versus pending risks assessment and public concerns

Environmental factors, oxidation and microorganisms contamination, are the major causes for food spoilage, which leads to sensory features alteration, loss of quality, production of harmful chemicals and growth of foodborne pathogens capable to cause severe illness. Synthetic preservatives, traditional conserving methods and food packaging (FP), although effective in counteracting food spoilage, do not allow the real-time monitoring of food quality during storage and transportation and assent a relatively short shelf life. In addition, FP may protect food by the spoilage caused by external contaminations, but is ineffective against foodborne microorganisms. FP preservative functionalities could be improved adding edible natural antioxidants and antimicrobials, but such chemicals are easily degradable. Nowadays, thanks to nanotechnology techniques, it is possible to improve the FP performances, formulating and inserting more stable antioxidant/antimicrobial ingredients, improving mechanical properties and introducing intelligent functions. The state-of-the-art in the field of nanomaterial-based improved FP, the advantages that might derive from their extensive introduction on the market and the main concerns associated to the possible migration and toxicity of nanomaterials, frequently neglected in existing reviews, have been herein discussed.

Transparent Polyimide/Organoclay Nanocomposite Films Containing Different Diamine Monomers

Poly (amic acid) s (PAAs) were synthesized using 4,4′-(hexafluoroisopropyl-idene) diphthalic anhydride (6FDA) and two types of diamines—bis(3-aminophenyl) sulfone (BAS) and bis(3-amino-4-hydroxyphenyl) sulfone (BAS-OH). Two series of transparent polyimide (PI) hybrid films were synthesized by solution intercalation polymerization and thermal imidization using various concentrations (from 0 to 1 wt%) of organically modified clay Cloisite 30B in PAA solution. The thermo-mechanical properties, morphology, and optical transparency of the hybrid films were observed. The transmission electronic microscopy (TEM) results showed that some of the clays were agglomerated, but most of them showed dispersed nanoscale clay. The effects of -OH groups on the properties of the two PI hybrids synthesized using BAS and BAS-OH monomers were compared. The BAS PI hybrids were superior to the BAS-OH PI hybrids in terms of thermal stability and optical transparency, but the BAS-OH PI hybrids exhibited higher glass transition temperatures (T_g) and mechanical properties. Analysis of the thermal properties and tensile strength showed that the highest critical concentration of organoclay was 0.50 wt%.

Colorless and Transparent Copolyimides and Their Nanocomposites: Thermo-Optical Properties, Morphologies, and Gas Permeabilities

A series of linear aromatic copolyimides (Co-PIs) were synthesized by reacting 4,4′-biphthalic anhydride (BPA) with various molar contents of 2,2′-bis(trifluoromethyl)benzidine (TFB) and p-xylylenediamine (p-XDA) in N,N′-dimethylacetamide (DMAc). Co-PI films were fabricated by solution casting and thermal imidization with poly(amic acid) (PAA) on glass plates. The thermo-optical properties and gas permeabilities of Co-PI films composed of various molar ratios of p-XDA (0.2–1.0 relative to BPA) were investigated. Thermal properties were observed to deteriorate with increasing p-XDA concentration. However, oxygen-transmission rates (O₂TRs) and optical transparencies improved with increasing p-XDA concentration. Co-PI hybrids with a 1:0.2:0.8 molar ratio of BPA:TFB:p-XDA and organically modified hectorite (STN) were prepared by the in situ intercalation method. The morphologies and the thermo-optical and gas permeation properties of the hybrids were examined as functions of STN loading (5–50 wt %). XRD and TEM revealed substantial increases in clay particle agglomeration in the Co-PI hybrid films as the clay loading was increased from 5 to 50 wt %. The coefficient of thermal expansion (CTE) and the O₂TR of a Co-PI hybrid film were observed to improve with increasing STN concentration; however, its optical transparency decreased gradually with increasing STN concentration.

Optically transparent high temperature shape memory polymers

Optically transparent shape memory polymers (SMPs) have potential in advanced optoelectronic and other common shape memory applications, and here optically transparent shape memory polyimide is reported for the first time. The polyimide possesses a glass transition temperature (Tg) of 171 °C, higher than the Tg of other transparent SMPs reported, and the influence of molecular structure on Tg is discussed. The 120 μm thick polyimide film exhibits transmittance higher than 81% in 450-800 nm, and the possible mechanism of its high transparency is analyzed, which will benefit further research on other transparent high temperature SMPs. The transparent polyimide showed excellent thermomechanical properties and shape memory performances, and retained high optical transparency after many shape memory cycles.