Xueyan Yun

Alternating poly(lactic acid)/poly(ethylene-co-butylene) supramolecular multiblock copolymers with tunable shape memory and self-healing properties

Alternating supramolecular multiblock copolymers with hard poly(lactic acid) (PLA) and soft poly(ethylene-co-butylene) (PEB) segments were prepared by terminal functionalization of PLA–PEB–PLA triblock oligomers with the 2-ureido-4[1H]-pyrimidinone (UPy) self-complementary quadruple hydrogen bonding units. Such supramolecular copolymers (SMPs) exhibit the characteristic properties of thermoplastic elastomers. The thermal, morphological, mechanical, shape memory, and self-healing properties of SMPs can be readily modulated by varying the composition, stereostructure, and crystallizability of PLA blocks. The prepared SMPs are shown as transparent and elastic films, while their PLA–PEB–PLA precursors are viscous or brittle solids. Crystallization of isotactic PLA blocks, i.e. poly(L-lactic acid) (PLLA), in SMPs is significantly impeded by the end-caped UPy motifs. The prepared SMPs show a well-defined microphase-separated structure, which varies from cylindrical to lamellar morphology with the increasing fraction of PLA blocks. Compared to the PLA–PEB–PLA precursors, SMPs exhibit improved mechanical strengths, modulus, elongation-at-break, good thermally-induced shape memory and light-triggered self-healing properties. The recovery ratios of SMPs containing atactic poly(D,L-lactic acid) (PDLLA) blocks are nearly 100%. The shape memory and self-healing properties of SMPs can be modulated by the stereostructure of PLA segments and they become worse when the isotactic, crystallizable PLLA segments are presented.

Gas Permeability and Permselectivity of Poly(L‐Lactic Acid)/SiOx Film and Its Application in Equilibrium‐Modified Atmosphere Packaging for Chilled Meat

A layer of SiOx was deposited on the surface of poly(L-lactic acid) (PLLA) film to fabricate a PLLA/SiOx layered film, by plasma-enhanced chemical vapor deposition (PECVD) process. PLLA/SiOx film showed Youngs modulus and tensile strength increased by 119.2% and 91.6%, respectively, over those of neat PLLA film. At 5 °C, the oxygen (O2 ) and carbon dioxide (CO2 ) permeability of PLLA/SiOx film decreased by 78.7% and 71.7%, respectively, and the CO2 /O2 permselectivity increased by 32.5%, compared to that of the neat PLLA film. When the PLLA/SiOx film was applied to the equilibrium-modified atmosphere packaging of chilled meat, the gas composition in packaging reached a dynamic equilibrium with 6% to 11% CO2 and 8% to 13% O2 . Combined with tea polyphenol pads, which effectively inhibited the microbial growth, the desirable color of meat was maintained and an extended shelf life of 52 d was achieved for the chilled meat.

Improved mechanical and barrier properties of PPC multilayer film through interlayer hydrogen bonding interaction

Poly(propylene carbonate) (PPC) is coated with cellophane (PT) via the intermolecular hydrogen bonds of chitosan (CS) with the both polymers, and their multilayer films are prepared in different weight ratios. The trace presence of chitosan on the surface of PT promotes the combination with PPC layers. After coating, the Young’s modulus and tensile strength of PPC are greatly improved, the storage modulus still keep a high value between ∼0–70°C. The oxygen barrier of PPC is increased at least 580 times, and the multilayer films still keep good water vapor barrier in some degree.

Studies on Comonomer Compositional Distribution of Poly(propylene carbonate-propylene oxide) Copolymer and Its Effect on the Thermal, Mechanical and Oxygen Barrier Properties of Fractions

Poly(propylene carbonate) (PPC) was synthesized by the alternating copolymerization of carbon dioxide and propylene oxide (PO). However, during the polymerization, two by-products tended to produce cyclic propylene carbonate (CPC) and a polyether (PE) segment. The excess PO repeat units (PE segment) can easily insert into the PPC backbone and eventually produce the PPC–PO copolymer. The production of CPC and PE segments affected the increase of polymer chain length. In order to investigate the effects of the existence of PE segments, CPC, and molecular weight of fractions on the physical properties of PPC–PO copolymer, a series of fractions with narrow molecular weight distribution were obtained by repeated fractionation. Based on a solvent/non-solvent (chloroform/n-heptane) mixture, an original PPC–PO sample was fractionated into nine fractions with number–average molecular weights (Mn) from 0.34 × 105 to 5.56 × 105 and PE content from 0.4 to 15 mol%. The Mn of PPC–PO fractions decreased with the increase of PE content in the PPC–PO backbone, and the thermal and mechanical properties of the PPC–PO copolymers were affected by their Mn and PE contents. Furthermore, the lower the PE content, the higher the Mn value. Higher Mn means better tensile and lower oxygen permeability of PPC–PO copolymer.

Fast Crystallization and Toughening of Poly(L-lactic acid) by Incorporating with Poly(ethylene glycol) as a Middle Block Chain

High molecular weight poly(L-lactic acid)-poly(ethylene glycol)-poly(L-lactic acid) (PLLA–PEG–PLLA; PLGL) triblock copolymers with various lengths of the PLLA blocks were synthesized by ringopening polymerization of L-lactide. The amorphous and crystalline PLLA and PLGL films were prepared by hot pressing with different temperature treatments. PLLA and PEG blocks exhibited good miscibility in the amorphous PLGL samples, while phase separation occurred in the crystalline ones. The flexible PEG blocks not only accelerated the crystallization rate of PLLA but also greatly improve its flexibility. The crystallization time of PLGL copolymers shorten to less than 5 min and copolymers showed much better flexibility than neat PLLA, the maximum fracture strain reached about 600% for amorphous sample. The processing time of PLLA was greatly shortened and the brittleness of material was improved.

Fabrication of high-barrier plastics and its application in food packaging

Good barrier packing is particularly important for foods and keeps a suitable atmosphere inside the packages that is condusive to the preservation of the nutrients in food and extend storage time. The development of new technologies has greatly promoted the fabrication of thin film with a certain barrier property and optimizes the internal atmosphere of packages. This chapter focuses on the enhancement of barrier performance of nondegradable and biodegradable plastics as well as their fabrication and application in food packaging. The fabrication techniques include the orientation, coating, nanocompositing, blending, and layer-by-layer (LBL) assemply of nanoparticles based on synthetic and biobased polymers. The major emphasis is on the enhancement of barrier properties to guard against oxygen, carbon dioxide, and water vapor, which is key for food packaging plastics. The fabrication of new high barrier plastics will also be discussed, especially the development and potential application of new biodegradable materials in the food packaging industry.