Thermal Stability of Poly (L-lactide): Influence of End Protection by Acetyl Group

Thermal stability of end-protected poly (L-lactide) (PLLA) was studied by dynamic thermal degradation and pyrolyzate analyses. The treatment of PLLA by acetic anhydride resulted in the acetylation of end hydroxyl groups, and at the same time a decrease in the residual Sn content in the polymer. The thermal degradation of the acetylated PLLA-Ac showed a shift to a 40-50°C higher degradation temperature range than that of untreated, high Sn content PLLA, but exhibited nearly the same degradation behavior as the untreated PLLA with a comparable Sn content. Purified metal-free PLLA-H showed good thermal stability, having the highest degradation temperature range. Interestingly, despite the end-protection, the acetylated metal-free PLLA-H/Ac decomposed at almost the same temperature as that of PLLA-H. From pyrolyzate and kinetic analyses, it was found that the contribution of the hydroxyl-end acetylation to the stability of PLLA was negligible, except for the stabilization effect due to the elimination of residual Sn during the acetylation process

Racemization on Thermal Degradation of Poly(L-lactide) with Calcium Salt End Structure

Poly(L-lactide) with calcium salt end structure (PLLA-Ca) is a promising material for PLLA recycling because of the ease of lactide recovery through the unzipping depolymerization process. However, the pyrolysis of PLLA-Ca also causes meso-lactide to form. In this article, the racemization in PLLA-Ca pyrolysis was analyzed in detail with Py-MS, Py-GC/MS, and a glass tube oven. The results suggested that at a temperature lower than 250°C, nucleophilic attack by a carboxylate anion end on an asymmetrical methyne carbon in a penultimate lactate unit occurred, resulting in the predominant formation of meso-lactide. On the other hand, also at temperatures over 320°C, by-reactions, such as enolization reactions, caused the meso-lactide to form, but not dominantly. In the temperature range of 250-320°C, L,L-lactide was produced exclusively, because unzipping depolymerization proceeded as the main reaction. This is a very significant result for PLLA recycling, because PLLA-Ca is an easily recyclable material, which depolymerizes based on the 1st-order weight loss process

Thermal degradation of poly(L-lactide): effect of alkali earth metal oxides for selective L,L-lactide formation

To achieve the feed stock recycling of poly(L-lactide) (PLLA) to L,L-lactide, PLLA composites including alkali earth metal oxides, such as calcium oxide (CaO) and magnesium oxide (MgO), were prepared and the effect of such metal oxides on the thermal degradation was investigated from the viewpoint of selective L,L-lactide formation. Metal oxides both lowered the degradation temperature range of PLLA and completely suppressed the production of oligomers other than lactides. CaO markedly lowered the degradation temperature, but caused some racemization of lactide, especially in a temperature range lower than 250 °C. Interestingly, with MgO racemization was avoided even in the lower temperature range. It is considered that the effect of MgO on the racemization is due to the lower basicity of Mg compared to Ca. At temperatures lower than 270 °C, the pyrolysis of PLLA/MgO (5 wt%) composite occurred smoothly causing unzipping depolymerization, resulting in selective L,L-lactide production. A degradation mechanism was discussed based on the results of kinetic analysis. A practical approach for the selective production of L,L-lactide from PLLA is proposed by using the PLLA/MgO composite

Thermal Degradation Behavior of Poly (lactic acid) Stereocomplex

Thermal degradation of poly(lactic acid) stereocomplex (scPLA) was investigated to clarify the pyrolysis mechanism. Three scPLA samples with different chain end structures were prepared, namely, as-polymerized scPLA-ap, precipitated-with-methanol scPLA-pr, and purified metal-free scPLA-H. From the analyses of thermal degradation kinetics and pyrolyzates of the scPLA samples, typical degradation mechanisms of these scPLAs were proposed as follows: The pyrolysis of scPLA-ap proceeds through main unzipping depolymerization caused by Sn-alkoxide chain ends with apparent Ea = 80-100kJ mol-1, showing zero-order weight loss behavior. The pyrolysis of scPLA-pr also proceeds via a zero-order weight loss process consisting of main Sn-catalyzed selective lactide elimination with apparent Ea = 100-120kJ mol-1 caused by Sn-carboxylate chain ends. The pyrolyzates from scPLA-ap and scPLA–pr were predominantly L,L-/D,D-lactides. In the case of scPLA-H, random degradation is a main process, producing a large amount of meso-lactide and cyclic oligomers. These degradation mechanisms were nearly the same as those of the corresponding PLLAs, except that the scPLA-ap pyrolysis started at higher temperature due to the higher melting point of scPLA

Effect of Sn Atom on Poly(Ｌ-lactic acid) Pyrolysis

Tin 2-ethylhexanoate is an indispensable component of commercially available poly(L-lactic acid) (PLLA). However, the thermal degradation kinetics of PLLA containing Sn have not yet clearly been established; in particular, whether the degradation mechanism is a 1st-order or a random reaction. To clarify the effects of residual Sn on PLLA pyrolysis, PLLA samples with different Sn contents from 20 to 607 ppm were prepared and subjected to pyrolysis analysed with pyrolysis-gas chromatography/mass spectroscopy (Py-GC/MS) and thermogravimetry (TG). The pyrolysis of PLLA Sn-607 (Sn content: 607 ppm) with Py-GC/MS in the temperature range of 40–400 °C selectively produced lactides. In contrast, the pyrolysis of PLLA Sn-20 (Sn content: 20 ppm) was accompanied by the production of cyclic oligomers. The dynamic pyrolysis of PLLA-Sn samples by TG clearly indicated that with an increase in Sn content there was a shift to a lower degradation temperature range and a decrease in activation energy Ea. The kinetic analysis of the dynamic pyrolysis data indicates that the Sn-catalyzed pyrolysis starts through a random degradation behaviour and then shifts to a zero-order weight loss as the main process. Three reactions were put forward as being possible mechanisms of the zero-order weight loss; one being an unzipping reaction accompanying a random transesterification, the other two being the Sn-catalyzed pseudo-selective and selective lactide elimination reactions from random positions on a polymer chain. The kinetic parameter values obtained could be adequately explained for each degradation process

Porous Collagen Scaffold Reinforced with Surfaced Activated PLLA Nanoparticles

Porous collagen scaffold is integrated with surface activated PLLA nanoparticles fabricated by lyophilizing and crosslinking via EDC treatment. In order to prepare surface-modified PLLA nanoparticles, PLLA was firstly grafted with poly (acrylic acid) (PAA) through surface-initiated polymerization of acrylic acid. Nanoparticles of average diameter 316 nm and zeta potential −39.88 mV were obtained from the such-treated PLLA by dialysis method. Porous collagen scaffold were fabricated by mixing PLLA nanoparticles with collagen solution, freeze drying, and crosslinking with EDC. SEM observation revealed that nanoparticles were homogeneously dispersed in collagen matrix, forming interconnected porous structure with pore size ranging from 150 to 200 μm, irrespective of the amount of nanoparticles. The porosity of the scaffolds kept almost unchanged with the increment of the nanoparticles, whereas the mechanical property was obviously improved, and the degradation was effectively retarded. In vitro L929 mouse fibroblast cells seeding and culture studies revealed that cells infiltrated into the scaffolds and were distributed homogeneously. Compared with the pure collagen sponge, the number of cells in hybrid scaffolds greatly increased with the increment of incorporated nanoparticles. These results manifested that the surface-activated PLLA nanoparticles effectively reinforced the porous collagen scaffold and promoted the cells penetrating into the scaffold, and proliferation

Scaffold Structural Microenvironmental Cues to Guide Tissue Regeneration in Bone Tissue Applications

In the process of bone regeneration, new bone formation is largely affected by physico-chemical cues in the surrounding microenvironment. Tissue cells reside in a complex scaffold physiological microenvironment. The scaffold should provide certain circumstance full of structural cues to enhance multipotent mesenchymal stem cell (MSC) differentiation, osteoblast growth, extracellular matrix (ECM) deposition, and subsequent new bone formation. This article reviewed advances in fabrication technology that enable the creation of biomaterials with well-defined pore structure and surface topography, which can be sensed by host tissue cells (esp., stem cells) and subsequently determine cell fates during differentiation. Three important cues, including scaffold pore structure (i.e., porosity and pore size), grain size, and surface topography were studied. These findings improve our understanding of how the mechanism scaffold microenvironmental cues guide bone tissue regeneration

INFLUENCE OF VIBRATION ON PERFORMANCE OF RECYCLED CONCRETE

In this study, the vibration mixing technology is researched, and the effect of the vibration on the performance of recycled concrete is investigated. Through the analysis of the strengthened mechanism on the recycled concrete adopting vibration mixing, the vibration could increase of collision numbers between aggregates so as to purify the surface of recycled aggregates, and improve the interface between the recycled aggregates and cement pastes, and realize the macroscopic and microscopic uniformity of recycled concrete to improve the performance of recycled concrete. The performance of recycled concrete mixed by ordinary forced mixing and vibration mixing respectively, was compared experimentally. And the results indicate that the vibration could increase the air contents of recycled concrete, improve the mechanical performance of the recycled concrete, provide a favorable environment in order to enhance the microscopic structure and strengthen the recycled concrete

Advanced application of collagen-based biomaterials in tissue repair and restoration

AbstractIn tissue engineering, bioactive materials play an important role, providing structural support, cell regulation and establishing a suitable microenvironment to promote tissue regeneration. As the main component of extracellular matrix, collagen is an important natural bioactive material and it has been widely used in scientific research and clinical applications. Collagen is available from a wide range of animal origin, it can be produced by synthesis or through recombinant protein production systems. The use of pure collagen has inherent disadvantages in terms of physico-chemical properties. For this reason, a processed collagen in different ways can better match the specific requirements as biomaterial for tissue repair. Here, collagen may be used in bone/cartilage regeneration, skin regeneration, cardiovascular repair and other fields, by following different processing methods, including cross-linked collagen, complex, structured collagen, mineralized collagen, carrier and other forms, promoting the development of tissue engineering. This review summarizes a wide range of applications of collagen-based biomaterials and their recent progress in several tissue regeneration fields. Furthermore, the application prospect of bioactive materials based on collagen was outlooked, aiming at inspiring more new progress and advancements in tissue engineering research. Graphical Abstrac