Τhe effect of silica nanoparticles on the thermomechanical properties and degradation behavior of polylactic acid

Structure-property relationships in renewable composites of poly(lactic acid) reinforced by low amounts of micro- and nano-kraft-lignin

We investigate the direct and indirect effects of micro- and nano-kraft lignin, kL and NkL, respectively, at a quite low amount of 0.5 wt%, in poly(lactic acid) (PLA)-based composites. These renewable composites were prepared via two routes, either simple melt compounding or in situ reactive extrusion. The materials are selected and prepared using targeted methods in order to vary two variables, i.e., the size of kL and the synthetic method, while maintaining constant polymer chain lengths, L-/D-lactide isomer ratio and filler amounts. The direct/indirect effects were respectively investigated in the amorphous/semicrystalline state, as crystallinity plays in general a dominant role in polymers. The investigation involves structural, thermal and molecular mobility aspects. Non-extensive polymer-lignin interactions were recorded here, whereas the presence of the fillers led to both enhancements and suppressions of properties, e.g., glass transition, crystallization, melting temperatures, etc. The local and segmental molecular dynamics map of the said systems was constructed and is shown here for the first time, demonstrating both expected and unexpected trends. An interesting discrepancy between the trends in the calorimetric measurement against the dielectric Tg is revealed, providing indications for 'dynamical heterogeneities' in the composites as compared to neat PLA. The reactive extrusion as compared to compounding-based systems was found to exhibit stronger effects on crystallizability and mobility, most, probably due to the severe enhancement of the chains' diffusion. In general, the effects are more pronounced when employing nano-lignin compared to micro-lignin, which is the expected beneficial behaviour of nanocomposites vs. conventional composites. Interestingly, the variety of these effects can be easily manipulated by the proper selection of the preparation method and/or the thermal treatment under relatively mild conditions. The latter capability is actually desirable for processing and targeted applications and is proved here, once again, as an advantage of biobased polyesters such as PLA.

Segmental mobility in sustainable copolymers based on poly(lactic acid) blocks built onto poly(butylene succinate) in situ

Two series of newly synthesized sustainable block copolymers based on poly(butylene succinate) (PBSu) and polylactide (PLA) were studied. The copolymers were synthesized by a ring-opening polymerization of PLA in the presence of two initial PBSu of low molar mass. We focused on the effects of the PBSu/PLA ratio (1/99 up to 15/85), chain length and initial PBSu length on the final thermal transitions in the copolymers with an emphasis on molecular mobility/dynamics and subsequently on crystallization. Both aspects are considered relevant to the final materials performance, as well as facilitation of polymer renewability. Calorimetry and dielectric spectroscopy were the main investigation tools. In the amorphous state (i.e., in which the direct effects of copolymer structure are assessable), the segmental mobility of neat PLA was significantly faster in the copolymers. Segmental mobility was monitored via the decrease in the calorimetric and dielectric (α relaxation) glass-transition temperatures, Tg and Tg,diel, respectively. The effect was systematic with an increase in the PBSu/PLA ratio, and was rationalized through the plasticizing role of PBSu (low-Tg component) and facilitated also by the simultaneous lowering of the chain length in the copolymers. Dielectric spectroscopy allowed evaluation of the dynamical fragility (cooperativity) of chains, which was strongly suppressed in the copolymers. This finding suggested an increase in free volume or a gradual increase of interchain distances. This phenomenon could favor the natural enzymatic degradation of the systems (compostability), which is limited in neat PLA. We recorded enhancement of nucleation and the crystalline fraction in the copolymers that was likely connected with faster chain diffusion. Further lowering of the Tg with the implementation of crystallization was noted (which seemed a controversial effect) but which indicated crystallization-induced phase separation.

Revisiting Non-Conventional Crystallinity-Induced Effects on Molecular Mobility in Sustainable Diblock Copolymers of Poly(propylene adipate) and Polylactide

This work deals with molecular mobility in renewable block copolymers based on polylactide (PLA) and poly(propylene adipate) (PPAd). In particular, we assess non-trivial effects on the mobility arising from the implementation of crystallization. Differential scanning calorimetry, polarized light microscopy and broadband dielectric spectroscopy were employed in combination for this study. The materials were subjected to various thermal treatments aiming at the manipulation of crystallization, namely, fast and slow cooling, isothermal melt- and cold-crystallization. Subsequently, we evaluated the changes recorded in the overall thermal behavior, semicrystalline morphology and molecular mobility (segmental and local). The molecular dynamics map for neat PPAd is presented here for the first time. Unexpectedly, the glass transition temperature, T_g, in the amorphous state drops upon crystallization by 8–50 K. The drop becomes stronger with the increase in the PPAd fraction. Compared to the amorphous state, crystallization leads to significantly faster segmental dynamics with severely suppressed cooperativity. For the PLA/PPAd copolymers, the effects are systematically stronger in the cold- as compared to the melt-crystallization, whereas the opposite happens for neat PLA. The local β_PLA relaxation of PLA was, interestingly, recorded to almost vanish upon crystallization. This suggests that the corresponding molecular groups (carbonyl) are strongly involved and immobilized within the semicrystalline regions. The overall results suggest the involvement of either spatial nanoconfinement imposed on the mobile chains within the inter-crystal amorphous areas and/or a crystallization-driven effect of nanophase separation. The latter phase separation seems to be at the origins of the significant discrepancy recorded between the calorimetric and dielectric recordings on T_g in the copolymers. Once again, compared to more conventional techniques such as calorimetry, dielectric spectroscopy was proved a powerful and quite sensitive tool in recording such effects as well as in providing indirect indications for the polymer chains’ topology.

A Review on Synthesis, Properties, and Applications of Polylactic Acid/Silica Composites

Polylactic acid (PLA)/silica composites as multifunctional high-performance materials have been extensively examined in the past few years by virtue of their outstanding properties relative to neat PLA. The fabrication methods, such as melt-mixing, sol–gel, and in situ polymerization, as well as the surface functionalization of silica, used to improve the dispersion of silica in the polymer matrix are outlined. The rheological, thermal, mechanical, and biodegradation properties of PLA/silica nanocomposites are highlighted. The potential applications arising from the addition of silica nanoparticles into the PLA matrix are also described. Finally, we believe that a better understanding of the role of silica additive with current improvement strategies in the dispersion of this additive in the polymer matrix is the key for successful utilization of PLA/silica nanocomposites and to maximize their fit with industrial applications needs.

Effect of ligustrazine nanoparticles on Th1/Th2 balance by TLR4/MyD88/NF-κB pathway in rats with postoperative peritoneal adhesion

Background

Postoperative peritoneal adhesion (PPA) is regarded as fibrous bands connecting both injured abdominal wall and organs or adjacent tissues. It is associated with T helper (Th)1 and Th2 differentiation. However, the critical role of the immunopathogenesis of adhesion formation was precisely unknown. The aim of this study was to investigate the effect of a new agent polylactic acid (PLA) nanoparticles loaded with ligustrazine, that is, ligustrazine nanoparticles (LN) on PPA and identify the potential mechanism.

Methods

Twenty-four Sprague–Dawley rats were randomly divided into the sham, model, LN, and sodium hyaluronate (SH) groups. The structure of LN, including entrapment efficiency (EE) and loading capacity (LC), and in vitro drug release were calculated. Adhesions were scored and the Masson's trichrome staining was used to determine the collagen deposition. The expressions of TLR4, MyD88, and NF-κB were measured by qRT-PCR, immunohistochemistry, and western blot assay. Moreover, Th1-related cytokines (IFN-γ, IL-12), Th2-related cytokines (IL-4, IL-6) in the cecum tissue and serum were conducted by ELISA.

Results

LN had good EE, LC, and control-release delivery characters with fairly uniform diameter and spherical morphology. It could effectively prevent adhesion formation after surgery. Besides, it could reduce collagen fibers accumulation, downregulate the expression levels of TLR4, MyD88, and NF-κB, and maintain Th1/Th2 balance.

Conclusions

Ligustrazine nanoparticles had effective effects on Th1/Th2 balance by regulating TLR4/MyD88/NF-κB pathway in PPA rats. It may be served as a promising therapy on postoperative adhesion formation.

Role of Surface-Treated Silica Nanoparticles on the Thermo-Mechanical Behavior of Poly(Lactide)

Surface-treated fumed silica nanoparticles were added at various concentrations (from 1 to 24 vol%) to a commercial poly(lactide) or poly(lactic acid) (PLA) matrix specifically designed for packaging applications. Thermo-mechanical behavior of the resulting nanocomposites was investigated. Field Emission Scanning Electron Microscopy (FESEM) micrographs revealed how a homogeneous nanofiller dispersion was obtained even at elevated filler amounts, with a positive influence of the thermal degradation stability of the materials. Modelization of Differential Scanning Calorimetry (DSC) curves through the Avrami–Ozawa model demonstrated that fumed silica nanoparticles did not substantially affect the crystallization behavior of the material. On the other hand, nanosilica addition was responsible for significant improvements of the storage modulus (E′) above the glass transition temperature and of the Vicat grade. Multifrequency DMTA tests showed that the stabilizing effect due to nanosilica introduction could be effective over the whole range of testing frequencies. Sumita model was used to evaluate the level of filler dispersion. The obtained results demonstrated the potential of functionalized silica nanoparticles in improving the thermo-mechanical stability of biodegradable matrices for packaging applications, especially at elevated service temperatures.

Recent advances in the application of mesoporous silica-based nanomaterials for bone tissue engineering

Silica nanomaterials (SNMs) and their composites have recently been investigated as scaffolds for bone tissue engineering. SNM scaffolds possess the ability to encourage bone cell growth and also allow the simultaneous delivery of biologically active biomolecules that are encapsulated in the mesopores. Their high mechanical strength, low cytotoxicity, ability to stimulate both the proliferation and osteogenic differentiation of progenitor cells make the SNMs appropriate scaffolds. Their physiochemical properties facilitate the cell spreading process, allow easy access to nutrients and help the cell-cell communication process during bone tissue engineering. The ability to deliver small biomolecules, such as dexamethasone, different growth factors, vitamins and mineral ions depends on the morphology, porosity, and crystallinity of SNMs and their composites with other polymeric materials. In this review, the abilities of SNMs to perform as suitable scaffolds for bone tissue engineering are comprehensively discussed.

Influence of Annealing and Biaxial Expansion on the Properties of Poly(l-Lactic Acid) Medical Tubing

Poly-l-lactic acid (PLLA) is one of the most common bioabsorbable materials in the medical device field. However, its use in load-bearing applications is limited due to its inferior mechanical properties when compared to many of the competing metal-based permanent and bioabsorbable materials. The objective of this study was to directly compare the influence of both annealing and biaxial expansion processes to improve the material properties of PLLA. Results showed that both annealing and biaxial expansion led to an overall increase in crystallinity and that the crystallites formed during both processes were in the α’ and α forms. 2D-WAXS patterns showed that the preferred orientation of crystallites formed during annealing was parallel to the circumferential direction. While biaxial expansion resulted in orientation in both axial and circumferential directions, with relatively equal sized crystals in both directions, Da (112 Å) and Dc (97 Å). The expansion process had the most profound effect on mechanical performance, with a 65% increase in Young’s modulus, a 45% increase in maximum tensile stress and an 18-fold increase in strain at maximum load. These results indicate that biaxially expanding PLLA at a temperature above T_cc is possible, due to the high strain rates associated with stretch blow moulding.