Roles of Conformational and Configurational Defects on the Physical Aging of Amorphous Poly(lactic acid)

Bulk enthalpy of melting of poly (l-lactic acid) (PLLA) determined by fast scanning chip calorimetry

The bulk enthalpy of melting of α-crystals of poly (l-lactic acid) (PLLA) has been evaluated by fast scanning chip calorimetry (FSC), by analysis of the correlation between the measured enthalpy of melting of sets of samples of different crystallinity and the corresponding heat capacity at 90°C, that is at a temperature higher than the glass transition temperature of the bulk amorphous phase and lower than the melting temperature. Extrapolation of this relationship for crystals formed at 140°C towards the heat capacity of fully solid PLLA yields a value of about 104.5±6 J/g when melting occurs at 180-200°C. The analysis is based on the presence of a two-phase structure, that is, absence of a vitrified rigid amorphous fraction (RAF) at the temperature of analysis of the solid fraction of the material (90°C). Formation and vitrification of an RAF was suppressed by avoiding both continuation of primary crystallization and secondary crystallization during cooling the system from the crystallization temperature of 140°C to 90°C, making use of the high cooling capacity of FSC. Small-angle X-ray scattering (SAXS) confirmed distinct thickening of initially grown lamellae which only is possible if these lamellae are not surrounded by a glassy RAF. Linear crystallinity values obtained by SAXS and calorimetrically determined enthalpy-based crystallinities agree close to each other. This article is protected by copyright. All rights reserved.

Degradation Mode of PBAT Mulching Film and Control Methods During Its Degradation Induction Period

:

The plastic film plays an important role in China's agricultural production. However, the large-scale use of plastic film has also produced a very serious problem of agricultural film pollution. Biodegradable polymers have attracted much attention because of the environmental pollution caused by traditional plastic mulching film. The most typical one is poly (butylene adipate co butylene terephthalate, PBAT). Poly (Butylene Adipate-co-Terephthalate) (PBAT) is a kind of aliphatic–aromatic polyesters with excellent biodegradability and mechanical processing properties. Therefore, it has been rapidly developed and widely used in the industry. However, there are clear requirements for the degradation period of agricultural film. At present, the degradable materials available on the market are difficult to meet the requirements of all crops for their degradation period. In this paper, the basic properties，degradation process and ways to delay the degradation of PBAT are reviewed to improve the degradation period of plastic film prepared by using this kind of material. Among them, the degradation process includes photodegradation, biodegradation and hydrolysis. The ways to delay the degradation include adding chain extender, light stabilizer, anti-hydrolysis agent and antibacterial agent. These can provide a theoretical basis for the research and development of biodegradable film with controllable degradation cycle. The future research and development of biodegradable polymers mainly focus on controllable degradation rate, stable degradation cycle, new materials and reducing research and development costs.

Structure-function assessment of 3D-printed porous scaffolds by a low-cost/open source fused filament fabrication printer

Additive manufacturing encompasses a plethora of techniques to manufacture structures from a computational model. Among them, fused filament fabrication (FFF) relies on heating thermoplastics to their fusion point and extruding the material through a nozzle in a controlled pattern. FFF is a suitable technique for tissue engineering, given that allows the fabrication of 3D-scaffolds, which are utilized for tissue regeneration purposes. The objective of this study is to assess a low-cost/open-source 3D printer (In-House), by manufacturing both solid and porous samples with relevant microarchitecture in the physiological range (100-500 μm pore size), using an equivalent commercial counterpart for comparison. For this, compressive tests in solid and porous scaffolds manufactured in both printers were performed, comparing the results with finite element analysis (FEA) models. Additionally, a microarchitectural analysis was done in samples from both printers, comparing the measurements of both pore size and porosity to their corresponding computer-aided design (CAD) models. Moreover, a preliminary biological assessment was performed using scaffolds from our In-House printer, measuring cell adhesion efficiency. Finally, Fourier transform infrared spectroscopy - attenuated total reflectance (FTIR-ATR) was performed to evaluate chemical changes in the material (polylactic acid) after fabrication in each printer. The results show that the In-House printer achieved generally better mechanical behavior and resolution capacity than its commercial counterpart, by comparing with their FEA and CAD models, respectively. Moreover, a preliminary biological assessment indicates the feasibility of the In-House printer to be used in tissue engineering applications. The results also show the influence of pore geometry on mechanical properties of 3D-scaffolds and demonstrate that properties such as the apparent elastic modulus (E_app) can be controlled in 3D-printed scaffolds.

Time-dependent failure of amorphous polylactides in static loading conditions

Polylactides are commonly praised for their excellent mechanical properties (e.g. a high modulus and yield strength). In combination with their bioresorbability and biocompatibility, they are considered prime candidates for application in load-bearing biomedical implants. Unfortunately, however, their long-term performance under static load is far from impressive. In a previous in vivo study on degradable polylactide spinal cages in a goat model it was observed that, although short-term mechanical and real-time degradation experiments predicted otherwise, the implants failed prematurely under the specified loads. In this study we demonstrate that this premature failure is attributed to the time-dependent character of the material used. The phenomenon is common to all polymers, and finds its origin in stress-activated segmental molecular mobility leading to a steady rate of plastic flow. The stress-dependence of this flow-rate is well captured by Eyring's theory of absolute rates, as demonstrated on three amorphous polylactides of different stereoregularity.We show that the kinetics of the three materials are comparable and can be well described using the proposed modeling framework. The main conclusion is that knowledge of the instantaneous strength of a polymeric material is insufficient to predict its long-term performance.

Conformational and microstructural characteristics of poly(L-lactide) during glass transition and physical aging

The glass transition and physical aging processes of poly(L-lactide) (PLLA) were studied by variable-temperature Fourier transform infrared (FTIR) spectroscopy and (13)C solid-state NMR spectroscopy. The glass transition temperature (T(g)) of PLLA can be well determined from the temperature-dependent FTIR intensity. Nearby T(g), a distinct change in the slope of spectral intensity versus temperature plot is detected. FTIR results suggest that the energy-favorable gauche-trans (gt) conformers rearrange into the less energy-favorable gauche-gauche (gg) counterparts with heating over the glass transition region, which becomes more distinct at temperature above T(g). Besides, the 1267 cm(-1) band, which shows different trends of variation from the other bands upon heating, was assigned to be more sensitive to the nu(as)(C-O-C)+delta(CH) vibration mode of the less energy-favorable gg conformers in PLLA. By comparing the FTIR spectra of the aged and deaged PLLA, it was demonstrated that the rearrangement from the high- to low-energy conformers, i.e., gg to gt, occurs with physical aging. (13)C spin-lattice relaxation measurements indicate that the relaxation rate distribution broadens with aging, which agrees with the previous suggestion that the locally ordered domains are formed during physical aging. Because of the larger variation in the conformational state and microstructure, the FTIR intensities vary much more abruptly for the aged sample with heating to nearby T(g).