Tamara M. Díez-Rodríguez

Several composites based on poly(3-hydroxybutyrate) (PHB) and mesoporous SBA-15 silica were prepared by solvent-casting followed by a further stage of compression molding. The thermal stability, phase transitions and crystalline details of these composites were studied, paying special attention to the confinement of the PHB polymeric chains into the mesopores of the silica. For that, differential scanning calorimetry (DSC) and real-time variable-temperature X-ray scattering at small angles (SAXS) were performed. Confinement was stated first by the existence of a small endotherm at temperatures around 20 °C below the main melting or crystallization peak, being later confirmed by a notable discontinuity in the intensity of the main (100) diffraction from the mesoporous silica observed through SAXS experiments, which is related to the change in the scattering contrast before and after the crystallization or melting of the polymer chains. Furthermore, the usual α modification of PHB was developed in all samples. Finally, a preliminary investigation of mechanical and relaxation parameters was carried out through dynamic–mechanical thermal analysis (DMTA). The results show, in the temperature interval analyzed, two relaxations, named α and β (the latest related to the glass transition) in order of decreasing temperatures, in all specimens. The role of silica as a filler is mainly observed at temperatures higher than the glass transition. In such cases, stiffness is dependent on SBA-15 content.

The deficient management of plastic waste has caused a serious worldwide environmental problem. Thus, one of the main challenges for the industry in the plastics sector in contributing to sustainability and a circular economy consists of providing a subsequent service life to this waste. For that purpose, the appropriate incorporation of antioxidants will play a key role in preventing or postponing the degradation of plastic waste, where the formation of radicals is initiated during its previous lifetime by the action of degrading agents. Functionalized particles, based on mesoporous MCM-41 silica with Irganox 1076, were prepared with two different protocols and were further incorporated into a material containing virgin PP and 30 wt.% of recycled PP, with the purpose of guaranteeing thermal stability during its next service life. A very significant increase in the thermal stability of the resulting composites was found, attributable to the synergistic action between the Irganox 1076 antioxidant and the MCM-41 particles. In addition, the presence of hybrid particles leads to an important nucleating effect for the crystallization of PP. Moreover, a reinforcing role was also played by these modified mesoporous silicas in the resultant systems. The presented methodology constitutes, therefore, a promising strategy for contributing to the circular economy—since the synergy between the Irganox 1076 antioxidant and MCM-41 particles was found to play an important role in the ultimate performance of recycled polyolefins.

Composites based on an L-rich poly(lactic acid) (PLLA) and MCM-41, either neat or modified with a silver (MCM-41@Ag), are achieved by solvent casting, being next processed by compression molding. Ag is mainly embedded as nanowires within the hybrid MCM-41@Ag particles, enabling its antimicrobial character. In these composites, the PLLA thermal stability, nucleation efficiency, and mechanical response are dependent on the MCM-41 nature and, to a lesser extent, on its content. Thus, differences in transitions of the PLLA matrix are noticed during cooling at 10 °C/min and in the subsequent heating when composites with neat or modified MCM-41 are compared. A very remarkable nucleation effect is played by pristine MCM-41, being inferior when MCM-41@Ag is incorporated into the PLLA. Wide angle X-ray scattering (WAXS) measurements using synchrotron radiation and performed under variable-temperature conditions in the composites containing MCM-41@Ag indicate that during cold crystallization, the disordered α′ polymorph is initially formed, but it rapidly transforms into ordered α crystals. A long spacing peak, clearly seen in pure PLLA, appears as a small shoulder in PLLAMCM@Ag4 and is undetectable in PLLAMCM@Ag9 and PLLAMCM@Ag20. Furthermore, an increase in MH with the silica content is found in the two sets of composites, the higher MH values being observed in the family of PLLA and MCM-41@Ag. Finally, remarkable antimicrobial features are noticeable in the composites with MCM-41@Ag since this modified silica transfers its biocidal characteristics into the PLLA composites.

Different materials, based on an L-rich polylactide (PLA) as matrix, acetyl tri-n-butyl citrate (ATBC) as plasticizer, and mesoporous Mobile Crystalline Material.41 (MCM-41) particles as nucleating agent, were attained by melt extrusion. These materials are constituted by (a) binary blends of PLA and ATBC with different contents of the latest; (b) a dual compound of PLA and a given amount of MCM-41 silica (5 wt.%); and (c) ternary composites that include PLA, ATBC at several compositions and mesoporous MCM-41 at 5 wt.%. Influence of the incorporation of the plasticizer and nucleating particles has been comprehensively analyzed on the different phase transitions: glass transition, cold crystallization, melt crystallization and melting processes. Presence of both additives moves down the temperature at which PLA phase transitions take place, while allowing the PLA crystallization from the melt at 10 °C/min in the composites. This tridimensional ordering is not noticeable in the pristine PLA matrix and, accordingly, PLA crystallization rate is considerably increased under dynamic conditions and also after isothermal crystallization from either the melt or the glassy state. An important synergistic effect of dual action of ATBC and MCM-41 has been, therefore, found.

Two L-rich polylactides (PLLA) with distinct contents in D isomer and their composites with an intermediate amount of mesoporous Santa Barbara Amorphous-15 (SBA-15) (about 9 wt.%) particles were attained by melt extrusion for the evaluation of the effect of content in D isomer and incorporation of mesoporous silica on the structural PLLA features and on their ultimate mechanical performance. For that, samples have been crystallized under dynamic and isothermal tests (from the melt and from the glassy states). The results from DSC and X-ray diffraction show obtainment of the pure α’ and α modifications at different intervals of crystallization temperature depending on the D steroisomer amount of the PLLA used. Furthermore, several phase transitions are observed depending on the crystallinity reached and the polymorphs developed during the isothermal crystallization from the glass: an additional cold crystallization, the α’/α transformation and the subsequent melting process, appearing all of them at temperatures clearly dependent on the D content. Rigidity, measured through microhardness in amorphous samples, is also affected by the D isomer and the presence of SBA-15 particles. Reinforcement effect of mesoporous silica is relatively more important in the matrix with the highest D content.

A study of different nanocomposites based on poly(ε-caprolactone) (PCL) and mesoporous SBA-15 silica that were prepared by melt extrusion was carried out by analyzing the possible effect of this filler on the crystalline details of PCL, on its mechanical behavior, and on the eventual observation of the confinement of the polymeric chains within the hollow nanometric silica channels. Thus, simultaneous Small-Angle and Wide-Angle X-ray Scattering (SAXS/WAXS) synchrotron experiments at variable temperature were performed on these PCL nanocomposites with different mesoporous silica contents. The importance of the morphological and structural features was assessed by the changes that were observed during the mechanical response of the final materials, which determined that the presence of mesoporous particles leads to a noticeable reinforcing effect.

In this study, nanocomposites based on polycaprolactone (PCL) and two types of mesoporous silicas, MCM-41 and SBA-15, were attained by melt extrusion. The effect of the silica incorporated within the PCL matrix was observed, firstly, in the morphological characteristics and degradation behavior of the resultant composites. DSC experiments provided information on the existence of confinement in the PCL–SBA-15 materials through the appearance of an additional small endotherm, located at about 25–50 °C, and attributed to the melting of constrained crystallites. Displacement to a slightly lower temperature of this endothermic event was observed in the first heating run of PCL–MCM-41 composites, attributed to the inferior pore size in the MCM-41 particles. Thus, this indicates variations in the inclusion of PCL chains within these two mesostructures with different pore sizes. Real-time variable-temperature small-angle X-ray scattering (SAXS) experiments with synchrotron radiation were crucial to confirm the presence of PCL within MCM-41 and SBA-15 pores. Accurate information was also deduced from these measurements regarding the influence of these two mesoporous MCM-41 and SBA-15 silicas on PCL long spacing. The differences found in these morphological and structural features were responsible for the ultimate mechanical response exhibited by the two sets of PCL nanocomposites, with a considerably higher increase of mechanical parameters in the SBA-15 family.

Several composites based on an L-rich poly(lactic acid) (PLA) with different contents of mesoporous Santa Barbara Amorphous (SBA-15) silica were prepared in order to evaluate the effect of the mesoporous silica on the resultant PLA materials by examining morphological aspects, changes in PLA phases and their transitions, and, primarily, the influence on some final properties. Melt extrusion was chosen for the obtainment of the composites, followed by quenching from the melt to prepare films. Completely amorphous samples were then attained, as deduced from X-ray diffraction and differential scanning calorimetry (DSC) analyses. Thermogravimetric analysis (TGA) results demonstrated that the presence of SBA-15 particles in the PLA matrix did not exert any significant influence on the thermal decomposition of these composites. An important nucleation effect of the silica was found in PLA, especially under isothermal crystallization either from the melt or from its glassy state. As expected, isothermal crystallization from the glass was considerably faster than from the molten state, and these high differences were also responsible for a more considerable nucleating role of SBA-15 when crystallizing from the melt. It is remarkable that the PLA under analysis showed very close temperatures for cold crystallization and its subsequent melting. Moreover, the type of developed polymorphs did not accomplish the common rules previously described in the literature. Thus, all the isothermal experiments led to exclusive formation of the α modification, and the observation of the α’ crystals required the annealing for long times at temperatures below 80 °C, as ascertained by both DSC and X-ray diffraction experiments. Finally, microhardness (MH) measurements indicated a competition between the PLA physical aging and the silica reinforcement effect in the as-processed amorphous films. Physical aging in the neat PLA was much more important than in the PLA matrix that constituted the composites. Accordingly, the MH trend with SBA-15 content was strongly dependent on aging times.