Influence of Content in D Isomer and Incorporation of SBA-15 Silica on the Crystallization Ability and Mechanical Properties in PLLA Based Materials

Abstract

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.

Synergistic Effect of Mesoporous SBA‐15 Particles and Processing Strategy for Improving PLA Crystallization Capability in their Composites

“…[ 29,30 ] Content in the PLA of D stereoisomer is the key parameter. [ 40 ] As mentioned above, the disordered α ′ modification is attained for this PLA grade at temperatures below ≈90 °C, [ 40 ] while the pure α form is achieved for temperatures above ≈110 °C, observing mixed proportions of the two polymorphs in the temperature interval in between those two values. From the results in Figure 3b, it can be anticipated that the PLA and the mmPLASBA8 should give, under the present cooling rates, a mixture of the two polymorphs, except for the cooling rate of 1 °C min −1 , where the ordered α modification could be expected.…”

“…It can be observed that the profiles for the solution‐mixed composites are close to the values reported for the ordered α modification of PLA. [ 40,53 ] Thus, the spacings for the main 110/200 diffraction are presented in Table 1 , being those found in the solution‐mixed samples very near to the one of 0.5286 nm described for the α lattice.…”

“…203) of the two pure modifications. [40,53] Table 1. Results for the different samples after cooling from the melt at 5 °C min −1 : DSC and X-ray crystallinity (f c DSC and f c WAXS ), and spacing and full width at half maximum (d 110/200 and FWHM 110/200 ) for the main (110/200) diffraction (peak value).…”

“…Location of T c in these two cooling runs at these rates ( β = 40 and β = 20 °C min −1 ) suggests that development of both α ′ and α crystallites has probably occurred. [ 35,36,40 ] The disordered α ′ modification is usually obtained for this PLA grade at temperatures below ≈90 °C, [ 40 ] while the pure α form is achieved for temperatures above ≈110 °C, with mixed proportions of the polymorphs at temperatures in between those two values.…”

“…The inset shows an amplification in the region of the main diffraction (110)/(200) for three selected samples: PLA, mmPLASBA8, and smPLASBA8. The vertical lines are the values reported for diffractions (110/200) and (203) of the two pure modifications [40,53].…”