Enhancing mechanical performance of polylactide by tailoring crystal morphology and lamellae orientation with the aid of nucleating agent

The Effect of Thermal History on the Fast Crystallization of Poly(l-Lactide) with Soluble-Type Nucleators and Shear Flow

The N₁,N₁'-(ethane-1,2-diyl)bis(N₂-phenyloxalamide) (OXA) is a soluble-type nucleator with a dissolving temperature of 230 °C in poly(l-lactic acid) (PLLA) matrix. The effect of thermal history and shear flow on the crystallization behavior of the PLLA/OXA samples was investigated by rheometry, polarized optical microscopy (POM), differential scanning calorimetry (DSC), wide angle X-ray diffraction (WAXD), and scanning electron microscopy (SEM). The crystallization process of the PLLA/OXA-240 sample (i.e., pre-melted at 240 °C) was significantly promoted by applying a shear flow, e.g., the onset crystallization time (tonset) of the PLLA at 155 °C was reduced from 1600 to 200 s after shearing at 0.4 rad/s for even as short as 1.0 s, while the crystallinity (Xc) was increased to 40%. Moreover, the tonset of the PLLA/OXA-240 sample is 60%⁻80% lower than that of the PLLA/OXA-200 sample (i.e., pre-melted at 200 °C) with a total shear angle of 2 rad, indicating a much higher crystallization rate of the PLLA/OXA-240 sample. A better organization and uniformity of OXA fibrils can be obtained due to a complete pre-dissolution in the PLLA matrix followed by shear and oscillation treatments. The well dispersed OXA fibrils and flow-induced chain orientation are mainly responsible for the fast crystallization of the PLLA/OXA-240 samples. In addition, the shear flow created some disordered α'-form crystals in the PLLA/OXA samples regardless of the thermal history (200 or 240 °C).

Thermal behavior of modified poly(L-lactic acid): effect of aromatic multiamide derivative based on 1H-benzotriazole

The goal of this work was to synthesis a novel aromatic multiamide derivative based on 1H-benzotriazole (PB) as an organic nucleating agent for poly(L-lactic acid) (PLLA), and investigate the effect of PB on the non-isothermal crystallization, melting behavior and thermal decomposition of PLLA. Here, PB was firstly synthesized through 1H-benzotriazole aceto-hydrazide and terephthaloyl chloride, then PB-nucleated PLLA was fabricated via melt-blending technology at various PB concentration from 0.5 wt% to 3 wt%. Finally, the thermal performances were evaluated through differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The high thermal decomposition temperature of PB indicated that PB possessed possibility as a nucleating agent for PLLA, and the non-isothermal crystallization behavior confirmed the crystallization accelerating effectiveness of PB for PLLA. Upon optimum concentration of 2 wt%, the onset crystallization temperature, the crystallization peak temperature and the non-isothermal crystallization enthalpy increased from 101.4°C, 94.5°C and 0.1 J·g-1 to 121.3°C, 115.8°C and 35.1 J·g-1, respectively. In addition, the non-isothermal crystallization behavior was also affected by the cooling rate and the final melting temperature. The melting behavior further evidenced the advanced nucleating ability of PB, and the competitive relationship between PB and the heating rate, the nuclear rate and crystal growth rate. Thermal stability measurement showed that PB with a concentration of 1 wt%–2 wt% could slightly improve the thermal stability of PLLA.

Facilely assess the soluble behaviour of the β-nucleating agent by gradient temperature field for the construction of heterogeneous crystalline-frameworks in iPP

Nucleating agent (NA) species with solubility and self-assembly abilities can readily and effectively manipulate the crystalline morphology of semicrystalline polymers through the construction of heterogeneous frameworks prior to the primary crystallization of basal resins. However, the solubility of NA species is difficult to assess by the current traditional methods. In this study, gradient temperature field (g-T field) was utilized for the first time to ascertain the dissolution and self-assembly behaviors of β-NA in the melts of isotactic polypropylene (iPP). The g-T field technique can facilely assess the soluble behavior of β-NA by determining the transformation between several NA frameworks, namely the needle-, flower- and dendrite-like supramolecular structures. Clarifying the soluble behavior of β-NA is of great significance to guide the formation of various crystalline frameworks under the homo-temperature fields and control the resultant crystalline morphology of β-modified iPP. Some interesting findings are summarized as follows: (1) an in situ observation under the g-T field clearly indicates the sequential occurrence of various nucleation and crystallization events in the same observed window, and proves the migration of well molten β-NA, (2) the exact correlation between Tf and framework type reveals that an abrupt transformation (over the narrow temperature range of 1 °C) occurred between needle-like and dendrite-like frameworks, (3) the primary crystallization of iPP is strongly dependent on the construction mode of the β-NA framework.

Enhanced crystallization kinetics of poly(lactide) with oxalamide compounds as nucleators: effect of spacer length between the oxalamide moieties

Crystallization kinetics of poly (lactide) were enhanced by using oxalamide compounds as nucleators and the nucleation efficiency can be well tailored by tuning the aliphatic spacer length between the oxalamide moieties.

High-performance poly(lactide) composites by construction of network-like shish-kebab crystals

Novel high-performance PLA composites were prepared by solid-state uniaxial stretching with the OXA as needle-like nucleation templates leading to a unique network-like shish-kebab crystal structures with a tensile strength above 120 MPa.

Effect of solubility of a hydrazide compound on the crystallization behavior of poly(l-lactide)

The influence of the annealing temperature on the solubility and nucleating efficiency of TMBH has been clarified.

Stereocomplex poly(lactic acid) nanoparticles crystallized through nanoporous membranes and application as nucleating agent

Stereocomplex crystallization of poly(l-lactic acid) (PLLA) and poly(d-lactic acid) (PDLA) was performed by flowing their blended solution through nano-channels of porous membranes.