Conformational Regulation and Crystalline Manipulation of PLLA through a Self-Assembly Nucleator.

Self-assembly nucleators have been increasingly used to manipulate the crystallization of PLLA due to their strong intermolecular interaction with PLLA, while the molecular mechanism of such interaction is still unrevealed. In present work, one special self-assembly nucleator (TMC-300) with relatively high solubility in PLLA matrix, is chosen to investigate how the interaction works at molecular level to promote the crystallization of PLLA mainly through time-resolved spectroscopy. The results indicate that due to the dipole-dipole NH···O═C interaction between dissolved TMC-300 and PLLA, PLLA chains are transformed into gt conformer before TMC-300 phase-separating from PLLA melt, resulting in low energy barrier to pass for the following formation of PLLA α-crystal (α-crystal is consisted of gt conformer). Once the dissolved TMC-300 starts to self-assemble into frameworks upon cooling, the transformed PLLA chains with high population of gt conformer form the primary nuclei on the surface of such self-assembling TMC-300 frameworks. For the first time, not only the heterogeneous nucleation but also the conformational regulation of PLLA chains are proved to be responsible for the high efficiency of the self-assembly nucleators (TMC-300) in promoting the crystallization of PLLA. Therefore, conformational regulation is proposed for crystalline manipulation of PLLA, and this work brings new insight on promoting the crystallization of PLLA even other polymers by regulating their molecular conformation.

Comb Polybutadiene with Long Polystyrene Side Chains: A Solution for Tunable Flowability and Enhancing Dielectric Properties in High-Frequency Printed Board Adhesive Films.

Liquid high-vinyl polybutadiene (PB) possessed excellent dielectric properties, rendering them suitable candidates for adhesive films of high-frequency printed boards. However, their inherent low molecular weights resulted in chain slippage and overflow during processing, thereby diminishing the performance of the adhesive films. To address this challenge, we synthesized comb PB with long polystyrene side chains via reversible addition-fragmentation chain transfer (RAFT) polymerization, effectively immobilizing the PB backbone and restricting relative chain slippage. Controlling the length and number of "comb teeth" (styrene side chains) efficiently regulated the flowability of comb PB, achieving distinct flow states. Simultaneously, molecular dynamics simulations revealed that the elongated and inflexible polystyrene side chains of comb PB could create minuscule cavities, which impeded close packing of molecules and led to low dielectric constants (2.39/2.01, 1 MHz/10 GHz) and ultralow dielectric losses (0.0071/0.0016, 1 MHz/10 GHz). Furthermore, a series of printed circuit boards were fabricated using a comb PB adhesive film, and the signal loss was significantly reduced to 48.8% (19 GHz) in comparison with a commercial epoxy adhesive. This study demonstrated the potential of comb PB with polystyrene side chains to achieve desirable flow and dielectric properties by introducing tangles, large volume potential resistance, and microporosity compared with block structures.

Selectively multilayered distribution of stereocomplex crystallite and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) ribbons to achieve highly ductile and strong poly(l-lactide) composites.

Blending poly(l-lactide) (PLLA) with elastic polymers is an efficient way to obtain highly ductile materials (> 300 %), but it is accompanied by a significant reduction in strength. In this work, a special alternating multilayered composites with alternating stereocomplex crystallite (SC) (PLLA/poly(d-lactide) (PDLA) layer) and highly oriented Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) ribbons (PLLA/PHBV layer) is in situ constructed during laminated structuring process. Experimental results show that in situ formed PHBV ribbons are limitedly distributed in the thickness direction and align parallel to the layer interfaces. More interestingly, not only highly oriented shish crystals but also sparse lamellae of PLLA, which are arrested by SC, shish crystals, and PHBV ribbons, are in situ formed. Compared with sea-island structured composites prepared by traditional compression molding, the alternating multilayered composites show an increase in elongation at break from 8.7 % to 345.1 % and an increase in yield strength from 61.4 MPa to 73.2 MPa. During the tensile testing, the PLLA/PHBV layers firstly form micro-fibrils and micro-voids, driving the molecular chains of the PLLA/PDLA layer to respond in time to external forces through stress transfer of rich continuous layer interfaces. Since shear yielding and plastic deformation can easily penetrate the entire matrix, the alternating multilayered composites go a brittle-ductile transformation and the ductility is improved significantly. The increased strength of the alternating multilayered material is ascribed to the stiff shish crystals and SC. This work provides important guidance for the durable application of strong and ductile PLLA-based materials.

In-situ constructing highly oriented ductile poly(3-hydroxybutyrate-co-3-hydroxyvalerate) nanoribbons: Towards strong, ductile, and good heat-resistant polylactic-based composites.

It remains a great challenge to manufacture polylactic (PLA) with high strength, ductility, and heat resistance simultaneously. Herein, PLA/poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) nanoribboned composites, the highly oriented PHBV nanoribbons decorated by the PLA lamella, are successfully achieved through the multistage stretching extrusion (MSE) system. SEM confirms that in-situ highly oriented PHBV nanoribbons are achieved by biaxial-stretching field during the MSE process. Through investigating crystalline architecture of PLA/PHBV nanoribboned composites, it is found that the stiff shish and sparse lamellae of PLA are obtained under the coupling effect of PHBV nanoribbons and biaxial-stretching field. DMA reveals partial compatibility between PLA and PHBV. Interestingly, during tensile test, PHBV nanoribbons show high flexibility and synergistically facilitate the stretch of semi-rigid chains of PLA by an effective interfacial interaction. Consequently, even they both are extremely brittle, PLA/PHBV nanoribboned composites exhibit excellent strength (82.9 MPa) and ductility (186.7 %), compared with pure PLA (71.4 MPa and 12.3 %). Additionally, due to the promotion of the crystallization of PLA, PLA/PHBV nanoribboned composites show excellent heat resistance (E'140°C > 350 MPa). The nanoribboned composites are of immense significance, which provide significant guidance for the simultaneous enhancement of ductility and strength of polymer materials.

Biocompatible Shape Memory Blend for Self-Expandable Stents with Potential Biomedical Applications.

Biocompatible poly(propylene carbonate) (PPC)/polycaprolactone (PCL) shape memory blends were fabricated using melt blending. The shape memory performance of these blends was found to depend remarkably on their components. On addition of 25 vol % PCL, one blend (PL-25) achieved an optimal shape-fixing ratio (Rf) and -recovery ratio (Rr). Specifically, its Rr considerably increased by 24.1 and 50.0% compared with those of pure PPC and PCL, respectively, because of the restricted irreversible deformation of the amorphous chains cross-linked by tiny crystals. After undergoing three thermomechanical cycles, Rf and Rr reached 97.0%. The PL-25 blend was further melt-processed into a stent, which showed a fast response and self-expansion at 37 °C. These results, along with those obtained from evaluating the material's blood compatibility, in vitro degradation and drug release behavior, demonstrated the great potential of PL-25 for biomedical applications.

In Situ Formation of Microfibrillar Crystalline Superstructure: Achieving High-Performance Polylactide.

As a biobased and biodegradable polyester, polylactide (PLA) is widely applied in disposable products, biomedical devices, and textiles. Nevertheless, due to its inherent brittleness and inferior strength, simultaneously reinforcing and toughening of PLA without sacrificing its biodegradability is highly desirable. In this work, a robust assembly consisting of compact and well-ordered microfibrillar crystalline superstructure (FCS) surrounded by slightly oriented amorphism, is achieved by a combined external force field. Unlike the classic crystalline superstructures such as shish-kebabs, cylindrites, and lamellae, the newfound FCS with diameter of about 100 nm and length of several tens of micrometers is aggregated with well-aligned crystalline nanofibers. FCS can serve as discontinuous fiber to self-reinforce the amorphous PLA; more importantly, FCS can also act as rivets to pin the propagating fibrillar crazes leading to the formation of dense fibrillar crazes during stretching, which dissipates much energy and translates the failure of PLA from brittle to ductile. Consequently, PLA with FCS exhibits exceptionally simultaneous enhancement in ductility, strength, and stiffness, outperforming normal PLA with increments of 728, 55, and 70% in elongation at break, strength, and modulus, respectively. Therefore, FSC exhibits competitive advantages in achieving high-performance PLA even for other semicrystalline polymers. More significantly, this newfound crystalline superstructure (FCS) provides a new structural model to establish the correlation between structure and performance.

Controlled release of diclofenac sodium from polylactide acid-based solid dispersions prepared by hot-melt extrusion.

In this paper, hot-melt extrusion was applied to prepare drug delivery systems using polylactide acid (PLA) as the matrix. Diclofenac sodium (DS) was used as a model drug. Polyethylene glycol (PEG, molecular weight is 6000) and sodium dodecyl sulfate (SDS) were used as the release rate modifiers. For the PLA/PEG/DS blends, the release of DS was enhanced with higher amounts of PEG and DS. After the addition of SDS to the PLA/PEG/DS blends, the dispersion of DS and PEG was significantly improved. Compared to the PLA/PEG/DS blends with the same drug loading, the drug release behavior of PLA/PEG/DS/SDS was remarkably suppressed due to the presence of SDS. And a controllable linear release of DS was achieved.