Supertough Polylactide Materials Prepared through In Situ Reactive Blending with PEG-Based Diacrylate Monomer

Supertough biocompatible and biodegradable polylactide materials were fabricated by applying a novel and facile method involving reactive blending of polylactide (PLA) and poly­(ethylene glycol) diacylate (PEGDA) monomer with no addition of exogenous radical initiators. Torque analysis and FT-IR spectra confirm that cross-linking reaction of acylate groups occurs in the melt blending process according to the free radical polymerization mechanism. The results from differential scanning calorimetry, phase contrast optical microscopy and transmission electron microscopy indicate that the in situ polymerization of PEGDA leads to a phase separated morphology with cross-linked PEGDA (CPEGDA) as the dispersed particle phase domains and PLA matrix as the continuous phase, which leads to increasing viscosity and elasticity with increasing CPEGDA content and a rheological percolation CPEGDA content of 15 wt %. Mechanical properties of the PLA materials are improved significantly, for example, exhibiting improvements by a factor of 20 in tensile toughness and a factor of 26 in notched Izod impact strength at the optimum CPEGDA content. The improvement of toughness in PLA/CPEGDA blends is ascribed to the jointly contributions of crazing and shear yielding during deformation. The toughening strategy in fabricating supertoughened PLA materials in this work is accomplished using biocompatible PEG-based polymer as the toughening modifier with no toxic radical initiators involved in the processing, which has a potential for biomedical applications

Significantly Accelerated Spherulitic Growth Rates for Semicrystalline Polymers through the Layer-by-Layer Film Method

The influence of a molten liquid polymer layer on the crystallization of the beneath semicrystalline polymer has been seldom considered. In the study, the nucleation and growth of spherulites for the beneath polylactide (PLA) layer in poly­(ethylene oxide)/polylactide (PEO/PLA) double-layer films during isothermal crystallization at various temperatures above the melting point of PEO have been investigated by using polarized optical microscopy, with the particular results compared with that for neat PLA and PLA/PEO blend films. It is interesting to find that the top covering molten PEO layer can greatly accelerate the spherulitic growth rate (<i>G</i>) of the beneath PLA layer. Another significant result is that the temperature for the measurable nucleation and spherulitic growth of PLA in the double-layer films can be eventually pushed down close to the glass transition temperature of neat PLA. The changes of glass transition temperature, <i>T</i>_g, for PEO/PLA multilayer films have been measured by using modulated differential scanning calorimetry and dynamic mechanical analysis, which reveal slight decreases of <i>T</i>_g for PLA layer due to the influence of PEO layer. The layer structures of fractured surface of the double-layer films are analyzed on the basis of the observation from scanning electron microscopy, and the existence of interdiffusion areas with irregular boundary between PEO and PLA layers is the key clue to understanding the significant acceleration of <i>G</i> for PLA. The layer-by-layer film method infers promising applications, which might be considered to well replace the blending method

Facile Synthesis of Hybrid Silica Nanoparticles Grafted with Helical Poly(phenyl isocyanide)s and Their Enantioselective Crystallization Ability

In this contribution, we report on the facile synthesis of hybrid silica nanoparticles grafted with helical poly­(phenyl isocyanide)­s via both “grafting from” and “grafting to” strategies. First, triethoxysilanyl functionalized alkyne–Pd­(II) initiator was anchored onto the surface of bare silica nanoparticles through silanization coupling reaction. Polymerization of phenyl isocyanide using the Pd­(II)–anchored silica nanoparticles lead to the formation of hybrid nanoparticles grafted with helical poly­(phenyl isocyanide)­s. The surface-initiated polymerization was revealed to proceed in a living/controlled chain-growth manner, afforded the hybrid nanoparticles with controlled thickness. ³¹P NMR analysis indicated the initiation efficiency of the surface-anchored Pd­(II) initiators is very high, and almost quantitative. The grafting density was determined to be ∼0.89 nm²/chain based on the thermal gravity analysis (TGA). Polymerization of optically active phenyl isocyanide bearing an l-alanine with a long decyl chain using the Pd­(II)-anchored silica nanoparticles formed chiral hybrid nanoparticles grafted with helical poly­(phenyl isocyanide) arms in preferred handedness. Second, the hybrid silica nanoparticles were prepared via “grafting to” strategy. Well-defined triethoxysilanyl terminated poly­(phenyl isocyanide) was prepared in controlled manners. The polymer was grafted to the surface of bare silica nanoparticles via the silanization coupling reaction, afforded hybrid silica nanoparticles grafted with helical poly­(phenyl isocyanide). TGA indicates the grafting density is ∼0.76 nm²/chain. Taking advantage of this synthetic method, left-handed helical poly­(phenyl isocyanide) was grafted to the surface of silica nanoparticles, generated chiral hybrid silica nanoparticles with high optical activity. Such chiral nanoparticle exhibited good performance in enantioselective crystallization of racemic Boc-alanine. The enantiomeric excess (ee) of the induced crystal is up to 95%

Synthesis and Characterization of Nanostructured Copolymer-Grafted Multiwalled Carbon Nanotube Composite Thermoplastic Elastomers toward Unique Morphology and Strongly Enhanced Mechanical Properties

Considering that multiwalled carbon nanotubes (MWCNTs) can be used as anisotropic and stiff nano-objects acting as minority physical cross-linking points dispersed in soft polymer grafting matrixes, a series of copolymer-grafted multiwalled carbon nanotube composite thermoplastic elastomers (CTPEs), MWCNT-<i>graft</i>-poly­(<i>n</i>-butyl acrylate-<i>co</i>-methyl methacrylate) [MWCNT-<i>g</i>-P­(BA-<i>co</i>-MMA)], with minor MWCNT contents of 1.2–3.8 wt % was synthesized by the surface-initiated activators regenerated by electron transfer for atom-transfer radical polymerization (ARGET ATRP) method. Excellent dispersion of the MWCNTs in the CTPEs was demonstrated by SEM and TEM, and the thermal stability properties and glass transition temperatures of the CTPEs were characterized by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), respectively. Mechanical property test results demonstrated that the CTPEs exhibit obviously enhanced mechanical properties, such as higher tensile strength and elastic recovery, as compared with their linear P­(BA-<i>co</i>-MMA) copolymer counterparts. The microstructural evolutions in the CTPEs during tensile deformation as investigated by in situ small-angle X-ray scattering (SAXS) revealed the role of the MWCNTs, which can provide additional cross-linking points and transform soft elastomers into strong ones

Incorporation of Heteroatoms in Conjugated Polymers Backbone toward Air-Stable, High-Performance <i>n</i>‑Channel Unencapsulated Polymer Transistors

Organic field-effect transistors (OFETs) without any encapsulation of polymer semiconductor layers that still exhibit unipolar <i>n</i>-type characteristics under air conditions are very rare. In this study, we use fluorinated bithiophene as a donor, and bis­(2-oxoindolin-3-ylidene)-benzodifuran-dione (BIBDF, <b>P1</b>) and aza-substituted BIBDF (<b>P2</b>) as acceptor units to develop air-stable and unipolar electron transport polymer semiconductors. Unencapsulated OFETs based on <b>P1</b> and <b>P2</b> were fabricated and directly evaluated under air conditions. The highest effective mobility (μ_e,max^eff) of 0.23 cm² V^–1 s^–1 was obtained for <b>P2</b>-based devices with high <i>I</i>_on/<i>I</i>_off ratio of >10⁶ and low threshold voltage of 1.1 V. Moreover, <b>P2</b> had high air stability and maintained unipolar electron transport with μ_e,max^eff of up 0.1 cm² V^–1 s^–1 and <i>I</i>_on/<i>I</i>_off ratio of >10⁶ during the 60 days of air storage. The work provides an effective molecular design strategy to develop air-stable and high-performance <i>n</i>-channel unencapsulated polymer transistors that can be directly operated under air conditions

Tuning the Energy Levels of Aza-Heterocycle-Based Polymers for Long-Term <i>n</i>‑Channel Bottom-Gate/Top-Contact Polymer Transistors

Conjugated polymer-based organic thin film transistors (OTFTs) have received tremendous attention due to their potential applications. In addition to their high performances, air stability is also essential for application and another main property that OTFTs have. In this paper, three aza-heterocycle (BABDF)-based polymers were designed and synthesized using strong donor thiophene–vinylene–thiophene (TVT), weak donor thiophene–cyanovinylene–thiophene (TCNT), and weak acceptor dithiazole (TZ) as co-units. The lowest unoccupied molecular orbital (LUMO)/highest occupied molecular orbital (HOMO) energy levels were effectively lowered by introducing TCNT and TZ units, especially for PBABDF-TZ, for which the too much deep LUMO/HOMO energy levels of −4.28/–6.06 eV were obtained. These levels are low enough for air-stable electron transport and large enough for the hole injection barriers in OTFTs. Consequently, the unencapsulated bottom-gate/top-contact (BG/TC) devices exhibited unipolar electron transport under air conditions. Furthermore, these devices had high air stability and maintained unipolar electron transport with a mobility of up to 0.01 cm² V^–1 s^–1 during the one-year characterization period. Very low LUMO and HOMO levels were necessary for electron transport and the hole barriers, respectively, and both were important for long-term, air-stable <i>n</i>-channel polymer transistors

Facile Preparation of Regioregular Poly(3-hexylthiophene) and Its Block Copolymers with π‑Allylnickel Complex as External Initiator

Simply prepared π-allylnickel complexes were used as external initiators for promoting the polymerization of 2-bromo-3-hexyl-5-chloromagnesiothiophene in a living/controlled chain growth manner to afford regioregular poly­(3-hexylthiophene) with an allyl terminus. The nickel species on the other chain end can initiate the block copolymerization of hexadecyloxylallene and 2-bromo-3-hexyl-5-chloromagnesiothiophene to give a well-defined triblock copolymer containing poly­(3-hexylthiophene) and poly­(hexadecyloxylallene) segments in one pot via mechanically distinct, sequential living polymerization. Furthermore, such π-allylnickel­(II) complexes can also catalyze the polymerization of a range of vinyl monomers, including styrene, 1-methoxy-4-vinylbenzene, and 1-chloro-4-vinylbenzene as well as <i>tert</i>-butyl acrylate, in living/controlled fashion. The active nickel unit at the growing chain end of these vinyl polymers can also initiate the block copolymerization of 2-bromo-3-hexyl-5-chloromagnesiothiophene to give a series of block copolymers containing vinyl polymer and poly­(3-hexylthiophene) segments. The new block copolymerizations have been demonstrated to proceed in living/controlled chain-extension manner. The well-defined conjugated block copolymers are isolated in high yield with controlled molecular weight and tunable compositions

Helix-Sense-Selective and Enantiomer-Selective Living Polymerization of Phenyl Isocyanide Induced by Reusable Chiral Lactide Using Achiral Palladium Initiator

Polymerization of phenyl isocyanide using achiral Pd­(II) initiator with the presence of chiral l- or d-lactide (l-LA or d-LA) as additive was found to proceed in helix-sense-selective manner. The polymerization of achiral phenyl isocyanide, 4-isocyanobenzoyl-2-aminoisobutyric acid decyl ester (<b>1</b>) by this method produced optically active helical poly-<b>1</b>_m(L), whose chirality was solely come from the helical conformation without containing of any other chiral atoms. The added chiral LA can be facilely recovered and reused in the helix-sense-selective polymerizations without significantly loss of its chiral induction, and the chiral economy of the polymerization is high. When enantiomerically pure phenyl isocyanide bearing an <i>R</i>- or <i>S</i>-alanine pendent with a long <i>n</i>-decyl chain (<b>1r</b> or <b>1s</b>) were polymerized by this method, the polymerization was found to proceed in a highly enantiomer-selective manner with one of the enantiomers preferentially polymerized over the antipode by a factor of 3.6. Single-handed helical polyisocyanides can be achieved when the chirality of the monomer was appropriately matched with the added LA

Multiple Stimuli-Responsive and White-Light Emission of One-Pot Synthesized Block Copolymers Containing Poly(3-hexylthiophene) and Poly(triethyl glycol allene) Segments

Conjugated block copolymers with tunable properties have attract considerable research interests in recent years. Herein, we report a series of novel block copolymers containing conjugated poly­(3-hexylthiophene) (P3HT) and poly­(triethyl glycol allene) (PTA) segments which were synthesized in one pot using nickel complex as a single catalyst via distinct polymerization mechanisms. Interestingly, the P3HT-<i>b</i>-PTA diblock copolymers exhibit excellent thermoresponsive properties in water, and the lower critical solution temperature (LCST) is dependent on polymer concentration and the block ratio. Moreover, the diblock copolymers showed pH-responsive properties in CHCl₃ with the emission color shuttled between orange and deep green upon the alternate additions of trifluoroacetic acid and triethylamine. Both P3HT-<i>b</i>-PTA and P3HT-<i>b</i>-PTA-<i>b</i>-P3HT block copolymers exhibit solvatochromism properties. The emission of the block copolymers can be facilely tuned through variation on solvents with the emission color spanned widely from red to blue. Very interestingly, white-light emission can be readily achieved from the P3HT-<i>b</i>-PTA-<i>b</i>-P3HT triblock copolymer in the mixture of THF and methanol with 1/3 volume ratio

Air-Stable (Phenylbuta-1,3-diynyl)palladium(II) Complexes: Highly Active Initiators for Living Polymerization of Isocyanides

A family of air-stable (phenylbuta-1,3-diynyl)­palladium­(II) complexes were designed and prepared in a facile synthetic procedure. Their structures were characterized by ¹H and ¹³C NMR, MS, and X-ray analysis. These Pd complexes were revealed to efficiently initiate the polymerization of phenyl isocyanides in a living/controlled chain growth manner, which led to the formation of poly­(phenyl isocyanide)­s with controlled molecular weights and narrow molecular weight distributions. ¹³C NMR analysis indicated the isolated poly­(phenyl isocyanide) was of high stereoregularity. The Pd unit at the end of the polymer chain could undergo further copolymerization with phenyl isocyanide monomers to give block copolymers. It was also found that incorporation of an electron-donating group on the phenyl group of the Pd complex could improve the catalytic activities. Furthermore, these Pd complexes were tolerant to most organic solvents and applicable to a wide range of isocyanide monomers including alkyl and phenyl isocyanides and even phenyl isocyanide with bulky substituents at the ortho position and diisocyanide monomers. Therefore, this polymerization system is versatile in the preparation of well-defined polyisocyanides with controlled sequence. Bi- and trifunctional Pd complexes with two and three Pd units incorporated onto the same phenyl ring were designed and synthesized. They were also able to initiate the living polymerization of phenyl isocyanide to afford telechelic linear and star-shaped polyisocyanides with controlled molecular weights and narrow molecular weight distributions