Junwu Zhang

Soft segment free thermoplastic polyester elastomers with high performance

A soft segment free thermoplastic polyester elastomer is fabricated by controlling the stereochemical structure of molecular chains with the utilization of the cis 1,4-cyclohexylene ring moiety (cis-CHRM) in poly(butylene 1,4-cyclohexanedicarboxylate) (PBC). PBC with 71% cis-CHRM exhibits good elasticity with shape recovery rates of 64% at 200% strain and 92% at break, tensile modulus, strength and elongation at break at 111 and 18 MPa and 1230%, respectively.

Synthesis and characterization of triblock copolymer PLA-b-PBT-b-PLA and its effect on the crystallization of PLA

A series of poly(L-lactide)-b-poly(butylene terephthalate)- b-poly(L-lactide) (PLA-b-PBT-b-PLA) triblock copolymers were synthesized via ring-opening polymerization of L-lactide with hydroxyl-functionalized poly(butylene terephthalate) (HOPBT) as macroinitiator and stanneous 2-ethylhexanoate (Sn(Oct)2) as the catalyst in 1,1,2,2-tetrachloroethane (TCE). The conversion of L-lactide was high and the PLA block in the copolymers showed high optical purity. Chemical structure and physical properties of the triblock copolymers were studied using NMR, GPC, DSC, and WAXD. The block structure of the copolymers was confirmed using NMR, and each block formed the same crystal type as the respective homopolymer, i.e., PLA or PBT as revealed by DSC and WAXD. The copolymer, even at a very low content (5 wt%), showed an unexpected favoring effect on the crystallization of PLA, which made it a potential polymeric nucleating agent for PLA.

Role of cis-1,4-cyclohexanedicarboxylic acid in the regulation of the structure and properties of a poly(butylene adipate-co-butylene 1,4-cyclohexanedicarboxylate) copolymer

A unique non-planar ring structure 1,4-cyclohexanedicarboxylic acid (CHDA) is introduced to synthesize a poly(butylene adipate-co-butylene 1,4-cyclohexanedicarboxylate) (PBAC) copolyester. The impact of the stereochemistry of CHDA on the structure and properties of PBAC, especially the role of cis-CHDA in tuning the thermal, tensile and elastic properties of PBAC is explored in depth. Instead of considering PBAC as a diblock random copolymer consisting of poly(butylene adipate) (PBA) and poly(butylene 1,4-cyclohexanedicarboxylate) (PBC), our results reveal that PBAC can be considered as a random copolymer consisting of actually three blocks, namely PBA, a PBC unit with only trans-CHDA (trans-PBC), and PBC unit with only cis-CHDA (cis-PBC). The role of cis-CHDA is found to be rigid which initiates a high modulus and strength, and soft which results in a decreased melting temperature and increased elongation at break and elasticity.

Incorporation of 1,4-cyclohexanedicarboxylic acid into poly(butylene terephthalate)-b-poly(tetramethylene glycol) to alter thermal properties without compromising tensile and elastic properties

Thermal and tensile properties of thermoplastic elastomers (e.g. poly(butylene terephthalate)-b-poly(tetramethylene glycol) (PBT–PTMG)) are usually tuned by changing the composition of hard and soft segment parts. Simply increasing the amount of soft segment results in a lower melting temperature and better elastic properties, but the thermal stability and tensile properties are inevitably sacrificed. In this work, by incorporation of an aliphatic ring structure (i.e. 1,4-cyclohexanedicarboxylic acid) (CHDA) to partially replace the aromatic ring (i.e. terephthalic acid) in PBT-PTMG, the properties of the material can be tuned in such a way that the melting temperature decreases, while the thermal stability, tensile and elastic properties are not compromised. Moreover, manipulation of the stereo-chemistry of the CHDA unit discloses the “elastic” nature of the non-planar ring structure. Samples with a greater amount of cis-CHDA tend to have better tensile and elastic properties compared with their trans-CHDA counterparts.

Synthesis of poly(butylene terephthalate)-poly(tetramethylene glycol) copolymers using terephthalic acid as starting material: A comparation between two synthetic strategies

Poly(butylene terephthalate)-poly(tetramethylene glycol) (PBT-PTMG) copolymer is prepared with terephthalic acid (PTA) rather than its dimethyl ester (DMT) as starting material by a two-step melt polycondensation. This process includes the synthesis of PBT prepolymer from PTA with 1,4-butanediol (BDO) in the first step, followed by the synthesis of PBT-PTMG copolymer from PBT prepolymer with PTMG in the second step. The molecular weight, composition as well as thermal and mechanical properties of the products from the two-step melt polycondensation are compared with the properties of the PBT-PTMG from the traditional one-step melt polycondensation. When the PTMG content is low, there is only slight difference in molecular weight, composition, thermal and mechanical properties among PBT-PTMG copolymers obtained from these two methods. However, when the PTMG content is high, only the two-step strategy is able to give high molecular weight products, and the products have comparable thermal and mechanical properties with those from traditional one-step strategy using DMT as starting material.

Living lamellar crystal initiating polymerization and brittleness mechanism investigations based on crystallization during the ring-opening of cyclic butylene terephthalate oligomers

The brittleness of polymerized cyclic butylene terephthalate (pCBT) is generally ascribed to its high crystallization degree and perfect crystal structure. Herein, a simple and effective method for improving the brittleness of pCBT is presented. Unlike other published toughening methods, this method did not decrease or destroy any of the natural advantages of cyclic butylene terephthalate oligomers (CBT) and its ring-opening polymerization (ROP). The melting behaviour and crystallization of CBT demonstrated that some CBT crystals were unable to melt completely during ROP. These un-melted crystals served as self-compatible nucleators and reduced the crystallization induction time of pCBT during ROP, they also plant many stress concentration points which result in the brittleness of pCBT, according to mechanical tests and crystallization kinetics. Finally, the “living lamellar crystal” initiating ROP and new brittleness mechanisms were proposed.

Heteroatom substituted naphthodithiophene–benzothiadiazole copolymers and their effects on photovoltaic and charge mobility properties

A novel series of heteroatom-substituted naphthodithiophene and dithienylbenzothiadiazole alternating copolymers namely PNTP, PNBO, PNfTB, and PNffTB were designed and synthesized in which heteroatoms including fluorine, oxygen or nitrogen were incorporated into electron-deficient benzothiadiazole units of the resulting copolymers. In general, with the incorporation of heteroatoms into the acceptor units of the polymer backbone, the absorption spectra would shift to a longer wavelength, indicating a decrease in the optical band gap. In addition, fluorine substitution can effectively lower both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of the resulting copolymers; on the other hand, the incorporation of nitrogen or oxygen atoms into the copolymer backbone can only stabilize the LUMO energy level of copolymers. The effect of heteroatoms on the photovoltaic and charge mobility properties of organic solar cells (OSCs) and organic field-effect transistors (OFETs) was also investigated. The bulk heterojunction (BHJ) OSCs fabricated from the blend of heteroatom-substituted copolymers and PC71BM using diiodooctane as a solvent additive with ZnO/Al as a double interlayer in an inverted device structure afforded a power conversion efficiency up to 5.05%. In addition, the solution-processed bottom gate bottom contact based OFETs fabricated from PNfTB exhibited an enhanced hole mobility of 0.14 cm(2) V-1 s(-1) with an on/off current ratio of 10(7). Our results suggested that incorporation of heteroatoms into the donor-acceptor polymer backbone would be a useful strategy to enhance the functional properties for applications in OSCs and OFETs.

InAs self-assembled quantum dots grown on an InP (311)B substrate by molecular beam epitaxy

Self-assembled InAs quantum dots (QDs) have been grown by solid-source molecular beam epitaxy on a (311)B InP substrate. Transmission electron microscopy clearly shows that a high density of smaller InAs islands can be obtained by using such a high index substrate. After introducing a lattice-matched underlying In0.52Al0.24Ga0.24As layer, the InAs QDs are much more uniform in size and form two-dimensional well ordered arrays. The photoluminescence (PL) spectra also confirm that the InAs QDs grown on underlying In0.52Al0.24Ga0.24As have a better quality than those grown in the In0.52Al0.48As matrix. A simple calculation indicates that the redshift of the PL peak energy mainly results from InAs QDs on underlying In0.52Al0.24Ga0.24As of large size