Nanoscale effects of TiO2 nanoparticles on the rheological behaviors of ultra-high molecular weight polyethylene (UHMWPE).

Considering the molar mass between entanglements to be an intrinsic property of ultra-high molecular weight polyethylene (UHMWPE), the number of entanglements per chain increases with increasing molar mass, correspondingly making the UHMWPE intractable. Herein, we dispersed TiO2 nanoparticles with different characteristics into UHMWPE solutions to disentangle the molecular chains. Compared with the UHMWPE pure solution, the viscosity of the mixture solution declines by 91.22%, and the critical overlap concentration increases from 1 wt% to 1.4 wt%. A rapid precipitation method was utilized to obtain UHMWPE and UHMWPE/TiO2 composites from the solutions. The melting index of UHMWPE/TiO2 is 68.85 mg, which is in sharp contrast to that of UHMWPE which is 0 mg. We characterized the microstructures of UHMWPE/TiO2 nanocomposites using TEM, SAXS, DMA, and DSC. Accordingly, this significant improvement in processability contributed to the reduction of entanglements and a schematic model was proposed to explain the mechanism by which nanoparticles disentangle molecular chains. Simultaneously, the composite demonstrated better mechanical properties than UHMWPE. In summary, we provide a strategy to promote the processability of UHMWPE without sacrificing its outstanding mechanical properties.

Co-Crystallization between Aliphatic Polyesters through Co-Inclusion Complexation with Small Molecule.

Crystalline/crystalline blends of polymer have shown advantages in the preparation of new polymeric materials. However, the regulation of co-crystallization in a blend is still full of challenges due to the preferential self-crystallization driven by thermodynamics. Here, an inclusion complex approach is proposed to facilitate the co-crystallization between crystalline polymers, because the crystallization process displays a prominent kinetics advantage when polymer chains are released from the inclusion complex. Poly(butylene succinate) (PBS), poly(butylene adipate) (PBA) and urea are chosen to form co-inclusion complexes, where PBS and PBA chains play as isolated guest molecules and urea molecules construct the host channel framework. The coalesced PBS/PBA blends are obtained by fast removing the urea framework and systematically investigated by differential scanning calorimetry, X-ray diffraction, proton nuclear magnetic resonance and Fourier transformation infrared spectrometry. It is demonstrated that PBA chains are co-crystallized into PBS extended-chain crystals in the coalesced blends, while such a phenomenon has not been detected in simply co-solution-blended samples. Though PBA chains could not be totally accommodated in the PBS extended-chain crystals, their co-crystallized content increases with the initial feeding ratio of PBA. Consequently, the melting point of the PBS extended-chain crystal gradually declines from 134.3 °C to 124.2 °C with an increasing PBA content. The PBA chains playing as defects mainly induce lattice expansion along the a-axis. In addition, when the co-crystals are soaked in tetrahydrofuran, some of the PBA chains are extracted out, leading to damage to the correlative PBS extended-chain crystals. This study shows that co-inclusion complexation with small molecules could be an effective way to promote co-crystallization behavior in polymer blends.

Rationalizing the Dependence of Poly (Vinylidene Difluoride) (PVDF) Rheological Performance on the Nano-Silica.

Research on the rheological performance and mechanism of polymer nanocomposites (PNCs), mainly focuses on non-polar polymer matrices, but rarely on strongly polar ones. To fill this gap, this paper explores the influence of nanofillers on the rheological properties of poly (vinylidene difluoride) (PVDF). The effects of particle diameter and content on the microstructure, rheology, crystallization, and mechanical properties of PVDF/SiO2 were analyzed, by TEM, DLS, DMA, and DSC. The results show that nanoparticles can greatly reduce the entanglement degree and viscosity of PVDF (up to 76%), without affecting the hydrogen bonds of the matrix, which can be explained by selective adsorption theory. Moreover, uniformly dispersed nanoparticles can promote the crystallization and mechanical properties of PVDF. In summary, the viscosity regulation mechanism of nanoparticles for non-polar polymers, is also applicable to PVDF, with strong polarity, which is of great value for exploring the rheological behavior of PNCs and guiding the process of polymers.

Promoting Co-Crystallization in Poly(butylene succinate) and Poly(butylene fumarate) Blends via End-Group Functionalization.

Co-crystallization plays a crucial role in the integration and regulation of thermal and mechanical properties in polymer blends, but the poor compatibility of the components in the crystal phase has always been a major obstacle to co-crystallization, which puts forward stricter requests for linkage and interaction between different entities. On the basis of the hydrogen-bonding interaction that can promote chain stacking and thus improve miscibility, we propose that crystalline/crystalline blends of 2-ureido-4[1H]-pyrimidinone (UPy)-functionalized poly(butylene succinate) and poly(butylene fumarate) (PBS-UPy/PBF-UPy) where UPy groups with quadruple hydrogen-bonding interaction are employed to connect different chain ends, could inhibit phase separation and improve co-crystallization. PBS-UPy/PBF-UPy blends exhibit complex component-dependent and cooling-rate-dependent co-crystallization behavior. A high level of co-crystallization occurs in the range of PBS-UPy-rich fractions, and the proportion could approach over 98% under optimized conditions with the aid of UPy quadruple hydrogen bonds interaction. This work enriches the understanding of co-crystallization in crystalline/crystalline polymer blends and provides more possibility for the design of structures and properties of polymer materials.

Poly(butylene succinate-co-butylene acetylenedicarboxylate): Copolyester with Novel Nucleation Behavior.

Big spherulite structure and high crystallinity are the two main drawbacks of poly(butylene succinate) (PBS) and hinder its application. In this work, a new type of copolyester poly(butylene succinate-co-butylene acetylenedicarboxylate) (PBSAD) is synthesized. With the incorporation of acetylenedicarboxylate (AD) units into PBS chains, the crystallization temperature and crystallinity are depressed by excluding AD units to the amorphous region. In contrast, the primary nucleation capability is significantly strengthened, without changing the crystal modification or crystallization kinetics, leading to the recovery of total crystallization rate of PBSAD under the same supercooling condition. The existence of specific interaction among AD units is found to be crucial. Although it is too weak to contribute to the melt memory effect at elevated temperature, the interaction continuously strengthens as the temperature falls down, and the heterogeneous aggregation of AD units keeps growing. When the aggregating process reaches a certain extent, it will induce the formation of a significant amount of crystal nuclei. The unveiled nucleation mechanism helps to design PBS copolymer with good performance.

Effect of cellulose nanocrystals on the crystallization behavior and enzymatic degradation of poly(butylene adipate).

Cellulose nanocrystals (CNCs) are nature-resourced nanoparticles and have been widely used to improve performance of biodegradable polyesters. Two types of CNCs respectively prepared by sulphuric acid hydrolysis (aCNCs) and ammonium persulfate oxidation (oCNCs) processes were incorporated into poly(butylene adipate) (PBA) matrix to regulate its crystallization behavior and enzymatic degradation performance. Thermal and X-ray diffraction analysis revealed that both aCNCs and oCNCs could promote the crystallization ability and lamellar thickening of α-form PBA, while oCNCs showed stronger promotion than aCNCs. Optical morphology study indicated that both two types of CNCs enhanced the nucleation ability of PBA. Furthermore, the mechanism of crystallization promotion of CNCs on PBA was further discussed and attributed to the "memory effect" in the melt state of PBA/CNCs composites, which was originated from the hydrogen-bonding interaction between CNCs and PBA chains. The enzymatic degradation testing proved that CNCs could slower down degradation rate of PBA and PBA/oCNCs composites possessed lower degradation rate than PBA/aCNCs composites.

Investigation of Structure and Crystallization Behavior of Poly(butylene succinate) by Fourier Transform Infrared Spectroscopy.

The detailed structure and crystallization behavior of poly(butylene succinate) (PBS) have been investigated by Fourier transform infrared (FTIR) and other methods systematically. For the first time, we confirmed that the C═O stretching modes of PBS can respond to three distinguish absorption bands in the FTIR spectrum, at around 1736, 1720, and 1714 cm-1 respectively. The 1736 cm-1 band is adopted as the stretching mode of C═O groups in free amorphous fraction (MAF); the 1714 cm-1 band which is relevant to more stable structure, displays more anisotropic in polarized FTIR spectra, and has been confirmed as stretching vibrations of hydrogen-bonded C═O groups in the crystalline phase. The 1720 cm-1 band is linked to crystallization but comes from less ordered structure. Moreover, the 1720 cm-1 band can be destroyed prior to 1714 cm-1 band during heating and constructed behind 1714 cm-1 band during cooling. Thus, the 1720 cm-1 band is reasonably ascribed to the C═O groups in rigid amorphous fraction (RAF) or intermediate phase which locates between MAF and crystalline phase. The corresponding investigation by differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD) further supports that the three particular C═O absorption bands indeed reveal the typical three-phase structure for PBS. More important, the FTIR spectrum of PBS is very sensitive to sample preparation process and measurement mode. The relative content of each band depends on the crystallization temperature (Tc) and measured thickness. The higher Tc, the more RAF content appears when measured at room temperature; the thinner penetration thickness of FTIR measurement, the less RAF content can be detected, and the penetration thickness-dependent behavior is suggested as the result of higher mobility of chains in the air/bulk surface. Additionally, the particular three absorption bands of C═O groups in PBS force us to carefully reconsider previous reports on structure and interaction state obtained by FTIR spectroscopy in PBS and its composites.

Characterizing the Structure and Phase Transformation of Poly(ethylene Oxide)-Urea Complexes Using Terahertz Time-Domain Spectroscopy.

Terahertz time-domain spectroscopy (THz-TDS) was employed to characterize the structure and αâ†’ß phase transformation of poly(ethylene oxide) (PEO)-urea complexes. While the THz responses of α- and ß-form complexes are both originated from hydrogen bonding interactions, they possess different THz absorption bands. The α-form PEO-urea complex shows two bands at 1.12 and 1.24 THz, which are vibration modes due to the hydrogen bonding among urea and between PEO and urea, respectively; the ß-form PEO-urea complex shows a band at 1.48 THz, which is a vibration mode due to the hydrogen bonding among urea. A polarized THz-TDS study demonstrates that the urea-urea hydrogen bonding direction rotates 90° during the ß→α transformation process. This study reveals the great potential of THz-TDS for investigating polymer-small molecule complexes.

Enhancing Stereocomplexation Ability of Polylactide by Coalescing from Its Inclusion Complex with Urea.

In this study, polylactide/urea complexes were successfully prepared by the electrospinning method, then the host urea component was removed to obtain a coalesced poly(L-lactide) (PLLA)/poly(D-lactide) (PDLA) blend. The crystallization behavior of the coalesced PLLA/PDLA blend (c-PLLA/PDLA) was studied by a differential scanning calorimeter (DSC) and Fourier transform infrared (FTIR) spectroscopy. The c-PLLA/PDLA was found to show better crystallization ability than normal PLLA/PDLA blend (r-PLLA/PDLA). More interestingly, the c-PLLA/PDLA effectively and solely crystallized into stereocomplex crystals during the non-isothermal melt-crystallization process, and the reason was attributed to the equally-distributing state of PLLA and PDLA chains in the PLLA/PDLA/urea complex, which led to good interconnection between PLLA and PDLA chains when the urea frameworks were instantly removed.

Development of poly(vinyl alcohol) porous scaffold with high strength and well ciprofloxacin release efficiency.

Hydrophilic porous polymer scaffolds have shown great application in drug controlled release, while their mechanical properties and release efficiency still need further improvement. In the current study, the porous scaffolds of polyvinyl alcohol (PVA) prepared by quenching in liquid nitrogen and freeze drying method from different original concentration aqueous solutions were fabricated. Among different PVA scaffolds, the scaffold stemming from 18wt.% PVA aqueous solution exhibited the best mechanical properties, 10.5 and 1.54MPa tensile strengths for the dry and hydrogel states respectively. The inner morphology of such PVA scaffold was unidirectional honeycomb-like structure with average microchannel section of 0.5µm, and the scaffold showed porosity of 71% and rather low ciprofloxacin (Cip) release efficiency of 54.5%. Then poly(ethylene glycol) (PEG) was incorporated to enhance the Cip release efficiency. The release efficiency reached 89.3% after introducing 10wt.% PEG, and the mechanical properties of scaffold decreased slightly. Various characterization methods demonstrated that, adding PEG could help to enlarge the microchannel, create extra holes on the channel walls, weaken the interaction between PVA chains and Cip, and miniaturize the crystal size of Cip. All these effects benefit the dissolution and diffusion of Cip from scaffold, increasing its release capability. Moreover, based on biocompatible material composition, PVA/PEG scaffold is a non-cytotoxicity and have been verified that it can promote cell growth. And PVA/PEG scaffolds loaded with Cip can completely inhibit the growth of microorganism because of Cip sustaining release. The PVA scaffold would have a good potential application in tissue engineering, demanding high strength and well drug release capability.

On the circular birefringence of polycrystalline polymers: polylactide.

The circular birefringence of polycrystalline polymers is invariably obscured by strong linear birefringence. To parse the two mechanisms of light retardation, polycrystalline spherulites of polylactide enantiomers were analyzed by Mueller matrix microscopy. Polymer films are barely optically active in normal incidence, but if illuminated obliquely they become strongly optically active. Opposite hemispheres have oppositely signed circular birefringence. The sign is independent of the enantiomer but dependent on the sense of the sample's tilt. These observations are consistent with light path inhomogeneities resulting from stacked, mis-oriented lamellae. Chiroptical commonalities based on symmetry arguments are discussed among polylactide, a single oriented water molecule, and microfabricated metamaterial arrays, as well as the first physical model of optical activity, Reusch's pile of mica plates. The latter model provides the best explanation of the circular birefringence of polylactide spherulites. The only data on the optical rotation of crystalline polymers to date come from ostensible single crystals of polylactide. The enormous, anisotropic optical rotations observed previously are in quantitative agreement with misoriented lamellae observed here. Limitations of parsing circularly birefringent systems into those showing 'natural optical activity' and those others, somehow 'unnatural', are discussed.

Biosynthesis and characterization of poly(3-hydroxydodecanoate) by ß-oxidation inhibited mutant of Pseudomonas entomophila L48.

A medium-chain-length (MCL) polyhydroxyalkanoates (PHAs) producer Pseudomonas entomophila L48 was investigated for microbial production of 3-hydroxydodecanote homopolymer. Pseudomonas entomophila L48 was found to produce MCL PHA consisting of 3-hydroxyhexanoate (3HHx), 3-hydroxyoctanoate (3HO), 3-hydroxydecanoate (3HD), and 3-hydroxydodecanoate (3HDD) from related carbon sources fatty acids. In this study, some of the genes encoding key enzymes in ß-oxidation cycle of P. entomophila such as 3-hydroxyacyl-CoA dehydrogenase, 3-ketoacyl-CoA thiolase, and acetyl-CoA acetyltransferase were deleted to study the relationship between ß-oxidation and PHA synthesis in P. entomophila. Among the mutants constructed, P. entomophila LAC26 accumulated over 90 wt % PHA consisting of 99 mol % 3HDD. A fed-batch fermentation process carried out in a 6 L automatic fermentor produced 7.3 g L(-1) PHA consisting of over 97 mol % 3HDD fraction. Properties of MCL PHA were significantly improved along with increasing 3HDD contents. P(2.1 mol % 3HD-co-97.9 mol % 3HDD) produced by P. entomophila LAC25 had the widest temperature range between T(g) and T(m), which were -49.3 and 82.4 °C, respectively, in all MCL PHA reported so far. The new type of PHA also represented high crystallinity caused by side-chain crystallization compared with short side chain PHA. For the first time, P(3HDD) homopolymers were obtained.

Microbial production of polyhydroxyalkanoate block copolymer by recombinant Pseudomonas putida.

Polyhydroxyalkanoate (PHA) synthesis genes phaPCJ(Ac) cloned from Aeromonas caviae were transformed into Pseudomonas putida KTOY06ΔC, a mutant of P. putida KT2442, resulting in the ability of the recombinant P. putida KTOY06ΔC (phaPCJ(A.c)) to produce a short-chain-length and medium-chain-length PHA block copolymer consisting of poly-3-hydroxybutyrate (PHB) as one block and random copolymer of 3-hydroxyvalerate (3HV) and 3-hydroxyheptanoate (3HHp) as another block. The novel block polymer was studied by differential scanning calorimetry (DSC), nuclear magnetic resonance, and rheology measurements. DSC studies showed the polymer to possess two glass transition temperatures (T(g)), one melting temperature (T(m)) and one cool crystallization temperature (T(c)). Rheology studies clearly indicated a polymer chain re-arrangement in the copolymer; these studies confirmed the polymer to be a block copolymer, with over 70 mol% homopolymer (PHB) of 3-hydroxybutyrate (3HB) as one block and around 30 mol% random copolymers of 3HV and 3HHp as the second block. The block copolymer was shown to have the highest tensile strength and Young's modulus compared with a random copolymer with similar ratio and a blend of homopolymers PHB and PHVHHp with similar ratio. Compared with other commercially available PHA including PHB, PHBV, PHBHHx, and P3HB4HB, the short-chain- and medium-chain-length block copolymer PHB-b-PHVHHp showed differences in terms of mechanical properties and should draw more attentions from the PHA research community.

Metabolic engineering for microbial production of polyhydroxyalkanoates consisting of high 3-hydroxyhexanoate content by recombinant Aeromonas hydrophila.

Polyhydroxyalkanoate synthase gene phaC(ah) in Aeromonas hydrophila strain 4AK4 was deleted and its function was replaced by phaC1(ps) cloned from Pseudomonas stutzeri strain 1317 which favors 3-hydroxyhexanoate (3HHx) and longer chain length monomers. Genes fadD and fadL encoding Escherichia coli acyl-CoA synthase and Pseudomonas putida KT2442 fatty acid transport protein, respectively, were introduced into the recombinant with new phaC1(ps). Accumulation of a series of novel medium-chain-length polyhydroxyalkanoates (mcl-PHA) consisting of 80-94 mol% 3HHx were observed. The recombinant accumulated 54% poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) in cell dry weight consisting of 94.5 mol% 3HHx or 51% poly(3-hydroxybutyrate-co-3-hydroxyhexanoate-co-3-hydroxyoctanoate) consisting of 82 mol% 3HHx and 16 mol% of 3HO during a two-step cultivation process under nitrogen limitation when grown on sodium hexanoate or sodium octanoate. The two polyesters containing high percentage of 3HHx are physically characterized. They could be used as biodegradable pressure sensitive adhesives, coatings, polymer binding agents in organic-solvent-free paints or a source for chiral R-3-hydroxyhexanoate.