Bao-Hua Guo

Study on styrene-assisted melt free-radical grafting of maleic anhydride onto polypropylene

Abstract Free-radical melt grafting of the multi-monomer system of maleic anhydride (MAH)/styrene (St) onto polypropylene (PP) was studied using a single-screw extruder. The effects of St and initiator concentrations on the grafting reaction were investigated. It was shown that the addition of St to the melt-grafting system as a comonomer could significantly enhance MAH graft degree onto PP. The maximum MAH graft degree was obtained when the molar ratio of MAH to St was approximately 1:1. However, the melt flow rate (MFR) value of the grafted PP was the highest at this ratio. This implies that the interaction and reaction between MAH and St monomers plays an important role in the grafting reaction. St improves the grafting reactivity of MAH and also reacts with MAH to form the St–MAH copolymer (SMA) before the two monomers graft onto PP. Grafting of SMA onto PP greatly enhanced the graft degree of MAH. When the molar ratio of MAH to St is 1:1, the main grafting reaction in the system is the grafting of SMA to PP; therefore, the graft degree of the grafted PP is the highest. When the concentration of MAH is higher than that of St, some MAH monomer reacts with St to form SMA, but others can directly graft onto PP macroradicals. When the amount of St added is higher than that of MAH, part of the St monomer may preferentially react with PP macroradicals to form more stable styryl macroradicals, while others copolymerize with MAH to yield SMA. The MFR value of the grafted PP is lower in this case.

An in situ-forming zwitterionic hydrogel as vitreous substitute

Zwitterionic polymers have shown ultra-low biofouling properties at surfaces and excellent biocompatibility as implant. In this study, an in situ-forming zwitterionic hydrogel was designed and evaluated, both in vitro and in vivo, as a long-term vitreous substitute. The zwitterionic polymer poly(MPDSA-co-AC) was designed as a copolymer of the sulfobetaine methacrylamide and acryloyl cystamine monomers, providing the zwitterionic components and the thiol functional groups, respectively. The in situ gelation was via the thiol-ene Michael addition reaction with α-PEG-MA as the crosslinker. The gelation time, rheological properties, swelling profiles, and the transparency of zwitterionic hydrogels were studied in detail. Two systems with different crosslinker concentrations were tested in a rabbit model, and the one with the thiol-ene ratio of 2 : 1 showed excellent biocompatibility in vivo, formed space-filling hydrogels and remained transparent in the vitreous cavity for the 2 month implantation period. Therefore, in situ-forming zwitterionic hydrogels represent a promising material system as a vitreous substitute and possibly for other soft tissue replacements.

How to regulate the isothermal growth rate of polymer spherulite without changing its molecular composition

Poly(butylene succinate-co-butylene fumarate) (PBSF) can work as a highly efficient polymeric nucleating agent for poly(butylene succinate) (PBS) and this is based on epitaxial crystallization, namely matching of the crystal lattice. Benefiting from the unique behavior in which PBSF can enhance the spherulite growth rate (G) of PBS, a new method combining the addition of a fixed amount of PBSF and self-nucleation has been proposed to regulate G of isothermally crystallized PBS for the first time.

Morphology and crystalline structure of inclusion compounds formed between poly(ethylene glycol) and urea

The poly(ethylene glycol) (PEG, with Mw 2000)-urea inclusion compound (IC) crystallized at high temperature region showed two typical orientations, flat-on and edge-on crystals. 2D-XRD and polarized FTIR spectroscopy revealed that the PEG chains within urea channels were perpendicular to the substrate in flat-on oriented crystals, while PEG chain axes were parallel to the substrate and lay along the growth direction in the edge-on crystals. FTIR absorption bands of PEG in the ICs are sensitive to orientation of the crystals. A scheme of PEG chain packing in the urea IC channel was proposed, which could explain the orientation of the crystal nucleus causing the two types of morphologies. Furthermore, functioning of PEG2000 chain end with analine had significantly influence on the morphology and orientation of the inclusion compound crystals, due to the defects caused by large terminal groups included in the urea channel.

The effect of polymer-substrate interaction on the nucleation property: Comparing study of graphene and hexagonal boron nitride Nanosheets

Hexagonal boron nitride nanosheets (BNNSs) can work as a more efficient nucleating agent for two polyesters compared to graphene. Studies on the crystallization and dewetting processes of two polyesters, poly(butylene succinate) and poly(butylene adipate), on the two substrate surfaces prove that the interaction between BNNSs and the polyesters is stronger than that between graphene and the polyesters. This strong interaction induces the pre-ordered conformation of molten PBA which has been identified by the in situ FTIR spectra. Thus BNNSs possess higher nucleation property than graphene. Finally, a new polymer-substrate interaction induced nucleation mechanism was proposed to explain the nucleation efficiency difference between graphene and BNNSs.

Interplay between crystallization and the Diels–Alder reaction in biobased multiblock copolyesters possessing dynamic covalent bonds

A series of biobased poly(2,5-furandimethylene succinate)-b-poly(butylene succinate) (PFS-PBS) multiblock copolyesters containing furan in the polymer main chains was synthesized, in which the thermo-reversible network structures are established via the furan/maleimide Diels–Alder (DA) reaction using bismaleimide (BMI) as a cross-linker. First, the DA reaction kinetics of PFS/BMI was investigated by in situ Fourier transform infrared spectroscopy (FTIR). The corresponding reaction rate coefficients at variable temperatures and the activation energy of the DA reaction were calculated. Furthermore, the interplay between crystallization and the DA reaction was studied combining differential scanning calorimetry (DSC) and in situ FTIR. The crystallization is suppressed by the DA reaction due to confinement of the formed DA network structure. On the other hand, the crystallization effect on the DA reaction is related to the FS sequence length of the copolyesters. The restriction effect of crystallization on the DA reaction is clearer in PFBS random copolyesters with a shorter FS sequence length than that in PFS-PBS multiblock copolyesters with a longer FS sequence length. Finally, the mechanical properties of PFS-PBS/BMIs were studied. The formation of a DA network structure significantly increases the modulus and tensile strength of PFS-PBS/BMIs despite the reduction in crystallinity. PFS-PBS/BMIs can be adjusted from soft to rigid and brittle materials by simply controlling the PFS and BMI content.

Study on melting and recrystallization of poly(butylene succinate) lamellar crystals via step heating differential scanning calorimetry

Differential scanning calorimetry (DSC) has been widely applied to study crystallization and melting of materials. However, for polymeric lamellar crystals, the melting thermogram during heating process usually exhibits a broad endothermic peak or even multiple endotherms, which may result from changes of metastability via recrystallization process. Sometimes, the recrystallization exotherm cannot be observed due to its overlapping with the melting endotherm. In this work, we employed a step heating procedure consisting of successive heating and temperature holding stages to measure the metastability of isothermally crystallized poly(butylene succinate) (PBS) crystals. With this approach we could gain the fraction of crystals melted at different temperature ranges and quantitatively detect the melting-recrystallization behavior. The melting-recrystallization behavior depends on the polymer chain structure and the crystallization temperature. For instance, PBS block copolymer hardly shows recrystallization behavior while PBS oligomer and high molecular weight PBS homopolymer demonstrate remarkable melting-recrystallization phenomenon. High molecular weight PBS isothermally crystallized in the low temperature range shows multiple melting-recrystallization while those isothermally crystallized at elevated temperatures do not exhibit observable recrystallization behavior. Furthermore, the melting endotherms were fitted via the melting kinetics equations. The original isothermally crystallized lamellae demonstrate quite different melting kinetics from the recrystallized lamellar crystals that melt at the highest temperature range, which is attributed to the different degrees of stabilization. Finally, the mechanism of melting-recrystallization is briefly discussed. We propose that apparent melt-recrystallization phenomenon be observed when melting of preformed lamellar crystals and recrystallization of thicker lamellae have similar free energy barrier.

Monodisperse erythrocyte-like and hollow erythrocyte-like silica nanoparticles prepared by a simple template-free and surfactant-free sol-gel route

This paper introduced a simple two-step aqueous method in which monodisperse erythrocyte-like silica nanoparticles could be prepared without surfactant and template by sol–gel method. KOH was used to form the asymmetrical silica nanoparticles through quickly condensation in rotation system. The erythrocyte-like hollow silica nanoparticles with the same external contour could be obtained by further hydrothermal. In addition, double-layer hollow erythrocyte-like and cage-like structures nanoparticles could also be acquired by hydrothermal reaction. A simple and green method was found to work in a dynamic process and produce asymmetrical structure getting rid of the minimization of interfacial energy.Graphical Abstract

Copolymerization with Polyether Segments Improves the Mechanical Properties of Biodegradable Polyesters

To improve the properties of poly(butylene succinate) (PBS), a series of poly[(butylene succinate)-co-poly(tetramethylene glycol)]s (PBSTMGs) with different poly(tetramethylene glycol) (PTMG) contents were successfully prepared by the catalyzed melt polycondensation process. The effect of introducing flexible PTMG segments on the properties was investigated, and they were compared to those of PBS. The differential scanning calorimetry results indicated that the melting temperature, crystallization temperature, and crystallinity of PBSTMG copolymers were slightly lower than those of PBS. Furthermore, these thermal parameters decreased gradually with the increase of PTMG content. Dynamic mechanical analysis showed that there was a significant decline of storage modulus (E′) in the overall temperature range of copolymers compared to that of PBS. The incorporation of PTMG did not modify the crystal lattice of PBS according to the wide-angle X-ray diffraction analysis. Because of copolymerization, the size of the spherulites was reduced at high PTMG contents. The soft domain in the copolymers might contribute to the enhanced tear strength of PBSTMG. The elongation at break and impact strength of PBSTMG copolymers were greatly improved as a result of the phase separation structure and lower degree of crystallinity. Especially, when the PTMG content was 10 mol %, the impact strength of the copolymer reached up to 4.5 times that of PBS. In addition, with more soft segments introduced, the biodegradability of the copolymers became much better than that of PBS.

Correction to A Catalyst-Free Epoxy Vitrimer System Based on Multifunctional Hyperbranched Polymer

The corrected Figure 4c is shown here. With this correction, the calculated activation energy (Ea) is 62.8 kJ/mol. This correction affects the originally calculated Ea of 29.5 kJ/mol, but it does not change the following conclusion remark on page 6795: “According to previously reported literatures, the Ea of most epoxy-based vitrimers is usually between 70 and 150 kJ/mol. In this work, the greatly lower Ea indicates a high rate of TER for the HBE/SA system”.