Zhong-Ren Chen

Flow-induced alignment of lamellar block copolymer melts

Toward the integrated synthesis and processing of functional block copolymer nanostructures, the physics of flow-induced alignment of block copolymers must be understood to predict the direction, rate and degree of alignment. In this review we focus on key issues regarding flow-induced alignment of lamellae. A three-dimensional mapping summarizes previous results on the selection of alignment directions (parallel or perpendicular) and their pathways in terms of three dimensionless parameters: frequency, temperature and strain amplitude. Trajectories, kinetics and structural evolution are explored in a fourth dimension (time). The challenge of developing adequate experimental methods for monitoring transient structure is discussed. A comprehensive experimental approach, which combines in-situ rheo-optical measurements and ex-situ structural characterization by electron microscopy and X-ray scattering, is presented as a new tool for tracking changes of microstructure and orientation during flow-induced alignment. Various mechanisms that have been proposed over the past two decades are reviewed and re-evaluated based on recent experimental results. Outstanding questions and new issues raised by ABC triblock copolymer nanstructures are discussed.

Tensile and impact behavior of polystyrene microcellular foams with bi-modal cell morphology:

Bi-modal PS foams with various volume fractions of large cells (fL), cell sizes and densities were prepared to investigate the effect of cell structures on the tensile and impact behaviors. The tensile results showed that for the similar density, the tensile strength and modulus decreased with the increase of fL, unless the cell size of large ones is smaller than 25 µm. Similarly, the impact experimental results showed that the impact strength decreased with increasing fL, unless the fL is in the range of 25–32%. It indicated that the bi-modal cell structure could lead to the better properties than that of uniform one, when the cell morphology was proper (fL in the range of 25–32% and the cell size of large ones smaller than 25 µm). The SEM images of impact-fractured surface of bi-modal foams further confirmed that the cell morphology with fL of 32% was more favorable to the absorption of impact energy during the fracture process.

Fabrication of honeycomb-structured porous film from polystyrene via polymeric particle-assisted breath figures method

AbstractPolymeric particles were utilized as a second component to assist polymer patterning via the breath figures (BF) method. Polymeric constituents lessen the incompatibility between particles and polymers, which brings new characteristics in both pattern fabrication and particle allocation. The influence of the experimental parameters for two kinds of polymeric particles on the pattern morphology of the obtained film was investigated. Different assembling characteristics of particles under different circumstances are also discussed. Polymeric particles are proven to be able to function as BF array stabilizers and assist in pattern formation. Also, polymeric particles enriching the patterned pores present unique assembling characteristics which are much different from previously reported inorganic particles. The possibilities for the functionalization of BF arrays utilizing functional microgels were also tested. These results demonstrate a study in the polymeric particle-assisted BF method, which may provide new insight into novel functional material preparation methods via self-assembly.

Crack growth mechanism of natural rubber under fatigue loading studied by a real-time crack tip morphology monitoring method

The present paper deals with the crack growth behavior of vulcanized natural rubber under fatigue loading. Our research focuses on the crack tip morphology evolution and its relationship with the crack growth rate. For this purpose, an original real-time monitoring method is applied to capture the crack tip morphology during standard fatigue testing. This method involves the use of a high speed microscopic camera and a dynamic mechanical analyzer with a crack growth testing unit. The tear energy (T) dependence of the crack tip morphology is discussed. It is revealed that there are two characteristic cracking morphologies: at T 600 J m−2, the power law slope of dc/dn versus T is 2. A ligament break-up mechanism dominates. This work gives us new opportunities to study the crack growth mechanism of rubbers from the viewpoint of real-time crack tip morphology investigation.

An NMR Investigation of Phase Structure and Chain Dynamics in the Polyethylene/Montmorillonite Nanocomposites

Novel exfoliated and interacted polyethylene (PE)/montmorillonite (MMT) nanocomposites prepared by in situ polymerization were characterized by solid-state nuclear magnetic resonance (NMR). The phase structure and molecular mobility were investigated by proton and carbon NMR under static and magic-angle spinning (MAS) conditions. The results showed that incorporation of MMT layer enhanced the polyethylene crystallinity behavior. The chain mobility of crystalline phase, interphase and amorphous phase was hindered in the nanocomposites. The phase structure and chain dynamics were also investigated upon changing the temperature. The orthorhombic and monoclinic phases were detected according to the 13CP/MAS NMR. Quantitative characterization of the phase structure was also conducted by 13C DP/MAS upon changing the temperature. Finally, the difference in the phase structure and chain dynamics in each phase of PE/nanocomposites was compared based on the NMR results when fiber filler was introduced.

New insight into stretch induced structural evolution of α trans-1,4-polyisoprene characterized by real time synchrotron WAXS and SAXS measurements

Real time synchrotron wide angle and small angle X-ray scattering (WAXS and SAXS, respectively) were used to characterize the stretch induced structural evolution of α trans-1,4-polyisoprene (trans-PI). 2D WAXS results indicated two ensembles of crystalline modifications with distinctive orientation modes coexisted during stretching. Stretching transformed part of the monoclinic α phase into highly oriented orthorhombic β phase at strain ε = ~0.4. The β phase had rather high orientational degree with polymer chains parallel to the stretching direction, while the orientational degree of α phase was much lower. Complemented by qualitative 2D SAXS analysis, it was found that amorphous layer deformation and intralamellar chain slip dominated at different stretching stage. The melt and recrystallization process of α phase which led to the formation of β phase was also investigated. Formation of two interpenetrating networks of crystalline skeleton (constructed by residual α and β crystals) and amorphous entanglement accounted for the stress-hardening in the late stage.

Study on the stretch induced phase transition of α trans-1,4-polyisoprene by in-situ SAXS and WAXS measurements

Trans-1,4-polyisoprene (trans-PI) is an important industrial polymer, which exhibits pronounced polymorphisms. However, understanding of the crystal form transition of trans-PI is rather limited. In the present work, the stretch-induced crystal form transition of α trans-PI at room temperature was investigated in detail. It was found that the β crystals with high molecular chain orientation appeared at strain = 0.,42 and the total crystallinity increased at the same time. Meanwhile, the average long spacing (lac), thickness of lamellae (lc), and amorphous layers (la) along the stretching direction had undergone multiple evolution processes during stretching. Results also showed that part of the amorphous phase and the recrystallization process of α crystals both contributed to the formation of the β form. Based on the combination analysis of the in-situ SAXS (small angle X-ray scattering), WAXS (wide angle X-ray scattering), differential scanning electron microscopy (DSC), and Fourier transform infrared spectroscopy (FTIR), it was proved that the β mesophase with layered structure emerged before the formation of β crystals. Finally, a possible multistage evolution mechanism was proposed to interpret the stretch-induced crystal form transition of α trans-PI.

Crack growth mechanism of styrene-butadiene rubber filled with silica nanoparticles studied by small angle X-ray scattering

The crack growth mechanism of rubber composites has not been explored clearly. Here, we focus on the microstructure evolution of styrene-butadiene rubber/silica (SBR/silica) composites during crack propagation. Two groups of SBR/silica composites are constructed. The silica nanoparticles used in the two groups have different sizes but the same surface properties. By crack propagation and small angle X-ray scattering measurements, the crack growth rate, nanocavitation and silica distribution morphology of the rubber composites are investigated. The role of damage structure nucleation and growth during crack propagation of the SBR/silica composites is discussed. It is demonstrated that the damage structure nucleation is the key factor for the crack growth at large tear energies, while the growth feature of damage structure dominates the crack growth at smaller tear energies. This work provides us with better understanding on the crack growth mechanism of rubber composites.

Exploration of selective decoration of Janus silica particles within polymeric patterned pore arrays

Janus particles, especially amphiphilic ones, are known to be highly active when involved in interfacial self-assembly processes. However, experimental reports on employing Janus particles as an interfacial stabilizer to establish functional structures are rather limited. In this paper, amphiphilic Janus silica particles were prepared by selective chemical treatment on the particle “colloidosomes”. Different ways (decoration of Au nanoparticles, interfacial adsorption test) were used to confirm the acquirement of the Janus character of the particles. Upon successful synthesis of amphiphilic Janus particles, they were employed in the breath figures method for the first time. It turned out that by using the Janus particles, a regularly arranged pore array could be easily obtained. Characterization of the morphology of the patterned structure shows that Janus particles were densely located on the interior wall of the entire holes, forming finer particle arrays compared to the case of isotropic particles. The result indicates that Janus particles show optimized behavior when they are involved in the interfacial self-assembly processes. With further work, by choosing Janus particles with functional features, selective functionalization of porous films could be achieved with high efficiency and great ease using this method.