Chunhai Li

Crystallization kinetics behavior, molecular interaction, and impact-induced morphological evolution of polypropylene/poly(ethylene-co-octene) blends: insight into toughening mechanism

In this paper, SEM, POM, DSC, FTIR, polarized FTIR, and part-impact test were performed to investigate the effect of dispersed POE phase on crystallization kinetics behavior, molecular interaction, and impact-induced morphological evolution in polypropylene/poly(ethylene-co-octene) (PP/POE) blends. The main focus was to establish a systematic and deep toughening mechanism from microscopic molecular interaction to macroscopic deformation. The results showed that the existence of POE particles played the role of an obstacle during the crystallization process of a PP matrix, which could increase the growth path of PP lamellae or ordered PP molecules and reduce the growth space of spherulites, resulting in a slower spherulite growth rate and smaller spherulite size. This behavior was explained by a crystallization model. Most interestingly, a coated structure was formed in the interface, which was a transition state structure of molecules with different morphologies. The as-formed coated structure can be considered the origin of the cavitation effect and impact-induced morphology evolution of POE particles during the impact process. Moreover, micro-plastic deformation in PP/POE blends during the fracture process was a multi-stage mechanism, in which the POE content played a decisive role.

Conformational Regulation and Crystalline Manipulation of PLLA through a Self-Assembly Nucleator

Self-assembly nucleators have been increasingly used to manipulate the crystallization of PLLA due to their strong intermolecular interaction with PLLA, while the molecular mechanism of such interaction is still unrevealed. In present work, one special self-assembly nucleator (TMC-300) with relatively high solubility in PLLA matrix, is chosen to investigate how the interaction works at molecular level to promote the crystallization of PLLA mainly through time-resolved spectroscopy. The results indicate that due to the dipole-dipole NH···O═C interaction between dissolved TMC-300 and PLLA, PLLA chains are transformed into gt conformer before TMC-300 phase-separating from PLLA melt, resulting in low energy barrier to pass for the following formation of PLLA α-crystal (α-crystal is consisted of gt conformer). Once the dissolved TMC-300 starts to self-assemble into frameworks upon cooling, the transformed PLLA chains with high population of gt conformer form the primary nuclei on the surface of such self-assembling TMC-300 frameworks. For the first time, not only the heterogeneous nucleation but also the conformational regulation of PLLA chains are proved to be responsible for the high efficiency of the self-assembly nucleators (TMC-300) in promoting the crystallization of PLLA. Therefore, conformational regulation is proposed for crystalline manipulation of PLLA, and this work brings new insight on promoting the crystallization of PLLA even other polymers by regulating their molecular conformation.

Simple and Consecutive Melt Extrusion Method to Fabricate Thermally Conductive Composites with Highly Oriented Boron Nitrides

In the region of thermally conductive polymer composites, forcing anisotropic fillers into the highly oriented structure is the most effective method to improve thermal conductivity and mechanical properties simultaneously. However, up to now, such highly oriented structure was mainly achieved in low viscosity polymer matrix or solutions. For the purpose of expanding the range of applications, in the present work, a new strategy, the consecutive and powerful shear flow field, was applied to introduce highly oriented boron nitride (BN) into high viscosity polymer matrix. Results indicated that BN was almost totally oriented along the extrusion plane; as a result, the anisotropic index and thermal conductivity of the composite filled with 40 wt % BN reached as high as 480% and 3.57 W/(m K), respectively. Furthermore, compared with the samples with randomly oriented BN, elongations at break were improved more than 50-fold at the same filler content. Finite element analysis was also applied to systematically investigate the effect of the orientation direction of BN on heat dissipation property of the composites, and results indicated that orienting the longitudinal direction of BN parallel to the heat source is the best way to reduce the heat source temperature to a low level. Therefore, the simple, consecutive, and environmentally friendly melt extrusion with powerful shear flow field is an outstanding method to fabricate high efficiency thermally conductive composites, and the simulative results also have important significance on designing such composites for different applications.

In Situ Formation of Microfibrillar Crystalline Superstructure: Achieving High-Performance Polylactide

As a biobased and biodegradable polyester, polylactide (PLA) is widely applied in disposable products, biomedical devices, and textiles. Nevertheless, due to its inherent brittleness and inferior strength, simultaneously reinforcing and toughening of PLA without sacrificing its biodegradability is highly desirable. In this work, a robust assembly consisting of compact and well-ordered microfibrillar crystalline superstructure (FCS) surrounded by slightly oriented amorphism, is achieved by a combined external force field. Unlike the classic crystalline superstructures such as shish-kebabs, cylindrites, and lamellae, the newfound FCS with diameter of about 100 nm and length of several tens of micrometers is aggregated with well-aligned crystalline nanofibers. FCS can serve as discontinuous fiber to self-reinforce the amorphous PLA; more importantly, FCS can also act as rivets to pin the propagating fibrillar crazes leading to the formation of dense fibrillar crazes during stretching, which dissipates much energy and translates the failure of PLA from brittle to ductile. Consequently, PLA with FCS exhibits exceptionally simultaneous enhancement in ductility, strength, and stiffness, outperforming normal PLA with increments of 728, 55, and 70% in elongation at break, strength, and modulus, respectively. Therefore, FSC exhibits competitive advantages in achieving high-performance PLA even for other semicrystalline polymers. More significantly, this newfound crystalline superstructure (FCS) provides a new structural model to establish the correlation between structure and performance.

The toughening behavior of the PP/POE alternating multilayered blends under EWF and impact tensile methods

The multilayered polypropylene (PP) and poly(ethylene-co-octene) (POE) sheets were prepared by the micro-layered co-extrusion system. The essential work of fracture (EWF) and the impact tensile methods have been successfully used to evaluate the toughening behaviors of the PP/POE multilayered blends under quasi-static and dynamic uniaxial tensile stress, respectively. The experimental results indicate that the multilayered structure plays a key role in the toughening behaviors. On increasing the layer number of the multilayered blends, the specific essential work of fracture, we, increases obviously. As for the βwp, there is no obvious variation in the multilayered blends with low POE content (6.79%), however, obvious enhancement is observed with increasing the layer number of the high POE content multilayered blends (16.57%). Compared with the conventional blends, the multilayered blends with 6.79% POE content are effective to increase the value of we. Additionally, the multilayered blends with high layer numbers present absolute advantage in improving the impact tensile values.