Chunjiang Xu

Insights into the nucleation role of cellulose crystals during crystallization of poly(β-hydroxybutyrate)

Cellulose crystals, including microcrystalline cellulose (MCC) and nanocrystalline cellulose (NCC), were used as the fillers to prepare green composites with poly(β-hydroxybutyrate) (PHB) by melt mixing for crystallization study. The results reveal that the spherulite morphology of PHB and its composites depends highly on the crystallization temperature, evolving from bundle shaped to ring-banded and finally to irregular or zigzag textures with increase of temperature. However, the ring-banded structure is strongly affected by the presence of cellulose crystals, and the average band space decreases evidently with the addition of MCC or NCC. Compared with PHB/MCC composite, PHB/NCC composite shows degraded spherulite structure with smaller band space and higher flocculation level of peak-to-valley height because of stronger unbalanced stresses in this system. Besides, cellulose crystals can act as good heterogeneous nucleating agent to accelerate the crystallization of PHB, which is further confirmed by the polarized optical microscopy observations and the kinetic analyses.

Rheological properties of nanocrystalline cellulose suspensions

Rheological behavior, including linear and nonlinear, as well as transient rheology of nanocrystalline cellulose (NCC) suspensions was studied in this work. Two kinds of polymer solutions, aqueous poly(vinyl alcohol) (PVA) with flexible chain structure and aqueous carboxymethyl cellulose (CMC) with semi-rigid chain structure, were used as the suspension media to further explore the role that the interactions among NCC and polymers played during shear flow. The results reveal that NCC has lower values of percolation threshold in the PVA solution than in the CMC one during small amplitude oscillatory shear (SAOS) flow because the flexible PVA chain has higher adsorbed level onto NCC particles than the negatively charged semi-rigid CMC chain, which is further confirmed by the Fourier transformed infrared (FT-IR) spectroscopy tests. As a result, the NCC suspension shows a weak strain overshoot in PVA solution during large amplitude oscillatory shear (LAOS) flow, which cannot be seen on the one in CMC solution. During startup shear flow, both of these two suspensions show evident stress overshoot behavior with the strain-scaling characteristics, indicating the formation of ordered long-term structure of rod-like NCC particles with self-similarity during flow. However, NCC suspension have far stronger stress overshoot response in CMC solution relative to the one in PVA solution. A possible synergy mechanism between NCC and CMC chain is hence proposed.

Polylactide/cellulose nanocrystal composites: a comparative study on cold and melt crystallization

Polylactide (PLA) composites with pristine cellulose nanocrystals (CNC) and acetylated one (aCNC) were prepared for the crystallization study. The roles of CNC and aCNC in cold and melt crystallization of PLA were explored. Both CNC and aCNC have good nucleation activity during cold crystallization of PLA, but also highly impede transport of adjacent chain segments to the growing surface, acting as the role of physical barrier in the glassy bulk. Within the experimental temperature range, growth dominates the overall kinetics, rather than nucleation. Therefore, barrier role overwhelms nucleation agent one and as a result, the cold crystallization rates of composites decrease as compared with neat PLA, accompanied by decreased degrees of crystallinity. During melt crystallization, although the presence of CNC and aCNC leads to sharply increased system viscosities, reducing chain mobility, nucleation is the dominant role as the systems crystallize from the melts. Thus, the presence of CNC and aCNC promotes melt crystallization of PLA, and the composites show far higher crystallization rates and degrees of crystallinity than neat PLA. Besides, the surface acetylation of CNC improves its nucleation ability during melt crystallization of PLA, and as a result, the composite with aCNC has denser fold surfaces relative to the one with CNC. But the presence of these two kinds of particles has no evident influence on the lamellar structure of PLA whether in the cold or in melt crystallization. This work can provide useful information on the crystallization control of PLA using CNC.

Crystallization of poly(ε-caprolactone) in its immiscible blend with polylactide: insight into the role of annealing histories

Polylactide/poly(e-caprolactone) (PLA/PCL) is a very promising blend material with biodegradable characteristics and tailorable performance because of the good property complementarity between the two components. However, PLA and PCL are asymmetric thermodynamically: PLA has far higher melting point than PCL, and crystallization temperature of PCL is even lower than glass transition temperature of PLA. But this also provides good opportunity to control final structure and properties of PCL/PLA blends through thermal annealing. In this work, two annealing routes were designed to control supermolecular structure of discrete PLA phase in the PCL-rich blends, and the two systems, the blend with discrete amorphous PLA domains and the one with discrete crystallized PLA phase, were obtained. The interfacial property alteration and crystallization behavior of continuous PCL phase were then studied. The results are very interesting. Relative to the amorphous PLA phase, the crystallized PLA domains have better affinity to the continuous PCL phase, showing stronger nucleating effect to the crystallization of PCL and higher impeding effect on the shear flow of blend system. But the presence of discrete PLA phase, whether in crystallized or amorphous state, has no evident influence on the crystal structure and lamellar thickness of PCL. These effects on the crystallization of PCL make the mechanical properties of blends very sensitive to the annealing histories. The blend sample with the crystallized PLA domains shows higher modulus and strength than the one with the amorphous PLA domains, and the values of modulus and strength increase by about 130% and 43%, respectively, relative to the neat PCL. This work provides a facile and green approach to well tailor the supermolecular structure and mechanical properties of the PCL/PLA blends through the simple control of annealing process.

Transcrystallization of polypropylene in the presence of polyester/cellulose nanocrystal composite fibers

Pristine cellulose nanocrystal (CNC) and acetylated one (aCNC) were used as the modifier to change the surface properties of poly(trimethylene terephthalate) (PTT) fibers for the transcrystallization study of polypropylene (PP). The results reveal that all three kinds of fibers, including the neat PTT, PTT/CNC and PTT/aCNC ones can induce PP transcrystallization. But the PTT/aCNC fiber-filled PP system shows the most remarkable transcrystallization behavior because of the highest nucleation density of PTT/aCNC fiber. The long period and lamellar thickness of two composite fiber-filled PP systems increase as compared with the neat PTT fiber-filled one, which is caused by the reduced system undercooling and higher surface energy level of composite fibers. Accordingly, the former two systems show the lower transcrystal growth rates than the latter, which is further analyzed by the secondary nucleation theory. This work can provide useful information on the control of PP transcrystallization using the CNC-filled polyester composite fibers.

The starch nanocrystal filled biodegradable poly(ε-caprolactone) composite membrane with highly improved properties

The platelet-like starch nanocrystal (SNC) particles were used to prepare composite membrane with biodegradable poly(ε-caprolactone) (PCL) for the potential packaging membrane application. The presence of SNC particles improves gas barrier properties, tearing strength, as well as the creep resistance of the PCL membrane evidently at the lower loading levels. This improvement effect can be further enhanced by the surface acetylation of SNC. The presence of acetylated SNC particles (1wt%) can reduce the oxygen transmission rate by about 70% and increase the tearing strength by about 68%, which is due to the improved phase adhesion. The strain failure stage of the membrane is also highly delayed in the presence of acetylated SNC, from hour-time scale to day-time one. Therefore, the biodegradable SNC is a promising candidate to be used as the filler to improve the key properties of biodegradable PCL membrane, but the chemically modified SNC is the better option.

Green poly(β-hydroxybutyrate)/starch nanocrystal composites: Tuning the nucleation and spherulite morphology through surface acetylation of starch nanocrystal

Starch nanocrystal (SNC) particles were used as the filler to prepare green composites with biodegradable poly(β-hydroxybutyrate) (PHB). An interesting way to tune the nucleation and banded morphology of composites by the surface acetylation of SNC was proposed. Pristine SNC acts as the nucleating agent, while acetylated SNC as the antinucleation one to PHB. This role switching is due to improved polymer-particle compatibility after surface acetylation of SNC particles. The banded structure of PHB spherulites degrades evidently in the presence of two kinds of SNC particles, showing decreased ring-band space, with deteriorated periodicity and increased flocculation of peak-to-valley height. But the two kinds of composites have different mechanisms on the degradation of their ring-bands because the two kinds of SNC particles, pristine SNC and acetylated one, have different influences on the PHB spherulite growth rates and system undercooling. This work also opens a new window for the applications of SNC particles.