Nucleation mechanism and kinetics
Nie, Y.-J.; Gao, H.-H.; Wu, Y.-X.; Hu, W.-B.* Thermodynamics of strain-induced crystallization of random copolymers. Soft Matter 10, 343-347(2014). Journal Link
[ABSTRACT] Industrial semi-crystalline polymers contain various kinds of sequence defects, which behave like non-crystallizable comonomer units on random copolymers. We performed dynamic Monte Carlo simulations of strain-induced crystallization of random copolymers with various contents of comonomers at high temperatures. We observed that the onset strains of crystallization shift up with the increase of comonomer contents and temperatures. The behaviors can be predicted well by a combination of Flory's theories on the melting-point shifting-down of random copolymers and on the melting-point shifting-up of strain-induced crystallization. Our thermodynamic results are fundamentally important for us to understand the rubber strain-hardening, the plastic molding, the film stretching as well as the fiber spinning.
Zha, L.-Y.; Wu, Y.-X.; Hu, W.-B.* Multi-component thermodynamics of strain-induced polymer crystallization. J. Phys. Chem. B 120(27), 6890-6896(2016). Journal link
[ABSTRACT] We developed a linear combination of two Flory’s melting-point theories, one for stretched and the other for solution polymers, to predict the melting point of stretched solution polymers. The dependences of the melting strains on varying temperatures, polymer volume fractions, and solvent qualities were verified by the onset strains of crystallization in our dynamic Monte Carlo simulations of stretched solution polymers under a constant strain rate. In addition, owing to phase separation before crystallization in a poor solvent, calibration of polymer concentration to the polymer-rich phase appears necessary for the verification. Our results set up a preliminary thermodynamic background for the investigation of the multicomponent effect on strain-induced crystallization of polymers in rubbers and gels as well as on shear-induced crystallization of polymers in solutions and blends.
Strong memory effect due to sequence-length segregation
Guan, X.-C.; Zha, L.-Y.; Wu, Y.-X.; Hu, W.-B.* Strong memory of strain-induced copolymer crystallization revealed by Monte Carlo simulations. Polymer 98, 282-286(2016). Journal link
[ABSTRACT] We performed dynamic Monte Carlo simulations of strain-induced crystallization of homopolymer and random copolymers under cyclic loading of strains. We found that since the second loading random copolymers shift down the onset strain of crystallization and raise up the crystallinity, in contrast to homopolymer. We attributed the strong memory to the remaining of sequence-length segregation raised by copolymer crystallization during the first loading of strains. The mechanism is consistent with that for the strong memory of copolymer crystallization under cyclic cooling, as revealed by previous experiments and simulations. Our results showed a new effect of chain-sequence defects on the cyclic loading performance of rubbers.
Nucleation mechanism and kinetics
Transition from intramolecular to intermolecular nucleation
Nie, Y.-J.; Gao, H.-H.; Yu, M.-H.; Hu, Z.-M.; Reiter, G.; Hu, W.-B.* Competition of crystal nucleation to fabricate the oriented semi-crystalline polymers. Polymer 54, 3402-3407(2013) Journal Link
[ABSTRACT] The mechanical performance of many polymeric materials, such as natural rubber tires, plastic bottles and bags, and textile fibers, depends crucially on the stretch-induced alignment of crystalline molecules in the course of processing. However, the underlying molecular mechanism to solidify the alignment is still poorly understood. We employed dynamic Monte Carlo simulations to unveil how at temperatures close to the melting point a homogeneous stretch of bulk polymers can affect crystal nucleation and leads to aligned crystalline molecules. We observed that upon the molecular strain increasing beyond a critical value, the emerging crystallites suddenly decrease their probability of chain-folding, corresponding to a transition from intramolecular chain-folding nuclei to intermolecular fringed-micelle nuclei. On the basis of the classical nucleation theory, the transition can be predicted well by the competition in the free energy barriers for these two coexisting nucleation mechanisms.
Chain-folding reveals the habits of shish-kebabs and plastic necking
Nie, Y.-J.; Gao, H.-H.; Hu, W.-B.* Variable trends of chain-folding in separate stages of strain-induced crystallization of bulk polymers. Polymer 55, 1267-1272(2014). Journal Link
[ABSTRACT] We performed dynamic Monte Carlo simulations of isothermal crystallization of bulk polymers at a high temperature, which was induced by a homogeneous stretching with a constant strain rate over a wide range of strains. We observed that the crystallites exhibit variable trends of chain folding in three sequential regions of strains, revealing hierarchical mechanisms of strain-induced polymer crystallization: in the first region, sporadic stretched segments initiate intermolecular crystal nucleation with less chain folding at higher strains; in the second region, massive less-stretched segments perform crystal growth with more chain folding at higher strains; in the third region, those folded chains extend via a melting-recrystallization process, again with less chain folding at higher strains. Different trends of chain-folding between crystal nucleation and growth appear intrinsic and ultimately lead to shish-kebab crystals. Our observations provided a molecular-level rationale to understand various experimental phenomena upon the processing for oriented semi-crystalline polymers.
Effects of molecular weight polydispersity
Zhang, M.-M.; Zha, L.-Y.; Gao, H.-H.; Nie, Y.-J.; Hu, W.-B.* How polydispersity of network polymers influences strain-induced crystal nucleation in a rubber. Chinese J. Polym. Sci. 32(9), 1218-1223(2014). Journal Link
[ABSTRACT] Network polymers in a rubber or a gel often contain non-uniform chain lengths. By means of dynamic Monte Carlo simulations of polymer mixtures with various compositions of two chain lengths, we investigated how the factor of polydispersity influences their strain-induced crystal nucleation. Under a high temperature and a high strain rate, the stretching of both polymers revealed that crystal nucleation is mainly accelerated by the presence of short-chain polymers; nevertheless, both polymers join together in the nucleation process. Further analysis proved that crystal nucleation is initiated from those highly stretched short segments, which are rich on the short-chain polymers.
Effect of comonomer sizes
Nie, Y.-J.; Gao, H.-H.; Wu, Y.-X.; Hu, W.-B.* Effect of comonomer sizes on the strain-induced crystal nucleation of random copolymers. European Polymer J 81, 34-42(2016). Journal link
[ABSTRACT] We performed dynamic Monte Carlo simulations of random copolymers containing variable sliding mobility of non-crystallizable comonomer sequences (reflecting their relative sizes) in the crystalline monomer regions. Upon raising strains, we observed that the comononer sliding mobility does not affect the strain evolution curves of crystallinity. However, in the middle temperature region, the low mobility causes a delay of switching from the intra-molecular to the inter-molecular modes of crystal nucleation in the copolymers holding low fractions of comonomers. We attributed the effect to the difficulty of comonomers to be excluded from the nucleating domains that are limited by sequence segregation in the oriented amorphous segments. The implication of this effect to the efficiency of crystal nucleation upon cyclic loading has been discussed.
Hu, W.-B.; Frenkel, D.; Mathot, V. B. F. Shish-kebab crystallites induced by a single pre-aligned macromolecule. Macromolecules 35, 7172-7174(2002).Journal link
Communication to the editor.
Precursor formation for oriented crystals
[ABSTRACT] We studied the athermal relaxation of bulk extended chains and the isothermal crystallization in an intermediately relaxed melt by means of dynamic Monte Carlo simulations of lattice−polymer systems. The melt contained a memory of chain orientations but no more crystalline order. We found that the athermal relaxation is continuous and homogeneous among the bulk chains, and its rate depends on the chain length. The orientational memory in the melt significantly enhances the rate of crystallization. Nevertheless, no precursor of crystallization occurs in the melt of uniform chain lengths. But, in a binary blend of different chain lengths, the crystallization of oriented long chains acts as the precursor to induce epitaxial crystallization of the relaxed short chains. This mechanism explains the formation of shish-kebab crystals observed frequently in the processing of semicrystalline polymers. The results suggest that in flow-induced polymer crystallization the orientational relaxation of chains decides a selection of the long-chain component and then the precursor formation.
Epitaxial on the fiber
Cheng, S.-J.; Hu, W.-B.;* Ma, Y.; Yan, S.-K. Epitaxial polymer crystal growth influenced by partial melting of the fiber in the single-polymer composites. Polymer 48, 4264-4270(2007). Journal link.
[ABSTRACT] We report dynamic Monte Carlo simulations of polymer melt crystallization induced by the same species of the fiber. We found that partial melting on the lateral surface of the fiber postpones the epitaxial crystal growth of the matrix, while the crystal growth rate has been little affected. The delay can be attributed to the recrystallization on the lateral surface of the fiber due to the prior melting. In addition, we observed the highly stretched status of those polymers melted from the fiber, which is responsible for an instant initiation of the recrystallization at a low temperature. The relevance of our results to the experimental observations of isotactic polypropylene has been discussed.
Tuning the nascent structure of spun fiber
Liu, Q.; Gao, H.-H.; Zha, L.-Y.; Hu, Z.-M.; Ma, Y.; Yu, M.-H.; Chen, L.; Hu, W.-B.* Tuning bio-inspired skin-core structure of nascent fiber via interplay of polymer phase transitions. Physical Chemistry Chemical Physics 16 (29), 15152-15157(2014). Journal Link
[ABSTRACT] The properties of polymer fibers are determined by their inner structures. We performed dynamic Monte Carlo simulations of early-stage solidification in the fluid filaments of stretched polymer solutions after extrusion into a coagulation bath upon fiber spinning. We observed that the radial temperature gradient dominates polymer crystallization to form an oriented crystalline skin (from single to multiple layers), while the radial non-solvent influx dominates phase separation to form a concentrated but less oriented core. The skin–core structure offers fibers a balanced performance between strength and toughness similar to plant stems, which can be tuned by the interplay of phase transitions. Our molecular-level observations facilitate a systematic understanding of the microscopic mechanism of fiber-spinning, which will pave a way towards making excellent polymer fibers.