Huanhuan Gao

Crystallization of Statistical Copolymers

Conventional polymers contain various chemical, geometrical, and stereo-optical sequence irregularities along the backbone chain, which can be treated as noncrystallizable comonomers in statistical copolymers. For statistical copolymers, the link between chemistry (copolymerization to characterize statistical copolymers) and physics (crystallization to determine structures and properties) has recently been enhanced. This review discusses how the crystallization behavior and resulting semicrystalline structure of statistical copolymers are affected by the various microstructure parameters of their comonomers, such as content, distribution along or even among polymer chains, and size (determining their inclusion in or exclusion from the crystallites). The discussion of crystallization is focused on its interplay with component segregation at three different length scales: monomer, monomer sequence, and macromolecule. The first two mainly occur in homogeneous copolymers, whereas the last one is only operative for heterogeneous copolymers. In addition, some unique phenomena such as strong memory effects and (cross)fractionation are discussed briefly.

How Polydispersity of Network Polymers Influences Strain-induced Crystal Nucleation in a Rubber

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.

Crystallization Kinetics of Lamellar Crystals Confined in Polymer Thin Films

We used dynamic Monte Carlo simulations to investigate the crystallization kinetics of flat-on lamellar polymer crystals in variable thickness films. We found that the growth rates linearly reduced with decreasing film thickness for the films thinner than a transition thickness dt , while they were constant for the films thicker than dt . Moreover, the mean stem lengths (crystal thickness) we calculated decreased with film thickness in a similar way to the growth rates, and the intramolecular crystallinities we calculated confirmed the film thickness dependence of the crytsal thickness. Besides, the crystal melting rates in thin films were calculated and increased with decreasing film thickness. We proposed a new interpretation on the film thickness dependence of the crystal growth rate in thin films, suggesting that it is dominated by the crystal thickness in terms of the driving force term (l–l min) expressed by Sadler, rather than the chain mobility based on experiments. The crystal thickness can determine whether a crystal grows or melts in a thin film at a fixed temperature, indicating the reversibility between the crystal growth and melting.

Fast-scan chip-calorimeter measurement on the melting behaviors of melt-crystallized syndiotactic polystyrene

We employed fast-scan chip-calorimeter (FSC) measurement (Flash DSC1) to study the melting of syndiotactic polystyrene after melt-crystallized at various cooling rates as well as after isothermally crystallized at various high temperatures. We attributed the observed double melting peak to a melting-recrystallization process of beta-form crystals upon heating, as evidenced by their variations with different cooling and heating rates. Our experiments demonstrated the advantages of FSC techniques in the investigation of crystallization and melting behaviors of polymer materials.

Free energy change of crystallisation in single copolymers

ABSTRACT We performed dynamic Monte Carlo simulations to calculate the free energy change of crystallisation in single linear and ring polymers containing one or more non-crystallisable sequence defects (comonomers) along the chain. We found that, similar to chain ends, the numbers of comonomers bring only a thermodynamic effect to the free energy barrier and shift down the melting points of single copolymers by following Flory’s thermodynamic equation. Furthermore, there exists a critical comonomer number (or sequence length) for the success of crystallisation, which explains the segregation of sequence lengths upon crystallisation in statistical copolymers. Our observations shed light onto the kinetic suppression of crystallinity for polymers containing various chemical, geometrical or stereo-optical sequence defects, as well as for protein molecules containing specific sequences. GRAPHICAL ABSTRACT

Role of chain ends in coil deformation of driven single polymer

Nonlinear elasticity of coil deformation is profound in polymer rheology. The structural asymmetry between the middle part of the chain and the chain end is supposed to be the origin of internal tension for coil deformation of driven single linear polymers. We performed dynamic Monte Carlo simulations on driven single ring polymers without the abovementioned asymmetry, and compared their deformation behavior to that of the linear counterparts of the same chain length. The comparison proved that the chain ends are not the cause of internal tension for coil deformation, and rather, their relatively high mobility suppresses the coil–stretch transition upon large coil deformation. Therefore, the “cracking-the-whip” effect, as an integration of local acceleration caused by dynamic heterogeneity of nearby monomers on the polymer chain, is the sole origin of internal tension for the observed coil deformation of driven single linear and ring polymers. This study opens an extensive investigation on the intramolecular source of nonlinearity for nonequilibrium polymer dynamics.

Combining Fast-Scan Chip Calorimetry with Molecular Simulations to Investigate Polymer Crystal Melting

Reversible and irreversible melting of lamellar polymer crystals have been studied by means of combining fast-scan chip calorimetry of polymorphic isotactic polypropylenes with dynamic Monte Carlo simulations of polymer chains on a lattice. Different polymorphic phases of polypropylenes are linked to variation of the chain mobility in the crystals of the same species, and this mobility appears as an adjustable parameter in parallel molecular simulations. Such a combination of two different approaches having complemental advantages facilitates a better understanding of the complex phase transition behaviors of lamellar polymer crystals.