Differential scanning calorimetry (DSC) has been widely applied to study crystallization and melting of materials. However, for polymeric lamellar crystals, the melting thermogram during heating process usually exhibits a broad endothermic peak or even multiple endotherms, which may result from changes of metastability via recrystallization process. Sometimes, the recrystallization exotherm cannot be observed due to its overlapping with the melting endotherm. In this work, we employed a step heating procedure consisting of successive heating and temperature holding stages to measure the metastability of isothermally crystallized poly(butylene succinate) (PBS) crystals. With this approach we could gain the fraction of crystals melted at different temperature ranges and quantitatively detect the melting-recrystallization behavior. The melting-recrystallization behavior depends on the polymer chain structure and the crystallization temperature. For instance, PBS block copolymer hardly shows recrystallization behavior while PBS oligomer and high molecular weight PBS homopolymer demonstrate remarkable melting-recrystallization phenomenon. High molecular weight PBS isothermally crystallized in the low temperature range shows multiple melting-recrystallization while those isothermally crystallized at elevated temperatures do not exhibit observable recrystallization behavior. Furthermore, the melting endotherms were fitted via the melting kinetics equations. The original isothermally crystallized lamellae demonstrate quite different melting kinetics from the recrystallized lamellar crystals that melt at the highest temperature range, which is attributed to the different degrees of stabilization. Finally, the mechanism of melting-recrystallization is briefly discussed. We propose that apparent melt-recrystallization phenomenon be observed when melting of preformed lamellar crystals and recrystallization of thicker lamellae have similar free energy barrier.

Crystallization of flexible polymer chains reveals distinct characters compared to small molecules, which provides a platform to study molecular self-assembly and morphogenesis. In this review, some examples, e.g., twisting chirality of polymer lamellar crystals, recognition of different chain units and competitive nucleation of different polymorphs and different lamellar thicknesses are briefly discussed. It is shown that the polymer crystallization process far from equilibrium is in practically minimization of the system free energy in local space and finite time, leading to formation of twisted crystals, metastable polymorphism and lamellar crystals with finite thickness. Though each molecule is blind to others, the peculiar ordered configurations with stronger long-range interactions are chosen from the enormous random trials. At the end, we list some remaining questions and outlook the perspectives.Download high-res image (111KB)Download full-size imagePolymer crystallization process far from equilibrium is in practically minimization of the system free energy in local space and finite time, leading to formation of twisted crystals, metastable polymorphism and lamellar crystals with finite thickness. Though each molecule is blind to others, the peculiar ordered configurations with stronger long-range interactions are chosen from the enormous random trials.

The detection of long-chain branches (LCB) in polyethylene is of considerable importance as the processing properties of polyethylene are strongly affected by even a small amount of LCB. While the conventional characterization techniques such as GPC-MALS and 13C NMR fail or take very long time to detect low content of LCB, we turn to the rheological method, which is more sensitive to LCB. In our study, we performed oscillatory shear test, creep test and stress relaxation test on two series of metallocene linear low density polyethylene (LLDPE), revealing that the resins with LCB show higher zero-shear-rate viscosity, retarded relaxation and higher flow activation energy than those without or with less LCB. The resins with LCB showed shear thinning at very low shear rate and their zero-shear-rate viscosities were obtained via creep test. The content of LCB was quantitatively estimated from the flow activation energy. In addition, the modulus-time curves during stress relaxation of melt of the different resins obeyed the power law. The exponent of the resins with more LCB was −0.7, different from that of the resins with less LCB, around −1.7.We performed oscillatory shear, creep and stress relaxation test on two series of metallocene linear low-density polyethylene (LLDPE), revealing that the A-series of resins show higher zero-shear-rate viscosity, more significant sensitivity to shear rate, retarded relaxation and higher flow activation energy. All of these rheological traits indicate the existence of LCB in A-series resins. The relaxation modulus versus time curves are presented in the double logarithmic coordinate and the slope of A-series is much lower than that of B-series, which can be attributed to the fact that long-chain branches considerably slow down the macromolecular motion.

Hexagonal boron nitride nanosheets (BNNSs) can work as a more efficient nucleating agent for two polyesters compared to graphene. Studies on the crystallization and dewetting processes of two polyesters, poly(butylene succinate) and poly(butylene adipate), on the two substrate surfaces prove that the interaction between BNNSs and the polyesters is stronger than that between graphene and the polyesters. This strong interaction induces the pre-ordered conformation of molten PBA which has been identified by the in situ FTIR spectra. Thus BNNSs possess higher nucleation property than graphene. Finally, a new polymer-substrate interaction induced nucleation mechanism was proposed to explain the nucleation efficiency difference between graphene and BNNSs.

Hexagonal boron nitride nanosheets (BNNSs) were prepared by a facile chemical exfoliation method. Then cosolution film casting method was used to obtain poly(butylene adipate) (PBA)/BNNSs nanocomposites. The incorporated BNNSs changed the formation condition of the polymorphic crystals of PBA. The results of differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD) showed that BNNSs significantly facilitated the formation of α-form crystals for isothermal and nonisothermal crystallization. Compared to pure PBA at 28 °C, the α-form crystals would not disappear until the temperature decreased to 13 °C with the addition of 0.5 wt % BNNSs. In addition, the excellent heterogeneous nucleation ability of BNNSs was identified by the nonisothermal and isothermal crystallization process. The enzymatic degradation experiments exhibited that PBA/BNNSs nanocomposites possessed a considerably lower degradation rate than neat PBA. This work demonstrates that BNNSs is an efficient material to regulate the formation of polymorphic crystals and the degradation behavior of PBA.

We present simulations of the mechanism of secondary nucleation of polymer crystallization, based on a new model accounting for the microscopic kinetics of attaching and detaching. As the key feature of the model, we introduced multibody-interaction parameters that establish correlations between the attaching and detaching rate constants and the resulting thickness and width of the crystalline lamella. Using MATLAB and Monte Carlo method, we followed the evolution of the secondary nuclei as a function of various multibody-interaction parameters. We identified three different growth progressions of the crystal: (i) Widening, (ii) thickening and (iii) simultaneously thickening and widening of lamellar crystals, controlled by the corresponding kinetic parameters.Via Monte Carlo simulation based on the microscopic kinetics model, we identified three different growth progressions of the crystal: Widening, thickening and simultaneously thickening and widening of lamellar crystals, controlled by the corresponding kinetic parameters.

Most of the pharmaceutical co-crystals are formed between drug molecules and small molecular compounds. Here, we demonstrated that a small molecular drug griseofulvin and poly(ethylene glycol) can also form co-crystals.

Stretch-induced structural changes of poly(butylene adipate) at various temperatures were systematically investigated by X-ray diffraction method. It was found that tensile deformation could lead to both occurrence of α to β and β to α crystal transition via solid–solid process. Experimental results shows that either α or β form can be adopted as the stable phase for PBA crystal depending on the competition between tensile strain and temperature. Higher temperature favors β to α crystal transformation, while larger strain would result in α to β crystal transformation. Due to the conversion of crystal stability during stretching, a unique “β to α to β” transformation phenomenon of PBA appears at appropriate temperature for original β spline. This study provides evidence that both temperature and tensile strain can alter the relative stability of polymorphic crystals of semi-crystalline polymers.

Strict isomorphism between butylene succinate and butylene fumarate in poly(butylene succinate-co-butylene fumarate) (PBSF) was revealed by DSC and wide-angle X-ray diffraction results. They adopt the same crystal modification, with only a little difference in crystal lattice parameters. The melting point of the copolymer increases linearly with increasing molar ratio of butylene fumarate and the melting enthalpy hardly changes, which meet the requirement of strict isomorphism. The introduction of unsaturated comonomer, fumaric acid, into PBS can enhance the total crystallization rate and the radial growth rate of spherulites. Consequently, PBF and PBSF are highly efficient polymeric nucleating agents for PBS and its copolymers. In this work, strict isomorphism provides us a new method to find polymeric nucleating agent.

The circular birefringence of polycrystalline polymers is invariably obscured by strong linear birefringence. To parse the two mechanisms of light retardation, polycrystalline spherulites of polylactide enantiomers were analyzed by Mueller matrix microscopy. Polymer films are barely optically active in normal incidence, but if illuminated obliquely they become strongly optically active. Opposite hemispheres have oppositely signed circular birefringence. The sign is independent of the enantiomer but dependent on the sense of the sample’s tilt. These observations are consistent with light path inhomogeneities resulting from stacked, mis-oriented lamellae. Chiroptical commonalities based on symmetry arguments are discussed among polylactide, a single oriented water molecule, and microfabricated metamaterial arrays, as well as the first physical model of optical activity, Reusch’s pile of mica plates. The latter model provides the best explanation of the circular birefringence of polylactide spherulites. The only data on the optical rotation of crystalline polymers to date come from ostensible single crystals of polylactide. The enormous, anisotropic optical rotations observed previously are in quantitative agreement with misoriented lamellae observed here. Limitations of parsing circularly birefringent systems into those showing ‘natural optical activity’ and those others, somehow ‘unnatural’, are discussed.

Various methods were employed to study the thermal behaviors of a novel microbial polyhydroxyalkanoate (PHA) terpolyester, namely, poly(3-hydroxybutyrate-co-3-hydroxyvalerate-co-3-hydroxyhexanoate) (PHBVHHx) compared with poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx). PHBVHHx showed higher crystallization rate and degree of crystallinity. PHBVHHx exhibited also different multiple melting behaviors from PHBHHx. The WAXD results demonstrated that the crystal lattice of PHBVHHx was more compact than that of PHBHHx, suggesting stronger interaction between chain stems. DSC and in-situ heating WAXD studies revealed that PHBVHHx showed a partial melting-lamellar thickening-remelting process during heating, while PHBHHx demonstrated a melting-rapid formation of new crystals-remelting process. It is proposed that the simultaneous introduction of 3-hydroxyvalerate and 3-hydroxyhexanoate monomers into poly(3-hydroxybutyrate) improves the mobility of chain stems along the chain direction, leading to easier intralamellar slip during heating or drawing, further resulting in improvement of mechanical properties, which was supported by the DMA tests. Consequently, we establish a relationship between the thermal behavior and the mechanical properties of biodegradable plastics, which we believe is applicable to other polymers as well.

Most of the pharmaceutical co-crystals are formed between drug molecules and small molecular compounds. Here, we demonstrated that a small molecular drug griseofulvin and poly(ethylene glycol) can also form co-crystals.