This review firstly gives an overview on the importance of crystallization in natural and synthetic polymers/macromolecules. Then it introduces the typical features that have been raised by chain-like macromolecules in crystallization, including anisotropic interactions in the thermodynamic driving forces, chain folding in the crystal morphologies, chemical confinement in the copolymer crystallization, and mechanical enhancement in the stretching processes. Four features separately cover the thermodynamics and the kinetics of polymer crystallization, as well as the crystallinity and the mechanical properties of semicrystalline polymers. The review ends up with how these features enhance specific functions of crystalline polymers, which demonstrates polymer crystallization as a challenging yet promising field in the future.

Fractal structures have attracted considerable scientific interest, and fractal geometry is an effective tool to study the complicated structure and dynamic processes, such as dilation symmetry of disorder structure, percolation, gelation, and metal-insulator transition. Here, we summarized our findings of fractal growth of giant amphiphiles in Langmuir-Blodgett (LB) film. Giant amphiphiles composed of polyhedral oligomeric silsesquioxanes (POSS) derivatives and poly(ethylene oxide) (PEO), with structural diversities of POSS head modification, POSS head number, PEO chain length, are precisely synthesized. The effect of molecular structures and operation parameters on the fractal growth patterns are discussed, in addition, other fractal growth patterns in LB films and in Langmuir films are included, aiming to understand the basic molecular principle of fractal growth in LB system.

Stereocomplex (SC) crystallization has been an effective way to improve the physical performances of stereoregular polymers. However, the competition between homo and SC crystallizations can lead to more complicated crystallization kinetics and polymorphic crystalline structure in stereocomplexable polymers, which influences the physical properties of obtained materials. Herein, we select the medium-molecular-weight (MMW) poly(_L-lactic acid)/poly(_D-lactic acid) (PLLA/PDLA) asymmetric blends with different PDLA fractions (f_D=0.01−0.5) as the model system and investigate the effects of f_D and crystallization temperature (T_c) on the crystallization kinetics and polymorphic crystalline structure. We observe the fractionated (i.e., multistep) crystallization kinetics and the formation of peculiar β-form homocrystals (HCs) in the asymmetric blends under quiescent conditions, which are strongly influenced by both f_D and T_c. Precisely, crystallization of β-form HCs is favorable in the MMW PLLA/PDLA blends with high f_D (≥0.2) at a low T_c (80−100 °C). It is proposed that the formation of metastable β-form HCs is attributed to the conformational matching between β-form HCs and SCs, and the stronger constrain effects of precedingly-formed SCs in the early stage of crystallization. Such effects can also cause the multistep crystallization kinetics of MMW PLLA/PDLA asymmetric blends in the heating process.

The macromolecular architecture is the crucial factor in determining the arrangement of the ordering structures, which, because of the multiscale feature, may exhibit distinct melting behaviors and induce the so-called memory effect to affect the following recrystallization. Until present, the correlation between the occurrence of memory effect and the intrinsic molecular structure is still far from the comprehensive understanding. In this work, four kinds of 1-butene/α-olefin random copolymers were designed and synthesized using the (pyridyl-amino) hafnium catalyst to introduce the different branches. The branch length was precisely controlled by the specific α-olefin comonomers, which include 1-hexene, 1-decene, 1-tetradecene, and 1-octadecene, while the branch density was tuned by the incorporation. As expected, the incorporation of α-olefin co-units to poly(1-butene) backbone decreases the non-isothermal crystallization kinetics and the degree of crystallinity. More interestingly, the resulting linear branch can induce the occurrence of memory effect and the threshold concentration of co-units (i.e., branch density) decreases with increasing the branch length. Based on the results of these 1-butene/α-olefin copolymers with designable branches, a direct correlation with the occurrence of memory effect and the fraction of amorphous region was established, which quantitatively indicates the degree of local segregation of the crystallized poly(1-butene) sequences by the α-olefin co-units.

We demonstrate here a novel method for the design of liquid crystals (LCs) via the cyclization of mesogens by flexible chains. For two azobenzene-4,4′-dicarboxylate derivatives, the cyclic dimer, cyclic bis(tetraethylene glycol azobenzene-4,4′-dicarboxylate) (CBTAD), shows LC properties with smectic A phase, while its linear counterpart, bis(2-(2′-hydroxyethyloxy)ethyl azobenzene-4,4′-dicarboxylate (BHAD), has no LC phase. The difference is ascribed to the shackling effect from the cyclic topology, which leads to the much smaller entropy change during phase transitions and increases the isotropic temperature greatly for cyclics. In addition, the trans-to-cis isomerization of azobenzene groups under UV-light is also limited in CBTAD. With the reversible isomerization of azobenzene groups, CBTAD showed interesting isothermal phase transition behaviors, where the LC phase disappeared upon photoirradiation of 365 nm UV-light, and recovered when the UV-light was off. Combined with the smectic LC nature, a novel UV-light tuned visible light regulator was designed, by simply placing CBTAD in two glass plates. The scattered phase of smectic LC was utilized as the “OFF” state for light passage, while the UV-light induced isotropic phase was utilized as the “ON” state. The shackling effect outlined here should be applicable for the design of cyclic LC oligomers/polymers with special properties.

Components of co-continuous phase can form an interpenetrating network structure, which has great potential to synergistically improve the mechanical properties of the blends, and to impart the functional blends superior electrical conductivity and permeability. In this work, the effects of shear rates (50−5000 s⁻¹) at different temperatures on the phase morphology, phase size and lamellar crystallites of biodegradable co-continuous polybutylene terephthalate (PBAT)/polybutylene succinate (PBS) blend are quantitatively investigated. The results show that the above features of the PBAT/PBS have a strong dependence on the shear flow and thermal field. The co-continuous phase of the blend is well maintained at 130 °C. Interestingly, this phase structure transforms into a “sea-island” structure at 160 °C, which gradually recovers to a co-continuous phase when the shear rate increases from 1000 s⁻¹ to 5000 s⁻¹. The phase size decreases with the increase of shear rate both at 130 °C and 160 °C due to the refinement and deformation of phase structures caused by strong shear stress. Unexpectedly, a unique phenomenon is observed that the shear-induced lamellar crystallites are oriented perpendicular to shear direction in the range of 500−5000 s⁻¹ at 130 °C, while the orientation of lamellar crystallites at 160 °C is along the shear direction within the whole range of shear rates. The degree of orientation for the PBAT/PBS blend crystals increases first and then decreases at both temperatures above. In addition, the range of shear rate has reached the level in the industrial processing. Therefore, this work has important guiding significance for the regulation of the co-continuous phase structure and the performance for the blend in the practical processing.

Co-crystallization of different monomeric units fundamentally contributes to the versatile and tunable performance of random copolymers. At present, the crystallization manners of random binary copolymers have been divided into three categories: isomorphism, isodimorphism and comonomer exclusion. Each category, however, has its own advantages and disadvantages. Therefore, it is challenging to design and prepare random copolymer sharing the advantages of isomorphism and isodimorphism through a new co-crystallization manner beyond the ones already exist. On the basis of previous study on poly(alkylene succinate-ran-alkylene fumarate) whose co-crystallization can be extensively and finely regulated by simply varying the chemical structure of alkylene, random copolymers of poly(propylene succinate-ran-propylene fumarate) (PPSF) are synthesized using 1,3-propanediol as the diol source. The thermal properties and crystal structure of PPSF are investigated, and, intriguingly, it is proved that PPSF is an isodimorphism system while displays similar composition-dependent thermal properties and crystallinity as isomorphism. That is, PPSF exhibits a novel co-crystallization behavior that has rarely been discovered, which would combine the advantages of both isomorphism and isodimorphism. Consequently, PPSF could be termed as a new-type, special composition-dependent polymorphism. Besides, the altering of PPS-like to PPF-like crystal structure of PPSF when changing chain composition has been proved to originate from the shift of dominant inter-segment interaction from van der Waals forces to strong hydrogen-bonding interaction. This work enriches the co-crystallization manner of random copolymers, leading to more diverse performance design of polymer materials.

Interfacial welding of thermosetting polymers has been a challenge, but vitrimers with dynamic covalent networks open numerous opportunities for welding and adhesion of these materials. In this work, we performed interfacial welding between epoxy-based vitrimers and an epoxy vitrimer and a thermoplastic polyurethane (TPU). Catalyst-controlled interfacial mechanical properties for both the vitrimer/vitrimer and vitrimer/TPU are observed, that is, the more efficient the catalyst for the transesterification reactions, the larger the interfacial fracture energy is, and the better the welding strength will be. The interfacial mechanical properties are also found to be independent of the original mechanical properties of the vitrimers. Even for a vitrimer with poor mechanical properties, both the welded vitrimer/vitrimer and vitrimer/TPU exhibit larger interfacial fracture energy than the one with better mechanical properties as long as the former uses more efficient catalyst.

Strong preshear flow stretches poly(butene-1) chains to form flow-induced precursors of fibrillar crystals. The formation leads to further growth of shear/normal stress after the stress reaches the steady state. In this study, stress relaxation measurements are performed after intervals of a strong flow during which the precursors form. The small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) measurements are conducted to characterize the crystalline structure formed after the preshear and relaxation processes, each of different periods. A combination of the stress relaxation and scattering results reveal that for the precursors formed within 5 s, the stress can fully relax and the orientation of the subsequent crystals (formed upon quenching the sample at different stress levels) decays gradually with the relaxation. In comparison, for those precursors formed after 5 s, the stress hardly relaxes during ~10⁴ s, and the orientation of the subsequent crystals hardly decays, suggesting that the precursors have been somehow stabilized during the shear. These features have been discussed with respect to the percolation of the precursors to form a reversible network.

Heavy metallic salts are capable to bind with proteins and cause detrimental fibrilization in living cells. Herein, we report a similar case of supramolecular polymerization and thus fibrilization from a liquid crystalline (LC) block copolymer (BCP) initiated by heavy metallic salts. Analogous to the naturally-occurring process, LC BCP “monomers” could bind with metallic salts to form small aggregates, which functioned as seeds to trigger the subsequent supramolecular polymerization of the rest BCP monomers, to produce highly uniform supramolecular polymers. The lengths of the resultant supramolecular polymer fibrils were linearly proportional to the ratios between the BCP and the metallic salts, and largely influenced by the choice of metallic cations, as well as the counterions. Lastly, this method was used to polymerize two different diblock copolymer “monomers” to produce pentablock supramolecular polymers in a one-pot manner.

In this work, phase segregation and localization of PBSU have been investigated with the combination of SAXS and DSC in its blend with PVDF. After stepwise crystallization of PVDF and PBSU, there are double melting peaks of PBSU in DSC and double scattering peaks in SAXS. It has been demonstrated that double peaks can be attributed to the localization of PBSU in interlamellar/interfibrillar region in pre-formed PVDF crystal framework. In the case of low content of PBSU in blend, PBSU is trapped into the interlamellar region of PVDF crystals, resulting in the alternating lamellae crystal of them and the first peak (with low-q) in SAXS. The enhanced confinement effect produces thinner PBSU lamellae, corresponding to the lower melting temperature in DSC. Upon increasing its content in blend, some PBSU segregates in interfibrillar regions in addition to the enrichment in interlamellar regions of PVDF crystal framework. The larger space and higher concentration of PBSU in interfibrillar-regions contribute to periodic lamellae structure of PBSU with higher thickness, which is the reason for the second peak (with high-q) in SAXS and DSC. Our results not only clarify the relationship between localization of PBSU in interlamellar/interfibrillar regions and double peaks in DSC/SAXS, but also provide a novel strategy to detect the interlamellar and interfibrillar segregation of low-T_m component in miscible crystalline/crystalline blend.

The shape accuracy and residual stress distribution of a nano-molded semicrystalline polymer are studied by molecular dynamics simulations. Semicrystalline polyethylene flakes are obtained by continuous cooling inside templates with four shapes. We find that the curvature of contour curve near template corners decreases with corner angle. A simple 2D shape model of minimum surface energy is proposed to understand the shapes for repulsive and attractive templates. The confinement of template induces highly ordered chain packing in surface region. According to the spacial distribution of local stress, we find the contracting stress caused by volume shrinkage during cooling concentrates on the chains perpendicular to the direction of relative stress principal. The distribution of von Mises stress indicates the outer layer of semicrystalline polymer flake has lower distortion. Our results provide a theoretical insight for better nano-molding techniques.

The nucleation of crystals is often a determining step in the phase transition of materials, but it remains a challenge to control the density and specific location of nuclei simultaneously. Here we fabricated the isolated single crystals of uniform size with controlled number density and spatial distribution by self-nucleation of patterned dendritic crystals. Imprint lithography creates the periodic void space on the surface of poly(ethylene oxide)-b-poly(2-vinyl pyridine) (PEO-b-P2VP) block copolymer thin films and provides spatial redistribution of polymers, leading to the preferential nucleation and subsequent oriented growth of dendrites in the periodic arrays of imprinted lines. The morphology and thermal stability of the patterned crystals can be adjusted by tuning embossing conditions (e. g., temperature and pressure). Furthermore, in the self-nucleation technique, the annealing temperature and heating rate are used as the feedback parameters to map the number density and spatial distribution of regrown single crystals. Such PEO-b-P2VP crystalline pattern can be used as a versatile template for large-area manufacturing of selective metal patterns for electronic devices and other applications.

The crystallization kinetics of semicrystalline polymers is often studied with isothermal experiments and analyzed by fitting the data with analytical expressions of the Avrami and Lauritzen and Hoffman (LH) theories. To correctly carry out the analysis, precautions in both experiments and data fitting should be taken. Here, we systematically discussed the factors that influence the validity of the crystallization kinetics study. The basic concepts and fundamentals of the Avrami and LH theories were introduced at first. Then, experimental protocols were discussed in detail. To clarify the impact of various experimental parameters, selected common polymers, i.e., polypropylene and polylactide, were studied using various experimental techniques (i.e., differential scanning calorimetry and polarized light optical microscopy). Common mistakes were simulated under conditions when non-ideal experimental parameters were applied. Furthermore, from a practical point of view, we show how to fit the experimental data to the Avrami and the LH theories, using an Origin^® App developed by us.

Ferroelectric poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE))/semiconducting poly(3-hexyl thiophene) (P3HT) blend systems have drawn great attention with their potential use for electronic applications, particularly non-volatile memory devices. It is essential to grasp a full understanding of the crystallization habits of the two polymers on different substrates for purposeful control of the structures of the blend and therefore the properties of the devices. Here, the effects of structure and morphology of the blend films generated at different substrate surfaces on the ferroelectric and switching properties of related devices are reported. It is identified that P(VDF-TrFE)/P3HT blend films prepared on graphene substrate show not only an obvious optimization in the ferroelectric behavior of P(VDF-TrFE), but also an enhancement of the charge transport within P3HT domains. By employing sandwich structure constructed by silver electrode and P3HT/P(VDF-TrFE) blend film on graphene substrate, high-performance ferroelectric memory devices have been obtained, which exhibit a great electrical switching behavior with high ON/OFF ratio of about 1000 and low coercive voltage of approximately 5 V. These findings provide useful guidance for fabricating high-performance ferroelectric memory devices.

When the size of the material is smaller than the size of the molecular chain, new nanostructures can be formed by crystallizing polymers in nanoporous alumina. However, the effect of pore wall and geometric constraints on polymer nanostructures remains unclear. In this study, we demonstrate three new restricted nanostructures {upright-, flat- and tilting-ring} in polybutylene terephthalate (PBT) nanorods prepared from nanoporous alumina. The dual effects of geometrical constraints and interfacial interactions on the formation of PBT nanostructures were investigated for the first time by using X-ray diffraction and Cerius² modeling packages. Under weak constraints, the interaction between pore wall and the PBT rings is dominant and the ring plane tends to be parallel to the pore wall and radiate outward to grow the upright-ring crystals. Surprisingly, in strong 2D confinement, a structural formation reversal occurs and geometrical constraints overpower the effect of pore wall. Rings tend to pile up vertically or obliquely along the long axis of the rod, so the flat- and tilting-ring crystals are predominate in the constrained system. In principle, our study of the nanostructure formation based on the geometrical constraints and the pore wall interfacial effects could provide a new route to manipulate the chain assembly at the nanoscale, further improving the performance of polymer nanomaterial.