Recent advances utilizing forced assembly multilayer coextrusion have led to the development of a new approach to study the structure–property relationships of confined polymer crystallization. Confinement of crystalline polymer materials in layer thicknesses ranging from hundreds to tens of nanometers thick, resulted in multilayer films possessing enhanced gas barrier properties. The enhanced gas barrier has been attributed to nanolayer confinement of the crystalline polymer resulting in a highly ordered intralayer lamellae orientation extending over micron or larger scale areas. Research into the confined crystallization mechanism of the multilayered polymer films has resulted in several material case studies as well as an understanding of the chemical and thermodynamic parameters that control the degree and rate of the confinement in multilayer polymer systems. This review highlights our recent studies on the confinement of poly(ethylene oxide), poly(ε-caprolactone), polypropylene, and poly(vinylidene fluoride) polymers in multilayered films.

Synchrotron time-resolved small-angle x-ray scattering studies were carried out on extruded sheets of linear low-density polyethylene (LLDPE) under tension. Stress–strain traces obtained simultaneously exhibited a double yield behavior. LLDPE initially exhibited lamellar morphology with a long period of 21.5 nm. Initial deformation increased the long period due to flow-induced crystallization. Between the first and second yield points, the lamellae were axially deformed by a slip process toward the tensile direction; off-meridional scattering was produced. In the second yield point region, there was a gradual rotation and thinning of the off-meridional scattering indicating that a shear process was destroying the lamellae. Simultaneously, amorphous scattering arise due to microvoids. Second, a weak meridional scattering was also produced. Strikingly, further deformation in the second yield region (SYR) increased the meridional long period suggesting a recrystallization process. From the cold-drawing region the long period decreased monotonically. The results suggest a melting and recrystallization process in the SYR.

Polypropylene–clay (Cloisite Na+) composites with clay contents in weight percentage (wt%) ranging from 1 to 15% were characterized for crystallization mechanism and kinetics. Combination of differential scanning calorimetry, transmission electron microscopy (TEM), and polarized light microscopy was used to investigate the crystallization behavior. Different crystallization mechanisms were observed in the matrix with 1–5 wt% nanoclay compared to the matrix with 10 and 15 wt% of nanoclay additive. TEM micrographs revealed intercalated and flocculated morphology for all the concentrates. At lower wt%, well-dispersed clay platelets acted as antinucleating agent and reduced polymer chain mobility. At high wt%, nucleation rate overcomes the slow diffusion rate. In the case of samples with higher wt% of nanoclay additives, segregation and precipitation of clay was observed in the interspherulite region. On the basis of crystallization kinetics and morphology results, a schematic model of the nanocomposite formation is proposed.

We studied the correlation between shear-induced crystallization and rheological behavior of syndiotactic polystyrene. It was found that after applying a steady shear flow around the nominal melting temperature (Tm = 270 °C), crystal growth rate was accelerated compared with the quiescent state and a morphology of oriented lamellae (kebabs) was observed. On the other hand, no obvious morphological change was observed when applying a shear flow with relatively slow shear rate. We discussed a possibility that the difference of crystal growth rate and morphology could be attributed to the competition between shear rate and relaxation time such as reptation time. Our rheological results suggested that when the imposed shear rate is close to the reciprocal of reptation time, oriented lamellae (kebabs) are observed but extended-chain crystals (shishs) cannot be formed since the chain segments between adjacent entanglements remain unstretched.

Poly(l-lactic acid) (PLLA) is one of the most studied biopolymers nowadays. Due to its good performance, it constitutes an alternative to petrochemical-derived polymers. It was largely studied by differential scanning calorimetry (DSC) and temperature-modulated DSC. Nevertheless, there is an ongoing debate of what happens at the overlapping melting processes. In the present work, the experimental setups are discussed. Different modulation conditions are proposed for the study of the glass transition, cold crystallization, and the two reported melting processes. Finally, the experimental results allowed to measure the heat capacity change at the cold crystallization and a correct interpretation of what happens at the reported double melting peak of PLLA, which involves the existence of three crystalline structures.

Heterogeneous copolymers contain diverse comonomer contents among copolymers, and the extremely diverse case becomes a binary polymer blend. We report a numerical study of crystallization in two series of heterogeneous copolymers that are separated with strong and weak heterogeneities of comonomer distributions, and both of which are composed of crystallizable monomers and noncrystallizable comonomers with various compositions. A comparison of simulation results between these two series of samples demonstrates that, something like a compatibilizer in an incompatible polymer blend, copolymer fractions with intermediate comonomer contents between two compositional extremities depress the prior liquid–liquid demixing on cooling, and hence weaken the subsequent crystallization behaviors. However, we found that in these intermediate fractions, comonomers distribute quite homogeneously on each chain and the amphiphilicity occurs on multiple short sequences, rather than like on a diblock copolymer.

In this study, crystal orientation and polymorphism formation in electrospun poly(vinylidene fluoride) (PVDF)/polyacrylonitrile (PAN) blend fibers after melt-recrystallization were studied. To achieve uniform alignment of electrospun fibers, mechanical stretching was applied to the as-spun nonwoven fibers at 110 °C. Pure ferroelectric β-PVDF crystals in the PAN matrix were achieved, and both polar β-PVDF and polar PAN crystals oriented with their chain axes parallel to the fiber axes. After melt-recrystallization of PVDF, a significant amount of ferroelectric β crystals was retained in addition to the formation of nonpolar α crystals. A polarized Fourier transform infrared study showed that the degree of orientation of ferroelectric β-PVDF crystals was higher than that of nonpolar α crystals, suggesting that the β-PVDF crystals should form at the PVDF/PAN interfaces because of strong dipolar and hydrogen bonding interactions between vinylidene fluoride and acrylonitrile units. The nonpolar α-PVDF crystals should form in the center of PVDF domains.

Poly(lactide) (PLA) composites filled with electrospun nylon 6 fibers were prepared. This allowed us to simultaneously improve the mechanical properties and tune the degradation of the PLA matrix. The interfacial adhesion between the PLA matrix and the nylon fibers was good. The major effect of electrospun fibers on the matrix was that of modifying the semicrystalline framework, thickening the polymer lamellae. This allowed an increase in the mechanical properties of the material, and on the other hand to modify its degradation behavior. The modulus of the composites was increased up to 3-fold with respect to neat PLA. The peculiar morphology of matrix–filler interaction moreover slowed down the degradation rate of the material and improved the dimensional stability of the specimens during the degradation process. This shows the potential of electrospun fibers as a way to tune the durability of PLA-based products, widening the range of application of this promising material.

Asphalt bitumens are complex colloidal systems of high viscosity and complex behavior, which are mainly used for making asphalt concrete for road surfaces. Thermal and rheological characterizations are needed to understand their complex behavior, particularly at the processing stage. Prediction of properties at short and long observation times is usually performed through time-temperature superposition (TTS) models, which make use of some calculated shift factors. The influence of crystallization-like transformation processes on the validity of these shift factors is investigated here by temperature-modulated differential scanning calorimetry (TMDSC). Four asphalt emulsions are considered in this work, each one with a specific transformation behavior. The structure-properties relationships are explained on the basis of the transformation profiles and rheological data.

Tetra(4-dihydroxyborylphenyl)germanium as the tetrahedral units and 1,2,4,5-tetrahydroxybenzene as linkers were selected to form a crystalline porous aromatic framework, CPAF-13, with the planar five-membered BO2C2 ring in its structure by a dehydration reaction. The crystallinity of CPAF-13 was confirmed by x-ray diffraction analysis. The Ar sorption measurement on activated CPAF-13 results in a surface area of 417 m2/g, using Brunauer Emmett Teller model. CPAF-13 also shows a considerable adsorption capacity of H2.