In this study, the effect of composition and melting time on the phase separation of poly(3-hydroxybutyrate)/poly(propylene carbonate) (PHB/PPC) blend thin films was investigated. Optical microscopy under phase contrast confirms the occurrence of phase separation of PHB/PPC blends at 190 °C. Polarized optical and scanning electron microscopies (POM and SEM) demonstrate that phase separation takes place along both horizontal and vertical film planes, which should be attributed to the two different interfaces and immiscible blends. A low PPC content (e.g. 30 wt %) results in the formation of compact PHB spherulites filling the whole space, whereas the noncrystallizable PPC spherical microdomains scatter in the PHB region, and their size shows a remarkable melting-time dependence. With the increasing PPC component and melting time, it is observed from POM that the separated PHB domains scattered in the continuous PPC phase, like the island structure. However, it can be revealed by SEM micrographs that the PHB thick domains are actually connected by its thin layer under the PPC layer. The real situation is, therefore, a large amount of PPC aggregates to the surface to form a network uplayer, whereas the PHB thick domains connected by its thin layer form a continuous PHB region, leading to a superimposed bilayer structure. There is also a small amount of PHB small domains scattered in the PHB phase. The PHB thick domains crystallize cooperatively with the PHB- or PHB-rich sublayer in a way just like the growth of pure PHB spherulites. This superimposed bilayer by interplay between phase separation and crystallization may provide availability to tailor the final structure and properties of crystalline/amorphous polymer blends.

The influence of atactic poly(3-hydroxybutyrate) (aPHB) and poly(vinyl phenol) (PVPh) on the crystallization, phase transition, and enzymatic degradation behaviors of poly(1,4-butylene adipate) (PBA) was studied. It was found that both aPHB and PVPh can lower the critical temperature of neat α-PBA crystallization from 34 °C for neat PBA to 32 °C for the blends. Also the critical temperatures of neat β-PBA crystallization decrease from 28 °C for neat PBA to 26 and 24 °C for the PBA/aPHB and PBA/PVPh, respectively. Moreover, the β-to-α phase transition can be accelerated by incorporation of PVPh and aPHB. The β-to-α phase transition completes at 55 °C during heating process for neat PBA, while the temperatures for a complete β-to-α transition of PBA in PBA/aPHB and PBA/PVPh are 50 and 45 °C, respectively. This result should be attributed to the decreasing melting point of PBA in its blends with aPHB or PVPh. Therefore, the melting of the original β-PBA and accompanied recrystallization into α ones should take place earlier and more quickly in the blends than that in neat PBA. The analysis of enzymatic degradation demonstrates that the degradation of PBA can be affected by crystalline morphology and the molecular chain mobility of PBA in the amorphous region. The restricted mobility of amorphous PBA imposed by aPHB and PVPh can slow down the degradation rate of PBA in the blends. The higher Tg and stronger intermolecular interaction between PVPh and PBA result in the slowest degradation of PBA in the PBA/PVPh blend. Furthermore, in neat PBA, PBA/PVPh, or PBA/aPHB, the degradation rate of α-PBA crystals obtained via annealing is slower than that of α-PBA prepared by isothermal crystallization and even slower than that of β-PBA.

The chain organization of poly(ε-caprolactone) (PCL) in its blend with poly(4-hydroxystyrene) (PVPh) in thin films (130 ± 10 nm) has been revealed by grazing incident infrared (GIIR) spectroscopy. It can be found that PCL chains orient preferentially in the surface-normal direction and crystallization occurs simultaneously. The morphology of the PCL/PVPh blends films can be identified by optical microscopy (OM). When crystallized at 35 °C, the blends film shows a seaweed-like structure and becomes more open with increasing PVPh content. In contrast, when crystallized at higher temperatures, i.e., 40 and 45 °C, dendrites with apparent crystallographically favored branches can be observed. This characteristic morphology indicates that the diffusion-limited aggregation (DLA) process controls the crystal growth in the blends films. The detailed lamellar structure can be revealed by the height images of atomic force microscopy (AFM), i.e., the crystalline branches are composed of overlayered flat-on lamellae. The branch width has been found to be dependent on the supercooling and PVPh content. This result differs greatly from pure PCL, in which case the crystal patterns controlled by DLA process developed in ultrathin film or monolayers of several nanometers. In the PCL/PVPh blends case, the strong intermolecular interactions and the dilution effect of PVPh should contribute to these results. That is to say, the mobility of PCL chains can be retarded and diffusion of them to the crystal growth front slows down greatly, even though the film thickness is far more than the lamellar thickness of PCL.

The miscibility of poly(propylene carbonate) (PPC) and poly(3-hydroxybutyrate) (PHB) blends was analyzed by differential scanning calorimetry (DSC) and polarized optical microscopy (POM). The results indicated that the blends are immiscible at most blending compositions, and a miscible blend can be obtained when the PHB content is as low as 10 wt%. The morphology of the PPC/PHB (70/30) blend film was characterized by POM, scanning electron micrography (SEM) and Fourier transform infrared spectroscopy (FTIR), and the development of a PPC-top and microporous PHB-bottom bilayer structure can be revealed. Different from the normal case, phase separation can take place on the normal direction of the film surface in the PPC/PHB (70/30) blend at 190 °C, attributed to the different surface energies of the two components. The continuous segregation of PPC to the top-layer can result in the crystallization of PHB at the bottom layer and conversely promote the complete development of a bilayer structure. Since the isotropic PPC layer is transparent with no birefringence, the PHB spherulite with a microporous structure at the bottom layer can be observed directly by POM. Moreover, the microporous structure of the bottom layer should be attributed to the solution cast procedure. Thus, some unique crystalline patterns may be created in the demixed crystalline/amorphous polymer blends, which differ greatly from those obtained from the miscible blend systems.

In the present study, the crystallization of commercial-grade isotactic polypropylene (iPP) was studied by optical microscopy, scanning electron microscopy (SEM) and wide-angle X-ray diffraction (WAXD). It was found that isothermal crystallization of used iPP at 135 °C leads to the formation of pure α-iPP crystals. During non-isothermal crystallization at a cooling rate of 10 °C min−1, growth transformation of α into β-iPP occurs at the growth front of α-iPP spherulites. This is not common and has only been found during crystallization in a temperature gradient. The observed αβ-iPP growth transformation is confirmed to be a result of the existence of some impurities, which exhibited their ability to nucleate β-iPP. The ability of the impurities to nucleate β-iPP is dependent on concentration and temperature. Therefore, interlamellar rejection of the impurities caused by fast cooling (40 °C min−1) cannot induce the αβ-iPP growth transformation. On the other hand, even though isothermal crystallization of iPP at 135 °C can result in the accumulation of impurities at the spherulite growth front, it still cannot induce the αβ-iPP growth transformation at a low concentration.

Growth transition from β-isotactic polypropylene (iPP) to its α phase during the stepwise crystallization process is investigated. β-iPP hexagonites and spherulites are prepared first at 135 °C by using pimelic calcium as a nucleation agent, and transformation of them to their α-counterparts is realized by continuous isothermal crystallization at elevated temperatures, e.g., 151 °C. The β to α growth transition regions are investigated by optical and scanning electron microscopies. The results show that β-iPP hexagonites and spherulites grow in thin and thick films, respectively, while α-iPP spherulites are always observed in both thin and thick films. After abrupt temperature jump from 135 to 151 °C, α-iPP spherulites grow continuously with edge-on lamellae extending smoothly from the region crystallized at 135 °C to the area grown at 151 °C. On the contrary, the growth of the β-iPP crystals stops. At the growth front of β-iPP crystals, α-iPP crystallization takes place via nucleation and subsequent crystal growth, leading to sporadically dispersed α-iPP crystals around the growth front of β-iPP crystals. The similar structure of the transformed α-iPP crystals with that produced directly by isothermal crystallization at 151 °C also supports this conclusion. Moreover, the same cross-hatched lamellar structure of α-iPP in contact with both the flat-on and edge-on β-iPP lamellae demonstrates that there is no fixed structure relationship between the newly formed α- and the original β-iPP crystals.

The intermolecular interaction between poly(vinylphenol) (PVPh) and polycaprolactone (PCL) and the crystallization behavior of PCL in PCL/PVPh blends with different compositions and under different conditions were investigated by Fourier transform infrared spectra (FTIR) and differential scanning calorimetry (DSC). It has been shown that the PCL in the blends with different blend ratios all exists in crystalline state after solution casting, even though the crystallinity decreases with increasing PVPh content. For the melt crystallized samples, PCL in its 80/20 PCL/PVPh sample can still crystallize. The crystallinity is, however, lower than that of the solution cast sample. For blends containing 50% or 20% PCL, the as-cast samples are semicrystalline and can change to compatible amorphous state after heat treatment process. FTIR analysis shows the existence of hydrogen bonding between PCL and PVPh and the fraction of hydrogen bonds increases remarkably after heat treatment process.

The effect of PBS on the morphological features of PVDF has been investigated by optical and atomic force microscopies under various conditions. It was found that neat PVDF forms large γ form spherulites with extraordinarily weak birefringence at 170°C. Adding 30% PBS makes PVDF exhibit intrigued flower-like spherulitic morphology. The growth mechanism was explained by the decrease of the supercooling and the materials dissipation. Increasing the PBS content to 70% favors the formation of ring banded spherulites. Temperature dependent experiments verify the γ→γ phase transition occurs from the junction sites of the α and γ crystals, while starts from the centers of α spherulites in the blends. Ring banded structures could be observed in neat PVDF, 70/30 blend and 30/70 blend when crystallized at 155°C, without γ crystals. The band period of PVDF α spherulites increases with crystallization temperature as well as the amount of PBS content. At 140°C, spherulites in neat PVDF lose their ring banded feature, while coarse spherulites consisting of evident lamellar bundles could be found in 30/70 blend.

The miscibility of poly(propylene carbonate) (PPC) and poly(3-hydroxybutyrate) (PHB) blends was analyzed by differential scanning calorimetry (DSC) and polarized optical microscopy (POM). The results indicated that the blends are immiscible at most blending compositions, and a miscible blend can be obtained when the PHB content is as low as 10 wt%. The morphology of the PPC/PHB (70/30) blend film was characterized by POM, scanning electron micrography (SEM) and Fourier transform infrared spectroscopy (FTIR), and the development of a PPC-top and microporous PHB-bottom bilayer structure can be revealed. Different from the normal case, phase separation can take place on the normal direction of the film surface in the PPC/PHB (70/30) blend at 190 °C, attributed to the different surface energies of the two components. The continuous segregation of PPC to the top-layer can result in the crystallization of PHB at the bottom layer and conversely promote the complete development of a bilayer structure. Since the isotropic PPC layer is transparent with no birefringence, the PHB spherulite with a microporous structure at the bottom layer can be observed directly by POM. Moreover, the microporous structure of the bottom layer should be attributed to the solution cast procedure. Thus, some unique crystalline patterns may be created in the demixed crystalline/amorphous polymer blends, which differ greatly from those obtained from the miscible blend systems.