Takayuki Ikehara

Poly(hydroxybutyrate)/poly(butylene succinate) blends: miscibility and nonisothermal crystallization

Four blends of poly(hydroxybutyrate) (PHB) and poly(butylene succinate) (PBSU), both biodegradable semicrystalline polyesters, were prepared with the ratio of PHB/PBSU ranging from 80/20 to 20/80 by co-dissolving the two polyesters in N,N-dimethylformamide and casting the mixture. Differential scanning calorimetry (DSC) and optical microscopy (OM) were used to probe the miscibility of PHB/PBSU blends. Experimental results indicated that PHB showed some limited miscibility with PBSU for PHB/PBSU 20/80 blend as evidenced by the small change in the glass transition temperature and the depression of the equilibrium melting point temperature of the high melting point component PHB. However, PHB showed immiscibility with PBSU for the other three blends as shown by the existence of unchanged composition independent glass transition temperature and the biphasic melt. Nonisothermal crystallization of PHB/PBSU blends was investigated by DSC using various cooling rates from 2.5 to 10 °C/min. During the nonisothermal crystallization, despite the cooling rates used two crystallization peak temperatures were found for PHB/PBSU 40/60 and 60/40 blends, corresponding to the crystallization of PHB and PBSU, respectively, whereas only one crystallization peak temperature was observed for PHB/PBSU 80/20 and 20/80 blends. However, it was found that after the nonisothermal crystallization the crystals of PHB and PBSU actually co-existed in PHB/PBSU 80/20 and 20/80 blends from the two melting endotherms observed in the subsequent DSC melting traces, corresponding to the melting of PHB and PBSU crystals, respectively. The subsequent melting behavior was also studied after the nonisothermal crystallization. In some cases, double melting behavior was found for both PHB and PBSU, which was influenced by the cooling rates used and the blend composition.

Melting behaviour of poly(butylene succinate) in miscible blends with poly(ethylene oxide)

The melting behavior of poly(butylene succinate) (PBSU) in miscible blends with poly(ethylene oxide) (PEO), which is a newly found polymer blends of two crystalline polymers by our group, has been investigated by conventional differential scanning calorimetry (DSC). It was found that PBSU showed double melting behavior after isothermal crystallization from the melt under certain crystallization conditions, which was explained by the model of melting, recrystallization and remelting. The influence of the blend composition, crystallization temperature and scanning rate on the melting behavior of PBSU has been studied extensively. With increasing any of the PEO composition, crystallization temperature and scanning rate, the recrystallization of PBSU was inhibited. Furthermore, temperature modulated differential scanning calorimetry (TMDSC) was also employed in this work to investigate the melting behavior of PBSU in PBSU/PEO blends due to its advantage in the separation of exotherms (including crystallization and recrystallization) from reversible meltings (including the melting of the crystals originally existed prior to the DSC scan and the melting of the crystals formed through the recrystallization during the DSC scan). The TMDSC experiments gave a direct evidence of this melting, recrystallization and remelting model to explain the multiple melting behavior of PBSU in PBSU/PEO blends.

Miscibility and crystallization of poly(ethylene oxide) and poly(ε-caprolactone) blends

Blends of poly(ethylene oxide) (PEO) with poly(e-caprolactone) (PCL), both semicrystalline polymers, were prepared by co-dissolving the two polyesters in chloroform and casting the mixture. Phase contrast microscopy was used to probe the miscibility of PEOB/PCL blends. Experimental results indicated that PEO was immiscible with PCL because the melt was biphasic. Crystallization of PEO/PCL blends was studied by differential scanning calorimetry and analyzed by the Avrami equation. The crystallization rate of PEO decreased with the increase of PCL in the blends while the crystallization mechanism did not change. In the case of the isothermal crystallization of PCL, the crystallization mechanism did not change, and the change in the crystallization rate was not very big, or almost constant with the addition of PEO, compared with the change of the crystallization rate of PEO.

Evidence for the formation of interpenetrated spherulites in poly(butylene succinate-co-butylene carbonate)/poly(l-lactic acid) blends investigated by atomic force microscopy

Abstract The spherulitic morphology in poly(butylene succinate- co -butylene carbonate)/poly( l -lactic acid) (PEC/PLLA) blends was investigated by atomic force microscopy (AFM) to obtain direct evidence for the formation of interpenetrated spherulites (IPS), where the spherulites of PEC penetrate into PLLA spherulites. The observation actually revealed that PEC crystals penetrated into interfibrillar regions of edge-on lamellae in a PLLA spherulite. The penetration process was also investigated by AFM with a temperature controller. An edge-on PLLA lamella or a fibril that ran nearly perpendicular to the growth direction of a PEC spherulite obstructed the growth of PEC spherulite. The PEC crystals filled the blocked space after growing around the PLLA lamella. These results showed that the spherulites of PEC and PLLA grow on the same layer instead of forming a layered structure of two spherulites. All the results supported the formation of IPS.

Observation of gellan gum by scanning tunneling microscopy

Abstract A bacterial polysaccharide, gellan gum, was deposited on highly oriented pyrolytic graphite and was studied by means of scanning tunneling microscopy. The sample was prepared in the presence of cations to make double helices of gellan associate into cation-mediated aggregates. This study is the first direct confirmation of the helix structure of gellan in the real space. An image of a cross-linking domain, observed in the ambient condition, showed that half a pitch of the double helix was ca 2.6nm and that the separation of strands was ca 2.3nm. The effect of co-existing cations on the strength of macroscopic gels was discussed in relation to the microscopic structures observed by STM. The difference of cations mainly influenced the length of strands of gellan.

Miscibility in blends of poly(3‐hydroxybutyrate) and poly(vinylidene chloride‐co‐acrylonitrile)

Miscibility behavior of poly(3-hydroxybutyrate) [PHB]/poly(vinylidene chloride-co-acrylonitrile) [P(VDC-AN)] blends have been investigated by differential scanning calorimetry and optical microscopy. Each blend showed a single Tg, and a large melting point depression of PHB. All the blends containing more than 40% PHB showed linear spherulitic growth behavior and the growth rate decreased with P(VDC-AN) content. The interaction parameter χ12, obtained from melting point depression analysis, gave the value of −0.267 for the PHB/P(VDC-AN) blends. All results presented in this article lead to the conclusion that PHB/P(VDC-AN) blends are completely miscible in all proportions from a thermodynamic viewpoint. The miscibility in these blends is ascribed to the specific molecular interaction involving the carbonyl groups of PHB.

Nanorheology measurement on a single polymer chain

The mechanical properties of a single chain of a synthesized polystyrene were measured by atomic force microscopy (AFM). In the conventional force-distance curve measurement, the stress-strain behavior of a chain was obtained. We also measured the dynamic sinusoidal response, as in macroscopic rheological studies, repeatedly at several extension lengths before full stretching or rupturing, by a “nanorheology AFM” that we constructed. It enabled us to design any required movements on a z-piezoelectric scanner, especially sinusoidal movements. The rheological properties of a single polymer chain were discussed from the frequency-dependent measurement.

Conductive‐filler‐filled poly(≥‐caprolactone)/poly(vinyl butyral) blends. II. Electric properties (positive temperature coefficient phenomenon)

The electric properties of poly(e-caprolactone) (PCL)/poly(vinyl butyral) (PVB) blends containing carbon black (CB) were studied as a functions of the PVB content and crystallization time. Comparison of the electric properties between the two cases (PCL/PVB blends and pure PCL) provided us useful information on the origin of the positive temperature coefficient (PTC) phenomenon of the resistivity. In this article, we report the influence of the morphology and the spherulitic structure on the distribution of CB, which results in the resistivity changes. Blending a small amount (up to 5%) of PVB caused significant changes in the electric property at a constant CB content. Both the resistivity and the intensity of PTC increased with the PVB content. These changes are ascribed to the change of CB distribution. A model is proposed to explain these results using Ohes theory.

Nanotribology of polymer blends

We investigated rheological and tribological properties of polystyrene (PS) and poly (vinyl methyl ether) blends by atomic force microscopy. Glass transition temperature measurements showed that the blends whose PS contents were 80% (PS80) and 60% (PS60) were in the glassy state and in the glass-rubber transition state at room temperature, respectively. The force-distance curves were consistent with these results. The dependence of the frictional force Ff of mica on the load indicated that only adhesional component contributed to Ff. In contrast, the viscoelastic deformation of the surface of the blends contributed to the additional component of Ff. PS60, which was in the transition state, exhibited the largest value of Ff because of the large deformation. The value of Ff of PS60 had a hysteresis in the loading and unloading processes because the tip penetration depth was larger in the unloading process. The large penetration depth of PS60 in the negative load region in the unloading process is ascribed t...

Primary and secondary crystallization processes of poly(ϵ-caprolactone)/styrene oligomer blends investigated by pulsed NMR

Abstract The crystallization process of poly(ϵ-caprolactone)/oligostyrene (PCL/OS) blends was analyzed by measuring the spin–spin relaxation time T2 by pulsed NMR. We examined: (i) the temporal changes of T2 and the fractional amount f of the crystalline, amorphous, and intermediate components in the primary and secondary crystallization processes; and (ii) the dependence of the behavior on the crystallization temperature Tc and the OS content φOS. The decrease of T2 of the intermediate and amorphous components with increasing φOS indicated that OS rejected during crystallization is mixed with the residual amorphous PCL as well as trapped in the interface between a lamellar crystal and an amorphous layer. The fraction of the crystal, which is the crystallinity based on the number of protons, decreased with increasing Tc. This is ascribed to the polydispersity of PCL. The values of f of the crystalline and amorphous components depended less on Tc and φOS after the secondary crystallization proceeded than at the end of the primary process. This indicates that the secondary crystallization has the effect of annealing. The fraction of the interface can be an index of regularity of the lamellar structure. Its change implied which process of lamellar thickening, lamella generation, and the rejection of OS was dominant in the secondary crystallization. We also discussed which region OS was rejected into. The changes of f of the three components in the secondary process showed extreme values at φOS≃5%. To discuss this behavior, we proposed two possibilities, namely the influence of the morphology of the spherulites and local phase separation during crystallization.