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September 14, 2005 Crystallization and Melting Behavior of Poly(ε-caprolactone) under Physical ConfinementR.-M. Ho1, Y.-W. Chiang1, C.-C. Lin2, and B.-H. Huang2 We studied the crystallization behavior of poly(ε-caprolactone) (PCL) in a physically confined system, the self-assembly of poly(ε-caprolactone)/polystyrene-b-poly(ethylenepropylene) (PCL/PS-PEP) blends, using simultaneous small-angle x-ray scattering (SAXS) and wide-angle x-ray diffraction (WAXD). The glassy PS-rich phases effectively confined the PCL crystallization due to the localization behavior of PCL. Contrary to a typical microphase-separated morphology of semi-crystalline copolymers (i.e. a chemically confined system), the physically confined system for the crystallization of PCL provides a representative system for understanding crystallization behavior under spatial confinement. With effective confinement, the crystalline chains of PCL appeared in a random orientation at low crystallization temperatures but in a parallel orientation at high crystallization temperatures.
Crystallization behavior under nanoscale confinement has drawn attention due to the necessity of having a basic understanding of crystallization in order to develop nanotechnology applications. In particular, the crystallization behavior of semi-crystalline block copolymers, in which at least one of the constituted blocks is crystallizable, has been thoroughly studied as illustrated in Figure 1a (namely, a chemical confinement). By contrast, the unique morphology with a crystallizable PCL component localized between the lamellar microdomains of PS-PEP gives rise to a specific crystallization environment in which the crystallization is carried out in a nanometer-scale confined environment without the restraint of a chemical connection (Figure 1b, a physical confinement). This unique morphology, a crystallizable PCL component localized favorably within PS-rich constituted lamellar in a PS-PEP block copolymer, has been obtained via melt-mixing in a MiniMax mixer. Shear (velocity), vorticity, and velocity gradient directions are labeled x, y, and z, respectively. Two-dimensional SAXS patterns along the x, y, and z directions indicate that
microphase-separated microdomains can be oriented after melt-mixing, as illustrated in Figure 2 for PCL11/PS-PEP blends (the Mw of PCL11 is 11000g/mol). Up to four orders of lamellar scattering peaks (q/q* = 1 : 2 : 3: 4) can be identified when the incident x-ray beams are along x and y, as shown in Figure 2a-2b. By contrast, we found no significant scattering peak along the z direction in the two-dimensional SAXS pattern (Figure 2c). These two-dimensional SAXS results indicate that microphase-separated lamellae of PCL11/PS-PEP are aligned parallel to the x-y plane (i.e. the shear plane). Moreover, the oriented microphase-separated lamellar microstructure was preserved after PCL crystallization so that the PCL is completely confined in the PS-PEP lamellar layer.
For PCL crystallization at low crystallization temperatures (for instance, at Tc= -20°C), the two-dimensional WAXD patterns exhibit a typical ring pattern in all directions, suggesting that PCL crystals appear randomly oriented under confinement. However, a specific orientation of the PCL crystals can be identified when the shear-aligned samples are crystallized at high crystallization temperatures (for instance, 40°C). Two-dimensional WAXD patterns along the x and y directions are practically identical, and exhibit oriented features (Figure 2d-2e). Only an isotropic ring pattern along the z direction was observed (Figure 2f). On the basis of the orthorhombic lattice structure of PCL crystals with a unit cell of a=0.749nm, b=0.498nm, c=1.703nm, and α = β = γ = 90°, the corresponding reflections were identified as {110} and {200}. The azimuthal profiles (Figure 3a) were obtained from the two-dimensional WAXD pattern (Figure 2d).
The intense {110} diffraction peaks are separated into four diffraction arcs and appear at Φ=56°, 124°, 236°, and 304°, and two {200} reflections appear at Φ=0°, 180°, respectively. According to the azimuthal results, the diffraction pattern is illustrated in Figure 3b. The fiber-pattern-like diffractions suggest a parallel-type orientation with PCL crystalline chains parallel to the microphase-separated lamellae (i.e. the x and y directions). Figure 3c shows the molecular disposition of crystalline PCL chains, and indicates their parallel orientation at high crystallization temperatures in a physically confined environment. The crystalline orientation is strongly dependent upon the crystallization temperature under physical confinement. As a result, the orientation of the PCL crystalline chain localized between the PS-PEP layers can be thoroughly understood by two-dimensional SAXS and WAXD techniques at a synchrotron light source. BEAMLINE FUNDING PUBLICATIONS FOR MORE INFORMATION |
The National Synchrotron Light Source at Brookhaven National Laboratory in New York, is a national user research
facility funded by the U.S. Department of Energy's Office of Basic Energy Science. The NSLS operates two electron
storage rings: an X-Ray ring and a Vacuum UltraViolet (VUV) ring which provide intense light spanning the electromagnetic
spectrum from the infrared through x-rays. Each year over 2300 scientists from universities, industries and government
labs perform research at the NSLS.
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