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Lynn Loo *01

Ph.D. Thesis Research:

Semicrystalline diblock copolymers --- comprising of both crystallizable and amorphous blocks --- exhibit characteristics of both crystalline homopolymers and conventional amorphous diblock copolymers.  Thermodynamically, two opposing driving forces dictate the final morphological structure of a semicrystalline block copolymer.  If the system is sufficiently segregated in the melt, crystallization can be confined within the system’s self-assembled melt microdomains.  Thus, the melt morphology is preserved on cooling.  This is analogous to the phase behavior of amorphous diblock copolymers.  However, if crystallization is the stronger driving force, the melt morphology can be disrupted on cooling resulting in spherulite formation (as in the case of semicrystalline homopolymers).

We have examined the final morphology and the crystallization kinetics of several strongly-segregated semicrystalline diblock copolymers.  Using small- and wide-angle X-ray scattering (SAXS/WAXS), we found that the interblock segregation alone cannot effectively confine crystallization to individual melt microdomains.  As such, crystallization is merely templated by the pre-existing melt microdomains.  In order to truly confine the crystallization, the system must have either spherical melt microdomains which are small in all dimensions or a glassy matrix at the onset of crystallization in addition to being strongly-segregated in the melt.  Crystallization kinetics of these semicrystalline diblock copolymers were studied using synchrotron-based time-resolved SAXS/WAXS  and in-situ differential scanning calorimetry (DSC).  When crystallization is truly confined within individual melt microdomains (in the case of spherical microdomains or a glassy matrix), the isothermal crystallization kinetics display first-order behavior.  This indicates that crystallization is isolated within individual melt microdomains, supporting prior static SAXS results.  However, in the case where crystallization is merely templated by the pre-existing melt morphology, the isothermal crystallization kinetics appear to be similar to that of semicrystalline homopolymers implying cross-talk among crystallizing microdomains.  
 To complete our probe of the final morphological structures of these strongly-segregated semicrystalline diblock copolymer systems, transmission electron microscopy (TEM) will be used to provide real space images of our fully crystallized samples.  Microscopy is a complementary technique to scattering experiments, providing us with images with which we can verify our scattering results.  Moreover, microscopy would be essential in determining the structure of a distorted or not highly regular system as is the case with templated crystallization.