Effects of Molecular Weight and Crystallization Temperature on the Morphology Formation in Asymmetric Diblock Copolymers with a Highly Crystalline Block

Abstract

ABSTRACT:The morphology formed in asymmetric poly(E-caprolactone)-block-polybutadiene (PCL-b-PB) copolymers has been investigated by differential scanning calorimetry (DSC), small-angle X-ray scattering (SAXS), and transmission electron microscopy (TEM) as a function of total molecular weight M,, (8000 s M,, s 62000) and crystallization temperature T, ( -20 s Tes 45°C). All the copolymers have a cylindrical or spherical microdomain structure in the melt with the highly crystalline PCL block inside. In PCL-b-PB copolymers with M,, s 19000, the SAXS result showed the morphological transition (microdomain structure -lamellar morphology) at each Te by the crystallization of PCL blocks. The repeating distance of the lamellar morphology increased significantly with increasing T" as usually observed in the crystallization of homopolymers. The PCL crystallinity Xe (i.e., the weight fraction of crystallized PCL blocks against total PCL blocks in the system) was 0.58-0.79, comparable to that of PCL homopolymer. In PCL-b-PB copolymers with M,, 2'. 44000, on the other hand, the morphology did not change by the crystallization at any T" and Xe was extremely reduced <xe < 0.15). The morphology formed in PCL-b-PB with M,,=30000 was a combination of above two cases; the repeating distance was independent of Te and equal to that of the microdomain structure but the morphological transition was confirmed by SAXS and TEM. The relationship between the stabilities of the microdomain structure (or M,,) and lamellar morphology (or Te) is qualitatively discussed.KEY WORDS Crystalline-Amorphous Diblock Copolymer/ Microdomain Structure / Lamellar Morphology I Molecular Weight/ Crystallization Temperature/ The morphology formation in crystalline block copolymers is very complicated by an interplay between the microphase separation and crystallization of constituent blocks. 1 Many studies have been performed so far on the final morphology of such copolymers as a function of crystallization condition and molecular characteristics.2-20 It is established from these studies 2 -s,i 3 -i 5 that the microdomain structure of low molecular weight copolymers is completely destroyed by the crystallization to form a lamellar morphology, an alternating structure consisting of lamellar crystals and amorphous layers, which is theoretically predicted to be favorable for crystalline block copolymers. 21 · 22 This morphological transition arises from the competition between two factors, that is, the strong crystallization force overwhelms the low stability of weakly segregated microdomains. There are, however, limited studies on the morphology of high molecular weight copolymers. Register et al. 14 have recently investigated the morphology of polyethyleneblock-poly(3-methyl-1-butene) (PE-b-PMB) copolymers as a function of molecular weight, where all the copolymers had a cylindrical morphology in the melt. They found that the sufficiently strong segregation could effectively constrain the crystallization in the existing microdomains. An important factor ...

Crystallization Process in Binary Blends of Poly(ε-caprolactone)-block-Polybutadiene Copolymers

“…The ratio of total molecular weights, M M30 /M M11 , is 2.7 and that of PCL molecular weights, M M30,PCL /M M11,PCL , is 1.6. It was confirmed in our previous study by SAXS and TEM 27 that the crystallization of PCL blocks both in M11 and M30 brought about the morphological transition from the cylindrical or spherical microdomain to the lamellar morphology. The crystallization of M11 is extremely faster than that of M30 at the same crystallization temperature, for instance, M11 crystallizes completely within 45 min at 25 • C while M30 takes more than 3 days to crystallize at 25 • C.…”

“…In our previous study by SAXS and TEM, 27 both M11 and M30 formed the lamellar morphology after the crystallization of PCL blocks, and therefore the lamellar morphology is naturally expected for the crystallized blends composed of M11 and M30. Details of this morphology can be evaluated quantitatively from the DSC and SAXS results.…”

Fractionated crystallization in semicrystalline polymers

“…2) Weakly segregated systems (Low values), TODT > Tc > Tg. In this case, there is little morphological restriction for the crystallization process, which allows a break out from the ordered melt structure, therefore the crystallization determines the final structure, usually crystalline lamellae are formed, erasing the previous structure of the melt [95,96,102,114,118,[143][144][145][146][153][154][155][156][157][158][159][160][161][162][163][164].…”

Poly(ε‐caprolactone)–poly(isobutylene): A crystallizing, hydrogen‐bonded pseudo‐block copolymer

“…Especially, the cases of either “templated” or “confined” crystallization are important, as now the Flory–Huggins parameter (χ N ), as well as the chain length of the polymer blocks are determining the crystallization process. A prominent example is poly(ε‐caprolactone‐ block ‐polybutadiene) BCPs (PCL‐ b ‐PB), where crystallization dominates over microphase separation at low molecular weights ( M n < 19,000 g/mol)8 (leading to “breakout‐crystallization”), whereas at significantly higher molecular weights ( M n > 44,000 g/mol −1 ) the microphase‐separated structure is retained and over‐rules crystallization 9. Similar cases have been described on a large variety of other BCPs (i.e., poly(ethylene)‐ b ‐poly(3 methyl‐1‐butene) (PE‐ b ‐PME),10, 11 poly(ethylene)‐ b ‐poly(styrene‐ethylene‐butene) (PE‐ b ‐PSEB),12 poly(ethylene oxide)‐ b ‐poly(butadiene) (PEO‐ b ‐PB13)) as well as BCP blends (i.e., poly(ethylene oxide)‐ b ‐poly(butylene oxide)/poly(butylene oxide) (PEO‐ b ‐PBO/PBO) 14.…”