Crystallization Kinetics in Microphase-Separated Poly(ethylene oxide)-block-poly(1,4-butadiene)

Abstract

We studied the crystallization kinetics in a diblock copolymer system exhibiting different mesophase structures in the melt. A symmetric poly(ethylene oxide)-block-poly(1,4-butadiene) (PEO-b-PB) was blended with a low molecular weight PB homopolymer to yield the block copolymer blends containing lamellar, cylindrical, and spherical PEO microdomains. The crystallization kinetics of PEO blocks in these nanoscaled microdomains was then studied by monitoring the development of crystallinity in the course of isothermal crystallization. In the lamellar melt, crystallization could occur at the normal undercooling, and its kinetics closely followed the classical Avrami model found in the spherulitic crystallization of homopolymers. Crystallinity developments in the cylindrical and spherical morphology obeyed a simple exponential function prescribed by the first-order kinetics. This first-order kinetic behavior along with the exceedingly large undercooling verified the homogeneous nucleation controlled kinetics in these two types of mesophases. Crystallization in the lamellar melt transformed the melt structure into a highly interconnected lamellar morphology due to the ability of the crystal growth fronts to repeatedly thrust into the microdomains yet to be crystallized. For the crystallization condition chosen (i.e., cooling at −5 °C/min from the melt), the melt structures associated with the cylindrical and spherical morphology were not totally disrupted and transformed into one-dimensionally stacked lamellae upon crystallization. The melt mesophases were not fully preserved either, suggesting that some intermediate structures may have been formed through the crystallization.

Ellipsometry as a probe of crystallization kinetics in thin diblock copolymer films

“…For our PB-PEO/PB films, T g of the PB matrix is well below the crystallization temperatures (T c 's), which means that the PB matrix is in the rubbery state for the temperature range accessed. However, as Chen et al 10 demonstrated, this diblock is strongly segregated, with confinement further strengthened by the presence of the PB homopolymer. Thus, crystallization is effectively confined within the spherical domains without breakout or domain coalescence occurring when T c 's are reached.…”

“…8,10 The transition occurs over a 5 8C range, which is expected because each domain must create its own nucleus, and this is a process that has been shown to occur stochastically. 8,28 In agreement with the findings of Röttele et al, 15 we also have observed a wide temperature range (%5 8C) over which melting occurs.…”

“…For systems in which the crystallization kinetics are driven by nucleation rather than crystal growth, the rate is proportional only to the number of domains yet to be crystallized, resulting in firstorder kinetics. 2,6,[8][9][10] A deviation from first-order kinetics occurs when confinement is not strong enough to contain crystallization. Studies have shown that when the matrix is rubbery as opposed to glassy, crystallization can break out and destroy the phase-separated melt morphology.…”

“…2,[11][12][13][14] However, if the blocks are strongly segregated, this may not be the case, and the phase-separated morphology can be preserved upon crystallization. 9 Chen et al 10 found that they could increase confinement by blending poly(butadiene-b-ethylene oxide) (PB-PEO) with a lowmolecular-weight polybutadiene (PB) homopolymer.…”

“…2,7,9,10,[14][15][16] Atomic force microscopy (AFM) studies by Reiter et al 8 have allowed for the direct visualization of nucleation within individual spherical domains of thin PB-PEO films. The use of AFM was particularly insightful because the random distribution of nucleated domains verified that crystallization was occurring in each sphere, independently of neighboring sites.…”