Competition between nucleation and confinement in the crystallization of poly(ethylene glycol)/ large aspect ratio hectorite nanocomposites

Abstract

The overall crystallization kinetics of polymer nanocomposites is determined by nucleation and crystal growth, which are both greatly affected by confinement. Heterogeneous nucleation is influenced by the interphase area between filler and polymer matrix. Starting with a homogeneous nematic aqueous dispersion of a mixture containing polyethylene glycol (PEG) and varying amounts of a high aspect ratio layered silicate (hectorite, Hec), nanocomposite films were casted displaying a systematic variation of the degree of PEG confinement. This is achieved by a partial phase segregation upon drying, where independently of filler content a thermodynamically stable, 1 dimensional crystalline hybrid with constant volume of intercalated PEG (0.81 nm corresponding to a fraction 75 wt% and 55 vol%, respectively) is formed. This intercalated hybrid phase is incorporated into segregated PEG domains. The segregation is a kinetically controlled process and the length scale of segregation increases with PEG available in surplus of the hybrid. Due to the very large lateral extension of the Hec, the segregated domains are increasingly two dimensional. As evidenced by transmission electron micrographs and powder X-ray diffraction, the segregation produces composite structures where, in dependency of filler content, PEG slabs of different thickness are separated by domains of the intercalated hybrid material. The crystallization behavior of these bi-phasic materials was investigated by Differential Scanning Calorimetry (DSC) and Polarized Light Optical Microscopy (PLOM). DSC results reveal a competition between the nucleating effect of Hec, which was particularly important at low amounts, and the PEG confinement effect at higher filler loadings. Applying a self-nucleation protocol, the nucleation efficiency of the hectorite was shown to be up to 67%. The isothermal crystallization kinetics accelerated at low Hec contents (nucleation), went through a maximum and then decreased (confinement) as Hec content increased. Additionaly, a clear correlation between filler content and the Avrami index was obtained supporting the increase in confinement as filler loading increased.