Effect of Hydrogen Bonding Strength on the Microstructure and Crystallization Behavior of Crystalline Polymer Blends

Combinations of differential scanning calorimetry, Fourier transform infrared spectroscopy, optical microscopy, and small-angle X-ray scattering were used to investigate the influence of hydrogen bonding strength on the crystallization kinetics and morphologies in poly(ε-caprolactone) (PCL) blends with three different well-known hydrogen bond donating polymers, i.e., phenolic, poly(vinylphenol) (PVPh), and phenoxy. The strength of the intercomponent interactions in the blend system depends on the hydrogen bond donor group and occurs, based on the Painter−Coleman association model, in the order phenolic/PCL > PVPh/PCL > phenoxy/PCL. Significantly reduced overall crystallization kinetics and crystal growth rate in PCL crystalline phase were also in the order phenolic/PCL > PVPh/PCL > phenoxy/PCL, which is consistent with the relative strengths of their intermolecular hydrogen bonding. Our experimental findings show that the hydrogen bonding strength has a greater effect on the rate of crystallization than does the influence of the blend's glass transition temperature, which is related to its chain mobility. In addition, values of the surface free energy of chain folding and crystalline thickness in PCL blends depend strongly on the relative ratio of the interassociation equilibrium constant and the self-association equilibrium constant ( K A / K B ). In phenolic/PCL and PVPh/PCL blends, the values of the surface free energies of chain folding in the PCL crystalline phase are increased with an increase in the content of the hydrogen bond donating polymer since the K A is greater than the K B in these two blend systems. In contrast, in the phenoxy/PCL blend system, the smaller K A relative to the K B induces a smaller value for the surface free energy of chain folding than that of pure PCL. Various miscible crystalline/amorphous binary polymer blends exhibiting either strong hydrogen bonding or weak interactions are also reviewed.