The role of interfacial energy in the heterogeneous nucleation of polyether crystallization
Authors: Cole J. H. , St. Pierre L. E. 
Published: June 2010

From photomicrography, both isotactic poly(propylene oxide) (iPPO) and poly(ethylene oxide) (PEO) crystallize with spherulitic morphology in a thin film. On the addition of silica filler, the average spherulitic diameter in a completely crystallized film is reduced; an iPPO composite containing 10 parts of untreated silica contains so many spherulites it has a granular appearance, which is in contrast with the unfilled polymer film which is composed of much larger, distinct spherulites. Similar effects were found for PEO composites. The Me3SiCl‐treated silica was found to be a less effective nucleant in both the iPPO and PEO crystallization, with the average spherulite diameter significantly larger for a treated silica composite than for an untreated silica composite containing the same amount of silica. The isothermal nucleation rates of unfilled iPPO and two iPPO composites were measured at five different temperatures. For all samples it was found that as the supercooling decreased, the total number of nuclei formed was lowered. Both iPPO and its composites exhibited “pseudohomogeneous” nucleation. The number of nuclei present increased with time over the initial time period, after which a constant value is reached. Using dilatometry to measure bulk crystallization rates, it was found that induction times and half‐times of primary crystallization were reduced by the addition of silica. At the same loadings, the effect of the untreated silica was greater than that of the Me3SiCl‐treated silica. Avrami constants for iPPO, PEO, and their respective composites were largely nonintegral, and had values ranging from 2.46 to 3.26. Using the Avrami equation and making the approximation that the nucleation was instantaneous, values for the total number of nuclei formed were found at five different temperatures. The high‐energy surface silica composites produced more nuclei than the corresponding low‐energy surface silica, but the latter did significantly increase the number of nuclei formed compared to the unfilled polymers. Increasing the crystallization temperature reduced the number of nuclei formed. The crystallization behavior of PEO composites of different loadings showed that as the filler loading increased, a progressively higher number of nuclei per unit loading were formed. It was concluded that the difference in nucleating ability of the silicas is caused by a change in the chemical nature of the surface through trimethylsilation of the acidic silanols on the untreated silica surface. The mechanism put forward for the nucleation by the untreated silica is that polymer segments from the melt are adsorbed specifically on the hydroxylated surface because of the interaction of the acidic silanols with the lone pair of the ether oxygen in the polyether backbone. In this way a length of a polymer chain has its motion restricted by the surface, and may be oriented by it. This increases the probability that a nucleus of critical size will be formed by the addition of other segments from the melt. On the treated silica surface, specific adsorption is not able to occur to any degree, and its nucleating effect can be tentatively attributed to the physical nature of the surface providing nucleation sites.