Crystallization is an important property of polymeric materials. In conventional viewpoint, the transformation of disordered chains into crystals is usually a spatially homogeneous process (i.e., it occurs simultaneously throughout the sample), that is, the crystallization rate at each local position within the sample is almost the same. Here, we show that crystallization of ultra-thin poly(ethylene terephthalate) (PET) films can occur in the heterogeneous way, exhibiting a stepwise crystallization process. We found that the layered distribution of glass transition dynamics of thin film modifies the corresponding crystallization behavior, giving rise to the layered distribution of the crystallization kinetics of PET films, with an 11-nm-thick surface layer having faster crystallization rate and the underlying layer showing bulk-like behavior. The layered distribution in crystallization kinetics results in a particular stepwise crystallization behavior during heating the sample, with the two cold-crystallization temperatures separated by up to 20 K. Meanwhile, interfacial interaction is crucial for the occurrence of the heterogeneous crystallization, as the thin film crystallizes simultaneously if the interfacial interaction is relatively strong. We anticipate that this mechanism of stepwise crystallization of thin polymeric films will allow new insight into the chain organization in confined environments and permit independent manipulation of localized properties of nanomaterials.

The question of how to scale the mobility gradient of polymer chains near a substrate in supported ultrathin polymer films is a great challenge. In this paper, a mobility gradient of poly­(ethylene terephthalate) (PET) chains near a substrate is characterized by cold crystallization. We found that either decreasing the PET film thickness or increasing the absorbed layer thickness consistently reveals three characteristic film thicknesses, which are all linearly dependent on the adsorbed-layer thickness. At the first thickness, the low-temperature peak of the top surface crystallization starts to shift toward the high-temperature peak of the bulk-like polymer crystallization; at the second thickness, it arrives there; and at the third thickness, crystallization is completely suppressed. The three kinds of film thicknesses characterize the depth profile of the local dynamics, reflecting the long-range effects of the substrate, which could be scaled by the thickness of the adsorbed layer.

No abstract

A facile method was developed to measure the utmost distance of polymer dynamics suppressed by a solid substrate by investigating the diffusion of a fluorinated tracerlabeled polymer in the bottom layer of bilayer samples. Here, a labeled layer of fluorinated tracer-labeled at the polymer chain end of variable thickness was supported on a substrate, and the corresponding homopolymer layer of fixed thickness was deposited on top of the bottom layer. From the critical time (t*) required for the fluorinated component to diffuse from the bottom layer to the bilayer film surface as a function of the thickness of the bottom layer, a critical thickness of the bottom layer (h c *), corresponding to the thickness at which t* started to increase, was obtained. This thickness was related to the distance over which polymer dynamics were altered by the substrate. The results showed that h c * was very sensitive to the surface chemistry of the substrate and exhibited both molecular weight and temperature dependencies.

The surface average conformation of poly(ethylene terephthalate) (PET) films was modulated by adjusting the water content in corresponding trifluoroacetic acid (TFA) spin-coating solutions, which was confirmed by surface-sensitive sum frequency generation (SFG) vibrational spectroscopy. Molecular dynamics simulation results showed that the added water molecules partly disrupt the hydrogen bond network between TFA molecules and PET chains and make it possible to preserve the statistical average bulk-like spherical conformation at the film surface during the formation of PET film. Slower surface crystallization kinetics, higher surface glass transition temperature (T g surface ), and suppressed T g -confinement effect were observed when the surface average conformation changed from oriented oblate ellipsoidal conformation to isotropic spherical conformation, showing that surface dynamics is very sensitive to chain conformation.