Breakup behavior of nanolayers in polymeric multilayer systems — Creation of nanosheets and nanodroplets
Introduction
Forced assembly by multilayer coextrusion was proven by Dow Chemical to be a reliable technique for producing nanolayers and microlayers in a continuous process [1]. The past two decades witnessed the rapid development of multilayer coextrusion which evolved from uniform layers to gradient layers, from one-dimensional structure to two-dimensional structure and from two-component systems to more complex systems [2]. The nano/microlayered films demonstrated highly tunable gas barrier [3], mechanical [4], optical [5,6] and electrical [7,8] properties arising from scaling and layer-layer interaction.
For many polymer multilayer systems, there existed a layer thickness below which the polymer layers lost their integrity. In these cases, the polymer layers break and transform into nanosheets and nanodroplets, similar to the dewetting phenomenon observed in thin films [9,10]. Such nanostructures were observed on several polymer pairs (polypropylene (PP)/polyethylene oxide (PEO) [11], polycarbonate (PC)/polyethylene terephthalate (PET) [12], PP/polystyrene (PS) [13] or PS/polymethyl methacrylate (PMMA) [14]). As such, the critical layer thickness was defined in a recent work by Sollogoub et al. as the smallest continuous layer observed in a given multilayer system, where they systematically studied the breakup behavior of nanolayers during coextrusion process of PS/PMMA [10,14]. They observed that this critical layer thickness was close to 7 nm for polystyrene and 10 nm for PMMA regardless of the processing route; suggesting it depends only on material parameters. It was speculated this critical layer thickness is due to small interfacial perturbations that are amplified by van der Waals disjoining forces that become dominant over stabilizing interfacial tension forces for thicknesses on the order of 10 nm.
In 2005, Baer et al. benefited from layer break-ups in nanolayered films to develop nanodroplets as polymer blends morphologies [[12], [13], [14]]. The nanolayered films were prepared from coextrusion and then thermally treated at high temperature. Even though the multilayer films had stable morphology at room temperature, the nanolayers were unstable at high temperature. The polymer nanodroplets were generated from the breakup and coalescence of nanolayers during thermal treatment. High density polyethylene and polypropylene were both made into well-dispersed nanodroplets in a PS matrix using this approach [15,16]. It was discovered that the size of the nanodroplets could be controlled by changing the layer thickness of films before thermal treatment [15,17]. Thermal treatment of thicker films yielded larger nanodroplets and thinner films produced smaller nanodroplets.
In this paper, the possibility of using a macroscopic analysis technique, oxygen permeability measurement, to track breakup phenomenon of polymer nanolayers was investigated, using two different multilayer systems: glassy/glassy (PS/PMMA) and glassy/semi-crystalline (PS/polycaprolactone (PCL)). The multilayer composite films with different nominal layer thicknesses were prepared by varying the layer number and polymer compositions. When the layer thickness was lower than the thickness at which the layers start to break (onset thickness for layer break-up) nanosheets and nanodroplets were directly formed during the coextrusion process (as opposed to the nanodroplets formerly produced from the annealing of continuous multilayer films [16,17,21]). Oxygen permeability and light transparency of the composite films were performed on the extruded films and revealed the layer breakups in both systems. Coupling these measurements with an in-depth morphological analysis of the PCL (via AFM, Wide-angle X-ray scattering, Differential Scanning Calorimetry and optical light transmission), we showed that it was possible to model the effective oxygen permeability of PCL to quantitatively obtain the ratio of nanosheets and nanolayers in the PS/PCL films.
Section snippets
Materials
The polystyrene (PS) used in this study was supplied by Americas Styrenics LLC (MC3650). It had a glass transition temperature of 95 °C and melt flow index of 13.0 g/10 min measured at 200 °C under a load of 5 kg. The poly (methyl methacrylate) (PMMA) was a poly (methyl methacrylate-co-ethyl acrylate) which contained 5%–10% ethyl acrylate to prevent depolymerization during extrusion, purchased from Arkema (Plexiglas VS). The glass transition temperature (Tg) of the PMMA was measured as 95 °C
Results and discussion
Fig. 1 showed the morphology of as-extruded PS/PMMA films. The PS phase was the dark color phase and the PMMA one the light color one in AFM phase images, as identified with varying PS/PMMA proportions in the films. Both PS and PMMA were continuous layers when the nominal layer thickness was 60 nm. At 20 nm nominal layer thickness, about half of the layers remain continuous as shown in the AFM images. Some PMMA layers broke up into elongated nanosheets which were parallel to the continuous
Conclusions
The breakup behavior of PMMA nanolayers in the PS/PMMA system and PCL nanolayers in the PS/PCL multilayer films had been systematically studied. The onset for the appearance of breakups in the layers was identified in both cases for nominal thicknesses of 30–40 nm. The cross-sectional AFM images showed that when the nominal layer thicknesses below this value were targeted, some of the continuous layers started to lose continuity and developed into nanosheets. The number of continuous layers
Acknowledgement
This research was generously supported by National Science Foundation under Grant No. DMR 0423914.
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