The diffusion of erucamide in single layer polymer films has been monitored previously using FT-IR microspectroscopy [Appl. Spectrosc., 53, 11 (1999)]. This research extends that work to multilayer films. In particular, erucamide migration is characterized in press-laminated bilayer and coextruded trilayer linear low-density polyethylene (LLDPE) films and also coextruded bilayer films of LLDPE and a propylene-ethylene copolymer (PEC). Results in bilayers and trilayers demonstrate that the erucamide prefers to migrate to a film surface rather than into an adjacent layer, even in the presence of steep concentration gradients at layer–layer boundaries. The same behavior was observed for bilayers of LLDPE and PEC where it was anticipated that erucamide may diffuse into the less dense PEC layer. Attenuated total reflectance FT-IR was also used to confirm that little or no erucamide diffused through an additive-deficient layer to reach a film–air interface in a bilayer structure. However, results clearly showed erucamide migration from a core layer through additive-deficient skin layers to reach the surfaces of a trilayer film.

The diffusion of additives in thick (approximately 500 microns) single layer and multilayer films has been characterized using FT-IR microspectroscopy. The objective of this research was to investigate additive migration and concentration profiles in coextruded multilayer films of industrially relevant thicknesses. In particular, the investigation focused on the migration of an erucamide slip agent in 50-micron-thick coextruded bilayer films of linear low-density polyethylene (LLDPE) and a polyolefin plastomer (POP). Erucamide concentration profiles were successfully mapped using synchrotron-based FT-IR microspectroscopy. The synchrotron radiation helped to achieve a higher spatial resolution for the thin films. Meticulous sample preparation was needed to map the thin film samples. Results with FT-IR microspectroscopy showed that the additive-concentration profiles were relatively uniform across the multilayer-film thickness irrespective of the intended initial additive distribution. For example, a bilayer planned for 1 wt % erucamide in an LLDPE layer and no erucamide in a POP layer showed significant additive migration into the POP layer at the extrusion rates used. FT-IR microspectroscopy results also showed that more erucamide migrated to the surface of a POP layer than an LLDPE layer. Attenuated total reflectance (ATR) FT-IR spectroscopy was used to confirm the time-dependent increase of erucamide surface concentration and that the increase was more pronounced at the surface of the POP layers.

This research focuses on mapping the concentration profile of erucamide in LLDPE film. Attenuated Total Reflectance (ATR) FTIR and FTIR microspectroscopy (FTIR-mS) have both been used previously to map concentration profiles in polymer films. However, ATR-FTIR spectroscopy is restricted to the near-surface region of a film while FTIR-mS works well in the film bulk but has deficiencies near film surfaces. Two other techniques, Raman microspectroscopy (R-mS) and FTIR photoacoustic spectroscopy (FTIR-PAS), reportedly work well for depth-profiling in polymers with micron or submicron resolution. Experiments were conducted with R-mS and FTIR-PAS to attempt to quantify the spatial distribution of erucamide in LLDPE film. The amide I carbonyl peak was identified from neat erucamide powder for both R-mS and FTIR-PAS. That peak was not observed with R-mS for film containing erucamide, even for films with a relatively high erucamide loading (1 wt.%). However, FTIR-PAS could detect statistically significant differences in erucamide concentration as a function of penetration depth, indicating that FTIR-PAS may be used as an effective depth-profiling tool for the erucamide-LLDPE system.

Barrier properties of polymer films can be improved in various ways, such as formation of multilayer structures by coextrusion, surface treatment, and coatings. This work explores the use of thousands of alternating layers of polymer (xyxy…) to modify the resistance to permeation. A model is presented to predict the number of layers needed in a laminate to change the flux of a permeant by a given amount. An important feature of the model is the species transport across the interdiffusion regions at the polymer‐polymer interfaces where diffusivity of the penetrant, Di, is assumed to be a constant or a function of the volume fractions (ϕ) of the interdiffusing polymers. For constant Di, the modeling results show that increasing the number of interfacial regions decreases the flux for a given condition, and a large number of layers are required to achieve appreciable flux reduction. For ϕ‐dependent Di a balanced interdiffusion region was modeled in which the interdiffusing polymers were assumed to interpenetrate one another equally. In this particular case, the flux was predicted to always increase with the number of layers.