Investigation of the Effects of Isotopic Labeling at a PS/PMMA Interface Using SIMS and Mean-Field Theory

Abstract

Isotopic labeling (deuteration) is known to affect the phase behavior of polystyrene (PS) and poly-(methyl methacrylate) (PMMA) blends, but little is known regarding the changes in the interfacial properties at the PS/PMMA interface due to deuteration of PS and/or PMMA. To investigate these potential changes, secondary ion mass spectrometry (SIMS) was used to measure real-space depth profiles of dPS in hPS:dPS/hPMMA bilayers, with the hPS:dPS blend being well within the single-phase region of the phase diagram. Profound changes in the thermodynamic behavior of this system at the polymer/polymer interface are observed in the form of significant segregation of dPS to the hPS:dPS/hPMMA interface. The observation of a depletion hole during the formation of an equilibrium excess of dPS implies that the energetic gain at the interface per dPS chain has to be >kT. These results cannot be described, even qualitatively, using previously reported changes in for PS/PMMA due to isotopic labeling. The previously reported values of for dPS/hPMMA and hPS/hPMMA actually predict a depletion of dPS at the hPS:dPS/hPMMA interface rather than the observed segregation. The observed interfacial excess is quantified by generating theoretical profiles, using self-consistent mean-field theory (SCMF), and fitting an effective interaction energy parameter ∆ p as a function of temperature. This parameter represents the asymmetry in dPS/hPMMA and hPS/PMMA interactions. The temperature dependency of ∆ p was found to be a factor of 3-4 greater than any of those reported for of PS/PMMA. It was also found that SCMF theory accurately describes the concentration dependency of dPS segregation at a constant dPS molecular weight using a concentration-independent ∆ p ; however, ∆ p was found to be dependent on dPS molecular weight.

Carbon-13 labeling for improved tracer depth profiling of organic materials using secondary ion mass spectrometry

“…The hPS:dPS blend is well within the single-phase region of the hPS:dPS phase diagram [10], so the observed phenomenon is not due to bulk phase separation. This behavior has been described in detail elsewhere, with identification of diffusion-controlled behavior at very early times [14]. Figure 2 shows profiles of the 12 C-PS/ 13 C-PS bilayer using both 6.…”

“…Because dPS has an energetic preference for hPMMA, there is an observed segregation of dPS to the hPS:dPS/hPMMA interface [14]. The hPS:dPS blend is well within the single-phase region of the hPS:dPS phase diagram [10], so the observed phenomenon is not due to bulk phase separation.…”

“…After 24 h of annealing at 125°C, there is no apparent segregation of 13 C-PS to the heterogeneous interface or free surface, as shown in Figure 3. Even though this is a lower temperature and annealing time than used with the hPS:dPS/hPMMA system (see Figure 1), it has been shown that a diffusion gradient and interfacial excess of 140 kDa dPS (5% vol/vol in 138 kDa hPS) were readily visible for an hPS:dPS/hPMMA system after 4 h at 128 °C [14]. The molecular weight dependency of diffusivity of entangled polymer chains in the melt (D ␣ M w Ϫ2 ) is well known [21].…”

“…hPMMA was cast from 1-chlorobenzene onto hydrogen-passivated Si (100) to a thickness of ca. 200 nm and annealed at 150°C for 3 h. PS (90% hPS ϩ 10% dPS vol/vol) was cast directly onto hPMMA from 1-chloropentane, a selective solvent for PS over PMMA, to a thickness of 210 nm and annealed for 94 h at 138°C, which is a much longer time than required for equilibrium segregation of this system [14]. A 50 nm (approximate) hPS sacrificial layer was added to the sample before analysis to avoid pre-equilibrium sputtering at the hPS: dPS surface.…”

“…For instance, it has been established that deuterium substitution can affect polymer/polymer phase behavior, with reported changes in both bulk [8 -10] and surface interactions [11,12], and deuterium substitution has even been shown to alter polymer-solvent phase behavior [13]. More recently, it has been found that deuterium substitution can have a profound effect on the properties of a polymer/polymer heterogeneous interface, with the observation of diffusion-controlled segregation of deuterium labeled polystyrene (dPS) to an hPS:dPS/ poly(methyl methacrylate) (hPMMA) interface [14]. This type of behavior could significantly impact several types of investigations, such as drug delivery mechanisms [15], with the potential for improper interpretation of experimental results due to labeling-induced artifacts.…”