Mobility of polymers at surfaces

The time scale that controls polymer chain relaxation has taken on a new importance as the paradigm for polymer innovation evolves from three dimensions to two. Nanotechnology initiatives, involving the design and fabrication of highly confined polymer layers, drive the need for a comprehensive description of chain dynamics, as bulk polymers become thinner and more surface-like. In this study we apply near-edge X-ray absorption fine structure (NEXAFS) to measure directly both surface and bulk segmental relaxation throughout a uniformly deformed polystyrene slab. Using this methodology, in a single experiment, chain relaxation is found to occur almost 50 % faster at the surface than in the bulk.

Partial electron yield (PEY, kinetic energy > 150 eV) and fluorescence yield (FY, carbon Kα 277 eV) carbon K-edge NEXAFS intensities, with probing depths of approximately 2 and 200 nm, respectively, were recorded simultaneously at the NIST/Dow soft X-ray Materials characterization facility at the National Synchrotron Light Source at Brookhaven National Laboratory. For each sample, PEY and FY were measured with the incident polarized X-ray beam normal to the sample surface and at two azimuthal sample orientations: with the electric field vector, E, parallel (ϕ = 0 deg) and perpendicular (ϕ = 90 deg) to the elongation direction as shown in Figure below.

illustration of sample measurements

Time evolution of the orientation factor, OF, from an elongated (oriented) polystyrene sample reveals that when annealed at 60 °C the surface chains relax to an equilibrium (unoriented) configuration faster than the bulk chains. OF is calculated from (Ir – Ip)/(Ir + Ip), where Ir and Ip are the 1s->π* resonance NEXAFS intensities collected with the sample elongation direction parallel (ϕ = 0 deg) and perpendicular (ϕ = 90 deg) to the electric vector of the soft X-ray beam, E, respectively. The blue circles denote OF from the partial electron yield NEXAFS signal (surface region) and the red circles represent the fluorescence yield NEXAFS signal (bulk region), measured simultaneously. The closed and open circles depict OF evaluated from data collected on elongated specimens and those not subjected to elongation, respectively. The solid lines represent exponential decay fits to the experimental data on the elongated samples; the rate decay constants are reported in the text. The schematic on the right details the sample geometry for the incident polarized soft X-ray beam normal to the polystyrene sample surface with the elongation direction parallel to E (f = 0 deg). The insets in the lower right represent chain configurations in oriented (before annealing) and unoriented (after annealing) elongated samples.

Orientation of the chain backbone was determined by monitoring the C=C phenyl ring 1s->π* NEXAFS resonance intensity at 285.5 eV, which involves the excitation of carbon 1s electrons to the unfilled p* antibonding orbitals of the phenyl ring. Enhancement of the 1s->π* resonance intensity was observed when E was parallel to the elongation direction. Since the phenyl p* orbitals are oriented normal to the phenyl rings, and the phenyl rings, free to rotate around the pendant bond, will have a component normal to the chain axis, the intensity of the π* signal has been shown to be an unambiguous signature of backbone orientation. This orientation, seen both in the PEY and the FY NEXAFS signals of the elongated samples, provides clear evidence of chain orientation at the outset of the experiment. A direct measure of the chain relaxation rates at the surface and in the bulk can be obtained by defining an orientation factor, OF (see Figure caption below) that is evaluated from the time dependence of the 1s->π* resonance intensity in the PEY (relative uncertainty ±0.01%) and FY (relative uncertainty ±0.05%) NEXAFS spectra, respectively, during annealing. Figure below shows that while the OF for both surface and bulk chains decays as a function of increasing annealing time at 60 °C, the surface orientation is initially greater and decays faster than for the bulk. Fitting the decay rates to exponential functions gives characteristic time constants of approximately 33 min and 50 min (with corresponding R2 values of 0.90 and 0.91) for the surface and the bulk, respectively.

These results show conclusively that polystyrene surface chain relaxation dynamics are significantly faster than the bulk. Our results are in accord with recent theory predicting that collective motion on chain loops extended to the sample surface is responsible for a rapid increase of chain mobility near the surface region of the film (top ca. 5 nm) [P. G. de Gennes, Eur. Phys. J. E 2, 201 (2000)]. The finding that the polystyrene surface segmental mobility greatly outpaces the bulk is expected to be a universal property of polymeric chains profoundly influencing the design, processing, and application of polymeric materials.