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The large-amplitude nonlinear shear rheology of polymer melts confined between strongly adsorbing surfaces (parallel plates of mica) was studied as a function of strain, frequency, and thickness of the polymer films. The shear strains varied from less than 0.1 (linear response) to over 30 (at which the film structure was strongly modified by the imposed shear). The measurements employed a surface forces apparatus modified for dynamic mechanical shear. The polymers were atactic poly(phenylmethylsiloxane) (PPMS), with chain lengths from 31 to 153 skeletal bonds. The nonlinear shear forces, decomposed into a Fourier series of harmonic frequencies, were always odd in the excitation frequency, as required by symmetry considerations. The in-phase and out-of-phase oscillatory shear responses at the same frequency as the excitation (the nonlinear storage and loss moduli G\ and Gi", respectively) were analyzed. Four principal conclusions emerged. First, from the frequency dependence of G\ and Gi" at constant strain, we conclude that relaxations were accelerated by large strain. Second, a marked decrease of both Gf and Gi" was observed with increasing strain at constant frequency, except at the smallest film thickness, «s40 Á, where Gi" passed through a maximum with increasing strain but Gi continued to display shearthinning. Third, the critical strain for onset of nonlinear response increased with the excitation frequency. Fourth, at sufficiently large strains (larger than 10), the shear moduli were independent of polymer molecular weight (comparisons made at fixed film thickness) and appeared to reach limiting strain-independent levels at sufficiently large strains. This final observation contrasts sharply with the linear response and is consistent with shear-induced loss of inter digitation between opposed adsorbed polymer layers, consistent with the tendency toward slippage of adsorbed polymer layers over one another.

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Narow track recording with MR head is being used for increasing areal density in longitudinal recording media. However, recording with MR head has shown track edge hooks with wide erase bands particulaty at high skew angles in circumferentially textured (anisotropic) media [I]. This not only reduces the available area for recording data, but also poses problem in servo tracking. To investigate this effect we prepared both isotropic and anisotropic media and investigated track edge noise at different head skew angles.CoCrTa longitudinal media with an Mrt of about 0.8 memu/cmz and a coercivity of 2400 Oe were prepared by using NiPlAl substrates. Magnetic film thickness and substrate roughness (Ra) were found to have significant influence on orientation ratios of Hc and Mr. The OR of Hc was found to decrease with decreasing magnetic film thickness and Ra of substrate. For the media prepared on smooth untextured surface ( fig.1 .a) the Hc and Mr were isotropic in the film plane, whereas for the medium prepared on circumferentially textured substrates (fig 1 .b) with an Ra of 47 A the OR of Hc was 1.3forthickerfilm (M,t=2.0 memulcm2) and 1.13 for thinnerfilm (Mrt = 0.8 memulcmz).The recording measurements were performed on these media with an MR head of 3 urn track width at 0 and 10 degree skew angles. The recording was done at different linear densities up to 180 kfci. No track edge hooks were observed but partial erasure was present at 140 kfci and above ( fig. 2.a &b). Standard low frequency TAA showed that isotropic media is about 15% lower in amplitude at both skew angles ( fig. 3a). On the other hand the overwrite was 3 to 4 dB higher for the isohpic media. RMS noise was measured (140 kfci) at various offtrack locations for both the disks at both skew angles. We found that the ontrack RMS noise is roughly proportional to the low frequency signal amplitude in all cases, which reveals in similar SNR. The offtrack noise is modelled by sum of track noise and edge noise. Here, track noise is defined as the noise contributed by the fraction of the track under the head assuming that it has the same noise property as the track center, which means that its power is proportional to the signal amplitude. The edge noise power is calculated by subtracting track noise power from the measured offtrack noise power. We found that the edge noise is significant in both disks that the noise does not reduce at the same time as signal when read head moves offtrack. However, when normalized with the ontrack low frequency signal amplitude, the RMS edge noise is quite similar in all cases except that the isotropic disks seems to have larger noise far from the track (fig. 3b).The study revealed that isotropic media can be prepared on smooth untextured substrates and when 0.R is small, both anisotropic and isotropic media show similar track edge noise characteristics independent of skew angles.