Thermally induced morphological changes in thin (1−5 μm) films of poly(α,α,α‘,α‘-tetrafluoro-p-xylylene, (C6H4CF2)
n
) (Parylene-F or PPX-F) are characterized using differential scanning calorimetry
(DSC), wide-angle X-ray diffraction (WAXD), and thermal stress measurements. A reversible crystalline
phase transition is observed between 360 and 400 °C, which to our knowledge has not been reported
previously. The transition is accompanied by an increase of the in-plane tensile stress of the film, which
is attributed to contraction of the in-plane polymer structure during the transition to the high-temperature
crystalline form. The stress and thermal behavior are qualitatively similar to those occurring in the more
extensively studied nonfluorinated material, poly(p-xylylene, (C6H4CH2)
n
) (Parylene-N or PPX-N), which
undergoes a shift in stress during the β1−β2 transition (270−300 °C).
Previous work on the performance analysis of IEEE 802.11p beaconing protocol has paid little attention to the varying number of contending nodes and the restricted channel access: Since each node is allowed to broadcast only one beacon frame per control channel (CCH), the number of contending nodes decreases as the CCH elapses. Thus, the performance of 802.11p MAC protocol varies with the number of contending nodes, and the expiration of CCH may cause the beacon messages to drop. In this paper, we propose a new mathematical model to analyze the performance of 802.11p MAC, which considers both the effects of changing number of contending nodes and the restricted channel access. Based on the analytic results, a random contention window scheme is proposed. Through conducting extensive simulations, we verify that the proposed scheme considerably outperforms the legacy 802.11p protocol.
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