Bottom-Up Multiscale Approach to Estimate Viscoelastic Properties of Entangled Polymer Melts with High Glass Transition Temperature

A multiscale computational method is presented for the prediction of the viscoelastic properties of entangled homopolymer melts with high glass transition temperatures. Starting from an atomistic model of a polymer, two coarser representations are introduced─a coarse-grained model and a slip-spring representation─which successively operate at longer time and length scales. The three models are unified by renormalizing the time and modulus scales, which is achieved through matching their normalized chain mean squared displacement and stress relaxation modulus, respectively. To facilitate the relaxation of entangled chains, the simulations are performed at temperatures higher than those accessible in experiments. Time–temperature superposition is then applied to extrapolate the viscoelastic properties calculated at high temperatures to experimentally accessible lower temperatures. This proposed approach can predict the linear rheology of the melt starting from an atomistic model and does not require experimental parameters as an input. Here, it is demonstrated for syndiotactic and atactic polystyrene, where good agreement with experimental measurements is observed

Optical Coherence Tomography Analysis of Attenuated Plaques Detected by Intravascular Ultrasound in Patients with Acute Coronary Syndromes

Background. Recent intravascular ultrasound (IVUS) studies have demonstrated that hypoechoic plaque with deep ultrasound attenuation despite absence of bright calcium is common in acute coronary syndrome. Such “attenuated plaque” may be an IVUS characteristic of unstable lesion. Methods. We used optical coherence tomography (OCT) in 104 patients with unstable angina to compare lesion characteristics between IVUS-detected attenuated plaque and nonattenuated plaque. Results. IVUS-detected attenuated plaque was observed in 41 (39%) patients. OCT-detected lipidic plaque (88% versus 49%, P < 0.001), thin-cap fibroatheroma (48% versus 16%, P < 0.001), plaque rupture (44% versus 11%, P < 0.001), and intracoronary thrombus (54% versus 17%, P < 0.001) were more often seen in IVUS-detected attenuated plaques compared with nonattenuated plaques. Conclusions. IVUS-detected attenuated plaque has many characteristics of unstable coronary lesion. The presence of attended plaque might be an important marker of lesion instability