Double Stress Overshoot in Start-Up of Simple Shear Flow of Entangled Comb Polymers.

We report on the unusual response of a well-characterized entangled comb polymer in simple shear flow. The polymer with highly entangled backbone (about 16 entanglements) and on average 29 long branches (about 3 entanglements each) has been extensively characterized by interaction chromatography and its rheology carefully studied under controlled conditions using a special cone partitioned-plate geometry. We observe that the start-up shear stress becomes roughly rate independent above a certain critical rate, related to the relaxation time of the branches. Within the rate-independent region, the start-up shear stress displays a double overshoot. We interpret these observations in light of tube-based pompom dynamics. The key idea is that for sufficiently long branches the main stress overshoot, which reflects backbone stretching and orientation, is preceded by the withdrawal of branches into the backbone tube. The excellent quantitative comparison between the simulations and experiments supports the proposed mechanism of the double stress overshoot.

Diffusive mixing of polymers investigated by Raman microspectroscopy and microrheology.

Diffusive mixing in a model polymer blend of limited miscibility (i.e., the pair polydimethylsiloxane/polyisobutene) is investigated. The diffusion process is followed in the actual droplet-based microstructure of the polymer blend, as opposed to the ideal planar geometry used in previous studies (Brochard et al. Macromolecules 1983, 16, 1638; Composto et al. Nature 1987, 328, 234). In our experiments we combine Raman microspectroscopy and video particle-tracking microrheology. The first technique allows us to monitor local concentration of the two polymers with high spatial resolution both inside and outside a micrometer-size droplet of the dispersed phase. In addition, microrheology enables to follow how the local viscosity inside the droplet changes during the diffusion. The polymer viscosity inside the droplet is determined by video tracking the Brownian motion of a polystyrene bead microinjected into the droplet. The microspectroscopic and microrheological data are combined to estimate the concentration dependence of the monomer friction factor of the two species, which is a key parameter to calculate the interdiffusion coefficient D. Numerical calculations based on such concentration-dependent interdiffusion coefficient D and several alternative models of the polymer diffusion are compared to the experimental concentration profiles. A satisfactory agreement is found for the so-called "slow theory" (Brochard et al.). A phenomenological model improving the agreement of the model with the experimental data is also presented.

Materials science. Polymers go with the flow.

The flow of polymer melts and solutions differs substantially from that of other liquids, because the long polymer chains can become entangled and cannot cross each other freely in their motion. In his Perspective, Marrucci provides an overview of theoretical attempts to explain polymer flow. He highlights the report by Bent et al., who have performed sophisticated experiments on linear polymers that prove key elements of the latest theory. Furthermore, the approach used by the authors will become an important experimental tool for investigating more complex polymer systems.

[Colorectal carcinoma in the elderly. Assessment of prognostic factors]. / Carcinoma colorettale nell'anziano. Valutazione dei fattori prognostici.

Between 1976 and December 1992 a total of 80 patients aged 80 or over underwent surgery for colorectal cancer at the Surgical Clinic of Pisa University. Twelve patients died, 18 presented non-lethal complications and 48 had a normal postoperative recovery. The type of surgery did not appear to be correlated with mortality. Urgency, associated with the advanced stage of cancer (according to Duke's classification modified by Aster-Coller), was the factor which most severely influenced mortality. A marked prognostic improvement can be obtained by an early diagnosis and careful evaluation of the patient's overall conditions.

[Importance of endoscopic follow-up in the diagnosis of metachronous colorectal cancer]. / Importanza del follow-up endoscopico nella diagnosi di cancro metacrono del colon-retto.

Between 1976 and December 1992 a total of 748 patients with colorectal cancer were observed at the Surgical Clinic of Pisa University; in 9 of these a second colorectal tumour appeared between 30 months and 18 years after the original tumor. Coloscopy is the treatment of choice for evaluating the colon operated due to cancer since it allows an early diagnosis of any metachronous tumours, reveals synchronous tumours which have been overlooked prior to the operation and performs a therapeutic and prophylactic function in terms of the endoscopic removal of adenomatous polyps which have appeared in the meantime.

Deactivation of free and stabilized acid phosphatase by urea.

Tests on acid phosphatase (E.G. 3.1.3.2) deactivation by urea have been performed at two pH values. Two conditions have been used: native enzyme operating batch-wise in dilute solution and stabilized enzyme in continuous flow ultrafiltration membrane reactor. Stabilization is achieved by confining the enzyme within a concentrated solution of a linear chain polymer that forms a polarization layer over the membrane. The results provide significant information on the kinetics and thermodynamics of the complex phenomena taking place during deactivation. Deactivation by urea is also compared with thermal deactivation.

Series mechanism of enzyme deactivation. Characterization of intermediate forms.

Acid phosphatase (E.C. 3.1.3.2) undergoes complex thermal deactivation phenomena, as revealed by the two-slope pattern of the enzyme logarithmic-specific-activity versus time curves. The native enzyme first decays toward an equilibrium distribution of less, but still active, intermediate structures and these, in turn, undergo a final degradation to a completely inactive form. The effect of the experimental conditions at which the enzyme is kept during the deactivation process on the characteristics of these intermediate enzymatic structures has been investigated. The kinetic parameters of p-nitro-phenyl phosphate hydrolysis, as catalyzed by some of these intermediate forms, have been determined and the results compared to those obtained with the native enzyme.

Acid phosphatase deactivation by a series mechanism.

Acid phosphatase (E.C.3.1.3.2.) thermal deactivation at pH 3.77 has been investigated by monitoring the enzyme activity as a function of time in the hydrolysis of p-nitrophenyl phosphate. The experimental curves obtained show a two-slope behavior in a log (activity)versus-time plot, which indicates that deactivation occurs via a complex mechanism. From the dependence of the kinetic parameters on both deactivation and hydrolysis temperatures, it is inferred that the deactivation mechanism involves intermediate, temperature-dependent, less-active forms of the enzyme. This interpretation is confirmed by the results of additional tests in which the temperature was suddenly changed during the deactivation process.

Chronic prostato-vesiculitis and sperm-cell mobility in male infertility.

The purpose of this research has been to value the importance of chronic inflammation of the prostatovesicular apparatus in sterile patients. The authors wish to underline the high incidence of chronic inflammation if this apparatus in such patients, the low incidence of bacterial forms and the high frequence of association with bradycinesia. The therapy of such forms gives excellent results.