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P Jacobsson

Publications and source records attributed to P Jacobsson.

9 recordsLinked to original sources
PubMed2005

Influence of chain length on the alpha-beta bifurcation in oligomeric glass formers.

A series of oligo(propylene glycol) dimethyl ethers has been investigated using dielectric spectroscopy in order to relate features of the glass transition dynamics to the number of monomer units N in the chain. The results show that (i) when scaled with the glass transition temperature, the beta relaxations systematically become faster for larger N whereas the alpha relaxations display nearly identical temperature evolutions, i.e., the alpha-beta bifurcation shifts towards shorter times for larger N, (ii) the bifurcation scenario displays a crossover in behavior at N approximately 10 monomer units, signaling the transition from oligomeric to polymeric behavior, and (iii) the beta relaxation has a cooperative nature.

Journal Article↗
PubMed2003

Power laws in polymer solution dynamics.

The dynamical screening length xi(h) in semidilute to highly concentrated polymer solutions of poly(methyl methacrylate) in propylene carbonate has been examined using photon correlation spectroscopy and pulsed field gradient nuclear magnetic resonance. A crossover between different concentration dependent regimes, xi(h) approximately phi(-alpha), where alpha is found to be approximately 0.5, approximately 1, and approximately 2, is observed when the local viscosity is taken into account. Here phi is the volume fraction of polymer in the solution. Well-defined crossovers between alpha=0.5 and alpha=1 corresponding to a transition from a marginal solvent to a theta solvent behavior have been predicted to occur due to the reduction of excluded-volume effects between the spatially correlated polymer segments with increasing polymer volume fraction. However, a clear experimental validation of the crossover has never been presented before. The third regime (alpha approximately 2) is observed in the highly concentrated region where the static screening length is comparable to the persistence length of the polymer. The observation indicates that the rigid rod model previously used to describe concentrated solutions is an oversimplification valid only in the very high concentration limit. The obtained results at high concentrations are discussed in the frame of a simple physical model where segments at the persistence length scale are treated as flexible rodlike segments.

Journal Article↗
PubMed2003

Chain-length-dependent relaxation scenarios in an oligomeric glass-forming system: from merged to well-separated alpha and beta loss peaks.

We have studied the relaxation dynamics of a homologous series of propylene glycol based dimethyl ethers in the supercooled regime by means of broadband dielectric spectroscopy. The system is chosen in order to minimize changes of the intermolecular interactions with varying molecular weight, M. A gradual transformation from a scenario of well-separated to one of merged alpha and beta loss peaks was observed with decreasing M. The results give strong evidence for the currently debated excess wing being due to an underlying beta relaxation. The study suggests that the main difference between glass formers with and without excess wings is the relaxation time at the merging temperature.

Journal Article↗
PubMed1993

Is profound peripheral insulin resistance in patients with pancreatic cancer caused by a tumor-associated factor?

Diabetes in patients with pancreatic cancer occurs in 70% to 80% of the patients and is characterized by high plasma levels of insulin. In type II diabetes that is not associated with pancreatic cancer, peripheral insulin resistance and impaired muscle glycogen synthesis are major pathogenic factors. We investigated peripheral insulin sensitivity in patients with pancreatic cancer before and after tumor removal. The effects of pancreatic tumor extracts on glycogen synthesis in skeletal muscle in vitro and the tumor content of pancreatic islet hormones were also investigated. Marked peripheral insulin resistance was found in the patients with pancreatic cancer and was more pronounced in the diabetic patients than in the nondiabetic patients. Insulin sensitivity was not correlated with weight loss, tumor size, or bilirubin levels but improved after surgery. Tumor extracts from diabetic patients with pancreatic cancer caused a marked reduction of glycogen synthesis in skeletal muscle in vitro. All tumors contained islet hormones but not in concentrations sufficient to explain the effect on glycogen synthesis. These findings indicate that a diabetogenic factor associated with pancreatic adenocarcinomas could be involved in the development of the profound peripheral insulin resistance and thereby could contribute to the high incidence of diabetes observed in patients with pancreatic cancer.

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