Molecular dynamics glass transition

Although molecular mobility is severely restricted below the glass transition temperature, the dynamic glass transition temperature (main transition or, conventionally -relaxation) in polymers as it have been described above, is usually accompanied by subglass secondary relaxations labeled as p, y, S, relaxations. The glass transition at low temperatures is assumed to be caused by the cooperative motion of many particles, while the secondary relaxations have a more localized molecular... 

In Eq. (4-31), the first three terms describe a simple damped harmonic oscillator the first term is due to molecular accelerations, the second is due to viscous drag, and the third is due to the restoring force. Qq is the oscillator frequency, which is of order 10 sec", and p is a viscous damping coefficient. The crucial term producing the dynamic glass transition is, of course, the fourth term, which has the form of a memory integral, in which molecular motions produce a delayed response. The kernel m(t — t ) is determined self-consistently by the time-dependent structure. One simple choice relating m(s) to the structure is ... 

Broadband dielectric spectroscopy enables one to analyse the dynamics of polar groups in polymeric systems. Due to its broad frequency range of more than 10 decades a manifold of different molecular fluctuations can be studied from the dynamic glass transition (spanning already more than 10 decades in times) to secondary relaxations. Additionally one finds in chiral liquid crystals cooperative processes like soft-and Goldstone modes. 

Dielectric spectroscopy enables one to measure directly the molecular fluctuations corresponding to the dynamic glass transition in a wide frequency and temperature range... 

In side-chain LC polymers, the molecular dynamics of the mesogenic groups is decoupled from the main chain by use of a flexible (aliphatic) spacer. Three relaxation processes typically take place. (1) The p-relaxation which is assigned to librational fluctuation of the mesogen arotmd the long molecular axis. (2) The 5-relaxation process corresponding to librational fluctuation around the short molecular axis. (3) And finally, the a-relaxation process corresponds to the motion of the polymer backbone at the dynamical glass transition. It should be... 

More recently, Class and Chu" extended the use of dynamic mechanical measurements to a systematic study of resin-elastomer blends which revealed the relationship between the structure, concentration and molecular weight of resins and their effect on the viscoelastic properties of elastomers. Dynamic mechanical data typical of that obtained from an elastomer or elastomer-resin blend is shown in Fig. 4. G is the elastic or storage modulus, G" is the viscous or loss modulus, and the ratio of G jG gives the tan 6 curve. The temperature at which the tan 6 curve shows a maximum corresponds to a dynamic glass transition temperature. Class and Chu showed that with these types of measurements, the effect of modifying resins on the viscoelastic properties of elastomers can be readily determined. Resins which are compatible with an elastomer will cause a decrease in the elastic modulus G at room temperature and an increase in the tan delta peak or glass transition temperature. Resins which are incompatible with an elastomer will cause an increase in the elastic modulus G at room temperature and will show two distinct maxima in the tan delta curve. 

AokI M I and Tsumuraya K 1997 Ab initio molecular-dynamics study of pressure-induced glass-to-crystal transitions In the sodium system Pbys. Rev. B 56 2962-8... 

Molecular dynamics simulations have also been used to interpret phase behavior of DNA as a function of temperature. From a series of simulations on a fully solvated DNA hex-amer duplex at temperatures ranging from 20 to 340 K, a glass transition was observed at 220-230 K in the dynamics of the DNA, as reflected in the RMS positional fluctuations of all the DNA atoms [88]. The effect was correlated with the number of hydrogen bonds between DNA and solvent, which had its maximum at the glass transition. Similar transitions have also been found in proteins. 

Tackifying resins enhance the adhesion of non-polar elastomers by improving wettability, increasing polarity and altering the viscoelastic properties. Dahlquist [31 ] established the first evidence of the modification of the viscoelastic properties of an elastomer by adding resins, and demonstrated that the performance of pressure-sensitive adhesives was related to the creep compliance. Later, Aubrey and Sherriff [32] demonstrated that a relationship between peel strength and viscoelasticity in natural rubber-low molecular resins blends existed. Class and Chu [33] used the dynamic mechanical measurements to demonstrate that compatible resins with an elastomer produced a decrease in the elastic modulus at room temperature and an increase in the tan <5 peak (which indicated the glass transition temperature of the resin-elastomer blend). Resins which are incompatible with an elastomer caused an increase in the elastic modulus at room temperature and showed two distinct maxima in the tan <5 curve. 

From the dynamic mechanical spectroscopy, an increase of PTMO molecular weight from 650 to 2000 results in a decrease in both the modulus and the glass transition temperature of the final product. The SAXS results indicate that a correlation distance exists in the samples, and this distance increases as PTMO molecular weight increases. A cluster model is thus suggested to account for the experimental results. 

Various types of power law relaxation have been observed experimentally or predicted from models of molecular motion. Each of them is defined in its specific time window and for specific molecular structure and composition. Examples are dynamically induced glass transition [90,161], phase separated block copolymers [162,163], polymer melts with highly entangled linear molecules of uniform length [61,62], and many others. A comprehensive review on power law relaxation has been recently given by Winter [164],... 

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