Relaxation processes of polymers

Anew experimental method based on the polarization-selective photochromic reactions is proposed to monitor extremely slow reorientation dynamics of molecular tracers in glassy polymer matrix. The correlations between the local relaxation processes of polymers and the reorientation dynamics of the tracers with different sizes are found from the experimental results obtained by this method. 

The fluorescence decay times of excited states are such that the fluorescence depolarization technique may only be used to examine relatively high frequency relaxation processes of polymers. Consequently fluorescence depolarization has been primarily limited to the study of relaxation processes of polymers in solution. The anisotropy of a system, r t), is derived from measurements of the fluorescence decays with polarizations parallel and perpendicular to the polarization of excitation ... 

The purpose of this paper is to establish the fundamental links between the glass transition, viscoelastic relaxation, and yield stress by investigating the relaxation processes in polymers. The relationship between temperature and relaxation time scale is represented by a shift factor (a). At temperature T... 

Many fundamental material properties are accessible in rheological and mechanical testing experiments [60]. Rheological properties are not only very relevant for the processing of polymers, they are also the basis for understanding chain motion and relaxation processes in (linear) polymers. Relatively few rheological studies have been reported on PPC, often only in combination with the processing of PPC [15,61]. 

Quantitative comparison of dipole relaxation characteristics of polymers listed in Table 10, shows that at temperatures Tr (i.e. lower than Tg) a relaxation process,... 

A detailed comparative study of dielectric behaviour of smectic and nematic polymers was carried out for polymers of acrylic and methacrylic series, containing identical cyanbiphenyl groups (polymers XI and XII) 137 138>. The difference in structural organization of these polymers consists in a more perfect layer packing of smectic polymer XI (see Chaps. 4.1 and 4.2) with antiparallel orientation of CN-dipoles. This shifts the relaxation process of CN-dipole reorientation to a low frequency region compared to nematic polymer XII. Identification of Arrhenius plots for dielectric relaxation frequencies fR shows that for a smectic polymer the value of fR is a couple of orders lower than for a nematic polymer (Fig. 21). Though the values... 

We do not intend to discuss aspects of the use of cyclic strain conditions to obtain information on viscous, visco-elastic, and relaxation properties of polymers, although this, by itself, may present a significant interest for theoretical and experimental research. The aim of this publication is to review recent works which provide a basis for various technical applications, i.e., facilitate rearrangement of molding processes. 

Of the diluents known to affect the dynamic relaxation behavior of polymers in the glassy state, water has so far received the greatest attention. Many polymers, which in the dry state are lacking any secondary relaxation process at temperatures from 77 to 273 K, e.g. poly(methyl methacrylate)135, polymethacrylamide136, cellulose and its derivatives137, collagen138, polysulfones139, poly(2,6-dimethylphenylene oxide)139, and others,... 

PTHPMA is a good example of the analysis of relaxational processes in polymers containing saturated and substituted side rings, and, allow to an understanding the origin of the molecular motions responsible of the fast relaxational complex processes in these systems. 

PTC) is a family of polymers whose thermal properties, Tg, unperturbed dimensions and partial specific volumes has been reported [248-252], The dielectric properties of these polymers were recently studied [253-256], Relaxational studies on poly(thiocarbonate)s are scarce, but on the contrary there is much information about relaxation processes of the analogues poly(carbonate)s (PC). Therefore the study of PTC is an interesting approach to get confidence about the motions responsible of the relaxational behavior of these polymers. 

Fig. 3.12 (a) A pom-pom with three arms at each branch point (q = 3). At short times the polymer chains are confined to the Doi-Edwards tuhe. Sc is the dimensionless length of branch point retraction into the tube X is the stretch ratio where L is the curvilinear length of the crossbar and Lq is the curvilinear equilibrium length, (b) Relaxation process of a long-chain-branched molecule such as LDPE. At a given flow rate e the molecule contains an unrelaxed core of relaxation times t > g 1 connected to an outer fuzz of relaxed material of relaxation t < g 1, behaving as solvent. [Reprinted by permission from N. J. Inkson et al., J. Rheol., 43(4), 873 (1999).]... 

Because Dcr is proportional to the content of Ag nanocrystals and Ai/2 is in inverse proportion to their mean size d [77, 78], one may conclude that the state and amount of Ag nanocrystals were not affected by the cryopolymerization. Sharp growth of DCI at heating of obtained metal-polymer films specifies that the main part of Ag at 77 K is in a form of small noncrystalline Ag clusters, which aggregate with formation of nanocrystals under action of thermal relaxation processes in polymer matrix. According to data in work [79], in UV vis spectra of PPX films on a background of the PPX absorption only absorption bands of Ag with n> 15 could be observed in open range of PPX spectrum at X >320 nm. Because in this spectral range... 

In the temperature interval of —70 to 0°C and in the low-frequency range, an unexpected dielectric relaxation process for polymers is detected. This process is observed clearly in the sample PPX with metal Cu nanoparticles. In sample PPX + Zn only traces of this process can be observed, and in the PPX + PbS as well as in pure PPX matrix the process completely vanishes. The amplitude of this process essentially decreases, when the frequency increases, and the maximum of dielectric losses have almost no temperature dependence [104]. This is a typical dielectric response for percolation behavior [105]. This process may relate to electron transfer between the metal nanoparticles through the polymer matrix. Data on electrical conductivity of metal containing PPX films (see above) show that at metal concentrations higher than 5 vol.% there is an essential probability for electron transfer from one particle to another and thus such particles become involved in the percolation process. The minor appearance of this peak in PPX + Zn can be explained by oxidation of Zn nanoparticles. 

For a polymer solution, p ss 10 2 Ps, p k, 1 g/cm3, and the size of macro-molecular coil is a k, 10-5 cm, which allow us to estimate the relaxation time r 10 1° s. Processes with relaxation times so small are not essential when compared to other relaxation processes in polymer solutions. 

Next, we discuss NMR results on the /1-process of polymers to ascertain similarities and differences of the secondary relaxation in molecular and polymeric glasses. 

Other complications may arise in dielectric relaxation spectra of polymers from chain branching, which may introduce a distinct relaxation process connected with molecular motion at a branch point, and from crosslinking, which greatly restricts certain kinds of molecular movement. 

Fig. 3.14 Generalised pattern of dielectric relaxation processes in polymers.

In the preceding sections, we have looked at the various types of relaxation processes that occur in polymers, focusing predominantly on properties like stress relaxation and creep compliance in amorphous polymers. We have also seen that there is an equivalence between time (or frequency) and temperature behavior. In fact this relationship can be expressed formally in terms of a superposition principle. In the next few paragraphs we will consider this in more detail. First, keep in mind that there are a number of relaxation processes in polymers whose temperature dependence we should explore. These include ... 

Chain relaxation processes of uniaxially stretched polymer chains an infrared dichroism study... 

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