Macromolecules melting

By introducing TMDSC [10-12,16] it was possible to examine directly the kinetics and thermodynamics of the reversibility and irreversibility of the latent heat of melting [12-18]. In contrast to small molecules, well-crystallized macromolecules melt irreversibly. The irreversible melting process of low molar mass POE [43] is illustrated in Figure 9.21. Results show lack of reversing heat capacity during melting of a well-crystallized poly-... 

Luengo G ef a/1997 Thin film rheology and tribology of oonfined polymer melts oontrasts with bulk properties Macromolecules 30 2482-94... 

Polyamides, like other macromolecules, degrade as a result of mechanical stress either in the melt phase, in solution, or in the soHd state (124). Degradation in the fluid state is usually detected via a change in viscosity or molecular weight distribution (125). However, in the soHd state it is possible to observe the free radicals formed as a result of polymer chains breaking under the appHed stress. If the polymer is protected from oxygen, then alkyl radicals can be observed (126). However, if the sample is exposed to air then the radicals react with oxygen in a manner similar to thermo- and photooxidation. These reactions lead to the formation of microcracks, embrittlement, and fracture, which can eventually result in failure of the fiber, film, or plastic article. 

R. Granek. Stress relaxation in polymer melts and solutions Bridging between the breathing and reptation regimes. Macromolecules 2<5 5370-5371, 1995. 

T. Matsuda, G. D. Smith, R. G. Winkler, D. Y. Yoon. Stochastic dynamics simulations of n-alkane melts confined between solid surfaces Influence of surface properties and comparison with Schetjens-Fleer theory. Macromolecules 28 65- 13, 1995. 

J. G. van Alsten, B. B. Sauer, D. J. Walsh. Polymer dynamics at the melt/solid interface Experimental evidence of reduced center of mass mobility. Macromolecules 25 4046-4048, 1992. 

S. K. Kumar, M. Vacatello, D. Y. Yoon. Off-lattice Monte Carlo simulations of polymer melts confined between two plates. 2. Effects of chain length and plate separation. Macromolecules 25 2189-2197, 1990. 

In conclusion, it may be said that a lot of literature has been published that favors the Frye and Horst mechanism of stabilization. Most of this is based on studies done on low-molecular weight model compound for al-lylicchlorines in PVC, i.e., 4-chloro-2-hexene. Although the large contribution of these studies toward understanding the mechanism of stabilization of PVC cannot be denied, the extrapolation of these results to the processes involved in the actual stabilization of the polymer should be done with extreme care. The polymer represents a complex mixture of macromolecules, which in the melt is not only physically a very different system compared to the low-molecular weight model compound, but invariably contains, apart from stabilizers, other additives, such as plasticizers, lubricants, processing aids, etc., that further complicate the situation. The criticism of the Frye and Horst mechanism is also based on solid experimental evidence, and hence, the controversy is still very much alive. 

The thickness of interphases in polymer melts and solutions may be much greater than in low-molecular matrices owing to the greater size of their macromolecules and existence in them of submolecular associates [51],... 

Two approaches to the attainment of the oriented states of polymer solutions and melts can be distinguished. The first one consists in the orientational crystallization of flexible-chain polymers based on the fixation by subsequent crystallization of the chains obtained as a result of melt extension. This procedure ensures the formation of a highly oriented supramolecular structure in the crystallized material. The second approach is based on the use of solutions of rigid-chain polymers in which the transition to the liquid crystalline state occurs, due to a high anisometry of the macromolecules. This state is characterized by high one-dimensional chain orientation and, as a result, by the anisotropy of the main physical properties of the material. Only slight extensions are required to obtain highly oriented films and fibers from such solutions. 

Many authors studying the formation of ECC from melts and solutions suggested that preliminary unfolding and extension of macromolecules occurs. Keller and Maehin25 have shown that in all known cases (including such extreme variants as the crystallization of natural rubber under extension and a polyethylene melt under flow) the same initial process of linear nucleation occurs and fibrillar structures is formed by the macromolecu-lar chains oriented parallel to the fibrillar axes27. ... 

The formation of ECC is not only an extension of a portion of the macromolecule but also a mutual orientational ordering of these portions belonging to different molecules (intermolecular crystallization), as a result of which the structure of ECC is similar to that of a nematic liquid crystal. After the melt is supercooled below the melting temperature, the processes of mutual orientation related to the displacement of molecules virtually cannot occur because the viscosity of the system drastically increases and the chain mobility decreases. Hence, the state of one-dimensional orientational order should be already attained in the melt. During crystallization this ordering ensures the aggregation of extended portions to crystals of the ECC type fixed by intermolecular interactons on cooling. 

We studied the effect of the mechanical stretching field on the conformations of the macromolecules in the melt. It is known that for a freely jointed chain the Maxwell distribution of end-to-end distances holds50). 

Figure 15 describes the decrease in the flexibility f of the macromolecules during melt stretching (corresponding to an increase in /3m) with x. According to Flory s criterion, the diminution of the flexibility of molecules to the value of f < 0.63 leads to a spontaneous transition of the system into the state of parallel order. It can be seen in Fig. 15 that f = 0. is attained at x = 30 or o = 0.6 x 107 n/m2 at these stresses, the melt is organized into a nematic state. 

Hyperbranched polymers generally have very low melt and indinsic viscosities. The large number of chain-end functional groups present in hyperbranched macromolecules have also been shown to dramatically affect physical properties... 

As we have seen previously the presence of crosslinks between macromolecules influences the way in which these materials respond to heat. Uncrosslinked polymers will generally melt and flow at sufficiently high temperatures they are usually thermoplastic. By contrast, crosslinked polymers cannot melt because of the constraints on molecular motion introduced by the crosslinks. Instead, at temperatures well above those at which thermoplastics typically melt, they begin to undergo irreversible degradation. 

When a stress is applied to the bulk polymer melt, the mass flows in the direction that relieves the stress. At the molecular level, the probability of a molecular jump becomes higher in the direction of the stress than in any other direction and hence these stress-relieving motions predominate, leading to the observed pattern of flow. There is evidence that the molecular unit of flow is not the complete macromolecule but rather a segment of the molecule containing up to 50 carbon atoms. Viscous flow takes place by successive jumps of such segments until the entire macromolecule has shifted. 

Winter, H.H. Evolution of rheology during chemical gelation. Prog. Colloid Polym. Scl,15,104—110,1987. Hempenius, M.A. et al. Melt rheology of arborescent graft polystyrenes. Macromolecules, 31, 2299, 1998. 

Leong, K. W., Simonte, V., and Danger, R., Synthesis of polyanhydrides Melt-polycondensation, dehydrochlorination, and dehydrative coupling. Macromolecules, 20. 705-712, 1987. 

Diffusion of flexible macromolecules in solutions and gel media has also been studied extensively [35,97]. The Zimm model for diffusion of flexible chains in polymer melts predicts that the diffusion coefficient of a flexible polymer in solution depends on polymer length to the 1/2 power, D N. This theoretical result has also been confirmed by experimental data [97,122]. The reptation theory for diffusion of flexible polymers in highly restricted environments predicts a dependence D [97,122,127]. Results of various... 

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