Relaxation transitions crystalline polymers

The temperature dependence of the compliance and the stress relaxation modulus of crystalline polymers well above Tf is greater than that of cross-linked polymers, but in the glass-to-rubber transition region the temperature dependence is less than for an amorphous polymer. A factor in this large temperature dependence at T >> TK is the decrease in the degree of Crystallinity with temperature. Other factors arc the reciystallization of strained crystallites ipto unstrained ones and the rotation of crystallites to relieve the applied stress (38). All of these effects occur more rapidly as the temperature is raised. 

Some polymorphic modifications can be converted from one to another by a change in temperature. Phase transitions can be also induced by an external stress field. Phase transitions under tensile stress can be observed in natural rubber when it orients and crystallizes under tension and reverts to its original amorphous state by relaxation (Mandelkem, 1964). Stress-induced transitions are also observed in some crystalline polymers, e.g. PBT (Jakeways etal., 1975 Yokouchi etal., 1976) and its block copolymers with polyftetramethylene oxide) (PTMO) (Tashiro et al, 1986), PEO (Takahashi et al., 1973 Tashiro Tadokoro, 1978), polyoxacyclobutane (Takahashi et al., 1980), PA6 (Miyasaka Ishikawa, 1968), PVF2 (Lando et al, 1966 Hasegawa et al, 1972), polypivalolactone (Prud homme Marchessault, 1974), keratin (Astbury Woods, 1933 Hearle et al, 1971), and others. These stress-induced phase transitions are either reversible, i.e. the crystal structure reverts to the original structure on relaxation, or irreversible, i.e. the newly formed structure does not revert after relaxation. Examples of the former include PBT, PEO and keratin. 

This paper presents some of our results on the synthesis and structure of thermotropic main-chain liquid crystalline polyethers based on bis(4-hydroxy-phenoxy)-p-xylene. It also deals with two areas in the field of liquid crystalline polymers that have received only little attention, namely the dielectric relaxation (5-10 and Gedde, U.W. Liu, F. Hult, A. Gustafsson, A. Jonsson, H. Boyd, R.H. Polymer submitted) and the kinetics of isotropic-mesomorphic state transitions (11-14. 32). They are both very important for the understanding of the nature of the mesomorphic state in polymers and for the understanding of similarities and differences of physical phenomena between liquid crystalline and semi-crystalline polymers. 

In crystalline polymers, the principal relaxation process is associated with melting. In polyethylene, a (1-. and 7-transit ions have been identified and, particularly in higb-tlensity polyethylene, the a-transition has been sub-divided into a and a. In ethylene-based polymers, the y-transitions at —120°C is generally associated with the amorphous phase, in particular, with crankshaft motion of methylene sequences. " However, based upon studies of solution grown lamellae, it has also been suggested that this may llien be associated with... 

Small molecular mass liquid crystals do not respond to extension and shear stress. Liquid crystalline polymers may exhibit a high elastic state at some temperature due to the entanglements. However, the liquid crystalline network itself is an elastomer, showing rubber elasticity. In the presence of external stress, liquid crystalline networks deform remarkably and then relax back after the release of stress. The elasticity of liquid crystalline networks is more complicated than the conventional network, such as the stress induced phase transition, the discontinuous stress-strain relationship and the non-linear stress optical effect, etc. 

A modulus value increase upon storage under ambient conditions is also reported for other semi-crystalline polymers like, for instance, polypropylene. Struik [11] measured for PP a continuously increasing dynamic stiffness at 20°C in combination with a decrease of the intensity of the glass-rubber (S) transition of PP (the temperature location of the S-transition did not change). Struik called this phenomenon an amorphous phase ageing effect a densification process of the amorphous PP phase due to a free volume relaxation effect. 

The shape of the mastercurve is related to the polymer microstructure. That for polystyrene at 100 °C (Fig. 7.5b) shows a transition from a glassy compliance at Is to a rubbery one at times exceeding 10 s. It continues to 10 ° s, so it can be used for extrapolation to times longer than those accessible by experiment. Time-temperature superposition for semi-crystalline polymers, such as polyethylene, may be successful for a limited temperature range, i.e. 20°C-80°C. As polyethylene starts to recrystallise if heated within 50 °C of T, and residual stresses may start to relax, data for higher temperatures will not superimpose. 

Polymers can be divided into two broad classes, amorphous and semicrystalline. If observations are made at a fixed frequency, or isochronally, crystalline polymers often exhibit three major transitions as the temperature is varied, usually labelled a, p and y in decreasing order of temperature, whereas amorphous polymers generally exhibit two major transitions, labelled a and p in decreasing order of temperature. If other relaxations are seen at lower temperatures, they are labelled y or 8, respectively. 

Boyer also reported a transition at still higher temperature, T p Tll + 50 1.47 g, which he named the intramolecular relaxation transition, which separates structured polymer melts from true liquids [Boyer, 1977, 1980a,b, 1985, 1987]. This high-temperature transition may be related to melting. Formally, liquids remain supercooled below the melting point, 1.5Tg [van Krevelen, 1997]. Detection of this temperature is contingent on the polymer achieving sulHcient crystalline content. For example, poly(vinyl chloride), usually treated as an amorphous polymer, shows Tm = 444 to 452 K when the crystalline content of the syndiotactic isomer is about 2wt% [Marshall, 1994]. 

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