Phase transformations in high polymers
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s in the solid state are by no means unknown in high polymers but, with some exceptions in biological systems, they are of m u c h less practlcal significance than corresponding transitions in metals and alloys. In synthetic polymers a phase transition of great importance is crystallization, in which case one of the two phases involved is liquid, either m e R or solution. F r o m the standpoint of atomic (or molecular) processes or of thermodynamics, there are no differences of principle in the treatment of atomic and of macromolecular sollds, nor would any be expected. The predominant cohesive forces are of different kinds and of different intensity; in terms of consequences other than those related to electrical 9 r thermal conduction, however, this is a relatively minor detail. The essential difference in character between materials of the two types derives from the connectedness of atoms, or groups of atoms, that belong to a given long chain molecule. R is our purpose here to indicate how this connectedness influences the properties of high polymers in the solid state and the transitions they exhibit. One of the more obvious consequences of the macromolecular character of polymers is their large viscosity in the melt. As a result, m a n y polymers that are capable of crystallizing m a y be supercooled very readily, even down to a temperature, known as the glass transition temperature, below which they remain indefinitely as hard glassy solids.* Polymers that do *The glass transition is a second order thermodynamic transition of httle interest m the present context. It will not be discussed further.
not c r y s t a l l l z e s o l l d l f y a s g l a s s e s of t h i s kind. The a b i l i t y of p o l y m e r m o l e c u l e s to c r y s t a l l i z e d e p e n d s upon t h e i r c h e m i c a l r e g u l a r i t y and s u i t a b i l i t y of p r o file. Crystallization itself always involves a side-bys i d e a g g r e g a t i o n of m o l e c u l a r c h a i n s in p a r a l l e l o r i entation, but not n e c e s s a r i l y of c h a i n s with f u l l y e x t e n d e d c o n f o r m a t i o n s . P o l y e t h y l e n e and s e v e r a l o t h e r p o l y m e r s of r e l a t i v e l y s i m p l e c h e m i c a l s t r u c t u r e c r y s t a l l i z e with e x t e n d e d m o l e c u l a r c o n f o r m a t i o n s H. D. KEITH is the Head of the Analytical Chemistry Department, Bell Laboratories, Murray Hill, N. J. 07974. This paper is based on a presentation made at a symposium on "Phase Transformations in Less Common Metals: A Dialogue," held at the Fall Meeting in Cleveland on October 16, 1972, under the sponsorship of the Phase Transformations Activity, Materials Science Division, American Society for Metals. METALLURGICAL TRANSACTIONS
having repeated trans conformations of C - C bonds. Very frequently, however, bulky sidegroups cause polymer chains to adopt helical conformations, and crystallization then involves lateral accretion of these helices. As is now well known, most polymers crystallize from
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