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synopsisThe predominant role of van der Waal interactions in determining the helical conformations of a simple synthetic linear polymer, as well as helical polypeptides, was pointed out in systematic studies by Liquori et a1.V In the case of homopolypeptides the conformational analysis carried out on the basis of a simple semiempirical function describing the van der Waal pairwise interactions between the non directly bonded atoms lead to the conclusion that only five helices are allowed (Ra, 8, y, 6, L01).%3In view of the close similarities with poly-Galanine, we have investigated by x-ray and conformational analysis the molecular conformation of poly(S-lactic acid) which has recently been described by Kleine and Kleine4 and Schuls and Schwaab5 and studied in solution by Goodman and D'Alagni.6 In fact, this polymer may be related to the polypeptide by the interchange of a peptide bond with ester bond along the main chain. This operation is expected to involve only a relatively small change in the steric interaction within the possible helical conformation, but obviously rules out any possibility of hydrogen bonding.

SynopsisA multiordering parameter model for glass-transition phenomena has been developed on the basis of nonequilibrium thermodynamics. In this treatment the state of the glass is determined by the values of N ordering parameters in addition to T a n d P; the departure from equilibrium is partitioned among the various ordering parameters, each of which is associated with a unique retardation time.These times are assumed to depend on T, P , and on the instantaneous state of the system characterized by its overall departure from equilibrium, giving rise to the well-known nonlinear effects observed in volume and enthalpy recovery. The contribution of each ordering parameter to the departure and the associated retardation times define the fundamental distribution function (the structural retardation spectrum) of the system or, equivalently, its fundamental material response function. These, together with a few experimentally measurable material constants, completely define the recovery behavior of the system when subjected to any thermal treatment. The behavior of the model is explored for various classes of thermal histories of increasing complexity, in order to simulate real experimental situations. The relevant calculations are based on a discrete retardation spectrum, extending over four time decades, and on reasonable values of the relevant material constants in order to imitate the behavior of polymer glasses. The model clearly separates the contribution of the retardation spectrum from the temperature-structure dependence of the retardation times which controls its shifts along the experimental time scale. This is achieved by using the natural time scale of the system which eliminates all the nonlinear effects, thus reducing the response function to the Boltzmann superposition equation, similar to that encountered in the linear viscoelasticity. As a consequence, the system obeys a rate (time) -temperature reduction rule which provides for generalization within each class of thermal treatment. Thus the model establishes a rational basis for comparing theory with experiment, and also various kinds of experiments between themselves. The analysis further predicts interesting features, some of which have often been overlooked. Among these are the impossibility of extraction of the spectrum (or response function) from experiments involving cooling from high temperatures at finite rate; and the appearance of two peaks in the expansion coefficient, or heat capacity, during the heating stage of three-step thermal cycles starting a t high temperatures. Finally, the theory also provides a rationale for interpreting the time dependence of mechanical or other structure-sensitive properties of glasses as well as for predicting their long-range behavior.As mentioned above, the set of differential equations (20) cannot be integrated analytically. Nevertheless, they represent an "autonomous system"20 subject to some simplification. Since at fixed T,aT is invariant and a6 has the same strictly positive value for every i , one ...

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