Mechanical Relaxations of Poly(,L-aspartate)s Xavier Ramis, 1 Montserrat Garcı ´a-Alvarez, 2 Sebastia ´n Mun ˜ oz-Guerra 2 1 Laboratori de Termodina `mica, ETSEIB, Universitat Polite `cnica de Catalunya, Diagonal 647, 08028 Barcelona, Spain 2 Departament d’Enginyeria Quı ´mica, ETSEIB, Universitat Polite `cnica de Catalunya, Diagonal 647, 08028 Barcelona, Spain Received 17 May 2005; accepted 23 June 2005 DOI 10.1002/app.22508 Published online in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: The dynamic mechanical thermal properties of a family of poly(-alkyl ,l-aspartate)s bearing various cyclic, linear, and branched alkoxycarbonyl groups in the side chain were studied. The measurements carried out by dynamic mechanical thermal analysis (DMTA) revealed the significant influence of the constitution of the side chain on mechanical relaxation phenomena. Three relaxations were observed, which are referred to as , , and , in increasing order of temperature. The first two,  and , are related to the local and global motions of the side chain, respectively. Relaxation  is related to the motion of the main chain. Relaxation , which is associated with the rotation of the side chain, is the most intense. The magnitude and temper- ature at which this relaxation occurs depends on the volume, the length, and the degree of branching of the ester group of the side chain. A comparison between the dynamic mechan- ical properties of poly(,l-aspartate)s and poly(,l-gluta- mate)s revealed that the two methylene groups spacing the ester group from the main chain provides the poly(-l- glutamate)s with greater mobility, and thus, relaxations  and  occur at lower temperatures. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 994 –1003, 2006 Key words: poly(,l-aspartate)s; mechanical relaxation; structure–property relations; poly(,l-glutamate)s INTRODUCTION Poly(-alkyl ,l-aspartate)s are nylon 3 derivatives. The alkoxycarbonyl group is stereoregularly attached to -carbon of the repeating unit. The significance of these substituted polyamides resides in the fact that their properties are intermediate between those of ny- lons and polypeptides. Poly(-isobutyl ,l-aspartate), which has been widely studied by our research group, crystallizes in helical structures with features similar to the -helix characteristic of polypeptides. 1,2 This polymer has been shown to have properties that are related to its helicoidal structure, such as piezoelec- tricity 3 and the formation of cholesteric liquid crys- tals. 4 Recent studies carried out on other poly(-alkyl ,l-aspartate)s with a great variety of side chains have shown that the formation of the helical structure is common to this family of compounds. 5–8 Further- more, poly(-alkyl ,l-aspartate)s 9 and poly(-alkyl ,l-glutamate)s 10 that have side chains of more than ten carbons have a complex biphasic structure in which the main chains form helices and the alkyl side chains are crystallized in a separated phase. No studies on the structural relaxation phenomena of poly(-alkyl ,l-aspartate)s have been carried out so far. On the contrary, a pretty wealth of research has been carried out on poly(-alkyl ,l-glutamate)s and poly(methacrylate)s, a couple of polymer families structurally related to polyaspartates. The formers are crystalline polypeptides with the main chain in the -helix conformation, whereas poly(methacrylate)s are amorphous polymers that resemble to poly(-alkyl ,l-aspartate)s, in that the alkoxycarbonyl side group is joined directly to the main chain, R being the alkyl group. Studies by means of mechanical and dielectric re- laxation in poly(methacrylates) and poly(-alkyl ,l- glutamate)s show that there are typically three relax- ations, referred to as ,  and , in increasing order of temperature. 10 –14 In both cases, relaxation  has been attributed to local motion in the alkyl end groups of the side chain. This motion usually occurs at temper- atures below -120°C and involves apparent activation energies of 4 –11 kcal mol -1 . These activation energies are similar in magnitude to the energy barrier that restricts rotation around the COC bond in simple paraffins. Relaxation  is attributed to the motions of the whole side chain, both the rotation of the ester group and cooperative motions between the side chain and groups that are adjacent to the main chain. The de- crease observed by several authors in the temperature of the relaxation , T  , both in terms of the dielectric 12 Correspondence to: X. Ramis (ramis@mmt.upc.edu). Contract grant sponsor: CICYT and FEDER; contract grant numbers: MAT2004 – 04,165-C02– 02, MAT2003– 06,955-C02– 01. Journal of Applied Polymer Science, Vol. 99, 994 –1003 (2006) © 2005 Wiley Periodicals, Inc.