Antiplasticization of a polyamide: a positron annihilation lifetime spectroscopy study A. Garcia a , M. Iriarte a , C. Uriarte a , J.J. Iruin a , A. Etxeberria a, * , J. del Rio b a Departamento de Ciencia y Tecnologia de Polimeros, Instituto de Materiales Polimericos (POLYMAT), Universidad del Pais Vasco UPV/EHU, P.O. Box 1072, 20080 San Sebastian, Spain b Departamento Fisica de Materiales, Facultad de Fisicas, Universidad Complutense de Madrid, 28040 Madrid, Spain Received 28 May 2003; received in revised form 28 January 2004; accepted 10 February 2004 Abstract In order to modify the transport properties of the amorphous polyamide Trogamid-Te, several additives with structural characteristics that produce antiplasticization have been tested. Positron annihilation lifetime spectroscopy (PALS) has been used to determine both the size of free volume holes and the number of holes (free fractional free volume). Polyvinyl phenol (PVPh), 1,5 dihydroxy naphthalene (Ndiol) and a hexafluorinated Bisphenol A (HFBA) were chosen according to the results of a preliminary study based on density and PALS measurements with 15% of additive blends. Their blends with Trogamid show different behaviours: HFBA behaves clearly as a plasticizer while the polyvinyl phenol acts as an antiplasticizer. Ndiol has a more special effect: blend densities are greater than additive whereas the hole size decreases. q 2004 Elsevier Ltd. All rights reserved. Keywords: Antiplasticization; Positron annihilation lifetime spectroscopy; Polyamide 1. Introduction The incorporation of small amounts of low molecular additives into glassy polymers often leads to an increase in stiffness as a result of the phenomenon that has been termed as antiplasticization. Variations of free volume have been used to explain the effects of this phenomenon [1,2] in polymeric materials. In that case, the inclusion of small molecules into a polymer raises the rigidity of the material with a subsequent suppression of motions in the polymer chain. Because of that, there are changes in physical properties of the polymeric materials. For instance, modulus and strength increase whereas ductility and permeability to gases decrease [3–13], there is a suppression of sub-T g relaxation processes [14], etc. Investigations of the antiplasticization phenomenon have been mainly carried out in poly(vinyl chloride) (PVC) and polycarbonate (PC) [6,7,15 – 18]. It is however known that certain kinds of polyamides [15], poly(methyl methacrylate) [15], polysulfone [19], polyhydroxyether [19] and poly- styrene [1,15] may be antiplasticized as well. The effect of various antiplasticizers on the gas permeability of poly- sulfone (PSF) and poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) was studied in detail by Maeda and Paul [10–12]. In a given polymer, the degree of the antiplasticizing effect, depends on the type and quantity of the additives used. Additives that act as antiplasticizers should be compatible with the polymer and their structures usually possess some of the following characteristics: they are relatively planar, rigid and contain specific atoms such as halogen, nitrogen, oxygen, etc. The majority of the existing antiplasticizers are low molecular weight substances and most studies of antiplasticization have been devoted to the polymer – diluent interactions. On the other hand, until few years ago, attributing antiplasticization to a decrease in free volume was solely based on density measurements and theoretical calculations. Now, positron annihilation lifetime spectroscopy (PALS) should be considered as the unique method that gives most direct and detailed information on free volume. The PALS technique has been widely used in the study of polymeric systems at molecular level in the recent years [20–23]. 0032-3861/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2004.02.045 Polymer 45 (2004) 2949–2957 www.elsevier.com/locate/polymer * Corresponding author. Tel.: þ 34-943-01-53-49; fax: þ 34-943-21-22- 36. E-mail address: popetlia@sq.ehu.es (A. Etxeberria).