Solution properties of poly(diphenoxyphosphazene) below the u temperature obtained by SEC/MALLS M.T.R. Laguna, E. Saiz, M.P. Tarazona * Departamento de Quõ Âmica Fõ Âsica, Universidad de Alcala Â, 28871 Alcala  de Henares, Spain Received 12 January 2000; accepted 23 February 2000 Abstract Dilute solution properties of poly(diphenoxyphosphazene), PDPP, in THF at 258C have been studied by size exclusion chromatography, using simultaneously multiangle light scattering and differential refractive index detectors. The anomalous elution behavior and the dependence of the dimensions, i.e. radius of gyration, of the polymer on the molecular weight are discussed. Coef®cients for the scaling law and unperturbed dimensions have also been calculated. Molecular Dynamics simulations of some model compounds were performed in order to analyze the conformational characteristics of the P±N±P and N±P±N pairs of skeletal bonds. The conformational model thus obtained indicates that the PDPP chain is mainly formed by sequences of about four skeletal bonds in the trans conformation separated by one bond in cis, with negligible incidence of cis ± cis orientations. The chain thus generated is very rigid so that the characteristic ratio of the unperturbed dimensions C N increases with the number of repeat units of the chain x, up to values as high as x < 800; reaching an asymptotic value of ca 30 when x ! 1; in very good agreement with the experimental result. q 2000 Elsevier Science Ltd. All rights reserved. Keywords: Solution properties; Size exclusion chromatography; Light scattering 1. Introduction Measurement of properties of polyphosphazenes in dilute solution is a powerful tool for both the characterization of these polymers and the study of their anomalous solution behavior. One of the problems frequently encountered in these measurements is the large polydispersity of the poly- mer samples due to the dif®culties observed in the fractio- nation techniques when they are applied to poly phosphazenes [1]. Thus, the molecular weight dependence of properties such as radius of gyration can be masked by the discrepancies between the different molecular weight averages used. Different polydispersity corrections or numerical procedures have been used in order to solve the problem [2]. On the other hand, the use of a widely employed techni- que, size exclusion chromatography (SEC) for the solution characterization of polyphosphazenes has the inconveni- ence that the reliability of the universal calibration techni- que [3] becomes questionable for these polymers. Both problems can be surmounted using coupled multi-angle light scattering (MALLS) and refractometric (DRI) detectors in SEC measurements [4,5]. This experimental setup allows absolute distributions for both molecular weight and root mean square radius of gyration for polypho- sphazenes from which information about their scaling law, ks 2 l 1=2  QM q ; and unperturbed dimensions, ks 2 l 1=2 0 and C N , can be deduced [6]. We have applied these methods to several copolymers of polyphosphazenes containing spiro groups [7] ®nding that the logarithmic plots of the scaling laws show a pronounced curvature at low molecular weights. This peculiar behavior has also been encountered in other polymeric structures [8,9]. Thus, it seemed interesting to study the behavior of a simpler homo polyphosphazenes, i.e. poly(diphenoxypho- sphazene), PDPP, and to compare the additional informa- tion obtained using this technique with previous literature results. Singler et al. [10] used viscosity and light scattering measurements to characterize a sample with broad molecu- lar weight distribution M n  1:3 £ 10 5 ; M w  6:07 £ 10 5 ; M w =M n  4:7 in THF solution at 308C. The behavior of the polymer in THF solution was indicative of near u condi- tions. The low values of the second virial coef®cient, A 2 , together with the observed incipient precipitation as the polymer concentration was raised, or the temperature lowered, suggested that THF was a poor solvent for the polymer. They also found an important feature of dimen- sions which appears in others polyphosphazenes: the Polymer 41 (2000) 7993±8000 0032-3861/00/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved. PII: S0032-3861(00)00169-5 * Corresponding author. Tel.: 1 34-91-885-4664; fax: 1 34-91-885- 4763. E-mail address: mpilar.tarazona@uah.es (M.P. Tarazona). www.elsevier.nl/locate/polymer