Macromolecules zyxwvu 1996,28, zyxwvu 589-595 689 Enthalpies of Mixing in Polymer Blends of Chlorinated Polymers: Application of a Group Contribution Method A. Etxeberria,* M. Iriarte, and J. J. Iruin Departamento de Ciencia y Tecnologia de Polimeros, Facultad de Quimica, Universidad del Pais Vasco, P.O. Box 1072,20080 San Sebastian, Spain Received April 1, 1994; Revised Manuscript Received October 17, 1994@ ABSTRACT: zyxwvutsr A modified Guggenheim quasichemical method (MGQ) has been applied to calculate enthalpies of mixing in mixtures based on chlorinated polymers such as poly(epichlor0hydrin) (PECH) or poly(viny1chloride)(PVC). The required parameters have been determined from experimental heats of mixing in mixtures of model compounds. Using the MGQ method, enthalpies of mixing of PECWester containing polymer mixtures and PVC/polyoxide blends have been simulated. In PECWester containing polymer blends, the MGQ method gives an adequate picture of the evolution of the miscibility with the polyester CHJCH2COO ratio. Similarly, in PVC/polyoxideblends, the variation of the heat of mixing along the polyoxide family seems to agree with the miscibility window of these blends. However, in PVC/poly(ethyleneoxide)blends, the MGQ method was not able to predict accurately the experimentally observed dependence of the miscibility on the composition. Introduction Different factors should be taken into account in considering the miscibility of polymer blends. The presence of specific interactions is particularly relevant due to the small contribution of the combinatorial entropy.lP2 However, other terms such as the noncom- binatorial entropy or the free-volume effects can play an important role in the free energy of mixing.3 In this sense, a magnitude such as the enthalpy of mixing allows us to know more about the nature of the interac- tions and their role in the miscibility of these systems. Unfortunately, it is not possible to measure accurately enthalpies of mixing in polymer blendsS4 Some alterna- tive ways have been proposed to solve this problem. The use of the Hess cycle516 and the use of mixtures of analogue compound^^-^ can provide an acceptable ap- proach to the heat of mixing and to the polymer- polymer miscibility. In this framework, Lai et al.1° have compared the possibilities of some group contribution methods to quantify thermodynamic magnitudes in mixtures with specific interactions. They have demonstrated the major ability of a modified Guggenheim quasichemical method (MGQ)11J2 to reproduce the enthalpy of mixing of polar mixtures. Like in any other group contribution method, each molecule has to be divided into some structural groups, and the formation of a contact (i-j) between two different groups (i-i and j-j) is defined through the quasichemical reaction 1 zyxwvutsrq AV%J i-j 1 zyxwvutsrq 2 2 - i-i + - j-j - where A1J and hE, are the exchange entropy and energy, respectively. In order to generalize the model, it is necessary to define an additional parameter, To, which takes into account the nonrandomness of the mixture. r, is defined by Panayiotou and Vera13 as where N, is the number of i-j contacts in the mixture and zyxwvutsr Nu has the same meaning but in a totally random mixture. TI, = NlJfN, (1) * To whom correspondenceshould be addressed. @ Abstract published in Advance ACS Abstracts, January 1, 1995. In a simplified picture of the thermodynamic factors involved in polymer-polymer miscibility, Lai et al.l0 identified the enthalpy of mixing, calculated by this method, with the so-called interaction energy density (B) of the polymer blend, In eq 2 V is the total volume of the mixture and & the volume fraction of component i. B is a magnitude directly related to the excess free energy of mixing. In writing eq 2 the combinatorial entropy is neglected as well as other contributions to the free energy of mixing such as free volume or noncombinatorialentropic terms. In spite of some successful r e s ~ l t s , ~ ~ J * J ~ this last assumption would probably fail in systems where these other contributions play an important role.16 A later combination of the MGQ method and the lattice fluid theory had tried to improve the possibilities of the method in predicting phase diagrams.17 In this work we will restrict the use of the MGQ method to calcula- tions of the enthalpy of mixing. Along this work, we will study systems containing a chlorinated component as the first component. More specifically, we will consider poly(viny1chloride) (PVC)/ polyoxide blends and poly(epich1orohydrin) (PECH) mixtures in which the second component is an ester- containing polymer such as acrylates, methacrylates, aliphatic esters, and dialkyl itaconates (the repeating units of PECH and itaconates are presented below). CH2COO(-CH2-),CH3 I I (-OCH2CH-), (- CHzC-), COO(-CH2-),CH3 I CHzCl PECH poly(itaconates); when zyx rn = 0; PDMI; rn = 1 ; PDEI, , , , We are interested in these systems for several rea- sons. The polymer-polymer mixture composed of PECH and poly(methy1 acrylate) (PMA)is a qualified member of the blend family involving PECH and polyesters, which has been extensively studied by Fernandes et a1.18-20 As far as the PECWPMA blend is concerned, it has been studied in the literature18*21122 and in our 1aborato1-y~~ in a wide range of temperatures and compositions. There is an open discussion in the literature about the nature of the interactions relating 0024-9297/95/2228-0589$09.00/0 0 1995 American Chemical Society