Reply:

Professor Han has three criticisms of ourarticle: our viscosity equation has no rheo-logical significance, our assumption of w = 0is unjustified, and we failed to reference his1991 article.

We agree that our viscosity equation is em-pirical. Viscosity does not enter into themodel except as a measure of mixing of thetwo diffusing polymers. As stated on page982 of our article we actually ‘‘used the ex-perimental data as a master curve, and athird-order spline interpolation method tofind the viscosity of the blends at any concen-tration.’’

We disagree that w, 0 is required for mis-cibility of two polymers. From Flory–Hug-gins theory with chains of equal length N, thecriterion for miscibility is wN = 2. However,

even if w for HDPE/LLDPE blends wereslightly negative this would not change theconclusion of our article, namely that a sin-gle, concentration dependent-diffusion coeffi-cient can model their mutual diffusion.On the last point we did review the Kim

and Han 1991 article in an early stage ofthis study. We could have included it forcompleteness on the subject of mutual dif-fusion in polymers. However, we do notfind that the subject of this article isdirectly related to our study, and nor wouldits results alter the discussion or conclu-sions of our article. In Kim and Han, theeffect of processing conditions on the inter-facial layer thickness between two misciblepolymer layers is considered. They solveunsteady state mutual diffusion into asemi-infinite space. In our article, we solvequasi-static, mutual diffusion between twomiscible polymers using 32 coextrudedlayers. Our diffusion problem is anunsteady-state finite boundary problem, i.e.mutual diffusion in the 32 layers boundedby two solid walls. Therefore, we do not

think our article simply ‘‘reinvents thewheel’’. With increasing number ofresearch groups using multilayer structuresto study mutual diffusion, we believe(along with the reviewers of the AIChEJournal) that our methodology is a valua-ble contribution.

Rui Zhao

ExxonMobil Chemical Company,

BTEC-West,

Specialty Elastomers Technology

Baytown, TX 77520

E-mail: rui.zhao@exxonmobil.com

Chris Macosko

Professor of Chemical Engineering

and Materials Science and Director

of I PRIME, The Industrial Partnership for

Research in Interfacial and Materials

Engineering, University of Minnesota,

421 Washington Av.

Minneapolis MN 55455

E-mail: macosko@umn.edu

AIChE Journal, Vol. 53, 3019 (2007)� 2007 American Institute of Chemical EngineersDOI 10.1002/aic.11284Published online September 17, 2007 in WileyInterScience (www.interscience.wiley.com).

AIChE Journal November 2007 Vol. 53, No. 11 Published on behalf of the AIChE DOI 10.1002/aic 3019