Length Scales Which Perturb Chain Packing in Amorphous Polymers Marcin Wachowicz, † Justyna Wolak, Hanna Gracz, Edward O. Stejskal, Stefan Jurga, † Elizabeth F. McCord, ‡ and Jeffery L. White* Department of Chemistry, Campus Box 8204, North Carolina State University, Raleigh, North Carolina 27695-8204 Received April 15, 2004; Revised Manuscript Received April 21, 2004 ABSTRACT: Direct spectroscopic probes of individual chain conformation and free volume are used to measure the increasing perturbation in the local glass-transition temperature of one polymer chain with decreasing length scale of mixing in binary polyolefin blends. Solid-state 2 H and 129 Xe NMR experiments reveal a compositional miscibility window in side-chain concentration for polyisobutylene (PIB)/poly- (ethylene-co-butene) (PEB) blends. A combination of pulsed-field gradient and chemical shift data for xenon gas absorbed in these polymer blends indicates that the presence of polymer chains within a radius of ∼35 nm of a different chain structure will perturb the intermolecular packing contribution to the total conformational energy of that chain, thereby changing its T g. Introduction Kumar and co-workers have recently published a theoretical paper in which they question the length scales that control dynamics in miscible polymer blends specifically and polymer chains in general. 1 Key con- siderations in that work include the influence of con- centration fluctuations, chain connectivity effects, the size of dynamic heterogeneities or cooperatively re- arranging regions, and their temperature dependencies. As evidenced by the large number of recent publications in this general area, these topics are relevant to structure-property relationships in both polymers and inorganic glasses, 2-5 the glass transition length scale, 6-11 the glass transition time scale, 12 and structure relax- ation. 13-15 While much of the recent published work has attempted to discern the size or length scale of coopera- tive motions giving rise to the glass transition, we feel an equally important question for blends of amorphous molecules has to do with what length scale of mixing in a binary mixture is required to change the glass transition characteristics (temperature or time scale) of either component relative to their pure states. Stated differently, one could ask what minimum length scale of concentration fluctuation (or domain size) is required to render all molecules in that region “dynamically perturbed” relative to those same molecules in their pure, bulk environment. The ramifications of this issue are clear for the physical properties of binary polymer blends but are also relevant to the general behavior of polymer interfaces, surfaces, and thin films. In this contribution, we address the question of length scales experimentally using local spectroscopic probes of polymer chain dynamics and free volume in binary blends of nonassociating polyolefins. Solid-state NMR experiments reveal that different perturbations of the chain-level T g take place in blends of polyisobutylene (PIB) and poly(ethylene-co-1-butene) (PEB), in which the 1-butene comonomer concentration is varied among the blends. Specifically, we monitor the change in local chain dynamics of PIB as the length scale of mixing with PEB changes in the blends. A combination of static 2 H (a noninvasive, direct probe of polymer chain dynamics) and 129 Xe (a noninvasive, direct probe of local chain packing) NMR experiments, as well as 129 Xe pulsed-field gradient diffusion (PFG) NMR, in these solid polymer blends reveals that concentration fluctuations of 60- 70 nm (several times R g ) lead to measurable changes in the dynamics of chains included in those regions, relative to their pure bulk dynamics. Experimental Section Copolymers of hydrogenated/perdeuterated polyisobutylene were prepared by cationic polymerization of isobutylene monomers. Copolymers of perdeuterated monomers and their hydrogenated analogues are referenced as follows: 80%-PIB- d 8 (Mn ) 466 000) copolymer denotes 80% perdeuterated monomers, and 20%-PIB-d8 (Mn ) 353 000) copolymer indi- cates 20% perdeuteration. Commercial PIB (Mw ) 1 000 000) obtained from ExxonMobil Chemical was used for the xenon experiments. The PEB-66 (M w ) 114 000) is the same polymer previously referenced as HPB66 by Graessley and co-workers and is a monodisperse ethylene-butene copolymer obtained by anionic polymerization of butadiene, followed by hydrogen- ation. 16-18 The degree to which the diene polymerizes 1,2 vs 1,4 addition determines the butene and ethylene concentra- tions, respectively, as has been extensively discussed in pre- vious papers. 19 The PEB-23 sample is a commerical ethylene- butene copolymer made via metallocene polymerization (Mw ) 79 000) and sold as Exact 4041 by ExxonMobil. The 50:50 wt % blends were prepared from dissolution in toluene for 24- 48 h, followed by solvent evaporation, and then vacuum-drying to 10 -2 Torr for 4 days or longer. Thermal analysis of each polymer by DSC revealed the following Tg’s: PIB )-68 °C; PEB-66 )-52 °C; PEB-23 )-39 °C; PIB/PEB-66 blend ) broad, poorly resolved transition at ca. -61 °C; PIB/PEB-23 blend ) two well-resolved transitions at -68 and -39 °C. All 2 H NMR measurements were performed on a Bruker CXP spectrometer operating at a magnetic field strength of 4.7 T, corresponding to 30.7 MHz deuteron resonance fre- quency. The spectra were recorded using the quadrupole-echo pulse sequence [(π/2) x-τ1-(π/2)y-τ2-acq] with a typical pulse width of 3.0 μs. Consequently, pure perdeuterated PIB spectra required 512 scans while the blends required up to 4000 scans to obtain spectra of sufficient sensitivity. The temperature † Department of Macromolecular Physics, Institute of Physics, Adam Mickiewicz University, ul. Umultowska 85, 61-614 Poznan, Poland. ‡ DuPont Central Research and Development, Experimental Station, Wilmington, DE. * To whom all correspondence should be addressed: e-mail Jeff_L_White@ncsu.edu. 4573 Macromolecules 2004, 37, 4573-4579 10.1021/ma049263u CCC: $27.50 © 2004 American Chemical Society Published on Web 05/21/2004