@article{wolak_white_2005, title={Factors that allow polyolefins to form miscible blends: polyisobutylene and head-to-head polypropylene}, volume={38}, ISSN={["1520-5835"]}, DOI={10.1021/ma052162r}, abstractNote={Solid-state CODEX NMR experiments directly probe chain dynamics in pure polyisobutylene (PIB), head-to-head polypropylene (hhPP), and their binary blend. The conclusions we draw from the results of the CODEX experiments, which are conducted over a temperature range of 210−300 K and which probe frequencies of 5−10 Hz, indicate that the overall chain dynamics of the blend system exceeds that of the two unmixed polymers. This is in agreement with our previous work on miscible PIB/polyethylene-co-butene blends [Macromolecules 2003, 36, 4844−4850], in which configurational entropy was shown to increase for miscible polyolefins. The CODEX experiments on PIB/hhPP, which offer the advantage of direct and simultaneous observation of both chains in the blend, show that conformational reorientation in hhPP increases significantly upon blending with PIB, while PIB chain dynamics are relatively unchanged. Rotating frame spin−lattice relaxation experiments support this conclusion. The benefits of direct chain observati...}, number={25}, journal={MACROMOLECULES}, author={Wolak, JE and White, JL}, year={2005}, month={Dec}, pages={10466–10471} }
 @article{wachowicz_wolak_gracz_stejskal_jurga_mccord_white_2004, title={Length scales which perturb chain packing in amorphous polymers}, volume={37}, ISSN={["1520-5835"]}, DOI={10.1021/ma049263u}, abstractNote={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 2H and 129Xe 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 Tg.}, number={12}, journal={MACROMOLECULES}, author={Wachowicz, M and Wolak, J and Gracz, H and Stejskal, EO and Jurga, S and McCord, EF and White, JL}, year={2004}, month={Jun}, pages={4573–4579} }
 @article{wolak_knutson_martin_boyle_sargent_white_2003, title={Dynamic disorder and conformer exchange in the crystalline monomer of polycarbonate}, volume={107}, ISSN={["1520-5207"]}, DOI={10.1021/jp036527q}, abstractNote={Direct measurement of chemical exchange events in the crystalline polycarbonate monomer 4,4‘- isopropylidenediphenol (bisphenol A, BPA) via 2D 13C solid-state NMR reveals slow, large-amplitude aromatic ring reorientations. X-ray diffraction, however, indicates a static crystalline structure. Experiments with multiple exchange times show that ring flips occur in all of the three unique conformers found in the crystalline unit cell of this compound, but in specific cases, two of the three unique molecules actually switch conformations. Kinetic analysis of the exchange data indicates that the average rate constant kex = 0.01 s-1 for ring flips and conformer interchange at room temperature. Differential scanning calorimetry and variable-temperature powder diffraction studies indicate a systematic volume expansion that accompanies this motion but no first-order phase transition. All room-temperature exchange events may be quenched at 213 K, at least on the time scale (up to several seconds) probed in this work...}, number={48}, journal={JOURNAL OF PHYSICAL CHEMISTRY B}, author={Wolak, JE and Knutson, J and Martin, JD and Boyle, P and Sargent, AL and White, JL}, year={2003}, month={Dec}, pages={13293–13299} }
 @article{jia_wolak_wang_white_2003, title={Independent calibration of H-1 spin-diffusion coefficients in amorphous polymers by intramolecular polarization transfer}, volume={36}, ISSN={["0024-9297"]}, DOI={10.1021/ma0215316}, abstractNote={We report a general method, based on intramolecular spin diffusion, for the measurement and calculation of spin-diffusion coefficients in amorphous polymers and their blends using only NMR data. The basic structural unit that defines 1H polarization density in polymers is the monomer unit. Using appropriately selected internal reference distances calculated from energy-minimized chain dimension simulations, time scales for the redistribution of 1H polarization within amorphous homopolymers may be used to independently calculate maximum values of the spin-diffusion coefficients D. This strategy represents an attractive alternative to current methods employed for domain size measurements in polymer blends, which require calibration of spin-diffusion coefficients based on comparisons of similar NMR data obtained on model compounds analyzed previously using scattering or microscopy techniques. In this way, many more polymer systems become amenable to study by NMR spin-diffusion methods, since X-ray scattering...}, number={3}, journal={MACROMOLECULES}, author={Jia, X and Wolak, J and Wang, XW and White, JL}, year={2003}, month={Feb}, pages={712–718} }
 @article{wolak_jia_gracz_stejskal_white_wachowicz_jurga_2003, title={Polyolefin miscibility: Solid-state NMR investigation of phase behavior in saturated hydrocarbon blends}, volume={36}, ISSN={["0024-9297"]}, DOI={10.1021/ma0301449}, abstractNote={Chain-level mixing in polyolefins is investigated for blends of polyisobutylene (PIB) and polyethylene-co-1-butene (PEB). Previous reports suggest that PIB exhibits unusual mixing behavior in certain saturated blends relative to other polyolefins, even though it is immiscible with most. Variable-temperature 1H, 2H, 13C, and 129Xe NMR experiments are used to characterize local PIB chain dynamics in blends with PEB in which the concentration of 1-butene comonomer units is 23 or 66 wt %. Results from 1D and 2D solid-state 13C exchange experiments, 1H relaxation measurements, and 2H line shape analysis indicate that local conformational dynamics of the PIB CH2 group in the polymer backbone increase significantly in blends with PEB copolymers containing 66 wt % butene comonomer (PEB-66). Even though the PEB-66 is a higher Tg polymer than PIB, PIB exhibits a lower effective Tg when the blend is formed relative to its pure state. Similar perturbations are not observed in the PIB/PEB-23 blend, indicating that thi...}, number={13}, journal={MACROMOLECULES}, author={Wolak, J and Jia, X and Gracz, H and Stejskal, EO and White, JL and Wachowicz, M and Jurga, S}, year={2003}, month={Jul}, pages={4844–4850} }
 @article{wolak_jia_white_2003, title={The  glass transition time scale and configurational entropy in polymers: An experimental molecular view}, volume={125}, DOI={10.1021/ja037736b}, number={45}, journal={Journal of the American Chemical Society}, author={Wolak, J. E. and Jia, X. and White, J. L.}, year={2003}, pages={13660–13661} }