@article{frankowski_khan_spontak_2007, title={Chain-scission-induced intercalation as a facile route to polymer nanocomposites}, volume={19}, ISSN={["1521-4095"]}, DOI={10.1002/adma.200600793}, abstractNote={Polymer/clay nanocomposites (NCs) prepared using melt intercalation must be properly formulated and processed to achieve intercalation or exfoliation. Organically modified layered silicates (OMLSs) are often used to permit polymer intercalation and thereby separate the silicate platelets. Prior modeling efforts suggest that high-molecular-weight (HMW) polymers tend to yield immiscible NCs unlike their low-molecular-weight (LMW) analogs, which commonly result in exfoliated or intercalated NCs. Although formulations containing LMW polar additives (e.g., maleic anhydride copolymers) have succeeded in producing at least partially exfoliated rather than immiscible NCs, a need persists for simpler HMW formulations in order to expedite industrial scale-up. The present study examines blends composed of HMW polystyrene (PS) and an OMLS that remains largely immiscible when annealed under an inert N2 atmosphere with little or no shear. Controlled thermal-oxidative chain scission of the HMW PS in an O2-rich environment promotes intercalation of short PS chains within the OMLS, thereby expanding and disordering the platelets as an intercalated NC develops. This scission-induced intercalation mechanism ensures that the resultant NC is in a thermodynamically stable state unlike NCs prepared using in situ polymerization, which may deintercalate during further processing. Moreover, scission intercalation may be used as a general means by which to promote intercalation or possibly exfoliation in previously reported immiscible NCs. Polymeric NCs containing clay have become an increasingly important topic in the development of lightweight, tough, and impermeable materials since the Toyota research laboratories polymerized nylon-6 in the presence of sodium montmorillonite (Na-MMT) for automotive applications in the late 1980s. In situ polymerization has enjoyed the most success in producing exfoliated NCs wherein individual clay platelets are uniformly dispersed in a polymeric matrix, because the polymer chains can grow within the silicate galleries. Another NC-fabrication strategy is solution intercalation, in which polymer, solvent, and clay are mixed together. In this case, confinement of polymer chains upon diffusion into the silicate galleries is compensated for by the entropic gain caused by desorbed solvent molecules. Both in situ polymerization and solution intercalation typically require relatively large quantities of organic solvent, whereas melt intercalation relies on annealing above the glass-transition (Tg) or melting (Tm) temperature of the polymer matrix under static or shear conditions. Melt intercalation constitutes an environmentally benign and commercially feasible process that successfully yields intercalated or exfoliated NCs from more polymer–clay pairs than either in situ polymerization or solution intercalation. Giannelis and co-workers have previously demonstrated that sieved PS powder can intercalate into fluorohectorite under static annealing in an inert N2 atmosphere at 180 °C and below. At molecular weights comparable to that of the PS used in the present study (weight-average molecular weight, Mw, 330 kg mol ), noticeable intercalation requires approximately six hours. This result reflects their observation that the diffusion of PS into the silicate galleries under quiescent conditions becomes increasingly difficult with increasing chain length. Complementary studies have shown that twin-screw extrusion of HMW PS and OMLS with optimized screw profiles can overcome diffusion limits associated with static annealing at high temperatures, as well as circumvent undesirable dispersion issues. Under routine operating conditions, however, we expect sufficient concentrations of O2 to be present [31] for thermal-oxidative chain scission to occur. Indeed, an increase in temperature or addition of clay have been independently found to increase the rate of PS chain scission in the melt. Although most studies do not report Mw for the polymer after extrusion, we propose that the sequential chain-scission–intercalation mechanism described below may be at least partially responsible for the intercalation of polymer into OMLSs and the corresponding formation of NCs from HMW PS. Our system was designed to be commercially relevant as resins are usually obtained in pellet form and the screw of an extruder may not always be optimized for clay intercalation because of multiple products being run on a common manufacturing line. Single-screw extrusion of HMW PS and Cloisite 15A OMLS at a melt temperature of 185 °C yielded an immiscible NC according to X-ray diffractometry (XRD). As discussed later, extruded PS/OMLS exhibited a principal peak C O M M U N IC A TI O N}, number={9}, journal={ADVANCED MATERIALS}, author={Frankowski, David J. and Khan, Saad A. and Spontak, Richard J.}, year={2007}, month={May}, pages={1286-+} }
 @article{wang_wang_frankowski_lam_welch_winnik_hartmann_manners_spontak_2007, title={Growth and crystallization of metal-containing block copolymer nanotubes in a selective solvent}, volume={19}, ISSN={["1521-4095"]}, DOI={10.1002/adma.200602230}, abstractNote={Due to their inherent ability to microphase-segregate into well-defined nanostructures, AB diblock copolymers serve as model surfactants to probe complex phase behavior [1] and precursor materials to nanoscale-templated motifs. [2] ^ In the melt, alone or in miscible blends with other (co)polymers, [3] AB molecules can spontaneously self-organize into a variety of nanoscale morphologies, the size and shape of which reflect the packing of A and B repeat units along the interface sepa-rating A-and B-rich spatial regions. Interfacial chain packing and, hence, curvature are strongly dictated by factors such as the size, number and incompatibility of the A and B repeat units along the copolymer backbone. [4] Most block copolymer morphologies of fundamental and technological interest [5] are periodic and are generally classified as either classic or complex, [6] depending on topological metrics such as symmetry, coordination and genus. [7] Aperiodic copolymer melt morphologies described as sponges [8,9] or bicontinuous emulsions [10] are also of contemporary interest. This wealth of block copolymer phase behavior is preserved when copolymer molecules microphase-separate in the presence of one or two selective solvents. [11] In addition to those copolymer nano-structures observed in the melt, however, novel solvated supramolecular aggregates (e.g., crew-cut micelles, [12] nano-tubes, [13] “hamburger” micelles, [14] polymersomes [15] and toroids) [16–18] and network nanostructures [19] likewise develop under favorable conditions. Solvated}, number={17}, journal={ADVANCED MATERIALS}, author={Wang, Xiaosong and Wang, Hai and Frankowski, David J. and Lam, Peter G. and Welch, Paul M. and Winnik, Mitchell A. and Hartmann, Juergen and Manners, Ian and Spontak, Richard J.}, year={2007}, month={Sep}, pages={2279-+} }
 @article{gil_frankowski_hudson_spontak_2007, title={Silk fibroin membranes from solvent-crystallized silk fibroin/gelatin blends: Effects of blend and solvent composition}, volume={27}, ISSN={["0928-4931"]}, DOI={10.1016/j.msec.2006.05.017}, abstractNote={Protein membranes have been prepared by mixing gelatin (G) with Bombyx mori silk fibroin (SF) and using aqueous methanol (MeOH) to induce SF crystallization. Amorphous blends of these polymers appear quasi-homogeneous, as discerned from visual observation, electron microscopy and Fourier-transform infrared (FTIR) spectroscopy. Upon subsequent exposure to aqueous MeOH, SF undergoes a conformational change from random-coil to β-sheet. This transformation occurs in pure SF, as well as in each of the G/SF blends, as discerned from FTIR spectroscopy and thermal calorimetry. The influence of MeOH-induced SF crystallization on structure and property development has been measured as functions of blend and solvent composition. By preserving a support scaffold above the G helix-to-coil transition temperature, the formation of crystalline SF networks in G/SF blends can be used to stabilize G-based hydrogels or generate SF membranes for biomaterial, pharmaceutical and gas-separation purposes. The present study not only examines the properties of G/SF blends before and after SF crystallization, but also establishes the foundation for future research into thermally-responsive G/SF bioconjugates.}, number={3}, journal={MATERIALS SCIENCE & ENGINEERING C-BIOMIMETIC AND SUPRAMOLECULAR SYSTEMS}, author={Gil, Eun S. and Frankowski, David J. and Hudson, Samuel M. and Spontak, Richard J.}, year={2007}, month={Apr}, pages={426–431} }
 @article{frankowski_capracotta_martin_khan_spontak_2007, title={Stability of organically modified montmorillonites and their polystyrene nanocomposites after prolonged thermal treatment}, volume={19}, ISSN={["0897-4756"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-34250177310&partnerID=MN8TOARS}, DOI={10.1021/cm061953k}, abstractNote={Melt intercalation of montmorillonite (MMT) into polymeric matrices to improve the mechanical properties of polymers has evolved into a subject of tremendous fundamental and technological interest....}, number={11}, journal={CHEMISTRY OF MATERIALS}, author={Frankowski, David J. and Capracotta, Michael D. and Martin, James D. and Khan, Saad A. and Spontak, Richard J.}, year={2007}, month={May}, pages={2757–2767} }
 @article{gil_frankowski_bowman_gozen_hudson_spontak_2006, title={Mixed protein mends composed of gelatin and bombyx mori silk fibroin: Effects of so solvent-induced crystallization and composition}, volume={7}, ISSN={["1526-4602"]}, DOI={10.1021/bm050622i}, abstractNote={Novel protein blends have been prepared by mixing gelatin (G) with Bombyx mori silk fibroin (SF) and using aqueous methanol (MeOH) to post-induce SF crystallization. When co-cast from solution, amorphous blends of these polymers appear homogeneous, as discerned from visual observation, microscopy, and Fourier-transform infrared (FTIR) spectroscopy. Upon subsequent exposure to aqueous MeOH, SF undergoes a conformational change from random coil to beta sheet. This transformation occurs in pure SF, as well as in each of the G/SF blends, according to X-ray diffractometry and thermal calorimetry. The influence of MeOH-induced SF crystallization on structure and property development has been ascertained in terms of preparation history and blend composition. Thermal gravimetric analysis reveals that the presence of beta sheets in SF and G/SF blends improves thermal stability, while extensional rheometry confirms that SF crystallization enhances the tensile properties of the blends. By preserving a support scaffold above the G helix-to-coil transition temperature, the formation of crystalline SF networks in G/SF blends can be used to stabilize G-based hydrogels for biomaterial and pharmaceutical purposes. The present study not only examines the properties of G/SF blends before and after SF crystallization, but also establishes the foundation for future research into thermally responsive G/SF bioconjugates.}, number={3}, journal={BIOMACROMOLECULES}, author={Gil, ES and Frankowski, DJ and Bowman, MK and Gozen, AO and Hudson, SM and Spontak, RJ}, year={2006}, month={Mar}, pages={728–735} }
 @article{gil_frankowski_spontak_hudson_2005, title={Swelling behavior and morphological evolution of mixed gelatin/silk fibroin hydrogels}, volume={6}, ISSN={["1526-4602"]}, DOI={10.1021/bm050396c}, abstractNote={Mixed protein-based hydrogels have been prepared by blending gelatin (G) with amorphous Bombyx mori silk fibroin (SF) and promoting beta-crystallization of SF via subsequent exposure to methanol or methanol/water solutions. The introduction of beta crystals in SF serves to stabilize the hydrogel network and extend the solidlike behavior of these thermally responsive materials to elevated temperatures beyond the helix-->coil (h-->c) transition of G. In this work, we investigate the swelling and protein release kinetics of G/SF hydrogels varying in composition at temperatures below and above the G h-->c transition. At 20 degrees C, G and G-rich mixed hydrogels display evidence of moderate swelling with negligible mass loss in aqueous solution, resulting in porous polymer matrixes upon solvent removal according to electron microscopy. When the solution temperature is increased beyond the G h-->c transition to body temperature (37 degrees C), these gels exhibit much higher swelling with considerable mass loss due to dissolution and release of G. The extent to which these properties respond to temperature decreases systematically with increasing SF content. The unique temperature- and composition-dependent properties of G/SF hydrogels dictate the efficacy of these novel materials as stimuli-responsive delivery vehicles.}, number={6}, journal={BIOMACROMOLECULES}, author={Gil, ES and Frankowski, DJ and Spontak, RJ and Hudson, SM}, year={2005}, pages={3079–3087} }
 @article{frankowski_fournier-bidoz_manners_ozin_khan_spontak_2004, title={Tunable microcellular morphologies from poly (ferrocenylsilane) ceramic precursors foamed in supercritical CO(2)}, volume={205}, ISSN={["1022-1352"]}, DOI={10.1002/macp.200400333}, abstractNote={Abstract}, number={18}, journal={MACROMOLECULAR CHEMISTRY AND PHYSICS}, author={Frankowski, DJ and Fournier-Bidoz, SB and Manners, I and Ozin, GA and Khan, SA and Spontak, RJ}, year={2004}, month={Dec}, pages={2398–2408} }