Preparation of monolithic polycaprolactone foams with controlled morphology

Polycaprolactone (PCL) foams were produced by thermally induced phase separation. Tetrahydrofuran/methanol (THF/MeOH) (solvent/non-solvent) mixture was used for the induction of liquid-liquid phase separation of PCL solutions at three different temperatures. Subsequent solvent exchange followed by vacuum drying yielded polymeric foams with different morphologies. Characterization of foams was obtained by scanning electron microscopy, x-ray diffractometry, mercury intrusion porosimetry and compression tests. Influence of polymer concentration (8, 10 and 12 wt%), quench temperature (4, −20 and −80 °C), and THF/MeOH ratio from (42/58) to (54/46) (wt/wt) on the foam formation, morphology and properties were investigated systematically. Lower PCL concentration, lower THF content and higher quench temperature lead to larger pore sizes in the foams obtained. Detailed discussions of the influence of processing parameters on foam structure and porosity, foam density, percent crystallinity and compressive properties are provided. By selectively tuning the process parameters, foams with controlled pore sizes (10–450 μm), porosity (83–91%) and morphology (cellular, bead-like, microspherical) were obtained

Multiscale Modeling of the Morphology and Properties of Segmented Silicone-Urea Copolymers

Molecular dynamics and mesoscale dynamics simulation techniques were used to investigate the effect of hydrogen bonding on the microphase separation, morphology and various physicochemical properties of segmented silicone-urea copolymers. Model silicone-urea copolymers investigated were based on the stoichiometric combinations of alpha,omega-aminopropyl terminated polydimethylsiloxane (PDMS) oligomers with number average molecular weights ranging from 700 to 15,000 g/mole and bis(4-isocyanatocyclohexyl)methane (HMDI). Urea hard segment contents of the copolymers, which were determined by the PDMS molecular weight, were in 1.7-34% by weight range. Since no chain extenders were used, urea hard segments in all copolymers were of uniform length. Simulation results clearly demonstrated the presence of very good microphase separation in all silicone-urea copolymers, even for the copolymer with 1.7% by weight hard segment content. Experimentally reported enhanced properties of these materials were shown to stem from strong hydrogen bond interactions which leads to the aggregation of urea hard segments and reinforcement of the PDMS

Understanding the influence of hydrogen bonding and diisocyanate symmetry on the morphology and properties of segmented polyurethanes and polyureas: Computational and experimental study

Quantum mechanical calculations (QMC) and dissipative particle dynamics (DPD) siniulations were utilized to understand the nature of the short and long-range hydrogen bonding and its influence on the microphase morphology in segmented polyurethanes and segmented polyureas prepared without chain extenders through the stoichiometric reactions of hydroxy or amine terminated poly(tetramethylene oxide) (PTMO-1000) with 1,4-phenylene diisocyanate (PPDI) and 1,3-phenylene diisocyanate (MPDI). The possibility of long-range connectivity due to a network of well-ordered hydrogen bonds between symmetrical PPDI and kinked MPDI based model urethane and urea compounds were also investigated. Special emphasis was given on the understanding of the influence of diisocyanate symmetry and nature of the hydrogen bonding between hard segments on the morphology development. QMC results obtained clearly indicated the possibility of long-range ordering of hydrogen bonds between PPDI based urethane and urea groups, while MPDI based systems did not display such a behavior. DPD results strongly supported the QMC studies and clearly demonstrated the possibility of long-range connectivity of hydrogen bonds between urethane and urea groups in PPDI based segmented copolymers, leading to the formation of microphase separated morphologies in these systems, which was not observed in the kinked MPDI based segmented urethane and urea copolymers. Computational results obtained strongly supported the experimental observations reported on the morphology and thermal and mechanical properties of these segmented polyurethanes and polyureas based on PPDI and MPDI

Geometric Confinement Controls Stiffness, Strength Extensibility, and Toughness in Poly(urethaneurea) Copolymers

Achieving a unique combination of stiffness, strength, extensibility, and toughness in sol-cast poly(urethane-urea) (PU) copolymer films is a challenge since these properties are-in general-mutually exclusive. Here we demonstrate that geometric confinement of the basic building blocks controls stiffness, strength, extensibility, and toughness in PU films. Our results suggest that the severity of geometric confinement can be tuned by adjusting (i) soft segment molecular weight (SSMW) and (ii) drying temperature (DT) thanks to their effects on the structure formation via microphase separation and/or (confined and/or bulk) crystallization. It is therefore possible to produce (i) soft (no notable confinement) and (ii) stiff, strong, extensible, and tough (severe confinement) materials without changing any other parameter except SSMW and DT. The former has a typical physically cross-linked network and shows a well-defined elastomeric behavior with an elastic modulus (E) of 5-20 MPa, a tensile strength (σmax) of 30-35 MPa, an extensibility (ϵ) of 1000-1300%, and a toughness (W) of 90-180 MJ m-3. The latter, on the other hand, possesses an elegant hierarchical structure containing tightly packed secondary structures (72-helix, 41-helix, and antiparallel β-sheets) and displays an elastoplastic behavior with an E of 400-700 MPa, a σmax of 45-55 MPa, an ϵ of 650-850%, and a W of 200-250 MJ m-3. Hence, our findings may be of interest in designing advanced materials containing synthetic replica of the secondary structures found in protein-based materials. The structure formation in the materials with this structural hierarchy is driven by the confined crystallization of helical poly(ethylene oxide) (PEO) chains in subnanometer urea channels, which-to the best of our knowledge-is a phenomenon well-known in host-guest systems but has not yet demonstrated in PU copolymers, and complemented by the "bulk"crystallization of PEO and/or the microphase separation

Effects of solvent on TEOS hydrolysis kinetics and silica particle size under basic conditions

In-situ liquid-state Si-29 nuclear magnetic resonance was used to investigate the temporal concentration changes during ammonia-catalyzed initial hydrolysis of tetraethyl orthosilicate in different solvents (methanol, ethanol, n-propanol, iso-propanol and n-butanol). Dynamic light scattering was employed to monitor simultaneous changes in the average diameter of silica particles and atomic force microscopy was used to image the particles within this time frame. Solvent effects on initial hydrolysis kinetics, size and polydispersity of silica particles were discussed in terms of polarity and hydrogen-bonding characteristics of the solvents. Initial hydrolysis rate and average particle size increased with molecular weight of the primary alcohols. In comparison, lower hydrolysis rate and larger particle size were obtained in the secondary alcohol. Exceptionally, reactions in methanol exhibited the highest hydrolysis rate and the smallest particle size. Ultimately, our investigation elaborated, and hence confirmed, the influences of chemical structure and nature of the solvent on the formation and growth of the silica particles under applied conditions

Geometric confinement controls stiffness, strength, extensibility, and toughness in poly(urethane-urea) copolymers

Achieving a unique combination of stiffness, strength, extensibility, and toughness in sol-cast poly(urethane-urea) (PU) copolymer films is a challenge since these properties are-in general-mutually exclusive. Here we demonstrate that geometric confinement of the basic building blocks controls stiffness, strength, extensibility, and toughness in PU films. Our results suggest that the severity of geometric confinement can be tuned by adjusting (i) soft segment molecular weight (SSMW) and (ii) drying temperature (DT) thanks to their effects on the structure formation via microphase separation and/or (confined and/or bulk) crystallization. It is therefore possible to produce (i) soft (no notable confinement) and (ii) stiff, strong, extensible, and tough (severe confinement) materials without changing any other parameter except SSMW and DT. The former has a typical physically cross-linked network and shows a well-defined elastomeric behavior with an elastic modulus (E) of 5-20 MPa, a tensile strength (σmax) of 30-35 MPa, an extensibility (ϵ) of 1000-1300%, and a toughness (W) of 90-180 MJ m-3. The latter, on the other hand, possesses an elegant hierarchical structure containing tightly packed secondary structures (72-helix, 41-helix, and antiparallel β-sheets) and displays an elastoplastic behavior with an E of 400-700 MPa, a σmax of 45-55 MPa, an ϵ of 650-850%, and a W of 200-250 MJ m-3. Hence, our findings may be of interest in designing advanced materials containing synthetic replica of the secondary structures found in protein-based materials. The structure formation in the materials with this structural hierarchy is driven by the confined crystallization of helical poly(ethylene oxide) (PEO) chains in subnanometer urea channels, which-to the best of our knowledge-is a phenomenon well-known in host-guest systems but has not yet demonstrated in PU copolymers, and complemented by the "bulk"crystallization of PEO and/or the microphase separation

Mechanical reinforcement and memory effect of strain-induced soft segment crystals in thermoplastic polyurethane-urea elastomers

An amorphous poly(urethane-urea) copolymer composed of 70 wt% poly(ethylene oxide) (PEO) soft segments (SS) (Mw = 2000 g mol−1) and 30 wt% cycloaliphatic hard segments (HS) was subjected to in-situ X-Rays during tensile deformation. Mechanical hardening at room temperature was attributed to strain induced crystallization (SIC) of the PEO SS through the multiplication of aligned crystallites. The permanent nature of these crystals after stress removal indicates a certain mechanical stability, which we related here to the concomitant effect of superstraining of the SS crystallites and the HS reorganization upon deformation. This is in marked contrast to previous studies which reported the crystalline phase to be temporary upon unloading. A manifestation of such enhanced stability is the memory effect evidenced by an increased crystallizability of PEO segments during incremental cyclic loading. These results offer a way to (i) tune the mechanical properties of TPUs via the formation of mechanically stable pre-oriented SS crystals and to (ii) tune the thermally/water activated shape memory properties of TPUs (shape fixity, kinetics of shape memory recovery)

Polyurethaneurea–silica nanocomposites: preparation and investigation of the structure–property behavior

Nanocomposites consisting of thermoplastic polyurethane-urea (TPU) and silica nanoparticles of various size and filler loadings were prepared by solution blending and extensively characterized by Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), thermal analysis, tensile tests, and nanoindentation. TPU copolymer was based on a cycloaliphatic diisocyanate and poly(tetramethylene oxide) (PTMO-2000) soft segments and had urea hard segment content of 20% by weight. TPU/silica nanocomposites using silica particles of different size (29, 74 and 215 nm) and at different loadings (1, 5, 10, 20 and 40 wt. %) were prepared and characterized. Solution blending using isopropyl alcohol resulted in even distribution of silica nanoparticles in the polyurethane-urea matrix. FTIR spectroscopy indicated strong interactions between silica particles and polyether segments. Incorporation of silica nanoparticles of smaller size led to higher modulus and tensile strength of the nanocomposites, and elastomeric properties were retained. Increased filler content of up to about 20 wt. % resulted in materials with higher elastic moduli and tensile strength while the glass transition temperature remained the same. The fracture toughness increased relative to neat TPU regardless of the silica particle size. Improvements in tensile properties of the nanocomposites, particularly at intermediate silica loading levels and smaller particle size, are attributed to the interactions between the surface of silica nanoparticles and ether linkages of the polyether segments of the copolymers

Stiff, strong, tough, and highly stretchable hydrogels based on dual stimuli-responsive semicrystalline poly(urethane-urea) copolymers

There has been a considerable interest in developing stiff, strong, tough, and highly stretchable hydrogels in various fields of science and technology including biomedical and sensing applications. However, simultaneous optimization of stiffness, strength, toughness, and extensibility is a challenge for any material, and hydrogels are well-known to be mechanically weak materials. Here, we demonstrate that poly(ethylene oxide)-based dual stimuli-responsive semicrystalline poly(urethane-urea) (PU) copolymers with high hard segment contents (30 and 40%) can be utilized as stiff, strong, tough, and highly stretchable hydrogels with an elastic modulus (4-10 MPa) tens to hundreds of times higher than that of conventional hydrogels (0.01-0.1 MPa), strength (7-13 MPa) and toughness (53-74 MJ·m-3) fairly comparable to those of the toughest hydrogels reported in the literature, and stretchability beyond 10 times their initial length (1000-1250%). In addition, the shape-memory program has been used to tune the room temperature stiffness and strength of the studied PU copolymers. Finally, the materials show fast shape recovery (less than 10 s) during both heat- and water-activated shape memory cycles, which can be adjusted by a simple modulation of the hard segment content and/or soft segment molecular weight. Our findings may be of interest in emerging biomedical and sensing applications

Effect of surface modification of colloidal silica nanoparticles on the rigid amorphous fraction and mechanical properties of amorphous polyurethane-urea-silica nanocomposites

Colloidal silica nanoparticles (NPs) modified with eight different silane coupling agents were incorporated into an amorphous poly(tetramethylene oxide)-based polyurethane-urea copolymer matrix at a concentration of 10 wt % (4.4 vol %) in order to investigate the effect of their surface chemistry on the structure-property behavior of the resulting nanocomposites. The rigid amorphous fraction (RAF) of the nanocomposite matrix as determined by differential scanning calorimetry and dynamic mechanical analysis was confirmed to vary significantly with the surface chemistry of the NPs and to be strongly correlated with the bulk mechanical properties in simple tension. Hence, nanocomposites with an RAF of about 30 wt % showed a 120% increase in Young's modulus, a 25% increase in tensile strength, a 15% decrease in elongation at break with respect to the neat matrix, which had no detectable RAF, whereas nanocomposites with an RAF of less than 5% showed a 60% increase in Young's modulus, a 10% increase in tensile strength and a 5% decrease in the elongation at break. (c) 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 201