Showing papers in "Macromolecules in 2007"

How Nano Are Nanocomposites

Abstract: Composite materials loaded with nanometer-sized reinforcing fillers are widely believed to have the potential to push polymer mechanical properties to extreme values Realization of anticipated properties, however, has proven elusive The analysis presented here traces this shortfall to the large-scale morphology of the filler as determined by small-angle X-ray scattering, light scattering, and electron imaging We examine elastomeric, thermoplastic, and thermoset composites loaded with a variety of nanoscale reinforcing fillers such as precipitated silica, carbon nanotubes (single and multiwalled), and layered silicates The conclusion is that large-scale disorder is ubiquitous in nanocomposites regardless of the level of dispersion, leading to substantial reduction of mechanical properties (modulus) compared to predictions based on idealized filler morphology

Toward High-Performance Polymer Solar Cells: The Importance of Morphology Control

Abstract: Polymer solar cells have the potential to become a major electrical power generating tool in the 21st century. R&D endeavors are focusing on continuous roll-to-roll printing of polymeric or organic compounds from solution-like newspapers-to produce flexible and lightweight devices at low cost. It is recognized, though, that besides the functional properties of the compounds the organization of structures on the nanometer level-forced and controlled mainly by the processing conditions applied-determines the performance of state-of-the-art polymer solar cells. In such devices the photoactive layer is composed of at least two functional materials that form nanoscale interpenetrating phases with specific functionalities, a so-called bulk heterojunction. In this perspective article, our current knowledge on the main factors determining the morphology formation and evolution is introduced, and gaps of our understanding on nanoscale structure-property relations in the field of high-performance polymer solar cells are addressed. Finally, promising routes toward formation of tailored morphologies are presented.

Polymorphous Crystallization and Multiple Melting Behavior of Poly(l-lactide): Molecular Weight Dependence

Abstract: The effect of molecular weight (MW) on the polymorphous crystallization and melting behavior of poly(l-lactide) (PLLA) were systemically studied by differential scanning calorimetry (DSC), polarized optical microscopy (POM), wide-angle X-ray diffraction (WAXD), and time-resolved Fourier transform infrared (FTIR) spectroscopy. It was found that the polymorphism of PLLA is not influenced much by MW, and the α‘- and α-form crystals are produced at low and high crystallization temperature (Tc), respectively, regardless of the MW. However, MW significantly affects the crystallization kinetics, and the crystallization rate reduces greatly with MW increasing. Moreover, the Tc- and MW-dependent melting behavior of PLLA was clarified with combining the DSC and FTIR results. It was found that the α‘- to α-crystalline phase transition occurs prior to the dominant melting in both the low- and high-MW PLLA crystallized at low Tc. Unlike the high-MW PLLA, in low-MW PLLA crystallized at low Tc, the α‘-form crystals only...

Large Strain Hysteresis and Mullins Effect of Tough Double-Network Hydrogels

Abstract: Systematic loading and unloading experiments, in uniaxial tension and uniaxial compression, have been performed on a double-network hydrogel exhibiting a very high toughness. We observed a significant hysteresis during the first loading cycle that increased strongly with the applied maximum deformation. A large hysteresis was not observed during a second loading cycle, implying that the initial hysteresis can be attributed to the fracture of covalent bonds in the primary network. We report this type of dissipative mechanism for polymer gels for the first time. Assuming that the entire energy dissipated during the hysteresis cycle can be attributed to the fracture of network strands by a Lake−Thomas mechanism, our results suggest that the fracture and unloading of only 1% of the bonds within the network leads to a decrease of up to 80% of the number of strands. These results also demonstrate the very large degree of heterogeneity within the hydrogel network. If such a dissipative mechanism is active at the...

Crystallization and Melting Behavior of Poly (l-lactic Acid)

Abstract: Effects of the crystallization temperature on the crystal structure and its melting behavior of poly (l-lactic acid) (PLLA) have been investigated by means of wide-angle (WAXS) and small-angle (SAXS) X-ray scattering, optical microscopy, and differential scanning calorimetory (DSC). PLLA was found to crystallize as the α form when the crystallization temperature Tc was higher than 120 °C, while significant change in lattice parameters was seen for Tc's below 120 °C. The ratio of the a- and b-axis lengths begins to decrease with Tc below 120 °C and is 31/2 below 90 °C, which suggests a new crystalline form with hexagonal packing, namely, the α‘ form. The possible reason for α‘ formation is discussed. High-temperature WAXS and SAXS measurements showed that α‘ crystal transforms into ordered a form during heating. The transition takes place at 150 °C without a decrease in scattering intensity and without heating rate dependence. The mechanism for the transition is discussed.

Effect of molecular weight on the mechanical and electrical properties of block copolymer electrolytes

Abstract: The relationship between ionic conductivity, morphology, and rheological properties of polystyrene-block-poly(ethylene oxide) copolymers (SEO) doped with a lithium salt, Li[N(SO2CF3)2], is elucidated. We focus on lamellar samples with poly(ethylene oxide) (PEO) volume fractions, φ, ranging from 0.38 to 0.55, and PEO block molecular weights, MPEO, ranging from 16 to 98 kg/mol. The low-frequency storage modulus (G‘) at 90 °C increases with increasing MPEO from about 4 × 105 to 5 × 107 Pa. Surprisingly, the conductivity of the SEO/salt mixtures with the molar ratio of Li to ethylene oxide moieties of 0.02 σ, also increases with increasing MPEO, from 6.2 × 10-5 to 3.6 × 10-4 S/cm at 90 °C. We compare σ with the conductivity of pure PEO/salt mixtures, σPEO, and find that σ/[φσPEO] of our highest molecular weight sample is close to 0.67, the theoretical upper limit for transport through randomly oriented lamellar grains.

Panchromatic Conjugated Polymers Containing Alternating Donor/Acceptor Units for Photovoltaic Applications

Abstract: We designed and synthesized a series of conjugated polymers containing alternating electron-donating and electron-accepting units based on (4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b‘]dithiophene), 4,7-(2,1,3)-benzothiadiazole, and 5,5‘-[2,2‘]bithiophene. These polymers possess an optical band gap as low as 1.4 eV (i.e., in the case of poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b‘]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)]), and their absorption characteristics can be tuned by adjusting the ratio of the two electron-donating units: (4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b‘]dithiophene) and 5,5‘-[2,2‘]bithiophene. The desirable absorption attributes of these materials qualify them as excellent candidates for light-harvesting materials in organic photovoltaic applications allowing for high short-circuit current. Electrochemical studies indicate sufficiently deep HOMO/LUMO levels that enable a high photovoltaic device open-circuit voltage when fullerene derivatives are used as ...

About the Phase Transitions in Aqueous Solutions of Thermoresponsive Copolymers and Hydrogels Based on 2-(2-methoxyethoxy)ethyl Methacrylate and Oligo(ethylene glycol) Methacrylate

Abstract: The phase transitions in water of well-defined copolymers of 2-(2-methoxyethoxy)ethyl methacrylate (MEO2MA) and oligo(ethylene glycol) methacrylate (OEGMA, Mn = 475 g mol-1) were studied by means of 1H NMR, dynamic light scattering (DLS) and turbidimetry. These copolymers were synthesized by atom transfer radical polymerization (ATRP) in order to obtain macromolecules with a homogeneous chain-to-chain comonomer composition and therefore with a narrow phase transition. As previously reported, copolymers containing 10 mol % of OEGMA per chain (Mn = 20 100 g mol-1 ; Mw/Mn = 1.35) exhibited a sharp LCST in water around 40 °C, whereas copolymers having 20 mol % of OEGMA per chain (Mn = 19 500 g mol-1; Mw/Mn = 1.32) precipitated slightly above 50 °C. DLS indicated that these phase transitions are most likely due to a reversible coil-to-globule transition. Moreover covalently cross-linked networks of MEO2MA and OEGMA were synthesized by ATRP in the presence of the cross-linker ethylene glycol dimethacrylate. The...

Thio-click modification of poly [2-(3-butenyl)-2-oxazoline]

Abstract: The radical addition of mercaptans (RSH) onto poly[2-(3-butenyl)-2-oxazoline], which is available through a living/controlled cationic isomerization polymerization, can proceed smoothly in the absence of side reactions, exhibiting the characteristics of a click reaction. The “thio-click” reaction can be performed under feasible ([RSH]/[CC] ∼ 1.2−1.5, no transition metal additives) and mild conditions (generation of radicals with UV light at room temperature) and goes to completion within a day. Hydrophobic fluoropolymers can be prepared in the same way as water-soluble (co-)polymers or glycopolymers, starting from readily available materials.

Morphology and Phase Segregation of Spin-Casted Films of Polyfluorene/PCBM Blends

Abstract: In this study the morphology of spin-casted films of polymers blended with [6,6]-phenyl C61-butyric acid methyl ester (PCBM) has been studied. It was found that the lateral structure formation in the films is favored by rapid solvent evaporation and strong polymer−PCBM repulsion. The formation of homogeneous films is favored by slow evaporation and weak polymer−PCBM repulsion. The effect of solvent evaporation rate is the opposite of what is found for spin-casting polymer−polymer blends. The results can be explained by the kinetics of phase separation and the phase behavior involving limited solubility and crystallization of PCBM.

Single Wall Carbon Nanotube/Polyethylene Nanocomposites: Thermal and Electrical Conductivity

Abstract: The thermal and electrical conductivities in nanocomposites of single walled carbon nanotubes (SWNT) and polyethylene (PE) are investigated in terms of SWNT loading, the degree of PE crystallinity, and the PE alignment. Isotropic SWNT/PE nanocomposites show a significant increase in thermal conductivity with increasing SWNT loading, having 1.8 and 3.5 W/mK at a SWNT volume fraction of φ ∼ 0.2 in low-density PE (LDPE) and high-density PE (HDPE), respectively. This increase in SWNT/HDPE is more than additive and suggests a reduction of the interfacial thermal resistance. Fitting the thermal conductivity data of the SWNT/HDPE nanocomposites with two models indicates that the thermal conductivity relies on a percolating SWNT network. Oriented SWNT/HDPE nanocomposites exhibit higher thermal conductivities, which are attributed primarily to the aligned PE matrix.

Aromatic Poly(ether ketone)s with Pendant Sulfonic Acid Phenyl Groups Prepared by a Mild Sulfonation Method for Proton Exchange Membranes

Abstract: The sulfonation selectivity of seven poly(ether ether ketone)s (PEEKs) was investigated, and several possessed targeted single- or double-substituted sites per repeated unit on pendant phenyl groups via the postsulfonation approach. The presence of the various pendant groups enabled postsulfonation to occur under mild reaction conditions, in much shorter times than required for the sulfonation of commercial PEEK. A series of poly(ether ketone)s (PEKs) with ion exchange capacity of 2.23−0.84 mequiv/g could be realized by controlling the length of unsulfonated segments of both homopolymers and copolymers. These side-group sulfonation polymers had excellent mechanical properties, good thermal and oxidative stability, and good dimensional stability in hot water. The methanol permeability values of Me-SPEEKK, Me-SPEEKDK, Ph-SPEEKK, and Ph-SPEEKDK at room temperature were in the range 3.31 × 10-7−9.55 × 10-8 cm2/s, which is several times lower than that of Nafion 117. Me-SPEEKK and Ph-SPEEKK also exhibited high...

Tuning the Thermoresponsive Properties of Weak Polyelectrolytes: Aqueous Solutions of Star-Shaped and Linear Poly(N,N-dimethylaminoethyl Methacrylate)

Abstract: We investigated the thermoresponsive behavior of aqueous solutions of star-shaped and linear poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA). The observed cloud points strongly decrease with increasing pH of the solution. This is explained by a weak charging of the star polymers with decreasing pH. A significant decrease of the cloud points with increasing molecular weight for high pH, i.e., for the almost uncharged state, was found to be virtually independent of the arm number and arm length. These findings are explained by classical Flory−Huggins theory. The increase of cloud points upon charging is captured by introduction of an effective degree of polymerization. Polymers with shorter arms show slightly higher cloud points at low pH than polymers with longer arms. The intramolecular segment density also influences the observed apparent pKb values, leading to higher values for stars with higher arm numbers.

A Comparison of Hydrogen Bonding and Order in a Polyurethane and Poly(urethane−urea) and Their Blends with Poly(ethylene glycol)

Abstract: The effect of hydrogen bonding on the phase behavior of a chemically similar polyurethane and polyurea and their blends with poly(ethylene glycol) is examined. The polyurethane and polyurea were synthesized from the same diisocyanate, 1,5-diisocyanato-2-methylpentane, using an aromatic diol and aromatic diamine, respectively. Fourier transform infrared spectroscopy was used to characterize the distribution of hydrogen bonds in these polymers and their blends. The distribution of hydrogen bonds in the polyurethane homopolymer was found to be quite similar to that found in an amorphous polyurethane studied previously in this laboratory. However, upon annealing, some sort of ordered structure was detected spectroscopically. The polyurea formed an equivalent ordered structure much more readily at room temperature. Ordered hydrogen-bonded domains were also detected in the spectra of the blends, either after an extended time at room temperature or after annealing at an elevated temperature. The formation of ord...

Synthesis of Well-Defined Cyclic Poly(N-isopropylacrylamide) via Click Chemistry and Its Unique Thermal Phase Transition Behavior

Abstract: We report on the preparation of well-defined cyclic poly(N-isopropylacrylamide) (cyclic-PNIPAM) via click chemistry and its unique thermal phase transition behavior as compared to the linear counterpart. α-Alkyne-ω-azido heterodifunctional PNIPAM precursor (linear-PNIPAM-N3) was prepared by atom transfer radical polymerization (ATRP) of N-isopropylacrylamide in 2-propanol using propargyl 2-chloropropionate as the initiator, followed by reacting with NaN3 to transform the terminal chloride into azide group. The subsequent end-to-end intramolecular coupling reaction under high dilution and “click” conditions leads to efficient preparation of narrow-disperse cyclic-PNIPAM. Gel permeation chromatography (GPC), 1H NMR, Fourier transform infrared (FT-IR), and matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry all confirmed the complete transformation of linear-PNIPAM-N3 to cyclic-PNIPAM. The thermal phase transition behavior of cyclic-PNIPAM was investigated by temperature-...

Polymer Micelles Stabilization on Demand through Reversible Photo-Cross-Linking

Abstract: The stability of polymer micelles, which affects the stability of encapsulation of guest molecules, may be a crucial condition for some controlled delivery applications. It is possible that polymer micelles are disintegrated after being administrated in the body due to an extreme dilution to below the critical micelle concentration of the polymer. If this happens, the entrapped guest such as a drug can leak out quickly, which renders any strategy for site-specific transport of the micelles useless. 1

Critical Role of Side-Chain Attachment Density on the Order and Device Performance of Polythiophenes

Abstract: High performance, solution processable semiconductors are critical to the realization of low cost, large area electronics. We show that a signature molecular packing motifside-chain interdigitationcorrelates to high performance for a large and important class of organic semiconductors. The side chains of recently developed high performance copolymers of poly(alkylthiophenes) can and do interdigitate substantially, whereas they do not in the most common form of the extensively studied, lower performance poly(alkythiophenes). Side-chain interdigitation provides a mechanism for three-dimensional ordering; without it, poly(alkylthiophenes) are limited to small domains and poor performance. We propose the synthetic design rule that three-dimensional ordering is promoted by side-chain attachment densities sufficiently low to permit interdigitation.

Tunable pH- and Temperature-Sensitive Copolymer Libraries by Reversible Addition−Fragmentation Chain Transfer Copolymerizations of Methacrylates

Abstract: Reversible addition−fragmentation chain transfer (RAFT) polymerizations have been performed on a Chemspeed Accelerator SLT100 automated synthesizer to polymerize N,N-(dimethylamino)ethyl methacrylate (DMAEMA) and poly(ethylene glycol) methyl ether methacrylate (PEGMA) at 70 °C. Azobis(isobutyronitrile) (AIBN) was used as source of radicals and 2-cyano-2-butyl dithiobenzoate (CBDB) as RAFT agent. A complete screening in composition of P(DMAEMA-stat-PEGMA) copolymers was elaborated from 0% of PEGMA to 100% of PEGMA. All polydispersity indices of the obtained copolymers are comprised between 1.11 and 1.30. The reactivity ratios have been determined by the extended Kelen−Tudos method (rDMAEMA = 0.93 and rPEGMA = 0.66). The behavior of the pH- and temperature-sensitive copolymers was studied in aqueous solution by measuring the lower critical solution temperature (LCST) by UV/vis spectroscopy. The measurements were performed at three different pH values (4, 7, and 10). At pH 7 and pH 10 it has been observed th...

Behavior of Surface-Anchored Poly(acrylic acid) Brushes with Grafting Density Gradients on Solid Substrates: 1. Experiment

Abstract: We describe experiments pertaining to the formation of surface-anchored poly(acrylic acid) (PAA) brushes with a gradual variation of the PAA grafting densities on flat surfaces and provide detailed analysis of their properties. The PAA brush gradients are generated by first covering the substrate with a molecular gradient of the polymerization initiator, followed by the “grafting from” polymerization of tert-butyl acrylate (tBA) from these substrate-bound initiator centers, and finally converting the PtBA into PAA. We use spectroscopic ellipsometry to measure the wet thickness of the grafted PAA chains in aqueous solutions at three different pH values (4, 5.8, and 10) and a series of ionic strengths (IS). Our measurements reveal that at low grafting densities, σ, the wet thickness of the PAA brush (H) remains relatively constant, the polymers are in the mushroom regime. Beyond a certain value of σ, the macromolecules enter the brush regime, where H increases with increasing σ. For a given σ, H exhibits a ...

Mechanics of Polymer−Clay Nanocomposites

Abstract: The mechanics of nanocomposites is critical in the design of nanomaterials with desirable properties In this paper, the mechanics of polymer−clay nanocomposites is studied using a designed polymer and solution nanocomposite synthesis A copolymer latex, with function groups that strongly interact with the surface of the clay nanoplatelet and glass-transition temperature lower than room temperature, was synthesized Uniformly dispersed nanocomposites were then generated using water as the intercalation agent through the solution process The chain mobility in the nanocomposites is greatly reduced as studied by dynamic mechanical thermal analysis (DMTA) and dielectric thermal analysis (DETA) The modulus of the composite increases significantly The modulus enhancement strongly relates to the volume of the added clay as well as the volume of the constrained polymer This modulus enhancement follows a power law with the content of the clay and is modeled well by Mooney's equation for this soft-polymer-based

Temperature-Induced Phase Transition of Well-Defined Cyclic Poly(N-isopropylacrylamide)s in Aqueous Solution

Abstract: Department of Chemistry and Faculty of Pharmacy,UniVersity of Montreal, CP6128 Succursale Centre Ville,Montreal, QC, Canada H3C 3J7, and Department ofPolymer Chemistry, Graduate School of Engineering, KyotoUniVersity, Katsura, Kyoto 615-8510, JapanReceiVed June 19, 2007ReVised Manuscript ReceiVed August 15, 2007The thermosensitivity of aqueous polymer solutions is thedriving force at work in various soft “intelligent” devices usedin biotechnology and nanotechnology.

Electrical and rheological percolation in polystyrene/MWCNT nanocomposites

Abstract: A systematic electrical and rheological characterization of percolation in commercial polydisperse polystyrene (PS) nanocomposites containing multiwall carbon nanotubes (MWCNTs) is presented. The MWCNTs confer appreciable electrical conductivities (up to ca. 1 S/m) to these nanocomposites at a concentration of 8 vol %. In addition to enhancing the electrical properties, even at small concentrations (ca. 2 vol %), MWCNTs significantly enhance the rheological properties of PS melts. At concentrations exceeding 2 vol %, a plateau appears in the storage modulus G‘ at low frequencies, indicating the formation of a percolated MWCNT network that responds elastically over long timescales. Network formation, in turn, implies a diverging complex viscosity vs complex modulus curve. A focus of this study is on the correlation between electrical and rheological properties at the onset of percolation. The experimental results indicate that the elastic load transfer and electrical conductivity are far more sensitive to ...

Graft Copolymers by a Combination of ATRP and Two Different Consecutive Click Reactions

Abstract: The epoxide ring in glycidyl butyrate (model compound) and in well-defined copolymers of glycidyl methacrylate (<40 mol %) and methyl methacrylate prepared by ATRP was efficiently opened with sodium azide in the presence of ammonium chloride in DMF at 50 °C. This click-type reaction led to the formation of the corresponding 1-hydroxy-2-azido compounds in high yields. The prepared azide-containing copolymers were functionalized in a second click reaction, the room temperature CuBr/N,N,N‘,N‘ ‘,N‘ ‘-pentamethyldiethylenetriamine-catalyzed 1,3-dipolar cycloaddition, of poly(ethylene oxide) methyl ether pentynoate to yield loosely grafted polymeric brushes with hydrophilic PEO side chains.

Effect of Solubility and Nucleating Duality of N,N'-Dicyclohexyl-2,6-naphthalenedicarboxamide on the Supermolecular Structure of Isotactic Polypropylene

Abstract: N,N‘-Dicyclohexyl-2,6-naphthalenedicarboxamide (NJS) is an efficient nucleating agent to prepare isotactic polypropylene (iPP) samples rich in the β-modification (β-iPP). However, NJS is not a selective β-nucleating agent, as the related samples always contain both α and β modification of iPP. Depending on the final temperature of heating (Tf), NJS may be partially or completely dissolved in the iPP melt. The dual nucleation ability of NJS was studied by polarized light microscopy (PLM) and calorimetry (DSC). It was established that the lateral surface of the needle crystals of NJS acts as α-nucleating agent. Because of the solubility and dual nucleating ability of NJS, a wide variety of supermolecular structures may form in its presence. The feature of the structure formed during the crystallization of iPP depends on the concentration of the nucleating agent, on the end temperature of heating, and on the thermal conditions during cooling and crystallization. In this study we observed an αβ-transcrystalli...

Polyacrylamide-clay nanocomposite hydrogels : Rheological and light scattering characterization

Abstract: The free-radical polymerization of acrylamide in aqueous clay dispersions and the structure of the resulting polymer-clay nanocomposite hydrogels have been investigated by rheometry using oscillatory deformation tests. Laponite was used as clay particles in the hydrogel preparation. The reactions were carried out with and without the presence of the chemical cross-linker N,N'-methylenebis(acrylamide) (BAAm). In the absence of BAAm, increasing clay concentration from 0.2 to 7% results in 3 orders of magnitude increase of the elastic modulus Gof the hydrogels. At a clay concentration of 5% or above, all the reaction systems, with or without BAAm, exhibit similar elastic moduli, indicating that clay mainly determines the rubber elasticity of the hydrogels. The loss factor tan ‰ was found to be around 0.1, indicating that the nanocomposite hydrogels are much more viscous than the conventional hydrogels. Increasing the amount of clay also increases the viscous, energy dissipating properties of the nanocomposite hydrogels, which are responsible for their improved mechanical properties. Dynamics of the nanocomposite hydrogels was also investigated by dynamic light scattering. The ensemble- averaged scattered intensity of the hydrogels varies nonmonotonically with the clay concentration due to the action of clay both as a cross-linker and as an ionic component during the formation of the nanocomposite hydrogels. shown that the scattering intensity from polymer gels is always larger than that from the solution of the same polymer at the same concentration. The excess scattering over the scattering from polymer solution is related to the inhomogeneous distribu- tion of the polymer material along the gel sample, which is

Solvent-Promoted Self-Healing Epoxy Materials

Abstract: Mary M. Caruso,† David A. Delafuente,† Victor Ho,‡ Nancy R. Sottos,§ Jeffrey S. Moore,†,§ and Scott R. White*,| Department of Chemistry and Beckman Institute, UniVersity of Illinois at UrbanasChampaign, Urbana, Illinois 61801, Department of Chemical Engineering, UniVersity of Texas at Austin, Austin, Texas 78712, Department of Materials Science and Engineering and Beckman Institute, UniVersity of Illinois at UrbanasChampaign, Urbana, Illinois 61801, and Department of Aerospace Engineering and Beckman Institute, UniVersity of Illinois at UrbanasChampaign, Urbana, Illinois 61801 ReceiVed September 4, 2007 ReVised Manuscript ReceiVed October 16, 2007

A Model of the Fracture of Double Network Gels

Abstract: I propose a very simple model accounting for the very high toughness of double network gels based on the assumption that the first, stiff network will break up forming multiple cracks when the stress is above a defined value. These cracks are held together by the second network. A multiply cracked damage zone will form around any macroscopic crack in the material, causing energy dissipation and shielding the second network. The toughness enhancement by this process is estimated to be about ×40. The effect of cross-linking of the second network is discussed and explained.

Effects of initiator structure on activation rate constants in ATRP

Abstract: Activation rate constants (kact) for a variety of initiators for Cu-mediated ATRP have been determined under the same conditions. The ratio of the activation rate constants for the studied alkyl (pseudo)halides exceeds 1 million times. The activation rate constants increase with initiator substitution (e.g., for primary, secondary, and tertiary α-bromoesters the ratios are ∼1:10:80), with the radical stabilizing α-substituent (e.g., alkyl bromides with −C(O)NEt2, −Ph, −C(O)OMe, and −CN groups the ratios are ∼1:4:8:600 but with both α-Ph and α-C(O)OEt ∼ 140 000), and with the leaving atom/group (e.g., for methyl 2-halopropionates: chloro:bromo:iodo ∼1:20:35, but benzyl bromide is ∼10 000 more reactive than the corresponding isothiocyanate/thiocyanate).