Energy-consuming micromechanisms in the fracture of glassy polymers. I. Chain scission in polystyrene

The number of chain scissions per unit area that occur during the fracture of partially annealed latex films from M_n ? 180,000 g/mol polystyrene particles of about 275 Å radius were measured and correlated to annealing times. A curve with four regimes was found. At short annealing times the curve is nearly flat, in what is called the chain pull-out regime. In the second regime, the number of chains broken per unit area increases with a 0.8 power of annealing time as entanglement of the diffusing polymer chains increases in neighboring host particles. This is in good agreement with Wool's theory which predicts a 0.75 power dependence. Then, after reaching a peak, the number of scissions decreases in the third regime, indicating a change in fracture mechanism. The number of chain scissions increases again in the fourth regime, as final healing of the film interface takes place. Fracture surface analysis reveals a rough surface for short annealing times and a smooth surface for longer annealing times. The number of polymer chain scissions per unit area of fracture surface showed no dependence on initial molecular weights for t ? τ_r where t and τ_r are annealing and relaxation times, respectively. The number of chain bridges crossing a unit area of interface was suggested as the basic molecular property. © 1992 John Wiley & Sons, Inc.     

Ion‐Induced Fragmentation of Amino Acids: Effect of the Environment

In general, radiation‐induced fragmentation of small amino acids is governed by the cleavage of the C? C_α bond. We present results obtained with 300 keV Xe²⁰⁺ ions that allow molecules (glycine and valine) to be ionised at large distances without appreciable energy transfer. Also in the present case, the C? C_α bond turns out to be the weakest link and hence its scission is the dominant fragmentation channel. Intact ionised molecules are observed with very low intensities. When the molecules are embedded in a cluster of amino acids, a protective effect of the environment is observed. The fragmentation pattern changes: the C? C_α bond becomes more protected and stable amino acid cations are observed as fragments of the molecular clusters. Evidently, the molecular cluster acts as a “buffer” for the excess energy, capable of rapidly redistributing excess energy and charge.     

Gas transport properties of thermotropic liquid-crystalline copolyesters. II. The effects of copolymer composition

Gas transport properties are reported for a series of compression-molded films prepared from copolyesters of hydroxybenzoic acid (HBA) and 2,6 hydroxynaphthoic acid (HNA) having 30/70, 58/42, 73/27, 75/25, and 80/20 mol % HBA/HNA. The mesomorphic and crystalline morphology of the materials was characterized using dynamic mechanical thermal analysis (DMTA), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and X-ray diffraction As evidenced by DMTA, the phenyl and naphthyl moieties of the HBA/HNA materials exhibit a significant degree of segmental mobility below the glass transition temperature. The nonlinear nature of the naphthyl unit leads to a more hindered rotation about the chain axis. Permeability measurements were made for He, H₂, O₂, N₂, Ar, and CO₂ at 35°C and the diffusivities were computed from time-lag data. As previously observed in these materials, the films exhibited excellent barrier properties resulting largely from very low gas solubility coefficients. The liquid-crystalline copolyester: (LCP) materials with the highest HNA content exhibit the best barrier properties. It appears that the more hindered motions of the naphthyl unit restrict penetrant mobility. The reduction in permeability with increased naphthyl unit content is accompanied by a very dramatic increase in selectivity between gas pairs. Fractional free volume analysis was used to correlate the transport properties of the LCP materials and other conventional polymers. A “two-phase” modification of the free volume correlation suggests that transport may likely occur in a small volume fraction of a less dense boundary phase.     

Polymer‐based fluoride‐selective chemosensor: Synthesis,sensing property,and its use for the design of molecular‐scale logic devices

A new styryl‐type monomer, 2‐(4‐vinylbenzyloxy)‐1 ‐naphthaldehyde thiosemicarbazone (VNT), was synthesized and then copolymerized with methyl methacrylate (MMA) by reversible addition fragmentation chain transfer polymerization affording a series of poly(MMA‐co‐VNT)s with different functional unit content, predetermined molecular weight, and narrow molecular‐weight distribution. The desired copolymers were structurally confirmed by various spectroscopic characterizations. Colorimetric and fluorescent titration spectra revealed that the copolymers are highly selective toward fluoride anions over other competitive species including Cl^?, Br^?, I^?, H₂PO₄^?, AcO^?, and HSO₄^?. On addition of F^?, a remarkable colorless‐to‐yellow color change is easily observed by naked eyes. The influence of the copolymer composition and molecular weight on its sensing capacity was then carefully investigated. The results showed that higher VNT‐incorporation amount within the copolymer chains leads to higher sensitivity toward F^? ions. Interestingly, the chromogenic process of the polymeric sensor can be switched back and forth by successively adding F^? and HSO₄^? anions into the dimethyl sulfoxide solution of the polymer, which may be represented by a complementary “IMPLICATION/INHIBIT” logic gate at molecular level using both the ions as the chemical inputs. Based on such a reversible and reproducible sensing system, we designed a molecular‐scale sequential information processing circuit displaying “writing–reading–erasing–reading” behavior and “multiwrite” function in the form of binary logic. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Kinetics of structural relaxation of poly(ethylene terephthalate)

The results of an experimental study of the kinetics of structural relaxation of amorphous poly(ethylene terephthalate) are reported. Samples were prepared by ultraquenching the melt on rotating stainless-steel discs. Two types of measurements by differential scanning calorimetry were made: (1) the dependence of the “fictive” (or “structural”) temperature T_f(q^?) introduced by Tool, on the cooling rate q^? and (2) the dependence of the glass transition temperature T_g on the heating rate q⁺. In this way the value x = 0.47 was obtained for the dimensionless parameter proposed by Narayanaswamy.     

Effect of reversible structural relaxation on diffusion in bulk metallic glasses

Long time relaxation at temperatures below the calorimetric glass transition causes reversible structural changes in metallic glasses. The resulting enthalpy recovery was measured by means of DSC in Zr_46.8Ti_8.2Cu_7.5Ni₁₀Be_27.5- and Pd₄₀Cu₃₀Ni₁₀P₂₀-bulk glass annealed for different times at 553 K and 542 K, respectively. Relaxation times of about 10⁶ s and 10⁴ s, respectively, were determined. The diffusion coefficients of B, Fe and Co were measured above and below the calorimetric glass transition temperature. Whereas the temperature dependence of these diffusion coefficients in the non-relaxed glasses shows “non-linear” Arrhenius behaviour with a break near the glass transition, the diffusion in the long time relaxed glasses follows a uniform temperature dependence over the entire temperature range with considerably reduced diffusion coefficients below the glass transition. This behaviour can be reversed by annealing the relaxed glasses again at higher temperatures indicating the strong effect of the reversible structural relaxation on the diffusion coefficients.     

NMR investigation of effect of dissolved salts on the thermoresponsive behavior of oligo(ethylene glycol)‐methacrylate‐based polymers

The synthesis of thermo‐ and ionic‐responsive copolymers based on polyethylene glycol methyl ether methacrylate (OEGMA) and 2,2,2‐trifluoroethyl acrylate (TFEA) via reversible addition‐fragmentation chain transfer polymerization is described. Reactivity ratios for the copolymerization of OEGMA and TFEA are r_OEGMA = 2.46 and r_TFEA = 0.22, indicating that OEGMA is incorporated more rapidly than TFEA monomers. The copolymers are thermosensitive and exhibit volume phase transitions (lower critical solution behavior) at temperature, which depend on copolymer composition and the presence of added salts in the aqueous solutions. It was found that the copolymers exhibited LCST transitions at temperatures below 353 K only in salt solutions. ¹H NMR measurements indicated that motion of the protons located in and near the hydrophobic main chain are more sensitive to temperature than protons in the hydrophilic OEGMA side chains. The hydrophilic side chains remain largely hydrated; however, the presence of two distinct conformations of the terminal groups of the side chains was confirmed. The influence of OEGMA side chain length, copolymer composition, and salt type on aggregation behavior and dynamics was examined in detail. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 2375–2385     

Random-walk theory for localization

A continuous-time random-walk theory has been developed for Anderson localization. On a continuous time scale random walks are performed along extended (i.e., propagating) and localized (i.e., trap) states. Complete information of disorder is contained in a distribution function called “hopping time distribution function” ψ_nm(t), which gives the probability per unit time for transition from state m to state n in time t. The “stay-put” probability ??(t = ∞), which is the probability to rediscover an excitation at a site “0” at time t = ∞ if it was there at t = 0, is obtained in terms of ψ_nm(t). Appropriate forms for ψ_nm(t) are constructed which are in conformity with the photoconductivity experiments on dispersive transport, and ??(∞) are calculated. The results indicate that the entire spectrum consists of three regimes, namely, those of (i) “diffusion,” (ii) “weak diffusion,” and (iii) “no diffusion,” which, respectively, designate the extension, the power-law localization, and the exponential localization of states. The results also shed light on the question of “continuous or discontinuous (?)” transition across the mobility edge.     

Synthesis of polyacrylonitrile mediated by manganese(III) acetylacetonate (Mn(acac)3) and 2‐cyanoprop‐2‐yl dithionaphthalenoate

2‐Cyanoprop‐2‐yl dithionaphthalenoate (CPDN) was successfully used as the chain transfer agent to prepare polyacrylonitrile in combination with manganese(III) acetylacetonate (Mn(acac)₃) as the initiator. The novel polymerization exhibited well “living”/controlled characteristics. The polymerization behavior was revealed to comply with features of reversible addition–fragmentation chain transfer polymerization process. Mn(acac)₃ played a key role as the initiator rather than the radical trapping agent in polymerization and exhibited better control performance than azo‐initiator. The narrowest molecular weight distribution was 1.31 under the condition of [AN]₀:[Mn(acac)₃]₀:[CPDN]₀ = 200:1:0.025 and AN:DMF = 1:1 (V/V). Various feed ratios of Mn(acac)₃ and CPDN were also investigated in detail. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 1305–1309     

Cooperative motions in PVC studied by thermally stimulated currents: Comparison with A.C. dielectric derivative analysis

The thermally stimulated current–thermal sampling (TSC–TS) technique was used to study the broadened glass transition in conventional “atactic” poly(vinyl chloride), PVC. The activated parameters obtained from the TSC–TS data, mainly the apparent activation energy (E_a), characterize the breadth of glass transitions in a very sensitive way. These results are compared with those values of E_a obtained from the literature, using a recently proposed method of analyzing a.c. dielectric constants and their derivatives, over the temperature range of −100–130°C. Both techniques detect weak cooperative glass transition-like relaxations well below the main glass transition of ca. 80°C. As is the case with “atactic” PMMA, the data suggest that compositional heterogeneity related to a small fraction of predominantly isotactic sequences contribute to the broad glass transition extending ca. 60°C below the main glass transition in atactic PVC. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 913–918, 1998     

Excited state decay of tetrahalomanganese(ii) complexes

The excitation and emission spectra and decay times of several MnX^2-₄ (X = Cl^?, Br^?, 1^?) complexes of various tetraalkylammonium, -phosphonium, and -arsonium salts have been measured for the pure solids at 298°K and 77°K. High luminescence quantum yields (0.3-1.0) reveal that lifetimes fairly accurately reflect radiative decay rates. An impressive correlation exists between the lifetime, τ, of the ⁴T₁ (G) →⁶ A₁ emission and the ligand, X: for X = Cl^?, τ = 1.2 - 3,5 x 10^?3 sec; X = Br^?, τ = 0.35 - 0.43 X 10^?3 sec; X =l^?, τ = 0.036 – 0.055 X 10^?3 sec. We attribute this decreasing lifetime largely to the enhanced spin-orbital coupling associated with the heavier halide. We find that direct population of high energy charge-transfer (CT) states gives smaller emission yields than excitations in the ligand-field (LF) region.     

Effect of Topology on the Properties of Poly(N-isopropylacrylamide) in Water and in Bulk

Three macrocyclic poly(N-isopropylacrylamide)s (PNIPAM) with molecular weight (MW) ranging from 6 to 19 kg/mol were synthesized by ‘click’ ring closure of the corresponding α-azido ω-propargyl telechelic linear PNIPAMs, themselves prepared by reversible addition fragmentation chain transfer (RAFT) polymerization of N-isopropylacrylamide. Differential scanning calorimetry (DSC) studies revealed that both the thermal phase separation in water and the glass transition in bulk of PNIPAM were affected by polymer topology. In aqueous solution, the cyclic polymers exhibit a higher phase separation temperature and broader phase transition range than the corresponding linear counterparts. In bulk, the cyclic polymers display a higher glass transition temperature of lesser molecular weight dependence, as compared to their linear precursors.     

Order of magnitude of the centrifugal distortion constants

A simple empirical relationship is proposed between the centrifugal distortion constants τ'_αααα and the corresponding rotational constants B_α. The properties and applications of this new relationship are discussed.     

The first automated 470.59 MHz 19F NMR‐controlled titration: dissociation constants and ion‐specific chemical shifts of 2‐amino‐4‐fluoro 2‐methylpent‐4‐enoic acid

2‐Amino‐4‐fluoro‐2‐methylpent‐4‐enoic acid, obtained as a 1 : 1 salt with trifluoro‐acetic acid, was characterized by ¹H and ¹⁹F high‐resolution NMR spectroscopy. High‐precision potentiometry led to the dissociation constants pK = 1.879 and pK = 9.054. The first automated 470.59 MHz ¹⁹F NMR‐controlled titration yielded the dynamic chemical shift 〈δ_F〉 as a function of pcH or τ and the ion‐specific chemical shifts: δ_F(H₂L⁺) = ?94.81 ppm, δ_F(HL) = ?94.21 ppm, δ_F(L^?) = ?92.45 ppm. The deprotonation gradients were found to be Δ₁ = ?0.60 ppm and Δ₂ = ?1.76 ppm. Copyright © 2002 John Wiley & Sons, Ltd.     

Preparation of ureido group bearing polymers and their upper critical solution temperature in water

Poly(2‐ureidoethylmethacrylate) (PUEM_n) was synthesized via reversible addition‐fragmentation chain transfer (RAFT) radical polymerization and following polymer reaction. We prepared two PUEM_n samples with different degrees of polymerization (n = 100 and 49). The polymers exhibited upper critical solution temperature (UCST) in phosphate‐buffered saline (PBS) solution. The phase separation temperature (T_p) in PBS can be controlled ranging from 17 to 55 °C by changing molecular weight of the polymer, polymer concentration, and adding NaCl concentration. The polymers in PBS formed coacervate drops by liquid–liquid phase separations below T_p. Results of the dielectric relaxation measurement, the hydration number per monomeric unit was 5 above T_p. Based on a fluorescence study, the polymer formed slightly hydrophobic environments below T_p. The liquid–liquid phase separation was occurred presumably because of weak hydrophobic interactions and intermolecularly hydrogen bonding interactions between the pendant ureido groups. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 2845–2854     

R‐RAFT approach for the polymerization of N‐isopropylacrylamide with a star poly(ε‐caprolactone) core

Reversible addition‐fragmentation chain transfer (RAFT) polymerization is a more robust and versatile approach than other living free radical polymerization methods, providing a reactive thiocarbonylthio end group. A series of well‐defined star diblock [poly(ε‐caprolactone)‐b‐poly(N‐isopropylacrylamide)]₄ (SPCLNIP) copolymers were synthesized by R‐RAFT polymerization of N‐isopropylacrylamide (NIPAAm) using [PCL‐DDAT]₄ (SPCL‐DDAT) as a star macro‐RAFT agent (DDAT: S‐1‐dodecyl‐S′‐(α, α′‐dimethyl‐α″‐acetic acid) trithiocarbonate). The R‐RAFT polymerization showed a controlled/“living” character, proceeding with pseudo‐first‐order kinetics. All these star polymers with different molecular weights exhibited narrow molecular weight distributions of less than 1.2. The effect of polymerization temperature and molecular weight of the star macro‐RAFT agent on the polymerization kinetics of NIPAAm monomers was also addressed. Hardly any radical–radical coupling by‐products were detected, while linear side products were kept to a minimum by careful control over polymerization conditions. The trithiocarbonate groups were transferred to polymer chain ends by R‐RAFT polymerization, providing potential possibility of further modification by thiocarbonylthio chemistry. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Synthesis and characterization of well‐defined block and statistical copolymers based on lauryl methacrylate and 2‐(acetoacetoxy)ethyl methacrylate using RAFT‐controlled radical polymerization

Four well‐defined diblock copolymers and one statistical copolymer based on lauryl methacrylate (LauMA) and 2‐(acetoacetoxy)ethyl methacrylate (AEMA) were prepared using reversible addition‐fragmentation chain transfer (RAFT) polymerization. The polymers were characterized in terms of molecular weights, polydispersity indices (ranging between 1.12 and 1.23) and compositions by size exclusion chromatography and ¹H NMR spectroscopy, respectively. The preparation of the block copolymers was accomplished following a two‐step methodology: First, well‐defined LauMA homopolymers were prepared by RAFT using cumyl dithiobenzoate as the chain transfer agent (CTA). Kinetic studies revealed that the polymerization of LauMA followed first‐order kinetics demonstrating the “livingness” of the RAFT process. The pLauMAs were subsequently used as macro‐CTA for the polymerization of AEMA. The glass transition (T_g) and decomposition temperatures (ranging between 200 and 300 °C) of the copolymers were determined using differential scanning calorimetry and thermal gravimetric analysis, respectively. The T_gs of the LauMA homopolymers were found to be around ?53 °C. Block copolymers exhibited two T_gs suggesting microphase separation in the bulk whereas the statistical copolymer presented a single T_g as expected. Furthermore, the micellization behavior of pLauMA‐b‐pAEMA block copolymers was investigated in n‐hexane, a selective solvent for the LauMA block, using dynamic light scattering. pLauMA‐b‐pAEMA block copolymers formed spherical micelles in dilute hexane solutions with hydrodynamic diameters ranging between 30 and 50 nm. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 5442–5451, 2008     

Interaction of organo-alkali compounds with unsaturated hydrocarbons

The stability of disodium tetramer of α-methylstyrene (“living” polymer) in THF and in a THF-α-methylstyrene mixture has been investigated by spectrophotometry. It was found that at 25°C and at concentrations lower than the equilibrium concentration α-methylstyrene greatly stabilizes the process leading to disappearance of the main absorption band (λ_max = 340 mμ) of “living” polymer. In this case isomerization of “living” polymer is accompanied by quantitative conversion of the compound having λ_max = 340 mμ into a new compound with λ_max = 430 mμ. The constants of the disappearance rate D₃₄₀ and the activation energies of the process were determined in THF and in a THF-α-methylstyrene mixture. The introduction of small amounts of α-methylstyrene into living polymer at 25°C markedly increases its activity in the course of propagation. The experimental results are considered from the standpoint of formation of complexes of living polymer with α-methylstyrene.     

Thermal and rheological properties of miscible polyethersulfone/polyimide blends

Blends of an aromatic polyethersulfone (commercial name Victrex) and a polyimide (commercial name Matrimid 5218), the condensation product of 3,3′,4,4′-benzophenone tetracarboxylic dianhydride and 5(6)-amino-1-(4′-aminophenyl)-1,3,3′-trimethylindane, were studied by differential scanning calorimetry, dynamic mechanical analysis, and rheological techniques. The blends appeared to be miscible over the whole range of compositions when cast as films or precipitated from solution in a number of solvents. After annealing above the apparent phase boundary, located above T_g, the blends were irreversibly phase separated indicating that the observed phase boundary does not represent a true state of equilibrium. Only a narrow “processing window” was found for blends containing up to 20 wt % polyimide. Rheological measurements in this range of compositions indicated that blending polyethersulfone with polyimide increases the complex viscosity and the elastic modulus of the blends. For blends containing more than 10 wt % polyimide, abrupt changes in the rheological properties were observed at temperatures above the phase boundary. These changes may be consistent with the formation of a network structure (due to phase separation and/or crosslinking). Blends containing less than 10 wt % polyimide exhibited stable rheological properties after heating at 320°C for 20 min, indicating the existence of thermodynamic equilibrium.     

Synthesis and characterization of liquid‐crystalline epoxy and its blend with conventional epoxy

Terephthaloyl chloride was reacted with 4‐hydroxy benzoic acid to get terephthaloylbis(4‐oxybenzoic) acid, which was characterized and further reacted with epoxy resin [diglycidyl ether of bisphenol A (DGEBA)] to get a liquid‐crystalline epoxy resin (LCEP). This LCEP was characterized by Fourier transform infrared spectrometry, ¹H and ¹³C NMR spectroscopy, differential scanning calorimetry (DSC), and polarized optical microscopy (POM). LCEP was then blended in various compositions with DGEBA and cured with a room temperature curing hardener. The cured blends were characterized by DSC and dynamic mechanical analysis (DMA) for their thermal and viscoelastic properties. The cured blends exhibited higher storage moduli and lower glass‐transition temperatures (tan δ_max, from DMA) as compared with that of the pure DGEBA network. The formation of a smectic liquid‐crystalline phase was observed by POM during the curing of LCEP and DGEBA/LCEP blends. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 3375–3383, 2003