Model polyurethane elastomers prepared from noncrystallizable poly(propylene oxide) chains

Elastomers of controlled molecular structure were prepared from hydroxyl-terminated atactic poly(propylene oxide) (PPO) chains having number-average molecular weights M_n in the range 800–4360 g mole^?1. The chains were end-linked into noncrystallizable trifunctional networks using a specially prepared aromatic triisocyanate. The networks thus obtained were studied with regard to their stress–strain isotherms in the unswollen state, in elongation at 25°C, and with regard to their equilibrium swelling in benzene at 61°C. Values of the modulus in the limit at high deformation were in good agreement with corresponding results previously obtained on networks of poly(dimethylsiloxane) (PDMS). This is of considerable importance since use of the widely used “plateau modulus” as a measure of interchain entangling would suggest that the networks of PPO would have a much higher density of such entanglements than would the corresponding networks of PDMS. The close similarity between the moduli of the two types of networks therefore argues against the idea that such entanglements make large contributions to the equilibrium elastomeric properties of a polymer network. These values of the high deformation modulus are also in good agreement with recent molecular theories as applied to the nonaffine deformation of a “phantom” network. The values of the low deformation modulus were considerably smaller than the values predicted for an affine deformation, however, suggesting that the junction points were not firmly embedded in the network structure. This is presumably due to the relatively low degree of chain-junction entangling in the case of relatively short network chains. The swelling equilibrium results were in very good agreement with the new theory of network swelling developed by Flory.     

Extraction and sorption studies using linear polymer chains and model networks

Model networks have been prepared by tetrafunctionally endlinking linear polydimethylsiloxane (PDMS) chains having molecular weights M_n in the range 2000–15,000 g mole^?1. The first series of networks were prepared from mixtures containing known amounts of unreactive linear PDMS chains with molecular weights M_d between 1000 and 16,000 g mole^?1. Rates of extraction were used to estimate diffusion coefficients; as expected, they were found to increase with increase in molecular weight M_c = M_n between crosslinks, but to decrease with increase in M_d. The ease with which all of such a diluent could be removed showed the same dependence on M_c and M_d. A second series of networks was prepared from the same reactive PDMS chains without diluents. Sorption and extraction studies using the same diluents were then carried out. The diffusion coefficients for sorption were found to be in the range (1.7–15.0) × 10^?12 m² s^?1 and depended on both M_c and M_d. The amount of diluent absorbed at equilibrium was between 10 and 70%, which is in good agreement with predictions from the Flory equation for dilation in networks, with account of constraints on crosslink fluctuations.     

Olefin copolymerization with metallocene catalysts. II. Kinetics,cocatalyst, and additives

The kinetics of ethylene/propylene copolymerization catalyzed by (ethylene bis (indeyl)-ZrCI₂/methylaluminoxane) has been investigated. Radiolabeling found about 80% of the Zr to be catalytically active. The estimates for rate constants at 50°C are k₁₁ = 1104 (M_s)^?1, k₁₂ = 430 (M_s)^?1, k₂₂ = 396 (M_s)^?1,k₂₁ = 1020 (M_s)^?1, and k^A_tr,1 + k^A_tr.2 = 1.9 × 10^?3 s^?1. Substitution of trimethylaluminum for methylaluminoxane resulted in proportionate decrease in polymerization rate. The molecular weight of the copolymer is slightly increased by loweing the [Al]/[Zr] ratio, or addition of Lewis base modifier but at the expense of lowered catalytic activity and increase in ethylene content in the copolymer. Lowering of the polymerization temperature to 0°C resulted in a doubling of molecular weight but suffered 10-fold reduction in polymerization activity and increase of ethylene in copolymer.     

Diffusion of polymer molecules into polymer networks: Effect of stresses and contraints

Fine threads of cis-1, 4-polyisoprene, diameter ca. 50 μm, were prepared by drawing from solution and drying. They were crosslinked by reaction with H₂S and SO₂ and then swollen with linear cis-polyisoprene liquids of varied molecular weight M_s, from 1,000 to 24,000 g/mol. Diffusion coefficients were determined from the initial rate of uptake, for both unrestrained and stretched threads. They were found to be in good agreement for stretches of up to about 300%. On the other hand, the equilibrium uptake increased markedly (> 100%) over this range of strain, similar to the increase in swelling observed with low-molecular-weight liquids. Values of diffusion coefficient were also obtained from the rate of deswelling upon release of swollen threads from tension, and from the rate of uptake of polymer liquids by a thin coating of crosslinked polymer, bonded onto glass fibers. Satisfactory agreement was obtained in all cases. A number of simple experiments thus give similar values for the diffusion coefficient of polymer liquids in lightly crosslinked polymer networks, in the range 10^?13?10^?16 m²/s, depending upon the molecular weight M_s of the polymer liquid approximately weight as M^?2_s. The amount of liquid absorbed was strongly dependent on its molecular weight, M_s, the degree of crosslinking of the host material, and applied stresses or constraints.     

Critical molecular weight for onset of non-newtonian flow and upper newtonian viscosity of polydimethylsiloxane

The flow curves of fractionated polydimethylsiloxanes of different molecular weights were obtained over a wide range of shear rates, from 3 × 10^?1 to 4.3 × 10⁶ sec^?1, by use of a gas-driven capillary viscometer designed to decrease the experimental error in high shear rate region. Non-Newtonian flow can occur at molecular weights below the critical molecular weight M_c for the entanglement of polymer chain. The critical molecular weight M′_c for the onset of the non-Newtonian flow is identical with that of the segment of viscous flow. For the polymer of molecular weights from M′_c to M_c, the upper Newtonian viscosity increases with an increase in molecular weight. Above M_c, the upper Newtonian viscosity is almost independent of the molecular weight.     

The use of model networks with reptating chains to study extraction efficiencies

It is possible to prepare “model” elastomeric networks having known values of the molecular weight M_c between crosslinks by endlinking functionally terminated polymer chains having number-average molecular weights M_n equal to the desired values of M_c. If chains having chemically inert groups at both ends are intentionally included during the preparation of such a system, they will remain unattached, merely reptating through the subsequently formed network structure. This technique was used to prepare a series of tetrafunctional polydimethylsiloxane (PDMS) networks having essentially the same degree of crosslinking (10^?3M_c = 11.3 g mol^?1) and constant amount of diluent in the form of unattached PDMS chains having molecular weights of 10^?3M_d = 26.4, 18.6, 15.8, 9.8, 6.7, 1.2, and 0.70 g mol^?1. Because of the very high mobility of PDMS, it was also possible to introduce essentially the same amount of the same diluents into already formed PDMS networks having the same M_c. Extractions carried out using tetrahydrofuran at room temperature showed that the diluent (“sol fraction”) introduced by swelling the network is more easily removed than that present during the endlinking, possibly because of less convoluted arrangements within the network structure. Chains with the largest values of M_d which were present during the endlinking were found to be very difficult to remove entirely. It is therefore extremely important to carry out exhaustive extractions to obtain reliable values of network sol fractions, particularly when such data are to be used to estimate extents of reaction in the preparation of end-linked elastomers.     

Studies of self-diffusion of poly(dimethylsiloxane) chains in PDMS model networks by pulsed field gradient NMR

We have measured the diffusion of poly(dimethylsiloxane) (PDMS) chains in PDMS model networks by using a pulsed field gradient NMR technique. The model networks have been prepared by tetrafunctional endliking of linear PDMS chains having molecular weights M_n of 3,700 and 7,400 g mol^?1. The diffusants have been incorporated in the networks by immersing pieces of them in PDMS linear chains with molecular weights M_n between 3,000 and 12,000 g mol^?1 and molecular weight distributions M_w/M_n between 1.1 and 1.7. Although spin-echo attenuation results were fitted to a model which takes into account polydispersity of the diffusant, these results did not exhibit any dependency upon the molecular weight distribution. The self-diffusion coefficients of PDMS chains in the PDMS model networks were found to be smaller than in the melt, and the exponents for the diffusion coefficient dependence on M_n in the networks were found to be about-1.3. Free diffusion will give an exponent equal to-1, whereas free volume contributions or behavior intermediate between free and entangled diffusion will increase the magnitude of the exponent.     

Chain dimensions in plastically deformed semicrystalline polymers

The effect of plastic deformation on the chain dimensions of polymers in the semicrystalline state was investigated using linear hydrogenated polybutadiene (HPB), a model ethylene/butene-1 copolymer having about 40% crystallinity at room temperature. Dilute blends of deuterium-labeled chains with various molecular weights (20,000 ≤ M ≤ 214,000) in the same unlabeled matrix (M = 95,000) were uniaxially stretched at 25°C to extension ratios of α = 2.4 and 4.4. Radius of gyration normal to the stretch direction R_⊥ was measured for the labeled chains by small-angle neutron scattering. The molecular extension ratio inferred from these data α_m = R/R was significantly smaller than α for short chains (M < 50,000) but increased to the affine range α_m = α for M > 100,000. This variation in α_m/α closely parallels the molecular weight dependence of mechanical strength and ductility in HPB over the same range.     

Ruthenium‐catalyzed fast living radical polymerization of methyl methacrylate: The R?Cl/Ru(Ind)Cl(PPh3)2/n‐Bu2NH initiating system

A fast living radical polymerization of methyl methacrylate (MMA) proceeded with the (MMA)₂? Cl/Ru(Ind)Cl(PPh₃)₂ initiating system in the presence of n‐Bu₂NH as an additive [where (MMA)₂? Cl is dimethyl 2‐chloro‐2,4,4‐trimethyl glutarate]. The polymerization reached 94% conversion in 5 h to give polymers with controlled number‐average molecular weights (M_n's) in direct proportion to the monomer conversion and narrow molecular weight distributions [MWDs; weight‐average molecular weight/number‐average molecular weight (M_w/M_n) ≤ 1.2]. A poly(methyl methacrylate) with a high molecular weight (M_n ～ 10⁵) and narrow MWD (M_w/M_n ≤ 1.2) was obtained with the system within 10 h. A similarly fast but slightly slower living radical polymerization was possible with n‐Bu₃N, whereas n‐BuNH₂ resulted in a very fast (93% conversion in 2.5 h) and uncontrolled polymerization. These added amines increased the catalytic activity through some interaction such as coordination to the ruthenium center. © 2002 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 40: 617–623, 2002; DOI 10.1002/pola.10148     

The effect of network chain-length distribution,specifically bimodality,on strain-induced crystallization

Polyurethane elastomers were prepared from a series of poly(ethylene oxide) samples by end-linking the chains into “model” trifunctional networks. The molecular weight M_c between crosslinks in such networks is simply the number-average molecular weight M_n of the precursor polymer. End-linking samples separately gave networks with unimodal distributions of network chain lengths, whereas end-linking mixtures of two samples having very different values of M_n gave bimodal distributions with average values of M_c equal to the average value of M_n for the two samples. Stress-strain isotherms in elongation were obtained for these networks, both unswollen and swollen to various extents. Strain-induced crystallization was manifested in elastic properties that changed significantly with changes in temperature. Swelling has more complicated effects, since it causes deformation of the network chains as well as melting of some of the crystallites. Comparisons among stress-strain isotherms at constant M_c indicate that bimodality facilitates strain-induced crystallization.     

Titanium complexes of dialkanolamine ligands as initiators for living ring‐opening polymerization of ε‐caprolactone

The titanium complexes with one ( 1a , 1b , 1c ) and two ( 2a , 2b ) dialkanolamine ligands were used as initiators in the ring‐opening polymerization (ROP) of ε‐caprolactone. Titanocanes 1a and 1b initiated living ROP of ε‐caprolactone affording polymers whose number‐average molecular weights (M_n) increased in direct proportion to monomer conversion (M_n ≤ 30,000 g mol^?1) in agreement with calculated values, and were inversely proportional to initiator concentration, while the molecular weight distribution stayed narrow throughout the polymerization (M_w/M_n ≤ 1.2 up to 80% monomer conversion). ¹H‐NMR and MALDI‐TOF‐MS studies of the obtained poly(ε‐caprolactone)s revealed the presence of an isopropoxy group originated from the initiator at the polymer termini, indicating that the polymerization takes place exclusively at the Ti–OⁱPr bond of the catalyst. The higher molecular weight polymers (M_n ≤ 70,000 g mol^?1) with reasonable MWD (M_w/M_n ≤ 1.6) were synthesized by living ROP of ε‐caprolactone using spirobititanocanes ( 2a , 2b ) and titanocane 1c as initiators. The latter catalysts, according MALDI‐TOF‐MS data, afford poly(ε‐caprolactone)s with almost equal content of α,ω‐dihydroxyl‐ and α‐hydroxyl‐ω(carboxylic acid)‐terminated chains arising due to monomer insertion into “Ti–O” bond of dialkanolamine ligand and from initiation via traces of water, respectively. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 1230–1240, 2010     

The influence of molecular weight of the donor polymer on the solid-state behavior of interchain EDA complexes

N-(2-hydroxyethyl)carbazolyl methacrylate (HECM) and N-ethyl-3-hydroxymethyl carbazolyl methacrylate (HMCM-2) were polymerized by group transfer polymerization to varying molecular weights of somewhat narrow molecular weight distribution. The thermal behavior of the homopolymers and of their EDA complexes with poly(β-hydroxyethyl-3,5-dinitrobenzoyl methacrylate) (PDNBM-2) was studied as a function of molecular weight. The T_g′s of both polymers and their miscible complexes increase steadily as molecular weight increases and then become constant at about M _n = 6000. Both the PHECM–PDNBM-2 and PHMCM-2—PDNBM-2 systems are thermally reversible miscible networks over all polymer molecular weights. Miscibility is thermodynamically controlled over the entire range of molecular weights in the first system and decomplexation does not occur below the decomposition temperature. However, miscibility is thermodynamically controlled in the second system when the molecular weight of PHMCM-2 is less than 5000, and kinetically controlled for higher molecular weights. The decomplexation temperature or LCST of the PHMCM-2–PDNBM-2 system occurs below the decomposition temperature and increases with decreasing PHMCM-2 molecular weight, in agreement with theoretical predictions on the dependence of LCST on polymer molecular weight.     

Chain dynamics in entangled polymers: Diffusion versus rheology and their comparison

This review article scrutinizes and reanalyzes the extensively available literature data on the tracer and self chain diffusion coefficients D_tr and D_s along with the corresponding zero‐shear viscosity η₀ to show that D_s ∝ M^?ν starts with ν > 2.0 and converges to the asymptotic scaling exhibited by D_tr ∝ M^?2.0 as the molecular weight M increases beyond M/M_e = 10–20, in contrast to the onset of the asymptotic scaling M³ for η₀ taking place typically for M/M_e ? 10–20. A coherent analysis of these observations leads to the suggestion that the observed crossover in D_s is due to the constraint release effect, which diminishes around M/M_e = 10–20 and is negligible in measurements of D_tr when the matrix molecular weight P is much greater than M. The contour length fluctuation (CLF) effect, which is believed to cause the molecular weight scaling of η₀ to deviate significantly from its limiting behavior of M³, has little direct influence on the chain diffusion. The absence of the CLF effect on D_s leads to a much stronger than linear dependence of the product η₀D_s on M, which has been observed previously. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1589–1604, 2003     

Living cationic polymerization of ethyl 2-(vinyloxy)ethoxyacetate: A vinyl ether with an ether and an ester function in the pendant

Ethyl 2-(vinyloxy)ethoxyacetate ( 4 ; CH₂?CH? OCH₂CH₂OCH₂? COOC₂H₅), a vinyl ether having both carboxylic acid ester and oxyethylene unit in its pendant, afforded well-defined living polymers when polymerized by the hydrogen iodide/iodine (HI/I₂) initiating system in toluene at ?40°C. The polymers possessed a narrow molecular weight distribution (M _w/M _n ≤ 1.15), and their molecular weight (M _n) increased proportionally to monomer conversion or the molar ratio of the monomer to hydrogen iodide. The polymer molecular weight also increased upon addition of a fresh feed of the monomer to a completely polymerized reaction mixture. Polymers of high molecular weights (M _n > 5 × 10⁵) and broad molecular weight distributions were obtained by BF₃OEt₂ in toluene at ?40°C. Polymerization rate of 4 with HI/I₂ is ca. 100 times greater than that of the corresponding alkyl vinyl ether, and thus 4 was found to be one of the most reactive vinyl ethers thus far studied. Alkaline hydrolysis of the pendant ester groups of the polymers gave a vinyl ether-based polymeric carboxylic acid 6 with a narrow molecular weight distribution.     

Dynamic mechanical properties of moderately concentrated polystyrene solutions

The storage (G′) and loss (G″) shear moduli have been measured in the frequency range from 0.04 to 630 Hz for solutions of narrow distribution polystyrenes with molecular weights (M) 19,800 to 860,000, and a few of poly(vinyl acetate), M = 240,000. The concentration (c) range was 0.014–0.40 g/ml and the viscosities of the solvents (diethyl phthalate and chlorinated diphenyls) ranged from 0.12 to 70 poise. Data at different temperatures (0–40°C) were combined by the method of reduced variables. Two types of behavior departing from the usual frequency dependence describable by the Rouse-Zimm-Tschoegl theories were observed. First, for M ? 20,000, the ratio (G″ ? ωη_s)/G′ in the neighborhood of ωτ₁ = 1 was abnormally large and the steady-state compliance J was abnormally small, especially at the lowest concentrations studied. Here ω is circular frequency, η_s solvent viscosity, and τ₁ terminal relaxation time. Related anomalies have been observed by others in undiluted polymers at still lower molecular weights. Second, at the highest concentrations and molecular weights, a “crossover” region of the logarithmic frequency scale appeared in which G″ ? ωη_s < G′. The width of this region is a linear function of log c; the frequency dependence under these conditions can be represented by a sequence of Rouse relaxation times grafted on to a sequence of Zimm relaxation times. For each molecular weight, the terminal relaxation time τ₁ was approximately a single function of c for different solvents of widely different η_s. At lower concentrations, τ₁ was close to the Rouse prediction of 6ηM/π²cRT, where η is the steady-flow viscosity; but at higher concentrations, τ₁ was proportional to η/c² and corresponded, according to a recent theory of Graessley, to an average molecular weight of 20,000 between entanglement coupling points in the undiluted polymer.     

Living cationic polymerization of isobutylene coinitiated by FeCl3 in the presence of isopropanol

A simple but effective FeCl₃‐based initiating system has been developed to achieve living cationic polymerization of isobutylene (IB) using di(2‐chloro‐2‐propyl) benzene (DCC) or 1‐chlorine‐2,4,4‐trimethylpentane (TMPCl) as initiators in the presence of isopropanol (iPrOH) at ?80 °C for the first time. The polymerization with near 100% of initiation efficiency proceeded rapidly and completed quantitatively within 10 min. Polyisobutylenes (PIBs) with designed number‐average molecular weights (M_n) from 3500 to 21,000 g mol^?1, narrow molecular weight distributions (MWD, M_w/M_n ≤ 1.2) and near 100% of tert‐Cl terminal groups could be obtained at appropriate concentrations of iPrOH. Livingness of cationic polymerization of IB was further confirmed by all monomer in technique and incremental monomer addition technique. The kinetic investigation on living cationic polymerization was conducted by real‐time attenuated total reflectance Fourier transform infrared spectroscopy. The apparent constant of rate for propagation (k_p^A) increased with increasing polymerization temperature and the apparent activation energy (ΔE_a) for propagation was determined to be 14.4 kJ mol^?1. Furthermore, the triblock copolymers of PS‐b‐PIB‐b‐PS with different chain length of polystyrene (PS) segments could be successfully synthesized via living cationic polymerization with DCC/FeCl₃/iPrOH initiating system by sequential monomer addition of IB and styrene at ?80 °C. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Separation of polar and steric effects on absolute rate constants and arrhenius parameters for the reaction of tert-butyl radicals with alkenes

Absolute rate constants and their temperature dependencies were measured for the reaction of tert-butyl radicals with 24 substituted ethenes and several other compounds in 2-propanol solution by time-resolved electron spin resonance. At 300 K the rate constants cover the range from 60 M^?1 s^?1 (1,2-dimethylene) over 16,500 M^?1 s^?1 (vinyl-chloride) to 460,000 M^?1 s^?1 (2-vinylpyridine). For the mono- and 1,1-disubstituted ethenes log k₃₀₀ increases and the activation energy decreases with increasing electron affinity of the olefins. The frequency factors are in the range log A/M^?1 s^?1 = 7.5 ± 1.0 as typical for addition reactions, with minor exceptions. Electron affinity (polar) and steric effects on reactivity are separated for the addition of tert-butyl to chloro- and methyl-substituted ethylenes. A comparison with rate data for methyl, ethyl, 2-propyl, and other radicals indicates both polar and steric effects on radical substitution.     

Viscometric properties of dilute polystyrene/dioctyl phthalate solutions

We report viscometric data collected in a Couette rheometry on dilute, single‐solvent polystyrene (PS)/dioctyl phthalate (DOP) solutions over a variety of polymer molecular weights (5.5 × 10⁵ ≤ M_w ≤ 3.0 × 10⁶ Da) and system temperatures (288 K ≤ T ≤ 318 K). In view of the essential viscometric features, the current data may be classified into three categories: The first concerns all the investigated solutions at low shear rates, where the solution properties are found to agree excellently with the Zimm model predictions. The second includes all sample solutions, except for high‐molecular‐weight PS samples (M_w ≥ 2.0 × 10⁶ Da), where excellent time–temperature superposition is observed for the steady‐state polymer viscosity at constant polymer molecular weights. No similar superposition applies at a constant temperature but varied polymer molecular weights, however. The third appears to be characteristic of dilute high‐molecular‐weight polymer solutions, for which the effects of temperature on the viscosity curve are further complicated at high shear rates. The implications concerning the relative importance of hydrodynamic interactions, segmental interactions, and chain extensibility with increasing polymer molecular weight, system temperature, and shear rate are discussed. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 787–794, 2006     

Molecular weight distribution and mechanical behavior of PBI fibers

Two different polybenzimidazole (PBI) samples have been investigated in order to correlate the differences in molecular weight distribution (MWD) with changes in the elastic modulus and strength of undrawn and drawn fibers. It has been found that within the weight-average molecular weight range (7,000 ≤ M_w ≤ 13,000) there was no obvious correlation with M_w and M_n. However, one sample had a narrow unimodal molecular weight distribution and the other a wide bimodal molecular weight distribution. The small percentage of high molecular weight present in the latter sample gave its fibers better mechanical properties. X-ray diffraction studies showed that the orientation in both drawn fiber samples was equal. This isolated the effects of the molecular weight distribution on mechanical properties.     

Rheological properties of poly(lactides). Effect of molecular weight and temperature on the viscoelasticity of poly(l‐lactic acid)

The dynamic viscoelastic behavior of Poly(l‐lactic acid) (PLLA), with molecular weights ranging from 2,000 to 360,000, have been studied over a broad range of reduced frequencies (approximately 1 × 10⁻³ s⁻¹ to 1 × 10³ s⁻¹), using time–temperature superposition principle. Melts are shown to have a critical molecular weight, M_c, of approximately 16,000 g/mol, and an entanglement density of 0.16 mmol/cm³ (at 25°C). PLLA polymers are noted to require substantially larger molecular weights in order to display similar melt viscoelastic behavior, at a given temperature, as that for conventional non‐biodegradable polymers such as polystyrene. The reason for this deviation is suspected to be due to steric hindrance, resulting from excessive coil expansion or other tertiary chain interactions. PLLA melts show a dependence of η₀ on chain length to the 4.0 power (M), whilst J is independent of M_W in the terminal region. Low molecular weight PLLA (∼ 40,000) shows Newtonian‐like behavior at shear rates typical of those achieved during film extrusion. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1803–1814, 1999