Epoxidized soybean oil modified using fatty acids as tougheners for thermosetting epoxy resins: Part 2—Effect of curing agent and epoxy molecular weight

A series of bio-rubber (BR) tougheners for thermosetting epoxy resins was prepared by grafting renewable fatty acids with different chain lengths onto epoxidized soybean oil at varying molar ratios. BR-toughened samples were prepared by blending BRs with diglycidyl ether of bisphenol A resins, Epon 828 and Epon 1001F, at different weight fractions and stoichiometrically cured using an amine curing agent, 4, 4′-methylene biscyclohexanamine (PACM). Fracture toughness properties of the unmodified and BR toughened polymer samples—including critical strain energy release rate (G_Ic), and critical stress intensity factor (K_Ic)—were measured to investigate the toughening effect of prepared BRs. It was found that the degree of phase separation and toughening were more controllable relative to similar polymers cured using the aromatic curing agent Epikure W, and the use of higher molecular epoxy resins produces a synergistic effect increasing the toughness much more than similar polymers made with lower molecular weight epoxy resins. Average BR domain sizes ranging from 200 to 900 nm were observed, and formulations with G_Ic, values K_Ic as high as 1.0 kJ/m² and 1.4 MPa m^1/2 were attained respectively for epoxy systems with T_g greater than 130°C.     

Deformation behavior of polystyrene as a function of molecular weight parameters

The mechanical deformation of polystyrene as it relates to molecular weight parameters was investigated. Mechanical testing consisted of uniaxial tension and compression experiments on a variety of polystyrenes. Such quantities as modulus, proportional limit, and various yield stress measurements were determined on polystyrene samples of controlled number-average molecular weight and molecular weight distribution. A basic tool for the mechanical behavior analysis was the use of a power law equation σ = K?ⁿ to examine the initial nonlinear region of each experimentally determined stress–strain curve. Correlations between mechanical deformation and molecular weight parameters were determined using statistical linear regression analysis. It was generally found for uniaxial tension that mechanical parameters in or near the elastic region were independent of M?_n and MWD, while at larger strains correlations were found. For uniaxial compression, stress maxima and the strain where this occurred increased with increasing MWD. Otherwise, mechanical parameter changes in uniaxial compression did not occur with changing M?_n and MWD. Finally, a direct comparison of tension versus compression showed only the initial moduli to be the same. All other mechanical parameters showed significantly differing values, indicating different deformation mechanisms operating in tension verus compression. The analysis of this behavior from both a mechanics and molecular weight viewpoint provides some insight about glassy polymer deformation processes on the microscopic level.     

Number-average molecular weight by size exclusion chromatography

It is now theoretically possible to obtain absolute accurate values of number-average molecular weight of complex polymers (e.g., branched polymers or copolymers) using size exclusion chromatography (SEC) with only a detector that measures the difference between the eluting polymer solution viscosity and the viscosity of the pure mobile phase (a differential viscometer [DV] detector). However, both precision and accuracy of these “DV M?_n” values are of concern. In this work, the precision of NBS 706 polystyrene was found to be two to three times worse for the DV M?_n than for the conventionally calculated M?_n. Also, regarding accuracy, the DV M?_n values were affected by the location of the universal calibration curve along the retention volume axis (a problem intimately associated with the problem of specifying the correct interdetector volume), the sensitivity of the DV detector to low molecular weights present in the sample, and axial dispersion. Each of these sources of error are examined in turn and two methods of calculating M?_n values are proposed. © 1994 John Wiley & Sons, Inc.     

Polyurethane–polystyrene interpenetrating polymer networks synthesized at low temperature: Effect of crosslinking level

Interpenetrating polymer networks (IPNs) of polyurethane (PU)–polystyrene (PS) containing 50 wt % PU were synthesized at low temperature with varying crosslink density of each component. PU was polymerized first, followed by the photopolymerization of PS at low temperature (0 and 40°C). The theoretical molecular weight between crosslink (M?_c) of PU ranged from 8200 to 2050, and the M?_c of PS varied from linear to 2000. The degree of mixing of the components in these IPNs was investigated using dynamic mechanical analysis, electron microscopy, and density measurement. The degree of mixing increased with decreasing M?_c and/or synthesis temperature. The crosslink density variation at low synthesis temperature is more effective in enhancing the miscibility of IPN than at high synthesis temperature, because both the temperature and crosslink density can affect the polymer chain mobility during the synthesis. The variation of PU network crosslink density shows the better effect in increasing the miscibility of IPN than that of the PS network. The morphology and the density behavior agree well with the dynamic mechanical result.     

Polymerization of formaldehyde and the physical properties of the polymerization products. I.

Polymerization of formaldehyde carried out in the solution method and gas supply method using various initiators at various temperatures and characterization of the polyoxymethylene obtained are discussed. In both modes of polymerization, polyoxymethylenes with sufficient molecular weight (M?_n) and small M?_w/M?_n were obtained. The toughness of polymer as determined by the M?_w/M?_n of the polymer with sufficient M_n was confirmed.     

Engineering plastics from lignin. XVII. Effect of molecular weight on polyurethane film properties

A series of five fractions with number average molecular weights (M?_n) between 1500 and 10,000 daltons were isolated from a Kraft hydroxypropyl lignin (HPL). From ¹H-NMR and UV analysis the chemical properties of the HPLs were found to vary slightly with molecular weight. The hydroxyl content decreased while the glass transition temperature (T_g) increased as the HPL molecular weight increased. The Fox-Flory equation adequately described the M?_n vs. T_g relationship. The HPL fractions were used as polyols for the preparation of solvent-cast polyurethane networks (PU) in film form. The T_g of the PUs increased from 40° to 120°C as the M?_n of the polyol rose from 1500 to 10,000 daltons. The molecular weight between crosslinks (M?_c) of the networks was determined by swelling. An observed decrease in M?_c with an increase in M?_n was related to the functionality of the system. The strength properties of films prepared from fractionated HPLs were superior to those prepared from nonfractionated HPLs.     

Structure and properties of cellulose graft copolymers. II. Cellulose–styrene graft copolymers synthesized by the ceric ion method

Styrene was graft-copolymerized onto wood cellulose by the ceric ion method of Mino and Kaizerman. The grafting reaction was found to depend strongly on the concentration of ceric ion in the grafting system and maximum grafting occurred in a narrow range of concentration of initiator, 1.0 × 10^?3-1.8 × 10^?3 mol/l, at 58 ± 1°C. A pretreatment technique, developed to enhance the monomer diffusion into cellulose, was found to increase the grafting considerably. The structures of the cellulose-styrene graft copolymers were studied by hydrolyzing away the cellulose backbone to isolate the grafted polystyrene branches. The molecular weight and the molecular weight distributions of the grafted polystyrene were determined using gel permeation chromatography. The number-average molecular weight (M?_n) ranged from 23,000 to 453,000 and the polydispersity ratios (M?_w/M?_n) varied from 2.5 to 8.0. The grafting frequencies calculated from the per cent grafting and molecular weight data were of the order of 0.05–0.4 polystyrene branches per cellulose chain.     

Thermal degradation of polyethylene in a nitrogen atmosphere of low oxygen content. II. Structural changes occuring in low-density polyethylene at an oxygen content less than 0.0005%

Samples of low-density polyethylene, free from additives, were heated at temperatures between 284° and 355°C under high-purity nitrogen. Changes in molecular weight distribution (MWD), molecular weight averages, and degree of long-chain branching (LCB) were followed by gel chromatography (GPC) and viscosity measurements. Other structural changes were investigated by infrared spectroscopy and differential scanning calorimetry (DSC). At 284° and 315°C, the MWD's were shifted toward higher molecular weights and the M?_w values increased. At 333° and 355°C, the MWD's shift toward lower molecular weight, but the high molecular weight, tail is largely retained. M?_w decreases slowly at 333°C. At 355°C, M?_w undergoes a rapid initial drop which levels off. M?_w/M?_n and the degree of LCB increase with heating time and temperature. Olefinic unsaturation increases. The vinyl groups show a larger relative increase than do the trans-vinylene and vinylidene groups. At 355°C, the peak of the unimodal DSC thermogram is shifted to ～3°C higher temperature. A lower melting peak then develops, and after 72 and 90 min the two peaks are about equal in size. The density increases from 0.922 g/cm³ to 0.930 g/cm³ for samples heated at 355°C, and the weight loss was 1.5% after 90 min. A reaction scheme for the thermal degradation of polyethylene is discussed. Initiation is suggested to be accomplished by scission of allylic C? C bonds. Propagation proceeds by both intra- and intermolecular hydrogen abstraction, followed by β-scission. Termination can occur by both combination and disproportionation. Combination reactions are suggested to account for the observed formation of LCB and high molecular weight material. Due to changes in the degree of LCB during the degradation, viscometry alone will not give a proper measure of the changes in molecular weight.     

Molecular weight and molecular weight distribution from dynamic measurements of polymer melts

A method for determining the molecular weight distribution (MWD) of a polymer melt has been developed using the dynamic elastic modulus (G'), plateau modulus (G), and zero shear complex viscosity (η). The cumulative MWD was found to be proportional to a plot of (G'/G)^0.5 vs. measurement frequency (ω). Frequency (ω) was found to be inversely proportional to (MW)^3.4, as expected. Results were scaled to absolute values using the empirical relationship η ∝ (M?_w)^3.4, where M?_w is the weight-average MW. M?_w, M?_n (number-average MW) and M?_w/M?_n calculated from melt measurements were found to agree with size exclusion chromatography usually well within 10 percent for broad and bimodal distribution samples. M?_w/M?_n tended to be approximately 20 percent higher for narrow distribution samples (M?_w/M?_n < 1.2) because we did not account for a finite distribution of relaxation times from a collection of monodisperse polymer chains. We also did not account for the plasticizing effect of short chains mixed with long ones which caused peak positions to be closer together for Theological vs, size exclusive chromatography (SEC) determinations of MW for bimodal distribution blends.     

Influence of molecular weight and molecular weight distribution on the tensile properties of amorphous polymers

Tensile property data for polystyrene samples of varying polydispersity are correlated with various parametric measures of molecular weight. Traditional measures of molecular weight, such as M?_n, M?_w, and M?_z, are shown to be unable to account for the variation of tensile properties with molecular weight. However, a new molecular weight parameter, termed the failure property parameter, is able to provide a single relationship between tensile strength and the parameter for both the broad and narrow distribution polymers. The form of this parameter is consistent with its having origins in the view that it is the entanglement network in an amorphous polymer that provides the observed strength properties. Specifically for polystyrene, the failure property parameter results indicate that material below 60,000 molecular weight does not contribute to polymer strength. Although the results of this investigation are specifically for polystyrene, the arguments used to develop the failure property parameter are not dependent on polymer chemical structure. Consequently, we believe that both the concepts and definition of this new parameter are applicable to all amorphous polymers.     

Shear degradation of poly(vinyl acetate) in toluene solutions by high-speed stirring

A poly(vinyl acetate) (PVAc) of M?_w 750,000 and M?_w/M?_n 5.10 in toluene solution was sheared in a Virtis-60 homogenizer. The polymer concentration was 3.0 to 12.0 g/100ml, and test temperature was 10 ± 0.5°C. The extent of degradation was measured by gel permeation chromatography (GPC). It was concluded that on shearing (i) the molecular weight decreases rapidly at the beginning of shearing and thereafter decreases ever more slowly toward a limiting value, (ii) the molecular weight distribution is narrowed, (iii) no degradation occurs up to 5000 rpm and thereafter increases with stirring speed, (iv) degradation is more at lower concentrations but concentration is not a sensitive variable, and (v) the chain scission occurs randomly. The Mark-Houwing relationship for PVAc in THF at 25°C was derived as [η] = 2.47 × 10^?4 × M?_v^0.644.     

Some properties of low molecular weight polybutadiene and polytetrahydrofuran

The proposition, that low molecular weight polymer fractions in good solvents behave as if they were under ? conditions, has been examined experimentally. Series of monodisperse hydroxy-terminated polytetrahydrofuran (PTHF), 82% 1,4-polybutadiene (PBD), and 30% 1,4-PBD were prepared, and values of M?_n obtained by vapor-pressure osmometry and endgroup analysis. The Mark–Houwink viscosity parameters K and ν were determined in a number of solvents. The general conclusion is that the proposition is invalid for these systems notwithstanding the fact that ν = 0.50 for one of them [82% 1,4-PBD in methyl ethyl ketone (MEK) at 25°C]. For this particular case, the following evidence suggests that these are actually ? conditions so that the apparent fulfilment of the proposition is fortuitous. (1) Cloud-point precipitation yields ? = 26 ± 3°C in MEK. (2) The value of K is close to that of K_? found elsewhere for PBD in a different solvent at a similar temperature. (3) Application of the Kurata-Stockmayer iterative procedure for estimating K_? from data in good and bad solvents yields a reasonably small discrepancy (10%) between the K_? values from data in toluene and MEK at 25°C for this polymer and only a 3% difference in the unperturbed dimensions (〈r₀²〉/M)^1/2 derived from them. Measured melting points T_m of PTHF (M?_n = 1000–13000), plotted as a function of chain length Z, viz., 1/T_m = 1/T_m0 + 2R/ZΔH_f, yield 43 ± 3°C and 1.6 kcal/submole, respectively, for the limiting melting point T_m0 and the heat of fusion ΔH_f. The former is in good agreement with the value obtained dilatometrically for high molecular weight polymer, while the latter indicates a degree of crystallinity of ca. 54%.     

Siloxane-modified polyethersulfideimide

BDSDA/APB, a novel linear polyethersulfideimide, was synthesized using siloxane units as flexible linkages in the backbone in an attempt to improve use properties and processability. The effect of these flexible linkages on molecular weight buildup (M _n and M _w), G_Ic, flexural strength and modulus, glass transition temperature, and melt-flow properties was determined.     

Plane stress and plane strain fractures in polypropylene

The concepts of Linear Elastic Fracture Mechanics (LEFM) are applied to polypropylene, a homopolymer and two copolymers, with a view to characterizing their brittle behavior at slow rates (0.5 cm/min) in terms of a fracture toughness, K_Ic. The effect of thickness, notch sharpness, and the mode of loading on K_Ic have been investigated in order to determine the plane strain toughness values, K_cI for the materials. The two types of material are compared in terms of their toughness values over a range of temperatures between +30 and ?160°C. Evidently, the small amounts of ethylene added to the copolymers show plasticizing effects, suppressing the yield stress and the ductile-brittle transition temperature. In addition, the copolymers exhibit a ductile-brittle region between ?100 and ?45°C where notch strengthening is apparent in the tension mode and a slow crack growth region between ?45 and ?30°C where slow growth precedes unstable fracture. The homopolymer, however, shows no clear evidence of such intermediate regions, except for slight amounts of slow growth above 0°C, and becomes ductile around 30°C.     

Cooking schedule of alkyd resin preparation. Part II. Effect of cooking schedule on molecular weight distribution of alkyd resin

The effect of cooking schedule on the molecular weight distribution of an alkyd resin was investigated. At the molar ratio of 1.03/0.43/1.00 for glycerin, lauric acid, and phthalic anhydride, two kinds of alkyd resin were prepared, one by maintaining the reaction temperature at 170°C for an hour and then raising it up to 230°C (sample I), the other by raising the temperature up to 230°C at a uniform rate of 33°C per 10 min. (Sample II). Sample I and sample II were fractionated into seven or eight fractions by adding n-heptane to 5 wt-% toluene solution at 30°C. Acid valued, hydroxyl value, intrinsic viscosity [η], and the number-averagè molecular weight M?_n were determined. The result showed that the molecular weight distribution curve obtained from sample I was much narrower than that from sample II. In addition, a relation was found between [η] and M?_n conforming to the equation [η] = KM^α, where different values of K and α were obtained for sample I and sample II. Based on the differences in the molecular weight distribution curves and in the rate constants for the esterification reactions, glycerin/lauric acid and glycerin/phthalic anhydride at 170°C and 230°C, the mechanism of condensation reaction of short oil alkyd resin was discussed.     

Synthesis and characterization of biodegradable lactic acid‐based polymers by chain extension

BACKGROUND: Poly(lactic acid) (PLA), coming from renewable resources, can be used to solve environmental problems. However, PLA has to have a relatively high molecular weight in order to have acceptable mechanical properties as required in many applications. Chain‐extension reaction is an effective method to raise the molecular weight of PLA. RESULTS: A high molecular weight biodegradable lactic acid polymer was successfully synthesized in two steps. First, the lactic acid monomer was oligomerized to low molecular weight hydroxyl‐terminated prepolymer; the molecular weight was then increased by chain extension using 1,6‐hexamethylene diisocyanate as the chain extender. The polymer was characterized using ¹H NMR analysis, gel permeation chromatography, differential scanning calorimetry and Fourier transform infrared spectroscopy. The results showed that the obtained polymer had a M_n of 27 500 g mol^?1 and a M_w of 116 900 g mol^?1 after 40 min of chain extension at 180 °C. The glass transition temperature (T_g) of the low molecular weight prepolymer was 47.8 °C. After chain extension, T_g increased to 53.2 °C. The mechanical and rheological properties of the obtained polymer were also investigated. CONCLUSION: The results suggest that high molecular weight PLA can be achieved by chain extension to meet conventional uses. Copyright © 2008 Society of Chemical Industry     

Polymer rheology: Influence of molecular weight and polydispersity

Literature data on the non-Newtonian flow of bulk polymer and of polymer solutions are correlated on the basis of a four-parameter equation, η = η_∞ + (η₀ ? η_∞)/[1 + (τD)^m], η being the viscosity at shear rate D, and η₀ and η_∞ limiting values at D = 0 and D = ∞, respectively. The parameters η₀, η_∞, and τ all show dependence on molecular weight, and in general there is good correlation between τ and η₀. There is evidence that τ is related to a molecular weight higher than the weight-average. The exponent m shows dependence on molecular weight distribution and approaches an upper limit of unity for a monodisperse linear polymer. For linear unblended polymers it may be expressed empirically by m = (M?_n/M?_w)^1/5.     

Comparaison des méthodes de fractionnement par chromatographie de perméation et par gradient d'elution pour la détermination des répartitions moléculaires des polypropylénes

Molecular weight distributions for polypropylene samples have been determined by a permeation fractionation method (GPC). Porous silica beads were used as a packing material for the columns. The set of columns allows a good separation of the polypropylene macromolecular chains in a range of molecular weights from 5000 to 1.5 × 10⁶, and the thermal and mechanical stabilities of these beads are very good. The calibration has been carried out with fractions of polypropylene of narrow molecular weight distribution prepared by a large-scale column fractionation. The molecular weights M?_w and M?_n and the ratios M?_w/M?_n calculated from the GPC curves show, in general, good agreement with the ones calculated from the column fractionation curves. However, the M?_w/M?_n ratios are always highter in the case of GPC fractionation. This could be due to diffusion phenomena.     

Head to head polymers IX: The rheological behavior of H-H and H-T atactic polystyrenes

The rheological behavior of a sample of H-H polystyrene of M_n of 41,000 and a M_w/M_n of 2 was compared at 160 and 190°C with a sample of H-T atactic polystyrene of similar molecular weight. The melt viscosity of H-H polymer (unlike the H-T polymer) was non-Newtonian at low stresses and decreased more rapidly with stress. This observation seems to indicate a stiffer polymer chain for the H-H polystyrene. The flow activation energy (E*) of H-H polystyrene was found to be dependent on the dynamic shear stress and decreased with increasing dynamic shear stress. The dynamic shear storage modulus of the H-H polymer has a smaller increase of G′ with ω than that of the H-T polystyrene.     

Cooking schedule of alkyd resin preparation. I. Effect of cooking schedule on viscosity-molecular weight relationship

The two constants in the equation log η = A + C′ M?_n^1/2 (η is the viscosity of molten alkyd and M?_n the number average molecular weight) were determined at 110°C for two kinds of alkyd resin prepared with the same formulation but with different cooking schedules. It was found that the magnitude of the slope C′ was larger for the alkyd which was prepared by raising the reaction temperature directly up to 230°C, in comparison with that of the alkyd which was prepared by maintaining the temperature at 170°C for an hour and then raising it up to 230°C. Measurements of η and M?_n were carried out until the gelation occurred. Both upward and downward breaks were observed in log η vs. M?_n^1/2 plot. Based on these viscometric data, the gel point mechanism was discussed. Disagreements between the molecular weight observed and that calculated from the Flory's theoretical equation became more remarkable as the esterification proceeded. This suggested that a large extent of intraesterification reaction is taking place in alkyd synthesis.