Vinylidene fluoride copolymers with fluoroolefins

Summary Copolymers of vinylidene fluoride (VDF) with fluoroolefins of the series CF₂CFX (X = F, Cl, Br, CF₃ and C₅F₁₁) have been synthesized in the 0–16 mole % CF₂CFX range. Results of physico-chemical characterisation by means of DSC techniques (T_g, T_M, H_f) and X-ray diffraction are examined and related to the nature of the substituent of the fluoroolefin. The T_g/T_M ratio varies from 0.5 for the VDF homopolymer to higher values at increased CF₂CFX content, although to a different extent for the various series of copolymers. The results show that the Beaman-Boyer empirical rule (T_g/T_M=2/3), developed for homopolymers, also applies to some copolymeric systems.     

Synthesis and properties of polyurethane elastomers based on renewable castor oil polyols

In this contribution, castor oil polyols with functionality of f = 2.7 and f = 2 are used as soft segments (SS) for synthesizing polyurethane elastomers (PUEs) without addition of petroleum-based polyol. The effect of molar ratio of castor oil polyols on structure and properties of PUEs has been investigated by Fourier transform infrared spectroscopy, differential scanning calorimetry, thermogravimetric analysis, dynamic mechanical analysis, X-ray diffraction, tensile, swelling, and water absorption tests. The results reveal that hydrogen bonding mainly exists in hard segments (HSs) and weakens with decreasing the molar percentage of castor oil polyol (f = 2.7) in SS. T _g of SS decreases while T _g of HS remains constant as molar percentage of castor oil polyol (f = 2.7) decreased. The initial degradation temperatures (T_5%) are above 300 °C and independent of the molar ratio of castor oil polyols. However, the temperature at 50% weight loss (T_50%) decreases significantly as molar percentage of castor oil polyol (f = 2.7) decreased. Moreover, PUEs exhibit very low water absorption rate, <1%, after immersing in water for 140 h at room temperature. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47309.     

Melt viscosity of poly(diethylene glycol-co-succinic acid): Molar mass dependence in the nonentangled regime

Experimental and theoretical results are reported on the dependence of melt viscosity on the molar mass of poly(diethylene glycol-co-succinic acid) (DEG-SA) in the nonentangled regime. A power law described the behavior with an exponent of 2.09 ± 0.10 at 30°C and of 2.59 ± 0.09 at 90°C. The modified Rouse theory was used to describe the relationship between melt viscosity and molar mass. The parameters related to the modified Rouse theory were determined by the WLF equation and the free-volume theory. The WLF parameters of various samples were checked at their T_g, which is the reference temperature. Results confirmed that the free volume and the monomeric friction coefficient (ξ₀) dominate the behavior of the melt. The agreement between the calculated and experimental melt viscosity is satisfactory. © 1994 John Wiley & Sons, Inc.     

A unifying approach for melt rheology of linear polystyrene

Several studies of melt rheological properties of polystyrene have been conducted over the past 50 years. Several approaches, including empirical models, have been developed to understand the behavior of materials using simple equations. The existing melt rheology models are best suited for high‐molecular‐weight polymers whose T_g does not vary. In this work, a semiempirical viscosity equation has been derived, including the effect of T_g dependence on molecular weight, to describe the melt rheology of low‐molecular‐weight polymers. The equation is derived based on a combination of well‐known concepts, such as the effects of free volume and molecular dynamics on polymer rheology. This provides a better understanding of the rheological behavior in the low‐molecular‐weight regime with respect to temperature and molecular weight. Because of the industrial trend towards lower molecular weight materials for applications such as high solids coatings, this unifying approach, based on the free volume theory with a simple expression, is of extreme practical significance. This equation can predict the zero shear viscosity behavior for different molecular weights, including low‐molecular‐weight regions, and temperatures. Viscosity calculations using the empirical equation agree with published experimental data. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 2597–2607, 2007     

Effect of Piperazine Concentration on the Properties of Lower Purity Dimer Acid-Synthesized Polyamide Hot-Melt Adhesive

Abstract

Polyamide hot-melt adhesive was synthesized from lower purity, i.e., 75% purity dimer acid, sebacic acid, ethylenediamine and piperazine. The effect of piperazine content on the properties of polyamides, such as thermal properties: heat of fusion (H _f), heat of crystallization (H _c), softening point (T _s) and glass transition temperature (T _g); mechanical properties: tensile strength and hardness; adhesion properties: lap shear strength (LSS) and T-peel strength (TPS); and rheological properties was investigated. Concentration of piperazine was varied from 12.5 to 37.5 molar %. It was found that H _f, H _c, T _s, T _g, tensile strength, hardness, LSS, TPS and viscosity all increased with decrease in molar percentage of piperazine. This is due to increase in crystallinity of the polyamide with decrease in piperazine, giving better packing of molecules in the adhesive, thus giving better properties.     

Co‐(POSS#‐styrene) nanocomposites reduced the glass‐transition temperature,rubbery modulus,and melt viscosity of entangled polystyrene

The addition of polyhedral oligomeric silsesquioxane‐styrene copolymers, co(POSS#‐sty), to entangled polystyrene (PS) reduced (1) the glass‐transition temperature, T_g,blend, (2) the rubbery modulus, and (3) the melt viscosity. POSS#‐sty copolymers with # = 15, 25, and 45 wt% POSS were blended with PS. The blends were miscible and T_g,blend decreased with POSS#‐sty content. Strikingly, POSS#‐sty copolymers also reduced the melt viscosity, up to an order of magnitude reduction. The reductions of T_g,blend and melt viscosity were driven by the type of POSS#‐sty copolymer, POSS45‐sty producing the largest decrease of T_g,blend. Linear viscoelasticity and the time–temperature superposition (TTS) principle (using T_ref = T_g + 50 K to ensure iso‐frictional conditions) revealed that POSS#‐sty induced up to an order of magnitude reduction of the rubbery modulus G_e. The increase of free volume f_g promoted by POSS#‐sty induced the reduction of T_g,blend and G_e, as revealed by TTS analysis. The increase of free volume promoted by POSS#‐sty induced chain intercalation (TEM showed that POSS domains were smaller than the molecular mesh) and these are key factors for the chain disentanglement with the consequent rubbery modulus and melt viscosity reductions. The use of low‐molecular weight polystyrene alone will not produce increase of free volume and tube dilation. POLYM. ENG. SCI., 59:2377–2386, 2019. © 2019 Society of Plastics Engineers     

Enthalpies of vaporization of oligomers of poly(hexamethylene sebacate) and esters of alkylcarboxylic acids

Enthalpies of vaporization for esters covering a molecular weight range of about 74–939 g/mol · [monocarboxylics; linear esters of sebacic series; branched esters of triglyceride series; and, oligomer esters of poly(hexamethylene sebacate)] and a temperature range of about 273.15–523.15 K have been empirically fitted to within about 5% to an equation of the following form: ΔHv(T,M) = S(T)f(M) + I₀(T), where S(T) = C Ln(T) + K₀, I₀(T) = aT + b₀, and f(M) = M/(1 + a₀M), M is the molecular weight (molar mass); T is in degrees Kelvin; and, C, K₀, a, b₀, and a₀ are constants. These results were used to determine the heat capacity difference, ΔC_p = C_p(l) − C_p(g), and compared to calculated values from functional relationships of C_p(l) and C_p(g), l is liquid g is gas. The heat capacity difference results in conjunction with C_p(l) were used to empirically calculate the heat capacity of the gas, C_p(g), over the molecular weight and temperature ranges investigated and compared to a group contribution method. The functional forms for ΔHv(T,M), ΔC_p(T,M), C_p(l), and C_p(g) were also found to be applicable for n-alkanes. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 731–746, 1998     

Parameters of the Fluctuation Free Volume Theory for Glasses in the Ge-As-Se System

The fraction of fluctuation free volume (f _g = V _f/V) frozen at the glass transition temperature T _g is determined from the temperature dependence of the viscosity in the glass transition range in terms of the Vogel-Fulcher-Tammann equation and the formula _f T _g T _g = f _gln(1/f _g). The fluctuation free volume fractions f _g obtained according to these two procedures for glasses in the Ge-As-Se system are in quite reasonable agreement. The energies E _h of formation of fluctuation microvoids and their volumes V _h are calculated. It is demonstrated that the quantities f _g, E _h, and V _h and the ratio of the microhardness H to T _g depend substantially on the glass structure and can serve as characteristics of the rigidity of the glass networks. It is noted that the fluctuation free volume fraction f _g is a nonmonotonic function of the mean coordination number Z _m and that it exhibits a specific dependence on the lattice Grüneisen parameter . The Poisson ratios are estimated from the fluctuation free volume fraction f _g with the use of the relationship f _gln(1/f _g) = . It is shown that the Poisson ratios thus obtained are close to those calculated from the data on the transverse (V _s) and longitudinal (V _l) velocities of ultrasound.     

Effect of dispersity on the Tll (>Tg) transition in polystyrene

The presence of a high temperature (>T_g) relaxation in amorphous polystyrene has been investigated further. In the previous work,¹ the techniques of differential thermal analysis (DTA) and torsional braid analysis (TBA) were employed to study polystyrene as a function of “monodisperse” molecular weight. The occurrence of the T_ll transition appeared to be associated with the attainment of a critical viscosity level with also corresponded with a free volume level. An entanglement network developed at a critical value of molecular weight, M_c, giving a break in the T_ll-versus-M plots. The present work deals with the influence of dispersity on the T_ll transition, below and above M_c. A series of binary blends of “monodisperse” anionically polymerized polystyrenes with systematic changes in M?_n and heterogeneity index (M?_w/M?_n) was tested by TBA. The results show that when both components have molecular weights below M_c, single and average values of T_g and T_ll are observed which are linearly related to M?_n^?1, as predicted by free volume arguments. Although a single T_g is observed when one component has a molecular weight above and the other has a molecular weight below M_c, the components appear to undergo the T_ll relaxation independently. The results indicate that both the glass transition and the T_ll transition are basically governed by the same type of molecular motion but at different length ranges.     

Physical aging in graphite/epoxy composites

Matrix-dominated mechanical behavior of a graphite/epoxy composite has been found to be affected by sub-T_g annealing. Postcured (±45°)_4S specimens of Thornel 300 graphite/Narmco 5208 epoxy were quenched from above T_g and given a sub-T_g annealing at 140°C for times up to 10⁵ min. The ultimate tensile strength, strain-to-break, and toughness of the composite material were found to decrease as functions of sub-T_g annealing time. No weight loss was observed during the sub-T_g annealing. The time-dependent change in mechanical behavior is explained on the basis of free-volume changes that are related to the physical aging of the nonequilibrium glassy network-epoxy. The results imply possible changes in composite properties with service time.     

Free volume of liquids and polymers from internal pressure studies

The free volume, v_f, of liquids is defined in many ways. Comparison of solid and liquid behavior indicates that the definition for free volume in terms of the internal pressure of the liquid (?E/?V)_T, is physically reasonable. Application of the definition of free volume, v_f = RT/(?E/?V)_T, to polymethylenes, coupled with surface energy values, leads to an evaluation of both polymer segmental volume, ?_s, and free volume per segment, (v_f)_s, as a function of temperature. These equilibrium thermodynamic measurements of ?_s and (v_f)_s lead to an energy of activation for viscous flow in good agreement with viscosity studies. Information of this type could be of great use in considering many current problems in polymer flow such as the effect of pressure on viscosity.     

Determination of the Fluctuation Free Volume in the Glass Transition Range from the Data on Viscosity and Thermal Expansion

The fraction of fluctuation free volume f _g at the glass transition temperature is determined by two methods, namely, from the data on the viscosity (f _g) and on the jump in the bulk expansion coefficient (f _g ^*). A comparison of the results obtained shows that the values of f _g and f _g ^* are in satisfactory agreement for silicate glasses and significantly differ for borate glasses. The origin of the difference between the mechanisms of viscous flow of silicate and borate glasses is discussed.     

Investigation of the Tu(>Tg) relaxation in homopolymers and block copolymers of styrene by torsional braid analysis

Investigation of the T_u (>T_g) relaxation in amorphous polymers of styrene by the technique of torsional braid analysis is reviewed. For the most part the relaxation behaves like the glass transition (T_g) in its dependence on molecular weight, on average molecular weight in binary polystyrene blends, and on composition in a polystyrene homogeneously plasticized throughout the range of composition. Diblock and triblock copolymers also display a T > T_g relaxation above the T_g, of the polystyrene phase. Two results in particular suggest that the T_u relaxation is molecularly based. (1) The T_u temperature is determined by the number average molecular weight for binary blends of polystyrene when both components have molecular weights below M_c. (the critical molecular weight for chain entanglements). (2) Homopolymers, and diblock and triblock copolymers of styrene, have a T > T_g relaxation at approximately the same temperature when the molecular weight of the styrene block is equal to that of the homopolymer.     

Experimental arguments on the conformation of molecules in molten and glassy polymers

In molten non-crystallising polymers we find normal random coil conformations of the molecules. This is inferred from the theoretically predicted and experimentally observed relationships between the properties of solutions (relaxational modulus, viscosity, osmotic pressure and specific volume) and melts. It is also derived from the intrinsic viscosities (η) and the diffusion constants of molecules with a very high molecular weight in polymeric solvents (M = 10⁴?10⁵) of the same chemical constitution. Moreover the properties of melts (relaxational modulus, flow curve, viscosity) depend on chemical structure and long chain branching in the manner predicted assuming a random coil conformation. At the glass temperature T_g the conformation resembles that in the melt as the temperature dependence of the conformation at T > T_g shows ((η) in polymeric solvents, diameters of the domains in block copolymers). However, on cooling below T_g an irreversible volume shrinking process may lead to an inhomogeneous distribution of densities and possibly to an extended conformation of molecules in grain boundaries. Experiments show that only strong deviations from random coil conformations decrease the ultimate strength of a glass.     

Polystyrene crosslinked with oligomeric and polymeric poly(dimethyl siloxane) derivatives. Thermal and dynamic mechanical studies

Styrene has been copolymerized with various bifunctional poly(dimethyl siloxane) cross-linkers having molar masses in the range 200–35,000. Calorimetric and rheological measurements show that the crosslinkers change T_g, T_ll, and T_exo, as well as melt viscosity and elasticity of polystyrene. These changes are strongly dependent on the molar mass of the crosslinker. Increasing the length of the siloxane crosslinker lowers T_g and melt viscosity, if the molar mass of the crosslinker is in a range 200–2000. Copolymerizing styrene with a bifunctional poly(dimethyl siloxane) having molar mass near 35,000 leads to a phase-separated polymer where polystyrene phase largely retains its original properties.     

Factors influencing the impact strength of high impact polystyrene

The relationship between synthesis factors and the impact resistance of high impact polystyrene (HIPS) is investigated in the light of its morphology and dynamic mechanical properties. A decrease in polymerization temperature results in an increase in T_g, melt viscosity and molecular weight of the continuous polystyrene phase as characterized by gel permeation chromatography. The separated, occluded polystyrene phase however shows an invariant T_g suggesting that the grafting and/or crosslinking effect overweighs the molecular weight effect. The observed high impact strength has been correlated with the homogeneous 1-2 μ rubber particle size distribution, a comparatively sharp rubber T_g transition at lower temperature, and a much lower occluded polystyrene content in the dispersed phase.     

The viscosity of concentrated polymer solutions containing low molecular weight solvents

The zero shear rate viscosities of polystyrene/ethylbenzene solutions having polymer weight fractions ranging from 0.5 to 1.0 have been measured using a novel sealed rheometer cell over a temperature range of 50 to 200°C. The concentration and temperature dependence of the solution viscosity has been found to be well described by the relation η₀ = K c^aM_w ^3.4ζ(c, T) where the monomeric friction coefficient ζ is determined by the free volume of the solution. Following the procedure of Berry, the free volume parameters, α_f(c)/γ and T_∞ (c), and the fractional free volume, f(c,T)/γ, have been determined. After using these parameters to account for the concentration dependence of the friction coefficient, the concentration exponent a has been evaluated and found to be in reasonable agreement with the value of 3.4 obtained by Berry and Fox for other polymer/solvent systems. A comparison of the relative conributions made by the friction coefficient and the term c^3.4 to the overall concentration dependence of the viscosity of these highly concentrated solutions shows the friction coefficient to be the dominant factor     

Predictors of glass transition in the biodegradable poly‐lactide and poly‐lactide‐co‐glycolide polymers

The biodegradable polylactide (PLA) and polylactide‐co‐glycolides (PLGAs) are being widely investigated for use as scaffolds in bone and ligament reconstruction. The glass transition temperatures (T_g) for these polymers are generally greater than 37°C, causing PLA and PLGA devices to possess brittle characteristics in physiological conditions. To evaluate the possibility of obtaining PLGA polymers with T_g values below 37°C, we evaluated the determinants of T_g in PLA and PLGA copolymers. The T_g, changes in specific heat capacity (ΔC_p), and enthalpic relaxation (ΔH_g) in two consecutive heating cycles were correlated with lactide/glycolide content and intrinsic viscosity [η] for PLA, PLGAs 90:10, 75:25, 65:35, and 50:50. A linear correlation was observed between T_g and intrinsic viscosity, with 0.1 dL/g increase in viscosity resulting in an increase in T_g by about 3.55°C. The selection of PLA and PLGA copolymers with [η] values <0.19 dL/g, corresponding to a viscosity average molecular weight of <70 kDa, will obtain PLA/PLGA polymers with T_g values below 37°C. The lowest attainable T_g values were found to be 28–30°C. Intrinsic viscosity also correlated with ΔC_p differences between aged and rapidly cooled polymers, and is therefore important in predicting free volume changes within these polymers upon aging. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 1983–1987, 2006     

Limits to expanding the PN‐F series of polyphosphazene elastomers

Mixed‐substituent fluoroalkoxyphosphazene polymers bearing ～15% 1H,1H,2H,2H‐perfluorooctan‐1‐oxy or 1H,1H,2H,2H‐perfluorodecan‐1‐oxy side groups together with trifluoroethoxy cosubstituent groups were synthesized. The low reactivity of the long‐chain fluoroalkoxides and their limited solubility in organic solvents prevented higher levels of substitution. Moreover, the sodium alkoxides with two methylene residues adjacent to the oxygen proved to be unstable in solution due to elimination of NaF and precipitation of side products, and this limited the time available for chlorine replacement reactions. The resulting cosubstituent polymers were characterized by proton nuclear magnetic resonance (¹H‐NMR), ³¹P‐NMR, ¹⁹F‐NMR, gel‐permeation chromatography, and differential scanning calorimetry. Unlike homo‐ or mixed‐substituent fluoroalkoxyphosphazene polymers, such as [NP(OCH₂CF₃)₂]_n (a microcrystalline thermoplastic, T_g ～ ?63°C, T_m ～ 242°C) or [NP(OCH₂CF₃)(OCH₂(CF₂)_xCF₂H)]_n (PN‐F, a rubbery elastomer, T_g ～ ?60°C, but no detectable T_m), the new polymers are gums (T_g ～ ?50°C, but no detectable T_m) with molecular weights in the 10⁵ g/mol rather than the 10⁶ g/mol range. POLYM. ENG. SCI., 54:1827–1832, 2014. © 2013 Society of Plastics Engineers     

Investigation of readily processable thermoplastic‐toughened thermosets. V. Epoxy resin toughened with hyperbranched polyester

This article describes the use of hyperbranched polyester oligomers (HBPs) as modifiers for epoxy thermosets. The effect of HBP molar mass, end group, and loading on prepolymer viscosity, thermoset fracture toughness, T_g, and high‐temperature dynamic storage modulus (E′) were measured. The HBP molar mass was systematically increased from nominal values of ∼ 1750 g mol (Generation 2, or G2) up to ∼ 14,000 g mol (Generation 5, or G5), which corresponds from a low of two layers of monomer up to a maximum of five layers of monomer around the central core. Toughness increased only modestly with the molar mass of the HBP. At 7% loading in the epoxy thermoset, the G5 HBP increased toughness by ∼ 60% over the untoughened control. Toughness increased to 82% above the untoughened control at a loading of 19% G5 HBP, but the toughness decreased at 28% HBP loading. The T_g and E′ were influenced by the HBP modifier, but the effect was not systematic and may have been due to competing effects of HBP molar mass and end group. The effect of the architecture of the thermoplastic modifier was investigated by introducing a linear aliphatic polyester (∼ 5400 g mol) with a repeat unit structure, which was similar to that of the HBP. At the molecular weight range investigated, neither the prepolymer viscosity nor the thermoset toughness of the HBP–epoxy was significantly different from that of the linear polyester in epoxy. Preliminary results are presented showing the effect of thermoplastic molecular weight and architecture on morphology. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 151–163, 1999