Blends of amorphous and crystalline polylactides with poly(methyl methacrylate) and poly(methyl acrylate): a miscibility study

Blends of amorphous and crystalline polylactides (PDLA and PLLA) with poly(methyl methacrylate) (PMMA) and poly(methyl acrylate) (PMA) have been prepared. Thermal behaviour and miscibility of these blends along the entire composition interval were studied by differential scanning calorimetry (d.s.c.). The results were compared with those obtained by dynamic mechanical analysis (DMTA). Only one T_g was found in PDLA/PMA and PDLA/PMMA blends, indicating a high degree of miscibility in both systems. Nevertheless, the PDLA/PMMA blend presented enlargements of the T_g width at high PMMA contents. In this case, additional evidence of complete miscibility was obtained by studying the evolution of the enthalpic recovery peaks which appear after different thermal annealing treatments. When the polylactide used was semicrystalline (PLLA), once the thermal history of the blends had been destroyed, crystallization of PLLA was disturbed in both blends PLLA/PMMA and PLLA/PMA, but in a rather different fashion: in the first case crystallization was almost prevented while in the second one it was favoured. This behaviour was explained in terms of the effect of the higher stiffness as indicated by the value of T_g for PMMA compared to that for PMA.     

Polar interaction in a cyanated poly(ether sulfone)-modified polycyanurate

Low molecular weight cyanated poly(ether sulfone) (CPES, Mn=3200) was prepared, and cured with bisphenol A dicyanate (BPADCy). The resulting polycyanurates of different compositions show an S-shaped T_g-composition curve. This unexpected S-shaped curve is interpreted in term of the polar interaction between poly(ether sulfone) and s-triazine rings formed by polycyclotrimerization of aromatic dicyanates. Separate study on u.v. spectra of mixture model compounds suggests that this polar interaction belongs to an n–π* interaction between the lone pair electrons of the N-atoms in the s-triazine ring and the π*-orbital of the phenylene rings neighbouring to the sulfone linkage. I.r. study on the cured polycyanurates indicates that this polar interaction causes the shift of the —C=N stretching from 1564 to 1580 cm⁻¹.     

The elastic modulus of the poly(phosphonitrilic chloride) crystal

The Young's modulus for a crystal of poly(phosphonitrilic chloride) (poly-dichlorophosphazene) (NPCl₂)_n has been calculated using force constants derived from spectroscopy. Assuming that the molecule is a uniform helix the value of the modulus is 1.38 × 10⁹ dyne cm⁻² [dyne cm⁻² = 0.1 N m⁻²]; the result is 1.66 × 10¹⁰ dyne cm⁻² if a cis-planar structure is assumed for the molecule. Neither value is close to those obtained experimentally (1.8 × 10⁶ to 6.5 × 10⁶ dyne cm⁻²). This is because experimental values relate to the amorphous polymer whereas the calculated values are those for the crystal. There is good agreement between the values calculated for the (NPCl₂)_n crystal and those for other polymer crystals.     

Preparation and characteristics of natural rubber/poly(ethylene oxide) salt hybrid mixtures as novel polymer electrolytes

Poly(ethylene oxide) (PEO) of molecular weight 1000 (PEO₁₀₀₀) containing lithium benzenesulfonate (LiBs) (PEO₁₀₀₀/LiBs), PEO derivatives having benzenesulfonate groups on both chain ends (PEO₁₀₀₀–(BSLi)₂), or 1-ethyl-2,3-dimethylimidazolium bromide (ImB), were each blended with natural rubber (NR). The ionic conductivity was measured from AC impedance values. The ionic conductivity of the mixture of NR and PEO₁₀₀₀/LiBs (40 wt%) was about 10⁻⁶ S cm⁻¹ at 50°C; this mixture retained rubbery physical characteristics. At NR content of 10 wt%, the ionic conductivity of the mixture (NR/PEO₁₀₀₀/LiBs) was 2.7×10⁻⁵ Scm⁻¹ at 50°C, approximately 10 times higher than that of the bulk PEO/LiBs mixture. For mixtures of NR and PEO₁₀₀₀–(BSLi)₂, no improvement in ionic conductivity by mixing was found. The ionic conductivity of the mixture of NR and ImB was about 10 times higher than for the bulk of PEO₁₀₀₀–(BSLi)₂ at a NR content of 10 wt%. We propose that the ionic conductivity of the mixture increases when an ion conducting matrix containing simple salt is added. On the other hand, the DSC curve for NR/PEO derivatives showed two T_gs based on the separate components, suggesting phase separation of the PEO derivative in the NR phase.     

Poly(o- and m-toluidine)–polystyrene blends: spectral, thermal and electrical properties

Poly(o-toluidine) (POT) and poly(m-toluidine) (PMT) blends with polystyrene of five different compositions were prepared by solution blending using THF as the solvent in which both the POT–HNO₃ and PMT–HNO₃ bases are almost completely soluble. The blends have been characterized by spectral, thermal and electrical measurements. The results suggest that blend formation occurs at all compositions presently studied. The thermal stability of the respective blends is higher than that of the POT–HNO₃ and PMT–HNO₃ salts. The maximum conductivity of the blends is 9.2×10⁻⁴ S cm⁻¹. The results show that POT/PMT can be blended with up to 30% wt/wt of polystyrene without a significant drop in its conductivity.     

The glass temperature of polymer blends

The entropy theory of glasses is used to derive the glass temperature, T_g, of a binary polymer blend in terms of the glass temperatures of the two substituents. The formula is T_g = B₁T_g1 + B₂T_g2, where B_i is the fraction of flexible bonds of substituent i. A bond is flexible if rotation about it changes the shape of the molecule. Bonds in side groups as well as in the backbone are to be counted. This formula assumes that the free volume, taken here to be the volume fraction of empty lattice sites, is the same for each of the three materials. It has no parameters. The above equation expressed in weight fractions, W_i, is (T_g − T_g1)W₁(γ₁/ω₁) + (T_g − T_g2)W₂(γ₂/ω₂) = 0, where ω_i is the weight of a monomer unit and gg_i is the number of flexible bonds per monomer unit. A more general treatment is given. One variation of the more general treatment which expresses the properties of the blend in purely additive terms gives T_g = B₁T_g1 + B₂T_g2 + KB₁B₂(T_g1 − T_g2)(V₀₁ − V₀₂), where V_0i are the free volume fractions of the homopolymers at their glass temperatures and K is a constant. The added term is usually small. The most general form of the equation requires the energy of interaction between the two unlike molecules, which can be estimated by volume measurements on the blend.     

Longitudinal volume viscosity of 1,2 syndiotactic polybutadiene

The volume flow of 1,2 syndiotactic polybutadiene (1,2 s-PB) ( , T_m = 373 K and T_g = 262 K) has been measured. The elastic modulus of the longitudinal wave, the longitudinal volume viscosity, the initial longitudinal volume viscosity and retardation times are described at compression rates of 1.0 × 10⁻⁵–200.0 × 10⁻⁵ s⁻¹, and at temperatures of 403–423 K and pressures up to 150 MPa. Longitudinal volume viscosity decreases with increasing compression rate, and with decreasing volume deformation, the behaviour being in all cases typical non-equilibrium behaviour. Longitudinal volume viscosity increases with increasing temperature, the volume flow activation energy being 56.3 kJ mol⁻¹.     

Physical aging behavior of miscible blends containing atactic polystyrene and poly(2,6-dimethyl-1,4-phenylene oxide)

The influence of blend composition on physical aging behavior was assessed for miscible blends of atactic polystyrene (a-PS) and poly(2,6-dimethyl-1,4-phenylene oxide) (PPO). At aging temperatures of 15 and 30°C below the midpoint glass transition temperature (T_g), the a-PS/PPO blends exhibited volume relaxation rates that were retarded compared to additivity based upon the aging rates for pure a-PS and PPO. This negative deviation diminished with increased undercooling, and eventually the volume relaxation rates displayed a nearly linear trend with respect to composition at the greatest undercooling of 60°C that was employed. The compositional nature of unaged glassy density and secondary relaxation intensity, both influenced by the presence of specific attractive interactions in the blend system, were likely causes for the variation of volume relaxation rate with composition and undercooling. For aging at 30°C below T_g, the dependence of enthalpy relaxation rate on composition was similar to that observed for volume relaxation. Mechanical aging rates determined from time–aging time superposition of creep compliance data showed significantly less than additive behavior for the blends aged at T_g−30°C, but unlike the volume relaxation results, this trend persisted at the 60°C undercooling.     

Preparation and properties of novel poly(urethane-imide)s

A series of poly(urethane-imide)s were prepared by a novel approach. Polyurethane (PU) prepolymer was prepared by the reaction of polyester polyol and 2,4-tolylenediisocyanate (2,4-TDI), and then end-capped with phenol. The PU prepolymer was blended with poly(amide acid) or oligo(amide acid) prepared from 2,2′-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA) and oxydianiline (ODA) at room temperature in various weight ratios. The blend films obtained by casting and then drying were not transparent, suggesting that phase separation occurred between the polyimide (PI) and PU components. The films became transparent, however, after thermal treatment at 100°C and then 200°C for 1 h each, irrespective of the ratio of the two components. The poly(urethane-imide) films showed good solvent-resistance. Dynamic mechanical analysis of the films showed that glass transition temperatures (T_g) shifted depending on the ratio of PI and PU components. This shift of T_g, along with the transparency of the films, suggests that the PU and PI components employed here are miscible to some extent and that domains of each phase by microphase separation are small. Tensile measurement of the blend films from poly(amide acid) showed that the films are plastic or elastic, depending on the ratio of the components. Thermal stability of the PU was found to increase by the incorporation of polyimide component.     

Water absorption by an epoxy resin and its effect on the mechanical properties and infra-red spectra

A filled epoxy resin used as a structural adhesive and based on the diglycidyl ether of bisphenol A cured with dicyandiamide has been subjected, in its bulk form, to ageing at 40, 55 and 70°C and ca. 100% relative humidity. Gravimetric, viscoelastometric and FTi.r. studies have been effected after various times of exposure. Water absorption in the polymer is essentially Fickian, although closer inspection reveals the finer behaviour to be sigmoidal. The activation energy for diffusion is of the order of 80 kJ mol⁻¹, but there appears to be no clear relationship between equilibrium absorption values and temperature. Viscoelastometry has shown that T_g diminishes from ca. 115°C before ageing to ca. 90°C at saturation, 1% of water uptake corresponding to ca. 8°C reduction in T_g. Reductions in Young's modulus were observed both in the glassy and rubbery states after ageing and the latter is associated with molecular chain scission. FTi.r. analysis has shown several modifications occurring due to water absorption, the main one being an increase in intensity of the band at 1740 cm⁻¹. It is concluded that water absorption leads both to plasticization effects and chemical modification of the epoxy resin.     

Glass transition and physical ageing in plasticized poly(vinyl chloride)

Several samples of poly(vinyl chloride) both unplasticized and plasticized with dioctyl phthalate, have been examined by differential scanning calorimetry. It was observed that, while the glass transition temperature T_g decreased as expected with increasing plasticizer content, a small portion of the sample appeared to be resistant to the plasticizer. This was manifest in the appearance of a second T_g corresponding to the unplasticized sample which remained unaffected by addition of plasticizer. The ageing behaviour of the samples was also examined using enthalpy relaxation measurements and it was observed that the presence of plasticizer accelerates the ageing process, probably due to the fact that there is greater mobility of the chains in the plasticized samples.     

Effect of chemical compositional distribution on solid-state structures and properties of poly(3-hydroxybutyrate-co-3-hydroxyvalerate)

Solid-state structures and crystallization kinetics were compared between poly(3-hydroxybutyrate-co-3-hydroxyvalerate) [PHB-HV] and PHB/PHB-HV blends exhibiting the cocrystallization. As cocrystallizable blends, both the blends showing complete cocrystallization, i.e. the PHB content in the crystalline phase is the same as that of the whole blends, and the blends forming a PHB-rich crystalline phase were used. The PHB and HV content in the cocrystalline phase were determined by high-resolution solid-state ¹³C NMR spectroscopy. In order to determine these contents with a minimum experimental error, site-specific ¹³C-labeled PHB/PHB-HV blends and PHB-HV copolymers were used. The crystallinity, lamellar structures, spherulite growth rate, and melting behavior were analyzed by wide-angle X-ray diffraction, small-angle X-ray scattering, polarized microscope, and differential scanning calorimetry, respectively. In these data, no difference was observed between the complete-cocrystallizable PHB/PHB-HV blends and the PHB-HV copolymers with the same overall HV content. On the other hand, the PHB/PHB-HV blends forming a PHB-rich crystalline phase has the amorphous layers thicker than that of the PHB-HV copolymers with the same overall HV content. Based on the collected data, the similarity and differences in the solid-state structures and properties between PHB-HV copolymers and cocrystallizable PHB/PHB-HV blends were discussed.     

Highly conductive poly(ethylene oxide)-poly(methyl methacrylate) blends complexed with alkali metal salts

Solid electrolytes of room temperature ionic conductivity exceeding 10⁻⁵ S cm⁻¹ were obtained by complexing NaI, LiI, LiBF₄ and LiClO₄ with blends prepared by thermal polymerization of methyl methacrylate in the presence of high molecular weight poly(ethylene oxide). All of the studied samples were thermally stable up to at least 60°C. Differential scanning calorimetry studies indicated that these electrolytes contained amorphous phases with very low glass transition temperatures. This is due to the plasticizing effect of the grafted copolymers formed during the polymerization. This is supposed to lead to high mobility of the charge carriers in the systems studied.     

Energy storage during inelastic deformation of glassy polymers

In this paper, aspects of the microstructural state of glassy polymers that evolve during physical ageing and inelastic deformation were studied. Differential scanning calorimetric (d.s.c.) measurements were performed on specimens of three glassy polymers: polystyrene (PS), polycarbonate (PC) and poly(methyl methacrylate) (PMMA). Materials were subjected to both a quenched and a well annealed heat treatment and subsequently deformed in compression to various levels of strain. Stress-strain curves and companion d.s.c. scans were compared.

The well known enthalpy overshoot at T_g was observed for the annealed samples, showing that ageing is accompanied by enthalpy relaxation. The annealed material was also found to require a higher stress to yield, and the additional work required to strain-soften the annealed polymer to the flow stress level of its quenched companion was found to correlate well with the area of the enthalpy overshoot of the annealed specimen.

Inelastic deformation was found to increase the specific enthalpy of both annealed and quenched specimens. In the annealed material, the enthalpy overshoot at T_g was found to decrease with inelastic strain and was completely erased by about −20% strain. Simultaneously, a pre-T_g exotherm was observed to develop with inelastic strain over a wide range of temperature. The pre-T_g exotherm was found to evolve until essentially reaching a steady-state profile at approximately −25% strain. This evolution coincided with the strain-softening phenomenon observed in the corresponding stress-strain results. A pre-T_g exotherm was also found to evolve with straining of the quenched material. Furthermore, the steady-state exotherms of the quenched and annealed materials were found to be nearly identical, as were their corresponding flow stress values after strain softening.

Finally, a second, post-T_g exotherm was found to develop with further straining beyond strains of −25%. This exotherm was found to increase with inelastic strain and coincided with the occurrence of strain hardening (due to chain orientation) in the materials.

The presence of two distinct and separately evolving exotherms in the inelastically deformed polymers indicates the existence of two separate deformation resistances in glassy polymers, one related to the initial yield and strain-softening behaviour, and the other to the orientation-induced strain hardening of the material. The observation that the pre-T_g exotherm is spread over a wide temperature range reflects the distributed nature of the structural state and may be quantified using a distribution in activation energy for the local rearrangements. The results therefore provide valuable information about the processes that must be accounted for in the development of accurate constitutive models of mechanical behaviour.     

Grazing incidence reflection absorption Fourier transform infrared (GIRA-FTIR) spectroscopic studies on the ferroelectric behavior of poly(vinylidene fluoride–trifluoroethylene) ultrathin films

Grazing incidence reflection absorption Fourier transform infrared (GIRA-FTIR) spectroscopy was used to characterize the ferroelectric behavior of a thin poly(vinylidene fluoride–trifluoroethylene) P(VDF–TrFE) copolymer. The lab-built GIRA-FTIR apparatus allowed the heating and corona poling process to be carried out whilst collecting the GIRA spectra of the thin polymer film. The Curie transition from the ferroelectric to paraelectric phase was confirmed from the abrupt change in intensity of the 849 cm⁻¹ band in the RA-FTIR spectrum. It was found that the Curie temperature dropped dramatically when the film thickness was lowered to below a certain critical value of approximately 100 nm. The switching of the CF₂ dipoles in the ferroelectric crystals after applying the external electric field could be determined by monitoring the change in the 849 cm⁻¹ band intensity. For the 600 nm thick P(VDF–TrFE) film, the switching of the dipoles appears to occur almost instantaneously, while the kinetics of dipole switching of the 75 nm thick film were significantly retarded. The repeated switchability of the CF₂ dipoles upon the application of a bipolar cyclic electric field was also confirmed. The bistability of the film due to remnant polarization was also confirmed from the absorbance of the 849 cm⁻¹ band after removing the applied voltage during corona poling.     

Preparation and properties of disyndiotactic poly(alkyl crotonate)s

Ethyl, n-propyl, iso-propyl and n-butyl crotonates were polymerized by group transfer polymerization (GTP) using ketene trialkylsilyl acetals as initiators in the presence of mercury (II) iodide as catalyst and iodotriethylsilane as co-catalyst. Predominantly disyndiotactic polymers with number-average molecular weights ranging from 56,000 to 90,000 were obtained almost quantitatively. Thermal and mechanical properties of poly(alkyl crotonate)s, such as glass transition temperature (T_g), refractive index, transmissivity, Izod impact strength, Young's modulus, hardness, tensile strength, tensile elongation at break, gas permeability coefficient and density were measured. All the polycrotonates have 65–90 °C higher T_g than the corresponding polymethacrylates.     

Poly(ethylene oxide)–LiX rubbery electrolytes with X–X disulfonamide anions having two or three ether groups in their structures

This work deals with poly(ethylene oxide), PEO–MX (M=Li, K and Cs) amorphous electrolytes with ⁻X–X⁻, [CF₃SO₂NCH₂(CH₂OCH₂)₂CH₂NSO₂CF₃]²⁻ (EDSA) and [CF₃SO₂NCH₂CH₂(CH₂OCH₂)₃CH₂CH₂NSO₂CF₃]²⁻ (TTSA) disulfonamide anions. These dianions have X⁻ end-groups identical to anions [CF₃SO₂N(CH₂)₂OCH₃]⁻ (MESA) and [CF₃SO₂N(CH₂)₃OCH₃]⁻ (MPSA), one of which (MPSA) was reported to yield chelate-like associated species (presumably LiX₂⁻ triplets) at concentrations above EO/Li=20 in PEO. This feature of LiMPSA, evidenced through glass transition temperature (T_g) measurements, does not apply to Li₂EDSA and Li₂TTSA. Though none of these lithium salts form crystalline intermediate compounds with PEO, the limit of solubility of LiMESA (EO/Li=16) does not allow a clarification of this point for this salt. At lower concentrations, however, a conductivity comparison with the potassium and caesium salts shows that the apparent degree of dissociation (=C_Li+/C_Li) of LiMESA is comparable to that of LiMPSA. As opposed to both these salts and to some extent to Li₂EDSA, a much greater dissociation takes place for Li₂TTSA, the anion of which contains an inner, third ether group in its structure.     

Effect of cooling rate and frequency on the calorimetric measurement of the glass transition

The glass transition of thermoplastics of different polydispersity and thermosets of different network structure has been studied by conventional differential scanning calorimetry (DSC) and temperature modulated DSC (TMDSC). The cooling rate dependence of the thermal glass transition temperature T_g measured by DSC, and the frequency dependence of the dynamic glass transition temperature T measured by TMDSC have been investigated. The relation between the cooling rate and the frequency necessary to achieve the same glass transition temperature has been quantified in terms of a logarithmic difference Δ=log₁₀[|q|]−log₁₀(ω), where |q| is the absolute value of the cooling rate in K s⁻¹ and ω is the angular frequency in rad s⁻¹ necessary to obtain T_g(q)=T(ω). The values of Δ obtained for various polymers at a modulation period of 120 s (frequency of 8.3 mHz) are between 0.14 and 0.81. These values agree reasonably well with the theoretical prediction [Hutchinson JM, Montserrat S. Thermochim Acta 2001;377:63 [6]] based on the model of Tool–Narayanaswamy–Moynihan with a distribution of relaxation times. The results are discussed and compared with those obtained by other authors in polymeric and other glass-forming systems.     

Differential scanning calorimetry and infra-red crystallinity determinations of poly(aryl ether ether ketone)

The differential scanning calorimetry (d.s.c.) heating thermograms of 12 poly(aryl ether ether ketone) (PEEK) samples of varying degrees of crystallinity have been recorded. The relation found between the degree of crystallinity as determined by specific gravity measurements, and the melting enthalpy of the polymer, shows that recrystallization is occurring during a heating scan rate of 10°C min⁻¹. This implies that d.s.c. is not a convenient technique to assess PEEK crystallinity. The infra-red absorbance spectra of the same samples have also been examined in the range from 1030 to 880 cm⁻¹. The 965 cm⁻¹ band, up to now considered as indicative of the PEEK crystallinity, is shown to be practically independent of the degree of crystallinity above 15%. However, there is evidence to support the existence of a true i.r. crystalline band located at 947 cm⁻¹. It is also suggested that the 965 cm⁻¹ band is due to a normal vibration mode of a short segmental conformation, whose presence is favoured in the crystalline phase, but also in the amorphous zones nearest to the crystallite surface.     

Kinetics and equilibrium swelling of gelatine gels

The swelling features of gelatine gels in water (good solvent) were studied as a function of thermodynamic conditions of sol—gel transition and ripening. It is shown that the degree of equilibrium swelling Q_e varies with the volume fraction of the polymer in a casting solution φ_o in accordance with the prediction of the classic theory: Q_e φ_o^−0.4. Q_c, as a function of the gelation temperature T_g, the ripening time t_r and φ_o, can be rescaled and described by the single empirical equation: Q_e T_g^−x t_r^−yφ_o^−0.4, where x = 0.1, y = 0.15 for wet gels and X = −0.5, y = 0.04 for dried gels. The kinetics of macroscopic swelling is described by the equation of Peters and Candau, with values of collective diffusion coefficients being in good agreement with values obtained by other workers via photon correlation spectroscopy.