Association of stereoregular poly(methyl methacrylates): 3. Thermal behaviour and composition of stereocomplex

Stereocomplexes of isotactic and syndiotactic poly(methyl methacrylate) (i- and s-PMMA) were prepared by mixing dilute i- and s-PMMA solutions in acetone or DMF at different temperatures. At 50°C the rate of complex formation is larger than at 20° or 70°C. Differential scanning calorimetry and X-ray powder diffraction measurements were performed on the material crystallized from mixed solutions. The thermograms showed 3 endotherms at temperatures not depending on the i-/s-PMMA ratio of the sample. The endo therm at ～280°C is caused by degradation of PMMA, the maximum at ～210°C by melting of the stereocomplex and the endotherm at ～185°C by melting of solvent stabilized s-PMMA. By comparing X-ray powder diffraction patterns of s-PMMA crystallized from borderline solvents with those of stereocomplexes of i-/s-PMMA with different drying histories it is concluded that the reflection at 2 θ = 4°10′ belongs to a solvent stabilized s-PMMA structure. Partial association of i- and s-PMMA chains in an i-/s-PMMA ratio of about 1:1 and crystallization of solvent rich s-PMMA is proposed as a mechanism for the complex formation under the present conditions.     

The phase diagram and morphology of blends of poly(vinylidene fluoride) and poly(ethyl acrylate)

The phase diagram and crystallization behaviour of the polymer blend system consisting of poly(vinylidene fluoride) (PVF₂) and poly(ethyl acrylate) (PEA) have been examined. The melt exhibits phase separation upon heating to 10°C–50°C above the melting point of the PVF₂, depending on the composition. The cloud point and equilibrium melting point curve (for α-PVF₂) intersect at about 180°C and a composition of 50% (by weight) PVF₂. The polymer-polymer interaction parameter, χ, was calculated from the equilibrium melting point depression data and found to be −0.16 (at 170°C). Spherulite growth rate data have been measured as a function of composition and temperature. Assuming regime II crystallization a value of the product of the surface free energies of the α-PVF₂ crystals was calculated to be 4.4 × 10⁻⁴J²m⁻⁴. In blends crystallized from the one phase melt the texture of spherulites becomes more open and the spherulite extinction ring spacing (due to lamaller twist) becomes larger with increasing crystallization temperature. In addition the ring spacing increases with PEA content at constant crystallization temperature.     

Multiple melting in blends of poly(vinylidene fluoride) with isotactic poly(ethyl methacrylate)

Multiple melting phenomena have been studied in blends of poly(vinylidene fluoride) (PVF₂) with low molar mass isotactic poly(ethyl methacrylate) (it-PEMA). In all blends, as well as in pure PVF₂, a transition (T₁) was observed prior to the main melting point (T₂). T₁ is probably connected with the melting of secondarily-crystallized material. In addition to this, a high temperature melting endotherm (T₃) was observed, which could be ascribed completely to recrystallization of PVF₂. The highest transition (T₄) was caused by melting of the σ form of PVF₂. From Hoffman-Weeks plots—T₂ vs. crystallization temperature, T_c — it could be concluded that no thermody amic depression of the melting point of PVF₂ occurred in the blends. The stabilities of PVF₂ crystallites in the various blends were derived from the slopes of Hoffman-Weeks plots and were in good agreement with lamellar thicknesses found from SAXS measurements.     

Computerized infra-red study of the interaction of poly(vinylidene fluoride) with stereoregular poly(methyl methacrylate)

A computerized infra-red study was carried out on blends of poly(vinylidene fluoride) (PVF₂) with isotactic or syndiotactic poly(methyl methacrylate) (it- and st-PMMA) to investigate the effect of tacticity of PMMA on the interaction with PVF₂. A calibration method for the infra-red spectra had to be developed to insure that sum and difference spectra were performed correctly. Any influence of stereoregular PMMA on the infra-red spectrum of PVF₂ could be ascribed merely to the loss of crystal-linity of PVF₂. In PMMA poor blends, PVF₂ crystallized in the α(II) modification. In some of these blends, recrystallization was observed, but this did not lead to another crystalline form. The infra-red spectra of it- and st-PMMA were influenced strongly by PVF₂, especially vibrations of the carbonyl group. A considerably larger spectral change in the homogeneous blends of amorphous PVF₂ with it-PMMA compared with blends with st-PMMA indicated that the interaction of the PVF₂ segments is stronger with the isotactic than with the syndiotactic segments. This is in agreement with our previous study on melting point depressions of PVF₂ and points to a better fit of it-PMMA with the dipoles of PVF₂ segments.     

The crystalline morphology of poly(vinylidene fluoride)/poly(methylmethacrylate) blends

Poly(vinylidene fluoride), PVF₂, as well as blends of PVF₂ with poly(methyl methacrylate), PMMA, develop a variety of crystalline morphologies at low undercoolings. Both the α and γ crystal forms grow from the melt and the former undergoes a solid-solid phase transition to the latter, though its morphology remains unaltered. Three melting temperatures which decrease with increasing PMMA content are observed. Hoffman-Weeks analysis shows the equilibrium melting points of the blends to be depressed. Using these equilibrium values, the thermodynamic interaction energy density is calculated to go from ?5.40 × 10⁶ to ?2.96 × 10⁷ j/m³ as the blend composition goes from 40.1 volume percent to pure PVF₂. The band periodicity in the α form spherulites increases with crystallization temperature and PMMA content and it appears to be from a lamellar reorientation process with an apparent activation energy of 322 cal/mole. Electron diffraction patterns taken along the radial direction in a given spherulite reveal lamellar twisting which causes the banded appearance. Light scattering results suggest that the lamellar are formed into rod-like structures on a local scale but that on a larger scale they develop a disoriented spherulitic morphology.     

Influence of tacticity of poly(methyl methacrylate) on the compatibility with poly(vinyl chloride)

Blends of conventional poly(vinyl chloride) (PVC) and poly(methyl methacrylate) (PMMA) with different tacticities were obtained both from the bulk and from solution. The glass transition temperatures (T_g) of the blends were determined with a differential scanning calorimeter and by dynamic mechanical measurements. Some turbidity measurements on films and viscosity measurements on mixed solutions supplied additional information about the state of mixing. From the results it appeared that isotactic (i-)PMMA and PVC form an incompatible system over the entire composition range (two T_g's), whereas blends of syndiotactic (s-)PMMA and PVC form a compatible system up to a composition corresponding with a monomer unit ratio of about 1:1. For higher s-PMMA contents phase separation is observed; one phase corresponding with the 1:1 s-PMMA—PVC associate and the other phase representing the excess of pure s-PMMA. This effect of tacticity is discussed in terms of the differences in chain conformation of i- and s-PMMA.     

Melting and crystallization behaviour of the poly(vinylidene fluoride)/poly(monobenzyl itaconate) blends

Summary Melting and crystallization behaviour of blends of poly(vinylidene fluoride)(PVF₂) and poly(monobenzyl itaconate) (PMBzI) have been analyzed as a function of the composition in the range 100-40% PVF₂. Using the Hoffman-Week plot we have not found an equilibrium melting point depression in all the blends studied. However, the addition of PMBzI to pure PVF₂ leads to an increase in its crystallization rate. The results suggest that both polymers are incompatible and that PMBzI acts as nucleating agent in the PVF₂ crystallization.     

Crystallization and melt behaviour of isotactic poly(2-vinylpyridine)

The crystallization and melting behaviour of highly isotactic poly(2-vinylpyridine) (it-P2VP) with $\text{M}\text{?}{}_{\mathrm{<mtext>v</mtext>}}\text{= 4 × 10}{}^5$ has been studied by microscopy and d.s.c.. The maximum spherulitic growth rate was found to be 250 × 10^?3μm/min at a crystallization temperature T_c of 165°C. Experimental data could be described by the growth rate theory for small supercooling, by taking the appropriate value of 75 for the constant c₂ of the WLF equation. The chain-folded surface free energy σ_e, was estimated at 39.5 × 10^?3 J m^?2. The melting curves showed 1,2 or 3 melting endotherms. At large supercooling, crystallization from the melt produced a small melting endotherm just above T_c. This peak may originate from secondary crystallization of melt trapped within the spherulites. The next melting endotherm is related to the normal primary crystallization process. Its peak temperature increased linearly with T_c, yielding an extrapolated value for the equilibrium melting temperature T°_m of 212.5°C. At the normal values of T_c and heating rate a third endotherm appeared with a peak temperature that was independent of T_c, but rose with decreasing heating rate. From the effects of heating rate and partial scanning on the ratio of peak areas, it is concluded that this peak arises from secondary crystallization by continuous melting and recrystallization during the scan. This crystallization and melting behaviour of it-P2VP is very similar to that of isotactic polystyrene.     

Association of stereoregular poly(methyl methacrylates): 2. Formation of stereocomplex in bulk

Blends of isotactic and syndiotactic poly(methyl methacrylate) (i- and s-PMMA) are obtained by precipitation from chloroform and acetone solutions. By differential scanning calorimetry and dynamicmechanical measurements the formation of stereocomplexes from i- and s-PMMA in bulk is demonstrated. After annealing of the blends at 130–160°C a melting endotherm is detected and it is established by X-ray analysis that this endotherm is caused by formation of crystalline stereocomplex. The rate of complex formation is maximal at 140°C and the extent of complex formation is maximal at an isotactic-rich composition. It appears that the difference in solvent history can be removed by heating to 240°C. The subsequent S-shaped course of glass transition temperature, T_g, with composition is explained by the occurrence of some complex formation during cooling from 240°C. Asymmetry and shift of the dynamic-mechanical damping curves after annealing are also explained by the formation of complexes. A mechanism is proposed with helical isotactic chains acting as nuclei for a fringed micelle type of complex formation.     

Miscibility of linear low density polyethylene (LLDPE)-graft-poly(methyl methacrylate) with poly(vinylidene fluoride) (PVF2) and its application as compatibilizer LLDPE/PVF2 blends

Differential scanning calorimetry (DSC), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS) were used to study the miscibility of blends of a graft copolymer of poly(methyl methacrylate) on linear low density polyethylene (LLDPE-g-PMMA, G-3) with poly(vinylidene fluoride)

1 Systematic name: poly(1,1-difluoroethylene).

(PVF₂) and the compatibilization of blends of LLDPE/PVF₂. The specific interaction between PMMA side chains and PVF₂ in G-3/PVF₂ binary blends is weaker than that between the homopolymers PMMA and PVF₂. There are two states of PVF₂ in the melt of a G-3/PVF₂ (60/40, w/w) blend, one as pure PVF₂ and the other interacting with PMMA side chains. The miscibility between PMMA side chains and PVF₂ affects the crystallization of PVF₂. LLDPE-g-PMMA was demonstrated to be a good compatibilizer in LLDPE/PVF₂ blends, improving the interfacial adhesion and dispersion in the latter. Diffusion of PMMA side chains into PVF₂ in the interfacial region reduces the crystallization rate and lowers the melting point (T_m) and the crystallization temperature (T_c) of PVF₂ in the blends.     

Solution properties and unperturbed dimensions of poly(vinylidene fluoride)

A commercial sample of poly(vinylidene fluoride) (PVF₂) was fractionated and characterized. The following relationships were found: (η) = 1.93 × 10^?4M^0.677 (in dimethylacetamide at 25°C); (η) = 2.13 × 10^?4M^0.62 (in acetophenone at 85°C); (η) = 6.86 × 10^?4M^0.50 (in benzophenone at 190°C), leading to a value of the steric factor: σ = (〈r²〉₀/〈r²〉_0f) = 1.66 ± 0.05, which is in agreement with the crystalline properties of the polymer. The concept of M(η) as a universal parameter for g.p.c. calibration was valid for PVF₂ in dimethylacetamide.     

LCST behaviour in blends of poly(vinylidene fluoride) and isotactic poly(ethyl methacrylate)

Blends of poly(vinylidene fluoride) (PVF₂) with isotactic, atactic or syndiotactic poly(ethyl methacrylate) (it-, at- or st-PEMA) were studied by calorimetry and light microscopy. The occurrence of single glass transitions over a broad composition, as well as the lowering of the crystallization temperature upon cooling from the melt, indicate a complete compatibility in the amorphous state in blends of PVF₂ with at- and st-PEMA up to high temperatures. With it-PEMA, however, phase separation took place when the temperature was raised, sugesting LCST behaviour. Cloud points appeared in the temperature range of 150–200°C, making this system suitable for phase separation studies. The occurrence of double glass transitions as well as double crystallization exotherms in some $\text{PVF}{}_2\text{it-PEMA}$ blends could be explained from the phase diagram and the slowness of phase mixing upon cooling.     

Polymer blends containing poly(vinylidene fluoride). Part I: Poly(alkyl acrylates)

The transitional behavior of poly(vinylidene fluoride) (PVF₂) blends with poly(methyl acrylate) and with poly(ethyl acrylate) was examined by differential thermal analysis and dynamic mechanical testing. Both blend systems were judged to be miscible on the bases of the presence of single, composition dependent glass transitions and of the strong melting point depression of the PVF₂ component, Blends of poly(isopropyl acrylate) with poly(vinylidene fluoride) were found to be immiscible. These results suggest that miscibility of the acrylate series depends on a specific attractive interaction between the PVF₂ and oxygen within the acrylate and the effect of this interaction is diminished as the hydrocarbon content of the ester is increased.     

Morphology and melting behaviour of poly(vinylidene fluoride) crystallized from the melt

The influence of crystallization temperature on the melting behaviour and the morphology of poly(vinylidene fluoride) (PVF₂) has been investigated. The DSC endotherms of PVF₂ crystallized from the melt show at least two peaks. The peak areas depend on the thermal history of the samples and the heating scan rate. The area of the first peak was found to increase as the crystallization temperature or the scan rate increased. The double peak configuration was attributed to a melting–recrystallization process. Electron microscopy supports these results, for which only one type of lamella was found in the spherulitic structure.     

Synthesis and characterization of poly(butylene terephthalate)‐co‐poly(butylene succinate)‐block‐poly(ethylene glycol) segmented block copolymers

Poly(butylene terephthalate)‐co‐poly(butylene succinate)‐block‐poly(ethylene glycol) segmented random copolymers, with poly(butylene succinate) (PBS) molar fraction (M_PBS) varying from 10 to 60 %, were synthesized through a melt polycondensation process and characterized by means of GPC, NMR, DSC and mechanical testing. The number‐average relative molecular mass of the copolymers was higher than 4 × 10⁴ g mol^?1 with polydispersity below 1.9. Sequence distribution analysis on the two types of hard segments by means of ¹H NMR revealed that the number‐average sequence length of PBT decreased from 2.80 to 1.23, while that of PBS increased from 1.27 to 4.76 with increasing M_PBS. The random distribution of hard segments was also justified because of the degree of randomness around 1.0. Micro‐phase separation structure was verified for the appearance of two glass transition temperatures and two melting points, respectively, in DSC thermograms of most samples. The crystallinity of hard segments changed with the crystallizability controlled by the average sequence length and reached the minimum value at an M_PBS of about 50–60 mol%. The results can also be ascribed to the co‐crystallization between two structurally analogous hard segments. Mechanical testing results demonstrated that incorporating a certain amount of PBS moieties (less than 30 mol%), at the expense of a minute depression of the elastic modulus, that higher relative elongation and more flexibility of polymer chain could be expected. Maximum equilibrium water absorption and faster degradation rates were observed on samples with higher M_PBS values and lower crystallinity of hard segments were better hydrophilicity of the polymer chain, through in vitro degradation experiments. Copyright © 2003 Society of Chemical Industry     

Transition behavior of poly(vinylidene fluoride)/poly(ethyl methacrylate) blends

Previous work has shown evidence that PMMA and PEMA are miscible with PVF₂. The present paper examines in detail the behavior of PEMA/PVF₂ blends by thermal analysis and dynamic mechanical testing. All transitions and relaxations are affected by blond composition but in a complex manner owing to the crystallization of PVF₂ from blends rich in this component. Inadequacies of the simple two-phase picture of semi-crystalline polymers is believed responsible for some of the transitional behavior observed here. The melting point depression observed for PVF₂ was found to be consistent with an exothermic heat of mixing for this pair comparable in value to that found for PPMA/PVF₂/All evidence here are consistent with the previous conclusion of miscibility for these systems.     

Non‐isothermal crystallization behavior of stereo diblock polylactides with relatively short poly(d‐lactide) segments from the melt

Stereo diblock polylactides (SDB‐PLAs) composed of relatively short poly(d ‐lactide) (PDLA) segments and relatively long poly(l ‐lactide) (PLLA) segments were synthesized to have a wide number‐average molecular weight (M_n) range of 2.5 × 10⁴–2.0 × 10⁵ g mol^?1 and d ‐lactyl unit content of 0.9–38.6%. The effects of incorporated short PDLA segments (M_n = 2.0 × 10³–7.7 × 10³ g mol^?1) on crystallization behavior of the SDB‐PLAs were first investigated during heating after complete melting and quenching or during slow cooling after complete melting. Stereocomplex (SC) crystallites can be formed at d ‐lactyl unit content as low as 4.3 and 5.8% for heating and slow cooling, respectively, and for M_n of PDLA segments as low as 2.0 × 10³ and 3.5 × 10³ g mol^?1, respectively. With decreasing M_n and increasing d ‐lactyl unit content, the cold crystallization temperature during heating decreased and the crystallization temperature during slow cooling increased. With increasing d ‐lactyl unit content, the melting enthalpy (ΔH_m) of SC crystallites during heating and the crystallinity (X_c) of SC crystallites after slow cooling increased, whereas ΔH_m of PLLA homo‐crystallites during heating and X_c of PLLA homo‐crystallites after slow cooling decreased. The total ΔH_m of SC crystallites and PLLA homo‐crystallites during heating and the total X_c after slow cooling became a minimum at d ‐lactyl unit content of 10–15% and gave a maximum at d ‐lactyl unit content of 0%. Despite the accelerated crystallization of some of SDB‐PLAs, the low values of total ΔH_m and X_c at d ‐lactyl unit content of 10–15% are attributable to the formation of two crystalline species of SC crystallites and PLLA homo‐crystallites.     

The morphology of poly(aryl-ether-ether-ketone)

The morphology and related properties are described for the aromatic thermoplastic poly(aryl-ether-ether-ketone) (PEEK) [C₆H₄OC₆H₄OC₆H₄CO]_n. Topics covered include crystallinity, crystallization and melting behaviour, Iamellar thickness and spherulitic structure. The data are used to derive the following material parameters $\text{T1}{}_{\mathrm{<mtext>m</mtext>}}\text{= 395°}\text{C}$, σ_e = 49 erg cm^?2, σ_s = 38 erg cm^? and ΔH_F = 130 kJ kg^?1. PEEK is closely analogous to poly(ethylene terephthalate) in its crystallization behaviour except that the main transitions occur about 75°C higher.     

General crystallization behaviour of poly(l-lactic acid)

A study has been made of the general crystallization behaviour of poly(L-lactic acid), PLLA, and is intended to be the basis for further work on fibre formation processes. PLLA is shown to be a semicrystalline polymer that may crystallize from the melt and from solution, and that may form fibres. Spherulites grown from the melt were negatively birefringent and grew at a rate similar to that for polypropylene. The equilibrium melting point and the glass transition temperature were found to be about 215°C and 55°C, respectively. Solution crystallization resulted in lamellar crystals about 10 nm thick and electron diffraction revealed a hexagonal unit structure with dimensions smaller than reported earlier. The equilibrium dissolution temperature in p-xylene and the end surface free energy at the fold surface amounted to 126.5°C and about 0.075 J m^?2 respectively. Fibres of PLLA were formed by precipitation in a non-solvent; some of the fibres were highly porous with a pore size in the range 0.1–0.6 μm.     

Crystallization of poly(vinylidene fluoride) by freeze-extracting method

Summary High crystallinity poly(vinylidene fluoride) (PVF₂) has been achieved by freezing its very dilute solution and followed by freeze-extracting the frozen solvent. FT-IR, WAXD, and DSC studies indicate that the freeze-extracted PVF₂ has higher crystallinity than those samples prepared by solution crystallization or by thermal annealing techniques.