Crystallization of segmented trans-1,4-polyisoprene/epoxidized trans-1,4-polyisoprene block copolymers from solution

The crystallization of segmented block copolymers of trans-1,4-polyisoprene (TPI)/expoxidized TPI from solution was investigated. One preparation with an average TPI block length of 14.5 and an average epoxidized TPI block length of 10 was crystallized from 2-pentanol at 20°C, 2-octanol at 20°C, and 2-pentanone at 0°C. A second preparation with an average TPI block length of 18 and the same average epoxidized TPI block length was crystallized from 2-octanol at 20°C and 2-pentanone at 0°C. The crystallization products were expoxidized in suspension a second time and then characterized by carbon-13 solution NMR to determine the average noncrystalline traverse length and the average crystalline stem length. The crystallized products were also studied by scanning electron microscopy, Fourier transform infrared spectroscopy, and differential scanning calorimetry to determine the morphology, the crystal form and crystallinity, and the melting point, respectively. The average noncrystalline traverse length was found to be highly dependent on the crystallization conditions and the crystalline stem length of the parent TPI lamellas; the composition of these traverses is discussed.     

Crystallization of trans-1,4-polyisoprene

Crystals of fractionated trans-1,4-polyisoprene (TPI) were grown from amyl acetate solution at two weight fractions, 5.7 × 10^?4 and 0.011; for the lower concentration a precooling followed by heating and then crystallization at temperatures in the 10–32°C range was used, while for the higher concentration this method and direct crystallization at a temperature T_C in the 0–32°C range were employed. The precooling method yielded samples crystallized in the α form, while direct crystallization led to formation of β-TPI at low T_C and α at higher T_C. The value for the DSC endotherm, characteristic of α-form melting, increased with increasing T_C, with a shift to lower values with increasing concentration for precooled samples. A β to α transformation was found to occur for synthetic unfractionated TPI when swollen with amyl acetate at 35°C for 17h. Swelling in n-butyl acetate for one day at 25°C or 17 h at 35°C also led to this transformation. From experimental results 74°C is chosen as the temperature at which the α and β forms coexist in the bulk, and this is used to calculate the enthalpy of fusion of β-TPI, yielding a value of 8.6 kJ mol^?1.     

Morphology of solution crystallized trans-1,4-polyisoprene

The film formation method has been successfully used to grow single crystals and other complex morphological features of low melting form (LMF) and high melting form (HMF) of trans-1,4-polyisoprene (TPIP). Below 40 °C dilute amyl acetate solution gave hexagonal shaped LMF crystals. Thick and elongated hexagonal shaped morphology was shown by HMF crystals at temperatures above 40 °C. Straight faces and sharp corners of the single crystals, and also of complex crystals, acquired round shapes when highly polydispersed TPIP was used for crystallization.     

Transitions in trans-1,4-polyisoprene

The melting transitions of both crystalline forms of trans-1,4-polyisoprene, as detected by differential thermal analysis, have been identified by attendant studies with optical microscopy and x-ray diffraction. The lower-melting (LM) form melts initially at a temperature which depends upon the crystallization temperature but which, under our experimental conditions, is between 45 and 53°C. If recrystallization is allowed to occur, the apparent final melting point, which depends upon the recrystallization temperature, is about 58°C. The initial melting point of the higher-melting (HM) form, also crystallization temperature-dependent, is upwards of 57°C. Under the most easily accessible experimental conditions, it may be obscured by the final melting of the LM-form. The apparent final melting point of the HM form is approximately 66°C. Conversion of the LM form into the HM form occurs only by fusion and crystallization. No evidence of a solid-solid transition was found. The rate of conversion is governed principally by the rate of nucleation at the conversion temperature. If fusion of the LM form is incomplete, recrystallization of the LM form takes place instead of conversion to the HM form.     

Dilute-solution properties of trans-1,4-polyisoprene

Trans-1,4-polyisoprene was fractionated by both fractional precipitation and preparative gel permeation chromatography to obtain possibly sharp fractions of narrow molecular weight distribution. Selected fractions were characterized by light scattering, viscosity, and gel permeation chromatography. Necessary corrections for molecular heterogeneity were applied. Some of the characteristic relations between [η] and M _w are [η] = 1.81 × 10^?4 M in benzene at 30°C, [η] = 1.38 × 10^?4 M in n-hexane at 30°C, which are found to be in good agreement with literature data when corrected for molecular heterogeneity.     

Mechanical relaxations in trans-1,4-polyisoprene crystals

Dynamic mechanical relaxation spectra were obtained for solution-grown crystals of trans-1,4-polyisoprene (TPI) in the α and β form. For single crystal mats three relaxations were observed. The highest temperature relaxation peak was characterized as due to the crystalline regions, whereas the intermediate peak was assigned to the primary amorphous relaxation which originates from the fold regions. The nature of the amorphous regions was elucidated by examining the effect of epoxidation on the lamellar fold surface. For an epoxidized single crystal mat, the intermediate relaxation maximum shifted to a higher temperature which corresponds to the glass transition of the almost completely epoxidized TPI. These results are discussed in terms of the fold structure of the TPI single crystals.     

Transcrystallinity and x-ray diffraction in trans-1,4-polyisoprene

Middle range fractions of mimusops balata, a natural trans-1,4-polyisoprene, exhibit transcrystallinity when crystallized in the 35–40°C range. The presence of transcrystallinity was established by x-ray diffraction and optical microscopy. Diffraction peak intensities change due to transcrystallinity, suggesting x-rays be used to detect transcrystallinity and that caution be exercised in using x-ray diffraction techniques to determine the phases present in bulk material and in calculating the degree of crystallinity. The tendency toward transcrystallinity may depend upon molecular weight.     

Study of crystallization kinetics of trans-1,4-polyisoprene

The concentrations and the growth rates of high- and low-melting type spherulites of trans-1,4-polyisoprene were measured in the temperature range 39–49°C. It was shown that above about 40°C., the crystallization rate of trans-1,4-polyisoprene is determined primarily by the radial growth rate of high-melting form (HMF) spherulites, whereas the predominance of the low-melting form (LMF) crystals below 40°C. can be attributed to the high rate of formation of LMF primary nuclei at lower crystallization temperatures. Temperature-independent rate parameters were calculated from optical and dilatometric measurements and were found to be in good agreement. Both the change in nucleation habit and spherulite growth rate with temperature can be explained on the basis of a lower end surface free energy of LMF crystals of trans-1,4-polyisoprene compared to that of the HMF crystals.     

Synthesis of well-defined telechelic trans-1,4-polyisoprene by oxidative cleavage

Synthesis of telechelic trans-1,4-polyisoprenes(TPI: trans-structure 95%) was evaluated based on two different methods of oxidative cleavage(indirect cleavage: first epoxidation of TPI, then the selective cleavage of epoxidized units in epoxidized trans-1,4-polyisoprene(ETPI) and direct cleavage of isoprene units in TPI). The influence of solvents and the ratio of oxidative agents was investigated by 1H-NMR and 13C-NMR. A series of well-defined telechelic TPI with double terminated functional groups and less side reaction(molecular weight distribution range: 1.96?2.26) were synthesized by indirect cleavage in chloroform. Telechelic TPI showed similar crystallization behavior with TPI and interesting cold crystallization behavior characterized by DSC.     

Fourier transform infrared and Raman spectral analysis of trans-1,4-polyisoprene.

Polyisoprene is the most widely used polymer in industry and commerce. Fourier transform infrared (FTIR) and Raman spectra of trans-1,4-polyisoprene have been recorded in the range of 4000-400 and 4000-100 cm(-1), respectively. In the present investigation, detailed assignments of the observed fundamental bands of trans-1,4-polyisoprene has been analysed in terms of peak positions and relative intensities. With the hope of providing more and effective information on the fundamental vibrations, a normal co-ordinate analysis has been performed on trans-1,4polyisoprene, by assuming Cs symmetry. The simple general valance force field (SGVFF) method has been employed in normal co-ordinate analysis and to calculate the potential energy distribution (PED) for each fundamental vibration. The PED contribution corresponding to each of the observed frequencies shows the reliability and accuracy of the spectral analysis. The validity of the SGVFF method as a practical tool for complete analysis of vibrational spectra, even for a polymer of this complexity, is confirmed in the present work.     

Study on viscoelastic properties of epoxidized trans-1,4-polyisoprene by rubber process analyzer

Two kinds of epoxidized trans-1,4-polyisoprene (ETPI) were prepared by surface selective epoxidation which products named as SETPI and solution random epoxidation which product named as RETPI, respectively. The viscoelastic behavior of unmodified trans-1,4-polyisoprene (TPI) and ETPI, both RETPI and SETPI, was studied with the use of a rubber process analyzer (RPA). The strain sweep, frequency sweep, temperature sweep and stress sweep indicated that the SETPI could be seen as a blend of the pure ETPI and TPI. The rheological behavior of TPI and ETPI was recorded by two kinds of shear rate. An obvious effect of epoxidation on the movement behavior of the melt of RETPI was observed. However, the effect of epoxy content of SETPI on ln??* is weak. The complex viscosity at various frequencies of SETPI is higher than that of RETPI.     

A thermally stable polybenzimidazole polybenzoxazole segmented block copolymer

m-phenyl benzoate end-capped pre-polybenzoxazole oligomer and a 3,4-diaminobenzene endcapped pre-polybenzimidazole were synthesized. These functionalized oligomers were then condensed to give a polybenzoxazole-polybenzimidazole segmented block copolymer. The functionalized oligomers were characterized by proton NMR, diffuse reflectance FTIR, and thermal gravimetric analysis. The final block copolymer was characterized by diffuse reflectance FTIR and thermal gravimetric analysis.     

Fabrication of Trans-1,4-polyisoprene Nanofibers by Electrospinning and Its Crystallization Behavior and Mechanism

Trans-1,4-polyisoprene(TPI) nanofibers have been fabricated successfully through electrospinning technology.Through the control of electrospinning parameters, highly crystallized TPI fresh fibers composed mainly of β phase were produced. Morphology and diameter of TPI nanofibers can be controlled by adjusting the electrospinning conditions. The in situ observations of FTIR spectra revealed that the crystallinity of the TPI fibers decreased with aging. While for TPI nanofibers aging at 45 °C for 24 h, a decrease in crystallinity as well as β to a transformation was observed with aging and these changings happened in the first 50 h during aging. The mechanism for β-TPI formation during electrospinning process and the reduced crystallinity with aging were proposed.     

Synthesis of N,N-diethyldithiocarbamate functionalized 1,4-polyisoprene, from natural rubber and synthetic 1,4-polyisoprene

N,N-Diethyldithiocarbamate functionnalized 1,4-polyisoprenes were prepared from 1,4-polyisoprenes (natural or synthetic). The syntheses were performed by nucleophilic addition of N,N-diethyldithiocarbamate salts upon oxirane rings of epoxidized units according to a SN2 mechanism with ring opening. Studies on model molecules of epoxidized 1,4-polyisoprene units (1,2-epoxy-1-methylcyclohexane and 4,5-epoxy-4-methyloctane) were previously achieved to develop the procedure. The best yields were obtained at low temperature in polar medium, and more especially in water with sodium N,N-diethyldithiocarbamate (DEDT-Na) as reagent. A diastereospecific addition was noted when reaction was performed in water with DEDT-Na. Afterwards, the developed procedure was successfully generalized to epoxidized synthetic polyisoprenes and epoxidized natural rubber (in THF, then in latex medium). Excellent results were obtained in latex medium with epoxidized natural rubber (ENR) latices. As with the models, a diastereospecific addition of sodium N,N-diethyldithiocarbamate trihydrate onto epoxidized 1,4-polyisoprene units of ENR was observed at the condition to bring the latex medium to pH 8 before introduction of DEDT-Na. Influence of temperature, drc, and DEDT-Na concentration were successively examined to determine the best conditions of the addition on ENR latices.     

Effect of molecular weight on the growth rates of low melting spherulites of trans-1,4-polyisoprene

Growth rates of G of low-melting spherulites in fractions of trans-1,4-polyisoprene have been measured. The data were analyzed by use of an equation, ln G = ln G₀ ? ΔF^*/RT_c, valid at temperatures close to the equilibrium melting point. Plots of ln G against a function of the critical free energy of nucleation ΔF^* result in a family of straight lines having a common intercept, ln G₀, which is independent of molecular weight. The slope of these lines is a measure of the interfacial free energy of the crystallites and increases with the molecular weight, reflecting increasing irregularity in the structure of the semicrystalline mass. Comparison of growth rates of low-melting and high-melting trans-1,4-polyisoprene indicates that G₀ does not, to a first approximation, depend on the nature of the crystals growing from the melt. The temperature at which spherulites of the two crystalline forms grow at equal rates has been calculated.     

The conformation and vibrational spectra of trans-1,4-dihalocyclohexanes: Part III. trans-1,4-Diiodo-, trans-1,4-bromoiodo- and trans-1,4-dibromocyclohexane

The IR spectra of trans-1,4-diiodo- and trans-1,4-bromoiodocyclohexane as solutes in various solvents, as pellets and as solids under high pressure are recorded in the region 4000–30 cm^?1. Additional spectra of the melts, amorphous and annealed crystalline solids at 90 K and dichroic spectra of oriented crystals are recorded above 200 cm^?1. Raman spectra of the amorphous and annealed solids at 90 K and as solutes in various solvents, are obtained, including polarization measurements. IR and Raman spectra of trans-1,4-di-bromocyclohexane in the temperature range 90–250 K are recorded. Equilibrium mixtures of ee and aa conformers of the title compounds are observed in solution, in the melts and in the amorphous solid at 90 K. The ee conformer only is present in the stable crystal, while the aa conformer predominates in apparently metastable crystals annealed to ca. 205 K. The concentration of the aa conformer increases under high pressure (50 kbar). Fundamental frequencies for both ee and aa conformers are assigned. A normal coordinate analysis is carried out, and the force Fields adjusted to nine halogenated cyclohexanes using the overlay technique.     

Preparation of block copolymer vesicles in solution

Block copolymer vesicles can be prepared in solution from a variety of different amphiphilic systems. Polystyrene‐block‐poly(acrylic acid), polystyrene‐block‐poly(ethylene oxide), and many other block copolymer systems can produce vesicles of a wide range of sizes; those in the range of 100–1000 nm have been explored extensively. Different factors, such as the absolute and relative block lengths, the presence of additives (ions, homopolymers, and surfactants), the water content in the solvent mixture, the nature and composition of the solvent, the temperature, and the polydispersity of the hydrophilic block, provide control over the types of vesicles produced. Their high stability, resistance to many external stimuli, and ability to package both hydrophilic and hydrophobic compounds make them excellent candidates for use in the medical, pharmaceutical, and environmental fields. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 923–938, 2004     

Properties of crystallized trans 1,4-polyisoprene

Stirrer crystallization of a trans-1,4-polyisoprene fraction(M?_n = 3 × 10⁵) was carried out from n-butyl acetate and from n-heptane solutions (2% w/v). Fibrous crystals in the ß form were obtained at temperatures of 46-48°C in the two solvents, respectively. At 36-46°C from n-butyl acetate and 25-35°C from n-heptane lamellar crystallization took place leading predominantly to the α form. Melting endotherms and densities for various samples were obtained. The maximum TENDO for α was 74°C and for ß 79°C and the maximum weight fraction crystallinity for ß was 0.78. The stability of performed α nuclei in n-butyl acetate and n-heptane using a fraction with M?_n = 2.5 × 10⁵ was monitored as a function of temperature. The dissolution temperature of fibrous ß-TPI with the maximum TENDO was measured in 13 liquids and the results analyzed in terms of the Flory-Huggins parameter. The heats of fusion for the α and ß forms, obtained by extrapolation of heat of fusion versus change in specific volume, were found to be 8.0 and 10 kJ mol^?1, respectively. The pressure coefficients of the melting temperature were calculated to be 38 and 43 K kbar^?1 and the fold-surface free energies recalculated to be 42 ± 1 and 53 ± 1 erg cm^?2 for the α and ß forms, respectively. The number of monomer units per average noncrystalline chain traverse for the most crystalline fibrous ß-TPI was estimated at 36.