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1.
During the freezing process, water is partially separated as ice and the solutes are concentrated in the unfrozen matrix (UFM). With further lowering of the temperature, the UFM becomes highly viscous. The high viscosity of the UFM prolongs ice formation and makes it difficult to accurately determine the glass transition (Tg′) and the concentration (Cg′) of the maximally freeze-concentrated matrix. In this study, a new method for the determination of the concentration of the UFM was developed using differential scanning calorimetry (DSC). Sugar solutions were frozen, annealed at temperatures slightly above the expected Tg′, rapidly cooled and then heated to 20 °C. The UFM concentrations of the annealed samples were obtained by estimating the solute concentration corresponding to the Tg at the respective annealing temperature. The dependence of the Tg on experimental conditions such as the annealing time, annealing temperature and cooling rate was studied in detail. Values for Cg′ and Tg′ were obtained by linear and quadratic extrapolations of the experimental data over a short temperature and solute concentration range. The maximal freeze-concentrations of glucose, sucrose and maltose were determined to be 79.9, 80.9 and 80.3% (w/w), respectively. Results of this study were in good agreement to previously published data.  相似文献   

2.
The crosslinking of poly(N-vinylcarbazole) PVK resulting from UV irradiation (λ > 300 nm) was studied by combining various techniques including infrared spectroscopy, size exclusion chromatography (SEC), differential scanning calorimetry (DSC), swelling ratio, positron annihilation spectroscopy (PAS) and thermoporosimetry. The results obtained from all of these techniques were correlated and allowed a complete monitoring of the crosslinking during irradiation, from the creation of the first crosslinks to the formation and densification of the three-dimensional network. From SEC analysis and determination of swelling ratios, it was demonstrated that crosslinking was the predominant mechanism during the first stages of irradiation. PAS and DSC analyses revealed a good correlation between variations of free volume and increase of glass transition (Tg) of the bulk material. In addition, the development of the polymer architecture shown by the growth of Tg was nicely correlated with the modifications of the chemical structure observed by infrared analysis. Thermoporosimetry allowed the determination of the mesh size distributions within the polymer for the longest irradiation times. Numerical relationships were established and their validity was checked.  相似文献   

3.
Protein crystals are usually grown in multi-component aqueous solutions containing salts, buffers and other additives. To measure the X-ray diffraction data of the crystal, crystals are rapidly lowered to cryogenic temperatures. On flash cooling, ice frequently forms affecting the integrity of the sample. In order to eliminate this effect, substances called cryoprotectants are added to produce a glassy (vitrified) state rather than ice. Heretofore, the quantity of cryoprotectant needed to vitrify the sample has largely been established by trial and error. In this study, differential scanning calorimetry (DSC) was used to measure the melting (Tm), devitrification (Td) and glass transition (Tg) temperatures of solutions with a range of compositions typical of those used for growing protein crystals, with the addition of glycerol as cryoprotectant. The addition of cryoprotectant raises the Tg and lowers the Tm of bulk solution thereby decreasing the cooling rates required for vitrification of protein crystals. The theoretical Tg value was calculated using the apparent volume fraction using the Miller/Fox equation extended for multi-component systems. The experimental values of Tg were within approximately ±4% of that predicted by the model. Thus, the use of the model holds the promise of a rational method for the theoretical determination of the composition of cryoprotectant requirement of protein crystallization solutions.  相似文献   

4.
In this report, we describe the synthesis and characterization of photosensitive poly(l-lactide) with a pendent cinnamate group. α,ω-Dihydoxy terminated poly(l-lactide) (PLLA-diol) [molecular weights (MW); 2000, 4000 and 9000 g/mol ] was chain-extended with a diacyl chloride of 5-cinnamoyloxyisophthalic acid (ICA) to obtain high-molecular-weight photocrosslinkable poly(l-lactide)s (ICA/PLLA). The resulting polyesters were characterized by proton nuclear magnetic resonance spectroscopy, gel permeation chromatography, ultraviolet–visible spectroscopy, differential scanning calorimetry, thermomechanical analysis and thermogravimetry. The glass transition temperature (Tg), the melting temperature (Tm) and the degree of relative crystallinity (Xc) of ICA/PLLAs increased with the increasing MW of the PLLA-diols. The photosensitive ICA/PLLAs were irradiated with a 400 W high-pressure mercury lamp (λ > 280 nm) for various times to produce the PLLA gel films without a photoinitiator. The crosslinking rate monitored by a UV–vis spectrum decreased with the increasing MW of the PLLA-diols. The crosslinking of the ICA/PLLA ?4000 film enhanced the Tg slightly and the tensile strength and Young’s modulus significantly, while reduced the Tm and Xc. The enzymatic degradation was measured by the weight loss of the films in a phosphate buffer solution with proteinaze-k. The crosslinking of the films decreased markedly the degradation rate.  相似文献   

5.
Poly (styrene-co-furfuryl methacrylate) networks were prepared by the Diels-Alder (D-A) reaction in solution at 25 °C between the linear copolymer and bismaleimide (BM). The resultant crosslinked polymers were swollen to equilibrium in toluene at 25 °C and swelling properties were studied by gravimetric and dimensional measurements. The swelling behaviour of these organogels depended on the composition of the copolymer and the concentration of BM used. Shear and Young’s moduli and the effective crosslinking densities (νe) were determined by compression (stress)-strain measurements. The theoretical crosslinking density was higher than the νe for all the crosslinked copolymers. An endothermic peak without Tg was observed on the DSC curve on heating the dry crosslinked polymer. On reheating a Tg at ≈98 °C was found, which is attributed to presence of linear copolymer produced by the retro D-A reaction in the first heating. The thermal stability of a crosslinked copolymer under nitrogen and air showed differences with the stability of the linear copolymer. The increase in viscosity of the solution during the D-A reaction was followed with time, for initial linear copolymers of different molecular weights. It was found that onset of gelation increased to longer reaction times the lower the molecular weight of copolymer.  相似文献   

6.
In this paper the synthesis and characterization of a new family reactive nematic oligomers based on 4-hydroxybenzoic acid (4-HBA) will be presented. We modified the backbone using para- and meta-substituted aromatic monomers such as terephthalic acid (TA), isophthalic acid (IA), hydroquinone (HQ), resorcinol (RS), 4,4′-bisphenol (BP) and 3-hydroxybenzoic acid (3-HBA). All oligomers, with a target Mn of 5000 and 9000 g mol−1, were end-capped with reactive phenylethynyl functionalities and synthesized using standard melt condensation techniques. Curing of the phenylethynyl reactive functionalities proceeds through chain extension and crosslinking, depending upon the temperature and time and can be carried out between 310 and 400 °C. Fully cured nematic thermosets could be obtained with glass-transition temperatures previously not accessible (Tg > 400 °C). The cured polymers exhibit excellent tensile properties, i.e. tensile strength (83 MPa) and elongation at break (9%). This approach allows us to prepare all-aromatic polymers with a combination of useful properties such as ease of processing, high Tg’s, and excellent mechanical properties.  相似文献   

7.
A series of copolyimides were prepared from benzophenone-3,3′,4,4′-tetracarboxylic dianhydride (BTDA) and various aromatic diamines which contain a fluorenyl group and/or alkyl substituents in ortho position to the amine groups. The effect of the chemical composition on the glass transition temperature (Tg), thermal stability as well as on the dielectric constant of these polymers was studied. High Tg polymers (Tg ranging from 260 °C to 370 °C), withstanding temperatures as high as 400 °C for 10 h and having a low dielectric constant (from 2.6 to 3.1) were successfully synthesized. All these polymers were able to crosslink under UV or thermal treatments.  相似文献   

8.
The dynamic mechanical Tg of styrene-poly(diethylene succinate fumarate) networks has been quantitatively related to the crosslink density. The crosslinking constants, derived from this analysis, were compared with those obtained from other network systems. It is concluded that the crosslinking constant of Di Marzio is not a universal constant; however, a related parameter was found to be very nearly material independent.  相似文献   

9.
The measurement of the maximum glass transitionT g∞ of a thermosetting resin is usually performed by differential scanning calorimetry in the second scan (T g2scan), after a previous scan by heating up the sample to a temperature where the exothermic curing reaction has been completed. However, this method can eventually produce thermal degradation, decreasing the crosslinking density and theT g of the sample. Values ofT g2scan between 95? and 102?C were found in an epoxy resin based on DGEBA cured with phthalic anhydride. Thermal degradation effects can be avoided if the measurement is performed by isothermal curing and further determination ofT g. AT g∞ value of 109?C is achieved, which is the maximum value ofT g according to the topological limit of conversion.  相似文献   

10.
The molecular motion of crosslinked polyepichlorohydrin (PECH) is studied qualitatively by NMR techniques. The results of temperature dependence of 1H T2 and T1 indicate that the crosslinking (crosslink density < 3%) restricts molecular motions of the polymer even far above its Tg. The 1H T1 minimum, corresponding to the large-scale chain-motion of crosslinked PECH, shifts to higher temperatures with increasing crosslink density. 1H T2 data also show that the crosslinking hinders free chain motions of the polymer above its Tg. The 13C T1 relaxation time is sensitive to such motional changes as well. 13C linewidths of crosslinked PECHs vary with the crosslink density in both the swollen state and the solid state. The mechanism of 13C linewidth broadening of crosslinked polymers is discussed in detail. In the case of PECH, the linewidth broadening is caused by changing molecular environment due to crosslinking (such as presence of various chemical shift structures and freezing effects in conformational environment as chain mobility decreases), rather than increasing correlation times, which shorten the relaxation time (T2) of polymer chains. © 1994 John Wiley & Sons, Inc.  相似文献   

11.
The thermal and rheological characterizations of seven random, low molecular weight (Mn ≅ 9500 g mol−1), H2N-ended polyethersulfone/polyetherethersulfone (PES/PEES) copolymers, at various PES/PEES ratios, were performed. The glass transition temperatures (Tg) were determined by DSC. Degradations were carried out in a thermobalance, under flowing nitrogen, in dynamic heating conditions from 35 °C to 650 °C. The initial decomposition temperatures (Ti) and the half decomposition temperatures (T1/2) were directly determined by TG curves, while the apparent activation energies of degradation (Ea) were obtained by the Kissinger method. In addition, the complex viscosities (η) of the molten polymers were determined in experimental conditions of linear viscoelasticity. Tg, Ea and η values increased linearly with PES% content, while Ti and T1/2 values showed opposite behaviour. In every case both PES and PEES homopolymers felt outside linearity. The results obtained are discussed and interpreted, and compared with those of corresponding Cl-ended copolymers previously studied.  相似文献   

12.
Films of block copolymers of polystyrene + isoprene, cast from cyclohexane at temperatures above and below a conformational transition temperature (Tp) derived from the plot of [η] vs. T, have been examined for glass transition temperatures. In every case, two glass transitions were found, Tg1 (polyisoprene) and Tg2 (polystyrene) or Tg1 and Tg (an intermediate species). Tg is assumed to be characteristic of a mixed phase thus providing further evidence that Tp marks a conformational transition from a phase-separated to a phase-mixed form.  相似文献   

13.
The rotational relaxation data for free nitroxyl radicals in poly(ethylene glycol) samples of mol. wt 3000-22,000 gave for the glass-transition temperature (Tp) a value of ?60°, independent of molecular weight. The rotational activation energy was ~ 2 kJ/mole below Tg and ~ 10 kJ/mole above Tg. This indicated that the mechanism of motion below Tg differs from the above Tg. Evidently the rotating units of polymer are much smaller below Tg than above it. The rotational relaxation time (T) was found to be dependent on the molar volume of PEG (V) and to follow the empirical equations t = A exp (?kV1) and t = B exp (?kV1) where Va and V1 are the molar volumes (cm3/mol) when T < Tg and T > Tg respectively, and k ~ 1. Therefore the defects in which radicals are located are perhaps the dominant factor determining the dynamic state of probe radicals in polymers at low temperatures.  相似文献   

14.
Poly(ether imide)s containing the 1,2-diphenylcyclopropane or diphenylacetylene moiety have been synthesized from two new dianhydrides by solution condensation. Characterization of and crosslinking studies on these polymers were carried out utilizing DSC, TGA, and NMR. The polymers can be thermally crosslinked when heated above 350°C and the glass transition temperatures (Tg) of the polymers increase after crosslinking. The resulting crosslinked networks are insoluble in all solvents tried. Thermogravimetric analysis shows that no significant weight loss accompanies the cross-linking reaction. © 1996 John Wiley & Sons, Inc.  相似文献   

15.
Se96Sn4 chalcogenide glass was prepared by melt quenching technique and exposed, at room temperature, to different doses of 4, 8, 12, 24 and 33 kGy of high-energy 60Co gamma irradiation. Differential scanning calorimeter (DSC) was used under non-isothermal condition to determine the glass transition temperature Tg, onset Tc and peak Tp temperatures of crystallization, of un-irradiated and γ-irradiated samples, at four different heating rates. The variation of Tg with heating rates was utilized to calculate the glass transition activation energy Et for un-irradiated and γ-irradiated glass, using the methods suggested by Kissinger and Moynihan. Based on the obtained values of the characteristic temperatures Tg, Tc and Tp, thermal stability was monitored through the calculation of the S parameter and the crystallization rate factor 〈Kp〉 for irradiated and un-irradiated glass. Results reveal that, as γ-dose increases Tg increases up to 12 kGy then decreases at higher doses but remains more than that of un-irradiated glass. Meanwhile, both Et and 〈Kp〉 attain their minimum values at the same dose of 12 kGy and the glass is thermally stable at this particular dose.  相似文献   

16.
Poly(4-vinylphenol) (PVPh) was used as thermoplastic modifier of epoxy resins. Binary epoxy/PVPh mixtures with high thermoplastic content rise high glass transition temperatures (T g) after heating, due to the epoxy-phenol reaction. Blends with low PVPh percentages reach high T g if 2-methylimidazole is added, which catalyses epoxy homopolymerisation and epoxy-phenol reaction. The cured blends do not present phase separation although the network structure depends on the epoxy/phenol ratio. At low PVPh percentage the main crosslinking reaction is epoxy-epoxy but, when the thermoplastic content increases, the epoxy-phenol reaction prevails, causing an important T g increase and becoming less brittle.  相似文献   

17.
UV-crosslinkable polyacrylates were synthesized for use as pressure sensitive adhesives (PSAs). These polyacrylates acted as polymeric photoinitiators due to the benzophenone incorporated into their backbones. Hydrogenated rosin epoxy methacrylate (HREM; based on hydrogenated rosin and glycidyl methacrylate) was also synthesized as a tackifier, and blended at different levels with the synthesized, UV-crosslinkable polyacrylates for use as PSAs. The effect of the new tackifier, HREM, on the properties of the UV-crosslinkable PSAs was examined in comparison with the properties exhibited by PSA/hydrogenated rosin blends. The characteristics of these PSA/tackifier blends were examined by Fourier-transform infrared (FTIR) spectroscopy, differential scanning calorimetry (DSC) and an advanced rheometric expansion system (ARES). In addition, the adhesion performance of the PSA blends was investigated using probe tack tests. DSC and ARES revealed all the PSA blends with HREM or hydrogenated rosin to be miscible at the molecular level. The glass transition temperature (Tg) of HREM was −25.6 °C, which is lower than that of other commercially available rosin tackifiers. FTIR revealed changes in the relative concentration of benzophenone groups in the PSAs at 1580 cm−1, which demonstrated that the crosslinking efficiency is proportional to the benzophenone content and UV dose, but decreases with increasing hydrogenated rosin content. However, the reduced crosslinking reaction efficiency was improved in the PSA/HREM blends due to the low Tg of HREM which only slightly increased the Tg of the PSA blends. Moreover, the relative initial decrease in the probe tack of the PSA/HREM blends was lower than that of the PSA/hydrogenated rosin blends after UV irradiation.  相似文献   

18.
A new implementation of copper-free thermal Huisgen 1,3-dipolar crosslinking reaction into a high Tg hyperbranched polyimide polymer in order to stabilize the electro-optic (EO) activity of second-order non linear materials is reported. Towards this goal, two different synthetic approaches were explored. The first strategy is based on the post-functionalization of the polymer with mixtures of DR1 azido derivative and propargylic alcohol, whereas, the second consists in the preparation of two complementary functionalized hyperbranched polymers that are mixed just before the preparation of films. Materials exhibit good second-order nonlinear optical coefficients (d33) close to 30 pm/V at the fundamental wavelength of 1064 nm. Moreover, the thermal stability of the NLO properties of these materials reaches temperatures as high as 150 °C, and probably higher. This represents the highest thermal stability of crosslinkable EO polymers based on the crosslinking Huisgen reaction.  相似文献   

19.
A novel method was developed to determine the ultra-low glass transition temperature (Tg) of materials through physical blending via differential scanning calorimetry. According to the Fox equation for polymer blends, a blend of two fully compatible polymers has only one Tg. The single Tg is a function of the Tgs of the two simple polymers. Thus, the ultra-low Tg of one material can be obtained from the Tgs of another polymer and their blends. The error of Tg measurements depends on the measurement error of the Tgs for the blends and another polymer. The method was successfully applied to determine the Tgs of acetyl tributyl citrate (ATBC), tributyl citrate (TBC) and poly(ethylene glycol)s (PEG)s with different molecular weights. The Tgs for ATBC, TBC, PEG-4000 and PEG-800 were ?57.0 °C, ?62.7 °C, ?76.6 °C and ?83.1 °C, respectively. For all the samples, the standard deviation of measurements was less than 3.3 °C, and the absolute error of measurements was theoretically not more than 5.3 °C. These results indicate that this method has acceptable precision and accuracy.  相似文献   

20.
The thermal and rheological behaviour of seven random Cl-ended aromatic PES/PEES copolymers (Mn ≈ 9500 g mol−1), at various PES/PEES repeating unit ratios, was studied. The glass transition temperatures (Tg), determined by DSC experiments, showed a dependence on copolymer composition significantly different from the ideal linear behaviour expected on the basis of Fox equation. Degradations were carried out in the scanning mode, under flowing nitrogen, in the temperature range 35-650 °C and a single degradation stage was observed for all copolymers. The initial decomposition temperatures (Ti) and the half decomposition temperatures (T1/2) were directly determined by TG curves, while the apparent activation energies of degradation (Ea) were obtained by the Kissinger method. In addition, the complex viscosity (η) of molten copolymers was determined in experimental conditions of linear viscoelasticity. Ti, T1/2, Ea, and η values were depending on copolymer composition, showing a trend similar to that of Tg values. The results obtained were discussed and interpreted.  相似文献   

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