Synthesis and gelation properties of PEG-PLA-PEG triblock copolymers obtained by coupling monohydroxylated PEG-PLA with adipoyl chloride

Ring-opening polymerization of D,L-lactide was carried out in the presence of monohydroxylated poly(ethylene glycol) (PEG) with Mn of 2000 and 5000, using zinc powder as catalyst. The resulting PEG-b-polylactide (PEG-PLA) diblocks with various ethylene oxide/lactyl (EO/LA) ratios were coupled with adipoyl chloride to yield PEG-PLA-PEG triblock copolymers. N-Dimethylaminopyridine (DMAP) was used as catalyst. The obtained PEG-PLA-PEG triblock copolymers were characterized by various analytical techniques such as IR, 1H NMR, size exclusion chromatography, X-ray diffraction, and differential scanning calorimetry. Data showed that all the copolymers were semicrystalline with the PEG-type crystalline structure, the crystallinity decreasing with increasing PLA block length. Bioresorbable hydrogels were prepared from the water-soluble triblock copolymers. Rheological measurements showed a gel-sol transition with increasing temperature and gelation was found to be thermoreversible. The copolymer solution behaves like a viscoelastic liquid above the gel point and like a viscoelastic solid below the gel point. The critical gelation concentration, the gel-sol transition temperature at a given concentration, and corresponding moduli depend on both the EO/LA ratio and the molecular weight of the copolymers. It is assumed that gelation results from interactions between PEG blocks at low temperatures and that these interactions are disrupted as the temperature is elevated. The shrinking of PEG blocks with increasing temperature seems to be in agreement with the variation of the gel-sol transition temperatures.     

Dynamic mechanical relaxation behavior of block copolymers at high temperatures

We carried out dynamic mechanical measurements to investigate three different examples of block copolymers: styrene–isoprene diblock copolymers and styrene–butadiene–styrene and styrene–(styrene butadiene)–styrene triblock copolymers. Isochronal and isothermal measurements of the real and imaginary parts of the complex shear modulus were performed over wide ranges of temperature and frequency. The measurements showed the presence of an additional relaxation process appearing at temperatures higher than those of the glass relaxation of the polystyrene phase, which has been misinterpreted by some authors as an order–disorder transition. The frequency dependence revealed that this process was a relaxation process and did not belong to a first‐order transition. Moreover, the influence of crosslinking via dicumylperoxide was measured, and we constructed complete master curves to confirm the presence of two relaxation processes. The high‐temperature relaxation process was strongly suppressed by crosslinking. Therefore, it was possible to detect the glass relaxation process of the polystyrene phase in a precise manner. The results were compared with those of homopolymers. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2198–2206, 2001     

可交联的PMMA型和PS型高热稳定性二阶非线性光学材料的合成与表征

采用封管反应的方法,以较高产率(80%以上)合成了一系列含环氧基团的可交联PMMA型和PS型极化聚合物材料,该材料具有很好的成膜性.用DSC和T_gA等方法研究了聚合物固化前后的热性能,结果表明,由于聚合物在极化后期经热固化使引入的环氧基团开环交联,聚合物的玻璃化转变温度(T_g)较固化前明显提高30～50K.同时,固化后的聚合物具有较高的热分解温度(T_d>543K).对聚合物的二阶非线性光学性质的测试结果表明,在室温下放置100h后,聚合物的电光系数r₃₃值均保持在初始值的75%以上.这是由于环氧基团的开环使固化后的聚合物本身产生一定程度的交联,导致取向后的发色团被聚合物的交联网禁锢而不易弛豫,从而使这类PMMA型和PS型二阶非线性聚合物材料的热稳定性能得以提高.     

Thermal and mechanical properties of radiation crosslinked poly(tetrafluoroethylene-co-perfluoromethyl vinyl ether)

The effect of irradiation temperature on the polymer properties was investigated for the fluoroelastomer poly(tetrafluoroethylene-co-perfluoromethylvinyl ether) (TFE/PMVE). TFE/PMVE samples were γ-irradiated to 150 kGy at temperatures ranging from 77 K to 373 K. Analysis of the sol/gel behaviour, tensile properties, and glass transition temperatures indicated that crosslinking commenced in the temperature range 195 to 263 K, for a dose of 150 kGy. The latter temperature was 13 K below the glass transition temperature. Crosslinking remained relatively constant to higher temperatures. Chain scission reactions were found to occur well below the glass transition temperature and increased at higher temperatures. The optimum temperature for the radiation crosslinking of TFE/PMVE, for the temperatures investigated, was 263 K.     

Effect of the reference chain dimension on the viscoelastic and stress–strain behavior of poly(n-butyl methacrylate) networks

The linear viscoelastic and stress-strain behavior of poly(n-butyl methacrylate) networks at a content of crosslinking agent (ethyleneglycol dimethacrylate) of c? 0–1 × 10^?4 mole/cm³ was investigated in the main transition and rubberlike region in the temperature interval from 20 to 150°C. The dependence of the unperturbed chain dimensions on temperature was determined from thermoelastic measurements in the rubberlike region; this dependence was unaffected by the content of crosslinking agent. Application of time–temperature superposition to the linear viscoelastic behavior did not give a continuous superimposed curve in the proximity of the rubberlike region; superposition within the whole time region required introducing the change of the unperturbed chain dimensions with temperature. This correction was sufficient for a sample with a higher content of the crosslinking agent. However, for loose networks (c< 0.1 × 10^?4 mole/cm³) it was insufficient, because of another relaxation mechanism in the region of high temperatures. It was found that the intensity and temperature dependence of this relaxation mechanism, which is probably due to a change of the number of entanglements with temperature, are connected with the magnitude and the temperature dependence of the C₂ constant of the Mooney-Rivlin equation.     

Synthesis and characterization of temperature-sensitive cellulose-graft-poly(N-isopropylacrylamide) copolymers

A new series of cellulose-graft-poly(N-isopropylacrylamide)(cellulose-g-PNIPAM) copolymers were prepared by atom transfer radical polymerization(ATRP) of N-isopropylacrylamide monomers from a cellulose-based macro-initiator, which was homogeneously synthesized in an ionic liquid 1-allyl-3-methylimidazolium chloride(Amim Cl). The composition of cellulose-g-PNIPAM copolymers could be adjusted by altering the feeding ratio and reaction time. The resultant copolymers with relatively high content of PNIPAM segments(molar substitution of PNIPAM ? 18.3) were soluble in water at room temperature. Aqueous solutions of cellulose-g-PNIPAM copolymers exhibited clear temperature-sensitive behavior, and their sol-to-gel phase transition properties were investigated by dynamic light scattering(DLS) and UV measurements. Compared with pure PNIPAM, the cellulose-g-PNIPAM copolymers possessed higher lower critical solution temperatures(LCST) in a range from 36.9 ?C to 40.8 ?C, which are close to normal human body temperature, and could be tuned by adjusting the content of PNIPAM segments in copolymers. Spherical structure of cellulose-g-PNIPAM copolymers formed at temperatures above LCST and its morphology was observed by TEM and SEM. These novel cellulose-g-PNIPAM copolymers may be attractive substrates for some biomedical applications, such as drug release and tissue engineering.     

Physical & thermal properties of model polysiloxane rubbers: impact of crosslink density and tin concentration

The influence of the tin octanoate catalyst on the physical and thermal properties of RTV 5370 polysiloxane rubbers has been studied. To assess the likely influence of crosslinking on a number of physical and thermal properties in polysiloxanes, “model siloxane networks” (representing networks of well defined composition/structure) have been formulated by the hydrosilylation of polysiloxane diols of known average molecular weight with tetraethoxysilane curing agent. It was found that linear swell and the crystallisation melting transitions of these systems were both significantly affected by changes in crosslink density. A selection of RTV5370 foamed rubbers with different tin concentrations were prepared in a similar manner to assess the influence of the tin catalyst. For these materials it was found that the area of the crystalline melting transition decreased with increasing tin concentration, an effect indicative of increased crosslinking. Samples with double the standard amount of tin (10% wt catalyst) show a shift in the crystallisation transition to higher temperature with no further effect beyond this concentration. Furthermore, the storage modulus (E') of the rubbers at room temperature appears to be independent of the tin catalyst concentration. Through correlation with the data from the model siloxanes, these results suggests that increasing the catalyst concentration appears to induce additional crosslinking interactions that are stable at low temperatures but not stable at room temperature.     

Syntheses and crosslinking reactions of self-curable copolymers containing pendant cyclic iminoethers and carboxylic acid groups

Soluble copolymers containing both pendant cyclic iminoethers such as 4,4-dimethyl-2-oxazoline or 4,4,6-trimethyl-4H-dihydro-1,3-oxazine and carboxylic acid were successfully synthesized by radical copolymerizations of 4,4-dimethyl-2-vinyl-2-oxazoline, 4,4-dimethyl-2-isopropenyl-2-oxazoline, or 4,4,6-trimethyl-2-vinyl-4H-dihydro-1,3-oxazine with methacrylic acid and styrene, methyl methacrylate, or ethyl acrylate using AIBN as an initiator in benzene or DMF at 60 or 80°C. The crosslinking reaction of the copolymers obtained did not occur by heating at 70°C. However, these copolymers quantitatively produced gel products by heating at 130°C. The rate of crosslinking reaction of the copolymer increased with increasing pendant cyclic iminoether and carboxylic acid groups. The rate of crosslinking was also affected by the molecular motion of the polymer chain. Our results show that the copolymers of more sterically hindered 2-vinyl-2-oxazolines are more stable and so they can be crosslinked in a controlled manner and at higher temperatures than the previously studied polyoxaziline system.     

Phase‐transition characteristics of amphiphilic poly(2‐ethyl‐2‐oxazoline)/poly(ε‐caprolactone) block copolymers in aqueous solutions

Amphiphilic diblock and triblock copolymers of various block compositions based on hydrophilic poly(2‐ethyl‐2‐oxazoline) (PEtOz) and hydrophobic poly(ε‐caprolactone) were synthesized. The micelle formation of these block copolymers in aqueous media was confirmed by a fluorescence technique and dynamic light scattering. The critical micelle concentrations ranged from 35.5 to 4.6 mg/L for diblock copolymers and 4.7 to 9.0 mg/L for triblock copolymers, depending on the block composition. The phase‐transition behaviors of the block copolymers in concentrated aqueous solutions were investigated. When the temperature was increased, aqueous solutions of diblock and triblock copolymers exhibited gel–sol transition and precipitation, both of which were thermally reversible. The gel–sol transition‐ and precipitation temperatures were manipulated by adjustment of the block composition. As the hydrophobic portion of block copolymers became higher, a larger gel region was generated. In the presence of sodium chloride, the phase transitions were shifted to a lower temperature level. Sodium thiocyanate displaced the gel region and precipitation temperatures to a higher temperature level. The low molecular weight saccharides, such as glucose and maltose, contributed to the shift of phase‐transition temperatures to a lower temperature level, where glucose was more effective than maltose in lowering the gel–sol transition temperatures. The malonic acid that formed hydrogen bonds with the PEtOz shell of micelles was effective in lowering phase‐transition temperatures to 1.0M, above which concentration the block copolymer solutions formed complex precipitates. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2400–2408, 2000     

The Influence of Temperature and Viscosity of Polyethylene Glycol on the Rate of Microwave-Induced In Situ Amorphization of Celecoxib

Microwaved-induced in situ amorphization of a drug in a polymer has been suggested to follow a dissolution process, with the drug dissolving into the mobile polymer at temperatures above the glass transition temperature (T_g) of the polymer. Thus, based on the Noyes–Whitney and the Stoke–Einstein equations, the temperature and the viscosity are expected to directly impact the rate and degree of drug amorphization. By investigating two different viscosity grades of polyethylene glycol (PEG), i.e., PEG 3000 and PEG 4000, and controlling the temperature of the microwave oven, it was possible to study the influence of both, temperature and viscosity, on the in situ amorphization of the model drug celecoxib (CCX) during exposure to microwave radiation. In this study, compacts containing 30 wt% CCX, 69 wt% PEG 3000 or PEG 4000 and 1 wt% lubricant (magnesium stearate) were exposed to microwave radiation at (i) a target temperature, or (ii) a target viscosity. It was found that at the target temperature, compacts containing PEG 3000 displayed a faster rate of amorphization as compared to compacts containing PEG 4000, due to the lower viscosity of PEG 3000 compared to PEG 4000. Furthermore, at the target viscosity, which was achieved by setting different temperatures for compacts containing PEG 3000 and PEG 4000, respectively, the compacts containing PEG 3000 displayed a slower rate of amorphization, due to a lower target temperature, than compacts containing PEG 4000. In conclusion, with lower viscosity of the polymer, at temperatures above its T_g, and with higher temperatures, both increasing the diffusion coefficient of the drug into the polymer, the rate of amorphization was increased allowing a faster in situ amorphization during exposure to microwave radiation. Hereby, the theory that the microwave-induced in situ amorphization process can be described as a dissolution process of the drug into the polymer, at temperatures above the T_g, is further strengthened.     

Effect of crosslinking on the photoinduced orientation of azo groups in thin polymer films

A series of amorphous copolymers containing disperse red 1 and crosslinkable acrylic groups were prepared. The crosslinked polymers were prepared in thin films by thermal polymerization of the acrylic groups in the copolymers. The orientation induced by irradiation with a linearly polarized laser was measured as birefringence at several temperatures, and the effect of crosslinking on the photoinduced orientation was investigated. Crosslinking enhanced the stability of the photoinduced birefringence. In particular, crosslinking helped to maintain the birefringence both at high temperatures and after the linearly polarized laser was turned off. The birefringence dynamics was analyzed with biexponential curve fitting. Crosslinking influenced not only the birefringence levels but also its rate of growth. The growth rate of the photoinduced birefringence decreased by crosslinking, whereas the relaxation was not significantly affected. Although crosslinking restrained the mobility of the azo chromophores, a certain fraction could orient or move randomly even in highly crosslinked polymer networks. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1686–1696, 2001     

Tannic acid: A sustainable crosslinking agent for high glass transition epoxy materials

Epoxy thermosets have revolutionized the coating, adhesive, and composite industries but the chemicals from which they are synthesized have significant effects on the environment and human health not only precure but also after crosslinking has occurred. In this study, we propose tannic acid (TA) as an alternative epoxy hardening agent for commercially available epoxy resin, the diglycidyl ether of bisphenol A (DGEBA). The resulting thermosets were characterized by Fourier transform infrared spectroscopy, optical microscopy, dynamic mechanical analysis, differential scanning calorimetry, compression testing, and thermogravimetric analysis. The results from this study showed that at temperatures above 100 °C, the compatibility of TA in DGEBA was significantly increased for loading levels up to 37% weight of TA in DGEBA, something that has not been seen before in literature. It was also discovered that at high loading levels, the resulting materials had glass transition temperatures at and above 200 °C. The resulting material was proposed as a more sustainable alternative to amine or acid hardened epoxy thermosets and was particularly useful in high‐temperature applications. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 1468–1480     

丙烯酸乙酯/烯丙基缩水甘油醚共聚物的热可逆共价交联

研究了双环戊二烯基二羧酸及其铵盐对丙烯酸乙酯 /烯丙基缩水甘油醚共聚物的交联反应 ,对比了二者的交联速度 .用DSC评价了它们的热可逆转化行为 .制得了热可逆部分达 79 7%的共价交联环氧型丙烯酸酯橡胶     

Thermosensitive superabsorbents based on poly(N,N-diethylacrylamide-co-sodium acrylate)

A series of poly(N,N-diethylacrylamide-co-sodium acrylate) with a degree of crosslinking of 1 mol%have been prepared as thermosensitive superabsorbents for water.The critical swelling temperatures or the volume phase transition temperature(VPTT) and the water absorption capacity of the polymers can be modulated by varying the amount of sodium acrylate(0-60 mol%) in the copolymers.The water absorption and swelling properties of the different hydrogels have been studied as function of temperature.The crosslinked copolymers can absorb large amounts of water at ambient temperatures and dehydrate at higher temperatures with relative ease,making the absorbent materials thermally responsive and thus reusable.The water absorption capacity of the copolymers depends on the pH of the media as the acrylate monomer has a higher water absorption in its deprotonated state.Added urea in the media raises and sharpens the VPTT values of the copolymers containing sodium acrylate.     

Long term environmental effects on physical properties of vinylester composite pipes

Glass fiber reinforced vinylester pipes (GFRP) have been exposed to outdoor conditions for 60 months. The effect of the outdoor exposure on the glass transition temperature (Tg) and the crosslinking index is discussed. The Tg and the degree of crosslinking has been determined using differential scanning calorimetry (DSC). DSC has become a preferred technique for rapid determination of transition temperatures and crosslinking for pure polymers, however, the use of this technique for composites is quite complex. This complexity arises from the fact that, in addition to the temperature overlap between the Tg and that of the crosslinking range, the amount of glass fibers in the GFRP is not constant. For this reason, a special measuring technique has been developed for the measurement of the amount of crosslinking of GFRP material. In this paper, it is illustrated that the DSC results clearly show the glass transition temperature and the exothermic crosslinking reaction of the vinylester composites. The results show that the glass transition temperature of the vinylester rapidly increases with respect to exposure time up to 12 months and then slightly decreases for longer exposure times. Similar trends were also observed for the change of the degree of crosslinking. The outdoor environment has also affected the weight of the exposed GFRP samples. It is observed that the weight of GFRP has gradually decreased with the exposure time. These physical changes of Tg, crosslinking and weight have impact on the tensile strength of the GFRP. For the initial outdoor exposure up to 24 months there is a 12% increase in strength from 250 MPa to 280 MPa. This increase is attributed to about 12% increase in crosslinking of the GFRP. However, for longer exposure the tensile strength has decreased back to 250 MPa due to slight decrease in crosslinking and small reduction of weight. It is observed that most of the weight loss has taken place at the surface of the GFRP samples.     

Intelligent drug delivery systems obtained by radiation

Radiation-induced polymerization of acryloyl-L-proline methyl ester, an -aminoacid-containing monomer, in the presence of a crosslinking agent and a hydrophilic monomer gave rise to polymer hydrogels whose water content at equilibrium was found to decrease as the swelling temperature increased. Some hydrogel samples were obtained with entrapped acetaminophen, an analgesic and antipyretic drug. It was ascertained that the release of the drug was controlled by both the hydrophilicity of the polymer matrices and the environmental temperature.     

Phase Transition and Micellization of Temperature Responsive Dextran—graft—poly（N—isopropylacrylamide）Polymers

Phase behavior and micellization of dextran-graft-poly(N-isopropylacrylamide)(PNIPAAm)polymers in aqueous solution are investigated in this paper using DSC and AFM methods.It is found that with the increase of grafting(G%) of the copolymers the endothermic enthalpy during the phase transition increases significantly and the transition temperature decreases slightly.The phase transition behavior of the copolymers is scanning rate dependent.Micelles are formed whenever the solution temperature is raised above the LCST of the copolymers.It is proposed that by using this thermal responsive property of the copolymers,drugs could be incorporated into the micelles without employing any organic solvent.     

Electron tunneling dynamics in anharmonic bath

Tunneling transition probability for a particle interacting with an anharmonic bath is found in a time-dependent Hartree approximation. The general expression is presented in terms of medium Keldysh functions that are assumed to be known. Furthermore, the transition probability is calculated in the noninteracting-blip approximation where the rate constant does not exhibit an activation dependence at high temperatures. The reorganization energy E(r) and the renormalized reaction heat epsilon are expressed in terms of the correlation matrix for a solvent and internal modes in both quantum and classical regimes. It is shown that E(r) and epsilon are temperature dependent.     

Thermoanalytical study of an epoxy resin crosslinked with an aliphatic polyamine

Samples of the cured resins were prepared in the form of cast sheets. The concentration of the amine curing agent (triethylenetetramine) in the epoxy resin (bisphenol-A diglycidylether) was varied between 25 and 100% of the stoichiometric quantity. The cured resins were examined by differential scanning calorimetry, penetration under load as a function of temperature, and dynamic mechanical analysis. It is found that all of these methods provide a useful means of monitoring crosslinking through changes in the glass transition temperature. The dependence of some characteristic secondary relaxation temperatures, and the change in heat capacity at the glass transition, on the concentration of the amine were also investigated.     

Polymers from multifunctional isocyanates 14. Alternating copolymers from isocyanato alkenes: Copolymerization of 2-propenyl isocyanate with trimethylsilyl methacrylate as electron acceptor monomer

The copolymerization of 2-propenyl isocyanate ( 1 ) with trimethylsilyl methacrylate ( 2 ) has been investigated. 1 is an electron donor monomer with little tendency to undergo homopolymerization, while 2 is an electron acceptor monomer, capable of free radical homopolymerization. Polymerization to low conversion in benzene gave copolymers with preferential incorporation of 2 and a tendency towards alternating copolymers with increasing amounts of 1 in the feed (1 : 1.13 with a 9 : 1 feed ratio of monomers 1 : 2 ). The glass transition temperatures of the amorphous polymers are in the range from 100–70°C, with a T_g of poly(trimethylsilyl methacrylate) being 135°C. Desilylation occurs in the presence of water, causing an exothermal reaction above the glass transition temperature probably with formation of amides, a reaction that can be used for crosslinking. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 611–616, 1998