共查询到20条相似文献,搜索用时 15 毫秒
1.
Stable macroradicals of methyl methacrylate were prepared by the azobisisobutyronitrile-initiated polymerization of methyl methacrylate in hexane whose solubility parameter value (δ) differed from that of the macroradical by more than 1.8 hildebrand units and in 1-propanol at temperatures below its theta temperature (84.5°C). The rates of heterogeneous polymerization in hexane and 1-propanol were much faster than that of the homogeneous polymerization in benzene. Stable macroradicals were not obtained in benzene which was a good solvent nor at temperatures above the glass transition temperature (Tt) of the macroradicals. Thus, stable macroradicals of butyl methacrylate (Tg20°C) and and methyl acrylate (Tg3°C) were not obtained at a polymerization temperature of 50°C. Good yields of block copolymers of methyl methacrylate and acrylonitrile were obtained by the addition of acrylonitrile to the methyl methacrylate macroradical in methanol, ethanol, 1-propanol and hexane at 50°C. The rate of formation of the block copolymer decreased in these poor solvents as the differences between the solubility parameter of the solvent and macroradical increased.The block copolymer samples prepared at temperatures of 50°C and above were dissolved in benzene which is a non-solvent for acrylonitrile homopolymer, but is a good solvent for poly(methyl methacrylate) and the block copolymer. The presence of acrylonitrile and methyl methacrylate in the benzene-soluble macromolecule was demonstrated by pyrolysis gas chromatography, infra-red spectroscopy and differential thermal analysis. 相似文献
2.
An investigation of the effect of physical aging on excess enthalpy of compatible polymer blends was carried out. Poly(methyl methacrylate) (PMMA) and poly(styrene-co-acrylonitrile) (SAN) were chosen for this study. Blends of different ratios of PMMA and SAN were physically aged at different times and temperatures below their glass transition (Tg) and then subjected to enthalpy relaxation measurement in a differential scanning calorimeter (DSC). An improved procedure was developed and, employed to analyze the data. The error associated with the calculation of the normalized deviation in enthalpy, known as the “Φ” function, was below 4%. The relaxation was observed to proceed faster at higher aging temperature. It was also found that at higher aging temperatures of Tg – 20 and Tg– 35°C, enthalpy relaxation in SAN-rich blends proceeds faster than in PMMA rich blends, while at the low aging temperature of Tg– 50°C the rate of relaxation becomes independent of the composition. 相似文献
3.
The low‐temperature physical aging of amorphous poly(L ‐lactide) (PLLA) at 25–50°C below glass transition temperature (Tg) was carried out for 90 days. The physical aging significantly increased the Tg and glass transition enthalpy, but did not cause crystallization, regardless of aging temperature. The nonisothermal crystallization of PLLA during heating was accelerated only by physical aging at 50°C. These results indicate that the structure formed by physical aging only at 50°C induced the accelerated crystallization of PLLA during heating, whereas the structure formed by physical aging at 25 and 37°C had a negligible effect on the crystallization of PLLA during heating, except when the physical aging at 37°C was continued for the period as long as 90 days. The mechanism for the accelerated crystallization of PLLA by physical aging is discussed. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010 相似文献
4.
Katsuhiro Inomata Keisuke Nakagawa Chieko Fukuda Yoshimi Nakada Hideki Sugimoto Eiji Nakanishi 《Polymer》2010,51(3):793-503
This paper describes a shape memory behavior of graft copolymers poly(methyl methacrylate)-graft-poly(ethylene glycol) (PMMA-g-PEG). In shape memory test, the sample was deformed from its original shape to a temporary shape above glass transition temperature (Tg), cooled below Tg to fix the temporary shape, and subsequently heated above Tg for spontaneous recovery to the original shape. By grafting PEG onto PMMA backbone, shape memory ability was drastically enhanced than PMMA homopolymer. The shape recovery ratio was decreased with the increase in the shape deformation temperature. With considering a good miscibility of backbone and side chain in PMMA-g-PEG, this shape memory ability may be related to a physically cross-linked network structure by chain entanglement of the comb-like graft copolymer. Stress relaxation measurements were investigated in order to confirm the effect of the graft chains on the shape memory behavior. 相似文献
5.
The physical aging characteristics of oriented poly(ethylene terephthalate) (PET), have been studied ad functions of storage time and temperature below the glass transition temperature (Tg) of PET. The free volume relaxation, associated with aging, has been characterized by the enthalpy at Tg, as measured by differential scanning calorimetry. The effects of the free volume relaxation on mechanical properties and the mode of failure have been investigated. It has been determined that a correlation exists between the enthalpy of relaxation and the ductile-to-brittle failure transition. Molecular orientation reduces significantly the enthalpy of relaxation, resulting in the disappearance of the ductile-to-brittle transition when highly oriented samples are aged over time. It has been established that a minimum amount of orientation is required to reduce or eliminate the effects of PET aging. Molecular orientation has also been found to reduce craze formation when oriented PET is exposed to a stress-cracking medium at constant stress. 相似文献
6.
Giao Nguyen Daniel Nicole Marek Matlengiewicz Denis Roizard Norbert Henzel 《Polymer International》2001,50(7):784-791
The glass transition temperature of a series of samples of the poly[(methyl methacrylate)‐co‐(ethyl acrylate)] copolymer, synthesized at low conversion, were calculated theoretically using the equations of Barton and Johnston. The values obtained are more precise when the probabilities of the compositional diads are derived from the 13C NMR data instead of the classical method utilizing reactivity ratios. This can be observed more clearly when the copolymer samples are synthesized at high conversion. Introduction of configuration (tacticity) at the diad level confirms the above observations and slightly improves the calculated values of Tg compared to the initial formulae which were only taking into account the compositional sequences of the copolymer. © 2001 Society of Chemical Industry 相似文献
7.
Yunlong Guo 《Polymer》2009,50(16):4048-1018
The long-term viscoelastic behavior of polymeric materials used below the glass transition temperature (Tg) is greatly affected by physical aging. In contrast to isothermal physical aging, long-term response under nonisothermal history has received far less attention. This paper reports experimental results and analytical methods of long-term creep behavior of polyphenylene sulfide (PPS) subjected to complex thermal histories in a temperature range below Tg. To characterize the effects of aging, creep tests were performed using a dynamic mechanical analyzer (DMA). Besides the long-term data, short-term creep tests in identical thermal conditions were also analyzed; these were utilized with effective time theory to predict long-term response under both isothermal and nonisothermal temperature histories. The long-term compliance after a series of temperature changes was predicted by the effective time theory using the KAHR-ate model to obtain nonisothermal physical aging shift factors. Comparison of theoretical predictions with experimental data shows good agreement for various thermal histories. 相似文献
8.
Reactive and non-reactive diblock copolymers based on polyethylene oxide (PEO) and a poly(glycidyl methacrylate) (PGMA, reactive) or polystyrene (non-reactive) block, respectively, are prepared via ATRP and those are incorporated into a cycloaliphatic epoxy matrix. Crosslinking of the matrix is then performed by cationic UV curing, producing modified thermosets. 1H NMR and SEC measurements are carried out and used to analyze the composition, the molar mass and dispersity of the prepared block copolymers. The viscoelastic properties and morphology of the modified epoxy are determined using DMTA and FESEM, respectively. The addition of 4 and 8 wt% of the reactive PEO-b-PGMA block copolymer into epoxy resin has only minor effects on the glass transition temperature, Tg. The reactive homopolymer PGMA significantly increases and the non-reactive block copolymer PEO-b-PS slightly decreases the glass transition temperature of the epoxy matrix. The non-reactive block copolymer PEO-b-PS causes a little decrease in Tg values. The measurement of the critical stress factor, KIC, shows that the fracture toughness of the composite materials is enhanced by inclusion of the non-reactive block copolymer. In contrary, the reactive block copolymer has negative effect on the fracture toughness especially in case of short PEO block. FESEM micrographs studies on the fracture surfaces sustain the microphase separation and the increase in surface roughness in the toughened samples, indicating more energy was dissipated. 相似文献
9.
The behavior of poly(ethylene terephthalate) (PET) and poly(butylene terephthalate) (PBT) on water aging has been studied above and below the glass transition temperature (Tg). The aging process is caused by: degradation of the matrix and an increase in crystallinity above Tg, and microcavitation at the amorphous/crystalline interface below Tg. Such behavior well explains the deviation of the sorption kinetics from the Fickian model. The apparent water diffusion coefficients and the transport activation energies of PET and PBT have been calculated at temperatures above and below Tg. The mechanical behavior of the two polymers on water aging has been measured by means of fracture mechanics and Izod impact tests at different stress concentration factors. An increase of toughness of PET at short aging times has been shown by mechanical tests and SEM analysis fracture surfaces of differently aged samples. Izod tests of PET and PBT composites reinforced by long glass fibers have shown the contribution of fibers to the total fracture energy. 相似文献
10.
《Polymer》1987,28(7):1177-1184
The phase behaviour for blends of various polymethacrylates with styrene-acrylonitrile (SAN) copolymers has been examined as a function of the acrylonitrile content of the copolymer. Poly(methyl methacrylate), poly(ethyl methacrylate) and poly(n-propyl methacrylate) were found to be miscible with SANs over a limited window of acrylonitrile contents while no SANs appear to be miscible with poly(isopropyl methacrylate) or poly(n-butyl methacrylate). These conclusions were reached on the basis of lower critical solution temperature (LCST) and glass transition temperature behaviour. All miscible blends exhibited phase separation on heating, LCST behaviour, at temperatures which varied greatly with copolymer composition. An optimum acrylonitrile (AN) level ranging from about 10 to 14% by weight resulted in the highest temperatures for phase separation which has important implications for selection of SANs to produce homogeneous mixtures by melt processing. The basis for miscibility in these systems is evidently repulsion between styrene and acrylonitrile units in the copolymer as explained by recent models. The excess volumes for all blends are zero within experimental accuracy which suggests that the interactions for miscibility are relatively weak even for the optimum AN level. This interaction becomes smaller the larger or more bulky is the alkyl side group of the polymethacrylate. 相似文献
11.
Controlled degradation of poly(ethyl cyanoacrylate-co-methyl methacrylate) (PECA-co-PMMA) copolymers
Copolymers of poly(ethyl cyanoacrylate-co-methyl methacrylate) (PECA-co-PMMA) with various compositions were synthesized by free radical bulk polymerization in an effort to control degradation and stability as well as glass transition temperature to overcome intrinsic poor processability of the poly(ethyl cyanoacrylate) (PECA) homopolymer. The copolymers were found to have an alternating random tendency, which was responsible for the efficient inhibition of the unzipping degradation from the polymer chain. Consequently, the stability of the copolymers at elevated temperature and in solution was significantly improved compared to the PECA homopolymer. The glass transition temperatures of the copolymers were lowered by the incorporation of the methyl methacrylate (MMA), thereby further widening the operating temperature range of the polymer. On the other hand, the copolymer films exhibited hydrolytic degradation in phosphate buffered saline (PBS) solution at 37 °C, which is promising for their use as novel biomaterials. 相似文献
12.
Poly[methyl methacrylate(81)/butyl acrylate(19)] copolymer was exposed to atmosphere of humidity for various times. The normal α and ρ peak, a third LL peak is observed in thermally stimulated depolarization current (TSDC) spectra of the copolymer. The α peak corresponds to the glass transition, the ρ peak originates from the detrapping of trapped carriers in the bulk amorphous structure, and the LL peak can be attributed to the charge detrapping related to the liquid–liquid transition of the copolymer. The three peaks all move to lower temperature with an increase of the moisture content, indicating that the flexible moisture content not only has an effect of plasticization on the glass transition and liquid–liquid transition, but also makes the trap depth of ρ peak shallower. The trap depth of ρ peak is affected by the introduction of moisture content and the degree of hydration of the copolymer. Based on analysis of calculated results, it was confirmed that the relaxation time of LL peak obeys VF equation when temperature is below T LL. 相似文献
13.
Poly(ethyl α‐benzoyloxymethylacrylate) (EBMA) and copolymers of methyl methacrylate (MMA) with EBMA have been prepared by free radical polymerization. Monomer precursors of ethyl α‐benzoyloxymethylacrylate have likewise been polymerized. Glass transition temperatures (Tg) of homo and copolymers have been determined by differential scanning calorimetry. The Johnston equation, which considers the influence of monomeric unit distribution on the copolymer glass transition temperature, has been used to explain the Tg behaviour. Tg12 has been calculated by the application of the Johnston equation, which gave a value markedly lower than the average value expected from the additive contribution of the Tg of the corresponding homopolymers. © 2000 Society of Chemical Industry 相似文献
14.
The developments of physical aging in phenolphthalein poly(aryl-ether-ketone) (PEK-C) and poly(aryl-ether-sulfone) (PES-C) with time at two aging temperatures up to 20 K below their respective glass transition temperatures (Tg = 495 and 520 K) have been studied using differential scanning calorimetry (DSC). Substantial relaxation within the aging course of several hours were observed by detecting (Tg) decreasing during physical aging process at the two aging temperatures. The relaxation processes of both polymers are extremely nonlinear and self-retarding. The time dependencies of their enthalpies during the initial stages of annealing were approximately modeled using the Narayanaswamy-Tool model. The structure relaxation parameters obtained from this fitting were used to predict the possibility of physical aging occurring at their respective using temperatures. © 1995 John Wiley & Sons, Inc. 相似文献
15.
The nanophase separation in diblock and triblock copolymers consisting of immiscible poly(n-butyl acrylate) (block A) and gradient copolymers of methyl methacrylate (MMA) and n-butyl acrylate (nBA) (block M/A) were investigated by means of their heat capacity, Cp, as a function of the composition of the blocks M/A and temperature. In all copolymers studied, both blocks are represented by their Cp and glass transition temperature, Tg, as well as the broadening of the transition-temperature range. The low-temperature transition of the blocks A is always close to that of the pure poly(n-butyl acrylate) and is independent of the analyzed compositions of the block copolymer, but broadened asymmetrically relative to the homopolymer due to the small phase size. The higher transition is related to the glass transition of the copolymer block of composition M/A. Besides the asymmetric broadening of the transition due to the phase separation, it decreases in Tg and broadens, in addition, symmetrically with increasing acrylate content. The concentration gradient is not able to introduce a further phase separation with a third glass transition inside the M/A block. 相似文献
16.
Poly(vinyl alcohol) was modified by UV radiation with dimethyl amino ethyl methacrylate (DMAEMA) monomer to get poly(dimethyl amino ethyl methacrylate) modified poly(vinyl alcohol) (PVADMAEMA) membrane. The PVADMAEMA membranes were characterized by Fourier transform infrared spectroscopy. The tensile strength and elongation of PVADMAEMA membranes were measured by Universal Testing Machine. The results of X‐ray diffraction (XRD) and differential scanning calorimetry (DSC) showed that (1) the crystalline area in PVADMAEMA decreased with increasing the content of poly(dimethyl amino ethyl methacrylate) in the membrane. (2) Only one glass transition temperature (Tg) was found for the various PVADMAEMA membranes. It means that poly(dimethyl amino ethyl methacrylate) and PVA are compatible in PVADMAEMA membrane. (3)The Tg of the membrane is reduced with increasing the content of poly(dimethyl amino ethyl methacrylate) in the membrane. The water content on the PVADMAEMA membranes was determined. It was found that the water content on the PVADMAEMA membrane increased with increasing the content of poly(dimethyl amino ethyl methacrylate). The changes of properties enhanced the permeability of 5‐Fluorouracil (5‐Fu) through the PVADMAEMA membranes. A linear relationship between the permeability and the weight percent of poly(dimethyl amino ethyl methacrylate) in the PVADMAEMA membrane is found. It is expressed as P (cm/s) = (9.6 ± 0.4) × 10?5 + (8.8 ± 0.6) × 10?5 W x , where P is the permeability of 5‐Fu through the membrane and Wx is the weight percent of poly(dimethyl amino ethyl methacrylate) in the PVADMAEMA membrane. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012 相似文献
17.
Using the copolymer of acrylonitrile (AN), methyl methacrylate (MMA), and poly(ethylene glycol) methyl ether methacrylate as a backbone and poly(ethylene glycol) methyl ether (PEGME) with 1100 molecular weight as side chains, comb‐like gel polymers and their Li salt complexes were synthesized. The dynamic mechanical properties and conductivities were investigated. Results showed that the gel copolymer electrolytes possess two glass transitions: α‐transition and β‐transition. Based on the time–temperature equivalence principle, a master curve was constructed by selecting Tα as reference temperature. By reference to T0 = 50°C, the relation between log τc and c was found to be linear. The master curves are displaced progressively to higher frequencies as the content of plasticizer is increased. The relation between log τp and the content of plasticizer is also linear. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 576–584, 2007 相似文献
18.
A series of high glass transition temperature copolymers based on poly(methyl methacrylate) (PMMA) were prepared by free radical copolymerization of methacrylamide and methyl methacrylate monomers in dioxane solvent. The thermal properties and hydrogen-bonding interactions of these poly(methacrylamide-co-methyl methacrylate) (PMAAM-co-PMMA) copolymers with various compositions were investigated by differential scanning calorimetry (DSC), Fourier transform infrared (FTIR) spectroscopy, and solid-state nuclear magnetic resonance (NMR) spectroscopy. A large positive deviation in the behavior of Tg, based on the Kwei equation from DSC analyses, indicates that strong hydrogen bonding exists between these two monomer segments. The FTIR and solid-state NMR spectroscopic analyses give positive evidence for the hydrogen-bonding interaction between the carbonyl group of PMMA and the amide group of PMAAM (e.g. by displaying significant changes in chemical shifts). Furthermore, the proton spin-lattice relaxation time in the rotating frame (T1ρ(H)) has one single value over the entire range of compositions of copolymers, and gives a value shorter than the average predicted. The proton relaxation behavior indicates the rigid nature of the copolymer. 相似文献
19.
Summary Poly(p-vinyl phenol) is miscible with poly(methyl methacrylate), poly(ethyl methacrylate), poly(n-propyl methacrylate), poly(isopropyl methacrylate), and poly(tetrahydrofurfuryl methacrylate), but is immiscible with poly(n-butyl methacrylate). Except for poly(p-vinyl phenol)/ poly(methyl methacrylate) blends, the other miscible blends show pronounced positive deviations in their glass transition temperatures. The Tg-composition curves of the five miscible blend systems can be described by the Gordon-Taylor and the Kwei equations. 相似文献
20.
Comb-like random copolymers of itaconic anhydride (ITA) and stearyl methacrylate (SM) copolymers were synthesized by free-radical polymerization. Both monomers are derived from natural and renewable resources. Ionomers (Na, Ca or Zn carboxylates) were prepared by partial neutralization of the copolymers. The incorporation of the ionic groups decreased the melting point, which was lower than the glass transition temperature (Tg) and crystallinity of the SM side-chains and increased the Tg of the backbone by 20-25 °C. Above Tg, the parent copolymer was a viscoelastic liquid, but the introduction of the ionic groups changed the properties to that of an elastic solid due to the physical crosslinks formed by intermolecular interactions of the ionic dipoles. 相似文献