乳酸-乙醇酸共聚物的制备及降解性能研究

以L-乳酸(L-LA)和乙醇酸(GA)为原料(L-LA和GA投料摩尔比为80∶20),二水合氯化亚锡和对甲苯磺酸(TSA)为复合催化剂,采用直接熔融缩聚法制OLGA预聚物,然后在扩链剂六亚甲基二异氰酸酯(HDI)存在下进行熔融扩链,最终得到较高分子量的乳酸-乙醇酸共聚物(PLGA),其数均分子量和重均分子量分别为5.5×104和1.3×105。将溶液浇铸法制备的PLGA薄膜在37℃、pH=7.3～7.4的磷酸盐缓冲液(PBS)中进行降解实验,采用pH分析、失重率测量、GPC、SEM、FT-IR等方法研究了PLGA薄膜的降解行为,并对降解机理进行了初步探讨。结果表明,随着降解的进行,PLGA薄膜的相对分子量和质量都会逐渐下降,在降解反应进行的前21天中,相对分子量的减小较明显,21天之后质量损失较明显。结合FT-IR谱图的研究,分析可知PLGA的降解主要是通过酯键的水解实现的。     

Maximum strength and drawing mechanism of hot drawn high molecular weight polyethylene

An investigation of the hot drawing of high molecular weight polyethylene fibres containing a substantial amount of solvent, and of fibres with a porous structure that were dried after solution spinning, was conducted. It was found that the tensile strength as well as the Young's modulus of the polyethylene fibres were linearly related to the applied stress up to values of 0.3 GPa. Drawing stresses exceeding 0.3 GPa could produce polyethylene fibres with a tensile strength of 3.7 GPa and a Young's modulus of 120 GPa. The ultimate mechanical properties appeared to be affected by the nature of the solvent, the polymer concentration and the spinning temperature. Chain extension in the hot drawing and the ultimate draw ratio depended substantially on the number of intermolecular entanglements initially present in the fibre.     

Brittle fracture of low molecular weight polymers

A new model is presented to predict the fracture energy of polymers with molecular weights smaller than the threshold value for the formation of chain entanglements. A fracture mechanism is assumed that calls for the sliding of the polymer chains in a microscopic craze at the crack tip. The plastic work is related to the chain interpenetration distance, which has been derived from the solution of the Fokker-Planck equation. The predictions of the model agree with experimental data of the fracture energy of polystyrene.     

A molecular theory of the fracture toughness of low molecular weight polymers

Recent experiments by Robertson show that the fracture toughness G _IC of glassy polystyrene PS does not decrease to the ideal brittle value 2 (where is the surface energy for PS) at molecular weights M _w below M _c the entanglement molecular weight. Instead G _IC is more than an order of magnitude above 2 at M _c and decreases slowly below M _c. It is postulated that a small craze exists at the crack tip in such low molecular weight glassy polymers. However, since entanglements do not occur single molecules must span this craze; if they do not the craze becomes unstable and the crack advances. Under these conditions a critical craze surface displacement exists and G _C can be computed to be G _IC=S _c(–1) R ^21/2, where and S _c are the craze fibril extension ratio and craze surface drawing stress observed in high molecular weight crazes (both quantities should be only weak functions of M _w) and R ^21/2 is the root mean square end-to-end distance of the PS molecule in the glass from neutron scattering measurements. The fracture toughness is predicted to decrease as M _w ^1/2 ; this prediction and the absolute magnitude of G _IC are in excellent agreement with experiment.     

Fatigue characteristics of ultra high molecular weight polyethylene with different molecular weight for implant material

In order to investigate the effect of molecular weight on fatigue characteristics in the ultra high molecular weight polyethylene (UHMWPE), tension–tension fatigue tests of notched specimens were carried out in the present study. The effects of frequency and stress ratio on the fatigue characteristics were also investigated and fractography was discussed. The fatigue strength does not increase with increasing molecular weight. The fatigue strength might be influenced by the high degree of crystallinity in spite of the decreased tie molecule density in this study. Almost no effect of frequency on the number of cycles to failure can be observed. However, the higher the frequency, the higher the crack tip temperature. The effects of heat and strain rate on the fatigue strength must be considered in polymer materials. At a high stress ratio, the stress–number of cycles to failure (S–N) curves shift to high number cycles to failure side. Both stress amplitude and mean stress influence the fatigue life of UHMWPE. © 2001 Kluwer Academic Publishers     

Mass spectrometric analysis during the vapor deposition of amorphous silicon

The gaseous species produced by the electron beam evaporation of silicon from conventional copper and carbon hearths were studied by utilizing a residual gas analyzer. At low evaporation rates about 13% of the silicon evaporates as molecules containing two to seven atoms. The percentage of clusters increases only slightly with rate, indicating that their heat of evaporation differs only slightly from that of the silicon atoms. Additional molecules, predominantly Si₂C, occur if vitreous carbon liners are used. The resulting carbon contamination of the amorphous silicon films (about 1.5 mol.%) was confirmed by electron microprobe analysis. A discussion is given concerning the influence of cluster deposition on the growth of amorphous silicon films.     

Super-molecular structure of dilute solutions of high molecular weight polymers which lead to reduced turbulent friction

Data are obtained which indicate a relationship between the viscoelastic properties of dilute solutions of high molecular weight polymers and their super-molecular network structures. The changes in structure are determined which result as the concentration and molecular weight of the polymer are increased.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 8, No. 1, pp. 49–55, January, 1990.     

Mathematical modeling of the nonisothermal degradation of high polymers and natural coals

A kinetic equation is proposed to describe the nonisothermal degradation of decomposing materials in a broad range of heating rates with allowance for the coking stage. The physical meaning of the parameters of the proposed equation is demonstrated, and examples of analysis of empirical data using the model are presented.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 42, No. 6, pp. 910–916, June, 1982.     

Mass spectra of molecular products formed during friction of polymers

The molecular compositions of products formed during the friction of polymers [poly(methyl methacrylate), polystyrene] in both symmetric (same materials) and asymmetric (different materials) polymer-on-polymer friction pairs have been determined by using mass spectrometry. In addition to monomers, which are formed during friction as a result of the depolymerization of the corresponding free macroradicals, the mass spectra reveal new products. The formation of these products is explained by the interaction of monomers with hydrogen molecules, which are also formed in the contact zone.     

The influence of molecular weight on the fracture of thermoplastic glassy polymers

Journal of Materials Science - Experimental evidence is presented in support of a previously proposed relationship between the fracture energy of glassy polymers and the square of the molecular...     

Quantitative analysis of synthetic polymers using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

Quantitative analyses of synthetic polymers were accomplished using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI TOF MS). Many factors have hindered the development of quantitative measurement of polymers via MALDI TOF MS, e.g., laser power, matrix, cation salt, and cocrystallization. By probing the optimal conditions, two sets of polymers were studied. Fair repeatability of the samples ensures acceptable results. In set 1, two poly(ethylene glycols) with different end groups showed equal desorption/ionization efficiencies. Two synthetic polymers in set 2 with different chemical properties resulted in different MALDI responses. Good linearity was achieved by plotting the relationship between the sample concentration ratio and the total signal intensity ratio in both sets.     

Sintering of ultra high molecular weight polyethylene

Ultra high molecular weight polyethylene (UHMWPE) is a high performance polymer having low coefficient of friction, good abrasion resistance, good chemical resistance etc. It is used in shipbuilding, textile industries and also in biomedical applications. UHMWPE is processed by powder processing technique because of its high melt viscosity at the processing temperature. Powder processing technique involves compaction of polymeric powder under pressure and sintering of the preforms at temperature above its melting point. In this study, we report our results on compaction and sintering behaviour of two grades of UHMWPE with reference to the powder morphology, sintering temperatures and strength development.     

Preparation and characterization of high molecular weight low bandgap polymers based on poly(2,7-carbazole)s for organic solar cells

We report the PCDTBT {Poly[N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)]}, an alternating copolymer of 2,7-carbazole and dithienyl-2,1,3-benzothiazole, has high molecular weight and narrow molecular weight distribution. Our PCDTBT can be successfully prepared as good yield by using tetrakis(triphenylphosphine)palladium [Pd(PPh3)4] catalyst instead of Pd2dba3/P(o-Tol)3 catalyst. From the UV/Vis absorption spectroscopy, we can observe that absorption bands of PCDTBT are bathochromically shifted by increasing the molecular weight, that is to say, our high molecular weight PCDTBT can absorb much longer wavelength light compare to low molecular weight PCDTBT. The best performance can be obtained from device based on the mixture of PCDTBT (polymer-30) and PC70BM {[6,6]-phenyl C71-butyric acid methyl ester} (1:4) as an active layer, which shows 4.50% of PCE with 10.1 mA/cm2 of short-circuit current density (J(SC)), 0.85 V of open-circuit voltage (V(OC)), and 52.3% of fill factor which is very similar with Leclerc's published result.     

Mass spectrometric determination of O-glycosylation sites using beta-elimination and partial acid hydrolysis

The present study demonstrates that treating O-glycosylated peptides with methylamine vapor followed by partial acid hydrolysis is an effective means for locating O-glycosylation site(s). The reaction with methylamine transforms the glycosylated Ser and Thr residues into stable methylamine derivatives with a mass increment of +13 Da relative to nonglycosylated Ser and Thr residues. Peptide sequencing based on partial acid hydrolysis followed by mass spectrometric analysis or in favorable cases by CID-MS/MS enables the determination of the formerly O-glycosylated sites.     

Mass spectrometric identification of degradation products of insulin and its long-acting analogues in human urine for doping control purposes

The search for target analytes to uncover the misuse of long acting insulin analogues (Lantus, Insulin Glargine; Levemir, Insulin Detemir) in doping control samples led to the identification of several degradation products of insulin or its synthetic analogues. Specimens obtained from healthy volunteers or patients and athletes suffering from diabetes mellitus contained DesB30, DesB24-30, and DesB25-30 human insulin or DesB30-32, DesB31-32, and DesB24-32 Lantus, respectively. Analytes were purified from urine by immunoaffinity chromatography (IAC) with subsequent liquid chromatography-tandem mass spectrometry analysis. The employed analytical procedure was validated for qualitative determination considering the main metabolic products DesB30 human insulin and DesB30-32 Lantus. The occurrence of the identified Lantus degradation products in urine provided the direct and unambiguous evidence for an administration of this insulin analogue. For the determination of surreptitious Levemir or recombinant human insulin applications, an unequivocal argument was not detected, but promising approaches based on a modified insulin degradation profile with altered relative intensities of metabolites are presented.     

Mass spectrometric analysis of mercury incorporation into proteins for X-ray diffraction phase determination

Heavy-atom incorporation is an essential and often rate-limiting step in the determination of phases for X-ray diffraction studies of protein structures. Until the present, there has been no practical method (short of the X-ray diffraction experiment itself) to judge the success and extent of incorporation. Here we show that mass spectrometry is an effective tool for determining the extent of heavy-atom incorporation in proteins. In particular, we demonstrate the utility of matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) and electrospray ionization mass spectrometry (ESI-MS) for assaying mercury derivatization of cysteinyl thiol groups in proteins. Each of these mass spectrometric methods has advantages and drawbacks. ESI-MS provides a more accurate quantitative measurement of the extent of mercury incorporation, while MALDI-MS provides a useful lower limit to the level of mercury incorporation. Conversely, MALDI-MS does not require removal of excess derivatization reagents, salts and buffers, thus permitting facile analysis of single protein crystals as well as rapid, semiquantitative evaluation of the extent of protein mercuration. The approaches described in the present paper have contributed to the successful X-ray analyses of several noteworthy protein structures.