聚乳酸基热固性聚氨酯及其形状记忆性能

通过聚乳酸二元醇和聚乳酸-聚己内酯共聚物二元醇与六亚甲基二异氰酸酯(HDI)三聚体交联反应合成了一系列生物基热固性聚氨酯(Bio-PUs)。利用傅里叶红外(FTIR)、差示扫描量热分析(DSC)、热失重分析(TGA)、万能拉伸机和细胞毒性等测试方法对获得的聚乳酸基聚氨酯进行了表征。结果表明,与聚乳酸二元醇相比,聚乳酸-聚己内酯共聚物二元醇降低了生物基热固性聚氨酯的玻璃化温度(Tg),提高了热固性聚氨酯的热稳定性;且聚乳酸-聚己内酯型聚氨酯的力学性能和形状记忆性能更为优异。其中,聚乳酸-聚己内酯共聚物二元醇分子量为3 000时得到的热固性聚氨酯(Bio-PU2-3000)的杨氏模量为277.7 MPa,伸长率为230%;聚乳酸-聚己内酯共聚物二元醇分子量为1 000得到的热固性聚氨酯(Bio-PU2-1000)在人体体温下的形变回复时间仅为93 s。另外,通过显微镜观察到细胞在含聚乳酸基热固性聚氨酯的培养液中生长状态良好,表明制备得到的生物基聚氨酯无细胞毒性。     

Degradation properties of co-continuous calcium-phosphate-polyester composites

Co-continuous composites consisting of a porous calcium phosphate matrix (hydroxyapatite, HA, or β-tricalcium phosphate, TCP) filled with poly(D,L-lactide) (PDLLA) were produced with two different methods: in situ polymerization of D,L-lactide monomer inside the matrix, or infiltration of the matrix with molten polymer. The influence of the calcium phosphate matrix as well as the manufacturing method on the degradation were investigated with accelerated in vitro studies at 42 °C in pH 7.4 phosphate-buffered saline (PBS), with some controls at 37 °C. The results show that samples produced with the infiltration method had higher initial molecular weights leading to a later onset of mass loss. Heterogenous polymer degradation was still present in the composites, as indicated by molecular weight distributions and glass transition temperature measurements. The calcium phosphate matrix delayed degradation, with evidence from X-ray microtomography suggesting that the polymer degrades more slowly in proximity to the matrix.     

Poly(epsilon-caprolactone) polyurethane and its shape-memory property

A series of segmented poly(epsilon-caprolactone) polyurethanes (PCLUs) were prepared from poly(epsilon-caprolactone) (PCL) diol, 2,4-toluene diisocyanate and ethylene glycol. The molecular weight (M(n)) of PCL was 500-10,000, and the soft-to-hard molar ratio was 1:2 to 1:6. Their shape-memory behaviors were investigated as a function of PCL molecular weight, PCLU composition, and thermal/mechanical history. When a deformation temperature 15-20 degrees C below T(m) was chosen, the lowest recovery temperature (LRT) was 15-18 degrees C below T(m), and the recovery ratio was 94-100% for tensile deformation of 300% and for compression of 2.7-fold. The reasons for this deformation-recovery procedure and the mechanism for this shape recovery below T(m) were discussed. The shape recovery was associated with the premelting of the crystals formed during the deformation and fixation, and, thus, it could be accomplished in the solid state. Its driving force was the inner stress stored in the deformed specimen during deformation and crystallization. Therefore, the LRT was a more practical temperature for shape-memory PCLU than T(m). It could be conveniently measured by means of thermal mechanical analysis. By adjusting the molecular weight of the PCL diol and the hard-to-soft ratio, the LRT of PCLU could be adjusted to the range of 37-42 degrees C, and reasonable rigidity could be retained after shape recovery, fulfilling the essential requirements of medical implantations.     

Building on the WLF/free volume framework: utilization of the coupling model in the relaxation dynamics of the gelatin/cosolute system

The onset of softening in the glass transition dispersion of the gelatin/cosolute system at 78% solids was examined using the stress relaxation modulus and dynamic oscillatory data on shear. Measurements were made between 5 and -70 degrees C, and isothermal runs were reduced to a master curve covering 21 orders of magnitude in the time domain. The sharpness with which the mechanical properties of our system changed with temperature was reflected in the shift factor a(T) used to pinpoint the glass transition temperature (T(g)). The prevalent analytical framework traditionally employed to follow the transition from the rubbery to glasslike consistency in biomaterials is that of the free volume theory in conjunction with the WLF equation. Increasingly, the combined WLF/free volume approach is challenged by the coupling model, which is able to provide additional insights into the physics of intermolecular interactions in synthetic materials at the vicinity of T(g). The model in the form of the Kohlrausch-Williams-Watts function described well the spectral shape of the local segmental motions of gelatin/cosolute at T(g). The analysis provided the intermolecular interaction constant and apparent relaxation time, parameters which depend on chemical structure. Results appear to be encouraging for further explorations of the dynamics of densely packed biomaterials at the glass transition region.     

Dynamic mechanical properties of elastin.

The dynamic mechanical properties of water-swollen elastin under physiological conditions have been investigated. When elastin is tested as a colsed, fixed-volume system, mechanical data could be temperature shifted to produce master curves. Master curves for elastin hydrated at 36°C (water content, 0.46 g water/g protein) and 55°C (water content, 0.41 g/g) were constructed, and in both cases elastin goes through a glass transition, with the glass transition temperatures of -46 and -21°C, respectively. Temperature shift data used to construct the master curves follow the WLF equation, and the glass transition appears to be characteristic of an amorphous, random-polymer network. For elastin tested as an open, variable-volume system free to change its swollen volume as temperature is changed, dynamic mechanical properties appear to be virtually independent of temperature. No glass transition is observed because elastin swelling increases with decreased temperature, and the increase in water content shifts elastin away from its glass transition. It is suggested that the hydrophobic character of elastin, which gives rise to the unusual swelling properties of elastin, evolved to provide a temperature-independent elastomer for the cold-blooded, lower vertebrates.     

Novel biodegradable shape memory material based on partial inclusion complex formation between alpha-cyclodextrin and poly(epsilon-caprolactone)

A novel shape memory material was prepared based on the formation of inclusion complexes between alpha-cyclodextrin (alpha-CD) and poly(epsilon-caprolactone) (PCL); the PCL-alpha-CD inclusion crystallites serve as a fixing phase, while free PCL crystallites serve as a reversible phase. The characteristics of the material were investigated and a mechanism for the shape memory behavior was proposed. This material showed good shape memory properties, with the recovery ratio exceeding 90% and the recovery time being less than 6 s at 90 degrees C. This PCL-alpha-CD partial inclusion complex lost about 50% (47.4 +/- 4.4%) weight within 45 days in presence of lipase, indicating its degradability. The shape memory and biodegradation properties of the well-designed polymer-alpha-CD complexes indicate great promise for this novel shape memory material.     

The effects of hydration on the dynamic mechanical properties of elastin

The dynamic mechanical properties of elastin have been quantified over a temperature and hydration range appropriate for a biological polymer. Composite curves of the tensile properties at constant water contents between 28.1 and 44.6% (g water/100 g protein) were typical of an amorphous polymer going through its glass transition. Water content had no effect on the shape of the curves, but shifted them a distance aC along the frequency axis. The combined effects of hydration and temperature are given in a series of isoshift curves where elastin's properties are constant along any one curve. A 1% change in hydration has the same effect as a 1 degrees-2 degrees change in temperature, depending on the initial water content and temperature. Theoretical isoshift curves that matched the experimental data were predicted using the WLF equation and coefficients determined from the data. These data form a basis to predict the role of elastin in arterial disease based on changes in its chemical and physical environment.     

Viscoelastic properties of pectin--co-solute mixtures at iso-free-volume states

Small-deformation oscillatory measurements were performed on pectin-sucrose-glucose syrup systems at a total level of solids of 81%, with the polysaccharide content being fixed at levels of industrial use (1%). The experimental temperature range was between 50 and - 50 degrees C. Analysis of the temperature dependence of viscoelastic processes by the equation of Williams, Landel, and Ferry provides values of fractional free volume for the temperatures covering the glass transition region. The shift factors used in the conversion of mechanical spectra into master curves were normalised at suitably different temperatures so that their temperature dependence becomes coincident. The treatment implies an iso-free-volume state and relates to changes in the monomeric friction coefficient with increasing levels of intermolecular interactions in the mixture. A free-volume related glass transition temperature was defined and manipulated markedly by introducing pectin of variable degrees of esterification to the sucrose-glucose syrup system.     

Water Content�CWater Activity�CGlass Transition Temperature Relationships of Spray-Dried Boroj�� as Related to Changes in Color and Mechanical Properties

The water content–water activity–glass transition temperature relationships of commercial spray-dried borojó powder, with and without maltodextrin, have been studied as related to changes in color and mechanical properties. The GAB and Gordon and Taylor models were well fitted to the sorption and glass transition data, respectively. The Boltzman equation adequately described the evolution of the mechanical parameter characterized in the samples with the difference between the experimental temperature and the glass transition temperature (T _g) of the sample. The color of the samples showed a sigmoid change with water activity. The changes in the mechanical properties of borojó powder related to collapse development started when the sample moved to the rubbery state and began to be significant at about 10 °C above T _g. The increase in the molecular mobility from this point on also favors browning reactions. Maltodextrin presence slows the caking kinetics but induces color changes to spray-dried borojó powder.     

Molecular motions of polysaccharides in the solid state: dextran, pullulan and amylose.

Dextran, pullulan and amylose have been investigated by differential scanning calorimetry, thermogravimetric analysis, dynamic mechanical and dielectric spectroscopy over a wide range of temperatures and frequencies. No melting or glass transition is seen below the range of thermal degradation (about 300 degrees C) for either amylose or pullulan; only dextran shows a Tg at 223 degrees C (delta cp = 0.40 J/g deg). The viscoelastic spectrum of the 'dry' polysaccharides is characterized by a low temperature relaxation that occurs at -94, -73 and -59 degrees C, at 1 kHz, (activation energy 32, 39 and 52 kJ/mol) in dextran, pullulan and amylose respectively and is assigned to small entity local motions of the polysaccharide backbone. Absorbed water strongly modifies the relaxation spectrum, inducing a new relaxation below room temperature and dissipation regions associated with water loss above room temperature. The former appears at temperatures higher than the relaxation characteristic of the dry polymer and moves to lower temperature with increasing water content. In normal 'room humidity' conditions (about 10% absorbed water) the water-induced relaxation, attributed to the motion of complex polymer-water relaxing units, is the only observable feature in the dynamic mechanical and dielectric spectrum below room temperature.     

Influence of sucrose and water content on molecular mobility in starch-based glasses as assessed through structure and secondary relaxation

Molecular mobility is known to be a key parameter in controlling the physical properties of materials and thus their quality and performance. Beyond glass transition related changes, attention should be called to the impact of local motions remaining in the glassy state. Gelatinized waxy maize starch at different sucrose contents (0-20% solids) was equilibrated between 0 and 14% water and sorption isotherms determined at 25 degrees C. The effect of water and sucrose content on the molecular mobility of glassy starch was investigated by differential scanning calorimetry through enthalpy relaxation studies and dynamical mechanical thermal analysis. The existence of sucrose-starch interactions was suggested by the sorption isotherms not following the expected additivity of the single component sorption curves. Contrary to the glass transition or associated alpha relaxation, water and sucrose affected differently the secondary relaxations. Indeed, the beta relaxation observed around -15 degrees C was shifted to lower temperature upon increasing hydration, and to higher temperature when sucrose content increased, suggesting a hindering of these local motions. Enthalpy relaxation of the ternary mixtures was studied following aging up to 668 h at Tg -15 degrees C. Ternary mixtures exhibited an enthalpy relaxation upon aging lower than starch alone as a sign of lower polymer mobility in the presence of small molecules, contrary to the free volume theory. Relaxation kinetics were characterized with the Cowie-Ferguson model and compared to literature data. The extent of the enthalpy relaxation appeared to be controlled by the distance between the aging temperature and the beta relaxation temperature.     

Influence of low-temperature nucleation on the crystallization process of poly(L-lactide)

The crystallization kinetics of poly(l-lactide), PLLA, is slow enough to allow a quasi-amorphous polymer to be obtained at low temperature simply by quenching from the melt. The PLLA crystallization process was followed by differential scanning calorimetry and optical microscopy after nucleation isothermal treatments at temperatures just below (53 degrees C) and just above (73 degrees C) the glass transition temperature. The crystallization exotherm shown in the heating thermograms shifts toward lower temperatures as the annealing time at 73 degrees C increases. The same effect is shown to a lesser extent when the sample nucleates at 53 degrees C, showing the ability to nucleate in the glassy state, already shown in other polymers. The shape of the DSC thermograms is modeled by using Avrami's theory and allows an estimation of the number of crystallization germs formed. The results of optical microscopy are converted to thermograms by evaluating the average gray level of the image recorded in transmission mode as a function of temperature and calculating its temperature derivative. The shape of such optical thermograms is quite similar to that of the DSC traces but shows some peculiarities after long nucleation treatments. Atomic force microscopy was used to analyze the crystal morphology and is an additional proof of the effect of nucleation in the glassy state. The crystalline morphology observed in samples crystallized after nucleation in the glassy state is qualitatively different from that of samples nucleated above the glass transition temperature, and the number of crystals seems to be much greater than what could be expected from the crystallization kinetics.     

Mechanical properties of laser cut poly(L-lactide) micro-specimens: implications for stent design, manufacture, and sterilization

BACKGROUND: The development of endoluminal stents from polymeric materials requires an understanding of the basic mechanical properties of the polymer and the effects of manufacturing and sterilization on those properties. METHODS: Pure poly(L-lactide) (PLLA) and PLLA containing varying amounts of triethylcitrate (TEC) as a plasticizer (5-10-15%) were studied. The specimens were solution-cast and CO2 laser-cut. Specimen dimensions were adapted to the strut size of polymeric vascular stents. The properties of the PLLA micro-specimens were assessed before and after sterilization (EtO cold gas, H2O2-plasma, beta- and gamma-irradiation). Tensile tests, and creep and recovery tests were carried out at 37 degrees C. Additionally the thermal and thermo-mechanical characteristics were investigated using dynamic-mechanical analysis (DMA) and differential scanning calorimetry (DSC). RESULTS: The results showed the dramatic influence of the plasticizer content and sterilization procedure on the mechanical properties of the material. Laser cutting had a lesser effect. Hence the effects of processing and sterilization must not be overlooked in the material selection and design phases of the development process leading to clinical use. Altogether, the results of these studies provide a clearer understanding of the complex interaction between the laser machining process and terminal sterilization on the primary mechanical properties of PLLA and PLLA plasticized with TEC.     

Biodegradable thermoplastic polyurethanes incorporating polyhedral oligosilsesquioxane

A new hybrid thermoplastic polyurethane (TPU) system that incorporates an organic, biodegradable poly(D, L-lactide) soft block with a hard block bearing the inorganic polyhedral oligosilsesquioxane (POSS) moiety is introduced and studied. Changes in the polyol composition made through variation of the hydrophilic initiator molecular weight show direct control of the final transition temperatures. Incorporating POSS into the hard segments allows for excellent elasticity above T(g), as evidenced with dynamic mechanical analysis, not seen in most other biodegradable materials. This elasticity is attributed to physical cross-links formed in the hard block through POSS crystallization, as revealed with wide-angle X-ray diffraction. Increasing the POSS incorporation level in the TPU hard block was observed to increase crystallinity and also the rigidity of the material. The highest incorporation, using a statistical average of three POSS units per hard block, demonstrated one-way shape memory with excellent shape fixing capabilities. In vitro degradation of this sample was also investigated during a two month period. Moderate water uptake and dramatic molecular weight decrease were immediately observed although large mass loss (approximately 20 wt %) was not observed until the two month time point.     

Acid catalyzed transesterification as a route to poly(3-hydroxybutyrate-co-epsilon-caprolactone) copolymers from their homopolymers

Copolymers of (R)-3-hydroxybutyric acid (HB) and epsilon-caprolactone (CL) with a composition ranging from 28 to 81 mol % of HB were synthesized by transesterification of the corresponding homopolymers in solution in the presence of 4-toluenesulfonic acid. The copolyesters were characterized with regard to their molecular weights, thermal properties, molar compositions, and average block length of repeating units by gel permeation chromatography (GPC), differential scanning calorimetry, (1)H NMR, and (13)C NMR, respectively. Random and microblock copolymers could be obtained depending on experimental conditions, with weight-average molecular weights of up to 20,000. The glass transition temperature decreased from 2 to -42 degrees C as the CL content was increased from 0 to 72 mol %. The melting temperature (T(m)) of the PCL phase decreased from 70 to 46 degrees C as the HB content changed from 0 to 47 mol %, while the T(m) of the PHB phase decreased from 177 degrees C to 163 degrees C as the CL content changed from 0 to 72 mol %. Matrix-assisted laser desorption ionization time-of-flight mass spectra of GPC fractionated samples allowed us to ascertain that copolymers rich in HB units have mostly hydroxyl and carboxyl end groups, while copolymers rich in CL units have mostly tosyl and carboxyl end groups.     

Glass transition and enthalpy relaxation of amorphous lactose glass

The glass transition temperature, T(g), and enthalpy relaxation of amorphous lactose glass were investigated by differential scanning calorimetry (DSC) for isothermal aging periods at various temperatures (25, 60, 75, and 90 degrees C) below T(g). Both T(g) and enthalpy relaxation were found to increase with increasing aging time and temperature. The enthalpy relaxation increased approximately exponentially with aging time at a temperature (90 degrees C) close to T(g) (102 degrees C). There was no significant change observed in the enthalpy relaxation around room temperature (25 degrees C) over an aging period of 1month. The Kohlrausch-Williams-Watts (KWW) model was able to fit the experimental enthalpy relaxation data well. The relaxation distribution parameter (beta) was determined to be in the range 0.81-0.89. The enthalpy relaxation time constant (tau) increased with decreasing aging temperature. The observed enthalpy relaxation data showed that molecular mobility in amorphous lactose glass was higher at temperatures closer to T(g). Lactose glass was stable for a long time at 25 degrees C. These findings should be helpful for improving the processing and storage stability of amorphous lactose and lactose containing food and pharmaceutical products.     

Novel conjugated linseed oil-styrene-divinylbenzene copolymers prepared by thermal polymerization. 1. Effect of monomer concentration on the structure and properties

A variety of novel opaque, white polymers ranging from rubbery materials to tough and rigid plastics have been prepared by the thermal polymerization at 85-160 degrees C of varying amounts of 87% conjugated linseed oil, styrene, and divinylbenzene. Gelation of the reactants typically occurs at temperatures higher than 120 degrees C, and fully cured thermosets are obtained after postcuring at 160 degrees C. The fully cured thermosets have been determined by Soxhlet extraction to contain approximately 35-85% cross-linked materials. The microcomposition of these polymers, as determined by 1H NMR spectroscopy, indicates that the cross-linked materials are composed of both soft oily and hard aromatic phases. After solvent extraction, the insoluble materials exhibit nanopores well distributed throughout the polymer matrixes. Dynamic mechanical analysis of these polymers indicates that they are phase separated with a soft rubbery phase having a sharp glass transition temperature of -50 degrees C and a hard brittle plastic phase with a broadened glass transition temperature of 70-120 degrees C. These polymers possess cross-link densities of 0.15-2.41 x 10(4) mol/m3, compressive Young's moduli of 12-438 MPa, and compressive strengths of 2-27 MPa. These materials are thermally stable below 350 degrees C and exhibit a major thermal degradation of 72-90% at 493-500 degrees C.     

Characterization of chemically treated bacterial (Acetobacter xylinum) biopolymer: some thermo-mechanical properties

Bacterial cellulose prepared from pellicles of Acetobacter xylinum (Gluconacetobacter xylinus) is a unique biopolymer in terms of its molecular structure, mechanical strength and chemical stability. The biochemical analysis revealed that various alkali treatment methods were effective in removing proteins and nucleic acids from native membrane resulting in pure cellulose membrane. The effect of various treatment regimens on thermo-mechanical properties of the material was investigated. The cellulose in the form of purified cellulose membranes was characterized by differential scanning calorimetry (DSC), thermo-gravimetric analysis (TGA) and dynamic mechanical thermal analysis (DMTA). The glass transition temperature (T(g)) of the native cellulose (untreated, compressed and dried pellicle) was found to be 13.94 degrees C, in contrast, the chemically treated cellulose membranes has higher T(g) values, ranging from 41.41 degrees C to 48.82 degrees C. Investigations on isothermal crystallization were carried out to study the bulk crystallization kinetics. Thermal decomposition pattern of the native as well as alkali treated cellulose was determined by obtaining thermo-gravimetric curves. At higher temperatures (>300 degrees C), the biopolymer was found to degrade. Nevertheless, the alkaline treated cellulose membrane was more stable (between 343.27 degrees C and 370.05 degrees C) in comparison to the native cellulose (298.07 degrees C). Further, the percentage weight loss in case of native cellulose was found to be 26.57%, in comparison to 6.45% for the treated material, at 300 degrees C. The DMTA revealed complex dynamic modulus of the material, at different temperatures and fixed shear stress, applied at a frequency of 5 Hz. The study delineated the effect of alkali treatment regimens, on the thermo-mechanical properties of bacterial cellulose for its application over a wide range of temperatures.     

Collapse temperature of freeze-dried Lactobacillus bulgaricus suspensions and protective media

Optimization of the freeze-drying process needs to characterize the physical state of frozen and dried products. A protocol to measure the collapse temperature of complex biological media such as concentrated lactic acid bacteria using freeze-drying microscopy was first elaborated. Afterward, aqueous solutions of one or several components as well as concentrated lactic acid bacterial suspensions were analyzed in order to study how the structure of these materials is degraded during freeze-drying. A similar behavior toward collapse was observed for all aqueous solutions, which was characterized by two temperatures: the "microcollapse" temperature (T(microc), beginning of a local loss of structure) and the "collapse" temperature (T(c), beginning of an overall loss of structure). For aqueous solutions, these two temperatures were close, differing by less than 3 degrees C. Nevertheless, when lactic acid bacteria were added to aqueous solutions, the collapse temperatures increased. Moreover, the interval between microcollapse and collapse temperatures became larger. Lactic acid bacterial cells gave a kind of "robustness" to the freeze-dried product. Finally, comparing glass transition, measured by differential scanning calorimetry (DSC) and collapse temperature for aqueous solutions with noncrystallizable solutes, showed that these values belonged to the same temperature range (differing by less than 5 degrees C). As suggested in the literature, the glass transition temperature can thus be used as a first approximation of the collapse temperature of these media. However, for lactic acid bacterial suspensions, because the difference between collapse and glass transition temperatures was about 10 degrees C, this approximation was not justified. An elegant physical appearance of the dried cakes and an acceptable acidification activity recovery were obtained, when applying operating conditions during freeze-drying in vials that allowed the product temperature to be maintained during primary drying at a level lower than the collapse temperature of lactic acid bacterial suspensions. Consequently, the collapse temperature T(c) was proposed as the maximal product temperature preserving the structure from macroscopic collapse and an acceptable biological activity of cells.     

Thermal transitions in erythrocyte membranes revealed by their permeability to ANS

A number of breaks were recorded on the curve of Arrhenius relationship of the rate constant of the dye 1-anilino-8-naphthalenesulphonate sodium salt (ANS) input into human erythrocytes of 20, 28, 36, 42 and 46 degrees C. Variations in the values of activation energies within the temperature range of 28-36 degrees and 42-46 degrees C obtained in various blood samples allow to consider these temperatures as those at which structural changes of the membranes take place. The values of activation energy of the process for temperature "conformers" of the erythrocyte membrane are 12(10-20 degrees C), 26.5 (20-28 degrees C), 34.2(36-42 degrees C) and 47 kcal/mol (t is greater than 46 degrees C). Within the temperature range of 28-36 degrees and 42-46 degrees C an irreversible decrease of permeability to ANS of the erythrocyte ghost after their incubation for 10 min at increased temperatures were observed. Thus the temperature regions of the change in erythrocyte permeability correspond to those at which the resealing of ghost takes place. The break in Arrhenius graph at 20 degrees C seems to characterize a highly cooperative "point" transition. The lipid nature of the initiator of structural transition within 28-36 degrees C is proved by a sharp increase of the permeability of liposomes prepared from erythrocyte membrane lipids to ANS at 28 degrees C. The nature of the initiators of two other thermal transitions is discussed.