This work deals with the study of the mesomorphic form or mesophase induced by tensile drawing from the amorphous state of a polylactide material containing 4 mol % of d-stereoisomer units. ...Investigations have been carried out over the draw temperature domain 45−90 °C, i.e. an interval spanning roughly ±20 °C about the glass transition temperature. In situ WAXS experiments during drawing, stress relaxation, and/or heating of stretched samples invariably showed the strain-induced occurrence of the mesophase as far as temperature did not exceed 70 °C. This seems to be the upper stability temperature of the mesophase identified in a previous study. DSC traces upon heating of drawn samples exhibit a post glass transition endothermic peak similar to the enthalpy relaxation phenomenon observed for aged polymers. The amplitude of this strain-induced endotherm proved to be strongly dependent on draw temperature and draw ratio. Draw ratio also appeared to strongly influence the temperature domain of cold crystallization. The quite different structural evolution of the drawn samples as a function of temperature, depending whether cold crystallization occurred close or far from the strain-induced endotherm, led us to the conclusion that this endotherm results from neither physical aging nor orientation relaxation but from “melting” of the mesophase. This proposal is thoroughly supported by the insensitivity of the endotherm enthalpy to the DSC scanning rate that gives evidence of a first order thermodynamic transition in contrast to the case of aging-induced endotherm. WAXS as a function of temperature on drawn samples annealed with free ends enabled to probe the persistence of chain orientation and the stability of the strain-induced structural changes in relation to drawing conditions.
Mucoadhesive materials adhere persistently to mucosal surfaces. A mucoadhesive delivery system could therefore facilitate the controlled release of drugs and optimize their bioavailability in mucosal ...tissues. Polysaccharides are the most versatile class of natural polymers for transmucosal drug delivery. We used microviscosimetry to explore the mucoadhesion of a library of polysaccharide families with diverse structural characteristics as a first step toward the rational design of mucoadhesive polysaccharide-based nanoformulations. Here we show that the magnitude of deviation between the viscosity of mixed polysaccharide–mucin solutions and the corresponding individual stock solutions can indicate underlying molecular interactions. We found that nonlinear monotonic curves predicted a correlation between the magnitude of interaction and the ability of polysaccharide coils to contract in the presence of salt (i.e., chain flexibility). Charge-neutral polysaccharides such as dextran and Streptococcus thermophilus exopolysaccharide did not interact with mucin. Synchrotron small-angle X-ray scattering (SAXS) data supported the previously described structural features of mucin. Furthermore, high-q scattering data (i.e., sensitive to smaller scales) revealed that when mucin is in dilute solution (presumably in an extended conformation) in the presence of low-M w alginate, its structure resembles that observed at higher concentrations in the absence of alginate. This effect was less pronounced in the case of high-M w alginate, but the latter influenced the bulk properties of mucin–alginate mixtures (e.g., hydrodynamic radius and relative viscosity) more prominently than its low-M w counterpart.
The phenomenon of cavitation generally appears close to yielding in the high-density polyethylene. It can affect the yield stress and the properties at large strains. The influence of the ...microstructural and molecular parameters on cavitation is not well established; it is not even clear whether the cavitation is a cause or a consequence of plasticity. In this work, we focus on the initiation of cavitation and on the nucleation rate. Various polyethylenes with a wide range of microstructural and molecular parameters have been obtained. The cavitation is followed up by SAXS in-situ tensile tests. It is found that, depending on the polyethylene, cavitation can be avoided or, on the contrary, appears before or after yielding. The stresses necessary to initiate cavitation and crystallite shearing have been relied respectively on stress transmitters (tie molecules, interphase, etc.) and crystallite thickness. Then the comparison between the materials has allowed predicting the various polyethylene behaviors. All of the latter have been explained by a simple model based on very few microstructural parameters. Surprisingly, our results have shown that all the scenarios of plasticity and cavitation are possible. One is the cause or the consequence of the other in accordance with the molecular topology and the microstructure.
A mapping of the initiation of the three major plastic deformation processes in polyethylene is established as a function of temperature and microstructure. In this aim, a collection of polyethylene ...samples covering large ranges of crystallinity 0.50 < X c < 0.80 and crystal thickness, 8 nm < L c <29 nm was prepared. The strain onsets of crystal shear, cavitation, and martensitic-like transformation were determined from in situ WAXS and SAXS experiments during uniaxial tensile tests. The construction of the map was made via the determination of the critical L c values at which a competition exists between two processes for various temperatures in the range 25–100 °C. These critical L c values were reported on a temperature versus crystal thickness diagram where the various regions are assigning the order of occurrence that the three plastic processes should obey during tensile testing. Examples are given for illustrating the use of the map for predicting the order of occurrence of the three processes as a function of temperature for a given microstructure. Extrapolation is proposed for the behavior of materials having a crystal thickness out of the range of the present study.
This study focuses on the first occurrence of either cavitation or crystal shear in relation to temperature and microstructure during the tensile drawing of polyethylene. Four high density ...polyethylenes covering a range of crystallinity have been thermally treated to generate different microstructures displaying a large range of crystal thickness from 8 to 29 nm. The testing temperature spanned the domain 25–100 °C. In-situ SAXS measurements on synchrotron have been performed to capture the initiation of cavitation in parallel with stress-strain measurements. Depending on microstructure and temperature the strain onset of cavitation proved to be either before or after yielding associated with homogeneous or localized cavitation regimes respectively. The transition between the two regimes can be defined by a critical value of lamella thickness at each temperature. A physical modeling based on a thermally activated nucleation process has been developed for predicting the macroscopic stress for generation of cavities as well as the one for initiating crystal shearing. This modeling accounts for both temperature and microstructure effects on yielding. It allows describing successfully the delayed apparition of cavitation with increasing temperature and decreasing crystal thickness. The observation of complete disappearance of cavitation at high temperature is also predicted by the model in relation to crystal thickness. The more relevant aspects as well as the shortcomings of the model are discussed in the conclusion.
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This work deals with the
in situ and
ex situ WAXS study of the strain-induced structural changes in relation to the mechanical behavior of a semi-crystalline polylactide containing 4% of
d-stereomer ...units. The crystals isothermally generated at 120
°C were in the α form. The draw temperature was in the range 65
°C
<
T
d
<
90
°C,
i.e. just above
T
g. In this
T
d range isothermal crystallization was so slow that only strain-induced crystals could be generated during the time scale of the experiments. This allowed studying samples with various initial levels of crystallinity,
X
cr. For
X
cr
=
5%, the early occurrence of strain-hardening contrasted with the case of the amorphous material. This was due to the physical crosslinking of the macromolecular network by the crystallites that prevented chain relaxation since the onset of drawing. Strain-induced α′ crystals developed in parallel with the mesophase, the respective amount of the two phases being dependent on
T
d. For
X
cr
=
24%, the drawing behavior was that of a crosslinked and filled elastomer. Again, both strain-induced α′ crystals and mesophase occurred in various proportions in relation to
T
d. For
X
cr
=
40%, it was shown that the initial α crystals underwent partial destruction and subsequent reorganization into mesophase and/or α′ crystals. Whatever
T
d and
X
cr, the total amount of ordered phases,
i.e. crystal
+
mesophase, did not exceed the value of 40% that was the maximum accessible crystal content for the present material. A mechanism of plastic deformation involving twinning is proposed from the WAXS analysis of the crystalline texturing in the case
X
cr
=
40%. AFM observations as a function of strain corroborate the proposed deformation scheme.
▪
The macroscopic tensile deformation behavior of bulk PA6 was investigated in the elastic strain range. Samples having predominant α and β crystal structures were prepared thanks to appropriate ...crystallization procedures. Both materials were studied above and below the glass transition temperature of the amorphous phase. In parallel, the microscopic deformation behavior of these samples was followed up by combined in situ wide-angle and small-angle X-ray scattering. The change of SAXS long period over the whole azimuthal distribution was used to assess the local deformation at the scale of the lamella stacks in all directions. The WAXS patterns were used to assess the local crystal strains. Local stresses were further computed thanks to the elastic constant borrowed from the literature. The whole set of data allowed to establish micro–macro strain correlations and to determine the local elastic properties in the crystalline and amorphous phases. The effective tensile modulus of the amorphous phase of PA6 within lamella stacks at 120 °C is found to be several orders of magnitude higher than that of bulk amorphous PA6 in the rubbery state, i.e., close to that of the glassy state. This gives evidence of a strong confinement or strong Poisson’s effect of the amorphous layers due to the form factor to the stiff crystalline lamellae.
The effect of peptide charge on the self-assembly, gelation behavior, and model drug release profiles has been explored here for three octa-peptides, VEVKVEVK (VEK2), VKVKVEVK (VEK3), and VEVEVKVE ...(VEK1), that carry a net charge of 0, +2, and −2 at neutral pH, respectively. Transparent, self-supporting hydrogels were found to form above a critical concentration when the peptide charge modulus was >1 and this was independent of the sign of the charge. TEM, SAXS, and shear rheology revealed that there were no differences in hydrogel structure or mechanical properties when the peptides were at the same concentration and carried the same charge modulus. All peptides were found to form dense fibrillar networks formed by β-sheet rich single fibers where lateral aggregation of the fibers occurred and increased with decreasing charge modulus. Such behavior was found to correlate with an increase in hydrogel mechanical properties, demonstrating that fiber lateral aggregation is inextricably linked with the mechanical properties of these hydrogels. Two hydrophilic model drug molecules, namely napthol yellow (NY) and martius yellow (MY), were subsequently incorporated within the VEK1 and VEK3 hydrogels at pH 7 and although they did not effect the self-assembly of the peptide at a molecular level, they did effect the level of lateral fiber aggregation observed and, therefore, the mechanical properties of the hydrogels. The release of each molecule from the hydrogels was monitored over time and shown to be controlled by Fickian diffusion where the diffusion rate, D, was dependent on the ratio between the overall effective charges carried by the peptide, i.e., the fibrillar network, and the overall charges carried by the guest molecules, but independent from the hydrogel concentration and mechanical properties within the ranges investigated. This work highlights the possibility of controlling the rate of release of small drug molecules by manipulating the charges on the guest molecules as well as the charged state of the self-assembling peptide.
The influence of crystalline micro structure and molecular topology on the strain-induced fibrillar transformation of semi-crystalline polyethylenes having various chemical structures including ...co-unit content and molecular weight and crystallized under various thermal treatments was studied by in situ SAXS at different draw temperatures. The long period of the nascent microfibrils, L^sub pf^, proved to be strongly dependent on the draw temperature but non-sensitive to the initial crystallization conditions. L^sub pf^ was smaller than the initial long period. Both findings have been ascribed to the strain-induced melting-recrystallization process as generally claimed in the literature. The microfibrils diameter, D^sub f^, was shown to depend on the draw temperature and initial micro structure in a different way as L^sub pf^. The evolution of D^sub f^ was shown to correlate with the interfacial layer thickness that mainly depends on the chemical structure of the chains. It was concluded that, in contrast to L^sub pf^, the microfibril diameter should not be directly sensitive to the strain-induced melting-recrystallization. The proposed scenario is that after the generation of the protofibrils by fragmentation of the crystalline lamellae at yielding, the diameter of the microfibril during the course of their stabilization should be governed by the chain-unfolding and subsequent aggregation of the unfolded chains onto the lateral surface of the microfibrils. The morphogenesis of the microfibrils should therefore essentially depend on the chemical structure of the polymer that governs its crystallization ability, its chain topology and subsequently its fragmentation process at yielding. This scenario is summed up in a sketch.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, ODKLJ, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
•Self-assembly of MH-b-PMMA was studied in bulk and thin film states.•10nm Scale periodical structures were obtained via thermal and solvent annealing.•Morphology was governed by the block volume ...fraction and annealing solvent.
This paper describes the self-assembly of oligosaccharide-based hybrid block copolymers (BCPs) consisting of maltoheptaose (MH) and poly(methyl methacrylate) (PMMA) into 10nm scale lamellar and cylindrical phases depending on the volume fractions of MH (ϕMH) and the annealing process. Time resolved SAXS study of the BCP bulk samples during thermal annealing indicated that the BCPs phase separate into 10nm scale periodical structures. The solvent vapor annealing induced self-organizations of the BCP into different phases depending on ϕMH and the weight fraction of THF/H2O. BCPs with relatively higher ØMH, MH-b-PMMA3k (ϕMH=0.27) and MH-b-PMMA5k (ϕMH=0.16) self-organized into lamellar phases while the BCP sample with relatively lower ϕMH, MH-b-PMMA9k (ϕMH=0.10), self-organized into cylindrical phase by using THF/H2O=1/4 (w/w). On the other hand, the solvent vapor annealing with larger fraction of THF, i.e. THF/H2O=2/3 (w/w), induced cylindrical phases for MH-b-PMMA3k and MH-b-PMMA5k.
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