Structural, mechanical and osmotic properties of injectable hyaluronan-based composite hydrogels

The osmotic and scattering properties of hyaluronan-based composite hydrogels composed of stiff biopolymer chains (carboxymethylated thiolated hyaluronan (CMHA-S)) crosslinked by a flexible polymer (polyethylene glycol diacrylate (PEGDA)) are investigated and analyzed in terms of the scaling theory. The total pre-gel polymer weight concentration is varied between 0.5 wt.% and 3.2 wt.%, while the mole ratio between the reactive PEG chain ends and the thiolated HA moieties is changed between 0.15 and 1.0. The shear modulus G of the fully-swollen gels exhibits a stronger dependence on pre-gel concentration than on the crosslink density. Osmotic deswelling measurements reveal that the osmotic mixing pressure depends on the weight ratio CMHA-S/PEGDA, and is practically unaffected by the pre-gel concentration. Small-angle neutron scattering observations indicate that the thermodynamic properties of these composite gels are governed by total polymer concentration, i.e., specific interactions between the two polymeric components do not play a significant role.     

Study of three different families of water-soluble copolymers: synthesis, characterization and viscoelastic behavior of semidilute solutions of polymers prepared by solution polymerization

Polymer chains consisting of water-soluble polyacrylamides, hydrophobically modified with low amounts of N,N-dialkylacrylamides (N,N-dihexylacrylamide (DHAM) and N,N-dioctylacrylamide (DOAM)) have been prepared via free radical solution polymerization, using two hydrophobic initiators derived from 4,4′-azobis(4-cyanopentanoic acid) (ACVA) containing long linear chains of 12 (C12) and 16 (C16) carbon atoms. This procedure resulted in polyacrylamides containing hydrophobic groups along the chain as well as at the chain ends. This class of polymers, termed ‘combined associative polymers’, has been studied and compared with the multisticker (with hydrophobic groups along the polymer chain) and telechelic (with hydrophobic groups at the chain ends) associative polymers, which were prepared with DHAM or DOAM and with the hydrophobic initiator (ACVA) modified with alkyl chains of two different lengths. The viscoelastic properties of these different families of associative polymers were investigated using steady-flow and oscillatory experiments. The effect of type, localization and concentration of the hydrophobic groups on the viscosity of the associative polymer solution was investigated. All viscosity curves clearly show two different regimes within the semidilute range: a first unentangled regime where the viscosity increases moderately; and a second entangled regime where the viscosity varies according to a power law, proportional to C⁴. The relaxation time, T_R, and the plateau modulus, G₀, showed relatively high values which increased with the number of carbon atoms in the hydrophobic groups. The combined associative polymer (PAM-co-DHAM/ACVA12) showed relaxation times that remained relatively constant along the concentrations studied, but very high values of G₀.     

Influence of shear deformation on carbon nanotube networks in polycarbonate melts: Interplay between build-up and destruction of agglomerates

Shear-induced destruction and formation of conductive and mechanical filler networks formed by multi-wall carbon nanotubes in polycarbonate melts were investigated by simultaneous time-resolved measurements of electrical conductivity and rheological properties under steady shear and in the quiescent melt. The steady shear experiments were performed at shear rates between 0.02 and 1 rad/s and for nanotube concentrations ranging from 0.5 to 1.5 wt%. The influence of thermo-mechanical history on the state of nanotube dispersion and agglomeration was studied in detail.For melts with well-dispersed nanotubes a shear-induced insulator-conductor transition was observed, which is explained by the agglomeration of nanotubes under steady shear and the formation of an electrical conductive network of interconnected agglomerates. Simultaneously, a drastic decrease of the shear modulus (G^∗ = G′ + iG″) during steady shear was observed, which can be related to a reduction of mechanical reinforcement due to agglomeration of dispersed nanotubes. These findings indicate a substantial difference in the nature of “electrical” and “mechanical” network and contradict earlier assumptions that steady (or transient) shear is always destructive for the conductive filler network in highly viscous polymer composites.It was also shown that after a certain time of steady shear the filler network asymptotically reaches its steady state characterized by the constant electrical conductivity and shear modulus of the composite melt. Such asymptotic behaviour of composite properties was experimentally shown to be related to the interplay of the destructive and build-up effects of steady shear. For modelling of the electrical conductivity in presence of steady shear a kinetic equation was proposed for filler agglomeration with shear-dependent destruction and build-up terms. This equation was coupled to the generalized effective medium (GEM) approximation for insulator-conductor transition.     

Shear thickening behavior of dilute poly(diallyl dimethyl ammonium chloride) aqueous solutions

Using dynamic light scattering (DLS) and capillary dynamic viscoelasticity (DVE) analyzer, we investigated dilute (0.5 mg/ml) poly(diallyl dimethyl ammonium chloride) (PDADMAC) aqueous solution properties for three different molecular weights of PDADMACs mixed with various concentrations of NaCl. The dependence of PDADMAC molecular chain conformations in aqueous solutions on polymer molecular weight and NaCl concentration were studied. By analyzing dynamic shear viscosity η′(ω), viscoelastic relaxation times t_r, and shear rate at tube wall ?_a(ω) of PDADMAC aqueous solutions in oscillatory flows, we proposed that polymer chain conformations varied with increasing shear frequency ω via the following steps: intra-polymer associations, dissociation of intra-polymer associations, stretching of polymer chains, inter-polymer aggregations, and dissociations of inter-polymer aggregations. The intra-polymer associations lowered the n′ exponent of storage modulus G′(ω) (G′(ω) ∼ ω^n′) with n′ < 2, and the polymer chain stretching and inter-polymer aggregations caused shear thickening (i.e. upturn of η′(ω)) of PDADMAC aqueous solutions. The behaviors of the lowering of n′ exponent with n′ < 2 and the shear thickening were favored by increasing ionic strength of solutions. By comparing η′(ω) data with DLS hydrodynamic radii (R_h) data, we also confirmed the possibility of inter-polymer aggregations in dilute solutions when polymer chains were stretched in oscillatory flows.     

Association and physical gelation of ABA triblock copolymer in selective solvent

Association behavior and physical gelation mechanism of ABA triblock copolymer dissolved in B-selective solvent have been studied systematically from dilute to moderately concentrated solutions. Static and dynamic light scattering and nuclear magnetic resonance measurements for dilute solutions of poly(methyl methacrylate)-block-poly(tert-butyl acrylate)-block-poly(methyl methacrylate) (PMMA-PtBuA-PMMA) in 1-butanol (PtBuA selective solvent) indicated that PMMA-PtBuA-PMMA chains are molecularly dissolved above 50 °C. With decreasing temperature, the triblock copolymers form associated micelles consisting PMMA associated core and PtBuA shell. Linear dynamic viscoelastic measurements for solutions with moderate concentration (3.9-12.0 wt%) revealed that the system was viscous sol state at 60 °C. Drastic increase of shear storage modulus (G′) occurred with decreasing temperature, and at 25 °C, G′ showed rubbery plateau with weak frequency dependency, means the formation of elastic physical gel. The consistency between the temperature for micelle formation and that at the increase in G′ indicates that the physical gelation is owing to the network formation as the result of the association of PMMA chains and the bridging PtBuA chains connecting the PMMA cores. Master curves for the dynamic moduli were derived by time-temperature superposition along the frequency axis. Just above sol-gel transition concentration (C_gel), the master curves suggest the existence of fairy amount of aggregate that is not incorporated in the macroscopic network. With the increase in polymer concentration, the master curves become to reveal Maxwell-type viscoelasticity with narrow relaxation time distribution, suggesting the formation of transient network with easily generation and destruction of crosslinks. Concentration dependency of the plateau modulus is stronger than the theoretically expected, means the macroscopic transient network grows with polymer concentration by increasing the fraction of elastically effective bridging PtBuA chain above C_gel.     

Comparative properties of hyaluronan and chitosan in aqueous environment

In this paper, our objective is to compare the physico-chemical properties of chitosan (CHIT) and hyaluronic acid (HA), two important polysaccharides often used in biomedical or cosmetic applications. Polymer-polymer interactions as well as polymer-solvent interactions, studied by experiments and by molecular modeling, are discussed for the two biopolymers in presence of aqueous environments. HA and CHIT have similar non-freezing water content, although HA retains more water than CHIT at large degree of relative humidity (RH). Thermal degradation is larger for HA. The specific viscosity at zero shear rate of CHIT deviates from the master curve, whereas that of HA superimposed nicely. The solution viscosity of concentrated solutions of CHIT continuously decreases in the shear rate range between 0.5 up to 100 s^?1, whereas HA shows a Newtonian plateau at low shear rates. Dynamic rheology of semi-diluted solutions of HA shows storage modulus G?? lower than the loss modulus G?? up to 30 g/L but that of CHIT at 20 g/L shows G?? > G?? at nearly the same overlap parameter. The viscosity of CHIT solution is less influenced by temperature than that of HA. These results clearly underline the importance of interchain interactions for CHIT in a good solvent. Molecular modelling is used to provide insights on both the aggregated state and solution state of the two polysaccharides. Interchain interactions in the organized models were predicted larger for CHIT than for HA whereas the interaction between the polysaccharide and the solvent molecules are larger for HA than for CHIT. This approach rationalizes the experimental observations: the higher solvation of HA and the higher ability to aggregate for CHIT.     

Ultrahigh-shear processing for the preparation of polymer/carbon nanotube composites

Poly(vinylidene fluoride) (PVDF)/multiwalled carbon nanotube (MWCNT) composites were prepared using a novel ultrahigh-shear extruder by directly mixing MWCNT with PVDF in the molten state. A special feedback-type screw was used to obtain a high shear field and obtain a very uniform dispersion of the nanotubes in the polymer matrix under a higher screw rotation speed. Raman spectroscopy and scanning electron microscopy were used to determine the interaction and dispersion of nanotubes in the PVDF. The linear viscoelastic behavior and electrical conductivity of these composites were investigated. At low-frequencies, the storage shear modulus (G′) becomes almost independent of the frequency as nanotube loading increases, suggesting the onset of solid-like behavior in these composites. By plotting G′ vs. nanotube loading and fitting with a power-law function, we found that the rheological threshold of high-shear processed composites is about 0.96 wt% whereas that of low-shear processed composites is about 1.76 wt%. The electrical percolation threshold of high-shear processed composites is lower than that of low-shear processed composites.     

Thermorheological complex behaviour of maltosyl-chitosan derivatives in aqueous solution

N-maltosyl-chitosans with different substitution degrees were prepared by reductive N-alkylation. Dynamic shear experiments were used to study the influence of degree of substitution (DS), polymer concentration and temperature on the viscoelastic properties of these semi-synthetic polysaccharides. The attachment of the disaccharide as side chains drastically changed solubility and rheological behaviour of the ‘native’ biopolymer. At lower DS extensive interchain associations may develop, yet allowing for water solubility, but originating temporary gel-like networks. At higher DS, the bulky of the disaccharide groups leads to less extensive hydrogen bonding or hydrophobic interactions and the viscoelastic profile resembles more to an entangled high-molecular weight polymer. The combined effect of different types of interactions among the polysaccharide chains, including topological entanglements, but also more specific hydrogen bonding and hydrophobic interactions, is responsible for several peculiar rheological characteristics, such as strain-induced structuring, complex relaxation processes, elastic plateaus at low frequencies, anomalous scaling behaviour on concentration regarding the relaxation times and modulus, and complex temperature effects, including departures to the time–temperature superposition principle, which are strongly dependent on the polymer concentration, DS and temperature. Therefore, the explored approach demonstrated that branched chitosan derivatives could be produced with varied and tailorable rheological properties in aqueous media with expected enhanced applications.     

A unimolecular nanocapsule: Encapsulation property of amphiphilic polymer based on hyperbranched polythreitol

Hyperbranched polythreitol (1) with different molecular weights (M_w,SLS: 1.18 × 10⁴ and 4.79 × 10⁴) was reacted with trityl chloride in DMF to afford a novel amphiphilic polymer (2) consisting of 1 as the hydrophilic core and the trityl groups as the hydrophobic shell. Compound 2 was tested for its ability to act as a unimolecular nanocapsule toward the water-soluble dye, rose bengal (RB). Their encapsulation and release properties were also evaluated by comparison with the degree of substitution (DS) of the trityl groups, i.e., the hydrophobic shell density. The polymers were found to have very good unimolecular nanocapsule characteristics even at extremely low concentrations. The average number of RBs per polymer molecule depended on the hydrophilic core size and the hydrophobic shell density. The increasing DS value led to a decrease in the encapsulated amount due to the decrease in the hydrophilic core space, while the low DS value (less than ca. 20 mol%) led to a destabilization as a unimolecular nanocapsule and a lower encapsulation ability. In particular, 2 with ca. 23% DS value showed an efficient encapsulation. Based on a release test of the RB-loaded unimolecular nanocapsules, the polymers showed a high RB-holding ability in water.     

Phase transition of thermosensitive amphiphilic cellulose esters bearing olig(oxyethylene)s

Summary A series of cellulose esters bearing olig(oxyethylene)s with different degree of substitution (DS) and different length of the oxyethylene chain were synthesized by a homogeneous reaction of cellulose with corresponding monofunctional acid chloride in a 10% LiCl-dimethyl acetoamide (DMAc) solution. The effect of total DS value on the solubility of the derivatives in aqueous solution was investigated. It was found that the lower limit DS value for both water-soluble and amphiphilic derivatives decreases with increasing length of oxyethylene chains. The amphiphilic derivatives, which are soluble in both water and chloroform, precipitate out of aqueous solution on heating without gel forming, such a phase transition behavior was studied in terms of DS value, length of oxyethylene and concentration. The precipitation temperature (T_p) of the amphiphilic derivatives is range from 54°C to 96°C. It decreases with increasing the total DS value, and increases with an increase in the length of oxyethylene chains. The T_p value of the derivatives was found to be almost independent in the concentration range of 1–15 wt %, however the T_p value increases sharply with decreasing polymer concentration when the concentration is lower than 1 wt%. Received: 29 August 2000/Revised version: 16 October 2000/Accepted: 31 October 2000     

Molecular brushes as super-soft elastomers

Brush copolymers synthesized by controlled radical polymerizations were crosslinked either covalently or physically, resulting in elastomers with an unusually low equilibrium shear modulus G_e, of order 1 kPa. Examples are given for both crosslink motifs, along with the dynamic viscoelastic properties of these materials. The results are discussed in terms of the effect of the side chains on the brush polymers, which behave in some respect as a low molecular weight diluent that cannot be leached from the sample.     

Study on properties of hydrophobic associating polymer as drag reduction agent for fracturing fluid

In the paper, the hydrophobic associating polymer ACS-210 was prepared by solution polymerization of acrylamide, acrylic acid, salt-resisting monomer and hydrophobic monomer. Chemical structure and properties of the polymer was characterized by FTIR, TGA and XRD. The rheological property of ACS-210 solution was investigated by rheometer. The frictional resistance of the ACS-210 solutions at different application condition was examined using friction testing system. Results showed that the thermal stability of polymer ACS-210 increases and crystallinity of ACS-210 declines after incorporating of hydrophobic monomer. The viscosity of ACS-210 solution of different concentration decreased with prolonging the shearing time and the retention rate of viscosity is relatively high after long shearing time. The relation curve between the viscosity of polymer solution and shear rate followed the power law model. When the concentration of ACS-210 aqueous solution was less than the critical associating concentrations, storage modulus G’ is less than loss modulus G”, the association was weaker between the molecular chains, and the effective spatial structure did not form. After increasing the concentration of the polymer solution, G’ is more than G”, the degree of association of polymer is stronger. The synthesized polymer has favorable drag reduction effect. The molecular weight is not the only factor to determine drag reduction efficiency. The hydrophobic association can also improve the drag reduction efficiency.     

Monitoring the chemical crosslinking of propylene polymers through rheology

The chemical crosslinking of propylene polymers is described; the reaction has been performed under dynamic conditions, using a peroxide and a furan or bis-maleimide-based coagent as crosslinking promoter; the reaction mechanisms have been investigated through FT-IR spectrometry. Crosslinking experiments have been carried out while keeping constant the amount and the nature of peroxide and increasing the coagent concentration; four different coagents have been used. The samples have been studied, in the molten state, by measuring the melt flow rates (MFR) and the values of G′ (storage modulus), G″ (loss modulus), Tg(δ) (ratio G″/G′), and η^∗ (complex viscosity), in the frequency range 10⁻²-10² rad s⁻¹. Another set of measurements has been performed on the same materials, collecting for each sample the values of η⁰ (zero shear viscosity) and J_e⁰ (steady state creep compliance) by creep-creep recovery experiments. The effects of crosslinking degree estimated by gel content and through rheological measurements allows one to evaluate the efficiency of the coagent both in terms of prevention of β-scission and promotion of the crosslinking reactions. These results are discussed with reference to the selection of proper conditions of the crosslinking process in order to achieve a material having desired MFR, rheological behavior and melt elasticity for selected application.     

Effects of silicalite-1 nanoparticles on rheological and physical properties of HDPE

The addition of silicalite-1 nanoparticles (0.2-20 wt%) increased slightly the crystallization temperature of HDPE with silicalite-1 content, at 20 wt% loading by ca. 2.5 °C, but it had little effect on the melting temperature. The nanocomposites displayed a little higher onset degradation temperature than pure polymer by 7-11 °C. The WAXD profiles showed that the intensity of diffraction peaks for HDPE was decreased with increasing silicalite-1 content from 5 wt% but that the peak position of every crystal plane did not shift in the presence of silicalite-1 nanoparticles. The incorporation of the nanoparticles increased the melt viscosity of HDPE with silicalite-1 content. It also increased both storage (G′) and loss modulus (G″). In the so-called Cole-Cole plot, pure HDPE showed a single master curve whose slope was 1.37, while the nanocomposites with 10 and 20 wt% silicalite-1 exhibited the inflection in the low frequency range before which the slopes were 1.22 and 1.02, respectively. Much more accelerated crystallization behavior under shear was observed with silicalite-1 content at the isothermal crystallization temperature of 125 °C than at 120 °C.     

Characterization and Solution Properties of a Novel Water-soluble Terpolymer For Enhanced Oil Recovery

Summary  To obtain the polymer with good ageing property and the application in medium- and low-permeability oil layers, the water-soluble hydrophobically acrylamide-modified terpolymer (PAAN) with sodium 2-acrylamido-2-methylpropane sulphonate (NaAMPS) and 2-vinylnaphthalene as a hydrophobic monomer (VN) was synthesized by the micellar copolymerization. The differential scanning calorimetry (DSC) and thermogravimetry (TG) results show that large aromatic groups are incorporated into the PAAN polymer in the form of microblock structure and can increase the rigidity of molecular chains, resulting in the good thermal property of the polymer. The measurement results for intrinsic viscosities of PAAN indicate that the molecular weights of polymers are low and the breakage of the polymer chains can not occur at high shear rate. The polymer exhibits salt-thickening, temperature-thickening, thixotropy and good anti-ageing property. The hydrophobic microdomains and associating three-dimensional networks in the aqueous solution of PAAN were observed respectively by flourescent probe and ESEM.     

Development of techniques to compare mechanical properties of reversible hydrogels with spherical, square columnar and ocular lens geometry

We have developed a new experimental technique for determining the elastic modulus of the ocular lens by using hydrogel phantoms. We successfully prepared disulfide-reversible polyacrylamide hydrogels in the forms of a square column, a sphere, and a lens. The M_n (number average molecular weight) of the reduced copolymer ranged from 696,800 to 870,900 Da. The physical and swelling properties of the gels were independent of shape. The M_c (molecular weight between cross-links) ranged from 1776 to 1887 compared with the theoretical M_c of 1638 Da. The gels exhibited non-linear rubber elasticity, but at low strains the elastic moduli (E) were 4680±150, 5010±280, and 4870±220 Pa for the square column, sphere, and lens, respectively. The shear modulus (G) was 1531±70 Pa with an E/G ratio of approximately 3:1, indicating an incompressible gel at low strains. At high strains (∼15%) the Mooney-Rivlin plot was linear and the magnitude of 2C₁ was 1515 Pa, which was comparable to the shear modulus of the gels. Finally, the Tatara mechanical model for large deformation of rubber spheres was successfully applied to extract the elastic modulus of the lens. The modulus of the lens obtained with this technique was consistent with the moduli for the square column and sphere. The new technique will be used to determine the mechanical properties of the ocular lens.     

Effect of crosslinking on the mobility of PDMS filled with polysilicate nanoparticles: Positron lifetime, rheology and NMR relaxation studies

We study the effect of adding trimethylsilyl-treated polysilicate nanoparticles (Rg ∼ 2.2 nm) to crosslinked poly(dimethylsiloxane) (PDMS) elastomers above the entanglement molecular weight. The results are compared to un-crosslinked PDMS of a similar molecular weight, reported in previous studies and filled with the same polysilicate nanoparticles.Three techniques are used and compared to assess the enhancement or reduction in mobility with addition of filler: positron annihilation lifetime spectroscopy (PALS), rheology and nuclear magnetic resonance (NMR) spin-spin relaxation (T₂) measurements. PALS measurements do not show any clear effect of the filler on the mobility of the chains, as assessed by the size of free volume holes, but reveal a net increase in free volume with temperature increase (from 30 °C to 60 °C). A reduction in the dynamic shear storage modulus (measured at 1 rad s⁻¹) is observed in the filled network relative to the unfilled polymer (from 63 kPa without filler to 44 kPa with 40 w/w% filler), attributed primarily to a partial inhibition of the chemical crosslinking reaction by the particles. The NMR relaxation measurements, instead, show a reinforcement of the polymer network with increasing addition of polysilicate particles, as revealed by the faster T₂ decays at higher filler loadings, caused by increasing polymer bridging and particle flocculation. Similar trends are observed at higher temperatures (up to 80 °C), with a higher overall mobility. The apparent disagreement between rheology and NMR stems from the fact that rheology reflects bulk mobility and is primarily sensitive to chemical crosslinks in the network, while NMR probes segmental dynamics, which are affected by the presence of particles.In un-crosslinked PDMS instead, both rheology and NMR show an initial increase in mobility at low filler content, followed by reinforcement with further particle addition. These results strongly suggest that entanglements and filler-induced packing disruption, rather than free volume, play a major role in polymer dynamics.     

Structure and mechanical properties of PGA crystals and fibres

The elastic constants of poly(glycolic acid) (PGA) crystals are reported on the basis of a commercial software package and the published crystal structure of the polymer. Due to the planar zigzag conformation of the molecular chains, very high elastic anisotropy is found with a tensile chain modulus of 294 GPa and a longitudinal shear modulus for a fibre of 6 GPa. A combination of small and wide angle X-ray scattering and differential scanning calorimetry are used to characterise the structure of highly oriented PGA fibres. The combination of long period data, crystal size and crystallinity measurements suggests a structure similar to the Prevorsek model, with alternating crystalline and amorphous regions along the fibre axis, and layers of amorphous material in parallel. A parallel-series Takayanagi model, using the theoretically calculated chain modulus, is shown to give good agreement with the experimental data in a wide temperature range.     

Evaluation on the degrading behavior of melt polyolefin elastomer with dicumyl peroxide in oscillatory shear flow by Fourier transform rheology

The degradation of melt polyolefin elastomer (POE) at the presence of dicumyl peroxides (DCP) was estimated at elevated temperature in oscillatory shear flow. Large amplitude oscillatory shear (LAOS) experiments followed by Fourier transform rheology (FTR) were carried out to detect and evaluate the branching architecture of the products. The third complex harmonic (I₃^∗) and other two parameters, small strain elastic shear modulus (M) and large strain elastic shear modulus (L), which describe the nonlinearity and elasticity of a material obtained from FTR, are mainly used to characterize the topological structure of polymer chains. The results indicate the degradation appeared just after a large amount of the long chain branches (LCB) created rather than as soon as the reaction started when the strain was applied within the linear viscoelastic regime of the original POE at high frequencies. This is different from our previous result that the dominant reaction was coupling in linear shear flow. The threshold strain for degradation decreased with the oscillatory frequency, and the frequency owned a different acting mechanism from the strain amplitude to cause the degradation reaction. Moreover, there is a kind of selectivity of shear rate on the polymer chains for degradation. Low frequency results in short linear scission segments and a long branched chain suffers from degradation more than once. At high frequency, the possibility of degradation at the sites near the branching points of LCB increases.