Tuan Q. Nguyen

Kinetics of ultrasonic and transient elongational flow degradation: a comparative study

Abstract The kinetics of ultrasonic degradation has been investigated for dilute polystyrene solutions in decalin. The increase in the scission rate constant ( K ) with initial molecular chain length was systematically investigated over 2 decades of molecular weight by using narrow polymer fractions with M = 50 × 10 3 − 8.5 × 10 6 Da. An empirical relation of the form K ∼ ( M - M lim ) 1.9 , with M lim = 30 000, was found for 50 × 10 3 ≤ M ≤ 2.2 × 10 6 . The rate constant becomes independent from M at higher molecular weight. This complex behaviour is rationalized by establishing a parallelism between transient elongational flow and ultrasonic degradation.

Synthesis of thiol end-functional polystyrene via atom transfer radical polymerization

Polymers obtained by Atom Transfer Radical Polymerization (ATRP) possess an ω-halogen end-group that may be converted to other functional groups by appropriate chemical transformations. Here we report a versatile new method for end-functionalizing polymers with a thiol. The bromine end-group of the polystyrene obtained from ATRP is converted into an isothiouronium salt by reaction with thiourea. Subsequent treatment of this salt with a base permits the cleavage to a thiol. This method allows the preparation of thiol functional polystyrene of narrow polydispersity with a high degree of functionalization and control of the molecular weight.

Chain extension and degradation in convergent flow

Rheology and birefringence measurements showed conclusively that quasi-full chain extension could be achieved in stagnant extensional flow above some critical strain rate of the order of the reciprocal of the chain characteristic time. Degradation in this highly expanded state could be rationalized with a model of chain scission through frictional loading in which the fracture event is controlled by the force balance between polymer solvent viscous friction and the breaking strength of the bond under consideration. In contrast to the process of chain extension, which could be observed only under rather restrictive conditions of flow, polymer degradation is ubiquitous and is readily detected in situations as diverse as convergent flow, ultrasonic irradiation, solution crystallization, turbulence and flow across porous media. Although some extensional flow component was always present in these types of degradation, the residence time in the high strain-rate region is so short that only a limited degree of coil expansion could be expected. Degradation in transient elongational flow indicated a behaviour definitely distinct from the degradation of fully extended chains under stagnant conditions. In particular, the following experimental facts cannot be explained by the model of fully extended frictionally loaded chains: dotef ≊ M−1 (where dote is the critical strain rate for chain fracture); dotef ≊ ηs−0.25 (where ηs, is the solvent viscosity); degradation yield is a fairly unique function of average fluid velocity or entrance pressure drop and is almost independent of differences in elongational strain rate introduced by nozzle geometry. These facts lead us to the conclusion that the fundamental parameter which governs the rate of degradation of partly deformed coils is not the magnitude of the local stress, but rather the deformation energy accumulated by the coil up to the moment of rupture.

Crack-healing in crosslinked styrene-coacrylonitrile

Abstract The crack-heating behaviour of γ-irradiated poly (styrene-co-acrylonitrile) (SAN) was investigated as a function of radiation dose over the range 0–800 Mrad. The predominant response of SAN to γ-irradiation is crosslinking as evidenced by the radiation yields Gx(crosslink) = 0.077 and Gs (main chain scission) = 0.055. The presence of crosslinks considerably increased the time required for complete reheal of compact tension (CT) specimens. Fracture mechanics testing on non-irradiated samples showed that the energy release rate, G1, increased linearly with the square root of the rehealing time, tp, until the original value of the energy release rate, G10, of the virgin material was restored. In contrast, for crosslinked samples, the initial increase in energy release rate, Gl, was followed by a plateau region at a value of Glp which was less than Glo. A further recovery was observed over a long time scale, some minutes to one hour after the first plateau (Glp) was reached. The ratio of the plateau value and the original fracture toughness ( G lp G lo ) was found to vary with the square root of the average molecular weight between cross-links, M x . This phenomenon may be correctly explained in terms of the average length of free chains formed during the fracture process. A simple double-layer diffusion model describes well the existence of the two recovery regions, it is not yet sufficient, however, since the form of the transition from the first to the second recovery mechanism is not adequately represented by this model.

Polymer solutions under elongational flow: 1. Birefringence characterization of transient and stagnation point elongational flows

Abstract Birefringence measurements are reported in transient and stagnation point elongational flows created by forcing a polymer solution through a narrow contraction. A strongly molecular weight dependent birefringence was observed and attributed to segmental orientation caused by individual chain extension. The birefringence was also found to depend on the position within the flow field, the solvent type, thermodynamic quality and viscosity, the polymer composition, and the type of flow field. Two distinct concentration regimes, corresponding to the dilute and semi-dilute polymer regions, were observed. The transition was marked by a dynamic critical overlap concentration which was lower than that observed under quiescent conditions. In the dilute regime, a laminar flow profile was observed and intermolecular interactions were negligible. The modest local orientation and overall chain deformation however indicate that intramolecular hindrances to extension exist. Transient elongational flows were characterized by a ‘tube-like’ birefringence which had a diameter equal to that of the orifice and extended several orifice diameters into the solution. In contrast, stagnation point elongation flows possessed both the tube-like birefringence and the classical highly localized birefringent ‘lines’. The transition between the tubular and linear birefringent zones is a function of the viscous coupling between the polymer coil and the deforming fluid element. This coupling is itself a complex function of temperature, molecular parameters and the flow conditions. Measurements in stagnation point elongational flows have also indicated that the maximum degree of local orientation does not occur at the stagnation point, and data indicate that chains continue to accelerate until a region very close to the orifice. In transient elongational flow the polymer dynamics follow a coil-to-deformed coil transition with birefringence saturation not observed.

Characterization of Hyperbranched Aliphatic Polyesters and their Trimethylsilylated Derivatives by GPC-Viscometry

Abstract Gel permeation chromatography (GPC) viscometry with universal calibration (UC) has been assessed as a means of characterizing the absolute molecular weight distribution of hydroxyl terminated hyperbranched aliphatic polyesters (HBPs) of aliphatic compounds. To generate a valid UC curve, it is necessary to work under conditions of strict size‐exclusion separation. This requirement was met when both dimethyl formamide (DMF) and tetrahydrofuran (THF) were used as eluants. All the HBPs were highly soluble in DMF at room temperature, but its use may necessitate special columns. Dissolution of the HBPs in THF, on the other hand, which is compatible with a variety of stationary phases, was relatively difficult, requiring prolonged heating above 50°C. As an alternative to direct characterization, the HBP hydroxyl end‐groups were end‐capped with trimethylsilane prior to the GPC measurements. Near quantitative replacement of the hydroxyl groups was achieved and the modified HBPs remained stable for several weeks under dry nitrogen. This permitted straightforward GPC analysis in THF at room temperature, giving results consistent with those obtained for the unmodified HBPs.

Polymer solutions under elongational flow: 2. An evaluation of models of polymer dynamics for transient and stagnation point flows

Abstract Models for the dynamics of dilute polymer solutions under elongational flow have been examined in the light of new experimental data. In particular birefringence measurements on the polystyrene-decalin and poly(ethylene oxide)—water systems in both stagnation point and transient elongational flows are the most revealing as to the nature of polymer chain uncoiling and fracture. Both the deformation and fracture behaviour have been observed to depend on the type of elongational flow (transient, stagnation point) and the chemistry of the polymer chain. The data on the critical strain rate for chain fracture along with observations of precise midchain scission (as estimated from size exclusion chromatography), a limited overall chain deformation, and the role of accumulated energy (strain) in mechanochemical degradation, tend to imply that a hybrid model of polymer dynamics in elongational flows may be realistic. One such model, a ‘multi-stranded yo-yo with surface energy dissipation and solid like stress response’ has been developed as a general hypothesis to account for the deformation, stretching and fracture behaviour of polymer chains in both transient and stagnation point elongation flows. This modified yo-yo is found to be an energetically favourable alternative to an affinely deforming chain over certain degrees of elongation. It has been shown to qualitatively explain the apparent difference in behaviour of polymer chains in both stagnation point and transient elongational flows. The viscous coupling between the solvent and the polymer chain is also used as a qualitative parameter to account for the deformation and fracture behaviour of polymer chains under transient and stagnation point elongation flows. Based on these analyses, a set of recommendations as to future experiments are presented.

Templating porosity in polymethylsilsesquioxane coatings using trimethylsilylated hyperbranched polymers

A series of trimethylsilyl end-functionalized aliphatic hyperbranched polymers has been used to template porosity in polymethylsilsesquioxane films prepared by heat treatment of a spin cast methylsilsesquioxane precursor. By varying the extent of the end-functionalization, closed pore foams with controlled pore sizes and pore contents of up to 40 vol% were obtained by chemically-induced phase separation and thermal degradation of the hyperbranched polymers during the heat treatment.

MOLECULAR WEIGHT DISTRIBUTION OF POLYAMIDES BY GPC-VISCOMETRY. A COMPARISON BETWEEN HIGH TEMPERATURE AND LOW TEMPERATURE ELUANTS

Synthetic polyamides (PA) owe many outstanding properties to their semicristalline morphology and to the intermolecular hydrogen bonding of the amide groups. Although the strength of the Van der Waals network confers an exceptional solvent resistance to this class of polymers, it may lead to serious difficulties in their solution characterization, such as the determination of molecular weight distribution (MWD). Over the years, a large number of dissolution schemes have been developed for the GPC characterization of PA. A recent IUPAC round-robin test on the GPC of aliphatic PAs nevertheless showed large variations in the experimental data not only between laboratories employing different methods, but also between laboratories adopting similar protocols. The purpose of the present investigation is to understand the origins of these disparities by comparing GPC-Viscometry data obtained with some frequently used eluant systems: a. benzyl alcohol at high temperature; b. hexafluoro-isopropanol, pure or mixed with dichloromethane, at low temperature; c. common GPC solvents (THF, CH2Cl2,) after N-trifluoracetylation of the PA. Apart from pure HFIP, all the methods tested show consistent MW results when measured in the same laboratory. Viscometry data reveal, however, that the expected linear viscosity law is obeyed only in benzyl alcohol. With the other eluants, significant departure from the Mark-Houwink relation denotes presence of non-size exclusion effects, rendering data evaluation imprecise with the Universal Calibration. The problems are particularly acute for polyamides with short methylene sequences which can only be dissolved in mixtures containing a high concentration of HFIP. To improve accuracy, it is proposed that GPC calibration for this class of polymer be performed with absolute detection (on-line viscometer, light scattering) or with a broad MWD standard of identical chemical composition to the analyzed sample.

GPC Data Interpretation in Mechanochemical Polymer Degradation

Abstract Molecular weight distribution obtained by gel permeation chromatography (GPC) conceals a wealth of mechanistic information which can be disclosed only after adequate data treatment. This paper discusses how GPC chromatograms may be used for the elucidation of polymer degradation kinetics. The case of ultrasonic degradation of dilute polystyrene solutions was investigated in detail. The extent of bond scission was determined as a function of sonication time and initial polymer molecular weight. Different bond scission mechanisms were evaluated by comparing calculated molecular weight distributions with experimental data. Best agreement was obtained when multiple fragmentation was included in the degradation scheme, a result experimentally supported by spectroscopic chain ends titration. The results are rationalized by establishing a parallelism between transient elongational flow and ultrasonic degradation.