Chain configuration of polystyrene arising from freeze-drying dilute solutions

McGrath, K. J.; Roland, C. M.; Weiß, R.

doi:10.1021/ma00074a039

Cited by 21 publications

(25 citation statements)

References 1 publication

Supporting

Mentioning

Contrasting

Order By: Relevance

“…The recovery of entanglements observed for freezedried polymer melts is highly retarded, even though small-angle neutron scattering on freeze-dried polymers shows the chains to have their normal size (no "compaction"), 39 and solid state 13 C NMR dipolar dephasing measurements show the chains to be at least partially interspersed. 40,41 There are likely other unexplained phenomena in polymer rheology having their origin in the fact that entanglements reform more slowly than suggested by the linear diffusion constant.…”

Section: ■ Discussionmentioning

confidence: 99%

Reentanglement Kinetics in Polyisobutylene

Roy

Roland

2013

Macromolecules

Self Cite

View full text Add to dashboard Cite

ABSTRACT:The rheological properties of a polyisobutylene (PIB) having a molecular weight five times its entanglement molecular weight were measured, along with interrupted shear flow experiments to quantify the time required for structural recovery following steady state shearing in the non-Newtonian regime. The reentanglement kinetics was substantially slower (by more than an order of magnitude) than expectations based on the linear relaxation time of the fully entangled material. This result is in accord with published interrupted shear flow results for concentrated polymer solutions. The retarded recovery of entanglements may underlie other anomalies, for example, the delayed development of autoadhesion (tack) in rubber, the behavior of freeze-dried dilute solutions and the limiting behavior of the molecular weight dependence of the viscosity. It also offers a route to more processable polymers and, being reversible, will not affect the ultimate physical properties. ■ INTRODUCTIONWhen a polymer melt or solution is sheared at rates sufficiently low that the perturbation of the chains is slow compared to the rate of Brownian motion, their equilibrium conformation is retained, and the viscosity is Newtonian. Shear rates exceeding the reciprocal of the linear relaxation time, however, induce chain stretching and a reduction of entanglements, causing a decrease of the steady-state viscosity, η ss ("shear-thinning"). 1−3During the initial phase of non-Newtonian flow, the stress and normal force both pass through a maximum versus time, as the chains disentangle prior to attaining steady state; 4 birefringence measurements reveal similar behavior.5 The magnitude of the overshoot peak increases with shear rate, occurring at strains around 2 to 3 2 with some increase at high rates. 4 Such behavior is characteristic of a viscoelastic material with time-dependent structure (entanglements in this case), although unentangled polymers can also exhibit stress overshoots and shear thinning, but the effects are weaker. 6,7Herein we study reentanglement kinetics using interrupted non-Newtonian shear flow experiments, in which after steady state is achieved, the flow is halted and the material held in a quiescent state for some time before resumption of the shearing. Since the flow-induced loss of entanglements is reversible, the magnitude of the stress overshoot during subsequent shearing has an amplitude governed by the time allowed for reentanglement. 4,8−15 The steady-state viscosity is unaffected by interruption of the flow, since it depends only on the shear rate. For rest times exceeding some characteristic time, the entanglements return to their equilibrium concentration, and the magnitude of the overshoot has its maximum (initial) value. For concentrated solutions, the time to recover the fully entangled state has been shown to have the same molecular weight dependence (ca. M 3.4 ) as the viscosity and linear relaxation time. 16 Herein we determine its dependences on temperature and shear rate for a polymer melt.Althou...

show abstract

Section: ■ Discussionmentioning

confidence: 99%

Reentanglement Kinetics in Polyisobutylene

Roy

Roland

2013

Macromolecules

Self Cite

View full text Add to dashboard Cite

show abstract

“…62,63 Contrarily, in the physical blend (ID), in which intimate contact is precluded by the method of preparation, the aromatic carbon intensity falls off rapidly, due to the absence of cross-polarization of the deuterated carbons. They are all distant from the protonated species.…”

Section: Differential Scanning Calorimetry Displayed Inmentioning

confidence: 99%

NMR Study of the Morphology of Polystyrene Linear/Microgel Blends

2000

Self Cite

View full text Add to dashboard Cite

The morphology of blends of polystyrene (PS) microgels with linear PS were studied using NMR spectroscopy. Three techniques were employed to assess the intimacy of mixing: isothermal crosspolarization/magic-angle-spinning (CP/MAS) dipolar dephasing 13 C NMR, temperature-dependent 13 C CP/ MAS NMR, and one-pulse 1 H MAS NMR. The experimental data reveal that linear PS can penetrate the core of soft microgels, whose network strands are substantially larger than the linear chains. However, when the microgel is highly cross-linked, such penetration is prevented. These results are in accord with Flory-Huggins swelling theory. Blends of linear PS with highly cross-linked microgels yield thermodynamically stable morphologies, in which the components are molecularly dispersed, yet segmental contacts are virtually absent. Such a phase morphology is novel and cannot be obtained with conventional polymer blends.

show abstract

“…[8][9][10][11][12][13][14][15] Freeze-drying of dilute polymer solutions has also been investigated as a route to achieving disentangled high-molecular weight polymers, 16 although at least for polystyrene the results are disappointing. [17][18][19] Among the various signatures of a reduced entanglement density, the magnitude of the overshoot in the transient shear stress is especially useful for laboratory studies. At sufficiently high shear rates, the stress passes through a maximum, prior to attaining a steady-state value that reflects the equilibrium degree of entanglement.…”

Section: Introductionmentioning

confidence: 99%

Recovery of Shear-Modified Polybutadiene Solutions

Roland

Robertson²

2006

Rubber Chemistry and Technology

Self Cite

View full text Add to dashboard Cite

We have investigated the recovery of the overshoot in the transient viscosity, the first normal stress coefficient, and the dynamic modulus for entangled polybutadiene solutions subjected to nonlinear shear flow. The molecular-weight dependences of the various time scales (linear viscoelastic relaxation time, entanglement recovery time, and timescale for decay of stress following cessation of shearing) are all consistent with the usual 3.4 power law. Nevertheless, the time for recovery of the stress overshoot and plateau value of the dynamic modulus were substantially longer (by as much as two orders of magnitude) than the linear viscoelastic relaxation time calculated from the Newtonian viscosity and the equilibrium recoverable compliance. These results indicate that complete entanglement recovery requires cooperative chain motions over a length scale exceeding that associated with linear relaxation. This persistence of a disentangled state means that a state of low viscosity and reduced elasticity is retained for an extended time, suggesting that shear modification can be used to facilitate the processing of polymers.

show abstract

Chain configuration of polystyrene arising from freeze-drying dilute solutions

Cited by 21 publications

References 1 publication

Reentanglement Kinetics in Polyisobutylene

Reentanglement Kinetics in Polyisobutylene

NMR Study of the Morphology of Polystyrene Linear/Microgel Blends

Recovery of Shear-Modified Polybutadiene Solutions

Contact Info

Product

Resources

About