Autoacceleration

When the concentrations of monomers are high in solution or bulk polymerizations, typical auto accelerations of the rates can be observed. This is known as the gel effect or as the Trammsdorff effect, or also, as the Norrish–Smith effect [66]. The effect has been explained as being caused by a decrease in the rate of termination due to increased viscosity of the medium. Termination is a reaction that requires two large polymer-radicals to come together and this can be impeded by viscosity. At the same time, in propagation the small molecules of the monomer can still diffuse for some time to the radical sites and feed the chain growth.

One should not mistake an acceleration of the polymerization reaction due to a rise in the temperature under nonisothermal conditions for a true gel effect from a rise in viscosity. The gel effect can occur when the temperature of the reaction is kept constant. A critical analysis of the gel effect suggests that the situation is complicated. In some polymerizations, three different stages appear to be present when RP/[M][I]1/2 is plotted against conversion or against time [95]. The plot indicates that during the first stage there is either a constant or a declining rate and during the second stage there is autoacceleration. During the third stage, there is again a constant or a declining rate [95]. Numerous publication made a substantial case for associating and/or attributing the gel effect to entanglement of polymerizing chain radicals, resulting in a marked reduction in the termination rate parameter, kT. This was often done by using the assumption that in the neighborhood of the gel effect kT is controlled by polymer self-diffusion, which in turn exhibits entangled polymer dynam ics. O’Neil et al. [96], however, argued against that opinion. They carried out a series of experiments involving bulk polymerizations of methyl methacrylate and styrene and feel that their data contradicts this widely held belief that the gel effect onset is related to the formation of chain entanglements. The experimental conditions used were such that they tended to delay or eliminate the formation of chain entanglement. These conditions were high initiator and/or chain transferring agent concentrations and additions of low molecular weight polymers prior to the reactions. The results indicated that the gel effect occurs readily in the absence of entanglement and that delaying the onset of entanglements does not necessarily delay the onset of the gel effect. Also, critical examination of the molecular weights produced in these experiments indicated values that were too low for entanglement formation in solution (polymer plus monomer) and sometimes even in bulk polymer, not only at the onset but also throughout the gel effect [96]. O’Neil et al. [96], found that even under conditions where entanglements are likely to exist, the gel effect onset does not correlate with polymer molecular weight of the chains produced in a manner consistent with entanglement arguments. Whether the kinetics during the gel effect may be affected by entanglements was left uncertain.

اخر الاخبار

اخبار العتبة العباسية المقدسة

المجمع العلمي يقيم محطة قرآنية على طريق الزائرين بذكرى شهادة الإمام الكاظم (عليه السلام) في واسط

قسم العلاقات العامّة ينظّم برنامجًا ثقافيًّا لأكثر من 100 طالب من حفظة القرآن الكريم في النجف

قسم شؤون المعارف يختتم دورة حول تحقيق المخطوطات في جامعة كربلاء

قسم الشؤون الطبية يطلق برنامجًا تدريبيًّا لملاكاته بالتعاون مع مستشفى الكفيل التخصصي

المزيد

الآخبار الصحية

6 فواكة تساعدك على الشعور بتحسّن عند المرض

حفاظا على صحة العين.. هذا أفضل وقت لتناول فيتامين أ

الاستخدام اليومي لسماعات الأذن.. أخطر مما تتصور !

هل توقيت نشاطك اليومي يؤثر على صحة دماغك؟.. العلماء يجيبون

المزيد

الآخبار التكنلوجية

ابتكار ألماني قد يغيّر مستقبل صناعة البلاستيك

تحديثات وأرباح عبر المشاهدات.. ما الذي تخطط له "لينكد إن"؟

"الكوكايين الوردي".. مخدر جديد يثير قلق السلطات الأميركية

دراسة: تغيّرات في أسلوب القيادة قد تكشف مبكرا عن الخرف !

المزيد

مواضيع ذات صلة

Ceiling Temperature

Effect of Reaction Medium

Steric, Polar, and Resonance Effects in the Propagation Reaction

Propagation

Capture of Free Radicals by Monomers

Optical Activity in Polymers

Spherulitic Growth

Crystallization from Solution

Crystallization from the Melt

The Crystalline State

Rheology and Viscoelasticity of Polymeric Materials

Thermodynamics of Elasticity