ACOMP measurements

Using derivatives of the onhne M (t) to compute the polymer mass distributions andpolydispersity indices. ACOMP measures the accumulated M (t) at each moment for most chain growth reactions, this involves an accumulation of dead chains, which form very quickly compared to the time scale for the total conversion of monomer. 

The instantaneous sequence length distribution, defined as the probability of having a sequence of k monomers in a row of type A, followed by a monomer B can be determined from ACOMP measurements at each point/[22, 23]. 

While 5(0 is an important parameter to be monitored, since it allows running the reaction to any degree of conversion desired, there was no model-independent route to obtaining conversion, in contrast to typical ACOMP measurements of polymerization reactions, where model-independent conversion is directly obtained from collected data. The authors proposed the use of a chemically specific detector, such as FTIR, for the direct measurement of conversion in future studies. 

This can be used for predicting values of Mj(t), which is what the multiangle light scattering in ACOMP measures directly. On the other hand, it can be valuable to use M t) from ACOMP to compute M (t) as follows ... 

Thanks to continuous ACOMP measurements of comonomer conversion, average instantaneous molar composition of chains being produced at each point in time can be calculated from the knowledge of polymer mass created at each point. This eliminates the use of the reactivity ratios for the predictive procedure ... 

In sharp contrast, in the industrial environment, ACOMP measurement repeatability, system reliability, and continuous, problem-free operation are the main requirements. The number of detectors is chosen to be the absolute minimum needed to provide the most valuable characteristics to be monitored. Oftentimes, the characteristics to be measured are a small subset of what an R D ACOMP is aimed at for example, an industrial system may just be needed to monitor monomer conversion and polymer reduced viscosity. In light of these goals, both the front-end and detector components are chosen with ruggedness and reliability in mind. 

Determination of [p] hence requires that of the sample solvent and the total viscosity of the fluid containing the macromolecules 7] be measured. In some analytical techniques, such as SEC and ACOMP, the concentration is usually low enough that to a good approximation [p] = where is measured by combining viscometer and concentration detector data. Importantly, [ )] is a direct measure of the ratio of a polymer s hydrodynamic volume to its molar mass M. 

Because the reaction medium is normally quite concentrated, however, rheological and other measurements often only indirectly measure molecular mass and other single chain properties, because the interactions between polymer chains often dominate signals from undiluted reactor contents. Automatic continuous online monitoring of polymerization reactions (ACOMP) provides a solution for this. ACOMP is covered in Chapters 11-13. 

Alb AM, Reed WF. Fundamental measurements in online polymerization reaction monitoring and control with a focus on ACOMP. Macromol React Eng 2010 4 470-485. 

Advantages of ACOMP include its versatility as a generalized approach, its ability to make fundamental measurements without recourse to empirical models and calibration, its capacity for providing a data-rich stream of complementary information from multiple independent detectors, yielding multifaceted characteristics of polymerization reactions, and its use of the front end to extract, dilute, and condition a sample stream that allows sensitive detectors to provide reliable data without exposing them to harsh reactor or sample conditions. Disadvantages include the mechanical complexity of the front end, the delay time between a continuous fluid element s extraction from the reactor and downstream measurement by the detector train, and a small but continuous waste stream. ACOMP is more invasive than probes that can be placed at an outside reactor window, but are no more invasive than in situ probes, in that in either case access to the reactor contents is required. 

ACOMP Delay Time and Response Time Because ACOMP involves continuous withdrawal, dilution, and conditioning of reactor liquid, there is inevitably both (i) a delay time between when a fluid element is withdrawn and when a measurement of its properties is made by the detector train, and (ii) a system response time associated with the various mixing processes involved. 

The signal measured by a detector D(t) is related to the actual physical signal in the reactor, S(t), such as monomer concentration, via the ACOMP system response function R(t) according to ... 

The inset to Figure 11.2b shows the ACOMP system response function Rit), which was determined experimentally by rapidly injecting a small volume of acrylamide (Am) into the reactor to cause a step function in concentration, shown in Figure 11.2a (measured by the UV absorbance in this case) ... 

Raman has been used to follow monomer concentrations [2, 66-68]. However, it has been shown that Raman spectra could be affected by particle size in emulsion polymerization [69, 70]. Many of the foregoing referenced works, and others, combine IR and Raman measurements [71, 72]. Raman has not yet been applied to ACOMP although such application is expected soon. 

For example, in situ conductivity probes were used to monitor incorporation of ionic comonomers in aqueous copolymerization reactions [73,74], and to directly monitor counterion condensation during copolymerization [75]. Conductivity probes immersed in flow-coupled mixing chambers were used on ACOMP-extracted streams during organic phase polymerization, and for aqueous copolymerization where the reactor ionic strength was too high for direct conductivity measurements. In situ pH probes have been coupled with ACOMP data to monitor hydrolysis during postpolymerization modifications. 

Based on continuous extraction and dilution of reactor content to produce a dilute stream through the detectors, the versatile ACOMP platform combines light scattering, spectroscopy, viscometry, and other methods to monitor, during polymeric materials synthesis, various features such as reaction kinetics, monomer conversion, composition drift, linear charge density, molar mass, intrinsic viscosity, and other characteristics. The examples discussed next illustrate a number of reaction contexts where fundamental measurements are used to gain a comprehensive picture of reaction characteristics. 

Keeping in mind that polymer characteristics provided by ACOMP M, are determined from direct measurements, whereas NIR furnishes monomer conversion data via empirical calibration, an advantage of the in situ NIR is that it furnishes immediate information on the conversion in the reactor, whereas ACOMP relies on continuous withdrawal and dilution of a small stream of reactor fluid, so that there... 

Light scattering is one of the standard methods for the determination of molecular weights of macromolecules. In the case of copolymers, it has historically required that measurements be made in at least three separate solvents of different index of refraction [14, 15]. Comprehensive studies are available elsewhere [16], and the ACOMP solution to the problem is given in Chapter 11. 

A method of finding M (f)—discussed also in Chapter 11—Abased on the measured values of absolute scattering I q, c) and the concentration values determined at any point using ACOMP platform was presented in Reference [17]. The evolution of the copolymer M was computed as follows ... 

Comonomer Concentrations An important innovation made by the addition of a full spectrum UV spectrophotometer into ACOMP detector train led to the determination of the instantaneous concentration of each comonomer during the reaction, in the case of the copolymerization of comonomers with similar spectral characteristics [19, 20], The working assumption was that a UV spectrum at any instant is a Unear combination of the normalized basis spectra of the comonomers, the copolymer, and any other UV absorbing species, and that the unknown comonomer concentrations can be foimd by minimizing the error between measured and computed spectra over many wavelengths, even when spectral diffaences are small at any iven wavelength. 

Various methods have been used to analyze aliquots from reactions [27, 28], Online measurements by dilatometry have also been reported [29], A significant advance in this field was the adaptation of ACOMP as a means for monitoring conversion and measures of polymer molar mass during the inverse emulsion polymerization of acrylamide, in order to understand both reaction kinetics and mechanisms and, potentially, to control them during the reaction [30],... 

The ACOMP detector train provides m(t), mjit) is computed via the previous equation. m (t) is then obtained from Equation 13.50, and c (t)=m (t)IV(t). Density changes, while measurable, are usually small (negligible in this section, due to the very low Am concentration), and are not corrected for in this work. They have been addressed in the ACOMP context earlier [49, 97]. 

The c (t) are measured directly from the ACOMP UV diode array data. 

Reed and coworkers developed a strategy for automatic continuous online monitoring of polymerization (ACOMP). The method may be used during the initial development of the polymerization process, its optimization and monitoring of the continuous reaction. ACOMP automatically dilutes samples from the reactor and measures its properties, e.g., refractive index, near infrared (NIR) spectra, LSc, [ ]], etc., from which it computes evolution of MW, MWD, degree of conversion, copolymer composition (in copolymerization) and others. The method has been applied to a variety of the free radical homo- and co-polymerizations, including the reactions in emulsion or suspension. ... 

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