Optimal operation strategies to control the molecular weight distribution of polymer products (2023)

The effect of nanoconfinement on the rate of isothermal polymerization of methyl methacrylate (MMA) polymerization is investigated using differential scanning calorimetry. Controlled pore glass (CPG) with pore diameters of 13, 50, and 111nm are used for the confinement of the reaction. Both hydrophilic and hydrophobic pore surfaces are studied. The effective reaction rate and the apparent activation energy at low conversions, prior to autoacceleration, are unchanged in hydrophobic pores. On the other hand, in hydrophilic pores, the reaction rate increases by as much as a factor of 8 in the smallest 13nm hydrophilic pores, and the effective activation energy decreases. For both pore surfaces, the time required to reach autoacceleration decreases with decreasing pore size, with the effect much more pronounced in the hydrophilic pores. The results are consistent with a model of nanoconfined free radical polymerization which accounts for suppressed termination due to a decrease in the diffusivity of nanoconfined chains.

The measurement and control of polymerization reactors is very challenging due to the complexity of the physical mechanisms and polymerization kinetics. In these reactors many important variables, which are related to end-use polymer properties, cannot be measured on-line or can only be measured at low sampling frequencies. Furthermore, end-use polymer properties are related to the entire molecular weight, copolymer composition, sequence length, and branching distributions. This paper surveys the instrumentation technologies, which are of particular interest in polymerization reactors with emphasis on, for example, measurement of viscosity, composition, molecular weight, and particle size. This paper presents a hierarchical approach to the control system design and reviews traditional regulatory techniques as well as advanced control strategies for batch, semibatch, and continuous reactors. These approaches are illustrated by focusing on the control of a commercial multiproduct continuous emulsion polymerization reactor. Finally, the paper captures some of the trends in the polymer industry, which may impact future development in measurement and reactor control.

We study the control of a solution copolymerization reactor using a model predictive control algorithm based on multiple piecewise linear models. The control algorithm is a receding horizon scheme with a quasi-infinite horizon objective function which has finite and infinite horizon cost components and uses multiple linear models in its predictions. The finite horizon cost consists of free input variables that direct the system towards a terminal region which contains the desired operating point. The infinite horizon cost has an upper bound and takes the system to the final operating point. Simulation results on an industrial scale methyl methacrylate vinyl acetate solution copolymerization reactor model demonstrate the ability of the algorithm to rapidly transition the process between different operating points.

The optimal temperature policy which minimizes the terminal time in a batch emulsion polymerization reactor of styrene and α-methylstyrene was determined by means of orthogonal collocation techniques combined with a general non-linear programming method. The constraints concern the final latex properties and the thermal limitations of the pilot plant. An experimental validation has been realized. The optimal temperature profile was tracked using a non-linear geometric control technique which is particularly adapted to polymerization reactor control. An extended Kalman filter was used to estimate the non-measured state variables. Experimental results showed excellent agreement with predictions for this complex system. A good temperature tracking was observed and the product quality was well predicted and controlled.

An experimental study on the control of polymer weight chain length distribution is presented for batch-free radical solution polymerization of methyl methacrylate. The weight chain length distribution is calculated using the method of finite molecular weight moments in which the weight fraction of polymers over a number of finite chain length intervals covering the theoretically infinite chain length domain is calculated. Control of a target polymer chain length distribution is achieved by first computing a discrete sequence of reactor temperature setpoints which lead to the best match of a given target weight chain length distribution at a final desired monomer conversion. During the polymerization, an on-line extended Kalman filter is used to incorporate infrequent and delayed off-line molecular weight measurements. The piecewise constant reactor temperature setpoints are taken as the decision variables in a nonlinear programming problem. They are recomputed and updated at each sampling point during the course of polymerization to match the final desired molecular weight distribution. It is demonstrated through simulations and experimentation that it is feasible to control the entire polymer chain length distribution in a batch polymerization process.

Two different calorimetric techniques have been used to determine thermokinetic parameters and to asses the thermal stability of bulk methyl methacrylate (MMA) polymerization. The results obtained with both methods are in agreement with those reported in the literature. New correlations based on experimental results have been proposed to describe the influence of diffusion phenomena on the dynamics of the process. A mathematical model based on the experimental results has been developed in order to execute an analysis of the dynamics of the process.

Journal of Photochemistry and Photobiology A: Chemistry, Volume 333, 2017, pp. 18-25

A novel permethylated β-cyclodextrin (PM-β-CD) derivative, 6^I-acryloyl ethylenediamine-6^I-deoxy-2^I, 3^I-di-O-methyl-hexakis (2^II−VII, 3^II−VII, 6^II−VII-tri-O-methyl)-β-cyclodextrin (6-acryl-en-PM-β-CD) was synthesized as host molecule to construct a pH-sensitive water-soluble supramolecular-structured photoinitiator (SSPI) with hydrophobic 1-hydroxycyclohexyl phenyl ketone (HCPK). The conformation, binding behavior and photopolymerization kinetics of 6-acryl-en-PM-β-CD with HCPK were investigated by using FT-IR, ¹H NMR, fluorescence spectra titration and UV–vis at various pH values. Experiment results obtained indicated that the polymerization efficiency of the resulting water-soluble SSPI was able to be controlled by pH stimulation where the intrinsic mechanism was discussed from the insight of the binding manner geometry transformation. Moreover, the GPC and SEM results further demonstrated that apparent morphology of polymer product had the potential to be regulated by the pH-sensitive SSPI.

Vacuum, Volume 195, 2022, Article 110703

First-principles calculation has been used to comparatively investigate that the stability of Re and Cr at the Fe/W interface, as well as cohesion characteristics and fracture toughness. It is found that Re atom prefers to situate at the Fe position at the interface layer, and the reason is due to the surrounding W–Re and Fe–Re bond lengths are much shorter than other substitution positions of Fe/W interface. Our calculations also reveal that the different effects of the additions of Re and Cr on the cohesion properties and fracture toughness of Fe/W interface, i.e.,substituting Re for interfacial Fe position could increase cohesion strength and fracture toughness, which appeared the volatile changes by the Cr substitutions for Fe. The derived results are deeply understood by means of electronic structures, and sufficiently compare with experimental observations in the literature.

Journal of Food Engineering, Volume 171, 2016, pp. 185-193

The objective of this research was to develop state diagrams of model food systems prepared with several fructose/glucose/sucrose mass fractions as a basis for studying products obtained from natural juices. A total of 16 sugar mixtures, including pure components as well as binary and ternary mixtures, with freezable and reduced moisture, were prepared and subjected to thermal analysis using differential scanning calorimetry (DSC), where freezing points (Tm and Tm′) and glass transitions values (Tg, Tg′) were determined. The expected decrease of Tg due to the plasticizing effect of water and the depression of Tm as a function of increasing solids concentration were adequately predicted (R²>0.94) using the Gordon–Taylor and Chen models, respectively. Results showed that Tg and Tm depression were influenced significantly by sugar composition (p<0.05). Tm′ and Tg′ values ranged from−42.8 to−32.4°C and from−55.9 to−43.1°C, respectively and were also sugar composition dependent (p<0.05). Constructed state diagram showed that solid mass fraction at the maximal-freeze-concentration condition (xs′) varied from 0.772 to 0.842 (g solids/g sample), however, significant effect of the sugar composition was not observed for xs′ values.

Journal of the Taiwan Institute of Chemical Engineers, Volume 133, 2022, Article 104264

Journal of Solid State Chemistry, Volume 277, 2019, pp. 441-447

A free-standing hybrid film combining graphene and zinc oxide nanoflakes used as electrode materials was prepared by facile filtration self-assembly and thermal reduction process. The as-prepared ZnO nanoflakes were inserted into the graphene sheets to form unique layer-layer structure. The hybrid film can be directly employed as binder-free and self-supported electrode material for supercapacitors (SCs). The max specific capacitance of fabricated zinc oxide nanoflakes/reduced graphene oxide (ZnO/rGO) SCs is as high as 167.2 F g⁻¹ at a scan rate of 5 mV s⁻¹, which is nearly 50% higher than pure rGO (111.4 F g⁻¹) and more than twice as much as ZnO nanoflakes (71.1 F g⁻¹). The enhanced electrochemical performance could be due to the efficient utilization of electroactive ZnO nanoflakes and high conductive graphene sheets network and the good synergistic effect between them. The free-standing hybrid film also exhibits an excellent cycling capability, retention of the initial capacity over 90% after 2000 cycles. The EIS analysis of the hybrids showed low electrical resistance, improving ion diffusion capacity and surface charge transfer performance of the working electrode. It holds a potential for high performance electrochemical energy storage applications.

Chemical Engineering Journal, Volume 358, 2019, pp. 1410-1420