Over the past two decades, a substantial case has been made associating and/or attributing the “gel” effect or autoacceleration in free radical polymerization and its onset to entanglement of polymerizing chain radicals, resulting in a marked reduction in the termination rate parameter, k t. This case has often been made by modeling a limited range of free radical polymerization data using the assumption that in the neighborhood of the gel effect k t is controlled by polymer self-diffusion which in turn exhibits entangled polymer dynamics. The present study provides critical experiments involving both bulk methyl methacrylate and styrene polymerizations which contradict the widely held belief that the gel effect onset is related to the formation of chain entanglements. By employing experimental conditions which tend to delay or eliminate the formation of chain entanglements, i.e., high initiator and/or chain transfer agent concentrations and addition of low molecular weight polymer prior to reaction, it is shown that the gel effect readily occurs in the absence of entanglements and that delaying the onset of entanglements does not necessarily delay the onset of the gel effect. Critical examination of the molecular weights produced in these and other experiments often indicates values 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. This fact illustrates the importance of understanding the relationship between molecular weight and entanglements in bulk polymer and solution in performing a critical examination of the relationship between the origin of the gel effect and entanglement formation. Furthermore, even under conditions where entanglements are likely to exist, the gel effect onset does not correlate with polymer molecular weight, either of the chains produced or the matrix, in a manner consistent with entanglement arguments. Although the present results indicate that the onset of the gel effect is unrelated to entanglement formation, whether the kinetics during the gel effect may be affected by entanglements if they do form remains an open question; experimental tests to investigate this issue are outlined. Other theories which purportedly explain the origin of the gel effect are also discussed in the context of data from this study and future experimental tests.

Optoelectronic consumer products that are widely employed in the office and home attract attention for optical sensor applications due to (1) their cost advantage over analytical instruments produced only in small quantities, (2) robustness in operation due to the detailed manufacturability improvements, and (3) ease of operation. We demonstrate here a new approach for quantitative chemical/biochemical sensing when analog signals are acquired from conventional optical disk drives, and these signals are used for quantitative detection of optical changes of sensor films deposited on conventional CD and DVD optical disks. Because we do not alter manufacturing process of optical disks, any disk can be employed for deposition and readout of sensor films. The optical disk drives also perform their original function of reading and writing digital content to optical media because no optical modifications are introduced to obtain the analog signal. Such a sensor platform is quite universal and can be applied for chemical and biological quantitative detection, as well as for monitoring of changes of physical properties of regions deposited onto a CD or DVD (e.g., during combinatorial screening of materials). As a model example, we demonstrate the concept using chemical detection of ionic species such as Ca2+ in liquids (e.g., blood, urine, or water). Colorimetric calcium-sensitive sensor films were deposited onto a DVD, exposed to water with different concentrations of Ca2+, and quantified in the optical disk drive. The developed lab-on-DVD system demonstrated a 5 ppm detection limit of Ca2+ determinations, similar or slightly better than that achieved using a conventional fiber-optic portable spectrometer. This detection limit corresponded to a 0.023 absorbance unit resolution, as determined by the measurement of the same colorimetric films with a portable spectrometer. Determinations of Ca2+ unknowns using the lab-on-DVD system demonstrated +/-5 ppm accuracy and 2-5% relative standard deviation precision in predicting 100 ppm Ca2+.

The most comprehensive study to date of the effects of size, shape, and flexibility on the translational diffusion of small probe molecules in polymer solutions has been completed by Taylor dispersion, which directly yields D probe, and phosphorescence quenching, which yields k q, the concentration dependence of which is identical to that of D probe for appropriate conditions. Diffusion of 16 probes ranging by a factor of 6 in molar volume was investigated using both Taylor dispersion in solutions of up to 400 g/L polystyrene in tetrahydrofuran and phosphorescence quenching in solutions of up to 700 g/L polystyrene in tetrahydrofuran, cyclohexane, and carbon tetrachloride. Results were compared quantitatively to modified Vrentas−Duda free volume theory for ternary solutions to obtain probe jumping unit sizes relative to the solvent, ξprobe,s, which correlate with probe volume. With the exception of 3,4-hexanedione (a highly flexible and small probe), the PS concentration dependencies of D probe and k q were approximately equal to or greater than that of solvent (0.9 ≤ ξprobe,s ≤ 1.75). The data fell into two types of behavior:  when ξprobe,s was plotted against the ratio of probe to solvent molar volume, Ṽ(0)probe/Ṽ(0)s, the vast majority of data fell around a line of slope 0.13, while for two of the probes ξprobe,s fell near a line of slope unity. Literature data for five probes in several polymer−solvent systems could also be described by these two types of behavior. The former behavior indicates that for most probes the concentration dependence can be described by modified free volume theory, with the understanding that the critical hole free volume for a jump unit for these probes is but a fraction of the probe molar volume. The apparent dichotomy in the probe volume dependence of ξprobe,s raises the question of whether only two dependencies are possible or whether, by virtue of the probes selected, only these two distinct behaviors are observable. Small effects of flexibility and shape on D probe for probes with large aspect ratios were also observed and discussed in terms of anisotropic diffusion. A comparison of concentration dependence data with limited temperature dependence data from the literature shows a consistency based on the modified free volume picture. This, along with an understanding of the “bimodal” ξprobe,s data, indicates that the modified free volume theory for ternary systems forms a reasonably robust picture by which to interpret probe diffusion in polymer solutions.

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This paper examines the use of free volume approaches to describe the gel effect in free radical polymerization, specifically testing the consistency of free volume in describing the effect of temperature on the critical onset conversion for the gel effect, X crit. Experimental polymerization results for both methyl methacrylate (MMA) and styrene in which the temperature, T, is varied while the molecular weight, M, is held nearly constant show that the critical onset conversion for the gel effect, X crit, is affected by T; i.e., a higher T leads to a higher X crit, consistent with free volume. Other studies purported to show a link between X crit and M did not account for variations in T in the data analyzed, and it is highly likely that part of the X crit trend reported to be caused by M was in fact related to T variations. Further examination of this issue via modeling indicates that the experimental results are consistent with the quantitative trend predicted by free volume for X crit as a function of T as well, indicating that free volume is an appropriate basis for modeling the gel effect as it adequately handles effects of temperature on termination. However, as it does not describe the concentration or potential molecular weight dependence of termination, free volume is not a complete theory for the gel effect. For this task, additional molecular-level insight is needed, a fact underscored by certain experimental trends not predicted by free volume, such as the effects of chain transfer and solvent quality on termination.