No abstract

METHACRYLATE) BY THE GEL MELTINGPOINT Sir:Most of the quantitative methods for determining the tacticity of stereoregular poly-(methyl methacrylate) depend on nearest-neighbor interactions and, therefore, except in special cases,2 measure the fraction of monomer placements which may be in one tactic configuration or another. This is the case with two of the more useful methods thus far described, high-resolution n.m.r.2 and infrared spectroscopy. 1,3,4 Since complete steric control of polymerization probably never is attained with methyl methacrylate, it is also of importance to learn the average length of the stereoregular sequences, and the distribution of these sequence lengths. We report here a new method, specific to poly-(methyl methacrylate), which we believe gives a relative measure of average syndiotactic sequence length.This method is based on the observation that when a solution of isotactic poly-(methyl methacrylate) in a solvent such as dimethylformamide is mixed with a solution of syndiotactic poly-(methyl methacrylate) in the same solvent, a rigid continuous gel is formed rapidly. If this gel is heated, it will melt sharply to a clear solution. This gel melting point is (1) reproducible to j = O . 5 O , (2) only slightly affected by changes in the original polymer concentration from about 1 to 9% by wt. or by changes in the ratio of isotactic to syndiotactic polymer from about 1:4 to 4:l by wt., (3) independent of the moje of preparation of the isotactic polymer if it has Mv > ca. 100,000, (4) only slightly affected by the molecula; weight of the syndiotactic polymer if it has Mv > ca. 50-100,000, ( 5 ) markedly dependent on the temperature of polymerization of the syndiotactic polymer. Both n.m.r. and infrared spectroscopy have established that the fraction of syndiotactic placements increases with decwasing temperature of polymerization. SYNDIOTACTIC POLY-(METHYL (1) T. G . I'ox, B.Standard conditions therefore can be established so that the gel melting point becomes entirely a function of some element of tacticity of the syndiotactic poly-(methyl methacrylate).A single is0 tac tic poly-(me thy1 met hacry late) sample was used as the isotactic component of all gels. Gels were prepared with equal weights of various syndiotactic polymers in dimethylformamide a t a total concentration of 5% by wt. The isotactic polymer4 was prepared by initiation with phenylmagnesium bromide in toluene at 3 O , and had M, = 130,000; the syndiotactic polymers6 were prepared by free radical initiation at various constant temperatures from -50" to +140°. The gel melting points were measured by determining the temperature a t which the gel, contained in the tip of a 1-ml. pipet inserted into a snug-fitting testtube, flowed readily from the tip. Some results are given : Temp. of polymerization, OC. @V X 1CF Gel melting point, O C . -50 8 . 0 92.7 -30 19.0 87.5 0 11.5 7 6 . 5 60 25.1 60.0 140 5 . 7 4 3 . 5Gornick'j has applied the Flory theory of copolymer crystallization7 to partially t'actic systems, and concludes that ...

A laboratory experiment was developed to investigate the rheological properties of a green calcium-cross-linked alginate gel as an alternative to the traditional borax-cross-linked poly­(vinyl alcohol) gel. As borax is suspected of damaging fertility and the unborn child, a safe, green alternative is necessary. The rheological properties of a sodium alginate solution were examined as a function of temperature using capillary viscometry. Gelation and degelation processes were followed using rotational viscometry. The non-Newtonian shear-thinning properties were also exemplified by determining the viscosity as a function of spindle speed using the rotational viscometer. Students making the transition from a first- to second-year undergraduate chemistry program within a natural sciences degree have successfully conducted this laboratory experiment.

This work aims to determine the roles of reactive oxygen species HO∙ and HO in the bleaching of melanins by alkaline hydrogen peroxide. Experiments using melanosomes isolated from human hair indicated that the HO∙ radical generated in the outside solution does not contribute significantly to bleaching. However, studies using soluble Sepia melanin demonstrated that both HO and HO∙ will individually bleach melanin. Additionally, when both oxidants are present, bleaching is increased dramatically in both rate and extent. Careful experimental design enabled the separation of the roles and effects of these key reactive species, HO∙ and HO. Rationalisation of the results presented, and review of previous literature, allowed the postulation of a simplified general scheme whereby the strong oxidant HO∙ is able to pre-oxidise melanin units to o-quinones enabling more facile ring opening by the more nucleophilic HO. In this manner the efficiency of the roles of both species is maximised.

n m z the course of studies on latex paint formulations, it became apparent that a method of evaluating leveling by other than brushing tests was necessary in order to relate one set of experiments to another.I t was concluded, after a thorough physical evaluation of the leveling phenomenon, that :1. Leveling proceeds as long as the driving forces of surface origin exceed the yield stress of the paint film.2. The rate of leveling is determined by the instantaneous value of the paint film viscosity.3. The viscosity and yield stress of all paints rise sharply as water is lost beyond a critical solids content. 4. Water loss by wicking into a porous substrate is probably the major factor in differentiating between paints as to rate of change of properties.Both an initial yield stress and wicking rate must be determined, in order to correlate paint performance with measurable properties. 5.Latex emulsion just before fusionInitially it was assumed that the most important property determining leveling quality was the yield stress of the paint. Yield stress as used in this study means the value of shearing stress obtained by visual extrapolation (to 0 rate) of a graph of shearing stress us. rotational speed of a modified Stormer viscomewr under arbitrarily specified conditions.After an analysis of the problem revealed the profound effect on leveling of water loss of a paint into a poroiis substrate, an empirical test was devised to measure this property. The test employed was one in which the rate of separation and rise of the aqueous phase into a strip of filter paper was determined (henceforth referred to as the wicking rate).Having two measurable properties available, a full factorial experiment was designed in an effort to relate the yield stress and wicking rate to observer leveling ratings. I n the study was included the effect of thickener type, thickener concentration, pigment volunie concentration, and extender. The paints were applied to a moderately poroiis substrate and given visual ratings for flow out.I t was found that the leveling phenomenon can be reasonably expressed ' as functions of yield stress and wickirig rate by the following equations:A statistical analysis showed that the calculated leveling values, L, arid L,, correlated well with visual ratings of flow out.Using these equations to analyze trends, further formulation variables were examined which extended the full factorial design. and, ConclusionsI t was concluded that the method outlined is applicable for determining the effects of formulation variables on leveling in general, and on the specific factors controlling the leveling phenornenon, the yield stress, and wicking rate.