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Surface tension is the property that allows water striders to glide across the surface of a pond, for razor blades to float in a glass, and for water to stream from a hose for a certain length before breaking up into droplets. It is caused by intermolecular forces holding molecules in a liquid together, resisting an externally applied force. In water, the strong hydrogen bonds give water a high surface tension relative to other liquids (72.8 mN/m vs. ~30 mN/m for many organic solvents); this hydrogen bonding also results in water having a higher boiling point than many organic solvents. Mercury is also known for having a high surface tension (480 mN/m), resulting in its propensities for forming balls when placed on a substrate. Surface tension and energy are sometimes used interchangably, although tension is only applied to liquids. High surface energy liquids will not wet, or spread out, onto substrate that has a lower surface energy. Surfactants reduce the surface tension of liquids, allowing them to wet substrates that would normally not be wet by the unmodified liquid.

Surface tension in liquids is commonly measured by the Du Nuoy ring method. In this approach, a precision platinum ring of known diameter is attached to a precision balance, or tensiometer, and is slowly lowered into the liquid with a vertical stage (see image below). The direction is then reversed, and a ring of fluid is slowly pulled up out of the liquid. The tensiometer measures the force required to break free of the liquid, which can be easily converted into a surface tension by knowning the diameter of the ring.

For solids, the surface energy is usually required. A single surface energy term is a bit more challenging to report. The contact angle is normally reported, either from the sessile drop method, in which a liquid of known surface energy is placed on the substrate and the degree of wetting is assessed by its contact angle (see below), or from a Wilhelmy plate approach, in which a piece of the substrate is lowered into a vessel of a liquid of known surface energy, and the force required to lower and raise the liquid is monitored as a function of the perimeter length of the substrate. This latter test gives the advancing and receding contact angle.

 By making measurements of contact angle on a single substrate with multiple liquids of different surface tensions, the critical surface energy of a substrate can be determined where the contact angle goes to zero, indicating good wetting of the liquid on the substrate. All liquids with a surface tension below this critical surface energy will wet the substrate.

Visit our website for more information on surface energy and surface tension measurements.

The increasing interest in material deformulation analysis and quantification of trace compounds in materials has led to the development of coupled analytical techniques. One of these techniques is TGA-FTIR, which combines the mass sensitivity of thermogravimetric analysis (TGA) with the compound identification ability of Fourier transform infrared spectroscopy (FTIR). In TGA-FTIR, the sample is placed in a conventional TGA, and the mass loss is monitored as a function of temperature. As materials evaporate or are combusted, the mass of the sample will change accordingly and is monitored quantitatively by the TGA. A gas transfer line connects the TGA to the FTIR, which reports the chemical signature of the volatile components as they leave the TGA. In this manner, the identity of the volatile species can often be determined, in addition to their concentration based on the TGA mass change.

In an example experiment, we ran a 60:40 mixture of water and glycerol in our TGA-FTIR. Water has a boiling point of 100C, whereas glycerol has a boiling point around 290C. In an experiment run in nitrogen, the mass change of the sample as a function of time is shown below, indicating 60% loss of water by the time the tempererature reaches 100C. The remaining glycerol starts evaporating, and is gone by the time the system reaches the boiling point of glycerol.

The FTIR spectrum as a function of time is shown below (the lower curve is the Gram Schmidt signal, showing overall absorption intensity). The slice taken at 5.8 minutes shown as Extraction 1 in the upper absorption spectra indicates that the material coming out of the TGA at this time point is indeed water.

 The slice taken around 24 minutes below shows that the spectrum is glycerol, matching the TGA results.

West Virginia's drinking water situation has been getting a lot of press in the past week. Two toxic chemicals have been identified in the water (polyglycol ethers (PPH) and 4-methylcyclohexanemethanol), introduced from a tank rupture at Freedom Industries, a coal-cleaning operation. Prior to yesterday, only the methylcyclohexanemethanol had been know to be in the water, since the water contamination analytical tests commonly used only look for species that are suspected to be in the water, rather than any species. When Freedom Industries indicated that PPH was also released, the water analytical labs knew to look for this species as well. A more general analytical technique used to look for contaminants in water is gas chromatography with mass spectroscopy (GC-MS), which is a sensitive technique that will separate and identify all the volatile components in contaminated water, known or unknown. GC-MS is commonly performed in at Cambridge Polymer Group to determine if unknown species are in solvents, elutions from materials or devices, or to identify unknown liquids. Identification and quantification is possible with GC-MS.

Please visit our web site for more information on GC-MS testing.

Committee F04 is seeking abstracts for a workshop on Additives in Biomedical Polymers, to be held on May 6, 2014 in Toronto, Ontario. This workshop is intended to elicit discussion on the benefits, potential hazards, testing, regulatory considerations, and opportunites for standards activities related to intentionally incorporated additives in biomedical polymers. Information on how to submit abstracts, which are due March 1, 2014, can be found by clicking on the link below.


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CPG researchers Yuri Svirkin, Adam Kozak, and Gavin Braithwaite will be presenting their work on thiol-modified PVA hydrogels at the Fall Materials Research Society Meeting in Boston at 4:45 pm on December 3rd (Paper E5.09).

Abstract

Poly(vinyl alcohol) (PVA) is a well-respected biomaterial and forms highly hydrated hydrogels. It has been used in a number of applications, such as tissue bulking and nerve-guides, but is not intrinsically biodegradable, nor substantially mucoadhesive. These features can be built in to the molecule through complex co-polymerizations, but here we describe a simpler route involving modification of existing off-the-shelf materials.
Biodegradable hydrogels based on PVA modified with thiol groups (TPVA) were prepared and characterized. The TPVA was synthesized by an esterification reaction of PVA with 3-mercaptopropionic acid and characterized by 1H NMR. The TPVA produced contained pendant chains with ester bonds linking the thiol groups to the PVA backbone. Further, hydrogels were synthesized from this TPVA in a reaction between the TPVA and acrylate derivatives of poly(ethylene glycol) using Michael-type addition. The gelation reaction between the TPVA and PEG-acrylate proceeded under physiological conditions in aqueous environment without radical initiators or irradiation. The kinetics of gelation, including gelation time and dynamic modulus, were determined by rheology. The properties of the final hydrogels and cure characteristics were investigated as a function of pH, polymer concentration, molecular weight, degree of PVA and PEG chemical modifications and their ratio in the composition. The gelation time varied from seconds to 30 minutes and the equilibrium elastic modulus (G’) was in the range 500 Pa to 10 kPa.
The hydrogels were rendered degradable by the presence of the ester groups, which are easily hydrolysable and do not require the presence of enzymes for degradation to occur. In addition, the thiol-functional groups impart mucoadhesive properties to the PVA hydrogels, as has been reported elsewhere. The level of mucoadhesion was controlled by the amount of free thiol functionalities remaining uncrosslinked after the hydrogel formation reaction between the TPVA and PEG-acrylate molecules.
The degradability and swellability of these PVA-PEG hydrogels was tested in 1xPBS under ambient conditions. The hydrogels at 3 wt % polymer solids started losing mechanical integrity after 18 days and completely dissolved after 35 days. In addition, a specialized peel test was developed to measure the mucoadhesive properties of the hydrated TPVA hydrogel.

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Density, or the ratio of the mass of an object to its volume, is a commonly reported material parameter. Density is influenced by the chemical composition of the material, crystallinity, and porosity. The chemical composition depends on the elements that make up the material. Plastics are normally composed of hydrocarbons (carbon and hydrogen), which have lower atomic masses and tend to have densities less than 1 g/ml. Metals, which have higher atomic masses, tend to have densities in excess of 1 g/ml. Water at room temperature has a density around 1 g/ml, which is why plastics tend to float, and metals tend to sink in water.

There are two common methods of measuring the density of solid materials. The first involves a density column, which is a tall cylinder containing a mixture of fluids, often alcohol and water, that establishes a gradient in density, with the most dense fluid at the bottom, and smoothly decreases in density as one moves up the column. Floats of known density are placed in the column, where they settle at a height in the column where the surrounding fluid is equal to their density. An unknown piece of a sample is then placed in the column, where it will descend until it reaches a point in the column where its density matches the column density. By interpolating its position relative to the calibration floats, the density of the sample can be determined. This method is described in ASTM D1505.

In the second method, an analytical balance is required. A piece of an unknown sample is weighed, providing its mass. The sample is then immersed in a fluid of known density, such as water or alcohol, where its mass is counteracted by buoyancy forces (see How to Measure Volume), allowing one to measure the volume. The density is then calculated from the ratio of mass to volume. This method is described in ASTM D792.