P YCNOMETRY

Pycnometry is a method of studying the density of a material. The difference between the density of packing between crystalline (more dense) and amorphous (less dense) regions with a polymer, means that this method may also be used to calculate sample crystallinity.

Density (ρ) is a measure of the degree of packing of a material, defined by the volume (v) and mass (m) of a sample, as shown in the equation:

𝜌 = 𝑚 (𝑔) 𝑣 (𝑐𝑚³)

Within the instrument (Figure 3.8), the sample chamber (of known volume) contains the sample cup and sits alongside the “expansion chamber” (also of known volume).

While sealed, pressurised gas enters the sample chamber through a fill valve, pressurising the chamber to a specified pressure (by user). The fill valve is then closed and the pressure (P1) is recorded. A second valve is then opened between the sample and expansion chamber, allowing the pressurised gas to spread between the two chambers. Once the pressure (P2) is recorded the gas can be expelled through a final venting valve. From these measurements the volume (V) of the sample chamber can be calculated using the ‘ideal gas law’, which states that PV = nRT, where n is the number of moles of gas, R is the ideal gas constant and T is the temperature of the

system. Since n and T remain constant when the gas is allowed to expand from the sample cell (cel) into the expansion chamber (exp), 𝑃₁(𝑉_𝑐𝑒𝑙− 𝑉_𝑠) = 𝑃₂(𝑉_𝑒𝑥𝑝+ 𝑉_𝑐𝑒𝑙− 𝑉_𝑠) from which the volume of the sample (s) can be calculated using the equation:

𝑉_𝑠 = 𝑉_𝑐𝑒𝑙 − 𝑉_𝑒𝑥𝑝 𝑃₁⁄𝑃₂− 1

The density of the sample is then calculated using this sample volume and the known mass of the sample.

Figure 3.8: Micromeritics AccuPyc II 1340 pynometer internal schematic

This schematic, taken directly from the Micromeritics Foampyc software (Micromeritics Instrument Corporation 2011), shows the internal layout of the pressurised chambers (sample chamber – with lid – to the left) and venting system.

Accurate measurement of the displacement volume relies on accurate calibration of the chambers. National Institute of Standards and Technology (NIST - USA) traceable calibration spheres allow the volume of both the sample and expansion chambers to be calculated.

In order to obtain an absolute value of crystallinity using density, the density values for fully amorphous (ρa) and totally crystalline (ρc) forms of that material must be known as well as the plasticiser content if this is present in the sample. According to Mishra (2000) the percentage crystallinity (by weight) of a dry (unplasticised) sample may be calculated from sample density (ρs) using the equation:

% 𝐶𝑟𝑦𝑡𝑎𝑙𝑙𝑖𝑛𝑖𝑡𝑦 = 𝜌_𝑐(𝜌_𝑠− 𝜌_𝑎)

𝜌_𝑠(𝜌_𝑐− 𝜌_𝑎)× 100

In the case of cellulose the density values occur within a range for fully amorphous and completely crystalline samples, an average of the range for each will be taken and the values quoted as 1.36 gcm^-3 and 1.59 gcm^-3 respectively (Sun 2005, Derecskei and Derecskei-Kovacs 2006). A complicating factor, however, is the water content of the cellulose samples. Most of the water will be removed prior to the test by the application of dry heat, but tightly bound water molecules associated with the samples may be difficult to remove. If the sample is not completely dry, then the volume of water remaining in the samples needs to be accurately measured.

The aim of this set of experiments is to use the measurement of density to rank various cellulose samples in order of their crystallinity.

MATERIALS AND METHODS

All density studies were carried out with a Micromeritics AccuPyc II 1340 pycnometer, running at 25ºC and using AR grade bottled helium as purge gas at a pressure of 19.5 psi. Sample mass was measured directly prior to testing and again at the conclusion of the run. The 1cm³ sample cup was used for all measurements, holding a mass of around 150-350 mg (depending on the sample type).

Samples used for density tests are: CF – cotton fibre (supplied by P Henry, CSIRO), TF – Tencel® fibre (Lenzing), VF – viscose fibre (supplied by M Pate, CSIRO).

Samples were dried by placing the sample in an oven at 105°C overnight. On removal from the oven, samples were cooled in a desiccator containing silica gel and then rapidly transferred into the aluminium pycnometer cup. The cup was then placed back into the oven for 15 mins to remove any moisture picked up on transfer into the cup. The sample and cup were then cooled again over silica gel. Sample and cup were then weighed and placed directly into the instrument.

RESULTS AND DISCUSSION

Dry samples of cotton, Tencel® and viscose were tested in the pycnometer and over the course of 100 cycles the sample density was determined. Due to instrument equilibration over the first (approximately 50 cycles), the average sample density as

collected over the last 40 cycles of the run was used to calculate the fibre’s cellulosic crystallinity (equation on page 59). The calculated cellulosic crystallinity of each sample is listed in Table III.

Table III: Density and calculated crystallinity of dry cellulosic samples

Fibre Measured Density (gcm^-3) Crystallinity (%)

CF 1.538 80

TF 1.517 72

VF 1.500 65

The density values are consistent with the literature which quotes values for the density of dry cellulose of 1.50-1.55 gcm^-3 (Hearle and Morton 2008). The order of crystallinity calculated from density is also consistent with the literature, listing cotton, Tencel® and viscose from most to least crystalline. This, along with the sorption isotherms in Section 1.4, supports the literature which suggests that chemical processing methods, as used on Tencel® and viscose result in a reduction in crystallinity. This type of chemical treatment would also have the effect of increasing the purity of the cellulose within the fibre. It is worth noting that although impurities such as lignin and pectin, which are of lower density than cellulose, are present in cotton, they are unlikely to impact heavily the calculation of crystallinity in mature fibres due to their minimal overall content within the fibre (Pettolino 2013).